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Chemistry Letters Vol.36, No.8 (2007)
A Multifunctional Heterogeneous Catalyst: Titanium-containing Mesoporous Silica Material
Encapsulating Magnetic Iron Oxide Nanoparticles
Kohsuke Mori,1 Yuichi Kondo,1 Shotaro Morimoto,2 and Hiromi Yamashitaꢀ1
1Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University,
2-1 Yamada-oka, Suita, Osaka 565-0871
2Facility of Pharmaceutical Science, Osaka Ohtani University,
3-11-1 Nishikiori-Kita, Tondabayashi, Osaka 584-8540
(Received May 30, 2007; CL-070583; E-mail: yamashita@mat.eng.osaka-u.ac.jp)
Magnetic iron oxide nanoparticles coated with mesoporous
silica involving single-site titanium oxide moiety have been first
developed by adopting a two-step coating method. The catalytic
performance and magnetically separable ability were demon-
strated in the oxidation of 2,6-di-tert-butylphenol using hydro-
gen peroxide.
reddish-brown powder. This color change is originated from
the transformation of Fe3O4 phase into ꢀ-Fe2O3. It is well
known that the coprecipitation of ferrous and ferric ions in a
basic aqueous solution first gives Fe3O4 nanoparticles, which
can be directly oxidized into ꢀ-Fe2O3 by aeration.5 Next, a
source of Ti-HMS (TEOS, TPOT, and dodecylamine) was
added to the FexOy@SiO2 dispersed in 2-propanol, followed
by calcination at 823 K for 5 h, giving FexOy@Ti-HMS (Si:
42.0, Fe: 7.4, Ti: 0.64 wt %).
The development of prominent heterogeneous catalysts with
well-defined surface structure that enable rapid and selective
chemical transformations and can be separated completely from
the product is a paramount challenge.1 Among several catalyst
materials, mesoporous molecular sieves featuring large surface
area as well as ordered hexagonal mesopore channels ranging
from 2–10 nm are classified as promising catalyst supports.2
These materials have often been doped with heteroatoms, e.g.,
Ti, Cr, Mo, etc., generating potentially active site that exhibits
the requisite activity and selectivity for specific catalytic
reactions.3 The transition-metal oxide moieties are considered
to be highly dispersed at the atomic level and also to be well-
defined catalysts, which exist in the specific structure of the
framework. However, the difficulties in recovering the small
mesoporous silica particles from the reaction mixture severely
circumvent their industrial applications. In order to overcome
the above drawbacks, the fabrication of mesoporous silica incor-
porating magnetic nanoparticles would be a promising strategy:
it is possible to recover the composite materials from the liquid
system using external magnet. Although the synthesis of mag-
netic mesoporous silica by grafting or encapsulating magnetic
nanoparticles have been reported recently, the coating material
is limited to pure silica so far.4
In the low-angle XRD pattern, the FexOy@Ti-HMS exhibits
a diffraction peak at around 2ꢁ ¼ 3ꢁ associated with the d100
spacing, indicating the presence of hexagonally packed mesopo-
rous structure (Figure 1A). The wide-angle XRD pattern shows
clear peaks due to either Fe3O4 or ꢀ-Fe2O3 phase or both at
around 30.3, 35.6, 43.7, 54.1, 57.5, and 63.2ꢁ, corresponding
to the (220), (311), (400), (422), (511), and (440) reflections,
respectively (Figure 1B). Distinguishing between Fe3O4 and
ꢀ-Fe2O3 phase by XRD is quite difficult because of the same
inverse spinel structure and similarity in their d spacing. The
Raman spectrum, which is often applied as a useful technique
for differentiating various iron oxide phases, showed a character-
istic band of ꢀ-Fe2O3 phase at around 1400 cmꢂ1 (Figure 1S).11
N2 adsorption–desorption showed the typical type IV
isotherm without significant hysteresis loop (Figure 2S).11 The
BET surface area was found to be 910 m2ꢃgꢂ1. The pore size
distribution calculated from desorption of the N2 isotherm by
the BJH method showed that an average pore size of 3.0 nm
with narrow size distributions.
TEM image showed the formation of the iron oxide nano-
particles having size between 5 and 13 nm, and the average
diameter of ca. 9.5 nm were found to be distributed within
the host silica matrix (standard deviation: ꢂ ¼ 0:2 nm, ꢂ=d ¼
21:7%) (Figure 3S).11
Herein, we first developed Ti-containing hexagonal meso-
porous silica (Ti-HMS) encapsulating magnetic iron oxide nano-
particles (FexOy@Ti-HMS). The designed architecture enables
the powerful combination of useful functions, superparamagnet-
ism, catalytically active site, and mesoporous structure.
The isothermal magnetization curve of the FexOy@Ti-HMS
at 300 K displayed a rapid increase with increasing applied mag-
We adopted the two-step coating method as a synthetic strat-
egy. Thus, a silica layer was first deposited on the surface of iron
oxide nanoparticles in order to protect and facilitate the forma-
tion of Ti-HMS phase. Then, the Ti-HMS layer was formed from
subsequent sol–gel polymerization of tetraethylorthosilicate
(TEOS), tetrapropylorthotitanate (TPOT), and dodecyamine.
The synthetic procedure is as follows. Initially, a mixture of
the ferrous and ferric salt solutions (Fe2þ/Fe3þ = 0.5) was in-
troduced into an NH4OH solution. To the resulting black nano-
particles solution, TEOS was added dropwise with mechanical
stirring. The resultant black powder was calcined at 523 K
for 2 h to give the silica-coated iron oxide (FexOy@SiO2) as a
A
B
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3
5
7
9
10
20
30
40
50
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80
2θ/degree
2
θ/degree
Figure 1. (A) Low-angle and (B) wide-angle XRD patterns of
FexOy@Ti-HMS.
Copyright Ó 2007 The Chemical Society of Japan
Mori, Kohsuke
