Fe-promotion of supported Rh catalysts for direct conversion of syngas to ethanol

The influences of support (silica or titania) and loading of Fe promoter on the activity and selectivity of Rh-based catalysts for the direct synthesis of ethanol from syngas were explored. The reaction was performed in a fixed-bed reactor system typically operating at 543 K, 20 atm, WHSV of 8000 cm3 g−1cat h−1 and H2:CO ratio of 1:1. Characterization by H2 chemisorption and electron microscopy indicated that rhodium was very highly dispersed on the supports and was in direct contact with the Fe promoter. Although little ethanol was produced over 2 wt% Rh on silica, a similar loading of Rh on titania was active for this reaction. Promotion of 2 wt% Rh/SiO2 by 1 wt% Fe produced a catalyst that exhibited a 22% selectivity to ethanol, with methane being the primary side-product. Addition of Fe to 2 wt% Rh/titania also improved the selectivity to ethanol with the highest selectivity being 37% for a sample with 5 wt% Fe. The effects of temperature, pressure and H2:CO ratio on the performance of 2 wt% Rh/TiO2 and 2 wt% Rh–2.5 wt% Fe/TiO2 were also studied. Although the influence of pressure and H2:CO ratio was moderate, higher temperatures clearly increased methane production at the expense of ethanol and methanol. Adsorption and thermal desorption of CO in Ar or H2 were also studied by DRIFTS spectroscopy on 2 wt% Rh/TiO2 and 2 wt% Rh–2.5 wt% Fe/TiO2. The gem-dicarbonyl species that was the primary species on these catalysts at room temperature after exposure to CO was more thermally stable on the Fe-promoted catalyst.