Selective Oxidation Of Aromatic Alcohols Catalyzed By Cadmium Sulfide Matrix Composites Under Visible Light

As one of the basic chemical raw materials,benzaldehyde（BAD）has been widely used in the fields of medicine and food.The traditionalproduction processes of benzaldehyde,such as benzyl chloride hydrolysis method have harsh reaction conditions and cause additional resource consumption and environmental pollution,which are not in line with the development of green sustainable development.Photocatalytic oxidation of benzyl alcohol（BA）technology has the advantages of sustainable energy,mild reaction conditions and low energy consumption,which provides a promising route for the green production of benzaldehyde.In recent years,CdS has attracted much attention in the field of photocatalysis due to its excellent visible light absorption,easily tunable electronic structure,and low cost.However,the photocatalytic performance of CdS is limited by high photonic carrier recombination efficiency and the presence of photocorrosion.Therefore,CdS was used as the photocatalytic material for the oxidation of aromatic alcohol in this research.According to the activity restriction factors,the photocatalysis efficiency of CdS was improved by building different systems.The main research contents were as follows:（1）N-CQDs/CdS composites were synthesized by in situ precipitation at room temperature.The BA conversion rate of 7.5N-CQDs/CdS is 60.4%,and the BAD selectivity is 99%,which is 2.2 times higher than pure CdS.It was proved that N-CQDs could be used as an"electronic warehouse",based on the test of pphotoelectrochemistry and the calculation of differential charge density.The photogenerated electrons of CdS were transferred to N-CQDs,resulting the surface of N-CQDs rich in electrons,which was beneficial to the formation of superoxide radicals（·O₂^-）and improved the separation efficiency of carriers.The photocatalytic activity enhancement mechanism of N-CQDs/CdS composites,benzyl alcohol oxidation mechanism（·O₂^-and hole（h+）can oxidize benzyl alcohol to benzaldehyde）were proposed by active species capture experiments,ESR test and adsorption energy calculation..The Calculation results provided a theoretical basis for improving the separation efficiency of the photocatalyst carrier and the selective oxidation performance of alcohols.（2）Ce-CdS composites were prepared by hydrothermal-calcination for the first time,and the photocatalytic properties of semiconductor materials were studied by photocatalytic oxidation of benzyl alcohol to the corresponding benzyl aldehyde.The conversion rate of benzool of 0.2Ce-CdS composites is 97%,and the selectivity of benzaldehyde is 99%.The percent conversion is 2.6 times higher than that of the original CdS,and the kinetic constant is 7 times that of pure CdS.A series of photochemical tests showed that Ce³⁺/Ce⁴⁺could further improve the separation efficiency of carriers and promote the single-electron reduction of oxygen to generate·O₂^-as an electron scavenger,which was the main reason for the improvement of photocatalytic activity.The capture experiments and ESR tests proved that both·O₂^-and h⁺can effectively oxidize benzyl alcohol to benzaldehyde;the selective oxidation of benzyl alcohol to benzaldehyde could be carried out by forming intermediate carbon center free radical.The work in this chapter provided some certain research idea for improving the separation efficiency of photocatalyst photonic carriers and the selective oxidation performance of benzyl alcohols.（3）CdS nanorods were prepared by solvothermal method,Fe OOH was modified on the surface of CdS by in situ impregnation,and 0D/1D Fe OOH/CdS composite photocatalyst was synthesized.The conversion rate of benzyl alcohol increased from 26%to 62%and the selectivity of benzaldehyde was 99%by visible light irradiation was 30minutes.In addition,Fe OOH/CdS also had good activity in the oxidation of para-substituted benzyl alcohol,indicating that the composite had a universal oxidation of benzyl alcohol.Transient photocurrent tests showed that Fe OOH could accumulate holes,improve the separation efficiency of photonic carriers,and reduced the oxidation of h⁺to S^2-.Finally,the mechanism of benzyl alcohol oxidation was discussed through capture experiments and ESR tests.Under visible light,the electrons on the CdS valence band were excited to the conduction band,and the photonic holes on the valence band were captured and transferred by Fe OOH.Benzyl alcohol molecules adsorbed on the surface of the photocatalyst were oxidized by h⁺to benzaldehyde or intermediate carbon center free radical（Ph⁺）.The electrons on the conduction band reduce O₂ to·O₂^-,and·O₂^-further oxidizes Ph⁺to benzaldehyde molecules.This work expanded the application of CdS and Fe OOH composite photocatalysts in alcohol oxidation.