Application-driven design of a biocatalyst for the continuous synthesis of levulinate esters from α-angelica lactone: synergistic effects of silica, triflate ionic liquid and lipase

Abstract

This study presents the design of a biocatalyst for the continuous synthesis of sustainable bioproducts with lignocellulosic biomass. The enzyme support was carefully designed through the immobilization of the methyltrioctylammonium triflate ionic liquid on the silica surface (3.43 wt% ionic liquid loading), ensuring that the cation responsible for providing the hydrophobicity is oriented outward. The modified silica with IL was characterized using TGA, FTIR, SEM-EDS, BET, and ¹⁹F solid state NMR. Computational simulations revealed interaction forces between the triflate anion and the silica and the key role of strong hydrogen bonding and hydrophobic microenvironments in enhancing stability. Sulfur K-edge XANES confirmed that the triflate anion remains in an intact S(VI) environment after immobilization, while spectral changes indicate a more electron-poor, strongly surface-bound triflate anion consistent with hydrogen-bond anchoring on silica. The formation of robust hydrogen bonds between the fluorine atoms and the silica hydroxyl groups led to the development of a highly stable support material, which was subsequently used as a matrix for the immobilization of Candida antarctica lipase B (6.06 wt% loading). The synergistic effect of silica, triflate ionic liquid and lipase was demonstrated in the biotransformation of α-angelica lactone with n-butanol, achieving a 98% yield under optimised batch conditions (room temperature, 120 min). In a continuous-flow system, the biocatalyst exhibited sustained performance, maintaining a yield of n-butyl levulinate above 95% for up to 144 h of operation without detectable enzyme leaching. The integration of enhanced catalytic activity and long-term stability in continuous-flow systems offers a pathway for the development of sustainable and economically viable enzymatic processes.