catalysts [1]. In order to develop more industrially friendly catalysts, many investigations have advanced the use of different bio‐based carbon materials for a wide range of catalytic applications. The studied bio‐based carbon catalysts and supports initially started with biochar, activated carbon, carbon nanotubes (CNTs), mesoporous carbons, and sugar catalysts, but have now been extended to include graphene and its derivatives. The crucial target for these developments is the upgrading of bio‐based carbon materials into direct catalysts or as catalyst supports to maximize catalytic efficiency. A simple process for the preparation of bio‐based carbon materials as well as achieving high conversion and highly desired product yield under mild reaction conditions are aimed for.
An understanding of the mechanisms of heterogeneous catalysis could address the appropriate characteristics of bio‐based carbonaceous catalysts. Generally, a heterogeneous catalytic reaction takes place through the following steps: (1) dispersion of the substrate from the bulk fluid to the pore entrance on the external catalyst surface; (2) diffusion of the substrate from the pore entrance into the internal catalyst pore; (3) adsorption of the substrate on the active catalyst site; (4) reaction of the substrate on the active site to generate a product; (5) desorption of the product from the active site; (6) diffusion of the product from the internal catalyst pore to the external surface of the catalyst; and (7) dispersion of the product from the external surface of catalyst into the bulk fluid [78].
The adsorption of the reactant on the active sites dispersed throughout the catalyst pores is the most important reaction step in heterogeneous catalysis. In case there is more than one reactant, competition in reactant adsorption on the active sites may occur. The concentration of each reactant on the active sites of the solid catalyst should be appropriate. The catalytic activity of the catalyst is based on the proportion of the reactants adsorbed on the active sites, and for each individual reaction, the chemical properties and functional groups of the bio‐based carbon catalyst should be designed to enable proper interactions with the adsorbed reactants. Diffusion steps 2 and 6 depend on the molecular size of the reactant and product as well as the pore morphology and catalyst particle size. Large catalyst particles diminish the reactant approaching the active sites inside the pores. So, the use of small catalyst particles can enhance the opportunity of a reactant to reach active positions within the pores. However, small catalyst particles cause a high pressure drop when used in a fixed‐bed reactor. To overcome this, the morphology of bio‐based carbon catalyst should be designed to have a higher external surface area such as a hollow cylindrical shape. Such information enables us to specifically control the catalyst characteristics and operating conditions of bio‐based carbon synthesis for catalysis applications.
2.5 Catalysis Applications of Selected Bio‐based Carbon Materials
An overview of high‐performance bio‐based carbon materials as catalysts and as carbon‐supported catalysts in various reactions are discussed here to present state‐of‐the‐art bio‐based carbon materials in a wide range of catalysis applications.
Chemical reactions can be categorized into various groups, and some important types of chemical reactions in which the performance of biomass‐derived carbon catalysts has been studied are exemplified in Figure 2.4. The details of each reaction will be presented based on the types of bio‐based carbon materials in the following sections.
2.5.1 Biochar
Biochar is a carbonaceous solid product created by thermochemical conversion of biomass in an oxygen‐free or oxygen‐poor atmosphere by carbonization, pyrolysis and gasification. Among the various bio‐based carbon materials, biochar has promising characteristics allowing it to be used as a heterogeneous catalyst and as catalyst support in numerous reactions. As discussed in Section 2.3.1, biochars have been gaining increased attention in catalysis applications due to their low cost, high porosity, stability, easy regeneration, and being more environmentally safe than other synthetic carbon materials. Biochars were reported to exhibit good catalytic performance in biodiesel production, steam reforming, pyrolysis, photocatalysis, bio‐oil upgrading processes, and biomass conversions into fuels and chemicals [4,6–9, 79].
Some outstanding physical and chemical features of an unmodified biochar enable its direct use as a catalyst. Although unmodified biochar received directly from the thermochemical conversion of biomass presents a modest surface area and porosity, it contains interesting surface functional groups and some mineral matters [80]. Functional groups such as C–O bonds as well as hydroxyl, carbonyl, carboxylic, and phenolic hydroxyl groups and also certain inorganic species significantly contribute to catalyst performance by promoting adsorption of reactants onto the active sites as well as the catalytic activity [81–83]. The catalytic performance of an unmodified biochar obtained from the gasification of poplar wood chips at 850 °C was investigated in the pyrolysis of low‐density polyethylene (LDPE) and high‐density polyethylene (HDPE) [8]. The main product in the gas phase from the catalytic pyrolysis of LDPE was propane instead of hydrogen since the surface functional groups on the unmodified biochar possibly reacted with the generated hydrogen radicals and hydrocarbon radicals. Besides, a great deal of wax was simultaneously produced. Addition of the unmodified biochar catalyst not only increased the gas yield from 18.3 to 25.4 wt% but also enhanced the light tar yield from 3.3 to 5.9 wt%, while the wax yield was decreased from 37.5 to 25.8 wt%. For the catalytic pyrolysis of HDPE, the gas yield was decreased from 20.1 to 15.7 wt% and the heavy tar yield also reduced from 35.5 to 29.5 wt%, but wax and light tar yields were increased in the presence of the catalyst. The unmodified biochar catalyst could not productively promote the cracking of heavy tar into light tar and gas products. However, it effectively promoted the polymerization during HDPE pyrolysis to form more wax with a combined ring structure. Surprisingly, the spent biochar catalyst in the pyrolysis of LDPE and HDPE had higher contents of hydroxyl groups and carbonyls in aldehydes since the oxygen species appeared to relocate from the reaction intermediates to the surface of the unmodified biochar catalyst. Crystallinity and minerals of the spent biochar catalyst were also enhanced compared to the fresh biochar. Considering biochar properties after the catalytic pyrolysis of polyolefin, the spent biochar catalyst may be applied as a soil amendment. Hence, it could economize on solid waste treatment cost. Vidal et al. revealed that the surface functional groups in the unmodified biochar had high thermal stability for the production of cyclic carbonates from CO2 and epoxides [84]. TGA showed that the unmodified biochar had a low decomposition temperature of 250 °C, while the oxidized biochar with nitric acid (HNO3) rapidly decomposed. The appearance of a lower decomposition temperature was attributed to the functional groups produced entirely on the surface of the unmodified biochar during its production. The oxidized biochar retained more residuals from the oxidation step, which were lost at high temperature. In addition, the carboxyl groups on the surface of the unmodified biochar could boost the polarization of the C–O bond resulting in catalyzing the epoxide ring opening followed by the formation of cyclic carbonates.
Figure 2.4 Examples of chemical reactions catalyzed by biomass‐derived carbons.
The existence of organic compounds in the renewable resource matrix, particularly animal wastes, brings about occurring minerals and inorganic alkalis in the structure of the produced biochar, such as K, Ca, Mg, N, P, and S [85–88]. These elements may be present in the form of chemical compounds such as CaCO3, KCl, or SiCl4 [8]. These minerals and inorganic alkalis can behave like a natural promoter of biochar activity in some catalyzed reactions. For example, the alkali and alkali earth metallic species could markedly promote the catalytic activity of biochar in tar reforming during biomass gasification [89]. An increasing biomass pyrolysis temperature resulted in enhanced fixed carbon and mineral contents in the produced biochar [83]. However, the number of surface functional groups within