Catalytic Performance For Aryl Ethers Deoxygenation Over Niobium-Based Solid Acids Supported Noble Catalysts

Lignin is the second most abundant organic substance in nature,the only renewable resource that can provide aromatic hydrocarbons in nature,which can effectively replace the fossil energy to produce naphthene,aromatic liquid fuels and fine chemicals.However,the high oxygen content and complex structure of lignin lead to its low heating value and instability,making it difficult to be utilized directly.Catalytic deoxygenation of lignin can effectively reduce the oxygen content and produce liquid fuels and fine chemicals.Niobic acid(Nb2O5·nH2O)is a potential solid acid for deoxygention due to its excellent water resistance and abundant acid sites.Based on the above considerations,a series of niobium-based solid acids supported noble metal catalysts were prepared by using niobic acid as niobium source,and their catalytic performance of deoxygenation of lignin 4-O-5,?-O-4,?-O-4 and monophenol aryl ethers were investigated.The main research contents and results are shown as follows:1)The Nb2O5/Al2O3 supported Pt catalyst was prepared via the precipitation method and used in the hydrodeoxygenation of lignin 4-O-5 aryl ether diphenyl ether.The results showed that the introduction of niobic acid enriched the acidity of the support and the Nb-OH-Al bonds were formed,which significantly promoted the breaking of C-O bonds in diphenyl ether.The selectivity of cyclohexane over Pt/Nb2O25/Al2O3 catalyst was up to 1.7 times than that of Pt/Al2O3 catalyst at 200? and 3.0 MPa H2.In order to further improve the selectivity of cyclohexane,a series of niobium-based solid acid nanosheets were prepared and combined with carbon nanotubes and used to support Pt catalyst.The conversion of diphenyl ether was up to 99.7%and the selectivity of cyclohexane reached 96.4%in hydrodeoxygenation of diphenyl ether over Pt/HNbWO6/CNTs at 200? and 3.0 MPa H2,which was mainly attributed to the synergistic effect between the metal sites and acid sites.Among them,hydrogenation of the benzene ring was promoted over metal sites,the nanosheets with the abundant acid sites and medium-strong acid sites can significantly improve the breaking of C-O bonds.In addition,it was found that the higher C-O bond dissociation energy and the steric hindrance of oxygen-containing functional groups were not conducive to the conversion of lignin model compounds.2)The catalytic transfer hydrogenolysis(CTH)of lignin ?-O-4 aryl ether 4-(benzyloxy)phenol over Pt/HNbWO6/CNTs was investigated with alcohol solvents as hydrogen sources.The best catalytic activity was achieved in isopropanol solvent and the conversion of 4-(benzyloxy)phenol reached 98.1%at 240?.The interaction between 4-(benzyloxy)phenol and isopropanol was the weakest and the solvation effect of 4-(benzyloxy)phenol was the smallest compared with other alcohol solvents system,thus the 4-(benzyloxy)phenol can fully interact with the catalysts and isopropanol can release more hydrogen atoms,thereby the CTH reaction was effectively promoted.With respect to the CTH of lignin over Pt/HNbWO6/CNTs,the relative oxygen content in the liquid product was decreased from 41.2%to 24.6%,meanwhile,the liquid products were mainly composed of phenolics,especially monophenols.3)The Nb2O5 supported noble catalysts(Pt,Pd,Rh and Ru)were prepared by the impregnation method and used in hydrodeoxygenation of lignin ?-O-4 aryl ether 2-phenethyl phenyl ether.The conversion of 2-phenethyl phenyl ether reached 99.3%and the selectivity of aromatics reached 98.9%over Rh/Nb2O5 catalyst at 260? and 0.1 MPa H2,the excellent deoxygenation performance was attributed to the synergistic effect between Rh and NbOx species,in which Rh was responsible for dissociation of hydrogen,while NbOx activated C-O bond and reduced C-O bond energy barrier.As for hydrodeoxygenation of lignin,the total mass yield of the liquid products was up to 39.5 wt.%,and the selectivity of aromatic compounds was 84.1%over Rh/Nb2O5 under 0.1 MPa H2.4)The Nb2O5 with different calcination temperatures supported Rh catalysts were prepared via the impregnation method and tested in hydrodeoxygenation of lignin monophenol aryl ether 2-methoxy-4-propylphenol.The results showed that the Rh/Nb2O5-400(calcined at 400?)catalyst exhibited highly efficient deoxygenation performance in hydrodeoxygenation of 2-methoxy-4-propylphenol with a yield of propylbenzene up to 98.0%at 260? and 0.5 MPa H2.As for the catalytic degradation of pine wood lignocellulose,the lignin oil was obtained after the reductive degradation of lignocellulose in methanol solution with an 8.9 wt.%mass yield of lignocellulose biomass;then the hydrodeoxygenation of lignin oil was carried out over Rh/Nb2O5-400 under 0.5 MPa H2,it was found that the oxygen of lignin oil was completely removed and the liquid products were composed of C6-C10 hydrocarbons with 100%selectivity,in which the molar yield of aromatic compounds reached 80.1 mol%.