Understanding effect of molecular structure of imidazole-based ionic liquids on catalytic performance for biomass inulin hydrolysis

Developing a green hydrolysis process of biomass for the production of the value-added compounds has been widely concerned. This article aimed at elucidating the structure-property relationship of ionic liquids (ILs) as catalysts, i.e., effect of the chemical structures of anion and cation of ILs on their acidity and catalytic properties of inulin hydrolysis. Twenty kinds of imidazole-based ILs and one kind of quaternary ammonium ILs were synthesized. Inulin hydrolysis for the reducing sugar production was investigated with a series of functionalized ILs under moderate temperature (75 °C) and atmospheric pressure. The chemical structure, acidity and catalytic property of ILs were experimentally characterized and theoretically analyzed. The possible catalytic hydrolysis mechanisms of polysaccharide or lignocellulose by ILs were detailedly analyzed. This work demonstrated that the catalytic performance of catalysts depends not only on its acidity, but also on the chemical structure. It was revealed that imidazole-based ILs showed better catalytic performance than quaternary ammonium ILs in the inulin hydrolysis system, and SO₃H-functionalized sulfonate ILs obtained more efficient catalytic property because the hydrolysis reaction was catalyzed by the synergy of anion and cation, compared with the sulfuric acid with stronger acidity, which can cause serious equipment corrosion in biomass hydrolysis process. In all these tested ILs, (1-(4-sulfonic acid)-butyl-3-ethylimidazolium hydrogen sulfate) ([C₂MIM-PS][HSO₄⁻]) exhibited the best catalytic performance, and the yield of reducing sugar was 100%. Understanding the structure-property relationship of ILs will be helpful to design the covalent immobilization strategy of ILs to improve its reusability and eliminate the potential harmfulness to the environment.