Kinetika i kataliz

Investigations on the development of homogeneous chloride-free two-stage process (1st stage – target reaction, 2nd stage – catalyst regeneration) for oxidizing С₃Н₆ to acetone with oxygen in the presence of catalyst \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + Mo-V-Р heteropoly acid (HPA-х, х is the number of V atoms) have been presented. The kinetic equation of the target reaction has been obtained, and its mechanism has been proposed. It has been shown that catalysts based on high-vanadium modified (non-Keggin) compositions HPA-x_m are the most efficient. The kinetics of C₃H₆ oxidation in the presence of Keggin HPA-x and HPA-x_m solutions has been the same, but only the \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + HPA-x_m catalysts have turned out to be technologically feasible. These have a high thermal stability (up to 180°C), which allows the catalyst to be rapidly regenerated with oxygen. This favorably distinguishes the catalysts based on HPA-x_m from Keggin HPA-x, the thermal stability of which is limited to 140°C. The possibility of rapid regeneration of the catalyst has made it possible to close the 2-stage catalytic cycle of C₃H₆ oxidation to acetone with oxygen and has opened up prospects for the practical use of the process in the presence of \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + HPA-x_m. The catalyst has been successfully tested for stability.

The influence of the formation of microheterogeneous soot particles on the gas-phase conversion of rich mixtures of methane with oxygen into synthesis gas in the temperature range from 1500 to 1800 K under the conditions of an adiabatic reactor was studied by kinetic modeling. The effect of CO₂ and H₂O additives on this process was studied. The appearance of soot particles is observed in rich mixtures, starting from the fuel excess factor ϕ = 3.33. At relatively low temperatures ~1500 K, a small amount of microheterogeneous soot particles is formed, which do not significantly affect the other components of the reacting system. A noticeable effect of soot particles at this temperature is observed at a higher value of ϕ = 8.0. This is most clearly manifested in the temperature profile of the process, in which, with the addition of water, two maxima are observed at times of the order of 0.01 and 0.1 s. In the case of CO₂ additions, the second maximum on the temperature profile is almost not pronounced. A complex temperature profile leads to the appearance of the second maximum concentration of OH hydroxyl radicals at times of ~0.1 s. The addition of H₂O and CO₂ makes it possible to vary the H₂/CO ratio in the synthesis gas over a wide range, which is necessary for the synthesis of various products. Since the added CO₂ under these conditions is actually involved in the chemical process of obtaining synthesis gas, its partial recirculation from the conversion products makes it possible to reduce its emission during the production of synthesis gas.

It is assumed that in the reaction of alcohol oxyethylation, the kinetically independent unit is not a monomeric alcohol molecule, but a linear chain alcohol associate, consisting, on average, of n alcohol molecules. Тhe reaction rate is of the first order not by the gross concentration of alcohol, but by the concentration of these associates. It has been suggested that first-order reactions with respect to the associated component, which are widespread in liquid-phase chemical kinetics, take place as a result of the release of the reaction product from the associate.

The effect of ascorbic acid and α-tocopherol on the radical chain oxidation of tetrahydrofuran was studied at a temperature of 309 K. The reaction was initiated by 2,2'-azo-bis-isobutyronitrile. The process rate was monitored by the oxygen absorption. The rate constant of the tetrahydrofuran peroxyl radical reaction with α-tocopherol were measured k₇ = (2.9 ± 0.6) × 10⁵ l mol^–1 s^–1. The effect of an aqueous solution ascorbic acid on the oxidation rate of tetrahydrofuran was studied. The rate constant of the tetrahydrofuran peroxyl radical reaction with ascorbic acid were measured k₇ = (10.2 ± 1.0) × 10⁴ l mol^–1 s^–1. The effect of ascorbic acid on the tetrahydrofuran oxidation inhibited by α-tocopherol is considered. It has been shown that when both inhibitors are used together, the induction period is equal to the sum of the induction periods of individual compounds. The effective inhibition rate constant in this case is equal to k₇ = (2.5 ± 0.5) × 10⁵ l mol^–1 s^–1.

The main reasons for the promoting effect of phosphorus on the properties of Pd–P/ZSM-5 palladium catalysts in the direct synthesis of H₂O₂ from H₂ and O₂ under mild conditions are considered based on data from X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and inductively coupled plasma mass spectrometry (ICP MS). It is shown that the introduction of phosphorus into the composition of the catalyst affects the dispersity, the electronic state of palladium in the surface layer, and the surface concentration of phosphate and phosphite ions. An increase in the H₂O₂ yield is favored by an increase in the dispersion of Pd–P-catalysts, inhibition of the side process of H₂O₂ decomposition by surface phosphate and phosphite ions, and a decrease in the solubility of hydrogen in solid solutions of phosphorus in palladium.

Pd-containing catalysts (1%Pd/Al₂O₃ and 5%Pd/Al₂O₃) deposited on aluminum oxide were studied in the decomposition reaction of hydrazine monohydrate. According to in situ IR spectroscopy, it was found that hydrazine monohydrate is adsorbed on the coordination unsaturated centers of the catalyst surface in a linear form. When the temperature rises, the adsorbed hydrazine monohydrate loses a water molecule, which is accompanied by a change in the geometry of the molecular complex. Adsorption of hydrazine on a support and its diffusion onto palladium clusters is a more advantageous process than direct adsorption on active centers. This circumstance shows that the hydrazine adsorbed on the support can be an intermediate of its decomposition process. The studied catalysts have a maximum activity in the temperature range of 100–120°C, while the ratio of hydrogen and nitrogen concentrations in the reaction products was equal to 2, which corresponds to 100% selectivity for hydrogen. As the reaction temperature increases, the selectivity decreases significantly. The explanation of the high selectivity for hydrogen at low temperatures is due to the fact that the adsorption of N₂H₄ is carried out through the formation of hydrogen–metal bonds. The hydrogen–metal bond strength in such a complex is higher than the nitrogen–metal bond strength, hence the barrier for breaking the N–H bond is lower than the barrier for breaking N–N bond, which leads to breaking N–H bond and preserving the N–N bond. At elevated temperatures, some of the hydrogen atoms formed recombine, the other reacts with the surface complexes of hydrazine to form the intermediate NH₃–NH₃, the breaking of the bond in which leads to the formation of ammonia molecules in the gas phase.

Mordenite and ferrierite (Zeolyst International, H-form, SiO₂/Al₂O₃ ≈ 20) were used to study the adsorption, thermally programmed desorption, and carbonylation of dimethyl ether (DME). The behavior of mordenite with Cu, Co, and Mg cations introduced by ion exchange has also been studied. DME carbonylation was carried out at 200°C, pressure 3 MPa, space velocity 8000 ml g^–1 h^–1 in a mixture, vol. %: ⁓2.2 DME, 92.8–95.5 CO, rest. N₂. After the induction period, the methyl acetate content is about 4–5 times higher for mordenite compared to ferrierite. The formation of water, methanol and hydrocarbons was observed in small quantities. The introduction of Cu, Co, Mg cations into mordenite by ion exchange (single ion exchange, cation/Al ratio no more than 35%) not only increased the stability, but also increased the activity in the DME carbonylation reaction. It was found that an increase in the content of copper (from 1.19 to 2.23 wt %) and Mg (from 0.62 to 1.8 wt %) differently affects the activity. In the case of copper, an increase in activity was observed, while in the case of magnesium, the activity decreased. Preliminary reduction of copper-exchange mordenite leads to a decrease in activity and the appearance of metallic copper particles on the surface of mordenite crystallites. According to in situ diffuse reflectance infrared spectroscopy, the introduction of magnesium cations by triple ion exchange leads to a noticeable decrease in the number of Brønsted acid sites (BACs) in both the 12-MR and 8-MR channels of mordenite. The catalytic characteristics of ferrierite practically do not change when copper and magnesium are introduced by ion exchange.

In the case of Au/C catalysts have been shown advantages of application SAXS with masking liquid technique for determining supported metal particle sizes compared with TEM and XRD as standard methods. According to SAXS data particle size distributions of gold in a wide range (1–50 nm) were obtained with full consideration of all size fractions of particles present in the samples under study. Also values of the mass fraction of “X-ray amorphous” gold particles with size less than 4 nm (W_SAXS) were determined. It has been found that the oxidative treatment of the carbon support before deposition of metallic gold precursor complexes has a significant effect on the size distribution of gold particles in the final catalyst. Comparison of the results of measuring the rate of CO oxidation by an excess of moist air at 40°C on Au/C catalysts with the (W_SAXS) values found for these catalysts showed that the catalytic activity increases exponentially as (W_SAXS) increases. High activity in CO oxidation reaction was demonstrated by Au/C catalysts with (W_SAXS) ≥ 80%.

The possibility of using ruthenium within the composition of palladium-rhodium catalysts used to neutralize the exhaust gases of vehicles with gasoline engines was investigated. Thermolysis of the supported precursor in oxidizing and reducing media leads to the formation of samples with different initial catalytic activity. Stability of trimetallic systems is compared under conditions of prompt thermal aging with reference samples of similar chemical composition, but obtained by mechanical mixing of monometallic catalysts. The obtained results allow one to conclude that the alloy nanoparticles are more stable, and the thermolysis of the precursor in the reducing medium contributes to a higher initial activity of the catalyst in the oxidation of CO. According to the X-ray photoelectron spectroscopy method, the high thermal stability of trimetallic catalysts is due to the constant ratio of metals on the surface of the support during the catalytic reaction.