Biocatalytic upgrading of levulinic acid to methyl levulinate in green solvents

Abstract

High value-added utilization of biomass-based platform chemicals has attracted increasing attention in recent years. The present study addresses the upgrading of levulinic acid (LA), an important biomass-based platform chemical, to methyl levulinate via a green and efficient enzymatic approach. Various lipases were assessed, with Novozym® 435 displaying the best results for the esterification of LA. Subsequently, the effect of reaction medium on Novozym® 435 catalyzed esterification was also studied. Compared with other reaction media, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) showed the greatest enhancement in yield by suppressing hydrolysis activity. The catalysts reusability was also improved by protection from [bmim][PF6] with 43.2% activity remained after 5 cycles, compared to that of 19.1% in 2-Me THF. Reaction temperature exerted an unusual double-peak relationship over methyl levulinate yield because Novozym® 435 can simultaneously catalyze the esterification of LA and hydrolysis of methyl levulinate, each of which has a different optimal reaction temperature: ca. 30 °C and ca. 40 °C, respectively. When the molar ratio of LA/methanol, and the volume ratio of methanol/[bmim][PF6] were 1:5 and 1:3, respectively, methyl levulinate was obtained in 93% yield at 30 °C after 24 h with 10 mg/mL catalytic loading.