Cooperative Photocatalysis Of TiO₂ With TEMPO Salts For Selective Oxidation Of Amines

Imines are of vital importance in the construction of fine chemicals and various N-heterocyclic compounds with pharmacological activities.Heterogeneous photocatalysis which utilizes visible light to drive the synthesis of imines under mild reaction conditions is a green and clean reaction pathway,being a promising synthesis technology.As a typical photocatalyst in heterogeneous photocatalytic systems,titanium dioxide(TiO₂)has the advantages of high reactivity,good stability,ecological non-toxicity and low cost.In this thesis,we started with TiO₂ semiconductor material and then modified it with different strategies.By combing with the option of suitable redox mediators,two cooperative visible light photocatalytic systems based on TiO₂ and 2,2,6,6-tetramethylpiperidin-1-oxoammonium salts(TEMPO salts)were constructed and applied for the selective aerobic oxidation reaction of amines.The study contents of this thesis are concretely shown below:(1)We find that anatase TiO₂ nanotubes(TNTs)prepared through a cost-effective hydrothermal treatment and ensuing annealing process have high specific surface area and longer photoelectron–hole pair lifetime,which are better platforms for O₂ activation than the precursor P25 TiO₂.Thus,a dye-sensitized TiO₂ cooperative system is created with alizarin red S(ARS),TNTs,and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate(TEMPO⁺BF₄^–)for selective aerobic oxidation of amines under green light-emitting diode(LEDs)irradiation.The electron transfer between oxidatively quenched ARS-TNTs and the redox mediator is the key to secure cooperative photocatalysis.The results prove that the activity of anatase TNTs could reach about 2.2 times that of the P25TiO₂ precursor.Due to increased polarity,TEMPO⁺BF₄^-serves more efficiently for electron transfer than(2,2,6,6-tetramethylpiperidin-1-yl)oxy(TEMPO),conferring above 1.5 times of activity that of TEMPO for the selective aerobic conversion of amines.(2)TiO₂ microspheres with distinct hierarchical architecture and high specific surface area were fabricated by a facile one-pot,template-free solvothermal process.Assembling amines on TiO₂ microspheres initiated by visible light can lead to a surface complex that captures visible light for further oxidation of amines.Furthermore,we find that the selective oxidation of amines could be boosted by fully exploring the surface polarity of TiO₂microspheres with more polar 2,2,6,6-tetramethylpiperidine-1-oxoammonium hexafluorophosphate(TEMPO⁺PF₆^–)instead of TEMPO as the redox mediator.Hence,a cooperative photocatalytic system was designed based on TiO₂ microspheres and TEMPO⁺PF₆^–for highly efficient and selective oxidation of amines in air induced by blue LEDs.Experimental data indicate that the activity of TEMPO⁺PF₆^–surpassed that of TEMPO in aiding the visible light-initiated selective oxidation of amines over TiO₂microspheres,reaching more than about 3 times of TEMPO in some cases.In conclusion,we have synthesized two different TiO₂ materials with intriguing morphology.Dye sensitization and self-assembling of N-containing molecules on the surface of TiO₂are the two strategies used for extending the absorption edge of TiO₂ to visible light.Then,in combination with proper redox mediators,two different cooperative visible-light photocatalytic systems have been tailored for the highly efficient selective aerobic oxidation of amines.Our works provide a reference for the photocatalytic application of TiO₂ and feature the potential of designing redox mediators in amplifying cooperative photocatalysis driven by visible light.