Nanocrystalline Boehmite Research Articles

Thermoluminescence characteristics of different phase transitions from nanocrystalline alumina

Nanocrystalline boehmite material was synthesized using the hydrothermal method. Different annealing temperatures have been used to transform boehmite into different alumina phases to study the effect of different phase transitions on the thermoluminescence properties of alumina. XRD analysis was carried out to investigate the crystal structure of the different alumina phases. The thermoluminescence glow curves for different alumina phases showed different structures; however, the sensitivity was almost constant for all the phase transitions of alumina over the applied dose ranging from 0.55 to 330 Gy.

Influence of the milling conditions on the thermal decomposition of Bayer gibbsite

A synthetic gibbsite (Al(OH)3) produced by the Bayer process was mechanically activated by attrition milling for 24 h with various grinding ball-to-powder ratios (BPR = 5, 10 and 20). Changes in its structure were studied by thermal analysis and X-ray diffraction. As a result of increasing ball-to-powder ratio, the grain size of gibbsite decreased while its specific surface area increased. Only for these materials, the formation of nanocrystalline boehmite (AlO·OH) and amorphous aluminium hydroxides was also observed. Upon having been heat treated between 200 and 1200 °C (2 h), boehmite was detected at the temperature range of 200 and 350 °C for the BPR10 sample, while the boehmite of BPR20 was transformed to γ- Al2O3 at 500 °C. Further, only α-Al2O3 was detected at 1200 °C for all samples. Finally, combining the TG-DSC, XRD and SEM results, it was proposed a mechanism for the thermal decomposition of the non-milled and milled samples.

Structural Chemistry of Akdalaite, Al10O14(OH)2, the Isostructural Aluminum Analogue of Ferrihydrite

As part of an effort to characterize clusters and intermediate phases likely to be encountered along solution reaction pathways that produce iron and aluminum oxide-hydroxides from Fe and Al precursors, the complete structure of Al10O14(OH)2 (akdalaite) was determined from a combination of single-crystal X-ray diffraction (SC-XRD) data collected at 100 K to define the Al and O positions, and solid-state nuclear magnetic resonance (NMR) and neutron powder diffraction (NPD) data collected at room temperature (~300 K) to precisely determine the nature of hydrogen in the structure. Two different synthesis routes produced different crystal morphologies. Using an aluminum oxyhydroxide floc made from mixing AlCl3 and 0.48 M NaOH, the product had uniform needle morphology, while using nanocrystalline boehmite (Vista Chemical Company Catapal D alumina) as the starting material produced hexagonal plates. Akdalaite crystallizes in the space group P63mc with lattice parameters of a = 5.6244(3) Å and c = 8.8417(3) Å (SC-XRD) and a = 5.57610(2) Å and c = 8.77247(6) Å (NPD). The crystal structure features Al13O40 Keggin clusters. The structural chemistry of akdalaite is nonideal but broadly conforms to that of ferrihydrite, the nanomineral with which it is isostructural.

Nanocrystalline boehmite obtained at room temperature

The single phase nanocrystalline boehmite is obtained for the first time at room temperature by a facile hydrolysis of Bayer liquor. Mesoporosity in boehmite powders was constructed using the glucose as a template. As-synthesized boehmite powders have high surface area (above 350 m2/g) and the average crystallite size less than 5 nm. Through the thermal dehydration at 500 °C, boehmite isomorphously transforms to γ-alumina retaining the lamellar morphology and mesoporosity of the parent phase.

Crystalline microstructure of boehmites studied by multi-peak analysis of powder X-ray diffraction patterns

Nanocrystalline boehmite (gamma-aluminium-oxyhydroxide) is a material of industrial importance, the functionality of which follows from its crystalline microstructure. A procedure for preparing boehmite nanoparticles, comprising the formation of a precipitate by the alkalization of an aqueous solution of aluminium nitrate and subsequent hydrothermal aging, was previously elaborated. The application of an additive (maltitol or tartaric acid) to control the sizes and shapes of crystallites in the produced polycrystalline powder of boehmite was developed. The aim of this work is a study of the effect of the hydrothermal treatment time on nanocrystalline characteristics of boehmite, both in absence and in presence of the additive. The obtained materials were investigated by using X-ray diffraction (XRD) as principal technique and additionally by scanning and transmission electron microscopy. The multi-peak analysis of powder XRD patterns was applied to determine the prevalent crystallite shape, volume-weighted crystallite size distribution, and second-order crystalline lattice strain distribution being principal quantitative characteristics of the crystalline microstructure. Based on these characteristics, three types of the microstructure correlated with the production procedures were observed and discussed in detail. The nanoparticles of boehmites were found to be monocrystalline grains with characteristic habits and sizes of order of ten nanometers weakly dependent on the hydrothermal treatment time.

Mechanically induced formation of metastable χ- and κ-Al2O3 from boehmite

Nanocrystalline boehmite (γ-AlOOH) powder was treated by high-energy planetary ball milling. With an increase in milling time and centrifugal force, boehmite was transformed into χ- and κ-Al2O3 as a metastable phase of alumina. These metastable alumina phases were confirmed by comparing with X-ray diffraction patterns of the thermally treated gibbsite (γ-Al(OH)3). Eventually, the milling of boehmite resulted in the formation of thermodynamically stable α-Al2O3. Although boehmite is normally decomposed into γ-Al2O3 by heating, the phase transformation route of boehmite induced by mechanical milling differed from that induced by thermal process. We have proposed that the different phase transformation route can be attributed to the crystal deformation of boehmite by milling.

Effect of process parameters on the morphology and adsorption properties of nanocrystalline boehmite

In the last decade, exploration of transition alumina phases with good adsorption properties has attracted a great research interest from both a fundamental and a practical point of view. The transition phases of alumina are metastable polymorphs of aluminum oxide formed through the thermal dehydration of aluminum trihydroxide and aluminum oxyhydroxide. Powder X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and low-temperature nitrogen absorption studies were employed to trace the formation of the transition phases of alumina. In this work transition alumina powders were synthesized starting from sodium aluminate solution prepared from Bayer liquor. The neutralization of sodium aluminate solution was performed with the use of sulphuric acid, while glucose was added in the starting solution. In this way, the single phase nanocrystalline boehmite was obtained. As-synthesized boehmite powders have high surface area (above 360 m2/g) and the average crystallite size less than 5 nm. The results showed that the properties of the powders (structure, morphology) are strongly influenced by the initial pH value of sodium aluminate solution, as well as by the duration of neutralization step.

Effect of Temperature on Structural and Optical Properties of Boehmite Nanostructure

The nanocrystalline boehmite, γ‐AlOOH, was synthesized by the hydrothermal method using AlCl3·6H2O and urea as precursors, and the effect of different annealing temperatures resulting in different phases of alumina (Al2O3) was obtained. The effects of different temperature on the phase and micrographs of the prepared γ‐AlOOH nanostructures were investigated. The obtained products were characterized by X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy techniques. The XRD results show that with the increasing temperature, the transformation of boehmite into well‐crystallized α‐Al2O3 and the morphology from nanoplatelets with spindle‐like edges to vermicular structure take place. The crystallite size and lattice parameters were calculated by Rietveld refinement. The convincing evidence for the crystal phase of the as‐prepared and annealed samples was provided by FTIR spectra. The Raman spectra unveil the change in vibrational modes of the phase transition alumina.

Effect of phase transitions on thermoluminescence characteristics of nanocrystalline alumina

Nanocrystalline boehmite (γ-AlOOH) was synthesized by hydrothermal method using AlCl3·6H2O and Urea as precursors. The material gets decomposed to form the γ-Al2O3 phase at around 873K on annealing in air. On annealing further at higher temperatures it gets converted into different phases, such as, δ, θ and the most stable α-phase. Not only the phase changes but the annealing has also changed the morphology of the nanomaterial, i.e. it has changed from spindle like edges to vermicular structures and also grew bigger in sizes. The formations of different phases were confirmed by the X-ray diffraction (XRD) patterns and the changes in the morphology were seen through the TEM images. Further the effect of different phases on the thermoluminescence (TL) glow curve structures was studied and it is also shown that the TL glow curves structures do change due to phase transformations. To investigate further and to determine trapping parameters, different glow curves have been theoretically deconvoluted using computerized glow curve deconvolution (CGCD method) into simple glow peaks. The values of different trapping parameters also change as the glow curve structures on phase transformations due to reorganization of energy levels and the stress/strain generated by some intermediate phases.

Control of the textural properties of nanocrystalline boehmite (γ-AlOOH) regarding its peptization ability

Kneading-extrusion is a key shaping process, especially used to prepare extrudates for catalytic applications. It usually involves several successive steps, mainly used to control textural properties such as pore diameter and pore volume of γ-alumina catalyst supports. The ability to successfully tune these properties is highly dependent on the boehmite precursor features. This work aims at understanding the influence of the peptization and neutralization steps on the textural properties of boehmite exhibiting similar crystallite size and shape but different peptization abilities. Three kinds of boehmite (low, medium and high peptization ability) obtained either by precipitation or by alkoxide hydrolysis were subjected to a peptization and neutralization steps in order to simulate the full shaping process of kneading extrusion. After peptization and neutralization, no significant changes were found in crystallite shape and size, but only in their spatial arrangement. For the boehmite exhibiting the lowest peptization ability, the particle packing seems to be frozen and no modifications result from peptization or neutralization. However for boehmite with the medium peptization ability, obtained by precipitation from the same precursors as the low peptizable sample, the particles were initially poorly organised and their spatial packing was very affected by the peptization and neutralization phases, giving a narrower pore size distribution than the synthesized powder. Though the third boehmite, obtained by alkoxide hydrolysis, was fully peptizable, this does not allow tailoring the textural properties, whatever the phase of the shaping process. Finally the partially peptizable boehmite seems to be the most versatile starting material for the control of the textural properties.

Synthesis of Nanocrystalline α-Al2O3 from Nanocrystalline Boehmite Derived from High Energy Ball Milling of Gibbiste

Dry milling of gibbsite has been carried out for 5 h in planetary ball mill to study the effect of mechanical activation on α-Al2O3 formation. Gibbsite undergoes phase transformation during milling and has resulted nanocrystalline boehmite after 5 h of milling. The average crystallite size and the BET surface area of the nanocrystalline boehmite resulted by 5 h milling of gibbsite are 8 nm and 140 m2/g, respectively. The nanocrystalline boehmite has shown reduction in the α-Al2O3 formation temperature as well as in the activation energy of α-Al2O3 formation. The average crystallite size of nanocrystalline boehmite derived α-Al2O3 is measured to be 100 nm by TEM analysis and the BET surface area of resulted nanocrystalline α-Al2O3 is 12 m2/g.

The effect of glucose on the formation of the nanocrystalline transition alumina phases

Series of alumina powders were synthesized starting from sodium aluminate solution prepared from Bayer liquor. The neutralisation of sodium aluminate solution was performed with the use of sulphuric acid. The influence of glucose as a non-surfactant additive on the structure of alumina powders at moderate pH was investigated. The results show that the properties of the powders are influenced by the initial pH value of the solution, as well as the duration of the neutralisation step. High pHs lead to the formation of powders with heterogeneous structure with bayerite as a dominant phase, which during calcinations converts to η-alumina with high surface area. Addition of glucose to the starting aluminate solution leads to the formation of nanocrystalline boehmite with estimated average crystallite size less than 3 nm and high surface area (above 300 m 2/g). After calcinations, boehmite transforms to γ-alumina. The results have shown that during the heat treatment, structural transformations proceeded simultaneously with the significant changes in the textural properties of the obtained mesoporous γ- and η-alumina powders.

Effects of boehmite synthesis conditions on the epoxidation of styrene

The influence of synthesis conditions on the properties of nanocrystalline boehmite catalysts was studied for styrene epoxidation reaction in liquid phase. At hydrothermal synthesis of boehmite, effects of pH, aging time and precursor aluminum salts were studied by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and total surface area. Characterization results showed that boehmite samples were in the nanosize range and single-phase with high crystallinity. Styrene epoxidation reaction in liquid phase was performed over boehmite catalysts at 80 °C in ethanol, using 30% aqueous hydrogen peroxide solution and urea + 30% aqueous hydrogen peroxide solution as oxidants. The progress of the reaction was followed by gas chromatography (GC). Higher styrene conversion and higher epoxide selectivities were achieved when urea was mixed with H2O2 solution. Boehmite catalyst samples aged for 8 h resulted in higher epoxide selectivities.

Preparation of Nano-Crystalline Boehmite as a Gamma-Alumina Source for the Production of Na.beta-Al2O3 Solid Electrolyte

Nano-crystalline boehmite (AlOOH) proposed as a precursor for g-Al2O3 production was prepared using precipitation process. The mixture of two starting solutions of AlN3O9.9H2O and Na2Al2O4 was formed and NaOH was used as pH adjustment. Various preparation conditions, starting pH and temperatures were applied. The results suggested that high purity AlOOH powder could be obtained at solution temperature above 60oC and starting pH was in range 7-11. Their crystalline size increased from 2.4 to 3.9 nm with increasing pH. According to the results, AlOOH from pH 8 and temperature 80oC was selected for producing the g-Al2O3 by calcination in the temperature range of 350oC-1100oC. Transformation from AlO(OH) to g-Al2O3 could be readily observed from the whole temperature range and g-Al2O3 crystalline size was increased with increasing calcination temperature. After the temperature reached 1100oC, transformation to a- and q- phases were also found. Morphology of g-Al2O3 powder shows a strong aggomeration.

Elaboration of boehmite nano-powders by spray-pyrolysis

Boehmite (γ-AlOOH) synthesis have been investigated using a spray pyrolysis (SP) device starting from a stable sol of Al-tri- sec-butoxide peptized by nitric acid. Free spherical particles from 100 to 500 nm have been elaborated. Particles sub-structure is made of nano-crystalline boehmite with very small average crystallite size (one crystal cell along the b axis). The nano-crystalline boehmite synthesized by SP at low temperature (200 °C) is spontaneously dispersible in water without any surface treatment. Boehmite powder may be transformed to transition γ-alumina by heat post-treatment. Powders of sub-micrometric and spherical γ-alumina particles may also be synthesized by SP at 700 °C.

Effect of synthesis parameters on precipitation of nanocrystalline boehmite from aluminate solutions

Nanocrystalline boehmite (γ-AlOOH) is a cost-effective material for the production of γ-Al 2O 3 finding many industrial applications as catalyst or catalyst support, membranes and adsorbents. The preparation conditions applied in the production step of nanocrystalline boehmite strongly affect its morphology, which in turn is reflected to the final transition alumina. In this work, a precipitation method for the production of nanocrystalline boehmite is described studying the effect of pH, temperature and ageing time on the morphology of the final precipitate. The experiments were performed at temperatures 30, 60 and 90 °C, under moderate pH conditions (5, 6, 7) and 1 week of ageing in the mother liquor. What are noteworthy in the performed experiments are the starting solution used and the mixing procedure. The starting solution was a supersaturated sodium aluminate solution (SSA) with concentration equal to the Bayer liquor, which is not usually used in such synthesis experiments. On the other hand, the mixing procedure did not follow the usual route of addition of the neutralization agent (acid) to the SSA solution; on the contrary the SSA solution was added to the neutralization agent. Amorphous boehmite was prepared at temperatures as low as 30 °C and pH 7 under prolonged ageing conditions. At 60 °C the formation of pure nanocrystalline boehmite with crystallites 3–8 nm was facilitated at pH 6 and pH 7 after ageing in the mother liquor, while at the higher temperature of 90 °C the formation of pure nanocrystalline boehmite with crystallite size between 3 and 13 nm was achieved at pH 5, pH 6 and pH 7. Ageing and temperature influenced the crystallinity of the precipitated phases, with prolonged ageing and high temperatures inducing high crystallinity. The pH conditions also had a strong effect on the crystallite size of precipitates. Actually, for the same temperature and ageing time the higher the pH the larger the crystallites of the precipitates.

On the paradox of transition metal-free alumina-catalyzed epoxidation with aqueous hydrogen peroxide

The aluminas obtained by calcination of nanocrystalline boehmite, prepared by the sol–gel method, in the range 200–1000 °C were characterized by powder X-ray diffraction, thermogravimetric analysis, Fourier transformed infrared spectroscopy, nitrogen adsorption–desorption, and scanning electron microscopy. The catalytic activity of these aluminas in the epoxidation of cis-cyclooctene with aqueous 70 wt% hydrogen peroxide shows, after 24 h, a profile with two maxima at 400 and 700 °C that corresponds to 85 and 80% yield, respectively. However, this profile cannot be directly related to the structural and textural properties of the calcined aluminas. Regarding the hydrophilicities of the calcined aluminas, determined by the amount of water molecules per nm 2, the epoxide yield of the aluminas can be segregated into three groups corresponding to the general crystallographic classification of these aluminas: boehmite, γ-series, and δ-series. The aluminas from the γ-series show the highest cyclooctene oxide yield after 1 h; however, due to surface modifications occurring during the reaction, the aluminas from the δ-series also give good yields after 24 h. The water adsorption capacity or hydrophilicity of these aluminas shows a strong correlation with epoxide productivity of the calcined aluminas at 24 h. Generally, better epoxide productivity is observed for lower hydrophilicities. The hydrogen peroxide efficiencies are typically 2.0–2.5 mmol of oxidant per mmol of epoxide formed after 24 h. However, the aluminas of the δ-series show poor efficiency at the beginning of the reaction that improves from an average of 4.1 to 2.3 mmol of oxidant per mmol of epoxide after 2 h due to modification of their surfaces.

Structure and thermal behavior of nanocrystalline boehmite

First, the structural features of nanocrystalline boehmite synthesized by hydrolysis of aluminum sec-butoxide according to the Yoldas method are reported. The nanosized boehmite consists of rectangular platelets averaging 8 by 9 nm and 2–3 nm in thickness which contain a large excess of water. Dehydration by heating under vacuum induced an increase in the specific surface area, down to a minimum water content (∼0.2 H 2O per Al 2O 3); values up to 470 m 2/g can be reached. However this enlargement of specific surface area only results from water loss, the surface area remaining constant. The particle morphology, the excess of water, as well as the specific surface area, depend on the amount of acid used for the peptization during the synthesis. Second, a comprehensive investigation of the dehydration kinetics is presented. The simulations of the non-isothermal experiments at constant heating rates show that thermally stimulated transformation of nanocrystalline boehmite into alumina can be accurately modeled by a 4-reaction mechanism involving: (I) the loss of physisorbed water, (II) the loss of chemisorbed water, (III) the conversion of boehmite into transition alumina, (IV) the dehydration of transition alumina (loss of residual hydroxyl groups). The activation energy of each step is found to be very similar for experiments done in various conditions (heating rate, atmosphere, kind of sample,…).

Quantitative relationships between boehmite and γ-alumina crystallite sizes

Nanocrystalline boehmite obtained by limited hydrolysis of aluminum tri-sec-butoxide or aluminum chloride was aged under different conditions before being calcined above the dehydroxylation temperature. When aging was carried out under hydrothermal conditions, the condensation of the structural units obeyed a first-order kinetic law with apparent activation energy of 12.2 kcal/mol. Under dehydroxylation conditions, the boehmite fragmentation is accounted for by a simple power law that links its volume to that of the resulting γ-alumina. The main variable is the volatile compounds content (water for instance) in the fresh sample. In terms of texture, a better organization of the initial nanoparticles in the boehmite means a lower surface area and larger pore diameter in the corresponding γ-alumina.

Raman studies of sol-gel alumina: Finite-size effects in nanocrystalline AlO(OH).

A systematic Raman-scattering investigation has been carried out on sol-gel alumina prepared by the hot-water hydrolysis and condensation of Al(${\mathrm{OC}}_{4}$${\mathrm{H}}_{9}$${)}_{3}$, the Yoldas process, as a function of process variables such as the time spent in the sol phase. Nanocrystalline boehmite, \ensuremath{\gamma}-AlO(OH), is the principal component of these materials. We have found small but systematic changes, as a function of sol aging time, in the line shape and position of the dominant boehmite Raman band observed in the alumina hydrogels. These spectral changes are interpreted in terms of nanocrystallinity-induced finite-size effects associated with the slow growth of AlO(OH) nanocrystals in the sol. X-ray-diffraction experiments were used to determine nanocrystal sizes (as small as 3 nm for gels prepared from fresh sols) and to estimate growth kinetics from the Raman-line-shape results. The Raman peak-position shift is proportional to ${\mathit{L}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\alpha}}}$, where L is the average nanocrystal size and \ensuremath{\alpha} is a Raman-versus-size scaling exponent. For AlO(OH) we find \ensuremath{\alpha} to be 1.0, close to the scaling-exponent values reported for graphite and boron nitride (BN) and different from the values (about 1.5) that describe the reported behavior of Si and GaAs.