Hyperpolarized Xe-129 NMR Spectroscopy Investigation of Metal Cation-Exchanged FAU Zeolites

Continuous-flow hyperpolarized Xe-129 NMR was used to investigate the influence of metal cation-exchanged faujasite (FAU) zeolites on the Xe-129 chemical shift. Xe-129 is a sensitive probe molecule to detect the local electric field in a confined space originating from substituted metal cations with different electronic structures such as Na+, Ag+, Cu+, Cu2+, and Cs+. It is found that Na+ has less effect on the electric field gradient of the supercage in FAU zeolites. The chemical shifts of Xe in NaX and NaY have similar behavior. For cation with the nd(10) electronic structure such as Ag+ or Cu+, there is specific d(pi)-d(pi) interaction between metal cations and the Xe atom, which leads to the upfield of Xe chemical shift. For Cu2+, its partial paramagnetism results in the broadening of Xe NMR spectra or even disappearance of the Xe-129 signal in the supercage. For Cs+, whose cation has the same electronic structure as the Xe atom, it may adsorb more Xe atoms in the supereage of FAU zeolites, leading to the downfield of Xe chemical shift.