Valorization of Cellulose and Chitin into Valuable Chemicals by Heterogeneous Catalysis

This review article describes our research on heterogeneous catalysis for valorization of cellulose and chitin, which are abundant plant and marine biomass. First the background and history of this topic are presented, and importance of synthesis of valuable chemicals is emphasized in countries like Japan where the amount of biomass resource is limited. In cellulose degradation, we discovered that Pt supported on alumina catalyzes hydrolytic hydrogenation of cellulose to sorbitol. Then we found that Pt supported on carbon (Pt/C) is more active and durable than Pt/Al2O3, and Pt/C is a bifunctional catalyst in which carbon works as solid acid. This finding led us to develop weakly acidic carbon catalysts for cellulose hydrolysis, and ball-milling of mixed cellulose and the carbon catalyst is imperative to increase physical contact between the solid substrate and the solid catalyst. This method enabled fast and highly selective synthesis of glucose from cellulose even in real biomass. In our mech-anistic study, it is found that aromatic surface of carbon adsorbs cellulose through CH-1r bonding and that weakly acidic sites on carbon attack glycosidic bonds in cellulose. In comparing with benchmarks, the heterogeneous carbon catalyst is superior to both cellulase enzyme and homogeneous sulfonic acid. The weakly acidic carbon is also effective for the synthesis of valuable cello-oligosaccharides, which work as biostimulants. We have also been studying the depolymerization of chitin to oligomers and N-acetylglucosamine (NAG). Hydrolysis of chitin was promoted by mechanocatalysis combining acid catalysis with mechanical force of ball-milling. NAG can be further converted to various organonitrogen compounds.