Degradation,Migration And Dissolution Of The Cell Wall Components Of Poplar During The Conjugate Acid-alkali Pretreatment

Pretreating lignocellulosic material before pulping to extract hemicellulose from the raw materials is an important mothd to realize the effective utilization of hemicellulose.During the pretreatment process,with the degradation,migration and dissolution of carbohydrates and lignin,the chemical composition and microstructure distribution of chemical constituents may change greatly,which will have an important effect on the subsequent pulping and papermaking processes.Therefore,in order to establish the biorefinery platform,understanding the principles of the degradation,migration and dissolution of the cell wall components during pretreatment process is one of the urgent problems needing to be solved.The reactions of the cell wall components,such as hydrolysis,degradation and condensation reaction have close relation with the pH of the pretreatment system.In order to prevent the degradation of oligosaccharides to monosaccharides,and then to the small molecules like furfural,and also to suppress the condensation reaction of lignin units and lignin with furfural during hydrothermal pretreatment,the content of hydrated hydrogen ions in the pretreatment system was controlled by acetic acid-sodium acetate conjugated acid-base buffer solution.The change rules of the contents of carbohydrate and their degradation products and the content of lignin and their degradation products in the prehydrolyzate of poplar chips were studied.The dissolution law of hemicellulose and lignin during the pretreatment was analyzed.Moreover,the surface chemical composition of the poplar chips was analyzed by XPS and the surface morphology of the chips was analyzed by SEM.The results showed that the acetic acid-sodium acetate conjugated acid-base pairs could resist the acidic substances generated during the prehydrolysis of poplar,controlling the pH value of the system within a certain range,which could regulate the dissolution of the cell wall chemical components and the form of the dissloved substance in the liquid phase.In the heating-up stage of pretreatment,there was much hemicellulose dissloved into the liquid phase.After 60 min of holding time,64.6% of polypentose had been removed from the wood chips for the acetic acid-sodium acetate system of buffer ratio of 0.1.Compared with the hot water pretreatment system,the content of oligosaccharides in the prehydrolyzate of the acetic acid-sodium acetate system was higher,while the contents of monosaccharide,especially the small-molecular degradation products such as furfural and 5-hydroxymethyl furfural,were relatively lower.Additionally,as the buffer ratio of acetic acid-sodium acetate increasing,the proportion of oligosaccharides to total sugar increased.Acetic acid-sodium acetate conjugatedacid-base pairs could regulate the hydrolysis intensity of hemicellulose,prevent excessive degradation of oligosaccharides and further degradation of monosaccharides,improving the yield of sugars.Maintaining the pH of pretreatment system at a relatively lower and suitable level by acetic acid-sodium acetate buffer solution was beneficial to remove lignin.Moreover,the acetic acid-sodium acetate system with relatively higher buffer ratio could reduce the content of vanillin,phenol,clove aldehyde,p-hydroxybenzoic acid and other lignin degradation products in the prehydrolyzate.During the pretreatment,about 20% of the lignin could be removed.As the holding time prolonged,the content of acid-insoluble lignin in the poplar chips decreased while the content of acid-soluble lignin increased.After pretreatment with acetic acid-sodium acetate buffer solution at high temperature,the outer surface of the chips was covered with many micro-globular substances,which were mainly lignin migrated from the inner to the surface and the redeposited lignin substances.And the content of lignin on the surface of the pretreated wood chips increased with prolonging the holding time.After 60 minutes of holding time,the surface of the chips was almost completely covered by lignin.Moreover,after pretreatment by acetic acid-sodium acetate buffer solution,the content of ash in the poplar chips increased slightly;and the content of organic solvent extract in the chips increased with the prolonged holding time.