Cobalt-based Catalysts for the Conversion of Biomass-derived Syngas to Ethanol and Higher Oxygenates

Abstract

Higher oxygenates synthesis by catalytic conversion of synthesis gas are the potential approaches to replace fossil fuels and produce chemicals for pharmaceutical manufacture, detergents and polymer industry. Cobalt-based catalysts are the most promising catalysts to replace noble metal catalysts for producing ethanol, acetaldehyde and higher oxygenates via CO hydrogenation. The formation of higher oxygenates requires active sites that can adsorb CO non-dissociatively and insert to the alkyl intermediate. The hydrogenation of the CO-inserted intermediates yields C2 and C2+ oxygenates. The main focus of this study is to investigate the active site for non-dissociative CO adsorption, and the modification of the catalysts to enhance the production rate for higher oxygenates. The effect of different structural promoters are investigated on copper-cobalt catalysts. Three cobalt–copper catalysts singly promoted with La, Zr, or Al were studied for catalytic conversion of syngas to higher alcohols. CO hydrogenation was carried out, and catalyst activity and selectivity to higher alcohols are the greatest on La promoted catalyst. La promoted catalyst differs from the other two in the first 10 h of time-on-stream, with the product distribution shifted to favor oxygenates formation. These results suggest changes on La promoted catalyst during the reaction. The DRIFTS study of CO adsorption behavior on La promoted copper-cobalt catalyst observed that CO linearly adsorbed on Co2C site. A cobalt carbide phase was formed in the reaction, and Co2C is able to adsorb CO associatively and insert to the intermediates. X-ray absorption techniques detected the existence of cobalt carbide in the lanthanum promoted cobalt-copper catalysts. A further reaction study on a promoter-free bulk Co2C catalyst also detected higher oxygenates as products. To improve the selectivity to ethanol and higher oxygenates and provide a stable catalyst for CO hydrogenation, a novel method using metal organic framework as precursors was performed to synthesize a series of potassium-promoted cobalt catalysts for higher oxygenates synthesis. The MOF-mediated synthesis method provide the catalyst with superior resistance to sintering and deactivation. The potassium promoters suppresses the formation of methane and higher hydrocarbons, while cobalt carbide is formed from the metallic cobalt and the carbon in the catalyst. As a result, the potassium-promoted catalyst synthesized by MOF-mediated method gives high yields to ethanol and higher oxygenates. CO hydrogenation was also carried out using real biomass-derived syngas. The low CO composition and high CO/H2 ratio in the biomass-derived syngas require special catalysts to carry out Fischer-Tropsch reaction and produce liquid fuels.