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Alkaline-earth metal oxide aerogels prepared by sol–gel method followed by autoclave drying are nanocrystalline mesoporous materials with high reactivity. Bulk solid-state reaction of MgO aerogels with CF2Cl2 takes place after a long induction period, during which the active sites are accumulated on the surface of the nanoparticles. It was found that vanadium addition has a promoting effect on this reaction accelerating the process of the active sites formation. A method for characterization of electron-acceptor sites by electron spin resonance spectroscopy using perylene as the spin probe was developed. A good correlation was observed between the rate of the CF2Cl2 destructive sorption and the concentration of weak electron-acceptor sites. Simplified models of such sites were suggested. The acid sites on the modified MgO surface were supposed to be originated from separation of the charged fragments resulting in the surface polarization. Uncompensated oxygen substitution for chlorine and/or fluorine ions leads to appearance of Lewis acid sites while HCl/HF chemisorption results in Bronsted acid sites formation.

Tetradecahydrophenazine (14HP) is a nitrogen-containing heterocycle compound with a high content of hydrogen that can be released during its dehydrogenation to phenazine (P). The high stability of the 14HP/P pair and relatively low dehydrogenation temperature make 14HP a promising organic hydrogen carrier. This manuscript is devoted to the investigation of hydrogen production by 14HP dehydrogenation over Pd supported on a series of magnesium-aluminum oxides prepared by the aerogel method. This technique made it possible to synthesize catalyst supports characterized by a high surface area and high concentration of surface active sites where active transition metals could be stabilized in a finely dispersed state. The synthesized aerogels had high specific surface areas and pore volumes. A surface area as high as 600 m2/g after calcination at 500 °C was observed for the mixed aerogel with an Mg:Al ratio of 1:4. An increase in the concentration of acidic electron-acceptor sites determined by EPR on the surface of the mixed magnesium-aluminum oxide supports with a high surface area prepared by the aerogel method was found to result in higher hydrogen production due to the faster dehydrogenation of sterically hindered nitrogen-containing tetradecahydrophenazine heterocycles. [ABSTRACT FROM AUTHOR]

Pd-containing catalysts based on highly dispersed aerogel-derived mayenite were prepared via two approaches. The Pd@C12A7 sample was obtained through the addition of Pd nitrate solution to a fresh Ca(OH)2-Al(OH)3 gel. Pd/C12A7 was synthesized through conventional wet impregnation of the aerogel-derived mayenite. The evolution of the textural characteristics of the support (C12A7) depending on the calcination temperature was investigated. Pd-containing samples were explored using transmission electron microscopy and spin probe EPR spectroscopy. Using the latter method, the presence of active oxygen species capable of producing nitroxyl radicals from diphenylamine was observed. The activity of these species and the reproducibility of their redox behavior were studied in three cycles of temperature-programmed reduction in both hydrogen and CO atmospheres. A prompt thermal aging technique was used to access and compare the activity of the samples towards CO oxidation. The state of Pd species before and after the aging procedure was studied via UV–Vis spectroscopy. It was found that the dispersion of PdO was higher in the case of the Pd/C12A7 catalysts compared to the Pd@C12A7 sample. This is why the Pd/C12A7 catalyst demonstrated higher activity in CO oxidation and better reducibility in TPR cycles. [ABSTRACT FROM AUTHOR]

The present paper continues the exploration of the physicochemical and catalytic properties of vanadia-mayenite composites. The samples were prepared by an impregnation of calcium aluminate Ca12Al14O33 (mayenite, C12A7) with a solution of vanadium precursor. Pure mayenite and V/C12A7 nanocomposites were characterized by Raman and diffuse reflectance UV–Vis spectroscopies. The reducibility of the samples was examined in temperature-programmed reduction experiments performed in a hydrogen atmosphere. The catalytic performance of vanadium-containing systems was studied in the non-oxidative dehydrogenation of ethane. As found, the low-loaded sample (5%V/C12A7 sample) contains vanadium predominantly in the form of Ca3(VO4)2, while for the 10%V/C12A7 sample, two types of calcium vanadates (Ca2V2O7 and Ca3(VO4)2) are registered. The presence of these phases defines the spectroscopic characteristics and the redox properties of nanocomposites. Both the samples, 5%V/C12A7 and 10%V/C12A7, exhibit comparable catalytic activity, which is mainly connected with the amount of the Ca3(VO4)2 phase. The uniqueness of the studied catalysts is their excellent tolerance toward coke formation under the reaction conditions. [ABSTRACT FROM AUTHOR]

Calcium aluminates (CA) with a mayenite structure have attracted a growing interest during the last decades. The present paper reports the preparation of vanadia-mayenite composites performed via an impregnation of pure CA with ammonium vanadate solution. The properties of the prepared materials were explored by a low-temperature nitrogen adsorption/desorption technique, X-ray diffraction analysis, transmission electron microscopy, and spin probe method. As revealed, the addition of vanadium significantly affects the textural properties and the porous structure of mayenite. The blockage of micropores by vanadium species is supposed. The spin probe electron paramagnetic resonance technique based on the adsorption of 1,3,5-trinitrobenzene, phenothiazine, and diphenylamine has been applied to study the active sites on the surface of the composite samples. The results demonstrated an increase in the concentration of weak electron-acceptor sites when the vanadium loading was 10 wt%. X-ray diffraction analysis and transmission electron microscopy studies showed that the composites consist of few phases including mayenite, CaO, and calcium vanadates. [ABSTRACT FROM AUTHOR]

С/Al2O3 and C/MgO carbon-mineral composites were synthesized by propane decomposition over Al2O3 at 600–650 °C and MgO carbonization in 1,3-butadiene at 600 °C. The synthesized carbon-mineral composites were studied by EPR, XRD and transmission electron microscopy. It was found that a mosaic structure of carbon clusters was formed on the internal surface of the oxides and gradually grew until it covered its whole surface. A method for synthesis of graphene materials by using MgO as a template followed by its dissolution in hydrochloric acid was suggested. A maximum was observed on the dependence of the graphene surface area on the carbon concentration in the composites. The highest specific surface area of the synthesized graphene about 1800–1900 m2/g was observed for the samples obtained from C-MgO composites containing 8–10 wt.% C. [ABSTRACT FROM AUTHOR]

A study is carried out on the reaction between CF2Cl with nanoscale MgO at 325 degree Celsius and higher temperature. The reaction results in the formation of MgFe2 as a predominant solid product, with CCl4, and CO2 formed as the main gaseous products when excess of halocarbons is used.

The HRTEM study indicates that the composition of the mineral constituent in the carbon-mineral materials was not changed by the carbon deposition. A singular balanced Lorentzian line owing to carbon deposits has been observed in the ESR range of the carbon-mineral materials.

Transformation of nanocrystalline Mg(OH)2 to MgO under dynamic vacuum in a temperature range of 570-770 K is studied by TGA-GCMS, FTIR, HREM, XRD, helium pycnometry, and nitrogen adsorption at 77 K. A new phenomenon of decrease of the interlayer distance in MgO crystallites with increase of the nanoparticle size was observed.

Alkaline-earth metal oxide aerogels prepared by sol-gel method followed by autoclave drying are nanocrystalline mesoporous materials with high reactivity. Bulk solid-state reaction of MgO aerogels with CF2Cl2 takes place after a long induction period, during which the active sites are accumulated on the surface of the nanoparticles. It was found that vanadium addition has a promoting effect on this reaction accelerating the process of the active sites formation. A method for characterization of electron-acceptor sites by electron spin resonance spectroscopy using perylene as the spin probe was developed. A good correlation was observed between the rate of the CF2Cl2 destructive sorption and the concentration of weak electron-acceptor sites. Simplified models of such sites were suggested. The acid sites on the modified MgO surface were supposed to be originated from separation of the charged fragments resulting in the surface polarization. Uncompensated oxygen substitution for chlorine and/or fluorine ions leads to appearance of Lewis acid sites while HCl/HF chemisorption results in Bronsted acid sites formation. [ABSTRACT FROM AUTHOR]

Cobalt-magnesium and vanadium magnesium oxide catalysts were prepared by aerogel method. The obtained nanocrystalline materials are characterized by uniform distribution of the active component, which is present in a finely dispersed state amorphous to X-rays. The oxides have unique morphology. The size of their nanoparticles does not exceed 10 nm. The aerogel synthesis method was successfully applied for synthesis of two-component CoO x·MgO and VO x·MgO catalysts active in CO oxidation and propane oxidative dehydrogenation, respectively. [ABSTRACT FROM AUTHOR]

Nanoscale MgO, prepared by an aerogel method (AP-MgO), has been found to be a good destructive adsorbent for dehydrochlorination of 1-chlorobutane. MgO is converted to MgCl2 during reaction, but conversion continues because MgCl2 itself is a good catalyst for dehydrochlorination. Pelletization will likely be required in industrial use of AP-MgO to ensure lower pressure drop. Pelletization, however, can lead to internal diffusion limitations. In addition, the phase transformation of MgO to MgCl2 inside the pellets leads to physical changes that may limit the reaction rate. This study reports the changes in AP-MgO physical properties upon pelletization and reaction with 1-chlorobutane. Pore diffusion limitations are noted for all pellets. The phase transformation of MgO to MgCl2 plays an important role in the process, as the physical characteristics substantially change with this transformation. A layer of MgCl2 forms on top of MgO, rendering much of the starting pellet inaccessible to reactants. [ABSTRACT FROM AUTHOR]

High surface area nanocrystalline MgO, Al2O3, MgO·Al2O3, 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. [Copyright &y& Elsevier]

Abstract: VO x /MgO aerogel catalysts were synthesized using three different preparation methods: by mixing the aerogel MgO support with dry ammonium vanadate, by vanadium deposition from a precursor solution in toluene, and by hydrolysis of a mixture of vanadium and magnesium alkoxides followed by co-gelation and supercritical drying. The latter aerogel technique allowed us to synthesize mixed vanadium–magnesium hydroxides with the surface areas exceeding 1300m2/g. The synthesized catalysts were studied by a number of physicochemical methods (XRD, Raman spectroscopy, XANES and TEM). A common feature of all synthesized samples is the lack of V2O5 phase. In all cases vanadium was found to be a part of a surface mixed V–Mg oxide (magnesium vanadate), its structure depending on the synthesis method. The VO x /MgO mixed aerogel sample had the highest surface area 340m2/g, showed higher catalytic activity and selectivity in oxidative dehydrogenation of propane compared to the catalysts prepared by impregnation and dry mixing. The addition of iodine vapor to the feed in 0.1–0.25 vol.% concentrations was found to increase to propylene yield by 40–70%. [Copyright &y& Elsevier]

Highlights: [•] CF2Cl2 reaction with mesoporous VO x /MgO is characterized by long induction period. [•] Vanadium incorporation promotes the activity of nanocrystalline MgO. [•] Mechanism of CF2Cl2 reaction with nanocrystalline VO x /MgO is suggested. [•] Formation of surface active sites is supposed to be the rate-determining stage. [ABSTRACT FROM AUTHOR]

Abstract: Several pure and doped alumina samples were characterized by EPR using spin probes and tested in ethanol dehydration. It was found that the concentrations of the electron acceptor sites and the catalytic activity increased when alumina was modified with chlorides and sulfates. Meanwhile, the number of the electron donor sites decreased. A very good correlation between the concentration of the weak acceptor sites and the catalytic activity of the acid-modified catalysts almost passing through the origin of the coordinates was obtained for all the studied samples. Alumina poisoning with sodium resulted in the decrease in the catalytic activity and the concentration of the weak acceptor sites. Therefore, it appears that the weak acceptor sites tested using anthracene are related to the sites active in the ethanol dehydration reaction. [Copyright &y& Elsevier]

[Display omitted] • Sulfated perfluorinated polymers were deposited on SiO 2 , carbon nanofibers and ZrO 2. • The performance of the carbon nanofibers based composites surpassed that other samples. • Zirconia-based composites had increased resistance to the polymer desorption. • Polymers with low-equivalent weight and high acidity can be deposited on ZrO 2. Several composite samples containing perfluorosulfonic acid polymer (PFSA; e.g. Nafion) over silica, carbon nanofibers or zirconia were synthesized to determine the effect of the support on the acid catalytic properties. The synthesized composites contained 5–25 wt.% of the perfluorosulfonic acid polymer with variable concentration of sulfonic groups (equivalent weight of PFSA). The acidity of the composites was investigated by titrimetric techniques, in a model catalytic reaction between resorcinol and ethyl acetoacetate, by EPR using spin probes and by temperature programmed desorption of propylene. It is shown that impregnation of silica with high surface area or mesoporous carbon nanomaterials yields samples with characteristics exceeding commercially available SAC (Nafion over silica) samples. A method for synthesis of zirconia-based catalytic composites was developed. It was shown that despite the relatively low catalytic activity (compared to PFSA/SBA-15 and PFSA/CNF samples) these samples have increased resistance to the PFSA desorption. So, polymers with low-equivalent weight characterized by higher acidity can be used. [ABSTRACT FROM AUTHOR]

Formation of radical anions after adsorption of 1,3,5-trinitrobenzene (TNB) on electron donor sites of fully oxidized Al(2)O(3) samples with different phase compositions is studied by EPR. It is shown that the maximum concentration of the radical anions does not substantially depend on the choice of solvent and reaction temperature, and can be used to measure the total concentration of the donor sites. The donor sites are observed in almost the same concentration about 5 × 10(16) m(-2) on all alumina polymorphs except for α-Al(2)O(3). The formation rate of the TNB radical anions and the activation energy of this process are found to depend on the donor properties of the solvent. The EPR in situ experiments showed that a substantial amount of the adsorbate forming a liquid phase is required for generation of the radical anions. These results prove that the sites measured by the formation of the TNB radical anions are not genuine electron donor sites capable of direct electron transfer to the adsorbed TNB molecules. A model of the observed paramagnetic species based on the obtained experimental data and the results of quantum chemical simulations is suggested. According to this model, a TNB radical anion substitutes a hydroxyl group forming a neutral ion pair with a surface aluminum cation. The suggested mechanism for the formation of such ion pairs involves the migration of simple radicals and does not require long-distance charge separation. It is supposed that the donor site where the process is initiated includes a negatively charged surface hydroxyl group.