Hyper-polarized 129Xe NMR Studies on the Pore Characteristics of Zeolitic Imidazolate Frameworks

Abstract

Metal-organic frameworks (MOFs) are hybrid porous materials that are formed by the coordination of metal ions with organic linkers. MOFs with various pore sizes, dimensions and morphologies have been materialized, and their performance such as storage, separation and catalysis for gases has been well recognized. In particular, zeolitic imidazolate frameworks (ZIFs), which adopt the various topologies observed in zeolites, are an interesting subgroup of MOFs due to their high porosity and selectivity of CO2 over other gases like CH4, N2, and CO. Xenon is one of the most useful nuclei to probe the pore property and local structure of porous materials, mainly due to its ability to penetrate almost any porous materials. In addition, the use of hyper-polarized (HP) 129Xe achieved by optical pumping can greatly enhanced the sensitivity of 129Xe NMR. Thus, the optical pumping system for HP 129Xe NMR was constructed and optimized in this study, and the pore characteristics of various ZIFs were probed by using the HP 129Xe NMR technique. HP 129Xe NMR was applied to ZIF-68 and ZIF-69 with a nanochannel having wave-like shape. HP 129Xe NMR spectra of two ZIFs are significantly different, where ZIF-68 gave rise to an isotropic peak whereas ZIF-69 showed a powder pattern. This result could be caused by their different pore and aperture sizes, and indicates that xenon is a good probe molecule to discriminate ZIFs with different nanochannel. Especially, the 129Xe NMR powder pattern of ZIF-69 corresponding to a wave-like nanochannel of small size shows quite different behavior with temperature and pressure from other nanochannels with smooth shape, presumably due to its unique shape of the interior wall of nanochannel. ZIF-8, ZIF-76 and ZIF-95 are porous ZIFs with cages and have been studied for the thermodynamics and kinetics of xenon adsorption. The variation of 129Xe chemical shift with temperature showed a similar pattern for all three ZIFs, indicating that they have a similar thermodynamic property for xenon adsorption. However, HP 129Xe 2D exchange (EXSY) NMR showed that their exchange rates are greatly different with the order of ZIF-76 ？？ ZIF-95 ？？ ZIF-8, suggesting that the effective aperture size might play an important role for the access of a guest molecule in ZIFs. In addition, the characteristic 129Xe chemical shift, ？？S, showed a decided difference corresponding to imidazolate linker on pore surface of ZIFs.