Kinetics and Catalysis

The temperature dependence of the Fe-HZSM-5 activity and selectivity in the process of catalytic oxidation of ethane by the excess of N2O at 250–350°C exhibits a pronounced hysteresis. The oxidized catalysts free from condensation products are active only in the complete oxidation of ethane. At low temperatures of the reaction of the C2H6 + N2O mixture with the catalyst, coke formation takes place and the coordination state of iron ions differs from the initial sample. Under these conditions, the process of complete oxidation of ethane is essentially suppressed and the process of oxidative dehydrogenation dominates. The catalytic properties of iron-containing zeolites prepared either by direct synthesis or by introduction of iron ions into the cationic positions of H[Al]ZSM-5 are quite similar, because irreversible formation of new iron species considerably different from the initial species takes place during the catalytic reaction on both series of samples. The activity of HZSM-5 containing trace amounts of iron is much lower than that of iron-containing samples.

Heteropoly compounds (HPCs) with the general formula CsMHPVMo11O40are prepared and tested as catalysts. The influence of elements entering the formula on the catalyst properties is studied: Cs defines the acidity and specific area, V controls the selectivity, and the transition metal M defines the mobility of oxygen in the bulk and the catalyst activity. The mechanism of methacrolein oxidation over HPCs is investigated. Using the response method and mass spectrometry of the reaction mixture, it is shown that only the catalyst oxygen atoms take part in the formation of methacrylic acid and that the transport of active oxygen to adsorbed methacrolein plays a key role in the oxidation process. A correlation between the HPC activity and the redox ability of the metal cation M n+ ↔ M n+ i (i= 1 or 2) is found. New catalysts for methacrolein oxidation to methacrylic acid are developed on the basis of this correlation. These are the salts of PVMo-poly acid with Cs, Cu, and the transition metal M as cations. These catalysts are more active (a conversion of up to 91%) and selective (up to 98%) compared to conventional catalysts for methacrolein oxidation to methacrylic acid.

The alkylation reaction of aniline with methanol on zeolites HY and CsOH/CsNaY was studied by in situ 13C NMR spectroscopy under flow and batch conditions. Attention was focused on the identification of intermediates and on the determination of the formation mechanisms of N-methylaniline, N,N-dimethylaniline, and toluidines. To refine the main steps of the reaction, the transformations of the following individual compounds and intermediates, which were detected in the course of alkylation, were studied: dimethyl ether, surface methoxy groups, methylanilinium ions, formaldehyde, and N-methyleneaniline. It was found that N-methylaniline and N,N-dimethylaniline were formed as a result of aniline methylation by methanol dehydration products (methoxy groups or dimethyl ether) on acidic zeolites or as a result of alkylation by formaldehyde or methoxy groups on basic zeolites. Toluidines were formed by the isomerization ofN-methylanilinium ions, which were produced only on acidic zeolites, rather than by the direct alkylation of aniline.

The kinetics of cyclohexene hydrocarbomethoxylation catalyzed by the Pd(PPh3)2Cl2-PPh3-p-toluenesulfonic acid (TSA) is reported. The reaction is first-order with respect to cyclohexene and TSA and of order 0.5 with respect to Pd(PPh3)2Cl2. The reaction rate as a function of CO pressure or methanol or PPh3 concentration passes through an extremum. The chloride anion inhibits the reaction. A mechanism involving cationic hydride complexes as intermediates is suggested. A rate equation is set up by the quasi-steady-state treatment of experimental data.

Nonwoven chitosan (CS) nanofiber mats were successfully prepared by the electrospinning of the mixture of CS and poly(ethylene oxide) (PEO) in acetic acid aqueous solution. The CS/PEO fiber mats were treated with glutaraldehyde aqueous solution to stabilize fibers in solution. The concentration of glutaraldehyde is important for incorporating swelling properties in the cross-linked CS/PEO fiber mats. The cross-linked CS/PEO fibers (CCS/PEO fibers) were then used as supports for palladium catalysts in the Mizoroki–Heck reaction. The results of the study demonstrated that the catalytic activities of Pd catalyst supported on CCS/PEO fiber (Pd-CCS/PEO fiber) were highly dependent on the concentration of glutaraldehyde in the cross-linking process. Density functional theory (DFT) calculations indicated that the Schiff bond formed between CS and glutaraldehyde could reduce the energy needed to form a chelate complex between the CCS/PEO fibers and palladium active species. This in turn could decrease the activation energy of the Mizoroki–Heck reactions which occur in the presence of the Pd-CCS/PEO fiber catalysts. The optimized Pd-CCS/PEO fiber catalyst was very efficient and stable in the Mizoroki–Heck reaction of aromatic iodides with olefins.

The inhibiting effect of multiatomic gases has been investigated by numerical simulation of hydrogen oxidation with account taken of the nonequilibrium state of the initial components, intermediates, and reaction products behind the shock wave in the framework of a vibrationally nonequilibrated model. The central feature of the model is successively taking into account the vibrational disequilibrium of the HO2 radical as the most important intermediate in the chain branching process. The inhibiting effect can be explained by the influence of the multiatomic gases on the rate of the vibrational relaxation of the vibrationally excited HO2 radical resulting from the reaction. Methane, tetrafluoromethane, fluoromethane, difluoromethane, chlorofluoromethane, formaldehyde, ethane, hexafluoroethane, ethylene, tetrafluoroethylene, and propane have been considered as inhibitory admixtures.

The potential of surface self-propagating high-temperature synthesis (SSHS) for obtaining (CuO-CeO2)/glass cloth catalysts is demonstrated. The dependence of the structural and catalytic properties of the catalysts on their preparation conditions (nature of the fuel component) is considered. X-ray diffraction, electron microscopy, and EXAFS data suggest that the short-term action of high temperature in the SSHS leads to the complete decomposition of the precursors and has an effect on the distribution of the resulting phases. According to H2 TPR and XPS data, the degree of dispersion of CuO and the electronic state of the reacting CuO and CeO2 phases depend on the choice of fuel. This is likely due to fuels varying in the amount of heat released in their combustion. The degree of dispersion of CuO and the total contribution from Cu1+ and Ce4+ to the electronic state of the active component increase as the standard enthalpy of combustion increases in the urea < glycerol < citric acid order. This leads to an increase in the catalytic activity of the (CuO-CeO2)/glass cloth system in selective CO oxidation.

The kinetics of isothermal water sorption by the CaCl2/silica gel composite initiated by a small stepwise pressure rise over the sample has been investigated at a constant underlying plate temperature of 35°C. The initial portion of the kinetic curves is consistent with Fick’s diffusion model: the amount of sorbed water increases in proportion to the square root of the sorption time. This makes it possible to determine the effective diffusivity of water (D eff). At small amounts of sorbed water (w < 0.19 g/g), D eff changes slightly. The diffusivity of water in the composite pores (D) calculated for the same conditions is close to the Knudsen diffusivity of water vapor in mesopores. The D eff value grows with an increasing water content of the composite; that is, sorbed water accelerates water transport in the pores. This is likely due to the appearance of an extra diffusion channel, namely, diffusion through the aqueous solution of the salt, whose formation begins on the silica gel surface at w > 0.1 g/g. The contribution from this channel increases markedly when the amount of adsorbed water is above 0.25 g/g. This can be explained by the formation of the “connected” phase of the solution in the pores.