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 the biochar decreased with increasing carbonization temperatures because hetero‐atomic functional groups containing such oxygen and nitrogen atoms were volatilized from the biomass structure [90]. The remaining elements in the biomass after the volatile compounds’ detachment rearranged themselves to form more aromatic structures, causing a reduction in the number of active sites.
2.5.2 Modified Biochar
To develop the physicochemical properties and the number of active species on biochar toward a higher activity and maximum catalytic performance, a large number of modification techniques have been investigated including metal modifications as well as chemical and physical treatments.
2.5.2.1 Tar‐reforming Processes
The production of syngas via biomass gasification has attracted a great deal of interest. However, this technology faces some challenges, the biggest one of which is excessive tar formation resulting in clogged up equipment. Consequently, the total cost of biomass‐derived syngas production is increased making it difficult to develop this process into industrial manufacturing. To overcome this drawback, tar removal technologies have been intensively researched with regard to their economic and environmental impacts. The catalytic thermochemical conversion using biochar catalysts has been reported as a capable technique for tar reforming, but currently still shows inferior performance compared to conventional metal‐supported catalysts. Particularly steam and CO2‐treated biochars are very powerful catalysts for tar reforming. The biochar prepared by pyrolysis from several biomass sources was activated with 15 vol% H2O mixed with argon or with CO2 at 800 °C for a short time. The treated biochar catalysts increased the catalytic activity in the steam or CO2 reforming of tar compared to a regular biochar. Treatment of the biochar resulted in an increased surface area and pore volume (both microporous and mesoporous), and high content of oxygenated functional groups [91, 92]. The CO2 generated more micropores in the biochar, whereas conversely, steam created more mesopores, which adds further importance for tar reforming. Even though micropores showed a greater initial tar conversion, these are rapidly deactivated due to coke deposition in the pores [4, 91, 93]. The steam‐activated biochar had more oxygenated functional groups in the aromatic C–O forms, which are more active sites for tar reforming, than those treated by CO2 [89, 92, 94]. Tar molecules
