Ionic Liquid-Strengthened Immobilized Rhizomucor miehei Lipase for Catalytic Esterification of Itaconic Acid in Aqueous Media

Dimethyl itaconate (DI), a promising biomass-derived intermediate, cannot be synthesized from itaconic acid (IA) in organisms due to the lack of metabolic pathways. It is necessary to develop a green and high-efficiency method to synthesis DI from IA. In this study, DI was, for the first time, produced from IA in an aqueous medium by an ionic liquid-strengthened enzymatic method. We found that when pretreated by the ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), the pH tolerance range of the lipase shrunk and the hydrolysis activity of it was greatly inhibited, while its esterification activity was not. Moreover, when the [bmim][PF6]-treated immobilized Rhizomucor miehei lipase (RML, 50 mg/mL) was employed in a reaction containing IA/methanol at a molar ratio of 1:30 and methanol/water in a volume ratio of 1:10 at 30 °C after 24 h, a 64.3% yield and 73.2% selectivity of DI was obtained. The results of HPLC and GC–MS revealed that in addition to the main product DI, other side products including acetic acid, propionic acid, and itaconic acid cyclolactone were also generated, which indicates that the RML-catalyzed esterification of IA and methanol has various side reactions, such as lactonization and self-decomposition of IA.