Heterogenization of polyoxometalates as solid catalysts in aerobic oxidation of glycerol†
A series of heterogeneous catalysts based on phosphomolybdic salts with different metals in counter or substituent positions, LnPMo12O40 (L = Al3+, Fe3+, Cr3+, Ti4+, Zr4+ and Zn2+, abbreviated as LPMo12) and HxPMo11LO39 (L = Zn2+, Cr3+, Fe3+, Al3+, Ti4+, for Ti4+; the amount of O is 40, abbreviated as HPMo11L), were prepared using a simple calcination treatment and were evaluated in aerobic oxidation of glycerol. After calcination at about 250 °C for 4 h, homogeneous LnPMo12O40·nH2O and HxPMo11LO39·mH2O became heterogeneous. Their specific surface areas were also enhanced as well as their activity and reusability. Similar to the homogeneous catalysts, AlPMo12 treated at 400 °C (AlPMo12-400) was found to be the most active catalyst in glycerol oxidation to lactic acid, with 96.1% yield and 98.6% conversion at 60 °C for 5 h with 1 MPa of O2; it gave a turnover number (TON; TON = [LA]/[catalyst]) of 2.4 × 102, higher than that of AlPMo12-250 treated at 250 °C (TON = 2.0 × 102). AlPMo12 behaved as a heterogeneous catalyst in glycerol oxidation and could be reused at least 12 times. AlPMo12-400 also performed well in crude glycerol oxidation, affording lactic acid yields as high as 86.8%.