Selective hydrogenolysis of glycerol to propylene glycol on hydroxycarbonate-derived Cu-ZnO-Al2O3 catalysts

Abstract

Three Cu-ZnO-Al2O3 catalysts with similar compositions were prepared by homogeneous coprecipitation (CZA-HP), deposition-precipitation (CZA-DP), and conventional coprecipitation (CZA-CP). A Cu-ZnO catalyst was also prepared by homogeneous coprecipitation (CZ-HP) for reference. X-ray diffraction results showed that the degree of mixing of Cu2+, Zn2+, and Al3+ ions in the Cu-Zn-Al hydroxycarbonate precursor followed the order of CZA-DP < CZA-HP < CZA-CP. A more homogeneous aluminum distribution and intimate contact between Al2O3 and ZnO led to smaller CuO and ZnO crystallites in the CuO-ZnO-Al2O3 composite oxides, and consequently smaller Cu and ZnO crystallites in the final Cu-ZnO-Al2O3 catalysts. However, the intimate ZnO-Al2O3 contact hindered the interaction between Cu and ZnO in the Cu-ZnO-Al2O3 catalysts. As a result of these effects of Al2O3, for the samples other than CZA-CP, the Cu particles on CZA-HP showed the highest redox activity, which was characterized by N2O chemisorption-H2 temperature-programmed reduction. These catalysts catalyzed glycerol hydrogenolysis to propylene glycol with high selectivities above 90% at 30% glycerol conversion (473 K and 6.0 MPa H2). Their activities normalized per exposed surface Cu atom to give turnover frequencies increased in the order CZA-DP < CZA-CP < CZ-HP < CZA-HP, which was the order of the redox ability of the Cu particles. The addition of alumina increased the stability of Cu-ZnO catalysts. After six cycles (6 h per run), the activity of CZ-HP decreased by 45% accompanied by an increase of Cu crystallite size from 13.2 to 45.2 nm, while that of CZA-HP decreased only by 10% with an increase of the Cu crystallite size from 8.3 to 19.0 nm. These results demonstrated the promoting effects of Al2O3 on the activity and stability of Cu-ZnO catalysts derived from the hydroxycarbonates.