Catalytic hydrogenation of 1,4-phenylenediamine to 1,4-cyclohexanediamine

Article abstract of DOI:10.1134/S1070427214030239

Catalytic hydrogenation of 1,4-phenylenediamine to 1,4-cyclohexanediamine using Ru/Al₂O₃ as a catalyst was carried out in water, and the results were compared with those in isopropanol and SC-CO₂. 80% 1,4-phenylenediamine conversion with 87% selectivity to 1,4-cyclohexanediamine was achieved on 5% Ru/Al₂O₃ catalyst at 90°C and H₂ pressure of 4 MPa. The hydrogenation of 1,4-phenylenediamine is influenced by the solvent. A systematic study of the hydrogenation of 1,4-phenylenediamine revealed that the reaction was consecutive. The longer the time, the lower was the CHDA selectivity. Also, the reaction temperature was an important parameter and played a vital role in preventing the formation of side products. Pleiades Publishing, Ltd., 2014.

Full text of DOI:10.1134/S1070427214030239

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 3, pp. 397−403. © Pleiades Publishing, Ltd., 2014.
VARIOUS
TECHNOLOGICAL SOLUTIONS
Catalytic Hydrogenation of 1,4-Phenylenediamine
to 1,4-Cyclohexanediamine*
Hongxian Ma and Jianguo Cai
Chemical Engineering Research Center, East China University of Science and Technology (ECUST), Shanghai 200237, China
e-mail: jgcai@ecust.edu.cn
Received December 21, 2013
Abstract—Catalytic hydrogenation of 1,4-phenylenediamine to 1,4-cyclohexanediamine using Ru/Al2O3 as a
catalyst was carried out in water, and the results were compared with those in isopropanol and SC-CO2. 80%
1,4-phenylenediamine conversion with 87% selectivity to 1,4-cyclohexanediamine was achieved on 5%
Ru/Al2O3 catalyst at 90°C and H2 pressure of 4 MPa. The hydrogenation of 1,4-phenylenediamine is
influenced by the solvent. A systematic study of the hydrogenation of 1,4-phenylenediamine revealed that
the reaction was consecutive. The longer the time, the lower was the CHDA selectivity. Also, the reaction
temperature was an important parameter and played a vital role in preventing the formation of side products.
DOI: 10.1134/S1070427214030239
INTRODUCTION
acids are used to increase the CHDA selectivity. Our
strategy is based on the systematic study of the effect
of various reaction parameters. The possible reaction
products are CHDA, CHA, cyclohexanol, and 4-amino-
cyclohexanol (Scheme 1). Therefore, it is necessary to
identify the conditions under which the best results from
the viewpoint of CHDA selectivity and PDA conversion
could be achieved in water.
The development of an efficient approach to the pro-
duction of amines is an interesting research area because
amines are an important class of compounds [1, 2]. From
the commercial standpoint, 1,4-cyclohexanediamine
(CHDA) is one of the most versatile intermediates. It can
be used in the production of textile, paper, and artificial
leather. Also, CHDA is an important intermediate in the
polyurethane industry, because it can be used for the
preparation of polyamide resin and aliphatic polyurethane
[3, 4]. It has two stereoisomers: trans and cis, and the
former is more stable than the latter. The stereoselective
control in catalytic hydrogenation is of great significance
for both the development of chemical processes and
academic research [5, 6]. Qinglin Liu studied the hydro-
genation of 1,4-phenylenediamine in water using Ru/C
catalysts, and the CHDA selectivity was 90%; however,
the temperature in the course of catalyst preparation is
difficult to control, as activated carbon is flammable.
Supercritical carbon dioxide is an ideal “replacement”
solvent for hydrogenation due to its environmentally
benign, nontoxic, and nonflammable nature, complete
miscibility with gases, adjustable dissolving power, clean
product separation, and weaker catalyst deactivation
under “green” and mild conditions [1, 7–13]. Thus, super-
critical carbon dioxide (SC-CO2) has gained considerable
attention for various types of chemical reactions. In this
work, we have attempted the hydrogenation of PDA in
SC-CO2 medium to find how SC-CO2 affects the CHDA
selectivity and the PDA conversion.
The synthesis of 1,4-cyclohexanediamine (CHDA)
from 1,4-phenylenediamine (PDA) is a tandem reaction
carried out in a single reactor. It involves several side
reactions (Scheme 1). No additives like ammonia or any
EXPERIMENTAL
Materials
1,4-Phenylenediamine and deionized water were used
as received. High-purity hydrogen (>99.9%) was pur-
* The text was submitted by authors in English.
397

398
HONGXIAN MA, JIANGUO CAI
chased from Shanghai Wujing Industrial Co., Ltd., China,
and directly used from a cylinder. Ruthenium(III) chloride
trihydrate (RuCl3·3H2O) andAl2O3 were purchased from
Strem Chemicals and Cambridge Isotope Laboratories,
respectively. NaOH (>96%) was purchased from Sino-
pharm Chemical Reagent Co., Ltd., China.
in a stream of hydrogen (240°C, 4 h) and then cooled in
a nitrogen atmosphere to room temperature.
Catalytic Hydrogenation
Hydrogenation of 1,4-phenylenediamine was carried
out in a 200-mL stainless steel batch reactor (Fig. 1).
0.1 g of 5% Ru/Al2O3 catalyst and 0.5 g of the reactant
were loaded into the reactor, after which the reactor was
sealed and flushed with N2 twice. The reactor was placed
in a heater to maintain the desired temperature. H2 of re-
quired pressure was introduced when the reactor reached
the required temperature. Finally, the liquid product was
separated from the catalyst by filtration. The quantitative
analysis of the samples was performed with an HP-5 gas
chromatograph equipped with a capillary column(50 m ×
0.32 mm × 0.5 μm) and a flame ionization detector.
Catalyst Preparation
The Ru/Al2O3 catalyst was prepared by incipient-
wetness impregnation method. The ruthenium content
was set between 1.0 and 5.0 wt %. RuCl3·3H2O was dis-
solved in a certain amount of water. Agitation was done
with a magnetic stirring bar. 25 g of Al2O3 was added to
the resulting solution with stirring.After 36 h, NaOH was
added to the above solution with stirring. After 36 h, the
above solution was filtered, and Al2O3 was dried. After
drying in air at 120°C for 10 h, the samples were reduced
For all results reported, the selectivity is defined as
follows:
1 to 5%, the conversion increased from 99.67 to 99.92%,
and the selectivity, from 43.9 to 63.7%. This may due to
larger amount of catalyst active sites becoming available
for hydrogenation of PDA to CHDA with an increase in
the catalyst loading. However, according to [13], when
the catalyst loading was above 5%, both the conversion
and the selectivity decreased with increasing the catalyst
RESULTS AND DISCUSSION
Influence of Catalyst Loading
The catalyst loading is an efficient variable parameter
for the reaction optimization. PDA hydrogenation was
carried out at different catalyst loadings (Fig. 2). We
found that, with an increase in the catalyst loading from
100.0
64
99.8
5
6
60
56
52
48
44
1
99.6
99.4
2
99.2
99.0
3
1
2
1
2
3
4
5
4
Catalyst loading, %
Fig. 1. Schematic of the reactor setup used for hydrogenation
experiments. (1) H2 cylinder, (2) N2 cylinder, (3) autoclave, (4)
magnetic stirrer, (5) pressure controller, and (6) temperature
controller.
Fig. 2. Influence of the catalyst loading on the (1) PDAconver-
sion and (2) CHDA selectivity. Reaction conditions: 0.1 g
of catalyst, 0.5 g of substrate, 150°C, PH2 = 8 MPa, time
45 min.
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CATALYTIC HYDROGENATION OF 1,4-PHENYLENEDIAMINE
399
Influence of Reaction Time
loading. This may be attributed to tendency of the cata-
lyst to agglomerate with a further increase in the catalyst
loading, which can lead to the catalyst deactivation. Thus,
all the reactions were performed with 5% Ru catalyst.
The influence of the reaction time on the hydrogena-
tion of 1,4-phenylenediamine is shown in Fig. 4. It can
be seen that the conversion and the CHDAselectivity are
strongly influenced by the reaction time. The conversion
of PDA increased linearly from 47 to 99.3% as the time
was changed from 30 to 90 min. On the other hand, the
CHDA selectivity initially increased, but as the conver-
sion increased, the CHDAselectivity decreased from 60 to
35%. Thus, we deal with a tandem reaction, in accordance
with Scheme 1. The deamination of CHDAincreased with
time. Therefore, the choice of an optimum reaction time
was crucial for all the measurements.
Influence of H2 Pressure
To evaluate the influence of the H2 pressure, we per-
formed PDA hydrogenation at fixed temperatures of 140
and 150°C with 5% Ru (Fig. 3). The H2 pressure was
varied from 2 to 8 MPa. The results showed that the PDA
conversion increased from 60.5 to 99.6% as the pressure
changed from 2 to 8 MPa (Fig. 3a). This result can be
generally attributed to an increase in the H2 solubility
in the liquid with pressure. So, higher concentration of
surface hydrogen becomes available on the active catalyst
sites, facilitating the hydrogenation of PDA to CHDA;
hence, the CHDA selectivity marginally increased from
30 to 60% (Fig. 3b). Thus, hydrogenation under the reac-
tion condition studied was dependent on the amount of
H2. The optimum pressure is 4–6 MPa. With increasing
temperature, the CHDA selectivity decreased (Figs. 3a,
3b). The effect of temperature will be described below.
Influence of Temperature
The temperature is also a critical parameter for the
hydrogenation. In this work, the hydrogenation of PDA
was studied in the temperature range 80–180°C.
As seen from Fig. 5, the CHDA selectivity was rela-
tively low. To optimize the reaction, the reaction can be
performed at lower temperature by prolonging the reac-
tion time to improve the conversion (Fig. 6). Comparison
of Figs. 5 and 6 shows that the lower temperature is
preferable.
(a)
60
100
1
50
40
90
80
70
60
At 80°C, the conversion was as low as 45%, and at
90°C it increased jumpwise to 94.69%; then, the value
increased gradually with temperature. As for the CHDA
selectivity, it decreased with increasing temperature. At
lower temperatures, the by-product cyclohexanol did
not form (Fig. 5), and the cyclohexanol yield tended to
increase with temperature. The CHAyield also increased
2
30
20
3
2
3
4
5
6
7
8
P, MPa
(b)
100.0
99.5
60
50
40
30
1
2
60
56
52
48
44
40
36
32
100
90
80
99.0
3
70
60
50
40
3
4
5
6
7
8
P, MPa
30 40 50 60 70 80 90
t, min
Fig. 3. Influence of the H2 pressure on the hydrogenation of
1,4-phenylenediamine. Reaction conditions: 0.1 g of catalyst,
0.5 g of substrate, time 45 min; temperature, °C: (a) 150 and
(b) 140. (1) Conversion, (2) CHDA selectivity, and (3) CHA
selectivity.
Fig. 4. Influence of the reaction time on the hydrogenation of
1,4-phenylenediamine. Reaction conditions: 0.1 g of catalyst,
0.5 g of substrate, 130°C, PH2 = 4 MPa.
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HONGXIAN MA, JIANGUO CAI
Scheme 1. Probable pathways of PDA hydrogenation under the reaction conditions studied.
(1-1)
1,4-Cyclohexanediamine (CHDA)
P
(1-2)
(1-3)
Cyclohexylamine (CHA)
4-Aminocyclohexanol
(1-4)
4-Aminocyclohexanol
(1-5)
reaction conditions. The results are shown in the table.
PDA was mainly hydrogenated to CHDA and CHA in
organic solvents under the same conditions, similarly to
the aqueous medium. In isopropanol, the PDAconversion
was about 38%, and the CHDAselectivity was about 90%.
The reaction rate was enhanced in the presence of water.
The use of water caused a marked increase in the product
conversion, whereas isopropanol can improve the selec-
tivity by prohibiting the side reaction [Scheme (1-3)],
leading to the reaction inhibition [Scheme (1-4)]. These
results suggest that the use of mixed water–isopropanol
solvent may improve both the PDA conversion and the
CHDA selectivity.
with temperature. Thus, relatively high temperature en-
hanced side reactions of CHDA. An attempt was made to
check the effect of elevated reaction temperature (180°C);
the result indicated a considerable change in the product
distribution.Although the PDAconversion reached 100%,
the CHDA selectivity decreased dramatically. The side
product dicyclohexylamine (DCHA) formed [Scheme (1-
1–1-5)]. Thus, the temperature was an important parameter.
Reaction in Different Solvents
To reveal the role of solvent in PDAhydrogenation, we
conducted the reaction in organic solvents under similar
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CATALYTIC HYDROGENATION OF 1,4-PHENYLENEDIAMINE
401
Variation of solvent in hydrogenation of 1,4-phenylenediamine.
Reaction conditions: 0.1 g of catalyst, 0.5 g of substrate, 90°C,
PH2 = 4 MPa, t = 4 h
110
1 0 0
90
60
50
40
30
20
Conversion, Selectivity, trans/cis
Solvent
%
%
ratio
0.74
0.78
80
20 mL of water
98.9
38.0
69.4
90.8
20 mL of isopropanol
70
10
2 mL of water + 18 mL
of isopropanol
77.6
86.8
0.81
120 130 140 150 160 170 180
T, °C
1
2
3
4
5
Fig. 5. Influence of temperature on the (1) conversion and
(2–5) selectivity of 1,4-phenylenediamine hydrogenation.
Reaction conditions: 0.1 g of catalyst, 0.5 g of substrate, time
45 min, PH = 4 MPa. (2) CHDA, (3) CHA, (4) cyclohexanol,
and (5) 4-a2minocyclohexanol; the same for Fig. 6.
(d) Under SC-CO2, the main by-product was 4-ami-
nocyclohexanol: the introduction of CO2 can activate the
benzene ring to accelerate the reaction. However, less
basic PDA itself cannot react spontaneously with CO2
to form carbamic acid/carbamate, whereas CHDA could
easily form solid carbamic acid (Scheme 3). This may
be due to the n–π conjugation effect: the electron density
from the amino group is withdrawn to the benzene ring,
Mixed water–isopropanol solvent. In order to gain
further insight into the role of water in PDA hydrogena-
tion, we performed the experiment with isopropanol–
water (V= 20 mL) solvent (Fig. 7). As expected, CHDA
was preferentially formed when the hydrogenation rate
was low (lower amount of water). The optimum solvent
composition from the viewpoint of PDA conversion
and CHDA selectivity was 2 mL of water and 18 mL of
isopropanol.
70
60
50
40
30
20
10
100
90
80
70
60
50
SC-CO2 solvent.As we mentioned above, the hydro-
genation in organic solvents was mainly dependent on the
H2 concentration and was often controlled by the transport
rate. Thus, it required more energy to get a reasonable
reaction rate. Supercritical carbon dioxide (SC-CO2)
has gained considerable attention for different types of
chemical reactions. In this work, we have attempted the
hydrogenation of PDA in SC-CO2 medium. However,
the result was unsatisfactory (Figs. 8 and 9). This may
be due to several reasons.
40
80
90 100 100 120
T, °C
Fig. 6. Influence of temperature on the (1) conversion and (2–5)
selectivity of 1,4-phenylenediamine hydrogenation. Reaction
conditions: 0.1 g of catalyst, 0.5 g of substrate, time 4 h, PH2
4 MPa.
=
100
90
80
70
60
50
40
30
80
60
40
20
0
(a) The PDA conversion and CHDA selectivity were
reduced considerably (Figs. 8 and 9) due to the dilution
effect: the dissolution of CO2 molecules caused the dilu-
tion of the reacting species in the solvent phase, resulting
in decreased reaction rate.
1
2
3
(b) The insufficient ability of water to dissolve gases,
in turn, might cause gas–liquid mass transport problems,
although the solubility of H2 should be slightly improved
in the presence of SC-CO2.
0
5
10
15
20
Vwater, mL
Fig. 7. Influence of the composition of water–isopropanol
mixed solvent on the (1) conversion and (2, 3) selectivity of
1,4-phenylenediamine hydrogenation. Reaction conditions:
0.1 g of catalyst, 0.5 g of substrate, time 4 h, 90°C, PH2 = 4 MPa.
(2) CHDA and (3) CHA.
(c) The formation of large chain molecules (Scheme 2)
increased steric hindrance, which should be responsible
for the decreased total conversion.
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HONGXIAN MA, JIANGUO CAI
50
45
40
35
30
25
20
14
12
10
8
so CO2 reacts with CHDA more readily than with PDA.
(e) NH3 can react with CO2, which was beneficial to
the side reactions.
1
(f) The main side product in SC-CO2 medium is
4-aminocyclohexanol. This may be attributed to the fact
that CO2 retarded the deamination of CHDA through
stronger interaction between the CO2 molecule and the
amino group of PDA, which lowered the reactivity of
the amino group.
2
6
4
2
0
140 150 160 170 180 190 200
Stereoselectivities to trans forms were further im-
proved in the presence of SC-CO2, though SC-CO2 sol-
vent was not found to be very effective for the selective
hydrogenation of PDAto CHDAover a carbon-supported
rhodium catalyst, compared with the traditional liquid-
phase hydrogenation in organic solvents. The trans ratios
obtained in various PDA hydrogenation experiments
in SC-CO2 were in the range 0.84–0.92, while those in
2-propanol and water were in the range 0.75–0.81 and
0.67–0.78, respectively. Thus, higher trans-CHDA ratio
was reached in SC-CO2. The decrease in the cis ratio at
higher carbon dioxide pressure can be caused by the en-
hancement of the CHDAflipping [14], but this conclusion
needs further studies.
T, °C
Fig. 8. Influence of temperature on the hydrogenation of
1,4-phenylenediamine under SC-CO2. Reaction conditions:
0.1 g of catalyst, 0.5 g of substrate, PCO = 8 MPa, PH2 = 4 MPa,
t = 6 h. (1) Conversion and (2) CHDA selectivity; the same for
2
Figs. 9 and 10.
100
90
■ 1
14
12
2
80
70
60
50
40
30
10
8
6
4
2
0
0
2
4
6
8
10
Catalyst Recycling
PCO
2
Recycling of the catalyst is the most important crite-
rion for heterogeneous catalytic reaction. A frequently
encountered problem of supported metal catalysts was
Fig. 9. Influence of PCO on the hydrogenation of 1,4-phenylene-
diamine under SC-CO22. Reaction conditions: 0.1 g of catalyst,
0.5 g of substrate, 170°C, PH2 = 4 MPa, t = 6 h.
Scheme 2. Probable pathways of PDA hydrogenation in SC-CO2.
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CATALYTIC HYDROGENATION OF 1,4-PHENYLENEDIAMINE
403
Scheme 3. Formation of cyclohexylcarbamic acid.
1
2
3
3.6
3.4
3.2
3.0
2.8
2.6
95
85
75
65
55
NH2
NH2CO3H
+
+
1
2
3
4
NH2
NH2CO3H
Number of recycling
Fig. 10. Influence of recycling on the hydrogenation of
1,4-phenylenediamine. Reaction conditions: 0.1 g of catalyst,
0.5 g of substrate, 90°C, PH2 = 4 MPa, t = 4 h. (1) Conversion,
(2) Selectivity, (3) trans : cis ratio.
the decrease in their activity in the course of operation
due to the leaching of metal particles. In this experi-
ment, the catalyst was recycled in four successive runs
with the separation by filtration; the results are shown
in Fig. 10. The CHDA yield decreased from 65 to 37%
after the fourth recycle. Furthermore, the trans : cis ratio
decreased from 3.5 to 2.6. This may be caused by the loss
and deactivation of catalyst.
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