Si-doped γ-Al2O3 from solid-phase grinding for highly efficient one-step production of l,l-lactide

Abstract

L,L-Lactide, the important precursor of biodegradable polylactic acid, has attracted significant attention. The one-step synthesis based on nanoconfinement primarily utilizes a porous catalyst to induce the directional cyclization of L-lactic acid dimers into L,L-lactide; however, the reported porous catalysts suffer from complex fabrication methods and high costs. We have developed a new Si-doped γ-Al₂O₃ catalyst via solid-phase grinding (SPG-Si-Al₂O₃), where the incorporation of trace Si precisely modulated the pore size to create an optimal spatial confinement environment for directing L-lactic acid conversion to L,L-lactide. After a 3 h 170 °C reaction of 0.5 g of 90 wt% L-lactic acid and 0.25 g of catalyst in 10 mL of o-xylene, L,L-lactide was obtained with a yield of 77.9% (determined using an HPLC external standard method), purity of 79.4% (determined by ¹H NMR), and 100% optical selectivity (analyzed by chiral GC). By increasing the amount of catalyst to 0.30 g, 82% yield and purity can be achieved; by prolonging the reaction time to 5 h, 92% yield and 83.3% purity were achieved. Furthermore, after mild leaching regeneration, the catalyst maintains an L,L-lactide yield of around 80% over 5 cycles. The proposed catalyst is simply synthesized and low-cost, and this work provides a simple, low-energy, and low-cost approach for L,L-lactide production.