ISSN:
1572-9001
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract Initially in this review the sodalite framework [T12O24]6− (T=Al, Si] is modeled with regular tetrahedra and disordered T atoms. Equations are given for calculating atomic coordinates from the unit cell parameter a and the T—O distancet; the expansion or contraction of the sodaliteβ-cage is related quantitatively to changes ina through the cooperative twistsφ of TO4 tetrahedra about 4 axes and changes in 〈 TOT bridge angles. The fully expanded cage hasφ=0° and the maximum value ofa. The equations are general for any framework formed by isomorphous substitution of T by atoms other than Al, Si and for any Si∶Al ratio. The model and equations are extended to the zeolite A framework, which can be built from fully expanded sodalite cages. With the cooperative tilt of the TO4 tetrahedra of zeolite A, described by Depmeier, the major variable features of the zeolite A framework are explained quantitatively. The faujasite framework has twistedβ-cages (φ〉0), as in sodalite examples, and is quantitatively modeled most conveniently from sodalite examples with similarβ-cage contents. The review is extended to structures with T-ordering and distorted tetrahedra. Methods are given for estimating a for sodalites from a knowledge of the cavity contents, especially the sizes of cations and anions, and so on, present. Ways of predicting cavity sites in zeolite A as a function of cation size are presented, and the principal cavity sites in the faujasiteβ-cage region are discussed. Finally the review considers isomorphous replacement of T atoms (Si or Al) by B, Be, Fe, Ga, Ge, and P; many of these substituted frameworks are stabilized by templates, or guest molecules, which reside in the cavities. Templates also stabilize Si, Al frameworks with high Si∶Al ratios. The modeling approach reviewed here is tested on a range of isomorphously substituted frameworks isotypic with sodalite; observed and calculated values of twist and 〈TOT angles are in good agreement except in one case. This exception concerns aluminate sodalites [Al12O24]12− with distorted tetrahedra; these are structures with Al-O-Al bridges, which violate Loewenstein's Rule.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00672017
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