Well-crystallized Boehmite Research Articles

Thermal stability of transition alumina nanocrystals with different microstructures

The preparation of well-crystallized boehmite nanoparticles with different morphologies, encompassing from aciculae or rods of 320, 150 and 70 nm of length to platelets of 50 nm in diameter, allowed a comparative study of their respective thermal evolutions as alumina precursors. Static thermal treatments of boehmite nanocrystals at 600, 1000 and 1200 °C and a dynamic, in situ synchrotron study between 100 and 1000 °C revealed that original boehmite microstructures, i.e. size and shape of the nanoparticles, were kept not only in γ-Al2O3 but also in transitional aluminas up to 1000 °C. Specifically, at that temperature, acicular samples presented θ-Al2O3 structure, while in platelet-like nanoparticles δ-Al2O3 was identified. Each precursor morphology favored the respective transition phase, extending the thermal stability range over the limits previously reported in the literature.The described methodology was successfully applied to tailor-make transitional Al2O3 nanocrystals with microstructures ranging from fibers and rods to platelets, enhancing their thermal stability and thus their potential applications. Different transition sequences were also reported for the different shapes of nano-boehmite precursors.

Phase Transformation and Morphology Evolution Characteristics of Hydrothermally Prepared Boehmite Particles

Under hydrothermal condition, benefiting from dehydration and in situ nucleation processes, well-crystallized 3-dimensional (3D) boehmite nanoparticles with an average diameter of around 400 nm was directly converted from gibbsite phase in aluminum oxyhydroxide powder without adding any kinds of other assistants. Through investigation the influences of different factors, it is concluded that, hydrothermal treating temperature, time and initial slurry pH play an important role in obtaining well-crystallized boehmite particles. Besides XRD and FE-SEM, TG/DTA, FT-IR and Raman spectra were also employed to follow and confirm the formation of boehmite, which is useful for optimizing the synthesis condition. Moreover, after the hydrothermal treatment under the optimum condition, because as-prepared 3D boehmite nanoparticles have abundant surface oxygen anionic vacancies (F and F+-centers), thus endowing it optical band gap around 5.41 eV. This technique is significant and useful for developing a new opportunity for large scale production of well-crystallized boehmite powder with high purity from aluminum oxyhydroxide powder.

Morphological Controlled Growth of Nanosized Boehmite with Enhanced Aspect Ratios in an Organic Additive-Free Cationic–Anionic Double Hydrolysis Method

Well-crystallized boehmite nanoparticles showing different sizes and morphologies were fabricated in an organic additive-free cationic–anionic double hydrolysis method using inorganic aluminum chloride salt and sodium aluminate as dual aluminum sources. By adjusting the molar ratios of Al3+/AlO2– in the synthesis recipe, the boehmite particles’ shapes could be controllably tuned from two-dimensional flakes to one-dimensional (1D) rods, needles, and even fibers with enhanced particles’ aspect ratios. Through X-ray diffraction and high resolution transmission electron microscopy measurements, details of the microstructural features for boehmite particles were gained, and thus the growth habits are discussed, where in strong alkaline synthesis medium with a low Al3+/AlO2– molar ratio, dispersed nanoflakes grew with (010) and (101) faces as basal and lateral surfaces, while 1D nanoparticles, i.e. nanorods, nanoneedles, and nanofibers preferentially grew along the [100] direction with (100) and (101) faces une...

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 γ-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.

Catanionic-surfactant templated synthesis of organized mesoporous γ-alumina by double hydrolysis method

Organized mesoporous γ-alumina samples were prepared by cationic-anionic double hydrolysis (CADH) method using the mixture of cationic and anionic surfactants as template. The intermediate aluminum oxyhydroxides were characterized by XRD, SEM, TEM, FT-IR and TG. Boehmite could easily form under the given synthetic conditions, where an increase in the crystallization temperature favored the formation of well-crystallized boehmite. After the calcination, organized mesoporous γ-Al2O3 was obtained by dehydroxylation between AlOOH octahedron structures. After being characterized by N2 adsorption–desorption, the obtained γ-Al2O3 was of excellent textural properties, i.e., large pore volume (0.79 cm3 g−1), and large pore size (12.1 nm) with narrow pore size distribution which can be used as good candidates for catalyst support in the processing of heavy petroleum. In this modified CADH method, the mixture of cationic and anionic surfactants plays a key role in the formation of relatively large mesopore.

Influence of hydrothermal temperature on structure and microstructure of boehmite

Boehmites (Al 2O 3· xH 2O) were hydrothermally prepared from aluminum alkoxide. The effect of temperature on preparation was studied in the range of 100–180 °C. The XRD analysis shows that with temperature increasing, the transformation of pseudo-boehmite into well-crystallized boehmite takes place. The micro-morphologies change also from irregular clew to thin cubic platelets. From the mass loss of the samples prepared at different temperatures, the value of x is estimated to vary between 1.06 and 1.67. γ-Al 2O 3 obtained by subsequent calcination of boehmite at 600 °C is also characterized by XRD and its morphology remains unchanged.

Preparação de sóis aquosos de pseudoboemita com fibrilas de comprimentos diferentes

O mineral boemita, de composição (AlO)OH, na forma sintética pode ter estruturas cristalinas denominadas "boemita bem-cristalizada", "pseudoboemita" e "pseudoboemita fibrilar", caracterizáveis por difração de raios X e por microscopia eletrônica de transmissão. É feita uma revisão das propriedades e das aplicações desses tipos de boemita em cerâmica, catalisadores e ab/adsorventes, enfatizando a especificidade do tipo nas diversas aplicações. É descrito um método de preparação de sóis e géis aquosos de pseudoboemita fibrilar, muito pura, usando matérias-primas nacionais: alumínio em pó e ácido acético, não precisando usar Hg2+ como "ativador" da reação. Controlando o tempo de envelhecimento térmico dos produtos da reação, obtêm-se fibrilas de comprimento médio desde 53 nm até 720 nm. O método permite também preparar sóis aquosos monofásicos e bifásicos, bem como cristais coloidais de pseudoboemita fibrilar. As fibrilas de pseudoboemita com até 168 h de envelhecimento térmico são "high surface materials" porque sua área de superfície específica é maior que 100 m²/g.

Preparation of alumina through a sol–gel process. Synthesis, characterization, thermal evolution and model of intermediate boehmite

Hydrated boehmite with nano-sized crystallites AlO(OH)·0.8H2O are obtained through a very reproducible sol–gel procedure and characterized using powder X-ray diffraction, in-situ IR spectroscopy, thermal analysis, BET surface area and photoelectron spectroscopy. The unit cell parameters, obtained after deleting the first shifted (020) diffraction peak, are a = 3.686 A, b = 12.179 A and c = 2.855 A. They are consistent with well-crystallized boehmite and with the position of the harmonic (080) diffraction peak. For the (020) peak, the correlation between position and peak width is confirmed for nano-crystallites. The average shape of the crystallites, determined from three peak widths, corresponds to slabs with dimensions 7.9 × 2.7 × 8.7 nm. Based on this crystallite shape, a model of hydrated boehmite is proposed and the formula corresponding to a full monolayer of chemically adsorbed water molecules is AlO(OH)·0.55H2O.The thermal evolution of hydrated boehmite leads first to hydrated γ-alumina Al2O3·0.33H2O = Al2O2.67(OH)0.66 and the number of remaining hydroxyl groups is critical for the porosity. Between Al2O3·0.33H2O and Al2O3·0.2H2O the surface area remains approximately constant (300 m2 g−1) and the hydroxyl density decreases deeply. For calcination temperature higher than 800 K, the loss of the remainder of the water leads to a strong decrease of this area. The limiting value corresponds to about 9–10 OH-groups nm−2 and can be related to the hydrogen spinel HAl5O8. The same type of isolated OH groups are present on hydrated boehmite and hydrated transition alumina (IR bands at 3670 and 3730 cm−1). A simple model of partial dehydroxylation of boehmite is proposed, in agreement with the remaining water and unit cell parameters.

Modifications in the microstructure of alumina porous materials by hydrothermal treatment

Thin films of microporous γ-alumina were prepared by hydrothermal treatment of boehmite which was obtained by a sol-gel process. Films of boehmite hydrothermally treated at different temperatures and times were studied by X-ray diffraction, Nitrogen Adsorption, DTA/TG Thermal Analysis and Scanning Electron Microscopy. The results showed that a well-crystallized boehmite with an anisotropic particle growth mainly in the (002) plane was obtained. The modifications introduced into the boehmite affected the microstructure, such as surface area, pore morphology, pore volume, pore size distribution and thermal stability of the corresponding alumina.

Organic Derivatives of Layered Inorganics Having the Second Stage Structure

The reaction of gibbsite (a modification of Al(OH)3) in glycol at elevated temperatures yielded the glycol derivative of boehmite together with a small amount of well-crystallized boehmite and a residue of unreacted gibbsite. The addition of water to the medium increased the reactivity of gibbsite and yielded a novel phase having a large basal spacing. Pure product of this phase was obtained by the reaction of microcrystalline gibbsite (<0.2 μm) in 75 vol % ethylene glycol−water or in 95% 1,4-butanediol−water at 250 °C for 2 h. XRD analysis showed that the products had the boehmite layer structure and that the difference between the basal spacings of the present products and those of the corresponding glycol derivatives of boehmite obtained in pure glycols was about the same as the thickness of a boehmite layer. IR spectra of the present products exhibited a band due to hydrogen bonding between the boehmite layers. These results suggest that the products are the glycol derivatives of boehmite having the second stage structure. 2-Methoxyethyl and 2-(dimethylamino)ethyl derivatives of boehmite having the second stage structure were also formed by the similar reactions in 2-methoxyethanol and N,N-dimethylethanolamine, respectively.

Characterization of Silica-Containing Aluminum Hydroxide and Oxide Aerogels

Pure and silica-containing Al hydroxide aerogels were prepared by the supercritical drying method. The samples were later calcined, giving rise to alumina and Si–Al mixed oxides. The materials were characterized from the points of view of their bulk and surface structures. The Si-free material before calcination is well-crystallized boehmite that converts to γ-alumina by calcination. The silica-containing hydroxides are composed of boehmite layers with silicates in the interlayer region. The resulting mixed oxides present silica essentially in the bulk. The surface structure of alumina seems poorly sensitive to silica addition. Surface silanol groups appear only for SiO2more than 4%. No Brønsted acidity appears. Silica addition allows mixed oxides with higher surface areas to be obtained.

The porosity of aluminum oxide phases derived from well-crystallized boehmite: Correlated electron microscope, adsorption, and porosimetry studies

The porosity of a series of well-characterized aluminas, prepared by the heat treatment of well-crystallized boehmite (γ-AIOOH), was investigated using techniques of nitrogen adsorption, mercury porosimetry, and electron microscopy. The complementary information available from each technique is used to present a more complete picture of the development and evolution of the pore structure, in terms of the size, shape, distribution, and accessibility of the pores. The applicability of the various techniques used for the characterization of porous materials is discussed.

Conditions of Kaolinite Formation During Alteration of Some Silicates by Water at 200°C

AbstractWhen a mineral of the montmorillonite group, saturated with Na cations, is placed in a dilute solution of hydrochloric acid, and maintained at 200°C, it should alter and produce the corresponding mineral of the kaolinite group, according to the following reaction: triphormic clay + H+ ⇌ diphormic clay + SiO2 + Na+. The formation of the diphormic clay should depend only on the value of the [Na+]/[H+] ratio, for the various temperatures used in the process. In a number of experiments, a few minerals of various types were subjected to this alteration process, carried out under a wide variety of conditions; namely, duration of the alteration treatment, clay and acid concentrations and value of the [Na+]/[H+] ratio. Formation of kaolinite, was not found but instead either an amorphous gel, or a well-crystallized boehmite, or else no dissolution at all of the initial mineral. We have already shown that a feldspar, when subjected to an alteration under similar conditions, never produces kaolinite, but forms a poorly crystallized boehmite instead. This intermediate product is the only one able to fix silica in its numerous active sites, and thus produce kaolinite. In this investigation we sought to induce a formation of poorly crystallized boehmite, from montmorillonites, with a view to a subsequent growth of kaolinite. This was achieved by inserting layers of aluminum hydroxy-polymer between the layers of a montmorillonite, followed by an alteration conducted under the same conditions as previously. At the end of a 17 hr treatment, a number of fibers of poorly crystallized boehmite (pseudo-boehmite b) appeared. At the end of a 15 days period, large amounts of kaolinite were formed, and kaolinite alone remained at the end of one month. These experiments substantiate the need of some ‘storage’ of the aluminum in an intermediate poorly crystallized mineral. This is an essential preliminary condition to any formation of clay. This clay can be formed only when the silica-monomer can be fixed on active sites of the intermediate product.

The Hydration and Crystallization of ρ-Alumina and Alumina-gel in Aqueous Solutions of Various Basic Reagents

Abstract Alumina gel, boehmite gel, ρ-alumina, η-alumina, and γ-alumina were treated with aqueous solutions of various inorganic and organic reagents at temperatures below 100°C. Boehmite gel, η-alumina and γ-alumina were found to show no change upon this treatment, while alumina gel and ρ-alumina either remained unchanged or changed into fairly well-crystallized boehmite, boehmite gel, gibbsite, bayerite, or nordstrandite, according to the treating conditions. It has been newly established that water-containing pyridine and aniline, the aqueous solution of hydrazine hydrate, and the aqueous solution of o, m-phenylenediamine are effective in forming fairly well-crystallized boehmite, nordstrandite, and gibbsite respectively.

Studies on pore systems in catalysts II. The shapes of pores in aluminum oxide systems

Complete adsorption-desorption curves of nitrogen have been measured on aluminum oxide systems, including boehmites of various origin and bayerite, as well as aluminas obtained by dehydrating these starting materials by heating at various temperatures. The hysteresis loops can be described according to a classification given in an earlier study; they belong to types A, B, or E, or combinations of them, of that classification. The conclusions with respect to the shapes of the pores which can be drawn from these curves are discussed. The pores of gelatinous boehmite and of the aluminas obtained from it consist of the spaces between the small fibrillar particles which form sheets. At higher temperatures the pores widen and the pore walls smoothen. Microcrystalline boehmite prepared by hydrothermal conversion of sheetlike gelatinous boehmite at 150 °C, is built up from very small irregularly shaped particles and its pores are formed by the spaces between these particles. Hydrothermal treatment at higher temperatures yields far more regularly shaped platelets, forming pores with approximately rectangular cross sections. Well-crystallized boehmite shows a different pore system; on heating very narrow pores are formed in addition to the wide original ones, but they disappear again by heating at higher temperatures. Bayerite preparations behave more or less as gibbsite, described in an earlier publication.