Catalytic synthesis of ethanol from biomass-derived syngas

Abstract

Syngas produced by gasification of biomass or coal can be converted directly to ethanol and higher alcohols by processes based on heterogeneous catalysts. Ethanol can be used as a neat fuel or fuel additive or as a hydrogen carrier. Rhodium-based catalysts have been found to be most selective for the formation of C2 oxygenates from the hydrogenation of CO when suitably promoted. Here, we explore the effects of Mn, Fe, and Li promoters on Rh/TiO2. Rh-TiO2, Rh-Li/TiO2, Rh-Mn/TiO2, Rh-Mn-Li/TiO2 and Rh-Mn-Li-Fe/TiO2 catalysts were tested for the hydrogenation of CO and a mixture of CO and CO2. Rh-Li/TiO2 is the most active and selective of these catalysts for ethanol formation from CO hydrogenation, due to the interactions between Li and Rh resulting in enhanced Rh dispersion, which decreases CO dissociative adsorption activity on the catalysts leading to increased CO insertion and hydrogenation of surface species. Mn promotion leads to a weakening of the Rh-CO bond, making more CO available for insertion but with limited hydrogenation resulting in higher acetaldehyde selectivity than ethanol. Multiple promotion leads to loss of overall activity although total oxygenates selectivity increases. Despite increased methanation as a result of the addition of CO2 to the feed, Rh-Mn-Li-Fe/TiO2 catalysts produced ethanol at a higher selectivity during the hydrogenation of a CO/CO2 mixture than for the hydrogenation of only CO, which was not observed on others. The Fe promoter is believed to increased reverse WGS reaction upon CO2 addition, resulting in increased CO and decreased hydrogen species on the surface, leading to higher CO insertion activity. The result is a higher increase in ethanol selectivity than in methanation activity, causing the EtOH/CH4 to increase. The selectivity to ethanol versus methane is limited in this work and even in literature. Although it might be anticipated that increasing H2/CO ratio during CO hydrogenation would favor methane, the kinetic studies in the literature, and our results reported here, show that the point selectivity for ethanol on Rh-Mn-Li/TiO2 actually increases with increasing H2/CO ratio on Rh-based catalysts. This may be attributed to the increased hydrogenation of the surface acetaldehyde intermediate to ethanol.