Incorporating Flexibility Effects into Metal–Organic Framework Adsorption Simulations Using Different Models

High-throughput calculations based
on molecular simulations to
predict the adsorption of molecules inside metal–organic frameworks
(MOFs) have become a useful complement to experimental efforts to
identify promising adsorbents for chemical separations and storage.
For computational convenience, all existing efforts of this kind have
relied on simulations in which the MOF is approximated as rigid. In
this paper, we use extensive adsorption–relaxation simulations
that fully include MOF flexibility effects to explore the validity
of the rigid framework approximation. We also examine the accuracy
of several approximate methods to incorporate framework flexibility
that are more computationally efficient than adsorption–relaxation
calculations. We first benchmark various models of MOF flexibility
for four MOFs with well-established CO2 experimental consensus
isotherms. We then consider a range of adsorption properties, including
Henry’s constants, nondilute loadings, and adsorption selectivity,
for seven adsorbates in 15 MOFs randomly selected from the CoRE MOF
database. Our results indicate that in many MOFs adsorption–relaxation
simulations are necessary to make quantitative predictions of adsorption,
particularly for adsorption at dilute concentrations, although more
standard calculations based on rigid structures can provide useful
information. Finally, we investigate whether a correlation exists
between the elastic properties of empty MOFs and the importance of
including framework flexibility in making accurate predictions of
molecular adsorption. Our results did not identify a simple correlation
of this type.