Green Synthesis Of Epoxides And Cyclic Carbonates

The epoxide is one of the most important chemical raw materials.The production of epoxide by the traditional chlorohydrin method is highly polluting to the environment,which is contrary to the concept of sustainable development.The development of green,efficient epoxidation processes for olefins to replace the traditional chlorohydrin method is of great significance to promote the technological transformation and upgrading of the epoxy industry.The design of a stable industrial catalyst is the key to the green epoxidation of olefins.The cyclic carbonates formed by the further reaction of epoxides with carbon dioxide can not only produce high value-added chemicals,but also consume greenhouse gas,which is important for environmental protection and resource recycling.However,due to the thermodynamics stability and kinetic inertia of carbon dioxide,its chemical conversion process is high energy consumption.It is one of the major challenges in the field of green chemistry to develop efficient catalytic systems for the activation and transformation of carbon dioxide under mild conditions.The one-pot synthesis of cyclic carbonates from olefins and carbon dioxide is an innovative way for the green conversion of carbon dioxide,which can reduce the cost of raw materials,simplify the production process,and avoid the separation and storage of intermediate epoxides.The reaction and chemical composition involved in this process are complex,and there is still lacking of efficient and highly selective catalytic system.Based on the development of green and efficient synthesis methods of epoxides and cyclic carbonates,this study has carried out research on green epoxidation of olefins,cycloaddition of epoxides and carbon dioxide at room temperature,and oxidative carboxylation of olefins and carbon dioxide under mild conditions,respectively.The specific research contents and innovations are as follows:(1)Phosphotungstic heteropoly acid is a catalyst for the green epoxidation of olefin with excellent performance.The traditional catalysts are mostly homogeneous systems in the presence of phase transfer reagents.The poor recovery performance of this catalytic system limits its application in industrial production.In this study,a novel heterogeneous catalyst was designed and synthesized.The resin was used as a carrier,which surface was modified with polyethylenimine.The amine group on the surface of the resin was enriched,which mad the surface of the resin strong cationic.The phosphotungstic heteropoly acid anion was used as the catalytic active site.The structure of the catalyst was characterized by infrared spectra,electron spectroscopy,and elemental analysis,et al.The reaction conditions were optimized using the epoxidation of cyclohexene as a probe.The effect of polyethylenimine modification on the stability and activity of the catalyst was confirmed by experiments,and 89.0%cyclohexene conversion and 94.3%cyclohexane selectivity were achieved in the amplification experiment.(2)In this study,the non-toxic and cheap Schiff base aluminum complex was used as catalyst,the spatial structure of the Schiff base aluminum is changed by adjusting the rigidity of the Schiff base ligand framework,thereby adjusting the activity of the catalysts.And for the first time,imidazolium hydrogen carbonate ionic liquids were used as cocatalysts,so that the catalytic system had active sites of Lewis acid and carbene that could activate epoxy and carbon dioxide respectively.The synthesis of cyclic carbonate was achieved by reacting carbon dioxide with epoxide under room temperature without solvents.The structure of the Schiff base aluminum catalyst and cocatalyst was screened,and the reaction conditions were optimized.Compared with the traditional catalytic system containing halogen quaternary ammonium salt or organic base,this catalytic system has higher catalytic activity.The experiments of substrate expansion and catalyst recovery show that the catalytic system has wide applicability and good recovery ability.Based on the experimental results,we proposed a reaction mechanism for the synergistic effect of Schiff base aluminum and imidazolium hydrogen carbonate ionic liquid.(3)Based on the above studies,we found that imidazolium hydrogen carbonate ionic liquid could be spontaneously converted into carbene-CO₂adduct which has a good capacity of carbon dioxide capture and activation.Based on the above knowledge,this study first proposed that imidazolium hydrogen carbonate ionic liquid was used as a metal-free bifunctional catalyst to catalyze the one-pot synthesis of cyclic carbonate from aromatic olefin and carbon dioxide under mild conditions,the highest aromatic olefin conversion of 91%and cyclic carbonate selectivity of 89%were obtained.The results of confirmatory experiments show that the HCO₃^-anion and carbene–CO₂ play catalytic roles in olefin epoxidation and CO₂ cycloaddition,respectively.The imidazolium hydrogen carbonate ionic liquid is simple to prepare and can maintain excellent catalytic activity after being recycled for four times.In this paper,the epoxidation of olefins and the cycloaddition of carbon dioxide were studied from the viewpoint of green chemistry.In the study of olefins epoxidation,an efficient and stable catalyst was designed,and the green epoxidation process with hydrogen peroxide as oxidant was studied in detail.In the study of carbon dioxide cycloaddition,a new catalytic system and catalytic mechanism were proposed,which efficiently catalyzed the reaction of epoxides with carbon dioxide to synthesize cyclic carbonates at room temperature and atmospheric pressure.Finally,the direct conversion of olefin and carbon dioxide was achieved under mild conditions by combining the epoxidation process with the carbon dioxide cycloaddition process.The above works are expected to provide theoretical guidance for the green and efficient production of epoxides and cyclic carbonates,and the basic data for further industrialization.