Ni-Cu/γ-Al2O3 catalyzed N-alkylation of amines with alcohols

Article abstract of DOI:10.1016/j.catcom.2012.03.010

A γ-Al₂O₃ supported Ni and Cu bimetallic nanoparticles catalyst (45 wt.% Ni, Ni/Cu mass ratio = 4.5/1.0) is prepared by electroless plating method for the N-alkylation of amines with alcohols under base and Lewis acidic cocatalyst conditions. The catalyst afforded fast conversions, high selectivity for amines and alcohols with various structures under an Ar atmosphere in o-xylene. Furthermore, catalyst still has a stable catalytic activity after two consecutive cycles regenerated.

Full text of DOI:10.1016/j.catcom.2012.03.010

Catalysis Communications 24 (2012) 30–33
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ni–Cu/γ-Al O catalyzed N-alkylation of amines with alcohols
2
3
Jian Sun, Xiaodong Jin, Fengwei Zhang, Wuquan Hu, Juntao Liu, Rong Li ⁎
The Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A γ-Al O supported Ni and Cu bimetallic nanoparticles catalyst (45 wt.% Ni, Ni/Cu mass ratio=4.5/1.0) is
2
3
Received 7 December 2011
Received in revised form 2 March 2012
Accepted 7 March 2012
prepared by electroless plating method for the N-alkylation of amines with alcohols under base and Lewis
acidic cocatalyst conditions. The catalyst afforded fast conversions, high selectivity for amines and alcohols
with various structures under an Ar atmosphere in o-xylene. Furthermore, catalyst still has a stable catalytic
activity after two consecutive cycles regenerated.
Available online 16 March 2012
©
2012 Elsevier B.V. All rights reserved.
Keywords:
A, Ni–Cu catalysts
B, Alkylation
C, Alcohols
D, Amines
1
. Introduction
transition metal-based catalysts, most of them require long reaction
time[27,28], high pressure [20], even special equipment [14,30].
Nitrogen-containing molecules, particularly amines and their de-
2 3
Here, we report our results about Ni–Cu/γ-Al O catalyzed cou-
rivatives are intermediates and products of enormous importance
for chemical and life science applications [1,2]. The general synthetic
methods for the synthesis of amines are amination of aryl/alkyl ha-
lides [3] and reductive amination of carbonyl compounds [4–7].
However, there can be problems with the toxicity of such alkylating
agents and the need for activation of aryl/ alkyl halides. The use of al-
cohols instead of aryl/alkyl halides to achieve the N-alkyl amines is an
attractive method because it does not generate harmful by-products.
Alcohols are inexpensive, readily available and the selectivity of
the reaction can be controlled with the catalyst. A variety of transition
metals such as ruthenium [8–12], iridium [13–18], platinum [19],
gold [20], silver [21,22], palladium [23,24], nickel [25,26], copper
pling reaction of amine with alcohol in the presence of base and
Lewis acidic additives at relatively short time. Meanwhile, it was
found that the catalyst showed a high activity for the N-alkylation re-
action, affording a high yield in all the case investigated.
2. Experimental
2.1. Preparation of catalysts
Ni–Cu/γ-Al
and a metal, such as Ag, was used to induce the plating. Ag/γ-Al
was synthesized as follows: An aqueous solution (425 mL) containing
AgNO (0.02 g, 0.118 mmol), ammonia (0.15 g, 8.824 mmol), NaOH
(0.005 g, 0.125 mmol), and formaldehyde (0.002 g, 0.067 mmol)
was prepared and then the Al (2.0 g) was added. After stirring at
40 °C for 4 h, the resulting Ag/γ-Al was washed with distilled water
and dried at 50 °C for 12 h. The amount of Ag in the support was found
to be 0.2% based on ICP analysis. For the preparation of Ni–Cu/γ-Al
2 3
O was prepared by electroless plating method [39],
2 3
O
[27,28], and iron [29–31] catalysts have been reported for the
3
N-alkylation of amines and alcohols. Although the majority of
reported homogeneous catalysts are active for this reaction, they are
significantly more expensive and non-recoverable. Consequently,
the development of economical and easily recyclable heterogeneous
catalysts is attracting more and more attention. In this context, het-
erogeneous systems have been employed for the N-alkylation of
amines. Although there are several reports on the N-alkylation
using heterogeneous catalysts [32–38] such as solid acids and
2 3
O
2 3
O
2 3
O ,
−
1
the Ag/γ-Al
2 3
O (4 g L ) support was added to the aqueous solution con-
−
1
−1
taining NiCl
2
·6H
(36 g L ), Cu(NO
0–13 for adjusting the particle size of Ni–Cu nanoparticles. After stirring
2
O
(16.8 g L ), NH
3
·H
2 2 4 2
O (10 g L ), N H ·H O
−
1
−1
3 2 2
) ·6H O (3.4 g L ) and NaOH to maintain a pH of
1
at 70 °C for about 3 h until no significant bubbles were observed. The
product was washed with distilled water thoroughly to give a pH of 7
and separated by filtration. The collected sample was dried in N
2
flow
⁎
Corresponding author. Tel.: +86 0931 891 2311; fax: +86 0931 891 2582.
E-mail address: liyirong@lzu.edu.cn (R. Li).
−
3
−1
−1
(100 cm
g
min ) at 80 °C for 12 h, and then treated in a 20%
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.03.010

J. Sun et al. / Catalysis Communications 24 (2012) 30–33
31
(
0.25 mmol), o-xylene (3.0 mL) and catalyst (0.1 g). The reaction was
performed at 160 °C for 12 h and was monitored by GC analysis. As-
signments of products were made by comparison with authentic
samples. Selected products were also analyzed by GC/MS (Agilent
6
,890 N/5,973 N). Since the ferromagnetic property of Ni compo-
nent, the catalyst was recovered in a facile manner from the reaction
mixture by using a permanent magnet. When the separated catalyst
was used directly, the yield of mono-alkylated N-benzylaniline 3 was
only 45%. This result may be due to the Ni–Cu nanoparticles on sur-
face of catalyst were partially oxidized. For further use, the separated
catalyst was washed with water for several times, followed by cal-
cination in air at 500 °C for 20 min, and by the reduction with 20%
2 2
H /N at 350 °C for 30 min.
3
. Results and discussion
Fig. 1 shows the XRD patterns between 20 and 80° of the samples.
Fig. 1a is the pattern of pure Al
2 3 2 3
O . In comparison with Cu/Al O , Ni/
Al and Ni–Cu/Al exhibit three different peaks at 2θ=44.6°, 52°
O
2 3
2 3
O
and 76°, corresponding to the reflections of (111), (200) and (220)
crystal planes of nickel, respectively. The diffraction pattern of the
Fig. 1. XRD patterns of the catalysts: (a) Al
e) Ni–Cu/Al
2 3 2 3 2 3 2 3
O ; (b) Cu/Al O ; (c) Ag/Al O ; (d) Ni/Al O ;
(
2
O
3
.
as prepared Ag/Al
77.6°) template is almost the same as that of Ni/ Al
Fig. 2 presents the TEM images of Al , Ag/Al
Ag nanoparticles synthesized by the reduction of Ag with formalde-
hyde are highly dispersed on Al . The average particle size of metallic
catalysts, Ag/Al and Ni–Cu/Al , is 10 and 20 nm, respectively. It
can be seen that Ni–Cu alloy nanoparticles are uniformly distributed
on the support (Fig. 2c), which is advantageous for the catalytic activity.
Initially, the reaction of aniline with benzyl alcohol was chosen as
a test reaction to optimize reaction conditions. In order to promote
the formation of the desired product, an excess amount of benzyl al-
cohol with respect to aniline was used (5:2). Using the Ni45Cu10
catalyst, the effect of temperature and base were studied (Table 1).
Without any additives, the yield of mono-alkylated N-benzylaniline
3 was only 26% (Table 1, entry 5). A basic additive, NaOH, did mark-
edly increase the yield of 3 (Table 1, entries 2–4). According to the ex-
perimental data, the yield of 3 at 160 °C is higher than 140 °C (Table1,
entries 1–2).
2 3
O (metallic silver, 2θ=38.2°, 44.3°, 64.6° and
H
2
/N
2
flow (100 cm−3g−1min−1) at 350 °C (ramping rate, 5 °C min−1)
2 3
O .
−
3
−1
−1
for 2 h and passivation (0.1% O
temperature for 4 h. The amount of Ag in the obtained catalyst was
found to be 0.02% based on ICP analysis. Ni–Cu nanoparticles exchanged
most of the silver on the surface of Al
aggregation.
The Ni/Cu mass ratio of the obtained catalyst was 45:10 (with a
loading amount of 45 wt.% Ni: 10 wt.% Cu), the catalyst was denoted
as Ni45Cu10. Ni
2 2
/ N , 40 cm
g
min ) at ambient
O
2 3
2 3 2 3
O and Ni–Cu/Al O .
+
2 3
O
O
2 3
2
O
3
and well-dispersed without
O
2 3
0 5 5 5 5
Cu40, Ni Cu , Ni15Cu , Ni30Cu , Ni30Cu30, Ni40Cu20
and Ni50Cu were prepared using the same procedure.
0
2
.2. Characterization of catalysts
All of the reagents and solvents were commercially available and
used without further purification. XRD measurements were performed
on a Rigaku D/max-2400 diffractometer using Cu-Kα radiation as the
X-ray source in the 2θ range of 10-80°. The conversion was estimated
by GC (P.E. AutoSystem XL) or GC-MS (Agilent 6,890 N/5,973 N).
Ni–Cu content of the catalyst was measured by inductively coupled
plasma (ICP) on IRIS Advantage analyzer. The morphology of the cata-
lyst was observed by a JEM-1200EX transmission electron microscopy
and samples were obtained by placing a drop of a colloidal solution
onto a copper grid and evaporating the solvent in air at room
temperature.
In contrast, the yield of 3 was increased by the addition of polyva-
lent metal salts, such as Lewis acidic additives (Table 2, entries 1–12).
The result indicates that the hydrogenation of the imine 4 is slow
under neutral to basic conditions and Lewis acidic additives promote
the imine hydrogenation step. Among the co-catalysts tested, CaCl ,
2
gave the best yield of 3, and the second recovered catalyst just
showed a slightly lower yield of 3 (84%) than the first run (90%)
(
Table 2, entry 12). The same reaction condition only produces an in-
2
.3. Catalytic tests
crease in the reaction time (18 h), obtaining the yield of 3 (92%) has a
slightly increased. The result confirmed that a longer reaction time
can increase the selectivity. From an economic perspective, 12 h
should be an optimal reaction time.
Under argon atmosphere, 10 mL round-bottomed flask was charged
with amine (2.0 mmol), alcohol (5.0 mmol), NaOH (0.5 mmol), CaCl
2
2 3 2 3 2 3
Fig. 2. TEM micrographs of (a) Al O , (b) Ag/Al O and (c) Ni–Cu/Al O .

32
J. Sun et al. / Catalysis Communications 24 (2012) 30–33
Table 1
Effect of base and temperature.
Entry
T
Base
Conv.
[%]
Yield [%]
3
[
°C]
4
1
2
3
4
5
140
160
160
160
160
NaOH
NaOH
tBuOK
63
70
99
86
55
11
44
34
13
26
52
26
65
73
11
K CO
2 3
–
Reaction conditions: Ni45Cu10 (0.1 g), Alcohol (5 mmol), aniline (2 mmol), o-xylene (3 mL). Conversions and selectivities were determined by gas chromatography.
Then influence of various catalyst parameters on the catalytic ac-
tivity for the N-alkylation of aniline 2 with benzyl alcohol 1 was stud-
ied under the optimized reaction conditions (Table 2). A series of
alcohol (entries 2–5) proceeded efficiently. The reactions of amines
with aliphatic alcohols (entries 6–9) proceeded to give moderate
yields. The reaction using different 4-substituted benzylic alcohols
with benzylamine gave nearly the same results (Table 4, entries 10,
14 and 15), independent of the electron-donating or withdrawing
character of the substituent. Furthermore, the reaction of aniline with
4-chlorobenzyl alcohol (entries 12) gave a higher yield than benzylamine.
The reaction can also be performed using electron-poor heteroaromatic
amines: for instance, the reaction with the 2-pyridyl derivative gave
quantitative yield, practically independent of the benzylic alcohol used
(entries 5, 11, 13 and 16).
Ni–Cu/Al
–9) were tested their activity for the reaction. Among the various cata-
lysts (such as Ni45Cu10, Ni Cu40, Ni Cu , Ni15Cu , Ni30Cu , Ni30Cu30
Ni40Cu20 and Ni50Cu ), Ni Cu40 (Cu nanoparticles) and Ni50Cu (Ni nano-
2 3
O catalysts with different Ni–Cu loading (Table 3, entries
2
0
5
5
5
5
,
0
0
0
particles) gave great conversions, but not good selectivity. Meanwhile, the
yield of 3 increased with the enhancement of Ni/Cu mass ratio in obtained
catalysts. When the Ni/Cu mass ratio up to 4.5/1.0, the catalyst (Ni45Cu10
)
showed the highest conversion (100%) and yield (90%) (Table 3, entry 7).
Meanwhile, 0.1 g catalyst displays a higher yield in comparison to 0.05 g.
Without catalyst, the yield of mono-alkylated N-benzylaniline 3 was only
From these essential observations and discussions on Ni–Cu based
catalyst, we proposed a catalytic cycle in Scheme 1 for one-pot
53% (Table 3, entry 1).
2 3
N-Alkylation of Amines with alcohols using Ni–Cu/Al O catalyst, and
Various amines were allowed to react with alcohols under opti-
mized conditions; the results are summarized in Table 4, from which
it can be observed that the reactions of amine derivatives with benzyl
named it as “borrowing hydrogen” mechanism [40–42]. The first step
of the reaction would involve the oxidation of an alcohol to the corre-
sponding carbonyl compound. Then the carbonyl intermediate would
readily react with amine to afford an imine. The catalyst with NaOH sig-
2
nificantly increased the initial step of the reaction. CaCl with higher
Lewis acidity enables faster hydride transfer to immonium cation, and
this effect result in the higher selectivity of amines.
Table 2
Reaction of 1 with 2 in the presence of catalyst under various conditions.
Table 3
Reaction of 1 with 2 using various catalysts.
Entry
Additive
Conv. [%]
Yield [%]
3
4
1
2
3
4
5
6
7
8
9
–
70
72
70
31
95
71
71
95
65
98
99
44
64
56
16
35
60
59
54
60
77
88
90
92
84
26
8
11
15
60
3
10
41
5
17
8
CoCl
FeCl
SnCl
FeCl
CaF
2
·6H
·6H
·5H
2
O
3
2
O
Entry
Ni/Cu
Conv. [%]
Yield [%]
3
4
2
O
O
4
2
·4H
2
2
1
2
3
4
5
6
7
–
59
99
93
91
93
96
100
53
82
71
71
78
73
90
85
88
12
6
NiCl
2
·6H
2
O
0/40
5/5
30/30
40/20
15/5
45/10
12
18
20
9
21
5
4
6
87
AgNO
BaCl ·2H
·2H
3
2
2
O
10
11
12
CuCl
CuCl
2
2
O
CaCl
2
100
5
4
4
a
a
1
9
00
6
97
b
8
9
30/5
50/0
99
99
Reaction conditions: Ni45Cu10 (0.1 g), Alcohol (5 mmol), aniline (2 mmol), o-xylene
3 mL), NaOH (0.5 mmol), additive (25 mmol%), reaction temperature 160 °C. Conver-
(
Reaction conditions: Catalyst (0.1 g), Alcohol (5 mmol), aniline (2 mmol), o-xylene
(3 mL), NaOH (0.5 mmol), CaCl (25 mmol%), reaction temperature 160 °C, under
sions and selectivities were determined by gas chromatography.
2
a
After 18 h.
1 atm of Ar. Conversions and selectivities were determined by gas chromatography.
b
a
Yield after second recycle.
Catalyst (0.05 g) was used.

J. Sun et al. / Catalysis Communications 24 (2012) 30–33
33
Table 4
(45 wt.% Ni, Ni/Cu mass ratio=4.5/1.0) for the direct N-alkylation
of amines with alcohols in the presence of base and a catalytic
amount of Lewis acid. The catalyst system shows excellent selectivity
and high yield in the N-alkylation of amines with aliphatic/aromatic
alcohols to provide the corresponding alcohols.
N-alkylation of alcohols with amines.
Entry
1
Alcohol
Amine
Conv.
%]
Yield [%]
amine
90
[
Imine
5
100
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Scheme 1. Borrowing hydrogen in amine alkylation.