Molecular Simulation Study Of Gas Mixtures Separation In Metal-Organic Frameworks

Metal-Organic Frameworks(MOFs), commonly recognized as "soft" analogues of zeolites, is a new class of nanoporous materials. Due to their large surface areas, adjustable pore sizes and controllable properties, as well as acceptable thermal stability, MOFs are promising candidates for a wide range of applications in gas storage, separation, catalyst, biochemistry and pharmacy, etc. The study of MOFs has become a research frontier area of materials and hotspots.First, a novel separation concept based on the thermodynamic stepped behaviors of adsorption in nanoporous materials was brought our work. For this study, five TRMOF materials (IRMOFs-8,-10,-12,-14 and-16) with different pore sizes and similar chemical compositions were adopted as the respresentative of MOF solid. Then Grand canonical Monte Carlo(GCMC) simulations were performed to investigate the separation of CO2/N2 mixture with a bulk composition CO2:Na=15:85 (typical for flue gas) in these MOF materials at six temperatures, as a function of the bulk pressure up to 2.0 MPa. The simulation results show that the stepped behaviors can significantly enhance the adsorption selectivity of CO2 from the mixture. Based on the analysis of the various interactions between the adosobate-adsorbate and adsorbate-MOFs as well as the dependene of the enthalpy difference between CO2 and N2 gases, the so-obtained underlying mechanism at the molecular level reveals that the stepped phenomenon is mainly caused by the electrostatic interactions between CO2 molecules. In addition, it is also found that this type stepped behavior is highly affected by the pore size of the materials and temerature. Further, based on IRMOF-16 with the largest pore size among the selected MOFs, we constructed a new hypothetical material Li-modified form (denoted as IRMOF-16-4Li) by substituting four hydrogen atoms on the aromatic rings with four O-Li groups. It is found that the Lithium-modified MOF can greatly reduce the pressure at which the stepped behavior occurs, and also enhance the selectivity of CO2 from CO2/N2 gas mixture. The new method for mixture separation is expected to be not only of interest in the physical and surface sciences but also applicable for the separation of other mixtures where one of the components can exhibit such stepped behaviors.Second, GCMC simulations were also performed to study the adsorption of pure SO2 gas and its gas mixture with CO2 in several ZIFs at 298 K. The results show that the adsorption of single component and the separation of gas mixture are influenced primarily by functionality effects rather than pore metrics; the fewer of the composition of SO2 from SO2/CO2 mixture, the higher of the selectivities for SO2 from SO2/CO2 mixture under the same pressure.