Synthesis Of Tungsten And Molmbdenum Carbides And Their Application In Catalytic Hydrogenation Of Alkali Lignin

Problems such as crisis of fossil fuel energy and environmental pollution are induced along with the high developed science and technologies,which seriously influence sustainable development of our society.In this situation,scientists are thinking of ways to utilize sustainable energies,for instance,solar energy,wind energy,tidal energy,biomass energy,etc.Among them,biomass energy has attracted much attention due to its abundant resources,wide distribution,and environmental-friendly merit.Lignin is one of the main components of biomass.It is also one of the most abundant polymers in nature.The structure of lignin contains various functional groups,such that provides a large number of reaction sites.Also,it's low oxygen content and high energy density make it valuable with multiple applications.To convert lignin into high value products requires the breaking of a large amount of aryl ether bonds and carbon-carbon bonds.High temperature and catalyst are the key to direct degrade lignin with high conversion.Due to the special crystal structure and electronic structure,transition metal carbides exhibit excellent catalytic activity in hydrogenation and can be used to substitute noble metals for methane reforming,hydrodenitrogenation,hydrodesulfurization and many other reactions.In order to improve the liquefaction conversion rate of lignin and bio-oil yield,molybdenum carbide and tungsten carbide were studied in this paper.Synthetic method and structural,physical,chemical properties of the catalysts were studied in depth.Moreover,aiming at better catalytic performance in the hydrogenation liquefaction of alkali lignin,structural modification was made and investigated.Specific work is described as follows.Chapter 2 studies how molybdenum carbide helps with hydrogenation liquefaction of alkali lignin.Both Mo₂C and Ni/Mo₂C were prepared by sol-gel and carbothermal reduction process.Considering metal-loaded catalysts always have problems such as poor stability,short life,agglomeration,and easy to loss active surface,different amount of B was used in this work to modify Ni/Mo₂C(xB-Ni/Mo₂C).B element was proved to exists in the form of elemental B,B_xC and partial oxidation states by characterization of XRD,Raman,SEM,etc.It was also found that Mo₂C forms a pore-like structure after Ni loading,and the specific surface area is greatly improved.The xB-Ni/Mo₂C catalyst has a high Ni/Mo ratio,and B modification effectively inhibits the decomposition of Mo₂C and has stronger weak acidity,which effectively promotes the adsorption of reactant and hydrogenation process.In addition,synergistic effect between B and Ni promotes metal dispersion,resulting in more adsorption sites.Consequently,the alkali lignin conversion and bio-oil yield are both improved significantly with xB-Ni/Mo₂C.Further,the optimal reaction conditions were confirmed by single factor optimization.Chapter 3 studies the preparation of tungsten carbide-based catalysts and their catalytic performance in hydrogenation liquefaction of alkali lignin.The catalysts were prepared by sol-gel and carbothermal reduction process.The effect of carbonization reduction temperature(800-1400 ^oC)on the tungsten carbide phase was investigated by XRD,and the optimum reaction temperature was determined at1200°C.Then,different metal(Pt,Ru,Cu)loading on tungsten carbide were prepared at 1200°C.Physical and chemical properties of all catalysts were studied with BET method,ICP,SEM,TEM,EDX-mapping and electronic structural were studied with XPS.It turns out that the tungsten carbide catalyst does not significantly improve alkali lignin conversion and bio-oil production.Metal-supported tungsten carbide catalysts slightly increase alkali lignin conversion,but inhibit the production of bio-oil.In order to explore the ability of tungsten carbide-based catalysts to break C-C and C-O bonds,the catalysts were then applied to hydrogenolysis of glycerol(compound with very simple structure).Results showed that the breakage of C-C bonds closely related to tungsten carbide,while the breakage of C-O bonds mainly leading by loading metals.Compared with Pt and Ru,Cu shows better performance to break C-O bond,but Pt and Ru can further promote the C-C breaking ability of tungsten carbide.Based discussion of molybdenum carbides and tungsten carbides in previous chapters,chapter 4 focuses on the composite of them((Mo_xW_1-x)C)and 10%Ni-loaded molybdenum/tungsten carbides composite(Ni/(Mo_xW_1-x)C).XRD proves that part of the composite exists as solid solution phase.The BET specific surface area(S_BET)of the composite itself((Mo_xW_1-x)C)has no significant improvement compared with Mo₂C and WC,but the Ni/(Mo_xW_1-x)C has a S_BET up to?200 m²g^-1,which result from the formed channel and hole structure of Mo₂C and WC,caused by Ni²⁺.H₂-TPR results show reduction peaks of Mo₂C,W₂C and WC.NH₃-TPD results show that the composite has higher total acidic amount.At the same time,its weak acidic sites at 150-250°C is increased with enhanced W proportion(W<50%).The catalysts in hydrogenation liquefaction of alkali lignin process show higher alkali lignin conversion(up to 77.5%)as well as bio-oil yield(up to 62.3%).This result mainly benefits from significantly increased guaiacol in the final product.Chapter 5 use a more objective and comprehensive experimental design method:fractional factorial design,combined with machine learning method,for high efficiency optimization of hydrogenation liquefaction of alkali lignin with Ni/(Mo_xW_1-x)C.Three factors:the Mo molar ratio(Mo/(Mo+W)),reaction temperature and initial hydrogen pressure were selected as study parameters,which show significant influence on alkali lignin conversion and bio-oil yield based on experiences.Variables were set at several levels and thus given a(7,3,3)parameter space.In this space,1/3 fractional factorial design allows a total number of1/3í7í3í3=21 experiments(three replicates in each group).Statistical analysis(deviation analysis,analysis of variance,sensitivity test,F test)were firstly applied,showing high statistical reliability.Then the support vector regression(SVR)algorithm using the radial basis kernel function(RBF)was used to perform Gaussian regression analysis on obtained experimental data to realize prediction of any point in the parameter space.In this process,cross validation method was used to determine hyperparameters and verify the predicted results,in order to avoid overfitting or underfitting.Results show that reaction temperature has little effect on the target within the range of 270-310°C.The higher alkali lignin conversion is obtained at high hydrogen pressure(>3 MPa)and high molybdenum content(>0.5).High bio-oil yields require hydrogen pressures in the range of 2-4 MPa and molybdenum contents in the range of 0.4-0.7.Finally,the second round of optimization was performed based on the first round of optimization,which narrowed the parameter range and also verified results from the first round.