Kinetics and Catalysis

The catalytic properties of samples of amorphous mesoporous aluminosilicate ASM with different Si/Al molar ratios (40, 80, 160) were studied in the synthesis of practically important pyridines (by the interaction of С₂–С₅ alcohols with formaldehyde and ammonia, cyclocondensation of acetaldehyde and propionic aldehyde with ammonia), dialkylquinolines and alkyltetrahydroquinolines (by reaction of aniline with C₃, C₄ aldehydes) and alkyldihydroquinolines (by interaction of aniline with ketones, acetone and acetophenone). It is found that mesoporous aluminosilicate ASM sample with a molar ratio of Si/Al = 40, which has the highest acidity among the studied samples, exhibits the highest activity and selectivity in these reactions.

The subject of this review is related to the development of research on the mechanism of interaction of platinum complex compounds, primarily, H₂[PtCl₆], with alumina supports and the role of this interaction in properties formation of the corresponding platinum catalysts. Based on the study of the composition of Pt(IV) chloride complexes adsorbed on the γ-Al₂O₃ surface, it was proposed that the complexes should be quantitatively discriminated as inner- and outer-sphere complexes in accordance with the nature of their binding with the support. The chloride ligands were found to be dominant in the bound outer-sphere Pt(IV) complexes, and adsorption of the complexes on the surface occurs due to the protonation of the hydroxyl groups of the support without any significant changes in the composition and geometry of the [PtCl₆]^2– complex anion. The binding of Pt(IV) in the outer-sphere complexes leads (after their reduction with hydrogen) to the formation of Pt particles with a relatively low dispersion containing platinum atoms mainly in a nearly Pt⁰ state, which are characterized by hydrogenating and dehydrogenating activity. The inner-sphere platinum(IV) complexes are the hydrolyzed forms of the chloride complexes formed by the replacement of chloride ligands by the hydroxyl groups of the support. Their reduction forms finely dispersed platinum particles containing a significant fraction of atoms in the ionic form, which increases the activity of the Pt/Al₂O₃ catalyst in dehydrocyclizations of n-alkanes. Several approaches were proposed that make it possible to vary the ratio between the outer- and inner-sphere platinum(IV) complexes (increasing the degree of hydrolysis of the complexes, or varying the nature of the surface OH groups of the alumina support) and hence to modify the properties of the catalyst. The possibility of selective synthesis of outer- and inner-sphere platinum(IV) complexes bound with other supports was shown for the Pt/MgAlO_x system. Using the structural features of layered aluminum-magnesium hydroxides makes it possible to bind platinum(IV) in the complexes of the given type at high selectivity and to change the electronic state of Pt atoms in Pt particles and their activity in alkane dehydrogenations without changing the chemical composition of the catalyst.

The antiradical activity of the associates of 5-hydroxy-6-methyluracil with succinic acid, whose compositions differed in the succinic acid content, in a model system for the initiated radical chain oxidation of 1,4-dioxane was quantitatively studied. The parameter characterizing the inhibitory activity of the test associates was measured: fk₇ = (3.2 ± 0.1) × 10⁴ L mol^–1 s^–1 (333 K). The antioxidant activities of 5-hydroxy-6-methyluracil and its associates with succinic acid were compared.

The kinetic and thermodynamic parameters of the decomposition of cumyl hydroperoxide in a chlorobenzene medium in the presence of calcium 2-ethylhexanoate and a complex of CaCl₂ with crown ether were experimentally obtained. The formation of intermediate complexes in the catalyst–hydroperoxide system was kinetically established. The formation of intermediate complexes was confirmed using quantum chemical modeling, their structure was optimized, and the thermodynamic characteristics of reactions with their participation were found. A mechanism of the stepwise decomposition of the intermediate was proposed.

X-ray photoelectron spectroscopy (XPS) was used to study the state of the surface layer of a Pd–P catalyst, which exhibits high selectivity in the hydrogenation of 2-ethyl-9,10-anthraquinone. The preparation of the catalyst from Pd(acac)₂ and white phosphorus in hydrogen in a toluene–1-octanol solution led to the formation of Pd–P nanoparticles whose surface was enriched in electron-deficient palladium, apparently, as a constituent of an amorphized 2D layer based on a solid solution of Pd and P. Octyl esters of phosphoric acids, adsorbed on the surface, acted as stabilizers. A relationship between the properties of the Pd–P catalyst in the hydrogenation of 2-ethyl-9,10-anthraquinone and the state of the surface layer was considered.

The interaction of rhodium foil with nitrogen dioxide over a temperature range of 30–450°C at three NO₂ pressures of 10^–6, 10^–5, and 10^–4 mbar was studied using X-ray photoelectron spectroscopy (XPS). At all of the three pressures, a three-dimensional surface oxide film with the Rh₂O₃ stoichiometry was formed on the foil surface starting from a temperature of 150°C. As the interaction temperature was increased at an NO₂ pressure of 10^–6 mbar, the film thickness d initially increased to reach a maximum of ~2.0 nm at 350°C and then decreased due to the decomposition of Rh₂O₃ to the metal. At pressures of 10^–5 and 10^–4 mbar, a monotonic increase in d up to 3.0 or 4.7 nm, respectively, was observed in a temperature range of 150–450°C. In the case of the interaction with the participation of molecular oxygen at a pressure of 10^–4 mbar and temperatures of 30–450°C, the Rh foil surface was not oxidized. The results of the study on the interaction of NO₂ with Rh were compared with the data obtained earlier for Pd and Pt under comparable conditions.

The structure and catalytic characteristics of a bimetallic catalyst containing Pd₁In₁ nanoparticles deposited on the surface of γ-Al₂O₃ were studied. The formation of intermetallic nanoparticles was determined by X-ray diffraction analysis and confirmed by X-ray photoelectron spectroscopy and IR spectroscopy of adsorbed CO. In the hydrogenation of acetylene in excess ethylene, PdIn/Al₂O₃ had significantly higher selectivity of ethylene formation (~86%) than monometallic Pd/Al₂O₃ (~35%). The high selectivity of PdIn/Al₂O₃ is explained by two factors: (1) the formation of monatomic Pd₁ sites isolated from one another by In atoms and (2) the change in the electronic state of Pd atoms in the intermetallic nanoparticles.

The Sr_{2 –x}La_xTiO₄ (x = 0–2.0) catalysts were synthesized based on strontium titanate with a layered perovskite structure. The effect of the degree of substitution of La for Sr on the physicochemical (phase composition and textural characteristics) and catalytic properties of oxides in the oxidative condensation of methane at temperatures of 700–800°C were studied. It was found that multiphase Sr_2 – xLa_xTiO₄ samples with the degree of substitution x = 0.8–1.8 were most active and selective in the test reaction; this was likely related to the presence of lanthanum oxide and strontium oxide impurities in them, their optimum distribution over the surface, and the specific surface area.