Synthesis And Characterization Of Complex Silver Based Catalysts For The Selective Oxidation Of Alcohols

Carbonyl compounds are important in fine chemical industries, Pharmaceuticals, agrochemicals, perfumes and flavors, which include the aldehyde and the ketone. With the development of petrochemical industry and the modern fine chemical industry, those carbonyls that possess more than one carbon atom have attracted many concerns. Nowadays, the focus is on two types of reactions. One is the selective oxidation of mono-alcohols, where the desired products are often those of moderate oxidation, i.e., aldehydes and ketones. The other is the selective oxidation of polyhydric alcohols, where the target products are often moderately oxidized compounds while the products of lean oxidation (a-hydroxyketone), and where over oxidation (carboxyl acid, CO2) or cracking should be avoided. Up to now several silver-based catalysts including bulk silver and supported silver catalysts have been developed for the oxidation of alcohols. However, some serious problems are still encountered in the application of such catalysts. For example, in the case of conventional bulk silver (e.g., industrial electrolytic silver), the low catalytic activity at low temperatures often leads to the formation of the byproducts of lean oxidation, such as a-hydroxyketone. On the other hand, at a higher reaction temperature (>500 °C), the products of cracking and/or over oxidation are mainly obtained. To improve the catalytic performance of silver-based catalysts, great efforts have been made by either adding additives to bulk silver catalysts, or dispersing the silver particles on suitable supports. Supported silver is expected to enhance the dispersion and stability (anti-sintering ability) of silver, and thus its catalytic activity at relatively lowtemperatures. However, compared with bulk silver catalysts, supported silver catalysts usually have a low heat conductivity, which has greatly restricted their practical application, especially in some strongly-exothermic oxidation reactions. Hence the design of a catalyst with the elaborate structure and better performance is still a challenge for the research in alcohol catalytic oxidations, especially for those with relatively complex molecular structures (e. g., polyhydric alcohols) where the current catalysts are inapplicable because of the complexity in selectivity and/or low activity at low temperatures.In this work, our research is concentrated on the following fields: 1) the study of the catalytic performance of electrolytic silver in the selective oxidation of alcohols; 2) the preparation of the complex La₂O₃/Ag catalyst via the electrolytic inlay method by enchasing the La₂O₃ particles into the interface of the electrolytic particles to improve its anti-sintering ability; 3) the preparation of different types of supported silver catalysts: Ag/SiO₂, Ag/ZrO₂, Ag/SBA-15 and their catalytic performance in the alcohol oxidation: 4) the fabrication of nano-silver/zeolite film/copper grid catalyst via the combination of seed-film method and the electrolytic process and its great catalytic performance for the selective oxidation of alcohols with different structures.1. The electrolytic silver catalyst was prepared through the electrolytic process and was adopted in the selective oxidation of 1,2-propylene glycol and 3,5,5-trimethyl hexanol. It is shown that the electrolytic silver catalyst exhibits good activity when reaction temperatures are above 400°C. However the activity in low temperatures and high temperatures is poor. In the case of 3,5,5-trimethyl hexanol, the catalytic performance of electrolytic silver is not ideal because the complicated molecular structure of 3,5,5-trimethyl hexanol restrains the enhancement of reaction temperature. According to the XRD and SEM investigation, the silver particles in the electrolytic silver are easy to sinter at high temperatures, which is the hurdle for it to be used in the selective oxidation of alcohols with lame molecular sizes.2. La₂O₃/Ag complex silver catalyst was prepared by ultrasonic electrolytic method. The catalysts with different loading weight of La₂O₃ were investigated by SEM, XRD and ICP. The crystal structure of silver was not destroyed after mixing with small amount of La₂O₃ in electrolytic process, but thermo-stability was improved obviously. The activity and stability of complex silver catalysts were compared in selective oxidation of 1,2-propylene glycol reaction system between 1%La₂O₃/Ag and pure electrolytic silver catalyst. The results revealed that at high temperature e.g. 600°C, the conversion of 1,2-propylene glycol over La₂O₃/Ag catalyst was 93.3% and the selectivity to methyl glyoxal was 28.7%; while those two values over the electrolytic silver catalysts are 99.1 % and 8.6%, respectively.3. Three types of supported silver catalysts were prepared: Ag/SiO₂, Ag/ZrO₂, Ag/SBA-15. According to the activity test, the selective oxidation of 1,2-propylene glycol, both Ag/ZrO₂ and Ag/SBA-15 exhibited good catalytic performance in low reaction temperatures, however the improvement of the activity of Ag/SiO₂ is slight. The investigation of the support effect shows that the crystal state of ZrO₂ has great influence over the catalytic behaviors of Ag/ZrO₂ catalysts. The monoclinic ZrO₂ will do most favor to the activity improvements. The UV-vis investigation revealed that in the Ag/m-ZrO₂ catalyst, there lived much more isolated silver ions than that in the other types of Ag/ZrO₂ catalysts. The XRD investigation over the Ag/SBA-15 showed that the introduction of silver would never destroy the specific ordered structure of SBA-15 molecular sieves. The large specific surface of Ag/SBA-15 catalysts provide the silver particles with better dispersion level, which will benefit the formation of active sites in the catalyst surface according to the characterization of UV-vis and XPS.4. The selective oxidation of a series of alcohols to their corresponding carbonyl products was carried out over a rationally designed in situ electrolytic nano-silver/zeolite film/copper grid (SZF) catalyst, which was prepared by a combination of the seed-film method for the fabrication of an ultrathin zeolite filmand the in situ electrolytic process for the formation of highly dispersed silver nanoparticles. At a relatively low reaction temperature (ca. 320°C), the SZF catalyst with highly dispersed in situ electrolytic silver nanoparticles exhibited a much higher activity for the oxidation of mono-alcohols and a higher selectivity for ketonic aldehyde in the oxidation of di-alcohols than the conventional bulk electrolytic silver catalyst. On the basis of the combination of diffuse reflectance ultraviolet visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and thermoanalysis, the remarkably high activity and selectivity of the SZF catalyst was attributed to the highly dispersed silver nanopartieles which were stabilized by the zeolite film against sintering and accordingly the large amount of Ag⁺ ions and Ag_n^δ+ clusters existed in the silver nanopartieles. The improvements of the catalytic performance of the SZF catalyst in a wide application extension will bring new concerns in both theoretical and applied fields.