Investigation On Preparing Aromatic Monomers Through Cornstalk Lignin Catalytic Depolymerization

The use of lignocellulose biomass,a green and renewable resource,as a raw material for the preparation of high value-added chemicals and fuels is regarded as a promising strategy.Lignocellulose biomass is composed of cellulose,hemicellulose and lignin.The strategy of"lignin-first depolymerization"can reduce energy consumption and condensation reactions by avoiding the extraction or separation of lignin,thus promoting the efficient conversion of lignin.Lignin depolymerization mainly involves hydrogenolysis and hydrogenation processes,in which metal catalysts have attracted wide attention.However,the structure of cornstalk lignin is complex,its depolymerization efficiency is poor and the active intermediates are prone to condensation reaction.In view of the above problems,the modified catalysts were used to convert the cornstalk lignin into aromatic monomers.The main work of this thesis includes the following aspects:（1）The effect of cornstalk lignin hydrogenolysis under the Ru/Al₂O₃ catalyst was investigated,and the depolymerization products and catalysts were characterized and analyzed.The activity of the metal sites and the stabilization of the active intermediates were discussed.By comparing the ability of hydrogen adsorption with different Ru/Al₂O₃,it is found that the higher the dispersion of metal components,the more active sites are exposed,which is conducive to contact with the reactants in the reaction.Subsequently,the effect of different reaction conditions on lignin hydrogenolysis was explored.When the reaction temperature was 260 ^oC,it could be seen that 83.7%of lignin could be liquefied with 24.6wt%monomer product yield,and the activity of the catalyst was not significantly reduced after recycling three times in the cornstalk lignin hydrogenolysis.（2）On the basis of the upper part of solid acid,the effect of cornstalk lignin hydrogenolysis under acid-modified Ru/C catalyst was investigated,giving nearly 42.9 wt%yield of total monomers at the temperature of 260 ^oC.The effects of surface modification of carbon supports on hydrogenolysis activity and product selectivity were focused on,and the effects of reaction conditions on hydrogenolysis were studied.The characterization of catalysts showed that the larger specific surface area of carbon compared to the solid acid catalysts contributes to the better metal dispersion.The acid sites in the acid-modified Ru/C catalyst were able to weaken the C-O bond between lignin.And the highly dispersed metal sites could provide a large number of active hydrogen species and greatly promote hydrogenolysis.Furthermore,the acid-modified Ru/C catalyst can stabilize the unsaturated groups of the intermediates and active phenolic monomers,thereby inhibiting the condensation of the depolymerization products.（3）Based on the previous study,the effect of cornstalk lignin hydrogenolysis under the Ru-based biochar catalyst was explored,and 31.2 wt%monomer product yield was obtained at 260 ^oC.Using biochar prepared from waste as the catalyst support can not only effectively reduce the cost,but also be easy to modify.The characterization showed that developed pore structures and nitrogen species on supports are key factors in their great hydrogenolysis performance.Among them,the Ru-based biochar catalyst with micropore-mesopore structure can not only have a large specific surface area,which is conducive to the dispersion of the metal sites,but also conducive to the mass transfer and diffusion processes in lignin hydrogenolysis to avoid condensation.The N doping modification forms a large number of nitrogen species,which facilitates the anchoring of the metal center,and the interaction and electron transfer between Ru and the support can promote the C-O bond breakage and improve the yield of monomeric aromatic products.