Aluminosilicate zeolites zeolite framework types

The surface character of the AlPO molecular sieves differs from that of the silica molecular sieves even though both framework types are neutral with no extra-framework cations. The molecular sieve silicalite is hydrophobic and the AlPO molecular sieves are moderately hydrophilic. Zeolites are hydrophilic due to the interaction of the dipole of the Hz0 molecule with the electrostatic fields of the anionic aluminosilicate framework and the balancing nonframework cations. The hydrophilicity of the AlPOi, materials is apparently due to the difference in electronegativity between Al(1.5) and P(2.1). Neither mechanism is possible with silica molecular sieves. The AlPOi, molecular sieves do exhibit less affinity for HzO than the hydrophilic zeolites such as Type A and Type X. 

A. These local geometrical parameters are in harmony with the conclusion drawn from the calculations of Shah et al. that HSAPO-34 is less acidic that its aluminosilicate analogue, the HSSZ-13. Note however, that this conclusion does not seem to be generalizable to other frameworks. Experimental evidence seem to support the view that the relative acidity of the zeolite and the SAPO belonging to the same zeotype varies from one framework type to another. 

Table 7 lists the currently known aluminosilicate zeolites based upon definition by their unique frameworks following Baerlocker, Baerlocher, Meier, and Olson. It also contains typical unit cell compositions, the lUPAC code, and type localities, but is restrictive in the sense that it does not consider the mineralogical aspects of the study of zeolites. The authoritative body for these is the Subcoimnittee for Zeolite NomenclaMe of the Conunission on New Minerals and Mineral Names of the International Mineralogical Association. Their guidelines have been cited in a recent book on natural zeolites by Tschemich, as follows. 

Hydrocarbon sorbate vibrations. IINS spectra have been recorded for a number of simple sorbate molecules within aluminosilicate zeolites, including hydrogen in A (40, 41). acetylene in X (4, ethylene in A (42) and X (44-46). and p-xylene (42) in X type materials. In addition to intramolecular modes, where interaction between the sorbate and the non-framework cations is strong (for example in the ethylene - silver zeolite A system (42)), vibrational transitions associated with sorbate motion with respect to the zeolite s internal surface can be observed. The latter modes, and the dependence of their frequencies on loading, structure and composition are of particular interest as they convey detailed information about the character of the zeolite - sorbate... 

The nomenclature used throughout this paper designates the structural type by the letter of the most closely related phosphorus-free synthetic zeolite as used in the Union Carbide laboratories (4, 5). Table II in Ref. 5 contains a description of the synthetic zeolites described here. For the aluminosilicophosphate zeolites, these letter designations are prefixed by P to indicate that the zeolite framework contains phosphorus substituted in the tetrahedral (Si, Al) site. For example, the phosphate zeolite structurally related to Type A zeolite is designated P-A P-R zeolite is the phosphate zeolite related to zeolite R, a synthetic sodium aluminosilicate zeolite with a chabazite type framework structure (5). The aluminosilicophosphate zeolites will be abbreviated to phosphate zeolites or phosphorus-substituted zeolites throughout this paper. 

Aluminophosphates denoted as AlPO-n (n means the framework structure) are a new family of molecular sieves with unique zeolitic pore structure. Since the first report on the synthesis of AlPO-n by Wilson et al., synthesis of various types of AlPO-n as well as the isomorphous substitution of metal cation into the AlPO-n structure have been reported. At present, more than 50 different crystal structures of AlPO-n have been reported, which is comparable to that of aluminosilicate zeolites, making the aluminophosphates an important class of microporous crystals." ... 

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