Preparation Of Au NPs Loaded Carbon Nitride Photocatalysts And Application In Oxidative Cracking Of Carbon-carbon Bonds

Oxidative cleavage of carbon-carbon bonds is an important method to convert macromolecular organics into high added value small molecule compounds,and to introduce oxygen-containing functional groups at appropriate positions.At present,the oxidative cleavage of carbon-carbon bonds in industry mainly uses oxidants such as ozone and hydrogen peroxide,which use high energy and have a certain risk of explosion.Recently,photocatalytic molecular oxygen oxidation reaction has become a hot research topic due to its mild reaction conditions,good atomic economy and in line with the development direction of green chemistry.C₃N₄（carbon nitride）,a polymeric semiconductor material composed of nitrogen element and carbon element,is cheap non-toxic,easy to recycle,and is an ideal class of photocatalysts.But C₃N₄ materials have defects such as easy recombination of photogenerated electron hole pairs and narrow spectral response range.In this paper,we prepared a recyclable heterogeneous photocatalyst Au/PHI by loading Au nanoparticles（Au NPs）with polyheptazimide structured carbon nitride（KNa-PHI）as the support,which significantly improved the photocatalyst performance.At the same time,the oxidative cleavage of alkyne C≡C bonds,o-diol C-C bonds and lignin C-C bonds was achieved with high efficiency under light driven oxygen as oxidant.The main content of the article is divided into the following four sections:1.The Au NPs loaded carbon nitride photocatalyst Au/PHI was prepared by a"cation exchange-freeze drying-in situ photoreduction"step using KNa-PHI,a bottom-up low temperature polymerized carbon nitride material,as the support.The synthesized composites were characterized by HRTEM,SEM,XRD,XPS and electrochemical workstation.The results show that Au cations are anchored by negative sites of KNa-PHI through cation exchange,which makes Au NPs formed by subsequent in-situ photoreduction have better particle size and dispersion.At the same time,the freeze-drying process significantly reduced the catalyst particle size,increased the surface ravines and wrinkles,and increased the specific surface area,which was conducive to the contact and collision of the catalyst and the substrate.In addition,Au NPs and KNa-PHI form a heterojunction that promotes the separation of photogenerated electrons and holes,and the local surface plasmon resonance effect can amplify the excitation wavelength to the visible range.The modified Au/PHI presented heterogeneous in aqueous system for convenient recovery.2.Oxidative cleavage of the C≡C bonds of arylalkynes was achieved using Au/PHI as photocatalyst with oxygen as oxidant and water as solvent at room temperature using xenon lamp（250 W）as the light source to produce 15 aryl acids in 71%-92%yields through substrate extension experiments.Moreover,this reaction system can undergo a tandem oxidation/esterification reaction when methanol is used as solvent and HCl O₄ is an acid catalyst,and 15 aryl ester products are obtained in 48%-82%yields.Scale up experiments show that this reaction can achieve gram scale conversion under sunlight irradiation,and the catalyst can be recycled for 10 and 6 times without significant loss of activity for oxidative cleavage and oxidation/esterification reactions,respectively.The superoxide radical（·O₂^-）and hydroxyl radical（·OH）were shown to be the major green oxidative active species during the reaction by mechanism studies.3.Modified Au/PHI was used as photocatalyst and oxygen as the oxidant in in acetonitrile solvent,using a xenon lamp（250 W）as the light source,the oxidative cleavage of the 1,2-diols C-C bonds was achieved to prepare 15 carboxylic acid compounds.This reaction system has a relatively good substrate scope,the acid product is obtained in a yield of 40%to 91%for both aromatic and aliphatic ortho-diol substrates.The reaction can achieve scale-up experiments on a gram scale under solar light irradiation,and the catalyst shows no obvious activity drop over10 reuse experiments.The reaction was found to be a radical process by probing the mechanistic experiments with superoxide radical（·O₂^-）and singlet oxygen（¹O₂）as the main active species.4.With the lignin model compound as the substrate,the Au/PHI catalyst precursor,Au-PHI as the catalyst,oxygen as the oxidant,and aqueous CTAB（0.6ωt.%）as solvent,the oxidative cleavage effect under the irradiation of xenon lamp（250 W）,the process without prior reduction of gold ions,can in situ prepare Au/PHI during the reaction and take part in the reaction.Studies have shown lignin in aqueous systemsβ-O-4 model compound C_α-C_βbonds can undergo complete oxidative cleavage within 8 h.The reaction system was suitable for all fiveβ-O-4 model compounds,and the catalyst remained highly active after 20 cycles.Mechanistic studies revealed that the main active species during the reaction was the superoxide radical（·O₂^-）.Meanwhile,that catalyst also has good catalytic effect on real lignin（sodium lignosulfonate）,and high-value aromatic monome compounds can be obtain in the yield of 25.5%.