illustration of esterification of acetic acid."/>
Figure 2.7 Esterification of acetic acid.
One way to further enhance the catalytic activity of bio‐based activated carbon is via impregnation with various metal ions. The Fe3O4 embedded on beetroot‐derived activated carbon catalyst (Fe3O4/BRAC) was examined for its performance in the reduction of nitroarenes as illustrated in Figure 2.8 [126]. The process yielded a high aromatic amines product (99%) even at 85 °C for 15 minutes. The reaction pathway as described in Veerakumar et al. [126] can be briefly summarized as follows: first, the nitrobenzene and isopropanol were chemisorbed on the surface of Fe3O4/BRAC. Hydrogen in the form of hydride ion was transferred from the isopropanol to the substrate. Subsequently, the existence of KOH and the Fe3O4/BRAC catalyst promoted the transformation of alcohol into alkoxide species. The nitro groups on the catalyst surface were reduced to nitrosobenzene, which reacted with hydrogen ion from isopropanol to form aniline. The product desorbed from the catalyst surface in the final step. The immobilization of Fe3O4 on the large surface area activated carbon facilitated the reaction under mild conditions in a short reaction time. The Fe3O4/BRAC catalyst boosted the hydrogen relocation from interacting isopropanol and KOH. Moreover, it presented high stability with less than 10% decrease in aniline yield after five repeated cycles. Importantly, separation of the Fe3O4 impregnated carbon catalyst from the products was readily achieved by an external magnetic field owing to the magnetic properties of Fe3O4.
Lignocellulosic waste‐derived activated carbons impregnated with ZnCl2 (giving Lewis acid sites) or H2SO4 (giving Brønsted acid sites) were very effective for the conversion of glucose to 5‐hydroxymethylfurfural (HMF) through isomerization and dehydration according to pathway 1 presented in Figure 2.9. These catalysts were more productive than homogeneous catalysts (H2SO4 and ZnCl2), even though these contained higher Lewis and Brønsted acid sites than the impregnated activated carbon [127]. This implies that the activated carbon itself had specific catalytic behavior for this reaction. Activated carbon possessed some Lewis and Brønsted acid sites but it had very high specific surface area and high amount of oxygenated functional groups. Therefore, the two latter predominant properties of the activated carbon strongly affected its catalytic efficiency. Its large specific surface area might increase the opportunity for adsorption of reactants on the active site. The large amount of oxygen functional groups on its surface might cooperate in the fructose dehydration to HMF.
It is well known that KOH activation before metal impregnation increases the surface area and pore volume of the carbon, particularly the micropore volume. The metallic K interposes itself between the carbon structure, leading to permanent enlargement of the carbon lattice after the removal of potassium metal. Thus, it is capable of increasing both the catalytic activity and the stability of the metal‐impregnated carbon catalysts in many reactions, e.g. Mizoroki–Heck, Suzuki, and Sonogashira coupling reactions, toluene steam reforming, and methanation [128–130]. Even if the KOH activator can intensively improve the surface area and porosity of carbon, it requires a relatively high activation temperature. Alternatively, H3PO4 is an interesting activator that requires a lower activation temperature [131]. The H3PO4 was used to activate olive stone at room temperature before carbonization, followed by wet impregnation with zirconium, and the resulting catalyst was employed in methanol dehydration to dimethyl ether [132]. This preparation method produced a superb mesoporous structure and large surface area (1190 m2 g−1) despite the activation occurring at room temperature. Additionally, the extensive formation of active zirconium phosphate functional groups over the surface area of carbon was observed. The catalyst presented both a high conversion in methanol dehydration and high selectivity to dimethyl ether. An increase in the H3PO4 activation temperature along with the addition of NaBH4 promoted the microporous area and the total surface area of the activated carbon (2559 m2 g−1) [133]. Activated carbon was widely employed as a support for several metal catalysts such as nanoparticles of Co, Ni, Cu, and noble metals. These catalysts were effective in bio‐oil hydrodeoxygenation and hydrogen production via both biomass hydrolysate reforming and ammonia borane hydrolysis [131, 134, 135]
Figure 2.8 Reduction of nitrobenzene.
Figure 2.9 Conversion of glucose to 5‐hydroxymethylfurfural.
2.5.4 Hydrothermal Bio‐based Carbons
A green hydrothermal carbonaceous material was developed from by‐products of sugar dehydration in hot compressed water at 150–250 °C [136, 137]. Hydrothermal carbon has been used not only in catalysis but also in agriculture, energy storage, and adsorption. Hydrothermal reaction mechanisms comprise dehydration and decarboxylation, and the process parameters such as reaction temperature and type of biomass need to be considered since these factors affect the catalytic property of the carbon. Originally, carbohydrate derivatives were utilized as a model structure for the preparation of hydrothermal carbons. Various researchers have prepared hydrothermal carbons from mono‐, di‐, and polysaccharides [138–140]. The hydrothermal carbon produced from glucose had a uniform spherical morphology with micron‐sized particles and a smooth surface [141]. The hydrothermal temperature had a stronger influence than the reaction time, not merely on the yield but also on the elemental compositions (i.e. C, H, and O) of the hydrochar. An increase in temperature resulted in a decrease in oxygen and hydrogen contents, while the carbon content increased [142]. The hydrothermal carbon possessed a large amount of oxygen‐containing functional groups including hydroxyl and aromatic C=O groups (carboxyl, carbonyl, ester, and quinone). These functional groups catalyze several reactions involved in the transformation of biomass to chemicals and organic pollutant degradation but the activity was not outstanding [143, 144]. Its porosity and surface area were also relatively restricted, which were less than 10 cm3 g−1 and 40 m2 g−1, respectively. Chemical modification with an oxidizing agent and metal immobilization are usually applied to enhance its catalytic activity and other properties. After modification with H2SO4, the hydrothermal carbon presented a rougher surface that was more accessible to the reactant, as well as a large amount of Brønsted acids and some Lewis acids [140]. The sulfonation created polycyclic aromatic carbon rings in an irregular form [145]. The improved properties of the hydrothermal carbon benefited the yield of ethyl levulinate and ethyl glucoside from the cellulose ethanolysis, and no tar or char was formed throughout the reaction.
Other than carbohydrates, raw biomass has been used as a possible feedstock for the production of hydrothermal carbon. The irregular shape and agglomerated particles of the hydrothermal carbon from biomass were observed [144, 145]. Its abundance of oxygen, especially in the form of ketonic groups, was similar to the hydrothermal carbon prepared from carbohydrate feedstocks. The appearance of several functional groups on the hydrothermal carbon confirmed that the hemicellulosic polysaccharide in biomass was destroyed by the hydrothermal treatment. However, its stability and reactivity in biomass conversion into chemicals were distinctly different from the monosaccharide‐derived hydrothermal carbon due to the difference in the hydrothermal carbon formation mechanism. The hydrothermal pathways of glucose feedstock were mostly via glucose dehydration to HMF, condensation,
