Biocatalytic upgrading of fusel oil from bioethanol production to levulinate esters as renewable fuel oxygenates

Abstract

The transition toward a sustainable bioeconomy requires efficient strategies for converting renewable biomass and industrial side streams into value-added chemicals and fuels. Fusel oil is an important coproduct of bioethanol fermentation and levulinic acid (LA), a key platform molecule derived from lignocellulosic biomass, can be upgraded into alkyl levulinates, which are promising green solvents, specialty chemicals, and fuel-related applications. In this study, a solvent-free enzymatic approach was developed for the synthesis of alkyl levulinates via lipase-catalyzed esterification of levulinic acid with bioethanol-derived fusel oil and its major alcohol components, including 2-methyl-1-butanol and 3-methyl-1-butanol, using immobilized Candida antarctica lipase B (Novozym® 435). Under optimized conditions (50 °C, LA:alcohol molar ratio 1:10, enzyme loading 10 wt% and molecular sieve addition equal to substrate mass), quantitative levulinic acid conversion and levulinate yield (~ 98–99%) were achieved within 5 h. In comparison, the secondary alcohol 2-butanol, evaluated for mechanistic comparison, reached a lower conversion (~ 70%) under otherwise identical conditions, consistent with its greater steric hindrance within the enzyme active site. Molecular docking simulations supported experimental observations by elucidating substrate–enzyme interactions and differences in binding affinity. The immobilized enzyme exhibited excellent operational stability, maintaining over 97% LA conversion over five consecutive cycles, and successful operation at 100 mL in a bioreactor highlighted the potential for further scale-up. The solvent-free system reduces process complexity, facilitates downstream processing and enhances process sustainability. Overall, this work demonstrates an efficient strategy for valorizing fusel oil side stream into high-value levulinate esters, contributing to circular bioeconomy approaches and the development of renewable fuel oxygenates and chemical platforms.