Volume 59, Nº 4 (2018)

The selective oxidation of ethane over pure SBA-15 and V/SBA-15 were theoretically studied by density functional theory. The cluster models of pure SBA-15 and V/SBA-15 were proposed. The structure properties of these two models were calculated and were found to be in good agreement with experimental values. The catalytic reaction pathways for the ethane oxidation to acetaldehyde and ethylene were determined. Our results show that the hydroxyl groups on pure SBA-15 can activate the gas-phase O₂ to form a peroxide species, which acts as the active site for the selective oxidation of ethane. The formation of ethylene is much more preferred than that of acetaldehyde over pure SBA-15. For V/SBA-15, the peroxide species also acts as the active center. The energy barrier of C–H bond activation over V/SBA-15 is by 14.63 kJ/mol lower than that over pure SBA-15. The formation of acetaldehyde is preferred than that of ethylene over V/SBA-15. On the basis of our results, the reaction mechanisms of ethane selective oxidation over pure SBA-15 and V/SBA-15 were systematically compared and discussed. The theoretical results in this study are in good agreement with our previous experimental results. They can reasonably explain the catalytic nature of pure SBA-15 and the effect of vanadium, opening new perspectives in the understanding of the chemistry of SBA-15.

ZnO and MgO/ZnO (mass ratio 2: 5, 5: 5 and 8: 5) heterostructure photocatalysts (Mg₂Zn₅, Mg₅Zn₅ and Mg₈Zn₅, respectively) were successfully synthesized via co-precipitation method. MgO/ZnO composites were characterized by scanning electron microscopy (SEM), X-Ray diffraction and low temperature nitrogen adsorption–desorption isotherm. According to SEM images all composites consisted of spherical granules with particle sizes of 30–50 nm. The band gap value of ZnO was found to be lower than that of MgO/ZnO composites as observed during the optical studies. Pure ZnO showed lower photocatalytic activity (38%) in the degradation of 2,6-dichlorophenol (2,6-DCP) than MgO/ZnO composites. Mg₅Zn₅ composite with a higher concentration of defects in crystallites was more active in the photocatalytic degradation (79.5%) than Mg₈Zn₅ (61.2%) and Mg₂Zn₅ (63.5%). High-resolution mass spectrometry-and UV-Vis spectroscopic analysis of the by-products, derived from model pollutant 2,6-DCP, proved the successful photocatalytic performance of Mg₅Zn₅ under the UV light. The synthesized composites are future candidates against other potential environmental pollutants.

A kinetic study of ethylene oxidation to acetaldehyde by p-benzoquinone in the Pd(OAc)₂–HClO₄−LiClO₄–CH₃CN–H₂O system has been carried out under conditions when palladium(II) cationic complexes exist at a molar fraction of water of 0.67 and 30°С. For a reaction that mostly lead to the formation ofPd(CH₃CN)(H₂O)₃²⁺ two-route mechanism and a kinetic model have been proposed that describe adequately the experimental dependence of the reaction initial rate on the concentration of p-benzoquinone, HClO₄, and palladium. The model takes into account previous findings on the H₂O/D₂O and C₂H₄/C₂D₄ kinetic isotope effects and the important role of Pd(0) quinone complexes.

The kinetics of the liquid-phase hydrogenation of 2-methyl-3-butyn-2-ol (MBI) on the thin films of Pd₅₀Zn₅₀/TiO₂, Pd₈₀Zn₂₀/TiO₂, and Pd/TiO₂ was studied in a temperature range of 308–333 K at MBI concentrations of 0.1–0.6 mol/L. The films were applied to the internal walls of silica capillaries with an inside diameter of 530 μm. The reaction of MBI hydrogenation at its initial stage was of first order with respect to hydrogen, and the orders of reaction with respect to MBI on Pd₅₀Zn₅₀/TiO₂, Pd₈₀Zn₂₀/TiO₂, and Pd/TiO₂ were 0.3, 0.4, and 0.5, respectively. The yield of the target product 2-methyl-3-buten-2-ol on the bimetallic films was higher than that on Pd/TiO₂. The highest yield (94%) was achieved on the film of Pd₅₀Zn₅₀/TiO₂ at a conversion of 99%, a temperature of 313 K, and a partial hydrogen pressure of 1 atm. The higher selectivity of the reaction, which occurred according to the Langmuir–Hinshelwood mechanism, on the bimetallic films was explained by a decrease in the alkene/alkyne and alkene/alkane ratios of adsorption constants and by a decrease in the rate constants of 2-methyl-3-buten-2-ol hydrogenation.

The reaction of propane dehydrogenation on platinum–tin catalysts supported onto different woven carriers (an aluminoborosilicate and two silica materials) was studied. It was found that the catalyst was rapidly deactivated by carbon deposits formed, and the rate of this reaction increased with the specific surface area of the glass-fiber woven material and the Pt content. It was established that the Pt: Sn ratio in surface platinum particles was about 6, and it increased to 39 after the reaction; this fact is indicative of a Sn loss, which led to an increase in the conversion of feed into carbon deposits that deactivated the catalyst. A mixture of propane and 5–10 vol % H₂ should be used for the stabilization of the catalytic system; in this case, the negative effect of hydrogen on the yield of propylene was minimal. On the catalyst supported onto a silica carrier under optimum conditions (550°C; propane space velocity, 480 h^–1), which correspond to minimum selectivity for the formation of carbon deposits, the yield of propylene was ~18%. The test glass-fiber woven catalyst was inferior to granulated platinum–tin catalysts in terms of catalytic activity; therefore, its use in the reaction of propane dehydrogenation is inexpedient.

The catalytic activity of a series of the perovskite-like oxides La_1–xCa_xCoO_3–δ (x = 0–1) prepared by the Pechini method (from polymer–salt compositions) in the reaction of methane oxidation was studied. The dependence of the activity and stability of samples on their composition and reaction temperature was revealed. It was found that an increase in the calcium content of the oxides initially led to an increase in the activity (to the values of x = 0.3) and then to a nonmonotonic decrease with an intermediate maximum at x = 0.6. A decrease in the activity of oxides in the course of testing, which was most pronounced in the samples with x = 0.3, 0.6, and 1, was found. The phase compositions, specific surface areas, and microstructures of the oxides were determined before and after tests. According to X-ray diffraction analysis data, only the samples with x = 0–0.4 were single-phase ones; the samples with x > 0.4 were two-phase samples containing the phases of perovskite and brownmillerite. With the use of high-resolution transmission electron microscopy, it was found that the surface of particles was covered with the nanosized particles of simple calcium and cobalt oxides, and the particles of brownmillerite were present in the samples with x ≥ 0.4. The observed increase in the catalytic activity of the samples in a region to x = 0.3 correlated with an increase in the concentration of weakly bound oxygen in the perovskites, and a decrease at x > 0.3, with the appearance of the less active phase of brownmillerite in the samples and an increase in its concentration. The phase composition and the specific surface area of the samples remained unchanged after tests; however, planar defects were detected in the particles of perovskite and the calcium content on the surface increased. This was caused by the appearance and ordering of cationic and anionic vacancies under the action of a reaction medium, which can explain the observed changes in the activity of the samples.

The kinetics of reversible solketal synthesis in the presence of sulfuric acid as a catalyst is studied. The parameters of kinetic equations describing direct and reverse reactions are found.

Catalytic properties of monometallic Ni and bimetallic Ru–Ni supported on Al₂O₃, CaO–Al₂O₃, and MgO–Al₂O₃ have been studied in mixed reforming of methane. Physicochemical properties of the catalytic systems have been studied by X-ray diffraction, scanning electron microscopy with energy dispersive spectroscope and temperature-programmed reduction by hydrogen. It has been shown that, of all the studied samples, the highest conversion of methane and carbon dioxide is achieved in the presence of the Ru−Ni/MgO–Al₂O₃ bimetallic catalyst. Temperature-programmed reduction has confirmed the effect of hydrogen spillower from ruthenium to NiO. The formation of Ru–Ni alloy has also been found.

The MgAl layered double hydroxides (LDHs) were prepared by two-stage synthesis, which included mechanochemical activation at the first stage and the interaction of the resulting sample with distilled water at the second stage. The influence of the material of grinding bodies (steel and ceramics), the conditions of activation (activation time and the centripetal acceleration of balls), and the nature of initial compounds on the phase composition of the resulting products was investigated. It was established that the formation of a single-phase MgAl LDH was observed upon mechanochemical activation with the use of steel grinding bodies at an acceleration of 1000 m/s² for 30 min. The samples prepared by a traditional coprecipitation method and a method that included a stage of mechanochemical activation possessed identical structural parameters. However, the mixed oxides formed upon the calcination of LDHs synthesized by mechanochemical activation were characterized by a more uniform pore space with a pore diameter of 4–5 nm with a developed specific surface.