Facile and efficient reductive N-alkylation of nitrobenzenes with alcohols catalyzed by Cu-Cr/γ-Al2O3

Article abstract of DOI:10.1007/s11164-014-1641-6

A facile and efficient method for reductive N-alkylation of nitrobenzenes with alcohols catalyzed by the bimetallic catalyst Cu₃₀Cr₅/γ-Al₂O₃, in a fixed-bed reactor, was successfully established. X-ray diffraction, temperature-programmed reduction, and X-ray photoelectron spectroscopy were used to characterize the catalyst. Introduction of Cr to Cu₃₀/γ-Al₂O₃ was found to substantially enhance the dispersion of Cu⁰ particles, which are believed to be the active sites for the reductive N-alkylation. The reaction conditions were optimized and a series of alcohols and nitrobenzenes were converted into N-alkylated anilines in more than 90 % yield by use of this catalyst.

Full text of DOI:10.1007/s11164-014-1641-6

Res Chem Intermed
DOI 10.1007/s11164-014-1641-6
Facile and efficient reductive N-alkylation
of nitrobenzenes with alcohols catalyzed
by Cu–Cr/c-Al O3
2
Ge Song
•
Jun Tian
•
Ligong Chen Yang Li
•
Received: 18 January 2014 / Accepted: 4 April 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract A facile and efficient method for reductive N-alkylation of nitrobenz-
enes with alcohols catalyzed by the bimetallic catalyst Cu Cr /c-Al O , in a fixed-
3
0
5
2 3
bed reactor, was successfully established. X-ray diffraction, temperature-pro-
grammed reduction, and X-ray photoelectron spectroscopy were used to charac-
terize the catalyst. Introduction of Cr to Cu /c-Al O was found to substantially
3
0
2 3
0
enhance the dispersion of Cu particles, which are believed to be the active sites for
the reductive N-alkylation. The reaction conditions were optimized and a series of
alcohols and nitrobenzenes were converted into N-alkylated anilines in more than
9
0 % yield by use of this catalyst.
Keywords Nitrobenzene ꢀ Alcohol ꢀ Reductive N-alkylation ꢀ
Cu–Cr/c-Al O
2
3
Introduction
N-Alkylamines are important compounds in the chemical industry and are
extensively used for production of pharmaceuticals, agrochemicals, and fine
chemicals [1, 2]. N-Benzylaniline, one of the most important N-alkylated anilines, is
used as a building block for synthesis of dyes and surfactants [3], typically by
alkylation of amines with alkyl halides [4–6] or reductive amination of aldehydes or
ketones [7–9]. However, these processes are problematic, because of the toxic
G. Song ꢀ J. Tian ꢀ L. Chen ꢀ Y. Li (&)
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,
People’s Republic of China
e-mail: liyang777@tju.edu.cn
G. Song ꢀ J. Tian ꢀ L. Chen ꢀ Y. Li
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,
People’s Republic of China
123

G. Song et al.
nature of alkyl halides, the generation of inorganic salts and harmful by-products,
and the use of expensive carbonyl compounds. Use of alcohols as alkylating agents
in green and economical processes has attracted much attention. Recently, N-
alkylation of amines with alcohols, in which water is produced as the sole
byproduct, has been demonstrated to be an alternative and environmentally friendly
route for synthesis of N-alkylated amines by use of borrowing-hydrogen (or
hydrogen-autotransfer) methodology. Several metal catalysts, for example iridium
[10–13], ruthenium [14–18], palladium [19, 20], gold [21], and silver [22–24] have
been reported to enable effective N-alkylation.
Because nitrobenzenes and alcohols are inexpensive and readily available, direct
reductive N-alkylation of nitrobenzenes with alcohols would be a more convenient
method for preparation of N-alkylated anilines. There are reports of direct synthesis
of N-alkylated anilines in the presence of heterogeneous catalysts. In these systems,
hydrogenation of nitroaromatics, dehydrogenation of alcohols to carbonyl com-
pounds, and reductive N-alkylation of arylamines with carbonyl compounds can
proceed in one reactor. Peng et al. [25] have reported the one-pot synthesis of N-
alkylated anilines from nitroarenes and alcohols by use of an Fe O -supported nano-
2
3
gold catalytic system. Schimizu et al. [26] investigated direct synthesis of N-
substituted anilines from nitroaromatics and alcohols by use of alumina-supported
silver catalysts. Another report by Tang et al. [27] described direct one-pot reductive
N-alkylation of nitroarenes with alcohols by use of supported gold catalysts. More
recently, Lee et al. [28] investigated the synthesis of alicyclic amines and
subsequent N-alkylation with alcohols using Ru/C-NaNO . In our previous work
2
[29–32] good performance was observed for copper-based catalysts in the amination
of amines and alcohols. In this context, we report a method for the preparation of N-
alkylated anilines from nitrobenzenes and alcohols catalyzed by Cu–Cr/c-Al O in a
2
3
fixed-bed reactor. The effect of doping of Cu/c-Al O with Cr was studied by X-ray
2
3
diffraction (XRD), H -temperature programmed reduction (H -TPR), and X-ray
photoelectron spectroscopy (XPS). The activity and stability of Cu Cr /c-Al O in
2
2
3
0
5
2 3
the reductive N-alkylation of nitrobenzene with benzyl alcohol were studied.
Experimental
Materials and catalysts
Nitrobenzene and benzyl alcohol were obtained from Tianjin Guangfu Fine-
chemical institute, Tianjin, China. Pseudo boehmite was provided by Jiangyan
Chemical Auxiliary Factory, Jiangyan, China. Commercially available solvents and
reagents were used without further purification.
The c-Al O -supported bimetallic catalysts were prepared by the coprecipitation-
2
3
kneading method. For example, to prepare Cu Cr /c-Al O a mixture of 45.6 g
3
0
5
2 3
Cu(NO ) ꢀ3H O and 15.4 g Cr(NO ) ꢀ9H O in 200 ml H O, and a solution of
3
2
2
3 3
2
2
2
temperature with mechanical stirring, to a beaker containing 200 ml H O over 1 h
8.3 g sodium carbonate in 200 ml H O were added dropwise, at ambient
2
2
then aged for another 1 h. During the process, the pH of the system was maintained
1
23

Facile and efficient reductive N-alkylation of nitrobenzenes
at approximately 8. The precipitate was isolated by filtration, washed with distilled
water, dried for 6 h at 110 °C, then kneaded with a mixture of 39 g pseudo-
boehmite and 60 ml distilled water. This mixture was then molded into bars by use
of an extruder. After drying in air for 6 h at 110 °C, the bars were calcined for 4 h at
5
00 °C then reduced at 240 °C in a hydrogen stream (1 MPa) for 4 h before later
use.
Catalyst characterization
XRD patterns of the samples, in the range 5–95°, were recorded with a Rigaka
D/max 2500 X-ray diffractometer with Cu-K (40 kV, 100 mA) radiation. Temper-
ature-programmed reduction with H was measured by use of a Micromeritics 2910
a
2
apparatus equipped with a thermal conductivity detector (TCD); calcined samples
were heated from ambient temperature to 900 °C at 10 °/min in a mixture of 5 % H2
in N . XPS measurements were recorded with a PHI 1600 spectrometer with an
2
Mg-K X-ray excitation source.
a
Catalytic reaction
The reaction was conducted in a fixed-bed tubular reactor, with an inner diameter of
1
5 mm and a length of 660 mm, which was loaded with 40.0 ml catalyst. A solution
of nitrobenzene in benzyl alcohol (1:10 molar ratio of nitrobenzene to benzyl
alcohol) was introduced into the reactor at a flow rate of 0.3 ml/min by use of a
syringe pump. Excess benzyl alcohol was used not only as the alkylating reagent but
also as the solvent. The temperature in the reaction zone was measured with a
thermocouple placed in the center of the tube and regulated by use of a PID cascade
controller. The hydrogen pressure in the reaction system was set by use of a
hydrogen regulator. The reaction mixture was analyzed by gas chromatography
(
GC) with a 60 m 9 0.32 mm 9 0.50 lm SE-30 capillary column. The composi-
tion of the reaction mixture was identified by GC–MS (HP-1 capillary column:
0 m 9 0.25 mm, 0.2 lm film thickness) with an ion-trap detector.
3
Results and discussion
Catalyst selection and characterization
The reductive N-alkylation of nitrobenzene with benzyl alcohol is depicted in
Scheme 1. Aniline is initially obtained by hydrogenation of nitrobenzene. The imine
intermediate (N-benzylideneaniline) is formed by nucleophilic addition of aniline to
benzaldehyde, which is produced by dehydrogenation of benzyl alcohol. N-
Benzylideneaniline is hydrogenated to yield the desired N-benzylaniline.
According to the reported mechanism [33, 34], dehydrogenation of benzyl
alcohol to benzaldehyde should be the rate-limiting step. Selection of a suitable
catalyst is crucial for this reaction. Because of the dehydrogenation and
hydrogenation nature of the reaction, transition metal-based catalysts are often
123

G. Song et al.
Scheme 1 Reductive N-alkylation of nitrobenzene with benzyl alcohol
Table 1 Reaction of nitrobenzene with benzyl alcohol with a variety of catalysts
Entry Catalyst
Nitrobenzene conversion (%) Selectivity (%)
N-Benzylaniline N-Benzylideneaniline
1
2
3
4
5
6
7
8
9
1
1
1
Ni30/c-Al
2
O
3
99.5
99.8
99.8
84.3
53.2
99.3
98.3
99.9
97.6
99.5
99.7
97.5
70.1
72.7
77.1
70.8
61.7
84.3
80.6
90.6
83.9
85.1
87.1
88.1
17.2
16.3
16.4
16.9
33.3
13.6
13.4
7.3
Co30/c-Al
Cu30/c-Al
Cu20/c-Al
Cu10/c-Al
2
O
2
O
2
O
2
O
3
3
3
3
Cu30La
5
/c-Al
/c-Al
/c-Al
/c-Al
/c-Al
/c-Al
Cu30Cr10/c-Al
2 3
O
Cu30Zn
5
2 3
O
Cu30Cr
5
2 3
O
Cu30Mg
5
2
O
3
11.3
12.8
8.9
0
1
2
Cu30Ca
Cu30Cr
5
2 3
O
2
2 3
O
2
O
3
7.7
Reaction conditions: temperature = 200 °C, hydrogen pressure = 3.0 MPa, flow rate = 0.3 ml/min,
nitrobenzene–benzyl alcohol (mol) = 1:10
used. Because nickel [35, 36] and copper [37–39] have been shown to enable
effective N-alkylation of amines with alcohols, a series of copper, nickel, and
cobalt-based catalysts were prepared and evaluated for reductive N-alkylation of
nitrobenzene with benzyl alcohol. The results are summarized in Table 1.
GC–MS analysis of the reaction mixture showed the other main product was N-
0
benzylideneaniline, N,N -dibenzylaniline was not observed. It is apparent that
selectivity was much better for Cu /c-Al O than for Ni /c-Al O and Co /c-
3
0
2
3
30
2
3
30
Al O . Copper was therefore chosen as the main catalyst for reductive N-alkylation
2
3
of nitrobenzene with benzyl alcohol. However, the catalytic activity of Cu /c-
3
0
Al O decreased quickly during the reaction. Shiau and Lee [40] reported that
deactivation of Cu/c-Al O was caused by sintering of the copper. To improve the
2
3
2
3
stability of the catalyst and selectivity for the desired product, the Cu/c-Al O was
2
3
doped with Cr, La, Zn, Mg, or Ca as second component. As shown in Table 1, the
best catalytic performance was obtained with Cu Cr /c-Al O , which was therefore
3
0
5
2 3
selected as catalyst for synthesis of N-benzylaniline. To study the effects of the Cr
on the Cu/c-Al O , Cu /c-Al O and Cu Cr /c-Al O were studied by XRD,
2
3
30
2
3
30
5
2 3
H -TPR, and TEM.
2
1
23

Facile and efficient reductive N-alkylation of nitrobenzenes
#
+
*
Cu
CuO
CuCr O
#
2
4
#
#
*
*
#
*
#
(c)
#
#
#
+
(
b)
a)
+
+
+
+
+
+
+
+
+
(
20
40
60
80
Two-Theta (degree)
Fig. 1 XRD curves for (a) unreduced Cu30Cr
Cu30Cr /c-Al
5
/c-Al , (b) reduced Cu30/c-Al O , and (c) reduced
2
O
3
2 3
5
2 3
O
Catalyst characterization
XRD
The XRD patterns for unreduced Cu Cr /c-Al O , reduced Cu /c-Al O , and
3
0
5
2
3
30
2 3
reduced Cu Cr /c-Al O are shown in Fig. 1. As shown by the diffraction lines
3
0
5
2 3
labeled ‘‘?’’ in Fig. 1a, Cu exists as CuO in the unreduced Cu Cr /c-Al O . The
3
0
5
2 3
XRD patterns contained the typical diffraction lines of elemental copper, at 43.3°,
0
5
0.4°, 74.2°, and 90.0° in Fig. 1b, c. Cu particle sizes for the (111) diffraction peak
(
43.3°) were calculated by use of the Scherrer equation. When Cr was doped into
Cu /c-Al O , the diameter of Cu particles decreased from 243 ± 8 A in the
reduced Cu /c-Al O to 178 ± 6 A in the reduced Cu Cr /c-Al O . This
0
˚
3
0
2 3
˚
3
0
2
3
30
5
2 3
indicated that doping with Cr made the Cu particles smaller and better dispersed.
XRD peaks of CuCr O were detected in the reduced Cu Cr /c-Al O , indicating
2
4
30
5
2 3
that addition of Cr may prevent copper from sintering. No peaks corresponding to
Cr O were identified by XRD, indicating that the particle size of the Cr O in the
catalyst was either too small to be detected or was present as an amorphous phase.
2
3
2 3
H -TPR
2
The reducibility of the copper species in the unreduced catalysts was evaluated by
H -TPR; the results are shown in Fig. 2. The reduction temperature of the main
2
peak shifts from 310 °C for the unreduced Cu /c-Al O to 258 °C for the
3
0
2 3
unreduced Cu Cr /c-Al O . This result indicated that addition of chromium to
3
0
5
2 3
Cu /c-Al O can promote the reduction of the catalyst and thus improve the
3
0
2 3
123

G. Song et al.
2
58°C
3
10°C
(
b)
a)
(
0
200
400
600
800
1000
o
Temperature ( C)
Fig. 2 H
2
2 3 5 2 3
-TPR curves for (a) unreduced Cu30/c-Al O and (b) unreduced Cu30Cr /c-Al O
0
dispersion of Cu particles in the catalyst. These results are in good agreement with
the XRD results.
XPS
The XPS spectrum of reduced Cu Cr /c-Al O is shown in Fig. 3. The peak at
3
0
5
2 3
9
32.31 eV, the binding energy of the Cu2p3/2 level, indicates that Cu mainly occurs
0
as Cu species in the reduced Cu Cr /c-Al O catalyst, as already observed in the
3
0
5
2 3
XRD pattern of reduced Cu Cr /c-Al O catalyst. The binding energy of the
3
0
5
2 3
3
?
Cr2p3/2 level at 576.24 eV could be assigned to Cr , which occurs as CuCr O .
2
4
0
The presence of CuCr O could be helpful to the dispersion of the Cu species.
2
4
These results are in agreement with the XRD and H -TPR analysis, and it can be
2
0
concluded that Cu is the active site of the catalyst.
Optimization of the process conditions
Hydrogen pressure
As shown in Scheme 1, hydrogen is involved in the hydrogenation of nitrobenzene
to aniline, dehydrogenation of benzyl alcohol to benzaldehyde, and hydrogenation
of the imine to the desired product. Thus, the effect of hydrogen pressure on the
reaction was investigated; the results are listed in Table 2. When the hydrogen
pressure was increased from 1 to 3 MPa, selectivity for N-benzylaniline increased
from 54.9 to 90.6 %. When the hydrogen pressure was increased further, the
selectivity for N-benzylaniline decreased. This may be because a hydrogen pressure
of 3 MPa was favorable for production of N-benzylaniline, by hydrogenation of N-
benzylideneaniline, whereas higher hydrogen pressure may inhibit the
1
23

Facile and efficient reductive N-alkylation of nitrobenzenes
5
3
2
2
1
1
5
.0x10
5
.5x10
5
.0x10
5
.5x10
5
.0x10
4
.0x10
0
.0
1
000
800
600
400
200
0
Binding energy(eV)
4
4
4
4
4
4
5
5
4
4
4
4
.2x10
.0x10
.8x10
.6x10
.4x10
.2x10
Energy: 932.31eV
Cu 2p
9
70
960
950
940
930
920
Binding energy (eV)
4
5
5
4
4
4
4
.2x10
Energy: 576.24eV
Cr 2p
4
.0x10
4
.8x10
4
.6x10
4
.4x10
4
.2x10
6
00
590
580
570
560
Binding energy(eV)
5 2 3
Fig. 3 XPS spectra of reduced Cu30Cr /c-Al O
123

G. Song et al.
5 2 3
Table 2 Effect of hydrogen pressure on the reaction over Cu30Cr /c-Al O
Hydrogen pressure (MPa)
Nitrobenzene conversion (%)
Selectivity (%)
N-Benzylaniline
N-Benzylideneaniline
1
2
3
4
99.5
99.6
99.9
99.7
54.9
81.1
90.6
40.1
15.6
13.3
7.3
37.4
Reaction conditions: temperature = 200 °C, flow rate = 0.3 ml/min, nitrobenzene–benzyl alcohol
mol) = 1:10
(
5 2 3
Table 3 Effect of temperature on the reaction over Cu30Cr /c-Al O
Temperature (°C)
Nitrobenzene conversion (%)
Selectivity (%)
N-Benzylaniline
N-Benzylideneaniline
1
1
2
2
60
80
00
20
99.7
99.8
99.9
99.7
46.8
86.3
90.6
69.7
50.4
12.1
7.3
21.5
Reaction conditions: hydrogen pressure = 3.0 MPa, flow rate = 0.3 ml/min, nitrobenzene–benzyl
alcohol (mol) = 1:10
dehydrogenation of benzyl alcohol. Taking the operational feasibility of the process
into consideration, 3 MPa was chosen as the optimum hydrogen pressure.
Reaction temperature
The effect of reaction temperature on reductive N-alkylation of nitrobenzene with
benzyl alcohol over Cu Cr /c-Al O was examined; the results are shown in
3
0
5
2 3
Table 3. It is apparent that conversion of nitrobenzene was independent of
temperature in the range 160 to 220 °C and remained approximately 100 %. In
response to the increasing temperature, selectivity for N-benzylaniline increased.
The highest selectivity, 90.6 %, was obtained at 200 °C; it decreased to 69.7 % at
2
20 °C. This indicates that enhancement of the reaction temperature can facilitate
dehydrogenation of benzyl alcohol, which led to the more desired product.
However, higher temperature may result in formation of the di-alkylated N,N-
dibenzylaniline. Taking the yield of N-benzylaniline into consideration, 200 °C was
selected as the optimum reaction temperature.
Reductive N-alkylation of nitrobenzenes and different alcohols over Cu Cr /c-
3
0
5
Al O
2
3
The scope of this Cu Cr /c-Al O -catalyzed system with regard to nitrobenzene
3
0
5
2 3
derivatives and alcohols was examined; the results are summarized in Table 4. The
1
23

Facile and efficient reductive N-alkylation of nitrobenzenes
5 2 3
Table 4 Reductive N-alkylation of nitrobenzenes with different alcohols over Cu30Cr /c-Al O
Yield
%)
Entry
1
Nitroaromatics
Alcohols
Product
Temperature (°C)
180
(
NO
2
HN
OH
93.4
NO
2
OH
HN
2
3
4
200
200
200
95.6
NO
2
HN
96.4
95.1
OH
NO
2
HN
OH
NO
2
HN
5
6
OH
180
200
90.8
90.6
NO
2
NH
OH
NO
2
7
HN
200
220
92.1
92.7
OH
NH
NO
2
8
9
OH
Cl
NH
Cl
NO
2
220
220
94.6
90.5
OH
OH
H
3
CO
NH
H
3
CO
NO2
10
Reaction conditions: hydrogen pressure = 3.0MPa, flow rate = 0.3ml/min, nitroaromatic–alcohol
mol) = 1:10
(
stability of imine and steric bulkiness on the amino group of aniline substantially
affected the yield of the product. Reaction of nitrobenzene with aliphatic secondary
alcohols proceeded in excellent yields (Table 4, entries 2–4). Because higher
123

G. Song et al.
temperature could give the undesired di-alkylated anilines, a lower temperature was
required when linear alkyl alcohols, for example n-propanol and n-hexanol, were
used as the alkylating agents (Table 4, entries 1 and 5). Aromatic primary alcohols,
including benzyl alcohol and 2-phenylethanol, could be transformed into the
corresponding product in high yields (Table 4, entries 6 and 7). Furthermore, under
the optimized reaction conditions, different 4-substituted nitrobenzenes were
benzylated in good yields to give secondary amines (Table 4, entries 8–10).
Lifetime of Cu Cr /c-Al O
3
0
5
2 3
Because reductive N-alkylation of nitrobenzenes with alcohols had proceeded well
in a fixed-bed reactor, the lifetime of the catalyst was measured under the optimum
conditions over 150 h. During this period, the stability of the catalyst was good and
the yield of N-alkylated anilines remained above 85 %. The catalyst had general
applicability in the reductive N-alkylation of nitrobenzenes with a variety of
alcohols.
Conclusion
A facile and efficient method for reductive N-alkylation of nitrobenzenes with
alcohols over Cu Cr /c-Al O catalyst in a fixed-bed reactor was successfully
established. Cu species were assumed to be the active sites of the catalysts, and
3
0
5
2 3
0
0
doping of Cu /c-Al O with chromium led to highly dispersed Cu particles. The
3
0
2 3
catalyst system is highly stable and yields are high in the reductive N-alkylation of
nitrobenzenes with a variety of aliphatic and aromatic alcohols.
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