Development Of The Catalyst Used In Aerobic Oxidation Of β-Isophorone And The Study Of Their Mechanism

Green oxidation of organic compounds has attracted much attention in the field of chemistry and chemical engineering. Molecular oxygen is the cheapest and cleanest oxidant, and when it is used in oxidation, its by-product is only H2O. Thus the best way for the conversion of hydrocarbons to oxygen-containing products is through molecular oxygen oxidation under mild conditions. However, since molecular oxygen is inert, it could not react with substrates directly. In fact, molecular oxygen must be activated firstly by appropriate catalysts. Therfore, the development of high active and selective catalysts is the key point for molecular oxygen oxidation. In the present work, the oxidation of (3-isophorone with molecular oxygen as the oxidant was studied. Several kinds of homogeneous and heterogeneous catalysts have been developed for the environmental and economical point of view. Meanwhile, the reaction mechanism was investigated by optimizing the reaction conditions, and the characterization of the intermediates.We firstly focused on the synthesis of a series of amino acid Schiff base catalysts (including salen-type Schiff bases and tridentate Schiff bases). All of them were applied in the oxidation of P-isophorone, and the reaction conditions have been optimized. The results indicated that all the obtained amino acid Schiff bases provided higher activity and selectivity than general Schiff bases used before. The tridentate Schiff bases were more simple, more stable, cheaper, and more facilitate to be synthesized, compared with the salen-type ones. On the other hand, detailed studies showed that the M-Ami-sal Schiff bases with different functional groups displayed an important role in the oxidation ofβ-isophorone. It was found that the tridentate Schiff bases containing hydroxyl group and/or carboxyl group on side chains was favorable for improving its activity, while increasing the alkyl length improved its selectivity. This result was valuable in designing of different Schiff base catalysts with special functions.Since the above homogeneous catalysts were difficult to be recycled and reused, we then developed the heterogeneous catalysts, including poros silica suppoted FeCl3 and di-block copolymers supported CuCl2. Both of them could be recycled through simple filtration from the reaction mixture. Further studies confirmed that pyridine modified silica could immobilize FeCl3 firmly. The active site (FeCl3) was isolated, and the silica support played the cooperation effect with the active site. Thus the supported catalysts provided higher selectivity than that using the support and the active site separately. On the other hand, the di-block copolymers were used as the ligand to coordinate with copper ions. Though PDMAEMA block decreased its dispersion ability due to the combination with copper ions, the PEG block in the periphery remained well dispersion ability. Thus the finely dispersed CuⅡ-PDMAEMA-b-PEG clusters and their nano-scaled particle size notably provided high activity for the oxidation ofβ-isophorone. Besides, the coordination of copper ions with PDMAEMA blocks resulted in a stable CuⅡ-N2O2 structure. Thus it presented as high selective homogeneous catalysts used before. Moreover, an important feature of the CuⅡ-PDMAEMA-b-PEG catalyst was that its particle size could be changed under different temperatures. Hence, it could be easily leached from the mixture and recovered through simple treatments, allowing for their reuse.All the above catalysts contained metal ions as the active site. Because the use of metal ions would produce much pollution to the environment, we developed the metal-free catalysis with nitroxyl radicals (such as PINO, TEMPO, etc.) as catalysts in the oxidation of P-isophorone for the first time. It was the most environmental benign method since no metal ions was participated in the oxidation reaction. Further studies found that only catalytic amounts of the nitroxyl radical catalysts were needed in the oxidation. They provided high efficiency (KIP yiled> 90%) in the absence of any metal-containing or non-metal containing co-catalysts. The oxidation mechanism was hypothesized and discussed with ESR, GC-MS and theoretical calculation methods. This oxidation mechanism well expained that why only catalytic amounts of the nitroxyl radicals without any metal-containing or non-metal containing co-catalysts, could efficiently catalyze the oxidation ofβ-isophorone.All the synthesized catalysts were not only used in the oxidation ofβ-isophorone. To check their applibility, all of them were used in the oxidation of some other cyclic olefins, includingα-pinene, cyclohexene, and their derivatives. The results confirmed that the novel synthesized catalysts provided higher activity and selectivity than general catalysts used before.The background of the present study was the green process of the traditional oxidations. Several kinds of homogeneous and heterogeneous catalysts were developed for the aerobic oxidation ofβ-isophorone. We have offered a new method for the synthesis, characterization and oxidation. These results provided more choises for the researches in other oxidations.