Molecular Design And Experimental Study On The Degradation Of Lignin With Deep Eutectic Solvent As The Catalyst

Lignin is a potential and renewable biomass resource which consisted of various phenylpropane monomers connected with C-C bonds and C-O bonds.In 2015,the high value-added utilization of lignin was determined as the core of the refining of biomass resource by U.S.Department of Energy.At present,the method of chemical degradation was the most commonly method of effective utilization of lignin which can break the bonds between the monomers to form the small molecular monophenol compounds.Deep eutectic solvent（DES）is a biodegradable and non-toxic type of solvent which mixed with hydrogen bonding acceptors and donors at a certain ratio.At present,DES was commonly used in the pretreatment and dissolution of biomass resources and DES was used as a catalyst to the degradation of lignin in this paper.While there are many kinds of DES,quantum chemical computation was used to select the catalysts in order to save the experimental cost and time.The molecular design was completed from a microscopic perspective.However,most DES are hydrophilic and they could not be easily separated after reaction in the aqueous phase.Therefore,the concept of temperature-responsive DES was proposed based on the temperature-responsive ionic liquids.The specific research contents of this paper are as follows:1.4 hydrogen bond acceptors（HBAs）and 4 hydrogen bond donors（HBDs）were selected to form 16 kinds of Br（?）nsted acidic DES.Meanwhile,3 HBAs and 2 HBDs were selected to form 6 kinds of Lewis acidic DES.Then the|HOMO-LUMO|gap and the interaction energy of the above 22 kinds of DES and the reaction system containing DES andβ-O-4 lignin model were calculated based on FMO and DFT theory,wherein the mixed basis group was used for the calculation in Lewis acidic DES.It meant that C,H,O,N and Cl atoms use 6-31+G basis group and Fe,Al and Cu atoms use LANL2DZ basis group for calculation.The performance of DES to the degradation of lignin was evaluated by the degree of the reduction of|HOMO-LUMO|gap and the stability of the interaction energy in the system.The results showed that Br（?）nsted acidic catalyst[BMIM][Cl][Ts OH]and Lewis acidic DES[Ch Cl][Fe Cl₃]were excellent in the reducing the|HOMO-LUMO|gap and improving the interaction energy.Meanwhile,they could be also efficient to the degradation of lignin.2.The experiment of the degradation of lignin was used to verify the reliability of the method of the simulation calculation and the results for the molecular design.Based on the results,[Ch Cl][Ts OH],[BMIM][Cl][Ts OH]and[Ch Cl][Fe Cl₃]were picked up as the catalysts to the degradation rate of lignin.Meanwhile,the degradation ratio of lignin and the yield of monophenol were used as the experimental indicators to prove the consistency of the simulation and experiment.The experimental result showed that[BMIM][Cl][Ts OH]and[Ch Cl][Fe Cl₃]were the excellent catalysts for the degradation of lignin in Br（?）nsted and Lewis acidic DES respectively,which was in line with the results of the calculation.Subsequently,the effect of reaction conditions such as the composition of DES,the mass ratio of catalyst to lignin,the volume of water,reaction time and the temperature on the degradation of lignin and the yield of monophenol were studied then the optimal conditions with different catalysts were determined as well as the optimal degradation rate and the yield of the product were also obtained.3.The two temperature-responsive DES（TRDES）were designed and synthesized as the catalysts to realize the easily separated from the system after the reaction and improving the reaction effect then TRDES was formed the reaction system with water to the degradation of lignin.Therefore,the system would be homogeneous with the reaction temperature while it would be two phases with the room temperature.Then the molecular design was carried out with the same method and solubility of DES in water under the different temperature was examined.Both the two TRDES could be effective to the degradation,wherein the degradation rate of lignin was reach 83.63%and the yield of monophenol was 51.22%when the[BMIM][Cl][EDTA]was used as the catalyst.The values were higher than that of[BMIM][Cl][Ts OH]and[Ch Cl][Fe Cl₃].Meanwhile,the catalysts could be recovered through a simple operation of dumping when the temperature was reduced to room temperature after the reaction and the performance of catalysts has no obvious deactivation under the 4 times of recycling.