Aluminosilicate Structures Research Articles

Sol-gel synthesis of micro-mesoporous aluminosilicates using oligomeric esters of orthosilicic acid

A method for the sol-gel synthesis of micro- and mesoporous aluminosilicates (MAS) using commercially available raw materials (mixtures of oligomeric esters of the orthosilisic acid of ethyl silicate-40 and water-alcohol solutions of aluminum nitrate) is developed. The effect of pH has on the porous structures of aluminosilicates with different Si/Al ratios is studied. It is shown that varying the pH when studying hydrolysis allows the synthesis of both micro- and mesoporous aluminosilicates with relatively narrow distributions of pores, changing the specific surface area from 341 to 684 m2/g, the specific pore volume from 0.2 to 0.95 cm3/g, the pore diameter from 0.17 to 40 nm, and the total acidity on ammonia from 226 to 405 μmol/g. The acid properties of the samples are studied at different SiO2/Al2O3 ratios, and their catalytic activities are measured in the model dimerization of methylstyrene (80°C; 10 wt % MAS; initial concentration of α-methylstyrene in the chlorobenzene, 3.3 mol/L). It is established that the highest conversion of α-methylstyrene (99.8 mol %) is observed for samples with SiO2/Al2O3 ratios of 5 and 20, for which the maximum concentrations of acid sites are characteristic. Micro-mesoporous aluminosilicates are promising for making acid-base type catalytic changes in different organic substances.

Behaviour of Amorphous Aluminosilicates towards Ageing by High-Temperature Steaming

The structure of amorphous aluminosilicates obtained by grafting silicon on alumina (Si/Al2O3) mostly remains the same after ageing by steaming. The rearrangement of surface silicon species on the facets of γ-alumina slightly decreases the number of Brønsted acid sites, which is determined through ethanol adsorption followed by thermogravimetry as well as m-xylene isomerisation. Al/SiO2 prepared by grafting aluminium on silica undergoes sintering of the silica support and the agglomeration of the surface aluminium species during steaming. This process leads to a sharp decrease in the number of Brønsted acid sites, which are associated with the dispersed aluminium species, and dehydroxylation of the silica surface.

Permeation and strength characteristics of macroporous supports for gas separation produced by co-sintering mixtures of α-alumina and kaolin

Porous aluminosilicate structures are being considered as supports for metallic and ceramic gas separation membrane supports. In the present work the flux and strength behaviour of partially sintered mixtures of α-alumina and kaolin powders were studied and applicable models explored. Powder compacts were sintered to between 1450°C and 1525°C to obtain open porosities from 18.7% to 3.3%. Application of a parallel flow model to stepped-pressure H2 flux measurements indicated effective pore radii from 180nm to 328nm and effective tortuosity factors from 9 to 23. Below 1500°C, SEM of the fracture surface provided evidence of uneven grain coalescence with only a small reduction in pore size indicative of liquid phase sintering. Samples sintered to 1525°C exhibited larger, more uniform grains and pore shapes and sizes indicative of coalescence. The flexural strength of the samples increased from 98MPa to 174MPa with increasing sintering temperature and was found to be independent of pore size, but highly dependent on porosity. This dependence, along with an indirect correlation with the tortuosity factor in structures produced at 1500°C and below, allows the parallel flow model to be reformulated in terms of strength.

Composition, structure, and sorption capacity of natural aluminosilicates

Comprehensive study of the composition, structure, and sorption properties of a typical representative of natural aluminosilicates has been performed. The efficiency of extraction of nickel cations from aqueous solutions with a concentration of 10−5 to 10−2 M has been determined. Such sorbent characteristics as chemical and mineralogical composition, specific surface, porosity, maximum sorption capacity, and number of active sites (as well as its estimate obtained by heterogeneous potentiometric titration) have been found to be mutually consistent.

Pulsed laser deposited alumino-silicate thin films and amorphous chalcogenide/alumino-silicate structures

Alumino-silicate coatings and structures formed from alumino-silicate and amorphous chalcogenide submicrometer layers were prepared by pulsed laser deposition. Fabricated thin films were characterized in terms of their structure, morphology, topography, chemical composition, optical properties, and basic anticorrosive functionality. Prepared coatings are amorphous, smooth, without micrometer-sized droplets, with chemical composition close to parent targets. Spectral dependencies of refractive indices and extinction coefficients were derived from variable angle spectroscopic ellipsometry data. Amorphous chalcogenide/alumino-silicate structures present large refractive index differences of individual layers (Δ n ~ 1.2 at 1550 nm) which could be useful for optical systems working at infrared telecommunication band wavelengths. Basic anticorrosion data of alumino-silicate layers show promising anticorrosion behavior.

Zeolite synthesis: an energetic perspective

Taking |D(H(2)O)(x)|[AlSiO(4)] based materials (where D is Li, Na, K, Rb or Cs) as an archetypal aluminosilicate system, we use accurate density functional theory calculations to demonstrate how the substitution of silicon cations in silica, with pairs of aluminium and (alkali metal) cations, changes the energetic ordering of different competing structure-types. For large alkali metal cations we further show that the formation of porous aluminosilicate structures, the so-called zeolites, is energetically favored. These findings unequivocally demonstrate that zeolites can be energetic preferred reaction products, rather than being kinetically determined, and that the size of the (hydrated) cations in the pore, be it inorganic or organic, is critical for directing zeolite synthesis.

Polyfunctional microspherical materials for long-term disposal of liquid radioactive wastes

The possibility of immobilizing liquid radioactive wastes into polyfunctional microspherical materials of the block and powdered types is demonstrated. These materials are intended for reprocessing radioactive wastes of different compositions and make it possible to perform a multistage process of conditioning radioactive wastes under relatively mild conditions (at temperatures below 1000°C) with the conversion of water-soluble cesium and strontium compounds into water-insoluble mineral forms in the course of solid-phase transformations. Owing to the aluminosilicate composition of microspherical components of energy ashes (cenospheres), the cenospheres can serve as precursors of aluminosilicate granitoid minerals. Different techniques are proposed and conditions are experimentally determined for transforming precursors into final mineral-like materials of the predicted structure types chosen in the framework of the geoecological approach. The framework structures of aluminosilicates and phosphates thus formed can fix cesium and strontium in the crystal lattice and ensure the geochemical equilibrium between the matrix and incorporating granitoid rocks under conditions of long-term disposal.

High temperature oxidation resistance of FeCrAl alloys covered with ceramic SiO2–Al2O3 coatings deposited by sol–gel method

One-, three- and five-layer SiO 2 –Al 2 O 3 coatings were deposited on a FeCrAl alloy basis by the sol–gel method. Sols in which the molar ratio of tetraethoxysilan to aluminium tri- sec -butoxide was 1:1 and 1:3 were used. As the samples were being soaked at T = 1200 °C for t = 700 h the mass of the samples increased. Thermal shock ( T = 1200 °C, 10,000 cycles) causes greater degradation of the surface than soaking at a constant temperature. The XPS and EDS results show that the composition of the top layer of the coatings changes during high temperature oxidation and thermal shock. The outward movement of aluminium cations results in surface enrichment with aluminium, particularly for the single-layer coatings. The measured energies of bonds Si 2p and Al 2p in the multilayer coatings indicate that a structure of aluminosilicates with a composition between that of mullite and that of sillimanite forms during sintering.

Incoherent Inelastic Neutron Scattering Studies of Nanoconfined Water in Clinoptilolite and Heulandite Zeolites

In a continued effort studying the role of water in ion exchange processes of zeolites, a synthetic series of alkali and alkaline earth metal cation variants (Na+, K+, Rb+, Mg2+, and Ca2+) of the hydrated clinoptilolite (Si/Al ≈ 5) and heulandite (Si/Al ≈ 3.5) aluminosilicate zeolites is examined by incoherent inelastic neutron scattering (IINS). The low-frequency librational modes of water reveal the impact of nanoconfinement and framework charge within these isostructural aluminosilicate structures. The experimental IINS spectra are correlated with power spectra derived through molecular dynamics simulations. The impact of ion−zeolite, ion−water, and water−zeolite interactions on the hindered rotations (librations) of water molecules is explored as a function of Si/Al substitution, cation identity, and subnanometer confinement. The results indicate that electrostatic charge on the overall framework has a stronger influence than charge density of a given ion in the channel and that these effects become m...

Interaction of the Phospholipid Head Group with Representative Quartz and Aluminosilicate Structures: An Ab initio Study

Silica dust particles in the form of quartz (but not kaolin) have been hypothesized to promote pulmonary diseases such as silicosis. The hypothesis is that quartz and kaolin have a comparable membranolytic potential on a specific surface area basis, and they have a comparable cytotoxic potential for lavaged pulmonary macrophages. Suppression of the cytotoxic activity occurs when these dust particles are treated with dipalmitoylphosphatidylcholine (DPPC), a common phospholipid component of the lung pulmonary surfactant. However, the enzyme phospholipase A2 is known to digest the phospholipid component more readily in the presence of quartz than kaolin. Since surface silanol (Si-OH) and aluminol (Al-OH) groups may interact differently with the phospholipid, an understanding of the selective removal of phospholipid by PLA2 may explain in vivo differences in cytotoxicity between quartz and kaolin. To develop some insight into this phenomenon, the interaction between a phospholipid and silica particles was examined by performing ab initio DFT calculations on clusters constructed with small (one or two silica tetrahedral units) representative parts of the silicate surface and the phospholipid head group. The clusters consisted of a phospholipid head group or functional groups from the head group complexed with Si(OSiH 3) 3OH, Al(OSiH 3) 3OH (-) or Al(OSiH 3) 3OH 2. Fully optimized geometries of the complexes were used to determine binding energies, -OH vibrational frequency shifts, and NMR chemical shieldings. Results indicate that interaction of the protonated aluminol group (Al-OH 2 (+)) with the phosphate portion of the head group is strongest, while interaction of the -OH 2 (+) group with the trimethyl-choline moiety of the head group is weakest. The presence of the choline moiety increased the magnitude of the -OH vibrational frequency shifts, and the shifts were significantly larger in complexes with protonated aluminol groups relative to silanol complexes. Analysis of ChelpG atomic charges shows that a net transfer of charge occurs from the silica unit to the head group within the complexes.

Nanostructure of Gel-Derived Aluminosilicate Materials

In the present work, aluminosilicate aerogels prepared under various conditions were compared with respect to their nanostructures and porosity. The purpose of this investigation was to find a suitable way to predict the final product structure and to tailor a required texture. Several Al and Si precursors (Al nitrate, Al isopropoxide, Al acetate, tetraethoxysilane (TEOS), and sodium silicate) were used in our examinations; the solvent content (water and alcohols), surfactants, as well as the catalysts were varied. In addition, the aerogels were subjected to various heat treatments. Hybrid aerogels were synthesized by the addition of different polymers (poly(acrylic acid), polyvinyl acetate, and polydimethylsiloxane). Aluminosilicate and hybrid aerogel structures were investigated by 27Al MAS NMR, SAXS, SEM, and porosity measurements. Loose fractal structures with a good porosity and high Al incorporation can be achieved from TEOS and Al nitrate or isopropoxide via a sol-gel preparation route. The use of Al acetate led to compact aerogel structures independently of the Si precursor, the pH, and the catalyst.

The contribution of quartz and the role of aluminum for understanding the AAR with greywacke

Precambrian Greywacke from Lower Lusatia (Germany) has been well known as an alkali sensitive aggregate for several years. It can cause considerable damages in concrete buildings due to an Alkali–Aggregate-Reaction. The investigations are focused on quartz as the main releaser of silica, its characterization and its behavior in an alkaline solution. But there are no relations between quartz properties and the alkali sensitivities of greywacke samples. To understand this fact the role of aluminum which greywacke releases in different amounts into the alkaline solution too must be considered. Aluminum affects the silica concentration by three different mechanisms. The result is always a decrease of the silica concentration in the solution caused by an alumino silicate formation. The silica bound by alumino silicate structures can be quantified by 27Al–NMR-spectroscopy. The expansions of concrete samples can now be described much well as a function of a so called “free” silica. Based on this results a direct test method for the assessment of the alkali sensitivity of greywackes could be created and suggestions for an inhibition can be given.

An experimental study of the replacement of leucite by analcime

Leucite and analcime have open framework aluminosilicate structures, where ion exchange by cation substitution has been previously used to explain the replacement of one phase by another. Using 18O-enriched NaCl solutions in hydrothermal reactions and run-product analyses using scanning electron microscopy, infrared and Raman spectroscopy, and time-of-flight secondary ion mass spectrometry, we show that the replacement of leucite by analcime is not a solid-state reaction involving cation exchange by volume diffusion. Textural features such as nano-pores and clusters, as well as the detection of high amounts of 18O in the framework of analcime, suggest that the reaction proceeds by dissolution of leucite and reprecipitation of analcime, where structural O atoms of the leucite framework are exchanged and a new analcime structure forms at a moving interface through the leucite parent crystal. The characteristic high porosity (on a nano-scale) in the analcime product phase results from some of the parent phase being lost to the solution to give a volume deficit reaction. However, external dimensions are maintained during the process to result in the pseudomorphic replacement of an open framework aluminosilicate structure by a coupled dissolution-reprecipitation mechanism.

Thermally induced rings formation in aluminosilicate structures

In the present study, we performed an investigation of phase transformation of zeolite Na-GIS in temperature interval from room temperature up to 1300°C. The collapse of S4R and S8R (secondary building units in zeolite Na-GIS) and the forming of pure sodium nepheline phase, consists of S6R, was examined. During the thermal transformation of Na-GIS zeolite symmetry of rings was changed and their deformation increases. The irregularity of rings causes the appearance of defect modes in the IR spectra of amorphous precursors. Investigated phases were characterized by XRPD, IR, NMR and DSC methods. A semiempirical method AM1 was used for calculation of vibrational spectrum of nepheline.

Electro-optical Parameters of Bond Polarizability Model for Aluminosilicates

Electro-optical parameters (EOPs) of bond polarizability model (BPM) for aluminosilicate structures were derived from quantum-chemical DFT calculations of molecular models. The tensor of molecular polarizability and the derivatives of the tensor with respect to the bond length are well reproduced with the BPM, and the EOPs obtained are in a fair agreement with available experimental data. The parameters derived were found to be transferable to larger molecules. This finding suggests that the procedure used can be applied to systems with partially ionic chemical bonds. The transferability of the parameters to periodic systems was tested in molecular dynamics simulation of the polarized Raman spectra of alpha-quartz. It appeared that the molecular Si-O bond EOPs failed to reproduce the intensity of peaks in the spectra. This limitation is due to large values of the longitudinal components of the bond polarizability and its derivative found in the molecular calculations as compared to those obtained from periodic DFT calculations of crystalline silica polymorphs by Umari et al. (Phys. Rev. B 2001, 63, 094305). It is supposed that the electric field of the solid is responsible for the difference of the parameters. Nevertheless, the EOPs obtained can be used as an initial set of parameters for calculations of polarizability related characteristics of relevant systems in the framework of BPM.

Precipitation of Nitrate−Cancrinite in Hanford Tank Sludge

The chemistry of underground storage tanks containing high-level waste at the Hanford Site in Washington State is an area of continued research interest. Thermodynamic models have predicted the formation of analcime and clinoptilolite in Hanford tanks, rather than cancrinite; however, these predictions were based on carbonate-cancrinite. We report the first observation of a nitrate-cancrinite [possibly Na8(K,Cs)(AlSiO4)6(NO3)2 x nH2O] extracted from a Hanford tank 241-AP-101 sample that was evaporated to 6, 8, and 10 M NaOH concentrations. The nitrate-cancrinite phase formed spherical aggregates (4 microm in diameter) that consisted of platy hexagonal crystals (approximately 0.2 microm thick). Cesium-137 was concentrated in these aluminosilicate structures. These phases possessed a morphology identical to that of nitrate-cancrinite synthesized using simulant tests of nonradioactive tank waste, supporting the contention that it is possible to develop nonradioactive artificial sludges. This investigation points to the continued importance of understanding the solubility of NO3-cancrinite and related phases. Knowledge of the detailed structure of actual phases in the tank waste helps with thermodynamic modeling of tank conditions and waste processing.

Vibrational spectra of aluminosilicate structural clusters

In the work vibrational spectra have been used to describe the framework aluminosilicate structures. Results of spectra calculations carried out for isolated, overtetrahedral structural units which occur in silicates and aluminosilicates are presented. Clusters composed of single and double silico- and alumino-oxygen rings with various numbers of members (4 and 6) containing terminal hydrogen atoms have been chosen for calculations. These clusters have constituted the model analogues of secondary building units (SBU) existing in real zeolite structures. The units selected for calculations have differed in the degree of substitution of silicon atoms with aluminum ones in tetrahedral positions. Visualization of individual vibrations performed has facilitated the band assignments. The results obtained have been used in the interpretation of experimental framework silicates (zeolites) spectra containing the same types of SBU in their structures. The spectra have been analyzed mainly in the ring vibrations region with the aim to determine the influence of aluminum on the spectrum.

Vibrational spectra of aluminosilicate structural clusters

In the work vibrational spectra have been used to describe the framework aluminosilicate structures. Results of spectra calculations carried out for isolated, overtetrahedral structural units which occur in silicates and aluminosilicates are presented. Clusters composed of single and double silico- and alumino-oxygen rings with various numbers of members (4 and 6) containing terminal hydrogen atoms have been chosen for calculations. These clusters have constituted the model analogues of secondary building units (SBU) existing in real zeolite structures. The units selected for calculations have differed in the degree of substitution of silicon atoms with aluminum ones in tetrahedral positions. Visualization of individual vibrations performed has facilitated the band assignments. The results obtained have been used in the interpretation of experimental framework silicates (zeolites) spectra containing the same types of SBU in their structures. The spectra have been analyzed mainly in the ring vibrations region with the aim to determine the influence of aluminum on the spectrum.

Evaluation of Uranium Coprecipitation with Sodium Aluminosilicate Phases

Batch laboratory experiments performed to evaluate uranium incorporation into aluminosilicate structures during synthesis are described. This research was conducted in response to plant problems related to the accumulation of uranium with aluminosilicates in low-level radioactive waste evaporators. Conditions that favor precipitation of aluminosilicates also foster uranium solid precipitation, so it is difficult to attribute problems with uranium accumulation to, for example, only the formation of the aluminosilicates. Infrared spectra show that sodium uranates, uranium silicates, and other uranium solids are formed during the synthesis of sodium aluminosilicates structures in the presence of uranium. Both amorphous and sodalite aluminosilicate phases, unlike the zeolite A phase, show appreciable affinity for uranium incorporation during their formation in the presence of uranium.

MAS NMR and FTIR spectra of framework aluminosilicates

In the work, the results of 27Al MAS NMR studies carried out for structures of different framework aluminosilicates are presented. The spectra of natural clinoptilolite and its sodium and hydrogen forms were measured. The zeolite spectra were obtained before and after incorporation of heavy metal cations (Pb 2+, Cd 2+, Ni 2+ and Cr 3+) into the zeolite structure. After decomposition of the spectra into component peaks differences caused by ion exchange were observed. Additionally, NMR spectra of kalsilite (framework aluminosilicate, not belonging to zeolite group) and other zeolites (chabazite, heulandite and analcime) were measured. 27Al MAS NMR spectra of all the samples studied were compared with the FTIR spectra measured in the middle infrared region. Based on the results obtained it has been proved that the interpretation of spectra obtained using these two different techniques can give some information concerning changes occurring in various framework aluminosilicate structures.