Equilibrium hydrogen transfer between benzene and C6 hydrocarbons over supported metal catalysts

Abstract

A kinetic method is presented for the study of the hydrocarbon-surface interaction taking place during the catalytic conversion of hydrocarbons. The method involves the study of the rate of redistribution of an isotopic tracer between hydrocarbon molecules in thermodynamic equilibrium with the catalyst surface. A general discussion of the approach is given; its potentialities and advantages in studying the individual steps of hydrocarbon reactions are analyzed. These ideas are applied to the study of the rate of hydrogen transfer between cyclohexane and other C6 hydrocarbons on the one side and benzene on the other over Pt, Pd, Ir, Rh, and Ru supported on Al2O3 and SjO2. The investigation covered the effect on the reaction rate of the ratio of C6 hydrocarbon to benzene and of metal particle size at 117 [deg]C.With few exceptions the transfer rate increased upon increasing the ratio of cyclohexane to benzene and upon addition of molecular H2 to the reactants. For the cyclohexane-benzene combination the relative activitv among the metals investigated was Pt > Pd > Ir > Ru > Rh. In the range of Pt crystallite size studied (12-2000 A) the hydrogen surface coverage was influenced by the particle size, but the reaction rate constant per unit of metal surface area was little dependent upon crystallite size. Reactive surface efficiencies were in the range 10-4 to 10-7. Experiments on the hydrogen transfer between benzene and other C6 hydrocarbons produced the following sequence of reaction efficiency: cyclohexene > cyclohexane > methyl-cyclopentane >n-hexane, 2,3-dimethylbutane. A surface equilibrium reaction and a rate-controlling step consistent with the experimental results are postulated. The nature of the reactive metal surface, particularly the role of the adsorbed hydrogen, is pointed out. The conclusions are analyzed in the framework of the present understanding of metal catalysis.