Structure mordenite framework

Figure 1. Mordenite framework structure viewed in the crdirection...

The mordenite framework structure (see Plate VII and Fig. 9.3) has a complex topology, containing unidirectional 12-ring channels together with... 

Structure of mordenite framework as viewed along c-axis (66). 

The variety of the different framework structures result in different adsorbent characteristics acid strength, size of molecule adsorbed, adsorption/desorption rate of different molecules, capacity and stability. As a result, these differences characterize the adsorbent s selectivity to a specific molecule and adsorbent-adsorbate interactions. Take for example, the difference in selectivity of BaY and Ba-Mordenite [24] to p-xylene (PX), m-xylene (MX) and o-xylene (OX) ... 

Five zeolite minerals have been considered identical with mordenite with space group Cmcm. The possibility of a family of structures is considered four related ordered structures including Cmcm are proposed. Two of these (Cmcm and Imcm) have a one-dimensional system of large pores. The remaining pair (Cmmm and Immm) have a two-dimensional pore system with a second set of smaller channels. X-ray diffraction results show that synthetic and most mineral specimens have the Cmcm structure but also reveal mixtures of Cmmm, Immm, and Imcm with the Cmcm structure in three mineral specimens. Electron diffraction examination of a ptilolite sample reveals the Cmcm structure with an inter growth of the idealized structure Cmmm. Further tentative evidence for the existence of more than one ((mordenitef) framework structure, based on physical property characteristics, is discussed. 

In the present studies we have postulated three additional framework structures closely related to the mordenite (Cmcm) structure (11) and have examined how these structures might be distinguished from mordenite by x-ray and electron diffraction studies. We then critically re-examined the published literature and various available specimens to verify the existence of these other structures. Since verification was established, we examined some specimens further and considered the consequences of the various structures. 

Hawkins (19) showed that synthetic Sr mordenite gave Sr2+/Ca2+ ion exchange separation factors ranging from 8.4 to 3.5 as compared with 0.5 to 1.6 obtained with his synthetic Ca mordenite. X-ray data for the synthetic Sr mordenite agree with the Oman pattern, but no data were presented for the synthetic Ca mordenite. Perhaps the Sr mordenite and Ca mordenite samples differed in their framework structures, and this was the reason for their different ion exchange selectivity. 

Sr mordenite synthesized by Barrer and Marshall (16) gave the Cmcm x-ray pattern and was C-centered orthorhombic by electron diffraction. However, other electron diffraction studies, also by Kerr (12), revealed some synthetic Sr mordenite crystals of the Immm structure prepared at similar synthesis conditions. Future work should examine synthetic Sr and Ca mordenite specimens further for the possible presence of other framework structures, and correlation of structures with Sr2+/Ca2+ ion exchange selectivity. 

The main channels of each of the framework structures described above should be comparable in cross section. However, the size of the main channels in mordenite depends upon the cation form (21). Since cation locations in the various framework structures are expected to differ, the size of the main channels of the structures (in the same cation form) may differ. Thus the various structures may exhibit different degrees of large-port or small-port behavior although in the decationized form all should have equal sized main channels, as observed (21). Future studies may reveal correlations between the sorption properties and the presence of the other framework structures. 

Other related zeolites may also exist as families of framework structures. Models of modified framework structures of some of the other members of the mordenite family—i.e., ferrierite, dachiardite, and epistil-bite—have been constructed. Such structures have unit cell dimensions essentially equal to the reported structures, but of course they exhibit different symmetries. 

The NMR and X-ray diffraction data are only consistent with substitution of boron into the framework structure of the mordenite. Although we prepared boron substituted mordenite directly from modified gels, direct synthesis has severe limitations. The solution chemistry of the substituting element can interfere with zeolite nucleation and crystallization, as... 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

Three examples will suffice to demonstrate this information Figure 3 shows the polyhedral units in the synthetic zeolite Linde Type A, which link to provide a three-dimensional interconnecting array of channels, Figure 4 illustrates the essentially two-dimensional system of channels in the mordenite framework, and Figure 5 shows the major channels in synthetic zeolite Linde Type L arranged as parallel one-dimensional channels and shown as a stereo pair. Table 6 lists the Atlas notations for these structures with explanations, including the symbols used in Tables 2-5. 

Figure t. Framework structures of the most commonly studied zeolitic structures (a) zeolite A (b) zeolite X/Y (c) mordenite (d) Z5M-5 (e) zeolite L. 

Mordenite is a naturally-occurring silica-rich zeolite (Si/Al 5) which also can be synthesized readily (34). Meier determined the structure of its fully hydrated sodium form (28). The mordenite framework is characterized by a micropore system composed essentially of parallel elliptical cylinders of maximum and minimum crystallographic free diameters of 7.0 and 5.7A, respectively, these main channels being interconnected by smaller side channels. 

We prepared boron substituted mordenite by direct synthesis from gel precursors and by post- synthetic substitution into dealuminated mordenite. Direct substitution is favored in aluminum deficient gels, but exacting crystallization requirements for mordenite formation limit the amount of boron that can be incorporated into the framework structure. Higher substitution levels were achieved using a post-synthetic treatment. Boron substituted zeolite Y could not be prepared by a similar direct synthetic method, but post-synthetic methods were effective at providing low substitution levels. This demonstrates the more general utility of post-synthetic substitution methods. The hexane cracking activity of... 

Figure 9.3 Section of framework structure of mordenite, showing the various extraframework cation sites (represented as spheres).

PLATE VII Molecular graphic representation of the framework structures of MAPO-36, NU-87, silicate and mordenite. Colour code yellow, Si Magenta, Al Green, P Red, O. 

The size, location, and structure of platinum clusters in H-mordenite were modeled by molecular mechanics energy minimization and molecular dynamics simulation techniques [96G1]. It was suggested that the relative stability of monoatomic platinum sites in aluminosilicate mordenites is related to the specific aluminum insertion in T sites of the framework structure. The structural features of the platinum cluster confined to the 12-ring main channel are almost independent of the total Pt content and strongly dependent upon the surrounding zeolite structural field. 

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