Preparation of ternary Cu/Co/Al catalysts by the amorphous citrate process II. The effect of the decomposition-calcination atmosphere

Abstract

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, N 2-N 2 500°C; III, N 2 500°C-air 500°C, and IV, N 2 280°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 m 2 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 m 2 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 Co 3O 4 oxides and Co(Cu)Al 2O 4 aluminates. Surface species were mostly CuO and Co 3O 4. The distribution of the metals was uniform and the surface area values were relatively high (60–65 m 2 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.