NH3-efficient ammoxidation of toluene by hydrothermally synthesized layered tungsten-vanadium complex metal oxides

Article abstract of DOI:10.1016/j.jcat.2016.10.013

Hydrothermally synthesized W[sbnd]V[sbnd]O layered metal oxides (W[sbnd]V[sbnd]O) are studied for the vapor phase ammoxidation of toluene to benzonitrile (PhCN). Under similar conversion levels at 400?°C, W[sbnd]V[sbnd]O shows higher selectivity (based on

Full text of DOI:10.1016/j.jcat.2016.10.013

Journal of Catalysis 344 (2016) 346–353
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
NH3-efficient ammoxidation of toluene by hydrothermally synthesized
layered tungsten-vanadium complex metal oxides
a
Yoshinori Goto a, Ken-ichi Shimizu a,b, , Kenichi Kon , Takashi Toyao a,b, Toru Murayama a,c
,
⇑
Wataru Ueda d,
⇑
a Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
b Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615-8520, Japan
c Research Center for Gold Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan
d Kanagawa University, Rokkakubashi 3-27-1, Yokohama 221-8686, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrothermally synthesized WAVAO layered metal oxides (WAVAO) are studied for the vapor phase
ammoxidation of toluene to benzonitrile (PhCN). Under similar conversion levels at 400 °C, WAVAO
shows higher selectivity (based on toluene) to PhCN and lower selectivity to COx than conventional V-
based catalysts (V2O5 and VOx/TiO2). Under the conditions of high contact time, WAVAO shows 99.7%
conversion of toluene and 93.5% selectivity to PhCN. Another important feature of WAVAO is high
NH3-utilization efficiency in ammoxidation, which originates from the lower activity of WAVAO for
NH3 oxidation than that of V2O5. In situ infrared (IR) study shows that toluene is oxidized by the surface
oxygen species of W83V17 to yield benzaldehyde which undergoes the reaction with adsorbed NH3 to
give benzonitrile. Model reaction studies with WAVAO suggest that the rate of NH3 conversion to
PhCN in the benzaldehyde + NH3 + O2 reaction is 3 times higher than the rate of NH3 oxidation to N2
in the NH3 + O2 reaction. It is shown that the high NH3-efficiency of WAVAO is caused by the preferential
reaction of NH3 in PhCHO + NH3 + O2 over NH3 + O2 reaction.
Received 30 May 2016
Revised 26 August 2016
Accepted 16 October 2016
Keywords:
Ammoxidation
Complex metal oxides
Hydrothermal synthesis
Vanadium
Tungsten
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
oxidation of ammonia (NH3) to N2. In order to establish a sustainable
ammoxidation process, NH3 consumption during the reaction
Ammoxidation of methyl-substituted aromatics such as toluene,
xylene and methylpyridine to their corresponding nitriles is an
industrially important reaction, and numerous reports have been
dealt with this reaction [1–4]. The vapor phase ammoxidation of
toluene to benzonitrile has been extensively studied as a model
reaction [5–16]. Various metal oxide-based catalysts, including
vanadium (V)-based catalysts (V2O5 [5], VOx/TiO2 [6,7], VOx/ZrO2
[8], V2O5/Nb2O5ATiO2 [9], (VO)2P2O7 [10–13]) and other catalysts
(MoOx/Nb2O5 [13], MoOx/ZrO2 [15], Fe-based mixed oxides [16])
have been reported to show moderate to good yields (44–77%) of
benzonitrile (based on toluene) at around 400 °C [5–16]. The cat-
alytic efficiency in the ammoxidation of toluene has been discussed
in terms of the selectivity or yield of benzonitrile based on toluene.
In addition to the non-selective oxidation of toluene to COx, another
undesired side-reaction in the ammoxidation is non-selective
should also be minimized. However, quite a few reports have dis-
cussed the benzonitrile selectivity based on the NH3 consumed, or
in other words, the efficiency of NH3 utilization in ammoxidation
of benzonitrile. Mechanistic studies on vanadium oxides-catalyzed
ammoxidation of toluene [6,7,11–13] suggested bifunctional catal-
ysis as a catalyst design concept. It is proposed that the VOx sites cat-
alyze partial oxidation of toluene to benzaldehyde-like intermediate
and acid sites act as adsorption site of NH3 [3,11–13]. If one designed
a V-based mixed oxide catalyst having NH3 adsorption sites (such as
acidic WOx sites) in close proximity to the redox sites (VOx), the
aldehyde intermediate formed on the redox site would have react
preferentially with NH3 on the acid site, resulting in high efficiency
of NH3 utilization in the ammoxidation reaction.
Our research group has focused on the hydrothermal synthesis
of single crystalline MoAVAO based catalysts [17–19]. Particularly,
single phasic orthorhombic Mo3VOx, having a microporous and
layered structure, is of importance because its structure is basically
the same as that of so-called ‘‘M1 phase” which is well known as
active phase in the industrial selective oxidation catalysts [18].
We have found that the MoAVAO catalysts, as a structurally well
⇑
Corresponding authors at: Institute for Catalysis, Hokkaido University, N-21,
W-10, Sapporo 001-0021, Japan (K.-i. Shimizu).
E-mail addresses: kshimizu@cat.hokudai.ac.jp (K.-i. Shimizu), uedaw@kanaga-
wa-u.ac.jp (W. Ueda).
http://dx.doi.org/10.1016/j.jcat.2016.10.013
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

Y. Goto et al. / Journal of Catalysis 344 (2016) 346–353
347
defined catalytic phase for ‘‘M1 phase”, catalyzed the selective
oxidative dehydrogenation of ethane at low temperature (300 °C)
[18]. The single phase MoAVAP catalyst with similar structure also
catalyzed the ammoxidation of propane [19]. Recently, we have
extended the hydrothermal synthetic methodology to metal oxides
consisted of various groups 5 and 6 elements [20–23], and pre-
pared a series of binary metal oxides (such as WATaAO [20,23],
WANbAO [21] and WAVAO complex oxides [22]) with similar
microporous and layered structure as the orthorhombic Mo3VOx
[18]. Considering the fact that WOx is a well known acidic co-
catalyst of V-based catalysts [24–27], we have hypothesized that
the WAVAO oxides act as effective catalysts for the ammoxidation.
In this regard, we have recently reported the WAVAO-catalyzed
highly selective ammoxidation of 3-picoline to 3-cyanopyridine
[22]. We report herein a highly selective gas-phase ammoxidation
of toluene by the hydrothermally prepared WAVAO layered oxi-
des. The catalysts show high efficiency of NH3 utilization in
ammoxidation as well as high selectivity of benzonitrile based on
toluene. In situ IR study shows that the reaction of benzaldehyde
with NH3 is a main pathway to benzonitrile on this catalytic sys-
tem. Model reaction studies are also conducted to discuss the rea-
son why the WAVAO catalyst shows high efficiency of NH3
utilization.
overnight and heating at 400 °C for 2 h under N2 flow. The Na con-
tent in Na-W83V17 (0.74 mmol gꢂ1) was determined by ICP-AES
analysis. WO3-supported vanadia (VOx/WO3) with W/V molar ratio
of 85/15 was prepared by impregnation method; a suspension of
WO3 (20.0 mmol) in an aqueous solution (50 mL) of NH4VO3
(3.5 mmol) was heated to 90 °C for 30 min to evaporate water, fol-
lowed by drying at 80 °C overnight, and by heating at 400 °C for 2 h
under N2 flow. The bulk compositions and surface area of the
WAVAO and VOx/WO3 catalysts are listed in Table 1. V2O5 and
WO3 for catalytic studies were commercially supplied from Wako
Pure Chemical Industries. Vanadia loaded TiO2 with V loading of
5.9 wt%, named VOx/TiO2, was prepared by impregnation method
[22]; a suspension of anatase TiO2 (4 g, Wako Pure Chemical Indus-
tries) in aqueous oxalic acid solution (50 mL) of NH4VO3
(4.9 mmol) was evaporated at 50 °C, followed by drying at
100 °C, and by heating at 450 °C for 6 h under air.
2.2. Catalyst characterization
The BET surface areas of the catalysts were determined by N2
adsorption at ꢂ196 °C using BELCAT (MicrotracBEL). Prior to the
measurement, the samples were heated under He flow at 400 °C
for 1 h. Powder X-ray diffraction (XRD) pattern of the catalysts
were recorded on a Rigaku MiniFlex II/AP diffractometer with Cu
Ka radiation. Scanning electron microscope (SEM) images were
taken using a JEOL JSM-7400F field emission scanning electron
2. Experimental
microscope.
The pyridine-adsorption infrared (IR) spectra were measured at
100 °C by a JASCO FT/IR-4200 spectrometer equipped with an MCT
detector. The IR disk (38 1 mg, / = 2 cm) of a catalyst in the flow-
type IR cell. (CaF2 windows), connected to a flow system, was first
dehydrated under He flow (30 cm3 minꢂ1) at 500 °C for 0.5 h, fol-
lowed by cooling to the measurement temperature under He. For
the adsorption of organic compounds, 0.3 mmol gꢂca1t of liquid pyri-
dine, toluene, benzaldehyde or benzonitrile was injected to the He
flow preheated at 200 °C, and the vaporized organic compound
was fed to the IR cell. For the adsorption of NH3, NH3(2%)/He
(50 cm3 minꢂ1) was fed to the sample. Then, the IR disk was purged
with He for 600 s, and IR measurement was carried out. Spectra
2.1. Catalyst preparation
Inorganic materials were purchased from Wako Pure Chemical
Industries. According to our previous report [22], the complex
metal oxide of W and V (WAVAO) with W/V molar ratio of
83/17, named W83V17, was prepared by a hydrothermal synthesis
method as follows. An aqueous solution (40 mL) of (NH4)6[H2W12
-
O
40]ꢀnH2O (10.4 mmol), VOSO4ꢀnH2O (4.16 mmol) and oxalic acid
(0.10 mmol) was introduced into a stainless steel autoclave with
a Teflon inner tube (50 mL), followed by filling the inner space of
the tube by Teflon thin sheet (50 mm ꢁ 1000 mm). Then, N2
(20 mL minꢂ1) was fed into the solution for 10 min to remove
residual oxygen. The autoclave attached to a rotating machine
was installed in an oven, and the mixture underwent hydrothermal
reaction at 175 °C for 24 h under mechanical rotation (1 rpm). The
solid formed was filtered, washed with ion-exchanged water (1 L),
dried at 80 °C overnight and then heated at 400 °C for 2 h under N2
flow. W64V36 with W/V molar ratio of 64/36 was prepared accord-
ing to the method in our previous study [22]. Bulk composition of
the catalysts (Table 1) was determined by an inductively coupled
plasma (ICP-AES) method (ICPE-9000, Shimadzu). Na+-exchanged
W83V17 (designated as Na-W83V17) was prepared by mixing
WAVAO (2.0 g) with 100 mL of aqueous solution of Na2SO4
(19.1 mmol) for 5 h at room temperature, followed by centrifuging
and washing with ion-exchanged water four times, drying at 80 °C
were measured accumulating 15 scans at a resolution of 4 cmꢂ1
.
A reference spectrum of the catalyst wafer in He taken at the mea-
surement temperature was subtracted from each spectrum.
2.3. Catalytic reactions
Vapor phase ammoxidation of toluene was carried out at atmo-
spheric pressure using a fixed-bed flow reactor (Pyrex glass tube)
with an inner diameter of 9 mm. Catalyst powders were pressed
to pellets, crushed, and sieved. To maintain a constant reaction
temperature, the catalyst pellets (0.25–0.50 mm size; typically
1.5 g (1.1 cm3) of W83V17) were diluted with quartz
(0.2–0.4 mm size; 1.5 cm3) to fill the reactor (2.6 cm3). Use of the
same volume (2.6 cm3) of the pellets with the same size range will
result in the same pressure drop for different catalyst loadings. The
reaction temperature was measured inside the catalyst bed by a
thermocouple, whose tip was inside the upper side of the catalyst
bed. The gas stream (NH3/O2/He) was fed to the reactor with mass
flow controllers. Toluene was fed continuously into the gas stream
at 150 °C from a syringe pump with a micro-feeder. The reactor
was fed with toluene/NH3/O2/He mixture in the molar ratio of
1/10/4/34 with total flow rate (F) of 49 mL minꢂ1. Under the low
NH3 concentration conditions in Fig. 7, the molar ratio of
toluene/NH3/O2/He was 1/5/4/39 (F = 49 mL minꢂ1). CH4 was fed
into outlet gas as external standard for GC-TCD analysis.
Table 1
Surface area, composition and reaction rates of the catalysts.
Catalyst
Sa (m2 gꢂ1
)
W/Vb Vc (mmol hꢂ1 gꢂ1
)
Vd (mmol hꢂ1 mꢂ2
)
W64V36
W83V17
27.7
41.9
64/36 1.51
83/17 1.40
83/17 1.29
85/15 1.12
0.055
0.031
0.005
0.041
0.631
0.003
Na-W83V17 38.5
VOx/WO3
V2O5
14.2
4.8
–
–
3.03
0.09
WO3
16.8
a
b
c
BET surface area determined by N2 adsorption.
Composition determined by ICP-AES.
Rate of BN formation per weight of the catalysts.
As a model reaction (Fig. 9), benzaldehyde + NH3 + O2 reaction
by 0.1 g of the catalyst, diluted with the quartz pellets (2.5 cm3),
d
Rate of BN formation per surface area of the catalysts.

348
Y. Goto et al. / Journal of Catalysis 344 (2016) 346–353
was carried out with the same reactor which was fed with ben-
zaldehyde/NH3/O2/He mixture in the molar ratio of 1/10/4/34
(F = 49 mL minꢂ1). A model reaction (Fig. 9) of NH3 + O2 by 0.1 g
of the catalyst was carried out using NH3/O2/He mixture in the
molar ratio of 10/4/35 (F = 49 mL minꢂ1).
The gas phase products (CO, CO2 and N2) in the outlet gas were
analyzed by GC-TCD (GL Sciences GC-3200, 6 m SHINCARBON-ST
packed column). Organic products, trapped in ethanol at 0 °C, fol-
lowed by adding n-octane as an external standard, were analyzed
with GC-FID (Shimadzu GC-14B with TC-5 capillary column). The
carbon balance values for all the catalytic results were in a range
of 95.9–101.6%.
corresponds to the difference in the amount of Brønsted and Lewis
acid sites. The spectrum for Na-W83V17 shows lower intensities of
these bands, which indicates that the H+/Na+ cation exchange
results in decrease in the number of Brønsted and Lewis acid sites
of W83V17. It should be noted that the results of XRD show that
the crystal structures of Na-W83V17 and W83V17 are similar to
each other, and the surface area (Table 1) of Na-W83V17
(38.5 m2 gꢂ1) is close to that of W83V17 (41.9 m2 gꢂ1). These
results indicate that the structure of W83V17 is essentially very
close to that of Na-W83V17 except for acidity; W83V17 has larger
amount of Brønsted and Lewis acid sites than Na-W83V17. The
result also shows that W83V17 has larger amount of Brønsted
and Lewis acid sites than V2O5.
For the ammoxidation of toluene in the present system, the
nitrogen containing products were N2 and benzonitrile (PhCN)
with very small amount of benzamide (amide). GC-TCD analysis
showed no formation of N2O. Thus, the efficiency of NH3 utilization
3.2. Catalytic performance
in ammoxidation (
g
NH3) is defined as
Fig. 4 shows the effects of contact time on the properties of
ammoxidation by W83V17 at 400 °C. Experiments were carried
out by changing the catalyst weight (0.15, 0.75, 1.9, 1.5, 2.0 g) with
the same inlet gas flow rate. The conversion of toluene increased
with the contact time. The increase in the conversion resulted in
slight decrease in the selectivity to benzonitrile (SPhCN) and slight
increase in the COx selectivity (SCOx). The selectivities to benza-
mide (Samide) were below 0.9%. Hence, NH3 can be consumed by
two of the competitive reactions: (1) ammoxidation with toluene
to produce benzonitrile and (2) NH3 oxidation to N2. Interestingly,
the increase in the contact time resulted in the increase in the NH3-
gNH3 ¼ yPhCN PhCN=ðyPhCNnPhCN þ yamidenamide þ yN2NnN2Þ;
n
where yPhCN, yamide and yN2 are the molar amounts of nitrogen con-
taining products, and nPhCN, namide and nN2 are the number of nitro-
gen atoms in each product.
3. Result and discussion
3.1. Catalyst characterization
efficiency (gNH3). This indicates a preferential promotion of the
In our recent report [22], we synthesized a WAVAO catalyst
with W/V ratio of 67/33 and studied its structure by various char-
acterization methods: XRD, scanning transmission electron micro-
scopy (STEM) and N2-adsorption isotherm. Briefly, the results
showed three structural features: (i) layered-type structure along
c-axis direction characterized by diffraction peaks at 2h = 23° and
46° (XRD), (ii) long rod-shaped crystal morphology due to stacking
of the layers along the c-axis by sharing the apex oxygen (STEM),
and (iii) the presence of micropore (N2-adsorption). Considering
the structural model that we have proposed for the similar binary
metal oxides consisted of groups 5 and 6 elements [17–23], the
structural model of WAVAO is proposed in Fig. 1. In this paper,
WAVAO catalysts with different compositions (W64V36 and
W83V17) were prepared. As shown in Fig. 1, their XRD patterns
have essentially the same feature: two sharp diffraction peaks
around 23° and 46° assignable to the (001) and (002) planes of
the layered structure along c-axis direction. The XRD pattern of
Na-W83V17 also has the same diffraction peaks at 23° and 46°.
Although the intensities of Na-W83V17 are lower than those of
W83V17, the XRD results indicate that the Na+-exchanged
W83V17 (Na-W83V17) has basically the same crystal structure
as W83V17. The SEM image of W83V17 (Fig. 2) showed the rod-
shaped crystal probably due to stacking of the layers along the c-
axis. Before calcination of the hydrothermally prepared binary
metal oxides NH+4 is located in the 6 and 7-membered ring pores,
and thermal desorption of NH3 from the precursor results in the
formation of Brønsted acid sites in the pores [18]. The proton is
exchangeable to various cations in aqueous solution [18].
ammoxidation by W83V17 over the non-selective NH3 oxidation.
We tested various V-based catalysts (1.5 g of W83V17,
W64V36, Na-W83V17, VOx/WO3 and WO3 and 0.5 g of V2O5) for
the ammoxidation of toluene. To compare the selectivities of
V2O5 and WAV complex oxide catalysts under the similar conver-
sion levels, the catalyst amount of V2O5 was decreased. For repre-
sentative catalysts, the effects of temperature on the toluene
conversion, selectivity to various products and NH3-efficiency are
plotted in Fig. 5. Under the similar conversion levels, W83V17
showed higher selectivity to benzonitrile (SPhCN) than the conven-
tional V-based catalysts (V2O5 and VOx/TiO2). V2O5 and VOx/TiO2
showed higher selectivity to COx than W83V17.
Table 2 compares the conversions of toluene and selectivities to
benzonitrile (SPhCN), benzamide (Samide), benzoic acid (Sacid), ben-
zene (SBz) and COx (SCOx) at 400 °C. A conventional catalyst, V2O5,
showed 71.2% selectivity to benzonitrile at a conversion level of
87.2%. WO3 showed only 9.2% conversion of toluene. This indicates
that vanadium is an indispensable element for this catalytic sys-
tem. W83V17, W64V36 and VOx/WO3 show higher selectivities
to benzonitrile than V2O5. This indicates that tungsten oxides as
co-catalysts increase the selectivity to benzonitrile. The hydrother-
mally prepared catalysts (W83V17 and W64V36) showed higher
selectivity to benzonitrile than VOx/WO3. Na-W83V17 showed
low conversion (12.6%) than W83V17 (85.1%). Combined with
the IR results in Fig. 3, we can conclude that Brønsted and/or Lewis
acid sites of W83V17 significantly improve the ammoxidation
activity in the present system.
Another remarkable feature of the hydrothermally prepared
WAVAO catalysts (W83V17, W64V36) is highly efficient utiliza-
tion of NH3 in ammoxidation. The results in Fig. 5 and Table 2
Fig. 3 shows IR spectra (the ring-stretching region) of pyridine
adsorbed on W83V17 and Na-W83V17. The spectrum for
W83V17 shows the adsorption band at 1536 cmꢂ1 due to pyri-
dinium ion (PyH+) produced by the reaction of pyridine with
Brønsted acid sites and the adsorption band at 1447 cmꢂ1 due to
coordinatively bound pyridine on Lewis acid sites [28]. The results
indicate that the surface of W83V17 has both Brønsted and Lewis
acid sites. Note that the weight of the catalyst disk for the IR exper-
iment was the same for different catalysts, so the differences in the
peak area at 1536 cmꢂ1 and at 1447 cmꢂ1 between samples
demonstrated that W83V17 showed high NH3-efficiency (gNH3
than other V-based catalysts. As shown in Table 2, the order of
the NH3-efficiency ( NH3) of V-based catalysts is as follows:
)
g
W83V17 (56.9%) ꢃ W64V36 (44.8%) > VOx/TiO2 (42.2%) > VOx/
WO3 (36.4%) > V2O5 (34.9%). The result indicates that the WAVAO
catalysts (W83V17, W64V36) preferentially promote the ammoxi-
dation over the non-selective NH3 oxidation, while the classical
and conventional V-based catalysts show moderate activity for

Y. Goto et al. / Journal of Catalysis 344 (2016) 346–353
349
c
Na-W83V17
W83V17
W64V36
10
20
30
40
50
60
2 / deg. (Cu-K
Fig. 1. XRD patterns (left) and polyhedral model (right) of microporous and layered WAVAO oxides. The top layer of the model is highlighted in yellow.
Fig. 2. SEM image of W83V17.
Fig. 4. Ammoxidation of toluene by W83V17 at different contact times (400 °C):
conversion of toluene (+), selectivities of benzonitrile (s), benzamide (}) and COx
(d) and NH3-efficiency, gNH3, (h): toluene/NH3/O2/He = 1/10/4/34.
Most of the previous reports on ammoxidation of toluene
adopted excess NH3 feed conditions. The reaction with lower level
of NH3 feed will lead to more sustainable and economical produc-
tion of benzonitrile. Fig. 6 shows the results for the W83V17-
catalyzed ammoxidation with different amounts of NH3 (2, 5, 10
and 15 equiv. with respect to toluene). The selectivity to benzoni-
trile decreased with decreasing NH3 concentration, but NH3-
efficiency (gNH3) increased with decreasing NH3 concentration. At
the NH3 molar ratio of 2 equiv. (NH3/toluene = 2/1), the NH3-
efficiency was 73.9% and the yield of benzonitrile was 70.2%. This
indicates that the present system can produce benzonitrile with
high NH3-efficiency and high yield under low NH3 feed conditions.
However, it is important to note that the ammonia levels required
for achieving good benzonitrile yields are still several times above
that required for the stoichiometric reaction. Further studies are
necessary to address this issue.
Fig. 3. IR spectra of pyridine adsorbed on the catalyst disk (38 1 mg) at 100 °C.
Fig. 7 shows the catalytic results by 1.5 g of W83V17 and 0.5 g
of V2O5 for the ammoxidation of toluene with lower NH3 molar
ratio (5 equiv.) than the standard conditions in Fig. 5 (10 equiv.).
The general trends were basically similar to those for the standard
conditions in Fig. 5, but the difference between W83V17 and V2O5
was more significant in Fig. 7. At 440 °C, where the toluene conver-
sion reached 100% for both catalysts, V2O5 showed 52.1% selectiv-
ity to benzonitrile and 29.6% of NH3-efficiency, while W83V17
the oxidation of NH3 to N2. The catalytic results in Table 2 are con-
verted to the rate of benzonitrile formation per surface area of the
catalysts listed in Table 1. The reaction rates for WAVAO catalysts
(W83V17, W64V36) are lower than those of V2O5. It is obvious that
the main advantages of the WAVAO catalysts are high selectivity
to benzonitrile (low selectivity to COx) and highly efficient utiliza-
tion of NH3.

350
Y. Goto et al. / Journal of Catalysis 344 (2016) 346–353
Fig. 5. Temperature dependence of the conversion of toluene (+), selectivities of benzonitrile (s), benzamide (}) and COx (d) and NH3-efficiency,
gNH3, (h) for ammoxidation
of toluene by 1.5 g of W83V17: toluene/NH3/O2/He = 1/10/4/34.
Table 2
Catalytic results at 400 °C under the conditions in Fig. 5.
Catalyst
Cat. wt. (g)
Conv. (%)
SPhCN (%)
Samide (%)
Sacid (%)
SBz (%)
SCOx (%)
gNH3 (%)
W83V17
W83V17a
Na-W83V17
W64V36
VOx/WO3
VOx/TiO2
V2O5
1.5
1.5
1.5
1.5
1.5
1.0
0.5
1.5
85.1
83.3
12.6
99.9
83.9
92.5
87.2
9.2
93.6
93.4
99.1
93.0
82.5
76.2
71.2
57.2
0.9
1.5
0
0.3
2.1
0.8
1.3
42.1
0
0
0
0
0
0
0
0.1
0
1.1
1.8
0
5.5
5.1
0.9
6.6
15.4
20.6
23.8
0.7
56.9
56.0
15.4
44.8
36.4
42.2
34.9
8.1
0
1.3
1.9
0
WO3
a
Water vapor was co-fed to the catalyst: toluene/NH3/O2/H2O/He = 1/10/4/10/24 (F = 49 mL minꢂ1).
showed 84.9% selectivity to benzonitrile and 80.5% of NH3-
efficiency. The results clearly demonstrate that W83V17 shows
high efficiency of NH3 utilization in ammoxidation as well as high
selectivity to benzonitrile based on toluene. For the reaction with
trile selectivity (90.2–90.4%) and NH3-efficiency (50.9–53.1%) were
nearly constant throughout the experiment. We also studied the
effects of co-feeding of water on the catalytic performance of
W83V17. As compared in Table 2, co-feeding of water vapor did
not essentially change the toluene conversion, the selectivity to
W83V17 at 420 °C, the yield of benzonitrile is 85.2%, corresponding
to the benzonitrile productivity of 129 g Lꢂ1
h
. The yield and the
benzonitrile and NH3-efficiency (gNH3). These results suggest that
ꢂ1
-cat
productivity of this system are not high compareꢂd1with the yields
(67–92%) and the productivities (52–342 g Lꢂ-c1at h ) of the bench-
mark V-based catalysts in the patents for ammoxidation of toluene
[1].
the present catalytic system is tolerant to water vapor.
3.3. Mechanistic and in situ IR studies
Fig. 8 shows the stability versus time-on-stream of W83V17-
catalyzed ammoxidation of toluene under the standard conditions
at 400 °C. The toluene conversion slightly decreased from 98.6% to
95.1% during the 45 h of the continuous reaction. Then, the catalyst
was heated at 500 °C for 1 h under He flow. The heat-treatment
increased the toluene conversion to 97.6%, and the catalytic system
showed high conversions (>95.9%) for the next 30 h. The benzoni-
Previous studies [3,11] on the mechanism of ammoxidation of
toluene by V-based catalyst suggest that benzonitrile is produced
via a benzaldehyde intermediate, which then reacts with NH3 on
the surface to yield benzylimine surface species. Finally, the ben-
zylimine intermediate undergoes oxidative dehydrogenation to
give benzonitrile [3,11]. We carried out a model reaction of ben-
zaldehyde + NH3 + O2 by W83V17 and V2O5 catalysts. The results

Y. Goto et al. / Journal of Catalysis 344 (2016) 346–353
351
could arise on the surface of W83V17 during the ammoxidation
of toluene. Gaseous toluene or benzaldehyde (0.3 mmol gꢂca1t) was
introduced to the flow-type in situ IR cell at various temperatures
under He flow, followed by purging with He flow for 600 s to
obtain the IR spectra in Fig. 10. The spectrum of toluene adsorbed
on W83V17 at ambient temperature showed two ring vibration
bands of aromatics (1498 and 1588 cmꢂ1) [29] but no band due
to C@O or CAO stretching modes, indicating that toluene was not
oxidized by the surface oxygen of W83V17 but physically adsorbed
on the surface at ambient temperature. At higher temperature
(200 °C), a weak band at 1705 cmꢂ1 assignable to C@O stretching
modes (mCO) of benzaldehyde [11] and ring vibration bands of aro-
matics (1498 and 1588 cmꢂ1) were observed. The mCO band at
1705 cmꢂ1 was also observed for benzaldehyde adsorbed on
W83V17 at ambient temperature. These results indicate that
toluene is oxidized by surface oxygen species of W83V17 to yield
benzaldehyde even at lower temperature (200 °C) than the reac-
tion temperature (400 °C) of the catalytic ammoxidation in this
study. In the spectrum of benzonitrile on W83V17, a band at
2262 cmꢂ1 due to C„N stretching modes (
mCN) of adsorbed ben-
zonitrile was observed.
Fig. 6. Effect of the amount to NH3 with respect to toluene on the conversion of
toluene (+), selectivities of benzonitrile (s), benzamide (}) and COx (d) and
As shown in Fig. 11, the adsorption of NH3 at 330 °C resulted in
the formation of IR bands due to protonated ammonia (NH+4) on
Brønsted acid sites at 1410 cmꢂ1 [28] and coordinated ammonia
on Lewis acid sites at 1256 cmꢂ1 [28]. Then, we studied the reac-
tion of the adsorbed NH3 species with toluene at 330 °C under
He flow. Soon after the introduction of gaseous toluene
(0.3 mmol gꢂca1t) to the NH3-preadsorbed W83V17, the band at
NH3-efficiency, gNH3, (h) for ammoxidation of toluene by 2 g of W83V17 at 400 °C:
toluene/NH3/O2/He = 1/10/4/34.
in Fig. 9 (left) were consistent with the proposed mechanism in the
literature [3,11]; the formation rates of benzonitrile by ammoxida-
tion of benzaldehyde were more than 6 times higher than those by
ammoxidation of toluene (right) for both catalysts. This suggests
that the ammoxidation of toluene by W83V17 is also driven by
the proposed mechanism in the literature [3,11], Eq. (1), including
the partial oxidation of toluene to benzaldehyde as a rate-limiting
step.
2250 cmꢂ1 due to
mCN of benzonitrile was observed, and the inten-
sity of the mCN band increased with time. The introduction of
toluene decreased the intensities of the bands of NH4+
(1410 cmꢂ1) and coordinated NH3 (1256 cmꢂ1). These results indi-
cate that benzonitrile is produced by the reaction of adsorbed NH3
ð1Þ
To verify the hypothetical mechanism, we studied formation
and reaction of intermediates on the W83V17 catalyst using
in situ IR spectroscopy. First, adsorption of toluene and benzalde-
hyde was studied in order to identify the adsorbed species that
and toluene with oxygen atoms on the surface of W83V17 at
330 °C.
Fig. 7. Ammoxidation of toluene by 1.5 g of W83V17 and 0.5 g V2O5 in the low NH3 concentration conditions (toluene/NH3/O2/He = 1/5/4/39): conversion of toluene (+),
selectivities of benzonitrile (s), benzamide (}) and COx (d) and NH3-efficiency, NH3, (h).
g

352
Y. Goto et al. / Journal of Catalysis 344 (2016) 346–353
Fig. 12 shows the reaction of toluene-derived surface species
with NH3 monitored by in situ IR at 200 °C. After the observation
of the benzaldehyde (1705 cmꢂ1) on W83V17, the flowing gas
was switched from He to 2% NH3/He and the IR spectra were
observed as a function of the time of NH3 flowing at 200 °C. The
mCN band of adsorbed benzonitrile species (2256 and 2232 cmꢂ1
)
appeared, and the intensity of the mCN band increased with time.
Summarizing the IR results in Figs. 10–12, the following pathway
is proposed; toluene is oxidized by the surface oxygen species of
W83V17 to yield benzaldehyde which undergoes the reaction with
adsorbed NH3 to give benzonitrile possibly via benzylimine. This
pathway is consistent with that proposed in the literature [3,11]
shown in Eq. (1).
Fig. 9 compares the reaction rates for benzaldehyde + NH3 + O2
(left), NH3 + O2 (center) and toluene + NH3 + O2 (right) reactions by
W83V17 and V2O5. From the results in Fig. 9 and the pathway in
Eq. (1), the origin of the higher NH3-efficiency (gNH3) of W83V17
than that of V2O5 is explained as follows. For V2O5, the rate of
NH3 conversion to PhCN in the benzaldehyde + NH3 + O2 reaction
is 1.5 times higher than the rate of NH3 oxidation to N2 in the
NH3 + O2 reaction. For W83V17, the rate of NH3 conversion to
PhCN is 3 times higher than the rate of NH3 oxidation to N2. These
results indicate that, especially on W83V17, NH3 consumption in
the condensation of benzaldehyde with NH3 is faster than NH3
Fig. 8. Time course of ammoxidation of toluene by 1.5 g of W83V17 at 400 °C:
conversion of toluene (+), selectivities of benzonitrile (s), benzamide (}) and COx
(d) and NH3-efficiency, gNH3, (h), toluene/NH3/O2/He = 1/10/4/34. After 45 h of the
reaction, the catalyst was heated at 500 °C for 1 h under He flow, followed by re-
starting the ammoxidation reaction.
The same NH3 adsorption experiment for V2O5 (Fig. 11B) shows
that the amount of adsorbed NH3 on V2O5 is negligible at 330 °C.
Comparison of the results for W83V17 and V2O5 indicates that
the acid sites originate from W in W83V17 act as adsorption sites
of NH3 during the ammoxidation reaction. The same experiment
for VOx/WO3 shows that it shows adsorbed NH3 mostly on
Brønsted acid sites probably on the surface of WO3. However, the
introduction of toluene to the adsorbed NH3 on VOx/WO3 did not
result in the formation of adsorbed PhCN. Considering the struc-
tural difference between the W83V17 (mixed oxide) and VOx/
WO3 (VOx islands supported on WO3), one could conclude that
proximity between W (acid) sites and V (redox) sites is important
for the surface reaction between the adsorbed NH3, toluene and a
reactive oxygen atom on the surface to give PhCN.
Fig. 10. IR spectra of adsorbed species on W83V17 at various temperatures.
Fig. 9. Formation rates of PhCN, COx and N2 for ammoxidation of benzaldehyde (left), oxidation of ammonia (center), and ammoxidation of toluene (right) by W83V17 (top)
and V2O5 (bottom) at 400 °C.

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353
Fig. 11. IR spectra of adsorbed species on various catalyst disks (38 1 mg) during the reaction of adsorbed NH3 species with toluene-derived species at 330 °C.
levels, WAVAO showed higher selectivity to benzonitrile and
lower selectivity to COx than conventional V-based catalysts
(V2O5 and VOx/TiO2). Additionally, WAVAO was less active for
the undesired oxidation of NH3 to N2, resulting in the high
NH3-utilization efficiency in ammoxidation than the conventional
V-based catalysts. WAVAO showed high durability and high toler-
ance to co-fed water vapor, demonstrating promising catalytic
properties of the present system. Model reaction studies suggested
that the high NH3-efficiency of WAVAO was caused by the prefer-
ential reaction of NH3 in PhCHO + NH3 + O2 over undesired
NH3 + O2 reaction.
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The layered-type WAVAO metal oxides (WAVAO), synthesized
by hydrothermal method, selectively catalyzed the gas-phase
ammoxidation of toluene to benzonitrile. Under similar conversion