On the mechanism of higher alcohol formation over metal oxide catalysts: I. A rationale for branching in the synthesis of higher alcohols from syngas

Abstract

A new mechanism for the synthesis of alcohols from synthesis gas is proposed based on recent observations from surface spectroscopy, catalysis, and synthetic organometallic chemistry. Stepwise transfer of hydrogens to coordinated CO and chain growth by CO insertion provide the primary pathway for the construction of higher alcohols over metal oxide catalysts. The insertion of a carbon monoxide into a surface-bound aldehyde is proposed as the primary carbon-carbon bond forming step in the chain growth. A competing carbon-carbon bond-forming step is the reaction of a surface n 3 -enolate with a surface alkoxide. This condensation reaction is critical for providing product distributions deviating from those predicted by simple polymerization schemes. A novel rationale is proposed to explain the selectivity to branched products. The relative stabilities of the enolate precursors to the various alcohols and the relative rates of the 1,2-shift reactions of methyl and hydrogen are the rate-controlling mechanistic features which regulate selective formation of branched higher alcohol products. The mechanism is related to the dehydration of secondary alcohols to 1-olefins catalyzed by basic metal oxides.