Synergetic effect of Lewis acid and base in modified Sn-β on the direct conversion of levoglucosan to lactic acid

Abstract

This work presents a novel method to selectively convert levoglucosan to lactic acid (LA) via initially adding LA as the Brønsted acid together with modified Sn-β as the Lewis acid. Sn-β catalysts exchanged with various cations significantly enhance the yield of LA, following the order Sn-β < Sn-β–Na < Sn-β–K < Sn-β–Mg, Sn-β–Ca. The role of exchanged alkali ions stems from removing the remaining Brønsted acid sites in the zeolite and tuning the framework oxygen with more Lewis base sites. The exchange with alkaline earth cations such as Ca²⁺ and Mg²⁺ appears to be more effective in promoting the retro-aldol condensation due to the synergetic effect of their stronger Lewis acid that can stabilize the oxygen atom of deprotonated alkoxides during the reaction. The LA yield from levoglucosan can reach up to 66% on Sn-β–Ca under the optimized conditions. We also demonstrated that such a synergetic effect of alkaline-earth ion exchanged Sn-β shows great universality toward enhanced retro-aldol condensation, which can significantly promote the yield of methyl lactate over glucose conversion in methanol solvent.