Challenge of Small Energy Differences in Metal–Organic Framework Reactivity

Metal–organic frameworks (MOFs) offer a promising
architecture
for designing heterogeneous catalysts, in which catalytically active
metal centers can be installed on a porous MOF structure and modified
using nonstructural ligands. Recently, it was shown that the activity
for ethylene hydrogenation of Ni deposited on the MOF NU-1000 can
be amplified 26-fold via the use of substituted nonstructural benzoate
ligands (Liu et al. ACS Catal. 2019, 9, 3198–3207). However, as profound as this result
is experimentally, this change in activity corresponds to an activation
energy difference of only about 2.4 kcal/mol, putting it at the edge
of the accuracy of density functional theory. In this study, we evaluate
various density functionals for their ability to provide a consistent
description of the experimental trend using a single-metal approximation
of the active site. Our findings demonstrate the difficulties faced
when calculating trends in small energy differences in MOFs, such
as variance between functionals and differences in predictions due
to subtle geometrical or rovibrational effects. In summary, we find
no agreement in even the sign of the trend between different functionals,
and only a minority of functionals are able to reproduce the experimental
trend. However, among the functionals we examined, a subset demonstrate
robustness in their predictions (either in agreement with or against
the experimental trend) and we recommend using them for the computation
of these small energy differences in MOF catalysis.