Study On The Preparation And Catalytic Performances Of Magnetically Recyclable Metal-ferrite Nanocomposites Catalysts

Catalysis plays an important role in fields of fine chemicals, pharmaceuticals, agriculture, environmental protection, etc. It is estimated that more than 90 % of the chemical products of the current commercial processes in the world are derived from heterogeneous catalysis. Traditional solid catalysts are widely used in heterogeneous catalysis, for their simple technique in application and facile separation from the catalytic systems. However, reactions proceed only at the interfaces between catalysts and reactants in heterogeneous catalysis, and that is disadvantageous to catalytic activity comparing with the case in homogeneous catalysis. The introduction of nanotechnology into catalysis brings brand new opportunity for its development, and subsequently comes forth the magnetically recyclable catalyst (MRC) by supporting catalytic active nano-components on magnetic nanomaterials, giving body to the perfect combination of high activity with facile magnetic separation. But as a new type of catalyst, there is still much for MRC to be explored in terms of synthesis, performance optimization, application expansion, structure-activity relationship and so on. In addition, according to the principles of green chemistry, simple preparation, low cost, mild and environmentally friendly reaction conditions as well as high activity are required for MRC. We conducted our study in such a context and a series of new MRCs are synthesized; their catalytic activity, recyclability and stability were tested in certain catalytic systems; a general and facile method for the preparation of MRCs from cheap and readily available precursors is established; what is more, we also explored the green chemistry of p-nitrophenol (PNP) hydrogenation system catalyzed by the as-synthesized MRCs.Firstly, Ag-Fe3O4 nanocomposites were synthesized from cheap and readily available precursors through a simple one-pot solvothermal process under the assistance of polyvinylpyrrolidone (PVP). The characterization results of structure and morphology both prove that Ag-Fe3O4 nanocomposites were formed from the simultaneously assembly of primary Ag and Fe3O4 particles with the assistance of PVP. Magnetization study shows that the as-prepared Ag-Fe3O4 nanocomposites are superparamagnetic at room temperature, ensuring their facile magnetic separation from the catalytic system when the reaction is finished. Study on the catalytic performance indicates that the as-prepared Ag-Fe3O4 nanocomposites exhibit high activity and selectivity for styrene oxide in the catalytic epoxidation of styrene; examination of the catalytic results in five cycles of catalysis confirmed the facile magnetic separation property and stability of the as-prepared Ag-based MRC; moreover, analysis of the catalytic mechanism reveals that certain synergy exist between Ag and magnetite support in enhancing the catalytic efficiency.In order to investigate the applicable scope of the simple one-pot solvothermal synthesis for the preparation of MRCs, other ferrite supports instead of magnetite were used, including Co-ferrite, Ni-ferrite, Mn-ferrite and Zn-ferrite. The magnetically collected products were washed thoroughly. Analysis results of structures, morphologies, contents of each component, as well as the magnetization and surface properties of the as-prepared products all suggest that we have successfully prepared a series of Ag-based MRCs, indicating the extendability of our simple on-pot solvothermal method for the preparation of nanocomposite MRCs. Long chain organic molecule PVP was found to function doubly in the formation of Ag-ferrites nanocomposite MRCs. On one hand, PVP plays as surfactant: it stabilizes the primary particles, preventing their aggregation or further growth into nanowires or other bulk structures. On the other hand, PVP plays the bridging role, linking several particles simultaneously in loss or tight manner to form integral Ag-ferrite nanocomposites. Systematic study on the catalytic behaviors of those Ag-ferrite MRCs (Ag-Fe3O4 included) in the epoxidation of styrene reveals that the support effects of ferrites supports comes from a combination of geometric factor and electronic factor. Firstly, the geometric factor is taken into consideration. The relative amounts of active planes in primary Ag particles are different in the presence of different ferrites supports. Generally, more active planes in Ag mean higher catalytic activity in nanocomposite MRCs. This geometric factor plays decisive role in determining the catalytic activity of Ag in the epoxidation of styrene. Additionally, the electronic structures of M (II) ions in ferrites also affect the catalytic activity (electronic factor) by stabilizing the reactive oxygen intermediate. Owing to stable full-filled d-orbitals of Zn (II), the Ag-based Zn-ferrite MRC with relatively more active planes among the as-prepared MRCs shows the lowest activity in the catalysis.Next, we further simplified the one-pot method and applied it in the synthesis of other catalytic active metal-magnetite nanocomposite MRCs (metal = Pd, Pt, Au, Cu, Ni). Analysis results of the basic characterization data show that a series of catalytic active metal-magnetite nanocomposite MRCs are successfully synthesized, demonstrating that metals other than Ag are applicable in the simple one-pot solvothermal synthesis. Study results on the scope of metals and magnetic supports that can be applied in the synthesis together suggest that a general method for the synthesis of metal-ferrite nanocomposite MRCs has been established. We investigated the catalytic performances of all the as-synthesized metal-magnetite nanocomposite MRCs in the model reaction of p-nitrophenol hydrogenation, and high catalytic reaction rate constants were obtained on most of the MRCs. Control experiments manifest that metals instead of magnetite supports or PVP in the nanocomposite act as the catalytic active components.p-Aminophenol (PAP) is an important intermediate for the synthesis of analgesics and antipyretics in pharmaceutical industry. The consumption of PAP around the world is increasing year by year, and more and more attentions are paid to the catalytic hydrogenation of PNP to produce PAP. However, the present industrial processes for the production of PAP generally proceed under high pressure, high temperature, and in organic solvent, which are far more from green ones. No total green process for this system has been reported in laboratory studies yet. In our study, we proposed a total green system for the catalytic hydrogenation of PNP to produce PAP based on the principles of green chemistry: with the facilely synthesized Pd-Fe3O4 nanocomposites from readily available and cheap precursors as MRC, the catalytic hydrogenation of PNP was conducted in aqueous at room temperature and one atmospheric pressure of hydrogen. Under identical test conditions, the activity of Pd-Fe3O4 nanocomposites is about eight times higher than that of commercial Pd/C, demonstrating the great potential for industrial usage of the proposed green system. Catalytic tests for other MRCs indicate that adjustment in synthesis is an efficient way for tailoring the catalytic performance.In conclusion, we have established a facile and general method for the synthesis of nanocomposite MRCs, which are perfect combination of high activity with easy recovery. This provides a methodological basis for the development of other MRCs. Studies on their catalytic behaviors in the corresponding epoxidation and hydrogenation systems greatly enriched the field of catalysis. A preliminary green system for the catalytic hydrogenation of PNP has been proposed, which is a successful practice of the principles of green chemistry and in line with the requirements of sustainable society. These studies are experimental basis for industrial practice, and also provide theoretical guidance for the green processes of other catalytic transformation systems.