129Xe NMR study of 12-tungstophosphoric heteropoly acid supported on silica

The structure of 12-tungstophosphoric heteropoly acid (H₃PW₁₂O₄₀) supported on silica has been studied by¹²⁹Xe NMR of adsorbed xenon. The¹²⁹Xe NMR spectra are found to depend on the surface HPA concentration. The¹²⁹Xe NMR data provide evidence for the presence of an organized microporous structure within HPA overlayers. These results are in agreement with nitrogen adsorption and capillary condensation (77 K) measurements. The high sensitivity of the¹²⁹Xe NMR method and its applicability for testing of silica-based HPA catalysts are demonstrated.A preliminary report of this work was presented to the Second European Congress on Catalysis EUROPACAT-II, Maastricht, The Netherlands, 3–8 September 1995.Deceased.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Bimodal Detection of Proteins by 129Xe NMR and Fluorescence Spectroscopy

A full understanding of biological phenomena involves sensitive and noninvasive detection. Herein, we report the optimization of a probe for intracellular proteins that combines the advantages of fluorescence and hyperpolarized ¹²⁹Xe NMR spectroscopy detection. The fluorescence detection part is composed of six residues containing a tetracysteine tag (−CCXXCC−) genetically incorporated into the protein of interest and of a small organic molecule, CrAsH. CrAsH becomes fluorescent if it binds to the tetracysteine tag. The part of the biosensor that enables detection by means of ¹²⁹Xe NMR spectroscopy, which is linked to the CrAsH moiety by a spacer, is based on a cryptophane core that is fully suited to reversibly host xenon. Three different peptides, containing the tetracysteine tag and four organic biosensors of different stereochemistry, are benchmarked to propose the best couple that is fully suited for the in vitro detection of proteins.     

High‐resolution solid‐state NMR and SEM study of the interaction behavior of poly(ethylene‐co‐vinyl acetate)/poly(vinyl acetate) blends

Binary blends formed by two types of ethylene‐co‐vinyl acetate (EVA), which have different vinyl acetate contents, and poly(vinyl acetate) (PVAc) were prepared in a Haake Rheocord 9000 plastograph. A series of samples were obtained varying the PVAc amount up to 50%. The studies were carried out employing solid‐state nuclear magnetic resonance spectroscopy (NMR) and scanning electronic microscopy (SEM). The xenon‐129 (¹²⁹Xe) and carbon‐13 (¹³C) NMR response together with the microscopy results showed that the systems are heterogeneous. Therefore, EVA with a higher vinyl acetate content presented some interaction between the polymer blend components. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 116–124, 2002     

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.     

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.     

Recent Advances in the Control and Characterization of the Porous Structure of Pillared Clay Catalysts

Several methods have been developed and applied in recent years in the characterization of the structure of microporous materials, in general, and of pillared clays, in particular. In the present article, the latest results obtained in the control of the microstructure developed in alumina‐pillared clays and various approaches used to the evaluation of the porosity of these materials are reviewed. These include the control of the microstructure developed in pillared clays by adjusting the several parameters involved in the synthesis process, and the evaluation of the porosity of pillared clays. Emphasis is given to the methods to evaluate the pore size and pore size distribution (PSD) of these materials. Two separate sections are devoted to review mathematical modeling studies and ¹²⁹Xe NMR spectroscopy of physisorbed Xe to investigate the microporous structural properties of these materials.     

Instability of zeolite Y supported cobalt sulfide hydrotreating catalysts in the presence of H2S

The influence of the sulfidation temperature of dehydrated ion exchanged CoNaY on the catalytic activity and structure was studied by thiophene HDS activity measurements, overall sulfur analysis, temperature programmed sulfidation, Xe adsorption measurements in combination with¹²⁹Xe NMR, EXAFS and ESR. It was shown that up to a sulfidation temperature of 573 K small highly active Co sulfide clusters were formed in the supercages. Sulfidation above 573 K led to decomposition of these Co sulfide particles by a protolysis reaction resulting in the formation of H₂Sand a blue colored Co compound having almost no HDS activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Multinuclear NMR and catalytic investigations of deactivation of zeolite H-ZSM-5

Coke formation and catalyst deactivation by the reaction of methanol over a zeolitic catalyst based on H-ZSM-5 for various stream gases (N₂, H₂ or C₄H₁₀) were studied by¹³C CP MAS NMR,²⁹Si MAS NMR,¹²⁹Xe NMR and catalytic test reactions with different stream gases (N₂, H₂, C₄ H₁₀). The rate of deactivation varies in the order N₂ < H₂ < C₄H₁₀, which can be explained by cracking of paraffinic, olefinic and alkylaromatic coke deposits under the influence of the hydrogenating properties of the stream gas.     

High pressure NMR and electrical conductivity studies in acid form NAFION membranes

The effect of high pressure on the electrical conductivity and proton, deuteron and ¹⁷O NMR in acid form NAFION-117 conditioned at various levels of relative humidity has been determined. NMR measurements of T₁ were carried out as a function of pressure up to 0.25 GPa. The complex impedance studies were made along the plane of the film at frequencies from 10 to 10⁸ Hz at room temperature and pressures up to 0.3 GPa. The electrical conductivity and proton and deuteron NMR measurements show that the activation volume for materials containing greater than about three water molecules per sulfonate is 2–3 cm³ mol⁻¹ and larger values are found as the amount of water decreases. The implications of these results are discussed. The ¹⁷O NMR results are unexplained at the present time.     

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.     

Kinetics, acid sites and deactivation of H-mordenite during the SCR of NOx with CH4

H-mordenites are active for the SCR reaction but they suffer irreversible partial deactivation after being on stream for one hour at 650°C. The reaction orders and activation energies are not significantly affected by deactivation. This indicates that deactivation originates in a decrease in the number of active sites due to dealumination and possible pore blockage. The NO disappearance rate correlates with TPD NH₃ between 300 and 700°C; FTIR confirms these results. ¹²⁹Xe NMR of adsorbed xenon shows that pore blockage occurs and is due to the presence of aluminum species in the main zeolite channels. The overall deactivation process and the role of acid sites is discussed in terms of the current literature. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.