Different Zeolite Framework Topologies

It has been reported by F.A. Mumpton (1978) that more than 1000 occurrences of zeolite minerals in over 40 countries have been discovered since 1950. In addition to the hydrated aluminosilicate species, new minerals related to zeolites have been discovered, including the porous clathrasUs such as Melonophlogite (a silica only framework). Species in which the aluminum or silicon has been replaced by other elements (such as phosphorus, iron, and berylhum) have also been discovered as exemplified by viscite, a sihcoaluminophosphate related to analcime. At the present there are 3 8 different natural framework topologies, as shown in Table 25. 

The coordination sequences and the vertex symbol are unique for a particular framework topology, i.e. they can be used to distinguish between different zeolite framework types unambiguously. In this way, frameworks with the same topologies can be easily identified. Currently, it is easier to calculate the coordination sequences and vertex symbol using computer program based on crystallographic data. 

The connectivity (topology) of the zeolite framework is characteristic for a given zeolite type, whereas the composition of the framework and the type of extra-framework species can vary. Each zeolite structure type is denoted by a three-letter code [4], As an example, Faujasite-type zeolites have the structure type FAU. The pores and cages of the different zeolites are thus formed by modifications of the TO4 connectivity of the zeolite framework. 

Tschortnerite (TSC) surely is the most remarkable novel zeohte mineral discovered [67]. Its unique framework topology contains five different cages D-6Rs, D-8Rs, sodahte cages, truncated cubo-octahedra and a unique 96-membered cage. Cu-containmg clusters are encapsulated within the truncated cubo-octahedra. The pore structure is three-dimensional with 8R charmels, and the framework density of 12.2 is among the lowest known for zeolites. The framework is alumina-rich with Si/Al = 1, unusual for zeohte minerals. 

The main result of extensive simulations of A1 placement in the FAU-framework topology is that random insertion of A1 into the structure, subject to Loewenstein s rule and to a weaker second neighbor Al-Al repulsion term, does not reproduce the measured Si-nAl distribution patterns [4]. The details of the aluminum distributions are therefore determined by additional or different factors. This is consistent with Melchior s model of FAU-framework construction from pre-formed 6-iing units [47,48], The simulation results also highlight the likely limitations of quantum mechanical studies of aluminum T-site preferences. If the factors controlling the aluminum distributions in zeolites X and Y are also at work in other systems, purely energetic arguments will likely have limited direct relevance for application to real materials. 

Table 2.5 Topological features of different zeolite-type frameworks...

The chemistry of natural zeolites may have important effects on their ion exchange properties, mainly in terms of selectivity. It is well known that selectivity is a function of various parameters, depending on (1) framework topology, (2) ion size and shape, (3) charge density on the anionic framework, (4) ion valence and (5) electrolyte concentration in the aqueous phase [51]. Within the same zeolite type, the variation of the framework composition (in practice, Si/Al ratio) and therefore of the framework charge density, affects the cation selectivity [52], as it has experimentally been proven for phillipsitc [53]. It is improper, stricto sensu, to compare with each other, in terms of selectivity behaviour, different zeolites having... 

We have investigated the photooxidation of toluene in several different zeolite hosts (BaX, BaY, CaY, BaZSM-5, and NaZSM-5) using in-situ Fourier Transform Infrared (FT-IR) spectroscopy and ex-situ Gas Chromatography (GC) to analyze product formation and product yields. This combined approach allows for a more detailed analysis of the product distribution. The product selectivity in these reactions appears to be governed by the presence of a small number of acid sites rather than by the framework composition or topology of the zeolite host. 

It turns out that the details of structures with the same framework topology sensitively depend on the partial charges chosen for lattice cations and anions. The difference in relative stability of a- and / -quartz are an example [7]. In view of our interest in the zeolitic chemical bond, the overabundance of potential parameters and the need to properly balance the electrostatic and covalent interactions, a comparison of several predicted and experimental properties is essential. By doing this an accurate determination of potential parameters has become possible. 

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