Fischer‐tropsch synthesis in near‐critical n‐hexane: Pressure‐tuning effects

Abstract

AbstractFor Fe‐catalyzed FischerTropsch (FT) synthesis with near‐critical n‐hexane (Pc, = 29.7 bar; Tc, = 233.7°C) as the reaction medium, isothermal pressure tuning from 1.2–2.4 Pc, (for n‐hexane) at the reaction temperature (240°C) significantly changes syngas conversion and product selectivity. For fixed feed rates of syngas (H2/C0 = 0.5; 50 std. cm3/g catalyst) and n‐hexane (1 mL/min), syngas conversion attains a steady state at all pressures, increasing roughly threefold in this pressure range. Effective rate constants, estimated assuming a first‐order dependence of syngas conversion on hydrogen, reveal that the catalyst effectiveness increases with pressure implying the alleviation of pore‐diffusion limitations. Pore accessibilities increase at higher pressures because the extraction of heavier hydrocarbons from the catalyst pores is enhanced by the liquid‐like densities, yet better‐than‐liquid transport properties, of n‐hexane. This explanation is consistent with the single α ( =0.78) Anderson‐Schulz‐Flory product distribution, the constant chain termination probability, and the higher primary product (1 ‐olefin) selectivities ( ∼ 80%) observed at the higher pressures. Our results indicate that the pressure tunability of the density and transport properties of near‐critical reaction media offers a powerful tool to optimize catalyst activity and product selectivity during FT reactions on supported catalysts.