Effect of Mg, La and Ca promoters on the structure and catalytic behavior of iron-based catalysts in Fischer-Tropsch synthesis

The effects of Mg, La and Ca promoters on the structure, surface area, reduction, carburization and catalytic behavior of precipitated Fe/Cu/SiO2 Fischer-Tropsch synthesis (FTS) catalysts were investigated. The catalysts are characterized using X-ray diffraction (XRD), H-2 and CO temperature-programmed reduction (TPR) techniques. The catalysts were assessed in terms of their FTS activity and product selectivity using two superimposed Anderson-Schulz-Flory (ASF) models. The performance of the catalysts was tested in a fixed-bed reactor at 563 K, 1.7 MPa and feed (syngas with H-2/CO molar ratio of 1) flow rates of 4.9 and 13.3 nl h(-1) g(Fe)(-1). It was found that Mg, La and Ca promoters have negligible effects on the textural properties of the catalyst. Promoter addition enhanced the surface basicity of the catalyst in the order Ca > Mg > La. Addition of the promoters with the atomic ratio of X/Fe = 0.02 (X = Ca, Mg and La) improved the reduction and carburization during the reduction and FTS reaction in the order of Ca > Mg > La. The promoters suppressed the rate of hydrogenation reaction in the order of Ca > Mg > La. The results also indicate that Mg, La and Ca promoters can improve the CO conversion and WGS reaction, suppress the formation of methane, enhance the selectivities to olefin and higher molecular weight products, enhance the break in ASF distributions in the order of Ca > Mg > La due to increasing the catalyst surface basicity and dropping the reducibility. (C) 2008 Elsevier B.V. All rights reserved.