Xenon-129 NMR of xenon adsorbed in Y zeolites at 144 K

Abstract

129Xe NMR spectra of xenon adsorbed in Na-Y, K-Y, Mg-Y, Ca-Y, and B-Y zeolites have been measured as a function of the xenon loading at 144 and 293 K. At low xenon loadings, the 129Xe chemical shift shows two types of dependence on the xenon density in the supercage: linearly increasing and paraboliclike with a minimum. At 144 K, all the zeolites except Ca-Y have a linear dependence, whereas at 293 K, both Ca-Y and Mg-Y show a paraboliclike curvature. These dependences can be explained in terms of rapid exchange between xenon atoms adsorbed at special sites in the supercage and those remaining in the gas phase in the cages. Strong xenon adsorption at the special sites leads to the paraboliclike behavior, whereas weak adsorption yields the linear dependence. Formation of a partial bond between the xenon atom and 2+ cation by donation of one of the xenon 5p electrons to the empty s-orbital of the cation is proposed to explain the strong xenon adsorption in zeolites with 2+ cations. The bond formation introduces low-lying electronic excited states which lead to a large paramagnetic contribution to the chemical shift. By variation of the rate by which a sample is cooled to 144 K, it is shown that there is only one special adsorption site per supercage in the Y zeolites. In Na-Y, Mg-Y, and K-Y, the site can accommodate one xenon atom while in Ca-Y and Ba-Y, two xenon atoms can share the same site. At 144 K, a gas-liquid phase transition is observed when the xenon loading is between 7 and 9 xenon atoms/cage. The transition is characterized by rapid increase in the chemical shift and collapse of the line width. Before the transition, there are large increases in the line width that may be interpreted as the inhomogeneous broadening due to large fluctuations in the xenon number density in the cages. Measurements at 144 K with high loadings of xenon indicate that the supercage in Y zeolite can hold between 10 and 11 xenon atoms. The resulting chemical shift is between 250 and 270 ppm downfield from the resonance of 2 atm of xenon gas (at 293 K). At even higher xenon loading, the excess xenon condenses on the exterior surface of the zeolite microcrystallites to form solid xenon, which has a chemical shift of 304 ppm downfield.

Characterization of Zeolites — Infrared and Nuclear Magnetic Resonance Spectroscopy and X-Ray Diffraction

“…[381] For zeolites exchanged with divalent cations, the 129Xe NMR shift of adsorbed xenon atoms shows a parabola-like dependence on the loading. Assuming an exchange rate between xenon atoms in the gas phase and strongly adsorbed xenon atoms much faster than the 129Xe NMR relaxation rate and using an approach similar to that for the Langmuir isotherm, Cheung et al [390] found the following dependence of the chemical shift 6(T,p): ( 4.49) where 6 8 and ~g (T) are the chemical shift of xenon atoms in the adsorbed phase and the temperature-dependent chemical shift gradient in the gaseous phase, At low levels of divalent cations « 55% exchange) there is a linear relationship between the chemical shifts and the coverage. In these samples the divalent cations are located in the sodalite cages and, therefore, are not accessible for xenon atoms.…”

129Xe-NMR of physisorbed xenon used as a probe for the study of microporous solids

“…Cheung et al [7] also used the fast site exchange model and expressed the chemical shift of xenon adsorbed in Y zeolites at temperature T and concentration p as:…”

Applications of highly spin-polarized xenon in NMR

“…The probability of finding a xenon atom at the surface is given by, rp.easured for xenon in NaY zeolite at 144 K. [ 176] In view of the much smaller heat of adsorption of xenon on the polymer as compared to a zeolite, this large value for 8 5 is surprising. However for NaY 8 5 increases from 58 ppm at room temperature, to 86 ppm at 144 K, the only low temperature measured, so it may increase further at even lower \ temperatures.…”