Tuning Binding Tendencies of Small Molecules in Metal–Organic Frameworks with Open Metal Sites by Metal Substitution and Linker Functionalization

Metal–organic
frameworks (MOFs) with open metal sites (OMS)
are known to be selective for ethylene relative to ethane. In practical
applications of this separation, the presence of other small molecules
such as H2O, CO, and C2H2 may affect
the suitability of sorbents. We used density functional theory (DFT)
calculations to compute the binding energies of H2O, CO,
C2H2, C2H4, and C2H6 in M-BTC (BTC = 1,3,5-benzenetricarboxylic acid)
with 12 different metals forming OMS (M = Mg, Ti, V, Cr, Mo, Mn, Fe,
Ru, Co, Ni, Cu, and Zn). To probe the generality of these results
for MOFs containing other ligands, we performed similar calculations
for metal-substituted MOFs based on four more materials with dimeric
Cu sites. Our results provide useful insights into the variations
in binding energies that are achievable by metal substitution in this
broad class of MOFs, as well as pointing toward feasible adsorption-based
separation strategies for complex molecular mixtures. Zn OMS MOFs
were predicted to have the highest C2H4/C2H6 selectivity, but the strong binding energy of
solvents and other small molecules in these materials may create practical
challenges. We used DFT calculations to examine whether functionalizing
linkers in these materials with electron withdrawing (−fluorine)
and donating (−methyl) groups offer a useful way to tune molecular
binding energies on OMS in these materials.