Hydrogenation of aniline to cyclohexylamine in supercritical carbon dioxide: Significance of phase behaviour

Article abstract of DOI:10.1016/j.apcata.2011.02.016

Hydrogenation of aniline to cyclohexylamine was carried out in supercritical carbon dioxide using a variety of noble metal (Pt, Pd and Rh) catalysts. At 80 °C and 8 MPa of CO₂ pressure, >95% of aniline conversion with 93% selectivity to cyclohexylamine was achieved on 5% Rh/Al₂O₃. A strong influence of phase behaviour related to the CO₂ pressure was found on the conversion and selectivity. Optimization of reaction parameters resulted in a higher overall activity in the biphase (liquid substrate + gaseous H₂ and CO₂) than in the single phase (liquid substrate-CO₂-H₂) condition. It has been found that the interaction of CO₂ with amine leads to the formation of solid carbamic acid, which enhanced the selectivity of cyclohexylamine, but reduced the conversion significantly. Furthermore, reaction temperature played a crucial role in preventing the formation of carbamic acid and also maintained a reasonably high reaction performance in terms of conversion and selectivity.

Full text of DOI:10.1016/j.apcata.2011.02.016

Applied Catalysis A: General 396 (2011) 186–193
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydrogenation of aniline to cyclohexylamine in supercritical carbon dioxide:
Significance of phase behaviour
∗
M. Chatterjee , M. Sato, H. Kawanami, T. Ishizaka, T. Yokoyama, T. Suzuki
Research Center for Compact Chemical System, AIST Tohoku, 4-2-1 Nigatake, Miyagino-ku, Sendai, 983-8551, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 October 2010
Received in revised form 14 February 2011
Accepted 15 February 2011
Available online 22 February 2011
Hydrogenation of aniline to cyclohexylamine was carried out in supercritical carbon dioxide using a
variety of noble metal (Pt, Pd and Rh) catalysts. At 80 C and 8 MPa of CO2 pressure, >95% of aniline
◦
conversion with 93% selectivity to cyclohexylamine was achieved on 5% Rh/Al2O3. A strong influence of
phase behaviour related to the CO2 pressure was found on the conversion and selectivity. Optimization
of reaction parameters resulted in a higher overall activity in the biphase (liquid substrate + gaseous
H2 and CO2) than in the single phase (liquid substrate–CO2–H2) condition. It has been found that the
interaction of CO2 with amine leads to the formation of solid carbamic acid, which enhanced the selectivity
of cyclohexylamine, but reduced the conversion significantly. Furthermore, reaction temperature played
a crucial role in preventing the formation of carbamic acid and also maintained a reasonably high reaction
performance in terms of conversion and selectivity.
Keywords:
Supercritical carbon dioxide
Phase behaviour
Heterogeneous catalysis
Hydrogenation
Aniline
Cyclohexylamine
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Rh/Al O , and the selectivity of CHA was low. Instead of noble metal
2
3
catalysts, Narayanan and Unnikrishnan [3] compared the activity
The development of an efficient method for production of amine
and selectivity of aniline hydrogenation over 50 wt% Ni/Al O3 and
2
is an interesting research area because amines are an important
class of compounds. Among them, cyclohexylamine (CHA) is one of
the most versatile intermediates from the commercial standpoint.
It can be used in the synthesis of artificial sweeteners (sodium or
calcium cyclamate), metal corrosion inhibitors, rubber vulcanizing
additives, dyestuff, plasticizers and extracting agents for natural
products [1]. Commercially, CHA may be produced via either of
two processes (i) reductive amination of cyclohexanol or phenol or
Co/Al O catalyst and in each case the obtained product was a mix-
2
3
ture of CHA, DCHA and N-phenylCHA; maximum selectivity of CHA
was ∼70%. Aniline hydrogenation has also been reported in the liq-
uid phase using batch or continuous mode, acidic and non-acidic
medium. Several patents were issued describing specific catalyst
formulations [4], many of which contain mixtures of Ru and Pd on
Al O supports. However, in most of the cases, objectionable quan-
2
3
tities of DCHA or other by-products were formed. To prevent the
formation of DCHA, the most used technique for heterogeneously
catalyzed reaction is application of ammonia. However, the disad-
vantages of using ammonia are pressurized storage and excessive
amount required which results in environmental and economi-
cal concerns. Again, when the reaction was conducted in acidic
medium to improve the selectivity of CHA, an additive was nec-
essary, whereas non-acidic medium required higher temperature
(
ii) hydrogenation of aniline. However, the main disadvantages are
formation of by-products and the use of a large excess of ammonia.
A variety of metals such as Ni, Co, Rh, Ru, Pd and Pt were used to
study the hydrogenation of aniline in vapour phase as well as in the
liquid phase condition. Different reaction parameters such as cata-
lyst, reaction temperature, solvent used and whether the reaction
was carried out in gas phase or in liquid phase strongly influenced
the product distribution. For instance, in vapour phase hydrogena-
tion, Moss and Kemball [2] described the formation of benzene
and ammonia instead of CHA over Pt foil catalyst. On the contrary,
◦
(100–250 C) and pressure up to 35 MPa. Such high pressure and
temperatures are unfavorable from the safety viewpoint. Instead
of organic solvent, Sokolskii et al. [5] studied the hydrogenation of
◦
dicyclohexylamine (DCHA) and CHA were formed at 200 C, using
aniline in water using Rh/Al O3 catalysts; however, the selectivity
2
of CHA was 64–79% with low conversion.
Supercritical carbon dioxide (scCO ) has gained considerable
2
attention for different types of chemical reactions. Over the past few
years, several heterogeneously catalyzed reactions have been suc-
cessfully carried out in scCO2 medium, often with higher reaction
∗ Corresponding author. Tel.: +81 22 237 5213; fax: +81 22 237 5388.
E-mail addresses: c-maya@aist.go.jp (M. Chatterjee), h-kawanami@aist.go.jp
H. Kawanami).
(
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.02.016

M. Chatterjee et al. / Applied Catalysis A: General 396 (2011) 186–193
187
O
NH2
NH2
NH2
HN
OH
+
3H2
2-1
CO2
(1-1)
+
Cyclohexylamine (CHA)
Aniline
cyclohexylamine
CHA)
H
N
(a)
(
2
-2
2
CHA
Scheme 1. Formation of cyclohexylcarbamic acid.
Dicyclohexylamine (DCHA)
rates and different product distributions, as well as high selectiv-
ity, in comparison with those available in conventional organic
solvents [6–11]. In the CO2 medium, enhanced reaction rate and
high product selectivity are mainly tuned by the pressure and
temperature. In practice, depending on pressure, the solubility of
the reaction components in CO2 is varied; this is called phase
behaviour. It has considerable importance for the reaction in CO2,
since sometimes reaction outcomes can be dictated by the phase
behaviour of the reaction components. Furthermore, CO2 has a
great potential to overcome difficulties associated with the con-
ventional homogeneous and heterogeneous catalysts such as mass
transfer limitation that frequently occur between the reactants in
gas and liquid phase. In particular, scCO2 can be regarded as an
ideal medium for hydrogenation because, unlike organic solvents,
Scheme 2. Probable reaction path of aniline hydrogenation under the studied reac-
tion condition.
ture, H2 of required pressure was introduced. After that the liquid
CO2 was charged using a high-pressure liquid pump (JASCO) and
then compressed to the desired pressure. Finally, the liquid product
was separated from the catalyst simply by filtration and identified
by GC–MS, followed by quantitative analysis using a GC (HP 6890)
equipped with a capillary column and a flame ionization detector.
A 30 m × 0.53 i.d. capillary column with a 0.25 ␮m film thickness
(
DB-wax; Agilent) was used. The oven temperature program was
◦
◦
set to the initial temperature of 50 C followed by 50–100 C (hold
◦
◦
time 5 min) and 100–230 C ramps at 10 C/min (hold time 4 min).
H2 is completely miscible in CO . At a given pressure of gas, the
◦
2
Injector and detector were set at 230 C. Two microlitres of sample
effective concentration of H2 can be nearly an order of magnitude
higher than in classical organic solvents, which will increase the
H2 concentration to the catalyst surface leading to the enhanced
reaction rate. Also, scCO2 is non-inflammable and non-toxic; these
physically and toxicologically inert properties have made it a most
promising solvent for organic synthesis.
In this work, we have attempted the hydrogenation of aniline in
scCO2 medium. No additives like ammonia or any acids were used
to increase the selectivity of CHA. Our strategy was based on the fact
was injected in split mode, with a split ratio of 5.6. Quantification of
products was obtained by a multipoint calibration curve using stan-
dard compounds. For all results reported, the selectivity is defined
as follows:
concentration of the product
%
selectivity = total
× 100
concentration of products
For experiments in an organic solvent, CO was replaced by 5 ml
2
of the organic solvent.
that CO has a tendency to interact with nucleophilic amine to form
2
carbamic acid. However, less basic aniline (pK = 9.40) by itself does
b
not react spontaneously with CO to form carbamic acid/carbamate
2.3. Phase behaviour studies
2
[
(
12], whereas CHA (pK = 3.34) could easily form solid carbamic acid
Scheme 1), which might prohibit the formation of DCHA, but at
b
Understanding of the phase behaviour is the key to study the
reaction in CO2. The reactor used in this experiment was closed;
thus, no observation could be made on the number of phases
present during the reaction. Therefore, to mimic the reaction con-
dition, we conducted visual observation of the phase behaviour of
aniline separately with a 10 ml high pressure view cell fitted with a
sapphire window. However, no catalyst was used in the view cell.
The cell was placed over a magnetic stirrer for stirring the content
and connected to a pressure controller to regulate the pressure
the same time, there could be the possibility of reduced conversion
because of the phase separation due to solid carbamic acid depo-
sition. Therefore, it is necessary to identify the conditions under
which the best performances in terms of selectivity of CHA and
conversion of aniline could be achieved in scCO2.
2
. Experimental
◦
inside the cell; the desired temperature of 80 C was maintained
2
.1. Materials
by a temperature controller. A 0.1 g (1.06 mmol) amount of ani-
line was introduced into the view cell at a constant H2 pressure
of 4 MPa while CO2 pressure was varied between 6 and 15 MPa
and the phase behaviour of aniline–H –CO system was monitored.
Aniline, CHA and DCHA (Wako Pure Chemicals) were used as
received. Pd/C, Pd/Al O , Pt/C, Pt/Al O and Rh/C were from Aldrich.
Rh/Al O was purchased from Wako. Carbon dioxide (>99.99%) was
supplied by Nippon Sanso Co. Ltd.
2
3
2
3
2
2
2
3
Similarly, phase behaviours of CHA and DCHA and the mixture of
aniline + CHA + DCHA have also been studied. Noteworthy, only a
very small amount of CHA (0.098 mmol) was taken, because for
The catalyst used here is commercially available Rh/Al O . The
2
3
particle size of Rh was determined from the X-ray diffraction pat-
tern using the Scherrer equation from the (1 1 1) peak of Rh at
2
1
.0 mmol of CHA, large numbers of solid carbamic acid crystals were
◦
found on the sapphire window of the view cell, due to the lower
temperature of the window wall in contrast to the reactor wall,
which prevented the observation.
ꢀ = 40 .
2
.2. Catalytic activity
3. Results and discussion
Hydrogenation of aniline was carried out in a 50 ml stainless
steel batch reactors; some details are given elsewhere [10]. Briefly,
.1 g of catalyst and 0.5 g of the reactant were introduced into the
reactor and placed in an oven with a fan heater to maintain the
desired temperature. When the reactor reached the set tempera-
Scheme 2 represents a typical reaction profile of aniline hydro-
genation. Generally, aniline is considered to be hydrogenated to
CHA, followed by the formation of DCHA.
0

1
88
M. Chatterjee et al. / Applied Catalysis A: General 396 (2011) 186–193
Table 1
NHCO H
2
Hydrogenation of aniline over different supported noble metal catalysts and in dif-
ferent reaction medium .
a
TOF (h 1
−
)
Conv. (%) Selectivity (%)
Entry Catalyst
Dispersion
d
(%)
carbamate
CHA
DCHA
CO2
1
2
3
4
5
6
7
8
9
1
1
5% Pd/C
5% Pd/Al2O3
1% Pd/MCM-41 5.6
5% Pt/C
5% Pt/Al2O3
1% Pt/MCM-41
5% Rh/C
5% Rh/Al2O3
1% Rh/MCM-41 30.0
9.1
7.0
50.6
81.6
22.0
63.1
110.3
128.0
105.4
223.3
57.4
24.2
30.0
1.3
15.0
40.0
8.8
45.2
95.8
18.7
52.4
49.6
30.8
10.3
100.0 0.0
100.0 0.0
61.7
100.0 0.0
100.0 0.0
93.0
65.2
59.5
60.2
69.2
89.7
NH2
NH
8.3
4.5
12.0
7.9
3.6
38.3
+
7.0
34.8
40.5
39.8
Cyclohexylamine (CHA)
imine
b
0
1
5% Rh/Al2O3
5% Rh/Al2O3
122.2
115.6
c
a
Reaction condition: Catalyst: 0.1 g, aniline = 0.5 g, PCO = 8 MPa, PH = 4 MPa,
2
2
◦
NH3
reaction time = 6 h; Temp. = 80 C; MCM-41 catalysts are prepared in our laboratory
[
Ref.: Catal. Lett. 61 (1999) 199]; entry 10, 11 = organic solvent (5 ml).
b
Ethanol.
Hexane.
c
d
H
N
Approximate expression of metal dispersion = 0.9/diameter (in nm); [Ref.:
M. Boudart, G. Djega-Mariadassou, Kinet. Heterogeneous Catal. React., Prince-
ton University Press: Princeton, NJ, 1984]; Particle diameter (r) was obtained
from Scherrer equation: r = Kꢁ/ˇ cos ꢀ, where K = Scherrer constant nearly equals
to 1, ꢁ = wavelength of X-ray, ˇ = FWHM in radians 2ꢀ; Turnover frequency
(
TOF) = number of molecules reacted/number of sites × time.
dicyclohexylamine (DCHA)
3.1. Catalyst screening
Scheme 3. Schematic representation of the formation of DCHA from CHA.
The hydrogenation of aniline was performed over different
and selectivity of CHA was Rh > Pt > Pd and Pt > Rh > Pd, respectively.
Independent to the metal ion, when the support changes from C to
Al O (entries 1, 2, 4, 5, 7 and 8), the dispersion was decreased, but
noble metal catalysts supported on Al O , C (activated carbon) and
2
3
MCM-41. Table 1 (entries 1–9) presents the results of aniline hydro-
genation. It is evident that Pt and Pd catalysts are less efficient than
Rh to achieve targeted high conversion and CHA selectivity. For
2
3
conversion increased under the same reaction condition. For exam-
ple, Rh/Al O3 catalyst was highly active, despite of having a very
2
example, DCHA was the major product over Pd/C and Pd/Al O cat-
2
3
low dispersion (3.6%; Table 1; entry 7). Results in Table 1 also pro-
vide a significant relationship between intrinsic activity in terms of
the turnover frequency (TOF) and metal dispersion. An increase in
TOF along with the decreasing metal dispersion was observed when
the support changes from Al O3 to C (TOFAl2O3 > TOF ), indepen-
alysts (entry 1 and 2). Furthermore, CHA was formed with very high
selectivity of 100% using Pd/MCM-41 catalysts, but with low con-
version (∼1%; entry 3). Similar behaviour was also observed for
Pt catalysts. Nevertheless, except Rh/MCM-41, all other Rh cata-
lysts were highly active and produced CHA with high selectivity
2
C
dent of the metal ion used. Thus, in each case the highest efficiency
(
entries 7 and 8). For instance, Rh/C (entry 7) exhibited 100% selec-
tivity of CHA with the conversion of 45.2%. Table 1 also suggests
that irrespective of the metal, Al O3 and C supported catalysts are
was achieved over Al O3 support, which might be attributed to
2
the presence of the Lewis acid sites preferably activating the aro-
matic ring as proved by several theoretical and experimental results
[14–17]. Regarding the selectivity, DCHA was the main product on
2
more efficient than MCM-41. Generally, Al O3 and C are consid-
2
ered acidic, whereas MCM-41 is neutral. It was proposed that acidic
sites on the support material can act as the adsorption site for
aromatic compounds and, in the region surrounding a metal par-
ticle, these adsorbed species can react with H2 spilt-over from the
metal, thus contributing to the overall activity [13]. The inactivity
for all the MCM-41 supported catalyst might be due to the neutral
nature of the support, or to the low metal content (fewer active
sites exposed). This will require a separate study because it is a
complicated issue.
Pd/C and Pd/Al O (Table 1; entry 1, 2). However, CHA was obtained
2
3
with high selectivity using Pt and Rh catalysts (Table 1; entries 4, 5,
and 8) irrespective of the support used. The difference in product
7
distribution between Rh and Pd catalysts was observed previously
during the hydrogenation of ␤-myrcene in CO [18]. Similarly, dur-
2
ing the liquid phase hydrogenation of aniline, Rh was stated to form
CHA (Scheme 3) [19], whereas Pd was strongly inhibited. Hence, no
detailed study was available over Pd catalyst. The most striking is
that the hydrogenation of aniline in CO also followed a very similar
2
trend when Rh catalyst was used. However, for Pd catalyst, instead
of total inhibition such as occurred in organic solvent medium, a
moderate conversion of ∼30% with the formation of DCHA was
3
.2. Metal dispersion
Table 1 presents dispersion of metal, which is an important
observed in CO . Thus, judging from the overall activity, we con-
2
factor to decide the availability of the active phase for reaction.
An approximate expression was used to determine the dispersion
from the particle diameter, derived using the Scherrer equation.
Among the studied catalysts (excluding MCM-41 because its metal
content was low), dispersion was varied from ∼4% to 10%. For C
sidered Rh/Al O as the most effective catalyst concerning the high
2
3
conversion and selectivity of CHA (conversion = 95.8%, selectivity
to CHA = 93%) and we choose it for further study to optimize differ-
ent reaction parameters. It has to be mentioned that no conversion
occurred without a catalyst.
∼
∼
supported catalysts, the dispersion was Pd = Pt = Rh. However, con-
version and the selectivity to CHA followed the order: Rh > Pd > Pt
3.3. Effect of CO2 pressure
∼
3
and Rh = Pt > Pd, respectively. On the other hand, metal dispersion
of Al O -supported catalysts shows the order of Pd > Pt > Rh. In con-
For optimization of reaction parameters, CO2 pressure is an
2
trast to the C support, the exhibited trend of the aniline conversion
effective variable. Aniline hydrogenation was carried out under dif-

M. Chatterjee et al. / Applied Catalysis A: General 396 (2011) 186–193
189
behaviour of aniline, CO2 and H2 in a view cell (details in the
experimental section) and images are shown in Fig. 2(a–f). Fig. 2a
presents the image of only aniline in the view cell. After the intro-
duction of CO , at the lower pressure (Fig. 2b and c), the system was
2
biphasic. On the other hand, the meniscus between two phases dis-
appeared and the system attained a single phase (aniline–CO –H )
2
2
(Fig. 2d and e) when the CO2 pressure was above 12 MPa. There-
fore, according to the phase behaviour, the highest conversion of
aniline and CHA selectivity were achieved in the biphasic condition.
Increased conversion and selectivity in the biphasic condition is an
unusual observation for hydrogenation in scCO . However, Hitzler
2
et al. [20] described a similar example of fast hydrogenation in a
biphasic mixture containing high-pressure CO . Chouchi et al. [21]
2
Fig. 1. Effect of CO2 pressure on the conversion and selectivity of aniline hydro-
genation. Reaction conditions: catalyst = 0.1 g, substrate = 0.5 g, temperature = 80 C,
PH2 = 4 MPa, time = 6 h.
provide another example of high reaction efficiency at the bipha-
◦
sic condition during the hydrogenation of pinene in scCO . Authors
2
interpreted their results based on the assumption that the reaction
is controlledbythe adsorptionofsubstrateon thecatalystsurface as
the substrate is more concentrated in the liquid rich phase around
the catalyst, which represents a good model of CO2-expanded liq-
uids [21,22]. It has been reported that CO2 has a tendency to add to
organic liquids [23] and the combination act as CO2-expanded liq-
uid. The solubility of H2 is greater in the CO2-expanded liquid than
ferent CO pressure values at the fixed hydrogen pressure of 4 MPa
2
◦
and the temperature of 80 C (Fig. 1). Results show that with the
increase in pressure from 6 to 8 MPa, the conversion was changed
from 40.2% to 95.8% and then decreased to 21.2% when the pres-
sure reached 14 MPa. To explain this observation, we studied phase
◦
◦
Fig. 2. Phase behaviour of aniline–H2–CO2 at 80 C; (a) aniline in the view cell, in presence of CO2 (b) 8 MPa, (c) 12 MPa, (d) 14 MPa, (e) 15 MPa and (f) 8 MPa at 35 C.

1
90
M. Chatterjee et al. / Applied Catalysis A: General 396 (2011) 186–193
a
250
(Fig. 4f) and aniline (Fig. 2d) show complete solubility. As men-
tioned before, the conversion of aniline was substantially decreased
at the higher pressure of CO . We speculated that the produced
2
2
1
1
00
50
00
CHA, which was present in the same phase of aniline, converted
to DCHA (Scheme 3) [19]. This explanation could be supported by
the phase behaviour image of aniline + CHA + DCHA corresponding
to the composition of 8 (Fig. 4g) and 14 MPa (Fig. 4h), respectively,
which indicates the difference in the number of phases present in
the system. Thus, the conversion and selectivity of aniline hydro-
genation were strongly influenced by the phase behaviour under
the studied reaction conditions.
5
0
0
In the absence of CO , the conversion of aniline was reduced
2
to 16% and almost a 1:1 mixture of CHA and DCHA was produced
0
2
4
6
8
10
12
14
(Fig. 1), under the similar reaction condition, which again con-
PCO2 (MPa)
firmed the potential of using CO2 as a reaction medium.
1
00
b
3
.4. Variation of reaction time
8
1
MPa
4 MPa
8
6
4
2
0
0
0
0
0
The effect of reaction time on the hydrogenation of aniline in
CO2 at 8 MPa (biphase) is shown in Fig. 5. It can be seen that the
conversion and selectivity were strongly influenced by the reaction
time. The conversion of aniline was increased linearly from 5.4% to
9
5.8% as time changes from 0.5 to 6 h. On the other hand, CHA was
sole product in the beginning, but as the conversion increased, the
selectivity of CHA decreased from 100% to 93% with the formation of
DCHA. An attempt was made to check the effect of prolonged reac-
tion time (18 h), the result indicated a considerable change in the
product distribution. Although the conversion of aniline reached to
100%, the selectivity of CHA and DCHA changed from 93% to 44%
and 7% to 56%, respectively. Thus, an optimum reaction time of 6 h
was chosen for all the measurements.
0
1
2
3
4
5
6
Time (h)
3.5. Influence of H2 pressure
Fig. 3. (a) TOF of aniline hydrogenation at various CO2 pressures and (b) comparison
of the yield (%) of CHA at 8 and 14 MPa of CO2 pressure.
To check the influence of H2 pressure in the biphasic condition,
◦
we performed aniline hydrogenation at a fixed temperature of 80 C
and the CO2 pressure of 8 MPa in accordance with the observed
that in the absence of CO2 [18,24], which can explain the increased
reaction efficiency in the biphasic condition. On the contrary, in
the single phase condition, conversion and selectivity were reduced
considerably, due to the dilution effect.
highest conversion and selectivity (Fig. 6a). H pressure was varied
2
from 1 to 4 MPa. Results show that the conversion of aniline was
increased from 25.2% to 95.8% as the pressure changes from 1 to 4
MPa. The reason behind the enhancement of conversion was gener-
Fig. 3a represents TOF of aniline hydrogenation at different CO2
pressures, and shows a faster reaction rate (higher TOF) in the
biphasic condition when aniline is present in the liquid rich phase.
Again, Fig. 3b is a plot of CHA yield (%) versus reaction time at 8
and 14 MPa pressure, which evidenced a clear difference in the
rate of formation of CHA. The reaction is much faster towards the
formation of CHA in the biphasic condition than in the diluted sin-
gle phase. Thus, hydrogenation of aniline to CHA in CO2 medium
prefers liquid rich aniline phase rather than single phase contacted
to the catalyst; this result fits the explanation of Chouchi et al. [21].
ally given as the increased solubility of H2 in CO -expanded liquid
2
[24,25]. Recently, it was suggested that the hydrogenation in CO2-
expanded liquid catalyzed by Rh and Ru is influenced by the phase
equilibrium, related to the distribution of H2 between vapour and
liquid and the volumetric expansion of the liquid substrate due to
the presence of CO2 [26]. However, plots of the yield of CHA versus
reaction time at 1 and 4 MPa H pressure (Fig. 6b) revealed that the
2
effect of H2 pressure was insignificant for the present case. Thus,
aniline hydrogenation under the studied reaction condition is inde-
According to the results, a change in CO pressure was also coupled
pendent of the amount of H , present in large excess in the reaction
2
2
with the drastic change in product selectivity. When the pressure
increased from 8 to 14 MPa, the selectivity of CHA and of DCHA
was changed from 93% and 7% to 30.5% and 69.5%, respectively. The
tuning of the pressure is related to the number of phases present
in the system. Under the biphasic condition, aniline hydrogenation
towards the formation of CHA seems to dominate and further con-
version of CHA to DCHA was minimized, but in the single phase
condition (∼14 MPa) selectivity of DCHA increased. To explain this
observation, phase behaviour of CHA, DCHA and the mixture of
medium.
3.6. Effect of temperature
Variation of temperature is also a critical parameter to describe
the aniline hydrogenation in scCO2 because (i) it affects the phase
behaviour and (ii) the formation of carbamic acid. In this work, the
hydrogenation of aniline was studied at 35, 50, 60, 70 and 80 C at
the fixed pressure of H2 (4 MPa) and CO2 (8 MPa) (Fig. 7). The tem-
◦
◦
aniline + CHA + DCHA corresponding to the composition of 8 and
perature was found to influence the conversion of aniline. At 70 C,
◦
1
4 MPa were studied separately at 80 C (Fig. 4). At 8 MPa, CHA
the conversion of 60% could be observed; this value dropped to
<20% as the temperature decreased. While it is normal for a reaction
conversion to decrease with decreasing temperature, the steep-
ness of the observed drop suggests that phase behaviour rather
than reaction kinetics is responsible. Visual inspection of the phase
existed as a separate phase (Fig. 4b) due to the low solubility in CO2.
Therefore, a phase separation at the lower pressure of 8 MPa could
be the cause of the high selectivity of CHA (Fig. 4b). Moreover, at a
higher pressure the solubility of CHA was increased, whereas DCHA

M. Chatterjee et al. / Applied Catalysis A: General 396 (2011) 186–193
191
Fig. 4. Visual observation of phase behaviour of CHA (a–c); (a) only CHA; in the presence of CO2 (b) 8 MPa, (c) 14 MPa and DCHA (d–f); (d) only DCHA; in the presence of CO2
◦
(
e) 8 MPa, (f) 14 MPa of at 80 C; mixture of aniline + CHA + DCHA (g) 8 and (h) 14 MPa.
behaviour revealed that the change in temperature is associated
with the change in the number of phases present in the system.
Fig. 2f evidences a clear difference in the phase behaviour of ani-
line hydrogenation preferred a biphasic system rather than a single
phase for higher conversion. Again, in the presence of catalyst, ani-
line was converted to cyclohexylamine, and might be deposited as
a white solid due to the lower temperature. A separate experiment
was conducted on the visual observation of the phase behaviour of
◦
◦
line between 35 and 80 C (Fig. 2b). At 35 C the system existed as
single phase (aniline–H –CO ) with the disappearance of all the
2
2
◦
◦
liquid phase, whereas at 80 C a biphasic system is revealed; ani-
CHA at 35 C (Fig. 8), the result clearly demonstrated the deposi-

1
92
M. Chatterjee et al. / Applied Catalysis A: General 396 (2011) 186–193
Fig. 5. Variation of reaction time on aniline hydrogenation; Reaction condition:
catalyst = 0.1 g, substrate = 0.5 g, temperature = 80 C, PCO2 = 8 MPa, PH2 = 4 MPa.
Fig. 7. Effect of temperature on the conversion and selectivity of aniline hydro-
genation; Reaction condition: catalyst = 0.1 g, substrate = 0.5 g, PCO2 = 8 MPa, PH2
MPa, time = 6 h.
◦
=
4
tion of solid carbamic acid through the interaction between CHA
◦
and CO , and this scenario continued until 60 C. The formation
2
of the solid, results diminished contact between aniline and the
catalyst, which inhibited the reaction rate. When the temperature
◦
was above 60 C, solid carbamate was converted to liquid and the
conversion increased sharply from 18.2% to 61%, as depicted in
Fig. 7. Temperature also played a prominent role in determining the
product distribution. Concerning the selectivity, only CHA (selectiv-
◦
Fig. 8. Formation and deposition of solid carbamic acid at low temperature of 35 C.
◦
ity = 100%) was observed from 35 to 60 C. Therefore, temperature
selection for optimization was a crucial part to the aniline hydro-
genation in CO . Due to the limitations of the reactor, measurement
over 80 C was not undertaken.
3.7. Reaction in organic solvent
2
◦
In order to gain farther insight into the role of CO2 in aniline
hydrogenation, we conducted the reaction in organic solvent under
similar reaction conditions; the results are shown in Table 1 (entry
1
0 and 11). Similar to the CO2 medium, aniline was hydrogenated
to CHA and DCHA in organic solvents under the same condition.
In ethanol and hexane the conversion of aniline was ∼50% and
the selectivity of CHA was ∼60% (entry 10 and 11), which was
lower in comparison to CO2 medium. The use of CO caused a
2
marked increase in the product selectivity and TOF (Table 1; entry
1
0 and 11), which again confirmed the efficiency of CO -expanded
2
substrate. Conventional solid catalyzed hydrogenation in organic
solvents is mainly H2 concentration dependent and is often con-
trolled by the transport rate, which requires more energy to get a
reasonable rate of reaction. Moreover, the liquid phase reaction also
suffers from the additional requirement of removing the solvents
from the product.
3.8. Catalyst recycling
Recycling of the catalyst is the most important criterion for het-
erogeneous catalytic reaction. The catalyst was recycled for four
successive runs after the reaction followed by separation through
filtrations; the results are shown in Table 2. The yield of CHA was
decreased from 89% to 84% after the 4th recycle.
Table 2
Recycling of the catalyst .
a
Entry
Catalyst
Yield (%)
CHA
DCHA
1
2
3
4
5
Fresh
89.0
89.4
87.4
85.6
84.4
6.7
6.5
6.5
6.6
6.5
Recycle-1
Recycle-2
Recycle-3
Recycle-4
Fig. 6. (a) Variation of H2 pressure on the hydrogenation of aniline; Reaction condi-
◦
a
Reaction conditions: Catalyst: 0.1 g, aniline = 0.5 g, P
= 8 MPa, PH2 = 4 MPa,
tion: catalyst = 0.1 g, substrate = 0.5 g, temperature = 80 C, PCO2 = 8 MPa, time = 6 h
CO2
◦
and (b) comparison of the yield (%) of CHA at 1 and 4 MPa of H2 pressure.
reaction time = 6 h; Temp. = 80 C.

M. Chatterjee et al. / Applied Catalysis A: General 396 (2011) 186–193
193
4
. Conclusions
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