Kinetics and hydrothermal transformation of gibbsite

The transformation of gibbsite have been studied under hydrothermal influence. Results of the X-ray diffraction analysis of the samples heated at different temperatures from 170°C to 265°C are also discussed in detail. The percentage of the boehmite phase formed, due to the transformation of the gibbsite, have been determined by comparing the intensity of the most strongest reflection of the boehmite phase with a fully transformed sample. The activation energy value calculated form X-ray diffractograms is almost comparable with the value calculated from main DTA endotherm of gibbsite.Es wurde die Umwandlung von Gibbsit unter hydrothermalem Einfluß untersucht. Weiterhin werden auch Ergebnisse der Röntgendiffraktionsanalyse an den bei verschiedenen Temperaturen zwischen 170°C und 265°C erhitzten Proben besprochen. Durch Vergleich der Intensität der stärksten Reflexion der Böhmitphase mit einer vollständig umgewandelten Probe wurde der prozentuelle Anteil der während der Umwandlung von Gibbsit gebildeten Böhmitphase bestimmt. Der Wert für die Aktivierungsenergie, berechnet aus den Röntgendiffraktogrammen ist annähernd vergleichbar mit dem aus der DTA von Gibbsit berechneten Wert.

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RRL Contribution No. 2028.     

Investigation of dehydroxylation of gibbsite into boehmite by DSC analysis

The dehydroxylation of gibbsite into boehmite was investigated by means of DSC analysis under non-isothermal conditions in the temperature range 453–673 K at heating rates from 2.5 to 20.0 K min^?1. Mathematical analysis of the experimental DSC curves revealed the mechanism and kinetics of the gibbsite dehydroxylation process. The kinetic curvesα=f(t) andα=f(T) are sigmoidal in shape; their inflection points and the ν_m point of the curvesν=f(T) andν=f(T) are interrelated and are defined by the concept of a stationary point. The activation energy for the first stage of gibbsite dehydroxylation in the temperature range 453–673 K is 132.92±8.33–142.26±8.33 kJ mol^?1.     

Emanation thermal analysis study of synthetic gibbsite

Emanation thermal analysis (ETA) was used for thermal characterization of microstructure changes taking place during heating of synthetic gibbsite sample in argon in the range of 25–1200°C. Microstructure development and the increase of the surface area under in-situ conditions of the sample heating were characterized. The increase of the radon release rate from 130–330°C monitored the increase of the surface area due to the dehydration of Al(OH)₃. During heating of the sample in the range 450–1080°C the ETA results characterized the annealing of surface and near surface structure irregularities of intermediate products of gibbsite heat treatment. The mathematical model for the evaluation of the ETA experimental results was proposed. From the comparison of the experimental ETA results with the model curves it followed that the model is suitable for the quantitative characterization of microstructure changes taking place on heating of gibbsite sample. This revised version was published online in July 2006 with corrections to the Cover Date.     

Desorption of nitric acid from boehmite and gibbsite

Solid-state Fourier transform infrared spectroscopy (FTIR), evolved gas analysis-FTIR (EGA-FTIR), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) have been used to investigate the desorption of nitric acid from boehmite and from gibbsite. Samples containing between 3 and 36% of adsorbed nitric acid by mass were prepared by placing the mineral in a 70% nitric acid solution or by the adsorption of nitric acid vapors in humid air. FTIR established that water-solvated nitrate was the main species adsorbed on the surface of either mineral under these conditions. The water-solvated nitrate vaporized as nitric acid at approximately 400 K with an enthalpy of desorption of approximately 50 kJ/mol for both surfaces. A second nitric acid desorption occurred at approximately 450 K and had an enthalpy of desorption of 85 kJ/mol (95 kJ/mol) for boehmite (gibbsite). This was assigned as desorption of partially solvated aluminum hydroxylated nitrate. Monodentate and bridging nitrate were also observed on the boehmite. These species desorbed at approximately 725 K as NO2 and O2 with an enthalpy of reaction of approximately 55 kJ/mol of NO2 desorbed.     

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.     

Macroporous zirconia particles prepared by subcritical water in batch and flow processes

Porous zirconia particles were synthesized through a low-temperature hydrothermal synthesis process. Under hydrothermal conditions, water can control the direction of crystal growth, morphology, particle size, and size distribution because thermodynamics and transport properties can be controlled by pressure and temperature. In a batch process, the hydrothermal synthesis was conducted at 200–300 °C and 30 MPa with an SUS-304 tube as the reactor. At the same reaction pressure, experiments were also performed for a flow process with temperatures of 180–200 °C. The synthesized products were calcined and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results showed that the macroporous zirconia particles that were formed had pore diameters around 419 nm. The XRD pattern indicated that the products were composed of zirconium oxide particles with monoclinic, tetragonal, and cubic structures.     

Pd nanoparticles immobilized on boehmite by using tannic acid as structure-directing agent and stabilizer: a high performance catalyst for hydrogenation of olefins

Boehmite-supported Pd nanoparticles (Pd–TA–boehmite) were successfully synthesized by a hydrothermal method using tannic acid as the structure-directing agent as well as stabilizer. The physicochemical properties of the Pd–TA–boehmite catalyst were well characterized by XPS, XRD, N₂ adsorption/desorption, and TEM analyses. Catalytic hydrogenation of olefins was used as the probe reaction to evaluate the activity of the Pd–TA–boehmite catalyst. For comparison, the Pd–boehmite catalyst prepared without tannic acid was also employed for olefin hydrogenation. For all the investigated substrates, the Pd–TA–boehmite catalyst exhibited superior catalytic performance than the Pd–boehmite catalyst. For the example of hydrogenation of allyl alcohol, the initial hydrogenation rate and selectivity of the Pd–TA–boehmite catalyst were 23,520 mol/mol h and 99 %, respectively, while those of the Pd–boehmite catalyst were only 14,186 mol/mol h and 93 %, respectively. Additionally, the hydrogenation rate of the Pd–TA–boehmite catalyst could still reach 20,791 mol/mol h at the 7th cycle, which was much higher than that of the Pd–boehmite catalyst (5,250 mol/mol h) at the 4th cycle, thus showing an improved reusability.     

The Study of Nonisothermal Kinetics of Dehydration of Gibbsite in a Mixture with Zinc Oxide

Nonisothermal kinetics of dehydration of gibbsite in a mixture with zinc oxide has been studied by Friedman analysis (differential method) and Flynn‐Wall‐Ozawa analysis (integral method). The values of the activation energy and preexponential factor depending on the decomposition extent of gibbsite to boehmite have been determined. It has been shown that both methods give similar results. It has been established that the activation energy has a maximum value of 150–170 kJ/mol in the start stages of thermolysis (for conversion extent of less than 0.3). During further dehydration, the activation energy is reduced to 100–110 kJ/mol. It has been found that comilling of the mixture results in decreasing activation energy to 40–50 kJ/mol for a conversion extent more than 0.8. This testifies to the transition of the dehydration process out of the kinetic mode to the diffusion mode. It was explained by the accumulation of mechanical energy in the form defects of crystal lattice of gibbsite at the comilling stage.     

Modified hydrotalcites application as precursors for (Na,K)Mg/Al spinel-type compounds formation

In this work, a synthesis route of (Na,K)Mg/Al spinel-type compounds, which combines hydrothermal synthesis at low temperatures (<200 °C) and solid-state sintering (>800 °C) methods, is presented. It was examined that NaOH and KOH additives induce the reaction between initial Mg and Al components and the formation of hydrotalcite during hydrothermal treatment. It should be noted that after 1 h of calcination of synthetic precursors at 850 °C spinel-type compounds are formed only in the samples with alkali addition. Meanwhile in the pure system only traces of the mentioned compounds are observed at 900 °C. Moreover, the increase in solid-state sintering temperature and duration lead to the higher-crystallinity (Na,K)MgAl₂O₄ spinel-type compounds. It should be noted that textural properties of formed (Na,K)Mg/Al spinel-type compounds depend on the chemical composition of precursors. The synthetic and calcined products are characterised by XRD, STA, FT-IR analyses and BET method.     

修饰晶种法合成MFI型分子筛膜包覆的活性炭颗粒

由于分子筛膜的分离和催化作用, 分子筛膜包覆的传统催化剂颗粒可以实现高效的催化过程. 活性炭颗粒作为一种常用的催化剂载体, 由于其表面的憎水性和不平整, 在不规则活性炭颗粒表面直接水热合成包覆一层分子筛膜非常困难. 为了克服上述缺点, 本文采用一种勃姆石凝胶修饰的晶种法在活性炭颗粒表面合成连续的分子筛膜. 以勃姆石溶胶为前驱体, 在活性炭颗粒表面通过喷涂预先形成一层相对平整的勃姆石凝胶层以改善活性炭表面. 在随后的晶种涂覆过程中, 晶种分散液加入部分勃姆石溶胶为胶粘剂, 所得的晶种层覆盖载体完全, 与载体结合牢固, 无需焙烧处理. 将晶种涂覆后颗粒在旋转动态水热175℃处理6 h, 得到分子筛膜包覆的活性炭颗粒, 所得材料分别用X射线衍射和扫描电镜进行表征. 包覆的分子筛膜为MFI 结构, 厚度约为5μm. 对比实验表明, 没有勃姆石溶胶修饰的活性炭颗粒上不能成膜. 这种勃姆石凝胶修饰晶种法为在各种惰性载体上合成分子筛膜提供了便捷的方法.     

Hydrogen bonds and local symmetry in the crystal structure of gibbsite

First‐principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the ²⁷Al MAS NMR resonances in gibbsite. The ²⁷Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished ²⁷Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al‐I site characterized by a C_Q = 4.6 MHz is surrounded by OH? groups participating in four intralayer and two interlayer hydrogen bonds, while the Al‐II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH‐bonds are aligned toward the interlayer gallery. In high‐resolution solid‐state ¹H CRAMPS (combination of rotation and multiple‐pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite‐like layer—intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis of (K,Na)NbO3 particles by traditional hydrothermal method and high-temperature mixing method under hydrothermal�Csolvothermal conditions

Pure (K,Na)NbO₃ (KNN) powders have been successfully prepared by using traditional hydrothermal method and high-temperature mixing method (HTMM) under solvothermal and hydrothermal conditions. The products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to show the change of phase, morphology, and size of the as-prepared particles with the alkalinity of the starting solution. Compared with the traditional hydrothermal method, smaller particles with higher phase purity are prepared using HTMM under hydrothermal conditions. It is found that the alkalinity has significant effects on the crystal size. The KNN grain size first increases and then decreases with increasing alkalinity. Typical samples solvothermally synthesized in a mixed solvent with isopropanol/deionized water ratio of 50/50 by volume were made of well-crystallized single-crystalline nanoparticles with size of about 500 nm.     

Charging behavior of the gibbsite basal (001) surface in NaCl solution investigated by AFM colloidal probe technique

The charging behavior of the gibbsite gamma-Al(OH)3 basal (001) surface in aqueous solution is important for correctly modeling the overall charging properties of gibbsite particles which controls surface phenomena such as adsorption and crystal growth. However, the question of whether the hydroxyl groups on the basal plane are proton active has been raised recently both from experimental and theoretical points of view. Using gibbsite crystals prepared from industrial Bayer process, the surface potentials of cleaved (001) surfaces were calculated from forces measured by the colloidal probe technique in 1 mM NaCl solution with differing pH. It was surprisingly found that the basal plane is proton active in pH less than 7 and protonation seems to level off at about pH 5. The potential-pH data was accurately fitted with a single pKa surface protonation model with pK(a) = 5.9 +/- 0.2.     

The conversion of an organometallic compound into an intercalated thin‐layer amorphous structure

A thin alumina‐hydrotalcite (Al‐HT) film was fabricated from the synthesized boehmite and HT sol samples. The sols were a Newtonian fluid within 12 h of the sol synthesis and pseudo‐plastic flow thereon. Co‐precipitated HT demonstrated poorly crystallized periclase and spinel structures and apparent doublet peak of hydrotalcite at 2θ = 39–44°, indicative of a disordered structure. The heated Al‐HT sample demonstrated highly amorphous structure with single hydrotalcite peak but barely observed γ‐alumina and γ‐boehmite phases. The exfoliation of the spinel, gibbsite and periclase in the Al‐HT was caused by the intercalation of boehmite into the HT layers that impeded the formation of the oxides phases. Copyright © 2009 John Wiley & Sons, Ltd.     

Dynamic hydrothermal synthesis of xonotlite from acid-extracting residues of circulating fluidized bed fly ash

This investigation was made to examine how the conditions of hydrothermal synthesis influence the crystal structure of xonotlite and its morphology. For synthesis, we used acid residues after extracting the alumina from circulating fluidized bed fly ash as raw material, as far as we know, that no one used before. Staring with Ca/Si = 1, hydrothermal temperatures were between 180 and 260 °C. The samples were characterized in terms of mineralogical composition (XRD), morphological analysis (SEM), and thermal gravimetric and differential thermal analyses. During the 72 h synthesis at 240 °C, well formed xonotlite fibers of 10–15 μm long were obtained. Under the same conditions, but at 200 °C, the obtained xonotlite was poorly formed. The results indicated that the residue could be used to prepare pure xonotlite. Different hydrothermal temperatures and holding times have a great impact on crystallinity and morphology of xonotlite.     

Template-free preparation of bunches of aligned boehmite nanowires

A simple method based on a hydrothermal process using alkali salts as mineralizers is proposed for the synthesis of aligned bunches of boehmite (gamma-AlOOH) nanowires without a template's assistance. Most bunches of aligned boehmite nanowires are constructed by two separated shorter bundles with widths of 700 to approximately 800 nm and lengths of about 1 microm. XRD patterns, FTIR spectra, and SEM and TEM images were used to characterize the products. The specific surface area and pore-size distribution of the obtained product as determined by gas-sorption measurements show that the boehmite bundles possess a high BET surface area and porosity properties. The importance of adding Na2B4O7 salts for the formation of bundle morphologies has been discussed.     

Synthesis of boehmite and hematite by joint hydrolysis of carbamide,aluminum chloride,and iron(III) chloride under hydrothermal conditions

The formation of boehmite and hematite in dependence of the conditions of joint hydrothermal hydrolysis of carbamide and a mixture of aluminum and iron(III) chlorides in the presence of K, Na, Ca, and Mg chlorides at T = 160–200°C and P = 0.6–1.6 MPa was studied. It was shown that the amount of boehmite and hematite being formed in hydrolysis of Al and Fe chlorides strongly depends on pressure, temperature, hydrolysis duration, and composition of the model mixture of Al, Fe, Mg, Ca, K, and Na chlorides. It was found that a complete hydrolysis of AlCl₃ and FeCl₃ with 99% yield of boehmite and hematite occurs at the stoichiometric ratio between carbamide and aluminum and iron chlorides in the starting solution, whereas mostly iron oxyhydroxide [goethite FeO(OH)] and aluminum oxychloride [Al₁₇O₁₆(OH)₁₆Cl₃] are formed at nonstoichiometric ratios.     

Effect of Solution Chemistry on Preparation of Boehmite by Hydrothermal Assisted Sol-Gel Processing of Aluminum Alkoxides

Various aluminum alkoxide precursors have been used for the preparation of boehmite by hydrothermal assisted sol-gel processing. The coordination status of aluminum in solution for all precursors employed for the preparation of boehmite phase was determined by ²⁷Al NMR and correlation between coordination status of aluminum atoms of precursors and development of boehmite phase has been investigated. Hydrothermal assisted hydrolysis of aluminum alkoxides where the aluminum atoms are four or five coordinated in solution resulted in the formation of boehmite. In contrast, hydrothermal hydrolysis of aluminum alkoxides where the aluminum atoms are six coordinated resulted in the formation of amorphous gel. Development of boehmite phase by hydrothermal hydrolysis of aluminum alkoxides at various temperatures was pursued by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy.     

Hydrothermal synthesis and characterization of nanorods of various titanates and titanium dioxide

The formation process of titania based nanorods during hydrothermal synthesis starting from an amorphous TiO2.nH2O gel has been investigated. Sodium tri-titanate (Na2Ti3O7) particles with a rodlike morphology were prepared by a simple hydrothermal process in the presence of a concentrated NaOH aqueous solution. The ion exchange reaction of the synthesized Na2Ti3O7 nanorods with HCl under ultrasonic treatment promotes a complete sodium substitution and the formation of H2Ti3O7 nanorods. Low-temperature annealing of the as-produced nanorods of Na2Ti3O7 and H2Ti3O7 leads to a loss of the layered crystal structure and the formation of nanorods of condensed framework phases-sodium hexa-titanate (Na2Ti6O13) and metastable TiO2-B phases, respectively. These transformations proceed without a significant change in particle morphology. The nanostructures were investigated by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), the Brunauer-Emmett-Teller (BET) method, thermogravimetric analysis (TGA), and Raman spectroscopy. The structural defects of the synthesized nanorods were investigated by high-resolution electron microscopy. The presence of planar defects can be attributed to the exfoliation of the zigzag ribbon layers into two-dimensional titanates as well as to the condensation of the layers of TiO6 octahedra into three-dimensional frameworks.     

Comprehensive Model of Synthetic Bayer Liquors. Part 3. Sodium Aluminate Solutions and the Solubility of Gibbsite and Boehmite

Summary. A Pitzer model representing the thermodynamics of alkaline sodium aluminate solutions is presented. It constitutes a key part of our 10-component synthetic Bayer liquor model communicated in this series of papers. The present model calculates thermodynamic properties, such as heat capacities, osmotic coefficients, and densities, of these solutions as well as the solubilities of gibbsite and boehmite over temperature and concentration ranges of industrial interest.