Authors: Anh Hoang Tu Nguyen Sorenson
Source: ScholarsArchive  (Brigham Young University) (article, 2020-03-26)
Field: Chemistry — Organic Chemistry

Immobilization of Copper Nanoparticles onto Various SupportsApplications in Catalysis Copper-based materials are one of the most promising catalysts for performing transformations of important organic compounds in both academic and industrial operations. However, it is challenging to consistently synthesize highly active and stable copper species as heterogeneous catalysts due to their relatively high surface energy. As a result, agglomeration usually occurs, which limits the catalytic activities of the copper species. The work presented in this dissertation shows different synthetic strategies for obtaining active and stable copper-based materials by modifying chemical/physical properties of copper nanoparticles (NPs). Emphasis is placed on discussing specific catalytic systems, including carbon-supported catalysts (monometallic and bimetallic copper-based heterogeneous catalysts) and titania-supported catalysts, and their advantages in terms of catalytic performance. In recent years, there has been increasing interest in using metal-organic frameworks (MOFs) as a sacrificial template to obtain carbon-supported NPs via a thermolysis process. The advantages of using MOFs to prepare carbon supported nanomaterials are a fine distribution of active particles on carbon matrix without post-synthesis treatments and corresponding increased catalytic activity and stability in many reaction conditions. To better understand the potential of this synthetic approach, MOF pyrolyzed products have been characterized. Then, they were applied as heterogeneous catalysts for several chemical reactions. In particular, the high energy copper-based MOF, CuNbO-1, was decomposed to obtain an amorphous copper species supported on carbon (a-Cu@C). This catalyst was found to be highly active for reduction, oxidation, and N-arylation reactions without further tuning or optimization. Higher catalyst turnover numbers for each of these transformations were obtained when comparing a-Cu@C activity to that of similar Cu-based materials. To improve catalyst performance, a secondary metal can be introduced to create synergistic effects with the parent copper species. In order to gain insights into the role of the second metal, a well-known Cu-MOF, HKUST-1, was doped with nickel, cobalt, and silver solutions, followed by a decomposition process with 2,4,6-trinitrotoluene (TNT) as additive. This additive was used to enhance the rapid thermolysis of the bimetallic MOFs. In these bimetallic systems, the addition of a second metal was found to help in dispersing both metals over the carbon composite support and in influencing the particle size and oxidation state of the metals. Catalytic performance showed that even <1% of a secondary metal increased the rate for nitrophenol reduction. Optimal catalytic performance was achieved using a Ni-CuO@C bimetallic catalyst. Another synthetic strategy for Cu-catalyst preparation involves using the deposition-precipitation method, in which a copper catalyst anchored on a titania support was synthesized at low weight % in order to obtain a single atom catalyst (1-Cu/TiO2). The higher copper loading catalyst, 5-Cu/TiO2, was synthesized as a benchmark catalyst for comparison. The copper structure in the synthesized catalysts was investigated by powder X-ray diffraction (PXRD), Raman, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX), X-ray photoelectron spectroscopy (XPS), N2 physisorption and inductively coupled plasma mass spectrometry (ICP-MS) in order to characterize physical and chemical properties. STEM-EDX observations showed single atom copper species less than 0.75 nm in size, as well as nanoparticles with an average diameter of ~1.31 nm. This catalyst was highly active in the reduction of nitro-aromatic compounds with NaBH4 at room temperature. The small to atomic level sizes of the Cu species and multiple oxidation states of Ti species were found to play a crucial role in the catalytic activity. Catalysis ; Bimetallic strip ; Copper ; Nanoparticle ; Nanomaterials ; Heterogeneous catalysis ; Carbon fibers ; Amorphous carbon ; Pyrolysis Nanomaterials for catalytic reactions ; Metal-Organic Frameworks: Synthesis and Applications ; Catalytic Processes in Materials Science