Catalyzing Cascade Production of Methyl Levulinate from Polysaccharides Using Heteropolyacids H_<i>n</i>PW₁₁MO₃₉ with Brønsted/Lewis Acidic Sites

A series of Lewis acid metals monosubstituted phosphotungstic acids H_<i>n</i>PW₁₁MO₃₉ (HPWM, M = Cu^II, Zn^II, Cr^III, Fe^III, Sn^IV, Ti^IV, and Zr^IV; for Ti and Zr, the number of oxygen is 40) was evaluated in direct production of methyl levulinate (ML) from cellulosic biomass in a cascade reaction. One of the solid catalysts, H₅PW₁₁TiO₄₀ (HPWTi), was found to be highly efficient for generation of ML from mono- or polysaccharides, reaching 51.3% ML yield directly from cellulose. And under microwave-assistance, the efficiency could be improved to a 62.6% ML yield within 2 h, which was almost the best result so far among reported solid catalysts. Identification of the reaction intermediates and the products provided some insight into the reaction mechanism and showed the requirement of certain Brønsted/Lewis acid ratio as 2.84/1 for HPWM. Moreover, the different metals in catalysts profoundly affected the Lewis or total acidity, and therefore, the catalytic activity and selectivity to ML or methyl glucosides (MG). HPWTi acted as a heterogeneous catalyst after being calcinated at 200 °C and showed high recyclability with minor loss of performance