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A Synopsis entitled Designing metal oxide supported nano-catalysts for applications in organic reactions Submitted to Rajiv Gandhi University For registration of Ph.D Degree in Chemistry By Miss. Shreemoyee Phukan, M.Sc. Department of Chemistry Rajiv Gandhi University, Rono Hills Doimukh – 791 112 (Arunachal Pradesh) INDIA 2014 Title: Designing metal oxide supported nano-catalysts for applications in organic reactions Name: Ms. Shreemoyee Phukan Origin of the research problem Transition metal nanoparticles (MNPs) have attracted increasing attention because of their well–defined large surface area, stability, and surface permeability.1 Such materials have potential applications in a number of areas including efficient catalysts, optoelectronic devices, sensors, drug-delivery carriers, energy-storage devices, chemical/ biological separation and sensing.1-8 Although a growing number of applications have appeared lately for MNPs, the catalytic application of such MNPs in different organic reactions has just begun to unfold.9-13 The potential advantages of using the MNPs in catalysis include high surface–to–volume ratio and their high surface energy and achieving better selectivity’s and reduced eluent problems. As in any heterogeneous catalysis, the parameters to control include particle composition, size and shape of the MNPs. In addition to this, composition of the support substrates and the organizational structure of the porous networks used as support are other vital parameters that have key role in supported MNPs catalyst.14,15 Although MNPs itself offer higher catalytic properties but in the most of the cases, they are dispersed in support materials. The advantages associated with such supported catalyst are: i. The catalyst is easily and safely handled compared to the particles in the colloidal phase, ii. The metal catalyst immobilized on a support could be easily separated from the product and effectively reused, iii. Because metal nanoparticles are well separated from each other, they do not grow in size by sintering when heated to high temperature in a reducing atmosphere, iv. The reactant molecules get more rooms for adsorption as metal oxide particles offer higher surface area depending on its morphology, and v. The support provides a means of bringing promoters into close contact with the particles. A lot of organic and inorganic supports, including microporous polymers,16,17 carbon materials,18 amorphous or mesoporous silica,19,20 metal oxides,21,22 and molecular sieves,23 have been explored in metal mediated catalysis, in which metal complexes or nanoparticles are involved as active specie. Among different support materials reported in literature, metal oxides in nanometer dimension such as SiO2, ZnO, FexOy, Al2O3, TiO2 or MgO are widely exploited to carry MNPs as they offer high thermal and chemical stabilities combined with a well–developed structure and high surface areas, meeting the requirement of applications in catalysis.22 Also metal oxide nanoparticles are considered to be non–toxic and hence are environmentally friendly.22 Among different reactions catalyzed by supported MNPs, the prominent reactions include epoxidation of alkene,24-26 oxidation of alcohol,27-30 allylic oxidation,31,32 CO oxidation,33-36 C–C coupling reaction37-40 and hydrogenation.41-43 For instance, Zhang et al. have reported the use of magnetic nanoparticles supported Pd catalyst in Suzuki–Miyamura coupling reaction. They further showed that the catalyst system is recyclable.37 Park et al. reported a catalyst system consists of SiO2 and Pd which was used in Suzuki coupling reaction aryl halides.39 Wu et al. reported a magnetic core-shell nanocomposite, Fe3O4@SiO2@Pd-Au synthesized by reducing palladium and gold salts previously bound to the amine-ligand modified surface of silica-encapsulated magnetic iron oxide nanoparticles. This system is served as highly efficient and easily-recyclable catalyst for liquid phase hydrodechlorination of 4-chlorophenol under mild conditions.44 A catalyst system made of subnanometer Ag aggregates on alumina supports was reported by Cheng et al. for use in epoxidation of propylene.24 Soomro et al. reported the use of supported Pd as catalyst for Suzuki coupling reaction in pure water.45 Singh et al. reported the synthesis of Pd supported on zinc ferrite. They further used them as active catalyst for ligand free Suzuki and Heck coupling reaction.40 Haruta and his co-workers’ discovery of gold nanoparticles that are very active for CO oxidation were supported on oxides of 3d transition metals of group VIII, namely, Fe, Co, and Ni.46 Lakshmi Kantam et al. synthesized MgO stabilized Pd NPs by counter ion stabilization of PdCl42- with nanocrystalline MgO followed by reduction.47 This catalyst system was found to be very active in the Suzuki cross-coupling of aryl bromides and iodides with several arylboronic acids in pure water at room temperature. Over the past decades, many efforts have been made in the development of different methods for the design and fabrication of supported MNPs such as wet chemical and physical techniques. Most of the reports use strategies such as adsorption and deposition-precipitation, impregnation, immobilization on surfaces functionalized with appropriate ligands, coprecipitation, sol–gel, vapor-phase deposition etc. For example, Su et al. synthesized metal sols by borohydride reduction method and subsequently supported the MNPs on TiO2 using a sol–immobilization method.48 Ge et al. reported the synthesis of different shaped iron oxide nanoparticles by precipitation techniques. Pd particle was then loaded on these synthesized iron oxide supports by deposition–precipitation method.14 Xu et al. reported the preparation of Ni/ ZrO2 catalyst system by impregnation method.15 First they synthesized ZrO2 sample by supercritical drying method and subsequently the Ni/ ZrO2 catalyst system were obtained by impregnation of aqueous Ni(NO3)2 onto the oxide precursors dried at 270 C. Considering the recent developments in the field of catalysis, the current focus of this proposal is to design a metal oxide supported MNPs system by simple chemical approach involving greener approach. In addition, a major focus will be given to the study of catalytic behavior of these supported MNPs towards various organic reactions. Although a lot of reports exist on supported MNPs based catalysis but the details study of the reaction mechanism occurring on supporting MNPs surface is still need attention. So, investigation of the catalytic reaction mechanism going on those catalyst surfaces will also be taking into consideration. A detail of our objectives is outlined below: Proposed aim and specific objectives of the investigation i. To design a methodology by which a diverse range of metal oxide NPs such as ZnO, ZrO2, SnO2, MgO, SiO2 etc. can be synthesized. ii. Incorporation of MNPs such as Pd, Ag, Au, Co, Ni etc. into the synthesized metal oxide nanoparticles by different wet chemistry techniques. iii. To tune the size, shape and morphology of metal oxide nanoparticles. iv. To characterize the synthesized metal oxide and supported metal nanoparticles by different microscopic, spectroscopic, diffractometric techniques. v. To study the catalytic properties of these metal oxide supported MNPs in different organic reactions. vi. To investigate the possible mechanism of the catalytic reaction. Significance of the Study It is well known that catalytic activities of a heterogeneous catalyst are dependent on the surface area of the catalyst. A variety of organic reactions are performed with transition metal ions as catalyst. These are homogeneous catalysis and are very difficult to recover for further use of the catalyst which is a major concern in terms of reusability. Instead of metal ions as catalyst, if the same reactions can be performed with metal nanoparticles catalyst, then it will be more advantageous. In some cases, supported MNPs are used as the support materials offer extra surface area and a small amount of MNPs are needed. The advantages associated with the use of such catalyst are: (i) being nanometer size, only small amount of catalyst will be required, (ii) more surface of the catalyst will be accessible to the reactants due to high surface–to–volume ratio of the nanoparticle and hence reaction rates expected to be increased, (iii) being heterogeneous catalyst, the nanoparticles can be recovered and further use them for subsequent sets of reactions, (iv) use of magnetic oxide nanoparticles will ease the recovery process of the catalyst as those magnetic nanoparticles can be recovered easily with the help of a magnet without hampering its physical and chemical properties and (iv) nanoparticle catalyst are environmentally safer than their corresponding metal ions. So, considering the above advantages, it is expected that use of supported MNPs will be environmentally benign and will reduce the cost of catalyst. As the process is scalable, industrial scale synthesis of catalyst is possible in the method. Methodology In the proposed project, I will focus on the design and synthesis of metal oxide supported MNPs for application in catalysis. The MNPs that would be considered for the current study will include Au, Ag and Pd. As support materials, metal oxides such as ZnO, MgO, SnO2 will be used. From the point of view of synthesis, I will adopt water based wet chemical methods such as co-precipitation, impregnation and sol–immobilization technique. A typical synthesis procedure is presented here schematically as shown in Scheme 1. The products and the intermediates will be characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet–visible (UV-vis) spectrometry, Fourier transform–infra red (FT–IR) spectrometer, dynamic light scattering (DLS), X–ray photo electron spectroscopy (XPS) and fluorescence spectrometer. 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