Aluminum oxide coordination number

The catalytic activity of aluminas are mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as well as to the high ionicity of the surface Al-O bond [67,92]. The number of such very strong Lewis sites present on aluminum oxide surfaces depends on the dehydroxylation degree and on the particular phase and preparation. Depending on the activation temperature, the density of the strongest Lewis acid sites tends to decrease as the calcination temperature of the alumina increases (i.e., upon the sequence y — 5 —> 9, which is also a sequence of decreasing surface area and increasing catalyst stability). 

Aluminum is the most abundant metal and the third most abundant element in the Earth s cmst, behind only oxygen and silicon. Its low weight and useful properties make aluminum and its alloys valuable materials for manufacturing and electrical applications. Inorganic compounds of aluminum are plentiful and used as absorbents, catalysts, ionic conductors, ceramics, and electrical materials. Organometalhc compounds of aluminum are also of great industrial importance and fundamental discoveries continue to be made regarding the variety of coordination numbers, structures, oxidation states, and reactivity exhibited by aluminum. ... 

Now we will overview some experiments that reveal the specificities of the Jahn-Teller effect in diluted crystals. First of all, we will discuss a justification of their relaxation origin. We have mentioned before that the first experiments were done on the crystals of aluminum oxide (corundum), yttrium aluminum garnet, yttrium iron garnet, and lithium gallium spinel doped with a number of 3d ions [10,11]. The main result was the discovery of attenuation maximum which was considered to be observed at cot 1 and reconstruction of the relaxation time temperature dependence. In some experiments reported later both the velocity and attenuation of ultrasound were measured as functions of the temperature. They were done on ZnSe and ZnTe crystals doped with transition metals. These crystals have the zinc-blende structure with the Jahn-Teller ion in tetrahedral coordination. The following... 

In alkali fluoroaluminate solid compounds, aluminum is present only in octahedral coordination with fluorine. According to Spearing et al. (1994) and Smith and Van Eck (1999), their Al chemical shifts range between -13 and -1.4 ppm and are typically more shielded than the AlOe octahedrons in oxide compounds. Only a few studies report lower coordination numbers for Al in fluorides. Kohn et al. (1991) have described the Al MAS NMR spectra of glasses of jadeite mixed with cryolite in terms of the 5-fold and 6-fold coordination of aluminum at 22 and -5 ppm, respectively. Herron et al. (1993) reported a Al chemical shift at 49 ppm for the tetrahedral anion AIFJ in a [l,8-bis-(dimethylamino) naphtathalene H ] [AIFJ] saturated solution. 

Other X-ray spectroscopic studies include cadmium adsorption on aluminum oxides (Papelis et al. 1995), where it was found that Cd-O distances and coordination numbers (6 oxygens) on two aluminas agree with Cd-O distances and coordination in aqueous solutions. In low sorption density samples, absence of cadmium... 

The oxidizing properties of thallium(III) in both aqueous and non-aqueous media obviously preclude the existence of a number of compounds analogous to those found for aluminum, gallium and indium in this oxidation state. Thus, while there is much richer chemistry for thallium in the +1 state, there is a concomitant decrease in the detailed information on thallium(III) coordination chemistry. 

It also seems of interest to compare the stabilities of tetrahedral states of aluminum and boron in terms of this simple qualitative picture. The calculations have revealed that the planar structure is preferential for B(OH)3 as well. The value of r for B(OH)3 is 30.1 kcal/mol, that is, it exceeds substantially the value for Al(OH)3. This is a manifestation of the difference in the covalent radii of B and Al. The value of Ec for the B(OH)3 + HzO system is no longer sufficient to compensate Er, that is, in this system the tetrahedral coordination of the boron atom is unfavorable. However, in the B(OH)3 + OH" system, Ec = 88.4 kcal/mol, that is, the presence of the highly electron-donating oxygen atom of OH- should make the tetrahedral state of boron preferential. Actually, the tetrahedral boron is known to exist in a number of mixed oxides. 

Unlike aluminum, titanium is tetravalent and can exhibit different oxidation states. Thus TS-1 is non-acidic if isomorphously substituted. TS-1 is relatively difficult to synthesize, which is probably one of the reasons the site structure was under debate for quite some time. TS-1 can only be synthesized with a maximum of 3 wt% Ti if more Ti is added extra-framework titanium is formed. Inihally, it was suggested from XAS that octahedral sites are formed in the silicalite framework [60]. However, later more and more groups suggested tetrahedral isomorphous substitution of the titanium sites in the MFl framework [61-63], It is now more or less generally accepted that the Ti is four-coordinate and has a Ti—O distance of 1.79-1.81 A. This is a significantly increased distance compared to the Si—O distance 1.605 A, which is constant for a vast number of oxides [4]. The increase in... 

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