3QMAS NMR Research Articles

Investigation of the structural and electrical conductivity properties in pure and cation exchanged rectorite

Clay minerals, as biofriendly and low-cost materials, are highly essential for the modern industrial applications including the production of clean energy, its storage and conversion. Here, the capabilities of pure and cation exchanged rectorite, a regularly interstratified phyllosilicate from Beatrix Mine (South Africa) were explored and discussed. A comprehensive characterization was performed by means of high resolution solid-state NMR, electrochemical impedance spectroscopy and powder X-ray diffraction. 23Na MAS and 3QMAS NMR spectroscopy was used to characterize accessibility of interlayer space in rectorite for exchangeable cations. Three Na sites attributed to easily exchangeable Na+ on the surface or in large pores, Na+ in dehydrated micaceous interlayers and Na+ in hydrated smectite interlayers were identified. Differences in hydration states in smectitic interlayers depending on the type of intercalation were detected using 1H MAS NMR. A higher amount of hydrating water molecules in pure and Mg-exchanged rectorites was attributed to the higher hydration energy of Ca2+ and Mg2+. The temperature dependences of electrical conductivity in this work were best described using the empirical Vogel-Tammann-Fulcher equation with the temperature-dependent effective activation energy parameter. Significantly stronger change of activation energy in Mg2+ exchanged rectorite in the temperature range of 10 °C to 50 °C as compared to pure rectorite and its Na+, Li+ and NH4+ modifications was related to an extensive H-bond network in the interlayers, which facilitated an effective proton transfer responsible for electric conductivity. The obtained resutls suggested a greater potential for the use of Mg-exchanged rectorite as ionic conductor at enhanced temperatures.

Cation Ordering in Spinel from Calcium-Aluminum-Rich Inclusions in Carbonaceous Chondrites NWA 2364 and NWA 6991 to Quantify Temperature in the Early Solar System

Abstract Minerals extracted from two calcium-aluminum-rich inclusions, one each from NWA 2364 and NWA 6991 CV3 chondrite meteorites, were examined using micro X-ray diffraction, 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and Triple Quantum (3Q) MAS NMR. In situ examination by micro X-ray diffraction was used to confirm the presence of spinel (MgAl2O4) and to identify co-existing minerals. Aluminum-27 3Q MAS NMR was used to confirm the identity of co-existing minerals by their NMR signature in this two-dimensional experiment and to ensure that their NMR peaks did not overlap with those attributed to spinel. Aluminum-27 MAS NMR was used to quantitatively measure cation ordering between the tetrahedral ([4]Al) and octahedral ([6]Al) sites in the spinel. The measured cation distribution was used to calculate the inversion parameter, x, for each sample: x = 0.077 ± 0.007 for NWA 2364, x = 0.027 ± 0.001 for NWA 6991 (gehlenite-rich fraction), and x = 0.052 ± 0.003 for NWA 6991 (Al-bearing diopside-rich fraction). The measured cation-ordering data were input into six literature calibration curves to estimate the temperature of formation or the most recent equilibration temperature of the spinel. The NWA 2364 sample yielded temperatures between 420 and 707 K and the NWA 6991 sample yielded temperatures between 153 and 615 K, depending on the calibration curve used. These temperatures are lower than expected for nebular condensation temperatures, however, reordering may have occurred during cooling from high temperatures, so these values may be taken to represent temperature minima. The calculated spinel-related temperatures may thus represent equilibration temperatures related to subsequent nebular or CV chondrite parent body processes.

Overcoming challenges in 67Zn NMR: a new strategy of signal enhancement for MOF characterization.

67 Zn solid-state NMR suffers from low sensitivity, limiting its ability to probe the Zn2+ surroundings in MOFs. We report a breakthrough in overcoming challenges in 67Zn NMR. Combining new cryogenic MAS probe technology and performing NMR experiments at a high magnetic field results in remarkable signal enhancement, yielding enhanced information for MOF characterization.

Probing the transformation paths from aluminum (oxy)hydroxides (boehmite, bayerite, and gibbsite) to metastable alumina: A view from high-resolution 27Al MAS NMR

Abstract Detailed knowledge of the structural evolution of diverse aluminum (oxy)hydroxides with varying temperatures up to ~300 °C provides insights into the dehydration processes involving transitions among metastable phases on Earth's surface and in the crust. Here, we report the high-resolution solid-state 27Al NMR spectra for three different types of aluminum (oxy)hydroxides (i.e., boehmite, bayerite, and gibbsite) with varying annealing temperatures up to 300 °C, revealing the effect of distinct precursor minerals on the stability of metastable alumina. 27Al MAS NMR results allow us to obtain the quantitative fractions and NMR parameters for each phase during transformation. The results demonstrate that each aluminum (oxy)hydroxide phase follows a unique transformation path to metastable alumina. The 27Al MAS and 3QMAS NMR spectra of boehmite show that a minor but observable [4]Al signal (~2%) is detected at ~50 °C, and the [4]Al fraction gradually increases up to 300 °C (~16%), indicating that the phase transformation from boehmite to γ-/η-Al2O3 occurs at a temperature as low as ~50 °C, significantly lower than earlier estimations based on XRD. Together with the [4]Al fraction, the [5]Al fraction increases from <1% at 50 °C to ~2.3% at 300 °C, whereas the NMR results of bayerite and gibbsite do not show the presence of [5]Al. In addition, the 27Al 3QMAS NMR spectra resolved the [6]Al site in boehmite and that in γ-/η-Al2O3, which could not be uniquely determined from 1D NMR spectra. The population of bayerite abruptly decreases from 100% (at 150 °C), through ~47% (at 200 °C), to 0% (at 250 °C), indicating that the phase transition from bayerite to boehmite + γ-/η-Al2O3 occurs within a narrow temperature range. As for gibbsite, while [4]Al is not observed in the spectra up to 200 °C, the [4]Al fraction of ~2% is observed in the spectra for gibbsite annealed at 250 °C, and the [4]Al fraction increases rapidly to ~15% as the annealing temperature increases to 300 °C, suggesting that the phase transformation into γ-/η-Al2O3 occurs at ~250 °C. The results confirm that the phase transformation paths (gradual vs. dramatic) depend on the type of precursor minerals. Particularly, the onset temperature of the phase transformation from boehmite to metastable alumina (~50 °C) is lower than those from other precursor minerals (>150 °C). Furthermore, the phase transformation from boehmite to γ-/η-Al2O3 occurs gradually within broad temperature ranges from ~50 °C. This is due to their configurational disorder as evidenced by the presence of [5]Al. The observed structural evolution in aluminum (oxy)hydroxides in the low-temperature range facilitates our understanding of the nature of phase transformation and dehydration of oxides and hydroxides in the Earth's surface environments.

Effect of composition on structural evolution and NMR parameters of quadrupolar nuclides in sodium borate and aluminoborosilicate glasses: a view from high-resolution 11B, 27Al, and 17O solid-state NMR

In this study, using 11B, 27Al, and 17O 3QMAS NMR spectroscopy, we investigated the compositional effect on the structures (particularly, topological evolution of boron species and their spatial proximity) and the NMR parameters of B, Al, and O configurations in binary sodium borate and sodium aluminoborosilicate glasses in NaBSiO4–NaAlSiO4 and NaBSi3O8–NaAlSi3O8 joins. Noble structural findings of the current study include the two distinct [4]B species in sodium borate glasses with low alkali content and the presence of Na–O–B peak in NaAl0.25B0.75SiO4 glasses. The proximity between [3]B and [4]B in NaAlSiO4-NaBSiO4 glasses was revealed via 11B double quantum MAS NMR. The changes in quadrupolar coupling constants and isotropic chemical shift for diverse coordination B and Al environments and oxygen clusters provide improved insights in the effect of composition on the changes in the 2nd nearest neighbors environments and the extent of topological distortion.

Vanadium incorporation from aqueous NH4VO3 solution into siliceous Beta zeolite determined by NMR with formation of V-single site zeolite catalysts for application in SCR of NO

The speciation of vanadium in initial aqueous NH4VO3 solutions as a function of pH and concentration, in supernatant and in wet solids were investigated by 51V static and MAS NMR. Two series of VSiBeta zeolite catalysts were prepared by a two-step postsynthesis procedure at pH = 2.5 and 6. 51V static and MAS NMR, 51V 3Q MAS NMR, DR UV–vis, XPS and EPR allowed determining the state of vanadium in both series of catalysts. The catalytic activity of VSiBeta catalysts in SCR of NO strongly depended on the state of the vanadium present. The V-single site V1.0SiBeta(I) and V1.4SiBeta(I) catalysts with isolated pseudo-tetrahedral V(V) were active in SCR of NO with NH3 process, with maximum NO conversion about 75 % at 773 K for V1.4SiBeta(I). In contrast, V1.0SiBeta(II) and V7.5SiBeta(I) catalysts containing pseudo-octahedral V(V) were much less active in SCR of NO and high amount of undesired N2O was produced.

Evidence of phase coexistence in hydrothermally synthesized K0.5Na0.5NbO3 nanofibers

The 2D 23Na 3QMAS NMR analyses provide new insights into the crystalline structure of K0.5Na0.5NbO3 nanostructures on the microscopic scale, which are critical to understand the functional properties of these nano-materials.

The degree of polymerization and structural disorder in (Mg,Fe)SiO3 glasses and melts: Insights from high-resolution 29Si and 17O solid-state NMR

Probing the degree of polymerization and structural disorder of (Mg,Fe)SiO3 glasses and melts remains a challenging problem, despite their strong implications for the properties of natural volcanic glasses and dense melts in Earth’s surface and interior. Here, we explore the iron-induced changes in silicon and the oxygen configurations of (Mg1−XFeX)SiO3 glasses (Fe3+/ƩFe = 0.22 ± 0.07) with varying XFe [=Fe/(Mg + Fe), containing up to 13.5 wt% FeO] using high-resolution 29Si and 17O MAS, and 2D 17O 3QMAS NMR, unveiling the effect of Fe2+ on the extent of disorder in these glasses. The 29Si MAS NMR spectra show the apparent peak broadening in less polymerized species (i.e., Q1) and an increase in the fraction of highly polymerized (i.e., Q3) species with increasing XFe. These changes imply the selective decrease in NMR signal intensity of Q1 and Q2 due to their spatial proximity to Fe2+ (i.e., paramagnetic effect), and slight increase in the degree of polymerization due to the presence of minor fraction of Fe3+. The 2D 17O NMR spectra revealed that the fraction of Si-O-Si apparently increases with XFe and more significant changes in peak width for Mg-O-Si are observed with varying XFe. The results indicate that Mg-O-Si is more strongly affected by Fe2+ than Si-O-Si, suggesting a moderate degree of preferential proximity between Fe2+ and non-bridging oxygen. The results allow us to conceptualize the scheme to account for paramagnetic interaction between the nuclear spins and Fe2+. The scheme provides a systematic protocol to interpret the NMR spectra of diverse iron-bearing silicate glasses. The current NMR results, in conjunction with the results in the previous study for Fe3+-dominant sodium silicate glasses, suggest that paramagnetic effect depends on the valence state of Fe, indicating the presence of a moderate degree of preferential interaction between Fe2+ and NBO and less polymerized Si units as well as that between Fe3+ and highly polymerized Si units and BO. Though qualitative, the current NMR results for (Mg,Fe)SiO3 glasses shed light on an opportunity to explore the detailed structures of iron-bearing natural volcanic glasses and can improve understanding of the transport properties of the basaltic melts.

Al Organization in the SSZ-13 Zeolite. Al Distribution and Extraframework Sites of Divalent Cations

SSZ-13 is a Si-rich (Si/Al > 5) small pore zeolite (chabazite topology) important for both acid and redox catalysis. Using a sample with Si/Al = 12, a new procedure involving 27Al (3Q) MAS NMR spectroscopy and extensive periodic DFT calculations with molecular dynamics, in addition to the standard methods based on bare Co(II) cations as probes monitored by FTIR spectroscopy and UV–vis spectroscopy, was employed. The placement of the Al–O–(Si–O)2–Al and Al–O–(Si–O)3–Al sequences in the zeolite framework was determined (Al–O–Si–O–Al sequences are absent). 54% of the framework Al atoms correspond to Al–O–(Si–O)3–Al sequences which cannot form cationic sites for bare divalent cations but are able to accommodate divalent Co(II) hexaaqua complexes. The corresponding Al–O–(Si–O)3–Al sequence is located in two double 6-ring cages with one Al located in the 4-ring connecting two double 6-ring units. Our study also reveals that 35% of the framework Al atoms can accommodate neither divalent Co(II) hexaaqua complexes...

Synthesis and characterization of bundle-shaped ZSM-22 zeolite via the oriented fusion of nanorods and its enhanced isomerization performance

ZSM-22 zeolites with needle-shaped, bundle-shaped, as well as bulk-shaped morphologies are prepared by tuning hydrothermal synthesis conditions. These as-synthesized zeolites have similar crystallinity, Si/Al ratios and micropores revealed by XRD, XRF and BET. However, the oriented fusion of nanorods in the bundle-shaped ZSM-22 zeolite leads to the formation of more framework aluminum atoms over T1 + T2 + T3 sites on the 10-membered rings than that of needle-shaped ZSM-22 zeolite as detected by the 27Al 2D 3Q MAS NMR. The acidity properties determined by Py-FTIR and DMPy-FTIR show that the interfacial effect between the adjacent nanorods can effectively suppress Brönsted acid strength at the pore mouths and external surface. Interestingly, more Lewis acid sites are generated due to the interfacial defects. As a result, the Pt/Z-BUN catalyst with the weaker acidity and high dispersion of Pt exhibits the highest isomerization selectivity due to the more balanced bifunctionality between acid sites and metal sites among the as-prepared catalysts.

Effect of pressure on the short-range structure and speciation of carbon in alkali silicate and aluminosilicate glasses and melts at high pressure up to 8 GPa: 13C, 27Al, 17O and 29Si solid-state NMR study

Despite the pioneering efforts to explore the nature of carbon in carbon-bearing silicate melts under compression, experimental data for the speciation and the solubility of carbon in silicate melts above 4 GPa have not been reported. Here, we explore the speciation of carbon and pressure-induced changes in network structures of carbon-bearing silicate (Na2O-3SiO2, NS3) and sodium aluminosilicate (NaAlSi3O8, albite) glasses quenched from melts at high pressure up to 8 GPa using multi-nuclear solid-state NMR. The 27Al triple quantum (3Q) MAS NMR spectra for carbon-bearing albite melts revealed the pressure-induced increase in the topological disorder around 4 coordinated Al ([4]Al) without forming [5,6]Al. These structural changes are similar to those in volatile-free albite melts at high pressure, indicating that the addition of CO2 in silicate melts may not induce any additional increase in the topological disorder around Al at high pressure. 13C MAS NMR spectra for carbon-bearing albite melts show multiple carbonate species, including [4]Si(CO3)[4]Si, [4]Si(CO3)[4]Al, [4]Al(CO3)[4]Al, and free CO32–. The fraction of [4]Si(CO3)[4]Al increases with increasing pressure, while those of other bridging carbonate species decrease, indicating that the addition of CO2 may enhance mixing of Si and Al at high pressure. A noticeable change is not observed for 29Si NMR spectra for the carbon-bearing albite glasses with varying pressure at 1.5–6 GPa. These NMR results confirm that the densification mechanisms established for fluid-free, polymerized aluminosilicate melts can be applied to the carbon-bearing albite melts at high pressure.In contrast, the 29Si MAS NMR spectra for partially depolymerized, carbon-bearing NS3 glasses show that the fraction of [5,6]Si increases with increasing pressure at the expense of Q3 species ([4]Si species with one non-bridging oxygen as the nearest neighbor). The pressure-induced increase in topological disorder around Si is evident from an increase in peak width of [4]Si with pressure. 17O NMR spectrum shows that the fraction of Na⋯O[5]Si in carbon-bearing NS3 glasses is less than that of carbon-free NS3 glasses at 6 GPa potentially due to the formation of bridging carbonate species. While its presence is not evident from the 17O NMR spectrum primarily due to low carbon concentration, 13C MAS NMR results imply the formation of bridging carbonates, [4]Si(CO3)[4]Si, above 6 GPa. The spin-lattice relaxation time (T1) of CO2 in albite melts increases with increasing pressure from 42 s (at 1.5 GPa) to 149 s (at 6 GPa). Taking the pressure-induced change in T1 of carbon species into consideration, total carbon content in carbon-bearing albite melts increases with pressure from ∼1 wt% at 1.5 GPa to ∼4.1 wt% at 6 GPa. The results also reveal a noticeable drop in the peak intensity of free carbonates in carbon-bearing NS3 melts at 6 GPa, implying a potential non-linear change in the carbon solubility with pressure. The current results of carbon speciation in the silicate melts above 4 GPa provide an improved link among the atomic configurations around carbon species, their carbon contents, and isotope composition of carbon-bearing melts in the upper mantle.

Influence of Levulinic Acid Hydrogenation on Aluminum Coordination in Zeolite-Supported Ruthenium Catalysts: A 27 Al 3QMAS Nuclear Magnetic Resonance Study.

The influence of a highly oxygenated, polar protic reaction medium, that is, levulinic acid in 2‐ethylhexanoic acid, on the dealumination of two zeolite‐supported ruthenium catalysts, namely Ru/H‐β and Ru/H‐ZSM‐5, has been investigated by 27Al triple‐quantum magic‐angle spinning nuclear magnetic resonance spectroscopy (3QMAS NMR). Upon use of these catalysts in the hydrogenation of levulinic acid, the heterogeneity in aluminum speciation is found to increase for both Ru/H‐ZSM‐5 and Ru/H‐β. For Ru/H‐ZSM‐5, the symmetric, tetrahedral framework aluminum species (FAL) were found to be mainly converted into distorted tetrahedral FAL species, with limited loss of aluminum to the solution by leaching. A severe loss of both FAL and extra‐framework aluminum (EFAL) species into the liquid phase was observed for Ru/H‐β instead. The large decrease in tetrahedral FAL species, in particular, results in a significant decrease in strong acid sites, as corroborated by Fourier transform infrared spectroscopy (FT‐IR). This decrease in acidity, evidence of the inferior stability of the strongly acidic sites in Ru/H‐β relative to Ru/H‐ZSM‐5 under the applied conditions, is considered as the main reason for differences seen in catalyst performance.

Na+/K+ ion exchange in silicate glasses: Results from 17O 3QMAS NMR

The effect of Na+/K+ ion exchange on the distribution of the modifier ions and on their interaction with the oxygen atoms in the network is investigated in ternary Na,Mg- and Na,Ca- silicate glasses using 17O triple-quantum magic-angle-spinning nuclear magnetic resonance spectroscopy. The results indicate that the K+ ions preferentially exchange with the Na+ ions present in {Na,Ca}-NBO and {Na,Mg}-NBO environments, while the Na-NBO environment is not significantly affected. Remelting of the ion-exchanged glasses leads to a homogenization of the modifier ion distribution around the NBO sites. The dynamical site preference for the Na+/K+ ion exchange has far-reaching implications in our understanding of the alkali ion transport and more specifically of the mixed-alkali effect in silicate glasses.

ECNU-10 zeolite: A three-dimensional MWW-Type analogue

ECNU-10, an analogue of MCM-49 with the 3-dimensional (3D) MWW topology, was synthesized using 1-adamantanamine as structure directing agent (SDA), which is the second example of 3D MWW zeolite obtained by direct hydrothermal synthesis. The synthesis condition was investigated and proton-type ECNU-10 showed similar Al distribution with conventional MCM-22 obtained by the interlayer condensation of a layer precursor, as revealed by 27Al 3Q MAS NMR analysis. The catalytic performance of ECNU-10 was evaluated in the alkylation and disproportionation reactions in comparison to another two MWW-type aluminosilicates of MCM-22 and SSZ-25.

Crystal structures of two oxygen-deficient perovskite phases in the CaSiO3–CaAlO2.5 join

The crystal structures of Ca(Al0.5Si0.5)O2.75 and Ca(Al0.4Si0.6)O2.8 ordered oxygen-deficient perovskite phases are synthesized at 7 and 11 GPa, respectively, and 1500 °C, and were studied using NMR and synchrotron powder X-ray diffraction. 29Si MAS NMR, 27Al MAS and 3Q MAS NMR measurements revealed a single tetrahedral Si and single octahedral Al peak for the Ca(Al0.5Si0.5)O2.75 phase, and a tetrahedral and an octahedral Si peak and a single octahedral Al peak for the Ca(Al0.4Si0.6)O2.8 phase. Using this structural information as constraints, the crystal structures were solved from synchrotron X-ray diffraction data by the structure determination from powder diffraction (SDPD) technique. To double-check the structures, first-principles calculations of NMR parameters (chemical shifts and electric field gradients) were also conducted after relaxing the obtained structures. The calculated NMR parameters of both phases are consistent with the observed NMR spectra. The crystal structures of both phases consist of a perovskite-like layer of (Al,Si)O6 octahedra and a double-layer of SiO4 tetrahedra that are stacked alternatively in the [111] direction of ideal cubic perovskite. The perovskite-like layer is made of a double-layer of Al octahedra for the Ca(Al0.5Si0.5)O2.75 phase, and a triple-layer with a Si octahedral layer sandwiched between two Al octahedral layers for the Ca(Al0.4Si0.6)O2.8 phase. A unique feature common to both structures is that each SiO4 tetrahedron has one terminal oxygen that is not shared by other Si or Al. Homologous relation among these phases and merwinite (Ca3MgSi2O8) in terms of different numbers (1–3) of octahedral layers within the perovskite-like layer is noted.

TNU‐9 Zeolite: Aluminum Distribution and Extra‐Framework Sites of Divalent Cations

The TNU-9 zeolite (TUN framework) is one of the most complex zeolites known. It represents a highly promising matrix for both acid and redox catalytic reactions. We present here a newly developed approach involving the use of 29 Si and 27 Al (3Q) MAS NMR spectroscopy, CoII as probes monitored by UV/Vis and FTIR spectroscopy, and extensive periodic DFT calculations, including molecular dynamics, to investigating the aluminum distribution in the TUN framework and the location of aluminum pairs and divalent cations in extra-framework cationic positions. Our study reveals that 40 and 60 % of aluminum atoms in the TNU-9 zeolite are isolated single aluminum atoms and aluminum pairs, respectively. The aluminum pairs are present in two types of six-membered rings forming the corresponding α and β (15 and 85 %, respectively, of aluminum pairs) sites of bare divalent cations. The α site is located on the TUN straight channel wall and it connects two channel intersections. The suggested near-planar β site is present at the channel intersection.

A solid state NMR study of layered double hydroxides intercalated with para-amino salicylate, a tuberculosis drug

Para-amino salicylate (PAS), a tuberculosis drug, was intercalated in three different layered double hydroxides (MgAl, ZnAl, and CaAl-LDH) and the samples were studied by multi-nuclear (1H, 13C, and 27Al) solid state NMR (SSNMR) spectroscopy in combination with powder X-ray diffraction (PXRD), elemental analysis and IR-spectroscopy to gain insight into the bulk and atomic level structure of these LDHs especially with a view to the purity of the LDH-PAS materials and the concentration of impurities. The intercalations of PAS in MgAl-, ZnAl-, and CaAl-LDH's were confirmed by 13C SSNMR and PXRD. Moreover, 13C MAS NMR and infrared spectroscopy show that PAS did not decompose during synthesis. Large amounts (20–41%) of amorphous aluminum impurities were detected in the structure using 27Al single pulse and 3QMAS NMR spectra, which in combination with 1H single and double quantum experiments also showed that the M(II):Al ratio was higher than predicted from the bulk metal composition of MgAl-PAS and ZnAl-PAS. Moreover, the first high-resolution 1H SSNMR spectra of a CaAl LDH is reported and assigned using 1H single and double quantum experiments in combination with 27Al{1H} HETCOR.

Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds.

This work determines the potential of solid-state NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na7HNb6O19·15H2O (1), K8Nb6O19·16H2O (2), and Na8Ta6O19·24.5H2O (3) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). (1)H MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated clusters AxH8-xM6O19·nH2O (A = alkali ion; M = Nb, Ta). (93)Nb MAS NMR studies at different fields and MAS rotation frequencies have been performed on 1. For the first time, the contributions of NbO5Oμ2H sites were clearly distinguished from those assigned to NbO6 sites in the hexaniobate cluster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), (93)Nb WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample 1. The (93)Nb NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO5Oμ2H sites). 1D (23)Na MAS and 2D (23)Na 3QMAS NMR studies of 1 revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The (23)Na MAS NMR study of 3 confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in 1 was also probed by multinuclear MAS NMR ((1)H, (23)Na, and (93)Nb).

(11)B MAS NMR and First-Principles Study of the [OBO3] Pyramids in Borates.

Borates are built from the [Bϕ3] planar triangles and the [Bϕ4] tetrahedral groups, where ϕ denotes O or OH. However, the [Bϕ4] groups in some borates are highly distorted to include three normal B-O bonds and one anomalously long B-O bond and, therefore, are best described as the [OBO3] pyramids. Four synthetic borates of the boracite-type structures (Mg3B7O13Br, Cu3B7O13Br, Zn3B7O13Cl, and Mg3B7O13Cl) containing a range of [OBO3] pyramids were investigated by multifield (7.05, 14.1, and 21.1 T) (11)B magic-angle spinning nuclear magnetic resonance (MAS NMR), triple quantum (3Q) MAS NMR experiments, as well as density functional theory calculations. The high-resolution (11)B MAS NMR spectra supported by theoretical predictions show that the [OBO3] pyramids are characterized by isotropic chemical shifts δiso((11)B) from 1.4(1) to 4.9(1) ppm and nuclear quadrupole parameters CQ((11)B) up to 1.3(1) MHz, both significantly different from those of the [BO4] and [BO3] groups in borates. These δiso((11)B) and CQ((11)B) values indicate that the [OBO3] pyramids represent an intermediate state between the [BO4] tetrahedra and [BO3] triangles and demonstrate that the (11)B NMR parameters of four-coordinate boron oxyanions are sensitive to local structural environments. The orientation of the calculated unique electronic field gradient tensor element Vzz of the [OBO3] pyramids is aligned approximately along the direction of the anomalously long B-O bond, corresponding to B-2pz with the lowest electron density.

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

(27) Al 3Q MAS NMR and UV/Vis spectroscopy with bare Co(II) ions as probes of Al pairs in the zeolite framework were employed to analyze the location of framework Al atoms in the channel system of zeolite ZSM-5. Furthermore, the effect of Na(+) ions together with tetrapropylammonium cation (TPA(+)) in the ZSM-5 synthesis gel on the location of Al in the channel system was investigated. Zeolites prepared using exclusively TPA(+) as a structure-directing agent (i.e., in the absence of Na(+) ions) led to 55-90% of Al atoms located at the channel intersection, regardless the presence or absence of Al pairs [Al-O-(Si-O)2 -Al sequences in one ring] in the zeolite framework. The presence of Na(+) ions in the synthesis gel did not modify the Al location at the channel intersection (55-95% of Al atoms) and led only to changes in i) the distribution of framework Al atoms between Al pairs (decrease) and single isolated Al atoms (increase), and ii) the siting of Al in distinguishable framework tetrahedral sites.