| Metal-organic frameworks (MOFs), as the new kind of emerging materials, have got a rapid development in the past ten years. This class materials have the advantages of high adsorption capacity, highly ordered porosity over the traditional porous materials, such as zeolites, carbon nano tubes (CNT) and activated carbons. Additionally, we can further modulate and control the properties of channel surface of the materials by modifying organic linkers, so that the interaction between the porous materials and guest molecules can be tuned. Due to the above described advantages, MOFs have great potential application in gas storage, separation, catalysis and biomedicine. However, to realize these industrial applications, it is necessary to study the microscopic diffusion mechanism and the separation performance of guest molecules in MOFs. Additionally, a good knowledgement of the MOFs stability to the adsorbed guest molecules is also necessary. Thus, the studies were carried out in this work as follows:(1) The concentration dependence of self-diffusivity of C2H6was systematically studied by molecular dynamics (MD) simulations, and the microscopic diffusion mechanism of C2H6molecules in UiO-66(Zr) was also studied. In this work, MD simulations were conducted with rigid and flexible UiO-66(Zr) framework, and it was found that the Ds values of C2H6present a nonmonotonic tendency with a maximum with the increase of loading in the comparison of simulated and experimental values. Furthermore, the comparison of simulated results based on flexible and rigid framework indicates that the results simulated with flexible framework are more agreement with experimented diffusivities in both the magnitude and the concentration dependence, and this highlights that the flexibility of framework is much more significant to study the diffusion of ethane molecules in porous materials. Additionally, the global microscopic diffusion mechanism involving a combination of intracage motions and jump sequences between tetrahedral and octahedral cages is observed, which is consistent with the diffusion of other small molecules in this material.(2) The transport diffusivities of CO2and N2in UiO-66(Zr) were studied using MD simulation, and results show that the diffusivities of N2are an order of magnitude larger than the ones of CO2. Then, the adsorption selectivity, diffusion selectivity and membrane selectivity of CO2/N2were calculated at300K, and it was found that the membrane selectivity can reach4.003, a value higher than the ones in other MOFs. Thus, UiO-66(Zr) is suitable for separating the CO2/N2mixture.(3) The influence of acid gas H2S on the stability of Cu-BTC framework was studied by means of quantum chemical calculations, and the adsorption mechanism was analyzed in this work. The process of structure optimization demonstrates that H2S molecules are coordinated on the open metal sites of Cu-BTC firstly, no matter whether there is water or not. When the open metal sites are occupied, the added H2S molecules will lead to the rupture of Cu-O in Cu-BTC, and the Cu ions will combine with HS-through Cu-S bond, and the O ions will combine with H+through O-H bond, then the structure of Cu-BTC will disintegrate finally. The binding energies of structure optimization were calculated, and found that the binding energy of coordination process is much larger than the one of disintegration process; this indicates that there are two ways for the adsorption of H2S in Cu-BTC. Firstly, H2S molecules are coordinated onto the open-metal sites of Cu-BTC, and this adsorption will not’ lead to the disintegration of framework. And then, when the number of H2S molecules is large enough, the redundant H2S molecules will disintegrate the Cu-BTC framework, and lead to the collapse of material finally. |
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