m-xylene transformation over a USHY zeolite at 523 and 723 K: Influence ofcoke deposits on activity, acidity, and porosity

The modes of coking and deactivation of a USHY zeolite during m-xylene tran sformation at 523 and 723 K were established. At both reactions temperature s, coke molecules result mainly from transformation of the diarylmethane in termediates of transmethylation reactions, olefinic products formed only at 723 K also participating in coke formation at this temperature. From the e ffect of the number of coke molecules on the accessible micropore volume, o n the hydroxyl bands, and on the number of protonic sites able to retain ad sorbed pyridine, it can be concluded that at both temperatures, deactivatio n is initially due to poisoning of the protonic sites, pore blockage by cok e deposits appearing at longer time-on-stream. The effect of this latter mo de of deactivation is more significant at 723 than at 523 K. No interaction of Lewis acid sites with coke molecules is observed, which suggests that t hese sites play no role in m-xylene transformation and coke formation. (C) 2000 Academic Press.