Selective Photocatalytic Oxidation Of Titanium Dioxide Based Materials

As an important intermediate,imines have been widely used in pharmaceutical synthesis and chemical production.However,in the traditional process of condensation of amine and carbonyl compounds,it is difficult to obtain the ideal yield as the carbonyl compounds are quite active.For this reason,the search for heterogeneous catalysts with mild reaction conditions,high selectivity,portable separation and superior recycling performance has become the focus of our attention.Titanium dioxide can selectively oxidize benzylamine to imine under solar light which becomes the main alternative catalyst for photocatalysis.However,the large bandgap width makes TiO2 exhibit inferior photocatalytic performance under the condition of ultraviolet light,which greatly limits its practical application.In terms of TiO2 modification,supporting photosensitive molecules on its surface can not only broaden the absorption wavelength of TiO2,but also significantly improve the catalytic activity of the material under visible light.Here,we loaded two different types of photosensitive molecules onto the surface of TiO2,utilized visible light irradiation as reaction condition,and constructed a reaction system of selective conversion of benzylamine to imine.The specific research results are as follows:(1)The gold nanoclusters were used as a photosensitizer to load onto the surface of TiO2,and the conversion of benzylamine to corresponding imine was realized under the irradiation of purple LED lamp.Glutathione-protected gold nanoclusters(Au-GSH)were synthesized by heat reduction method and supported on TiO2 by dipping method.The reaction system of selective oxidation from imine to benzylamine was constructed by the oxygen in the air as oxidant and the violet LED lamp as the light source.The study shows that the synthesized Au-GSH nanoclusters possess the characteristics of photosensitive molecules thus can absorb visible light effectively.Through systematic study of the mechanism,it was found that under the irradiation of the purple LED lamp,Au-GSH nanoclusters would undergo energy level transition and form an excited state,and the electrons would be directly injected into the TiO2 conduction band.After receiving the injected electrons,TiO2 acts as an electronic transport and reaction platform,converting oxygen in the air into superoxide anions.The hole of Au-GSH nanoclusters convert benzylamine into benzylamine cation,then obtain the intermediate imine by the following degenerating the proton process.The unstable imine is immediately coupled with the unreacted benzylamine to form the final product imine.At the same time,it was found that TEMPO acted as a co-catalyst in the system and could improve the conversion rate to some extent.(2)In order to further broaden the reaction wavelength,perylene compounds were used as photosensitizer to support onto the surface of TiO2 to realize the conversion of benzylamine to imine under blue light.Under alkaline conditions,3,4,9,10-perylene tetrarboxylic acid dihydride was hydrolyzed and the solution was adjusted to acid with dilute hydrochloric acid.After filtration and drying,3,4,9,10-perylene tetrarboxylic acid(PTCA)was obtained.3,4,9,10-perylene tetrarboxylic acid(PTCA)was loaded onto the surface of TiO2 by ultrasonic method to obtain PTCA-TiO2 complex.With the synergy of blue light and TEMPO,a reaction system was constructed to selectively oxidize the imine into benzylamine with the help of oxygen in the air.The mechanism showed that PTCA,as a photosensitizer,absorbed blue light and injected electrons into TiO2 conduction band to play a sensitization role.After receiving injected electrons,TiO2 acts as a platform for electron transport and reaction,and converts oxygen in the air into superoxide anions.At the same time,TEMPO is converted to TEMPO+ by the positive PTCA+to complete the PTCA electron cycle.After that,TEMPO+ oxidizes benzylamine to the intermediate imine,and the unstable imine is immediately coupled with the unreacted benzylamine to produce the final product imine.The TEMPO-OH formed by TEMPO+ after oxidizing imine is converted into TEMPO-OH again under the action of superoxide anion to complete its own electron cycle.As an electronic transmission medium,TEMPO accelerates the reaction while completing the PTCA electron cycle.The results show that PTCA and TEMPO cooperate with each other and play an important role in electron transfer.In conclusion,the strategies of ultlizing gold nanoclusters and perylene compounds as photosensitizer were used respectively.With oxygen in the air as oxidizer,visible light irradiation as reaction condition and TEMPO as catalyst,the conversion of organic benzylamine to imine with high selectivity and high reaction rate was realized.It provides a new way to synthesize organic compounds under mild conditions.