Synthesis of Mixed La–Al Oxides by Treatment in a Water Fluid Medium and Their Catalytic Properties in Methane Oxidation

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Abstract
The synthesis of mixed La–Al oxide systems with the atomic ratio La : Al = 1 : 1 was studied. At the first stage, a dried mass containing starch and La and Al nitrates was burned. Heat treatment of the resulting amorphous product at a temperature of ≥700°C gave LaAlO3 aluminate with a cubic perovskite structure. The composition and morphology of the product formed by subsequent treatment in a water fluid (WF) medium (density 0.2 g/cm3, 415°C) depend on the degree of ordering of the precursor. Crystalline LaAlO3 (cubic) in the WF medium undergoes additional ordering with a decrease in the specific surface area (Ssp) while maintaining the crystalline structure; further calcination at 900°C did not lead to a change in either Ssp or the structure. Treatment of an amorphous precursor in the WF medium increased Ssp and produced a mixture of LaAlO3 aluminates of cubic and orthorhombic structures and La and Al hydroxides. Further calcination at 900°C gave a mixture of LaAlO3 (cubic), La2O3 (hexagonal), and, possibly, X-ray amorphous Al2O3. The synthesized systems were studied as catalysts for methane oxidation. There was no correlation between activity and selectivity for oxidative coupling products (ethane + ethylene) with the Ssp value; they were found to depend on the phase composition of the mixed La–Al oxide. The most efficient systems turned out to be those that underwent intermediate treatment in the WF medium and contained the LaAlO3 (cubic) and La2O3 (hexagonal) phases. The results obtained indicate the high structural sensitivity of the process characteristics.

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