Isomerisation of n-hexane over sulphated zirconia modified by noble metals

Isomerisation of n-hexane has been studied at industrially relevant conditions over sulphated zirconia promoted with Pt, Rh, Ir, and Ru. The Pt-promoted sample showed the highest activity, and it was considerably more active than a commercial, zeolite-based catalyst. The samples promoted with rhodium and iridium showed a lower, and similar activity, while ruthenium was less efficient and gave a less stable catalyst. The selectivities were similar for all the samples, with the methylpentanes and 2,3-dimethylbutane as main and primary products. Measured kinetic parameters for the reaction were in agreement with a conventional bifunctional mechanism: Positive order in n-hexane, negative in hydrogen and an activation energy around 90 kJ/mol. Prereduction of the samples was necessary to achieve a high activity, but if the activation temperature was too high the activity was lost, probably due to the loss of sulphate groups through excessive reduction.