Design, Synthesis And Catalytic Applications Of Metal-Organic Frameworks-Based Nanocomposites

Compared with traditional inorganic materials, metal-organic frameworks (MOFs), as new organic-inorganic hybrid materials, possess high specific surface area, highly ordered porous structure, designable structure types, functional pore space and other unique properties. In recent two decades, MOFs have shown potential applications in lots of fields, including gas adsorption and separation, sensing, drug delivery, catalysis, etc. Especially, MOFs have been widely used as supports for metal nanoparticles (M-NPs) to afford heterogeneous catalysts. In this thesis, we have synthesized several MOFs-stabilized M-NPs composites, which have been applied to catalyze the hydrolytic dehydrogenation of ammonia borane (NH3BH3) under mild conditions and as heterogeneous catalysts for some organic reactions. In addition, we also utilized the MOFs as precursors to afford porous carbon materials for electrochemistry. The main research results are as follows:1. As a promising material for chemical hydrogenation storage, NH3BH3 has been traditionally catalyzed for releasing hydrogen over noble metal catalysts. Although noble M-NPs exhibit excellent catalytic performance for many reactions, the high cost and limited storage restricts their large scale applications in industry. In reverse, non-noble M-NPs have low price and can meet the large demand for us, while they usually perform inferior catalytic activity than noble M-NPs. In order to reduce costs, one possible solution is to minimize the amount of noble metal by introducing a non-noble metal (transition metal) to form bimetallic NPs. MIL-101 with ultrahigh surface area, appropriate pore size and the hydrophilic nature of the inner pore surface was selected as host matrix for M-NPs in this work. The Pd(NO3)2 and CoCl2 aqueous solution was easily introduced into the pores of activated MOF by capillary force. Due to different reduction potentials of the two metal ions, Pd2+ was first reduced to Pd NPs by NH3BH3 and the produced Pd-H behaved as in situ seeds for the subsequent reduction of Co2+ to give Co shell NPs. The first prepared tiny Pd@Co NPs confined in MIL-101 exhibits superior catalytic performance for hydrolytic dehydrogenation of NH3BH3 under mild conditions, compared with the monometallic NPs, alloy and Pd@Co loaded on the MIL-101.2. Based on the work above, in order to further reduce the consumption of noble metals while maintaining excellent catalytic efficiency, we have developed a novel synthesis method to prepare transition metals with excellent catalytic performance under mild conditions. As we all know, the transition metal precursors have been traditionally reduced with a strong reductant or under harsh conditions due to then-low reduction potential. However, most of supports can hardly sustain these conditions, resulting in large M-NPs. Therefore, for the first time, we develop a general and simple on-step synthesis approach-seed mediated method under mild conditions by taking advantage of MIL-101 as support, a trace amount of noble metal ions as an initiator and NH3BH3 as an appropriate reducing agent. The noble metal ions were first reduced and the produced M-H species with strong reducibility could be able to reduce transition metal ions. The obtained transition M-NPs stabilized by MIL-101 using this method not only reduce cost greatly and display superior catalytic activity for the hydrolytic dehydrogenation of NH3BH3 under room temperature.3. Given the containing catalytic active sites in framworks, MOFs could also be applied as heterogeneous catalysts for some catalytic reactions except as supports for M-NPs. Although many research works have been focused on MOFs-stabilized M-NPs applied for catalysis, their cooperative catalysis for one-pot cascade reactions has been reported very rare. In this work, we first prepared Pd@MIL-101 bifunctional catalyst by rational double-solvent approach (DSA), which presents excellent performance in tandem catalysis based on the synergistic catalytic performance of the Lewis acidity of MIL-101 and the hydrogenation property of Pd NPs. Followed that, we prepared PdAg@MIL-101 multifunctional catalyst with ultrafine PdAg alloy NPs (～1.5 nm). The resultant catalyst, for the first time, has realized a one-pot multistep cascade reaction of nitrobenzene and benzaldehyde and exhibited superior catalytic activity and selectivity. The excellent catalytic performance and recycling stability of PdAg@MIL-101 was attributed to the combination of host-guest coorperation and bimetallic synergy, in which MOF provides Lewis acidity, Pd affords hydrogenation activity while Ag improves selectivity toward target product.4. MOFs have shown potential applications in many fields. In fact, MOF-derived porous carbon materials also inherit many advantages of MOFs and some may show new features. For example, we found that N-doped carbon material derived from N-MOFs performs good hydrophility. Recently, lignocellulosic biomass derived biofuels by pyrolysis have attracted widespread attention as a substitute for fossil fuel sources, while bio-oils usually have a low calorific value, immiscibility with traditional fuels and instability due to high amounts of oxygen components. Therefore, bio-oils should be upgraded for oxygen removal in order to be fully used. One effective approach is hydrodeoxygenation (HDO) reaction. However, the unprocessed biofuels usually perform hydrophilicity owing to the involved hydrophilic groups, which makes them hardly contact with the conventional hydrophilic catalysts, such as Pt/C. Herein, ZIF-8, a representative MOF, constructed by N-based organic ligand and Zn has been converted to N-doped porous carbon material (NPC-ZIF-8) with high surface area and good hydrophilicity via calcination at high temperature. The obtained NPC-ZIF-8 was firstly used as support for ultrafine Pd NPs to afford Pd/NPC-ZIF-8 catalyst, which exhibited superior catalytic activity, excellent selectivity and high cycling stability toward the hydrogenation reaction of vanillin. The excellent catalytic performance of Pd/NPC-ZIF-8 was mainly attributed to the high surface area, tiny Pd NPs, high amount of graphitic-N, electron-rich Pd sites and favorable hydrophilicity.5. In recent years, MOFs-derived porous carbons have been widely used as excellent electrode materials and have attracted intensive attention. However, it still faces great challenges to simultaneously realize a lot of advantages contained in one excellent porous carbon electrode material, even for MOF-derived carbon. For example, the electrochemical research about the carbon materials derived from Zn-based ZIF-8 or Co-based ZIF-67 have been reported. Although the ZIF-8 has been demonstrated to afford porous carbons with high surface area, it cannot provide Co(Fe)-Nx activated sites and high graphitized carbon. In reverse, ZIF-67 could realize these but can only offer carbon with relative low surface area. In this work, we have rationally designed and successfully synthesized a series of bimetallic ZIFs (BMZIFs) based on ZIF-67 and ZIF-8. The BMZIFs-derived porous carbons effectively inherit both merits of carbons independently derived from ZIF-67 and ZIF-8, and simultaneously possess the following advantages:ordered porous structure, superior surface area, uniform N dopant, high graphitized degree and abundant CoNz sites. Therefore, the optimal CNCo-20 from BMZIF-20 via adjusted Zn/Co molar ratio performs excellent catalytic performance toward ORR. Upon additional P doping, the P-CNCo-20 exhibits the superior ORR activity than commercial Pt/C and most of reported noble metal free catalysts. In addition, P-CNCo-20 possesses extraordinary long-term stability and ultra-high tolerance to methanol.