Preparation Of Magnetic Fe₃O₄@SiO₂ Supported Co(?)-Salen And Catalytic CO₂ Ring The Study Of Addition Reactions

The chemical utilization of CO₂ can not only minimize the dependence of human beings on non-renewable fossil fuels,but can also slow the pace of global climate change partly caused by atmospheric excessive CO₂ emissions.A variety of catalysts have been reported for the cycloaddition of epoxides and CO 2,including metal complexes of immobilized catalysts,Salen-type ligands(M(Salen)),metal-organic frameworks(MOFs)and ionic liquids.M(Salen)catalysts amongst appear increasing interest owing to the advantages of easy preparation and high catalytic activity.Nevertheless,the separation of them was usually a time consuming and cumbersome procedure.A typical way to solve these problems is heterogenization of homogeneous catalysts.Therefore,a lot of efforts have been devoted to immobilize the M(Salen)complexes onto various supports,such as nanoparticles,inorganic supports and polymers,the heterogeneous catalysts can be obtained with the advantages of easy separation,reusability and eco-friendly.Magnetic materials with high chemical stability,easy-functionalized and magnetic separation,are a promising option to immobilize M(Salen)complexes.Therefore,Fe₃O₄ nanoparticles were used as magnetic component,and the Fe₃O₄@SiO₂ nano-composites was applied as supports for the immobilization of Co(III)-Salen.The geometry of complexes was optimized and the catalytic mechanism were proved based on density functional theory(DFT).The main content is as follows:(1)Core-shell structural Fe₃O₄@SiO₂ magnetic nanoparticles were prepared in Triton X-100/ hexanol/cyclohexane/water based inverse microemul sion system by one-step method and two-step method respectively.The effects of stirrin g mode and ultrasonic condition on the structure and morphology of one-step method products were investigated and compared with the two-step method.The results indicated that nanoparticles obtained by one-step method aggregated strongly and no obvious core/shell structure was observed under TEM,and the stirring mode and ultrasonication had little influence on the microstructure of the products.Moreover,regular core-shell structural Fe₃O₄@SiO₂ nanoparticles can be obtained by the two-step method,and the hysteresis loop at room temperature showed a superparamagnetic behavior with saturation magnetization of 37.8 emu/g.(2)The Salen ligand were prepared by dehydration reaction between salicylaldehyde and(3-aminopropyl)triethoxysilane(APTES).Co(III)-Salen-OAc complex was synthesized by the coordination of the Salen ligand with Co(II).Then,the complex was covalent connected with Fe₃O₄@SiO₂ nanoparticles,and the Fe₃O₄@SiO₂/Co(III)-Salen catalyst were prepared.The structures of samples were characterization by TEM?FT-IR?XRD?BET?ICP?VSM?TGA?DLS?XPS? EDS.The results have shown that the catalyst possesses an obvious core-shell structure,abundant porosity and excellent thermostability,the saturation magnetization of the catalyst was 27.2 emu/g,the particle size was 29 nm.The various characterizations confirmed that the favourable performances and the intact structure of the catalyst.(3)The prepared catalyst was used for the cycloaddition of CO₂ and epichlorohydrin into cyclic carbonates.The effects of time,temperature,pressure and the ratio of epichlorohydrin/catalyst on the yield and selectivity of cycloaddition reactions were investigated.The products were evaluated by nuclear magnetic resonance(NMR).The results showed that temperature has a significant influence on the yield of the product,the higher the temperature,the higher the yield.In addition,the longer the reaction time and the greater the amount of catalyst,the higher the yield.In the range of 0.5–2.5 MPa of CO₂ pressure,the best yield can be obtained in 1 MPa.The catalyst exhibited high yield of 99% with selectivity of 100% for the ring-opening addition reaction of CO₂ and epoxide at 90? in 6 h.A plausible reaction mechanism was proposed that the Co(III)-Salen and bromine ion played synergetic roles in promoting cycloaddition reactions.A detailed evidence for the reaction mechanism were provided by the density functional theory(DFT)calculations.The magnetic Co(III)-Salen nanocomposites could be easily recovered by using an external magnet and reused for 5 times without significant leaching of met al center and loss of catalytic activity.