Simulation of the Mechanism of Gas Sorption in a Metal–Organic Framework with Open Metal Sites: Molecular Hydrogen in PCN-61

Grand canonical Monte Carlo (GCMC) simulations were performed
to
investigate hydrogen sorption in an rht-type metal–organic
framework (MOF), PCN-61. The MOF was shown to have a large hydrogen
uptake, and this was studied using three different hydrogen potentials,
effective for bulk hydrogen, but of varying sophistication: a model
that includes only repulsion/dispersion parameters, one augmented
with charge-quadrupole interactions, and one supplemented with many-body
polarization interactions. Calculated hydrogen uptake isotherms and
isosteric heats of adsorption, Qst, were
in quantitative agreement with experiment only for the model with
explicit polarization. This success in reproducing empirical measurements
suggests that modeling MOFs that have open metal sites is feasible,
though it is often not considered to be well described via a classical potential function; here it is shown that such systems
may be accurately described by explicitly including polarization effects
in an otherwise traditional empirical potential. Decomposition of
energy terms for the models revealed deviations between the electrostatic
and polarizable results that are unexpected due to just the augmentation
of the potential surface by the addition of induction. Charge-quadrupole
and induction energetics were shown to have a synergistic interaction,
with inclusion of the latter resulting in a significant increase in
the former. Induction interactions strongly influence the structure
of the sorbed hydrogen compared to the models lacking polarizability;
sorbed hydrogen is a dipolar dense fluid in the MOF. This study demonstrates
that many-body polarization makes a critical contribution to gas sorption
structure and must be accounted for in modeling MOFs with polar interaction
sites.