Controlled Synthesis of L-Lactide Using Sn-Beta Zeolite Catalysts in a One-Step Route

Polylactic acid (PLA), as a biodegradable and renewable polymer, has been widely used in biomedical and commercial applications. The energy-efficient synthesis of lactide from lactic acid (La) via a one-step route is highly desired for megaton-scale PLA production. In this work, stereo-pure L-lactide was synthesized with high yield (88.2-95.8%) via a one-step route catalyzed by Sn-beta zeolites. Sn-O-Si bonds on tin-silica catalysts are indeed important for a high L-Iactide yield. The Sn4+ ions with four isolated coordinates in tetrahedral configuration of Sn-beta zeolites are more active for L-lactide synthesis than those with hexacoordinated polymeric Sn-O-Sn-type species. L-La was first converted into a lactic acid dimer (L(2)a) and then formed L-lactide catalyzed by a Sn-beta zeolite, and the average reaction rate in first 20 min (R-20min(av)) increased linearly with increasing Sn content. Importantly, lactic acid trimers (L(3)a) formed during the reaction could also be converted to L-Iactide in the later stage of the reaction, and the diffusion of L(3)a to the active site within the Sn- beta zeolite pore is the rate-limiting step. The lactic acid oligomer (L(n)a, n >= 4) was limited to form lactide with an R-20min(av) of only 0.013 mmol min(-1) g(cat)(-1).

Automated summary

In this study, stereo-pure L-lactide was successfully synthesized with high yield via a one-step route catalyzed by Sn-beta zeolites. The Sn-O-Si bonds on the tin-silica catalysts play a crucial role in achieving high L-lactide yield. Furthermore, Sn4+ ions with four isolated coordinates in tetrahedral configuration are more active for L-lactide synthesis compared to other polymeric species.