Atomistic Model for Water Adsorption in Mg-MOF-74: Quantum Chemical Prediction of Structures and Isotherms

The design of improved metal–organic frameworks
(MOFs) for
water harvesting requires the reliable prediction of adsorption isotherms,
i.e., Gibbs free energies of adsorption, with no other input than
the atomic positions. We employ density functional theory (DFT) and
show that, in Mg-MOF-74, well-defined adsorption structures exist
for water loadings of n = 1, 2, 3, 4, and 5 molecules
per Mg2+ ion. The first water molecule attaches to the
open metal site, while on adsorption of subsequent molecules, structures
with an increasing number of hydrogen bonds per molecule form: dimers
(n = 2), chains in pore direction (n = 3), and a monolayer on the pore wall (n = 4).
For n = 5, a tube-like stack of water trimers connected
to the monolayer fills the pore completely, and all water molecules
are 4-fold coordinated. For isotherm predictions, we use a Multisite Langmuir model with Gibbs free energies of −33,
−19, −13, −10, and −21 kJ/mol for the
steps leading to adsorption states with n = 1, 2,
3, 4, and 5 molecules, respectively. The close agreement of the predicted
total isotherm with experiment corresponds to an accuracy of ±2
kJ/mol for Gibbs free energies. This is achieved only after adding
“high-level” Coupled Cluster corrections (0, 3, 9, 8,
and 11 kJ/mol for n = 1, 2, 3, 4, and 5, respectively)
to the DFT energies. We show that the large variations observed between
different experimental isotherms can be explained by sample imperfections
or incomplete evacuation of samples before isotherm measurements.