Alkalis in iron-based Fischer-Tropsch synthesis catalysts: distribution, migration and promotion

BACKGROUND: Alkali metals are considered important chemical promoters used in Fischer-Tropsch synthesis (FTS) iron-based catalysts. The purpose of this study is to distinguish the relative effects of alkali metals on iron based FTS catalysts. RESULTS: It is found that Li and Na promoters can easily diffuse into/out the bulk of catalysts while K, Rb and Cs mostly concentrate on the catalyst surface. Alkalis also inhibited H-2 adsorption, improved CO adsorption and dissociation, and suppressed the hydrogenation of carbon species on the surfaces of iron or iron carbides. Li and Na decreased the FTS activity while K, Rb and Cs largely increased the FTS activity of iron catalysts. The selectivities to methane and alkane were decreased while those to olefin and heavier hydrocarbons were increased by alkalis. Among these alkalis, potassium showed an optimal promotional effect on FTS performances. CONCLUSION: The distributions of alkalis in catalysts were quite different owing to their atomic radii. The majority of Li and Na promoters diffused into the bulk of catalysts while K, Rb and Cs mostly concentrated on the catalyst surface. The selectivity of methane exhibits a parabola-like trend with increasing atomic number of alkalis and reaches a minimum at KFeSi catalyst. (C) 2016 Society of Chemical Industry