Mechanistic insights into Ti/SiO2-catalyzed gas-phase synthesis of glycolide from methyl glycolate

Gas-phase catalytic synthesis of glycolide (GL) from methyl glycolate (MG) offers a promising route for sustainable production of polyglycolic acid, yet suffers from unfavorable catalytic performances due to limited mechanistic understandings. Here, a series of Ti/SiO 2 catalysts supported on mesoporous materials with distinct pore structures were synthesized to elucidate the structure–performance–mechanism relationship. Among them, Ti/KIT-6 exhibited the highest MG conversion and GL yield, attributed to its large pore size, three-dimensional interconnected mesostructure, and well-dispersed tetrahedral Ti(IV) species. Comprehensive structure characterizations and mechanistic studies establish a linear correlation for MG conversion rate and GL formation rate with the tetrahedral Ti(IV) content, pointing to Ti(IV) as the intrinsic active sites. Further combining with temperature-programmed and in situ spectroscopy experiments, a possible reaction mechanism is proposed for MG-to-GL on the active site. This study provides mechanistic insights to guide design of Ti-based catalysts for gas-phase ester cyclization reactions.