【作者】 刘超；

【导师】 李友明；

【作者基本信息】 华南理工大学 ， 制浆造纸工程， 2021， 博士

【摘要】 木质素是一种富苯环结构的生物质资源,而碱木质素（AL）作为主要的工业木质素之一,其复杂的分子结构,与普通溶剂难以形成均相体系等问题严重制约着AL的研究进展及高值化应用。利用合适的溶剂体系并结合AL自身结构特点及优势开发木质素基的纳米材料及其衍生的功能性产品,对于解决以上两大问题以及开拓木质素高值化利用新途径具有重要意义。本论文以工业碱木质素为研究对象,以“绿色溶剂”离子液体为溶剂体系,解析了木质素结构在碱性离子液体体系中的溶解行为变化,研究了离子液体体系中AL的微结构变化、聚集行为及微纳米球的制备,并以此为基础,通过不同的功能设计,探讨了碱木质素基微纳米球在吸附重金属离子和电化学领域的应用性能。主要的研究进展和内容如下:首先通过UV-vis与Hammett值法定量表征系列咪唑基离子液体的碱性强度,探究温和条件下AL在不同咪唑基离子液体（1-丁基-3-甲基咪唑醋酸盐（[Bmim][Ac]）,1-戊基-3-甲基咪唑醋酸盐（[C5mim][Ac]）,1-乙基-3-甲基咪唑醋酸盐（[Emim][Ac]）和1,3-二甲基咪唑磷酸二甲酯盐（[Mmim]DMP））中的溶解性能,解析木质素在溶解过程中的结构变化,揭示木质素含氧基团在溶解过程中的行为变化机制。结果表明:咪唑基离子液体碱性不仅与咪唑环上烷基链的长短有关,而且与阴离子的离解强弱有关;碱性较强的咪唑醋酸盐离子液体相比弱碱性[Mmim]DMP离子液体具有更优异的木质素溶解性能,溶解度可达33%w/w;木质素溶解过程中,离子液体主要攻击木质素侧链上的含氧基团及芳基醚键,其中甲氧基含量降低,β-O-4’键断裂明显,β-β’键部分降解,β-5’键相对稳定,并有不饱和键生成,说明木质素溶解过程中可能发生脱甲氧基和脱水反应;碱性离子液体溶解过程中木质素大分子降解为相对较小且均匀的结构单元,导致其相对分子量降低,但木质素的芳香环主体结构保持稳定。其次以草类碱木质素为研究对象,基于弱碱性[Mmim]DMP溶剂体系,探究了AL溶解过程中的聚集行为,通过向[Mmim]DMP-木质素溶液中滴加反向溶剂可以得到粒径在200 nm至1.5μm之间的球形木质素微纳米粒子（LM/NPs）;基于木质素溶解性更好的[Emim][Ac]溶剂体系,实现了结构可控的碱木质素纳米微球（ALNPs）的自组装制备,其粒径范围为51.6-210.8 nm,ALNPs的粒径可通过控制木质素初始浓度、搅拌速度和反向溶剂滴加速度等因素进行控制,当木质素浓度较低时,ALNPs的产率随木质素初始浓度的提高而增加。在ALNPs制备过程中,木质素的芳香环主体结构没有被破坏,ALNPs在p H值为4-10的范围内具有较高的稳定性。以不同来源AL（草类,阔叶木和针叶木碱木质素）和乙酰化碱木质素（ACAL）为对象,研究木质素结构中羟基对[Emim][Ac]离子液体中AL聚集行为的影响。结果表明:羟基对溶剂体系中木质素的聚集影响很大,不同来源AL经乙酰化后,约有85%以上的酚羟基发生了反应,其中草类碱木质素的羟基更容易被乙酰化;AL在聚集的过程中,由于分子间和分子内作用力较大,更容易聚集成粒径大的聚集体,但单分子的微球粒径较小。由于羟基的去除,ACAL的分子间和分子内作用力减小,使得ACAL的单分子构象较为伸展,不同羟基对木质素聚集行为的影响规律如下:缩合酚羟基>愈创木基和邻苯二酚羟基>脂肪族羟基。然后基于木质素在碱性离子液体溶剂体系中的聚集行为,在[Emim][Ac]溶剂体系中,通过微波辅助改性和反向溶剂两步法制备了一种羧基改性的碱木质素纳米微球（CLNPs）,对其吸附溶液中Pb（Ⅱ）重金属离子的性能进行了研究。结果表明:羧基改性后的CLNPs酚羟基明显下降,CLNPs呈均匀的球形形貌,粒径较小,平均粒径为73.9 nm,不易聚集。羧基含量（1.8 mmol/g）明显高于原碱木质素的0.95 mmol/g,其比表面积为8.63 m²/g,是原木质素的3.2倍。CLNPs对Pb（Ⅱ）的吸附容量最高可达333.26 mg/g,高于现有报道的大部分木质素基吸附剂及传统商业吸附剂。吸附动力学和吸附等温线模型表明,CLNPs对溶液中Pb（Ⅱ）的吸附符合准二级动力学模型,吸附过程为单层表面的化学吸附。另外,CLNPs表现出良好的回收性能,在连续10次吸附-解吸循环后,吸附容量仅损失27.0%。最后通过对[Mmim]DMP-木质素溶液进行预氧化和碳化处理,制备氮、磷掺杂的碱木质素基碳微球（MLCM）,并将其作为超级电容器电极材料进行电化学性能的研究。结果表明:相对于直接碳化的碱木质素碳（LC）,MLCM呈球形结构,具有更高的比表面积（938.1 m²/g）和孔体积（0.64 cm³/g）。氮、磷元素成功掺杂到MLCM中,其掺杂含量分别为2.28 at.%和1.23 at.%,杂原子的掺杂能够有效的提高法拉第效应和碳材料的导电性。MLCM材料相比于LC材料,具有更加优异的电化学性能,在1mol/L H₂SO₄电解液体系中,电流密度为0.8 A/g的条件下,其最高比电容为338.2 F/g,电流密度为10 A/g时,电容保持率可达70.2%。进一步将MLCM作为正负极材料组装成对称超级电容器研究其两电极电化学性能,在1A/g的电流密度下,比电容为194 F/g。器件在2 A/g的电流密度下冲放电5000次后仍保持优异的循环稳定性,电容保持率可达97.3%。木质素基碳微球在能量储存领域具有很好的应用前景。更多还原

【Abstract】 Lignin is a kind of biomass resource with rich benzene ring structure.As one of the main industrial lignin,alkali lignin（AL）has complex molecular structure and is difficult to form homogeneous system with common solvents,which seriously restricts the research progress and high-value utilization of AL.The development of lignin-based nano-functional materials according to the structural characteristics of AL and suitable solvent system is of great significance for solving the above problems.In this dissertation,industrial alkali lignin was used as the raw material,and the"green solvent"ionic liquids（ILs）were used as the solvent system.The dissolution behavior of lignin in basic ILs was analyzed.The microstructure changes and aggregation behavior of AL and the preparation of lignin micro/nanospheres based on ILs were investigated.The application properties of alkali lignin-based micro/nanospheres in adsorption of heavy metal ions and electrochemical field were discussed through different functional design.The detailed results were as follows:Firstly,the basic strength of the series of imidazole-based ILs was quantitatively characterized by UV-vis and Hammett value method.The solubility of AL in different imidazolium ILs（[Emim][Ac],[Bmim][Ac],[C5mim][Ac]and[Mmim]DMP）under mild conditions was investigated.The structural changes and oxygen-containing groups of lignin in the dissolution process were discussed.The results showed that the basicity of the imidazole-based ILs was not only related to the length of the alkyl chain in the imidazole unit,but also related to the dissociation strength of anions.Compared with weakly basic[Mmim]DMP ionic liquid,imidazole acetate ionic liquid with stronger basicity had higher lignin solubility,and the solubility reached 33%w/w.During the dissolution process,ILs mainly attacked the oxygen-containing groups and aryl ether bonds on the side chain of lignin,in which the content of methoxy groups decreased,theβ-O-4’bond was broken obviously,theβ-β’bond was partially degraded,theβ-5’bond was relatively stable,and unsaturated bonds were formed.These results indicated that demethylation and dehydration reactions occured during the dissolution process of lignin.The lignin macromolecules were degraded into smaller and uniform structural units,resulting in the decrease of relative molecular weight.However,the main structure of the lignin aromatic ring remained stable.Secondly,using grass alkali lignin as raw materials,the aggregation behavior of AL was investigated based on the weakly basic[Mmim]DMP solvent system.By dropping anti-solvent into the[Mmim]DMP-lignin solution,spherical lignin micro-nano particles（LM/NPs）with a particle size between 200 nm and 1.5μm were obtained;Then,based on the solvent system of[Emin][Ac]with better lignin solubility,the self-assembly of the alkali lignin nanospheres（ALNPs）with controllable structure was realized.The AL formed uniform nanospheres with a particle size range of 51.6-210.8 nm.The particle size of the ALNPs could be controlled by controlling the initial lignin concentration,stirring speed and anti-solvent drop speed.When the concentration of lignin was low,the yield of ALNPs increased with the increase of the initial concentration of lignin.During the preparation process of the ALNPs,the main structure of the aromatic ring of the lignin was stable,and the nanospheres had high stability in the p H range of 4-10.Thirdly,using different sources of AL（grass,hardwood and softwood lignin）and acetylated alkali lignin（ACAL）as raw materials,the effect of hydroxyl on the aggregation behavior of AL in[Emim][Ac]was investigated.The results showed that the hydroxyl group had a great influence on the aggregation of lignin in the solvent system.AL from different sources had been acetylated to remove more than 85%of the phenolic hydroxyl groups,and the hydroxyl groups of grass alkali lignin were more likely to be acetylated.In the process of the aggregation of AL,due to the large intermolecular and intramolecular forces,AL was easier to aggregate into larger aggregates,but the particle size of single molecule spheres was smaller.However,due to the removal of hydroxyl groups,the intermolecular and intramolecular forces of ACAL were reduced,making the single-molecule conformation of ACAL more extended.Furthermore,the effects of different hydroxyl groups on the aggregation behavior of lignin were as follows:condensed phenolic hydroxyl>guaiacyl and catechol hydroxyl>aliphatic hydroxyl.Fourthly,based on the aggregation behavior of lignin in basic ILs solvent system,carboxyl modified alkali lignin nanosphere（CLNPs）was designed by microwave-assisted modification and anti-solvent dropping in[Emim][Ac]solvent system,which could adsorb Pb（Ⅱ）ions in solution.The results showed that the phenolic hydroxyl groups of CLNPs decreased significantly.The CLNPs had a uniform spherical morphology with a small particle size and an average particle size of 73.9 nm,which was not easy to aggregate.The carboxyl content（1.8 mmol/g）of CLNPs adsorbent was higher than that of original alkali lignin（0.95mmol/g）.The specific surface area of CLNPs was 8.63 m²/g,which was about 3-fold higher than that of the original lignin.It was found that the maximum adsorption capacity of CLNPs for Pb（II）was 333.26 mg/g,which was higher than that of most lignin-based adsorbents and traditional commercial adsorbents.The adsorption kinetics and adsorption isotherm models showed that the adsorption of Pb（Ⅱ）in solution by CLNPs conformed to the quasi-second-order kinetic model,and the adsorption process was single-layer chemical adsorption.In addition,CLNPs showed an excellent recycling performance,with only 27.0%adsorption capacity loss after 10 consecutive adsorption-desorption cycles.Finally,nitrogen and phosphorus dual-doped alkali lignin-based carbon microspheres（MLCM）were prepared by pre-oxidation and carbonization of[Mmim]DMP-lignin solution,which was used as a supercapacitor electrode material.The results showed that compared with the directly carbonized alkali lignin carbon（LC）,MLCM had a spherical structure with higher specific surface area（938.1 m²/g）and pore volume（0.64 cm³/g）.Nitrogen and phosphorus were successfully doped into MLCM,with a content of 2.28 at.%and 1.23 at.%respectively.The doping of heteroatoms effectively improved the Faraday effect and the conductivity of carbon materials.Compared with LC materials,MLCM materials showed superior electrochemical performance.In the 1mol/L H₂SO₄ electrolyte system,MLCM showed the highest specific capacitance of 338.2 F/g at a current density of 0.8 A/g.The capacitance retention rate reached 70.2%at a current density of 10 A/g.Furthermore,MLCM was used as a positive and negative electrode material to assemble a symmetrical supercapacitor to test the electrochemical performance of its two electrodes.The device showed a specific capacitance of 194 F/g at a current density of 1A/g.In addition,the device maintained excellent cycle stability after 5000 times of charging and discharging at a current density of 2 A/g,and the capacitance retention rate reached 97.3%.Lignin-based carbon microspheres have good application prospects in the field of energy storage.更多还原