The Selective Catalytic Conversion Of Cellulose To Levulinic Acid

Since the beginning of the twenty-first century, the gradual decrease of fossil resources, continued growth of energy demand and serious environment problems brought by overuse of fossil resources urge people to search for clean, renewable and sustainable resource. In recent years, great attention of scientists from different countries has been paid on a renewable carbon resource—lignocellulosic biomass. Lignocellulosic biomass resources refer to all the organic materials derived from photosynthesis of green plants, including agricultural wastes generated from industrial and agricultural production, stalks, grasses and trees.Lignocellulosic biomass mainly contains cellulose, lignin and hemicellulose. Among the three, cellulose is the most abundant, accounting for ca.45%. Cellulose is a polysaccharide composed of glucose units via beta-1,4-glycosidic bonds. The hydrogen bonds formed among the chains of cellulose offers cellulose recalcitrant structure with high crystallinity degree, and therefore cellulose is difficult to be decomposed to small molecules. In view of the structural feature of cellulose, a study was carried out on the decomposition of cellulose based on design of solid acid catalyst and catalytic medium.In recent years, extensive research has been conducted on the cellulose decomposition by various solid acid catalysts. The main decomposition products of cellulose include glucose, sugar alcohol,5-hydroxymethylfurfural and levulinic acid, etc. The cellulose could not be effectively hydrolyzed to small molecules by the traditional solid acid catalysts, even though they have enough acid sites. This is due to that the cellulose is not soluble in reaction medium, and therefore the acid sites of catalysis could not effectively reach the cellulose. In addition, the acid sites of traditional solid acid catalysts are easy to leach into the reaction medium, leading to serious deactivation of catalysts. For most traditional solid acid catalysts, the acid sites are fixed on the catalysts by treating solid supports with concentrated sulfuric acid at high temperature. The drawback of this fixation method of acid site is that the acid site on the catalysts is not stableTherefore, we considered to design a mimic enzyme solid acid catalyst with not only stable and abundant acid sites but also affinity sites capable to draw cellulose closer to the catalysts. Chloromethyl polystyrene (CP resin, known as chloromethylated bead, the amount of chlorine:4.7mmol/g) was chosen as catalyst support. A novel polymeric solid acid catalyst was synthesized by partially substituting chlorine of the CP resin with sulfonic group via chemical modification. The sulfonic group was strong enough since it was introduced to the catalyst support through stable covalent bond. Furthermore, it’s easy to adjust the sulfonic group/chlorine ratio by controlling reaction condition, such as reaction time and temperature. The cellulose was adsorbed to the surface of catalyst through hydrogen bonds formed between chlorine on the catalyst and hydroxyl groups of cellulose, and then was hydrolyzed to small molecules. A levulinic acid yield of33%was obtained by hydrolyzing cellulose in water medium, and a levulinic acid yield of65%was achieved with90wt%γ-valerolactone/10wt%water as reaction medium.