MAS-NMR investigations of the crystallization behaviour of lithium aluminum silicate (LAS) glasses containing P2O5 and TiO2 nucleants

Lithium aluminum silicate (LAS) glass of composition (mol%) 20.4Li₂O-4.0Al₂O₃-68.6SiO₂-3.0K₂O-2.6B₂O₃-0.5P₂O₅-0.9TiO₂ was prepared by melt quenching. The glass was then nucleated and crystallized based on differential thermal analysis (DTA) data and was characterized by ²⁹Si, ³¹P, ¹¹B and ²⁷Al MAS-NMR. XRD and ²⁹Si NMR showed that lithium metasilicate (Li₂SiO₃) is the first phase to c form followed by cristobalite (SiO₂) and lithium disilicate (Li₂Si₂O₅). ²⁹Si MAS-NMR revealed a change in the network structure already for the glasses nucleated at 550 °C. Since crystalline Li₃PO₄, as observed by ³¹P MAS-NMR, forms concurrently with the silicate phases, we conclude that crystalline Li₃PO₄ does not act as a nucleating agent for lithium silicate phases. Moreover, ³¹P NMR indicates the formation of M-PO₄ (M=B, Al or Ti) complexes. The presence of BO₃ and BO₄ structural units in all the glass/glass-ceramic samples is revealed through ¹¹B MAS-NMR. B remains in the residual glass and the crystallization of silicate phases causes a reduction in the number of alkali ions available for charge compensation. As a result, the number of trigonally coordinated B (BO₃) increases at the expense of tetrahedrally coordinated B (BO₄). The ²⁷Al MAS-NMR spectra indicate the presence of tetrahedrally coordinated Al species, which are only slightly perturbed by the crystallization.     

MAS-NMR studies of lithium aluminum silicate (LAS) glasses and glass-ceramics having different Li2O/Al2O3 ratio

Emergence of phases in lithium aluminum silicate (LAS) glasses of composition (wt%) xLi₂O-71.7SiO₂-(17.7−x)Al₂O₃-4.9K₂O-3.2B₂O₃-2.5P₂O₅ (5.1≤x≤12.6) upon heat treatment were studied. ²⁹Si, ²⁷Al, ³¹P and ¹¹B MAS-NMR were employed for structural characterization of both LAS glasses and glass-ceramics. In glass samples, Al is found in tetrahedral coordination, while P exists mainly in the form of orthophosphate units. B exists as BO₃ and BO₄ units. ²⁷Al NMR spectra show no change with crystallization, ruling out the presence of any Al containing phase. Contrary to X-ray diffraction studies carried out, ¹¹B (high field 18.8 T) and ²⁹Si NMR spectra clearly indicate the unexpected crystallization of a borosilicate phase (Li,K)BSi₂O₆, whose structure is similar to the aluminosilicate virgilite. Also, lithium disilicate (Li₂Si₂O₅), lithium metasilicate (Li₂SiO₃) and quartz (SiO₂) were identified in the ²⁹Si NMR spectra of the glass-ceramics. ³¹P NMR spectra of the glass-ceramics revealed the presence of Li₃PO₄ and a mixed phase (Li,K)₃PO₄ at low alkali concentrations.     

Calcium aluminates hydration in presence of amorphous SiO2 at temperatures below 90 °C

The hydration behaviour of Ca₃Al₂O₆, Ca₁₂Al₁₄O₃₃ and CaAl₂O₄ with added amorphous silica at 40, 65 and 90 °C has been studied for periods ranging from 1 to 31 days. In hydrated samples crystalline phases like katoite (Ca₃Al₂(SiO₄)_3−x(OH)_4x) and gibbsite, Al(OH)₃, were identified, likewise amorphous phases like Al(OH)_x, calcium silicate hydrates, C-S-H, and calcium aluminosilicate hydrates, C-S-A-H, were identified. The stoichiometry of Ca₃Al₂(SiO₄)_3−x(OH)_4x (0?3−x?0.334), which was the main crystalline product, was established by Rietveld refinement of X-ray and neutron diffraction data and by transmission electron microscopy.     

Characteristics and formation of [AlO4Al12(OH)24(H2O)12]7+ in electrolysis process

[AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃) formation in electrolysis process is studied. The results detected by²⁷Al NMR spectroscopy show that high content of Al₁₃ polymer is formed in the partially hydrolyzed aluminum solution prepared by controlled electrolysis process. In the produced electrolyte of total Al concentration ([Al_T]) 2.0 mol · L⁻¹ with a basicity (B = OH/Al molar ratios) of 2.0, the content of Al₁₃ polymer is over 60% of total Al. Dynamic light scattering shows that the size distribution of the final electrolyte solutions ([Al_T] = 2.0 mol · L⁻¹) is trimodal with B = 2.0 and bimodal with B = 2.5. The aggregates of Al₁₃ complexes increase the particle size of partially hydrolyzed aluminum solution.     

Potentiometric determination of the 'formal' hydrolysis ratio of aluminium species in aqueous solutions

The ‘formal’ hydrolysis ratio (h = C(OH⁻)_added/C(Al)_total) of hydrolysed aluminium-ions is an important parameter required for the exhaustive and quantitative speciation-fractionation of aluminium in aqueous solutions. This paper describes a potentiometric method for determination of the formal hydrolysis ratio based on an automated alkaline titration procedure. The method uses the point of precipitation of aluminium hydroxide as a reference (h = 3.0) in order to calculate the initial formal hydrolysis ratio of hydrolysed aluminium-ion solutions. Several solutions of pure hydrolytic species including aluminium monomers (AlCl₃), Al₁₃ polynuclear cluster ([Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺), Al₃₀ polynuclear cluster ([Al₃₀O₈(OH)₅₆(H₂O)₂₆]¹⁸⁺) and a suspension of nanoparticulate aluminium hydroxide have been used as ‘reference standards’ to validate the proposed potentiometric method. Other important variables in the potentiometric determination of the hydrolysis ratio have also been optimised including the concentration of aluminium and the type and strength of alkali (Trizma-base, NH₃, NaHCO₃, Na₂CO₃ and KOH). The results of the potentiometric analysis have been cross-verified by quantitative ²⁷Al solution nuclear magnetic resonance (²⁷Al NMR) measurements. The ‘formal’ hydrolysis ratio of a commercial basic aluminium chloride has been measured as an example of a practical application of the developed technique.     

Synthesis,structure and characterization of [Mg7(μ3-OCH2CH2OMe)6(μ-OCH2CH2OMe)6][Al(n-Bu)4]2 – A cationic heptanuclear magnesium complex consisting of discrete cations and tetrabutylaluminate anions

Dibutylmagnesium (contaminated with Al(n-Bu)₃; n_Mg:n_Al ca. 1:0.2) was found to react with MeOCH₂CH₂OH followed by the addition of PhSCH(Me)Ph in the presence of 0.2 equiv n-butyllithium yielding [Mg₇(μ₃-OCH₂CH₂OMe)₆(μ-OCH₂CH₂OMe)₆][Al(n-Bu)₄]₂ (1) as the principal product (yield 40–45% referred to MeOCH₂CH₂OH). The single-crystal X-ray diffraction analysis revealed that the centrosymmetric cationic heptamagnesium complex is built up from seven edge-shared MgO₆ octahedra. The [Al(n-Bu)₄]⁻ anions adopt approximately a tetrahedral AlC₄ symmetry. ¹H, ¹³C and ²⁷Al NMR spectroscopic measurements showed that in THF solution the structures both of the heptamagnesium complex and the tetrabutylaluminate anion are preserved and that there are no cation–anion interactions reducing the symmetry. The ²⁷Al resonance (151.6 ppm) was found to be very sharp (w_1/2 = 5 Hz), the coupling constant ¹J(²⁷Al,¹³C) amounts to 72.3 Hz.     

Substitution clustering in a non-stoichiometric celsian synthesized by the thermal transformation of barium exchanged zeolite X

The thermal transformation of Ba exchanged zeolite X to celsian has been studied by ²⁷Al and ²⁹Si MAS NMR spectroscopy. Evidence for the degradation of the zeolite framework is present in the ²⁹Si NMR spectra after thermal treatment at 850 °C. Confirmation is provided by the ²⁹Si NMR data that synthesis of celsian via the decomposition of Ba exchanged zeolite leads to a single defect phase. Clustering of the isomorphous replacement of aluminium by silicon must occur to explain the observed ²⁹Si chemical shifts. The ²⁷Al NMR data show distorted aluminium co-ordination sites upon the thermal transformation of Ba exchanged zeolite X. The distortions present in the amorphous matrix are greater than those present in the monoclinic and hexagonal crystalline phases of celsian.     

MAS-NMR study of lithium zinc silicate glasses and glass-ceramics with various ZnO content

Lithium zinc silicate glasses of composition (mol%): 17.5Li₂O-(72−x)SiO₂-xZnO−5.1Na₂O−1.3P₂O₅−4.1B₂O₃, 5.5?x?17.7, were prepared by conventional melt-quenched technique and converted to glass-ceramic by controlled crystallization process. ²⁹Si and ³¹P MAS-NMR was used to characterize the structure of both glass and glass-ceramic samples. Despite the complex glass composition, Q², Q³ and Q⁴ sites are identified from ²⁹Si MAS-NMR, which relative intensities are found to vary with the ZnO content, indicating a network depolymerization by ZnO. Moreover, well separated Q³ and Q⁴ resonances for low ZnO content indicates the occurrence of phase separation. From ³¹P MAS-NMR, it is seen that phosphorus is mainly present in the form of ortho-(Q⁰) and pyro-phosphate (Q¹) structural units and variation of ZnO content did not have much effect on these resonances, which provides an additional evidence for phase separation in the glass. On conversion to glass-ceramics, lithium disilicate (Li₂Si₂O₅), lithium zinc ortho-silicate (Li₃Zn_0.5SiO₄), tridymite (SiO₂) and cristobalite (SiO₂) were identified as major silicate crystalline phases. Using ²⁹Si MAS-NMR, quantification of these silicate crystalline phases is carried out and correlated with the ZnO content in the glass-ceramics samples. In addition, ³¹P spectra unambiguously revealed the presence of crystalline Li₃PO₄ and (Na,Li)₃PO₄ in the glass-ceramics.     

Hydration of portland cement in the presence of aluminum-containing setting accelerators

Portland cement hydration in the presence of two different aluminum-containing compounds, highly dispersed amorphous Al(OH)₃ and aqueous solution of aluminum hydroxosulfate Al(OH)_1.78(SO₄)_0.61, was studied by solid-state ²⁷Al and ²⁹Si NMR spectroscopy.     

Elaboration and Characterisation of Hybrid Materials of Polymethylhydrosiloxane Modified by Diamines of Various Lengths

The polymethylhydrosiloxane (PMHS) modified by bifunctional organic compounds (diamines), offer the possibility of producing organic-inorganic hybrid materials. These materials present excellent opto-electronic properties and find numerous applications such as the manufacture of electroluminescent diodes and ion or radiation sensors.This work shows that monolithic and transparent hybrid gels were obtained by reaction at room temperature of PMHS with diamines in tetrahydrofuran, using hexachloroplatinic acid (H₂PtCl₆·6H₂O) as catalyst. The products have been characterized by infrared and ²⁹Si MAS-NMR spectroscopy. The results show that the diamines have reacted with the PMHS leading to the monolithic and transparent gels in which both organic-inorganic —Si—(H)N—(CH₂) _n —N(H)—Si— bridges are formed (n = 3, 4 and 6). The thermal analysis of the xerogels was determined by TGA and DTA. The structure and texture of the obtained materials, were studied by Chemical Analysis and the Brunauer-Emmett-Teller (BET) method.     

Room Temperature Synthesis and Characterization of Hybrid Materials of Polymethylhydrosiloxane Modified by Hydroxide Organic Compounds

New monolithic and transparent hybrid gels were obtained by reaction at room temperature of polymethylhydrosiloxane (PMHS) with 1,3,5-trihydroxybenzene (DO_1,3,5) and 2-hydroxybenzoic acid (DO₂), in tetrahydrofuran, using hexachloroplatinic acid (H₂PtCl₆·6H₂O) as catalyst. The products have been characterized by infrared and ²⁹Si MAS-NMR spectroscopy. The results show that the organic compounds have reacted with the PMHS, leading to monolithic and transparent gels in which organic-inorganic bridges were formed. An appropriate thermal treatment procedure was determined from TGA and DTA curves recorded on the hybrid gel powder after drying at 70 °C. The morphology and structure of the materials obtained were studied by scanning electronic microscopy (SEM) and X-ray powder diffraction (XRD).     

Reflections upon and recent insight into the mechanism of formation of hydroxyaluminosilicates and the therapeutic potential of silicic acid

The chemistry of mono or ortho silicic acid (Si(OH)₄) is barely considered in most chemistry texts. Mention is usually only made of its autocondensation in forming hydrated amorphous silica and its reaction with ammonium molybdate in forming the molybdosilicic acid complex. Reference should now be made to its unique inorganic chemistry with aluminium (Al) and specifically aluminium hydroxide (Al(OH)_3(s)) in forming hydroxyaluminosilicates (HAS_(s)). The competitive condensation or substitution of Si(OH)₄ into a framework of Al(OH)_3(s) results in the formation of either HAS_A or HAS_B. Which type of HAS_(s) predominates depends upon the ratio of Si:Al in preparative solutions with the formation of HAS_B requiring a two-fold excess of Si(OH)₄ over Al. The Si:Al ratio of HAS_A is 0.5 and the existence of HAS_A is a prerequisite to the formation of HAS_B in which the ratio of Si:Al is 1.0. HAS_A is composed of only octahedrally co-ordinated Al, Al^VI, whereas HAS_B is composed of equal quantities of Al^VI and tetrahedrally coordinated Al, Al^IV, and is formed by a Si(OH)₄-fuelled dehydroxylation reaction. HAS_(s) are significantly more ‘kinetically’ stable than Al(OH)_3(amorphous) with HAS_B predicted to predominate at pH > 4.0 and [Si(OH)₄] > 0.1 mmol/L. HAS_(s) are critical secondary mineral phases in the biogeochemical cycle of Al and Si(OH)₄ and the formation of HAS_(s) have played a major role in precluding Al³⁺_(aq) from biochemical evolution. In the future Si(OH)₄ and the formation of HAS_(s) are predicted to be of significant importance in providing protection for humans against a potentially burgeoning exposure to biologically available Al.     

Sol–gel preparation of mesoporous Al2O3–SiO2 glasses: structural evolution monitored by solid state NMR

Glasses along the composition line 0.5Al₂O₃–xSiO₂ (1 ≤ x ≤ 6) were prepared via a novel sol–gel route using tetraethylorthosilicate and aluminum lactate as precursors. The structural evolution from solution to gel to glass is monitored by standard ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopies, revealing important insights about molecular level mechanisms occurring at the various stages of glass formation. Under the experimental conditions reported, silica and alumina precursors undergo homoatomic condensation processes when the gel is heat treated at about 100 and 300 °C, respectively, and only little heteroatomic co-condensation occurs in this temperature range. The latter is promoted only upon elimination of the residual lactate and water ligands upon annealing the gels above 300 °C. Following calcination at 650 °C, mesoporous glasses are obtained, having average pore diameter of about 3 nm and a surface areas near 500 m²/g. Si–O–Al connectivities are detected by ²⁹Si magic angle spinning (MAS)-NMR. ²⁷Al MAS-NMR spectra reveal aluminum in four-, five- and six-coordination. The spectra differ significantly from those of other sol–gel derived Al₂O₃–SiO₂ materials prepared from different precursor routes, suggesting that the lactate route results in a higher degree of compositional homogeneity.     

New silica hybrids elaborated by sol-gel process from bifunctional thiadiazole and 1,2,4-triazole precursors

New hybrid materials were synthesized from an inorganic precursor, (tetraethoxysilane: TEOS) and bifunctional organic precursors through the sol-gel process. Indeed, monolithic and transparent hybrid gels were obtained by the reaction of tetraethoxysilicate (TEOS) with 1,2,4-thiadiazole-3,5-diamine (1), 5-amino-1,3,4-thiadiazole-2-thiol (2), and 1H-1,2,4-triazole-3-thiol (3) in ethanol as solvent, using chloric acid (HCl 10^?1M) as catalyst. It is shown that TEOS has been cross-linked with bi-functional heterocyclic compounds: (1), (2) and (3) leading to the formation of transparent and colorous gels. The characterization was realized by infrared, ²⁹Si and ¹³C CP MAS NMR. The xerogels morphology and texture were studied by scanning electron microscopy (SEM) and Brunauer – Emmet – Teller method (BET). The obtained results show that organic adducts (1), (2) and (3) react with TEOS leading to gels in which Si–N and Si–S bridges were formed. According to N₂-physisorption results, xerogels are mesoporous with specific surface area varied from 105 to 312 m²g^?1 and their isotherms are classified as type IV. The optic and magnetic properties of hybrids were studied respectively by UV-Vis spectroscopy and RPE. This study showed that all materials are paramagnetic semiconductors.     

Nouveaux Matériaux hybrides organiques–inorganiques obtenus par action du tétraethoxysilane avec des molécules hétérocycliques bi-fonctionnelles

New hybrid organic–inorganic materials have been obtained by the reaction of tetraethylorthosilicate (TEOS) with bi-functional heterocyclic molecules in ethanol using HCl as a catalyst at room temperature. This reaction has led to the formation of colored and transparent gels. The characterization of xerogels has been carried by infrared, ²⁹Si and ¹³C CP MAS-NMR, DTA–TGA, scanning electron microscopy, Brunauer–Emmet–Teller method (BET), and DRX. The obtained results show that organic molecules react with TEOS leading to gels, in which Si–N, Si–O, and Si–S bridges are formed. According to N₂-physisorption results, isotherms seem to have a similar sit, and classified as type IV according to the International Union of Pure and Applied Chemistry, synthesized X₁, X₂, and X₄ are mesoporous solids but X₃ is microporous. The BET surface area of these materials is important. It varied from 47 to 302 m²g^?1. The optic and magnetic properties of hybrids are also studied by Ultraviolet–visible spectroscopy and résonance para-électronique. The obtained results show that all materials are semiconductors, and X₂, X₃, and X₄ are paramagnetic but X₁ is diamagnetic.     

Revisiting Y2Si2O7 and Y2SiO5 polymorphic structures by Y MAS-NMR spectroscopy

This paper describes the ⁸⁹Y MAS-NMR spectra for all the established polymorphs of Y₂Si₂O₇ (y, α, β, γ and δ) and Y₂SiO₅ (X1 and X2). The combination of our spectroscopic data with the structural information published up to now from diffraction data permits the revision and correction of mistakes which appear in the literature. Finally, the influence of different structural factors, such as yttrium coordination number and Y-O distances on the ⁸⁹Y NMR isotropic chemical shift is analyzed.     

Preparation and photoluminescence property of a loose powder, Ca3Al2O6:Eu by calcination of a layered double hydroxide precursor

A novel red light-emitting material, Ca₃Al₂O₆:Eu³⁺, which is the first example found in the Ca₃Al₂O₆ host, was prepared by calcination of a layered double hydroxide precursor at 1350 °C. The precursor, [Ca_2.9−xAl₂Eu_x(OH)_9.8](NO₃)_2+x·2.5H₂O, was prepared by coprecipitation of metal nitrates with sodium hydroxide. The material is a loose powder composed of irregular particles formed from aggregation of particles of a few nanometers, as shown in scanning electron microscope (SEM) images. It was found that the photoluminescence intensity reached the maximum when the calcination temperature was 1350 °C and the concentration of Eu³⁺ was 1.0%. The material emits bright red emission at 614 nm under a radiation of λ=250 nm.     

Zur Koordination des Aluminiums in den Calciumaluminathydraten 2 CaO · Al2O3 · 8 H2O und CaO · Al2O3 · 10 H2O

On the Coordination of Al in the Calcium Aluminate Hydrates 2 CaO · Al₂O₃ · 8 H₂O and CaO · Al₂O₃ · 10 H₂O By investigations with high-resolution ²⁷Al-NMR in solids it is shown that in the compound 2 CaO · Al₂O₃ · 8 H₂O the Al merely exist in octahedral coordination. According to this and considering its structural relationship with 4 CaO · Al₂O₃ · 19 H₂O the dicalcium aluminate hydrate is proposed to be formulated as [Ca₂Al(OH)₆][Al(OH)₃ (H₂O)₃]OH. Likewise for the compound CaO · Al₂O₃ · 10 H₂O the octahedral coordination of the Al is proved by ²⁷Al-NMR. This result corresponds with literature according to which a constitution as cyclohexaaluminate Ca₃[Al₆(OH)₂₄] · 18 H₂O is proposed.     

Evolution of the Texture and Structure of SiO2–Al2O3 Xerogels and Aerogels as a Function of the Si to Al Molar Ratio

SiO₂–Al₂O₃ aerogels and xerogels with a Si to Al molar ratio r _Si/Al varying from 0.25 to 20, were made by sol-gel process in acidic conditions at pH 2 and respectively dried by the CO₂ supercritical method and by solvent evaporation. The Al precursor was also chelated with ethylacetoacetate, which made it possible to study the structure and texture of such gels in conditions favorable to the formation of mixed Al–O–Si bonds. Nitrogen adsorption isotherms according to the Brunauer, Emmett and Teller method (BET), ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS-NMR), Fourier Transformed Infrared Spectroscopy (FTIR) and Infrared absorbance spectra after Temperature Programmed Desorption (TPD) of pyridine, showed that the Si–O infrared asymmetric stretching vibration and the Bronsted acidity relative to the Lewis acidity, depended on the ratio of Al^IV to Si atoms.     

系列硅铝比纳米薄层ZSM-5分子筛的合成和表征

以自制不对称双子季铵盐表面活性剂为模板, 在水热合成体系中控制合成系列硅铝比纳米薄层ZSM-5分子筛.采用X射线衍射(XRD)、N₂吸附-脱附、X射线荧光光谱(XRF)、扫描电镜(SEM)和²⁷Al魔角旋转核磁共振(²⁷Al MAS-NMR)对合成的样品进行了表征. 详细研究了晶化温度、晶化时间、结构导向剂(SDA)用量、碱度等对合成的影响和纳米薄层ZSM-5分子筛的形成过程. 结果表明: 分子筛硅铝比越高, 结构导向剂用量越大, 所需的晶化时间越短; 晶化温度越高, 晶化时间越短; 且不同硅铝比纳米薄层ZSM-5分子筛的形貌规整度、比表面积和介孔/微孔孔容比例随着硅铝比而变化.