Heteroligand Zr-MOFs\nExplored by Xenon: Active Site\nRecognition by Synchrotron PXRD, Hyperpolarization NMR, and GCMC Simulations

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