【作者】 柯飞；

【导师】 朱俊发；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2014， 博士

【摘要】 金属-有机骨架(Metal-Organic Frameworks,简称MOFs)是一种具有永久孔洞结构的新颖晶体材料,其结构是由金属离子和多功能的有机配体在合适的溶剂中通过配位键自组装形成的。与传统的无机多孔材料相比,MOFs具有超大的比表面积、可控的孔洞尺寸和可调的内表面性质等优势,这些鲜明的优势使其在气体储存和分离、药物载体、传感和催化等领域具有潜在的应用前景。除了以上优势,MOFs骨架中还具有高密度的配位不饱和金属活性位点和大的孔隙率使其可以应用于多相催化,尤其是当金属纳米颗粒负载到MOFs孔洞中后,催化活性大大提高。到目前为止,已经有一些关于MOF负载金属纳米颗粒或双金属合金纳米颗粒在催化方面的研究报道。但是,基于MOF核-壳结构催化剂的研究还是非常的少。本论文提出了一种简单的合成基于MOF核-壳结构催化剂的方法,并研究了其在催化方面的应用。此外,对材料的核-壳结构以及壳层厚度与其催化性能之间的关系进行了初步探讨,为新型高效多孔核-壳催化材料的开发提供一些借鉴意义。具体的研究内容与成果主要有以下几个方面：1.通过简单的层层自组装方法制备了一种新型的基于MOF的Fe3O4@MIL-100(Fe)磁性核-壳多孔催化剂。制备的磁性核-壳催化剂在Claisen-Schmidt缩合反应中不仅表现了高的催化活性和选择性,而且利用一个简单的外界磁场就可以方便的将催化剂从反应溶液中分离、回收起来。催化结果显示,经过多次循环催化使用后,催化剂的催化反应活性并没有明显的降低。因此,与其它报道的Claisen-Schmidt缩合反应催化剂相比,这类基于MOF的磁性核-壳催化剂既绿色环保又便宜,更适合于大规模的工业应用。2.制备了一种新型的多孔Au@MIL-100(Fe)核-壳纳米催化剂,核-壳结构中MIL-100(Fe)的壳层厚度可以通过自组装的循环次数进行有效的调控。此外,我们还研究了所合成的核-壳纳米催化剂在催化还原对硝基苯酚为对氨基苯酚反应中的催化性能。催化结果表明,在相同的反应条件下,Au@MIL-100(Fe)核-壳纳米催化剂比纯的Au和MIL-100(Fe)具有更高的催化反应活性,说明了核-壳结构中Au核与MIL-100(Fe)壳在催化反应中具有协同催化的作用。而且,经过多次循环催化反应之后,核-壳纳米催化剂的催化活性几乎没有改变。3.由于经济和环境的原因,不管是多相催化还是均相催化,对昂贵催化剂的回收和再循环使用都是非常重要的。我们报道了一种多功能的基于Au的磁性核-壳催化剂,这个催化剂不仅制备简单而且在催化还原对硝基苯酚反应中具有非常高的催化活性。制备的Au-Fe3O4@MOF催化剂是由超顺磁性的Au-Fe3O4核和厚度可控的多孔MOF壳组成的,Au纳米颗粒催化剂像三明治似得均匀的分散在Fe304核和MOF壳之间。Au-Fe3O4@MOF磁性核-壳催化剂在催化反应中具有协同催化的作用,其在催化还原对硝基苯酚的反应中比报道的其它基于Au催化剂的催化活性都高。此外,因为它具有超顺磁性的核,所以回收非常方便。4.甲酸是一种安全方便的氢气能源材料,其在化学合成和可持续的能源储存方面具有极大的应用前景,但是合成一种在室温下就可以可控的和有效的催化分解甲酸制氢的催化剂具有非常大的挑战。我们通过一步快速的合成方法制备了一种新型的AgPd@MIL-100(Fe)核-壳纳米颗粒,而且第一次把它应用于室温下、不加任何添加剂的条件下催化分解甲酸制氢反应。非常有意义的是,在所有制备的催化剂中,具有7nm壳层厚度的AgPd@MIL-100(Fe)核-壳纳米颗粒表现了最高的催化反应活性,说明了核-壳结构中MIL-100(Fe)壳层对催化反应活性具有非常重要的影响,而且其壳层厚度是影响催化结果的一个重要因素。这个工作说明了基于MOF的核-壳材料在甲酸分解制氢的催化应用中具有潜在的应用前景。5.第一次采用一种无表面活性剂的共还原方法将基于非贵金属的CoAuPd三金属合金纳米颗粒成功的负载到了多孔MIL-101骨架上,金属Co0在纳米合金结构的保护下稳定性提高,使其成功的应用于甲酸分解制氢反应。更有趣的是,在室温不添加任何添加剂的条件下,所制备的具有低消耗贵金属的Co0.3Au0.35Pd0.35/MIL-101纳米复合催化剂在分解甲酸制氢的反应中展现了100%的制氢选择性、最高的催化活性和非常好的稳定性。因此,利用多孔MOFs作为金属纳米颗粒的载体,尤其是负载基于非贵金属纳米颗粒,这将为今后发展成本便宜且高效的分解甲酸制氢催化剂带来了机遇。6.可见光催化具有成本低、相对安全而且环境友好等特点,所以在有机合成中引起了广泛的关注。我们以多孔的MIL-100(Fe)为载体,醋酸镉为CdS的前驱体,通过简单的水热合成方法制备了一种新型的CdS-MIL-100(Fe)纳米复合可见光催化剂,并研究了其在选择性光催化氧化苯甲醇为苯甲醛中的催化性能。相比于纯的CdS, CdS-MIL-100(Fe)纳米复合光催化剂在该光催化反应中具有更高的光催化效率,复合光催化剂光催化性能的提高可以归属为对光的吸收能力增强、更有效的分离光电子-空穴对和具有更大的比表面积等因素的共同作用。这个结果说明了基于MOF的纳米复合材料在光催化有机合成中具有非常好的应用价值。更多还原

【Abstract】 Metal-organic frameworks (MOFs) are permanently porous materials synthesized by assembling metal ions with organic ligands in appropriate solvents. Compared with conventional inorganic porous materials, MOFs possess extraordinarily high surface areas, tunable pore size, and adjustable internal surface properties. These distinct characteristics make them very promising for a variety of applications, including gas storage and separation, drug delivery, sensing, and catalysis. Because of the large density of active sites in which the constitutional metal nodes have free or exchangeable coordination positions, and the high porosity of these materials, their logical application, especially when metal nanoparticles (NPs) are embedded into their pores, could be for heterogeneous catalysis. To date, there have been a few pioneering studies on the MOF supported metal NPs or bimetallic alloy NPs for heterogeneous catalysis. By contrast, researches on MOF-based core-shell catalysts with functional NPs as core and MOFs as shell have not been popular to date.In this dissertation, we present a simple encapsulation strategy for the fabrication of well-defined MOF-based core-shell heterogeneous catalysis, which shows a novel enhanced catalytic property. In addition, we have also investigated the relationship between the catalytic properties and core-shell structure, as well as thickness of the MOF shell. The results obtained will be helpful for designing and constructing new classes of high-performance core-shell porous catalysts. The details are summarized briefly are as follows:1. A novel type of MOF-based core-shell magnetic porous catalysts, i.e., Fe3O4@MIL-100(Fe) magnetic microspheres, have been fabricated by a simple step-by-step assembly strategy. These magnetic catalysts not only show high catalytic activities and selectivity towards the Claisen-Schmidt condensation reactions, but remarkably, they can be easily separated and recycled without significant loss of catalytic efficiency after used for many times by simply applying external magnetic fields. Therefore, compared to other reported catalysts used in the Claisen-Schmidt condensation reactions, such MOF-based core-shell magnetic catalysts are green, cheap and more suitable for large scale industrial applications.2. Novel Au@MIL-100(Fe) core-shell nanocatalysts with a controllable MIL-100(Fe) shell thickness were fabricated by using a versatile step-by-step fashion. Moreover, we have tested the catalytic properties of the core-shell nanocatalysts using the liquid-phase reduction of4-nitrophenol (4-NP) to4-aminophenol (4-AP). Catalytic studies show that the Au@MIL-100(Fe) core-shell nanocatalysts exhibit much higher catalytic activity than the pure Au and MIL-100(Fe) NPs, suggesting that the MIL-100(Fe) shell enhances the catalytic activity via a synergistic effect. Furthermore, this high catalytic activity remains almost unchanged after a number of reaction cycles.3. Recovery and reuse of expensive catalysts are important in both heterogeneous and homogenous catalysis due to economic and environmental reasons. This work reports a novel multifunctional magnetic core-shell gold catalysts which can be easily prepared and shows remarkable catalytic properties in the reduction of4-NP. The novel Au-Fe3O4@MOF catalysts consist of a superparamagnetic Au-Fe3O4core and a porous MOF shell with controllable thickness. The gold catalyst NPs are sandwiched between the Fe3O4core and porous MOF shell. Catalytic studies show a strong synergistic effect of core-shell structured Au-Fe3O4@MOF, which have much higher catalytic activities than other reported Au-based catalysts toward the reduction of4-NP. Moreover, the Au-Fe3O4@MOF core-shell magnetic catalysts could be easily recycled due to their superparamagnetic core.4. Formic acid (FA) has tremendous potential as a safe and convenient source of hydrogen for sustainable chemical synthesis and renewable energy storage, but controlled and efficient dehydrogenation of FA by heterogeneous catalysts at room temperature constitutes a major challenge. Here, we report a facile one-pot method for the fabrication of a novel core-shell AgPd@MIL-100(Fe) NPs, which were used for the first time as high performance MOF-based bimetallic catalysts for hydrogen production from FA without using any additive at room temperature. Remarkably, the resulting core-shell AgPd@MIL-100(Fe) NPs with a shell thickness of7nm shows the highest activity among all the prepared catalysts, suggesting that the MIL-100(Fe) shell exhibits a significant influence on the catalytic activity and moreover, the shell thickness is a key factor in determining the test results. This work demonstrates that MOF-based core-shell materials hold great promises in the practical application of hydrogen production from FA.5. CoAuPd alloy NPs based on a non-noble metal were successfully immobilized to MIL-101for the first time by a surfactant-free co-reduction method. The elevated stability of Co0in the protected alloy NPs makes its application in FA dehydrogenation successful. More interestingly, the resulting Coo.3Auo.35Pd0.35/MIL-101composites with the lower consumption of noble metals exhibit the100%hydrogen selectivity, highest activity and excellent stability toward hydrogen generation from FA without any additive at room temperature. Therefore, the present results open up new avenues for developing cost effective and high-performance catalysts for the generation of hydrogen from FA by using porous MOFs as hosts for metal NPs, especially non-noble metal based NPs.6. Visible-light initiated organic transformations have received much attention because they have advantages in terms of low cost, relative safety, and environmental friendliness. We report a novel type of visible-light-driven photocatalysts, namely, CdS-MIL-100(Fe) nanocomposites, which were prepared by a simple solvothermal method in which porous MIL-100(Fe) served as the support and cadmium acetate (Cd(Ac)2) as the CdS precursor. Using selective oxidation of benzyl alcohol to benzaldehyde as the probe reaction, the results show that the introduction of MIL-100(Fe) into the semiconductor CdS can remarkably enhance the photocatalytic efficiency at room temperature as compared to that using pure CdS. The enhanced photocatalytic performance can be attributed to the integrative effects of enhanced light absorption intensity, more efficient separation of the photogenerated electron-hole pairs, and increased surface area of CdS due to the presence of MIL-100(Fe). This work demonstrates that MOF-based materials hold great promises in the applications of solar energy conversion into chemical energy.更多还原