Cages, aluminum-oxygen

Cr(H20) 3. It consists of a central chromium joined to six H20 molecules, exactly as the fluorines are arranged around aluminum in Figure 22-2. The oxygen portion of each H20 molecule is turned toward the central chromium and the H portions point away from the center. The corners of the cage that holds the central Cr atom are occupied by six oxygen atoms, each of... 

The following review is concerned with the synthetic and structural chemistry of molecular alumo-siloxanes, which combine in a molecular entity the elements aluminum and silicon connected by oxygen. They may be regarded as molecular counterparts of alumo-silicates, which have attracted considerable attention owing to their solid-state cage structures (see for example zeolites).1 3 Numerous applications have been found for these solid-state materials for instance the holes and pores can be used in different separation techniques.4,5 Recently the channel and pore structures of zeolites and other porous materials have been used as templates for nano-structured materials and for catalytical purposes.6 9... 

Figure 3. Variation of contact angle and atomic percent oxygen as a function of time of treatment ((, 15% Ft, no cage (A., A) 3% Ft, aluminum cage (9, O) 15% F, aluminum cage reaction cortditions 3.0 mm, 40 cc/min, 50 W).

The structures contain channels in one, two, or three directions. When one channel intersects another, there may be a larger cage. The channels contain the alkali or alkaline earth metal cations (i.e., the exchangeable cations and water). Diagram 6.2690 shows the channels of ZSM-5. Structure 6.2791 is in zeolites X and Y. Each corner represents a silicon or aluminum atom. There is an oxygen atom on each line between them. I and II indicate where cations might be. 

The crystal structures of 4 ammonium exchanged, heat-treated faujasites were determined from x-ray powder data. Structure I, often called decationated Y, has lost 15 framework aluminum atoms and 21 framework 0(1) atoms (bridging oxygen atoms) per unit cell, and 15 Al(OH)2+ ions are present in the sodalite cages. Structure 11, called ammonium-aluminum Y hydrate, shows a complete rehydroxyla-tion of the vacant 0(1) positions. Structure III, called ultrastable Y, shows the same 15 framework aluminum atoms absent, and the removal of 25 0(3) and 13 0(h) framework oxygen atoms. Structure TV, which is a repetitive exchanged and heat-treated version of Structure 111, has a mean Si-O bond length of 1.610 A, which indicates that little framework aluminum is present. 

In Structure II, which we call ammonium-aluminum Y hydrate, the occupancy factors for all the framework positions were unity. Site Sn is occupied by 13.4 aluminum ions, and Sm sites are filled by 15.0 hydroxyl groups per unit cell. The 5.9 ions located in Sv sites are thought to be ammonium ions. The (approximately) 2 aluminum ions present per sodalite cage are coordinated to the same 2 hydroxyl groups from Sm sites at a distance of 1.77 A. The nearest framework oxygens are 3 0(3) oxygens at 3.02 A. The mean Si,Al-0 bond length is 1.65 A. 

Figure 1 Ball-and-stick reprentation of nitrate sodalite. Black spheres represent oxygens, white spheres represent nitrogens, dark gray spheres represent sodium. The sodalite cages are represented by sticks. Only silicon and aluminum atoms are shown.

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