INFLUENCE OF COKE DEPOSITS ON THE SELECTIVITY OF M-XYLENE TRANSFORMATION AND ON THE ISOMERIZATION MECHANISM

m-Xylene transformation was carried out at 623 K over a series of comm ercial HFAU zeolites with framework Si/Al ratios between 4 and 100. Al l the samples deactivate rapidly owing to the formation of coke. The l ower the Si/Al ratio the faster the deactivation, which can be explain ed by the increase in acid strength when the Si/Al ratio decreases and by the faster deactivation of the strong acid sites shown by the effe ct of cake on the intensity of the various OH IR bands. Deactivation c auses a significant increase in the disproportionation/isomerization r ate ratio (D/I) and a significant decrease in the para/ortho selectivi ty (p/o). These selectivity changes can be related to a greater select ivity of the strong acid sites in favor of the classical monomolecular mode of isomerization at the expense of disproportionation and of the bimolecular mode of isomerization (through m-xylene disproportionatio n followed by m-xylene-trimethylbenzenes transalkylation). This was co nfirmed by the fact that pyridine poisoning and deactivation by coke d eposition have an identical effect on D/I and p/o selectivities. The r apid transformation into coke of the diphenylmethane intermediates of disproportionation and of bimolecular isomerization on the strong acid sites was proposed in order to explain the low selectivity of these s ites for these reactions. (C) 1998 Elsevier Science B.V.