An effective medium of H2O and low-pressure CO2 for the selective hydrogenation of aromatic nitro compounds to anilines

Article abstract of DOI:10.1039/c0gc00246a

Chemoselective hydrogenation of water-insoluble aromatic nitro compounds can be achieved over Ni catalysts in a H₂O-compressed CO₂ system at 35-50 °C without using any environmentally harmful solvent. The effective CO₂ pressure is much lower than the critical pressure of CO₂. The hydrogenation of nitro group should be the rate-determining step. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00246a

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COMMUNICATION
An effective medium of H2O and low-pressure CO2 for the selective
hydrogenation of aromatic nitro compounds to anilines†
Xiangchun Meng,‡a Haiyang Cheng,b Shin-ichiro Fujita,a Yancun Yu,b Fengyu Zhao*b and Masahiko Arai*a
Received 22nd June 2010, Accepted 24th December 2010
DOI: 10.1039/c0gc00246a
Chemoselective hydrogenation of water-insoluble aromatic
nitro compounds can be achieved over Ni catalysts in a
H2O–compressed CO2 system at 35–50 ◦C without using
any environmentally harmful solvent. The effective CO2
pressure is much lower than the critical pressure of CO2.
The hydrogenation of nitro group should be the rate-
determining step.
conversion. The rate of hydrogenation depends on CO2 pressure
and is maximized at high pressures of ca. 9–12 MPa.3 Here, we
report that the combination of H2O and CO2 is more beneficial
for these selective hydrogenation reactions and the effectiveness
of CO2 pressurization can appear at a lower pressure of 0.8
MPa. The rate-determining step is the transformation of NB in
the present reaction instead of the hydrogenation of PHA.
Fig. 1 gives the results of NB hydrogenation in different
reaction media using a 41 wt% Ni/Al2O3 catalyst (the details of
catalyst preparation and hydrogenation run are given in ESI†).
The influence of CO2 pressure on the conversion of NB depends
on the medium used (Fig. 1a). In the absence of CO2, the
conversion was larger in ethanol and n-hexane (66% and 29%)
than that in H2O and solvent-less NB (6% and 3%). However,
The selective hydrogenation of aromatic nitro compounds to the
corresponding anilines is an industrially important reaction.1
The proposed reaction pathways (Scheme 1) include direct (I–
II–III and IV–III) and indirect (VI–VII–VIII–IX) routes.2 The
reduction of PHA to AN is the common rate-determining step
in the direct route.2 The authors studied the hydrogenation of ni-
trobenzene (NB) and chloronitrobenzene (CNB) over supported
Ni catalysts in compressed CO2.3 The use of compressed CO2
and supported Ni catalysts is effective for producing the aniline
compounds with almost 100% selectivity in the whole range of
Scheme
1
Possible reaction pathways for the reduction of ni-
trobenzene. NB: nitrobenzene, NSB: nitrosobenzene, PHA: N-
phenylhydroxylamine, AN: aniline, AOB: azoxybenzene, AB: azoben-
zene, HAB: hydrazobenzene.
aDivision of Chemical Process Engineering, Faculty of Engineering,
Hokkaido University, Sapporo, 060-8628, Japan.
E-mail: marai@eng.hokudai.ac.jp; Fax: +81 11 706 6594; Tel: +81 11
706 6594
bState Key Laboratory of Electroanalytical Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, China. E-mail: zhaofy@ciac.jl.cn; Fax: +86 431
8526 2410; Tel: +86 431 8526 2410
† Electronic supplementary information (ESI) available: Experimental
procedures, phase behavior observations, and hydrogenation data. See
DOI: 10.1039/c0gc00246a
‡ X. Meng is currently with School of Chemical Engineering,
Changchun University of Technology, Changchun 130012, China.
Fig. 1 (a) Influence of CO2 pressure on the conversion of NB after
30 min reaction and (b) the yield of AN against the conversion of NB in
different reaction systems. (Ni/Al2O3 0.1 g, NB 19.5 mmol, H2 6 MPa,
50 ◦C).
570 | Green Chem., 2011, 13, 570–572
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Table 1 Hydrogenation of NB, NSB, and PHAa
Selectivity (%)b
Substrate Conv. (%) AN NSB PHA
Entry Medium
1
2
3
4
5
6
H2O
NB
21
61.9 9.0
1.9
—
d
NSBc
PHA
8
20.4
94.6 —
100e
55
d
H2O–CO2 (3 MPa) NB
NSB
PHA
95.1 2.4
2.4
d
13
36.9
95.3 —
—
100e
d
a Conditions: H2O 10 cm3, Ni/Al2O3 0.1 g, substrate 9.75 mmol, H2 6
MPa, 50 ◦C, 10 min. b The other byproduct was AOB. c 30 min. d Not
detected. e PHA decomposed partly into NSB, AN, and AOB during GC
analysis (Fig. S4 in ESI†). Both PHA and NSB were not detected during
GC analysis of the product, indicating PHA is consumed completely
after 10 min of reaction. The total pressure in the reactor dropped to a
constant value in 6–8 min, further confirming the complete conversion
of PHA.
Fig. 2 FTIR spectra of PHA in the v(NO) region.
propose that H2O can promote the reaction step III accepted
as the rate-determining step in organic solvents, enhancing the
reaction rate and the selectivity to aniline.
the conversion was significantly enhanced when the H2O–NB
mixture was compressed by CO2 at a low pressure such as 0.8
MPa; the conversion was comparable to that obtained in the
supercritical 12 MPa CO2–NB mixture. For the H2O–NB, the
conversion gradually increased with CO2 pressure up to 17 MPa.
The pressurization with CO2 had a drastically negative impact
and a slightly positive effect on the conversion of NB in ethanol
and in n-hexane, respectively. Note that the selectivity to AN was
almost 100% in the whole range of conversion for the reactions
in scCO2, CO2–n-hexane, and CO2–H2O systems. In ethanol
and n-hexane, undesired PHA and AOB were formed in large
quantities, resulting in small yields of the desired product of AN
(Fig. 1b and Fig. S1 in ESI†). Hence, the combination of H2O–
CO2 medium and Ni catalyst is a better reaction system for the
selective hydrogenation of NB to AN under mild conditions (low
temperature and CO2 pressure) without any organic solvent.
To examine the features of our H2O–CO2 reaction media,
hydrogenation runs were conducted with NB, NSB, and PHA in
H2O and H2O–CO2 systems (Table 1). In both systems, the rate
of hydrogenation followed the order PHA > NB > NSB. For
NB hydrogenation in pure H2O, the selectivity to AN was ca.
62% and NSB and AOB were the main byproducts (entry 1). In
the H2O–CO2 system, however, the selectivity to AN was 95%;
NSB and AOB were little formed in the hydrogenation of NB
(entry 4) although the conversion of NSB was very slow (entry
5). It is likely, therefore, that in this H2O–CO2 system the NB
transforms directly to PHA and this step is the rate-determining
one. The couping of NSB and PHA should be suppressed due
to their low concentrations. In pure H2O, in contrast, the NB
should also change to PHA via NSB in addition to the direct
transformation to PHA. The coupling reaction bewteen NSB
and PHA is also possible to occur.
To further inspect the features of the present H2O–CO2
system, NB was hydrogenated in a H2O–compressed N2 (7
MPa) medium, but no positive effects on the conversion and
the selectivity to AN were observed. It is known that CO
may be formed via the reverse water gas shift reaction during
hydrogenation reactions in the presence of H2 and CO2.6
Although no CO was detected for the gas phase by GC analysis
in our reaction, an attempt was made to add 0.2 MPa 9.9%
CO/He into the reaction mixture of NB–H2O–CO2 (3 MPa).
This was found to cause a significant decrease in the conversion
from 62% to 15%. Thus, in the present hydrogenation reactions,
the formation of CO was unlikely.
The enhancement of NB conversion in NB–H2O–CO2 system
can be explained by a few different factors. CO2 is soluble in
H2O, its mole fraction being 0.25% and 2.0% at 0.8 and 9.4
◦
MPa, respectively, at 50 C.7 The dissolution of CO2 enhances
the solubility of a gaseous reactant H2 in the H2O and NB
phases. The H2O phase of H2O–CO2 mixture was acidic (ca.
pH 38). A reaction run was also conducted in a 0.8 mol
dm-3 NaHCO3 buffered H2O solution at 3 MPa CO2 (pH >
68b); the conversion (48%) was slightly lower than that in the
unbuffered system (62%), while the high selectivity to AN did
not change. NaHCO3 can also be hydrogenated to NaHCO2
with Ru complex or supported Pd catalyst;9 but supported
Ni catalysts do not give significant NaHCO2 concentrations.9a
When the concentration of NaHCO3 was changed from 0.8 to
0.2 mol dm-3, the hydrogenation of NB gave the same conversion,
indicating that the influence of NaHCO3 hydrogenation on the
reaction of NB is marginal in our systems. Thus, the acidic
nature of the H2O phase might be one of positive factors, but its
effect is small. Our previous in situ FTIR show that compressed
CO2 interacts with the reacting species, NB, NSB, and PHA,
decreasing the reactivity of NB but increasing the reactivity of
NSB and PHA.3 These molecular interactions should also be
important in the H2O–CO2–NB reaction system. In addition,
the solid catalyst granules were well dispersed in the H2O phase
but not in the NB phase, due to hydrophilic nature of the oxide
support materials (Fig. S2 in ESI†). NB is sparingly soluble
(ca. 17 mmol dm-3 at 25 ◦C) and PHA is soluble in H2O (ca.
82 mmol dm-3 in saturated salt solution at 0 ◦C10). Therefore, the
hydrogenation of NB and PHA is likely to occur at the NB–H2O
The molecular interactions of H2O with a hydrophilic inter-
mediate of PHA were examined by FTIR (experimental details
are in the ESI†). Fig. 2 shows a slight red-shift of v(NO) in
the H2O compared to that in the gas phase. The N–O bond
of PHA is weakened through interactions with H2O, possibly
via OH ◊ ◊ ◊ O and OH ◊ ◊ ◊ N bonding.4 Several authors reported
the promotion effects of H2O on the hydrogenation of aromatic
nitro compounds in organic solvents, but the reasons are still
unclear.5 Now, the results of Table 1 and Fig. 2 allow us to
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interfacial layer and in the H2O phase. In the H2O-CO2 system,
the rate of AN formation is controlled by the conversion of NB
into PHA. Interfacial H2O and NB molecules form hydrogen
bonding, OH ◊ ◊ ◊ ONO,11 which may weaken the N–O bond of
NB. Previously, a striking rate increase was observed in some
reactions on water and this was ascribed to the hydrogen bonds
between interfacial water molecules and reactants or transition
state.12 In addition, da Rocha et al. studied the molecular
structure of the H2O–compressed CO2 interface and observed
excess accumulation of the fluids on both sides of the interface.13
The local density enhancements can have a large impact on the
chemical reactions.14
After the reaction, the H2O phase was easily separated from
the organic product phase. This H2O phase containing 0.35 mol
dm-3 AN was further used for the second reaction, giving the
same results as that using pure H2O. Hence, the H2O phase can
be recyclable without any post-purification, which is of practical
significance.
o-CNB similar to NB but the conversion increased with CO2
pressure through to 13 MPa. The positive effect of H2O was
also observed in the hydrogenation of m- and p-CNB over 16
wt% Ni/TiO2. In Fig. 3b the yield of CAN is plotted against the
total conversion of o-, m-, and p-CNB substrates (the change
of conversion and selectivity with reaction time is shown in ESI
Fig. S3†). For all the isomers, no dehalogenation and coupling
occurred and so the selectivity to CAN was almost 100% at any
conversion, confirming the effectiveness of the present reaction
system including H2O and low-pressure CO2 for the selective
hydrogenation of aromatic nitro compounds to anilines.
In conclusion, the interactions of CO2 and H2O with the
reacting species, the in situ formed acidity, and the better
dispersion of Ni catalyst in the H2O phase are responsible for
the fast and selective hydrogenation of NB in the H2O–CO2
system, in which the conversion of NB into PHA may be the
rate-determining step.
Furthermore, the potential of the H2O–CO2 medium was
examined for the hydrogenation of CNB at 35 ◦C, which is
less soluble in H2O than NB. A 9 wt% or 16 wt% Ni/TiO2
catalyst was used since the selectivity to chloroaniline (CAN)
was slightly better than that obtained with the Ni/Al2O3 (Table
S1 in ESI†). Fig. 3a shows the conversion of o-CNB over 9
wt% Ni/TiO2 as a function of CO2 pressure for the reaction
mixtures in the presence and absence of H2O. One can see
again that the H2O significantly promoted the hydrogenation of
Acknowledgements
We thank the financial support from the One Hundred Talent
Program of CAS, NSFC 20873139, KJCX2, YW.H16, Japan
Society for the Promotion of Science with Grant-in-Aid for Sci-
entific Research (B) 18360378, and the CAS-JSPS international
joint project GJHZ05.
Notes
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Fig. 3 (a) Influence of CO2 pressure on the conversion of o-CNB over 9
wt% Ni/TiO2 after 50 min of hydrogenation in the systems of H2O–CO2
and compressed CO2 alone; (b) CAN yield against CNB conversion
during the hydrogenation of CNB isomers in the H2O–CO2 (6 MPa)
system. (CNB 9.52 mmol, Ni/TiO2 0.15 g, H2 4 MPa, 35 ◦C).
12 Y. S. Jung and R. A. Marcus, J. Am. Chem. Soc., 2007, 129, 5492–
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572 | Green Chem., 2011, 13, 570–572
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