| Graphene, the new carbon based nanomaterials, has been widely used in sensors, solar cells, transistors, and hydrogen storage due to its exceptional mechanical, thermal, optical and electrical properties since its discovery in 2004. The study about graphene has attracted great attention of scientists. It is very difficult to characterize their properties due to the nanosize of graphene, while molecular dynamic(MD) simulation method has much more advantages on studying the nanosized materials. Therfore, the main purpose of this article is to study the gas separation performance of porous graphene by using MD simulations. We will explore the influence factors, describe the separation process and also give the separation mechanism.First, the separation process of CO2 from a mixture of CO2 and N2 using a porous graphene membrane is investigated using MD simulations based on the COMPASS force-field. The effects of chemical functionalization of the graphene sheet and pore rim on the gas separation performance of porous graphene membranes were examined. It was found that chemical functionalization of the graphene sheet can increase the adsorption ability of CO2. This may be because the insertion of functional groups increases the active surface area of the porous graphene membrane, thereby increasing uptake. The –OH modi?ed graphene membrane exhibits the best adsorption ability towards carbon dioxide. While the chemical functionalization of the pore rim can signi?cantly improve the selectivity of CO2 over N2. The results show that the porous graphene membrane with all-N modi?ed pore-16 exhibits a higher CO2 selectivity over N2(11) due to the enhanced electrostatic interactions compared to the unmodi?ed graphene membrane. The in?uence of pore size and partial pressure on the CO2/N2 separtion performance of porous graphene membrane were also investigated and the number of the passing gas will increase as increasing the pore size or partial pressure. Therefore, this study demonstrates the potential use of functionalized porous graphene membrane for CO2 and N2 separation. We provide an effective way to improve the gas separation performance of porous graphene membranes, which may be useful for designing new concept membranes for other gases.Then, we simulate the separation process of CO2/CH4 using a N-doepd porous graphene membrane. The effects of pore size, N doping sites, initial gas pressure and feed gas compositions on CO2/CH4 separation performance of N-doped porous graphene membrane have been also investigated. It is found that CO2 permeability increases with increasing initial gas pressure or the feed gas percentages. Pore rim modifications with nitrogen atoms(pyridinic N) can significantly improve selectivity of CO2 over CH4 owing to the enhanced electrostatic interactions compared to the unmodified one and the all-N modified pore-16 shows the highest CO2 selectivity over CH4(~29), which demonstrates the potential use of N-doped porous graphene membrane for CO2 and CH4 separation. Doping N atoms on the graphene sheets(quaternary N) can increase the CO2 adsorption ability but have little effect on the CO2 selectivity. In addition, we also found that N-doped porous graphene can be used to separate CO2 from other CO2 mixture components(e.g. N2, H2, O2 and CO) and confirmed that the proper adsorption ability of N functionalized pore towards CO2 is the main reason for separation.Our simulation results have broadened our understanding on the effects of chemical functionalization on the the interaction between graphene and gas molecues at the micro level. It can also provide some theoretical guidance for designing graphene membrane and its application in gas separations. MD simulation methods can reduce the research period and the cost. Besides, it can also reduce the workload, effectively give the experimental direction and verify the accuracy and the precision of the experimental results. Our study also make it possible to use this kind of porous graphene in gas separation ereas. |
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