The Mechanism Research Of Selective Depolymerization Of Lignin By Reactive Oxygen Species

Lignin is the only biomacromolecule with aromatic structural units among renewable resources in nature,obtaining small-molecule aromatic compounds by breaking the chemical bonds between the aromatic structural units of lignin that is considered to be a way to realize high-value utilization of lignin resources.Previous studies have found that using electrochemical oxygen reduction reaction(ORR)to continuously provide reactive oxygen species(ROS)for selective breaking of chemical bonds between lignin structural units is an efficient and highly selective lignin depolymerization scheme.However,the type of ROS and its bond breaking mechanism are not yet clear,which is not conducive to the further optimization of the depolymerization process.Therefore,in this paper,guaiacol glycerol-?-guaiacyl ether(GGE)containing both ?-O-4 bond and C?-C? bond was selected as the lignin model compound.Based on the stabilizing effect of aprotic ionic liquids on ROS,1-butyl-3-methylimidazole tetrafluoroborate([BMIM]BF4)ionic liquid was selected as the supporting electrolyte to control the ORR electrolysis conditions to continuously produce ROS for degradation GGE.The mechanism of ROS in GGE bond breaking reaction is speculated based on the qualitative and quantitative analysis of the degradation products of GGE.The main research results in the paper are as follows:(1)The ORR electrochemical behavior of O2 molecules in ionic liquid[BMIM]BF4 supporting electrolyte was studied by means such as cyclic voltammetry(CV)and rotating ring and disk electrode(RRDE).The study found that under proton-free conditions,the number of electrons transferred by the ORR electrode process is 1,the main ROS product is ·O2-;while under the coexistence of a controlled amount of protons,the number of ORR transferred electrons becomes 2,the main ROS product is HO2-.GGE is degraded under the synergistic effect of indirect oxidation and anodic oxidation of ROS(2)In the qualitative and quantitative analysis of GGE and its degradation products in HPLC method,this thesis adopts liquid-liquid extraction and solid phase adsorption to carry out sample pretreatment to exclude the influence of ionic liquid.Butyl acetate as an extractant has a good effect on liquid-liquid extraction pretreatment of samples.Under optimized conditions,the GGE and its degradation products coniferyl alcohol,vanillin and guaiacol in the ionic liquid achieved the best separation effect.(3)In[BMIM]BF4,the continuous degradation experiments of GGE and product analysis results under different electrolysis conditions show that different GGE degradation products are obtained by controlling the ORR process to produce different types of ROS.When ROS is dominated by OOH,GEE breaks the C?-C? bond and ?-O-4 bond,generating vanillin,coniferyl alcohol,guaiacol and aldehyde small molecule compounds;when HO2-is the main ROS,C?-C? bond breaks,generating vanillin,guaiacol and aldehyde small molecule compounds.