Highly Selective Room Temperature Monoreduction of Dinitro-arenes by Hydrogen Sulfide under Liquid–Liquid Biphasic Catalysis

Article abstract of DOI:10.1002/kin.21136

Selective reduction of one of the nitro group present in dinitro aromatic compounds by a novel Zinin reagent, H₂S-laden N-methyldiethanolamine (MDEA) solution, has been explored in the presence of tetra-n-butyl phosphonium bromide as a phase transfer catalyst under the liquid–liquid mode of reaction. Under the room temperature reaction condition, reduction of 2,4-dinitrotoluene (2,4-DNT) with H₂S-laden MDEA leads to the selective reduction of one nitro group present either at the fourth position to obtain 4-amino-2-nitrotoluene (4A2NT) or at the second position to get 2-amino-4-nitrotoluene (2A4NT). The reaction was very fast to achieve 100% conversion, and the selectivity of 4A2NT is much higher than the 2A4NT. A detailed parametric study was performed to analyze the effect of parameters on 2,4-DNT conversion and selectivity of both the isomers. The apparent activation energy was found to be as high as 46.25 kJ/mol, and the reaction was found to be kinetically controlled. An empirical kinetic model has been developed to correlate with the conversion version time data obtained experimentally. The present system dealt with an industrial problem in dealing with H₂S, present in by-product gaseous streams of many petroleum and natural gas industries. Novelties in the selective monoreduction lie in that fact that the reaction was done at room temperature (303 K), with a novel reagent, H₂S-laden MDEA solution. Therefore waste-minimization was effected to yield value-added fine chemicals, that is, amines.

Full text of DOI:10.1002/kin.21136

Highly Selective Room
Temperature
Monoreduction of
Dinitro-arenes by Hydrogen
Sulfide under Liquid–Liquid
Biphasic Catalysis
UJJAL MONDAL, SUJIT SEN
Catalysis Research Laboratory, Department of Chemical Engineering, National Institute of Technology Rourkela, Rourkela,
769 008, India
Received 29 May 2017; revised 10 October 2017; accepted 10 October 2017
DOI 10.1002/kin.21136
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Selective reduction of one of the nitro group present in dinitro aromatic compounds
by a novel Zinin reagent, H2S-laden N-methyldiethanolamine (MDEA) solution, has been
explored in the presence of tetra-n-butyl phosphonium bromide as a phase transfer catalyst
under the liquid–liquid mode of reaction. Under the room temperature reaction condition,
reduction of 2,4-dinitrotoluene (2,4-DNT) with H2S-laden MDEA leads to the selective reduction
of one nitro group present either at the fourth position to obtain 4-amino-2-nitrotoluene
(
4A2NT) or at the second position to get 2-amino-4-nitrotoluene (2A4NT). The reaction was
very fast to achieve 100% conversion, and the selectivity of 4A2NT is much higher than the
A4NT. A detailed parametric study was performed to analyze the effect of parameters on 2,4-
2
DNT conversion and selectivity of both the isomers. The apparent activation energy was found to
be as high as 46.25 kJ/mol, and the reaction was found to be kinetically controlled. An empirical
kinetic model has been developed to correlate with the conversion version time data obtained
experimentally. The present system dealt with an industrial problem in dealing with H2S,
present in by-product gaseous streams of many petroleum and natural gas industries. Novelties
in the selective monoreduction lie in that fact that the reaction was done at room temperature
Correspondence to: Sujit Sen; e-mail: sensujit@nitrkl.ac.in.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
ꢀC
2017 Wiley Periodicals, Inc.

2
MONDAL AND SEN
(
303 K), with a novel reagent, H2S-laden MDEA solution. Therefore waste-minimization was
effected to yield value-added fine chemicals, that is, amines. ꢀC 2017 Wiley Periodicals, Inc.
Int J Chem Kinet 1–16, 2017
INTRODUCTION
available on the solubility study of acid gas mixture
CO2 and H2S) [13,14] or pure H2S in diethanolamine
(
Hydrogen sulfide (H2S) gas is a major source of impu-
rity in a gasification process of fossil fuels and found in
many tail-gas streams of biogas plant, syngas produc-
tion plant, and petrochemical plant [1,2]. H2S gas is
highly corrosive, toxic, and odorous in nature. H2S
concentration between 500 and 1000 ppm or more
can be fatal for any living organism, and maximum
allowable daily exposure limit without a significant
chance of detrimental effects is only 1.4 ppb. It is
very much obligatory to remove H2S from gas streams
as it can damage mechanical and electrical compo-
nents of any control system and corrode energy gener-
ation and heat recovery units and pipelines [3,4]. The
high content of sulfur in heavy crude oil generates a
high amount of H2S gas in downstream of petroleum
and natural gas–processing industries. Strict environ-
mental regulations have been imposed on the release
of this by-product stream into the environment. This
by-product gas stream is conventionally treated in an
amine-treating unit (ATU) for the removal of H2S gas,
and then regenerated H2S gas is oxidized in the Claus
unit to produce elemental sulfur. The Claus process is
facing the danger of extinction because of many rea-
sons such as (i) underutilization of valuable hydrogen
source, (ii) requirement of highly precise air rate con-
trol, (iii) the presence of trace sulfur compounds in the
spent air, etc. Several reports can be found in the liter-
ature for the production of hydrogen and sulfur from
H2S gas, through thermal [5], photochemical [6], elec-
trochemical [7–9], and thermochemical processes [10].
Syngas (H2, CO), which has been reported to be pro-
duced by pyrolysis of acid gases (H2S, CO2), can be
used as fuel for gas engines [11]. All the processes
mentioned above are an arduous process with the ex-
pensive mode of operation and leads to excess pro-
duction of elemental sulfur. Therefore, an economical,
ecofriendly, and energy-saving process for better uti-
lization of harmful H2S gas is in high demand.
or monoethanolamine solution [13–15]. H2S gas can
be selectively removed from the gases produced in re-
finery and coal gasification unit by an aqueous solution
of N-methyldiethanolamine (MDEA) [16–19]. In the
present study, aqueous MDEA solution is therefore uti-
lized for selective absorption of H2S as among all other
alkanolamines tried in industries; MDEA has the mini-
mum corrosion effect and causes a minute solvent loss
due to its less vapor pressure, chemically stable, and
economically viable.
Reduction of aromatic nitro compounds, with at-
tached a multifunctional group on the aromatic ring, to
aromatic amines, is a commercially important reaction.
High-value chemical intermediates can be produced
from aromatic amines, which are widely used as a raw
material for many utility chemicals, like pharmaceuti-
cals, fiber, explosives, polymers, cosmetics, pesticides,
dyes, etc. [20–25]. Reduction of 2,4-dinitrotoluene
(2,4-DNT) leads to the formation of 2-amino-4-
nitrotoluene (2A4NT), 2-nitro-4-aminotoluene, and 2,
4-diaminotoluene. These products are industrially used
as an intermediate for the production of dyes, artificial
pigments [26,27].
Some commercially and academically well-known
process for the reduction of nitroaromatic compounds
are catalytic hydrogenation [28], Bechamp reduction
[29], and Zinin reduction [30]. Reduction of dinitro
aromatics like 2,4-DNT can give rise to a reduction
of either both the nitro groups or a single nitro group
selectively. In the reduction of 2,4-dinitrotoluene into
2,4-diaminotolune, several kinds of reagents have been
tried out such as a Pd-pol catalyst in the presence
of NaBH4 [25], Ni(OAc)2·4H2O in the presence of
NaBH4 [31], and zinc in ammonium formate/HCOOH
[32]. Hydrogen at different pressure levels has been
employed in selective monoreduction of 2,4-DNT in
the presence of various catalysts such as Pt/C [33],
Au [34], and Ni-supported zeolite [35]. Hydrogena-
tion of p-nitro-phenol has been carried out in the mi-
crobubble reaction system utilizing Pd/SN-ON(x) cat-
alysts [36]. Yang et al. have observed 3 to10 fold
of reaction rate of hydrogenation of a number of ni-
troarenes in the presence of Pt/C&PMO catalyst [37].
The Pickering emulsification technique has been used
for the hydrogenation of m-nitrotoluene in the pres-
ence of solid catalyst (Pd/SM) [38]. The shortcomings
Nowadays, in the petroleum and natural gas indus-
tries, removal of sour gases containing H2S and CO2
is usually done by employing an aqueous solution of
alkanolamines. Alkanolamines are most popular for
sour gas absorption as they have less vapor pressure
and can be used in a broad range of operating condi-
tions (regarding temperature, pressure, concentration),
and recyclable [12]. A copious amount of literature is
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

HIGHLY SELECTIVE ROOM TEMPERATURE MONOREDUCTION OF DINITRO-ARENES
3
associated with all these processes are as follows: (1)
extreme reaction conditions at the various level of pro-
cess parameters, (2) tough process handling conditions
and high safety concerns, (3) reduction of other func-
tional groups present on the aromatic ring, (4) expen-
sive mode of operation, and (5) undesired by-product
formation. Monoreduction of polynitro-arenes can also
be affected by negative divalent sulfur in the form of
sulfide, hydrosulfide, and polysulfide and is called the
Zinin reduction [39]. Many researchers employed am-
monium sulfide or sodium sulfide as a source of sulfide
and hydrosulfide ions in the Zinin reduction for re-
ducing nitroaromatic compounds like chloronitroben-
zene [38,40], nitrotoluene [17], nitroanisoles [41], 4-
nitro-o-xylene [42], and 2,6-dinitrobenzoic acid [43].
Utilization of H2S gas for the reduction of mononitro-
arenes has also been tried out [41,42], but the selective
reduction of dinitro aromatic compounds by industri-
ally important H2S-laden MDEA has never been ex-
plored. In the present work, H2S-laden MDEA is em-
ployed as a novel Zinin reagent for selective monore-
duction in dinitro aromatics.
The current investigation uses a biphasic reaction
system where the Zinin reagent, aqueous H2S-laden
MDEA forms the aqueous phase and the reactant dini-
trotoluene, dissolved in solvent toluene, and forms the
organic phase. To enhance the reaction rate between
two immiscible phases, phase transfer catalysts (PTC)
can be used. The reaction rate gets accelerated when
PTC helps in transferring aqueous phase inorganic an-
ions to the organic phase crossing the interface, and
that promotes the reaction between organic reactants
with the inorganic anions [44]. Biphasic reaction sys-
tems are of two types: liquid–solid and liquid–liquid
ternary phosphonium salt, was used in Zinin reduction,
and the conversion achieved is comparable to the am-
monium salt [40]. Tetra-n-butylphosphonium bromide
(TBPB), another example of quaternary phosphonium
salt, has been successfully employed as PTC in several
different types of systems such as in the synthesis of
alkyl and aryl thioglycoside and thiodisaccharide, in
the one-pot synthesis of pyrano- and furanoquinolines,
and in synthesis of o-nitrophenyl octyl ether [54–56].
For the above reasons, TBPB has been tried out as PTC
in the current study, for selective reduction of dinitro
aromatic compounds by H2S-laden MDEA as a reduc-
ing agent at room temperature.
MATERIALS AND METHODS
Materials
2
,4-DNT was purchased from Sigma-Aldrich (St.
Louis, MO). TBPB was obtained from Sigma-Aldrich
Hong Kong, China), and potassium iodide (KI) and
(
sodium hydroxide pellets (NaOH) were acquired from
Rankem (New Delhi, India). Sodium thiosulfate pen-
tahydrate (Na2S2O3·5H2O) and toluene (ࣙ99%) of an-
alytical grade were obtained from Hi-Media (Mum-
bai, India). Starch soluble and sulfuric acid (ࣙ98%)
were acquired from Rankem (India). Potassium io-
date (KIO3), ferrous sulfide sticks (FeS), and MDEA
(
(
ࣙ99%) of analytical grade were procured from Merck
Mumbai, India).
Experimental Setup and Procedure
(
L–L) types [44–53]. The L–L PTC system (which is
For aqueous phase preparation, H2S gas was bubbled
through 35 wt% aqueous MDEA solution kept in an
ice bath. The gas bubbling process was continued till
desired sulfide concentration was attained. The sul-
fide concentration was determined by the iodometric
titration method [57]. The main reaction was carried
out in an isothermal baffled stirred batch reactor of
150 mL capacity in an inert nitrogen atmosphere. The
reactor was provided with a digital speed regulation
system and a PID temperature controller (± 1°C). The
reaction system consisted of an equal volume of the
aqueous phase (H2S-laden aqueous MDEA) and an
organic phase (toluene used an organic solvent to dis-
solve 2,4-DNT) and a catalytic amount of quaternary
phosphonium salt (TBPB). The aqueous phase was first
introduced into the reactor followed by stirring the so-
lution till the desired temperature was reached. After
stopping the stirrer, the organic phase along with the
TBPB was added into the reactor. The reaction com-
menced as soon as the stirrer was switched on again.
the case in the present study) is the most simple and
widely accepted system, because of its several advan-
tages such as operation simplicity, less requirement of
solvent and raw materials compared to other processes,
mild operating conditions, the higher selectivity of the
product, and enhanced reaction rate.
Different types of PTCs have been used in multi-
phase reactions, like phosphonium salts, ammonium
salts, crown ethers, cryptands, polyethylene glycols
(PEG), etc. [47]. Quaternary phosphonium and ammo-
nium salts are widely used among other PTCs for their
higher reactivity, stability at a higher temperature, and
less price. The quaternary phosphonium salts are more
beneficial than the latter. The phosphonium cation is
soft acid, and hydrosulfide anions (HS ) are soft bases
and as stated in Pearson’s HSAB theory, transport of
HS ions to the organic phase after binding with phos-
phonium cations is swifter than ammonium cation [52].
Ethyltriphenylphosphonium bromide (ETPPB), a qua-
−
−
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

4
MONDAL AND SEN
Table I Optimized Parameters for Each Experimental Run
Parameter (Unit)
Optimized Value
Range
Stirring speed (rpm)
Temperature (K)
Catalyst concentration (kmol/m )
Reactant concentration (kmol/m )
Sulfide concentration (kmol/m )
1500
303
0.0232
0.549
2.5
500–2500
303–328
0–0.093
0.2745–1.0981
1–2.5
2.02–5.02
constant
3
3
3
3
MDEA concentration (kmol/m )
Volume of organic and aqueous phase (m )
3.04
3 × 10
3
−5
0.1 mL of sample was collected from the upper or-
ganic phase layer after turning off the stirrer to allow
the phase separation. Based on the earlier studies, range
and optimized values of the operating parameters have
been chosen to carry out the current Zinin reduction,
and they are listed in Table I.
Method of Analysis
Samples withdrawn from the organic phase were ana-
lyzed by GC on an Agilent GC 7890B model. GC was
fitted with a capillary column DB-5MS, 2 m × 3 mm,
Elemental sulfur was found as a by-product of the
reduction reaction of p-nitrophenylacetic acid with
aqueous ammonium sulfide as shown in Eq. (2) [60].
Thiosulfate or elemental sulfur is generated as a by-
product of two different reactions as shown in Eqs. (1)
and (2). In the presence of a base, ammonia, the disso-
ciation equilibrium favors toward more ionization [61]
and as the concentration of ammonia increases the con-
and nitrogen was used as a carrier gas at a flow rate of
3
1
.6 cm /min. A flame ionization detector was used at
a temperature of 573.15 K.
RESULTS AND DISCUSSION
Two products namely 4-amino-2-nitrotoluene
2−
centration of sulfide ions (S ) becomes more than that
(
(
4A2NT) and 2A4NT were identified by GC-MS
Agilent 5977A model). No diamines or other products
−
of hydrosulfide (HS ) ions. Sodium disulfide was also
used as the reducing agent, and the overall stoichiome-
try is shown in Eq. (3) [60]. Zinin reduction of 2,4-DNT
with H2S-laden MDEA is assumed to follow either
Eq. (1) or (2).
like nitroso or hydroxylamine derivative were found.
So, the overall reaction can be written as follows:
Aqueous Phase Equilibrium. In the aqueous
2−
−
MDEA phase, sulfide (S ) and hydrosulfide (HS )
ions exist in an ionic equilibrium, which is affected by
the concentration of the MDEA [62]. The formation of
both the anions is shown in Scheme 1 from Eqs. (4)–
(6). Similar results were shown by ammonium sulfide
solution [63].
Proposed Mechanism of Reduction of
Phase Transfer Catalysis in Biphasic L–L
System. A general reaction mechanism of current
reduction reaction has been proposed based on the
current work and some previously published work
(Scheme 1) [39,40,51,52]. Valency of sulfur can
vary in a wide range (−2 to +6), and thus
the presence of sulfur in different anionic forms
2
,4-DNT under L–L PTC
As proposed by Zinin in 1842, the overall stoichiome-
try of the reduction reaction of nitroarenes by aqueous
ammonium sulfide is given by Eq. (1) [39]. Sodium sul-
fide as a reducing agent for the reduction of aromatic
nitro compounds also follows the same stoichiome-
try [39,40,57–59]:
−
−
−
−
(HS , HSO , HSO2 , HSO3 ) in the reaction media is
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

HIGHLY SELECTIVE ROOM TEMPERATURE MONOREDUCTION OF DINITRO-ARENES
5
Scheme 1 Proposed mechanism of reduction of 2,4-DNT by H2S-laden MDEA under L–L PTC.
very much possible and a proven fact. These anions can
bind to a single quaternary cation very quickly [41].
The catalyst used for our study was TBPB, which
is soluble in both the phases. The L–L PTC reaction
pathway can be easily explained by Stark’s extraction
mechanism, according to which the PTC partitions into
both phases. The catalyst cations present in the aqueous
phase transfer the inorganic anions to the organic phase
where it takes part in the reaction with the organic
substrate.
the samples was not able to detect those intermediates,
which may be due to the fast appearance and disap-
pearance of those intermediates [35,39,40,61,63]. The
selectivity of 2A4NT is less compared to 4A2NT. The
reason is the nitro group present at the second posi-
tion is sterically hindered by the methyl group present
on the aromatic ring. So the reduction of nitro group
present at the fourth position is easier than the sec-
ond position, and as a result selectivity of 4A2NT is
higher [35]. A few water molecules are also transferred
+
−
The L–L PTC system is a cyclic process; it starts
to the organic phase along with the Q HS ion pair
and taking part in the reaction (Eqs. (14)–(16)). The
+
at the aqueous phase where quaternary cations (Q )
−
+
−
rapidly pair with hydrosulfide ions (HS ) to form
quaternary ion pair gets inactivated (Q HSO ) after
3
+
−
Q
HS ion pair, and then the ion pair transfers to
the formation of the product (Eq. (17), and when it is
transferred back to the aqueous phase it gets regener-
the organic phase. After that, it takes part in some
complex elementary reaction as shown in Scheme 1
+
−
2−
ated (Q HS ) after reacting with S (Eq. (9)). The
catalytic cycle continues when the reactivated ion pair
is transferred back to the organic phase again.
(Eqs. (12)–(17)). Without the presence of PTC, the
similar reactions also occur, but it is rather a slow
reaction. The end product of the reduction reaction
of 2,4-DNT is 4-amino-4-nitrotoluene (4A2NT) and
The developed mechanism for the reduction of 2,4-
+
−
DNT suggests that the Q HS is the abundantly avail-
able ion pair among all other ions available in the
aqueous phase. In the aqueous phase, eight reactions
(Eqs. (4)–(11)) take place, and remaining reactions
(Eqs. (12)–(17)) occur sequentially in the organic
phase.
2A4NT. The reduction reaction of the aromatic nitro
compound to aromatic amines is an electron transfer
reaction, which requires six electrons transfer through
the formation of intermediates—nitrosobenzene and
benzene hydroxylamines [61,62]. GC-MS analysis of
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

6
MONDAL AND SEN
Figure 1 Effect of the stirring speed on the reaction rate.
Temperature = 303 K, organic phase volume = 30 mL, con-
centration of 2,4-DNT in the organic phase = 0.549 M, con-
centration of TBPB = 0.0232 M, the aqueous phase volume
Figure 2 Conversion–time plot obtained experimentally of
dinitrotoluenes. Stirring speed = 1500 rpm, temperature =
3
03 K, organic phase volume = 30 mL, concentration of reac-
tant in the organic phase = 0.549 M, concentration of TBPB
=
30 mL, concentration of MDEA in the aqueous phase =
=
0.0232 M, aqueous phase volume = 3 mL, concentration
3.04 M, and concentration of sulfide in the aqueous phase =
of MDEA in the aqueous phase = 3.04 M, and concentration
of sulfide in the aqueous phase = 2.5 M. [Color figure can
be viewed at wileyonlinelibrary.com]
2
.5 M.
Parametric Studies
Effect of Agitation Intensity. The effect of the stirring
speed on the reaction rate of 2,4-DNT disappearance in
the L–L PTC system has been explored in the range of
500–2500 rpm in the presence of PTC. It is very clear
from Fig. 1 that the variation of the initial reaction
rate is very small due to change in the stirring speed.
So it can be concluded that the reaction is kinetically
controlled and free from any mass transfer resistance
[41]. For elimination of the mass transfer effect, the
stirring speed has been fixed at 1500 rpm during other
parameter variation studies.
Comparison of Conversion between Different
Dinitrotoluenes. In this study, the effectiveness of
the developed reduction process by using H2S-laden
MDEA has been analyzed on different isomers of dini-
trotoluenes, as shown in Fig. 2. For the current study,
Figure 3 Effect of different catalysts on the conversion of
2
,4-DNT. Stirring speed = 1500 rpm, temperature = 303 K,
2,4-DNT and 2,6-dinitrotoluene (2,6-DNT) have been
organic phase volume = 3 mL, concentration of 2,4-DNT
in the organic phase = 0.549 M, concentration of TBPB =
used as a study material. From Fig. 2, it is very clear
that reactivity of 2,4-DNT is higher than 2,6-DNT and
0
.0232 M, aqueous phase volume = 3 mL, concentration of
2,4-DNT was selectively reduced to 4A2NT (79.6%)
MDEA in the aqueous phase = 3.04 M, and concentration of
sulfide in the aqueous phase = 2.5 M. [Color figure can be
viewed at wileyonlinelibrary.com]
and 2A4NT (20.4%). Reduction of 2,6-DNT results
only 2-amino-6-nitrotoluene (2A6NT), and conversion
achieved was 76.23% after 120 min of reaction. The
nitro group present at the second and sixth positions
that are sterically hindered by the methyl group at-
tached to the aromatic ring. So, the reduction of nitro
group present at the fourth position is swifter com-
pared to the second and sixth positions. As a result,
reactivity of 2,4-DNT was found of being higher than
Effect of Different Phase Transfer Catalysts. The
selective reduction of 2,4-DNT was studied with
different PTC as shown in Fig. 3. Catalysts taken
for the comparison studies are tetrabutylammonium
bromide (TBAB), tetrabutylphosphonium bromide
(TBPB), tetramethylammonium bromide (TMAB),
2,6-DNT.
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

HIGHLY SELECTIVE ROOM TEMPERATURE MONOREDUCTION OF DINITRO-ARENES
7
polarity of the solvent is low then the catalyst is un-
able to ionize, and hence reactivity will be very low,
and the distribution of the catalyst in both the phases
is also affected by the solvent [64]. The maximum de-
viation in the conversion of 2,4-DNT was found to be
7% because of the change of a solvent. So, the effect
of solvent is not significant, and therefore the solvent
effect can be ignored. Therefore, in kinetic modeling,
we did not consider the solvent effect. The dielectric
constant of the solvents used is in order of chloroform
>
benzene > toluene. And the conversion (and reactiv-
ity) of 2,4-DNT in the solvents follows the following
order: benzene > toluene > chloroform. Thus, in the
current reaction system, toluene has been used as an
effective solvent. Like in our present case, toluene was
also found to be a better solvent in other biphasic phase
transfer catalytic reaction systems [65].
Effect of Temperature of the Reaction. To study the
effect of temperature on the conversion of 2,4-DNT in
the L–L PTC system, the temperature of the reaction
was varied in the range of 313–343K and rest of all
reaction conditions were kept constant (Fig. 5a). From
the theory of transition state, the rate of a reaction in-
creases with the rise in temperature because increasing
temperature provides the energy required to overcome
the reaction barrier. It is evident from the figure that
the temperature variation has a great impact on reac-
tivity (conversion) of 2,4-DNT. With increasing tem-
perature, the collision between reactant molecules is
becoming more frequent and thus the reaction rate gets
enhanced.
The initial rates at different temperatures were de-
termined, and the Arrhenius plot of ln (initial rate)
2,6-DNT versus 1/T is drawn (Fig. 6). The slope of
the best fitted straight line gives us apparent activation
energy as 46.25 kJ/mol. As the value of the apparent
activation energy is high, the reaction is kinetically
controlled. The activation energy of the same order of
magnitude has been obtained in the selective reduction
of other nitro compounds, and it was also proposed to
be kinetically controlled one [39,60]. As the conver-
sion reached 100% after 60 min of reaction for each
temperature study, a selectivity plot (Fig. 5b) is drawn
based on the data of 30 min of reaction. The selec-
tivity of the two isomers of aminonitrotoluene follows
different trends with increasing temperature, although
there was an enhancement of the overall conversion of
2,4-DNT. From the plot (Fig. 5c), it is clear that with
increasing temperature the selectivity of 4A2NT has
decreased.
Figure 4 Effect of different solvents on the conversion of
2
,4-DNT. Stirring speed = 1500 rpm, temperature = 303 K,
organic phase volume = 30 mL, concentration of 2,4-DNT
in the organic phase = 0.549 M, concentration of TBPB =
0
.0232 M, aqueous phase volume = 30 mL, concentration of
MDEA in the aqueous phase = 3.04 M, and concentration of
sulfide in the aqueous phase = 2.5 M. [Color figure can be
viewed at wileyonlinelibrary.com]
tetrabutylammonium iodide (TBAI), ETPPB, and
tetrapropylammonium bromide (TPAB). The order of
the reactivity of these catalysts is TBPB > TBAB >
TPAB > ETPB > TBAI > TMAB. With a higher
number of carbon atom in the alkyl group of PTC,
the lipophilicity and extraction rate of PTC enhances,
which in turn generates higher productivity. ETPPB is
having a higher carbon number although it is show-
ing less reactivity than other ammonium and phospho-
nium salts (TBPB, TBAB, and TPAB). As the only
single methyl group and three big benzyl group are
attached to the quaternary cation in ETPPB, the qua-
ternary cation is not easily accessible for anions, and
the reaction rate gets slower. The phosphonium salt
(TBPB) has shown better reactivity than ammonium
salts (TBAB, TBAI).
Effect of Other Organic Solvents. In this study,
most common organic solvents such as chloroform,
benzene, and toluene were employed to analyze the
effect of their polarities (dielectric constant) on the re-
activity of the reduction reaction as shown in Fig. 4.
According to the Stark’s mechanism, PTC must trans-
fer the reactive anions into the organic phase to react
with the organic substrate. If the reactive anions are
firmly held by a cation, the reaction rate will be hin-
dered. Strong solvation (including hydration) of the
anions in the solvent also affects the reactivity. Non-
polar solvents promote higher reactivity by reducing
the extent of solvation of anions and by increasing the
concentration of PTC in the organic phase. But if the
Effect of Catalyst Concentration. To study the ef-
fect of the catalyst concentration, at five different cat-
alyst concentrations (0–0.093 M), the conversion of
2,4-DNT was determined and is shown in Fig. 7a. The
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

8
MONDAL AND SEN
Figure 6 Arrhenius plot of ln (initial reaction rate) versus
/T. All other conditions are same as in Fig. 5. [Color figure
1
can be viewed at wileyonlinelibrary.com]
conversion of 2,4-DNT, as well as the reaction rate,
enhances when the catalyst concentration increases.
Reactant conversion was found to be 100% with
0
7
.0232 M of catalyst loading, whereas it was about
0% without catalyst after 60 min of reaction. The
highest enhancement factor of 5.2 was achieved with
the catalyst concentration of 0.093 M, as shown in
Table II. The influence of catalyst loading on the se-
lectivity of 4A2NT and 2A4NT is shown in Fig. 7b
after 30 min of reaction. From Fig. 7c, it is evident
that with the catalyst concentration of 0.0465 M, the
highest selectivity of 4A2NT was attained and with
further catalyst loading the selectivity of 4A2NT has
been found to decrease gradually.
To determine the order of reaction with respect to
the catalyst concentration, the initial reaction rate was
calculated at different catalyst concentrations. A plot
of ln (initial rate) against ln (catalyst concentration)
was made and is shown in Fig. 8. From the slope of
the linear fit line, the order of reaction was determined.
The order of the reaction was found out to be 0.64 with
respect to the TBPB concentration.
Effect of the 2,4-DNT Concentration. The influ-
ence of the concentration of 2,4-DNT, on the kinetics
of the reaction, was studied in the concentration range
Figure 5 Effect of temperature on (a) the conversion of
,4-DNT, (b) the selectivity of 4A2NT and 2A4NT with re-
3
of 0.2745–1.0981 kmol/m and is presented in Fig. 9a.
2
spect to temperature, and (c) the selectivity of 4A2NT with
respect to reaction time. Stirring speed = 1500 rpm, or-
ganic phase volume = 30 mL, concentration of 2,4-DNT
in the organic phase = 0.549 M, concentration of TBPB =
It can be understood from the figure that with the in-
crease of the concentration of 2,4-DNT, the conversion
of 2,4-DNT has reduced. The selectivities of 4A2NT
and 2N4AT found at different concentrations of 2,4-
DNT are shown in Fig. 9b. According to Fig. 9c, the se-
lectivity of 4A2NT is increasing gradually with the in-
crease in the concentration of 2,4-DNT. At the highest
concentration of 1.0981 M of reactant, the selectivity
0
.0232 M, aqueous phase volume = 30 mL, concentration of
MDEA in the aqueous phase = 3.04 M, and concentration of
sulfide in the aqueous phase = 2.5 M. [Color figure can be
viewed at wileyonlinelibrary.com]
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

HIGHLY SELECTIVE ROOM TEMPERATURE MONOREDUCTION OF DINITRO-ARENES
9
Table II Effect of the Catalyst Concentration on the
Initial Reaction Rate
TBPB
Concentration
3
(
kmol/m of
Initial Reaction
Rate (kmol/m s)
Enhancement
3
Organic Phase)
Factor
0.0000
0.0232
0.0465
0.0697
0.0930
0.0136
0.0264
0.0536
0.0613
0.0696
1.0
2.0
3.0
4.5
5.2
All other conditions are same as in Fig. 7.
of 4A2NT was 82.26%. From Fig. 10, the order of re-
action with respect to the 2,4-DNT concentration was
obtained as 0.47.
Effect of Concentration of Sulfide Ion in the Aque-
ous Phase. The conversion of 2,4-DNT is greatly influ-
enced by the sulfide ions concentration in the aqueous
phase as shown in Fig. 11a. For this study, the sulfide
concentration in the aqueous phase was varied in the
range of 1.0–2.5 M. As the concentration of sulfide in-
creases, the conversion of 2,4-DNT also gets enhanced.
Figure 11b shows how the selectivity of 4A2NT is af-
fected by the change in the sulfide concentration, and it
is found from Fig. 11c that with a higher concentration
of sulfide selectivity of 4A2NT is gradually decreased.
From the plot of ln (initial rate) against ln (sulfide con-
centration) (Fig. 12), the slope of the linear fit line is
found to be 1.93.
Effect of MDEA Concentration. There is no direct
influence of the MDEA concentration on the reaction
kinetics, but it still affects the chemical equilibria of
the MDEA–H2O–H2S system. As shown in Scheme 1,
Figure 7 Effect of the TBPB concentration on (a) the
conversion of 2,4-DNT, (b) the selectivity of 4A2NT and
2A4NT with respect to catalyst loading, and (c) the selectiv-
ity of 4A2NT with respect to reaction time. Stirring speed
=
=
1500 rpm, temperature = 303 K, organic phase volume
30 mL, concentration of 2,4-DNT in the organic phase =
0
.549 M, concentration of TBPB = 0.0232 M, aqueous phase
volume = 30 mL, concentration of MDEA in the aqueous
phase = 3.04 M, and concentration of sulfide in the aqueous
phase = 2.5 M. [Color figure can be viewed at wileyonlineli-
brary.com]
Figure 8 Plot of ln (initial reaction rate) versus ln (catalyst
concentration). All other conditions are same as in Fig. 7.
[Color figure can be viewed at wileyonlinelibrary.com]
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

1
0
MONDAL AND SEN
Figure 10 Plot of ln (initial rate) versus ln (2,4-DNT). All
other conditions are same as in Fig. 9. [Color figure can be
viewed at wileyonlinelibrary.com]
active anions like sulfide (S2 ) and hydrosulfide (HS )
are formed in the aqueous phase and both the active an-
ions take part in the reactions as shown in Eqs. (1) and
−
−
(2). Owing to the basic nature of MDEA, ionization
of H2S in the aqueous phase is easier and availability
2−
of sulfide ions (S ) is more than hydrosulfide ions
−
(
HS ). MDEA variation with a fixed sulfide concen-
tration can give some idea about the existence of both
the reactions.
The color of the aqueous solution changes from
greenish yellow to orange and then finally to reddish
brown during the whole process of the reaction, and
this phenomenon indicates the progress of the reac-
tion. The reddish brown color is due to the formation
of polysulfide ions, which came into existence as the
reaction proceeds further [66].
In the present study, after 60 min of run time,
87.34% conversion of 2,4-DNT was achieved with the
highest MDEA concentration of 5.02 M with a sulfide
concentration 1.37 M in the aqueous phase, as shown
in Fig. 13a. These results support that the reaction fol-
lows Eq. (1). Similar observations were found when
scientists used aqueous ammonium sulfide solution for
the reduction of nitroaromatic compounds [39,58]. The
highest conversion achieved during MDEA variation
was 87.34%, and the current result is much higher than
the conversion obtained according to Eq. (1) (approxi-
mately 60%) or Eq. (2) (approximately 30%). It can be
explained by assuming that the reaction either follows
both Eq. (1) and (2) or it follows Eq. (3). As reaction
proceeds, polysulfide ions formed (as color intensifies
to red), when sulfide ions react with elemental sulfur
(produced in Eq. (2)), and higher conversion can be
achieved.
Figure 9 Effect of the 2,4-DNT concentration on (a)
the conversion of 2,4-DNT, (b) the selectivity of 4A2NT
and 2A4NT with respect to the reactant concentration, and
(c) the selectivity of 4A2NT with respect to reaction time.
Stirring speed = 1500 rpm, temperature = 303 K, organic
phase volume = 30 mL, concentration of TBPB = 0.0232 M,
aqueous phase volume = 30 mL, concentration of MDEA in
the aqueous phase = 3.04 M, and concentration of sulfide in
the aqueous phase = 2.5 M. [Color figure can be viewed at
wileyonlinelibrary.com]
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

HIGHLY SELECTIVE ROOM TEMPERATURE MONOREDUCTION OF DINITRO-ARENES
11
Figure 12 Plot of ln (initial rate) versus ln (sulfide concen-
tration). All other conditions are same as in Fig. 11. [Color
figure can be viewed at wileyonlinelibrary.com]
So it can be concluded that the increase in the
MDEA concentration with the fixed sulfide concen-
tration enhances the number of sulfide ions, and that
enhances the conversion of 2,4-DNT. Figures 13b and
13c show the effect of the MDEA concentration on the
selectivity of 4A2NT. From the figure, it is clear that
there is a slender influence of the MDEA concentra-
tion on the selectivity of either 4A2NT or 2A4NT. With
increasing MDEA concentration from 2.02 to 5.02 M,
the conversion of 2,4-DNT increases but the selectivity
of 4A2NT decreases slightly from 82.5% to 80.2%.
Effect of Elemental Sulfur Loading. Owing to ad-
dition of sulfur powder, a change in the color of H2S-
laden MDEA solution was observed from dark green
to orange. A similar phenomenon of color change was
noticed during MDEA variation. The effect of sulfur
powder loading on the reactivity of 2,4-DNT is shown
in Fig. 14, and the “S” nature of the curve can be ob-
served. An initial hike in the reaction rate is noted in
the figure because of the addition of elemental sulfur,
but as the reaction proceeds the reaction rate became
slower. It can be assumed that polysulfide ions (Sn2
−
,
where 2 ࣘ n ࣘ 6) were formed, among which disulfide
ions were in majority (Eq. (7)); these ions can easily
be transferred to the organic phase in comparison with
Figure 11 Effect of the sulfide concentration on (a) the
conversion of 2,4-DNT, (b) the selectivity of 4A2NT and
2− −
sulfide (S ), hydrosulfide (HS ), and other polysul-
2−
2A4NT with respect to the sulfide concentration, and (c) the
fide ions (Sn , where 3 ࣘ n ࣘ 6). The reduction rate
of 2,4-DNT with disulfide ions is faster [67]. It is an
established fact that polysulfide is a selective reducing
agent for nitro reduction [66]. Conversion of 2,4-DNT
in the run without elemental sulfur addition became
higher after 15 min of reaction. The sole reason for
this crossover may be the formation of elemental sul-
fur as the reaction proceeds (Eq. (2)). The sharp rise in
the conversion of reactant for no sulfur addition case
selectivity of 4A2NT with respect to reaction time. Stirring
speed = 1500 rpm, temperature = 303 K, organic phase
volume = 30 mL, concentration of 2,4-DNT in the organic
phase = 0.549 M, concentration of TBPB = 0.0232 M, aque-
ous phase volume = 30 mL, concentration of MDEA in the
aqueous phase = 3.04 M, and concentration of sulfide in
the aqueous phase = 2.5 M. [Color figure can be viewed at
wileyonlinelibrary.com]
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

1
2
MONDAL AND SEN
Figure 14 Effect of elemental sulfur addition on the con-
version of 2,4-DNT. Stirring speed = 1500 rpm, temperature
=
303 K, organic phase volume = 30 mL, concentration
of 2,4-DNT in the organic phase = 0.549 M, concentration
of TBPB = 0.0232 M, aqueous phase volume = 30 mL,
concentration of sulfide in the aqueous phase = 2.5 M, and
concentration of MDEA in the aqueous phase = 3.04 M.
[Color figure can be viewed at wileyonlinelibrary.com]
is due to the in situ production of elemental sulfur and
consequent production of disulfide and other polysul-
fides that enhance the reduction process.
Kinetic Modeling of L–L PTC
A kinetic model has been developed based on the pro-
posed mechanism shown in Scheme 1. The reaction
occurring at interphase between the aqueous and or-
ganic phases is also contributing significantly to the
overall reaction. From Fig. 7a the without catalyst run
shows 70% conversion.
It is further assumed that the overall L–L PTC re-
action follows both Eqs. (1) and (2) where 2,4-DNT
present in the organic phase is selectively reduced by
sulfide and hydrosulfide anions present in the aque-
ous phase to yield 4A2NT and 2A4NT as the main
products.
To eliminate the mass transfer effect, the reaction
mixture was agitated with at an optimum stirring speed
of 1500 rpm. As the reaction commences, the PTC cat-
Figure 13 Effect of the MDEA concentration on (a)
the conversion of 2,4-DNT, (b) the selectivity of 4A2NT
and 2A4NT with respect to the MDEA concentration, and
+
−
−
−
alyst ion-pair Q X (X = Cl ) exchanges its anions
(c) the selectivity of 4A2NT with respect to reaction time.
−
with HS ions very rapidly in the aqueous phase to
Stirring speed = 1500 rpm, temperature = 303 K, organic
phase volume = 30 mL, concentration of 2,4-DNT in the
organic phase = 0.549 M, concentration of TBPB = 0.0232
M, aqueous phase volume = 30 mL, and concentration of
sulfide in the aqueous phase = 2.5 M. [Color figure can be
viewed at wileyonlinelibrary.com]
+
−
transform into active catalyst ion pair (Q HS ). The
active catalyst ion pair is then transferred to the organic
phase crossing the interface. After that the reaction
proceeds in the organic phase, and different transition
anions are formed. Finally, the active catalyst became
−
inactive as the HSO ion gets attached to quaternary
3
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

HIGHLY SELECTIVE ROOM TEMPERATURE MONOREDUCTION OF DINITRO-ARENES
13
+
+
−
3
Table III Rate Constants of the Model
cations Q . Q HSO ions are transferred to the aque-
2−
+
ous phase and react with S to get reactivated as Q
−4
−4
Temperature
K)
k1 × 10
k2 × 10
−
3
−3.04 −1
3 −4.04 −1
HS again.
(
((kmol/m )
s
)
((kmol/m ) s )
The elemental sulfur produced in the reaction is en-
hancing the reaction rate initially, and it is the reason
for the “S” nature of the conversion versus time curve
3
3
3
3
03
08
13
18
7.10
15.80
11.82
9.02
13.60
22.42
30.27
(Fig. 14). The formation of polysulfide occurs when el-
6.11
emental sulfur reacts with H2S-laden MDEA solution.
Previously, it was discussed that the overall conver-
sion is decreased with the addition of elemental sulfur
due to the formation of polysulfide in a large number
in comparison with disulfide ions, in which the cata-
lyst cation cannot be transferred easily to the organic
2−
phase. According to Eq. (3), the disulfide (S ) ions
+
2−
+
form ion pair with two catalyst cations (Q S Q ) in
the aqueous phase and transfer to the organic phase.
Development of a mathematical model based on the
mechanism proposed is quite difficult because of the
complexities involved in the current reaction as the
selective reaction of dinitro compound gives us two
products, that is, 4A2NT and 2A4NT. So in our cur-
rent study, an empirical kinetic model has been devel-
oped to correlate with the conversion version time data
obtained experimentally. In our current study, the reac-
tion order with respect to catalyst, reactant, and sulfide
concentration has been evaluated and based on these
the rate of the reduction reaction of 2,4-DNT (–rR) is
expressed by Eq. (18)
0
.47 1.93 0.64
0.47 1.93 0.64
−
rR = k1C
C
C
+ k2CR
C
C
C
CE
R
S
C
S
(18)
where CR and Cc are the concentration of 2,4-DNT
and catalyst (TBPB) in the aqueous phase. Owing to
the formation of elemental sulfur in a reaction medium,
the curve is of “S” nature and it is defined by the second
term of the reaction rate (Eq. (18)). The concentration
of sulfide (CS) and elemental sulfur (CE) in the aqueous
phase is derived from the overall mass balance based
on the stoichiometry of Eq. (2), and these are shown in
the following expressions:
Figure 15 Arrhenius plot of (a) ln (rate constant, k1) versus
1/T and (b) ln (rate constant, k ) versus 1/T. [Color figure can
2
be viewed at wileyonlinelibrary.com]
constants k1 and k2 were calculated at different tem-
peratures by minimizing the objective function (E) as
given by the following equation:
CS = CSO − 3f (CRO − CR)
(19)
(20)
CE = f (CRO − CR)
ꢀn ꢁꢂ
ꢃ
ꢂ
ꢃ ꢄ
2
E =
(−r)pred i − (−r)expt
(21)
i
i=1
where CSO and CRO are the concentration of the initial
sulfide concentration in the aqueous phase and 2,4-
DNT concentration added in the organic phase, f is the
volume ratio between the organic and aqueous phase
and parameter estimation is done by a nonlinear re-
gression algorithm. The optimum values of the rate
In Table III, the calculated values of k1 and k2 are
listed. From the slope of the Arrhenius plots shown in
Figs. 15a and 15b, the activation energies are calculated
for both rate constants (k1 and k2) and it is 75.65 and
48.72, respectively.
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

1
4
MONDAL AND SEN
cessfully validated against the experimental data. The
activation energy was found to be 46.25 kJ/mol from
experimental findings.
Dr. Mondal is thankful for a doctoral fellowship from Min-
istry of Human Resource and Development (MHRD), India
during the tenure of the work.
NOMENCLATURE
aq
QX
C
QX concentration in the
aqueous phase, kmol/m3
QX concentration in the or-
ganic phase, kmol/m3
QHS concentration in the or-
ganic phase, kmol/m3
org
QX
C
Figure 16 Comparison between calculated conversion and
experimental conversion of 2,4-DNT at different tempera-
tures after 60 min of reaction. [Color figure can be viewed at
wileyonlinelibrary.com]
org
QHS
C
aq
QHS
C
QHS concentration (also
+
−
[
Q
HS ]a) in the aqueous
3
phase, kmol/m
Kinetic Model Validation
aq
QSQ
C
QSQ concentration in the
aqueous phase, kmol/m3
QSQ concentration in the or-
ganic phase, kmol/m3
QS2Q concentration in the
aqueous phase, kmol/m3
From the regression model, the predicted reaction rate
is evaluated and based on the predicted rates a com-
parison study between the predicted conversion versus
experimental conversion is plotted in Fig. 16. Excel-
lent agreement was observed between the predicted
and experimental conversion.
org
QSQ
C
C
C
aq
QS2Q
org
QSHO3
QSHO3 concentration (also
+
−
Q SHO3 ) in the aqueous
3
phase, kmol/m
CONCLUSIONS
aq
QSHO3
C
QSHO3 concentration in the
3
aqueous phase, kmol/m
The current study shows the novelties of L–L PTC in
the selective reduction of 2,4-DNT to the correspond-
ing amino compound by the novel Zinin reagent, H2S-
laden MDEA under milder reaction conditions, thereby
eliminating the use of a costly metallic catalyst, high-
temperature, and high-pressure reaction. As the reac-
tion was very fast, 100% conversion was achieved in
a very short time at room temperature (303 K). Se-
lective reduction leads to the formation of two mono-
nitro amine isomers: 2A4NT and 4A2NT. Selectivity
of 4A2NT was found to be higher than 2A4NT. The
detailed parametric study suggested that with the in-
crease of temperature, catalyst concentration, sulfide
ion concentration, MDEA concentration, the selectiv-
ity of 4A2NT decreases and that of 2A4NT increases.
The highest selectivity of 4A2NT was observed as
aq
S2O3
2−
C
S2O3 concentration in the
3
aqueous phase, kmol/m
aq
S2
2
− 2
C
C
C
[S ] concentration in the
3
aqueous phase, kmol/m
org
ArNO2
ArNO2 concentration in the
3
organic phase, kmol/m
∗
Catalyst (QX) concentration
initially fed to the aqueous
QX
3
phase, kmol/m
C
C
QT
Total catalyst Q concentra-
tion in the organic phase,
3
kmol/m
∗
Total reagent (ArNO2) con-
centration added in the or-
ArNO
2
3
ganic phase, kmol/m
aq
QSHO
C
82.26% when the reactant concentration in the reactor
3
3
K1 = Corg
3
QSHO
was 1.0981 kmol/m . A suitable reaction mechanism
has been proposed, and a kinetic model has been de-
veloped based on the mechanism. The kinetic model
based on the first-order approximation has been suc-
aq
aq
C
C
k1
QHSO3
S2O3
org
QS2Q
K2 = k
ꢁ
=
aq
QHSO
aq
S
2
C
1
C
C
3
aq
QX
org
C
K3
C
QX
International Journal of Chemical Kinetics DOI 10.1002/kin.21136

HIGHLY SELECTIVE ROOM TEMPERATURE MONOREDUCTION OF DINITRO-ARENES
15
org
C
10. Noringt, J. O. N. E.; Fletchers, E. A. Energy 1982, 7,
QHS
K4
K5
aq
C
HS
651–666.
org
C
C
C
C
QSQ
oq
11. Groisil, M.; Ibrahim, S.; Gupta, A. K.; AlShoaibi, A.
QSQ
Energy Proc. 2015, 75, 3066–3070.
org
QS
aq
QS
Q
2
2
K6
12. Kohl, A.; Nielsen, R. Gas Purification; Gulf Publishing
Houston,TX, 1997.
3. Lawson, J. D.; Garst, A. W. J Chem Eng Data 1976, 21,
0–30.
4. Isaacs, E. E.; Otto, D.; Alan, E. Engineering 1980, 118–
20.
Q
:
Kapp
Apparent first-order reaction
rate constant, m /(mol of
1
1
1
3
2
catalyst·min)
k 1
Forward
constant
reaction
[m /(mol
rate
of
aqueous
1
3
5. Lee, J. I. I.; Otto, F. D.; Mather, A. E. J Chem Eng Data
1976, 21, 207–208.
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phase
16. Astarita, G.; Savage, D. W.; Bisio, A. Gas Treating with
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17. Maity, S. K.; Pradhan, N. C.; Patwardhan, A. V. Appl
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1
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constant
3
[m /(mol
of
aqueous
catalyst·min)]
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k3
k3
Reaction
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organic phase
Reaction rate
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2
2
2
0. Catino, S. C. Farris, E . Concise Encyclopedia of Chem-
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