Theoretical Investigations of CO₂ and H₂ Sorption in Robust Molecular Porous Materials

Molecular simulations of CO₂ and H₂ sorption were performed in MPM-1-Cl and MPM-1-TIFSIX, two robust molecular porous materials (MPMs) with the empirical formula [Cu₂(adenine)₄Cl₂]Cl₂ and [Cu₂(adenine)₄(TiF₆)₂], respectively. Recent experimental studies have shown that MPM-1-TIFSIX displayed higher CO₂ uptake and isosteric heat of adsorption (Q_st) than MPM-1-Cl [Nugent, P. S.; et al. J. Am. Chem. Soc. 2013, 135, 10950-10953]. This was verified through the simulations executed herein, as the presented simulated CO₂ sorption isotherms and Q_st values are in very good agreement with the corresponding experimental data for both MPMs. We also report experimental H₂ sorption data in both MPMs. Experimental studies revealed that MPM-1-TIFSIX exhibits high H₂ uptake at low loadings and an initial H₂ Q_st value of 9.1 kJ mol^-1. This H₂ Q_st value is greater than that for a number of existing metal-organic frameworks (MOFs) and represents the highest yet reported for a MPM. The remarkable H₂ sorption properties for MPM-1-TIFSIX have been confirmed through our simulations. The modeling studies revealed that only one principal sorption site is present for CO₂ and H₂ in MPM-1-Cl, which is sorption onto the Cl^- counterions within the large channels. In contrast, three different sorption sites were discovered for both CO₂ and H₂ in MPM-1-TIFSIX: (1) between two TIFSIX groups within a small passage connecting the large channels, (2) onto the TIFSIX ions lining the large channels, and (3) within the small channels. This study illustrates the detailed insights that molecular simulations can provide on the CO₂ and H₂ sorption mechanism in MPMs.