SELECTIVE ISOMERIZATION OF NORMAL-HEXANE OVER NICKEL ON SILICA-ALUMINA CATALYSTS

The isomerization of n-hexane to branched chain isomers has been investigated over a range of nickel on silica-alumina catalysts containing between 5 and 16% nickel by weight. It is found that a 7% Ni catalyst has a high activity and selectivity for isomerization, but that catalysts containing more than about 10% Ni produce methane as the main product at temperatures above 633 K. The 7% Ni catalyst has an activity and selectivity which is comparable to a commercial platinum catalyst operating under the same experimental conditions. Hydrogen reduction experiments show that the nickel is present as zerovalent nickel, but X-ray and magnetic measurements do not correspond to bulk nickel metal. It is suggested that the active site in the 7% Ni catalyst is a zerovalent nickel atom probably attached to an aluminium ion. A mechanism is proposed for the isomerization reaction in which a nickel atom first abstracts a hydride ion from a hydrocarbon molecule forming a secondary carbonium ion, which then isomerizes to a tertiary carbonium ion on an adjacent oxide ion, before, finally, addition of the hydride ion gives the saturated isomer. © 1979.