Synthesis And Catalytic Properties Of Heterogeneous Composite Catalysts For CO₂ Chemical Fixation

The concentration of carbon dioxide（CO₂）in the atmosphere is rising rapidly with the intensification of human activities.As the main greenhouse gas,the excessive emission of CO₂ has led to a series of environmental problems,such as sea level rise,seawater acidification and so on.In addition,CO₂ is also the most abundant C₁ resource on earth.Hence,the capture and conversion of CO₂ into valuable chemicals has received extensive attention from researchers.Among them,the synthesis of cyclic carbonates by cycloaddition of CO₂ and epoxides is an ideal solution in terms of atom economy and environmental friendliness.However,traditional catalytic systems often have drawbacks such as low catalytic activity,tedious preparation progress,high cost,single active site,and difficult catalyst separation.Therefore,it is still a meaningful research direction to develop and use novel,high-efficiency heterogeneous catalysts that meet the concept of energy saving and emission reduction for CO₂ chemical fixation.In this thesis,several novel and efficient heterogeneous composite materials were synthesized for the cycloaddition reaction of CO₂ and epoxides.The structural properties of these materials were characterized and the effect of the interrelationship between CO₂ adsorption capacity and active sites on the catalytic reaction was further explored.The main research contents are as follows:1.Given the issue of expensive preparation and single active site of most catalysts for CO₂cycloaddition reaction,this paper was selected biomass waste peanut shells as raw materials,and used zinc chloride（ZnCl₂）pyrolysis to prepare low-cost,high-stability,Zinc oxide（ZnO）/porous carbon composite（PSC）material with excellent activity.The chemical activation of ZnCl₂ greatly increased the adsorption capacity of ZnO/PSC composites compared with that of PSC obtained by direct pyrolysis of peanut shells.Among them,the surface area of ZnO/PSC can reach up to 337m² g^-1.The CO₂ adsorption capacity of the composite material can reach up to 68 mg g^-1.In addition,the composites exhibited stronger CO₂ adsorption at lower pressures,indicating a clear interaction between CO₂ molecules and ZnO/PSC.The results show that the synergistic effect between the hydroxyl groups on biomass porous carbon and ZnO and its better CO₂ adsorption capacity enable ZnO/PSC-4 to perform well under mild conditions（1 bar CO₂,≤60℃）and in the presence of cocatalysts.With the help of the cycloaddition reaction of epichlorohydrin（ECH）with CO₂,it exhibits excellent catalytic activity,and the yield of chloropropene carbonate can reach 99%,and there is no obvious loss of activity in 5 consecutive cycles.This work provides a new idea for the application of biomass waste-based composites in catalytic organic reactions by integrating the properties of different components into the composites.2.In the previous work,porous carbon materials were used for catalytic reactions.When these powder materials are used in organic catalytic reactions,additional energy consumption is required for catalyst separation,and it is easy to bring about the problem of product pollution.However,most heterogeneous catalysts such as metal-organic frameworks（MOFs）had this problem.To solve this problem,this paper uses a metal template-guided growth strategy to grow metal oxides on metal substrates.The MOFs were grown in situ after the array to construct macroscopic MOF composites.The results show that the catalyst has a core-shell structure.The unique MOF array structure can effectively expose the catalytic active sites of MOFs and improve the mass transfer process in catalysis.The significantly improved MOFs The high processability ensures the easy recycling of the catalyst.With the help of cocatalysts,ZIF-8 arrays exhibit excellent catalytic activity for CO₂ chemical immobilization:under mild conditions（1 bar CO₂,≤60℃）,ZIF-8arrays catalyze the cycloaddition reaction of ECH with CO₂,The yield of chloropropene carbonate can reach 96%.It is worth mentioning that the catalytic performance of the catalyst has almost no decrease in catalytic activity after 10 catalytic cycles.In order to be more suitable for industrial application,the catalytic system can still achieve 90%cyclic carbonate yield under simulated flue gas conditions（15%CO₂）and after water vapor treatment,and it can be reused 3 times under this condition,the catalytic activity did not decrease afterward.The chemical fixation of CO₂ by photothermal means is a sustainable method,and according to the material properties,the catalytic activity of the ZIF-8 array is 7 times higher than that of the ordinary ZIF-8 system under photothermal conditions.This work provides an idea for the practical application of MOF-based materials in the field of organocatalysis.3.In the above two works,the composite materials cannot avoid the use of cocatalysts in the process of catalyzing the cycloaddition reaction.In this regard,this thesis set out to design the encapsulation of nucleophilic ion-containing ionic liquids（ILs）in the pore cavity of MOF to construct a catalytic system without the use of cocatalysts.Based on this,imidazolyl-based ILs were rationally grown in situ in the pore cavity of MOF materials by bottle boat method and post-synthesis modification method,,and two composite catalysts of ILs/MIL-101 were obtained,IL@MIL-101 and poly ILs@MIIL-101 respectively.In addition,this method solves the disadvantage that most MOF materials have a single active site and require harsh reaction conditions in the catalytic CO₂ cycloaddition reaction.The results show that IL@MIL-101 exhibits extraordinary CO₂ adsorption performance:110 cm³ g^-1 at 273 K and 1 bar,and 65 cm³ g^-1 at 298K and 1 bar.In addition,IL@MIL-101 with high mobility of active sites exhibits significantly higher catalytic activity than poly ILs@MIIL,based on hierarchical porous structure,efficient CO₂enrichment,and effective synergy between different active sites,under mild conditions without cocatalyst（1 bar CO₂,≤70℃）,the yield of cyclic carbonate catalyzed by the cycloaddition reaction of CO₂ and ECH can reach 94%.And there is no obvious loss of activity in 5 consecutive cycles.