First-principles Study On The Adsorption And Separation Performance Of Two-dimensional Metalloporphyrin Organic Thin Film

In recent years,biological porphyrins with abundant p electrons and largeπbonds have shown obvious properties of contributing electrons.Based on this property,single-layer porphyrin-based films with transition metal atoms as active centers have a wide range of applications,such as electrocatalysis,gas sensors and spintronics.According to the research of our research group,there is an obvious charge transfer phenomenon between the metal atoms and the adsorbed molecules in the two-dimensional metalloporphyrin-based film,which shows excellent activity.Therefore,based on the structural unit of porphyrin molecule,we designed a kind of metal porphyrin-based two-dimensional porous organic film,and explored its physical and chemical properties from the theoretical level using the first-principles calculation method:（1）as a high-density metalloporphyrin membrane to adsorb and detect harmful gases;（2）as molecular sieve material to separate and purify C₆ alkanes energy molecules.In the first part,firstly,we proved that the metal porphyrin grid structure（MPP-Grid,M=Fe,Co,Zn）with single metal atom Fe,Co,Zn as the active site is stable.Next,the stable adsorption structures of three harmful gas molecules,CO,NO and NO₂,on the substrate were determined.The calculated adsorption energy showed that Fe PP-Grid was the best adsorbent for adsorbing the three molecules among the three substrates.According to the calculation of Gibbs free energy at different temperatures,the increase of temperature would hinder the adsorption,but even at 900 K,the adsorption capacity of Fe PP-Grid to three molecules was still strong.According to the calculation of magnetic properties and substrate recovery time,since NO and NO₂adsorbed on Zn PP-Grid substrate existed different magnetic moment values,they could be used to detect two kinds of gas molecules,and because the time required for molecular desorption was very short(10^-9-10^-4 s),the substrate could recover to its original state in a short time,which was conducive to recycling.Similarly,Fe PP-Grid substrate could detect and distinguish three kinds of molecules,and it was easier to recover to its initial state under high temperature.This study can be applied to the green environmental protection field of reducing air pollution.In the second part,firstly,the mechanical stability of the structure of Zn PP-Grid molecular sieve material had been proved.Secondly,the diffusion barriers of four C₆alkanes energy molecules passing through the pore area of Zn PP-Grid molecular sieve were calculated based on CI-NEB method.The results showed that straight-chain hexane molecule could freely penetrate the pore without energy barrier,while double-branched 2,2-dimethylbutane could not pass through.Thirdly,the analysis of electronic properties of the transition state structures illustrated that the larger the overlapping charge density between molecules and the atoms at the edge of pore,or the more transferred electrons,the higher the diffusion barrier.Then,the geometric deformation and energy change of C₆ alkanes and Zn PP-Grid molecular sieve film during molecular penetration processes were analyzed.It could be seen that Zn PP-Grid membrane had high flexibility.Finally,by calculating the relative selectivity of molecules,it can be found that in the temperature range of 150 K-400 K,the relative selectivity of Zn PP-Grid molecular sieve membrane for linear hexane molecule was very high(～10⁸–10⁸⁰),so hexane molecule can be easily separated from the mixture,and the remaining molecules can be used to produce high-octane gasoline.In addition,the relative selectivity of mono-branched 3-methylpentane/di-branched 2,2-dimethylbutane mixture can reach 10²⁹,so the two isomers can also be distinguished.As a result,Zn PP-Grid molecular sieve film has good application prospects in the energy field.The paper is divided into the following five chapters:The first chapter describes the development status and research significance of two-dimensional nanomaterials,the structural design process of high-density two-dimensional organic films,the background and research content of this topic.In chapter 2,the density functional theory and the first-principles calculation method applied in this project are described in detail.In the third chapter,the adsorption,removal and detection of harmful gas molecules CO,NO and NO₂ by 2D high-density metalloporphyrin membrane are discussed in detail,and the feasibility is also analyzed.In chapter 4,the separation and purification of C₆ alkanes energy molecules by 2D Zn PP-Grid molecular sieve membrane are discussed in detail,and the separation properties of Zn PP-Grid membrane are given by analyzing diffusion barrier,electronic properties,selectivity and permeance.The fifth chapter summarizes this topic,and draws the conclusion that2D organic MPP-Grid（M=Fe,Co,Zn）membranes are promising single atom adsorption membrane and molecular sieve materials.