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-MOF-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.