Hypothetical Zeolite Frameworks

Potentially therefore we may be able to generate an unlimited number of possible zeolitic frameworks. Of these, only a portion is likely to be of interest as having desirable properties, with an even smaller fraction being amenable to synthesis in any given composition. It is this last problem, the feasibility of hypothetical frameworks, which is the key question in any analysis of such structures. The answer is not a simple one, since the factors that govern the synthesis of such materials are not fully understood. Zeolites are metastable materials, as shown by the calculated lattice energies reported in Table 1. Aside from this thermodynamic constraint, the precise identity... 

For zeolites, the simulated annealing method has been used to solve the structures of some new materials [35], including the product that results from a framework reconstruction on dehydration of Na6(ZnAs04)6.8H20 with the sodalite structure at 190°C [43] and the novel aluminosilicate UiO-7 [44], Analogously to the impact of simiilated annealing in structure solution sind hypothetic framework structure development, simulation will also be an exciting medium for new approaches to structure completion and refinement. 

Zeolite beta and its mineral analog tschernichite are intergrown materials and BEA represents the framework of a hypothetical end member. Simulated powder patterns for... 

A moment analysis of Si MAS spectra of zeolites is shown to provide direct information on the number of second-nearest-neighbor Al-Al pairs. Monte Carlo computer calculations are described of randomized A1 distributions in zeolite frameworks, under restrictions of Loewenstein s and Dempsey s rules. The method is applied to a hypothetical square planar lattice which allows the various A1 distribution patterns to be visualized in simple displays, and to the zeolite X and Y framework. The results are compared with experimental data taken from the literature. 

The simulated annealing method can generate large numbers of hypothetical structures in a short time frame. A non-exhaustive series of simulations based on the unit celL sizes and symmetries of 64 known zeolite structures produced over 5000 hypothetical structures [36]. Many of these structures are not 3-dimensional frameworks, instead forming discrete cages, chains or layers. However, for each different set of input data, each derived structure has a unique set of coordination sequences for the inequivalent T-atoms and therefore has a distinct topology. 

Except for the known zeolite frameworks mentioned above, a number of hypothetical structures can be also generated during the simulations (structures T1-T10), which are shown in Figure 7.13. Each structure possesses a 3-D framework containing cages or channels. More details could be found in Reference [36]. 

Atomic Adsorption Spectroscopy (AAS) of aluminum and Fourier Transform Infrared Spectroscopy (FT-IR) of both samples indicate that there must exist a large amount of EFA species in the zeolite (Tab. 1). By measuring the outer T-O-T stretching frequency, we can calculate with help from the equations disclosed in [10] that only about one Al atom per unit cell is incorporated into the zeolite framework of catalyst B (Tab. 5). On the other hand, our experimental results derived from Al-AAS show that hypothetically a higher number, i.e. altogether four Al atoms, could be present in the unit cell on the assumption that all Al is incorporated in the framework. This difference tells us that most of the aluminum must exist in form of EFA. 

Tab. 22.2 Proton transfer energy, A pj, deprotonation energy, E yp, hypothetical binding energy of NH4 on the deprotonated zeolite surfaces, E p(SH ), and energy of ammonia adsorption, ad( 3) ( J for Bronsted sites in different zeolite frameworks [13].

Typical unit cell compositions for the phosphate zeolites are given in Table V. Compositions for hypothetical phosphate zeolite frameworks calculated on the basis of the substitution reactions above are compared in Table XI with the unit cell compositions for the phosphate zeolites (calculated from chemical analysis). The substitution mechanism assigned to each phosphate zeolite species below can be understood best by referring to Table XI. 

Zeolites and related microporous materials offer abundant chemical diversity. Over the past three decades, many new synthetic zeolites have been discovered. The Structure Commission of The International Zeolite Association have approved 145 framework types (November 2003). In addition, there are numerous other zeolites with structures that are not yet known or are only hypothetical. The goals of this article are to briefly mention the main categories and principal industrial uses of zeolites, before providing more detailed discussions of molecular sieving and redox reactions. 

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