Fischer-Tropsch synthesis over cobalt supported catalysts
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The following catalysts have been studied by different characterization techniques and by kinetic experiments at FT conditions: 12 % Co/γ-Al2O3, 12 % Co - 0.5 % Re/γ-Al2O3, 12 % Co/SiO2, 12 % Co - 0.5 % Re/SiO2, 12 % Co/TiO2 and 12 % Co - 0.5 % Re/TiO2. Water is one of the products in the Fischer-Tropsch synthesis and will always be present in smaller or larger amounts during synthesis. The effect of water on the activity and selectivity of Co supported Fischer-Tropsch catalysts, using different supports, has been studied in a fixed bed reactor at 20 bar and 483 K (H2/CO = 2.1). The amount of water inside the reactor was varied by changing the conversion and adding different amounts of water vapour to the feed (pH2/pH2O = 0.38 ≈20 % and pH2/pH2O = 0.76 ≈30 %). As a consequence of the water addition, the partial pressures of CO and H2 were decreased. The effect of water on the activity depended on the type of support. For the alumina supported catalysts, a slight increase in the reaction rate was observed when the conversion was increased, and the rate of deactivation was enhanced. Adding ~20 % water resulted in a decrease in the reaction rate and more deactivation. Further water addition (~33 %) decreased the reaction rate even more and increased the rate of deactivation. The rate of deactivation was larger for the CoRe/Al2O3 catalyst than for the unpromoted Co/Al2O3 catalyst. Part of the deactivation was recovered after water removal. For the silica supported catalysts, increased reaction rate and increased rate of deactivation were observed as the conversion was increased. By ~20 % water addition, the reaction rate and rate of deactivation were further increased. Adding larger amounts of water resulted in a decrease in the reaction rate and an additional deactivation. A minor part of the deactivation was recovered upon water removal. For the titania supported catalysts, increased reaction rate was obtained both as the conversion was increased and when ~20 % water was added. When adding ~33 % of water, the reaction rate was decreased and a small deactivation was observed. Very little permanent deactivation was observed after water removal for the TiO2 supported catalysts. Common for all the catalysts was an increase in the C5+ selectivity and a decrease in the CH4 selectivity at increased conversion or by external water addition. Promoting with Re increased the reaction rate [gHC/gcat,h] and the C5+ selectivity for all catalysts. The effect of Re on the selectivities was less pronounced for the alumina than for the silica and titania supports. The C5+ selectivities were found to be dependent on the support, increasing in the order: Al2O3 < SiO2 < TiO2. The dependency of the C5+ selectivity on the support was connected to different pore sizes and/or particle sizes for the various supports. The catalysts were characterized by TPR, H2-chemisorption, O2-titration, N2- adsorption, XRD and STEM. The cobalt particle sizes increased with increasing average pore diameter of the supports, being smallest on alumina and largest on the titania support. On the large surface area supports, γ-Al2O3 and SiO2, Co3O4 was found as agglomerates (clusters) of smaller particles. Single Co3O4 particles, evenly distributed, were found on low surface TiO2 support. From element analysis in STEM, rhenium seemed to be in closer connection with cobalt on silica and titania, compared to on -alumina. The activity and selectivity results are discussed in terms of the findings from the characterization techniques. A microkinetic model for formation of C1 and C2 olefins and paraffins during methanation conditions has been constructed. By using the unity bond index-quadratic exponential potential (UBI-QEP) method for calculation of activation energies, the CO insertion mechanism was found to be energetically favoured compared to the carbide mechanism.