129Xe NMR to probe microscopic mixing state of composite catalysts

¹²⁹Xe NMR spectra obtained from microporous solid mixtures such as NaY-Pt/NaY and NaY-Pt/Al₂O₃ have indicated that the NMR peak pattern can depend greatly on the mixing method.¹²⁹Xe NMR can thus be a very simple and useful technique to probe the physical mixing state of many composite catalysts.     

Analysis of gas transport properties of PPO/PS blends by Xe NMR spectroscopy

Relationships among variations of microvoids and gas transport properties for miscible poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)/polystyrene (PS) blends in the glassy state have been investigated by Xe sorption, Xe permeation, and ¹²⁹Xe NMR measurements. Xe sorption isotherms of the blends can be interpreted successfully on the basis of the dual-mode sorption model. Decrease in the permeability of Xe is attributed to the decrease in the diffusivity of that in the Langmuir site. ¹²⁹Xe NMR spectra of ¹²⁹Xe in the blends show non-linear low-field shift with increasing sorption amount of Xe because of a fast exchange of Xe atoms between Henry and Langmuir sites. From the analysis of ¹²⁹Xe NMR chemical shifts, it is found that the mean volume of individual microvoids varies with a negative deviation against volume fraction of PPO in the blend. For PPO/PS blends, it has been clarified that the contraction of individual microvoids occurs by blending and highly affects gas transport properties.     

129Xe NMR spectroscopy on molecular sieves SAPO-37, AIPO4-5, SAPO-5 and SSZ-24

Xe NMR spectroscopy is used to study SAPO-37, NaY, AlPO₄-5, SAPO-5, and SSZ-24. Of these samples, those with FAU topology show that the¹²⁹Xe chemical shift is dependent upon framework composition while the materials with API topology do not. Values of the¹²⁹Xe NMR chemical shifts at zero xenon pressure cannot be correlated with the size of the framework void space.     

Proton sites in Keggin heteropoly acids from17O NMR

The structure of the bulk proton sites in dehydrated solid Keggin-type heteropoly acids (HPA) H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ was determined by¹⁷O NMR. The terminal W=O oxygen atoms in Keggin heteropolyanions were found to be the predominant protonation sites. The protonation of HP As in solution is also discussed on the basis of¹⁷O NMR data.     

Comparison of (non)-sulfided NiNaY zeolite catalysts prepared by ion-exchange and impregnation by xenon adsorption and129Xe NMR

Ni²⁺ exchanged and NiCl₂ impregnated non-sulfided and sulfided NaY zeolites were characterized by xenon adsorption isotherms,¹²⁹ Xe NMR and thiophene hydrodesulfurization. The nickel species are located mainly inside the micropores of the zeolites in all samples. Ion-exchange results in a more homogeneous distribution of these species than impregnation. This explains their lower hydrodesulfurization activity compared to the ion-exchanged samples.On leave of absence from the Institute of Isotopes, Budapest, Hungary.     

17O NMR determination of proton sites in solid heteropoly acid H3PW12O40.31P,29Si and17O NMR,FT-IR and XRD study of H3PW12O40 and H4SiW12O40 supported on carbon

¹⁷O MAS NMR spectra for solid heteropoly acid (HPA) H₃PW₁₂O₄₀ are reported. Comparison of solid-state and solution¹⁷O resonances shows that in the solid dehydrated H₃PW₁₂O₄₀ terminal W=O oxygen atoms are the predominant protonation sites. H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ supported on chemically activated carbon have been studied by means of NMR, FT-IR and XRD. The carbon-supported HPA's retain their Keggin structure and form finely dispersed HPA species. No HPA crystal phase is developed even at an HPA loading as high as 45 wt%.³¹P,²⁹Si and¹⁷O MAS NMR spectra for bulk and carbon-supported HPA's indicate interaction of the HPA Keggin units with the carbon surface, causing large line broadening in the NMR spectra.     

First evidence of interconnected micro and mesopores in CMK-3 materials

The porous structure and the pore interconnections of mesoporous carbon of CMK-3 type have been studied by means of hyperpolarized ¹²⁹Xe NMR spectroscopy.Parallel studies of purely microporous carbon and CMK-3 have unambiguously revealed the presence of micropores inside the CMK-3 structure. In addition, ¹²⁹Xe 2D-exchange NMR experiments have clearly shown direct exchange of Xe atoms between the micro and the mesoporosity. This indicates the presence of micropores inside the carbon rods constituting the mesoporous structure of CMK-3 materials.     

Number of metallic clusters in Y zeolites obtained from129Xe NMR

We present a simple model which permits to obtain the number of metallic clusters entrapped in Y zeolites from the curves of the chemical shifts of¹²⁹Xe NMR against the pressure of Xe. From the metal loading the average number of atoms per cluster can also be calculated. We apply the model to Pt/NaY and the bimetallic Pt-Cu/NaY.     

Quantitative high-resolution13C and1H nuclear magnetic resonance of ω3 fatty acids from white muscle of atlantic salmon (Salmo salar)

High-resolution¹³C nuclear magnetic resonance (NMR) spectra have been obtained and used to define the ω3 (n-3) fatty acid distribution in lipid extract and white muscle from Atlantic salmon (Salmo salar). The¹³C spectrum of lipid extracted from muscle gives quantitative information about the individual n-3 fatty acids, 18:2n-6, 20:1/22:1 and groups of fatty acids. The quantitative data compare favorably with those obtained by gas-liquid chromatography. The¹H NMR spectrum of the lipid extract gives information about the amount of 22:6n-3 and the total content of n-3 fatty acids. The¹³C NMR technique also revealed the positional distribution (1,3- and 2-acyl) of the important 20:5n-3 and 22:6n-3 acids in the triacylglycerol molecules. In the quantitative¹³C NMR spectrum of white muscle, the methyl region of the acyl chains of triacylglycerols gave rise to sufficiently resolved signals to permit estimation of the total concentration of lipids and the n-3 fatty acid content. The NMR data are in good agreement with corresponding data obtained by traditional methods.     

Chemisorption of H2 on supported Pt clusters probed by129Xe NMR

The surface of Pt clusters with average size between 1 and 8 nm supported on SiO₂, -Al₂O₃ or Y-zeolite was probed by¹²⁹Xe NMR as the Pt surface coverage with hydrogen, , was increased. A distinct change in the structure of the hydrogen overlayer at 0.3 was inferred from the NMR spectra. This change is believed to take place when the chemisorbed hydrogen fills all the next nearest neighbor metal sites and the nearest neighbors start to be occupied. These new observations clarify previously reported determinations of the average number of Pt atoms in supported clusters by means of Xe NMR and other techniques. It also appears that interfacial metal-support interactions may be probed by Xe NMR.     

Effect of pretreatment temperature on the surface modification of diatomite with trimethylchlorosilane

Diatomite samples from Costa Rica were purified using acidic treatments with hydrochloric acid, thermally treated (400–1000 °C) and then silylated with trimethylchlorosilane in toluene under inert atmosphere. The purification process allows to decrease the concentration of metals presented in the crude diatomite, as is confirmed by X-ray Fluorescence (XRF) Analysis. The silylated materials were analyzed by using Hyperpolarized ¹²⁹Xe Nuclear Magnetic Resonance Spectroscopy (HP ¹²⁹Xe NMR), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), rehydration tests, and contact angle measurements. XRD measurements indicate that diatomite is mainly amorphous, but presents several crystalline phases (kaolinite, cristobalite, and quartz). Pretreatments at high temperatures cause changes in those crystalline phases, resulting in more amorphous materials. However, there is no difference in the overall structure of purified and thermally treated diatomite samples with respect to the silylation products. In addition, SEM measurements show no effect over the pore structure of the materials. On the other hand, TGA measurements and rehydration tests show lower losses of water for silylated materials prepared using higher pretreatment temperatures. Moreover, HP ¹²⁹Xe NMR, FTIR, and contact angle measurements evidence a modification due to covalent attachment of Si(CH₃)₃-groups to the surface, which increases for higher pretreatment temperatures. The results provide valuable information about external factors that influence the surface modification of diatomite. This can be useful to control modifications that can be achieved in a similar way.     

Zeolite ZnY catalysts prepared by solid-state ion exchange

Zeolites ZnY with various overall zinc contents were prepared from mixtures of zeolite NH₄Y and crystalline zinc chloride by solid-state ion exchange. The obtained materials were investigated with carbon monoxide, xenon, and nitrogen adsorption as well as with ¹²⁹Xe NMR and XRF spectroscopy. From the results of these measurements, the zinc cation distributions between the different types of cages of the faujasite framework as well as between the crystallographic positions SIII and SII within the large voids (supercages) were quantitatively determined. The concentrations of zinc cations in the supercages of the presently prepared zeolites are considerably higher than in materials obtained from NaY by conventional wet ion exchange using aqueous zinc salt solutions. Experimental evidence is provided for salt inclusion under certain conditions of preparation. This revised version was published online in August 2006 with corrections to the Cover Date.     

129Xe NMR as a probe of the effect of crosslinking on the amorphous phase structure of solid polymers

Summary ¹²⁹Xe NMR of xenon adsorbed in a solid EPDM rubber shows the presence of at least four distinct regions in the amorphous phase of this polymer. Changes in the relative amounts of these environments are observed upon crosslinking indicating that changes in the amorphous phase structure of the polymer have occurred.¹²⁹Xe NMR provides a direct probe of the formation of a more condensed amorphous phase of the polymer matrix due to the crosslinking reaction.     

Solid-state nuclear magnetic resonance (NMR): Application to precursors of ceramics—Polycarbosilane

The chemical structures of oxidation- and electron irradiation-cured polycarbosilane fibers has been studied by IR and chemical analysis, but its structure has not been identified in detail. In this work, the chemical structure and curing mechanism was examined by solid-state high-resolution NMR spectroscopy. From the analysis of NMR spectra, it is explained that (i) in oxidation curing of PCS fibers, oxygen attacks first the SiC₃H bond and forms the SiC₃O bond and, next, the SiC₄ backbone bond and forms the SiC₂O₂ units; (ii) in electron irradiation curing, the signal intensity of SiC₃H units decreases with the increase in dose, the increase in the signal being due to the formation of SiC₄ units; (iii) solid-state²⁹Si high-resolution (CP/MAS) NMR spectroscopy is a powerful tool for investigating the chemical structure and curing mechanism of PCS fibers complementing infrared and solid-state¹³C high-resolution NMR spectroscopy; and (iv)¹H CRAMPS NMR spectroscopy is very useful for investigating the chemical structure and curing mechanism of PCS fibers.     

Xe-NMR study of free volume in amorphous perfluorinated polymers: comparsion with other methods

The ¹²⁹Xe-NMR method was used for evaluation of the sizes of free volume elements in amorphous glassy materials—random copolymers of tetrafluoroethylene and perfluorodioxoles of different structures. ¹²⁹Xe chemical shifts were measured at different pressures up to 1060 Torr. The zero pressure chemical shifts obtained by extrapolation were used for calculation of the diameters of microcavities having spherical and cylindrical shapes. The sizes of microcavities are consistent with those found by means of positron annihilation lifetimes and inverse gas chromatography methods.     

A comparison of 2H and 129Xe NMR as a probe of the effect of crosslinking in rubbery materials

The paper describes the investigation of butadiene rubber networks varying in crosslink density by means of xenon (¹²⁹Xe) and deuteron (²H) NMR spectroscopy. ¹²⁹Xe NMR spectra give an evidence of structural inhomogeneities inside the lower crosslinked network which were already observed in previous deuteron NMR studies under uniaxial deformation. The results are discussed in comparison with theoretical models and simulations. It appears that experiment, theory and also simulation are now coming together to give a detailed picture of the behaviour of these rubbery materials.     

Structural analysis of polyacenic semiconductor (PAS) materials with Xenon NMR measurements

Structural analysis of the polyacenic semiconductor (PAS) material prepared by the pyrolysis of phenol-formaldehyde resin at relatively low temperature (680 °C) has been performed by applying ¹²⁹Xe nuclear magnetic resonance (NMR) measurements. One can obtain information on the microporous structure of the PAS material through adsorption of Xe atoms, since a ¹²⁹Xe nucleus is a very sensitive probe of its microscopic environment. All the introduced Xe atoms were adsorbed on the internal surface of the pure PAS sample, which indicated remarkably large surface area of the PAS material. The average pore size of the pure PAS sample has been determined to be 7.7 ± 1.6 Å from the pressure dependence of the Xe NMR chemical shift. In connection with the application of the PAS material to the electrode of the Li rechargeable battery, changes in the Xe NMR spectrum brought about by extrinsic additives such as binder, electrolyte solvent, and the doped Li have been investigated. In particular, it has been found that the Li-doping entirely prevents Xe atoms from entering into the micropores of the PAS material, probably due to adsorption of the solvent molecules on the internal surface of the micropores.     

Investigation by EQCM of the electrosynthesis and the properties of polypyrrole films doped with sulphate ions and/or a Keggin-type heteropolyanion, SiMo12O40

The first part of this paper describes in detail the electrochemical synthesis of polypyrrole (PPy) films monitored by electrochemical quartz crystal microbalance (EQCM). These films are either doped only with a Keggin-type heteropolyanion (HPA), SiMo₁₂O₄₀⁴⁻, or co-doped with such polyanions and sulphate ions. It is evidenced that this HPA catalyses the electropolymerisation of the pyrrole since it is able to oxide pyrrole monomer, and on the other hand, that HPAs are definitively entrapped into the polypyrrole film as shown by their electrochemical response. All the PPy film is electroactive. When the co-doped film is obtained by potentiodynamic method, its anion composition is not homogeneous in depth, there is a concentration gradient of the doping anions. In the second part of this paper, the morphology of the films is shown as well as their electrochemical characterisations that were carried out by EQCM in order to focus on the charge compensation process that occurs during SiMo₁₂ reduction and re-oxidation. In a wide domain of potentials, only the HPA response is observed, the polymer remains in its conducting form. The SiMo₁₂O₄₀⁴⁻ doped PPy films are cationic films. When several types of cations are present in the medium, the influence of the pH on the nature of the cations implied in the charge compensation that occurs during SiMo₁₂ reduction and re-oxidation is important. When the PPy film is co-doped, it is possible to obtain a mixed PPy film. Another important finding is the stability of trapped SiMo₁₂ ions in contact with solutions of pH higher than 4, on the contrary to what occurs when they are in solution.     

13C NMR analysis and gas uptake measurements of pure and mixed gas hydrates: Development of natural gas transport and storage method using gas hydrate

¹³C NMR spectra were obtained for pure CH₄, mixed CH₄+THF, and mixed CH₄+Neohexane hydrates in order to identify hydrate structure and cage occupancy of guest molecules. In contrast to the pure CH₄ hydrates, the NMR spectra of the mixed CH₄+THF hydrate verified that methane molecules could occupy only the small portion of 5¹² cages because the addition of THF, water-soluble guest component, to aqueous solution prevents the complete filling of methane molecules into small cages. Furthermore, from these NMR results one important conclusion can be made that methane molecules can’t be enclathrated at all in the large 5¹²6⁴ cages of structure II. In addition, gas uptake measurements were carried out to determine methane amount consumed during pure and mixed hydrate formation process. The moles of methane captured into pure CH₄ hydrate per mole of water were found to be similar to the full occupancy value, while the moles of methane captured into the mixed CH₄+THF hydrate per moles of water were much lower than the ideal value. The overall results drawn from this study can be usefully applied to storage and transportation of natural gas.     

Preparation of nanosize Pt clusters using ion exchange of Pt(NH3) 4 2+ inside mesoporous channel of MCM-41

Mesoporous molecular sieve MCM-41 with a Si/Al ratio of 35 was obtained by hydrothermal synthesis using a gel mixture with a molar composition of 6 SiO₂0.1 Al₂O₃1 hexadecyltrimethylammonium chloride 0.25 dodecyltrimethylammonium bromide 0.25 tetrapropylammonium bromide0.15 (NH₄)₂O1.5 Na₂O300 H₂O. The MCM-41 sample was calcined in O₂ flow at 813 K and subsequently ion exchanged with Ca²⁺. A small Pt cluster has been supported on the MCM-41 sample following a procedure using ion exchange of Pt(NH₃) ₄ ²⁺ . The Pt(NH₃) ₄ ²⁺ ion supported on MCM-41 has been activated in O₂ flow at 593 K and subsequently reduced with Fh flow at 573 K, in the same way used for the preparation of a Pt cluster entrapped inside the supercage of zeolite NaY. The resulting Pt cluster supported on the MCM-41 shows hydrogen chemisorption oftotal two H atoms per Pt at 296 K (based on the total amount of Pt) and high catalytic activity for hydrogenolysis of ethane. The chemical shift in¹²⁹Xe NMR spectroscopy of adsorbed xenon indicates that the Pt cluster is located inside the mesoporous molecular sieve.