Trisodium citrate synergises with Zn-BTC-modified ZIF-8 immobilised enzyme for green epoxidation of α-pinene in acidic media

This study addresses challenges in the chemoenzymatic epoxidation of α-pinene, such as lipase deactivation, carrier decomposition in acidic environments, and product ring-opening side reactions, by developing a core-shell structured metal-organic framework (MOF)-based immobilized enzyme catalyst, CALB@ZIF-8@Zn-BTC. Candida antarctica lipase B (CALB) was encapsulated into ZIF-8 channels via in situ encapsulation. Trisodium citrate was introduced to accelerate the aggregation and co-precipitation of ZIF-8, while 1,3,5-benzenetricarboxylic acid (H₃BTC) coordinated with surface zinc ions of ZIF-8 to form a core-shell composite with enzyme protection and acid-buffering functionalities. Experimental results demonstrated that sodium citrate significantly enhanced acid scavenging capacity, suppressing acidic side reactions and achieving 96.6 % epoxidation selectivity. The Zn-BTC shell improved ZIF-8 stability in acidic systems, retaining over 60 % activity after 7 cycles. Under optimized conditions (3.35 mol/L ethyl acetate, 0.5 mol/L H₂O₂, 0.1 g sodium citrate), α-pinene conversion reached 83.1 %. Compared to the commercial catalyst Novozym435, CALB@ZIF-8@Zn-BTC exhibited superior catalytic efficiency, selectivity, and reusability. Structural characterization (FESEM, EDS, nitrogen adsorption) confirmed enhanced mass transfer and stability due to the composite’s unique architecture and uniformly distributed sodium citrate. This work provides an innovative solution for green and efficient α-pinene epoxidation and offers theoretical and practical insights for designing MOF-based catalysts in acidic environments.