**4. Conclusion**

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>

**(m2 g−1)**

) 2 h 426–1992 [114]

1273–1834 11.9% N-doping

600–900°C (Ar) 535–1815 VMeso/VTotal = 15–60% [123]

850°C 1381–1589 5.3–6.1% N-doping [117]

) 1 h 1770–2900 5.9–7.7% N-doping

**Observations Ref.**

[115]

[116]

[119]

VMeso/VTotal = 69.2–97.5%

) 1000–2160 VMeso/VTotal > 50% [105]

Pore structure dependent on NaCl proportion (micro +

mesopores)

Micropores

) 2 h 1067 4.28% N-doping [118]

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

It is also possible to found reports on highly mesoporous carbon materials obtained by


 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

activation mechanism when very high amount of

porous carbon materials with valuable graphene-like (2D) structures [125–128].

ZnCl<sup>2</sup>

ZnCl<sup>2</sup>

probably the result of the complex ZnCl<sup>2</sup>

the one accepted for conventional chemical activation.

**Reagents Thermal treatment BET area** 

700°C (N<sup>2</sup>

800 °C (N<sup>2</sup> ) two-step regime

900 °C (N<sup>2</sup>

650–850°C (N<sup>2</sup>

350°C 2 h + 900°C 1 h (N<sup>2</sup>

Tofu + LiCl/KCl + LiNO<sup>3</sup> 850°C (Ar) 2 h 1200 1.54–4.72% N-doping

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

Melamine and terephthalaldehyde + salt mixture (KCl/ZnCl<sup>2</sup>

56 Porosity - Process, Technologies and Applications

)

Glucose + eutectic and noneutectic

Phloroglucinol + glyoxylic acid +

Lignin from beech wood + nitration + eutectic salt mixture (KCl/ZnCl<sup>2</sup>

Adenine + eutectic and noneutectic

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

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

NaCl, KCl); pH control

salt mixture (NaCl/ZnCl<sup>2</sup>

Wheat straws + salt mixture (LiCl/KCl) + LiNO<sup>3</sup>

pluronic F127 + H<sup>2</sup>

)

)

O + salts (LiCl,

)

)

at

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 energy costs and minimizing wastes).

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 remove chemical compounds clog the porosity.

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 (e.g. glycerol).

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 conductivity.

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 by the discovery of completely novel processes.
