Consorted Vinylene Mechanism for Cobalt Fischer–Tropsch Synthesis Encompassing Water or Hydroxyl Assisted CO-Activation
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A mechanistic model for Fischer–Tropsch synthesis is proposed. The model contains four key steps. The first step is hydrogen assisted CO activation to formyl. This is followed by hydrogen transfer from either water or hydroxyl to a hydroxycarbene intermediate to generate CH* surface species on cobalt as chain building blocks. Further, chain propagation is by adding methylidyne (CH*) to an alkylidene (CHCH2R) chain to create a vinylene (CHCHR) chain end unit. Last, hydrogenation finalizes the chain-building step or terminates the chain depending on whether hydrogen is added to the β- or α-carbon atom, respectively. The result is that the main product is α-olefins. Chain growth probability is independent of hydrogen partial pressure and depends solely on the surface coverage of CH* monomers. Short-chain (Me) branching and variation in chain growth with chain length are also rationalized. Derived kinetic equations depend on details of the mechanism, but generally encompass water in the expressions and can account for both positive and negative water responses. The expression for the Anderson–Schulz–Flory (ASF) α is dependent on CO, and can contains the water vapor pressure as well, in a way consistent with higher α when more CO and water are present for production of CH* chain growth monomers.
KeywordsFischer–Tropsch Cobalt Mechanism Water CO-activation Kinetics
An important part of this paper is inspired by the presentation in Tromsø by Heiko Oosterbeek at the 11th Natural Gas Conversion Symposium in June 2016.
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