Hydrogen transfer steps in the catalytic hydrogenolysis of cyclohexane

Abstract

The rate of the isotopic exchange reaction: cyclo- ∗C 6H 12(g) + C 6(g) → cyclo-C 6H 12(g) + ∗C 6(g) (1) was studied over metal and metal oxide catalysts. C 6 included: n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. The catalysts tested were: Pt supported on acidified and nonacidified Al 2O 3, Cr 2O 3, Re-Cr 2O 3, Cr 2O 3-Al 2O 3 and Re-Al 2O 3,. The temperature range was 230 to 427 °C. The rate of reaction (1) was studied as a function of the ratio p c 6 p CHA in the range 3 × 10 −2 to 1 × 10 2. Since the reaction steps underlying reaction (1) include hydrogen transfer and rearrangement of the molecular structure, the experimental results have been analyzed in terms of these steps and information on their rate and thermodynamic and stoichiometric characteristics has been derived. The analysis suggested a mechanism for the transfer step that is influenced by catalyst acidity and hydro-carbon structure. Models for the reactive surface layer in metal and metal oxide catalysts are put forward and employed to indicate possibilities for the active sites at the surface. The models are also useful to interpret the observed phenomena of inversion in surface reactivity as a function of gas-phase composition. Finally, the experimental results and theoretical deductions are viewed in the framework of present information on the hydrogenolysis of cycloalkanes and isomerization of C 6 paraffins. The advantages of reaction (1) for fundamental studies on the hydroconversion of cycloalkanes are pointed out.