The Application Of Hydrotalcite-like Compounds As Catalysts In Baeyer-Villiger Oxidation

The investigation and exploitation of acid and base solid catalysts have been recently received much attention by catalysis researchers. Hydrotalcite-like ion clays and their composite oxides with certain basicity have better catalytic activity and selectivity in the aldol condensations, the polymerization of olefin oxide, etc. under environmental friendly conditions. On the other hand, hydrotalcite-like compounds with typical laminated structure have high surface areas, favorable hydro-thermo stability and capability of insertion, so they can be used as catalyst carriers. At the same time, the reaction selectivity can be increased via adjustment of the space between layers. Therefore, the application of hydrotalcite-like compounds and their derivate mixed oxides in catalysis is the research hotspot in recent years.Baeyer-Villiger oxidation is a valuable method to convert ketones into the corresponding esters and lactones, which are with high value and difficulty to be synthesized by other way. At present, Baeyer-Villiger oxidation has been a general designation for converting ketones into esters and lactones, because these reactions are sill very important and fundamental in organic chemistry. The research area of this reaction gives rise to the interests of many chemists.Our research work consists of three parts: Firstly, some hydrotalcite-like compounds (HTLcs) were prepared and characterized by FT-IR, XRD, DTA, UV-Raman, Vis-UV/DR techniques to investigate the relationship between structure composition and catalytic activity. Secondly, the Baeyer-Villiger oxidation of 6-methoxy-l-tetralone and 1-tetralone were examined under the reaction system of basic HTLcs oxidized by air and the reaction system of Bronsted acids oxidized by H2O2. Thirdly, the possible reaction mechanisms above were proposed.The main contents of this paper are as follows:The first chapter is a review based on 67 references. In this chapter, it describes the summarization of the preparation methods of the HTLcs, the type ofoxidants and the different catalytic systems of Baeyer-Villiger oxidation to make the research progress at home and abroad area. At the end of this chapter, the research object and content of this paper are also described concretely.The second chapter mainly introduces the preparation of HTLcs by the technique of coprecipitation at low supersaturation, and the application of HTLcs to the Baeyer-Villiger oxidation of 6-methoxy-l-tetralone and 1-tetralone by using a combination system of molecular oxygen and benzaldehyde. The Zn-Cr hydrotalcites were found to be the best catalyst to give high yields of the corresponding lactones.In the third chapter, it describes characterization of part of the samples as synthesized and their calcined products by bulk chemical analyses, power XRD, FT-IR, Vis-UV/DR, UV-Raman, and TG-DTA. The crystalline hydrotalcite-like structures for the as-synthesized Zn-Cr-HT sample, formation of chromate-like species and mixed oxides for the calcined sample have been confirmed by powder X-ray diffraction. It suggests the structure was destroyed for Zn-Cr-HT calcined at 450°C, and it was very difficult to come back to the original structure even if it was treated with vapor under nitrogen atmosphere. The anion between layers is CO32" namely the carbonate type hydrotalcites is confirmed by FT-IR. The DG-DTA results show hydrotalcite-like compounds have three thermal decomposition courses. The first band at about 100°C is assigned to the removal of water molecules from the interlayer space and the dehydration of the physisorbed water on the surface. The second band is due to removal of water molecules (from condensation of hydroxyl groups from the brucite-like layers) and of carbon dioxide (from interlayer carbonate anions) at about 200°C, thus leading to collapsing of the layered structure. The system changes from out-of-order to in order and forms new species because there is an exothermic band at about 500°C.In the fourth chapter, it gives the results of the Baeyer-Villiger oxidation of 6-methoxy-l-tetralone and 1-tetralone by using hydrogen peroxide as oxidant, Br0nsted acids as catalyst in the solvent of 2,2,3,3-tetrafluoro-l-propanol. The influences at different reaction conditions were discussed. The results indicate thatTFP is the best solvent and HC1 has the best catalytic activity among all the reagents selected.In the final chapter, the possible reaction mechanisms of 6-methoxy-l-tetralone and 1-tetralone mentioned in chapter two and four are discussed. As the reaction was catalyzed by basic hydrotalcite-like compounds, the product was only lactone. But two products were detected as the reaction was carried out in the solvent of 2,2,3,3-tetrafluoro-l-propanol catalyzed by acids with hydrogen peroxide. Lactone structure could not be kept under acid medium and might be hydrolyzed to the corresponding hydroxyphenyl substituted butyric acid. The hydroxyphenyl substituted butyric acid with -OCH3 group in the benzene ring would have higher reactivity and finally be rearranged to be another tetralone. It is interesting to get a dicarbonyl compound by the oxidation of 1-tetralone. The possible reaction pathways are described in detail in this chapter.