Study On The Directional Cleavage Of C-O Bond Of Lignin Model Compounds Over Solid Acid Catalyst

Lignin is the only source of a large number of renewable aromatic or cycloalkane chemicals.The selective cleavage of aryl ether C-O bonds to convert lignin into high-quality chemicals is one of the key problems in the conversion and utilization of lignin with high value-added.In this thesis,the lignin model compounds are used as research objects to investigate the catalytic hydrogenation performance and reaction mechanism of Ni and Ru supported solid acid catalysts under different reaction conditions.Then the catalysts are directly used in the catalytic reaction of lignin,and the composition and relative content of the light components in the liquid product after the reaction are detected and analyzed to provide theoretical basis and technical support for the directional preparation of high-quality chemicals by lignin hydrocracking.Nb₂O₅-based catalysts supported by different metals(Ni/Nb₂O₅,Co/Nb₂O₅,Cu/Nb₂O₅and Ru/Nb₂O₅)were prepared by impregnation method,and the lignin model compound diphenyl ether(DPE)was used as the probe to detect the catalytic performance of these catalysts.The results showed that,among general metal catalysts,Ni/Nb₂O₅ had higher catalytic activity than Co/Nb₂O₅ and Cu/Nb₂O₅.This was due to the fact that the average particle size of metallic Ni was smaller than that of Co and Cu,and the dispersibility would be better.Due to the extremely weak acidity of the catalyst,the hydrodeoxygenation(HDO)reaction is difficult to take place,and the oxygen atoms in the oxygen-containing products cannot be removed.Compared with general metal catalysts,Ru/Nb₂O₅ had better catalytic activity.When heating,DPE was mainly converted to produce cyclohexane and cyclohexanol via the cleavage of C-O bonds.At room temperature(30 ^oC),DPE mainly underwent aromatic ring hydrogenation to generate oxybicyclohexane(OCE).Because the acidity of Ru/Nb₂O₅ catalyst was insufficient,the HDO reaction cannot occur.The Ru/C-HZ5 catalyst was prepared by impregnating metal Ru on the commercial C-HZ5 support,which was used in the Catalytic Hydrogenation Reaction of DPE.The results showed that the catalyst could not only effectively convert DPE,but also achieve the removal of oxygen atoms in the product,and finally obtained the only product cyclohexane due to the special structure of the catalyst including highly dispersed active metal Ru,relatively more acidic sites and so on.Then the catalyst was improved and the Ga-doped Ru/Ga@S-HZ5 catalyst was prepared by combining in-situ doping and impregnation methods.By enhancing the interaction between the active metal and the support in the catalyst,the structure of the catalyst could be optimized and the catalytic activity could be improved.The characterization showed that the element Ga was uniformly dispersed in the framework of S-HZ5 and Ru was uniformly dispersed on the surface and pores of Ga@S-HZ5 support.Elemental Ga doping could increase the acid sites required for the HDO reaction,especially the midstrength acid sites.The highly active Ru/Ga@S-HZ5 catalyst could completely convert DPE into the only product cyclohexane under milder reaction conditions(180 ^oC,2 h,1 MPa H₂)and effectively remove oxygen atoms in the product.Ru/C-HZ5 and Ru/Ga@S-HZ5 catalysts were applied in the catalytic hydrogenation of lignin.It was found that when no catalyst was added,not only the liquid yield after lignin conversion was very low,but also there were a large number of oxygen-containing compounds in the product,and the catalytic effect was very poor.After adding Ru/C-HZ5and Ru/Ga@S-HZ5 catalysts,the liquid yield after lignin conversion was greatly improved and most of the oxygen-containing compounds in the product were converted into oxygen-free aliphatic hydrocarbons,which had a good effect on the removal of oxygen atoms.In contrast,the activity of Ru/Ga@S-HZ5 catalyst modified by the doping element Ga was higher than that of Ru/C-HZ5 prepared by the active metal Ru directly supported on the commercial C-HZ5 support.The research realized the directional conversion of lignin to produce high-quality oxygen-free chemicals and liquid fuels.The paper has 70 pictures,23 tables,126 references.