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

Abstract

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, Q st , 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.

Understanding Hydrogen Sorption in In-soc-MOF: A Charged Metal-Organic Framework with Open-Metal Sites, Narrow Channels, and Counterions

“…The development of the force field for In-soc-MOF was performed according to the procedure described in previous work. 12,24,25,[29][30][31][32][33][34][35][36][37][38][39][40][41] Nevertheless, a brief outline is given here. For all C, H, and N atoms on the organic linker in In-soc-MOF, the Lennard-Jones 12-6 parameters, representing van der Waals interactions between the sorbate molecules and the MOF atoms, were acquired from the Optimized Potentials for Liquid Simulations -All Atom (OPLS-AA) force field.…”

Investigating H2 Sorption in a Fluorinated Metal–Organic Framework with Small Pores Through Molecular Simulation and Inelastic Neutron Scattering

“…This has been validated through various studies that pinpoint the location of the sorbed H 2 molecules in these materials, such as neutron scattering spectroscopy (elastic and inelastic) [22][23][24][25] and computational modeling. 26,27 Tuning the pore sizes in MOFs to allow for optimal interactions between the H 2 molecule and the framework is also an encouraging method to increase the Q st for H 2 in these materials. [28][29][30] MOFs with small pore sizes often display a higher H 2 Q st than those with larger pore sizes because smaller pores permit the H 2 molecules to interact with many components of the framework at the same time.…”

“…27,38,50,51 Note, the error bars for all simulated state points considered are extremely small, and thus, they have been omitted for clarity.…”

Real space electrostatics for multipoles. II. Comparisons with the Ewald sum

“…27 As a result, the DSF method has seen increasing use in molecular systems with relatively uniform charge densities. [40][41][42][43][44][45][46] …”