Tungstophosphoric acid supported on metal/Si-pillared montmorillonite for conversion of biomass-derived carbohydrates into methyl levulinate

Biomass-derived carbohydrates exploited toward the synthesis of alkyl levulinates is very promising, but still limited by the applicable catalyst. In this study, novel metal/Si-pillared montmorillonite supported phosphotungstic acid catalysts (SG-HPW-XSiMt-y, where X = La, Ce, Er, Cu, Al, Ti, Zr) were synthesized by sol-gel method. The catalytic performance of SG-HPW-XSiMt-y were comprehensively studied for glucose methanolysis to methyl levulinate (ML), focusing on the effects of metal acidity, HPW contents and reaction conditions. The appropriate Lewis acidity of metal ions effectively facilitated the isomerization of glucose/methyl-glucoside to fructose/methyl-fructoside; the strong Brønsted acidity of incorporated HPW was capable of suppressing the side reactions; and the large specific surface area and hierarchical pores of the mesoporous structure allow the reaction to take place both on surface and inside the porous framework. Among these as-synthesized catalysts, the SG-HPW-ZrSiMt-20 showed the most superior activity and achieved 65.8% yield of ML from glucose at 170 °C for 4 h. Alternative biomass-derived carbohydrates, including fructose, sucrose and cellobiose, also gave excellent ML yields under such catalytic system. The SG-HPW-ZrSiMt-20 had impressively stable reusability with merely loss of catalytic activity after eight cycles, owing to the uniform dispersion and stable bonding of HPW with ZrSiMt support. Generally, this catalyst system realizes an efficient and sustainable conversion of biomass-derived carbohydrates to ML production.