Yurii D. Ivakin

Thermovapour treatment (TVT) of SnO₂ and CaO oxides in an autoclave at 673 K leads to the formation of isometric CaSnO₃ perovskite-type crystals. TVT at 623 K leads to a previously unknown CaSn₂O₄(OH)₂ phase with lamellar crystals. The structure of the new compound CaSn₂O₄(OH)₂ was determined using powder X-ray diffraction and characterized with scanning electron microscopy, thermogravimetry–differential scanning calorimetry (TG-DSC) analysis, and IR spectroscopy. CaSn₂O₄(OH)₂ crystallizes in space group Aba2 with Z = 4. The crystal structure consists of double layers formed from [SnO₆] octahedra alternating with layers formed from [CaO₈] tetragonal antiprisms. The TG-DSC study shows that the new compound CaSn₂O₄(OH)₂ is stable up to 973 K. The thermodynamic parameters of CaSn₂O₄(OH)₂ analysized using DFT in comparison with CaSn(OH)₆ and CaSnO₃. The synthesized lamellar CaSn₂O₄(OH)₂ exhibits high catalytic activity and selectivity to C₆₊ products in the aldol condensation of acetone at 673 K and 12.0 MPa.

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).

Objectives. To obtain porous corundum ceramics using an innovative cold sintering process starting from different phase modifications of aluminum oxohydroxide — boehmite γ-AlOOH and diaspore α-AlOOH; to study the phase and structural properties of the resulting materials; and to assess their permeability to water.Results. Cold sintering enables the formation of single-phase corundum ceramics with an open porosity of 47.9% directly from the initial boehmite powder with the addition of 5 wt % corundum in the presence of 20 wt % water at a temperature of 450°C, mechanical pressure of 220 MPa, and isothermal exposure for 30 min. Under the same conditions of cold sintering, a mixture of diaspore and boehmite was transformed into α-AlOOH ceramics. This then turned into corundum with an open porosity of 39% when calcined in air at 600°C for 1 h. The resulting materials had permeability for pure water above 5000 L/(m²∙h∙bar).Conclusions. Cold sintering is a promising approach to producing porous corundum ceramics which can be used in filtration systems. Compared to traditional ceramic technology, the new approach reduces energy, time, and labor costs in the material manufacturing. It also eliminates the need to use auxiliary substances (binders, pore-forming agents, etc.).

Porous α-Al₂O₃ ceramics are a highly sought-after material with a multitude of applications; for example, they are used as filters, substrates, biomedicine materials, etc. Despite the availability of raw materials, a challenge associated with this technology is the high energy budget caused by sintering above 1500 °C. For the cold sintering processing (CSP) of ceramics, lowering the α-Al₂O₃ sintering temperature is one of the most urgent challenges in the background of its rapid development. This paper is the first to demonstrate a solution to this problem using the CSP of α-alumina ceramics in the presence of pure water as a transient liquid. The manufactured materials were examined using XRD analysis; the evolution of their microstructures during CSP was revealed by SEM; and the porosity was evaluated using the Archimedes method. Ceramics with an open porosity up to 36% were produced at 380–450 °C and 220 MPa in 30 min. An increase in the pressure was found to impede α-Al₂O₃ formation from γ-AlOOH. The development of the microstructure was discussed within the framework of the dissolution–precipitation model and homogenous nucleation. The results of the SEM study pointed to the coalescence of γ-AlOOH grains during CSP.

Porous α-Al2O3 ceramics is a highly demanded material for multitude applications such as filters, substrates, biomedicine materials, etc. Despite the availability of the raw materials, a challenge of their technology is in a high energy budget caused by the sintering above 1500 °C. For a cold sintering processing (CSP) of ceramics, the lowering of α-Al2O3 sintering temperature is one of the most urgent challenges at the background of its rapid development. The current paper for the first time demonstrates the solution of the mentioned problem by the CSP of α-alumina ceramics in a presence of pure water as a transient liquid. The manufactured materials were examined by XRD analysis; the evolution of their microstructures during the CSP was revealed by SEM, and the porosity evaluated by the Archimedes method. The ceramics with an open porosity up to 36 % was produced at 380-450 °C, 220 MPa in 30 min. An increase in the pressure was found to impede α-Al2O3 formation from γ-AlOOH. The development of the microstructure was discussed within the frameworks of the dissolution-precipitation model and the homogenous nucleation. The results of the SEM study pointed to the coalescence of γ-AlOOH grains during the CSP.