Metal-organic frameworks (MOFs) are built from metal nodes (SBU) connected by organic spacers (linker) and represent a class of crystalline, highly porous materials. The modular design allows an easy and effective way to vary the cavity sizes, geometries and apertures. MOFs owe an ever-expanding research interest over the past number of years to this fact. Especially its great variability can be used to address the challenging requirements concerning the use as support in heterogeneous catalysis. Furthermore, the high functionalizability, the high inner surface area as well as the good accessibility of the pores contribute to the great success of those materials in the field of heterogeneous catalysis. Their porous structures allow the incorporation and stabilization of metallic nanoparticles inside the pores of MOFs. The deposition of a suitable metal precursor within the cavities, using the MOCVD method, eliminates several disadvantages of liquid-phase loading methods – for example, the precursor molecules do not compete with solvent molecules inside the pore. Due to this fact, high metal loadings as well as high control over the loading process can be achieved. In the present work, the loading of MIL 101 with a MOCVD precursor for the catalytic versatile metal Iridium was realized for the first time. The screening of different precursor materials led to an optimum in volatility, stability and reducibility towards to generation of pore size conform, metallic nanoparticles. This novel material, denoted as Ir@MIL-101, was characterized by PXRD, HAADF-STEM, TEM, ICP-OES as well as nitrogen adsorption measurements. The reduction of multiple bonds and ketones as well as the dehydrogenation of alcohols gave an insight in the catalytic performance of this catalyst. Catalysis involving metallic nanoparticles is highly active, yet the generation of selectivity profiles can often be realized easier using homogenous catalysts. Therefore, the immobilization of homogenous catalysts by SBU-grafting offers a great opportunity for the introduction of further catalytic functionalities within MOFs. The removal of auxiliary ligands at the SBUs of MIL-101 and the subsequent recoordination of the model substrate 1-octanol appeared to be an easy way of functionalization. By applying this concept, the specific macroscopic properties of MIL-101 can be modified. In a concrete example, the SBU-grafting with the aliphatic alcohol 1-octanol resulted in the hydrophobization of MIL-101, allowing the compatibilization with polyethylene in different concentrations. The distribution of MIL-101 particles within the polyethylene occurred to be quite homogenous. In the foaming application, the MOF revealed a positive effect as a foam cell nucleating agent. Those foams exhibit a higher density reduction as well as a more homogenous foam cell distribution and foam cell size distribution. Considering the example of 1-octanol, the grafting of hydroxygroups turned out to be a suitable tool for the functionalization of MIL-101. Therefore, one ligand of a well-established photocatalyst was redesigned to incorporate hydroxy groups which were then grafted to free coordinations sites of the MOF. The subsequent synthesis of the photocatalyst inside the pores of MIL-101 was successfully proven by IR-spectroscopy. The photocatalytic activity of the resulting material (denoted as [Ir]@MIL-101) was demonstrated by aza-Henry reaction. This MOF-based photocatalyst was characterized using cyclic voltammetry and nitrogen adsorption measurements. After the functionalization the inner surface area of [Ir]@MIL-101 turned out to be over 1.000 m²/g. In the last part of this work, the remaining inner surface was used to incorporate metallic nanoparticles inside the pores of the MOF additional to the photocatalyst. Therefore, we used the newly developed loading of iridium as well as the well-established MOCVD-loading of palladium, nickel and platinum. The catalytic activity of the photocatalyst combined with metallic nanoparticles within the same MOF, was investigated using the water reduction reaction. The combination of platinum or nickel with [Ir]@MIL-101 showed increased hydrogen evolution rates. While Pt/[Ir]@MIL-101 showed a raised activity by about 35 %, the activity of the Ni/[Ir]@MIL-101 raised by 650 %. The localization of both catalysts in the same pores leads to a spatial proximity, which could be responsible for the increased activity, based on a synergistic effect. The bimetallic Ni/[Ir]@MIL-101 was characterized by PXRD and EDX-mapping, which showed a homogenous distribution within the MIL-101 crystallites for the photocatalyst as well as for the nickel particles. Furthermore, the successful reduction of the nickel precursor towards pore size conform metallic nanoparticles was proven. Ni/[Ir]@MIL-101 showed a significantly increased activity compared to catalysts based on commercial available catalysts.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/4(E), Dissertation Abstracts International|
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