PCCP: high pressure 129Xe NMR to probe effects of pressure and density

Phys. Chem. Chem. Phys., 2006, (Advance Article)
DOI: 10.1039/b604633a

Local structure of xenon adsorbed in the nanospaces of zeolites as studied by high-pressure 129Xe NMR

Hironori Omi, Takahiro Ueda, Noriko Kato, Keisuke Miyakubo and Taro Eguchia

aDepartment of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan

bThe Museum of Osaka University, Toyonaka, Osaka, 560-0043, Japan. E-mail: ueda@museum.osaka-u.ac.jp; Fax: +81-6-6850-5785; Tel: +81-6-6850-5778
Received (in CAMBS) 30th March 2006, Accepted 30th May 2006

Pressure (0–10 MPa) and local density dependence of 129Xe NMR chemical shift of xenon in various microporous materials was investigated using an in situ high-pressure probe. The density dependence of the chemical shift was analyzed using virial expansion of the chemical shift by xenon density. Results indicate that the second virial coefficient depends on the pore size and shape, and that the void space affects xenon–xenon interaction in both microporous and mesoporous materials. Furthermore, to interpret the magnitude of the virial coefficient in terms of the local structure of the adsorbed xenon, we analyzed the local structure of adsorbed xenon in molecular sieve 5A using Xen clusters, thereby allowing description of the density dependence of the chemical shift. We also demonstrated the cluster models validity by applying it to molecular sieves 13X and ZSM-5. The latter showed that the adsorbed xenon exists as a xenon monomer up to the filling of about 0.6 in micropores. Larger xenon clusters up to n= 4 have been grown with increasing filling of xenon. According to analyses using the Xen cluster model, the second virial coefficient is related closely with the xenon cluster size, which contributes greatly to the chemical shift in the low loading region.