First-principles studies of early-stage CO₂ capture in diamine-appended metal organic frameworks

mmen-M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn) metal-organic frameworks (MOFs) are phase-change adsorbents with significant potential for CO₂ capture due to their high working capacities and strong selectivity resulting from a novel cooperative adsorption mechanism. Despite prior work on this system, our understanding of the early stages of CO₂ capture in these MOFs is still lacking. Herein, we investigate CO₂ capture in mmen-M₂(dobpdc) MOFs with first-principles density functional theory (DFT) and ab-initio molecular dynamics (MD) calculations. Our van der Waals-corrected DFT calculations show that CO₂ prefers to bond to the free N ion of the mmen ligand (in a so-called ‘tail’ geometry) instead of the metal-bonded N ion (or ‘head’ geometry). We find that tail geometry is more stable then head geometry by about 100 kJ/mol and even more stable than a previously-proposed early-stage CO₂ capture geometry (the so-called ‘pair’ geometry). In addition, our ab-initio MD calculations show that the tail geometry can undergo a transition to a chain geometry, the known ground state structure above a threshold CO₂ partial pressure.