Cell Wall Deconstruction And Enzymatic Hydrolysis During Main Non-cellulosic Components Removal Of Poplar

Efficient conversion of sustainable lignocellulosic biomass to biofuels has attracted widespread attention due to the shortage of energy supply and the detrimental of fossil fuels on environment.Lignin and hemicelluloses are considered key factors that hinder the high value utilization of cellulose.Much attention has been paid to the influence of content and distribution of lignin and hemicelluloses on the biomass recalcitrance and cellulose hydrolysis.In the present study,ultrastructural and topochemical changes of poplar cell wall in the process of dilute alkali pretreatment were studied with the combination of chemical and microscopic analysis.Impact of lignin and hemicelluloses removal on enzymatic hydrolysis was also investigated to provide theoretical guidance for industrial production of biomass fuels and chemicals.The main results and conclusions are as follows:Enzymatic hydrolysis of poplar samples was enhanced with the dissolve of lignin and hemicelluloses after dilute alkali pretreatment.The results indicated that hemicelluloses were removed gradually(7.86-68.88%)while the degree of delignification changed a little(21.14-22.61%)with the increase of alkali concentration from 0.5%to 5%.Meanwhile,the cellulose crystallinity increased gradually with the removal of amorphous part(mainly lignin and hemicelluloses).Consequently,the highest cellulose conversion of 69.2%was obtained for dilute alkali treated samples which was only 15.8%for untreated sample.In conclusion,delignification enhanced the enzymatic hydrolysis efficiency when the alkali concentration was 0.5%.However,with the increase of alkali concentration,hemicellulose removal contributed a lot.The poplar cell walls were significantly deconstructed during dilute alkali pretreatment.In microscale,cell corner middle lamellar(CCML)and compound middle lamellar(CML)were loosed due to delignification,which was more contributed to the penetration of cellulase solution into the interior cell wall.Nanoscale analysis showed that the space between microfibrils decreased due to the swelling of microfibrils.The strength of cell wall was declined and the coated microfibrils were fully exposed with the removal of lignin.Whereas the linkages between the microfibrils were reduced,thus separated the interconnected microfibrils,and increased the distance between microfibrils due to hemicelluloses removal.The cell walls became deformed,especially for 5%alkali treated sample.Results from Raman spectra showed that the secondary wall was subjected to tensile forces during the dissolution of hemicelluloses and lignin,which was beneficial to the loosen of the internal cell wall,thus increased the distance between microfibrils.The mechanism of excessive removal of lignin inhibits enzymatic hydrolysis was elucidated.The results showed that cracks appeared between the cellulose microfibrils and the microfibrils were fully exposed when 86.22%of the lignin was removed with acidified sodium chlorite.Meanwhile,the porosity and specific surface area(increased from 1.871 m2/g to 2.698 m2/g)of the substrate were also increased.The compact cell wall was deconstructed,resulting in a high cellulose conversion of 68.26%.However,the cellulose conversion decreased to 55.19%when more lignin of 96.58%was removed.This is attributed to the reduction of cell wall strength and collapse of the inner skeleton structure of cell wall after the loss of supporting lignin.At the same time,the adjacent cellulose microfibrils were reaggregated by hydrogen bonds and xylan linkages on the surface of microfibrils.The synergistic interaction made the internal cell wall more compact and decreased the specific surface area to 2.583 m2/g,thus impeded the penetration of the enzymatic hydrolysate and reduced the binding sites of the microfibrillar hydrophobic surfaces,ultimately inhibited the enzymatic hydrolysis.The adsorption behavior of cellulase and structural changes of poplar samples during the process of enzymatic hydrolysis were revealed.The results showed that sample with 86.22%of lignin removal owned the strongest adsorption capacity for cellulase,which was 5-fold more than that of the untreated poplar.The adsorption behavior of cellulase conforms to adsorption-desorption-resorption which is a dynamic process.After the adsorption of the enzyme to the specific binding site and finished the hydrolysis,the enzyme desorbs into the cellulase solution subsequently,and then hunts for a new binding site to reabsorb.Enzymatic hydrolysis rate during enzymatic hydrolysis was influenced by the crystallinity changes of the substrate.It was mainly the degradation of easily accessible amorphous cellulose during the initial stage(0-4 h)resulted in the increase of crystallinity.The hydrolysis of crystalline cellulose which was proved to be difficult leaded to the decrease of enzymatic hydrolysis rate in the later period.In terms of microstructural changes,the enzyme molecules degraded cellulose microfibrils networks through both shear and penetration,resulting in the disruption of microfibrils and the generation of concave or cavities in microfibrillar networks.The mechanism of hemicelluloses removal enhances the enzymatic hydrolysis of poplar was studied.Lignin was removed firstly with acidified sodium chlorite before removal of hemicelluloses with dilute alkali at room temperature.The cellulose crystallinity increased with the chemical and crystalline structure of cellulose remained unchanged after the selective removal of hemicelluloses as concluded from the FTIR and XRD results.Cracks between the microfibrils emerged with the removal of hemicelluloses,thereby increasing the specific surface area of poplar cell wall.In addition,diminution in the diameter of the microfibrils indicated that the hemicelluloses on the surface of the microfibrils were removed.Consequently,surface of the microfibrils is sufficiently exposed to generate more sites for specific binding of enzymes,thereby promoting the hydrolysis of cellulose.