Photocatalytic C-C Coupling Of Aromatic Aldehydes With Metal-doped ZnIn₂S₄

With the development of industrial globalization,the sharp increase of fossil fuel consumption leads to the increasingly serious energy crisis.Lignin is a renewable biological resource with abundant benzene ring structure.The depolymerized products can be used to prepare biofuels and high-value chemicals.However,the carbon atom content and oxygen atom content of the products obtained by depolymerization of lignin are low,which is inconsistent with the requirements of high calorific value and high density of biofuels.Therefore,these depolymerization products need to increase the carbon atom content through controllable C-C coupling reaction,and finally reduce the oxygen content of the products by hydrodeoxygenation reaction,so as to realize the conversion from lignin to biofuel.The traditional C-C coupling has poor selectivity and will produce a large amount of metal waste and cause environmental problems.Therefore,using sunlight and cheap photocatalyst to carry out aromatic aldehyde C-C coupling reaction under mild conditions has a good development prospect.In this paper,aromatic aldehydes,the depolymerization product of lignin and its model compounds,were used as raw materials,and C-C coupling reaction was carried out under mild simulated sunlight to obtain a series of Ortho glycol with high added value.The research contents and main results of this paper are as follows:（1）In order to overcome the shortcomings of short carrier life and low light absorption capacity of Zn In₂S₄,Zn In₂S₄（ZIS）was doped with copper by a simple one-step hydrothermal method to obtain Cu-Zn In₂S₄（Cu-ZIS）photocatalytic material.The characterization of morphology,structure and photoelectric properties show that Cu-ZIS has more active sites,wide light absorption range and excellent photoelectric properties.Compared with pure ZIS,Cu-ZIS photocatalytic material has smaller conduction band potential and stronger reduction ability,which provides a theoretical basis for Cu-ZIS photocatalytic C-C coupling of aromatic aldehydes.（2）The modified Cu-ZIS material was applied to the photocatalytic benzaldehyde C-C coupling reaction.The effects of photocatalytic hydrothermal time,copper doping amount,triethylamine dosage,solvent ratio and illumination time on the photocatalytic benzaldehyde C-C coupling effect were investigated.Under the optimum reaction conditions,the conversion of benzaldehyde is 100%,the yield of hydrobenzoin is100%,and Cu-ZIS still has excellent catalytic activity after 5 cycles.It indicates Cu-ZIS has excellent selectivity and stability for photocatalytic C-C coupling of benzaldehyde.The general applicability of substrates showed that 17 kinds of substrates could be coupled to ortho glycol under this reaction condition.The active species capture experiment shows that the key active species of photocatalytic benzaldehyde C-C coupling reaction is photogenerated electrons.On the basis of the above experiments,the C-C coupling mechanism of benzaldehyde on Cu-ZIS material is proposed.（3）Aiming at the problem that vanillin,the depolymerization product of lignin,cannot be successfully coupled in the triethylamine reaction system,a more reducing sodium sulfite reaction system was designed to successfully couple vanillin under light.The effects of photocatalyst,light source,sodium sulfite concentration,solvent ratio,substrate concentration and illumination time on vanillin C-C coupling reaction were investigated.Under the optimum reaction conditions,the conversion of vanillin was50.0%and the yield of hydrovanillin was 44.4%.The active species capture experiment shows that the key active species of vanillin C-C coupling reaction is hydrated electron,which is consistent with the conclusion of photocatalytic benzaldehyde coupling activity capture experiment.The general applicability of substrates showed that ortho glycol could be obtained by coupling five aromatic aldehydes under this reaction condition.（4）Aiming at the instability of hydrogenated vanillin in sodium sulfite reaction system,a more effective ascorbic acid reaction system was proposed to successfully C-C couple vanillin under photocatalysis.Meanwhile,the effects of photocatalyst,ascorbic acid concentration,solvent type and ratio,substrate concentration and light time on the photocatalytic vanillin C-C coupling reaction were investigated.Under the optimum reaction conditions,the conversion of vanillin was 91.1%,the yield of hydrovanillin was 60.4%,and hydrovanillin had good stability in the reaction system.Cyclic experiments show that Cu-ZIS also has good cyclic practicability in the photocatalytic coupling of vanillin.The general applicability of substrates showed that the reaction conditions were also suitable for the photocatalytic C-C coupling reaction of syringaldehyde.