CATALYSIS OF ETHANOL SYNTHESIS FROM SYNGAS

Catalytic synthesis of ethanol and other higher alcohols from CO hydrogenation has been a subject of significant research since the 1980s. The focus of this research is to establish a better fundamental insight into heterogeneous catalysis for CO hydrogenation reactions, in an attempt to design the best catalysts for ethanol synthesis. It has been reported widely that promoted Rh-based catalysts can exhibit high selectivity to C2+ oxygenates during CO hydrogenation. The doubly promoted Rh-La-V/SiO2 catalysts exhibited higher activity and selectivity for ethanol and other C2+ oxygenates than singly promoted catalysts. The better performance appears to be due to a synergistic promoting effect of the combined La and V additions through intimate contact with Rh. The kinetic study carried out in this study shows that, in general, increasing H2 pressure resulted in increased activities while increasing CO partial pressure had an opposite effect. Langmuir-Hinshelwood rate expressions for the formation of methane and of ethanol were derived and compared to the experimentally derived power law parameters. It was found that the addition of different promoters appeared to result in different rate limiting steps. Strong metal-oxide interactions (SMOI) of Rh and vanadium oxide (as a promoter) supported on SiO2 was studied. It was found by SSITKA (steady-state isotopic transient kinetic analysis) that the concentration of surface reaction intermediates decreased on Rh/V/SiO2 as the reduction temperature increased, but the activities of the reaction sites increased. The results suggest that Rh being covered by VOx species is probably the main reason for the decreased overall activity induced by high reduction temperature, but more active sites appear to be formed probably at the Rh-VOx interface. The mechanism of C1 and C2 hydrocarbon and oxygenate formation during CO hydrogenation on Rh/SiO2 was for the first time investigated in detail using multiproduct SSITKA. Based on SSITKA results, methanol and CH4 appeared to be produced on different active sites. It is possible that C2 products share at least one intermediate with CH4, but not with methanol. Moreover, C2 hydrocarbons are not likely to be formed from adsorbed acetaldehyde