Alumino framework structure

Oliver, S. Kuperman, A. Ozin, G. A, A new model for alumino-phosphate formation Transformation of a linear chain alumi-nophosphate to chain, layer and framework structures. Angew. Chem., Int. Ed. 1998, 37. 46,... 

The discussion in the previous sections should suffice to indicate the importance of the transformations of molecular compounds to materials. Studies of such transformations are only making a beginning, and there appears to be a great future for such investigations, Thus, the recent synthesis of a sodalite-related structure from a molecular zinc phosphate is noteworthy, besides the transformation of the centrosymmetric, tetrameric, molecular alumino-phosphate to open-framework structures. Preliminary investigations53 have shown that interesting transformations also occur in metal carboxylates. Thus, molecular zinc oxalate monomers and dimers are found to transform to chain or three-dimensional structures on heating with piperazine in an aqueous medium, The chain structure transforms to a layer structure, We believe that it would... 

Zeolitic catalysts are constituted of [TO4] tetrahedrons which are assembled by sharing oxygens to form one of several topologically distinct framework structures. In the case of alumino-silicates, T=Si with some A1 substitutions, while in silico-alumino-phosphates (SAPO), T sites are occupied by alternating A1 and P atoms with some Si substitutions. The substitutions destroy the electro-neutrality of the framework and necessitate the presence of compensating cations. While mono- or bivalent metallic cations usually occupy crys-tallographically well-characterized sites in the cavities, protons are chemically bonded to framework oxygens. 

When tripropylamine or tributylamine is used instead of the corresponding Alk N salt, ZSM-5/11 mixed phases (intergrowths ) are formed, suggesting that the Alk N species are less efficient in directing a specific structure. Unexpectedly, Bu N yields essentially a ZSM-5-rich phase while ZSM-11-rich phases are preferentially obtained with Pr N (XRD data). When an organic molecule acts as template towards (alumino)silicate species to form an ordered zeolitic framework, the latter is supposed to organize itself around the host organic species in such a way that a complete... 

As well as occurring naturally, alumino-silicates are manufactured. Their structure is that of a framework of silicon, aluminium and oxygen atoms. If the framework contains water, then this may be driven off by heating, leaving a porous structure, access to which is controlled by windows of precise molecular dimensions. Larger molecules are excluded, hence the description molecular sieve as discussed in Chapter 17. 

Lewis, D. W., Catlow, C. R. A. and Thomas, J. M. Influence of organic templates on the structure and on the concentration of framework metal ions in microporous alumino-phosphate catalysts, Chem. Mater., 1996, 8, 1112-1118. 

Compared with microporous phosphates, there are few microporous arsenates. The syntheses and structures of arsenates are similar to those of phosphates when divalent metal cations such as Zn + and Be + are employed as framework tetrahedral atoms. However, there is very tittle similarity between alumino- (or gallo-) arsenates and the corresponding phosphates. In the intermediate range where both divalent and trivalent metal cations are present, some similarities between arsenates and phosphates have been observed. ... 

For solids containing covalent bonds, including salts of molecular ions and alumino-silicate framework and similar structures, potential terms akin to those used in molecular mechanics calculations on organic molecules are used. These are bond, angle and torsion terms. A simple harmonic model is often used. 

Flanigen s review describes the structures, chemistry, and potential applications of alumino-phosphate molecular sieves (ALPO4-S) [41]. There are many ALPO4 structures with a wide variety of pore sizes and shapes. The Al/P ratio in the ALPO4 framework is always 1. All ALPO4-S are therefore neutral, have no ion-exchai e capacity, and cannot be made acidic. This excludes acid catalytic applications except where the acid sites should be extremely weak. Olefin isomerization, certain aromatic alkylations, and MTO are potential applications [42]. 

The acidic properties of mesoporous molecular sieves rely on the presence of active sites in their framework. In the case of MCM-41 active sites are generated by the introduction of heteroatoms into the structure. In particular, Bronsted acid sites are introduced by isomorphous substitution of A1 for Si which is achieved by hydrothermal synthesis in which charged quaternary ammonium micelles are used as the template for charged alumino-silicate inorganic precursors. 

Fig. 25. (a) Nepheline structure as an example of alumino-silicates with framework, (b) Crystal structure of aluminate calcium C12A7. New-type superior properties such as superconductor were designed from this structure. 

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