PREPARATION OF TERNARY CU/CO/AL CATALYSTS BY THE AMORPHOUS CITRATE PROCESS .2. THE EFFECT OF THE DECOMPOSITION CALCINATION ATMOSPHERE

The effect of the decomposition-calcination procedure of Cu/Co/Al citrate precursors on the characteristics of the resulting mixed oxides and on the catalytic activity and selectivity for synthesis gas conversion to methanol and higher alcohols was studied. Cu/Co/Al amorphous precursors were prepared by means of the citric complexing method. Four different decomposition-calcination procedures were employed: I, air-air 500°C; II, N2-N2 500°C; III, N2 500°C-air 500°C, and IV, N2 280°C-air 500°C. The physicochemical properties of the mixed oxides (major surface species, porous structure, binary phases) were influenced by the decomposition-calcination procedure employed. The mixed oxides obtained through procedure II had low surface areas (8 m2 g-1), large particles of Cu and Co in metallic state, and high amounts of residual carbon. As a consequence, the catalytic activity was low and the selectivity to hydrocarbons was enhanced up to 70 wt%. Procedure III eliminated the residual carbon and regenerated metal oxides phases. However, the surface area was still low (14 m2 g-1) and a Cu-rich layer was formed at the sample surface, thereby causing an inhomogeneous distribution of the metal ions. These catalysts exhibited poor higher alcohol selectivities. The mixed oxides obtained through procedures I and IV exhibited similar physicochemical properties and catalytic activities. XRD and TPR characterization identified the formation of CuO and Co3O4 oxides and Co(Cu)Al2O4 aluminates. Surface species were mostly CuO and Co3O4. The distribution of the metals was uniform and the surface area values were relatively high (60-65 m2 g-1). These catalysts exhibited higher catalytic activity and were selective for the formation of alcohols, producing 52-54 wt% of total alcohols and 23-25 wt% of ethanol and higher alcohols. The catalytic results suggest that the homogeneous distribution of the metallic elements is crucial to higher alcohol synthesis. © 1992.