MOFs-Based Catalysts Towards Thermal/Photothermal-Driven CO₂ Cycloaddition With Epoxides

The excessive emission of carbon dioxide（CO₂）is one of the main causes of global warming,but CO₂ is also the most abundant C1 resource on the earth.Therefore,capturing and converting CO₂ into valuable chemicals is one of the effective measures to solve the problem of CO₂pollution.Among them,the preparation of cyclic carbonate by cycloaddition reaction of CO₂ and epoxide is a scheme of high value utilization of CO₂ in line with atomic economy and environmental friendliness.However,the traditional catalytic system often has some problems,such as low catalytic activity,complicated preparation,high cost,single active site,and difficulty in catalyst separation.Therefore,it is of great research value and social significance to develop and use new,efficient,energy-saving and emissive heterogeneous catalysts for efficient CO₂cycloaddition reaction under mild conditions.This thesis is mainly based on metal-organic framework materials（MOFs）,designed a series of thermal and photothermally driven CO₂cycloaddition catalysts under mild conditions,and characterized the structural characteristics of these materials,explored the relationship between CO₂ adsorption capacity,active site,pore structure and performance,and discussed the thermal and photothermally driven catalytic reaction mechanism.This study provides design experience and theoretical reference for the design of efficient MOFs-based CO₂ cycloaddition catalysts.The main research contents of this paper are as follows:In the first part of the thesis（Chapter 2）,the bifunctionalization of ZIF-8 was achieved by the method of iron ion doping and amino group introduction.Among them,iron ion doping can optimize the Lewis acidity of ZIF-8,while amination modification can improve the CO₂adsorption ability of the sample and help to activate CO₂,thereby promoting the reaction.The experimental results showed that the modified sample（Fe-ZIF-8-NH₂）showed higher catalytic activity for the cycloaddition reaction of CO₂ and epichlorohydrin under relatively mild（0.1 MPa CO₂,≤80℃）and solvless reaction conditions catalyzed by the cocatalyst.After 24 h of reaction,the yield of the target product（allyl chloride carbonate）was up to 92%,much higher than that of Fe-ZIF-8（80%）,ZIF-8-NH₂（62%）,and pure ZIF-8（40%）.In addition,Fe-ZIF-8-NH₂ showed a good conversion efficiency for the cycloaddition of CO₂ and epichlorohydrin under water vapor treatment（80℃and 100%relative humidity）.After 24 h of reaction,the yield of allyl chloride carbonate could reach 91%-93%.In addition,Fe-ZIF-8-NH₂ also exhibited high chemical stability and good recycling ability,with no significant loss of activity after 6 consecutive cycles.In order to solve the problem that MOFs catalyst particles are easy to agglomerate in the reaction process and difficult to recover after the reaction,the second part of the thesis（chapter3 of the thesis）continued to use Carbonized melamine spong（CMS）as the carrier to support MOFs materials,and prepare the corresponding composite materials with rich channel network and high nitrogen doping.The three-dimensional porous structure of CMS itself is not only conducive to the loading and material transport of MOFs particles,but also its own nitrogen doping,especially pyridine nitrogen can be used as Lewis base to improve the CO₂ adsorption and activation ability of the composite sample.The loading of MOFs could optimize the pore structure,increase the microporous structure and strengthen the Lewis acidity of CMS.Due to the synergistic effect of CMS and MOFs,CMS@MOFs（i.e.CMS@MIL-88-NH₂ and CMS@ZIF-8-NH₂）exhibited high CO₂ adsorption capacity（33 to 54 cm³/g）and provided a large number of Lewis acid-base active sites to aid epoxide and CO₂ activation.The results showed that under mild conditions（0.1 MPa CO₂ and 80℃oil bath heating,tetrabutylammonium bromide as cocatalyst）,the yield of allyl chloride carbonate of CMS@MIL-88-NH₂ and CMS@ZIF-8-NH₂ can reach 96%and 91.5%,respectively,after 24 h of reaction.Even when a low concentration of CO₂（CO₂/N₂=15:85 vol.%）diluted with nitrogen is used in the reaction system,the CMS@MIL-88-NH₂ sample can still achieve a high yield of 80-82%.In addition,because the catalyst used is a composite material,the catalyst can be easily separated and recovered from the reaction system,which has good recyclability.The results show that the yield of allyl chloride carbonate can still reach 88%after 6 catalytic cycles,showing good cycle stability.The above reaction is carried out under thermal catalysis,and considering the photothermal effect of CMS itself,the last part of the paper（Chapter 4）studies the use of light energy instead of traditional heating,and studies the CO₂ cycloaddition reaction of CMS@MOFs materials under photothermal driving.Solar power is considered the cleanest,cheapest and most sustainable energy source.Therefore,CO₂cycloaddition driven by solar energy has unique advantages in terms of energy and environment.By using CMS as the substrate material and loading MOFs（such as Fe-ZIF-8,MIL-88-NH₂ and ZIF-8-NH₂）,the composite material makes full use of the absorption spectrum of CMS and the advantages of high photothermal conversion efficiency,and uses the strong photothermal conversion ability of CMS to provide energy to the system through illumination.The results show that 71%yield of allyl chloride carbonate can be obtained from the composite material CMS@MIL-88-NH₂ under the light irradiation（light intensity:～350 m W/cm²）at room temperature for 6 h,and the yield of allyl chloride carbonate can exceed 89%after the light irradiation is extended to 8 h.In addition,considering that the reaction needs to be catalyzed with the assistance of additives,the experiment further attempts to introduce ionic liquid into the MOFs material to investigate its performance of photodriven CO₂conversion without additives.The experimental results show that 66%CO₂ conversion and 84%selectivity can be achieved by the reaction of CO₂ with epichlorohydrin for 8 h under the same light condition and helpless catalyst.