V. B. Kazanskii

IR Spectroscopic Study of Ethylene Adsorption and Oligomerization on the Hydrogen Form of Mordenite

Diffuse-reflectance IR spectroscopy is used to study adsorption and oligomerization of ethylene on the hydrogen form of mordenite at room temperature. Ethylene adsorbs on bridging acid hydroxyl groups of the zeolite and forms π-complexes with a firm hydrogen bond. The interaction with hydroxyl groups most strongly excites composite vibrations in adsorbed molecules. These vibrations are a combination of the stretching vibration of a double bond and the deformational vibrations of the CH2 group. A conjecture is drawn that these composite vibrations correspond to the reaction coordinate of ethylene transformation to the ethoxy groups. Their further reactions with weakly adsorbed molecules result in ethylene oligomerization. A linear oligomer is formed, grafted on the zeolite surface and filling the pores of zeolites.

Alkylation of isobutane with C 4 olefins under conventional and supercritical conditions

The solid acid alkylation of isobutane with butylenes on the ultrastable zeolite Y is studied in the temperature range from 120 to 150°C and in a pressure range of 20–120 atm. The catalyst service life becomes longer on passing from the conventional (liquid- and gas-phase) conditions of alkylation to supercritical conditions. The maximum period of complete butylene conversion at 150°C and 120 atm is 3.5 h. The composition of the reaction products is determined by the phase state of the reaction mixture, the reaction time, and the conversion of the C4 olefins. When the alkylation is carried out under supercritical conditions, the C8 hydrocarbon selectivity varies between 30 and 40%. Thermoanalytical data suggest that the surface of the spent catalyst contains carbon deposits indicating the formation of oligomeric and cyclic structures.

IR spectroscopic study of alkane interaction with the Brønsted acid sites of hydrogen-exchanged zeolites

Diffuse-reflectance IR spectroscopy is used to study the interaction of C3 and C6 alkanes (propane, n-hexane, 3-methylpentane, and cyclohexane) with the Brønsted acid sites of hydrogen-exchanged mordenite, ferrierite, ZSM-5, and faujasite. It is found that a shift of the absorption band of the stretching vibrations of acidic Si(OH)Al groups toward lower frequencies (ΔνOH) due to the formation of a hydrogen bond with adsorbed alkanes increases in the following series: ΔνOH(propane) < ΔνOH(n-hexane) = ΔνOH(3-methylpentane). The accessibility of Si(OH)Al groups to alkane molecules is determined by the dimension of rings through which molecules enter zeolite channels and cavities. It follows from the measured ΔνOH values that the strength of Brønsted acid sites decreases in the following series: HZSM-5 > H-mordenite ≈ H-ferrierite ≫ HY. The difference between the three high-silica zeolites is not great. The results obtained are compared with the published IR data on Si(OH)Al groups of zeolites with adsorbed alkanes and other weak bases.

Gas-phase and supercritical n-pentane isomerization on H-mordenite

The skeletal isomerization of supercritical n-pentane on the H form of mordenite under flow conditions was studied for the first time. It was found that the conversion of supercritical n-pentane was 30–35% at 90% selectivity for isopentane at 260°C, 130 atm, and a liquid hourly space velocity of 30 h−1. The catalyst was deactivated as the temperature was increased above 280°C. According to differential thermal analysis data, the deactivation was related to the deposition of condensation products on the surface. The resistance of the H form of the zeolite to poisoning in n-pentane isomerization in a gas phase at 1–8 atm was lower than that under supercritical conditions. It was found that H-mordenite deactivated under gas-phase reaction conditions at 260°C and 8 atm can be regenerated by passing to supercritical isomerization conditions (260°C and 130 atm).

Use of the IR spectra of adsorbed ethane and propane molecules to characterize the strength of active sites in zeolites and to analyze the activation of C-H bonds in these paraffins

The adsorption and activation of ethane and propane on the hydrogen and cationic forms of mordenite, zeolite ZSM-5, and zeolite Y were studied by diffuse-reflectance IR spectroscopy. The effect of the polarization of these molecules by adsorption sites on the intensities and shifts of absorption bands due to C-H stretching vibrations were studied. It was found that weak adsorption species were formed on the hydrogen forms of the above zeolites. In this case, both the intensity distributions and the positions of absorption bands due to C-H stretching vibrations were almost independent of the nature of zeolites. However, both absorption band maximum positions and relative intensity distributions changed upon paraffin adsorption on the cationic forms. It was also found that relative intensity distributions and shifts of absorption bands due to C-H stretching vibrations strongly depended on the nature of cations and zeolites. In this case, the initially totally symmetrical C-H vibrations were found most strongly disturbed. The low-frequency shifts and relative intensities of absorption bands due to these vibrations for various cations and zeolites were found to increase in the following orders: H < Na < Ca < Mg < Zn and zeolite Y < Mord ≈ ZSM-5. The experimental results suggest that ethane and propane molecules can be used as molecular probes for acquiring information on the nature and properties of acidbase sites in zeolites. In this case, both the low-frequency shifts and the relative intensities of absorption bands due to C-H stretching vibrations can be used as measures to characterize the nature of cations and zeolites. However, the latter was found to be much more sensitive to the nature of active sites.

Proton Solvation in Nonaqueous HF as a Main Factor That Determines Its Superacidic Properties

The Hartree–Fock method and electron correlation at the MP2 level in the 6-31++G** basis set is used together with the combined method for taking into account solvation (the discrete model of microsolvation and the polarizable continuum model (PCM)) to study the following molecular systems in the medium of liquid HF: [HFH]+, [HFHFH]+, [HF...HFH...FH]+, [HF...HFHFH...FH]+, and [HF...HF...HFH...FH...FH]+. The results of calculation make it possible to explain the anomalously high activity of protons in nonaqueous HF by a lower solvation energy of protons in liquid HF (by 48 kcal/mol) than in water. A conclusion is drawn that the combined method used in this work (the discrete model of microsolvation and PCM) is an efficient tool in estimating the contribution from the solvation to the thermodynamic parameters of the reaction in liquid HF.

Diffuse reflectance IR spectra of molecular hydrogen and deuterium adsorbed on zinc oxide

Diffuse reflectance IR spectroscopy is used to study hydrogen and deuterium adsorption on zinc oxide at room temperature and 77 K. At room temperature, H2 and D2 molecules are dissociatively adsorbed with the formation of hydrides and hydroxy groups of three types. At 77 K, diffuse reflectance spectra reveal the bands from molecular hydrogen and deuterium in addition to the dissociatively adsorbed forms. The presence of several bands of stretching H–H and D–D vibrations points to the nonuniformity of adsorption sites. This nonuniformity is also confirmed by the fact that, after heating zinc oxide from 77 K to room temperature in an atmosphere of hydrogen, only an insignificant portion of adsorbed molecular hydrogen dissociates. Most of dissociatively adsorbed hydrogen is formed without a molecular precursor. The dissociation of H2 and D2 most likely occurs on very active adsorption species so rapidly that the molecular precursor is not observed. The bond energy in molecular deuterium precursors of dissociation estimated from the fundamental vibration frequency and the overtone of D–D vibrations suggests moderate excitation of the bond. This agrees well with the conclusion that the dissociative adsorption of hydrogen and deuterium occurs without a molecular precursor.

Photocatalytic removal of nitrogen oxides from air on TiO2 modified with bases and platinum

The efficiency of TiO2 (Degussa P-25) modified with an alkaline admixture (urea, BaO), sulfuric acid, or platinum in the photocatalytic oxidation of NO (50 ppm) with a flowing 7% O2 + N2 mixture under UV irradiation in a flow reactor at room temperature and atmospheric pressure is reported. Because of the progressive blocking of active sites of the photocatalyst by the reaction products (NO2, NO3−), it is impossible to realize prolonged continuous removal of NOx (NO + NO2) from air without catalyst regeneration at elevated temperatures. The efficiency of the photocatalysts is characterized by specific photoadsorption capacity (SPC) calculated from the total amount of NOx adsorbed during 2-h-long irradiation. Modification of TiO2 with 5% BaO or 5% urea raises the SPC of the catalyst by a factor of 2–3. Presumably, this promoting effect is due to the basic properties of these dopants, which readily sorb NO2 and NO3−. A considerable favorable effect on SPC is also attained by adding 0.5% Pt to (5% BaO)/TiO2. The SPC of the (0.5% Pt)/TiO2 catalyst depends on the state of the platinum. The samples calcined in air at 500°C, which contain Pt+ and Pt2+, have an approximately 2 times higher SPC than unpromoted TiO2 and ensure a much larger NO2/NO ratio at the reactor outlet. Conversely, the samples reduced in an H2 atmosphere at 200°C, whose platinum is in the Pt0 state, show a lower SPC than the initial TiO2 and cause no significant change in the NO2/NO ratio.

Catalytic activity of H-forms of zeolites in the isomerization of supercritical n-pentane and their physicochemical properties

The acidic properties of the H-forms of zeolites ZSM-5, Beta, Y, and mordenite are studied by diffuse reflectance IR spectroscopy using n-pentane as a probe molecule. The decreasing order of Brønsted acid site strengths is constructed. The isopentane selectivity in n-pentane isomerization under supercritical conditions (260°C, 130 atm) increases in the order H-ZSM-5 < H-Beta < H-mordenite(11) ≈ H-Y with decreasing strength of Brønsted sites. Catalytic data are analyzed jointly with the results of physicochemical studies of H-mordenite (temperature-programmed ammonia desorption, benzene adsorption, and IR spectroscopy). Under the supercritical conditions, the conversion of n-pentane on mordenite is determined by the total acidity of the zeolite and also by the accessibility of the acid sites inside the zeolite channels to the reactant.

Quantum chemical calculation of the catalytic reaction of ethane dehydrogenation on gallium oxide-hydroxide binuclear clusters in oxidized GaO/ZSM-5 zeolite

The catalytic activity of oxidized GaO/HZSM-5 in the reaction of alkane dehydrogenation can be due to hydrogenated gallium oxide clusters stabilized in the cationic positions of the zeolite. The binuclear gallium oxide clusters [Ga2O2]2+ in oxidized gallium-substituted high-silica zeolite HZSM-5, which are isomeric to two gallyl ions [GaO]+ stabilized on two spatially separated lattice aluminum ions, were considered using the DFT method within the framework of a cluster approach. It was found that, even in the case of a relatively large distance between these aluminum ions, gallium oxide particles in oxidized GaO/HZSM-5 can occur as charged planar [Ga2O2]2+ four-membered rings. These cluster particles exhibited a high affinity to hydrogen, and they were readily hydrogenated with the retention of their structural integrity. It was demonstrated that this partially hydrogenated cluster could be responsible for the catalytic process of ethane dehydrogenation. In the first step, ethane dissociatively added to the [Ga2O2H2]2+ cluster. Then, the ethylene molecule was eliminated from the resulting intermediate to leave the [Ga2O2H4]2+ cluster. The cycle was closed by the elimination of a hydrogen molecule with the formation of the initial structure of [Ga2O2H2]2+.