Synthesis Of Heteroatom Doped Carbon Materials Derived From Natural Biomass Materials For The Catalytic Oxidation Of Furfural To Maleic Acid

Maleic acid is a very important chemical,which is widely used in the preparation of polyester resins,lubricating additives,coatings,plasticizers and pharmaceuticals.Furfural can be converted into maleic acid through catalytic oxidation.There are some problems about the catalytic material for oxidation of furfural to prepare maleic acid,such as low furfural conversion efficiency,poor stability,fuzzy reaction pathways and mechanisms.Designing and researching low-cost,green,and efficient catalytic materials is of great significance for achieving large-scale production of biomass-based maleic acid and reducing the dependence of chemical production on fossil resources.The heteroatom-doped carbon materials with excellent catalytic properties have been applied in various catalytic oxidation reaction systems.The types and amounts of heteroatoms,the types and distribution of active species,and the specific surface area of materials have important effects on the catalytic performance and reaction pathways of heteroatom-doped carbon materials.These properties are closely related to the preparation methods of heteroatom-doped carbon materials.Based on the above reasons,this thesis conducted the following research work:（1）A hierarchical porous nitrogen-doped carbon material for catalytic oxidation of furfural to maleic acid:A transparent folic acid-zinc-potassium bromide hydrogel（FA-Zn/KBr）was synthesized by using low cost and green biomass folic acid as carbon and nitrogen precursor,potassium bromide molten salt as pore-forming agent,coordination between folic acid and zinc ion and hydrogen bonding between folic acid molecules.A hierarchical porous nitrogen doped carbon material,PNC-800,was obtained by the calcination of FA-Zn/KBr xerogel at 800℃in an inert gas atmosphere and elution of KBr in the calcined product by washing.Based on the characterization results,the formation mechanism of FA Zn/KBr hydrogel was analyzed.The influence of calcination temperature on the morphology,structure and composition of the prepared hierarchical porous nitrogen-doped carbon materials was investigated.It was determined that the melting of potassium bromide at high temperature played a crucial role in the formation of hierarchical porous nitrogen-doped carbon materials with high specific surface area.The experimental results of PNC-800 catalyzing the conversion of furfural to maleic acid indicate that the graphite nitrogen（N-Q）contained in PNC-800 is the active site for catalytic oxidation of furfural to prepare maleic acid.In a mixed solvent of water and acetonitrile（2:3）,when the concentration of furfural is 100 m M,the conversion of furfural and the yield of maleic acid reach 100%and 70.2%,respectively.Based on experimental data and reaction kinetics analysis,it is inferred that5-hydroxy-furan-2（5H）-one（HFONE）is the main intermediate product in the catalytic conversion of furfural to maleic acid over PNC-800.The main reaction pathway for PNC-800 catalyzing the oxidation of furfural to maleic acid is the decomposition of hydrogen peroxide into hydroxyl radicals,which oxidize furfural to furfural radicals.The furfural radicals combine with adjacent carbon atoms of graphite nitrogen（N-Q）in PNC-800 to form a furfural adduct,which then undergoes multiple reactions to form HFONE.（2）Alginic acid-phosphoric acid hydrogel-derived phosphorus-doped carbon materials for catalytic oxidation of furfural to maleic acid:Alginic acid-phosphoric acid（AA-H₃PO₄）hydrogel was prepared from sodium alginate and phosphoric acid in aqueous solution at room temperature.After calcination of AA-H₃PO₄ xerogel under N₂,the phosphorus-doped carbon material,PC-700,was synthesized.The hydrogen bond interaction and the ester bond between phosphoric acid and alginic acid（AA）make phosphoric acid highly dispersed in AA-H₃PO₄ xerogel.Due to the high dispersion of phosphoric acid,the phosphorus-doped carbon materials based on AA-H₃PO₄xerogel possess rich defects,high phosphorus content,and evenly distributed phosphorus species.When the concentration of furfural aqueous solution is 100 mM,PC-700can obtain 79%of maleic acid yield after 5 hours for the catalytic oxidation of furfural.Based on experimental data and reaction kinetics analysis,it is determined that the C₃P=O group in PC-700 is active species for the conversion of furfural to maleic acid.Theβ-formylacrylic acid（β-FA）is the main intermediate for PC-700 catalytic conversion of furfural to maleic acid.PC-700 catalyzed the decomposition of hydrogen peroxide into hydroxyl radicals HO˙.Theβ-FA can be directly oxidized to maleic acid by HO˙,and it can also be isomerized to HFONE and then converted to maleic acid.The rate of PC-700 catalyzing the conversion of HFONE to maleic acid is lower than direct oxidation ofβ-FA to maleic acid.（3）Nitrogen and phosphorus-codoped carbon materials for catalytic oxidation of furfural to prepare maleic acid:Nitrogen and phosphorus-co-doped carbon material were obtained by calcination of the mixture of melamine and alginate phosphoric acid xerogel at 700℃in nitrogen atmosphere for 3 h.The characterization results demonstrate that NPC contains abundant N-Q and C₃P=O active species.When the concentration of furfural aqueous solution was 100 mM,NPC catalyzed the oxidation of furfural to prepare maleic acid,resulting in a maleic acid yield of 85.7%.NPC showed no significant decrease in activity after 6cycles.The reaction and experimental data confirm that the catalytic oxidation of furfural to maleic acid by NPC is related to the production of hydroxyl radicals.The synergistic effect of N-Q and C₃P=O active groups contained in NPC enables the nitrogen and phosphorus-codoped carbon material to efficiently catalyze the conversion of furfural to maleic acid in aqueous solution.