A combined statistical mechanical and ab initio approach to understanding H2O/CO2 co-adsorption in mmen-Mg2(dobpdc)

We study the effects of H2O on CO2 adsorption in an amine-appended variant of the metal-organic framework Mg2(dobpdc), which is known to exhibit chaining behavior that presents in a step-shaped adsorption isotherm. We first show how the presence of different levels of local H2O affects this chaining behavior and the energetics of CO2 adsorption, based on a series of ab initio calculations, giving insight into the atomic-scale environment. In particular, we predict a novel adsorbed configuration, in which H2O and CO2 intertwine to make a braided chain down the MOF pore. We then show how an existing lattice model can be adapted to incorporate the effect of water and predict the CO2 isotherms for the various water levels, observing a sharp shift in the uptake at low partial pressures. The manifestation of this braided chain in the lattice model points to the potential emergence of a shift from cooperative capture to that of a phase transition. In addition to the physical insights, this work may serve as a launching off point for further work on this and related materials.