Heterogenization of polyoxometalates as solid catalysts in aerobic oxidation of glycerol

Abstract

A series of heterogeneous catalysts based on phosphomolybdic salts with different metals in counter or substituent positions, L_nPMo₁₂O₄₀ (L = Al³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺, Zr⁴⁺ and Zn²⁺, abbreviated as LPMo₁₂) and H_xPMo₁₁LO₃₉ (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 L_nPMo₁₂O₄₀·nH₂O and H_xPMo₁₁LO₃₉·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%.