LITMIR.BIZ

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both with and without modifications, has been explored for its effectiveness in various reactions. Graphene produced from biomass commonly contains a high content of impurities that could either act as a poison or as a natural promoter in catalysis [150]. The imperfect graphene contaminated with nitrogen dopant was active and stable for the selective CO₂ hydrogenation to methane. For example, the natural N‐doped graphene prepared by pyrolysis of chitosan at 500 °C showed a higher CO₂ conversion than other dopants and 99.2% selectivity toward CH₄ [151]. Alternatively, external N doping into graphene was employed as a metal‐free catalyst which also showed high stability and selectivity in hydrogenation [77]. Huang et al. [152] synthesized N‐doped graphene via the carbonization of biomass guanine along with colloidal silica at 1000 °C under N₂ atmosphere followed by the addition of hydrofluoric acid to remove the silica template. It was observed that its electrochemical activity in the oxygen reduction reaction under acid and base conditions was remarkable due to its porous structure and high N doping [152]. Other than N doping onto graphene, an Ru metal‐supported graphene synthesized via carbonization and calcination of a mixture of glucose, FeCl₃, and RuCl₃ also demonstrated to be highly effective for levulinic acid hydrogenation to γ‐valerolactone (see Figure 2.11) with 100% conversion and 96% yield [153]. In addition, a catalyst based on graphene‐encapsulated Fe₃C embedded in CNTs was successfully produced using the inexpensive co‐pyrolysis of a mixture of biomass (glucose, xylitol, or sucrose), melamine, and iron compound [77]. The functionalized CNTs were stable and efficient in the hydrogenation of nitrobenzene to anilines. The 100% conversion of nitrobenzene along with nearly 99% selectivity toward anilines was achieved with the catalyst that was carbonized at 700 °C. The catalytic activity of this catalyst was mainly based on the pyrolysis temperature; the Fe₃C, which represents the active site for this reaction, could be destroyed when the pyrolysis temperature reached over 700 °C.

2.6 Summary and Future Aspects

The exponentially growing development in the transformation of biomass into carbon‐based materials is presented in a wide variety of publications for more than a decade. Various examples related to the utilization of biomass‐derived carbon materials with applications in catalysis have been presented and discussed in this chapter. Several outstanding features of bio‐based carbon materials such as various physical properties, chemical functionalities, facile activation and functionalization, and environmental friendliness allow them to straightforwardly act as a catalyst and catalyst support. It is important to consider issues that take into account the variety and complication of agricultural biomasses utilized as raw materials in carbon synthesis. The surface area and pore diameter of carbon materials as well as surface properties including hydrophobicity, hydrophilicity, acidity, basicity, and chemical functional groups could be accommodated by the selection of the biomass and control of the preparation conditions. Biochar is a simple bio‐based carbon material commonly having diverse surface functional groups. In order to acquire a high catalytic activity and high stability of biochar, a physical or chemical modification step is required. Hydrochar also possesses a large amount of oxygen‐containing functional groups that are beneficial for the production of bio‐based chemicals and the degradation of organic pollutants. The abundance of mesopores and large surface area of activated carbon could lead to higher efficiency by providing more active sites. Novel crystalline bio‐based carbon materials including graphene and CNTs are currently focused on as high‐efficiency materials in catalysis. Even though current bio‐based carbon materials have accomplished much in catalysis applications relating to product yield, selectivity, and stability, most are not quite ready for production on an industrial scale. To accomplish a high catalytic efficiency, the production cost of biomass‐derived carbon catalysts is still higher than that of conventional catalysts. Thus, a more intensive investigation is required to decrease the production cost together with enhancing the catalytic efficiency of the bio‐based carbon materials. With the intention of moving the production and utilization of biomass‐derived carbon materials from laboratory scale to commercialization, techno‐economic and environmental feasibility should also be analyzed.

Figure 2.11 Hydrogenation of levulinic acid.

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