A structure-activity relationship study using DFT analysis of Bronsted-Lewis acidic ionic liquids and synergistic effect of dual acidity in one-pot conversion of glucose to value-added chemicals

The catalytic conversion of biomass-derived carbohydrates to value-added chemicals, such as 5-hydroxymethylfurfural, levulinic acid, and formic acid, is a commercially important reaction and requires the use of both Lewis and Bronsted acids. Multifunctional ionic liquids (ILs) with both types of acidity show promise as useful catalysts as well as solvents for one-pot glucose conversion to value-added chemicals. In this study, a series of chlorometallate ILs were synthesized and characterized, and their catalytic activity towards glucose conversion was investigated. The structure-activity relationship of chlorometallate ILs was investigated by qualitative comparison of their structural parameters obtained using density functional theory and catalytic activity studies. The acidity and basicity of these ILs were correlated with theoretically estimated parameters such as electrostatic surface potential maxima and minima, average local surface ionization energy, Fukui functions, and stabilization energy. Metal atoms showed a significant effect in controlling both the Lewis and Bronsted acidity of these ILs by withdrawing electron density from the cationic counterpart. Fukui function values showed the presence of significant amount of Lewis basicity in the metal halide-containing anionic part of the ILs. The ZnCl3-containing IL with highest electrostatic surface potential maxima, representing more electron-deficient region in the cationic part of the IL, was found to be highly Lewis acidic. Furthermore, it showed maximum catalytic activity in glucose conversion resulting in yields of 13.4%, 23.8%, and 67.1% of 5-hydroxymethyl furfural (HMF), levulinic acid (LA), and formic acid (FA), respectively. Hence, Lewis acidity was found to be the controlling parameter for glucose conversion to value-added chemicals. These multifunctional chlorometallate ILs were found to be recyclable with no loss of metal chloride from the anion of the ILs.