Appling Two Heteropoly Acid In Preparation Of Aromatic Compounds From Lignin Via Catalytic Oxdiation

With the increasing demand of energy and resource,problems such as resource depletion,environmental pollution,anomalies in the climate and ecological deterioration have become more serious.Exploring the alternative and renewable energy sources is vital for the develop of society.Biomass is a good choice for this problem.About 170 billion tons of biomass are produced each year,but only 3%are consumed by humans.Xinjiang province is one of the most important cotton production bases in China.As a by-product of cotton,more than 10million tons rods are produced every year,but most of the rods were crushed,burned or discarded.Cotton rod cotains about 25 wt%lignin.As a renewable aromatic polymer,lignin can be covert to aromatic compounds such as vanillin through selective catalytic oxidation.The key of lignin conversion is the catalyst.Hyterpoly acid is the earliest catalyst for the degradation of lignin with good performance in homogeneous system.However,their applications are limited due to the diffcult desperating and recycling.Therefore,the immobilization of HPA?tungstophosphoric acid and phosphomolybdic acid?on various supports was studied in this paper.Also the mechanism of catalytic oxidizing lignin was investigated.In this thesis,lignin model molucule was prepared firstly.Then the effects of supports on the catalytic performance were investigated,incluing graphene oxide,activated carbon,carbon nanotubes,silica,and SBA-15 zeolite.The optimal preparation method and conditions were obtained.Lastly,the catalysts were applied in the catalytic oxidation of real lignin and model molucules.The mechanism and optical conditions were discussed.NMR,XRD,FT-IR,HPLC,UV-vis DRS,GC-MS and other analytical methods were used to analyze the composition and structure of raw material,reacts and catalyst.It was found that Keggin-type tungstophosphoric acid can effectively catalyze the oxidation of lignin and its model molecules.The catalytic mechanism was speculated and further verified through experiments.The optimal condition of CH₃OH:H₂O?mL:mL?=50:50 was obtained.At 100°C,oxygen pressure 1.5 MPa,reaction time 3 h,the conversion rate of the model molecule reached 91.3%.Under the same conditions,the real lignin was subjected to catalytic oxidation experiments,and the product distribution was similar to that of the model molecular catalytic oxidation product.It was found that Keggin-type phosphomolybdic acid can produce a good synergistic effect with phosphomolybdic acid in?-valerolactone?GVL?solvent,thereby contributing to catalytic oxidation of lignin.At the optimal catalytic conditions of GVL:H2O?mL:mL?=70:30,reaction temperature 150°C,oxygen pressure 1.5 MPa,and reaction time 4 h,the conversion rate of the model molecule reached 92.6%.The real lignin has also been subjected to a catalytic oxidation experiment,and the product distribution is similar to that of the model molecule catalytic oxidation product.Gamma-valerolactone is a good solvent for lignin,which can dissolve large-molecular-weight lignin,and its polarity is appropriate.It is expected that?-valerolactone will be more deeply applied in the catalytic oxidation of lignin.To overcome the problems of homogeneous,difficult recovery and recycling of heteropolyacid catalysts,the immobilization of the heteropolyacid catalysts was studied.The catalytic performance of the carrier including graphene oxide,activated carbon,carbon nanotubes,silica,and SBA-15 zeolite was compared.The results show that graphene oxide as support has the best performance.Besides,the effect of impregnation,co-sedimentation,hydrothermal method and gel method on the firmness of catalyst loading was also studied.The results show that the catalyst and carrier obtained via hydrothermal method were solid.Furthermore,H₃PMo₁₂O₄₀/GO catalyst and H₃PW₁₂O₄₀/GO catalyst were prepared by hydrothermal method,respectively,and their repeated use experiments showed that the performance of the catalyst was relatively stable.The cotton lignin ethanol lignin was catalyzed and oxidized experimentally under the optimal conditions of homogeneously catalyzed lignin model molecules.The experimental results show that the product distribution is similar as that of model molecular catalytic oxidation products.Therefore,it can catalyze the real lignin.The mechanism of depolymerization is the same as that of model molecules.In conclusion,tungstophosphoric acid and phosphomolybdic acid can be immobilized with supports with high stabilty and good performace via hydrothermal method.The resutls of this work provides theoretical guidance and application reference for the high value-added transformation of cotton lignin.The comprehensive utilization of cotton stalks was further improved and enriched.It will helpful for the production of aromatic compounds from biomass?...