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 (R_20min^av) 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 (L_na, n ≥ 4) was limited to form lactide with an R_20min^av of only 0.013 mmol min^–1 g_cat^–1.