Boehmite

Infrared emission spectroscopy has been used to study the dehydroxylation behavior over the temperature range from 200 to 750 degrees Celsius of three major Al-minerals in bauxite: gibbsite (synthetic and natural), boehmite (synthetic and natural) and diaspore. A good agreement is found with the thermal analysis and differential thermal analysis curves of these minerals. Loss in intensity of especially the hydroxyl-stretching modes of gibbsite, boehmite and diaspore as function of temperature correspond well with the observed changes in the TGA/DTA patterns. The DTA pattern of gibbsite clearly indicates the formation of boehmite as an intermediate shown by a endotherm around 500 degrees Celsius. Dehydroxylation of gibbsite is followed by a loss of intensity of the 3620 and 3351 cm-1 OH-stretching bands and the corresponding deformation band around 1024 cm-1. Dehydroxylation starts around 220 degrees Celsius and is complete around 350 degrees Celsius. Similar observations were made for boehmite and diaspore. For boehmite dehydroxylation was observed to commence around 250 degrees Celsius and could be followed by especially the loss in intensity of the bands around 3319 and 3129 cm-1. The DTA pattern of diaspore is more complex with overlapping endotherms around 622 and 650 degrees Celsius. The dehydroxylation can be followed by the decrease in intensity of the OH-stretching bands around 3667, 3215 and 2972 cm-1. Above 550 degrees Celsius only a single band is observed that disappears after heating above 600 degrees Celsius corresponding to the two endotherms around 622 and 650 degrees Celsius in the DTA

Near-infrared (IR) spectroscopy has been used to distinguish between alumina oxo and hydroxy phases. Two near-IR spectral regions are identified for this function: (1) the high-frequency region between 6400 and 7400 cm−1, attributed to the first overtone of the hydroxyl stretching mode, and (2) the 4000–4800 cm−1 region attributed to the combination of the stretching and deformation modes of the AlOH units. Near-IR spectroscopy allows the study and differentiation of the hydroxy and oxo(hydroxy) alumina phases, since each phase has its own characteristic spectrum. The spectrum of bayerite resembles that of gibbsite, whereas the spectrum of boehmite is similar to that of diaspore. Bayerite has four characteristic near-IR bands at 7218, 7128, 6996, and 6895 cm−1. Gibbsite shows five major bands at 7151, 7052, 6958, 6898, and 6845 cm−1. Boehmite displays three near-IR bands at 7152, 7065, and 6960 cm−1. Diaspore shows a prominent band at around 7176 cm−1. The use of near-IR reflectance spectroscopy to study alumina surfaces has a wide application, particularly with thin films and surfaces. The technique is rapid and accurate. Near-IR, because of its sensitivity, can be used in reflectance mode for the on-line processing of bauxitic minerals.

Alumina has a variety of technological uses and has been utilized in many fields due to its versatility and abundance. The properties of aluminas, such as pore size, acid site concentration, and the amount of adsorbed water, can be tailored for a specific application using a variety of synthetic techniques. Doping alumina with silica has been shown to improve the thermal stability of the material and allows it to be used in higher temperature applications. Several synthetic routes to γ-Al2O3 involves the formation of a boehmite precursor that is later calcined at different temperatures to form various porous transitional aluminas. Silica-doped alumina can be prepared from a silica-doped boehmite precursor. We have measured the constant pressure heat capacities of a pure boehmite and a silica-doped boehmite alumina with the chemical formulas AlOOH·0.202H2O and AlOOH·0.045SiO2·0.284H2O, respectively. Molar heat capacities were measured from 1.8 K to 300 K using a Quantum Design Physical Property Measurement System (PPMS), and the data was fit to a theoretical functions below 15 K, orthogonal polynomials from 10 K to 60 K, and a combination of Debye and Einstein functions above 50 K. These fits were then used to generate Cp,m°, Δ0TSm°, Δ0THm°, and Φm° values at smoothed temperatures from 0 K to 300 K for all samples. The differences in the thermodynamic functions between both samples is attributed to the presence of silica in one of the samples. Results from this study are consistent with the results from a previous study comparing silica-doped γ-Al2O3 samples to pure γ-Al2O3 samples.

The dehydroxylation of boehmite has been studied by the application of infrared emission spectroscopy over the 200 to 750 °C temperature range. The dehydroxylation is followed by the loss of intensity of the hydroxyl stretching frequencies observed at 3478, 3319, and 3129 cm−1 and by the loss of intensity of the hydroxyl deformation modes at 1140 and 1057 cm−1. Dehydroxylation starts at 250 °C and is complete by 450 °C. No difference was found between the synthetic and natural boehmite dehydroxylation. The hydroxyl stretching frequencies show a pronounced blue shift, while the hydroxyl deformation modes show a pronounced red shift. Infrared absorption bands were observed at 3413, 3283, and 3096 cm−1 for the hydroxyl stretching frequencies and at 1161 and 1071 cm−1 for the hydroxyl deformation frequencies. Low-frequency infrared absorption bands are observed at 749, 635, and 542 cm−1 and infrared emission bands at 811, 716, 611, and 456 cm−1. The infrared emission low-frequency bands moved to higher frequencies upon thermal treatment. Spectral changes in the low-frequency bands confirm that dehydroxylation commenced at 250°C. Infrared emission spectroscopy allows the phase changes of the Al2O3–H2O alumina system to be studied in situ at the elevated temperatures.

A model for pseudoboehmite crystallite packing formed during the hydrolysis of trisecbutoxyaluminium is postulated. The model describes platelike crystallites of pseudoboehmite stacked in a sharing edges only configuration. With this type of stacking, the pore sizes detected are approximately equal to the crystallite sizes of the hydrolysates. The hydrolysates age via a dissolution re-precipitation reaction. This increases the size of the crystallite size of the pseudoboehmite formed, speeding peptization by allowing nitrate ions to enter pores and access the surfaces of the crystallites. This type of model also allows an explanation for the peptization kinetics of systems containing sec-butanol formed during the hydrolysis of trisecbutoxyaluminium.

Abstract:  Alumina based composite material could be used as porous absorbing layer for coatings on ceramic or metallic substrates in order to manufacture the catalytic supports. The paper presents some of our results referring to the preparation and characterisation of alumina hydrate (boehmite) that are to be used as binder for the preparation of alumina coatings on metallic substrates. Thermal analysis, X-ray diffraction and FTIR investigation were used to characterise the prepared alumina samples.

High‐quality Raman spectra were used for the characterization of alumina phases of gibbsite, bayerite, diaspore and boehmite. The Raman spectrum of gibbsite shows four strong, sharp bands at 3617, 3522, 3433 and 3364 cm−1 in the hydroxyl stretching region. The spectrum of bayerite shows seven bands at 3664, 3652, 3552, 3542, 3450, 3438 and 3420 cm−1. Five broad bands at 3445, 3363, 3226, 3119 and 2936 cm−1 and four broad and weak bands at 3371, 3220, 3085 and 2989 cm−1 are present in the Raman spectrum of the hydroxyl stretching region of diaspore and boehmite. The hydroxyl stretching bands are related to the surface structure of the minerals. The Raman spectra of bayerite, gibbsite and diaspore are complex whereas the Raman spectrum of boehmite shows only four bands in the low‐wavenumber region. These bands are assigned to deformation and translational modes of the alumina phases. A comparison of the Raman spectrum of bauxite with those of boehmite and gibbsite showed the possibility of using Raman spectroscopy for on‐line processing of bauxites that contain a mixture of alumina phases.

Ga-substituted boehmites were synthesized using a soft-chemistry route in the presence of poly(ethylene oxide) surfactant under hydrothermal conditions. The effect of Ga-substitution was studied by X-ray diffraction and X-ray Photoelectron Spectroscopy. Initial precipitates were amorphous (minor bayerite). After hydrothermal treatment XRD showed broad reflections at low Ga-substation indicating low crystallinity and formation of pseudoboehmite. This was confirmed by high resolution XPS of Al 2p, Ga 2p and O 1s. The Al 2p binding energy of 74.03 eV was closer to boehmite than  pseudoboehmite, probably caused by the Ga-substitution in the structure. The O 1s showed the presence of O, OH and H2O. The ratio of (total O)/(Al+Ga) showed an excess of O as expected for pseudoboehmite. The ratio of (O+OH)/Al+Ga) was also slightly larger than the theoretical value of 2 for boehmite. High concentrations of added Ga (10%, 20%) resulted in amorphous material after hydrothermal treatment.

The aging of the aluminum tridecamer in acidified aluminum sec-butoxide is studied with 27Al nuclear magnetic resonance. At 20°C the tridecamer content passes through a maximum of 19.7% after 133 h of aging to decline subsequently to a final content of approximately 12%. At 90°C the tridecamer disappears within 21 h of aging due to clustering. After 91 h colloidal gibbsite forms, while the pH drops continuously from 4.3 to 3.5. Simultaneously the monomer concentration increases to approximately 6.6%. The increase is possibly due to liberation of the central fourfold-coordinated Al from the tridecamer that rearranges to hexameric rings, and to dissolution of some gibbsite in the acid solution. The hexameric rings are considered to be the precursor for the crystallization of gibbsite and boehmite. Increase of the preparation temperature from 20°C to 80°C has no effect on the amount of tridecamer, although at 90°C the amount increases from approximately 9% to 14.1%. An increase of the preparation temperature leads to changes in the second-order quadrupole effects, which results in a line sharpening from 19.5 to 9.8 Hz for the central fourfold-coordinated Al signal of the tridecamer at 63.3 ppm.

235 σ.Πρόδρομο υλικό για την παρασκευή γ-Al2O3, καθώς και όλων των υπολοίπων μεταβατικών ή ενεργών αλουμινών (η-, δ- και θ- Al2O3) που χρησιμοποιούνται σε καταλυτικές εφαρμογές, αποτελεί τόσο το υδροξείδιο του αργιλίου (γιββσίτης, γ-Al(OH)3) όσο και το οξυ-υδροξείδιο του αργιλίου (βαιμίτης, γ-AlOOH). Η απόδοση των προϊόντων αυτών ενισχύεται όταν παρασκευάζονται από νανο-δομημένες σκόνες. Τα διαλύματα που προκύπτουν κατά τη διεργασία Bayer είναι υπέρκορα αργιλικά διαλύματα (ΥΑΔ) και οδηγούν στη σύνθεση γιββσίτη. Ο γιββσίτης αυτός αποτελεί την κυρίαρχη πρώτη ύλη και συγχρώνως την παλαιότερη για την παρασκευή εμπορικών προϊόντων ενεργής αλούμινας, παρ’όλο που για τη μετατροπή του στις προαναφερθείσες ενεργές αλούμινες απαιτείται ο μετασχηματισμός του σε βαιμίτη. Στόχος της παρούσας διδακτορικής διατριβής, ήταν από τα διαλύματα αυτά να παραχθεί νανο-κρυσταλλικός βαιμίτης με ομοιογενή καταβύθιση (απουσία φύτρων) σε θερμοκρασία χαμηλότερη από 100οC. Για τα πειράματα χρησιμοποιήθηκε συνθετ...

Abstract In this article, nanostructures of boehmite (γ-AlOOH) have been successfully synthesized in a time varied, facile and cost effective hydrothermal route. The samples have been characterized by using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and Fourier Transform Infrared (FTIR) spectroscopy. Our samples exhibit remarkably high photoluminescence emission intensity and dielectric constant (3.71 × 106 for BH12 sample) with a nominal dielectric tangent loss. A detailed growth mechanism including the nucleation and self assembly of our nanomaterial has been discussed. Moreover, we are the first to report here the electrical properties of the boehmite nanoparticles. Present investigation also explores temperature dependent dielectric property and ac conductivity of these samples. The role of crystallinity in the optical and electrical enhancement of our samples has been illustrated with proper analytical details. Not only high emission intensity in the blue and green region of the spectrum makes our sample a strong candidate for bio-imaging and bio-sensing applications, but also high dielectric constant makes them desirable for cost-effective energy storing devices, such as supercapacitors and solar cells.