In situ synthesis of gold nanoparticles inside the pores of MCM-48 in supercritical carbon dioxide and its catalytic application

Article abstract of DOI:10.1002/adsc.200606137

Gold nanoparticles are deposited into the channels of MCM-48 through a simple H₂-assisted reduction of HAuCl₄ (aqueous solution) in supercritical carbon dioxide medium at 70 °C within 2-4 h. The nanoparticles were characterized by powder X-ray diffraction (PXRD), N ₂ adsorption-desorption, transmission electron microscopy (TEM), and UV-Vis spectroscopy. The particle size of the synthesized material is tunable with the pressure (density) of the supercritical carbon dioxide medium. At the fixed temperature (70°C) and hydrogen pressure [P(H₂) = 2 MPa], the Au particle size varies from ca. 25 nm to ca. 2 nm with the change in CO₂ pressure from 7 MPa to 17 MPa. At the low solvent density conditions, larger particles of ～25 nm were obtained. On the contrary, a high solvent density of CO₂ slows down particle aggregation, resulting in the small particle size within the range of 2-5 nm. This change in particle size with CO₂ pressure and the interaction of the particles with the silica support were correlated well with long-range van der Waals interactions and consequently the Hamaker constant for the gold nanoparticle-CO₂ (A₁₃₁) and silica-gold core-CO ₂ (A₁₃₂), respectively. Supercritical carbon dioxide alone can provide a unique environment for stabilizing gold nanoparticles in the channels of the cubic mesoporous MCM-48 support and exquisite control of the particle size without perturbing the support structure. The synthesized material is highly stable, recyclable and no metal nanoparticle leaching was observed. The selective hydrogenation of crotonaldehyde with the synthesized material provides convincing evidence that the particles are inside the pores and available to the reactant molecules.

Full text of DOI:10.1002/adsc.200606137

FULL PAPERS
DOI: 10.1002/adsc.200606137
In situ Synthesis of Gold Nanoparticles inside the Pores of MCM-
48 in Supercritical Carbon Dioxide and its Catalytic Application
a
a, b,
a
a
M. Chatterjee, Y. Ikushima, * Y. Hakuta, and H. Kawanami
a
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology, 4-2-1
Nigatake, Miyagino-ku, Sendai 983-8551, Japan
Fax : (+81)-22-237-5215; e-mail: y-ikushima@aist.go.jp
CREST (Japan) Science and Technology Corporation (JST), Kawaguchi 332-0012, Japan
b
Received: March 24, 2006; Revised: June 13, 2006; Accepted: June 27, 2006
Abstract: Gold nanoparticles are deposited into the of the particles with the silica support were correlat-
channels of MCM-48 through a simple H -assisted ed well with long-range van der Waals interactions
2
reduction of HAuCl (aqueous solution) in supercrit- and consequently the Hamaker constant for the gold
4
ical carbon dioxide medium at 708C within 2–4 h. nanoparticle-CO2 (A131) and silica-gold core-CO2
The nanoparticles were characterized by powder X- (A132), respectively. Supercritical carbon dioxide
ray diffraction (PXRD), N2 adsorption-desorption, alone can provide a unique environment for stabiliz-
transmission electron microscopy (TEM), and UV- ing gold nanoparticles in the channels of the cubic
Vis spectroscopy. The particle size of the synthesized mesoporous MCM-48 support and exquisite control
material is tunable with the pressure (density) of the of the particle size without perturbing the support
supercritical carbon dioxide medium. At the fixed structure. The synthesized material is highly stable,
temperature (708C) and hydrogen pressure [P(H )= recyclable and no metal nanoparticle leaching was
2
2
MPa], the Au particle size varies from ca. 25 nm to observed. The selective hydrogenation of crotonalde-
ca. 2 nm with the change in CO2 pressure from hyde with the synthesized material provides convinc-
MPa to 17 MPa. At the low solvent density condi- ing evidence that the particles are inside the pores
7
tions, larger particles of ~25 nm were obtained. On and available to the reactant molecules.
the contrary, a high solvent density of CO slows
2
down particle aggregation, resulting in the small par-
ticle size within the range of 2–5 nm. This change in Keywords: gold; mesoporous materials; nanoparti-
particle size with CO pressure and the interaction cles; selective hydrogenation; supercritical fluids
2
Introduction
the availability of the metal precursor in terms of
high volatility, perturbation of the support surface
In the past decades, metal nanoparticles have attract- structure, non-uniform distribution of the particles
ed much attention because of the unique size depend- and particle aggregate formation. In a recent ap-
ence of their optical, catalytic, semi-conductive and proach, a sonochemical method was used to diffuse
magnetic properties.[
1–5]
These distinct properties may the nanoparticle through the solvent inside the pores
be attributed to the quantum confinement phenomen- of the mesoporous material, which was undesirably
[
14]
on derived from the change in density and effective slow due to the low diffusivity of the medium.
band gap of the electronic energy level, as well as to The potential use of supercritical carbon dioxide
the high ratio of surface to bulk atoms. Therefore, (scCO ) has attracted much interest as a sustainable
2
one of the key challenges is the synthesis of nanopar- and “green” medium for material synthesis. It can be
ticles of specific size and shape that would provide handled easily because it is non-toxic, non-flammable
significant opportunities in various applications. In and inexpensive. The unique properties of scCO2
particular, gold nanoparticles are very attractive for combine the advantages of liquid phase and gas phase
nanotechnology research because of their exciting op- processes. The tunable density of scCO can be con-
2
tical properties and catalytic activities.[
6–8]
Generally, trolled to match that of the liquid phase, which ena-
gold nanoparticles are synthesized by the wet impreg- bles the medium to dissolve the metal precursor. On
[
9]
[10]
nation method,
bly
synthetic insertion,
and a surface sol-gel modification method.
However, these processes have some difficulties with eliminate precursor volatility constraints and facilitate
co-assem- the other hand, low viscosity, high diffusivity and zero
[
11]
[12]
surface tension, which are closer to the gas phase,
1580
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Synthesis of Gold Nanoparticles inside the Pores of MCM-48
FULL PAPERS
the particle deposition within the confined geome- which correlates well with the particle size and the
tries. The miscibility of scCO2 with the reducing variation of CO pressure. For example, purple to rich
2
gasses like H mitigates the mass transfer limitation, pink colored samples are obtained at 7–10 MPa of
2
which is common in liquid phase reductions.
CO whereas, above 10 MPa the sample color changes
2
Several metal nanoparticles[
15–17]
were prepared in from light pink to colorless (see Figure 2, panel a).
scCO . The conventional approaches for the synthesis The color scenario of the synthesized material was ex-
2
of nanoparticles are the rapid expansion of supercriti- pected to give some primary idea about the size of
cal solution (RESS) and the supercritical antisolvent the gold metal particles, like <2 nm for colorless par-
(
SAS) techniques. These techniques, however, face ticles, rose pink for 2–20 nm and purple for >20 nm
the difficulties of dispersibility due to very low van particles as observed on tailor-made gold nanoparti-
[
11]
der Waals forces and the polarizability of the cles inside the mesoporous channel.
medium. To resolve this, Johnston and Kogel
Hence, the ap-
[
18]
used pearance of the synthesized materials suggested a
expensive and non-biodegradable fluorinated ligands, considerable influence of CO pressure on particle
2
but the problems of carrier solvent solubility and size.
ligand by-products formation still remains. In another
Figure 1 panel a compares the XRD pattern of the
approach, water in a CO micro-emulsion as a “nano- parent MCM-48 with that of Au-MCM-48 material
2
reactor” was used to synthesize metal nanoparti- (17 MPa). All the materials show typical low angle
[
19,20]
cles.
Even though the size, shape and particle (211) reflections and also the (220) reflections along
morphology can be controlled, it is difficult to collect with the smaller Braggꢁs peaks in their scattering pat-
the prepared sample owing to aggregation and coales- tern, which are the characteristics of the cubic space
cence. With regard to catalytic activities, these metal group Ia3d. The presence of the similar peaks of the
nanoparticles often suffer from modest activity and parent material confirmed that the overall structure
decreased life due to aggregation. In a recent ap- of MCM-48 was retained after gold nanoparticle for-
proach, pre-synthesized gold nanoparticles (size range mation. For the parent material, the most intense
2.2–3.4 nm) were impregnated successfully into the (211) XRD line appeared at 2q=2.528, whereas after
channels of different mesoporous silica supports in the incorporation of gold nanoparticles, the main
[21]
scCO -toluene medium.
However, the pre-synthe- peak is shifted to the lower 2q=2.068 (17 MPa) with
2
sized particles need isolation from the by-products, a decrease in the intensity as well. The reduced scat-
and cleaning before impregnation. Therefore, the in tering intensities of the Bragg reflections might be
situ syntheses of nanocrystals within the channels of caused by the introduction of scattering materials into
porous material without perturbing the support struc- the pores[
22,23]
or by the partial collapse of the or-
tures still remain a challenge.
dered cubic structure due to the presence of gold
In this work, we have successfully applied the H2- nanoparticles. However, the scattering pattern of Au-
assisted reduction of an aqueous solution of HAuCl4 MCM-48 (Figure 1, panel a) shows a series of Braggꢁs
to the formation of gold nanoparticles inside the peaks originating from the cubic structure of MCM-
pores in scCO medium. The key advantages of our 48. Therefore, the reduced peak intensity of Au-
2
method are the use of an aqueous solution of MCM-48 might be due to the presence of gold parti-
HAuCl , which prevented the problem of ligand ad- cles inside the channel, rather than the collapse of the
4
sorption, unwanted by-product formation and main- cubic structure. In order to check the stability of the
tained the support structure after nanoparticle forma- Au-MCM-48 structure in terms of the amount of the
tion. Moreover, this method offers a simple and con- gold incorporation in scCO , the gold content was in-
2
venient synthesis process with the good control over creased from ca. 1 to ca. 5 wt% at the fixed pressure
the particle size, devoid of any harsh reducing agent, of CO (17 MPa) and H (2.0 MPa). As a result, the
2
2
organic solvent-free synthesis and no further antisol- main XRD peaks are shifted towards the lower angle
vent or stabilizing surfactant is necessary. The catalyt- and the corresponding d spacing of Au-MCM-48
ic activity of the synthesized material to the selective sample increases with increasing gold content. The
hydrogenation of crotonaldehyde with respect to the Au-MCM-48 (17 MPa) retained its structure up to the
gold particle size has also been monitored to validate gold loading of ca. 5 wt%. The unit cell parameters
p
the nanoparticle deposition inside the channels of (a =d
6) for the parent material and Au-MCM-48
0
211
MCM-48.
obtained at different CO2 pressure are shown in
Table 1. It has been found that the value of a changes
0
from 8.46 nm for the parent material to 10.49 nm for
Au-MCM-48 (17 MPa). This increase in unit cell pa-
rameters is attributed to the pore filling of the host
Results and Discussion
The formation of gold nanoparticles on MCM-48 was material, which consequently reduces the scattering
[
24,25]
confirmed by the change of color from white to pink. contrast between the pores and the walls.
Gold nanoparticles exhibit a clear optical signature
Adv. Synth. Catal. 2006, 348, 1580 – 1590
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M. Chatterjee et al.
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Figure 2. UV-Vis spectra of Au-MCM-48 obtained at various
CO pressures.
2
(
2q=308 to 708) and are displayed in Figure 1, panel
b. Two distinctive Bragg reflections (111) and (200)
of fcc gold are clearly observed for all the samples
treated at or below 12 MPa of CO at 2q=38.1 and
2
44.4. However, these peaks are extremely weak for
the materials obtained above 12 MPa, indicating that
the nanoparticles are not within the detection limit.
Using the Scherrer diffraction formula, the particle
size was obtained from the FWHM of the gold (111)
peak and is presented in Table 1. An increase in
FWHM of the gold (111) peak corresponding to the
decrease in particle size (~25 nm to<2 nm) with in-
creasing CO pressure (7 MPa to 17 MPa) was ob-
2
served. At the low density (low pressure) conditions,
the formation of larger particles were favored, where-
as smaller particles need a high density (higher pres-
sure) to be deposited. This happens because the
larger particles possesses greater inter-particle van
der Waals attractions and are formed at low solvation
conditions (lower density), whereas the smaller parti-
Figure 1. X-ray diffraction patterns of (a) the parent materi-
al and Au-MCM-48. (b) Wide angle XRD pattern of Au-
cles are obtained at higher solvation conditions
MCM-48 at different CO pressures.
2
[26]
(
higher density).
Hence, the tunable solvent densi-
ty of the scCO medium has the unique advantage to
2
To detect the gold nanoparticles, XRD measure- obtain the desired size of the metal nanoparticles.
ments were carried out at the higher angle region This result is in the good agreement with the color
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Synthesis of Gold Nanoparticles inside the Pores of MCM-48
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Table 1. Physical properties of Au-MCM-48, pressure-dependent gold nanoparticle-CO Hamaker constant (A131) and Ha-
2
maker constant (A132) between silica and gold core mediated by scCO2.
Material (CO pres- a0
Pore volume BJH pore di-
S
Average particle di- Color of the A131
A132
2
BET
2
3
À1
À1
[a]
sure in MPa)
(nm) (cm g )
ameter (nm)
(m g ) ameter (nm)
material
(eV) (eV)
No gold
8.56 0.88
9.62 0.81
10.47 0.72
10.56 0.70
10.52 0.65
3.4
3.3
3.2
3.2
3.1
1050
981
898
862
790
-
white
rich purple
purple
light Pink
light pink to 2.020 0.646
colorless
-
-
Au-MCM-48 (7)
Au-MCM-48 (10)
Au-MCM-48 (12)
Au-MCM-48 (14)
25.1
10.2
4.0
2.5
2.345 0.858
2.233 0.784
2.083 0.683
Au-MCM-48 (17)
10.49 0.61
3.1
760
<2
light pink to 1.465 0.291
colorless
[
a]
Particle size as obtained from Scherrer diffraction formula (particle size=kl/FWHM cosq, where l is the wavelength of
the Cu Ka1 line (0.154 nm), q is the angle between the incident beam and the reflection lattice planes, and FWHM is the
width (in radians) of the diffraction peak) using the line width of the gold (111) peak.
scenario of Au-MCM-48 as shown in Figure 2, panel
a.
Figure 2, panel b shows the optical absorption spec-
tra of Au-MCM-48 recorded as a function of CO2
pressure. From the absorption spectra, we observed a
monotonic increase in intensity of the surface plas-
mon resonance with the decrease in CO pressure.
2
According to the Mie theory,[ the absorption band
is due to the excitation of the surface plasmon vibra-
tion. This gives the particles their characteristics pink/
purple color depending on the particle size. The ab-
sorbance peaks of all the materials that were obtained
27]
[
28]
below 10 MPa with large size of the nanoparticles,
exhibit distinct absorption spectra in the visible
region of the electromagnetic spectrum (ca. 545 nm),
responsible for the striking purple color (Figure 2,
panel b). However, a blue shift in the corresponding
peak position (ca. 510 nm) was observed on change of
CO pressure to>10 MPa. The shifting of the surface
2
plasmon resonance is due to the decrease in particle
size; and these small metallic particles are attributed
to quantum size effects, which leads to the formation
of quantized energy states. The blue shift in the sur-
face plasmon vibration with increasing CO pressure
2
was strongly supported by the color of the obtained
material.
Figure 3. FT-IR spectra of (a) the parent material and (b)
Au-MCM-48 (17 MPa)
Figure 3, traces a and b show the spectra of the
parent MCM-48 material and Au-MCM-48 (17 MPa)
À1
in the region 1300–400 cm , respectively. The parent poration of gold nanoparticles and confirmed the
À1
material shows the vibrational band at ~1090 cm
XRD observation. However, the displacement of the
that is assigned as the anti-symmetric stretching (SiÀ lattice vibrational bands can be observed in the wave-
À1
OÀSi) vibration, while the peak at 810 cm is attrib- number of the (SiÀOÀSi) band which decreases from
À1
À1
uted to symmetrical stretching vibration of SiÀOÀSi. 810 cm (purely siliceous MCM-48) to 795 cm for
The IR spectrum remains nearly unchanged after the Au-MCM-48 (17 MPa).
incorporation of gold nanoparticles. Although the
Figure 4 shows the TEM images of Au- MCM-48
wavenumber of (SiÀOÀSi) band decreases from and the corresponding particle size distribution at dif-
À1
1090 cm
087 cm
(in the purely siliceous sample) to ferent CO pressures of 7 MPa to 12 MPa. The influ-
2
À1
1
[Au-MCM-48 (17 MPa)], this shift is ence of CO2 pressure as observed in the XRD
within the spectral resolution. It indicates that the (Figure 1, b) becomes obvious from the inspection of
structure of the support is maintained after the incor- the TEM images. The number of the larger particles
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M. Chatterjee et al.
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Figure 4. TEM images of Au-MCM-48 obtained at (a) 7 MPa, (b) 10 MPa, and (c) 12 MPa.
dispersed on the outer surface was more at the lower diffusivity of the scCO tends to diffuse the particles
2
[
29]
pressure of CO2 (7 MPa; Figure 4, a), while the towards the center of the pore.
number decreases at the higher pressure of CO2 when particles are included inside the pores, mesopo-
12 MPa; Figure 4, b). No larger particles are found at rous materials have a strong tendency to form regular
In addition to that
(
1
7 MPa (Figure 4, c), which proves that the smaller pores by expansion,[ which was confirmed by the
11]
gold nanoparticles are totally confined within the increase in center-center spacing (a , Table 1) of the
0
pores of MCM-48. Apparently a five-fold increase in pores. It is very difficult to show any evidence by
CO density leads to a decrease of 88% in the parti- TEM for the presence of the very small (<2 nm) gold
2
cle size. The average inter-particle distance appears to nanoparticles produced at ca. 17 MPa inside the
decrease with increasing particle size, possibly due to pores/channels.
the stronger attractive van der Waals forces between
The N adsorption-desorption isotherms along with
2
the larger particles and having a less homogenous size the pore size distribution of the parent material, Au-
distribution compared to the smaller one. Above MCM-48 (7 MPa) and Au-MCM-48 (17 MPa) are pre-
1
2 MPa of CO pressure, where the particle size is sented in Figure 5, panels a, b and c, respectively. The
2
close to the pore diameter, an increase in the number isotherm of the parent material indicates a linear in-
of particles inside the pores can be expected. Al- crease in the amount of adsorbed N at low relative
2
though the chances of pore blockage were there, high pressure (P/P =0.3–0.4) associated with capillary con-
0
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The position of the inflection point depends on the di-
ameter of the mesopores, and the sharpness indicates
the narrow pore size distribution. The height and
sharpness of the N condensation provides clear evi-
2
dence of the high quality of the parent material with
2
a pore diameter of 3.4 nm, a surface area of 1050 m g
3
and a pore volume of 0.88 cm g (Table 1) which cor-
[
30]
responds well to the literature data.
The N adsorp-
2
tion-desorption isotherm of Au-MCM-48 also shows a
similar type IV adsorption isotherm (Figure 5, panels
b and c). In support of the XRD data, a narrow pore
size distribution was evident for all of the samples, in-
dicating that the highly ordered structures are main-
tained after incorporation of the gold. No micropores
were observed in the isotherm. The BET surface
area, BJH pore diameter of the parent material and
Au-MCM-48 obtained at different CO pressure are
2
summarized in Table 1. Compared to the parent mate-
rial, there was no systematic decrease in the pore
volume and the pore diameter for the samples ob-
tained at or above 12 MPa. A slight decrease of pore
3
volume (0.61 cm g) was observed for Au-MCM-48
(
17 MPa). This suggests that the pores of the mesopo-
rous channels are still accessible and could be fruitful
for catalytic reaction to allow a guest molecule to dif-
fuse into the silica in order to reach the active nano-
particles.
Supercritical CO deposition is a new approach for
2
metallization. An attempt was made to visualize the
bulk phase behavior of scCO at different pressures
2
(
fixed pressure of H =2 MPa) in the presence of an
2
aqueous solution of gold using a high-pressure view
cell. A significant phase transition was observed. At
the higher pressure of 17 MPa a continuous phase
was formed, which indicates the formation of highly
dispersed smaller particles. On the other hand, at the
lower pressure of 7 MPa, two phases were clearly visi-
ble, which might be due to the possible formation of
the larger particles. Although CO is considered as a
2
non-polar solvent, there is always some finite solubili-
ty of CO in water depending on temperature and
2
pressure. Moreover, from the MD simulation, Sato et
[
31]
al.
identified a hydrogen bond between oxygen in
CO and hydrogen in H O to explain the dissolution
2
2
of CO in H O. The mechanism behind the nanoparti-
2
2
cle formation probably consists of three steps. First,
the gold aqueous solution is reduced by hydrogen,
and then in the next step the reduced gold aqueous
solution reacts further. In this step, the metal solution
and CO adsorb on the surface of MCM-48, followed
2
by diffusion into the pores because of the higher den-
Figure 5. N2 adsorption-desorption isotherm of (a) parent
Si-MCM-48 and (b) Au-MCM-48 (7 MPa) and (c) 17 MPa.
sity of CO inside the pores compared to the bulk
2
[
29]
phase.
Finally, CO at that high density interacts fa-
2
vorably with the metal, and water desorbs from the
densation in the channels of the MCM-48 structure. surface easily and diffuses into scCO , leaving the
2
According to IUPAC nomenclature the resulting iso- pure metal in the channels of the support.
therm can be classified as Type IV with hysteresis.
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M. Chatterjee et al.
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We have experimentally observed that the adjust- from 7 MPa to 17 MPa at 708C and results are shown
ment of the solvent properties of scCO can induce a in Table 1. It has been observed that the Hamaker
2
change in the particle size, which will be explained by constant decreased from 2.345 eV to 1.465 eV with
the interaction of the solvent with the metal particles. the increase in CO pressure (density). According to
2
A theoretical approach of a soft sphere model was the DLVO (Derjaguin–Landau–Verwey–Overbeek)
proposed by Shah et al.[ to determine the total in- theory, the Hamaker constant is proportional to the
teraction energy. The total interaction energy is ob- van der Waals attraction. On the other hand, the van
tained by the summation of attractive and repulsive der Waals attractive forces (Fvdw) between particles
32]
forces [Eq. (1)]:
increase with the square of the ratio of the particle
radius and are inversely proportional to the center-to-
center separation distances. Therefore, the significant
decrease of the Hamaker constant with the increase
Ftotal ¼ Fvdw þ Fosm þ Felas
ð1Þ
The attractive energy is the van der Waals attractive in CO pressure leads to lower F
and consequently
2
vdw
force resulting from the dipole-induced dipole inter- to the increase in separation distance, which corre-
action and its magnitude is reduced at larger distan- sponds well with the smaller core-core interaction for
ces. The repulsive forces are conposed of the osmotic the smaller particles obtained at higher pressures. To
term Fosm and the elastic term Felas. When particles the contrary, theFvdw increases with the increase in
are small, typically in the range of micrometers or the particle size, the largest nanoparticles are ob-
less, and are dispersed in a solvent, van der Waals at- tained at lower solvent density conditions.
traction forces and Brownian motion play important
We have elucidated the pressure/density depend-
roles. The combination with Brownian motion would ence of nanoparticle formation in scCO . We will now
2
further result in the agglomeration of the nanoparti- try to unravel the van der Waals forces between nano-
cles. The attractive forces (that arise when dipoles in- particles and silica surface. The Hamaker constant
teract across an intervening medium) between two A132 for a gold particle (1) and silica surface (2)
nanoparticles are related to the particle radius R, across the scCO medium (3) may be given as follows
2
[
34]
center to center separation distance d and the Ha- [Eq. (5)]:
maker constant A by the following equation [Eq. (2)]:
pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi
A132 ¼ ð A À A Þð A À A Þ
ð5Þ
11
33
22
33
ꢂ
ꢀ
ꢁꢃ
2
2
2
2
A131
2R
2R
d
d À 4R
where A22 and A33 are the Hamaker constants of the
silica surface and CO , respectively, obtained from
the Lifshitz theory as described in Eq. (4). The values
Fvdw ¼ À
þ
þ ln
ð2Þ
2
2
2
2
6
d À 4R
d
2
where the negative sign represents the attraction of A11 for gold,[33] e (3.91) and n (1.45) were taken
nature of the interaction between two particles. The from the literature.[
36,37]
The strong pressure-depen-
Hamaker constant A131 is a proportionality factor dent Hamaker constant demonstrates the ability to
that accounts for the interaction between two nano- describe adsorption of the nanoparticles into the
particles of same material (1) through the solvent (3) channels of the mesoporous silica support in the com-
and can be determined from the pure component pressed fluid medium. The Hamaker constant for the
[
Eq. (3)]:
gold-silica interaction decreases (0.858 eV to
0.291 eV) considerably with the increase in the CO2
pressure from 7 to 17 MPa. Accordingly, when the
dispersed particles are much smaller than the sizes of
pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi
2
A131 ¼ ð A À A Þ
ð3Þ
11
33
where, A =Hamaker constant for pure gold (4” the pores, they might be only partially bonded to the
1
1
À19 [33]
[38]
10
J)
and A33 was obtained from the Lifshitz pore walls.
and any effect from the stiff surround-
[34]
theory following the relation [Eq. (4)]:
ings cannot take place. However, when the particle
size is large enough, the silica matrix would exert a
ꢀ
ꢁ
[39]
2
2
3
2
vac
2
great effect on the enclosed particles,
the large
3
e À e
3hn ðn À n
Þ
3
vac
vac
e
A33 ¼ 4 k T
þ
pffiffi
ð4Þ
B
A132 at lower pressure of CO explains this phenom-
2
3
2
vac
3=2
2
e þ e
3
16 2 ðn þ n
Þ
enon well. Therefore, the strong density dependence
of A132 can be used as a probe to correlate the parti-
where e is the dielectric constant, n is the refractive cle mobility and channel accessibility of the nanopar-
index, k is Bolzmanꢁs constant, T is the temperature, ticles.
B
h is Planckꢁs constant and n is main absorption fre-
The catalytic activity of the Au-MCM-48 was tested
e
quency, typically 3”1015 s . The values for e
À1
and by the selective hydrogenation of crotonaldehyde
vac
nvac are considered to be 1, while e and n were ob- (Scheme 1) in scCO medium. Figure 6 panels a and 6
3
3
2
tained from the literature.[
34,35]
From Eq. (4), the A131 show the results of Au-MCM-48 (10 MPa) and Au-
for gold in scCO was calculated at different pressures MCM-48 (17 MPa), respectively. Under the studied
2
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FULL PAPERS
reaction conditions [P(H )=4 MPa; temperature=
2
508C; reaction time=4 h] the hydrogenation reaction
path from crotonaldehyde to crotyl alcohol was more
active than the formation of butyraldehyde. The selec-
tivity to the crotyl alcohol increases with an increase
in pressure from 7–14 MPa and then decreases. The
change in selectivity along with CO pressure can be
2
correlated well with the density of the medium. The
true reason for this effect may be a debatable issue,
but there is no doubt that the pressure effect has a
strong influence on the selectivity. On the other hand,
it has been observed that the hydrogenation of croto-
naldehyde in scCO medium is sensitive to particle
2
size. For instance, the selectivity to the crotyl alcohol
decreases from ca. 90% to ca. 50% as the gold parti-
cle size changes from 10 nm [Au-MCM-48 (10 MPa)]
to<2 nm [Au-MCM-48 (17 MPa)]. Bailie et al[
studied the influence of particle size (4 nm to 20 nm)
on the selectivity in the Au/ZnO system and proposed
that the active site for the selective hydrogenation of
crotonaldehyde to crotyl alcohol was associated with
40]
Scheme 1. Hydrogenation of crotonaldehyde in supercritical
carbon dioxide.
Figure 6. CO pressure dependence on conversion and selectivity of crotonaldehyde hydrogenation using (a) Au-MCM-48
2
(
7 MPa) and (b) Au-MCM-48 (17 MPa) in scCO at fixed pressure of H (4 MPa) and (c) comparison with organic solvent
2 2
[43]
and literature data. (d) Effect of hydrogen pressure on conversion and selectivity.
Adv. Synth. Catal. 2006, 348, 1580 – 1590
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1587

M. Chatterjee et al.
FULL PAPERS
the presence of larger gold particles. Moreover, for Conclusions
smaller gold particles, it is likely that a marked
change in the electronic character of nanosized gold A successful approach to the formation of gold nano-
particles occurs with the change in the particle size particles into the channels of mesoporous material in
and that the structure sensitivity originates from a scCO medium using a hydrogen reduction technique
2
[41]
quantum-size effect.
The literature data regard- is reported. It was possible to control the desired par-
[
42,43]
ing
crotonaldehyde hydrogenation in convention- ticle size by simple tuning of the solvent density, with-
al solvents revealed that the silica support does not out disturbing the mesoporous structure. The varia-
favor the formation of crotyl alcohol. However, in our tion of particle size with CO pressure can be corre-
2
[
44,45]
previous work
it was found that the C=O hydro- lated well with the Hamaker constant, consequently
genation was favored in scCO medium compared to with van der Waals attractions. The CO medium can
2
2
the conventional organic solvent. The catalytic activi- act as an effective medium for the synthesis of gold
ty of the synthesized materials indicates that the nanoparticles because of its low polarizability. An N2
pores are still accessible for the reactant molecule adsorption-desorption isotherm study shows that the
after gold incorporation. This is in good agreement pore blockage in MCM-48 after gold nanoparticle in-
with the result obtained by N adsorption desorption corporation is minimum and pores are accessible,
2
analysis.
which was supported by the Hamaker constant be-
To compare the conversion and crotyl alcohol selec- tween the silica surface and the gold core mediated
tivity of Au-MCM-48 (10 MPa), we have carried out by scCO . The catalytic activity provides conclusive
2
the same reaction in conventional organic solvents evidence that the particles are inside the pores and
and the result is shown in Figure 6, panel c. Although are available for the reactant molecules. Therefore,
the conversion and selectivity in the organic solvent the method described here is simple, reproducible
were lowered compared to the scCO , they are still and convenient. It will surely open a new trend for
2
higher than with the Au/SiO catalyst obtained by the the synthesis of gold nanoparticle of desired size by
2
[43]
chemical vapor deposition method.
The occurrence tuning the solvent properties in a “green” and clean
of metal leaching was not observed, which is absolute- medium.
ly undesirable for a true heterogeneous catalyst.
The hydrogen pressure dependence study over Au-
MCM-48 (10 MPa) revealed that the highest selectivi- Experimental Section
ty of crotyl alcohol was obtained at 4 MPa of hydro-
gen (Figure 6, panel d). The high solubility of hydro- Materials
gen in scCO increases the conversion along with the
2
increase in hydrogen pressure. The product distribu- Tetraethyl orthosilicate (TEOS) as the silica source and
sodium hydroxide were purchased from Wako Pure Chemi-
tion for the crotonaldehyde hydrogenation is inde-
cals. Cetyltrimethylammonium bromide (CTAB) and
pendent on the studied temperature. The reactions
were conducted at 258C, 35 8C, 50 8C and 808C at
HAuCl were from Aldrich Chemical Co. All materials were
4
used as received. CO2 (>99.99%) was provided by the
Nippon Sanso Co., Ltd.
1
2 MPa of CO with the constant hydrogen partial
2
pressure of 4 MPa using Au-MCM-48 (10 MPa) be-
cause the high selectivity to the crotyl alcohol was ob-
tained with Au-MCM-48 (10 MPa). With an increase
in temperature, the conversion of crotonaldehyde in- Synthesis of MCM-48
creases, while the product selectivity remains con-
stant.
A typical recycle experiment for the hydrogenation
of crotonaldehyde showed that the catalyst is truly
stable even after 5 recycles without losing its activity
and selectivity. Elemental analysis for Au was done
by ICP before and after recycling, and indicates
The MCM-48 mesoporous silica was obtained by the proce-
dure described in the literature.[
46]
Typically, CTAB and
sodium hydroxide were added to deionized water and stir-
red until they dissolved. After that, TEOS was added slowly
under stirring conditions. The molar composition of the re-
sultant gel was 1 M TEOS:0.25 M Na O:0.65 M C H -
2
16 33
(
CH ) NBr:0.62 M H O. The final gel was autoclaved with
3 3 2
<
0.02% of Au leaching, confirming negligible leach-
heating at 1408C for 2 days. The solid product was filtered
and washed thoroughly, followed by oven drying at 608C. Fi-
ing of gold from the catalyst. The reaction mixture
was also tested after separation from the solid cata- nally, the product was calcined at 5508C for 8 h in air.
lyst; it was inactive, further supporting that the leach-
ing of gold is negligible. Au-MCM-48 obtained from
CO medium was highly stable and can be recycled
2
several times without losing activity and selectivity.
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Synthesis of Gold Nanoparticles inside the Pores of MCM-48
FULL PAPERS
Synthesis of Gold Nanoparticles in scCO Medium
temperature of 508C. The reactor was sealed and flushed
2
with 2 MPa of CO for 3–4 times to remove the air and kept
2
for 90 min to attain a constant temperature. A prescribed
amount of hydrogen was first introduced into the reactor.
The synthesis of gold containing MCM-48 (referred to as
Au-MCM-48 in the text) in scCO medium was conducted
2
After that, liquid CO was charged using a high-pressure sy-
in a 50 mL stainless steel autoclave. In a typical experiment,
2
ringe pump, and then compressed to the desired pressure. A
back-pressure regulator was used to maintain the constant
pressure of the system. The reaction was conducted while
stirring the mixture with a magnetic stirrer for 2 h. The reac-
tor was then cooled to room temperature with an ice bath
and depressurized to atmospheric pressure. Finally, the
liquid product was separated from the catalyst by filtration
then identified by GC/MS and analyzed quantitatively by
GC (HP 6890) equipped with flame ionization detector.
Quantification of the products was obtained by a multipoint
calibration curve for each product.
5
g of calcined mesoporous support and 10 mL of HAuCl4
solution were loaded in the reactor before it was sealed. The
closed reactor was placed in a constant temperature circulat-
ing oven of 708C and allowed to equilibrate. After the ther-
mal equilibrium was reached, H was introduced in the reac-
tor and pressurized to 2.0 MPa. Then the reaction vessel was
2
charged with CO to the desired pressure using a high pres-
2
sure syringe pump (JASCO scf-bpg). The vessel contents
were mixed mechanically using a magnetic stirrer. The reac-
tion was allowed to proceed for 2–4 h. During the synthesis,
a back-pressure regulator maintained the constant reaction
pressure. After the reaction, the reactor was placed in ice-
cold water and depressurized very slowly and carefully. The
samples were recovered from the container, dried at room
temperature followed by the characterization.
The stability of the heterogeneous catalyst was evaluated
by recovering it from the hot reaction mixture (temperature
5
08C) by filtration and analysis by ICP-AES. On the other
hand, the filtrate has been used in a new reaction but no
catalytic activity was observed.
Characterization Methods
References
The powder X-ray diffraction pattern was recorded on a
Rigaku RAD-X system using monochromatized Cu Ka ra-
diation (l=1.542 Š). In general, the diffraction data were
collected using the continuous scan mode with a scan speed
of 2 deg/min over the scan range 2q=1.5–158 and for wide-
angle (2q=30–708) measurements. Transmission electron
microscopy (TEM) of the material was performed on a Phi-
lips Technai operating at 200 kV. The powder was suspended
in ethanol by an ultrasonic method. A drop of this solution
was placed on the grid with a holey carbon/copper film and
[
1] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.
Genet, B. Delmon, J. Catal. 1993, 144, 175.
[
2] H. Sakurai, M. Haruta, Appl. Catal. A General 1995,
1
27, 93.
[
[
3] M. Takagi, J. Phys. Soc. Jpn. 1954, 9, 359.
4] S. Link, M. A. El-Sayed, J. Phys. Chem. B 1999, 103,
8
410.
[
[
[
5] J. H. Hodak, I. Martini, G. V. Hartland, S. Link, M. A.
El-Sayed, J. Chem. Phys. 1999, 108, 9210.
6] J. Jia, K. Haraki, J. Kondo, K. Domen, K. Tamaru, J.
Phys. Chem. B 2000,104, 11153.
7] G. C. Bond, D. T. Thompson, Catal. Rev.-Sci. Eng.
then allowed to dry, covered by a watch glass. N adsorp-
2
tion-desorption isotherms were obtained at À1968C with a
Micromeritics ASAP 2000 instrument. The sample was first
outgassed at 3008C for 24 h immediately prior to data col-
1
999, 41, 319.
[8] G. C. Bond, P. A. Sermon, Gold Bull. 1973, 6, 102.
[9] a) Y.-J. Han, J. M. Kim, G. D. Stucky, Chem. Mater.
2000, 12, 2068; b) K. B. Lee, S. M. Lee, J. Cheon, Adv.
Mater. 2001, 13, 517.
[10] a) Z. Konya, V. F. Puntes, I. Kiricsi, J. Zhu, J. W. Ager
III, M. K. Ko, H. Frei, P. Alivisatos, G. A. Somorjai,
Chem. Mater. 2003, 15, 1242; b) P. Mukherjee, C. R.
Patra, A. Ghosh, R. Kumar, M. Sastry, Chem. Mater.
2002, 14, 1678.
[11] W. Yan, B. Chen, S. M. Mahurin, E. W. Hagaman, S.
Dai, S. H. Overburry, J. Phys. Chem. 2004, 108, 2793.
[12] S. R. Hall, S. A. Davis, S. Mann, Langmuir 2000, 16,
1454.
[13] W. Chen, W. Cai, L. Zhang, G. Wang, L. Zhang, J. Col-
loid Interface Sci. 2001, 238, 291.
lection. The volume of the adsorbed N was normalized to
2
standard temperature and pressure. The surface area and
the pore size distribution were calculated by applying the
BET equation[
47]
and BJH formula from the adsorption
[48]
branch of the isotherm,
respectively, to avoid the pres-
ence of artificial maxima on the pore size and other undesir-
able effects.[ The UV-Vis spectra of the materials were re-
corded on a Shimadzu spectrophotometer operating in the
reflection mode at a resolution of 2 nm using aluminum
oxide as standard. The FT-IR spectrum in the framework
49]
À1
À1
region (1300 cm to 400 cm ) was recorded on a Horiba
FT-720 using a KBr pellet. About 1 mg of finely powdered
sample was mixed with 300 mg of dried KBr and then press-
ed to obtain the pellet.
[
[
[
14] P. S. Shah, S. Husain, K. P. Johnston, B. A. Krogel, J.
Phys. Chem. B 2001, 105, 9433.
15] H. Wakayama, N. Setoyama, Y. Fukushima, Adv.
Mater. 2003, 15, 742.
16] a) A. Kameo, T. Yoshimura, K. Esumi, Colloids Surf. A
2003, 215, 181; b) A. Kameo, S. Sarashina, K. Esumi,
Langmuir 2004, 20, 5189.
Catalytic Activity
The hydrogenation of crotonaldehyde was carried out in a
stainless steel batch reactor (50 mL) in scCO2 medium.[
50]
100 mg of the catalyst and 6.5 mmol of the reactant were
[17] B. Korgel, K. P. Johnston, P. S. Shah, Abstracts of
loaded into the reactor which was placed in an oven at a
Papers of ACS, 223, u663, 242, April 7, 2002.
Adv. Synth. Catal. 2006, 348, 1580 – 1590
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
1589

M. Chatterjee et al.
FULL PAPERS
[
[
[
[
[
18] M. Ji, X. Chen, C. M. Wai, J. L. Fulton, J. Am. Chem.
[35] T. Moriyoshi, T. Kita, Y. Uosaki, Ber. Bunsenges. Phys.
Chem. 1993, 97, 589.
[36] G. J. Besserer, D. B. Robinson, J. Chem. Eng. Data
Soc. 1999, 121, 2631.
19] J. D. Holmes, P. A. Bhargava, B. A. Korgel, K. P. John-
ston, Langmuir 1999, 15, 6613.
20] G. Gupta, P. S. Shah, X. Zhang, A. E. Saunders, B. A.
Korgel, K. P. Johnston, Chem. Mater. 2005, 17, 6728.
21] W. Hammond, E. Proujet, S. D. Mahanti, T. J. Pinnavia,
Microp. Mesop. Mater. 1999, 27, 19.
1973, 18, 137.
[37] A. Hourri, J. M. St-Arnaud, T. K. Bose, J. Chem.
Phys.1997, 106, 1780.
[
[
[
38] R. C. Burns, C. Graham, A. R. M. Weller, Mol. Phys.
1
986, 59, 41.
22] a) M. H. Lim, C. F. Blanford, A. Stein, Chem. Mater.
39] A. L. Pan, H. G. Zheng, Z. P. Yang, F. X. Liu, Z. J.
Ding, Y. T. Qian, J. Phys.: Condens. Matter 2003, 15, 9.
40] J. E. Bailie, H. A. Abdullah, J. A. Anderson, C. H. Ro-
chester, N. V. Richardson, N. Hodge, J. G. Zhang, A.
Burrows, C. J. Kiely, G. J. Hutchings, Phys. Chem.
Chem. Phys. 2001, 3, 4113.
1
998, 10, 467; b) M. Frçba, R. Kçhn, G. Bouffaud,
Chem. Mater. 1999, 11, 2858.
[23] J. Sauer, F. Marlof, B. Spliethoff, F. Schꢂlth, Chem.
Mater. 2002, 36, 217.
[24] H. Parala, H. Winkler, M. Kolbe, A. Wohlfart, R. A.
Fischer, R. Schmechel, H. von Seggern, Adv. Mater.
[
41] S. Schimpf, M. Lucas, C. Mohr, U. Rodemerck, A.
Brꢂkner, J. Radnik, H. Hofmeister, P. Claus, Catal.
Today 2002, 72, 63.
2
000, 12, 1050.
[
[
25] C. M. Yang, P. H. Liu, Y. F. Ho, C. Y. Chiu, K. J. Chao,
Chem. Mater. 2003, 15, 275.
[
[
[
[
42] J. E. Bailie, G. J. Hutchings, Chem. Commun. 1999,
26] M. C. McLeod, M. Anand, C. Kitchens, C. B. Roberts,
Nano Lett. 2005, 5, 461.
2
151.
43] M. Okamura, T. Akita, M. Haruta, Catal. Today 2002,
4, 265.
44] M. Chatterjee, FY Zhao, Y. Ikushima, Catal. Lett. 2002,
2, 141.
[27] G. Mie, Ann. Phys. 1908, 25, 377.
7
[
28] a) J. J. Storhoff, A. A. Lazarides, R. C. Mucic, C. A.
Mirkin, R. L. Letsinger, G. C. Schatz, J. Am. Chem.
Soc. 2000, 122, 4640; b) R. Elghanian, J. J. Storhoff,
R. C. Mucic, R. L. Letsinger, C. A. Mirkin, Science
8
45] S. Schimpf, M. Lucas, C. Mohr, U. Rodemerck, A.
Brꢂkner, J. Radnik, H. Hofmeister, P. Claus, Catal.
Today 2002, 72, 63.
1
997, 277, 1078.
29] M. S. Schneider, J. D. Grunwaldt, A. Baiker, Langmuir,
004, 20, 2890.
[
[
[
[
[
[
[
46] A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margo-
lese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, P.
Petroff, A. Firouzi, M. Janicke, B. F. Chmelka, Science
2
30] K. Schumacher, P. I. Ravikovitch, A. D. Chesne, A. V.
Neimark, K. K. Unger, Langmuir, 2000, 16, 4648.
31] H. Sato, N. Matubayashi, M. Nakahara, F. Hirata
Chem. Phys. Lett. 2000, 323, 257.
32] P. S. Shah, J. D. Holmes, K. P. Johnston, B. A. Krogel, J.
Phys. Chem. B 2002, 106, 2545.
33] V. A. Parsegian, G. H. Weiss, J. Colloid Interface Sci.
1
993, 261, 1299.
[
[
47] S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem.
Soc. 1938, 60, 309.
48] E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem.
Soc. 1951, 73, 373.
1
981, 81, 285.
[49] M. Kruk, M. Jaroniec, A. Sayari, Langmuir 1997, 13,
34] J. N. Israelachvili, Intermolecular and surface forces:
with application to colloidal and biological systems,
Academic Press, London, Orlando, 1985.
6267.
[50] M. Chatterjee, FY Zhao, Y. Ikushima, Adv. Synth.
Catal. 2004, 346, 377.
1590
www.asc.wiley-vch.de
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1580 – 1590