Faujasite topology

The formation and location of the active complex is obviously strongly dependent on the zeolite topology and composition indeed, among the many zeolite structures examined, only the faujasite topology with the charge density of zeolite Y was found stabilize a maximum number of active sites. 

The Benne11/Schomaker method is not easy to describe and can be illustrated by the first example (see the Appendix) which derives the idealized faujasite topology. The method derives starting x,y,z parameters for the tetrahedral atom of (.125,-.050,.038), which compare with refined parameters of (.125,-.054,.037). The second example derives parameters for the RHO topology of ( A,. 102,. 398) as compared to refined parameters of (%,.1014,.3986). The third example derives idealized parameters for AlPO -16 of (.114,.114,.114) as compared to refined parameters of (.1139,.1139,.1139) and (.1156,.1156,.1156). In these three examples the derived parameters are more than adequate to completely describe the idealized framework topology and can even be used as a starting set in a structure refinement. 

THE IDEALISED FAUJASITE TOPOLOGY. The following assumptions are made 1) the space group is Fd3m (second setting) and 2) all 192 T atoms are in general positions. The cell dimensions are known and the T-T interatomic distance is assumed to be 3-1A. For convenience the atom coordinates (x,y,z ) are expressed in Angstroms, and not in fractions of the unit cell dimensions. 

Structural models of zeolite with faujasite topology (A) crystallographic unit cell with the Fd3 m symmetry containing 576 atoms (192 T atoms) (B) a smaller rhombohedral lower-symmetry unit cell with 144 atoms (48 T) (C) a cluster model containing sixT-atoms (D) representing a local structure of a single cation site and a respective hybrid 12 T cluster embedded in the rhombohedral faujasite unit cell (E) (adapted from Ref. [28]). 

Usually, the zeolite inner surface characteristics are rather complex as a consequence of the (3D) character of the porous topologies of most of the zeolite types. The porous framework is a (3D) organization of cavities connected by channels. Inner surfaces are composed of several sorption sites characterized by their local geometry and curvature. Illustrative examples of such inner surface complexity are represented on Figures 1 and 2 they concern the Faujasite and Silicalite-I inner surfaces respectively. 

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. 

The topological structure of X- and Y-type zeolites (also known as faujasites) consists of an interconnected three-dimensional network of relatively large spherical cavities termed supercages (diameter of about 13 A Figure 12). Each supercage is connected tetrahedrally to four other super-... 

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 9.6 Topologies of zeolite structure types, (a) Sodalite (b) Linde type A (c) faujasite (zeolite X and Y) (d) AlP04-5 and (e) ZSM-5. The vertices represent the positions of AI04 or Si()4 letrahedra while straight lines represent Si-O-Si or Si-O-Al linkages. (Reproduced with permission from [5]).

From the topological point of view, with the truncated octahedra of faujasite, a large number of structures can be derived considering a close packing of hexagonal layers of sodalite cages (171. 

The lower energy of the more open structures relates to the decrease in Madelung energy. However, as Figure 2 shows, local topological effects also play a role. See, for instance, the difference in energy calculated between zeolite A and faujasite. 

Both ZK4 and faujasite exhibit short range statistical order which can be rationalized in terms of a competition between the statistics of the step in which the sodalite cage is formed and the tendency of the system to avoid Al, Al next nearest neighbor interactions. The basic difference between the two systems can be understood in terms of the difference in the severity of the constraint imposed by Loewenstein s rule for the formation of a sodalite cage from four ordered 6R s in the case of faujasite or from six 4R s or D4R s in the case of ZK4. The local order in faujasite can be considered as "frozen in" by the topological constraint imposed by the ordered 6R sub-units. As such this order is more a property of the sub-unit than a manifestation of next nearest neighbor interactions in the faujasite crystal. By contrast the next nearest neighbor interaction in ZK4 cannot be... 

The important qualitative features of the various geometric descriptions of the void space are that the topologies of the internal surface of the faujasite represent an interlinked three dimensional network for diffusion, i.e., if a molecule is of appropriate size to diffuse throughout the ftamework, it can move from any initial site to any other site on the internal surface. The important quantitative features of the void space are related to size and shape features of diffusing species relauve to the size/shape features of the void space. The interactions of size/shape features of the zeolite with size/shape features of the adsorbed molecules are critical in determining the rotational and diffusional characteristics of species within the internal surface. 

FIGURE 3. Building blocks and simple topological representation of the framework structure of faujasite zeolites. 

Among the many possible candidates for catalyst support, some zeolite topologies constitute a particular group of carriers. It seems that Romanovski et al. [4] were the very first to report an in situ synthesis of transition metal Pc (MePc) complexes in the supercages of faujasite type zeolites. The synthesis was later successfully repeated by Schulz-Ekloff et al. [5] and Herron [6]. It is assumed that formation of MePc out of four 1,2-dicyanobenzene (DCB) molecules in the supercages of Me-exchanged faujasite (FAU)-type zeolites is accompanied by a two-electron oxidation of (residual) water molecules ... 

Comparison of results. Typical refined tetrahedral atom parameters for a faujasite structure are (0-125,-0-054,0-037), so the parameters derived by the Bennett/Schomaker technique are more than adequate to both build the topology and to be used as a starting set in a refinement. 

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