Selective ammoxidation of isobutylene to methacrylonitrile on a new family of crystalline Re-Sb-O catalysts

Article abstract of DOI:10.1006/jcat.2001.3198

The catalytic properties of a new family of crystalline Re-Sb-O compounds SbRe₂O₆, SbOReO₄·2H₂O, and Sb₄Re₂O₁₃ in selective ammoxidation of isobutylene to methacrylonitrile (MAN) have been studied and compared with those of a coprecipitated SbRe₂O_x catalyst, an Sb₂O₃-supported Re₂O₇ catalyst, bulk Re oxides, and bulk Sb oxides. The Re-based catalysts were more or less active for MAN synthesis with selectivities of 47.9-83.6% at 673 K, whereas bulk Sb oxides (Sb₂O₃ and Sb₂O₄) showed no activity. The results demonstrate that Re is prerequisite for the ammoxidation catalysis of Re-Sb-O systems. In catalytic systems the presence of Sb also contributes to the ammoxidation catalysis for MAN synthesis. Among these catalysts, SbRe₂O₆ was most active and selective (83.6%) for MAN formation at 673 K. No structural change in the bulk and surface of SbRe₂O₆ was observed after i-C₄H₈ ammoxidation by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and confocal laser Raman microspectroscopy. The good performance of SbRe₂O₆ may be ascribed to its specific crystal structure composed of alternate octahedral (Re₂O₆)^3- and (SbO)⁺ layers. Pulse reaction results suggested that adsorbed NH₃ species on the SbRe₂O₆ catalyst facilitated the adsorption and subsequent activation of isobutylene. Increasing reaction temperature and decreasing GHSV did not give rise to increasing formation of by-products CO₂ and acetonitrile, while increasing the i-C₄H₈ conversion. Thus the crystalline SbRe₂O₆ compound may be a new promising catalyst for ammoxidation of light hydrocarbons.

Full text of DOI:10.1006/jcat.2001.3198

Journal of Catalysis 200, 69–78 (2001)
doi:10.1006/jcat.2001.3198, available online at http://www.idealibrary.com on
Selective Ammoxidation of Isobutylene to Methacrylonitrile on a New
Family of Crystalline Re–Sb–O Catalysts
1
2
Haichao Liu, Hideo Imoto, Takafumi Shido, and Yasuhiro Iwasawa
Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received August 10, 2000; revised January 2, 2001; accepted January 7, 2001; published online April 18, 2001
exhibited unique activities in selective hydrogenation of
The catalytic properties of a new family of crystalline Re–Sb– organic compounds, hydrodesulfurization of heavy crude
O compounds SbRe2O6, SbOReO4 � 2H2O, and Sb4Re2O13 in se- oil, and selective dehydroaromatization of methane to
lective ammoxidation of isobutylene to methacrylonitrile (MAN)
have been studied and compared with those of a coprecipitated
SbRe2Ox catalyst, an Sb2O3-supported Re2O7 catalyst, bulk Re ox-
ides, and bulk Sb oxides. The Re-based catalysts were more or less
active for MAN synthesis with selectivities of 47.9–83.6% at 673 K,
because Re oxides possess redox properties comparable to
whereas bulk Sb oxides (Sb2O3 and Sb2O4) showed no activity. The
results demonstrate that Re is prerequisite for the ammoxidation
benzene (1–10). As for selective oxidation, however, Re
has found hitherto few applications to the oxidation of
methanol and ethanol (7, 11–13). Notwithstanding this fact,
Re can be a key element in catalytic oxidation processes
those of V, Mo, and W oxides (14), which are extensively
employed as main components in various oxidation cata-
lysts (15–21). Such limited uses of Re in selective oxida-
catalysis of Re–Sb–O systems. In catalytic systems the presence of
Sb also contributes to the ammoxidation catalysis for MAN synthe-
sis. Among these catalysts, SbRe2O6 was most active and selective tion reactions are most likely relevant to the instability of
83.6%) for MAN formation at 673 K. No structural change in the Re oxides involving facile volatilization under usual pre-
(
bulk and surface of SbRe2O6 was observed after i-C4H8 ammox- treatment and reaction conditions. Therefore, to develop
idation by means of X-ray diffraction, X-ray photoelectron spec-
troscopy, scanning electron microscopy, and confocal laser Raman
microspectroscopy. The good performance of SbRe2O6 may be as-
and understand new catalytic properties of Re, it is es-
sential to discover a new Re mixed oxide system in which
Re oxides are stabilized by coexisting second metal oxide
species.
The survey of numerous catalyst formulations for se-
lective oxidation and ammoxidation of light hydrocarbons
nominates Sb as a good element to form mixed oxide phases
with Re due to its unique property as a promoter element in
cribed to its specific crystal structure composed of alternate octahe-
dral (Re2O6) 3 and (SbO) layers. Pulse reaction results suggested
that adsorbed NH3 species on the SbRe2O6 catalyst facilitated the
adsorption and subsequent activation of isobutylene. Increasing
reaction temperature and decreasing GHSV did not give rise to
increasing formation of by-products CO2 and acetonitrile, while
�
+
increasing the i-C4H8 conversion. Thus the crystalline SbRe2O6 many mixed oxides such as V–Sb–O, Sn–Sb–O, Mo–Sb–O,
compound may be a new promising catalyst for ammoxidation of Fe–Sb–O, and U–Sb–O (15–23). In these catalysts, Sb
light hydrocarbons.
�c 2001 Academic Press
oxides are considered to facilitate the abstraction of an
Key Words: Re–Sb oxide catalysts; SbRe2O6; selective ammoxida- �-hydrogen from hydrocarbon molecules as well as the sub-
tion; isobutylene; methacrylonitrile; X-ray diffraction; X-ray photo- sequent oxygen or nitrogen insertion into allylic interme-
electron spectroscopy; scanning electron microscopy; Raman; pulse
reaction.
diate species. Our recent results concerning the Pt/SbOx
catalyst for the selective oxidation ofisobutane and isobuty-
lene to methacrolein also show that the Sb6O13 subox-
ide phase formed under the catalytic oxidation conditions
contributes to the oxygen insertion into allylic interme-
1
. INTRODUCTION
diates in a synergetic manner with Pt particles (24–27).
These considerations tempted us to endeavor to develop
Re-Sb mixed oxides as a new family of selective oxidation
catalysts.
Rhenium-based catalysts have received much attention
in many industrial processes such as metathesis of alkenes
and reforming of petroleum feedstocks. They also have
Recently, we employed three crystalline Re–Sb mixed
4
.5+
3+
oxides—SbRe2O6 (mixed valent Re , Sb ), SbOReO4 �
2
H2O (Re7 , Sb ), and Sb4Re2O13 (Re , Sb )—to exa-
+ 3+ 7+ 3+
1
Present address: Department of Applied Chemistry, Faculty of Engi-
mine their performance in the selective oxidation of
i-C H and i-C H to methacrolein (MAL) (28–30). They
neering, Utsunomiya University, Yoto, Utsunomiya 321-8585, Japan.
2
To whom all correspondence should be addressed. Fax: 81-3-5800-
4
8
4
10
6
892. E-mail: iwasawa@chem.s.u-tokyo.ac.jp.
displayed promising activities, but a problem encountered
69
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

7
0
LIU ET AL.
under the catalytic oxidation conditions was partial decom- (6 mm in diameter). The typical composition of reaction
position of the catalysts, resulting in the loss of Re oxides. feed was 10% i-C4H8, 15% NH3, and 20% O2, with the
More recently, we have found that the catalytic i-C4H8 balance He (mol% ). The catalysis was conducted typically
ammoxidation to methacrylonitrile (MAN) proceeds with 0.15 g of catalyst diluted with 1 g of quartz sand to
efficiently on SbRe2O6 without any decomposition of the avoid temperature gradients and hot spots in the reactor.
crystalline compound (31). In this paper, we report the Prior to each run, the catalyst was pretreated in a He flow
�
1
details of the selective i-C4H8 ammoxidation catalysis (40 ml min ) at 673 K for 1 h. Then the reaction feed was
of the three crystalline Re–Sb–O compounds and their introduced into the reactor at a gas hourly space velocity
�
1
characterization by means of X-ray diffraction (XRD), (GHSV) of 20,000 h using digital mass-flow controllers.
X-ray photoelectron spectroscopy (XPS), scanning elec- The reactants and products were analyzed using two on-
tron microscopy (SEM), and in situ confocal laser Raman line gas chromatographs equipped with three columns of
microspectroscopy (LRM).
Unibeads C at 423 K for O2, CO, and CO2, Gaskuropack 54
at 423 K for methacrylonitrile and other oxygenates, and
VZ-10 at 348 K for i-C4H8 and other hydrocarbons. The
steady-state kinetic data were collected over the catalysts
after 2 h of time-on-stream.
A blank test at 673 K was conducted in an empty reac-
tor as well, and no i-C4H8 conversion was observed in the
homogenous gas phase of the reactor.
2
. EXPERIMENTAL
2
.1. Preparation of Catalysts
Three crystalline Re–Sb–O compounds—SbOReO4 �
2
H2O, SbRe2O6, and Sb4Re2O13—were synthesized by pro-
cedures similar to those reported previously (13, 28–30, 32,
3
3). Briefly, SbOReO4 � 2H2O was synthesized hydrother-
2
.3. Pulse Reactions
mally as follows: Re2O7 2.8 g (Soekawa Chemicals, pu-
rity >99.99% ) was dissolved in deionized water (2 ml)
Pulse reactions on the SbRe2O6 catalyst were carried out
in a Teflon-lined autoclave. To the resultant perrhenic at 673 K in an apparatus in which pulse- and continuous-
acid solution, 1.7 g of Sb2O3 (Soekawa Chemicals, pu- flow reactions can be alternatively operated at atmospheric
rity >99.99% ) was added under vigorous stirring (Sb/Re pressure, and He was used as carrier gas at a flow rate of
�
1
atomic ratio = 1/1). Afterward, the autoclave was sealed 40 ml min . After the SbRe2O6 catalyst (0.15 g) was pre-
and maintained at 423 K for 24 h and then at room tem- treated at 673 K under He for 1 h, reaction feeds with
perature for 6 days. By drying the resultant compound different compositions were pulsed into the reactor via
under vacuum at 343 K, SbOReO4 � 2H2O was obtained a six-port gas sampling valve. The volume of each pulse
as a white powder. SbRe2O6 was synthesized through a was ca. 1 ml. The products were analyzed using on-line
solid-state reaction between SbOReO4 � 2H2O and metal- GC.
lic Re (Soekawa Chemicals, purity >99.99% ) in a 9/5 mo-
lar ratio at 773 K for 3 days in a sealed silica tube. The
black powder obtained was identified to be SbRe2O6 by
XRD and X-ray fluorescence (XRF). The synthesis of
Sb4Re2O13 was conducted similarly to that of SbRe2O6. A
mixture of SbOReO4 � 2H2O and Sb2O3 in a 2/1 molar ra-
tio was heated at 773 K for 6 days in an evacuated silica
tube to yield gray crystals of Sb4Re2O13. The BET surface
area of the three Re–Sb–O compounds was approximately
2
.4. Characterization of Catalysts
XRD patterns were measured in air on a Rigaku Miniflex
goniometer using CuK� radiation (� = 1.5418 A˚ ) at 30 kV
�
�
and 15 mA. The 2� angles were scanned from 5 to 60 at a
�
� 1
.
rate of 2 min
X-Ray photoelectron spectra were recorded on a Rigaku
XPS 7000 spectrometer using MgK� radiation (1253.6 eV)
operated at 200 W. The binding energies were referred to
the adventitious C 1s peak at 284.6 eV. To minimize expo-
sure of the samples to air, after the pretreatments under
He or NH3/He or after the ammoxidation under a mixture
of i-C4H8/NH3/O2/He, the samples were rapidly cooled to
room temperature under the gas flow, followed by sealing
of the reactor. Then, in a N2-filled glove box, the samples
were pressed into disks and attached to XPS sample hold-
ers with thin double-sided tapes, and transferred to the XPS
chamber within 1 min.
2
� 1
1
m g .
For comparison, an Sb2O3-supported Re2O7 catalyst
(
denoted hereinafter as Re2O7/Sb2O3; Re loading: 10 wt% )
was prepared by an impregnation method using an aque-
ous solution of NH4ReO4 (Soekawa Chemicals, purity
>99.9% ). A coprecipitated SbRe2Ox catalyst (denoted as
copr. SbRe2Ox) was also prepared by a coprecipitation
method using an ethanol solution of ReCl3 and SbCl3, fol-
lowed by washing with water to eliminate the residual Cl
from the catalyst.
Scanning electron micrographs were taken on a Hitachi
S-4500 microscope equipped with a field emission gun,
which was operated at 15 kV and 10 �A. Different places
2
.2. Catalytic Performance
Catalytic ammoxidation reactions were carried out at at- of the samples were measured to obtain common features
mospheric pressure in a Pyrex glass fixed-bed flow reactor of SEM images.

Re–Sb–O CATALYSTS FOR ISOBUTYLENE AMMOXIDATION
TABLE 2
71
LRM images were recorded on a JASCO NRS2100 SA
+
spectrometer equipped with an Ar laser (514.5 nm) and a
Catalytic Performance of SbRe2O6 after Different
�
1
liquid N2-cooled CCD detector. The resolution was 5 cm
,
a
Pretreatments at 673 K
and the laser power was set to 2 mW. Raman shifts for all
�
1
i-C4H8 conversion Selectivity to MANb
the samples were measured in the range 1100–185 cm . To
evaluate the micrometer-scale homogeneity of the surface,
about 30 spectra were measured at different spots of a sam-
ple. The samples were analyzed in situ similarly to those
described previously (28).
(
% )
(% )
He pretreatment
5.2
5.8
6.0
3.1
83.6
85.2
84.5
70.8
NH3/He pretreatment
Reaction feed pretreatment
O2/He pretreatment
3
. RESULTS
a
� 1
.
i-C4H8/NH3/O2/He = 10/15/20/55 (mol% ); GHSV = 20,000 h
b
MAN, methacrylonitrile.
3
.1. Catalytic Performance of Different
Re–Sb–O Catalysts
1
0% i-C4H8 + 15% NH3 + 20% O2 + 55% He. As shown
in Table 2, after the pretreatments under He, NH3/He, and
the reaction feed, the i-C4H8 conversions and MAN selec-
tivities at the steady state were nearly the same, whereas
the conversion and selectivity were lower with the pretreat-
ment with O2/He. Hence, the SbRe2O6 sample subjected to
catalytic performance and characterization in this study was
pretreated in a flow of He at 673 K.
Table 1 lists the conversions, reaction rates, and selectivi-
ties of three crystalline Re–Sb–O compounds—SbRe2O6,
SbOReO4 � 2H2O, and Sb4Re2O13—in i-C4H8 ammoxida-
�
1
tion at 673 K (GHSV: 20,000 h ), in which the performance
of copr. SbRe2Ox, Re2O7/Sb2O3, Sb2O3, Sb2O4, Re2O7,
ReO3, and ReO2, was also listed for comparison. The Re–
Sb–O catalysts as well as bulk Re oxides (Re2O7, ReO3,
and ReO2) were more or less active and selective for the
ammoxidation of i-C4H8 to MAN, whereas bulk Sb oxides
Figure 1a shows the conversion in the i-C4H8 ammoxida-
tion on SbRe2O6 at 673 K as a function of time-on-stream
(
Sb2O3 and Sb2O4) showed no activity under the identi-
cal conditions. Among the samples examined in Table 1,
SbRe2O6 exhibited the best performance. The steady-state
i-C4H8 conversion was 5.2% and the selectivity to MAN
was 83.6% at 673 K.
3
.2. Kinetic Behavior on SbRe2O6
In the selective oxidation of i-C4H8 to MAL, the catalytic
property of SbRe2O6 depended largely on the pretreatment
atmospheres (28–30). To investigate the effect of pretreat-
ment of SbRe2O6 on i-C4H8 ammoxidation to form MAN,
SbRe2O6 was pretreated at 673 K for 1 h under the follow-
ingfour different atmospheres:100% He, 15% NH3 + 85%
He (mol% ), 20% O2 + 80% He, or a reaction mixture of
TABLE 1
Isobutylene Ammoxidation on Different Catalysts at 673 Ka
Activity
Selectivity (% )
Conversion
% )
(�mol
�
1
� 1
h )
MANb CH3CN CO2
(
g-cat
SbRe2O6
SbOReO4 � 2H2O
Sb4Re2O13
copr.SbRe2Ox
Re2O7/Sb2O3
Re2O7
ReO3
ReO2
Sb2O3
5.2
0.9
1.9
1.4
1.2
1.6
1.4
3.1
0
3712.8
642.6
1356.6
999.6
856.8
1142.4
999.6
2213.4
0
83.6
66.8
70.7
69.8
76.8
47.9
51.9
50.6
—
6.1
3.9
4.6
11.1
6.8
13.2
17.1
17.8
—
9.8
29.1
24.2
19.1
16.4
38.9
31.0
31.6
—
Sb2O4
0
0
—
—
—
FIG. 1. Reaction profiles of i-C4H8 conversion (᭺) and selectivity to
MAN (᭝) with time-on-stream on SbRe2O6 at 673 K under GHSV of
a
� 1
.
i-C4H8/NH3/O2/He = 10/15/20/55 (mol% ); GHSV = 20,000 h
b
� 1 � 1
20,000 h (a) and 10,000 h (b).
MAN, methacrylonitrile.

7
2
LIU ET AL.
vantageous property of SbRe2O6. The Arrhenius plots for
the i-C4H8 conversion rate are shown in Fig. 2b, where there
�
1
� 1
is a break: 103 kJ mol at T < 673 K and 35 kJ mol at
T > 673 K.
The dependencies of the reaction rate and selectivity on
the partial pressure of i-C4H8, NH3, and O2 at constant
�
1
GHSV (20,000 h ) are shown in Fig. 3. At low partial
FIG. 2. (a) Catalytic performance of SbRe2O6 in i-C4H8 ammoxida-
tion as a function of reaction temperature. ᭺, i-C4H8 conversion; ᮀ, se-
lectivity to MAN + MAL; ᭝, selectivity to MAN; ᭞, selectivity to MAL;
�
1
,
selectivity to CO2. (b) Arrhenius plot of ln k against T for i-C4H8
ammoxidation.
at GHSV of 20,000 h 1. It was found that the conversion
of i-C4H8 initially increased slightly with time and then
reached a constant value at 5.2% . The selectivity to MAN
remained almost unchanged at 83.6% during the test. When
�
�
1
the GHSV decreased to 10,000 h , the i-C4H8 conversion
increased to 12.4% , while the selectivity to MAN (82.6% )
did not change so much as compared with that at the GHSV
�
1
of 20,000 h as shown in Fig. 1b.
Figure 2a shows the variation of the conversion and
selectivity of the i-C4H8 ammoxidation with reaction tem-
perature in the range 623–773 K. i-C4H8 conversion in-
creased with increasing temperature, while keeping the
MAN selectivity at 83–84% up to 698 K. Above 700 K,
another selective oxidation product, MAL, appeared in
addition to MAN. The selectivity to MAL reached 28%
at 773 K. As a result, the selectivity to MAN decreased
above 723 K, while the total selectivity to the sum of
MAL + MAN increased slightly. By-products acetonitrile
FIG. 3. Catalytic i-C4H8 ammoxidation on SbRe2O6 at 673 K as a
function of (a) partial pressure of i-C4H8 (15% NH3, 20% O2, balance
He), (b) partial pressure of NH3 (10% i-C4H8, 20% O2, balance He), and
(
c) partialpressure ofO2 (10% i-C4H8, 15% NH3, balance He). ᭝, Reaction
and CO2 tended to decrease above 723 K, which is an ad- rate for MAN formation; ᮀ, selectivity to MAN; ᭞, selectivity to MAL.

Re–Sb–O CATALYSTS FOR ISOBUTYLENE AMMOXIDATION
73
pressures of i-C4H8, the rate of MAN formation nearly
increased linearly with increasing pressure. After passing
through a maximum value around 10 kPa, the rate de-
creased with a further increase in i-C4H8 pressure. As the
i-C4H8 pressure was higher than 15 kPa, MAL was formed
and its selectivity increased from 0 to 8.5% with increas-
ing pressure. Inversely, the selectivity to MAN decreased
at the higher i-C4H8 pressures, but the selectivity to the
sum of MAN + MAL was almost independent of the par-
tial pressure of i-C4H8. Similar dependence of the reac-
tion rate on the partial pressure of NH3 was observed in
Fig. 3b where the maximum rate was reached around 22 kPa
of NH3. The selectivity to MAN initially increased and
then remained constant with further increasing NH3 pres-
FIG. 5. MAN yields as a function of the number of pulses of
sure. MAL formation decreased steeply with the increase i-C H /O /He (=10/20/70, mol% ) on NH -preadsorbed SbRe O at 673 K
4
8
2
3
2
6
in NH3 pressure and eventually reached zero at NH3 pres- (curve a) and as a function of the number of pulses of NH3/O2/He
(
=15/20/55) on i-C4H8-preadsorbed SbRe2O6 at 673 K (curve b).
sures higher than 5 kPa. In contrast to these behaviors in
Figs. 3a and 3b, the rate of MAN formation increased with
increasing O2 pressure and attained a saturation value at
higher pressures, while selectivity to MAN was almost in- to the pulse of i-C4H8/NH3/He mixture without O2. The
dependent of O2 pressure in the range 3–50 kPa (Fig. 3c). MAN yield in the first pulse was as low as 0.7% . MAN for-
No MAL was formed under these conditions.
mation diminished with further pulses. After i-C4H8 con-
version became negligible, the i-C4H8/NH3/O2/He mixture
was pulsed into the reactor again, resulting in the recovery
of MAN yield as shown in Fig. 4.
3
.3. Pulse Reactions
Four types of pulse experiments were conducted on
Figure 5a shows the MAN yield with i-C4H8/O2/He mix-
ture pulses on the NH3-pretreated SbRe2O6 catalyst. A
fresh SbRe2O6 catalyst was treated with NH3 pulses to
saturate the surface with ammonia. The NH3-saturated
catalyst was exposed to the i-C4H8/O2/He pulses. It was
found that the first pulse yielded MAN at the high conver-
sion, but the yield decreased sharply by the second and third
pulses, followed by a gradual decrease. Figure 5b shows
the MAN yield in pulsing a mixture of NH3/O2/He on the
i-C4H8-saturated SbRe2O6 catalyst. MAN was produced by
the reaction of the preadsorbed i-C4H8 with NH3/O2, but
the yield was one order of magnitude lower than the steady-
state result.
the SbRe2O6 catalyst: (i) pulses of the i-C4H8/NH3/O2/He
10/15/20/55, mol% ) mixture, (ii) pulses of the i-C4H8/
NH3/He mixture (10/15/75), (iii) pulses of the i-C4H8/O2/He
mixture (10/20/70) on NH3-pretreated SbRe2O6, and
(
(
iv) pulses of the NH3/O2/He mixture (15/20/65) on i-C4H8-
preadsorbed SbRe2O6.
Pulsing the mixture of i-C4H8/NH3/O2/He on a fresh
SbRe2O6 catalyst yielded MAN, and the MAN yield in-
creased with the number of pulses to reach a constant value
of ca. 6.5% as shown in Fig. 4. Then the pulse was switched
3
.4. Characterization of Catalysts
.4.1. XRD. Figure 6 shows XRD patterns of the
3
SbRe2O6 catalyst before and after the pretreatments under
He and NH3/He at 673 K, as well as after i-C4H8 ammox-
idation reactions at 673 and 773 K for 3 h. It was found
that the XRD patterns after these treatments and the i-
C4H8 ammoxidation were identical to that for the fresh
sample. Figure 7 shows XRD patterns of Sb4Re2O13 and
SbOReO4 � 2H2O before and after i-C4H8 ammoxidation at
6
73 K. For Sb4Re2O13, the XRD pattern did not change
essentially after the i-C4H8 ammoxidation at 673 K for
�
3
h, but a new peak appeared at a 2� angle of 27.8 . The
new peak was attributable to Sb2O3. A dramatic change in
crystal structure was observed with SbOReO4 � 2H2O after
the i-C H ammoxidation at 673 K as shown by the XRD
FIG. 4. MAN yields on SbRe2O6 at 673 K as a function of the number
of pulses of i-C4H8/NH3/O2/He (=10/15/20/55, mol% ) (curves a and c) and
i-C4H8/NH3/He (=10/15/75) (curve b).
4
8

7
4
LIU ET AL.
FIG. 6. XRD patterns for SbRe2O6 before (a) and after pretreatments
under He (b) and NH3/He (c) at 673 K for 1 h and i-C4H8 ammoxidation
at 673 K (d) and 773 K (curve e) for 3 h.
patterns in Fig. 7 (a and c). The XRD pattern in Fig. 7c
was similar to that for Sb4Re2O13 after that ammoxidation
FIG. 8. Re 4f XPS spectra for SbRe2O6 before (curve a) and after
pretreatment under He (curve b) and NH3/He (curve c) at 673 K for 1 h
and i-C H ammoxidation at 673 K (curve d) and 773 K (curve e) for 3 h.
(
Fig. 7d).
4
8
3
.4.2. XPS spectra. Figure 8 shows Re 4f XPS spectra
for the SbRe2O6 catalysts before and after the pretreat- ments with He and NH3/He at 673 K, as well as after
the i-C4H8 ammoxidation at 673 and 773 K. In Fig. 8a for
fresh SbRe2O6 three peaks were observed at binding en-
ergies of 42.3, 44.8, and 47.7 eV. The peak at 42.3 eV is
assigned to Re 4f7/2 for Re4 . The peak at 47.7 eV is as-
+
signed to Re 4f5/2 probably for Re6 as discussed hereafter.
+
Thus the most intense peak around 44.8 eV should be su-
perimposed by Re 4f5/2 of Re4 and Re 4f7/2 of Re . We
+
6+
have deconvoluted the XPS spectra as shown in Fig. 8. The
4+
6+
Re 4f5/2 and Re 4f7/2 peaks were separated at 44.7 and
5.3 eV, respectively. The SbRe2O6 sample after pretreat-
4
ment under He at 673 K exhibited no significant difference
from the XPS spectra for the fresh sample. After treatment
with NH3/He at 673 K for 1 h, the intensity of the peaks at
4
7.7 and 45.3 eV decreased, while the intensity of the peaks
at 44.7 and 42.3 eV increased relatively (Fig. 8c). The XPS
spectra for the samples after i-C4H8 ammoxidation at 673
and 773 K for 3 h were similar to those for the fresh and
He-treated samples (Figs. 8d and 8e). The spectra for fresh
SbOReO4 � 2H2O and Sb4Re2O13 were similar, exhibiting
two peaks centered at 45.7 and 48.1 eV. The peaks are at-
tributable to the Re 4f7/2 and Re 4f5/2 levels of Re7 species,
+
respectively. After the ammoxidation reaction at 673 K for
FIG. 7. XRD patterns for fresh SbOReO4 � 2H2O (a) and Sb4Re2O13
b) and for SbOReO4 � 2H2O (c) and Sb4Re2O13 (d) after i-C4H8 ammox-
3
4
h, the spectra changed drastically to show three peaks at
2.3, 44.9, and 47.4 eV. We have deconvoluted the spectra by
(
idation at 673 K for 3 h. The asterisks show the XRD peak for the Sb2O3
phase.
6+
4+
assuming Re and Re species, but the fitting has never

Re–Sb–O CATALYSTS FOR ISOBUTYLENE AMMOXIDATION
75
FIG. 9. Re 4f XPS spectra for fresh SbOReO4 � 2H2O (curve a) and
Sb4Re2O13 (curve b) and for SbOReO4 � 2H2O (curve c) and Sb4Re2O13
curve d) after i-C4H8 ammoxidation at 673 K for 3 h.
(
reproduced the observed spectra. As a consequence, the
fitting has been performed by assuming three different oxi-
dation states of Re. The results are shown in Figs. 9c and 9d.
The deconvoluted peaks at 42.3 and 44.7 eV are assigned to
4+
Re 4f7/2 and Re 4f5/2 peaks for Re , respectively, the peaks
at 45.1 and 47.5 eV are assigned to Re 4f7/2 and Re 4f5/2
FIG. 10. In situ confocal laser Raman microspectra for fresh SbRe2O6
(
curve a) and for SbRe2O6 after pretreatment under He (curve b) and
NH3/He (curve c) at 673 K for 1 h and i-C4H8 ammoxidation at 673 K
curve d) and 773 K (curve e) for 3 h.
FIG. 11. Scanning electron micrographs for fresh SbRe2O6 (a) and
for SbRe2O6 after i-C4H8 ammoxidation at 673 K for 3 h (b). (c) Drawing
of SbRe2O6 (100) plane.
(

7
6
LIU ET AL.
peaks for Re6 , and the peaks at 45.9 and 48.3 eV are as- which reveal neither change nor modification in bulk and
+
7+
signed to Re 4f7/2 and Re 4f5/2 peaks for Re , respectively. surface ofSbRe2O6. Thusthe SbRe2O6 crystalisa promising
It is to be noted that for the three Re–Sb–O compounds Sb catalyst for ammoxidation. The fact that the performance
4
d and Sb 3d3/2 bands appeared around 34.5 and 539.8 eV, of SbRe2O6 depends on the crystal structure and oxidation
3+
respectively, which are the binding energies typical of Sb
independent of the pretreatments and the reaction.
,
state of Re is also confirmed by the decrease in both activ-
ity and selectivity of the SbRe2O6 sample pretreated under
O2/He (Table 2). It has been demonstrated that destruction
of the SbRe2O6 surface takes place by a partial decomposi-
tion of SbRe2O6 to Sb4Re2O13 and Re2O7 under O2/He at
3
.4.3. LRM spectra. The SbRe2O6 samples before and
after the pretreatments under He and NH3/He at 673 K,
as well as after the i-C4H8 ammoxidation at 673 and
6
73 K (28–30).
7
73 K, were also investigated by in situ LRM. As shown in
7
+
The crystalline compounds SbOReO4 � 2H2O (Re
,
Fig. 10. The SbRe2O6 samples possessed Raman features
at 226 (most intense peak), 238s, 278w, 306w, 346w, 388m,
4
range 1100–185 cm . After the pretreatments under He
and NH3/He, and the i-C4H8 ammoxidation, the Raman
shifts essentially remained unchanged.
3+
7+
3+
Sb ) and Sb4Re2O13 (Re , Sb ) were built up from
�
+
�
1
tetrahedral (ReO4) anions and cationic (SbO) layers (13,
32). In accordance with their formulas, the XPS spectra in
Figs. 9a and 9b show the oxidation states of Re and Sb to
be 7+ and 3+, respectively. After i-C4H8 ammoxidation
at 673 K, the XPS spectra for SbOReO4 � 2H2O and
Sb4Re2O13 became complicated ones that were repro-
11m, 424m, 456w, 524m, 643w, and 756m cm in the
�
1
3.4.4. SEM images. Figures 11a and 11b are the scan-
7+
ning electron micrographs for SbRe2O6 before and after
i-C4H8 ammoxidation at 673 K. Fresh SbRe2O6 was com-
posed of crystals possessing square basal faces with dimen-
sions of 0.5–3 �m and about 100 nm thick. The basal (100)
faces were smooth and had sharp and regular edges. After
the i-C4H8 ammoxidation, the SbRe2O6 crystals exhibited
morphology similar to that for the fresh sample.
duced only by the sum of the 4f7/2 and 4f5/2 peaks for Re
,
6+
4+
Re , and Re species (Figs. 9c and 9d). In addition,
Sb2O3 was observed by XRD in Fig. 7. It is evident that
reduction of the two Re–Sb–O compounds occurred
under the ammoxidation conditions. Although the fresh
SbOReO4 � 2H2O and Sb4Re2O13 possess Re species in the
6+
7
+ oxidation state, they show surface Re species (Re and
4+
Re ) similar to those for SbRe2O6 after ammoxidation at
73 K. Nevertheless, the SbRe2O6 catalyst was superior to
6
4
. DISCUSSION
the SbOReO4 � 2H2O and Sb4Re2O13 catalysts. There may
It was found that the Re-based oxides such as the three be two possible explanations for the difference in their
crystalline Re–Sb–O compounds SbRe2O6, SbOReO4 � performance. The Sb2O3 produced by the decomposition
2
H2O, and Sb4Re2O13, copr.SbRe2Ox, Re2O7/Sb2O3, and of SbOReO4 � 2H2O and Sb4Re2O13 catalysts may cover the
bulk Re oxides, catalyzed the selective ammoxidation of catalyst surfaces, resulting in a decrease in performance.
isobutylene to MAN at 673 K, whereas the bulk Sb oxides This can explain the low activities of the two catalysts
were completely inactive under identical reaction condi- (Table 1). However, the low selectivities compared with
tions (Table 1). These results indicate that rhenium in the that for SbRe2O6 cannot be explained by the presence of
Re–Sb–O samples is a prerequisite element to the ammoxi- Sb2O3 at the surfaces because Sb2O3 never contributes to
dation catalysis. On the other hand, by supporting Re2O7 on the catalytic process and hence does not change the selec-
the inert Sb2O3 (Re2O7/Sb2O3), MAN selectivity increased tivity much (Table 1). It is more plausible that the difference
from 47.9 to 76.8% , and the copr. SbRe2Ox catalyst gave a in catalytic performance of the three crystalline Re–Sb–O
selectivity (69.8% ) that was also higher than those for bulk catalystsmaybe due not to the difference in their surface Re
Re oxides (Re2O7, ReO3, and ReO2) (47.9–51.9% ). The oxidation states, but to the difference in their surface struc-
results indicate that the presence of Sb oxide in the samples tures. This is entirely different from the finding in the se-
contributes positively to an increase in the performance of lective oxidation of i-C4H10 and i-C4H8 to MAL that the
MAN synthesis. Furthermore, among the Re–Sb–O sam- activities of the three crystalline Re–Sb–O compounds are
ples, SbRe2O6 was much superior to other samples in both ascribed to cooperation between Re2O7 and Sb4Re2O13,
activity and selectivity. The difference in catalytic perfor- both being formed under oxidizing conditions (28–30).
mance may provide information on a key structural issue
for i-C4H8 ammoxidation catalysis of Re–Sb–O samples.
In relation to the importance of the structural property
of SbRe2O6 in i-C4H8 ammoxidation, it is noteworthy that
4.5+
3+
The most active compound SbRe2O6 (Re , Sb ) con- while the conversion of i-C4H8 on the SbRe2O6 catalyst in-
sists of alternate octahedral (Re2O6)3 and (SbO) layers, creased with increasing temperature, formation of the un-
which are connected to each other along the (100) plane as desired degradation product acetonitrile and combustion
shown in Fig. 11c (33). This crystal structure is maintained product CO2 decreased above 723 K (Fig. 2a). Figure 1 also
under the catalytic ammoxidation conditions as evidenced shows that while i-C4H8 conversion increased by decreasing
by XRD, XPS, in situ LRM, and SEM (Figs. 6, 8, 10, and 11), GHSV, MAN selectivity did not change significantly. It was
�
+

Re–Sb–O CATALYSTS FOR ISOBUTYLENE AMMOXIDATION
77
found that steady-state catalytic performance remained al- indicating that the reduction and oxidation processes of Re
most unchanged without any deactivation over at least 10 h species at the catalyst surface are reversible. The amounts
on stream. The catalytic performance was reproduced in of lattice oxygen atoms consumed in the reactions with NH3
the repeated experimental runs. These results demonstrate and with i-C4H8 were estimated to be approximately 2.3 �
17
the potential advantage and characteristic property of the 10 , which corresponds to about 15% of the surface lattice
18
crystalline SbRe2O6, which provides a promising catalyst oxygen atoms (ca. 1.5 � 10 ) of the catalyst (0.15 g), as-
1
9
for the ammoxidation reaction.
Arrhenius plots (Fig. 2b) showed a break in the line and
suming the surface oxygen density to be roughly 10 atoms
m
�
2
(37). As the replenishment of the surface oxygen atoms
hence a change in the activation energy Ea between 623- by bulk oxygen atoms at 673 K is slow, the consumed sur-
�
1
� 1
and 773 K: 103 kJ mol at T < 673 K and 35 kJ mol
face oxygen atoms are replenished rapidly by gas-phase
at T > 673 K. This reflects a change possibly in the rate- oxygen. The gas-phase oxygen does not promote i-C4H8
determining step in i-C4H8 ammoxidation on SbRe2O6. At combustion to CO2 as shown in Fig. 3c. The active oxy-
6+
T < 673 K, the reaction rate may be limited by activation gen atoms may be bound to the Re species that are con-
of a C–H bond in i-C4H8 adsorbed at the surface, while nected to the Sb3 –O–Sb chains at the SbRe2O6 surface.
the adsorption of i-C4H8, NH3, or O2 may determine the A portion of the surface oxygen atoms are replaced by NH
reaction rate above 673 K. At temperatures higher than species under the ammoxidation conditions as suggested by
+
3+
7
00 K, MAL appeared and the selectivity for its formation FT-IR (38).
increased along with the parallel decrease in selectivity to The experiments with the alternating pulses of NH3 and
MAN, while keeping the selectivity to the sum of MAL + i-C4H8 on SbRe2O6 in Fig. 5 show that MAN is produced
MAN almost constant. This may be interpreted as a result mainly by the i-C4H8 pulse after the NH3 pulse rather than
of the NH3 coverage decrease at higher temperatures, thus by the NH3 pulse after the i-C4H8 pulse. It is supposed
increasing the probability of oxygen incorporation into the that the adsorption of NH3 to form Re–NH species on
methallyl intermediate to yield MAL. This is in agreement SbRe2O6 facilitates the adsorption and activation of i-C4H8.
with the result shown in Fig. 3b where MAL was produced The proposition also accounts for the observation that the
at low NH3 pressures. MAL was also produced at high activity of SbRe2O6 for i-C4H8 ammoxidation to MAN in
i-C4H8 pressures as shown in Fig. 3a, which indicates that the presence of NH3 is higher than that for the i-C4H8 selec-
NH3 adsorption is inhibited by i-C4H8 at higher pressures. tive oxidation to MAL in the absence of NH3 under similar
Conversely, when NH3 pressure increased above 25 kPa, reaction conditions. This is in agreement with the results
the ammoxidation rate decreased as shown in Fig. 3b. over Bi–Mo oxide catalysts of Wragg et al. (39). The pro-
Thus it is probable that adsorption of i-C4H8 and NH3 moting effect of NH3 is much more significant in isobutane
takes place competitively at the same sites on the catalyst ammoxidation on SbRe2O6, where no C–H bond breaking
surface.
It is generally accepted that selective oxidation and am-
moxidation of hydrocarbons on mixed metal oxides occur
following a Mars–van Krevelen type of redox mechanism
in which lattice oxygen atoms react with the hydrogen of
of isobutane occurs in the absence of NH3 (31, 38, 40).
. CONCLUSIONS
1. The three crystalline Re–Sb–O compounds SbRe O ,
5
2
6
hydrocarbons to form water and the consumed lattice oxy- SbOReO � 2H O, and Sb Re O catalyzed the selective
4
2
4
2
13
gen atoms are replenished by gas-phase oxygen (23, 34–36). ammoxidation of i-C H to MAN at 673 K. SbRe O was
4
8
2
6
The important contribution of lattice oxygen to the am- superior to the other two compounds in catalytic perfor-
moxidation of i-C4H8 to MAN on SbRe2O6 can be seen in mance.
Fig. 4. Because the lattice oxygen atoms are consumed
first by reaction with NH3, which is indeed evidenced by catalysts was much worse than that of the SbRe O catalyst.
2 x 2 7 2 3
2. The performance of copr. SbRe O and Re O /Sb O
2
6
the formation of N2 and H2O on pulsing NH3 or the
3. Re was prerequisite to the ammoxidation catalysis
i-C4H8/NH3/He mixture on SbRe2O6, the MAN yield for of Re–Sb–O catalysts, while the presence of Sb also con-
the first pulse of the i-C4H8/NH3/He mixture on the tributed positively to the MAN synthesis.
steady-state SbRe2O6 catalyst was as low as 10% of the
4. No structural change in the bulk and surface of
steady-state value (Fig. 4). However, when O2 was intro- SbRe O was observed after i-C H ammoxidation. The
2
6
4
8
duced into the reactor by pulsing the reaction mixture of good performance of SbRe O may be relevant to its spe-
2
6
3�
i-C4H8/NH3/O2/He, consumed lattice oxygen atoms were cific crystal structure with alternate octahedral (Re O )
rapidly replenished and steady-state activity was recovered and (SbO) layers which connect to each other through
Fig. 4). The XPS spectra in Fig. 8 also demonstrate partial Re–O–Sb bonds.
reduction of Re species to Re species when SbRe2O6 is
5. The i-C H ammoxidation on SbRe O took place by
4 8 2 6
pretreated with NH3/He at 673 K and recovery of the orig- a redox mechanism involving the surface oxygen atoms on
2
6
+
(
6+
4+
6+
inal species under the ammoxidation reaction conditions, Re
.

7
8
LIU ET AL.
6
. The adsorption of NH3 on SbRe2O6 facilitated the ad- 14. Wachs, I. E., Deo, G., Andreini, A., Vuurman, M. A., and de Boer, M.,
J. Catal. 160, 322 (1996).
sorption and activation (C–H bond breaking) of i-C4H8.
. Increasing reaction temperature and decreasing
1
1
5. Grasselli, R. K., Catal. Today 49, 141 (1999).
6. Albonetti, S., Cavani, F., and Trifiro, F., Catal. Rev.-Sci.-Eng. 38, 413
7
GHSV increased i-C4H8 conversion to form MAN, but did
not give rise to the increase in the formation of by-products
(
1996).
17. Centi, G., and Trifiro, F., Catal. Rev.-Sci. Eng. 28, 165 (1986).
CO2 and acetonitrile, which marks the crystalline SbRe2O6 18. Nilsson, J., Land-Canovas, A. R., Hansen, S., and Andersson, A.,
J. Catal. 186, 442 (1999).
9. Nilsson, R. Lindblad, T., and Andersson, A., J. Catal. 148, 501 (1994).
0. Berry, F. J., Adv. Catal. 30, 97 (1981).
as a promising catalyst for hydrocarbon ammoxidation.
1
2
2
2
ACKNOWLEDGMENTS
1. Moro-oka, Y., and Ueda, Adv. Catal. 40, 233 (1994)
2. Pillep, B., Behrens, P., Schubert, U.-A., Spengler, J., and Kn o¨ zinger,
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This work was supported by Core Research for Evolutional Science
and Technology (CREST) of the Japan Science and Technology Corpo-
ration (J.S.T.). The SEM measurements were conducted in the Electron
Microbeam Analysis Facility of the Mineralogical Institute, University of
Tokyo.
2
2
2
2
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