Molecular design of efficient SO3H-functionalized ionic liquid to catalyse chitin into levulinic acid: NMR and DFT study

Although chitin is the second abundant natural biomass after cellulose, investigations on chitin conversion are still limited. Levulinic acid (LA) is versatile biorefinery platform chemical. The SO3H-functionalized ILs present excellent catalytic activity to transfer biomass into LA. To improve the yield of LA and select the IL with higher catalytic activity, the relationship between the structure of SO3H-functionalized ILs and their catalytic capacities for LA production from chitin are explored with the 1H NMR and density functional theory (DFT) methods in this work. First, the catalytic activities of the selected ILs were predicted by the DFT methods. Then, they were confirmed by the 1H NMR experiments. The order of the catalytic activity of the ILs for chitin conversion into LA is consistent with the sequence of the gap of the LUMO-HOMO of the most stable IL- N-acetylglucosamine (NAG) complexes. The imidazolium-based ILs show the best catalytic activity, followed by the pyridinium-based and ammonium-based ILs. The ILs with butyl chain have stronger catalytic activity than those with propyl chain. Based on the 1H NMR and DFT results, the [C4SO3Hmim]HSO4 was selected to catalyse chitin conversion into LA. The maximum yield of LA is 74.7 % achieved at 5 h reaction time, 190 ℃, 6.0 g water, 1.5 g [C4SO3Hmim]HSO4 catalyst and 0.1 g chitin.