CHEMCATCHEM
FULL PAPERS
DOI: 10.1002/cctc.201400067
Optimized Dispersion and Stability of Hybrid Fe3O4/Pd
Catalysts in Water for Suzuki Coupling Reactions: Impact
of Organic Capping Agents
Hyunje Woo, Kyoungho Lee, and Kang Hyun Park*[a]
Pd nanoparticles were easily immobilized on Fe3O4 micro-
spheres without any treatment such as functionalization with
organic groups. The Fe3O4 microspheres were synthesized by
using different capping agents, and the effect of the capping
agents on the dispersion stability and catalytic activity of the
Fe3O4/Pd composite for Suzuki coupling reactions in water
were evaluated. The catalytic activity varied depending on the
dispersion stability of Fe3O4/Pd catalysts in water. The Pd nano-
particles immobilized onto Fe3O4 microspheres capped with tri-
sodium citrate showed strong dispersion stability in water over
the reaction duration of seven days (at pH 7), high catalytic
activity, and recyclability under mild conditions.
Introduction
In recent years, a number of nanoparticles (NPs) have been
prepared by using various capping agents,[1–3] because capping
agents are essential to control shape, size, and crystal structure
in nanomaterial synthesis. Furthermore, as the effective disper-
sion of NPs in various solvents is important for biological and
pharmaceutical applications,[4,5] the use of hydrophilic or hy-
drophobic capping agents and surface modification of NPs
have been attempted to control the dispersion stability in or-
ganic and nonorganic solvents.[6,7] Surface modification of
magnetic NPs has received particular attention because of
their utility in various state-of-the-art technological applica-
tions and the fundamental scientific interest in magnetic NPs,
including their use as magnetic storage media,[8] in medical di-
agnosis and therapy,[9,10] and as contrast agents in magnetic
resonance imaging.[11,12] Considerable effort has been made in
the development of hybrid catalysts by using Fe3O4 micro-
spheres and transition-metal NPs.[13,14] Hybrid magnetic cata-
lysts offer the advantage of facile separation from the products
by using an external magnet, and thus, lend themselves to ef-
fective recycling. To functionalize the surface of Fe3O4 micro-
spheres in the hybrid catalyst, Fe3O4 microspheres have been
coated with various materials such as SiO2, polymers, and
carbon.[15–17] Functional groups such as 3-aminopropyl triethyl-
silane and hyperbranched polyglycerol have been coated onto
the surface of Fe3O4 microspheres to immobilize monodisperse
transition-metal NPs.[18,19] These functional groups conferred
dramatic stabilization to the metal NPs; however, multiple
steps are required to synthesize the catalysts. Therefore,
a simple technique for the synthesis of stable hybrid magnetic
catalysts without the requirement for the treatment of Fe3O4
microspheres is desired. To fabricate a suitable hybrid magnet-
ic catalyst, it is quite essential to develop techniques for con-
trolling the dispersion of NPs in various solvents to apply them
to functional materials. According to the classical Derjaguin–
Landau–Verwey–Overbeek (DLVO) theory, the stability of a parti-
cle dispersion is determined by the sum of repulsive electro-
static forces and attractive van der Waals forces.[20] Thus, sever-
al approaches have been made for surface modification of NPs
or microspheres to facilitate dispersion in organic solvents, and
many organic capping agents have been used for enhancing
the dispersion stability.[21,22] Furthermore, a dispersion of NPs in
physiological solutions for biological in vitro and in vivo stud-
ies has been reported.[23] A variety of surfactants have been uti-
lized to promote the solubility of iron oxide NPs or micro-
spheres in various solvents.[7,16]
CÀC coupling reactions such as the Mizoroki–Heck and
Suzuki coupling reactions are important in many types of or-
ganic syntheses, as well as in the chemical, pharmaceutical,
and agricultural industries.[24–26] Immobilization of Pd NPs on in-
organic supports such as silica, alumina, or mesoporous mate-
rials, as well as carbon, has been known to produce active cat-
alysts for CÀC coupling reactions.[27–29] Many researchers have
also reported hybrid magnetic catalysts exhibiting highly effi-
cient catalytic activity and recyclability.[30,31] Nevertheless, im-
proving the yield of Suzuki coupling reactions in water still
remains a major challenge.
Until now, attempts to synthesize well-defined hybrid mag-
netic catalysts have mainly focused on the synergetic effect of
the hybrid catalyst and recyclability, however, an adequate ex-
planation of the surface modification effect by using various
capping agents for the catalytic reaction has rarely been pro-
vided. In this paper, we present the design and synthesis of Pd
NPs immobilized onto Fe3O4 microspheres coated with differ-
ent capping agents such as Na3Cit (Cit=citrate), poly(ethylene
glycol) (PEG), and poly(vinylpyrrolidone) (PVP), comparing
[a] H. Woo, K. Lee, Prof. K. H. Park
Department of Chemistry and Chemistry Institute for Functional Materials
Pusan National University
Busan 609-735 (Republic of Korea)
Supporting information for this article is available on the WWW under
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 7
&
1
&
ÞÞ
These are not the final page numbers!