Erionite, framework structure

Figure 7.7. Schematic representation showing framework structures of (a) zeolite A, (b) zeolites X and Y, (c) erionite and (d) chabazite.

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

In addition, structural similarities can often be determined from careful interpretation of XRD powder patterns. The powder patterns of offretite and erionite look quite different, but are easily understood in terms of the crystallographic consequences of a change in the ordered layer stacking sequence (11), cf. Figure 4. In offretite, the layers are stacked in an AAB sequence, while in erionite, they are ordered in an AABAAC arrangement that doubles one of the crystallographic unit cell parameters. The doubled c-parameter is readily deduced from an analysis of the XRD powder pattern of erionite. Another framework structure effect, isomorphous substitution, can result in changing unit cell sizes, observed as shifts in XRD line positions for such systems as X and... 

Chabazite, gmelinite, levynite and erionite form a group of porous crystals which exhibit certain similarities. Structures have previously been proposed for the anionic frameworks of chabazite,5 gmelinite and erionite. A structure is now proposed for levynite. On the basis of the crystallographic data, dilfusion anisotropy and molecular sieve behaviour of the four types of framework have been examined and the channel systems described and compared. 

This may indicate that in both the (Si,Al,0)-framework of K,Na-gels and crystal lattice of erionite, similar structural elements are present, formation of which requires the participation of potassium ions. Potassium ions will play the dominant role in the erionite structure and in the gel skeleton on potassium deficit in the initial mixture. Such structural elements can be built from 6-membered rings of tetrahedra like columns of erionite structure. Potassium ions are blocked inside the elements of erionite structure and owing to this are nonexchangeable (37, 40). 

FIGURE 1.6. Schematic representation showing framework structures of a) zeolite A, (b) zeolites X and Y, (c) erionite and (d) chabaziie. (To translate these schematic diagrams into the actual structure, a Si or Al is placed at each vertex and an 0 at or near the center of each line.)... 

Framework Model of Erionite Structure Containing Two Quinuclidine Molecules... 

Increasing attention has been given to the structures of zeolitic crystals, not only on account of their practical value as selective sorbents, but also because of the remarkable pore systems which have been revealed. As a result considerable new information exists about the anionic frameworks, although the disposition of the relatively mobile intracrystalline water and cations is intrinsically more difficult to determine. Four structures which have certain related features, and which are of interest as molecular sieves, are those of chabazite, gmelinite, levynite and erionite, for which hexagonal unit cells may be given as follows ... 

Anionic frameworks have been proposed for chabazite. gmelinite 5 and erionite. In this paper we suggest an anionic framework for levynite, and compare, on the basis of the proposed frameworks, the ease and degree of anisotropy of molecule diffusion, and the possible molecular sieve behaviour, for the four zeolites. It has already been shown that diverse intracrystalline channel systems can arise in structures such as analcite, nosean-sodaUte minerals, cancrinite, faujasite, and Linde Sieve A.9> 10... 

Cu ions introduced into the MFI, mordenite, erionite and beta zeolites were characterized by discrete bands of the Cu luminescence and IR spectra of NO adsorbed on Cu. The conditions necessary to obtain semiquantitative analysis of the Cu siting in zeolites are given. Four different Cu sites with defined coordinations, controlled by the local A1 arrangements in the framework, have been found, regardless of the matrix structure and total Si/Al ratio. The Cu sites differ substantially in reducibility and in their population in the matrix. 

Synthetic zeolites which have been resolved include the zeolite type L (14) and the zeolite Q (13), both of which have open frameworks and should have important catalysis application. Ion exchange studies (71) have contributed to an understanding of the synthetic zeolite T (19), which appears to be structurally related to the minerals offretite and erionite. 

The major crystal structures of AlPO-n and AlPO-n-based molecular sieves are summarized in Table 1 At present, there are 31 AlPO-n materials whose crystal structure has been determined. They include 18 novel structures as well as 13 structures with framework topologies related to those found in the zeolites such as chabazite (n=34, 44, 47), erionite (n=17), gismondine (n=43), levynite (n=35), Linde type A (n=42), faujasite (n=37), and sodalite (n=20). Among these, the crystal structure of AlPO-5 has been the most widely studied. Figure 1 shows the pore structure of AlPO-5. The structure of... 

IR framework spectra were used as a diagnostic tool by Occelli et al. [260] in detecting the presence of offretite (via a band at 600-610 cm ) and erionite (bands at 410-425,550-610,655-685 cm ) in mixtures of these two structures. Roessner et al. [261 ] considered, in their IR spectroscopic work on the cation distribution in dehydrated calcium-exchanged erionite, also the framework vibrations of Ca-erionite besides OD vibrations, CO adsorption and DRIFT spectroscopy in the NIR region. They were able to show that the Ca + cations were selectively located in the supercages in front of the six-membered rings. Similar to the features encountered with Y-type zeolites and mordenite (vide supra), also with offretite a sufficiently linear relationship was found between the wave-numbers of the asymmetric and symmetric T-0 vibrations and the number of framework Al atoms per unit cell [262]. 

When B is introduced in the zeolite framework instead of Al, the acid strength decreases and the resultant materials are selective catalysts to produce olefins from methanol. Using a borosilicate of the MFI structure, propylene is selectively prepared from methanol (221). Modification of the borosilicate by HF, HCl, or extrusion with amorphous silica-alumina, changes the product distribution giving more Q-Q olefins. In the case of mordenite and erionite, the introduction of B in the framework improves the formation of olefins from methanol (222). 

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