Contrast between nickel and platinum catalysts in hydrogenolysis of saturated hydrocarbons

Abstract

In order to contrast the reaction mechanism of hydrogenolysis on nickel catalysts with that on platinum catalysts, the detailed analysis of initial reaction products in hydrogenolysis of five hexane isomers and methylcyclopentane was ensured by means of a pulse technique, using hydrogen as carrier gas for a gas Chromatographic microreactor. The hydrogenolytic products from reactants on nickel and platinum catalysts showed a very interesting contrast with each other at low conversion. Analogous to other reactions in hydrogen atmosphere, the reaction intermediates in hydrogenolysis are presumed to be normal alkyls -CH 2(CH 22) n CH 3 on nickel catalysts, which are selectively hydrocracked owing to the successive α-scission to give methane as a main product. On the contrary, a carbonium ion mechanism has been proposed for platinum catalysts to interpret both the characteristic distribution of the initial hydrogenolytic products and the considerable skeletal isomerization during hydrogenolysis. Since heterolytic splitting of a carbon-hydrogen bond of saturated hydrocarbon gives a carbonium ion, the carbon-hydrogen heterolysis at the stage of adsorption on platinum catalysts may be due to the large stability of platinum-hydride coordination, which cannot be expected for the nickel-hydride case. The contrast between nickel and platinum catalysts in hydrogenolysis of saturated hydrocarbons is thus correlated with the nature of the carbon-hydrogen splitting, homolytic or heterolytic, at the stage of adsorption, which is understandable only in terms of the softness of the catalyst metals.