Design Of Multifunctional Catalyst For Carbon Dioxide Conversion Into Cyclic Carbonates And Catalytic Mechanism

CO₂ is one of the main greenhouse effect gases,and has been attracting much attention as an abundant,inexpensive,nontoxic,and renewable C1 resource.For benefitting the eco-environment and developing new renewable energy,the realization of an efficient catalytic process for CO₂ mitigation and transformation into value-added products is of great strategic significance.One of the most promising endeavors is synthesis of cyclic carbonates by cycloaddition of CO₂ to epoxides.The cyclic carbonate has wide potential applications ranging from the electrochemistry to pharmaceutical industries,also it is green in term of 100%atom efficiency and represents an alternative to the traditional phosgene process.Because CO₂ has inherent thermodynamic stability and kinetic inertness,the key to realizing its conversion demands design of efficient catalyst system for CO₂ activation.Based on the mechanism of CO₂ activation and conversion,a series of facile Lewis acid–base binary catalyst systems,including N-heterocyclic compound/Zn Br₂,melamine/Zn I₂ and dicationic ionic liquids?ILs?/Zn I₂ were designed and used for the catalytic coupling of CO₂ with epoxides to cyclic carbonates.The influence of different catalyst components,reaction parameters on catalytic activity were systematically investigated.The reaction kinetic studies were also undertaken and a Lewis acid–base synergistic catalytic mechanism was proposed.Wherein,Lewis acid played the role of activating epoxide,Lewis base activating CO₂,and the synergetic effects promoted the reaction smoothly.To replace zinc halide in the reaction,the homogeneous single-component Zn-based task-specific ILs?Zn-TSILs?catalysts with hydrogen bond donor?HBD?sites were developed.The prepared Zn-TSILs exhibit multiple functionalities of Lewis acid property,HBD ability and nucleophilicity.Under the optimized reaction conditions?120 ^oC,2.0 MPa,1 h?,excellent PC yields and selectivity were achieved with high TOF values up to 794 h^-1.The activation ability of HBD sites to epoxide decreased in the order of-OH>-COOH>-NH₂.The Zn-TSILs were proved to be versatile catalysts.The rate constants and the activation energies for the reaction catalyzed by Zn-TSIL and TSIL were comparatively determined.The Zn-TSIL reduced the activation energy value by 14.7 k J·mol^-1,which was attributed to the synergistic effects of the zinc species,HBD sites,and halogen anions.In addition,for Zn-TSILs,easy recyclability by extraction without significant loss of activity,and superior hydrothermal stability were found,overcoming the drawback of water sensitivity for zinc halide used as an separate co-catalyst.Transition metal ions with high-activity have been widely applied in catalytic reactions,the environmental problems are concerned.Several metal-free urea derivative-based ionic liquids?UDILs?with superior thermal stability were facilely synthesized.It represents a substantially greener alternative to the transition metal ions for epoxide activation by introducing HBD groups.The results showed a much higher CO₂ capture capacity with 2.62 mmol of CO₂ per mmol UDIL absorbent than traditional reported ionic liquids.Additionally,UDIL exhibited outstanding catalytic activity for the conversion of CO₂ and various epoxides to cyclic carbonates under metal-and cocatalyst-free conditions.Combining the kinetic studies,a cation-anion synergistic catalytic mechanism was proposed.The UDILs overcome the drawbacks of low synthesis efficiency and high energy consumption for traditional ionic liquids,and display a bi-functionality for both CO₂ capture and conversion.To realize highly efficient separation of the catalyst,novel heterogeneous metal-free tri-s-triazine terminal-linked ionic liquids and periodic mesoporous organosilica with a basic urea-derived framework?PMO-UDF?were successfully synthesized with UDILs.The materials as prepared possess multiple functionalities of hydrogen bond donor ability,Lewis base property,and nucleophilicity,which are vital to the ring-opening of epoxide and the activation of CO₂.They can efficiently catalyze the coupling of CO₂ and epoxides to form cyclic carbonates under mild and solvent-free conditions,realizing the heterogeneous catalytic conversion of CO₂.In addition,the developed PMO-UDFs exhibit enhanced CO₂ capture capacity under ambient conditions??0.1 MPa,273K?due to the cooperative effects of the high surface area and anchoring Lewis base units.The CO₂ capture capacity is comparable or higher than the partial MOFs and nitrogen doped carbon adsorbents.Density functional theory?DFT?was employed to simulate the structures of reactants,intermediates,transition states and products in 1-?2-hydroxyl-ethyl?-3-buthylimidazolium bromide?HEBim Br?and HEBim Br/EG catalytic system to calculate the energy barrier.EG addition could remarkably reduce energy barrier of ring-closing step,and change the rate-determining step to promote the reaction.Furthermore,the activation abilities of HBD groups and transition metal ions to epoxide were elucidated by designing various metal?Zn,Fe,Co,Ni?modified SBA-15 catalyst to replace silanol groups on SBA-15.The results showed that the ring-opening reaction was the rate-determining step over unmodified SBA-15 catalysts.After modified with Zn sites,the energy barrier of ring-opening reaction was significantly reduced,and the ring-closing reaction became the rate-determining step.As to epoxide activation,Lewis acid Zn sites exhibit a stronger role than–OH.This work provides theoretical basis for further development of catalysts.