Crystalline Aluminium Borates with the Mullite Structure: A <Superscript>11</Superscript>B and <Superscript>27</Superscript>Al Solid-State NMR Study

²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were acquired at 8.45, 14.1 and 16.45 T for a series of aluminium borates with the mullite structure (Al_6−x B _x O₉, where x has nominal values of 1 to 4) augmented with ²⁷Al multiple-quantum MAS NMR spectra at 8.45 T. Even though the ²⁷Al NMR spectra are complex, simulation of the combined set of data produced a relatively well-defined set of parameters (e.g., quadrupolar interaction, isotropic chemical shift, etc.) for each site. The ¹¹B MAS NMR spectra of the same compounds were also acquired at 14.1 T. Linear changes in the X-ray a-, b- and c-cell parameters with composition suggest that these compounds constitute a continuous series. Based on a Rietveld structural refinement of the compound synthesized as Al₄B₂O₉, the resulting site occupancies and relative site distortions allow the identification of particular sites with specific NM resonances. Changes in the ²⁷Al and ¹¹B MAS NMR spectra of the related compounds with x = 1–4 show at the lowest Al contents a greater degree of asymmetry in the Al sites of the octahedral chains. A fairly distorted cross-linking tetrahedral site, which persists throughout the composition range, is accompanied in the lower Al compositions by two 5-fold coordinated Al–O units which are replaced by two more-regular tetrahedral Al–O sites as the Al content increases. In the compounds of lowest Al composition (i.e., highest B content) both the tetrahedral and trigonal cross-linking sites are distinguishable, but as the Al content increases, the BO₄ units progressively disappear. Authors' address: Kenneth J. D. MacKenzie, School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand     

Double rotation27Al NMR in molecular sieves

Although high-resolution NMR spectra can be obtained in solids, the use of²⁷Al NMR to investigate the structure of aluminosilicate and aluminophosphate molecular sieves has been severely limited because anisotropic second-order quadrupolar interactions, responsible for spectral broadening, cannot be eliminated by conventional magic angle spinning (MAS) or multiple pulse techniques. Here we give the principles of the double rotation (DOR) NMR technique which can remove not only the first-order broadenings but also the second-order broadenings in the NMR spectra of quadrupolar nuclei in solids. High-resolution²⁷Al NMR using DOR is capable of resolving discrete framework aluminum sites in aluminophosphate molecular sieves, permitting quantitative investigation of site-specific adsorbate-host interactions, and of discriminating different aluminum species in zeolites.     

Application of27Al NMR techniques to structure determination in solids

There have been great improvements in the quality of²⁷Al NMR spectra from solids over the last decade. The impact of this technique on structure determination for a wide range of solids is briefly reviewed. Emphasis is placed on the effects arising from the presence of the quadrupolar interaction and strategies that allow unambiguous spectral interpretation. It is demonstrated that in addition to the chemical shift, the quadrupolar interaction itself can be a rich source of local information about solids. The insight obtained from²⁷Al NMR into atomic level structure often contains surprises, such that previous models of structure and properties have to be reassessed.     

Basics of NMR line shape in quasicrystals

The measurement and analysis of broad nuclear magnetic resonance (NMR) spectra of quasicrystals require experimental methods and theoretical interpretations different from NMR investigations of regular periodic crystals. Frequency- and field-sweep methods for recording quasicrystalline NMR spectra are described and compared with the measurement of²⁷Al NMR spectra of icosahedral AlPdMn and decagonal AlNiCo quasicrystals. The nuclear spin interactions that determine the NMR line shape are the same for both types of the above Al-based quasicrystals, where the electric quadrupolar interaction with the broad distribution of its electric field gradient parameters predominantly determines the shape of the broad satellite “background” intensity. The essential observations are an almost isotropic²⁷Al NMR spectrum of the icosahedral quasicrystals and a strong angular dependence of the spectrum of decagonal quasicrystals.     

NMR Studies of Heat-Induced Transitions in Structure and Cation Binding Environments of a Strontium-Saturated Swelling Mica

In this work, we combine ²⁷Al, ²⁹Si, ¹⁹F, and ²³Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) to characterize the structure and interlayer cation environments in a strontium-saturated member of the swelling mica family before and after a heat-induced collapse of the interlayer space. The ²⁷Al and ²⁹Si MAS NMR demonstrate that the sample consists mainly of swelling mica, though the composition does not match the ideal structural formula. Aluminum NMR also shows that a portion of the aluminum shifts from a tetrahedral to an octahedral coordination environment upon heating. Changes in the ²⁹Si and ¹⁹F NMR after heating are consistent with a structural rearrangement of the tetrahedral sheet to permit the binding of larger cations in the ditrigonal cavity. The ²³Na MAS NMR results indicate the presence of three unique sodium environments before and after heating. The heat-invariant resonance is consistent with the presence of sodium carbonate. The other two resonances are associated with interlayer sodium and reflect a migration of sodium to a dominantly anhydrous ditrigonal binding structure with heating. Quantitative elemental analysis and NMR data presented here suggest strontium is bound deep within the ditrigonal cavity of the collapsed micas. Authors' address: Karl T. Mueller, 104 Chemistry Building, Penn State University, University Park, PA 16802, USA     

TRAPDOR double-resonance and high-resolution MAS NMR for structural and template studies in zeolite ZSM-5

A combination of ²⁷Al magic-angle spinning (MAS)/multiple-quantum (MQ) MAS, and ²⁷Al–{¹⁴N} TRAnsfer of Population in DOuble-Resonance (TRAPDOR) nuclear magnetic resonance (NMR) was used to study aluminium environments in zeolite ZSM-5. ²⁷Al–{¹⁴N} TRAPDOR experiments, in combination with ¹⁴N NMR were employed to show that the two tetrahedral peaks observed in the ²⁷Al MAS/3Q-MAS spectra of as-synthesized ZSM-5 are due to aluminium atoms occupying crystallographically inequivalent T-sites. A ¹³C–{²⁷Al} TRAPDOR experiment was used to study the template, tetrapropyl ammonium bromide (TPABr), in the three-dimensional pore system of ZSM-5. The inequivalency of the methyl groups of TPA was observed in the ¹³C–{²⁷Al} TRAPDOR spectra of as-synthesized ZSM-5 and the motion of the methyl end of the propyl chain appeared to be more restricted in the sinusoidal channel than in the straight channel.     

Characterization of Al(III) complexes with hematein in artistic alum logwood inks

The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were characterized by Raman and ²⁷Al NMR (nuclear magnetic resonance) spectroscopies. Raman spectra of the crystallized complexes and of the compounds applied on a paper substrate are presented and assigned based on published data for the parent compounds. These Raman spectra show that the coordination of the hematein to the Al(III) ions takes place in both cases through the carbonyl and hydroxyl groups in the molecule, and that the aromatic rings are also involved in the interaction. The Raman spectra of the pure hematein–Al(III) complexes were found to be consistent with those recorded for a logwood ink prepared following a late 19th century recipe, using logwood chips instead of pure hematein, and applied on a paper substrate. These spectra can be used as references for the noninvasive identification of the compounds in works of art. ²⁷Al solid‐state NMR showed that the coordination of the Al(III) atoms in the crystallized powder is predominantly octahedral, while when applied on a paper substrate the colorant is present mainly as a tetrahedral complex, with an octahedral coordination also present in a smaller proportion. The fact that the predominant coordinations for the complexes in the crystallized material and for the ones present on the paper substrate are different is relevant for the study of the lightfastness and thermal stability of works of art bearing these media. Copyright © 2009 John Wiley & Sons, Ltd.     

<Superscript>29</Superscript>Si, <Superscript>27</Superscript>Al, <Superscript>1</Superscript>H and <Superscript>23</Superscript>Na MAS NMR Study of the Bonding Character in Aluminosilicate Inorganic Polymers

²⁹Si, ²⁷Al, ¹H and ²³Na solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to relate nominal composition, bonding character and compressive strength properties in aluminosilicate inorganic polymers (AIPs). The ²⁹Si chemical shift varies systematically with Si-to-Al ratio, indicating that the immediate structural environment of Si is altering with nominal composition. Fast ¹H MAS and ²⁹Si T _SiH/T _1ρ relaxation measurements demonstrated that occluded pore H₂O mobility within the disordered cavities is slow in comparison with H₂O mobility characteristics observed within the ordered channel structures of zeolites. The ²⁷Al MAS NMR data show that the Al coordination remains predominantly 4-coordinate. In comparison with the ²⁹Si MAS data, the corresponding ²⁷Al MAS line shapes are relatively narrow, suggesting that the AlO₄ tetrahedral geometry is largely unperturbed and the dominant source of structural disorder is propagated by large distributions of Si–O bond angles and bond lengths. Corresponding ²³Na MAS and multiple-quantum MAS NMR data indicate that Na speciation is dominated by distributions of hydration states; however, more highly resolved ²³Na resonances observed in some preparations supported the existence of short-range order. New structural elements are proposed to account for the existence of these Na resonances and an improved model for the structure of AIPs has also been proposed. Authors' address: John V. Hanna, NMR Facility, Institute of Materials and Engineering Science, Lucas Heights Research Laboratories, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia     

Parallelizing acquisitions of solid-state NMR spectra with multi-channel probe and multi-receivers: Applications to nanoporous solids

A five-channel (¹H, ¹⁹F, ³¹P, ²⁷Al, ¹³C) 2.5 mm magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) probe is used in combination with three separate receivers for the parallel acquisitions of one (1D) and two-dimensional (2D) NMR spectra in model fluorinated aluminophosphate and porous Al-based metal-organic framework (MOF). Possible combinations to record simultaneously spectra using this set-up are presented, including (i) parallel acquisitions of quantitative 1D NMR spectra of solids containing nuclei with contrasted T₁ relaxation rates and (ii) parallel acquisitions of 2D heteronuclear NMR spectra. In solids containing numerous different NMR-accessible nuclei, the number of NMR experiments that have to be acquired to get accurate structural information is high. The strategy we present here, i.e. the multiplication of both the number of irradiation channels in the probe and the number of parallel receivers, offers one possibility to optimize this measurement time.     

Soft-Pulsed Aluminum-27 Quadrupolar Central Transition NMR Studies of Ovotransferrin

We have employed soft (Gaussian) pulses to examine²⁷Al NMR signals arising from Al³⁺bound to ovotransferrin. In addition to enhancing the general detectability of²⁷Al signals from a practical standpoint, this technique makes it possible to invert the central component of the protein-bound²⁷Al signals, providing a route for, to our knowledge, the first inversion-recovery measurements on such systems. The suitability of this approach for gaining insights into the nature of metal ion binding sites in large metalloproteins is critically assessed.     

Tetrahedral cation ordering in layer silicates by 29Si NMR spectroscopy

²⁹Si NMR spectra of layer silicates with a wide range of tetrahedral composition (Si/Al ratios from 7.7 to 2.7) indicate that Si, Al distribution is mainly controlled by the electrostatic requirement of homogeneous dispersion of Al, confirming our earlier conclusions derived from the analysis of natural samples of micas with Si/Al ratios near to 3. This requirement includes, as a partial aspect, the Loewenstein's rule which forbids the presence of Al in neighbouring tetrahedra.     

Structural characterisation of aluminium layered double hydroxides by Al solid-state NMR

²⁷Al solid-state NMR has been applied to study the local structure of pristine and chemically modified aluminium layered double hydroxides (LDH). The pristine LDH only shows six-fold coordinated, octahedral, aluminium, while the calcined and subsequently surfactant treated LDH sample shows a significant fraction of four-fold coordinated tetrahedral aluminium. The co-existence of two types of octahedral sites with different quadrupolar parameters is clearly observed in both samples. Quadrupolar coupling constants and isotropic chemical shifts have been measured from the ²⁷Al triple-quantum MAS NMR allowing to fit the ²⁷Al MAS spectra and quantify the different species in the samples. The quantitative analysis reveals that 30% of the aluminium is in four-fold coordination in the surfactant-modified LDH. We show that this chemical modification retains the two types of AlO₆ sites with a decreased intensity of the site showing the lowest quadrupolar coupling constant.     

C–Al TRAPDOR and REDOR Experiments for the Detection of C–Al Dipolar Interactions in Solids

We report ¹³C–²⁷Al double resonance experiments (REDOR and TRAPDOR) on several aluminum organic compounds with the aim of detecting ¹³C–²⁷Al dipolar couplings and distances in solids. The ¹³C and ²⁷Al pulses are applied to the same probe channel because their resonance frequencies are in close proximity. The different possibilities of controlling the efficiency of the TRAPDOR approach (by varying the ²⁷Al RF amplitude and the MAS frequency) are investigated. The results indicate that TRAPDOR is superior to REDOR in resolving differences in ¹³C–²⁷Al distances when choosing the proper experimental conditions. Where known, the crystal structure data are in qualitative agreement with the distance information extracted from our experiments. The experiment should be very valuable in different fields of solid state chemistry, where the interaction of organic and inorganic sample fractions is of fundamental importance.     

129Xe,29Si and27Al NMR studies of dealuminated Y zeolite

The dealumination of a commercial Y zeolite by contact with SiCl₄ is monitored with NMR spectroscopy of²⁹Si,²⁷Al and¹²⁹Xe. The results are interpreted in terms of the changes in the material as silicon is incorporated in the framework and aluminum is deposited in the pores.     

Studies of heterogeneous catalysis using high-resolution solid state NMR

Recent results obtained at the institute of Catalysis from studies of heterogeneous catalysis using high-resolution¹H,¹³C,¹⁵N,²⁷Al,²⁹Si,³¹P and⁵¹V solid state NMR have been summarized. Emphasis is made on: (1) structural studies of active in catalysis compounds and sites in supported oxides, hydrides and inorganic acids; (2) studies of structures and properties of surface OH groups active in Bronsted acid catalysis; (3) studies of Lewis acidity of heterogeneous catalysts using¹⁵N NMR of adsorbed N₂O and (4) studies of adsorption mechanisms for molecules of reactants over various catalysts.     

Biaxial Q-shearing of 27Al 3QMAS NMR spectra: Insight into the structural disorder of framework aluminosilicates

In this contribution, we present the application potentiality of biaxial Q-shearing of ²⁷Al 3QMAS NMR spectra in the analysis of structural defects of aluminium units in aluminosilicates. This study demonstrates that the combination of various shearing transformations of the recorded ²⁷Al 3QMAS NMR spectra enables an understanding of the broadening processes of the correlation signals of disordered framework aluminosilicates, for which a wide distribution of ²⁷Al MAS NMR chemical shifts and quadrupolar parameters (i.e., second-order quadrupolar splitting and quadrupole-induced chemical shifts) can be expected. By combining the suitably selected shearing transformation procedures, the mechanisms of the formation of local defects in aluminosilicate frameworks, including Al/Si substitution effects in the next-nearest neighbouring T-sites, variations in bond angles, and/or variations in the physicochemical nature of charge-balancing counter-ions, can be identified. The proposed procedure has been extensively tested on a range of model aluminosilicate materials (kyanite, γ-alumina, metakaolin, analcime, chabazite, natrolite, phillipsite, mordenite, zeolite A, and zeolite Y).     

Study of structural modification of sodium aluminophosphate glasses with TiO2 addition through Raman and NMR spectroscopy

Structural modification of sodium aluminophosphate (NAP) glasses with TiO₂ addition has been investigated using Raman and MAS-NMR (³¹P and ²⁷Al) spectroscopy. TiO₂ incorporated NAP glasses having composition (mol%): 40Na₂O-10Al₂O₃-xTiO₂-(50−x)P₂O₅ (x=0-20), are prepared by conventional melt quench method. The low-frequency Raman spectrum suggests an increase in the average ionic character of phosphate glass network with addition of TiO₂. Raman and ³¹P MAS-NMR revealed that the glasses without TiO₂, consist mainly metaphosphate (Q²) structural units. These are gradually converted into pyrophosphate (Q¹) and orthophosphate (Q⁰) structural units along with the formation of P-O-Ti/P-O-Al linkages. ²⁷Al MAS-NMR revealed the change in coordination of Al from octahedral (AlO₆) to tetrahedral (AlO₄) for TiO₂ above 10 mol%. Raman spectra indicate that TiO₂ enters the network in the form of octahedral (TiO₆) and tetrahedral (TiO₄) structural units and at high concentration of TiO₂, tetrahedral structural units are more favourable. Various thermo-physical properties e.g. density (ρ), molar volume (V_m), glass transition temperature (T_g), microhardness (MH), and thermal expansion coefficient (TEC) have been measured as a function of TiO₂ content. Variations in the thermo-physical properties are correlated with these structural modifications in the phosphate structural units and consequently changes in the average ionic character of phosphate glass network.     

Auflösungsgrenze des elektrostatischen Energie-Analysators und hochauflösende Elektronen-Spektrometrie

Using an electrostatic analyzer-lens as a monochromator the energy-widthΔE of the thermionically emitted electrons was significantly reduced to 10 meV at 10 keV primary energy. According to theoretical conclusions there results a dependence ofΔE with primary energyE asE ^3/2. The arrangement was used for investigations of energy-loss-spectra of solids (Al and Ag) and gases (Ar).     

1-D and 2-D NMR of broad-line solids: an application to quasicrystals

Nuclear magnetic resonance (NMR) techniques for measuring one-dimensional absorption spectra and two-dimensional exchange spectra of solids with extremely inhomogeneously broadened lines are discussed. Among various “broad-line” solids, quasicrystals represent alloys of metallic elements, the structures of which include “forbidden” symmetry elements. NMR absorption lines of quasicrystals exhibit a strong electric-quadrupole-induced inhomogeneous broadening that originates from the lack of translational periodicity of the otherwise perfectly long-range-ordered quasiperiodic lattice. Recording an NMR spectrum of a quasicrystalline sample requires a magnetic field-sweep technique. The two-dimensional exchange experiment on quasicrystals can be performed on selectively excited portions of the NMR spectrum only. Due to the off-resonance effects in a selective excitation, the use of a simple three-pulse stimulated-echo exchange sequence is preferred. The²⁷Al spectra of the Al-Pd-Mn and Al-Pd-Re families show interesting features like temperature-dependent frequency shifts and exchange effects due to atomic motion.     

Al Multiple-Quantum MAS NMR of Mechanically Treated Bayerite (α-Al(OH)3) and Silica Mixtures

Two-dimensional ²⁷Al multiple-quantum magic angle spinning (MQMAS) NMR experiments are used to study mixtures of bayerite (α-Al(OH)₃) with either silicic acid (SiO₂.nH₂O) or silica gel (SiO₂) that have been ground together for varying lengths of time. This mechanical treatment produces changes in the ²⁷Al MAS and MQMAS NMR spectra that correspond to the formation of new Al species. Mean values of the quadrupolar interaction (P_Q) and isotropic chemical shift (δ_CS) are extracted from the two-dimensional ²⁷Al NMR spectra for each of these species. The presence of significant distributions of both ²⁷Al quadrupolar and chemical shift parameters is demonstrated and the effect of grinding duration on the magnitudes of these distributions is discussed.