AlCl3 immobilized on silicic acid as efficient Lewis acid catalyst for highly selective preparation of dicyclohexylamine from the vapor phase hydroamination of cyclohexene with cyclohexylamine

Article abstract of DOI:10.1016/j.catcom.2020.106112

An efficient and stable Lewis acid catalyst silicic acid (SA)-immobilized AlCl₃ (AlCl₃-SA) has been successfully prepared by the chemical bonding method in this work. The results indicated that the immobilized 15percentAlCl₃-SA exhibited excellent catalytic performance and stability in the vapor phase hydroamination of cyclohexene with cyclohexylamine. 58.5percent cyclohexene conversion with 98.7percent selectivity to dicyclohexylamine was still maintained after running for over 150 h, and the space time yield of dicyclohexylamine was 142.6 mol/h·m³. The developed AlCl₃-SA catalyst had the advantages of low cost and long-time stable activity. Maybe this work provides a promising approach for hydroamination of olefins to amines.

Full text of DOI:10.1016/j.catcom.2020.106112

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AlCl3 immobilized on silicic acid as efficient Lewis acid catalyst
for highly selective preparation of dicyclohexylamine from the
vapor
phase
hydroamination
of
cyclohexene
with
cyclohexylamine
Jingbin Wen, Kuiyi You, Fangfang Zhao, Jian Jian, Pingle Liu,
Qiuhong Ai, He'an Luo
PII:
S1566-7367(20)30188-6
DOI:
https://doi.org/10.1016/j.catcom.2020.106112
CATCOM 106112
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 April 2020
17 June 2020
3 July 2020
Please cite this article as: J. Wen, K. You, F. Zhao, et al., AlCl3 immobilized on silicic
acid as efficient Lewis acid catalyst for highly selective preparation of dicyclohexylamine
from the vapor phase hydroamination of cyclohexene with cyclohexylamine, Catalysis
Communications (2020), https://doi.org/10.1016/j.catcom.2020.106112
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AlCl3 immobilized on silicic acid as efficient Lewis acid catalyst for
highly selective preparation of dicyclohexylamine from the vapor
phase hydroamination of cyclohexene with cyclohexylamine
Jingbin Wena, Kuiyi Youa,b,*, Fangfang Zhaoa, Jian Jianc, Pingle Liua,b,
Qiuhong Aia,b and He’an Luoa,b,*
a School of Chemical Engineering, Xiangtan University, Xiangtan 411105,
P.R. China
b
National & Local United Engineering Research Center for Chemical
Process Simulation and Intensification, Xiangtan University, Xiangtan
411105, P. R. China
c School of Chemistry and Chemical Engineering, Hunan University of
Science and Technology, Xiangtan 411201, P.R. China
*Corresponding Author:
E-mail: kuiyiyou@xtu.edu.cn (K. You); hluo@xtu.edu.cn (H. Luo).
Tel: +86-731-58293545; Fax: +86-731-58298267.
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Abstract
An efficient and stable Lewis acid catalyst silicic acid
(SA)-immobilized AlCl3 (AlCl3-SA) has been successfully prepared by
the chemical bonding method in this work. The results indicated that the
immobilized 15%AlCl3-SA exhibited excellent catalytic performance and
stability in the vapor phase hydroamination of cyclohexene with
cyclohexylamine. 58.5% cyclohexene conversion with 98.7% selectivity
to dicyclohexylamine was still maintained after running for over 150
hours, and the space time yield of dicyclohexylamine was 142.6 mol/h·m3.
The developed AlCl3-SA catalyst had the advantages of low cost and
long-time stable activity. Maybe this work provides a promising approach
for hydroamination of olefins to amines.
Keywords: Immobilized AlCl3-SA catalyst, Dicyclohexylamine,
Cyclohexene, Catalytic hydroamination, Vapor phase.
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1. Introduction
The organic amines as important chemical raw materials and
intermediates dominated a key role in fine chemicals, pharmaceuticals,
pesticides, dyestuffs and petrochemical industries. For example,
dicyclohexylamine (DCHA) was used as a useful chemical intermediate
for the synthesis of excellent vapor phase corrosion inhibitors [1], rubber
vulcanization accelerators, textiles, varnishes and commercial insecticide
[2]. It can be also employed to absorb acidic gases, to plasticize casein
and neutralize plant and insect poisons. Its fatty acids and sulfuric acid
salts have soap and detergent properties used in the printing and textile
industries. Several methods are employed for the manufacture of DCHA,
which include the hydrogenation of equimolar amounts of cyclohexanone
and cyclohexylamine [3], the hydrogenation of nitrobenzene [4] and the
hydrogenation of nitrobenzene with phenol [5]. At present, DCHA is
industrially manufactured by vapor phase catalytic hydrogenation of
aniline at elevated temperature and pressure [6]. However, DCHA is hard
to be separated from the high boiling residues comprised of unreacted
aniline, N-phenylcyclohexylamine and DCHA in this process. Therefore,
developing an alternative method for the convenient production of DCHA
is imperative. Fortunately, the direct hydroamination of olefins to amines
is a desirable process, which offers a simple, efficient, and high
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atom-economical access. Hence, one-step hydroamination of cyclohexene
and cyclohexylamine to DCHA with high atom-economical efficiency is
regarded as a promising method.
Previous studies for the catalysts of hydroamination mainly focused
on the Brönsted acids [7, 8], zeolites [9-15], noble metals [16] and
transition metals [17, 18]. It worth be mentioned that the pentasil zeolites
[13, 14] are the only type of catalysts currently applied in the
commercialization of the direct amination, and have an extremely high
service time and last for years without deactivation. Based on these
patents reported by Wolfgang Hölderich, the very important intermediate
tertbutyl-amine was commercialized in three BASF units, such as
Freeport USA (10.000 t/a), Antwerp NL (10.000 t/a) and Nanjing China
(30.000 t/a). However, mentioned-above these catalysts more or less had
some disadvantages (Table S1 in Supporting Information (SI)).
Therefore, developing a cheap, highly efficient and stable catalyst for
direct hydroamination of olefins to amines is highly desirable. To our
delight, the Lewis acid catalysts have been widely applied in some
organic reaction fields due to its environmental, practical and economical
considerations. Especially, some typical Lewis acid catalysts were
introduced into the liquid phase hydroamination of olefin to amines
[19-24], and an acceptable amine yield was achieved. However, they
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were scarcely used in vapor phase hydroamination of olefin to amines.
Herein, we had successfully prepared a new mesoporous silicic acid
(SA)-immobilized AlCl3 (AlCl3-SA) as a highly efficient and stable
Lewis acid catalyst by chemical bonding method. The mesoporous
AlCl3-SA catalyst exhibited excellent catalytic performance and stability
in the vapor phase hydroamination of cyclohexene with cyclohexylamine
to DCHA (Scheme 1). The obtained results from the vapor phase
hydroamination process will be reported in details in this paper.
NH2
H
AlCl3-SA
vapor phase
+
+
N
N
( DCHA )
( CCA )
Scheme 1 One-step catalytic hydroamination of cyclohexene to DCHA
over immobilized mesoporous AlCl3-SA.
2. Experimental Section
2.1 Reagents and instruments
Cyclohexene (AR) and cyclohexylamine (AR) was purchased from
Shanghai Macklin Biochemical Co., Ltd, China. Other chemicals (AR)
used were obtained from Sinopharm Chemical Reagent Co., Ltd., China.
The gas chromatograph (GC) (Agilent 7890B) equipped with a HP-5
capillary (30mx0.25mmx0.25μm) column and a hydrogen flame
ionization detector (FID) was employed as the quantitative analysis of
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products. The gas chromatography-mass spectrometer (GCMS-QP2010
Plus Shimadzu) was employed as the qualitative analysis of products by
the internal standard method (toluene was used as internal standard
substance).
2.2 Preparation process of immobilized AlCl3-SA catalyst
The immobilized Lewis acid catalysts were prepared by chemical
bonding method. Typically, the preparation process of immobilized
AlCl3-SA catalyst (Scheme 2) was as follows:
The preparation of SA: Firstly, 35.0 g sodium silicate was
dissolved in 200 ml deionized water. Then, 30.8 g hydrochloric acid was
added into the solution under stirring. The resulting gel was aged for 24 h
at room temperature. Finally, the gel was washed until the filtrate became
neutral, and the obtained gel (silicic acid) was dried at 100 oC for 12 h in a
vacuum oven.
The preparation of immobilized AlCl3-SA catalyst: Firstly, in
order to avoid the hydrolysis of anhydrous AlCl3, a certain amount of
anhydrous AlCl3 was rapidly dissolved in 20 mL anhydrous ethanol. Then,
2 g of dried silicic acid as precursor was added into the above solution.
o
The resulting suspension was vigorously stirred and refluxed at 80 C for
4 h in N2 atmosphere. After then, the solvent was removed by rotary
o
evaporated at 60 C under vacuum. Finally, the obtained white solid
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powder was calcinated at 300 oC for 4 h in N2 atmosphere. The resulting
catalysts impregnated with 0.1g, 0.2 g, 0.3 g, 0.4 g, 0.5 g and 0.6 g of
anhydrous AlCl3 were named as 5%AlCl3-SA, 10%AlCl3-SA,
15%AlCl3-SA, 20%AlCl3-SA, 25%AlCl3-SA and 30%AlCl3-SA (wt%),
o
respectively. The spent catalyst was regenerated by calcination at 550 C
in air for 4 h.
AlCl3
HCl
O
O
Cl
Cl
Cl
Na2SiO3
Al O Si
O
Al
HO Si OH
80 oC, 4 h
Cl
SA
Immobilized AlCl3-SA
Scheme 2 Preparation of immobilized AlCl3-SA catalyst
2.3 Typical experimental procedure
The vapor phase hydroamination of cyclohexene with
cyclohexylamine was carried out in a fixed bed straight tubular reactor of
850 mm length with 10 mm inner diameter. 3.5 g catalyst was placed at
the center of the reactor. The hydroamination reaction was performed at a
set reaction temperature under atmospheric pressure. The mixture of
cyclohexene and cyclohexylamine were introduced into the reactor using
a high pressure constant flow pump with 0.02 ml/min flow rate, and the
flow of N2 as carrier gas was controlled by a mass flow meter. The
mixtures from the outlet of reactor were condensed with a
o
low-temperature cooling liquid around 2-6 C. The liquid products were
identified and quantified by GC-MS and GC, respectively. Toluene was
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used as internal standard sample for the quantitative analysis of products.
The conversion of cyclohexene and selectivity to DHCA and
N-cyclohexylidenecyclohexanamine (CCA) were calculated using the
following equations:
the mole of cyclohexene reacted
=
Conversion of cyclohexene
Selectivity to DCHA=
×
100%
the mole of cyclohexene added
the mole of DCHA
×
100%
100%
the mole of cyclohexene reacted
the mole of CCA
×
Selectivity to CCA=
the mole of cyclohexene reacted
2.4 Characterization of catalysts
X-ray diffraction (XRD) patterns of samples were recorded on a
Japan Rigaku D/Max-2550VB+ 18 KW powder diffractometer with a
monochromatic Cu Kα radiation source (λ=1.5418 Å) at 40 kV voltage
and 300 mA electric current. The infrared spectra of pyridine adsorption
(Py-FTIR) of samples were examined on a Thermo Nicolet 380 FT-IR
spectrometer in the range of 1400-1650 cm-1. The N2
adsorption-desorption isotherms of samples were measured on an
American Mike Instruments 3Flex Physisorption analyzer at 77 K. The
temperature-programmed desorption of ammonia (NH3-TPD) of samples
were performed on a CHEMBET-3000 instrument from 100 to 700 oC at
a
heating rate of 10 oC/min. Thermogravimetric/derivative
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thermogravimetric (TG/DTG) analysis of samples were conducted on a
Mettler TGA/DSC1/1600HT thermogravimetric analyzer under 40
ml/min of nitrogen flow and 10 oC/min of heating rate from 30 to 800 oC.
3. Results and discussion
3.1 Catalytic performance of various catalysts in the vapor phase
hydroamination reaction
In an effort to seek the suitable catalyst for selective hydroamination
of cyclohexene with cyclohexylamine to DCHA, several typical catalysts
were screened under the identical reaction conditions at atmospheric
pressure, and the obtained results were summarized in Table 1. As
expected, DCHA was not obtained in the absence of catalyst. The
cyclohexene conversion and selectivity to DCHA were improved in
different degrees as the microporous molecule sieves with strong
Broensted acidic sites (such as HZSM-5, HY) and the weakly acidic
catalysts with surface hydroxyl groups (such as SA and γ-Al2O3) were
introduced into the hydroamination reaction. In addition, compared with
strongly acidic HZSM-5, the weakly acidic catalysts, iron modified
ZSM-5 (Fe/ZSM-5), cobalt modified ZSM-5 (Co/ZSM-5) and boron
modified ZSM-5 (B/ZSM-5) obviously improved the cyclohexene
conversion and selectivity to DCHA. The cyclohexene conversion and
selectivity to DCHA were futher improved over typical Lewis acid
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catalysts (such as AlCl3, FeCl3 and ZnCl2). Unfortunately, these Lewis
acid catalysts have poor stability and are easy to be hydrolyzed in air,
which makes the difficulties in operation. To our delight, an acceptable
cyclohexene conversion and selectivity to the desired product DCHA
were achieved over the immobilized Lewis acid catalysts. Especially, the
immobilized 15%AlCl3-SA catalyst gave the satisfactory results with
41.6% of cyclohexene conversion and 97.3% of selectivity to DCHA.
These results showed that the catalyst played a crucial role in the vapor
phase hydroamination of cyclohexene to DCHA.
Table 1 Comparison of catalytic performance of various catalysts in the
hydroamination reactiona
Selectivity (%)
Conversion of cyclohexene
Catalyst
(%)
DCHA CCA
None
HZSM-5
HY
-
-
-
4.3
85.4
14.6
6.4
88.8 11.28
γ-Al2O3
SA
17.6
18.3
10.5
8.7
71.6
75.4
88.3
86.5
88.7
97.1
95.8
90.6
91.1
28.4
24.6
11.7
13.5
11.3
2.9
Fe/ZSM-5
Co/ZSM-5
B/ZSM-5
12.5
28.5
26.8
27.6
36.9
b
AlCl3
FeCl3
ZnCl2
4.2
9.4
15%H2SO4/SiO2
8.9
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15%FeCl3-SA
15%ZnCl2-SA
41.5
44.9
41.6
35.9
91.4
71.3
97.3
8.6
28.7
2.7
15% AlCl3-SA
15%AlCl3-MCM-41
97.2
2.8
a Reaction conditions: the reaction temperature is 280 C, the molar ratio
of cyclohexene to cyclohexylamine is 1:1.5, residence time is 14.6 s, time
on stream is 6 h, and GHSV is 246.6 h-1.
o
b The reaction temperature is 190 oC.
Moreover, the hydroamination reaction of other amines and olefins
over immobilized AlCl3-SA catalyst were also examined, and some
satisfactory results were summarized in Table 2. 42.6% cyclohexene
conversion and 98.5% selectivity to N-methylcyclohexylamine was
obtained in the hydroamination of cyclohexene and methylamine. In
addition, for the hydroamination of styrene and diethylamine, 55.3% of
styrene
conversion
with
94.1%
of
the
selectivity
to
N,N-diethylphenethylamine was achieved. Furthermore, an acceptable
conversion and selectivity to the desired product were also obtained over
AlCl3-SA catalyst in the hydroamination of isobutylene with ethylamine,
and cyclohexene with aniline. These results demonstrated that the
immobilized AlCl3-SA catalyst exhibits good catalytic performance in the
hydroamination of olefins and amines.
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Table 2 Hydroamination reaction results of other amines and olefins over
15%AlCl3-SA catalysta
Conversion Selectivity
Olefins
Amines
Products
(%)
42.6
55.3
20.1
(%)
98.5
94.1
96.7
90.2
cyclohexene methylamine N-methylcyclohexylamine
styrene diethylamine N,N-diethylphenethylamine
isobutylene ethylamine
cyclohexene aniline
N-tert-butylethylamine
N-cyclohexylaniline
15.3
o
a Reaction conditions: the reaction temperature is 280 C, the molar ratio
of cyclohexene to cyclohexylamine is 1:1.5, residence time is 14.6 s, time
on stream is 6h, and GHSV is 246.6 h-1.
3.2. Optimization of reaction conditions
In order to find out optimal reaction conditions in hydroamination
reaction, a single factor experiment was performed in present work.
Table S2 in SI showed the effects of immobilized AlCl3-SA catalysts
with different AlCl3 loadings on the catalytic hydroamination reaction of
cyclohexene with cyclohexylamine. Fig. S1 in SI depicted the effects of
reaction temperature on the catalytic hydroamination reaction. The results
indicated that properly increasing the reaction temperature was beneficial
to the generation of target product DCHA. Fig. S2 in SI showed the
effects of residence time on the catalytic hydroamination reaction. It can
be clearly seen that the cyclohexene conversion gradually increased with
prolonged the residence time, and the selectivity to DCHA was basically
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unchanged. However, further prolonging the residence time more than 20
s, the cyclohexene conversion increased slowly, while the selectivity to
CCA obviously increased. The formation mechanism of CCA may be a
follow-up dehydrogenation reaction of DCHA over solid acid catalyst
under the longer residence time. Fig. S3 in SI displayed the effects of the
molar ratio of cyclohexene to cyclohexylamine on the hydroamination
reaction. The results showed that enhancing concentration of
cyclohexylamine facilitated the formation of DCHA. From the
single-factor experiment results, 58.5% conversion of cyclohexene with
98.7% selectivity to DCHA was achieved over immobilized
15%AlCl3-SiO2 catalyst under the optimal reaction conditions that 270 oC
of reaction temperature, 1:4 of the molar ratio of cyclohexene to
cyclohexylamine, 16.7 s of residence time and 215.6 h-1 of gas hourly
space velocity (GHSV).
3.3 Stability test of immobilized 15%AlCl3-SiO2 catalyst in the vapor
phase hydroamination reaction
It is well known that the catalyst stability is one of the crucial issues
in the development of efficient catalysts. Therefore, in order to verify the
stability of immobilized 15%AlCl3-SA catalyst, a long-term performance
test was performed under the optimal reaction conditions, and the
obtained results were depicted in Fig. 1. It was clearly seen that 58.5% of
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cyclohexene conversion with 98.7% selectivity to dicyclohexylamine was
still maintained after running for over 150 hours under the optimal
conditions. These results demonstrated that the immobilized
15%AlCl3-SA catalyst exhibited excellent stability and activity in the
vapor phase hydroamination of cyclohexene with cyclohexylamine to
DCHA. Furthermore, in order to further investigate the leaching problem
of immobilized 15%AlCl3-SA catalyst, the second and third run over the
regenerated catalyst were performed in this work, and the obtained results
were depicted in Fig. S4 and Fig. S5 in SI. These results demonstrated
that there were no leaching problems over the time for the immobilized
15%AlCl3-SA catalyst, and the thermos-stability of AlCl3 is significantly
improved by the immobilization methods.
100
80
60
40
Conversion of cyclohexene
Selectivity to DCHA
Selectivity to CCA
20
0
0
15 30 45 60 75 90 105 120 135 150
Time on stream (h)
Fig. 1 Stability test of immobilized 15% AlCl3-SA catalyst in the
hydroamination of cyclohexene with cyclohexylamine
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3.4 Characterization of catalysts
The XRD patterns for the wide-angle of SA and immobilized
15%AlCl3-SA samples were displayed in Fig. S6 in SI. Obviously, a
characteristic broad diffraction peak at 2θ=22o was observed in all
samples, which belonged to the typical amorphous structure [25].
Moreover, the diffraction peaks of AlCl3 were hardly observed in the
immobilized 15%AlCl3-SA samples, indicating that no free AlCl3
molecular species existed in the catalyst. The N2 adsorption-desorption
isotherms and pore diameter distribution curves of SA and 15%AlCl3-SA
samples were presented in Fig. S7 in SI, and the textural properties
(surface area, pore volume, and pore diameter) were summarized in
Table S3 in SI. The BET surface area of SA is 588 m2/g, while the BET
surface area decreased to 534 m2/g after impregnation. Fig. S8 in SI
depicted the thermo-stability of SA, AlCl3 and 15%AlCl3- SA samples.
The results demonstrated that the thermo-stability of AlCl3 was
significantly improved after immobilizing on the mesopores SA by
forming the stronger bond of Si-O-Al-Cl2.
To further verify the acidic sites and acid strength of catalysts, the
FT-IR spectra of adsorbed pyridine and NH3-TPD curves of SA and
15%AlCl3-SA catalysts were measured, and the obtained results were
depicted in Fig. 2. As shown in Fig. 2 (A), no significant IR signals were
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observed in the range of 1400-1650 cm-1 in the pyridine adsorbed FT-IR
spectra of SA. While AlCl3 was introduced to mesoporous SA, four
characteristic peaks were observed at 1447, 1490, 1547 and 1609 cm-1.
The high-intensity peaks appeared at 1447 and 1609 cm-1 were ascribed
to the characteristic adsorption of pyridine on strong Lewis acidic centers
[26], the low-intensity peak at 1547 cm-1 was attributed to the adsorption
of pyridine on Brønsted acidic centers [27], and the sharp peak at 1490
cm-1 was assigned to the interaction of pyridine adsorbed on both Lewis
and Brønsted acidic sites [28]. These results demonstrated that the Lewis
acidity of catalyst was significantly improved after AlCl3 was
immobilized on the SA, and 15%AlCl3-SA catalyst possessed the strong
Lewis acidic sites.
0.006
Fresh 15% AlCl3-SA
Spent 15% AlCl3-SA
SA
B
A
SA
Fresh 15%AlCl3-SA
Spent 15%AlCl3-SA
Regenerated 15%AlCl3-SA
Regenerated 15% AlCl3-SA
0.004
0.002
0.000
-0.002
170
1547
520
1447
1490
1609
100
200
300
400
500
600
700
800
1620 1590 1560 1530 1500 1470 1440 1410
Temperature (oC)
Wavenumber (cm-1)
Fig. 2 FT-IR spectra (A) of adsorbed pyridine and NH3-TPD curves (B)
of SA and 15%AlCl3-SA samples
Fig. 2 (B) described the NH3-TPD profiles of SA and 15%AlCl3-SA
samples. Obviously, the immobilized 15%AlCl3-SA samples exhibited
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the strong NH3 desorption peak while SA only showed the weak NH3
o
desorption peak at around 170 C, which was assigned to the desorption
of ammonia bonded on weak acid centers [29]. The results indicated that
the acid amounts of the weak acid centers of catalyst were remarkably
improved as AlCl3 was immobilized on SA. Moreover, a new NH3
desorption peak at around 520 oC was observed in 15%AlCl3-SA samples,
which was ascribed to the desorption of ammonia bonded on strong acid
centers [30]. This acid sites may be created by the interaction between Al
species and the Brönsted acid sites [31]. In addition, the intensities of
desorption peaks for spent 15%AlCl3-SA catalyst was almost unchanged,
indicating that the acid amounts and acid strength of catalyst were well
maintained during the hydroamination reaction. Furthermore, the FT-IR
spectra of adsorbed pyridine and NH3-TPD curves of regenerated catalyst
also indicated that there was no obviously loss in chloride and aluminum
after regeneration.
3.5 The possible reaction pathways for the hydroamination of
cyclohexene with cyclohexylamine over Lewis acid AlCl3-SA catalyst
In general, the reaction pathways for vapor phase catalytic
hydroamination of olefin with amine catalyzed by Lewis acid catalyst are
regarded as the electrophilic addition [24, 32, 33]. According to the
results obtained in this work, the possible mechanistic pathways of the
hydroamination of cyclohexene with cyclohexylamine over Lewis acid
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AlCl3-SA catalyst were proposed in Scheme 3. Firstly, cyclohexene and
cyclohexylamine were adsorbed on the immobilized AlCl3-SA surface.
Since Al3+ has higher affinity to cyclohexylamine than cyclohexene
because of the electronegativity of the lone pair electron of nitrogen atom,
the adsorption step results in the activation of cyclohexylamine [34].
Thus, the Al-coordinated amine species was formed, which generated the
proton as electrophilic reagent [35]. Then, the carbocation of cyclohexene
was generated by the electrophilic addition between Al-coordinated
amine species as protic acid and cyclohexene. The formed carbocation of
cyclohexene was attacked by the lone electron pair of Al-coordinated
amine species to generate the intermediate. Finally, the free Lewis acid
site of catalyst was regenerated again by the release of DCHA product.
H
NH2
NH2
δ
HN
Cl
O
Si
O
Si
O
Si
Cl
Cl
Cl
Cl
Cl
δ
Al
Cl
Al
O
O
Al
Al
O
O
Al
O
O
Al
Cl
Cl
Cl
Cl
Cl
Immobilized AlCl3-SA
adsorption activation
H
electrophilic addition
H
HN
HN
Cl
O
Si
O
H
N
Cl
Cl
Cl
Cl
Al
O
O
Al
Al
O
Si
O
Al
desorption
Cl
Cl
Cl
H
intermediate
electrophilic addition
( DCHA )
Scheme 3 Possible catalytic hydroamination reaction mechanism of
cyclohexene with cyclohexylamine over Lewis acid AlCl3-SA catalyst
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4. Conclusions
In summary, a simple and efficient approach for the selective
catalytic hydroamination of cyclohexene and cyclohexylamine to DCHA
in vapor phase has been successfully developed in this work. The
immobilized AlCl3-SA catalyst with strong Lewis acid sites was
successfully prepared by the chemical bonding between AlCl3 and
surface silicon hydroxyl groups (Si-OH). 58.5% of cyclohexene
conversion with 98.7% of selectivity to DCHA was obtained under the
optimized reaction conditions. Moreover, the immobilized 15% AlCl3-SA
catalyst exhibited high thermal-stability and activity in the
hydroamination reaction. Finally, the possible catalytic hydroamination
reaction mechanism of cyclohexene with cyclohexylamine over Lewis
acid AlCl3-SA catalyst was suggested. Maybe the present work provides a
promising strategy for the hydroamination of olefin to valuable amines
employing the inexpensive immobilized Lewis acid as catalyst, and has
important industrial application prospects.
Acknowledgements
We are grateful for the financial support by the National Natural
Science Foundation of China (21676226), the Natural Science
Foundation for Distinguished Young Scholars in Hunan Province
(2018JJ1023), Key Research and Development Program in Hunan
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(2019GK2041), Hunan Key
Province
Laboratory
of
Environment-Friendly Chemical Process Integrated Technology and
Collaborative Innovation Center of New Chemical Technologies for
Environmental Benignity and Efficient Resource Utilization.
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Credit Author Statement
Jingbin Wen: Experimental, Methodology, Characterization,
Writing-Original draft
Kuiyi You*: Supervision, Methodology, Investigation, Reviewing
and revising
Fangfang Zhao: Experimental, Characterization, Reviewing and
revising
Jian Jian: Experimental, Characterization,
Pingle Liu: Reviewing, Revising
Qiuhong Ai: Experimental
He’an Luo*: Supervision, Methodology, Investigation, Reviewing
and revising
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Declaration of Interest Statement
We declare that we have no any financial and personal relationships with
other people or organizations that can inappropriately influence our work,
there is no professional or other personal interest of any nature or kind in
any product, service and/or company that could be construed as
influencing the position presented in, or the review of, the manuscript
entitled, “AlCl3 immobilized on silicic acid as efficient Lewis acid
catalyst for highly selective preparation of dicyclohexylamine from the
vapor phase hydroamination of cyclohexene with cyclohexylamine”.
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Cl
Cl
Cl
Al
Si
Al
O
O
Cl
O
Graphic abstract:
Meso immobilized AlCl3-SA
29