Mechanisms of xylene isomerization over acidic solid catalysts

In this review paper, the gas-phase isomerization of xylene over fresh acidic solid catalysts is shown to occur through the two mechanisms proposed to explain this reaction with Friedel-Crafts catalysts: the well-known intramolecular mechanism which proceeds through methyl shifts in benzenium-ion intermediates and an intermolecular one, involving successively xylene disproportionation followed by transalkylation between the trimethylbenzene and reactant xylene molecules produced. When steric constraints in the vicinity of the acid sites inhibit (medium-pore size zeolites) or limit (large-pore zeolites such as EMT, BEA, etc.) the formation of the bulky diphenylmethane intermediates of transalkylation, xylene isomerization only occurs through the intramolecular mechanism. This is also the case when the catalyst has very strong acid sites because of a rapid transformation of the diphenylmethane intermediates into coke. On the other hand, the intermolecular mechanism becomes predominant when the catalyst has only,weak acid sites localized in large cages or in long, non-interconnected channels (e.g., MCM-41). On the example of HFAU catalysts, it is shown that the relative significance of the intra- and intermolecular pathways can be simply determined from the para-/ortho-xylene ratio obtained in the isomerization of meta-xylene. (C) 2000 Elsevier Science B.V. All rights reserved.