Acetic Acid Prehydrolysis,Enzyme Hydrolysis Of Lignocellulose For Xylooligosaccharides And Fermentable Sugars Production

In this thesis,xylooligosaccharides(XOS)were prepared by selective hydrolysis of the xylan and agricultural straw,using acetic acid that was very mild and fully recyclable.The optimal acidic hydrolysis conditions of xylan from viscose fiber processing and alkali-extracted corncob,corncob,corn straw and wheat straw were investigated,respectively.Next,enzymatic hydrolysis of the corncob pretreated with acetic acid was evaluated.The separation and purification technology of XOS was examined as well by using vacuum evaporation and nanofiltration to refine of XOS and utilize of lignocellulose efficiently and comprehensively.The main results were as follows:A maximum XOS yield of 45.86% was achieved on hydrolysis of 33.77 g/L waste xylan from viscose fiber processing using 20% acetic acid for 20 min at 140 ?.39.70% XOS yield could be obtained when 30 g/L of alkali-extracted corncob xylan was treated with 10% acetic acid at 140 °C for 30 min.Reaction temperature was found to be the biggest influencing factor on the hydrolysis of waste xylan,whereas reaction time and acetic acid concentration were secondary factors.Corncob was directly hydrolyzed for 30 min at 150 °C using 20% acetic acid,more than 42% XOS yields,corresponding to 12% based on corncob,were achieved at the solid loading of 1:3.5-7,with relatively few byproducts.This is the highest level in the current field.In contrast,the XOS yields of wheat stalk and corn stalk reached 36.60% and 25.08%,corresponding to 8.78% and 5.85% based on dry material,respectively,and the byproducts were somewhat high.HPAEC-PAD analysis of the component profiles obtained from xylan and agricultural straw indicated that the main components in the XOS product were xylobiose,xylotriose,and xylotetraose,followed by xylopentaose and xylohexaose.In fact,small amounts of presumably xyloheptaose and xylooctaose components,which could not be quantified,were obtained in the product.In the case of enzymatic hydrolysis,XOS,mainly with degree of polymerization(DP)in the range of 2-3,was obtained.The quantities of XOS found in acetic acid prehydrolyzate were higher compared to sulfuric acid and hydrochloric acid prehydrolyzates at the same pH.And acetic acid pretreatment can substantially increase the cellulolytic digestibility of corncob.Larger surface areas and significant micropore quantities on the cellulosic fibers in acetic acid pretreated corncob may offer more surfaces for action by cellulases,improving fermentable sugar yield from enzymatic hydrolysis.More than 91% conversion of cellulose was achieved in a batch-hydrolysis using only a cellulase loading of 20 FPU/g cellulose and even a high solid loading of 20% for 72 h.This is the highest level in the area of cellulase hydrolysis.Efficient bioconversion of hemicellulose and cellulose in agricultural straw to xylooligosaccharides and fermentable sugars could be achieved by combined acetic acid prehydrolysis and enzyme hydrolysis.After acetic acid hydrolysis at pH 2.70 and 150 ? for 30 min,1 kg corncob produced 139.8 g of XOS,and the solid residue was next enzymatically-digested into 328.1 g glucose,25.1 g cellobiose,and 73.1 g xylose by using an enzyme dosage of 15 FPU/g cellulose after 72 h of hydrolysis.The yields of fermentable sugars and XOS obtained from cellulose and hemicellulose reached 96.1% and 45.9%,respectively.When the concentration multiple was up to 10,a maximum recovery rate of 97.3% was obtained by vacuum evaporation at 0.01 MPa and 70 ?.Then using nanofiltration can recycle almost 100% acetic acid and effectively remove formic acid,furfural,HMF and more than 75.0% xylose from XOS.