  The importance of co-adsorption for applications of porous materials in gas
separation has motivated fundamental studies, which have initially focused on
the comparison of the binding energies of different gas molecules in the pores
(i.e. energetics) and their overall transport. By examining the competitive
co-adsorption of several small molecules in M-MOF-74 (M= Mg, Co, Ni) with
in-situ infrared spectroscopy and ab initio simulations, we find that the
binding energy at the most favorable (metal) site is not a sufficient indicator
for prediction of molecular adsorption and stability in MOFs. Instead, the
occupation of the open metal sites is governed by kinetics, whereby the
interaction of the guest molecules with the MOF organic linkers controls the
reaction barrier for molecular exchange. Specifically, the displacement of CO2
adsorbed at the metal center by other molecules such as H2O, NH3, SO2, NO, NO2,
N2, O2, and CH4 is mainly observed for H2O and NH3, even though SO2, NO, and
NO2, have higher binding energies (~70-90 kJ/mol) to metal sites than that of
CO2 (38 to 48 kJ/mol) and slightly higher than water (~60-80 kJ/mol). DFT
simulations evaluate the barriers for H2O->CO2 and SO2->CO2 exchange to be - 13
and 20 kJ/mol, respectively, explaining the slow exchange of CO2 by SO2,
compared to water. Furthermore, the calculations reveal that the kinetic
barrier for this exchange is determined by the specifics of the interaction of
the second guest molecule (e.g., H2O or SO2) with the MOF ligands.
