CO Hydrogenation to Higher Alcohols over Cu–Co-Based Catalysts Derived from Hydrotalcite-Type Precursors

Cu–Co-based catalysts derived from hydrotalcite (HT)-type precursors were applied in higher alcohol synthesis (HAS) at 280 °C, 60 bar and a H₂/CO ratio of 1/1. Catalysts with higher Cu/Co ratios were found to provide the best trade-off between selective alcohol formation and moderate Fischer–Tropsch synthesis (FTS) activity. Within the alcohols and hydrocarbons formed the productivities decreased exponentially with increasing chain length according to the ASF distribution indicating a chain growth mechanism. Thermal analysis revealed the presence of different bivalent cations in one single HT-type precursor phase. After calcination at lower temperatures (T_calc < 600 °C) a carbonate-modified ZnAl₂O₄ matrix was obtained. Within this amorphous matrix Cu²⁺ and Co²⁺ were found to be partially embedded resulting in an impeded ion reduction. After HAS the presence of bulk Co₂C was detected by XRD. Both close contact of Cu⁰ and Co⁰ as well as Co₂C–Co⁰ interfaces are known to provide the mechanistic requirements for higher alcohol formation. For comparison HAS was performed over a physical mixture consisting of the Al-containing HTs of Cu, Co or Zn. For the simultaneously co-precipitated samples the major roles of Cu are to decrease the FTS activity of metallic Co and to lower the alcohol chain growth probability by intimate Cu⁰–Co⁰ interactions. With increasing Cu content the alcohol selectivities were found to increase at the expense of high conversion, with ethanol being the major oxygenate product for all HT-based catalysts.