Mechanistic Insights into Glycerol Ketalization to Glycerol Levulinate Ketal over USY Molecular Sieve

Abstract

Efficient ketalization of glycerol and alkyl levulinate to high-value-added glycerol levulinate ketal (GLK) and insights into the catalytic mechanism are of great significance but remain challenging without a clear understanding of active sites. Herein, the ketalization of glycerol with methyl levulinate (ML) and ethyl levulinate (EL) to methyl glycerol levulinate ketal (MLK) and ethyl glycerol levulinate ketal (ELK) over USY molecular sieves was reported. Under the optimal conditions, the yields of MLK and ELK can reach 96%. A kinetic study indicated that the ketalization of alkyl levulinate and glycerol to GLK followed the pseudo-second-order kinetic model, and the activation energy for MLK synthesis (154.87 kJ·mol^–1) over USY(6) was lower than that of ELK (168.03 kJ·mol^–1) over USY(10). Density functional theory calculations revealed that AlOOH sites and Al³⁺ sites in USY exhibit identical catalytic activity during the synthesis of MLK (or ELK). Furthermore, the formation of an ether intermediate (IM2) from the acyl group of ML attacked by the GLY-terminated hydroxyl was the rate-determining step of MLK production on the AlOOH sites, while the intramolecular cyclization of an intermediate IM3 containing a carbocation to form ELK was the rate-determining step of EL ketalization on the Al³⁺ sites.