Faujasite, dealumination procedure

FIGURE 2.36 " Al MAS NMR spectra at 104.2 MHz obtained on faujasite samples at various stages of the SiCU dealumination procedure (a) starting faujasite sample, (h) intact sample after reaction with SiCU before washing, (c) sample (b) after washing with distilled water, and (d) after several washings. 

These dealumination procedures result in Al deficient zeolites of the high thermal stability. In the case of faujasite Y, the so called ultra-stable Y zeolite (US-Y) is used as a catalyst for cracking catalysis (47). It was reported that the total acidity of (48) Al-deficient zeoITtes was less than that of the parent zeolite buT with stronger acidic sites (49). For mordenite, this acidity decreases linearly with Al content (50, 51). However calorimetric measurement of the NH- heat of adsorption has shown that when the total number of acidic sites decreases regularly with dealumination, as could be reasonably expected, the strength of the strongest acid sites is enhanced (52). 

Mesoporosity is a characteristic of hydrothermally dealuminated zeolites identified in the early 1980 s. Since then a considerable amount of TEM (19, 20) and adsorption (21) evidence has accumulated concerning the size and surface area of the mesopores as well as the potential activity and selectivity benefits associated with mesoporosity. It appears that mesoporosity is formed during the initial period of rapid dealumination. Although the use of slow, low temperature dealumination procedures do produce some mesoporosity, the amount is much less than is produced by high temperature rapid hydrothermal dealumination techniques. This is illustrated in Figure 2 where the mesoporosity development of two sieves, one hydrothermally dealuminated to 2.454 nm unit cell is compared to the same zeolite dealuminated using silicon hexafluoride to 2.452 nm. Both USY faujasites were subsequently treated hydrothermally to further reduce the unit cell size. There is a significant difference in the amount of mesoporosity developed after both the initial and subsequent hydrothermal treatments. 

In a cooperative effort, Linde Research and Union Carbide Nuclear Co. prospected for and located deposits of natural zeolites in Western United States. No deposits of A, X, Y, or faujasite were found. Numerous and extensive deposits of other useful zeolites were located (chabazite, erionite, mordenite, clinop-tilolite), claimed and at a later date some were mined and sold for special uses. We learned how to dealuminate zeolites while maintaining crystal structure, opening the pore and increasing the silica/alumina from 10 to about 20 in mordenite. Procedures for synthesizing A, X, and Y from clays were discovered. 

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