Dffusion Beaviors Of CH₄，CO₂and Their Mixtures In Zeolitic Imidazolate Frameworks-8Explored By Molecular Simulations

Natural gas is a kind of high-quality energy. It has large number of advantages:large calorific value, convenient transportation, complete combustion, smoke-free, noresidue, cheap price and so on. Therefore, it is widely used in the field of internalcombustion engines, automobiles, steel, heat treatment, printing and dyeing, textile oftransportation, metallurgical, power, light industry. At the same time as an importantfuel supply for residents. But natural gas is a gas mixture of hydrocarbon and variousimpurities, including acidic gas such as hydrogen sulfide, carbon dioxide and nitrogen,helium, water which can not only influence gas calorific value but also corrodepipeline. It can be applied after separation and purification. So the separation ofnatural gas has received the widespread attention. Zeolitic imidazolate frameworks（ZIFs） material is the sub-family of Metal organic frameworks （MOFs）.This kind ofmaterial has tunable pore geometry as MOFs and has special chemical stability andthermal stability, especially can be stable in water. Molecular simulation one of themost popular computer simulation technologies is widely applied in the field of gasadsorption and diffusion behaviors in porous materials. It is able to research thechemical and physical properties that the experiment is difficult to detect at themolecular level.At present, the literature of molecular modeling study is mostly focus on theadsorption behavior of gas molecules in ZIFs materials. However, it is still veryscarce for related diffusion behavior research. Therefore, this paper using molecularsimulation technology study the diffusion behavior of CH₄, CO₂and mixed gas inZIF-8. It plays a guiding role for the preparation of the effective gas separationmaterials. This thesis mainly research contents are as follows:On the one hand, GCMC was used to explore CO₂adsorption and adsorptionmechanism at298K at1,5,10,15bar in flexible ZIF-8. Then, we studied diffusionbehavior of single-component CO₂gas in ZIF-8cage. Through the simulation, theadsorption quantity in the simulation results can be agreement well with experimentresults which indicated the accuracy of this work. Through density distribution of CO₂gas in ZIF-8material, we can conclude that there were two adsorption sites: one waslocated in the both sides of the six-member ring and the other was located on the top of the four-member ring. Then MD method was used to study diffusion behaviors ofpure CO₂in ZIF-8. The sublinear variations of MSD curves mean that CO₂moleculesin ZIF-8exhibited two-stage diffusion: one was intra-cage free diffusion, the otherwas inter-cage sub-diffusion. The most important is that it was the first time therelevant mean residence time molecules can be up to several tens of picoseconds inthe pressure range of1to15bar.On the other hand, the diffusion behaviors of CO₂/CH₄mixtures were studied byMD method under different loadings. In order to contrast the results of the mixture,we also calculated the same components of pure CO₂and CH₄gas moleculardiffusion behavior in ZIF-8. We concluded that whether in the pure components ormixtures CO₂diffusion was faster than CH₄. At the same time, the diffusion rate ofCO₂in both single component and mixture were higher than the diffusion rate of CH₄.At the same time, we found that with the change of loadings the diffusion rate of CO₂presented a similar trendency as pure component. However, diffusion rate of CH₄inthe mixture was different from pure components largely. It indicated CO₂moleculedominated the diffusion behavior of CO₂/CH₄gas mixture in ZIF-8. Based on thedensity distribution of CO₂in ZIF-8, we found that CO₂adsorped near the window ofsix-member ring of ZIF-8hindered the effective diffusion of CH₄molecules. Werevealed the diffusion behavior from the molecular level. It provided theoreticalguidance for further synthesis of new materials using in the separation of natural gasindustry.