Mesoporous Nitrogen-doped Carbon Supported Nickel-based Catalyst For Catalytic Depolymerization Of Lignin To Prepare Monophenolic Compounds

Lignin is the most abundant renewable aromatic naturally resources,which is composed of three types of phenylpropane units,and has great potential in the production of aromatic compounds and further preparation of high-value chemicals and fuels.Due to the complex structure and its inert chemical properties,lignin has not been fully utilized so far.Catalytic depolymerization is considered to be one of the most effective methods to achieve high-value utilization of lignin.Therefore,it is of great significance to prepare an economic,efficient and stable metal catalyst to hydrodepolymerize lignin for obtaining aromatic monomer compounds.In this study,a Nickel based catalyst supported on nitrogen-doped carbon,which was synthesized by hydrothermal method with glucosamine hydrochloride as the carbon source,ethylenediamine as the nitrogen source and nano-silica as the template.The prepared catalyst Ni_10wt%/NHC（22）-1 with adjustable mesopore size was used to depolymerize lignin for providing the theoretical basis and application approach for the efficient utlization of lignin.The influence factors on the morphology and catalytic performance of the catalyst were explored,including mesopore size and nitrogen doping amount,and the catalyst was characterized by BET,XRD,SEM,TEM,H₂-TPR,etc.When the mass ratio of ethylenediamine and glucosamine hydrochloride was 0.25 and the metal nickel loading was10 wt%,the catalyst Ni_10wt%/NHC（22）-1 using the template nano-silica（22 nm）as porogen has flake structure with abundant and uniform mesoporous size,and has a larger specific surface area(646.8 m²·g^-1),larger pore size（23.2 nm）and large pore volume(1.98 cm²·g^-1).By adjusting the size of the mesopores,it is found that the mesoporous channel with a size of20-25 nm was most suitable for the depolymerization and diffusion of lignin molecules.The addition of ethylenediamine was conducive to the formation of a rich defect structure on the carbon support.There is an electronic interaction between the doped nitrogen atom in the support and metal Ni,which reduced the reduction temperature of Ni and effectively prevented agglomeration of Ni nanoparticals.Highly dispersed Ni nanoparticles with a smaller particle size（4.4 nm）provided more active sites,favourable for efficiently depolymerizing lignin.The optimal process variables for the catalytic degradation of lignin over catalyst Ni_10wt%/NHC（22）-1 were explored through single-factor experiments,and the distribution of depolymerization products was analyzed by GC-MS,and GPC.The results indicateed that the optimal reaction temperature,catalyst dosage,reaction time and hydrogen pressure were300℃,0.06 g,120 min and 1.0 MPa,respectively.Under these conditions,the yields of aromatic monomers and bio-oil are 32.93%and 73.27%,respectively.Compared with the commercial catalysts of Pd/C and Ru/C,the nitrogen-doped carbon-supported Ni-based catalyst prepared in this work has the advantages of low-cost,easily prepared and excellent catalytic activity,and has great potential for promoting the efficient depolymerization of lignin to prepare aromatic monomers.