| 张震,冯军宗,姜勇刚,刘平,张秋华,卫荣辉,陈翔,冯坚.利用离子液体制备无机气凝胶的研究进展[J].材料导报,2018,32(9):1469-1476 |
| 利用离子液体制备无机气凝胶的研究进展 |
| Progress in the Preparation of Inorganic Aerogels from Ionic Liquids |
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| DOI：10.11896/j.issn.1005-023X.2018.09.011 |
| 中文关键词: 离子液体 常压干燥 二氧化硅气凝胶 二氧化钛气凝胶 炭气凝胶 |
| 英文关键词: ionic liquid, ambient pressure drying, silica aerogel, titania aerogel, carbon aerogel |
| 基金项目:国家自然科学基金(51302317)；湖南省自然科学基金(14JJ3008) |
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| 中文摘要: |
| 气凝胶具有三维纳米多孔网络结构,独特的结构使它具有低密度、高比表面积和高孔隙率等性质以及低热导率、低介电常数和低声传播速率等性能,在隔热、介电、隔声、催化、吸附等领域具有广阔的应用前景。然而,溶剂-凝胶法作为目前制备气凝胶最成熟、应用最广的技术,需要使用大量的有机溶剂,严苛而危险的超临界干燥工艺进一步推高了成本,限制了气凝胶的大规模工业化生产和应用,因此,降低成本和在常压干燥条件下制备高比表面积的块状气凝胶是气凝胶产业急需解决的问题。离子液体被称为21世纪的绿色溶剂,具有低蒸气压、低表面张力、高催化性和高溶解性等特殊性质。离子液体与气凝胶材料的发展几乎同步,但直到2000年两种材料才产生交集。离子液体作为模板剂具有微观结构导向作用,使纳米孔结构均一化,其不挥发性和低表面张力保证了老化和常压干燥过程中纳米孔结构不会因毛细管力而坍塌破坏,另外其催化作用可以缩短凝胶时间。因此,离子液体为常压干燥合成气凝胶提供了新的工艺路线。目前,有关借助离子液体制备SiO2气凝胶、TiO2气凝胶、SiO2-TiO2复合气凝胶、炭气凝胶等无机气凝胶的探索均已展开,其中制备SiO2气凝胶的研究最多,涉及工艺、微观结构、掺杂和应用等方面。通过常压干燥可获得比表面积高达677 m2/g的块状气凝胶,通过选用不同的离子液体还可以控制纳米孔的微观形貌, 所得SiO2气凝胶产物在电化学、生物、吸附等领域有较高的应用潜力。利用离子液体替代有机溶剂可以使得到的TiO2气凝胶不经煅烧即含有锐钛矿相,通过金属原子Ag、Fe、Ge等掺杂改性,可进一步提高锐钛矿相的结晶度,提升其光催化性能。利用离子液体制得的SiO2-TiO2复合气凝胶具有一定强度和良好的光催化活性。此外,除在传统的溶胶-凝胶法中用作模板剂或催化剂外,离子液体还可作为新型的炭源用于制备炭气凝胶,即通过熔盐法高温炭化裂解离子液体“自上而下”直接制备。这种方法可以制备杂原子在原子水平上均匀分布的功能化炭气凝胶,无需制备有机气凝胶前驱物,极大缩短制备周期,并且炭气凝胶产物的比表面积相对更高,得到了科研界的广泛关注。本文介绍了离子液体在气凝胶材料合成过程中的作用原理,归纳了借助离子液体制备前述几种重要的无机气凝胶的国际研究状况。 |
| 英文摘要: |
| Aerogels have three dimensional nanoporous network structure and the unique structure endows them with the characteristics of low density, high specific surface area and high porosity, and the properties of low thermal conductivity, low dielectric constant and low transmission rate. Thus aerogels have found promising application in the fields of acoustic insulation, thermal insulation, dielectric materials, catalyts, adsorption and so on. However, sol-gel method, as the most mature and the most widely used aerogel fabrication technique, suffering the problems of excess use of hazardous organic solvent and severely dangerous and costly supercritical drying process, only has limited large-scale industrial production and application potential. Thus, reducing cost and preparing monolithic aerogels with high specific surface area under ambient pressure conditions are the most urgent subjects. Ionic liquids, known as green solvents in the 21st century, have many exceptional properties such as low vapor pressure, low surface tension, high catalytic and high solubility. The development of ionic liquids and aerogels were almost simultaneous, but not until 2000 did two materials meet. Ionic liquids serving as template agents offer microstructure orientation effect, and homogenize the resultant pore structure. The nonvolatility and low surface tension of ionic liquid templates help to avoid capillary effect and consequently ensure the intactness of nanometer pore structure during aging and ambient pressure drying, and moreover, the catalytic effect of ionic liquids can shorten gel time. All of the above open up a new avenue to produce aerogels by ambient pressure drying with the assistance of ionic liquids. By now, researchers have conducted extensive research on applying ionic liquids to the preparation of inorganic aerogels such as SiO2 aerogels, TiO2 aerogels, SiO2-TiO2 composite aerogels and carbon aerogels. Among them, SiO2 aerogels acquire the most research endeavors, from the perspectives of procedure, microstructure, doping and application, etc. The monolithic aerogels with specific surface area up to 677 m2/g and controllable (by adopting different ionic liquids) pore microstructure can be obtained through ambient pressure dying, and the resultant products have great electrochemical, biological and adsorptional application prospects. Ionic liquids instead of organic solvents can be used to synthesize TiO2 aerogels with anatase phase without calcination. Notably, through the doping modification of metal atoms, e.g. Ag, Fe, Ge, the degree of crystallinity of anatase phase can be further enhanced, thereby improving the aerogel’s photocatalytic performance. SiO2-TiO2 composite aerogels prepared by using ionic liquids display relatively high strength and favorable photocatalytic activity. Besides acting as template agent or catalyst in the conventional sol-gel process, ionic liquids can be used as a new type of carbon source in preparing carbon aerogels, i.e. the “top-to-bottom” direct fabrication by pyrolyzing ionic liquids through the molten salts approach. This method is capable to produce functionalized carbon aerogels with higher specific area, atomic level uniform distribution of the heteroatoms, and furthermore, can exempt the procedure from prepa-ring organic aerogel precursors. This paper introduces the action mechanism of ionic liquids within the synthesis process of aerogels, and provide a comprehensive summary over the research status of applying ionic liquids to the fabrication of the above mentioned representative inorganic aerogels. |
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