Post-synthetic Modification Of Metal-organic Frameworks And Related Separation Membranes For Carbon Capture

The excess emissions of CO₂ will cause various environmental problems such as ocean acidification and global warming.The capture and recovery of CO₂ from flue gas is of great significance.Compared with absorption and distillation,adsorption and membrane separation have environment-friendly and energy-efficient for carbon capture,which have received considerable attention in academia and industry.Metal-organic frameworks?MOFs?are porous crystalline materials formed by the coordination of metal nodes and organic ligands.MOFs show great potential in gas adsorption and membrane separation due to its large surface areas,regular and adjustable pores,excellent adsorption performance,diverse structure and unique affinity.Various MOFs have been designed as adsorbents and as membrane materials for CO₂ adsorption and separation applications.The adsorption and separation of CO₂ follow the mechanism of adsorption and diffusion,which mainly depends on the pore sizes of MOFs and the interaction between MOFs and permeating molecules.In this context,we focused on developing of controlling adsorption and diffusion.To overcome the synthetic limitation of conventional MOF materials,MOF materials and membranes were post-synthetic modified to change and optimize the pore structures,microenvironment and diffusion process,thereby improving the CO₂ capture selectivity and separation selectivity of MOF materials and related separation membranes.The main research contents and results are as follows:?1?A vapor phase linker exchange?VPLE?method for MOFs modification was developed.Through solvent-free and environmentally friendly VPLE processing,the parent MOFs are directly exposed to the gas phase daughter linkers for linker exchange the parent MOFs,to achieve post-synthetic modification of the MOF materials.Based on daughter and parent linkers with similar physicochemical properties,various daughter linkers with functional groups but not for MOF crystallization are integrated into the parent MOF framework in the form of daughter building blocks.Besides single exchange for preparing MOFs with dual linkers,VPLE can also perform linker exchange through multi-step linker exchange to obtain MOF materials with multiple linkers and functional groups.The adsorption performance of MOFs mainly depends on the pore properties of MOF materials and the characteristics of the target adsorbed molecules.Since the halogen incorporation strengthen the polarizabilities and quadrupole moments of the MOF materials,the interaction of frameworks and gas molecules is reinforced and the modified MOF materials maintain good porosity,the prepared MOF materials exhibited excellent carbon capture performance.The improved CO₂/N₂ selectivity of the VPLE-treated ZIF materials is approximately double that for unmodified materials,from 15.0 to 31.1.?2?A versatile post-modification strategy based polydopamine?PDA?coating was reported to improve the CO₂ separation performance of MOF membranes.Due to strong bio-adhesion,PDA can be deposited on the Ui O-66 membrane by a simple and mild process.Meanwhile,due to PDA impregnation in invalid nanometer-sized pinholes and grain boundaries of the MOF membrane suppress non-selective gas diffusion,the PDA/Ui O-66 membrane displayed enhanced CO₂/N₂ and CO₂/CH₄ selectivities of 51.6 and 28.9,respectively,which are two to three times higher than the reported MOF membranes with similar permeance.Because of the mild modification and ultra-thin coating of PDA,the intrinsic pores and submicron membrane thickness of Ui O-66 were maintained after modification.Therefore,the CO₂ permeance is 2-3 orders of magnitude larger than those membranes with similar selectivity,up to 3.7×10^-7 mol m^-2 s^-1 Pa^-1?1115 GPU?.High selectivity and permeance have satisfied the performance requirements of industrial flue gas capture.The separation process of the membrane in the present study is attributed to adsorption-diffusion model.After modification,the adsorption performance of the membrane to different molecules only displayed a small change,but the suppression of diffusion in invalid pinhole caused the significant enhancement in the diffusion selectivity.In addition,the PDA/Ui O-66 membrane with good reproducibility displayed excellent long-term stability for CO₂ capture in 36-h measurement under humid conditions.