Solvent-free synthesis of imines via N-alkylation of aromatic amines with alcohols over Co2+-exchanged zeolites

Article abstract of DOI:10.1016/j.catcom.2013.10.008

Co²⁺-exchanged zeolite catalysts were prepared by an ion-exchange route and firstly applied in the N-alkylation of aromatic amines with alcohols to imines. For the N-alkylation of aniline with benzyl alcohol, the aniline conversion of 89.8 mol% with an imine selectivity of 100% was achieved over Co-13X at 433 K under optimal conditions. The results showed that many factors including the Co loading, the support, the temperature, the alkali, could influence the reactions. Investigations carried out by XRD, SEM and recycling studies indicated that the Co-13X catalyst still remained porous structures of 13X zeolite and possessed a stable catalytic activity.

Full text of DOI:10.1016/j.catcom.2013.10.008

Catalysis Communications 43 (2014) 213–217
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Solvent-free synthesis of imines via N-alkylation of aromatic amines with
2+
alcohols over Co -exchanged zeolites
Y.-W. Sun a,b, X.-H. Lu a,b,⁎, X.-L. Wei a,b, D. Zhou a,b, Q.-H. Xia a,b,
⁎
a
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei,
b
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR China
a r t i c l e i n f o
a b s t r a c t
Co2+-exchanged zeolite catalysts were prepared by an ion-exchange route and firstly applied in the N-alkylation
of aromatic amines with alcohols to imines. For the N-alkylation of aniline with benzyl alcohol, the aniline
conversion of 89.8 mol% with an imine selectivity of 100% was achieved over Co-13X at 433 K under optimal
conditions. The results showed that many factors including the Co loading, the support, the temperature, the
alkali, could influence the reactions. Investigations carried out by XRD, SEM and recycling studies indicated
that the Co-13X catalyst still remained porous structures of 13X zeolite and possessed a stable catalytic activity.
Article history:
Received 8 August 2013
Received in revised form 8 October 2013
Accepted 10 October 2013
Available online 18 October 2013
Keywords:
Aromatic amines
Alcohols
©
2013 Elsevier B.V. All rights reserved.
Solvent-free N-alkylation
Imines
Co-zeolite catalysts
1
. Introduction
that the development of simple heterogeneous catalysts becomes one
of the significant tasks for researchers.
The N-alkylation of amines or ammonia with alcohols is a valuable
So far, several heterogeneous non-noble metal catalysts have been
reported, inclusive of the supported copper catalysts [19], Ni–Cu/γ-
Al
O catalyst [20], and NiCuFeOx catalyst [21], which required solvent
2 3
route for the synthesis of N-alkyl amines [1], which are valuable
intermediates broadly used in the manufacturing of pharmaceuticals,
dyestuffs, synthetic rubbers, herbicides, insecticides and agrochemicals
and long reaction time. Molecular sieves as heterogeneous catalysts
have been a subject of growing interests in many areas, which exhibit
superior catalytic performance [22]. For instance, Anand et al. reported
the gas-phase N-alkylation of aniline with ethanol over HY zeolites [23].
This work firstly reports catalytic application of simply Co2 -exchanged
zeolites by an ion-exchange route for the liquid-phase synthesis of imines
via N-alkylation of aromatic amines with alcohols in a solvent-free
system.
[
2]. Among those nitrogen-containing compounds, imines or secondary
amines are of particular importance. Traditional routes for this
preparation are amination of aryl/alkyl halides [3] and reductive
amination of carbonyl compounds [4–7], which have some drawbacks
like use of toxic alkylating agents and expensive carbonyl compounds,
and generation of inorganic salts. Since alcohols are nontoxic,
inexpensive and readily available, thus, using alcohols instead of the
above-stated reagents to produce N-alkyl amines is much more
concerned. Various kinds of homogeneous transition-metal catalysts
including ruthenium [8], iridium [9], platinum [10], gold [11],
silver [12], palladium [13] complexes have been reported for the
N-alkylation of amines with alcohols. In comparison with the
homogeneous catalysts, heterogeneous ones have advantages with
respect to isolation of product from catalyst, catalyst reuse, and
operational handling. Some heterogeneous catalysts have been lately
explored, such as solid acids [14] and transition metal-based catalysts
+
2. Experimental
2.1. Materials
All organic reagents included benzyl alcohol (N99%, Tianjin),
cyclohexanol (N99%, Aladdin), pentanol (N99%, Tianjin), 2-
phenylethanol (99%, Shanghai), benzylamine (99%, Aladdin), p-
chloroaniline (98%, Aladdin), which were directly used without further
purification. However, aniline, which was purchased from Sinopharm
Chemical Reagent Co. Ltd., underwent re-distillation under reduced
pressure to obtain a purity of 99.9%. Inorganic salts containing cobalt
[
15]; however, most of which required strict conditions [16–18], so
⁎
Corresponding authors at: Ministry-of-Education Key Laboratory for the Synthesis
and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR
China. Fax: +86 27 88663043.
E-mail addresses: xinhuan003@aliyun.com (X.-H. Lu), xiaqh518@aliyun.com
Q.-H. Xia).
(
II) acetate (99.5%, Co(Ac)
Ni(NO · 6H O), ferric (III) nitrate (98.5%, Fe(NO
chromic (III) nitrate (99.0%, Cr(NO · 9H O), potassium tert-
butoxide (99.5%, Bu OK) and sodium methoxide (99.5%, CH ONa)
2
· 4H
2
O), nickelous (II) nitrate (N98%,
)
3 2
2
)
3 3
· 9H O) and
2
)
3 3
2
t
(
3
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.10.008

214
Y.-W. Sun et al. / Catalysis Communications 43 (2014) 213–217
were purchased from Aladdin, sodium hydroxide (96%, NaOH),
potassium carbonate (97%, K CO ) and sodium bicarbonate (98%,
NaHCO ) were purchased from Tianjin. Na-MOR, 13X, Y, 5A, 4A, 3A,
2
3
c
3
Na-ZSM-5, Na-β zeolites were purchased from Nankai University,
China. SAPO-5 and SAPO-34 were synthesized in our lab.
2
+
Co cations or other metal ions were introduced into zeolites or
molecular sieves by an ion-exchange route. Typically, 2.0 g of zeolite
was treated with an aqueous solution (100 ml) of metal salts
b
(
1.0 mmol) at 363 K overnight while stirring. Then, the solid powder
was recovered by filtration, washed several times and followed by
drying at 373 K for 10 h.
2
.1. Characterization of catalysts
a
XRD (X-ray diffraction) patterns of solid samples were recorded on a
Rigaku D/MAX-IIIC diffractometer with Cu Kα (λ=1.54184Å) radiation
operating at 30kV and 25mA. The scanning range was from 5 to 65° 2θ
with a scanning speed of 2°/min. The particle sizes of samples were
calculated based on SEM images, which were determined on a JEOL
JSM-6510A scanning electron microscope. Cobalt content in each
sample was analyzed on a Shimadzu XRF-1800 spectrometer. The BET
specific surface area was calculated using the BET equation in the
1
0
20
30
40
50
60
Fig. 1. XRD patterns of a (13X), b (Co-13X), and c (Co-13X recovered after 5 recycles).
The scanning electron micrographs of catalysts are shown in Fig. 2,
which indicates that the morphology and particle size of Co-13X that
has undergone ion-exchange treatment are still similar to those of the
parent 13X.
range of relative pressures (p/p
.2. Catalytic reactions
The liquid-phase synthesis of imines via N-alkylation of aromatic
0
) between 0.05 and 0.25.
2
3
3
.2. Catalytic reactions
amines with alcohols in a solvent-free system was performed in a 25-ml
round-bottom glass flask equipped with a tap-water condenser at desired
temperatures under atmospheric pressure. In a typical run, 2.0 mmol of
amine, 10.0 mmol of alcohol and 40 mg of catalyst were added to the
reactor and mixed vigorously by a magnetic stirrer, followed by addition
of 45mg of basic promoter. After the completion of reactions, the reaction
mixture was cooled down to room temperature and diluted with 10 ml
of dichloromethane. The catalyst was recovered by filtration from the
liquid phase. The reaction mixture was analyzed by gas chromatograph
.2.1. Effect of different catalysts
As shown in Table 2, the influence of different M-zeolite catalysts on
the N-alkylation of aniline with benzyl alcohol was tested. When no
catalyst was added, the conversion of aniline was only 32.4%. Co-13X
was the most active for this reaction, achieving the highest aniline
conversion of 89.8 mol% with an imine selectivity of 100% under the
same conditions. However, the conversions of aniline over other M-
zeolite catalysts such as Ni-zeolites, Cr-zeolites, Cu-zeolites, Fe-zeolites
were b80 mol%, lower than those over Co-zeolites.
The effect of various Co-zeolites on the this reaction is compared in
Table 3, manifests that only Co-13X obtained 89.8 mol% conversion of
aniline, accompanied with 100% of imine selectivity. Although the
surface areas and pore diameters of various catalysts can be arranged
in the order of Co-13X N Co-Y N Co-SAPO-5 N Co-Beta N Co-MOR N Co-
ZSM-5 N Co-SAPO-34 N Co-5A N Co-4A N Co-3A, there is no obvious
correlation between the catalytic activity and both of the surface area
and pore size.
(Shimadzu GC-2010) with a DB-1 column (30 m × 0.25 mm × 0.25 μm)
and an FID detector, wherein chlorobenzene was used as the internal
standard. The conversion (mol%) of amine and the selectivity (%) to
imine were calculated as follows, e.g. the conversion of aniline = yield
of N-benzylideneaniline (mol%) + yield of N-phenylbenzylamine
(mol%) + yield of N,N-dibenzylaniline (mol%), and the selectivity of
imine = yield of N-benzylideneaniline/conversion of aniline × 100%.
3
. Results and discussion
3
.1. Characterization of catalysts
Table 1
a
b
Physicochemical characterization of zeolite catalysts. ( Result obtained from XRF. Values
obtained from N
2
-adsorption results.)
Metal contenta
(wt.%)
The XRD patterns of 13X and Co-13X have been shown in Fig. 1. Very
clearly, the diffraction pattern of Co2 -exchanged 13X shows the
characteristics of highly crystalline materials, with observable five
XRD peaks at 2θ=5.02°, 15.12°, 23.82°, 27.14° and 30.72°, typical
reflections of faujasite structures. However, the intensity of diffraction
lines of Co-13X in the range of 5–45° shows a slight reduction, as
compared to that of the parent 13X, probably due to the removal of
+
a
b
Catalyst
SiO
ratios
2
/Al
2
O
3
Surface area
(m /g)
Pore size
(Å)
2
13X
0
2.3
4.0
862.4
750.4
764.1
748.2
821.3
720.4
731.2
718.7
249.3
235.6
240.1
230.9
239.5
556.5
496.3
487.9
10
Co-13X
Ni-13X
Cr-13X
Na-Y
Co-Y
Ni-Y
3.05
3.11
3.07
0
3.12
3.08
3.10
0
3.03
2.98
3.06
3.09
0
2+
4.7
7.4
6.6
partial Al from the framework of 13X during the Co -exchanged
2+
process. The majority of Co species in the Co -exchanged sample exists
2
+
in the form of Co cations, as characterized by the appearance of only
Cr-Y
Na-MOR
Co-MOR
Ni-MOR
Cr-MOR
Cu-MOR
Na-Beta
Co-Beta
Fe-Beta
20.6
one peak with the binding energy of ca. 782 eV in the X-ray
photoelectron spectra, characteristic of Co2+-containing zeolites [24].
BET surface areas and pore sizes of various zeolites determined by BET
analysis are given in Table 1. It can be seen that such an ion-exchange
treatment has led to a slight decrease in the surface area of catalysts.
The XRF results have shown that the metal contents of all M-zeolite
catalysts prepared by ion-exchange are in the range of 2.98–3.15 wt.%.
25.0
6.7
3.04
3.15

Y.-W. Sun et al. / Catalysis Communications 43 (2014) 213–217
215
Fig. 2. SEM images of 13X (a) and Co-13X (b).
Table 2
1
00
100
80
60
40
20
0
Effect of M-zeolites loaded with different metals.
Entry
Catalyst
Conversion (mol%)
Selectivity (%)
8
6
4
0
0
0
1
2
3
4
5
6
7
8
9
–
32.4
42.5
79.6
76.0
89.8
63.9
51.9
81.9
72.9
43.5
73.1
63.7
84.3
73.1
82.9
87.4
100
96.6
100
100
100
97.4
100
100
100
96.7
91.5
82.7
13X
Ni-13X
Cr-13X
Co-13X
Ni-Y
Cr-Y
Co-Y
Ni-MOR
Cr-MOR
Co-MOR
Cu-MOR
Co-Beta
Fe-Beta
20
1
1
1
1
1
0
1
2
3
4
0
0
.5
2.5
4.5
6.5
8.5
10.5
Co loading /%
Fig. 3. Effect of different Co loadings on the N-alkylation of aniline with benzyl alcohol over
Co-13X: (♦) aniline conversion, (□) imine selectivity. (Reaction conditions: Aniline,
mmol; alcohol, 20 mmol; catalyst, 40 mg; Bu OK, 45 mg; temperature 433 K; duration
h).
t
Reaction conditions: Alcohol (20 mmol), aniline (2 mmol), catalyst (40 mg), Bu OK
(45 mg), temperature 433 K, time 6 h.
t
2
6
The effect of different Co loadings on the catalytic activity of Co-13X
was studied. As presented in Fig. 3, when 1.0wt.% Co was loaded on 13X,
the conversion of aniline and the selectivity to imine were 67.1 mol%
and 100%, lower than 71.8 mol% for 1.5 wt.% Co loading. When the Co
loading was increased to 3.05 wt.%, the conversion of aniline and the
selectivity to imine reached the maxima of 89.8 mol% and 100%. Along
with the continuous increase of the Co loading to 5.0 wt.% and
As one can see from Fig. S1, the aniline conversion over Co-13X
(ca. 3.05 wt.%) was increased from 78.3 to 93.0 mol% with increasing
the reaction temperature from 393 to 473 K. However, the selectivity
of imine was first increased from 85.6% at 393 K to 100% at 433 K, then
drastically dropped to ca. 77.6% at 453 K, and to 45.3% at 473 K, for
which 433 K was the most appropriate temperature, and too high
temperature would enhance side reactions. It seems that a suitable
temperature favored the formation of imine [25].
10.0 wt.%, the conversion of aniline remained basically unchanged but
the selectivity to imine abruptly dropped to 89.6% and 88.5%. This
shows that the excess cobalt loading did not favor the N-alkylation of
aniline with benzyl alcohol [22].
3.2.2. Effect of different basic promoters
According to our studies, the combined use of Co2+-zeolites with a
basic promoter played a key role in activating the desired reaction.
When no base was added, only 22.2 mol% of aniline was converted;
however, a basic promoter could greatly improve the efficiency of the
Table 3
Effect of various Co-zeolites.
Entry
Catalyst
Conversion (mol%)
Selectivity (%)
Table 4
1
2
3
4
5
6
7
8
9
Co-13X
Co-Y
Co-MOR
Co-ZSM-5
Co-5A
Co-4A
Co-3A
Co-Beta
Co-SAPO-5
Co-SAPO-34
89.8
81.9
63.1
67.1
82.3
82.0
81.4
84.3
72.4
76.1
100
97.4
100
100
91.1
92.2
93.2
91.5
N99
N99
Effect of different basic promoters.
Entry
Base
Conversion (mol%)
Selectivity (%)
1
2
3
4
5
6
–
22.2
89.8
82.8
81.3
80.8
52.2
60.2
100
92.7
30.3
97.8
100
t
Bu OK
CH
NaOH
CO
NaHCO
3
ONa
K
2
3
1
0
3
t
Reaction conditions: aniline (2 mmol), alcohol (20 mmol), catalyst (40 mg), Bu OK
Reaction conditions: Aniline (2mmol), alcohol (20mmol), catalyst (40mg), base (45mg),
temperature 433 K, time 6 h.
(45 mg), temperature 433 K, time 6 h.

216
Y.-W. Sun et al. / Catalysis Communications 43 (2014) 213–217
conv.
sele.
means the vital role of the added basic promoter in enhancing the
conversion of aniline and controlling the selectivity to imine.
1
00
100
80
3.2.3. Recycling studies
80
60
40
20
0
For the N-alkylation of aniline with benzyl alcohol, the recycling uses
of the catalyst Co-13X were carried out five times at 433 K while
keeping other conditions unchanged. For each recycle, the catalyst
was recovered by filtration, washing with ethanol and drying, and
then re-used in the next reaction. As revealed in Fig. 4, the conversion
of aniline and the selectivity to imine showed only a small drop from
89.8 to 84.4 mol% and from 100 to 95.3% with recycling. Chemical
analyses of the reaction mixture did not detect any visible amount of
cobalt in the solution, indicating that the leaching of cobalt was
negligible during reactions. The XRD patterns in Fig. 1 testify that the
faujasite structure of recovered Co-13X was still retained even if it had
been reused 5 cycles. This proposes the stability and recyclability of
Co-13X.
60
40
2
0
0
1
2
3
4
5
Number of recycle
Fig. 4. Recycling studies of Co-13X catalyst.
reaction. Indeed, different bases exerted obvious impacts, as shown in
3.2.4. Catalytic synthesis of various imines from aromatic amines with
alcohols
Table 4, where the promoting effect of five alkali compounds can be
t
put in the order of Bu OK N CH
3
ONa N NaOH N K
CO
2 3
N NaHCO
3
. In the
As illustrated in Scheme 1, various aromatic amines can be allowed
to react with different alcohols under optimal conditions to form imines.
One can observe from Table 5, the N-alkylations between aromatic
amines and aromatic alcohols could proceed more readily than those
of aromatic amines with both linear and cyclic alcohols. In the cases of
the former, the conversions of aromatic amines could surpass
80 mol%; however, for the latter two reactions the conversions of
amines were extremely low b10.8 mol%. Especially, the conversion of
aniline in the reaction between aniline and benzyl alcohol could reach
t
organic synthesis, Bu OK, as a moderate intense alkali, can be effective
for the α-position alkylation by a carbonyl group [26], which may be
t
the reason that the participation of Bu OK promoted the formation of
alkylated imine.
As illustrated in Fig. S2, the increase of the base amount from 0 to
6
9
0 mg led to almost a linear growth of aniline conversion from 22.2 to
0.2 mol% at 433 K. However, the excess increase of the base amount
would decrease the selectivity of imine from 100 to 54.6%, which
Scheme 1. Synthetic routes of imines via N-alkylation of aromatic amines with alcohols.

Y.-W. Sun et al. / Catalysis Communications 43 (2014) 213–217
217
Table 5
Acknowledgment
N-alkylation of various aromatic amines with alcohols.
Entry
1
Amine
Alcohol
Conv./%
83.0
Imine sele./%
70.9
The authors thank the financial supports by the National Natural
Science Foundation of China (Nos. 20901023, 21173073, 21273064).
Appendix A. Supplementary data
2
3
81.8
85.6
74.5
91.3
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.10.008.
References
[
[
1] X.J. Cui, X.C. Dai, Y.Q. Deng, F. Shi, Chem. Eur. J. 19 (2013) 3665–3675.
2] S.A. Lawrence, Amines: Synthesis, Properties, and Applications, Cambridge Univ.
Press, Cambridge, 2004.
4
5
6.0
48.8
58.3
[
3] A. Dhakshinamoorthy, P. Visuvamithiran, V. Tharmaraj, K. Pitchumani, Catal.
Commun. 16 (2011) 15–19.
10.8
[
[
4] S. Bhattacharyya, J. Org. Chem. 60 (1995) 4928–4929.
5] A.F. Abdel Magid, K.G. Carson, B.D. Harris, C.A. Maryanoff, R.D. Shah, J. Org. Chem. 61
(1996) 3849–3862.
[
6] S. Bhn, S. Imm, L. Neubert, M. Zhang, H. Neumann, M. Beller, ChemCatChem 3
(2011) 1853–1864.
t
Reaction conditions: amine (2mmol), alcohol (20mmol), Co-13X (40mg), Bu OK (45mg),
temperature 433 K, time 6 h.
[7] T. Mizuta, S. Sakagushi, Y. Ishii, J. Org. Chem. 70 (2005) 2195–2199.
[
8] S.A. Moya, M. Vidal, K. Brown, C. Negrete-Vergara, G. Abarca, P. Aguirre, Inorg. Chem.
Commun. 22 (2012) 146–148.
[
9] S. Michlik, R. Kempe, Chem. Eur. J. 16 (2010) 13193–13198.
[
10] T. Ohshima, Y. Miyamoto, J. Ipposhi, Y. Nakahara, M. Utsunomiya, K. Mashima, J. Am.
Chem. Soc. 131 (2009) 14317–14328.
[11] T. Ishidaa, R. Takamura, T. Takei, T. Akita, M. Haruta, Appl. Catal. A Gen. 413–414
(2012) 261–266.
12] X.J. Cui, Y. Zhang, F. Shi, Y.Q. Deng, Chem. Eur. J. 17 (2011) 1021–1028.
13] G. Reyes-Rios, Juventino J. García, Inorg. Chim. Acta 392 (2012) 317–321.
14] Sagar P. Pathare, Krishnacharya G. Akamanchi, Appl. Catal. A Gen. 452 (2013) 29–33.
the highest value of 89.8 mol%. This may be ascribed to the formation of
stable intermediate benzaldehyde and the easy condensation between
aniline and benzaldehyde, because the reaction has involved two steps:
[
[
[
(
1) the catalytic dehydrogenation of alcohol to benzaldehyde, (2) the
condensation between aniline and benzaldehyde plus subsequent
dehydration [1,2,20].
[15] D.L. Guo, H. Huang, J.Y. Xu, H.L. Jiang, H. Liu, Org. Lett. 10 (2008) 4513–4516.
[
[
16] A. Martínez-Asencio, D.J. Ramón, M. Yus, Tetrahedron Lett. 51 (2010) 325–327.
17] L. He, X.B. Lou, J. Ni, Y.M. Liu, Y. Cao, H.Y. He, K.N. Fan, Chem. Eur. J. 16 (2010)
13965–13969.
[
18] W.X. Zhang, X.C. Dong, W.L. Zhao, Org. Lett. 13 (2011) 5386–5389.
4
. Conclusions
[19] X.J. Cui, F. Shi, M.K. Tse, D. Gördes, K. Thurow, M. Beller, Y.Q. Deng, Adv. Synth. Catal.
51 (2009) 2949–2958.
3
[
[
20] J. Sun, X.D. Jin, F.W. Zhang, W.Q. Hu, J.T. Liu, R. Li, Catal. Commun. 24 (2012) 30–33.
21] X.J. Cui, X.C. Dai, Y.Q. Deng, F. Shi, Chem. Eur. J. 19 (2013) 3665–3675.
In summary, we firstly demonstrate that Co2+-exchanged zeolites
could act as heterogeneous catalysts for direct liquid-phase synthesis
of imines via N-alkylation of aromatic amines with alcohols in a
solvent-free system in the presence of basic promoter. Under optimal
conditions, the reaction between aniline and benzyl alcohol could
achieve the highest aniline conversion of 89.8 mol% with an imine
selectivity of almost 100% over Co-13X. Recycling studies showed the
recyclability of Co-13X as a heterogeneous catalyst.
[22] B. Tang, X.H. Lu, D. Zhou, P. Tian, Z.H. Niu, J.L. Zhang, X. Chen, Q.H. Xia, Catal.
Commun. 31 (2013) 42–47.
[
23] R. Anand, S.S. Khaire, R. Maheswari, K.U. Gore, V.R. Chumbhale, Appl. Catal. A Gen.
42 (2003) 171–177.
[24] D. Zhou, X.H. Lu, J. Xu, A.A. Yu, J.Y. Li, F. Deng, Q.H. Xia, Chem. Mater. 24 (2012)
160–4165.
25] G. Xu, Q.H. Xia, X.H. Lu, Q. Zhang, H.J. Zhan, J. Mol. Catal. A Chem. 266 (2007)
80–187.
[26] B. Movassagh, A. Rakhshani, Chin. Chem. Lett. 22 (2011) 1179–1182.
2
4
[
1