Molecular Dynamics Simulation On The Separation Of Gas Mixtures By TBPB Semi-clathrate Hydrate

Hydrate functions as molecular sieve for gas separation,which is based on the selective adsorption of small gas molecules on hydrate cages.Tetra-n-butylphosphonium bromide?TBPB?can form a semi-clathrate hydrate;the crystal structure contains dodecahedron cavities and has the good molecular sieve characteristics.However,few researchers investigated the mechanisms.This paper utilized TBPB hydrate as molecular sieve to separate gas mixtures,compared the ability of hydrate cages to capture different gas molecules and provided a theoretical basis for the wide application of hydrate as molecular sieve.All molecular dynamic simulations were performed using the open source package,GROMACS software.NPT molecular dynamics simulation ran for 100 ns at pressure 5 MPa and temperate 275K.Molecular dynamics simulation was employed to capture for H₂,N₂,CO₂,CH₄,C₂H₆,and separate CO₂/N₂,CH₄/C₂H₆,CO₂/CH₄ and H₂/C₂H₆ mixtures.Snapshots of system configurations,the density of gas molecules,the radial distribution function of water around the gas molecules,and the adsorption energy of hydrate cages on gas molecules were used to characterize the sieve effect of TBPB semi-clathrate hydrate.Results showed that the cell crystal structure of TBPB hydrate contained six empty dodecahedral cages.Based on the difference of their position and structure,these cages were divided into open R cage,open D cage,R cage and D cage.When TBPB hydrate captured H₂,N₂,CO₂,CH₄ and C₂H₆,respectively,the hydrogen bonding structure composed of open cages on the hydrate surface was destroyed,gas molecules entered the open cages.The adsorption affinity of open cages on gas molecules was weak.The gas molecules then diffused into the hydrate and were absorbed into the intact R cages or D cages to form the stable hydrate structure.TBPB hydrate had different adsorption capacity for gas molecules,it captured N₂ and CH₄ with the best effect,CO₂ and H₂ took second place,and macromolecule C₂H₆ can hardly enter the hydrate empty cage.The separation effect of the gas mixtures by TBPB semi-clathrate hydrates is affected by the sizes of gas molecules.For CO₂/N₂ mixtures,N₂ preferred to enter into TBPB hydrate.The separation factor is 21.For CH₄/C₂H₆ mixtures,R cages and D cages exhibited stronger adsorption on CH₄,so CH₄ was preferentially adsorbed on the empty cages.The separation factor is 28.5.For CO₂/CH₄ mixtures,R cages preferred to adsorb CH₄,D cages preferred to adsorb CO₂,but CO₂ easily escaped from hydrate cages.The separation factor of CH₄ is 12.For H₂/C₂H₆ mixtures,H₂ preferred to enter into the hydrate cages,the separation factor is 27.It is found that R cages and D cages exhibited the strongest adsorption affinity on the solutes with a diameter with 0.373 nm,open cages exhibited the stronger adsorption on the solutes with a diameter between 0.473 to 0.573 nm.Other size gas molecules had weak adsorption affinity with TBPB hydrates.The van der Waals interaction between the guest molecules with medium size and the hydration cage was stronger,larger or smaller solutes do not have the energetic stabilization in hydrate cages.