(758c) Influence of Support and Promoter on the Activity and Selectivity of Rh Catalysts for the Direct Conversion of Syngas to Ethanol
Increased use of coal, natural gas, and biomass resources to make syngas for the production of liquid fuels and chemicals is in the national interest. The conversion of syngas directly to ethanol and other higher alcohols is therefore an important reaction. In this work, the influence of support and promoter on the activity and selectivity of Rh-based catalysts for the direct synthesis of ethanol from syngas was explored. The reaction was performed in a fixed-bed reactor system typically operating at 543 K, 20 atm, and a WHSV of 133 mL (STP) syngas g cat-1 min-1 with a H2/CO ratio of 1:1. The catalysts were prepared either by incipient wetness or deposition-precipitation methods using rhodium nitrate as the precursor for rhodium. Catalyst characterization by H2-chemisorption and HRTEM suggests that rhodium is very highly dispersed on supports. Although little ethanol was produced over Rh particles (2%) on silica or ceria, a similar loading on Rh on titania was active for this reaction. Promotion of 2%Rh/SiO2 by 1%Fe produced a catalyst with substantial selectivity to ethanol with methane being the primary side-product. Addition of Fe to 2%Rh/titania catalyst also improved the production of ethanol and a 2%Rh-2.5%Fe/TiO2 was found to be optimal in terms of activity and selectivity. The influence of temperature, pressure, and H2/CO ratio on the performance of a 2% Rh/TiO2 and 2%Rh-2.5%Fe/TiO2 catalyst was also studied. Higher temperatures and higher H2/CO ratios increased methane production at the expense of ethanol. CO adsorption and thermal desorption in Ar and dihydrogen was also studied by DRIFTS spectroscopy for a 2% Rh/TiO2 and a 2%Rh-2.5%Fe/TiO2 catalyst. The results suggest that gem-dicarbonyl is the primary species on these catalysts at room temperature after exposure to CO, and is more stable on the Fe-promoted catalyst.