Probing the adsorption sites for CO2 in metal organic frameworks materials MIL-53 (Al, Cr) and MIL-47 (V) by density functional theory

A comprehensive DFT study of the possible CO2 adsorption geometries in the MIL-53 (Al, Cr) and MIL-47 hybrid organic-inorganic materials has been performed, as a preliminary step to gain a deeper understanding of the CO2 adsorption mechanism in these systems and to help explain the "breathing" effect displayed by the MIL-53 materials. This technique allows us to explore the possible spatial configurations of the CO2 molecules in the MIL-53 systems depending on the size of the pore opening at different loadings. Our results show that the replacement of the mu(2)-OH groups by oxo moieties in the MIL-47 material leads to fewer, weaker adsorption sites for CO2 onto the framework itself, whereas a larger number Of CO2 geometries are possible in both the large and narrow versions of MIL-53. In the narrow pore form, the double interaction, where a CO2 molecule bridges the pore to simultaneously coordinate with 2 mu(2)-OH groups on opposite sides of a pore wall, was predicted to be the most energetic arrangement at the initial stage of adsorption. Additional configurations include coordination with both inorganic and organic parts of the framework. When the number Of CO2 molecule increases, our calculations indicate that the resulting increase in the intermolecular interactions between the adsorbate molecules lead to a significant modification of the CO2 arrangement within the pore and should be an important feature in the explanation of the "breathing" of this material. In the large pore form, an interaction reflecting the opening of the pore is not possible, and so the most likely interaction is one where a CO2 interacts with a single mu(2)-OH group, in a number of different orientations.