Preparation And Characterization Of Highly Active Iron Oxide-based Catalysts And Their Application In Alcohol Oxidation And Oxidative Coupling Reactions

Alcohol is a kind of very important industrial block and fine chemicals, which was widely used in organic synthesis, energy and chemical industry. It is an efficient way to increase the additional value of alcohols by transforming them into aldehyde, ketone, carbonyl as well as imine compounds via selective oxidation or oxidative coupling reactions. Oxygen is one of the ideal oxidants. Catalytic oxidation of alcohols with molecular oxygen or air as oxidant, only water is produced as a by-product, is a general and environment friendly chemical technology. So far, a series of effective noble metal-based homogeneous and heterogeneous catalysts have been reported in this transformation. Non-noble metal based heterogeneous catalyst, however, usually requiring drastic conditions, such as high temperature and pressure as well as relative long reaction time. To meet the requirements of low cost in industrial application, it is desirable to design and synthesize highly active non-noble metal catalysts which could work under relative mild conditions.Iron is the second most abundant metal in the Earth’s crust. The advantages of ready available, low cost, and low toxicity make iron an ideal metal for catalysis. Traditionally, iron oxides were regard as inert under mild condition, particular for molecular oxygen activation. In this thesis, various synthetic strategies were employed to tailor the redox property of iron oxide, aiming to developing highly active iron oxide catalysts for alcohols oxidation under mild conditions(< 373 K). Mesoporous carbon materials with different pore structure and surface property were employed as supports, and a series of iron oxide catalysts with different redox property were obtained by controlling the dispersion and crystal phase of iron oxide particles. These iron oxide based catalysts exhibit relative high redox activity for molecular oxygen activation and high catalytic activity in selected oxidation of alcohol and oxidative coupling of alcohol and amine to imine. The physicochemical properties(structure, surface, and crystal phase etc.) of the obtained iron oxide catalysts were deeply studied based on a series of characterization. In addition, the correlation between the catalytic activity and physic-chemical properties of the catalysts as well as the catalytic mechanism of the reaction was also discussed. The main contents and results are as follows:1. Highly dispersed iron oxides on mesoporous carbon for alcohol oxidation and oxidative coupling reactionsHighly dispersed iron oxide supported catalysts(Fe Ox/HCMK-3) were prepared with a traditional wet impregnation method using HNO3-treated CMK-3(HCMK-3) as support. Characterization results show that iron species present as quite small particles highly dispersed on the HCMK-3 support. When using CMK-3 without any treatment as a support, iron oxide particles with diameters of 5-20 nm was obtained. Fe Ox/HCMK-3 exhibits relative high redox activity for molecular oxygen activation under mild condition. It could work as a catalyst for benzyl alcohol oxidation with air as sole oxidant. A 71.8 % conversion of benzyl alcohol was obtained after 8 h reaction at 80 oC, which is much higher than that of bulk iron oxides catalyst as well as carbon supported iron oxides with the particle size of 5-20 nm(the benzyl alcohol conversion is 0.9 % and 16.3 %, respectively). Fe Ox/HCMK-3 also exhibited high catalytic activity and stability in oxidative coupling of alcohol and amine to imine under mild condition. When benzyl alcohol and aniline were used as reagents, a 98.8 % yield of imine was obtained after 6 h reaction with air as oxygen source. The catalyst could be reused for four times without significant loss of activity and selectivity. Characterization result shows that iron oxides in Fe Ox/HCMK-3 are facilely reduced compared with carbon supported 5-20 nm iron oxide nanoparticle and bulk iron oxide. The results above show that decreasing the particle size is related to the higher concentration of surface defect and low coordinated metal atoms in the small iron oxide particles. And it should be responsible for the facile changing oxidation state of iron oxides and relative high catalytic activity in the alcohol oxidation reactions.2. Effects of microstructures of carbon support on the catalytic performance of supported iron oxide catalysts in air oxidation of alcoholsAccording to above works, the intrinsic properties of carbon support seem to be important factors in the catalyst design. In this section, the concrete role of the microstructure of carbon support especially pore structure and surface properties in the catalysts design and the catalytic performance of iron oxides catalyst was investigated in detail. We adopted a modified hard template method for the fabrication of mesoporous carbon with abundant surface functional groups and tunable pore size. Using this carbon as support, supported iron oxide catalysts with small particle size(< 2 nm) and high dispersion state were obtained. Catalytic results show that the activity of these supported iron oxide catalysts were obviously enhanced compared with that of Fe Ox/HCMK-3. The catalysts also show relative high stability and recyclability in the reaction process.Systematic investigation results show that the porous structure of carbon support is not the key factor for the enhancement of catalytic performance, whereas the surface property of carbon support is of vital importance in highly active catalyst design. To understand the role of surface oxygen functional groups in constructing iron oxide active sites, the surface property of carbon support was tuned by different passivating treatment. With the decrease of surface oxygen groups on the support, the aggregation degree of iron oxide particle increased and the catalytic activity of the obtained catalyst decreased in alcohol air oxidation obviously. In the process of catalyst preparation, the oxygen functional groups could render the carbon surface negatively charged and serve as strongly active sites for anchoring positively charged Fe3+ ions, resulting in high dispersion of small iron oxide particles. These functional groups also provide a suitable coordinate environment for small iron oxide, increasing the electron density of iron centers and forming active sites for the oxidation of benzyl alcohol with molecular oxygen.3. Preparation of magnetic iron oxide-based catalysts and their application in alcohol oxidation and oxidative coupling reactionsThe application of magnetic nanocatalyst could simplify the recovery process of catalysts from liquid reaction mixtures by using an external supermagnet. In this section, two magnetic iron oxide based catalysts, carbon supported Fe3O4 nanoparticles(Fe3O4/C) and γ-Fe2O3 nanocatalyst, were prepared with different synthetic strategy. They could serve the dual role of catalysts and magnetically recoverable entity in alcohol oxidation and oxidative coupling reactions. Fe3O4/C was prepared via an ammonia-assisted precipitation method. A kind of mesoporous carbon was used as the carbon support and the nucleation rate and crystal phase of iron oxide could be controlled by employing ethanol and ammonia as solvent and alkali precipitator, respectively. Fe3O4/C exhibits relative high activity in selective oxidation of various benzyl alcohols with air as the oxidant. Bulk Fe3O4, however, showed very poor catalytic activity under the same condition. Characterization results show that Fe3O4 in Fe3O4/C is more reducible than that in bulk Fe3O4, which should be due to the small particle size of iron oxide. Meanwhile, the obtained Fe3O4/C possesses relatively high superparamagnetism, it could be easily recovered by using an external supermagnet and reused for more than five times.In another work, γ-Fe2O3 nanocatalyst was synthesized by thermal treating a kind of Fe3O4 nanoparticles. The catalyst has a pure gama phase structure and relative high surface area. γ-Fe2O3 nanocatalyst showed a relative high catalytic activity and stability in imine synthesis from oxidative coupling of alcohol and amine with air as oxidant. Besides that, γ-Fe2O3 nanocatalyst possesses stronger magnetic property than that of Fe3O4/C, which could facilitate its recovery and reutilization in oxidation reactions. Characterization results showed that the lattice oxygen should participate in the reaction and the imine synthesis over γ-Fe2O3 was proved obeying a redox mechanism. Meanwhile, the catalytically active γ-Fe2O3 possessing both redox site and lewis acid site and the synergistic effect these sites should be responsible for the high activity in the oxidative coupling reaction.4. Correlation between the structure and surface properties of graphite oxides and their catalytic behavior in air oxidation of benzyl alcoholIn recent years, nanostructured carbon materials have shown promising abilities in many transformations as one kind of metal-free heterogeneous catalysts. Nanocarbon materials with different structure and surface property were successfully developed with various methods. Many efforts were devoted to investigate the stability and catalytic mechanism of these nanocarbon catalysts in specific reactions. In this section, graphite oxides(GO) were chosen as nanocarbon catalysts, and thermal treatment was carried out to prepare GO catalysts with different structure, texture and surface properties. Benzyl alcohol oxidation with air as oxidant source was carried out to detect the catalytic behaviors of different GO materials. Characterization results show that the structures of GO materials change gradually from multilayer sheets to a transparent ultrathin 2D structure of the carbon sheets. Meanwhile, the concentration of surface oxygen functional groups decrease significantly upon treating temperature increasing, particular for hydroxyl and carboxyl groups. Catalytic results show that the benzyl alcohol conversion decreases obviously with the increase of thermal treated temperature. Combined with catalytic results, it shows that GO could catalyze the alcohol oxidation reaction, and the surface carboxylic groups should function as active sites. In the catalytic process, alcohol was firstly oxidized by reacting with carboxylic groups and the active sites recovered via being re-oxidized by molecular oxygen. In addition, because of the low stability of surface functional groups on GO, the catalytic activity decreased gradually in cycle process.