Facile and high-yield synthesis of methyl levulinate from cellulose†
Efficient production of chemicals from cellulose provides sustainable routes for the utilization of natural renewable resources to meet the requirements of human society. Herein, we reported a highly efficient and simple metal salt catalyst, Al2(SO4)3, for cellulose conversion to methyl levulinate (ML) under microwave conditions. A highest ML yield of 70.6% was obtained at 180 °C within a very short time of 40 min. The introduction of water could reduce humin/coke formation and solvent consumption, and could also switch the reaction pathway via the more reactive intermediate glucose. Kinetic and mechanistic studies of the subreactions showed that both cellulose hydrolysis and alcoholysis pathways were involved in the cellulose conversion to ML, with the former as the main route in the presence of water. The Lewis acid species [Al(OH)x(H2O)y]n+ and the Brønsted acid species H+, generated by in situ hydrolysis of Al2(SO4)3, were responsible for the reaction conversions. The reaction with microwave heating showed accelerated reaction rates of 25 times the reaction with conventional oil heating, and even more times for the rates of glucose and methyl glucoside (MG) dehydration, resulting in higher reaction selectivity toward ML production. The catalyst was also successfully recycled and applied to the conversion of cellulose to other alkyl levulinates, as well as the conversion of raw biomass to ML with high yields. The homogeneous nature of Al2(SO4)3, together with its high efficiency and excellent recyclability, make it a potential catalyst for the large-scale production of ML in industry.