Effects of Intrinsic Flexibility on Adsorption Properties of Metal–Organic Frameworks at Dilute and Nondilute Loadings

Molecular
simulation of adsorption in nanoporous materials has
become a valuable complement to experimental studies of these materials.
In almost all cases, these simulations treat the adsorbing material
as rigid. We use molecular simulations to examine the validity of
this approximation for the adsorption in metal–organic frameworks
(MOFs) that have framework flexibility without change in their unit
cells because of thermal vibrations. All nanoporous materials are
subject to this kind of framework flexibility. We examine the adsorption
of nine molecules (CO2, CH4, ethane, ethene,
propane, propene, butane, Xe, and Kr) and four molecular mixtures
(CO2/CH4, ethane/ethene, propane/propene/butane,
and Xe/Kr) in 100 MOFs at dilute and nondilute adsorption conditions.
Our results show that single-component adsorption uptakes at nondilute
conditions are only weakly affected by framework flexibility, but
adsorption selectivities at both dilute and nondilute conditions can
be significantly affected by flexibility. The most dramatic impacts
of framework flexibility occur for adsorption uptake in the limit
of dilute adsorption. These results suggest that the importance of
including framework flexibility when attempting to make quantitative
predictions of adsorption selectivity in MOFs and similar materials
may have been underestimated in the past.