New insight on the mechanism of the catalytic hydrogenation of nitrobenzene to 4-aminophenol in CH3CN-H2O-CF3COOH as a reusable solvent system.

Article abstract of DOI:10.1016/j.apcata.2014.01.033

The preparation of 4-aminophenol via hydrogenation of nitrobenzene in a single liquid phase has been carried in the presence of different precious metal catalysts. The liquid phase is composed of CH₃CN-H ₂O-CF₃COOH, which can be easily distilled at low temperature, thus avoiding work-up operation. The yield of 4-aminophenol is in the range 45% and in the presence of sulfolane as promoter reaches almost 50%. The best result has been obtained in the presence of Pt/C as hydrogenation catalyst. The role of the solvent is strictly related to the selectivity to 4-aminophenol, since CH₃CN decreases the hydrogenation activity compared to other solvent, CF₃COOH promotes the formation of the desired product both via Bamberger rearrangement in solution as well by a surface catalyzed reaction, while H₂O is responsible for 4-aminophenol formation in both reactions. Even though, nitrosobenzene and phenylhydroxylamine have not been observed, their reactivity suggest a complex pattern of reactions occurring either on the catalyst surface or in the solution. Indeed, formation of 4-aminophenol may occur both in solution, via acid catalyzed Bamberger rearrangement and on the catalyst surface by the formation of a surface Pt-nitrenium complex, which undergoes surface nucleophilic attack by H₂O.

Full text of DOI:10.1016/j.apcata.2014.01.033

Applied Catalysis A: General 475 (2014) 169–178
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
New insight on the mechanism of the catalytic hydrogenation
of nitrobenzene to 4-aminophenol in CH CN–H O–CF COOH
3
2
3
as a reusable solvent system. Hydrogenation of nitrobenzene
catalyzed by precious metals supported on carbon
∗
Giuseppe Quartarone, Lucio Ronchin , Alice Tosetto, Andrea Vavasori
Department of Molecular Sciences and Nanosystems, University Ca’ Foscari of Venice, Dorsoduro 2137, 30123Venezia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation of 4-aminophenol via hydrogenation of nitrobenzene in a single liquid phase has
been carried in the presence of different precious metal catalysts. The liquid phase is composed of
CH3CN–H2O–CF3COOH, which can be easily distilled at low temperature, thus avoiding work-up opera-
tion. The yield of 4-aminophenol is in the range 45% and in the presence of sulfolane as promoter reaches
almost 50%. The best result has been obtained in the presence of Pt/C as hydrogenation catalyst. The role
of the solvent is strictly related to the selectivity to 4-aminophenol, since CH3CN decreases the hydro-
genation activity compared to other solvent, CF3COOH promotes the formation of the desired product
both via Bamberger rearrangement in solution as well by a surface catalyzed reaction, while H2O is
responsible for 4-aminophenol formation in both reactions. Even though, nitrosobenzene and phenylhy-
droxylamine have not been observed, their reactivity suggest a complex pattern of reactions occurring
either on the catalyst surface or in the solution. Indeed, formation of 4-aminophenol may occur both in
solution, via acid catalyzed Bamberger rearrangement and on the catalyst surface by the formation of a
surface Pt-nitrenium complex, which undergoes surface nucleophilic attack by H2O.
Received 14 November 2013
Received in revised form 9 January 2014
Accepted 13 January 2014
Available online 22 January 2014
Paper dedicated to Prof. Luigi Toniolo for
celebrating his long research and teaching
activity in the field of Catalysis and
Industrial Chemistry after his mandatory
retirement.
Keywords:
Nitrobenzene hydrogenation
4
-aminophenol
©
2014 Elsevier B.V. All rights reserved.
Trifluoroacetic acid
Reusable solvent
Hydrogenation mechanism
1
. Introduction
-Aminophenol is an important raw material for several prod-
in continuous stirred tank reactor in which the biphasic reaction
medium is used to accomplish simultaneously the Pt catalyzed
hydrogenation of nitrobenzene and the acid catalyzed Bamberger
rearrangement of the intermediate N-phenylhydroxylamine (see
4
ucts in the field of dyes, polymer and pharmaceutics [1–3].
For instance, paracetamol (N-acetyl-4-aminophenol) is a widely
employed analgesic and antipyretic whose production is in con-
tinuous growth, particularly in the Far East region [4–9]. Industrial
synthesis of paracetamol is based on 4-aminophenol, which is
obtained through four different routes: (i) nucleophilic substitu-
tion of the chlorine of the 4-chloronitrobenzene, (ii) reduction of
Scheme 1) [4–6,8,9]. The aqueous solution of H SO4 is the key
2
for obtaining high selectivity to the 4-aminophenol. The suc-
cess is due to the easy protonation of the phenylhydroxylamine
(
pka = 1.9 [10]), which is extracted from the organic phase. In
−1
the acid solution (H SO4 concentration 0.6–1 mol L ), the sul-
2
fate of the N-phenylhydroxylamine is formed and it undergoes
fast rearrangement to the corresponding 4-aminophenol salt [11].
Moreover, in aqueous phase, the competitive hydrogenation of the
phenylhydroxylamine is negligible, being the hydrogenation cata-
lyst hydrophobic. In fact, only the fraction present in the organic
phase is hydrogenated to aniline [12].
From an environmental point of view, the major drawback of
the process is the neutralization of the acidic phase, with the con-
sequent by-production of sulfate salts, which are undesired low
values products and/or wastes. In addition, diluted sulfuric acid
causes huge corrosion concern, with the consequent increased
costs [1,2,4–6,8,9].
4
-nitro-phenol, (iii) selective hydrogenation of nitrobenzene, (iv)
Beckmann rearrangement of 4-hydroxyacetophenone oxime.
Till now, the selective hydrogenation of nitrobenzene is likely
to be the most convenient from both economical and environmen-
tal point of view [1,2,4–9]. The major concern of this process is,
however, the use of H SO , which poses corrosion, safety, purifi-
2
4
cation and environmental problems. The reaction is carried out
∗ Corresponding author.
E-mail address: ronchin@unive.it (L. Ronchin).
0
926-860X/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2014.01.033

1
70
G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
the precious metal catalyst in order to depress the reduction of
N-phenylhydroxylamine to aniline [23].
NO
2
Here, we present some new results on the hydrogenation of
nitrobenzene to 4-aminophenol in a single liquid phase composed
of solvent-H O–CF COOH and in the presence a precious metal
2
3
H2 Pt/C
hydrogenation catalyst. Furthermore, we propose a reaction path,
which allows an explanation for the several features of this reaction.
2. Experimental
Pt/C
H2
2
.1. Materials
NH
2
NHOH
organic phase
Nitrobenzene, aniline, 4-aminophenol, 2-aminophenol, trifluo-
roacetic acid, sulfolane were all Aldrich products, their purity
were checked by the usual methods (melting point, TLC, HPLC, GC
and GC–MS) and employed without any purification, acetonitrile
HPLC gradient grade was supplied by BDH, 1,4-dioxane, methanol,
nitromethane, dimethylformamide and dimethyl sulfoxide are ACS
reagent supplied by Aldrich.
H+
2 4
aqueous H SO
HO
NH
3
2
NH OH
H+
Catalysts used were commercial materials supplied by Engel-
hard (now Basf Catalysts): Pd/C 5%: Escat 10, Pt/C 3%, Ru/C 5% Escat
4
0 and Rh/C 5%.
2.2. Equipment
Products were identified by gas chromatography (GC), gas
Scheme 1. Hydrogenation of nitrobenzene in biphasic liquid system.
chromatography coupled mass spectrometry (GC–MS) and high
performance liquid chromatography (HPLC). GC and GC–MS anal-
ysis were carried out with a Agilent 7890A equipped with FID or
MS detector (Agilent 5975 C and a HP 5 column (I.D. 320 ␮m 30 m
long), helium was employed as carrier under the following condi-
Therefore, removal of H SO4 use is likely to be the most impor-
2
tant target for the improvement of the process. Several researchers
have recently proposed the use of a biphasic liquid system (water-
nitrobenzene) employing Pt supported on solid acid in the presence
of surfactant and promoters, which, however, cause problems in the
recycle and/or disposal of the aqueous phase [13–15].
Besides, gas phase reaction catalyzed by bifunctional Pt-zeolites
catalysts has been proposed. The yields to the 4-aminophenol are
interesting for practical purposes, however extensive catalyst deac-
tivation limits its synthetic utility [16].
Starting from these considerations, a single liquid phase process
could be a catwalk to a more sustainable process by using easily
reusable solvents and catalysts [17–20]. When products are solids
or high boiling point liquids the use of a much lower boiling point
solvent can be the key to improve the environmental sustainability
of the whole process, because of products separation, catalyst and
solvents reuse is made easy by filtration and evaporation. In this
−
1
tions: injector 523 K, detector 543 K, flow 1 mL min , oven 333 K
−
1
for 3 min 523 K 15 K min and 523 K for 15 min. Due to the ther-
mal instability of the products, routine analysis were carried out
by HPLC (PerkinElmer 250 pump, LC 235 diode array detector and
a C 8, 5 ␮m, 4 mm i.d. 25 cm long column) analysis were carried
out with CH CN–H O as mobile phase in isocratic 70% of CH CN
3
2
3
−
1
at 1 mL min . Conversion yield and selectivity are calculated by
the calibration with standard solutions of the pure products. N2
physisorption and CO chemisorption has been carried out with a
Micromeritics ASAP 2010C automatic adsorption analyzer.
2.3. Catalyst characterization
Chemisorption of carbon monoxide was carried at 308 K with
the double isotherm method and 1 min of equilibration time [24].
The chemisorption stoichiometry was set 1 (1 molecule of CO for
1 surface atom of metal) only for comparative purpose. Before
the analysis, the catalyst was pretreated in a flow of hydrogen
way, the high cost of CF COOH is counter balanced by its recycling,
3
making the process sustainable also from an economical point of
view.
−
1
The bases of the present research are the results in the Beck-
mann rearrangement of ketoximes to the corresponding amides in
CH CN–CF COOH as solvent catalytic system [17–20]. The analogy
(20 mL min ) at 473 K for 3 h and for 5 h under vacuum at the same
temperature in order to ensure total reduction of the precious metal
particles average diameter of catalyst particles (40 ␮m) and appar-
3
3
−
1
between the Beckmann rearrangement with the process to 4-
aminophenol via nitrobenzene hydrogenation originates from the
idea that the N-phenylhydroxylamine (the intermediate of the pro-
cess) (see Scheme 1) may undergo acid Bamberger rearrangement
ent density (0.54 g mL ) were given by the supplier and are the
same for all the catalysts (see Table 1).
2.4. Hydrogenation of nitrobenzene
very easily in non-aqueous CF COOH system.
3
The rearrangement is well known from long time and the
commonly accepted mechanism is via nitrenium ion intermedi-
ate followed by nucleophilic attack of a water molecule (Scheme 2)
Some preliminary reactions have been carried out in several sol-
vents (1,4-dioxane CH NO , (CH ) NCHO and (CH ) SO CH OH).
3
2
3
2
3
2
3
The kinetic runs were carried out in a well stirred glass reactor
thermostatted by circulation bath in the range 323–353 K, using
CH CN–H O as a solvent, CF COOH as an acid catalyst and a pre-
[
11,21,22].
Several solvents can be used for the hydrogenation reac-
3
2
3
tion, but those to be considered in this research possess low
nucleophilicity in order to avoid side reaction in the Bamberger
rearrangement [21,22]. In addition, suitable solvents (for instance
CH CN and (CH ) SO) have to lower the hydrogenation activity of
cious metal catalyst (Pt, Pd, Rh, Ru) supported on carbon. Hydrogen
was fed continuously at constant pressure (c.a. 0.12 MPa) by a gum
balloon (see supplementary information). Blank reactions in the
absence of nitrobenzene have been carried out in order to verify
3
3 2

G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
171
NHOH
NHOH
NH
NH
2
2
NH
H+
2
H O
-
H+
2
-H O
OH
Scheme 2. Bamberger rearrangement of phenylhydroxylamine.
Table 1
and chloroanilines, while in the presence CH SO H we observe ani-
3
3
Properties of precious metal catalysts supported on carbon black: metal dispersion,
total surface area.
line and a complex mixtures of not identified products. In addition,
we found the formation of two liquid phases and precipitation of
salts in both reactions.
Catalyst
Metal
dispersion (%)
CO uptake
BET surface
area (m gcat
−
1
2
−1
)
(mL gcat
)
Pt/C 3%
Pd/C 5%
Rh/C 5%
Ru/C 5%
65
27
24
23
2.5
3.2
2.9
2.8
920
880
890
910
3
.2. Influence of the catalyst type on reaction rate and selectivity
The results in the previous section suggest that the best sol-
vent is CH CN, coupled with CF COOH as acid catalyst. Because of
3
3
these evidences we will exclusively show results of experiments
carried out in the mixture CF COOH–CH CN, which is actually a
the stability of the solvent. All the solvents employed did not show
any products of reduction at GC analysis.
3
3
CF COOH–H O–CH CN system since H O is contained either in the
3
2
3
2
In a typical experiment 2.5 mL of acetonitrile, with suspended
the Pd/C catalyst (typically 50 mg), was introduced into the reac-
tor. After that the reactor was closed, purged before with nitrogen
and then with hydrogen, pressurized at 121 kPa, finally heated at
solvent or in the reagent. In addition, H O is a reagent in the Bam-
berger rearrangement for this reason its role in the reaction path
will be studied.
2
The comparison of various precious metal catalysts is reported
in Table 2. It is noteworthy that the best performing one, in terms
of both catalytic activity and yield to 4-aminophenol, is Pt/C. This
gives the best results also with the biphasic sulfuric acid nitroben-
zene system [1–6]. Both Pd/C and Rh/C show high hydrogenation
activity, but 4-aminophenol yields (in both cases) is lower than
that observed in the presence of Pt/C. Ru/C shows both poor hydro-
genation activity and negligible 4-aminophenol yield, because of
its intrinsic low hydrogenation activity observed in these reactions
3
53 K under stirring for 2 h in order to activate the catalyst. The
temperature of reaction (range 323–363 K) was left to stabilize,
than the preheated solution of acetonitrile water trifluoroacetic
acid and nitrobenzene (c.a. 22.5 mL, concentration nitrobenzene
− −1
.075 mol L , CF COOH 0.2–0.8 mol L ) was added to reactor.
3
1
0
After the addition of this solution, the computing of the reac-
tion time starts and the liquid phase is periodically sampled
and analyzed by GC, GC–MS and HPLC, during reaction course.
−
1
Nitrobenzene at a concentration of 0.075 mol L is completely sol-
uble in the H O–CH CN–TFA till 5 g of water in 25 mL of solution.
(Table 3).
2
3
Fig. 1 shows that the trends of 4-aminophenol yield vs. time
are similar in the presence of different metal catalysts. These
trends show that the formation of 4-aminophenol decreases as
the reaction proceeds suggesting an influence of the products on
the 4-aminophenol yield. As a matter of fact, amines reduce the
overall acidity of the solution, thus likely decreasing the rate of
rearrangement of the N-phenylhydroxylamine and the overall yield
in 4-aminophenol. Besides, it is likely that the better performance
3
. Results and discussion
3.1. Influence of the solvent system on the activity and selectivity
The type of solvent and the acidity of the system are parame-
ters of paramount importance for activity, selectivity and resistance
to deactivation of the precious metal phase in a catalyzed reac-
tion. This is especially true in the nitrobenzene conversion to
60
4
-aminophenol, since both reactions of the process (consecutive
hydrogenation and Bamberger rearrangement) are sensitive to the
nature of the solvent [23,25]. For this reason a screening of various
solvents is important in order to select the best one for the reaction.
Pt/C
Pd/C
Rh/C
Ru/C
50
40
We tested several solvents in the presence of Pt/C, however, CH CN
3
gives the best results (see Table 2 and supplementary materials).
The reason of the higher yield in 4-aminophenol by using CH CN
as a solvent is likely due to a simultaneous and synergic effect of
3
3
2
1
0
0
0
0
CH CN in deactivating the stage of hydrogenation and in promoting
3
the rearrangement of the N-phenylhydroxylamine. As a matter of
fact, in single phase reactions when CH CN is not the solvent much
3
lower yield in 4-aminophenol is obtained. For instance, in the pres-
ence of 1,4-dioxane, methanol and nitromethane the reaction is
selective to aniline, while it shows a complex mixture of products
in dimethylformamide. On the contrary, the reactions are practi-
cally inhibited by using dimethyl sulfoxide or sulfolane as solvents,
since they are both deactivating substances for hydrogenation pre-
cious metal catalysts [26]. Moreover, the reaction has been tested in
0
200
400
600
800
1000
1200
time (minutes)
Fig. 1. Influence of the catalyst type on 4-aminophenol yield. Run conditions: T:
353 K, kPa, PH2 121 kPa, nitrobenzene 0.075 mol L , CF COOH 0.6 mol L , H O 2.2 g,
the presence of HCl and CH SO H as acid systems. In the presence
−1 −1
3
3
3
2
of HCl, as expected, the reaction proceed to the formation of aniline
CH3CN 22 g, catalyst 50 mg.

1
72
G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
Table 2
Influence of the solvent on 4-aminophenol and aniline yield after 20 h. Run conditions: T: 353 K, PH2 121 kPa, nitrobenzene 0.075 mol L 1, acid catalyst 0.6 mol L−1, H2O 2.2 g,
−
solvent 22 g, catalyst Pt/C 3% 50 mg.
Solvent-acid
Conversion (%)
Yield 4-APa (%)
Yield ANb (%)
Yield others (%)
Notes
CH3CN–CF3COOH
CH3CN–HCl
65
20
20
98
88
42
13
10
12
90
60
10
10
8
3
13
Azoxy - azo benzene
Azoxy - azo benzene, chloroanilines
Azoxy - azo benzene
trifluoroacetanilide
Azoxy - azo benzene, 2-EtO-aniline and
anilides
Unidentified products
At 378 K 1 MPa reaction does not
proceed
Practically no reaction
Trifluoroacetanilides, 2-EtO-aniline
and anilides, Azoxy - azo benzene
traces
traces
5
CH3CN–CH3SO3H
c
C4H4O2 –CF3COOH
CH3NO2–CF3COOH
15
(
(
CH3)2NCHO–CF3COOH
CH3)2SO–CF3COOH
92
<1
10d
–
40
–
42
Traces
(
CH2)4SO2e –CF3COOH
<1
96
Traces
5
Traces
60
Traces
31
CH3OH–CF3COOH
a
4
-AP = 4-aminophenol.
AN = aniline.
,4-Dioxane.
b
c
1
d
e
mainly N-trifluoroacetyl-4-aminophenol.
Sulfolane.
Table 3
Influence of the catalyst type on nitrobenzene conversion, 4-aminophenol yield minutes and initial reaction rate after 180 min. Run conditions: T: 353 K, PH2 121 kPa,
−1
−1
nitrobenzene 0.075 mol L , CF3COOH 0.6 mol L , H2O 2.2 g, CH3CN 22 g, catalyst 50 mg.
Yield 4-APa (%)
Yield AN (%)
Yield others (%)
r0 (mol L min gmet )
−1
−1
−1
Catalyst
Conversion (%)
Pt/C 3%
Pd/C 5%
Rh/C 5%
Ru/C 5%
41
42
46
0.4
26
14
16
Traces
2
26
29
0.19
12
1
1
0.13
0.078
0.080
0.0007
0.2
a
These products are a mixture of 4-aminophenol and trifluoroacetylated compounds of 4- aminophenol.
of Pt/C catalyst is due to a specific selectivity of the catalyst to
and nitrosobenzene, but these products are present in very small
amount and only after few minutes of reaction. It is noteworthy that
the products “other” pass through a maximum and their decrease
is accompanied with the steepest aniline increase. This behavior is
consistent with the Haber reduction scheme of nitrobenzene where
azobenzene and azoxybenzene are intermediates to aniline [27].
4
-aminophenol.
3.3. Time vs. concentration profiles
In Fig. 2 reaction shows time vs. concentration profiles of
the species observed in the hydrogenation of nitrobenzene
in CH CN–H O–CF COOH. The concentration of 4-aminophenol
3
2
3
3.4. Influence of catalyst amount on the initial reaction rate
increases smoothly with the time, on the contrary, the yield to
aniline has a significant increase only after a long time. Other
products have been identified, but not precisely quantified and
are signed as “other”. The main side products (in agreement with
the Haber reduction scheme of nitrobenzene [27]) are azoxy-
benzene and azo-benzene, which are hydrogenated to aniline after
a longer time. Besides, there are traces of N-phenylhydroxylamine
Fig. 3 shows the influence of the catalyst amount on the initial
reaction rate and on the yield of 4-aminophenol. It appears (Fig. 3a)
that external mass transfer poorly influences the reaction kinetics
because of the small value of the intercept [28]. However, a com-
plete evaluation of the influence of rate of diffusion of reagents and
products on the reaction kinetics is beyond the scope of the present
work, in which initial reaction rate data are given only for com-
parative purpose. The yield of 4-aminophenol seems to be faintly
influenced by the catalyst amount (Fig. 3b), furthermore, catalyst
deactivation affects the overall reaction kinetics, being reached a
plateau, in any case.
8
nitrobenzene
4
-aminophenol
7
6
5
4
3
2
1
0
aniline
other
3.5. Influence of temperature on reaction rate and selectivity
The Arrhenius plot of the initial reaction rate (see supplemen-
tary materials) presents an almost linear trend, with a temperature
−
1
coefficient (or apparent activation energy) of 91 kJ mol . Even
though, kinetic data are referred to initial reaction rate, the value
of the apparent activation energy together with the results of the
previous section suggest the kinetic of reaction is poorly influenced
of by external diffusion limitation [28]. In additions, the inspection
of Wheeler–Weisz criterion implies little influence of the internal
diffusion, since values in the range of 0.05–0.11 are calculated for
hydrogen and nitrobenzene, respectively [28]. These results are in
agreement with other hydrogenation reaction carried out under
similar conditions by using Pd/C catalysts with similar porosity and
granulometry [24].
0
200
400
600
800
1000
1200
time (minutes)
Fig. 2. Time concentration profile of nitrobenzene hydrogenation to 4-
−
1
aminophenol. Run conditions: T: 353 K, PH2 121 kPa, nitrobenzene 0.75 mol L
,
CF3COOH 0.6 mol L−1, H2O 2.2 g, CH3CN 22 g, catalyst Pt/C 50 mg.

G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
173
(
a) 600
(b) 50
500
400
300
200
100
0
40
30
20
10
0
catalyst 25 mg
catalyst 50 mg
catalyst 80 mg
0
10
20
30
40
50
0
200
400
600
800
1000
1200
-1
1
/wcat (g )
time (min)
Fig. 3. Influence of catalyst loading on the initial reaction (a) rate and 4-aminophenol yield (b): T: 353 K, PH2 121 kPa, nitrobenzene 0.75 mol L 1, CF3COOH 0.6 mol L−1, H2O
−
2
.2 g, CH3CN 22 g, catalyst Pt/C 25–80 mg.
(
a) 60
(b) 60
3
3
23 K
33 K
323 K
333 K
353 K
358 K
50
40
30
20
10
0
50
40
30
20
10
0
353 K
58 K
3
0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
time (minutes)
time (minutes)
Fig. 4. Influence of temperature on 4-aminophenol (a) and aniline (b) yield. Run conditions: T: 323–358 K, PH2 121 kPa, nitrobenzene 0.075 mol L 1, CF3COOH 0.6 mol L−1
−
,
H2O 2.2 g, CH3CN 22 g, catalyst Pt/C 50 mg.
The trends of 4-aminophenol and aniline yield (observed at
different temperature and reported in Fig. 4) show that high tem-
peratures favor the formation of 4-aminophenol, while aniline is
the preferred product at the lower ones. This behavior suggests
intermediates whose reactivity is strongly influenced by tem-
perature and the kinetics of formation of 4-aminophenol has an
activation energy higher than that of aniline.
3.7. Influence of water concentration on initial rate of reaction
and 4-aminophenol yield
In Table 4 the influence of water concentration after 180 min
of reaction is reported. The sensitivity of the reaction to water
concentration is noteworthy, since only in a narrow range of
concentration (c.a. 10 wt.%) both high initial reaction rate and
high yield in 4-aminophenol is achieved. The reasons of such a
behavior are not straightforward because of the large number of
3
.6. Influence of CF COOH concentration on reaction rate and
3
selectivity
0.14
The influence of the concentration of CF COOH on the reaction
3
rate is reported in Fig. 5. The increase of the acid concentra-
tion corresponds to a small increase of the initial rate of reaction
both at 333 K and at 353 K, thus suggesting the acid concentra-
tion slightly influences the overall kinetics of reaction. It is likely
that the first step of nitrobenzene hydrogenation is the rate deter-
mining step, for which the influence of the acid is negligible. The
small increase of the initial rate with acid concentration could be
interpreted as a solvent effect of the acid on the polarity of the
solvent [23,29].
0.12
0.10
0.08
0.06
0.04
0.02
3
3
33 K
53 K
On the contrary, Fig. 6 shows that the selectivity is strongly
influenced by the CF COOH concentration. The increase of the
3
acid concentration causes an increase of 4-aminophenol yield and
a decrease of the yield in aniline. Such a behavior is in agree-
ment with the formation of an intermediate whose reactivity is
sensitive to the acid concentration. Besides, it is likely that the prac-
tically negligible concentration of N-phenylhydroxylamine (only
traces), observed during the reaction course, is due to all the simul-
taneous reactions involved both in solution and on the catalyst
surface.
0.0
0.1
0.2
0.3
0.4
-1
0.5
0.6
CF COOH (mol L )
3
Fig. 5. Influence of CF3COOH concentration on initial reaction rate. Run conditions:
T: 353 K, PH2 121 kPa, nitrobenzene 0.075 mol L , CF3COOH 0–0.6 mol L , H2O
2.2 g, CH3CN 22 g, catalyst Pt/C 50 mg.
−1
−1

1
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G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
(b) 55
(
a) 5
0
0
0
0
0
0
50
45
4
3
2
1
40
35
30
25
20
15
10
5
0
0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
time (minutes)
time (minutes)
Fig. 6. Influence of CF3COOH concentration on 4-aminophenol (a) and aniline (b) yield. Run conditions: T: 353 K, PH2 121 kPa, nitrobenzene 0.075 mol L 1, CF3COOH
−
−
1
0
–0.6 mol L , H2O 2.2 g, CH3CN 22 g, catalyst Pt/C 50 mg.
60
equilibria in which water is involved. Indeed, water may com-
pete with the reagents for the adsorption on Pt surface and may
reduce the protonation ability of the trifluoroacetic acid, which is
likely to be related to N-phenylhydroxylamine rearrangement to
4
-aminophenol [11,20,21]. On the other hand, water is involved in
40
20
0
the formation of 4-aminophenol, being the required nucleophile
for obtaining the product [11,20,21].
3
4
.8. Influence of sulfolane and dimethyl sulfoxide as promoters in
-aminophenols formation
-1
-1
-1
sulfolane
r0 = 0.16 mol L min gmet
-1
-1
-1
no promoters r0 = 0.13 mol L min gmet
dimethyl sulfoxide no reaction
The influence of sulfolane and dimethyl sulfoxide as promoters
for 4-aminophenols formation is reported in Fig. 7. In the pre-
liminary study on the solvent effect (Section 3.1) both sulfolane
and dimethyl sulfoxide used as the solvent inhibit the reaction
completely. Such a strongly deactivating effect of these sulfur com-
pounds as solvent towards the Pt catalyst, suggests their potential
use as selective modifier of the platinum surface, in order to try
an enhancement of the selectivity to 4-aminophenol [8,26]. As a
matter of fact, the presence of sulfolane increases both reaction
0
200
400
600
800
1000
1200
time (minutes)
Fig. 7. Influence of promoters on 4-aminophenol yield. Run conditions: T: 353 K, PH2
21 kPa, nitrobenzene 0.075 mol L , CF3COOH 0.6 mol L , H2O 2.2 g, CH3CN 22 g,
catalyst Pt/C 50 mg, sulfolane and dimethyl sulfoxide concentration 0.03 mol L
−
1
−1
1
−1
.
Table 4
Influence of water on nitrobenzene hydrogenation. Run conditions: T: 353 K, PH2 121 kPa, nitrobenzene 0.075 mol L 1, CF3COOH 0.6 mol L−1, CH3CN 22 g, catalyst Pt/C 3%
−
5
0 mg.
Water
Conversion
Yield 4-APa
(%)
Yield ANb
Yield others
(%)
r0
−
1
−1
−1
(
mg)
(%)
10
60
42
16
1
(%)
2
3
3
(mol L min gmet )
c
d
e
2
0
6
2
0.02
0.18
0.13
0.03
0.001
d
e
1300
2200
3400
4000
10
40
12
8
d
e
26
6d
–
2
e
e
Traces
1
a
b
c
4
-AP = 4-aminophenol.
AN = aniline.
H2O contained in the CH3CN measured by HPLC analysis [30].
These products are a mixture of 4-AP and trifluoroacetylated compounds of 4-AP.
These products are mainly recognized as azobenzene and azoxybenzene and trifluoroacetanilide.
d
e
Table 5
Reactivity of nitrosobenzene in the presence of phenylhydroxylamine and aniline under N2 after 120 min of reaction. Run conditions T: 353 K, nitrosobenzene 0.02 mol L−1,
H2O 2.2 g, CH3CN 22 g.
Hydrogen donor
Pt/C 3% (mg)
CF3COOH (mol L−1)
Conversion (%)
Yield azoxybenzene + azobenzene (%)
Phenylhydroxylaminea
–
50
–
–
50
–
0.6
0.6
–
0.6
0.6
–
99
99
99
97
99
2
95
95
95
95
95
Traces
–
Phenylhydroxylaminea
Phenylhydroxylamine
Aniline
Aniline
Aniline
–
–
50
0.6
0.6
–
–
–
–
a
The reaction is quantitative in few minutes at 298 K, and the rest of the mass balance is recognized by GC–MS analysis as condensation products mainly benzidine and
its oxidation products.

G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
175
This is confirmed by an evident stench of sulfides clearly smelt only
in the presence of dimethyl sulfoxide and not with sulfolane. The
higher initial rate of reaction, in the presence of sulfolane could be
related with adsorption equilibria of the species in contact with
the catalyst. In particular, Pt surface can be modified by sulfolane
adsorption thus increasing both reation rate and selectivity, but
these are conjectures needing further investigations. Despite of
the higher reaction rate and yield to 4-aminophenol compared
to the reactions carried out in the absence of sulfolane, the pres-
ence of small amount of this high boiling point compounds, is a
problem both for solvent recovery and for product purifi-
cation, thus suggesting to avoid its use for practical
purposes.
80
60
40
20
0
various products
-aminophenol
aniline
4
0
10
20
30
40
50
60
3.9. Reactivity of nitrosobenzene and N-phenylhydroxylamine
time (minutes)
Fig.
8 reports the hydrogenation of nitrosobenzene in
Fig.
8. Hydrogenation
of
nitrosobenzene,
initial
reaction:
CH CN–H O–CF COOH in the presence of Pt/C catalyst. At
3
2
3
−
1
−1
−1
r0 = 0.28 mol L min gmet . Run conditions: T: 353 K, PH2 121 kPa, nitrosoben-
−1
−1
difference of what is found by Jackson and coworkers [31,32],
the reaction proceed faster (r = 0.27 mol L min gmet and
zene 0.06 mol L , CF3COOH 0.6 mol L , H2O 2.2 g, CH3CN 22 g, catalyst Pt/C 3%
0 mg.
−
1
−1
−1
5
0
conversion is almost quantitative after 1 h) than that of nitroben-
zene hydrogenation, but the yield to 4-aminophenol and aniline
is 10% and 5%, respectively, the rest is a complex mixture of
products, mainly azoxybenzene, azobenzene and several uniden-
tified products. As a matter of fact, several reactions occur
between nitrosobenzene and the products of hydrogenation (for
instance N-phenylhydroxylamine and aniline) thus diminishing
the selectivity to 4-aminophenol and aniline. This is confirmed
by the data of Table 5 where the reactivity of nitrosoben-
zene is checked under different conditions. Nitrosobenzene is
rate and yield in 4-aminophenol, while dimethyl sulfoxide inhibits
completely the reaction. It is likely that the effect of the dimethyl
sulfoxide is due to its reduction to dimethyl sulfide (stench of sul-
fides when reaction is ended), which is known to be a strong poison
for Pt catalysts [8,26]. On the contrary, sulfolane is quite strongly
adsorbed on catalyst surface, thus inhibiting the reaction when
used as a solvent, but it is not hydrogenated to sulfide, which is the
actual poison of the catalyst in the presence of dimethyl sulfoxide.
(a)100
(b) 100
80
60
40
20
0
80
60
40
20
0
0
20
40
60
80
100
120
140
160
0
20
40
60
80
100
120
140
160
time (minutes)
time (minutes)
Fig. 9. N-phenylhydroxylamine rearrangement, conversion (a) and yield (b) of 4-aminophenol vs. time in the presence of Pt/C-N2, Pt/C-H2 and N2. Run conditions: T: 353 K,
−
1
−1
PH2 121 kPa, N-phenylhydroxylamine 0.02 mol L , CF3COOH 0.6 mol L , H2O 2.2 g, CH3CN 22 g, catalyst Pt/C 3% 50 mg.
(
a) 20
(b)100
1
5
0
5
0
80
60
40
20
0
1
0
20
40
60
80
100
120
140
160
0
50
100
150
time (minutes)
time (minutes)
Fig. 10. N-phenylhydroxylamine rearrangement, yield of aniline (a) and other byproducts (b) vs. time in the presence of Pt/C-N2, Pt/C-H2 and N2. Run conditions: T: 353 K,
−
1
−1
PH2 121 kPa, N-phenylhydroxylamine 0.02 mol L , CF3COOH 0.6 mol L , H2O 2.2 g, CH3CN 22 g, catalyst Pt/C 50 mg.

1
76
G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
NH
2
NO2
NO
NHOH
NO
NHOH
O
N
N
N
N
HN NH
+
Scheme 3. Haber reaction path for nitrobenzene reduction.
stable in CH CN–H O–CF COOH and CH CN–H O–CF COOH Pt/C
catalyst), giving mainly azoxybenzene and azobenzene [31]. In fact,
both N-phenylhydroxylamine and aniline are reductants toward
nitrosobenzene, but N-phenylhydroxylamine is much more reac-
tive.
3
2
3
3
2
3
at 358 K, but when aniline or N-phenylhydroxylamine are present,
it reacts almost instantaneously (particularly fast reaction occurs
in the presence of N-phenylhydroxylamine, also without any
NO
dehydrogenation
-
H2
desorption
Pdsurf
NOH
NHOH
NH
2
NO2
steps
steps
Pd/C
desorption
Pd/C
H2
H
2
surface reaction
desorption
-
H O
2
N
O
N
steps
steps
N
N
steps
H
N
H
N
Scheme 4. Jackson reaction path for nitrobenzene hydrogenation. Pdsurf: metallic surface of the Pd/C catalyst.

G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
177
CH CN-H O-CF COOH + H
3
2
3
2
liquid phase
NO
N
N
N
fast
+
O
N
PhNHOH
PhNO
fast
dehydrogenation
-H2
surface reaction
NO2
steps
H2
CF COOH
3
H
H
H
H
H
NOH
Pt Pt
-H O
N
N
2
Pt
Pt
Pt Pt
Pt
Pt Pt
Pt/C
solid phase
hydrogenation
several steps
hydrogenation
Nu = H O
2
NHOH
NH2
NH2
OH
-
PhNO
fast
+
H / slow
Scheme 5. Proposed reaction path for nitrobenzene hydrogenation in CH3CN–H2O–CF3COOH. Species outside the rectangle are in liquid phase with the relative homogeneous
reactions, those inside are adsorbed on Pt/C catalyst with the relative surface reaction, dotted lines indicate adsorption and desorption steps.
As mentioned above, the reaction rate of nitrosobenzene hydro-
genation is higher than that observed in nitrobenzene, this could
be ascribed to a solvent effect, which competes with nitrobenzene
during the direct hydrogenation of nitrobenzene. This suggests an
alternative path for 4-aminophenol formation in the hydrogena-
tion reactions involving species generated step by step on the Pt
surface, directly. On the contrary, desorption of nitrosobenzene
and N-phenylhydroxylamine will cause reactions of condensation
and decomposition giving mainly byproducts. Besides, this is in
agreement with the evidence that, except during the first minutes,
neither nitrosobenzene nor N-phenylhydroxylamine is detected
during nitrobenzene hydrogenation.
and nitrosobenzene in the adsorption on Pt sites (CH CN is a coor-
3
dinating solvent). In this case, it is likely that nitrosobenzene could
be favored in the first step of hydrogenation because of its ability
to form Pt-nitroso complexes more stable than the nitrobenzene
ones [33].
Figs. 9 and 10 show the reactivity of N-phenylhydroxylamine
under different reaction conditions. The initial reaction rate is
comparable to that of nitrobenzene hydrogenation, even though
substrate concentration is about 4 times lower than in nitroben-
zene hydrogenation. It is noteworthy that the highest reaction rate
3.10. Discussion on the reaction mechanism
Taking into account the results shown in the previous sec-
is observed in the presence of Pt/C 3% and in the absence of H2,
while the slowest one occurs without the Pt catalyst (see Fig. 9a).
The highest yield to 4-aminophenol is achieved in the presence of
tions, we will attempt a discussion on the whole reaction path for
nitrobenzene hydrogenation catalyzed by Pt/C in the presence of
CH CN–H O–CF COOH solvent acid system.
3
2
3
CF COOH without Pt catalyst (Fig. 9b). On the contrary, the yield
3
The generally accepted Haber mechanism for nitrobenzenes
reduction consists in a multistep reaction path (Scheme 3),
which comprises the step of reduction of nitro-group to nitroso-
of aniline in various systems (see Fig. 10a) is poorly influenced
by the type of reaction environment, thus suggesting that its for-
mation occurs through a parallel path, which does not directly
involve the catalysis of the Pt during the first 160 min of reaction. N-
phenylhydroxylamine in the presence Pt/C catalyst gives in small
amount 4-aminophenol and mainly condensation products, such
as azoxybenzene and azobenzene (compare Fig. 9b and Fig. 10b).
Starting from these evidences, it is likely that the homogeneous
acid catalyzed Bamberger rearrangement is not the only responsi-
ble for 4-aminophenol formation (or at least it is not the main way)
,
hydroxyamino- and amino-, as well as the condensation of the
intermediates to azoxybenzenes and azobenzenes, being, in turn,
reduced to hydrazine and finally to aniline [27].
The mechanism [27] has been generally accepted for long time
but recently several authors have criticized some aspects, which
do not explain the reactivity of the intermediates and of the
specificity of different type of catalyst [31,32,34–38]. In particu-
lar, Jackson and co-workers showed that the hydrogenation rate of

1
78
G. Quartarone et al. / Applied Catalysis A: General 475 (2014) 169–178
nitrobenzene is higher than that of nitrosobenzene, and no
nitrosobenzene accumulation is observed. In addition, a large iso-
rearrangement in solution, as well as on promoting the formation
of a Pt-nitrenium complex on the catalysts surface, which gives
4-aminophenol.
topic effect is evidenced, when D2 is used instead of H , for
2
nitrobenzene while for nitrosobenzene this effect is not observed
[
31]. On the basis of these results they suggested a reaction path
Acknowledgements
involving a surface Pd-hydroxyamino species from that starts sev-
eral parallel reactions as exemplified in Scheme 4 [31].
Financial support by Ca’ Foscari University of Venice is gratefully
acknowledged (ADIR fund 2011). A special thank to Mr. Claudio
Tortato for the helpful discussions and the valuable suggestions.
The results of the present research do not exclude the for-
mation of all intermediates of the Haber Scheme, but suggest
that reactivity of the catalyst surface may play the major role,
being in agreement with Jackson path, too. In fact, it is likely
that reaction steps are influenced by all the intermediates of the
Haber mechanism, but it is also probable that a Pt-hydroxyamino
surface complex, similar to that suggested by Jackson and
co-workers, may play a fundamental role for 4-aminophenol
formation. Indeed, the results, discussed in Section 3.9. on the
reactivity of nitrosobenzene and N-phenylhydroxylamine, support
a mechanism mediated by Pt-surface species, as reported in
Scheme 5. The Pt-hydroxyamino surface complex is protonanted
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.01.033.
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1
(
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3
2
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