Hypothetical Zeolites

Using 3G and 4-31G type basis sets (39-41), ab initio quantum chemical calculations have been carried out for several small structural units of zeolites, with a variety of observed and hypothetical Si-Al distributions (29-32). The results of these studies can be summarized in a series of hypothetical Si - A1 exchange reactions within these structural units. The calculated internal energy changes for the reactions involving two neighbouring tetrahedra, are as follows ... 

Such Al-O-Al linkages in zeolites, however, are not altogether impossible, if suitable counter ions are present, as indicated by the calculations on the following hypothetical reactions, analogous to reactions (2). .. (6). 

Foster, M.D. and Treacy, M.M.J. (2009) A Database of Hypothetical Zeolite Structures, http //www.hypotheticalzeo-lites.net (accessed June 2009). 

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 rational zeolite design challenge the design and synthesis of a hypothetical 14 MR zeolite related to the high-silica zeolite SSZ-48... 

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. 

This paper reports the results obtained for two actual connectivity schemes Zeolite X or Y and the square planar lattice. The latter can be viewed as a hypothetical zeolite because it has the same topological features as real zeolites. 

Three well known zeolite structure types - Linde L(18), mazzite(19) and the hypothetical omega-structure(20) - have essentially identical hexagonal lattice constants with =18A and .=7.5A, but only mazzite has a 7.5A x 18A layer that contains an n-glide plane operator the thickness of this layer is -15.5A. If the hexagonal overgrowth is mazzite, the electron micrograph interpretation implies that ECR-1 comprises mazzite sheets interconnected with some other subunit to form the observed 26A repeat distance. The... 

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... 

When the crystallographic residual (a quantitative measure of the degree of match between experimental and simulated diffraction data, in the zeolite case powder diffraction data) was added to the energy expression the number of hypothetical but inappropriate structural models was dramatically reduced [36]. The next step which, althoiigh seemingly obvious at the time,... 

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. 

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]. 

Table 7.5 Atomic coordinates of hypothetical zeolite structures. Reproduced with permission from [37], Copyright (2003) American Chemical Society...

Hypothetical Zeolites - Enumeration Research (http //www.hypotheticalzeolites.net/)... 

Hypothetical Zeolite Database (http //mezeopor.jlu.edu.cn/hypo/)... 

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