Synthesis Of Zn,Co Coordination Polymers Of Bisbenzimidazole And Catalytic Properties For Cycloaddition Of Carbon Dioxide

Carbon dioxide(CO₂)is regarded as abundant,economical,and non-toxic renewable carbon source that can be used to produce high value-added chemicals.The effective utilization of CO₂ can not only reduce the dependence on traditional petrochemical resources,but also achieve carbon emission reduction and carbon neutralization.Cycloaddition reaction by heterogeneous catalytic is an ideal method to realize CO₂ conversion.This method has high atomic efficiency,is one of the ideal ways to realize green synthesis and solve environmental crisis,and has broad application prospects.However,it is difficult for most catalysts to realize the cycloaddition reaction of CO₂ and epoxide under mild conditions,and the catalytic efficiency can not meet the requirements of practical application.The process of cycloaddition reaction involves epoxide activated,ring opened,CO₂ added and other reaction steps,which have very strict requirements for catalysts.The catalyst needs not only a stable structure,but also effective active sites.In addition,easy preparation and recyclability are also important features of catalysts for practical applications.At present,it is still facing great challenges to design a stable,efficient and cheap catalyst for CO₂ cycloaddition.Coordination Polymers(CPs)as a kind of crystalline material with diverse structures and excellent properties,have attracted great interest.In recent years,the research of CPs in academia and industry is mainly focused on the close relationship between their structure and properties.In this work,several stable CPs have been successfully synthesized by bisbenzimidazole,carboxylic acids and transition metals Zn(?)and Co(?).As heterogeneous catalysts for cycloaddition of CO₂ with epoxides under mild conditions,the properties of these CPs were investigated.The specific contents are as follows:1.Zn-CP-1 was successfully synthesized by hydrothermal method using 1,3-bis(5,6-dimethylbenzimidazol-1-yl)-2-propanol,1,3,5-benzenetricarboxylic acid and transition metal Zn(?)as raw materials.The as-prepared Zn-CP-1 can be used as a catalyst for the cycloaddition of CO₂ with propylene oxide.The experimental results showed that Zn-CP-1 has good stability and contains Lewis acid and Brn(?)sted acid bifunctional active sites.Zn-CP-1 can produce synergistic effect with the co-catalyst TBAB during the catalytic reaction process.They can efficiently catalyze the CO₂cycloaddition reaction.In addition,the reaction mechanism was deduced by using Density Functional Theory(DFT)method,and the catalytic effect of Zn-CP-1 in the reaction was discussed.This work can provide new ideas and methods for design and synthesis of efficient catalysts,and also provide a new effective way to explore the active center and reaction mechanism of catalysts from the atomic scale.2.Zn-CP-2 with two-dimensional wavy layered structure was prepared by solvothermal method using 1,3-bis(2-methylbenzimidazol-1-yl)-2-propanol,1,4-dicarboxybenzene and transition metal Zn(?).Due to the steric hindrance and configuration between ligands,Zn-CP-2 has open metal active sites.Moreover,the electron density around the N atom on the imidazole ring was increased by methyl substituents in 1,3-bis(2-methylbenzimidazol-1-yl)-2-propanol.So,the N atom is easy to coordinate with Zn(?).On the other hand,the Lewis acidity of Zn-CP-2 is also promoted.Zn-CP-2 has high thermal stability and chemical stability.As a Lewis acid heterogeneous catalyst,Zn-CP-2 can catalyze the cycloaddition of CO₂ to propylene oxide under mild conditions with an efficiency of 94.8%.The catalytic efficiency of Zn-CP-2 did not decrease significantly after five cycles.3.Zn-CP-3 was prepared rapidly by ultrasonic assisted synthesis technology at room temperature,which is composed of Zn(?)ion,1,3-bis(benzimidazol-1-yl)-2-propanol and 5-methyisophthalic acid.Zn(?)ion is closely linked in the framework with four coordinated structure.The as-synthesized Zn-CP-3 has micron rod shape and uniform size.The ultrasonic assisted synthesis need short time,get high yield and consume low energy.The effects of ultrasonic power and reaction time on the size and morphology of Zn-CP-3 were investigated in detail.Then Zn-CP-3 was used as Lewis acid catalyst for the cycloaddition of CO₂with propylene oxide at room temperature.The high catalytic activity of Zn-CP-3 is due to the unsaturated coordination active metal sites of Zn(?)in the framework.On the other hand,there are many active sites exposed because of the small size of Zn-CP-3,and Zn-CP-3 has good dispersion in solution which can fully contact with the reactants.Based on the good stability,Zn-CP-3 can be recycled for many times.4.Micron sheet Co-CP-4 was prepared by ultrasonic assisted synthesis technologyusing1,6-bis(5,6-dimethylbenzimidazol-1-yl)hexane,4,4'-oxybis(benzoate)and Co(?)ion as raw materials.The effects of initial reagent concentration,ultrasonic power and reaction time on the size and morphology of Co-CP-4 were evaluated.The results showed that ultrasonic can promote the rapid nucleation of crystal,shorten the reaction time,increase the yield and purity of the product.1,6-bis(5,6-dimethylbenzimidazol-1-yl)hexane contains a long chain of six carbon atoms,which can more fully reflect the steric hindrance effect and contribute to the formation of unsaturated metal sites.The Co(?)ion in Co-CP-4 has a four coordinated unsaturated configuration with empty orbit to accept electron pairs,and thus can be used as a Lewis acid catalyst for CO₂ cycloaddition.Becase a large number of active sites are exposed,the Co-CP-4 sheet prepared by ultrasonic method can realize the efficient catalysis of CO₂ cycloaddition at room temperature.In addition,Co-CP-4 has strong framework and stable structure which can be recycled.