Preparation and characterization of lanthanum-promoted cobalt–copper catalysts for the conversion of syngas to higher oxygenates: Formation of cobalt carbide

Abstract

A series of La-promoted cobalt–copper catalysts with various Co:Cu ratios have been used to study the conversion of syngas to oxygenates and hydrocarbons. In particular, the effect of the Co:Cu composition on the selectivity to oxygenates versus hydrocarbons has been examined.Three bulk catalysts were synthesized by coprecipitation, reduced in H2/He flow, and then cobalt carbide was formed during CO hydrogenation. The composition of the catalysts was as follows: Cu:Co=12:9, 7:13, and 0:21 (cobalt only). CO hydrogenation tests were performed at differential conversions and 30bar, H2/CO=2/1 and 250°C.The C1 selectivity (methane+methanol+CO2) was ∼64% for the two catalysts containing Co and Cu, and slightly less for the Co-only catalyst (52%). These products are formed by three mechanisms: (1) CH4: hydrogenation of dissociatively adsorbed CO at metallic cobalt sites, (2) CH3OH: hydrogenation of associatively adsorbed CO at copper sites, and (3) CO2: water gas shift, also at the copper sites. C2+ alcohol selectivity for the two Cu-containing catalysts is greater than for the Co-only catalyst, while the Co-only catalyst has the highest selectivity to acetaldehyde. The formation of C2+ oxygenates is consistent with the CO insertion mechanism, in which associatively adsorbed CO is inserted into the CHx species and forms the first C–C bond, producing a CHxCO intermediate that can be hydrogenated into ethanol or acetaldehyde.