Metal oxide-catalyzed ammoxidation of alcohols to nitriles and promotion effect of gold nanoparticles for one-pot amide synthesis

Article abstract of DOI:10.1016/j.apcata.2012.03.006

Transition metal oxides (MnO₂, Co₃O₄, and NiO) are catalytically active for the ammoxidation of alcohols to nitriles. In particular, MnO₂ exhibited remarkably high catalytic activity and selectivity for the ammoxidation of alcohols to produce nitriles. Benzyl alcohol could also be directly converted to benzonitrile by MnO₂ catalyst by the one-pot ammoxidation and the hydration with water which was formed by the first ammoxidation step. The deposition of gold nanoparticles (Au NPs) onto MnO₂ did not enhance the ammoxidation of benzyl alcohol but promoted the hydration of benzonitrile to produce benzamide with high selectivity. In contrast, Au NPs supported on Al₂O₃, CuO, and CeO ₂ catalyzed the ammoxidation of benzyl alcohol, whereas these metal oxides themselves were inactive for the ammoxidation or showed low catalytic activity. These results have demonstrated that gold is intrinsically active as a catalyst for the ammoxidation of alcohols.

Full text of DOI:10.1016/j.apcata.2012.03.006

Applied Catalysis A: General 425–426 (2012) 85–90
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Metal oxide-catalyzed ammoxidation of alcohols to nitriles and promotion effect
of gold nanoparticles for one-pot amide synthesis
Tamao Ishidaa,c,∗, Hiroto Watanabeb, Takashi Takeib,c, Akiyuki Hamasakia,c, Makoto Tokunagaa,c
,
Masatake Harutab,c,∗∗
a Department of Chemistry, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
b Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan
c Japan Science and Technology Agency (JST), CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Transition metal oxides (MnO2, Co3O4, and NiO) are catalytically active for the ammoxidation of alcohols
to nitriles. In particular, MnO2 exhibited remarkably high catalytic activity and selectivity for the ammox-
idation of alcohols to produce nitriles. Benzyl alcohol could also be directly converted to benzonitrile by
MnO2 catalyst by the one-pot ammoxidation and the hydration with water which was formed by the
first ammoxidation step. The deposition of gold nanoparticles (Au NPs) onto MnO2 did not enhance the
ammoxidation of benzyl alcohol but promoted the hydration of benzonitrile to produce benzamide with
high selectivity. In contrast, Au NPs supported on Al2O3, CuO, and CeO2 catalyzed the ammoxidation of
benzyl alcohol, whereas these metal oxides themselves were inactive for the ammoxidation or showed
low catalytic activity. These results have demonstrated that gold is intrinsically active as a catalyst for
the ammoxidation of alcohols.
Received 29 December 2011
Received in revised form 26 February 2012
Accepted 2 March 2012
Available online 10 March 2012
Keywords:
Metal oxides
Gold nanoparticles
Ammoxidation
Nitrile hydration
Alcohol
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
corresponding nitriles has also been known by using stoichio-
metric oxidants such as Ag2O and Pb(OAc)2. One of the other
Nitriles and amides are important class of organic compounds
for chemical industries. Nitriles are generally produced by the
ammoxidation of alkenes or alkanes over Bi–Mo mixed oxides in
gas phase [1]. Some aromatic nitriles such as m-tolunitrile and
cyanopyridines (nicotinonitrile) are synthesized by the ammox-
idation of methyl groups connected to the aromatic rings over
vanadium-based oxide catalysts in gas phase [2]. However, these
ammoxidation methods are hardly applied to the production of
aliphatic nitriles. They are generally produced by the condensation
of carboxylic acids or esters with ammonia (NH3) to form amides
followed by the dehydration of amides over metal oxides at tem-
peratures above 300 ◦C [3].
methods is the ammoxidation of aldehyde in the presence of stoi-
chiometric oxidants such as MnO2 with MgSO4 [4]. Since aldehydes
are produced by the oxidation of alcohols, the direct conver-
sion of alcohols to produce nitriles is preferred and has been
studied by using NiSO4/K2S2O8–NaOH [5], I2 with 1,3-diiodo-5,5-
dimethylhydantoin [6], nickel–copper formate with (Bu4N)2S2O8
[7], and MnO2 with MgSO4 [8]. However, these methods have draw-
backs that they need stoichiometric amount of oxidants.
Recently, Mizuno et al. [9,10] reported the catalytic ammoxida-
tion of alcohols into nitriles followed by the hydration of nitriles
with an excess amount of water to yield amides over ruthe-
nium hydroxide supported on Al2O3. Since Au catalysts have been
reported to be active for alcohol oxidations in the presence of amine
to yield imine [11,12] and for direct amide synthesis from alco-
hol with amine [12–15], we have attempted to apply Au catalysts
to alcohol oxidation in the presence of NH3 to produce nitrile and
amide. During the course of our investigation, we found that MnO2,
Co3O4, NiO, CuO catalyzed the ammoxidation of alcohols with-
out the deposition of Au. Manganese dioxide is well known as a
stoichiometric oxidant that can be used for nitrile synthesis from
alcohols with NH3. However, a large excess (more than 15 equimo-
lar amount of substrates) of MnO2 has to be used [8]. In addition, the
substrates were limited to benzylic alcohols and unsaturated alco-
hols. Whereas metal oxides such as MnO2 [16–20], Co3O4 [21,22],
In laboratory, on the other hand, there are many synthetic
routes for nitriles such as the substitution of alkyl halides with
metal cyanides. Oxidative conversion of primary amines to the
∗
Corresponding author at: Department of Chemistry, Graduate School of Sci-
ences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
Tel.: +81 92 642 7528; fax: +81 92 642 7528.
∗∗
Corresponding author at: Department of Applied Chemistry, Graduate School of
Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa,
Hachioji, Tokyo 192-0397, Japan. Tel: +81 42 677 2852; fax: +81 92 677 2852.
E-mail addresses: tishida@chem.kyushu-univ.jp (T. Ishida),
haruta-masatake@center.tmu.ac.jp (M. Haruta).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2012.03.006

86
T. Ishida et al. / Applied Catalysis A: General 425–426 (2012) 85–90
and NiO [23] have been revealed to be catalytically active for the
aerobic oxidation of alcohols, catalytic ammoxidation of alcohol
over these metal oxides is very limited. Very recently, Mizuno and
his co-workers demonstrated that the amide synthesis from pri-
mary alcohol was catalyzed over manganese oxide based molecular
sieves via the ammoxidation of alochols to nitriles [24]. However,
excess amount of Mn (ca. 2 equiv.) was still required.
In this paper, we report the catalytic performance of MnO2,
Co3O4, and NiO for the ammoxidation of alcohols including unac-
tivated alcohols. Since MnO2 can be used for hydration of nitriles
[25–30], the one-pot synthesis of an amide from an alcohol over
MnO2 was also investigated. We also found that metal oxide-
supported Au catalysts promoted the ammoxidation of an alcohol
to produce a nitrile and an amide.
solution (500 mL) of Mn(NO3)2·6H2O (0.3 mol) at room tempera-
ture [31]. The mixture was stirred at room temperature for 1 h. The
precipitate was collected by filtration, dried at 65 ◦C overnight, and
then calcined in air at 300 ◦C for 4 h.
2.3.2. Deposition of gold onto metal oxides
Gold on NiO and Au/CuO were prepared by the co-precipitation
(CP) [32]. Gold on Co3O4 was prepared by a similar manner to
Au/NiO except for precipitaion and aging at room temperature
for aging. Gold on CeO2 and Au/Al2O3 were prepared by the
deposition–precipitation (DP) method [33].
Gold on MnO2 was prepared by the homogeneous
deposition–precipitation (HDP) with urea [34]. To an aqueous
solution (200 mL) of HAuCl4 (0.3 mmol) and urea (60 mmol) was
added MnO2 (1.1 g, Au 5 wt%) at room temperature. The suspension
was gradually warmed to 90 ◦C and stirred for 16 h. The solid was
washed with water, dried at 65 ◦C overnight, and then calcined in
air for 4 h.
2. Experimental
2.1. Materials
Reagent grades, Mn(NO3)2·6H2O, KMnO4, Co(NO3)2·6H2O,
Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Na2CO3, NaOH, urea, were used
as received. Molybdenum oxide (MoO3) was supplied by Kojundo
Chemical Laboratory Co., Ltd. High purity nanoparticulate CeO2,
TiO2 (P-25), and Al2O3, were supplied by Daiichi Kigenso Kagaku
Kogyo, Nippon Aerosil Co., Ltd., and JGC Catalysts and Chem-
icals Ltd., respectively. Vanadium oxide (V2O5) was purchased
from Kanto Chemical Co. Tetrachloroauric acid tetrahydrate
(HAuCl4·4H2O) was purchased from Tanaka Kikinzoku KKK
and used as received. Benzyl alcohol, cinnamyl alcohol, 2-
pyridine methanol, 2-thiophene methanol, 1-octanol, and toluene
(dehydrated grade) were purchased and used without further
purifications.
2.4. Catalytic test
2.4.1. Ammoxidation of alcohol to nitrile
To an autoclave was charged benzyl alcohol (50 ␮L, 0.5 mmol),
toluene (3 mL), metal oxide catalysts (20 mg), and a magnetic stir-
ring bar. The autoclave was purged and filled with NH3 until the
pressure reached 0.85 MPa. Then O2 was introduced until the total
pressure reached to 1.35 MPa (pO2 0.5 MPa). The reaction mix-
ture was stirred at a controlled temperature. After the reaction,
the mixture was extracted with Et2O and filtered. The filtrate was
analyzed by GC–MS and GC by using anisole as an internal stan-
dard. For recycling tests, the catalyst was filtered after the reaction,
washed with acetone for three times, dried under vacuum, cal-
cined in air at 300 or 400 ◦C for 4 h, and then used for the next
run.
2.2. Instruments
Specific surface area was obtained from nitrogen adsorption
measurements. The samples were pretreated under vacuum at
200 ◦C for 2 h. Then, N2 adsorption isotherms were measured at 77 K
with a SHIMADZU Tristar. X-ray powder diffraction (XRD) patterns
were obtained by Rigaku RINT-TTR III at a scanning rate of 2◦/min
and a sampling angle interval of 0.02◦ in 2ꢀ ranging from 10◦ to 80◦
with high-intensity Cu K␣ radiation (ꢁ = 0.151478 nm). The oper-
ating voltage and current were 50 kV and 300 mA. The phase of
components was identified by matching diffraction patterns to the
JCPDS powder diffraction file. The conversions and product yields
were measured by gas chromatography by using Agilent GC-7890A
with HP-5 capillary column (0.32 mm i.d., 30 m, 0.25 ␮m thick-
ness) using anisole as an internal standard. Qualitative analysis
was performed with a GC–MS (SHIMADZU PARVUM2 and GC-2010
with Shinwa Chemical ULBON HR-1 capillary column, 0.25 mm i.d.,
30 m).
2.4.2. One-pot synthesis of benzamide from benzyl alcohol
After the nitrile formation at 100 ◦C for 1 h, the reactant gases
were released. Then the autoclave was purged and filled with N2.
The reaction mixture was stirred again at 100 ◦C. The mixture
was extracted with Et2O and toluene, and then filtered. The fil-
trate was measured by GC–MS and GC using anisole as an internal
standard.
3. Results
3.1. Ammoxidation of benzyl alcohol to benzonitrile over metal
oxides
Various kinds of metal oxides have been screened for the
ammoxidation of benzyl alcohol (1) (Table 1). Although V2O5 (entry
3), Fe2O3 (entry 9) and MoO3 (entry 13) are active for the gas
phase ammoxidation of alkanes and alkenes, they can not catalyze
the ammoxidation of an alcohol in liquid phase. Among the metal
oxides, MnO2 (entry 4) and Co3O4 (entry 10) promoted the reac-
tion to yield benzonitrile (2) with remarkably high selectivity. The
reaction also proceeded in the presence of NiO. However, ben-
zaldehyde (3) and N-benzylidenebenzylamine (imine, 5) were also
formed as by-products (entry 11). In particular, MnO2 exhibited
superior catalytic activity and selectivity to the nitrile. Namely, the
ammoxidation reaction completed within 30 min over MnO2 (entry
5) while it completed after 4 h in the presence of Co3O4 (entry 10).
The catalytic activity order was as the same as that for alcohol oxi-
dation in the absence of NH3 (data are not shown). Toluene, which
was used as a solvent, was not converted to benzonitrile under our
conditions.
2.3. Catalyst preparation
2.3.1. Metal oxides
Ferric oxide (Fe2O3), Co3O4, CuO, and NiO were prepared
by the neutralization method. Briefly, an aqueous solution of
Ni(NO3)2·6H2O (0.1 M, 200 mL) was rapidly added into an aque-
ous solution of Na2CO3 (0.1 M, 240 mL) at 70 ◦C and the mixture
was stirred at 70 ◦C for 1 h. The precipitate was washed with dis-
tilled water, filtered, dried at 100 ◦C overnight, and then calcined in
air at 300 ◦C for 4 h. Cobalt oxide (Co3O4) was prepared in a similar
manner except for aging at room temperature.
For the preparation of MnO2, an aqueous solution (500 mL) of
KMnO4 (0.2 mol) and NaOH (0.6 mol) was added to an aqueous

T. Ishida et al. / Applied Catalysis A: General 425–426 (2012) 85–90
87
Table 1
Catalytic performance of metal oxides for the ammoxidation of benzyl alcohol.a
Entry
Catalyst
Temp.
(◦C)
Time
(h)
Conv.
(%)
Yield (%)b
2
3
4
5
1
2
3
Al2O3
TiO2
100
100
100
100
100
100
60
4
4
4
4
0.5
0.5
1
1
4
4
4
4
4
4
4
0
2
0
0
0
0
0
2
0
1
0
0
1
1
0
0
8
0
0
0
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
14
0
0
0
V2O5
MnO2
MnO2
MnO2
MnO2
MnO2
Fe2O3
Co3O4
NiO
4
100
100
86
56
56
0
96
67
23
0
96
100
78
48
47
0
94
34
11
0
5
6c
7
8d
9
60
100
100
100
100
100
100
100
10
11
12
13
14
15
CuO
MoO3
PdO
7
19
4
0
CeO2
0
a
Conditions: benzyl alcohol (0.5 mmol), metal oxide (20 mg), toluene (3 mL), pNH3 (0.85 MPa), pO2 (0.5 MPa).
Calculated based on GC analysis using anisole as an internal standard.
MnO2 (5 mg),
b
c
d
Ammoxidation of the reaction solution (entry 7) after the removal of MnO2 by filtration. Conditions: pNH3 (0.85 MPa), pO2 (0.5 MPa).
Further, lesser amount of MnO2 (Mn 12 mol%) was
enough to promote the reaction (entry 6), proving that
the reaction proceeded catalytically. Furthermore, metal-
time yield (molnitrile molMn−1 h−1
was recorded to be
13.4 molnitrile molMn−1 h−1 which was much higher than that
of Ru(OH)x/Al2O3 (1.4 molnitrile molRu−1 h−1
[9]. In order to
investigate whether the reaction proceeded heterogeneously or
by leached Mn(IV) species, the ammoxidation was carried out
after the removal of MnO2 at a conversion of 56% (entry 7). After
the filtration of MnO2, the conversions and the yields were the
same as those in entry 7, suggesting that the reaction took place
heterogeneously (entry 8).
Effect on calcination temperatures of MnO2 was investigated
(Table 2). An increase in calcination temperatures led to a decrease
in both specific surface area and catalytic activity of MnO2, when
the yield of benzonitrile after 10 min was compared (entries 1, 2,
6, and 10). With regard to the recycling test, the catalytic activity
of MnO2 calcined at 300 ◦C was slightly decreased for the third run
(entry 5). Although the initial catalytic activity was better for the
calcination at 300 ◦C than 400 ◦C, MnO2 calcined at 400 ◦C showed
better recyclability after the third run (entry 9).
b could not be assigned due to very broad diffraction patterns. For
MnO2 calcined at 400 and 500 ◦C, diffraction peaks were observed
at 12◦, 18◦, 29◦, 38◦, and 50◦ (Fig. 1e and f). These two MnO2 sam-
ples had the same crystalline phase and were assigned to ␣-MnO2
(JCPDS #44-0141) [35]. It can be said that a high calcination temper-
ature caused an increase in crystallinity and a decrease in specific
surface area without the change of crystalline phase. Thus, it can be
concluded that the difference in catalytic activity of MnO2 calcined
at different temperatures was derived from the amount of active
sites on MnO2 surface. XRD patterns of MnO2 (calcined at 300 ◦C)
after the reaction (Fig. 1c) and the re-calcined MnO2 at 300 ◦C after
the reaction (Fig. 1d) were almost identical to the fresh MnO2
(Fig. 1b), suggesting that the crystalline nature of MnO2 catalyst
did not change during the reaction.
a
)
)
Manganese dioxide was also applicable to other primary alco-
hols (Scheme 1). Cinnamyl alcohol was converted to cinnamonitrile
without the isomerization and the hydrogenation of C C dou-
ble bonds (Scheme 1a). Heteroaromatic alcohols also reacted to
form the corresponding nitriles (Scheme 1b and c). Taylor et al. [8]
reported that MnO2 only oxidized activated alcohols such as benzyl
alcohol even large excess amount of MnO2 was used. In contrast,
MnO2 prepared by the reaction of KMnO4 with Mn(NO3)2 catalyzed
the ammoxidation of unactivated alcohols such as 1-octanol to
yield octanenitrile with high yield (Scheme 1d).
3.2. Characterization of MnO2
Fig. 1 shows the X-ray diffraction (XRD) patterns of MnO2 cal-
cined at different temperatures. For MnO2 calcined at 250 and
300 ◦C, weak and broad diffraction peaks at around 2ꢀ of 12◦ and
38◦ were observed (Fig. 1a and b). Crystalline phases of Fig. 1a and
Scheme 1. Ammoxidation of other primary alcohols: cinnamyl alcohol (a), 2-
pyridine methanol (b), 2-thiophene methanol (c), 1-octanol (d). Reaction conditions:
substrate (0.5 mmol), toluene (3 mL), MnO2 (20 mg), pNH3 (0.85 MPa), pO2 (0.5 MPa),
100 ◦C. Calculated based on GC analysis using anisole as an internal standard.

88
T. Ishida et al. / Applied Catalysis A: General 425–426 (2012) 85–90
Table 2
Catalytic properties of MnO2 calcined at different temperatures for the ammoxidation of benzyl alcohol.a
Entry
Calcined temp.
(◦C)
Specific surface area
(m2/g)
Reaction time
(min)
Conv.
(%)b
Yield (%)b
Nitrile
Aldehyde
1
2
3
250
300
300
300
300
400
400
400
400
500
107
70
–
–
–
51
–
–
–
19
10
10
30
30
30
10
30
30
30
10
100
88
100
100
92
87
100
96
98
82
100
100
90
78
95
94
93
0
0
0
0
1
1
0
0
0
5
4c
5d
6
7
8c
9d
10
98
31
19
a
Conditions: benzyl alcohol (0.5 mmol), toluene (3 mL), MnO2 (20 mg, 0.23 mmol), pNH3 (0.85 MPa), pO2 (0.5 MPa), 100 ◦C.
Calculated based on GC analysis using anisole as an internal standard.
Second run.
Third run.
b
c
d
Longer reaction time (12 h) gave benzamide in 11% yield, suggest-
ing that the sequential ammoxidation and hydration of the nitrile
occurred (entry 1). Lower pressures of NH3 and O2 (0.4 MPa each)
improved the amide yield, indicating that the hydration was sup-
pressed under high NH3/O2 pressures (entry 2). Thus, the gases
(NH3 and O2) were released after the ammoxidation step for 1 h
and filled with atmospheric pressure of N2, and then the reac-
tion mixture was heated again to 100 ◦C to promote the hydration
for additional hours (condition B). As a result, the yield of amide
reached to 57% after 24 h (entry 4) which was higher than that for
the hydration of benzonitrile over MnO2 (entry 7). When the ini-
tial NH3/O2 pressure was reduced to 0.1/0.1 MPa of NH3/O2, the
ammoxidation of alcohol completely proceeded whereas amide
yield was not improved (entry 5). Increase in the amount of MnO2
was not effective for the hydration efficiency (entry 6).
3.4. Effect of gold on ammoxidation and one-pot amide synthesis
Since it has been reported that MnO2 supported Au NPs exhib-
ited high catalytic activity for alcohol oxidation [36], the effect of
Au NPs on the ammoxidation of alcohol was investigated (Table 4).
The deposition of Au NPs onto MnO2 did not enhance the reaction
rate of the ammoxidation (entry 1) as compared to that for MnO2
(entry 6 in Table 1). In contrast, a promotion effect of Au NPs was
observed for the hydration step, i.e., the yield of amide increased
to 58% (entry 2) from 34% for MnO2 (entry 6 in Table 3). The cat-
alytic activity of Au/MnO2 for the hydration of benzonitrile itself
was slightly higher (entry 4) than that of MnO2 (entry 7 in Table 3).
Consequently, optimized reaction conditions achieved 81% amide
yield for the one-pot amide synthesis without further addition of
water (entry 3).
Fig. 1. XRD patterns of MnO2 calcined at 250 ◦C (a), 300 ◦C (b), 300 ◦C after the reac-
tion (c), MnO2 treated by re-calcination in air at 300 ◦C for 4 h after the reaction (d),
400 ◦C (e), and at 500 ◦C (f).
3.3. One-pot benzamide formation from benzyl alcohol
As shown in Table 4, Au/Al2O3 (entry 5) and Au/CeO2 (entry 8)
catalyzed the ammoxidation although support metal oxides alone
did not catalyze the reaction (entries 1 and 15 in Table 1). In
contrast, neither ammoxidation nor alcohol oxidation to aldehyde
underwent over Au/TiO2 (entry 6), suggesting that Au/TiO2 might
be deactivated by NH3. In addition, the product selectivity strongly
depended on the kind of supports, i.e., Au/CeO2 and Au/Al2O3 gave
benzamide together with benzonitrile, N-benzylidenbenzylamine,
and triphenyl triazine as by-products, whereas Au/CuO gave ben-
zonitrile as a major product with high selectivity (90%).
One-pot, two-steps amide synthesis from alcohol by the ammx-
oidation of alcohols followed by the hydration of nitriles with the
addition of excess amount of water at the second step has been
reported over Ru(OH)x/Al2O3 [9]. However, the removal of water
after the reaction is a high-cost process and minimizing the amount
of water is strongly desired. When the ammoxidation of an alco-
hol to a nitrile proceeded, three equimolar amount of water is
produced (Scheme 2). Thus, we tried to use the water generated
by the nitrile formation for the next hydration step (Table 3). A
4. Discussions
4.1. Reaction pathway for MnO2 catalyzed ammoxidation
Since benzaldehyde was obtained as a by-product over MnO2
and Co3O4, benzyl alcohol was firstly oxidized to benzaldehyde and
Scheme 2. Ammoxidation of benzyl alcohol over metal oxide catalysts.

T. Ishida et al. / Applied Catalysis A: General 425–426 (2012) 85–90
89
Table 3
One-pot benzamide synthesis from benzyl alcohol over MnO2 catalysts.
Entry
Catalyst
NH3/O2
Time
(h)
Conditionsa
Conv.b
(%)
Yield(%)b
Nitrile
(MPa/MPa)
Amide
1
2
3
4
5
6c
7
MnO2
0.85/0.5
0.4/0.4
0.4/0.4
0.4/0.4
0.1/0.1
0.1/0.1
0/0
12
12
12
24
12
12
24
A
A
B
B
B
B
C
100
100
100
100
100
100
45
85
73
57
41
61
59
–
11
21
38
57
30
34
40
a
Reaction condition A: benzyl alcohol (0.5 mmol), toluene (3 mL), MnO2 (20 mg), 100 ◦C. Condition B: the reaction was performed under the pressure of NH3/O2 for 1 h
and then for additional hours under N2 atmospheric pressure. Condition C: benzonitrile (0.5 mmol) was used as a substrate. Catalyst (20 mg), toluene (3 mL), H2O (1.5 mmol),
atmospheric pressure of N2, 100 ◦C.
b
Calculated based on GC analysis using anisole as an internal standard.
MnO2 (50 mg).
c
that Mn, Co, and Ni oxides catalyzed not only alcohol oxidation but
also imine oxidation to form nitrile. For MnO2-catalyzed aerobic
oxidation of alcohols, a redox mechanism between MnO2 and MnOx
was assumed [18]. In the case of MnO2-catalyzed ammoxidation of
alcohols, similar redox mechanism might be involved; Mn(IV)O2
oxidized an alcohol to an aldehyde and an imine to a nitrile. Then,
Mn(IV) was reduced to Mn(III), which was re-oxidized to Mn(IV)
by molecular oxygen.
Scheme 3. Plausible reaction pathway for the ammoxidation of alcohol over MnO2.
4.2. Catalytic activity of supported gold catalysts
then imine was formed by dehydrative condensation of aldehyde
with NH3 via the hemiaminal formation (Scheme 3). It was con-
firmed that Fe2O3 catalyzed neither benzyl alcohol oxidation nor
the ammoximation of benzaldehyde with NH3. Thus, it appeared
For Au-catalyzed ammoxidation of benzyl alcohol, the selection
of supports, which are not deactivated by NH3, is a key factor. In
addition, the reaction route could be controlled by the selection of
Scheme 4. Plausible reaction pathway for the ammoxidation and amidation over supported Au catalysts.
Table 4
Effect of gold deposition on metal oxides in the ammoxidation of benzyl alcohol and amide synthesis.
Entry
Catalyst
NH3/O2
Temp.
(◦C)
Time
(h)
Conditionsa
Conv.b
(%)
Yield (%)b
Nitrile
(MPa/MPa)
Amide
1
2
3
4
5
6
7
8
Au/MnO2
0.85/0.5
0.1/0.1
0.1/0.1
0/0
0.85/0.5
0.85/0.5
0.85/0.5
0.85/0.5
100
100
100
100
150
120
120
120
0.5
12
72
24
23
23
23
23
A
B
B
C
87
100
100
56
100
15
77
33
9
–
22
0
0
58
81
51
45
0
Au/Al2O3
Au/TiO2
Au/CuO
Au/CeO2
Ac
Ac
Ac
Ac
61
61
55
10
0
28
a
Reaction condition A: benzyl alcohol (0.5 mmol), toluene (3 mL), catalyst (50 mg). Condition B: the reaction was performed under the pressure of NH3/O2 for 1 h and
then for additional hours under N2 atmospheric pressure. Condition C: benzonitrile (0.5 mmol) was used as a substrate. Catalyst (20 mg), toluene (3 mL), H2O (1.5 mmol),
atmospheric pressure of N2, 100 ◦C.
b
Calculated based on GC analysis using anisole as an internal standard.
Au 2 mol%, toluene (2 mL).
c

90
T. Ishida et al. / Applied Catalysis A: General 425–426 (2012) 85–90
suitable supports for Au NPs and reaction conditions. Au/CuO (solid
line in Scheme 4) did not enhance the hydration, giving benzoni-
trile as a major product. On the other hand, Au/Al2O3 and Au/CeO2
would enhance the hydration of benzonitrile or the direct oxi-
dation of hemiaminal to produce benzamide as a major product
(dashed line in Scheme 4). According to the fact that benzonitrile, N-
benzylidenebenzylamine, and 2,4,6-triphenyl-1,3,5-triazine were
obtained as by-products over Au/Al2O3 and Au/CeO2, the one-pot
ammoxidation followed by the hydration is more likely. In our
experiments, the ammoxidation was carried out under the pres-
surized NH3 in organic solvents in contrast to the direct amidation
of alcohols in aqueous solution of NH3 in the presence of inorganic
base [15]. These conditions facilitated preferentially the dehydra-
tion of hemiaminal to form imine which was easily oxidized to
nitrile.
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We have found that MnO2, Co3O4, and NiO were catalyti-
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unactivated alcohols to produce nitriles. In particular, MnO2 exhib-
ited remarkably high catalytic activity and excellent selectivity to
nitriles. The MnO2 catalyst could be reused at least for three times.
We have also found that Au NPs deposited on MnO2 promoted the
hydration of benzonitrile to form benzamide without further addi-
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for the ammoxidation of benzyl alcohol to produce benzonitrile and
benzamide, presenting a new type of catalysis by Au NPs.
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This work was financially supported by JST-CREST and a Grant-
in-Aid for Young Scientists (B) (No. 21750160) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
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