Tuning molecular adsorption in metal-organic frameworks through coadsorption and temperature-dependent diffusion barriers*

We report a novel strategy to increase the gas adsorption selectivity of metal organic frameworks (MOFs) by coadsorbing other molecules. Specifically, we find that addition of tightly bound NH₃ molecules in 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. Most remarkably, C₂H₄ is now almost completely excluded from entering the MOF once C₂H₂ has been loaded. This finding suggests a new route to tune the adsorption behavior of MOFs through harnessing the interactions among coadsorbed guests. Furthermore, in the same MOF, we report a temperature-induced variation in a capping-layer gate-opening mechanism through a combination of in situ infared experiments and ab initio simulations of the capping layer. An atypical acceleration and increase in the loading of adsorbed molecules upon raising the temperature above room temperature is observed. This finding shows the discovery of novel temperature-dependent kinetics that goes beyond standard kinetics and suggest a new avenue for tailoring selective adsorption by thermally tuning the surface barrier.