THE EFFECT OF CYCLIC DEALUMINATION OF MORDENITE ON ITS PHYSICOCHEMICAL AND CATALYTIC PROPERTIES

Commercial Na mordenite was ion-exchanged with both hydrochloric acid and ammonium chloride. The exchanged samples were subjected to three cycles of hydrothermal/acid treatments, the hydrothermal treatments being performed at 723, 823, and 923 K. Samples having a silica-to-alumina ratio (SAR) in the range 10.7-100 were prepared. They were characterized by XRD, XRF, Si-29, and Al-27 solid-state n.m.r., t.p.d. of NH3, nitrogen physisorption, and XPS N.m.r. results showed that most but not all extraframework alumina (EFAL) could be leached with no damage to the zeolite framework. Considerable mesopore formation in the range 30-50 Angstrom was observed. Also, the EFAL was shown to block micropores, mesopores, and strong acid sites. Steaming promoted migration of EFAL to the external surface, whereas leaching rendered the samples rather homogeneous. The activity for m-xylene isomerization displayed a maximum as the aluminum content decreased. Catalytic stability increased with aluminum removal for both n-heptane cracking and m-xylene isomerization.