Vapour-phase condensation of methyl glycolate into glycolide over oxide catalysts at lowered pressure

Abstract

Glycolide is now considered as a promising monomer for production of biodegradable polyglycolate. Due to its high gas impermeability, mechanical strength and thermal stability, this polymer can be used in oil and gas industry, in medicine as biocompatible implants and surgical suture material, and as an ecological packaging material. Traditionally, glycolide is produced from glycolic acid. First, it is polycondensed into a low-molecular-weight oligomer, which is then depolymerized into a monomer. In this work, the vapor-phase conversion of methyl glycolate over several supported oxides without the use of an inert carrier gas under lowered pressure was studied. Condensation occurs according to the reaction 2С₃H₆O₃ → C₄H₄O₄ + 2CH₃OH. First, methyl glycolyl glycolate is formed from two molecules of methyl glycolate, which is further condensed into glycolide. The reaction was carried out at 250-300 °C, a pressure of 25-150 mbar and loads on a catalyst of 7-25 mmol MG/g_cat/h at a contact time of less than 1 sec. It was shown that more effective is supported TiO₂/SiO₂ catalyst, which provides 44 % methyl glycolate conversion with a selectivity towards glycolide of 64 % at  270 °C/25 mbar. Glycolide productivity of this catalyst achives to 4.9 mmol/g_cat/h. The main by-products are methanol, methyl glycolyl glycolate, methyl methoxyacetate and methoxymethanol.