Investigation Of The Design And Gas Separation Performance Of The Modified Graphdiynes And Graphynes With Heteroatoms

Membrane separation is considered as a promising technology and have been widely applied into the gas separation industy because of its advantages of no pollution,low energy consumption,high efficiency and convenient operation.The selection of new membrane materials is the key issues in the gas membrane separation field.Since the experimental synthesis of graphdiyne in 2010,the graphyne family has always been considered as nature membrane materials due to their uniformly high-density intrinsic pores as well as one-atom thickness.Too large or too small pore diameters in nature graphyne family have largely limited their application scope and performance as separation membranes.In this thesis,we designed three modified graphdiynes（GDY_H,GDY_F,and GDY_O）and two modified graphynes（GYN and GYH）by substituting some acetylenic linkages in pristine graphdiyne and graphyne with different heteroatoms,and further confirmed their stabilities by the calculated cohesive energies and phonon dispersion spectra.Then,the gas separation performance of the designed graphdiyne-like and graphyne-like monolayers was investigated by using both first-principle density functional theory（DFT）and molecular dynamic（MD）simulations.Firstly,we studied the CO₂/N₂/CH₄ separation performance of the modified graphdiyne monolayers.Both the DFT and MD calculations demonstrated that the GDY_H membrane has poor selectivity for CO₂/N₂/CH₄ gases because of its too large pore size,and the three gases can penetrate through the membrane by overcoming barriers of only 0.06⁰.16 eV;the GDY_F and GDY_O membranes have excellently high selectivities for CO₂/CH₄ and N₂/CH₄（idea selectivities reach 10³¹0⁵ at 298 K）,and can separate CO₂ and N₂ from CH₄ in a wide temperature range;moreover,the CO₂/N₂ mixture can be effectively separated by GDY_O at room temperature（idea selectivities is 1.7?10² at 298 K）.Besides,extremely high permeances were found for CO₂ and N₂ passing through the GDY_X membranes(10^-410^-2 mol/m² s Pa at298 K)and exceeded the industrial standard by 5⁷ orders,which surgests the GDY_F and GDY_O have high efficiency for CO₂ and N₂ separation.Then,we explored the H₂ purification performance of the modified graphynes from the mixed gases（H₂/H₂O/CO₂/N₂/CO/CH₄）.The results suggested the designed GYN and GYH membranes both can purify H₂ from mixed gases in a wide temperature range,the H₂ ideal selectivities over H₂O,CO₂,N₂,CO,and CH₄ are higher than 10¹⁰,10¹³,10²¹,10¹⁸,and 10⁴⁶,respectively,at room temperature.Besides,the moderate diffusion energy barriers（0.63 and0.68 eV）suggest that the H₂ permeance penetrating through the GYN and GYH membranes can reach the industrial standard at the temperature of 400 K and 425 K,respectively,and exceed the industrial standard by 3⁴ orders at 600 K.In addition,the MD simulations indicate that the GYN and GYH membranes only allow H₂ to pass through even at the temperature of600 K,which further confirmed the super high H₂ selectivity of GYN and GYH membranes.