Pretreatment Of Triploid Populus Tomentosa Carr. For The Enhancement Of Cellulose Bioconversion

Lignocellulosic materials have been considered as a potential sustainable source of sugars for biofuels, biochemicals and other biomaterials. However, the intricate structure of plant cell wall, the high ordered cellulose structure, and the seals of hemicellulose and lignin contribute to the recalcitrance of lignocellulosic materials and respond for the high cost of lignocellulose conversion. In this paper, the effect of dilute acid pretreatment, white-rot fungal pretreatment, ionic liquid pretreatment on the structural changes and the enzymatic digestibility of triploid poplar cellulose were comparatively studied. The role of hemicelluloses on the cellulose digestibility was also investigated. In addition, xylan preparation of triploid poplar was also converted to xylooligosaccharides (XOS) by xylanase for the integrational ulitization of lignocellulosic materials. The results of this study were summarized as follows:1. Pretreatment of triploid poplar with0.5wt%H2SO4were conducted at various temperatures (100-200℃), and the effect of temperature on cellulose structure and enzymatic digestibility was evaluated. The results indicated that79.9%of the hemicellulosic polymers were degraded at130℃. With the increasing of pretreatment temperature, the mono-sugars in the hydrolyzete were further degraded to formic, acetic and levulic acids, furfural and5-hydroxymethyl furfural (5-HMF). The content of residual lignin increased from20.8%to50.4%. Taking both of feedstock recovery and cellulose digestibility into considerateon, the optimum pretreatment temperature was found to be120℃. In this case,84.4%lignocellulose was recovered and achieved60.8%of cellulose conversion via enzymatic hydrolysis. Further fractation of the residue from dilute acid pretreatment with1.5%NaOH-70%aqueous ethanol solution increased the bioconversion of cellulose to69.8%. Alkaline fractionation also recovered17%-30%Klason lignin from the dilute acid pretreated residues. However, dilute acid pretreatment led to the degradation of β-O-4bonds and condensation of lignin units.2. Pretreatment of triploid poplar with white-rot fungus Trametes velutina D10149(T. velutina D10149) and1.0%(w/v) NaOH-70%(v/v) ethanol aqueous were proposed to reduce the recalcitrance of lignocelluloses. The results suggeseted that syringly type lignin substrate was more susceptible to white-rot fungus for degradation than guaiacyl type. The effect of biologic pretreatment was enhanced by the subsequent alkaline fractionation. Incultivation with Trametes velutina D10149for8weeks and subsequent alkaline fractionation could effectively facilitated the carbohydrate availability and achieved the maximum cellulose enzymatic digestibility as63.0%. After the stepwise pretreatment, the BET surface area of lignocellulose increased from1.7m2/g to10.7m2/g.3. AmimCl, BmimCl and EmimAc were used to dissolve ball-milled triploid poplar, and the cellulose-rich preparations were subsequently recovered via incubation in5%NaOH aqueous solution and filtration. After pretreatment, the recoveries of cellulose were70.8%,71.1%and73.4%, respectively. Dissolution with these three ionic liquids also decreased the molecular weight of cellulose from317000g/mol to214000g/mol,258000g/mol and229000g/mol, respectively. After reconstitution, the digestibility of the three cellulosic preparations was all improved, yielding87%cellulose conversion, which were1.3-fold times as that without IL pretreatment (67.2%for5%NaOH pretreated sample).4. The enzymatic digestibilities of poplar cellulosic preparations with different contents of hemicelluloses were investigated. The results indicated that cellulose digestibility increased with the decreasing of hemicelluloses. The cellulose conversion increased from64.2%to85.8%as the xylan content decreased from24.7%to4.1%. Moreover, supplementation of xylanase can hydrolysis xylan and improve the cellulose conversion in return. The result indicated that5IU/g xylans additional xylanase supplementation was effective for enhancing cellulose digestibility, increasing the cellulose conversion from50.0%to59.5%.5. The production of xylooligosaccharides (XOS) from lignocellulosic material was performed by combined chemical-enzymatic methods. Xylans were prepared from triploid Populas tomentosa, and were conversed into XOS by crude xylanase solution obtained from Pichia stipitis. Under the optimal condition of the xylanase (25IU/g substrate and pH5.4at50℃),36.8%of the xylans preparation were converted to XOS after14h. Xylobiose, xylotriose and xylotetrose were analyzed to be the main products of the enzyme hydrolysis, which accounted98.0%of the released oligosaccharides. In addition, pretreatment of the xylans preparation with sonication in1%NaOH for30min increased the xylans conversion to43.8%. XOS with DP2-4were also the main products of the enzymatic hydrolysis, accounting98.0%of the total XOS.