Elucidation of the Underlying Mechanism of CO Capture by Ethylenediamine-Functionalized M(dobpdc) (M = Mg, Sc-Zn).

We, for the first time, systematically investigated the crystal structures, adsorption properties, and microscopic mechanism of CO capture with ethylenediamine (en)-appended isostructural M(dobpdc) materials (M = Mg, Sc-Zn), using spin polarized density functional theory (DFT) calculations. The binding energies of en range from 142 to 210 kJ/mol. The weakest binding materials are en-Cr(dobpdc) and en-Cu(dobpdc). Two typical models, the pair model and the chain model, have been considered for CO adsorption. Generally, the chain model is more stable than the pair model. The CO adsorption energies of the chain model are in the range of 30-96 kJ/mol, with a strong metal dependence. Among these, the en-Sc(dobpdc) and en-Cu(dobpdc) have the highest and lowest CO adsorption energies, respectively. Moreover, the dynamic progress of CO adsorption has been unveiled via exploration of the full reaction pathway, including transition states and intermediates. First, the CO molecule interacts with en-MOFs to form a physisorbed complex with a shallow potential well. This is followed by overcoming a relatively large energy barrier to form a chemisorbed complex. Finally, ammonium carbamate is formed along the one-dimensional channels within the pore with a small energy barrier for configuration transformation. These results agree well with the experimental observations. Understanding the detailed microscopic mechanism of CO capture is quite crucial for improving our fundamental knowledge base and potential future applications. This work will improve our understanding of CO adsorption with amine functionalized MOFs. We expect our results to stimulate future experimental and theoretical research and advance the development of this field.