【作者】 肖领平；

【导师】 孙润仓；

【作者基本信息】 北京林业大学 ， 林产化学加工工程， 2014， 博士

【摘要】 石油资源的短缺,特别是近年原油价格的飙升,大大推动了利用农林生物质资源为原料的炼油厂的发展。然而由于植物细胞壁结构的复杂性,致使生物质炼制过程仍存在诸多关键障碍。为了解决这一问题并实现生物质的多组分利用,本论文主要进行了三方面的研究工作：一是采用水热技术对木质生物质进行预处理以提高纤维素酶解得率；二是采用水热预处理技术将林业废弃物制备无胶胶合板；三是采用水热碳化技术制备生物质炭和生物油。所得主要结论如下：等温水热处理红柳能有效提高纤维素酶解得率。在200℃水热预处理3h后,92h的纤维素酶解率可达88%。通过扫描电镜和原子力显微镜的观察,发现水热处理过程有效破坏红柳生物质的抗降解屏障,导致细胞壁结构变得疏松。当高温液态水温度高于140℃时,随着半纤维素的降解,木质素溶出、重聚、迁移并沉积在纤维表面形成直径0.2-70μm的微小球颗粒。非等温水热处理红柳制备低聚木糖的最优条件：预处理强度logR02.70, TMAX190℃,低聚木糖含量达到最高为92.7g/kg原料,半纤维素转化率为42%。此时预水解液中的分子量通过凝胶渗透色谱测定为11900g/mol。酶解得率的最优条件为：预处理强度logR03.58, TMAX220℃,72h的纤维素酶解得率最高为74.4%。研究发现纤维素酶解得率与半纤维素的脱除、比表面积和孔隙度的增大有直接的关系。水热预处理能有效破坏红柳纤维致密的结构,促进半纤维素的降解和部分木质素的溶解。这些物理障碍的去除增大了纤维素的比表面积和孔隙度,使得纤维底物更加“开放”,促进了纤维素酶与底物的有效接触,从而提高其水解得率。为了探索灌木红柳自水解过程中木质素结构变化机理,分别采用Bjorkman方法分离出磨木木质素(MWL)及碱木质素并同未处理原料的MWL进行对比研究。首先通过红外光谱、半定量二维HSQC核磁共振波谱和热解质谱分析鉴定了红柳木质素属于典型的GS4型阔叶木木质素,其主要键连接为p-0-4’醚键结构(74%),其次为p-β’(15%),接下来是少量的对羟基肉桂醇端基结构(6%)和螺旋二烯酮结构(2%)以及苯基香豆满结构(1%),S/G比值为1.7。通过定量碳谱和分子量测定对水热处理前后的MWL进行分析,结果发现水热预处理过程导致木质素发生均裂降解,38%的p-O-4’连接键断裂,分子量由3750g/mol减少到3440g/mol,但p-p’和p-5’等缩合型链接键则相对增加了27%。此外,分析发现分离得到的碱木质素组分糖含量和分子量相对低(Mw3950g/mol-2690g/mol),但S/G比值相对未处理的MWL增大到2.6,β-0-4’降低到57%。该结果说明水热处理过程导致木质素碳水化合物链接键(LCC结构)的断裂和半纤维素降解,同时木质素分子量降低但缩合度增大。通过对巨尾桉原料、水热处理后的浆料及胶合板的样品分离出纤维素酶解木质素(CEL),揭示了无胶巨尾桉胶合板热压自胶合机理。结果表明水热处理过程使巨尾桉纤维素结晶度升高,氢键强度增大。水热环境中酸催化条件下p-0-4’链接键和酯键的断裂是导致木质素降解的主要原因。热压过程导致无胶胶合板木质素含量减少以及玻璃化转变温度降低,从而使得木质素更容易迁移到纤维表面上。热处理过程中桉木纤维素和半纤维素糖甙键断裂及部分木质素的降解增加了活性羟基的数量,提高了木质素的自粘结作用,从而利于热压过程中形成氢键,同时增加无胶胶合板的强度。根据本研究结果,说明水热和热压处理能促使木质素发生软化、提高活性酚羟基含量并增加木质素缩合结构,从而有利于木质纤维原料发生自胶合作用。选用农林废弃物玉米秸秆和灌木红柳为原料,在高压反应釜中250℃条件下水热碳化4h,分别对固体组分和液体组分进行分析。结果表明水热碳化过程彻底破坏了生物质的细胞壁结构,纤维素和半纤维素几乎全部降解,最后得到富含碳元素的褐煤状生物质炭。这些生物质炭石墨化程度低均为无定型碳,具有核壳结构且表面富含氧官能团,包括羟基、羧基、羰基以及芳香族官能团等。玉米秸秆和红柳经过水热碳化处理后,热值分别提高了66.8%和58.3%达到29.2MJ/kg和28.4MJ/kg,且比表面积和总孔积增大4～11倍。作为生物油的水热碳化液体通过气质联用仪检测发现,其主要组分包括酮类、醛类和酚类化合物。玉米秸秆生物油主要为2,6-二甲氧基丁酯,2-甲基丙基酯基-1,2-苯二羧酸和4-乙氧基-2,5-二甲氧基苯甲醛等C8-C16芳香类化合物,而红柳生物油主要为2,6-二甲氧基苯酚,3-甲氧基-1,2-苯二酚,对-二甲苯和苯酚等C6-C8酚类化合物。这些芳香类化合物的含量约为3.1-3.6mg/mL。基于以上讨论,说明水热碳化是将生物质转化为更高能量密度形式的碳的一种有效途径,也是制备生物质炭材料和生物油的重要方法。更多还原

【Abstract】 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.更多还原