(510a) Crucial Role of Blocking Inaccessible Cages in the Simulation of Gas Adsorption in a Paddle-Wheel Metal-Organic Framework
H2 used in industry is generally produced from steam-methane reformation followed by water-gas shift, and is required to be separated from the effluent gas for production of high-purity H2 and for pre-combustion CO2 capture. In the last decade, metal-organic frameworks (MOFs) have emerged as a new family of hybrid porous materials. The large surface area, high porosity, and tunable structure place them at the frontier for CO2/H2 mixture separation. Numerous experimental and simulation studies have been reported for gas adsorption in MOFs. In the simulation studies of gas adsorption in MOFs, the predicted capacities are usually compared with available experimental data. The deviations between simulation and experiment are generally attributed to two reasons: impurities or defects present in experimental samples, and the inaccuracy of force fields used to describe gas-MOF interactions. However, a less discussed possible reason is that inaccessible cages existing in MOFs might not be taken into account by simulation. We report a molecular simulation study to investigate the adsorption of CO2 and H2 in a recently synthesized paddle-wheel Cu-based metal-organic framework (Cu-MOF). The Cu-MOF consists of three types of cages, and type-III cages are restricted by small windows and inaccessible to gas molecules. Without blocking the inaccessible cages, the simulated adsorption isotherms are much higher than experimental data. However, the agreement is substantially improved after blocking. For CO2/H2 mixture, the adsorption uptakes predicted from the ideal-adsorbed solution theory (IAST) are nearly identical to simulation results. The selectivity increases with increasing pressure due to the enhanced cooperative interactions between CO2 molecules, which is also predicted by the IAST. Furthermore, the breakthrough profiles are evaluated for CO2/H2 mixture in a fixed-bed packed with the Cu-MOF. The breakthrough times are estimated to be 2.8 and 85.2 for H2 and CO2, respectively, implying the efficient separation of CO2/H2 mixture. This simulation study demonstrates that blocking inaccessible cages is crucial to simulate gas adsorption in the Cu-MOF, and the IAST can be applied to the Cu-MOF with inaccessible cages. In new MOFs or other porous materials, one should conduct structural analysis to determine if there exist inaccessible cages before starting the simulation of gas adsorption.
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