Probing the mechanism of the conversion of methyl levulinate into γ-valerolactone catalyzed by Al(OiPr)3 in an alcohol solvent: a DFT study

Abstract

Biomass-derived γ-valerolactone (GVL) is a versatile chemical that can be used in various fields. As an efficient, cheap, and sustainable catalyst, Al(OiPr)₃ has been successfully used in the conversion of methyl levulinate (ML) to GVL in the solvent isopropanol (IPA). However, the molecular mechanism of this conversion catalyzed by Al(OiPr)₃ remains ambiguous. To investigate the mechanism of the conversion of ML to GVL catalyzed by Al(OiPr)₃, the reaction pathways, including the transesterification, Meerwein–Ponndorf–Verley (MPV) hydrogenation, and ring-closure steps, were probed using density functional theory (DFT) calculations at the M062X-D3/def2-TZVP level. Among the elementary steps, it is found that ring-closure is the rate-determining step and that Al³⁺ can coordinate with the oxygen of 2-hydroxy-isopropyl levulinate (2HIPL) to catalyze the last ring-closure step. A four-centered transition state can be formed, and Al(OiPr)₃ shows a strong catalytic effect in the two steps of the ester exchange reaction. The center of Al(OiPr)₃ mainly coordinates with the carbonyl oxygen atom of the ester to catalyze the reaction. The present study provides some help in understanding the conversion mechanism of ML to GVL and designing more effective catalysts for use in biomass conversion chemistry.