Effect of time on stream and temperature on upgraded products from Fischer–Tropsch synthesis when zeolite is added to iron-based activated-carbon-supported catalyst

• Adding ZSM-5 to a Fe-based FT catalyst alters stability and effect of temperature.
• FT and WGS rates are unaffected by ZSM-5, and pass through maxima at 300 °C.
• Individual products show temperature effects based on competing activation energies.
• ZSM-5 and FT function losses are caused by coke deposited and decrease in active iron.

In a continuation of previously reported work, the effect of temperature on products obtained when zeolite ZSM-5 is added to a supported iron-based Fischer–Tropsch (FT) catalyst was studied. Further, the stability of the combination was investigated for up to 240 h time on stream at the optimum temperature. From 280 °C to 320 °C, amounts of small-chain hydrocarbons (C1–5) increase with increase in temperature because of higher FT rates. Liquid hydrocarbon production rates increase at 300 °C due to an increase in the FT rate. A further increase in the temperature causes these rates to drop, to give small-chain hydrocarbon (C1–4) species. Aromatic yields are also found to reach a maximum at 300 °C in the separate bed. Alcohol formation for the FT catalyst increases with increase in temperatures. The lowest temperature used, 280 °C, is sufficient for the ZSM-5 to dehydrate all the alcohols formed by the FT catalyst to hydrocarbons. Further, the FT catalyst and ZSM-5 were tested for 240 h time on stream (TOS) at 300 °C. The catalyst shows FT stability up to 120 h and a slow decline in activity up to 240 h. A progressive loss of the ZSM-5 activity occurs with TOS. Deposition of coke and/or coke precursors on ZSM-5, confirmed by energy-dispersive X-ray spectroscopy, decreases the aromatic compound yield, and increases olefin selectivity with time on stream due to reduction in oligomerization reactions.

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