Tuning the Adsorption Properties of Metal-Organic Frameworks through Coadsorbed Ammonia.

In this work, we report a novel strategy to increase the gas adsorption selectivity of metal organic framework materials by coadsorbing another molecular species. Specifically, we find that addition of tightly bound NH3 molecules in the well-known metal-organic framework MOF-74 dramatically alters its adsorption behavior of C2H2 and C2H4. Combining in situ infrared spectroscopy and ab initio calculations, we find that-as a result of coadsorbed NH3 molecules attaching to the open metal sites-C2H2 binds more strongly and diffuses much faster than C2H4, occupying the available space adjacent to metal-bound NH3 molecules. Most remarkably, C2H4 is now almost completely excluded from entering the MOF once C2H2 has been loaded. This finding dispels the widespread belief that strongly coadsorbed species in nanoporous materials always undermine their performance in adsorbing or separating weakly bound target molecules. Furthermore, it suggests a new route to tune the adsorption behavior of MOF materials through harnessing the interactions among coadsorbed guests.