Volume 60, Nº 2 (2019)

The kinetics of thermal decomposition of bis(η³-allyl) nickel complexes in various media is studied. The specific features of the mechanism are determined, including the combination of the stages of trans-cis isomerization of Niall₂ and the bimolecular decomposition of the cis-isomer with diallyl formation. The effect of autocatalytic decomposition of complexes with metallic nickel is been detected. The qualitative dependences of the process rate on the nature of the solvent and the structure of the allyl ligand are determined. The activation parameters of individual steps, consistent with quantum chemical calculations, are found.

The review is a brief summary of the great scientific heritage of Georgii Konstantinovich Boreskov, the member of the Russian Academy of Sciences and a famous Russian scientist in the field of catalysis, chemical kinetics, and chemical technology. Boreskov was a staunch proponent of the chemical theory of catalysis. He developed the main ideas of this theory in heterogeneous catalysis. These primarily include: the chemical nature of intermediate interaction between reactants and a catalyst; the key role of the bond energy of intermediate surface compounds in the rate and direction of reactions; dynamic unity of the system “reactants–catalyst”; approximate constancy of the specific catalytic activity of substances of the same chemical composition and structure. Boreskov made a significant contribution to the theory of kinetics of catalytic reactions and the development of the method of mathematical modeling. He showed the importance of the porous structure in the preparation of practical catalysts. Technologies of many processes were developed with his lead or direct participation. Ideas of the Boreskov’s theory of catalysis form the basis for the modern science of catalysis and drive its development.

Kinetic study of cholesterol oxidation has been studied using a series of N-cetylpicolinium dichromates (CPDC), a class of phase transfer oxidants, in acetic acid medium under first order conditions with respect to oxidant. Rate constants were calculated in the temperature range 290–300 K. The kinetics was followed spectrophotometrically; cholest-5-en-3-one is found to be the only oxidation product. Unlike the previously reported lipopathic oxidant containing cetyltrimethylammonium ions, these oxidants show a direct variation of rate with the oxidant concentration ruling out any reversed micellar organization of the oxidant molecules. From the experimental data formation of an unstable cyclic transition state followed by intra-molecular proton transfer has been proposed. Solvent isotope effect for the cholesterol oxidation (\({{{{k}_{{{\text{CHC}}{{{\text{l}}}_{3}}}}}} \mathord{\left/ {\vphantom {{{{k}_{{{\text{CHC}}{{{\text{l}}}_{3}}}}}} {{{k}_{{{\text{CDCC}}{{{\text{l}}}_{3}}}}}}}} \right. \kern-0em} {{{k}_{{{\text{CDCC}}{{{\text{l}}}_{3}}}}}}}\) = 1.52) indicated a carbon-hydrogen cleavage rather than a carbon-carbon cleavage. Variation of solvent polarity is found to impose a remarkable impact on the rate of oxidation: more polar reaction environment favours the oxidation by β-CPDC oxidant to a higher extent, compared to the other two oxidant isomers, α-CPDC and γ-CPDC.

The exchange process between carbon monoxide and nitrous oxide CO + N₂O \( \rightleftarrows \) СO₂ + N₂ resulting in СO₂ formation is studied. The methods of quantum mechanics are used to calculate transition states, vibration frequencies and reaction pathways. The direct process of oxygen atom transfer and a multiple-step process via several intermediate complexes are considered. In the framework of the transition state theory, the constants of the forward and backward reactions are calculated. A critical analysis of experimental data available in the literature is carried out based on the comparison with the results of calculations The recommended rate constants are presented in the general Arrhenius form for a wide range of temperatures: 300–2500 K.

The work describes an efficient one-pot synthesis of 2,4,6-trisubstituted pyridine derivatives through a three-component catalytic reaction. The procedure involves mesoporous organic polymer based on calix[4]resorcinarene in and functionalized by N-propyl sulfamic acids a solid acid catalyst. Excellent yields, short reaction times, catalyst reusability, and easy reaction handling under solvent-free and mild acidic conditions are the most important advantages of the present protocol. The acidic catalyst was characterized by a number of spectroscopic methods such as Fourier-transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and elemental CHNS analysis.

A series of Nb modified CeAlO_x catalysts were prepared by hydrothermal method and tested in the selective catalytic reduction of NO with NH₃. The catalyst with an Nb/Al molar ratio of 1 : 1 exhibited the best catalytic activity and high resistance to sulfur poisoning. Nitrogen adsorption measurements, XRD, NH₃-TPD, H₂-TPR and XPS analyses as well as in situ DRIFTS technique were applied to characterize the catalysts. The characterization results showed that the introduction of Nb enhanced the redox power and increased surface acidity. Moreover, the modification enriched the catalyst surface with adsorbed O_α and Ce³⁺ species. In addition, in situ DRIFTS experiments demonstrated that doping with Nb increased catalyst adsorption capacity for NH₃ while decreased the adsorption capacity for NO + O₂. All of these factors played important roles in improving NH₃-SCR performance of the modified catalysts.

The effect of preparation procedures (a polymer ester precursor method and incipient wetness impregnation) on the physicochemical and functional properties of Ni/CeO₂ catalysts with different nickel contents (0–15 wt %) was studied in order to develop highly active and carbonization-resistant catalysts for hydrocarbon reforming. Based on the results of studying the samples by low-temperature nitrogen adsorption, X-ray phase analysis, Raman spectroscopy, transmission electron microscopy and temperature-programmed reduction with hydrogen, it was found that the textural, structural, and redox properties of the materials depend on the method of their synthesis. As compared with the samples prepared by impregnation, the Ni/CeO₂ catalysts obtained by the polymer ester precursor method were characterized by different active component stabilization forms (a Ce_{1 –x}Ni_xO_y solid solution phase and NiO particles <5 nm in size vs. a NiO phase with a particle size of 5–50 nm), a smaller average size of CeO₂ crystallites (5.5 vs. 11 nm), a high specific surface area (105 vs. 75 m²/g), a defect structure, and a decreased reducibility. It was found that the samples of both series provided comparable yields of hydrogen (to 50% at 600°C) in an autothermal ethanol reforming reaction, but the Ni/CeO₂ catalysts synthesized by the polymer ester precursor method were more resistant to the formation of carbonaceous deposits.

The physicochemical properties of alumina–platinum catalysts with supports prepared by mixing aluminum and zirconium hydroxides have been studied. The chemisorption and nonoxidative conversion of methane in the presence of the above systems have been investigated. The effect of the zirconium content on the activity and selectivity of the conversion of chemisorbed CH₄ and n-C₅H₁₂ into aromatic hydrocarbons has been established.