Investigation On Mechanism Of CO₂ Capture And Separation In Flexible Metal-organic Framework

In order to alleviate the huge threat to the global climate and environment caused by the increase of CO₂ concentration,CO₂ capture and separation technology has received more and more attention than ever.The most important issue is the development of adsorption separation materials for low concentration and high flow rate CO₂ of fixed carbon source points.Among them,metal organic frameworks（MOFs）have their advantages of ultra-high porosity,adjustable pore size,rich functional sites,large specific surface area,etc.especially flexible MOFs structures exhibiting dynamic changes in response to external stimuli.Its dynamically changing pore size under external stimuli enables efficient separation of different gas molecules.Therefore,this thesis combines density functional theory（DFT）and grand canonical Monte Carlo simulation method（GCMC）to study the effect of pore size on CO₂ capture separation and the influence of functional on flexible MOFs and the integration of adsorption sites and pore size on CO₂ capture and separation during structural transformation.First,the effect of the MFM-300 with nanopores on different adsorption and separation of CO₂,CH₄ and N₂ was studied by replacing different metal centers.Studies have shown that the uptake of CO₂ follows the order of MFM 300（In）>MFM-300（A1）>MFM-300（Ga）>MFM-300（In-3N）at low pressure.At 298 K and 1.0 bar,the uptake of CO₂ reached up to 8.0 mmol g^-1 in MFM-300（In）and exhibited a distinct"ramp-up"selectivity at low pressure.The adsorption heat analysis showed that the larger adsorption heat difference between CO₂ and N₂/CH₄,the greater selectivity of CO₂ relative to N₂ and CH₄.The interaction of CO₂-frameworks plays a leading role,and the interaction between CO₂ molecules also greatly promotes the adsorption amount and selectivity.The increase of temperature reduces the tightness of gas molecules and has an adverse effect on the adsorption efficiency,indicating that the multi-wall cross-interaction and intermolecular interaction is provided in the restricted pores.Second,combined with DFT and GCMC,the mechanism of breathing MOFs structure 1B on CO₂/N₂ adsorption separation was studied.Studies have shown that 1B had the best state of adsorption separation structure during dynamic transformation.As the expands of 1B structure,specific surface area,the porosity and the effective pore volume tend to increase.During the process of structure expansion,the number of adsorption sites is gradually increasing,and the optimal adsorption sites were transferred.The combination of adsorption sites and pore sizes in structure 1B＿13.1 showed the highest selectivity.The interaction of CO₂-framework of narrow pore and the larger pore state depends on the competation of the proper pore and the adsorption site,so it can be confirmed that the MOFs with breathing exhibit conducive to the change for gas adsorption.Reasonable control the state of structure is great significance for the application of breathing MOFs in gas adsorption separation.Finally,the effects of functionalization on gate-opening effect and the mechanism of CO₂/N₂ mixed competitive adsorption in ZIF-8,0GPa-90 and 0GPa-65 were investigated.The results show that the rotation of the imidazole ligand in ZIF-8 leads to a larger window size under high pressure,and the modified structure has a tendency to decrease in specific surface area.In low pressure region（1 bar）,the CO₂ uptake of 1.47GPa-65 reached 2.81 mmol g^-1.It is better than that of the other five structures,and the CO₂ uptake of the modified structure is higher than that of the unmodified structure at low pressure.0GPa-65 has the highest selectivity,and the pressure increase selectivity decreases.Reasonable control of adsorption pressure can regulate the selective efficiency of ZIF-8,0GPa-65 and 0GPa-90.In addition,strong polarity functional has a great effect on the selectivity of the gas.