Study On The Preparation Of Ethyl Levulinate By Alcoholysis Of Cellulose Catalyzed By γ-valerolactone Cosolven

Catalytic conversion of cellulose and other carbohydrate resources to produce value-added chemicals could effectively alleviate human dependence on non-renewable resources.Cellulose is the most abundant organic macromolecule in lignocellulosic biomass.Due to the inherent characteristics of cellulose such as the van der Waals forces and intramolecular/intermolecular hydrogen bonds,the glucose molecular chains orderly accumulate to form a highly crystalline structure.This is the main factor restricting the effective conversion of cellulose.In recent years,the use of acid-catalyzed alcoholysis technology to convert cellulose to prepare levulinic acid esters is a new energy chemical has attracted much attention.Among them,the organic co-solvent-mediated strategy is considered as an important way to further modulate the reactivity and increase the catalytic reaction rate in addition to the limited effect of catalyst and reaction conditions.Based on this,this thesis systematically carried out the study ofγ-valerolactone co-solvent promoted acid-catalyzed alcoholysis of cellulose for the preparation of ethyl levulinate,including the development of an efficient acid-catalyzed catalytic system for the preparation of ethyl levulinate from cellulose in the co-solventγ-valerolactone/ethanol solvent,on the basis of which the mechanism ofγ-valerolactone co-solvent promoted acid-catalyzed cellulose preparation of ethyl levulinate was investigated;then for further in-depth analysis ofγ-valerolactone on the solubilization behavior of cellulose in the process of alcoholysis reaction,the effect ofγ-valerolactone on the alcoholic conversion of different crystalline cellulose was investigated;finally,the co-solvent was introduced in the process of eucalyptolysis to investigate the process characteristics ofγ-valerolactone co-solvent promoted catalytic alcoholysis of liquefied of eucalyptus wood.Firstly,the influence of different kinds of co-solvents on the alcoholysis conversion of cellulose were investigated to optimize the alcoholysis reaction conditions,andγ-valerolactone was chosen as the reaction co-solvent.Following this,the kinetic behaviors of the alcoholysis conversion of cellulose and several key intermediates（such as cellobiose,glucose and fructose）to ethyl levulinate were experimentally compared,and the experimental results showed that the reaction rate constants of ethyl levulinate formation from cellulose,cellobiose and glucose were all more than two times higher when alcoholysed in ethanol/γ-valerolactone mixed solvents than when they were in pure ethanol solvents.Analysis of the modeling of the alcoholysis process of cellulose in two reaction media revealed that the activation energy of cellulose alcoholysis to ethyl glucoside reaction decreased from 126.9 k J/mol to 96.1 k J/mol after the addition ofγ-valerolactone in pure ethanol solvent.The relative solubility of cellulose reached 12.2%when cellulose was treated in ethanol/γ-valerolactone solvent at 180℃for 3 h,while cellulose was almost undissolved in ethanol.It was found that the addition ofγ-valerolactone to ethanol solutions containing Al（OTf）₃ decreases the p H of the solvent system.Quantum chemical calculations revealed that the charge density increases（from 518.3 to 526.4）whenγ-valerolactone is substituted for ethanol as a ligand in the Al（III）-solvent complex,suggesting the formation of a harder metal Lewis acid,which in turn induces enhanced Br（?）nsted acidity.In conclusion,a combination of experimental investigations and Density functional theory calculations revealed thatγ-valerolactone enabled the generation of Al species with higher acidity and catalytic activity in the alcoholysis reaction system.Cellulose III was prepared using natural microcrystalline cellulose I as the parent,and the characteristics of alcoholysis conversion behavior of different crystalline cellulose types inγ-valerolactone co-solvent were investigated as a way to gain insight into the structural effects ofγ-valerolactone in promoting cellulose solubilization conversion process.It was found that the reaction performance ofγ-valerolactone on the alcoholysis of cellulose of different crystalline forms was significantly improved.The liquefaction rates of cellulose I and III catalyzed by alcoholysis reaction in ethanol were 26.9%and 11.6%at a reaction time of 60 min at 160℃,while the liquefaction rates were 31.6 and 19.1%after the addition ofγ-valerolactone in the solvent,respectively.In addition,γ-valerolactone promoted the cellulose reaction from the surface to its whole swollen region,so as to accelerate the cellulose solubilization and transformation process.Since cellulose is mainly derived from biomass feedstock,the effect ofγ-valerolactone on the alcoholysis process of raw wood material was further investigated.The effects ofγ-valerolactone content and reaction temperature on the liquefaction rate and product distribution of eucalyptolysis were investigated by applyingγ-valerolactone co-solvent to the process of eucalyptolysis,and the solid residues before and after the reaction were characterized by various technical means.Among them,γ-valerolactone exhibited a good promoting effect as a co-solvent for the alcoholic liquefaction of eucalyptus.The liquefaction rate of eucalyptus in the reaction medium containing 40%γ-valerolactone reached 88.6%at 170℃for 120 min,compared with74.3%in the pure ethanol medium.At 170°C,150 min,the yield of ethyl levulinate in pure ethanol was 21%.In contrast,the yield of ethyl levulinate increased to 37%when20 vol.%γ-valerolactone was added into pure ethanol,respectively.The experimental results indicated that the introduction ofγ-valerolactone in the reaction medium could accelerate the degradation of hemicellulose and lignin solubilization in eucalyptus raw materials,and also had a positive effect on the alcoholysis conversion of cellulose.