Radiation-Thermal Processes at the Surface of Pyrogenic γ-Alumina

The effects of ⁶⁰Co -radiation, 75-keV electrons, and thermal history (295–973 K) on the surface properties of pyrogenic -Al₂O₃ were studied by thermal desorption mass spectrometry. It was found that the state of a hydroxyl cover and the concentrations of surface acid–base sites and chlorine impurity atoms can be regulated by changing the conditions of oxide irradiation.     

Surface Properties of γ-Alumina Deposits on Highly Permeable Cellular Carriers

The electron–acceptor properties and the hydroxyl cover of the surface of a secondary support, -Al₂O₃ deposited on highly porous permeable cellular materials of corundum and Nichrome, were studied. The effect of lanthana additives on the acid–base properties of the surface of this support was also studied. The surface properties of the secondary support are typical of -Al₂O₃. The type of highly permeable porous cellular materials used and the modifying additives influence both the acidity of hydroxyl groups and the strength and concentration of electron-acceptor sites.     

Studies on the Catalytic Properties of Magnesia Supported on γ-Alumina

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Methane Adsorption and Plasma-Assisted Catalytic Conversion on the Surface of γ-Alumina

Diffuse scattering IR spectroscopy was applied to study the adsorption and plasma-assisted catalytic conversion of methane (deuteromethane) on the surface of -alumina. It was found that CH₄ adsorption sites on the -Al₂O₃ surface were formed by molecular and dissociative mechanisms under the action of an electric-discharge plasma at room temperature. As distinct from -radiolysis, plasma treatment leads to the formation of acetylene in addition to other hydrocarbon products, aluminum hydrides, and surface hydroxyl groups. Plausible mechanisms were proposed for plasma-assisted catalytic reactions in the -Al₂O₃–adsorbed CH₄(CD₄) system.     

Coupling of Methanol and Isobutanol to Ethers over γ-Alumina Catalyst. Kinetic Studies

The kinetic of methanol (MeOH) and isobutanol (iBuOH) coupling to ethers has been studied. Catalytic tests were carried out in the gas phase in a fixed-bed flow microreactor, in the temperature range of 463-533 K, under atmospheric pressure, using -Al₂O₃ as the catalyst. The quasi-homogeneous model for the process was assumed. The reaction rates were described by simple kinetics.     

Preparation of YSZ Microporous Membranes on Porous α-Alumina with Metal chlorides as Precursor

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Influence of Moderate Temperature Treatment under Oxidation Atmosphere on Acidic Site Distribution of γ-Alumina

In order to elucidate the role of acidity in reaction of selective NO reduction by hydrocarbons in oxygen-rich circumstances, a commercial -alumina was treated under atmosphere at moderate temperature range and the acidic sites of treated alumina were determined by differential NH3 adsorption heat at 473 K. The alumina has little impurity of Na, Fe, Si. The BET surface area and average pore width are 350 m2/g and 4 nm respectively. Air treatment at moderate temperature has no serious effect on the total amount of acidic sites above 70 kJ/mol, but influences the acidic site distribution at the initial and the tail energy sides. Treatment at higher temperature gives rise to a higher initial adsorption heat due to activation of alumina surface. Disappearance of carbon-contained species and appearance of distinct surface hydroxyl groups under air treatment above 673 K are plausibly observed by FT-IR. Degree of dehydration has a tremendous effect on development of Brönsted acidic sites. The relationship of changes in acidic site distribution at moderate temperature and De-NOx activity at the same temperature was discussed.     

γ-Alumina: The Essential and Unexpected Role of Water for the Structure, Stability, and Reactivity of "Defect" Sites

Combining experiments and DFT calculations, we show that tricoordinate Al(III) Lewis acid sites, which are present as metastable species exclusively on the major (110) termination of γ- and δ-Al(2)O(3) particles, correspond to the "defect" sites, which are held responsible for the unique properties of "activated" (thermally pretreated) alumina. These "defects" are, in fact, largely responsible for the adsorption of N(2) and the splitting of CH(4) and H(2). In contrast, five-coordinate Al surface sites of the minor (100) termination cannot account for the observed reactivity. The Al(III) sites, which are formed upon partial dehydroxylation of the surface (the optimal pretreatment temperature being 700 °C for all probes), can coordinate N(2) selectively. In combination with specific O atoms, they form extremely reactive Al,O Lewis acid-base pairs that trigger the low-temperature heterolytic splitting of CH(4) and H(2) to yield Al-CH(3) and Al-H species, respectively. H(2) is found overall more reactive than CH(4) because of its higher acidity, hence it also reacts on four-coordinate sites of the (110) termination. Water has the dual role of stabilizing the (110) termination and modifying (often increasing) both the Lewis acidity of the aluminum and the basicity of nearby oxygens, hence the high reactivity of partially dehyxdroxylated alumina surfaces. In addition, we demonstrate that the presence of water enhances the acidity of certain four-coordinate Al atoms, which leads to strong coordination of the CO molecule with a spectroscopic signature similar to that on Al(III) sites, thus showing the limits of this widely used probe for the acidity of oxides. Overall, the dual role of water translates into optimal water coverage, and this probably explains why in many catalyst preparations, optimal pretreatment temperatures are typically observed in the "activation" step of alumina.     

Formation of α-Alumina Around 500°C in Alkoxy-Derived Alumina Gels Under Ambient Pressure—Effects of Starting Solution Composition and Seeding

The hydrolysis of aluminum-sec-butoxide (ASB) in the ASB-H₂O-C₂H₅OH-HCl solutions at 0°C for 18 h and subsequent aging of the sol for 30 days at 5°C provided an alumina gel which was partially (20–30 wt%) transformed to -Al₂O₃ when heat-treated at 500°–600°C under ambient pressure. In addition to the hydrolysis and aging conditions, the amounts of water and HCl in the starting solution were found to be very important factors for the low temperature formation of -Al₂O₃ in the resultant gel. Some difference in the chemical environment of involved OH groups was found between the alumina gel forming -Al₂O₃ at 500°–600°C and the conventional gel. Corundum seeding was found to be effective; thus, 1 wt% seeding brought about an increase in the fraction of -Al₂O₃ formed at 600°C to 40–50 wt%.     

Thermodynamic study of Iron (III) removing by the synthesized α-Alumina powder and evaluating the corresponding adsorption isotherm models using Response Surface Method

In this study, α-Alumina as an adsorbent was initially synthesized by combustion method and characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), EDAX and Brunauer–Emmett–Teller (BET) techniques. Then the efficiency of the synthetized adsorbent for iron (III) removal was investigated and the effect of corresponding parameters such as adsorbent dose, contact time, initial concentration, temperature and solution pH on the adsorption capacity were examined and the optimums values of these parameters were concluded upon the surfaces response method. By using response surface methodology (RSM), the adsorption experimental design was performed and the statistical analysis showed that the quadratic model as well as the model terms were significant. In addition, the experimental results were examined with some suitable models, such as Langmuir, Freundlich isotherm models, where Freundlich model fitted better our experimental results. Finally, the thermodynamic behavior of the studied adsorption process was considered and the thermodynamic functions of the process were evaluated. The results showed that the Fe (III) ion adsorption onto the synthesized adsorbent is exothermic and spontaneous at the experimental conditions.     

γ-Alumina as a recyclable catalyst for the four-component synthesis of 6-amino-4-alkyl/aryl-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles in aqueous medium

γ-Alumina as an expedient and recyclable catalyst for the synthesis of 6-amino-4-alkyl/aryl-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles in water via a four-component reaction is established. Comparative study of the efficacy of γ-alumina, basic alumina and KF-alumina is also discussed. The method presented is mild, environment friendly, inexpensive and functionality tolerable to give the products in good to excellent yields.