Coordinated phosphate silicon

Other framework structures based on zeolites have also been synthesized which contain atoms other than aluminium and silicon, such as boron, gallium, germanium, and phosphorus, which are tetrahedrally coordinated by oxygen. Such compounds are known as zeotypes. Pure aluminium phosphate, commonly called ALPO, and its derivatives, can take the same structural forms as some of the zeolites such as sodalite (SOD), faujasite (FAU), and chabazite (CHA) (e.g., ALPO-20 is isostructural... 

Silicon and aluminum, of course, are not unique in their ability to form tetrahedrally coordinated oxide networks. The element phosphorus, at the right of silicon in the periodic table, frequently assumes tetrahedral coordination with oxygen. With phosphorus in the +5 oxidation state as phosphate, aluminum phosphate possesses many structural similarities to silica 1) A1P0 is isoelectronic with S120 . 2) The average of the ionic radii of... 

It will help here to mention the structure of polar counterparts to SiC)2. These are constructed exactly as aluminum phosphide is constructed from silicon in the simple tetrahedral solid. The process is illustrated for the simple molecular lattice in I ig. 11-5, which shows how the structure for SiOj can be transformed to the structure for aluminum phosphate. Transferring an additional proton leads to magnesium sulphate. Indeed, the counterpart of each AB tetrahedral semiconductor, ABO4, is po.ssible in principle. The structures may be obtained from Wyck-hoff (1963), and if the structure has twofold-fourfold coordination, it can be analyzed by the methods outlined here. 

The synthesis and characterization of a mesoporous silicoalumino phosphate are given employing typical structure directing reagent viz. cetyltrimethyl ammonium bromide. The Si MAS NMR showed that silicon is found in coordination with 1, 2, 3 and 4 A1 ions through oxygen bridges. The sorption characteristics of this new mesoporous SAPO material are also presented. Keywords Silico aluminophosphate, Mesoporous, ordered molecular arrays, MAS NMR. 

Other framework structures containing atoms such as aluminium and phosphorus tetrahedrally coordinated by oxygen have been synthesised and are given the generic name zeotypes. Pure aluminium phosphate (commonly known as ALPO) and its derivatives have been found to take the same structural forms as some of the zeolites, such as sodalite and faujasite, as well as some novel structures. The metal-aluminium phosphates can be formed with metals such as Li, Be, Mg, Mn, Fe and Zn replacing some of the aluminium and these are called MeALPOs. If the compound contains silicon or silicon and a metal, partially replacing aluminium or phosphorus leads to SAPOs and MeSAPOs. 

Silicon phosphates. These compounds contain silicon with an octahedral coordination of O atoms, as in Si2P207, Si50(P04)g and (NH4)2SiP40i3. In these compounds measured Si-0 distances are about 10% greater than when tetrahedral coordination is involved, and ionic character for the bonds, with SP+ present, is indicated (Chapter 3.4). Neither P nor S has any tendency to form such octahedral arrangements, although arsenic has. 

Figure 2, 3D comparison of the small SRP RNA domain. Models were constructed on a Silicon Graphics Indigo 2 Extreme workstation with the program ERNA-3D (S) with considerations as described in the text. Views of the small domains of the SRP RNAs of Methanococcus jannaschii (Met. jan.) and Homo sapiens (Horn, sap.) are shown with the sugar-phosphate backbones as narrow tubes and the base paired helical sections as cylinders. Labeling of the RNA ends, the helices, and the pseudoknot are precisely as in Figure 1. RNA sections of the M jannaschii model that correspond to nucleotides absent in the human model are shown dark-gray. (The PDB coordinates of the three-dimensional models are available from the SRP database at the internet address http //www.uthct.edu/SRPDB/SRPDB.html (2).)...

The effect of phosphoric acid on silica has been examined by Mitsyuk (193), who found that HjPO is adsorbed on silica gel from water. The amount corresponds to about one HjPO molecule per surface silicon atom, indicating formation of a surface compound of silicon phosphate. Once treated with as little as 0.06% HjPO, the rate of dissolution of the silica in water is greatly reduced. The addition of H2SO to the dilute H,PO< solution prevents the formation of an adsorbed film and accelerates dissolution. In strong HjPO there is further reaction as silica is converted to SiPjOr in which silicon is coordinated with six oxygen atoms. 

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