CeO2 nanorods anchored on mesoporous carbon as an efficient catalyst for imine synthesis

Article abstract of DOI:10.1039/c6cc05496j

CeO₂ nanorods anchored on mesoporous carbon exhibit high activity and stability in aerobic oxidative coupling of alcohols and amines to imines. The abundant surface Ce³⁺ and the suitable interaction between CeO₂ nanorods and the carbon support should be responsible for the excellent catalytic behaviors.

Full text of DOI:10.1039/c6cc05496j

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CeO2 nanorods anchored on mesoporous carbon as an efficient
catalyst for imine synthesis
Longlong Geng,a† Jinling Song ,a† Yahui Zhou,b Yan Xie,c Jiahui Huang,c Wenxiang Zhang,a Luming
Peng,*b and Gang Liu*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
CeO2 nanorods anchored on mesoporous carbon exhibit high surface-to-bulk atomic ratio, altered surface energies,
activity and stability in aerobic oxidative coupling of alcohol and quantum confinement effects etc.24-30 These features could
amine to imine. The abundant surface Ce3+ and the suitable significantly influence the redox properties of metal oxides.
interaction between CeO2 nanorods and carbon support should be Some successes have been achieved on the synthesis of CeO2
responsible for the excellent catalytic behaviors.
with controllable particle size and morphology, and examples
with enhancing redox catalytic activities were also reported.31-
34
Ceria (CeO2) is an interesting metal oxide that has wide
applications in fields such as catalysis, electrochemistry and
optics.1-5 In heterogeneous catalysis, materials based on CeO2
have been practically used in the production and purification
of hydrogen, and the purification of exhaust gases in three-
way automotive catalytic converters.6-10 CeO2 can also be
applied in catalyzing organic synthesis.11-14 Recently, an
interesting low temperature catalytic behavior of CeO2 was
reported in oxidative coupling imine synthesis.13 Imine is a kind
of nitrogen sources containing a reactive C=N moiety, which is
widely used in biological, agricultural and pharmaceutical
synthetic processes.15-17 Aerobic oxidative coupling of alcohol
and amine to imine is an environmentally friendly synthesis
route, in which air is used as oxygen source and only water is
produced as a by-product.18-23 CeO2 could catalyze this redox
However, the increase in surface energy with reduction in
particle size and exposing special facet usually gives rise to
aggregation and shape changes during a catalytic reaction,35, 36
which subsequently limit the recyclability of nanocrystal
catalysts. Most of these nanocrystal catalysts were confined to
fundamental researches. For practical application, much more
efforts are still needed to develop nanocrystal catalysts with
both high activity and high stability.
In this work, it was found CeO2 nanorods anchored on
mesoporous carbon (denoted as CeO2/MC) exhibit excellent
activity and stability in imine synthesis. The activity of
CeO2/MC is much higher not only than that of bulk CeO2, but
also than that of naked CeO2 nanorod catalysts. It is interesting
that the morphology of CeO2 nanorods of CeO2/MC could be
maintained during the reaction process, and the activity of
used CeO2/MC has no obvious change compared with that of
fresh one, exhibiting a relatively high stability.
reaction even at 30 ºC without using any inorganic or organic
base as additives.13 The shortcoming is that the activity of
current CeO2 materials is relatively low and not satisfied for
large-scale application.
CeO2/MC was prepared with a wet impregnation method
followed by a fine thermal-treatment in argon condition
(details see in supporting information). The loading amount of
CeO2 is 10 wt%. A kind of mesoporous carbon (MC) with pore
size of 6.5 nm and surface area of 1392 m2·g-1 was used as
carbon support. This carbon material was prepared with a
modified hard-template route, which was reported by our
group previously.37, 38 Fig. 1 shows the SEM and TEM images of
CeO2/MC. It reveals that CeO2 nanorods were with a diameter
of 20-35 nm and a length within 200-400 nm. These CeO2
nanorods are highly dispersed on the surface of carbon
support. HRTEM image demonstrated that CeO2 nanorods
possess well crystallinity (Fig. 1d). From the Fourier transform
(Fig. 1d inset) analysis, three kinds of lattice fringe directions
attributed to (111), (002), and (220) were observed for the
anchored CeO2 nanorods. The interplanar spacing of 0.32 and
Controlling size and morphology are regarded as efficient
strategies to tune the catalytic performance of metal oxides. It
usually brings some exceptional qualities, including high
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B
A
MC
CeO2/MC
CeO2/MC
CeO2 nanorod
CeO2 nanorod
CeO2
× 1/3
× 1/3
CeO2
MC
JCPDS 34-0394
200
70
400
600
)
10
20
30
40
(deg)
50
60
Raman shift (cm-1
2θ
Peak II
O 1s
Ce 3d
Peak III
ꢀ'
D
C
ν'
ꢀ''
ꢀ
Peak I
ꢀ'''
ν
ν'''
ν''
CeO2/MC
CeO2/MC
CeO2 nanorod
CeO2
CeO2 nanorod
CeO2
920
910
900
890
880
538
536
534
532
530
528
Fig. 1 SEM (a, b), TEM (c) and HRTEM (d) images of CeO2/MC catalyst.
Bindng energy (EB/eV)
Binding energy (EB/eV)
Fig. 2 (A) XRD patterns and (B) Raman spectra of MC support, CeO2/MC, CeO2
nanorod and bulk CeO2 catalysts. (C, D) Ce 3d and O 1s XPS spectra of CeO2/MC, CeO2
nanorod and bulk CeO2 catalysts.
0.28 nm was identified in the HRTEM image, which is
corresponded to the lattice fringes of the (111) and (002)
facets, respectively. This result is consistent with that of CeO2
nanorods prepared by hydrothermal method,39 indicating
CeO2/MC has similar exposed crystal facets with CeO2
nanorods.
respective peaks, the fractions of the surface Ce3+ followed the
trend of CeO2/MC (0.48) > CeO2 nanorods (0.13) > bulk CeO2
(0.06). As for the O 1s spectra, the two peaks centered at
530.1 eV (peak I) and 531.9 eV (peak II) could be ascribed to
“O2-” ions in lattices and surface -OH or oxygen vacancies. The
new peak (peak III) at 533.5 eV for CeO2/MC can be attributed
to the oxygen groups remained on MC support. The area ratio
of peak II/ peak I is 7.17 for CeO2/MC, which is much higher
than that of CeO2 nanorods (0.76) and bulk CeO2 (0.25).
Fig. 2 shows the XRD patterns of CeO2/MC, MC support,
reference CeO2 nanorods and bulk CeO2. As for CeO2/MC, the
broad peak centered at 24.5° and 44° can be assigned to the
diffractions from (002) and (100) graphite planes of carbon
support.40 The small peaks at 28.5 and 47.5° could be assigned
to the diffractions from (111) and (220) planes of CeO2.36 As
for reference samples of CeO2 nanorods and bulk CeO2, five
relatively sharp diffraction peaks at 2θ value of 28.5, 33.1, 47.5,
56.3, and 59.1° can be observed, which match well with the
diffractions from (111), (200), (220), (311) and (222) lattice
planes of cubic phase CeO2 (fluorite structure, JCPDS 34-
0394).32 These results suggest that CeO2 in all these samples
possesses cubic structure.
Oxidative coupling of benzyl alcohol and aniline to imine
was carried out to detect the catalytic performance of
CeO2/MC, naked CeO2 nanorods and bulk CeO2. Fig. 3 shows
that all these samples are active in the title reaction. The
samples with nanorod morphology exhibit much higher
activities than bulk one. The reactivity trend in this system
follows CeO2/MC > CeO2 nanorods > bulk CeO2 > MC. As for
CeO2/MC, a 99.2 % yield of imine was obtained after 2 h
Raman spectra showed that all these CeO2 samples had a
strong band at ~455 cm-1 and a weak band at ~600 cm-1. The
band at ~455 cm-1 is ascribed to the Raman-active vibrational
mode (F2g) of the fluorite-type structure, while the band at
~600 cm-1 demonstrates the intrinsic oxygen vacancies due to
the existence of Ce3+ in CeO2.41 As for CeO2/MC, the red shift
of the later band to 645 cm-1 should be due to the presence of
interaction between CeO2 nanorod and MC support. According
to the literature, the ratio of the integral area of the two peaks,
i.e. A600/A455, can be used to evaluate the difference of oxygen
vacancies.42, 43 The value of A600/A455 is 0.12, 0.05, and 0.03 for
CeO2/MC, CeO2 nanorods and bulk CeO2. This result suggests
that CeO2/MC possesses much more oxygen vacancies than
other two samples. XPS measurement was also carried out to
detect the chemical state of Ce on the surface CeO2/MC, CeO2
nanorods and bulk CeO2. According to the literature,44, 45 eight
peaks associated with the 3d electrons of Ce can be fitted,
which corresponded to four pairs of spin-orbit doublets. Peaks
marked as u/v, u’’/v’’ and u’’’/v’’’ are assigned to the Ce4+
species, while peaks denoted as u’/v’ are attributed to the Ce3+
species. Calculated from the integrated areas of the
reaction at 80
ºC. In addition, the catalytic performance of
100
80
CeO2/MC
CeO2 nanorod
60
40
CeO2
MC
20
0
0
2
4
6
8
Reaction time (h)
Fig. 3 Time dependence of imine formation from oxidative coupling of
benzyl alcohol and aniline over CeO2/MC, CeO2 nanorod, bulk CeO2 and MC
catalysts. Reaction conditions: benzyl alcohol (1 mmol), aniline (2 mmol),
catalyst (0.3 g), solvent (toluene 10 mL), 80 ºC, air (1 bar).
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Table. 1 Oxidative coupling of alcohols and amines to imines over CeO2/MC
catalyst a
Fig. 4 (A) Reusability of CeO2/MC in the imine synthesis processes and SEM
images (B) of the fresh and reused (four cycles) CeO2/MC catalysts.
99 %
98 %
94 %
96 %
90 %
CeO2/MC at different temperature was also detected (Fig. S3).
The yield of imine was 77.6% at 60 ºC and 46.5% at 40 ºC,
respectively. This result is much higher than other non-noble
metal catalysts and CeO2 with other morphologies in the imine
synthesis (Table S1 and Fig. S4). Based on the initial reaction
rate, the apparent activation energy (Ea) of reaction over
CeO2/MC, CeO2 nanorods and bulk CeO2 was also calculated
(Fig. S5). The Ea of CeO2/MC is about 26.04 kJ·mol-1, which is
much lower than that of naked CeO2 nanorods (57.89 kJ·mol-1)
and bulk CeO2 (64.86 kJ·mol-1).
93 %
92 %
89 % b
95 %
88 %
In order to confirm the heterogeneous nature of the
CeO2/MC catalyst, a hot filtration test was carried out. The
result showed that no subsequent conversion of benzyl alcohol
was detected in the filtrate, proving that CeO2/MC is a really
heterogeneous catalyst (Fig. S6). In addition, multiple aerobic
oxidation cycles were also carried out to examine the
recoverability of CeO2/MC (Fig. 4A). After four cycles test, the
catalyst still maintains a relatively high activity and no obvious
changes can be observed. SEM measurement shows that CeO2
still maintains the morphology of nanorod and highly
dispersed on the surface of carbon support. No aggregation of
CeO2 species could be observed in the CeO2/MC (Fig. 4B). All
these results suggest that CeO2 nanorod in the CeO2/MC be
stable during the reaction. This should be an impressive
advantage of CeO2/MC, since the aggregation of nanorod
during a catalytic reaction even the preparation process is a
major drawback limiting their application.
90 %
92 %
a Reaction conditions: alcohol (1 mmol), amine (2 mmol), catalyst (0.3 g), solvent
(toluene 10 mL), 80 ºC, air (1 atm). Numbers in parentheses are imine yield.
b
Reaction time: 6 h.
(0.92 mmol·g-1), lactonic (0.91 mmol·g-1), phenolic groups (1.40
mmol·g-1). These functional groups may serve as anchor sites
for CeO2 nanorod. In this work, 13C cross polarization (CP)
magic angle spinning (MAS) NMR and 1H MAS NMR
experiments were used to confirm the fact that a variety of
oxygen-containing functional groups are present on the
surface of MC (Fig. S7). The 13C NMR spectrum of MC shows a
major peak at 127 ppm with a shoulder at approx. 170 ppm,
corresponding to sp2 carbon in the bulk part of MC and -
COOH/-COOR environments of MC, respectively. In addition,
relatively weak signals at lower frequencies, i.e., 10~70 ppm
can be observed. These peaks can be ascribed to the alkyl
groups in the surface -COOR sites. The absence of the alkyl
groups in the 13C NMR spectrum of CeO2/MC (10-70 ppm) may
indicate the reaction between carboxylic and ester sites of MC
and rich hydroxyl groups of CeO2.46 Large amount of Ce3+
should form in this process due to the reduction behaviour of
carbon support in the inert atmosphere. And this interaction
between CeO2 and MC is very important for the improvement
of catalytic performance.
The general application of CeO2/MC catalyst was also
investigated. At least twelve kinds of imines could be obtained
by oxidative coupling of different alcohols with amines. Table 1
shows that CeO2/MC catalyst is active in the reactions using
benzyl alcohol derivatives with either electron-rich- or
electron-poor- substituent groups, and excellent imine yields
were obtained especially for 4-chlorobenzyl alcohol (98 %
imine yield after 2 h reaction) and p-methyl benzyl alcohol (96 %
imine yield after 8 h reaction). Notably, CeO2/MC catalyst also
worked well in catalysing oxidative coupling of benzyl alcohol
with substituted anilines, and obtained a high imine yields. In
addition, aliphatic amines such as cyclohexane and n-
butylamine can also reacted efficiently with benzyl alcohol to
afford the desired imines in good yields. The above catalytic
tests clearly show that CeO2/MC is a robust catalyst in imine
synthesis with oxidative coupling route.
The Ce3+ species are tightly correlated with redox
properties and then the catalytic performance of Ce-based
catalysts. In our case, O2-TPD showed an improvement of
oxygen adsorption capacity over CeO2/MC (Fig. S8). H2-TPR
reflects that CeO2/MC possess a relatively high reduction
temperature compared with that of CeO2 nanorods and bulk
Our previous work has shown that large amount of oxygen-
containing functional groups (detected by Boehm titration)
present on the surface of MC support, including carboxylic
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improve the catalytic performance for molecular O2 activation
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the relatively low reaction temperature. The oxidation of
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This work was supported by the National Science
Foundation of China (Grant No. 21473073, 21473074 and
21573103) and "13th Five-Year" science and technology
research of the Education Department of Jilin Province
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