First principles Monte Carlo simulations of unary and binary adsorption: CO2, N2, and H2O in Mg-MOF-74.

ABSTRACT

Dative bonding of adsorbate molecules onto coordinatively-unsaturated metal sites in metal-organic frameworks can lead to unique adsorption selectivities. However, the distortion of the electron density during dative bonding poses a challenge for force-field-based simulations. Here, we report first principles Monte Carlo simulations with the PBE-D3 functional for the adsorption of CO2, N2, and H2O in Mg-MOF-74, and obtain accurate predictions of the unary isotherms without any of the adjustments or fitting often required for systems with strong adsorption sites. Simulations of binary CO2/N2 and H2O/CO2 mixtures yield selectivities of 200 and 160, respectively, and indicate that predictions from ideal adsorbed solution theory need to be viewed with caution.