(722c) Fischer-Tropsch Synthesis On Co/Al2O3 Catalyst – Effect of Gas Space Velocity

Due to high activity of cobalt metal in syngas conversion to higher hydrocarbons, supported cobalt catalysts have been widely used for Fischer-Tropsch synthesis (FTS) reaction. Cobalt catalyst is very active to produce long chain normal paraffin and also it shows high resistance to deactivation. [1, 2]. In this study we investigated effect of gas space velocity on catalytic performance of Co/Al₂O₃ catalyst. Catalyst containing 15wt% Co was prepared by incipient wetness impregnation of g-alumina support with aqueous solution of Co(NO₃)₂.6H₂O and calcined in air at 300 °C for 10 h. Catalyst was characterized by different methods such as BET, XRD, TPR, TGA and H₂-TPD. Catalytic activity tests were performed in a fixed-bed reactor at the following conditions: 220 °C, 300 psig, H₂/CO =2 and gas space velocities of 2.8, 3.0, 3.75 and 6.0 Nl/g-cat/h. The catalyst (60-100 mesh) was activated in situ with hydrogen at 375 °C for 12 h. BET surface areas of the support and catalyst were 180 m²/g and 154 m²/g, respectively. Decrease of pore volume from 0.6 cm³/g to 0.42 cm³/g and average pore radius from 4.6 nm to 4.4 nm indicates that there was no significant pore blocking during impregnation and calcination steps. Also results of pore size distribution (PSD) measurements show that there are no significant differences in the PSD between the support and the catalyst. XRD pattern of calcined catalyst showed existence of cubic Co₃O₄ in addition to g-Al₂O₃ phase. XRD pattern of reduced catalyst shows the presence of metallic cobalt (Co°). Also it can be concluded that metallic cobalt species is active during reaction. XRD pattern of catalyst after reactivity test shows existence of metallic cobalt, Co₃O₄, g-Al₂O₃ and wax. TPR and TGA results show existence of two step reduction of Co₃O₄. Degree of reduction was about 93% (H₂-TPD followed by pulse re-oxidation) and dispersion 8.6%, both of which contributed to high activity of the catalyst during FTS. During reactivity test, CO conversion varied from 92% at the lowest space velocity (2.8 Nl/g-cat/h) to about 35% at the highest space velocity (6 Nl/g-cat/h). Selectivities of light hydrocarbon products (C₁ to C₄) increased with increase in space velocity, whereas selectivity of liquid and wax (C₅₊ hydrocarbons) decreased. Olefin content of gaseous hydrocarbons (C₂ to C₅) increased slightly with decrease in conversion. This was even more pronounced for 1-olefins, which are primary products of FTS. Total olefin and 1-olefin selectivities decreased with increase in carbon number (C₃-C₅ hydrocarbons), whereas ethylene selectivity was lower than that of propene.

Acknowledgement: This research was supported in part by a grant from the Qatar National Research Fund.

References: [1] W. Chu, P.A. Chernavskii, L. Gengembre, G.A. Pankina, P. Fongarland and A.Y. Khodakov, J. Catal. 252 (2007) 215-230. [2] E. Iglesia, Appl. Catal. A: General 161 (1997) 59-78.