Theoretical Investigations of CO₂ and H₂ Sorption in an Interpenetrated Square-Pillared Metal–Organic Material

Simulations of CO₂ and H₂ sorption and separation were performed in [Cu­(dpa)₂SiF₆-i], a metal–organic material (MOM) consisting of an interpenetrated square grid of Cu²⁺ ions coordinated to 4,4′-dipyridylacetylene (dpa) rings and pillars of SiF₆^2– ions. This class of water stable MOMs shows great promise in practical gas sorption/separation with especially high selectivity for CO₂ and variable selectivity for other energy related gases. Simulated CO₂ sorption isotherms and isosteric heats of adsorption, Q_st, at ambient temperatures were in excellent agreement with the experimental measurements at all pressures considered. Further, it was observed that the Q_st for CO₂ increases as a function of uptake in [Cu­(dpa)₂SiF₆-i]. This suggests that nascently sorbed CO₂ molecules within a channel contribute to a more energetically favorable site for additional CO₂ molecules, i.e., in stark contrast to typical behavior, sorbate intermolecular interactions enhance sorption energetics with increased loading. The simulated structure at CO₂ saturation shows a loading with tight packing of 8 CO₂ molecules per unit cell. The CO₂ molecules can be seen alternating between a vertical and horizontal alignment within a channel, with each CO₂ molecule coordinating to an equatorial fluorine MOM atom. Calculated H₂ sorption isotherms and Q_st values were also in good agreement with the experimental measurements in [Cu­(dpa)₂SiF₆-i]. H₂ saturation corresponds to 10 H₂ molecules per unit cell for the studied structure. Moreover, there were two observed binding sites for hydrogen sorption in [Cu­(dpa)₂SiF₆-i]. Simulations of a 30:70 CO₂/H₂ mixture, typical of syngas, in [Cu­(dpa)₂SiF₆-i] showed that the MOM exhibited a high uptake and selectivity for CO₂. In addition, it was observed that the presence of H₂O had a negligible effect on the CO₂ uptake and selectivity in [Cu­(dpa)₂SiF₆-i], as simulations of a mixture containing CO₂, H₂, and small amounts of CO, N₂, and H₂O produced comparable results to the binary mixture simulations.