Iionic Liquid As Effcient Catalyst For Direct Conversion Of Carbohydrate Biomass Into Levulinic Acid Compounds

Biomass is one of the most common and valuable renewable resources that can serve as an alternative resource for producing biofuels and chemicals.In recent years,the synthesis of platform chemicals from biomass have emerged as one of important research fields,5-hydroxymethylfurfural（HMF）and levulinic acid（LA）have important application as platform chemicals.At present,hydrolysis of biomass to levulinic acid（LA）has been put into effect using inorganic solid catalysts and liquid acids.Solid catalyst has poor catalytic performance on the hydrolysis of cellulose and other biomass,while liquid catalyst has poor thermal stability and poor performance.It was very important to solve the above problems via developing efficient catalysts.Ionic liquids（ILs）were considered as a green catalyst and solvent owing to their low vapor pressure,high thermal stability,easy product recovery and environmentally friendly.Most of reported ILs showed poor performance when employed to convert biomass into LA.This research was synthesized a series of ILs containing different anions or different cations,their structures have been identified by means of nuclear magnetic spectroscopy（NMR）,mass spectroscopy,and Fourier transform infrared spectroscopy（FT-IR）.The effect of the anion on the catalytic performance was fully investigated.In addition,by designing ILs with different cations,the influence of acid anion and halogen anion in ionic liquid on the catalytic effect was fully studied.Biomass such as cellulose,sucrose,starch,cellobiose,chitin,chitosan and rice husk was used as substrates for catalytic hydrolysis into LA,and the yield of LA is above 50%.This result shows that the series of ILs have high catalytic performance.Meanwhile,the polymer polystyrene sulfonic acid is used as catalyst to hydrolysis of glucose and fructose into HMF,and its catalytic performance is poorer than that of ILs.Hence,this work mainly focused on the biomass conversion.（1）A series of SO₃H-functionalized ILs were synthesized and successfully applied as the catalysts for converting cellulose into LA.The structure,thermal stability,and Hammett acid（H₀）of ILs were characterized,and all the ILs showed high thermal stability.The H₀of ILs was almost determined by the anions,and followed the order H₂PO₄^-<Ts O^-<CF₃SO₃^-<HSO₄^-.Meanwhile,the H₀ of ILs have an obvious influence on the yield of LA,and the yield of LA using[BSim]H₂PO₄,[BSim]Ts O,[BSim]CF₃SO₃,[BSim]HSO₄ as catalyst were 25%,55%,56%,and65%,respectively,which is in good agreement with their H₀ order.Moreover,catalyst[BSim]HSO₄ also showed great catalytic performance for converting other biomass such as sucrose,chitin,and cellobiose into LA.The[BSim]HSO₄ was recycled up to five times with a 44%yield of LA,and the NMR spectra showed that the recycle IL was high purity after recycling four times.（2）A series of new SO₃H-functionalized ILs containing different halogen-anions have been successfully synthesized and used as the catalyst to converting cellulose into LA.The yield of LA was increased dependent on the anions and follow the tendency:F^-<Cl^-<Br^-,which was consistent with the acidity order.Thermogravimetric analysis（TGA）showed that these SO₃H-functionalized ILs have good thermal stability in the range of reaction temperature.Among the SO₃H-functionalized ILs,[BSim]Br showed high catalytic activity with 76.5%yield of LA without using any additional catalyst for only 20 min reaction time.More importantly,[BSim]Br catalyst also exhibited high performance for conversion other biomass such as sucrose and cellobiose to LA,and the[BSim]Br catalyst can easily be recycled without structural changes,achieving 74.5%LA yield at the fifth cycle,indicating that[BSim]Br has high thermal stability in the reaction process.（3）A series of SO₃H-functionalized polymer ionic liquids（PILs）was successfully synthesized and used as catalysts for cellulose hydrolysis into LA.The structure,thermal stability,and H₀ of PILs were tested,and all of these PILs showed higher thermal stability than small molecular ILs.The LA yields were 57.1%,79.4%,70.7%,and 59.8%using[Pim]HSO_4,[Pim]CH₃SO₃,[Pim]Br_0.5（CH₃SO₃）_0.5,and[Pim]Cl_0.5（CH₃SO₃）_0.5 as catalysts,respectively.Bromide and chlorine anions can accelerate the rate of product LA in the reaction,and[Pim]CH₃SO₃ exhibited the best catalytic performance among these PILs.The LA yield at the optimal reaction conditions（100 mg cellulose,1.0 g[Pim]CH₃SO₃,0.8 g H₂O,180℃,and 1.5 h）was 79.4%.More importantly,[Pim]CH₃SO₃ has high catalytic activity on other biomass such as cellobiose and starch.The product LA was easily separated by extraction from the reaction mixture,and[Pim]CH₃SO₃ were recycled up to five times with an LA yield of 68.9%.The ¹H NMR revealed that the[Pim]CH₃SO₃ has high purity even after five recycles.In addition,FT-IR and SEM measurement found that some humins were produced during the cellulose conversion.（4）A series of acidic ILs based on tetramethylpropanediamine have been synthesized,the H₀ of ILs is related to the anion,and increased follow the order of Cl^-<Br^-<Ts O^-<CH₃SO₃^-<HSO₄^-<1,5-NS^-,TGA analysis shows that these ILs have good thermal stability,which was used as the catalyst to catalyze the cellulose and chitin conversion to LA.Among them,the IL[TMPDAPS]Br containing with bromine anion shows the best catalytic performance for the cellulose and chitin as substrates,the yield of LA can reach 62.7%when using chitin as substrates after 3 h.This yield is one of the highest for IL catalyzed chitin to produce LA.[TMPDAPS]Br showed good catalytic performance and stability in the recycle experiment.After four catalytic reactions,the yield of LA was 35.6%,and the structure of the recovered IL catalyst remained unchanged.（5）The PIL[Pim]CH₃SO₃ was used to catalyze the hydrolysis of rice husk to prepare LA.Under the optimal reaction conditions:400 mg rice husk,1.0 g[Pim]CH₃SO₃,3.0 g water,180℃and 3hours,the LA yield is as high as 74.6%,which is the highest value for the ILs catalyzed hydrolysis of lignocellulose to prepare LA.When the added amount of rice husk was increased to800 mg,the yield of LA was 69.6%,indicating that[Pim]CH₃SO₃ can efficiently hydrolysis rice husk to produce LA.In addition,we used TGA,FT-IR and SEM to analyze the solid residue after the reaction,the result indicated that the solid residue was humins.（6）Poly（4?Styrenesulfonic Acid）（PSS）was considered as an efficient liquid and Br（?）nsted acid catalyst for the conversion of carbohydrates to HMF.The effects of different reaction parameters on the conversion of fructose and glucose to HMF were systematically studied.PSS efficiently catalyzed the conversion of fructose to HMF with the yield up to 86.8%at 130℃for 3 h with NMP as the solvent.In addition,it can effectively catalyze the conversion of glucose to HMF,and the HMF yield reached 48.5%at 130°C for 6 h using DMSO as the solvent.The catalyst showed good reusability after being used five times.This efficient catalytic system will provide a new way for the conversion of carbohydrates to platform chemical HMF.