Faujasitic zeolites, relation

In all cases, the high activity and selectivity of this zeolite as compared to other three-dimensional zeolites, such as beta or dealuminated Faujasite zeolite, is related to the easier and/or faster diffusion of the products and to minimization of undesired consecutive reactions. 

Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained first by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we will show how these selectivity differences can be related to structural differences of the various zeolites. 

It is interesting to compare structural features of the zeolites mor-denite and faujasite in relation to aging. Mordenite, with ncnintercon-necting channels, can be visualized as a bundle of tubes with elliptical (6.96 X 5.81A) cross-section. If an obstruction develops within such a tube, access to catalytic sites can be decreased (Scheme lOA) or completely blocked (Scheme lOB), depending on its location. In faujasite. 

Figure 1.7 Diagrammatic structure of faujasite and related synthetic zeolites (Reproduced by permission from J. Dwyer and A. Dyer, Chem. Ind. (London), 1984, 237)...

All the books and reviews on zeolites which have been published deal mainly with X and Y. Even so, two books, one on all aspects of zeolites except catalysis and the other on catalysis over zeolites and related topics, are essential texts for all workers on non-faujasitic zeolites. Other useful books and reviews, including the Proceedings of the 3rd and 4th International Molecular Sieve Conferences and the Symposium on Zeolites at Szeged, have appeared. 

Since the first " Xe NMR study of xenon adsorbed on a zeolite, this technique has been shown to be of interest for the investigation of the distribution and the size of supported metal particles, the quantitative distribution of phases chemisorbed on these particles, the dimensions of the void spaces of zeolites, the detection of structure defects, the location of cations and the effect of electric fields they create [i,2 ]. We report here some typical applications related to the study of intra-and inter-crystallite diffusion of cations in faujasite zeolites. 

Related to this aim is the preparation of MgO powders by chemical vapor deposition [6], and the dispersion of MgO on Si02 [7] and within the cavities of faujasite zeolites [8]. Noticeably, in the last case, the location of the basic oxide inside a porous matrix was intended for the introduction of an additional shape selectivity effect related to the host structure. However, the access to the faujasite cavities with a diameter of ca. 7 A is limited, and bulky reactant cannot reach the inner catalytic active centers. 

Jackson and Catlow [12-13] studied the stabilities of various zeolites using the static lattice energy simulation technique. In addition, the energetic distribution of non-framework cations and the relative stabilities as a function of Si/Al ratio in faujasite, zeolite A and silicalite were also studied. Reliable interatomic potentials necessary for such calculations were derived empirically in collaboration with Sanders [14]. These potentials and related force fields commonly used in zeolite computational studies are discussed later in this section. The adsorption of various molecules in zeolites leading to... 

Hoppe, R., Schulz-Ekloff, G., Wohrle, D., Kirschhock, C., Fuess, H. (1994). Location and photostability of faujasite-incorporated methylene blue. In Zeolites and Related Microporous Materials State of the Art 1994, Weitkamp, J., Karge, H.G., Pfeifer, H. and Holderich, W. (eds), Vol. 84, pp. 821-827. Elsevier Science BV, Amsterdam... 

One of the most widely used methods for determining the pore size and surface area of zeolites is nitrogen physisorphon. From the shape of the nitrogen adsorption and desorption isotherm the presence and shape of the mesopores can be deduced. As shown in Figure 4.41 a faujasite without mesopores have a type I isotherm since the micropores fiU and empty reversibly, while the presence of mesopores results in a combination of type I and IV isotherms. The existence of a hysteresis loop in the isotherms indicates the presence of mesopores while the shape of this hysteresis loop is related to their geometric shape. 

The naming of zeolites and related structures has been somewhat unsystematic. Some structures were named after the parent minerals (e.g., sodalite, faujasite), while others were named by researchers, or after the projects which synthesized them (e.g., ZSM [Zeolite Socony Mobil]). Unfortunately, this led to the same zeolites synthesized by different routes and bearing different names—in some cases, up to 20 different trade names ... 

Faujasite-type zeolite structures have maximum symmetry Fd3m, and all the 192 T atoms per unit cell of the A structure are symmetrically equivalent. The observed Si/Al ratios of synthetic faujasite-type species vary within a range from slightly over 1 up to 2.5 (and occasionally above). Unmodified species thus normally contain between 48 and almost 96 A1 atoms per unit cell. The almost continuous range in A1 content does not by itself rule out any kind of Si, A1 order. Discontinuities in the plot of the cell dimensions against the number of A1 atoms per unit cell have been reported by several investigators (11, 12). The observed discontinuity at around 64 Al, in particular, has been related to Si, A1 ordering (12). Full details and references on faujasite-type zeolite structures can be found in the comprehensive and critical review by Smith (13). 

Most of the published information regarding surface acidity and its relation to catalytic activity has involved zeolites of the faujasite structure as found in zeolites X and Y. A smaller number of investigations of mor-denite have been reported. This discussion will concentrate on studies of these two types of zeolites because their acidic and catalytic properties have been most widely investigated, and because they are both of significant industrial importance. 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

Xenon Adsorption Isotherms and 129Xe NMR Measurements. Figure 1 displays the room temperature (22 °C) xenon adsorption isotherms of the coadsorbed xenon for the three different zeolite samples loaded with various amounts of benzene. A consistent decrease of adsorption with increasing 6 was found for each benzene/zeolite system. By comparing the slope at low xenon pressures, i.e. in the Henry s Law region, we obtained for the adsorption strength NaX(1.23) > NaY(2.49) > NaY(2.70). Moreover, the saturation benzene concentration in faujasite-type zeolites with different Si/Al ratios follows the relation NaX(1.23) < NaY(2.49) < NaY(2.70). 

The explanation of the procedure for the measurement of the lattice parameters by x-ray diffraction is given in Chapter 4. In Table 3.6, the Si/Al relation, determined with the assistance of the interdependence between the unit cell parameter and the aluminum contents in the zeolite framework for faujasite, that is, using the so-called Breck-Flanigen relationship, is reported [52]... 

The lower energy of the more open structures relates to the decrease in Madelung energy. However, as Figure 2 shows, local topological effects also play a role. See, for instance, the difference in energy calculated between zeolite A and faujasite. 

In view of what precedes, it has been the aim of the present work to identify the Ru-species present in faujasite-type zeolites activated under WGS-conditions, making use of the avail-albe literature data. The activation procedure of Ru(III)hex-ammine in NaY has been related to its catalytic performance as low temperature WGS-catalvst. Subsequently, the basicity of the material was related to its catalytic behavior in the same reaction, by changing the nature of the parent complex, of the charge compensating cations and of the aluminum content of the faujasite-type zeolite. 

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