EFFECT OF STEAMING ON THE DEFECT STRUCTURE AND ACID CATALYSIS OF PROTONATED ZEOLITES

The catalytic properties of ultrastable Y (USY) are directly influence d by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment. A new picture of the formati on and evolution of mesopores during hydrothermal treatment has emerge d from recent electron microscopy studies on hydrothermally dealuminat ed USY materials. Laboratory steam treatments give rise to an inhomoge neous distribution of mesopores, which occurs concomitantly with furth er zeolite dealumination. Such inhomogeneities are observed among diff erent USY grains as well as within single grains. In regions with high defect concentration, mesopores ''coalesce'' to form channels and cra cks which, upon extended hydrothermal treatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate o f aluminum ejected from lattice sites appears to be closely associated with dark bands which often decorate these newly formed fracture boun daries. High-silica Y materials, having little or no nonframework Al, exhibit poor catalytic activity. The results of recent studies provide compelling evidence that the critical nonframework Al species are (1) highly dispersed, and (2) quite possibly exist as cationic species in the small cages of dealuminated H-Y. Investigations of Lewis acidity in mildly dealuminated zeolites indicate that the origin of the high c atalytic activity is a synergistic interaction between Bronsted (frame work) and highly dispersed Lewis (nonframework) acid sites. The enhanc ed cracking, isomerization activity associated with the presence of hi ghly dispersed nonframework Al species is not reflected in direct meas ures of solid acidity, as, for example, by calorimetry, or by NMR spec troscopy. The enhanced activity of mildly steamed protonated zeolites is not due to an increase in acidity of the bridging hydroxyl or Brons ted sites.