Experimental and theoretical studies of single gas component are well established for CO2 adsorption into metal organic frameworks (MOFs). However, understanding co-adsorption of muticomponent mixtures CO2/(H2O, N2, CH4, N2) is critical and required to evaluate gas separation and purification methods. For example, the presence of water was shown to be detrimental to CO2 capture (i.e. lowering CO2 loading), especially for the MOFs with unsaturated open metal sites such as MOF-74, which has the highest reported CO2 uptake at 296 K and 1 Bar among all MOFs. Combining infrared absorption spectroscopy and ab initio density functional theory (DFT) calculations, we have derived a detailed picture of the co-adsorption process of mixtures of CO2 with H2O, N2, CH4, or O2 in MOF-74 with different metal centers (Ni, Co, Mg). While each molecule has been shown to preferentially adsorb at the metal site, as evidenced for instance by single component neutron/X-ray diffraction studies, we find a competition between mixture components for the metal adsorption site. In the case of CO2/H2O, where CO2 is initially preadsorbed, water molecules are found to diffuse into the MOF-74 unit cells, gradually displacing preadsorbed CO2. Spectroscopic data confirm that CO2 is initially in the metal sites and then moves out in adjacent sites (possibly the center of the channel) when water binds to the metal site, eventually diffusing out of the MOF crystal all together at room temperature (i.e. their binding energy is much lower if they are not at the metal sites). Our temperature dependent experiments suggest that the exchange process is activated (i.e. characterized by a kinetic barrier), as confirmed by our DFT modeling. Furthermore, our NEB DFT calculations find the explicit pathway for H2O displacing CO2 molecules. For the other gases such as O2, CH4, N2, there is no competitive adsorption as CO2 remains at the metal site. The joint investigation combining vibrational spectroscopy and DFT calculation provides a microscopic understanding for the co-adsorption mechanism in this class of materials.

Co-adsorption of CO2, H2O, O2, CH4, N2 adsorption in MOF-74 (Mg, Ni, Co)

247th National Meeting (2014)

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