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.