An insight into the changes in the thermal analysis curves of boehmite with mechanical activation

This study deals with the changes in the thermal transformation behaviour of boehmite with mechanical activation (MA), carried out in planetary mill. Observed changes in the TG-DTG–DTA curves are: shifting of the desorption of physically adsorbed water to higher temperature, decrease in the γ-Al₂O₃ transformation temperature and its peak area, formation of α-Al₂O₃, not observed for unmilled boehmite upto 1,200 °C, for milling time ≥60 min. Reasons for such changes are explored on the basis of physicochemical changes occurring as a result of high energy milling. Structural degradation is found to increase with increase in milling time. As a consequence of structural changes, Al–OH bonds get stronger, whereas the hydrogen bonds get weaker. Stronger Al–OH bonding and enhanced surface energy increase water affinity and delays its removal. Decreased hydrogen bond strength, easy exit of dehydroxylation product (water) and displacement of Al to tetrahedral positions make the γ-Al₂O₃ transformation easier. Ease of removal of residual hydroxyls from small crystallite transition alumina from MA boehmite, as a result of shorter diffusion path, ensures α-Al₂O₃ transformation at lower temperature.     

Kinetic Analysis Crystallization of α-Al2O3 by Dynamic DTA Technique

The phase transformation of seeded (5 mass% Fe₂O₃ as a Fe(NO₃)₃ solution) boehmite derived alumina gel to α-Al₂O₃ was studied with DTA technique and compared with unseeded and α-Al₂O₃ seeded boehmite gels. Data for kinetic analysis of α-Al₂O₃ crystallization were obtained from quantitative DTA curves. The kinetic parameters were analysed by traditional Kissinger analysis and Friedman and Ozawa-Flynn-Wall methods using the Netzsch Thermokinetics program. Results of the comparison of values of activation energies for all three gels and methods are the process of α-Al₂O₃ transformation for originally γ-AlOOH/Fe(NO₃)₃ gels goes like that of unseeded boehmite gels,only under lower temperatures (lower about 200°C). This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation of unsupported alumina membrane by sol-gel techniques

Unsupported alumina membranes were prepared by sol-gel technique using aluminum isoproxide. The influence of the hydrolysis conditions, the type and concentration of peptizant acid on the boehmite sols has been studied. The suitable hydrolysis temperature for the aluminum isoproxide was above 50°C. Crack-free unsupported alumina membranes were obtained by rapid gelation processing of sols. The boehmite gel membrane and γ-Al₂O₃ membrane formed exhibited (020) and (440) preferred orientation.     

Biomolecule-Assisted Synthesis of Hierarchical Multilayered Boehmite and Alumina Nanosheets for Enhanced Molybdenum Adsorption

The effective utilization of various biomolecules for creating a series of mesoporous boehmite (γ-AlOOH) and gamma-alumina (γ-Al₂O₃) nanosheets with unique hierarchical multilayered structures is demonstrated. The nature and concentration of the biomolecules strongly influence the degree of the crystallinity, the morphology, and the textural properties of the resulting γ-AlOOH and γ-Al₂O₃ nanosheets, allowing for easy tuning. The hierarchical γ-AlOOH and γ-Al₂O₃ multilayered nanosheets synthesized by using biomolecules exhibit enhanced crystallinity, improved particle separation, and well-defined multilayered structures compared to those obtained without biomolecules. More impressively, these γ-AlOOH and γ-Al₂O₃ nanosheets possess high surface areas up to 425 and 371 m² g⁻¹, respectively, due to their mesoporous nature and hierarchical multilayered structure. When employed for molybdenum adsorption toward medical radioisotope production, the hierarchical γ-Al₂O₃ multilayered nanosheets exhibit Mo adsorption capacities of 33.1–40.8 mg g⁻¹. The Mo adsorption performance of these materials is influenced by the synergistic combination of the crystallinity, the surface area, and the pore volume. It is expected that the proposed biomolecule-assisted strategy may be expanded for the creation of other 3D mesoporous oxides in the future.     

Thermal decomposition of nickel aluminium mixed hydroxides and formation of nickel aluminate spinel

Various nickel aluminium mixed hydroxide samples of different compositions were prepared by co-precipitation from their nitrate solutions using dilute NH₄OH. Additional samples were prepared by impregnation of hydrated Al₂O₃, preheated at 600 and 900°C, with nickel nitrate solution in an equimolar ratio. The thermal decomposition of different mixed solids was studied using DTA. The X-ray investigation of thermal products of the mixed solids was also studied.The results obtained revealed that the presence of NiO up to 33.3 mole % with aluminium oxide much enhanced the degree of crystallinity of the γ-Al₂O₃ phase. In contrast, the presence of Al₂O₃ much retarded the crystallization process of the NiO phase. With the exception of samples containing 20 mole% NiO, all the mixed hydroxide samples, when heated in air at 900°C, led to the formation of well-crystalline Ni Al₂O₄ spinel, alone, or together with either NiO or γ-Al₂O₃, depending on the composition of the mixed oxide samples. The solid containing 20% NiO and heated at 900°C was constituted of amorphous NiO dispersed in γ-Al₂O₃. Heating the nickel nitrate-impregnated Al₂O₃ in air at 800–1000°C led to the formation of Ni Al₂O₄ together with non-reacted NiO and γ-Al₂O₃. The degree of crystallinity of the spinel was found to increase by increasing the calcination temperature of the impregnated solids from 800 to 1000°C and by increasing the preheating temperature of the hydrated Al₂O₃ employed from 600 to 900°C.     

Real structure of metastable forms of aluminum oxide

Differences in the real structure of γ-Al₂O₃ samples obtained by the thermal decomposition of pseudoboehmite and boehmite prepared by the hydrothermal treatment of bayerite were found. The transformations of these structures during their conversion to δ-Al₂O₃ as the treatment temperature increased were studied. The rate of conversion of metastable alumina species into the stable α-Al₂O₃ phase significantly depends on the real structure of samples. The rate of this transformation is drastically retarded in the presence of extended defects in the oxides originated from boehmite, and the stability of metastable alumina species increased as the degree of surface dehydroxylation increased.     

Synthesis of α-alumina from a less common raw material

A nanostructured α-Al₂O₃ with particle size lower than 100 nm was obtained from a hazardous waste generated in slag milling process by the aluminium industry. The route developed to synthesize alumina consisted of two steps: in the first one, a precursor of alumina, boehmite, γ-AlOOH was obtained by a sol–gel method. In the second step, the alumina was obtained by calcination of the precursor boehmite (xerogel). Calcination in air was performed at two different temperatures, i.e. 1,300 and 1,400 °C, to determine the influence of this parameter on the quality of resulting alumina. X-Ray diffraction patterns and transmission electron microscopy images of calcined powers revealed beside corundum the presence of transition aluminas and some rest of amorphous phase in the sample prepared at 1,300 °C. The increase of the calcinations temperature to 1,400 °C favors the formation of an almost single-phase corundum powder. The transition of θ- to α-Al₂O₃ was followed by means of infrared spectroscopy, since it is accompanied by the disappearance of the IR band frequencies associated with tetrahedral sites (AlO₄ sites), giving rise to a spectrum dominated by Al³⁺ ions in octahedral sites (AlO₆) characteristic of corundum.     

The dehydration of boehmite, γ-AlOOH,to γ-Al2O3

Electron microscopic observations of microstructure, together with studies of the evolution of X-ray and electron diffraction patterns, have been used to provide mechanistic information on the dehydration of boehmite to γ-Al₂O₃. Firing boehmite in air at 400°C produces slow development of a fine, lamellar, porous microstructure, oriented parallel to (001)_γ, the dimensions of which are consistent with the loss of one-quarter of the oxygen atoms of the boehmite lattice. The mechanism proposed for the dehydration is controlled by diffusion in a direction determined by the hydrogen bond chains in the boehmite structure and involves countermigration of the Al cations and protons with crystallographic formation of voids in a coherent cubic close-packed matrix. Final reorganization to give the spinel structure of γ-Al₂O₃ is suggested to involve gradual filling of the tetrahedral cation sites.     

Thermoanalytical study on the chlorination of hydrated aluminas and transition aluminas

The chlorination processes of four hydrated aluminas (bayerite, gibbsite, pseudoboehmite, boehmite) and four transition aluminas (η-,γ-,δ-,θ-Al₂O₃) were studied in the presence of active carbon by means of a gas-flow DTA apparatus. In the case of hydrated alumina systems three exothermic peaks appeared at about 230, 460 and 500°C or above, which corresponded to the formation of hydrogen chloride, white needle-like crystals and the chlorination of hydrated aluminas, respectively. On the other hand, in transition alumina systems, only one exothermic peak due to the chlorination of transition alumina appeared at 580–670°C. The relationship between the chlorination behavior and structure of transition aluminas was discussed.     

A New Method of Synthesis of Nanosized Boehmite (AlOOH) Powders with a Low Impurity Content

A new method of synthesis of nanosized aluminum oxyhydroxide (AlOOH, boehmite) powders has been suggested through a hydrothermal treatment of nanosized γ-Al₂O₃ powder in water and a 1.5 wt % HCl solution at different temperatures. It has been found that hydrothermal treatment in a 1.5 wt % HCl solution leads to the purification of the starting material; different treatment durations allow one to obtain boehmite particles of different shape. It has been demonstrated that a nanosized boehmite powder is obtained upon the hydrothermal treatment of a nanosized γ-Al₂O₃ in water above 80°С. The nanosized boehmite powders synthesized at different temperatures have been studied by various methods.     

Thermal and morphological study of Al2O3 nanofibers derived from boehmite precursor

The boehmite nanofibers were prepared by using NaAlO₂ and Al₂(SO₄)₃ as the starting materials without any surfactant. The phase transitions of the boehmite nanofibres against different temperature were studied and various phases were derived from well-crystallized boehmite nanofibers. All these phases had the same morphology even after high temperature calcination. In addition, the retention of specific surface area of the samples were very high because of the limited aggregation occurred in calcinations for each sample. For instance, the ??-Al₂O₃ obtained at 500?°C had the specific surface area (208.56?m²/g) with an average pore diameter of 6.0?nm. With the further increase of the calcination temperature, the nanofibers became shorter and coarsening, which resulted in the decrease of the specific surface area. It is worthwhile to notice that the BET surface areas (40.97?m²/g) and the pore volume (0.27?cm³/g) of the fibrous structures obtained after 1200?°C calcination are substantially higher than that of the non-fibrous alumina because of the morphology maintenance.     

Thermal stability of aluminium hydroxycarbonates with monovalent cations

Dawsonite-type compounds of formula MAl(OH)₂CO₃ (M = Na. K, NH₄) as well as a laminar hydrotalcite-type hydroxycarbonate of composition [Al₂Li(OH)₆]₂CO₃·4H₂O. have been hydrothermally synthesized The thermal decomposition of these compounds was monitored by DTA and TG, and the resulting products have been studied by X-ray and IR techniques. Sodium and potassium dawsonites are destroyed at 335°C. yielding a poorly crystalline compound in which part of the overall carbonate is present; the remaining carbonate is lost between 600 and 700°C, yielding NaAlO₂ and KAlO₂, respectively Ammonium dawsonite and lithium hydrotalcite are less stable, their thermal decomposition occurring at about 240°C. The ammonium dawsonite heated at 680°C shows the presence of A1₂O₃ with a poorly ordered structure, while lithium hydrotalcite yields poorly crystalline γ-Al₂O₃ at 500°C and a mixture of γ-LiAlO₂ and LiAl₅O₈ when the compound is heated at higher temperatures ( ~ 1000°C).     

Preparation and characterization of supports with a synthesized layer of catalytic filamentous carbon: III. Synthesis of carbon nanofibers on nickel supported onto aluminum oxide

The synthesis of catalytic filamentous carbon (CFC) on catalysts prepared by supporting Ni²⁺ compounds onto the surface of various alumina modifications (macroporous α-Al₂O₃ and mesoporous ?-Al₂O₃ and δ-Al₂O₃) using two procedures (impregnation and homogeneous precipitation) was studied. The texture characteristics (specific surface area and pore structure) of the parent supports and adsorbents with a CFC layer were compared. The effect of the supporting procedure on the surface morphology of Ni/Al₂O₃ catalysts and the synthesized CFC layer was studied by scanning electron microscopy. It was found that the carbon yield on a macroporous catalyst prepared by homogeneous precipitation was higher than that on a catalyst prepared by impregnation by a factor of ～2. The CFC layer exhibited a mesoporous structure because of a chaotic interlacing of carbon nanofibers, and the synthesis of CFC on macroporous supports resulted in the formation of a bidisperse pore structure of the adsorbent. Active and stable heterogeneous biocatalysts were prepared by the adsorptive immobilization of enzymatically active substances (glucoamylase and nongrowing baker’s yeast cells) on CFC.     

Sol–gel synthesis, characterization and catalytic activity of γ-alumina with bimodal mesopore distribution

In this study, boehmite sols were used for preparation of mesoporous γ-alumina with bimodal mesopore distribution. Superfine nanospheres of poly(methyl methacrylate) (PMMA) prepared by water based emulsion polymerization method were used as a template. Nitrogen sorption revealed that aluminas prepared using this approach demonstrated bimodal mesopore size distribution with maxima at 3.8 and 25.7 nm, respectively. Catalytic tests showed that bimodal mesopore distribution within γ-Al₂O₃ prepared with PMMA nanospheres as a template provides improved catalytic activity in the methanol dehydration reaction.     

Dynamic study of ammonium dawsonite doped with yttrium transformation at elevated temperatures

Co-precipitation of alumina/YAG precursor from aluminum and yttrium nitrates solution with ammonium carbonate results in dawsonite (NH₄Al(OH)₂CO₃). Its crystallographic parameters differ from the compound precipitated without the yttrium additive. It indicates that yttrium ions become incorporated into the dawsonite structure. The DSC/TG and X-ray measurements show decomposition of dawsonite at elevated temperature resulting in γ-Al₂O₃ which transforms to δ and θ modifications at still higher temperatures. The full transformation to α-Al₂O₃ and YAG occurs at temperatures higher than 1,230 °C.     

In situ XRD study of nanocrystalline cobalt oxide reduction

The reduction of nanocrystalline cobalt oxide samples (single-phase and supported on γ-Al₂O₃) was studied using in situ X-ray diffraction (XRD) analysis. The atomic structures of single-phase and supported Co₃O₄ samples were refined, and the occurrence of cationic vacancies was demonstrated. A set of methods (XRD, temperature-programmed reduction, and differential dissolution) was used to find that the reduction of supported and unsupported model cobalt oxide was considerably different. The single-phase sample was reduced in undiluted hydrogen to cobalt metal with a hexagonal closely packed structure. The reduction of the supported sample (unlike the single-phase sample) occurred through the formation of a crystalline CoO phase to the formation of cobalt metal with a face-centered cubic structure. Interaction of cobalt oxide with the γ-Al₂O₃ support, which hinders the reduction to cobalt metal, was detected.     

Subtle interactions in silica-alumina mixtures

Object of the study in this paper was the mechanical mixtures of amorphous silica and α-Al₂O₃ with different precursors (gibbsite, boehmite, and γ-Al₂O₃). The results obtained revealed that measurable interactions exist in different binary systems without previous thermal treatment. These interactions could be explained by the existence of attractive and repulsive forces which appear between the OH⁻ groups present on the surfaces of alumina and silica constituents. In thermally treated samples, the interactions are not driven by intermolecular forces but rather by polymorphic transformations of alumina and silica, which are followed by sintering.     

Alumina nanofibers obtained via electrospinning of pseudo-boehmite sol/PVP solution

Alumina nanofibers were fabricated by calcination of the polyvinylpyrrolidone (PVP)/pseudo-boehmite nanocomposite precursor fibers formed by electrospinning PVP/ethanol solution of dispersed pseudo-boehmite nanoparticles with and without additive of silica. The evolution of the phase, mechanical property and morphological features of the calcined fibers were studied and the effect of adding SiO₂ on the phase transformation of alumina was discussed. Adding SiO₂ can retard the phase transformation of γ-Al₂O₃ to α-Al₂O₃ and therefore inhibit the growth of alumina grains during calcination. Upon calcining the precursor fibers with 4 wt% SiO₂ additive at 1,300 °C, continuous alumina nanofibers with diameter ranging from 300 to 800 nm were obtained. These continuous nanofibers exhibited good flexibility and could be very promising for applications in filtration and catalyst support.     

KF promoted mesoporous γ-Al2O3 with strong basicity: Preparation,characterization and catalytic activitiy for transesterification to biodiesel

The present paper describes the preparation of KF/M-γ-Al₂O₃, efficient mesoporous solid bases. The procedure involves loading KF into a crystalline mesoporous γ-Al₂O₃ that was synthesized by the self-assembly of poly-4-vinylpyridine (P4VP) with Al³⁺ species. The synthesis is based on the strong acid-base interaction, hydrothermal treatment at 180°C and calcination at 550°C. Characterizations using XRD analysis and low temperature N₂ adsorption indicated that different amounts of KF could be introduced into crystalline mesoporous γ-Al₂O₃ to obtain catalysts with high BET surface areas, large pore volumes and uniform pore size distribution. Based on SEM images, KF/M-γ-Al₂O₃ catalysts have rough surface character and a large nanopore volume. CO₂-TPD curves registered for KF/M-γ-Al₂O₃ contain high temperature peaks, indicating strong basicity of the catalysts. Under the same reaction conditions KF/M-γ-Al₂O₃ catalysts exhibit much better activities for transesterification to biodiesel than KOH, NaOH, H₂SO₄, hydrotalcite and CaO. Enhanced activities appear to arise from strong basisity and large BET surface areas.     

Surface reactions of dimethyl ether on γ-Al2O3: 1. Adsorption and thermal effects

Reactions of dimethyl ether (DME) over γ-Al₂O₃ at 250°C have been investigated in a flow catalytic reactor. The main products of the interaction between DME and alumina are methanol and water. Heat evolution is observed as DME is passed over alumina, and replacing DME with nitrogen gives way to heat absorption. Calcination of alumina before the reaction considerably strengthens the exotherm, which is due to DME adsorption, while the endotherm is due to the desorption of weakly bound DME. The role of the hydroxyl groups of γ-Al₂O₃ in methanol and water formation has been elucidated. Treating alumina with water vapor after bringing it into contact with DME completely restores the hydroxyl cover and replaces strongly adsorbed DME with hydroxyl groups.