Computational Chemistry Study On Gas Transportation Through Molecular Sieve Membranes

Computational chemistry is based on the theory of quantum mechanics,using effective mathematical descriptions and computer programs to calculate the properties of molecules.It is used to explain and achieve a variety of essential chemical problems.The four main approaches to calculating molecular properties are ab initio methods,semiempirical methods,the density functional theory（DFT）method,and the molecular mechanics（MM）method.From a longitudinal view,there are two main techniques for generating an ensemble,molecular dynamics（MD）and Monte Carlo（MC）.While computational results normally complement the information obtained by chemical experiments,it can in some cases predict hitherto unobserved chemical phenomena.It is widely used in the design of new materials and drugs.Examples of such properties are structure（i.e.,the expected positions of the constituent atoms）,absolute and relative（interaction）energies,electronic charge density distributions,dipoles and higher multipole moments,vibrational frequencies,or other spectroscopic quantities,reactivity,cross sections for collision with other particles,and quantitative structure–property relationship（QSPR）or quantitative structure–activity relationship（QSAR）.Nanoporous materials in polymer materials have high specific surface area,high porosity and various topological pore structures,which have high potential industrial application value.Among them,carbon molecular sieve membranes（CMSMs）or amorphous carbon（a-C）membranes porous material contains different hybrid types of carbon atoms,is a common material in chemical,optical and electronic industries,used in gas separation,wear-resistant coating,solar collector surface,magnetoresistance,field emission,etc.Metal–organic frameworks（MOFs）are used in separation,catalysis,adsorption,fluorescent probes,sensors,photoelectromagnetic materials,drug transport,biological entity protection technology and other fields.In this work,computational chemistry for porous materials was studied.CMSMs and CAU-1（a kind of MOFs）membrane for gases separation systems were selected to calculate using molecular dynamics,quantum chemistry etc.method.（1）CMSMs with different densities could be successfully constructed by our DCP-NEMD method.Melt-quenching technique was adopted under different temperature and pressure conditions.Energies of the system showed reasonable behavior by the Stillinger-Weber（SW）two-body and three-body potentials to reach a steady state.The real-time temperature and energy changes of various formation configurations were monitored.The rationality investigation of the configuration was investigated,including radial distribution function（RDF）,bond angle distribution（BAD）and pore size distribution（PSD）,etc.Time mesh was a very important factor to generate different structures at the same density.The smaller time mesh and a little more running time were set,the more homogeneous membrane was obtained.The desired CMSM configuration(density of 1.8 g·cm^-3 and pore size of 0.75 nm at time mesh of 1fs)was selected for further gas permeation calculation.The permeation behaviors of common industrial small molecule gases（He,Ne,H₂,CO₂,N₂,CH₄ and SF₆）were investigated by DCP-NEMD method.Comparation with Knudsen diffusion,different gas species molecules permeation showed little different activities.As a result,the number of permeated particles was depended on not only molecular dynamics diameter,but also weight,shape and the interactions of gas molecules.The permeances almost decreased with gas molecular weights through CMSMs of low density.As for SF₆,the permeation mechanisms through CMSMs were from Knudsen diffusion to surface diffusion and then to active diffusion.And the permeation order was H₂>He>CH₄>Ne>N₂>CO₂>SF₆.（2）The initial CAU-1 membrane structure was from the experimental single crystal XRD（x-ray diffraction pattern）results through Rietveld refined analysis in Cambridge Structural Database.Its crystal growth morphology was inferred by using BFDH（Bravais-Friedel Donnay-Harker method）crystal modeling.After fine pretreatment and geometry optimization,the characteristics of CAU-1 supercell were further analyzed including the topology,XRD,PSD,free volume,electrostatic potential（ESP）colored van der Waals（vd W）surface map,etc.The reasonable structure for simulation system was obtained.Then,DCP-NEMD method was used to investigate the permeation behavior of gas molecules through CAU-1 membrane.Pure gas systems and binary mixtures were calculated under the grand canonical ensemble（μVT）from 298 K to 600 K.Gas molecules were considered as Lennard-Jones（LJ）particles with periodic boundary conditions（PBC）in x-and y-coordinates.As for pure gas systems（CO₂,N₂,CH₄,H₂,He,Kr and Xe）,the computational permeances of 0.62～2.62×10^-3 mol?m^-2?s^-1?Pa^-1were about 3 orders of magnitude bigger then experimental ones as the function of the thickness of membranes.H₂ has the highest permeance from Knudsen diffusion to surface diffusion.Ideal H₂/CO₂separation factor of 2.1 was consistent with the experiment result.As for the binary mixtures（CO₂/N₂,CO₂/CH₄,CO₂/H₂ and H₂/CH₄,and rare gas mixture Kr/Xe）,all the input molar ratio were 1:1.For H₂/CH₄,the trend of H₂ permeance with temperature dependency was similar to that of single component gas.At 298 K,the H₂/CH₄ separation factor of 1.9 was consistent with the experimental one.The separation factor of the mixture was slightly lower than the ideal one,suggesting that there was a competitive adsorption relationship on the same site in CAU-1,and it was not obvious.The diffusion rate of CO₂ is faster than that of N₂,which may be due to its characteristic adsorption.Because the framework pores were not able to carry more gas molecules through.For CO₂/CH₄ and CO₂/H₂,the simulated separation factors reached 23.0 and 12.9 respectively when considering electrostatic and van der Waals interactions,which are closer to the experimental results than the ideal separation factors.Because there were stronger interactions between the quadrupole moment CO₂ molecule and the NH₂-on the framework wall of CAU-1 than that of other gas molecules,resulting in CO₂molecule passing through preferentially.For the Kr/Xe binary system,the permeances of Kr was nearly constant at 3.1×10^-4 mol?m^-2?s^-1?Pa^-1,while Xe of 4.1?which is almost same as the PLD of CAU-1,could hardly pass through.That showed obvious molecular sieve effect.Therefore,DCP-NEMD method can be used to predict that CAU-1 membrane material has extremely excellent selectivity and separation performances for radioactive gases Kr,Xe or Kr/Xe in practical application.Through those calculations,the expected configuration models and gases permeation results are obtained and used to guide the industrial applications.DCP-NEMD method provides an advantageous MD method to simulate gas permeation properties.