Zeolite A, structure

The logical next step in this sequence of calculations was to perform simulations that included the extraframework cations Na and Ca in the zeolite A structure (56). The cations were situated near the hexagonal faces of the sodalite units in zeolite A such that the geometrical hindrance to methane molecules passing through the 8-ring windows was small. Interactions between the methane molecule and the cations were derived from the data of Ruthven and Derrah (58), and the same two sets of parameters were used (A and B). Long simulations were performed, 25 ns with a time step of 10 fs. 

Another example of the sensitivity of 13C MAS NMR to zeolite structure is the work of Jarman and Melchior (331) who could distinguish between TMA+ cations trapped in the ol and / (sodalite) cages in zeolite A structure in the course of crystallization from a precursor gel. If, therefore, the zeolite which is to be analyzed by 13C NMR, so as to evaluate the extent of intergrowth or variable cage environments is synthesized using TMA + cations as templates, then this method, as we show below, seems viable. 

The secondary structure unit in zeolites A. X, and V is the truncated octahedron. These polyhedral units are linked in three-dimensional space through the four- or six-membered rings, The former linkage produces the zeolite A structure, and the latter the topology of zeolites X and Y and of the mineral faujasite. 

Figure 5.7 Zeolite structure, (a) 6-ring containing two aluminium and four silicon tetrahedral centres, (b) Zeolite A structure. Each of the eight sodalite units depicted contains 24 aluminium or silicon tetrahedral centres arranged to give six 4-rings plus eight 6-rings...

Thallium and Silver Zeolite-A The zeolite-A structure is constructed from simple aluminosilicate cubeoctahedra, connected to form a three-dimensional, cubic network by so-called double-4 rings (Figure 1). This arrangement leads to the formation of large a-cages whose internal diameter is approximately 11 A. 

A corresponding Fm3c refinement of the silver zeolite-A structure, using a sample with a Si/Al ratio of 1.02, has also been achieved. The final profile R-factor is 12.3 percent and a bimodal distribution of Si-0 and Al-0 bond lengths is again obtained, confirming once more the alternation of Si and Al. Our analysis of the cation positions is not yet complete and we are examining the results for evidence of silver clusters, as reported by Uytterhoeven and co-workers (17). 

Figure 18 Motion of sodium cations in 6-rings and 8-rings of dehydrated zeolite A structure. Data from Ref. 161, reprinted with permission of the American Chemical Society.

The zeolite A structure is produced by linking the ji-cages via their quadratic surfaces (over cubes). Linking over the. six-corned surfaces with hexagonal prisms leads to zeolite X and Y, which correspond to the mineral faujasite. The cationic sites are not shown in these figures. The complicated structure of ZSM 5 zeolite systems is characterized, see Fig. 5.1-7 by two crossing channel systems one linear and the other zig-zag. 

Figure 1.1 Microporous (zeolitic) crystal structures showing the sodalite cage (bottom), zeolite A structure (top), sodalite structure (left), and zeolite Y (right).

The zeolite A structure has not been observed in gallosilicates. Possibly, a stable precursor of the cube type cannot be formed because of the larger gallium. Instead, the trimer is formed immediately, leading to the faujasite structure. 

Tetramethylammonium ions appear to stabilize the precursors that cause the zeolite A structure to crystallize. Thus, a zeolite A structure (ZK-4, ZK-21, ZK-22) is obtained from a reaction mixture that would yield a zeolite of faujasite structure if the only cations present were sodium (9, 16). 

G. H. Kuhl I have never encountered phosphate substitution in the framework. However, the zeolite A structure has been found with up to one P per sodalite cage intercalated. 

Based on such variations in the framework of zeolites, a structure code has been assigned to each one of them, for the sake of simplicity in their identification. In fact, as many as 191 types of structure codes (i.e.. Framework Type Code, FTC) have been proposed by the Structure Commission of the International Zeolite Association (IZA-SC) [5]. It is notable that several zeolites exhibit similarities in their structures which can be grouped together to form a iso-structural group of zeolites. Table 2.9 represents details of the common groups based on the type of structure of zeohtes [8]. 

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