Preparation Of Ni/Nb₂O₅-based Catalysts And Its Catalytic Vanillin Hydrodeoxygenation In Aqueous Phase

Aqueous hydrodeoxygenation process,as a green and efficient upgrading technology,can convert biomass and its derivatives into high value-added chemicals and renewable fuels.However,the catalysts in aqueous phase are prone to problems such as metal leaching and carrier collapse,resulting in a decrease in the activity of the catalysts.Therefore,it is of great significance to design and develop the supported catalysts with high water resistance for biomass hydrodeoxygenation（HDO）in aqueous phase.In this paper,a novel Ni/Nb₂O₅-based catalyst suitable for hydrodeoxygenation of biomass derivatives（vanillin）in aqueous phase was prepared through Nb₂O₅crystal control and carbon doping strategies.Influence of properties of specific surface area,oxygen vacancy,acid sites,metal-support interaction on the catalytic activity of Ni/Nb₂O₅-based catalysts were investigated.The main research contents are as follows:First,by adjusting the calcination temperature（500℃,700℃,900℃）,Ni/X-Nb₂O₅catalysts with different crystal structures（Ni/TT-Nb₂O₅,Ni/T-Nb₂O₅,Ni/H-Nb₂O₅）were synthesized,and the effect of catalyst crystal form on the hydrodeoxygenation of vanillin in aqueous phase was studied.Compared with Ni/TT-Nb₂O₅and Ni/H-Nb₂O₅,the Ni/T-Nb₂O₅catalyst exhibited higher catalytic performance.With the increase of water volume in the reaction system（water/ethanol from 0/20 to 8/12）,the yield of product MMP over Ni/T-Nb₂O₅catalyst increased significantly.Under the conditions of 180℃,1 MPa H₂,water/ethanol=8/12,and 1 h,the vanillin conversion over Ni/T-Nb₂O₅catalyst was 96.1%,and the selectivity of 2-methoxy-4-methylphenol（MMP）of the target product is 79.2%.The catalysts were characterized by means of SEM,TEM,BET,XRD,Raman,XPS,H₂-TPR,NH₃-TPD,etc.It was found that the abundant oxygen vacancies in Ni/T-Nb₂O₅could adsorb and activate hydroxy of vanillyl alcohol（VA）,thereby promoting the formation of MMP.In addition,water formed a stronger transition state with VA,which further reduceed the energy of C-OH bond breaking.Therefore,the synergistic effect of oxygen vacancies and water on the Ni/Nb₂O₅catalyst promoted the hydrodeoxygenation of vanillin in the aqueous phase,achieving high yields of MMP.Second,a series of Ni/T-Nb₂O₅@C composite supporter catalysts were successfully prepared by the carbon doping strategy（adjusting the doping ratio and the type of carbon source）.The characterization results of TEM,BET,XPS,FT-IR,NH₃-TPD,etc.show that the doping ratio（C/Nb=0.5,1,2,3）and the type of carbon source（fructose,tannin,xylose）can effectively change the pore structure,specific surface area,the concentration of oxygen vacancy,metal-support interaction and surface acidity of the catalyst.When tannin was used as the carbon source and C/Nb=2,the prepared Ni/T-Nb₂O₅@C₂-2 catalyst exhibited excellent catalytic performance for the hydrodeoxygenation of vanillin in aqueous phase.Under the conditions of 150℃,1 MPa H₂,water/ethanol=8/12,1 h,the vanillin conversion of 99.9%and the MMP selectivity of 96.6%were obtained.Combining the experimental and characterization results,it was found that the oxygen vacancies and acid sites on the surface of the Ni/T-Nb₂O₅@C₂-2 catalyst significantly improved the hydrodeoxygenation performance of the catalyst.In addition,the presene of water lowered the activation energy barrier for the VA hydrogenolysis process and improving the selectivity of MMP.Meanwhile,the Ni/T-Nb₂O₅@C₂-2 catalyst exhibited good catalytic stability and universality in the aqueous hydrodeoxygenation of biomass derivatives.