| There are three main steps for lignocellulose bioconversion,that is,pretreatment,enzymatic saccharification and fermentation.Among them,the high costs of pretreatment and saccharification process are the key factors hindering the industrialization of lignocellulose bioconversion.Due to the high delignification efficiency,high cellulose recovery,lack of toxicity,and ease of recovery,ethanol was frequently used as solvent for lignocellulose pretreatment.However,the pretreatment with ethanol was generally conducted under high reaction temperature for effective dissolving out lignin and achieving high saccharification efficiency,resulted in serious degradation of hemicellulose during the pretreatment,which affected the total yields of fermentable sugars and economic benefit.In addition,high enzyme dosage and low saccharification efficiency led to the high cost in subsequent enzymatic saccharification process.Based on the above background,paper proposed a new pretreatment process,KMnO4 assisted EtOH pretreatment(KMnO4+EtOH),to overcome the disadvantages of EtOH pretreatment process.At the same time,the enzyme system components used in enzymatic hydrolysis were optimized and effect of surfactants on enzymatic hydrolysis was studied to enhance enzymatic hydrolysis.Using solid residue from enzymatic saccharification as raw material,lignocellulosic nanofibrils(LCNFs)were also prepared.This study provides theoretical reference for further promoting the application of EtOH pretreatment process and production of high concentration sugar,thus help to comprehensive utilization of lignocellulose biomass.The main research contents and results of this paper were as follows:1.Effect of KMnO4 assisted EtOH pretreatment on enzymatic hydrolysis of corn stover and mechanism of improving enzymatic digestibilityA KMnO4 assisted EtOH pretreatment process was proposed to improving enzymatic digestibility of corn stover.In the process,a simple KMnO4 pretreatment was used before EtOH pretreatment.The results showed that the KMnO4 assisted EtOH pretreatment effectively reduced the degradation of hemicellulose and increased the recovery of carbohydrates,at the same time,decreased the formation of inhibitors.The enzymatic digestibility of corn stover was also effectively improved by the pretreatment under lower temperature compared to the single EtOH pretreatment.Process conditions of the KMnO4 assisted EtOH pretreatment were optimized and found that,when corn stover was firstly treated by 1%(w/v)KMnO4 at 40℃ for 60 min,then treated by 50%EtOH at 185℃ for 90 min,the recoverys of cellulose and hemicellulose reach to 94.28%and 58.45%respectively.Using commercial cellulase produced by Penicillium oxalicum to hydrolyze the pretreated corn stover,the conversion of cellulose and hemicellulose at 72 h of enzymatic hydrolysis reached 89.45%and 85.30%respectively.Further,mechanism of improvement of enzymatic hydrolysis of corn stover after the KMnO4 assisted EtOH pretreatment was investigated.It was found that,after the KMnO4 assisted EtOH pretreatment,the cell wall surface contained more carbohydrates,less lignin and extract,which increased the accessibility of enzyme to substrate,thus improving the efficiency of enzymatic saccharification.In general,compared to single EtOH pretreatment,the KMnO4 assisted EtOH pretreatment process can reduce energy consumption and hemicellulose loss while improve the total output of fermentable sugar.2.Effect and mechanism of surfactants on enzymatic saccharification of pretreated corn stoverIt is known that the addition of surfactant can improve the efficiency of saccharification during enzymatic hydrolysis of lignocellulose.However,it is necessary to search for cheap and efficient surfactants for reducing the cost of saccharification process.In this study,effects of several surfactants including nonionic surfactants AEO-23 and LAE-24,anionic surfactants SSL and cationic surfactants CTAB on the saccharification process of KMnO4 assisted EtOH pretreated corn stover(KEPCS),diluted H2SO4 pretreated corn stover(HPCS)and NaOH pretreated corn stover(NPCS)were studied.The results showed that the effect of adding surfactants on the improvement of saccharification efficiency was not only related to the type of surfactant and the properties of substrate,but also related to the dosage of surfactant,the concentration of substrate and saccharification process.Among them,nonionic surfactant AEO-23 showed better promoting effect on the saccharification of the three pretreated corn stover than tween-80,showing a good application potential.The possible mechanism of surfactant AEO-23 promoting enzymatic saccharification was further analyzed.It was found that addition of AEO-23 during enzymatic hydrolysis could reduce the unproductive adsorption of enzymes onto lignin,increase free enzyme concentration and activities of free endoglucanase and free cellobiohydrolase in hydrolysis system.From the perspective of the functional group structure and electronic effect of AEO-23 on lignin,it is speculated that negatively charged oxygen atoms in the hydrophilic polyoxyethylene((CH2CH2O)23-)of AEO-23 may linked to the methoxyl,phenylpropanyl and hydroxyl groups of lignin through various hydrogen bond bonding modes,thus increasing the hydrophilicity of lignin.The steric hindrance formed by the AEO macromolecule bound to lignin could also hinder the binding of enzymes to lignin and reduce the nonproductive adsorption of enzymes onto lignin.3.Optimization of saccharification process of pretreated corn stover by KMnO4 assisted EtOH processThe saccharification efficiency of pretreated corn stover using commercial cellulase,cellulases from three Trichoderma reesei strains and four Penicillium oxalicum strains were compared.It was found that the conversions of cellulose and hemicellulose were highest when the cellulase from Trichoderma reesei SCB18 was used to hydrolyze the pretreated corn stover compared to above other cellulases.It was also found that the cellulases from T.reesei QXC and P.oxalicum CXy13-4 could enhance hydrolysis efficiency of the cellulase from Trichoderma reesei SCB18,and the crude enzyme solution from strain SCB18,QXC and CXy13-4 respectively was mixed in the ratio of 8:2:2(based on PFU,and total dosage was 12 FPU/g-substrate)and used to hydrolyze pretreated corn stover,high sugar yield was obtained.By further adding endoglucanase from strain cEES-XM and xylanase,the saccharification efficiency of pretreated corn stover was further improved.Using fed batch saccharification process,the saccharification efficiency of substrate at high solid loading was improved,and the total fermentable sugars yield increased.The total fermentable sugar of 107.97 g/L,in which,the glucose concentration and xylose concentration was 84.47 g/L and 23.50 g/L respectively,was obtained in enzymatic saccharification stage under the conditions of 13 FPU/g-substrate of total enzyme dosage and 20%solid loading,4.Preparation of Lignocellulosic nanofibers(LCNFs)from solid residue from saccharification stepUsing the solid residues from enzymatic saccharification of KEPCS as raw materials,lignocellulosic nanofibers(LCNFs)with different morphology,diameter and thermal stability were successfully prepared.It was found that the LCNFs prepared by homogenization treatment had a wide diameter distribution and obvious heterogeneity,and there were phenomena such as serious flocculation,superposition and interweaving among nanofibers.For the solid residues with high lignin content,increasing homogenization strength can reduce the average diameter of LCNFs,improve the dispersion of LCNFs,and the diameter distribution becomes more uniform.For the solid residues with high cellulose content,however,increasing homogenization strength reduced the average diameter of LCNFs,but the improvement effect on dispersion and diameter distribution was not obvious.Ultrasonic pretreatment prior to homogenization treatment had little influence on the morphology and diameter distribution of LCNFs under experiment conditions used in the paper.However,enzymatic pretreatment before homogenization affected the morphology and diameter distribution of LCNFs,and the LCNFs with more small average diameter,more uniform diameter distribution and good dispersion performance was obtained by suitable enzymatic treatment.Addition of appropriate amount of xylanase during enzymatic pretreatment with cellulase enhanced enzymatic pretreatment effect,and improved diameter distribution uniformity,thermal stability and dispersion performance of LCNFs,and reduce the interweaving and flocculation phenomenon among LCNFs. |
PDF Full Text Request