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Silica-Supported Cobalt Catalysts for Fischer-Tropsch Synthesis: Effect of Support Surface Area, Calcination Temperature and Preparation Medium

Alaa Kababji, Babu Joseph, and John Wolan. Chemical Engineering, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620

Supported cobalt catalysts are preferred for FTS when high per pass conversion, low water–gas shift activity, long life-times, and paraffinic nature of the resulting synthetic crude are needed. This work is focused on the Co/support interaction and its influence on the activity and performance of the catalyst. The effects of support surface area, drying and calcination temperatures as well as the Co precursor/silica mixing medium (water or ethanol) are explored.

Silica-supported Co catalysts were prepared by the incipient wetness impregnation. Co loading was maintained at 20wt%. Samples were reduced at high temperatures, using H2 at high linear velocities. Reactions were carried out in a fixed-bed catalytic reactor operating at 8 bars, 240°C with a H2/CO ratio of 2:1. FTIR equipped with a fixed 1.2m path length gas-cell and an online GC were used to analyze the products. CO conversion, CO2, CH4 and +C5 selectivities were calculated. XPS was used to determine Co phases and oxidation state. XRD was used to determine the presence of mixed Co oxides. These oxides are considered desirable and easily reduced to active metallic Co particles [1]. ATR-FTIR was used to investigate the presence of Co mixed-oxide phases as well as the formation of amorphous high surface area cobalt silicate. Cobalt silicate is regarded as an undesired phase since it's irreducible at FTS reaction temperatures [2]. Cobalt silicate forms from the interaction of CoO phase with silica support in the presence of water [1]. BET was used to study the loss of surface area as a result of catalyst sintering or Co particle agglomeration, high temperature calcinations as well as the filling of the support pores with Co particles, which is countered by the formation of high surface area amorphous Co silicate. In some of the samples prepared, ethanol was used instead of water as a medium during preparation to study its effect on the formation of Co silicate [3].

Our studies confirm the formation of cobalt silicate and its detrimental effect on FTS conversion and yield.

1)I. Puskas, T.H. Fleisch, P.R. Full, J.A. Kaduk, C.L. Marshall and B.L. Meyers, Applied Catalysis A: General, 311, 146–154 (2006).

2)H. Ming and G. Baker, Applied Catalysis A: General, 123, 23-36 (1995).

3)Y. Zhang, Y. Liu, G Yang, S. Sun and N. Tsubaki, Applied Catalysis A: General, 321, 79–85 (2007).