Specific role of aluminum site on the activation of carbonyl groups of methyl levulinate over Al(OiPr)3 for γ-valerolactone production

Abstract

The high-efficiency synthesis of biofuel γ-valerolactone (GVL) from biomass-derived levulinates is a challenging task. The Meerwein-Ponndorf-Verley (MPV) reduction with its extraordinary chemoselectivity is advantageous for the hydrogenation process, compared to the molecular-hydrogen-based process using noble metal catalysts. Therefore, we used a classical Al-based isopropoxide to catalyze transfer hydrogenation (CHT) of methyl levulinate (ML) to GVL. A high yield of GVL up to 97.6% could be achieved using 2-proponal as the H-donor and solvent under mild conditions (150 °C, 30 min). Besides, three reaction stages were observed in the conversion, including transesterification, hydrogenation and cyclization. LC/MS analysis and the density functional theory (DFT) caculations revealed that Al atom of Al(OiPr)3 as the electron transfer center activated ester carbonyl of the substrate via four-membered transition states before activating the ketone carbonyl, resulting in the occurrence of transesterification prior to the hydrogenation. In addition, 2-propanol as proton transfer carrier assisting the cyclization process was proved to be the lowest-energy pathway. Our work shed light on the role of Al (OiPr)3 in the MPV reduction of ML, providing a comprehensive understanding on the metal alkoxide catalysis mechanism for GVL production.