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气凝胶是在保持凝胶网络结构不变的条件下，除去其中的液体溶剂得到的一种三维多孔材料。纤维素气凝胶以其独特的氢键网络结构、优秀的生物相容性和可降解性，弥补了传统无机气凝胶和聚合物气凝胶的缺点，成为一种具有广阔应用前景的环境友好型多孔材料。由于纤维素来源广、储量大、可降解，而且纤维素链含有许多活泼的羟基，可以通过分子内和分子间氢键作用获得稳定的三维网络结构，因此纤维素气凝胶被赋予许多特殊的性质。但现有的纤维素气凝胶制备技术还存在一些问题，如生物质利用方法受限、制备过程复杂、反应条件苛刻、溶剂毒性和腐蚀性较强及溶剂回收困难等问题，阻碍了纤维素气凝胶的应用。针对以上问题，本论文旨在利用离子液体体系，用不同原料制备纤维素气凝胶，开发设计绿色温和的制备方法，获得高性能纤维素基气凝胶材料，探索其形成机理并拓展纤维素气凝胶的应用领域。主要创新性工作及成果如下：（1）制备了纳米纤维素气凝胶及再生纤维素气凝胶。首先制备长度100-200 nm，直径10-15 nm的棒状纳米纤维素，纳米纤维素可均匀分散在水中形成稳定的胶体，ζ电势为±35 mV；当浓度为12%时纳米纤维素胶体发生凝胶化，通过冷冻干燥得到纤维素 Ⅰ 型的纳米纤维素气凝胶。其次，用离子液体制备了纤维素 Ⅱ 型的再生纤维素气凝胶，通过对离子液体的筛选，发现[Amim]Cl离子液体在70 °C下，仅用30 min即可溶解纤维素，再生率高达93.2%，再生纤维素气凝胶的压缩强度为63.7 N/cm2；用微晶纤维素、棉纤维和秸秆分别制备了再生纤维素气凝胶，其中微晶纤维素和棉纤维溶解时间较短且气凝胶形貌规整，形成纤维互相交错的三维孔道结构，但因秸秆中含有大量的木质素和半纤维素，导致再生纤维呈包覆状，使气凝胶形成多级孔道结构。（2）制备了纤维素/ZnO复合气凝胶并对其原位合成机理进行研究。以离子液体和二甘醇为基础，在纤维素气凝胶上原位合成了分布均匀、大小可调的球形ZnO纳米颗粒，用温和绿色的方法制备了纤维素/ZnO气凝胶。解决了传统制备过程中溶剂不环保且ZnO过度聚集的问题。探索了ZnO合成机理，通过模拟发现Zn2+与葡萄糖-OH之间有一定的相互作用，可实现ZnO颗粒在纤维素上的原位合成。研究发现，随着Zn(OAc)2前驱体浓度和水解时间的增长，ZnO纳米颗粒逐渐成核长大并产生聚集。过大的颗粒堵塞气凝胶孔道，导致复合气凝胶比表面积降低。通过调节Zn(OAc)2浓度和水解时间，可获得高比表面积的复合气凝胶（271 m2/g），且ZnO球型纳米颗粒分布均匀，大小均一。纤维素/ZnO气凝胶可以作为可回收催化剂，对聚对苯二甲酸乙二醇酯进行有效降解，其中对苯二甲酸双羟乙基酯单体转化率为100%，收率为78%，具有高效的催化效果。且三次循环后气凝胶没有收缩破碎，与传统催化剂相比，复合气凝胶可直接取出无需分离，便于循环使用。这项工作为制备纤维素/金属氧化物复合气凝胶提供一种简单温和的原位合成方法，并探索了气凝胶材料用作催化剂的潜在应用价值。（3）以秸秆为原料制备富纤维素气凝胶。基于卤素类离子液体+氨基磺酸溶剂体系，用简单温和的方法从秸秆中一步去除木质素，制备富纤维素气凝胶。研究了不同卤素类离子液体溶剂体系对气凝胶组分、结构、热稳定性和比表面积等的影响。研究发现，[Amim]Cl离子液体与氨基磺酸溶剂体系可以一步去除秸秆中的木质素得到富纤维素气凝胶，其中纤维素含量高达到88.11%。制备出的富纤维素气凝胶拥有较高的比表面积（201 m2/ g），足够的热稳定性和压缩强度，而且对染料刚果红和考马斯亮蓝均有良好的吸附效果，饱和吸附量分别为549.13 mg/g及301.58 mg/g。对其吸附机理进行了模拟，结果表明：刚果红中的Na与葡萄糖中的-OH有很强的相互作用，优于考马斯亮蓝-HSO3-和葡萄糖的氢键作用及π-π相互作用。这项工作为木质纤维素原料制备纤维素气凝胶提供了一种简单高效的方法，并探索了纤维素气凝胶材料在染料吸附领域的应用。（4）以涤棉混纺织物为原料制备了纤维素气凝胶隔膜用于超级电容器。以废旧涤棉混纺织物为原料，通过醇解和离子液体溶解法，制备了纤维素气凝胶隔膜，并合成高热稳定性[Emim]TFSI离子液体电解液，制备超级电容器。涤棉混纺织物经过醇解可得到棉纤维，其回收率可达86.59±6.47%。考察了不同浓度的纤维素气凝胶隔膜的性质，研究发现，当纤维素浓度为2%时，制备出的隔膜中纤维交错堆叠出多级孔结构，直径在10-20 nm，拥有最高的比表面积（210 m2/g）和断裂伸长率（28.4%）。通过分析纤维素气凝胶隔膜和商用PP隔膜的性能，发现纤维素气凝胶隔膜在160 °C下收缩率小于1%，而商用PP隔膜收缩率在39%左右，纤维素气凝胶隔膜良好的稳定性保证了超级电容器的安全。纤维素气凝胶隔膜比PP隔膜拥有更好的电解液浸润性。制备的超级电容器的循环性能好，阻抗小且拥有很好的倍率特性。;Aerogel is a kind of three-dimensional porous material obtained by removing the liquid solvent without changing the network. Cellulose aerogels, possessing the unique hydrogen bond network structure, excellent biocompatibility and biodegradability, make up the shortcomings of traditional inorganic aerogels and polymeric aerogels and become promising environment-friendly porous materials. Cellulose is widespread, abundant and degradable, and the cellulose chains contain numerous active hydroxyl groups which can form a stable three-dimensional structure through the intramolecular and intermolecular hydrogen bonding interactions. So cellulose endows aerogel with a variety of special properties. However, many limitations impede the application of cellulose aerogels, including the limited methods of biomass utilization, the complex approaches of cellulose aerogel fabrication, the toxicity and corrosiveness of the used solvents, and the difficulty of solvent recovery. This dissertation aims to fabricate cellulose aerogels from different raw materials based on ionic liquid systems, and develop green and facile preparation approaches. Furthermore, through exploring the formation mechanisms to obtain high performance cellulose-based aerogels and expand the applications of cellulose aerogels. The main innovation and achievements are as follows:(1) Nano-cellulose aerogel and regenerated cellulose aerogel were prepared. Firstly, the rod nanocellulose with a length of 100-200 nm and a diameter of 10-15 nm was prepared. The nanocellulose was uniformly dispersed in water to form colloids. The ζ potential of nanocellulose colloid was ±35 mV which is stable enough to store for a long time without aggregation or precipitation. When the concentration of nanocellulose is 12%, the colloid becomes gelation. The nanocellulose aerogel (cellulose Ⅰ) obtained by lyophilization. Second, the regenerated cellulose aerogel (cellulose Ⅱ) was prepared using ionic liquid. through the screening of ionic liquids, [Amim]Cl was found to be a good cellulose solvent which can dissolve cellulose under 70 °C in 30 min and the cellulose regeneration rate is as high as 93.7%. The compressive strength of cellulose aerogel is 63.7 N/cm2；The regenerated cellulose aerogels were prepared from microcrystalline cellulose, cotton fiber and corn stalk, respectively. (2) The preparation and mechanism of cellulose composite aerogels were studied. The cellulose/ZnO aerogel was fabricated through a facile approach based on ionic liquid and diethylene glycol. The uniform and adjustable spherical ZnO nanoparticles were synthesized in situ on the cellulose which solves the problem occurred in the traditional fabrication of cellulose/ZnO aerogel materials, such as the solvents were not environment-friendly and the ZnO particles accumulated excessively. The mechanism of synthesize ZnO in situ was simulated and confirmed further. The simulation showed there was interaction between Zn2+ and –OH of glucose. With the increase of zinc acetate concentration and hydrolysis time, ZnO nanoparticles gradually nucleate and grow up. Large particles blocked the aerogel pores, resulting in a decrease of specific surface area of aerogel. By adjusting the concentration of zinc acetate and the hydrolysis time, the cellulose/ZnO aerogel had high specific surface area (271 m2/g), and the ZnO spherical nanoparticles are uniformly distributed with uniform size. Cellulose/ZnO aerogel can be used as a recoverable catalyst to effectively degrade polyethylene terephthalate. The conversion of bis(2-hydroxyethyl) terephthalate monomer was 100% and the yield was 78% which showed a high catalytic effect. After the three degradation cycles, the aerogel showed nearly no shrinkage and deformation. Compared with traditional catalysts, aerogels can be taken out without separation and make it easy to reuse in subsequent cycles. This work provides a facile approach to fabricate cellulose/metal oxide composite aerogel and explores the potential application of aerogels as catalysts.(3) The preparation of cellulose aerogel from stalks was studied. Based on the halogen ionic liquid and sulfamic acid solvent systems, the cellulose-rich aerogel was prepared by a facile and simple approach, whereby remove the lignin from the stalks in one step. The effects on cellulose aerogel properties were investigated, including different ionic liquid solvent systems, component content, morphology, thermal stability and specific surface area. [Amim]Cl and sulfamic acid solvent system can remove the lignin from corn stalks in one step, and obtain a cellulose-rich aerogel in which the cellulose content was as high as 88.11%. The prepared cellulose-rich aerogel has three-dimensional structure with high specific surface area (201 m2/g), and possess sufficient thermal stability and compression strength. It has good adsorption effect on anionic dye (Congo red) and cationic dye (Coomassie brilliant blue), and the saturated adsorption capacity is 549.13 mg/g and 301.58 mg/g, respectively. The adsorption mechanism was simulated. The results showed that Na in Congo red and -OH in glucose displayed the strongest hydrogen bond than the π-π interactions and the interaction between -HSO3- of Coomassie brilliant blue with glucose. This work provides a simple and facile approach to prepare of cellulose aerogel from lignocellulosic biomass and explores the application of cellulose aerogel materials in the field of dye adsorption.(4) The preparation of cellulose aerogel separators from polyester-cotton blended fabrics was studied. Based on alcohol hydrolysis and ionic liquid dissolution, the cellulose aerogel separators were prepared from polyester-cotton blended fabric which was used in supercapacitors. The polyester-cotton blended fabric can be degraded in ethylene glycol under the action of a catalyst to obtain cotton fiber, wherein the cotton fiber recovery rate was as high as 86.59±6.47%. By investigating the properties of cellulose aerogel separators with different cellulose concentration, it was found that the separator with 2% cellulose has the optimal properties. This aerogel separator consists of interlaced cellulose with the diameter of 10-20 nm, and possesses high specific surface area (210 m2/g) and elongation at break (28.4%). The performance of cellulose aerogel separators and commercial PP separators were compared. The shrinkage rate of cellulose aerogel separator was less than 1% under 160 °C, while that of PP separator was approximately 39%. The excellent stability of cellulose aerogel separator ensures the safety of the supercapacitor. Moreover, the high thermal stability [Emim]TFSI ionic liquid electrolyte was synthesized, and the cellulose aerogel separators have better electrolyte wettability than PP separators. Furthermore, the prepared supercapacitor has good cycle performance, low impedance and good characteristics.
|李晓前. 基于离子液体的纤维素气凝胶制备及应用研究[D]. 中国科学院大学,2019.|
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