Fischer-Tropsch Synthesis: Influence of CO Conversion on Selectivities, H2/CO Usage Ratios, and Catalyst Stability for a Ru Promoted Co/Al2O3 Catalyst Using a Slurry Phase Reactor

The effect of CO conversion on hydrocarbon selectivities (i.e., CH4, C5+, olefin and paraffin), H-2/CO usage ratios, CO2 selectivity, and catalyst stability over a wide range of CO conversion (12-94%) on 0.27% Ru-25% Co/Al2O3 catalyst was studied under the conditions of 220 degrees C, 1.5 MPa, H-2/CO feed ratio of 2.1 and gas space velocities of 0.3-15 NL/g-cat/h in a 1-L continuously stirred tank reactor (CSTR). Catalyst samples were withdrawn from the CSTR at different CO conversion levels, and Co phases (Co, CoO) in the slurry samples were characterized by XANES, and in the case of the fresh catalysts, EXAFS as well. Ru was responsible for increasing the extent of Co reduction, thus boosting the active site density. At 1%Ru loading, EXAFS indicates that coordination of Ru at the atomic level was virtually solely with Co. It was found that the selectivities to CH4, C5+, and CO2 on the Co catalyst are functions of CO conversion. At high CO conversions, i.e. above 80%, CH4 selectivity experienced a change in the trend, and began to increase, and CO2 selectivity experienced a rapid increase. H-2/CO usage ratio and olefin content were found to decrease with increasing CO conversion in the range of 12-94%. The observed results are consistent with water reoxidation of Co during FTS at high conversion. XANES spectroscopy of used catalyst samples displayed spectra consistent with the presence of more CoO at higher CO conversion levels.