Dehydration mechanism of fructose to 5-hydroxymethylfurfural catalyzed by functionalized ionic liquids: a density functional theory study

Abstract

Ionic liquids (ILs) have shown great catalytic effects in the conversion of fructose to 5-hydroxymethylfurfural (HMF) but the dehydration mechanism remains ambiguous. In this work, density functional theory (DFT) calculations have been carried out for the mechanism of the dehydration of fructose to HMF, catalyzed by functionalized ILs. ILs with different kinds of functionalized imidazolium-based cations and SO₃ group-based anions have been used to investigate the catalytic effects of different anions and cations on the dehydration of fructose. The whole reaction process of fructose to HMF includes three dehydration steps. The overall rate-determining step is the second dehydration, and the functionalized cations showed good proton transfer ability to catalyze the reaction in the dehydration steps. The proton shuttling ability of cations is better than that of anions. The anions can form strong H-bonds with the fructose and cations and provide a polar environment to stabilize intermediates and transition states. The high catalytic activity is attributed to the synergetic catalysis of cations and anions to complete the dehydration of fructose. The dehydration reaction mechanism from fructose to HMF catalyzed by [BmimHSO₃][HSO₄]-functionalized ILs has been proposed by DFT calculations. This study emphasizes the catalytic performance of functionalized ILs replaced by different substituent groups and the dehydration mechanism of fructose to HMF.