*3.2.2. Carbonization of organic salts*

yield of the obtained acidchar that attained 32% (wt.) for the reaction at 95°C during 10 h when

prepared under distinct acid concentrations present similar elemental analysis, the BET area

the particles and consequently the development of the pore network in the inner particle [87]. In a recent publication, Cui and Atkinson [89] systematically investigated liquid glycerol AMC

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>

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

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

In parallel with the studies centered in the activation of hydrochars, in the last few years, the scientific community also started to explore new synthesis routes to obtain a porous structure during the HTC step, being prepared porous carbons with BET surface areas up to 700m<sup>2</sup> g−1. The methodologies proposed avoid the need of further thermal or chemical activation, may enable the synthesis of heteroatom-doped solids [90] and can also allow the synthesis of hier-

Fechler et al. reported the synthesis of porous carbon materials with BET areas between 425 and 672 m<sup>2</sup> g−1 through HTC (180°C overnight) of glucose mixed with several eutectic salt mix-

[90]. The authors proved that both the amount of water added and the salt composition are determinant for the successful synthesis of materials, which are formed by very small particles

carbon materials with the highest surface area. When 2-pyrrol-carboxyaldehyde was added as

, and KCl/ZnCl<sup>2</sup>

576 m<sup>2</sup> g−1 was obtained. Fellinger et al. used glucose as carbon source and borax (Na<sup>2</sup>

aggregation, identical to aerogels. The use of the eutectic mixture LiCl/ZnCl<sup>2</sup>

SO<sup>4</sup>

even higher heteroatom doping with phosphorus content between 2.04 and 4.34%.

PO<sup>4</sup>

tion of carbon nanoparticles during carbonization, limiting the uniform H<sup>3</sup>

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

between 990 and 2470 m<sup>2</sup> g−1 and tailored porosity were obtained. The H<sup>3</sup>

PO<sup>4</sup>

and pore volumes increase as the concentration of the H<sup>2</sup>

rationalized by the authors considering that high H<sup>2</sup>

solution was used in both hydrolysis and carbonization [88]. Regarding the effect of

concentration (42–72%) during acid carbonization onto the textural properties of the

SO<sup>4</sup>

SO<sup>4</sup>

, HCl and CH<sup>3</sup>

activation, the authors concluded that although acidchars

). Upon activation, materials with BET area values ranging

activation only 22–25% mesopore volume. The elemental analysis of

PO<sup>4</sup>

SO<sup>4</sup>

decreases [87]. These findings were

COOH) aiming to study the effect

PO<sup>4</sup>


carbonization revealed the presence of

and activated with CO<sup>2</sup>

) in the presence of a small amount of water

, a material with 3% of nitrogen and BET area of

impregnation of


attained

originated the

B4 O7 ) as

. The

concentrations promote the aggrega-

PO<sup>4</sup>

72% H<sup>2</sup>

the H<sup>2</sup>

for H<sup>2</sup>

SO<sup>4</sup>

volume, and the CO<sup>2</sup>

archical materials [91].

tures (i.e. LiCl/ZnCl<sup>2</sup>

SO<sup>4</sup>

activated carbon obtained by H<sup>3</sup>

52 Porosity - Process, Technologies and Applications

using various acid catalysts (i.e. H<sup>2</sup>

and 50% yield for H<sup>3</sup>

AMC, while the steam activation of the H<sup>2</sup>

the nanoporous carbon materials obtained by H<sup>2</sup>

0.35–0.71% of sulfur and the materials carbonized with H<sup>3</sup>

**3.2. Other strategies to synthetize nanoporous carbons**

*3.2.1. Variations of hydrothermal carbonization (HTC) process*

, NaCl/ZnCl<sup>2</sup>

co-reagent of the mixture glucose and ZnCl<sup>2</sup>

SO<sup>4</sup>

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

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 volume) [95, 96].

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

#### *3.2.3. Ionothermal approaches*

The synthesis of porous carbon materials *via* ionothermal concept derives from the methodologies proposed in 2004 by Morris and co-workers to prepare zeolite analogues (i.e. inorganic materials) using ionic liquids or eutectic mixtures as both solvents and inorganic structure directing agents (templates) [100]. The ionothermal approach is analogous to hydrothermal and solvothermal processes, where the solvents are predominantly molecular (water or nonaqueous solvents, respectively) but, as the name means, it refers to processes occurring in ionic solvents (e.g. ionic liquids or eutectic mixtures) [100]. The low vapor pressure of the ionic solvents is a great advantage of ionothermal reaction over the hydrothermal or solvothermal concepts, since it allows avoiding the safety concerns connected with the high pressures required to prevent molecular solvent evaporation [101].

**Reagents Thermal treatment BET area** 

open air

180°C 24 h

1000°C (N<sup>2</sup>

200°C 24 h

850°C (N<sup>2</sup>

850°C (N<sup>2</sup>

1000°C (N<sup>2</sup>

reactor

550 °C Schlenck-type

900–1000°C (N<sup>2</sup>

550–1000°C 5 h (N<sup>2</sup>

(N<sup>2</sup> )

(autoclave) + 600–900°C

two-step regime

600–1300°C (N<sup>2</sup>

Autoclave + 750°C (N<sup>2</sup>

1000°C or 1400°C (N<sup>2</sup>

) linear or

) 4 h

Imidazolium-based ionic liquids 800 °C (N<sup>2</sup>

Imidazolium-based ionic liquids 800 °C (N<sup>2</sup>

Glucose, fructose, xylose, or starch + iron containing ionic liquid

N-containing and N,B-containing ionic liquids + eutectic salt mixtures (alkaline metal and zinc

Glucose, cellulose, or lignin + eutectic salt mixture (KCl/ZnCl<sup>2</sup>

Glucose + molten salt LiCl/ KCl + activating oxysalts (KOH,

Glucose, cellulose, or sugar cane bagasse + metal free ionic liquids

, CaCl<sup>2</sup>

Imidazolium ionic liquid + salt mixture (NaCl/ZnCl<sup>2</sup>

Molten ZnCl<sup>2</sup> + common organic solvents (e.g. ethanol, acetonitrile, dimethylsulfoxide, glycerol)

ZnCl<sup>2</sup> + glucose or glucosamine

O (+ 2-thiophenecarboxylic acid (TCA))

Glucose and melamine + eutectic salt mixture (LiCl/KCl)

dissolved in H<sup>2</sup>

Imidium ionic liquid + eutectic salt

)

)

Peanut shell + salt mixtures

mixture (KCl/ZnCl<sup>2</sup>

 or KClO<sup>3</sup> )

chlorides)

NaBO<sup>2</sup> , K<sup>2</sup> CO<sup>3</sup> , KNO<sup>3</sup>

K2 SO<sup>4</sup>

(Na<sup>2</sup> CO<sup>3</sup> /K<sup>2</sup> CO<sup>3</sup> , Li<sup>2</sup> CO<sup>3</sup> /Na<sup>2</sup> CO<sup>3</sup> /

K2 CO<sup>3</sup> , CaCl<sup>2</sup>

Ionic liquid + glucose or fructose 160–200°C, 2–20 h,

)

, KH<sup>2</sup> PO<sup>4</sup> ,

/NaCl)

**(m2 g−1)**

44–155

160–404

) 1100–2000 5% N-doping

) 997–1912 Oxysalt influences

) 1 h 640–780 Micropores or micro +

) 1 h <100 11.4–17.6% N-doping [108]

Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters

magnetic

amount

morphology

dependent

Micro + mesopores

16–627 2.8–6.6% N-doping

) 1 h 316–408 [120]

S-doping

750–1650 14% N-doping or 13% S-doping

on solvent

1.8%S)

Carbon aerogels

) 387–1190 6–24% N-doping [106]

Aerogel, nanosheet or hyperbranch morphology dependent

N,S-doping (5.6% N and

) 2 h 1410–1770 3.7–4.5% N-doping [111]

) 2 h 1056 2.8% N-doping and 5.16%

) 881–1246 Hierarchical pores

**Observations Ref.**

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

[107]

55

[103]

[109]

[113]

[104]

[110]

[121]

[122]

mesopores depending of ionic liquid used 2–3% N-doping

6–288 Micro + mesopores [102]

Hierarchical pores and

Ionic liquid triple role: salt template, solvent, catalyst

6% N + 6% B double doping Pore network dependent on salt mixture and

866–2025 VMeso/VTotal = 63–92% [112]

VMeso/VTotal = 21–52% oxysalt

The preparation of carbon materials following the ionothermal principles is reported in the literature as both ionothermal and molten salt synthesis processes. Nowadays, there is still not a generally accepted terminology for these processes what turns difficult to understand the classifications and underlying procedures followed by the distinct authors. In fact, the use of ionothermal/molten salt process can be linked either to the preparation of carbon materials with incipient porosity obtained at temperatures ≈ 200°C (ionothermal carbonization—ITC) [102] or to the preparation of porous carbons by a two-step process including the previously mentioned ITC followed by a thermal treatment of the ionothermal derived carbon at high temperatures (attaining 1000°C or more) [103, 104]. Ionothermal/molten salt process is also considered in the case where the mixture of the carbon precursor and the ionic solvent is directly thermally treated at high temperature [105, 106].

The first studies reporting the preparation of porous carbon materials through ionothermal process used ionic liquids as carbon precursors. In 2009, Lee et al. synthesized N-doped materials (2–3%) with BET area values between 640 and 780 m<sup>2</sup> g−1, and the authors demonstrated the influence of the ionic liquid nature on the development of mainly microporous or micro + mesoporous materials [107]. In a further work, the authors report a similar process for obtaining materials with up to 17% of nitrogen content, although with lower porosity development [108]. In 2010, other approach of the same research group gathered an ionic liquid with simple carbohydrates allowing to obtain carbon materials with a highly developed mesopore network after treatment at only 200°C during 20 h in a nonpressurized chamber [102]. Xie et al. reported the synthesis of magnetic hierarchical porous carbons by using several carbohydrates and an iron containing ionic liquid, and the authors proposed that the ionic liquid has a triple role: salt template, solvent and catalyst [103].

In the subsequent research in ionothermal approaches (**Table 3**), the introduction of salts as co-reagents became common and generally accepted as a hard template route. Zinc chloride is by far the most frequently reported salt, used alone or in mixtures (eutectic or not) with other salts. This strategy allowed to synthesize porous carbons with ultrahigh surface area (easily around 2000 m<sup>2</sup> g−1), hierarchical structure and high heteroatom doping (i.e. nitrogen (>5%) and sulfur). All these features are of fundamental importance for boosting the application of these materials in energy storage processes. Actually, in the great majority of publications, the authors report high performance of the ionothermal-derived porous carbons as supercapacitors. While initially nitrogen-containing ionic liquids were used as carbon precursors [109–111], along the years, the number of studies exploring carbohydrates, or even biomass, as carbon source has increased [105, 106, 112–120].


*3.2.3. Ionothermal approaches*

54 Porosity - Process, Technologies and Applications

The synthesis of porous carbon materials *via* ionothermal concept derives from the methodologies proposed in 2004 by Morris and co-workers to prepare zeolite analogues (i.e. inorganic materials) using ionic liquids or eutectic mixtures as both solvents and inorganic structure directing agents (templates) [100]. The ionothermal approach is analogous to hydrothermal and solvothermal processes, where the solvents are predominantly molecular (water or nonaqueous solvents, respectively) but, as the name means, it refers to processes occurring in ionic solvents (e.g. ionic liquids or eutectic mixtures) [100]. The low vapor pressure of the ionic solvents is a great advantage of ionothermal reaction over the hydrothermal or solvothermal concepts, since it allows avoiding the safety concerns connected with the high

The preparation of carbon materials following the ionothermal principles is reported in the literature as both ionothermal and molten salt synthesis processes. Nowadays, there is still not a generally accepted terminology for these processes what turns difficult to understand the classifications and underlying procedures followed by the distinct authors. In fact, the use of ionothermal/molten salt process can be linked either to the preparation of carbon materials with incipient porosity obtained at temperatures ≈ 200°C (ionothermal carbonization—ITC) [102] or to the preparation of porous carbons by a two-step process including the previously mentioned ITC followed by a thermal treatment of the ionothermal derived carbon at high temperatures (attaining 1000°C or more) [103, 104]. Ionothermal/molten salt process is also considered in the case where the mixture of the carbon precursor and the ionic solvent is directly thermally treated at high temperature [105, 106]. The first studies reporting the preparation of porous carbon materials through ionothermal process used ionic liquids as carbon precursors. In 2009, Lee et al. synthesized N-doped materials (2–3%) with BET area values between 640 and 780 m<sup>2</sup> g−1, and the authors demonstrated the influence of the ionic liquid nature on the development of mainly microporous or micro + mesoporous materials [107]. In a further work, the authors report a similar process for obtaining materials with up to 17% of nitrogen content, although with lower porosity development [108]. In 2010, other approach of the same research group gathered an ionic liquid with simple carbohydrates allowing to obtain carbon materials with a highly developed mesopore network after treatment at only 200°C during 20 h in a nonpressurized chamber [102]. Xie et al. reported the synthesis of magnetic hierarchical porous carbons by using several carbohydrates and an iron containing ionic liquid, and the authors proposed that the ionic liquid has a triple role: salt template, solvent and catalyst [103]. In the subsequent research in ionothermal approaches (**Table 3**), the introduction of salts as co-reagents became common and generally accepted as a hard template route. Zinc chloride is by far the most frequently reported salt, used alone or in mixtures (eutectic or not) with other salts. This strategy allowed to synthesize porous carbons with ultrahigh surface area (easily around 2000 m<sup>2</sup> g−1), hierarchical structure and high heteroatom doping (i.e. nitrogen (>5%) and sulfur). All these features are of fundamental importance for boosting the application of these materials in energy storage processes. Actually, in the great majority of publications, the authors report high performance of the ionothermal-derived porous carbons as supercapacitors. While initially nitrogen-containing ionic liquids were used as carbon precursors [109–111], along the years, the number of studies exploring carbohydrates, or even biomass,

pressures required to prevent molecular solvent evaporation [101].

as carbon source has increased [105, 106, 112–120].


For more information regarding the preparation of porous carbon materials *via* ionothermal/ molten salt approaches and also their main applications, several review works are available

Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters

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

57

The technological relevance of porous carbon materials continues to prompt the scientific community and companies to explore alternative routes to the conventional methods, in order to develop specialized materials or improve the production process (e.g. optimizing

The great majority of conventional and innovative processes allowing the synthesis of porous carbons involve heat treatments at moderate-to-high temperatures for carbonization to occur and subsequent formation of the porous carbon skeleton. In the case of chemical activation or ionothermal/molten salt processes, a washing step with water or 10% HCl is required to

Conventional methods are based on a solid and structured carbon material and therefore activation occurs by gasification, selective oxidation of the most reactive carbon atoms and heteroatoms or intercalation processes. Thus, the conventional methodologies are considered top-down processes. Regarding innovative approaches both top-down and bottom-up routes are reported in the literature. HTC or inonothermal reactions promote a top-down process when the carbon precursor is a biomass, but is a bottom-up route when discrete entities (e.g. carbohydrates or ionic liquids) are the starting materials. The acid-mediated degradation of biomass is a top-down route that converts biomass in a carbohydrate-rich acid liquor that is carbonized in a bottom-up approach. Some of the advantages of these novel approaches over the conventional ones are related to the possibility of producing highly porous carbon materials with easier pore size distribution tuning, high amounts of heteroatoms in the surface, and fine control of morphology (e.g. sponge-like, aerogel-like, spherical, sheets (2D)). In light of a more sustainable and circular economy, it is also relevant that some of the novel synthetic approaches (e.g. HTC) allow enabling future largescale production based on high moisture containing biomass residues and even liquids

The driving force for the development of new porous carbons throughout these nonconventional methods is the search for high performing materials for electrochemical applications and energy storage, which request hierarchically porous structures ideally doped with electron-rich nonmetallic elements (e.g. nitrogen, sulfur) to increase the

The carbon atom is a versatile element that since the Stone Age has reinvented itself. So, the overlook of this chapter allows to predict that in the near future the number of novel synthesis routes to feed the demand for even more specialized porous carbons will continue to increase. This may occur by revisiting synthesis routes established for other classes of materials or

in the literature [132–136].

energy costs and minimizing wastes).

remove chemical compounds clog the porosity.

by the discovery of completely novel processes.

**4. Conclusion**

(e.g. glycerol).

conductivity.

**Table 3.** Chronological overview of porous carbon synthesis by ionothermal approaches (2009–2017).

More elaborated synthesis schemes have been reported, as is the case of the work developed by Chang et al. [121] where organic solvents were added dropwise to molten ZnCl<sup>2</sup> at 550°C. By changing the solvent, the authors were able to obtain nitrogen (14%) or sulfur (13%) doped porous carbons with different morphologies (i.e. aerogel, nanosheets or hyperbranch). There are also reports on ITC of biomass with nitrogen-containing ionic liquid followed by conventional KOH activation to yield nitrogen doping up to 1.59% and apparent surface areas of 2838 m<sup>2</sup> g−1 [124]. The ionothermal approach is actually a powerful route to synthesize porous carbon materials with valuable graphene-like (2D) structures [125–128].

It is also possible to found reports on highly mesoporous carbon materials obtained by ZnCl<sup>2</sup> -mediated ionothermal/molten salt synthesis [129–131]. Although these routes are presented as novel synthesis procedures, the high mesopore volumes reported are most probably the result of the complex ZnCl<sup>2</sup> activation mechanism when very high amount of ZnCl<sup>2</sup> is added. As it was previously mentioned, the behavior of chemical activating agents as templates during carbonization cannot be disregarded, and for the particular case on ZnCl2, the results obtained by Molina-Sabio and Rodríguez-Reinoso [30] allowed the authors to conclude that this chemical acts as template for the creation of porosity. So, although new materials are being produced under "novel" synthesis procedures with appealing names, in some cases, the experimental route and underlying mechanism for pore creation seems to be the one accepted for conventional chemical activation.

For more information regarding the preparation of porous carbon materials *via* ionothermal/ molten salt approaches and also their main applications, several review works are available in the literature [132–136].
