Preparation And Characterization Of Au-supported N-doped Biomass Carbon-based Catalyst And The Performance In Selective Catalytic Oxidation

At present,the industrial production of various chemicals and synthetic polymers relies heavily on fossil resources.In the process of seeking alternatives to renewable resources,biomass and biomass-derived materials have become the hottest research hotspots in the contemporary era due to their low prices and wide sources.Among them,researches on the preparation of in situ heteroatom-doped biomass-based carbon materials by pyrolysis,and the conversion of biomass polysaccharide units to prepare high value-added chemicals attract much attention.Xylonic acid and gluconic acid are important research products of sugar acid platform compounds,and are widely used in food,agriculture,medicine,construction and other fields.Besides,xylonic acid is also a pharmaceutical intermediate and acts as a green solvent to catalyze the Biginelli reaction.Therefore,it is of great significance to develop an efficient,simple,environmentally friendly and energy-saving catalyst with universal applicability to prepare both xylonic acid and gluconic acid.In this paper,from the perspective of high value-added utilization of biomass,natural N-doped basic polymer chitosan is used as a raw material to prepare a Au NPs supported carbon-based catalyst,and its pentose D-xylose,hexose sugar D-glucose universally and selectively oxidized in the alkali-free aerobic catalysis for the preparation of sugar acids are explored.The obtained results are as follows:1.Using the mechanical ball mill to physically grind the chitosan urea mixture.By adjusting the ratio of different chitosan urea,it is prepared by carbonization to obtain NC-1,NC-2,NC-3,NC-4 and NC-5 series N-doped biomass carbon materials.The analysis and research of the NC-x series of N-doped biomass porous carbon materials by different characterization methods revealed that only the N-doped biomass carbon-based material NC-3with a ratio of chitosan to urea of 1:4 showed a hierarchical porous fibrous structure with a specific surface area of 653.82 m²g^-1,the average pore diameter is 1.65 nm,and the crystalline carbon structure of NC-3 is mainly T-type carbon and G-type carbon.In addition,NC-3 has the most N-doped defect vacancies,which The contents of graphite nitrogen and pyridine nitrogen were 30.56%and 34.00%,respectively.2.Using the sol-immobilization method to load the Au-particles of the NC-x series of N-doped biomass carbon-based materials prepared above,and apply it to the alkali-free catalytic oxidation of D-xylose and D-glucose to prepare high value-added sugars acid reaction,where Au/NC-3 shows excellent catalytic performance and high selectivity.The Au/NC-3 catalyst had an Au loading of 0.97%,and the average diameter of Au nanoparticles was 3.87 nm.The optimal reaction conditions for Au/NC-3 catalytic base-free aerobic oxidation of D-xylose into D-xylonic acid under 0.20 g xylose,reaction temperature 100?,reaction time 120 min,oxygen pressure 3 bar,xylose/Au=524:1 mild condition.The yield of D-xylonic acid was 98.87%,and the conversion rate of D-xylose was 99.86%;the optimal conditions for Au/NC-3 to base-free catalyze and oxidize D-glucose to prepare gluconic acid were as follows:D-glucose 0.20g,reaction temperature 100?,reaction time 120 min,oxygen pressure 2 bar,glucose/Au=437:1.The yield of D-gluconic acid was 97.62%,and the conversion rate of D-glucose was98.76%.The catalyst remained relatively stable in the first three cycles,and decreased significantly after five cycles.The main reason for the decrease in catalyst activity was the agglomeration of Au nanoparticles,and the mean diameterof Au NPs was 6.74 nm after 5 used.After activation,the O content increased significantly,and the N species content and nitrogen oxide consumption also affected the catalyst activity.3.Using the density functional theory to establish Au₁₃ nanoclustered plate model supported by N species doped graphene,preferential adsorption sites for Au nanoparticles in N-doped biomass carbon structure,substances in the reaction system and reaction intermediates simulation and analysis of the stable structure at the catalytically active site.It is obtained that Au nanoparticles are preferentially adsorbed on the pyridine nitrogen vacancies.D-xylose and D-glucose are preferentially adsorbed on the surface of Au nanoparticles over oxygen and H₂O molecules.D-xylonic acid and D-gluconic acid are preferentially replaced by D-xylose and D-glucose and desorbed on the catalyst surface.The mechanism of alkali-free catalytic oxidation was discussed,and the reaction mechanism of free radical-driven oxidation of monosaccharides to sugar acids on the catalyst surface was obtained,moreover,the Au NPs can stabilize the reaction intermediate predicted by the reaction process.