Structure of intrazeolite molybdenum oxide clusters and their catalysis of the oxidation of ethyl alcohol

Article abstract of DOI:10.1039/b108639c

Molybdenum oxide clusters confined in zeolite supercages were synthesized using a precursor Mo(CO)₆ adsorbed in the pores of zeolite and characterized by XAFS, HREM, LRS and XPS. The structure of the intrazeolite Mo oxide clusters and their catalysis of the oxidation of ethyl alcohol to acetaldehyde were investigated. It was found by Mo K-edge XAFS that intrazeolite Mo(VI) oxide dimer species with an octahedral symmetry of Mo are formed by thermal oxidation of Mo(CO)₆ entrapped in an NaY zeolite irrespective of the Mo loading up to two Mo atoms per supercage. HREM and XRD detected no degradation of the crystallinity of the host zeolite. The dimer catalyst showed a specific activity for the oxidation of ethyl alcohol 10 times higher than an impregnation catalyst, in which isolated tetrahedral Mo oxide species are formed. The Mo oxide dimer species were transformed to monomeric species by a heat treatment at 673 K. This led to a considerable decrease in catalytic activity. The Mo oxide clusters prepared by thermal oxidation of Mo(CO)₆ encaged in the cavity of a high silica FAU zeolite (Si/Al = 630) showed a significantly higher specific activity than the Mo oxide clusters in NaY. From the Mo K-edge EXAFS analysis, it was found that Mo oxide clusters containing several Mo atoms are constructed in the pores of the high silica zeolite. It is concluded that the structure and size of Mo oxide clusters encaged in zeolite are controlled by the zeolite composition and heat treatment and that larger Mo oxide clusters exhibit a higher specific activity for the partial oxidation of ethyl alcohol.

Full text of DOI:10.1039/b108639c

View Article Online / Journal Homepage / Table of Contents for this issue
Structure of intrazeolite molybdenum oxide clusters and their catalysis
of the oxidation of ethyl alcohol
a
Yasuaki Okamoto,* Nobuyuki Oshima, Yasuhiro Kobayashi, Osamu Terasaki,
a
b
c
d
Tetsuya Kodaira and Takeshi Kubota
a
a
Department of Material Science, Shimane University, Matsue, 690-8504, Japan.
E-mail: yokamoto@riko.shimane-u.ac.jp
Department of Chemical Engineering, Osaka University, Toyonaka, 560-8531, Japan
Department of Physics, Tohoku University, Sendai, 980-8578, Japan
Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and
Technology, Tsukuba, 305-8565, Japan
b
c
d
Received 21st September 2001, Accepted 18th March 2002
First published as an Advance Article on the web 8th May 2002
Molybdenum oxide clusters confined in zeolite supercages were synthesized using a precursor Mo(CO)
6
adsorbed in the pores of zeolite and characterized by XAFS, HREM, LRS and XPS. The structure of the
intrazeolite Mo oxide clusters and their catalysis of the oxidation of ethyl alcohol to acetaldehyde were
investigated. It was found by Mo K-edge XAFS that intrazeolite Mo(VI) oxide dimer species with an octahedral
6
symmetry of Mo are formed by thermal oxidation of Mo(CO) entrapped in an NaY zeolite irrespective of the
Mo loading up to two Mo atoms per supercage. HREM and XRD detected no degradation of the crystallinity
of the host zeolite. The dimer catalyst showed a specific activity for the oxidation of ethyl alcohol 10 times
higher than an impregnation catalyst, in which isolated tetrahedral Mo oxide species are formed. The Mo oxide
dimer species were transformed to monomeric species by a heat treatment at 673 K. This led to a considerable
6
decrease in catalytic activity. The Mo oxide clusters prepared by thermal oxidation of Mo(CO) encaged in the
cavity of a high silica FAU zeolite (Si/Al ¼ 630) showed a significantly higher specific activity than the Mo
oxide clusters in NaY. From the Mo K-edge EXAFS analysis, it was found that Mo oxide clusters containing
several Mo atoms are constructed in the pores of the high silica zeolite. It is concluded that the structure and
size of Mo oxide clusters encaged in zeolite are controlled by the zeolite composition and heat treatment and
that larger Mo oxide clusters exhibit a higher specific activity for the partial oxidation of ethyl alcohol.
2
4–26
absorption fine structure (XAFS).
However, no catalytic
Introduction
properties of intrazeolite Mo oxide catalysts have been reported
until now. It is of great interest to investigate the relation
between the catalytic properties and the structure of Mo oxide
clusters in zeolite. In the present study, we report the prepara-
tion of Mo oxide clusters in zeolite by thermal oxidation of
Uniform and well-defined catalytically active species are antici-
pated to be formed when uniform supports and well-character-
ized metal precursors are utilized in the preparation of
catalysts. Zeolites are widely appreciated as potential media
for these purposes because of their uniform, homogeneous
crystalline structures having unique, ordered pore systems on
Mo(CO) encapsulated therein and the effect of the zeolite com-
6
position. The structure of the Mo oxide clusters is correlated to
the catalytic activity for the oxidative dehydrogenation of ethyl
alcohol to acetaldehyde as a test reaction.
1
–5
a molecular level.
In the petrochemical industry, oxidation is by far one of the
most important reactions and metal oxide catalysts have been
used for selective oxidation reactions of alcohol and hydrocar-
bons. Molybdenum oxide based catalysts are widely used for
6
,7
Experimental
these purposes. It is well established that the catalysis of
8
,9
Mo oxides depends strongly on their surface structure. It
might be possible to control the structure of metal oxide clus-
ters and, accordingly, their catalytic properties, by varying the
zeolite structure and composition, when a transition metal
complex encaged in zeolite is used to prepare the oxide clus-
ters.
Catalyst preparation
NaY zeolite (JRC-Z-Y5.5, a reference catalyst provided by the
Catalysis Society of Japan, Si/Al ¼ 2.8) and a high silica FAU
zeolite (HSi-FAU, Tosoh, Si/Al ¼ 630), prepared by dealumi-
nation of NaY, were used as host zeolites. To synthesize zeolite
supported Mo oxide catalysts, we employed two preparation
methods; a chemical vapor deposition (CVD) method and a
conventional impregnation method. In the CVD method, zeo-
1
0,11
Many workers have employed molybdenum hexacarbo-
nyl Mo(CO) encaged in cation exchanged zeolites to prepare
Mo clusters with well-defined structure, such as Mo sul-
6
1
1–22
23–26
27
28
oxy-carbide and metal. It was shown
ꢀ3
fide,
that the structure and size of the Mo sulfide clusters depend
oxide,
lite (0.1 g) was evacuated (<10 Pa) at 673 K for 1.5 h and
then exposed to Mo(CO)6 vapor at room temperature for
16 h, unless otherwise noted, followed by evacuation for 10
min at room temperature to remove Mo(CO)6 molecules
physisorbed on the external surface of the zeolite. Thermal
1
4,15,17
on the pore structure and composition of the host zeolite.
The preparation and structure of intrazeolite Mo oxide clusters
have been studied by a variety of techniques including X-ray
2
852
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862
This journal is # The Owner Societies 2002
DOI: 10.1039/b108639c

View Article Online
oxidation of Mo(CO)
6
/zeolite was carried out at 353 K in the
presence of 20 kPa of O using a closed circulation system
tal threshold energies. The empirical backscattering amplitude
F(k) and phase shift f(k) for a Mo–Mo atomic pair were
extracted from the EXAFS data for MoS with R(Mo–
2
2
3
(
volume : 0.20 dm ). In order to ensure complete oxidation
of Mo(CO) to Mo(VI) oxides, the O gas was evacuated and
replaced every 3 h with 20 kPa of fresh O until almost no for-
mation of CO and CO was detected (usually 3–4 times). The
Mo oxide catalyst thus prepared is denoted CVD-MoO /zeo-
6
2
Mo) ¼ 0.316 nm and N ¼ 6. The empirical parameters for
2
an Mo–O bond were extracted from the EXAFS data for
Na MoO
2
2
4
ꢁ2H O with R(Mo–O) ¼ 0.1772 nm and N ¼ 4. In
2
3
both extractions of the empirical parameters, s and E were
0
lite. The Mo content was 10 wt.%, unless otherwise noted. The
Mo content is shown by the number in parentheses, if neces-
tentatively set at 0.006 nm and 0 eV respectively.
sary, as CVD-MoO
ing 5.3 wt.% Mo. CVD-MoO
73 K for 2 h is denoted CVD-MoO
tent was analyzed by X-ray fluorescence (XRF) (Rigaku, RIX-
3
/NaY (5) for CVD-MoO
/NaY treated in vacuum at
/NaY (ev). The Mo con-
3
/NaY contain-
XPS and XRD measurements. The X-ray photoelectron (XP)
spectra of the catalysts were measured on a Hitachi 507 (Al-
Ka1,2 : 1486.6 eV). The sample was transferred to the pretreat-
3
6
3
ment chamber using a N -filled glovebag. The binding energies
were referenced to the Si 2p level from the host zeolite (NaY:
2
2
000).
An NaY-supported Mo oxide catalyst (10 wt.% Mo) was
1
02.6 eV and dealuminated high silica FAU: 104.0 eV), which
also prepared by a conventional impregnation technique using
an aqueous solution of (NH ) Mo O ꢁ4H O. The catalyst was
were separately determined for the corresponding untreated
zeolites using the C 1s level at 285.0 eV due to adventitious car-
bon. The X-ray powder diffraction patterns of the catalyst
samples were obtained on a RINT 2000 (Rigaku) after expos-
ing to air just before the measurements.
4
6
7
24
2
dried at 373 K and then calcined at 773 K for 3 h in an electric
furnace. The Mo oxide catalyst thus prepared is denoted imp-
3
MoO /NaY.
Catalytic reaction of ethyl alcohol oxidation
LRS measurements. Fourier transform Raman spectra were
obtained at room temperature on a Bruker RFS-100 instru-
ment equipped with an FRA-106 sample stage. The power of
the excitation light (1064 nm) from Nd-YAG laser was 10
mW for all measurements.
The catalytic oxidation of ethyl alcohol was carried out at 433
K using a closed circulation system. The initial partial pres-
sures of C H OH and O were 2.7 and 5.3 kPa, respectively.
2
5
2
With CVD-MoO
3
/zeolite, the reaction gas was introduced
after evacuation at 433 K. With imp-MoO /NaY, the reaction
3
was carried out after a pretreatment at 673 K for 1 h in the pre-
sence of 13.3 kPa of O , followed by evacuation and cooling to
the reaction temperature, 433 K. The reaction gas was ana-
lyzed by an on-line gas chromatograph (TCD) using a column
packing of Porapac PS (GL Science).
HREM observations. High-resolution electron microscopic
(HREM) images of CVD-MoO /NaY and CVD-MoO /
HSi-FAU were taken on a 400 kV EM, JEM-4000 EX, with
2
3
3
a spherical aberration constant C ¼ 1.00 mm, along the
s
h100i direction. An NaY zeolite (Tosoh, Si/Al ¼ 2.75), which
had been confirmed by HREM to possess complete crystalli-
nity even at the edge of the zeolite particles, was used for the
Catalyst characterization
preparation of CVD-MoO /NaY for HREM observations
3
instead of JRC-Z-Y5.5. The sample was briefly exposed to
air just before the HREM measurements.
XAFS measurement. Mo K-edge EXAFS and X-ray absorp-
tion near-edge structure (XANES) spectra for the Mo oxide/
zeolite samples and reference compounds were measured in
transmission mode at room temperature at BL-10B in the
Photon Factory of Institute of Materials Structure Science,
High Energy Accelerator Research Organization (KEK-
IMSS-PF) with 2.5 GeV ring energy and 250–290 mA stored
current. The synchrotron radiation was monochromatized by
a Si (311) channel-cut monochromator. The intensities of inci-
dent and transmitted X-rays were counted by ion chambers
with the applied voltages being 500 V. The XANES energy
was calibrated by using the absorption edge of Mo metal at
Results
Catalytic oxidation of ethyl alcohol
The reaction products were exclusively acetaldehyde and
diethyl ether. No reaction was observed to occur at 433 K over
the host zeolite. The formation rate of acetaldehyde was signif-
icantly reduced when the reaction was carried out without gas-
2
0 003 eV. The energy resolution was 1.0 eV in the XANES
eous O
by oxidative dehydrogenation of ethyl alcohol as observed for
2
, indicating that acetaldehyde is catalytically produced
region. The number of scans was three for each sample and
the sum spectrum was used for further analysis.
2
9–37
the oxidation of alcohol over Mo oxide catalysts.
other hand, the rate of diethyl ether formation was hardly
affected by the presence of O , suggesting that diethyl ether
On the
The EXAFS data were analyzed using an analysis program
developed by Technos (Osaka, Japan) and assuming a spheri-
cal wave approximation and a single scattering model. The
analysis involves pre-edge extrapolation and background
removal by a spline smoothing method to extract EXAFS
oscillation. The EXAFS data were Fourier transformed from
k-space (40–150 nm ) to R-space. The Fourier transforms
were analyzed in R-space by a curve fitting procedure to obtain
the coordination number N, bond distance R, Debye–Waller
2
3
8
is mainly produced by acidic sites. Accordingly, we focus
on the acetaldehyde formation as the oxidation activity of
the Mo oxide catalyst hereafter.
The catalytic activities of the zeolite supported Mo oxides
(10 wt.% Mo) are summarized in Table 1 for the oxidation
ꢀ
1
of ethyl alcohol. It is evident that CVD-MoO
an activity for the formation of the aldehyde 10 times higher
than imp-MoO /NaY. However, when CVD-MoO /NaY
3
/NaY shows
factor s and inner potential E
0
using eqn. (2).
3
3
was treated in a vacuum at 673 K for 2 h, the activity was
decreased to 65% of the fresh catalyst. The reduced activity
was not recovered by heat treatment at 673 K in the presence
0
02
2
0
0
0
2
wðkjÞ ¼ SjNjFðkj Þexpðꢀ2kj sj Þsinð2kj Rj þ fðkj ÞÞ=kj Rj
ð2Þ
is the wave vector of a photoelectron and is expressed
2 3
of gaseous O . It is remarkable that CVD-MoO /HSi-FAU
0
where k
by eqn. (3).
j
exhibits an activity 16 times higher than the NaY-supported
counterpart. All these results show that the activity of the
Mo oxide/zeolite catalyst depends strongly on the zeolite used,
the preparation method and the heat treatment.
0
2
2
1=2
kj ¼ ðkj ꢀ 2mDE0j=h Þ
ð3Þ
where m and DE0j represent, respectively, the mass of an elec-
tron and the difference between the theoretical and experimen-
In order to examine the effect of the Mo content on the
activity, the Mo content in CVD-MoO /NaY was varied by
3
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862
2853

View Article Online
Table 1 Catalytic activities of Mo oxides supported on zeolite for the oxidation of ethyl alcohol at 433 K
ꢀ1
ꢀ1
g
Rate of formation/mmol min
Catalyst
Mo content (wt.%)
Acetaldehyde
Diethyl ether
CVD-MoO
CVD-MoO
CVD-MoO
3
3
3
/NaY
/NaY (ev)
/HSi-FAU
10
10
10
10
1.4
0.88
22
0.40
0.60
2.5
imp-MoO
3
/NaY
0.12
0.10
changing the exposure time of degassed NaY to a vapor of
Mo(CO) at room temperature. Fig. 1 shows the Mo content
MoO
in the fresh catalyst (MoO
the peak at 228.5 eV due to Mo(CO)
complete oxidation of the carbonyl species. The formation of
Mo(VI) oxides was also confirmed for CVD-MoO /HSi-FAU
(Mo 3d5/2 : 232.9 eV, figure not shown). The slightly blue color
of CVD-MoO /HSi-FAU (weak absorption bands at 865 and
690 nm) suggests the presence of a small fraction of Mo(V)
3
/NaY, indicating the formation of Mo(VI) oxide species
; 233.1 ꢂ 0.2 eV). The absence of
(Fig. 3(a)) indicates a
6
3
as a function of the exposure time. After 12 h, the Mo content
was saturated at 10 wt.% (2Mo/supercage) in line with the
6
1
0,15,24,27,28
results of other workers.
10) was exposed again to Mo(CO)
content increased to 13 wt.% Mo. Fig. 2 shows the catalytic
When CVD-MoO
3
/NaY
3
(
6
vapor for 16 h, the Mo
3
sh
activity of CVD-MoO /NaY for the oxidation of ethyl alcohol
3
against the Mo content. The activity to acetaldehyde forma-
tion increases proportionally as the Mo content increases to
3
9
species. However, the present XPS measurements could not
detect them within experimental accuracy. When CVD-
1
0 wt.%, with an abrupt increase at 13 wt.%. On the other
3
MoO /NaY was evacuated at 673 K for 2 h, the sample chan-
hand, the activity to diethyl ether formation increases linearly
with increasing Mo content to 13 wt.%.
ged from white to brown in color to form CVD-MoO /NaY
3
XPS characterization
3 3
CVD-MoO /NaY and CVD-MoO /HSi-FAU were white and
slightly blue in color, respectively, after preparation. As shown
in Fig. 3(b), the Mo 3d5/2 binding energy is 233.1 eV for CVD-
3
Fig. 1 Mo content in CVD-MoO /NaY as a function of the expo-
sure time of NaY to a vapor of Mo(CO)
6
at room temperature.
Fig. 3 X-Ray photoelectron spectra of the Mo 3d level for (a)
6 3 3
Mo(CO) /NaY, (b) CVD-MoO /NaY (10), (c) CVD-MoO /NaY
Fig. 2 Catalytic activity of CVD-MoO /NaY for the oxidation of
ethyl alcohol at 433 K as a function of Mo content. (X) acetaldehyde
3
(ev), (d) CVD-MoO /NaY H -treated at 673 K for 2 h, (e) CVD-
3
MoO /NaY H -treated at 873 K for 2 h and (f) imp-MoO /NaY
3 2 3
2
H -treated at 873 K for 2 h.
2
and (N) diethyl ether.
2
854
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862

View Article Online
ꢀ
1
(
ev). As shown in Fig. 3(c), the XPS spectrum was only slightly
cm , indicating the formation of isolated Mo oxide species
in tetrahedral configurations.
broadened but no significant shift in the Mo 3d5/2 binding
energy was observed (232.9 eV) on evacuation. Although the
change in color suggests reduction of a part of the Mo(VI) to
lower valence Mo species such as Mo(IV), during the evacua-
tion, the fraction of reduced Mo may be very small. On heat
XRD results
2
5
Fig. 5 compares the XRD patterns for CVD-MoO
imp-MoO /NaY with that for the host zeolite NaY. With
CVD-MoO /NaY (a), neither an XRD peak ascribable to
crystalline MoO nor degradation of the crystal structure of
/NaY and
3
treatment of CVD-MoO
(20 kPa), the sample remained white in color.
When CVD-MoO /NaY was treated at 673 K for 2 h in a
stream of atmospheric pressure of H , the reduction of Mo(VI)
species was apparently observed as depicted in Fig. 3(d). When
CVD-MoO /NaY was H -treated at 873 K for 2 h (Fig. 3(e)),
3
/NaY at 673 K in the presence of
3
O
2
3
3
3
the NaY are observed. Scrutinizing the XRD pattern (a) indi-
cates that the relative intensities of a few diffraction peaks such
as (111) and (220) are remarkably reduced by the accommoda-
tion of Mo oxides, strongly suggesting that Mo oxides are
2
3
2
the weak hump around 234 eV observed in Fig. 3(d) disap-
pears. It is most likely, on the basis of the spectral width, that
the Mo species with the Mo 3d5/2 binding energy of 229.2 eV
1
5
located inside the zeolite pores. In the case of imp-MoO /
3
NaY, however, the XRD pattern (b) shows a considerable
degradation of the crystallinity of the host zeolite, in line with
the observations of other workers.
are predominantly formed after the H
2
-treatment at 873 K.
4
4–47
These species are assigned possibly to Mo(III) species on the
4
0
basis of the published binding energies (Mo(VI): 232.7 eV,
Mo(IV): 229.6 eV, Mo(III): 228.8 eV, Mo(II): 228.2 eV). Fig.
HREM observations
3
3
(f) also presents the reduction behaviour of imp-MoO /
3
Fig. 6 presents the HREM image of CVD-MoO /NaY.
NaY at 873 K. It is obvious from an unresolved broad spectral
feature that several kinds of Mo species are simultaneously
formed in imp-MoO /NaY in contrast to the predominating
Obviously, it is concluded that the crystal structure of the host
zeolite is not destroyed at all by the accommodation of
Mo(CO) and successive oxidation at 353 K. In addition, no
3
Mo(III) species in CVD-MoO
ent reduction behaviour of the Mo oxide species in CVD-
and imp-MoO /NaY indicates the formation of distinctly
3
/NaY. The remarkably differ-
6
agglomerated Mo oxide species are observed on the external
3
different Mo oxide species in these catalysts.
LRS characterization
Fig. 4 depicts the LRS spectra for CVD-MoO /NaY (a),
3
CVD-MoO
spectrum for MoO /SiO (6.7 wt.% Mo) is also presented (d)
3 3
/HSi-FAU (b) and imp-MoO /NaY (c). The
3
2
to show the LRS spectrum characteristic of MoO
3
(995 and
ꢀ
1
18 cm ). The bands at 972 (NaY) and 997 cm
ꢀ1
8 (HSi-
FAU) are ascribed to Mo=O fundamental stretching vibra-
tions. It is likely that the broad structures around 820–830
ꢀ
1
cm
kages.
mation of MoO
sharp peaks due to MoO
(c)), a sharp band characteristic of MoO
are due to the stretching mode of Mo–O–Mo lin-
4
1–43
In both LRS spectra for the CVD-catalysts, the for-
is entirely excluded by the absence of the
. With imp-MoO /NaY (Fig.
interacting with
is observed at 938
3
3
3
2ꢀ
4
4
4
1–43
a surface (Mo=O stretching band)
3 3
Fig. 5 XRD patterns of (a) CVD-MoO /NaY, (b) imp-MoO /NaY
and (c) NaY.
Fig. 4 FT-Raman spectra of (a) CVD-MoO
MoO /HSi-FAU, (c) imp-MoO /NaY and (d) MoO
3
/NaY, (b) CVD-
/SiO
3
Fig. 6 An HREM image of CVD-MoO /NaY (10); 400 kV EM,
[110] incidence.
3
3
3
2
.
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862
2855

View Article Online
surface of the host zeolite. Accordingly, it is inferred that the
Mo oxide species are located inside the zeolite cages, as sug-
gested by the XRD results. The HREM observations were
consistent with the XRD results, showing no degradation of
the crystallinity of the zeolite and no diffraction peaks due to
formation of Mo
XPS results which indicate the formation of Mo(VI) oxides.
With CVD-MoO /NaY (5), an essentially identical Fourier
transform (Fig. 8(b)) and structural parameters are obtained
(Table 2).
Fig. 8(d) shows the k -weighted Fourier transform of Mo K-
2
O
6
dimer oxide clusters on the basis of the
3
3
MoO
not shown) led to the same conclusions as for CVD-MoO
NaY.
3
. The HREM observations for CVD-MoO
3
/HSi-FAU
(
3
/
3
edge EXAFS for CVD-MoO /HSi-FAU. Two Fourier trans-
form peaks are observed at 0.30 and 0.32 nm (phase shift:
uncorrected) in the region of Mo–Mo. The structural para-
meters are also shown in Table 2. The parameters for Mo–
XAFS characterization
Mo indicate that in CVD-MoO
having a size larger than a dimer are constructed in the pores.
With imp-MoO /NaY, the structural parameters are close to
those for Na MoO . In conformity with the LRS results, the
3
/HSi-FAU, Mo oxide species
The structure of Mo oxides prepared in the zeolite cavities was
studied by means of EXAFS and XANES. Fig. 7 shows k -
3
3
weighted Mo K-edge EXAFS oscillations for some representa-
tive catalyst systems; imp-MoO /NaY, CVD-MoO /NaY,
2
4
Mo oxide species in imp-MoO /NaY are assigned to mono-
3
3
3
meric Mo oxides in tetrahedral configurations.
Thermal stability of the Mo dimer species in CVD-MoO3/
CVD-MoO /NaY (ev) and CVD-MoO
3
3
/HSi-FAU. The
Fourier transforms for the catalysts and some reference com-
pounds are presented in Fig. 8 and Fig. 9, respectively. From
NaY was examined by EXAFS. As shown in Fig. 11, the inten-
sity of the Fourier transform peak at 0.28 nm due to Mo–Mo
is significantly reduced after evacuation at 673 K (b), suggest-
ing a degradation of the Mo oxide dimer species to monomeric
species or to non-uniform Mo oxide clusters considerably dis-
ordered in the structure. Similarly, the Fourier transform peak
at 0.28 nm was considerably decreased in intensity when CVD-
a comparison of the Fourier transforms for Mo(CO)
and the CVD-catalysts, it is concluded that Mo(CO)
zeolite is completely converted to Mo oxide species, in agree-
ment with the XPS results (Fig. 3(a) and (b)). Comparison of
6
/NaY
in the
6
the Fourier transforms (Fig. 8(b)–(d)) for the MoO
catalysts derived from Mo(CO)6 with that for MoO3 (Fig.
(a)) or Na MoO (Fig. 9(b)) evidently indicates the formation
3
/zeolite
MoO /NaY was heat treated at 673 K in the presence of gas-
3
9
2
4
eous O
structure of the dimer species is considerably unstable at 673
3
K even in the presence of gaseous O . When CVD-MoO /
2
(Fig. 11(c)). Accordingly, it is concluded that the
of Mo oxide species having distinctly different structures from
the reference compounds. On the other hand, the Fourier
transform for imp-MoO /NaY (Fig. 8(a)) is close to that for
2
3
NaY was H -reduced at 873 K for 2 h (Fig. 11(d)), the Fourier
2
Na
2
MoO
4
. The structural parameters of the zeolite-supported
MoO catalysts, as derived from the EXAFS analysis by using
transform changed greatly and a strong peak due to Mo–Mo
appeared. The Mo–Mo atomic distance being larger than that
for Mo metal and the presence of two kinds of Mo–O bonds
indicate the formation of agglomerated, low valence Mo oxide
species rather than a mixture of Mo oxides and Mo metal.
Fig. 12 shows the Mo K-edge XANES spectra for CVD-
3
the empirical back-scattering amplitudes and phase shifts, are
listed in Table 2. The representative curve-fitting results are
shown in Fig. 10.
With the Fourier transform for CVD-MoO /NaY (10), the
3
peak at 0.28 nm (phase shift: uncorrected) in Fig. 8(c) is
assigned to a Mo–Mo second shell, indicating the formation
of Mo oxide clusters in NaY zeolite cages rather than mono-
meric species. The Mo–O region was reasonably curve-fitted
only by assuming two Mo–O bonds. The shorter Mo–O bond
at 0.172 nm is ascribed to a Mo=O double bond, while the
longer one at 0.193 nm to a Mo–O single bond. The coordina-
tion number (0.8) of Mo–Mo is close to unity, suggesting the
MoO
and imp-MoO
Na MoO and MoO
3
3
/NaY, CVD-MoO
/NaY. The Mo K-edge XANES spectra for
are also presented in Fig. 12 as reference
3 3
/NaY (eV), CVD-MoO /HSi-FAU
3
2
4
compounds. It is well established that XANES spectra vary
with the symmetry of the local structure of the X-ray absorb-
4
8
ing atom (Mo). A weak peak appearing in the pre-edge
region (20 005–20 010 eV) is assigned to a 1s ! 4d quadrupole
transition. When Mo atoms have no inversion center, as in
Na
by hybridization of the 4d states with p-type wave functions.
Thus slightly distorted octahedral MoX gives only a weak
s ! 4d pre-edge peak as observed for MoO , whereas a sig-
nificantly distorted octahedral or tetrahedral MoX shows a
2
MoO
4
, the 1s ! 4d pre-edge peak intensity is enhanced
6
1
3
n
2 4
stronger pre-edge peak as exemplified by Na MoO . A relative
pre-edge peak intensity for the Mo oxides in Fig. 12 is sum-
marized in Table 3, together with the peak position. The pre-
edge peak energy for Na MoO is slightly lower than that
2
4
3
for MoO .
The first derivatives of the XANES spectra in Fig. 12 are
compared in Fig. 13 to more clearly characterize the CVD-
MoO /zeolite catalysts. The peak position around 20.00 keV
3
corresponds to the edge energy and is summarized in Table
3
. The edge energies for the MoO /zeolite catalysts are
3
2 4 3
between those for Na MoO and MoO . However, it is note-
worthy from the spectral feature that the peak at about 20.00
keV for the catalyst consists of a single peak rather than a con-
voluted one of the peaks due to Na MoO and MoO . This
2
4
3
allows us to conclude that the Mo oxide species in the catalyst
are composed of mainly specific Mo species rather than a mix-
2ꢀ
ture of MoO
3
clusters and MoO
4
species.
With imp-MoO /NaY, the intensity and position of the
3
1
s ! 4d pre-edge peak are very close to the values for
3
Fig. 7 Background-subtracted, k -weighted EXAFS functions of the
Na MoO , indicating that the Mo species have tetrahedral
2 4
symmetry. On the other hand, the intensity and position of
the pre-edge peak for CVD-MoO /NaY are between those
Mo K-edge for (a) imp-MoO
3
3
/NaY, (b) CVD-MoO /NaY, (c) CVD-
/HSi-FAU.
MoO /NaY (ev) and (d) CVD-MoO
3
3
3
2
856
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862

View Article Online
3
ꢀ1
Fig. 8 Fourier transforms of k -weighted EXAFS modulations (Dk ¼ 40–150 nm ) of the Mo K-edge for (a) imp-MoO
3
/NaY, (b) CVD-
MoO /NaY (5), (c) CVD-MoO /NaY (10) and (d) CVD-MoO /HSi-FAU.
3
3
3
for Na
a considerably distorted octahedral symmetry. The pre-edge
peak intensity of CVD-MoO /NaY is slightly increased by
the evacuation at 673 K. It is considered that the Mo species
in CVD-MoO /NaY (ev) is still in a distorted octahedral sym-
metry, although the local structure is considerably altered.
With CVD-MoO /HSi-FAU, a weak pre-edge peak evidently
indicates that the local symmetry of the Mo atoms in the clus-
ters is very close to that in MoO . In addition, the XANES
2
MoO
4
and MoO
3
, indicating that the Mo species have
the formation of Mo oxide clusters having a local structure
similar to that of MoO3 .
3
Discussion
3
Location and structure of intrazeolite Mo oxide clusters
3
The location of the Mo oxide species, that is, inside or out-
side the zeolite pores, is of great importance to the control
of the size and structure of the clusters and to the catalytic
3
spectrum for CVD-MoO /HSi-FAU (Fig. 12) shows three
3
peaks, although much less resolved, at about 200 026,
00 038 and 200 052 eV characteristic of MoO , suggesting
3
performance. The HREM image in Fig. 6 for CVD-MoO /
2
3
NaY clearly demonstrates that the crystal structure of the
host zeolite is not degraded by the incorporation of the Mo
oxide clusters and that no agglomeration of Mo oxides takes
place on the external surface of the zeolite. The LRS and
XRD results support the HREM observations. In our pre-
1
5
vious study, the pore volume of CVD-MoO
measured by using benzene as an adsorbate. The pore volume
3
/NaY was
3
ꢀ1
of CVD-MoO
3
/NaY (10) was 0.169 cm
g
compared to
ꢀ1
for NaY. The decrease in the pore volume
3
.298 cm g
0
3
ꢀ1
of 0.13 cm g is even greater than that expected from the
volume of MoO (ca. 0.06 cm g ), indicating that Mo oxide
3
ꢀ1
3
species are formed inside the zeolite pores. It is concluded
that, with CVD-MoO /NaY, the Mo oxide species are
3
formed in the pores of NaY. It is inferred from the HREM
observations that, with CVD-MoO /HSi-FAU, Mo oxide
3
species are also formed in the supercage of the host zeolite.
The location of the Mo oxide species in imp-MoO /NaY is
3
not clear in the present study. However, taking into consid-
eration the predominant formation of tetrahedral Mo oxide
interaction species and a low external surface area of zeolite,
it is likely that most of the Mo oxide species migrate into the
pores of the host zeolite, with considerable destruction of the
zeolite structure.
On the basis of the XPS results, it is concluded that the che-
mical state of the Mo atoms in the freshly prepared CVD-
3
Fig. 9 Fourier transforms of k -weighted EXAFS oscillations
MoO
3 6
/NaY is Mo(VI) oxide species. The Mo(CO) molecules
ꢀ1
encapsulated in the NaY zeolite pores thermally decompose
at 330–400 K to form subcarbonyl species, Mo(CO)
(
Mo(CO) /NaY, (b) Na MoO O and (c) MoO
Dk ¼ 40–150 nm ) of the Mo K-edge for reference compounds. (a)
6
2
4
ꢁ2H
2
3
.
x
(x ¼ 5,
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862
2857

View Article Online
a
Table 2 Structural parameters as derived from the Mo K-edge EXAFS for molybdenum oxides encaged in zeolites
2
ꢀ6
2
Catalyst
Treatment
Fresh
Atom pair
R/nm
CN
E
0
/eV
Ds /10 nm
CVD-MoO
3
/NaY (10)
Mo–O
Mo–O
Mo–Mo
Mo–O
Mo–O
Mo–O
Mo–O
Mo–Mo
Mo–O
Mo–O
Mo–Mo
Mo–O
Mo–O
Mo–Mo
Mo–Mo
Mo–O
Mo–C
0.172
0.193
0.322
0.171
0.170
0.165
0.208
0.278
0.170
0.194
0.321
0.176
0.193
0.334
0.358
0.172
0.207
0.322
2.9
2.3
0.8
3.7
4.2
1.2
2.8
3.2
3.4
1.9
0.7
2.8
2.7
1.6
0.8
4.5
6.3
5.7
ꢀ10.3
ꢀ11.2
ꢀ6.6
ꢀ9.1
ꢀ13.6
9.3
12
12
ꢀ6
41
40
58
40
33
26
16
ꢀ6
38
74
47
17
17
6
Evac. at 673 K
O
2
-treat. at 673 K
2
-treat. at 873 K
H
ꢀ5.7
ꢀ0.9
ꢀ7.5
ꢀ2.7
0.6
CVD-MoO
3
3
/NaY (5)
Fresh
Fresh
CVD-MoO
/HSi-FAU
ꢀ13.4
ꢀ13.9
ꢀ0.3
8.7
imp-MoO /NaY
Mo(CO) /NaY
3
Fresh
Fresh
ꢀ10.9
0.1
0.1
6
Mo–CO
0
Reference compound
MoS
Na MoO
2
Mo–Mo
Mo–O
Mo–Mo
0.316
0.177
0.2726
6
4
8
6
2
4
ꢁ2H
2
O
Mo metal
0
.3147
a
R: bond distance, CN: coordination number, E
0
: inner potential, s: Debye–Waller factor.
1
0,49–53
4
, 3).
In the present preparations at 353 K, the resultant
The structure of the Mo oxide species encaged in zeolite is
discussed on the basis of the XAFS results. It is concluded
from the XANES derivative spectra in Fig. 13 that the Mo
oxide species encaged in the zeolite pores are not composed
of a simple mixture of tetrahedral Mo oxide species (as
subcarbonyl species readily react with gaseous O
oxide species. In the case of photooxidation of Mo(CO)
at room temperature, Mo(CO) is decarbonylated by UV-irra-
diation to subcarbonyl species, which subsequently react with
gaseous O to form Mo(VI) oxide species (Mo 3d5/2 : 233.0
2
to form Mo
6
/NaY
6
2
observed in imp-MoO
CVD-MoO /NaY (10), the structural parameters indicate
the formation of Mo oxide dimer species (Mo–Mo CN: 0.8).
3
In conformity with this, careful sulfidation of CVD-MoO /
3 3
/NaY) and MoO clusters. With
2
6
54,55
eV). According to Almond et al.,
W subcarbonyl species
3
react with O molecules to form monomeric W(VI) oxide spe-
2
cies in a liquid Ar matrix (10 K). It is deduced that the forma-
tion of the subcarbonyl species and subsequent oxidation
under moderate conditions are the most important processes
to synthesize intrazeolite Mo oxide species.
1
4
NaY produced Mo sulfide dimer species encaged in NaY.
The formation of intrazeolite W oxide dimer species in a tetra-
2
3,24,56–59
hedral coordination was reported by Ozin et al.,
W(CO) /NaY was photooxidized. According to Ozin
et al.,
when
6
4,25
2
however, monomeric Mo oxide species in an octahe-
dral symmetry were synthesized by photooxidation (80 kPa of
O , 450 W, high pressure Xe arc lamp) of Mo(CO) /NaY in
contrast to the dimer species formed by thermal oxidation at
2
6
2
6
3
7
53 K (present work) or by photooxidation (20 kPa of O2 ,
5 W, high pressure Hg lamp). It is conjectured that the differ-
ences in the structure of the Mo oxide species reside in the dif-
ference in the severity of the oxidation conditions of Mo(CO)
the oxidation of Mo(CO) was completed within 1 h in the case
6
:
6
2
4,25
of Ozin et al.
mal oxidation and 50 h in the previous photooxidation.
Obviously, it is considered that oxidation of Mo(CO) under
in contrast to about 16 h in the present ther-
2
6
6
mild conditions is favorable for the formation of the Mo oxide
dimer clusters.
The XANES spectrum for CVD-MoO /NaY (10) clearly
3
indicates that the Mo atoms are in a distorted octahedral sym-
metry. The sum of the coordination numbers of the Mo–O
bonds at 0.172 and 0.193 nm is 5.2, being consistent with the
octahedral symmetry of Mo. Taking into account the coordi-
nation numbers of Mo–O and Mo–Mo (XAFS), the presence
of Mo=O and Mo–O–Mo bonds (LRS and XAFS) and the
chemical state of Mo(VI) (XPS), it is rational to propose that
2 6
Mo O clusters (Mo(VI) oxide dimer) are formed, interacting
with the zeolite lattice oxygens to form a molecular structure
as illustrated in Fig. 14. A similar dimer unit of edge-shared
3
Fig. 10 Fourier transforms of k -weighted EXAFS modulations
ꢀ1
distorted MoO
compounds; e.g., in (NH
6
octahedra is found in some Mo oxide
(Dk ¼ 40–150 nm ) of the Mo K-edge and the best curve-fittings.
3
9
60
(
a) CVD-MoO /HSi-FAU and (b) CVD-MoO /NaY.
3
3
4
)
2
Mo
2
O
7
,
where R(Mo–Mo) ¼
2
858
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862

View Article Online
3
ꢀ1
Fig. 11 Fourier transforms of k -weighted EXAFS modulations (Dk ¼ 40–150 nm ) of the Mo K-edge for (a) CVD-MoO
MoO /NaY (ev), (c) CVD-MoO /NaY treated at 673 K for 1 h with 13 kPa of O , (d) CVD-MoO /NaY H
Mo metal (reference).
3
/NaY, (b) CVD-
2
-treated at 873 K for 2 h and (e)
3
3
2
3
0
.3146 nm, R(Mo=O) ¼ 0.1734 and 0.1722 nm, R(Mo–
the dimer are in tetrahedral symmetry in contrast to the pre-
sent octahedral configuration.
When CVD-MoO /NaY is evacuated at 673 K, the Fourier
O) ¼ 0.2069, 0.2311, 0.2166 and 0.1840 nm. The structural
parameters of the Mo oxide dimer species are in conformity
3
with these values. Table 2 shows that CVD-MoO
has essentially identical structural parameters with CVD-
MoO /NaY (10). It is concluded that the Mo dimer clusters
are formed, irrespective of the Mo content, corresponding to
3
/NaY (5)
transform is considerably changed and the peak due to the
Mo–Mo second shell almost disappears. However, major Mo
species are still in a Mo(VI) state, although some portion of
Mo is reduced to lower valence species as evidenced by a
less-resolved Mo 3d band (Fig. 3(c)) and a color change from
white to brown. These findings suggest a transformation of the
dimer structure to a monomeric Mo(VI) species or to dimer
structures so greatly distorted as to have a broad distribution
of the Mo–Mo atomic distance. Taking into consideration
the uniformity of the zeolite structure, however, it is most
likely that the dimer oxide clusters are transformed to mono-
3
2
for W oxide
Mo atoms per supercage, in agreement with the observations
5
6–59
14,15
and Mo sulfide
clusters encaged in zeolite.
prepared a Mo oxide dimer species
by using Mo (C . The Mo atoms in
3
3–35
Iwasawa et al.
attached to SiO
2
2
3 5 4
H )
meric oxide species. The XANES spectrum of CVD-MoO
NaY (ev) indicates that the Mo species are in a considerably
distorted octahedral symmetry. When CVD-MoO /NaY is
3
/
3
treated at 673 K in the presence of gaseous O , a similar degra-
2
dation of the dimer species to monomeric one is observed with-
out a color change. A plausible structure is proposed in Fig.
1
et al.
4. The structure is similar to that proposed by Ozin
for freshly prepared intrazeolite Mo oxide species.
2
4,25
These findings indicate relatively low thermal stability of the
intrazeolite Mo oxide dimer species.
It is considered from the structural parameters in Table 2
that the Mo oxide clusters in CVD-MoO /HSi-FAU consist
3
of several Mo atoms. A possible structure is a dimer of the
Mo oxide dimers, taking into account the two kinds of Mo–
Mo contributions with coordination numbers of one and
two. The Mo atoms are in slightly distorted octahedral symme-
5
7,59
tries. Ozin et al.,
proposed a similar structure for intrazeo-
lite W oxide clusters prepared by photooxidation of W(CO)6
encaged in NaY (4 W/supercage).
Larger Mo oxide clusters are formed in CVD-MoO /HSi-
3
FAU than in CVD-MoO
observed for the preparation of intrazeolite Mo sulfide clusters
by using Mo(CO) as a precursor, that is, the Mo sulfide clus-
ters in a USY zeolite (Si/Al ¼ 3.9) are larger in size than Mo
3
/NaY. A similar trend has been
Fig. 12 XANES spectra of the Mo K-edge for the Mo oxide catalysts
and reference compounds.
6
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862 2859

View Article Online
Table 3 The position and relative intensity of the 1s ! 4d pre-edge peak and the edge energy of Mo K-edge absorption
a
Peak position /eV
a
Edge energy /eV
Catalyst/reference compound
Relative intensity
Na
2
MoO
4
1.00
0.88
0.77
0.71
0.66
0.61
20 005.4
20 005.5
20 007.1
20 007.6
20 009.0
20 008.3
20 003.1
20 003.1
20 004.1
20 004.6
20 005.2
20 004.6
Imp-MoO
3
/NaY
CVD-MoO
CVD-MoO
CVD-MoO
3
3
3
/NaY (eV)
/NaY
/HIS-FAU
MoO
3
a
The accuracy of the peak position and edge energy is ꢂ0.5 eV.
1
5
sulfide dimer clusters in NaY (2.1). It is considered that the
cluster size of the intrazeolite Mo oxides or sulfides is corre-
lated to the interaction strength of intermediate Mo subcarbo-
nyl species with the host zeolite. The basic strength of the
framework oxygens of the host zeolite decreases as the Si/Al
6
1,62
ratio increases.
to the Mo subcarbonyl species as solid ligands to form
Since the framework oxygens coordinate
5
1–53,63
Mo(CO)6-x(Oz)
x
(Oz: framework oxygen),
the decrease
in the basic strength results in weaker interactions between
the Mo subcarbonyl species and the framework oxygens and
accordingly leads to their higher mobility during the reactions
with O or H S, accompanying the growth in the cluster size. It
2 2
is also probable that the resultant Mo oxide or sulfide clusters
interact less strongly with less basic framework oxygens
(weaker anchoring effect), leading to a larger tendency toward
agglomeration.
In contrast to the intrazeolite Mo oxide clusters prepared
from Mo(CO)
analyses that monomeric Mo oxide species in a tetrahedral
symmetry are formed in imp-MoO /NaY, accompanying a
6
, it is concluded from the EXAFS and XANES
3
partial degradation of the zeolite crystallinity. The formation
of the tetrahedral species is also substantiated by the LRS
2ꢀ
results (Fig. 4(c)). It is proposed that MoO4 species are com-
bined with extra-framework Na(I) or Al(III) cations and/or Al
oxides.
Structure and activity relationship
As summarized in Table 1, the specific activity of Mo oxide
clusters for the oxidative dehydrogenation of ethyl alcohol
Fig. 13 The first derivatives of the XANES spectra in Fig. 12.
decreases in the order: CVD-MoO
MoO /NaY > CVD-MoO
/HSi-FAU ꢃ CVD-
3
3
3
/NaY (ev) ꢃ imp-MoO
3
/NaY. As
can be deduced from Fig. 2, the formation of acetaldehyde is
catalyzed by the Mo oxide species and not by the host zeolite.
It is considered that the activity sequence is correlated to the
size and local structure of the clusters and their interactions
with the zeolite. The tetrahedral Mo oxide species in imp-
MoO /NaY strongly interact with the zeolite and are chemi-
3
3
cally robust, as observed for the Mo oxide species in MoO /
6
4,65
14,15
Al O having a low Mo content.
Okamoto et al.
Vorbeck et al. showed that the Mo oxide species in imp-
and
2
3
1
8
MoO /NaY show high resistance against reduction and sulfi-
3
dation at 673 K. Since SiO -attached monomeric Mo oxide
2
species with a tetrahedral configuration show a comparable
specific activity for the oxidation of ethyl alcohol with the
3
4
Mo oxide dimer species attached to SiO2 , it is considered
that the low activity of imp-MoO /NaY results from the
3
strong interaction with the host zeolite rather than from its tet-
rahedral structure of the Mo species.
The Mo oxide dimer species in CVD-MoO /NaY exhibit a
3
higher specific activity than the monomeric species in CVD-
MoO /NaY (ev). According to Iwasawa and Tanaka,
3
4
3
SiO
2
-attached Mo oxide dimer species prepared using
3
Mo (Z -C H ) as a precursor showed a specific activity simi-
2
3
5 4
lar to Mo monomer species for ethyl alcohol oxidation at 420
3
4
K. It was shown, however, that the former species show an
activity per site 1.5 times higher than the latter species. Accord-
ingly, the high specific activity and, in particular, considerably
high activity per site (ca. 3 times higher activity per site than
Fig. 14 Structures of the Mo oxide dimer species and monomeric
species formed in CVD-MoO /NaY and CVD-MoO /NaY (ev),
respectively. Oz represents the framework oxygen of the host zeolite.
3
3
2
860
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862

View Article Online
the monomeric species) of the present intrazeolite dimer spe-
cies are ascribed to cooperative catalysis of the two Mo atoms
ful to Mr. T. Kawabata (Shimane University) for the
measurements of the XRD patterns.
3
4
as proposed by Iwasawa et al. In the intrazeolite Mo oxide
dimer species, two Mo atoms are strongly bonded to each
other through the Mo–O–Mo bonds, while the two Mo atoms
3
3–35
References
in the SiO
2
-attached dimer species
are only weakly inter-
acting, on the basis of the EXAFS results showing no
Mo–Mo contributions at 200–300 K. The difference in the
structures of these dimer species leads to the different catalytic
activities relative to the corresponding monomeric species. It is
1
2
3
G. D. Stucky and J. E. MacDougall, Science, 1990, 247, 669.
G. A. Ozin, Adv. Mater., 1992, 4, 612.
G. A. Ozin, A. Kuperman and A. Stein, Angew. Chem., Int. Ed.
Engl., 1989, 28, 358.
noteworthy that CVD-MoO
ity 16 times higher than CVD-MoO
high activity of CVD-MoO /HSi-FAU may be due to the
increased size of the Mo oxide clusters. It is considered that
in the larger clusters, more effective cooperation between the
Mo atoms takes place in a redox mechanism.
As shown in Fig. 2, the oxidation activity of CVD-MoO3/
NaY increases proportionally as the amount of Mo increases
to 2 Mo atoms per supercage. This finding evidently demon-
strates that the identical Mo oxide dimer clusters are formed
irrespective of the average number of Mo atoms in the pores
up to 2 Mo atoms per supercage, in conformity with the
EXAFS analysis. The activity, however, significantly increases
3
/HSi-FAU shows a specific activ-
4
5
6
W. M .H. Sachtler and Z. Zhang, Adv. Catal., 1993, 39, 129.
B. C. Gates, Chem. Rev., 1995, 95, 511.
3
/NaY. The significantly
B. C. Gates, J. R. Katzer, G. C. A. Schuit, Chemistry of Catalytic
Processes, MacGraw-Hill, New York, 1979, ch. 4.
(a) Y. Iwasawa, Stud. Surf. Sci. Catal., 1996, 101, 21; (b) Y. Iwa-
sawa, Adv. Catal., 1987, 35, 187; (c) Y. Iwasawa, Catal. Today,
1993, 18, 21.
J. C. Volta, J. M. Tatibouet, C. Phichitkul, J. E. Germain, Pro-
ceedings of the 8th International Congress on Catalysis, Dechema,
Frankfurt, 1984, p. IV-451.
3
7
8
9
0
J. C. Volta and B. Morawek, J. Chem. Soc., Chem. Commun.,
1980, 338.
R. F. Howe, in Tailored Metal Catalysts, ed. Y. Iwasawa, Reidel,
1
Dortrecht, 1986, p. 1141.
11 Y. Okamoto, Catal. Today, 1997, 39, 45.
1
2
Y. Okamoto, A. Maezawa, H. Kane, T. Imanaka, in Proceedings
of the 9th International Congress on Catalysis, ed. M. J. Phillis
and M. Ternan, The Chemical Institute of Canada, Ottawa,
3
on further addition of Mo to CVD-MoO /NaY (10). This may
be a consequence of the formation of Mo oxide clusters larger
than the dimer species, although a possible activity increase
due to agglomeration of the added Mo oxide species on the
external surface of the zeolite cannot be ruled out. In contrast
to the oxidation reaction, the formation of diethyl ether
increases linearly as the Mo content increases to more than 2
Mo/supercage. This may suggest that different active sites
are involved in the oxidative dehydrogenation and dehydra-
tion of ethyl alcohol. It is considered that acidic sites are gen-
erated on or near the Mo oxide species and that the acidity is
less affected by the structure of the Mo oxide clusters than the
oxidation activity. In line with this, the ether formation in
Table 1 does not depend so much on the preparation as the
acetaldehyde formation.
1
988, vol. 1, p. 11.
1
3
Y. Okamoto, A. Maezawa, H. Kane and T. Imanaka, J. Mol.
Catal., 1989, 52, 337.
14 Y. Okamoto and H. Katsuyama, Ind. Eng. Chem. Res., 1996, 35,
834.
1
1
5
Y. Okamoto, H. Katsuyama, K. Yoshida, N. Nakai, M. Matsu-
moto, Y. Sakamoto, J. Yu and O. Terasaki, J. Chem. Soc., Fara-
day Trans., 1996, 92, 4647.
Y. Okamoto, H. Okamoto, T. Kubota, H. Kobayashi and O. Ter-
asaki, J. Phys. Chem. B, 1999, 103, 7160.
1
6
17 Y. Okamoto and T. Kubota, Catal. Surveys from Japan, 2001,
, 3.
5
1
8
G. Vorbeck, W. J. J. Welters, L. J. M. van de Ven, H. W. Zanber-
gen, J. W. de Haan, V. H. J. de Beer and R. A. van Santen, Zeo-
lites and Related Microporous Materials: State of the Art, Elsevier,
Amsterdam, 1994, p. 1617.
1
2
2
9
0
1
W. J. J. Welters, PhD Thesis, Eindhoven University of Technol-
ogy, The Netherlands, 1994.
P. de Pont, PhD Thesis, Delft University of Technology, The
Netherlands, 1998.
Conclusions
The salient findings in the present study are:
M. J. Vissenberg, PhD Thesis, Eindhoven University of Technol-
ogy, The Netherlands, 1999.
M. Laniecki and W. Zmierczak, Zeolites, 1991, 11, 18.
1
. Intrazeolite Mo(VI) oxide dimer clusters in a distorted
octahedral configuration are synthesized at 353 K by thermal
oxidation of Mo(CO) encaged in an NaY zeolite. The Mo
22
23
24
6
¨
G. A. Ozin and S. Ozkar, Chem. Mater., 1992, 4, 511.
oxide dimers are transformed to monomeric, octahedral Mo
oxide species at 673 K.
¨
G. A. Ozin, S. Ozkar and R. A. Prokopowicz, Acc. Chem. Res.,
1992, 25, 553.
25 R. Jelinek, S. Ozkar and G. A. Ozin, J. Phys. Chem., 1992, 96,
¨
2
. The thermal oxidation of Mo(CO) encaged in HSi-FAU
6
5
949.
Y. Okamoto, Y. Kobayashi and T. Imanaka, Catal. Lett., 1993,
0, 49.
K. Asakura, Y. Noguchi and Y. Iwasawa, J. Phys. Chem. B, 1999,
03, 1051.
(
Si/Al ¼ 630) leads to the formation of intrazeolite Mo oxide
2
2
6
7
clusters containing several Mo atoms in octahedral coordina-
tions.
2
3
. Monomeric Mo oxide species in a tetrahedral symmetry
are predominantly formed by the impregnation method.
. The structure and size of the intrazeolite Mo oxide clus-
1
28 T. Komatsu, S. Namba, T. Yashima, K. Domen and T. Onishi,
J. Mol. Catal., 1985, 33, 345.
4
29
30
31
M. Niwa, M. Mizukami, M. Takahashi and Y. Murakami,
J. Catal., 1981, 70, 14.
J. Tatibouet, J. M. Germain and J. C. Volta, J. Catal., 1983, 82,
240.
J. C. Chung, R. Miranda and C. O. Bennett, J. Catal., 1988, 114,
ters are controlled by the zeolite composition and the heat
treatment after preparation.
5. The specific activity of the Mo oxide species for the oxida-
tion of ethyl alcohol depends strongly on the cluster size and
interaction strength with the host zeolite. The activity
398.
32 C. Louis, J. M. Tatibouet and M. Che, J. Catal., 1988, 109, 354.
decreases in the order: CVD-MoO /HSi-FAU ꢃ CVD-
3
3
3
3
4
Y. Iwasawa and M. Yamagishi, J. Catal., 1983, 82, 373.
Y. Iwasawa and H. Tanaka, in Proceeding of the 8th International
Congress on Catalysis, Dechema, Frankfurt, 1984, p. IV-381.
Y. Iwasawa, Tailored Metal Catalysts, ed. Y. Iwasawa, Reidel,
Dortrecht, 1986, p.1.
MoO
3
/NaY > CVD-MoO
3
/NaY (ev) ꢃ imp-MoO
3
/NaY.
3
5
6
Acknowledgement
3
M. Niwa, H. Yamada and Y. Murakami, J. Catal., 1992, 134,
3
31.
This work was supported by CREST, Japan Science and Tech-
nology Corporation. This work was also supported by a
Grant-in-Aid for Science Research from the Ministry of Edu-
cation, Science, Sports and Culture (09650860). We are grate-
3
3
3
7
8
9
T. J. Yang and J. H. Lunsford, J. Catal., 1987, 103, 55.
K. Tanabe, Solid Acids and Bases, Kodansha, Tokyo, 1970.
(a) F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry,
Wiley, New York, 1988, pp. 807–819; (b) N. N. Greenwood and
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862
2861

View Article Online
¨
A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford,
984, ch. 23.
52 S. Ozkar, G. A. Ozin, K. Moller and T. Bein, J. Am. Chem. Soc.,
1990, 112, 9575.
1
4
0
1
(a) R. B. Quincy, M. Houalla, A. Proctor and D. M. Hercules, J.
Phys. Chem., 1990, 94, 1520; (b) M. Yamada, J. Yasumaru, M.
Houalla and D. M. Hercules, J. Phys. Chem., 1991, 95, 7037.
F. D. Hardcastle and I. E. Wachs, J. Raman Spectrosc., 1990, 21,
53 Y. Okamoto and T. Kubota, Microporous Mesoporous Mater.,
2001, 48, 301.
54 M. J. Almond, J. A. Crayston, A. J. Downs, M. Poliakoff and J. J.
Turner, Inorg. Chem., 1986, 25, 19.
4
6
83.
55 M. J. Almond and J. Downs, J. Chem. Soc., Dalton Trans., 1988,
809.
42
43
44
W. P. Griffith and P. J. B. Lesniak, J. Chem. Soc. A, 1969, 1066.
H. Kn o¨ zinger and H. Jeziorowski, J. Phys. Chem., 1978, 82, 2002.
R. Cid, F. J. Gil Lalambias, J. L. G. Fierro, A. Lopez Agudo and
J. Villasenor, J. Catal., 1984, 89, 478.
¨
56 G. A. Ozin, S. Ozkar and P. Macdonald, J. Phys. Chem., 1990, 94,
6939.
57 G. A. Ozin and S. Ozkar, J. Phys. Chem., 1990, 94, 7556.
¨
¨
58 K. Moller, T. Bein, S. Ozkar and G. A. Ozin, J. Phys. Chem.,
4
4
4
4
5
6
7
8
R. Cid, F. Ovellana and A. Lopez Agudo, Appl. Catal., 1987, 32,
3
27.
J. L. G. Fierro, J. C. Conesa and A. Lopez Agudo, J. Catal., 1987,
08, 334.
1991, 95, 5276.
59 G. A. Ozin, R. A. Prokopowicz and S. Ozkar, J. Am. Chem. Soc.,
1992, 114, 8953.
60 A. W. Armour, M. G. B. Drew and P. C. H. Mitchell, J. Chem.
Soc., Dalton Trans., 1975, 1493.
61 W. J. Mortier and R. A. Schooheydt, Prog. Solid State Chem.,
1985, 16, 1.
62 Y. Okamoto, Crit. Rev. Surf. Chem., 1995, 5, 249.
63 Y. Okamoto, T. Imanaka, K. Asakura and Y. Iwasawa, J. Phys.
Chem., 1991, 95, 3700.
¨
1
J. Thoret, C. Marchal, C. Doremieux-Morin, P. P. Man,
M. Gruia and J. Fraissard, Zeolites, 1993, 13, 269.
(a) J. C. J. Bart, Adv. Catal., 1986, 34, 203; (b) S. Yoshida and T.
Tanaka, in X-Ray Absorption Fine Structure for Catalysts and Sur-
faces, ed. Y. Iwasawa, World Scientific, Singapore, 1996, p. 304.
P. Gallezot, G. Coudurier, M. Primet and B. Imelik, ACS Symp.
Ser., 1977, 40, 144.
49
50
51
Y. Yong-Sing and R. F. Howe, J. Chem. Soc., Faraday Trans. 1,
64 C. P. Li and D. M. Hercules, J. Phys. Chem., 1984, 88, 456.
65 H. Kn o¨ zinger, Proceedings of the 9th International Congress on
Catalysis, ed. M. J. Phillips and M. Ternan, The Chemical Insti-
tute of Canada, Ottawa, 1988, vol. 5, p. 20.
1986, 82, 2887.
Y. Okamoto, A. Maezawa, H. Kane, I. Mitsushima and T. Ima-
naka, J. Chem. Soc., Faraday Trans. 1, 1988, 84, 851.
2
862
Phys. Chem. Chem. Phys., 2002, 4, 2852–2862