Local Structure and Xenon Adsorption Behavior of Metal−Organic Framework System [M2(O2CPh)4(pyz)]n (M = Rh and Cu) As Studied with Use of Single-Crystal X-ray Diffraction, Adsorption Isotherm, and Xenon-129 NMR

Abstract

The local structure and xenon adsorption behavior of a metal−organic framework system [M(II)2(bza)4(pyz)]n (bza and pyz = benzoate and pyrazine, M = Rh (1a) and Cu (1b)) were investigated by using single-crystal X-ray diffraction, xenon adsorption isotherm, and 129Xe NMR measurements. Single-crystal X-ray diffraction analysis revealed that rare gas atoms were accommodated in a one-dimensional (1D) nanochannel with the dimer structure. Xenon adsorption reached saturation at the xenon uptake of 1.93 Xe per molecular unit in 1a and of 1.85 Xe in 1b, suggesting accommodation of two Xe atoms per host formula unit. Analysis of the xenon adsorption isotherm based on the Fowler−Guggenheim equation gave the xenon−xenon interaction and the isosteric heat of adsorption. The 129Xe NMR spectrum suggested that the environment of the adsorption site for xenon in 1a is very tight and anisotropic. Furthermore, the temperature dependence of the 129Xe chemical shift was explained by using xenon loading, supporting the xenon dimer as the local structure of xenon in 1a. These aspects revealed that cooperative adsorption of xenon to 1a and 1b occurs according to the xenon−xenon interaction. Xenon is stabilized in the nanochannel through formation of a dimer structure.