Direct Synthesis of Lactide from Lactic Acid by Sn-beta Zeolite: Crucial Role of the Open Sn Site

Sn-beta zeolite is a promising catalyst for the direct synthesis of lactide from lactic acid; here, we clarify the relationship between its local structure and activity through the interplay of experiments and DFT calculations. The open sites (HO-Sn-(OSi)₃ with adjacent Si–OH) play a key role in the high activity of Sn-beta zeolite, where the reaction rate significantly decreases from 0.2258 mmol min^–1 g_cat^–1 to 0.0724 mmol min^–1 g_cat^–1 if the open sites were masked with Na⁺. Density functional theory (DFT) calculations show that the decrease in reaction rate comes from the weaker interaction between substrate and the open Sn site by Na⁺ exchange, which leads to the significant increase in the Gibbs energy barrier from 34.0 kcal/mol to 57.2 kcal/mol (rate-determining step). Other activities of heteroatom M-beta zeolites (where M = Ti, Zr, Hf) were also investigated, where the open sites were found to be crucial for the high activity in the direct synthesis of lactide as well. The information provided by this work is valuable for precise design of efficient catalysts for the one-step synthesis of lactide.