Characterization of the Metal−Organic Framework Compound Cu₃(benzene 1,3,5-tricarboxylate)₂ by Means of ¹²⁹Xe Nuclear Magnetic and Electron Paramagnetic Resonance Spectroscopy

Abstract

129Xe NMR measurements of adsorbed xenon are shown for the first time to be a suitable tool to characterize the porosity and the properties of the metal-organic framework Cu3(BTC)2(H2O)3 (BTC = benzene 1,3,5-tricarboxylate). The NMR experiments are performed at room temperature and over a wide range of xenon pressure and on two different synthesized Cu3(BTC)2 samples. 129Xe NMR results reveal that in dependence on the kind of the synthesis pathway either one or two signals are observed which can be attributed to two kinds of fast exchange of xenon atoms in two pores with different pore sizes. Coadsorption experiments of xenon and ethylene demonstrate that the xenon atoms prefer to fill the greater pores of the material because the smaller pores are occupied with residual molecules from the synthesis procedure and additionally adsorbed ethylene. Besides the NMR experiments a series of electron paramagnetic resonance (EPR) measurements are performed to estimate the state of copper having a strong influence on the chemical shift of the adsorbed xenon. The EPR experiments demonstrate that spin exchange between the interconnected copper dimers is taking place across the BTC linker molecules in the Cu3(BTC)2 framework.

Computational screening of metal-organic frameworks for xenon/krypton separation

“…The smaller pockets have diameters of 0.50 nm, whereas the larger cavities have diameters of 1.06 and 1.24 nm. The window into these small pockets is $0.46 nm, and previous experimental work using 129 Xe NMR spectroscopy 30 has demonstrated that these sites are accessible to xenon. Based on the size of krypton and xenon, whose Lennard-Jones diameters are 0.3636 and 0.4100 nm, respectively, these pockets should be accessible as adsorption sites.…”

Solid-State NMR Spectroscopy of Metal–Organic Framework Compounds (MOFs)

“…Such parameters are the chemical shift, the line width, the chemical shift anisotropy, and the longitudinal relaxation time T 1 . A first application of 129 Xe NMR spectroscopy to MOFs has been reported in 2006 [179]. Subsequently, other authors made use of the favorable properties of 129 Xe NMR in order to study MOFs [23,180,181,182,183].…”

On the importance of a precise crystal structure for simulating gas adsorption in nanoporous materials

“…1). 1,2,8,14,15 The next gas adsorption sites that become occupied are the four windows that lead into this pocket (Fig. Many previous works on HKUST-1 have shown that for Kr and Xe, the most favorable site is at the center of the small pockets in the structure (Fig.…”