Oxidative coupling of alcohols and amines to an imine over Mg-Al acid-base bifunctional oxide catalysts

Article abstract of DOI:10.1016/S1872-2067(17)63006-7

A series of Mg-Al mixed oxide catalysts are prepared and introduced as efficient irreducible catalysts for the oxidative coupling of alcohols and amines to imine. The structure and surface properties of Mg-Al oxides are modulated by changing the Mg/Al ratios, calcination temperature and treatment with probe molecules. Detailed characterization, including X-ray diffraction, ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy, N₂-adsorption, NH₃-temperature-programmed desorption, CO₂-temperature-programmed desorption and X-ray photoelectron spectroscopy are carried out to determine the physicochemical properties of these catalysts. The Mg-Al oxides with Mg/Al = 3 exhibit the highest activity in the reaction, which possess a large number of surface weak basic sites and a relatively small number of weak acidic sites. The role of the acidic and basic sites in the reaction process is systematically investigated, and are shown to serve as adsorption and activation sites for amines and alcohols, respectively. Under the synergistic effect of these acid-base centers, the oxidative coupling process successfully occurs on the surface of Mg-Al mixed oxides. Compared with the acidic sites, the weak basic sites play a more important role in the catalytic process. The acidic sites are the catalytic centers for the benzyl alcohol activation, which control the reaction rate of the oxidative coupling reaction.

Full text of DOI:10.1016/S1872-2067(17)63006-7

Chinese Journal of Catalysis 39 (2018) 309–318
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Oxidative coupling of alcohols and amines to an imine over Mg‐Al
acid‐base bifunctional oxide catalysts
Jinling Song a, Guiyang Yu a, Xi Li a, Xuwei Yang a, Wenxiang Zhang a,*, Wenfu Yan b, Gang Liu a,#
a Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun 130012, Jilin, China
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, Jilin, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of Mg‐Al mixed oxide catalysts are prepared and introduced as efficient irreducible cata‐
lysts for the oxidative coupling of alcohols and amines to imine. The structure and surface proper‐
ties of Mg‐Al oxides are modulated by changing the Mg/Al ratios, calcination temperature and
treatment with probe molecules. Detailed characterization, including X‐ray diffraction, 27Al magic
Received 13 November 2017
Accepted 25 December 2017
Published 5 February 2018
angle
spinning
nuclear
magnetic
resonance
spectroscopy,
N2‐adsorption,
NH3‐temperature‐programmed desorption, CO2‐temperature‐programmed desorption and X‐ray
photoelectron spectroscopy are carried out to determine the physicochemical properties of these
catalysts. The Mg‐Al oxides with Mg/Al = 3 exhibit the highest activity in the reaction, which possess
a large number of surface weak basic sites and a relatively small number of weak acidic sites. The
role of the acidic and basic sites in the reaction process is systematically investigated, and are
shown to serve as adsorption and activation sites for amines and alcohols, respectively. Under the
synergistic effect of these acid‐base centers, the oxidative coupling process successfully occurs on
the surface of Mg‐Al mixed oxides. Compared with the acidic sites, the weak basic sites play a more
important role in the catalytic process. The acidic sites are the catalytic centers for the benzyl alco‐
hol activation, which control the reaction rate of the oxidative coupling reaction.
Keywords:
Oxidative coupling
Imine synthesis
Alcohol oxidation
Mg‐Al mixed oxides
Acid‐base catalysis
© 2018, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
coupling of alcohols and amines to imines in the presence of air
or oxygen is a more benign synthetic route, which is currently
Owing to the presence of the reactive C=N moiety, imines
are an important nitrogen source in the fields of biology, medi‐
cine and agriculture [1–5]. A series of valuable nitro‐
gen‐containing derivatives can be synthesized through some
simple chemical processes [6–8]. In industrial processes, most
imine‐containing compounds are obtained by condensing car‐
bonyl compounds and primary amines under strongly acidic
conditions [9–11]. Compared with this route, direct oxidative
being intensively researched [12,13].
Supported noble metal catalysts, such as Pd, Pt and Au, ex‐
hibit certain catalytic properties in the presence of alkali as
additives or by using basic oxides as supports [14–20]. Consid‐
ering the cost and environmental friendliness of catalysts, sig‐
nificant research has been devoted to developing transition
metal oxide catalysts. Some reducible metal oxides, such as
MnO2, Fe2O3, V2O5 and CeO2, have been reported to catalyze this
* Corresponding author. Tel: +86‐431‐85155390; Fax: +86‐431‐88499140; E‐mail: zhwenx@jlu.edu.cn
# Corresponding author. Tel: +86‐431‐85155390; Fax: +86‐431‐88499140; E‐mail: lgang@jlu.edu.cn
This work was supported by the National Natural Science Foundation of China (21473073, 21473074), the Development Project of Science and
Technology of Jilin Province (20170101171JC), "13th Five‐Year" science and technology research of the Education Department of Jilin Province
(2016403) and the Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (201703).
DOI: 10.1016/S1872‐2067(17)63006‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 39, No. 2, February 2018

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Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
reaction [21–25]. Our recent work showed that the reducibility
of the transition metal oxides, which could directly influence
the activation of molecular oxygen, was an important factor
affecting the activity of the catalyst [26,27]. The activity can be
improved by optimizing the particle size or morphology of the
metal oxides. However, the influence of other factors, such as
the surface acidity and basicity of catalysts have been ignored
in this reaction previously [28]. Some valuable solid acid‐base
catalysts composed of environmentally‐friendly non‐reducible
metals have also been ignored in the previous work.
From the viewpoint of the reaction mechanism, a solid ac‐
id‐base catalyst should also be suitable for this reaction. Gener‐
ally, the title reaction undergoes two consecutive steps: (1)
oxidative dehydrogenation of the alcohol to benzaldehyde and
(2) condensation of benzaldehyde with aniline [23,29,30]. Cur‐
rent research based on transition metal oxides have shown that
step (1) mainly occurs on the redox center, while step (2)
mainly occurs on the acidic center [6,31]. It is known that a
solid base can also be used as an effective dehydrogenation
active center in many catalytic processes [32]. Therefore, it is
possible to construct a solid acid‐base catalyst suitable for the
above reaction by regulating the surface acid‐base center of the
catalyst.
Based on this hypothesis, Mg‐Al mixed oxides were intro‐
duced as catalysts in the oxidative coupling of alcohols and
amines to imine. The surface acid‐base properties of Mg‐Al
oxides were modulated by changing the Mg/Al ratios, calcina‐
tion temperature and treatment with probe molecules. Under
the optimized conditions, Mg‐Al mixed oxides with Mg/Al = 3
exhibited a relatively high activity in this reaction, which was
comparable with that of supported FeOx catalysts reported
previously [26]. Various characterizations, including X‐ray dif‐
fraction (XRD), 27Al magic angle spinning nuclear magnetic
resonance spectroscopy (MAS NMR), N2‐adsorption,
NH3‐temperature‐programmed desorption (TPD), CO2‐TPD
and X‐ray photoelectron spectroscopy (XPS) were performed
to investigate the structure and surface properties of Mg‐Al
oxides. A series of detailed treatment conditions and catalytic
tests were also carried out to clarify the influence of the surface
acid‐base properties on the catalytic performance for the oxi‐
dative coupling of alcohols and amines.
Al2O3 were also prepared using the same method as the Mg‐Al
mixed oxides.
2.2. Catalyst characterization
The X‐ray diffraction (XRD) patterns were recorded on a
Rigaku X‐ray diffractometer using Cu Kα radiation (λ = 0.15418
nm) at 40 kV and 40 mA. N2 adsorption‐desorption isotherms
were measured at –196 °C, using a Micromeritics ASAP 2010N
analyzer. Samples were degassed at 200 °C for 20 h before
measurements. Specific surface areas were calculated using the
Brunauer‐Emmett‐Teller (BET) model. Pore size distributions
were evaluated from desorption branches of nitrogen iso‐
therms using the Barret‐Joyner‐Halenda (BJH) model. Scanning
electron microscopy (SEM) was performed using an HITACHI
SU8020 instrument with 30 kV. XPS data were obtained on a
Thermo ESCA LAB 250 system with a Mg Kα source (1254.6
eV). TPD was performed on a ChemBET Pulsar TPR/TPD in‐
strument (Quantachrome Instruments). Before detection, the
catalysts (50 mg) were first treated in a N2 (99.99%) flow at
500 °C for 0.5 h. The 27Al MAS spectra were recorded on a
Bruker AVANCE III 400 WB spectrometer equipped with a 4
mm standard bore CP MAS probe‐head whose X channel was
tuned to 104.27 MHz for 27Al, where pulse sequence was one
pulse (0.9 μs, π/12), using a magnetic field of 9.39 T at 24 °C.
The dried and finely powdered samples were packed in the
ZrO2 rotor closed with a Kel‐F cap and were spun at 12 kHz
rate. A total of 100 scans were recorded with a 2 s recycle delay
for each sample. All 27Al MAS chemical shifts were referenced
to the resonances of [Al(H2O)6]3+ with standard (d = 0.00).
2.3. Catalyst screening
Catalytic screening for the oxidative coupling of alcohols and
amines was carried out as follows: a 50‐mL two‐neck flask was
charged with toluene (10 mL), catalyst (0.3 g), 1.0 mmol benzyl
alcohol and 2.0 mmol aniline. The flask was placed in an 80 °C
oil bath and connected with a balloon full of air. The products
were extracted with a sampling pipe equipped with a filter, and
the analyses were performed on
a gas‐chromatograph
equipped with an HP‐5 column and FID detector. The corre‐
sponding alcohols and amines and imines were used as stand‐
ards.
2. Experimental
The oxygen‐free experiment was carried out in a Schlenk
tube. First, the catalyst was put in the Schlenk tube and de‐
gassed for 1.5 h under vacuum. The tube was then charged with
nitrogen (99.99%). The above procedure was repeated three
times. Then the degassed reagents were introduced into the
Schlenk tube and the tube was placed in an 80 °C oil bath. For
the recycling experiment, the solid catalyst was separated by
filtration after reaction without any washing treatment. Before
another reaction, the catalyst was calcined at 500 °C for 1 h to
remove the adsorbed reactants.
2.1. Catalyst preparation
An Mg‐Al mixed oxides catalyst was prepared by a
co‐precipitation method. In a typical process, a certain amount
of Mg(NO3)2·6H2O and Al(NO3)3·9H2O was dissolved in 200 mL
deionized water. After 0.5 h stirring, 12.5% NH3·H2O added to
the mixture to adjust the pH value to 9.0. After continuous stir‐
ring for 2 h, the mixture was filtrated and washed with deion‐
ized water. The obtained solids were dried in an oven at 80 °C
overnight, and then calcined at 500 °C for 4 h in a muffle fur‐
nace. The Mg/Al ratio of Mg‐Al mixed oxides was adjusted from
0.33 to 5.0 by changing the amount of Mg(NO3)2·6H2O and
Al(NO3)3·9H2O in the initial process. For comparison, MgO and
To understand the role of the acid‐base sites in the reaction,
pyridine and pyrrole poisoning experiments were carried out
over the Mg‐Al mixed oxide with Mg/Al = 3. The detailed pro‐
cess was as following: the catalyst (0.3 g) was added to a mix‐

Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
311
ture of toluene (10 mL) and pyridine/pyrrole (100 μL). The
mixture was stirred for 3 h at room temperature. The catalyst
was then filtered out and washed with toluene several times.
3. Results and discussion
Mg/Al=5
Mg/Al=3
Fig. 1 shows the XRD patterns of pure Al2O3, MgO and Mg‐Al
oxides with different Mg/Al ratios. All these samples were pre‐
pared with the same precipitation method, and ammonia solu‐
tion was used as the pH adjusting agent. The samples were all
calcined at 500 °C for 4 h before use as catalysts and character‐
ization. As for Al2O3, the weak diffraction peaks centered at 2θ
= 37.5°, 39.5°, 45.9° and 66.7° could be assigned to the (311),
(222), (400), (440) planes of the γ‐phase Al2O3, which was con‐
sistent with the JCPDS Card 10‐0425 [33]. As for MgO, three
diffraction peaks centered at 2θ = 37.0°, 43.0° and 62.5° could
be observed, which were assigned to the (111), (200) and
(220) planes of MgO (JCPDS Card 77‐2364) [34]. As for Mg‐Al
oxides with different Mg/Al ratios, a mixed phase existed in all
the samples. According to the JCPDS Card 33‐0853, the main
diffraction peaks of MgAl2O4 are located at 2θ = 36.8°, 44.8° and
65.6°, which are associated with the (311), (400) and (440)
crystal planes of MgAl2O4. When Mg/Al≤1, both γ‐Al2O3 and
MgAl2O4 phases existed in the sample. The low intensity and
broad peaks showed that the crystallinity of both γ‐Al2O3 and
MgAl2O4 phases was very low. When 2≤Mg/Al≤4, the diffrac‐
tion peak at approximately 2θ = 36.8° and 44.8° broadened,
and no obvious diffraction peaks assigned to the MgO phase
could be observed. This result indicated that the Al3+ cations
remained closely associated with the Mg‐O and the local struc‐
ture was amorphous. As for the sample with Mg/Al = 5, addi‐
tional peaks assigned to the MgO crystal phase could be ob‐
served.
Mg/Al=1
Mg/Al=0.33
150 100
50
0
-50 -100 -150
Chemical shift (ppm)
Fig. 2. 27Al MAS NMR spectra of Mg‐Al mixed oxides with different
Mg/Al ratios.
nated to oxygen, respectively [36–38]. With the increase of the
Mg/Al ratio, the peaks of Alo and Alt were slightly shifted down‐
field. This result suggested that electronegative Al3+ was being
replaced by more electropositive Mg2+ as the nearest neighbor
cation bonded to the Al‐O polyhedral [39]. Different results
were obtained for the Mg/Al = 5 sample, in which the peak
assigned to Alt split into two peaks and the peak assigned to Alo
moved to a lower chemical shift. This result mainly arose from
the formation of the MgO and MgAl2O4 crystal phases at this
Mg/Al ratio [40].
Combined with the results of XRD and 27Al MAS NMR, it was
shown that as the Mg content increased, more and more Mg2+
cations bonded to the Al‐O polyhedral forming Mg‐O‐Al. When
Mg/Al≤4, although some MgO and MgAl2O4 units were present
in the sample, the samples still generally possessed the prop‐
erty of the amorphous state. The relatively wide XRD diffrac‐
tion peaks supported this deduction. When Mg/Al was in‐
creased to 5, the formed MgO crystal phase broke the original
bonding balance. Both MgO and MgAl2O4 crystalline phases
could be clearly observed in the XRD patterns. Furthermore,
the 27Al MAS NMR spectra also showed that Al3+ cations in all
the sample preferentially located in the octahedral positions.
Comparing these samples, the Alo/Alt values decreased from
2.89 for Mg/Al = 0.33 to 2.04 for Mg/Al = 1.0, and then re‐
mained approximately constant when the Mg/Al ratio was in‐
creased further.
NMR is a powerful local‐probe measurement that can detect
all the structures regardless of crystalline or amorphous phase
[35]. It can effectively complement Powder XRD by probing
amorphous phases where direct information could only be
obtained from samples with crystalline structure. Fig. 2 shows
the 27Al MAS NMR spectra of Mg‐Al oxides with four typical
Mg/Al ratios. There were two resonance peak in the areas
around 8–10 ppm and 67–81 ppm, which could be assigned to
Al3+ cations octahedrally (Alo) and tetrahedrally (Alt) coordi‐
Fig. 3(a) shows the N2 adsorption‐desorption isotherms of
pure Al2O3, MgO and Mg‐Al oxides with different Mg/Al ratios.
All these samples exhibited a type IV isotherm, which indicated
a mesoporous character [41,42]. Most of the hysteresis loops
located at a relatively high pressure (p/p0) zone, which indi‐
cated the presence of a relatively large pore size. Pore size dis‐
tribution plots showed that the most integrable pore size of
Mg‐Al oxides with different Mg/Al ratios was centered at
around 10–15 nm (Fig. 3(b)). The specific surface area showed
a decreasing trend with the increase of the Mg/Al ratio (see
Table 1). As for pure Al2O3, a relatively small pore size (around
5 nm) could be observed. As for pure MgO, the wide pore size
distribution meant that the pores in the sample were mostly
derived from the accumulated pores between particles. SEM
MgAl O

2
4


MgO

MgO


Mg/Al=5



Mg/Al=4
Mg/Al=3
Mg/Al=2
Mg/Al=1
Mg/Al=0.33
Al2O3
10
20
30
40
2/(o )
50
60
70
Fig. 1. XRD patterns of Mg‐Al mixed oxides with different Mg/Al ratios.

312
Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
1000
800
600
400
200
Al2O3
Mg/Al=0.33
(a)
(b)
Mg/Al=1
Mg/Al=3
Mg/Al=5
Mg/Al=2
Mg/Al=4
MgO
D
V
+100
0.8
0.0
0.2
0.4
0.6
1.0
0
10
20
30
40
50
Relative pressure (p/p0)
Pore diameter (nm)
Fig. 3. N2 adsorption‐desorption isotherms (a) and pore size distribution plots (b) of Mg‐Al mixed oxides with different Mg/Al ratios.
images further confirmed the results from N2‐adsorption that a
large amount of mesopores arose from the packing holes be‐
tween the particles (Fig. 4). When the Mg/Al ratio was in‐
creased to 5, relatively large particles and pores could be clear‐
ly observed. This mainly arose from the remarkable crystalliza‐
tion occurring at such a high Mg/Al ratio.
of basic centers with different strengths were present on the
surface of all these samples. They separately came from OH−
groups, Mg‐O pairs, Al‐O pairs and O2− anions [43–45]. As for
the Mg‐Al oxides, with an increase of the Mg content, the total
number of surface basic sites increased at first then decreased.
The largest number of surface basic sites was present in the
Mg/Al = 3 samples (89.8 μmol·g–1). A large number of weak
basic centers existed on the surface of the sample (25.9
μmol·g–1), which accounted for 28.8% of the surface basic cen‐
ters (Table 2).
The above results showed that the change of surface acidity
and basicity was closely related to the Mg/Al ratio of the sam‐
ples. However, it should be noted that the composition of sur‐
face Mg/Al was not consistent with that of the body. Table 1
shows that the surface Mg/Al was much lower than that of the
stoichiometric ratio, which indicated that more Mg atoms en‐
tered into the bulk of the samples. This part of the Mg atoms
indirectly affected the acid‐base properties of the catalyst sur‐
The surface acidic and basic properties of the samples were
investigated by TPD measurements using NH3 and CO2 as
probe molecules, respectively. Fig. 5(a) shows the NH3‐TPD
profiles of pure Al2O3, MgO and Mg‐Al oxides with different
Mg/Al ratios. All these samples exhibited a broad desorption
peak in the detection region, which indicated the relatively
complex surface acidic nature of these samples. The TPD pro‐
files were deconvoluted into four desorption peaks (Fig. 5(a)
and Table 2). According to the literature, the low‐temperature
peak could be assigned to a reversible H‐bonded adsorption on
Brönsted sites, and the high‐temperature peak could be at‐
tributed to the irreversible coordinated adsorption on the
Lewis sites [43,44]. These Lewis sites included Al3+ cations in
Al3+‐O2–‐Mg2+ and Al3+‐O2– species, and Mg2+ cations in Mg2+‐O2‐
species [45–48]. From the perspective of total acidity, the larg‐
est surface acidity appeared in the Mg/Al = 0.33 samples. With
the increase of the Mg/Al ratio, the total surface acidity de‐
creased first and then increased. The sample of MgO exhibited
the lowest surface acidity.
(b)
(a)
The CO2‐TPD profiles (Fig. 5(b)) showed that the surface
basic properties of these samples were also complex. A series
Table 1
Texture properties of Mg‐Al mixed oxides with different Mg/Al ratios.
(d)
SBET
(m2·g–1)
Pore volume Pore size a Surface Mg/Al
(c)
Sample
(cm3·g–1)
(nm)
ratios b
Al2O3
296
203
165
171
168
176
125
92
0.39
0.39
0.26
5.2
7.6
6.8
—
Mg/Al=0.33
Mg/Al=1
Mg/Al=2
Mg/Al=3
Mg/Al=4
Mg/Al=5
MgO
0.06
1.03
—
1.38
—
0.40
0.36
0.33
0.32
9.4
8.6
7.6
10.3
11.6
2.32
—
0.27
a Calculated from adsorption branches using BJH model. b Detected by
Fig. 4. SEM images of Mg‐Al mixed oxides with different Mg/Al ratios:
(a) Mg/Al=0.33, (b) Mg/Al=1, (c) Mg/Al=3, and (d) Mg/Al=5.
XPS.

Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
313
(a)
II
I
(b)
III
IV
MgO
MgO
Mg/Al=5
Mg/Al=5
III
Mg/Al=3
Mg/Al=1
IV
II
Mg/Al=3
Mg/Al=1
I
Mg/Al=0.33
Al2O3
Mg/Al=0.33
Al2O3
100
200
300
400
500
100
200
300
400
500
Temperature (C)
Temperature (C)
Fig. 5. NH3‐TPD (a) and CO2‐TPD (b) profiles of Mg‐Al mixed oxides with different Mg/Al ratios.
Table 2
Characterization of the acid‐base properties of Al2O3, MgO and Mg‐Al mixed oxides with different Mg/Al ratios.
Total acid amount
(μmol·g–1)
CO2‐TPD (μmol·g–1)
Total base amount
(μmol·g–1)
NH3‐TPD (μmol·g–1)
Sample
PeakⅠ Peak Ⅱ Peak Ⅲ Peak Ⅳ
PeakⅠ
4.0
6.6
8.2
25.9
9.7
PeakⅡ
—
Peak Ⅲ
15.0
Peak Ⅳ
Al2O3
17.4
4.4
6.8
5.9
4.0
—
21.6
43.4
4.1
11.6
15.6
5.3
15.8
11.9
12.8
4.1
11.1
1.2
66.4
75.4
29.1
34.8
49.2
27.5
23.1
15.9
22.3
18.8
22.9
11.8
42.0
36.2
72.0
89.8
74.4
78.7
Mg/Al=0.33
Mg/Al=1
Mg/Al=3
Mg/Al=5
MgO
—
13.7
21.3
12.2
30.9
18.5
24.1
32.9
10.8
34.3
8.5
24.2
8.7
16.3
9.2
17.6
14.1
face through bonding with Al species (detected from NMR).
With the increase of the Mg content, the O 1s XPS spectra (Fig.
6) shifted slightly toward a lower binding energy, which was
consistent with the change of the surface basic properties. It
should be noted that the total base amount of the Mg‐Al oxides
in our case was lower than that in some reports [41–43,49,50].
This was closely related to the distribution of surface Mg/Al
elements. XPS results clearly showed that the sample prepared
under our conditions possessed relatively low surface Mg/Al
ratios compared with the samples reported previously.
Mg/Al = 3, but it was still higher than that of the MgO and Al2O3
samples. It should be pointed out that a trace amount of
by‐product, azoxybenzene, could be detected by GC. It was
identified with Gas chromatography‐mass spectrometry and a
standard reagent. This by‐product arose from the condensation
of a small amount of aniline.
As for the sample with Mg/Al = 3, a hot filtration test was
carried out to confirm the heterogeneous nature of the sample.
The results showed that Mg‐Al oxides behaved as an opera‐
tionally heterogeneous catalyst, since the imine concentration
in the filtrate did not increase (Fig. 7(b)). Almost no Mg and Al
Fig. 7(a) shows the catalytic activity of the above samples in
the oxidative coupling of benzyl alcohol and aniline to an imine
at 80 °C. The mole ratio of benzyl alcohol to aniline in the rea‐
gent was 1:2. The yield of imine was calculated using benzyl
alcohol as the standard. The selectivity calculated based on
benzyl alcohol was larger than 99%. Therefore, the conversion
of benzyl alcohol was almost the same as the yield (not shown
here). Fig. 7(a) shows that both MgO and Al2O3 exhibited a low
activity in this reaction. Only a 19.2% and 22.7% yield of imine
could be observed over MgO and Al2O3 after reaction for 8 h. As
for the Mg‐Al oxides, the introduction of a small amount of Mg
significantly improved the activity of the sample. When Mg/Al =
3, the yield of imine could reach to 93.5% (with benzyl alcohol
and aniline as substrates) after reaction for 8 h. The activity
could be further improved when the reaction temperature was
increased from 80 °C to 100 °C. This result was comparable
with that of the supported FeOx catalysts reported previously
[26]. When Mg/Al was further increased, the activity of the
samples decreased compared with that of the sample with
O 1s
Mg/Al=5
Mg/Al=3
Mg/Al=1
Mg/Al=0.33
540
535
530
525
Binding energy (eV)
Fig. 6. O 1s XPS spectra of Mg‐Al mixed oxides with different Mg/Al
ratios.

314
Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
100
100
80 C 100 C
Mg/Al=3
100
80
60
40
20
0
(c)
(b)
(a)
4
2
1
80
60
40
20
0
80
60
40
20
0
hot filtration
0.33

5
Al2O3
MgO
0
2
4
6
8
0
2
4
6
8
1
2
3
4
Reaction time (h)
Reaction time (h)
Run times
Fig. 7. (a) Catalytic performances of Mg‐Al mixed oxides with different Mg/Al ratios at 80 °C. Reaction conditions: benzyl alcohol (1 mmol), aniline (2
mmol), catalyst (0.3 g), toluene (10 mL), air (0.1 MPa). (b) Leaching experiment of Mg‐Al mixed oxides (Mg/Al = 3) by continuing the reaction after
filtration of the catalyst. The 2–7 h reaction result shows the imine yield after removing the catalysts. (c) Reusability of Mg‐Al mixed oxides (Mg/Al=3)
for the reaction at 80 °C and 100 °C. Reaction conditions: benzyl alcohol (1 mmol), aniline (2 mmol), catalyst (0.3 g), toluene (10 mL), air (0.1 Mpa).
species in the filtration could be detected by ICP measurement.
Furthermore, multiple reaction cycles were performed to in‐
vestigate the recoverability of this sample. Two separate reac‐
tions were carried out at 80 and 100 °C, respectively (Fig. 7(c)).
After four cycles, the catalyst in the reaction at 100 °C still re‐
tained a high activity, whereas the activity of the catalyst in the
reaction at 80 °C decreased significantly. It could be concluded
that the adsorption and desorption of reactants and products
on the catalyst surface have an important effect on the cyclic
use of the catalyst. The adsorbed species could destroy the
structure of Mg‐Al oxides during the following thermal treat‐
ment. The surface area detected by N2‐adsorption decreased
from 168 m2·g–1 to 39 m2·g–1. A relatively high reaction tem‐
perature was favorable for the desorption of products, which is
beneficial for the recycling of catalysts.
It is known that Mg‐Al oxides, as well as MgO and Al2O3, are
all irreducible metal oxides. The catalytic process should be
quite different from the reducible metal oxide catalysts re‐
ported previously [21–26]. Combined with the above charac‐
terization results, it could be proposed that the surface ac‐
id‐base sites of Mg‐Al oxides catalyst should be the main active
sites for the oxidative coupling of benzyl alcohol and aniline.
The influence of Mg/Al ratio on the catalytic performance was
also mainly manifested by the change of the surface acidity and
basicity of the catalyst. To understand the role of the acid‐base
properties of the catalyst in the title reaction, we attempted to
use pyridine (basic molecule) and pyrrole (weak acidic mole‐
cule) to poison part of the surface acidic and basic active sites
of the Mg‐Al oxide catalyst (Mg/Al = 3). The detailed
pre‐treatment process has been described in the Experimental
section. Fig. 8 shows the catalytic performance of the poisoned
catalysts. Compared with the untreated sample, the catalytic
activity of both poisoned samples decreased. The sample
treated with pyrrole exhibited a much lower activity than that
treated with pyridine. This result indicated that the surface
basic sites should play a more important role in the reaction.
NH3‐TPD and CO2‐TPD results showed that the sample with
Mg/Al = 3 possessed a small number of surface acidic sites
(34.8 μmol·g–1, Table 2) and a large number of surface basic
sites (89.8 μmol·g–1, Table 2). Combined with the catalytic re‐
sult, we could further confirm the important role of the surface
basic sites in the oxidative coupling of benzyl alcohol and ani‐
line.
To further understand the role of the surface acid‐base
properties in the catalytic process, we attempted to tune the
surface properties of the sample (Mg/Al = 3) by changing the
calcination temperature and then investigating their catalytic
performance. XRD patterns (Fig. 9(a)) showed that the sample
calcined at 400 °C (denoted as MgAl‐400) exhibited a similar
crystal structure to that calcined at 500 °C (the catalyst studied
above, denoted as MgAl‐500). The sample calcined at 800 °C
(denoted as MgAl‐800) contained more MgAl2O4 crystalline
phases compared with the other two samples. N2‐adsorption
results showed that MgAl‐400 exhibited a similar specific sur‐
face area to MgAl‐800, which was 97.1 and 102.1 m2·g–1, re‐
spectively. These were a little lower than that of MgAl‐500
(167.9 m2·g–1, Table 3). The catalytic result showed that
MgAl‐500 exhibited a higher activity than MgAl‐800. Whereas
the MgAl‐400 sample showed a very low activity under the
same reaction conditions (Fig. 9(b)). NH3‐TPD results (Fig.
10(a)) showed that all these samples possessed a certain num‐
ber of acidic sites on the catalyst surface. Among them,
100
(1)
80
60
40
20
0
(2)
(3)
0
2
4
6
8
Reaction time (h)
Fig. 8. Catalytic performance of Mg‐Al mixed oxides (Mg/Al = 3) treated
under different conditions: (1) fresh sample, (2) pre‐treated with pyri‐
dine, (3) pre‐treated with pyrrole. Reaction conditions: benzyl alcohol
(1 mmol), aniline (2 mmol), catalyst (0.3 g), toluene (10 mL), air (0.1
Mpa).

Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
315
100
500 C
600 C
800 C
MgAl2O4
(b)
(a)
MgO




80
60
40
20
0


800 C


600 C
500 C
400 C
400 C
0
2
4
6
8
10
20
30
40
2/(o )
50
60
70
Reaction time (h)
Fig. 9. (a) XRD patterns and (b) catalytic performances of Mg‐Al mixed oxides with Mg/Al = 3 calcined at different temperatures. Reaction conditions:
benzyl alcohol (1 mmol), aniline (2 mmol), catalyst (0.3 g), toluene (10 mL), 80 °C, air (0.1 MPa).
which is a weak acidic reagent. Aniline (pKa = 4.63) was mainly
adsorbed on the surface acidic sites of catalyst.
Table 3
Texture properties of Mg‐Al mixed oxides with Mg/Al = 3 calcined at
different temperatures.
Our previous work has shown that oxidative dehydrogena‐
tion of benzyl alcohol is the rate‐determining step in the reac‐
tion [26]. Over reducible oxide catalysts, the reaction proceeds
in two consecutive steps, including oxidative dehydrogenation
of alcohol to benzaldehyde and imine formation by reaction of
benzaldehyde with aniline [23]. In this case, the step of oxida‐
tive dehydrogenation of alcohol to benzaldehyde hardly occurs
on the irreducible Mg‐Al oxides in the absence of aniline (Fig.
11(a)). This means that oxidative coupling of benzyl alcohol
and aniline over Mg‐Al oxides should experience a different
reaction path. Molecular O2 was demonstrated to be involved in
this reaction since little imine can be observed in the absence of
molecular O2 (Fig. 11(b)). However, the activation of molecular
oxygen and benzyl alcohol is a relatively complex process,
which needs participation by aniline. This point is quite differ‐
ent from that over the reducible oxides catalysts, such as sup‐
ported FeOx and CeO2, reported previously [23,26]. The cou‐
Sample
SBET (m2·g–1) Pore volume (cm3·g–1) Pore size a (nm)
MgAl‐400
MgAl‐500
MgAl‐600
MgAl‐800
97.1
167.9
128.3
102.1
0.23
0.36
0.28
0.36
9.4
8.6
8.8
14.1
a Calculated from adsorption branches using BJH model.
MgAl‐400 possessed the largest amount of weak acidic sites.
The CO2‐TPD (Fig. 10(b)) results showed that MgAl‐400 was
almost absent of weak basic sites, which was quite different to
that for MgAl‐500, MgAl‐800 and the other samples studied
above. The low activity of MgAl‐400 in the title reaction should
mainly arise from the absence of weak basic sites on the cata‐
lyst surface. Combined with above results, it can be concluded
that weak basic sites play a very important role in the oxidative
coupling of benzyl alcohol and aniline. These weak basic sites
should mainly provide adsorption sites for benzyl alcohol,
(b)
(a)
I
II
II
III
III
IV
IV
800 °C
800 °C
I
500 °C
400 °C
500 °C
400 °C
100
200
300
400
100
200
300
400
500
Temperature (C)
Temperature (C)
Fig. 10. (a) NH3‐TPD and (b) CO2‐TPD profiles of Mg‐Al mixed oxides with Mg/Al = 3 calcined at different temperatures.

316
Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
Table 4
Characterization of acid‐base properties of Mg‐Al mixed oxides with Mg/Al = 3 calcined at different temperatures.
Total acid amount
(μmol·g–1)
CO2‐TPD (μmol·g–1)
Total base amount
(μmol·g–1)
NH3‐TPD (μmol·g–1)
Sample
PeakⅠ
13.5
5.9
PeakⅡ Peak Ⅲ Peak Ⅳ
PeakⅠ
1.0
25.9
9.6
PeakⅡ
12.4
Peak Ⅲ
10.5
Peak Ⅳ
—
18.8
MgAl‐400
MgAl‐500
MgAl‐800
22.9
8.5
19.4
3.3
16.3
21.0
—
4.1
9.7
39.6
34.8
53.5
23.9
89.8
49.7
12.2
32.9
3.5
12.2
17.9
10.0
100
80
60
40
20
100
100
80
60
40
20
0
(a)
(c)
(b)
(1)
(2)
(1)
80
60
40
20
0
(2)
0
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
Reaction time (h)
Reaction time (h)
Reaction time (h)
Fig. 11. (a) Benzyl alcohol oxidation (1) in the presence of aniline and (2) without aniline. (b) The coupling reaction of benzyl alcohol and aniline
under (1) air (molecular oxygen) and (2) N2 (inert) conditions. (c) The coupling reaction of benzaldehyde and aniline in air.
pling of benzaldehyde and aniline is a fast step over Mg‐Al ox‐
ide catalysts (Fig. 11(c)). Therefore, the activation of benzyl
alcohol is still the rate‐determining step for this oxidative cou‐
pling reaction although this process is not directly achieved by
forming benzaldehyde. Further work is still needed to clarify
the details of this process.
Furthermore, the general applicability of the Mg‐Al oxides
catalyst was also investigated. At least six kinds of imines could
be obtained by oxidative coupling of amine with different alco‐
hols. Table 5 shows that the Mg‐Al mixed oxide catalyst was
active in the reactions using benzyl alcohol derivatives with
either electron‐rich‐ or electron‐poor‐substituent groups, es‐
pecially for 4‐methoxy benzyl alcohol, 4‐nitro benzyl alcohol
and p‐methyl benzyl alcohol (the yield of imine was 92.2 %,
85.6% and 80.1% after 12 h reaction, respectively).
4. Conclusions
Mg‐Al mixed oxides with various Mg/Al ratios have been
prepared by a co‐precipitation method. They can effectively
catalyze the oxidative coupling of alcohols and amines to imine.
The sample with Mg/Al = 3, which possesses a large amount of
weak basic sites and a relatively small amount of weak acidic
sites, exhibits the highest activity in the reaction. These acidic
and basic sites serve as adsorption and activation sites for
amines and alcohols, respectively. Among them, the weak basic
sites play a more important role than the acidic sites in the
catalytic process. They are the reactive centers for benzyl alco‐
Table 5
Aerobic oxidative coupling of various alcohols and aniline over Mg‐Al mixed oxide catalysts.
Entry
Imine
Time (h)
Alcohols
Yield (%)
1
2
8
92.2 %
80.1%
12
3
4
12
12
60.2 %
85.6 %
5
6
12
12
76.1%
68.5 %
Reaction conditions: alcohols (1 mmol), aniline (2 mmol), catalyst (0.3 g), toluene (10 mL), 80 °C, air (0.1 MPa).

Jinling Song et al. / Chinese Journal of Catalysis 39 (2018) 309–318
317
Graphical Abstract
Chin. J. Catal., 2018, 39: 309–318 doi: 10.1016/S1872‐2067(17)63006‐7
Oxidative coupling of alcohols and amines to an imine over Mg‐Al acid‐base bifunctional oxide catalysts
Jinling Song, Guiyang Yu, Xi Li, Xuwei Yang, Wenxiang Zhang *, Wenfu Yan, Gang Liu *
Jilin University
O2
H2O
Mg‐Al mixed oxides with acid‐base bifunctional properties can effectively catalyze the aerobic coupling of alcohols and amines to imine.
Yu, Chem. Eur. J., 2011, 17, 13308–13317.
hol activation, and the activation of benzyl alcohol is the
rate‐determining step for this oxidative coupling reaction. This
work is very helpful to understand the role of the surface ac‐
id‐base properties in the oxidative coupling reaction at low
temperature, and will provide a new method for the develop‐
ment of irreducible acid‐base catalysts for the imine synthesis.
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镁铝酸碱双功能氧化物催化剂上醇胺氧化耦合合成亚胺性能
宋金玲a, 于贵阳a, 李 玺a, 杨胥微a, 张文祥a,*, 闫文付b, 刘 钢a,#
a吉林大学化学学院, 表面与界面化学吉林省重点实验室, 吉林长春130021
b吉林大学无机合成与制备化学国家重点实验室，吉林长春130012
摘要: 亚胺是一类重要的含氮有机化合物, 由于具有不饱和C=N双键, 可以作为一种有效的氮源, 用于一系列含氮衍生物
的合成. 目前合成亚胺的工艺路线主要是通过羰基化合物和一级胺在强酸条件下缩合; 与该路线相比, 醇和胺在空气或氧
气存在条件下耦合是一条更为绿色的工艺路线, 其副产物只有水产生. 目前的报道表明, 一些具有氧化还原性质的催化剂,
如负载型贵金属催化剂和负载型过渡金属氧化物催化剂在该反应中表现出一定催化性能, 但很少关注表面酸碱性质对该
反应性能的影响. 在本工作中, 我们尝试将具有酸碱双功能性质的Mg-Al复合氧化物作为催化剂用于该反应中, 考察了
Mg/Al比、焙烧温度和后处理条件对催化性能的影响. 结果显示, Mg/Al = 3的催化剂在反应中表现出最优的催化活性;
NH3-TPD和CO2-TPD显示, 随着镁含量的增加, 样品表面碱性中心的数量呈现出先增加后减少的趋势, 其中Mg/Al = 3的样
品表面酸、碱总量最大, 而且该样品表面弱碱中心数量也最多; 我们通过焙烧和探针分子吸附等后处理手段进一步调控了
催化剂表面的酸碱性质, 初步结果表明在酸碱中心的协同作用下可以有效地催化醇和胺的氧化耦合反应; 其中弱碱性位
可能是活化醇的主要活性中心, 而醇的氧化是该反应的速控步骤, 因此可以推测表面弱碱中心的数量在一定程度上决定着
催化剂在该反应中的性能.
关键词: 氧化耦合; 亚胺合成; 醇氧化; 镁铝复合氧化物; 酸碱催化
收稿日期: 2017-11-13. 接受日期: 2017-12-25. 出版日期: 2018-02-05.
*通讯联系人. 电话: (0431)85155390; 传真: (0431)88499140; 电子邮箱: zhwenx@jlu.edu.cn
#通讯联系人. 电话: (0431)85155390; 传真: (0431)88499140; 电子邮箱: lgang@jlu.edu.cn
基金来源: 国家自然科学基金(21473073, 21473074); 吉林省科技发展计划项目(20170101171JC); 吉林省教育厅“十三五”科学技
术研究项目(2016403); 国家重点实验室无机合成和制备化学的开放项目(201703).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).