Local structure of pseudoboehmites

Summary The structure of pseudoboehmite was studied by the method of wide-angle X-ray scattering (WAXS). The samples of pseudoboehmite with various characteristics produced by different methods were examined in order to obtain the most complete information about their structure. In contrast to reference data, it was found that modifications in the structure of pseudoboehmite layers occur viathe insertion of additional water molecules.     

XPS study of the major minerals in bauxite: gibbsite, bayerite and (pseudo-)boehmite

Synthetic corundum (Al2O3), gibbsite (Al(OH)3), bayerite (Al(OH)3), boehmite (AlO(OH)) and pseudoboehmite (AlO(OH)) have been studied by high resolution XPS. The chemical compositions based on the XPS survey scans were in good agreement with the expected composition. High resolution Al2p scans showed no significant changes in binding energy, with all values between 73.9 and 74.4 eV. Only bayerite showed two transitions, associated with the presence of amorphous material in the sample. More information about the chemical and crystallographic environment was obtained from the O1s high resolution spectra. Here a clear distinction could be made between oxygen in the crystal structure, hydroxyl groups and adsorbed water. Oxygen in the crystal structure was characterised by a binding energy of about 530.6 eV in all minerals. Hydroxyl groups, present either in the crystal structure or on the surface, exhibited binding energies around 531.9 eV, while water on the surface showed binding energies around 533.0 eV. A distinction could be made between boehmite and pseudoboehmite based on the slightly lower ratio of oxygen to hydroxyl groups and water in pseudoboehmite.     

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.     

Structural features of finely dispersed pseudoboehmite obtained by a sol-gel method

Simulation of diffraction patterns, radial distribution function analysis, and electron microscopy are applied to study the features of the structure of nanosized pseudoboehmite obtained by the sol-gel method. It is found to consist of plate-like particles with a thickness of one lattice constant in the [010] direction. Such a structure of the pseudoboehmite particles results in the absence of the 020 diffraction peak in the diffraction pattern.     

Preparation of aqueous sols containing well-shaped boehmite crystals

The paper describes the preparation of aqueous sols of crystalline boehmite, containing about 70 grams of AlOOH per liter from three different starting materials; gels of Al(OH)₂Ac are obtained and are hydrolyzed by thermal aging into sols of fibrillar pseudoboehmite. The pseudoboehmite fibrils are hydrothermally recrystallized at 200°C producing well-shaped hexagonal laths of boehmite, also as stable sols.     

Solution-based chemical synthesis of boehmite nanofibers and alumina nanorods

This article reports an easy chemical method of synthesizing boehmite nanofibers by a modified sol-gel process involving aluminum isopropoxide precursor. Nanorods of gamma-alumina have been successfully prepared after dehydration of the viscous sol at 600 degrees C for 4 h in air. The boehmite nanofibers and gamma-alumina nanorods were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy for surface chemistry and functional groups, scanning electron microscopy, high-resolution transmission electron microscopy with selected area electron diffraction, and energy-dispersed spectroscopy for morphology and structure identification. The length of the boehmite nanofibers was found to be more than 10 mum with a crystalline lattice structure. The mechanism of formation of the boehmite nanofibers included the preferential growth along the longitudinal axis due to interaction between the solvent molecules and the surface OH- groups of hydrogen bonds. It is also suggested that the boehmite nanofibers may have formed due to the inherent instability of the planar structure of the boehmite lattice. The diameter of the gamma-alumina nanorods was found to be less than 10 nm with a varying length in the range of 50-200 nm. Boehmite to gamma-Al2O3 transformation was attributed to the loss of water molecules by internal condensation of protons and hydroxyl ions.     

Relationship between formation conditions, properties, and crystallite size of boehmite

Boehmites (gamma-AlOOH) with a variety of crystallite sizes were prepared under various precipitation conditions from solutions of aluminum nitrate and sodium aluminate and also by hydrothermal treatment of a commercial boehmite powder. The relationships between the crystallite size and other properties of the boehmites were investigated. Crystalline boehmite was formed in the pH range 7 to 10 while gels precipitated below pH 7 and bayerite (Al(OH)3) formed at pH 11. The crystallite size of the boehmite increased at higher pH, higher temperature of the precursor solution, and longer aging time. As the crystallite size of the boehmite increased, the specific surface area, pore volume, and excess water decreased. Boehmites of small crystallite size, called pseudoboehmite, showed broadened X-ray peak widths and increased d-spacing of the 020 reflection. These changes are attributed to the small crystallite size (smaller numbers of stacking layers) and also to excess water in the interlayers of the boehmite structure.     

由单层薄水铝石制备的活性氧化铝处理水中氟离子

The preparation of high-efficiency and low-cost adsorbents for the defluoridation of drinking water remains a huge challenge. In this study, single-layer and multi-layer boehmite were first synthesized via an organic-free method, and active alumina used for fluoride removal from water was obtained from the boehmite. The advantage of a single layer is that more aluminum is exposed to the surface, which can provide more adsorption sites for fluoride. The active alumina adsorbent derived from single-layer boehmite exhibits a high specific surface area and excellent adsorption capacity. The high surface area ensures a high adsorption capacity, and the organic-free synthesis method lowers the preparation cost. The as-prepared adsorbent was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FTIR) and nitrogen adsorption-desorption analysis. The single-layer structure of boehmite was determined from the simulated XRD diffraction pattern of single-layer boehmite. The disappearance of the (020) diffraction peak of boehmite illustrates that the dimensions in the b direction are extremely small, and according to the XRD simulation results, the single-layer structure of boehmite could be determined. Single-layer boehmite with a surface area of 789.4 m²·g^-1 was formed first. The active alumina obtained from the boehmite had a surface area of 678.4 m²·g^-1, and the pore volume was 3.20 cm³·g^-1. The fluoride adsorption of the active alumina was systematically studied as a function of the adsorbent dosage, contact time, concentration, co-existing anions, and pH. The fluoride adsorption capacity of the active alumina obtained from the single-layer boehmite reached up to 67.6 mg·g^-1, which is higher than those of most alumina adsorbents reported in the literature. The adsorption capacities of the active alumina are related to the specific surface area and the number of hydroxyl groups on the surface. Dosages of 0.6, 1.0, and 2.6 g·L^-1 of active alumina were able to lower the 10, 20, and 50 mg·L^-1 fluoride solutions, respectively, below the maximum permissible limit of fluoride in drinking water in China (1.0 mg·L^-1), suggesting that the active alumina synthesized in this work is a promising adsorbent for defluoridation of drinking water. In addition, the fluoride adsorption is applicable in a wide pH range from 4 to 9 and is mainly interfered by SO₄^2- and PO₄^3-. Further investigation suggested that the fluoride adsorption of the active alumina follows the pseudo second-order model and Langmuir isotherm model     

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.     

Die Bildung von kristallinem Böhmit in gemeinsam gefällten AlFe(III)-Hydroxiden unter hydrothermalen Bedingungen

Under mild hydrothermal conditions originally amorphous aluminium hydroxide coprecipitated with iron(III)ions as Al_0,5Fe_0,5(OH)₃ transforms into aluminium hydroxide crystalline phases which are not observed in products of ageing of pure aluminium hydroxide.By X-ray diffraction analysis, IR spectroscopic studies and microscopic observations it has been found that due to the presence of iron(III) hydroxide it is possible to stabilize the trihydoxide of aluminium i.e. bayerite until 130–140°C. Moreover, with increasing temperature well crystallized boehmite is obtained instead of the usually formed pseudoboehmite.Experiments revealed that the formation of crystalline boehmite does not take place as a process of pseudoboehmit crystallites growth but only as a result of bayeriteboehmite transformation.

     

Molecular structure of self-assembled chiral nanoribbons and nanotubules revealed in the hydrated state

A detailed molecular organization of racemic 16-2-16 tartrate self-assembled multi-bilayer ribbons in the hydrated state is proposed where 16-2-16 amphiphiles, tartrate ions, and water molecules are all accurately positioned by comparing experimental X-ray powder diffraction and diffraction patterns derived from modeling studies. X-ray diffuse scattering studies show that molecular organization is not fundamentally altered when comparing the flat ribbons of the racemate to chirally twisted or helical ribbons of the pure tartrate enantiomer. Essential features of the three-dimensional molecular organizations of these structures include interdigitation of alkyl chains within each bilayer and well-defined networks of ionic and hydrogen bonds between cations, anions, and water molecules between bilayers. The detailed study of diffraction patterns also indicated that the gemini headgroups are oriented parallel to the long edge of the ribbons. The structure thus possesses a high cohesion and good crystallinity, and for the first time, we could relate the packing of the chiral molecules to the expression of the chirality at a mesoscopic scale. The organization of the ribbons at the molecular level sheds light on a number of their macroscopic features. Among these are the reason why enantiomerically pure 16-2-16 tartrate forms ribbons that consist of exactly two bilayers, and a plausible mechanism by which a chirally twisted or helical shape may emerge from the packing of chiral tartrate ions. Importantly, the distinction between commonly observed helical and twisted morphologies could be related to a subtle symmetry breaking. These results demonstrate that accurately solving the molecular structure of self-assembled soft materials--a process rarely achieved--is within reach, that it is a valid approach to correlate molecular parameters to macroscopic properties, and thus that it offers opportunities to modulate properties through molecular design.     

X-ray diffraction study of human serum

Blood serum from people suffering from cancer and healthy subjects was subjected to a comparative study by X-ray diffraction. The diffraction patterns were referred to that of pure water. The patterns obtained for blood serum of healthy subjects were similar to that of pure water, while those of cancer patients (two kinds of cancer changes) were different. The former similarity is accounted for by the dominant interaction of water molecules in healthy blood serum with non-polar side chains of amino acids, stabilising the serum structure. In samples from cancer patients the structure of water in blood serum is destroyed because of enhanced interactions of water with polar molecules of conformationally changed proteins. This observation suggests X-ray examination of blood serum from cancer patients and healthy subjects, indicating X-ray diffraction as a diagnostic tool for the occurrence of cancer. The results of this work have shown that the presence of protein affected by cancer has a destructive effect on the structure of water in human serum. The results reported confirm the earlier finding relating cancer changes to optical circular birefringence effects.     

Anisotropic molecular structure in dip-coated films of linear poly(ethylene imine) studied by infrared multiple-angle incidence resolution spectrometry

Molecular structure in dip-coated films of linear poly(ethylene imine) (LPEI) on a germanium (Ge) substrate in dried and hydrated conditions have been analyzed by infrared multiple-angle incidence resolution spectrometry (IR MAIRS). The MAIRS-IP (in-plane) and -OP (out-of-plane) spectra of the dried film exhibited largely different patterns from each other, which indicated that LPEI molecules had an apparent molecular orientation with respect to the substrate surface. Although the film exhibited no peak in X-ray diffraction patterns, the low-crystallinity film has been found to have highly oriented molecular structure. Many key bands indicated that the molecules were involved in the double-stranded helix structure, which is specific to the anhydrate crystal of LPEI, with nearly perpendicular orientation. The Davydov splitting of the NH stretching vibration mode was readily captured by the IR MAIRS spectra, which also supported the helix standing model. When the film was stored in a humid condition, on the other hand, IR MAIRS spectra revealed that the helix was resolved to be straight chains, but the perpendicular orientation was kept unchanged. In addition, the MAIRS spectra also revealed molecular orientation of the water molecules of crystallization. The unique molecular arrangements are understandable by considering that the stabilization energy in the polymer monolayer directly attached on the substrate surface is minimized by the standing-molecule arrangements.     

Ions in water: the microscopic structure of concentrated NaOH solutions

A neutron diffraction experiment with isotopic H/D substitution on four concentrated NaOH/H(2)O solutions is presented. The full set of partial structure factors is extracted, by combining the diffraction data with a Monte Carlo simulation. These allow to investigate both the changes of the water structure in the presence of ions and their solvation shells. It is found that the interaction with the solute affects the tetrahedral network of hydrogen bonded water molecules in a manner similar to the application of high pressure to pure water. The solvation shell of the OH(-) ions has an almost concentration independent structure, although with concentration dependent coordination numbers. The hydrogen site coordinates a water molecule through a weak bond, while the oxygen site forms strong hydrogen bonds with a number of molecules that is on the average very close to four at the higher water concentrations and decreases to about three at the lowest one. The competition between hydrogen bond interaction and Coulomb forces in determining the orientation of water molecules within the cation solvation shell is visible in the behavior of the g(NaHw)(r) function     

Ions in water: the microscopic structure of a concentrated HCl solution

A neutron diffraction experiment with isotopic H/D substitution on a concentrated HCl/H2O solution is presented. The full set of partial structure factors is extracted, by combining the diffraction data with a Monte Carlo simulation. This allows us to investigate both the changes of the water structure in the presence of ions and their solvation shell, overcoming the limitations of standard diffraction experiments. It is found that the interaction with the solutes affects the tetrahedral network of hydrogen bonded water molecules, in a manner similar to the application of an external pressure to pure water, although HCl seems less effective than other solutes, such as NaOH, at the same concentration. Consistent with experimental and theoretical data, the number of water molecules in the solution is not sufficient to completely dissociate the acid molecule. As a consequence, both dissociated H+ and Cl- ions and undissociated HCl molecules coexist in the sample, and this mixture is correctly reproduced in the simulation box. In particular, the hydrated H+ ions, forming a H3O+ complex, participate in three strong and short hydrogen bonds, while a well-defined hydration shell is found around the chlorine ion. These results are not consistent with the findings of early diffraction experiments on the same system and could only be obtained by combining high quality experimental data with a proper computer simulation.     

Influence of peptization on the properties of alumina produced from boehmite sols

Formation of boehmite sols from aluminium alkoxides was studied by X-ray diffraction, IR-spectroscopy, DLS (Dynamic Light Scattering) and other methods. It was found that the distortion of the boehmite crystal lattice occurs under the electrolyte sol formation processes. The surface charges were evaluated to explain the observed changes in X-ray patterns and IR-spectra of the boehmite samples and the causes that bring about these changes. The influence of the peptizing process on -Al₂O₃ specific surface was also studied.     

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.     

Spectroscopic,thermal and X-ray crystal structure studies of some bis-(pyrazine-2-carboxylato) nickel(II) complexes

Three nickel(II) complexes [Ni(PC)₂(H₂O)₂], 1, [Ni(PC)₂(TU)₂], 2 and [Ni(PC)₂(ABI)₂]· 2(H₂O), 3, PC?=?pyrazine-2-carboxylate, TU?=?thiourea and ABI?=?2-aminobenzimidazole, were synthesized and characterized by elemental analysis, IR spectroscopy and thermal analysis. The complexes were also investigated by single crystal X-ray diffraction analysis. Structures of the monomeric complexes showed nickel(II) chelated to two PC ions from an oxygen atom of carboxylate ion and the adjacent hetero nitrogen atom. The three complexes were crystallized in a monoclinic system with P2 ₁ /c space group for 1, while 2 and 3 had C2/c space groups. The structure of 1 showed two coordinated water molecules occupying the trans positions of a slightly elongated octahedron. The structure of 2 consisted of two PC ions and two thiourea molecules in trans positions. The structure of 3 was different and showed a highly distorted octahedron with two ABI molecules chelated to the nickel ion in cis positions through their hetero nitrogen atoms. Two water molecules of crystallisation were shown in the structure of 3.     

X-Ray Structural Analysis of Human Serum

The structure of human serum was studied by X-ray diffraction method and the patterns were referred to those of pure water. For two groups of serum samples, the patterns differed significantly from those of pure water. One of the distinguished groups came from people suffering from neoplasmic diseases - the samples were studied directly after centrifugation. The other distinguished group included the samples studied after precipitation of proteins. The results obtained confirmed the earlier supposition that the dominant interactions in water low-concentration solutions of proteins in the serum of healthy people are those between water molecules and non-polar chains of side amino acids. For the samples of serum with precipitated proteins or obtained from hemolysed blood, the diffraction pattern shows a sharp maximum, testifying to the formation of a significant number of water-ion complexes disturbing the structure of pure water. The results have confirmed a possibility of using serum diffraction patterns as a diagnostic tool.     

X-ray crystal structure of a p-hydroxycalix[6]arene derivative

The first molecular structure of a p-hydroxycalix[6]arene 6 has been determined by a single crystal X-ray diffraction study. The calix[6]arene molecule assumes a 1,2,3-alternate conformation with all OH groups at the upper rim engaged in H-bonds with pyridine molecules. The stacking of molecules of p-hydroxycalix[6]arene 6 along the a and c axes gives rise to a solvent pseudo-cylindrical cavity at the centre of the cell.