Carbon Nitride And Graphyne As The Membran For Separating Small Molecules

Membrane separation is considered to be most promising, which mainly stemsfrom the high energy efficiency, pollution-free, and low investment cost compared tothe other conventional counterparts. High-performance membrane materials are thecore of the high efficiency membrane separation technology. The nature of membranematerial directly affects the separation and permeability. Ideal membrane materialshould not only have good physical and chemical stability, but also should have goodpermeability and high selectivity. But nowadays, the film material is very difficult tohave both high selectivity and good permeability. The permeability of the membraneis inversely proportional to the thickness of the membrane. The graphene-basedmembranes are of particular interest and great potential for hydrogen separationbecause of its one-atom thickness, and consequently high efficiency. However, acost-effective method of preparing graphene with precisely controlled pore structureis yet to come. Therefore, we studied some new membrane materials, which have aflat two-dimensional homogeneous porous structure, such as carbon nitride andgraphyne.1. We simulated the process of H2and He molecules pass through g-C3N4. Theenergy barrier for H2and He penetrating the pore of g-C3N4is0.870and0.399eV. So,g-C3N4can be used to filter He form mixed gas molecules, but it is not suitable forscreening H2under normal experimental conditions. We have investigated the effectof tensile strain on the energy barrier for gas molecules passing through the pore ofg-C3N4. The results show that the energy barrier for gas molecules passing throughthe pore of g-C3N4decreased linearly. Especially, the energy barrier for H2penetratingthe pore of g-C3N4will be reduced to0.484eV with tensile strain of6%. At the sametime, g-C3N4with tensile strain of6%exhibits a high selectivity for hydrogen overother gas molecules. Therefore, g-C3N4with tensile strain of6%can be applied forefficient separation of H2from mixed gas molecules. We also calculated the relativeenergy of total system when the gas molecules penetrating the pore of g-C10N13. Theresults show that the energy barrier for H2, O2, Ne and He penetrating the pore ofg-C10N13is0.257,0.298,0.164and0.08eV, respectively. Then we calculated the ratioof diffusion rate of gas molecules. After analysis, we find that g-C10N13can be used tofilter O2, H2, Ne and He form mixed gas molecules. These conclusions have important theoretical value to design and application of molecular sieve membrane material.2. We studied the process of H2penetrating the pore of graphyne. The resultsshow that the energy barrier for H2penetrating the pore of graphyne is2.31eV. Thehydrogen molecule is difficult to pass through graphyne under normal experimentalconditions. So, graphyne is not suitable for screening H2. We also simulated theprocess of nine kinds of gas molecules pass through graphdiyne, and obtained theenergy barrier for gas molecules passing through the pore of graphdiyne. The energybarrier for H2, Ne, and He is0.163,0.164, and0.07eV, respectively. Then wecalculated the ratio of diffusion rate of these gas molecules, and found that graphdiynecan be used as molecular sieve membrane material to filter O2, H2, and He formmixed gas molecules...