Controllable Synthesis And Catalytic Properties Of Core-Shell Structured Catalysts Based On MOFs

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.