Pore structure octahedral/tetrahedral

Nature of Vanadium Species. The data on the characterization of V- containing silicalite indicate the presence of various types of vanadium species (i) a polynuclear V-oxide containing V in various valence states (V , V and V ), (ii) octahedral sites, preferentially interacting with OH groups localized inside the pore structure of the zeolite crystals, (iii) nearly symmetrical tetrahedral species, attributed to a V species directly interacting with the zeolite framework, and (iv) after reduction, V species in a nearly tetrahedral environment. 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

Zeolites. A1 n.m.r. has been used to discriminate between framework (tetrahederally co-ordinated) and extra-framework (octahedrally co-ordinated) A1 in a number of natural and synthetic zeolites with different framework structures.65,72 7S 78 94 95 It appears that signal intensities can be used quantitatively to determine the relative amounts of the two forms of A1 present, but some caution must be exercised for non-ordered materials.78 Heat treatment of zeolites tends to remove the tetrahedrally co-ordinated A1 from the framework and deposit it in the zeolite pores as the octahedrally coordinated form A1(H20)63 +. This process has been monitored by 27A1 n.m.r.75... 

Curved structures are not only limited to carbon and the dichalcogenides of Mo and W. Perhaps the most well-known example of a tube-like structure with diameters in the nm range is formed by the asbestos mineral (chrysotil) whose fibrous characteristics are determined by the tubular structure of the fused tetrahedral and octahedral layers. The synthesis of meso-porous silica with well-defined pores in the 2-20 nm range was reported by Beck and Kresge.6 The synthetic strategy involved the self-assembly of liquid crystalline templates. The pore size in zeolitic and other inorganic porous solids is varied by a suitable choice of the template. However, in contrast to the synthesis of porous compounds, the synthesis of nanotubes is somewhat more difficult. 

Solid phosphates show a huge variety of crystal structures, and it is not practical to classify them in terms of structural types as is done with simple oxides, halides, etc. However, some general classes of metal phosphate structures will be considered three-dimensional frameworks of linked phosphate tetrahedra and tetrahedrally or octahedrally coordinated cations, layered phosphates, and phosphate glasses. In all of these materials the size and topology of pores within the structure are of importance, as these determine the ability of ions and molecules to move within the structure, giving rise to useful ion exchange, ionic condnction, or catalytic properties. Ion exchange can also be nsed to modify the properties of the host network, for example, the nonlinear optical behavior of potassium titanyl phosphate (KTP) derivatives. 

Mesoporous TiOa has been synthesized in the presence of a non-ionic surfactant by assembly, using a Ti alkoxide as Ti source. The porous structure partially collapses upon calcination. However, this fact can be avoided by extraction of the surfactant with boiling acid/ethanol mixtures. Thus, TiOa samples with surfaces areas up to 470 m /g and pore sizes in the range 2-6 nm have been obtained. DR UV-Vis spectra of the as-synthesized samples show the presence of Ti species with both tetrahedral and octahedral coordinations, which resemble that of anatase powder. 

Figure 2.17 The large pore (12MR) aluminophosphate structures (top left) AIPO4-5 and (top right) AIPO4-36 and the extra large pore (18MR) VPI-5 (below) possess one-dimensional channel systems. In the as-prepared VPI-5 (shown) aluminium exists in both tetrahedral and octahedral coordination, the octahedral coordination being made up by two coordinated water molecules.

In general, the surface of pure silicate mesostructures is weakly acidic. It is found that the incorporation of metal ions into the framework can introduce acidic and ion-exchange functionality and catalytically active sites. Various metal ions, such as Al +, Ti " ", V +, Ga +, and Fe +, have been incorporated into S BA-15 to enhance its catalytic performance. In contrast to zeolites, which have crystalline structures, the incorporation of metal ions in mesoporous silicates caimot be strictly defined as intra- or extra-framework incorporation since these ions are highly dispersed on the framework. A wide range of compositions with different coordination numbers and chemical environments can contribute to amorphous framework structures. For example, both tetrahedrally and octahedrally coordinated aluminum in S BA-15 are involved in the formation of the amorphous pore walls, and may be defined as intraframework Al. The former may exist inside the pore walls, while the latter may be located on the pore surface. 

Ions in Clays, Fig. 1 Structure of montmorillonite clays. An octahedral (O) aluminuin oxide layer is sandwiched between two tetrahedral (T) silicon oxide layers. Substitutions of A1(III) by Mg(II) in the T layer result in a pmnanent negative charge compensated by counterions (here sodium Na ), located in the interlayer pores and can be hydrated by water molecules. Al and Mg atoms are in green. Si ia yellow, Na in blue, oxygen in red, and hydrogen in white... 

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