【作者】 李明飞；

【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， 2012， 博士

【摘要】 面对资源短缺和能源紧缺所带来的严峻挑战,木质纤维素原料制备生物基材料、能源以及化学品的研究日益受到研究者的重视。基于生物炼制的木质纤维素组分分离,可实现木质纤维素资源的高效利用。本研究采用温和条件分离竹材主要组分,并对竹材进行了苄基化改性。球磨后竹粉采用超声辅助乙醇处理、低温氢氧化钠／尿素处理、乙醇结合二氧六环抽提以及二甲亚砜溶剂抽提,分离得到乙醇溶出组分、木质素、溶出聚糖和纤维素组分。采用有机酸在常压下将竹材分离为纤维素组分、降解碳水化合物以及木质素。分别在氯化锂/二甲亚砜体系和氢氧化钠／尿素体系中实现了竹材苄基化改性。采用氢氧化钠/尿素体系预处理结合有机溶剂连续抽提方法分离竹材,发现：(1)超声辅助乙醇抽提过程中,随着超声时间的延长,乙醇溶出组分得率增加,木质素含量提高。所得木质素主要由G、S和H型单元组成。溶出木质素的H型单元含量较高,随着超声时间的延长,G型和S型单元的相对含量增加。(2)低温氢氧化钠／尿素预处理后竹粉采用二氧六环和乙醇抽提,所得木质素主要为β-O-4’连接。木质素中的酯键在低温碱性溶液中被破坏。采用二甲亚砜抽提所得聚糖组分,主要单糖为葡萄糖(50%-55%),其次为木糖(41%-44%)。核磁共振波谱分析表明聚糖组分主要由α-(1→4)-D-葡聚糖(淀粉)和含有取代基的β-(1→4)-D-聚木糖组成。(3)球磨竹粉经过连续处理,所得纤维素组分得率为75.1%-77.7%。对比纤维素组分以及球磨竹粉,发现连续预处理导致部分半纤维素类聚糖和木质素溶出,所得纤维素组分的总糖含量高于原本球磨样品。处理后得到的纤维素组分结构更加松散,并且具有较大的表面积。这表明低温氢氧化钠／尿素溶液有效地破坏了竹材的水解障碍,生成具有高反应活性的纤维素原料,可用于酶水解生产生物乙醇。采用有机酸在常压条件下分离竹材的研究表明：(1)乙酸常压分离竹材的适宜条件为乙酸浓度90%,温度114℃,盐酸用量4.0%,处理时间2 h。原料中木质素的脱出率达到95.28%,所得纤维素组分的得率为47.55%(残留木质素的含量为2.11%)。与磨木木质素相比,乙酸木质素杂质含量低(碳水化合物含量2.48%-4.56%)。乙酸木质素的重均分子量为4870～5210g/mol,低于磨木木质素的一半。由于乙酸木质素在抽提过程中连接键断裂形成大量游离酚羟基,乙酸木质素具有较强的抗氧化性。(2)以乙酸／甲酸／水为溶剂,采用微波辅助加热可有效提取竹材木质素。升高温度能够显著提高木质素的得率。对抽提后木质素进行结构表征,发现木质素单元之间的醚键(主要为β-O-4’连接)大量断裂。此外,随着微波加热强度的增加,木质素中碳水化合物含量降低,木质素分子量降低。微波辅助有机酸分离竹材木质素的优化条件为：微波加热处理温度109℃,时间60 min,木质素得率为17.98%。所得木质素的糖含量为1.81%,重均分子量为6070g/mol,游离基捕获指数为1.15,高于BHT(0.29)。(3)甲酸及添加催化剂的甲酸体系可用于竹材的制浆,获得纸浆、木质素和降解碳水化合物。竹材甲酸木质素可采用连续溶剂分级方法分离,所用溶剂为乙醚、乙酸乙酯、甲醇、丙酮和二氧六环/水。采用乙酸乙酯分级所得木质素的抗氧化性较高,可用做稳定剂以防止高分子材料老化。热稳定性分析表明,使用木质素制备木质素基共混材料温度应低于170℃。苄基化改性发现：(1)在氯化锂/二甲亚砜溶剂体系中,竹粉可不经丝光化处理直接进行苄基化。较之传统浓氢氧化钠溶液体系(改性温度100℃以上),该体系中苄基化可在较低温度条件下进行。由于无定形组分的降解,苄基化后样品的结晶度略有降低,热稳定性有所提高,但仍低于纯纤维素。(2)在7%氢氧化钠/12%尿素体系中实现了竹粉苄基化改性。较之传统浓氢氧化钠体系中的改性,本研究的改性温度较低。适宜条件为：氯化苄对竹粉羟基的摩尔比3,反应温度80℃,时间2 h。原本球磨竹粉在苄基化改性后结晶结构发生了破坏。改性所得苄基化竹粉含有较大非极性基团且结晶度较低,可用于制备可降解生物复合材料。(3)采用氢氧化钠/尿素体系溶解纤维素,并合成了苄基纤维素。与传统方法合成的苄基纤维素相比,在氢氧化钠/尿素体系中合成的苄基纤维素,即使取代度较低(取代度≤0.51),也能溶解于常见的有机溶剂。综上所述,本研究采用基于氢氧化钠/尿素预处理的有机溶剂抽提,实现了温和条件下木质纤维素主要组分的有效分离。率先提出了一种以有机酸为分离介质的高效清洁分离木质纤维素原料新方法,并系统研究了所得木质素结构与抗氧化性。采用氯化锂/二甲亚砜溶剂体系和低温氢氧化钠/尿素溶剂体系,首次实现了木质纤维素温和条件下苄基化改性和纤维素直接溶解后苄基化改性。温和条件下木质纤维素组分分离与化学改性,为木质纤维素在替代石油化学品、生物能源和生物基材料等领域的应用奠定了基础。该博士研究生以第1作者身份发表SCI收录期刊论文10篇(其中JCR一区和二区论文共4篇),目前影响因子总和22.8；被国外专家邀请参与编写英文专著2部。更多还原

【Abstract】 In response to the severe challenges of resource scarcity and energy shortage, the preparation of biobased materials, energy, and chemicals has attracted increasing attention. Fractionation of lignocelluloses based on the concept of biorefinery technology is of vital importance for their effective utilization. In this study, bamboo utilization was investigated with fractionation (sequential treatments and organic acid extraction) and benzylation under mild conditions. The sequential treatments involved ultrasonic irradiation extraction in ethanol, cold NaOH/urea pretreatment, ethanol/dioxane extraction, and DMSO extraction, yielding ethanol-soluble fractions, lignin, soluble polysaccharides, and cellulose fractions. In addition, bamboo was fractionated into cellulosic pulp, degraded carbohydrates, and lignin with organic acid extractions under atmospheric pressure. Benzylation of bamboo was carried out in two systems, LiCl/DMSO and NaOH/urea.Investigation of the sequential treatments of ball-milled bamboo based on 7% NaOH/12% urea pretreatment and successive organic solvent extraction indicated that:(1) There was a slight increase of the yield and the molecular weight of the lignin with the increase of the ultrasonic irradiation time. Lignin was mainly composed of guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) type units. Lignin rich in H unit was preferentially released in ethanol solution, whereas an increase of the ultrasonic irradiation time resulted in a slight increase of the dissolution of lignin with G and S units.(2) The lignin isolated with dioxane and ethanol from the cold NaOH/urea pretreated bamboo had a major proportion ofβ-O-4’ ether linkages. In addition, the ester groups of lignin were largely cleaved in the cold alkaline solution. For the polysaccharide fractions obtained by DMSO extraction, glucose (50-55%) was the major sugar component, and xylose (41-44%) was the second major sugar component in all cases. NMR spectral analysis suggested that the polysaccharide fractions were mainly composed of (1,4)-linkedα-D-glucan from amylose and (1,4)-linkedβ-D-xylan attached with minor amounts of branched sugars from hemicelluloses.(3) The ball-milled bamboo was subjected to successive pretreatments to obtain cellulose fractions with yields of 75.1-77.7%. The comparative characterization of the cellulose fractions and the original ball-milled bamboo showed that the successive pretreatments resulted in partial removal of hemicelluloses and lignin. The total sugar content in the cellulose fractions was higher than that in the original ball-milled sample. The characteristics of the obtained cellulose fractions suggested that the cold sodium hydroxide/urea based pretreatments effectively disrupted the recalcitrance of bamboo, generating highly reactive cellulosic materials for enzymatic hydrolysis to produce bioethanol.The organic acid extraction under atmospheric pressure indicated that:(1) Under the optimal conditions for acetic acid extraction, i.e.,4.0% HCl dosage in 90% aqueous acetic acid at 114℃,95.28% of the lignin in the raw material was released accompanying a cellulosic pulp yield of 47.55% and the residual lignin content in the pulp was 2.11%. Compared to milled wood lignin (MWL), acetic acid lignins had lower impurities (carbohydrates 2.48-4.56%). Acetic acid lignins had Mw ranging from 4 870 g/mol to 5 210 g/mol, less than half of that of MWL. The stronger antioxidant activity of acetic acid lignins observed was attributed to a large reduction of aliphatic hydroxyl groups together with a significant increase of free phenolic hydroxyl groups.(2) Bamboo lignin was successfully extracted by microwave heating in acetic acid/formic acid/water system. An increase of the extraction temperature markedly increased the yield of the extracted lignin. Structural investigation of the extracted lignin indicated that there was an increase of the phenolic hydroxyl content in the lignin due to the cleavage of the linkages between lignin units (mainlyβ-O-4’linkages). In addition, an increase of the severity of the microwave-assisted extraction resulted in a decrease of bound carbohydrates and a lower molecular weight of the lignin. Under the optimal conditions, i.e., microwave-assisted extraction at 109℃for 60 min, a lignin yield of 17.98% was achieved. The obtained lignin fraction had a low bound sugar content (1.81%) together with a low molecular weight (Mw 6 070 g/mol), and showed good antioxidant activity (RSI 1.15), which was higher than that of BHT (0.29).(3) Aqueous formic acid with and without the addition of reagents (HCl or H2O2) was successfully applied to separate pulp, lignin, and degraded carbohydrates from bamboo. Bamboo organosolv lignin was fractionated into five preparations by sequential solvent extractions with ether, ethyl acetate, methanol, acetone, and dioxane/water. The lignin fraction extracted with ethyl acetate had a high RSI value, making it a promising stabilizer material to prevent polymer aging. Thermal stability investigation indicated that the working temperature for the preparation of thermoplastics and organosolv lignin based polymer blends should be lower than 170℃.Benzylation of bamboo suggested that:(1) Benzylated bamboo can be synthesized in LiCl/DMSO solution without mercerization via concentrated NaOH solution. The preferential mild reaction conditions suggested that benzylation of lignocellulose could be achieved in nonaqueous solution at a relatively low temperature. The highly benzylated bamboo had a porous surface with small irregular slices ascribed to the disruption of intermolecular and intramolecular hydrogen bonds. The crystallinity of bamboo decreased negligibly after modification due to the degradation of amorphous components, and the thermal stability slightly increased but was still lower than that of cellulose.(2) Bamboo was successfully benzylated in 7% NaOH/12% urea aqueous solution under various reaction conditions. It was found that benzylation can be accomplished at a relatively lower temperature, compared to the conventional benzylation in concentrated alkali. The optimal conditions were found to be the molar ratio of benzyl chloride to OH group in bamboo 3, reaction temperature 80℃, and reaction time 2 h. It was found that the crystalline structure of the native ball-milled bamboo was damaged after benzylation modification. The benzylated bamboo obtained had a low crystallinity as well as large non-polar groups and can be used for the preparation of biobased composite material.(3) Benzyl cellulose was successfully synthesized starting from cold NaOH/urea aqueous solution. Compared to the benzyl cellulose synthesized in heterogeneous systems, the benzyl cellulose product with a low DS (≤0.51) obtained in the homogeneous system in the present study dissolved in many organic solvents.In conclusion, a mild and effective protocol based on the NaOH/urea pretreatment and organic solvent extraction was proposed for the fractionation of the major components of lignocelluloses. For the first time, a clean and effective method using organic acids as separation media was put forward to fractionate lignocelluloses, and the relationship between lignin structure and its antioxidant capacity was investigated. Bamboo was initially benzylated with LiCl/DMSO and NaOH/urea systems under mild conditions, and cellulose was successfully benzylated after dissolution in cold NaOH/urea system. The mild fractionation and modification of lignocelluloses lay the foundation of their potential application in the fields of chemicals, energy, and materials.The doctor candidate has published 10 papers in SCI international journals as first author, among which 4 papers are published in journals of JCR 1 and 2. The total impact factor is 22.8 at present. In addition, the candidate has been invited and joined for writing 2 books published in English.更多还原