Breaking the Si/Al Limit of Nanosized β Zeolites: Promoting Catalytic Production of Lactide

Abstract

Efficient production of lactide (LT) from highly concentrated lactic acid (LA) based on β zeolite catalysts is highly desired for an economically sustainable polylactic acid (PLA) industry. Like any zeolite, the Si/Al ratio of nanosized β zeolite needs to be adjusted to fit the requirements of different industrial catalytic processes. However, varying its Si/Al ratio above 100 or below 20 while keeping the crystal size within 100 nm remains a challenge. We successfully prepared nanosized β (10–106 nm) with broad Si/Al ratios (6–300) by l-lysine-assisted two-step crystallization in a concentrated gel system. The crystallization process of the nanosized β zeolite was studied. Significantly, the as-prepared β-15-10 catalyst (Si/Al = 15.5, size = 10.1 nm) with the lowest Si/Al ratio and the smallest particle size shows the highest LnA conversion (n = 1–3) and LT yield (74%) in the conversion of highly concentrated LA (105 wt %). Importantly, not only LA/L2A but also L3A existing in the highly concentrated LA could be converted to LT. This is the result of a fast LT production promoted by the increased Brønsted acid site density and its rapid diffusion out of the nanosized crystals avoiding secondary reactions, namely, decyclization and oligomerization. The reaction pathways from L3A/L2A to LT were elucidated via density functional theory (DFT) calculations.