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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.
For the application purpose, the supported metal catalyst will be used for various
organic reactions such as C–C coupling, epoxidation of alkene and oxidation reactions. The
catalysis reactions will be monitored by GC spectrometry techniques. A typical procedure for
catalysis reaction involving oxidation/ epoxidation is shown in Scheme 2 below.
Scheme 1.
Scheme 2.
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