Design Synthesis Of Squarate-based Metal-organic Frameworks For Gas Adsorption And Separation

In order to achieve carbon neutrality and increase the utilization of non-renewable energy,the separation and purification of carbon dioxide（CO₂）and low-carbon hydrocarbons（C₁-C₃）from flue gas becomes crucial.Metal-organic frameworks（MOFs）provide a feasible solution for gas storage and separation due to their adjustable pore size and structural diversity.Among them,the squarate-based MOFs constructed by cheap and small-sized rigid squaric acid ligands exhibits ultra-high stability and excellent gas adsorption and separation performance.Because the synthesis is concentrated in an alkaline environment,the synthesis conditions are too harsh and single,so the number of squarate-based MOFs reported for gas adsorption and separation is not large.Therefore,it is of great significance to explore new squarate-based MOFs and study their gas adsorption and separation performance.In this paper,several novel squarate-based MOFs were successfully designed and synthesized by using acid reaction environment and metal ion regulation.Finally,novel squarate-based ZnOF and Co OF were selected to explore their gas adsorption and separation performance,which provided new ideas for exploring squarate-based MOFs.By selecting transition metal element Mn and rare earth metal element Sm as metal nodes,two new squarate-based Mn OF and one new squarate-based Sm OF were synthesized under non-alkaline conditions.However,due to the blockage of guest molecules（dimethylamine cations and coordinated water molecules）in the framework or the formation of too small pore size,the above three new squarate-based MOFs cannot be studied for gas adsorption and separation performance,but it lays a foundation for the subsequent exploration of the synthesis of squarate-based MOFs in acidic environment.In the hexafluorozirconic acid system,a new type of squarate-based ZnOF（Zn-SA）was synthesized by hydrothermal method.It has a cubane-type[Zn₄（μ₃-F）₄]secondary building unit（SBU）,and is connected by squaric acid ligands to form a pcu topological type network framework.Zn-SA has a pore size of 4.8?,and the fluorine atoms in its SBU point to the pores,which can form a strong interaction with gas molecules.The IAST selectivity of Zn-SA for CO₂/N₂（15/85）and C₂H₂/CO₂（50/50）were 125 and 3.76,respectively,which proved that Zn-SA could effectively separate CO₂/N₂and C₂H₂/CO₂.In addition,other hydrocarbon gas adsorption separation tests were carried out on Zn-SA,and the results showed that it was also expected to be used for the separation of C₂H₂/C₂H₄,C₃H₄/C₃H₆and C₃H₆/C₃H₈.In order to further improve the gas separation performance and stability of such squarate-based MOFs,a novel squarate-based Co OF（FJUT-3）was successfully obtained by replacing metal nodes and adjusting the synthesis environment.FJUT-3 has a[Co₄（μ₃-OH）₄]SBU structural unit and a smaller pore size（4.7?）.The structure of FJUT-3 remained unchanged after soaking in different p H solutions and in water for 40 days,indicating that it had excellent acid-base stability and water stability.Importantly,FJUT-3 exhibited excellent adsorption performance for CH₄/N₂,CO₂/N₂and other hydrocarbon gases（such as C₂H₂/CO₂,C₂H₂/C₂H₄,C₂H₄/C₂H₆and C₃H₄/C₃H₆）.In addition,FJUT-3 can be scaled up to gram scale.FJUT-3 is expected to become a potential adsorption material in practical industry due to its outstanding stability,low cost and easy synthesis.