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

TL;DR

This study combines statistical mechanics and ab initio calculations to understand how water influences CO2 adsorption in a metal-organic framework, revealing new adsorption configurations and predicting shifts in adsorption behavior.

Contribution

It introduces a novel combined approach to model H2O/CO2 co-adsorption, including a new braided chain configuration and an adapted lattice model for better prediction.

Findings

Water affects CO2 adsorption energetics and chaining behavior.

Predicted a new H2O-CO2 braided chain configuration.

Observed a sharp shift in CO2 uptake at low partial pressures.

Abstract

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 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.