Preparation And Catalytic Properties Of Supported Au Nanocatalysts For The Selective Oxidation Of Alcohols In Liquid-phase

The selective oxidation of alcohols to carbonylic compounds is one of the most important reactions in both laboratory and industrial synthetic chemistry, as the resulting compounds serve as the important and versatile intermediates in the manufacture of fine chemicals and pharmaceutical industry. From the viewpoints of green chemistry and sustainable development, it is the goal of academia and industry that this kind of reaction is performed by heterogeneous catalysts using oxygen or air as the oxidant under mild conditions, as well as to replace traditional alcohol oxidation processes in which toxic and expensive inorganic oxidants are required. Due to the extraordinary catalytic activity and selectivity, many supported gold nanocatalysts have been developed and used for liquid oxidation of alcohols. It is the key for the selective oxidation of alcohols to design and to prepare the efficient gold catalysts. Based on the factors that affects the performances of gold catalysts,supported gold catalysts are fabricated through the construction of the boundary between gold and the support, the regulation of the size and composition of the support, respectively, which can modulate the electronic of Au nanoparticles and structural properties of the catalysts. By the characterization techniques such as Powder X-ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM), X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) and Ultraviolet-Visible Spectroscopy (UV-Vis),the resulting supports and catalysts are well characterized.These catalysts are evaluated in the oxidaiton of alcohols, and then their structure property relationships and the reaction mechanisms are discussed and determined.The main results are summarized as follows:1. Au-nanoparticles half-encapsulated in nano iron oxide are prepared and loaded on alumina as supported catalysts. The results show that an exposed gold nanoparticle is embedded in an iron oxide nanoparticle in a majority of the resulting nanocomposites. The donation of electrons from nano iron oxide to Au nanoparticles through their boundaty surface is detected and both the properties of gold and iron oxide are adjusted by the donation. The properties are different from the bulk iron oxide supported gold catalysts, in which iron oxide is little influenced by the electronic interaction between the two components. The catalyst shows noticeably promoted activity for the aerobic oxidation of 1-phenylethanol than Au/Al2O3 and Au/bulk FeOx. The enhanced catalytic behavior should result from the cooperative effect between Au nanoparticles and nano iron oxide.2. On the basis of the above results, using nano Fe2O3 as the support, Au/nano Fe2O3 catalysts were prepared by the simpler method, characterized and evaluated in the aerobic oxidation of 1-phenylethanol. The results indicate that the novel Au core/nano Fe2O3 shell formed in which 5 um Au nanoparticles are surrounded by the comparably nanosized Fe2O3, while 3 nm Au nanoparticles are highly dispersed on the surface of bulk Fe2O3 in Au/bulk Fe2O3. Au/nano Fe2O3 shows the enhanced activity for the aerobic oxidation of 1-phenylethanol than Au/bulk Fe2O3.The promoted catalytic behavior is related to the comparably nanosized Fe2O3 and the more boundaries between Au and nano Fe2O3. In comparison with Au nanoparticle size effect, which has attracted considerable attention in current literatures, our results disclose that the support particle size has greater influence on the activity of Au catalysts.3. The mixed NiAl-oxide with varied composition is obtained from the calcination of the corresponding layered double hydroxide (NiAl-LDH) prepared by coprecipitation method. Au/NiAl-oxide is prepared through the deposition precipitation method and used in the selective oxidation of alcohols. The experimental results show that Au/NiAlO-2.5 catalysts exhibit the highest performances. In addition to the electron interaction between Au and the support, the enhanced catalytic property of the Au catalysts is mainly attributed to their structure cooperation. The transformation of a small amount of NiAlO-2.5 adjacent to Au nanoparticles to NiAl-LDH-2.5 occurs, and the catalytic system constituted of Au nanoparticles,NiAlO-2.5 and its precusor promotes the rapid removal of H2O as the byproduct from the Au catalytic sites, which will accelerate the conversion of alcohols and is regarded as the dynamics synergy. The Au/NiAlO-2.5 catalysts are very active for the oxidation of aromatic alcohols and linear aliphatic alcohols, indicative of the high versatility of the catalyst, and also show the good stability.