Effect of steaming on the defect structure and acid catalysis of protonated zeolites

The catalytic properties of ultrastable Y (USY) are directly influenced by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment. A new picture of the formation and evolution of mesopores during hydrothermal treatment has emerged from recent electron microscopy studies on hydrothermally dealuminated USY materials. Laboratory steam treatments give rise to an inhomogeneous distribution of mesopores, which occurs concomitantly with further zeolite dealumination. Such inhomogeneities are observed among different USY grains as well as within single grains. In regions with high defect concentration, mesopores “coalesce” to form channels and cracks which, upon extended hydrothermal treatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate of aluminum ejected from lattice sites appears to be closely associated with dark bands which often decorate these newly formed fracture boundaries. 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 catalytic activity is a synergistic interaction between Brønsted (framework) and highly dispersed Lewis (nonframework) acid sites. The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework Al species is not reflected in direct measures of solid acidity, as, for example, by calorimetry, or by NMR spectroscopy. The enhanced activity of mildly steamed protonated zeolites is not due to an increase in acidity of the bridging hydroxyl or Brønsted sites.     

One-step hydroxylation of benzene to phenol. II. Gas phase N2O oxidation over Mo/Fe/borosilicate molecular sieve

The Fe/Mo/partially deboronated borosilicate molecular sieve catalyst prepared by the chemical vapor deposition (CVD) method was active for the selective formation of phenol by gas-phase N₂O oxidation of benzene. The impregnated counterpart exhibited lower activity than the CVD catalyst. The borosilicate molecular sieve itself also was active. Two mechanistic paths are postulated based on reactive oxygen species such as O^–, which can be generated via interaction of N₂O with the iron sites in the CVD-borosilicate molecular sieve catalyst, or OH⁺, which can be generated by Brønsted sites on the borosilicate molecular sieve itself.