How to Improve the Selectivity of Zeolitic Catalysts in C8Aromatic Cut Isomerization

The catalysts used in the isomerization processes of the C8 aromatic cut (xylenes and ethylbenzene) generally contain platinum loaded on a mixed binder/zeolite support. Improving these catalysts consists in increasing the ethylbenzene conversion, which is the most difficult step in this transformation, while minimising the secondary side reactions such as ethylbenzene disproportionation or dealkylation, naphthene ring opening and consecutive cracking of the resulting paraffins. Most of the various adopted approaches concern optimisation of the zeolitic phase. The selectivity gain remains modest on conventional mordenite-based catalysts and it seems more appropriate to explore new zeolitic structures. In this context, interesting results have been obtained on EU-1-based catalysts, which selectivate during the stabilisation period and work then in pore mouth. It has been possible to extend this pore mouth catalysis concept to other one-dimensional zeolitic structures of intermediate pore size, such as ZSM-22 and ferrierite. In addition, the selectivity of EU-1 zeolite can be optimised by varying the acidic sites location through the use of special organic templates. Another approach consisted in considering the entire catalyst (zeolitic and metallic phases, possibly associated with an acidic alumina support such as chlorinated alumina), and in identifying the sites responsible for the paraffin formation. The results demonstrate that paraffin cracking mainly follows a bifunctional mechanism, suggesting that the most promising way to reduce the paraffin yield is to act on the zeolite.