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J. Phys. Chem. B 2000, 104, 7306-7310
Applications of Hyperpolarized Xenon to Diffusion in Vycor Porous Glass† Igor L. Moudrakovski,§ Anivis Sanchez,§,| Christopher I. Ratcliffe,§ and John A. Ripmeester*,§,| Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario K1A 0R6, Canada, and Department of Chemistry, Carleton UniVersity, Ottawa, Ontario K1S 5B6, Canada ReceiVed: March 1, 2000; In Final Form: May 9, 2000
Applications of hyperpolarized xenon to the measurement of diffusion in porous materials were tested on porous Vycor glass. Ingression of hyperpolarized xenon into Vycor was followed both by measuring the changes in intensity of adsorbed xenon and by measuring the distribution in a selected slice of Vycor by NMR imaging. The resulting diffusion coefficients, 2.4 × 10-8 and 2.2 × 10-8 m2/s, respectively, compared well with each other, as well as with results obtained from pulsed field gradient measurements with thermally polarized xenon, 1.9 × 10-8 m2/s. Temperature-dependent measurements were used to derive an activation energy for diffusion of 4.1 kJ/mol. The experiments with hyperpolarized xenon gave large savings in experimental time.
Introduction The development of optical pumping techniques for the production of large nonequilibrium magnetization in noble gases1 has made possible a wide variety of new NMR and MRI applications. The most spectacular achievements in this area have taken place in medical applications of magnetic resonance imaging with hyper-polarized (abbreviated to HP from here on) 3He 2 and 129Xe,3,4 where much effort has been expended to develop new diagnostic tools for the study of pulmonary diseases.4 The application of spectroscopy with HP noble gases to the material sciences is another rapidly growing area. Here the main emphasis has been on the transfer of high nonequilibrium polarization from the noble gas to other nuclei, especially on surfaces, with the aim of increasing the total sensitivity of NMR spectroscopy in surface applications.5-8 Several different approaches to transferring polarization have been explored, including low field level-crossing,5 high field cross-polarization6 and the so-called SPINOE (spin polarization induced Nuclear Overhauser Effect) transfer.7,8 The high sensitivity of HP xenon also has been used to probe low surface area materials with 129Xe NMR 9,10 where the high sensitivity of the chemical shift to the environment of the xenon atom was exploited. Some efforts to carry out NMR imaging of materials using HP gases have also been undertaken.11,12 Several other recent publications that can be mentioned are the works by Brunner et al.13 and Kaiser et al.,14 on the visualization of gas flow and the imaging of porous materials using HP xenon produced in a closed circulating system, and by Mair et al.,15 where HP Xe was used to evaluate time dependent gas diffusion in glass bead packs. Xenon has a number of properties that make it an attractive probe of pore space, namely its chemical inertness, wide range of chemical shifts and high sensitivity to the local environment. Xenon, with a diameter of 4.4 Å, is thus a very good model †
Issued as NRCC no: 43842. * Author for correspondence. Mailing address: 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada. Phone: (613) 993 2011. Fax: (613) 998 7833. E-mail:
[email protected]. § National Research Council. | Carleton University.
gas for studying the diffusion of small hydrocarbon molecules of similar size such as methane, also for the evaluation of the accessibility of pores and their interconnectivity. Diffusion can dramatically affect processes in microporous materials, for instance, in connection with chemical reactors for heterogeneous catalysis. Whereas the general theory of diffusion is well developed, the accurate prediction of diffusion properties, particularly of heterogeneous materials, is still a very complicated task, and reliable data concerning diffusion usually are obtained from experimental work. Several techniques, including NMR, are currently used in studies of diffusion. In the current study we explore the diffusion of xenon gas in Vycor porous glass using HP 129Xe. Vycor is a good choice as, first, it is a heterogeneous system with a relatively narrow distribution of interconnecting pores, and second, it is available in large pieces (on a scale of cm), for which the effects of interparticle diffusion will be minimal. Third, chemical reactors made of Vycor porous glass recently have received considerable interest for the oxidative coupling of methane 17 and from the point of view of membrane reactors in general.18,19 In such a reactor, the rate of oxygen transfer to a catalyst can be controlled accurately, resulting in a large effect on the product selectivity.17,18 Evaluation of the diffusion properties of the reactor’s materials is an important step in process optimization and improvement. Some effort, including 129Xe NMR and MD simulations, has been expended in characterizing the dynamics of gas in porous Vycor,20,21 although we note that, so far, only estimates of the diffusion coefficient of xenon in Vycor are available without there being any experimental data. Two different approaches to probing diffusion with HP xenon have been tested in this work. The first consisted simply of measuring the uptake of HP xenon by the porous glass. For the second, we used 1-D NMR imaging to register directly the distribution of the gas in the porous glass during adsorption. The combination of NMR imaging with the high sensitivity of HP xenon permitted us to visualize the diffusion directly. The results obtained are compared with Pulsed Field Gradient (PFG) measurements, also performed in the course of this work. These new applications further extend the utility of xenon NMR to the study of the structure of adsorbents and catalysts.
10.1021/jp000812h CCC: $19.00 Published 2000 by the American Chemical Society Published on Web 06/29/2000
Xe Applied to Diffusion in Porous Glass
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7307
Figure 1. 1D imaging pulse sequence.
Experimental Section The Vycor porous glass (Corning 7930) used in this work has a mean pore size of 42 Å and a specific surface area of 200 m2/g (N2 BET). The sample was machined as a 36 mm long cylinder of 7.35 mm i.d. Care was taken to preserve the integrity of the glass during machining. The porous Vycor, as purchased, contained large amounts of organic impurities. Before use, it was cleaned by boiling for 24-48 h in a 1:1 mixture of concentrated H2SO4 and 30% H2O2, then washed for 3 days in fresh lots of deionized water, slowly dried in air and then calcined at 550 °C in flowing oxygen. Before experimental work, the sample was evacuated at 500 °C for 12 h. Xenon gas (99.95% pure, purchased from Matheson) with the natural isotope distribution of 129Xe (26.4%) was polarized in batch mode using spin-exchange with optically pumped Rb in cylindrical cells equipped with high vacuum Teflon valves, in much the same way as described by earlier workers.1 The HP xenon was then transferred to the samples on a glass vacuum line assembled directly in the bore of the magnet as described in ref 10. Polarization levels of about 6-7% were achieved, as found from the comparison of signal intensities with those from sealed samples of xenon-oxygen mixtures. Two sealed samples of xenon in Vycor were also made for conventional PFG measurements using thermally polarized xenon. The equilibrium pressure of xenon in these two samples was 700 and 2600 Torr and about 50 Torr of oxygen was added to each to reduce the relaxation time. The Vycor for these samples was in the form of 2-3 mm chunks. All spectra and MR images were obtained on a Bruker DSX400 instrument (magnetic field 9.4 T, 129Xe-resonance frequency 110.7 MHz) equipped with Bruker microimaging accessories. A 10 mm birdcage resonator and water-cooled gradient system, capable of producing a maximum gradient of 1 T/m, were used. The uptake of HP xenon was sampled in a single scan with short (
