Crucial role of blocking inaccessible cages in the simulation of gas adsorption in a paddle-wheel metal-organic framework

A molecular simulation study is reported to investigate the adsorption of CO2 and H-2 in a recently synthesized paddle-wheel Cu-based metal-organic framework (Cu-MOF). The Cu-MOF consists of three types of cages; the type-III cages are restricted by narrow windows and inaccessible to gas molecules. By blocking the inaccessible type-III cages, the simulated adsorption isotherms for pure CO2 and H-2 agree well with experimental data. Ideal-adsorbed solution theory (IAST) is used to predict the adsorption of a CO2/H-2 mixture, and the predicted isotherms and selectivities are consistent with simulated results. Furthermore, the breakthrough profiles are evaluated for a CO2/H-2 mixture in a fixed-bed packed with the Cu-MOF. The breakthrough times are estimated to be 2.8 and 85.2 for H-2 and CO2, respectively, implying the efficient separation of the CO2/H-2 mixture. The simulation study reveals the crucial role of blocking inaccessible cages in the proper simulation of gas adsorption in the Cu-MOF, and the capability of IAST applied to the Cu-MOF with inaccessible cages.