Surface Modified γ-Alumina Supports For Cobalt Fischer-Tropsch Catalysts
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Fischer-Tropsch synthesis over cobalt based catalysts is an important route for the conversion of syngas, derived from natural gas, to liquid fuels. Fischer-Tropsch performance of cobalt catalysts depends on a number of factors, for instance the type and structure of support have a significant effect on the particle size, dispersion, reducibility. These, in turn, affect the activity of the cobalt supported catalysts. Silica supports surface modified with different silanes, have been studied for cobalt Fischer-Tropsch synthesis. Modification of gamma-alumina with silanes to yield hydrophobic surfaces has been carried out. However, use of silane modified gamma-alumina as a support for cobalt Fischer-Tropsch synthesis has not been studied until now. Herein, a systematic study was performed to investigate the effect of silane modification on activity and selectivity of gamm-alumina supported cobalt Fischer-Tropsch catalysts. Six silane modified catalysts were prepared by organic phase silylation with various silanes differing with respect to number and type of active and organic groups. Catalysts with 20 wt % Co promoted with 0.5 wt % Re were prepared by one step aqueous incipient wetness impregnation. Catalysts were characterized by H2 chemisorption, N2 adsorption, temperature programmed reduction (TPR), thermal gravimetric analysis (TGA), Fourier transform infra-red spectroscopy (FT-IR) and X-ray diffraction (XRD). Activity and selectivity of catalysts was determined in a fixed bed reactor at 20 bar, 210 oC and H2/CO = 2.1. %. Effective surface organic modification of support was indicated by water absorption and FT-IR. Most promising results in terms of Fischer-Tropsch activity and selectivity, were displayed by methoxytrimethylsilane (TMMeS) modified catalyst. H2 chemisorption, XRD and TPR revealed enhanced dispersion, similar cobalt crystallite size and reducibility as reference catalyst. TG suggested partial decomposition below calcination temperature. Site-time yield (STY) increased to 0.071 s-1 and C5+ selectivity slightly decreased to 85.15 % in comparison with 0.064 s-1 and 85.69 % of reference catalyst respectively. Chlorotrimethylsilane, dichlorodimethylsilane and chlorodimethyloctylsilane modified catalysts had lower cobalt crystallite sizes than the reference catalyst. A positive correlation was observed between crystallite sizes and amount of silane present. Despite of smaller cobalt crystallite size, modified catalysts have similar reducibility attributed to lower cobalt support interaction as a result of surface modification. Dispersion measured from hydrogen chemisorption was apparently lower than actual. Smaller cobalt crystallite and lower dispersion were ascribed to presence of chlorine and it needs further investigation. Relatively small changes were observed in C5+ selectivity but STY decreased significantly. Catalyst modified with methoxydimethyoctylsilane exhibited similar cobalt crystallite size and observed a similar increase in STY to 0.071 s-1 as TMMeS modified catalyst but C5+ selectivity decreased to 83.04 % in comparison with reference catalyst. Modification with trichloroctylsilane had the most severe impact with STY decreasing to 0.02 s-1. The results indicate a positive effect of methoxy-silanes modification on the activity of catalysts, ascribed to hydrophobicity induced by surface modification. Effect of chlorosilane on dispersion, cobalt crystallite size and activity of catalysts need further investigation.