Catalytic Depolymerization Of Formaldehyde-pretreated Lignin From Poplar Wood To Prepare Phenolic Compounds

Monophenols are important basic organic chemicals,which are mainly prepared from fossil resources such as coal tar.With the gradual depletion of fossil resources,it is particularly important to find a sustainable source of monophenols.Lignin is the only renewable source of aromatic compounds in nature,and high value-added phenolic compounds can be obtained by depolymerizing lignin.In this thesis,the extraction of poplar lignin and its catalytic depolymerization to prepare monophenols were explored,in order to provide a new approach and experimental basis for the conversion and utilization of lignin.Firstly,extraction of polar lignin from poplar powder with organic solvents was studied.In the extraction process,the poplar powder was first pretreated with formaldehyde considering the formaldehyde can react with the a and y diols of lignin to form a dioxane structure,which is beneficial to improve the yield of monophenols in the catalytic depolymerization process.Factors including amount of organic solvent,extraction temperature and extraction time were optimized by single factor experiment and orthogonal experiment.The extracted poplar lignin was characterized by GPC,FT-IR infrared spectroscopy and 2D HSQC.Under the optimal conditions of formic acid:acetic acid:water(3.5:5:1.5),extraction temperature of 90?,extraction time of 4.5 hours,and rotation speed of 500 rpm,65.6%of lignin extraction rate can be obtained,and the average molecular weight of extracted lignin is 1939.Secondly,catalytic hydrogenolysis of extracted poplar lignin using commercially available precious metal catalysts(Pd/C and Ru/C)was investigated.Qualitative and quantitative analysis of monomer products was carried out by GC-MS,LC-MS,HPLC,as well as optimization of the hydrogenolysis conditions.The reaction was carried out in the supercritical ethanol at temperature of 260? and pressure of 9 MPa for 4 hours,using Pd/C as a catalyst.The yield of monophenols starting from formaldehyde-treated lignin was 32.1%,which was 2.04 times higher than that on lignin without pretreatment by formaldehyde.Additionally,the yield of monophenols from formaldehyde-treated lignin in the Ru/C system was 43.4%,which was 1.76 times higher than that without formaldehyde.Moreover,the introduction of a common acid,base or water as a co-catalyst into the Pd/C system revealed that the acid and water as co-catalysts can significatnly increase the monophenol yield to 41.3%and 41.5%.Furthermore,Ni/C and Ni-Co/C catalysts were prepared using MOF materials synthesized by gallic acid as ligands as a precursor.The catalyst was characterized by specific surface area analysis,scanning electron microscopy,high-resolution transmission electron microscopy,X-ray powder diffraction,and X-ray photoelectron spectroscopy.The effects of calcination temperature and different Ni:Co ratio on the performance of Ni/C and Ni-Co/C catalysts were investigated.The cycle performance and catalytic mechanism of the catalyst were investigated.The Ni/C catalyst obtained at the calcination temperature of 700? has the highest catalytic efficiency.On the other hand,when the Ni:Co ratio is 1:1,the catalytic efficiency of the Ni-Co/C bimetallic catalyst is the highest.Meanwhile,the yield of monophenols of depolymerization by poplar lignin can reach 55.2%,and the guaiacol selectivity as high as 70.3%.The catalytic mechanism of Ni/C and Ni-Co/C was investigated by using guaiacol-based glycerol-?-guaiacyl ether as a model compound.