Fischer-Tropsch synthesis: Effect of solvent on the H2–D2 isotopic exchange rate over an activated nickel catalyst

• Ni catalysts during FTS yields lighter product range shifted towards methanation.
• Low activity of Ni catalysts in FTS due to the formation of volatile carbonyls.
• Lower exchange rates were observed under conditions of pore filling with solvent.
• The exchange rate decreases with increasing molecular weight of solvent.
• Higher molecular weight solvents inhibit adsorption and restrict diffusion rates.

The effect of solvent on the hydrogen-deuterium isotopic homomolecular exchange rate over a 26.4% Ni/Al2O3 catalyst was investigated using a plug flow reactor at room temperature (23 °C) and at atmospheric pressure. In this study, two different molecular weight solvents were tested: n  -pentane (MW¯(average molecular weight) = 72), and n  -hexadecane (MW¯ = 226). At dry (without addition of solvent) and n-pentane solvent addition conditions, similar amounts of H2 and D2 are consumed and the concentration of the HD produced is two times the consumption of H2 or D2. Thus, these conditions exhibited around 100% exchange with the formation of H2:HD:D2 having a 1:2:1 ratio. With n-hexadecane solvent, the initial exchange rate was approximately 45%, but the exchange rate decreased with time and reached a steady state rate after 60 min on-stream. The lower exchange rate of the n-hexadecane solvent might be due to the limited mobility of reactant molecules in the liquid-filled pores of the catalyst. The exchange rate of the catalyst is decreased with increasing molecular weight and density of the solvent. An isotopic partitioning preference was not observed for the nickel catalyst under various co-fed solvents, and similar findings were observed in our previous work (Catalysis Letters 143 (2013) 1368–1373).

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