Synthesis of higher alcohols on copper catalysts supported on alkali-promoted basic oxides Original Research Article

K–CuyMg5CeOx and Cs–Cu/ZnO/Al2O3 are selective catalysts for the synthesis of alcohols from an H2/CO mixture at relatively low pressures and temperatures. CO2 produced in higher alcohol synthesis and water–gas shift (WGS) reactions reversibly inhibits the formation of methanol and higher alcohols by increasing oxygen coverages on Cu surfaces and by titrating basic sites required for aldol-type chain growth steps. Inhibition effects are weaker on catalysts with high Cu-site densities. On these catalysts, the abundance of Cu sites allows reactants to reach methanol synthesis equilibrium and maintain a sufficient number of Cu surface atoms for bifunctional condensation steps, even in the presence of CO2. The addition of Pd to K–Cu0.5Mg5CeOx weakens CO2 inhibition effects, because Pd remains metallic and retains its hydrogenation activity during CO hydrogenation. Basic sites on Mg5CeOx are stronger than on ZnO/Al2O3 and they are more efficiently covered by CO2 during alcohol synthesis. K and Cs block acid sites that form dimethylether and hydrocarbons. Alcohol addition studies show that chain growth occurs predominantly by aldol-type addition of methanol-derived C1 species to ethanol and higher alcohols, following the rules of base-catalyzed aldol condensations. The initial C–C bond formation required for ethanol synthesis, however, proceeds directly from CO, at least on K–CuyMg5CeOx catalysts. A detailed kinetic analysis shows that chain growth probabilities are very similar on K–CuyMg5CeOx and Cs–Cu/ZnO/Al2O3 catalysts. The growth probabilities of C1 chains to ethanol and of iso-C4 chains to higher alcohols are much lower than for other chain growth steps.