Structural Elucidation Of The Main Components Of Lignocellulosic Biomass And Its Enzymatic Hydrolysis During Synergistic Pretreatment Process

Increasing concerns about global climate change and diminishing fossil resources have increased attention in the development of alternative forms of biofuels and materials from renewable resources. An alternative and sustainable source for the their productions is lignocellulose, and its fractionation based on the concept of biorefinery technology is crucial to its effective utilization. Nowadays, the most promising biorefinery is to integrate biofuel production into the process in which hemicelluloses and lignin in lignocellulose can also be converted into high value-added products. Therefore, an effective pretreatment technology is necessary to separate cellulose,hemicelluloses and lignin, and enhance enzyme digestibility of cellulose. In this paper, the effect of hydrothermal, steam explosion, and ionic liquid pretreatments on the chemical composition, structural features and the enzymatic digestibility of different lignocelluloses were comparatively studied. For comprehensive understanding of the fractionation process, the chemical structures and properties of the hemicelluloses and lignin isolated were also investigated in detail, which is helpful for the high value-added utilization of lignocelluloses. The results were summarized as follows:The effect of different solvents on the enzymatic hydrolysis of furfural residue was investigated with respect to delignification and structural features. Hot water, aqueous ethanol, sodium hydroxide, alkaline ethanol, and alkaline hydrogen peroxide solution (AHP) were selected as the delignification solvents. The structure, morphology, and enzymatic hydrolysis of the original and treated samples were comparatively studied. Meanwhile, the structural features and antioxidant activity of the isolated lignin were also investigated. Results indicated that the crystallinity index of furfural residue linearly increased with the decrease of the total lignin to cellulose ratio. After pretreatment, the compact structure of fiber was damaged to different degrees, and the pores and cracks formed on the fibril introduced a higher level of surface exposure. In all the solvents used, sodium hydroxide and AHP can remove lignin effectively, especially AHP. After AHP pretreatment, the ratio of the total lignin to cellulose content in furfural residue and the absorbance ratio of lignin to cellulose (A1508/A1057) on the sample surface were reduced from 0.99 to 0.13 and from 0.40 to 0.04, respectively. However, as compared to NaOH treatment, the processing condition of AHP was severe and the processing cost was high. Meanwhile, the severe degradation of lignin during AHP occurred and its antioxidant activity was low. Take into consideration production cost, enzymatic hydrolysis rate, recovery rate of lignin and its antioxidant activity, NaOH pretreatment has a great application prospect for biorefinery.A two-step pretreatment using hydrothermal pretreatment at various temperatures and alkali fractionation was performed on eucalyptus fiber. (1) The hydrothermal pretreatment (100-220℃) significantly degraded hemicelluloses, resulting in an increased crystallinity of the pretreated fibers. However, as the pretreatment temperature reached 240℃, partial cellulose was degraded, resulting in a reduced crystallinity of cellulose. As compared to the hydrothermal pretreatment alone, a combination of hydrothermal and alkali treatments significantly removed hemicelluloses and lignin, resulting in an improved enzymatic hydrolysis of the cellulose-rich fractions. As compared with the raw fiber, the enzymatic hydrolysis rate increased 1.1 to 8.5 times as the hydrothermal pretreatment temperature increased from 100 to 240℃. Interestingly, after a combination of hydrothermal pretreatment and alkali fractionation, the enzymatic hydrolysis rate increased 3.7 to 9.2 times. Taking into consideration the consumption of energy and the production of xylo-oligosaccharides and lignin, an optimum pretreatment condition was found to be hydrothermal pretreatment at 180℃ for 30 min and alkali fractionation with 2% NaOH at 90℃ for 2.5 h, in which 27.19 kg fermentable glucose,7.65 kg xylo-oligosaccharides, and 8.14 kg lignin for 100 kg of initial fiber were recovered. (2) When the hydrothermal temperature was ranged between 100 and 160℃, a higher temperature resulted in the isolation of the residual hemicelluloses with less branches. However, when the hydrothermal temperature was 180℃, a longer time led to the hemicelluloses with more branches. (3) The hydrothermal pretreatment facilitated the separation of alkali lignin from the pretreated fibers. It was found that the linkages of β-O-4’,β-β’, and β-5’ decreased gradually with the increase of hydrothermal severity. Furthermore, decreased molecular weights, associated carbohydrate contents and aliphatic OH contents, and increased phenolic OH contents and thermal stability of the alkali lignins were observed with the increase of the hydrothermal severity.An environmentally friendly steam explosion process (SEP) of dried and water-immersed bamboo chips, followed by alkali and alkaline ethanol delignification, was developed to improve the enzymatic digestibility for an efficient bioethanol production. (1) It was found that the SEP alone significantly removed partial hemicelluloses, while the synergistic treatment by SEP and alkaline delignification removed most hemicelluloses and lignin. Results from enzymatic hydrolysis showed that SEP alone improved the enzymatic hydrolysis rate by 7.9-33.1%, while the synergistic treatment by SEP and alkaline delignification enhanced the rate by 45.7-63.9%. (2) Hemicelluloses extracted with alkali had relatively lower degree of branches than those extracted with alkaline ethanol. For the water-immersed samples, the alkali soluble hemicelluloses had relatively higher molecular weight than the alkaline ethanol hemicelluloses. Furthermore, an increment of incubation pressure resulted in a decreased thermal stability of hemicelluloses obtained from water-immersed sample. (3) After steam explosion the lignins isolated showed relatively low carbohydrate contents (0.55-1.76%) and molecular weights (780-1050 g/mol). For each steam-exploded sample, alkali-extracted lignins presented higher phenolic OH values, p-coumaric acid to ferulic acid ratios, and syringyl to guaiacyl ratios than those from alkaline ethanol-extracted lignins. The lignins isolated consisted mainly of β-O-4’ linkages combined with small amounts of β-β’, β-5’, and a-O-4’/β-O-4’linkages. In addition, the isolated lignins from the steam exploded bamboo had the high antioxidant activity, and the alkali-extracted lignins obtained in the initial extraction process had higher antioxidant activity than alkaline ethanol-extracted lignins obtained during the second extraction process.Corncob was submitted to pretreatments with 1-ethyl-3-methylimadazolium acetate (EMIMAc) and water/organic solvents (DMSO, DMF, and DMAc) followed by extraction with alkali to isolate hemicelluloses and lignin. The hemicelluloses obtained demonstrated higher thermal stability as compared to the hemicelluloses without pretreatment. Sugar analysis and structural characterization indicated that all the hemicelluloses had similar structures, which are composed of a (1â†'4)-linked β-D-xylopyranosyl backbone substituted with arabinofuranosyls attached to C-2 and C-3 and with 4-O-methyl-a-D-glucuronic acid also linked to C-2. (2) Lignin fractions isolated with EMIMAc and water/organic solvents had higher yields (4.04-9.78%, based on the dried material) than the alkaline lignin fraction (2.75%) without pretreatment. Especially, a maximum yield of 85.04%(based on the original lignin) was achieved for LDMSO fraction obtained by pretreatment with the EMIMAc/DMSO. As compared to MWL, lignins isolated with EMIMAc/organic solvents had similar molecular weights (Mw,2050-2430 g/mol) and higher purities (i.e. carbohydrates content 0.48-1.40%), while lignins obtained with EMIMAc/water had a higher molecular weight and lower purity. Based on the results, the structures of the hemicelluloses and lignin isolated with the EMIMAc/DMSO pretreatment were not destroyed, and their yields and purities were high, thereby they can served as raw materials for the production of value-added products.