Ethylene activity and Fischer-Tropsch synthesis: new perspectives in reaction mechanism

Abstract

With the aim of studying the reactivity of ethylene and consequently understanding and elucidating aspects of the Fischer-Tropsch (FT) reaction mechanism, a large number of experiments were conducted using feed gas mixtures with different proportions of CO/H2/C2H4/inert gas (N2 or Ar) over a conventional cobalt-based FTS catalyst (15% Co/TiO2) at both normal (180-220 °C) and low operating temperatures (100-160 °C). Different kinds of reactions, including C2H4 hydrogenation, C2H4 dimerization, hydroformylation, hydrogenolysis, and the FT chain growth reaction (including the normal FT chain growth and a C2H4 initiated chain growth) were observed. After comparing the experimental results, we concluded that: 1. There are no reactions between C2H4 molecules at any temperature (100-220 °C), based on the results for feed mixtures of C2H4/N2. 2. C2H4 could form both chain growth monomers and initiators with the assistance of H2; these, in turn, reacted to produce longer chain hydrocarbons. The products of this reaction (C3-6) fitted a typical Anderson–Schulz–Flory (ASF) product distribution. 3. At extremely low reaction temperatures (140-160 °C), CO/H2 mixtures did not react, indicating that CO could not dissociate in the presence of H2. However, CO reacted when co-feeding C2H4 with syngas via the CO insertion mechanism to form long-chain hydrocarbons. 4. Co-feeding a small amount of C2H4 into the syngas promoted the FTS reaction, especially at low reaction temperatures; however, co-feeding CO into the C2H4 hydrogenation system suppressed ethylene reactivity. 5. The paraffin to olefin (P/O) ratio of the products for all the CO/H2/ C2H4 experiments fitted the “Yao plot,” which is a linear relationship between Pn+1/On+1 and Pn/On. A competitive reaction equilibrium was hypothesized to explain this linear relationship. We concluded that the CO insertion mechanism and the CO dissociation mechanism might exist and compete under normal FTS reaction conditions by investigating the changes in the product distribution and the carbon chain-growth probability for different feed gas mixtures. The CO insertion mechanism is dominant at low reaction temperatures, while the CO dissociation mechanisms dominate at higher reaction temperatures. Furthermore, co-feeding C2H4 could assist the conversion of CO via the CO insertion mechanism.