Direct synthesis of lactide from concentrated lactic acid catalyzed by hierarchical Sn-beta zeolite

Abstract

<p indent="0mm">Direct synthesis of lactide from concentrated lactic acid (LA) is an energy-efficient method, which is of great significance to reduce the cost of polylactic acid production. It is difficult to produce lactide with high yield from commercial concentrated LA due to the great quantity of LA oligomers. Herein, the hierarchical Sn-beta (Sn-beta-M) zeolite was prepared as a catalyst for the conversion of 97 wt% LA to lactide with yield of 86.5% in 3 h. The mesoporosity in Sn-beta-M characterized by N₂ adsorption/desorption test shortened the diffusion path way in the micropores, which facilitated the fast escape of lacide avoiding decyclization and oligomerization. L₃A and L_{<italic>n</italic>}A’s conversion to lactide may be attributed to the Sn sites present in mesopores. The Sn⁴⁺ coordinated in Sn-beta-M framework, exhibited strong Lewis acidity and could act as the active center for the synthesis of lactide. We speculated that the Sn and the surrounding O in Sn-beta-M acted as strong Lewis acid sites and weak base sites respectively in mediating C–O bond formation of lactide from L₂A and L_{<italic>m</italic>}A (<italic>m</italic>≥3).