Catalysts used in pentane or hexane isomerisation processes require two types of reaction site – the platinum sites where dehydrogenation and subsequent hydrogenation occur and acid sites, provided by the supports, where isomerisation of the dehydrogenated product takes place. An undesirable side-reaction, hydrocracking to lower molecular weight paraffins, has been studied by C. G. Myers and G. W. Munns, Jr., of the Socony Mobil Oil Co. Inc. Their results are published in a recent paper (Indust. & Eng. Chem., 1958, 50, (12), 1727–1732).

The hydrocracking of n -pentane, -hexane and -heptane was studied using isomerisation catalysts comprising platinum on alumina and platinum on silica-alumina. The hydrocracking of n -pentane can be correlated with the dehydrogenation activity of the catalyst – a function of the platinum sites – but is not directly related to the nature of the acidic support. The products of the cracking reaction are evenly distributed over the range C.−C₄ hydrocarbons and this distribution is not affected by altering the catalyst support. However, the change in pentane hydrocracking activity per unit change in dehydrogenation activity is less for platinum-alumina than for platinum-silica-alumina catalysts, with the result that the hydrocracking activity is in general less for platinum-alumina when the two catalysts have the same dehydrogenation activity. This may be explained by chemical interaction between platinum and the siliceous support. These results suggest that hydrocracking of n -pentane is catalysed by the platinum sites.

Hydrocracking of n -hexane and n -heptane does not give rise to an even distribution of cracking products but to a higher proportion of hydrocarbons derived from cracking at the centre-bond. This is a characteristic of cracking at acid sites. It is shown conclusively in the case of n -heptane that the more acidic silica-alumina support favours centre-bond cracking to a greater extent than the alumina support.

The results obtained indicate that on dual-function isomerisation catalysts two types of cracking can occur depending on the chain length of the hydrocarbon. Platinum-catalysed hydrocracking, giving an even distribution of reaction products, occurs alone in n -pentane and together with acid-catalysed cracking in n -hexane. Acid site hydrocracking alone occurs in n -heptane. It is suggested that platinum-catalysed hydrocracking occurs via the olefin intermediate produced in the isomerisation reaction by dehydrogenation at the platinum sites.