Noble-metal Nanoparticles Decorated G-C₃N₄ On The Photocatalytic Nitration Of Aromatics

The nitration products of aromatics are commercially applied as the fine intermediates and other related chemical materials.They are currently produced by the traditional process in nitro-sulfur acid system,that performs a high yield and poor selectivity,generating an amount of unfriendly waste acids.It is costly and unsafe within the post separation and treatment.Therefore,it is absolutely needed to develop greener nitrification,for example,the energy-saving and environmental photocatalysis.Utilizing the clean solar energy and visible-light illumination,the semiconductor could produce the redox-active electrons and holes,participating and accelerating the mild organic synthesis.The present study,a noble-metal nanoparticles decorated over the g-C₃N₄ composite was prepared via a simple process.Its performance would be improved owing to the localized surface plasmon effect（LSPR）of metal nanoparticles.The catalyst was deeply characterized and applied on the selective nitration of bromophenols and nitrogen-containing heterocycles,using the nitrite as the nitro-source.Their electrostatic potential（ESP）diagrams were further calculated by the density functional theory（DFT）to explain the stereoselective mechanism of the photocatalytic nitration.Our results provided certain theoretical and technical exploration for the innovation of green nitrification.The main research of this paper is as follows:（1）Initially,the composite Ag/g-C₃N₄ photocatalyst was prepared by hydrothermal and photo-reduced processes.Ag nanoparticles were proved to be successfully loaded on the surface of g-C₃N₄ by SEM,TEM,XRD and XPS.The improvement of the photocatalytic activity due to the introduction of Ag nanoparticles was determined by the spectral analysis of UV-Vis,PL,TRPL and the photoelectronic analysis.Then,a train of bromophenols were picked for the photo-driven nitration with nitrite as nitrogen-source using Ag/g-C₃N₄ as photocatalyst.The heavy touch with the different substituents of bromophenol was examined and discussed.The results showed that fluorine,chlorine and bromine substituted bromophenol showed higher nucleophilic potential than alkyl and alkoxy substituted bromophenol.From the inhibition experiment and EPR analysis,the photo-generated superoxide and active holes were believed to function as a crucial role in the nitrification process.The electrostatic potential maps of all bromophenols were calculated using the Gaussian software to explain the possible mechanisms.（2）The photocatalyst was changed into the Pd nanoparticles loaded on g-C₃N₄,that is called Pd/g-C₃N₄ composites.Its electronic band structure may be favorable for Ag/g-C₃N₄,which produces more active electrons and holes in the presence of light.The photocatalytic nitration of bromophenols was carried out using copper nitrate hydrate as the nitration reagent.The results showed that the composite Pd/g-C₃N₄ normally performed better than Ag/g-C₃N₄.Through control experiments and radical inhibition experiments,it was found that the active h⁺played a key role in the photo-catalyzed nucleophilic process.The regioselective mechanism of bromophenols was explained by the calculation of their electrostatic potential diagrams.It was found that the reaction trend remained nucleophilic,while the nitrification ability in the control experiment was mainly due to the fact that light was the key factor of the whole reaction,and even without the catalyst,the reaction produced unstable nitrate in the excited state,which in turn released nitrogen dioxide radicals to attack the brominated aromatics.（3）Finally,the above two different nitration systems were utilized in the photocatalytic nitration of brominated nitrogen-containing heterocycles.The results demonstrated that the brominated nitro-heterocycles exhibited excellent nitration performance compared with that of the above brominated phenols.That is due to their electrostatic potential diagrams showing the advantageous potential than those of brominated aromatics.Using the 3-bromopyrazole and3,5-dibromopyrazole as models,the crossover experiments with both nitrating regents displayed the unbeneficial results and only the best performance took place over the nitration systems composed of Pd/g-C₃N₄ and copper nitrate,Ag/g-C₃N₄ and sodium nitrite.This further confirmed the difference between the two systems.The calculation of the electrostatic potential of nitro-heterocycles was used to make understandable the trend of mono-nitration.It is regrated that the existing research has not yet achieved the multiplex nitrification.