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 (Qst) 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 Qst 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 Qst value of 9.1 kJ mol-1. This H2 Qst 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.