New reaction pathways for high selectivity synthesis of methyl lactate via SnClx(OH)2-x-catalyzed cellulose conversion in water-containing methanol solution

Abstract

Catalytic conversion of cellulose into lactic acid (LA) is paramount for the emerging biomass-based chemical industry. With emphasis on the reaction mechanism, the catalytic cellulose conversion to methyl lactate (ML) was systematically investigated in water-containing methanol solution. The role that water plays and catalytic mechanism of the acidic and basic sites were analyzed and discussed. It was found that water played an important role in modulating hydrolysis of SnCl2 to release H+ protons and basic SnClx(OH)2-x species. These basic species are very efficient for the isomerization and [3+3] retro aldol condensation in terminal glucose units of cellulose chain, while H+ protons facilitate the ring-opening of the ends of cellulose chains to trioses rather than hydrolysis of the glucoside bonds to form glucose. Based on advanced characterization techniques including ESI-MS, GPC, FT-IR, XRD, XPS and CP-MAS 13C NMR as well as DFT calculations, two new reaction pathways were disclosed for the first time, which are distinct from the well-known traditional pathway that goes through the formation of glucose. Owing to the exclusion of the formation of glucose during the cellulose conversion, the selectivity towards ML can be greatly enhanced. Under optimal conditions, the total selectivity could reach to 91.5 % with a ML and LA yield of 66.3 %. These findings provide new insights into the catalytic conversion of cellulose into value-added chemicals and help explore effective catalysts.