| The photocatalytic molecular oxygen selective oxidation of C-H bonds occupies a very important position in the field of organic catalysis.Due to its mild reaction conditions and no secondary pollution,it has been favored by the majority of researchers and has become one of the current hot spots in the field of catalysis.Anthraquinone and its derivatives(AQs)are natural products,which are widely distributed in nature,and they are widely used in fine chemical industries such as dyes.However,the traditional anthraquinone(AQ)synthesis method has disadvantages such as high energy consumption,strong toxicity,equipment corrosion and serious environmental pollution.In addition,because AQs have unique photophysical and chemical properties,they are widely used as catalysts in photooxidation reactions.In the redox process,the substituents of AQ play a particularly important role in regulating the kinetics of the electron and proton transfer reaction.The electron-deficient substituents can enhance the catalytic performance of AQ.However,there are few reports about the effect of substituents on the optical properties and photocatalytic performance of AQ.For this reason,the study of this paper takes the cheap 9,10-dihydroanthracene(DHA)and its oxidation product AQ and its derivatives as the catalyst,molecular oxygen(O2)as the oxidant,visible light as the driving force,and carries out the oxidation of organic compounds by O2 driven by visible light under mild conditions.The main innovations are as follows:1.Developed a synthetic method that uses visible light to drive O2 to directionally oxidize DHA to AQ under mild conditions,which can achieve quantitative AQ yield(100%)without any extra catalysts and additives under normal temperature and pressure.Furthermore,this protocol has a good compatibility for the photooxidation of several other compounds with a similar structure to DHA.Based on a series of control experiments and free radical quenching and EPR spin trapping results.The photo-autooxidation of DHA is likely initiated by its photo-excited state DHA*and the latter can activate O2 to a superoxide anion radical(O2-·)via a transfer of its electron.Subsequently,this photooxidation is gradually dominated by the oxygenated product AQ as an active photo-catalyst,obtained from the oxidation of DHA by O2-·,and it is accelerated with the gradually accumulation of AQ.2.The selective photooxidation of ethylbenzene(EB)to acetophenone(ACP)driven by the photo-autooxidation of DHA under visible light illumination is realized,which can achieve 87.7%EB conversion and 99.5%acetylacetone(ACP)selectivity under ambient conditions.Furthermore,this protocol has a good adaptability for the photooxidation of other organic substrates to their corresponding oxygenates.A series of control and quenching tests,combined with EPR spectra,suggest that the photo-excited DHA can transfer its photo-electron to O2 to yield a superoxide radical anion(O2-·),then DHA is preferentially oxidized to AQ by the active O2-· owing to its high reactivity.Finally,the in situ generated AQ as an active photo-catalyst can achieve the photooxidation of EB and other organic compounds by O2.3.By changing the substituents on the AQ skeleton to fine-tune and improve the optical properties and photocatalytic performance of AQ at the molecular level,and the electron-withdrawing substituents exhibit a better improvement effect than the electron-donating one.The best catalyst AQ-COOH can achieve 70.9%PX conversion and 88.2%p-toluic acid selectivity.The addition of benzenesulfonic acid in the above photocatalytic system can further increase PX conversion to 86.7%,along with a significant improvement in the selectivity of p-carboxybenzaldehyde and terephthalic acid,which likely originates from the unique recombination behavior of the photo-generated charge carriers of the catalyst and the additive driven by the strong interactions between their aromatic skeletons and acidic substituents.Furthermore,AQ-COOH can achieve almost quantitative yield of acetylacetone in ethylbenzene photooxidation and the regulating effect of toluene's para-position substituents on its reactivity is also reported in the AQ-COOH photocatalysis system.Based on quenching tests and EPR spectral characterizations,the oxygen vacancies(h+),superoxide free radicals(O2-)and singlet oxygen(1O2)as the photo-generated active species are involved in the proposed photo-catalysis mechanism.4.The optical properties and photocatalytic performance of AQ-COOH can be fine-tuned and improved at the molecular level through additives.It is found that PhSO3H can significantly promote the improvement of the photocatalytic performance of AQ-COOH.When adding anhydrous PhSO3H as an additive to the reaction system based on AQ-COOH photocatalytic oxidation of toluene,the conversion of toluene was significantly increased from 41.9%to 70.1%.In addition,the photooxidation system has good adaptability for photooxidation of other organic substrates.For aromatic substrates,after adding PhSO3H as an additive,the catalytic efficiency of AQ-COOH is generally improved,but the promotion of oxidation of non-aromatic substrates is not obvious.In addition to the recombination of photo-generated charge carriers between the catalyst and additives,this promotion may also be caused by AQ-COOH combined with the protons provided by PhSO3H to form HAQ-COOH+cations,which are excited to form the excited state(HAQ-COOH+*)after light illumination.This excited state can realize the photoelectron transfer to O2 to form O2-,and the positive charge is transferred to the aromatic substrate with a benzene ring to form a benzyl radical cation,the H+in the latter is abstracted by O2-to generate benzyl radical and hydroperoxy radical HOO·.Benzyl radicals are finally converted into oxidation products benzaldehyde(BA)and benzoic acid(BC).Free radical quenching experiments and EPR spectra show that active species such as electron h+,e-,O2-· and 1O2 exist in the photocatalytic oxidation system of toluene. |
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