129Xe - NMR

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

X. H. Ren, M. Bertmer, H. Kuhn, S. Stapf, D. E. Demco, B. Blumich, C. Kem, A. Jess 2002, ( H, 13C and 129Xe NMR study of changing pore size and tortuosity during deactivation and decoking of a naphtha reforming catalyst), NATO Sci. Ser. ITMath., Phys. Chem. 7b, 603. 

R. W. Mair, R. L. Walsworth 2005, (Study of gas-fluidization dynamics with laser-polarized 129Xe), Magn. Reson. /mag. 23, 203-207 (b) R. Wang 2005, (Study of gas flow dynamics in porous and granular media with laser-polarized 129Xe NMR), Ph.D. Thesis, Department of Nuclear Engineering, Massachusetts Institute of Technology, February 2005. 

Hyperpolarized 129Xe NMR Spectroscopy, MRI and Dynamic NMR Microscopy for the In Situ Monitoring of Gas Dynamics in Opaque Media Including Combustion Processes... 

One of the resulting 129Xe NMR spectra is shown in Figure 5.3.8 (solid line 2) in comparison with the spectrum of the same initial mixture without combustion (dashed line 1). Referenced with 0 ppm is the gas phase peak at room temperature. 

Fig. 5.3.8 Photograph of the detection region of the NMR probe with radiofrequency coil. A methane—air mixture was ignited above the zeolite pellets. The mixture also contained xenon for NMR detection. Hp-129Xe NMR spectra with 30% xenon (from high-density xenon optical pumping) in 70% methane is depicted. (1) The spectrum in the absence of combustion and (2) the spectrum during combustion. Adapted from Ref. [2],...

Fig. 5.3.10 (A) Polarization obtained contin- lasers (squares). (B) Actual intensities as uous-flow hp-xenon experiment as a function of measured by 129Xe NMR spectroscopy at xenon partial pressure for two different laser 110.69 MHz using 29-W laser power (triangles) powers, i.e., one 30-W diode array laser (tri- and full laser power (squares). Adapted from angles) and two combined 30-W diode array Ref. [16].

Figure 1. Top Original "tennis ball" monomer 1, its syn- the guests and 129Xe-NMR spectrum of a solution of 1 1 thesis, and analogs 2 and 3. Bottom left X-ray structure of with Xe added. Due to the aromatic centerpieces of the cap-...

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

Important NMR data of the pentafluorophenylxenonium cation in aHF are presented in Figure 3. The resonances of the fluorine atoms F2,6 contained well separated 129Xe-satellites (20). The V(Xe,F) and 5J(Xe,F) coupling had to be determined from the 129Xe NMR signal. It is worth mentioning, that the coupling constant ./(C-l,Xe) in aHF (85 Hz) is remarkably smaller than in MeCN (113 Hz). 

Moudrakovski, I.L. Sanchez, A.A. Ratcliffe, C.I. Ripmeester, J.A. (2001). Nucleation and Growth of Hydrates on Ice Surfaces New Insights from 129Xe NMR Experiments with Hyperpolarized Xenon. J. Phys. Chem. B, 105, 12338-12347. 

A number of recent developments in 129Xe NMR spectroscopy are presented with direct applications to the study of mesopore space in solids. This includes the establishment of a relationship between pore size and chemical shifts for a number of controlled pore glasses and the exploration of hyperpolarized (HP) xenon for a number of NMR and microimaging applications to porous solids. With HP xenon, the increase in experimental sensitivity is remarkable. Experiments illustrated include the rapid characterization of the void space in porous solids, including the in-situ study of processes such as diffusion and dehydration, and imaging with chemical shift resolution. 

The 129Xe NMR spectra for xenon in contact with the CPG samples consisted of a gas line near 0 ppm, a low field line characteristic of xenon in the pores, and, in some cases, a weak line at an intermediate position characteristic of a population of xenon atoms... 

Figure 2. Time dependence of 129Xe NMR spectrum (left) and intensity (inset) as HP Xe diffuses into a cylinder of porous Vycor glass (right). The stronger line close to 0 ppm is due to Xe gas.

Fig. 4 shows a series of 129Xe NMR spectra taken from flowing HP gas in contact with a sample of pillared montmorillonite in the spinner of a MAS NMR probe. In the air-dry sample there is a weak line beside the gas line ( 0ppm) that can be attributed to xenon in... 

Figure 4. Variable temperature flowing HP 129Xe NMR MAS spectra of xenon adsorbed in montmorillonite pillared with pyridinium ions. The Xe/N2/He mixture flow rate was 300 cc/min. The sequence of temperature steps progresses from bottom to top. The spectrum at 373K was after drying in a stream of flowing helium for 3 hrs at 373K.

Fig. 69. 129Xe NMR spectra at 24.9 MHz of xenon adsorbed on zeolite Na-Y containing finely dispersed metal particles with and without preadsorbed ethylene (341). Spectrum 1, Pt4-Na-Y 2, Pt4-Na-Y + C2H4 at 25°C 3, Pt4-Na-Y + C2H4 at 60°C. Chemical shifts are in ppm from xenon gas at zero pressure.

Hydrate structures containing hydrate former + Xe help gas have been confirmed by 129Xe NMR spectroscopy by Ripmeester and Ratcliffe (1990a). 

Figure 6.12 Determination of ratios of xenon atoms in large and small hydrate cages using 129Xe NMR Spectroscopy at77 K. (Reproduced fromRipmeester, J.A., Ratcliffe, C.I., J. Phys. Chem., 94, 8773 (1990).

---