【作者】 张旺；

【导师】 俞书宏；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2016， 博士

【摘要】 金属-有机框架(Metal-Organic Frameworks, MOFs)是由金属离子或者金属簇和有机配体构成的一种新颖的多孔固体材料。由于该类材料的多孔性、高比表面积、可裁剪性、多活性位点等特点,使其在气体储存、二氧化碳捕集、分子分离、催化、药物或者其它材料载体等领域具有极其重要的应用。近年来,因MOFs材料的多孔及高度有序结构,可以获得高比表面积的多孔碳以及金属氧化物,而广泛应用于能量储存和转换领域。当前,对于该类材料的研究往往集中在直接合成获得的单分散微晶或者纳米晶粉末,以纳米孔的MOFs晶体颗粒为结构单元构筑其在不同维度的组装体及宏观的高级结构鲜有报道,同时其衍生材料缺乏形貌和孔结构的有效调控。本论文在实验室近年来发展的多重模板法制备系列一维纳米结构材料的工作基础上,集中阐述了模板指引的方法制备一维金属-有机框架纳米纤维及其复合材料,并研究了其衍生材料的电化学性能。我们选择了多种MOFs材料作为研究对象,成功获得了多种多样的MOFs纤维以及复合物纤维,并且进一步组装成三维的MOFs纤维基的气凝胶。本论文深入讨论了不同形貌结构对其衍生多孔纳米碳和金属氧化物的电化学性能影响,为设计和制备高活性的一维多孔非贵金属电催化剂以及电极材料提供了新的思路。另外,通过置换活泼的模板金属碲,获得一维贵金属@MOFs的复合纳米结构,并讨论这种新颖的一维贵金属在MOFs纤维结构限域中的催化活性。本论文所取得的主要研究成果如下：1.采用高质量的超细碲纳米线(TeNWs)作为模板,成功地指引一种经典的金属-有机框架材料(ZIF-8)的成核和生长,首次制备了均一、高长径比以及直径可控的MOFs纳米纤维。模板法合成的ZIF-8纳米纤维通过高温煅烧可以直接转化为多孔氮掺杂碳纳米纤维。这种新颖的纤维状的纳米碳展现出复杂的网络结构、多级孔道和超高比表面积等多重优点。通过阴极电催化氧还原反应测试研究表明,这种多孔碳纳米纤维的电催化活性远高于直接碳化ZIF-8纳米晶制备的微孔碳。进一步掺磷,获得的氮磷共掺杂多孔碳纳米纤维展现出比商业铂碳催化剂更加优异的电化学氧还原活性,半波电位达到～-0.161 V(/Ag/AgCl)。该研究工作为制备新颖的金属有机框架组装结构及其衍生的多孔碳材料或金属氧化物纳米材料提供了一种有效的合成路径。2,设计和合成出一种新颖的基于一维ZIF-8纳米纤维的气凝胶。这种特殊的气凝胶结构可以显著避免ZIF-8纤维之间的接触,因此在高温煅烧过程中,减少纤维之间的聚集,最终形成高分散的中空多孔碳纳米纤维。这种高分散的纤维作为电催化剂和电极材料,可以显著地增强其电化学性能。基于高分散、中空以及多级孔的结构,这种碳纳米纤维作为载体可以装载质量比超过70%的硫,成为稳定和高容量(1337 mA h g-1)的锂-硫电池电极材料。另外,通过同时加入锌和钴元素,一步合成双金属混合的金属-有机框架纳米纤维基的气凝胶。进一步煅烧可以获得Co/N/C的超高活性的氧还原催化剂,半波电位达到～-0.133 V (/Ag/AgCl)。3,发展和制备了一种基于碳质纳米纤维(CNF)及其气凝胶的CNF@MOFs复合纳米纤维和气凝胶,选择三种经典的MOFs材料作为研究对象(HKUST-1, ZIF-8, MIL-100-Fe),通过层层自组装的方法,成功获得一维的CNF@MOFs纳米纤维及其气凝胶,并且研究了它们的气体吸附性能。进一步通过煅烧CNF@ MIL-100-Fe,获得了高分散的三氧化二铁纳米管。并研究其在锂离子电池中的电化学行为,发现在高倍率条件下,具有很好的稳定性和高容量。4,设计和制备出了基于一维贵金属及其合金的复合MOFs纳米纤维。通过碲元素到贵金属转化,分别获得Pd或Pt及其合金的纳米线、纳米管的复合UIO-66和UIO-66-NH2勺纳米纤维。进一步研究了这种特殊一维贵金属线和管的MOFs纤维结构基于分子尺寸选择和气体富集在增强有机催化和光催化等方面的应用。5,发展了一种简单的气相沉积策略在金属-有机框架材料表面修饰疏水的聚二甲基硅氧烷(PDMS)以提高材料的水／水蒸汽稳定性。三种常见的水稳定性差的MOFs材料作为研究对象(MOF-5、[Zn(bdc)(ted)0.5]·2DMF·0.2H2O和HKUST-1),在MOFs晶体的表面上沉积了一层约十几个纳米厚的PDMS保护层,使得原本亲水的MOFs材料,转变成具有高度疏水的特性,从而阻隔了水分子的侵入。研究发现,这种表面保护层的形成不会影响金属有机框架材料原有的晶体结构、多孔性以及催化活性位点的可达性,使得修饰后的MOFs能在一定的湿度条件下甚至在水中,极好地维持其既有的气体吸附和催化能力。研究表明,该方法可广泛应用于多种MOF材料的表面疏水修饰,有效增强其对水(或湿度)的稳定性。更多还原

【Abstract】 Metal-organic frameworks (MOFs) are a novel class of porous solid, which is consist of metal ions or metal clusters and multifunctional organic ligands. MOFs have shown wide applications in gas storage, CO2 capture, molecule separation, catalysis, drug or other materials support due to their porosity, high surface area, tailorability, abundant unsaturated sites, etc. Recently, MOF-derived porous carbon and metal oxide materials have attracted tremendous attention for energy storage and conversion applications owing to porous and ordered structure of MOFs. To date, many studies in this research field are focused on MOF micro-or nanocrystals, and it lacks effective control of the resulted morphology and pore texture of their derivatives. Meanwhile, it is difficult to control synthesis of the hierarchically structured MOF architectures based on the assembly of MOF nanoparticles.Based on the continued research in template-directed organization of nanoparticles in our lab, the present dissertation will focus on nanowire-directed templating synthesis of one-dimensional MOFs nanofibers and their complex, and even MOF nanofibers-based aerogels were successfully prepared. Here, we discuss the enhanced electrochemical performance of their derived materials with novel one-dimensional structure. This strategy provide a new route for rational design and synthesis of one-dimensional non-precious metal porous electrocatalysts and electrode materials with high activity. On the other hand, diverse noble metal@MOFs nanofibers were synthesized by element replacement reaction, and further discuss their organic catalytic activity based on the size selectivity and gas molecule enrichment. The main results can be summarized as follows:1. A nanowire-directed templating method was developed for preparation of high-quality MOF nanofibers. Ultrathin tellurium nanowires (TeNWs) with excellent dispersivity was employed as template to induce growth and assembly of ZIF-8 nanocrystals (one typical MOF), resulting in the formation of uniform ZIF-8 nanofibers. Further calcination, these ZIF-8 nanofibers as carbon source can be conveniently converted into nanofibrous doped porous carbon, which exhibit high surface area, hierarchical pores and complex network structure. Compared with bulk carbon by direct carbonization of MOF crystals, the as-obtained nanofibrous doped carbon exhibit better electrocatalytic performance. Further doped by phosphorus (P) species, the co-doped carbon nanofibers exhibit high ORR activity with half-wave potential of ～-0.161 V, which is even better than the benchmark of the commercial Pt/C catalyst. This synthetic route will offer new opportunities for rational design and synthesis of diverse MOF nanofibers and their derived porous carbon or metal oxide materials.2. A new nanofibers-based MOF aerogel was synthesized by ice freeze-drying technology. Such aerogel as perfect model can effectively reduce contact among MOF nanofibers in high temperature calcination, resulting well-dispersed hollow porous carbon nanofibers. The final products can be as electrocatalyst and electrode material for enhanced their electrochemical performance. Furthermore, well-dispersed carbon nanofibers was introduced into sulfur support for the encapsulation of sulfur into their hierarchical micro- and mesopores as well as hollow nanostructure. The final composite exhibited excellent electrochemical features, good cycling stability and high coulombic efficiency as the cathode electrode materials for lithium-sulfur batteries. Meanwhile, an ultrahigh ORR active catalyst with half-wave potential of ～-0.133 V was prepared by carbonized a bimetal MOF nanofibers-based aerogel.3. Carbonaceous nanofibers (CNF)-based MOF composite nanofibers and aerogel were prepared by step-by-step route. Three classical MOF materials (HKUST-1, ZIF-8, MIL-100-Fe) were selected as research model for synthesis of CNF@MOF composite nanofibers and gel, and their gas sorption capacity were studied. CNF@ MIL-100-Fe was further converted into well-dispersed Fe2O3 nanotube by pyrolysis in air, and further investigated their performance in Li-ion battery.4. Several noble metal nanowire or nanotube and MOF nanofibers composite were designed and synthesis. Based on element replacement reaction from tellurium to palladium and platinum, these noble metal or alloy@UIO-66 and UIO-66-NH2 nanofibers were obtained and further discussed their organic catalytic activity based on the small molecule selectivity and gas enrichment and photocatalytic activity based on one-dimensional co-catalyst.5. A facile and general polydimethysiloxane (PDMS)-coating technology to enhance moisture/water stability of metal-organic frameworks has been successfully developed. Here, we employed three vulnerable MOFs (MOF-5, HKUST-1 and [Zn(bdc)(ted)0.5]·2DMF·0.2H2O) to investigate their water stability before and after PDMS coating. The surface of these MOFs were successfully coated by PDMS, which form a hydrophobic protective layer. Upon moisture/water treatment, the porosity of PDMS-coated MOFs exhibits nearly 100% retained while the pristine MOFs lose their porosity completely. Compared with other post-modified methods, such PDMS-coating approach is not only facile but also shows significant superiority regarding porosity preservation. Remarkably, the PDMS-coated MOF have well retained porosity and surface area, as well as the active sites that remain accessible to substrates after water treatment.更多还原