Investigations into enhanced wax production with combustion synthesized Fischer–Tropsch catalysts

• Higher FT activity of Co/SDA (silica-doped-alumina) catalysts over Co/Al2O3 catalysts.
• Low metal-support interaction & high dispersion for combustion synthesis (CS) catalysts.
• 75–80% increase in C5+ yield with Co/SDA-CS catalysts compared to Co/SDA-IWI.
• Wax formation for Al2O3 catalysts and C10–C24 hydrocarbon for SDA supported catalysts.
• Higher acidic sites on SDA support hinder wax formation as favoured on Alumina support.

Combustion synthesized (CS) cobalt catalysts deposited over two supports, alumina and silica doped alumina (SDA), were characterized and tested for its Fischer–Tropsch (FT) activity. The properties of CS catalysts were compared to catalysts synthesized by conventional impregnation method (IWI). The CS catalysts resulted in 40–70% increase in the yield of C6+ hydrocarbons compared to IWI catalysts. The FT activity for CS catalysts showed formation of long chain hydrocarbon waxes (C24+) compared to the formation of middle distillates (C10–C20) for IWI synthesized catalysts, indicating higher hydrocarbon chain growth probability for CS catalysts. This is ascribed to the smaller crystallite sizes, increased degree of cobalt reduction and consequentially, a higher number of active metal sites exposed over the catalyst surface. Additionally, 12–13% increase in the overall C6+ hydrocarbon yield is realized for SDA-CS catalysts, compared to Al2O3-CS catalysts. The improved performance of CS-SDA catalysts is attributed to 48% increase in cobalt dispersion compared to Al2O3 supported CS catalysts, which is again caused by the decrease in the cobalt-support interaction for SDA supports. The metal support interactions were analyzed using XPS and H2 TPR–TPD experiments. Combustion method produced catalysts with smaller crystallite size (17–18 nm), higher degree of reduction (∼∼92%) and higher metal dispersion (16.1%) compared to the IWI method. Despite its enhanced properties, the CS catalysts require prominently higher reduction temperatures (∼∼1100–1200 K). The hydrocarbon product analysis for Al2O3 supported catalyst showed higher paraffin wax concentrations compared to SDA supported catalysts, due to the lower surface basicity of Al2O3. This work reveals the impact of the CS catalysts and the nature of support on FT activity and hydrocarbon product spectrum.

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