Structural Changes And Deposition Of Poplar Wood Lignin During Acetic Acid-sodium Acetate Buffer Pretreatment

In the process of biorefinery based on pulp and paper platform,pretreating the lignocellulosic materials to extract hemicellulose is an important reaction unit for the utilization of all components of the lignocellulosic materials,playing a key role in the subsequent pulping process.Prehydrolysis is an effective method for the extraction of oligosaccharides from the lignocellulosic materials.However,during the prehydrolysis of high intensity,the dissolved sugars will further degrade into monosaccharide,furfural and other small molecules,and the condensation reaction will take place betwwen the lignin units or between the lignin and carbohydrate degraded by-products,which will not only reduce the saccharide yield,but also have adverse effect on the subsequent process.Since the degradation,condensation and deposition behavior of the fiber cell wall components is closely related with the pH,the acetic acid/sodium acetate buffer solution was used as the medium to prehydrolyze the poplar chips in order to improve the efficiency of hemicellulose extraction and prevent excessive degradation of oligosaccharides and condensation reaction of lignin.The results showed that the acetic acid/sodium acetate buffer solution could maintain the pH value of the prehydrolysis system in a certain range,affecting the degradation and dissolution of hemicellulose and lignin and also the exist form of the dissloved substances in the the liquid phase.The dissloved substances in the the hydrolysate mainly existed in the form of DS.Approximately 20%of lignin aggregated into small particles with the oligosaccharides or existed in colloidal form.During the prehydrolysis process,the conductivity and particle size of the hydrolysate increased gradually with the holding time increased,while the turbidity and zeta potential?absolute value?of the hydrolysate firstly increased and then decreased.The conductivity of the hydrolysate of the acetic acid/sodium acetate buffer system was higher than that of the autohydrolysis system.Moreover,under the condition of high intensity prehydrolysis,the decrement of the turbidity of the acetic acid/sodium acetate sysetm was lower than that of the autohydrolysis system.During the process of prehydrolysis,there was a kind of micro spherical substances deposited onto the surface of the wood chips,which mainly came from the condensation products of lignin units and lignin with furfural,5-hydroxymethyl furfural,et al.The content of lignin on the surface of the wood chips increased.With the holding time increased,the amount of surface sediments increased,which turned into large flakes from small microspheres.The decrease of temperature during the solid-liquid separation was the main cause of the deposition of hydrophobic substances on the surface of wood chips.When the temperature of solid-liquid separation cool down to 80?,the deposition of the hydrophobic substances onto the chip surfaces almostly completed.When the temperature continued to cool,the dissolved oligosaccharides and lignin began to aggregate into hydrophobic particles due to the reduce of their solubility in the liquid phase.And the hydrophobic substances would no longer deposit on the surface of wood chips.The extent of the deposition on the surface of the wood chips was also related to the amount of lignin migrated to the surface of the chips and also the concentration of the dissolved lignin and condensation products of lignin in the liquid phase.However,compared with the autohydrolysis system,the deposition of substances on the surface of the wood chips during pretreatment by the acetic acid/sodium acetate buffer solution was lower.During the pretreatment of poplar wood chips by acetic acid/sodium acetate buffer solution,the methoxy groups in lignin structure was partially removed and a part of?-O-4 linkages broke.The total phenolic hydroxyl and condensated phenolic hydroxyl in the lignin increased,while the aliphatic hydroxyl reduced and the ratio of syringyl OH/guaiacyl OH increased.The molecular weight of lignin increased and their polydispersity index decreased.The condensation reaction between the lignin units mainly occurred between C_?and C₅,which increased the amount of?-5 linkages in the lignin structure.However,compared with the autohydrolysis system,the condensation degree of lignin units in the acetic acid/sodium acetate buffer system was lower.