Study On Acid/Alkali-Catalyzed Hydrothermal Pretreatment And Enzymatic Hydrolysis Of Lignocellulosic Biomass

Objective:Hydrothermal pretreatment was widely considered as an effective technique to enhance enzymatic hydrolysis of lignocellulosic biomass.The main limitations of the hydrothermal pretreatment at high temperature were its high energy consumption and undesirable delignification.In the present study,acid-catalyzed hydrothermal pretreatment(AHP)and alkali-catalyzed hydrothermal pretreatment(ALHP)at mild temperature were systematically investigated to develop an efficient pretreatment strategy.The composition and microstructure of substrates in response to these pretreatments were investigated to better understand the exact roles of each pretreatment in subsequent enzymatic hydrolysis enhancement.The fermentable sugar production through the whole process from pretreatment to enzymatic hydrolysis of biomass sample were optimized based on the acid and/or alkali-catalyzed hydrothermal pretreatment,opening a promising avenue for efficient biofuel production from lignocellulosic biomass.Methods:To enhance the fermentable sugar recovery from both pretreatment and enzymatic hydrolysis,firstly we optimized the pretreatment conditions in AHP process including acid type,acid concentration and temperature and pretreatment time.Three energy crop species,including Miscanthus sinensis,Arundo donax Linn and Pennisetum alopecuroides,were chosen as the substrates.Yields of soluble sugars in the pretreatment stage(YSP),the enzymatic hydrolysis stage(YSEH)and the whole process(YST)were used as indicators of fermentable sugar recovery efficiency by different pretreatments.Then we investigated effects of ALHP with or without surfactant addition on enzymatic hydrolysis.The composition and microstructure of substrates in response to AHP and ALHP pretreatments were investigated to better understand the exact roles of each pretreatment in subsequent enzymatic hydrolysis enhancement.Considering different characteristics of AHP and ALHP at recovering sugar,a two-step AHP-ALHP process was further designed to maximize the fermentable sugar recovery.Results:(1)An AHP method has been developed for obtaining a high YSP in the pretreatment process.Despite of its low lignin removal,AHP using FeCl3 or oxalic acid was effective at removing hemicellulose components and changing microstructure of the sample,thereby enhancing the subsequent enzymatic hydrolysis to some extent.Acid-catalyzed hydrothermal pretreatment(2.5%oxalic acid or 1%FeCl3,140?,30 min)appeared to be the most efficient method for obtaining an attractive YSP of 208.2 mg/g?217.0 mg/g and relatively high YSEH.(2)An ALHP method has been developed for obtaining a significant improvement of YSEH in the enzymatic hydrolysis process.Despite of its lower hemicellulose removal,ALHP induced much higher delignification and stronger morphological changes of the biomass as well as cellulose swelling as compared to AHP.As a result,a remarkable increase of efficiency was observed in the enzymatic hydrolysis of the ALHP sample pretreated by 1.2%NaOH at 120? for 30 min.The YSEH increased to more than 400%and 200%of that observed in the untreated stalk and AHP sample,respectively.(3)A two-step AHP-ALHP process has been developed for improving both the YSP and YSEH levels.A high YSP was obtained in the first stage of AHP using FeCl3 or oxalic acid,while the AHP sample still had high lignin content.An effective delignification of the AHP sample was observed in the second ALHP stage,thereby obviously improving the YSEH in the subsequent enzymatic hydrolysis.Among all one-step and two-step process tested,the highest YST,together with a 36%decrease of cellulase consumption,was observed in the optimized condition(1st AHP:2.5%oxalic acid,140?,30 min;2nd ALHP:1.6%NaOH,1%PEG-2000,120?,30min).Conclusions:(1)Oxalic acid(or Ferric chloride)-catalyzed hydrothermal pretreatment at mild condition was effective at removing hemicellulose components and recover them from the pretreated liquid,indicating a significant improvement of YSP.At the same time,enzymatic hydrolysis of the pretreated sample was also improved to some extent.(2)While the YSP is not very attractive,the YSEH were significantly improved in enzymatic hydrolysis of the stalk pretreated by ALHP.ALHP can effectively remove lignin and hemicellulose and swell cellulose,hence reducing recalcitrance of biomass and improving the subsequent enzymatic hydrolysis.(3)A two-step AHP-ALHP process(using oxalic acid or ferric chloride in AHP),in which both the YSP and YSEH were remarkably enhanced,has been successfully developed to maximize the fermentable sugar recovery at a reduced quantity of celluase.