Isomerisation of xylenes over a silica–alumina catalyst

The catalytic vapour phase isomerisation of m‐xylene into o‐ and p‐xylenes was studied in a flow system in the temperature range 460–700 °C, and over a wide range of space velocities on a silica–alumina catalyst. The isomeric distribution of the products, as determined by a combination of spectroscopic and chromatographic methods, was found to be consistent with calculated equilibrium values. Reduced pressures tended to suppress undesirable side reactions such as disproportionation and favoured the isomerisation. First order kinetics appeared to be applicable and the overall activation energy for both isomerisation reactions (i.e. meta‐ to para‐, and meta‐ to ortho‐) was found to be close to 25 kcal/mol, being of the same order of magnitude in both cases. Isomerisation of o‐, m‐ and p‐xylenes failed to occur even at 600 °C under non‐catalytic thermal treatment, although formation of polynuclear ring systems (“coke”) in small quantities was observed. The results appear to indicate that the mechanism of isomerisation of xylenes over acidic catalysts involves essentially ionic intermediates. Based on the evidence obtained, it is concluded that the vapour phase isomerisation of xylenes apparently proceeds via the formation of a localised π‐complex, leading to a 1,2‐intramolecular shift of the methyl group.