| The uncertainties in the continuous supply of fossil fuels from the crisis-ridden oil-rich region of the world is fast shifting focus on the need to utilize lignocellulosic biomass and develop more efficient technologies for its conversion to fuels and chemicals. Biorefineries are sustainable biomass conversion processes to make bio-based fuels and chemical products. The dissertation work is directly aimed at supporting the commercial production of biofuels and value-added products from lignocellulosic biomass. The prime objective of this study was using aqueous phase reforming, hydrothermal pretreatment and hydrothermal carbonization to recovery hemicelluloses as oligosaccharides, enhance enzymatic digestibility, manufacture binderless boards, and produce hydrochar as well as bio-oil from lignocellulosic biomass.The main effect of hot compressed water (HCW) pretreatment under isothermal conditions was the removal of hemicelluloses, which resulted in enriched cellulose and lignin content. The enzymatic saccharification of HCW pretreated solids at the severest conditions assayed at200℃resulted in an enzymatic hydrolysis yield of88%, which was improved by3.4-fold in comparison with the untreated raw material. Microscopy studies of SEM and AFM revealed that HCW pretreatment resulted in breakage of the matrix fibrous polymers network and partial defibrillation. It was evidently observed that the deposition of lignin droplets were produced during pretreatment under hot water conditions, and above140℃they can migrate out of the cell wall and redeposit as spherical droplets on the residual surfaces. These observed droplets coalesced into lignin globules of various sizes ranging from0.2to70μm in diameterTo optimize the non-isothermal hydrothermal treatment (HTT), T. ramosissima were pretreated in a batch reactor using hot compressed water. The result showed that the severity at logR02.70(TMAX190℃) produced the maximum concentrations of xylose and xylooligosaccharides in the treatment liquor (3.9and92.7g/kg, respectively) without formation of significant amounts of inhibitor products. Under these conditions, the dissolved oligosaccharides consisted mainly of xylooligosaccharides (58wt%) and the autohydrolysates had a high molecular weight of11990g/mol. On the other hand, the optiumal condition at logR03.28(TMAX220℃) reached the higher cellulose digestibility (74.4%) as compared to the untreated material (8.6%). The enhancement in cellulose digestibility was directly associated with increased surface area and pore volume. This finding demonstrates that the removal of the physicochemical barriers of hemicelluloses and lignin leads to an increase of the pore volume and surface area of the solid residue, thus improving the access of cellulase to the "open" cellulose structure. The efficient utilization of hemicelluloses will improve the process economics in a forest-based biorefinery for the production of green chemicals.In order to obtain a more comprehensive understanding of structural changes in lignin occurring under autohydrolysis conditions as a pretreatment for enzymatic hydrolysis, milled wood lignin (MWL) and alkaline lignin were isolated from untreated and pretreated Tamarix ramosissima. From the results of FT-IR, semi-quantitative of HSQC NMR, and Py-GC/MS analyses, it was found that the MWL of T. ramosissima was a typical GS4type hardwood lignin with a weight-average molecular weight of3750g/mol and a syringyl to guaiacyl (S/G) ratio of1.7. Tamarix lignin was the high predominance of β-O-4’ interunit linkages (74%), followed by resinol (β-β’,15%) and a small percentage of p-hydroxycinnamyl alcohol terminal structures (6%) and β-l’ linkages (2%), together with a low percentage (1%) of phenylcoumaran substructures. Based on quantitative13C NMR spectra of the MWLs, it was found that the main reaction responsible for the lignin degradation was the homolytic cleavage of aryl-ether bonds resulting in a reduced amount of β-O-4’ interunit linkages (38%decreased) and, as a consequence, an elevated amounts of β-β’ and β-5’linkages (27%increased). The MWL isolated from the pretreated solid residue was more condensed and had a lower molecular weight (Mw3440g/mol) than those of the untreated MWL. The alkaline lignin fractions recorved from the hydrothermal pretreated solids possessed low carbohydrate contents, lower molecular weights (Mw2690～3950g/mol) together with low proportions of β-O-4’ linkages, demonstrateing that hydrothermal pretreatment broke down the lignin-carbohydrate complex (LCC) structures and the lignin macromolecules to a certain extent. However, the lignin condensation was observed to be increased when the hydrothermal pretreatment was performed under more severe conditions. Lignin monomeric composition analyzed by Py-GC/MS indicated that HTT increased the S/G ratio from1.67(MWL) to2.64(L200).Furthermore, the research evidenced that the combination of autohydrolysis and alkaline ethanol process could potentially turn the recovered lignin fractions into value added products being in accordance with the "biorefinery" concept.To make clear the self-bonding mechanism of the binderless boards manufactured from Eucalyptus grandis, the structural characteristics of cellulolytic enzyme lignin (CEL) isolated from Eucalyptus wood, its hydrothermal pretreated fibers, and binderless boards were thoroughly investigated by chemical and spectroscopic methods. The result showed that hydrothermal pretreatment and hot pressing process could change cellulose crystalline structures by disrupting inter/intra hydrogen bonding of cellulose chains. During the hydrothermal pretreatment of Eucalyptus wood, acid-catalyzed cleavage of β-O-4’ linkages and ester bonds were the major mechanisms of lignin cleavage. This degradation pathway led to a more condensed lignin which has a high average molecular weight and more phenolic hydroxyl groups than the control. The hot pressing process resulted in the binderless boards with reduced lignin contents and decreased the glass transition temperature, thus making the lignin more accessible to the fiber surface. CEL isolated from the binderless boards showed an increased S/G ratio but a lower molecular weight than those of the untreated Eucalyptus wood and the hydrothermal pretreated fibers. Based on the finding of this study, it is suggested that the combination of hydrothermal pretreatment and hot pressing process is a good way for conditioning hardwood sawdust for the production of binderless boards. The thermal softening of lignin, rich in phenolic hydroxyl groups, and increased condensed lignin structure contributed in the self-bonding formation of lignocellulosic materials.Hydrothermal carbonization (HTC) is a novel thermochemical conversion process to convert lignocellulosic biomass into value-added products. In the last charpter of this thesis, HTC processes were studied using two different biomass feedstocks (corn stalk and T. ramosissima) at250℃for2h. The results showed that the treatment brought an increase of the higher heating values up to29.2and28.4MJ/kg for corn stalk and T. ramosissima, respectively, corresponding to an increase of66.8%and58.3%as compared to those of the raw materials. The BET surface area increased from2.3to10.8m2/g and1.0to11.3m2/g, which were4.8times and10.9times greater for the hydrochars, respectively. The resulting lignite-like solid products contained mainly lignin-like material with a high degree of aromatization and a large amount of oxygen-containing groups. Liquid products extracted with ethyl acetate were analyzed by GC-MS. It was found that the main phenol monomers of the ethyl acetate extracts from corn stalk consisted mainly of2,6-dimethoxyl, butyl2-methylpropyl ester-1,2-benzenedicarboxylic acid, and4-ethoxy-2,5-dimethoxybenzaldehyde, which ranged from C8to C16. On the other hand, the major hydrocarbons of the lignin-derived compounds of T. ramosissima were2,6-dimethoxyphenol,3-methoxy-1,2-benzenediol,p-xylene, and phenol, which were in the range of C6to C8. The concentrations of these identified phenolic compounds were in the range of3.1mg/mL to3.6mg/mL, which may be desirable feedstocks for biodiesel and chemical production. Based on these results, HTC is considered to be a potential treatment in a lignocellulosic biomass refinery. |
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