Design And Synthesis Of Porous Organic Polymers Based On Thiazolo[5,4-d] Thiazole Units For CO₂ Cycloaddition

In recent years,the greenhouse effect caused by the excessive emission of greenhouse gases represented by carbon dioxide（CO₂）has increasingly seriously affected the earth’s climate and become a serious environmental problem faced by all mankind.In response to climate change,China made a solemn commitment at the 75th United Nations General Assembly in 2020:“strive to peak CO₂ emissions by 2030 and strive to achieve carbon neutrality by 2060”.On the other hand,CO₂ is also a kind of abundant and cheap C1 resources,which can be used as an important raw material for the transformation and production of a variety of high value-added chemicals.Among the various conversion schemes of CO₂,the strategy of preparing cyclic carbonate through cycloaddition reaction with epoxide not only meets the requirements of green chemistry concept and sustainable development strategy,but also is of great significance to the strategic objectives of“peak CO₂ emissions”and“carbon neutralization”.The generated cyclic carbonate could be applied in lithium-ion batteries,petrochemical additives,polymer precursors and other important fields.Because the C=O bond energy of CO₂ is up to 803 kJ mol^-1 and has high chemical stability,the reaction usually needs high temperature and pressure conditions.The research and development of catalysts that can efficiently activate CO₂ is still a difficulty and of great significance.In industry,the synthesis route of cyclic carbonate mainly employs metal halides or quaternary ammonium salts as catalysts,which still exhibits some problems,such as harsh reaction conditions（temperature,pressure,etc.）,the need for additional solvents,difficult product purification and difficult catalyst recovery.Therefore,the research and development of novel efficient heterogeneous catalysts with stability has become a hot issue in the research field of CO₂ cycloaddition.Porous organic polymers（POPs）composed of light elements such as C,H and N and connected by strong covalent bonds is a significant kind of porous materials.In particular,due to its easy preparation,convenient modification,high catalytic activity and stability,POPs has been widely studied as a heterogeneous catalyst for CO₂cycloaddition,and shows an excellent application prospect.However,POPs catalysts with high catalytic activity always need to introduce metal active centers to activate CO₂ or add co-catalysts to improve their catalytic performance,which increases their cost and energy consumption in practical application.Therefore,the research and development of new heterogeneous POPs catalysts with high catalytic activity without additional co-catalysts is still of great significance.Thiazolo[5,4-d]thiazole（TzTz）is an important class of bicyclic aromatic units with good affinity for CO₂ molecules.It has been used in the field of CO₂ adsorption and separation and photocatalytic methanol production.From the reaction mechanism,TzTzunits could serve as nucleophilic units to activate CO₂ and accelerate the CO₂ cycloaddition with epoxide.Therefore,we consider introducing it into the POPs structure and creatively use it as a heterogeneous catalyst that can efficiently activate CO₂.Based on the above considerations,this paper takes TzTz-based POPs as the research object and takes the catalysis of CO₂cycloaddition with epoxide as the functional orientation.The main purpose is to design and synthesize novel heterogeneous catalyst of TzTz-based POPs with multiple active centers,non-metallic and high catalytic activity without external co-catalyst.The main research contents and results of this paper are as follows:In Chapter 2,we prepared a series of TzTz-linked conjugated microporous polymers（TZCPs）with good thermal stability and adjustable porosity through hydrothermal method by adjusting different monomers,and investigated the catalytic performance of TZCPs as catalysts for CO₂ cycloaddition under mild conditions without additional reagents for the first time.In addition,we used the method of organic iodine modification to obviously improve the catalytic activity of TZCPs for CO₂cycloaddition.I_x@TZCPs showed better catalytic activity compared with the corresponding TZCPs.Among them,I_x@TZCP-4 shows the best catalytic activity.It can catalyze the CO₂cycloaddition with a variety of epoxides at 100℃,1 MPa CO₂and without any additives（solvent,cocatalyst,etc.）,and obtain 80.0-90.1%yield of cyclic carbonate in 24 h.Although the cycle stability of the catalyst needs to be further improved,this work proves for the first time the catalytic activity of TzTzunits for the CO₂cycloaddition with epoxide and the practicability of organic iodine modification method.In Chapter 3,we successfully designed and synthesized a series of hydroxyl-rich TzTz-linked pyridine doped conjugated microporous polymers（TZPs）,which could be used as catalyst for CO₂ cycloaddition without any additives（solvent,cocatalyst,etc.）.Through comparative experiments,it is proved that the introduction of hydroxyl and pyridine groups can improve the catalytic activity of CO₂ cycloaddition.We further modified TZPs by ionization method.The prepared TZIPs showed significantly higher catalytic activity than the corresponding TZPs,which proved the effectiveness of the ionization method.In order to simplify the synthesis route of the catalyst,TZIP3-EtI-OP was synthesized by one pot method,which showed better catalytic activity than TZIP3-EtI under the same experimental conditions.Under the conditions of 100℃,1MPa CO₂and without any additives,TZIP3-EtI-OP can give 87.1-99.0%yield of cyclic carbonate for the cycloaddition reaction of CO₂ with various epoxides（20 mmol）in 12 h,and there is no obvious yield loss after recycling for 5 times.Based on the previous chapter,this chapter designed and synthesized a series novel high-efficiency CO₂ cycloaddition catalyst with better performance than TZCPs by introducing hydroxyl,pyridine,halogen anion and other active sites,which proved the development potential and application prospect of TzTz-linked conjugated microporous polymer as a catalyst for CO₂ cycloaddition.In Chapter 4,we synthesized a series of TzTz-linked pyridine grafted ionic hyper crosslinked porous polymers（TZIHCPs）,and studied their catalytic properties as catalysts for CO₂ cycloaddition reaction.Among them,TZIHCP-1 showed the best catalytic activity for CO₂ cycloaddition reaction.Under the conditions of 100℃,1 MPa CO₂and without any additives（solvents,cocatalysts,etc.）,the yield of CO₂cycloaddition with styrene oxide catalyzed by TZIHCP-1 reached 99.9%within 10 h,and showed good recyclability and substrate adaptability.Compared with similar hyper crosslinked porous polymer catalysts,TZIHCP-1 has the advantages of milder reaction conditions,no additives,non-metallic and less catalyst dosage.On the other hand,we synthesized TzTz-linked imidazole grafted ionic hyper crosslinked porous polymers（TZImHCPs）with different halogen anions.TZImHCP-I has the highest catalytic activity and shows good substrate adaptability and recycling.This work proves the excellent catalytic activity of ionic hyper crosslinked porous polymer with TzTzunits,and provides a reference for the design and synthesis of new CO₂ cycloaddition catalyst based on hyper crosslinked polymer.