In the Laboratory
129Xe
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NMR of Zeolite NaY in the Inorganic Chemistry Laboratory Tanja Pietraß Department of Chemistry, New Mexico Tech, Socorro, NM 87801;
[email protected] Procedure An NMR tube is prepared by sealing one end a of mediumwalled, 5-mm o.d. Pyrex glass tube, which is then loaded with about 200 mg of zeolite NaY powder. A small plug of clean glass wool prevents the sample from accidentally entering the vacuum system. The sample is evacuated while being heated to 673 K overnight, and a known amount of Xe gas is adsorbed onto the zeolite by immersing the sample tube in liquid nitrogen. The amount of xenon is determined by measuring, on the vacuum rack, the pressure of xenon contained in a predetermined volume. The sample tube is then flame-sealed and can be used in any standard 5-mm high-resolution NMR probe. The students were divided into teams and each team prepared a sample with a different xenon loading. The chemical shifts of these samples were plotted versus xenon loading. Chemical shifts can be referenced against a gas sample of known density (4 ). Extrapolation to zero loading yields the intrinsic chemical shift of the zeolite (2). Techniques The students learn about using a vacuum rack with a nitrogen trap, handling gases, gas adsorption in porous solids, 492
Xe NMR Chemical Shift
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Some classes of inorganic compounds that play an important role in industrial chemistry are too difficult to synthesize with the equipment and time available to an undergraduate laboratory class in inorganic chemistry. Therefore, we chose NMR spectroscopy to introduce our students to the striking properties of those materials, using 129Xe NMR spectroscopy to demonstrate the porous structure of zeolites. Zeolite NaY, an ¯ aluminosilicate, has a cubic structure with space group Fd 3m. Each unit cell contains eight sodalite (truncated octahedron) and eight α-cages. Only the latter, which have an inside diameter of 11.8 Å, are accessible to xenon with its 4.4 Å van der Waals diameter (1) (see top inset in Fig. 1). Strong interactions between the xenon atom and the walls of the cage give rise to a downfield chemical shift when compared to the free gas. Collisions with other xenon atoms further increase the shift, so that the shift characteristic of a given zeolitic material must be obtained from a series of xenon densities and extrapolation to zero (2). This experiment is suitable for students in their senior year, since some prior knowledge of NMR spectroscopy is required. The experiment combines aspects of advanced stoichiometric calculations, the basic physical chemistry of gases, inorganic structural chemistry, and spectroscopy and is thus also ideally suited for an integrated laboratory approach. It can be extended by incorporating powder X-ray crystallography and BET adsorption isotherm analysis (3) to determine the pore size and internal surface area of the zeolite.
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Xe Atoms/α-Cage Figure 1. 129Xe NMR chemical shift of xenon adsorbed in NaY zeolite samples with different Xe loadings at ambient temperature. The linear fit yields an intrinsic chemical shift of 60.5 ppm (Si/Al = 2.4). Top inset: schematic of xenon enclosed in a zeolite structure. Bottom inset: 129Xe NMR spectrum of the sample loaded with three Xe atoms per α-cage, acquired on an Apollo (Tecmag)/Bruker MSL 400 spectrometer at a Larmor frequency of 110.668 MHz for 129Xe (512 scans, 3-s recycle delay, 24-µs pulse width (90° pulse), 40-kHz spectral width).
and 129Xe NMR spectroscopy. Xenon is particularly useful because it freezes readily in liquid nitrogen, and 129Xe has very good NMR properties (nuclear spin I = 1/2, a gyromagnetic ratio slightly larger than that of 13C, and a natural abundance of 26.4%). The students were provided merely with the elemental analysis of the zeolite as obtained from the manufacturer. They had to calculate both the Si/Al ratio and the necessary pressure of xenon in a known volume in order to yield a certain xenon loading per α-cage. Xenon adsorption is fairly slow (several minutes) and can be monitored with a pressure gauge. NMR acquisition is straightforward: a spectrum of satisfactory signal-to-noise ratio can be acquired in about 20 minutes (see bottom inset in Fig. 1). Hazards Zeolite NaY dust is irritating and should not be inhaled. Loading of the zeolite powder into the glass tube is best done in a fume hood. In reference to an empty sample tube, the samples prepared in this experiment all have pressures higher than 1 × 105 Pa, although the presence of the zeolite reduces the equilibrium pressure to very low values. Nevertheless, safety goggles should be worn at all times when handling the sealed samples. We recommend that the samples be transferred
Journal of Chemical Education • Vol. 79 No. 4 April 2002 • JChemEd.chem.wisc.edu
In the Laboratory
directly after sealing into a Styrofoam container precooled with liquid nitrogen, which is then placed in a closed fume hood. Warming of the sample thus occurs slowly and prevents sudden exposure of a potentially fragile seal to high gas pressures. Xenon is an anaesthetic and may affect the nervous system, so inhalation is strongly discouraged.
Equipment
Concepts
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This experiment involves a wide variety of concepts. Evacuating and heating of the zeolite demonstrates the omnipresence of water and can be used to introduce applications of zeolites as drying agents and molecular sieves. Calculating the amount of Xe needed to achieve a certain Xe loading per α-cage is a good example of the application of the ideal gas law. Freezing of the xenon can prompt discussions about phase diagrams and BET adsorption isotherms. Pulsed NMR is an excellent example of a coherent spectroscopy technique based on the principles of quantum theory, and the chemical shift data are used to discuss the structure of zeolites.
Instructions for students and notes for the instructor are available in this issue of JCE Online.
Typical Results The dependence of the chemical shift on xenon loading as obtained by three groups of students is plotted in Figure 1. The inset shows a typical 129Xe NMR spectrum. The intrinsic chemical shift of 60.5 ppm for a Si/Al ratio of 2.4 agrees very well with published data (2).
This experiment requires access to a multinuclear, highfield NMR spectrometer, a vacuum rack with liquid nitrogen trap, pressure gauge, vessel of known volume, and a glassblowing torch. Zeolite NaY is commercially available (e.g. Aldrich), and xenon gas can be purchased from a local supplier. Supplemental Material
Acknowledgments Partial support for this work was provided by the National Science Foundation’s Course, Curriculum and Laboratory Improvement Program under grant DUE-9979271 and by The Camille and Henry Dreyfus Special Grant Program in the Chemical Sciences under grant SG-99-058. Literature Cited 1. Labouriau, A.; Pietraß, T.; Weber, W. A.; Gates, B. C.; Earl, W. L. J. Phys. Chem. 1999, 103, 4323–4329, and references therein. 2. Ito, T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225–5229. 3. Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309–319. 4. Jameson, C. J.; Jameson, A. K.; Cohen, S. M. J. Chem. Phys. 1973, 59, 4540–4546.
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