Study On The Mechanism Of Conversion Of Lignin In Ionic Liquids

Lignin is an abundant renewable resource in nature which has a rich aromatic structure,and is expected to produce high value-added aromatic products as an alternative of the limited fossil resource.Selectively cleaving the C-C/C-O bonds among lignin aromatic units while retaining its aromatic structure is the key step in lignin conversion.However,due to the high bond dissociation energy of the target C-C/C-O bonds,it is difficult to cleave them and upgrade lignin resource.Hence,exploring efficient metal-free catalytic system,which could lower the energy barrier of lignin conversion by activating substances,is of great academic significance and application value,Ionic liquids open a plethora of opportunities to achieve the above-mentioned goal with the promising characteristics such as low vapor pressure,high chemical stability,special hydrogen bond network and designability in catalytic active sites.The utilization of ionic liquids could contribute to activate the reactant molecules by regulating their structure,interaction sites and hydrogen bond donor/acceptor ability.Herein,this work developed a high-efficiency ionic liquid catalytic system for mild and metal-free lignin conversion.By adjusting the anion/cation structure of ionic liquids and forming a multi-site interaction between ionic liquids and lignin,the reaction system could effectively activate lignin interunit linkages and promote the selective cleavage of the C-C/C-O bonds.Basic researches have been carried out on the structural properties of ionic liquids,the interaction between ionic liquids and lignin linkages,and the mechanism of ionic liquids to activate lignin molecules,providing guides for the development of efficient,green and energy efficient lignin conversion technologies.The main research content and innovative achievements of this thesis are as follows:(1)Based on the chromophores in lignin,lignin autologous radicals were induced under ultraviolet light radiation by building multiple interactions of ionic liquids with lignin ?-O-4-ketone linkages.In this case,the cleavage of lignin C-C/C-O bonds could be achieved at room temperature and metal-free conditions.The effects of ionic liquids structure and various experimental conditions on the cleavage of lignin ?-O-4-ketone model compound were investigated,and the kinetics of the conversion were studied.Quantum chemistry calculations and free radical detection experiments were conducted to study the photocatalytic conversion mechanism of lignin.It was found that anion of 1-propyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide([PMim][NTf2])could interact with lignin ?-O-4 linkages to activate C-H to generate free radicals,which then initiate the free radical chain transfer reaction and lead to the cleavage of lignin C-C/C-O bonds.The calculated apparent activation energy was as low as 47.6 kJ/mol,indicating the low energy barrier for their breakages.The conversion of reactants can reach up to 98.4 mol%with the maximum product yield of 93.3 mol%at room temperature,in atmosphere with 100 mW/cm2 of ultraviolet radiation and Br(?)nsted acid.(2)Based on the characteristics of the nitrile group that is rich in electrons to form hydrogen bonds and the good performance of anion[NTf2]-,the structure of the nitrile functionalized ionic liquid is regulated to form multiple hydrogen bonds between ionic liquids and lignin ?-O-4 linkages.In this case,lignin C-C/C-O bond oxidative cleavage could be realized under atmosphere without metal and ultraviolet light present.The effects of ionic liquids structure on the efficiency of lignin ?-O-4-ketone model compounds oxidation were investigated.Quantum chemical calculations and spectrum analysis were carried out to study lignin oxidative conversion mechanism.It was found that the nitrile functionalized caion[CPMim]+in 1-propylronitrile-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide([CPMim][NTf2])could form triple hydrogen bonds with the oxygen-containing groups on both sides of the ether bond,which lower the C-H bond dissociation energy to produce free radicals and further trigger the C-C/C-O bond rupture.The conversion of reactants reaches up to 98.3 mol%with the maximum product yield of 91.7 mol%at 80?,atmospheric pressure air and Br(?)nsted acid conditions.(3)Aiming at the conversion of more comprehensive ?-O-4-alcohol structure with higher bond dissociation energy,the photo-thermal synergism is used to lower reaction energy barrier.Meanwhile,the multi-site interactions of ionic liquids with lignin ?-O-4-alcohol linkages are regulated to affect the charge distribution of lignin and stabilize the reaction intermediate.In this case,lignin C-C/C-O bonds could be cleaved under mild and metal-free conditions.The influences of ionic liquids structure and photo-thermal synergism on the conversion of lignin ?-O-4-alcohol model compounds were investigated.Quantum chemical calculations and free radical scavenging experiments were carried out to study the mechanism of lignin conversion.It was found that the ionic liquid 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide([BMim][NTf2])could interact with lignin ?-O-4-alcohol linkages to increase the charge density of the 'O' atom on the C?-OH and stabilize the generated carbocation intermediate,which contribute to the cleavage of lignin C?-O bond and subsequently initiate the fragmentation of the Ca-C? bond via Norrish type I reaction under photo-thermal synergism.All the reactants could be converted with the maximum product yield of 79.4 mol%at 50?,atmospheric pressure air with 100 mW/cm2 of ultraviolet light radiation and Br(?)nsted acid conditions.(4)To avoid the application of Br(?)nsted acid,ionic liquids oxidizing structure is adjusted,which,in combination with ionic liquids-photo-thermal effects synergism,could realize lignin C-C/C-O bonds cleavage under acid-free,metal-free and mild conditions.The effects of ionic liquid structure and the photo-thermal synergism on alkali lignin conversion efficiency were investigated,and the product distribution at various temperatures were tracked over time.The photo-thermal conversion mechanism of alkali lignin in ionic liquids was studied by carrying out lignin model compounds conversion experiments.It was found that the ionic liquid 1-butyl-3-methylimidazolium perchlorate([BMim][ClO4])could significantly promote the conversion of alkali lignin.The C-C/C-O bonds in lignin linkages could be cleaved under the combined effects of[BMim][ClO4],photo-thermal synergism and oxygen The reaction can occur under mild conditions of 80? and 150 mW/cm2 of ultraviolet light radiation with low energy consumption.The total yield of aromatic monomer reaches up to 4.4 wt%with vanillin and acetosyringone as the main products.There are only ionic liquid and alkaline lignin in the reaction system without any metal or acid catalyst,indicating that the technology is simple and green.