Highly Active and Stable Silica-Doped, Alumina-Supported Fe Fischer-Tropsch Catalyst

Typical commercial iron Fischer-Tropsch (FT) catalysts are unsupported and consist of precipitated iron promoted with copper, potassium, and SiO₂. However, most precipitated iron catalysts are structurally too weak to be used in slurry-bubble column reactors (SBCR), the most thermally efficient and economical reactors for FTS [1]. The use of a carrier or support has the potential to greatly increase the strength and longevity of these catalysts; however, previous research on silica- and alumina-supported Fe has not been promising as it has shown activity and productivity values ½ and ¼, respectively, of those for unsupported iron [2].

            To overcome the low activity and attrition resistant problems, in this study we explored using 4 different alumina supports, including an original silica-doped g-alumina (AlSi). We found that AlSi after thermal treatment at 1100°C still maintained excellent pore properties (200 m²/g, 1.06 cm³/g, 24 nm) which allowed higher iron loadings (40%) without pore blocking or iron aggregation.  The 4 catalysts were prepared by co-impregnation with non-aqueous solution, were tested in a fixed bed reactor, and were compared to unsupported Fe catalysts from the literature. A St. Gobain alumina-based catalyst and the AlSi-based catalyst showed the best activity.   Both were nearly three-fold more active than the silica-supported catalyst reported by Bukur [2], and 6-fold more active than his alumina-supported catalyst. More importantly, the activity of our catalysts at 260°C was nearly 20% higher than Bukur's most active unsupported catalyst.  In addition the AlSi based catalyst was tested for 700 h and showed excellent stability. In fact its activity after 700 h is about 15% higher than its initial activity.

      The results of the present work indicate that the preparation protocol used here produces supported iron catalysts of high extent of reduction, high dispersion, very high FTS activity, moderate hydrocarbon selectivity, and excellent stability.  In addition it was shown that our high pore volume SiO₂ doped alumina (AlSi) support served as a superior support for supported Fe FT catalysts.

References:

1. Bukur, D.B. and X. Lang, Highly Active and Stable Iron Fischer-Tropsch Catalyst for Synthesis          Gas Conversion to Liquid Fuels. Ind. Eng. Chem. Res., 1999. 38(9): p. 3270–3275.

2. Bukur, D.B. and C. Sivaraj, Supported iron catalysts for slurry phase Fischer–Tropsch synthesis.       Applied Catalysis A: General, 2002. 231(1-2): p. 201-214.