One pot catalytic NO2 reduction, ring hydrogenation, and N-alkylation from nitroarenes to generate alicyclic amines using Ru/C-NaNO 2

Article abstract of DOI:10.1016/j.catcom.2013.09.012

A report to produce alicyclic amines and subsequent N-alkylation with alcohols using Ru/C-NaNO₂ catalyzed facile transformation of nitrobenzene was investigated. Effects of solvent, temperature, pressure, reaction time, and molar-ratio of substrate/catalyst on product composition were also studied. These mechanistic studies explain that nitrobenzene undergoes hydrogenation reaction in the following order; -NO₂ reduction to -NH₂, aromatic ring-hydrogenation to alicyclic, and from the reaction of alcohol to give N-alkylated amines. This investigation shed lights on possible application to polyurethane chemistry since these amines are used as important precursors for diisocyanates.

Full text of DOI:10.1016/j.catcom.2013.09.012

Catalysis Communications 43 (2014) 79–83
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
One pot catalytic NO2 reduction, ring hydrogenation, and N-alkylation
from nitroarenes to generate alicyclic amines using Ru/C-NaNO2
Seung Geun Oh a,b,1, Vivek Mishra b,1, Jin Ku Cho b, Baek-Jin Kim b, Hoon Sik Kim c, Young-Woong Suh d,
Hyunjoo Lee e, Ho Seok Park a, Yong Jin Kim b,
⁎
a
Department of Chemical Engineering, College of Engineering, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea
Green Process Material Research Group, Korea Institute of Industrial Technology, Chungcheongnam-do 331-822, Republic of Korea
Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, Seoul 130-701, Republic of Korea
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
b
c
d
e
Clean Energy Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2013
Received in revised form 11 September 2013
Accepted 12 September 2013
Available online 20 September 2013
A report to produce alicyclic amines and subsequent N-alkylation with alcohols using Ru/C-NaNO2 catalyzed fac-
ile transformation of nitrobenzene was investigated. Effects of solvent, temperature, pressure, reaction time, and
molar-ratio of substrate/catalyst on product composition were also studied. These mechanistic studies explain
that nitrobenzene undergoes hydrogenation reaction in the following order; –NO2 reduction to –NH2, aromatic
ring-hydrogenation to alicyclic, and from the reaction of alcohol to give N-alkylated amines. This investigation
shed lights on possible application to polyurethane chemistry since these amines are used as important precur-
sors for diisocyanates.
Keywords:
Hydrogenation
Nitrobenzene
© 2013 Elsevier B.V. All rights reserved.
Cyclohexylamine
Ruthenium catalyst
1. Introduction
of aromatic amines due to its high activity, selectivity, and reproducibil-
ity [10–13]. The N-alkylation of amines using non-halide reagent is also
The efficient synthetic pathway for producing amines has been
extensively investigated due to the fact that they are widely used as
significant intermediate as well as final product in various chemical
industries [1,2]. Especially, the cyclohexylamine (CHA) finds various
applications in pharmacology or as agrochemicals in terms of its being
raw material for carbamates and isocyanates. Among these, alicyclic
diamines also more substantially used feedstock for the synthesis of
non-yellowing polyurethanes.
Conventionally, the CHA has been produced from the following
two step; i) reduction of nitrobenzene (NB) to aniline [3–9], ii) ring hy-
drogenation of aniline to CHA [1,2,10–13], which makes the process
complicated. Therefore, development of a facile one step synthetic path-
way to alicyclic amines from nitroarenes is of great importance with
respect to process efficacy.
an important process because conventional halogenated substances
may generate harmful by-products such as HCl [15–17]. To overcome dis-
advantages, alcohols can be employed as direct alkylating agent for the N-
alkylation of amines [18–21]. Since there has been no report on one-pot
synthesis of CHA or N-alkylated amines from the reaction of NB with H2
and alcohols, this approach might provide much greener pathway from
the view point of process efficiency as well as cost saving.
In this context, a direct synthesis of alicyclic amines and their subse-
quent production of N-alkylated amines from the reaction of nitroarenes
with H2 and alcohols in the presence of a catalyst system comprising
carbon supported Ru and NaNO2 are presented.
2. Experimental
As to the industrial processes to manufacture aniline through NB hy-
drogenation, the process generally employs above 240 °C with copper-
catalysts in two-stage bed reactor [14]. Recently, Langer et al. reported
a low-pressure process for the hydrogenation of aniline to produce
CHA with high selectivity using rhodium catalysts [11]. The literature in-
dicates that ruthenium is the most used catalyst for ring-hydrogenation
All the chemicals and catalyst were purchased from Aldrich (South
Korea) and used without further purification.
2.1. Hydrogenation reaction
All the hydrogenation reactions were carried out in a 100 ml pres-
surized reactor with a magnetic stirrer and an electrical heater (Fig. 1).
Detailed procedure of hydrogenation reactions is given in supporting
information.
⁎
Corresponding author. Tel.:+82 415898469; fax: +82 415898580.
E-mail address: yjkim@kitech.re.kr (Y.J. Kim).
These authors contributed equally to this work.
1
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.09.012

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S.G. Oh et al. / Catalysis Communications 43 (2014) 79–83
Fig. 1. Process diagram for alicyclic amines from NB.
3. Results and discussions
(entries 2–3, Fig. 2). In contrast, THF, diethyl ether, and diisopropyl
ether produced considerable amount of CHA and dicyclohexylamine
(DCHA) without the existance of aniline (entries 4–6, Fig. 2), giving a
clue that they facilitate the reaction to undergo ring hydrogenation as
well as –NO2 reduction. From these results, NMP was found to be the
most favorable polar aprotic solvent at 170 °C in terms of production
of CHA and the rate of hydrogenation reaction is much influenced
according to the solvent used.
However, when we employed polar protic solvents, the products
were identified as CHA, DCHA, and N-alkyl CHA whilst the aniline was
not detected, suggesting that due to high solubility of hydrogen gas in al-
cohols, these solvents accelerate both the ring hydrogenation and –NO2
reduction. Among those solvents, it is interesting to note that IPA showed
best result for obtaining ring-hydrogenated product, N-isopropyl CHA
(IP-CHA) almost quantitatively at 170 °C (entry 9, Fig. 2). The facile syn-
thesis of N-alkyl CHA is also significant since they are used as important
intermediates in the fine chemical industry, especially when obtained
directly from nitroarenes.
3.1. Effect of polar aprotic and protic solvents
Catalytic reduction of –NO2 group and ring hydrogenation reaction
of NB were carried out over Ru/C catalyst in the presence of various
polar aprotic and alcoholic solvents at 170 °C. This temperature was
used to figure out all the possible products from these harsh reaction
conditions. The results summarized in Fig. 2 show that NB conversion
was observed as 100% (data not shown) regardless of solvent used
probably due to the high temperature. Among polar aprotic solvents,
only NMP showed 100% selectivity toward CHA (entry 1, Fig. 2). On
the other hand, DMF and acetonitrile delivered aniline as major product,
which suggests that reaction is retarded in these solvents thus reaction
did not continue further (entries 2–3, Fig. 2). It is generally accepted that
the reaction rate of the reduction of –NO2 group is regarded as faster
than that of ring hydrogenation and it is somewhat supported from
the results of very small amount of CHA in the reaction mixtures
Fig. 2. Effect of various solvents on product composition: NB (7.5 mmol), 5 wt.% Ru/C (0.19 mmol of Ru), solvent (15 ml), T = 170 °C, P = 8.3 MPa, t = 4 h.

S.G. Oh et al. / Catalysis Communications 43 (2014) 79–83
81
pressure, and the molar ratio of substrate/catalyst on the product compo-
sition were investigated.
3.2. Effect of temperature
Fig. 3 shows the effect of temperatures on product composition
produced from hydrogenation of NB using IPA as solvent. The main prod-
ucts were found to be hydrogenated compounds such as aniline, CHA,
and IP-CHA.
The NB conversion and aniline selectivity were 79.6% and 100%, re-
spectively at 80 °C whilst 100% of NB was converted to CHA at 90 °C, in-
dicating that temperature barrier between the products of –NO2 reduced
and ring-hydrogenated is very slight. As temperature increased above
90 °C, CHA decreased gradually and IP-CHA started to generate, and
eventually, the CHA was completely converted to IP-CHA at 170 °C.
These results suggest that the hydrogenation of NB undergoes i) aniline
formation, ii) CHA formation, and iii) N-alkylated CHA formation path-
way, therefore all intermediate compounds such as aniline, CHA, and
IP-CHA can be synthesized selectively by simply varying the reaction
temperature in IPA.
Fig. 3. Effect of temperature on product composition; NB (7.5 mmol), 5 wt.% Ru/C
(0.19 mmol of Ru), IPA (15 ml), P = 8.3 MPa, t = 4 h.
3.3. Effect of pressure
To investigate the effect of pressure on the product composition,
same hydrogenation reaction of NB was carried out in IPA at 90 °C for
4 h with varying pressure. Fig. 4 described that NB conversion gradually
increased up to 100% with increasing pressure ranging from 2.06 to
5.50 MPa. At this pressure range, a sharp decrease in the selectivity of
aniline was observed with a simultaneous increase in the formation of
CHA, and finally, the 100% CHA selectivity was achieved without the
formation of IP-CHA even at 8.3 MPa, probably ascribing to the relative-
ly low temperature (90 °C).
3.4. Effect of reaction time
To study the product compositional change as a function of time,
hydrogenation reaction was performed in IPA at 90 °C and 8.3 MPa
with varying reaction time. As shown in Table 1, the conversion of NB
remained 50.3% until 1 h, producing aniline with 100% selectivity.
When the reaction time is retained for 2 h, conversion reached 100%
with a parallel disappearance of aniline while the CHA was formed
with 89% selectivity, and after 3 h, the CHA was obtained almost quan-
titatively (both conversion and selectivity = 100%).
These results are again clearly supporting that hydrogenation of NB
undergoes following step: i) reduction of –NO2 to –NH2; ii) aromatic
ring hydrogenation to alicyclic; iii) alkylation with alcohol to give corre-
sponding N-alkylated amine as shown in Scheme 1.
Fig. 4. Effect of pressure on product composition; NB (7.5 mmol), 5 wt.% Ru/C (0.19 mmol
of Ru), IPA (15 ml), T = 90 °C, t = 4 h.
Table 1
Effect of reaction time on the product compositional change.a
Time
Conversion and selectivity (%)
NB conversion
Aniline selectivity
CHA selectivity
IP-CHA selectivity
1 h
2 h
3 h
4 h
49.7
100
100
100
0
0
0
89
100
100
0
0
0
0
3.5. Effect of catalyst molar ratio
100
0
To investigate the effect of catalyst loading, various molar ratios (NB/
catalyst) ranging from 40:1 to 2000:1 were tested at 140 °C, 8.3 MPa for
4 h (Fig. 5). The substrate conversion retained 100% until the molar ratio
was 400, and thereafter a sharp decrease was observed. The selectivity
of aniline increased very rapidly until the molar ratio reached 1000
and thereafter a gradual decrease was observed while the selectivity
of CHA increased sharply until the molar ratio of 200:1 then deep
decrease was observed. The overall decrease in the NB conversion as
a
Reaction conditions: NB (7.5 mmol), 5 wt.% Ru/C (0.19 mmol of Ru), IPA (15 ml),
T = 90 °C, P = 8.3 MPa.
These experimental results led us to a hypothesis that a fine tuning of
the reaction conditions using IPA as solvent may result in producing pre-
ferred amine compounds selectively, e.g., aniline, CHA, and N-alkyl CHA.
Thus to correlate above hypothesis, the effects of solvents, temperature,
Scheme 1. Reaction pathway for the hydrogenation of NB.

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S.G. Oh et al. / Catalysis Communications 43 (2014) 79–83
very low selectivity toward ring-hydrogenated product even at elevated
temperatures.
3.7. Reusability of catalyst for hydrogenation
The recycling study revealed that the catalyst was able to be reused
five times without any loss of its original reactivity, at sixth recycling,
however, the yield of CHA reduced slightly down to 94.1% (Table S1).
Details are given in supporting information.
4. Conclusions
Ru/C-NaNO2 catalyst system is found to catalyze various nitroarenes
to produce corresponding alicyclic or aromatic amines in high yield. The
hydrogenation reaction undergoes following steps; NO2 reduction, ring
hydrogenation, and continuing to N-alkylation depending on the reac-
tion conditions used. The more severe reaction conditions, the further
the reaction proceeded thus any compound out of the three, e.g., aniline,
CHA, N-alkylated CHA, could be produced almost quantitatively by sim-
ply altering the reaction conditions. The effects of solvent, temperature,
pressure, reaction time, and molar ratio on the product composition
were studied using NB as substrate. Among the reaction parameters,
temperature showed highest dependency on product composition.
The catalyst reuse study revealed that this catalyst system was able to
catalyze 5 times without any loss of activity. Various aromatic dinitro
and diamines were tested using same catalyst system which produced
corresponding aromatic diamines in high yield but not delivered alicy-
clic diamines.
Fig. 5. Effect of molar ratio on product composition; 5 wt.% Ru/C (0.19 mmol of Ru), IPA
(15 ml), T = 140 °C, P = 8.3 MPa, t = 4 h.
the increase in the molar ratio is mainly due to the fact that the chances of
the interactions between catalyst with substrate are greatly diminished
at the molar ratios higher than 200 under these reaction conditions.
3.6. Scope of hydrogenation reaction for dinitroarenes and aromatic diamines
To broaden the hydrogenation scope, various dinitroarenes and
diamines were examined for the formation of alicyclic diamines. The re-
sults in Table 2 show that conversions were mostly high but this catalyst
system was not able to properly hydrogenate dinitroarenes to give corre-
sponding alicyclic diamines whereas 2,4 and 3,4-dinitrotoluenes were re-
duced to give a quantitative yield (100%) of corresponding diamino
toluenes (entries 1,2). The low selectivity of 2,6-diaminotoluene (66.4%,
entry 3), is likely ascribed to the steric hindrance arising from the 2,6 po-
sition of nitro group. Among the diamino compounds, 4,4 -methylene
dianiline (MDA) was converted to di(4-aminocyclohexyl)methane in ex-
cellent yield even at 110 °C (entry 4) while others (entries 5,6) showed
Acknowledgment
This work was supported by the “Fusion Research Program for Green
Technologies (NRF-2012M3C1A1054497)” funded by the Ministry of
Education, Science and Technology, Korea.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.09.012.
Table 2
Hydrogenation of various aromatic dinitro and diamine compounds.a
Entry
1
Substrate
T.
Conv.(%)
100
Selectivity (%)
–NH2
Ring-hydrogenated
0
N-alkylated
110
100
0
2
3
150
130
100
100
100
0
0
0
0
66.4
4
5
110
150
100
100
N/A
N/A
100
0
trace
22.9
6
170
100
N/A
24.8
45.2
a
Substrate (7.5 mmol), 5 wt.% Ru/C (0.19 mmol of Ru), IPA (15 ml), P = 8.3 MPa, t = 4 h.

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