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-lactide 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 (L2a) and then formed l-lactide catalyzed by a Sn-beta zeolite, and the average reaction rate in first 20 min (R20minav) increased linearly with increasing Sn content. Importantly, lactic acid trimers (L3a) formed during the reaction could also be converted to l-lactide in the later stage of the reaction, and the diffusion of L3a to the active site within the Sn-beta zeolite pore is the rate-limiting step. The lactic acid oligomer (Lna, n ≥ 4) was limited to form lactide with an R20minav of only 0.013 mmol min–1 gcat–1.