Understanding Hydrogen Sorption in a Metal–Organic Framework with Open-Metal Sites and Amide Functional Groups

Grand canonical Monte Carlo (GCMC)
studies of the mechanism of
hydrogen sorption in an rht-MOF known as Cu-TPBTM
are presented. The MOF is a decorated/substituted isostructural analogue
to the unembellished rht-MOF, PCN-61, that was studied
previously [Forrest, K. A. J. Phys.
Chem. C 2012, 116, 15538−15549.]. The simulations were performed
using three different hydrogen potentials of increasing complexity.
Simulated hydrogen sorption isotherms and calculated isosteric heat
of adsorption, Qst, values were in excellent
agreement with the reported experimental data for only a polarizable
model in one of four experimentally observed crystal structure configurations.
The study demonstrates the ability of modeling to distinguish the
differential sorption of distinct strucures; one configuration is
found to be dominant due to favorable interactions with substrates.
In addition, it was discovered that the presence of polar amide groups
had a significant effect on the electrostatics of the Cu2+ ions and directs the low-pressure physisorption of hydrogen in the
MOF. This is in contrast to what was observed in PCN-61, where an
exterior copper ion had a higher relative charge and was the favored
loading site. This tunability of the electrostatics of the copper
ions via chemical substitution on the MOF framework can be explained
by the presence of the negatively charged oxygen atom of the amide
group that causes the interior Cu2+ ion to exhibit a higher
positive charge through an inductive effect. Further, control simulations,
taking advantage of the flexibility afforded by theoretical modeling,
include artificially modified charges for both Cu2+ ions
chosen equal to or with a higher charge on the exterior Cu2+ ion. This choice resulted in distinctly different hydrogen sorption
characteristics in Cu-TPBTM with no direct sorption on the open-metal
sites. Thus, this study demonstrates both the tunable nature of MOF
platforms and the possibility for rational design of sorption/catalytic
sites and characteristics through the active interplay of theory and
experiment. The ability of accurate, carefully parametrized and transferable
force fields to model and predict small molecule sorption in MOFs,
even including open-metal sites, is demonstrated.