Gold Catalysts Supported On Mesoporous Al₂O₃and Metal Microfibers For The Aerobic Catalytic Oxidation Of Alcohols Towards Green Aldehyde/Ketone Synthesis

Because aldehydes/ketones are valuable both as intermediates and as high-value components for the perfume and medicine industry, the selective oxidation of alcohols into aldehydes/ketones is one of the most pivotal processes and challenging reactions in green chemistry, but exigently needs a fundamental shift from methods based on toxic and expensive inorganic oxidants (notably Cr reagents) to greener and more atom-efficient methods that adopt recyclable heterogeneous catalysts and O2as an oxidant. Such aerobic oxidation of alcohols can proceed in the liquid/gas-phase and mainly depend on the thermal stability and volatility of reagents and products. Because of the extraordinary high catalytic performance of gold nanoparticles (Au NPs), many gold-based catalysts loaded on various supports have been developed for liquid-phase oxidation of alcohols, especially for the complex alcohols with high boiling point. For the gas-phase oxidation, the present-used catalysts suffer from the weak heat-transfer ability and bad low-temperature activity. To resolve the present-existing problems, several series of novel and highly efficient catalysts of Au-based catalysts were designed, prepared and characterized. With the help of series of characterization techniques, including electron microscopic, spectroscopic, thermal and adsorption/desorption analysis, the catalysis nature of these catalysts was investigated.●Preparation of the catalyst Au/meso-γ-Al2O3by deposition-precipitation with ureamethod and its catalytic performance for the liquid-phase oxidation of alcoholsA highly active and selective catalyst Au/meso-γ-Al2O3has been successfully developed for the liquid-phase selective oxidation of alcohols by depositing Au NPs onto a home-made mesoporous γ-Al2O3(noted as meso-γ-Al2O3, prepared via cation-anion double hydrolysis (CADH) method) using deposition-precipitation with urea (DPU) method. The experimental testing results indicated that the formation of small Au NPs of～2.0nm definitely contributes to obtaining an excellent catalytic activity, which can be realized by accurately controlling the DPU parameters (including time, temperature and Au/urea molar ratio) and calcination temperature. We anticipate that our results will make the DPU method more useful for developing high-performance gold catalysts, especially ones using an "inert" support and/or others that are difficult to prepare by other methods.In spite of the excellent activity for the liquid-phase oxidation of alcohols with high boiling point, but the catalyst Au/meso-γ-Al2O3and the other reported catalysts of the same kind suffer from low reaction rate and catalyst/solvent separation issues. From the industrial point of view, gas-phase oxidation would be a more effective route because of the convenience of catalyst separation, solvent-free conditions, and much higher production efficiency for the bulk aldehydes and ketones, such as benzaldehyde. Therefore, the microfibrous structured catalysts were developed for the gas-phase oxidation of alcohols.Preparation of the microfibrous supported Au/8μm-Cu-fiber by galvanic reaction, and its catalytic performance and nature for the gas-phase oxidation of alcoholsA highly efficient catalyst Au/8μm-Cu-fiber, with excellent low-temperature activity and heat conductivity, was successfully prepared for the gas-phase selective oxidation of various alcohols. The Au/8μm-Cu-fiber catalysts were obtained by galvanic deposition of Au onto a thin-sheet microfibrous structure consisting of5vol%8-μm Cu-fiber and95vol%voidage. The best catalyst is Au-3/8μm-Cu-fiber-200(Au-loading:3wt%; calcined at200℃in air), which is effective for acyclic, benzylic and polynary alcohols using a high WHSV of20h-1. For example, benzyl alcohol conversion of86%was obtained with99%selectivity to benzaldehyde at220℃. A small AT of<10℃between catalyst bed and reactor external wall was observed in the selective oxidation of benzyl alcohol, owing to the enhanced heat transfer ability. Special AuCu(alloy)-Cu2O active composites are formed during reaction and their cooperative effect contributes to promoting the low-temperature activity. By nature, the AuCu alloy could catalyze oxidation of Cu2O-H species with O2to release active Cu2O sites. What to be noted is that the copper microfiber is easily oxidized and powdered in the long time reaction process, which reduces its catalytic performance and mass/heat transfer ability.Preparation of the microfibrous supported Au/8μm-Ni-fiber by galvanic reaction and its catalytic performance for the gas-phase oxidation of alcoholsAnother catalyst Au/8μm-Ni-fiber, with excellent low-temperature activity, heat conductivity and good stability, was successfully prepared for high-efficiency gas-phase oxidation of alcohols. The Au/8μm-Ni-fiber catalysts were obtained by galvanically depositing Au particles onto a thin-sheet microfibrous structure consisting of5vol%8-μm Ni-fiber and95vol%voidage. The best catalyst is Au-4/8μm-Ni-fiber-300(Au-loading:4wt%; calcined at300℃in air), being effective for acyclic, benzylic and polynary alcohols using a high WHSV of20h-1. For benzyl alcohol, the conversion of>98%was achieved with>98%selectivity to benzaldehyde at280℃, while a very small ΔT of<10℃between catalyst bed and reactor external wall was observed. At250℃, a single-run lifetime of230h and total660h lifetime after two regenerations could be obtained with benzyl alcohol conversion of95%and>99%selectivity to benzaldehyde, showing promising stability characteristics and industrial application.●Origin of the excellent low-temperature activity of Au/8μm-Ni-fiber catalyst:Evidence for NiO@Au Nanostructure and Ni2O3-Au+Hybrid Active SitesLast but not least, the origin of the excellent low-temperature activity of this catalyst Au/8μm-Ni-fiber was investigated as the extension of third part. Firstly, with the help of SEM, TEM, and XPS, it was found that an interesting date-cake-like NiO@Au nano-composites (i.e., partial coverage of20-30nm Au particles, like cake, with NiO segments, like dates) is formed in the pre-activation process. The experiments over a series of contrastive catalysts demonstrated that the NiO@Au nano-composites are essential to the excellent low-temperature activity of the catalyst. Then, the O2-TPD and in-situ FTIR results showed that there are abundant of active oxygen species over the NiO@Au nano-composites. Finally, the XPS and XAFS techniques indicated that the NiO@Au nano-composites provide unique synergistic effect, by nature, inducing a high surface concentration of Ni2O3-Au+hybrid active sites. Ni2O3specimens not only promote the formation of Au+cations and stabilize them but also serve as active O species reservoir. It was also proposed that O2is activated on the oxygen vacancy of Ni2O3and then the adsorbed O atoms spillover onto Au+cations to react with alcohols. Ignited by this active nanostructure, the other transition-metal oxides were selected to partially cover the gold particles. Similarly, these catalysts exhibited excellent low-temperature activity, indicating that such active nanostructure and the proposed catalysis nature is reasonable and popular in the catalytic selective oxidation of alcohols.