Effects Of Boron Cross-linking Agents On The Melting And Pyrolysis Process Of Lignin And Carbonization Regulatory Mechanisms

Lignin pyrolysis to produce high-quality functional carbon materials is a feasible way to realize its resourceful and high-value utilization.However,lignin softening and agglomeration pose a primary technical challenge for scale-up of the pyrolysis system,and also affect the subsequent regulation of biochar structure.In this work,industrial lignin from paper black liquor was used as the research object,and boron crosslinking agents were used to inhibit lignin softening,and combining with ultrafast-freezing pretreatment to prepare B-doped nano-carbon materials.The main works are shown as follows:The mechanism of lignin softening and pyrolysis was explored through the examination of the physical and chemical changes in the matrix and the release of volatiles during pyrolysis of two types of fractionated lignin using in situ high-temperature ¹H NMR,high-temperature rheology,and fixed bed reaction system.The results showed that lignin underwent stages of glass transition,swelling,softening,apparent solidification,substantial solidification and carbonization.The pyrolysis process of lignin involved solid-liquid,gas-liquid and gas-solid interphase reactions.Furthermore,it was observed that the higher the molecular weight,the lower theβ-O-4 bond content,and the more difficult the softening and decomposition of lignin.To suppress the agglomeration of char particles during lignin pyrolysis,the effects and mechanisms of borates and metal hydroxides on lignin softening and pyrolysis properties were investigated.Both methods can avoid agglomeration and foaming of solid char by skipping the softening region.Metal hydroxides lower the glass transition temperature of lignin,while borates suppress the glassy transition of lignin.Metal hydroxides inhibit the polymerization of functional groups such as phenol hydroxyls,carboxyls and aldehydes of lignin by forming phenates or carboxylates,while borates react with hydroxyls or carboxyls of lignin to limit molecular chains movement.The mechanism of the ammonium borate on the slow pyrolysis of lignin was investigated using multiple in-situ analysis methods to monitor the physical and chemical changes and volatiles release behavior of boron-modified lignin.There are two kinds of binding forms between borate and lignin hydroxyl groups,i.e.borate ester crosslinking structure or non-covalent hydrogen bonds.With the increase of boron content,the mobility of lignin decreased,and the deformation and viscoelastic modulus of the matrix decreased significantly and tended to be stable.Borates promoted the carbonization of lignin,increased the pyrolysis activation energy,and decreased the release of small molecule gases and organic volatiles.The effects of boron content and pyrolysis temperature（450～650°C）on the fast pyrolysis of lignin were studied in a fixed bed to determine the regulation mechanism of ammonium borate on the properties of lignin pyrolysis products.High temperature and high boron content promoted the conversion of methoxy phenols into simple phenols,especially catechol.The content of heavy components in the lignin pyrolytic oil decreased.The introduction of boron promoted the conversion of high-oxygen species（O₄）to low-oxygen species（O₂）,the specific surface area of biochar and the number of meso-and macropores were also increased.In addition,the oxygenated groups content and carbon sequestration rate also increased significantly.To prepare high-quality lignin-based carbon materials,ammonium borate was used to modify lignin,K₃PO₄ as template agent,and with the aid of ultra-fast freezing pretreatment,lignin-based boron doped porous carbon with morphology controllable was prepared,and the synergistic regulation mechanism on physical and chemical structure of carbon materials was explored.K₃PO₄ can promote the self-assembly of lignin in aqueous solution and acts as a mesoporous template.The ultra-fast freezing pretreatment can construct the precursor nanostructure and micropores.Ammonium borate can ensure that the nanostructure of the precursor is stably inherited to the carbon product and form boron doping structure.The maximum equilibrium adsorption capacity of boron-doped carbon nanosheets on methylene blue solution reached 473.9 mg/g.The construction of nano-morphology and hierarchical pore structure greatly improved the adsorption rates of dye molecules.