Molecular Simulation On H₂/CO₂/CH₄ Storage Properties Of Five Novel Covalent Organic Frameworks With The Higher Valency

Currently,the combustion of fossil fuels emits large amounts of carbon dioxide,which exacerbates global greenhouse effects.The search for new clean energy sources to replace traditional ones is urgent.Hydrogen has features such as cleanliness,high heat value,and recyclability that make it a promising new energy gas to use and has been widely studied by researchers.At present,safe,efficient,low-cost storage and transportation in the hydrogen energy industry chain are an urgent problem to be solved.Meanwhile,during the process of industrial production of hydrogen gas,inevitably some impurity gases such as CO₂ and CH₄will be mixed in which not only affects the purity of hydrogen but also poses a certain threat to its safety storage.Gas adsorption storage based on porous materials with reversible adsorption and simple operation has become a research hotspot in recent years due to its advantages.Covalent organic framework（COF）material is a crystalline porous material composed entirely of non-metallic elements with characteristics such as low density,large specific surface area,high porosity,adjustable pore structure,and high chemical and hydrothermal stability.Therefore,it has become one of the most promising gas adsorption separation materials.However,due to the low valence state（connectivity）of the building units for most COFs materials being mostly 3 and 4-valent states,there are few types available.If COFs can be constructed according to the valence state of their building units increased significantly in number which may lead to even better performance in gas adsorption separation.Based on the above situation,this article focuses on five newly reported high-valence boron-phosphorus COFs（BP-COFs）and uses density functional theory（DFT）and molecular simulation methods to study their structural characteristics,hydrogen storage performance,and adsorption separation performance for H₂/CO₂/CH₄ mixed gases.Firstly,the DFT method is used to optimize the structure of the material,followed by theoretical characterization of its structure using numerical Monte Carlo（NMC）method.Then,the grand canonical Monte Carlo（GCMC）method is employed to the adsorption performance of five BP-COFs for H₂ in the pressure range of 0.1 Bar to 100 Bar at 77 K,the adsorption performance of H₂,CO₂,and CH₄ pure gases in the pressure range of 0.1 Bar to 100 Bar at298 K,and the adsorption separation performance of H₂/CO₂/CH₄ binary and ternary mixed gases in the pressure range of 0.1 to 50 Bar at 298 K.The results showed that BP-COF-4 and BP-COF-5 had higher H₂,CO₂,and CH₄ adsorption capacities than BP-COF-1,BP-COF-2,and BP-COF-3 under the investigated conditions.At 298 K,the equal adsorption heat of CO₂and CH₄ molecules in BP-COFs was greater than that of H₂ molecules,indicating that the interaction between the material and CO₂ and CH₄ molecules was stronger,suggesting that BP-COFs materials can adsorb more CO₂ and CH₄ from H₂/CO₂/CH₄ mixed gases to achieve hydrogen purification.The adsorption and separation study of mixed gases showed that,under the conditions of 298 K,BP-COF-1 and BP-COF-4 had better adsorption and separation ability for CH₄/H₂ binary mixed gas.Among the five materials,the separation selectivity of CO₂/H₂binary mixed gas followed the order of BP-COF-2>BP-COF-3>BP-COF-1>BP-COF-4>BP-COF-5.The structure-activity relationship analysis showed that the pore limit diameter（PLD）and maximum pore diameter（LCD）of the material were the main factors affecting the adsorption selectivity of the material:within a certain range,the smaller the pore diameter of the material,the stronger the interaction with specific molecules,and the higher the corresponding separation selectivity.For H₂/CO₂/CH₄ ternary mixed gas,the adsorption capacity of BP-COF-4 was 0.72 mg/g,68.44 mg/g,292.28 mg/g,and the adsorption capacity of BP-COF-5 was 2.06 mg/g,105.96 mg/g,245.46 mg/g,respectively.The difference in adsorption capacity for different gases showed the broad application prospects of BP-COF-4and BP-COF-5 in the field of hydrogen purification.The study of the center of mass density distribution of the adsorbate molecules showed that the optimal adsorption sites for CO₂,CH₄,and H₂ were concentrated in the corners formed by the organic ligand and the hexagonal B₄P₄O₁₂ unit or the benzene ring（-C₆H₄-）in BP-COFs.In addition,due to the presence of fluorine atoms and methyl groups in BP-COF-2 and BP-COF-3,obvious adsorption sites appeared at the corresponding positions,which could provide some reference for further improving the gas adsorption capacity of COFs materials.This research not only reveals the excellent gas adsorption and separation properties of the five high-valence BP-COFs materials but also provides a theoretical reference for the experimental development of new high-valence gas adsorption and separation COFs materials.