Over
14 000 porous, three-dimensional metal–organic
framework structures are compiled and analyzed as a part of an update
to the Computation-Ready, Experimental Metal–Organic Framework
Database (CoRE MOF Database). The updated database includes additional
structures that were contributed by CoRE MOF users, obtained from
updates of the Cambridge Structural Database and a Web of Science
search, and derived through semiautomated reconstruction of disordered
structures using a topology-based crystal generator. In addition,
value is added to the CoRE MOF database through new analyses that
can speed up future nanoporous materials discovery activities, including
open metal site detection and duplicate searches. Crystal structures
(only for the subset that underwent significant changes during curation),
pore analytics, and physical property data are included with the publicly
available CoRE MOF 2019 database.
The very large number of distinct structures that are known for metal-organic frameworks (MOFs) and related materials presents both an opportunity and a challenge for identifying materials with useful properties for targeted applications. We show that efficient computational models can be used to evaluate large numbers of MOFs for kinetic separations of light gases based on finding materials with large differences between the diffusion coefficients of adsorbed gas species. We introduce a geometric approach that rapidly identifies the key features of a pore structure that control molecular diffusion and couple this with efficient molecular modeling calculations that predict the Henry's constant and diffusion activation energy for a range of spherical adsorbates. We demonstrate our approach for >500 MOFs and >160 silica zeolites. Our results indicate that many large pore MOFs will be of limited interest for separations based on kinetic effects, but we identify a significant number of materials that are predicted to have extraordinary properties for separation of gases such as CO(2), CH(4), and H(2).
Breathe in, breathe out: Efficient methods are introduced for assessing the role of framework flexibility on molecular diffusion in metal‐organic frameworks (MOFs) that does not require defining a classical forcefield for the MOF. These methods combine ab initio MD of the MOF with classical MD simulation of the diffusing molecules. The effects of flexibility are shown to be large for CH4, but not for CO2, in ZIF‐8 (see picture).
Electrostatic interactions are a critical factor in the adsorption of quadrupolar species such as CO(2) and N(2) in metal-organic frameworks (MOFs) and other nanoporous materials. We show how a version of the semiempirical charge equilibration method suitable for periodic materials can be used to efficiently assign charges and allow molecular simulations for a large number of MOFs. This approach is illustrated by simulating CO(2) and N(2) adsorption in ~500 MOFs; this is the largest set of structures for which this information has been reported to date. For materials predicted by our calculations to have promising adsorption selectivities, we performed more detailed calculations in which accurate quantum chemistry methods were used to assign atomic point charges, and molecular simulations were used to assess molecular diffusivities and binary adsorption isotherms. Our results identify two MOFs, experimentally known to be stable upon solvent removal, that are predicted to show no diffusion limitations for adsorbed molecules and extremely high CO(2)/N(2) adsorption selectivities for CO(2) adsorption from dry air and from gas mixtures typical of dry flue gas.
The air-free reaction of CoCl2 with 1,3,5-tri(1H-1,2,3-triazol-5-yl)benzene
(H3BTTri) in N,N-dimethylformamide
(DMF) and methanol
leads to the formation of Co-BTTri (Co3[(Co4Cl)3(BTTri)8]2·DMF), a sodalite-type
metal–organic framework. Desolvation of this material generates
coordinatively unsaturated low-spin cobalt(II) centers that exhibit
a strong preference for binding O2 over N2,
with isosteric heats of adsorption (Qst) of −34(1) and −12(1) kJ/mol, respectively. The low-spin
(S = 1/2) electronic configuration of the metal centers
in the desolvated framework is supported by structural, magnetic susceptibility,
and computational studies. A single-crystal X-ray structure determination
reveals that O2 binds end-on to each framework cobalt center
in a 1:1 ratio with a Co–O2 bond distance of 1.973(6)
Å. Replacement of one of the triazolate linkers with a more electron-donating
pyrazolate group leads to the isostructural framework Co-BDTriP (Co3[(Co4Cl)3(BDTriP)8]2·DMF; H3BDTriP = 5,5′-(5-(1H-pyrazol-4-yl)-1,3-phenylene)bis(1H-1,2,3-triazole)),
which demonstrates markedly higher yet still fully reversible O2 affinities (Qst = −47(1)
kJ/mol at low loadings). Electronic structure calculations suggest
that the O2 adducts in Co-BTTri are best described as cobalt(II)–dioxygen
species with partial electron transfer, while the stronger binding
sites in Co-BDTriP form cobalt(III)–superoxo moieties. The
stability, selectivity, and high O2 adsorption capacity
of these materials render them promising new adsorbents for air separation
processes.
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