129Xe Nuclear Magnetic Resonance Spectra Research Articles

Porosity of Molecularly Imprinted Polymers Investigated by 129Xe NMR Spectroscopy.

Molecularly imprinted polymers (MIPs) display intriguing recognition properties and can be used as sensor recognition elements or in separation. In this work, we investigated the formation of hierarchical porosity of compositionally varied MIPs using 129Xe Nuclear Magnetic Resonance (NMR) and 1H Time Domain Nuclear Magnetic Resonance (TD-NMR). Variable temperature 129Xe NMR established the morphological variation with respect to the degree of cross-linking, supported by 1H TD-NMR determination of polymer chain mobility. Together, the results indicate that a high degree of cross-linking stabilizes the porous structure: highly cross-linked samples display a significant amount of accessible mesopores that instead collapse in less structured polymers. No significant differences can be detected due to the presence of templated pores in molecularly imprinted polymers: in the dry state, these specific shapes are too small to accommodate xenon atoms, which, instead, probe higher levels in the porous structure, allowing their study in detail. Additional resonances at a high chemical shift are detected in the 129Xe NMR spectra. Even though their chemical shifts are compatible with xenon dissolved in bulk polymers, variable temperature experiments rule out this possibility. The combination of 129Xe and TD-NMR data allows attribution of these resonances to softer superficial regions probed by xenon in the NMR time scale. This can contribute to the understanding of the surface dynamics of polymers.

129Xe NMR analysis reveals efficient gas transport between inborn micro-, meso- and macropores in geopolymers

Geopolymers are alumino-silicates with inborn mesoporous structures. For construction applications, the knowledge on the porous structure and fluid dynamics is important. In this work, the interconnected pore structures of metakaolin-based geopolymers and dynamics of guest molecules were studied by 129 Xe nuclear magnetic resonance (NMR), by exploiting the high sensitivity of the 129 Xe chemical shift to its local environment. The 129 Xe NMR spectra revealed three different pore structures experienced by xenon gas in the geopolymers: the first associated with microporous zeolite-analogous cages (≤2 nm), the second with the mesopores (2–50 nm) and the third with free Xe in larger voids. The sizes of mesopores, heat of adsorption as well as pore connectivity were determined as a function of water-to-solid ( w / s ) ratio. The results show that the pore structures can be tailored by changing the w / s ratio, providing tunability both for potential construction, catalysis, and adsorbent applications.

Hyperpolarized129Xe Nuclear Magnetic Resonance Studies of Si Nanocomposite Electrode Materials

Porosity and interconnectivity of pores play an important role in the performance of composite electrodes in Li ion batteries. Hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) is powerful for probing porosity and interconnectivity of nanopores. This study represents the first time that temperature-dependent HP 129Xe NMR has been successfully applied to explore the pore structure and interconnectivity in composite electrodes made of nano Si powder, super P carbon, and sodium carboxyl methyl cellulose (CMC) binder. In particular, this work focuses on the influence of the binder content on the porosity and pore interconnectivity in Si composite electrode (Si + C):CMC materials. Our data from HP 129Xe NMR spectra show that the CMC content has an effect on the porosity and pore interconnectivity of Si nanocomposite electrode materials. We determined parameters, such as pore size, heat of adsorption, and characteristic chemical shift, of (Si + C):CMC samples based on the temperature-dependent HP 129Xe...

How accessible is the specific surface area of minerals? A comparative study with Al-containing minerals as model substances

Abstract The specific surface area (SSA) is a principal characteristic for the assessment of the sorption capacity of minerals. In addition to size, the accessibility of the surface area is of importance, which is primarily affected by porosity. In particular the micropore surface area of soil minerals may be inaccessible due to too small pore entrances or their clogging by organic material. In this study, SSA and micropore surface area of three minerals (Al oxide, Al hydroxide and kaolinite), acting as model materials for oxides, hydroxides and clay minerals, respectively, were measured by gas adsorption methods with both N 2 at 77.35 K or CO 2 at 273.15 K and the retention of the polar liquid ethylene glycol monoethyl ether (EGME). Moreover, porosity was assessed by 129 Xe nuclear magnetic resonance (NMR) spectroscopy. A NaY zeolite was used as microporous reference material. 129 Xe NMR spectroscopy and the determination of SSA by physisorption of N 2 (BET-N 2 SSA) were performed after sample pretreatment at various outgassing temperatures because BET-N 2 SSA and porosity are significantly affected by the outgassing temperature due to the loss of adsorbed water molecules. For all minerals investigated, the highest SSA was obtained by EGME retention. The gas adsorption methods exhibited differences in SSA and micropore surface area depending on the gas and pretreatment temperature. Thereby, increased pretreatment temperatures resulted in increased SSA and micropore surface area. 129 Xe NMR spectra revealed that this increase in surface area was mainly ascribed to an increase in porosity. It appeared that the accessibility of surface area was limited by surface hydration. This implies that at environmental conditions, partly saturated pores will yield a reduced sorption capacity compared to the one that may be expected from SSA measurements. Consequently, porosity and hydration of the mineral surfaces have to be considered for an improved assessment of the sorption capacity of minerals and soils.

A 129Xe Nuclear Magnetic Resonance Study on Xenon Trapped in Fully Dehydrated Mesoporous Silica and Molecular Sieves 5A and 13X under Atmospheric Pressure

129Xe nuclear magnetic resonance (NMR) spectra of naturally abundant xenon trapped in fully dehydrated mesoporous materials with pore diameters of 2 nm or less were observed under atmospheric pressure in the temperature range of 168–373 K. The diameters of the three porous materials considered in this study were about 0.5, 1, and 2 nm for the molecular sieves 5A and 13X and synthesized mesoporous silica, respectively. The samples were fully dehydrated using an ultrahigh-vacuum system, and xenon gas was introduced into a precooled sample at 168 K, slightly above its boiling point. The 129Xe NMR spectra were observed while increasing the temperature, and the 129Xe shift, linewidth, and area intensity were observed at each temperature for the three samples under atmospheric pressure. The behaviors of xenon atoms in small pores observed in equilibrium states can provide important information on the relationship between the pore structure and the 129Xe chemical shift, linewidth, and signal intensity.

Behavior of a Thermotropic Nematic Liquid Crystal Confined to Controlled Pore Glasses as Studied by 129Xe NMR Spectroscopy

The behavior of nematic liquid crystal (LC) Merck Phase 4 confined to controlled pore glass (CPG) materials was investigated using 129Xe nuclear magnetic resonance (NMR) spectroscopy of xenon gas dissolved in the LC. The average pore diameters of the materials varied from 81 to 2917 A, and the measurements were carried out within a wide temperature range (approximately 185-370 K). The spectra contain lots of information about the effect of confinement on the phase of the LC. The theoretical model of shielding of noble gases dissolved in liquid crystals on the basis of pairwise additivity approximation was applied to the analysis of the spectra. When pore diameter is small, smaller than approximately 150 A, xenon experiences on average an isotropic environment inside the pore, and no nematic-isotropic phase transition is observed. When the size is larger than approximately 150 A, nematic phase is observed, and the LC molecules are oriented along pore axis. The orientational order parameter of the LC, S, increases with increasing pore size. In the largest pores, the orientation of the molecules deviates from the pore axis direction to magnetic field direction, which implies that the size of the pores (approximately 3000 A) is close to magnetic coherence length. The decrease of magnetic coherence length with increasing temperature is clearly seen from the spectra. When the sample is cooled rapidly by immersing it in liquid nitrogen, xenon atoms do not squeeze out from the solid, as they do during gradual freezing, but they are occluded inside the solid lattice, and their chemical shift is very sensitive to crystal structure. This makes it possible to study the effect of confinement on the solid phases. According to the measured 129Xe NMR spectra, possibly three different solid phases are observed from bulk liquid crystal in the used temperature region. The same is also seen from the samples containing larger pores (pore size larger than approximately 500 A), and the solid-solid phase-transition temperatures are the same. However, no first-order solid-solid phase transitions are observed from the smaller pores. Melting point depression, that is, the depression of solid-nematic transition temperature observed from the pores as compared with that in bulk LC, is seen to be very sensitive to the pore size, and it can be used for the determination of pore size of an unknown material.

129Xe NMR spectroscopy of adsorbed xenon as an approach for the characterisation of soil meso- and microporosity

Abstract 129 Xe nuclear magnetic resonance (NMR) spectroscopy of adsorbed xenon was applied for the characterisation of soil meso- and microporosity. Model systems (Ca-montmorillonite, quartz sand) and three soil types (Luvisol Alh, Bt and Cv horizons; Gleysol Go horizon; Podzol Bvs horizon) were studied. For Ca-montmorillonite, the average intercrystallite pore size has been evaluated. In natural soils, 129 Xe resonance parameters were shown to be affected by different factors: pore heterogeneity, influence of organic functional groups, possible presence of paramagnetic compounds, occurrence of xenon exchange between inter- and intraparticle void spaces. The effect of those factors on the pattern of 129 Xe NMR spectra was tested. In the three soils studied, no micropores within the mineral phase available for xenon adsorption were found. The most probable reason is that such pores are occupied by small molecules of the soil organic matter (SOM). Variable extent of accessibility of mesopores within the mineral phase of the various soils was revealed. It was highest in the Podzol. Here, xenon exchange between different adsorption zones (i.e., pores of differing size, e.g., internal and external void spaces) was slow on an NMR time scale that allowed to detect separate signals, each characterising xenon behaviour in the respective adsorption zone. The pore system of the soil organic matter was shown to be not accessible for xenon, as it is accepted for N 2 and other nonpolar adsorbates. Based on analysis of the spectra, a model for the possible mutual location of organic matter and iron compounds in natural soils was suggested. According to this model, a certain part of organic matter species can form the layers above iron species, thus masking them and preventing 129 Xe NMR spectra from significant low-field shifts and signal broadening.

Synthesis of dealuminated zeolites NaY and MOR and characterization by diverse methodologies: 27Al and 29Si MAS NMR, XRD, and temperature dependent 129Xe NMR

Zeolites NaY and mordenite have been dealuminated by SiCl4 and steaming methods and carefully characterized during and after the various synthetic stages by 27Al and 29Si magic angle spinning nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) to ensure sample quality. 129Xe NMR spectra for xenon adsorbed in NaX, siliceous Y, mordenite, and dealuminated mordenite have been obtained for very low xenon loadings and as a function of temperature. Data obtained for faujasites indicate that 129Xe NMR chemical shifts are very sensitive to the Si/Al ratio reflecting the presence or absence of strong adsorption sites. 129Xe NMR chemical shift data for the mordenite samples show that Xe dwells preferentially in the side pockets which do not seem to be affected by the dealumination process. In addition, the 129Xe NMR data clearly reveal information not available from 29Si and 27Al NMR or XRD regarding structural integrity of the zeolites. The findings presented here point to the need for characterization of siliceous zeolites by several techniques before conclusions regarding zeolite structural integrity can be drawn.

Adsorption of xenon and CH4 mixtures in zeolite NaA. 129Xe NMR and grand canonical Monte Carlo simulations

Investigation of competitive adsorption is carried out using the Xe–CH4 mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the CH4 molecules are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of nine samples of varying Xe and CH4 loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average methane and xenon occupancies. The distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of CH4 molecules are obtained directly from the relative intensities of the Xen peaks. From the observed 129Xe chemical shift of each Xen peak can be obtained the average number of CH4 molecules in the same cavity as n Xe atoms. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and CH4 in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all nine samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 80 to 230 ppm and are in good agreement with experiment. We also consider a very simple strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component molecules are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–CH4 system are compared. The ideal adsorbed solution theory gives a first approximation to the selectivity of the adsorption of the Xe and CH4 from a mixture of gases, but starts to fail at high total pressures, especially at low CH4 mole fraction in the bulk.

Perfluorocarbon emulsions as intravenous delivery media for hyperpolarized xenon.

The use of perfluorooctyl bromide (PFOB) emulsions as delivery media for hyperpolarized xenon has been investigated. Emulsion droplet size was controlled by varying the content of egg yolk phospholipid (EYP), which served as an emulsifier. Hyperpolarized 129Xe nuclear magnetic resonance (NMR) spectra of the dissolved gas were obtained. The NMR spectra were found to be correlated strongly with the emulsion droplet size distribution. The NMR line width is determined by xenon exchange between the PFOB droplets and the aqueous environment. Our findings show that, in a 1.5-Tesla field, relatively narrow 129Xe NMR spectra are obtained for droplet sizes larger than 5 microm. Preliminary results on animal models show that PFOB emulsions have potential as hyperpolarized 129Xe carriers for in vivo magnetic resonance applications.

29Si and 129Xe NMR of Mn 2+ doped silica xerogels

Abstract Paramagnetic Mn2+ ions were incorporated in silica gel during the preparation of the sol. Doped silica xerogels were obtained by heat treatment at different temperatures and studied by 29Si and 129Xe nuclear magnetic resonance (NMR) experiments. Comparison between 29Si NMR magic angle spinning (MAS) spectra of doped and undoped xerogels showed that there is a significant effect of the paramagnetic probe on Q2, Q3 and Q4 peaks. Reduction of the 29Si-T1 relaxation time in the presence of the Mn2+ probe was observed. 129Xe NMR spectra were recorded on the xerogels at variable Xe pressures. Results were consistent with 29Si NMR data and confirmed the localization of the paramagnetic probe in surface sites as shown by electron paramagnetic resonance (EPR) of Mn2+.

Competitive adsorption of xenon and argon in zeolite NaA. 129Xe nuclear magnetic resonance studies and grand canonical Monte Carlo simulations

Investigation of competitive adsorption is carried out using the Xe–Ar mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the Ar atoms are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of 12 samples of varying Xe and Ar loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average argon and xenon occupancies. The detailed distributions, f(XenArm), the fractions of cages containing n Xe atoms and m Ar atoms cannot be observed directly in this system, that is, individual peaks for XenArm mixed clusters are not observed in the NMR spectrum. This information is, however, convoluted into the observed 129Xe chemical shifts for the Xen peaks and the distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of Ar atoms, obtained from their relative intensities. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Ar in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all 12 samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 75 to 270 ppm and are in good agreement with experiment. The GCMC results are compared with a strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model introduced here provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–Ar system are compared.

Competitive adsorption of xenon and krypton in zeolite NaA: 129Xe nuclear magnetic resonance studies and grand canonical Monte Carlo simulations

Investigation of competitive adsorption is carried out using the Xe–Kr mixture in zeolite NaA as a model system. The XenKrm clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the nuclear magnetic resonance (NMR) spectrum for the clusters. The 129Xe nuclear magnetic resonance spectra of several samples of varying Xe and Kr loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average krypton and xenon occupancies. The detailed distributions, f(XenKrm), the fractions of cages containing n Xe atoms and m Kr atoms can be observed directly in this system from the relative intensities since individual peaks for XenKrm mixed clusters are observed in the NMR spectrum. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Kr in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts. The agreement with experiment is excellent. The calculated absolute chemical shifts for the Xen peaks and XenKr peaks at 300 K are in good agreement with experiment. A strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion provides a limiting case for the distributions, with which the GCMC simulations and the properties of the actual Xe–Kr system may be compared. The selectivity coefficients of the Xe–Kr mixture in zeolite NaA is well described by the ideal adsorbed solution model.

129Xe nuclear magnetic resonance studies of supported platinum-alumina catalysts

129Xe nuclear magnetic resonance spectra of xenon adsorbed on supported platinum-alumina catalysts have been examined. The state of metallic platinum particles (Pto) was shown to be different for Pt/Al2O3 and Pt/Al2O3(Cl) samples. Some assumptions about the mutual arrangement of supported metallic particles have been made.