Crucial factors for catalyst aggregation and deactivation on Co/Al2O3 in a slurry-phase Fischer-Tropsch synthesis

The different behaviors for catalyst deactivation by forming aggregated catalyst lumps were investigated on the phosphorous-modified Co/P-Al2O3 and the unmodified Co/Al2O3 Fischer-Tropsch synthesis (FTS) catalysts in a slurry-phase continuous stirred tank reactor. The lower catalytic activity at steady-state with a fast deactivation rate was observed on the phosphorous-unmodified Co/Al2O3 catalyst due to the facile formation of aggregated catalyst lump with a high hydrophilic property of support and a significant hydrocarbon deposition on catalyst surface. The amount of adsorbed water and the deposited hydrocarbons are strongly related with aggregated catalyst lump formation. The adsorbed water could possibly transform the local gamma-Al2O3 surface to pseudo-boehmite (Bronsted acidic Al2OH sites) material which is characterized as a low attrition resistance and accelerates the formation of fine powders during FTS reaction. The formed fine powder during FTS reaction could be easily aggregated and it is resulted in catalyst deactivation due to the difficult intraparticular diffusion of reactants. The possible mechanisms of phase transformation of gamma-Al2O3 to pseudo-boehmite and formation of aggregated catalyst lump were suggested by measuring the deposited hydrocarbons and the concentration of adsorbed water on catalyst surface with the help of the characterizations such as X-ray photoelectron spectroscopy, temperature-programmed surface reaction, diffuse reflectance infrared Fourier transform and water-sorption method. (C) 2011 Elsevier B.V. All rights reserved.