Alexander F. Bedilo

Oxidative dehydrogenation of butane over nanocrystalline MgO, Al2O3, and VOx/MgO catalysts in the presence of small amounts of iodine

Abstract High surface area nanocrystalline MgO, Al 2 O 3 , MgO·Al 2 O 3 , commercial MgO, and a series of 10% V/MgO samples were used as catalysts in one-step selective oxidative dehydrogenation of butane to butadiene in the presence of oxygen and iodine. Molecular iodine shifts the equilibrium of the dehydrogenation reactions to the right and makes it possible to achieve high butane conversion with high selectivity to butadiene. When excess oxygen is present in the feed, iodine is successfully regenerated and can be recycled. Butadiene selectivity as high as 64% has been achieved in the presence of small amounts of iodine (0.25 vol%) over a vanadia–magnesia catalyst at 82% butane conversion. The best performance was observed over a catalyst containing the magnesium orthovanadate phase.

Synthesis of carbon-coated MgO nanoparticles

Carbon-coated MgO nanoparticles, with carbon forming a porous coating on the surface of MgO nanoparticles, have been prepared by two different techniques. Resorcinol has been found to be an efficient agent for the modification of magnesium methoxide leading to carbon-coated MgO nanocrystals of small crystallite size and high surface area. Decomposition of dry magnesium methoxide under an inert gas flow proved to be another efficient and economical way to synthesise carbon-coated MgO. The carbon coating acts as a hydrophobic barrier partially protecting the core metal oxide from water adsorption and conversion to magnesium hydroxide. However, destructive adsorption reactions can still proceed on the metal oxide surface, as evidenced by the dehydrochlorination of 2-chloroethyl ethyl sulfide (2-CEES) and 1-chlorobutane. The overall stability of the material in the presence of water vapor is significantly improved in comparison with non-coated nanocrystalline MgO.

ESR study of nanocrystalline aerogel-prepared magnesium oxide

Two signals have been observed in the ESR spectra of nanocrystalline MgO synthesized by an aerogel technique. These signals have been assigned to F+ centers in the nanocrystals and –O–˙CH2 radicals formed from residual methoxide groups present at the points of contact between the nanocrystals. The signal of F+ centers was found to be thermally stable up to 500 °C in oxidizing atmosphere. The second signal was somewhat less stable, and could be removed upon heating to 300 °C in the presence of oxygen. Similar organic radicals could be generated by UV irradiation of aerogel prepared (AP) samples, while their concentration appeared to follow the concentration of residual organics in the materials. Irradiation of the samples in the presence of oxygen resulted in the formation of 4-coordinated surface [O−˙O2] radical anions, while no signal typical for 3-coordinated radical anions was observed. Strong Lewis acid and basic sites on the surface of AP-MgO were characterized using the adsorption of 2,2,6,6-tetramethyl-4-oxo-1-oxyl-pyperidine (TEMPONE) and dinitrobenzene, respectively.

Superoxide radical anions on the surface of zirconia and sulfated zirconia: formation mechanisms, properties and structure

In situ ESR spectroscopy has been used for direct comparison of different thermal and light-induced processes leading to generation of superoxide radical anions on the surface of various zirconia and sulfated zirconia materials. For materials of both types the magnetic resonance parameters of the radical anions were found to be practically independent of the generation method, except for oxygen coadsorption with NO that yields radicals with somewhat smaller gz values. The parameters appear to depend mostly on the state of the surface zirconia cations stabilizing the radical anions, so that the g tensor anisotropy is significantly smaller over sulfated zirconia. It is shown that light-induced formation of superoxide radical anions in the presence of coadsorbed hydrocarbons can be initiated with visible light. Original SIET reaction mechanisms are suggested for the formation of superoxide radical anions by coadsorption with hydrocarbons and illumination after such coadsorption to extend the previously known ones to account for the observed phenomena. Cluster model DFT calculations of magnetic resonance parameters of O2- radical anions stabilized on the surface of zirconium dioxide showed that the adsorption complexes have a -shape rather than linear structure. The magnetic resonance parameters obtained by calculations practically match experimental data and adequately describe their changes after the surface modification with sulfates.

CATION RADICAL INTERMEDIATES IN BENZENE AND TOLUENE POLYCONDENSATION ON SULFATED ZIRCONIA

Benzene dimer cation radicals were detected on sulfated zirconia at low temperature and benzene coverage. Two temperature regions for their transformations were observed. Below 473 K the radical concentration remained stable reflecting the amount of strong surface acceptor sites. Above 623 K it increased drastically.

Formation of tetramethylethylene radical cations after pentane adsorption on sulfated zirconia

Tetramethylethylene radical cations have been registered by ESR after pentane adsorption on sulfated zirconia. The radical cations proved to be not stable in the presence of oxygen, only molecular oxygen radical anions being registered. Tetramethylethylene formation and pentane disproportionation are shown to occur under illumination within the same spectral region proving that the former is formed as a by-product of pentane transformations.

Suppression of radical-cationic benzene oligomerization on sulfated zirconia

The effect of catalyst activation conditions on transformations of benzene cation radicals on sulfated zirconia has been studied by in situ ESR spectroscopy. After the catalyst activation at 300°C there was no oligomerization of the cation radicals to biphenyl and heavier products, which is observed after catalyst calcination at 500°C. It has been suggested that the oligomerization is suppressed by the presence of strong Brönsted sites on the surface.

Nanocrystalline carbon coated alumina with enhanced phase stability at high temperatures

A comparative investigation of the phase stability at high temperatures of nanocrystalline Al2O3 and carbon-coated Al2O3@C systems was performed using a set of physicochemical and spectroscopic methods. The obtained data demonstrate that the carbon coating hinders the sintering of the δ-Al2O3 phase and its transformation to the α-Al2O3 phase at 1250 °C. Without the carbon coating, the δ-Al2O3 sinters and becomes completely converted to corundum at noticeably lower temperatures. The stabilization of the nanosized oxide particles in the Al2O3@C system was shown to be the decisive factor preventing their transformation to the α-Al2O3 phase. The thermal stability of the Al2O3@C samples calcined within a range of 1180–1250 °C in an argon atmosphere followed by the calcination in air to remove the carbon coating was found to exceed that of pure δ-Al2O3. Such samples are characterized by the presence of carbon–alumina interfaces, when carbon is encapsulated in small amounts at the places of contact between the oxide nanoparticles. Such interfaces hinder the sintering of alumina nanoparticles. It is important that the active sites present on the surface of the oxide core in Al2O3@C samples calcined in air are similar to those known for pure alumina. The high concentration of such sites after thermal treatment at elevated temperatures makes this class of materials promising for use as catalysts or catalyst supports capable of operating at high temperatures.

Dehydrochlorination of 1-Chlorobutane Over Nanocrystalline MgO: The Role of Electron-Acceptor Sites

Nanocrystalline MgO aerogels possess high surface areas and outstanding reactivity with respect to variety of harmful organic substances. The catalytic activity of MgO aerogels in dehydrochlorination of 1-chlorobutane substantially increases with time due to partial MgO conversion to MgCl2. This catalytic activity increase and surface area decrease indicate that more active catalytic sites are formed on the surface during this reaction. The catalytic activity was found to increase significantly during the 1-chlorobutane dehydrochlorination reaction, which is accompanied by the MgO modification with chloride ions. The activation energy of this reaction over partially chlorinated MgO was found to be equal to 135 kJ/mol at T < 225 °C and 63 kJ/mol at T > 225 °C. For the first time the concentrations of electron-acceptor sites were measured during a catalytic reaction. No electron-acceptor sites were observed on the surface of initial AP–MgO samples. They appeared only during the reaction. A good correlation was observed between the catalytic activity and the concentration of weak electron-acceptor sites tested using perylene. The obtained results indicate that they may be the active sites accounting for 1-chlorobutane dehydrochlorination in the active state of the catalyst.

Synthesis and Characterization of Nanocrystalline M-Mg-O and Carbon-Coated MgO Systems

Two different approaches to modify the structure and reaction ability of nanocrystalline MgO have been discussed. In the first case, a series of two-component x%MOx–MgO systems (M = Fe, Co and Ni, x = 1–45 wt.%) was synthesized via sol–gel technique. Aqueous solution of inorganic salt precursor was used as a hydrolyzing agent. Samples obtained were characterized by number of physicochemical methods (differential thermal analysis, X-ray diffraction analysis, low-temperature adsorption, etc.). It was shown that presence of inorganic salt in magnesium hydroxide matrix shifts the temperature of decomposition of latter towards lower values. Structural and textural characteristics of MgO-based oxide systems were found to be strongly affected by the presence of additive and their concentration. Formation of joint phase was observed in the case of cobalt oxide only. Second way of MgO modification was represented by creating a controlled carbon coating over its surface via decomposition of C4H10 at 400 °C. The obtained x%C/MgO (x = 1–10 wt.%) samples were shown to possess the improved reaction ability in destruction sorption of CF2Cl2 as well as in catalytic dehydrochlorination of 1-chlorobutane in presence of water vapors.