Effects Of Green Liquor Pretreatment On Lignin And Enzymatic Hydrolysis Of Poplar

A method of green liquor (GL) pretreatment on biomass was used in this study.The effects of GL pretreatment on the chemical compositions and enzymaticsaccharification of poplar (Populus deltoids), masson pine (Pinus massoniana) andmoso bamboo (Phyllostachys) were investigated. GL is a mixture of sodium sulfideand sodium carbonate, which can be completely recovered after combustion of theblack liquor in the recovery boiler in a kraft pulp mill. GL pretreatment has beenproved to be an effective pathway to improve the enzymatic digestibility. GLpretreatment technology can reduce the pollutant emissions and costs of equipment andchemical reagent.In this work, the effect of green liquor pretreatment on the chemical compositionand enzymatic hydrolysis of poplar, masson pineand moso bamboo, were investigated.The residual lignin in GL-pretreated poplar was characterized by alkaline nitrobenzeneoxidation (NBO). To understand the structural changes in the process of GLpretreatment, cellylolytic enzyme lignin (CEL) samples isolated from poplar with andwithout GL pretreatment were comparitively characterized by chemistry andspectroscopic analysis. The results are listed as follows.During the GL pretreatment of poplar, the increasing of H-factor and totaltitratable alkaline (TTA) charge resultedin enhanced delignification, whereas anincreased degradation of carbohydrate as well, in which hemicellulose acted moresusceptible than cellulose. The sugar yield of enzymatic hydrolysis went up rapidlywith the increasing TTA charge in GL pretreatment, while the effect of differentH-factor on the sugar yield is unnoticeable. The pretreated solid recovery was75.5%(lignin removal was29.2%) under optimized conditions (TTA charge20%, sulfidity25%and H-factor400). The sugar yield of glucan, xylan and total sugar of GLpretreated poplar in enzymatic hydrolysis reached up to89.9%,65.5%and82.8%,respectively at cellulase loading of40FPU/g-cellulose.The ratio of lignin removal increased with the increasing of TTA charge in GLpretreatment of masson pine, whereas an increased degradation of carbohydrates aswell. GL-pretreated pulp was hydrolyzed by the enzyme of Cellic CTec2. The sugaryields of enzymatic hydrolysis were not to go up notablely with the increasing TTAcharge in GL pretreatment. The method of GL pretreatment was not an effective way toimprove the enzymatic hydrolysis conversion of masson pine.Lignin can be effectively removed with the increasing of TTA charge andtemperature in the GL pretreatment of moso bamboo, whereas an increased degradation of polysaccharides as well. GL-pretreated pulp was hydrolyzed by the enzyme ofCellic CTec2. The sugar yields of enzymatic hydrolysis went up with the increasingTTA charge and temperature in GL pretreatment. The pretreated solid recovery was68.3%(lignin removal was44.6%) under optimized conditions (TTA charge16%,sulfidity25%,160°C and remained1h). The sugar yield of glucan, xylan and totalsugar of GL pretreated moso bamboo in enzymatic hydrolysis reached up to64.9%,48.0%and58.1%, respectively at cellulase loading of40FPU/g-cellulose.The residual lignin in GL-pretreated poplar was characterized by NBO. Theresults illustrated that the yields of poplar total NBO products were2.61mmol/g (basedon total lignin)，and the ST/VTmolar ratio and mass fractionof uncondensation partwere2.0and45.4%, respectively. The yields of total NBO products, as well as theST/VTmolar ratio decreased with the increasing of TTA charge and H-factor in GLpretreatment process. It could be deduced that the condensation degree of residuallignin in pretreated solid increased with the lignin removal, and the syringyl lignin wasmore degradable as compared to guaiacyl type during GL pretreatment.Isolated lignin samples of GL-pretreated solids and raw material were prepared bythe method of cellulolytic enzyme lignin (CEL). The yield of GL-pretreated poplarisolated lignin was much higher than that of poplar at ball-milling time of4h. Theyields of ball-milling poplar total NBO products, as well as the ST/VTmolar ratio ofuncondensation part were decreased. The total NBO product yields and ST/VTmolarratio of GL-pretreated poplar isolated lignin decreased with the increasing of TTAcharge and H-factor. The number-average molecular weight and weight-averagemolecular weight of GL-pretreated poplar isolated lignin decreased with the increasingof TTA charge and H-factor. GL-pretreated poplar isolated lignin had a widerpolydispersity than raw materials. The FT-IR spectra of isolated lignin showed thatguaiacyl type of lignin and syringyl type of lignin decreased with the increasing of TTAcharge and H-factor. The degree of the unconjugated carbonyl breaking increased withthe increasing of TTA charge and H-factor. The raw materials isolated lignin exhibitiedthe typical UV spectrum with a maximum around at273~275nm, the maximum UVabsorbance of the GL-pretreated isolated lignin moved to the long wavelength side.The13C-NMR spectra of acetylated lignin showed that5-5′, β-5, β-β′structures werenot be broken obviously. The cinnamaldehyde type of lignin was removed, and thephenolic hydroxyl group of lignin was increased during GL pretreatment.