Aerobic oxidation of glycerol catalyzed by M salts of PMo12O403-(M = K+, Zn2+, Cu2+, Al3+, Cr3+, Fe3+)

Abstract

Glycerol is a co-product in the manufacture of biodiesel from vegetable oil and animal fat. A significant increase in biodiesel production results in adramatic overproduction of glycerol. The oxidation of glycerol to lactic acid (LA) by O2 is considered to be a promising technology to increase the value of glycerol. Studies on the fundamental reaction mechanism vary for different catalytic systems. In this work we have chosen Keggin polyoxometalates (POMs) as an exemplary family of water-soluble hydrolytically and oxidatively-stable, fully-inorganic complexes with transition-metal and non-transition-metal counter-cations (M in the formula, MPMo12O40, where M = H+, K+, Zn2+, Cu2+, Al3+, Cr3+, Fe3+ in appropriate number to counterbalance the 3- charge of PMo12O403−). Reactions involving hydrogen peroxide, an intermediate during glycerol oxidation, produce free radicals. A kinetic analysis reveals that the radical chain length under typical conditions is about 20–30. After minimal optimization, the yield of LA was 88% at glycerol conversion > 97% (1.0 M aqueous solution of glycerol, 4.0 mM of the Al salt of PMo12O403−, henceforth “AlPMo”, 10 bar O2 at 60 °C, reaction time 6.0 h).