Photocatalytic Selective Oxidation Of 5-hydroxymethylfurfural Over G-C₃N₄-based Composite Photocatalyst

At present,the world is facing a severe situation of depletion of fossil resources and the increasingly serious greenhouse effect.Biomass resources are considered as alternatives to fossil energy because of their sustainability and cheapness.Among them,5-hydroxymethylfurfural?HMF?is considered as one of the most attractive and promising biomass platform compounds with multiple active groups,which can be used as intermediates to convert into high value-added chemicals such as2,5-furandiboxaldehyde?DFF?and 5-formyl-2-furancarboxylic acid?FFCA?through oxidation,hydrogenation and other ways.In the early research of HMF oxidation,the traditional stoichiometric oxidant was mainly used,but this method has the disadvantages of low atomic utilization rate,high cost,and the generation of toxic and harmful substances.In contrast,molecular oxygen has attracted more attention because of its advantages of environmental protection and low cost.However,the currently used heterogeneous catalytic system has shortcomings such as low activity or stability,demanding reaction conditions or environmentally unfriendly organic reagents.Therefore,it is of great significance for environmental protection and economy to develop environmentally friendly ways to achieve efficient conversion from HMF to DFF,FFCA.Photocatalysis technology has the advantages of no pollution,green environmental protection and low cost,and has become a hot spot in current research.Therefore,using light as the reaction driving force and molecular oxygen as the oxidant to selectively oxidize HMF is an ideal reaction path,which is of great significance to get rid of the current dilemma of energy depletion.In this study,we prepared two g-C₃N₄-based composite photocatalysts for photocatalytic selective oxidation of HMF to 5-formyl-2-furancarboxylic acid?FFCA?and2,5-furandialdehyde?DFF?,and studied their physicochemical properties and photocatalytic performance.1.The g-C₃N₄/NaNbO₃ photocatalyst was synthesized by simply calcining the mixture of NaNbO₃ and melamine.The synthesized composite exhibits high photocatalytic performance in the selective oxidation of HMF to FFCA when using water as solvent.The structure and composition of g-C₃N₄/NaNbO₃ photocatalysts were characterized by TG,XRD,SEM,UV-Vis,FT-IR and XPS methods,and the optical and electrochemical properties were investigated by EIS,PC and PL techniques.·O₂^-was inferred to be the primary active species in this process based on the active species trapping experiment.Heterostructure formation of g-C₃N₄/NaNbO₃composites efficiently promoted the separation of photo-generated electron-hole pairs and accelerated the electron transfer rate,thus reduced the formation of·OH,and sequentially improved the selectivity of FFCA.The highest HMF conversion of35.8%with FFCA selectivity of 87.4%was achieved on C/N-59.6 photocatalyst under the irradiation of visible light.In addition,the reaction pathway and the action mechanism of the catalyst were also speculated.2.The CdS/mpg-C₃N₄ composite photocatalyst was prepared by a simple precipitation method which was used in the reaction system of photocatalytic selective oxidation of HMF to DFF.The structure and photoelectrochemical properties of CdS/mpg-C₃N₄ samples were analyzed by XPS,XRD,UV-Vis,FT-IR,EIS,PC and PL characterization methods.At the same time,we also investigated the effects of different CdS content,reaction time,reaction solvent and other parameters on the reaction,and determined the optimal conditions.The results show that the CdS?7.5%?/mpg-C₃N₄ catalyst exhibits the best activity in the selective oxidation of HMF to DFF.The HMF converstion of 73.4%with DFF selectivity of 84.5%was obtained under visible light for 6 h,using 2 mL of trifluorotoluene and 3 mL of acetonitrile as solvents.In addition,the active species were identified through scavenger experiments.