Study Of Au/UiO-66 Structure Regulation And Photocatalytic Alcohols Oxidative Esterification

As ester compounds are widely used in solvents,plasticizers,surfactants,and drugs,the esterification reaction occupies an important position in organic synthesis.Direct oxidative esterification of alcohols to synthesize ester compounds has the advantages of high atom utilization,low raw material cost,and few operation steps.Currently,most of the catalysts used for alcohol esterification are metal-supported catalysts,which are composed of metals(such as platinum,palladium,cobalt,etc.)and a carrier,and mainly use the excellent catalytic activity of metal nanoparticles to achieve catalytic conversion.At present,most of the catalytic systems are simple composites of metals and carriers and through traditional thermal catalytic pathways,complete conversion can be achieved under high temperature and high pressure conditions,and the catalytic efficiency is low.By absorbing and using incident light,photocatalysis can significantly improve the catalytic performance and achieve a mild and efficient conversion of the reaction,which is an effective measure to improve the catalytic efficiency of the alcohol esterification reaction.Gold(Au)nanoparticles exhibit outstanding catalytic oxidation performance,but also have a plasmon resonance effect,can absorb and utilize visible light to significantly enhance their catalytic activity,which provides the possibility of achieving a mild conversion of alcohol esterification reaction.Studies have shown that the factors affecting the catalytic activity of Au-supported catalysis mainly involve the particle size,characteristic morphology,electronic state,and carrier effect of Au.These factors can be controlled by metal-support interaction and material preparation methods.In this regard,this article uses metal-organic framework materials(UiO-66)to load Au.By controlling the defects of UiO-66,the electronic state of gold is changed to the negative electron state,and the photocatalytic conversion of alcohol esterification reaction under mild and atmospheric pressure is achieved.In addition,by changing the precursor content of Au during the photo-deposition process to orderly adjust the size distribution of Au particles,a graph of the relationship between catalytic activity and particle size of Au particles was obtained to find the optimal Au particle size and further improve the catalytic efficiency of Au.On the other hand,by performing NH₂-modification on UiO-66,Au/NH₂-UiO-66 was tried to catalyze the alcohol esterification reaction without alkalization.The specific research contents are as follows:1.Material defects can be created by adding modulator HCl to form UiO-66-HCl during UiO-66 synthesis.N₂ adsorption,XPS and NH₃-TPD confirmed the structural defects of UiO-66-HCl with a missing linker.In UiO-66-HCl,the unsaturated central metal clusters interact with the supported Au and undergo electron transport,changing the electronic structure of Au to the negative electron state.while the Au loaded on defect-free UiO-66-DF(defect free)cannot interact with the central metal cluster,and its electronic state is the same as that of conventional Au.Using benzyl alcohol oxidative esterification to generate methyl benzoate as a probe reaction,Au/UiO-66-HCl can achieve fast photocatalytic conversion of benzyl alcohol under normal temperature and pressure conditions,but there is no catalytic activity in Au/UiO-66-DF.The photocatalysis activity is related to the plasmon resonance effect of Au.The reason for the difference in catalytic activity is that Au in the negative electron state can better activate oxygen to generate superoxide radicals,which is conducive to the catalytic reaction.2.By changing the HAuCl₄ content in the solution during the Au deposition process,the particle size of Au can be effectively adjusted.As the theoretical loading increased from 1 wt%to 6 wt%,the size of Au NPs gradually increased from 2.39 nm to 9.08 nm.During the esterification of alcohols,the catalytic activity(turnover frequency,TOF)of Au NPs showed a"volcanic"trend with the continuous increase of Au nanoparticles and showed the best catalytic activity at2.54 nm.Therefore,in the Au/UiO-66-HCl catalyzed alcohol esterification system,Au in the negative electron state exhibits the best catalytic activity at a particle size of 2.54 nm.3.For Au/UiO-66-HCl to catalyze the reaction of benzyl alcohol to methyl benzoate requires the assistance of an external base,we tried to modify the basic functional group-NH₂ on UiO-66-HCl to form NH₂-UiO-66-HCl,loaded with Au to form Au/NH₂-UiO-66.Combining the strong oxidizability of Au and the basicity of-NH₂,we tried to realize the non-alkalinization of the photocatalytic alcohol esterification process.However,the results show that Au interacts with-NH₂ in NH₂-UiO-66 to change the electronic state,and the catalytic reaction cannot proceed at room temperature and normal pressure.In addition,in the ligand-deficient NH₂-UiO-66-HCl,the basicity of NH₂-is masked by the Lewis acid of the unsaturated coordination center metal cluster,and the alkalization of alcohol esterification cannot be achieved.