Preparation And Properties Of Two-dimensional Porphyrin-based Covalent Organic Frameworks And Membranes In Gas Separation

With the development of industrialization,people’s material and spiritual civilization has been greatly satisfied,but problems such as energy exhaustion and environmental pollution have emerged in an endless stream.It is urgent to find renewable and clean energy and build a sustainable ecosystem.Hydrogen is considered as a new generation of clean and renewable energy source,which can effectively alleviate energy crisis and environmental problems.However,in industrial hydrogen production processes such as steam-methane reforming,the H₂ product usually contains a proportion of other gases such as CO₂.Therefore,it is necessary to seek economical and reliable separation technology to purify the hydrogen produced.On the other hand,in the CO₂ capture and storage technology,the main capture process in pre-combustion is the separation of H₂/CO₂ mixture.In general,gas separation,especially H₂/CO₂ separation technology,plays an indispensable role in providing clean energy and improving environmental pollution.Membrane separation technology is considered to be very suitable for H₂/CO₂separation,which ensures high efficiency,small carbon footprint,and low energy consumption for separation applications,and thereby fulfills the needs of new sustainable industrial processes.Currently,most commercial gas separation membranes are still conventional polymer membranes which meet the large area and low cost requirements for their industrial application.However,polymer membranes have a balance between productivity（permeability）and efficiency（selectivity）,known as the Robeson upper bound.Membranes based on molecular sieving mechanism are expected to overcome this limitation by distinguish gas molecules based on the pore size and shape.Two-dimensional covalent organic frameworks（COFs）are ideal platforms for constructing molecular sieve membranes due to their well-ordered pores,lamellar structures which are easy to exfoliate and designability at the molecular level.In this dissertation,starting from the structural design of COFs and the preparation methods of ultrapermeable molecular sieving membrane,through in-depth research on the pore structure elements,gas separation mechanism and membrane preparation methods of COF membranes,a nitrogen-rich COF was synthesized and successfully applied in ultrapermeable membrane gas separation.The main contents are as follows:In the first part,based on the principle of structural topology design,a two-dimensional porphyrin-based covalent organic framework,denoted JU-COF-368,was successfully synthesized under solvothermal conditions by using aldehyde-based and amino-based porphyrin monomers as building blocks.Single-component sorption isotherms of CO₂,CH₄,N₂,H₂ were collected at 273 K and 298 K,and the breakthrough experiments were performed to evaluate the performance of JU-COF-368 in an adsorptive dynamic separation process.The results demonstrated that due to the nitrogen-rich groups in the skeleton,JU-COF-368exhibited a good adsorption capacity for CO₂ molecules and showed high potential for CO₂/CH₄,CO₂/N₂ separation,indicating a good application prospect in the treatment of CO₂and other greenhouse gas emissions.In the second part,based on 2D JU-COF-368 which has been synthesized previously,atomic-thick nanosheets were obtained through the top-down strategy,and an ultrathin 2D COF membrane was prepared on the alumina substrate by hot-drop coating method.Firstly,COF powder was exfoliated into COF nanosheets by chemical exfoliation method with the help of ultrasonication.Then,the well-dispersed COF nanosheet colloidal suspension was dropped on the alumina substrate by hot-drop coating method,and the nanosheets were self-assembled into an ultrathin 2D COF membrane.The gas permeation behavior and long-term stability of the membrane were tested.The 2D COF nanosheet-based molecular sieve membrane demonstrated a good separation selectivity for H₂/CO₂ and a fairly high H₂permeance,and the separation performance was maintained for up to 7 days.These results indicated that the ultrathin 2D COF membranes prepared by hot-drop coating method hold great potential in hydrogen purification.