Hydrogenolysis of saturated hydrocarbons IV. Kinetics of the hydrogenolysis of ethane, propane, butane, and isobutane over nickel

Kinetics of catalytic hydrogenolysis of ethane, propane, butane, and isobutane over 5 wt% Ni on dealuminated silica-alumina have been investigated in a flow reactor at atmospheric pressure. These hydrocarbons can be classified according to their increasing rate of hydrogenolysis at 200 °C in the sequence ethane, isobutane, propane, and butane, but their differences of reactivity over nickel are much lower than on platinum. The kinetic orders and apparent activation energies have been determined. All the results fit the rate equation derived from the kinetic scheme proposed by A. Cimino, M. Boudart and H. S. Taylor (J. Phys. Chem. 58, 796 (1954)). When possible, the rate constant of the rupture of the CC bond and a value proportional to the equilibrium constant of adsorption have been calculated. The low differences in reactivity observed for the hydrocarbons come from the rate constant differences of the hydrogenolysis step while the equilibrium constants remain nearly the same. These results are in complete opposition to what had been found on platinum. The much higher activity of nickel compared to that of platinum in hydrogenolysis is due to a huge increase in the reactivity of the adsorbed species reflected by an increase of the CC bond scission rate constant.