**3. Nanoporous carbon synthesis**

There are a large range of carbon-rich precursors namely from vegetable (e.g. wood, fruit shells, stones or peels) or petrochemical origin that can be successfully transformed into activated carbons. However, the economically viable large-scale production of activated carbons requests the use of raw materials that gather high availability and constancy with high density and hardness, low inorganic content and, last but not the least, low cost. Thus, commercially available activated carbons are mainly produced from coal (i.e. lignite, sub-bituminous, bituminous and anthracite), coconut shell charcoal and wood charcoal. The raw material is a key issue for activated carbon producers since the development of the porous network occurs by consumption of the (char)coal precursor and the preparation yields can reach values as low as 10% (wt.). Consequently, assuring the supply of high quality precursors at a cost-effective price is of paramount importance to control the quality of the final material and the production cost. In 2015, the global activated carbon market was valued by Markets and Markets at USD 4.74 billion corresponding to 2743.7 kton being expected to continue to growth, mainly driven by more stringent government regulations to assure human and environmental protection. As already mentioned, (char)coal is the most important raw material for activated carbon producers but they are not the major (char)coal consumer. In fact, activated carbons are only a "by-product" of (char)coal production, which have the energy sector as major market (mainly for metallurgical, industrial and cooking fuel) that actually controls demand and prices. This reality, allied to the fact that most of raw materials came from the Asian continent, and that external factors (e.g. natural disasters) may compromise the quality, availability and cost of (char)coal, continues to drive the research for finding new, and more sustainable, raw materials and preparation routes for activated carbon production.

sulfur or phosphorus containing groups. These heteroatoms are mainly located at the edges of the basal planes due to the presence of unsaturated carbon atoms that are highly reactive. **Figure 1** summarizes the most relevant oxygen, nitrogen, and sulfur-containing surface groups that may be present in the surface of nanoporous carbon materials, being important to mention that oxygen surface groups are, by far, the most common ones. These materials may also contain inorganic matter, sometimes attaining 20% (wt.), being this ash content mainly

The elemental composition and type of surface groups of a nanoporous carbon influence its performance in both gaseous and liquid phase processes, due to specific interactions with the adsorptive and also the solvent in the case of adsorption from solution. Properties like hydrophobicity/hydrophilicity or acid/base behavior are highly dependent on the surface chemistry of these materials. Carbons are in general hydrophobic, but the presence of increasing amounts of oxygen surface functionalities favors the adsorption of water molecules due to the formation of hydrogen bonds and, consequently, its wettability. This ability of porous carbon materials may be advantageous, or not, depending on the desired application. For example, in the impregnation of carbon supports with catalysts in aqueous phase, a high wettability will increase the impregnation degree; but in the adsorption of organic compounds from diluted aqueous solutions, higher wettability may led to the formation of water molecule clusters that hinder the diffusion of the target compounds

towards the adsorption active sites (the same being valid for gas steams purification).

qualitative information about the surface of the nanoporous carbons [23].

acterization methods, several reference texts are available [8, 21, 23–27].

common ones), electrical conductivity, hardness and density.

**3. Nanoporous carbon synthesis**

Regarding acid/base character, nanoporous carbons are considered amphoteric materials due to the presence of both acid and basic sites in their surface. Thus, depending on the amount and strength of all the surface groups, the materials may present net acid, basic or neutral surfaces. The surface chemistry of nanoporous carbons can be assessed through numerous techniques, and the best way to get a good characterization is to employ complementary techniques and combine the analysis of the results. For example, Boehm and potentiometric titrations provide qualitative and quantitative information on the surface of the nanoporous carbons, while temperature programmed desorption (TPD) detects more oxygen groups than Boehm titration, although with less quantitative information. On the other hand, X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared spectroscopy (DRIFT) provide

Other properties of nanoporous carbon materials that may play important roles in specific applications are the morphology (powdered (PAC) and granular (GAC) forms are the most

For more specific information regarding properties of nanoporous carbon materials and char-

There are a large range of carbon-rich precursors namely from vegetable (e.g. wood, fruit shells, stones or peels) or petrochemical origin that can be successfully transformed into activated carbons. However, the economically viable large-scale production of activated carbons

inherited from the carbon precursor [12].

42 Porosity - Process, Technologies and Applications

Conventional methods for activated carbon production request the use of high temperature kilns where, under a controlled atmosphere, the carbon-rich raw materials are transformed into nanoporous carbons. There are numerous procedures for the production of activated carbons, although the great majority of synthetic routes published and patents registered worldwide, including those allowing nowadays industrial production, fall in two major categories: the selective gasification of raw materials' carbon atoms (physical or thermal activation), and the co-carbonization of the precursor with oxidizing and/or dehydrating agents (chemical activation). The number of steps requested for the production of activated carbons depends on the characteristics of the raw material, morphological specifications of the final product, and on the type of activation. A general process flowsheet is presented in **Figure 2**.

The most commonly used **pre-treatments** are those aiming to obtain a given particle size or shape, washing steps for dirt removal and/or for the reduction of the inorganic content (i.e. acid washing) and pre-oxidation to prevent fluidization of coking coal during carbonization [8, 12]. The raw material can be crushed and sieved to obtain powdered or granular particles with specific dimensions. Milling, binder addition and briquetting are other common options when working with precursors presenting low volatile content (e.g. medium-high rank coals,

**Figure 2.** General flowsheet for conventional physical (thermal) and chemical activation processes.

volatiles <20–30% (wt.)). To assure an efficient diffusion of the activating agent to all the individual coal particles of the briquette, it is common to mill and subsequently briquette the particles to obtain granular materials with a well-developed network of transport pores (i.e. meso and macropores) [8]. Direct activation of granular coal with low volatile content will not allow an uniform activation of the granules due to the incipient network of transport pores.

occurs along with templating by the activating agent, thus justifying the porosity evolution

Chemical activation has advantages over physical process related with the use of a single heating step and lower temperatures, usually higher yields, shorter activation time (hours) and higher surface area and pore volumes. All these add-ons came with a price since this

vating agent, a time-consuming washing step is needed to remove chemicals from the newly

declining due to the environmental issues associated with zinc residues [12], low recovery

Zinc chlorine and phosphoric acid lead to higher activation yields (≈40%) compared to physical activation since the use of these compounds inhibit the formation of tar and other by-products formed in physical activation [12]. In the case of potassium hydroxide, the more complex activation mechanism gathering K intercalation in carbon lattice and gasification allows to prepare activated carbon with a large range of yields depending on the precursor: from 80 and 70% [31–33] to values, as low as, 20–10% [34–36]. KOH allows obtaining superactivated carbons (over 3000 m<sup>2</sup> g−1) when high KOH/precursor weight ratios (i.e. between 2 and 4) are used.

**Table 1** gathers information regarding the most suitable precursors for the commonly used activating agents, as well as, the type of porosity obtained, and the effect of experimental conditions on the pore size distribution. It is important to emphasize that the effect of experimental conditions on pore size distribution presented in **Table 1** is a general trend; thus, for an in-depth knowledge of the effect of activating agent and experimental conditions on the porous characteristics

The above-mentioned precursors and activating agents are those more representative for industrial scale production of activated carbons. However, regarding the production of specialty carbons (low volume processes) and for research works, the number of options increases exponentially. Regarding raw materials, hard biomass residues like shells and stones allow to obtain high quality PACs or GACs; phenol formaldehyde polymers yield high surface area porous carbons, while polyacrylonitrile (PAN), acrylonitrile textile or rayon are adequate to synthetize activated carbon fibers or cloths [24]. When aiming nitrogen doped materials, for example for the adsorption of sulfur species, the use of polymers containing nitrogen (e.g.

PAN) is a common synthetic route. In what concerns activating agents, besides ZnCl<sup>2</sup>

At industrial scale, steam activation continues to be the most widely used method to produce activated carbons. The advantages of steam activation are related with the fact that does not request post-activation work-up, namely a final washing step, is less expensive and has less environmental constrains than the chemical activation. Besides, for precursors with less than 10% of ash, carbon materials with surface areas of 1000 m<sup>2</sup> g−1 can be easily obtained with activation yields of 50% [8], thus allowing a good compromise between production cost and

) can also be used [24].

CO<sup>3</sup> , Na<sup>2</sup> CO<sup>3</sup>

formed pore network and, if possible, to recover the activating agent. Actually, ZnCl<sup>2</sup>

efficiencies, corrosion problems and presence of residual zinc in final carbon [8].

or KOH) than the physical one, and activating

http://dx.doi.org/10.5772/intechopen.72476

). Also, regardless the acti-

use is

45

, H<sup>3</sup> PO<sup>4</sup>

) and metal chlorides (e.g.

and H<sup>3</sup>

PO<sup>4</sup>

Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters

impregnated.

of a given raw material, a comprehensive study is always needed.

and KOH, alkali hydroxides and salts (e.g. NaOH, K<sup>2</sup>

Cl, CuCl<sup>2</sup>

AlCl<sup>3</sup>

, FeCl<sup>3</sup>

porosity development.

, NH<sup>4</sup>

activation process is more corrosive (e.g. ZnCl<sup>2</sup>

agents can be hazardous for the environment (i.e. ZnCl<sup>2</sup>

with the amount of ZnCl<sup>2</sup>

**Physical or thermal activation** is generally made in two consecutive heating steps: carbonization under inert atmosphere (usually nitrogen) to devolatilize the raw material, followed by activation that consists in the partial gasification of the obtained char with oxidizing agents (i.e. steam, carbon dioxide or a mixture of both) leading to the formation of the porous network. While the carbonization usually occurs at temperatures between 400 and 600°C, the gasification requests temperatures ranging from 800 to 1000°C. Depending on the raw material, it is also possible to avoid the carbonization step and proceed directly to the thermal activation. Activation with CO<sup>2</sup> must occur in conditions that assure chemical control (slow activation rate—days) instead of diffusional control which is faster but leads to external particle burning and, consequently, to poor porosity development [8]. The reactions of steam and carbon dioxide with carbon are endothermic, thus thermal activation needs external energy supply to maintain the requested high temperature. Oxygen (or air) is not commonly used as oxidizing agent since its reaction with carbon is highly exothermic and fast, being difficult to control and assure porosity development instead of particle consumption [8, 12, 24]. Due to this and to the safety issues related to the temperature control, oxygen activation is scarcely used. However, low amounts of oxygen (or air) can be added during thermal activation with steam or carbon dioxide to help maintaining the high temperatures by reacting with the gases produced during activation (i.e. CO and H<sup>2</sup> ) [24]. This approach has the advantage of lowering the pressure of CO and H<sup>2</sup> , both inhibiting gases for the activation, and increasing the partial pressure of the activating agent [24].

**Chemical activation** usually requests only one heating step: the raw material is mixed with an activating agent (e.g. ZnCl<sup>2</sup> , H<sup>3</sup> PO<sup>4</sup> , KOH) and is further treated under controlled atmosphere at temperatures from 400 to 900°C, depending on the selected activating agent. The solid product obtained need to be exhaustively washed with water to remove the chemicals and dried before storage. In this process, the mechanism of pore formation is dependent on the chemical agent: zinc chloride promotes the removal of water molecules from the lignocellulosic structures of the raw material while phosphoric acid combines chemically with them [24]. In none of these processes, the selective removal of carbon atoms occurs. With potassium hydroxide, the process is more complex since there is the disintegration of the structure followed by intercalation of the metallic potassium [24, 28, 29] into the "graphitic" laminar structure, breaking down particles and limiting granular activated carbons synthesis. Simultaneously, there is also some gasification of carbon atoms due to reaction with CO<sup>2</sup> and H2 O, resulting from the redox reaction of carbon with potassium compounds [30]. During carbonization (i.e. heat treatment under inert atmosphere), the lignocellulosic precursor losses volume by contraction, but when chemical activation is applied there is incorporation of the activating reagent inside the particles inhibiting the expected contraction, that is, the activating agent may behave as a template for the formation of the microporosity [24]. In fact, in 2004, Molina-Sabio and Rodríguez-Reinoso [30] published an enlightening research that allowed to conclude that the dehydration of the carbon precursor (i.e. peach stones) by ZnCl<sup>2</sup>

occurs along with templating by the activating agent, thus justifying the porosity evolution with the amount of ZnCl<sup>2</sup> impregnated.

volatiles <20–30% (wt.)). To assure an efficient diffusion of the activating agent to all the individual coal particles of the briquette, it is common to mill and subsequently briquette the particles to obtain granular materials with a well-developed network of transport pores (i.e. meso and macropores) [8]. Direct activation of granular coal with low volatile content will not allow an uniform activation of the granules due to the incipient network of transport pores. **Physical or thermal activation** is generally made in two consecutive heating steps: carbonization under inert atmosphere (usually nitrogen) to devolatilize the raw material, followed by activation that consists in the partial gasification of the obtained char with oxidizing agents (i.e. steam, carbon dioxide or a mixture of both) leading to the formation of the porous network. While the carbonization usually occurs at temperatures between 400 and 600°C, the gasification requests temperatures ranging from 800 to 1000°C. Depending on the raw material, it is also possible to avoid the carbonization step and proceed directly to the thermal

activation rate—days) instead of diffusional control which is faster but leads to external particle burning and, consequently, to poor porosity development [8]. The reactions of steam and carbon dioxide with carbon are endothermic, thus thermal activation needs external energy supply to maintain the requested high temperature. Oxygen (or air) is not commonly used as oxidizing agent since its reaction with carbon is highly exothermic and fast, being difficult to control and assure porosity development instead of particle consumption [8, 12, 24]. Due to this and to the safety issues related to the temperature control, oxygen activation is scarcely used. However, low amounts of oxygen (or air) can be added during thermal activation with steam or carbon dioxide to help maintaining the high temperatures by reacting with the gases

**Chemical activation** usually requests only one heating step: the raw material is mixed with

sphere at temperatures from 400 to 900°C, depending on the selected activating agent. The solid product obtained need to be exhaustively washed with water to remove the chemicals and dried before storage. In this process, the mechanism of pore formation is dependent on the chemical agent: zinc chloride promotes the removal of water molecules from the lignocellulosic structures of the raw material while phosphoric acid combines chemically with them [24]. In none of these processes, the selective removal of carbon atoms occurs. With potassium hydroxide, the process is more complex since there is the disintegration of the structure followed by intercalation of the metallic potassium [24, 28, 29] into the "graphitic" laminar structure, breaking down particles and limiting granular activated carbons synthesis. Simultaneously, there is also some gasification of carbon atoms due to reaction with CO<sup>2</sup>

O, resulting from the redox reaction of carbon with potassium compounds [30]. During carbonization (i.e. heat treatment under inert atmosphere), the lignocellulosic precursor losses volume by contraction, but when chemical activation is applied there is incorporation of the activating reagent inside the particles inhibiting the expected contraction, that is, the activating agent may behave as a template for the formation of the microporosity [24]. In fact, in 2004, Molina-Sabio and Rodríguez-Reinoso [30] published an enlightening research that allowed to conclude that the dehydration of the carbon precursor (i.e. peach stones) by ZnCl<sup>2</sup>

must occur in conditions that assure chemical control (slow

) [24]. This approach has the advantage of lower-

, KOH) and is further treated under controlled atmo-

and

, both inhibiting gases for the activation, and increasing the

activation. Activation with CO<sup>2</sup>

44 Porosity - Process, Technologies and Applications

produced during activation (i.e. CO and H<sup>2</sup>

partial pressure of the activating agent [24].

, H<sup>3</sup> PO<sup>4</sup>

ing the pressure of CO and H<sup>2</sup>

an activating agent (e.g. ZnCl<sup>2</sup>

H2

Chemical activation has advantages over physical process related with the use of a single heating step and lower temperatures, usually higher yields, shorter activation time (hours) and higher surface area and pore volumes. All these add-ons came with a price since this activation process is more corrosive (e.g. ZnCl<sup>2</sup> or KOH) than the physical one, and activating agents can be hazardous for the environment (i.e. ZnCl<sup>2</sup> and H<sup>3</sup> PO<sup>4</sup> ). Also, regardless the activating agent, a time-consuming washing step is needed to remove chemicals from the newly formed pore network and, if possible, to recover the activating agent. Actually, ZnCl<sup>2</sup> use is declining due to the environmental issues associated with zinc residues [12], low recovery efficiencies, corrosion problems and presence of residual zinc in final carbon [8].

Zinc chlorine and phosphoric acid lead to higher activation yields (≈40%) compared to physical activation since the use of these compounds inhibit the formation of tar and other by-products formed in physical activation [12]. In the case of potassium hydroxide, the more complex activation mechanism gathering K intercalation in carbon lattice and gasification allows to prepare activated carbon with a large range of yields depending on the precursor: from 80 and 70% [31–33] to values, as low as, 20–10% [34–36]. KOH allows obtaining superactivated carbons (over 3000 m<sup>2</sup> g−1) when high KOH/precursor weight ratios (i.e. between 2 and 4) are used.

**Table 1** gathers information regarding the most suitable precursors for the commonly used activating agents, as well as, the type of porosity obtained, and the effect of experimental conditions on the pore size distribution. It is important to emphasize that the effect of experimental conditions on pore size distribution presented in **Table 1** is a general trend; thus, for an in-depth knowledge of the effect of activating agent and experimental conditions on the porous characteristics of a given raw material, a comprehensive study is always needed.

The above-mentioned precursors and activating agents are those more representative for industrial scale production of activated carbons. However, regarding the production of specialty carbons (low volume processes) and for research works, the number of options increases exponentially. Regarding raw materials, hard biomass residues like shells and stones allow to obtain high quality PACs or GACs; phenol formaldehyde polymers yield high surface area porous carbons, while polyacrylonitrile (PAN), acrylonitrile textile or rayon are adequate to synthetize activated carbon fibers or cloths [24]. When aiming nitrogen doped materials, for example for the adsorption of sulfur species, the use of polymers containing nitrogen (e.g. PAN) is a common synthetic route. In what concerns activating agents, besides ZnCl<sup>2</sup> , H<sup>3</sup> PO<sup>4</sup> and KOH, alkali hydroxides and salts (e.g. NaOH, K<sup>2</sup> CO<sup>3</sup> , Na<sup>2</sup> CO<sup>3</sup> ) and metal chlorides (e.g. AlCl<sup>3</sup> , FeCl<sup>3</sup> , NH<sup>4</sup> Cl, CuCl<sup>2</sup> ) can also be used [24].

At industrial scale, steam activation continues to be the most widely used method to produce activated carbons. The advantages of steam activation are related with the fact that does not request post-activation work-up, namely a final washing step, is less expensive and has less environmental constrains than the chemical activation. Besides, for precursors with less than 10% of ash, carbon materials with surface areas of 1000 m<sup>2</sup> g−1 can be easily obtained with activation yields of 50% [8], thus allowing a good compromise between production cost and porosity development.


the raw material (i.e. coal or biomass) in a carbon-rich material (charcoal) that will be subsequently activated to improve the pore network. This thermochemical conversion is particularly important when using biomass whose carbon content usually is between 30 and 50% (wt.) [25]. However, the need for more specialized porous carbon materials, as well as the growing interest in developing more sustainable and greener processes, has prompted the research community to explore alternatives to the conventional carbonization step which, in

Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters

http://dx.doi.org/10.5772/intechopen.72476

47

In the category of alternative methods to obtain carbon-rich materials for further activation *via* conventional methodologies, processes as distinct as sol-gel polymerization reaction, hydrothermal carbonization (HTC) and acid-mediated carbonization (AMC) can be grouped. While sol-gel process involves the use of synthetic compounds—resorcinol and formaldehyde—as reagents for the polycondensation reaction in the presence of a catalyst, both HTC and AMC

The first resorcinol-formaldehyde organic gel was synthetized by Pekala in 1989 being the aqueous polycondensation performed under alkaline conditions [40]. One of the major advantages of this process is its flexibility since the main properties of the gel can be tuned during the synthesis and drying process. The synthesis starts with the formation of the wet gel and, during this step, the most important parameters controlling the properties of the final gel are the precursors' concentration, the catalyst type and concentration and the time and temperature of curing [41]. After drying, aerogels (supercritical drying), xerogels (subcritical drying), and cryogels (freeze drying) can be obtained. The drying procedure is one of the most important steps regarding the porous properties of the organic gel and is also crucial when the preparation of a thermally stable carbon gel is foreseen. The organic gels are mainly mesoporous materials thus if the preparation of a micro + mesoporous material is envisage a thermochemical step involving carbonization and/or activation must be considered. The first works focused on the carbonization and activation of organic gels were published during the 1990s, and nowadays, these methodologies continue to be studied to better

A great advantage of conventional activation of organic gels over the previously mentioned carbon precursors is the possibility of coupling the mesopore network of the organic gels with micropores created during carbonization or activation step. Tamon et al. studied the effect of carbonization on the porous properties of aerogels and cryogels proving that both precursors preserved the mesoporous structure after the thermal treatment and that cryogels are more prone to develop micropores [42]. Regarding activation, literature data reveal that the

and increasing the volume of micropores [43, 44]. Moreno-Castilla and co-workers reported the activation of a mesoporous aerogel with CO<sup>2</sup> showing that for a 22% burn-off the increase and widening of the precursor micropore network practically did not change the mesopore structure [44]. In a recent work, Ania and co-workers [45] evaluated the effect of carbonization and activation (physical and chemical) of xerogels which, due to their drying in subcritical conditions, may suffer stronger changes upon thermal treatment. In fact, both carbonization

activation lead to the increase of the micropore volume at the expense of a severe

allows retaining the mesopore network of the organic gels

some cases, also allow to make feasible the use of novel precursors.

are mainly applied to transform biomass into a coal-like material.

tune the pore network of these materials and explore novel applications.

gasification of aerogels with CO<sup>2</sup>

and CO<sup>2</sup>

**Table 1.** Appropriate precursors, kinetic of activation, and type of porosity usually obtained for the most common activating agents (information gathered from reference literature [8, 12, 24, 29]).

In general, the above-described activating methodologies produce activated carbons with wide pore size distributions, so the need for carbon materials with more regular porosity prompted the research on the synthesis of ordered mesoporous carbons (OMCs) by applying hard or soft templating approaches. The advantages of OMCs over conventional activated carbons are related with the ordered and hierarchical pore network, but their multi-step preparation procedures are time consuming, use hazardous chemicals to remove the inorganic templates and, consequently, have very low atomic economy and high production costs. This class of nanostructured materials is out of the scope of the present chapter since the first works were published in the late 1990s, and comprehensive reviews are available in the literature, providing a broad overview and up-to-date information on synthesis and properties of this class of porous carbon materials [37–39]. In the following, the recent developments regarding new synthesis approaches—i.e. precursors, activating agent and routes—for preparing nanoporous carbon materials are presented.

#### **3.1. Conventional activation of gels and chars obtained by novel approaches**

As already mentioned, classical preparation of activated carbons usually requests an initial thermochemical process (carbonization) to remove other elements than carbon and transform the raw material (i.e. coal or biomass) in a carbon-rich material (charcoal) that will be subsequently activated to improve the pore network. This thermochemical conversion is particularly important when using biomass whose carbon content usually is between 30 and 50% (wt.) [25]. However, the need for more specialized porous carbon materials, as well as the growing interest in developing more sustainable and greener processes, has prompted the research community to explore alternatives to the conventional carbonization step which, in some cases, also allow to make feasible the use of novel precursors.

In the category of alternative methods to obtain carbon-rich materials for further activation *via* conventional methodologies, processes as distinct as sol-gel polymerization reaction, hydrothermal carbonization (HTC) and acid-mediated carbonization (AMC) can be grouped. While sol-gel process involves the use of synthetic compounds—resorcinol and formaldehyde—as reagents for the polycondensation reaction in the presence of a catalyst, both HTC and AMC are mainly applied to transform biomass into a coal-like material.

The first resorcinol-formaldehyde organic gel was synthetized by Pekala in 1989 being the aqueous polycondensation performed under alkaline conditions [40]. One of the major advantages of this process is its flexibility since the main properties of the gel can be tuned during the synthesis and drying process. The synthesis starts with the formation of the wet gel and, during this step, the most important parameters controlling the properties of the final gel are the precursors' concentration, the catalyst type and concentration and the time and temperature of curing [41]. After drying, aerogels (supercritical drying), xerogels (subcritical drying), and cryogels (freeze drying) can be obtained. The drying procedure is one of the most important steps regarding the porous properties of the organic gel and is also crucial when the preparation of a thermally stable carbon gel is foreseen. The organic gels are mainly mesoporous materials thus if the preparation of a micro + mesoporous material is envisage a thermochemical step involving carbonization and/or activation must be considered. The first works focused on the carbonization and activation of organic gels were published during the 1990s, and nowadays, these methodologies continue to be studied to better tune the pore network of these materials and explore novel applications.

In general, the above-described activating methodologies produce activated carbons with wide pore size distributions, so the need for carbon materials with more regular porosity prompted the research on the synthesis of ordered mesoporous carbons (OMCs) by applying hard or soft templating approaches. The advantages of OMCs over conventional activated carbons are related with the ordered and hierarchical pore network, but their multi-step preparation procedures are time consuming, use hazardous chemicals to remove the inorganic templates and, consequently, have very low atomic economy and high production costs. This class of nanostructured materials is out of the scope of the present chapter since the first works were published in the late 1990s, and comprehensive reviews are available in the literature, providing a broad overview and up-to-date information on synthesis and properties of this class of porous carbon materials [37–39]. In the following, the recent developments regarding new synthesis approaches—i.e. precursors, activating agent and routes—for pre-

**Table 1.** Appropriate precursors, kinetic of activation, and type of porosity usually obtained for the most common

**Appropriate precursors Porosity General trend of experimental conditions on** 

degree

Micro (+ meso)

Micro + meso

Micro + meso

**pore size distribution (PSD)**

high activation temperatures

of Zn/precursor ratio

temperature (<450°C)

temperature

volume of micropores with similar pore size





Micro - KOH/precursor ratio has more influence on

adsorption capacity and PSD than activation


Micro - High activation degree leads to high

distribution (PSD)

paring nanoporous carbon materials are presented.

**Type of activation** **Activating agent**

46 Porosity - Process, Technologies and Applications

Physical CO<sup>2</sup> Coals and, in less extend,

Chemical ZnCl<sup>2</sup> Lignocellulosic materials

H3

hard lignocellulosic

(high volatile and oxygen

(high volatile and oxygen

volatile and high carbon

materials

Steam Coals and, in less extend, hard lignocellulosic

materials

content)

content)

content)

activating agents (information gathered from reference literature [8, 12, 24, 29]).

KOH High rank coals (low

PO<sup>4</sup> Lignocellulosic materials

**3.1. Conventional activation of gels and chars obtained by novel approaches**

As already mentioned, classical preparation of activated carbons usually requests an initial thermochemical process (carbonization) to remove other elements than carbon and transform A great advantage of conventional activation of organic gels over the previously mentioned carbon precursors is the possibility of coupling the mesopore network of the organic gels with micropores created during carbonization or activation step. Tamon et al. studied the effect of carbonization on the porous properties of aerogels and cryogels proving that both precursors preserved the mesoporous structure after the thermal treatment and that cryogels are more prone to develop micropores [42]. Regarding activation, literature data reveal that the gasification of aerogels with CO<sup>2</sup> allows retaining the mesopore network of the organic gels and increasing the volume of micropores [43, 44]. Moreno-Castilla and co-workers reported the activation of a mesoporous aerogel with CO<sup>2</sup> showing that for a 22% burn-off the increase and widening of the precursor micropore network practically did not change the mesopore structure [44]. In a recent work, Ania and co-workers [45] evaluated the effect of carbonization and activation (physical and chemical) of xerogels which, due to their drying in subcritical conditions, may suffer stronger changes upon thermal treatment. In fact, both carbonization and CO<sup>2</sup> activation lead to the increase of the micropore volume at the expense of a severe decrease in the mesopore volume and shrinkage of the average pore size. However, under controlled experimental conditions (i.e. impregnation methodology and temperature), the chemical activation of the xerogel with KOH or K<sup>2</sup> CO<sup>3</sup> allowed to suppress the shrinkage and structural collapse, forming a micropore structure associated with the enlargement and/ or preservation of the mesopore network of the pristine xerogel [45]. For further discussion and information regarding the control of the properties of resorcinol-formaldehyde organic and carbon gels, the reader can refer to the reviews by Al-Muhtaseb and co-workers [41, 46].

**Activating agent**

H3

**Carbon precursor BET area** 

O2

Furfural, glucose, starch, cellulose,

Potato starch, eucalyptus sawdust

Cellulose, potato starch, eucalyptus

Eucalyptus sawdust, *Paeonia lactiflora* (flowering plant) *Sargassum fusiforme*

eucalyptus sawdust

(+ melamine)

sawdust

(seaweed)

Coconut shell (+ H<sup>2</sup>

**(m2 g−1)**

Tobacco stem 297–1347 [69]

Rattan sawdust (*Calamus gracilis*) 1151 [70]

Glucose 1283 Spherical particles [59] Starch, cellulose, sawdust 1260–2850 [71]

Glucose, cellulose, rye straw 891–2250 Controlled PSD [58] Spruce and corncob hydrolysis products 2220–2300 [74] Hazelnut 1700 [75] Paper pulp mill sludge 1470–2980 Sponge-like particles [76] Sucrose 1169–2431 Controlled PSD [35] Glucose, sucrose 1312–3152 [60] Jujum grass, *Camellia japonica* 1050–3537 [77] Bagasse waste + sewage sludge 2296 Hierarchical porosity [78] Sucrose 1635–3036 [79] Sucrose 1534 [80]

Tobacco rods 1761–2115 VMeso/VTotal = 79–89% and

NaOH Glucose, sucrose 532–2129 Spherical particles [60]

Tobacco stem 217–501 [69] Microalgae (*Spirulina platensis*) + glucose 1260–2370 N-doped [63] Algae (*Nannochloropsis salina*) + glucose 747–1538 N-doped [62]

Palm date seed 1282 [82] Rattan stalks (*Lacosperma secundiflorum*) 1135 VMeso/VTotal = 72% [55]

) 1652–1744 VMeso/VTotal = 50–60% [52]

N,S-doped

Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters

3280–3420 VMeso/VTotal = 50–55% and N-doped

N-doped

1202–2783 [81]

1200–2370 [72]

2125–2967 [73]

ZnCl<sup>2</sup> *Salix psammophila* 839–1308 [68]

Sewage sludge 417–519 VMeso/VTotal = 53–66% and

PO<sup>4</sup> Glucose, sucrose 1750–2120 Spherical particles [60]

KOH D-Glucosamine 600 N-doped [51]

**Observations Ref.**

http://dx.doi.org/10.5772/intechopen.72476

[53]

49

[56]

[54]

Hydrothermal carbonization (HTC) is probably one of the most promising alternatives to conventional carbonization of biomass, as it is clearly shown by the ever-increasing number of publications focused in this process since the beginning of the twentieth century. HTC is inspired in the natural processes of coal formation that take millions years and request temperature and pressure in a nonoxidizing atmosphere to transform biomass into a carbon-rich material. Interestingly, the first research work on HTC was published in 1913 by Friedrich Bergius, a Nobel Laureate in recognition of his studies regarding carbon-water reactions at high pressure and mild temperature to successfully produce H<sup>2</sup> by avoiding CO generation [47]. When performing the experiments, Bergius noticed that when peat was used as carbon source the solid residue obtained had an elemental composition similar to that of coal, leading him to investigate the HTC decomposition of plant-based compounds into coal-like materials [48]. As reported in [49], with the exception of the works by Berl and Schmidt (1932) and van Krevelen et al. (1960), the interest in the solid carbon materials obtained from HTC process was forgotten until the work published by Wang et al. [50] in 2001. These authors reported the synthesis of carbon spheres from sugar under hydrothermal conditions (190°C and self-generated pressure in a high-pressure vessel). HTC is a cost effective and eco-friendly process; since to convert biomass into carbon-rich materials, it uses water as solvent, mild temperatures, self-generated pressure and occurs in few hours with no CO<sup>2</sup> emission. Since 2001, the interest in this carbonization process has been exponentially increasing, and in 2016, around 400 papers mentioning hydrothermal carbonization were published, and the works focusing in this process received more than 11,000 citations (source Web of Science, Sept 2017). Some advantages of HTC-derived materials, commonly known as hydrochars, over charcoals obtained by conventional carbonization is their high content in oxygenated groups and the possibility of morphological control (e.g. spherical morphology).

Hydrochars have an incipient pore network; thus, conventional activation has been commonly employed to obtain specialized nanoporous carbon materials, mainly regarding surface functionalization, morphology features and ultrahigh surface areas and pore volumes. The use of hydrochars as activated carbon precursors was first reported by Zhao et al. [51] aiming to prepare a nitrogencontaining porous carbon material to be used as supercapacitor. The authors performed HTC of D-glucosamine (carbon source) and the hydrochar containing 6.7% of nitrogen was further activated with KOH allowing to obtain a nitrogen-doped porous carbon with a BET area of 600 m<sup>2</sup> g−1.

As it can be clearly seen in the overview of hydrochar-derived activated carbons presented in **Table 2**, the great majority of studies concerning hydrochar activation used KOH as activating agent, mainly because, in general, this oxidizing agent allows obtaining the most developed microporosities reaching BET surface areas higher than 3000 m<sup>2</sup> g−1. It is also interesting to notice that there are studies reporting the use of hydrochars for the synthesis of micro + mesoporous


decrease in the mesopore volume and shrinkage of the average pore size. However, under controlled experimental conditions (i.e. impregnation methodology and temperature), the

and structural collapse, forming a micropore structure associated with the enlargement and/ or preservation of the mesopore network of the pristine xerogel [45]. For further discussion and information regarding the control of the properties of resorcinol-formaldehyde organic and carbon gels, the reader can refer to the reviews by Al-Muhtaseb and co-workers [41, 46]. Hydrothermal carbonization (HTC) is probably one of the most promising alternatives to conventional carbonization of biomass, as it is clearly shown by the ever-increasing number of publications focused in this process since the beginning of the twentieth century. HTC is inspired in the natural processes of coal formation that take millions years and request temperature and pressure in a nonoxidizing atmosphere to transform biomass into a carbon-rich material. Interestingly, the first research work on HTC was published in 1913 by Friedrich Bergius, a Nobel Laureate in recognition of his studies regarding carbon-water reactions at

[47]. When performing the experiments, Bergius noticed that when peat was used as carbon source the solid residue obtained had an elemental composition similar to that of coal, leading him to investigate the HTC decomposition of plant-based compounds into coal-like materials [48]. As reported in [49], with the exception of the works by Berl and Schmidt (1932) and van Krevelen et al. (1960), the interest in the solid carbon materials obtained from HTC process was forgotten until the work published by Wang et al. [50] in 2001. These authors reported the synthesis of carbon spheres from sugar under hydrothermal conditions (190°C and self-generated pressure in a high-pressure vessel). HTC is a cost effective and eco-friendly process; since to convert biomass into carbon-rich materials, it uses water as solvent, mild tempera-

interest in this carbonization process has been exponentially increasing, and in 2016, around 400 papers mentioning hydrothermal carbonization were published, and the works focusing in this process received more than 11,000 citations (source Web of Science, Sept 2017). Some advantages of HTC-derived materials, commonly known as hydrochars, over charcoals obtained by conventional carbonization is their high content in oxygenated groups and the

Hydrochars have an incipient pore network; thus, conventional activation has been commonly employed to obtain specialized nanoporous carbon materials, mainly regarding surface functionalization, morphology features and ultrahigh surface areas and pore volumes. The use of hydrochars as activated carbon precursors was first reported by Zhao et al. [51] aiming to prepare a nitrogencontaining porous carbon material to be used as supercapacitor. The authors performed HTC of D-glucosamine (carbon source) and the hydrochar containing 6.7% of nitrogen was further activated with KOH allowing to obtain a nitrogen-doped porous carbon with a BET area of 600 m<sup>2</sup> g−1. As it can be clearly seen in the overview of hydrochar-derived activated carbons presented in **Table 2**, the great majority of studies concerning hydrochar activation used KOH as activating agent, mainly because, in general, this oxidizing agent allows obtaining the most developed microporosities reaching BET surface areas higher than 3000 m<sup>2</sup> g−1. It is also interesting to notice that there are studies reporting the use of hydrochars for the synthesis of micro + mesoporous

CO<sup>3</sup>

allowed to suppress the shrinkage

by avoiding CO generation

emission. Since 2001, the

chemical activation of the xerogel with KOH or K<sup>2</sup>

48 Porosity - Process, Technologies and Applications

high pressure and mild temperature to successfully produce H<sup>2</sup>

tures, self-generated pressure and occurs in few hours with no CO<sup>2</sup>

possibility of morphological control (e.g. spherical morphology).


sludge, tobacco rods or bamboo shoots—were selected this methodology allows retaining higher percentages of heteroatoms compared to conventional carbonization, and thus assures

Following a somewhat different approach, White et al. reported the preparation of nitrogendoped carbogel materials by reacting glucose and ovalbumin (secondary biomass precursor) in HTC conditions [64]. The novelty was the saturation of the monolithic hydrochar with ethanol followed by extraction under supercritical conditions, resembling the last step of the synthesis of organic aerogels. The final material was obtained after carbonization (350–900°C). When using 550°C, a material with an interconnected 3D pore system, BET area of 476 m<sup>2</sup> g−1

Very recently, Sevilla et al. proposed an alternative to KOH activation of biomass-derived

vated carbons with high yields (40–46%) [65]. The materials attained BET areas near 3000 m<sup>2</sup> g−1, presenting mainly micropores (70%) and nitrogen content between 0.5 and 0.9% irrespectively the amount of melamine used. Although the mixture of potassium oxalate/melamine is presented by the authors as an alternative to the corrosive KOH, it is important to highlight that this synthesis route is restricted for lab-scale (under restricted security conditions) since the KCN present in the solid will generate dangerous toxic vapors of HCN during the washing with HCl.

For a deeper overview regarding the HTC process and derived carbon materials, a recent book chapter by Titirici et al. [48] and a minireview [66] are recommended, and for deeper analysis on hydrochar-derived activated carbons, the following review [67] can be consulted.

Acid-mediated carbonization (AMC) is so far the less studied alternative method to obtain a carbon-rich material. However, considering that acid catalysis is a common practice to extract sugars from lignocellulosic biomass [86] and that, as just discussed, sugars can be successfully used as activated carbon precursors (i.e. *via* HTC followed by activation), this process must also be explored. This methodology allows processing largely available and hopefully low cost biomass residues. Moreover, AMC occurs at atmospheric pressure and, in some cases, at low temperature (≈ 100°C) allowing processing carbon precursors that do not gather the adequate properties to be carbonized through the conventional process or by HTC. In fact, AMC is able to maximize the availability of cellulose and hemicellulose in carbon precursors with high inorganic content (e.g. rice husk) [87, 88] and is also efficient for producing carbonrich materials from liquid carbon precursors, as is the case of glycerol [89]. Depending on the

) and melamine to obtain N-doped superacti-

Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters

http://dx.doi.org/10.5772/intechopen.72476

51

), the obtained oxygen-rich chars also contain rel-

[87, 88] and KOH [88] to produce nanoporous

SO<sup>4</sup>

C2 O4

relevant amounts of these atoms after the activation step [53, 54, 57, 62, 63].

and 5–7% (wt.) nitrogen content was obtained.

hydrochars by using potassium oxalate (K<sup>2</sup>

acid catalyst used (e.g. H<sup>2</sup>

final activated carbon.

SO<sup>4</sup>

chars. These materials were activated with H<sup>3</sup>

and/or H<sup>3</sup>

PO<sup>4</sup>

evant amounts of sulfur or phosphorus groups, thus enhancing the reactivity for subsequent activation and allowing to join a well-developed pore network with heteroatom doping in the

Wang et al. evaluated sulfuric acid hydrolysis of rice husk followed by the dehydration, polymerization and carbonization of the sugars to yield chars, which can be named as acid-

PO<sup>4</sup>

carbons with BET areas higher than 2400 m<sup>2</sup> g−1, and in the case of phosphoric acid activation, materials with micro + mesopore networks. In 2010, the authors reported the effect of H<sup>2</sup>

concentration during carbonization step, as well as the reaction temperature and time onto the

**Table 2.** Overview of the properties of hydrochar-derived activated carbons (2010–2017).

materials *via* chemical activation with zinc chloride [52, 53] or hydroxides [54–56], with the mesopore volume corresponding to at least 50% of the total pore volume, i.e. textural characteristics that are not easy to obtain by the most conventional carbonization + activation process. A recent work reports the preparation of an N-doped nanoporous carbon with similar volumes of micro and mesopores and a BET area near 1000 m<sup>2</sup> g−1 by using HTC of bamboo shoots in the presence of water and 0.5 cm<sup>3</sup> of H<sup>2</sup> SO<sup>4</sup> followed by the carbonization of the obtained hydrochar, so in the absence of any activating agent [57]. The work by Fuertes and Sevilla enables the synthesis of superactivated porous carbons with 50% of mesopores by adding melamine during the HTC process, thus the final materials also present 1.3–1.7% of nitrogen [56]. Besides opening the possibility of preparing mesoporous carbon materials, the use of hydrochars as activated carbon precursors also allows better control of the micropore size distribution [34, 35, 58].

When starting from simple carbohydrates, the use of HTC followed by activation may enable preparing carbons with spherical morphology (diameters in the micrometer range) and smooth surfaces [34, 35, 59, 60]. It is also possible to obtain a nanoporous carbon material with spherical shape, but with blackberry morphology, by adding acrylic acid to starch during the HTC step [61].

The use of water as solvent constitutes a major advantage of HTC over convention carbonization that requests dry biomass. In fact, HTC makes feasible the use of wet biomass as is the case of algae [62, 63] or sewage sludge [53] that after being carbonized can be activated to obtain a carbon material with well-developed pore structure and also heteroatom-rich surface (i.e. nitrogen and sulfur). HTC is, in fact, a valuable process when envisaging heteroatom-doped carbon materials, since if precursors with nitrogen and/or sulfur—e.g. glucosamine, algae, sewage sludge, tobacco rods or bamboo shoots—were selected this methodology allows retaining higher percentages of heteroatoms compared to conventional carbonization, and thus assures relevant amounts of these atoms after the activation step [53, 54, 57, 62, 63].

Following a somewhat different approach, White et al. reported the preparation of nitrogendoped carbogel materials by reacting glucose and ovalbumin (secondary biomass precursor) in HTC conditions [64]. The novelty was the saturation of the monolithic hydrochar with ethanol followed by extraction under supercritical conditions, resembling the last step of the synthesis of organic aerogels. The final material was obtained after carbonization (350–900°C). When using 550°C, a material with an interconnected 3D pore system, BET area of 476 m<sup>2</sup> g−1 and 5–7% (wt.) nitrogen content was obtained.

Very recently, Sevilla et al. proposed an alternative to KOH activation of biomass-derived hydrochars by using potassium oxalate (K<sup>2</sup> C2 O4 ) and melamine to obtain N-doped superactivated carbons with high yields (40–46%) [65]. The materials attained BET areas near 3000 m<sup>2</sup> g−1, presenting mainly micropores (70%) and nitrogen content between 0.5 and 0.9% irrespectively the amount of melamine used. Although the mixture of potassium oxalate/melamine is presented by the authors as an alternative to the corrosive KOH, it is important to highlight that this synthesis route is restricted for lab-scale (under restricted security conditions) since the KCN present in the solid will generate dangerous toxic vapors of HCN during the washing with HCl.

For a deeper overview regarding the HTC process and derived carbon materials, a recent book chapter by Titirici et al. [48] and a minireview [66] are recommended, and for deeper analysis on hydrochar-derived activated carbons, the following review [67] can be consulted.

materials *via* chemical activation with zinc chloride [52, 53] or hydroxides [54–56], with the mesopore volume corresponding to at least 50% of the total pore volume, i.e. textural characteristics that are not easy to obtain by the most conventional carbonization + activation process. A recent work reports the preparation of an N-doped nanoporous carbon with similar volumes of micro and mesopores and a BET area near 1000 m<sup>2</sup> g−1 by using HTC of bamboo shoots in the

**(m2 g−1)**

*Salix psammophila* 964–1469 [68] Golden shower pods 812–903 [83] Tobacco stem 355–553 [69]

Sunflower stem, walnut shells, olive stones 379–438 [85] Glucose, sucrose 923–2555 Spherical particles [60]

Sucrose 814 Spherical particles [35]

conc.) 972 VMeso/VTotal = 46–54% and

N-doped

CO<sup>2</sup> Pinewood sawdust and rice husk 292–569 [84]

Air Sunflower stem, walnut shells, olive stones 213–434 [85]

**Observations Ref.**

[35]

[61]

[57]

controlled PSD

blackberry morphology

char, so in the absence of any activating agent [57]. The work by Fuertes and Sevilla enables the synthesis of superactivated porous carbons with 50% of mesopores by adding melamine during the HTC process, thus the final materials also present 1.3–1.7% of nitrogen [56]. Besides opening the possibility of preparing mesoporous carbon materials, the use of hydrochars as activated carbon precursors also allows better control of the micropore size distribution [34, 35, 58].

When starting from simple carbohydrates, the use of HTC followed by activation may enable preparing carbons with spherical morphology (diameters in the micrometer range) and smooth surfaces [34, 35, 59, 60]. It is also possible to obtain a nanoporous carbon material with spherical shape, but with blackberry morphology, by adding acrylic acid to starch during the

The use of water as solvent constitutes a major advantage of HTC over convention carbonization that requests dry biomass. In fact, HTC makes feasible the use of wet biomass as is the case of algae [62, 63] or sewage sludge [53] that after being carbonized can be activated to obtain a carbon material with well-developed pore structure and also heteroatom-rich surface (i.e. nitrogen and sulfur). HTC is, in fact, a valuable process when envisaging heteroatom-doped carbon materials, since if precursors with nitrogen and/or sulfur—e.g. glucosamine, algae, sewage

followed by the carbonization of the obtained hydro-

 of H<sup>2</sup> SO<sup>4</sup>

SO<sup>4</sup>

**Carbon precursor BET area** 

CO<sup>3</sup> Sucrose 694–1375 Spherical particles and

Steam Starch (+ acrylic acid) 785–1148 Spherical particles with

VMeso and VTotal, microporous and total pore volume, respectively; PSD, pore size distribution.

**Table 2.** Overview of the properties of hydrochar-derived activated carbons (2010–2017).

presence of water and 0.5 cm<sup>3</sup>

— Bamboo shoots (+ H<sup>2</sup>

50 Porosity - Process, Technologies and Applications

HTC step [61].

**Activating agent**

K2

Acid-mediated carbonization (AMC) is so far the less studied alternative method to obtain a carbon-rich material. However, considering that acid catalysis is a common practice to extract sugars from lignocellulosic biomass [86] and that, as just discussed, sugars can be successfully used as activated carbon precursors (i.e. *via* HTC followed by activation), this process must also be explored. This methodology allows processing largely available and hopefully low cost biomass residues. Moreover, AMC occurs at atmospheric pressure and, in some cases, at low temperature (≈ 100°C) allowing processing carbon precursors that do not gather the adequate properties to be carbonized through the conventional process or by HTC. In fact, AMC is able to maximize the availability of cellulose and hemicellulose in carbon precursors with high inorganic content (e.g. rice husk) [87, 88] and is also efficient for producing carbonrich materials from liquid carbon precursors, as is the case of glycerol [89]. Depending on the acid catalyst used (e.g. H<sup>2</sup> SO<sup>4</sup> and/or H<sup>3</sup> PO<sup>4</sup> ), the obtained oxygen-rich chars also contain relevant amounts of sulfur or phosphorus groups, thus enhancing the reactivity for subsequent activation and allowing to join a well-developed pore network with heteroatom doping in the final activated carbon.

Wang et al. evaluated sulfuric acid hydrolysis of rice husk followed by the dehydration, polymerization and carbonization of the sugars to yield chars, which can be named as acidchars. These materials were activated with H<sup>3</sup> PO<sup>4</sup> [87, 88] and KOH [88] to produce nanoporous carbons with BET areas higher than 2400 m<sup>2</sup> g−1, and in the case of phosphoric acid activation, materials with micro + mesopore networks. In 2010, the authors reported the effect of H<sup>2</sup> SO<sup>4</sup> concentration during carbonization step, as well as the reaction temperature and time onto the yield of the obtained acidchar that attained 32% (wt.) for the reaction at 95°C during 10 h when 72% H<sup>2</sup> SO<sup>4</sup> solution was used in both hydrolysis and carbonization [88]. Regarding the effect of the H<sup>2</sup> SO<sup>4</sup> concentration (42–72%) during acid carbonization onto the textural properties of the activated carbon obtained by H<sup>3</sup> PO<sup>4</sup> activation, the authors concluded that although acidchars prepared under distinct acid concentrations present similar elemental analysis, the BET area and pore volumes increase as the concentration of the H<sup>2</sup> SO<sup>4</sup> decreases [87]. These findings were rationalized by the authors considering that high H<sup>2</sup> SO<sup>4</sup> concentrations promote the aggregation of carbon nanoparticles during carbonization, limiting the uniform H<sup>3</sup> PO<sup>4</sup> impregnation of the particles and consequently the development of the pore network in the inner particle [87].

both catalyst and structure-directing agent to prepare hierarchical structured carbogels under HTC conditions [91]. Further carbonization under nitrogen at 550 or 1000°C allowed to obtain

Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters

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53

In 2010, Zhou and co-workers developed mesoporous carbons by the carbonization of organic salts (magnesium and barium citrates) evaluating the influence of the temperature of the thermal treatment (600–800°C) [92]. In the case of magnesium salt, BET areas up to 2322 m<sup>2</sup> g−1 were attained, and the increase of temperature resulted in a progressive increase of the mesopore volume percentage (from 50 to 100%). The barium citrate-derived materials are mainly mesoporous (>90%) and, independently of the temperature, pores between 10 and 20 nm are obtained. Calcium citrate was tested by other authors who reported the paramount impor-

Atkinson and Rood proposed the use of dichloroacetates of alkaline metals as carbon precursors for the synthesis of nanoporous carbons, being the pore networks, once again, dependent on the cation [94]. The pore structure is produced by fast pyrolysis (15 s to 7 min) under nitrogen flow at temperatures between 300 and 1100°C, and materials attained BET areas of 740 m<sup>2</sup> g−1. This methodology allowed the synthesis of microporous materials when lithium salt was used and micro + mesoporous solids for the other two metals. In the same research line, Xu and co-workers reported the pyrolysis of EDTA salts to obtain nitrogendoped porous carbons with BET areas reaching 1800 m<sup>2</sup> g−1 and porosity characteristics dependent on the thermal treatment temperature (higher the temperature, higher the mesopore

The protocol of organic salts carbonization was extended to gluconates and alginates along with citrates to understand the mechanism of the porosity development [97–99]. The results shown that the textural properties of the porous carbon materials obtained by this methodology are heavily dependent on the type of the cation in the organic salt: while potassium salts originate essentially microporous solids, for sodium and calcium the amount of mesopores is also relevant [98]. In the case of calcium citrate-derived material, the development of the mesoporosity was attributed to the formation of CaO nanoparticles, which act as endotemplates during the carbonization. It was also shown that the nature of the organic salt has a great impact on the morphology, with sodium gluconate leading to the formation of large carbon nanosheets, while sodium citrate originates sponge-like particles. The synthesis of nitrogendoped porous carbons was also explored by mixing potassium gluconate with melamine, which allowed to obtain a microporous material gathering 22.9% of nitrogen with 660 m<sup>2</sup> g−1 of BET area [97]. The endotemplate approach on the carbonization of organic salts was further explored with iron, calcium and zinc citrates [99]. The carbonization of these nonalkali organic salts produces mesoporous materials with BET areas between 950 and 1610 m<sup>2</sup> g−1 and distinct pore size distributions: monomodal distribution centered at 11 nm for calcium citrate, bimodal distribution centered at 9 and 20 nm for iron citrate, and bimodal distribution centered at 3 and 10 nm for iron citrate. These carbons can be post-functionalized by heat treatment in the presence of melamine to obtain materials gathering high BET area and mesopore

a carbon material with a BET area of 614 m<sup>2</sup> g−1 and 70% of mesopore volume.

tance of temperature in the textural properties of the mesoporous carbons [93].

*3.2.2. Carbonization of organic salts*

volume) [95, 96].

volume with high nitrogen content (8–9%).

In a recent publication, Cui and Atkinson [89] systematically investigated liquid glycerol AMC using various acid catalysts (i.e. H<sup>2</sup> SO<sup>4</sup> , H<sup>3</sup> PO<sup>4</sup> , HCl and CH<sup>3</sup> COOH) aiming to study the effect of the acid carbonization conditions onto the textural properties of glycerol-derived nanoporous carbon materials obtained by subsequent physical activation with steam and CO<sup>2</sup> . The AMC of glycerol was made under nitrogen between 400 and 800°C, and the highest carbonization yields were obtained at 400°C for 10:3 volumetric mixtures glycerol:acid (30% yield for H<sup>2</sup> SO<sup>4</sup> and 50% yield for H<sup>3</sup> PO<sup>4</sup> ). Upon activation, materials with BET area values ranging between 990 and 2470 m<sup>2</sup> g−1 and tailored porosity were obtained. The H<sup>3</sup> PO<sup>4</sup> -char originated micro + mesoporous carbon materials with the volume of mesopores being more than 50% of the total porosity regardless the physical activating agent and the amount of acid during the AMC, while the steam activation of the H<sup>2</sup> SO<sup>4</sup> -char lead to materials with 40–44% mesopore volume, and the CO<sup>2</sup> activation only 22–25% mesopore volume. The elemental analysis of the nanoporous carbon materials obtained by H<sup>2</sup> SO<sup>4</sup> carbonization revealed the presence of 0.35–0.71% of sulfur and the materials carbonized with H<sup>3</sup> PO<sup>4</sup> and activated with CO<sup>2</sup> attained even higher heteroatom doping with phosphorus content between 2.04 and 4.34%.
