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Selective toluene disproportionation to benzene and xylenes is investigated over HZSM‐5 and modified HZSM‐5 zeolites. The zeolites are modified with nickel, magnesium, phosphorus and boron, characterised by sorption, acidity measurement and X‐ray photoelectron spectroscopy, and their activity towards toluene disproportionation is discussed in the light of their physico‐chemical properties. The two types of active sites have been identified; one promoting disproportionation and the other cracking and dealkylation of toluene. It is found that the latter sites are suppressed due to incorporation of the modifiers. The modified zeolites are found to be para‐selective which appears to be consistent with the commonly accepted view that modifiers like magnesium, phosphorus and boron reside in the channels and react with Brønsted acid sites, leading to partial blockages and thus favouring the formation of p‐xylene. The effect of poisoning the zeolite with 1‐methylisoquinoline is also investigated. In the case of poisoned catalyst, p‐xylene is produced at levels up to 99%. The total elimination of other isomers from the products is suggested due to the selective poisoning of the non‐selective sites (on the external crystal surface) and to partial blocking of the pore openings.

The transalkylation reactions of a mixture of toluene and C9+ aromatics were studied over HZSMS catalysts of varying SiO2:AI2O3 ratios (30-800) as well as over a NiHZSMS catalyst. As the Si02 : Al2O3 ratio is increased, the rate of transalkylation is decreased. Among the C9 aromatics that were present in the feed, the ethyl toluenes were extensively de-ethylated to toluene. The shape-selective characteristics of the ZSMS zeolite were revealed through the relative order of reactivity of the three trimethyl benzene isomers 1.2.4>1.2.3>1.3.5. The stable life of the catalyst can be prolonged by the incorporation of nickel. The influences of process parameters such as temperature, pressure, space velocity and H2 : hydrocarbon ratio are also presented. It was observed that the transalkylation of C9 aromatics with toluene to yield xylenes occurs mainly on the external surface of the ZSMS crystals. The disproportionation of toluene to yield benzene and xylene, however, takes place predominantly in the pore system.

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