Controlled Synthesis of l-Lactide Using Sn-Beta Zeolite Catalysts in a One-Step Route

Abstract

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 Sn 4+ 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 (L 2 a) and then formed Llactide 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 (L 3 a) formed during the reaction could also be converted to L-lactide in the later stage of the reaction, and the diffusion of L 3 a to the active site within the Snbeta zeolite pore is the rate-limiting step. The lactic acid oligomer (L n a, n ≥ 4) was limited to form lactide with an R 20min av of only 0.013 mmol min −1 g cat −1.

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

“…27 To further ascertain the influence of the coordination state of Sn on the acidity and catalytic activity of the catalyst, we compared the activity of Sn-beta zeolite with supported SnO 2 /Si-beta complex that possess extra-framework SnO x species. The supported SnO 2 /Si-beta complex has only some Lewis acid sites, and its catalytic activity (yield, 47.7%) is much lower than that of Sn-beta zeolite, 26 framework Sn is crucial for lactide synthesis. However, there are two forms of Sn sites in the Sn-beta zeolite framework.…”

“…Inspired by such reasoning, our group applied Sn-beta zeolite to catalyze the one-step synthesis of lactide and the yield is up to 86%−95.6%, consistent with our expectation. 26,27 Such interesting results encouraged us to investigate the mechanism of the reaction and elucidate the active sites in the Sn-beta zeolite. The pyridine-probed FT-IR suggests that the incorporation of Sn into the beta zeolite framework leads to generating both Lewis acid and Bronsted acid sites that play a dominant role in the synthesis of lactide.…”

Salt-Promoted Water Removal from Reflux Toluene for Efficient One-Step Lactide Synthesis

“…Polylactic acid (PLA) is one of the most promising biodegradable polymers for making plastic bottles and medical devices, − with expected market value of 6.5 billion USD by 2025 . The ring-opening polymerization of lactide (LD), a cyclic dimer of lactic acid (LA), is the most important commercial method for the production of PLA. , Given that the quality of LD is a crucial parameter for the polymerization process, LD with high chemical and optical purity is highly desired. − Nowadays, LD is manufactured from LA by a complicated two-step method, and its severe reaction conditions lead to the loss of optical purity, unwanted side products, and high operating costs. , Recently, Sels et al pioneered a simple one-step LD synthesis method via esterification of LA under mild conditions, in which 68% yield of LD with high optical purity (99%) was obtained over a shape-selective H-β zeolite at 140 °C for 3 h. The present studies mainly focus on catalyst development for this reaction. − However, few studies investigate the effect of water in the solvent on one-step LD synthesis, which indeed plays a significant role in esterification reactions. − …”

Confinement effect catalysis with covalent organic frameworks for highly efficient synthesis of lactide

“…[11][12][13] Most of the current industrial process to synthesize lactide has to go through a step of pre-polymerization of the LA to achieve PLA oligomer with low molecular weight (MW) first. 14,15 Then the endothermic transesterification with the help of nonrecoverable soluble metal salts results in the production of racemic lactide with many optical isomers and polymeric waste residues. The shortcomings of the current strategies to make lactide are obvious and greatly influence the industrialization process of PLA.…”

“…19 Zeolite catalysts, in particular, are well known for product selectivity control due to pore confinement effects. [20][21][22][23] The confinement effects of the porous and brønsted acidic zeolite catalysts lead to a direct conversion of LA to lactide in a high yield (~79%) and avoid both racemization and side-product formation. 19 The success of this strategy attracted the greatest attention of us to the unique advantages of the porous materials in confinement catalysis and encouraged us to explore the lactide preparation catalyzed by different crystalline porous materials.…”