Selective conversion of concentrated glucose to 1,2-propylene glycol and ethylene glycol by using RuSn/AC catalysts

A series of RuSn/AC catalysts with 5% Ru and 1-6% Sn loadings were synthesized for converting concentrated (10 wt%) sugars to 1,2-propylene glycol (1,2-PG) and ethylene glycol (EG) in a semi-continuous autoclave reactor. The catalysts structure and the product distributions are found to be highly dependent on the tin loading. At tin contents below 2%, RuSn alloy was formed coexisting with small amounts of highly dispersed SnO2 in the catalyst, and hexitols were the main products due to the high activity of catalysts for hydrogenation. At a tin content above 5%, more SnO2 species were present and covered RuSn alloy particles, leading to notable amounts of humins in the reaction due to the depressed hydrogenation activity. As for the optimal catalyst of 5%Ru3%Sn/AC, 77% of tin formed RuSn alloy with ruthenium and 23% of tin existed in the form of highly dispersed SnO2 under reaction conditions. The catalyst gave 25% and 26.9% yields of 1,2-PG and EG in the glucose conversion at 513 K for 10 min. The glycols yield was insensitive to the feeding rate of glucose, which could be increased to 10 mL/min with a glycols productivity of nearly 180 g L-1 h(-1). The catalysts structure and reaction networks were proposed according to pseudo in situ characterizations and conditional experiments. The highly dispersed SnO2 was effective to sugar isomerization and the RuSn alloy played the major role in retro-aldol condensation and hydrogenation reactions. Over the 5%Ru3%Sn/AC catalyst, the rates of cascade reactions of isomerization, retro-aldol condensation and hydrogenation matched well due to the balanced activity of RuSn alloy and highly dispersed SnO2, which afforded a high yield of 1,2-PG and EG.