Design, Synthesis And Application Of Green Air Oxidation Catalysts

A new and reasonable method to design the oxidation catalyst with high efficiency was built. Instructed by this method, the structure-reactivity relation of the oxidation reaction catalysts was found and a way was proposed to enhance the catalytic ability of traditional acetylacetone catalysts by supporting them on ionic liquid. Basing on this, a new kind of cheap and high effective air oxidation catalysts (ionic liquid-supported acetylacetone catalysts) were designed and synthesized, which overcomes the poor catalytic ability of traditional acetylacetone catalysts and solves the recycle problems of them. These new ionic liquid-supported acetylacetone catalysts were used for the oxidation of the intermediates of Vitamin E (2,3,5-trimethylphenol and 3,5,5-trimethylcyclohex-3-en-l-one). It not only checked the rationality of the design, but also established the base for the green oxidation of corresponding reactions.The purposes of this thesis are not limited in preparing and applying the new catalysts. How to build a reasonable model and find a feasible method to design the catalysts accurately and effectively is more important. Hence, this thesis also concerns the basis of designing catalysts. We found that the rate determing step of some oxidation reactions was a proton transfer process. Based on this common ground, a relationship estimating the barrier of different substrates oxidation was built. The influence brought by water for the oxidation reactions was found, by which some previous experimental phenomena were explained. At the same time, a new kind of catalysts (all-metal aromatic catalyst) was proposed. This new kind of catalysts was predicted to be more powerful than porphyrin metal catalyst.All of above findings were concluded step by step in two research sections, they are: (1) design new air oxidation catalysts; (2) synthesize and apply the new air oxidation catalysts.The fist part of the thesis concerns the design of new air oxidation catalysts. Because the rate determining steps of alkane, alcohol and aldehyde oxidation are all proton transfer processes, the proton transfer process itself was carefully investigated first. Generally, there is water produced in the oxidation reaction. So, we paied much attention to the influence of water molecules on the proton transfer process. The obtained results indicated that in the vicinity of the catalysts, there were two absolutely opposite regions. In one of the regions, water molecules can reduce the barrier of the rate determining step, whereas in another region, water molecules can heighten the barrier of the rate determining step. Using this conclusion, we can explain many experiment phenomena, such as why the reactivity of porphyrin metal catalyst can be enhanced by adding some additional water.Basing on the obtained results, a new kind of oxidation catalysts with simpler structure and more powerful catalytic ability was predicted (comparing with porphyrin metal catalyst). We named this new kind of catalysts as all-metal aromatic catalyst. The all-metal aromatic compounds were proven to have catalytic ability for the first time. Our results indicated that all-metal aromatic catalyst can active the oxygen absorbed on its surface and the actived oxygen has similar character with the active oxygen ion ¹O₂^2-. Further study indicated that the actived oxygen can oxidize the methane to methanol effectively, which demonstrated that all-metal aromatic compound was potential powerful catalyst for oxidation reaction. The reaction barriers of rate determining step of methane, ethane, propane and propene oxidation catalyzed by all-metal aromatic catalyst Al₄Fe are 25.9, 24.0, 22.7 and 21.1 kcal/mol, respectively. The catalytic ability of all-metal aromatic catalyst is probably more powerful than these of porphyrin metal catalyst and most typical inorganic catalysts for oxidation reaction.Acetylacetone metal is one of the cheapest catalysts among organometalliccatalysts. However, the catalytic ability of acetylacetone metal is very weak for oxidation reaction. Furthermore, it is not easy to recycle the catalyst because of its homogeneity. Hence, this part focused on how to design high-effective and reusable acetylacetone metal. Supporting the acetylacetone on ionic liquid was established for the purpose of catalyst recycling. The following works focused on what kind of supporting methods can enhance the catalytic ability of acetylacetone.Because the catalytic mechanism of acetylacetone metal still remains unknown, the reaction mechanisms of alkane, alcohol and aldehyde oxidation catalyzed by acetylacetone metal were investigated first. The obtained results indicated that the rate determining step of these three kinds of substrates oxidation was proton transfer porcess and the barrier of the proton transfer in the alkane oxidation were 9.1 and 12.2 kcal/mol higher than those in the oxidations of alcohol and aldehyde, respectively. Further research revealed that for the same catalyst, there was a good linear relation between the barriers of the rate determining step of different substrates oxidation and the C-H bond dissociation energies. This indicated that the results obtained in designing the catalyst for methane oxidation can be qualitatively used for other systems, which can increase the accuracy and efficiency synchronously.Basing on above researches, we designed a series of ionic liquid supported acetylacetone metal catalysts and non ionic liquid supported acetylacetone metal catalysts. The catalytic abilities of these catalysts were evaluated by using them to catalyze the methane oxidation reaction. Comparation between the designed catalyst and the traditional acetylacetone metal catalyst was made to test the effect of the modification. It was indicated that the following method can enhance the reactivity of the catalysts obviously: the ionic liquid connects with the C₃ position of acetylacetone directly (For example, the catalytic ability of traditional acetylacetone ferrum catalyst can be enhanced by 196 times by supporting it on methylamine ionic liquid).However, the following methods can not enhance the reactivity of the catalysts: the ionic liquid connects with the C₃ position of acetylacetone indirectly; the ionic liquid connects with the C₁ position of acetylacetone; not ionic liquid but simple amine connects with the C₁ or C₃ position of acetylacetone. Further research revealed that there had a good linear relation between the spin densities carried by the metal/0 and the reactivity of the catalysts. As the spin density carried by the O atom increases or the spin density carried by the metal decreases, the reactivity of the catalyst is enhanced. Such structure-reactivity relation is helpful for designing more powerful catalyst.The second part of the thesis concerns the synthesis and application of the new air oxidation catalysts. Three kinds of ionic liquid were used to support the acetylacetone at the C3 position directly according to the method indicated above. These synthesized ionic liquid supported acetylacetone catalysts include imidazolium salts ionic liquid supported acetylacetone catalysts, pyridinium salts ionic liquid supported acetylacetone catalysts and ammonium salts ionic liquid supported acetylacetone catalysts. All these catalysts were characterized by ¹H-NMR, ¹³C-NMR, IR and elementary analysis. These catalysts were applied in the oxidation of 2,3,5-trimethylphenol and 3,5,5-trimethylcyclohex-3-en-1-one, which are two important intermediates for synthesizing Vitamin E in two different synthesis routes. The obtained results proven that the ionic liquid support did bring more powerful acetylacetone catalyst comparing with the traditional acetylacetone catalyst. At the same time, the ionic liquid supported acetylacetone catalyst was also more selective for the oxidation reaction comparing with the traditional one. Taking advantage of better solubility and thermal stability of ionic liquid, the ionic liquid supported acetylacetone catalyst can be simply recycled by water washing or vacuumdistillation.The background of this research is renovation on the traditional oxidation reactions by green chemistry principle. The technology used for this purpose is air oxidation. The key point of this research is to design and develop effective and cheap catalyst. The research methods of this thesis are theoretic research for catalyst design combining with experimental research for checking the results of the design. On experiment, the aims of this research are synthesis and application of new powerful and nontoxic catalysts. On theory, the aim is to find out the metod to design catalyst accurately and effectively. This project comes from the industry and the obtained results are preponderant comparing with some of the previous methods. It provides more choices for the industry. This thesis deals with not only experiments but also theories. Especially, it concerns how to build reasonable model to enhance the accuracy and efficiency of the design process synchronously. The methods used in this thesis are universal. It provides some feasible methods and ideas for the catalyst designing process. Some new kinds of catalysts were brought forward, such as all-metal aromatic catalysts, whose catalytic ability is more powerful than that of porphyrin metal catalysts. It also provides a new method to enhance the catalytic ability of traditional catalysts by supporting them on ionic liquids. These results provide more choise for the researches in catalyst, catalyzing with all-metal aromatic complexes and ionic liquid supported acetylacetone catalysts.