Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 44

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, and aromatization. Whereas for the biomass feedstock, there are probably two reaction pathways: (1) partially by biomass hydrolysis, glucose dehydration, condensation, and aromatization, and (2) partially by direct biomass condensation and aromatization. The higher stability and aromaticity in the hydrothermal carbon from biomass was ascribed to the direct reaction pathways [144].