Impact of cobalt-based catalyst characteristics on the performance of conventional gas-phase and supercritical-phase Fischer-Tropsch synthesis

This study covers the performance of three cobalt-based catalytic systems (high surface area 15% CO/SiO(2), low surface area 15% CO/SiO(2), and 15% Co/Al(2)O(3)) of different characteristics in Fischer-Tropsch synthesis (FTS) in both conventional gas-phase and supercritical hexane (SCH) phase. The reaction was conducted in a high-pressure FTS reactor setup at reaction temperature of 240 degrees C, syngas (H(2)/CO ratio of 2) flow rate of 50 sccm/g(cat), and total pressures from 20 to 65 bar. The surface characteristics of these catalysts were measured by N(2) physisorption using a TriStar 3000 gas adsorption analyzer. Room temperature X-ray diffraction (XRD), temperature and magnetic field (H) variation of the magnetization (M), and low-temperature (5 K) electron magnetic resonance were used for determining the electronic states (Co(0), CoO, Co(3)O(4), CO(2+)) of cobalt for the calcined, reduced (before the reaction), and used samples (after the reaction). Correlations of the catalyst activity and selectivity with the catalysts surface characteristics reveal that pore radius of the catalyst has an influence on both syngas and CO conversions in gas-phase FTS. However, no such correlation was observed in the case of SCH-FTS, indicating an alleviation of that mass transfer limitations typically controlled by interparticle characteristics of the catalyst. This is attributed to the higher solubility of heavy products in the SCH medium that inhibits the condensation of those products inside the catalyst pores and enhances their in situ extraction. The XRD and magnetic characterizations of the used catalysts reveal that in situ reducibility of the Co(3)O(4) to hcp-Co(0) or fcc-Co(0) is taking place during the FTS reaction. However, minimal in situ reduction was observed in the case of gas-phase FTS, whereby significant changes in the reduced cobalt electronic state and support (alumina, and silica) phase were detected under SCH-FTS conditions. As a result, both the activity and selectivity of the cobalt catalyst was found to be very stable and recoverable during SCH-FTS for relatively long time-on-stream (15 days). (c) 2005 Elsevier B.V. All rights reserved.