Theoretical Investigations of CO₂and H₂Sorption in an Interpenetrated Square-Pillared Metal–Organic Material

Simulations of CO 2 and H 2 sorption and separation were performed in [Cu(dpa) 2 SiF 6 -i], a metal–organic material (MOM) consisting of an interpenetrated square grid of Cu 2+ ions coordinated to 4,4′-dipyridylacetylene (dpa) rings and pillars of SiF 6 2– ions. This class of water stable MOMs shows great promise in practical gas sorption/separation with especially high selectivity for CO 2 and variable selectivity for other energy related gases. Simulated CO 2 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 CO 2 increases as a function of uptake in [Cu(dpa) 2 SiF 6 -i]. This suggests that nascently sorbed CO 2 molecules within a channel contribute to a more energetically favorable site for additional CO 2 molecules, i.e., in stark contrast to typical behavior, sorbate intermolecular interactions enhance sorption energetics with increased loading. The simulated structure at CO 2 saturation shows a loading with tight packing of 8 CO 2 molecules per unit cell. The CO 2 molecules can be seen alternating between a vertical and horizontal alignment within a channel, with each CO 2 molecule coordinating to an equatorial fluorine MOM atom. Calculated H 2 sorption isotherms and Q st values were also in good agreement with the experimental measurements in [Cu(dpa) 2 SiF 6 -i]. H 2 saturation corresponds to 10 H 2 molecules per unit cell for the studied structure. Moreover, there were two observed binding sites for hydrogen sorption in [Cu(dpa) 2 SiF 6 -i]. Simulations of a 30:70 CO 2 /H 2 mixture, typical of syngas, in [Cu(dpa) 2 SiF 6 -i] showed that the MOM exhibited a high uptake and selectivity for CO 2 . In addition, it was observed that the presence of H 2 O had a negligible effect on the CO 2 uptake and selectivity in [Cu(dpa) 2 SiF 6 -i], as simulations of a mixture containing CO 2, H 2, and small amounts of CO, N 2, and H 2 O produced comparable results to the binary mixture simulations.