Effects Of Acetic Acid On Lignin Structures And Enzymatic Hydrolysis Of Poplar

Poplar,one of the main afforestation tree species in China,is a widely used biomass material with short growth cycle and wide planting area.It has become a promising energy crop for biomass conversion and biofuel production.Abundant poplar sawdust is remained from the processing of poplar tree leading to resources waste and low utilization which can be used to produce high value-added products such as ethanol by bio-refinery to maximize the utilization of poplar resources.Acetic acid pretreatment can be used for poplar to produce xylooligosaccharide to compensate for the low utilization rate of poplar.However,the presence of lignin in poplar inhibits the efficiency of enzymatic hydrolysis,which reduces the efficiency of biomass feedstock conversion.Therefore,it is important to explore the factors affecting enzymatic hydrolysis after acetic acid pretreatment.In this study,poplar was pretreated with 5% acetic acid at 160 °C,170 °C and 180 °C,respectively.The lignin samples under various severities of acetic acid pretreatment were isolated by extensive enzymatic hydrolysis.Then,the chemical composition,structural characteristics and enzymatic hydrolysis properties of pretreated poplar and lignin samples were analyzed.The results showed that the cellulose content of poplar was increased from 48.96 to59.45% pretreated at 170 °C with 5% acetic acid.And xylan removal of acetic acid pretreated poplar was up to 90.4% at 180 °C.The crystallinity and the surface lignin content of poplar increased after pretreatment with acetic acid.And as the pretreatment temperature increased,the surface lignin content increased.However,the hydrophobicity and the maximum adsorption capacity of cellulase on poplar decreased.It was also observed that the surface of poplar was covered by lignin droplets as confirmed by SEM analysis.In addition,the hydrolysis yield of poplar increased from 11.7 to 81.3% after the ball milling.After acetic acid pretreatment,the hydrolysis yield of poplar was increased to 81.5%-84.3%.The hydrolysis yield increased when adding Tween 80,β-glucosidase or xylanase.The promotion effect was better with the addition of β-glucosidase,and the hydrolysis yield increased by4%-18%.Furthermore,the effects of lignin samples were studied.The results showed that the enzymatic hydrolysis lignin samples had strong inhibitory effect on the hydrolysis of Avicel using Celluclast 1.5L+β-glucosidase,but it was found that enzymatic hydrolysis lignin samples had no negative effect on Avicel hydrolyzed by Celluclast 1.5L,Cellic CTec 2 or Cellic CTec 2+β-glucosidase.Additionally,the adsorption properties,Zeta potential and hydrophobicity of enzymatic hydrolysis lignin samples were investigated.The cellulase Cellic CTec 2 could be absorbed by the enzymatic hydrolysis lignin samples,and the lignin isolated by treatment at 180 °C had the largest adsorption capacity on cellulase.Zeta potential and hydrophobicity might affect the adsorption of lignin samples on cellulase.The enzymatic hydrolysis lignin samples was mainly composed of syringyl(S)and guaiacyl(G)lignin units.The content of β-O-4 bond decreased from 67.3 to 26.8% because of the cleavage of β-O-4 bond,and the lignin S/G ratio increased from 1.60 to 5.96%.