Redox properties of CeO2at low temperature: The direct synthesis of imines from alcohol and amine

Article abstract of DOI:10.1002/anie.201409601

We disclosed the redox properties of CeO₂ in organic reactions at low temperature of 303 K. CeO₂ works as the most effective heterogeneous catalyst for imine formation from benzyl alcohol and aniline at 303 K among various metal oxides and showed more than 38-fold higher activity than other simple metal oxides. CeO₂ is applicable to the reaction of various alcohols and amines and gives high yields (80-98%) and high selectivities (89- > 99%). Kinetic measurements, MS, and FTIR analyses demonstrated that the high activity of CeO₂ is a result of reactive oxygen species at the redox sites on CeO₂. This discovery can help to create a new field in metal oxide catalysis.

Full text of DOI:10.1002/anie.201409601

Angewandte
Chemie
DOI: 10.1002/anie.201409601
Direct Imine Formation
Redox Properties of CeO2 at Low Temperature: The Direct Synthesis
of Imines from Alcohol and Amine
Masazumi Tamura* and Keiichi Tomishige
Abstract: We disclosed the redox properties of CeO2 in organic
reactions at low temperature of 303 K. CeO2 works as the most
effective heterogeneous catalyst for imine formation from
benzyl alcohol and aniline at 303 K among various metal
oxides and showed more than 38-fold higher activity than other
simple metal oxides. CeO2 is applicable to the reaction of
various alcohols and amines and gives high yields (80–98%)
and high selectivities (89– > 99%). Kinetic measurements, MS,
and FTIR analyses demonstrated that the high activity of CeO2
is a result of reactive oxygen species at the redox sites on CeO2.
This discovery can help to create a new field in metal oxide
catalysis.
property of CeO2 will lead to new applications of metal oxide
catalysis.
Imines are very important intermediates in the synthesis
of various biological, agricultural, and pharmaceutical com-
pounds, because they can undergo versatile transformations
(reductions, additions, condensations, and multicomponent
reactions).[5] Traditionally, imines are synthesized by the
condensation reaction of aldehydes or ketones with amines
in the presence of an acid catalyst. Recently, versatile
alternative methods have been reported such as the oxidation
or dehydrogenation of secondary amines,[6] the dimerization
of primary amines,[7] the direct coupling of alcohol and
amine,[8] and the hydroamination of alkynes with amines.[9]
Among these methods, the direct coupling of alcohol and
amine is one of the most promising approaches, because
alcohols are readily available and inexpensive, and only
hydrogen and/or water is produced as a by-product. Various
effective homogeneous[8a–g] and heterogeneous[8h–t] catalysts
have been reported. However, most reaction systems require
a strong inorganic or organic base such as KOH, NaOH, and
DABCO as a catalyst or co-additive, which is undesirable,
because a large amount of salts is produced. Unlike these
reaction systems, the groups of Milstein[8g] and Schomaker[8c]
reported effective homogeneous Ru complex catalysts with-
out using base under N2 atmosphere, which, however, had the
drawbacks of high required temperature (> 373 K) and low
activity. With regard to reusability and durability, heteroge-
neous catalysts are desirable and some catalysts working
without additives were reported such as Au,[8l,s] Ru,[8q] Pd,[8r]
and Pt[8o] catalysts. However, these catalysts are based on
noble metals and require the use of pure O2. Therefore, the
development of an inexpensive and effective heterogeneous
catalyst that does not require additives and works under mild
reaction conditions such as air and low temperature is
desirable.
M
etal oxides are used in various fields of chemistry due to
both their acid–base and redox properties and constitute the
largest family of catalysts in heterogeneous catalysis. If metal
oxides could be used to catalyze organic reactions at low
temperature by making use of their redox properties, they
might substitute transition metal complexes and supported
metal catalysts. Among metal oxides, CeO2 is particularly
promising, because of its redox and acid–base bifunctional
properties and therefore attracts much attention in the fields
of catalyst and biological chemistry.[1] However, these proper-
ties of CeO2 are generally utilized at high temperature
(> 473 K) in catalysis such as the conversion of exhaust gases.
Recently, it was reported that the unique acid–base bifunc-
tionality of CeO2 at low temperature (ꢀ 373 K) is related to
high catalytic activity in liquid-phase organic reactions,[2] but
the redox property of CeO2 at low temperature (ꢀ 373 K) in
organic reactions has never been reported. On the other hand,
direct observation of the oxygen vacancies on CeO2 has been
investigated. Noncontact atomic force microscopy (NC-
AFM) studies demonstrated that oxygen vacancies on CeO2
are mobile at 303 K,[3] which is related to the presence of the
surface site with redox ability at low temperature. In contrast,
the combination of scanning tunneling microscopy (STM)
analysis and DFT calculations suggested that the oxygen
vacancies on CeO2 are immobile at room temperature.[4]
Therefore, clarification of the redox property of CeO2 at
low temperature (ꢀ 373 K) is of great importance to under-
stand the unique properties of CeO2. New findings regarding
organic reactions catalyzed at low temperature by the redox
Herein, we report that CeO2, a simple metal oxide, is the
most effective heterogeneous catalyst among various metal
oxides for imine synthesis from alcohol and amine even at low
temperature of 303 K under air. This is the first report that
CeO2 exhibits the redox property at 303 K in organic
reactions. Kinetic measurements, MS, and FTIR analyses
confirmed that the high activity is derived from the reactive
oxygen species at the redox sites of CeO2.
Initially, the catalytic activity of various metal oxides for
imine synthesis from benzyl alcohol and aniline was inves-
tigated at 303 K under air (Table 1). CeO2 showed activity in
this reaction, but other simple metal oxides are nearly
inactive. The activity of CeO2 was more than 38-fold higher
than those of other simple metal oxides in terms of both the
catalyst amount and surface area, and the yield of benzylidene
[*] Dr. M. Tamura, Prof. Dr. K. Tomishige
Graduate School of Engineering, Tohoku University
Aoba 6-6-07, Aramaki, Aoba-ku, Sendai, 980-8579 (Japan)
E-mail: mtamura@erec.che.tohoku.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409601.
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Table 1: Imine synthesis from benzyl alcohol and aniline over various
metal oxides.[a]
Metal oxide
S[b]
t [h] Yield V[c]
Vꢀ103[d]
[m2 gÀ1
]
[%]
[mmolhÀ1 gÀ1
]
[mmolhÀ1 mÀ2
]
CeO2
CeO2
CeO2
MnO2
Al2O3
Y2O3
Pr6O11
La2O3
Nb2O5
ZrO2
MgO
TiO2
Dy2O3
Ta2O5
Fe2O3
CaO
ZnO
84
84
84
64
182
62
33
8.0
47
91
37
48
2.5
3.2
3.3
12
5
12
75
96
0.46
–
–
0.0088
0.0046
0.003
0.002
0.001
0.001
0.001
5.3
–
–
0.14
0.025
0.04
0.06
0.08
0.02
0.01
48
24
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
24
[e]
2.1
1.1
0.6
0.5
0.2
0.2
0.2
Figure 1. a) Time dependence for imine formation from benzyl alcohol
~
^
and aniline over CeO2 ( : conversion, &: imine yield, : aldehyde
yield). Conditions: benzyl alcohol 1.0 mmol, aniline 2.0 mmol, mesity-
lene 1.5 g, CeO2 50 mg, 333 K, air. b) Time dependence for imine
0.1 <0.001
0.1 <0.001
0.0 <0.001
0.0 <0.001
0.0 <0.001
0.0 <0.001
0.0 <0.001
0.0 <0.001
0.0 <0.001
4.1
2.4
1.2
0.01
0.01
*
formation from benzaldehyde and aniline with/without CeO2 ( : imine
*
yield with CeO2, : imine yield without CeO2). Conditions: benzalde-
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.59
hyde 1.0 mmol, aniline 2.0 mmol, mesitylene 1.5 g, CeO2 50 mg, 333 K,
air.
50
formation of benzyl alcohol to benzaldehyde and 2) the imine
formation of benzaldehyde with aniline.
SiO2
530
560
54
SiO2-Al2O3
CeO2-ZrO2
(Ce/Zr=4)[f]
CeO2-ZrO2
(Ce/Zr=1)[f]
CeO2-ZrO2
(Ce/Zr=0.25)[f]
Generally, the imine formation readily proceeds upon
heating. To estimate the contribution of CeO2 to imine
formation, the reaction was performed in the absence and
presence of the CeO2 catalyst (Figure 1b). CeO2 clearly
catalyzed the reaction (136 mmolhÀ1 gÀ1), resulting in about
10-fold higher activity than that without CeO2. This indicates
that CeO2 works as an effective catalyst for the imine
formation of benzaldehyde with aniline. Therefore, we
concluded that CeO2 catalyzed both the steps of the imine
synthesis from benzyl alcohol and aniline. The reaction rate of
step 2 (136 mmolhÀ1 gÀ1) is more than 60 times higher than
that of the overall reaction (2.2 mmolhÀ1 gÀ1), indicating that
step 1 is the rate-determining step.
0.034
0.020
0.010
55
57
24
24
0.36
0.18
[a] Conditions: benzyl alcohol 1.0 mmol, aniline 2.0 mmol, mesitylene
1.5 g, metal oxide 50 mg, 303 K, air. [b] Specific surface area determined
by the Brunauer—Emmett—Teller (BET) method. [c] Formation rate per
catalyst amount. [d] Formation rate per surface area. [c,d] Formation
rates were measured under the conditions at which the conversion was
below 20%. [e] 333 K. [f] Molar ratio of Ce to Zr.
aniline reached 75% in 48 h. When the temperature was
333 K, CeO2 provided the highest yield of 96% in 24 h. In
addition, mixed metal oxides of CeO2–ZrO2 were also tested
in the reaction. The formation rate drastically decreased upon
the introduction of Zr. These results indicate that pure CeO2
is the most effective catalyst for the imine synthesis among the
tested metal oxides.
The time course of the imine synthesis over CeO2 at 333 K
was investigated (Figure 1a). The reaction proceeds smoothly
to afford the corresponding imine in 96% yield with 97%
selectivity in 24 h, and the initial reaction rate was
2.2 mmolhÀ1 gÀ1. Benzaldehyde was observed at the initial
stage and the amount slowly decreased with time, suggesting
that benzaldehyde is an intermediate and that the reaction
proceeds by two consecutive steps (Scheme 1); 1) the trans-
To clarify the mechanism of the step 1, the effect of
oxygen concentration on the reaction rate was examined
(Figure S1). The reaction without O2 (under Ar) provided no
product, suggesting that oxygen is essential for the process.
The reaction rate increased linearly below ca. 20% with
increasing O2 concentration and became constant between 20
and 100%. Considering that the reaction is conducted under
air (O2 content is ꢁ 21%), molecular O2 is not involved in the
rate-determining step.
If the reaction proceeds by the oxidative dehydrogenation
mechanism, H2O is produced in step 1 as a by-product. The
behavior of H2O production was investigated by mass
spectrometry by the addition of 5 mL benzyl alcohol
(0.048 mmol) to CeO2 (0.58 mmol) under He at 373 K,
followed by the introduction of 14% O2 (Figure 2a). CeO2
was preheated at 873 K to remove surface H2O and CO2. The
introduction of benzyl alcohol provided a sharp signal of H2O
under He, but no change of H2 signal. The amount of detected
H2O is estimated to be 0.014 mmol before O2 introduction,
which corresponds to about 29% of the introduced benzyl
alcohol. This indicates that H2O is formed by CeO2 under He
and that oxygen atoms of CeO2 are used for the production of
Scheme 1. Reaction pathway for imine synthesis.
2
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Figure 2. a) MS profiles of H2O and H2 for the addition of benzyl alcohol to CeO2 under He and O2/He. b) FTIR spectra of methanol adspecies at
323 K on CeO2 calcined at different temperatures (873–1373 K). c) Reaction rate as a function of the amount of redox sites on CeO2. The values in
the graph are calcination temperatures in K.
H2O. Therefore, we conclude that step 1 proceeds by the
oxidative dehydrogenation mechanism. After O2 introduc-
tion, the second signal of H2O was observed and the amount
of H2O was estimated to be 0.027 mmol, corresponding to
about 56% of the introduced benzyl alcohol. When O2 was
introduced, the reaction proceeded to reach 85% conversion
of benzyl alcohol, which supports the above result that O2 is
necessary for the catalytic reaction progress. In addition, the
similar experiment was conducted on time-resolved in situ
FTIR (Figure S2). Benzyl alcohol was adsorbed on CeO2, and
a part of the adsorbed benzyl alcohol reacted on CeO2 under
He to afford benzaldehyde adspecies. Introduction of O2
resulted in the progress of the reaction, which is in good
agreement with the results of MS analyses.
A plausible reaction mechanism of imine formation from
benzyl alcohol and aniline on CeO2 is proposed (Scheme 2):
oxidative dehydrogenation of benzyl alcohol with oxygen
species occurs on the redox sites of CeO2 and Ce will be
reduced from tetravalent (+ 4) to trivalent (+ 3). The
produced benzaldehyde reacts with aniline over CeO2 to
afford the corresponding imine. CeO2 is regenerated by
oxidation of the reduced CeO2 with O2.
Most likely, oxygen species at the redox sites of CeO2 are
the reactive oxygen species. It is known that the concentration
of redox sites on CeO2 can be estimated by FTIR spectros-
copy with CH3OH as a probe molecule.[10] According to
a previous report by Lavalley and co-workers, methanol is
dissociated into methoxy species and H+ on CeO2 and the
band of methoxy species around 970–1085 cmÀ1 is assigned to
the potential redox sites on CeO2. To examine the correlation
between the reaction rates and the amount of redox sites, we
performed catalytic tests and FTIR experiments with CeO2
samples (Table S1 and Figure S3) calcined at different
temperatures (Figure 2b). The bands at 1107, 1052, and
1032 cmÀ1 were observed on all CeO2 samples and assigned
Scheme 2. Proposed reaction mechanism over CeO2.
Finally, the scope of alcohols and amines in the CeO2-
catalyzed imine synthesis was examined (Table 2). Benzyl
alcohol and substituted benzyl alcohols bearing an electron-
donating or -withdrawing group (entries 1 and 6–9) reacted
with aniline to afford the corresponding imines in high yields.
o-Methyl benzyl alcohol (entry 9) required a longer reaction
time, which could be due to the steric hindrance of the methyl
group at the o-position. Moreover, the imine formation from
benzyl alcohol and aniline could be performed under the
stoichiometric condition of amine/alcohol = 1 (entry 2) or
under nonsolvent conditions (entry 3). Aniline derivatives
bearing an electron-donating or -withdrawing group also
reacted to afford the corresponding imines in high yields
(entries 10–13). Furthermore, CeO2 can be reused without
remarkable loss of yield and selectivity at least three times
(entries 1, 4, and 5). By inductively coupled plasma atomic
emission spectroscopy (ICP-AES) no Ce species was detected
in the solution after the reaction (< 0.1%). Therefore, CeO2 is
an intrinsically reusable heterogeneous catalyst.
À
to n(C O) at on-top, doubly bridged, and triply bridged
sites, respectively. The redox site amount (mmolgÀ1) on
CeO2 was calculated from the band around 970–1085 cmÀ1
and the absorption coefficient of the methoxy species
(6.9 cmmmolÀ1).[10] A good linearity between the reaction
rates and redox site amount was obtained (Figure 2c). This
indicates that oxygen species at the redox sites of CeO2 are
active species for the oxidative dehydrogenation of benzyl
alcohol, and that this step is the rate-determining step. Above
all, we demonstrated for the first time that the redox property
of CeO2 can function in the liquid-phase organic reaction at
low temperature.
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Alcohol
Amine
Yield [%]
Selc. [%]
1
96(93[g])
80
99
90
92
97
89
>99
97
98
2[b]
3[c]
4[d]
5[e]
6
98
91
95
83
96
98
95
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10
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90
91
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12
98
92
99
96
13[f]
[a] Conditions: alcohol 1.0 mmol, amine 2.0 mmol, mesitylene 1.5 g,
CeO2 50 mg, 333 K, 24 h, air. [b] Amine/alcohol=1. [c] No solvent. [d] 1st
reuse. [e] 2nd reuse. [f] 48 h. [g] Isolated yield.
In summary, we have demonstrated that CeO2 acts as
a reusable and efficient heterogeneous catalyst for the imine
formation from benzyl alcohol and aniline even at 303 K and
the activity is more than 38-fold higher than those of other
simple metal oxides. Based on kinetic measurements as well
as MS and FTIR spectroscopy, the high activity can be
attributed to the redox properties of CeO2 at low temper-
ature. This is the first report of the redox property of CeO2
functioning in organic reactions at low temperature of 303 K.
These results could lead to new applications of CeO2 in
organic syntheses and bring about new design strategy of
metal oxide catalysts.
Received: September 29, 2014
Revised: October 22, 2014
Published online: && &&, &&&&
Keywords: alcohols · amines · cerium oxide · imine ·
.
redox chemistry
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Direct Imine Formation
M. Tamura,*
K. Tomishige
&&&& — &&&&
Redox Properties of CeO2 at Low
Temperature: The Direct Synthesis of
Imines from Alcohol and Amine
The amazing redox property of CeO2 is
utilized in the direct imine formation
from alcohols and amines even at 303 K.
The catalytic activity of CeO2 is more than
38-fold higher than that of other simple
metal oxides. CeO2 is a reusable hetero-
geneous catalyst and can be applied to
various alcohols and amines, resulting in
high yields (75–98%) and high selectiv-
ities (89–>99%).
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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www.angewandte.org
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