Theoretical Investigation Of The Topology And Gas Adsorption Of The Ionic Covalent Organic Frameworks(ICOFs)

Porous materials have attracted much attention in recent years for their potential applications in gas storage,separation and catalysis.In order to improve the performance of porous materials in gas adsorption,ion functional groups have been introduced into porous materials to form a kind of ionic covalent organic framework?ICOF?.This type of ICOF has excellent thermal and chemical stability,outstanding contribution in proton/ion conduction and ion exchange fields,and good performance in gas storage and separation.In this paper,theoretical research is carried out on two new types of ICOF.The main results are as follows:1.A novel type of ICOFs with spiroborate linkage has been recently designed and synthesized by Zhang and coworkers.The spiroborate-linked ICOFs exhibit high BET surface areas and significant amount of H₂ and CH₄ uptakes,combined with excellent thermal and chemical stabilities.Inspired by the novel properties of the ICOFs,with the aim to gain better understanding on the structure of such spiroborate-linked ICOFs,a series of potential 3D network configurations of ICOFs connected with tetrahedral[BO₄]^-node were proposed,assuming the[BO₄]^-node in spiroborate segments takes a tetrahedral configuration.These ICOFs,in terms of 2D and 3D topologies via torsional energy of the[BO₄]^-fragment,pore-size distribution,total energy of the structure,and gas adsorption properties were compared and systematically investigated by density functional theory?DFT?calculations,molecular mechanics?MM?,and well-established Grand Canonical Monte Carlo?GCMC?simulations.The results indicate that spiroborate-linked ICOFs are likely a mixture of various topologies.Among these architectures,5-fold interpenetrating model shows the lowest energy and reasonable gas uptakes,therefore,it is considered to be the most possible structure.More importantly,the 5-fold interpenetrating model,showing high CH₄ uptakes compared with several classic porous materials,represents a promising CH₄ storage material.2.The gas adsorption and separation properties of PyTTA-BFBIm-ICOF ionic covalent organic skeleton were studied by GCMC simulations.In order to enhance its adsorption performance,several PyTTA-BFBIm-ICOF derivatives were designed?PBX-ICOF,X=F,Cl,HPW12O40?by changing the counter ions in skeleton.We found that PBX-ICOF(X=F,Cl,HPW₁₂O₄₀)can enhance the adsorption ability of CO₂,and revealed the gas adsorption sites in PBX-ICOF?X=F,Cl,HPW12O40?through the gas-channel interaction energies by DFT.Moreover,compared with the PyTTA-BFBIm-ICOF,the selectivity of designed derivatives PBX-ICOF(X=F,Cl,HPW₁₂O₄₀)towards CO₂/N₂,CO₂/CH₄,and CO₂/H₂ can be greatly enhanced.Therefore,the proposed derivatives PBX-ICOF?X=F,Cl,HPW12O40?are promising candidates to separate CO₂/N₂,CO₂/CH₄,and CO₂/H₂.