High-Performance Materials from Bio-based Feedstocks. Группа авторов

with surface areas of 640–970 m² g⁻¹ depending on the biowaste source. Under optimal conditions, conversion of up to 94% could be achieved for esterification [120]. Slow reaction rates, the requirements of high methanol‐to‐oil molar ratios, and high reaction temperatures are, however, weaknesses of the sulfonated carbon catalyst for biodiesel production [122]. A further application with regard to the upgrading of bio‐oil via reactive distillation including water removal, catalytic esterification, and neutralization steps was carried out with p‐toluene sulfonic acid embedded on an activated carbon obtained by activation of sawdust pellets [123]. The catalyst was applied to the catalytic esterification in the reactive distillation procedure. After water removal from the oil, the esterification reaction took place at 80 °C under atmospheric pressure with the acid‐modified activated carbon catalyst and zeolite. The heating value of the upgraded bio‐oil obtained from fast pyrolysis was improved from 14.80 to 23.13 MJ kg⁻¹ and was ascribed to the presence of higher ester and acetic acid contents in the oil according to the reaction in Figure 2.7. The properties of the upgraded oil were consistent with the diesel standard. Interestingly, sulfonated activated carbon catalysts have the potential for bio‐jet fuel production through co‐pyrolysis of biomass and plastic waste [124]. Prior to the sulfonation step, an activated corncob was converted by microwave‐assisted carbonization. The sulfonation temperature was a major factor that influenced the number of SO₃H groups on the catalyst. These active groups benefited the co‐pyrolysis oil yield of nearly 98% at 500 °C for 15 minutes. The obtained oil consisted of aromatic and C₉–C₁₆ alkanes, which was qualified as a bio‐jet fuel.

      A drawback of the sulfonated bio‐based activated carbon catalyst is that its preparation process is environmentally unfriendly owing to the heating of carbon materials in concentrated sulfuric acid for an extended time. Apart from sulfuric acid, tribasic potassium phosphate (K₃PO₄) was recently used to activate carbon materials. The K₃PO₄ impregnated on the activated carbon catalyst benefited the biodiesel production from waste cooking oil. With 5% catalyst loading, the biodiesel yield was 98% in a 12 : 1 methanol‐to‐oil molar ratio at 60 °C for four hours. Only 20% reduction in biodiesel yield was observed when the catalyst was used for five reaction cycles [125].

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