Bifunctional Condensation Reactions of Alcohols on Basic Oxides Modified by Copper and Potassium

Alcohol dehydrogenation and condensation reactions are involved in chain growth pathways on Cu/MgCeOx promoted with potassium. These pathways lead to the formation of isobutanol with high selectivity via reactions of higher alcohols with methanol-derived C1 species in reaction steps also relevant to higher alcohol synthesis from CO/H2 mixtures at higher pressures on K–Cu/MgCeOx catalysts. Ethanol reactions on K–CuyMg5CeOx show that both Cu and basic sites participate in alcohol dehydrogenation and aldol condensation steps leading to n-butyraldehyde and acetone. Chain growth occurs by condensation reactions involving a metal-base bifunctional aldol-type coupling of alcohols. Reactions of C2H5OH–C2H4O mixtures show that direct condensation reactions of ethanol can occur without requiring the intermediate formation of gas phase acetaldehyde. Reactions of C2H5OH/D2 mixtures show that Cu sites increase the rate of aldol condensation by introducing recombinative desorption sites that remove hydrogen atoms formed in C–H activation steps leading to the unsaturated aldol-type species required for chain growth. Reactions of acetaldehyde and 13C-labeled methanol lead predominantly to 1-13C-propionaldehyde and 2-13C-isobutyraldehyde, both of which lead to isobutanol during CO/H2 reactions. Mixtures of propionaldehyde and 13C-labeled methanol lead to singly-labeled isobutyraldehyde. Chain growth to C2+ alcohols occurs via addition of a methanol-derived C1 species to adsorbed oxygen-containing intermediates. The gradual appearance of 13C in the unlabeled reactant within these mixtures shows that aldol coupling reactions are reversible. Reverse aldol condensation reactions after intramolecular hydride transfer lead to the formation of acetone from ethanol. Isobutyraldehyde is a preferred end-product of aldol-type chain growth reactions of alcohols because it lacks the two α-hydrogens required for subsequent chain growth. c © 1998 Academic Press