Molecular Dynamics Study On The H₂ Separation Ability Of Graphene Membranes Embedded With Crown Ethers

As the consumption of fossil energy by human society is accompanied by ecological destruction and environmental pollution,especially the increase of harmful particles in the atmosphere and the global greenhouse effect,accelerating the construction of a cleaner and more efficient energy structure system is a key measure to address climate change and environmental issues.Due to its excellent characteristics of high energy density and zero combustion pollution,the application of hydrogen energy has received increasing attention.It is well known that electrolytic water can produce hydrogen,but it is costly and energy consuming.Therefore,industrial hydrogen production is mainly achieved through reforming petroleum and natural gas and coal gasification.The by-product of industrial production,coke oven gas,contains 54%to 59%H2 and 21%to 28%CH4,as well as a small amount of N2,CO,CO2,and other gases.In order to obtain high-purity H2,we need to separate it from a mixture containing H2.Currently,the main methods for separating H2 include membrane separation,pressure swing adsorption,and cryogenic separation.Membrane separation technology is popular due to its advantages such as simple operation,high efficiency,and low energy consumption.The key to membrane separation technology is the selection of suitable separation membranes.Due to the unique carbon atom arrangement and high strength mechanical properties,two-dimensional nano porous materials,especially graphene derivatives,have great potential in the field of membrane separation.Due to its dense structure,monolayer graphene cannot serve as a gas separation membrane.We consider embedding crown ethers into graphene and using 18-crown-6 crown ethers as key components of the separation membrane to simulate the transport of H2 gas mixtures,in order to demonstrate the feasibility of graphene crown ethers as H2 separation membranes.In this paper,a combination of molecular dynamics simulation and density functional calculation is used to simulate the gas permeation process of single and double layer graphene crown ether membranes using combinations of H2/CH4,H2/N2,H2/CO2,and H2/N2/CH4.Our molecular dynamics results show that under a single gas simulation,H2 can be efficiently transported to the other side of the graphene crown ether membrane at room temperature and normal pressure,while CH4,N2,and CO2 are completely excluded from the original region.We found that H2 has the characteristics of rapid passage through crown ethers under mild conditions.In the simulation of mixed systems,H2 can quickly pass through H2/CH4,H2/N2,and H2/CH4/N2 to the other side of the graphene crown ether,while CH4 and N2 are still completely excluded from the original region.This result has higher H2 selectivity and permeability compared to previous studies,and can spontaneously proceed at normal temperature.However,under the mixed simulation of H2/CO2,it was found that the transport capacity of H2 significantly decreased compared to the other side of the membrane,and a small amount of CO2 permeated to the other side of the membrane as the temperature increased.We found that there was a strong interaction between CO2 and the crown ether pore,leading to adsorption and blocking of the crown ether pore.Using density functional theory to calculate the energy barrier of four gases passing through graphene crown ethers,we found that H2 has the lowest value among the four gases,while the values of CO2,N2,and CH4 increase in turn,which explains why H2 is more likely to pass through graphene crown ethers and other gases are excluded.Compared to single-layer graphene crown ether membranes,double-layer graphene crown ether membranes can slightly reduce the amount of H2 penetration,but still maintain a good effect.In summary,our research shows that graphene crown ether is a good gas separation membrane,especially used for the separation of H2 in coke oven gas.