CO<sub>2</sub> Capture
by Metal–Organic Frameworks
with van der Waals Density Functionals
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Abstract
We use density functional theory calculations with van
der Waals corrections to study the
role of dispersive interactions on the structure and binding of CO<sub>2</sub> within two distinct metal–organic frameworks (MOFs):
Mg-MOF74 and Ca-BTT. For both classes of MOFs, we report calculations
with standard gradient-corrected (PBE) and five van der Waals density
functionals (vdW-DFs), also comparing with semiempirical pairwise
corrections. The vdW-DFs explored here yield a large spread in CO<sub>2</sub>–MOF binding energies, about 50% (around 20 kJ/mol),
depending on the choice of exchange functional, which is significantly
larger than our computed zero-point energies and thermal contributions
(around 5 kJ/mol). However, two specific vdW-DFs result in excellent
agreement with experiments within a few kilojoules per mole, at a
reduced computational cost compared to quantum chemistry or many-body
approaches. For Mg-MOF74, PBE underestimates adsorption enthalpies
by about 50%, but enthalpies computed with vdW-DF, PBE+D2, and vdW-DF2
(40.5, 38.5, and 37.4 kJ/mol, respectively) compare extremely well
with the experimental value of 40 kJ/mol. vdW-DF and vdW-DF2 CO<sub>2</sub>–MOF bond lengths are in the best agreement with experiments,
while vdW-C09<sub>x</sub> results in the best agreement with lattice
parameters. On the basis of the similar behavior of the reduced density
gradients around CO<sub>2</sub> for the two MOFs studied, comparable
results can be expected for CO<sub>2</sub> adsorption in BTT-type
MOFs. Our work demonstrates for this broad class of molecular adsorbate-periodic
MOF systems that parameter-free and computationally efficient vdW-DF
and vdW-DF2 approaches can predict adsorption enthalpies with chemical
accuracy