Study On The Catalysts And Catalytic Mechanism For Hydrodeoxygenation Of Lignin-oxygenates

The effective utilization of biomass resources is one of the important topics in the field of sustainable development and green chemistry.The related studies are of great significance for optimizing the current energy structure,easing the over-dependence on fossil energy and reducing environmental pollution.Lignin,rich in aromatic rings,can be depolymerized into oxygen-containing compounds with low molecular weight(mainly phenolic derivatives),which can then be converted into value-added chemicals via hydrodeoxygenation reaction.This thesis focused on the hydrodeoxygenation reaction of lignin-derived guaiacol,aiming at obtaining phenol and other aromatic compounds through designing highly efficient catalyst and understanding catalytic mechanism.Several supported metal catalysts were designed and fabricated for the activation of C-O bond and the inhibition of the hydrogenation of aromatic rings.Herein,the surface structure of heterogeneous catalysts was studied in detail and the reaction pathway,reaction mechanism and the structure-activity relationship of the catalyst were discussed.The results obtained would provide the fundamental data and theoretic supports for the rational design of highly efficient catalysts applied in the hydrodeoxygenation of lignin compounds under mild conditions.The main research contents and results obtained are as follows:1.Cl-modified Ru/TiO2 catalystThe supported Ru/TiO2 catalyst was modified with heteroatom Cl and evaluated in the hydrodeoxygenation reaction of guaiacol to produce phenol as the main product.The key factors affecting the product distribution of hydrodeoxygenation reaction were discussed.Cl-modified Ru/TiO2 catalyst could effectively hinder the hydrogenation of benzene ring of reactant and the product,thus exhibiting higher selectivity toward phenol.The introduction of Cl species changed the electronic states of active metal Ru and that of TiO2,inducing the formation of electron-rich Ru and Ti species,and the coordinately unsaturated Ti-related defects were also formed.These defects,located at the Ru-TiO2 interface and the surface of TiO2,could anchor O atoms and activate the methoxy group of guaiacol,catalyzing the dissociation of C-O bond to yield phenol;besides,Ruδ-species could inhibit the adsorption and hydrogenation of benzene ring,and phenol was produced as the main product which desorbed from the catalyst surface easily.Over the Ru/TiO2 catalyst,however,the benzene ring of guaiacol was strongly adsorbed on Ru surface,which can be hydrogenated to produce 2-methoxycyclohexanol.The cleavage of C-O bond in guaiacol could also occur to produce phenol.The strong adsorption of aromatic ring of phenol on Ru/TiO2 caused the occurrence of ring hydrogenation,resulting in the poor selectivity of phenol.2.Ti-decorated Ru/SiO2 catalystRu catalyst supported on Ti-modified SiO2(Ru/Ti-SiO2)was designed and investigated in the hydrodeoxygenation of guaiacol to under low hydrogen pressure.The prepared Ru/Ti-SiO2 catalyst presented high performance in the cleavage of CO bond and the promoting role of Ti modification in activation of C-O bond as well as the catalytic mechanism was investigated.Compared with Ru/SiO2 and Ru/TiO2 catalysts,Ru/Ti-SiO2 showed higher deoxygenation activity.At the guaiacol conversion of 83.6%,the selectivity of phenol and benzene reached 70.4%and 10.0%,respectively.The presence of Ti-O-Si linkages in the Ti-modified SiO2 support promoted the formation of defect(Ti3+)-rich and highly-dispersed TiOx clusters and particles.Ti3+ defects could effectively anchor Ru particles and increased their dispersion(geometric effect);besides,Ti species interacted with Ru electronically,donating electrons and promoting the formation of surface Ruo species(electronic effect).The interface between Ru and TiOx species(Ru0-Ti3+)of the Ru/Ti-SiO2 catalyst was confirmed to be the main active sites for the conversion of guaiacol,in which Ru0 was responsible for E2 activation and Ti3+ for the activation of C-O bond.Ru/Ti-SiO2 was highly active under low H2 pressure due to the presence of abundant active sites for deoxygenation reaction.3.Defect-rich Pt/TiO2-D catalystDefective TiO2 support(denoted as TiO2-D)supported Pt catalyst was designed and fabricated,and was applied in the hydrodeoxygenation of guaiacol under low temperature and ambient pressure to produce aromatic products.The amount of defective sites,the capacity of H2 activation and H spillover and the dynamic change of catalyst during the reaction were investigated.Experimental and characterization results confirmed that the presence of abundant defects(Ti3+ and oxygen vacancy)in TiO2-D could promote the formation of Ptδ+ species.The interfacial sites between TiO2-D and Ptδ+ facilitated the heterolytic cleavage of H2 and these surface defects of TiO2-D ensured the occurrence of hydrogen spillover even under low hydrogen pressure.During the reaction,H2O molecule(solvent)was activated on the surface of Pt/TiO2-D,and this process induced the transfer of activated H species to the bulk phase of TiO2-D.The insertion of H species induced the formation of TiHxOy surface/subsurface layer and further caused lattice distortion in the bulk of TiO2-D,which finally led to the formation of a new titanium oxide phase(Ti3O5).The ability of H2 activation and spillover was increased for Pt/TiO2-D catalyst due to the creation of more defects during reaction.It was confirmed that guaiacol was adsorbed on the surface of TiO2-D and reacted with spilt-over hydrogen,thus accomplishing the cleavage of C-O bond under mild reaction condition.