Study On Visible-light-driven Photocatalytic Oxidative Homocoupling Of Alkynes Over Copper-incorporated Hydrotalcite-like Compounds

Conjugated 1,3-butadiyne is an important class of organic compounds,mainly synthesized by the Glaser self-coupling reaction of terminal alkynes,that is,a thermal catalytic system composed of copper salts,alkali promoters or ligands.In view of the advantages of easy separation and reusability of heterogeneous catalysts,the recent use of supported copper catalysts for the thermal catalytic synthesis of 1,3-butadiyne has become the focus of research.Due to the visible light-induced localized surface plasmon resonance?LSPR?effect of nano-copper particles?CuNPs?,supported nano-copper catalysts are gradually being used in the field of photocatalytic synthesis.However,because of the problems of CuNPs being easily oxidized and lost,there was no report of heterogeneous catalysts containing CuNPs for visible light catalyzed oxidative coupling reaction of alkyne.In this paper,three kinds of catalysts for different positions of nano-copper in hydrotalcite-like laminates were prepared by co-precipitation-reduction method.Mg₃Al-CuY?Y is ethylenediaminetetraacetic acid?catalyst was mainly used to achieve the photocatalytic oxidative coupling of terminal alkyne to 1,3-butadiyne compound under room temperature,alkali-free,high-efficiency and catalysts reusable under visible light irradiation.The main results were as follows:1.The structure and composition of the catalysts were characterized by XRD,IR,ICP,BET,SEM,TEM,UV-Vis,ESR,XPS,etc.The effect of the position of nano-copper on the structure and photocatalytic properties of hydrotalcite-like catalysts was investigated.The results show that the nano-copper in the Mg₃Al-CuY catalyst was successfully inserted into the hydrotalcite-like layer.It has good visible light catalytic activity and stability,much higher than the thermal catalytic reaction.The order of activity and stability of the three catalysts is Mg₃Al-CuY>Cu/Mg₃Al-CO₃>Mg₅CuAl₂-CO₃.At the same time,the Mg₃Al-CuY catalyst has good substrate universality for photocatalytic oxidative homocoupling reaction of alkynes.2.The photocatalytic oxidation coupling reaction of phenylacetylene was used as a model to investigate the effect of Mg₃Al-CuY catalyst composition and reaction conditions on the catalytic reaction.The results show that the Mg₃Al-CuY catalyst with CuY^2-intercalation and Mg/Al ratio of 3:1 has the best catalytic activity.The photocatalytic reaction has no induction period.The polar solvent methanol helps the reaction proceed.Catalyst basicity and O₂ have an important influence on the catalytic reaction.This reaction is directly induced by visible light.3.By studying the effects of kinetic isotope effect?KIE?and free radical scavengers,a reasonable reaction mechanism is proposed.Cu⁰ is the active center of the reaction.The hot electrons generated by the LSPR effect of CuNPs activate O₂ to O₂^-species.The activated O₂^-species and the catalyst alkaline O-H promote the alkyne C-H bond cleavage.The dehydrogenation of alkynes is not the rate limiting step.At the same time,the presence of carbon radicals and the action of EDTA ligands make CuNPs relatively stable.