Sodalite cage occupancy

Sodalite Cage Occupancy in Y. The sodalite cage occupancy by A of three Y zeolites synthesized in Series 1 was determined by C... 

The results suggest that the sodalite cages of Y synthesized in the presence of TMA are filled by a random process the fit for two sodium ions/cage is slightly better than for three sodium ions/cage. Sodalite cage occupancy by approximately two sodium ions is consistent with many XRD studies(13). 

It is also of interest to use MAS NMR for the study of the thermal treatment of zeolites which are not in the ammonium-exchanged form. In an X-ray study, Pluth and Smith (179) found electron density at the center of the sodalite cages in dehydrated zeolites Ca-A and Sr-A and attributed this to a partial occupancy of these sites by a four-coordinated aluminous species. No such effect was found in zeolite A exchanged with monovalent cations. Corbin et al. (180) used 27A1 MAS NMR to examine commercial samples of K-A, Na-A and (Ca,Na)-A, as received (see Fig. 41). For K-A and Na-A, only framework tetrahedral Al species were observed, with chemical shifts of 57 and 52 ppm respectively. However, in (Ca,Na)-A an additional intense resonance at 78 ppm, typical of AlfOH) but definitely not due to framework aluminum, was also found (see Fig. 41). A much weaker signal, also at 78 ppm, was detected in zeolite Sr-A its intensity increased greatly on heating the sample to 550°C. Freude et al. (183) came to very similar conclusions in their NMR study of heat-treated zeolite Ca-A. They found that maximum framework dealumination occurs at 500°C and corresponds to ac. 17% of total Al. 

One way to simplify the problem is to consider only a small structural unit (e.g., one or two sodalite cages) and to assume that all other units in the crystal are identical and/or related through symmetry. In several publications (2., 3., 4), this approach led to specific patterns of A1 occupancies which had excellent agreement with experimental NMR data. Other theoretical investigations considered random A1 distributions of... 

N. Gobeltz-Hautecoeur, A. Demortier, B. Lede, J.P. Lelieur and C. Duhayon (2002) Inorganic Chemistry, vol. 41, p. 2848 - Occupancy of the sodalite cages in the blue ultra-marine pigments . 

In Structure II, which we call ammonium-aluminum Y hydrate, the occupancy factors for all the framework positions were unity. Site Sn is occupied by 13.4 aluminum ions, and Sm sites are filled by 15.0 hydroxyl groups per unit cell. The 5.9 ions located in Sv sites are thought to be ammonium ions. The (approximately) 2 aluminum ions present per sodalite cage are coordinated to the same 2 hydroxyl groups from Sm sites at a distance of 1.77 A. The nearest framework oxygens are 3 0(3) oxygens at 3.02 A. The mean Si,Al-0 bond length is 1.65 A. 

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