On the Mn promoted synthesis of higher alcohols over Cu derived ternary catalysts†
This work provides insight into the promotional effect of Mn on the synthesis of higher alcohols over Cu-based ternary catalysts through XPS and in situ DRIFTS and powder XRD. These revealed that the surface of K-CuZnAl, an active catalyst for CO hydrogenation to methanol, possesses Cu+ sites able to adsorb CO associatively and Cu0 sites for H2 dissociation. Here we show that exchanging Zn with Mn induces a strong interaction between Cu and Mn that decreases the overall copper surface area and increases the Cu+/Cu0 ratio. In situ DRIFTS showed that electronic modification of Cu+ sites by proximate Mn favors dissociative CO chemisorption, resulting in the formation of C and O adspecies that are precursors to higher alcohol formation. The decrease in metallic copper limits available sites for H2 dissociation, and hence retards the hydrogenation of oxygen-containing intermediates, thereby further promoting carbon-chain growth. Mn also increases the dispersion of the K promoter over the catalyst surface, providing abundant basic sites for aldol-type condensation reactions to branched oxygenates.