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Kinetics of reaction of benzyl chloride with H2S-rich aqueous monoethanolamine selective synthesis of dibenzyl sulfide under liquidЦliquid phase-transfer catalysis.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2011; 6: 257–265
Published online 19 March 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.430
Research Article
Kinetics of reaction of benzyl chloride with H2S-rich
aqueous monoethanolamine: selective synthesis of
dibenzyl sulfide under liquid–liquid phase-transfer catalysis
Sujit Sen, Narayan C. Pradhan* and Anand V. Patwardhan
Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721 302, India
Received 23 June 2009; Revised 18 December 2009; Accepted 18 January 2010
ABSTRACT: The development of viable alternative processes for the conversion of hydrogen sulfide (H2 S) to produce
commercially important chemicals is important in process industries, particularly in refineries handling large quantity
of sour crude. This work was undertaken to synthesize value-added chemicals such as dibenzyl sulfide (DBS) and
benzyl mercaptan (BM) utilizing H2 S from various by-product gas streams. This process is a viable alternative to
the expensive Claus process, which produces only the less valuable elemental sulfur product from H2 S. The reaction
between benzyl chloride (BC) and H2 S-rich aqueous monoethanolamine (MEA) was carried out in an organic solvent,
toluene, using tetra-n-butylammonium bromide as phase-transfer catalyst. Two products, DBS and BM, were identified
in the reaction mixture and both chemicals have many industrial uses. The conversion of BC and the selectivity of DBS,
were maximized by considering the effect of various parameters such as stirring speed, catalyst loading, concentration
of BC, concentration of MEA, concentration of sulfide, and temperature. The highest selectivity of DBS obtained
was about 99% after 480 min of reaction with excess BC at 60 ◦ C. The apparent activation energy for the kinetically
controlled reaction was found to be 51.3 kJ/mol. The MEA/H2 S mole ratio was found to have a significant effect on
the selectivity of DBS and BM.  2010 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: monoethanolamine; hydrogen sulfide; dibenzyl sulfide; benzyl mercaptan; phase-transfer catalysis;
kinetics
INTRODUCTION
Petroleum and natural gas processing industries produce
hydrogen sulfide (H2 S) in one or more gaseous streams.
As H2 S is corrosive to process equipment and a
potential environmental pollutant, it is separated from
the gaseous streams and then converted to harmless
forms. Generally, H2 S from the gaseous streams is
removed through an amine treating unit and then
processed in the Claus unit to produce elemental
sulfur.[1] However, there are several disadvantages of
air oxidation of H2 S to elemental sulfur such as loss of
a valuable hydrogen source, the requirement of precise
air rate control, the removal of trace sulfur compounds
from spent air, and a limit on the concentration of H2 S
in the feed gas stream. Therefore, the development of
a viable alternative process for the conversion of H2 S
to produce commercially important chemicals, is very
much welcome in the process industry, particularly in
*Correspondence to: Narayan C. Pradhan, Department of Chemical
Engineering, Indian Institute of Technology, Kharagpur 721 302,
India. E-mail: ncp@che.iitkgp.ernet.in
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
the refineries handling large quantities of sour crude.
The present work was undertaken to synthesize valueadded chemicals such as dibenzyl sulfide (DBS) and
benzyl mercaptan (BM) utilizing H2 S of various byproduct gas streams.
The DBS finds many applications as additives for
extreme pressure lubricants, anti-wear additives for
motor oils, stabilizers for photographic emulsions, in
refining and recovery of precious metals, and in different anti-corrosive formulations.[2] BM is useful as a raw
material for the synthesis of herbicides in the thiocarbamate family.[3] It is mainly used for the synthesis of
herbicides like esprocarb, prosulfocarb, tiocarbazil, etc.
The preparation of DBS and BM using various types
of reagents and starting materials are well documented.
For example, the kinetics of synthesis of DBS by the
reaction of benzyl chloride (BC) with sodium sulfide
was reported using phase-transfer catalysts (PTCs) in
liquid–liquid and solid–liquid modes[2] and unimpregnated inorganic solid catalyst like basic alumina and
amberlyst A27 (Cl− form) anion exchange resins under
solid–liquid mode.[4] The preparation of DBS from
BC was also reported using polymer-supported sulfide
258
S. SEN, N. C. PRADHAN AND A. V. PATWARDHAN
anions.[5] There are reports in the literature on the preparations of DBS by the reduction of disulfide using zinc
powder in the presence of AlCl3 [6 – 8] and deoxygenation
of sulfoxide using various reducing agents[9 – 11] . However, the reduction of sulfoxides suffers from serious
disadvantages, such as use of expensive reagents, difficult workup of the reaction mixture, harsh acidic conditions, very high reaction temperatures and long reaction
times. In addition, the preparation of DBS by the reduction of the corresponding sulfoxide is impractical as
sulfoxide is usually prepared by the oxidation of the
sulfide. DBS was also reported to be prepared under liquid–liquid–liquid phase transfer catalysis from BC and
aqueous sodium sulfide using tetra-n-hexylammonium
bromide as PTC.[12]
The preparation of BM from BC was also reported
in the literature using various types of reagents such
as methanolic ammonium hydrosulfide (NH4 SH),[3]
aqueous ammonium hydrosulfide,[13] sodium hydrosulfide salt under hydrogen sulfide atmosphere,[14] and
polymer-supported hydrosulfide.[15] BM was also prepared by Pd-catalyzed methanolysis of thioacetates
with borohydride exchange resin.[16] However, use
of industrially relevant reagent, H2 S-rich aqueous
monoethanolamine (MEA), for preparation of DBS and
BM was not reported earlier.
Although both ammonia- and alkanolamine-based
processes are used for the removal of acid constituents
(H2 S and CO2 ) from gas streams, alkanolamine-based
process has received widespread commercial acceptance
as the preferred gas treatment method, because of its
advantages of low vapor pressure (high boiling point)
and ease of reclamation.[1] The low vapor pressure of
alkanolamines can make the operation more flexible,
in terms of operating pressure, temperature, and concentration of alkanolamine, in addition to negligible
vaporization loses. Among the various alkanolamines,
MEA has been used widely because of its high reactivity, low solvent cost, ease of reclamation, low absorption of hydrocarbons, and low molecular weight (which
results in high solution capacity at moderate concentrations). The H2 S-rich aqueous MEA, which could be
obtained from the conventional scrubbing step of the
amine treatment unit, was therefore used in the present
study. Moreover, in this process, the costly regeneration
of the H2 S-rich amine solution can also be avoided.
The applications of PTCs have been discussed in
many reports, mostly from the point of view of
the scientific features and the potential of the catalysis in the field of synthesis of chemicals.[17 – 19]
These catalysts are highly valuable in most heterogeneous chemical processes, including liquid–liquid,
solid–liquid, gas–liquid, solid–liquid–liquid, and liquid–liquid–liquid types of reaction, where more than
one phase are involved. The reaction between two mutually insoluble phases can be promoted by use of PTCs
under mild operating condition to give products of high
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
yield or selectivity. This catalyst is capable of dissolving or extracting the reagent into the organic phase, in
the form of an ion pair, where the reaction with the
substrate takes place.
Organic soluble quaternary ammonium or phosphonium cations were found to be excellent agents for the
transport of anions from aqueous phase to an organic
phase.[20] However, quaternary ammonium salts are
most preferred, for their better activity and ease of availability. Tetra-n-butylammonium bromide (TBAB) has
been reported to be the most active PTC among six
different catalysts used to intensify the reaction of BC
with solid sodium sulfide.[2] The same catalyst, TBAB,
was therefore used in the present study.
Recently, authors of the present work reported
the preparation of DBS and BM from BC using
aqueous ammonium sulfide under liquid–liquid PTC
conditions.[21] The use of aqueous ammonium sulfide
and H2 S-rich aqueous alkanolamines for liquid–liquid
PTC-catalyzed reduction of aromatic nitro compounds
to produce value-added aromatic amines were also documented in the literature by co-authors of the present
work.[22 – 26] Considering the industrial importance of
H2 S capture and making it harmless, the present work
was undertaken to synthesize DBS in high selectivity by
reacting BC with industrially relevant H2 S-rich aqueous MEA in the presence of a PTC, TBAB. Moreover,
a suitable mechanism has been formulated based on
the experimental findings to explain the course of the
reaction.
MATERIALS AND METHODS
Materials
Toluene (≥99%), MEA (≥98%), and BC (≥99%) of
synthesis grade were procured from Merck (India)
Ltd., Mumbai, India. TBAB (≥99%) was obtained
from SISCO Research Laboratories Private Limited,
Mumbai, India. All chemicals were used as such without
further purification.
Experimental set-up
The reactions of BC with H2 S-rich aqueous MEA were
performed batch-wise in a fully baffled mechanically
agitated three necked glass reactor with a capacity
of 250 cm3 (6.5 cm I.D.). The reactor was equipped
with a four-leg vertical baffle and a vertical reflux
condenser. A 2.0-cm diameter six-bladed glass-disk
turbine impeller with the provision of speed regulation,
located at a height of 1.5 cm from the bottom of
the reactor, was used to stir the reaction mixture.
Throughout the course of the reaction, the reactor was
Asia-Pac. J. Chem. Eng. 2011; 6: 257–265
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
KINETICS OF REACTION OF BC WITH H2 S-RICH AQUEOUS MEA
kept immersed in a constant-temperature water bath, the
temperature of which could be controlled within ±1 ◦ C.
products, DBS and BM, used in this study are defined
as the fraction of BC converted to a particular product
divided by the total conversion of BC.
Preparation of H2 S-rich aqueous MEA solution
For the preparation of H2 S-rich aqueous MEA, ∼20
wt% MEA was prepared first by adding a suitable
quantity of MEA in distilled water. H2 S gas was
then bubbled through this aqueous MEA in a 250 cm3
standard gas bubbler. Liquid samples were withdrawn
from time to time after stopping the gas bubbling and
the samples were then analyzed for sulfide content.
The gas bubbling was continued until the desired
sulfide concentration was obtained in the aqueous MEA
solution.
RESULTS AND DISCUSSION
The reactions of BC with aqueous H2 S-rich MEA were
carried out in batch mode both in the absence and in
the presence of PTC. DBS and BM were detected as
the products from the reaction mixture by GLC. No
benzyl alcohol or dibenzyl disulphide was detected in
the reaction mixture even after a batch time of 8 h.
Effect of speed of agitation
Experimental procedure
In a typical run, 50 cm3 of the aqueous phase containing
a known concentration of sulfide was introduced into
the reactor and kept well agitated until the constant
reaction temperature was attained. The organic phase
containing a measured amount of BC, catalyst TBAB,
and solvent-toluene, kept separately at the reaction
temperature, was then charged into the reactor at zero
time. The reaction mixture was then agitated at a
constant speed. Approximately 0.3 cm3 of the organic
layer was withdrawn at a regular interval after stopping
the agitation and allowing the phases to separate.
For any kinetic study, elimination of mass-transfer resistance during the reaction is very important to obtain true
reaction kinetics. To determine the role of mass-transfer
resistance, the effect of stirring speed on the conversion
of BC was studied in the range 1000–2000 rpm under
otherwise identical experimental conditions in the presence of PTC as shown in Fig. 1. As it is evident from
the figure, the variation of conversion of BC with speed
of agitation in the range studied is so small that the reactions may be considered to be free from mass-transfer
resistance. All other experiments were performed at
1500 rpm with negligible effect of mass-transfer resistance on the reaction kinetics.
Analytical technique
All the samples from the organic phase were analyzed by gas–liquid chromatography (GLC) using a
2 m × 3 mm stainless steel column packed with 10%
OV-17 on Chromosorb W(80/100). A gas chromatograph (Chemito Model 8610 GC) interfaced with a
data processor (Shimadzu C-R6A Chromatopac) was
used for the analysis. The column temperature was
programmed with an initial temperature of 150 ◦ C for
1 min, increased at a rate of 20 ◦ C/min up to 300 ◦ C,
and maintained at 300 ◦ C for 4 min. Nitrogen was used
as carrier gas with a flow rate of 20 cm3 / min. An injector temperature of 250 ◦ C was used during the analysis.
An flame ionization detector was used at a temperature
of 320 ◦ C. The products were characterized by GLC
and infrared spectra. The composition of the samples
being analyzed was calculated by direct comparison of
the peak areas against a calibration curve. The initial
sulfide concentrations were determined by the standard
iodometric titration method. The aqueous phase sulfide
concentrations during the reaction were obtained from
the overall mass balance. The term selectivity of the two
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 1. Effect of stirring speed on conversion of BC.
Volume of organic phase = 5.0 × 10−5 m3 ; concentration
of BC = 2.6 kmol/m3 organic phase; concentration of
toluene = 6.6 kmol/m3 organic phase; volume of aqueous
phase = 5.0 × 10−5 m3 ; concentration of catalyst = 1.0 ×
10−1 kmol/m3 organic phase; concentration of sulfide =
1.88 kmol/m3 ; MEA/H2 S mole ratio = 1.74; temperature
= 333 K.
Asia-Pac. J. Chem. Eng. 2011; 6: 257–265
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S. SEN, N. C. PRADHAN AND A. V. PATWARDHAN
Figure 2.
Arrhenius plot. Volume of organic phase
= 5.0 × 10−5 m3 ; concentration of BC = 2.6 kmol/m3
organic phase; concentration of toluene = 6.6 kmol/m3
organic phase; volume of aqueous phase = 5.0 × 10−5 m3 ;
concentration of catalyst = 1.0 × 10−1 kmol/m3 organic
phase; concentration of sulfide = 1.88 kmol/m3 ; MEA/H2 S
mole ratio = 1.74.
Asia-Pacific Journal of Chemical Engineering
Figure 3. Effect of catalyst loading on conversion of BC.
Volume of organic phase = 5.0 × 10−5 m3 ; concentration of
BC = 2.6 kmol/m3 organic phase; volume of aqueous phase
= 5.0 × 10−5 m3 ; concentration of sulfide = 1.88 kmol/m3 ;
MEA/H2 S mole ratio = 1.74; temperature = 333 K; stirring
speed = 1500 rpm.
Effect of temperature
The effect of temperature was studied at five different
temperatures in the range 313–353 K. Initial rate of
reaction of BC was calculated at different temperatures
and an Arrhenius plot of logarithm of initial rate versus
1/T was made as shown in Fig. 2. As observed form the
figure, the rate of reaction of BC increases with increasing temperature as expected. The apparent activation
energy for the reaction of BC was calculated from the
slope of the best fitted straight line as 51.3 kJ/mol. The
observed high apparent activation energy confirms that
the reaction is kinetically controlled.
Effect of catalyst loading
The effect of catalyst (TBAB) loading on the conversion
of BC was studied in the concentration range of
0–0.15 kmol/m3 of organic phase, as shown in Fig. 3.
With increase in catalyst concentration, the conversion
of BC as well as reaction rate increases. Only by
increasing the catalyst concentration, BC conversion of
more than 98% was achieved, whereas it was about
92% without catalyst even after 480 min of reaction
under otherwise identical conditions. Fig. 3 also shows
that over certain concentration of the catalyst, ca
0.10 kmol/m3 of organic phase, the conversion of BC
becomes constant. This could be attributed to interface
saturation, which means that mass-transfer of the active
species into organic phase reaches a maximum value.
The selectivity of DBS increases with increase in
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 4. Effect of catalyst loading on selectivity of DBS.
Volume of organic phase = 5.0 × 10−5 m3 ; concentration of
BC = 2.6 kmol/m3 organic phase; volume of aqueous phase
= 5.0 × 10−5 m3 ; concentration of sulfide = 1.88 kmol/m3 ;
MEA/H2 S mole ratio = 1.74; temperature = 333 K; stirring
speed = 1500 rpm.
catalyst concentration as shown in Fig. 4. Therefore,
the selectivity of BM decreases with catalyst loading.
For liquid–liquid two-phase reactions, the overall rate
of reaction is governed by rate of transportation of
anions from aqueous phase to organic phase. In the
presence of PTC, the transportation of anions (in the
present case S2− and HS− ) is facilitated and the reaction
becomes organic-phase limited. The hydrosulfide (HS− )
and sulfide (S2− ) ions present in the aqueous phase
readily form ion pairs [Q+ HS− and Q+ S2− Q+ ]
Asia-Pac. J. Chem. Eng. 2011; 6: 257–265
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Asia-Pacific Journal of Chemical Engineering
KINETICS OF REACTION OF BC WITH H2 S-RICH AQUEOUS MEA
with quaternary cation [Q+ ], and are transferred to the
organic phase and then react with BC. With increased
catalyst concentration, more amount of [Q+ ]2 S2− ion
pair is formed and transferred to the organic phase
to react with BC to form DBS. The selectivity of
DBS, therefore, increases with increase in catalyst
concentration.
Table 1 shows the enhancement factor, which is the
ratio of rate of reaction in the presence of TBAB to
that in the absence of TBAB for a fixed conversion
of BC (40%), for various catalyst concentrations. The
enhancement factor increases with increasing catalyst
concentration as observed from the table. A maximum
rate enhancement factor of 2.68 was obtained with catalyst concentration of 0.15 kmol/m3 of organic phase.
However, a marginal change in enhancement factor was
observed at conversion levels >80% (2.95 at 90% conversion of BC).
Effect of sulfide concentration
Figure 5 shows the effect of sulfide concentration in
the aqueous phase on the conversion of BC at MEA
concentration of 3.274 kmol/m3 . As evident from the
figure, the conversion of BC increases with increase
in the concentration of sulfides. Keeping all other
conditions fixed, a conversion of more than 98% was
achieved after 480 min of run. However, an opposite
trend was observed (Fig. 6) for selectivity of DBS. A
selectivity of DBS of more than 98% has been obtained
with a sulfide concentration of 0.474 kmol/m3 after
480 min of run under otherwise identical conditions.
Further increase of sulfide concentration results in
decrease of selectivity of DBS as observed from the
figure.
With increase in sulfide concentration, selectivity of
DBS decreases. This can be explained by considering
the fact that although MEA as such does not participate
in the reaction with BC, it does affect the equilibrium
Table 1. Effect of catalyst TBAB loading on the reaction
rate of benzyl chloridea .
Concentration
of TBAB
(×102 kmol/m3 of
organic phase)
0
5
10
15
Reaction rate
(×104 kmol/m3 × s)
Enhancement
factor
7.004
14.095
15.292
18.739
–
2.01
2.18
2.68
a
Matching BC conversion is 40%, Volume of organic phase = 5.0 ×
10−5 m3 ; concentration of BC = 2.607 kmol/m3 organic phase;
concentration of toluene = 6.609 kmol/m3 organic phase; volume
of aqueous phase = 5.0 × 10−5 m3 ; concentration of sulfide =
1.884 kmol/m3 ; MEA/H2 S mole ratio = 1.737.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 5. Effect of sulfide concentration on conversion
of BC. Volume of organic phase = 5.0 × 10−5 m3 ;
concentration of BC = 2.6 kmol/m3 organic phase; volume
of aqueous phase = 5.0 × 10−5 m3 ; concentration of
MEA = 3.27 kmol/m3 ; concentration of catalyst = 9.98 ×
10−2 kmol/m3 organic phase; temperature = 333 K; stirring
speed = 1500 rpm.
Figure 6. Effect of sulfide concentration on selectivity
of DBS. Volume of organic phase = 5.0 × 10−5 m3 ;
concentration of BC = 2.6 kmol/m3 organic phase; volume
of aqueous phase = 5.0 × 10−5 m3 ; concentration of
MEA = 3.27 kmol/m3 ; concentration of catalyst = 9.98 ×
10−2 kmol/m3 organic phase; temperature = 333 K; stirring
speed = 1500 rpm.
among MEA, H2 S, and water, which results in two
active anions, sulfide (S2− ) and hydrosulfide (HS− ), in
the aqueous phase. These two active anions participate
in two different reactions. In the presence of a base
(MEA), the dissociation equilibrium shifts toward more
ionization and the concentration of sulfide ions, relative
to hydrosulfide ions in the aqueous phase, increases as
the MEA concentration increases. Therefore, only by
changing the MEA concentration with constant sulfide
Asia-Pac. J. Chem. Eng. 2011; 6: 257–265
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S. SEN, N. C. PRADHAN AND A. V. PATWARDHAN
Asia-Pacific Journal of Chemical Engineering
concentration in the aqueous phase, it would be easy to
prove the existence of two different reactions.
Effect of MEA concentration
Although MEA, as such, does not participate in the
reaction with BC, it does affect the equilibrium among
MEA, H2 S, and water, which results in two active
anions, sulfide (S2− ) and hydrosulfide (HS− ), in the
aqueous phase. These two active anions participate in
two different reactions. With increase in the concentration of a base, MEA, the dissociation equilibrium shifts
toward more ionization and the concentration of sulfide
ions relative to hydrosulfide ions in the aqueous phase
increases.
To study the effect of MEA concentration, H2 Srich aqueous MEA of different MEA concentrations
(but constant sulfide concentration) was prepared by
taking 30 cm3 of H2 S-rich aqueous MEA (with known
sulfide and MEA concentration) and then adding various
proportions of pure MEA and distilled water to it in
such a way that the total volume became 50 cm3 in all
the cases.
As seen from the Figs 7 and 8, both the conversion
of BC and selectivity of DBS increase with increase
in MEA concentration under otherwise identical experimental conditions. The concentration of sulfide ions
(S2− ) relative to hydrosulfide ions (HS− ) increases as
the concentration of MEA increases for a fixed sulfide
concentration. Thus, with increase in MEA concentration, there is an increase in the conversion of BC via
the transfer of sulfide ions resulting in higher selectivity
of DBS at higher MEA concentration.
Figure 8. Effect of MEA concentration on selectivity of DBS.
Volume of organic phase = 5.0 × 10−5 m3 ; concentration of
BC = 2.6 kmol/m3 organic phase; volume of aqueous phase
= 5.0 × 10−5 m3 ; concentration of sulfide = 1.13 kmol/m3 ;
concentration of catalyst = 1.0 × 10−1 kmol/m3 organic
phase; temperature = 333 K; stirring speed = 1500 rpm.
Figure 9. Effect of BC concentration on conversion of BC.
Volume of organic phase = 5.0 × 10−5 m3 ; concentration
of catalyst = 1.0 × 10−1 kmol/m3 organic phase; volume of
aqueous phase = 5.0 × 10−5 m3 ; concentration of sulfide
= 1.24 kmol/m3 ; MEA/H2 S mole ratio = 2.64; temperature
= 333 K; stirring speed = 1500 rpm.
Effect of concentration of benzyl chloride
Figure 7. Effect of MEA concentration on conversion of BC.
Volume of organic phase = 5.0 × 10−5 m3 ; concentration of
BC = 2.6 kmol/m3 organic phase; volume of aqueous phase
= 5.0 × 10−5 m3 ; concentration of sulfide = 1.13 kmol/m3 ;
concentration of catalyst = 1.0 × 10−1 kmol/m3 organic
phase; temperature = 333 K; stirring speed = 1500 rpm.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
The effect of concentration of BC on its conversion
and selectivity of DBS was studied at three different concentrations as shown in Figs 9–11. The selectivity of DBS increases with increase in the concentration of BC as shown in Fig. 10. Therefore, the
selectivity of BM decreases with the concentration of
Asia-Pac. J. Chem. Eng. 2011; 6: 257–265
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
KINETICS OF REACTION OF BC WITH H2 S-RICH AQUEOUS MEA
With low BC concentration in the organic phase,
almost complete conversion of BC was achieved. This
resulted in very low selectivity of DBS, i.e. high selectivity of BM. With excess BC, higher DBS selectivity was achieved with efficient utilization of sulfide in the aqueous phase although the BC conversion
remained low.
MECHANISM
Figure 10. Effect of BC concentration on selectivity of DBS;
Volume of organic phase = 5.0 × 10−5 m3 ; concentration
of catalyst = 1.0 × 10−1 kmol/m3 organic phase; volume of
aqueous phase = 5.0 × 10−5 m3 ; concentration of sulfide
= 1.24 kmol/m3 ; MEA/H2 S mole ratio = 2.64; temperature
= 333 K; stirring speed = 1500 rpm.
Figure 11. Relationship between conversion of BC and
selectivity of DBS under different BC concentration. All the
conditions are same as in Figure 9 and 10.
Two different mechanisms, extraction and interfacial,
are generally used to explain the liquid–liquid phase
transfer catalysis based on the lipophilicity of PTC
used. The extraction mechanism is useful to explain
the course of the reaction when the PTC is not highly
lipophilic one so that it can distribute themselves
between the organic and the aqueous phase.[17 – 18]
According to this mechanism, the PTC exchanges
anions with hydrophilic reactant in the aqueous phase
and forms a lipophilic ion pair. The active catalysts thus
formed are then transferred to the organic phase and
react with lipophilic reactants there. In the interfacial
mechanism, catalysts remain entirely in the organic
phase because of their high lipophilicity and exchange
anions across the liquid–liquid interface. The reaction
of BC with H2 S-rich aqueous MEA was studied in
presence of TBAB, which is not so highly lipophilic
one and, therefore, the reaction can be represented by
extraction mechanism as shown by Scheme 1.
In the aqueous phase, there exist ionic equilibria
among MEA, H2 S, and water, which result three active
anions: hydroxide (HO− ), hydrosulfide (HS− ), and sulfide (S2− ) as represented by Eqns (1)–(4) in Scheme 1.
These ions are capable of producing the ion pairs (Q+
OH− , Q+ SH− and Q+ S2− Q+ ) with quaternary ammonium cation, Q+ [(n-C4 H9 )4 N+ ]. However, no benzyl
alcohol, C6 H5 CH2 OH (substitution product of BC with
QOH), was identified in the GC analysis from the
two-phase reaction in the presence of TBAB. This is
RNH2 + H2O
H 2S
BC. From the plot of selectivity of DBS versus conversion of BC (Fig. 11), it is seen that there is a
sharp increase of slope of the curve with increase in
the concentration of BC. Because the reaction leading
to the formation of BM is very fast compared with
that of DBS, at low BC concentration there will be
insufficient quantity of BC present to produce DBS,
which results in low selectivity of DBS. It is also
seen from Fig. 9 that with increase in the concentration of BC, the conversion of BC decreases because
of limited quantity of sulfide present in the aqueous
phase.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
RNH3+ + OH− … (1)
H+ + HS− … (3)
HS−
H+ X− + RNH3+OH−
H2O
H+ + HO− … (2)
H+ + S2− … (4)
Aqueous
Phase
RNH3+ X− + H2O
Q+ + X −
QSH
Q+ + HS−
Q+ + S2−
QSQ
Interface
QSH + ΦX
ΦSH + ΦX
ΦSH + QX
ΦSΦ + HX
QX + ΦSQ
QSQ + ΦX
QX + ΦSΦ
ΦSQ + ΦX
R=HOCH2CH2−
Organic
Phase
X = Br/Cl; Φ = C6H5CH2−; Q+= (n-C4H9)4N+
Scheme 1. Mechanism of liquid–liquid phase transfer
catalyzed reaction of BC with H2 S-rich aqueous MEA.
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because of the fact that the active catalyst, QOH, is
more hydrophilic in nature and not easily transferred
to the organic phase[27] and therefore the hydrolysis of
BC under weak alkaline medium of aqueous MEA is
slow.[28] However, only two species (Q+ SH− and Q+
S2− Q+ ) are generated by the phase transfer catalysis.
In some of the research articles, it was reported
that the sulfide ions present in the aqueous phase
are first converted to the hydrosulfide ions (S2− +
H2 O ↔ HS− + OH− ) and are then transferred to
the organic phase via phase transfer catalysis.[29] If
only hydrosulfide ions are transferred, the present
reaction system becomes a case of series reactions
and the DBS should form only by the reaction of
BM with BC. However, it was observed from an
independent experiment with sodium sulfide that no BM
was formed. Also it was observed that with increase in
MEA concentration, the selectivity of DBS increases as
discussed earlier. These observations confirm that both
sulfide and hydrosulfide ions present in the aqueous
phase are simultaneously transferred to the organic
phase in the form of active catalysts, Q+ SH− and Q+
S2− Q+ and react with BC to produce DBS and BM,
respectively.
Figure 12 shows the concentration profile for a typical batch. It is seen from the figure that concentration of
BM reaches a maximum and then falls gradually with
time. Therefore, BM is converted to DBS whose concentration increases with time. Probably, BC reacts with
BM to produce DBS and hydrochloric acid. Because,
the hydrochloric acid (strong acid) is formed from a
weak acid, BM, this reaction is expected to be slow and
is favored only because of the presence of MEA, which
Asia-Pacific Journal of Chemical Engineering
reacts with hydrochloric acid irreversibly to produce
methanolamine hydrochloride in the aqueous phase.
CONCLUSIONS
The reaction of BC with H2 S-rich aqueous MEA is of
great industrial relevance, which could lead to different
products, DBS and BM, of high commercial value. This
reaction has been studied in detail under liquid–liquid
phase transfer catalysis conditions. The various process
parameters (stirring speed, catalyst concentration, reactants concentration, and temperature) have been optimized. The observed variations of conversion of BC
and selectivity of products (DBS and BM) with the
process parameters were used to establish a mechanism with cyclic phase transfer initiation step in the
heterogeneous liquid–liquid system. The reaction has
been found to be kinetically controlled with an apparent
activation energy value of 51.3 kJ/mol. The MEA/H2 S
mole ratio has been found to have enormous effect on
the selectivity of DBS and BM. The higher ratio favors
DBS while the lower ratio favors BM. The change in
the temperature and the catalyst concentration (beyond
a certain value) only changes the reaction rate without
significantly affecting the selectivity. The selectivity of
DBS increases with excess BC in the organic phase
although the conversion of BC remains low. However,
the opposite trend was observed for BM.
The process involves a complex mechanism. The
existence of ionic equilibriums among MEA, hydrogen
sulfide, and water producing sulfide (S2− ) and hydrosulfide (HS− ) ions in the aqueous phase was established.
The two active ion pairs (Q+ S2− Q+ and Q+ SH− )
formed in the aqueous phase are first transferred to the
organic phase and then react with BC to produce DBS
and BM, respectively. The DBS is also formed by the
reaction of BM with BC.
Acknowledgement
Financial support for this work from the Council of
Scientific and Industrial Research (CSIR), New Delhi,
India is gratefully acknowledged.
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Figure 12.
Concentration profile for a typical run.
Volume of organic phase = 5.0 × 10−5 m3 ; concentration
of BC = 2.6 kmol/m3 organic phase; concentration of
catalyst = 9.98 × 10−2 kmol/m3 organic phase; volume of
aqueous phase = 5.0 × 10−5 m3 ; concentration of sulfide
= 1.63 kmol/m3 ; concentration of MEA = 3.27 kmol/m3 ;
temperature = 333 K; stirring speed = 1500 rpm.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
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benzyl, reaction, liquidцliquid, selective, dibenzyl, kinetics, h2s, chloride, phase, synthesis, sulfide, catalysing, transfer, aqueous, monoethanolamine, rich
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