**Native Forest and Climate Change — The Role of the Subtropical Forest, Potentials, and Threats**

Silvina M. Manrique and Judith Franco

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62030

#### **Abstract**

Conference, 2007 (INTELEC 2007); September 30 2007 to October 4 2007; Rome: IEEE;

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024.

54 Greenhouse Gases

The subtropical rainforest of Argentina, called Yungas, has been subjected to rapid de‐ forestation and degradation processes in recent years, especially in the lower district: "Pedemontana Jungle" (PJ; ≤900 m.a.s.l.). In Salta, in the north of the country, the rate of deforestation is around three times higher than the world average. The disappear‐ ance of PJ significantly limits the area of contact between Yungas and Chaco forest, which could have important consequences for natural and cultural biodiversity in the region (the largest number of aboriginal ethnic groups live here, most of which de‐ pend on native forest for their existence and identity). In addition, the loss and degra‐ dation of forests is the second largest sector of greenhouse gas (GHG) emissions to the atmosphere (about 18%), affecting the world climate. We present a synthesis of differ‐ ent studies developed in PJ forests, observing its role as reservoirs of carbon and dis‐ cussing issues that could influence the total capacity of carbon sequestration of the same. This will contribute to build the reliable database on the sequestration potential, which will facilitate standardization of units, reduction of uncertainties, and contribu‐ tion to a more efficient strategy to limit the GHG emission to the environment, provid‐ ing some learning and useful recommendations.

**Keywords:** biomass, carbon sequestration, edge effect, fragmentation, native forest

#### **1. Introduction**

#### **1.1. Deforestation, fragmentation, and climate change**

According to recent studies, the forests covering about 30% of the earth's surface [1] contain 80% terrestrial biomass and provide habitat for about half of the world's known species of plants and animals [2]. Forests provide a wide range of ecological, economic, and social assets, as well as services such as climate regulation through the storage of carbon in complex physical,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

chemical, and biological processes [3–5]. Despite a wide recognition of the importance of native forests, recent data show that the loss of forest cover over the planet (deforestation) in 2000– 2012 was 2.3 million km2 , while the gain (grown or planted) was 0.8 million km2 [6]. Conversely, Keenan et al. reported a rate of 0.08% of forest loss in 2010–2015, while farmland continued expanding in 70% of the countries [1].

Native forests have been affected in terms of not only the total amount of existing surface (deforestation) but also the quality of the remaining fragments (degradation) [5, 7], therefore the biomass availability and its derived flow, which means a source of ecosystem goods and services, has been doubly modified. Several of them, such as soil protection, gas and climate regulation, water regulation, nutrient cycling, providing habitat and refuge, food production, raw materials and genetic resources, the provision of medicinal and ornamental resources, and others related to culture (recreation, aesthetics, and spirituality), are associated with biomass existence and generation [2, 4, 5]. Similarly, there are an increasing number of studies showing the interrelationship between the aboveground and subterranean processes, and particularly among the aboveground biomass (AGB) and soil, links that determine the abundance of species, coexistence, and succession [8, 9]. Therefore, any changes in the biomass, including degradation – although it is a hardly measureable phenomenon [10] – will affect soil charac‐ teristics, which, in turn, will modify reproduction patterns and survival of typical plants in the ecosystem in question and their associated fauna [2, 8, 9].

Deforestation and fragmentation of forests, have been an object of study of the scientific community for many years, but attention to these phenomena has begun to rise from the perspective of their contribution to global warming by greenhouse gas emissions [3, 10–15]. It is recognized that the change in land use (including forest degradation and deforestation) is the second sector of global importance in terms of GHG emissions (so-called LULUCF or land use, land use change and forestry) and is responsible for 20% of total emissions [16]; therefore, it is an important component of human impact on global climate.

Variations in the soil cover are one of the natural and anthropogenic forces that operate on different scales, influencing changes in regional and global climates [3, 13, 16]. Malhi et al. [13] document some interrelations in the Amazon forests, noting that they have a great influence on regional and global climates. They mention that the extraction of water from the soil, through the tree roots up to 10 m depth, and its return to the atmosphere ("perspiration service") is, perhaps, the most important regional ecosystem service. Therefore, the removal of trees through deforestation can become a driver for climate change and a positive feedback for externally forced climate change. In agreement with the other authors, forest loss also results in (i) decreased cloud cover and an increase in insulation; (ii) increase in the reflectance of the earth's surface, approximately offsetting the effect of clouds; (iii) changes in the aerosol loading of the atmosphere from a hyperclean "green ocean" atmosphere to a smoky and dusty continental atmosphere that can modify rainfall patterns; and (iv) changes in the surface roughness (and therefore the wind speed) and a large-scale convergence of atmospheric humidity, which generates precipitation [14, 15]. These large-scale interrelations repeat on lesser scales, although they have not been sufficiently studied.

Deforestation and fragmentation could increase the vulnerability of forests to climate change [2, 3, 17, 18], being two interlinked processes, since deforestation to open up new land for cultivation is concentrated in the periphery of existing forest fragments, reducing them in size and/or making them disappear. Both processes have been recognized as important drivers of biodiversity loss [2, 4, 5, 19–21].

#### **1.2. Climate change in Argentina**

chemical, and biological processes [3–5]. Despite a wide recognition of the importance of native forests, recent data show that the loss of forest cover over the planet (deforestation) in 2000–

Keenan et al. reported a rate of 0.08% of forest loss in 2010–2015, while farmland continued

Native forests have been affected in terms of not only the total amount of existing surface (deforestation) but also the quality of the remaining fragments (degradation) [5, 7], therefore the biomass availability and its derived flow, which means a source of ecosystem goods and services, has been doubly modified. Several of them, such as soil protection, gas and climate regulation, water regulation, nutrient cycling, providing habitat and refuge, food production, raw materials and genetic resources, the provision of medicinal and ornamental resources, and others related to culture (recreation, aesthetics, and spirituality), are associated with biomass existence and generation [2, 4, 5]. Similarly, there are an increasing number of studies showing the interrelationship between the aboveground and subterranean processes, and particularly among the aboveground biomass (AGB) and soil, links that determine the abundance of species, coexistence, and succession [8, 9]. Therefore, any changes in the biomass, including degradation – although it is a hardly measureable phenomenon [10] – will affect soil charac‐ teristics, which, in turn, will modify reproduction patterns and survival of typical plants in the

Deforestation and fragmentation of forests, have been an object of study of the scientific community for many years, but attention to these phenomena has begun to rise from the perspective of their contribution to global warming by greenhouse gas emissions [3, 10–15]. It is recognized that the change in land use (including forest degradation and deforestation) is the second sector of global importance in terms of GHG emissions (so-called LULUCF or land use, land use change and forestry) and is responsible for 20% of total emissions [16]; therefore,

Variations in the soil cover are one of the natural and anthropogenic forces that operate on different scales, influencing changes in regional and global climates [3, 13, 16]. Malhi et al. [13] document some interrelations in the Amazon forests, noting that they have a great influence on regional and global climates. They mention that the extraction of water from the soil, through the tree roots up to 10 m depth, and its return to the atmosphere ("perspiration service") is, perhaps, the most important regional ecosystem service. Therefore, the removal of trees through deforestation can become a driver for climate change and a positive feedback for externally forced climate change. In agreement with the other authors, forest loss also results in (i) decreased cloud cover and an increase in insulation; (ii) increase in the reflectance of the earth's surface, approximately offsetting the effect of clouds; (iii) changes in the aerosol loading of the atmosphere from a hyperclean "green ocean" atmosphere to a smoky and dusty continental atmosphere that can modify rainfall patterns; and (iv) changes in the surface roughness (and therefore the wind speed) and a large-scale convergence of atmospheric humidity, which generates precipitation [14, 15]. These large-scale interrelations repeat on

, while the gain (grown or planted) was 0.8 million km2 [6]. Conversely,

2012 was 2.3 million km2

56 Greenhouse Gases

expanding in 70% of the countries [1].

ecosystem in question and their associated fauna [2, 8, 9].

it is an important component of human impact on global climate.

lesser scales, although they have not been sufficiently studied.

In 2015, Argentina presented its Third National Communication (TNC) on Climate Change [22], with an updated GHG inventory as part of the fulfillment of their assumed commitments to the United Nations Framework Convention on Climate Change (UNFCCC). They inform that national emissions in 2012 imply a 0.88% participation in global emissions (429,437 Gg CO2eq). The six sectors surveyed were as follows: (1) energy (43% of total emissions), (2) industrial processes (3.6%), (3) use of solvents and other products (0%), (4) agriculture and livestock (27.8 %), (5) land use change and forestry (LUCF) (21.1%), and (6) waste (6%). Within the LUCF sector – the third most important – the subsector of "forest and other land conver‐ sion" contributes 67% of emissions.

Of the total native forests, in 2002 (33 million ha), Yungas occupied 11.2% of the surface (3.7 million ha). A TNC report mentions that the loss of native forests in 2002–2010 was 3.5 million hectares (computed in "conversion of forests and other land") corresponding to the 8% loss of Yungas (280,300 ha), which caused a reduction of 7.5% of the total area. The rest of the removed area corresponded to the Chaco region, whose surface involved 70% of the total forests in the country (larger ecosystem) that year.

In effect, the Intergovernmental Panel on Climate Change (IPCC), in which more than 300 scientists from all over the world participate, warned that, in 2014, 4.3% of global deforestation occurred in Argentina [16]. At a local level, the Secretary of Environment for the Nation published, in the same year, the report "Monitoring of the area of native forest in Argentina," pointing out that between November 2007 (when the National Forest Act was enacted) and the end of 2013, 1.9 million hectares were removed – an average of 1 ha/2 min. Eighty percent of the deforestation is concentrated in four provinces: Santiago del Estero, Salta, Formosa, and Chaco [23].

At the same time, variations in local and regional climate had begun to be noticed in the country. The average annual temperature increased from 1960 to 2010 in almost all the northwest subregions (and Cuyo); in many areas (more than 0.5°C), the most notable changes were observed in spring. From 1950 to 2010, the annual average temperature, through the region was 0.6ºC and it reached 0.7°C in Salta and Jujuy [22]. At a national level, the average temperature increases from ½ to 1ºC. The possibility of increasingly intense heat waves has been forecast. In the northwest, an increase of 4–5º is projected by 2030, one of the highest on the planet. In the west and, notably, in the north of the country, there has been a shift toward the extension of dry winters. This could be generating problems with water availability for the populace, more favorable conditions for wildfires in forests and grasslands, as well as stress on livestock. This could bring implications on the biodiversity of the native forest remnants in Yungas [22], and, at the same time, the disappearance of such remnants, which could provide feedback for those changes that are taking place at an atmospheric level.

Improving the understanding of biomass and carbon stocks in forests, therefore, provides valuable information for use land planning and designing comprehensive strategies in the context of global climate change. The purpose of this chapter is to present a synthesis of some of the different works developed in the subtropical forest of the Pedemontana Jungle, based on years of studies in the area. Studies were focused on the northern of the country, noting its role as carbon reservoirs and discussing factors that could influence the carbon sequestration total capacity of the same. The information presented here, without doubt, will contribute to the construction of a reliable database of this potential, which will facilitate standardization of units, reduction of uncertainties, and contribution to a more efficient strategy to limit GHG emissions, providing some learning and useful recommendations. Inasmuch as this ecosystem extends to Venezuela, the results obtained will provide a frame of reference for future studies on this ecological zone. This information is also necessary to improve the understanding of the distribution patterns of biomass and carbon at the global level and to describe patterns of land use. The results presented could guide in designing plans and management policies for these types of forests, at national and international levels.

#### **2. Materials and methods**

#### **2.1. The Yungas ecosystem: Pedemontana Jungle**

The phytogeographic Yungas province borders the Andes mountain range from Venezuela to Argentina [24]. The Argentine Yungas, which constitute a vital habitat for the fundamental role in the regulation of the water basins and protection against erosion, have been subjected to a long history of anthropogenic interventions, especially in low-lying areas, called the Pedemontana Jungles, which have a high agricultural potential [25].

The history of Pedemontana Jungle in the north of Argentina has been closely tied to the railway expansion, necessary for the transport of precious wood, tropical crops, and sugar. More recently, from the 1990s, soybeans won the major role, expanding rapidly in the foothills landscape and its transition to the Chaco plain. The deterioration from the advance of the agricultural frontier, coupled with logging, the commercial bird catching and poaching – among others – are causes for concern because of the almost 5 million hectares that cover the Argentine Yungas, the effectively protected area is only 5% of the total [21].

The Pedemontana Jungle, which stretches from 450 to 900 m.a.s.l. – other authors mention minor ranges [24] – and which represents 25% of the Yungas, has been considered as an ecosystem in danger of extinction, and its deforestation would eliminate 30% of the total Yungas biodiversity [25]. In this region, 120 species of mammals and 8 of the 10 species of neotropical cats are represented. Also, approximately 583 species of birds inhabit it, which represent 60% of the species in Argentina [26]. Likewise, in the Pedemontana Jungle of Argentina and Bolivia, they were identified 18 AICBA (Areas of Importance for the Conser‐ vation of the Birds of Argentina), noting that the AICBA including sectors of the Pedemontana Jungle, have a diversity of birds comparable to the cloud mountain forests (ecological zone of higher altitude than the PJ) and higher than the Chaco forests that surround them [27].

The physiography varies from submountain foothills to alluvial descents, presenting a hilly and wavy topography. The soils present in the study area are according to the taxonomic classification of FAO soils of the Phaeozem Haplic and Luvic type [28]. Soils of luvisol calcium were recorded only on the Coronel Moldes site.

The climograph and altitude for each of the studied sites are shown in Figure 1.

Figure 1. Annual average rainfall (mm), annual average temperature (°C), and altitude (m.a.s.l.) for different sites studied in the Pedemontana Jungle in northern Argentina (Salta and Jujuy provinces). Source: http://es.climate‐data.org/location/145171/ **Figure 1.** Annual average rainfall (mm), annual average temperature (°C), and altitude (m.a.s.l.) for different sites stud‐ ied in the Pedemontana Jungle in northern Argentina (Salta and Jujuy provinces). Source: http://es.climate-data.org/ location/145171/

#### **2.2. Case studies**

0

10

Yungas [22], and, at the same time, the disappearance of such remnants, which could provide

Improving the understanding of biomass and carbon stocks in forests, therefore, provides valuable information for use land planning and designing comprehensive strategies in the context of global climate change. The purpose of this chapter is to present a synthesis of some of the different works developed in the subtropical forest of the Pedemontana Jungle, based on years of studies in the area. Studies were focused on the northern of the country, noting its role as carbon reservoirs and discussing factors that could influence the carbon sequestration total capacity of the same. The information presented here, without doubt, will contribute to the construction of a reliable database of this potential, which will facilitate standardization of units, reduction of uncertainties, and contribution to a more efficient strategy to limit GHG emissions, providing some learning and useful recommendations. Inasmuch as this ecosystem extends to Venezuela, the results obtained will provide a frame of reference for future studies on this ecological zone. This information is also necessary to improve the understanding of the distribution patterns of biomass and carbon at the global level and to describe patterns of land use. The results presented could guide in designing plans and management policies for

The phytogeographic Yungas province borders the Andes mountain range from Venezuela to Argentina [24]. The Argentine Yungas, which constitute a vital habitat for the fundamental role in the regulation of the water basins and protection against erosion, have been subjected to a long history of anthropogenic interventions, especially in low-lying areas, called the

The history of Pedemontana Jungle in the north of Argentina has been closely tied to the railway expansion, necessary for the transport of precious wood, tropical crops, and sugar. More recently, from the 1990s, soybeans won the major role, expanding rapidly in the foothills landscape and its transition to the Chaco plain. The deterioration from the advance of the agricultural frontier, coupled with logging, the commercial bird catching and poaching – among others – are causes for concern because of the almost 5 million hectares that cover the

The Pedemontana Jungle, which stretches from 450 to 900 m.a.s.l. – other authors mention minor ranges [24] – and which represents 25% of the Yungas, has been considered as an ecosystem in danger of extinction, and its deforestation would eliminate 30% of the total Yungas biodiversity [25]. In this region, 120 species of mammals and 8 of the 10 species of neotropical cats are represented. Also, approximately 583 species of birds inhabit it, which represent 60% of the species in Argentina [26]. Likewise, in the Pedemontana Jungle of Argentina and Bolivia, they were identified 18 AICBA (Areas of Importance for the Conser‐

feedback for those changes that are taking place at an atmospheric level.

these types of forests, at national and international levels.

Pedemontana Jungles, which have a high agricultural potential [25].

Argentine Yungas, the effectively protected area is only 5% of the total [21].

**2.1. The Yungas ecosystem: Pedemontana Jungle**

**2. Materials and methods**

58 Greenhouse Gases

60 70 80 ha) Yungas forest Chaco forest All the studies summarized in this chapter were carried out in the province of Salta, in northern Argentina, with the exception of case II, which was developed in the province of Jujuy. The province of Salta has an area of 155,500 km², occupying the sixth position at the national level, and with a value similar to the surface of Nepal, has a population of 1.2 million, making it the eighth most populous out of 23 at the national level.

40 50 Stored (tC/ Of the entire surface occupied by the Yungas ecosystem in the country, 61% of it extends through this province, making it essential to focus on studies in this particular region. Also, Salta has 23% of the total surface of the country's native forests, and deforestation in this province is triple the world average [29].

20 30 CarbonSome of the assumptions, which have been evaluated from various case studies (always focusing on the Pedemontana Jungle), are as follows (more detailed in Table 1):

AGB‐10 AGB‐0 BGB LUV HUV LI SOC

Figure 2. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0,

BGB, HUV, LUV, LI and SOC are explained in the text.



The acronyms AGB10, AGB0, HUV, LUV, LI, SOC, MIC are explained in the text.

**Table 1.** Methodological differences and similarities between the case studies.

#### **2.3. Sampling design**

The methodology used for each of the case studies presented in the next section shows some differences that are summarized in Table 1. In most cases, the data were collected following a random sampling design. Only in case V, the sampling was systematic.

The experimental design used was nested plots. Main plots had a total area of 100 m2 and were rectangular plots. The criterion used to determine sample size for each stratum was an estimation of AGB of trees with a diameter at breast height (dbh) ≥ 10 cm during pre-sampling (90% probability and 20% mean standard error).

**i.** The subtropical rainforests of the country have a greater capacity for carbon seques‐

**ii.** Carbon sequestration in forests disturbed by human activity is lower than in forests less seized by humans, releasing the difference of carbon into the atmosphere. **iii.** The carbon stock, in legally protected forest sectors, is higher than in other sectors without protection located at identical latitude and under similar conditions. **iv.** The potential for carbon sequestration in the Pedemontana Jungle is less if latitude

**v.** The fragmentation of the Pedemontana Jungle generates microclimatic changes at the

Wildlife Reserve of Acambuco and Campo Pizarro

 (AGB0); 250 soil plots (SOC)

50 main plots (AGB10); 50 plots of

50 m2

AGB0 AGB10 BGB SOC

The methodology used for each of the case studies presented in the next section shows some differences that are summarized in Table 1. In most cases, the data were collected following a

Aguaray and General Pizarro

50 main plots (AGB10); 50 plots of

 (AGB00); 250 soil plots (SOC) and 500 microclimatic instantaneous records (MIC)

50 m2

AGB0 AGB10 BGB SOC MIC

Colonia Santa Rosa

; 468 soil plots

78 main plots (AGB10); 78 plots of

50 m2

AGB0 AGB10 BGB SOC MIC

(SOC); 156 microclimatic instantaneous records (MIC)

tration than subtropical dry forests at identical latitude.

edges, which could affect carbon sequestration.

Calilegua

20 main plots (AGB10); 20 plots of

and LI); 120 soil plots (SOC)

 (AGB0); 40 plots of 1 m2

(HUV

50 m2

AGB0 AGB10 LI HUV BGB SOC

**Case** I II III IV V **Legal protection** No Yes Yes and no No No

increases.

60 Greenhouse Gases

**Plot number** 23 main plots

**Carbon pool** AGB0

**2.3. Sampling design**

**Site** Coronel Moldes National Park

(AGB10) for each ecosystem; 23 plots

(AGB0); 23

(LUV);

The acronyms AGB10, AGB0, HUV, LUV, LI, SOC, MIC are explained in the text.

**Table 1.** Methodological differences and similarities between the case studies.

random sampling design. Only in case V, the sampling was systematic.

of 50 m2

AGB10 LI HUV LUV BGB SOC

plots of 5 m2

46 plots of 1 m2 (HUV and LI); 138 soil plots (SOC)

Carbon represents about 50% of the total oven-dried biomass present in forests [32]. Estimation of carbon pools in forests necessarily involves studying the different strata of biomass present in them. In the different studies, the following carbon pools and variables were measured:


Wet weight was recorded on site for LUV, HUV, and LI fractions. Dry weight was determined in the lab (registered after drying in an oven at 80°C until constant weight). The equation introduced by Cairns and coworkers [31], for tropical forest and lower latitudes than 25°, was used. The AGB fraction, also called as "biomass density" when expressed as tons of oven-dried weight per ha [32], is the main source of total biomass in a forest ecosystem. Its relevance as a GHG mitigation option is therefore crucial [11–13]. This fraction was thoroughly assessed using a nondestructive methodology: allometric equations (Table 2).



**k.** Soil humidity (%): This was estimated by two soil samples taken at 10 cm depth per plot.

AGB = tree aboveground biomass (kg oven-dry); SB = stem biomass; S = wood density (oven-dried biomass per green volume, in t/m3 ); D = diameter at breast height (1.3 m above ground, in cm); D30 = diameter at 30 cm above ground; H = total height (m); BGB = belowground biomass (t/ha); OC = concentration of organic carbon in the soil (%); BD = soil bulk density (g/cm3 ) and D = depth of soil (cm); V = total tree volume, in dm3 , included stem, bark, and branches.

**Table 2.** Allometric equations used in this chapter.

The last five parameters called "microclimate factors" were measured in each preset distance, for each transect study, always at midday between 12 p.m. and 2 p.m. In the case of values per site, the different measurements taken were averaged per plot.

#### **2.4. Estimation of biomass and carbon**

weight per ha [32], is the main source of total biomass in a forest ecosystem. Its relevance as a GHG mitigation option is therefore crucial [11–13]. This fraction was thoroughly assessed

**h.** Air relative humidity (%): This was recorded using a psychrometer or hygrometric probe Vaisala HM 34. Reading is immediate and accuracy is ±2%. The sensor is the Humicap

**i.** Air relative temperature (°C): This was registered with a Vaisala HM 34 probe, with temperatures ranging from −20 to +60°C. Measurements were taken at 1.5 m from ground

**j.** Soil temperature (°C): This was measured with a FLUKE 54 II digital thermometer with accuracy ranging from 0.05% + 0.3°C. Measurements were taken at 10 cm depth.

**k.** Soil humidity (%): This was estimated by two soil samples taken at 10 cm depth per plot.

**Authors Carbon pool Equation No.**

Diameter (30 cm)=1.235×dap + 0.002×(dap)2

BGB BGB=exp(−1.0587 + 0.8836×ln(AGB)) (5)

SOC SOC=OC× BD×D (6)

AGB=exp( <sup>−</sup>7.114 <sup>+</sup> 2.276\*ln(D30)) (4)

AGB10 and AGB0 AGB=exp( −2.977 + ln(S.D2

AGB10 and AGB0 AGB=0.112(S.D2

AGB10 and AGB0 AGB=0.0673 + (S.*D* <sup>2</sup>

) and D = depth of soil (cm); V = total tree volume, in dm3

AGB10 and AGB0 AGB=exp( −2.4090 + 0.9522 ln(S.D2

Sevola (1975) V *V* = −2.2910 + 0.0558×log (*D* <sup>2</sup> ×*H* ) (7) Sevola (1975) V *V* = −3.2794−0.0734×log*H* <sup>2</sup> + 1.0580×log(*D* <sup>2</sup> ×*H* ) (8) Sevola (1975) V *V* = −2.4385 + 0.9560×log(*D* <sup>2</sup> ×*H* ) −0.80350×log(*H* / *D*) (9)

AGB = tree aboveground biomass (kg oven-dry); SB = stem biomass; S = wood density (oven-dried biomass per green

= total height (m); BGB = belowground biomass (t/ha); OC = concentration of organic carbon in the soil (%); BD = soil

); D = diameter at breast height (1.3 m above ground, in cm); D30 = diameter at 30 cm above ground; H

): A LICOR 250 pyranometer was used with a silicon sensor

. The measures of global radiation readings are precise to

.H)) (1)

.H.)0.916 (3)

.*H* )0.976 (10)

, included stem, bark, and branches.

.H.)) (2)

using a nondestructive methodology: allometric equations (Table 2).

type. Measurements were taken at 1.5 m from ground level.

**g.** Solar radiation intensity (W/m2

±5%.

62 Greenhouse Gases

level.

Chave et al. (2005)

Brown et al. (1989)

Chave et al. (2005)

Gehring et al. (2004)

Cairns et al. (1997)

Macdicken, (1997)

Chave et al. (2014)

volume, in t/m3

bulk density (g/cm3

AGB

**Table 2.** Allometric equations used in this chapter.

with a resolution of 0.1 W/m2

Once field measurements were carried out, the data were computed clerically, carrying out the biomass estimate for each compartment, transforming it into carbon values (factor of 0.5 [32]) and achieving the sum of all the carbon pools. All equations used are shown in Table 2. Equation (1) was developed by Chave et al. [33] for "moist forest stand," while equation (3), by the same authors, was developed for "dry forest stands" (applied to the Chaco). Equation (10) was recently developed by these authors and was applied to the *Anadenatnhera colubrina* and *Cedrela angustifolia* species, for which no specific equations were found. Equation (4) was applied only in vines and required converting the dbh into diameters at 30 cm height, and then entering that value into the equation [34]. In the case of volumetric equations (7, 8, and 9) [35], the total biomass conversion was carried out by multiplying the total volume by the basic density of each species. Equation (7) was then applied to the *Calycophyllum multiflorum* species, equation (8) to *Phyllostylon rhamnoides*, and equation (9) to *Astronium urundeuva*, all equations being developed in the region.

The basic wood densities (dry) for different species were obtained from Ref. [36]. A basic density value obtained from the weighted average of the densities of each site's species was used for the species that for various reasons could not be identified. For estimation of SOC (soil organic carbon), equation (6) was used [30]. For data analysis, the nonparametric type test was chosen. We used the INFOSTAT® software, and a value of 0.05 was considered significant.

#### **3. Results and discussion**

#### **3.1. Effect of temperature and humidity on the carbon stock: dry and humid subtropical forests at the same latitude**

Contributors: Manrique, S.M. and Franco, J.

#### **The subtropical moist forests of the country have a greater capacity of carbon sequestration than subtropical dry forests at identical latitude.**

As was mentioned, the Chaco ecosystem is the largest surface area at the national level. It was interesting to compare facets of this ecosystem with the Yungas Pedemontana Jungle with regard to the potential for carbon sequestration at the same latitude. The work was carried out in the municipality of Coronel Moldes (25°16′00″ South latitude and 65°29′00″ West longi‐ tude), 60 km south of the capital of the province of Salta.

The province's climate is defined as subtropical mountainous with a dry season. However, the topography does allow the development of contrasting environments. Thus, the moist winds from the southeast enter the province and release their moisture from submountainous ranges that make up the sub-Andean hills in the north-central region of the country. This allows the spread of vegetation, which is a unique environment that runs along elevations in different altitudes, forming a north–south strip. The Chaco ecosystem develops on the plain that extends from the center of the country to the East, and in Salta two districts are exhibited: the semiarid Chaco and the mountain Chaco. Precipitation decreases as it moves eastward, shrinking from more than 650–700 mm per year in the Pedemontana Jungle ecosystem to values of less than 460 mm in the Chaco ecosystem. Temperatures also suffer a slight increase as it moves west away from the mountains, which have the moisture [37], marking isotherms in the range of tenths of degrees, as the distance between the mountains and the eastern point increases.

The starting points are corroborated in this study: the most humid ecosystem shows a carbon stock 43% larger than that stored in the driest ecosystem (Table 3). In the case of Yungas, the AGB fraction means almost 80% of the total biomass, although the greater fraction (AGB10) alone implies 71%, leaving the AGB0 a reduced participation. The BGB means more than 16% of total biomass, and the rest if divided between the LI (about 3%), the LUV (with almost 2%) and lastly, the negligible participation of HUV (0.1%). For the Chaco, the fraction AGB provides more than 71% of the total biomass, where the trees of larger diameters (AGB10) mean 66% of this contribution. In this environment, the BGB takes on greater importance (with more than 21%), and is followed by – in the identical order shown in the Yungas environment – LI fraction (3.2%), LUV (2.6%), and HUV (0.6%) (Figure 2). Figure 1. Annual average rainfall (mm), annual average temperature (°C), and altitude (m.a.s.l.) for different sites studied in the Pedemontana Jungle in northern Argentina (Salta and Jujuy provinces). Source: http://es.climate‐data.org/location/145171/

Figure 2. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, **Figure 2.** Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, BGB, HUV, LUV, LI and SOC are explained in the text.

BGB, HUV, LUV, LI and SOC are explained in the text.

Clearly, the AGB and SOC fractions are the two largest contributors in the two ecosystems. In Yungas, the AGB represents 48% of the total fixed carbon, while the SOC contributes 39%. In the case of the Chaco, 33% of the total carbon in the ecosystem is concentrated in AGB, while 54% remains captured in SOC. Soil is an important reservoir of carbon, becoming the most important fraction in dry environments. However, when we compare the absolute values of SOC in both environments, the soil shows a significant relationship with the vegetation found on the surface. In Yungas, it is 63 tC/ha, while in Chaco it is 50 tC/ha.

that make up the sub-Andean hills in the north-central region of the country. This allows the spread of vegetation, which is a unique environment that runs along elevations in different altitudes, forming a north–south strip. The Chaco ecosystem develops on the plain that extends from the center of the country to the East, and in Salta two districts are exhibited: the semiarid Chaco and the mountain Chaco. Precipitation decreases as it moves eastward, shrinking from more than 650–700 mm per year in the Pedemontana Jungle ecosystem to values of less than 460 mm in the Chaco ecosystem. Temperatures also suffer a slight increase as it moves west away from the mountains, which have the moisture [37], marking isotherms in the range of tenths of degrees, as the distance between the mountains and the eastern point increases.

The starting points are corroborated in this study: the most humid ecosystem shows a carbon stock 43% larger than that stored in the driest ecosystem (Table 3). In the case of Yungas, the AGB fraction means almost 80% of the total biomass, although the greater fraction (AGB10) alone implies 71%, leaving the AGB0 a reduced participation. The BGB means more than 16% of total biomass, and the rest if divided between the LI (about 3%), the LUV (with almost 2%) and lastly, the negligible participation of HUV (0.1%). For the Chaco, the fraction AGB provides more than 71% of the total biomass, where the trees of larger diameters (AGB10) mean 66% of this contribution. In this environment, the BGB takes on greater importance (with more than 21%), and is followed by – in the identical order shown in the Yungas environment – LI fraction

Figure 1. Annual average rainfall (mm), annual average temperature (°C), and altitude (m.a.s.l.) for different sites studied in the Pedemontana Jungle in northern Argentina (Salta and Jujuy

> Yungas forest Chaco forest

Figure 2. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0,

**Figure 2.** Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, BGB, HUV, LUV, LI

Clearly, the AGB and SOC fractions are the two largest contributors in the two ecosystems. In Yungas, the AGB represents 48% of the total fixed carbon, while the SOC contributes 39%. In the case of the Chaco, 33% of the total carbon in the ecosystem is concentrated in AGB, while

AGB‐10 AGB‐0 BGB LUV HUV LI SOC

BGB, HUV, LUV, LI and SOC are explained in the text.

0

and SOC are explained in the text.

10

20

30

Carbon

Stored (tC/ ha)

40

50

60

70

80

64 Greenhouse Gases

(3.2%), LUV (2.6%), and HUV (0.6%) (Figure 2).

provinces). Source: http://es.climate‐data.org/location/145171/


**Table 3.** Carbon stock (tC/ha) in both ecosystems, Chaco and Yungas, in Coronel Moldes, Salta, Argentina.

Viglizzo and Jobbágy [21] point out that the carbon stocks in the biomass and in the organic fraction of the soil in Argentina vary from one ecoregion to another. The carbon stock in biomass is directly associated with the availability of vegetation biomass. In the tropical and subtropical regions of Argentina (e.g., Yungas), more than 50% of total carbon is found stored in the AGB fraction, which makes this element vulnerable and easily appropriable by humans. This relationship falls dramatically in areas dominated by grasslands/pastures (e.g., Chaco), and even more (without reaching 10%) in intensively cultivated ecosystems.

In Yungas, the average height was 11 m and average dbh was 17.6 cm, both higher than those for Chaco, although still lower than figures cited for pristine Yungas ecosystem [24, 25, 38]. Estimations made for tropical humid forests around the world range from 150 to 192 t/ha for closed, undisturbed forests and around 50 t/ha for open forests [39]. Certainly, different factors may be influencing these differences (rainfall, soil type and site features, topography, etc.) [32, 33, 39, 40]. Moreover, the structure of the forest in the Yungas area included in this study was clearly disturbed by humans and livestock. Numerous recent and decomposing stumps were found and there were unambiguous signs of wandering animals and persons. *Solanum riparium* was also abundant in this area, a species normally dispersed by wild animals or cattle. The appearance of typically Chaco species in sections of Yungas forest is probably a sign of human intervention in this region [24, 38].

Our results suggest that forest degradation is detectable not only in Yungas but also in Chaco. In environments similar to Chaco, discrepancies between these results (lower) and estimations made in similar environments in other forests of the world might be due to structural differ‐ ences, altitude, latitude and humidity, gradients (24, 32, 33). However, in our case the level of degradation exerted by human activity in this environment might also be responsible for the discrepancies [20, 21, 41] (further details refer to [43]).

Economic activities such as agriculture and logging, which take place in these ecosystems, are arguably not respecting their carrying capacity. Local institutions do not seem to be capable of stopping, controlling, or regulating these activities. Whether entering into a market-based system like the one promoted by the Kyoto Protocol will be part of the solution to the problem of deforestation and conservation of local native forests remains to be seen. Decisions are highly political and many times the relevant decision makers are thousands of kilometers away. No decisions affecting the future of these forests should be taken until agreements on this issue are reached or until judiciary processes are properly finished. Competing claims on the ownership of the forestland, the products of the forests, and the provision of ecosystem services must be taken into consideration in a comprehensive forest management.

#### **3.2. Effect of human influence on the carbon stock in forests**

Contributors: Gallucci, G.B. and Manrique, S.M.

#### **Carbon sequestration in forest disturbed by human activity is lower than in forests less seized by humans.**

In case I, we identified that studied forest sectors clearly show human influence as a factor of degradation of the original structure of the same type. In this case study, it was interesting, particularly, to assess this difference and try to quantify it for samples of the same Pedemon‐ tana Jungle ecosystem, but this time as a protected area: Calilegua National Park (23°27'–23°45' South latitude and 64°33'–64° 52'0" West longitude). The park was created in 1979 to protect a representative sector of the Yungas and to protect the headwaters of the Calilegua streams, which are a part of the San Francisco River basin, and provide water to neighboring crops in the protected area. With an area of 76,320 ha, it is the largest national park in the Argentine Northwest. It is approximately 165 km from the city of Salta.

We studied two areas of the park (north and south sectors) separated by only 50 km but which have different accessibility to human influence. The north sector surrounding the town of Caimancito has been invaded by oil companies, which have conducted exploration activities in the area, and therefore have dissected the forest, leaving open "choppings" or paths of prospecting. This has led to the accessibility of nearby residents who have taken advantage of the forest and even have led their animals to graze there. In the south, on the other hand, exploration activities were not carried out and therefore, even if villagers could have accessed the site, on its more sheltered side (the other side of rivers that flow through the park), a better conservation has been maintained, which can be seen in the large, heavily wooded trees, and the high forest value that is still there. Surely, the presence of Park Rangers (Aguas Blancas section) in this sector has helped much in this protection.

Two sectors that maintain homogeneous topographic, edaphic, and climatic conditions were selected. Both sectors were compared through analysis of average annual rainfall records (56 years series) without finding statistically significant differences (*H* = 0.01, *p* > 0.999). Records of minima and maxima were also analyzed. The series of annual average temperatures were not statistically different (*H* = 0.16, *p* = 0.686). In the case of edaphic variables, existing carto‐ graphic studies allowed us to associate both sectors with the same series of soils. Organic matter samples taken in the area showed no significant differences (*H* = 4.71, *p*=0.210). It was assumed that both sectors had identical site conditions.

We evaluated the same carbon pools as in case I with the exception of LUV, which had no relevant participation in the previous case, and therefore it was not included in the pursuit of reducing the fieldwork effort and costs.

The obtained results allow us to advance with the basic assumption: the north sector, subject to anthropogenic influence, it showed a carbon stock 23% lower than the south sector, which had less accessibility and a better state of conservation (Table 4). These differences were statistically significant (*H* = 11.20, *p* < 0.001) only for the AGB stratum, but not for the other strata studied nor for the total carbon stock. Under similar conditions of climate, soil, geo‐ morphology, altitude, and latitude, the human influence could explain these differences, as the AGB stratum is the easiest to appropriate by humans [10, 17, 19, 21]. The AGB make the largest contribution in both sectors to the carbon stock (53, 55%), followed by SOC (28–31%) and finally BGB (8–10%) depending on the sector analyzed (Figure 3). provinces). Source: http://es.climate‐data.org/location/145171/ Figure 2. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, BGB, HUV, LUV, LI and SOC are explained in the text.

different sites studied in the Pedemontana Jungle in northern Argentina (Salta and Jujuy

away. No decisions affecting the future of these forests should be taken until agreements on this issue are reached or until judiciary processes are properly finished. Competing claims on the ownership of the forestland, the products of the forests, and the provision of ecosystem

**Carbon sequestration in forest disturbed by human activity is lower than in forests less**

In case I, we identified that studied forest sectors clearly show human influence as a factor of degradation of the original structure of the same type. In this case study, it was interesting, particularly, to assess this difference and try to quantify it for samples of the same Pedemon‐ tana Jungle ecosystem, but this time as a protected area: Calilegua National Park (23°27'–23°45' South latitude and 64°33'–64° 52'0" West longitude). The park was created in 1979 to protect a representative sector of the Yungas and to protect the headwaters of the Calilegua streams, which are a part of the San Francisco River basin, and provide water to neighboring crops in the protected area. With an area of 76,320 ha, it is the largest national park in the Argentine

We studied two areas of the park (north and south sectors) separated by only 50 km but which have different accessibility to human influence. The north sector surrounding the town of Caimancito has been invaded by oil companies, which have conducted exploration activities in the area, and therefore have dissected the forest, leaving open "choppings" or paths of prospecting. This has led to the accessibility of nearby residents who have taken advantage of the forest and even have led their animals to graze there. In the south, on the other hand, exploration activities were not carried out and therefore, even if villagers could have accessed the site, on its more sheltered side (the other side of rivers that flow through the park), a better conservation has been maintained, which can be seen in the large, heavily wooded trees, and the high forest value that is still there. Surely, the presence of Park Rangers (Aguas Blancas

Two sectors that maintain homogeneous topographic, edaphic, and climatic conditions were selected. Both sectors were compared through analysis of average annual rainfall records (56 years series) without finding statistically significant differences (*H* = 0.01, *p* > 0.999). Records of minima and maxima were also analyzed. The series of annual average temperatures were not statistically different (*H* = 0.16, *p* = 0.686). In the case of edaphic variables, existing carto‐ graphic studies allowed us to associate both sectors with the same series of soils. Organic matter samples taken in the area showed no significant differences (*H* = 4.71, *p*=0.210). It was

We evaluated the same carbon pools as in case I with the exception of LUV, which had no relevant participation in the previous case, and therefore it was not included in the pursuit of

services must be taken into consideration in a comprehensive forest management.

**3.2. Effect of human influence on the carbon stock in forests**

Northwest. It is approximately 165 km from the city of Salta.

section) in this sector has helped much in this protection.

assumed that both sectors had identical site conditions.

reducing the fieldwork effort and costs.

Contributors: Gallucci, G.B. and Manrique, S.M.

**seized by humans.**

66 Greenhouse Gases

Figure 3. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, **Figure 3.** Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, BGB, HUV, LI and SOC are explained in the text.


**Table 4.** Carbon stock (tC/ha) in both sectors, north and south, in Calilegua, Jujuy, Argentina.

BGB, HUV, LI and SOC are explained in the text.

Against the results, there is an urgent need to review the administration and safeguards for the Calilegua National Park, with a reinforcement of the Corps of Rangers in the area (currently with few people that must patrol the whole park). Other authors are agreed that the declaration to protect does not always mean adequate protection [43, 44]. The acquisition of more financial resources for the protected areas should be carried out in the light of a strict management plan and monitoring. Poaching, livestock grazing, and logging without authorization – with the thinning out of valuable wood species – must be eradicated from the core area, so that the Park can fulfill its role with the conservation of biodiversity, which has been included in the international statement "Yungas Biosphere Reserve."

#### **3.3. Effect of legal protection on the ecosystem**

Contributors: Manrique, S.M.; Vacaflor, P.; Fernández, M. and Franco, J.

#### **The carbon stock in legally protected forest sectors is higher than in unprotected sectors located at the same latitude and under the same conditions.**

In case II, two sectors of the legally protected Pedemontana Jungle were analyzed, which clearly show differences between them in their accessibility to human influence. It became interesting to continue in this line of study, exploring if the trend found in the former case could be due to a particular situation in the Calilegua National Park. In this case study, we sought to observe comparative sectors inside and outside legally protected regions located at the same latitude and altitude, and under the same conditions. We started to identify protected areas in the province which shelter samples from the Pedemontana Jungle. We finally worked in and out of the Provincial Reserve of Flora and Fauna of Acambuco (PRFFA) (22°12'38.5" South latitude and 63°56'23.1" West longitude) and in the National Reserve of Campo Pizarro (NRCP) (24°11'54.87 and 24°14'21.7" South latitude, and 64° 7'27.00" and 64° 9'23.79" West longitude). The creation of PRFFA dates back to 1979, and currently has an area of 32,000 ha. It is approximately 470 km from the city of Salta to PRFFA. In the case of NRCP, it was created in late 1995 with an area of 25,000 ha, and soon after a process of reversal and social conflict, the NRCP ended up with an area of 21,000 ha. It is approximately 280 km from Salta. In this study, efforts were concentrated in the carbon pools considered most significant in the prior cases, eliminating HUV and LI from the samples.

The results show that, on average, the carbon stock is similar in protected and nonprotected areas (Table 5). Having considered the average of all sectors included in the Reserves and the average of all the studied sectors not protected in them, no significant differences were found (*H* = 0.85, *p*= 0.356), by even analyzing just AGB separately (*H* = 0.98, *p* = 0.322). The initial assumption cannot be confirmed: no case shows that the legal protection has caused differences in the ecosystem it protects, neither favoring nor against. Yet we see different values if we consider the samples of the north sector and south sector separately, as will be discussed in the following section.


**Table 5.** Carbon stock (tC/ha) in both sectors, protected and unprotected forest, in Acambuco and Campo Pizarro, Salta, Argentina.

In terms of the importance of each of the studied carbon pools (Figure 4), the carbon fixed at the fraction of AGB returns to be larger than the fixed carbon in the soil (SOC). Figure 3. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, BGB, HUV, LI and SOC are explained in the text.

Figure 1. Annual average rainfall (mm), annual average temperature (°C), and altitude (m.a.s.l.) for different sites studied in the Pedemontana Jungle in northern Argentina (Salta and Jujuy

Figure 2. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0,

provinces). Source: http://es.climate‐data.org/location/145171/

BGB, HUV, LUV, LI and SOC are explained in the text.

and monitoring. Poaching, livestock grazing, and logging without authorization – with the thinning out of valuable wood species – must be eradicated from the core area, so that the Park can fulfill its role with the conservation of biodiversity, which has been included in the

**The carbon stock in legally protected forest sectors is higher than in unprotected sectors**

In case II, two sectors of the legally protected Pedemontana Jungle were analyzed, which clearly show differences between them in their accessibility to human influence. It became interesting to continue in this line of study, exploring if the trend found in the former case could be due to a particular situation in the Calilegua National Park. In this case study, we sought to observe comparative sectors inside and outside legally protected regions located at the same latitude and altitude, and under the same conditions. We started to identify protected areas in the province which shelter samples from the Pedemontana Jungle. We finally worked in and out of the Provincial Reserve of Flora and Fauna of Acambuco (PRFFA) (22°12'38.5" South latitude and 63°56'23.1" West longitude) and in the National Reserve of Campo Pizarro (NRCP) (24°11'54.87 and 24°14'21.7" South latitude, and 64° 7'27.00" and 64° 9'23.79" West longitude). The creation of PRFFA dates back to 1979, and currently has an area of 32,000 ha. It is approximately 470 km from the city of Salta to PRFFA. In the case of NRCP, it was created in late 1995 with an area of 25,000 ha, and soon after a process of reversal and social conflict, the NRCP ended up with an area of 21,000 ha. It is approximately 280 km from Salta. In this study, efforts were concentrated in the carbon pools considered most significant in the prior

The results show that, on average, the carbon stock is similar in protected and nonprotected areas (Table 5). Having considered the average of all sectors included in the Reserves and the average of all the studied sectors not protected in them, no significant differences were found (*H* = 0.85, *p*= 0.356), by even analyzing just AGB separately (*H* = 0.98, *p* = 0.322). The initial assumption cannot be confirmed: no case shows that the legal protection has caused differences in the ecosystem it protects, neither favoring nor against. Yet we see different values if we consider the samples of the north sector and south sector separately, as will be discussed in

**Sector Average Standard deviation**

**Table 5.** Carbon stock (tC/ha) in both sectors, protected and unprotected forest, in Acambuco and Campo Pizarro,

Protected forest 203 74 Unprotected forest 213 82

international statement "Yungas Biosphere Reserve."

Contributors: Manrique, S.M.; Vacaflor, P.; Fernández, M. and Franco, J.

**located at the same latitude and under the same conditions.**

**3.3. Effect of legal protection on the ecosystem**

68 Greenhouse Gases

cases, eliminating HUV and LI from the samples.

the following section.

Salta, Argentina.

**Figure 4.** Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, BGB and SOC are explained in the text.

The inclusion of Pedemontana Jungle sectors within legal protection figures has not resulted in benefits in terms of their ability to sequester carbon in the different carbon pools. However, this consideration is not conclusive in the role of protected areas. Pedemontana Jungle sectors, within and without the protected areas, could be similar in their capacity to sequester carbon in two possible situations: (i) a good general ecosystem condition level, which still remains a certain continuity of ecosystem, and therefore, either inside or outside the Reserve, is of similar forest samples and show no particular features nor different structural configurations; (ii) a poor state of conservation, which has equally affected protected and nonprotected areas, imprinting similar features in the different sectors, by simultaneous intervention in the different forest sectors. A more in-depth study of other ecosystem variables would perhaps lean toward one alternative or another. However, the different log of measured carbon stock in case II in Calilegua National Park, or the degradation features in the two types of ecosystems (Yungas and Chaco), which were observed in case I, clearly indicates that the forests have not received the attention they should have over the years.

Therefore, these thoughts should be a trigger to continue with deeper and more comprehensive evaluation and to draw attention to the need to review and update control schemes and monitoring of native forests – mainly protected areas. The global community has recognized the importance of forests for biodiversity, and has prioritized the preservation of forest biodiversity and ecosystem functions through multiple multilateral agreements and processes. For example, the Aichi Biodiversity Targets established by the Convention on Biological Diversity (CBD) in its strategic plan include halving the rate of loss of natural habitats including forests (target 5) and conserving 17% of terrestrial areas through effectively and equitably managed, ecologically representative, and well-connected systems of protected areas (target 11). Currently, designating protected areas is one of the primary strategies for conserving biodiversity. Different authors have discussed the increase in protected areas over the past century; however, they find that many key biodiversity areas are not adequately covered by protected area status [44].

The always-protected system areas will be limited to preserve all the original diversity, but even so, it is imperative that these areas exist and continue to expand with scientific criteria.

#### **3.4. Effect of latitude and altitude on the carbon stock**

Contributors: Manrique, S.M.; Vacaflor, P. and Fernández, M.

#### **The potential of carbon sequestration in the Pedemontana Jungle is less if the latitude increases.**

In case III, the average carbon stock in protected and unprotected areas was approximately similar, although with different values from cases I to II. This led to the analysis of the position within the ecosystem of the studied sites. In case I, with 162 tC/ha, the forest is at 25°16 and 65°29. In case II, with 221–272 tC/ha, the forest is approximately between 23°27 and 64°33. Analyzing other sectors located at different latitudes could confirm the trend of higher values of biomass and the north sector of the Pedemontana Jungle (e.g., Calilegua showing a value 100% greater than the Coronel Moldes value) and values that decrease toward the south. It was interesting, therefore, to explore case III results separately, taking as a northern sector, the plots carried out near Aguaray (22°12 and 63°56) and, as a southern sector, those carried out near General Pizarro (24°11 and 64°7).


RI = radiation intensity; RH = relative humidity; RT = relative temperature; SH = soil moisture; ST = soil temperature. Mean and range for each variable. Means followed by different letters (a, b) within the same column indicate statistically significant differences (P <0.05).

#### **Table 6.** Average climatic conditions.

The loss of species diversity and conditions of humidity and altitude from north to south, along the gradient in which the Pedemontana Jungle extends within Argentina, has been previously documented [27, 40]. Therefore, it was interesting to know in this case if this trend was also clearly reflected in the carbon stocks of the studied sectors of the Pedemontana Jungle. If the previous studies, the participation of the HUV and LUV strata was between 0.01% and 0.02% and the LI carbon pool was between 1.5% and 3%. Therefore, in this study efforts were concentrated in the strata of AGB, BGB, and SOC. The chosen sectors show the average weather conditions (for the same season, day, and year), which differ significantly in air relative temperature (RT), moisture and soil temperature (SM and ST, respectively) (see Table 6).

Diversity (CBD) in its strategic plan include halving the rate of loss of natural habitats including forests (target 5) and conserving 17% of terrestrial areas through effectively and equitably managed, ecologically representative, and well-connected systems of protected areas (target 11). Currently, designating protected areas is one of the primary strategies for conserving biodiversity. Different authors have discussed the increase in protected areas over the past century; however, they find that many key biodiversity areas are not adequately covered by

The always-protected system areas will be limited to preserve all the original diversity, but even so, it is imperative that these areas exist and continue to expand with scientific criteria.

**The potential of carbon sequestration in the Pedemontana Jungle is less if the latitude**

In case III, the average carbon stock in protected and unprotected areas was approximately similar, although with different values from cases I to II. This led to the analysis of the position within the ecosystem of the studied sites. In case I, with 162 tC/ha, the forest is at 25°16 and 65°29. In case II, with 221–272 tC/ha, the forest is approximately between 23°27 and 64°33. Analyzing other sectors located at different latitudes could confirm the trend of higher values of biomass and the north sector of the Pedemontana Jungle (e.g., Calilegua showing a value 100% greater than the Coronel Moldes value) and values that decrease toward the south. It was interesting, therefore, to explore case III results separately, taking as a northern sector, the plots carried out near Aguaray (22°12 and 63°56) and, as a southern sector, those carried out

**) RH (%) RT (°C) SM (%) ST (°C)**

(0.02–0.44) (21.7–86.7) (16.6–36.55) (3.15–17.25) (20.15–25)

(0.02–0.13) (16.35–55) (25.4–37.8) (2.3–14.4) (22.85–30.8)

RI = radiation intensity; RH = relative humidity; RT = relative temperature; SH = soil moisture; ST = soil temperature. Mean and range for each variable. Means followed by different letters (a, b) within the same column indicate statistically

The loss of species diversity and conditions of humidity and altitude from north to south, along the gradient in which the Pedemontana Jungle extends within Argentina, has been previously documented [27, 40]. Therefore, it was interesting to know in this case if this trend was also clearly reflected in the carbon stocks of the studied sectors of the Pedemontana Jungle. If the

North sector 0.065 a 43.55 a 26.94 a 9.9 a 22.09 a

South sector 0.065 a 36.9 a 31.66 b 7.24 ab 24.2 b

protected area status [44].

**increases.**

70 Greenhouse Gases

**3.4. Effect of latitude and altitude on the carbon stock**

near General Pizarro (24°11 and 64°7).

**Sector RI (W/m2**

significant differences (P <0.05).

**Table 6.** Average climatic conditions.

Contributors: Manrique, S.M.; Vacaflor, P. and Fernández, M.

Studies of carbon stock results show that the two sectors are clearly separated in terms of their potential. The northern sector has the largest records of total carbon with an average of 242 tC/ ha, while the southern sector registers an average 28% lower (Table 7) with statistically significant differences (*H* = 12.38; *p* < 0.01). These values can be associated with different microclimates, possibly generated by a latitude effect, whose influence on climatic variables can be seen in Table 6. In all cases, the differences are in favor of a cooler, more humid climate in the northern sector and warmer and drier in the southern. Although the number of analyzed sectors in a latitudinal gradient in the Pedemontana Jungle (narrow strip of north–south direction), are not representative of the whole distribution, the data can be interpreted in light of the existing scientific studies in the area [25, 27, 40].

Once again, the two carbon pools that make a greater contribution to the total ecosystem carbon stock are AGB and SOC, being greater in the case of the northern sector, meaning 52% and 34% of the total carbon stock, respectively (Figure 5). This implies that more than 86% of total carbon is concentrated in these two fractions. In the southern sector, the participation of these carbon pools is 47% and 38% for AGB and SOC, respectively, but with greater involvement of the SOC carbon pool in this case.

BGB, and SOC are explained in the text. **Figure 5.** Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0, BGB, and SOC are explained in the text.

Radiation Intensity (RI) Relative Humidity (RH) Soil Temperature (ST) Soil Moisture (SM)

Figure 5. Carbon stock and contribution of each carbon pool studied. The acronyms AGB10, AGB0,

Relative Temperature (RT)

Figure 6. Microclimatic variables studied in fragments from edge to inside. Values are expressed in relative terms as a percentage of value at the edge (considered as 100%). The units are the

0 13 26 52 105 210

Distance from the edge (m)

following: RI= W/cm2; ST= °C; RT=°C; RH= %; SM= %.

0

100

200

300

400

Percentage (%)

500

600

700

800


**Table 7.** Carbon stock (tC/ha) in both sectors, north and south, in Salta, Argentina.

However, always considering the altitude of the Pedemontana Jungle, as the latitude increases, the altitude decreases in general terms. It has been recognized that the floral changes are influenced by complex interactions of weather and edaphic variables in Yungas altitude ranges [24, 25, 40]. Beyond the fact that the associated variables of increasing altitude (which in this study varies between 22° S and 24° S) and/or altitude (which varies between 500 and 700 m.a.s.l. in the southern sector and between 700 and 900 m.a.s.l. in the north) would more or less determine overt changes at the level of species and ecosystems, such variations exist without doubt, and they are defining two sectors of the same forest in terms of carbon sequestration potential.

These observations indicate that it is essential to preserve sectors of different latitudes and altitudes in the Pedemontana Jungle, since there are intrinsic factors that are defining differ‐ ential features in the biomass and carbon stock, as well as, in every ecosystem functions associated with these particular conditions [40]. Other authors have already pointed out that the recommendation in all cases is to maintain connectivity of Yungas in distribution, safe‐ guarding different sectors of the Pedemontana Jungle, varying in latitude and altitude [27].

Human influence, not analyzed in this study, will, no doubt, imprint differential features over time if their presence is not restricted, since we have observed signs of livestock and logging in the different studied areas. In the southern sector, where the Pedemontana Jungle has been deeply fragmented and immersed in an array of crops, it is considered that there might be a microclimatic influence on the fragments by the existence of rough edges [7, 18]. This aspect will be dealt with in the following section.

#### **3.5. Effect of fragmentation on the carbon stock**

Contributors: Manrique, S.M.

#### **Fragmentation of the Pedemontana Jungle generates microclimate changes at its edge, which could affect the sequestration of the carbon stock.**

The fragmentation of forests, reducing surface and insulation, exposes organisms, which remain in the fragment, to conditions differing from their ecosystem, which is primarily manifested in the contact between two different environments, which has been defined as "edge effect" [18], and that impact toward the forest interior.

Microclimatic changes caused as a consequence of contrasting conditions between the remnant forest and the adjacent field, subjected to different uses (cultivation, planting, and pastures), would seem to be the most immediate and apparent fragmentation changes [7]. Several authors have recognized that, at the edge of the fragments is an environmental gradient toward the interior: generally brightness, evapotranspiration, temperature, and wind speed decrease, while soil moisture and humidity increase toward the interior of the fragment. Biological changes could then arise as a result of these changes in the microcli‐ mate of the fragment edges [7, 18].

**Sector Average Standard deviation**

Forest in the north (22° latitude) 242 96 Forest in the south (24° latitude) 174 68

However, always considering the altitude of the Pedemontana Jungle, as the latitude increases, the altitude decreases in general terms. It has been recognized that the floral changes are influenced by complex interactions of weather and edaphic variables in Yungas altitude ranges [24, 25, 40]. Beyond the fact that the associated variables of increasing altitude (which in this study varies between 22° S and 24° S) and/or altitude (which varies between 500 and 700 m.a.s.l. in the southern sector and between 700 and 900 m.a.s.l. in the north) would more or less determine overt changes at the level of species and ecosystems, such variations exist without doubt, and they are defining two sectors of the same forest in terms of carbon sequestration

These observations indicate that it is essential to preserve sectors of different latitudes and altitudes in the Pedemontana Jungle, since there are intrinsic factors that are defining differ‐ ential features in the biomass and carbon stock, as well as, in every ecosystem functions associated with these particular conditions [40]. Other authors have already pointed out that the recommendation in all cases is to maintain connectivity of Yungas in distribution, safe‐ guarding different sectors of the Pedemontana Jungle, varying in latitude and altitude [27].

Human influence, not analyzed in this study, will, no doubt, imprint differential features over time if their presence is not restricted, since we have observed signs of livestock and logging in the different studied areas. In the southern sector, where the Pedemontana Jungle has been deeply fragmented and immersed in an array of crops, it is considered that there might be a microclimatic influence on the fragments by the existence of rough edges [7, 18]. This aspect

**Fragmentation of the Pedemontana Jungle generates microclimate changes at its edge,**

The fragmentation of forests, reducing surface and insulation, exposes organisms, which remain in the fragment, to conditions differing from their ecosystem, which is primarily manifested in the contact between two different environments, which has been defined as

Microclimatic changes caused as a consequence of contrasting conditions between the remnant forest and the adjacent field, subjected to different uses (cultivation, planting, and pastures), would seem to be the most immediate and apparent fragmentation changes [7]. Several authors have recognized that, at the edge of the fragments is an environmental

**Table 7.** Carbon stock (tC/ha) in both sectors, north and south, in Salta, Argentina.

potential.

72 Greenhouse Gases

will be dealt with in the following section.

Contributors: Manrique, S.M.

**3.5. Effect of fragmentation on the carbon stock**

**which could affect the sequestration of the carbon stock.**

"edge effect" [18], and that impact toward the forest interior.

This study sought to analyze and quantify the possible microclimatic changes generated in the fragment edges of the Pedemontana Jungle, also observing the distribution of five represen‐ tative tree species (by their frequency [24]). The studied species were as follows: (i) *Calyco‐ phyllum multiflorum Griseb, Castelo*, (ii) *Phyllostylon rhamnoides J.Poiss., Taub*, (iii)*Astronium urundeuva Engl*., (iv) *Anadenanthera colubrina Vell., Brenan,* and (v) *Cedrela angustifolia DC*. It was estimated that the typical species, "climax" or more conservative ones of the population (e.g., those that have higher demands for their germination or growth requirements and with low tolerance for humidity fluctuations), could be more easily eliminated like those selected for this study. These species, which have a high degree of integration, complexity, and efficient energy use, are recognized as more susceptible to edge changes [18]. Therefore, in fragmented environments, the survival advantage is given to those pioneer species with a maximum tolerance for a wide range of environmental conditions.

Five forest sectors in the Colonia Santa Rosa municipality were worked (23°20'00 south latitude and 64º 30'15" west longitude): four, clearly turned into fragments, and one continuous (not fragmented) taken as a standard for comparison. The fragments were of distinct sizes: two large (sites 1 and 2 between 160 and 180 ha) and two small (3 and 4 between 3 and 5 ha). The distance from the city of Salta is 250 km.

The results of microclimatic records (taken from the edge toward the inside of the fragments, except in the site 5 as it was not considered the same edge but worked in an inside sector, looking for original ecosystem conditions) suggest that (Figure 6):


Changes are not manifested with identical magnitude in all cases. The smaller fragments tend to register values higher or lower for the measured variables (results not shown).

Figure 6. Microclimatic variables studied in fragments from edge to inside. Values are expressed in relative terms as a percentage of value at the edge (considered as 100%). The units are the following: RI= W/cm2; ST= °C; RT=°C; RH= %; SM= %. **Figure 6.** Microclimatic variables studied in fragments from edge to inside. Values are expressed in relative terms as a percentage of value at the edge (considered as 100%). The units are the following: RI= W/cm2 ; ST= °C; RT=°C; RH= %; SM= %.

50 60 70 80 90 100 stored (tC/ha) Site 1 Site 2 Site 3 Site 4 Site 5 Microclimatic variables are interrelated. Thus, for example, the RH and the RT are inverse and strongly related; the RI and RT relate directly and the RH and RI in reverse. This means that the intensity of radiation reaching the edge of the plot is influencing the relative temperature directly (higher radiation and higher relative temperatures) and inversely with relative humidity (greater radiation and lower relative humidity). In addition, the relative humidity and temperature inversely influence themselves (where there are higher values of relative temperature, there are lower values of relative humidity).

Figure 7. Carbon stock and contribution of each carbon pool studied (AGB includes only five species studied). The acronyms AGB10, AGB0 and SOC are explained in the text. 0 10 20 30 40 AGB‐10 AGB‐0 SOC Carbon In the AGB case, the relative participation of each species to the biomass stock varies according to site between 9% and 22 % for *C. multiflorum*, 5% and 79% for *P. rhamnoides*, 0% and 15% for *A. urundeuva*, 11% and 48% for *A. colubrina,* and between 0% and 23% for *C. angustifolia*. In general, the best-represented species is *P. rhamnoides*, followed by *A. colubrine*, and *C. multi‐ florum*. The fraction of ≤10 cm dbh ("sprout") contributes to their maximum values up to 6% of total AGB per site. AGB decreases significantly (*H* = 53.66; *p* < 0.001) from site 5 (179 ± 36 t/ ha) to site 1 (116.4 ± 32.2 t/ha), site 2 (106 ± 44.6 t/ha), site 4 (16 ± 6.7 t/ha), and lastly site 3 (10.37 ± 4.1 t/ha). The studied species represent approximately 86–90% of the total in the case of the forest (according to plot). In the fragments, the five studied species not only have lower AGB but also have proliferated heliophyllum species, typical of open environments, and species composition has changes (results not shown). It cannot be concluded that carbon sequestration in vegetation is less because of the microclimatic edge effect. Although there are clear differ‐ ences in the AGB10, the correlation of different distance values does not give significant values (*r* = 0.03; *p* = 0.804), nor in the AGB0 (*r* = 0.20; *p* = 0.134). The AGB of key species differs among fragments, but it cannot be said that a whole biomass has declined, since other shrubs and herbaceous species have proliferated. Larger studies are necessary to evaluate this aspect in depth.

Carbon sequestration in SOC, estimated up to 10 cm depth, increases from 19.3 ± 5 tC/ha in the site 3 (small forest fragment) to 23.4 ± 5 tC/ha in the site 4 (small forest fragment), 28.8 ± 7.5 tC/ha in the site 1 (large forest fragment), 28.9 ± 12.2 tC/ha in the site 2 (large forest fragment), and 34.8 ± 8.8 tC/ha in the forest or site 5 (Figure 7). Figure 6. Microclimatic variables studied in fragments from edge to inside. Values are expressed in relative terms as a percentage of value at the edge (considered as 100%). The units are the

following: RI= W/cm2; ST= °C; RT=°C; RH= %; SM= %.

Figure 7. Carbon stock and contribution of each carbon pool studied (AGB includes only five species studied). The acronyms AGB10, AGB0 and SOC are explained in the text. **Figure 7.** Carbon stock and contribution of each carbon pool studied (AGB includes only five species studied). The acronyms AGB10, AGB0 and SOC are explained in the text.

Table 1. Methodological differences and similarities between the case studies. It can be assumed that the influence of these changes will affect, in the middle or long term, the composition and facilitate the establishment the different species, according to their requirements. Mainly, the dominant tree species (climax) could result in changes in its germination and survival, promoting the success of pioneers species implantation, and altering the original composition and structure of the forest [18].

No Yes Yes and no No No

Wildlife

Aguaray and

and 500 climatic

Colonia Santa

climatic instantaneous

Case I II III IV V

National Park

and LI); 120 soil plots (SOC)

plots of 5 m2 (LUV); 46 plots

#### protection Site Coronel **4. Main remarks**

Legal

Figure 6. Microclimatic variables studied in fragments from edge to inside. Values are expressed in relative terms as a percentage of value at the edge (considered as 100%). The units are the

**Figure 6.** Microclimatic variables studied in fragments from edge to inside. Values are expressed in relative terms as a

Microclimatic variables are interrelated. Thus, for example, the RH and the RT are inverse and strongly related; the RI and RT relate directly and the RH and RI in reverse. This means that the intensity of radiation reaching the edge of the plot is influencing the relative temperature directly (higher radiation and higher relative temperatures) and inversely with relative humidity (greater radiation and lower relative humidity). In addition, the relative humidity and temperature inversely influence themselves (where there are higher values of relative

In the AGB case, the relative participation of each species to the biomass stock varies according to site between 9% and 22 % for *C. multiflorum*, 5% and 79% for *P. rhamnoides*, 0% and 15% for *A. urundeuva*, 11% and 48% for *A. colubrina,* and between 0% and 23% for *C. angustifolia*. In general, the best-represented species is *P. rhamnoides*, followed by *A. colubrine*, and *C. multi‐ florum*. The fraction of ≤10 cm dbh ("sprout") contributes to their maximum values up to 6% of total AGB per site. AGB decreases significantly (*H* = 53.66; *p* < 0.001) from site 5 (179 ± 36 t/ ha) to site 1 (116.4 ± 32.2 t/ha), site 2 (106 ± 44.6 t/ha), site 4 (16 ± 6.7 t/ha), and lastly site 3 (10.37 ± 4.1 t/ha). The studied species represent approximately 86–90% of the total in the case of the forest (according to plot). In the fragments, the five studied species not only have lower AGB but also have proliferated heliophyllum species, typical of open environments, and species composition has changes (results not shown). It cannot be concluded that carbon sequestration in vegetation is less because of the microclimatic edge effect. Although there are clear differ‐ ences in the AGB10, the correlation of different distance values does not give significant values

0 13 26 52 105 210

Radiation Intensity (RI) Relative Humidity (RH) Soil Temperature (ST) Soil Moisture (SM)

Distance from the edge (m)

Site 1 Site 2 Site 3 Site 4 Site 5

; ST= °C; RT=°C; RH= %;

Figure 7. Carbon stock and contribution of each carbon pool studied (AGB includes only five species

AGB‐10 AGB‐0 SOC

studied). The acronyms AGB10, AGB0 and SOC are explained in the text.

following: RI= W/cm2; ST= °C; RT=°C; RH= %; SM= %.

temperature, there are lower values of relative humidity).

percentage of value at the edge (considered as 100%). The units are the following: RI= W/cm2

Relative Temperature (RT)

0

Carbon

stored

(tC/ha)

SM= %.

100

200

300

400

Percentage (%)

500

600

700

800

74 Greenhouse Gases

Moldes Calilegua Reserve of Acambuco and Campo Pizarro General Pizarro Rosa Plot number 23 main plots 20 main plots 50 main plots 50 main plots 78 main plots The studies presented in this chapter offer insight into the varied potential of the Pedemontana Jungle for sequestration of atmospheric carbon, and how this potential can be influenced by human intervention in processes of deforestation, degradation, and fragmentation.

(AGB10) for each ecosystem; 23 plots of 50 m2 (AGB0); 23 (AGB10); 20 plots of 50 m2 (AGB0); 40 plots of 1 m2 (HUV (AGB10); 50 plots of 50 m2 (AGB0); 250 soil plots (SOC) (AGB10); 50 plots of 50 m2 (AGB00); 250 soil plots (SOC) (AGB10); 78 plots of 50 m2; 468 soil plots (SOC); 156 The carbon stock estimated for the Pedemontana Jungle ranges from 162 tC/ha (in Coronel Moldes) to 272 tC/ha (in Calilegua). In all cases, greater carbon storage occurs in the AGB fraction (from 47% to 55% of the total), where AGB0 fraction provides between 6 and 10% of

the total stock. Soil (SOC) constitutes the second most important carbon pool. Its contributions range from 28% to 39% according to the site.

The Pedemontana Jungle sequesters 43% more carbon than the Chaco forest at the same latitude. Moreover, the potential of carbon sequestration in the Pedemontana Jungle increases as the latitude decreases, sequestrating 28% more carbon at 22° than 24° south latitude.

Carbon sequestration in the Pedemontana Jungle sectors least affected by humans (degrada‐ tion) is 23% higher than in more degraded areas. There are no advantages for sites that are legally protected (i.e., carbon sequestration is approximately similar). Forest degradation practices such as unsustainable timber production, overharvesting of fuel wood, extensive cattle ranching, and fires at the edge of forest fragments are less easily observed than defor‐ estation, but they can contribute substantially to emissions. Forest degradation can also be a precursor to deforestation. These multiple changes in land use and forest area need to be monitored at the national level.

The Pedemontana Jungle sectors that have been left isolated are subject to edge effect, with changes clearly visible in microclimatic variables. The AGB in fragments is notably reduced for the main five tree species studied, but the species composition has also changed.

The potential impact of climate change on forest remnants is still unpredictable and depends on each one's resilience, on the remnant's adaptive capacity to climate change, and the magnitude and intensity of the phenomenon manifested in each area. At the same time, deforestation, degradation, and fragmentation of the Pedemontana Jungle could be affecting its ecological and social integrity, and the ability to provide ecosystem services of supply and regulation in the long term, and therefore its ability to respond to the global climate change impact.

Human management has taken over the ecosystem services that sustain the most important production systems from an economic standpoint. For example, irrigation water for cattle pastures and soil for agriculture. In many forests, such as the Pedemontana Jungle, other ecosystem services, for example, cultural or climatic regulation, are subordinated to these major objectives. The consequences of this imbalance in handling are shown negatively in the middle and long term, whereas in the short term, it cannot be seen most of the time. Vulnerable ecosystems are thus generated from the biophysical and social point of view, with a reduced capacity to respond to additional disturbances such as global climate change.

Forests require immediate support, with long-term policies independent of the ideologies, and management plans developed on technical bases, which are based on compliance with Article 41 of the National Constitution, "*All citizens enjoy the right to a healthy and balanced environment, suitable for human development and for productive activities that meet present needs without compro‐ mising those of future generations; and have the duty to preserve it*..." Land use plans should prioritize the conservation of ecosystems of high ecological value, such as the Pedemontana Jungle or Chaco, moreover, in a province where the natural biodiversity is accompanied by cultural biodiversity (with nine aboriginal ethnic groups), and where the forests are the principal sustainers of life.

#### **Acknowledgements**

the total stock. Soil (SOC) constitutes the second most important carbon pool. Its contributions

The Pedemontana Jungle sequesters 43% more carbon than the Chaco forest at the same latitude. Moreover, the potential of carbon sequestration in the Pedemontana Jungle increases as the latitude decreases, sequestrating 28% more carbon at 22° than 24° south latitude.

Carbon sequestration in the Pedemontana Jungle sectors least affected by humans (degrada‐ tion) is 23% higher than in more degraded areas. There are no advantages for sites that are legally protected (i.e., carbon sequestration is approximately similar). Forest degradation practices such as unsustainable timber production, overharvesting of fuel wood, extensive cattle ranching, and fires at the edge of forest fragments are less easily observed than defor‐ estation, but they can contribute substantially to emissions. Forest degradation can also be a precursor to deforestation. These multiple changes in land use and forest area need to be

The Pedemontana Jungle sectors that have been left isolated are subject to edge effect, with changes clearly visible in microclimatic variables. The AGB in fragments is notably reduced

The potential impact of climate change on forest remnants is still unpredictable and depends on each one's resilience, on the remnant's adaptive capacity to climate change, and the magnitude and intensity of the phenomenon manifested in each area. At the same time, deforestation, degradation, and fragmentation of the Pedemontana Jungle could be affecting its ecological and social integrity, and the ability to provide ecosystem services of supply and regulation in the long term, and therefore its ability to respond to the global climate change

Human management has taken over the ecosystem services that sustain the most important production systems from an economic standpoint. For example, irrigation water for cattle pastures and soil for agriculture. In many forests, such as the Pedemontana Jungle, other ecosystem services, for example, cultural or climatic regulation, are subordinated to these major objectives. The consequences of this imbalance in handling are shown negatively in the middle and long term, whereas in the short term, it cannot be seen most of the time. Vulnerable ecosystems are thus generated from the biophysical and social point of view, with a reduced

Forests require immediate support, with long-term policies independent of the ideologies, and management plans developed on technical bases, which are based on compliance with Article 41 of the National Constitution, "*All citizens enjoy the right to a healthy and balanced environment, suitable for human development and for productive activities that meet present needs without compro‐ mising those of future generations; and have the duty to preserve it*..." Land use plans should prioritize the conservation of ecosystems of high ecological value, such as the Pedemontana Jungle or Chaco, moreover, in a province where the natural biodiversity is accompanied by cultural biodiversity (with nine aboriginal ethnic groups), and where the forests are the

capacity to respond to additional disturbances such as global climate change.

for the main five tree species studied, but the species composition has also changed.

range from 28% to 39% according to the site.

monitored at the national level.

principal sustainers of life.

impact.

76 Greenhouse Gases

Supported by the Erasmus Mundus Action 2 Programme of the European Union. Special thanks to CONICET and the Research Council of National University of Salta, both of Argen‐ tina, and National University of Salamanca, Spain. The Municipality of Coronel Moldes and Colonia Santa Rosa are gratefully acknowledged. The authors thank Sandra Brown, Milena Segura, and Angelina Martínez-Yrízar for information, suggestions, and comments. Andrés Tálamo is acknowledged for his statistical advice. Thanks to the National Parks Administration and Environment Secretary of the province of Salta, for entry permits to protected areas and logistical support. This work could not have been completed without the invaluable help of the students who assisted during field trips.

### **Author details**

Silvina M. Manrique and Judith Franco

\*Address all correspondence to: silmagda@unsa.edu.ar

Non Conventional Energy Resources Investigation Institute (INENCO) of National Univer‐ sity of Salta (UNSa) and National Council of Scientific and Technical Research (CONICET), Salta, Argentina

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Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63154

#### **Abstract**

According to the recent information, CO2 concentration in the atmosphere reached 402 ppm at the beginning of 2016. On the other hand, fossil fuels remain as the major source to produce energy. The International Energy Agency estimate that those fuels will remain as the most used source during coming decades.

Carbon capture and storage technology is the most promising technology to significantly decrease CO2 emissions. Nevertheless, it may be possible to use CO2 as a raw material for other industrial uses. In this chapter, authors explain both ways to decrease CO2 emis‐ sions.

**Keywords:** CCS technology, CO2 capture technologies, CO2 storage, CO2 uses, macrofoul‐ ing

#### **1. Introduction**

The Fifth Assessment Report from the Intergovernmental Panel on Climate Change states that human influence on the climate system is clear [1]. The CO2 concentration in the atmosphere is continuously growing. The latest value is 402.52 ppm (January 2016, Mauna Loa Observa‐ tory), which is 2 pmm higher than the value registered in January 2015 [1].

Carbon capture and storage (CCS) is a way of 'decarbonising' fossil fuel power generation. It involves capturing carbon dioxide (CO2) emitted from high-producing sources, transporting it and storing it in secure geological formations deep underground, to mitigate the effect of greenhouse emissions on climate change [2].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The transported CO2 can also be reused in processes such as enhanced oil recovery (EOR) or in the chemical industry, a process sometimes known as carbon capture and utilisation (CCU). CCS can be applied to fossil fuel power plants (coal and gas-fired power stations) and to industrial CO2-emitting sources such as oil refineries or cement, chemical and steel plants. Rather than being a single technology, CCS is a suite of technologies and processes. While some of these have been operated successfully for decades, progress in applying large-scale CCS to power generation globally has been slow (Figure 1). The transported CO2 can also be reused in processes such as enhanced oil recovery (EOR) or in the chemical industry, a process sometimes known as carbon capture and utilisation (CCU). CCS can be applied to fossil fuel power plants (coal and gas-fired power stations) and to industrial CO2-emitting sources such as oil refineries or cement, chemical and steel plants. Rather than being a single technology, CCS is a suite of technologies and processes. While some of these have been operated successfully for decades, progress in applying large-scale CCS to power generation globally has been slow (Figure 1).

Figure 1. Shares of global anthropogenic greenhouse gas emissions (GHG) and world CO2 emissions from fuel combustion by fuel (Mt of CO2) [3, 4]. **Figure 1.** Shares of global anthropogenic greenhouse gas emissions (GHG) and world CO2 emissions from fuel com‐ bustion by fuel (Mt of CO2) [3, 4].

Carbon capture and storage (CCS) is likely to be a crucial part of the least-cost path to decarbonisation. It can provide a back-up role for variable renewables and help to manage swings in demand. CCS also has a crucial role in decarbonising heavy industry where there are limited options, and in the longer term would help to maximise the emission reduction obtained from scarce supplies of sustainable bioenergy as well as opening up other decarbonisation pathways. The European Commission has also emphasised that 'CCS may be the only option available to reduce Carbon capture and storage (CCS) is likely to be a crucial part of the least-cost path to decar‐ bonisation. It can provide a back-up role for variable renewables and help to manage swings in demand. CCS also has a crucial role in decarbonising heavy industry where there are limited options, and in the longer term would help to maximise the emission reduction obtained from scarce supplies of sustainable bioenergy as well as opening up other decarbonisation path‐ ways.

direct emission from industrial processes at the large scale needed in the longer term'. In this chapter, authors review the carbon capture, storage technology (including the CO2 transport The European Commission has also emphasised that 'CCS may be the only option available to reduce direct emission from industrial processes at the large scale needed in the longer term'.

through pipeline), and CO2 utilisation technologies. **2. CO2 capture** In this chapter, authors review the carbon capture, storage technology (including the CO2 transport through pipeline), and CO2 utilisation technologies.

This process consists of the separation of CO2 from flue gas produced during the combustion of fossil fuels

#### and can be applied to large flue gas stationary sources as thermal power stations and industrial processes. **2. CO2 capture**

Current CO2 capture technology (first generation) is adapted from gas separation processes already in industrial use. There are several technologies and strategies to capture CO2 from stationary sources: precombustion, post-combustion and oxy-fuel (Figure 2). This process consists of the separation of CO2 from flue gas produced during the combustion of fossil fuels and can be applied to large flue gas stationary sources as thermal power stations and industrial processes.

Current CO2 capture technology (first generation) is adapted from gas separation processes already in industrial use. There are several technologies and strategies to capture CO2 from stationary sources: pre-combustion, post-combustion and oxy-fuel (Figure 2).

**Figure 2.** Summary of CO2 capture technologies (adapted from IPCC) [2].

#### **3. First generation of capture technologies**

#### **3.1. Post-combustion capture**

The transported CO2 can also be reused in processes such as enhanced oil recovery (EOR) or in the chemical industry, a process sometimes known as carbon capture and utilisation (CCU). CCS can be applied to fossil fuel power plants (coal and gas-fired power stations) and to industrial CO2-emitting sources such as oil refineries or cement, chemical and steel plants. Rather than being a single technology, CCS is a suite of technologies and processes. While some of these have been operated successfully for decades, progress in applying large-scale

The transported CO2 can also be reused in processes such as enhanced oil recovery (EOR) or in the chemical industry, a process sometimes known as carbon capture and utilisation (CCU). CCS can be applied to fossil fuel power plants (coal and gas-fired power stations) and to industrial CO2-emitting sources such as oil refineries or cement, chemical and steel plants. Rather than being a single technology, CCS is a suite of technologies and processes. While some of these have been operated successfully for decades, progress in applying large-scale CCS to power generation globally has been slow (Figure 1).

Figure 1. Shares of global anthropogenic greenhouse gas emissions (GHG) and world CO2 emissions from fuel

Carbon capture and storage (CCS) is likely to be a crucial part of the least-cost path to decarbonisation. It can provide a back-up role for variable renewables and help to manage swings in demand. CCS also has a crucial role in decarbonising heavy industry where there are limited options, and in the longer term would help to maximise the emission reduction obtained from scarce supplies of sustainable bioenergy as well as

Carbon capture and storage (CCS) is likely to be a crucial part of the least-cost path to decar‐ bonisation. It can provide a back-up role for variable renewables and help to manage swings in demand. CCS also has a crucial role in decarbonising heavy industry where there are limited options, and in the longer term would help to maximise the emission reduction obtained from scarce supplies of sustainable bioenergy as well as opening up other decarbonisation path‐

**Figure 1.** Shares of global anthropogenic greenhouse gas emissions (GHG) and world CO2 emissions from fuel com‐

The European Commission has also emphasised that 'CCS may be the only option available to reduce

The European Commission has also emphasised that 'CCS may be the only option available to reduce direct emission from industrial processes at the large scale needed in the longer term'. In this chapter, authors review the carbon capture, storage technology (including the CO2

In this chapter, authors review the carbon capture, storage technology (including the CO2 transport

This process consists of the separation of CO2 from flue gas produced during the combustion of fossil fuels and can be applied to large flue gas stationary sources as thermal power stations and industrial processes.

This process consists of the separation of CO2 from flue gas produced during the combustion of fossil fuels and can be applied to large flue gas stationary sources as thermal power stations

Current CO2 capture technology (first generation) is adapted from gas separation processes already in industrial use. There are several technologies and strategies to capture CO2 from

stationary sources: pre-combustion, post-combustion and oxy-fuel (Figure 2).

Current CO2 capture technology (first generation) is adapted from gas separation processes already in industrial use. There are several technologies and strategies to capture CO2 from stationary sources: pre-

direct emission from industrial processes at the large scale needed in the longer term'.

transport through pipeline), and CO2 utilisation technologies.

CCS to power generation globally has been slow (Figure 1).

combustion by fuel (Mt of CO2) [3, 4].

bustion by fuel (Mt of CO2) [3, 4].

**2. CO2 capture**

**2. CO2 capture**

and industrial processes.

ways.

82 Greenhouse Gases

opening up other decarbonisation pathways.

through pipeline), and CO2 utilisation technologies.

combustion, post-combustion and oxy-fuel (Figure 2).

Post-combustion capture follows the conventional application of a specific purification unit applied for a particular pollutant removal (CO2 in this case). Figure 3 illustrates a typical block diagram of the post-combustion process that offers a great feasibility and versatility in terms of operating conditions and process integration.

**Figure 3.** Simplified scheme of a fossil-fuel power plant using a post-combustion capture unit [5].

CO2 concentration in the flue gas from a combustion process varies from 4 to14% in natural gas and coal-power plants, while other industries such as cement, iron and steel and petro‐ chemical produce flue gas ranging between 14 and 33%. The key drawbacks hindering the large-scale implementation of this technology lies in the large volume of gas that should be treated and the low CO2 concentration of the flue together with high energy requirements, mainly related to CO2 desorption process. The presence of large amounts of dust, O2, SOx, NOx and trace pollutants such as Hg and the relatively high temperature of the flue gas, typically between 120 and 180°C, are also design challenges that have significant impact on the capture costs.

The technologies currently available for post-combustion capture are classified into five main groups: absorption, adsorption, cryogenics, membranes and biological separation. The most mature and closest to market technology and so, the representative of first generation of postcombustion options, is capture absorption from amines.

#### **3.2. Chemical absorption from amines**

Post-combustion capture using chemical absorption by aqueous alkaline amine solutions has been used for CO2 and H2S removal from gas-treating plants for decades [6]. Amines react rapidly, selectively and reversibly with CO2 and can be applied at low CO2 partial pressure conditions. Amines are volatile, cheap and safe in handling. They show several disadvantages as they are also corrosive and require the use of resistant materials. Furthermore, amines form stable salts in the presence of O2, SOX and other impurities such as particles, HCl, HF and organic and inorganic Hg trace compounds that extremely constrain the content of those compounds in the treated gas.

The most widely used amine is monoethanolamine (MEA), which is considered as a bench‐ mark solvent because of its high cyclic capacity, significant absorption-stripping kinetic rates at low CO2 concentration and high solubility in water. Some other amine-based solvents such as diethanolamine (DEA), triethanolamine (TEA), diglycolamine (DGA), N-methyldiethanol‐ amine (MDEA), piperazine (PZ), 2-amino-2-methyl-1-propanol (AMP) and N-(2-aminoeth‐ yl)piperazine (AEP) have also traditionally been utilised.

A typical chemical absorption scheme is shown in Figure 4. A low CO2 concentrated flue gas is introduced in the absorber in crosscurrent with lean solvent from the stripper at 50–55°C and ambient pressure. CO2 reacts with amines in the absorber according to the overall reaction:

$$\rm{CO}\_2 + 2R\_1R\_2NH \leftrightarrow R\_1R\_2NCOO^- + R\_1R\_2NH\_2^+ \tag{1}$$

As CO2 is absorbed, rich amine from the absorber bottom is fed into a cross-exchanger with lean amine before it is introduced into the stripper. The stripping temperature varies between 120 and 150°C, and the operating pressure reaches up to 5 bar. A water saturated CO2 stream is released from the top and is subsequently ready for transport and storage, while lean amine leaving the stripper is pumped back into the absorber.

The high energy penalty related to amines regeneration (a high-intensive energy process because of the stripper operating conditions and solvent used) and solvent degradation are the issues most hindering a large deployment of this technology.

**Figure 4.** Diagram of a conventional CO2 capture process using amine-based chemical absorption.

#### **3.3. Pre-combustion capture**

chemical produce flue gas ranging between 14 and 33%. The key drawbacks hindering the large-scale implementation of this technology lies in the large volume of gas that should be treated and the low CO2 concentration of the flue together with high energy requirements, mainly related to CO2 desorption process. The presence of large amounts of dust, O2, SOx, NOx and trace pollutants such as Hg and the relatively high temperature of the flue gas, typically between 120 and 180°C, are also design challenges that have significant impact on the capture

The technologies currently available for post-combustion capture are classified into five main groups: absorption, adsorption, cryogenics, membranes and biological separation. The most mature and closest to market technology and so, the representative of first generation of post-

Post-combustion capture using chemical absorption by aqueous alkaline amine solutions has been used for CO2 and H2S removal from gas-treating plants for decades [6]. Amines react rapidly, selectively and reversibly with CO2 and can be applied at low CO2 partial pressure conditions. Amines are volatile, cheap and safe in handling. They show several disadvantages as they are also corrosive and require the use of resistant materials. Furthermore, amines form stable salts in the presence of O2, SOX and other impurities such as particles, HCl, HF and organic and inorganic Hg trace compounds that extremely constrain the content of those

The most widely used amine is monoethanolamine (MEA), which is considered as a bench‐ mark solvent because of its high cyclic capacity, significant absorption-stripping kinetic rates at low CO2 concentration and high solubility in water. Some other amine-based solvents such as diethanolamine (DEA), triethanolamine (TEA), diglycolamine (DGA), N-methyldiethanol‐ amine (MDEA), piperazine (PZ), 2-amino-2-methyl-1-propanol (AMP) and N-(2-aminoeth‐

A typical chemical absorption scheme is shown in Figure 4. A low CO2 concentrated flue gas is introduced in the absorber in crosscurrent with lean solvent from the stripper at 50–55°C and ambient pressure. CO2 reacts with amines in the absorber according to the overall reaction:

As CO2 is absorbed, rich amine from the absorber bottom is fed into a cross-exchanger with lean amine before it is introduced into the stripper. The stripping temperature varies between 120 and 150°C, and the operating pressure reaches up to 5 bar. A water saturated CO2 stream is released from the top and is subsequently ready for transport and storage, while lean amine

The high energy penalty related to amines regeneration (a high-intensive energy process because of the stripper operating conditions and solvent used) and solvent degradation are

2 12 1 2 12 2 *CO R R NH R R NCOO R R NH* 2 - + +« + (1)

combustion options, is capture absorption from amines.

yl)piperazine (AEP) have also traditionally been utilised.

leaving the stripper is pumped back into the absorber.

the issues most hindering a large deployment of this technology.

**3.2. Chemical absorption from amines**

compounds in the treated gas.

costs.

84 Greenhouse Gases

In pre-combustion CO2 capture, CO2 separation occurs prior to fuel combustion and power generation (Figure 5). The fuel reacts at high temperature and pressure with either oxygen or/ and steam under sub-stoichiometric conditions, and thereby a gas stream primarily composed of CO and H2 is obtained. This CO/H2 gas mixture is commonly known as *synthesis gas* or **syngas**.

In general, steam is utilised in case fuel is solid, namely gasification, whereas sub-stoichio‐ metric oxygen is used with liquid and gaseous fuels. Both reactions occur at elevated temper‐ ature (1,400°C) and pressure (3–7 Mpa), as seen in Equations 2 and 3.

Steam reforming:

$$\rm{xC}\_{x}H\_{y} + xH\_{2}O \leftrightarrow xCO + \left(x + \frac{y}{2}\right)H\_{2}; \Delta H\_{r} > 0\tag{2}$$

Partial oxidation:

$$\rm{xCO}\_{y} + xO\_{2} \leftrightarrow xCO + \left(\frac{y}{2}\right)H\_{2}; \Delta H\_{r} < 0\tag{3}$$

Steam reforming needs a secondary fuel to provide the energy supply necessary for the reaction that occurs and a catalysts to improve the kinetic of this process. In Equation (3), the primary fuel is partially oxidised by a limited amount of oxygen. Partial oxidation produces less H2 per fuel unit than stream reforming, but the kinetic reaction is faster, it requires smaller reactors and neither catalyst nor energy supply from a secondary fuel.

Once particulate matter is removed, the syngas passes through a two stages catalytic reactor, where CO reacts with steam to produce CO2 and further yield H2: *water-gas-shift (WGS) reaction*.

WGS reaction:

$$\text{CO} + \text{H}\_2\text{O} \leftrightarrow \text{CO}\_2 + \text{H}\_2; \Delta H\_r = -41 \text{kJ/mol} \tag{4}$$

The syngas resulted is mainly composed of CO2, ranging from 15 to 40%v/v, and H2 at elevated pressure from which CO2 can be easily separated by a physical absorption mechanism and then CO2 can be easily released by simply dropping pressure.

Before the syngas from WGS reactor is separated into its primary components, the sulphur compounds, mainly in COS and H2S form, are removed to avoid its emission to the atmosphere. Sulphur is then recovered in either as solid in a Claus plant or as sulphuric acid.

The sulphur-free syngas has a high CO2 concentration and an elevated pressure (2–7 MPa), thus making physical absorption highly recommended for CO2 separation, although adsorp‐ tion process such as pressure swing adsorption (PSA) is also utilised.

The remaining nearly pure H2 stream could be burned in a combined cycle power plant to generate electricity, but H2 turbines require further development. Power fuel cells and transportation fuels are alternative options for using H2 in the future, currently under devel‐ opment.

**Figure 5.** Simplified scheme of an integrated gasification combined cycle (IGCC) coupled with a pre-combustion CO2 capture and storage unit using a physical absorption process [5].

#### **3.4. Oxy-combustion capture**

Oxy-combustion or oxy-fuel capture is considered as one of the most promising CCS technol‐ ogies that would be economically competitive in fossil-fuel power plants and industrial facilities. It has been developed for both new designs and retrofitting of existing plants, although it is best adapted to newly designed power plants. A basic process flow diagram is given in Figure 6. Oxy-combustion technology is based on the use of high purity O2 as oxidiser in an O2/CO2 mixture instead of air during the combustion process. It has been first proposed for coal boilers and gas turbines but can be applied to any type of fossil fuel utilised for thermal power production. As burning with O2 at high concentration can produce high flame temper‐ atures in the boiler, part of the exhaust gas from the boiler, mainly CO2 and water vapour (FGR flue gas recirculation stream), is recycled to control temperatures to levels compatible with available boiler materials. The flue gas obtained from this system consists mainly of CO2 and H2O and are accompanied by minor quantities of N2, SOx, NOx, Ar and Hg. Water can be easily removed by condensation, producing a highly CO2 concentrated flue gas. The CO2 content varies from 70 to 95%v/v, depending on the process configuration, air in-leakages, fuel characteristics and the purity of O2.

Once particulate matter is removed, the syngas passes through a two stages catalytic reactor, where CO reacts with steam to produce CO2 and further yield H2: *water-gas-shift (WGS) reaction*.

The syngas resulted is mainly composed of CO2, ranging from 15 to 40%v/v, and H2 at elevated pressure from which CO2 can be easily separated by a physical absorption mechanism and

Before the syngas from WGS reactor is separated into its primary components, the sulphur compounds, mainly in COS and H2S form, are removed to avoid its emission to the atmosphere.

The sulphur-free syngas has a high CO2 concentration and an elevated pressure (2–7 MPa), thus making physical absorption highly recommended for CO2 separation, although adsorp‐

The remaining nearly pure H2 stream could be burned in a combined cycle power plant to generate electricity, but H2 turbines require further development. Power fuel cells and transportation fuels are alternative options for using H2 in the future, currently under devel‐

**Figure 5.** Simplified scheme of an integrated gasification combined cycle (IGCC) coupled with a pre-combustion CO2

Oxy-combustion or oxy-fuel capture is considered as one of the most promising CCS technol‐ ogies that would be economically competitive in fossil-fuel power plants and industrial facilities. It has been developed for both new designs and retrofitting of existing plants, although it is best adapted to newly designed power plants. A basic process flow diagram is given in Figure 6. Oxy-combustion technology is based on the use of high purity O2 as oxidiser in an O2/CO2 mixture instead of air during the combustion process. It has been first proposed

Sulphur is then recovered in either as solid in a Claus plant or as sulphuric acid.

tion process such as pressure swing adsorption (PSA) is also utilised.

capture and storage unit using a physical absorption process [5].

**3.4. Oxy-combustion capture**

then CO2 can be easily released by simply dropping pressure.

2 22 ; 41 *CO H O CO H H kJ mol <sup>r</sup>* + « + D =- (4)

WGS reaction:

86 Greenhouse Gases

opment.

**Figure 6.** A simplified scheme of a fossil-fuel power plant based on the oxy-combustion concept [5].

Oxy-combustion requires large amounts of high purity (95–99%) O2 for power production. A typical 500 MWe fossil-fuel power plant would need 9,000–10,000 t/d to operate under oxycombustion conditions [7]. Currently, cryogenic distillation is the only available technology that can supply those amounts of O2. An air separation unit (ASU) can provide around 4,500– 7,000 t/d of oxygen, while other alternative technologies such as vacuum pressure swing adsorption (VPSA) units and membranes can only produce one order of magnitude below ASU production.

The ASU would consume up to 60% of the total electricity required for carbon capture and reduces the overall efficiency of the power plant by about 7–9%, reaching up to 15% in some cases. Furthermore, the availability and rapid response of the ASU to load changes have been noted as crucial challenges for the global oxy-combustion plant operation and feasibility. New technologies for O2 production as ion transport membranes (ITM) or VPSA have shown promising results related to energy consumption, but the large amounts of O2 required in power plant operation avoid currently its commercial deployment.

The CO2 stream obtained from oxy-fuel combustion shows high levels of water vapour, sulphur compounds, N2, O2 and impurities such as mercury in the flue gas. NOx emission is low when compared with air combustion.

The CO2 gas quality has significant impact on the capture cost by this technology, and uncertainties on the future regulatory requirements of CO2 quality for its transport and storage has influence on the process configuration of the oxy-combustion plant, gas cleaning unit performance, overall CO2 recovery capacity and on the energy requirements for CO2 com‐ pression and purification.

#### **4. Emerging technologies for CO2 capture**

The most promising emerging technologies applied to carbon capture are discussed in this section to complete the overview of the CO2 capture technologies currently under research.

#### **4.1. Chemical looping combustion**

Chemical looping combustion (CLC) is a promising technology for fuel combustion, which can be beneficial in carbon capture applications. It is based on the use of an oxygen carrier, typically a metal oxide, to supply the O2 needed for the fuel combustion process, producing a highly CO2 concentrated exhaust gas. Iron, nickel, cobalt, copper, manganese and cadmium are commonly used as oxygen carriers in CLC.

**Figure 7.** A simplified scheme of a chemical looping for oxy-combustion.

CLC consists of two fluidised bed reactors, namely reducer and oxidiser. In the reducer reactor, fuel is fed along with the metal oxide containing oxygen, which is transferred from the metal oxide to the reactor as the combustion occurs (Figure 7). A flue gas containing over 99%v/v of CO2 can be obtained by a simply condensation stage because of the fact that the exhaust gas at the reducer outlet is primarily formed by CO2 and water vapour. This stream is then sent to further compression and permanent storage.

Reducer:

The CO2 gas quality has significant impact on the capture cost by this technology, and uncertainties on the future regulatory requirements of CO2 quality for its transport and storage has influence on the process configuration of the oxy-combustion plant, gas cleaning unit performance, overall CO2 recovery capacity and on the energy requirements for CO2 com‐

The most promising emerging technologies applied to carbon capture are discussed in this section to complete the overview of the CO2 capture technologies currently under research.

Chemical looping combustion (CLC) is a promising technology for fuel combustion, which can be beneficial in carbon capture applications. It is based on the use of an oxygen carrier, typically a metal oxide, to supply the O2 needed for the fuel combustion process, producing a highly CO2 concentrated exhaust gas. Iron, nickel, cobalt, copper, manganese and cadmium

**MeO**

**Me**

CLC consists of two fluidised bed reactors, namely reducer and oxidiser. In the reducer reactor, fuel is fed along with the metal oxide containing oxygen, which is transferred from the metal oxide to the reactor as the combustion occurs (Figure 7). A flue gas containing over 99%v/v of CO2 can be obtained by a simply condensation stage because of the fact that the exhaust gas at the reducer outlet is primarily formed by CO2 and water vapour. This stream is then sent to

**Fuel Reactor**

> **Fuel CnHm**

**Combustion Products CO2, H2O** 

pression and purification.

88 Greenhouse Gases

**4.1. Chemical looping combustion**

are commonly used as oxygen carriers in CLC.

**Air Reactor**

**Oxygen Depleted Air N2 , O2**

> **Air O2, N2**

**Figure 7.** A simplified scheme of a chemical looping for oxy-combustion.

further compression and permanent storage.

**4. Emerging technologies for CO2 capture**

$$(2n+m)M\_yO\_x + \text{C}\_nH\_{2m} \leftrightarrow (2n+m)M\_yO\_{x-1} + mO + nCO\_2.\tag{5}$$

Oxidiser:

$$\boldsymbol{M}\_{y}\boldsymbol{\mathcal{O}}\_{x-1} + \frac{1}{2}\boldsymbol{\mathcal{O}}\_{2} \leftrightarrow \boldsymbol{M}\_{y}\boldsymbol{\mathcal{O}}\_{x} + \boldsymbol{N}\_{2} + \boldsymbol{\mathcal{O}}\_{2} \text{(excess)}.\tag{6}$$

#### **4.2. Hydrate-based separation**

This separation approach is based on the hydrate formation from high pressure water in contact with the flue gas containing CO2. Hydrates are crystalline under suitable low temper‐ ature and high pressure conditions. A pure CO2 stream is then obtained as CO2 is released from the hydrates, achieving up to 99% of CO2 recovery.

#### **4.3. Calcium looping**

Calcium looping is based on the reversible reaction between CaO and CO2 to form calcium carbonate.

Calcium looping consists of two fluidised bed reactors, namely carbonator and calciner. In the carbonator, primary fuel is burned and CaO reacts with the CO2 formed from the fuel com‐ bustion following the reaction seen in Equation (7). Carbonator temperature is within 650– 700°C, depending on the system pressure.

Carbonator:

$$\text{CaO}(s) + \text{CO}\_2(g) \leftrightarrow \text{CaCO}\_3(s). \tag{7}$$

Calciner:

$$\text{CaCO}\_3\text{(s)} \leftrightarrow \text{CaO}\text{(s)} + \text{CO}\_2\text{(g)}.\tag{8}$$

CaCO3 is then heated by secondary fuel combustion in the calciner. CaO is regenerated and CO2 is released for storage according to the reaction in Equation (8). The calciner temperature can reach 900°C, depending on the CO2 partial pressure.

This technology shows benefits for carbon capture. Limestone is cheap and widely available, and there is a potential for process integration, which can lead to low energy penalties, i.e., heat released from carbonisation can be utilised in a steam cycle or the heat used in the calciner reactor can be recovered in the carbonation process.

#### **4.4. Partial oxy-combustion**

The energy consumption required for solvent regeneration and high purity oxygen production is the major drawback of post-combustion and oxy-combustion technologies. A new hybrid concept has been proposed to reduce the energy requirements associated with CO2 capture step combining a partial oxy-fuel combustion (using oxygen-enriched air instead of high purity oxygen as oxidiser) and a CO2 separation process treating a flue gas with a higher CO2 concentration than in conventional air combustion (Figure 8).

**Figure 8.** A simplified scheme of a partial oxy-combustion plant.

The combination of a less-constrained ASU for oxygen production and a carbon capture process using membranes instead of amine solvents can conduce to a minimal energy require‐ ment associated with an oxygen purity ranging between 0.5 and 0.6 molar fraction.

#### **4.5. Biological CO2 capture**

Biological CO2 capture from a gas mixture is based on natural reactions of CO2 with living organism, mainly enzymes, generally proteins and (micro)algae. Enzymes catalyse CO2 chemical reaction and enhance CO2 absorption rate in water. Enzymes can be also immobilised at the gas-liquid interface to promote CO2 dissolution from the bulk gas. In this sense, carbonic anhydrase enzyme supported in a hollow fibre with liquid membrane has been reported as a potential method applied to CO2 capture, achieving up to 90% CO2 capture associated with low energy requirements in the regeneration process at laboratory-scale experiments. Carbonic anhydrase promotes carbonic acid formation from dissolved CO2 and enhances CO2 absorp‐ tion from gas phase using and extremely low CO2/enzyme ratio. CO2 separation using enzymes must incorporate a tailored regeneration process to produce a high concentrated CO2 exhaust stream. Membrane boundary, fouling, long-term operation and pore wetting are identified as the most relevant technical issues to be addressed before the scale-up of this CO2 capture approach.

The use of algae is also considered a promising CO2 capture option among natural occurring reactions. Algae consume CO2 through photosynthesis mechanism. The use of algae in CO2 capture would avoid subsequent CO2 compression and storage stages, but there are some key issues that must be addressed for its large-scale deployment. In fact, algae require excessive amount of water and large gas-liquid interface surfaces that drastically limit their application in carbon capture. Algae are also highly susceptive to changes in operating conditions and to the presence of impurities such as vanadium and nickel.

#### **4.6. Ionic liquid absorption**

**4.4. Partial oxy-combustion**

90 Greenhouse Gases

**Fuel**

approach.

**O2 enriched Air**

**4.5. Biological CO2 capture**

The energy consumption required for solvent regeneration and high purity oxygen production is the major drawback of post-combustion and oxy-combustion technologies. A new hybrid concept has been proposed to reduce the energy requirements associated with CO2 capture step combining a partial oxy-fuel combustion (using oxygen-enriched air instead of high purity oxygen as oxidiser) and a CO2 separation process treating a flue gas with a higher CO2

**Boiler Energy**

**Rich Amine Lean Amine**

**Heat Exchanger**

**Cleaned Gas CO2 to** 

**CO2 Stripping** **secuestration**

**CO2 Absorption**

The combination of a less-constrained ASU for oxygen production and a carbon capture process using membranes instead of amine solvents can conduce to a minimal energy require‐

Biological CO2 capture from a gas mixture is based on natural reactions of CO2 with living organism, mainly enzymes, generally proteins and (micro)algae. Enzymes catalyse CO2 chemical reaction and enhance CO2 absorption rate in water. Enzymes can be also immobilised at the gas-liquid interface to promote CO2 dissolution from the bulk gas. In this sense, carbonic anhydrase enzyme supported in a hollow fibre with liquid membrane has been reported as a potential method applied to CO2 capture, achieving up to 90% CO2 capture associated with low energy requirements in the regeneration process at laboratory-scale experiments. Carbonic anhydrase promotes carbonic acid formation from dissolved CO2 and enhances CO2 absorp‐ tion from gas phase using and extremely low CO2/enzyme ratio. CO2 separation using enzymes must incorporate a tailored regeneration process to produce a high concentrated CO2 exhaust stream. Membrane boundary, fouling, long-term operation and pore wetting are identified as the most relevant technical issues to be addressed before the scale-up of this CO2 capture

ment associated with an oxygen purity ranging between 0.5 and 0.6 molar fraction.

concentration than in conventional air combustion (Figure 8).

**Figure 8.** A simplified scheme of a partial oxy-combustion plant.

Significant progress has been made in the application of ionic liquids (ILs) as alternative solvents to CO2 capture because of their unique properties such as very low vapour pressure, a broad range of liquid temperatures, excellent thermal and chemical stabilities and selective dissolution of certain organic and inorganic materials. ILs are liquid organic salts at ambient conditions with a cationic part and an anionic part.

ILs have the potential to overcome many of the problems of associated with current CO2 capture techniques. ILs are particularly applicable in absorption of CO2 while effectively avoiding the loss of sequestering agents. Other advantage of ILs is that they can be combined into polymeric forms, increasing the CO2 sorption capacity compared with other ILs and conventional solvents and greatly facilitates the separation and ease of operation.

### **5. CO2 transport**

Currently there are more than 6,500 km of CO2 pipelines worldwide. Most of them deliver CO2 to EOR operations in the United States, but there is also a growing number under development for CO2 storage projects

The relative development of the infrastructure to transport CO2 is still in its early stages. This is reflected by the low number of existing infrastructures developed to transport CO2 from stationary sources into geological structures. Table 1 provides an overview of the current developments for CO2 transportation globally. All of these examples have been developed in relation to the EOR technique, where the CO2 source is found mainly in natural reserves. In Europe, only a few projects are in operation, but there are plans to deploy an extended CO2 pipeline network along Europe to optimise CO2 storage structures.

These examples may be used to study CO2 conditions; in addition, many CO2 pipeline projects are based on well-known designs and materials commonly used in natural gas pipeline specifications. The most profitable way to transport CO2 is in its dense phase [9].

To avoid two phases, it has been suggested that the most efficient way to transport CO2 is as its supercritical phase [8, 9], which occurs at a pressure higher than 7.38 MPa and a temperature of more than 31.1 °C. To maintain these conditions, this type of transportation may require the use of booster stations in the pipeline layout to maintain the required pressure and tempera‐ ture.


**Table 1.** Current CO2 pipelines. The first long distance CO2 pipeline was in the 1970s. Main utilisation of the natural & anthropogenic CO2 is EOR activities [8].

Material selection should be compatible with all states of the CO2 stream. They should be defined to prevent corrosion and maximum material stress. In addition, eligible materials need to be qualified for the potential low temperature conditions that may occur during a pipeline depressurisation situation.

The design of a pipeline should meet the requirements set by appropriate regulations and standards. CO2 pipelines shall be designed according to applicable regulatory requirements. The Recommended Practice for Design and Operation of CO2 refers to the following pipeline standards: ISO 13623:2009, DNV-OS F101:2012 and ASME B31.4 or ASME B31.8.

Usually CO2 pipelines are designed using existing national standards for gas and liquid transportation pipes, while additional CO2 specific design issues are taken into consideration by the pipeline construction/operation companies to guarantee the reliable and safe operation of a given pipeline.

The use of carbon steels (e.g., with API X-60 and X-65) for the transportation of CO2 streams has been ongoing for more than 30 years, as required in EOR projects. During the 2002–2008 period, 18 incidents were reported with no fatalities and/or injuries.


**Table 2.** Summary of the current parameters considered in the CO2 transport phase.

The cost of pipeline transportation will be determined by the pipeline route, in which physical and social geography will be crucial conditions.

The three major cost elements for pipelines are (1) construction costs (e.g., materials, labour, booster station, if needed, and others), (2) operation and maintenance costs (e.g., monitorisa‐ tion, maintenance, energy costs) and (3) other costs (design, insurance, fees, and right-of-way).

### **6. CO2 storage**

**Pipeline Location Length (Km) Diameter (inches) Estimated**

Cortez US 808 30 23.6 Sheep Mountain US 656 NA 11.0 Bravo US 351 20 7.0

Choctaw US 294 20 7.0 Bairoil US 258 NA 23.0 Central Basin US 230 16 4.3 Canyon Reef Carriers US 224 16 4.3 Comanche Creek US 193 6 1.3 Centerline US 182 16 4.3 Delta US 174 24 11.4 Snohvit Norway 153 NA 0.7 Borger US 138 4 1.0 Coffeyville US 112 8 1.6 OCAP The Netherlands 97 NA 0.4 Beaver Creek US 85 NA NA Anton Irish US 64 8 1.6 El Mar US 56 6 1.3 Chaparral US 37 6 1.3 Doliarhide US 37 8 1.6 Lacq France 27 NA 0.1 Adair US 24 4 1.0 Cordona Lake US 11 6 1.3

Dakota Gasification/Weyburn US/Canada 328 14 2.6

**Table 1.** Current CO2 pipelines. The first long distance CO2 pipeline was in the 1970s. Main utilisation of the natural &

Material selection should be compatible with all states of the CO2 stream. They should be defined to prevent corrosion and maximum material stress. In addition, eligible materials need to be qualified for the potential low temperature conditions that may occur during a pipeline

The design of a pipeline should meet the requirements set by appropriate regulations and standards. CO2 pipelines shall be designed according to applicable regulatory requirements. The Recommended Practice for Design and Operation of CO2 refers to the following pipeline

standards: ISO 13623:2009, DNV-OS F101:2012 and ASME B31.4 or ASME B31.8.

anthropogenic CO2 is EOR activities [8].

92 Greenhouse Gases

depressurisation situation.

**maximum (106**

**year)**

**t/**

At present, there are three possible geological structures that may be considered for CO2 storage: depleted hydrocarbon and production, deep saline aquifers, and coal seams.

#### **6.1. Depleted hydrocarbon fields**

The CO2 can be stored in supercritical conditions, rising by buoyancy and can be physically held in a structural or stratigraphic trap, the same way as the natural accumulation of hydro‐ carbons occurs. The advantage of the capacity of containment system has been demonstrated by the retention of oil for millions of years. If the site is in production, it is used to increase the recovery of oil or gas (EOR recovery – enhanced oil, gas-enhanced recovery – EGR). These operations, EOR/EGR, provide an economic benefit that can offset the costs of the capture, transport and storage of CO2.

#### **6.2. Deep saline aquifers**

They are the best options for storing large volumes of CO2 because of its size and found more than 800 meters below the surface. The supercritical CO2 is 30–40% less dense than typical saline water from these formations, which means that the CO2 naturally rise by buoyancy through the reservoir until it is caught or becomes longer solution term. They require an impermeable cap rock to ceiling (shales or layers of evaporites) and a porous and permeable rock store (sandstone or limestone) that promotes the injection, migration and trapping.

#### **6.3. Coal seams**

CO2 in gaseous form is injected into the coalbed, 300 to 600 metre depth, and adsorbed on the matrix pores, releasing the existing CH4 in the same (two molecules of CO2 adsorbed by each CH4 molecule that travels). This has led to the possibility of storing CO2 in coal seams, while CH4 recovered is valued. This technique is called 'enhanced coalbed methane production' (ECBM).

Coal properties (range, degree and permeability) determine the suitability of the site, either for storage or storage with only CH4 recovery.

#### **6.4. Site selection and exploration**

Figure 9 represents a proposed work flow for any CO2 storage project. It is possible to determine three mayor phases: pre-injection, injection and post-injection phases.

**Figure 9.** Work flow proposed for basin screening (Definition phase) [10].

In general, most of the areas that could be suitable for storing CO2 are not well explored geologically. For this reason, pre-injection phase is crucial to decrease the inherent risk in subsurface exploration.

**6.2. Deep saline aquifers**

94 Greenhouse Gases

**6.3. Coal seams**

(ECBM).

for storage or storage with only CH4 recovery.

**Figure 9.** Work flow proposed for basin screening (Definition phase) [10].

**6.4. Site selection and exploration**

They are the best options for storing large volumes of CO2 because of its size and found more than 800 meters below the surface. The supercritical CO2 is 30–40% less dense than typical saline water from these formations, which means that the CO2 naturally rise by buoyancy through the reservoir until it is caught or becomes longer solution term. They require an impermeable cap rock to ceiling (shales or layers of evaporites) and a porous and permeable rock store (sandstone or limestone) that promotes the injection, migration and trapping.

CO2 in gaseous form is injected into the coalbed, 300 to 600 metre depth, and adsorbed on the matrix pores, releasing the existing CH4 in the same (two molecules of CO2 adsorbed by each CH4 molecule that travels). This has led to the possibility of storing CO2 in coal seams, while CH4 recovered is valued. This technique is called 'enhanced coalbed methane production'

Coal properties (range, degree and permeability) determine the suitability of the site, either

Figure 9 represents a proposed work flow for any CO2 storage project. It is possible to

determine three mayor phases: pre-injection, injection and post-injection phases.

Screen phase could be differentiated by the data recompilation task and the multicriteria decision tool. It is integrated as a preliminary phase, and it is connected with a second phase called *characterisation phase,* which corresponds to site maturation and testing.

Those criteria should comprise both technique and socio-economic criteria, and they should answer several questions such as where, how much quantity, and which conditions. All of these criteria and questions will contribute to solve and select the most suitable emplacement for storing CO2 [10].

There are different examples and analogues that can be useful for the definition of criteria. Analogues can be natural (releases and resources) and industrial.

Assess or characterisation task is related to three major ways to explore the sub-surface: outcrops, geophysics and wells.

To decrease the inherent risk of exploration, it is necessary to consider all of the three subphases:


Wells will provide real information of the storage and caprock formation in sub-surface

conditions. Test will provide information about geomechanical, hydrogeological properties and it may be

Considering the injection phase, control of the behaviour of injected CO2 is one of the most important

Demonstrate that the injected CO2 is stored in the selected reservoir and therefore must be a

Check for surface environmental effects occur, and therefore, you must provide the affected

The monitoring strategy should not be limited to the operational and post-operational periods but has an important role during the pre-operational stage by conducting the baseline of the injection site [11]. This baseline defines the set of physical, geochemical and biological processes operating in the storage area before any activity injection. The baseline is critical because especially in the early stages of injection, the changes are not evident, both in depth and on the surface, and comparison with the undisturbed condition is needed. The development of the baseline may have added value; for example, building trust and showing the population from the beginning that the project is under control and that any anomaly is detected. Numerous methods have been proposed for monitoring CO2 in geological repositories. Of these, one can clearly distinguish two types: (a) to detect the evolution of CO2 injected into deep and (b) for leakage from storage. In the first type, these methods are generally based on geophysical techniques, while in the second type, the range of methods is broader, including geochemical, physical and biological techniques. Therefore, the final selection of the monitoring strategy should take into account the following

Compliance with these requirements will be conditioned by the type of store and its area of influence. Clearly, monitoring techniques will be very different in stores on-shore and off-shore, and within a storage type, geological, hydrological and even ecological characteristics will favour the implementation of a

guarantee that the company responsible for fulfilling its commitment to reducing emissions.

Check that no intrusion occurs in other exploitable aquifers and water resources.

Figure 10. Seismic survey based on vibroseis. Example of a seismic profile. **Figure 10.** Seismic survey based on vibroseis. Example of a seismic profile.

possible to test interaction between the rock and CO2.

tasks. For instance, the control and monitoring strategy must:

population security and peace on the operations of injection.

Efficiency in detecting small changes in behaviour warehouse

Implementation in large tracts of land

Reasonable economic cost

methodology or other.

**7. Monitoring techniques**

aspects [12, 13]:

**•** Wells will provide real information of the storage and caprock formation in sub-surface conditions. Test will provide information about geomechanical, hydrogeological properties and it may be possible to test interaction between the rock and CO2.

#### **7. Monitoring techniques**

Considering the injection phase, control of the behaviour of injected CO2 is one of the most important tasks. For instance, the control and monitoring strategy must:


The monitoring strategy should not be limited to the operational and post-operational periods but has an important role during the pre-operational stage by conducting the baseline of the injection site [11]. This baseline defines the set of physical, geochemical and biological processes operating in the storage area before any activity injection. The baseline is critical because especially in the early stages of injection, the changes are not evident, both in depth and on the surface, and comparison with the undisturbed condition is needed. The develop‐ ment of the baseline may have added value; for example, building trust and showing the population from the beginning that the project is under control and that any anomaly is detected. Numerous methods have been proposed for monitoring CO2 in geological reposito‐ ries. Of these, one can clearly distinguish two types: (a) to detect the evolution of CO2 injected into deep and (b) for leakage from storage. In the first type, these methods are generally based on geophysical techniques, while in the second type, the range of methods is broader, including geochemical, physical and biological techniques. Therefore, the final selection of the monitor‐ ing strategy should take into account the following aspects [12, 13]:


Compliance with these requirements will be conditioned by the type of store and its area of influence. Clearly, monitoring techniques will be very different in stores on-shore and offshore, and within a storage type, geological, hydrological and even ecological characteristics will favour the implementation of a methodology or other.

The monitoring deployment is based on the following aspects: (a) characterisation of the area, (b) establishing a base line CO2, (c) establishment of potential areas of migration and release of CO2 (and other gases) and (d) validation and development of techniques for monitoring CO2.


**Table 3.** Possible types of leakage of CO2 in a geological storage [13, 14, 15].

#### **8. CO2 uses**

**•** Wells will provide real information of the storage and caprock formation in sub-surface conditions. Test will provide information about geomechanical, hydrogeological properties

Considering the injection phase, control of the behaviour of injected CO2 is one of the most

**•** Demonstrate that the injected CO2 is stored in the selected reservoir and therefore must be a guarantee that the company responsible for fulfilling its commitment to reducing emis‐

**•** Check for surface environmental effects occur, and therefore, you must provide the affected

The monitoring strategy should not be limited to the operational and post-operational periods but has an important role during the pre-operational stage by conducting the baseline of the injection site [11]. This baseline defines the set of physical, geochemical and biological processes operating in the storage area before any activity injection. The baseline is critical because especially in the early stages of injection, the changes are not evident, both in depth and on the surface, and comparison with the undisturbed condition is needed. The develop‐ ment of the baseline may have added value; for example, building trust and showing the population from the beginning that the project is under control and that any anomaly is detected. Numerous methods have been proposed for monitoring CO2 in geological reposito‐ ries. Of these, one can clearly distinguish two types: (a) to detect the evolution of CO2 injected into deep and (b) for leakage from storage. In the first type, these methods are generally based on geophysical techniques, while in the second type, the range of methods is broader, including geochemical, physical and biological techniques. Therefore, the final selection of the monitor‐

Compliance with these requirements will be conditioned by the type of store and its area of influence. Clearly, monitoring techniques will be very different in stores on-shore and offshore, and within a storage type, geological, hydrological and even ecological characteristics

The monitoring deployment is based on the following aspects: (a) characterisation of the area, (b) establishing a base line CO2, (c) establishment of potential areas of migration and release of CO2 (and other gases) and (d) validation and development of techniques for monitoring CO2.

**•** Check that no intrusion occurs in other exploitable aquifers and water resources.

and it may be possible to test interaction between the rock and CO2.

important tasks. For instance, the control and monitoring strategy must:

population security and peace on the operations of injection.

ing strategy should take into account the following aspects [12, 13]:

**•** Efficiency in detecting small changes in behaviour warehouse

will favour the implementation of a methodology or other.

**•** Implementation in large tracts of land

**•** Reasonable economic cost

**7. Monitoring techniques**

sions.

96 Greenhouse Gases

#### **8.1. Current uses of CO2**

Nowadays different applications are known that can be used for demonstrating that CO2 is a useful, versatile and safe product. Figure 11 illustrates most of the current and potential uses of CO2.

**Figure 11.** CO2 uses. Different pathways for utilisation CO2.

There are many classifications that can be made about the use or valuation of large-scale CO2 and including the three categories proposed by Vega [16] for type of uses, which also is used by the PTE-CO2, 2013 (Technology Platform Spanish CO2). To wit:


#### **8.2. Direct use of CO2**

**Figure 11.** CO2 uses. Different pathways for utilisation CO2.

98 Greenhouse Gases

There are many classifications that can be made about the use or valuation of large-scale CO2 and including the three categories proposed by Vega [16] for type of uses, which also is

used by the PTE-CO2, 2013 (Technology Platform Spanish CO2). To wit:

#### *8.2.1. Enhanced oil recovery (CO2-EOR)*

As much as two-thirds of conventional crude oil discovered in U.S. fields remain unproduced, left behind because of the physics of fluid flow. In addition, hydrocarbons in unconventional rocks or that have unconventional characteristics (such as oil in fractured shales, kerogen in oil shale or bitumen in tar sands) constitute an enormous potential domestic supply of energy.

Carbon dioxide is used in oil wells for oil extraction and to maintain pressure within a formation.

There are many methods for EOR and each has differences that make it more useful based on specific reservoir challenges and other parameters. Choosing the right method by screening the reservoir and fluid properties can ultimately reduce risk by eliminating inefficiencies.

CO2 EOR is an 'EOR' technology that injects CO2 into an underground geologic zone (oil/ hydrocarbon containing 'reservoir') that contains hydrocarbons for the purpose of producing the oil. The CO2 is produced along with the oil and then recovered and re-injected to recover more oil.

When the maximum amount of oil is recovered from the reservoir, the CO2 is then 'sequestered' in the underground geologic zone that formerly contained the oil and the well is shut-in, permanently sequestering the CO2.

CO2 injection is a technology successfully used from more than 50 years. The first patent for CO2-EOR appeared in 1952 and in 1964 began field trials. In the first commercial project of CO2-EOR in Texas, in 1972 (SACROC project), CO2 was supplied from a gas plant, where the CO2 was eliminated in the production of ammonia At present the CO2 is sent from geological formations (natural) from Bravo Dome in Colorado, and Mc Elmo Dome in New Mexico.

Nowadays, two techniques are largely used for CO2-EOR:


**Figure 12.** CO2-EOR operation diagram. CO2 injection into reservoir to 'flood'. Diagram courtesy of Dakota Gasifica‐ tion Company.

viscous, allowing the oil to be extracted more easily from the bedrock. The CO2 used to increase oil recovery can be naturally occurring, or an effective means of sequestering an industrial by-product. In this case, carbon dioxide, under pressure, is injected between oil wells to freeing the stranded oil. CO2 is a superior agent in recovering stranded oil as the CO2 naturally reduces the surface tension that traps the liquid oil to in the oil reservoir. When the oil is recovered from the production well, CO2 is also produced, but is easily separated from the crude oil because the CO2 reverts back to its gaseous state when the pressure is removed.

#### *8.2.2. Fire suppression*

Some fire extinguishers use CO2 because it is denser than air. Carbon dioxide can blanket a fire, because of its heaviness. It prevents oxygen from getting to the fire and as a result, the burning material is deprived of the oxygen it needs to continue burning.

#### *8.2.3. Supercritical CO2 uses*

When CO2 is at suitable temperature and pressure above the critical point (Figure 13), it is called supercritical CO2.

This state emphasises its capacity to dissolve chemicals and natural substances of similar way as do different organic solvents such as hexane, acetone or dichloromethane. There‐ fore, the first applications focused on **the extraction of natural substances** as an alterna‐ tive to using organic solvents. Thus, **removal of caffeine** (coffee or tea) with supercritical CO2 is the most mature application at industrial level and is also used in the **extraction of hops or cocoa fat.**

**Figure 13.** CO2 phases diagram.

viscous, allowing the oil to be extracted more easily from the bedrock. The CO2 used to increase oil recovery can be naturally occurring, or an effective means of sequestering an industrial by-product. In this case, carbon dioxide, under pressure, is injected between oil wells to freeing the stranded oil. CO2 is a superior agent in recovering stranded oil as the CO2 naturally reduces the surface tension that traps the liquid oil to in the oil reservoir. When the oil is recovered from the production well, CO2 is also produced, but is easily separated from the crude oil because the CO2 reverts back to its gaseous state when the

**Figure 12.** CO2-EOR operation diagram. CO2 injection into reservoir to 'flood'. Diagram courtesy of Dakota Gasifica‐

Some fire extinguishers use CO2 because it is denser than air. Carbon dioxide can blanket a fire, because of its heaviness. It prevents oxygen from getting to the fire and as a result, the

When CO2 is at suitable temperature and pressure above the critical point (Figure 13), it is

This state emphasises its capacity to dissolve chemicals and natural substances of similar way as do different organic solvents such as hexane, acetone or dichloromethane. There‐ fore, the first applications focused on **the extraction of natural substances** as an alterna‐ tive to using organic solvents. Thus, **removal of caffeine** (coffee or tea) with supercritical CO2 is the most mature application at industrial level and is also used in the **extraction of**

burning material is deprived of the oxygen it needs to continue burning.

pressure is removed.

*8.2.3. Supercritical CO2 uses*

called supercritical CO2.

**hops or cocoa fat.**

*8.2.2. Fire suppression*

tion Company.

100 Greenhouse Gases

The dry cleaning with CO2 is one of the most popular applications of supercritical fluids in the textile sector. This method is characterised by removing stains from the fabrics and garments where no harmful organic solvents for the average ambient, such as perchlorethylene (PER), common in conventional dry cleaning processes are used and without causing discoloration or shrinkage and without leaving odour.

One of the main advantages of supercritical CO2 is that its solubility can easily be controlled suitably adjusting the pressure and temperature, allowing **fractionate mixtures** where all components are soluble.

Supercritical CO2 extraction coupled with a fractional separation technique is used by pro‐ ducers of **flavours and fragrances** to separate and purify volatile flavour and fragrance concentrates. Like any solvent, supercritical CO2, it allows processing chemicals by precipita‐ tion or recrystallisation, obtaining particles of controlled size and shape, without excessive fines without thermal stresses and controlling the shape of a polymorphic substance.

It is, therefore, a cutting-edge technology with great potential, because it is a new way to obtain natural products; it allows the adaptation of new high quality products with appropriate value to consumer habits; enables the development of new non-polluting processes and initiate the development of a tertiary sector led to the new technology.

#### *8.2.4. Food and beverages*

Liquid or solid CO2 is used for quick freezing, surface freezing, chilling and refrigeration in the transport of foods. In cryogenic tunnel and spiral freezers, high pressure liquid CO2 is injected through nozzles that convert it to a mixture of CO2 gas and dry ice 'snow' that covers the surface of the food product. As it sublimates (goes directly from solid to gas states), refrigeration is transferred to the product.

Carbon dioxide gas is used to carbonate soft drinks, beers and wine and to prevent fungal and bacterial growth. CO2 has an inhibitory effect on bacterial growth, especially those that cause discoloration and odours.

CO2 has an inhibitory effect on bacterial growth, especially those that cause discoloration and odours (Figure 14). CO2 has an inhibitory effect on bacterial growth, especially those that cause discoloration and odours (Figure 14).

It is used as an inert 'blanket', as a product-dispensing propellant and an extraction agent. It can also be **Figure 14.** CO2 applications in food.

Figure 14. CO2 applications in food.

used to displace air during canning. Cold sterilisation can be carried out with a mixture of 90% carbon dioxide and 10% ethylene oxide, the It is used as an inert 'blanket', as a product-dispensing propellant and an extraction agent. It can also be used to displace air during canning.

carbon dioxide has a stabilising effect on the ethylene oxide and reduces the risk of explosion. **8.2.5. Water treatment** Cold sterilisation can be carried out with a mixture of 90% carbon dioxide and 10% ethylene oxide, the carbon dioxide has a stabilising effect on the ethylene oxide and reduces the risk of explosion.

#### Carbon dioxide can change the pH of water because of its slightly dissolution in water to form carbonic *8.2.5. Water treatment*

because of excessive aggressive CO2.

environment by preventing other chemicals:

handling mineral acids

bicarbonates, calcium carbonate inlaid and the CO2 added.

meaningful in water treatment plants, because soft water is indigestible.

Safe neutralisation. Avoiding risk of over-acidification with strong acids

acid, H2CO3 (a weak acid), according to Equation 9:

**(9)** Carbon dioxide can change the pH of water because of its slightly dissolution in water to form carbonic acid, H2CO3 (a weak acid), according to Equation 9:

$$\text{CO}\_2 + \text{H}\_2\text{O} \rightarrow \text{H}\_2\text{CO}\_3\tag{9}$$

**(10)**

bicarbonate ion HCO3- , according to Equation 10: Carbonic acid reacts slightly and reversibly in water to form a hydronium cation H3O+ , and the bicarbonate ion HCO3 - , according to Equation 10:

Carbonic acid reacts slightly and reversibly in water to form a hydronium cation H3O+, and the

$$H\_2CO\_3 + H\_2O \to HCO\_3^- + H\_3O^+ \tag{10}$$

approximately 5.5 when it has been exposed to air. This chemical behaviour explains why water, which normally has a neutral pH of 7 has an acidic pH of approximately 5.5 when it has been exposed to air.

This chemical behaviour explains why water, which normally has a neutral pH of 7 has an acidic pH of

At the moment, CO2 technology is widely introduced in treatments such as sewage water, industrial water or drinking water remineralisation. At the moment, CO2 technology is widely introduced in treatments such as sewage water, industrial water or drinking water remineralisation.

The increased requirements of drinking water in large cities becomes necessary to use sources of very soft water and because of its low salinity and pH are very aggressive and can bring on corrosion phenomena in the pipes of the pipeline, with the appearance of colour and turbidity when these pipes are made of iron, The increased requirements of drinking water in large cities becomes necessary to use sources of very soft water and because of its low salinity and pH are very aggressive and can bring on corrosion phenomena in the pipes of the pipeline, with the appearance of colour and turbidity

and by undermining these ones made with cement fibre by dissolving the calcium carbonate (CaCO3),

The introduction of carbon dioxide in the pipes regulates a state of equilibrium between dissolved

Therefore, for the treatment of soft or aggressive waters, the use of CO2 in combination with lime or calcium hydroxide is advisable to increase water hardness. This process is called **remineralisation** and is

The use of CO2 in wastewater neutralisation, Figure 15, offers great advantages in the operation and the

Better working conditions. Eliminate the risk of burns, toxic fumes and other injuries from

when these pipes are made of iron, and by undermining these ones made with cement fibre by dissolving the calcium carbonate (CaCO3), because of excessive aggressive CO2.

The introduction of carbon dioxide in the pipes regulates a state of equilibrium between dissolved bicarbonates, calcium carbonate inlaid and the CO2 added.

Therefore, for the treatment of soft or aggressive waters, the use of CO2 in combination with lime or calcium hydroxide is advisable to increase water hardness. This process is called **remineralisation** and is meaningful in water treatment plants, because soft water is indiges‐ tible.

The use of CO2 in wastewater neutralisation, Figure 15, offers great advantages in the operation and the environment by preventing other chemicals:


**•** Economy

Carbon dioxide gas is used to carbonate soft drinks, beers and wine and to prevent fungal and bacterial growth. CO2 has an inhibitory effect on bacterial growth, especially those that cause

CO2 has an inhibitory effect on bacterial growth, especially those that cause discoloration and

CO2 has an inhibitory effect on bacterial growth, especially those that cause discoloration and odours

It is used as an inert 'blanket', as a product-dispensing propellant and an extraction agent. It can also be

It is used as an inert 'blanket', as a product-dispensing propellant and an extraction agent. It

Cold sterilisation can be carried out with a mixture of 90% carbon dioxide and 10% ethylene oxide, the carbon dioxide has a stabilising effect on the ethylene oxide and reduces the risk of

Cold sterilisation can be carried out with a mixture of 90% carbon dioxide and 10% ethylene oxide, the

Carbon dioxide can change the pH of water because of its slightly dissolution in water to form carbonic

Carbon dioxide can change the pH of water because of its slightly dissolution in water to form

**(9)**

*CO H O H CO* 2 2 23 + ® (9)


**(10)**

, and

carbon dioxide has a stabilising effect on the ethylene oxide and reduces the risk of explosion.

Carbonic acid reacts slightly and reversibly in water to form a hydronium cation H3O+, and the

, according to Equation 10:

This chemical behaviour explains why water, which normally has a neutral pH of 7 has an acidic pH of

*H CO H O HCO H O* 23 2 3 3

Carbonic acid reacts slightly and reversibly in water to form a hydronium cation H3O+

At the moment, CO2 technology is widely introduced in treatments such as sewage water, industrial water

At the moment, CO2 technology is widely introduced in treatments such as sewage water,

This chemical behaviour explains why water, which normally has a neutral pH of 7 has an

The increased requirements of drinking water in large cities becomes necessary to use sources of very soft water and because of its low salinity and pH are very aggressive and can bring on corrosion phenomena in the pipes of the pipeline, with the appearance of colour and turbidity when these pipes are made of iron, and by undermining these ones made with cement fibre by dissolving the calcium carbonate (CaCO3),

The increased requirements of drinking water in large cities becomes necessary to use sources of very soft water and because of its low salinity and pH are very aggressive and can bring on corrosion phenomena in the pipes of the pipeline, with the appearance of colour and turbidity

The introduction of carbon dioxide in the pipes regulates a state of equilibrium between dissolved

Therefore, for the treatment of soft or aggressive waters, the use of CO2 in combination with lime or calcium hydroxide is advisable to increase water hardness. This process is called **remineralisation** and is

The use of CO2 in wastewater neutralisation, Figure 15, offers great advantages in the operation and the

Better working conditions. Eliminate the risk of burns, toxic fumes and other injuries from

, according to Equation 10:

acidic pH of approximately 5.5 when it has been exposed to air.


industrial water or drinking water remineralisation.

carbonic acid, H2CO3 (a weak acid), according to Equation 9:

discoloration and odours.

odours (Figure 14).

102 Greenhouse Gases

Figure 14. CO2 applications in food.

**8.2.5. Water treatment**

*8.2.5. Water treatment*

explosion.

bicarbonate ion HCO3-

used to displace air during canning.

**Figure 14.** CO2 applications in food.

acid, H2CO3 (a weak acid), according to Equation 9:

can also be used to displace air during canning.

approximately 5.5 when it has been exposed to air.

bicarbonates, calcium carbonate inlaid and the CO2 added.

meaningful in water treatment plants, because soft water is indigestible.

Safe neutralisation. Avoiding risk of over-acidification with strong acids

or drinking water remineralisation.

the bicarbonate ion HCO3

because of excessive aggressive CO2.

environment by preventing other chemicals:

handling mineral acids

(Figure 14).

**Figure 15.** Dosing system for sewage.

#### *8.2.6. Carbonate mineralisation*

The alkaline waste management presents significant problems, mainly because of its volume and its geochemical properties that do not allow disposing in conventional landfills. Therefore, the accelerated carbonation of this waste is another technological uses of CO2.

Carbonate mineralisation refers to the conversion of CO2 to solid inorganic carbonates. Naturally occurring alkaline and alkaline-earth oxides react chemically with CO2 to produce minerals, such as calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). These minerals are highly stable and can be used in construction or disposed of without concern that the CO2 they contain will release into the atmosphere. One problem is that these reactions tend to be slow, and unless the reactions are carried out in situ, there is a large volume of rocks to move. Carbonates can also be used as filler materials in paper and plastic products.

#### *8.2.7. Biological utilisation*

Green plants convert carbon dioxide and water into food compounds, such as glucose and oxygen. This process is called photosynthesis (Equation 11).

$$\text{CO}\_2 + \text{6H}\_2\text{O} \xrightarrow{\text{H}\_2\text{O}} \text{C}\_6\text{H}\_{12}\text{O}\_6 + \text{6O}\_2\tag{11}$$

The reaction of photosynthesis is as follows: Biological applications are based primarily on the use of CO2 as food for plant growth. In a similar way as the plants take advantage of sunlight and CO2 for biomass, or other products, 'imitating' nature, improving its results. Therefore, this technology is also known as biomimetic transformation.

There are two main ways in the biological utilisation process: greenhouses carbonic fertilisa‐ tion and growth of microalgae.

#### *8.2.7.1. Greenhouses carbonic fertilisation*

CO2 is found naturally in the atmosphere and, therefore, in the **greenhouse** environment. It is essential for plant growth, since it represents the carbon source for organic compounds they need, in short, for compounds that constitute their biomass (leaves, stems, fruits, etc.).

CO2 is not the only factor involved in photosynthesis, so that for its use, other factors must be at levels that do not limit the process. Light, temperature, amount of available nutrients and the relative humidity are other environmental factors affecting photosynthetic activity.

During photosynthesis, plants capture light energy and CO2 through the leaves, and water and nutrients through the roots. Thanks to these elements and chlorophyll leaves, plants get synthesise sugars and various organic compounds required for their development. Photosyn‐ thesis is responsible for plant growth. Therefore, favouring photosynthesis we managed to promote the development of the plants and agriculture in our case.

Yields of plant products grown in **greenhouses** can increase by 20% by enriching the air inside the greenhouse with carbon dioxide. The target level for enrichment is typically a carbon dioxide concentration of 800 ppm – or about two-and-a-half times the level present in the atmosphere (Figure 16).

In the CENIT SOST-CO2 project that includes the entire life cycle of CO2, researching technology uses as chemical and biological uses, the following results were confirmed, among others [18]:

various organic compounds required for their development. Photosynthesis is responsible for plant growth. Therefore, favouring photosynthesis we managed to promote the development of the plants and

Figure 16. Carbon fertilisation in hydroponics culture greenhouses. **Figure 16.** Carbon fertilisation in hydroponics culture greenhouses.


#### *8.2.7.2. Growth of microalgae*

with better quality.

agriculture in our case.

minerals, such as calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). These minerals are highly stable and can be used in construction or disposed of without concern that the CO2 they contain will release into the atmosphere. One problem is that these reactions tend to be slow, and unless the reactions are carried out in situ, there is a large volume of rocks to

Green plants convert carbon dioxide and water into food compounds, such as glucose and

The reaction of photosynthesis is as follows: Biological applications are based primarily on the use of CO2 as food for plant growth. In a similar way as the plants take advantage of sunlight and CO2 for biomass, or other products, 'imitating' nature, improving its results.

There are two main ways in the biological utilisation process: greenhouses carbonic fertilisa‐

CO2 is found naturally in the atmosphere and, therefore, in the **greenhouse** environment. It is essential for plant growth, since it represents the carbon source for organic compounds they need, in short, for compounds that constitute their biomass (leaves, stems, fruits, etc.).

CO2 is not the only factor involved in photosynthesis, so that for its use, other factors must be at levels that do not limit the process. Light, temperature, amount of available nutrients and the relative humidity are other environmental factors affecting photosynthetic activity.

During photosynthesis, plants capture light energy and CO2 through the leaves, and water and nutrients through the roots. Thanks to these elements and chlorophyll leaves, plants get synthesise sugars and various organic compounds required for their development. Photosyn‐ thesis is responsible for plant growth. Therefore, favouring photosynthesis we managed to

Yields of plant products grown in **greenhouses** can increase by 20% by enriching the air inside the greenhouse with carbon dioxide. The target level for enrichment is typically a carbon dioxide concentration of 800 ppm – or about two-and-a-half times the level present in the

In the CENIT SOST-CO2 project that includes the entire life cycle of CO2, researching technology uses as chemical and biological uses, the following results were confirmed, among

2 2 6 12 6 2 *CO H O C H O O* + ¾¾® + 6 6 (11)

move. Carbonates can also be used as filler materials in paper and plastic products.

oxygen. This process is called photosynthesis (Equation 11).

Therefore, this technology is also known as biomimetic transformation.

promote the development of the plants and agriculture in our case.

*8.2.7. Biological utilisation*

104 Greenhouse Gases

tion and growth of microalgae.

atmosphere (Figure 16).

others [18]:

*8.2.7.1. Greenhouses carbonic fertilisation*

Microalgae are photosynthetic microorganisms that can grow in diverse areas mainly in water media where the forced culture can be carried out in diverse type of reactors in concordance with its design and operation. The advantage of this process is that microalgae are a microor‐ ganism with a high production rate (some species are able to duplicate their biomass in 24 hours), and therefore with increased demand for CO2 conventional terrestrial plants. **8.2.7.2. Growth of microalgae** Microalgae are photosynthetic microorganisms that can grow in diverse areas mainly in water media where the forced culture can be carried out in diverse type of reactors in concordance with its design and operation. The advantage of this process is that microalgae are a microorganism with a high production rate (some species are able to duplicate their biomass in 24 hours), and therefore with increased demand for CO2 conventional terrestrial plants.

The investigation of microalgae culture for different purposes began in the middle of last century, when the United States launched the 'Aquatic Species Program'. At that time, the research focused on the possibility of obtaining biofuels from microalgae: mainly methane and hydrogen, but after the oil crisis in the 1970s the biodiesel was also considered. The investigation of microalgae culture for different purposes began in the middle of last century, when the United States launched the 'Aquatic Species Program'. At that time, the research focused on the possibility of obtaining biofuels from microalgae: mainly methane and hydrogen, but after the oil crisis in the 1970s the biodiesel was also considered.

Biofixation of CO2 by microalgae, especially as an option for the utilisation of flue gases from power plants, has been the subject of extensive investigations in the United States, Japan and Europe (IEA-GHG Biofixation Network). However, none of the related projects have demon‐ strated the feasibility of the concept at a pre-industrial level. What is more, CO2 fixation efficiency is quite low because of the photobioreactors used in those pilot plants (raceway or open-ponds) (Figure 17). Biofixation of CO2 by microalgae, especially as an option for the utilisation of flue gases from power plants, has been the subject of extensive investigations in the United States, Japan and Europe (IEA-GHG Biofixation Network). However, none of the related projects have demonstrated the feasibility of the concept at a pre-industrial level. What is more, CO2 fixation efficiency is quite low because of the photobioreactors used in those pilot plants (raceway or open-ponds) (Figure 17).

The current production of microalgae is mainly focused around a few species, such as *Spirulina*, *Chlorella*, *Dunaliella* or *Haematococcus* for nutritional purposes (for humans) and animal feed (especially aquaculture). Other sectors, such as cosmetics, effluent treatment and bioenergy, have shown interest, incorporating these or other species of microalgae and cyanobacteria into commercial products. Currently, 95% of the production of microalgae is based on open systems

Figure 17. Microalgae culture in open system (raceway) and close photobioreactor (Almeria University and Palmerillas Research Center). The current production of microalgae is mainly focused around a few species, such as *Spirulina*, *Chlorella*, **Figure 17.** Microalgae culture in open system (raceway) and close photobioreactor (Almeria University and Palmerillas Research Center).

(raceways or circular open ponds). These systems have a low rate of CO2 fixation and it is estimated to be around 20–50% of the injected gas is effectively set by microalgae [17]. *Dunaliella* or *Haematococcus* for nutritional purposes (for humans) and animal feed (especially aquaculture). Other sectors, such as cosmetics, effluent treatment and bioenergy, have shown interest, incorporating these or other species of microalgae and cyanobacteria into commercial products. Currently,

95% of the production of microalgae is based on open systems (raceways or circular open ponds). These

#### *8.2.8. Use of CO2 in chemicals* systems have a low rate of CO2 fixation and it is estimated to be around 20–50% of the injected gas is effectively set by microalgae [17].

Carbon dioxide gas is used to make urea (used as a fertiliser and in automobile systems and medicine), methanol, inorganic and organic carbonates, polyurethanes and sodium salicylate. Carbon dioxide is combined with epoxides to create plastics and polymers. **8.2.8. Use of CO2 in chemicals** Carbon dioxide gas is used to make urea (used as a fertiliser and in automobile systems and medicine),

Corn-to-ethanol plants have been the most rapidly growing source of feed gas for CO2 recovery. methanol, inorganic and organic carbonates, polyurethanes and sodium salicylate. Carbon dioxide is combined with epoxides to create plastics and polymers.

Corn-to-ethanol plants have been the most rapidly growing source of feed gas for CO2 recovery.

#### *8.2.8.1. Artificial photosynthesis*

catalysis [21].

Because CO2 is a practically inert molecule, artificial photosynthesis of CO2 involves the use of large amounts of energy so it must use a clean source of energy (such as solar radia‐ tion).Therefore, the use of catalytic agent to facilitate the process allowing even take place at ambient temperature and pressure is necessary. In this case, it is also called as photocatalysis or photoreduction. **8.2.8.1. Artificial photosynthesis** Because CO2 is a practically inert molecule, artificial photosynthesis of CO2 involves the use of large amounts of energy so it must use a clean source of energy (such as solar radiation).Therefore, the use of catalytic agent to facilitate the process allowing even take place at ambient temperature and pressure is necessary. In this case, it is also called as photocatalysis or photoreduction.

In photocatalysis two processes occur: CO2 reduction and oxidation of other compounds. Early works on the photocatalytic reduction of CO2 in aqueous solution were published between 1978 and 1979 ([19, 20]), and later numerous investigators have studied the mechanism and efficiency of the process using different catalysts (oxides of titanium, zinc and cadmium, cadmium sulphide, silicon carbide), and reducing (water, amines, alcohols) and R light sources (lamps xenon, mercury, halogen). Thus, it has been shown that by using specific semiconduc‐ tors and reducing agents, can be obtained a great variety of products (methane, methanol, formaldehyde, formic acid, ethanol, ethane, etc.). In photocatalysis two processes occur: CO2 reduction and oxidation of other compounds. Early works on the photocatalytic reduction of CO2 in aqueous solution were published between 1978 and 1979 ([19, 20]), and later numerous investigators have studied the mechanism and efficiency of the process using different catalysts (oxides of titanium, zinc and cadmium, cadmium sulphide, silicon carbide), and reducing (water, amines, alcohols) and R light sources (lamps xenon, mercury, halogen). Thus, it has been shown that by using specific semiconductors and reducing agents, can be obtained a great variety of products (methane, methanol, formaldehyde, formic acid, ethanol, ethane, etc.). Along with thermodynamics, catalysis is one of the core technologies for an economically interesting use of CO2 as feedstock in chemical processes. This is one of the areas most sophisticated and complex of

Along with thermodynamics, catalysis is one of the core technologies for an economically interesting use of CO2 as feedstock in chemical processes. This is one of the areas most modern chemical research. It is one of the major challenges for the scientific and technological developments related to the fields of energy and catalysis, as was highlighted in the report to officiate Sciences US Department basic Energy: more than 85% of all products are produced using chemical

Photocatalysis involve the production of reactions because of the incidence of light on a semiconductor material. Unlike metals, these materials have a forbidden energy band, which extends from the top of the

so-called valence band to the bottom of the conduction band (Figure 18).

sophisticated and complex of modern chemical research. It is one of the major challenges for the scientific and technological developments related to the fields of energy and catalysis, as was highlighted in the report to officiate Sciences US Department basic Energy: more than 85% of all products are produced using chemical catalysis [21].

Photocatalysis involve the production of reactions because of the incidence of light on a semiconductor material. Unlike metals, these materials have a forbidden energy band, which extends from the top of the so-called valence band to the bottom of the conduction band (Figure 18).

**Figure 18.** Diagram of behaviour of a semiconductor, TiO2, in light presence and participation in the photocatalytic CO2 reduction organic products.

The main disadvantage in these cases remains in the low process efficiency.

In general, the process of photocatalytic reduction of CO2 requires a milder conditions and lower energy consumption than chemical reduction [22].

#### **8.3. Chemical conversion**

(raceways or circular open ponds). These systems have a low rate of CO2 fixation and it is estimated to be around 20–50% of the injected gas is effectively set by microalgae [17].

Figure 17. Microalgae culture in open system (raceway) and close photobioreactor (Almeria University and Palmerillas

**Figure 17.** Microalgae culture in open system (raceway) and close photobioreactor (Almeria University and Palmerillas

The current production of microalgae is mainly focused around a few species, such as *Spirulina*, *Chlorella*,

*Dunaliella* or *Haematococcus* for nutritional purposes (for humans) and animal feed (especially aquaculture). Other sectors, such as cosmetics, effluent treatment and bioenergy, have shown interest, incorporating these or other species of microalgae and cyanobacteria into commercial products. Currently, 95% of the production of microalgae is based on open systems (raceways or circular open ponds). These systems have a low rate of CO2 fixation and it is estimated to be around 20–50% of the injected gas is

Carbon dioxide gas is used to make urea (used as a fertiliser and in automobile systems and medicine), methanol, inorganic and organic carbonates, polyurethanes and sodium salicylate.

Corn-to-ethanol plants have been the most rapidly growing source of feed gas for CO2

Carbon dioxide gas is used to make urea (used as a fertiliser and in automobile systems and medicine), methanol, inorganic and organic carbonates, polyurethanes and sodium salicylate. Carbon dioxide is

Corn-to-ethanol plants have been the most rapidly growing source of feed gas for CO2 recovery.

Because CO2 is a practically inert molecule, artificial photosynthesis of CO2 involves the use of large amounts of energy so it must use a clean source of energy (such as solar radia‐ tion).Therefore, the use of catalytic agent to facilitate the process allowing even take place at ambient temperature and pressure is necessary. In this case, it is also called as photocatalysis

Because CO2 is a practically inert molecule, artificial photosynthesis of CO2 involves the use of large amounts of energy so it must use a clean source of energy (such as solar radiation).Therefore, the use of catalytic agent to facilitate the process allowing even take place at ambient temperature and pressure is

In photocatalysis two processes occur: CO2 reduction and oxidation of other compounds. Early works on the photocatalytic reduction of CO2 in aqueous solution were published between 1978 and 1979 ([19, 20]), and later numerous investigators have studied the mechanism and efficiency of the process using different catalysts (oxides of titanium, zinc and cadmium, cadmium sulphide, silicon carbide), and reducing (water, amines, alcohols) and R light sources (lamps xenon, mercury, halogen). Thus, it has been shown that by using specific semiconduc‐ tors and reducing agents, can be obtained a great variety of products (methane, methanol,

In photocatalysis two processes occur: CO2 reduction and oxidation of other compounds. Early works on the photocatalytic reduction of CO2 in aqueous solution were published between 1978 and 1979 ([19, 20]), and later numerous investigators have studied the mechanism and efficiency of the process using different catalysts (oxides of titanium, zinc and cadmium, cadmium sulphide, silicon carbide), and reducing (water, amines, alcohols) and R light sources (lamps xenon, mercury, halogen). Thus, it has been shown that by using specific semiconductors and reducing agents, can be obtained a great variety of products (methane,

Along with thermodynamics, catalysis is one of the core technologies for an economically interesting use of CO2 as feedstock in chemical processes. This is one of the areas most

Photocatalysis involve the production of reactions because of the incidence of light on a semiconductor material. Unlike metals, these materials have a forbidden energy band, which extends from the top of the

Along with thermodynamics, catalysis is one of the core technologies for an economically interesting use of CO2 as feedstock in chemical processes. This is one of the areas most sophisticated and complex of modern chemical research. It is one of the major challenges for the scientific and technological developments related to the fields of energy and catalysis, as was highlighted in the report to officiate Sciences US Department basic Energy: more than 85% of all products are produced using chemical

Carbon dioxide is combined with epoxides to create plastics and polymers.

necessary. In this case, it is also called as photocatalysis or photoreduction.

so-called valence band to the bottom of the conduction band (Figure 18).

*8.2.8. Use of CO2 in chemicals*

effectively set by microalgae [17].

**8.2.8. Use of CO2 in chemicals**

combined with epoxides to create plastics and polymers.

*8.2.8.1. Artificial photosynthesis*

**8.2.8.1. Artificial photosynthesis**

formaldehyde, formic acid, ethanol, ethane, etc.).

methanol, formaldehyde, formic acid, ethanol, ethane, etc.).

or photoreduction.

catalysis [21].

recovery.

Research Center).

106 Greenhouse Gases

Research Center).

Large quantities are used as a raw material in the chemical process industry, especially for urea across CO2 reaction with NH3 and later dehydration of the formed carbamate. Urea is the product most used as agricultural fertiliser. It is used in feed for ruminants, as carbon cellulose explosives stabiliser in the manufacture of resins and also for thermosetting plastic products, among others.

Methanol production, where CO is added as additive, is very a well-known reaction. The production is carried out in two steps. The first step is to convert the feedstock natural gas into a synthesis gas stream consisting of CO, CO2, H2O and hydrogen. This is usually accomplished by the catalytic reforming of feed gas and steam. The second step is the catalytic synthesis of methanol from the synthesis gas. If an external source of CO2 is available, the excess hydrogen can be consumed and converted to additional methanol.

consumption than chemical reduction [22].

**8.3. Chemical conversion**

CO2 is also used, to make inorganic and organic carbonates, carboxylic acids, polyurethanes and sodium salicylate. Carbon dioxide is combined with epoxides to create plastics and polymers (Figure 19). CO2 is also used, to make inorganic and organic carbonates, carboxylic acids, polyurethanes and sodium salicylate. Carbon dioxide is combined with epoxides to create plastics and polymers (Figure 19).

gas and steam. The second step is the catalytic synthesis of methanol from the synthesis gas. If an external source of CO2 is available, the excess hydrogen can be consumed and converted to additional methanol.

Methanol production, where CO is added as additive, is very a well-known reaction. The production is carried out in two steps. The first step is to convert the feedstock natural gas into a synthesis gas stream

In general, the process of photocatalytic reduction of CO2 requires a milder conditions and lower energy

Large quantities are used as a raw material in the chemical process industry, especially for urea across CO2

reaction with NH3 and later dehydration of the formed carbamate. Urea is the product most used as agricultural fertiliser. It is used in feed for ruminants, as carbon cellulose explosives stabiliser in the

manufacture of resins and also for thermosetting plastic products, among others.

**9. New ways for CO2 uses Figure 19.** Different products made with CO2 derivatives.

Figure 19. Different products made with CO2 derivatives.

#### to other storage approaches, such as geologic storage. Thus, more exploratory technological investigations are needed to discover new applications and new reactions. **9. New ways for CO2 uses**

Many challenges exist for achieving successful CO2 utilisation, including the development of technologies capable of economically fixing CO2 in stable products for indirect storage. In general, the area of CO2 utilisation for carbon storage is relatively new and less well known compared to other storage approaches, such as geologic storage. Thus, more exploratory technological investigations are needed to discover new applications and new reactions.

In general, the area of CO2 utilisation for carbon storage is relatively new and less well known compared

Significant innovation and technical progress are being made across a number of utilisation technologies. The **electrochemical reduction** could be really attractive because it is an excellent way for renewable Many challenges exist for achieving successful CO2 utilisation, including the development of technologies capable of economically fixing CO2 in stable products for indirect storage.

energy storage. **9.1. Power to gas technology (P2G)** Significant innovation and technical progress are being made across a number of utilisation technologies. The **electrochemical reduction** could be really attractive because it is an excellent way for renewable energy storage.

#### In the **3rd Carbon Dioxide Utilisation Summit, October 2014 in Bremen, Germany**, ETOGAS GmbH presented its turn-Key plan and technology Power-to-Gas for SNG through electrolysis processes [18]. **9.1. Power to gas technology (P2G)**

This technology uses CO2 as a feed gas for the production of carbon products with Etogas methanation plant (Figure 20), which are reactor systems for conversion of H2 and CO2 to methane (synthetic natural In the 3rd Carbon Dioxide Utilisation Summit, October 2014 in Bremen, Germany, ETOGAS GmbH presented its turn-Key plan and technology Power-to-Gas for SNG through electrolysis processes [18].

gas). The produced gas is DVGW- and DIN-compliant synthetic natural gas and can be used directly, e.g., as a fuel for a CNG vehicle. This technology uses CO2 as a feed gas for the production of carbon products with Etogas methanation plant (Figure 20), which are reactor systems for conversion of H2 and CO2 to methane (synthetic natural gas). The produced gas is DVGW- and DIN-compliant synthetic natural gas and can be used directly, e.g., as a fuel for a CNG vehicle.

**Figure 20.** SNG schematic process. Source: ETOGAS Project.

#### **9.2. Electrochemical CO2 utilisation**

CO2 is also used, to make inorganic and organic carbonates, carboxylic acids, polyurethanes and sodium salicylate. Carbon dioxide is combined with epoxides to create plastics and

salicylate. Carbon dioxide is combined with epoxides to create plastics and polymers (Figure 19).

CO2 is also used, to make inorganic and organic carbonates, carboxylic acids, polyurethanes and sodium

In general, the area of CO2 utilisation for carbon storage is relatively new and less well known compared to other storage approaches, such as geologic storage. Thus, more exploratory technological investigations

Many challenges exist for achieving successful CO2 utilisation, including the development of technologies

In general, the area of CO2 utilisation for carbon storage is relatively new and less well known compared to other storage approaches, such as geologic storage. Thus, more exploratory technological investigations are needed to discover new applications and new reactions.

Significant innovation and technical progress are being made across a number of utilisation technologies. The **electrochemical reduction** could be really attractive because it is an excellent way for renewable

Many challenges exist for achieving successful CO2 utilisation, including the development of technologies capable of economically fixing CO2 in stable products for indirect storage.

Significant innovation and technical progress are being made across a number of utilisation technologies. The **electrochemical reduction** could be really attractive because it is an excellent

In the **3rd Carbon Dioxide Utilisation Summit, October 2014 in Bremen, Germany**, ETOGAS GmbH presented its turn-Key plan and technology Power-to-Gas for SNG through electrolysis processes [18].

This technology uses CO2 as a feed gas for the production of carbon products with Etogas methanation plant (Figure 20), which are reactor systems for conversion of H2 and CO2 to methane (synthetic natural gas). The produced gas is DVGW- and DIN-compliant synthetic natural gas and can be used directly, e.g.,

This technology uses CO2 as a feed gas for the production of carbon products with Etogas methanation plant (Figure 20), which are reactor systems for conversion of H2 and CO2 to methane (synthetic natural gas). The produced gas is DVGW- and DIN-compliant synthetic

natural gas and can be used directly, e.g., as a fuel for a CNG vehicle.

In the 3rd Carbon Dioxide Utilisation Summit, October 2014 in Bremen, Germany, ETOGAS GmbH presented its turn-Key plan and technology Power-to-Gas for SNG through electrolysis

Methanol production, where CO is added as additive, is very a well-known reaction. The production is carried out in two steps. The first step is to convert the feedstock natural gas into a synthesis gas stream consisting of CO, CO2, H2O and hydrogen. This is usually accomplished by the catalytic reforming of feed gas and steam. The second step is the catalytic synthesis of methanol from the synthesis gas. If an external source of CO2 is available, the excess hydrogen can be consumed and converted to additional methanol.

In general, the process of photocatalytic reduction of CO2 requires a milder conditions and lower energy

Large quantities are used as a raw material in the chemical process industry, especially for urea across CO2

reaction with NH3 and later dehydration of the formed carbamate. Urea is the product most used as agricultural fertiliser. It is used in feed for ruminants, as carbon cellulose explosives stabiliser in the

manufacture of resins and also for thermosetting plastic products, among others.

polymers (Figure 19).

108 Greenhouse Gases

Figure 19. Different products made with CO2 derivatives.

**9. New ways for CO2 uses**

**Figure 19.** Different products made with CO2 derivatives.

**9.1. Power to gas technology (P2G)**

way for renewable energy storage.

**9.1. Power to gas technology (P2G)**

are needed to discover new applications and new reactions.

capable of economically fixing CO2 in stable products for indirect storage.

**9. New ways for CO2 uses**

energy storage.

as a fuel for a CNG vehicle.

processes [18].

consumption than chemical reduction [22].

**8.3. Chemical conversion**

According to DNV GL, electrochemical CO2 utilisation presents some advantages as follows:

Production de-coupled from the sun (flexibility in renewable energy source); land use is minimised and no limitation with respect to geography; no competition with food (corn, sugar); flexibility in end fuel – ethanol, butanol or diesel (depending on the organism used); flexibility in electrochemical process (matching to supply/demand of renewable energy); and significant net reduction in CO2 emission (Figure 21).

**Figure 21.** Electrochemical production of formic acid (HCOOH) and CO. Source: third Carbon Dioxide Utilisation Summit. DNV GL.

#### **9.3. Polymers production**

**Bayer MaterialScience** (Germany) in the Project "Dream Production" combines part of waste streams of coal-fired power plants, CO2, with the production of polymers. The target is the design and development of a technical process able to produce CO2-based polyether polycar‐ bonate polyols on a large scale. The first step was to convert the CO2 in new polyols, and these polyols showed similar properties such as products already on the market and can be proc‐ essed in conventional plans as well (Figure 22).

**Figure 22.** Target product polyurethanes – All rounder among plastics. Source: 3rd Carbon Dioxide Utilisation Sum‐ mit. Courtesy: Bayer.

The CO2 thus acts as a substitute for the petroleum production of plastics. Polyurethanes are used to produce a wide range of everyday applications. When they are used for the insulation of buildings, the polyurethane saves about 80% more energy than it consumes during production. Light weight polymers are used in the automotive industry, upholstered furniture and mattress manufacturing.

#### **9.4. Macrofouling control in industrial facilities**

In the past years, several projects have been focused in the **direct use of flue gases** from Combined Cycle Power Plants for developing different applications. In this way, the project CENIT SOST-CO2 has demonstrated the use of flue gases from CCPP in a direct way to control the pH in the cooling water systems with refrigeration tower and Iberdrola has developed an application for power plants.

Another application for the future will be "CO2 for Zebra Mussel Control". A project developed by Iberdrola and the University of Salamanca shows that carbonic acidification just in the moment when the larva of zebra mussel are in the adequate phase (pediveliger) causes a much greater lethality than inorganic acids because of the synergistic effect of the lethal hypercapnia by physiological changes in cell metabolism of the larvae. (*CDTI Project: Seguimiento de la incidencia del mejillón cebra (Dreissena polymorpha) en el Ciclo Combinado de Castejón 2009-2011. Iberdrola – Universidad de Salamanca).*

The project LIFE13 ENV/ES/000426. CO2FORMARE [23], "Use of CO2 as a substitute of chlorine-based chemicals used in O&M Industrial processes for macrofouling remediation", led by Iberdrola Generación, seeks to demonstrate the viability of using CO2 from combustion gases to control macrofouling *(fouling caused by larger organisms, such as oysters, mussels, clams and barnacles*) in a thermal power plant (Castellon CCPP), cooled by sea water. First estimates indicate that a 400 MW CCPP (Figure 23) may be necessary to use up to 50,000 t CO2 yr–1, [23].

**Figure 23.** Castellon Power Plant. Courtesy: Iberdrola.

#### **10. Others**

**9.3. Polymers production**

110 Greenhouse Gases

mit. Courtesy: Bayer.

and mattress manufacturing.

application for power plants.

**9.4. Macrofouling control in industrial facilities**

essed in conventional plans as well (Figure 22).

**Bayer MaterialScience** (Germany) in the Project "Dream Production" combines part of waste streams of coal-fired power plants, CO2, with the production of polymers. The target is the design and development of a technical process able to produce CO2-based polyether polycar‐ bonate polyols on a large scale. The first step was to convert the CO2 in new polyols, and these polyols showed similar properties such as products already on the market and can be proc‐

**Figure 22.** Target product polyurethanes – All rounder among plastics. Source: 3rd Carbon Dioxide Utilisation Sum‐

The CO2 thus acts as a substitute for the petroleum production of plastics. Polyurethanes are used to produce a wide range of everyday applications. When they are used for the insulation of buildings, the polyurethane saves about 80% more energy than it consumes during production. Light weight polymers are used in the automotive industry, upholstered furniture

In the past years, several projects have been focused in the **direct use of flue gases** from Combined Cycle Power Plants for developing different applications. In this way, the project CENIT SOST-CO2 has demonstrated the use of flue gases from CCPP in a direct way to control the pH in the cooling water systems with refrigeration tower and Iberdrola has developed an

Another application for the future will be "CO2 for Zebra Mussel Control". A project developed by Iberdrola and the University of Salamanca shows that carbonic acidification just in the The Carbon Storage Program of the NETL (National Energy Technology Laboratory) of US Department of Energy supports four main CO2 utilisation research areas: **cement**, **polycar‐ bonate plastic, mineralisation** and **enhanced hydrocarbon recovery.** Several projects on active CO2 utilisation focused in these areas receive Department of Energy (DOE) funds that aim to obtain the goals for the Carbon Storage Program.

#### **Author details**

Bernardo Llamas1,2\*, Benito Navarrete3 , Fernando Vega2 , Elías Rodriguez4,5, Luis F. Mazadiego1 , Ángel Cámara1 and Pedro Otero6

\*Address all correspondence to: bernardo.llamas@upm.es

1 Escuela de Ingenieros de Minas y Energía, Universidad Politécnica de Madrid (UPM), Madrid, Spain

2 INERGYCLEAN Technology, Almería, Spain

3 Escuela Técnica Superior de Ingeniería de Sevilla, Sevilla, Spain

4 IBERDROLA Generación, Madrid, Spain

5 Westec Environmental Solutions, llc, Chicago, USA

6 Es.CO2. Centro de Desarrollo de Tecnologías de Captura de CO2, CIUDEN, León, France

Authors, Elías Rodriguez and Pedro Otero, retired from their affiliations.

#### **References**


[10] B. Llamas, P. Cienfuegos: *Multicriteria Decision Methodology to Select Suitable Areas for Storing CO*2. Energy & Environment. 2012. Vol. 23, 2–3, 249–264.

**Author details**

112 Greenhouse Gases

Luis F. Mazadiego1

Madrid, Spain

**References**

Bernardo Llamas1,2\*, Benito Navarrete3

, Ángel Cámara1

2 INERGYCLEAN Technology, Almería, Spain

5 Westec Environmental Solutions, llc, Chicago, USA

4 IBERDROLA Generación, Madrid, Spain

\*Address all correspondence to: bernardo.llamas@upm.es

3 Escuela Técnica Superior de Ingeniería de Sevilla, Sevilla, Spain

Authors, Elías Rodriguez and Pedro Otero, retired from their affiliations.

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Vol. 34, 9–11, 1095–1103.

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, Fernando Vega2

and Pedro Otero6

1 Escuela de Ingenieros de Minas y Energía, Universidad Politécnica de Madrid (UPM),

6 Es.CO2. Centro de Desarrollo de Tecnologías de Captura de CO2, CIUDEN, León, France

[1] http://www.esrl.noaa.gov/gmd/ccgg/trends/ [Accessed on 1 February 2016].

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[7] Toftegaard, Maja Bøg et al. *Oxy-fuel combustion of solid fuels*. Progress in Energy and

[8] P. Noothout, F. Wiersma, O. Hurtado, P. Roelofsen, D. Macdonald: CO2 pipeline in‐

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[24] http://www.co2formare.eu/ [Accessed on 12 December 2015].

## **Review of Recent Developments in CO2 Capture Using Solid Materials: Metal Organic Frameworks (MOFs)**

Mohanned Mohamedali, Devjyoti Nath, Hussameldin Ibrahim and Amr Henni

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62275

#### **Abstract**

[24] http://www.co2formare.eu/ [Accessed on 12 December 2015].

114 Greenhouse Gases

In this report, the adsorption of CO2 on metal organic frameworks (MOFs) is comprehen‐ sively reviewed. In Section 1, the problems caused by greenhouse gas emissions are ad‐ dressed, and different technologies used in CO2 capture are briefly introduced. The aim of this chapter is to provide a comprehensive overview of CO2 adsorption on solid mate‐ rials with special focus on an emerging class of materials called metal organic frame‐ works owing to their unique characteristics comprising extraordinary surface areas, high porosity, and the readiness for systematic tailoring of their porous structure. Recent liter‐ ature on CO2 capture using MOFs is reviewed, and the assessment of CO2 uptake, selec‐ tivity, and heat of adsorption of different MOFs is summarized, particularly the performance at low pressures which is relevant to post-combustion capture applications. Different strategies employed to improve the performance of MOFs are summarized along with major challenges facing the application of MOFs in CO2 capture. The last part of this chapter is dedicated to current trends and issues, and new technologies needed to be addressed before MOFs can be used in commercial scales.

**Keywords:** CO2 capture, solid sorbent, MOFs, ZIFs

#### **1. Introduction**

#### **1.1. Environmental problem and CO2 emissions**

The increasing level of CO2 emission is considered one of the major environmental challenges that our planet is facing today. The concentration of greenhouse gases in the atmosphere reached a new record in 2013, with CO2 at 396 ppm which represents 142% of the concentration of the pre-industrial era [1]. Findings of a recent global atmosphere watch reported in a

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

greenhouse gas bulletin [1] revealed that CO2 concentration has increased between 2012 and 2013, more than any other year since 1984, which was attributed to the reduced uptake by the earth's biosphere. This alarming level of CO2 shows the urgency for taking immediate actions to prevent serious repercussions of climate change. On December 2015, at the Paris Climate Conference (COP21), 195 countries adopted a historical and the first legally binding global climate agreement to keep the increase in global average temperature to well below 2o C above pre-industrial levels. The discovery of new fossil fuel reserves, combined with rising energy demand, led to an increase in the number and capacities of power plants worldwide. This situation is expected to extend into the future due to various factors such as industrial development and economic growth, especially in developing nations, which in turn is expected to further contribute to increasing levels of greenhouse gas emissions in years to come. According to a recent report by the Energy Information Administration, energy consumption is projected to rise by 56% between 2010 and 2040. Fossil fuels will continue to supply about 80% of the world energy through 2040. Industrial energy consumption represents the greatest share of emissions and is projected to consume more than 50% of the energy delivered in 2040. According to currently implemented regulations regarding fossil fuels, CO2 emissions from power plants is projected to increase by 46% compared to emission level in 2010 [2].

Among several approaches that could be used to overcome the greenhouse gas effect is the utilization of clean energy alternatives which could be the ultimate solution to the climate change problem in terms of reducing CO2 emissions. However, these green technologies still require significant modifications to the current energy framework. The great challenges facing these green technologies lie in the difficulty for implementation at industrial scale, which makes it economically infeasible when compared to fossil fuel-based power plants. This implies that unless green energy alternatives and energy infrastructure for the commerciali‐ zation and the implementation of these new technologies are attained, the pursuit of new CO2 emission reduction technologies will continue to be the most practical method to address greenhouse gas effects until the advancement in clean energy technologies reaches commercial stages.

There are three different strategies to reduce emissions of CO2 from fossil fuel-based power plants. These include post-combustion capture in which CO2 is separated from the combustion flue gas stream that is mainly composed of nitrogen and some other minor components such as water vapor and oxygen. The separation process in this scheme is a downstream unit which allows for an easy retrofit of a post-combustion capture unit to an existing power plant. However, the limitations of this technique include a low CO2 partial pressure, relatively high flue gas temperature and large quantities of CO2 in the flue gas stream [3, 4]. In the precombustion capture scenario, the fossil fuel is treated under certain temperature and pressure to gasify the fuel and produce hydrogen. This method offers streams with high CO2 partial pressure and thus easy separation by utilizing variety of solvents; however, it requires significant modifications to the power generation plant. The last scenario is called the oxy-fuel capture in which the fuel is burned under a pure oxygen environment which requires the separation of oxygen/nitrogen from an air stream. The process produces pure CO2 and water vapor which can be easily recovered through a simple condensation unit. Each separation scenario requires a different capture technology, and therefore the properties, characteristics, and operation of the separation process are also entirely different among the three strategies. The most advanced process for implementation in the field is post-combustion. We will therefore, in this chapter, focus on the post-combustion separation applications.

#### **1.2. Existing technologies for CO2 capture**

greenhouse gas bulletin [1] revealed that CO2 concentration has increased between 2012 and 2013, more than any other year since 1984, which was attributed to the reduced uptake by the earth's biosphere. This alarming level of CO2 shows the urgency for taking immediate actions to prevent serious repercussions of climate change. On December 2015, at the Paris Climate Conference (COP21), 195 countries adopted a historical and the first legally binding global

pre-industrial levels. The discovery of new fossil fuel reserves, combined with rising energy demand, led to an increase in the number and capacities of power plants worldwide. This situation is expected to extend into the future due to various factors such as industrial development and economic growth, especially in developing nations, which in turn is expected to further contribute to increasing levels of greenhouse gas emissions in years to come. According to a recent report by the Energy Information Administration, energy consumption is projected to rise by 56% between 2010 and 2040. Fossil fuels will continue to supply about 80% of the world energy through 2040. Industrial energy consumption represents the greatest share of emissions and is projected to consume more than 50% of the energy delivered in 2040. According to currently implemented regulations regarding fossil fuels, CO2 emissions from

C above

climate agreement to keep the increase in global average temperature to well below 2o

power plants is projected to increase by 46% compared to emission level in 2010 [2].

stages.

116 Greenhouse Gases

Among several approaches that could be used to overcome the greenhouse gas effect is the utilization of clean energy alternatives which could be the ultimate solution to the climate change problem in terms of reducing CO2 emissions. However, these green technologies still require significant modifications to the current energy framework. The great challenges facing these green technologies lie in the difficulty for implementation at industrial scale, which makes it economically infeasible when compared to fossil fuel-based power plants. This implies that unless green energy alternatives and energy infrastructure for the commerciali‐ zation and the implementation of these new technologies are attained, the pursuit of new CO2 emission reduction technologies will continue to be the most practical method to address greenhouse gas effects until the advancement in clean energy technologies reaches commercial

There are three different strategies to reduce emissions of CO2 from fossil fuel-based power plants. These include post-combustion capture in which CO2 is separated from the combustion flue gas stream that is mainly composed of nitrogen and some other minor components such as water vapor and oxygen. The separation process in this scheme is a downstream unit which allows for an easy retrofit of a post-combustion capture unit to an existing power plant. However, the limitations of this technique include a low CO2 partial pressure, relatively high flue gas temperature and large quantities of CO2 in the flue gas stream [3, 4]. In the precombustion capture scenario, the fossil fuel is treated under certain temperature and pressure to gasify the fuel and produce hydrogen. This method offers streams with high CO2 partial pressure and thus easy separation by utilizing variety of solvents; however, it requires significant modifications to the power generation plant. The last scenario is called the oxy-fuel capture in which the fuel is burned under a pure oxygen environment which requires the separation of oxygen/nitrogen from an air stream. The process produces pure CO2 and water vapor which can be easily recovered through a simple condensation unit. Each separation

In order to locate metal organic frameworks (MOFs) on the map of the technologies used for CO2 capture applications, we will briefly describe the major technologies that have been employed and discuss their advantages and limitations. Figure 1 shows the different technol‐ ogies used for CO2 capture, whereas MOFs are used under the category of membranes and adsorbents.

The most widely investigated technology for CO2 capture from flue gas is absorption using aqueous amine solutions such as monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA), as well as blends of different amines [5–7]. Amine scrubbers are considered a well-developed technology and is available in commercial scale for postcombustion capture applications [8]. The major limitations of this technology include the high energy required for solvent regeneration, stability of the amine system at the regeneration conditions, and the negative influence of impurities present in the flue gas that might signifi‐ cantly affect the stability and performance of the solvent [9, 10].

Under the category of absorption technology and in order to overcome the limitations of amine-based technologies, aqueous ammonia as a solvent for CO2 separation has also been widely used to benefit from the low heat of absorption of ammonia-based solvents as compared to amine systems. Besides, the ammonia can also absorb other impurities existing in the gas stream such as NO and SOx. The major drawback of ammonia-based solvents lies in the need to cool the flue gas prior to introducing it to the absorption column to prevent ammonia losses to the gas stream. This adds a huge energy requirement considering the large volume of flue gas stream that typically needs to be treated [11]. The chilled ammonia process faces similar issues in addition to fouling of the heat exchanger by ammonium bicarbonate deposition from saturated solutions [4].

Great efforts have been made to find new and efficient materials for absorptive CO2 separation. Ionic liquids (ILs) are liquid salts composed of cations and anions, have been proposed as promising solvents to replace the existing amine-based solvents. ILs possess several remark‐ able properties that make their application in CO2 separation one of the hottest research topics in the last few years [12–14]. These properties include low volatility, high CO2 solubility, good thermal stability, and the possibility of systematically tuning the structure toward certain properties [15–17]. Several review papers reporting experimental data related to CO2 solubil‐ ity, selectivity, effect of ILs structure on performance, and the stability of ILs are available [12, 18]. Recent developments on the application of the amine-modified ILs, known as task-specific ILs (TSILs), are also widely discussed in the literature [19, 20], including both physical and chemical interactions with CO2. Unfortunately, many ILs and TSILs suffer from a common problem of high viscosity after CO2 absorption. Even though some recent reports mentioned the availability of ILs with low viscosities, it is still evident that much work has to be done to

overcome this limitation. Finding cheap routes for the synthesis of these materials is one of the greatest challenges facing researchers working in this area [21]. In this chapter, a great portion will be dedicated to the incorporation of ILs into the pores of MOFs to improve their CO2 capture capabilities.

**Figure 1.** Different technologies used for CO2 capture [22]**.**

### **2. CO2 capture using solid sorbents**

#### **2.1. Criteria for the evaluation of solid sorbents**

In order to evaluate solid materials for their performance in CO2 separation from flue gases, some important performance criteria must be met. These include:

**• Adsorption capacity**: it is a key criteria in evaluating solid sorbent performance. It provides information on the amount of CO2 that could be adsorbed by a given solid material. It can be represented in terms of gravimetric uptake which is the amount of CO2 adsorbed per unit mass sorbent (gram CO2/gram sorbent, or cm3 CO2/gram sorbent). The volumetric uptake is another measure for capacity, and it reports the CO2 uptake per volume of sorbent material (gram CO2/cm3 sorbent, or cm3 CO2/cm3 sorbent). This criterion is of great importance because it represents the amount of sorbent needed for a particular duty and therefore the size of the adsorption bed. It is also considered a crucial factor in determining the energy requirement during the regeneration step.


In the following sections, we describe the main solid sorbents used for CO2 capture, their applications, major attributes, and limitations.

#### **2.2. Zeolites**

overcome this limitation. Finding cheap routes for the synthesis of these materials is one of the greatest challenges facing researchers working in this area [21]. In this chapter, a great portion will be dedicated to the incorporation of ILs into the pores of MOFs to improve their CO2

In order to evaluate solid materials for their performance in CO2 separation from flue gases,

**• Adsorption capacity**: it is a key criteria in evaluating solid sorbent performance. It provides information on the amount of CO2 that could be adsorbed by a given solid material. It can be represented in terms of gravimetric uptake which is the amount of CO2 adsorbed per unit mass sorbent (gram CO2/gram sorbent, or cm3 CO2/gram sorbent). The volumetric uptake is another measure for capacity, and it reports the CO2 uptake per volume of sorbent material

because it represents the amount of sorbent needed for a particular duty and therefore the size of the adsorption bed. It is also considered a crucial factor in determining the energy

sorbent). This criterion is of great importance

capture capabilities.

118 Greenhouse Gases

**Figure 1.** Different technologies used for CO2 capture [22]**.**

**2. CO2 capture using solid sorbents**

(gram CO2/cm3

**2.1. Criteria for the evaluation of solid sorbents**

requirement during the regeneration step.

some important performance criteria must be met. These include:

sorbent, or cm3 CO2/cm3

Zeolites are porous crystalline aluminosilicate materials available naturally, but can also be prepared synthetically. The zeolite framework is composed of tetrahedral T atoms where T could be Si or Al, connected by oxygen atoms to form rings of different pore structures and sizes. The pore size of the zeolite framework varies between 5 and 12 Å [23]. They are widely used as catalysts in the refining industry [24, 25], fine chemicals synthesis [26, 27], and in gas separation applications [28, 29]. Zeolites are considered promising candidates in CO2 capture application as has been widely reported in the literature [30–32]. CO2 can be adsorbed on zeolites through different mechanisms, such as molecular sieving effect based on the difference in size [33, 34]. Separation can also take place based on polarization interactions between the gas molecule and the electric field on the charged cations in the zeolite framework [33]. Accordingly, CO2 removal with zeolites can be controlled by changing the pore size, polarity, and the nature of the extra framework cation. Among the different zeolites investigated for CO2 capture applications, zeolite 13X is the most widely studied sorbent, and is considered the benchmark technology for solid sorbents [35, 36]. Research on the use of zeolites as sorbents for CO2 capture can be categorized into different areas depending on the approach and the techniques adopted to address the improvement in capture performance. These categories comprise tuning the pore size, designing zeolites with controlled polarities, investigating novel zeolites, optimizing the cation exchange, and most recently incorporating amine moieties and other chemical functions into the zeolite frameworks. Ocean et al. [37] have studied the selectivity to adsorb CO2 by controlling the pore size of an NaKA zeolite through the synthesis of nanosized NaKA zeolites. Overall, the adsorption kinetics on the nanosized crystals was fast enough for CO2 capture applications; however, the formation of a thin layer on the nanosized NaKA zeolite, due to intergrowth on the surface, did not considerably improve the adsorption kinetics. In contrast, Goj et al. [38] performed atomistic simulations for silicalite, ITQ-3, and ITQ-7, and reported a positive effect on CO2 uptake and selectivity by tuning the pore apertures. Sravanthi et al. [39] provided a novel approach to control the pore size and volume by utilizing pore expansion agents and obtained average pore size around 30 nm. The application of the pore-expanded MCM-41 in CO2 separation resulted in the uptake of about 1.2 mmol/g.

Several studies have been conducted to control zeolite affinity toward CO2, which can be realized by tuning the polarity of the zeolite through alteration of the Si/Al ratio and the nature of the cation. Remy et al. [33] studied the selective separation of CO2 on low-silica KFI zeolite (Si/Al = 1.67) by employing ion exchange with Na, Li, and K. Li-exchanged KFI has shown the highest CO2 uptake which was attributed to the large pore volume as compared to Na and K cations. In comparison with high-silica KFI sample (Si/Al = 3.57–3.67), Li–KFI had the highest capacity at low pressure due to the strong electrostatic field. The overlap between pore size and polarity effects is also strongly observed for amine-supported zeolites, which have gained considerable attention in the last few years [40–45]. For instance, Ahmad et al. [46] have impregnated melamine into β-zeolite and obtained dynamic CO2 uptake of 3.7 mmol/g at atmospheric pressure and temperature of 25 ° C. The major challenge facing amine-modified zeolites is the tradeoff between the increased affinity toward CO2 (strong interaction with the sorbent) and the reduction in pore volume, and consequently the uptake, especially, at low pressures. Factors such as amines loading, distribution, and the nature of the cation can play a vital role to avoid the blockage of the porous structures with the bulky amine moieties [42, 47]. Kim et al. [48] have performed a rigorous investigation through the simulation of thousands of zeolites to evaluate the adsorption properties of these materials and identify the optimum structures for improved CO2 separation attributes. This study provides a systematic approach to rank and select appropriate zeolites for the required capture objectives. However, important factors such as stability under humid environment, adsorbent and process cost, and the availability of zeolite structures were not taken into consideration.

The hydrophilic nature of most zeolite structures is considered a major drawback of zeolites especially for post-combustion CO2 applications [49, 50]. Water competes with CO2 on the available sorption sites and might influence the zeolite structure and framework [51]. As explained earlier, the presence of the exposed cation sites increases CO2 uptake. In a recent study by Serena et al. [52], the relationship between the water content of the zeolite and the density of the cations was investigated, and a linear relationship was found to describe the decrease of the cation population with increasing water content. This observation highlights the detrimental effect of the presence of water vapor on the adsorption of CO2 on zeolites.

#### **2.3. Carbon-based CO2 capture**

and the nature of the extra framework cation. Among the different zeolites investigated for CO2 capture applications, zeolite 13X is the most widely studied sorbent, and is considered the benchmark technology for solid sorbents [35, 36]. Research on the use of zeolites as sorbents for CO2 capture can be categorized into different areas depending on the approach and the techniques adopted to address the improvement in capture performance. These categories comprise tuning the pore size, designing zeolites with controlled polarities, investigating novel zeolites, optimizing the cation exchange, and most recently incorporating amine moieties and other chemical functions into the zeolite frameworks. Ocean et al. [37] have studied the selectivity to adsorb CO2 by controlling the pore size of an NaKA zeolite through the synthesis of nanosized NaKA zeolites. Overall, the adsorption kinetics on the nanosized crystals was fast enough for CO2 capture applications; however, the formation of a thin layer on the nanosized NaKA zeolite, due to intergrowth on the surface, did not considerably improve the adsorption kinetics. In contrast, Goj et al. [38] performed atomistic simulations for silicalite, ITQ-3, and ITQ-7, and reported a positive effect on CO2 uptake and selectivity by tuning the pore apertures. Sravanthi et al. [39] provided a novel approach to control the pore size and volume by utilizing pore expansion agents and obtained average pore size around 30 nm. The application of the pore-expanded MCM-41 in CO2 separation resulted in the uptake of about

Several studies have been conducted to control zeolite affinity toward CO2, which can be realized by tuning the polarity of the zeolite through alteration of the Si/Al ratio and the nature of the cation. Remy et al. [33] studied the selective separation of CO2 on low-silica KFI zeolite (Si/Al = 1.67) by employing ion exchange with Na, Li, and K. Li-exchanged KFI has shown the highest CO2 uptake which was attributed to the large pore volume as compared to Na and K cations. In comparison with high-silica KFI sample (Si/Al = 3.57–3.67), Li–KFI had the highest capacity at low pressure due to the strong electrostatic field. The overlap between pore size and polarity effects is also strongly observed for amine-supported zeolites, which have gained considerable attention in the last few years [40–45]. For instance, Ahmad et al. [46] have impregnated melamine into β-zeolite and obtained dynamic CO2 uptake of

amine-modified zeolites is the tradeoff between the increased affinity toward CO2 (strong interaction with the sorbent) and the reduction in pore volume, and consequently the uptake, especially, at low pressures. Factors such as amines loading, distribution, and the nature of the cation can play a vital role to avoid the blockage of the porous structures with the bulky amine moieties [42, 47]. Kim et al. [48] have performed a rigorous investigation through the simulation of thousands of zeolites to evaluate the adsorption properties of these materials and identify the optimum structures for improved CO2 separation attributes. This study provides a systematic approach to rank and select appropriate zeolites for the required capture objectives. However, important factors such as stability under humid environment, adsorbent and process cost, and the availability of zeolite structures were not taken into

The hydrophilic nature of most zeolite structures is considered a major drawback of zeolites especially for post-combustion CO2 applications [49, 50]. Water competes with CO2 on the

C. The major challenge facing

3.7 mmol/g at atmospheric pressure and temperature of 25 °

1.2 mmol/g.

120 Greenhouse Gases

consideration.

Carbon-based adsorbents have been used for CO2 separation in different forms including activated carbons (ACs), carbon nanotubes (CNTs), and graphenes. Activated carbons have an amorphous porous structure with high surface areas that are readily available for CO2 uptake. They have been widely investigated as sorbents for CO2 removal due to their low cost and the availability of raw materials [53–55]. However, there are no active sites to bond with the adsorbed CO2 as cations in zeolite sorbents. This weak interaction results in lower enthalpy and therefore lower energy requirement for regeneration. On the contrary, ACs have very low CO2 uptake at low pressures due to the absence of the electric field on the surface. Kacem et al. [56] performed a comparison between the performance of ACs and zeolite for CO2/N2 and CO2/CH4 separation based on their capacity, regeneration capacity, and reusability. It was concluded that at higher pressures (above 4 bars), the CO2 uptake for ACs was much higher than zeolites. Also, the recovered CO2 after the regeneration of ACs had higher purity than in the case of zeolites. When compared to zeolites, ACs maintain their adsorption stability even in the presence of water vapor which does not cause any framework failure [57].

In order to enhance the adsorption capacity on ACs, several studies have been conducted in order to improve the affinity toward CO2 by introducing amine-based functional groups [58– 61]. In a recent study, Maria et al. [62] described a systematic surface modification of micro‐ porous ACs through a stepwise chemical treatment. They were successful in grafting amine and amide functional groups on the surface of ACs with only 20% loss of surface area. Gibson et al. [63] studied the polyamine-impregnated porous carbons and achieved 12 times higher CO2 capacity than bare porous carbon. Chitosan and triethylenetetramine have been success‐ fully impregnated onto the surface of ACs and have shown 60 and 90% increased CO2 uptake at 298 K and 40 bars. In addition to amine functional groups, ammonia-modified ACs, at atmospheric pressure and a temperature range from (303 to 333) K, have been studied [64]. Authors report that an enthalpy of 70.5 kJ/mol was obtained compared to 25.5 kJ/mol for the pristine ACs, suggesting the possibility of chemisorption. Another report has also supported the improved adsorption capacity and selectivity by employing NH3 at high temperature and has considerably improved CO2 uptake from 2.9 mmol/g for the bare AC to 3.22 mmol/g for the modified one at 303 K and 1 bar.

Several studies have been dedicated to the application of amine-modified carbon nano tubes (CNTs) as solid sorbents for CO2 separation [65–69]. Industrial grade CNTs have been functionalized with tetraethylenepentamine (TEPA) by Liu et al. [65], and the effects of amine loadings on the CO2 uptake, heat of adsorption, and adsorbent regenerability were investi‐ gated. TEPA-impregnated CNTs have shown an enhanced capacity of 3.09 mmol/g at 343 K. Similar studies were also reported using different amines such as (3-aminopropyl)triethoxy‐ silane (APTES) [70], polyethyleneimine (PEI) [67], and other amines (primary, secondary, tertiary, diamines, and tri-amines) [71].

Graphene is a planar sheet of carbon atoms extended in two dimensions, and was discovered in 2004 [72]. Graphite-based capture was recently introduced (after 2011) as a promising candidate for CO2 capture applications, and research is growing rapidly in this area [73–77]. A recent review by Najafabadi is available on the current status and research trends of using graphene and its derivatives as solid sorbents for CO2 capture [78]. Research in this area involves grafting various functional groups on graphene such as N-doped graphene compo‐ sites (surface area = 1336 m2 /g), as reported by Kemp et al. [79], which showed a reversible CO2 capacity of 2.7 mmol/g at 298 K and 1 atm as well as enhanced stability for repeated adsorption cycles. Borane-modified graphene was also reported by Oh et al. [80], obtaining a CO2 uptake of 1.82 mmol/g at 298 K and 1 atm. Some novel hybrid materials have also been introduced to obtain better improvements in the adsorption properties, including mesoporous graphene oxide (GO)-ZnO nanocomposite [81], mesoporous TiO2/graphene oxide nanocom‐ posites [82], Mg–Al layered double hydroxide (LDH), graphene oxide [83], MOF-5 and aminated graphite oxide (AGO) [84], UiO-66/graphene oxide composites [85], and MIL-53(Al) and its hybrid composite with graphene nanoplates (GNP) [86].

#### **2.4. Metal organic frameworks**

A more recent class of porous materials was manufactured and named metal organic frame‐ works. They represent one of the promising adsorbents and have gained significant attention during recent years for gas separation applications [87, 88]. MOFs are composed of metal ions or clusters (nodes) bridged by organic ligands (connecters) to form various structures and networks. MOFs are well recognized for their extraordinary surface areas, ultrahigh porosity, and most importantly the flexibility to tune the porous structure as well as the surface functionality due to the presence of organic ligands that can easily be chemically modified [89, 90]. One main advantage of MOFs over other solid materials is the possibility to tailor the pore size and functionality by rational selection of the organic ligand, functional group, metal ion, and activation method.

Several review papers are available in the literature for gas separation using MOFs [91–96]; however, great progress has been achieved during the past four years (2012 onward). In order to address the limitations of MOFs and investigate new structures, novel functional groups, in addition to hybrid systems and technologies, more studies are needed to explore the mechanisms involved and to improve the uptake capacity in a humid environment. For these reasons, considerable effort has been observed during the past decade to address gas separa‐ tion and adsorption using MOFs. Figure 2 shows the number of publications on CO2 capture and separation using MOFs during the past 15 years, which reflects the growing interest of MOFs as efficient solid sorbents.

Review of Recent Developments in CO2 Capture Using Solid Materials: Metal Organic Frameworks (MOFs) http://dx.doi.org/10.5772/62275 123

**Figure 2.** Number of publications on CO2 capture using MOFs (based on Web of Science database)

#### CO2 capture performance of different MOFs will be comprehensively reviewed in terms of their capacity, selectivity, **3. Adsorption of CO2 on metal organic frameworks**

provided. The next section is dedicated to review the most recent studies of CO2 capture and separation on MOFs, and we will mainly target the works published in the last four years. **5. Evaluation of MOFs in CO2 Capture**  As introduced earlier, capacity, selectivity, and heat of adsorption are considered the main criteria for the evaluation CO2 capture performance of different MOFs will be comprehensively reviewed in terms of their capacity, selectivity, heat of reaction, and major challenges facing researchers, and some ideas to approach these challenges will also be provided. The next section is dedicated to review the most recent studies of CO2 capture and separation on MOFs, and we will mainly target the works published in the last four years.

heat of reaction, and major challenges facing researchers, and some ideas to approach these challenges will also be

**Figure 2.** Number of publications on CO2 capture using MOFs (based on Web of Science database)

#### pressure corresponds to post-combustion applications. The gravimetric uptake of CO2 is indicative of the ability of **3.1. Evaluation of MOFs in CO2 Capture**

pressure data.

**4. Adsorption of CO2 on metal organic frameworks** 

gated. TEPA-impregnated CNTs have shown an enhanced capacity of 3.09 mmol/g at 343 K. Similar studies were also reported using different amines such as (3-aminopropyl)triethoxy‐ silane (APTES) [70], polyethyleneimine (PEI) [67], and other amines (primary, secondary,

Graphene is a planar sheet of carbon atoms extended in two dimensions, and was discovered in 2004 [72]. Graphite-based capture was recently introduced (after 2011) as a promising candidate for CO2 capture applications, and research is growing rapidly in this area [73–77]. A recent review by Najafabadi is available on the current status and research trends of using graphene and its derivatives as solid sorbents for CO2 capture [78]. Research in this area involves grafting various functional groups on graphene such as N-doped graphene compo‐

CO2 capacity of 2.7 mmol/g at 298 K and 1 atm as well as enhanced stability for repeated adsorption cycles. Borane-modified graphene was also reported by Oh et al. [80], obtaining a CO2 uptake of 1.82 mmol/g at 298 K and 1 atm. Some novel hybrid materials have also been introduced to obtain better improvements in the adsorption properties, including mesoporous graphene oxide (GO)-ZnO nanocomposite [81], mesoporous TiO2/graphene oxide nanocom‐ posites [82], Mg–Al layered double hydroxide (LDH), graphene oxide [83], MOF-5 and aminated graphite oxide (AGO) [84], UiO-66/graphene oxide composites [85], and MIL-53(Al)

A more recent class of porous materials was manufactured and named metal organic frame‐ works. They represent one of the promising adsorbents and have gained significant attention during recent years for gas separation applications [87, 88]. MOFs are composed of metal ions or clusters (nodes) bridged by organic ligands (connecters) to form various structures and networks. MOFs are well recognized for their extraordinary surface areas, ultrahigh porosity, and most importantly the flexibility to tune the porous structure as well as the surface functionality due to the presence of organic ligands that can easily be chemically modified [89, 90]. One main advantage of MOFs over other solid materials is the possibility to tailor the pore size and functionality by rational selection of the organic ligand, functional group, metal ion,

Several review papers are available in the literature for gas separation using MOFs [91–96]; however, great progress has been achieved during the past four years (2012 onward). In order to address the limitations of MOFs and investigate new structures, novel functional groups, in addition to hybrid systems and technologies, more studies are needed to explore the mechanisms involved and to improve the uptake capacity in a humid environment. For these reasons, considerable effort has been observed during the past decade to address gas separa‐ tion and adsorption using MOFs. Figure 2 shows the number of publications on CO2 capture and separation using MOFs during the past 15 years, which reflects the growing interest of

and its hybrid composite with graphene nanoplates (GNP) [86].

/g), as reported by Kemp et al. [79], which showed a reversible

tertiary, diamines, and tri-amines) [71].

sites (surface area = 1336 m2

122 Greenhouse Gases

**2.4. Metal organic frameworks**

and activation method.

MOFs as efficient solid sorbents.

MOFs to adsorb CO2 and, therefore, we have reported CO2 uptake along with MOF surface area, and other properties for MOFs published after 2012 which could be added to the published reviews that have listed these data in a table format. Table 1 represents the properties of MOFs at high-pressure applications, while Table 2 presents the low-As introduced earlier, capacity, selectivity, and heat of adsorption are considered the main criteria for the evaluation of MOFs for CO2 separation. CO2 uptake is a proportional function

**Table 1.** Adsorption capacities at high pressure

of MOFs for CO2 separation. CO2 uptake is a proportional function of pressure in the gas phase, where the low

of pressure in the gas phase, where the low pressure corresponds to post-combustion appli‐ cations. The gravimetric uptake of CO2 is indicative of the ability of MOFs to adsorb CO2 and, therefore, we have reported CO2 uptake along with MOF surface area, and other properties for MOFs published after 2012 which could be added to the published reviews that have listed these data in a table format. Table 1 represents the properties of MOFs at high-pressure applications, while Table 2 presents the low-pressure data.



**Table 1.** Adsorption capacities at high pressure

of pressure in the gas phase, where the low pressure corresponds to post-combustion appli‐ cations. The gravimetric uptake of CO2 is indicative of the ability of MOFs to adsorb CO2 and, therefore, we have reported CO2 uptake along with MOF surface area, and other properties for MOFs published after 2012 which could be added to the published reviews that have listed these data in a table format. Table 1 represents the properties of MOFs at high-pressure

applications, while Table 2 presents the low-pressure data.

**Surface Area (m2**

Common name BET Langmuir Capacity

Cu3(H2L2

124 Greenhouse Gases

Cu3(H2L2

(NO3)2}n

(BF4)2}n

{Ag3[Ag5(l3-3,5-Ph2tz)6]

{Ag3[Ag5(l3-3,5-tBu2tz)6]

**/g)**

(wt%)

UiO(bpdc) 2646 2965 72.5 20 303 [97] ZJU-32 3831 49 40 300 [98] UPG-1 410 514 11.9 9.8 298 24 24 [99]

)(bipy)2.11H2O 6.4 8.5 298 [100]

)(etbipy)2.24H2O 4.7 9.6 298 [100]

NU-111 4932 61.8 30 298 **23** [101] HTS-MIL-101 3482 52.8 40 298 [102] DGC-MIL-101 4198 59.8 40 298 [102] UTSA-62a 2190 43.7 55 298 16 [103] ZIF-7 312 355 20.9 10 298 33 [104]

Basolite® C 300 1706.42 41.9 224.99 318 18 [106] Basolite® F300 1716.46 24.1 224.99 318 19 [106] Basolite® A100 1524.8 26.9 224.99 318 9 [106] IRMOF-8 1599 1801 7.8 1 298 21.1 [107] IRMOF-8-NO2 832 926 3.8 1 298 35.4 [107] MIL-101(Cr) 2549 24.2 30 303 [108] HKUST-1 1326 26.3 30 303 [108] DMOF 1980 38.1 20 298 12a 20 [109] DMOF-DM1/2 1500 27.5 20 298 [109] DMOF-Br 1320 24.3 20 298 [109] DMOF-NO2 1310 32 20 298 [109]

Pressure (bar)

Temp.

12.3 10 298 10.5 19.1 [105]

5.4 10 298 14 15 [105]

(K) Selectivity Qst

(kJ/mol)

Ref.



**Surface Area (m2**

Common name BET Langmuir Capacity

Zn(5-mtz)(2-eim).(guest)

Zn(5-mtz)(2-pim).(guest)

[ZTIF-1]

126 Greenhouse Gases

[ZTIF-2]

Cu3(H2L1

Cu3(H2L2

Cu3(H2L2

[Zn2(BME-bdc)x(DBbdc)2\_xdabco]n

**/g)**

(wt%)

UTSA-49 710.5 1046.6 13.6 1 298 95.8 [118] ZJNU-40 2209 16.4 1.01 296 18.4 [119] UPG-1 410 514 2.1 1 298 24 24 [99] InOF-8 6.9 1 295 45.2 [120]

)(bipy)2.9H2O 2.5 1 195 [100]

)(bipy)2.11H2O 2.3 1 298 [100]

)(etbipy)2.24H2O 0.5 1 298 [100]

UiO-66(Zr100) 1390 1644 6.2 1 298 26 [121] UiO-66(Ti32) 1418 1703 6.4 1 298 28 [121] UiO-66(Ti44) 1749 2088 7.2 1 298 34 [121] UiO-66(Ti56) 1844 2200 8.8 1 298 37 [121] NU-111 4932 4.8 1 298 23 [101] JLU-Liu1 145 221 5.9 1 298 47.7 [122] HTS-MIL-101 3482 12.3 1 298 [102] DGC-MIL-101 4164 14.5 1 298 [102] UNLPF-1 13.9 1 273 [123] UTSA-62a 2190 8.1 1 298 16 [103]

Zn-DABCO 1870 1902 7.2 1 298 22.4 [125] Ni-DABCO 2120 2219 8.1 1 298 25.8 [125] Cu-DABCO 1616 1678 6.2 1 298 22.4 [125] Co-DABCO 2022 2095 4.1 1 298 29.8 [125] ZnAcBPDC 920 11.7 0.9 293 [126] ZnBuBPDC 850 7.6 0.89 293 [126] Mg/DOBDC 1415.1 25 1 298 47 [127] Co/DOBDC 1089.3 21.6 1 298 37 [127]

Pressure (bar)

1430 1981 8.2 1 295 81 22.5 [117]

1287 1461 3.8 1 295 20 [117]

Temp.

21.7 0.91 195 [124]

(K) Selectivity Qst (kJ/

mol)

Ref.



**Table 2.** Adsorption capacities at low pressure

#### **3.2. Strategies to Improve the CO2 Capture Performance on MOFs**

Several strategies have been adopted to improve the performance of MOFs in CO2 capture applications. The ability to precisely tune the MOF structures has led to versatile approaches that can be utilized to enhance CO2 uptake, selectivity, and the affinity toward CO2. These methods could be classified into effects of open metal sites, pre-synthetic modifications of the organic ligand, and post-synthetic functionalization schemes.

#### *3.2.1. Open Metal Sites*

**Surface Area (m2**

Common name BET Langmuir Capacity

128 Greenhouse Gases

a

IAST selectivity

**Table 2.** Adsorption capacities at low pressure

**3.2. Strategies to Improve the CO2 Capture Performance on MOFs**

Several strategies have been adopted to improve the performance of MOFs in CO2 capture applications. The ability to precisely tune the MOF structures has led to versatile approaches

**/g)**

(wt%)

DMOF-NO2 1310 9.9 1 298 [109] DMOF-TM1/2 1210 8.1 1 298 [109] DMOF-TF 1210 3.3 1 298 9a 18 [109] DMOF-Cl2 1180 8.8 1 298 17a 21 [109] DMOF-OH 1130 9.6 1 298 [109] DMOF-DM 1120 1 298 23a 23 [109] DMOF-TM 1050 13.3 1 298 28a 29 [109] DMOF-A 760 10.6 1 298 [109] CPM-33a 966 1257 12.6 1 298 22.5 [133] CPM-33b 808 1119 19.9 1 298 25 [133] Ni3OH(NH2bdc)3tpt 805 1115 14.8 1 298 21.5 [133] Ni3OH(1,4-ndc)3tpt 222 310 4.6 1 298 25.3 [133] Ni3OH(2,6-ndc)3tpt 1002 1392 7.9 1 298 24.7 [133] Ni3OH(bpdc)3tpt 724 1009 5.5 1 298 18.7 [133] ZIF-7-S 150 3.7 1 303 [134] ZIF-7-D 25 9 1 303 [134] ZIF-7-R 5 8.7 1 303 34 [134] HKUST-1 2203 12.8 1 313 [135] Fe-MIL-100 2990 6.6 1 313 [135] Zn(pyrz)2(SiF6) 10.8 1 313 [135] Mg2(dobpdc) 1940 23.8 1 313 [135] Ni2(dobpdc) 1593 21.2 1 313 [135] mmen-Mg2(dobpdc) 15.8 1 313 [135] mmen-Ni2(dobpdc) 7.3 1 313 [135] mmen-CuBTTri 11.3 1 313 [135]

Pressure (bar)

Temp.

(K) Selectivity Qst (kJ/

mol)

Ref.

Open metal sites in MOFs are formed by the removal of a solvent molecule coordinated to the metal nodes by applying vacuum and/or heat after the synthesis of framework in a process called "activation." The presence of open metal sites on the MOF framework has a great impact on the selectivity toward CO2 as well as on the binding energy between the adsorbed CO2 molecules and the surface of MOF sorbents. These coordinately open metal centers act as binding sites where CO2 molecules can attach and bind to the pore surface by the induction of dipole–quadrupole interactions. Allison et al. [136] have developed a systematic procedure to precisely understand the interactions between the CO2 molecule and the force field generated by the open metal sites in MOF-74. The developed method allows for accurate estimation of adsorption isotherms using computational approach which enables the evaluation of different hypothetical open metal sites. These observations confirm previous findings of Kong et al. on understanding CO2 dynamics in MOFs with open metal centers [137]. Among the MOF family, HKUST-1, M-MIL-100, M-MIL-101, and M-MOF-74 are the most widely studied frameworks with open metal sites (M represents the metal site). However, to precisely investigate the influence of the open metal sites, we need to isolate the effects of the nature of organic ligands, the synthesis route, and functional groups present in the framework. It was observed that utilizing light metal sites provides higher surface areas, and therefore improve CO2 uptake at low pressures for MOF-74 [138]. Several studies have reported the effects of metal centers using computational approach as reported for M-MOF-74 [138–140] where noble metals such as Rh, Pd, Os, Ir, and Pt are considered promising candidates for CO2 capture (see Figure 3).

Casey et al. [141] studied the isostructural series of HKUST-1 for various metal centers (Mo, Ni, Zn, Fe, Cu, and Cr) to get insights into the adsorption mechanism and the force field created by different metal types. It was found that the presence of divalent metals such as Mg2+ significantly increased CO2 binding strength and resulted in higher selectivity toward CO2. In addition to the nature of the metal nodes, it was found that the activation method plays a vital role in determining CO2 uptake and affinity toward CO2 which was in agreement with Llewellyn et al. [142] for MIL-100 and MIL-101, where various activation methods resulted in different CO2 loadings and heat of adsorption.

In a recent study, Cabelo and coworkers [143] investigated the interaction between CO2 and the unsaturated Cr(III), V(III), and Sc(III) metal sites in MIL-100 framework using variable temperature infrared spectroscopy. The enthalpy of adsorption for Cr(III), V(III), and Sc(III) were amounted to be (−63, −54, and −48) kJ/mol, respectively, which are considered among the highest values for CO2 adsorption on MOFs with open metal centers to date. The synthesis and characterization of an M-DABCO series (M = Ni, Co, Cu, Zn) were described by Sumboon et al. [125] to systematically evaluate the effect of the metal identity on surface area, pore volume, and CO2 uptake. It was concluded that Ni-DABCO has shown the highest pore volume and specific surface area due to the high charge density present at the metal center. Comparison

**Figure 3.** Top: Δ*E* for CO2 adsorption (in kJ/mol) in M-MOF-74. Bottom: Magnitude of the adsorption energy of CO2 relative to H2O. A positive value in this plot means that CO2 binds more strongly than H2O (Adapted from [139]).

of the M-DABCO with activated carbons and MIL-100(Cr) revealed that the unsaturated cations possess exceptional CO2 uptake of 180 cm3 /g at 1 bar and 298 K [as compared to 30 cm3 /g for ACs and 60 cm3 /g for MIL-100(Cr),144].

#### *3.2.2. Pre-synthetic Modifications of MOFs*

Organic ligands are the linkers that connect the metal nodes together and therefore determine the framework structure, pore volume, pore window, and surface area which are very crucial characteristics in CO2 separation process. Ligand functionalization is considered to be a powerful tool to improve the adsorption of CO2 on MOFs due to the wide range of functional groups and the ease of modifying the organic ligand through strong covalent interactions. In a recent computational work by Torrissi et al., the impacts of various functional groups attached to the ligand part were investigated by density functional theory (DFT) [145]. The incorporation of amine functional moieties to the organic ligands has witnessed much attention in recent years, due to the proven positive effect of the presence of open nitrogen sites on the MOF frameworks [146]. Keceli et al. [147] studied four biphenyl ligands modified with amide groups of different chain lengths. Varying the length of the alkylamide group has shown a great impact on the porosity, surface area, and CO2 capacity. It was also evident that the activation procedure has great influence on the surface area of the resulting material which is attributed to the different mechanisms of solvent removal from the MOF framework. Three amino-functionalized MOFs have been prepared from 2-aminoterephthalate (ABDC) and three different metals (Mg, Co, and Sr). Despite a low surface area (63, 71, and 2.5 for Mg, Co, and Sr, respectively) and a relatively low CO2 uptake (1.4 mmol/g at 1 bar and 298 K), the prepared MOFs had exceptional selectivity toward CO2 (396 was recorded for Mg-ABDC) and exhibited high heat of adsorption [148]. Shimizu and coworkers [149] used 3-amino-1,2,4 triazole ligands to design a 3D structure MOF with 782 m2 /g surface area and 0.19 cm3 /g pore volume that is capable to achieve CO2 uptake of 4.35 mmol/g at 1.2 bar and 273 K. Moreover, the as-synthesized MOF has shown enthalpy of adsorption of 40.8 kJ/mol at zero coverage which was comparable to the commercial zeolite NaX (48.2 kJ/mol). In a similar study, Xiong et al. [118] used triazole ligands to prepare a new framework called UTSA-49 by incorporating nitrogen atoms and methyl functional groups on 5-methyl-1H-tetrazole ligands which recorded 13.6 wt% CO2 uptake at 1 bar and 298 K and 27 kJ/mol enthalpy (Figure 4). These observations were in agreement with work reported by Gao et al. for the influence of triazolate linkers [150]. It is essential to understand the synergistic effect between the multiple functional groups on the pore surface and their size exclusion effects which are considered potential approaches to optimize the performance of functionalized MOFs. Table 3 summarizes CO2 capture properties of MOFs modified with different amino functional groups.

of the M-DABCO with activated carbons and MIL-100(Cr) revealed that the unsaturated

**Figure 3.** Top: Δ*E* for CO2 adsorption (in kJ/mol) in M-MOF-74. Bottom: Magnitude of the adsorption energy of CO2 relative to H2O. A positive value in this plot means that CO2 binds more strongly than H2O (Adapted from [139]).

Organic ligands are the linkers that connect the metal nodes together and therefore determine the framework structure, pore volume, pore window, and surface area which are very crucial characteristics in CO2 separation process. Ligand functionalization is considered to be a powerful tool to improve the adsorption of CO2 on MOFs due to the wide range of functional groups and the ease of modifying the organic ligand through strong covalent interactions. In a recent computational work by Torrissi et al., the impacts of various functional groups attached to the ligand part were investigated by density functional theory (DFT) [145]. The incorporation of amine functional moieties to the organic ligands has witnessed much attention in recent years, due to the proven positive effect of the presence of open nitrogen sites on the MOF frameworks [146]. Keceli et al. [147] studied four biphenyl ligands modified with amide groups of different chain lengths. Varying the length of the alkylamide group has shown a great impact on the porosity, surface area, and CO2 capacity. It was also evident that the activation procedure has great influence on the surface area of the resulting material which is attributed to the different mechanisms of solvent removal from the MOF framework. Three amino-functionalized MOFs have been prepared from 2-aminoterephthalate (ABDC) and three different metals (Mg, Co, and Sr). Despite a low surface area (63, 71, and 2.5 for Mg, Co,

/g for MIL-100(Cr),144].

/g at 1 bar and 298 K [as compared to 30

cations possess exceptional CO2 uptake of 180 cm3

/g for ACs and 60 cm3

*3.2.2. Pre-synthetic Modifications of MOFs*

cm3

130 Greenhouse Gases

**Figure 4.** (a) Adsorption (solid) and desorption (open) isotherms of carbon dioxide (red circles), methane (blue squares), and nitrogen (green triangles) on UTSA-49a at 298 K. (b) Mixture adsorption isotherms and adsorption selec‐ tivity predicted by IAST of UTSA-49a for CO2 (50%) and CH4 (50%) at 298 K. (c) Mixture adsorption isotherms predict‐ ed by IAST of UTSA-49a for CO2 and N2 (10:90, 15:85, and 20:80) at 298 K. (d) Mixture selectivity predicted by IAST of UTSA-49a for CO2 and N2 (10:90, 15:85, and 20:80) at 298 K. Adapted from [118].


**Table 3.** CO2 uptake for MOFs modified with amine containing ligands

Apart from amine groups, there are other functional moieties that are proven to be effective in enhancing the performance of MOFs in CO2 capture. Phosphonate and sulfonate organic ligands have gained tremendous attention recently due to their significant improvements in MOF stability toward water [157]. Several studies are reported based on the use of phosphonate and sulfonate ligands, for instance, the selective CO2/N2 separation over nitrogen-containing phosphonate MOFs was studied by Marco et al. [100], and the synthesis, stability, porosity of the phosphonate MOFs [158], and their major applications were reported for water stability studies [159–161]. The shielding effect exhibited by phosphonate groups were responsible for the improved stability under humid conditions up to 90% relative humidity at 353 K as observed for CALF-30 [161]. The enhanced water stability of these MOFs was attributed to the kinetic blocking effect which makes the framework completely hydrophobic [159].

MOFs containing nitrogen-donor building blocks were also widely investigated, particularly adenine group which was extensively used due to framework robustness, richness in nitrogen sites, and framework diversity [162]. Song et al. [163] reported the preparation of three new adenine-based MOFs by controlling the adenine coordination with Cd metal sites. This study has provided insight into the controlled synthesis of MOFs by controlling the structure building units (SBU) which can be utilized to extend the idea to include multiple building units within the same framework. Similar studies are also available based on adenine groups as building units, where the effect of the adenine functionalization on framework topology, porosity, and adsorption behavior was investigated [164]. The use of Zn-adeninate SBU led to the discovery of highly porous Bio-MOF-11 to 14 series [165] and Bio-MOF-100 [166] with exceptional surface area (4300 m2 /g) and very large pore volume (4.3cm3 /g); however, the framework stability of these materials still needs to be addressed as the material tends to lose its porosity under harsh activation environment. This issue has been tackled by Zhang et al. [167] to prepare more stable adenine-based PCN-530 structure. Lin et al. have observed high density of open nitrogen-donor sites on 1,3,5-tris(2H-tetrazol-5-yl)benzene (H3BTT) which was responsible for the enhanced CO2 capacity [146] through the improvement of the frame‐ work porosity and the utilization of nitrogen sites readily available to adsorb CO2. However, the richness of nitrogen atoms in the framework does not necessarily favor CO2 adsorption, as reported by Gao et al. [110] for the case of tetrazolate-based rth-MOF that has more exposed nitrogen sites as compared to pyrazolate-based rht-MOF and yet was showing less CO2 uptake attributed to the strong electric field observed on the pyrazolate-based rht-MOF.

**MOF name Type of functional group**

132 Greenhouse Gases

2-Aminoisonicotinate and

**Table 3.** CO2 uptake for MOFs modified with amine containing ligands

adeninate

Zn(ad)(ain)

**CO2 uptake (wt. %)** **Enthalpy of adsorption (kJ/mol)**

ZIF-10 IM 20.9 14.9 0.9 298 - [151] ZIF-68 (bIM)(nIM) 41.3 33.3 0.9 298 1220 [151] ZIF-69 (cbIM)(nIM) 38.1 25.9 0.9 298 1070 [151] ZIF-71 dcIM 18.0 19.4 0.9 298 - [151] Cu2(**L**)(H2O)2 Pyrazol 32 - 1 195 844.5 [152] [Zn2(**L**)] Pyrazol 37.4 - 1 195 1075.4 [152] [Cd2(**L**)] Pyrazol 24.6 - 1 195 571.7 [152] [Co2(**L**)(H2O)6] Pyrazol 31.6 - 1 195 734.6 [152] Zn4(bpta)2-1 Bipyridine pillar ligands 8.2 34.82 1.2 298 413 [153] Zn4(bpta)2-1 Bipyridine pillar ligands 3.1 27.69 1.2 298 51 [153] Cu2L (DMA)4 Acrylamide 22.2 35 1 296 1433 [154]

bio-MOF-11 Adenine 22.2 33.1 1 273 1148 [156] bio-MOF-12 Adenine 16.2 38.4 1 273 - [156] bio-MOF-13 Adenine 10.4 40.5 1 273 - [156] bio-MOF-14 Adenine 8 - 1 273 17 [156] Cu(tba)2 Triazol 7.3 36 1 293 - [131]

Apart from amine groups, there are other functional moieties that are proven to be effective in enhancing the performance of MOFs in CO2 capture. Phosphonate and sulfonate organic ligands have gained tremendous attention recently due to their significant improvements in MOF stability toward water [157]. Several studies are reported based on the use of phosphonate and sulfonate ligands, for instance, the selective CO2/N2 separation over nitrogen-containing phosphonate MOFs was studied by Marco et al. [100], and the synthesis, stability, porosity of the phosphonate MOFs [158], and their major applications were reported for water stability studies [159–161]. The shielding effect exhibited by phosphonate groups were responsible for the improved stability under humid conditions up to 90% relative humidity at 353 K as observed for CALF-30 [161]. The enhanced water stability of these MOFs was attributed to the

kinetic blocking effect which makes the framework completely hydrophobic [159].

MOFs containing nitrogen-donor building blocks were also widely investigated, particularly adenine group which was extensively used due to framework robustness, richness in nitrogen sites, and framework diversity [162]. Song et al. [163] reported the preparation of three new

**Pressure Temperature**

9.2 40 1 298 399 [155]

**Surface area (m2 /g)**

**Ref.**

Other ligand modifications are also reported in literature by deploying several types of functional groups such as hydroxyl groups (OH) on Zn(BDC) [168], (CH3)2, (OH), and (COOH) on MIL-53(Al) [145], NO2 on IRMOF-8 [107] as well as alkyl and nitro groups grafted on DUT-5 [169]. Based on the contribution by Yaghi's group [170], several studies were dedicated to understand the effects of ligand extension on the pore size, surface area, and the sorption behavior of MOFs [98, 109, 133, 171–173]. Recently, zeolite-like MOFs denoted as ZTIFs have attracted great interest due to their unique characteristics for tuning the structure toward various applications [174, 175]. New frameworks (ZTIF-1 and ZTIF-2) were recently reported based on the incorporation of tetrazolates into Zn-Imidazolate structures [176], with similar structures. UTSA-49 was also reported by Chen and coworkers for the selective separation of CO2/N2 mixture [117].

Lately, the idea of mixed ligand approach for the synthesis of MOFs with tunable properties has gained much attention which allows for incorporating several functionalities within each ligand to target certain properties such as improving the stability and the capacity for CO2 simultaneously [177]. For instance, the water stability issue was tackled by Marco and coworkers [178] by utilizing two heterocyclic N-donor-mixed phosphonate-based organic ligands. The designed MOF has shown great water stability and achieved CO2 uptake of 77 cm3 /g at low pressure and 195 K. By deploying the mixed-ligand approach, Liu et al. [179] have successfully prepared Co-based MOFs containing both benzenetricarboxylic- and triazolebased ligands by using a solvo-thermal synthesis technique. The synthesized MOFs displayed CO2 uptake of up to 15.2 wt. % at 1 bar and 295 K as well as remarkable selectivity toward nitrogen. A detailed investigation of mixed ligand approach in the design of MOFs is available in literature [180, 181]; however, further work is still needed to optimize the synthesis condi‐ tions and correlate the observed performance to the appropriate constituents on the organic ligands. Recent work by Yaghi et al. provided tools to quantitatively map different functional groups incorporated into the same MOF structure [182].

#### *3.2.3. Post-synthetic Functionalization of MOFs*

As mentioned previously, tuning the affinity of the framework functionalities toward CO2 is crucial for improving adsorptive capacities. The aim is to decorate the pore surface in order to have high adsorption selectivity and capacity and yet minimize the regeneration energy. In addition to the pre-synthetic modification of the organic linker, post-synthetic functional‐ ization of MOFs (PSM) is considered a viable route to insert functionalities into the MOF structure after the formation of the basic framework. This approach can overcome the limitations observed in pre-synthetic functionalization, for instance precise control of the synthesis conditions is needed to preserve the functional groups during the solvo-thermal synthesis conditions. Note that some functional groups are not stable under synthesis conditions which require a narrow range of conditions to prepare the MOFs. Others, however, cannot be introduced to the synthesis mixture due to solubility issues, hindrance effects, and they might participate in the crystallization process and yield unwanted materials. Besides, inserting functional groups on the metal sites prior to the synthesis of the framework might intervene in the formation of the building units which can result in the deterioration of the crystal structure [183–185]. PSM is therefore considered an attractive pathway to tailor the properties of MOFs toward better CO2 capture performance.

In order to make use of high amine affinity toward CO2, several amine moieties were selected for the modification of various solid sorbents [186–189] including MOFs [190, 191]. Ethyl‐ ene diamine (en) is considered the most commonly used type of amine for PSM of MOFs for CO2 capture application. In 2014, Lee et al. [192] reported grafting the diamine into the expanded MOF-74 or Mg(dopbdc) structure at amine loadings of 16.7 wt. % at room temperature which exhibited very high CO2 uptake of 13.7 wt% at 0.15 bar higher than the 12.1 wt. % capacity reported by McDonald et al. for N,N'-dimethylethylenediamine (mmen) grafted on Mg(dopbdc) [173]. The isosteric heat of adsorption was recorded to be 49 to 51 kJ/ mol indicating chemisorption of CO2 molecules which was further confirmed by the formation of carbamic acid probed by the in situ Fourier transform infrared spectroscopy (FTIR) experiments. The en grafted Mg(dopbdc) was further evaluated for the multicycle adsorption, and it has only lost 3% of its CO2 uptake after five cycles. Moreover, en-Mg(dopbdc) has also shown stable structure and capacity after exposure to different moisture contents, and therefore this material has a potential for large-scale CO2 capture (see Figure 5). NH2, CH2NH2, CH2NHMe along with other functional groups were recently grafted on IRMOF-74, and it was found that IRMOF-74-III-CH2NH2 displayed CO2 capacity of 3.2 mmol/ g at 1 bar [132]. The sodalite-type structure Cu-BTTri was also grafted by en functional group [193] which showed chemisorption interaction with the adsorbed CO2 molecule as can be observed from the high isosteric heat of adsorption (90 kJ/mol). However, the en-Cu-BTTri has only shown improved capacity at low pressure while the unmodified MOF shows higher uptakes at high pressure which is attributed to the significantreduction in Brunauer–Emmett– Teller (BET) surface area from 1770 to 345 m2 /g due to the pore blocking effect of the en

group. In an attempt to address this issue, McDonald at al. functionalized mmen group on Cu-BTTri and preserved a BET surface area of 870 m2 /g with 96 kJ/mol isosteric heat of adsorption, nitrogen selectivity of 327, and CO2 uptake of 9.5 wt.% under 0.15 bar CO2/0.75 bar N2 mixture at 25 °C. The negative impact of alkylamine functional groups on reducing the surface area was evident, and one approach to overcome this issue is to introduce ligand extension prior to the introduction of the amine group so as to increase the MOF porosity and avoid the pore-blocking problem during PSMs [132]. Also, a deep insight into the mechanism of CO2 adsorption on alkylamine-grafted MOFs is crucial to further under‐ stand the interactions for improved structural design and amine loadings [194]. Other amine functionalities such as piperazine were also grafted into Cu-BTTri [128] and exhibited 2.5 times higher CO2 uptake as compared to bare Cu-BTTri, while the heat of adsorption confirms the chemisorption interactions. The area reduction was also evident as it was reduced from 1700 m2 /g to 380 m2 /g (similar to ethylendiamin, (en)- Cu-BTTri [195]). Pyridine was also grafted on Ni-DOBDC to improve the water stability and increase the hydrophobicity of the material [196]. Experimental observations supported by simulations results confirmed the enhanced water stability for the Pyridine-Ni-DOBDC samples while maintaining the CO2 uptake at atmospheric conditions and low pressures. It was also concluded that the amine moiety was grafted on the unsaturated metal sites of the framework, which makes this approach desirable for amine functionalization. From a combined experimental and simula‐ tion study, it was found that pyridine modification of an MOF can reduce H2O adsorption while retaining considerable CO2 capacity at conditions of interest for flue gas separation. This indicates that post-synthesis modification of MOFs by coordinating hydrophobic ligands to unsaturated metal sites may be a powerful method to generate new sorbents for gas separation under humid conditions. Amine functionalization to target the water stability of MOFs will be further discussed in the next section.

ligands. Recent work by Yaghi et al. provided tools to quantitatively map different functional

As mentioned previously, tuning the affinity of the framework functionalities toward CO2 is crucial for improving adsorptive capacities. The aim is to decorate the pore surface in order to have high adsorption selectivity and capacity and yet minimize the regeneration energy. In addition to the pre-synthetic modification of the organic linker, post-synthetic functional‐ ization of MOFs (PSM) is considered a viable route to insert functionalities into the MOF structure after the formation of the basic framework. This approach can overcome the limitations observed in pre-synthetic functionalization, for instance precise control of the synthesis conditions is needed to preserve the functional groups during the solvo-thermal synthesis conditions. Note that some functional groups are not stable under synthesis conditions which require a narrow range of conditions to prepare the MOFs. Others, however, cannot be introduced to the synthesis mixture due to solubility issues, hindrance effects, and they might participate in the crystallization process and yield unwanted materials. Besides, inserting functional groups on the metal sites prior to the synthesis of the framework might intervene in the formation of the building units which can result in the deterioration of the crystal structure [183–185]. PSM is therefore considered an attractive pathway to tailor the

In order to make use of high amine affinity toward CO2, several amine moieties were selected for the modification of various solid sorbents [186–189] including MOFs [190, 191]. Ethyl‐ ene diamine (en) is considered the most commonly used type of amine for PSM of MOFs for CO2 capture application. In 2014, Lee et al. [192] reported grafting the diamine into the expanded MOF-74 or Mg(dopbdc) structure at amine loadings of 16.7 wt. % at room temperature which exhibited very high CO2 uptake of 13.7 wt% at 0.15 bar higher than the 12.1 wt. % capacity reported by McDonald et al. for N,N'-dimethylethylenediamine (mmen) grafted on Mg(dopbdc) [173]. The isosteric heat of adsorption was recorded to be 49 to 51 kJ/ mol indicating chemisorption of CO2 molecules which was further confirmed by the formation of carbamic acid probed by the in situ Fourier transform infrared spectroscopy (FTIR) experiments. The en grafted Mg(dopbdc) was further evaluated for the multicycle adsorption, and it has only lost 3% of its CO2 uptake after five cycles. Moreover, en-Mg(dopbdc) has also shown stable structure and capacity after exposure to different moisture contents, and therefore this material has a potential for large-scale CO2 capture (see Figure 5). NH2, CH2NH2, CH2NHMe along with other functional groups were recently grafted on IRMOF-74, and it was found that IRMOF-74-III-CH2NH2 displayed CO2 capacity of 3.2 mmol/ g at 1 bar [132]. The sodalite-type structure Cu-BTTri was also grafted by en functional group [193] which showed chemisorption interaction with the adsorbed CO2 molecule as can be observed from the high isosteric heat of adsorption (90 kJ/mol). However, the en-Cu-BTTri has only shown improved capacity at low pressure while the unmodified MOF shows higher uptakes at high pressure which is attributed to the significantreduction in Brunauer–Emmett–

/g due to the pore blocking effect of the en

groups incorporated into the same MOF structure [182].

properties of MOFs toward better CO2 capture performance.

Teller (BET) surface area from 1770 to 345 m2

*3.2.3. Post-synthetic Functionalization of MOFs*

134 Greenhouse Gases

It is evident from the previous discussion that amine impregnation into MOFs always sacrifices the surface area of the final product. Therefore, the choice of the amine that can improve the affinity toward CO2 and attain high surface area simultaneously is a trade-off issue. MIL-101 materials were reported to have the highest pore volume and surface area among MOFs to date (BET = 3125 m2 /g and 1.63 cm3 /g). Hence they allow the incorpora‐ tion of amines with longer alkyl chains such as polyethyleneimine while at the same time maintaining relatively high surface area (1112.6 m2 /g after 75 wt% amine loading). PEIloaded MIL-101 prepared by Lin et al. [197] exhibited remarkably high CO2 uptake of 4.2 mmol/g at 0.15 bar and 298 K with exceptional CO2/N2 selectivity of 770 at 25 ° C.

Optimization of amine loadings and distribution within the MOF structure is a detrimental factor for the impact of these functionalities on the performance in CO2 capture process. Precise control of the different factors during the grafting process is crucial to append these groups exactly on the unsaturated metal centers, while avoid blocking the pores and hindering access to the interior volume. Improving the PSM methods is considered one of the means to achieve the ideal grafting and amine distribution [191].

target the water stability of MOFs will be further discussed in the next section.

adsorbed CO2 molecule as can be observed from the high isosteric heat of adsorption (90 kJ/mol). However, the en-Cu-BTTri has only shown improved capacity at low pressure while the unmodified MOF shows higher uptakes at high pressure which is attributed to the significant reduction in Brunauer–Emmett–Teller (BET) surface area from 1770 to 345 m2/g due to the pore blocking effect of the en group. In an attempt to address this issue, McDonald at al. functionalized mmen group on Cu-BTTri and preserved a BET surface area of 870 m2/g with 96 kJ/mol isosteric heat of adsorption, nitrogen selectivity of 327, and CO2 uptake of 9.5 wt.% under 0.15 bar CO2/0.75 bar N2 mixture at 25 °C. The negative impact of alkylamine functional groups on reducing the surface area was evident, and one approach to overcome this issue is to introduce ligand extension prior to the introduction of the amine group so as to increase the MOF porosity and avoid the pore-blocking problem during PSMs [132]. Also, a deep insight into the mechanism of CO2 adsorption on alkylamine-grafted MOFs is crucial to further understand the interactions for improved structural design and amine loadings [194]. Other amine functionalities such as piperazine were also grafted into Cu-BTTri [128] and exhibited 2.5 times higher CO2 uptake as compared to bare Cu-BTTri, while the heat of adsorption confirms the chemisorption interactions. The area reduction was also evident as it was reduced from 1700 m2/g to 380 m2/g (similar to ethylendiamin, (en)- Cu-BTTri [195]). Pyridine was also grafted on Ni-DOBDC to improve the water stability and increase the hydrophobicity of the material [196]. Experimental observations supported by simulations results confirmed the enhanced water stability for the Pyridine-Ni-DOBDC samples while maintaining the CO2 uptake at atmospheric conditions and low pressures. It was also concluded that the amine moiety was grafted on the unsaturated metal sites of the framework, which makes this approach desirable for amine functionalization. From a combined experimental and simulation study, it was found that pyridine modification of an MOF can reduce H2O adsorption while retaining considerable CO2 capacity at conditions of interest for flue gas separation. This indicates

**Figure 5.** Top: Adsorption isotherms of CO2 for 1-en at the indicated temperatures. Bottom: Adsorption–desorption cy‐ cling of CO2 for 1-en showing reversible uptake from (a) simulated air (0.39 mbar CO2 and 21% O2 balanced with N2) and from (b) simulated flue gas (0.15 bar CO2 balanced with N2). (c) time-dependent CO2 adsorption for porous materi‐ als (A = 1-en, B = mmen-Mg2(dobpdc), C = 1, D = Mg-MOF-74, E = Zeolite 13X, F = MOF-5). (d) CO2 adsorption ratio of 1-en in flue gas (after 6 min exposure to 100% RH at 21 °C) to 1-en in flue gas (Adapted from [192]).

#### **4. Recent Advances and Current Trends**

#### **4.1. Hybrid Systems Based on MOFs**

For more efficient utilization of MOFs sorbents, several hybrid systems based on MOFs with other solid sorbents have been investigated in the literature. The objective of having hybrid materials is to utilize the synergism between the two sorbents and therefore ultimately improve the overall performance in CO2 separation. Moreover, sorbents such as activated carbons, graphenes, and CNTs provide the added feature of high surface area and easily functionalized sites which contribute to the tuning of the final properties of the composite material. CNTs represent one of the effective candidates that can improve the properties of MOFs for gas adsorption applications. Zhu et al. [198] incorporated HKUST-1 in the interspace of CNTs. The designed composite exhibited superior selectivity and a CO2 saturation capacity of 7.83 mmol/g at 298 K, which was attributed to the high porosity and surface area. In a similar study, multiwall CNTs, well dispersed in MIL-101 (Cr), were successfully prepared and maintained the same framework and crystal structure as MIL-101. An increase of 60% in CO2 uptake was observed for the MWCNT-MIL-101 composite which was attributed to the increased porosity as a result of incorporating CNTs [199], as was confirmed by similar work on MWCNT-MIL-53(Cu) composite [200].

Graphene oxide composites with different MOFs are extensively reported in the literature such as HKUST-1 [201], MOF-5 [202], and Cu-BTC [203]. Graphite oxide (GO) is considered a stabilizing agent for MOFs under humid environment, and it has shown remarkable CO2 capacity of 3.3 mmol/g and great stability under simulated flue gas conditions for GO/Cu-BTC composite [203]. The synthesis of Cu-based MOFs composite with aminated graphite oxides (GO) was carried out and fully characterized by Zhao et al. [204]. The composite exhibited 50% enhanced porosity as compared to the parent MOF and displayed unique structure and pore sizes effective for size exclusion separation of CO2 from the flue gas. Silica aerogel (SA) was also investigated as a promising candidate for hybrid systems with ZIF-8 [205]. The detailed characterizations of the SA/ZIF-8 confirmed the presence of the two phases in the composite after sol–gel synthesis procedure with different ZIF-8 loadings and mild BET surface area [205].

Several composite materials have been reported for various applications; however, utilizing these hybrid systems in CO2 adsorption might be a promising route for improving the CO2 capture process. Ahmed et al. [206] published a review of information related to the synthesis and adsorption applications of MOF composite materials.

#### **4.2. Ionic Liquids/MOF Composites**

**4. Recent Advances and Current Trends**

target the water stability of MOFs will be further discussed in the next section.

136 Greenhouse Gases

For more efficient utilization of MOFs sorbents, several hybrid systems based on MOFs with other solid sorbents have been investigated in the literature. The objective of having hybrid materials is to utilize the synergism between the two sorbents and therefore ultimately improve the overall performance in CO2 separation. Moreover, sorbents such as activated carbons, graphenes, and CNTs provide the added feature of high surface area and easily functionalized sites which contribute to the tuning of the final properties of the composite

**Figure 5.** Top: Adsorption isotherms of CO2 for 1-en at the indicated temperatures. Bottom: Adsorption–desorption cy‐ cling of CO2 for 1-en showing reversible uptake from (a) simulated air (0.39 mbar CO2 and 21% O2 balanced with N2) and from (b) simulated flue gas (0.15 bar CO2 balanced with N2). (c) time-dependent CO2 adsorption for porous materi‐ als (A = 1-en, B = mmen-Mg2(dobpdc), C = 1, D = Mg-MOF-74, E = Zeolite 13X, F = MOF-5). (d) CO2 adsorption ratio of

1-en in flue gas (after 6 min exposure to 100% RH at 21 °C) to 1-en in flue gas (Adapted from [192]).

adsorbed CO2 molecule as can be observed from the high isosteric heat of adsorption (90 kJ/mol). However, the en-Cu-BTTri has only shown improved capacity at low pressure while the unmodified MOF shows higher uptakes at high pressure which is attributed to the significant reduction in Brunauer–Emmett–Teller (BET) surface area from 1770 to 345 m2/g due to the pore blocking effect of the en group. In an attempt to address this issue, McDonald at al. functionalized mmen group on Cu-BTTri and preserved a BET surface area of 870 m2/g with 96 kJ/mol isosteric heat of adsorption, nitrogen selectivity of 327, and CO2 uptake of 9.5 wt.% under 0.15 bar CO2/0.75 bar N2 mixture at 25 °C. The negative impact of alkylamine functional groups on reducing the surface area was evident, and one approach to overcome this issue is to introduce ligand extension prior to the introduction of the amine group so as to increase the MOF porosity and avoid the pore-blocking problem during PSMs [132]. Also, a deep insight into the mechanism of CO2 adsorption on alkylamine-grafted MOFs is crucial to further understand the interactions for improved structural design and amine loadings [194]. Other amine functionalities such as piperazine were also grafted into Cu-BTTri [128] and exhibited 2.5 times higher CO2 uptake as compared to bare Cu-BTTri, while the heat of adsorption confirms the chemisorption interactions. The area reduction was also evident as it was reduced from 1700 m2/g to 380 m2/g (similar to ethylendiamin, (en)- Cu-BTTri [195]). Pyridine was also grafted on Ni-DOBDC to improve the water stability and increase the hydrophobicity of the material [196]. Experimental observations supported by simulations results confirmed the enhanced water stability for the Pyridine-Ni-DOBDC samples while maintaining the CO2 uptake at atmospheric conditions and low pressures. It was also concluded that the amine moiety was grafted on the unsaturated metal sites of the framework, which makes this approach desirable for amine functionalization. From a combined experimental and simulation study, it was found that pyridine modification of an MOF can reduce H2O adsorption while retaining considerable CO2 capacity at conditions of interest for flue gas separation. This indicates that post-synthesis modification of MOFs by coordinating hydrophobic ligands to unsaturated metal sites may be a powerful method to generate new sorbents for gas separation under humid conditions. Amine functionalization to

**4.1. Hybrid Systems Based on MOFs**

Ionic liquids as solvents for the absorption separation of CO2 from flue gas are discussed in Section 1.2 in order to overcome the limitations related to the poor dynamics of CO2 separation in ILs due to their high viscosity. MOFs can act as an ideal support material for the incorpo‐ ration of ILs into their porous structure while preserving their unique properties. The concept of immobilization of ILs into solid sorbents has been reported for various applications. For instance, ILs immobilization on mesoporous silica was reported for the catalytic esterification reaction [207], ILs addition into polymer gels for ionic conductivity applications [208], ILs/ Zeolite composites [209], in addition to several review papers available on this topic indicating the widespread use of this new approach over the past years [210, 211]. Computational investigation of the theoretical possibility of incorporating ILs into MOFs was studied by Jiang's group for (BMIM)PF6 IL supported on IRMOF-1 for CO2 capture applications [212]. The confinement effects of the narrow pore on the ILs and the ionic interactions between [BMIM]+ favor the open pore while the anion, [PF6]<sup>−</sup> , was attached to the open metal sites, was observed in a simulation study. It was ascertained that CO2 was favorably attached to the [PF6]<sup>−</sup> anions sites. The study demonstrated that IL/MOF composites are a potential candidate for CO2 adsorption and have displayed significantly high CO2/N2 selectivity. To the best of our knowledge, the first report on an experimental attempt to immobilize ILs into MOF structures was published by Liu et al. [213] for the insertion of Bronsted acidic ILs (BAIL) into the pores of MIL-101 using post-synthetic approach with triethylene diamine (TEDA) or imidazole (IMIZ) as a solvent assisting during the functionalization process. Nitrogen adsorption isotherms of bare MIL-101, TEDA-BAIL/MIL-101, and all (IMIZ-BAIL/MIL-101) samples showed type-I isotherm indicating the microporous nature of the composite. BET surface area was 1873 m2 /g for the bare MIL-101 which was slightly decreased to 1728 m2 /g and 1148 m2 /g for IMIZ-BAIL/MIL-101 and TEDA-BAIL/MIL-101, respectively. Following this leading report, Jhung's group [214] successfully grafted up to 50 wt.% acidic chloroaluminate IL on MIL-101 which reduced the BET surface area of the bare MIL-101 by around 60%. The incorporation of ILs with basic nature which is favorable for CO2 adsorption was for the first time reported by Kitagawa et al. [215]. A detailed characterization and investigation of the phase behavior of the immobilized 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonylamide) denoted as EMI-TFSA ILs into ZIF-8 was presented in this study. A reduction of 29% in pore volume was measured in N2 adsorption isotherm experiments and computational calculations. The EMI-TFSA/ZIF-8 composite has shown distinctive ion conductivity at low temperature as reported in a second paper by the same group [216]. The prospect of IL/MOF composite for gas separation is still under computational investigation with no reported experimental studies of CO2 adsorption on these composites. Recently, Vincent-Luna et al. [15] investigated the effects of adding room temperature ILs (RTILs) into the pores of Cu-BTC structures. The adsorption of CO2, N2, CH4, and their mixtures were studied by utilizing various RTILs having the same cation 1-ethyl-3-methylimidazolium [EMIM]+ and different anions such as bis[(trifluorometh‐ yl)-sulfonyl]imide [Tf2N]<sup>−</sup> , thiocyanate [SCN]<sup>−</sup> , nitrate [NO3]<sup>−</sup> , tetrafluoroborate [BF4]<sup>−</sup> , and hexafluorophosphate [PF6]<sup>−</sup> . The RTIL/Cu-BTC composite has shown enhanced CO2 uptakes at low pressures with high CO2/N2 selectivity due to the polarization driving force rendering these materials as a promising system for post-combustion CO2 capture. Another application of IL/MOF composite as a precursor for the preparation of nitrogen and boron–nitrogen (Nand BN-)-decorated porous carbons was recently reported by Aijaz et al. [217] as a novel synthesis strategy.

#### **4.3. Ab/Adsorption in Ionic Liquids/MOF Slurry System**

Another approach to utilize the combined synergistic advantages of MOF and IL composites is through a novel hybrid adsorption–absorption technology. This novel technology can provide an efficient approach to utilize the high capacity, selectivity, and low heat of adsorp‐ tion of the solid sorbents along with the advantages of having a continuous flow process that allows for better heat integration and separation rates in contrast to the conventional batch process used in adsorption-only process. Mass transfer enhancement due to the dispersion of fine solids in liquid solvents was studied and insight into the mechanism and the analysis of different mass transfer resistances were described by Zhang and coworkers [218] which was in agreement with previous findings [219–221]. As far as enhancement of CO2 capture in slurry systems is concerned, a study dealing with AC particles dispersed in K2CO3 aqueous solution was reported by Sumin et al. [222] to investigate the influence of the hydrodynamics on the mass transfer improvements. In a similar work by Rosu et al. [223], AC particles were also

Review of Recent Developments in CO2 Capture Using Solid Materials: Metal Organic Frameworks (MOFs) http://dx.doi.org/10.5772/62275 139

knowledge, the first report on an experimental attempt to immobilize ILs into MOF structures was published by Liu et al. [213] for the insertion of Bronsted acidic ILs (BAIL) into the pores of MIL-101 using post-synthetic approach with triethylene diamine (TEDA) or imidazole (IMIZ) as a solvent assisting during the functionalization process. Nitrogen adsorption isotherms of bare MIL-101, TEDA-BAIL/MIL-101, and all (IMIZ-BAIL/MIL-101) samples showed type-I isotherm indicating the microporous nature of the composite. BET surface area

/g for the bare MIL-101 which was slightly decreased to 1728 m2

for IMIZ-BAIL/MIL-101 and TEDA-BAIL/MIL-101, respectively. Following this leading report, Jhung's group [214] successfully grafted up to 50 wt.% acidic chloroaluminate IL on MIL-101 which reduced the BET surface area of the bare MIL-101 by around 60%. The incorporation of ILs with basic nature which is favorable for CO2 adsorption was for the first time reported by Kitagawa et al. [215]. A detailed characterization and investigation of the phase behavior of the immobilized 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonylamide) denoted as EMI-TFSA ILs into ZIF-8 was presented in this study. A reduction of 29% in pore volume was measured in N2 adsorption isotherm experiments and computational calculations. The EMI-TFSA/ZIF-8 composite has shown distinctive ion conductivity at low temperature as reported in a second paper by the same group [216]. The prospect of IL/MOF composite for gas separation is still under computational investigation with no reported experimental studies of CO2 adsorption on these composites. Recently, Vincent-Luna et al. [15] investigated the effects of adding room temperature ILs (RTILs) into the pores of Cu-BTC structures. The adsorption of CO2, N2, CH4, and their mixtures were studied by utilizing various RTILs having the same

/g and 1148 m2

and different anions such as bis[(trifluorometh‐

, tetrafluoroborate [BF4]<sup>−</sup>

, nitrate [NO3]<sup>−</sup>

at low pressures with high CO2/N2 selectivity due to the polarization driving force rendering these materials as a promising system for post-combustion CO2 capture. Another application of IL/MOF composite as a precursor for the preparation of nitrogen and boron–nitrogen (Nand BN-)-decorated porous carbons was recently reported by Aijaz et al. [217] as a novel

Another approach to utilize the combined synergistic advantages of MOF and IL composites is through a novel hybrid adsorption–absorption technology. This novel technology can provide an efficient approach to utilize the high capacity, selectivity, and low heat of adsorp‐ tion of the solid sorbents along with the advantages of having a continuous flow process that allows for better heat integration and separation rates in contrast to the conventional batch process used in adsorption-only process. Mass transfer enhancement due to the dispersion of fine solids in liquid solvents was studied and insight into the mechanism and the analysis of different mass transfer resistances were described by Zhang and coworkers [218] which was in agreement with previous findings [219–221]. As far as enhancement of CO2 capture in slurry systems is concerned, a study dealing with AC particles dispersed in K2CO3 aqueous solution was reported by Sumin et al. [222] to investigate the influence of the hydrodynamics on the mass transfer improvements. In a similar work by Rosu et al. [223], AC particles were also

. The RTIL/Cu-BTC composite has shown enhanced CO2 uptakes

/g

, and

was 1873 m2

138 Greenhouse Gases

cation 1-ethyl-3-methylimidazolium [EMIM]+

**4.3. Ab/Adsorption in Ionic Liquids/MOF Slurry System**

, thiocyanate [SCN]<sup>−</sup>

yl)-sulfonyl]imide [Tf2N]<sup>−</sup>

synthesis strategy.

hexafluorophosphate [PF6]<sup>−</sup>

**Figure 6.** Top left: schematic of the slurry system. (a) Comparison of selectivity toward N2. (b) ab/adsorption enthalpy. (c) CO2 uptake at 303.15 K (Adapted from [225]).

found to improve the absorptive CO2 capture process. The unique characteristics of MOFs in CO2 adsorption and their recent applications in aqueous solution environment [224] have opened the door toward the possibility of immersing MOF particles in various physical and chemical solvents for CO2 separation application. This novel unit operation process can overcome the limitations reported for conventional adsorption on MOFs such as high pressure drop and the necessity for formulating the powders into different shapes and sizes which affects their structural stability and reduces the active surface area. Liu et al. [225] reported, for the first time, the preparation of ZIF-8/glycol and ZIF-8/glycol/2-methylimidazole slurries (Figure 6). CO2 uptake of 1.25 mmol/L was recorded for the slurry system with CO2/N2 selectivity of 394 at 1 bar, and most importantly a very low enthalpy of 29 kJ/mol. In a similar work by Lei et al. [226], ILs [EMIM, TF2N] and [OMIM, PF6] were used to prepare slurry systems with ZIF-8 and ZIF-7. CO2 adsorption in the slurry system has shown a promising performance with isosteric heat of adsorption less than 26 kJ/mol. Following these two studies, the solubility of CO2 in physical solvents such as methanol mixed with ZIF-8 was also investigated [227]. The study revealed that ZIF-8 can significantly improve the low pressure-CO2 uptake in physical solvents and can dramatically reduce the solvent losses by evaporation to the gas phase at the top of the absorber. Increasing ZIF-8 loadings has shown further enhancement of the CO2 capacity, as observed previously [225]; however, it is worth noting that a high solid loading in the slurry system was not recommended from process engineering point of view as it might cause some problems during the pumping of the slurry mixture and increases the solid losses in the multicycle separation process [225].

For future studies on MOF-based slurry systems, there is basic selection of criteria that needs to be satisfied by both MOF and the liquid solution. The selection of the MOF possessing the appropriate pore size for the preparation of the slurry system is very important to guarantee that the size of the liquid is large enough and does not occupy the pores which leaves no space for CO2 to adsorb. Moreover, the structural stability of the MOF in the aqueous solution is essential so that it does not lose its porous framework nor its surface area. The selection of the liquid candidate is crucial, as it should not provide any extra mass transfer resistance for CO2 molecules. Further, experimental and computational investigations are still required to understand the separation mechanism in slurry mixtures and to have insight into the different types of interactions between the gas, liquid, and solid materials.

#### **5. Challenges and Outlook**

In conclusion, MOFs are considered the largest growing research area in CO2 capture, with great achievements and developments. Due to their versatile structures and possibilities for various functionalization approaches, the door is still open for further improvements and advancements of their performance under real flue gas conditions, and in large-scale applica‐ tions. Although we have reported MOFs with distinguished properties and exceptional CO2 capacity, selectivity, and stability, there are still some concerns that need to be addressed before reaching commercial scale level. The lack of information about the performance of MOFs under real gas mixture conditions is one of the key issues to understand the actual working uptakes and identify any possible limitations. Further experimental testing of MOFs using, for example, a gas mixture containing all the impurities that might be present in an actual flue gas is needed high-throughput technique. Computational gas mixture studies can provide essential infor‐ mation in this regard; however, experimental investigation is still considered the most reliable approach. MOF stabilities in humid conditions, high temperature, and harsh mechanical stress situations must be given much attention. Several studies were performed to target MOF stability, and great achievements were recorded in this field [228, 229], as reviewed in refer‐ ences [160, 230]. Finally, in the following section, we focus on water stability studies as it is one of the main drawbacks of MOFs.

#### **5.1. Water Stability of MOFs**

Water stability is a major challenge that has to be overcome before metal organic framework can be used in removing carbon dioxide from flue gas. The core structure of MOF reacts with water vapor content in the flue gas leading to severe distortion of the structure and even failure. As a consequence, the physical structure of MOF is changed, e.g., reduction of porosity and surface area, etc. that decreases the capacity and selectivity for CO2. Complete dehydration of flue gas increases the cost of separation. It is therefore essential for MOFs to exhibit stability in the presence of water up to certain extent [91].

Metal–ligand coordination bond, which is the most significant part of MOF, is hydrolyzed with water, resulting in the displacement of ligand bond; and as a consequence, the whole structure usually collapses [91]. The stability of MOF in the presence of water depends on the strength of metal ligand bond. p*K*<sup>a</sup> values of the ligand atom can be considered as the strength of this metal- ligand bond. Since the hydrolysis reaction between MOF and water molecule is governed by Gibbs free energy and activation energy of the reactant and product molecules, thermodynamics and kinetics factors have great influence on the water stability of MOF [160]. Insight into the molecular structure, more specifically the metal–ligand strength, the weakest part of MOF, and thermodynamics as well as kinetics study of hydrolysis reaction are very important to improve water stability. Several strategies based on these two important aspects have been taken into consideration.

For future studies on MOF-based slurry systems, there is basic selection of criteria that needs to be satisfied by both MOF and the liquid solution. The selection of the MOF possessing the appropriate pore size for the preparation of the slurry system is very important to guarantee that the size of the liquid is large enough and does not occupy the pores which leaves no space for CO2 to adsorb. Moreover, the structural stability of the MOF in the aqueous solution is essential so that it does not lose its porous framework nor its surface area. The selection of the liquid candidate is crucial, as it should not provide any extra mass transfer resistance for CO2 molecules. Further, experimental and computational investigations are still required to understand the separation mechanism in slurry mixtures and to have insight into the different

In conclusion, MOFs are considered the largest growing research area in CO2 capture, with great achievements and developments. Due to their versatile structures and possibilities for various functionalization approaches, the door is still open for further improvements and advancements of their performance under real flue gas conditions, and in large-scale applica‐ tions. Although we have reported MOFs with distinguished properties and exceptional CO2 capacity, selectivity, and stability, there are still some concerns that need to be addressed before reaching commercial scale level. The lack of information about the performance of MOFs under real gas mixture conditions is one of the key issues to understand the actual working uptakes and identify any possible limitations. Further experimental testing of MOFs using, for example, a gas mixture containing all the impurities that might be present in an actual flue gas is needed high-throughput technique. Computational gas mixture studies can provide essential infor‐ mation in this regard; however, experimental investigation is still considered the most reliable approach. MOF stabilities in humid conditions, high temperature, and harsh mechanical stress situations must be given much attention. Several studies were performed to target MOF stability, and great achievements were recorded in this field [228, 229], as reviewed in refer‐ ences [160, 230]. Finally, in the following section, we focus on water stability studies as it is

Water stability is a major challenge that has to be overcome before metal organic framework can be used in removing carbon dioxide from flue gas. The core structure of MOF reacts with water vapor content in the flue gas leading to severe distortion of the structure and even failure. As a consequence, the physical structure of MOF is changed, e.g., reduction of porosity and surface area, etc. that decreases the capacity and selectivity for CO2. Complete dehydration of flue gas increases the cost of separation. It is therefore essential for MOFs to exhibit stability

Metal–ligand coordination bond, which is the most significant part of MOF, is hydrolyzed with water, resulting in the displacement of ligand bond; and as a consequence, the whole structure

types of interactions between the gas, liquid, and solid materials.

**5. Challenges and Outlook**

140 Greenhouse Gases

one of the main drawbacks of MOFs.

in the presence of water up to certain extent [91].

**5.1. Water Stability of MOFs**

Jasuja et al. [231] performed a study on the effects of functionalization of the organic ligand in a series of isostructural MOFs in the Zn(BDC-X)-(DABCO)0.5 family on water stability. In this experiment, they cyclically stabilized an unstable parent structure in humid conditions through the incorporation of tetramethyl-BDC ligand. The results of molecular simulation disclosed that the kinetic stability is improved due to the carboxylate oxygen in the DMOF-TM2 structure which acted as a shield to prevent hydrogen-bonding interactions and subse‐ quent structural transformations. Hence, electrophilic zinc atoms in this structure became inaccessible to the nucleophilic oxygen atoms in water, resulting in prevention of the hydrol‐ ysis reactions for the displacement ligand. They also performed another study to evaluate the effect of strength of metal–ligand coordination bond and catenation in the framework on water stability [232]. According to their results, the non-interpenetrated MOFs constructed from a pillar ligand of higher p*K*a exhibited higher stability; however, interpenetrated MOFs con‐ structed from a pillar ligand of lower p*K*a values exhibited less stability. The interpenetration in MOF with incorporation of ligands of relatively high basicity exhibited good water stability. By considering the results of previous experiment, they synthesized cobalt-, nickel-, copper-, and zinc-based, new pillared MOFs of similar topologies which exhibited good water stability [233]. The grafted methyl group on the benzene dicarboxlate (BDC) ligand introduced steric factors around the metal centers; consequently, water stability of MOF drastically improved. The basicity of BTTB-based MOFs synthesized with bipyridyl pillar ligands had lower basicity than DABCO; however, they exhibited better stability in the presence of humid condition.

Bae et al. [114] performed a study to modify Ni-DOBDC with pyridine molecules. The study showed that pyridine molecule made the normally hydrophilic internal surface more hydro‐ phobic; as a result, water absorption was reduced, while substantial CO2 capture capacity was retained to a certain level. Fracaroli et al. [132] improved the interior of IRMOF-74-III by covalently functionalizing it with a primary amine, and used a MOF, IRMOF-74-IIICH2NH2, for the selective capture of CO2 in 65% relative humidity.

Zhang et al. [234] performed a study to modify the surface of the MOF hydrophilic to hydro‐ phobic to improve water stability. They demonstrated a new strategy to modify hydrophobic polydimethysiloxane (PDMS) on the surface to significantly enhance their water resistance by a facile vapor deposition technique. In this study, they successfully coated three vulnerable MOFs according to the water stability (MOF-5, HKUST-1, and ZnBT), while the porosity, crystalline characteristics, and surface area were unchanged.

All these studies demonstrated that water stability of MOFs can be improved by incorporating specific factors (e.g., metal–ligand strength, thermodynamic and kinetic factors, etc.) which govern the structural stability of the framework.

#### **Author details**

Mohanned Mohamedali, Devjyoti Nath, Hussameldin Ibrahim and Amr Henni\*

\*Address all correspondence to: amr.henni@uregina.ca

Industrial/Process Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, SK, Canada

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