Zeolites aluminum site density

For zeolites having a high Si/Al ratio, these sites should be sufficiently remote from each otho- so they do not experience mutual interactions. Thus, protons associated with these aluminum ions should all be of the same acid strength and the acidity of such a zeolite should be defined solely by its acid site density, i.e., its extensive acidity. 

The number of very strong acid sites was found to rise, pass through a pronounced maximum and then decrease with increasing aluminum content in the framework [62] (see Fig. 7b). These results are in good agreement with those of Macedo et al. [151] and the concept of aluminum topological density of zeolites [152]. 

Zeolites exhibit much higher acidity than the earlier amorphous siUca-alumina catalysts. Also, zeolites present both Bronsted and Lewis acid sites. The density of these acid sites is related to the SAR in the zeolite framework. Such density increases with the aluminum content. Nevertheless, as the SAR increases and the density of sites decreases, the strength of isolated sites increases, attaining a maximum. In general, the following parameters affect the acidity of a zeolite ... 

Zhixiang et al. [107] compared typical characteristics of aliunimun-deficient Y zeolites prepared by steaming followed by add leaching and produced by the reverse sequence of the two steps. The sample obtained by the reverse method showed, in contrast to the other one, aluminum eiuichment on the external surface of the crystals and lower charge and add site density on the intracrystalline and mesopore surface. 

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. 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

However, it should be emphasized that the measurement and quantification of the acid/base strength of zeolites is complex and that it is difficult to directly compare the aciti base strength of a solid with that of a liquid. This results from the feet that the stabilization of carbocations and carbanions in a microporous solid differs from that in strongly polar acid and base solutions. For zeolites, it can be stated that the concentiatlbn of aluminum in the lattice is directly proportional to the concentration of acid sites and the polarity of the lattice and to a first approximation indirectly proportional to the strength of acid sites [40]. For a given chemical composition of the zeolite, the polarity of the lattice increases with decreasing framework density [41],... 

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