Hydrogenolysis and isomerization of n-pentane on group VIII transition metals

Abstract

The reaction of n-pentane in the presence of hydrogen was studied using supported Group VIII transition metals as catalyst. The specific activity for hydrogenolysis referred to unit surface area of the metal was examined and the following sequence was obtained: Ru > Rh > Pt > Ni > Co > Ir > Pd ~ Fe. Hydrogenolysis on nickel proceeded via successive demethylation at the terminal carbon-carbon bond of adsorbed hydrocarbon, whereas on cobalt and iron, the desorption step of surface residues was so slow that the exact mode of hydrogenolysis could not be determined. On Group VIII noble metals, all the carbon-carbon bonds of n-pentane cracked almost statistically. On ruthenium, rhodium, and iridium, multiple bond breaking (extensive degradation) occurred at low partial pressure of hydrogen, although at high partial pressure of hydrogen, the amounts of methane and ethane were equal, respectively, to those of butane and propane. The main features of the product distributions of hydrogenolysis on palladium and on platinum catalysts could be accounted for by assuming that only one carbon-carbon bond was ruptured at the residence of a molecule on the surface. Carbon-supported catalysts produced methane and butane in higher proportion than silica gel supported catalysts. Among all the Group VIII transition metals with the exception of osmium, only palladium and platinum were found to be active for isomerization of n-pentane to isopentane. The selectivity of platinum for isomerization was found to decrease with increasing partial pressure of hydrogen.