Reducing meso-lactide formation from racemic lactic acid using shape-selective H-Beta zeolite†

Abstract

The utilization of racemic lactic acid (rac-LA) produced via chemocatalytic conversion for polylactic acid (PLA) synthesis remains limited due to the formation of undesired mesomeric lactide (meso-LD), a by-product that arises during the conversion process. This study proposes the use of shape-selective H-Beta zeolite to reduce the formation of meso-LD in lactide (LD) by leveraging its selectivity towards different lactic dimers (L₂A) in a one-step catalytic conversion of rac-LA to LD. Experimental results from various LA solutions with different L₂A content indicate that the L₂A content is critical for maximizing the selectivity of racemic lactide (d/l-LD), thus confirming the shape-selective nature of H-Beta zeolite towards L₂A. Additionally, it is found that increasing the reaction temperature and employing structurally reconstructed H-Beta zeolite with more regularized channels as a catalyst can further decrease meso-LD formation, and a LD yield of 60% with a d/l-LD/meso-LD ratio of 2.26 can be achieved under optimal catalyst and reaction conditions. Despite the coexistence of d/l-LD and meso-LD in the final product, this study marks a significant step towards controlling the stereoisomer distribution of LD synthesized from rac-LA derived from chemical conversion.