Dealuminated synthetic faujasite

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

The use of traditional and new techniques to elucidate the structure of synthetic faujasites with different silica alumina ratios, dealuminated by steaming and chemical treatment, and with and without faulting will be described. The migration and fixation of cations and the role of aluminum in the dealumination of the zeolite will be discussed. 

Si-MAS-NMR spectra of the synthetic faujasites indicate a much lower concentration of non-framework Al when dealuminated with SiCl4 than in the case of the hydrothermally treated samples. The difference between the Si/Al ratio as obtained by 29Si-MAS-NMR and elemental analysis is much smaller, indicating that much of the Al is completely removed from the lattice. 

Cotterman et al. (34) showed that hexadecane-cracking activity of AFS and USY zeolites appeared to be a function of total Al content, independent of method of dealumination, implying that hexadecane cracking occurs over both framework- and extra-framework-acid sites. Hence, extra-framework material in mildly steamed synthetic faujasite, USY, makes a significant contribution to catalyst activity, as previously reported (32). Gasoline selectivity is influenced by both the method of dealumination and steam treatment, and depends on both framework-acid sites and the presence of extra-framework material. 

In the first paper dealing with this method [ 147] it was reported that attempts to use this technique for the dealiunination of L zeolite in its as-synthesized K,Na-form and Na-mordenite failed. Later it was claimed [170] that synthetic large-pore Na-mordenite is partially (24%) dealuminated with SiCl4 vapor at 700 °C. That is for this type of reaction an extremely high temperature and resulted, at least in the case of faujasites, in complete destruction of the lattice. Though it was stated that the treated material retained high crystallinity, the crucial point was not pointed out in more detail, i.e., it was not evidenced that the relatively low dealumination was not accompanied by a similarly slight loss in crystallinity. 

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