Effects of Polarizability on the Adsorption of Noble Gases at Low Pressures in Monohalogenated Isoreticular Metal–Organic Frameworks

A systematic investigation of the effects of linker polarizability on the adsorption properties of weakly interacting gases (N2, Ar, Kr, and Xe) is reported. Experimental and simulated adsorption properties were measured for a complete isoreticular series of monohalogenated metal–organic frameworks (MOFs). Variations on IRMOF-2, in which one linker hydrogen is replaced with −F, −Cl, −Br, or −I, comprise the series. Both experimental and simulated results indicate that increasing linker polarizability correlates with increased gas uptake. Evidence of increased adsorbate interaction with increased linker polarizability is also observed in the Kr/N2, Xe/N2, and Xe/Kr selectivity data and in isosteric heats of adsorption. Unexpectedly, comparison between simulated and experimental isotherms reveals that the agreement between the two improves with the size of the adsorbate, with essentially identical results for Xe. This is apparently due to the creation of regions inaccessible to any of the noble gases as a result of halogen functionalization. Simulated adsorption isotherms are also reported for radon, which is difficult to measure experimentally due to its radioactivity.