Conversion Of CO₂ Into Cyclic Carbonates Catalyzed By Supported Ionic Liquids

Carbon dioxide emission is one of the major cause of greenhouse effect.Chemical conversion of CO₂ has attracted wide attention among all different CO₂ recovery strategies on account of its high economic value.The fixation of CO₂ into cyclic carbonates is one of the routes.The reaction has high value and no by-products,which is according to the thought of green chemistry.However,traditional catalysts（such as metallic salts）have the problems of catalytic inefficiency,environmental pollution and difficult product separation.In this paper,a new supported ionic liquid catalyst was developed for CO₂ cycloaddition reaction,which is to establish a green,efficient,solvent-free,mild,durable and easily separated catalytic system.The main works are summarized as follows:（1）A new supported ionic liquid catalyst,DBU@SBA-15 was prepared by chemical grafting.By adjusting the mass ratio of DBU/SBA-15（1:10～1.5:1）,the ionic liquid loading（21.68%）was optimized.The prepared catalyst was characterized by N₂-adsorption,TGA and FT-IR.The results showed that the ionic liquid has been successfully grafted into the pores of SBA-15.The thermal stability of the catalyst was great（the initial decomposition temperature is 273°C）.Abundant mesoporous structure（specific surface area 200.86 m²/g）was maintained and pore blockage was not found.（2）The catalytic activity of DBU@SBA-15 for CO₂ cycloaddition reaction with epichlorohydrin was investigated by batch method.The catalyst amount（25～150 mg）,temperature（40～100°C）,pressure（0～3 MPa）and reaction time（6～15 h）was studied.Under mild conditions（cat.100 mg,80°C,2 MPa,12 h）,the yield and selectivity of the reaction were both>99%,which was better than most heterogeneous catalytic systems.After being reused for4 times,the catalyst still maintained a high yield of 92.45%,showing good stability.When propylene oxide and styrene oxide were used as substrates,good catalytic activity（>99%and21.68%）was also obtained,indicating that the catalyst was suitable for different substrates.（3）Technology of fixed bed continuous conversion of CO₂ into propylene carbonate was established.The packing structure of catalyst（no packing,tablet,plastic filler,metal filler,quartz）was investigated and quartz was selected as the best filler.The reaction temperature（45～90°C）,pressure（1～4 MPa）and CO₂ flow（5～100 m L/min）were studied.Under the optimal conditions（cat.100 mg,90°C,2 MPa,liquid flow 0.1 m L/min,and CO₂ flow 15m L/min）,the product yield and selectivity reached 51.95%and>99%,respectively.After 24 h continuous reaction,the product yield could still reach 16.86%,suggesting the high stability of the catalyst.（4）The mechanism of CO₂ cycloaddition reaction catalyzed by DBU@SBA-15 was established.Density functional theory was utilized to optimize the configuration of reactants,catalysts,intermediates and products.Transition states of each step were searched.The calculation of energy variation during the reaction suggested that the ring open process of propylene oxide showed the maximum energy barrier（2.9851 e V）,which was the rate limiting step.