Biocatalytic upgrading of levulinic acid to methyl levulinate in green solvents

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((R)) 435 displaying the best results for the esterification of LA. Subsequently, the effect of reaction medium on Novozym((R)) 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((R)) 435 can simultaneously catalyze the esterification of LA and hydrolysis of methyl levulinate, each of which has a different optimal reaction temperature: ca. 30 degrees C and ca. 40 degrees 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 degrees C after 24 h with 10 mg/mL catalytic loading.