SYNTHESIS OF HIGHER ALCOHOLS ON COPPER-CATALYSTS SUPPORTED ON ALKALI-PROMOTED BASIC OXIDES

K-CuyMg5CeOx and Cs-Cu/ZnO/Al2O3 are selective catalysts for the synth esis of alcohols from an H-2/CO mixture at relatively low pressures an d temperatures. CO2 produced in higher alcohol synthesis and water-gas shift (WGS) reactions reversibly inhibits the formation of methanol a nd higher alcohols by increasing oxygen coverages on Cu surfaces and b y titrating basic sites required for aldol-type chain growth steps. In hibition effects are weaker on catalysts with high Cu-site densities. On these catalysts, the abundance of Cu sites allows reactants to reac h methanol synthesis equilibrium and maintain a sufficient number of C u surface atoms for bifunctional condensation steps, even in the prese nce of CO2. The addition of Pd to K-Cu0.5Mg5CeOx weakens CO2 inhibitio n effects, because Pd remains metallic and retains its hydrogenation a ctivity during CO hydrogenation. Basic sites on Mg5CeOx are stronger t han on ZnO/Al2O3 and they are more efficiently covered by CO2 during a lcohol synthesis. K and Cs block acid sites that form dimethylether an d hydrocarbons. Alcohol addition studies show that chain growth Occurs predominantly by aldol-type addition of methanol-derived C-1 species to ethanol and higher alcohols, following the rules of base-catalyzed aldol condensations, The initial C-C bond formation required for ethan ol synthesis, however, proceeds directly from CO, at least on K-CuyMg5 CeOx catalysts. A detailed kinetic analysis shows that chain growth pr obabilities are very similar on K-CuyMg5CeOx and Cs-Cu/ZnO/Al2O3 catal ysts. The growth probabilities of C-1 chains to ethanol and of iso-C-4 chains to higher alcohols are much lower than for other chain growth steps. (C) 1998 Elsevier Science B.V.