Aluminates structure

Aluminum forms mixed oxides with many other metals which either replace Al3+ in octahedral positions or occupy tetrahedral sites. The spinel structure of MgAl204, an important prototype, has been investigated by photoelectron spectroscopy.122 Vibrational and solid state NMR methods are also useful for studying aluminate structures.12 125... 

Singh, R.N. Fracture strength of a porous lithium aluminate structure for application in molten carbonate fuel cells. Proc. Ceram. Sci. Eng. 1981, 1 (7-8)(B), 500-507. 

Sim, J.W. Singh, R.N. Kinoshita, K. Testing of sintered lithium aluminate structures in molten carbonate fuel cells. J. Electrochem. Soc. 1980, 127 (8), 1766-1768. 

On the other hand, Kato et al. (1988) studied the addition of different rare-earth oxides to V-AI2O3 in a 5/95 ratio. They found that in those systems where (1-aluminates were detected after firing at 1473 K the surface area was always higher than in those cases where only (X-AIO3 was detected. They concluded that the common feature of thermally resistant supports is that they have a layered aluminate structure which retards support sintering. 

One of the keys to the properties of the hexa-aluminates is the crystal structure. The hexa-aluminate structure consists of blocks of spinel-structure divided by mirror planes in which the large A ions are situated. This results in a layered structure. There are two different structure types, the magnetoplumbite and P-alumina structure where, for example. La and Sr in the A-position will yield magnetoplumbite while Ba yields the fl-alumina structure. ... 

Several ions have been substituted into the hexa-aluminate structure both in the A and B positions. The B ions seem to have the most pronounced effect on the catalytic activity of the hexa-aluminate materials. This catalytic effect seems to be closely correlated to the change in oxidation state of B ions between +2 and +3. Several different B ions have been studied. " For BaMAlnOi9 (where M is a transition metal) the trend for methane combustion, measured as the temperature for 10% conversion, was Mn, Cuhigh activity for the Mn could be correlated to its ability to shift oxidation state. ... 

The parallelism between the decomposition of the dried ammonium-exchanged zeolite by heat depicted in the two previously discussed reactions and the acid exchange of the ammonium zeolite discussed previously is obvious. The aluminate structure on losing the stabilizing positive ion shifts into the stable crystalline form under the conditions of both transformations. 

Classical MPV reduction (Al(Oi-Pr)3 and i-PrOH system) has several disadvantages. That is, stoichiometric amount of Al(Oi-Pr)3 and excess amount of 2-propanol as hydride donor are needed for smooth reaction. In addition, reaction rate is relatively slow. Since these reasons, development of effective catalysts for MPV reduction and/or Oppenauer oxidation is an important task in synthetic organic chemistry. Recently, to solve these problems highly active aluminum catalysts were reported. Maruoka and coworkers reported that bidentate Lewis acids (75a) and (75b) derived from bis-aluminated phenoxide (17) and various secondary alcohols effectively catalyze MPV reductions (Scheme 6.58). In these catalysts, bis-aluminated structure, which is coordinated by one carbonyl oxygen in a double coordination manner, is crucial for high catalytic activity [76]. 

Figure 3.10 The stability fields for dioctahedral (a) and trioctahedral (b) 2 1 structures as a function of layer charge and the site of the layer charge. The figures are for aluminous structures (a) and magnesian structures (b).

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