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.     

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 γ-Alumina Membranes From Sulphuric Electrolyte Anodic Alumina and Its Transition to α-Alumina

Gamma-alumina membrane was prepared from anodic (amorphous) alumina (AA) obtained in a sulphuric acid electrolyte. The transformation scheme, i.e., the crystallization to form metastable alumina polymorphs and the final transition to α-Al₂O₃ with heating was studied by TG-DTA and X-ray diffraction (XRD) using fixed time (FT) method. When heating at a constant rate, the crystallization occurred at 900°C or higher and the final formation of α-Al₂O₃ occurred at 1250°C or higher, which temperatures were higher than the case of using anodic (amorphous) alumina prepared from oxalic acid electrolyte. Relative content of S of the products was obtained by transmission electron microscope (TEM)-energy dispersive spectroscopy (EDS). The proposed thermal change of anodic alumina membrane prepared from sulphuric acid is as follows: 1. At temperatures lower than ca 910°C: Formation of a quasi-crystalline phase or a polycrystalline phase (γ-, δ- and θ-Al₂O₃); 2. 910–960°C: Progressive crystallization by the migration of S toward the surface within the amorphous or the quasi-crystalline phase, forming S-rich region near the surface; 3. 960°C: Change of membrane morphology and the quasi-crystalline phase due to the rapid discharge of gaseous SO₂; 4. 960–1240°C: Crystallization of γ-Al₂O₃ accompanying δ-Al₂O₃; and 5. 1240°C: Transition from γ-Al₂O₃ (+tr. δ-Al₂O₃) into the stable α-Al₂O₃. The amorphization which occurs by the exothermic and the subsequent endothermic reaction suggests the incorporation of SO₃ groups in the quasi-crystalline structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

Coating of Sol Particles for Membrane Applications

For improving chemical and thermal stability of γ-Al₂O₃ membranes boehmite (AlOOH) sol-particles are coated with Zr⁴⁺-species with two techniques. These techniques are heterogeneous precipitation (HP-method) and “surface-reaction-followed-by-polycondensation” (SRPC-method). A continuous coating layer is formed at relative low Zr⁴⁺-concentration, about one monolayer coverage of boehmite particles, and with the HP-method. For large concentrations and for the SRPC-method small particles (order 1 nm) of Zr⁴⁺-species are formed on the surface of the boehmite particles. After drying and calcination up to 1000°C no continuous layer of a zirconia phase could be detected for all samples. However the thermal stability of the porous structure is improved. Phase transitions of alumina occur at temperatures of 1100°C and the porous structure of the membrane material is then destabilized.     

Catalytic properties of Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts in the aqueous-phase reforming of ethylene glycol: Effect of the alumina support

The influence of the alumina support on the catalytic activity of Pt/Al₂O₃ catalysts in aqueous phase reforming of ethylene glycol to hydrogen was studied. The catalysts were prepared by impregnation of γ-, δ-, and α-alumina with H₂PtCl₆. The highest rate of hydrogen production (452 μmol min⁻¹ g⁻¹) obtained with the Pt/α-Al₂O₃ catalyst can be related to the highest extent of dispersion of Pt on α-Al₂O₃. XPS, TEM-EDX and TPR-H₂ measurements showed the absence of chloride-containing surface complexes in the Pt/α-Al₂O₃ catalyst. However, chloride-containing entities were found on the surface of Pt/γ-Al₂O₃ and Pr/δ-Al₂O₃ catalysts. When chloride ions are removed chlorinated Pt species facilitate the sintering of Pt crystallites and in this way affect the extent of Pt dispersion. Moreover, depending upon the particular crystalline form, alumina atoms have different coordination and alumina surfaces contain varying amounts of OH groups of different nature which affect the interaction between Pt and the support.     

Crystallization of Anodic Alumina Membranes Studied by Simultaneous TG-DTA/FTIR

The thermal change of anodic alumina (AA), particularly the exothermic peak followed by the endothermic peak at ca 950°C was studied in detail by mainly using simultaneous TG-DTA/FTIR. The gradual loss of mass up to ca 910°C is attributed to dehydration. When heated at a constant rate by using TG-DTA, an exothermic peak with subsequent endothermic peak is observed at ca 950°C, but the exothermic peak becomes less distinct with decreasing heating rate. It has been found that gaseous SO₂ accompanying a small amount of CO₂ is mainly discharged at this stage. The reaction in this stage can be considered roughly in two schemes. The first scheme can be said collectively as crystallization, in which the migration of S or C trapped inside the crystal lattice of the polycrystalline phase (γ-, δ-, and θ-Al₂O₃, which presumably accompanies a large amount of amorphous or disordered phase) occurs. In the second scheme, the initial polycrystalline (+amorphous) phase crystallizes into a quasi-crystallineγ-Al₂O₃-like metastable phase after amorphization. Conclusively,after the distinct exo- and endothermic reactions, the amorphous phase crystallizes intoγ-Al₂O₃, presumably accompanying small amount of δ-Al₂O₃. It is also found that, when maintained isothermally, the metastable phases undergo transformation into the stable α-Al₂O₃ at 912°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural thermal and textural studies on cobalt oxide/alumina supported catalysts

CoO/Al₂O₃ catalysts containing amounts of cobalt ranging form 2 to 20% were prepared atpH 11 from neutral mesoporous alumina composed of γ-Al₂O₃ and poorly crystalline boehmite, and were then dried at 80?C. X-ray diffraction, DTA and TG techniques were used to study the structural changes produced upon thermal treatment up to 700?C. Soaking of the alumina in cobalt ammine complex solutions for a period of 10 days (the time required for equilibrium) resulted in a series of catalyst samples (I–V). Another sample (III-a) was soaked for a period of 5 days only in order to study the effect of the soaking time upon the equilibrium conditions. Cobalt aluminate (CoAl₂O₄) bands were characterized in all catalyst samples except III-a. They increased in intensity with increasing cobalt content. Surface species appeared in samples heated to 80?C, and others persisted at 150?C. Heating to temperatures above 200?C resulted in the formation of cobalt oxides, due to decomposition of the surface compounds. DTA and TG studies showed that this was more pronounced at higher concentrations of cobalt. Samples heated at 500?C and above did not undergo any further structural changes, except that the boehmite in the support was converted to γ-Al₂O₃. The variations in the surface parameters followed the same pattern as found previously [1], demonstrating that the catalyst samples are mesoporous, with retention of two ranges of pore size in most cases.     

Effect of silica on the stability of the nanostructure and texture of fine-particle alumina

The effect of the modification of aluminum oxide with silicon oxide on the stability of fine-particle Γ- and δ-Al₂O₃ phases upon heat treatment in the wide temperature range of 550–1500°C was studied. It was found that the Γ- and δ-Al₂O₃ phases modified with silica are thermally stable up to higher temperatures than pure aluminum oxide. This is due to changes in the real structure of the modified samples, specifically, an increase in the concentration of extensive defects stabilized by hydroxyl groups bound to not only aluminum atoms but also silicon atoms. It is likely that Si-OH groups, which are thermally more stable than Al-OH groups, stabilize the microstructure of Γ- and δ-Al₂O₃ to higher temperatures, as compared with aluminum oxide containing no additives. Simultaneously, an increase in the thermal stability of the modified samples is accompanied by the retention of a high specific surface area and a developed pore structure at higher treatment temperatures.     

Rapid solidification of cryolite and cryolite–alumina melts

Abstract  

Rapid solidification processing (with a cooling rate in the interval 10⁵–10⁶ K s⁻¹) was used to prepare deeply undercooled cryolite–alumina melts. These samples were analyzed by XRD, infrared, and Raman spectroscopy. Besides cryolite, the amorphous phase and a low amount of ι-Al₂O₃ were detected. Annealing of the quenched sample revealed the transformation of metastable amorphous phases into different products depending on the annealing conditions. The results obtained showed that all of the elements (Na, Al, O, and F) are probably present in the amorphous parts of the quenched samples.     

Preparation of aluminum nitride fibers from alumina gel fibers by nitridation in ammonia

Aluminum nitride (AlN) fibers were prepared from alumina gel fibers and by heat-treatment in ammonia. The influence of silica on the formation of AlN was investigated. It was shown that phase transformation of alumina (γ-Al₂O₃ to α-Al₂O₃) and nitridation reaction took place above 1,100 °C for pure alumina fiber. The addition of a small amount of silica (3 wt%) suppressed the formation of α-Al₂O₃ and preserved the highly reactive metastable alumina, and nitridation rate was enhanced. Fine grain (~20 nm) AlN fibers were obtained for pyrolysis at 1,150–1,250 °C for 3 h in ammonia, and AlN was identified as the sole crystalline phase.     

Synthesis of corundum doped with cerium in supercritical water fluid

The properties of fine crystalline corundum doped with cerium (α-Al₂O₃: Ce³⁺) during synthesis in a supercritical fluid have been studied. The synthesis of corundum has been carried out by the thermal treatment of hydrargillite, Al(OH)₃, at T = 415°C and {ie290-1} MPa in reaction media that contained from 0.001 to 0.25 wt % of cerium. Cerium ions are incorporated into the boehmite lattice during the transformation of hydrargillite into boehmite, which forms fine monocrystals of the doped corundum with a size from 20 to 50 μm. The size of the corundum crystals increases with increasing pressure and increasing concentration of cerium. The synthesized α-Al₂O₃: Ce³⁺ reveals a luminescent band in the UV region at 352 nm and a blue band at 421 nm. The intensity of the cerium ion luminescence in corundum increases with an increase in the water fluid pressure during synthesis. The follow-up annealing of α-Al₂O₃: Ce³⁺ at 1400°C in a vacuum leads to a decrease in the luminescence. It has been concluded that spectrally active complex structures that include cerium ions, oxygen vacancies, and hydroxyl groups are produced in the media of a supercritical water fluid upon the synthesis of boehmite and corundum. Exposure to high temperatures causes their transformation.     

Sol–gel synthesis,characterization and catalytic activity of mesoporous γ-alumina prepared from boehmite sol by different methods

In this study, boehmite sols were used as alumina precursors for preparing mesoporous γ-aluminas by two different methods. In one case polyethylenimine was used as a structure-directing agent, and in another case ultrasound treatment was applied. Nitrogen physisorption showed that aluminas that had been prepared by these methods demonstrated different porous structures. The sample obtained without additional treatment had closely packed spherical particles and pores had ink-bottle neck morphology. Ultrasound treatment led to the transformation of ink-bottle pores into cylindrical form and to the increase in surface area and pore volume. Aluminas prepared using polyethylenimine as a template showed larger cylindrical wormhole-like mesopores with a broader pore size distribution, high surface area and pore volume. Catalytic tests showed that textural properties as well as crystallite size were very important parameters of synthesized samples which affected the catalytic activity in the methanol dehydration reaction. It was found that γ-Al₂O₃ prepared by ultrasound treatment had large crystallite size and demonstrated high catalytic activity.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Precipitated boehmites had high values of S _BET determined from nitrogen adsorption (220–300 m²g^–1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al₂O₃, and delayed transformation of γ phase into α-Al₂O₃. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties. The online version of the original article can be found at     

Dry Pressing Boehmite Gels for the Fabrication of Monolithic α-Al2O3

It is demonstrated that dense alpha alumina can be fabricated by pressing ethanol-dispersed, α-Al₂O₃ seeded boehmite. Green bodies were formed by uniaxially pressing either water or ethanol dispersed gel powder and then isostatically pressing the compacts at 280 MPa. Green microstructure and alumina densification are correlated with gel agglomerate size and dispersing medium. The ethanol-dispersed specimens showed better sinterability than the water-dispersed specimens, and sintered to more than 99% theoretical density at 1300°C. It is shown that softer gel agglomerates were obtained by alcohol dispersion and resulted in better green microstructure and higher sintered density.     

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.     

A kinetic study of the system γ-AlOOHAl2O3

A time-temperature study has been made of the system: $\text{γ-}\text{AlOOH (boehmite)}\text{Al}{}_2\text{O}{}_3$. The isothermal TTT diagram produced demonstrates the course of the transformation sequence, which involves a number of metastable transition alumina intermediate phases. This type of treatment allows calculation of kinetic parameters for the various transformation processes, and these may be correlated with the mechanisms operating. Kinetic aspects of variations in microstructure and crystal structure have also been studied. In particular, the progress of cation migration processes involved in the transformation γ → δ-Al₂O₃ has been followed in terms of the continuous variation of the spinel subcell parameters.     

Chemical composition optimization and characterization of the NiO/B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system as a catalyst for ethylene oligomerization

The samples of the NiO/B₂O₃-Al₂O₃ system with NiO contents from 0.48 to 38.30 wt % were synthesized by the impregnation of borate-containing alumina (20 wt % B₂O₃). It was found that nickel oxide occurred in an X-ray amorphous state in the samples containing to 23.20 wt % NiO. At a NiO content of 4.86 wt % or higher, the support was blocked by the modifier to cause a decrease in the specific surface area from 234 to 176 m²/g and in the amount of acid sites from 409–424 to 333 μmol/g. An extremal character of the dependence of catalyst activity in ethylene oligomerization on NiO content was found with a maximum in the range of 4.86–9.31 wt %. Based on spectroscopic data, it was found that ethylene activation on the NiO/B₂O₃-Al₂O₃ catalyst can be associated with the presence of Ni²⁺ cations, which chemically interact with the support. The catalyst containing 4.86 wt % NiO at 200°C, a pressure of 4 MPa, and an ethylene supply rate of 1.1 h⁻¹ provided almost complete ethylene conversion at the yield of liquid oligomerization products to 90.0 wt %; the total concentration of C⁸⁺ alkenes in these products was 89.0 wt %.     

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.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The paper concerns aluminium hydroxides precipitated during hydrolysis of aluminium acetate in ammonia medium, as well as aluminium oxides obtained through their calcination at 550, 900 or 1200°C for 2 h. The following techniques were used for analysing of obtained materials: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption-desorption of benzene vapours and scanning electron microscopy. Freshly precipitated boehmite/pseudoboehmite had high value of S _BET, very good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by a decrease of specific surface determined from nitrogen adsorption, decrease of sorption capacity for benzene vapours and weakening of the hydrophilic character. Calcination of all hydroxides at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ-, δ-and θ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C were characterised with high values of specific surface area and displayed quite high heat resistance, probably due to a specific morphology of starting hydroxides. The process of ageing at elevated temperature developed thermal stability of aluminium oxides.     

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.