Selective Hydrodeoxygenation Of Mixed Phenolics In Bio-oil To Aromatic Hydrocarbons And Their Competitive Reaction Mechanism

Bio-oil produced by rapid thermal cracking of biomass contains a large amount of phenolics,which are prone to phenolic condensation side reactions,leading to difficulties in quality extraction.Hydrogenation is one of the important means for bio-oil upgrading,however,the poor hydrogenation selectivity causes high hydrogen consumption.Bio-oil phenolics hydrodeoxygenation（HDO）to aromatics is an effective method to prepare high quality fuel oil,but the development of efficient catalysts is still a challenging topic.To this end,two catalysts were developed and their catalytic mechanism of competitive adsorption in the catalytic bio-oil phenol HDO process was investigated in the thesis.Firstly,hierarchical Nb₂O₅（H-Nb₂O₅）was synthesized by hydrothermal method and Pd/H-Nb₂O₅was prepared by impregnation method loaded with Pd.SEM,PXRD,TEM characterization showed that H-Nb₂O₅has a unique structure of two-dimensional nanoplate,uniformly dispersed Pd clusters,and obvious interlaced lattice surfaces at the interface of H-Nb₂O₅.NH₃-TPD,Py-IR,XPS,and ESR analyses demonstrate the existence of a large number of oxygen defects and acidic sites on the surface of Pd/H-Nb₂O₅,which are favorable for the electron transfer between Pd clusters and H-Nb₂O₅.the interlaced lattice surfaces and electron transfer between Pd clusters and H-Nb₂O₅reflect the existence of metal-carrier interactions in Pd/Nb₂O₅.Density functional theory（DFT）calculation analysis revealed that the metal-carrier interaction between the Pd cluster and the carrier Nb₂O₅originates from the oxygen defect of the carrier.The catalyst performance of Pd/H-Nb₂O₅was investigated and compared with previous literature reports.The Pd/H-Nb₂O₅catalyst exhibited higher phenol conversion（74.2%）and benzene selectivity（100%）.It was found that the adsorption mode of phenols on the catalyst surface is the key to determine the hydrogenation selectivity:the"nonplanar"adsorption of phenolic oxygen groups favors HDO,while the"plane"adsorption of aromatic rings favors aromatic ring hydrogenation.H₂molecules are activated by the Pd clusters and migrate to the vicinity of the carrier oxygen defects to hydrogenate the C_Ar-O bonds to produce benzene.Non-precious metal catalysts Mo O_x/SBA-15 were synthesized by impregnation method,and Mo O_x/Ba O@SBA-15 was prepared by modifying Ba O on the surface of Mo O_x/SBA-15,which achieved the modulation of acidity and basicity on the catalyst surface and enhanced the competitive adsorption of phenols.The characterization by TEM and EDX showed that Mo O_xwas highly dispersed on the surface of SBA-15;PXRD,PXRD,XPS,H₂-TPR,ESR and other characterizations confirm that the metal active component undergoes Mo O₃-Mo₄O₁₁-Mo O₂phase evolution during hydrogen activation,and the accompanying large number of oxygen defects have excellent"nonplanar"adsorption ability of phenolic compounds,thus enhancing the HDO selectivity.The experimental results showed that the selectivity of phenol conversion to benzene catalyzed by Mo O_x/SBA-15 can reach 100%;the hydrogenation process of guaiacol will be hydrolyzed to phenol and methanol first,and the strong adsorption of methanol on the catalyst surface will inhibit the continued HDO of phenol to aromatics.Kinetic fitting analysis showed that the apparent activation energies of guaiacol C_Ar-OH and C_Ar-OCH₃rupture were 48.57 k J×mol^-1and 24.68 k J×mol^-1,respectively,confirming that the hydrogenation of guaiacol would preferentially produce phenol and methanol rather than anisole and water.DFT theoretical calculations indicated that the presence of methanol strongly inhibited the HDO of phenol to benzene.The Ba O surface-modified Mo O_x/Ba O@SBA-15 catalyzed HDO showed 100%conversion of guaiacol and 75%total aromatic selectivity;the total aromatic yield was 57.6%when the mixed phenol solution containing phenol,cresol and guaiacol was hydrodeoxygenated.These findings fully indicate that the introduction of basic sites enhanced the competitive adsorption ability of phenol,and at the same time strengthened the reaction of phenol with methanol alkylation to form cresol,which weakened the inhibitory effect of methanol on phenol HDO.The research work in this paper not only provides new ideas for the design and preparation of new HDO catalysts,but also lays a good theoretical foundation for the development of bio-oil phenol HDO to high quality aromatic fuel technology.