Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High-Surface-Area Mesoporous CeO2 with Exceptional Redox Property

Article abstract of DOI:10.1002/chem.202003915

High-surface-area mesoporous CeO₂ (hsmCeO₂) was prepared by a facile organic-template-induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO₂ and hsmCeO₂ after different thermal treatments were also investigated. XRD, N₂ physisorption, UV-Raman, H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, EPR spectroscopy, and X-ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO₂ calcined at 400 °C shows the highest specific surface area (158 m² g^?1), the highest fraction of surface coordinatively unsaturated Ce³⁺ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×10¹⁵ spins g^?1). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO₂ catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol (Formula presented.) h^?1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram-scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO₂ catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Full text of DOI:10.1002/chem.202003915

Accepted Article
Accepted Article
Title: Efficient Imine Formation via Oxidative Coupling at Low
Temperature Catalyzed by High-Surface-Area Mesoporous CeO2
with an Exceptional Redox Property
Authors: Shipeng Wu, Yinghao Wang, Qiue Cao, Qihua Zhao, and
Wenhao Fang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003915
Link to VoR: https://doi.org/10.1002/chem.202003915
01/2020

10.1002/chem.202003915
Chemistry - A European Journal
Efficient Imine Formation via Oxidative Coupling at Low
Temperature Catalyzed by High-Surface-Area Mesoporous
CeO2 with an Exceptional Redox Property
Shipeng Wu,[a] Yinghao Wang,[a] Qiue Cao,[ab] Qihua Zhao,[a] and Wenhao Fang[ab*]
[a]
S. Wu, Y. Wang, Prof. Q. Cao, Prof. Q. Zhao, Dr. W. Fang
School of Chemical Science and Technology, Key Laboratory of Medicinal Chemistry for
Natural Resource - Ministry of Education, Functional Molecules Analysis and
Biotransformation Key Laboratory of Universities in Yunnan Province, Yunnan University, 2
North Cuihu Road, 650091 Kunming, China
Email: wenhao.fang@ynu.edu.cn
[b]
Prof. Q. Cao, Dr. W. Fang
National Demonstration Center for Experimental Chemistry and Chemical Engineering
Education, Yunnan University, 650091 Kunming, China
Supporting information for this article is available on the WWW under http://######
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Abstract
A high-surface-area mesoporous CeO2 (hsmCeO2) is prepared by a facile organic template-induced
homogeneous precipitation method and shows excellent catalytic activity in imine synthesis in
absence of base from primary alcohols and amines in air atmosphere at low temperature. In
comparison, an ordinary CeO2 and hsmCeO2 after different thermal treatments are also investigated.
XRD, N2 physisorption, UV-Raman, H2-TPR, O2-TPD, EPR and XPS are used to mainly unravel
their structural and redox properties. hsmCeO2 calcined at 400 C shows the highest specific surface
area (158 m2 g1), the highest fraction of surface coordinatively unsaturated Ce3+ ions (18.2%), and
the highest concentration of reactive oxygen vacancies (2.41015 spins g1). In the model reaction of
oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the
hsmCeO2 catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol gceria1 h1) based on
a  99% yield at 60 C and 80 C respectively in air with no base additives. It can also work
effectively at room temperature of 30 C, as well as for the gram-grade synthesis. This is one of the
best results among all the ceria benchmark catalysts in literature. Moreover, the hsmCeO2 catalyst
displays a wide scope towards primary alcohols and amines with good to excellent yield of imines.
The influence of reaction parameters, the reusability of catalyst, and the reaction mechanism are
investigated.
Keywords
ceria catalyst; high surface area; imine synthesis; oxidative coupling; oxygen vacancy
Introduction
The fluorite-type ceria (CeO2) has been widely used as heterogeneous catalyst for various reactions
by taking advantages of its oxygen storage and/or oxygen diffusion properties.[1] The release and
uptake of oxygen by ceria enable the redox reactions and the formation of reactive oxygen
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vacancies (Vö) due to the easily reversible redox cycle between Ce3+ and Ce4+ ions, which has been
shown to be important for catalytic activity.[2] Ceria also exhibits good stability owing to its firm
fluorite structure. Therefore, the unique surface properties of ceria, i.e., abundant oxygen vacancy
and strong oxophilicity, can give rise to an enhancement of adsorption and activation of molecular
O2 and oxygen-containing organic compounds. However, such redox property of ceria is barely
used in catalysis at low temperature for organic reactions,[3] even though oxygen vacancies on ceria
are demonstrated to be mobile at room temperature.[4] In order to enhance the surface redox ability
of ceria, various strategies have been used, such as controlling morphologies,[5] modulating crystal
planes,[6] changing cerium precursors,[7] doping metals,[8] and treating with plasma atmospheres.[9] It
is well known for solid catalysts that high surface area usually contain more reactive sites. Thus,
preparing high-surface-area ceria can be probably a straighter and simpler way to enrich surface
Ce3+ ions and oxygen vacancy on ceria, but this remains much challenging. To the best of our
knowledge, very few attempts have been successful till date.[10]
Imines as a major class of nitrogen-containing organic intermediates are widely used in the
fields of agriculture, biology, chemistry and pharmacy.[11] Among various chemical syntheses of
imines, the direct coupling of alcohol and amine is regard to be the most promising route.[12]
Because alcohols are usually cheap and readily available and the byproduct of this reaction is water
and/or hydrogen. These are in line with the principle of sustainable chemistry. However, how to
effectively activate CH bond in an alcohol and simultaneously stabilize C=N bond in an imine
under mild reaction conditions is rather difficult. Metal oxides (Mg, Al, Mn, Fe, Co and Cu) as
inexpensive heterogeneous catalysts are highly desirable to oxidative coupling of alcohol and amine,
by making full use of their acidic-basic and/or redox properties to activate alcohol and O2.[13]
Interestingly, ceria catalysts have been recently explored for imine synthesis on the model
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reaction of oxidative coupling of benzyl alcohol and aniline.[3, 6-7, 10b, 14]. M. Tamura et al. first
reported the use of redox property of CeO2 for this reaction at low temperatures of 30 C and 60 C
but with moderate to high yields of benzylideneaniline (75% and 96%) in air atmosphere.[3a] They
recently demonstrated in a continuous work on the reaction mechanism that the reactive lattice
oxygen (Olatt.) species were mobile ones at the redox sites of CeO2, and the high mobility of Olatt.
was responsible for the high activity in the reaction.[14a] Z. Zhang et al. investigated the effect of
CeO2 crystal planes and disclosed the (110) plane to be the best among the (110), (100) and (111)
planes due to its strongest redox ability and highest concentration of oxygen vacancy. 97% yield of
imine was obtained under O2 atmosphere at 60 °C.[6] L. Geng et al. discovered CeO2 nanorods
anchored on mesoporous carbon (CeO2/MC) as an efficient catalyst, i.e., > 99% yield at 80 C in O2
atmosphere. The high activity was dependent on the abundant surface Ce3+ ions and the suitable
interaction between CeO2 nanorods and MC support, but unsupported CeO2 nanorods could only
afford 77.6% yield.[14b] J. Zhang et al. compared three different inorganic cerium precursors for
preparation of CeO2 catalyst and cerium nitrate was found to be the optimal choice probably related
with the highest proportion of oxygen vacancy on the prepared CeO2.[7] H. Zhang et al. elucidated
the effect of five morphologies for ceria catalysts on their behaviors in the model reaction. The
amorphous CeO2-5 catalyst prepared by precipitation at room temperature can achieve  99% yield
of imine in air atmosphere at 30 °C and 60 C. The higher acidic sites, specific surface area and
proportion of surface oxygen were demonstrated to be responsible for its high activity.[3b] Recently,
they reported a CeO2-Pfa-7 catalyst derived from Ce-based organometallic precursor (Ce-MOFs)
induced mesoporous CeO2, giving 92% yield of imine at 60 °C in air. The catalytic activity was
ascribed to the defect-enriched CeO2 with high specific surface area.[10b] Y. Long et at. developed
mesoporous hollow CeO2 microspheres (mh-CeO2) catalyst by a novel protective roasting strategy.
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This mh-CeO2 catalyst showed superior activity (98% yield at 60 °C in air) due to its super high
specific surface area and mesoporous hollow structure.[14c] Nonetheless, the reported catalytic
systems still have some obvious shortcomings, such as unsatisfying conversion or selectivity,
difficulty in reuse of catalysts, complicated morphology-controlled synthesis, and requirement of
pure O2. Moreover, the deep understanding of the reaction mechanism of ceria catalysis at low
temperature still remains unclear.
Herein, a high-surface-area mesoporous CeO2 (hsmCeO2) catalyst was successfully prepared by
a facile organic template-induced homogeneous precipitation method. The cheap surfactant
hexadecyltrimethylammonium bromide was used as a soft template. This catalyst showed
exceptional redox property which can significantly boost imine synthesis from the direct oxidative
coupling of alcohol and amine at low temperature in air atmosphere. The catalyst efficiency in form
of productivity toward imine ranked among the top for ever-reported ceria catalysts in literature.
The influence of preparation method (i.e., organic temperate and calcination temperature) and
reaction parameters (i.e., reaction temperature and time, amine/alcohol ratio, reusability and
scaling-up test) on the redox property of the hsmCeO2 catalyst was investigated. The
structure-activity relationship by combining the catalytic kinetic results and the role of Ce3+ ions
and oxygen vacancy in CeO2 was elucidated, which allowed openly discussing the reaction
mechanism of CeO2 catalysis in molecular level.
Results and Discussion
Comprehensive characterizations of hsmCeO2
The structures of CeO2 and hsmCeO2-T are analyzed by XRD and N2 physisorption. As shown in
Figure 1a, both CeO2 and hsmCeO2 present the characteristic pattern due to the cubic CeO2 phase
(JCPDS #43-1002). The diffraction peaks corresponding to the (111), (200), (220), (311), (222),
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(400), (311), (420) and (422) planes of CeO2 are well resolved. The mean crystal size of hsmCeO2 is
calculated to be of about 9.8 nm, which is smaller than that of the ordinary CeO2 (11.9 nm, Table 1).
As shown in Figure 1b, when the temperature of thermal treatment increases from 120 C to 600 C,
the crystal structure of hsmCeO2 remains stable, but as expected the crystallinity and the grain size
gently increase (Table 1).
(111)
(b)
(a)
(111)
(220)
(220)
(311)
(311)
(222)
(200)
(200)
hsmCeO2
(331)
(400)
(331)
(400)
(422)
(420)
(422)
(420)
600C
(222)
500C
400C
CeO2
300C
120C
CeO2: JCPDS 43-1002
10
20
30
40
50
60
70
80
90
10
20
30
40
50
60
70
80
90
2 theta ()
2 theta ()
Figure 1. XRD patterns of (a) CeO2 and hsmCeO2, (b) hsmCeO2 after different thermal treatments.
For the microstructure, all the hsmCeO2-T compounds, as well as CeO2, present a unique type
IV isothermal curve and an obvious H2 hysteresis loop at a relative pressure of 0.40.9 (Figure 2),
which is indicative of mesoporous structure. Furthermore, the pore size distributions confirm the
relatively uniform mesoporous structures of the series ceria. As listed in Table 1, as the thermal
treatment temperature increases from 120 C to 600 C, the specific surface areas and the pore
volumes of hsmCeO2-T first quickly rise and then dramatically fall, while the pore sizes slightly
decrease. The SBET and VP of hsmCeO2-400 reach the highest values of 158 m2 g1 and 0.18 cm3 g1
respectively, which are much larger than those of CeO2 (87 m2 g1 and 0.14 cm3 g1). This reveals
the successful preparation of mesoporous CeO2 with high specific surface by the organic
template-induced homogeneous precipitation method. And moreover, the calcination temperature is
found to strongly affect the textural property.
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(a)
(b)
hsmCeO2
hsmCeO2
CeO2
CeO2
25
0
5
10
15
20
30
0
0.2
0.4
0.6
0.8
1
Pore width (nm)
Relative pressure (P/P0)
(c)
(d)
600°C
500°C
600°C
500°C
400°C
400°C
300°C
300°C
120°C
120°C
0
5
10
15
20
25
30
0
0.2
0.4
0.6
0.8
1
Relative pressure (P/P0)
Pore width (nm)
Figure 2. N2 adsorption-desorption isotherms and the corresponding pore size distribution of (a, b)
CeO2 and hsmCeO2, (c, d) hsmCeO2 after different thermal treatments.
Table 1. The mean crystal size and the textural parameters of CeO2 and hsmCeO2-T
Catalyst
dcrystal [nm]a)
DP [nm]
6.2
SBET [m2 g1] VP [cm3 g1]
CeO2
11.9
87
63
0.14
0.10
0.16
0.18
0.13
0.08
hsmCeO2-120
hsmCeO2-300
hsmCeO2-400
hsmCeO2-500
hsmCeO2-600
9.3
6.3
9.6
135
158
98
4.7
9.8
4.8
10.0
4.8
10.4
48
5.6
a) Calculated by the Scherrer equation from the (111) plane of CeO2.
The redox properties of CeO2 and hsmCeO2 (calcined at 400 C as default) are
comprehensively analyzed by UV-Raman, H2-TPR, O2-TPD, EPR and XPS. The resonance Raman
effect is very sensitive to detect defects on the surface of ceria. As displayed in Figure S1, both
CeO2 and hsmCeO2 show a strong band at about 460 cm1, which is characteristic of F2g band of
fluorite nanocrystalline ceria.[15] In comparison with CeO2, the F2g band of hsmCeO2 shifts to a
lower frequency (465 cm1 vs 460 cm1) and becomes broader, which can be caused by the smaller
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particle size of hsmCeO2,[16] in consistent with XRD analysis. This phenomenon has been also
attributed to the presence of higher lattice distortion and oxygen vacancy (Vö) in ceria.[17] Besides,
the secondary bands due to the nanostructured ceria are also observed at about 260 cm1 and 330
cm1.[16] Notably, the band at 600 cm1 is characteristic of local vibration of oxygen vacancy on
Ce3+Vö complex in ceria lattice,[17] and a stronger intensity is obtained for hsmCeO2. Raman
results imply on the surface of hsmCeO2 that there is a higher amount of oxygen vacancies attached
to the coordinatively unsaturated Ce3+ ions.
Combined H2-TPR and O2-TPD are used to further investigate the redox properties of CeO2
and hsmCeO2. As reported in Figure 3a, hsmCeO2 shows three reduction peaks that can be
respectively attributed to the reduction of weakly adsorbed physical and/or chemical oxygen species
(Oads.), Ce4+ to Ce3+ on surface and Ce4+ to Ce3+ in bulk,[3b] from low to high temperatures. However,
CeO2 shows only two peaks related to the reduction of ceria. Notably, the reduction temperatures of
Ce4+ to Ce3+ from surface and bulk for hsmCeO2 are found to be lower than those of CeO2 (i.e., 495
C vs 512 C, 749 C vs 770 C). In addition, hsmCeO2 presents a higher total consumption of H2
(about 1.5 folds) and the fraction of surface consumption of H2 out of the total is measured to be 64%
(Table S2), i.e., about 2 folds over CeO2. These results clearly show an enhanced surface
reducibility of hsmCeO2. Moreover, As shown in Figure 3b, three types of oxygen species can be
identified, which can be assigned to physically adsorbed and weakly chemisorbed oxygen species

(Oads.) on ceria surface ( 180 C), surface reactive oxygen species (Osurf., i.e., O2 , O22 and O)
originated from oxygen vacancy (180550 C), and lattice oxygen species (Olatt.,  550 C),[18]
respectively. Notably, the desorption intensity of Osurf. species on hsmCeO2 is stronger. This
phenomenon reveals the presence of abundant reactive Osurf. species on CeO2 and hsmCeO2 and an
enhanced oxygen mobility of Osurf. for hsmCeO2, which is well consistent with Raman and TPR
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results.
350
(a)
(b)
Surf.
Ce4+Ce3+
Oads.
Osurf.
Olatt.
2
O2, O2
O
Bulk
Ce4+Ce3+
105
Reduction of Oads.
220
750
123
486
244
hsmCeO2
470
hsmCeO2
446
305
265
CeO2
800
CeO2
0
200
400
Temperature (C)
600
0
100 200 300 400 500 600 700 800
Temperature (C)
Figure 3. (a) H2-TPR and (b) O2-TPD profiles of CeO2 and hsmCeO2.
EPR is regarded to be powerful to identify and measure oxygen vacancy (Vö) in ceria-based
materials. Thereby, a deep determination of oxygen vacancy in CeO2 and hsmCeO2 is carried out. A
signal at about 3515 G (Figure 4a) is obtained and the free electron factor g is of 2.003 (Figure 4b),
which confirm the existence of oxygen vacancies in CeO2 and hsmCeO2.[19] The EPR spectrum of
hsmCeO2 grows more intense, which can be indicative of a higher amount of oxygen vacancy. The
quantitative analysis evidences that hsmCeO2 contains obviously higher concentration of oxygen
vacancy compared to CeO2 (2.4  1015 vs 1.7  1015, spins g1).
hsmCeO2
C (Vö): 2.41015 spins g1
(b)
(a)
Vö
g = 2.003
hsmCeO2
CeO2
1.99
CeO2
C (Vö): 1.71015 spins g1
3460
3480
3500
3520
3540
3560
1.98
2
2.01
2.02
2.03
Magnetic field (G)
g factor
Figure 4. EPR spectra of CeO2 and hsmCeO2.
Following that, the surface features of CeO2 and hsmCeO2 are further analyzed by XPS.
Particularly, hsmCeO2-600 with unusual structural and textural information is compared so as to
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unravel the influence of calcination temperature on its redox property. As shown in Figure 5, the Ce
3d spectra registered on different ceria can be resolved into three spin-orbits doublets Ce 3d3/2-Ce
3d5/2, respectively. The binding energy (BE) at about 885.3 eV (v1) and 903.9 eV (u1) are attributed
to Ce3+ species, while the other peaks are assigned to Ce4+ species.[3a] The O 1s spectra can be
resolved into two peaks. The peak at a lower BE of about 529.2 eV is characteristic of Olatt. species
(O2), while the peak at a higher BE of about 530.7 eV can be attributed to the reactive Osurf. species

(O2 and O) that are mainly associated with the active oxygen vacancy.[20] As listed in Table S2, the
BE values of different valence states for Ce and O show barely variations regardless of the use of
organic template or the calcination temperature. However, the quantitative analysis reveals that the
optimal hsmCeO2 calcined at 400 C shows the highest proportions of Ce3+ species (18.2%) and
Osurf. species (36.2%), followed by CeO2 and hsmCeO2-600. This implies that the template-induced
precipitation method is beneficial to create more active coordinatively unsaturated Ce3+ species and
Osurf. species on the surface of ceria and the calcination at higher temperature ( 500 C) would
inhibit those desirable reactive sites. These data are in good agreement with TPR results.
Ce 3d
v0
O 1s
u3
v3
u0
Olatt.
hsmCeO2-600
Osurf.
(O2, O)
（O2
)
u2
u1
v2
v1
hsmCeO2-600
hsmCeO2-400
hsmCeO2-400
CeO2
CeO2
535
533
531
B.E. (eV)
529
527
525
920 915 910 905 900 895 890 885 880 875
B.E. (eV)
Figure 5 Deconvoluted XPS spectra for Ce 3d and O 1s core levels of CeO2, hsmCeO2-400 and
hsmCeO2-600.
Oxidative coupling of benzyl alcohol and aniline
The oxidative coupling of benzyl alcohol and aniline is selected as a model reaction for evaluating
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the prepared ceria catalysts. As reported in Table 2, the ordinary CeO2 catalyst (entry 1) shows 70%
conversion of benzyl alcohol and > 99% selectivity of benzylideneaniline with benzaldehyde (< 1%)
as the only byproduct. The dried and calcined hsmCeO2 catalysts (entries 26) maintain the
excellent selectivity of imine but demonstrate different conversions of benzyl alcohol. On the
catalysts thermally treated between 120 C and 600 C, conversion of benzyl alcohol increases from
58% to 99% and then decreases to 36%, and productivity of imine also follows the similar tendency
from 1.59 mmol g1 h1 up to 2.75 mmol g1 h1 and then down to 1.00 mmol g1 h1. The optimum
result is obtained on the hsmCeO2-400 catalyst. Therefore, a volcano-like curve can be plotted
between the thermal treatment temperature and the activity of hsmCeO2-T catalysts. The hsmCeO2
catalyst (calcined at 400 C as default) shows about 1.5 folds of productivity toward imine over
CeO2. These catalytic results are found to be consistent with the specific surface areas of the series
ceria catalysts and can be probably related with their redox properties.
Reaction parameters including the molar ratio of aniline to benzyl alcohol, the reaction
temperature and the scale-up test are further investigated on the hsmCeO2 catalyst. As reported in
entries 4, 7 and 8, decreasing the ratio from 1.5/1 to 1/1 brings about a decline in both conversion of
benzyl alcohol (99% to 85%) and selectivity of imine (99% to 91%), while selectivity of
benzaldehyde increases (1% to 9%). This shows that properly excessive aniline is necessary to
promote the oxidative coupling reaction, which has been observed on other CeO2-based catalysts
and may be related with the weak basicity of aniline to facilitate selective oxidation of benzyl
alcohol to benzaldehyde.[14c, 21] Interestingly, the hsmCeO2 catalyst can work at room temperature of
30 C (entry 9). An excellent yield (98%) to imine can be obtained by simply extending the reaction
time to 24 h. In addition, a higher reaction temperature at 80 °C enables to greatly improve the
productivity of imine up to 5.55 mmol g1 h1 (entry 10), which is two folds of that obtained at
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60 °C and is also the highest activity ever reported for ceria catalysts in literature. Furthermore, the
hsmCeO2 catalyst shows a superior performance in the scale-up test (entry 11). A gram-grade
production of imine (1.79 g) with an excellent yield (99%) can be obtained at 60 C after 12 h.
For heterogeneous catalysis, the stability and reusability of a catalyst is important for potential
practical applications. The used hsmCeO2 catalyst after the post-treatment (see experimental details)
is evaluated in the consecutive oxidative coupling reactions under the same conditions. As shown in
Figure S2, the hsmCeO2 catalyst remains a stable yield of imine (99%) after five cycles, showing
the same activity as the fresh catalyst dose. However, sometimes the possible saturation of catalytic
activity may exist under the optimized conditions. To exclude that interference, conversion of
benzyl alcohol in the kinetic controlled region (i.e., the initial stage after 1 h) is further examined.
To one’s satisfaction, the hsmCeO2 catalyst demonstrates a considerable stability and reusability.
Table 2. Oxidative coupling of benzyl alcohol and aniline over CeO2 and hsmCeO2-T catalysts
Prod. 3 [mmol g1 h1]
Entry
Catalyst
Conv. 1 [%]
Select. 3 [%]
> 99
> 99
> 99
> 99
> 99
> 99
97
Select. 4 [%]
CB [%]
99.8
99.6
99.9
99.8
100
1
CeO2
70
58
82
99
73
36
98
85
99
100
99
< 1
< 1
< 1
< 1
< 1
< 1
3
1.91
1.59
2.26
2.75
2.00
1.00
2.64
2.14
0.46
5.55
0.91
2
hsmCeO2-120
hsmCeO2-300
hsmCeO2-400
hsmCeO2-500
hsmCeO2-600
3
4
5
6
99.7
99.9
99.9
100
a)
7
hsmCeO2
b)
8
hsmCeO2
91
9
c)
9
hsmCeO2
99
1
d)
10
11
hsmCeO2
> 99
> 99
< 1
< 1
99.8
99.7
e)
hsmCeO2
Reaction conditions: benzyl alcohol, 1 mmol; aniline, 1.5 mmol; catalyst, 90 mg; toluene, 5 mL; air, 1 atm; temperature,
60 C; time, 4 h. Carbon balance (CB) is reported.
a) Aniline, 1.25 mmol.
b) Aniline, 1 mmol.
c) Temperature, 30 C; time, 24 h.
d) Temperature, 80 C; time, 2 h.
e) Benzyl alcohol, 10 mmol; aniline (1.08 g), aniline, 15 mmol (1.40 g); catalyst, 900 mg; toluene, 10 mL; time, 12 h.
Comparison with benchmark catalysts
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The prepared hsmCeO2 catalyst is compared with typical ceria catalysts that have been so far
reported for oxidative coupling of benzyl alcohol and aniline. As summarized in Table 3, three ceria
catalysts including hsmCeO2 allow working in air atmosphere at room temperature of 30 °C (entries
13).[3] Nonetheless, the CeO2-5 catalyst deactivates (about 20% of yield) after four recycles even
though it can afford the highest productivity of imine based on a 99% yield.[3b] Though the
hsmCeO2 catalyst shows the second highest productivity of imine, the excellent yield (99%) is very
well remained during catalyst reuse. Another CeO2 catalyst produces a moderate yield of imine
(75%) with benzaldehyde as byproduct.[3a] In comparison with other ceria catalysts working at
60 °C (entries 410), the hsmCeO2 catalyst also exhibits the second highest productivity of imine.
Notably, the mh-CeO2 and the CeO2-Pfa-7 catalysts present very high productivity as well, but the
mesoporous hollow CeO2 microspheres[14c] and the organometallic precursor induced
defect-enriched mesoporous CeO2 catalysts[10b] have to be prepared by complex and well-defined
methods, and the exceptional productivity of imine largely depends on the unique morphologies of
ceria. On other ceria catalysts, much lower productivities of imine are obtained. Moreover, a low to
moderate formation of benzaldehyde is still detected and pure O2 is necessarily needed.[6, 14b]
Working at a higher temperature of 80 C, the hsmCeO2 catalyst can offer an excellent productivity
of imine in a fast response (entries 11 and 12). Therefore, the hsmCeO2 catalyst prepared in this
work by a simple template-induced homogeneous precipitation method is one of the best among all
the ever-reported ceria catalysts in terms of activity and stability.
Table 3. Comparison of oxidative coupling of benzyl alcohol and aniline to benzylideneaniline over typical ceria
catalysts
Prod. [mmol g1 h1]
Entry Catalyst
Oxidant [1 atm] Time [h] Yield [%]
Stab. Ref.
Temp. [C]
1
2
3
4
5
CeO2-5
hsmCeO2
CeO2
30
30
30
60
60
air
air
air
air
air
36
24
48
12
4
0.55
0.46
0.31
4.04
2.75
no
[3b]
 99
99
yes
n.d.
yes
yes
This work
[3a]
75
mh-CeO2
hsmCeO2
98
[14c]
99
This work
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6
CeO2-Pfa-7 60
air
air
air
O2
O2
O2
air
8
93
2.30
0.83
0.80
0.43
0.38
1.65
5.55
no
[10b]
[3b]
7
CeO2-5
CeO2
60
60
24
24
6
no
 99
96
8
n.d.
yes
yes
yes
yes
[3a]
9
CeO2/MC 60
CeO2 rod 60
CeO2/MC 80
hsmCeO2 80
77
[14b]
[6]
10
11
12
12
2
92
99
[14b]
This work
2
> 99
Substrate scope
Table 4 reports the oxidative coupling of aniline with various primary alcohols over the hsmCeO2
catalyst under the optimal reaction conditions. This catalyst exhibits good universality and activity
in converting substituted benzyl alcohols bearing electron-donating (MeO, Me and tBu) or
electron-withdrawing (NO2 and Cl) substituents, and high conversions ( 94%) of alcohols and high
yields ( 93%) of imines are obtained (3a-3f). Still, there are some minor differences in the
conversion and yield, which may be related with the property and location of the substituents. The
electrophilicity of benzylic carbon is reduced because the electron-donating groups can increase its
electron density to reduce its reactivity with nucleophilic amines. The electron-withdrawing group
is beneficial to remove hydrogen from the hydroxyl group. The deprotonated alcohol or alkoxide
can be oxidized and dehydrogenize benzyl carbon to form carbonyl compounds and react with
aniline immediately to form imines. In addition, the influence of substituent position (3b and 3c)
and steric hindrance (3d) should not be neglected. Besides substituted benzyl alcohols, some other
aliphatic alcohols, aromatic alcohols, and even alcohols containing heteroatoms are also examined
(3g-3l). Cyclohexylmethanol and pyridin-4-methanol afford 93% (3g) and 91% (3h) yields to the
desired imines, respectively. The activation of unbranched linear alcohol is rather difficult, but the
oxidative coupling of aniline with 1-octanol over the hsmCeO2 catalyst gives a satisfactory yield
(77%) of the corresponding imine (3i) when reaction time is extended to 8 h. In addition, the
hsmCeO2 catalyst can activate several complex alcohols including 3,4-(methylenedioxy)benzyl
alcohol, 2-naphthalenemethanol and 2-thiophenemethanol to offer a moderate to low yield of the
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corresponding imines (3j-3l) at 60 C in 8 h.
Table 4. Oxidative coupling of aniline with different primary alcohols over the hsmCeO2 catalyst
3a: 4 h, 100%, 99% 3b: 4 h, 99%, 98%
3c: 4 h, 97%, 96%
3d: 4 h, 96%, 95% 3e: 4 h, 100%, 99% 3f: 4 h, 94%, 93%
3g: 4 h, 94%, 93% 3h: 4 h, 92%, 91%
3j: 8 h, 75%, 74% 3k: 8 h, 63%, 62%
3i: 8 h, 78%, 77%
3l: 8 h, 29%, 28%
Reaction conditions: alcohol, 1 mmol; aniline, 1.5 mmol; catalyst,
90 mg; toluene, 5 mL; air, 1 atm; temperature, 60 C. Reaction time,
conversion of alcohol and yield of imine were reported.
Table 5 reports the oxidative coupling of benzyl alcohol with various primary amines over the
hsmCeO2 catalyst under the optimal reaction conditions. Similarly, this catalyst demonstrates good
universality and activity for the substituted anilines (3m-3q) and a high yield (≥ 93%) of desired
imines are obtained. As expected, the nucleophilicity of aniline can be enhanced by both
electron-withdrawing (Cl) and electron-donating (MeO and Me) groups. And the enhanced
nucleophilicity can promote nucleophilic attack, although there are some minor differences due to
the property and the location of substituents. In companion with aniline, the oxidative coupling of
benzyl alcohol with benzyl amine is examined and a high yield (96%) of imine (3r) is obtained,
which may be due to the high basicity of benzyl amine. Notably, the linear aliphatic octan-1-amine
would afford a high desired imine production (3s, 94%) in 4 h, compared with the yield of 3i (77%)
in 8 h. This implies that converting an alcohol to the aldehyde can be probably the rate-limited step
in the oxidative coupling reaction. t-Butylamine and cyclohexylamine afford a moderate yield to the
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desired imines (3t and 3u). In addition, pyridin-4-amine shows as expected a much lower yield to
the imine 3v (40%), which is maybe affected by the basicity of pyridine group. In summary, the
hsmCeO2 catalyst shows a wide scope and good to excellent activity for imine synthesis from the
direct oxidative coupling of various primary alcohols and amines. The electronic effect of
substituents, the nucleophilicity or electrophilicity of substrates, and even the acidic-basic
properties of substrates and catalysts are worthy of a comprehensive study in future work.
Table 5. Oxidative coupling of benzyl alcohol with different primary amines over the hsmCeO2 catalyst
3m: 4 h, 100%, 99% 3n: 4 h, 100%, 99% 3o: 4 h, 99%, 98%
3p: 4 h, 96%, 95%
3q: 4 h, 94%, 93%
3t: 4 h, 52%, 51%
3r: 4 h, 97%, 96%
3u: 4 h, 29%, 28%
3s: 4 h, 91%, 90%
3v: 4 h, 41%, 40%
Reaction conditions: benzyl alcohol, 1 mmol; amine, 1.5 mmol;
catalyst, 90 mg; toluene, 5 mL; air, 1atm; temperature, 60 C.
Reaction time, conversion of benzyl alcohol and yield of imine were
reported.
Discussion on the structure-activity relation and the plausible mechanism
In order to understand the reaction pathway, the time-course performances of the hsmCeO2 catalyst
are investigated by a series of controlled experiments. As plotted Figure 6a, oxidative coupling of
benzyl alcohol and aniline to benzylideneaniline shows a fast response. Conversion of alcohol and
yield of imine almost linearly increase within 2 h and 1.5 h at 60 C and 80 C respectively,
reaching 68% and 87%. > 99% yield of imine can be obtained at 4 h and 2 h, respectively.
Meanwhile, only traces of benzaldehyde (select. < 1%) are detected in the whole reaction process.
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As plotted in Figure 6b, condensation of benzaldehyde and aniline to benzylideneaniline also
proceeds very fast. Yield of imine reaches >99% in Ar or O2 atmosphere at 60 C within 2 h and at
80 C within 1 h, respectively. Notably, the same tendencies and results are obtained in the absence
of the hsmCeO2 catalyst. This reveals that condensation of benzaldehyde and aniline over hsmCeO2
is a non-catalytic process, which can be determined by the reaction temperature. This phenomenon
has been also observed by other reports.[12c, 22] As plotted in Figure 6c, aerobic oxidation of benzyl
alcohol over the hsmCeO2 catalyst proceeds gently. Conversion of benzyl alcohol gradually
increases within 3 h and reaches about 55%. Selectivity of benzaldehyde is of 100%. This catalytic
performance is stabilized by extending reaction time to 5 h. Thus a chemical equilibrium may be
reached under the given reaction conditions. Interestingly, adding 1.5 equivalent of aniline into the
reactor can give rise to a further oxidation of benzyl alcohol, and the conversion obviously increases
up to 87% by extending reaction time to 8 h. Notably, benzylideneaniline (98% in select.) rather
than benzaldehyde is found to be the dominant product. This result evidences that the presence of
excessive aniline can promote aerobic oxidation of benzyl alcohol to benzaldehyde, which is
consistent with the influence of the molar ratio of aniline to benzyl alcohol (Table 2). Once
benzaldehyde is produced from benzyl alcohol, it can be immediately condensed with aniline to
yield imine. This shows that benzaldehyde is an intermediate of the oxidative coupling reaction, and
aerobic oxidation of benzyl alcohol to benzaldehyde is considered to be the rate-determining step,
as proposed for CeO2 catalysts.[3, 14a] As plotted in Figure 6d, coupling benzyl alcohol and aniline
can also proceed in Ar atmosphere over the hsmCeO2 catalyst. But conversion of benzyl alcohol to
benzaldehyde stops after 3 h in inert gas, the conversion stabilizes at 52% after 4 h. When Ar is
replaced by air atmosphere, the oxidation process is initiated again and conversion of benzyl
alcohol rapidly goes up to 85% within 1 h. This observation indicates that both molecular O2 and
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Osurf. species play important roles in the rate-determining step. Osurf. species on the hsmCeO2
catalyst can drive oxidation of benzyl alcohol in inert gas and are gradually consumed till a full
depletion, then the reaction cannot proceed. When air is provided, molecular O2 can be activated on
the oxygen vacancy to generate Osurf. species that trigger the oxidation process.
(a)
(b)
100
80
60
40
20
0
100
80
60
40
20
0
hellow symbols: Ar
solid symbols: O2
□,: with catal. at 80C
○,: without catal. at 80C
◇,: with catal. at 60C
△,: without catal. at 60C
□: Conv.(PhCH2OH) at 80C
◇: Yield (PhC=NPh) at 80C
╳: Yield (PhCHO) at 80C
△: Conv.(PhCH2OH) at 60C
○: Yield (PhC=NPh) at 60C
╋: Yield (PhCHO) at 60C
Catal.
Ar
+
Catal.
+
+
Air
0
0.5
1
1.5
2
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
(c)
(d)
100
100
80
60
40
20
0
Benzyl alcohol
+ aniline
Benzyl alcohol
air
Ar
□: Conv.(PhCH2OH)
△: Yield (PhCHO)
○: Yield (PhC=NPh)
80
60
40
20
0
□: Conv.(PhCH2OH)
○: Yield (PhC=NPh)
0
1
2
3
4
5
0
2
4
6
8
Reaction time (h)
Reaction time (h)
Figure 6. (a) Time course for oxidative coupling of benzyl alcohol and aniline at different
temperatures, (b) time course for condensation of benzaldehyde and aniline in Ar or O2 with or
without catalyst, (c) time course for aerobic oxidation of benzyl alcohol without or with addition of
aniline, (d) time course for coupling of benzyl alcohol and aniline in Ar or air. Reaction conditions:
benzyl alcohol, 1 mmol; benzaldehyde, 1 mmol; aniline, 1.5 mmol; the hsmCeO2 catalyst, 90 mg;
toluene, 5 mL; air or Ar, 1 atm; temperature, 60 C unless specified.
In order to elucidate the structure-activity relationship, the apparent activation energy (Ea) of
oxidative coupling of benzyl alcohol and aniline by numerical regression assuming a valid
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Arrhenius law is measured for the typical catalysts. Before that, the influence of mass transfer is
investigated. As listed in Table S3, the default stirring rate (800 rpm) is found to be beyond the
mass-transfer limited range. Thereby, all the reactions are conducted under chemical control and the
mass transfer limitation can be excluded. The kinetic data in details can be found in Table S4. As
displayed in Figure 7a, the hsmCeO2 catalyst shows a much lower Ea of 39.2 kJ mol1, followed by
the ordinary CeO2 catalyst (55.4 kJ mol1) and the hsmCeO2-600 catalyst (65.0 kJ mol1), which is
well consistent with their catalytic performances (Table 2). Combined with the characterization
results, the catalytic activities of these ceria catalysts can be related with their surface areas and
specifically be greatly affected by their redox properties. Thereby, the productivity of imine is
plotted against the proportion of surface Ce3+ ions and Osurf. species and a good relationship can be
established (Figure 7b). The productivity of imine is found to be closely related with the proportion
of coordinatively unsaturated Ce3+ and reactive Osurf. on ceria catalyst. The highest specific surface
area of the hsmCeO2 catalyst is beneficial to afford more surface Ce3+ ions and oxygen vacancies
which can adsorb and activate O2 to generate more reactive Osurf. species as the active sites for the
oxidative coupling reaction.[3b, 14c] However, CeO2 and hsmCeO2-600 catalysts with inadequate
redox properties can only exhibit obviously lower catalytic activities.
40
(b)
(a)
3.5 R² = 0.9984
3
2.5
2
hsmCeO2-400
Ea: 39.2 kJmol−1
30
20
10
0
2.5
2
R² = 0.9995
CeO2
1.5
1
Ea: 55.4 kJ mol−1
R² = 0.9992
1.5
1
□: Prod./%
hsmCeO2-600
Ea: 65.0 kJ mol−1
0.5
0
△: Osurf./(Olatt.+Osurf.)/%
○: Ce3+/(Ce3++Ce4+)/%
0.5
2.8
2.9
1000/T (K1
3
3.1
)
Figure 7. (a) Arrhenius plot for conversion of benzyl alcohol. Reaction conditions: benzyl alcohol,
1 mmol; aniline, 1.5 mmol; catalyst, 90 mg; toluene, 5 mL; air, 1 atm; temperature, 6080 C; time,
19 / 24
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1 h. (b) The relationship between productivity of benzylideneaniline and the surface proportion of
Osurf. and Ce3+ species.
Therefore, as proposed in Scheme 1, a high concentration of oxygen vacancies (Vö) attached to
the coordinatively unsaturated Ce3+ ions can be created in the hsmCeO2 catalyst by using a
template-induced homogenous precipitation method. The Ce3+Vö complex in ceria lattice is of
great importance to the redox properties and the oxidation performances of ceria-based catalysts.[14a,
23] Besides, the higher proportion of Ce3+ ions in the hsmCeO2 catalyst can correspond to the lower
redox potential between Ce4+ and Ce3+ ions, which is characteristic of the catalyst with an enhanced
mobility of chemisorbed oxygen.[18b, 23] Oxygen vacancy is highly reactive to adsorb and activate


molecular O2 to generate Osurf. species (O2 , O) that are usually called oxygen deficiency sites. O2
and O species are well known to be the highly reactive electrophilic radicals and can give rise to
the promotion of oxidation reactions.[24] The Osurf. species on hsmCeO2 are proposed to be the active
sites for oxidative coupling of benzyl alcohol and aniline, in agreement with other ceria-based
catalytic systems.[3b, 14a, 23] Osurf. species can efficiently convert benzyl alcohol to benzaldehyde,
which is considered to be the rate-determining step. A high proportion of Osurf. already exist in the
hsmCeO2 catalyst, and such active sites can be simultaneously created on oxygen vacancy by
adsorption and activation of molecular O2. Subsequently, aniline can react immediately with the
produced benzaldehyde to rapidly yield benzylideneaniline via a non-catalytic step.
H2O
H2O
O2
O
O
Ce3+
O
O
O
O
Ce4+

O2 , O
O
O
O
O
O
oxygen vacancy (Vö)
Scheme 1. The plausible reaction mechanism of oxidative coupling of benzyl alcohol and aniline
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over the hsmCeO2 catalyst.
Conclusions
In conclusion, the hsmCeO2 catalyst with exceptional redox property is reported for efficient
oxidative coupling of primary alcohols and amines to imines in air atmosphere at low temperatures
without base additives. This catalyst shows a good reusability, a wide substrate scope, and a
potential scaling-up synthesis of imine. The template-induced homogeneous precipitation method
allows creating a high surface area and a uniform mesoporous structure for CeO2, on whose surface
abundant oxygen vacancies attached to the coordinatively unsaturated Ce3+ ions are generated. It is
demonstrated that molecular O2 can be activated and transformed on oxygen vacancy to the highly
reactive Osurf. species which can selectively convert benzyl alcohol to benzaldehyde, i.e., the
proposed rate-determining step. The subsequent condensation of benzaldehyde with aniline to imine
is a fast and uncatalyzed step. Compared to the ordinary CeO2, the hsmCeO2 catalyst bearing the
optimal proportion of Ce3+ ions and oxygen vacancy can significantly boost the catalytic oxidation
performance.
Experimental
The experimental details including chemicals, hsmCeO2 preparation, characterization methods, and
oxidative coupling reaction can be found in the supporting information.
Conflicts of interest
There is no conflict to declare.
Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 21763031 and
22062025), National Special Funds of China (No. C176220100063), Yunnan Fundamental Research
Projects (No. K264202007120), and Program for Excellent Young Talents of Yunnan University.
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10.1002/chem.202003915
Chemistry - A European Journal
Efficient Imine Formation via Oxidative Coupling at Low Temperature Catalyzed by
High-Surface-Area Mesoporous CeO2 with an Exceptional Redox Property
Chem. Euro. J. Year, Volume, Page – Page
Shipeng Wu,[a] Yinghao Wang,[a] Qiue Cao,[ab] Qihua Zhao,[a] and Wenhao Fang[ab*]
A high-surface-area mesoporous CeO2 catalyst prepared by an organic template-induced
homogeneous precipitation method shows an exceptional redox property and can boost imine
formation via the direct oxidative coupling of primary alcohols and anilines in air atmosphere at
low temperature.
H2O
H2O
air
O
O
Ce3+
O
O
O
O
Ce4+
O2, O
O
O
O
O
O
oxygen vacancy (Vö)
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