M. I. Shilina

Adsorption and oxidation of carbon monoxide on Au and Ni nanoparticles deposited on Al2O3 by laser electrodispersion

Dissociative adsorption of CO on Au and Ni nanoparticles deposited on Al2O3 by laser electrodispersion (LED) was for the first time observed using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The average particle size was 4.0 nm for the Au nanoparticles and 1.5–2.0 nm for the Ni nanoparticles. The process on alumina-supported Au nanoparticles of similar size, prepared by ion exchange proceeds with lower efficiency. The Ni/Al2O3 catalyst prepared by LED exhibits unusually high activity in the oxidation of CO at T > 600 K. When the catalyst was reused, the temperature of CO oxidation decreased to 450 K. The activity of the Au- and Ni-catalysts prepared by LED is higher than that of the catalysts of identical composition prepared by traditional methods, namely, ion exchange and impregnation. The DRIFT and X-ray photoelectron spectroscopy (XPS) data are used to analyze the structure of the catalysts prepared by LED that is associated with specific features of their adsorption and catalytic properties.

Quantum chemical study of butane isomerization on clusters of aluminum and cobalt chlorides. Mixed complexes

Activated complexes and routes of the model catalytic process, viz., butane isomerization by the aluminum and cobalt chloride complexes, were calculated by the DFT/PBE/TZ2p quantum chemical method. Alkanes are activated via the alkyl mechanism to form binuclear bimetallic alkyl clusters, where the Co atoms are linked by the metal-metal bonds. The revealed binuclear complexes can transform into bimetallic alkyl clusters with similar energy in which the transition metal atoms are linked by bridges of the Cl atoms. The full model of the catalytic cycle was developed for the maximum multiplicity (7), and particular key regions related to the cleavage and formation of the C-C bonds were calculated with a lowered multiplicity (5 and 3). The sequence of mutual rearrangements of the polynuclear complexes provides the possibility of C-C bond cleavage in alkanes and formation of the metal-carbon bonds. The calculated energy barriers of particular stages of the cyclic catalytic process of butane isomerization are not higher than 29 kcal mol−1 for multiplicity 7 and by ∼10 kcal mol−1 lower for a lower multiplicity.

Aluminium Chloride Complexes with Nitrobenzene: Low-Temperature Synthesis, Structure, and Stability

Low-temperature IR spectroscopy was used to study intermolecular interactions in the aluminium chloride–nitrobenzene system. The 1 : 1, 2 : 1, and 1 : 2 molecular complexes of aluminium chloride with nitrobenzene were discovered. The spectral parameters were found to correlate with the structure of the complexes and their thermal stability. Above 170 K, the 1 : 2 complexes become unstable and self-ionize to give particles with the general formula [AlCl2(RNO2)m]+AlCl–4. The quantum-chemical calculations carried out at the HF/6-31G level confirm the existence of three types of molecular complexes of 1 : 1, 2 : 1, and 1 : 2 composition. The structure of the title complexes was proposed on the basis of spectral parameters and calculation results.

Theoretical and low-temperature IR spectroscopic studies of catalytic complexes of aluminum chloride with nitroalkanes and nitrobenzene

Abstract This paper demonstrates the possibilities and advantages of low-temperature infrared (IR) spectroscopy in investigating active labile catalytic complexes, intermediates, and reaction mechanisms, taking aluminum halide complexes as an example. The intermolecular interactions between aluminum chloride and nitromethane, 1-nitropropane and nitrobenzene were studied both by the low-temperature IR spectroscopy and quantum-chemical methods. Three kinds of molecular species have been found in these systems: complexes of monomer AlCl 3 1:1 and 1:2 composition and 2:1 dimer Al 2 Cl 6 ones, existing as trans - and gauche -conformers. Their structure, thermal stability, and catalytic activity were investigated. The aluminum chloride complexes with nitroalkanes are more active than the ones with nitrobenzene in catalytic hydrocarbon cracking.

APPLICATION OF LOW TEMPERATURE IR SPECTROSCOPY FOR STUDIES OF CATALYST PROPERTIES AND DYNAMICS ON THE EXAMPLE OF ALUMINUM HALIDE COMPLEXES

Abstract This paper shows the benefits of applying IR spectroscopy to solid state studies of active labile catalytic complexes, intermediates and reaction mechanisms taking aluminum halide complexes as an example. Aluminum halide complexes with ethylene, hydrogen chloride and organic nitrocompounds were prepared under codeposition of reagents on a cooled surface and then films obtained were studied at 80–150 K. Semi-empirical and ab initio calculations were used to analyze spectral data. A limited molecular mobility and special features of the solid state make it possible to investigate the nature and properties of different types of complexes. In addition, direct observation of catalytic species unstable in solutions becomes possible. Structure and reactivity of a number of molecular and ionic monomer and dimer aluminum halide complexes were studied. Dimer complexes are more active and may catalyze hydrohalogenation of ethylene and dehydration of nitrocompounds even at low temperatures.

Peculiarities of the structure and catalytic behavior of nanostructured Ni catalysts prepared by laser electrodispersion

The peculiarities of the structure and catalytic behavior of nickel nanoparticles deposited onto an Al2O3 surface by laser electrodispersion (LED) with subsequent activation in carbon monoxide atmosphere has been considered. The reduction of these nanoparticles by in situ treatment in a catalytic cell in Ar + 5% Н2 atmosphere at 150–450°C has been studied using X-ray photoelectron spectroscopy (XPS). It is shown that formation of metal nickel starts by reduction in hydrogen at 300°C. A comparison of catalytic activity of the Ni/Al2O3 systems in the catalytic oxidation of CO is carried out. It is found that the preliminary treatment of Ni/Al2O3 sample by carbon monoxide leads to an increase in the catalyst efficiency and decrease in the reaction temperature by 50–100°C.

Quantum chemical study of butane metathesis on mixed aluminum and cobalt chloride complexes

Activated complexes and routes of the model catalytic butane metathesis in the presence of mixed complexes of aluminum and cobalt chlorides were calculated by the quantum chemical DFT/PBE/TZ2p method. The earlier predicted bimetallic alkyl clusters containing Co-Co dimeric bonds are key intermediates leading to the formation of both isomerization products and products of alkane metathesis (pentane and propane molecules in the case of butane). The sequence of mutual rearrangements of the polynuclear complexes provides a possibility of cleavage and formation of C-C bonds in alkanes and metal-carbon bond formation. The full model of the catalytic cycle was constructed for the maximum multiplicity (7) of the catalytic system, and individual key regions related to C-C bond cleavage and formation were calculated with a lowered multiplicity (5 and 3). The obtained energy barriers of individual stages of the cyclic catalytic process of butane metathesis are not higher than 33 kcal mol−1 for multiplicity 7 and are lower by ∼15 kcal mol−1 for a lower multiplicity.

Synergism in the actions of a transition metal cation and a Lewis acid in the liquid- and gas-phase catalytic conversion of alkanes over modified ZSM-5 zeolites under mild conditions

Bimetallic catalysts have been prepared by successively modifying the high-silica zeolite ZSM-5 with cobalt and aluminum salts. The performance of these catalysts in the conversion of higher alkanes at medium temperatures (130–190°C) depends on whether the reaction is conducted in the liquid or gas phase. In both cases, the transition metal and surface-anchored aluminum chloride act synergetically. In the liquid-phase reactions of n-heptane and n-dodecane, the activity of the bimetallic systems is more than one order of magnitude higher than the activity of the hydrogen form of the initial zeolite. New adsorption and catalytic sites resulting from the introduction of the two modifiers into the zeolite have been discovered by diffuse reflectance IR spectroscopy. In particular, the modifiers generate a new state of cobalt in which the transition metal atoms are linked with aluminum atoms through chlorine or oxygen atoms. The liquid-phase conversion of alkanes over the modified zeolites is unlikely to proceed via a carbocationic mechanism.

Adsorption and catalytic properties of sulfated aluminum oxide modified with cobalt ions

The adsorption properties of sulfated aluminum oxide (9% SO42-/γ-Al2O3) and a cobalt-containing composite (0.5%Сo/SO42-/γ-Al2O3) based on it are studied via dynamic sorption. The adsorption isotherms of such test adsorbates as n-hydrocarbons (C6–C8), benzene, ethylbenzene, chloroform, and diethyl ether are measured, and their isosteric heats of adsorption are calculated. It is shown that the surface sulfation of aluminum oxide substantially improves its electron-accepting properties, and so the catalytic activity of SO42-/γ-Al2O3 in the liquid-phase alkylation of benzene with octene-1 at temperatures of 25–120°C is one order of magnitude higher than for the initial aluminum oxide. It is established that additional modification of sulfated aluminum oxide with cobalt ions increases the activity of this catalyst by 2–4 times. It is shown that adsorption sites capable of strong specific adsorption with both donating (aromatics, diethyl ether chemosorption) and accepting molecules (chloroform) form on the surface of sulfated γ-Al2O3 promoted by cobalt salt.

Cryosynthesis, structure, and IR spectra of aluminum chloride—nitromethane molecular complexes

The products of codeposition of aluminum chloride and nitromethane were studied by low-temperature IR spectroscopy (80—200 K). Density functional (B3LYP/6-31G* and B3LYP/6-31+G**) quantum-chemical calculations of the geometry and vibrational frequencies of aluminum chloride, nitromethane, and AlCl3·MeNO2 and 2AlCl3·MeNO2 complexes were carried out. Comparison of the experimental and calculated IR spectra suggests that the preferred geometry of the 2AlCl3·MeNO2 complex is such that one AlCl3 molecule is coordinated to an O atom of the nitro group, while the other AlCl3 molecule forms an Al...Cl bridge.