m-Xylene transformation over H-MCM-22 zeolite: 1. Mechanisms and location of the reactions

The transformation of m-xylene was investigated at 350 °C over a MWW zeolite sample. The distributions of the desorbed products and of the nondesorbed products (“coke”) were established for a large range of contact times, hence of conversions (from 4.8 to 43.7%), and for different times on stream (TOS). A rapid deactivation can be observed during the first 10 min of reaction, followed by a quasi plateau in activity. On the fresh sample (TOS=2 min), m-xylene was transformed into isomers and disproportionation products (toluene and trimethylbenzenes or TMB); TMB underwent various secondary reactions leading to ethyltoluenes, C2–C4 alkanes and alkenes, toluene, and coke. Deactivation causes a small decrease in isomerization and suppresses disproportionation and secondary TMB transformations. These latter reactions were demonstrated to occur only in the supercages. On the other hand, approximately 30% of isomerization took place in the supercages and 70%, without any deactivation, in the sinusoidal channels and in the external pockets.