| Aromatic aldehydes play an important role in many fields such as medicine,chemical synthesis,and food,etc.The use of energy-saving and environmentally friendly photocatalytic technology to oxidize aromatic alcohols is one of the important ways to the efficient synthesis of aromatic aldehydes.Polyoxometalates(POMs)have abundant adjustable structural types,which are chemically stable,have redox reversibility.And they are also excellent electron acceptors,which can effectively improve the separation efficiency of photo-generated carriers and promote photocatalysis performance.Graphitic carbon nitride(g-C3N4)is a non-metallic polymer-based organic semiconductor photocatalyst.It’s non-toxic,easy to prepare,stable in physical and chemical properties,and absorbs visible light,which is one of the popular materials in semiconductor photocatalysts.However,the bulk g-C3N4 has a small specific surface area and photo-generated carriers are easy to recombine.In this manuscript,a series of heteropolyacid-based composite materials were obtained by loading POMs onto g-C3N4 nanosheets through electrostatic interaction.And we systematically explored its photocatalytic performance for the efficient oxidation of aromatic alcohols in the aqueous phase.1.Urea was used as the precursor for the synthesized of bulk g-C3N4 and the unstack g-C3N4 nanosheets(UCNS)were facilely prepared by exfoliating bulk g-C3N4 via hydrothermal treatment and sonication.Subsequently,the UCNS were further modified by phosphoric acids and PW12-P-UCNS was finally prepared after the incorporation of tungstophosphoric acid(PW12).The morphology and structure of PW12-P-UCNS were characterized by TEM,HRTEM,XRD,and other methods.The results showed that the bulk g-C3N4 was successfully exfoliated into a nanosheet with a thickness of 10 nm,and the PW12 nanoparticles were evenly dispersed on the surface of the carbon nitride nanosheets.PW12-P-UCNS has shown excellent performance in the selective photo-oxidation to produce benzaldehyde in the aqueous phase.After the reaction of 2 h,the conversion of benzyl alcohol is about 58.3%and the selectivity of benzaldehyde is maintained at 99.5%.The excellent photocatalytic performance is attributed to the introduction of the excellent electron acceptor PW12 to improve the separation efficiency of photo-generated carriers;the phosphoric acid modified carbon nitride nanosheets P-UCNS can enhance the adsorption of O2 and produce more superoxide radicals;the close integration between PW12 and P-UCNS can promote effective charge separation.In addition,the PW12-P-UCNS almost retained the original performance after five consecutive cycles,which proves that the composite material has good catalytic stability.2.Based on the previous chapter,to solve the problem of the low utilization rate of catalyst for light absorption,we used thermal oxidation etching to adjust the morphology and energy band structure of bulk g-C3N4,which obtained ultrathin carbon nitride nanosheets(HUCNS)and stronger redox ability under visible light irradiation.Finally,PW12 was loaded on carbon nitride nanosheets through silicate bridges and a new composite catalyst PW12-Si-HUCNS was constructed.The morphology and chemical structure of PW12-Si-HUCNS were characterized by TEM,HRTEM,XRD,and other methods.It was used to oxidize aromatic alcohols in the aqueous phase under visible light to prepare the corresponding aromatic aldehydes.After the reaction of 3 h,the conversion of benzyl alcohol was about 40.3%and the selectivity was 91.8%,and the benzyl alcohol derivative can also achieve the selectivity above 85%.This is due to the introduction of polyoxometalates to accelerate the separation of photo-generated hole-electron pairs.The silicate bridges facilitate the transfer of electrons.Besides,the carbon nitride ultra-lamellar structure can expose more active sites and facilitates better dispersion in aqueous solutions.The composite also has excellent reusability,and almost retained original performance after four consecutive cycles. |
PDF Full Text Request