Study On Preparation Of Levulinic Acid From Biomass Sugar Via Br(?)nsted-lewis Acidic Ionic Liquids Functional Catalysts

Levulinic acid?LA?is a novel platform chemical containing carboxyl and carbonyl functional groups.Nowadays,studying how to produce LA from biomass resources has become one of the hot areas.But,inorganic acid catalysts have problems such as corrosion of equipment and not recyclable of catalysts.Solid acids also have problems such as active site inactivation and coking.Br?nsted-Lewis acidic ionic liquids?ILs?have the advantages of stable structure,acidity and easy recovery.Therefore,the study of Br?nsted-Lewis acidic ILs is crucial to improve the productivity of LA and protect environment.?1?Some kinds of Br?nsted-Lewis acidic ILs were designed and synthesized based on triethylamine,imidazole and pyridine.The structure of Br?nsted-Lewis acidic ILs was characterized by FT-IR,¹H NMR,UV-Hammett and FT-IR spectra of ILs using acetonitrile/pyridine as probe.The results showed that the synthesized Br?nsted-Lewis acidic ILs were the target product.The acidity of Br?nsted acidic ILs as follows:[HO₃S-?CH₂?₃-mim]Cl>[HO₃S-?CH₂?₃-py]Cl>[HO₃S-?CH₂?₃-NEt₃]Cl.The Lewis acidic as follows:ZnCl₂>FeCl₃>CuCl>CrCl₃.?2?The effect of ILs as a catalyst on the results of reaction was investigated when glucose,cel obiose and cellulose were used as raw materials.Glucose was used as raw material.Under the optimum conditions:[HO₃S-?CH₂?₃-mim]Cl-ZnCl₂?x=0.60?0.62 mmol,glucose 0.50 g,water dosage 30 mL,T=180 ^oC,t=9 h,the yield of LA was 88.0%.The LA yield was maintained at 84.0%when ILs after 5 cycles.Characterization of the solid residues indicated that they were humins.When[HO₃S-?CH₂?₃-mim]Cl-FeCl₃?x=0.60?catalyzed cellobiose,water dosage30 mL,catalyst 0.62 mmol,cellobiose 0.50 g,T=180 ^oC,t=10 h,the highest yield of LA was 67.5%.The LA yield was maintained at 62.0%when ILs after 5 cycles.Characterization of the solid residues indicated that they were humins.Cellulose was used as raw material.Under the optimum conditions:[HO₃S-?CH₂?₃-mim]Cl-FeCl₃?x=0.60?0.62 mmol,cellulose 0.50 g,t=10 h,T=180 ^oC,water dosage 30 mL,the yield of LA was 49.5%.The LA yield was maintained at 44.0%when ILs after 5 cycles.At the same time,there were humins covering the surface of the solid residues.Carbon materials were prepared based on solid residues and their electrochemical properties were investigated.It was found that the specific capacitance of the carbon material in the acidic electrolyte and the alkaline electrolyte reached132.0 F/g and 85.6 F/g after activation by NH₃.?3?The synergistic catalytic mechanism of the Br?nsted-Lewis acidic ILs was proposed based on its catalytic performance for glucose,cellobio se and cellulose.Cl^-could form hydrogen bonds with the hydroxyl groups of cellulose and cellobiose in order to promote the formation of glucose by cleavage of glycosidic bonds.Then glucose could generate LA under the action of H⁺,and glucose also could be isomerized to fructose under Lewis catalysis,and then Br?nsted acidic catalyzed the production of LA from fructose.Glucose isomerized to fructose was the control step of the whole reaction.The structure and formation mechanism of humins were inferred based on the characterization of humins structure.Humins were mainly formed by hydrolysis,isomerization,dehydration,aldol condensation and aromatization.The core of humins was mainly composed of stable groups?ether,quinone and pyrone?,while the outer shell of humins was mainly composed of reactive/hydrophilic?hydroxyl,carbonyl,carboxyl and ester groups?oxygen-containing functional groups.To sum up,Br?nsted-Lewis acidic ILs were synthesized,which had stable structure and acidity.ILs were used to catalyze the preparation of LA from biomass,which exhibited good synergistic catalytic effects and stable reusability.This study inferred the structure and formation mechanism of the solid residue s.The carbon material was prepared based on the solid residues,which had showed good reversibility after being activated by NH₃.Therefore,the research played a key role in the development of environmentally-friendly catalysts,the rational use of biomass resources and the protection of the environment.