Heteroligand Zr-MOFs Explored by Xenon: Active Site Recognition by Synchrotron PXRD, Hyperpolarization NMR, and GCMC Simulations

The formation of porous crystalline structures of a series of Zr-MOFs comprising two coligands (dimethyl-terphenylene and diphenylanthracene dicarboxylates, TP and DPA, respectively) with varying ratios offered the opportunity to investigate systematically the modulation of the accessible pore volumes. These MOF structures exhibit large octahedral cavities of common diameter, while the tetrahedral cavity sizes vary in the series and are gradually reduced with increasing anthracene-based ligand. At low loadings, xenon atoms concentrate in the most energetically favorable corners of the tetrahedral cavities. The application of multiple investigation techniques, such as synchrotron-radiation X-ray diffraction of Xe-loaded crystalline powders, Xe adsorption isotherms, and grand canonical Monte Carlo (GCMC) simulations, allows for determining gas location and interaction energy with the pore walls. The remarkable Xe adsorption energy of 25.4 kJ/mol at low coverage was obtained for the more effective confinement-sites of homoligand Zr-DPA. Additionally, hyperpolarized Xe-129 NMR chemical shifts, collected at very low Xe partial pressure (2% Xe dilution), increased from 78.5 to 94.0 ppm, with increasing DPA ligand content. GCMC sorption simulations of the gas mixture with the same composition as that used for the hyperpolarized NMR experiments show remarkable Xe capture and 99% Xe located in the tetrahedral corners.