Preparation Of PH-responsive Lignin-based Surfactants And Their Applications In The Enzymatic Hydrolysis Of Lignocellulose And Cellulase Recovery

The energy crisis,environmental pollution and the greenhouse effect have driven people to look for a renewable resource that can replace petrochemical resources.Biomass energy is chemical energy obtained by plants through photosynthesis.Because of its huge reserves,renewability and the characteristics of being directly converted into chemical and liquid fuels,it has received widespread attention and research in various countries.Lignocellulose is the main form of biomass,enzymatic conversion of lignocellulose into fermentable sugars and further conversion into liquid fuels and chemicals is one of the important ways of biorefinery.Enzymatic hydrolysis of lignocellulose has the advantages of environmental friendliness,strong specificity,low equipment requirements and mild conditions,but its large-scale industrialization still has these problems:low substrate hydrolysis efficiency,high cost of cellulase and difficulty in high-value utilization of lignin.In order to solve these problems,this work creatively proposes to promote the hydrolysis of lignocellulose by modifying enzymatic hydrolysis lignin(EHL)into pH-responsive surfactants,and use them to recover cellulase.This method not only alleviates the problem of difficulty in high-value utilization of lignin,but also promotes the hydrolysis of lignocellulose and saves the amount of cellulase.At first,the anionic surfactant(Shixian sodium lignosulfonate,abbreviated as SXSL)and the cationic surfactant(cetyltrimethylammonium bromide,abbreviated as CTAB)were compounded to study its effect on the enzymatic hydrolysis of lignocellulose,which can provide a theoretical basis for the synthesis of pH-responsive lignin amphoteric surfactant(pH-LAS).The results show that the combination of SXSL and CTAB is more effective than SXSL alone to improve the enzymatic hydrolysis efficiency of lignocellulose.Through surface charge testing and adsorption experiments,it was found that CTAB can neutralize part of negative charge on the surface of SXSL,so the electrostatic repulsion among SL molecules or between SL and lignin decreased,thereby increasing the adsorption of SXSL on lignin in substrate,forming an effective steric hindrance and hydration film,reducing the non-productive adsorption of cellulase on lignin,and promoting the hydrolysis of cellulose.Then,sodium lignosulfonate quaternary ammonium salt(SLQA)was synthesized by grafting(3-chloro-2-hydroxypropyl)trimethylammonium chloride(CHPTAC)on SXSL,and effect of SLQA with different degrees of quaternization on lignocellulosic hydrolysis and cellulase adsorption was studied.As the degree of quaternization increases,the promotion of SLQA on the hydrolysis of lignocellulose gradually increased,and its adsorption on lignin film gradually increases.The lignin film adsorbed SLQA is more hydrophilic than that adsorbed SXSL,so SLQA can form a more effective steric hindrance and hydration membrane to reduce the non-productive adsorption of cellulase on lignin,thereby improving the digestibility of cellulose.After hydrolysis,SLQA can be recovered by adjusting pH,and the recovered SLQA can still promote the enzymatic hydrolysis of lignocellulose.However,because the amount of sulfonic acid groups in SLQA is much higher than the amount of quaternary ammonium groups,its acid precipitation yield is low and pH response sensitivity is not high enough,which need to be further improved.Next,CHPTAC was grafted on low-sulfonated sodium lignosulfonate(SAL)to synthesize the sensitive pH-responsive lignin carrier SAL-Nx(x is the mass percentage of CHPTAC to SAL).Among them,the isoelectric point of SAL-N28 is 2.3.When the pH of the solution drops from 5.0 to 3.0,more than 95%of SAL-N28 is precipitated from 5 m M acetate buffer,and at the same time,cellulase in the solution can be precipitated and recovered.SAL-N28 has different recovery ability for different cellulase components,it can recover most of?-glucosidase(?-GL),endoglucanase(EG)III and xylanase(Xyn)in CTec 2,and the recovery rate of exoglucanase(CBH)I and CBH II also exceeded 50%.The electrostatic effect is the main force of SAL-N28 to adsorb cellulase.During the enzymatic hydrolysis of Avicel,SAL-N28 can recycle cellulase by adjusting the pH of hydrolysate and save about 70%of cellulase.In the enzymatic hydrolysis of corn cob residue(CCR),SAL-N28 can not only increase the glucose yield from 78%to 93%,but also save 50%of cellulase.Considering that grafting a large number of anionic and cationic groups on lignin will weaken its hydrophobic interaction with cellulase,so a certain of nonionic segment was grafted on lignin to weaken its inhibition on cellulase activity and used to recover cellulase by utilizing the pH responsiveness of lignin itself.In order to facilitate the reaction control,the experiment grafted monomethoxy polyethylene glycol(m PEG)onto lignin through epichlorohydrin(ECH)to obtain L-m PEGx(x is the mass percentage of m PEG to lignin).With the increase of grafted m PEG,the inhibition of L-m PEG on cellulase activity decreases,and its promoting effect on the hydrolysis of lignocellulose increases.Compared with the lignin zwitterionic surfactant SAL-N28,the pH-responsive group in L-m PEG40 molecules is mainly a weak acid group carboxyl group,so it has a higher yield and a more sensitive pH-responsiveness.Moreover,the grafting rate of hydrophilic group in L-m PEG40 is much smaller than that of SAL-N28,so its hydrophobic effect with cellulase is stronger,and more cellulase can be recovered.During the enzymatic hydrolysis of CCR,L-m PEG40 can increase the yield of glucose by 17.2%,saving more than 60%of cellulase,and more than 96%of L-m PEG40 can be recycled.In order to further improve the ability of lignin surfactant to flocculate and precipitate cellulase,polyethylene glycol(PEG)was used to cross-link lignin to synthesize L-PEG_x-y(x is the molecular weight of PEG,y is the mass percentage of PEG to lignin)to increase the molecular weight of the surfactant.Compared with L-m PEG,L-PEG synthesized under the same conditions has a larger molecular weight.As the larger molecular weight is conducive to the formation of effective steric hindrance on the lignin,L-PEG can more effectively enhance the enzymatic hydrolysis of lignocellulose.And the larger molecular weight is beneficial to enhance the interaction force between L-PEG and cellulase during flocculation and precipitation,thereby increasing the recovery of cellulase.Adding 3 g/L of L-PEG₁₀₀₀-40 can increase the digestibility of dilute acid-treated eucalyptus with high lignin content from 38.2%to 84.5%,and save 40%of cellulase.The application of pH responsive lignin surfactants in the enzymatic hydrolysis of lignocellulose not only improves the conversion rate of cellulose,reduces the amount of cellulase,but also realizes the comprehensive utilization of lignocellulose,which is of great significance to optimize the hydrolysis process of lignocellulose.