One-pot synthesis of Pd-promoted Ce-Ni mixed oxides as efficient catalysts for imine production from the direct: N -alkylation of amine with alcohol

Article abstract of DOI:10.1039/c8cy01857j

Ce-Ni mixed oxides with different Ni/Ce molar ratios, promoted by a rather low amount of Pd (0.2 wt%), were prepared by a simple one-pot precipitation method. The thus-synthesized CeNi_XO_Y and Pd-CeNi_XO_Y catalysts were applied respectively to the direct N-alkylation of amine and alcohol via oxidative coupling, under O₂ in the absence of basic additives. The CeNi_XO_Y catalyst could provide a >99% yield of imine in the model alkylation reaction of aniline with benzyl alcohol at 100 °C. The Pd-promoted Pd-CeNi_XO_Y catalyst surprisingly showed an enhanced imine yield (>99%) at a mild temperature of 60 °C. This catalyst exhibited good reusability, and moreover, demonstrated high performance towards imine synthesis from various amines with alcohols. The presence of Pd species and the Ni/Ce molar ratio showed a synergistic impact on the conversion of aniline, as well as on the product selectivity, which was believed to be related to the improvement on the intrinsic activity of the oxidation of benzyl alcohol to benzaldehyde. Various physicochemical techniques, including ICP-MS, XRD, N₂ adsorption-desorption, UV-Raman, H₂-TPR, TEM, EPMA and XPS, were employed to study mainly the composition, structure and redox properties of the catalysts. The proportion of Ce³⁺ species and oxygen vacancies, which can be manipulated by the Ce-Ni redox system and the interaction between Ce and Ni, was crucial to the selective activation of alcohols in the presence of reactive amines. The activation of alcohol on the highly reactive Pd⁰ species was the other crucial factor.

Full text of DOI:10.1039/c8cy01857j

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One-pot synthesis of Pd-promoted Ce–Ni mixed
oxides as efficient catalysts for imine production
from the direct N-alkylation of amine with alcohol
Cite this: DOI: 10.1039/c8cy01857j
Mengyuan Zhang,†a Shipeng Wu,†a Longchun Bian,a
ab
Qiue Caoab and Wenhao Fang
*
Ce–Ni mixed oxides with different Ni/Ce molar ratios, promoted by a rather low amount of Pd (0.2 wt%),
were prepared by a simple one-pot precipitation method. The thus-synthesized CeNiXOY and Pd–CeNiXOY
catalysts were applied respectively to the direct N-alkylation of amine and alcohol via oxidative coupling,
under O2 in the absence of basic additives. The CeNiXOY catalyst could provide a >99% yield of imine in
the model alkylation reaction of aniline with benzyl alcohol at 100 °C. The Pd-promoted Pd–CeNiXOY cata-
lyst surprisingly showed an enhanced imine yield (>99%) at a mild temperature of 60 °C. This catalyst
exhibited good reusability, and moreover, demonstrated high performance towards imine synthesis from
various amines with alcohols. The presence of Pd species and the Ni/Ce molar ratio showed a synergistic
impact on the conversion of aniline, as well as on the product selectivity, which was believed to be related
to the improvement on the intrinsic activity of the oxidation of benzyl alcohol to benzaldehyde. Various
physicochemical techniques, including ICP-MS, XRD, N2 adsorption–desorption, UV-Raman, H2-TPR, TEM,
EPMA and XPS, were employed to study mainly the composition, structure and redox properties of the cat-
alysts. The proportion of Ce3+ species and oxygen vacancies, which can be manipulated by the Ce–Ni re-
dox system and the interaction between Ce and Ni, was crucial to the selective activation of alcohols in
the presence of reactive amines. The activation of alcohol on the highly reactive Pd0 species was the other
crucial factor.
Received 5th September 2018,
Accepted 20th November 2018
DOI: 10.1039/c8cy01857j
rsc.li/catalysis
proach is the direct N-alkylation of amines with alcohols to
synthesize alkylamines, because alcohols are inexpensive and
1. Introduction
The N-alkylation of amines is regarded as an important or-
ganic reaction in synthetic chemistry.1–6 Imines, as well as
secondary amines, represent important synthetic intermedi-
ates that have been widely applied to agrochemicals, bioactive
compounds, dyes, pharmaceuticals, and polymers.7–13 The
N-alkylation process mostly used in industry is the coupling
of amines with alkyl halides with the assistance of stoichio-
metric amounts of organic bases.14 However, this process
usually leads to environmental issues due to the usage of haz-
ardous reagents and/or generation of hazardous by-products.
Meanwhile it often receives criticism from a lack of selectivity,
i.e., a mixture of primary, secondary and tertiary amines is
likely produced. Alternatively, an environmentally friendly ap-
largely available and moreover, only water is generated as a
clean by-product. The so-called “borrowing hydrogen” mecha-
nism via dehydrogenative coupling transfers temporarily a H2
molecule to the catalyst from an alcohol to yield an aldehyde
that behaves more reactively towards amines, and the catalyst
thereafter returns the “borrowed H2” by reducing the imines
to secondary amines.15–19 Previously, the reductive amination
of carbonyls proceeded via an intermediate imine or enamine
yielding water as the sole by-product.20 In spite of that, unlike
reductive amination, no extra reductive H2 is required for im-
ine or enamine hydrogenation. The borrowing hydrogen
mechanism can overcome the lack of reactivity of the reduc-
tive amination catalysts by the temporary removal of H2 from
the alcohol by the catalyst. However, this route usually based
on organometallic homogeneous catalysts requires basic addi-
tives, high reaction temperature, and long reaction time.21,22
On the other hand, oxidative coupling with a faster response
and a lower temperature undergoes a tandem mechanism of
the aerobic oxidation of alcohols followed subsequently by
the nucleophilic substitution of the reactive CO group of
the intermediate aldehydes by the NH2 group of amines to
a 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 Prov-
ince, Advanced Analysis and Measurement Center, Yunnan University, 2 North
Cuihu Road, 650091 Kunming, China. E-mail: wenhao.fang@ynu.edu.cn
b National Demonstration Center for Experimental Chemistry and Chemical
Engineering Education, Yunnan University, China
† M. Zhang and S. Wu contributed equally to this work.
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yield imines.7–11 Discovery of cost-effective chemical transfor-
mations is one of the most important objectives in catalysis
science. It is generally needed to develop smart catalysts and
advanced technologies to efficiently produce target chemicals
and to simultaneously minimize by-products.
As a fluorite-type oxide, ceria (CeO2) has been applied to
many different reactions by taking advantage of its oxygen
storage and/or oxygen diffusion properties.23 The release and
uptake of oxygen by ceria allows it to participate in redox re-
actions, and the enhancement of the formation of oxygen va-
cancies due to the reduction of cerium cations has been
shown to be important for catalytic activity.24–26 Recently,
ceria and ceria-based materials have been reported as active
precipitation synthesis and the diverse applications of ceria-
containing catalysts call for a new preparation method and
compound formulation toward imine synthesis, in addition
to the well-developed CeO2 systems. Herein, we reported that
Ce–Ni mixed oxides can selectively catalyze the direct
N-alkylation of aniline and benzyl alcohol without basic addi-
tives at 100 °C in the presence of O2 for N-benzylideneaniline
production. When the CeNiXOY oxides were promoted by a
very small amount of Pd (0.2 wt%) using a simple one-pot
synthesis, the thus-prepared Pd–CeNiXOY catalyst showed at a
low temperature (60 °C) an enhanced yield of imine (>99%)
and a good reusability (4 times). The Pd–CeNiXOY catalyst
demonstrated high yields towards imine synthesis from vari-
ous amines and alcohols. To the best of our knowledge, there
has been no report on the preparation and application of
such Pd–CeNiXOY materials. The influence of the Ni/Ce molar
ratio, the reaction time and the aniline/benzyl alcohol molar
ratio on the catalytic performance was studied. The potential
activity of the Pd–CeNiXOY catalyst towards imine production
from the direct N-alkylation of various amines and alcohols
was investigated. The possible role of Pd and Ni in relation
to the redox properties of the Pd–CeNiXOY catalyst was stud-
ied, and the relationship between the catalyst properties and
activity was elucidated.
and
reusable
heterogeneous
catalysts
for
N-benzylideneaniline formation from the direct condensation
of benzyl alcohol and aniline, the most representative model
reaction of imine synthesis.27–31 M. Tamura et al. showed
that CeO2 with redox properties due to reactive oxygen spe-
cies could catalyze this reaction in air at low temperature.27
Such CeO2 can be applied to the alkylation of various alco-
hols and amines but with relatively lower activities. Z. Zhang
et al. reported the effect of CeO2 crystal planes on the oxida-
tive coupling of alcohols and amines and disclosed that the
(110) crystal plane of CeO2 was more active than the (100)
and (111) planes due to its higher concentration of oxygen va-
cancies.28 Moreover, H. Zhang et al. prepared five different 2. Experimental
nanocerias by using various cerium sources and methodolo-
2.1. Materials and reagents
gies and compared their performances for the direct coupling
All the chemicals were obtained from commercial sources
of alcohol and amine to the corresponding imines under air
atmosphere at low temperature.29 CeO2 prepared by co-
precipitation at room temperature showed the best catalytic
activity due to its higher acidic sites, specific surface area
and proportions of surface oxygen. However, the morphology-
controlled synthesis of ceria remains complicated and diffi-
cult. J. Zhang et al. studied the influence of different cerium
precursors on the number of surface oxygen vacancies of
CeO2.30 The results indicated that CeO2 derived from cerium
nitrate showed the best catalytic activity for imine synthesis,
which could be related to the highest proportion of surface
oxygen vacancies. L. Geng et al. reported CeO2 nanorods an-
chored on mesoporous carbon as an efficient and stable cata-
lyst for imine synthesis.31 The abundant surface Ce3+ species
and the suitable interaction between CeO2 and carbon were
assumed to be responsible for the high activity. In addition
to imine synthesis via the oxidative coupling, A. Corma et al.
and Z. Yan et al. respectively reported that Au/CeO2 (ref. 32)
and Pd/CeO2 (ref. 33) catalysts allowed production of second-
ary amines via the direct N-alkylation of a primary amine with
an alcohol by a borrowing-hydrogen methodology.
and used as received. PdCl2 (60% on Pd basis) and NaBH4
(98%) were purchased from Alfa Aesar. CeIJNO3)3·6H2O
(99.5%), NiIJNO3)2·6H2O (98%), aniline (AR) and benzyl alco-
hol (AR) were obtained from Acros Organics. Trimethylamine
(AR), methanol (AR) and toluene (GR) were purchased from
Shanghai Aladdin Bio-Chem Technology Co., LTD.
2.2. Catalyst preparation
Pd promoted Ce–Ni mixed oxides denoted as Pd–CeNiXOY
were synthesized by a one-pot precipitation method. Typi-
cally, in 100 mL of methanol, PdCl2 (0.2 wt%) and CeIJNO3)3
·6H2O (10 mmol) were dispersed. And then NiIJNO3)2·6H2O
(Ni/Ce = 0, 0.02, 0.1, 0.3, 0.5, 1 and 5, in mol) was added and
dissolved. The metal precursors were afterwards precipitated
by trimethylamine till pH = 9. The suspension was stirred at
room temperature for 1 h and then aged at 65 °C under a re-
flux condenser for 18 h. The solids were separated and
washed with excess deionized water to remove trimethyl-
amine. The collected solids were thereafter dispersed in de-
ionized water (60 mL) and in situ reduced by a freshly made
NaBH4 solution (NaBH4/Pd = 5 in mole, concentration: 0.5
mol L−1). The solids were recovered by filtration and washed
with excess deionized water to remove Na+ ions. The Pd–
CeNiXOY catalyst was finally dried at 80 °C under vacuum for
5 h. This protocol was also applied to the synthesis of
CeNiXOY oxides (Ni/Ce = 0, 0.02, 0.1, 0.3, 0.5, 1 and 5, in mol)
but without reduction treatment.
As is known to all, mixed oxides comprising transition
metal and rare earth ions usually demonstrate a large num-
ber of surface and bulk oxygen vacancies and show enhanced
oxygen storage capacity.26,34,35 Adding cheap transition
metals to ceria leads most often to different mixed oxides
which show unique redox properties largely depending on
the tunable chemical compositions. Moreover, the simple co-
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2.3. Characterization methods
namine,0  namine,t
The precise metal loadings (i.e., Pd, Ce and Ni) were mea-
sured by the inductively coupled plasma mass spectrometry
(ICP-MS) technique on an Agilent 7500a apparatus. Specific
surface areas were determined by N2 adsorption–desorption
at 77 K on a Quadrasorb evo™ surface area and pore size
analyzer using the Brunauer–Emmett–Teller (BET) method.
X-ray diffraction (XRD) analysis was conducted using a
Rigaku TTR III diffractometer with Cu Kα radiation (λ =
1.5418 Å) and a beam voltage of 40 kV. The patterns were
registered in the 2θ domain (5–90°) with a measured step of
Conv. % 


namine,0
nalkylamine
Select. % 


namine,0  namine,t
nimine
Yield % 


namine
0.02° and
a time integration of 0.2 s. Temperature-
programmed reduction in H2 (H2-TPR) was carried out on a
Micromeritics Autochem II chemisorption analyzer. The 3. Results and discussion
sample was treated in 5% H2/Ar (in vol) mixed gas with a
3.1. Characterization of the Pd–CeNiXOY catalysts
flow rate of 30 mL min−1. The temperature was increased to
850 °C at a rate of 10 °C min−1. UV-Raman spectra were
recorded on a LabRam Infinity Raman spectrometer (Horiba
Jobin Yvon) using a visible laser with an output laser wave-
length of λ = 532 nm. X-ray photoelectron spectroscopy
(XPS) analysis was performed under ultra-high vacuum on a
Thermo Scientific Escalab 250Xi system provided with Al Kα
radiation. The binding energy shift due to the surface charg-
ing was adjusted using a reference to the C 1s line at 284.5
eV. High resolution transmission electron microscopy (HR-
TEM) analysis in bright field and dark field was conducted
on a Philips-FEI Tecnai F30 field emission electron micro-
scope with an acceleration voltage of 300 kV. Electron probe
microanalysis (EPMA) was carried out using a JXA-8230
EPMA with an accelerating voltage of 20 kV for back
scattered electron imaging and for the Ce, Ni and O ele-
mental X-ray mapping.
ICP-MS and N2 adsorption–desorption. Table 1 reports the
precise Pd loading and Ni/Ce molar ratios of the Pd–CeNiXOY
catalysts determined by ICP-MS. Both the Pd loading and Ni/
Ce ratios are found to be very close to the theoretical values
that are used in the synthetic protocol, which suggests that
the employed one-pot precipitation is effective. All the cata-
lysts display relatively large specific surface areas between 65
and 125 m2 g−1, which depend on the Ni/Ce molar ratio. The
Pd–CeNi1OY catalyst shows the highest surface area. Note that
the parent CeNiXOY mixed oxides display highly similar sur-
face areas to Pd-containing catalysts. This indicates that the
presence of the Pd promoter may not modify the porosity of
the parent oxides. Therefore, this simple and fast one-pot
precipitation method allows sufficient dispersion of Pd spe-
cies with a rather low loading in the CeNiXOY mixed oxides
with a certain formulation.
XRD. Fig. 1 shows the XRD patterns of the Pd–CeNiXOY
catalysts with different Ni/Ce molar ratios. When the Ni/Ce
molar ratio ≥ 0.5, the Pd–CeNiXOY catalyst exhibits a mixed
phase of cerianite (CeO2, JCPDS file #34-0394) and NiIJOH)2
(JCPDS file #22-0444). When the Ni/Ce ratio is lower than 0.5,
the catalyst corresponds to a Ce–Ni solid solution, i.e., the
CeO2 phase remains unchanged but the NiIJOH)2 phase can-
not be detectable. Furthermore, it is found that the diffrac-
tion peaks of ceria not only become relatively broader but
also slightly shift to lower 2θ values with the addition of Ni
to ceria. This can be attributed to the substitution of Ce4+
ions with Ni2+ ions inside the CeO2 lattice and has been
interpreted as the evidence in the literature of the formation
of Ce–Ni solid solution.36 In fact, the ionic radius of Ni2+
(0.07 nm) is smaller than that of Ce4+ (0.09 nm). No Pd-
related peaks can be observed over any of the analyzed cata-
lysts, which could be probably due to the rather low Pd load-
ing and/or very high dispersion of Pd nanoparticles, beyond
the detection limit of the XRD technique. Therefore, the Pd–
CeNiXOY catalysts can be ascribed to a mixture of nano-
particles comprising Pd, CeO2 and NiIJOH)2 when the Ni/Ce
molar ratio ≥ 0.5 and to a Pd-containing solid solution of Ce
and Ni species when the Ni/Ce molar ratio < 0.5. The highest
proportion of the Ce–Ni solid solution may exist when the Ni/
2.4. Catalytic reaction
N-Alkylation of amines with alcohols was carried out in a
WP-TEC-1020HC parallel reactor from WATTCAS™. Typi-
cally, aniline (0.5 mmol), benzyl alcohol (0.75 mmol) and
the CeNiXOY or the Pd–CeNiXOY catalyst (75 mg) were added
successively. And then the reaction mixture was stirred (600
rpm) at 60 °C or 100 °C under oxygen (1 atm, O2 balloon)
for 16 hours. The tube was immediately cooled in ice water.
The reaction mixture was analyzed using a Thermo Scien-
tific TRACE 1310 gas chromatograph (GC) equipped with a
flame ionization detector (FID) and a TR-5 column (30 m
length, 0.32 mm inner diameter, 0.25 μm film thickness).
Each reaction was repeated twice at least to guarantee a re-
producible result. The quantification was carried out using
the external standard method. The conversion of aniline,
the selectivity to alkylamines and the yield of imine were
calculated, on the basis of a carbon balance. For the reus-
ability test, the catalyst was separated by centrifugation
(8000 rpm for 10 min), washed with ethanol and water,
dried at 80 °C under vacuum, and was used for the next
run using the same procedure as mentioned above.
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Table 1 Elemental analysis, specific surface areas, and crystallite sizes of the Pd–CeNiXOY catalysts
SBET (m2 g−1
)
c
c
b
Pd loadinga
(wt%)
Ni/Cea
(mol)
d-CeO2
(nm)
d-NiIJOH)2
(nm)
Entry
Catalyst
With Pd
Without Pd
1
2
3
4
5
6
7
Pd–CeO2
0.19
0.18
0.20
0.21
0.18
0.20
0.21
0
65
70
75
64
71
74
8.3
6.5
6.1
5.5
6.6
6.2
6.9
—
Pd–CeNi0.02OY
Pd–CeNi0.1OY
Pd–CeNi0.3OY
Pd–CeNi0.5OY
Pd–CeNi1OY
Pd–CeNi5OY
0.02
0.11
0.29
0.50
1.02
4.95
n.d.
n.d.
n.d.
n.d.
14
80
80
112
125
86
110
126
88
16
a
Determined by ICP-MS. b Measured by N2 adsorption–desorption using the Brunauer–Emmett–Teller method. c Calculated from XRD patterns
by using the Scherrer equation.
Ce molar ratio is distributed between 0.3 and 0.5, which is in
agreement with the analysis in the literature of the CeNiXOY
compounds.37 The mean crystallite sizes of CeO2 and NiIJOH)2
nanoparticles are calculated by the Scherrer equation from
the (111) plane of CeO2 (2θ = 28.55°) and the (200) plane of
NiIJOH)2 (2θ = 38.96°), respectively. The size of CeO2 crystal-
lites is ca. 6 nm for the Ni-containing Pd–CeNiXOY catalysts
and 8.3 nm for the Pd/CeO2 catalyst without Ni (Table 1),
which seems dependent on the Ni/Ce molar ratio. Note that a
smallest CeO2 size of 5.5 nm is obtained over the Pd/
CeNi0.3OY catalyst. NiIJOH)2 crystallites with a mean size of ca.
15 nm can be measured over the Pd/CeNi1OY and Pd/CeNi5OY
catalysts. It has to be recalled, for the lower Ni content (Ni/Ce
≤ 0.5), that some small-sized (<2 nm) crystallites of Ni spe-
cies have been well observed in the CeNiXOY compounds.37
UV-Raman. Fig. 2 shows the UV-Raman spectra of the Pd–
CeNiXOY catalysts with different Ni/Ce molar ratios. It is usu-
ally more sensitive to probe the surface defect sites in ceria
due to the resonance Raman effect. A strong frequency lo-
cated at 460 cm−1 is the characteristic F2g band due to the
fluorite phase of ceria nanocrystals.38 This peak shifts to
lower frequency and becomes broadened for the Pd–CeNiXOY
catalysts with increasing Ni content. The highest shift is
found over the Pd–CeNi0.3OY catalyst (445 cm−1), which is
comparable to that reported for the Ce1−XNiXO2−Y solid solu-
tion (443 cm−1).39 This phenomenon can be related to the sol-
ubility of Ni into ceria, which is in agreement with the forma-
tion of Ce–Ni solid solution. In addition to that, two bands
located at ca. 550 and 620 cm−1 are well observed on the Pd–
CeNi0.1OY and Pd–CeNi0.3OY catalysts. The band at ca. 566
cm−1 is characteristic of oxygen vacancies in the ceria lat-
tice.38 It has been reported that a couple of bands with weak
frequency located between 500 and 600 cm−1 (D band) can be
associated with doped ceria and attributed to oxygen vacan-
cies created due to the incorporation of the dopant.38 More-
over, the two bands of the Pd–CeNiXOY catalysts centered at
550 and 620 cm−1 could be attributed to the local vibrations
of different oxygen vacancy complexes (VO). The creation of
Ce3+–VO complexes in the ceria lattice has been identified by
the band at ca. 600 cm−1.39 Hence, for the series of Pd–
CeNiXOY catalysts the distortion of the ceria lattice and the
Fig. 1 XRD patterns of (a) CeO2, (b) Pd–CeO2, (c) Pd–CeNi0.02OY, (d)
Pd–CeNi0.1OY, (e) Pd–CeNi0.3OY, (f) Pd–CeNi0.5OY, (g) Pd–CeNi1OY, and
(h) Pd–CeNi5OY.
Fig. 2 UV-Raman spectra of (a) Pd–CeO2, (b) Pd–CeNi0.02OY, (c) Pd–
CeNi0.1OY, (d) Pd–CeNi0.3OY, (e) Pd–CeNi0.5OY, (f) Pd–CeNi1OY, and (g)
Pd–CeNi5OY.
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Fig. 3 (A) H2-TPR profiles of (a) Pd–CeNi0.02OY, (b) Pd–CeNi0.1OY, (c) Pd–CeNi0.3OY, (d) Pd–CeNi0.5OY, (e) Pd–CeNi1OY, and (f) Pd–CeNi5OY. (B)
Dependence of H2 consumption during TPR analysis on the Ni molar proportion in all the metals of the Pd–CeNiXOY catalysts.
generation of oxygen vacancies are much higher when the
CeNiXOY (X = 0.1 and 0.3) solid solution is formed.
mainly assigned to the reduction of larger NiIJOH)2
nanoparticles.
H2-TPR. Fig. 3 shows the H2-TPR profiles of the Pd–
CeNiXOY catalysts with different Ni/Ce molar ratios. A small
reduction peak below 100 °C can be observed for all the ana-
lyzed catalysts, which can be assigned to the reduction of Pd
nanoparticles.33 For a very low Ni content (X = 0.02), no obvi-
ous reduction peak can be observed. It is commonly accepted
that ceria shows two TPR peaks at ca. 470 °C and 750 °C,
which can be attributed to the reduction of surface and bulk
Ce4+ to Ce3+.40 When the Ni/Ce ratio increases to 0.1, a very
small peak at ca. 320 °C related to the reduction of Ni species
could be detected. When the Ni/Ce ratio further increases to
0.3, a strong peak emerges at ca. 270 °C. This peak appar-
ently shifts to higher temperatures and centers at ca. 315 °C
on the Pd–CeNi5OY catalyst. Moreover, a small shoulder peak
at ca. 195 °C is also observed and slightly shifts in the same
manner with increasing Ni content. A linear-like relationship
can be plotted between the amount of H2 consumed below
500 °C in the TPR procedure and the Ni molar proportion in
all the metals of the Pd–CeNiXOY catalysts, which reveals that
the consumed H2 in this temperature range is supposed to
reduce the Ni species in the Pd–CeNiXOY catalysts. This was
reported for the reductive procedure of NiIJOH)2 in TPR by
one step at ca. 370 °C.41 Hence, the reductive properties of
the Ni species in the Pd–CeNiXOY catalysts can be tuned by
the Ni content due to the different interactions between the
Ni species and Ce species. The TPR peaks located at low tem-
peratures (≤270 °C), especially for the Pd–CeNi0.3OY catalyst,
can be assigned to the Ni species belonging to the Ce–Ni
solid solution and/or to the very small easily reducible
NiIJOH)2 nanoparticles. Note that some of these species would
be re-oxidized at the same time by reducing Ce4+ species in
their vicinity into Ce3+ species when a redox system is
established, as described in eqn (1) and (2).42 And then the
TPR peak at relatively higher temperatures (≥300 °C) can be
Ni2+ + 2OH− + H2 → Ni0 + 2H2O + □ (□: anionic vacancy) (1)
2Ce4+ + Ni0 ↔ 2Ce3+ + Ni2+
(2)
HR-TEM and EPMA. Fig. 4-(a) shows the high-resolution
TEM image of the Pd–CeNi0.3OY catalyst. Nanostructured
ceria particles with a mean size of ca. 5.4 0.2 nm can be
identified, in addition to an amorphous composition that
can be probably due to the Ce–Ni solid solution. Both phases
are found to be in close interaction. Moreover, the statistics
show that the number of the (220) crystal planes of ceria is
higher than those of the (100) and (111) planes, which sug-
gests possibly in the Pd–CeNi0.3OY catalyst a higher propor-
tion of oxygen vacancies.28 These direct observations are in
good agreement with the XRD analysis. Fig. 4-(b) shows the
HR-TEM image of the used catalyst after regeneration in H2
at 200 °C. The size of ceria nanoparticles is well maintained,
demonstrating a very similar morphology to the fresh cata-
lyst. Moreover, the elemental mapping EPMA-EDX images in
Fig. 5 of the Pd–CeNi0.3OY catalyst clearly show that cerium
and nickel are homogeneously dispersed with oxygen in the
integrity structure of the catalyst. However, the loading of
palladium is too low to be measured by the EPMA technique.
XPS. Fig. 6 shows the XPS spectra of the Pd3d, Ce3d, Ni2p
and O1s levels obtained from the series of Pd–CeNiXOY cata-
lysts. The Pd3d levels cannot be well resolved due to the
rather low loading. The presence of metallic Pd species on
the surface could be speculated as all the catalysts have been
reduced in situ by NaBH4 during preparation. The Ce3d spec-
trum of a typical ceria can be resolved generally into three
spin–orbit doublets 3d3/2–3d5/2 denoted as (u, v), (u′, v′) and
(u″, v″),37 respectively. The Pd–CeNiXOY catalysts show these
characteristics that can be attributed broadly to the 3d
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Fig. 4 HR-TEM images and size distributions of ceria nanoparticles of (a) the fresh Pd–CeNi0.3OY catalyst and (b) the used Pd–CeNi0.3OY catalyst
after regeneration in H2 at 200 °C.
envelope to Ce4+ species in ceria-like compounds. Note that
Ce3+ species corresponding to BEs ca. 885.5 eV and 904.0 eV
are found to co-exist with the characteristic Ce4+ species on
the surface of the catalysts. The band shapes of the Ni2p3/2
level are identical to those of Ni2+ species, i.e., a main emis-
sion peak with its characteristic satellite peak.43,44 The BE ca.
856.5 eV in all the catalysts can be attributed to the Ni2+ spe-
cies reported for the Ce–Ni mixed oxides.24,37 When the Ni
content increases, a broadening effect is observed. It has
been shown that Ni2+ species mainly incorporate into ceria to
form a solid solution and the surface nickel species may co-
exist in different forms for a high Ni/Ce ratio.24 The O1s
levels show two bands at BEs ca. 529.6 eV and 531.3 eV. The
former peak at the lower BE is characteristic of the typical
O2− lattice oxygen species, while the latter peak at the higher
BE can be ascribed to the surface oxygen species mainly due
species, on the surface as a function of the Ni/Ce molar ratio.
The same tendency can be also obtained for surface oxygen
species. The Pd–CeNi0.3OY catalyst shows the optimum pro-
portions of Ce3+ and O2 species among the series of
−
catalysts.
3.2. N-Alkylation of aniline with benzyl alcohol
Table 2 reports the direct N-alkylation of aniline with benzyl
alcohol over the CeNiXOY catalysts at 100 °C. The lab-made
CeO2 shows a low conversion of aniline (20%), to yield
mainly imine (i.e., 81% benzylideneaniline in selectivity), as
well as secondary amine (19% benzylaniline in selectivity) as
the by-product. Adding nickel to ceria not only alters the con-
version of aniline but also minimizes the by-product forma-
tion. Both the conversion of aniline and the selectivity to im-
ine start to increase when a very small amount of Ni is doped
to form the CeNi0.02OY solid solution as the catalyst. When
Ni/Ce ≥ 0.1 the selectivity to imine is found to be higher than
99%, i.e., no more secondary amine is produced. Interest-
ingly, the conversion of aniline first increases and then de-
creases, and an optimum value (67%) is obtained over the
CeNi0.3OY catalyst after 16 h without the aid of basic
to superoxide O2 and/or O−.45,46 Some quantitative analysis
−
would allow a better understanding of the surface features of
the catalysts. The surface Ni/Ce molar ratio obtained from
XPS follows well linear evolution to the bulk ratio determined
by ICP-MS, which indicates a satisfactory dispersion of the Ni
species in ceria. Moreover, a volcano-like curve can be plotted
between the proportions of the Ce3+ species in all the cerium
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Fig. 5 EPMA of the Pd-CeNi0.3OY catalyst: the EDX mapping and Ce, Ni, O elemental mapping images.
additives. It is confirmed that the CeNi0.3OY–water suspension
treated under the reaction conditions (in the absence of reac-
tants) presents a neutral-like pH value of 6.6. Doubling the
catalyst mass (150 mg) would lead to a higher conversion of
aniline (85%). Total conversion can be achieved when the re-
action time further extends to 26 h. It has to be recalled that
the CeNiXOY catalyst is found to be inactive in the presence
of argon for benzylaniline production, i.e., a secondary
amine, from the dehydrogenative coupling of aniline with
benzyl alcohol under the current reaction conditions.
Table 3 reports the direct N-alkylation of aniline with ben-
zyl alcohol over the Pd–CeNiXOY catalysts at 60 °C. It is obvi-
ously easy to reach higher conversion of aniline by adding a
very low amount of Pd (0.2 wt%) to the CeNiXOY mixed oxides.
It is important to emphasize that the reaction temperature
can be lowered to 60 °C, a mild temperature. In fact, the pres-
ence of Pd already enables the selectivity to benzylaniline to
be minimized to as low as ca. 5% compared to those obtained
over the bare CeO2 and CeNi0.02OY catalysts. In the meantime
the conversion of aniline is largely promoted. The evolution
of aniline conversion depending on the Ni/Ce molar ratio (X
≥ 0.1) is observed to be similar to that obtained over the Pd-
free CeNiXOY catalysts. The selectivity to imine higher than
99% is also maintained. As expected, the Pd–CeNi0.3OY cata-
lyst shows the best activity of a complete conversion of aniline
to benzylideneaniline with benzyl alcohol after 16 h at 60 °C.
Working under an Ar atmosphere the Pd–CeNi0.3OY com-
pound could catalyze via the hydrogen-borrowing route the
dehydrogenative coupling of aniline with benzyl alcohol. But
the sole product is imine instead of a secondary amine, which
indicates that the Pd–CeNi0.3OY catalyst cannot further reduce
the produced imine to a secondary amine by using the
“borrowed H2”. Moreover, the dehydrogenative coupling over
such a catalyst is found to be much more difficult (36% in
conversion) than the oxidative coupling.
Fig. 7 shows the influence of reaction time and reactant
composition (i.e., the molar ratio of aniline to benzyl alcohol)
on the performance of the Pd–CeNi0.3OY catalyst. It can be
found that the Pd–CeNi0.3OY catalyst is highly selective to im-
ine production. No by-product is formed regardless of the re-
action time. The conversion of aniline increases almost line-
arly during the first 4 hours and already reaches 64%. When
the reaction time is prolonged the increase of aniline conver-
sion becomes gentle, reaching 91% and 99% after 16 hours
and 24 hours, respectively. Altering the molar ratio of aniline
to benzyl alcohol leads to the variation of the conversion of
aniline but not the selectivity to benzylideneaniline.
A
volcano-like curve can be plotted between the conversion of
aniline and the reactant composition. The optimum conver-
sion of aniline (91%) is obtained when benzyl alcohol is
slightly in excess (aniline/alcohol = 0.66, as used elsewhere in
this work). The reaction using the equimolar amount of ani-
line and alcohol decreases the conversion to 81%. An excess
of either benzyl alcohol or aniline would eventually retard the
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Fig. 6 XPS spectra and quantitative analysis of the Pd–CeNiXOY catalysts. (a) Pd–CeNi0.02OY, (b) Pd–CeNi0.1OY, (c) Pd–CeNi0.3OY, (d) Pd–CeNi0.5OY,
(e) Pd–CeNi1OY, and (f) Pd–CeNi5OY.
transformation of benzyl alcohol to benzaldehyde,47 although
the catalyst can still drive the condensation of benzaldehyde
with aniline.
a high-speed centrifuge (8000 rpm for 10 min) from the reac-
tion mixture, washed with water and ethanol in turn three
times, and finally dried at 80 °C under vacuum for 5 h. The
post-treated catalyst needs to be regenerated with H2 at 200
°C for 30 min before its next use. It has been confirmed that
there are no changes in the morphology and particle size of
CeO2 after the H2 treatment. After consecutive use for four cy-
3.3. Reusability of the Pd–CeNiXOY catalyst
The Pd–CeNi0.3OY catalyst shows a considerable stability dur-
ing recycling (Fig. 8). The spent catalyst was separated using
cles, the Pd–CeNi0.3OY catalyst still maintains a stable
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Table 2 N-Alkylation of aniline with benzyl alcohol over the CeNiXOY catalysts
Conv. (%)
Select. (%)
Yield (%)
Entry
Catalyst
1
3
4
3
1
2
3
4
5
6
7
8
9
CeO2
20
29
58
67
85
100
55
47
43
81
89
19
11
0
0
0
0
0
0
0
18
26
58
67
85
>99
55
47
43
CeNi0.02OY
CeNi0.1OY
CeNi0.3OYa
CeNi0.3OYab
CeNi0.3OY
CeNi0.5OY
CeNi1OY
>99
>99
>99
>99
>99
>99
>99
CeNi5OY
Reaction conditions: aniline, 0.5 mmol; benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm; temperature, 100 °C; time, 16 h.
a
Catalyst, 150 mg. b Time, 26 h.
Table 3 N-Alkylation of aniline with benzyl alcohol over the Pd–CeNiXOY catalysts
Conv. (%)
Select. (%)
Yield (%)
Entry
Catalyst
1
3
4
3
1
2
3
4
5
6
7
8
9
Pd–CeO2
30
59
84
91
100
36
79
75
57
94
95
6
5
0
0
0
0
0
0
0
29
55
84
91
>99
36
79
75
57
Pd–CeNi0.02OY
Pd–CeNi0.1OY
Pd–CeNi0.3OYa
Pd–CeNi0.3OYab
Pd–CeNi0.3OY
Pd–CeNi0.5OY
Pd–CeNi1OY
Pd–CeNi5OY
>99
>99
>99
>99
>99
>99
>99
Reaction conditions: aniline, 0.5 mmol; benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm; temperature, 60 °C; time, 16 h.
a
Catalyst, 100 mg. b Ar, 1 atm (O2-free).
conversion of aniline (ca. 91%) and a relatively high selectiv-
ity to imine (>99%), behaving as good as the fresh catalyst.
This may be due to the existence of strong interaction be-
tween cerium and nickel species and to the regeneration of
the active Pd0 species.
with about 20% loss of the imine yield.29 The remaining
catalysts (entries 3–5) all provide high imine yields (≥93%)
at 60–80 °C with good reusability.28,31 However, a small
amount of benzaldehyde is still observed for a CeO2 cata-
lyst.28 The average formation rate of imine (mmol gcat h−1)
−1
allows rough comparison of the activity of various CeO2-
based catalysts between different substrate amounts, catalyst
masses and reaction times.27 The Pd–CeNi0.3OY catalyst is
found to be highly active under mild conditions and compa-
rable to the best catalysts reported in the literature. To be
noted, the mesoporous carbon-supported CeO2 nanorod
(CeO2/MC) catalyst shows the fastest formation rate of im-
ine due to a rather short reaction time (2 h).31 This excep-
tional rate must be obtained necessarily at the highest tem-
perature (80 °C), because the aerobic oxidation of benzyl
alcohol to benzaldehyde is endothermic and appears to be
3.4. Comparison of catalytic performances between various
CeO2-based catalysts
Table 4 compares the Pd–CeNi0.3OY catalyst developed in
this work to the so-far reported typical CeO2-based bench-
mark catalysts. Two CeO2 catalysts (entries 1 & 2) could
work at a temperature as low as 30 °C for imine forma-
tion,27,29 but one produces a relatively high amount of benz-
aldehyde as the by-product and the reusability of ceria re-
mains unclear.27 The other deactivates after four recycles
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Fig. 7 N-Alkylation of aniline with benzyl alcohol over the Pd–CeNi0.3OY catalyst as a function of (a) the reaction time and (b) the aniline/benzyl
alcohol molar ratio. Reaction conditions (a): aniline, 0.5 mmol; benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm; temperature,
60 °C. Reaction conditions (b): aniline, 0.5 mmol; benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm; temperature, 60 °C; time,
16 h.
very sensitive to temperature.48 On the other hand, only the
CeO2 nanorods with controllable particle size and morphol-
ogy prepared by a fine hydrothermal method could show
such a high concentration of Ce3+ ions and oxygen vacan-
cies which improve the catalytic performance for molecular
O2 activation and the subsequent oxidation of benzyl alco-
hol. Moreover, the isolated state of CeO2 nanorods on the
surface of the carbon support could make the facet of CeO2
have more opportunity to be in contact with reagents.
Meanwhile the second effective CeO2 catalyst unfortunately
suffers from deactivation. In summary, all the reported
CeO2 catalysts have to follow complex and well-defined syn-
thetic methodology, and their high catalytic activity largely
depends on the unique particle size, morphology and/or
facet. Therefore, the facile and reproducible one-pot precipi-
tation with a cheap Ni component allows obtaining efficient
and stable Pd-promoted CeNiXOY catalysts for a stoichiomet-
ric yield of imine at a fast formation rate.
3.5. Relationship between catalyst properties and activity
Fig. 9-(a) shows the dependence of the imine yield on the Ni/
Ce molar ratio in the CeNiXOY and Pd–CeNiXOY catalysts for
the N-alkylation of aniline with benzyl alcohol. Very similar
volcano-like curves can be plotted for these two series of cata-
lysts, which indicates that the incorporation of Ni into ceria
has a strong impact on the formation of imine. The optimum
yield is obtained when the Ni/Ce ratio is 0.3. When a very low
amount of Pd (0.2 wt%) is added, the yield of imine obtained
on each Ni/Ce ratio is obviously increased. Note that the reac-
tion temperature used for the Pd–CeNiXOY catalyst is much
lower than that for the CeNiXOY catalyst (i.e., 60 °C vs. 100
°C). This clearly shows that the presence of Pd species is criti-
cal to lower the activation energy required for the formation
of imine. To be rigorous, the intrinsic activity of both cata-
lysts needs to be compared under the defined conditions.
Fig. 9-(b) shows the dependence of the initial reaction rate
over the CeNiXOY and Pd–CeNiXOY catalysts on the Ni/Ce mo-
lar ratio for the N-alkylation of aniline with benzyl alcohol at
60 °C. The initial rate is defined as the amount of the sub-
strate converted per gram catalyst per minute in the initial
stage (60 min), presented in the form of μmol gcat min−1
.
−1
The obtained initial rates of aniline show an analogous
volcano-like dependence on the Ni/Ce molar ratio, which re-
veals the synergistic effect between the CeNiXOY mixed oxides
and the Pd species. The composition of the CeNiXOY mixed
oxides (i.e., Ni/Ce ratio) could determine the initial reaction
rate. The trend of the initial rate could be well correlated
with the evolution of the surface proportion of Ce3+ species
and active oxygen species (Fig. 6). And moreover, the initial
rate would be largely facilitated by the presence of Pd species.
Fig. 8 Reusability of the Pd–CeNi0.3OY catalyst for N-alkylation of
aniline with benzyl alcohol. Reaction conditions: aniline, 0.5 mmol;
benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm;
temperature, 60 °C; time, 16 h.
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Table 4 Comparison of N-alkylation of aniline with benzyl alcohol over various CeO2-based catalysts
Oxidant
(1 atm)
Temp.
(°C)
Time
(h)
Yield 3
(%)
Formation rate 3
Entry
Catalyst
(mmol gcat−1 h−1
)
Reuse times
Stability
Ref.
1
2
3
4
5
CeO2
CeO2
CeO2
Pd–CeNi0.3OY
CeO2/MC
Air
Air
O2
Air
Air
30
30
60
60
80
48
36
12
16
2
75
98
93
99
99
0.31
0.55
0.38
0.31
1.65
Not done
Unknown
No
Yes
Yes
Yes
27
29
28
4
4
4
4
This work
31
Fig. 9 (a) Yield of imine versus the Ni/Ce molar ratio in the CeNiXOY and Pd–CeNiXOY catalysts. Reaction conditions: aniline, 0.5 mmol; benzyl
alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm; time, 16 h. (b) Initial rate of N-alkylation of aniline with benzyl alcohol versus the Ni/
Ce molar ratio in the CeNiXOY and Pd–CeNiXOY catalysts. Reaction conditions: aniline, 0.5 mmol; benzyl alcohol, 0.75 mmol; catalyst, 75 mg;
toluene, 5 mL; O2, 1 atm; temperature, 60 °C; time, 1 h. (c) Initial rate of aerobic oxidation of benzyl alcohol to benzaldehyde versus the Ni/Ce
molar ratio in the CeNiXOY and Pd–CeNiXOY catalysts. Reaction conditions: benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm;
temperature, 60 °C; time, 1 h. (d) Aerobic oxidation of benzyl alcohol to benzaldehyde versus the reaction time over the CeNi0.3OY catalyst.
Reaction conditions: benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; O2, 1 atm; temperature, 60 °C.
For instance, the initial rate on the optimum CeNi0.3OY cata-
lyst increases ca. 2.5 fold when promoted by Pd. Further in-
vestigation on the initial rate of the aerobic oxidation of ben-
zyl alcohol to benzaldehyde also shows a highly similar
volcano relationship for the two series of catalysts (Fig. 9-(c)).
But this oxidation step is observed to be very slow using the
mixed oxides without Pd and even the CeNi0.3OY catalyst
would only trigger this reaction after 4 hours (Fig. 9-(d)). On
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the other hand, both series of catalysts are inactive in
converting aniline in the presence of oxygen. And moreover,
it is confirmed in this work that the coupling of aniline with
benzaldehyde proceeds very fast even without any catalyst,
which is in agreement with the report of Z. Yan et al.33 A stoi-
chiometric yield of benzylideneaniline can be obtained at 60
°C after 1 hour under oxygen or argon. The produced benzal-
dehyde as the intermediate can be condensed immediately
with aniline to produce imine. Thus the formation of imine
over the CeNiXOY catalysts may be probably promoted even
though the rate-determining step is very slow. Therefore, the
aerobic oxidation of benzyl alcohol to benzaldehyde can be
considered as the rate-determining step for the N-alkylation
of aniline with benzyl alcohol. The Pd promoter can signifi-
cantly improve the yield of imine by accelerating the rate-
determining step because noble metal catalysts like Pd-based
catalysts have been well recognized due to their high perfor-
mances for activating alcohols by oxidation or
dehydrogenation.48–52 It is widely accepted that the hydroxyl
group in benzyl alcohol is prone to adsorb onto the metallic
Pd nanoparticles to form a metal alkoxide intermediate
which is further converted to an aldehyde via β-hydrogen
elimination. Moreover, it has been shown that the Pd species
in a Pd/CeO2 catalyst accelerate the formation of surface oxy-
gen vacancies and enhance the oxygen storage capacity and
therefore the surface activity of ceria.53 The Pd–ceria interac-
tion could decrease the formation energy of oxygen vacancies
and then enhance the lower temperature activity of the cata-
lyst.53 On the other hand, O2 plays an important role in the
rate-determining step. A time-online plot experiment shows
that the reaction ‘stops’ under inert gas and then adding O2
into the reactor leads obviously to an increase in the conver-
sion of benzyl alcohol, as shown in Fig. 10. This may confirm
the contribution of the surface oxygen species in the CeNiXOY
mixed oxides, because such reactive oxygen species could be
progressively consumed and then, after full depletion, the re-
action would probably stop. When gaseous O2 is provided, it
could be dissociated at the vacancy site to active surface oxy-
gen. Hu et al. confirmed the result for O2 adsorption into a
surface vacancy and found that on a subsurface vacancy only
one electron was transferred from a Ce3+ ion to O2, yielding
−
an O2 species.34 Conesa et al. also confirmed the formation
−
of O2 species by EPR experiments when O2 was adsorbed on
activated CeO2 with a high surface area.54 Preda et al. studied
the interaction of O2 with ceria nanoparticles and showed
−
that the superoxide O2 radical formed upon direct interac-
tion of O2 with low-coordinated Ce3+ ions on the reduced
ceria nanoparticles.54 As shown in this work by characteriza-
tion, the Ni/Ce molar ratio has a strong effect on the struc-
ture and the redox properties of the final CeNiXOY mixed ox-
ides. When the Ni/Ce molar ratio is 0.3, the formed CeNi0.3OY
solid solution shows the highest proportion of Ce3+ species
and oxygen vacancies due to a reversible redox system be-
tween Ce3+ and Ni2+ species that can be reduced and
reoxidized easily because of the existence of strong interac-
tion between Ni and Ce cations. This unique feature could
contribute to the optimum catalytic performance of the
CeNi0.3OY catalyst.
3.6. Scope of substrates for the Pd–CeNiXOY catalyst
Table 5 reports the direct N-alkylation of aniline with various
primary alcohols over the Pd–CeNi0.3OY catalyst at 60 °C. The
substituted benzyl alcohols are first investigated (entries 1–
6). The Pd–CeNi0.3OY catalyst shows good performance in ac-
tivating different alcohols. The benzyl alcohol substituted by
both the electron-withdrawing groups (NO2 and Cl) and the
electron-donating groups (MeO, tBu and Me) can be effec-
tively converted, and high yields (≥92%) of imines are
obtained. On the other hand, some differences in the yield
and the required reaction time are indicative of the effect of
the substituent properties and location. The electron-
withdrawing groups like NO2 could facilitate removal of hy-
drogen from the hydroxyl group, which has been observed on
halogenated benzyl alcohols.28 The deprotonated alcohol or
alkoxide may undergo hydride abstraction from benzylic car-
bon via oxidative dehydrogenation to yield a carbonyl com-
pound that reacts immediately with aniline to form imine.1
The electron-donating groups such as MeO and tBu may re-
duce the electrophilicity of benzylic carbon by enriching its
electron density, leading consequently to a low reactivity with
the nucleophilic amine. Besides, the effect of steric hin-
drance (3d) and the location (3e and 3f) of substituent groups
cannot be disregarded. Pyridin-4-methanol (entry 7) and
cyclohexylmethanol (entry 8) are also examined in this reac-
tion, to afford the desired imines in high yields, i.e., 99% of
3g and 90% of 3h, respectively. The linear aliphatic alcohol
like octan-1-ol (entry 9) shows as expected a relatively lower
yield to the imine 3i (75%), which has been widely attributed
to the difficulty in activating non-branched fatty alcohols.55
Moreover, several more complex alcohols including 3,4-
(methylenedioxy)benzyl alcohol (entry 10), 2-naphthalene-
Fig. 10 Time course for N-alkylation of aniline with benzyl alcohol
over the Pd–CeNi0.3OY catalyst. Reaction conditions: aniline, 0.5 mmol;
benzyl alcohol, 0.75 mmol; catalyst, 75 mg; toluene, 5 mL; Ar or O2, 1
atm; temperature, 60 °C.
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Table 5 N-Alkylation of aniline with various primary alcohols over the Pd–CeNi0.3OY catalyst
Entry
1
Product code
Product structure
Reaction time (h)
20
Yield (%)
3a
>99
2
3b
24
92
3
4
5
6
3c
3d
3e
3f
22
24
24
24
>99
95
98
95
7
8
3g
3h
24
24
>99
90
9
3i
3j
24
24
75
72
10
11
12
3k
3l
24
24
58
24
Reaction conditions: aniline, 0.5 mmol; alcohol, 0.75 mmol; Pd–CeNi0.3OY, 100 mg; toluene, 5 mL; O2, 1 atm; temperature, 60 °C.
methanol (entry 11) and 2-thiophenemethanol (entry 12)
could be activated and converted by the Pd–CeNi0.3OY catalyst
to the corresponding imines (3j–3l) with low to moderate
yields.
Table 6 reports the direct N-alkylation of various amines
with benzyl alcohol over the Pd–CeNi0.3OY catalyst at 60 °C.
As is previously disclosed the activation of benzyl alcohol is
the key step in the tandem alkylation process. Thereby, the
nature of amines may probably have an impact on the con-
version and product selectivity. The Pd–CeNi0.3OY catalyst
shows high imine yields to 3m and 3q (≥93%) from the
substituted anilines (entries 1–5). As expected, the electron-
donating groups (MeO and Me) and the electron-withdrawing
groups (Cl) could enhance the nucleophilicity of aniline and
thus facilitate the nucleophilic attack, despite that some mi-
nor variations can be observed due to the properties and the
location of substituents. Compared with aniline, the alkyl-
ation of benzyl amine with benzyl alcohol (entry 6) also yields
a high imine production (3r, 95%), which may be due to the
high basicity of benzyl amine and/or the possibility of its
self-coupling reaction to form imine.27 The linear aliphatic
octan-1-amine (entry 7) would offer a high desired imine
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Table 6 N-Alkylation of benzyl alcohol with various primary amines over the Pd–CeNi0.3OY catalyst
Entry
1
Product code
Product structure
Reaction time (h)
Yield (%)
3m
22
>99
2
3
3n
3o
24
24
98
93
4
3p
24
95
5
6
3q
3r
24
24
>99
95
7
3s
24
90
8
3t
24
24
24
40
49
27
9
3u
3v
10
Reaction conditions: amine, 0.5 mmol; benzyl alcohol, 0.75 mmol; Pd–CeNi0.3OY, 100 mg; toluene, 5 mL; O2, 1 atm; temperature, 60 °C.
production (3s, 90%). Therefore, it is evident again that, 4. Conclusion
when comparing with the yield of 3i, the conversion of an al-
cohol to an aldehyde is the rate- and yield-determining step
in the cross-coupling of alcohols with amines to imines.
Pyridin-4-amine (entry 8) affords a low imine yield to 3t
(40%), which is probably influenced by the basicity of the
pyridine group. In addition, tert-butylamine and cyclohexyl-
amine (entries 9 & 10) can also react with benzyl alcohol to
afford the desired imines (3u and 3v) in low to moderate
yields. In summary, the Pd–CeNi0.3OY catalyst shows a wide
scope in activating various amines and alcohols for imine
synthesis via the oxidative coupling mechanism. The electron
effect of substituent groups, the nucleophilicity or electrophi-
licity of substrates, and even the acidic–basic properties of
substrates and catalysts are worth studying comprehensively
in future work.
Pd-promoted Ce–Ni mixed oxides were reported as efficient
and reusable heterogeneous catalysts for imine synthesis
from the direct N-alkylation of various amines with alcohols.
The Pd species and the Ni/Ce molar ratio in the Pd–CeNiXOY
catalysts played vital roles in the selective activation of alco-
hols in the presence of reactive amines. In the model alkyl-
ation reaction of aniline with benzyl alcohol, the metallic Pd
species can significantly improve the formation of imine un-
der mild conditions by accelerating the aerobic oxidation of
benzyl alcohol to benzaldehyde that was disclosed to be the
rate-determining step. A simple one-pot precipitation method
allowed synthesis of highly-dispersed Pd-promoted Ce–Ni
solid solution and/or mixed oxide which showed a reversible
redox process between Ce3+ and Ni2+ species due to the
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existence of a strong interaction between Ni2+ and CeO2. The
Ni/Ce molar ratio strongly influenced the structure and the
redox properties of the final CeNiXOY mixed oxides. The Pd–
CeNi0.3OY catalyst bearing the highest proportion of Ce3+ spe-
cies and oxygen vacancies, together with the synergistic effect
of the Pd promoter, showed an enhanced yield (>99%) of im-
ine at 60 °C by facilitating the initial reaction rate of aniline
with benzyl alcohol.
16 K.-i. Shimizu, N. Imaiida, K. Kon, S. M. A. Hakim Siddiki
and A. Satsuma, ACS Catal., 2013, 3, 998–1005.
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20 K. S. Hayes, Appl. Catal., A, 2001, 221, 187–195.
21 G. Guillena, D. J. Ramón and M. Yus, Chem. Rev., 2010, 110,
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There are no conflicts of interest to declare.
22 Q. Yang, Q. Wang and Z. Yu, Chem. Soc. Rev., 2015, 44,
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This work was supported by the Natural Science Foundation
of China (Nos. 21603187, 21763031), the National Special
Funds of China (No. C6176102) and the Program for Innova-
tive Research Team (in Science and Technology) in the Uni-
versity of Yunnan Province. The authors sincerely thank Mr.
G.-H. Zhu from the Advanced Analysis and Measurement Cen-
ter (YNU) for the ICP-MS analysis.
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