Competitive co-adsorption of CO2 with H2O, NH3, SO2, NO, NO2, N2, O2, and CH4 in M-MOF-74 (M= Mg, Co, Ni): the role of hydrogen bonding

摘要

The importance of co-adsorption for applications of porous materials in gasseparation has motivated fundamental studies, which have initially focused onthe comparison of the binding energies of different gas molecules in the pores(i.e. energetics) and their overall transport. By examining the competitiveco-adsorption of several small molecules in M-MOF-74 (M= Mg, Co, Ni) within-situ infrared spectroscopy and ab initio simulations, we find that thebinding energy at the most favorable (metal) site is not a sufficient indicatorfor prediction of molecular adsorption and stability in MOFs. Instead, theoccupation of the open metal sites is governed by kinetics, whereby theinteraction of the guest molecules with the MOF organic linkers controls thereaction barrier for molecular exchange. Specifically, the displacement of CO2adsorbed 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, andNO2, have higher binding energies (~70-90 kJ/mol) to metal sites than that ofCO2 (38 to 48 kJ/mol) and slightly higher than water (~60-80 kJ/mol). DFTsimulations evaluate the barriers for H2O->CO2 and SO2->CO2 exchange to be - 13and 20 kJ/mol, respectively, explaining the slow exchange of CO2 by SO2,compared to water. Furthermore, the calculations reveal that the kineticbarrier for this exchange is determined by the specifics of the interaction ofthe second guest molecule (e.g., H2O or SO2) with the MOF ligands.