Fischer–Tropsch synthesis over alumina-supported molybdenum carbide catalyst

Alumina-supported molybdenum carbide catalyst prepared by temperature-programmed carburization has been evaluated for Fischer–Tropsch synthesis between 473 K and 515 K with feed gas containing H2/CO = 1:5–5:1. Optimum CO consumption rate was found at H2 mole fraction of 0.67 for all temperatures suggesting that similar FT mechanism was operative over the temperature range studied. Associated Arrhenius parameters revealed that activation energy values varied between 51 kJ mol−1 and 158 kJ mol−1. Olefin-to-paraffin ratio (ROP) initially rose with carbon number, n, to a maximum value followed by a tailing drop. A mechanism involving molecular CO chemisorption with gas phase H2 attack to yield enolic species was used to develop a kinetic model that adequately captured individual hydrocarbon species as well as the behaviour of the ROP with composition. Combination of the kinetic model with the nonlinear ROP model permitted, for the first time, estimation of the separate intrinsic activation energy values for termination to olefin and paraffin on the catalyst surface.

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► Alumina-supported Mo carbide has decent FT activity under usual conditions.
► H2 gas phase attack of molecularly chemisorbed CO gave a fitting kinetic model.
► Active site appeared to be an oxycarbide centre.
► Kinetic model and an ROP model permitted general hydrocarbon rate prediction.
► Ea for termination to olefin and paraffin are 71 and 104 kJ mol−1 respectively.