Zeotypes structure

Zeolite/zeotype Structure type code No. of tetrahedra in ring Window diameter (pm) Cavity diameter (pm)... 

Related to guanidinium sulfonates are analogous cation phosphonate structures which can adopt either pillared or zeotype structures. Recently novel tubular morphologies have also been discovered for both classes of compound which may have promise for improving porosity in these types of material. At present, like the guanidinium sulfonates, few phosphonates, are really porous and removal of the guest template generally leads to the collapse of the structures.15... 

The relation between SDAs and the zeotype structures they direct is not simply bijective. Usually, the formation of a certain zeotype structure can be directed by more than one SDA [5]. For example, the sodalite structure shown in Fig. 1 can also be directed by molecules other than trioxane, namely, 1,3-dioxolane [15], 1,3-dioxane [16], ethylene glycol [17], ethanolamine [18], and ethylenediamine [18]. Also, depending upon the reaction conditions, an organic molecule may direct the formation of different frameworks [8, 9]. It has therefore been proposed that the term "template", formerly used instead of SDA, should now be reserved for cases of strong structure direction and strong host-guest relationships, in line with its usage in other areas of the natural sciences [5]. A strict condition for a true "template" is that it should not exhibit disorder in the voids of the silica host. 

More than one hundred different zeotype structures, based on different topologies of the linkage of corner-sharing [TO4/2] tetrahedra (T = Al, Si, P. . . ), have been discovered so far. The various topologies are compiled in the "Zeolite Atlas" [21] under their respective three-letter code (e.g., LTA for "Linde Type A" or zeolite A, MFI for "ZSM-5" or "Mobil Five"). Apart from the framework composition, the high variability of framework structures is related to the fact that all of these compounds are metastable [14] and are formed by kinetic control under the influence of various SDAs. 

Organometallics as a novel class of SDAs also offer the chance to generate new framework topologies, and this has in fact been accomplished in one case (see below) In particular, it might also be possible to discover the "Holy Grail" of synthetic zeolite chemistry, namely, the preparation of a chiral zeotype structure by using metal complexes as SDAs with a chiral shape. 

Figure 7.14 (a) Comparison between the LTL-framework and a hypothetical 12-ring channel zeotype-structure predicted by the AASBU method, (b) A second zeotype structure is predicted, which contains GME cages and has channels surrounded by 24 tetrahedra. Reproduced with permission from [35], Copyright (2000) Wiley-VCH... 

Although tin is an important element in both adsorbent applications and catalysis science, procedures for the incorporation of Sn" into zeotype structures at framework positions were not readily evident (147). The postsynthesis modification of zeohtes using Snp4 or SnCl4 was attempted, but the crystalline structures of the zeohtes were damaged (4,148). However, Mai et al. (66a) incorporated Sn" into silicahte (MFI) and silicahte-2 (MEL) structures. Tin incorporation into the frameworks of MTW (66a, 149), beta (68), and MCM-41 (150) has also been accomplished. 

The success of syntheses designed to incorporate gallium into zeotype frameworks depends on the synthesis conditions, as is true for other framework metal-containing zeotype materials. Several researchers have claimed gaUium incorporation into the frameworks of zeotype structures (5b, 166). 

Several different titanium-containing zeotype structures have been invest ted for the ammoximation of cyclohexanone, for example TS-1 (221,224), TS-2 (177,226), Ti-beta (227), Ti-MOR (228), Ti-MCM-41 (229), and Ti-MCM-48 (227a). Le Bars et al. (227a) evaluated zeotype materials with various pore sizes and concluded that the best performance and highest turnover in ammoximation is achieved with TS-1 as catalyst. The superiority of this material is believed to arise from its hydrophobicity, in combination with the three-dimensional framework of straight charmels that facilitate the diffusion of reactants and products in the catalyst pores. 

Sever and Root used DPT methods to investigate the reaction mechanism of Sn-catalyzed BV oxidation reactions (239). Their approach was to determine the mechanisms for a generic active tin center (modeled as Sn(OH)4 cluster) rather than for tin in a specific zeotype structure. The... 

The mechanism of the BV oxidation of cyclohexanone with hydrogen peroxide catalyzed by tin in a zeotype structure, Sn-beta, has been investigated by computational chemistry (240) and compared with that of the... 

Based on the lUPAC nomenclature, microporous molecular sieves have pores with dimensions up to 2.0 nm in dimension. The mesopore dimension is between 2.0 and 50 nm, while macropores exhibit pores larger than 50 nm. Shown in Fig. 2 are the pore size and schematic structure of several zeolites, zeotypes, and mesoporous molecular sieves. Evidently, a large number of different zeolite or zeotype structures and mesoporous materials are available from which the optimum one for the given application can be selected. 

A great deal of research effort aimed at enlargement of the zeolite and zeotype structures resulted in numerous papers on the synthesis, properties, and characterization of delaminated and pillared zeolitic materials and mesoporous molecular sieves with amor-... 

The preceding examples demonstrate that the incorporation of heteroatoms in the aluminophosphate frameworks can stabilize the zeolite framework and induce novel zeotype structures. 

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