Microscopic Origins of Enhanced Gas Adsorption and Selectivity in Mixed-Linker Metal-Organic Frameworks

We use molecular simulations to study the gas adsorption properties of metal organic framework (MOF) materials composed of mixtures of linker groups, focusing on the prototypical MTV-MOE-5 systems. While MOF functionalization is well-known to influence gas uptake, we show that the absolute gas uptake is frequently not merely a sum of linear contributions from its constituent functionalities but rather there exists a synergistic enhancement that arises due to cooperative adsorbate linker interactions involving multiple functionalities. In certain mixed-linker MOFs, such cooperativity yields increased gas uptake over any possible corresponding pure "parent" compound. Considering a model system based on ZIF-8, we are able to clearly demonstrate the microscopic origin of this synergy, arising from the strong, simultaneous interactions of multiple linker groups with a single adsorbate. We also provide a concrete example of a mixed-linker MOF that exhibits gas adsorption superior to that of any of its pure parent compounds. We conclude that such cooperativity should be a fairly general phenomenon and suggest some design guidelines that can be exploited to synthesize synergistically enhanced mixed MOFs.