Vol 60, No 3 (2019)

Norbornadiene allylation by allyl formate in the presence of palladium catalytic systems has a number of features associated with different directions of β-hydride transfer in key intermediates, which can be carried out with the participation of allyl, norbornenyl, or formyl fragments. The value of the kinetic isotope effect and the nature of the limiting step in the reaction of hydroallylation of norbornadiene are determined using deuterated reagents.

The kinetics of the interaction of glutathione (GSH) with hydrogen peroxide (H₂O₂) was studied. It was shown that the rate of GSH consumption nonlinearly depended on reactant concentrations and the process was accompanied by the appearance of radicals with a relatively low rate, which was a fraction of a percent of the rate of GSH consumption. Based on the experimental results and literature data on the reactions of GSH with H₂O₂ and thiyl radicals, a kinetic model of the complex interaction of GSH and H₂O₂ in an aqueous solution at 37°C was proposed. The model includes 15 quasi-elementary reactions with corresponding rate constants, including the formation of the intermediate complex GSH–H₂O₂ and its subsequent reactions with the formation of final products. Computer simulation based on the model developed satisfactorily described the reaction kinetics in a wide range of reactant concentrations.

The paper presents data on the effect of iron metal complexes (ferrocene and dimer cyclopentadienyldicarbonyl iron) on the homo- and copolymerization of methyl methacrylate and butyl methacrylate initiated by benzoyl peroxide. It was shown that, in the presence of the metal complexes, the shapes of kinetic curves changed, and the compositions of copolymers and the initial mixtures of monomers almost coincided; that is, the resulting copolymer composition corresponded to the azeotropic composition. The copolymerization constants were close to unity. The presence of metallocenes affected the microstructure of the copolymers and their molecular-weight characteristics. These changes were explained by the formation of macromolecules with the participation of both free radicals and coordination active centers formed in the presence of the metal complexes.

Catalytic systems 2 wt % Pd/Al₂O₃ were prepared using noncalcined boehmite (NC) and two types of alumina support: one was prepared by the calcination of boehmite at 600°C (C) and the other produced by Engelhard (E). To prepare 2 wt % Pd/HPC–Al₂O₃ samples, these supports were modified by impregnation by a heteropoly compound (HPC) (20 wt % Н₈[Si(W₂O₇)₆] ⋅ 6Н₂О). The effect of the Al₂O₃ structure and its modification by the heteropoly compound on the physicochemical properties, activity, selectivity and stability of catalysts in the reaction of multiphase hydrodechlorination of 1,3,5-trichlorobenzene (TCB) was studied. All catalysts showed activity in the considered reaction with the predominant formation of benzene but were deactivated in the reaction medium. Modification by the heteropoly compound resulted in increased stability and was especially effective for catalyst supported on Al₂O₃(E). The method of scanning electron microscopy (SEM) was used to determine the morphological differences of supports. According to the data of transmission electron microscopy, all catalysts contained palladium in the form of particles less than 20 nm in size. The particle size and width of the size distribution increases in the series Pd/Al₂O₃(NC) < Pd/Al₂O₃(C) < Pd/Al₂O₃(E). Modification by the heteropoly compound was favorable for the decrease in the size of palladium particles. The method of temperature-programmed reduction with hydrogen (TPR-H₂) showed that all catalysts included in their composition palladium hydride along with more strongly surface-bound metal forms that are reduced at elevated temperatures, and their content decreases after modification by the heteropoly compound and increases after catalytic tests. According to diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the deposition of a heteropoly compound leads to a change in the type of Lewis acid sites on the alumina surface and in the electronic state of palladium. According to the results of infrared spectroscopic studies of adsorbed CO, the relatively large particles of Pd⁰ are the main form on the surface of nonmodified catalysts. The catalysts modified by the heteropoly compound contain single Pd⁺ and Pd²⁺ cations, and the fraction of Pd⁰ is substantially smaller. The specific features of the Lewis acidity of the catalyst surface determine the possibility of 1,3,5-trichlorobenzene adsorption and activation on the support and the spillover of hydrogen from Pd⁰. An increase in the catalyst stability as a result of support modification by the heteropoly compound can be explained by the appearance of new active sites in the interaction of palladium with the heteropoly compound or its thermal decomposition products.

Using the dependence of the activation energy of 4-alkyl-1,3-dioxane formation on the pore diameter of zeolites, the catalytic activity of the latter in the Prins reaction was theoretically explained. The stabilization of a prereaction π-complex in the cavities of a number of zeolites (\({\text{Na}}_{x}^{ + }\)(H₂O)_y)[Al_aSi_bO_c], Al_aP_bO_c, and Ca_x[H₂O]_yAl_aSi_bO_c) is studied by molecular dynamics methods. It is shown that the dependence of the stabilization energy of the π-complex and the transition state on the diameter of the cavity has an extremal shape. Taking into account the stabilization energies of the π-complex and the transition state in the zeolite cavity, the change in the activation energy of the 4-alkyl-1,3-dioxane formation is determined depending on the pore size. A comparison is made of the nature of changes in the transition state stabilization energy and the activation energy of the cavity diameter. It is demonstrated that the dependence of the activation energy on the diameter gives a narrower interval of optimal pore sizes.

This paper presents the results of a kinetic study on the step of activation of aryl iodides and aryl bromides under conditions of ligand-free catalysis in the direct arylation of indole at a C–H bond performed using an analysis of the integral kinetic curves of consumption of the competing aryl halides. In the course of an experiment under the conditions of competition between a pair of aryl halides having different reactivity, an acceleration in the consumption of a less active substrate was observed, which coincided in time with the almost complete consumption of a more active one. The experimental data unambiguously indicated a rapid character of the stage of oxidative addition with the participation of not only reactive aryl iodides but also nonactivated aryl bromides.

The effect of selenium additives on the structure, physicochemical, and catalytic properties of VMoTeNbO catalysts in the oxidative dehydrogenation of ethane was studied. It was shown that the introduction of selenium on the first stage of catalyst preparation (mixing of the initial solutions) makes it possible to obtain in the course of subsequent heat treatment highly crystallized samples, which mainly include the M1 phase. Despite the fact that, in the process of heat treatment, selenium is completely removed from the samples, the prepared catalysts differ markedly from unmodified samples and demonstrate a higher selectivity to ethylene at a conversion of ethane of 93%. The observed effects are due to the influence of selenium on the process of formation of the M1 phase resulting in increase of its crystallinity.

Based on the Sibunit carbon composite, Ru–Cs(Ba)/Sibunit catalysts with Cs(Ba) : Ru molar ratios of 0.5, 1.5, and 2.5 were obtained. The catalytic activity of the obtained promoted catalysts in the decomposition of ammonia and their thermal resistance to methanation are compared. In the reaction of ammonia decomposition, the specific activity (W_sp) of barium-containing catalysts proved to be 2 times lower than the specific activity of cesium-containing samples with the same molar M : Ru ratio. Thus, W_sp at 500°С was 29.4 mmol H₂ min^–1 for 0.5Cs–Ru/Sibunit and 15.4 mmol H₂ g^–1_cat min^–1 for 0.5Ba–Ru/Sibunit. It was shown that the introduction of barium at molar ratios Ba : Ru = of 0.5–2.5 significantly increases the thermal stability of the samples defined as the ratio of the weight of hydrogen obtained on the catalyst to the weight of carbon subjected to methanation. There is 52 g H₂ for nonpromoted Ru/Sibunit catalyst, 370 g H₂ for the 0.5Ba–Ru/Sibunit sample, 200 g H₂ for the 1.5Ba–Ru/Sibunit sample, and 150 g H₂ for the 2.5Ba–Ru/Sibunit sample per 1 g of carbon loss.