Mechanism of the Low-Temperature Interaction of Hydrogen with α-Oxygen on FeZSM-5 Zeolite

The mechanism of a low-temperature reaction of hydrogen with the radical anion surface oxygen species (-oxygen, ) formed by decomposing N₂O over FeZSM-5 zeolite was studied using kinetic and isotope techniques. It was found that the reaction is of first order with respect to hydrogen and the rate of the reaction is proportional to the concentration of . The activation energy of the reaction, which was measured for ₂ or D₂ over a temperature range from +20 to –;;100°, is equal to 3.2 or 5.3 kcal/mol, respectively. The reaction occurs with a considerable kinetic isotope effect (k /k _D), which varies over the range of 3.4–;;41 depending on the temperature. This fact indicates that the rate-limiting step of the reaction includes the dissociation of the hydrogen molecule. The temperature dependence of the isotope effect gave a value of 2.1 kcal/mol, which is close to the difference between the zero bond energies in the molecules of ₂ and D₂; this fact suggests that a tunnel effect does not significantly contribute to the reaction. The dissociative mechanism is consistent with data obtained by in situ IR spectroscopy. The interaction of hydrogen with -oxygen is accompanied by the formation of new hydroxyl groups (absorption bands at 3635 and 3674 cm^–;;1) at the surface of the zeolite. The identification of these groups was supported by an isotope shift either on the replacement of ₂ by D₂ or on the replacement of ¹⁶ by ¹⁸. The stoichiometric ratio ₂ : is consistent with the previously drawn conclusion on the paired arrangement of -sites.     

Hydration of dimethyl ether to methanol over solid acids

The hydration of dimethyl ether (DME) to methanol over various solid acids was studied. The acidity of the catalysts is determined by FTIR spectroscopy. The hydration is found to occur predominantly on the Br?nsted acid sites. Among the catalysts studied, WO_x/ZrO₂ and H-ZSM-5 appeared to be most active and selective.     

Relation between the catalytic and acidic properties of zeolite catalysts

Influence of the strength and concentration of non protic centers on the catalytic activity of metal-zeolite catalysts in isomerization of n-butane has been studied. A relationship between the specific activity and the center strength is suggested.

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Catalytic and Physicochemical Properties of Oxidative Condensation Products in the Oxidative Dehydrogenation of Propane by Sulfur Dioxide on SiO2

The effect of the deposition of oxidative condensation products in the reaction of oxidative propane dehydrogenation in the presence of SO₂ on the catalytic, acid–base, and texture characteristics of silica was studied. It was found that the oxidative condensation products exhibited high catalytic activity in this reaction. The carbonization of silica from 0 to 40 wt % was accompanied by an increase in the yield of propylene from 3.4 to 46 mol % (640°C; a C₃H₈/SO₂/He + N₂ mixture, 10 : 10 : 80 vol %). Further accumulation of condensation products resulted in a considerable decrease in the pore volume and radius; this imposed diffusion limitations on both propane conversion and selectivity to propane conversion products. The nature of active and deactivated condensation products was studied by DRIFT spectroscopy, diffuse-reflectance UV–VIS spectroscopy, EPR spectroscopy, XPS, thermal analysis, and electron microscopy.     

Investigation of proton-acceptor properties of oxide surfaces by IR spectroscopy of hydrogen-bonded complexes

Strength of the basic centers on catalyst surfaces is suggested to be characterized by the IR spectra of adsorbed chloroform. The existence of centers with different proton-acceptor properties on several oxides is shown experimentally.

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Mechanochemical Synthesis and Catalytic Properties of the Calcium Ferrite Ca2Fe2O5

The formation of the real structure of calcium ferrite prepared by the calcination of a mechanochemically activated hydroxide mixture at 600–1100°C was studied by X-ray diffraction analysis, electron microscopy, thermal analysis, Moessbauer spectroscopy, IR spectroscopy, small-angle X-ray scattering, and secondary-ion mass spectrometry. It was found that low-temperature calcium ferrite is an anion-modified oxide, in which the ordering of oxygen vacancies was incomplete. Regions with a disordered structure were detected on the surface of crystallites. As the calcination temperature was increased, the brownmillerite crystal structure was improved and the intercrystalline boundaries were formed and then annealed. At the surface, these processes were accompanied by a change in the predominant form of adsorbed NO from nitrosyl to dinitrosyl species. An increase in the specific catalytic activity of samples with calcination temperature can be associated with the perfection of the brownmillerite structure and with a change in the state of adsorption centers.     

Conversion of dimethyl disulfide in the presence of zeolites

Dimethyl disulfide conversion in the presence of zeolites was studied at atmospheric pressure and T = 190–350°C. For all catalysts, the products of the reaction at T = 190°C—methanethiol, dimethyl sulfide, and hydrogen sulfide—result directly from dimethyl disulfide. The relative reaction rate and the dimethyl sulfide selectivity decreases in the order HZSM-5 ≥ CoHZSM-5 > HNaY > NaX, NaY. The methanethiol formation selectivity changes in the reverse order. The highest methanethiol selectivity at T = 190°C is shown by the sodium zeolites; the highest dimethyl sulfide selectivity, by the high-silicz zeolite HZSM-5. Raising the reaction temperature increases the reaction rate and changes the process route: at high temperatures, dimethyl disulfide decomposes to methanethiol, which then condenses to yield dimethyl sulfide and hydrogen sulfide. The observed regularities are explained in terms of the different acidic properties of the zeolite surfaces.     

Development of contactless methods for directly measuring the temperature of nanoparticles of the active component in operating supported catalysts

In situ X-ray diffraction and spectroscopic methods for direct contactless measurement of the temperature of nanoparticles of the active component in operating supported catalysts are considered. These methods have provided convincing evidence that the temperature of nanoparticles of the active component may differ significantly from the support temperature during a very exothermic reaction.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 302–315.Original Russian Text Copyright © 2005 by Plyasova, Averyanov, Paukshtis, Kriger, Khasin, Parmon.     

The effect of chemical modification on the chromatographic properties of polymeric adsorbents polysorb-1 and hisep Q and the diatomite carrier chromosorb P NAW

Polytrimethylsilylpropyne with the decomposition temperature 205°C was synthesized and characterized. The effect of chemical modification by polytrimethylsilylpropyne on the gas chromatographic properties of polymeric adsorbents and the diatomite carrier for the selective separation of C₁–C₁₀ hydrocarbons, alcohols and aromatic and sulfur-containing compounds was studied. Original Russian Text ? E.Yu. Yakovleva, V.Yu. Belotserkovskaya, V.P. Fadeeva, G.S. Litvak, E.A. Paukshtis, D.E. Babushkin, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 5, pp. 972–977.     

Physicochemical Properties of Mesoporous Mesophase Silicate Materials Formed via the Electrostatic S+I– Mechanism

The formation of the structure of a highly organized silicate mesoporous mesophase material (MMM) with hexagonal packing via the S⁺I^–reaction pathway and MMM-based aluminosilicates (Al,Si)-MMM and titanosilicates (Ti,Si)-MMM with different concentrations of the elements are considered. The structural, textural, and catalytic properties of the materials are studied.