Alcohol synthesis over cesium promoted copper/zinc oxide catalysts: Surface species, mechanistic pathways, and catalyst lifetime

Cesium formate promoted copper/zinc oxide catalysts that are effective for the synthesis of methanol and higher oxygenates from carbon monoxide and hydrogen have been studied.; The surface species formed from synthesis gas on the surface of the catalysts have been observed by infrared spectroscopy. Over the undoped catalysts, bidentate formate, formaldehydic and methoxide surface species were identified. Addition of cesium resulted in a unique bidentate formate species associated with the alkali cations, evidence that the highly basic cesium centers promote CO activation and hence increase the synthesis rates.; Under methanol synthesis conditions, cesium salt addition to the Cu/ZnO catalyst promoted the synthesis rates of methanol, methyl formate and ethanol. Using {dollar}sp{lcub}13{rcub}{dollar}CH{dollar}sb3{dollar}OH and {dollar}sp{lcub}12{rcub}{dollar}CO/H{dollar}sb2{dollar}, {dollar}sp{lcub}13{rcub}{dollar}C-NMR analysis of the products synthesized evidenced the formation of methyl formate by the direct carbonylation of methanol whereas ethanol was formed by coupling of oxygenated C{dollar}sb1{dollar} intermediates formed from the enriched methanol. The experimental evidence also supported the presence of kinetically significant formyl and formaldehyde surface species.; Under higher alcohol synthesis conditions, cesium promotion of the Cu/ZnO catalyst increased the synthesis rates of the higher oxygenates, especially 2-methyl-1-propanol. Injection of carbon-13 enriched ethanol into the synthesis gas demonstrated that the synthesis of higher alcohols involves pathways of linear and branched carbon chain growth. The growth is dominated by C{dollar}sb1{dollar} oxygenate addition to the {dollar}beta{dollar}-carbons of oxygenated intermediates as well as by linear addition. Over the undoped Cu/ZnO catalyst, only the linear chain growth of ethanol was observed, CH{dollar}sb3sp{lcub}13{rcub}{dollar}CH{dollar}sb2{dollar}OH + CO/H{dollar}sb2{dollar} {dollar}to{dollar} CH{dollar}sb3sp{lcub}13{rcub}{dollar}CH{dollar}sb2{dollar}CH{dollar}sb2{dollar}OH while cesium promoted {dollar}beta{dollar}-carbon addition, CH{dollar}sb3sp{lcub}13{rcub}{dollar}CH{dollar}sb2{dollar}OH + CO/H{dollar}sb2{dollar} {dollar}to{dollar} {dollar}sp{lcub}13{rcub}{dollar}CH{dollar}sb3{dollar}CH{dollar}sb2{dollar}CH{dollar}sb2{dollar}OH. The isotope labeling evidenced the retention of the oxygen associated with the C{dollar}sb1{dollar} intermediate formed from CO/H{dollar}sb2{dollar} and loss of the oxygen from the {dollar}sp{lcub}13{rcub}{dollar}CH{dollar}sb2{dollar}OH group of ethanol. The mechanism has been proposed to proceed via a {dollar}beta{dollar}-ketoalkoxide intermediate, the mechanism termed as aldol coupling with oxygen retention reversal. Higher oxygenate synthesis also proceeds by oxygen retention reversal and normal oxygen retention in the coupling reactions, the pathways observed attributed to steric effects involving the {dollar}beta{dollar}-alkoxide intermediates.; Catalytic lifetime tests were performed under higher alcohol synthesis conditions. Careful control of the testing conditions demonstrated the intrinsic stability of the cesium promoted Cu/ZnO catalyst.