Preparation Of Nitrogen-doped Carbon Materials Loaded With Metal Catalysts And Catalytic Conversion Of Straw Lignin

Lignin,an important component of lignocellulosic biomass,is considered a potentially renewable aromatic resource due to its benzene ring structure.The main source of lignin is the paper industry.Currently,more than 50 million tons of lignin are produced globally each year.As annual lignin production increases,it is expected to rise to 225 million tons by 2030,but the current effective lignin utilization rate is less than 2%.Therefore,it is crucial to increase the added value of lignin and to expand its application areas.Lignin has a complex and dense structure,and there are multiple reaction pathways in the catalytic conversion process,with many conversion products and easy repolymerization.In this thesis,the design and characterization of catalysts and the study of the reaction mechanism were carried out firstly with typical monomeric derivatives of lignin as the target.A polydopamine(PDA)coated SiO2 carrier(PDA@SiO2)was prepared,and a metal Ni-loaded catalyst Ni/PDA@SiO2 was prepared using the impregnation method.The catalytic conversion of guaiacol to 1-methyl-1,2-cyclohexanediol in hexane solvent with high selectivity was achieved by adding a nitrogen source(urea)to adjust the nitrogen content in the catalyst with a selectivity of 63.97%with 100%conversion.The catalyst was characterized and the results showed that the addition of urea not only increased the content of pyridine nitrogen but also increased the specific surface area and porosity of the catalyst,which induced the formation of a large number of defective sites in the catalyst,which could anchor the nickel atoms and promote their dispersion,thus enhancing the catalytic activity of the catalyst.The presence of pyridine nitrogen can increase the active site Ni0 through electron transfer.An in-depth study on the catalytic conversion of lignin oil was carried out.Firstly,straw lignin was pyrolyzed to lignin oil,and the composition of lignin oil was characterized by GC-MS.The main phenolic monomer products in lignin oil were phenol,guaiacol,4-ethylphenol,4-ethyl guaiacol,2,6-dimethoxyphenol,and 4-ethylcatechol.Ru,a metal with stronger hydrogenation activity than Ni,was selected,and the one-pot method was preferred by comparing the activity of two different preparation methods(impregnation and onepot methods)for the synthesis of Ru-loaded nitrogen-carbon catalysts(Ru/NC-X).By direct pyrolysis of a mixture of ruthenium trichloride and melamine,highly dispersed,small-sized Ru nanoparticles(NPs)loaded with nitrogen-carbon carrier Ru/NC-M catalyst was obtained,which can effectively convert lignin oil.The lignin oil was completely converted under the reaction conditions of 240℃ and 1 MPa H2,and the selectivity of the product cyclohexane was 36.58%.The morphology,specific surface area,and pyridine nitrogen content of the catalyst could be regulated by changing the precursor of the nitrogen-carbon carrier.The percentage of pyridine nitrogen in the catalysts prepared with melamine as the nitrogen-carbon precursor(31.35%)was much higher than that of catalysts prepared with urea(16.47%)and dicyandiamide(8.20%)as the nitrogen-carbon precursors.The presence of pyridine nitrogen not only serves as a ligand site to make Ru nanoparticles uniformly dispersed and stable but also regulates the electron density of Ru NPs and increases the active site Ru0 through electron transfer,so Ru/NC-M possesses a strong ability to break C-O bonds.The interaction between the metal and the carrier and the synergistic effect of the acidic sites on the NC carrier make Ru/NC-M possess high catalytic activity.Based on the previous study,the bimetallic catalyst Ni2Ru1/NCM was prepared by a one-pot method with selected metals Ni,Ru,and nitrogen-carbon precursors(melamine),which could effectively catalyze the conversion of straw lignin through the synergistic effect of bimetallic active centers as well as acidic sites,resulting in 82.96%conversion of straw lignin,and the products were mainly 4-ethylphenol and 2,6-d imethoxyphenol.The yield of 4-ethylphenol and 2,6-dimethoxyphenol was 11.04%and 5.70%,respectively.It was shown that the electron transfer between Ni and Ru and the presence of acidic sites in the nitrogencarbon carrier together enhanced the catalytic activity of the Ni2Ru1/NCM catalyst In addition,the causes of catalyst deactivation after the reaction were investigated in depth.The possible reaction pathways for lignin degradation were systematically investigated and revealed that the C-C and C-O bonds were firstly broken to form intermediate products,and then the C-O bonds were further broken to form monophenols,namely 4ethylphenol and 2,6-dimethoxyphenol.