PFG NMR Research Articles

PFG NMR diffusion measurements of CH4 and CO2 through large ZSM-58-crystals

Large crystals of zeolite ZSM-58 having the DDR-topology have been synthesised. The crystals are diamond shaped and 100–160μm long. Basic characterisation involving X-ray diffraction and N2-porosimetry proved good structure quality and microporosity of the sample. 1H and 13C PFG NMR self-diffusion studies of single gas and binary 1:1 mixtures with CO2 and CH4 show that – in agreement with the topology of the ZSM-58 – the intracrystalline molecular diffusion occurs in a two dimensionally interconnected micropore space. The self-diffusion coefficients of the smaller carbon dioxide exceed those of the larger methane molecules by about two orders of magnitude, which is clearly the effect of increased diffusional resistance and shape selectivity within the narrow 8-ring window system of the ZSM-58 micropores. Carbon dioxide diffusion is rather constant and methane diffusion increases with increasing total loading. For methane this indicates that the narrow windows act as barriers which are easier passed if additional interactions with adsorbed molecules are present. These trends for the loading dependences in both the single-component and the binary mixture diffusion studies confirm earlier predictions by kinetic MC and MD computer simulations. However, at low loadings the predicted values for methane self-diffusion are significantly smaller than the experimental NMR data.

A diffusion study of small hydrocarbons in DDR zeolites by micro-imaging

Intracrystalline diffusion of the light hydrocarbons methane, ethane, ethylene and propylene in three different species of zeolite DDR has been investigated by recording the transient concentration profiles during uptake and release. The measured profiles approximate the form of the appropriate solution of the diffusion equation for radial diffusion in an infinite cylinder. This is as expected since, for an ideal DDR crystal with its 2D pore structure, there should be no flux in the axial direction. Over the range of guest molecules considered, the diffusivities are found to decrease, with increasing critical molecular diameter, over more than three orders of magnitude, i.e. over about one order of magnitude on comparing ethane, ethylene and ethane, and over, additionally, more than two orders of magnitude for propylene. The observed diffusivities are found to be in reasonable agreement (with differences of up to a factor of three) between the three different DDR specimens as well as with values from the literature, obtained mainly from transient macroscopic measurement or microscopic PFG NMR self-diffusion measurements. In the present study, all measurements were performed by the same experimental technique (micro-imaging by interference microscopy), and the derived diffusivities provide a coherent picture of the diffusion process.

Filter diagonalization method for processing PFG NMR data

Obtaining diffusion coefficients from PFG NMR diffusion (a.k.a DOSY) data is, in the general case, an ill-posed problem. Numerous methods for processing such data have therefore been developed, each with different constraints and assumptions. The Regularized Resolvent Transform (RRT) is a proven and robust method for spectral inversion. In earlier papers RRT, albeit very slow, was argued to be superior for DOSY processing to a related algorithm, the Filter Diagonalization Method (FDM). Here FDM is revisited and a new regularization method is implemented, which drastically improves the performance and provides spectra of comparable or better quality to those provided by RRT. Both the RRT and the FDM for DOSY processing have been implemented as options in the free and open source DOSY Toolbox.

The structure and morphology of gold nanoparticles produced in cationic gemini surfactant systems

Potential applications of gold nanoparticles (GNP) are strictly connected with their size and shape. The influence of different dicationic (gemini) surfactants, alkyloxymethylimidazolium derivatives derivatives, on the structure and morphology of GNP was studied. The synthesis of nanoparticles was performed in the presence of various gemini surfactants—dodecyloxymethylimidazolium nitrate (IMI_NO3_C4_C12), propionate (IMI_PROP_C4_C12) and 3,3'-[1,9-(2,8-dioxanonane)]bis-(1-nonyloxymethylimidazolium) chloride (IMI_Cl_oxyC7_C9), used as stabilizers and templates for obtaining different size and shape of gold nanoparticles. The samples obtained were examined using transmission electron microscopy (TEM), small angle scattering of synchrotron radiation (SAXS), UV–vis spectroscopy and NMR PFG spectroscopy. For the obtained solutions of nanoparticles the plasmon resonance was observed at the wavelengths corresponding to the presence of gold nanoparticles of sizes ranging from 5–100nm, with a significant shift towards higher wavelength for the samples prepared with addition of dicationic surfactants. TEM images evidence the presence of gold nanoparticles with tetrahedral and spherical morphology in solutions prepared with the surfactants IMI_PROP_C4_C12, IMI_NO3_C4_C12, and those of spherical morphology, but strongly aggregated, in the solution with the cationic surfactant IMI_Cl_oxyC7_C9.

Premicellar interaction of PEO-PPO-PEO triblock copolymers with partially hydrophobic alcohols: NMR study

The interactions of three alcohols, namely, 2-butanol (BuOH), 3-methyl-2-butanol (MeBuOH), and 3,3-dimethyl-2-butanol (Me2BuOH) with propylene oxide octamer (PO8) and the copolymers (EO)8(PO)13(EO)8(L35) and (EO)13(PO)30(EO)13(L64) in D2O were studied using (13)C NMR spectra and relaxations and (1)H PFG NMR diffusion measurements. For L64, it was shown that the temperature at which the PO chain starts to change its conformation under dehydration decreases by 6 K for each additional methyl group in the alcohol molecule (i.e. with increasing its hydrophobicity), and the analogous conformation states are attained at temperatures approximately 10 K lower compared using ketonic analogs of the alcohols under the same conditions. Also, the first signs of L64 aggregation, according to the normalized diffusion coefficients, are at temperatures 7, 10, and 13 K lower for BuOH, MeBuOH, and Me2BuOH, respectively. These effects are much weaker for (PO)13 in L35 or nonexistent for (PO)8 in PO8, thus showing the role of cooperativity in dehydration and aggregation processes. According to diffusion measurements, the molar fraction of the alcohol hydrogen bonded to L64 increases with its hydrophobicity and, in an apparent conflict with thermodynamics, with increasing temperature at which also higher NOE can be observed. Strong hydrogen bond interaction, which is in cooperation with hydrophobic interaction, does not preclude the exchange between bound and free states of the alcohol, however. Using (13)C transverse relaxation, its correlation time is shown to be of the order of 10 ms.

Ion and water mobility in hydrated Li-LSX zeolite studied by 1H, 6Li and 7Li NMR spectroscopy and diffusometry

Crystallites of zeolite LSX with a diameter of about 10μm were synthesized. Crystals of this size are shown to allow the simultaneous investigation of intracrystalline mass transfer phenomena of water molecules and lithium ions in hydrated zeolite Li-LSX by NMR diffusometry. By MAS NMR spectroscopy with the 1H and 6Li nuclei, the water molecules and lithium ions are found to yield two signals, a major and a minor one, which may be attributed to locations in the sodalite cages and the supercages, respectively. By 1H and 6Li exchange spectroscopy the mean residence times in the sodalite cages at 373K are found to be about 150ms for the water molecules and about 40ms for the lithium cations. PFG NMR self-diffusion measurements at 373K yield a diffusivity of about 2×10−11m2s−1 for the lithium ions, which is about one order of magnitude below the water diffusivity.

Molecular‐Dynamics Simulation of Self‐Diffusion of Molecular Hydrogen in X‐Type Zeolite

The self‐diffusion of hydrogen in NaX zeolite has been studied by molecular‐dynamics simulations for various temperatures and pressures. The results indicate that in the temperature range of 77–293 K and the pressure range of 10–2700 kPa, the self‐diffusion coefficients are found to range from 1.61 × 10−9 m2·s−1 to 3.66 × 10−8 m2·s−1 which are in good agreement with the experimental values from the quasielastic neutron scattering (QENS) and pulse field gradients nuclear magnetic resonance (PFG NMR) measurements. The self‐diffusion coefficients decrease with increasing pressure due to packing of sorbate‐sorbate molecules which causes frequent collusion among hydrogen molecules in pores and increase with increasing temperature because increasing the kinetic energy of the gas molecules enlarges the mean free path of gas molecule. The activated energy for hydrogen diffusion determined from the simulation is pressure‐dependent.

The interplay between inter- and intra-molecular dynamics in a series of alkylcitrates

The inter- and intra-molecular dynamics in a series of glass-forming alkylcitrates is studied by a combination of Broadband Dielectric Spectroscopy (BDS), Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR), Fourier-Transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC). Analyzing the temperature dependencies of specific IR absorption bands in terms of their spectral position and the corresponding oscillator strengths enables one to unravel the intramolecular dynamics of specific molecular moieties and to compare them with the (primarily dielectrically) determined intermolecular dynamics. With decreasing temperature, the IR band positions of carbonyls (part of the core units) and H-bonded moieties of citrates show a red shift with a kink at the calorimetric glass transition temperature (Tg) while other moieties, whose dynamics are decoupled from those of the core units, exhibit a blue shift with nominal changes at Tg. The oscillator strength of all units in citrates depicts stronger temperature dependencies above Tg and in some, the ester linkage and H-bonded units show a change of slope at a temperature where structural and faster secondary relaxations merge. By that, a wealth of novel information is obtained proving the fundamental importance of intramolecular mobility in the process of glass formation, beyond coarse-grained descriptions.

Tetragonal Li10GeP2S12 and Li7GePS8 – exploring the Li ion dynamics in LGPS Li electrolytes

Tetragonal Li10GeP2S12 (LGPS) is the best solid Li electrolyte reported in the literature. In this study we present the first in-depth study on the structure and Li ion dynamics of this structure type. We prepared two different tetragonal LGPS samples, Li10GeP2S12 and the new compound Li7GePS8. The Li ion dynamics and the structure of these materials were characterized using a multitude of complementary techniques, including impedance spectroscopy, 7Li PFG NMR, 7Li NMR relaxometry, X-ray diffraction, electron diffraction, and 31P MAS NMR. The exceptionally high ionic conductivity of tetragonal LGPS of ∼10−2 S cm−1 is traced back to nearly isotropic Li hopping processes in the bulk lattice of LGPS with EA ≈ 0.22 eV.

NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures

Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework-8 (ZIF-8) were performed by 1H and 13C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible frameworks.

The impact of freeze-drying on microstructure and rehydration properties of carrot

The impact of freeze-drying, blanching and freezing rate pre-treatments on the microstructure and on the rehydration properties of winter carrots were studied by μCT, SEM, MRI and NMR techniques. The freezing rate determines the size of ice crystals being formed that leave pores upon drying. Their average size (determined by μCT) can be predicted in a quantitative manner by considering dendritic growth and freezing rates. Blanching as a pre-treatment, however, did not affect pore size distribution induced by freeze-drying. Upon rehydration of the freeze-dried carrots, PFG NMR and MRI show that cellular compartments were not restored and instead a porous network with permeable barriers is formed. Blanching pre-treatment introduced a less connected and more anisotropic porous network if followed by fast freezing, indicating that more of the native cell wall morphology is preserved.

The gamma distribution model for pulsed-field gradient NMR studies of molecular-weight distributions of polymers

Self-diffusion in polymer solutions studied with pulsed-field gradient nuclear magnetic resonance (PFG NMR) is typically based either on a single self-diffusion coefficient, or a log-normal distribution of self-diffusion coefficients, or in some cases mixtures of these. Experimental data on polyethylene glycol (PEG) solutions and simulations were used to compare a model based on a gamma distribution of self-diffusion coefficients to more established models such as the single exponential, the stretched exponential, and the log-normal distribution model with regard to performance and consistency. Even though the gamma distribution is very similar to the log-normal distribution, its NMR signal attenuation can be written in a closed form and therefore opens up for increased computational speed. Estimates of the mean self-diffusion coefficient, the spread, and the polydispersity index that were obtained using the gamma model were in excellent agreement with estimates obtained using the log-normal model. Furthermore, we demonstrate that the gamma distribution is by far superior to the log-normal, and comparable to the two other models, in terms of computational speed. This effect is particularly striking for multi-component signal attenuation. Additionally, the gamma distribution as well as the log-normal distribution incorporates explicitly a physically plausible model for polydispersity and spread, in contrast to the single exponential and the stretched exponential. Therefore, the gamma distribution model should be preferred in many experimental situations.

Exploring the hierarchy of transport phenomena in hierarchical pore systems by NMR diffusion measurement

The pulsed field gradient technique of NMR (PFG NMR) is applied for exploring molecular diffusion in different specimens of zeolite NaCaA, notably in samples containing “hierarchical” pore systems where the micropores are traversed by mesopores. Choosing ethane (capable of permeating both micro- and mesopores) and cyclohexane (unable to penetrate the micropores) as guest molecules and varying purposefully the accessibility and mobility in the mesopores by temperature variation and pore blocking, the diffusivities in the two pore spaces are measured separately from each other. It is shown that the presence of the mesopores may give rise to dramatically enhanced intracrystalline diffusivities while a blockage of the mesopores reduces the intracrystalline diffusivities by an order of magnitude.

NMR Investigation of Li+ Ion Interactions in Some Precursors of the Lithium Polymer Battery System

High resolution NMR has been used to reveal interactions between components in prepared electrolyte material prior to polymerization that was aimed for use in lithium polymer battery research. Previous results indicated a decrease in lithium mobility with increased amount of methoxysilyl groups contained in the sample. 1H, 13C, 7Li, and 29Si NMR spectra as well as pulsed field-gradient (PFG NMR) measurements of the self-diffusion of lithium ions were obtained and a preferred coordination of lithium ions to the methoxysilyl groups was proved. Although this interaction seems quite weak, based on the small shifts observed in the spectra, a clear decrease in lithium diffusion dependent on the increased content of methoxysilyl groups was shown.

Monitoring Molecular Mass Transfer in Cation-Free Nanoporous Host Crystals of Type AlPO-LTA

Micro-imaging is employed to monitor the evolution of intra-crystalline guest profiles during molecular adsorption and desorption in cation-free zeolites AlPO-LTA. The measurements are shown to provide direct evidence on the rate of intra-crystalline diffusion and surface permeation and their inter-relation. Complemented by PFG NMR and integral IR measurements, a comprehensive overview of the diffusivities of light hydrocarbons in this important type of host materials is provided.

Exploring Mass Transfer in Mesoporous Zeolites by NMR Diffusometry.

With the advent of mesoporous zeolites, the exploration of their transport properties has become a task of primary importance for the auspicious application of such materials in separation technology and heterogeneous catalysis. After reviewing the potential of the pulsed field gradient method of NMR (PFG NMR) for this purpose in general, in a case study using a specially prepared mesoporous zeolite NaCaA as a host system and propane as a guest molecule, examples of the attainable information are provided.

Interaction of Premicellar States of a PEO-PPO-PEO Triblock Copolymer with Partially Hydrophobic Substances: NMR Study

According to (1)H and (13)C NMR spectra, relaxations, and PFG NMR self-diffusion measurements, partially hydrophobic additives methyl-ethyl ketone (MEK), methyl-isopropyl ketone (MIPK), and methyl-t-butyl ketone (MTBK) facilitate the conformation change and subsequent self-association of the copolymer Pluronic L64. The correlation time (4-9 ms) and activation energy (43-52 kJ/mol) of transition between its conformation states decrease with the increasing hydrophobicity and bulkiness of the additive. The temperature of the first PPO self-association decreases in the same order (by 4 K for MTBK). The interaction of the additives was indirectly proved by the decrease of their rotational and translational mobility in the presence of L64. The rotational correlation time τ(c) is between 3 and 6 × 10(-11) s, whereas that of the same molecules in the absence of L64 is lower than 6 × 10(-12) s. The normalized self-diffusion coefficient decreases to about 0.7 of its original value in the presence of L64. The interaction of the additive with the PPO block is transient but effective enough to facilitate its conformational change and self-association. Its mediation by a water molecule bound to PPO as a possible mechanism is suggested.

In situ nondestructive sediment characterization and resuspendability evaluation of concentrated aqueous paliperidone palmitate suspensions in prefilled syringes by low-field one-dimensional pulsed-field gradient NMR profilometry

This work aims to demonstrate the usefulness of a low-field one-dimensional pulsed-field gradient NMR (1D pfg NMR) profilometry technique to enable in situ nondestructive sediment characterization and resuspendability quantification of concentrated prefilled injectable suspensions. Aqueous paliperidone palmitate suspensions were used as model samples and low-intensity centrifugation was evaluated as a long-term gravity simulation approach. The low-field 1D pfg NMR technique allowed a detection zone of 2.5 cm in height for water content measurement of syringe samples using a Teflon syringe holder. Thus, the sediment compactness could be deduced from its water content. Quantitative evaluation of resuspendability was realized by front tracking of the NMR profile signals, which yielded the exponential sediment volume decay constant as a resuspendability quantification parameter. The study shows that both active ingredient particle size distribution and storage temperature had significant effects on the sedimentation rate and the resuspendability of the suspensions. The centrifugation method proved to be useful as a long-term gravity simulation and screening method, although the results should be interpreted with caution due to its higher acceleration and compression force imposed on the active ingredient particles.

Understanding the Ion Jelly Conductivity Mechanism

The properties of the light flexible device, ion jelly, which combines gelatin with an ionic liquid (IL) were recently reported being promising to develop safe and highly conductive electrolytes. This article aims for the understanding of the ion jelly conductive mechanism using dielectric relaxation spectroscopy (DRS) in the frequency range 10(-1)-10(6) Hz; the study was complemented with differential scanning calorimetry (DSC) and pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The room temperature ionic liquid 1-butyl-3-methylimmidazolium dicyanamide (BMIMDCA) used as received (1.9% w/w water content) and with 6.6% (w/w) of water content and two ion jellies with two different ratios BMIMDCA/gelatin/water % (w/w), IJ1 (41.1/46.7/12.2) and IJ3 (67.8/25.6/6.6), have been characterized. A glass transition was detected by DSC for all materials allowing for classifying them as glass formers. For the ionic liquid, it was observed that the glass transition temperature decreases with the increase of water content. While in subsequent calorimetric runs crystallization was observed for BMIMDCA with negligible water content, no crystallization was detected for any of the ion jelly materials upon themal cycling. To the dielectric spectra of all tested materials, both dipolar relaxation and conductivity contribute; at the lowest frequencies, electrode and interfacial polarization highly dominate. Conductivity, which manifests much more intensity relative to dipolar reorientations, strongly evidences subdiffusive ion dynamics at high frequencies. From dielectric measures, transport properties as mobility and diffusion coefficients were extracted. Data treatment was carried out in order to deconvolute the average diffusion coefficients estimated from dielectric data in its individual contributions of cations (D(+)) and anions (D(-)). The D(+) values thus obtained for IJ3, the ion jelly with the highest IL/gelatin ratio, cover a large temperature range up to room temperature and revealed excellent agreement with direct measurements from PFG NMR, obeying to the same VFT equation. For BMIMDCA(6.6%water), which has the same water amount as IJ3, the diffusion coefficients were only estimated from DRS measurements over a limited temperature range; however, a single VFT equation describes both DRS and PFG NMR data. Moreover, it was found that the diffusion coefficients and mobility are similar for the ionic liquid and IJ3, which points to a role of both water and gelatin weakening the contact ion pair, facilitating the translational motion of ions and promoting its dissociation; nevertheless, it is conceivable the existence of a critical composition of gelatin that leads to those properties. The VFT temperature dependence observed for the conductivity was found to be determined by a similar dependence of the mobility. Both conductivity and segmental motion revealed to be correlated as inferred by the relatively low values of the decoupling indexes. The obtained results show that ion jelly could be in fact a very promising material to design novel electrolytes for different electrochemical devices, having a performance close to the IL but presenting an additional stability regarding electrical measurements and resistance against crystallization relative to the bulk ionic liquid.

Highly proton conducting sulfonic acid functionalized mesoporous materials studied by impedance spectroscopy, MAS NMR spectroscopy and MAS PFG NMR diffusometry

Proton conducting mesoporous Si-MCM-41 materials with different amounts of SO3H-groups were prepared by the co-condensation method and investigated by NMR spectroscopy, NMR diffusometry and impedance spectroscopy. The successful incorporation of mercaptopropyltrimethoxysilane (MPMS) into the mesoporous framework was proven by 29Si NMR measurements. The deconvolution of the 29Si MAS NMR spectra shows that Si atoms of the functionalizing silane are linked to the MCM-41 framework via three or two ≡Si–O–Si≡ bonds. 29Si MAS NMR proves that almost all MPMS was incorporated into the mesoporous framework, since 36.9% of the signal belongs to T groups, whereas the concentration of MPMS in the synthesis solution amounts 40%. 13C CP MAS spectroscopy confirms that the majority of the organic functional groups remained intact after the oxidation in 30 wt.-% H2O2. The proton conductivity was investigated by impedance spectroscopy (IS). Drastic differences were found for different degrees of functionalization. The maximum conductivity was found for the sample with maximum loading of sulfonic acid groups (40% functionalization) as σdc=0.19 S cm−1 at 413K. The large conductivity differences between 20% and 40% functionalization result in large differences in the diffusion coefficients of the charge carrier by application of the Nernst–Einstein equation. But only small differences of the water self-diffusion coefficients were measured by magic-angle spinning pulsed field gradient (MAS PFG) NMR diffusometry. The comparison of IS and MASPFG NMR results allows the determination of the Haven factor, which amounts 5 660 for 20% and 329 for 40% functionalization. We explain the proton conductivity in functionalized MCM-41 by structure diffusion. The drastic increase of conductivity (at 353K from 9.51 to 260×10−5Scm−1) from 20% to 40% functionalization is caused by the reduction of the activation energy of the charge relocation in a denser lattice of proton donator sites.