One-Pot Synthesis of Secondary Amines from Nitroarenes and Aldehydes on Supported Copper Catalysts in a Flow Reactor: The Effect of the Support

Article abstract of DOI:10.1134/S0023158418050014

The effect of the support on the properties of copper catalysts supported on γ-Al₂O₃, SiO₂, and TiO₂–SiO₂ with a ~5 wt % Cu content was studied in the one-pot synthesis of N-heptyl-p-toluidine from p-nitrotoluene and n-heptanal. The catalysts were characterized by elemental analysis, X-ray diffraction analysis, transmission electron microscopy, temperature-programmed reduction, and low-temperature nitrogen adsorption. The reaction was carried out in a flow reactor with the use of molecular hydrogen as a reducing agent. It was established that the nature of the support exerts a profound effect on the yield of the target secondary amine; in this case, 5%Cu/Al₂O₃ was found the most active catalyst. A combination of high catalyst activity in the hydrogenation of a nitro group to an amino group with the presence of acid sites, which facilitate imine formation as a result of the interaction of n-heptanal with p-toluidine, on the catalyst surface is necessary for reaching the greatest yield of N-heptyl-p-toluidine. The study of reaction mechanism on the 5%Cu/Al₂O₃ catalyst showed that p-nitrotoluene inhibits the hydrogenation of n-heptanal, and aldehyde hydrogenation into alcohol begins only after the conversion of the major portion of p-nitrotoluene as a result of the selective adsorption of the nitroarene under the conditions of the simultaneous presence of p-nitrotoluene and n-heptanal in the reaction mixture.

Full text of DOI:10.1134/S0023158418050014

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 5, pp. 593–600. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © E.A. Artyukha, A.L. Nuzhdin, G.A. Bukhtiyarova, E.A. Derevyannikova, E.Yu. Gerasimov, A.Yu. Gladkii, V.I. Bukhtiyarov, 2018, published in Kinetika
i Kataliz, 2018, Vol. 59, No. 5, pp. 583–590.
One-Pot Synthesis of Secondary Amines from Nitroarenes
and Aldehydes on Supported Copper Catalysts in a Flow Reactor:
The Effect of the Support
a
a,
a
a
a
E. A. Artyukha , A. L. Nuzhdin *, G. A. Bukhtiyarova , E. A. Derevyannikova , E. Yu. Gerasimov ,
a
a, b
A. Yu. Gladkii , and V. I. Bukhtiyarov
a
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
b
Novosibirsk State University, Novosibirsk, 630090 Russia
*
e-mail: anuzhdin@catalysis.ru
Received March 1, 2018
Abstract—The effect of the support on the properties of copper catalysts supported on γ-Al O , SiO , and
2
3
2
TiO –SiO with a ~5 wt % Cu content was studied in the one-pot synthesis of N-heptyl-p-toluidine from
2
2
p-nitrotoluene and n-heptanal. The catalysts were characterized by elemental analysis, X-ray diffraction anal-
ysis, transmission electron microscopy, temperature-programmed reduction, and low-temperature nitrogen
adsorption. The reaction was carried out in a flow reactor with the use of molecular hydrogen as a reducing
agent. It was established that the nature of the support exerts a profound effect on the yield of the target sec-
ondary amine; in this case, 5%Cu/Al O was found the most active catalyst. A combination of high catalyst
2
3
activity in the hydrogenation of a nitro group to an amino group with the presence of acid sites, which facili-
tate imine formation as a result of the interaction of n-heptanal with p-toluidine, on the catalyst surface is
necessary for reaching the greatest yield of N-heptyl-p-toluidine. The study of reaction mechanism on the
5
%Cu/Al O catalyst showed that p-nitrotoluene inhibits the hydrogenation of n-heptanal, and aldehyde
2 3
hydrogenation into alcohol begins only after the conversion of the major portion of p-nitrotoluene as a result
of the selective adsorption of the nitroarene under the conditions of the simultaneous presence of p-nitrotol-
uene and n-heptanal in the reaction mixture.
Keywords: one-stage synthesis, secondary amines, nitroarenes, aldehydes, flow reactor, molecular hydrogen,
supported copper catalysts, support effect
DOI: 10.1134/S0023158418050014
INTRODUCTION
aldehydes on heterogeneous catalysts in the presence
of molecular hydrogen [7–25]. This process includes
the following three reactions: the selective hydrogena-
tion of nitroarenes to corresponding primary aromatic
amines, the interaction of these latter with aldehydes
with the formation of imines, and, finally, the hydro-
genation of imines into target secondary amines. All of
these reactions are carried out consecutively in one
reaction vessel. This approach makes it possible to
decrease the number of stages and the amount of
wastes because of the use of an environmentally
friendly reducing agent. The application of flow reac-
tors provides an opportunity to rapidly control process
parameters and to increase the efficiency of reaction
because of a notable increase in the rate of heat and
mass transfer, which favorably affects selectivity for
the formation of a target product or its yield [26, 27].
Secondary amines are an important class of
organic compounds, which are widely used in the
industrial manufacture of the products of fine organic
synthesis, agrochemicals, and pharmaceutical prepa-
rations [1]. Traditional methods for their production
include the reduction of nitroarenes to corresponding
anilines and their subsequent reactions with alkyl
halides, alcohols, and carbonyl compounds in the
presence of reducing agents [2, 3]. The disadvantages
of this approach include the toxicity of reducing
and/or alkylating agents, the insufficient process
selectivity at the stage of monoalkylation, and the for-
mation of a large amount of wastes.
In accordance with contemporary tendencies in
the development of methods for the production of
functionalized molecules with an atomic accuracy [4],
a search for new economically effective and environ-
Earlier, the behavior of a wide range of metallic
mentally appropriate methods for the preparation of catalysts based on noble metals (Pd, Pt, Ir, Au, and
secondary amines is of considerable current interest Ag) [7–20], Cu [21], Fe [22], and Co [23–25] in the
[
5, 6]. One of these methods is the one-pot synthesis course of the one-pot synthesis of secondary amines
of secondary amines by a reaction of nitroarenes with was studied and the catalytic effectiveness of Au, Ag,
593

5
94
ARTYUKHA et al.
and Cu nanoparticles supported onto γ-alumina in the were dried at 60°C in a vacuum and calcined in air at
reactions of nitroarenes with aldehydes in a flow reac- 300°C for 3 h [21]. The samples were stored in an
tor was demonstrated [18–21]. At the same time, sys- atmosphere of nitrogen and reduced in a reactor
tematic studies of the effect of the support on the directly before the onset of an experiment.
activity and selectivity of catalysts with the use of this
method for the synthesis of secondary amines are cur-
Physicochemical Properties of the Catalysts
rently not available.
In this work, we compared the catalytic properties
The concentrations of copper and titanium were
of copper-containing catalysts on different supports determined by X-ray fluorescence spectroscopy on an
(
γ-Al O , SiO , and TiO –SiO ) in the one-pot syn- ARL instrument (Thermo Scientific, the United
2 3 2 2 2
States). The textural characteristics of the supports
and catalysts were studied using an ASAP-2400 system
thesis of secondary amines from nitroarenes and alde-
hydes (based on the example of p-nitrotoluene and
n-heptanal) in a flow reactor. The aim of our experi-
ments was to determine the roles of the support and
the concentration of an active component and to
establish the mechanism of secondary amine forma-
tion from nitroarenes and aldehydes in an atmosphere
of hydrogen.
(
Micromeritics, the United States) based on the iso-
therms of the low-temperature (–196°C) adsorption
of nitrogen. X-ray diffraction (XRD) analysis was car-
ried out on a Bruker D8 Advance diffractometer
(
Bruker, Germany) with CuK radiation using a Lynx-
α
Eye position-sensitive detector. The measurements
were performed in a range of 2θ angles from 15° to 80°
with a step of 0.05° and an accumulation time of 3 s per
point. The ICDDPDF-2 powder database was used in
the phase analysis. The average size of copper crystal-
lites (the coherent scattering region (CSR)) was evalu-
ated by the Rietveld method (LVol-IB, Bruker TOPAS
V4.2) [28]. The samples were also studied by high-res-
olution transmission electron microscopy (TEM) on a
JEM-2200FS-CS electron microscope (JEOL,
Japan). The temperature-programmed reduction
EXPERIMENTAL
Reagents and Materials
In this work, copper(II) nitrate trihydrate (99%),
p-nitrotoluene (99%), n-heptanal (95%), and n-dec-
ane (99%) from Acros Organics; tetraethyl orthotita-
nate (Sigma); and KSKG silica gel (OOO Khro-
manalit) were used. Toluene (high-purity grade,
99.5%; ZAO EKOS-1) was used as a solvent.
(
TPR) of the samples was carried out on a ChemBET
Pulsar TPR/TPD instrument (Quantachrome Instru-
ments, the United States). The samples were purged with
argon at room temperature for 30 min; thereafter, they
were heated at a rate of 10 K/min in a flow containing
Support Preparation
The γ-alumina was prepared by the extrusion of
paste obtained upon the mixing of aluminum oxide
1
0 vol % H in Ar (flow rate, 20 mL/min).
2
(
PuraloxTH 100/150, Sasol) with aluminum hydrox-
ide (Disperal, Sasol) in a 30 : 70 ratio by weight in the
presence of an aqueous solution of nitric acid (0.5 wt %).
The granules obtained were dried at a temperature of
Catalytic Properties
The catalytic properties of the samples were stud-
ied in an H-Cube Pro flow system (Thalesnano, Hun-
gary) equipped with reactors of CatCart®30 stainless
steel (length, 30 mm; inside diameter, 4 mm) [18–21].
Before the onset of reaction, the copper catalysts were
reduced in toluene with hydrogen at a pressure of
1
10°C and calcined at 550°C for 4 h [19]. Then, they
were ground and sifted to obtain particles with a diam-
eter of 0.25–0.50 mm. The silica gel intended for its
modification with titanium dioxide and for the prepa-
ration of catalysts was also used as granules of size
0
.25–0.50 mm.
5
.0 MPa and a temperature of 120°C for 1 h (the flow
In order to prepare the TiO –SiO support, the
2
2
rates of toluene and H were 0.5 and 60 mL/min,
2
preliminarily dried silica gel (2.0 g) was impregnated
with a solution of tetraethyl orthotitanate (0.56 mL) in
isopropanol in a vacuum rotary evaporator at a pres-
sure of 0.086 bar and a temperature of 60°C. After the
removal of the solvent, the sample was hydrolyzed in
water at room temperature, dried at 120°C for 2 h, and
calcined at 400°C for 3 h.
respectively) [21]. Then, after the reduction of the cat-
alyst and the establishment of a specified temperature,
a reaction mixture containing a solution of p-nitrotol-
uene (0.025 mol/L) and n-heptanal (0.0375 mol/L) in
toluene was introduced into the reactor. The reaction
was carried out at a temperature of 115°C, a pressure of
5
.0 MPa, and reaction mixture and hydrogen flow
rates of 0.5 and 60 mL/min, respectively. The test
samples were taken 30–34 min after the supply of the
reaction mixture. Previously, we found that, under the
Catalyst Preparation
The supported copper catalysts were prepared by conditions indicated, external and internal diffusion
the incipient wetness impregnation of the supports exerted no effect on the conversion of nitroarenes [29].
(
γ-Al O , SiO , and TiO –SiO ) with an aqueous The test samples were analyzed on an Agilent 6890N
2
3
2
2
2
solution of copper(II) nitrate. The granules obtained gas chromatograph (Agilent, the United States) with a
KINETICS AND CATALYSIS
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ONE-POT SYNTHESIS OF SECONDARY AMINES FROM NITROARENES
Table 1. Physicochemical properties of the supports and catalysts
595
2
3
Sample
Cu content, wt %
SBET, m /g Vpore, cm /g
dpore, nm
а, Å
D*, nm
γ-Al O3
–
179
300
271
172
169
150
147
260
253
0.61
0.81
0.64
0.60
0.59
0.53
0.52
0.74
0.63
13.7
10.6
9.5
–
–
–
–
2
SiO2
TiO –SiO2
–
–
–
–
2
3
5
7
9
5
5
Cu/Al O3
3.1
5.3
6.7
9.4
5.4
5.0
13.7
13.8
14.1
14.1
11.0
9.9
**
**
6
2
Cu/Al O3
3.62
**
2
Cu/Al O3
**
**
17
12
2
Cu/Al O3
**
2
Cu/SiO2
3.62
3.63
Cu/TiO –SiO2
2
*
Average size of copper nanoparticles found by XRD analysis (error, ±1 nm).
** Not determined.
1
9091S-416 HP 5-MS capillary column (length, 60 m; 300°C will be referred to as 5CuO/Al O , 5CuO/SiO ,
2 3 2
inside diameter, 320 μm; and film thickness, 0.25 and 5CuO/TiO –SiO , and the above samples after
2
2
μm). The reaction products were identified by chro-
matography–mass spectrometry on an Agilent 7000B
Triple Quad System instrument (Agilent). The con-
version of reactants was evaluated by the internal stan-
dard method, and n-decane was used as an internal
standard.
reduction will be referred to as 5Cu/Al O , 5Cu/SiO ,
2
3
2
and 5Cu/TiO –SiO , respectively.
2
2
Figure 1 shows the powder X-ray diffraction pat-
terns of the samples 5CuO/Al O , and 5CuO/SiO
2
3
2
and the catalysts 5Cu/Al O , 5Cu/SiO , and
2
3
2
5
Cu/TiO –SiO reduced in a flow reactor in an atmo-
2 2
sphere of hydrogen and toluene at a temperature of
20°C and a pressure of 5.0 MPa for 1 h. The X-ray
RESULTS AND DISCUSSION
1
Physicochemical Properties of the Samples
diffraction patterns of the reduced catalysts exhibited
signals at 2θ = 43.2° and 50.4°, which can be
attributed to the peaks (111) and (200) of the metallic
copper phase (ICDDPDF-2, no. 04-009-2090). The
absence of a signal due to the phase of CuO from the
spectrum of 5Cu/TiO –SiO together with the pres-
Table 1 summarizes the physicochemical proper-
ties of the supports and catalysts used in this work.
Earlier, alumina [29–31], silica [30], and titania [32–
3
4] were used for the preparation of metal catalysts for
2
2
the hydrogenation of nitroarenes. However, it is diffi-
cult to form granules from individual titanium dioxide;
therefore, we synthesized silica gel modified with tita-
nium dioxide (TiO –SiO ), which contained 8.8 wt %
ence of the characteristic reflections of the metallic
copper phase indicated the complete reduction of sup-
2
2
TiO according to the results of chemical analysis, in
2
Intensity, counts
order to perform the reaction in a flow reactor. The
introduction of titanium dioxide somewhat decreased
the specific surface area, the volume, and the mean
diameter of pores of the initial silica gel; this fact may
be explained by the formation of a titanium dioxide
layer on the internal surface of pores (Table 1). The
2
1
0000
8000
16000
4000
2000
0000
*—TiO2
v—CuO
Cu
*
Cu
Cu
*
5Cu/TiO2–SiO2
1
1
1
v
v
5Cu/SiO2
diffractogram of the TiO –SiO sample (Fig. 1) exhib-
v
2
2
v
v
v
v v
5CuO/SiO2
v
ited weakly intense signals at 2θ = 25.28° and 48.05°,
which are characteristic of a titanium dioxide phase
with the anatase structure (ICDD PDF-2, no. 00-
8000
6000
4000
5
Cu/Al2O3
v v
5CuO/Al2O3
0
21-1272). The TEM images of the TiO –SiO sup-
2 2
port indicated that TiO occupied approximately a half
2000
2
of the silica gel surface, and it predominantly occurred
in the form of particles with a diameter of 2–3 nm
20
30
40
50
60
70
80
2θ, deg
(Fig. 2a).
We studied the effect of the support on the activity
Fig. 1. X-ray diffraction patterns of the samples
CuO/Al O and 5CuO/SiO and the reduced catalysts
of copper catalysts using the samples containing about
5
2
3
2
5
wt % Cu. Henceforth, these samples calcined at
5Cu/Al O , 5Cu/SiO , and 5Cu/TiO –SiO .
2 3 2 2 2
KINETICS AND CATALYSIS Vol. 59 No. 5 2018

5
96
ARTYUKHA et al.
SiO2
TiO2
CuO
(а)
20 nm
(b)
500 nm
Ti + Cu
1 μm
(c)
0.5 μm
(d)
Fig. 2. TEM images that illustrate the mutual arrangement of CuO and TiO particles on the surface of SiO in the sample
2
2
5
CuO/TiO –SiO : (a) the TiO –SiO support, (b) the sample 5CuO/TiO –SiO , (c) the STEM image of the sample
2 2 2 2 2 2
5
CuO/TiO –SiO , and (d) EDX mapping Ti and Cu.
2
2
ported CuO particles [21]. The TEM study of the sam- appear as a result of the oxidation of metallic copper in
ple 5CuO/TiO –SiO showed that the lamellar parti- contact with air. The reflections of a metallic copper
2
2
cles of CuO with sizes from 10 to 50 nm evenly covered phase were also present in the X-ray diffraction pattern
the catalyst surface (Fig. 2b). However, in view of an of the sample 5Cu/Al O ; however, it was impossible
2
3
insufficiently contrast image, it is impossible to reli-
ably detect the localization of CuO particles in massive
to estimate the degree of reduction of copper(II) oxide
particles supported onto γ-alumina on the basis of
these data because signals from the CuO phase over-
lapped with the intense signals of the support phase.
SiO agglomerates. The EDX mapping of this region
2
in the STEM regime (Figs. 2c and 2d) made it possible
to conclude that titanium dioxide was evenly distrib-
uted over the surface of silica gel and its distribution
In order to compare the reducibility of the samples
did not correlate with the distribution of copper. In 5CuO/Al O , 5CuO/SiO , and 5CuO/TiO –SiO , we
2
3
2
2
2
general, the results obtained were indicative of a uni-
form distribution of the active component on the sur-
used a TPR method (Fig. 3). As is evident, the peak of
hydrogen absorption by the samples 5CuO/Al O and
2
3
face of TiO –SiO .
2
2
5CuO/TiO –SiO occurred in the region of lower
2 2
temperatures than that of the corresponding peak of
In the sample 5Cu/SiO , a small increase in the
2
the sample 5CuO/SiO ; that is, the first two catalysts
2
intensity of signals at 2θ = 35.6° and 38.7° corre-
sponding to the (002) and (111) peaks of the phase of
copper(II) oxide (ICDD PDF-2, no. 00-048-1548),
was noted in addition to the presence of signals from
were reduced under milder conditions. Because from
the XRD analysis data it follows that the reduced sam-
ple 5Cu/SiO did almost not contain the copper oxide
2
metallic copper; this copper(II) oxide phase could phase, it is believed that the CuO particles supported
KINETICS AND CATALYSIS
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2018

ONE-POT SYNTHESIS OF SECONDARY AMINES FROM NITROARENES
597
on the surface of γ-alumina underwent complete the surface of aluminum oxide to facilitate a more uni-
reduction under the experimental conditions.
form distribution of the active constituent. The modi-
fication of the surface of silica gel with titanium diox-
ide decreased the average particle size of copper from
The average size of copper crystallites was evalu-
ated based on the XRD analysis of the catalysts con-
taining ~5 wt % Cu. Table 1 summarizes the found lat-
tice parameters (a) and the CSR sizes (D) of a metallic
copper phase. The average size of copper particles in
the sample 5Cu/Al O was ~6 nm, and the other cata-
1
7 to 12 nm; this could also be related to the interac-
tion of the active constituent with TiO [32, 34]. In this
2
case, the textural characteristics of the support
changed insignificantly.
2
3
lysts contained larger particles. The average sizes of
copper particles in the samples 5Cu/TiO –SiO and
Catalytic Properties of the Samples
In order to explain the role of the support, we com-
2
2
5
Cu/SiO were 12 and 17 nm, respectively. On the sur-
2
face of γ-Al O , the formation of smaller particles than pared the catalytic properties of the test samples in the
2
3
those on the surface of silica gel can be explained by following reduction reaction of p-nitrotoluene (1)
the stronger interaction of the active constituent with with n-heptanal (2) in a flow reactor:
HN
NO2
NH2
N
PH2 = 5 MPa,
Toluene
Cu-Cat.
O
+
+
+
2
5
1
3
4
Under comparable conditions, the catalysts noted: we did not detect the intermediate formation of
5
Cu/Al O and 5Cu/TiO –SiO ensured the quantita- imine 4, which is rapidly hydrogenated with the for-
2
3
2
2
tive conversion of p-nitrotoluene, whereas its conver- mation of target secondary amine 5. However, the
sion was only 34% with the use of SiO as the support yield of N-heptyl-p-toluidine was close to zero on the
2
catalyst 5Cu/SiO , which exhibited considerably lower
(
Table 2, rows 1, 3, and 4). On the one hand, this can
2
be due to a higher average particle size in the catalyst activity in the hydrogenation reaction of a nitro group
5
Cu/SiO (Table 1) because a decrease in the disper- and did not contain acid sites.
2
sity of the metal is accompanied by a decrease in the
quantity of accessible metallic centers. On the other
hand, with the use of gold catalysts as an example, it
was demonstrated that catalysts on Al O and TiO
2
3
2
Hydrogen absorption
exhibit higher activity in the hydrogenation reactions
of nitroarenes than that of the samples on other sup-
ports because of the specific features of the interaction
of a nitro group with TiO and Al O [32–34].
2
2
3
5
CuO/Al O
2 3
The highest yield of N-heptyl-p-toluidine (86%)
was obtained on the catalyst 5Cu/Al O . An increase
2
3
in the yield of target product 5 (to 91%) was achieved
by decreasing the feed rate of the reaction mixture
5
CuO/SiO2
(
Table 2, rows 1 and 2). At the same time, the yield of
target product 5 on the copper catalysts supported on
SiO and TiO –SiO was considerably lower (Table 2,
2
2
2
rows 3 and 4). Thus, in the presence of 5Cu/TiO –
2
SiO , the yield of secondary amine upon the complete
5CuO/TiO2–SiO2
2
hydrogenation of p-nitrotoluene was only 35%; this
may be explained by the absence or an insufficient
quantity of acid sites, which are capable of catalyzing
imine formation in a reaction of n-heptanal with
p-toluidine, on the surface of TiO –SiO . These sites
1
00
200
300
400
500
Temperature, °С
2
2
can be the coordinatively unsaturated cations of tita-
Fig. 3. TPR curves of the samples 5CuO/Al O ,
CuO/SiO , and 5CuO/TiO –SiO .
nium as the constituents of TiO . At the same time, the
2 3
2
5
high hydrogenating activity of this catalyst should be
2 2 2
KINETICS AND CATALYSIS Vol. 59 No. 5 2018

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98
ARTYUKHA et al.
Table 2. Reaction of p-nitrotoluene with n-heptanal on the supported copper catalysts
Conversion
of p-nitrotoluene
Conversion
of n-heptanal
Yield*, %
Entry
Catalyst
3
4
5
%
1
2
3
4
5
6
7
5Cu/Al O3
100
100
34
100
95
100
100
23
91
75
14
9
0
0
23.5
0
3
0
86
91
0.5
35
50
85
85
2
**
***
****
5Cu/Al O3
2
5Cu/SiO2
10
65
42
15
15
5Cu/TiO –SiO2
2
3Cu/Al O3
2
7Cu/Al O3
100
100
100
100
2
9Cu/Al O3
0
2
Reaction conditions: [p-nitrotoluene] = 0.025 mol/L; [n-heptanal] = 0.0375 mol/L; catalyst sample weight, 200 mg; solvent, toluene;
3
temperature, 115 °C; pressure, 5.0 MPa; reaction mixture and hydrogen flow rates, 0.5 and 60 cm /min, respectively; and reaction time,
30–34 min.
*
Yields of products on a p-nitrotoluene conversion basis.
3
*
* Reaction mixture flow rate, 0.35 cm /min; reaction time, 40–45 min.
*** Catalyst sample weight, 156 mg.
**** Catalyst sample weight, 170 mg.
A decrease in the copper content of the catalyst
Reaction Mechanism
Cu/Al O to 3.1 wt % considerably decreased the yield
2
3
In order to study in detail the reaction mechanism
of secondary amine 5 (Table 2); this fact was explained
by the low activity of the sample in the reaction of nitro of the one-pot synthesis of secondary amines, we car-
group hydrogenation. At the same time, the yield of ried out the reaction of p-nitrotoluene with n-heptanal
N-heptyl-p-toluidine on the catalysts Cu/Al O with on the catalyst 5Cu/Al O by varying the contact time.
2
3
2
3
copper contents of 6.7 and 9.4 wt % was almost the Figure 4 shows the dependence of selectivity for the
same as that on 5Cu/Al O (Table 2, rows 1, 6, and 7).
2
3
formation of n-heptanal conversion products on the
conversion of p-nitrotoluene. It is evident that the
hydrogenation of n-heptanal began only after the con-
version of more than 71% p-nitrotoluene. Additional
experiments showed that, in the absence of p-nitrotol-
uene, n-heptanal was quantitatively converted into
Thus, in order to obtain a high yield of target sec-
ondary amine 5 in a flow reactor, the catalyst should
contain acid sites and manifest high activity in the
reaction of nitro group hydrogenation. The catalysts
Cu/Al O with copper contents from 5.3 to 9.4 wt %
2
3
n-heptanol on the catalyst 5Cu/Al O . Consequently,
possess these qualities.
2
3
p-nitrotoluene strongly inhibited the hydrogenation of
n-heptanal. Hence, it is possible to draw the conclu-
Selectivity, %
sion that the hydrogenation of –NO and –C=O
2
100
groups occurred at the same active sites; if the reaction
mixture simultaneously contained p-nitrotoluene and
n-heptanal, the selective adsorption of the nitroarene
occurred at these sites toward the nitro group [33, 34].
8
0
0
0
0
0
Imine
Secondary amine
n-Heptanol
6
4
2
It is well known that a hydrogenation reaction on
heterogeneous metallic catalysts includes a step of the
dissociative chemisorption of molecular hydrogen
34, 35]. Because hydrogen spillover from metallic
particles to an unreducible support (such as γ-Al O ) is
[
2
3
50
70
80
90
100
energetically unfavorable [35], it is reasonable to
assume that the hydrogenation of nitroarenes and
imines occurs at the interface between copper
nanoparticles and γ-alumina. In this case, a hypothet-
ical mechanism of the one-pot synthesis of N-heptyl-
p-toluidine on the catalyst Cu/Al O can be repre-
Conversion of p-nitrotoluene, %
Fig. 4. Dependence of selectivity for the formation of
n-heptanal conversion products on the catalyst 5Cu/Al O
2
3
upon the conversion of p-nitrotoluene. Reaction condi-
tions: [p-nitrotoluene] = 0.025 mol/L; [n-heptanal] =
2
3
0
.025 mol/L; temperature, 100 °C; pressure, 5.0 MPa; and
catalyst weight, 100–200 mg.
sented as the following reaction scheme:
KINETICS AND CATALYSIS
Vol. 59
No. 5
2018

ONE-POT SYNTHESIS OF SECONDARY AMINES FROM NITROARENES
H2
599
H
C H
O
H
6
13
N
H
C6H13
O
6
C H
H
C H
C H
13
O
6
13
6
13
H
H
+
HCu
H
C H
6 13
Cu
O
Cu
O
N
N
H
H
C H
6
13
H2
+
Cu
N+
C6H13
O
O
O-
Cu
H H
H
H
H
H
N+
C H
6
13
C6H13
H
N
H
Active site
of the catalyst
Cu
O
O-
Cu
H
H
Cu
N
Cu
N
The reaction begins with the selective adsorption of the course of the reaction. At the same time, the yield
p-nitrotoluene toward the nitro group on the active of secondary amine on the catalysts that possess weak
sites of the catalyst situated on the perimeter of metal- acidic properties (5Cu/SiO and 5Cu/TiO –SiO )
2
2
2
lic particles. The subsequent hydrogenation of p-nitro- and/or low hydrogenating activity (5Cu/SiO and
2
toluene with hydrogen atoms leads to the formation of
p-toluidine, which reacts with n-heptanal at the acid
sites of γ-alumina. The interaction of p-toluidine with
n-heptanal can also occur in solution (in the absence
of catalyst), as evidenced by imine formation on the
3
Cu/Al O ) under comparable conditions did not
2 3
exceed 50%.
The study of the reaction mechanism on the cata-
lyst 5Cu/Al O showed that the hydrogenation of
2
3
n-heptanal is strongly inhibited by p-nitrotoluene. On
the simultaneous presence of p-nitrotoluene and n-hep-
tanal in the reaction mixture, the selective adsorption of
the nitroarene occurs at the active centers of the catalyst
toward the nitro group; because of this, aldehyde hydro-
genation into alcohol comes into play only after the con-
version of a larger portion of p-nitrotoluene.
sample 5Cu/SiO , which does not contain acid sites
2
(Table 2). The side reaction of n-heptanal hydrogena-
tion to n-heptanol begins to play a noticeable role only
after the consumption of the major portion of p-nitro-
toluene. Because of the competition that appears upon
the adsorption of nitroarene, imine, and aldehyde
molecules and their conversion products at the active
sites, the contribution of the hydrogenation reaction to
the conversion of n-heptanal in the course of the one-
pot synthesis of N-heptyl-p-toluidine is relatively
small.
ACKNOWLEDGMENTS
This work was performed within the framework of
a state contract at the Boreskov Institute of Catalysis,
Siberian Branch, Russian Academy of Sciences (proj-
ect no. AAAA-A17-1170 41710083-5).
CONCLUSIONS
Thus, in this work, we studied the effects of the
support and the active component content on the
properties of supported copper catalysts in the one-pot
synthesis of N-heptyl-p-toluidine from p-nitrotoluene
and n-heptanal in a flow reactor with the use of
molecular hydrogen as a reducing agent. A high yield
of target product (to 91%) was reached on the catalysts
Cu/Al O with copper contents from 5.3 to 9.4 wt %
due to their high activity in the reaction of nitro group
hydrogenation and the presence of acid sites, which
are responsible for imine formation a result of the
interaction of n-heptanal with p-toluidine formed in
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Translated by V. Makhlyarchuk
KINETICS AND CATALYSIS
Vol. 59
No. 5
2018