Green Electrochemical Pretreatment To Promote Fractionation And Valorazation Of Biomass Components

With the increasing depletion of fossil resources and increasingly prominent environmental problems,the refining of using renewable lignocellulosic biomass to produce biofuels,functional materials and bulk chemicals has attracted great attention.Among lignocellulosic biomass,straw as a kind of agricultural and forestry waste has advantages of abundance,wide source and low cost,and has a broad application in biorefinery.However,the natural relactrant barrier constituted by the heterogeneity and complexity of biomass components and structures seriously hinders the green separation and efficient conversion of components.In this paper,a novel two-stage pretreatment combined seawater hydrothermal with electro-assisted pretreatment method is proposed for comprehensive utilization of straw,that is,seawater is selected as the only solvent in the first-stage hydrothermal pretreatment which aims to degrade hemicellulose into xylo-oligosaccharides.Electrogenerated alkaline hydrogen peroxide pretreatment is introduced in the second-stage to efficiently remove lignin and obtain cellulose-rich substrates for the production of fermentable sugars and small molecular weight lignin.The research results are as follows:The electrogenerated alkaline hydrogen peroxide（EAHP）pretreatment method is proposed.A graphite felt/carbon black electrode with low price,simple process and high hydrogen peroxide yield is firstly prepared for in situ production of H₂O₂ synergied with straw pretreatment.The pretreatment process was explored,and the optimal process conditions were established(30 m A·cm^-2,70℃,4 h and 100 sccm).Under this pretreatment condition,the best enzymatic hydrolysis efficiencies of glucan and xylan were 83.7%and 90.8%,respectively,and the delignification reached 76.3%.In addition,the cyclability of the self-made electrodes was also evaluated.The results showed that the current efficiency decreased by only 8.2%（78.9%to 70.7%）after 10 cycles under the current density of 30 m A·cm^-2,indicating that the electrode has excellent stability.In order to further realize the efficient separation and the comprehensive utilization of cmprehensive components,a new two-stage method combined seawater hydrothermal with electro-assisted pretreatment was studied and proposed.Firstly,the first-stage seawater hydrothermal pretreatment for production of xylo-oligosaccharides was studied,including the effects of hydrothermal time and temperature on the yields of xylo-oligosaccharides and fermentable sugars.The optimal conditions were established as follows:the reaction temperature was 170°C and the reaction time was 40 min.Under these conditions,89.9%of hemicellulose was removed,and the yield of xylo-oligosaccharide in the hydrolyzate was 49.4%.Under the optimal conditions explored above,the removal rates of lignin and hemicellulose from the substrate after two-step pretreatment were as high as 94.9%and 95.9%,respectively.The relative content of cellulose was 85.4%,and the enzymatic hydrolysis efficiency of glucan reached 91.2%,which is 5.3 times of the enzymatic hydrolysis efficiency of raw materials.In addition,both delignification rate（R~2=0.91）and the removal rate of hemicellulose（R~2=0.96）were positively correlated with glucan hydrolysis efficiency.The change of the chemical structure of lignin after the two-step pretreatment process was studied.The results showed that the content ofβ-O-4,β-β,β-5 andβ-1bonds in lignin decreased with the increase of pretreatment intensity,indicating that the two-stage pretreatment effectively destroyed the main structural units of lignin.The S/G ratio gradually increased,indicating that the G-type unit was more easily degraded than the S-type unit,which was beneficial to improve the enzymatic hydrolysis efficiency.In addition,the mass-average molecular weight of lignin was reduced from 12916 g/mol of raw materials to 1929 g/mol and it had lower polydispersity,making lignin potential application in bio-based materials.