Application Of TiO₂-based Photocatalytic System In The Selective Conversion Of Benzaldehyde From Benzyl Alcohol

Green chemistry is always advocated,and photocatalysis is a very important branch of green chemistry.Its green and non-polluting design concept and mild conversion process are important technology reserves for the national"double carbon"strategic goal,which can play a significant role in industrial and agricultural production.This thesis focuses on the use of photocatalysis to selectively oxidize benzyl alcohol to produce benzaldehyde,avoiding the use of toxic or corrosive oxidants（Cl₂,KMnO₄,K₂Cr₂O₇）and the harsh reaction conditions associated with the traditional benzyl alcohol production process.In this thesis,a series of TiO₂-based photocatalysts were designed using different modification methods to address the problems associated with TiO₂ in the oxidation of benzyl alcohol,such as narrow absorption range of solar wavelengths,high photogenerated carrier complexation rate and excessive oxidation.The main research of this thesis is as follows.（1）In this chapter,polyacrylonitrile（PAN）was used to prepare a controlled diameter nanofibrous template by electrostatic spinning,and the template was combined with tetrabutyl titanate（TBOT）to prepare TiO₂ precursors by natural hydrolysis,and then TiO_2-X nanotubes enriched with different oxygen vacancy concentrations were finally prepared by calcination and vacuum temperature gradient annealing to establish an efficient and stable photocatalytic benzyl alcohol a highly efficient and stable photocatalytic benzyl alcohol selective oxidation reaction system was established.As a typical one-dimensional structure,TiO_2-Xnanotubes have a high specific surface area and length-to-diameter ratio,which are conducive to charge transfer.At the same time,the presence of oxygen vacancies is not only beneficial to improve the catalyst’s ability to adsorb O₂ to·O₂^-conversion,but also to improve the light absorption utilization of the catalyst to a certain extent.The catalyst（T₃）prepared by vacuum annealing at 300°C was tested to have the best photocatalytic activity.Under simulated sunlight（full spectrum wavelength）irradiation,benzyl alcohol conversion of 78%and benzaldehyde production selectivity of 88%could be achieved,which were 2.6 and1.8 times higher than those of the pristine TiO₂ nanoparticles,respectively,which also showed good cycling stability.（2）Based on the preparation of the photocatalytically active best catalyst in the previous chapter,CdS quantum dots were loaded to prepare CdS quantum dots modified TiO_2-X nanotubes composite photocatalyst.The whole system because of CdS quantum dots photosensitization effect and heterojunction effect can achieve rapid separation of photogenerated electrons and holes at the same time,it also makes to expand the light absorption range of the composite catalyst,which can utilize more visible light part of sunlight.The photosensitization effect and heterojunction can simultaneously enhance the photocatalytic activity of the composite catalysts.We prepared T₃ nanotubes modified with 7.5%mass fraction of CdS quantum dots（7.5%CS QDs/T₃）with the highest photocatalytic activity,which could drive the conversion of benzyl alcohol to 90%and the selectivity of the target product benzaldehyde production to 99%.The 7.5%CS QDs/T₃ composite catalyst still maintained the high photocatalytic activity after four cycles of experiments under visible light.（3）Based on the previous chapter,Ti₃C₂ MXene quantum dots（TC QDs）were introduced,which can be used as a cheap co-catalyst to participate in the selective oxidation of benzyl alcohol as a substitute of noble metal materials.TC QDs can effectively transfer photogenerated electrons to the surface of the composite catalyst as an electron acceptor thus accelerating the selective oxidation of benzyl alcohol.The structure and performance of the prepared composite catalysts were characterized.It is known that we successfully prepared the composite catalyst 7.5%CS QDs/T₃/TC QDs,which can induce 98%conversion of benzyl alcohol and 99%selectivity of benzaldehyde production of the target product in the photocatalytic benzyl alcohol selective oxidation reaction.The composite catalyst 7.5%CS QDs/T₃/TC QDs maintained stable photocatalytic activity after three cycles of experiments under visible light.