Green Route for the Preparation of p-Aminophenol from Nitrobenzene by Catalytic Hydrogenation in Pressurized CO2/H2O System

Article abstract of DOI:10.1021/acs.oprd.5b00307

The preparation of p-aminophenol from nitrobenzene by one-pot catalytic hydrogenation and in situ acid-catalyzed Bamberger rearrangement was first realized in a pressurized CO₂/H₂O system. By employing Pt-Sn/Al₂O₃ as catalyst, nitrobenzene could be converted to p-aminophenol with selectivity as high as 85% when the reaction was carried out at 140°C under 5.5 MPa CO₂ and 0.2 MPa H₂. This new protocol is environmentally benign because it is fully rid of the use of mineral acid by the application of self-neutralizable carbonic acid.

Full text of DOI:10.1021/acs.oprd.5b00307

Article
pubs.acs.org/OPRD
Green Route for the Preparation of p‑Aminophenol from
Nitrobenzene by Catalytic Hydrogenation in Pressurized CO2/H2O
System
Tingting Zhang, Jingyang Jiang,* and Yanhua Wang
State Key Laboratory of Fine Chemicals, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian
University of Technology, Dalian 116024, P. R. China
S
* Supporting Information
ABSTRACT: The preparation of p-aminophenol from nitrobenzene by one-pot catalytic hydrogenation and in situ acid-
catalyzed Bamberger rearrangement was first realized in a pressurized CO2/H2O system. By employing Pt−Sn/Al2O3 as catalyst,
nitrobenzene could be converted to p-aminophenol with selectivity as high as 85% when the reaction was carried out at 140 °C
under 5.5 MPa CO2 and 0.2 MPa H2. This new protocol is environmentally benign because it is fully rid of the use of mineral
acid by the application of self-neutralizable carbonic acid.
It is known that carbonic acid formed in pressurized CO2/H2O
system is an attractive environmental benign alternative to
conventional permanent mineral acid, and the system has been
applied in various reactions such as hydrolysis,15 dehydration,16
diazotization,17 and reductions.18 In our previous studies, the
reduction of nitroarenes to the corresponding aromatic amines
by iron proceeded successfully in this system with high yield and
high chemoselectivity.19 Also, the preparation of N-arylhydroxyl-
amines by the partial reduction of nitroarenes using zinc under
0.1 MPa CO2 has been successfully accomplished with the yield
from 88% to 99%.20 Furthermore, the Bamberger rearrangement
of PHA was also realized in CO2/H2O system with a PAP’s
selectivity of 80%.21 Thus, it is significant to make further efforts
to investigate the hydrogenation of NB to PAP in the system. In
this paper, the preparation of PAP from NB by catalytic
hydrogenation in pressurized CO2/H2O system is demonstrated.
INTRODUCTION
■
p-Aminophenol (PAP) is a significant intermediate in the
synthesis of pharmaceuticals, pesticides, dyestuffs, and photo-
graphic chemicals.1 PAP can be prepared from p-nitrophenol
using Fe/HCl as reducing agents, but in the process, a large
amount of Fe/FeO sludge (1.2 kg/kg of product) is generated.2
PAP can also be prepared by hydrogenation of nitrobenzene
(NB) to N-phenylhydroxylamine (PHA) combined with PHA’s
Bamberger rearrangement3 in the presence of acid,4 and this
route has been industrialized by Mallinckrodt Inc. (U.S.A.)5 with
aniline (AN) produced by PHA’s further hydrogenation as the
main byproduct. For the hydrogenation of NB to PHA in this
reaction, various supported noble catalysts6 have been reported,
even Au supported on TiO2 has recently been proved to be
capable of catalyzing the reaction with a PAP’s selectivity of
81%.7 Among the above-mentioned catalysts, Pt catalysts are
considered as the best for the selective formation of PAP.8
Usually sulfuric acid is used for the rearrangement of PHA to
PAP in this reaction. However, it not only causes corrosion of the
reactor but also inevitably needs a neutralization step to separate
PAP from the reaction mixture during which a large volume of
effluents is generated. Therefore, several alternative methods
have been tried to eliminate the above-mentioned problem.
Chaudhari et al.9 used ion-exchange resins to replace sulfuric acid
to obtain PAP from NB using Pt−S/C catalyst, but the selectivity
of PAP was only 14%. Komatsu et al.10 introduced Pt supported
on H-ZSM-5 to carry out the hydrogenation of NB to PAP and
obtained PAP with a selectivity of 66%. Pt−S2O82−/ZrO211 and
Pb−Pt/MgAPO-512 have also been prepared and applied by
Zhao’s group in the hydrogenation of NB to PAP, and the
selectivity turned out to be 46.6% and 71%, respectively. Simple
mechanical mixture of Pt supported on MgLa mixed oxide13 with
ZrSAD-3 solid acid has been tested by Abhay et al.14 for the
hydrogenation of NB to PAP, and the selectivity has been
claimed to be 94%. All the above-mentioned works applied a
common strategy to use solid acid to promote the rearrangement
of PHA to PAP, but coke formation10 occurring on the acid sites
of the catalyst will restrict the catalysts’ performance and lifetime.
RESULTS AND DISCUSSION
■
Effect of Different Catalysts on the Hydrogenation of
NB to PAP. From the results shown in Table 1, it can be seen
that the Ni or Co catalyst is not good for the formation of PAP
(entries 17 and 18). When Ru catalyst is applied, the selectivity of
PAP is only 13% (entry 16). The selectivity of PAP is found to be
promising when Pt or Pd catalysts are used (entries 1, 4, 7, 10,
and 12).
To further improve the selectivity of PAP, the dimethyl
sulfoxide (DMSO), which has been reported as an effective
additive in terms of enhancing PAP’s selectivity of the
reaction,22,23 is added into the system. From the results shown
in entries 2, 5, 11, and 13, it is observed that the addition of
DMSO leads to the increase of PAP’s selectivity by 10−15%, but
it also results in the decrease of NB’s conversion because of its
toxic effect on the catalysts.22
Received: September 28, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.oprd.5b00307
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Table 1. Effect of Different Catalysts for the Hydrogenation of
a
NB to PAP
c
products
[%]
catalyst (metal
loading: wt %)
n
metal
/n
conv. of
sel. of
NB b
c
entry
NB [%] PAP AN PAP [%]
1
Pt/C(Pt:5%)
10000
10000
10000
10000
10000
37
25
28
37
26
20
35
27
17
14
16
16
15
12
18
16
20
11
12
21
11
8
46
56
57
43
56
61
51
59
d
2
Pt/C(Pt:5%)
3
4
Pt−Pb/C(Pt:5%)
Pt/SiO2(Pt:2%)
Pt/SiO2(Pt:2%)
d
5
6
7
8
Pt−Pb/SiO2(Pt:2%) 10000
Pt/Al2O3(Pt:0.5%)
10000
17
11
Pt−Sn/
10000
Al2O3(Pt:0.5%)
9
Pt−Sn/
10000
26
18
8
69
Al2O3(Pt:0.04%)
10
Pd/C(Pd:2%)
Pd/C(Pd:2%)
Pd/C(Pd:5%)
Pd/C(Pd:5%)
10000
10000
10000
10000
10000
58
48
57
50
37
22
25
19
25
23
36
23
38
25
14
38
53
34
49
62
d
11
12
d
13
14
Pd-CaCO3−PbO/
PbAc2 (Pd:5%)
e
15
Pd-BaSO4-
Quinoline
(Pd:5%)
10000
41
24
17
59
16
17
18
Ru/C(Ru:5%)
10000
90
14
11
12
1
78
13
10
13
7
Figure 1. Hydrogenation of NB under different CO2 pressure by using
Pt−Pb/SiO2 catalyst. Reaction condition: NB = 5 mmol, Pt−Pb/SiO2
Raney Ni(Ni:⩾90%) 10
Raney Co(Co: >
99%)
10
1
9
(Pt: 0.04%; molar ratio of Pb and Pt: 3; 2 μmol Pt), H2O 50 = mL, PH
=
2
0.5 MPa, T = 140 °C, stir rate = 1000 r/min, t = 4 h, PCO : initial pressure
2
a
Reaction condition: NB = 10 mmol, H2O = 60 mL, PCO = 5.0 MPa
2
at room temperature. Hydrogenation of NB under different CO2
pressure by using Pt−Sn/Al2O3 catalyst. Reaction condition: NB 5
mmol, Pt−Sn/Al2O3 (Pt: 0.04%; molar ratio of Sn and Pt: 3; 2 μmol Pt),
(initial pressure at room temperature), PH = 0.2 MPa, T = 140 °C, stir
rate = 1000 r/min, t = 3 h. Molar ratio of NB and metal. Determined
by HPLC. Dimethyl sulfoxide 1.0 mg. Quinoline 0.05 mL.
2
b
c
d
e
H2O 50 mL, PH = 0.5 MPa, T = 140 °C, stir rate = 1000 r/min, t = 4 h,
2
P
CO : initial pressure at room temperature.
2
The addition of DMSO is effective on enhancing the selectivity
of PAP, but it may create problems in the subsequent product
isolation. Therefore, Pb or Sn, which has been reported as an
effective metal modifier to influence the catalyst’s performance in
selective hydrogenation reactions,24 is applied. The results show
that the selectivity when the reaction is catalyzed by modified Pd
or Pt catalysts (entries 3, 6, 8, 9, 14, and 15) can be 10−28%
higher than those by monometallic catalysts (entries 1, 4, 7, and
12).
Among the catalysts examined in Table 1, Pt−Sn/Al2O3
catalyst, which is also effective for the hydrogenation of
ketones,25 aldehydes,26 and N-alkylation of amines, exhibits to
be the best for the selective formation of PAP,27 with AN and a
small amount of o-aminophenol (OAP, the concentration ratio of
PAP to OAP is around 15−20 in the reaction mixture according
to quantitative analysis by HPLC) as byproducts.
Hydrogenation of NB under Different CO2 Pressure.
Because the Bamberger rearrangement of the intermediate PHA
to PAP is acid-catalyzed and CO2 pressure is an important factor
to determine the acidity of the CO2/H2O system, different CO2
pressures were applied for the hydrogenation of NB to PAP by
using two kinds of catalysts: Pt−Pb/SiO2 and Pt−Sn/Al2O3, with
results presented in Figure 1A and Figure 1B, respectively.
Without CO2 in the system, there is hardly any PAP produced by
employing Pt−Pb/SiO2 as catalyst, indicating that carbonic acid
is essential for PAP’s formation (Figure 1A). By using Pt−Sn/
Al2O3 as catalyst, a PAP selectivity of 47% is obtained due to the
support Al2O3’s acidity (Figure 1B). With the increase of initial
CO2 pressure from 0 to 3 MPa, the pH value of the system
decreases from 3.9 to 3.5 as reported,28 and the selectivity by
using Pt−Pb/SiO2 and Pt−Sn/Al2O3 catalyst increase gradually
to 56% and 65%, respectively. However, when the initial CO2
pressure is higher than 3 MPa, the pH value does not change as
rapidly and apparently as it does when the CO2 pressure is
around 0−3 MPa, therefore, the selectivity of PAP hardly
increases. These results are just correlated with the relationship
between the CO2 pressure and the acidity of the system which we
previously studied,28 and also in accordance with the fact that
stronger acidity favors the rearrangement of PHA to PAP.
Effect of Hydrogenation Temperature on the For-
mation of PAP. The reaction was studied within a range of
temperatures from 100 to 160 °C. It is observed from Figure 2
that higher reaction temperature favors the formation of PAP;
the selectivity of PAP increases from 50% to 65% while the
temperature changes from 100 to 160 °C. There are two factors
accounting for the results. First, the increase of reaction
temperature favors PHA’s desorption from the catalyst surface
to acidic aqueous phase where it undergoes an acid-catalyzed
rearrangement process away from further hydrogenation.
Second, the rearrangement of PHA to PAP catalyzed by a
weaker acid should be conducted at a relatively high temperature
to obtain a high selectivity of PAP, which is already verified by
Wei et al.; they have investigated the rearrangement of PHA
catalyzed by three different kinds of acids: H2SO4, HCl, and
H3PO4.29 Their research shows that when the temperature is
lower than 60 °C, the selectivity of PAP in the catalytic reaction
B
DOI: 10.1021/acs.oprd.5b00307
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Article
Figure 2. Effect of hydrogenation temperature on the formation of PAP.
Reaction condition: NB = 5 mmol, Pt−Sn/Al2O3 (Pt:0.04%; molar ratio
Figure 4. Hydrogenation of NB under different H2 pressure. Reaction
conditions: NB = 20 mmol, Pt−Sn/Al2O3 (Pt: 0.04%; molar ratio of Sn
of Sn and Pt: 3; 2 μmol Pt), H2O = 50 mL, PCO = 5.5 MPa (at reaction
and Pt: 3; 2 μmol Pt), H2O = 50 mL, PCO = 5.5 MPa (at reaction
2
2
temperature, 3 MPa initially at room temperature), T = 140 °C, stir rate
= 1000 r/min, t = 4 h.
temperature, 3 MPa initially at room temperature), PH = 0.5 MPa, stir
2
rate = 1000 r/min, t = 4 h.
are further hydrogenated to give more AN, leading to the decline
of PAP’s selectivity.
using strong acid H2SO4 (or HCl) is higher than that using the
moderate strong acid H3PO4. However, if the catalytic reaction is
carried out under a temperature of 90 °C, the selectivity of PAP
using H3PO4 is the best.
Reaction Results with Different nNB/nPt (Molar Ratio
of NB to Pt). Keeping the other reaction conditions constant,
the catalytic hydrogenation of NB with different nNB/nPt was
examined. From the results presented in Figure 3, it can be seen
Hydrogenation of NB at Different Stir Rates. Because the
hydrogenation of NB to PAP is a complex four-phase
heterogeneous reaction including gas, solid catalyst, aqueous,
and organic phases,30 the change of reaction parameters which
can accelerate the diffusion and enhance the mass transfer will
benefit the formation of PHA, and those which can facilitate
PHA’s desorption from catalyst and efficient transport to acidic
phase will enhance PAP’s selectivity. There’s no doubt that the
increase of stir rate would favor the selectivity of PAP, and it is
also observed that the selectivity rises dramatically from 67% to
85% when the stir rate increases from 500 to 1000 rpm in Table
2. The selectivity does not change much when the stir rate is
beyond 1000 rpm, so 1000 rpm is chosen for the following
studies.
a
Table 2. Hydrogenation of NB at Different Stir Rates
b
products
[%]
b
entry stir rate/(r/min) conv. of NB [%] PAP AN sel. of PAP [%]
1
2
3
4
500
9
6
3
3
3
3
67
81
85
85
Figure 3. Reaction results with different nNB/nPt. Reaction condition:
Pt−Sn/Al2O3 (Pt: 0.04%; molar ratio of Sn and Pt: 3; 2 μmol Pt), H2O =
750
16
20
20
13
17
17
1000
1250
50 mL, PCO = 5.5 MPa (at reaction temperature, 3 MPa initially at room
2
temperature), PH = 0.5 MPa, T = 140 °C, stir rate = 1000 r/min, t = 4 h.
2
a
Reaction condition: NB = 20 mmol, Pt−Sn/Al2O3 (Pt:0.04%; molar
ratio of Sn and Pt: 3; 2 μmol Pt), H2O = 50 mL, PH = 0.2 MPa, PCO
=
2
2
5.5 MPa (at reaction temperature, 3 MPa initially at room
temperature), T = 140 °C, t = 4 h. Determined by HPLC.
that the selectivity of PAP increases from 65% to 80% with the
increase of nNB/nPt from 2500 to 12500. More NB’s competitive
adsorption on the catalyst surface might favor the intermediate
PHA’s desorption into the aqueous phase to rearrange into PAP,
afterward leading to more PAP produced.
Hydrogenation of NB under Different H2 Pressure. The
results in Figure 4 show that both the conversion of NB and the
yields of PAP and AN increase with the increase of H2 pressure,
but the selectivity of PAP declines gradually. It is known that the
increase of H2 pressure will lead to the increase of the
hydrogenation rate. With the increase of H2 pressure, the
formation rate of PHA, which will convert to PAP or AN,
increases; therefore, both the yield of PAP and the yield of AN
increase. However, PHA’s desorption rate from the catalyst is
constant with the temperature maintained at 140 °C, and the
generated PHA, which cannot desorb from the catalyst in time
b
Results of NB’s Hydrogenation with Different Reaction
Time. Figure 5 shows that the conversion of NB and the yields of
PAP and AN increase with the prolongation of reaction time, but
the selectivity of PAP gradually decrease. With the consumption
of substrate NB, there would be lower nNB/nPt along with lower
selectivity of PAP. These results are consistent with the results in
Figure 3. After the NB has been consumed for 6 h, the nNB/nPt
of the system is nearly 7500, and the selectivity of PAP turns out
to be about 70% in Figure 5. Besides, with the reaction time
prolonging, the color of the reaction liquid changes gradually
from pale yellow to dark brown, as a result of the generated PAP’s
conversion into other byproducts31 such as 4, 4′-diaminodi-
phenyl ether (detectable in the reaction liquid when the reaction
C
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Article
catalyst shows a total turnover number of 5400 mol PAP/mol Pt
after 4 successive runs.
EXPERIMENTAL SECTION
■
Materials and Reaction Procedure. Pt−Sn/Al2O3 cata-
lysts were donated by Dalian institute of chemical physics,
Chinese academy of sciences and prepared by the vacuum
complex impregnation method.32 Pt−Pb/C (5%) and Pt−Pb/
SiO2 (5%) catalysts were prepared as follows: Carbon black
(Vulcan XC-72, 2 g) or silica (Aerosil 300, 2 g) was impregnated
with an aqueous solution of H2PtCl6 (0.5 mmol Pt) and
(CH3COO)2Pb (1.5 mmol Pb) for 24 h, dried at 120 °C,
calcined at 500 °C for 4 h and finally reduced in hydrogen
atmosphere at 300 °C for 3 h. The other catalysts and all the
chemicals were purchased from commercial sources.
Figure 5. Results of NB’s hydrogenation with different reaction time.
Reaction condition: NB = 20 mmol, Pt−Sn/Al2O3 (Pt: 0.04%; molar
A 100 mL autoclave (Parr 4842) was applied to carry out the
reaction of NB to PAP. The nitrobenzene, catalyst, and water
were introduced into the autoclave which was then purged three
times with 0.2 MPa CO2. Then CO2 was gradually charged into
the autoclave to maintain a designated pressure. H2 was charged
when the autoclave reached the designated temperature under
constant stirring. The total operating pressure was maintained
constant by continuous recruitment of H2 during the reaction
process. After the reaction, the gas was released slowly. The
suspension was filtered, the solid was washed with methanol, and
then the filtrate was dissolved in methanol in order to obtain a
250 mL homogeneous solution. The above solution was diluted
for 10 times and quantitatively analyzed by high-performance
liquid chromatography.
ratio of Sn and Pt: 3; 2 μmol Pt), H2O = 50 mL, PCO = 5.5 MPa (at
2
reaction temperature, 3 MPa initially at room temperature), PH = 0.2
2
MPa, T = 140 °C, stir rate = 1000 r/min.
time is longer than 6 h). The byproduct may be another cause of
PAP selectivity’s decrease.
Reusability of the Catalyst. In order to test the reusability
of the catalyst, the used Pt−Sn/Al2O3 was collected by
centrifugation, washed with methanol, dried in vacuum
atmosphere at 120 °C, and then recharged to the reactor for
the next run. NB was added according to the weight of the reused
catalyst to keep the initial nNB/nPt constant. The results in Figure
6 indicate that the conversion of NB decreases by 4%, and the
Product Analysis. The analytical method used was similar to
that reported.12 Agilent 1100 series high-performance liquid
chromatography (HPLC) instrument equipped with a UV
detector was used with the following conditions: Mobile phase
(vacuum filtered before use): (A) 70 mmol/L aqueous solution
of ammonium acetate; (B) methanol. Gradient: t = 0 min 70% A
30% B, t = 20 min 0% A 100% B; flow rate: 1.0 mL/min; column:
Agilent TC-C18 column, 5 μm, 4.6 mm × 250 mm; column
temperature: 30 °C; UV detector: 254 nm.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.oprd.5b00307.
■
S
Figure 6. Reusability study of the catalyst. Reaction condition: nNB/nPt =
10 000, Pt−Sn/Al2O3 (Pt: 0.04%; molar ratio of Sn and Pt: 3; 2 μmol
Pt), H2O = 50 mL, PH = 0.2 MPa, PCO = 5.5 MPa (at reaction
The HPLC method used in product analysis was described
in detail (PDF)
2
2
temperature, 3 MPa initially at room temperature), T = 140 °C, stir rate
= 1000 r/min, t = 4 h.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jyjiang@dlut.edu.cn. Phone (Fax): 0411-84986033.
■
Notes
selectivity of PAP decreases by 7% after 4 runs, which may be
caused by the adsorbed species32 on the catalyst’s surface. The
catalyst shows a total turnover number of 5400 mol PAP/mol Pt
after 4 successive runs.
The authors declare no competing financial interest.
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CONCLUSION
■
In conclusion, the preparation of p-aminophenol from nitro-
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DOI: 10.1021/acs.oprd.5b00307
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