Kinetics of bifunctional isomerization over carbides (Mo, W)

Kinetics of isomerization of n-heptane was studied over three types of catalysts: a W2C-type carbide and two composites of molybdenum carbide and tungsten oxide. The precursors were carburized using ethane (10 vol.% C2H6/H2) following a temperature programmed reaction up to 863 K. The catalytic test conditions were as follows: total pressure of 1 or 6 bar, temperature range between 543 and 623 K, and H2/nC7 molar ratio equal to 14.8. The selectivity towards isomers was at least equal to 89% for the full range of conversion. The global consumption of n-heptane is a first order reaction to the reactant. By simulating the global kinetics of the reaction, the experimental curves lead us to calculate rate constants. We show that, at conversion higher than 60%, the reaction becomes a two-way process. The 2mC6/3mC6 molar ratio was close to 1, corresponding to a bifunctionnal isomerization. Isomerization mechanism can be explained by three cycles: the first cycle, corresponding to the dehydrogenation of the n-alkane on metallic sites, is closed to equilibrium; the second corresponds to the isomerization of n-alkene into iso-alkene on acidic sites; finally, the third cycle corresponds to the hydrogenation of iso-alkene into iso-alkane on metallic sites. We showed that the global kinetics of the reaction only occurs on acidic sites.