Photocatalytic CO₂ Conversion From Epoxides To Cyclic Carbonates Over Metal-Organic Framework

With the continuous development of industrialization,the environmental and energy problems caused by the burning of a large number of fossil fuels have been widely concerned by people.The burning of fossil fuels inevitably leads to massive emissions of carbon dioxide,and therefore global warming and sea level rise are inevitable consequences.Both climate change and energy shortages can be attributed to carbon dioxide.At the same time,carbon dioxide are abundant,cheap,low toxic and sustainable.The fixation and conversion of carbon dioxide into organic matter is a process of turning waste into treasure,which has very important significance.On the stage of carbon dioxide conversion,carbon dioxide conversion to cyclic carbonate plays an indispensable role.As a good polar solvent,cyclic carbonate is widely used in electrolytes of batteries,especially lithium ion batteries,and it is also the raw material for synthesis of some chemical products.However,the reaction usually requires high temperature and high pressure,which result in high energy consumption.How to reduce the energy consumption of the reaction and enable it efficient proceed under normal temperature and pressure is an important issue.As a kind of renewable green energy,solar energy is inexhaustible.In fact,scientists have long been continuing to explore how to convert solar energy into chemical energy,in order to solve energy and environmental problems.Using photocatalysis to reduce the concentration of CO2 is a wise choice.In recent years,it has been found that semiconductor catalysts can effectively absorb light energy,generate photogenerated electrons and holes,and the photogenerated electrons can effectively activate carbon dioxide.Therefore,it is reasonable to assum that light can promote the conversion of carbon dioxide under mild conditions.MOFs(Metal-Organic Frameworks)is a new kind of porous materials,which consist of metal and organic ligands.In addition,MOFs display superiorities in catalytic applications as well due to their large specific surface area,adjustable pore structure and high density of catalytic active sites.This thesis introduces Lewis acidic metals into MOFs by post modification,with the aim to promote the activation of epoxide.Meanwhile,we also introduce photocatalysis into the carbon dioxide cycloaddition reaction,which is traditionally driven by heat energy.It is found that light irradiation can activate carbon dioxide,leading to the decrease of energy barrier of the carbon dioxide cycloaddition reaction.Ultimately,the carbon dioxide cycloaddition proceeds efficiently under mild conditions.In addition,the photocatalytic carbon dioxide cycloaddition reaction mechanism was studied.The specific contents are as follows:(1)Bi-PCN-224 material was designed and synthesized by post-modification method.Bi atoms were introduced into PCN-224 material to increase the Lewis acid catalytic site of the material and improve the photogenerated carrier separation efficiency of the material.For carbon dioxide cycloaddition reaction,this work creatively introduces light into the reaction system,showing excellent performance in photocatalytic carbon dioxide cycloaddition;At the same time,the mechanism of photocatalytic cycloaddition reaction of carbon dioxide was studied in detail,and the reaction path are different from that of traditional thermal catalysis.(2)On the basic of the first work,the influence of the photophysical properties of materials on the photocatalytic cycloaddition reaction of carbon dioxide was further studied.Zn or Ce is successfully introduced into UiO-bpydc to obtain UiO-bpydc(M)to obtain UiO-bpydc(M),M=Zn or Ce.The effect of metal on the CO2 cycloaddition efficiency is systematically investigated with respect to CO2 and propylene oxide activation.This results suggest that UiO-bpydc(Zn)displays an optimized result,as introduction of Zn can not only activate propylene oxide effectively,but also promote the LMCT process,which leads to efficient CO2 activation.Hence this work finds an optimal way to improve the photocatalytic carbon dioxide cycloaddition reaction efficiency and provides an effective method to improve efficiency.