Investigation On Mechanism Of CO₂ Capture And Separation In Pillar-layered Flexible Metal-Organic Frameworks

In order to effectively alleviate the impact of the increasing carbon dioxide（CO₂）concentration in the atmosphere on the climate and environment,carbon capture and separation technology is regared as a promising technology,because it can effectively reduce the CO₂concentration in the atmosphere while meeting the current energy system’s demand for fossil fuels in the short to medium term.The most important strategy is to develop adsorbent materials with a high storage capacity and a high separation capacity for practical applications in CO₂capture and separation technology.Metal-organic framework materials（MOFs）have been investigated by many researchers due to the advantages of its ultra-high porosity,adjustable pore size,abundant functional sites,large specific surface area.Among them,the flexible MOFs structure can exhibit a dynamically changing pore structure in response to the stimulus of external conditions,thereby exhibiting an efficient separation ability for different gas molecules.In this thesis,the grand canonical Monte Carlo（GCMC）,density functional theory（DFT）and molecular dynamics（MD）simulation methods were employed to systematically explore the effects of subnetwork displacement,breathing,and linker rotation on CO₂ capture and separation behavior in flexible MOFs.First,the influence of interlayer expansion effect of kgm on the CO₂/N₂ adsorption and separation performance were investigated by using GCMC.The research results show that the interlayer expansion effect can expose more metal adsorption sites,and effectively improve the adsorption and separation of CO₂molecules.The CO₂ adsorption capacity in the expanded kgm-3 structure reached up to 13.2 mmol/g at 298 K and 20 bar,which is 4.5 times larger than that in the unexpanded kgm-1 structure.At the same time,the competitive selectivity of CO₂/N₂ in kgm-3 is 130.3,which is also improved compared to that in the structure before expansion.This chapter clarified the improvement mechanism of CO₂adsorption and separation performance by analyzing the pore size characteristics,isosteric heat,coulomb/non-coulomb interaction,and adsorption site analysis.Next,the breathing effect of the flexible MOFs structure x-pcu-n series and its influence mechanism on CO₂/N₂ adsorption and separation were studied by using MD and GCMC.Studies have shown that the threshold pressures of the breathing effect in the four structures of the x-pcu-n series are between 20-40 k Pa,and the uptake of CO₂ adsorption increases sharply within this pressure range.Through the flexibility analysis on the structures and the interaction analysis between CO₂ and the framework during the adsorption process,it is found that during the breathing expansion of the structure,the pore structure gradually increases,resulting in the exposure of the adsorption sites of the pore structure.This proves that MOFs with breathing properties can be beneficial to gas adsorption during respiratory expansion.Therefore,the rational control on the framework structure of the respiratory characteristics of MOFs is of great significance to the application of gas adsorption and separation.Finally,the influence of the structural transformation caused by the rotation of L ligand in the SIFSIX-23-Cu on the CO₂capture and separation performance is explored,and the change of temperature on the structural transformation threshold pressure is analyzed.The results show that the CO₂ adsorption isotherm of the framework material at a pressure of 0-30 bar presents a stepped shape,and there are two threshold pressures where the adsorption capacity increases sharply.With the increase of temperature,the threshold pressure of the framework increases significantly.The rotation of the L ligand causes the SIFSIX-23-Cu structure to produce multipolar phase transitions of different forms,which gradually increases the specific surface area and accessible pore volume of the framework material,thereby enhancing the adsorption capacity of CO₂ gas.Through the analysis of adsorption sites and interactions,the intrinsic changes of the intrinsic effect of the ligand rotation on the CO₂ adsorption are explained in detail.Therefore,the reasonable control of adsorption temperature and pressure can regulate the selective adsorption capacity of SIFSIX-23-Cu structure for CO₂.