Phosphate Alumina Process by Sol−Gel: Textural and Fractal Properties (original) (raw)
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Structural Evolution of Phosphated Alumina during Sol−Gel Synthesis
Journal of Physical Chemistry B, 2005
Phosphated alumina gels were prepared by the sol-gel method. Gels were aged from 1 to 8 days in air. Gel structure evolution, as time went on, was followed by 27 Al magic angle spinning nuclear magnetic resonance, X-ray diffraction, and small-angle X-ray scattering. It is concluded that the aging time is a crucial parameter in the formation of coordinately unsaturated sites of aluminum (Al IV and Al V ). The gel network is shown to have a fractal structure.
Microstructure Evolution of Nonhydrolytic Alumina Gels
Journal of Sol-gel Science and Technology - J SOL-GEL SCI TECHNOL, 1997
Nonhydrolitic sol-gel processes of aluminum chloride and aluminum bromide with isopropyl ether and aluminum sec-butoxide were performed at various temperatures. Based on the Arrhenius type variation of the gelation time with temperature, activation energies for the gelation were found to be in the range 19–25 Kcal/mol range. The energies were found to be sensitive to the nature of the aluminum ligands and the chemical scheme. Due to the large activation energy, it is possible to stop the reaction at any time before gelation by cooling the sol to room temperature. Small angle X-ray scattering (SAXS) of sols from the AlClAlCl3/Pr <img src="/fulltext-image.asp?format=htmlnonpaginated&src=TR20414223805470_html\10971_2004_Article_136473_TeX2GIFIE1.gif" border="0" alt=" _2^i " />O system shows unique development of a fractal like structure with nanometer scale order, demonstrated by discrete peaks in the SAXS data. A fractal dimension D = 1.64 was ...
Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths
Journal of Colloid and Interface Science, 2009
Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol-gel reaction. Starting from AlCl 3 Á6H 2 O and poly(ethylene oxide) (PEO) dissolved in a H 2 O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H 2 O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H 2 O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO.
Journal of Non Crystalline Solids, 2008
Transparent and homogeneous aluminophosphate gels and glasses have been widely synthesized through an aqueous sol-gel route, extending significantly the glass-forming range compared to that accessible via the melt-cooling route. Different phosphorus precursors, sodium polyphosphate (NaPO 3 ) and orthophosphate species (NaH 2 PO 4 and/or H 3 PO 4 ) were compared with regard to the macroscopic properties and the microscopic structure of the resultant gels and glasses as characterized by extensive high-resolution liquid-and solidstate NMR. Sodium polyphosphate solution results in a substantially wider composition range of homogenous gel formation than orthophosphate solutions, and the two routes produce significant structural differences in the sol and xerogel states. Nevertheless, the structures of the glasses obtained upon gel annealing above 400°C are independent of the P-precursors used.
Gelcasting of alumina nanopowders based on gelation of sodium alginate
The alumina nanopowder was synthesized via the sol–gel method. u-Alumina with crystallite size in the range of 25–110 nm was crystallized by calcination of the powder at 900 8C for 1 h. Sodium alginate, a natural innoxious polymer, was applied for in situ forming process of an Al2O3 green body, using calcium phosphate as a solidifier agent. Sodium hexa metaphosphate was also utilized as a chelator. Rheological and gelation behaviors of resultant slurry were analyzed. The viscosity of slurries with 15 vol.% alumina and 1.8 vol.% calcium phosphate dispersed by 1 wt.% sodium alginate solution, was less than 800 mPa s. The green bodies from the gelcasting process were dried at room temperature for 36 h and pressureless sintered at 1500 8C for 3 h. A uniform microstructure without huge grain growth was revealed by SEM.
Synthesis of Alumina Gels in Amphiphilic Media
Journal of Sol-gel Science and Technology, 1997
This work deals with the synthesis of alumina gels from aqueous solutions of aluminum chloride in the presence of cationic surfactant molecules. The effect on the sol-gel transition of the reagent concentrations and of the synthesis temperature are first studied. The structure of the resulting wet and dried gels and the formation of liquid crystal mesophases are studied by X-ray diffraction. The thermal and structural evolutions of the gels are then characterized by thermogravimetric and X-ray diffraction measurements. Finally, nitrogen adsorption isotherms are used to investigate the porous texture of the thermally treated materials up to the transformation into α-Al2O3.
Physicochemical and Catalytic Properties of Sol−Gel Aluminas Aged under Hydrothermal Conditions
Langmuir, 2003
Sol-gel boehmite was aged under hydrothermal conditions at 200°C using water as mineralizer agent. Samples were characterized with X-ray powder diffraction, nitrogen adsorption, and transmission electron microscopy. Calcined solids were also studied by infrared spectroscopy of adsorbed pyridine. Their catalytic properties were determined in the 2-propanol decomposition. The hydrothermal treatment ordered the atoms of the sol-gel boehmite mainly in two dimensions forming thin crystallites, which grew as treatment time increased. Due to the pseudomorphic transformation of boehmite into γ-alumina, the arrangement of crystallites in the corresponding boehmite determined alumina particle morphology and porosity. The pore size distribution was narrow, and the pore size shifted to larger values as the time of hydrothermal treatment increased. The strength and number of acid sites depended also on the treatment time. The catalytic activity correlated well with the acidity and specific surface area.
Journal of Non-Crystalline Solids, 2010
The SiO 2-Al 2 O 3 powders of various chemical composition were synthesized via the alkoxide sol-gel route from solutions containing aluminium tri-sec-butoxide (Al(OC 4 H 9) 3 ; TBA) and tetraethyl orthosilicate (Si(OC 2 H 5) 4 ; TEOS) as the precursors. The composition and morphology of the obtained gels were examined by TG/DTA, XRD, FTIR, SEM and isothermal nitrogen adsorption (BET). In this research, all the gels after heating at 500°C were amorphous and became crystalline after sintering at 1200°C. The results show that the crystallinity and structure of the phases formed depend on the molar ratio of the TEOS/TBA precursors, i.e. the atomic ratio Si:Al. In the powder obtained from the sol in which the atomic ratio Si:Al = 1:3 (as in mullite) only one crystalline phase was observedan orthorhombic mullite. In both the powders poorer and richer in silica (in comparison with the stoichiometric ratio of mullite), the crystallization of the mullite phase is less efficient and is preceded by spinel crystallization. The obtained powders show a low value of specific surface development. The specific surface area of the silica-alumina powder obtained from the sol with stoichiometry 2SiO 2 3Al 2 O 3 after thermal treatment (1000°C; 3 h) is the highest (10.5 m 2 g −1) while that of the other powders is considerably smaller and almost identical (~1.1 m 2 g −1).
Journal of the Serbian Chemical Society, 2010
Active porous alumina was prepared via a sol-gel method and subjected to thermal treatment in the temperature range 500-1200 °C. The addition of lanthanum effectively inhibited the surface area loss of the aluminas. Fractal analysis from nitrogen adsorption isotherm was used to study the pore surface roughness of alumina samples with different chemical compositions (PEG, PEG and lanthanum) and calcinations conditions in terms of the surface fractal dimension, d. The Mahnke and Mögel (MM) model was used to determine the value of d of La(III)-doped alumina. Following the MM model, the d value of the activated aluminas increased as the calcination temperature increased from 500 to 700 °C but decreased after calcination at 1000, 1100 and 1200 °C. The addition of polyethylene glycol (PEG 5600) to the boehmite sol reduced the surface fractal of the activated alumina due to the heterogeneous distribution of the pores. With increasing La(III) concentration from 0.015 to 0.045 mol La(III)/ /mol Al(III), the d value of La-modified alumina samples decreased, indicating a smoother surface. The obtained PEG+La-doped boehmite sol can be used as a precursor dispersion for the deposition of mesoporous alumina coatings on stainless steel foil, by the spray pyrolysis method.
Microporous and Mesoporous Materials, 2008
Solid phase (gel) separated from freshly prepared sodium aluminosilicate hydrogel as well as during its hydrothermal treatment at 80°C was analyzed by different methods such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal gravimetry (DTG), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Analysis of the obtained results have shown that the freshly prepared gel is mainly composed of disc-shaped primary particles, but also partially or even fully crystalline entities were observed by AFM analysis. AFM analysis of the solids separated from the hydrogel at various stages of its hydrothermal treatment (heating at 80°C) indicates that the particles of the partially and/or fully crystalline phase are nuclei for further crystallization of zeolite. Although it is assumed that the nuclei have structure of faujasite rather than zeolite A, further growth of zeolite A was readily explained by the fact that regardless of the ''structure'' of nuclei, the type of zeolite to be formed is determined by the concentrations of silicon and aluminum in the liquid phase of the crystallizing system.