Investigation of K-promoted Cu-Zn-Al, Cu-X-Al and Cu-Zn-X (X = Cr, Mn) catalysts for carbon monoxide hydrogenation to higher alcohols

Abstract

In this study, we report the effect of Cu/Zn/Al chemical composition and Zn and/or Al substitution by Mn and/or Cr in K-promoted Cu-Zn-Al catalysts for the hydrogenation of CO to higher alcohols. In terms of higher alcohols, the synthesis yielded preferentially primary 2-methyl branched alcohols with up to five carbon atoms, namely 2-methyl-1-propanol and 2-methyl-1-butanol, together with ethanol and propanol. Variation of the Cu/Zn/Al chemical composition showed that the catalyst with a Cu/Zn atomic ratio of 1 and low Al content (K-Cu45Zn45Al10) exhibits the optimum performance in terms of both activity and selectivity to higher alcohols. High Al contents on the other hand favor methanol production at the expense of higher alcohols. It is postulated that due to its acidic nature, alumina reduces the basic sites on the catalyst, thereby retarding the C1→C2 step. Substitution of Zn and/or Al by Mn and/or Cr was found to reduce activity by ∼50%, probably due to the lower exposed copper surface area as indicated by the formation of larger CuO crystals. In terms of selectivity, the most appreciable changes were recorded for the K-Cu45Mn45Al10 catalyst, where a 50% increase in higher alcohol formation was measured, rendering the catalyst the most selective among the investigated materials. Characterization of the catalysts provided some insight on the beneficial influence of Zn substitution by Mn in the K-Cu45Mn45Al10 sample. Inverse correlation between acidity and higher alcohols selectivity was evidenced, in accordance with the general notion that higher alcohol formation requires basic sites and indicates that reduced acidity is needed for the aldol-type condensation reactions. The replacement of Mn by Zn in K-Cu-Mn-Al reduced acidity and thus promoted the production of the desired higher alcohol products.