Hydrothermal conversion of glycerol to lactic acid catalyzed by Cu/hydroxyapatite, Cu/MgO, and Cu/ZrO2 and reaction kinetics

MgO-, ZrO2-, and hydroxyapatite (HAP)-supported metallic copper catalysts prepared by the incipient wetness impregnation method were used to catalyze the alkaline hydrothermal conversion of glycerol to lactic acid. The supported copper catalysts were characterized by XRD, XPS, BET, TPR, and CO2-TPD techniques. CuO species was completely reduced to metallic copper (Cu-0), which was the active site for the catalytic conversion of glycerol. Cu/MgO and Cu/HAP catalysts with high basicity exhibited higher catalytic activities for the hydrothermal conversion of glycerol to lactic acid than Cu/ZrO2 catalyst with low basicity. The lactic acid selectivity of 90% was obtained at the glycerol conversion of 91% in the hydrothermal conversion of glycerol over Cu(16)/HAP catalyst at 230 degrees C for 2 h with the initial glycerol and NaOH concentrations of 1.0 and 1.1 mol L-1, respectively. The Cu(16)/HAP catalyst exhibited good recycling performance in the hydrothermal conversion of glycerol to lactic acid. A power-law expression kinetic equation was used to evaluate the effect of glycerol and NaOH concentrations and reaction temperature on the alkaline hydrothermal conversion of glycerol to lactic acid over the Cu(16)/HAP catalyst. The intrinsic reaction activation energy, E-a, was 117.2 kJ mol(-1). (C) 2015 Elsevier B.V. All rights reserved.