A. V. Fionov

Nitroxide complexes with the solid Lewis acids

The possibilities of the paramagnetic probe method for investigating Lewis acid sites (LASS) on the surfaces of oxide catalysts (Al2 O3, Al2O3SiO2, zeolites etc.) are discussed. Nitroxides of different structures were used as paramagnetic probes. The experimental and calculated results have clarified the structure of nitroxide complexes with LASs and have led to an assessment of the impact of steric hindrance on the orientation and mobility of complex-bonded radicals. It has been demonstrated that the method is highly advantageous for the quantitative and qualitative control of surface acidity.

The application of paramagnetic complexes of probe molecules for the investigation of the Lewis acidity of aluminas

The Lewis acidity of aluminas formed by dehydration of aluminium hydroxides is studied by means of the EPR method, using paramagnetic complexes of indicators (nitrogen oxide, 2,2,6,6-tetramethylpiperidine-N-oxyl, anthraquinone) with coordinatively unsaturated aluminum cations at the solid surface. The nature, concentration and mutual disposition of Lewis acid sites on the oxide surfaces are determined. A correlation is shown to exist between the EPR data and the IR spectroscopy data of adsorbed carbon monoxide. The role of the alumina crystal structure in the formation of Lewis acid sites is discussed.

Catalytic conversion of C(2)-C(3) alcohols on detonation nanodiamond and its modifications

The catalytic activity of detonation nanodiamond and its modifications obtained through treatment with hydrogen or air at elevated temperatures is studied in the conversion of C2-C3 alcohols. The catalysts were characterized by means of electron microscopy, optical (FTIR) spectroscopy, elemental analysis and pulse microcatalytic method. It has been established that nanodiamond exhibits high catalytic activity in the conversion of alcohols. The oxidizing and reducing treatment of nanodiamond changes its activity and selectivity, and the activity of oxidized nanodiamond is considerably higher than that of reduced nanodiamond.

The oxidative destruction of lignin in the ozonation of wood

The oxidative destruction of lignin in the ozonation of aspen wood was studied. The kinetic curves of ozone consumption for samples with different contents of water were obtained. The consumption of ozone increased as the content of water grew. The second derivatives of the UV absorption spectra of lignin were obtained to show that the principal direction of lignin transformations under the action of ozone was the destruction of its aromatic constituents with the formation of carboxyl- and carbonyl-containing compounds. Measurements of the UV diffuse reflectance and EPR spectra of wood showed that the ozonation of wood caused the destruction of lignin quinoid structures. Part of lignin remained unchanged under the action of ozone. A key role in the destruction of wood lignin was played by ozone dissolved in water. Varying the content of water in wood samples allows various lignin transformation products to be obtained through ozonation.

PARAMAGNETIC COMPLEXES OF THE PROBE MOLECULES WITH ELECTRON ACCEPTING SITES ON THE SURFACE OF M2O3-ZRO2 : (M = AL, GA)

Abstract The electron accepting and Lewis acid properties of dehydroxylated surfaces of M 2 O 3 –ZrO 2 (M=Al, Ga) were investigated. The investigation was carried out by EPR with 2,2,6,6-tetramethylpiperidine- N -oxyl (TEMPO) and anthraquinone probe molecules. With the help of TEMPO two types of Lewis acid sites on the Al 2 O 3 –ZrO 2 (Ga 2 O 3 –ZrO 2 ) surface were defined: coordinatively unsaturated aluminium cation Al 3+ cus or gallium cation Ga 3+ cus and coordinatively unsaturated zirconium cation Zr 4+ cus . It was found out that electron accepting sites which are able to ionize anthraquinone are presented on the zirconia and Al 2 O 3 –ZrO 2 (Ga 2 O 3 –ZrO 2 ) surfaces as well. In the case of Al 2 O 3 –ZrO 2 obtained results can be explained with the presence of amorphous Al 2 O 3 that covered microcrystals of metastable tetragonal zirconia. In the case of Ga 2 O 3 –ZrO 2 surface Lewis acid properties are in accordance with the assumption of solid solution formation.

Lewis Acid Properties of Alumina Based Catalysts: Study by Paramagnetic Complexes of Probe Molecules

Lewis acid properties of LiAl 5 O 8 /Al 2 O 3 (2 wt.% Li) and MgAl 2 O 4 /Al 2 O 3 (3 wt.% Mg) catalysts were studied by EPR of adsorbed probe molecules anthraquinone and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). The lesser (in comparison with γ-Al 2 O 3 ) concentration and the strength of Lewis acid sites (LAS) formed on the surface of aluminate layer has been shown. The stability of this layer plays important role in the change of Lewis acid properties during the calcination of modified alumina. The lithium aluminate layer was stable at used calcination temperature, 773 K. meanwhile magnesium aluminate layer observed only at calcination temperature below 723 K. The increase of the calcination temperature to 773 K caused the segregation of MgAl 2 O 4 on the surface resulted in the release of alumina surface and recovery of the Lewis acid properties. The differences in the LAS manifestations towards TEMPO and anthraquinone was discussed. The mechanism of the formation of anthraquinone paramagnetic complexes with LAS-three-coordinated aluminum ions-was proposed. This mechanism includes the formation of anthrasemiquinone, and then anthrasemiquinone ion pair or triple ion. Fragments like -O Al-O play the role of cations in these ion pairs and triple ions. Proposed mechanism can also he applied for the consideration of similar anthraquinone paramagnetic complexes on the surface of gallium oxide containing systems.

ENDOR Study of Anthraquinone Paramagnetic Complexes with Lewis Acid Centers of Aluminium Oxide

Paramagnetic complexes with charge transfer formed upon adsorption of anthraquinone on the surface of aluminium oxide were studied with EPR and ENDOR methods. A hyper-fine structure due to the interaction of unpaired electron with one and two27Al nuclei of surface Lewis acid centers was observed. The process of complex decomposition in the presence of water on the surface was studied.

Effect of the Detonation Nanodiamond Surface on the Catalytic Activity of Deposited Nickel Catalysts in the Hydrogenation of Acetylene

A comparative study is performed of the catalytic activity of nanosized nickel deposited on detonation synthesis nanodiamond (DND) and coal (CSUG) produced by burning sugar and crystalline quartz in the hydrogenation of acetylene. Nanosized nickel is obtained through the thermal decomposition of nickel formate under a dynamic vacuum. The catalysts are studied by means of scanning electron and transmission electron microscopy, X-ray fluorescence, IR-spectroscopy, X-ray diffraction, and pulse microcatalytic method. It is shown that Ni/DND is an active catalyst of acetylene hydrogenation, considerably surpassing Ni/quartz and Ni/CSUG. The apparent activation energy of the hydrogenation of acetylene is calculated, and the region of the reaction are determined for all catalysts. It is found that the influence of the structure and nature of a functional coating of nanodiamond on the catalytic activity of Ni/DND deposited catalyst in the hydrogenation of acetylene. The ability of Ni/DND to hold active hydrogen is detected.

Quantum chemical analysis of coordination of nitroxide probes to acid sites on the surfaces of oxides

Quantum chemical analysis of coordination of nitroxides on the surfaces of alumina, gallia, and silica is performed. Structural, magnetic resonance and energy characteristics of coordination are calculated. Different cluster models of the surface acid sites (AS) are tested. The results of ESR experiments are interpreted. The comparison of Lewis acid sites (LAS) of alumina and gallia surfaces is carried out. Different ways of coordination of nitroxides with two electron-donating groups are analyzed.

Paramagnetic complexes of 9,10-anthraquinone on zeolite surfaces and their thermal transformations

The concentration of one-electron transfer sites on the surface of H-ZSM-5, H-Y, H-mordenite, and H-β zeolites was measured by EPR using 9,10-antrhraquinone as a probe. It has been found that the hyperfine structure from four protons typical for one-centered complexes of anthraquinone with one acidic site was observed in the EPR spectra after anthraquinone interaction with a zeolite surface in the temperature range of 373 to 423 K. It has been established that an elevated temperature of 473 K promoted the decomposition of the adsorbed anthraquinone and the disappearance of the hyperfine structure. It has been shown that the thermal instability of anthraquinone adsorbed on zeolites changed in the series H-β > H-Y > H-ZSM-5 ∼ H-mordenite; the coke-forming ability of zeolites with regard to n-decane at 443 K changed in a similar manner. It has been established that the presence of air promoted coke-forming processes upon interaction between n-decane and zeolites.