STRUCTURE AND REACTIVITY OF COPPER-ZINC-CADMIUM CHROMITE CATALYSTS

Different mixed oxides containing Cu, Zn, Cd and Cr were obtained by heating hydroxycarbonate precursors at various temperatures and in different atmospheres, and characterized by using X-ray diffraction (XRD), infrared, diffuse reflectance spectroscopy, thermogravimetry, and BET and copper surface area analysis. The reduction process and the phase evolution during reduction of the mixed oxides was followed by temperature-programmed reduction and XRD. The mixed oxides were activated in an H-2/N-2 flow and then tested as catalysts in the vapour-phase hydrogenation of gamma-butyrolactone (GEL) and a solution of maleic anhydride (MA) in GEL (60:40 w/w). Depending on composition, heating temperature and atmosphere, the samples contained different phases, such as CuO, CdO, Cr2O3, CuCrO2, CuCrO4, alpha-CdCrO4, cubic and tetragonal CuCr2O4, and cubic Cu-Zn and Cu-Cd spinel-type phases. The partial substitution of Cu2+ ions with Zn2+ or Cd2+ ions stabilizes the cubic form of the spinel-type phases, which form via intermediate chromate phases. The final product of reduction in all cases is metallic copper. Cubic CuCr2O4 is more reducible than the corresponding tetragonal phase, which reduces to metallic copper through the intermediate formation of CuCrO2. The presence of zinc or cadmium promotes or inhibits, respectively, the copper reducibility both in CuO and in the copper chromite spinel. In the hydrogenation of GBL, the Cu/Cr catalysts obtained by calcination show similar behaviours, with a small increase in activity as a function of the copper content. At the lower temperatures investigated the main products are tetrahydrofuran (THF) and n-butanol, while at 548 K significant amounts of ethanol are observed, favoured by increasing copper content. On the other hand, the Cu/Cr catalyst obtained by heating under a reduced oxygen atmosphere, for which only tetragonal CuCr2O4 is detected before reduction, shows a decrease in catalytic activity, Partial substitution of the CU2+ ions gives rise to a decrease in GEL conversion, with a considerable deactivation when Cd2+ ions are present. Decreasing the H,IC, molar ratio gives rise to a decrease in activity for all samples, without any significant change in the trend observed. With the MA/GBL solution, the partial substitution of Cd2+ ions for Cu2+ ions also gives rise to considerable deactivation. For the other catalysts, the main products at the lower temperatures are GEL and succinic anhydride (SA), while at 548 K overhydrogenation and hydrogenolysis reactions predominate. The presence of Zn2+ ions gives rise to an increase in yield in GEL, due to an increase in the hydrogenation activity toward SA, but the consecutive transformation of GEL at high temper ature to low cost by products is not inhibited. On the other hand, the presence in the Cu/Cr catalyst of only CuCr2O4 increases the yield in GEL, as a consequence of a strong inhibition of the overhydrogenation and/or hydrogenolysis reactions of GEL. With the MA/GBL solution, similar behaviours are observed when the H-2/C-4 molar ratio decreases. At the same time, however, the lack in the carbon balance increases and a displacement towards high temperature of the maximum yield in GEL is found.