Study On Salen-based Materials Catalyzing The Ring-Opening Addition Reaction Of CO₂ And Epoxy Compounds

Carbon dioxide(CO₂),the main greenhouse gas responsible for global warming,is rapidly accumulating in the atmosphere with an increasing rate.Therefore,it is urgent to develop new technologies to reduce the hazardous impact CO₂.In recent years,more attention has been paid in the conversion of CO₂ into high value-added products.The ring-opening addition of CO₂ with epoxides to synthesize cyclic carbonates is a practical method for CO₂ conversion.However,due to the inertness of CO₂,there is an urgent need to develop advanced catalysts to promote the ring-opening addition reaction of CO₂ with epoxides.Salen complexes have the advantages of simple preparation,insensitivity to water and air,easy structural modification and phenolic hydroxyl groups.These are considered to be able to catalyze the cyclic carbonate synthesis.Moreover,cellulose is a natural green polymer and a renewable resource.It contains a large number of hydroxyl groups,which are conducive to the modification of raw cellulose and in the preparation of supported catalysts.It is considered to be a sustainable strategy in green catalysis.In this paper,a metal-free homogeneous Salen ligand(H₂L)catalytic material was prepared and used in the ring-opening addition reaction of CO₂ and propylene oxide to synthesize propylene carbonate.The structure and thermal stability of H₂L were characterized by relevant characterization methods.Through the investigation of single factor,the optimal range of catalytic parameters was found.The optimum catalytic parameters of H₂L were optimized by response surface optimization(RSM)technique.The optimum catalytic parameters of H₂L were obtained:the reaction temperature was 102?,the CO₂ pressure was2MPa and the reaction time was 7h.The yield reached 98.05%.Secondly,a metal-free heterogeneous cellulose-based Salen complex catalytic material(Cell-H₂L)was prepared and used in the ring-opening addition reaction of CO₂ with epoxides.The Cell-H₂L material was characterized by infrared spectroscopy(FTIR),elemental analyzer(EA),X-ray photoelectron spectroscopy(XPS),solid-state nuclear magnetic resonance(Solid-state NMR),scanning electron microscope(SEM),specific surface area/pore size analysis(BET)and synchronous thermal analyzer(TGA).The study of structure and composition showed that the metal-free Salen complex was successfully supported on chlorofibrinol.The results of catalytic activity showed that the Cell-H₂L catalytic material showed excellent catalytic performance under solvent-free conditions,the yield of propylene carbonate was 98.7%and the selectivity was 100%.The catalytic material was effectively reused for 5 times without obvious loss of catalytic activity,it also showed high catalytic efficiency for other terminal epoxides.Finally,the kinetics and thermodynamics of the ring-opening addition reaction of CO₂and propylene oxide catalyzed by Cell-H₂L/TBAB were studied by using Arrhenius equation and Eyring equation.The results show that the activation energy(Ea)of the reaction is 49.9k J.mol^-1,the pre-exponential factor(A)is 5.706×10⁶min^-1,and the kinetic equation of the catalytic reaction is:Rate=5.706×10⁶[PO]e^-49.9/(RT).The enthalpy change(?H)of the thermodynamics of the catalytic reaction is 46.7858k J/mol,and the entropy change(?S)is-122.6268J/mol.The interaction between Salen ligand material and CO₂ and propylene oxide was simulated by density functional theory(DFT).The results show that there is a strong hydrogen bond between phenolic hydroxyl group in H₂L material and propylene oxide,and the phenolic hydroxyl group in H₂L promotes the ring opening of propylene oxide.Based on the results of various studies,the possible mechanism of ring-opening addition reaction of CO₂ and propylene oxide catalyzed by Cell-H₂L/TBAB is deduced.It is proposed that the synergistic effect of hydroxyl groups and Lewis basic groups plays an important role in the catalytic reaction.