23Na Magic-angle Spinning Research Articles

Alkali Metal Oxides, Peroxides, and Superoxides: A Multinuclear MAS NMR Study

The many roles of alkali metal−oxygen phases as catalyst components and promoters motivated a survey of the NMR properties of these compounds. Solid state 7Li, 23Na, 39K, 87Rb, and 133Cs magic angle spinning NMR spectra are reported for the following alkali metal oxides, peroxides and superoxides: Li2O, Li2O2, Na2O, Na2O2, NaO2, KO2, Rb2O2, RbO2, Cs2O2, and CsO2. Commercial samples labeled as “Cs2O” proved to be mixtures of Cs2O2 and CsO2. The superoxide (O2-) anion is paramagnetic, and this property leads to highly temperature-dependent chemical shifts for the alkali metal nuclei in all of the superoxides. All four superoxides also show phase transitions between polymorphic forms of cubic and lower symmetry, and these transitions profoundly affect the alkali metal NMR spectra. For CsO2 and RbO2, purely ionic bonding restricts the paramagnetic shift to the through-space dipolar mechanism, and large temperature-dependent shifts in the tetragonal phases vanish upon transition to the cubic phases. 13C in si...

Interstratification in Expandable Mica Produced by Cation-Exchange Treatment

AbstractA unique interstratified expandable mica was obtained by cation exchange treatments using an expandable mica synthesized from talc. The 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum of the expandable mica used as a starting material showed that it had 2 kinds of Na+: one was exchangeable and the other was not exchangeable. Half of the Na+ per unit cell of the expandable mica was replaced with Mg2+ by cation exchange treatments. The X-ray powder diffraction (XRD) analysis of the Mg2+-exchanged expandable mica, after heating at 73°C, indicated that Na+ in the interlayer sheets was exchanged with Mg2+ in every second layer and that it had an interstratified structure with a 12.5-Å layer thickness and a 9.6-Å layer thickness. The structure of the Mg2+-exchanged expandable mica was changed into a unique interstratified structure by the calcination at 600°C; one component had a stacking sequence of talc and a small amount of OH−, but the other had a different stacking sequence from talc and no structural OH—.

Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (C q) for 23Na. Off-resonance nutation experiments gave a mean C q = 1.75 ± 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, δ iso, and to provide an independent measurement of the values of C q. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean C q is essentially constant at 2.1-2.2 ± 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23Na NMR data of Kohn et al. 1989a and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H 2O, but the other conclusions of Kohn et al. 1989a and the principal features of the dissolution mechanism developed by Kohn et al. 1989a, Kohn et al. 1992, Kohn et al. 1994 are essentially unchanged.

Location of Na + and Cs + cations in CsNaY zeolites studied by 23Na and 133Cs magic-angle spinning nuclear magnetic resonance spectroscopy combined with X-ray structure analysis by Rietveld refinement

Solid-state 23Na and 133Cs nuclear magnetic resonance (NMR) spectroscopy was used in conjunction with X-ray powder diffraction (XRD) to study the location of Na + and Cs + cations in dehydrated CsNaY zeolites. Rietveld refinement of the structure of a 72% cesium-exchanged CsNaY zeolite dehydrated in vacuo at 623 K revealed sodium cations at SI and SII′ sites while cesium cations were found at SI, SI′ SII, and SIII sites. The results of the Rietveld refinement agree with the cation sitings derived from the 23Na and 133Cs magic-angle spinning (MAS) NMR spectra of this sample. Four lines identified in the 133Cs MAS NMR spectra of CsNaY zeolites were attributed to the distinct cation sites. In addition, the potential of 133Cs MAS NMR for the quantitative determination of site populations is demonstrated. 23Na and 133Cs MAS NMR is used to locate sodium and cesium cations in a series of CsNaY zeolites with different degrees of cesium exchange and dehydrated at different temperatures. With increasing Cs + content, the Na + cations at SI′/SII sites are exchanged first for Cs +. In addition, Na + cations at SI sites are replaced by Cs + cations at high cesium exchange levels. Upon increasing the calcination temperature of the 72% cesium-exchanged sample from 473 K to 773 K, further migration of Cs + and Na + to SI sites is observed. Calcination and rehydration of a 72% cesium-exchanged CsNaY zeolite leads to a partial migration of sodium cations from the sodalite cages into the supercages making them accessible to further ion exchange. Cesium exchange levels up to 90% were achieved by a three-fold exchange-calcination cycle of the 72% cesium-exchanged sample without damage to the zeolite structure.

Line shapes and widths of MAS sidebands for 27Al satellite transitions. Multinuclear MAS NMR of tugtupite Na 8Al 2Be 2Si 8O 24Cl 2

A multinuclear 9Be, 23Na, 27Al, and 29Si magic-angle spinning (MAS) NMR study has been performed for the mineral tugtupite (Na 8Al 2Be 2Si 8O 24Cl 2). The extremely well-resolved spectra allow observation of separate spinning sidebands (ssb's) from the inner ( ± 1 2 , ± 3 2 ) and outer ( ± 3 2 , ± 5 2 ) 27Al satellite transitions, and are utilized in a detailed analysis of the line shapes and widths of the individual ssb's from simulations. The line widths of the ssb's from the inner and outer 27Al satellite transitions are found to decrease systematically with increasing order of the ssb's across the spectrum. Accurate values for the 9Be, 23Na, and 27Al quadrupole coupling parameters and isotropic chemical shifts are obtained from simulations of the manifolds of ssb's from the satellite transitions. MAS NMR of the 9Be satellite transitions for tugtupite, BeO, and beryl (Al 2Be 3Si 6O 18) shows that these transitions are particularly useful for determination of 9Be quadrupole couplings because of the small 9Be quadrupole moment. The 29Si shielding anisotropy of δ σ = 48 ppm in tugtupite is the largest determined so far for a framework SiO 4 tetrahedron. Finally, the crystal structure of the tugtupite sample has been refined by single-crystal X-ray diffraction, and correlations between the multinuclear NMR parameters and structural data are reported.

Examination of the mixed-alkali effect in (Li,Na) disilicate glasses by nuclear magnetic resonance and conductivity measurements

Results from 29Si, 23Na and 7Li magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, 7Li NMR relaxation and electrical conductivity in a series of [Li (1 − x) · Na x ] 2O · 2SiO 2 (disilicate) glasses are used to investigate the mixed-alkali effect. From the 29Si NMR spectra there is relatively little change of the network with alkali composition. 23Na and 7Li NMR linewidths and shifts change continuously as a function of composition, indicating that the alkali ions are intimately and uniformly mixed rather than separated into lithium and sodium-rich domains. The activation energy from electrical conductivity shows a distinct maximum at the central composition ( x = 0.5), whereas the local activation energy for lithium motion determined from NMR shows only a smaller but monotonic increase as the lithium-content decreases.

Solid-state 23Na, 139La, 27Al and 29Si nuclear magnetic resonance spectroscopic investigations of cation location and migration in zeolites LaNaY

23Na Magic-angle spinning (MAS), double rotation (DOR) and two-dimensional nutation nuclear magnetic resonance (NMR) and static 139La NMR spectroscopy were applied to study the location and migration of sodium and lanthanum cations in faujasites. Generally, 23Na MAS NMR spectroscopy of as-exchanged and hydrated zeolites LaNaY was used for the quantitative determination of non-localized Na + in the large cavities at a 23Na NMR shift of −9 ppm and of sodium cations observed at −13 ppm. The latter originate from Na + ions located on position SII in the large cavities, on position SI in the hexagonal prisms and on positions SII′ and/or SI′ in the sodalite cages. The 23Na MAS NMR signal at about −13 ppm was found to be caused by two coonents. The component that is characterized by a quadrupolar interaction causing a field-dependent shift and a signal at v 1 = 2 v rf in the two-dimensional quadrupolar nutation spectra is attributed to Na + enclosed in the sodalite cages. The 23Na MAS NMR spectra of dehydrated lanthanum-exchanged faujasites are characterized by a low-field Gaussian line of Na + located on SI positions in the hexagonal prisms and a high-field quadrupole pattern of Na + located on positions SII and SI′. The migration of lanthanum cations from the large cavities to position SI′ in the sodalite cages was monitored by 139La NMR spectroscopy and verified by a theoretical estimation of the electric field gradient. The lanthanum migration was found to be coupled with a strain of SiOT and AlOT angles observed by 29Si and 27Al MAS NMR high-field shifts, respectively.

Structural characterisation of Na 2ZrO 3

A combination of X-ray and electron diffraction, electron microscopy and solid-state nuclear magnetic resonance (NMR) has been used to elucidate the structure and the ordering of Na 2ZrO 3. The diffraction data confirm a monoclinic crystal structure. A sample prepared by a conventional solid-state reaction of the components is shown by both X-ray diffraction and electron microscope imaging to have an extremely high concentration of planar defects associated with stacking disorder of the planes along the c-axis. The incidence of these defects is significantly reduced in a sample recrystallised from a bismuth oxide flux. NMR indicates that the local coordinations are well defined in both samples but with some sharpening of the spectra from the recrystallised sample indicative of the increase of long-range order. The 23Na magic angle spinning (MAS) NMR spectra clearly show three distinct sites with widely differing quadrupolar interaction parameters that can be related to the known site symmetries. Two distinct oxygen resonances are observed in the MAS NMR spectrum from an 17O-enriched sample while the static 91Zr NMR spectrum can be simulated with one set of interaction parameters.

23Na Magic-angle spinning nuclear magnetic resonance of central and satellite transitions in the characterization of the anhydrous, dihydrate, and mixed phases of sodium molybdate and tungstate

23Na Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of pure phases for Na 2MoO 4, Na 2MoO 4·2H 2O, Na 2WO 4, and Na 2WO 4·2H 2O have led to the determination of accurate values for the quadrupole coupling parameters and isotropic chemical shifts for all Na sites. The analysis of the spectra involves a combination of simulations of the line shapes for the central transitions and the manifold of spinning sidebands for the satellite transitions. The spectral parameters for the pure phases represent a prerequisite for a correct assignment and quantitative evaluation of 23Na MAS spectra at different magnetic field strengths observed for mixtures of the anhydrous and dihydrate phases. Such phase mixtures are observed, for example, for some commercial samples of Na 2MoO 4 or may be generated by (i) exposure of the anhydrous phases to a humid atmosphere or (ii) gently heating the dihydrates. The quadrupole coupling parameters for the two Na sites in the dihydrates are tentatively assigned to the two crystallographically distinct Na atoms in the asymmetric unit by calculations of an approximate dependency of the electric field gradient tensor on the local geometry for the Na sites.

23Na NMR chemical shifts and local Na coordination environments in silicate crystals, melts and glasses

In order to decipher information about the local coordination environments of Na in anhydrous silicates from 23Na nuclear magnetic resonance spectroscopy (NMR), we have collected 23Na magic angle spinning (MAS) NMR spectra on several sodium-bearing silicate and aluminosilicate crystals with known structures. These data, together with those from the literature, suggest that the 23Na isotropic chemical shift correlates well with both the Na coordination and the degree of polymerization (characterized by NBO/T) of the material. The presence of a dissimilar network modifier also affects the 23Na isotropic chemical shift. From these relations, we found that the average Na coordinations in sodium silicate and aluminosilicate liquids of a range of compositions at 1 bar are nearly constant at around 6–7. The average Na coordinations in glasses of similar compositions also vary little with Na content (degree of polymerization). However, limited data on ternary alkali silicate and aluminosilicate glasses seem to suggest that the introduction of another network-modifier, such as K or Cs, does cause variations in the average local Na coordination. Thus it appears that the average Na coordination environments in silicate glasses are more sensitive to the presence of other network-modifiers than to the variations in the topology of the silicate tetrahedral network. Further studies on silicate glasses containing mixed cations are necessary to confirm this conclusion.

Lead exchange into zeolite and clay minerals: A 29Si, 27Al, 23Na solid-state NMR study

Chabazite, vermiculite, montmorillonite, hectorite, and kaolinite were used to remove Pb, through ion exchange, from 0.01 M aqueous Pb(NO 3) 2 solutions. These minerals contained 27 (Nachabazite), 16, 9, 9, and 0.4 wt% of Pb, respectively, after equilibration with the solutions. Ion exchange reached equilibrium within 24 h for Na-chabazite and vermiculite, but in less than 5 min for montmorillonite and hectorite. Na-chabazite took up more Pb than natural (Ca, Na)-chabazite (7 wt% Pb), whereas no such difference was observed in different cation forms of the clay minerals. Calcite impurities, associated with the clay minerals, effectively removed Pb from the aqueous solutions by the precipitation of cerussite (PbCO 3). 29Si, 27Al, and 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR), 23Na double rotation (DOR) NMR, and 23Na variable-temperature MAS NMR were used to study the ion exchange mechanisms. In Na-chabazite, cations in all three possible sites take part in the fast chemical exchange. The chemical exchange passes from the fast exchange regime to the slow regime at −80 to −100°C. One site contains a relatively low population of exchangeable cations. The other two more shielded sites contain most of the exchangeable cation. The exchangeable cations in chabazite and vermiculite were found to be close to the SiO 4 and AlO 4 tetrahedra, while those in the other clay minerals were more distant. Two sites (or groups of sites) for exchangeable cations were observed in hectorite. Lead tended to occupy the one which corresponds to the −8 ppm peak on the 23Na MAS NMR spectrum. The behaviour of the exchangeable cations in the interlayer sites was similar in all the clay minerals studied.

Isomorphous substitution in non-linear optical KTiOPO4. Powder diffraction and magic angle spinning nuclear magnetic resonance study of (K½Na½)TiOPO4and (Rb½Na½)TiOPO4

The structures of (K½Na½)TiOPO4 and (Rb½Na½)TiOPO4 have been determined from Rietveld analysis of powder diffraction data, the former from a joint analysis of time-of-flight neutron and X-ray data, the latter from constant wavelength neutron data: Pn21a, (K½Na½)TiOPO4a= 12.75613(7)Å, b= 10.60426(6)Å, c= 6.31869(4)Å; (Rb½Na½)TiOPO4a= 12.8273(6)Å, b= 10.6052(5)Å, c= 6.3347(3)Å. 31P and 23Na magic angle spinning NMR spectra have also been obtained for the title compounds and the appropriate end members. Complete ordering of Na in the K(1) site and K or Rb in the K(2) site is demonstrated by both the diffraction and the NMR results. The second-harmonic generation properties of these compounds relative to KTiOPO4 are discussed.

Local structure in gallium‐ and germanium‐substituted silicate glasses investigated by magic angle spinning nuclear magnetic resonance

Local structural effects occurring with the substitution of gallium for aluminum and germanium for silicon in synthetic glasses with albite, jadeite, and nepheline analogue compositions are studied by 29Si, 27Al, and 23Na magic angle spinning nuclear magnetic resonance. There are systematic shifts to low field of the single broad resonance peak in each spectrum with progressive substitution by the larger cations. The effect of substitution of the next nearest neighbor tetrahedral cations (and the change in local structure) on chemical shift is calculated using model structures simulated with a computer graphics structure‐modeling program. Chemical shifts calculated for a random arrangement of tetrahedral cations are in closer agreement with observed chemical shifts than those calculated for ordered local structures complying with aluminum/gallium avoidance and having sodium coupled with aluminum/gallium cations. The observed shifts to low field with the substitution of aluminum or gallium for silicon or germanium are explainable only in part by the next nearest neighbor effect. A decrease in mean bond angle at the bridging oxygen atom, attributable to an increase in the number of small‐membered rings of TO4 tetrahedra, could account for further low‐field shifts, thus corroborating the findings of Henderson et al. (1985) from Raman spectroscopy.

The short range structure of sodium phosphate glasses I. MAS NMR studies

A series of x(Na 2O + H 2O)·(1 − x)P 2O 5 glasses have been characterized by 31P and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. High-resolution 31P NMR spectra reveal the presence of Q 2-(2 bridging oxygen/tetrahedron) and Q 3-(3 bridging oxygen/tetrahedron) tetrahedral sites in glasses with x < 0.5, and Q 2- and Q 1-tetrahedral sites in glasses with x > 0.5. Quantitative measurements of the respective NMR site populations are in excellent agreement with Van Wazer's predictions for the structures of ionic phosphates and illustrate the depolymerizing effects of residual H 2O on the glass structure. The systematic change in the Q 2 31P chemical shift is consistent with an increase in the average π-bond character of the phosphorus-nonbridging oxygen bond as x decreases.