The Mechanism Of Pretreatments With Different Solvent Systems For Promoting Components Fractionation And Enzymatic Conversion Of Cellulose In Bamboo

Energy and the environment are the basic guarantees for the survival of human society.The conversion of renewable woody biomass into biofuels is an effective means of addressing the energy and environmental crises that human society facing now.Due to the high energy consumption and generated pollution in the chemical conversion process,biochemical treatment(such as enzyme-catalyzed hydrolysis of cellulose to produce fermentable sugar and sugar fermentation to produce fuel)has gradually developed into the most promising biomass conversion means.However,due to the "resistance" formed by the multidimensional structure of woody biomass,chemical pretreatment has become a necessary pretreatment alternative to improve the accessibility of enzymes(cellulase)to cellulose.Because of the efficient removal of non-cellulosic components(hemicellulose and lignin),organic solvent and hydrotropy agent system pretreatment has become one of the most effective pretreatment methods.Thus in this thesis,with the aim of developing more economical and efficient pretreatment methods,we studied the effects of organic solvent pretreatment from three aspects:the removal efficiency of three major components of woody biomass,the dissolution kinetics of components,and finally,the physicochemical properties of solid substrates and the hydrolysis efficiency of cellulase in solid matrix.Firstly,using bamboo as raw material,taking acid-free ethanol-water(EW)pretreatment as an example,the effects of solvent composition,pretreatment temperature and time on the removal kinetics of the components were analyzed.In the acid-free catalytic EW pretreatment process,the formation of xylan,xylooligosaccharide and xylose were consistent with the pseudo first-order kinetic equation.The kinetic equation for the formation of xylan dissolution,xylooligosaccharides and xylose can be expressed as:Rx(%)=100dx[1-exp(-k1t)];(?);(?)(?).The introduction of dx can significantly improve the accuracy of the pseudo first-order kinetic model for predicting Rx,Cxy-olig and Cxy-Mono.The square of the correlation coefficient between the experimental measured value and the model predictive value is greater than 99%.Dx is a composite function of xylan dissolution activation energy,pretreatment temperature and ethanol concentration in pretreatment liquid,and its value is related to pretreatment temperature and pretreatment liquid.By data fitting,glucan removal rate,acetic acid and furfural concentration can be accurately predicted based on xylan removal rate.Secondly,during acid-catalyzed organic solvents microwave pretreatment of bamboo,in addition to the pretreatment temperature and acid dosage,the Hildebrand solubility parameter(δ)of the mixed solvent is the most important factor affecting the component removal efficiency.Compared to other solvent systems(water,glycerol-water,dimethyl sulfoxide-water,toluene-water and n-pentane-water),the δ value of n-pentanol(80%,v/v)-water system is 26.9(J/cm3)-1/2 is the closest to the range of 8 value of lignin which was reported as 25.6～29.9(J/cm3)-1/2.Under the optimal pretreatment condition(130℃、20min、4%H2SO4(w/w),the hemicellulose and lignin removal of n-pentanol(80%,v/v)-water system pretreatment were 96.4%and 88.1%respectively,which were 2 and 6 times of those in water pretreatment,and also far higher than other pretreatment systems((hemicellulose removal 1.2～1.9 times,lignin removal 1.2～11.9 times).When cellulase and cellobiase addition are 20 FPU/g glucan and 30 CBU/g glucan,respectively,the enzymatic hydrolysis conversion rate of cellulose(SED)and glucose yield(GY)in the solid matrix obtained from n-pentanol(80%,v/v)-water system pretreatment were as high as 96.9%and 87.5%,respectively,which were six times higher than those in the water pretreatment system,and also significantly higher than other solvent systems(SED 1.4～3 times and GY 1.3～2.8 times).Under the optimized pretreatment conditions,the porosity and specific surface area of the solid matrix produced by n-pentanol(80%,v/v)-water pretreatment are significantly higher than other solvent systems,but its Slig which is the dominant factor affecting cellulose SED is lower than other solvent systems.Meanwhile,in this thesis,the hydrotropy system pretreatment is further developed based on the conventional organic solvent pretreatment.Under the same pretreatment conditions(95 ℃,30 min,H+5.99 mol/L),lignin and hemicellulose removal of benzenesulfonic acid(PSA,80%,w/w)-water(W)system were 92.0%and 90.7%,respectively,which were significantly higher than those of W and ethanol(ET)(80%,w/w)-W system.When cellulase and cellobiase addition were 10FPU/g glucan and 20 CBU/g glucan,respectively,the SED of the solid matrix obtained by PSA(80%,w/w)-W system was the highest,reaching 78.0%,which is 1.7,1.9,2.4,3.3 and 11.4 times of that of PTL(80%,w/w)-W,ET(80%,w/w)-W,GLY(80%,w/w)-W,DMSO(80%,w/w)-W and W system respectively.After shielding the deposit lignin or pseudo lignin on the surface of solid substrates by BSA(80mg/g glucan),the SED and GY of the solid matrix pretreated by PSA(80%,w/w)-W system were increased to 91.6%and 79.2%respectively compared to 34.6%of SED of the matrix that without BSA addition.The SED and GY of solid matrix obtained from PSA(80%,w/w)-W system and added with BSA were 2 times high than those of other solvent systems。Characterized by SEM,porosity,CP MAS 13C NMR and XRD of the solid substrate,the porosity derived from the removal of non-cellulosic components and the amount of surface lignin deposition are confirmed to be the dominant factors affecting the hydrolysis efficiency of solid matrix enzymes obtained by pretreatment of organic solvents and hydrotropy systems.In general,the PSA(80%,w/w)-W system is a more economical and efficient pretreatment method compared to conventional solvent pretreatment,it inhibits the re-deposition of dissolved lignin,increases the lignin removal which indirectly promotes the removal of hemicellulose,the two former factors synergistically promote the increase of effective porosity(high porosity,but less lignin de-deposition),and ultimately significantly enhance the SED and GY of solid matrix.