Understanding the Carbon Dioxide Binding Potential Landscape near Open Metal Sites in M‑MOF-74 (M = Mg, Mn, Fe, Ni, Zn)

Metal–organic
frameworks (MOFs), especially those with undercoordinated
open metal sites (OMS), are attractive candidates for applications
involving adsorption of small molecules, including CO2.
Among MOFs with OMS, MOF-74 variants have drawn considerable interest
due to a high gravimetric density of such adsorption sites. The electronic
nature of adsorbate–adsorbent interactions at OMS typically
results in the accuracy of conventional force fields being insufficient
to describe adsorption energetics, motivating the need for more accurate
predictive methods. Toward this goal, we have utilized density functional
theory (DFT) to explore the binding characteristics of CO2 in M-MOF-74s. We focus specifically on examining how various parameters,
such as binding distance, variation in adsorption angles, and dihedral
(precession) angles influence adsorption energetics. We consider M-MOF-74
variants across a range of d-band occupations, including
Mg (3d0), Mn (3d5), Fe(3d6), Ni (3d8), and Zn (3d10) in this
work, illustrating the impact of electronic structure of the OMS metal
on the energetics of CO2 adsorption both at and in the
vicinity of the primary adsorption sites at the OMS. Our findings,
particularly at the minimum energy binding configurations, closely
align with experimental structures and isosteric heats of adsorption.
Our exploration of potential energy surfaces (PES) at OMS in off-minimum
binding configurations uncovers more complicated interaction mechanisms,
highlighting local minima and maxima within the PES. Developing these
comprehensive energy profiles provides vital insights, potentially
aiding in the creation of force fields that can accurately capture
adsorbate–adsorbent interactions in MOFs with OMS.