| Semiconductor photocatalysis has wide application prospects in organic conversion.Constructing a coupling reaction system by using relatively independent oxidation half-reaction of photogenerated holes and reduction half-reaction of photogenerated electrons in the photocatalytic process simultaneously is a green and atom-economic way to realize the conversion of organic compounds.Ullmann coupling reaction is a very important way to construct chemical bonds such as C-C.Its initial step is the activation of C-X(X is halogen)bond,which is essentially a reduction reaction.Coupling the selective oxidation reaction of benzyl alcohol to benzaldehyde with it not only can efficiently circulates the reaction,but also obtains valuable oxidation products.How to design and prepare photocatalyst is the key to realize this coupling reaction system.Therefore,based on thermodynamics and kinetics analysis,considering the semiconductor support,active metal and their interfacial interaction comprehensively,we constructed carbon nitride supported nano-noble metal bifunctional catalyst,developed a new green reaction approach by coupling the aromatic alcohol selective oxidation with Ullmann C-C coupling reaction,and tried to elucidate the relationship between the structure and performance of catalysts.The major contents are summarized as follows:(1)We deposited well-dispersed Pd nanoparticles with small size on crystallized carbon nitride(CN-C)by using a methanol reduction method to construct a Schottky-type Pd/CN-C hybrid for photocatalyzed Ullmann C-C homo-coupling of aryl halides under visible light irradiation at room temperature.The results show that Pd6.0/CN-C photocatalyst presents excellent photocatalytic activity for Ullmann C-C homo-coupling reaction with 94%yield,which is much higher than photocatalytic activity over Pd NPs supported g-C3N4(Pd6.0/g-C3N4,7%yield).Upon visible light irradiation,more photogenerated electrons from CN-C flow across the Schottky junction to metallic Pd and trigger the Ullmann C-C coupling of aryl halides.Moreover.Pd/CN-C has general applicability for various substrates and shows excellent stability and reusability for more than nine cycles.(2)The crystal structure and microstructure of carbon nitride were controlled by adjusting the calcination temperature,and thus O-doped crystalline carbon nitride containing N defects(CN)was synthesized.Then,Schottky Pd/CN heterojunction photocatalyst with lower Pd content was synthesized via an impregnation-reduction method.Two photocatalytic processes including Ullmann C-C coupling and aromatic alcohol oxidation reaction were coupled to form a complete photocatalytic coupling cycle.The experimental results show that the addition of aromatic alcohols not only improves the efficiency of Ullmann coupling reaction but also obtains simultaneously useful oxidation products aromatic aldehydes or ketones.N defects and O doping can enhance the visible light absorption of carbon nitride and promote the migration and separation of photogenerated charge carriers.Moreover,increased crystallinity enlarges? electron delocalization and improves electron transport ability along the in-plane direction.These cause the effectively improvement in photocatalytic activity.Pd/CN has wide applicability to various halogenated hydrocarbons and aromatic alcohols,and the reaction system has good stability and repeatability.This study is of theoretical and practical significance for the development of new high-efficient Ullmann C-C coupling green process and the enrichment of photocatalytic organic synthesis chemistry. |
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