Mg-Al spinel

Solid-state reactions are also used to produce new materials. For example, although equimolar Mg-Al spinel (i.e., MgAl204 or Mg0 Al203) powder is available commercially, nonequimolar Mg-Al spinel powders may be less easy to obtain. (A similar processing method is used to produce many of the spinel-structured ferrites.) These spinel powders may have useful properties since they can be used to produce polycrystalline compacts that deform more readily than the equimolar material. Such materials can be prepared by bring an intimate mixture of the equimolar spinel with high-purity alumina powder. A closely related process (structurally the opposite) occurs when y-alumina is transformed to a-alumina, for example, when alumina is prepared from boehmite. 

Li JG, Ikegami T, Lee JH, Mori T, Yajima Y (2001) Synthesis of Mg-Al spinel powder via precipitation using ammonium bicarbonate as the necipitant J Eur Ceram Soc 21 139—148... 

Reynen et al. [164] published a seminal account of the synthesis of ceramic powders by the emulsion technique, in which detailed comments were made on the synthesis of Mg-Al spinel particles. Special attention has been paid in this work on the mode of removal of water from ordinary precipitates or emulsions for preparation of particles. In the emulsion method, salts of Mg and Al in the desired molar ratio were dissolved in water, and the solution mixed with an equal amount of alkanes which, depending on the drying method, could be low or high boiling. For ensuring some stability of the emulsion, a surfactant (Span 80 for sulfate solutions and Bry for acetate and formate solutions 0.1-1%) was added. The necessary stirring was accomplished by very high shear forces. 

Zn-Al spinel was prepared via the wet mechanical mixing of ZnO powders with pseudoboehmlte. In some cases Sn02 powder was added to the mixed suspension. Mg-Al spinel was obtained via coprecipitation from the solutions of their nitrates. 

Ruhler et al. considered that Ba-Ru/MgO catalysts have high activities and stabilities. The researchers in Tops Company, Denmark developed series of ruthenium based ammonia synthesis catalysts support on different supports, with electronic and structural promoters. In these studies, the ruthenium catalysts supported on the Mg Al spinel and high surface area graphite show promising activity (see Table 6.6). However, the stability has some problems under industrial conditions. ... 

A more general study of the redox properties of the V2O5 oxide as a function of the metal oxide support (AI2O3, MgO, Mg-Al-hydrotalcite and Mg-Al-Spinel) was carried out using EPR. The different reducibilities of the vanadium species were related to the acid-base character of the metal oxide support. While was easily reduced to on acidic metal oxides, the reduction was hindered on basic supports, as EPR evidenced the preferential formation of from... 

Chatteijee N. (1987). Evaluation of thermochemical data on Fe-Mg olivine, orthopyroxene, spinel and Ca-Fe-Mg-Al garnet. Geochim. Cosmochim. Acta, 51 2515-2525. 

Needle like nanoparticles of Mg-Al mixed spinel catalysts synthesized under hydrothermal conditions were used for the synthesis of 1-methylimidazole by the gas phase imidazole methylation with methanol performed at atmospheric pressure [113]. High yield and selectivity to desired product were obtained at a temperature range between 320 and 350°C. It was proposed that one of the nitrogen atoms participates in the bonding of the imidazole with the basic site of the catalyst, and the second nitrogen atom is accessible for the reaction with electrophilic methyl species formed from methanol on acid site of the catalysts. 

Taftp, J. (1982) The cation-atom distribution in a (Cr,Fe,Al,Mg)304 spinel as revealed from the channeling effect in electron-induced X-ray emission. J. Appl. Crystallogr., 15, 378-81. 

The magnesium aluminate spinels that we have tested are MgAl204 and Mg Al Og. The latter is a solid solution (2,3) of pure spinel (MgAl204) and MgO. The spinel structure (4) is. based on a cubic close packed array of oxide ions. Typically, the crystallographic unit cell contains 32 oxygen atoms one eighth of the tetrahedral holes are occupied by the divalent metal ions (Mg2+), and one half of the octahedral holes are occupied by the trivalent metal ions (Al3+). 

Mullite Anorthite Gehlenite Lime Mullite Anorthite Lime (tr.) Periclase( ) Crist. Anorthite Mg-Al-Si No.2 Spinel Crist. Mg-Al-Si No.2 Spinel Anorthite Cordierite( ) Diopside( )... 

Figure 4 Covariation of Cr/(Cr + Al) and Fe/(Fe -t-Mg) in spinels from mantle xenoliths in kimberlites and other volcanic rocks. The data points are representative of the extremities of fields for a database of 2.5 X 10 analyses (source Barnes and Roeder, 2001).

Barj et al." and Pommier et al. examined thermal decomposition of Mg[Al(0-iec-Bu)4]2 in ethanol decomposition starts at 283°C, yielding an essentially amorphous product, while the reaction at 360°C yields partially crystallized MgAl204 spinel. The product consisted of secondary particles with a size of 3 pm, which is easily disaggregated by ultrasonic treatment into 0.02 pm primary particles. They also reported that thermal dehydration of ethanol becomes significant at temperatures higher than 360°C. Therefore crystallization of this product seems to occm with the aid of water formed by dehydration of the solvent. 

Millard et al. 1992 Maekawa et al. 1997), because of the distinct chemical shift ranges for " Al and Al in oxides. In spinel, " Al gives a narrow peak near +70 ppm, whereas the Al gives a broader peak near +10 ppm (Fig. 27). The tetrahedral site in spinel has cubic point symmetry which requires Cq = 0, but crystal defects and the [Mg,Al] disorder results in a small distribution of EFG s at the Al site, so that the peak at 70 ppm contains only the central transition. The earlier studies suffered from low MAS rates, which yield spinning sidebands that overlap the centerbands, and unequal excitation of the two Al NMR signals. 

In the hydrolysis of Mg Al(0-/-Pr)4 2, carried out in the presence of etha-nolamine during the preparation of the polycrystalline MgAl204 (spinel) by the SG procedure, a careful 27 A1 NMR study (557) showed that the basic framework of the bimetallic alkoxide, Mg /x-0-/-Pr)2Al(0R)2 2, does not break down, at least in the initial stages of the hydrolysis. This result confirmed that the conversion took place at the molecular level in the process. Similar indications were obtained in the conversion of the bimetallic alkoxide Mg Nb(OC2H4OMe)6 2 to the perovskite phase of PNM (558). 

Figure 1. PXRD patterns of samples Ni/Mg/Al (left), Rh/Ni/Mg/Al (right). HTlc precursors BOl and B02 (a), SAl and SA2 (b). Calcined samples BOl and B02 (c), SAl and SA2 (d). After reaction samples BOl and B02 (e), SAl and SA2 (f). MgO-type phase o spinel-type phase Ni metal phase...

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