Understanding Carbon Monoxide Binding and Interactions in M‑MOF-74 (M = Mg, Mn, Ni, Zn)

Small molecules may adsorb strongly in metal–organic
frameworks
(MOFs) through interactions with under-coordinated open metal sites
(OMS) that often exist within these structures. Among adsorbates,
CO is attractive to study both for its relevance in energy-related
applications and for its ability to engage in both σ-donation
and π-backbonding interactions with the OMS in MOFs. Concomitant
with strong adsorption, structural changes arise due to modifications
of the electronic structure of both the adsorbate and adsorbent. These
structural changes affect the separation performance of materials,
and accurately capturing these changes and the resulting energetics
is critical for accurate predictive modeling of adsorption. Traditional
approaches to modeling using classical force fields typically do not
capture or account for changes at the electronic level. To characterize
the structural and energetic effects of the local structural changes,
we employed density functional theory (DFT) to study CO adsorption
in M-MOF-74s. M-MOF-74s feature OMS at which CO is known to adsorb
strongly and can be synthesized with a variety of divalent metal cations
with distinct performance in adsorption. We considered M-MOF-74s with
a range of metals of varied d-band occupations (Mg (3d0), Mn (3d5), Ni (3d8), and Zn (3d10)) with various structural constraints ranging from geometrically
constrained adsorbent and adsorbate ions to fully optimized geometries
to deconvolute the relative contributions of various structural effects
to the adsorption energetics and binding distances observed. Our data
indicate that the most significant structural changes during adsorption
correlate with the greatest π-backbonding behaviors and commensurately
result in a sizable binding energy change observed for CO adsorption.
The insights built from this work are relevant to two longstanding
research challenges within the MOF community: rational design of materials
for separations and the design of force fields capable of accurately
modeling adsorption.