(643e) Gas–to–Liquid Conversion On Co/Al2O3 Catalyst – Pretreatment Effect Studies

Due to high activity of cobalt metal in syngas conversion to higher hydrocarbons, supported cobalt catalysts have been widely used for gas-to-liquid (GTL) conversion. Pretreatment conditions have significant effect on catalyst activity and product selectivity. In this study we investigated effect of cobalt phases, determined by X-ray diffraction (XRD), on catalytic performance of unpromoted cobalt on alumina catalyst (15 wt%Co/Al₂O₃) in a fixed bed reactor. Catalytic performance has been correlated with characterization results from XRD and temperature-programmed experiments coupled with mass spectroscopic measurements (TPH/TPO-MS).

 It has been known [1] that carbidization of Co⁰ fcc produce mainly Co₂C, which is inactive in FTS, but the latter can be hydrogenated and transformed to a very active Co⁰ hcp form [2,3]. The challenge is to achieve industrially relevant carbidization-reduction conditions that yield the highest amount of Co⁰ hcp without producing free carbon and/or Co₂C. In the present study we have used two variations of a two-step activation procedure (H₂ reduction, followed by CO carbidization and H₂ reduction) in addition to a standard (baseline) H₂reduction procedure (648 K, 1 bar for 12 h).

It was found that the baseline activation procedure results in the highest activity (59. 1% CO conversion) and the lowest methane selectivity (8.9%). The other two activation procedures yielded CO conversions of 48.4 – 54.8%, and methane selectivities of approximately 11%. H₂-CO-H₂ activations resulted in the formation of Co⁰ hcp, but their activity/selectivity was affected by the presence of surface carbide sites, Co_xC (x = 2, 3).

References

[1]        Hofer, L.J.E., Crystalline phases and their relation to Fischer-Tropsch synthesis, in Catalysis, P.H. Emmett, Editor. 1954, Reinhold Pub. Corp.: New York. p. 373-441.

[2].       Karaca, H., O.V. Safonova, S. Chambrey, P. Fongarland, P. Roussel, A. Griboval-Constant, M. Lacroix, and A.Y. Khodakov, Journal of Catalysis, 2011. 277(1): p. 14-26.

[3].       Ducreux, O., B. Rebours, J. Lynch, M. Roy-Auberger, and D. Bazin, Oil & Gas Science and Technology - Rev. IFP, 2009. 64(1): p. 49-62.