Effects Of Supercritical/subcritical Carbon Dioxide And Multiple Sulfonate Ionic Liquids On Biomass Carbohydrate Catalytic Conversion

With the rapid development of society,non-renewable resources such as petrochemical resources are increasingly exhausted,at the same time,excessive use of fossil resources caused serious pollution of the environment.As a common by-product of the sugar industry,bagasse is discarded and incinerated in production,causing further environmental pollution.Therefore,how to effectively convert lignocellulosic biomass into an important platform material 5-hydroxymethylfurfural（5-HMF）in the chemical industry has become a hot issue in resource utilization.In order to investigating more efficiently utilize of biomass resources,pretreatment of lignocellulosic biomass and synthesis of novel catalysts were combined to research effect on biomass carbohydrate catalytic conversion.Two types of multiple sulfonate ionic liquid were synthesized to investigate their catalytic effect in conversion fructose to 5-HMF in present work,which can provide a theoretical basis for subsequent biomass degradation.Then supercritical/subcritical carbon dioxide（SC/Sub-CO₂）was used as the catalyst for the reaction for the further study of effecting on fructose conversion.At last,supercritical carbon dioxide was used for the lignocellulosic biomass（sugarcane bagasse）pretreatment and used the multiple sulfonate ionic liquids as a catalyst to catalyze the conversion of biomass to5-HMF.The main research methods and details are as below:（1）The ionic liquid Qu-SO₃H-IL and Bi-SO₃H-IL were synthesized by two-step method and confirmed by FT-IR and ¹H-NMR,then their Hamilton acidity was determined.They were used as catalyst to conversing fructose to 5-HMF in different solvents,four factors of reaction time,temperature,solvent and catalyst were studies.The result shows that,Qu-SO₃H-IL is more effective for the conversion at 70-100℃for 0.5-5 h due to its higher acidity.At lower temperatures（70,80℃）,the ionic liquid[Bmim]Cl is preferred as a solvent.But in higher temperature environments（90,100℃）,[Bmim]Cl and DMSO perform similar solvent effects,the maximum yield of5-HMF is higher when using DMSO as a solvent.When the temperature is 100℃,the yield reached 95.64±0.56%after 2.5 h.The optimal conditions for the catalytic system are as follows:solvent DMSO reacted at 100℃for 2.5 h.At the same time,the catalytic system has high recyclability and repeatability.After repeated used for 6times,the whole system still has highly catalytic performance and the yield of 5-HMF was only reduced by 2.35%.（2）In order to make the catalytic system more green and sustainable,the catalytic ability SC/Sub-CO₂ to catalyze the production of 5-HMF in fructose was studied.The single factor analysis of temperature and time showed the higher reaction pressure has a significant effect on the growth of fructose yield.The high temperature in the range also improves the catalytic effect,but after the range is exceeded,the high temperature has an adverse effect on the yield of the target product.Therefore,the optimum reaction condition of the SC-CO₂/[Bmim]Cl system is 9 Mpa,and the reaction at 120°C is 0.5 h,the maximum yield is 87.21±2.52%.When the solvent is H₂O,the higher temperature is very important for the conversion of fructose in a non-polar solvent,and the acidity of the catalytic system is proportional to the pressure of CO₂,but after a certain range,the target product is prone to serious side reactions.From the decrease of the yield of the target product,when the reaction temperature is 180°C,the environment with pH 4 is the optimum condition for producing 5-HMF,and the corresponding CO₂ pressure is about 6 Mpa.It is concluded that the optimum conditions for the reaction of Sub-CO₂/H₂O system are 6Mpa and the reaction at 180°C for 2 h,the maximum yield is 48.50±5.11%.（3）Pretreatment of supercritical carbon dioxide with common lignocellulosic biomass bagasse as raw material and 1-butyl-3-methylimidazolium acetate（[Bmim]OAc）as solvent.The bagasse before and after pretreatment were characterized by FT-IR,XRD,TG,SEM and ¹³C-NMR.Then the enzymatic hydrolysis and acid hydrolysis of the regenerated bagasse were carried out.The yield of monosaccharides,polysaccharides and total reducing sugar yield in the hydrolysate were measured by HPLC and DNS.Finally,the biomass was conversed to 5-HMF by the multiple sulfonate ionic liquids synthesized.The results shows that after pretreatment with SC-CO₂/[Bmim]OAc,the lignin and hemicellulose removal rates in sugarcane is 41.31%and 19.20%,but the structural characterization of bagasse showed that the primary structure of the bagasse fiber is not changed.After pretreatment,the yield of reducing sugars obtained by enzymatic hydrolysis and acid hydrolysis increased by 12.57%and 22.04%,respectively,and under the catalysis of ionic liquid Qu-SO₃H-IL and Bi-SO₃H-IL,the 5-HMF yield increased by 61.26%and21.03%,further indicating that pretreatment has a huge effect on the degradation of biomass.