Ekaterina A. Ponomareva

The formation of mixed oxides in La₂O₃/MO–Al₂O₃ systems (M is a group IIA element: Mg, Ca, Sr, Ba) subjected to a high-temperature treatment and treatment in a water fluid (WF) medium is studied. It is shown that the high-temperature treatment of a group IIA element aluminate with supported lanthanum nitrate leads to the formation of LaAlO₃ only in the case of an Mg-containing system. In other cases, original aluminates do not undergo transformations, and lanthanum oxide is formed. The treatment of La/M–Al ternary systems in a WF medium leads to the formation of LaAlO₃ in all cases; in addition, in all samples, except for the Ba-containing system, phases of free oxides of group IIA metals appear. The increase in the activity and selectivity of La/M–Al ternary oxide systems in the oxidative coupling of methane compared to the respective parameters of M–Al binary systems is attributed to the presence of an individual La₂O₃ oxide phase.

The effect of the method used to synthesize Ln–Al (Ln = La, Ce, Pr) mixed oxide systems with an Ln : Al atomic ratio of 1 : 1 on the formation of their phase composition and their catalytic properties in the oxidative coupling of methane (OCM) is studied. The precursors are prepared by impregnating ashless filter paper with mixed solutions of nitrates of the respective metals by the incipient wetness impregnation method and subsequent drying and combustion of the resulting mass in air. Further treatment is conducted by combining calcination at 600 and 900°C with a treatment in a water fluid (WF) or water–ammonia fluid (WAF) medium. The laws governing the transformation of the amorphous precursor of Pr–Al oxides during treatment in WF and WAF media and high-temperature synthesis are similar to those observed for the La–Al system. In both cases, the amorphous precursor in a water-containing fluid is transformed into LnAlO₃ with a cubic perovskite structure with an admixture of AlO(OH) (boehmite) and basic REE carbonate phases. The subsequent treatment in air at 900°C leads to the formation of a mixture containing LnAlO₃ aluminates and free La₂O₃ or PrO₂ oxides. Single-phase samples containing exclusively lanthanum and praseodymium aluminates are synthesized by heating amorphous precursors in air at 900°C. The treatment of the Ce–Al system in a WF or WAF leads to the formation of a well-crystallized CeO₂ oxide instead of aluminate or Ce-containing hydroxides, while the Al-containing component remains X-ray amorphous. Cerium aluminate CeAlO₃ is synthesized by treating a mixture of cerium and aluminum oxide precursors in a hydrogen stream. It is found that, due to differences in the 4th ionization potential (IP4) values of the La, Ce, and Pr atoms (49.9, 36.7, and 39.0 eV, respectively), completely different synthesis conditions are required to form LnAlO₃ aluminates with a perovskite structure that contain REE in the (3+) oxidation state. The catalytic properties of the synthesized samples in the OCM are studied. The efficiency and stability of isostructural LnAlO₃ aluminates in the OCM decreases in the following order: La > Pr > Ce. Despite the fact that PrAlO₃ is the most active of these aluminates, LaAlO₃ exhibits the highest selectivity for OCM products (ethane + ethylene).

The catalytic properties in the oxidative coupling of methane (OCM) and the structural features of a series of lanthanum aluminum mixed oxides with a constant La : Al atomic ratio of 1 : 1 were studied. Samples were prepared from precursors containing lanthanum and aluminum nitrates and an organic component: filter paper or starch. After drying and burning of the organic component, the samples were subjected to additional heat treatment and treatment with water fluid (WF) with a density of 0.2 g/cm³ at 415°C. After calcination at 900°C, all the samples contained a phase of lanthanum aluminate LaAlO₃ with a cubic perovskite structure. Varying the type of organic component and the sequence of procedures and treatment conditions gave samples with different morphologies and catalytic properties. The minimum activity, selectivity for OCM products, and stability over time was demonstrated by the sample obtained under conditions conducive to the formation of the most ordered structure, namely, the sample sequentially calcined at 900°C, treated in a WF medium, and recalcined at 900°C. The most efficient and stable sample turned out to be the one whose crystal structure was formed mainly in the WF medium. It was noted that there is no correlation between the morphology of the particles and the specific surface area of the samples, on the one hand, and their catalytic properties, on the other. It was assumed that the catalytic properties are determined by the type and number of point defects in the crystal structure—primarily cation vacancies and, as a consequence, hole sites of the [O^–] type in the anion sublattice.

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 (S_sp) while maintaining the crystalline structure; further calcination at 900°C did not lead to a change in either S_sp or the structure. Treatment of an amorphous precursor in the WF medium increased S_sp 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 S_sp 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.

Metal–hypercrosslinked polystyrene (HCP) composites active in the hydrogenation of unsaturated hydrocarbons are obtained by impregnation of HCP with palladium and rhodium compounds in a supercritical carbon dioxide (SC-CO₂) medium followed by their reduction to the metallic state with molecular hydrogen. The influence of the conditions of preparation of the composites on their activity and selectivity in the hydrogenation of diphenylacetylene (DPA) is evaluated. The adsorption of palladium hexafluoroacetylacetonate on HCP from an SC-CO₂ medium is studied for the first time using supercritical fluid chromatography; it is shown that the adsorption equilibrium is described by the Langmuir equation.

The formation of mixed oxides in the Al₂O₃–MO (M = Mg, Ca, Sr, Ba) systems by heat treatment and treatment with water fluids (WFs) is studied. It is shown that during the high-temperature treatment of mixtures of aluminum hydroxide Al(OH)₃ with group 2 metal nitrates, mainly MAl₂O₄ aluminates are formed, sometimes with an admixture of aluminates of a different composition. If the same mixtures are pretreated in WF (400°C, fluid density 0.2 g/cm³), the composition of the sample is more complex because of dehydration and partial structuring of the Al-containing component during the treatment in the WF medium. It is demonstrated that the addition of ammonia to WF has a stimulating effect on the formation of double oxides, primarily due to the alkaline hydrolysis of the Group II metal salts and the formation of their hydroxides; and the reduction of nitrates and nitrites with ammonia is likely to be a secondary process.

The effect of the synthesis method on the formation of the structure and composition of composites produced by the deposition of lanthanum oxide compounds on porous alumina is studied. The effect of the morphology of porous alumina (α-modification) on the formation of various compounds (La₂O₃ oxide, as well as LaAlO₃ and La₁₀Al₄O₂₁ aluminates) during the thermal treatment of a support impregnated with a solution of lanthanum nitrate is shown. It is found that when systems are treated in a water fluid (WF), the processes of structuring and phase formation proceed at a much lower temperature and more intensely than during the thermal treatment, which is due to the mobility of the structural elements in the hydrated state. It is shown that the formation of LaAlO₃ aluminate is limited by the hydration of the initial aluminum oxide, which proceeds more intensely in the case of its γ-modification. Varying the sequence and conditions of the stages of the treatment in a WF and the thermal treatment makes it possible to obtain composites that have different morphologies and contain various oxide compounds and phases.