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.
A series of nanosized “rice ball” like hierarchical beta zeolite aggregates (RBZ x A y , x = 6, 9, 12 or 15; y = 2, 4 or 6) with hierarchical structures are successfully synthesized via gel aging and hydrothermal method without mesoporous template. The as-prepared RBZ x A y zeolites possess two ranges of inter-crystalline mesoporous that formed by zeolite nanoparticles aggregation compared to the non-aged samples (B 12 Z 4 ). The effects of Si/Al ratio and crystallization time on the morphology, structure and surface acidic properties of RBZ x A y were systematically investigated. Meanwhile, RBZ x A y as catalysts were applied in the conversion of lactic acid (LA) to lactide (LT) to investigate their catalytic performance. Benefitting from smaller crystal size, hierarchical structures and higher acid content, the representative catalyst (RBZ 12 A 4 ) exhibited outstanding catalytic activity that the LA conversion was high to 98 % and the LT yield up to 94 %. In addition, RBZ 12 A 4 presented perfect regeneration ability after 10 times recycles. This work provides an effective and facile route to directly synthesis nanocrystals aluminosilicate zeolite with hierarchal structure, and constructs an approach for efficiently converting LA to LT, it has greatly significance for the development of polylactic acid industry.
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