Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 47

Graphene, a member of the graphite family, is a flat monolayer of carbon atoms arranged into a 2D honeycomb structure. Graphene and graphene‐based materials (e.g. graphene oxides) have been extensively exploited because of their excellent thermal, mechanical, and electronic characteristics. Graphene, 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.