Preparation Of MOF@g-C₃N₄ Composites And Their Application In CO₂ Cycloaddition

As a cheap,non-toxic and renewable C1 resource,CO₂ can replace phosgene and CO as raw materials for the synthesis of high value-added chemicals.Among them,the preparation of cyclic carbonates by coupling CO₂ and epoxides has attracted widespread attention because of its 100%atomic economy and compliance with the principles of green chemistry.Cyclic carbonates are widely used in printing and dyeing,textile,polymer synthesis and electrochemistry.At present,homogeneous KI systems are generally used as catalysts for synthesizing cyclic carbonates in industrial production,but there are many problems such as low catalytic activity,harsh reaction conditions,the need for a large amount of organic solvents,and the difficulty of separating catalysts from products.Therefore,the development of heterogeneous catalysts with high activity,high stability and excellent cyclability has become the primary research goal of CO₂cycloaddition reactions.Based on the above arguments,this paper had carried out the following work:（1）Synergy of multiple active sites is a powerful means to improve catalytic activity.Firstly,ZIF-8（Zn/Co）and g-C₃N₄ were prepared into composite materials with multiple active centers by physical composite method,and FT-IR,XRD,XPS,N₂ adsorption-desorption and TGA were used to to characterize its structure means.ZCNs had high specific surface area（732-947 m~2/g）,Lewis acid-base bifunctional groups and abundant microporous structure,which could greatly improve their CO₂ adsorption and catalytic conversion performance.Through structural optimization,the adsorption capacity of ZCN-20 for CO₂ reached 1235μmol/g at 273 K and 1 bar;ZCN-60 exhibited the best catalytic performance in the coupling reaction of CO₂ with propylene oxide（PO）,under the optimized reaction conditions（90℃,2.0 MPa,6 h）,without adding additional solvent,the yield of propylene carbonate（PC）was as high as 98%,the selectivity reached more than 99%.In addition,the material also exhibited good cycling stability and universality to a variety of different epoxides.（2）On the basis of previous work,ZIF-8（Zn/Co）and mesoporous g-C₃N₄（mg-C₃N₄）were prepared into composites（mg-C₃N₄@ZIF-8（Zn/Co））by in-situ composite method.The synthesized composites contain metal ions as Lewis acid sites and Lewis basic triazine groups.The high specific surface area（941 m~2/g）of mg-C₃N₄@ZIF-8（Zn/Co）and the synergistic effect of Lewis acid-base resulted in good CO₂ adsorption capacity（804μmol/g at 273 K）and efficient catalytic activity for the cycloaddition reaction of CO₂ and epoxides.By regulating the catalyst structures and reaction conditions,the 60%mg-C₃N₄@ZIF-8（Zn/Co）can exhibit efficient activities toward the cycloaddition of CO₂ to various epoxides under mild conditions without solvents.In addition,the 60%mg-C₃N₄@ZIF-8（Zn/Co）also showed better recycling performance and easy separation in the CO₂ cycloaddition reaction.Finally,based on literature reports and experimental results,the reaction mechanism of Lewis acid-base synergistic catalysis was proposed.（3）In order to further improve the catalytic performance of the materials,a series of magnetic multifunctional carbon materials（MDCs）were prepared and characterized by pyrolysis method using ZIF-8（Zn/Co）and mg-C₃N₄ as precursors.The effects of different precursor ratios and pyrolysis temperatures on the structure of MDC were investigated.The results show that the MDC materials have high specific surface area,abundant mesoporous structure and Lewis acid-base active groups.By studying the performance of MDC on CO₂ adsorption and catalytic conversion,and it was found that MDC-30%-700 exhibited an excellent adsorption capacity for CO₂ at 273 K and 1 bar,reaching 1575μmol/g;using TBAB as cocatalyst,MDC-90%-700 could catalyze the cycloaddition reaction of CO₂/epoxide with high efficiency and selectivity under the reaction conditions of 70 ^oC and 2.0 MPa,and obtain the corresponding cyclic carbonate.In addition,MDC-90%-700 is easy to achieve separation,and it still has high catalytic activity after being recycled for 5 times.The solvent-free,bifunctional and easily separable functional carbon materials developed in this paper shows potential application in the field of CO₂ capture and utilization.