Higher alcohol and oxygenate synthesis over cesium-doped Cu/ZnO catalysts

The synthesis of higher (C{sub 2}{sup +}) alcohols and esters has been studied over cesium-doped Cu/ZnO catalysts. Under high alcohol synthesis conditions, e.g., 583 K, 7.6 MPa, and gas hourly space velocity = 3260 liters (STP)/kg cat/hr with a H{sub 2}/CO = 0.45 synthesis gas, the presence of cesium promoted the formation of higher oxygenates, especially 2-methyl-1-propanol. The yields of products passed through distinct maxima at cesium nominal concentrations of 0.3-0.5%. The principal role of cesium was to increase the ethanol synthesis rate and to provide an even greater enhancement in the rate of ethanol conversion to 1-propanol and subsequently to higher alcohols. To obtain insight into the mechanism of the carbon chain growth and linear versus branched carbon chain growth, a {sup 13}C-NMR study of the C{sub 2}-C{sub 4} product formed over Cu/ZnO and 0.4 mol% Cs/Cu/ZnO catalysts was performed. The presence of Cs effected a mechanistic switch and promoted {beta}-carbon addition, CH{sub 3}{sup 13}CH{sub 2}OH + CO/H{sub 2} {yields} {sup 13}CH{sub 3}CH{sub 2}CH{sub 2}OH. The position of the {sup 13}C label in the CH{sub 3} group of propanol provides evidence for retention of oxygen associated with the C{sub 1} intermediate, formed from CO/H{sub 2}, and loss of oxygen associated with {sup 13}CH{sub 2}OH group of ethanol. Mechanistically, such a retention is favored by a {beta}-ketoalkoxide intermediate that is bonded to the cesium centers via its anionic oxygen. This unique mechanism is termed here in as aldol coupling with oxygen retention reversal and is specific to the presence of the cesium salt dopant. 35 refs.