MAS NMR Spectroscopy Research Articles

Deciphering the structural origins of high sulfur solubility in vanadium-containing borosilicate glasses

The addition of V2O5 has been long known to increase the sulfur (as SO42-) solubility in borosilicate glasses. However, the mechanism governing this effect is still unknown. Although several studies have been published in the past two decades attempting to decipher the structural origins of increasing sulfur solubility as a function of V2O5 in borosilicate glasses, most of these studies remain inconclusive. The work presented in this paper attempts to answer the question, “Why does V2O5 increase sulfur solubility in borosilicate glasses?” Accordingly, a series of melt-quenched glasses in the system [30 Na2O – 5 Al2O3 – 15 B2O3 –50 SiO2](100-x) – xV2O5, where x varies between 0 – 9 mol.%, have been characterized for their short-to-intermediate range structure and the redox chemistry of vanadium using 11B, 27Al, 51V MAS NMR, Raman, and XPS spectroscopy. The impact of V2O5 on sulfur solubility in glasses has been followed by ICP-OES. The addition of ≤ 5 mol.% V2O5 results in a linear increase in sulfur solubility in the investigated glass system. Based on the results, we hypothesize that adding vanadium to the glasses increases their network connectivity, but reduces the network rigidity by replacing stronger Si–O–Si linkages with weaker Si–O–V linkages and forming (VO3)n-single chains. These modifications to the glass structure increase the flexibility of the network, thus making it possible to accommodate SO42− in their voids/open spaces.

Effect of Silver Cations on Propene Aromatization on H-ZSM-5 Zeolite Investigated by 13C MAS NMR and FTIR Spectroscopy

Effect of Silver Cations on Propene Aromatization on H-ZSM-5 Zeolite Investigated by ¹³C MAS NMR and FTIR Spectroscopy

Tuning Al sites in Y-zeolite for selective production of ϒ-valerolactone from levulinic acid

Converting biomass derived levulinic acid (LA) to γ-valerolactone (GVL) through catalytic transfer hydrogenation, using Lewis acidic catalysts and alcohols as hydrogen donors, is an important route for biomass valorization. Zeolites containing Lewis acidic metal ions have been widely studied for catalytic transfer hydrogenation of LA. On the other hand, zeolites themselves have Al-Lewis acid sites, inherent to their aluminosilicate framework, and these sites can be generated and tuned through simple post-synthesis methods. The present study shows that properly tuned Al sites in Y zeolites can catalyze LA to GVL conversion, and the key requirement for this conversion is creating more penta-coordinated Al sites in the catalyst. In order to establish this, a series of thermally-treated (500–800 °C) and steam-treated (500 and 700 °C) Y zeolites were prepared, characterized by X-ray diffraction, N2 adsorption, temperature programmed desorption of NH3, 27Al MAS NMR spectroscopy and pyridine adsorption – FTIR spectroscopy and then tested for LA to GVL conversion. Thermal dealumination of NH4Y zeolite at 700 °C (TY700) gave the highest percentage (21%) of penta-coordinated Al and it was found to be more selective (∼94%) in forming GVL than other catalysts studied under the optimized conditions. The turnover frequency of formation GVL, (TOFGVL) from isopropyl levulinate (IPL) on TY700 was three times higher than that of TY500 (NH4Y thermally treated at 500 °C). During the reaction, acid sites of the zeolite catalyze esterification of LA with isopropyl alcohol and hence LA is readily converted to isopropyl levulinate and then Meerwein–Ponndorf–Verley (MPV) reduction of IPL leads to the formation of isopropyl 4-hydroxy pentanoate, which undergoes lactonization to form GVL. Based on the experimental results, a plausible reaction mechanism involving the penta-coordinated Al Lewis acid sites and -Al-OH group was proposed.

Chloride effect on sulfate attack in hydrated Portland-limestone cement assessed by 29Si NMR spectroscopy and thermal analysis

Structural alterations in the C-S-H phase of hydrated Portland-limestone cements, caused by magnesium sulfate attack, were quantitatively assessed using 29Si MAS NMR spectroscopy, considering the presence of sodium chloride in the corrosive medium and the limestone content in cement. Quantitative information about crystalline deterioration products (ettringite, thaumasite, gypsum, brucite) was retrieved from thermogravimetric analysis. Scanning electron microscopy combined with elemental analysis indicated their accumulation areas. Formation of cross-linked silicate chains and reduction of bridging/pairing SiO4 tetrahedra in C-S-H was noted. The presence of chlorides decreased cross-linking of SiO4 tetrahedra and reduced gypsum formation and Ca depletion from the matrix. Precipitation of brucite at the edge of the specimens possibly inhibited the promotion of the attack process. Chlorides penetrated deeper into the matrix compared to sulfates. Increasing limestone content in cement led to a higher mean chain length of cross-linked chains and AlO4/SiO4 ratio in the sulfate-bearing environment. The extent of C-S-H degradation in the cement pastes with the highest limestone content was independent of the exposure conditions. Cement with approx. 12% limestone content can be considered of equal performance to ordinary Portland cement in severe sulfate–chloride solutions.

Reaction Pathways of Methanol Reforming over Pt/α-MoC Catalysts Revealed by In Situ High-Pressure MAS NMR

Methanol reforming to hydrogen is a promising route for releasing hydrogen when it is used as a hydrogen carrier, but the reaction pathways are still in dispute. Herein in situ high-pressure MAS NMR rotors have been developed to reveal the reaction pathways of methanol reforming over Pt/α-MoC catalysts. Time-resolved in situ 1H and 13C MAS NMR spectroscopy was carried out to quantitatively monitor the evolution of H2, CO2, and reaction intermediates. Notably, for Pt/α-MoC, in addition to CO derived from direct decomposition of methanol proposed for group 10 metal catalysts, formaldehyde is also an active intermediate that readily reacts with methanol or H2O and further transforms into methyl formate or formic acid, respectively, under high-pressure and lower-temperature conditions. Like the Cu-based catalyst, CO2 and H2 were produced mainly through a formic acid mediated mechanism. Moreover, our present work has proven multinuclear high-pressure in situ MAS NMR to be a powerful tool for exploring reactions containing multiple phases (solid, liquid, and gas), and the vital insights related to the reaction mechanism.

Nanoarchitectonics of polymeric crown-ether analog of Tröger base combined with potassium iodide and acids synergy in fixation of CO2 and epoxides

Tröger's bases (TB) containing two steric nitrogen atoms, are alternated as building block for constructing molecular acceptors to form covalent bonds with acids, making TB a promising catalyst. Herein, TB analogs fabricated with crown ethers were knitted by condensation reaction and combined with potassium iodide as a brand-new catalytic system (CATB-KIs), which were characterized using FT-IR, 13C CP MAS NMR, SEM, TGA, and XPS spectroscopy respectively. The generated CATB-KIs exhibited excellent performance for CO2 fixation reaction with epoxides to produce cyclic carbonates under mild conditions, and the catalytic activity could be further improved by coordinating with acids beforehand, in which HBr performs fascinated activity. After 12 h, the corresponding cyclic carbonate from epichlorohydrin with 95.1% yield was detected at mild conditions with solvent-free in the presence of CATB-KI-2 and HBr. A series of terminal epoxides were converted into desired cyclic carbonates with desirable yields. The catalyst could be recovered successfully and reused six times without significant loss of activity. Finally, dynamics study on catalytic mechanism has been accomplished and the activated energy was calculated as 72.1 kJ/mol. A synergistic catalytic mechanism of hydrogen bonding and I anion has been proposed to account for the excellent catalytic activity of this catalysts.

How Solid Surfaces Control Stability and Interactions of Supported Cationic CuI(dppf) Complexes─A Solid-State NMR Study

Organometallic complexes are frequently deposited on solid surfaces, but little is known about how the resulting complex-solid interactions alter their properties. Here, a series of complexes of the type Cu(dppf)(Lx)+ (dppf = 1,1'-bis(diphenylphosphino)ferrocene, Lx = mono- and bidentate ligands) were synthesized, physisorbed, ion-exchanged, or covalently immobilized on solid surfaces and investigated by 31P MAS NMR spectroscopy. Complexes adsorbed on silica interacted weakly and were stable, while adsorption on acidic γ-Al2O3 resulted in slow complex decomposition. Ion exchange into mesoporous Na-[Al]SBA-15 resulted in magnetic inequivalence of 31P nuclei verified by 31P-31P RFDR and 1H-31P FSLG HETCOR. DFT calculations verified that a MeCN ligand dissociates upon ion exchange. Covalent immobilization via organic linkers as well as ion exchange with bidentate ligands both lead to rigidly bound complexes that cause broad 31P CSA tensors. We thus demonstrate how the interactions between complexes and functional surfaces determine and alter the stability of complexes. The applied Cu(dppf)(Lx)+ complex family members are identified as suitable solid-state NMR probes for investigating the influence of support surfaces on deposited inorganic complexes.

SiCO Ceramics as Storage Materials for Alkali Metals/Ions: Insights on Structure Moieties from Solid-State NMR and DFT Calculations.

Polymer-derived silicon oxycarbide ceramics (SiCO) have been considered as potential anode materials for lithium- and sodium-ion batteries. To understand their electrochemical storage behavior, detailed insights into structural sites present in SiCO are required. In this work, the study of local structures in SiCO ceramics containing different amounts of carbon is presented. 13 C and 29 Si solid-state MAS NMR spectroscopy combined with DFT calculations, atomistic modeling, and EPR investigations, suggest significant changes in the local structures of SiCO ceramics even by small changes in the material composition. The provided findings on SiCO structures will contribute to the research field of polymer-derived ceramics, especially to understand electrochemical storage processes of alkali metal/ions such as Na/Na+ inside such networks in the future.

Effect of ultra-fast pyrolysis on polymer-derived SiOC aerogels and their application as anodes for Na-ion batteries

In the last decade, Sodium-Ion-Batteries (SIB) started to gain interest as a possible complementary candidate to support the overburdened lithium technology, but the manufacturing of a proper anode material is one of the challenging factors for the development of performing SIB. Among others, porous polymer-derived ceramics have been widely explored as suitable anodes despite the production of such materials being time and energy-consuming. In this work, we investigate the feasibility of adopting a low-cost ultra-fast high-temperature pyrolysis for the ceramic conversion of a polymer-derived SiOC aerogel to be employed as anode material. A comprehensive study including N2 physisorption, 29Si MAS NMR and Raman spectroscopy provides the insights of the effect of ultra-fast and conventional heating rates (i.e., 200 °C·s−1 vs. 5 °C·min−1) on the microstructural features and ceramic yield of the SiOC aerogels. As a consequence of the ultra-fast heating rate, a compositional drift towards oxygen-rich SiOC is observed and discussed. The electrochemical performance of both ceramics has been tested and related to the observed compositional differences, revealing a stable capacity of 103 mAh·g−1 for the ultra-fast pyrolyzed SiOC anode, and 152 mAh·g−1 for SiOC ceramized at 5 °C·min−1.

Cation and anion ordering in synthetic lepidolites and lithian muscovites: influence of the OH ∕ F and Li ∕ Al ratios on the mica formation studied by NMR (nuclear magnetic resonance) spectroscopy and X-ray diffraction

Abstract. A large number of lepidolites K(LixAl3−x)[Si2xAl4−2xO10](OH)yF2−y and Li-muscovites K(LixAl2-x/3□1-2x/3)[Si3AlO10](OH)yF2−y were synthesised by a gelling method in combination with hydrothermal syntheses at a pressure of 2 kbar and a temperature of 873 K. The nominal composition ranged between 0.0≤x≤2.0 and 0.0≤y≤2.0, i.e. from polylithionite K[Li2.0Al][Si4.0O10](OH)yF2−y over trilithionite K[Li1.5Al1.5][AlSi3.0O10](OH)yF2−y to muscovite K[Al2.0□][AlSi3.0O10](OH)yF2−y. 1H, 19F, 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) and 27Al multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy has been performed to investigate the order and/or disorder state of Si and Al in the tetrahedral layers and of Li, Al, OH and F in the octahedral layer. The synthetic mica crystals are very small, ranging from 0.1 to 5 µm. With increasing Al content, the crystal sizes decrease. Rietveld structure analyses on 12 samples showed that nearly all samples consist of two mica polytypes (1M and 2M1) of varying proportions. In the case of lepidolites, the 1M / 2M1 ratio depends on the Li/Al ratio of the reaction mixture. The refinement of the occupancy factors of octahedral sites shows that lepidolites (1.5≤x≤2.0) represent a solid solution series with polylithionite and trilithionite as the endmembers. In the case of the Li-muscovites (0.0≤x≤1.5), the 1M / 2M1 ratio depends on the number of impurity phases like eucryptite or sanidine depleting the reaction mixture of Li or Al. There is no solid solution between trilithionite and muscovite; instead, the Li-muscovite crystals consist of domains differing in the relative proportions of muscovite and trilithionite. The overall composition of the synthesised micas which consist of two polytypes can be characterised by 29Si, 1H and 19F MAS NMR spectroscopy. The Si/Al ratio in the tetrahedral layers and thus the content of [4]Al were calculated by analysing the signal intensities of the 29Si MAS NMR experiments. The Li content xest was calculated from the measured tetrahedral Si/Al ratio of the 29Si MAS NMR signals. The calculated Li contents xest of samples between polylithionite and trilithionite agree with the expected values. The F-rich samples show slightly increased values and the OH samples lower values. Lepidolites with only F (x = 1.5 to 2.0, y = 0.0), but not lepidolites with only OH (x = 1.5 to 2.0 and y = 2.0), were observed after synthesis. With decreasing Li content, x≤1.2, Li-muscovites containing mostly hydroxyl (y>1.0) are formed. It was possible to synthesise fluorine containing micas with a Li content as low as 0.3 and y = 0.2 to 1.8. The 19F and 1H MAS NMR experiments reveal that F and OH are not distributed statistically but local structural preferences exist. F is attracted by Li-rich and OH by Al-rich environments. The quadrupolar coupling constant which represents the anisotropy of the Al coordination is low for polylithionite with CQ=1.5 MHz and increases to CQ=3.8 MHz for trilithionite. For tetrahedral Al a smaller increase of CQ from 1.7 to 2.8 MHz is observed. Advancing from trilithionite to muscovite both quadrupolar coupling constants decrease to 2.5 MHz for octahedral and 1.5 MHz for tetrahedral Al. In polylithionite there is the most isotropic environment for octahedral Al; there are only Li2Al sites coordinated by F in the octahedral sheets and O from the tetrahedral sheets which are regular, containing only Si. The distortion and anisotropy for Al in tetrahedral as well as octahedral sheets increases with rising Al content. The most anisotropic environment can be found in trilithionite, especially for octahedral Al.

An Amorphous Teflate Doped Aluminium Chlorofluoride: A Solid Lewis‐Superacid for the Dehydrofluorination of Fluoroalkanes

AbstractAn anion‐doped aluminium chlorofluoride AlCl0.1F2.8(OTeF5)0.1 (ACF‐teflate) was synthesized. The material contains pentafluoroorthotellurate (teflate) groups, which mimic fluoride ions electronically, but are sterically more demanding. They are embedded into the amorphous structure. The latter was studied by PDF analysis, EXAFS data and MAS NMR spectroscopy. The mesoporous powder is a Lewis superacid, and ATR‐IR spectra of adsorbed CD3CN reveal a blue‐shift of the adsorption band by 73 cm−1, which is larger than the shift for SbF5. Remarkably, ACF‐teflate catalyzes dehydrofluorination reactions of monofluoroalkanes to yield olefins in C6D6. In these cases, no Friedel‐Crafts products were formed.

A prediction of Young's modulus for tin containing phosphate glasses using quantitative structural information

A rigid unit packing fraction (RUPF) model was used to better understand the influence of local structural units on the Young's elastic modulus (E) of binary and ternary tin phosphate glasses. Quantitative analyses of the units that constitute the glass structure, obtained from X-ray/neutron diffraction and 31P MAS-NMR spectroscopy, were used to calculate polyhedral packing fractions that, with tabulated bond dissociation energies, were used to predict E based on a modification of the Makashima-Mackenzie relationship, which uses ion sizes to calculate packing fractions. Predictions based on the RUPF model are better than those based on ion sizes, and extending the RUPF model to all cation-polyhedra accounts for the compositional dependence of the Sn-coordination number.

Spectroscopic identification of Ca-bearing uranyl silicates formed in C–S–H systems

Portland cement-based grouts used for radioactive waste immobilisation contain a Ca- and Si-rich binder phase, known as calcium–silicate–hydrate (C–S–H). Depending on the blend of cement used, the Ca/Si ratio can vary considerably. A range of C–S–H minerals with Ca/Si ratios from 0.6 to 1.6 were synthesised and contacted with aqueous U(VI) at 0.5 mM and 10 mM concentrations. Solid-state 29Si MAS-NMR spectroscopy was applied to probe the Si coordination environment in U(VI)-contacted C–S–H minerals and, in conjunction with U LIII-edge X-ray absorption spectroscopy analysis, inferences of the fate of U(VI) in these systems were made. At moderate or high Ca/Si ratios, uranophane-type uranyl silicates or Ca-uranates dominated, while at the lowest Ca/Si ratios, the formation of a Ca-bearing uranyl silicate mineral, similar to haiweeite (Ca[(UO2)2Si5O12(OH)2]·3H2O) or Ca-bearing weeksite (Ca2(UO2)2Si6O15·10H2O) was identified. This study highlights the influence of Ca/Si ratio on uranyl sequestration, of interest in the development of post-closure safety models for U-bearing radioactive waste disposal.

1H NMR spectroscopy of degraded perfluorosulfonic acid membranes: A simple methodology for detecting onset of degradation

Perfluorosulfonic acid (PFSA) polymer electrolyte membranes (PEMs) used in fuel cells undergo chemical degradation when exposed to radicals formed during electrochemical reactions. One of the products or intermediates formed during the degradation was reported to be proton-exchangeable groups such as CF2OH and COOH. However, the 1H magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of Nafion PEMs degraded chemically through the Fenton reaction revealed that CF2H was generated. Furthermore, the accompanying depletion of SO3H in the Nafion PEMs reduced the water contents of Nafion PEMs even under water saturation conditions. Our results are consistent with recent thermal degradation results of CF2H generation via SO3H substitution with H. While very simple, the 1H MAS NMR spectroscopic analysis was demonstrated to be a powerful technique for determining degradation of PFSA membrane such as Naion and Aquivion when other techniques, such as 19F MAS NMR spectroscopy, failed to show any degradation evidence. Hence, this method is potentially useful for investigating various PFSA PEMs for initial degradation evidence before analyzing their in-depth degradation mechanisms.

Amorphous aluminosilicates as efficient ion exchangers for ammonium cation removal from aqueous solutions

The efficient removal of ammonium cation, one of the main causes of the eutrophication of water sources, has been achieved by using amorphous aluminosilicates as low-cost ion exchangers. The materials with a Si/Al ratio that varied from 1 to 5 were prepared by the coprecipitation method just by mixing an acidic aluminium sulphate solution with a basic sodium silicate solution, and the products were characterized by XRD, SEM and 27Al MAS NMR spectroscopy. All the amorphous solids presented a higher cation exchange capacity (CEC) than a synthetic mordenite (Si/Al = 9.5) and a natural zeolite (Si/Al = 4.6). The ammonium uptake was investigated as a function of contact time and initial ammonium concentration; additionally, the presence of Na+ and K+ as coexisting cations in the solutions was also evaluated. Langmuir and Freundlich models were applied to describe the data obtained in the ammonium exchange isotherms. The amorphous aluminosilicate with Si/Al = 3 was regenerated with a KCl aqueous solution and was reused 5 times keeping a stable removal efficiency. The reusability of the amorphous aluminosilicate combined with the superior cation exchange capacity manifests the promising use of this type of materials as ion exchangers to remove ammonium cations from wastewater.

The electrostatic basis of diacylglycerol pyrophosphate-protein interaction.

Phosphatidic acid (PA) is an anionic lipid with a simple phosphate headgroup that recruits multiple proteins for its diverse functional roles in all eukaryotes. Its charge, shape and Electrostatic Hydrogen bond switch mechanism forms the basis of its interaction with membrane components. PA is phosphorylated to Diacylglycerol pyrophosphate (DGPP) in many signaling events in plants, yeast, and some parasites. DGPP has a pyrophosphate headgroup with similar ionization properties to PA, except that its charge at any given physiological pH is higher. The synthesis of DGPP in the signaling cascade potentially alters membrane dynamics and properties due to the differences in the physicochemical parameters between the two lipids. Thus, DGPP is expected to differ from PA in its interaction with membrane components. However, the overall effect on the biophysical properties of DGPP's interactions with proteins and ions are unexplored. In this study, we investigated the effect of the interactions of DGPP with model peptides KALP23 and WALP23 by 31P MAS NMR spectroscopy. In addition, we also analyzed the effect of DGPP with two positively charged lipids, dioleoyl trimethylammonium propane (DOTAP) and ethyl -phosphatidylcholine (EtPC) as well as with Magnesium and Calcium cations. Results from this study highlight a unique model for DGPP-protein interactions and has important implications in how DGPP interacts with membrane components in the signaling pathway.

Propene transformation on Cu-modified ZSM-5 zeolite: Aromatization and oxidation

Propene transformation on copper-modified ZSM-5 zeolite has been studied by 13C MAS NMR and FTIR spectroscopy. Two major reaction pathways are identified, aromatization to the simple aromatic hydrocarbons (benzene, toluene) and oxidation to acrolein. Copper-allyl species is inferred to be the key intermediate in both processes. π-Complex of propene with copper sites is the primary intermediate, preceding the formation of copper-allyl species. The role of different active sites is established. Cu2+–O2– Lewis acid-base pairs are responsible for the allylic species formation, whereas [Cu3(μ-O)3]2+ oxo-clusters provide the oxidation of copper-allyls. The results obtained are of importance for developing zeolite-based catalysts for propene aromatization to the simple aromatic compounds and partial oxidation to acrolein.

Investigation on fluorinated alumina catalysts prepared by the fluorination of γ-Al2O3 with CHF3 for the isomerization of E-1-chloro-3,3,3-trifluoropropene

As an environmentally friendly cleaning agent, Z-1-chloro-3,3,3-trifluoropropene (HCFO-1233zd(Z)) can realize industrial production by the isomerization of HCFO-1233zd(E). Herein, various isomerization catalysts with different structures, compositions and acid properties were prepared through the fluorination of γ-Al2O3 with CHF3 by controlling the activation conditions. For the first time, the catalysts for HCFO-1233zd(E) isomerization were systematically studied by XRD, N2 physisorption, XPS, SEM, MAS NMR spectroscopy, NH3-TPD and py-IR. The active site of this reaction was revealed and the catalyst deactivation mechanism was proposed. It turned out that HCFO-1233zd(E) isomerization prefers to occur at Lewis acid sites with moderate density and strength. The amorphous particles composed of γ-Al2O3, Al2O3(F) and α-AlF3 (R3¯c) at 450 °C exhibit highly catalytic activity for HCFO-1233zd(E) isomerization. After 100 h evaluation, it was found that residual γ-Al2O3 can be fluorinated by HCFO-1233zd(E) to form needle-like structures which could induce the amorphous Al2O3(F) particles to reorganize and give cubic structures, where the Lewis acid properties were enhanced, leading to catalyst deactivation.

Quantitative 67Zn, 27Al and 1H MAS NMR spectroscopy for the characterization of Zn species in ZSM-5 catalysts.

67Zn MAS NMR spectroscopy was used to characterize the state of Zn in Zn-modified zeolites ZSM-5. Two 67Zn enriched zeolite samples were prepared: by solid-state exchange with metal 67Zn (Zn2+/ZSM-5 sample) and by ion exchange with zinc formate solution (ZnO/H-ZSM-5 sample), both containing ca. 3.8 wt% Zn. The elemental analysis, TEM, and quantitative BAS and aluminum analyses with 1H and 27Al MAS NMR have shown that Zn2+/ZSM-5 contains zinc in the form of Zn2+ cations, while both ZnO species and Zn2+ cations are present in ZnO/H-ZSM-5 besides BAS. 67Zn MAS NMR has detected the signal of Zn in a tetrahedral environment from ZnO species for both the activated and hydrated ZnO/H-ZSM-5 zeolite. The signal of Zn in an octahedral environment was detected for the hydrated Zn2+/ZSM-5 and ZnO/H-ZSM-5 zeolites. This signal may belong to zinc cation [HOZn]+ or Zn(OH)2 species surrounded by water molecules. Quantitative 67Zn MAS NMR analysis has shown that only 27 and 38% of zinc loaded in the zeolite is visible for the activated and hydrated ZnO/H-ZSM-5 zeolite, and 24% of Zn is visible for the hydrated Zn2+/ZSM-5. Zinc in the form of ZnO species is entirely visible in both the activated and hydrated ZnO/H-ZSM-5 zeolite, while Zn2+ cations are not detected at all for the activated sample and only 29% of Zn2+ cations is visible for the hydrated zeolite. Detection of only a part of Zn2+ cations in the form of [HOZn]+ or Zn(OH)2 species in octahedral environment presumes only partial hydrolysis of the bond of Zn2+ cation with framework oxygen and further solvation of the Zn species formed at hydrolysis by the adsorbed water.

Self-diffusion in polycrystalline Li1+x Ti2-x Al x (PO4)3 (0.2 ≤ x ≤ 0.4) samples followed by 7Li PFG (pulse field gradient) NMR spectroscopy.

Short and long range lithium motions in powder Li1+x Ti2-x Al x (PO4)3 (LTAP) NASICON compounds prepared by ceramic (x = 0.2 and 0.4) and sol-gel (x = 0.3 and 0.4) routes are discussed. ND diffraction and MAS-NMR spectroscopy were previously used to investigate structural features of these compounds. In particular, Fourier map differences showed that the amount of Li atoms allocated at M3 increases at the expense of M1 sites when the Li content increases. In this work, PFG-NMR results show that diffusion coefficients rise with the amount of lithium and temperature. The restricted diffusion inside NASICON particles is compared with "free" diffusion processes. At 300 K, diffusion coefficients D PFG ∼ 5 × 10-12 m2 s-1 have been deduced in ceramic x = 0.2 and 0.4 samples, decreasing with diffusion time Δ used in PFG experiments. In sol-gel samples, diffusion coefficients are near those of ceramic samples, but decrease faster with diffusion Δ times, as a consequence of the Li confinement inside sub-micrometric crystallites. The NMR spin-echo signal displays minima at specific q(γgδ) values that are related to the crystallite size. From R dif ∼ q m -1 distances, calculated from the position of minima, and from diffusion coefficients deduced for high Δ values, the mean crystallite size was estimated. Finally, from the temperature dependence of conductivity and diffusion coefficients, the activation energy and charge carriers concentrations were determined.