ABSTRACT
The effect of CO conversion, resulting either from deactivation or from adjusting contact time, on selectivities (CH4, C5+, CO2, paraffin and olefin content, and 1-olefin and 2-olefin in the C2-C4 and C5-C10 ranges) of Ru-promoted and unpromoted 25%Co/Al2O3 catalysts was studied using a 1 L continuously stirred tank reactor (CSTR) at typical Fischer-Tropsch synthesis (FTS) conditions. The freshly activated and used unpromoted 25%Co/Al2O3 catalysts were characterized by BET, hydrogen chemisorption with O2 titration, extended x-ray absorption fine structure/ x-ray absorption near-edge structure (EXAFS/XANES), and scanning transmission electron microscope-electron energy loss spectroscopy (STEM-EELS) in order to elucidate the possible deactivation mechanisms of the catalyst during kinetic testing over a long period of time. Decreasing CO conversion in the range 66%–10%, regardless of whether it was caused by deactivation or by changing contact time at steady-state conditions, resulted in marked increases in CH4 selectivity and 1-olefin content and in decreases in C5+ selectivity and 2-olefin contents. However, the deactivation of the cobalt catalyst after 320 h (i.e., XCO from 40% to 20%) did not result in substantial changes in the content of total olefins, total paraffins, and CO2 selectivity; in contrast, decreasing CO conversion without deactivation by changing the contact time for a Ru-promoted 25%Co/Al2O3 catalyst led to pronounced decreases in total paraffin contents and CO2 selectivity and in a marked increase in total olefin content. Different mechanisms for selectivity trends obtained during deactivation or by changing space velocity were proposed. The data verified enhanced secondary reactions (i.e., hydrogenation, methanation reaction, and water-gas shift reaction) on cobalt oxide species that were derived from cobalt metals with deactivation, and the phase changes were primarily responsible for the selectivity changes; in contrast, the intrinsic relationship between the selectivity and CO conversion on cobalt catalyst was ascribed primarily to contact time and variation of partial pressures of reactants. Results of STEM-EELS, XANES, and EXAFS of the freshly reduced 25%Co/Al2O3 catalyst consistently showed that a significant fraction of cobalt metal was oxidized to cobalt oxide species such as CoO and CoAl2O4 during kinetic testing over a long period of time, which was likely a major deactivation path of the cobalt catalyst. Meanwhile, the STEM images revealed that small amounts of amorphous carbon are also present on the cobalt surface; however, this is not thought to be a dominant deactivation mechanism in the current context. Therefore, it was not likely responsible for the observed changes in hydrocarbon selectivities for the cobalt catalyst studied.
