Preparation Of Lignin-based Carbon Supported Metal Catalysts And Their Reductive Depolymerization Of Lignin

As a renewable biomass resource with abundant reserves and wide sources,lignocellulose has great potential to be converted into high-value carbon-based fuel and chemical products,and has received a lot of attention owing to shortage of fossil resources and"carbon peak"and"carbon neutral"double carbon economy transition.In terms of chemical composition,lignocellulosic biomass is mainly composed of cellulose,hemicellulose and lignin.Due to the essential structural differences,the utilization of lignin is usually underused compared with the other two polysaccharides components and is generally burned directly as a by-product of the pulp and paper industry.The depolymerization of lignin into monomeric phenolic compounds is an important way for its high value utilization,but the complex structure and low reactivity of lignin lead to its low depolymerization efficiency.Therefore,the development of catalysts with high activity is important for the efficient depolymerization of lignin.In this work,nitrogen-doped lignin-based carbon supported metal catalysts were prepared for the depolymerization of poplar organosolv lignin by catalytic transfer hydrogenation,and the main studies are as follows.（1）Nitrogen-doped lignin-based carbon supported nickel catalyst and its application in mild and efficient hydrolysis of lignin.The nitrogen-doped lignin-based carbon supported nickel catalyst（Ni@LNC）was prepared by calcination with a simple mixture of melamine after the formation of lignin-nickel complexes with nickel ions from sulfate lignin as raw material,and the catalytic transfer hydrogenation experiments were carried out on poplar organosolv lignin,and also compared with the commercial activated carbon loaded nickel（Ni/C）prepared by the incipient-wetness impregnation method and the lignin-based carbon supported nickel without nitrogen doping（Ni/LC）catalysts were compared.The results showed that Ni@LNC had the best catalytic activity compared to Ni/C and Ni/LC at the same loading,reaching the highest monophenol yield of 22.08%at 160 ^oC for 1 h.The high activity was maintained after five cycles of magnetic separation recovery.The characterization revealed that the high activity of Ni@LNC originated from the folded and fluffy,pore-rich(271.63 m^2.g^-1)structure with ultrafine nickel nanoparticles（1-3 nm）.The high specific surface area of the carriers is due to the pore-opening effect caused by the escape of volatile gases during the thermal decomposition of melamine,while the excellent dispersion and stability of the Ni nanoparticles originate from the dual effect of complexation of the lignin-terminal reactive groups and the anchoring and capturing of nitrogen.In contrast,Ni/C and Ni/LC exhibit poor catalytic activity due to the agglomeration effect of metallic nickel particles and the limitation of carbon encapsulation phenomenon of nickel nanoparticles,respectively.Due to the high efficiency and stability,simple synthesis and relatively low cost,the Ni@LNC catalyst developed in this work has a greater potential to be applied to the preparation of phenolic monomers by mild and efficient hydrolysis of lignin.（2）Nitrogen-doped lignin-based carbon supported nickel-copper bimetallic catalysts and its application in mild and efficient hydrolysis of lignin.Bimetallic catalysts have certain synergistic effects than monometallic catalysts,which in turn exhibit better activity in catalytic reactions.Lignin-bimetallic complexes were obtained by flocculation of mixed solutions of bimetallic salts with lignin,and nitrogen-doped lignin-based carbon supported bimetallic catalysts were prepared by pyrolysis of mixtures of lignin-bimetallic complexes with melamine and used for organosolv lignin hydrogenolysis catalysis.The Ni-Cu catalysts were selected from four different bimetallic catalysts by lignin catalytic hydrolysis experiments,and Ni-Cu catalysts with different metal loading ratios were prepared.The results showed that Ni₁Cu₁@LNC had the best catalytic activity and reached the highest monophenol yield of 30.77%at 200 ^oC,1 h.The activity did not show any substantial change after five cycles of catalyst use and remained stable overall.It was found by characterization that the best metal synergistic effect on the catalyst carrier was achieved when the Ni:Cu ratio reached 1:1.The adding of Cu promoted the dispersion of Ni,and the metal nanoparticles maintained excellent dispersion while forming a Ni-Cu alloy phase;the bimetallic also did not affect the carrier morphology,and the g-C₃N₄/C carrier morphology maintained a fluffy and folded,pore-rich structure.It provides a simple preparation,cheap and easily available raw materials and synergistic effect for the efficient conversion of lignin to phenolic monomers with dual non-precious metal catalysts.