Theoretical Study On Two-dimensional Porous Materials As Molecular Sieves

As an inert gas,helium(He)plays a significant role in multiple fields,such as scientific research,medical treatment and industrial manufacture,which all depend on continuous helium supply and economic helium production.With the continuous development and progress of society,the global demand for helium continues to grow,and the shortage of helium is still an ongoing problem.Presently,natural gas is the most reliable source of helium.So far,the prevailing industrial techniques for He purification are cryogenic distillation and pressure-swing adsorption,which are either energetically costly or have a limited production capability.Therefore,searching for feasible approaches to produce and purify helium gas requires serious and urgent attention.Membrane separation technology is a new gas separation technology.It refers to the selective separation technology when the mixture of molecules with different particle sizes passes through the separation membrane.Compared with traditional gas separation methods,membrane-based separation is an emerging and desirable technology due to its merits of low energy consumption,zero pollution and easy operation.Membrane is the key of this technology,as it defines selective barriers for gas molecules.An ideal membrane should be as thin as possible to maximize flux,mechanically robust to prevent rupture and have well-defined pore size to increase selectivity.Therefore,two-dimensional membrane materials with one-atom thickness and uniform pore distribution have a great prospect in gas separation.Based on the first-principles calculations and molecular dynamics simulation,we conduct a systematic theoretical study on the application of IGP and P2C3 materials membrane in the field of helium separation,which reveals the atomic structure of two-dimensional membrane materials and the microscopic mechanism of gas separation of.It provides a theoretical basis for the research of related experiments and the application of industry.The research content and conclusions of this paper are as follows:The IGP material membrane is structurally similar to graphene in that it has high porosity and excellent thermal,mechanical and chemical stability,which means that it may be a possible candidate membrane for gas separation.Using first-principles calculations combined with molecular dynamics simulations,we have theoretically demonstrated that the IGP membrane has both high selectivities and high permeance for separating He from other impure gas molecules in natural gas(H2O,CO2,CO,CH4 and N2),as well as other noble gas molecules(Ne and Ar).Our simulations and potential mean force analysis indicated that He has a rather small kinetic energy barrier(-6.4 kBT),compared to that of Ne(-25.5 kBT)and CO2(～128.1 kBT),which largely favors the selection of the He molecules over other impure molecules by the IGP membrane.In particular,molecular dynamics simulations demonstrated that high helium permeance(approximately 10-4 mol m-2 s-1 Pa-1)can be achieved over a wide range of temperatures(100 to 500 K)with high selectivity over other gas molecules.Our computational results provide a promising He separation membrane for industrial applications.The P2C3 membrane is a porous material with a single atomic thickness and uniform distribution of natural pore size.By employing first-principles calculations and molecular dynamics simulations,we have theoretically demonstrated that the P2C3 membrane can be used to separate He from other impure gas molecules in natural gas with high selectivity and high permeance.Moreover,our MD simulation has shown that the P2C3 membrane exhibits good selectivity and permeance for He/H2 separation,which is quite attractive as most commercial membranes fails to separate them.The P2C3 membrane has good selectivity(27)and permeance(8.2×10-4 mol m-2 s-1 Pa-1)for the separation of He/H2.Incorporating the zero-point-energy with quantum tunneling effects,our quantum analysis indicated that the 3He/4He selectivity by the P2C3 membrane can achieve 76 with a 3He permeance～10-24 mol cm-2 s-1 bar-1 at 20 K,indicating the probability of 3He separation by the P2C3 membrane.In summary,our results have proposed a promising membrane for helium separation and a possible quantum sieving membrane for 3He separation.