Hochfeld-27Al-NMR-Untersuchungen zur Aluminiumkoordination in kristallinen Aluminiumphosphaten

High-field ²⁷Al-NMR Investigations on the Aluminium Coordination in Crystalline Aluminium Phosphates The ²⁷Al NMR spectra of crystalline aluminium phosphates obtained at high-magnetic fields under magic-angle spinning (MAS) conditions allow a direct determination of the aluminium coordination. So for five new Al phosphates it is shown that, as it is almost exclusively the case for Al phosphates of known structures, the Al is octahedrally coordinated, too. Chemical shifts between ?13 and ?21 ppm for isolated AlO₆ octahedra, and 39 ppm for isolated AlO₄ tetrahedra (in crystalline AlPO₄), were determined. In comparison to the results on aluminates and aluminum oxides these values are generally upfield-shifted about 30 ppm. The stronger magnetic shielding is attributed to the influence of the phosphorus atoms present in the second coordination sphere of the aluminum.     

Über basische Aluminiumsalze und ihre Lösungen. VII. Zum Einfluß der Herstellungsbedingungen,der Konzentration und der Alterungszeit auf die Zusammensetzung von Lösungen basischer Aluminiumsalze

Basic Aluminium Salts and their Solutions. VII. Influence of Preparation, Concentration, and Aging on the Constitution of Solutions of Basic Aluminium Salts 0.1 to 4 molar basic aluminium chloride solutions – prepared by dissolving aluminium metal in substoichiometric quantities of hydrochloric acid and 10^?4 to 0.2 molar basic solutions of aluminium chloride and perchlorate – prepared by adding alkali to the solutions of the neutral salts were investigated for the kinetics of their reactions with ferrone and by ²⁷Al NMR spectroscopy. In all solutions the contents of polymeric species decreases at equal basicity with increasing aluminium concentration. On the other hand the Al₁₃O₄₀ complex is only formed in solutions prepared by addition of alkali. The differences of composition are confirmed by the aging behaviour of the solutions.     

Über basische Aluminiumsalze und ihre Lösungen. (XII). Vergleichende Untersuchungen an unterschiedlich dargestellten Basischen Aluminiumchloridlösungen

Basic Aluminium Salts and their Solutions. XII. Comparative Studies of Basic Aluminium Chloride Solutions Prepared in Different Ways Basic aluminum chloride solutions were prepared from solutions of the neutral salt by adding alkali or by dissolution of metallic aluminum. The solutions prepared by both methods contain monomeric, tridecameric, and polymeric aluminum cations, as was shown by the ferrone method and by ²⁷Al NMR spectroscopy. The formation of tridecameric ions is favoured by low temperature, low basicity, and in diluted solution; at higher temperatures and basicities they are transformed to polymeric and monomeric forms.     

Über basische Aluminiumsalze und ihre Lösungen. X. NMR-Untersuchungen am tridekameren Al-oxo-hydroxo-Kation

Basic Aluminum Salts and their Solutions. X. NMR-Investigations on the Tridecameric Al-Oxo-Hydroxo Cation Using solid-state high resolution ²⁷Al-NMR it is shown that in the basic aluminum chloride with a degree of alkalinity r = OH/Al = 2.5 cations of the [Al₁₃O₄(OH)₂₅(H₂O)₁₁]⁶⁺ type exist. The ¹H-NMR detects besides the constitutional water still an amount of mobile, enclosed H₂O molecules which can be removed by thermal treatment at 104°C. The ²⁷Al spectrum of the aqueous solutions of the chloride shows only one peak at 62.5 ppm which is characteristic for the tridecameric cation. Adding HCl to the solution causes a decomposition to low-molecular species (peak at 0 ppm). Adding NaOH to the aqueous solution of the chloride effects a rearrangement of the tridecameric cations to higher molecular particles which are no more accessible by ²⁷Al-NMR.     

Über basische Aluminiumsalze und ihre Lösungen. VI. Darstellung und Charakterisierung eines wasserlöslichen Al13O40-Chlorides

Basic Aluminium Salts and their Solutions. VI. Preparation and Characterization of a Water-soluble Al₁₃O₄₀ Chloride The water-soluble chloride is prepared from the sparingly soluble basic aluminium sulfate [Al₁₃O₄(OH)₂₅(H₂O)₁₁](SO₄)₃ · xH₂O by reaction with barium chloride. From kinetic and ²⁷Al NMR measurements is concluded, that the solid chloride as well as its aqueous solution contain exclusively the Al₁₃O₄₀ cation. It is shown by differential thermal analysis that the solid chloride includes water in different linkage.     

Über basische Aluminiumsalze und ihre Lösungen. XIX. Zur Art der Al-Kationen in hochbasischen,hochkonzentrierten Aluminiumchloridlösungen

Basic Aluminium Salts and their Solutions. XIX. Nature of Aluminium Cations in Highly Basic Highly Concentrated Aluminium Chloride Solutions It is shown that in highly concentrated basic aluminium chloride solutions (c_Al ≈ 7.7 mol · l^?1; r = OH/Al = 2.48), prepared by dissolving aluminium metal in substoichiometric quantities of hydrochloric acid, polymeric aluminium cations (Al_poly) dominate. By dilution of these solutions also increasing amounts of monomeric cations (Al_mono) occur. On aging Al_mono reacts with Al_poly forming small portions of tridecameric Al₁₃O₄₀ cations (Al₁₃). On the basis of these results literature data, after which corresponding commercial basic aluminium chloride solutions contain essentially Al₁₃ cations, are critically discussed.     

27Al-MAS-NMR- und IR-spektroskopische Untersuchungen zur Aluminium-Koordination in röntgenamorphen SiO2 ?Al2O3-Festkörpern

²⁷Al MAS N.M.R. and I.R. Investigations on the Aluminium Coordination in Amorphous Silica-Alumina Gels Amorphous silica-alumina of different Si/Al ratios have been investigated by ²⁷Al magic-angle spinning n.m.r. (MAS n.m.r.) and i.r. spectroscopy. Aluminosilicate framework structures were found for the Na⁺- and NH₄⁺-exchanged forms of the investigated gels. The thermal deammoniation of the NH₄⁺-exchanged gels, producing the H⁺ forms, causes an irreversible damage of the aluminosilicate framework. This process is accompanied by a change of the coordination state of a part of the aluminium, converting AlO₄ into AlO₆ units. The progress of the solid-state reaction upon further thermal treatment of the H⁺ forms is reflected by a considerable broadening of the n.m.r. spectra and a decrease of the intensity. The possibility that well-known results, including those of nonframework aluminium, obtained for comparable zeolitic systems may also be valid for amorphous solids, is discussed.     

Synthesis of polyaluminocarbosilane with low branched molecular structure using liquid polysilacarbosilane and aluminum acetylacetonate by high‐pressure method

The synthesis of polyaluminocarbosilane (PACS) using liquid polysilacarbosilane (PSCS) and aluminum acetylacetonate [Al (acac)₃] by a high‐pressure method is reported for the first time. The effects of reaction time, temperature and feed ratio on the structure of PACS are investigated in detail by gel permeation chromatography, Fourier transform‐infrared, ¹H‐NMR, ²⁹Si‐NMR and ²⁷Al‐NMR methods. It was found that the molecular weight and its polydispersity, as well as the branching degree of the molecular structure of PACS, increase with reaction time and temperature. Increasing the weight percentage of Al (acac)₃ has a similar effect as temperature. Combined with the gas chromatography–mass spectroscopy results, the reaction mechanism is proposed, which contains three main reactions: (i) cleavage and rearrangement reaction of PSCS; (ii) silicon‐free radicals react with Al (acac)₃, leading to cleavage of O=C and/or O‐C bonds and formation of AlO_x ligands; and (iii) conversion reaction of Al ligands from AlO₆ into AlO₅ and AlO₄. It is also found that PACS prepared by high‐pressure method has a lower branched molecular structure in comparison to its analog prepared under ambient pressure conditions, and it is achieved to increase the molecular weight and ceramic yield of PACS, which is beneficial for the processing and overall quality of the final product.     

27Al NMR diffusometry of Al13 Keggin nanoclusters

Although nanometer-sized aluminum hydroxide clusters (i.e., ϵ-Al₁₃, [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺) command a central role in aluminum ion speciation and transformations between minerals, measurement of their translational diffusion is often limited to indirect methods. Here, ²⁷Al pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has been applied to the AlO₄ core of the ϵ-Al₁₃ cluster with complementary theoretical simulations of the diffusion coefficient and corresponding hydrodynamic radii from a boundary element-based calculation. The tetrahedral AlO₄ center of the ϵ-Al₁₃ cluster is symmetric and exhibits only weak quadrupolar coupling, which results in favorable T₁ and T₂ ²⁷Al NMR relaxation coefficients for ²⁷Al PFGSTE NMR studies. Stokes–Einstein relationship was used to relate the ²⁷Al diffusion coefficient of the ϵ-Al₁₃ cluster to the hydrodynamic radius for comparison with theoretical simulations, dynamic light scattering from literature, and previously published ¹H PFGSTE NMR studies of chelated Keggin clusters. This first-of-its-kind observation proves that ²⁷Al PFGSTE NMR diffusometry can probe symmetric Al environments in polynuclear clusters of greater molecular weight than previously considered.     

Structural studies of sialon ceramics by high-resolution solid-state NMR

High-resolution solide-state ²⁷Al NMR with magic-angle spinning (MASNMR) readily monitors the quantity and coordination (four- and six-fold) of aluminium in two ceramic materials of the SiAlON system. Sialon X-phase, of approximate composition Si₃Al₆O₁₂N₂, contains aluminium-centred octahedra and tetrahedra in the ratio ca. 1.9:1.0, while another sample containing a mixture of sialon polytypoids shows AlO₆ octahedra and a large quantity of what is most probably nitrogen-coordinated tetrahedral aluminium. In addition, ²⁹Si MASNMR detects two different kinds of silicon in the latter sample in a 2:1 ratio. These observations are interpreted satisfactorily in terms of the crystal structures of the compounds and provide further examples of the potential of MASNMR in the investigation of complex ceramic systems.     

27Al-NMR-Untersuchungen an Alkalifluoroaluminaten

²⁷Al NMR Studies on Alkali Fluoroaluminates The ²⁷Al NMR spectra of the alkali fluoroaluminates sensitively reflect the kind of condensation of the AlF₆ octahedra. Whereas the chemical shift of the octahedral Al is shown to be rather independent on structural details, clear differences in the electric field gradients of isolated and condensed AlF₆ octahedra were found. In contrast to the isolated octahedra which show only weak effects of quadrupole interaction, both for chains and layers of AlF₆ octahedra axial EFG tensors result with quadrupolar coupling constants of between 10 and 13 MHz; for the threefold chains in CsAlF₄ only a value of 7.5 MHz is obtained.     

Effect of phenyl group on the structure and formation of transitional alumina from Al (OPh)<Subscript>3</Subscript>

The structure of freshly prepared Al(OPh)₃, its decomposition product, the hydrolyzed products and their structural evolution were investigated employing ²⁷Al MAS NMR spectroscopy, PXRD, TGA/DTA/DSC/FTIR techniques. In the ²⁷Al MAS NMR spectrum of the aluminium phenoxide, three signals with the chemical shift at 3.78, 21 and 45 ppm were observed. The chemical shift at 3.78 and 45 ppm revealed the presence of four and sixfold coordinated aluminum. The signal at 21 ppm corresponded to fivefold coordinated aluminium. When the aluminium phenoxide was directly decomposed in air at 600 °C, it resulted in amorphous product as evidenced from the PXRD pattern. The observed signals with chemical shifts at 10.1, 42, 73.6 ppm in the ²⁷Al MAS NMR spectrum indicated the presence of 6, 5 and 4 coordination for the aluminium atoms suggesting disordered transitional γ-alumina to be the product. The hydrolysis studies of Al(OPh)₃ with excess of water at 70 °C yielded bohemite (γ-AlOOH). The alumina obtained after dehydration at 600 °C was X-ray amorphous. The dehydrated product at 600 °C showed the presence of four and six coordinated aluminium atoms in the ²⁷Al MAS NMR spectrum confirming it to be ordered γ-Al₂O₃. Crystalline γ-Al₂O₃ was obtained on further heating at 800 °C.     

Über basische Aluminiumsalze und ihre Lösungen. (XIII). Kristalline basische Aluminiumchloride als Produkte der Rehydratisierung und Hydrochlorierung von Übergangstonerden

Basic Aluminum Salts and their Solutions. XIII. Crystalline Basic Aluminum Chlorides Formed as Products of Rehydration and Hydrochlorination of Transition Aluminas By reaction of transition aluminas with hydrochloric acid in sealed tubes at temperatures above 100°C crystalline basic aluminum chlorides were prepared. From the aluminum oxides used (χ-Al₂O₃, χ-Al₂O₃, and χ-Al₂O₃) χ-Al₂O₃ was most suitable for preparation. The homogeneous basic salts are characterized by a certain variability in composition as phases. After a treatment with water vapor there is a remarkable change in the X-ray diagrams of the compounds. It is shown, that at this alteration the lattice was only changed unessentially. Therefore the existence of intercalation compounds with a 3D host lattice is assumed; the water molecules are located at defined positions in the voids.     

NMR Chemical Shift and Quadrupolar Interaction Parameters of Carbon‐Coordinated 27Al in Aluminium Carbide,Al4C3

Aluminium carbide, Al₄C₃, was characterised by ¹³C and ²⁷Al solid‐state NMR spectroscopy. The ¹³C NMR spectra display two resonances with an intensity ratio of 1:2, which is in agreement with the reported crystal structure. The ²⁷Al NMR spectra of Al₄C₃ under both static and MAS conditions were deconvoluted into two spectral components, belonging to the two aluminium species Al1 and Al2 in the crystal structure of Al₄C₃. The spectral fit allowed for determination of the relatively large quadrupolar coupling constants (χ ≈? 16 MHz) of both ²⁷Al species. One aluminium species displayed a tendency of having a χ of slightly smaller magnitude compared to the other. By carrying out DFT calculations of the EFG tensor at the ²⁷Al sites using the Wien2k software, we could tentatively assign the smaller χ site to be the crystallographic Al1 species. Also, the isotropic chemical shift for the carbon‐coordinated aluminium in Al₄C₃ could be determined, being in the range of 111 to 120 ppm. This is somewhat larger than those shift values observed for ²⁷Al in nitrogen coordination.     

Über basische Aluminiumsalze und ihre Lösungen. XIV. Zur Strukturellen Funktion des Wassers in basischen Aluminiumchloriden

Basic Aluminum Salts and their Solutions. XIV. Structural Function of Water in Basic Aluminum Chlorides The structural function of water in basic aluminum chlorides as intercalation compounds was examined by X-ray, thermoanalytical, and solid state ¹H-NMR investigations in connection with charging experiments in water vapor atmosphere. The basic salts were prepared by hydrothermal reaction of χ-Al₂O₃ with hydrochloric acid. It was shown, that the structural changes caused by loading with water vapor are reversible; the original state of the compounds may be recovered by a treatment in vacuum or by heating up to 130°C. The ¹H-NMR spectrum of the H₂O-loaded compounds consists of three components, caused by protons (a) of fixed coordination water or of OH^? ions, (b) of adsorbed water molecules with possibility of random motion, (c) of water molecules with hindered mobility. The development of spectra as a function of specimen temperature and of specimen treatment shows, that the third component is bound to intercalated water, located in voids, the dimensions of which are in the order of magnitude of water molecules.     

Silicon/aluminum and oxygen/nitrogen ordering in lanthanum U−phase (La3Si3Al3O12N2)

High-resolution ²⁹Si, ²⁷Al, ¹⁵N, and ¹⁷O MAS NMR spectra have been obtained for both glass and crystalline samples of lanthanum U-phase, La₃Si₃Al₃O₁₂N₂. Previous X-ray single-crystal studies have shown that this phase is iso-structural with rare-earth gallogermanates of the type Ln₃Ga₅GeO₁₄, the atomic arrangement consisting of layers of [(Si,Al)(O,N)₄] tetrahedra in the x, y plane of the hexagonal unit cell, linked together in the z-direction by [AlO₆] octahedra, with rare-earth cations occupying the large holes in this network. However, previous studies obtained only a limited amount of information about cation and anion ordering, mainly deduced from bond-length data. NMR spectra have not only enabled the change in structure from glass to crystalline ceramic to be monitored, but have also given detailed ordering information about the latter, indicating partial disorder of both (Si,Al) and (O,N) on their respective sites in the tetrahedra.     

Al NMR studies of Ce-Al mixed oxides: origin of 40 ppm peak

CeO₂-γ-Al₂O₃ mixed oxides have been prepared by using both co-precipitation and impregnation methods followed by calcination at 650°C and investigated by ²⁷Al MAS NMR, powder X-ray diffraction and temperature programmed reduction techniques to understand the nature of chemical interaction existing between CeO₂ and γ-Al₂O₃. The ²⁷Al NMR spectra of CeO₂-containing samples showed an additional peak placed at 40 ppm along with the two peaks at 68 and 6 ppm which originate from the tetrahedrally and octahedrally coordinated Al³⁺ ions present in γ-Al₂O₃. As the concentration of CeO₂ in the mixed oxide increased, the intensity of the 40 ppm peak increased and this was the prominent peak for CeO₂-rich mixed oxide samples. The origin of this 40 ppm peak is discussed and it is inferred that this peak is due to Al³⁺ ions, which are present in CeO₂ lattice, forming a solid solution.     

On the stability of Al<Subscript>13</Subscript> Keggin cation in aqueous hydrogen peroxide solutions

We report the first attempt to study the behavior of the [AlO₄Al1₂(OH)₂₅(H₂O)₁₁]⁶⁺ (Al₁₃) Keggin cation (KC) in water–peroxide solutions. Addition of hydrogen peroxide into an aqueous solution containing the Al₁₃ KC reduces pH due to the acidity of hydrogen peroxide. According to the ²⁷Al NMR studies of water–peroxide solutions prepared just before the NMR experiment, with their pH adjusted to the initial value of 5.5 with aqueous NaOH, the Al₁₃ KC concentration decreases immediately once hydrogen peroxide is added to the initial system. Addition of 18.2 wt % hydrogen peroxide to the initial 0.88 mM Al₁₃ solution gives rise to a fourfold decline in Al₁₃ polyoxo cation concentration to 0.22 mM. Then, the KC concentration in the test system remains unchanged for 1 week. Large hydrogen peroxide amounts (27.9 wt % or higher) added to the initial system almost completely degrade the KC. Sodium sulfate added to the initial water–peroxide solution of Al₁₃ chloride where the hydrogen peroxide concentration is 5.5 wt % precipitates the earlier described Al₁₃ sulfate [AlO₄Al₁₂(OH)₂₅(H₂O)₁₁](SO₄)₃ · 16H₂O, where the aluminum polyoxo cation does not contain coordinated hydrogen peroxide molecules, peroxo or hydroperoxo groups as shown by X-ray diffraction.     

Synthesis and crystal structures of dialkyl[1,1-bis(alkylchloroalanyl)organylmethyl]phosphine•dialkylchloroalane(1/1) complexes

Dialkyl[1,1-bis(alkylchloroalanyl)organylmethyl]phosphine?dialkylchloroalane(1/1) complexes (1a–1d) were synthesized and fully characterized. In 1a–1d, dative bonding between phosphorus or chlorine as a donor atom and aluminum as an acceptor atom results in a bicyclic system. The ³¹P{¹H} NMR spectra of all compounds dissolved in d₆-benzene indicate the presence of several isomers in solution. The ²⁷Al{¹H} NMR spectra of 1a–1d dissolved in d₆-benzene as well show very broad singlets between 177 and 140 ppm. For all compounds, crystal structures consist of two fused four- and five-membered rings. The 1λ³-phosphaalkyne reacts at the Al–C bond of the starting material, whereas the Al–Cl moiety remains intact. The heterocycle isolated is a molecular complex of the underlying insertion compound and a third equivalent of dialkylaluminum chloride. The four- and five-membered rings both contain two chlorine-bridged aluminum atoms, Al3 and Al1, slightly more symmetrical than that between Al1 and Al6. In the four-membered ring the two aluminum atoms Al1 and Al6 approach each other at an average distance of 289.1 pm which tallies with the element–element distance (286.3 pm) in aluminum metal.     

Ultra‐wideline 27Al NMR Investigation of Three‐ and Five‐Coordinate Aluminum Environments

Ultra‐wideline ²⁷Al NMR experiments are conducted on coordination compounds with ²⁷Al nuclei possessing immense quadrupolar interactions that result from exceptionally nonspherical coordination environments. NMR spectra are acquired using a methodology involving frequency‐stepped, piecewise acquisition of NMR spectra with Hahn‐echo or quadrupolar Carr–Purcell Meiboom–Gill (QCPMG) pulse sequences, which is applicable to any half‐integer quadrupolar nucleus with extremely broad NMR powder patterns. Despite the large breadth of these central transition powder patterns, ranging from 250 to 700 kHz, the total experimental times are an order of magnitude less than previously reported experiments on analogous complexes with smaller quadrupolar interactions. The complexes examined feature three‐ or five‐coordinate aluminum sites: trismesitylaluminum (AlMes₃), tris(bis(trimethylsilyl)amino)aluminum (Al(NTMS₂)₃), bis[dimethyl tetrahydrofurfuryloxide aluminum] ([Me₂‐Al(μ‐OTHF)]₂), and bis[diethyl tetrahydrofurfuryloxide aluminum] ([Et₂‐Al(μ‐OTHF)]₂). We report some of the largest ²⁷Al quadrupolar coupling constants measured to date, with values of C_Q(²⁷Al) of 48.2(1), 36.3(1), 19.9(1), and 19.6(2) MHz for AlMes₃ , Al(NTMS₂)₃ , [Me₂‐Al(μ‐OTHF)]₂ , and [Et₂‐Al(μ‐OTHF)]₂ , respectively. X‐ray crystallographic data and theoretical (Hartree–Fock and DFT) calculations of ²⁷Al electric field gradient (EFG) tensors are utilized to examine the relationships between the quadrupolar interactions and molecular structure; in particular, the origin of the immense quadrupolar interaction in the three‐coordinate species is studied via analyses of molecular orbitals.