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 degrees C for 4 h, homogeneous LnPMo12O40 center dot nH(2)O and HxPMo11LO39 center dot mH(2)O 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 degrees 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 degrees C for 5 h with 1 MPa of O-2; it gave a turnover number (TON; TON = [LA]/[catalyst]) of 2.4 x 10(2), higher than that of AlPMo12-250 treated at 250 degrees C (TON = 2.0 x 10(2)). 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%.