Fischer-Tropsch synthesis of hydrocarbons over mesoporous Co/SBA-15 catalysts:: the influence of metal loading, cobalt precursor, and promoters

The influence of cobalt loading (10-40 wt% Co), cobalt precursor, and promoters (Re, Mn) on the physico-chemical and catalytic properties of mesoporous Co/SBA-15 catalysts for the Fischer-Tropsch Synthesis (FTS) reaction (T = 493 K, P = 20 bar, H-2/CO = 2) has been investigated. Catalysts were characterized by N-2 and Ar adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). For Co/SBA-15 catalysts prepared from Co(II) nitrate the dispersion decreased and the extent of cobalt reduction increased with cobalt loading. A maximum CO conversion was found for the sample with ca. 30 wt% Co loading, though the intrinsic activity of Co remained constant in the range of Co loading studied. More methane and less C5+ n-paraffins were produced over the less reducible 10 wt% Co loading sample. The addition of ca. 1 wt% Re enhanced the reducibility of cobalt oxides and increased the catalyst activity, though the intrinsic activity of cobalt sites was not altered. Rhenium also favored the formation of long-chain n-paraffins (C10+) while decreasing methane selectivity. Promotion of cobalt with ca. 2 wt% Mn significantly improved cobalt dispersion but decreased its reducibility, producing catalysts that were less active than the unpromoted one. At similar cobalt loading (ca. 20 wt%), a much better dispersion and a stronger cobalt-support interaction leading to the formation of low-reducible cobalt silicates was observed for oxidized samples prepared from acetate and acetylacetonate precursors as compared to that derived from cobalt(II) nitrate, as evidenced by TEM, XPS, and TPR. The former catalysts were characterized by a low FTS activity and a product distribution shifted toward the formation of lighter products. Finally, at comparable Co loading Co/SBA-15 catalysts (nitrate precursor) were about 1.5 times more active per weight of total Co than a Co/SiO2 sample, with only minor differences in product selectivity. (C) 2003 Elsevier Inc. All rights reserved.