MQMAS NMR Research Articles

Multiple Ca 2+ environments in silicate glasses by high-resolution 43Ca MQMAS NMR technique at high and ultra-high (21.8 T) magnetic fields

We here show the 43Ca 5QMAS NMR spectra at high field (16.4 T) and the first 7QMAS spectrum at ultra-high field (21.8 T) for geologically important Ca-containing glasses. The high-resolution MQMAS spectra present a clear evidence of multiple Ca sites in the amorphous structures that have never been identified by other analytical methods. The present study suggests that the Ca 2+ ions are mainly in 7- and 8-fold coordination sites. This will offer valuable insights for dynamic properties of magmatic liquids. The MQMAS NMR technique at high magnetic field is a unique tool to understand the detailed structural information on a specific element in solids including organic and inorganic compounds.

Local Al site distribution in aluminosilicate glasses by 27Al MQMAS NMR

Coordination and local environment around Al in glasses and liquids is a long-standing question, which has been a controversial issue in geochemistry and glass science. Nuclear magnetic resonance at high-field (750 MHz) was employed to investigate the geochemically and industrially important CaO–Al 2O 3–SiO 2 system. We show that Al remains mainly in tetrahedral position in glasses, however, the presence of five-fold coordinated aluminum is the general rule throughout the ternary CaO–SiO 2–Al 2O 3 system, except for the low silica percalcic region. The proportion of five-fold (Al V) and six-fold (Al VI) coordinated Al was quantified to determine the effects of composition overall the peralkaline part. Moreover our results indicate that Al may occupy more polymerized positions than Si. These two findings do put new questions to the modeling/understanding of these glasses and their parent liquids.

Modification of H-ZSM-5 zeolites with phosphorus. 1. Identification of aluminum species by 27Al solid-state NMR and characterization of their catalytic properties

Aluminum species in P-ZSM-5 zeolites were identified and quantified using 27Al MAS and MQMAS NMR methods. Samples containing between 0% and 15% P 2O 5 were studied after calcination or calcination followed by steaming. In addition to the tetrahedral framework aluminum, Al tet-f, and the octahedral aluminum, Al oct, we also observed significant quantities of highly distorted aluminum with tetrahedral coordination, Al tet-dis, and octahedrally coordinated aluminum in aluminum phosphate, Al oct-O-P. Furthermore, small quantities of the pentahedrally coordinated species Al pent-O-P were seen in samples with high phosphorus content. There was a good correlation between the amount of framework aluminum and acidity measured by TPD of n-propylamine. In contrast, the aluminum phosphate was shown to be catalytically inactive both by acidity measurements and catalytic tests.

Effects of Adsorbate Molecules on the Quadrupolar Interaction of Framework Aluminum Atoms in Dehydrated Zeolite H,Na-Y

The effect of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms with the electric field gradient in dehydrated zeolite H,Na-Y has been studied by (27)Al MAS NMR and (27)Al MQMAS NMR spectroscopy at magnetic fields of 9.4 and 17.6 T. Upon adsorption of molecules interacting with bridging OH groups by hydrogen bonds (acetonitrile and acetone), the quadrupole coupling constant of framework aluminum atoms was found to decrease from 16.0 MHz (unloaded zeolite) to 9.4 MHz. Adsorption of molecules, which cause a proton transfer from the zeolite framework to the adsorbates (ammonia and pyridine), reduces the quadrupole coupling constant to 3.8 MHz for coverages of 0.5-2 molecules per bridging OH group. The experiments indicate that the quadrupole coupling constant of framework aluminum atoms in dehydrated zeolite H,Na-Y reflects the chemical state of adsorbate complexes formed at bridging OH groups. In agreement with earlier investigations it was found that a proton affinity of the adsorbate molecules of PA = 812-854 kJ/mol is necessary to induce a proton transfer from the zeolite framework to the adsorbed compounds. This proton transfer is accompanied by a strong improvement of the tetrahedral symmetry of zeolitic framework AlO(4) tetrahedra and a decrease of the electric field gradient.

27Al NMR experiments and quadrupolar parameter ab initio calculations: Crystallographic structure refinement of β-Ba 3AlF 9

27Al quadrupolar parameters are determined for the three aluminium sites in β-Ba 3AlF 9 from the reconstruction of experimental SATRAS and MQ-MAS NMR spectra. These quadrupolar parameters are then computed using the DFT linearized augmented plane wave method implemented in the WIEN2k code. A fine agreement between experimental and calculated values is obtained only after optimizing the β-Ba 3AlF 9 structure. The optimized structure is more satisfactory than the previously published one from a crystallographic point of view (bond lengths and angles). This approach which allows to refine crystallographic structures especially when they lack accuracy may be extendable to other compounds containing quadrupolar nuclei.

Reaction of layered sodium disilicate SKS-6 with calcium chloride solution as revealed by solid state NMR spectroscopy: Exploring the calcium ion extracting mechanism of SKS-6

Solid-state 1H, 29Si NMR and 23Na MQMAS NMR spectroscopy combined with X-ray powder diffraction (XRD) technique was used to study the reaction between layered sodium disilicate SKS-6 and calcium chloride solution with different concentrations. Based on the experimental results the calcium ion extracting mechanism of SKS-6 was demonstrated to be via ion replacement, during which calcium ions replace sodium ions and combine with the non-bridging oxygen in SKS-6 for charge compensation, and simultaneously sodium ions leave the interlayer. This ion-replacement process was greatly influenced by the substitution of sodium ions by protons as well as the calcium ions concentration. Increasing calcium ions concentration favors the replacement of sodium ions by calcium ions, but prevents sodium ions from being substituted by protons.

Influence of glass composition and alteration solution on leached silicate glass structure: A solid-state NMR investigation

A multinuclear solid-state NMR investigation of the structure of the amorphous alteration products (so called gels) that form during the aqueous alteration of silicate glasses is reported. The studied glass compositions are of increasing complexity, with addition of aluminum, calcium, and zirconium to a sodium borosilicate glass. Two series of gels were obtained, in acidic and in basic solutions, and were analyzed using 1H, 29Si, and 27Al MAS NMR spectroscopy. Advanced NMR techniques have been employed such as 1H– 29Si and 1H– 27Al cross-polarization (CP) MAS NMR, 1H double quantum (DQ) MAS NMR and 27Al multiple quantum (MQ) MAS NMR. Under acidic conditions, 29Si CP MAS NMR data show that the repolymerized silicate networks have similar configuration. Zirconium as a second nearest neighbor increases the 29Si isotropic chemical shift. The gel porosity is influenced by the pristine glass composition, modifying the silicon–proton interactions. From 1H DQ and 1H– 29Si CP MAS NMR experiments, it was possible to discriminate between silanol groups (isolated or not) and physisorbed molecular water near Si (Q 2), Si (Q 3), and Si (Q 4) sites, as well as to gain insight into the hydrogen-bonding interaction and the mobility of the proton species. These experiments were also carried out on heated samples (180 °C) to evidence hydrogen bonds between hydroxyl groups on molecular water. Alteration in basic media resulted in a gel structure that is more dependent on the initial glass composition. 27Al MQMAS NMR data revealed an exchange of charge compensating cations of the [AlO 4] − groups during glass alteration. 1H– 27Al CP MAS NMR data provide information about the proximities of these two nuclei and two aluminum environments have been distinguished. The availability of these new structural data should provide a better understanding of the impact of glass composition on the gel structure depending on the nature of the alteration solution.

Synthesis, characterisation and catalytic performance of boron substituted SBA-15 molecular sieves

Boron substituted mesoporous SBA-15 molecular sieves with varying boron content were synthesised by hydrothermal method. The effects of incorporation of boron in the framework on the structure, crystal parameters, framework vibrations and textural properties of SBA-15 were discussed based on the results from XRD, FT-IR and nitrogen adsorption studies. The morphology changes due to boron incorporation were monitored by SEM analysis. The coordination environment of boron in the framework was revealed by 11B MAS NMR and MQMAS NMR techniques. The type of acid sites created by boron incorporation was analysed by DRIFT study of pyridine adsorbed catalysts. All the characterisation results indicate the successful incorporation of boron in the silica framework of SBA-15 in both tetra and tri coordination. Isopropylation of naphthalene with isopropyl alcohol was carried out to test the acidity of the synthesised materials. The effects of reaction temperature, feed ratio and feed space velocity on naphthalene conversion and products selectivity were discussed. The higher activity in naphthalene conversion and selectivity of di-isopropylated naphthalenes were accounted in terms of the increased acid sites due to boron incorporation in the framework.

Al coordination and speciation in calcium aluminosilicate glasses: Effects of composition determined by 27Al MQ-MAS NMR and Raman spectroscopy

The structure and properties of glasses and melts in the CaO–Al 2O 3–SiO 2 (CAS) system play an important role in earth and material sciences. Aluminum has a crucial role in this ternary system, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al Nuclear Magnetic Resonance on CAS glasses obtained by classic and rapid quenching methods. We propose an Al/Si tetrahedral distribution in the glass network in different Q n species. In this system, we show that Al and Si are mainly in Q 4 species along the join R = CaO/Al 2O 3 = 1, and in depolymerized Q 2 and Q 3 units at high CaO content for other joins ( R = 1.57 and 3). Five- ( [5]Al) and six-fold ( [6]Al) coordinated aluminum can be detected in the peraluminous glasses ( R < 1) in agreement with the deficit of charge compensators, Ca 2+ions, near Al atoms. Unexpectedly, between 5% and 8% of [5]Al is also observed for percalcic glasses ( R > 1), except for glasses with low silica and high CaO content. The presence of [5]Al is related to viscous flow mechanisms while, in highly depolymerized glasses, the absence of [5]Al may indicate different mechanisms for melts to flow. This systematic study on the CAS system modifies the simple picture of aluminosilicate glasses, and the existence of [5]Al should be included by geochemists, geophysicists and glass scientists to model appropriately the physical properties of aluminosilicate glasses and melts.

Thermal stability and siting of aluminum in isostructural ZSM-22 and Theta-1 zeolites

Conventional 29Si and 27Al MAS NMR techniques and 27Al two-dimensional Multiple Quantum Magic Angle Spinning (2D MQMAS) NMR technique were used to characterize two isostructural TON type zeolites (Theta-1, ZSM-22). Nonuniform distribution of framework aluminum and the preferential population of T3 and T4 sites by the aluminum atoms in the TON material were established. The aluminum sites of Theta-1 sample showed higher thermal stability compared to ZSM-22 sample, despite of the higher aluminum content and higher calcination temperature. It was shown that 27Al 2D MQMAS NMR technique can be used for monitoring the minute differences in the surrounding of aluminum T sites, caused by the different templates and the post-synthesis treatments.

Synthesis, Crystal Structure, and Solid‐State NMR Spectroscopy of a Salt‐Inclusion Stannosilicate: [Na3F][SnSi3O9

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A combined [formula omitted] RAPT and MQ-MAS NMR study of l-leucine

We report the application of rotor-assisted population transfer (RAPT) to measure the quadrupolar coupling constant ( C q ) for spin 5 2 nuclei. Results from numerical simulations are presented on the magnitude of enhancement factor as a function of frequency offsets, i.e. the RAPT profile. Experimental O 17 RAPT profile is traced for the amino acid l-leucine. In addition, results from MQ-MAS experiments are incorporated to determine the quadrupolar asymmetry parameter ( η q ) . Unlike previous reports, the O 17 NMR parameters for an amino acid, l-leucine, is reported at a relatively low field of 9.4 T.

Synthesis, Crystal Structure, and Solid-State NMR Spectroscopy of a Salt-Inclusion Stannosilicate: [Na3F][SnSi3O9

A salt-inclusion stannosilicate, [Na3F][SnSi3O9], has been synthesized using a flux-growth method and characterized by single-crystal X-ray diffraction. The structure consists of six-membered silicate rings linked via corner sharing by SnIVO6 octahedra to form a 3-D framework that delimits two types of channels. The F atoms and Na atoms are located in the structural channels and form a dimer with the anti-Al2Cl6(g) structure. This stannosilicate adopts a new structure and is the first metal silicate that contains both Na+ and F- ions in the channels. The 19F and 29Si MAS NMR and 23Na MQMAS NMR spectra are consistent with the crystallographic results.

Toward the aluminum coordination in dealuminated mordenite and amorphous silica–alumina: A high resolution 27Al MAS and MQ MAS NMR study

27Al MAS and MQ MAS NMR spectroscopy were used to study the aluminum coordination in dealuminated mordenite. A 27Al MAS NMR signal ascribed to distorted tetrahedrally coordinated Al (DTetrAl) was detected. By comparison with amorphous silica–alumina and alumina materials, in which no such signal was seen, the presence of DTetrAl species could be a typical feature in crystalline zeolites after activation and dealumination. An ammonium–water solution treatment under pressure in an autoclave was introduced; besides realumination of the activated zeolite, this treatment can transform six- and five- coordinate Al species to a tetrahedral coordination state, both in dealuminated zeolites and in amorphous silica–alumina, but not in alumina. From these experimental results we suggest that: (i) amorphous silica–alumina is a better model for the extra-framework material in dealuminated zeolites than alumina; (ii) DTetrAl species, which are absent in thermally treated amorphous silica–alumina, might be regarded as transitional from the crystalline framework lattice state to the amorphous silica–alumina state during the process of dealumination.

Multiplex MQMAS NMR of quadrupolar nuclei

A multiplex phase cycling method (N. Ivchenko et al., J. Magn. Reson. 160 (2003) 52–58) has been used to record two-dimensional MQMAS spectra with a very short phase cycling. A straightforward procedure has been developed to easily process the data. Combining this Multiplex approach and the new Soft-Pulse-Adding-Mixing (SPAM) method considerably increases the signal-to-noise ratio of the conventional MQMAS experiment. The Multiplex acquisition procedure is much simpler than the echo/anti-echo method recently proposed, and has been applied with success to record 87Rb spectra of RbNO 3 and 27Al 3Q and 5Q MQMAS NMR of microporous aluminophosphate AlPO 4 -14.

Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B0= 9.4 to 17.6 T

27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated gamma-Al2O3 and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (delta(iso) = 70 +/- 10 ppm, C(QCC) = 15.0 +/- 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 65 +/- 5 ppm, C(QCC) = 8.0 +/- 0.5 MHz). The residual two signals were attributed to aluminum cations (delta(iso) = 35 +/- 5 ppm, C(QCC) = 7.5 +/- 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 10 +/- 5 ppm, C(QCC) = 5.0 +/- 0.5 MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined.

Al environment in tectosilicate and peraluminous glasses: A 27Al MQ-MAS NMR, Raman, and XANES investigation

Tecto-aluminosilicate and peraluminous glasses have been prepared by conventional and laser heating techniques, respectively, in the CaO-Al 2O 3-SiO 2 system. The structure of these glasses were studied using Raman spectroscopy, X-ray absorption at the Al K-edge and 27Al NMR spectroscopy with two different high fields (400 and 750 MHz). Raman spectroscopy and X-ray absorption are techniques sensitive to the network polymerization and, in particular, show different signal as a function of silica content. However, these two techniques are less sensitive than NMR to describe the local aluminium environment. For tectosilicate glasses, aluminium in five-fold coordination, [5]Al, was found and a careful quantification allows the determination of a significant amount of [5]Al (7% in the anorthite glass). The proportion of [5]Al increases for the peraluminous glasses with small amounts (<2%) of six-fold coordination, [6]Al. The presence of [5]Al agrees with previous observations of the existence of nonbridging oxygens (NBOs) in tectosilicate compositions. However, the proportion of [5]Al in the present study indicates that no major proportion of triclusters (oxygen coordinated to three (Si,Al)O 4 tetrahedra) is required to explain these NBOS.

Zeolite Nanoclusters Coated onto the Mesopore Walls of SBA-15

Ultrahigh-field 27Al MAS and MQMAS NMR and 129Xe NMR were used to characterize the zeolite-coated SBA-15 materials. This study shows evidence that zeolite nanoclusters are coated on the mesopore surface of SBA-15. These techniques are useful for the detection of zeolite nanoclusters and different aluminum environments in zeolite/aluminosilicate composites, which are difficult or even impossible to detect by conventional techniques.

Probing through bond connectivities with MQMAS NMR

Probing through bond connectivities with MQMAS NMR

Local Order of Amorphous Zeolite Precursors from 29Si{H} CPMAS and 27Al and 23Na MQMAS NMR and Evidence for the Nature of Medium-Range Order from Neutron Diffraction

Amorphous aluminum silicates, isolated from hydrothermal reactions used to form crystalline zeolite A, have been studied using a number of techniques. NMR studies using 29Si{1H} cross-polarization (CP) MAS and 27Al and 23Na multiple-quantum (MQ) MAS methods provide information about the local atomic structure of the solids. Materials isolated at the early stages of reaction are silicon rich, and some of the aluminum is found in six-coordinate sites as in the amorphous alumina starting material. 29Si{1H} CPMAS NMR spectra show that a range of silicon environments are present in the amorphous solids. The line widths in the 27Al MQMAS NMR spectra of the amorphous materials suggest that a range of Al−O distances and/or Si−O−Al angles is present, rather than the well-defined units present in zeolite A. Although the average 23Na isotropic chemical shift suggests a coordination number of close to six for all sodium throughout the crystallization, the sodium in the amorphous precursor phase can be distinguished f...