Theoretical study on the gas adsorption capacity and selectivity of CPM-200-In/Mg and CPM-200-In/Mg–X (–X = –NH2, –OH, –N, –F)†
The adsorption capacities of a heterometallic metal–organic framework (CPM-200-In/Mg) to VOCs (HCHO, C2H4, CH4, C2H2, C3H8, C2H6, C2H3Cl, C2H2Cl2, CH2Cl2 and CHCl3) and some inorganic gas molecules (HCN, SO2, NO, CO2, CO, H2S and NH3), as well as its selectivity in ternary mixture systems of natural gas and post-combustion flue gas are theoretically explored at the grand canonical Monte Carlo (GCMC) and density functional theory (DFT) levels. It is shown that CPM-200-In/Mg is suitable for the adsorption of VOCs, particularly for HCHO (up to 0.39 g g−1 at 298 K and 1 bar), and the adsorption capacities of some inorganic gas molecules such as SO2, H2S and CO2 match well with the sequence of their polarizability (SO2 > H2S > CO2). The large adsorption capacities of HCN and HCHO in the framework result from the strong interaction between adsorbates and metal centers, based on analyzing the radial distribution functions (RDF). Comparing C2H4 and CH4 molecules interacting with CPM-200-In/Mg by VDW interaction, we speculate that the high adsorption capacities of their chlorine derivatives in the framework could be due to the existence of halogen bonding or strong electrostatic and VDW interactions. It is found that the basic groups, including –NH2, –N and –OH, can effectively improve both the adsorption capacities and selectivity of CPM-200-In/Mg for harmful gases. Note that the adsorption capacity of CPM-200-In/Mg–NH2 (site 2) (245 cm3 g−1) for CO2 exceeded that of MOF-74-Mg (228 cm3 g−1) at 273 K and 1 bar and that for HCHO can reach 0.41 g g−1, which is almost twice that of 438-MOF and nearly 45 times of that in active carbon. Moreover, for natural gas mixtures, the decarburization and desulfurization abilities of CPM-200-In/Mg–NH2 (site 2) have exceeded those of the MOF-74 series, while for post-combustion flue gas mixtures, the desulfurization ability of CPM-200-In/Mg–NH2 (site 2) is still comparable to those of the MOF-74 series at 303 K and 4 MPa. We hope that the current theoretical study could guide experimental research in the future.