Highly Selective Catalytic Conversion Of Carbon-oxygen Bond In Glycerol And 5-Hydroxymethylfurfural

The use of renewable biomass resources to produce high value-added fine chemicals and new biofuels is an effective means to replace traditional fossil energy sources and achieve green sustainable development.At present,the utilization of biomass resources has been extended to the conversion of biomass-based platform compounds to downstream value-added products,most of which involve the selective activation of C-O bond.Heterogeneous catalysis has attracted more attention in the catalytic conversion of C-O bond in platform compounds due to its simple equipment operation,green-high efficiency and easy separation of substrates.However,the characteristics of the C-O bond are high bond energy,complex bond type and multiple atomic orbital hybridization,which lead to a key problem for designing a highly selective catalytically active central structure to match the molecular orbital of the C-O bond group in the field of heterogeneous catalysis.Hererin,the selective oxidation of glycerol and hydrodeoxygenation of 5-hydroxymethylfurfural(5-HMF)as probes were used to systematically design and modulate the structure of nanocatalysts,and the influence of the active site of the catalyst on the catalytic performance were explored.The activation and dissociation nature of the reactant over the active site were revealed,which will provide new ideas for the development of other catalysts.Firstly,two Au/ZnOsol and Au/ZnODP-Air catalysts were prepared by sol-gel and precipitation deposition methods,then applied to the direct oxidation of C-O bond of glycerol to produce dihydroxyacetone(DHA).The catalytic activity of two catalysts were close(95%glycerol conversion),and the product distribution were both glyceric acid and glycolic acid under alkaline conditions.However,the catalytic activity of Au/ZnOsol catalysts for the oxidation of glycerol was very limited under alkaline-free conditions(the conversion of glycerol was<3.7%,5 h),and its oxidation product was mainly DHA(yield of 2.9%).Under the same conditions,the Au/ZnODP-Air catalyst showed higher catalytic activity(86.4%glycerol conversion),and the yield of DHA was 51.5%.The study of structure-activity relationship showed that the Au-ZnOx interface structure was formed on the surface of Au nanoparticles with the amorphous ZnOx overlayer in the Au/ZnODP-Air catalyst,and the stable Au step site and abundant oxygen vacancy defects were formed at this interface,the C-O bond of secondary alcohol in the glycerol molecule can achieve targeted activation with the combined action of the two sites under alkali-free conditions.In addition,After the regulating of the pretreatment atmosphere in Au/ZnO catalyst,the catalytic performance of air-treated Au/ZnO was much higher than that of hydrogen-nitrogen mixed and nitrogen-treated catalyst,which showed the strong metal and support interaction.It indicates that Au-ZnOx catalytically active center structure can be effectively constructed by air treatment.Combined with the experiment of O2 activation and the adsorption of glycerol secondary alcohol,it is believed that the vacancy defect is more favorable to the C-O bond adsorption and activation of secondary alcohol,then a possible reaction mechanism is proposed.Based on the fact that oxygen vacancy defects can be used as a highly efficient catalytic center for the selective activation of C-O bond,Co3O4 catalysts with oxygen vacancy defects were prepared,then applied to the hydrodeoxygenation of 5-HMF for preparing 2,5-dimethylfuran(DMF).The structure of the Co3O4 catalysts was further investigated by modulating different exposed crystal faces.The results of HRTEM and SEM showed that regular hexagonal Co3O4 nanoplates and irregular Co3O4 nanosheets catalysts with {111} and {112} crystal plane exposed were prepared under the protection of surfactants,respectively.The results showed that the best catalytic activity and the breaking ability of C-O bond over the hexagonal Co3O4 nanoplates with {111} crystal plane exposed.When the reaction time was 3 h,the conversion of 5-HMF was close to 100%,corresponding to DHA selectivity 92.5%.The results of XRD,Raman and XPS characterization show that the ?111} crystal plane has a higher proportion of Co2+/Co3+ and more oxygen vacancy defects.The relationship of the structure-activity showed that the Co2+/Co3+ cycle can achieve the activation of H2,while the C-O bond is adsorbed and activated at the oxygen vacancy to achieve the hydrodeoxygenation reaction in the Co3O4 catalyst.In addition,the circulation of Co2+/Co3+can generate more oxygen vacancies to participate in the adsorption and activation of C-O bond,further promoting the increase of catalytic efficiency.In summary,the effects of oxygen vacancy defects on the adsorption of C-O bond in the biomass platform compound molecules were deeply studied,and the catalytic efficiency of the catalyst was improved by the control of nanostructure of the Au/ZnO catalysts and Co3O4 catalysts.The results of this study provide a new strategy and theoretical basis for exploring the nature of the catalytic active center structure in the construction and recombination of C-O bond on the platform compounds,which benefits for achieving efficient conversion of biomass.