Molecular Simulation Study On The Gas Separation Performance Of Metal-Organic Frameworks

Metal-organic frameworks （MOFs） constructed by inorganic clusters and organic linkers through coordination bonds have become one of the hotspots in the research on porous materials over the last decades. In comparison with the other porous materials, MOFs have some unique advantages such as large free volume, high specific surface area, diversity of structures, multifunctionality of pores and channels, designable structure and pore size, and these features make MOFs have promsing applications in energy gas storage, gas/liquid separation, membrane separation, catalysis, sensing and drug delivery. Up to now, more than twenty thousand of MOFs have been synthesized. Due to the large numeber of MOFs, it is time-consuming to study the properties of MOFs using solely experimetal method. Computational chemistry, emerging as a complement to experimental method, is widely used in obtaining the microscopic mechanism, exploring new phenomenon, building structure-property relationships of MOFs, screening MOFs for specific application as well as designing new materials. Especially, the combination between computational chemistry method and experimental method greatly promote the development of MOF materials.In this paper, we explore the performance of MOFs and ionic liquid/MOF composites for gas seapration and membrane separation using molecular simulations. The main contents are as follows:1. Molecular simulations were performed to investigate the performance of 151 MOFs with large chemical and topological diversity on CO₂/CH4 separation via temperature swing adsorption （TSA） process. The thermal regeneration energy was firstly adopted as an evaluation criterion and combined with other three commonly used ones to fully estimate the separation ability of MOFs and the structure-property relationships were bulit for the separation process. The results show that the four evaluation criteria exhibit intimate correlations with the difference of adsorbility （△AD） of adsorbates but with non-concerted changing tendency. Under a certain range of this parameter, it can be used as a good indicator for the preliminary screening of MOFs and the tailoring of new materials. Besides, through analyzing the common structural characters of MOFs with good separation performance, we also provide useful information to design new MOFs with good performance for natural gas purification.2. A systematic computational study was performed to investigate the dispersion behavior of ionic liquid （IL） in MOF as well as the separation performance of the resulting composites for H2S/CH4 mixture. Cu-TDPAT was selected as supporter for four types of ILs combined from identical cation [BMIM]+ with different anions. The results show that the type of ions and the loading of ILs introdued into the pores of Cu-TDPAT effect the dispersion behavior of ILs. Then, the dispersion behavior and loading of ILs affect the separation performance of IL/Cu-TDPAT composites for H2S/CH4 mixture. The adsorption affinity towards H2S is significantly enhanced by incorporating each IL into the pores of Cu-TDPAT, and the H2S/CH4 adsorption selectivities of each composite are significantly higher than those of the pristine Cu-TDPAT within the pressure range examined. Besides, the selectivity of [BMIM][C1]/Cu-TDPAT composite with a loading of 25 IL molecules per unit cell can reach about 1302 and 715 at 1.0 and 10.0 bar, respectively, which are much higher than those reported in the literature. In both VSA and PSA processes, the H2S working capacity of the IL/Cu-TDPAT composites is also higher than that of Cu-TDPAT. On the basis of the results obtained in current study, it can be expected that incorporating ILs into the pores of MOFs is an alternative efficient strategy to generate new promising adsorbents for practical H2S separation related to natural gas purification.3. A new "cavity occupying" concept for tailoring the diffusion properties of guest moleculs in cage-type MOFs, namely, modification of the microenvironment of MOF’s nanocages via introduction of ILs ascavity occupants, was proposed to enhance the membrane separation property. The adsorption separation and membrane separation of CO₂/N₂ mixture for ZIF-8 and IL/ZIF-8 composites were computationally examined. The results show that the selectivity of CO₂/N₂ mixture for IL/ZIF-8 composites is greatly enhanced. As membrane materials, IL/ZIF-8 composites also show high membrane selectivity and CO₂ permeability, which surpass the Robeson’s upper bound for polymer membrane. Besides, the membrane selectivity of IL/ZIF-8 composites is higher than that of the other MOFs. Furthermore, the IL/ZIF-8 composites used as filler particles in mixed matrix membranes （MMMs） can enhance the CO₂ permeation performance of pure polymers with moderate separation ability. It can be conculed that using IL as the pore ornament and controller of MOFs is a efficenit way to increase the adsorption and permeation performance of MOFs.4. Flue gas usually contains trace impurities such as H2O and SO₂ molecules. Hence, it is valuable to investigate the separation performance of MOFs in the presence of these impurities. Moleculer simulations were performed to explore the effect of H2O and SO₂ on the separation performance of CO₂/N₂ mixture for UiO-66 and IL/UiO-66 compsoties. The CO₂/N₂ adsorption selectivities of IL/UiO-66 are significantly higher than those of the pristine UiO-66 within the pressure range examined, and the selectivity generally shows an increasing trend with increasing the loading of ILs. The presence of H2O affects the CO₂ adsorption on UiO-66 and IL/UiO-66 in two opposite ways:the negative effect of competitive adsorption of the impurity gases on the adsorption sites over CO₂, and the positive effects of formation of new adsorption sites for CO₂ increasing the interaction between CO₂ and MOF. In [BMIM][Cl]/UiO-66 and [BMIM][PF6]/UiO-66 composites, the competitive adsorption of H2O over CO₂ is strong leading to the decrease of selectivity. The presence of SO₂ did not affect the selectivity of CO₂/N₂ mixture for UiO-66. However, the selectivity of CO₂/N₂ mixture for IL/UiO-66 compsoties increase with increaing the content of SO₂ due to the increasing of the isosteric heat of adsorption between adsorbates and adsorbates.