Solid-state 27Al Magic-angle Spinning Research Articles

Spectroscopic evaluation of UVI-cement mineral interactions: ettringite and hydrotalcite.

Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and hydrotalcite (Mg6Al2(OH)16CO3·4H2O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing UVI from radioactive waste streams. U LIII-edge X-ray absorption spectroscopy (XAS) was used to probe the UVI coordination environment in contact with these minerals. For the first time, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these UVI-contacted minerals and make inferences on the UVI coordination, in conjunction with the X-ray spectroscopy analyses. The U LIII-edge XAS analysis of the UVI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with 27Al NMR analysis, indicated that a disordered Ca- or Al-bearing UVI secondary phase also formed. For the UVI-contacted hydrotalcite phases, the XAS and 27Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.

Environmental assessment of metal-organic framework DUT-4 synthesis and its application for siloxane removal

In this study, the effect of different solvents in metal-organic framework (MOF) synthesis was evaluated in a technical study and an environmental study. The technical study considered the octamethylcyclotetrasiloxane (D4) adsorption on MOF DUT-4 for biogas purification, while the environmental study was implemented using the Life Cycle Assessment (LCA) methodology. In addition, the obtained DUT-4 adsorbents were characterized by X-ray diffraction, N2 adsorption-desorption isotherms, solid-state 27Al magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and Fourier transform infrared spectroscopy techniques. The results indicated that technically similar materials are obtained through solvothermal synthesis with similar D4 siloxane adsorption capacities (9.8 mg/g). Conversely, hydrothermal synthesis led to a low D4 siloxane adsorption capacity (1.8 mg/g). The environmental study demonstrated that the solvothermal synthesis had the highest contribution to the environmental damage due to the use of toxic solvents in the synthesis, cleaning, and solvent exchange stages (environmental score factor up to 244 mPt). In addition, a techno-environmental impact factor was introduced to identify the most suitable synthesis route of DUT-4. The overall results pointed out that solvothermal synthesis is the best scenario for DUT-4 production due to its low environmental impact/adsorption capacity ratio (11.44). This study is a step toward a more holistic approach integrating technical and environmental criteria in MOF synthesis for adsorption applications.

Viscosity and structure evolution of CaO–SiO2-based mold fluxes with involvement of CaO–Al2O3-based tundish fluxes

In this study, the effect of involvement of CaO–Al2O3–SiO2–MgO tundish flux into CaO–SiO2–Al2O3–Na2O–CaF2 mold flux on viscosity and structure of mold flux was investigated through employing the rotating viscometer and associated structure analysis including high resolution Raman spectroscopy and solid-state 27Al magic angle spinning nuclear magnetic resonance (27Al MAS-NMR). The results showed that the viscosity continuously increases with increasing involvement of tundish flux. The apparent activation energy obtained from the temperature dependence of the viscosity presents an increase from 128.5 kJ/mol to 149.1 kJ/mol with increase of involvement ratio from 0 to 0.75. Semi-quantitative analysis using the deconvoluted Raman spectra and 27Al MAS-NMR spectra revealed an increase in amounts of [AlO4] and Si–O–Al units, but in the meanwhile a decrease in amounts of [AlOnF4-n] and [AlO5] units, this indicates the structure transformation from prevalent silicate structure to complex alumino-silicate structure with increasing involvement ratio. A self-developed structurally-based viscosity model can predict the viscosity of mold flux with varying involvement ratio and presents a smallest deviation of 15.8% compared to those prevalent viscosity models despite of the great transformation of the melt from silicate system to alumino-silicate system.

Ettringite-based binder from ladle slag and gypsum – The effect of citric acid on fresh and hardened state properties

The retardant effects of citric acid on the properties of ettringite binders are not yet fully understood. This investigation provides insights into the effects of citric acid on both the fresh and hardened state properties of the binder produced from the hydration between ladle slag and gypsum (LSG). The results revealed that LSG's initial setting time could be controlled via different citric acid contents and could range from 20 min to 2 h. Citric acid is effective at relatively high dosages; in its absence, ettringite formed almost immediately directly following exposure to water. The presence of citric acid alters the hydration reactions in LSG. AFm was found to be one of main hydration products; citric acid promoted the conversion from ettringite to monosulfate, detected by using solid-state 27Al magic-angle spinning nuclear magnetic resonance spectroscopy. In addition, citric acid increased the compressive strength of LSG mortars by up to 45%.

The cation-dependent effects of formate salt additives on the strength and microstructure of CaO-activated fly ash binders

This study investigated whether three formate salt additives (Ca(HCOO)2, NaHCOO, and KHCOO) could improve the strength of a CaO-activated fly ash (FA) system. Various tests were conducted to verify the effects of the formate salts including compressive strength, X-ray diffraction (XRD), thermogravimetric (TG) analysis, solid-state 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy, and mercury intrusion porosimetry (MIP). The results showed that the addition of Ca(HCOO)2 significantly strengthened the FA binder system by (1) substantially increasing C-S-H formation and (2) measurably reducing overall pore size. However, adding NaHCOO or KHCOO was much less advantageous to strength improvement because although helpful in dissolving FA, the high pH environment caused by their reaction with CaO inhibited further dissolution of CaO, which was the main activator used in this study.

Coalification and coal alteration under mild thermal conditions

Coalification temperatures are often considered to be approximately 100–170 °C for bituminous coal and 170–275 °C for anthracite. However, our micropetrographic observations, solid state 27Al magic-angle spinning nuclear magnetic resonance measurements, interpretation of δ13C values for whewellite in pelosiderite concretions from Carboniferous sediments, and assessment of whewellite thermal stability show that coalification temperatures can be significantly lower. Also the temperatures of coal alteration may be substantially lower than is stated. Ordinarily, high-temperature alteration is reported, but microthermometric measurements of fluids temperatures and micropetrographic observations show that the coal alteration can take place at low temperatures. For this reason, coals from the Kladno–Rakovník Basin, part of Late Paleozoic continental basins of the Czech Republic, were analyzed. Regarding coalification, micropetrographic characterizations of unaltered coals, the presence of thermally unstable Al complexes in the coal organic mass documented using 27Al MAS NMR method, and proven occurrence of whewellite in pelosiderite concretions suggest a lower coalification temperature, max. ~ 70 °C. Regarding coal alteration, micropetrographic observations revealed (a) the weaker intensity of fluorescence of liptinite, (b) mylonitic structures and microbreccia with carbonate fluid penetration, and (c) high oxygen content in coals (37–38 wt.%). These phenomena are typical for thermal and oxidative alteration of coal. As the temperature of carbonate fluids inferred from fluid inclusion analysis was evaluated as ~ 100–113 °C, the temperature of coal alteration was suggested as ~ 113 °C; the alteration was caused by hot hydrothermal fluids.

Elementary Steps of Faujasite Formation Followed by in Situ Spectroscopy

Ex situ and in situ spectroscopy was used to identify the kinetics of processes during the formation of the faujasite (FAU) zeolite lattice from a hydrous gel. Using solid-state 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), the autocatalytic transformation from the amorphous gel into the crystalline material was monitored. Al X-ray absorption near-edge structure shows that most Al already adopts a tetrahedral coordination in the X-ray-amorphous aluminosilicate at the beginning of the induction period, which hardly changes throughout the rest of the synthesis. Using 23Na NMR spectroscopy, environments in the growing zeolite crystal were identified and used to define the processes in the stepwise formation of the zeolite lattice. The end of the induction period was accompanied by a narrowing of the 27Al and 23Na MAS NMR peak widths, indicating the increased level of long-range order. The experiments show conclusively that the formation of faujasite occurs via the continuous formation and...

Influence of morphology of hollow silica\u2013alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane

Hollow silica–alumina composite spheres were prepared by a polystyrene (PS) template method using various amounts of PS suspension. Homogeneous hollow spheres prepared using 40 g were found to be with a diameter of about 300 nm in scanning electron microscopy, and transmission electron microscopy demonstrated their hollow sphere morphology. From the nitrogen adsorption isotherm results, the homogeneous hollow spheres prepared using 40 g of the PS suspension were found to be an ordered pore structure. The activities of the hollow spheres prepared using various amounts of the PS suspension for hydrolytic dehydrogenation of ammonia borane were compared. The results showed that 10, 7, and 6 mL of hydrogen were evolved from the aqueous ammonia borane solution in about 40 min in the presence of the hollow spheres prepared using 40, 80, and 120 g of PS suspension, respectively. The homogeneous hollow spheres with an ordered pore structure showed the highest activity among all the hollow spheres. The amount of acid sites and the coordination number of aluminum active species were characterized using neutralization titration and solid-state 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The homogeneous hollow spheres with an ordered pore structure had high amount of acid sites and 4-coordinated aluminum species. The relative proportion of 4-coordinated aluminum species was related to the dispersion of aluminum species. These results indicate that the homogeneous hollow spheres with an ordered pore structure showed the high activity because of high amount of acid sites induced by the highly dispersed aluminum species.

Low Temperature Oligomerization of Ethylene over Ni/Al-KIT-6 Catalysts

In this paper, we have studied the oligomerization of ethylene with a liquid heptane solvent over bifunctional Ni catalysts in a continuous flow reactor. We have prepared an Al-containing KIT-6 silica that was used as a support after calcination in the temperature range of 300–900 °C. The Ni/Al-KIT-6 catalysts had uniform mesopores with diameters in the range of 5.4–6.3 nm, excepting Ni/Al-KIT-6 (900). The calcination temperature of Al-KIT-6 support changed the surface acidity as well as the interaction of Ni2+ and acid sites for the Ni catalysts, as determined by temperature-programmed desorption of ammonia, temperature-programmed reduction, infrared spectroscopy after the adsorption of pyridine, solid-state 27Al magic-angle spinning nuclear magnetic resonance spectroscopy, and X-ray adsorption spectroscopy. Among the tested catalysts, the Ni/Al-KIT-6 (300) showed the highest ethylene conversion because of the increased intimate contact between Ni2+ and acid sites. The strong interaction of Ni2+ species and the support is not effective in increasing active sites for ethylene conversion. The Ni/Al-KIT-6 catalysts produced internal linear C4 and C6 olefins with high selectivity. The Ni/Al-KIT-6 (300) had 2.2–6.1 times lower selectivities toward 2-ethyl-1-butene than other catalysts at similar ethylene conversions. The reaction product mixture showed that the Ni/Al-KIT-6 catalysts shifted the product distribution towards acid-catalyzed oligomerization/cracking/realkylation products (i.e. C3, C7, C7, and C8+ olefins) as the concentration of Bronsted acid sites increased. Among the tested catalysts, the Ni/Al-KIT-6 (300) showed the highest yield of C4 and C6 olefins (78.3%).

Effect of Pt Particle Size on Propane Combustion Over Pt/ZSM-5

The effect of Pt particle size on propane combustion was investigated over Pt/ZSM-5 catalysts. The particle size of Pt was controlled by adjusting the SiO2/Al2O3 ratio in H-ZSM-5, a cation in ZSM-5, and an aging temperature. Several techniques such as inductively coupled plasma-atomic emission spectroscopy, N2 physisorption, CO chemisorption, C3H8 chemisorption, solid-state 27Al magic-angle spinning nuclear magnetic resonance spectroscopy, transmission electron microscopy, and infrared spectroscopy after CO chemisorption were used to characterize the catalyst. Among Pt catalysts supported on H-ZSM-5 with different SiO2/Al2O3 molar ratios, Pt/H-ZSM-5 (SiO2/Al2O3 = 150) having small Pt particles and large external surface area showed the highest specific reaction rate for propane combustion. The specific reaction rate decreased with an increase in the Pt particle size in Pt/H-ZSM-5 (SiO2/Al2O3 = 23) catalysts with different Pt particle sizes. Although Pt/Na-ZSM-5 catalysts had larger Pt particles than Pt/H-ZSM-5 catalysts, the former showed a lower apparent activation energy for propane combustion than the latter.

From Layered Double Hydroxides to ZnO-based Mixed Metal Oxides by Thermal Decomposition: Transformation Mechanism and UV-Blocking Properties of the Product

Thermal decomposition of layered double hydroxides (LDHs) is a way of fabricating mixed metal oxide (MMO) nanocomposite materials composed of metal oxide and spinel phases. A detailed understanding of the mechanism of the transformation of the LDH precursor to the MMO should allow the properties of the resulting MMO nanocomposites to be tailored to specific applications. Here we report a systematic investigation of the structure, composition, and morphology evolution from ZnAl-LDHs to ZnO-based MMO nanocomposites composed of ZnO and ZnAl2O4 on calcination at different temperatures. The nucleation and oriented growth of ZnO crystallites and the formation of ZnAl2O4 were monitored by high resolution transmission electron microscopy (HRTEM) combined with selected-area electron diffraction (SAED), in situ X-ray diffraction (XRD), solid-state 27Al magic-angle spinning nuclear magnetic resonance (27Al MAS NMR), and thermogravimetric and differential thermal analyses (TG−DTA). The layered structure of the LDH pr...

Effect of Aluminum Speciation and Structure Characterization on Preferential Removal of Disinfection Byproduct Precursors by Aluminum Hydroxide Coagulation

Humic acid was coagulated by aluminum chloride and pokyaluminum (PACl) at different pH conditions. Aluminum speciation was investigated in details through combining electrospray ionization mass spectrometer (ESI-MS) with solid-state 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR). Freeze-dried flocs from coagulation precipitates were analyzed by Fourier transform infrared (FTIR) and solid-state 13C cross-polarization magic angle spinning (CPMAS) NMR. This study also evaluated removal of disinfection byproduct (DBP) precursors by PACI and AlCl3 coagulation during coagulation process. Particular emphasis was paid to the role of aluminum speciation and structure characterization in the preferential removal of precursors by coagulation. The results indicated that prehydrolyzed or in situ formed Al13 species during coagulation process exhibited preferential removal for corresponding DBP precursors with relative structures. On one hand, for DBP precursors with aromatic and carboxylic structures, in situ Al13 species were preferentially bound to these DBP precursors. Thus, more DBP precursors with aromatic and carboxylic structures were removed by AlCl3 than those by PACl at pH 5.0. On the other hand, for DBP precursors with aliphatic structures, aluminum coagulation was not responsible for reducing these precursors. However, oligomeric aluminum species and preformed Al13 species were weak preferentially bound to DBP precursors with aliphatic structures.

A spectroscopic study of calcium aluminate gels obtained from aluminium sec-butoxide chelated with ethyl acetoacetate in various ratios

Calcium aluminate (CaAl2O4, CA) powders were prepared by sol–gel technique at low sintering temperatures. Aluminium-sec-butoxide (Al(OsBu)3, Asb) and calcium nitrate tetrahydrate (Ca(NO3)2 × 4H2O) were used as starting materials. Ethyl acetoacetate (C6H10O3, Eaa) was used as a chelating agent in order to control the rate of hydrolysis of Al(OsBu)3. Three gels with Eaa/Asb molar ratios of 1/1, 3/2 and 2/1 were prepared. The dried gels and thermally treated samples were characterized by means of Fourier Transform Infrared spectroscopy (FTIR), 1H, 13C Nuclear Magnetic Resonance (NMR) spectroscopy, solid-state 27Al Magic Angle Spinning (MAS) NMR, 3Q MAS NMR spectroscopy, and X-Ray Diffraction (XRD). From the results obtained, the effect of modification of the starting Asb on the hydrolysis process, hydrolyzed gel structure and crystallization behavior is discussed. It has been established that Eaa reacts completely with Asb forming chelate. Various chelate units are formed including trichelated, Al(Eaa)3 units. Spontaneous gellation has been observed in the sols slowly hydrolyzed by exposing to air moisture. The reactivity towards hydrolysis of chelated alkoxide depends on the number of chelating ligands bonded to aluminium. Sec-butoxy groups were primarily hydrolyzed; ethyl acetoacetate groups in less chelated units are much less susceptible to hydrolysis, while trichelated units were not hydrolyzed. Thus, in hydrolyzed gels a partially chelated oligomers and trichelated Al(Eaa)3 units exist. The crystal phase, not described previously, related to Al(Eaa)3 chelate crystallizes in the gels with higher Eaa/Asb ratio. Hydrolysis leads to formation of three kinds of Al coordination sites: six coordinated Al(Eaa)3, and five and six coordinated Al atoms in oligomers. The Eaa/Asb ratio strongly influences the relative ratios between the various coordination states of aluminium atom. After thermal treatment of the gels at 1,000 °C for 2 h, CA was obtained along with minor CaAl4O7, (CA2) and Ca12Al14O33(C12A7) compounds. Thermal treatment at higher temperature increases the amount of CA and decreases the amount of minor components.

Visible light photoreduction of Cr(VI) in aqueous solution using iron-containing zeolite tubes

In this study, a novel visible light photocatalyst consisting of iron-loaded zeolite tubes was designed to reduce Cr(VI) to Cr(III) in aqueous solution. Iron was incorporated into hollow, porous ZSM-5 tubes using two different methods. In the first method, the iron-encapsulated ZSM-5 tubes were prepared by introducing iron into the mesoporous silica template prior to the formation of the hollow ZSM-5 tubes resulting in an encapsulated iron species. In the second method, iron was ion-exchanged into the ZSM-5 shell during a post-synthesis treatment. The iron-loaded ZSM-5 structures were extensively characterized by powder X-ray diffraction (XRD), elemental analysis, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), electron microscopy with energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy (XPS). The iron in the iron-exchanged ZSM-5 tubes tended to aggregate compared to the iron in the iron-encapsulated ZSM-5 tubes. Both the iron encapsulated and iron-exchanged ZSM-5 hollow tubes were active for the visible light photoreduction of Cr(VI) to Cr(III) in aqueous solution. The iron-exchanged ZSM-5 tubes were very active initially but suffered from substantial iron leaching after the first use in aqueous Cr(VI) photoreduction relative to the iron-encapsulated ZSM-5 tubes which showed very little leaching of the iron.

Structural Characterization of Al10O6iBu16(μ-H)2, a High Aluminum Content Cluster: Further Studies of Methylaluminoxane (MAO) and Related Aluminum Complexes

The first structurally characterized isobutyl-containing aluminoxane compound is presented. The Al10O6iBu16(mu-H)2 (I) cluster is produced from neat octakis-isobutyltetraluminoxane (Al4O2iBu8) at 80 degrees C in 6-8 h followed by slow crystallization. The crystal is triclinic (space group P1) with the molecule lying on an inversion center. This aluminoxane contains both nearly linear, 154(2) degrees, aluminum-bridging hydrides and three-coordinate aluminum sites. Solid-state 27Al magic-angle spinning (MAS) NMR experiments were done at 19.6 and 40 T (833 MHz and 1.703 GHz, 1H) and at 30-35 kHz spinning speeds, leading to the determination of the Cq and eta values for the two four-coordinate Al sites and a lower limit of Cq for the three-coordinate Al site. Geometry-optimized restricted Hartree-Fock calculations at the double-zeta level of an idealized structure (methyl substituted, D2h geometry) yielded Cq and eta in close agreement with experiment; Cq agrees within 3 MHz.

Fate of coagulant species and conformational effects during the aggregation of a model of a humic substance with Al 13 polycations

A model of a humic substance (MHS) obtained from auto-oxidation of catechol and glycine, was aggregated at pH 6 and 8 with Al 13 polycations. The fate of Al 13 coagulant species upon association with MHS functional groups was studied using solid state 27Al Magic-angle spinning (MAS) NMR and CP-MAS 13C NMR. Electrophoretic measurements and steady-state fluorescence spectroscopy with pyrene as a fluoroprobe, were combined to investigate structural re-organization of humic material with aluminum concentration. MAS 27Al NMR revealed that the coagulant species are Al 13 polycations or oligomers of Al 13 units at both pHs. CP MAS 13C spectra indicated that, at low Al concentration, hydrolyzed aluminum species bind selectively to carboxylic groups at pH 6 and to phenolic moieties at pH 8. At higher coagulant concentrations, the remaining functional groups also interact with hydrolyzed Al to yield similar CP MAS 13C spectra in the optimum concentration range. Negative values of electrophoretic mobility were obtained at optimum coagulant concentrations even though an overall charge balance was achieved between MHS anionic charge and Al 13 cationic charge at pH 6. The polarity-sensitive fluorescence of pyrene revealed that the interaction of Al 13 coagulant species with MHS functional groups induces the formation of intramolecular hydrophobic microenvironments. Such structural changes were reversed upon further addition of Al 13 polycations.

Electrochemical and solid-state NMR studies on LiCoO 2 coated with Al 2O 3 derived from carboxylate-alumoxane

The surface of LiCoO 2 cathodes was coated with various wt.% of Al 2O 3 derived from methoxyethoxy acetate-alumoxane (MEA-alumoxane) by a mechano-thermal coating procedure, followed by calcination at 723 K in air for 10 h. The structure and morphology of the surface modified LiCoO 2 samples have been characterized with XRD, SEM, EDS, TEM, BET, XPS/ESCA and solid-state 27Al magic angle spinning (MAS) NMR techniques. The Al 2O 3 coating forms a thin layer on the surface of the core material with an average thickness of 20 nm. The corresponding 27Al MAS NMR spectrum basically exhibited the same characteristics as the spectrum for pristine Al 2O 3 derived from MEA-alumoxane, indicating that the local environment of aluminum atoms was not significantly changed at coating levels below 1 wt.%. This provides direct evidence that Al 2O 3 was on the surface of the core materials. The LiCoO 2 coated with 1 wt.% Al 2O 3 sustained continuous cycle stability 13 times longer than pristine LiCoO 2. A comparison of the electrochemical impedance behavior of the pristine and coated materials revealed that the failure of pristine cathode performance is associated with an increase in the particle–particle resistance upon continuous cycling. Coating improved the cathode performance by suppressing the characteristic structural phase transitions (hexagonal to monoclinic to hexagonal) that occur in pristine LiCoO 2 during the charge–discharge processes.

The detailed structure and origin of clay minerals at the Cretaceous/Tertiary boundary, Stevns Klint (Denmark)

AbstractThe structure of an illite-smectite (I-S) sample (HSI) from the Tertiary–Cretaceous boundary layer at Stevns, Denmark, i.e. the so-called Fish Clay, is investigated in order to discuss the origin of this clay and its similarity to clays from Maastrichtian-Danian chalk of Denmark and the North Sea. The phase compositions and layer sequences have been determined by X-ray diffraction. The structural formulae are determined from chemical analysis and solid-state 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The octahedral vacancy pattern has been determined by thermal analysis and the particle shape by atomic force microscopy. The HSI sample I-S consists of two phases, a high-smectitic (HS) phase (70%) having 95% smectite and 5% illite (leucophyllite) layers, and a low-smectitic (LS) phase (30%) having 50% smectite and 50% illite (leucophyllite) layers. Both phases have trans-vacant dioctahedral sheets and contain only traces of IVAl, the charge of the illite (leucophyllite) layers being provided predominantly by octahedral Mg for Al substitution. The structure of the tetrahedral and octahedral sheets for the HSI are compared to the structure of an I-S sample (GRI) from the Maastrichtian chalk at Stevns below the Tertiary-Cretaceous boundary and to the standard smectite SAz-1 from Arizona. For all three samples the 27Al MAS NMR spectra show the presence of two resolved IVAl resonances, which indicate the presence of two different IVAl sites. The NMR, infrared spectroscopy and chemical data show that both HSI and GRI have highly ordered Mg for Al substitution in the octahedral sheet, each Al having one Mg and two Al neighbours, whereas substitution of Mg for Al in SAz-1 is random. For the HSI sample, both the HS and LS phases are probably formed from volcanic ash. The structural similarity of the phases in HSI and GRI shows that GRI formed from a material similar to HSI by illitization of the HS phase and chloritization of the LS phase.

Structural Characterization of AION by27 Al Mas NMR and Quantum Chemistry Method

AbstractAluminum oxynitride (AlON) is a nitrogen-stabilized defective spinel phase of alumina (y-Al2O3). Its structure has not been well understood so far. Solid-state27 Al magic-angle spinning nuclear magnetic resonance and quantum chemistry calculation are used to characterize y-Al2O3 and AMON ceramics in the present study. The resonance spectra clearly show different units (e.g. [AlN4], [AlO4] and [AlO6]) and vary with composition. The calculation shows that the vacancy located at octahedral site is more stable than that located at the tetrahedral site, and nitrogen atoms preferentially replace oxygen atoms in the tetrahedral site, which is in agreement with the experimental analysis by nuclear magnetic resonance.

Single-phase and diphasic aerogels and xerogels of mullite: Preparation and characterization

Single-phase mullite composition gels have been synthesized using tetraethoxysilane [Si(OC 2H 5) 4] and aluminum nitrate nonahydrate as precursors. Diphasic mullite gels have been prepared using colloidal silica and boehmite as precursors. Xerogels and aerogels of both the above gels have been obtained by ordinary drying in air at 60 °C and critical point drying in methanol, respectively. Single-phase xerogels show an intense exothermic peak at about 980 °C while their counterparts, aerogels, do not show any detectable exotherm by differential thermal analysis (DTA). These results suggest that the structure of single phase gels changed during critical point drying and the structural changes were investigated by solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. No differences between diphasic xerogels and aerogels could be detected by DTA because there was little or no effect of drying on the discrete silica and alumina phases. The tetrahedral coordination of Al in single-phase mullite gel changed to octahedral coordination as detected by MAS NMR during critical point drying, which suggests that the alumina component segregated. Both single-phase and diphasic aerogels of mullite composition showed very high surface areas in the temperature range 1000–1400 °C and these may be useful for high temperature catalytic applications.