Adsorption and Separation of Xe in Metal-Organic Frameworks and Covalent-Organic Materials

We use grand canonical Monte Carlo simulations to investigate adsorption and separation of metal organic frameworks and covalent organic materials for the noble gas Xe. Results indicate that PAF-302 among these materials studied shows not only the highest gravimetric excess uptake of 4009 mg/g but also the largest volumetric uptake of 216 V(STP)/V, due to its large pore size and high accessible surface area. The gravimetric excess uptake of Xe at intermediate pressure follows the order of PAF-302 > UMCM-1 > IRMOF-1 > Cu-BTC > COP-4 > ZIF-8, which is entirely consistent with the accessible surface areas. Moreover, the maximum gravimetric excess uptakes of Xe in different materials exhibit an entirely linear correlation with the accessible surface area. However, at low pressure of p < 1 bar, the Cu-BTC shows the highest Xe uptake of 500 mg/g owing to its exposed metal sites and small side pockets. For the binary mixtures, the selectivity of Xe/N-2 follows the order of Cu-BTC > ZIF-8 > COP-4 > IRMOF-1 > UMCM-1 > PAF-302, which is actually the same with the order of difference of isosteric heats (DIH), and is in excellent agreement with our previous conclusion (i.e., the selectivity of a material is closely related to the DIH). In particular, the selectivity of Cu-BTC for Xe over N-2 reaches 80, which is an excellent candidate for Xe separation. In short, this work indicates that PAF-302 is an excellent candidate for Xe storage at intermediate pressure, whereas Cu-BTC is an excellent material for Xe separation.