Novelties of selective triphasic synthesis of bis-(p-chlorobenzyl) sulfide using hydrogen sulfide and reusable phase transfer catalyst

Article abstract of DOI:10.1016/j.molcata.2016.03.030

The present investigation is based on the advancement in hydrogen sulfide (H₂S) capture and utilization (HSCU). Commercially H₂S is absorbed efficiently using alkanolamines such as methyldiethanolamine (MDEA). Aqueous H₂S-rich MDEA is proposed to act as a sulfiding agent for aromatic halides such as p-chlorobenzyl chloride (p-CBC) to synthesize value-added thioethers. The objective of the present investigation is to synthesize bis-(p-chlorobenzyl) sulfide (BPCBS), a value-added thioether, selectively using p-CBC and H₂S-laden aqueous MDEA. For the immiscible bi-phasic system, reusable solid phase transfer catalyst, polymer-bound tributylmethylammonium chloride (PBTBMAC) was employed under liquid-liquid-solid (L-L-S) mode in the presence of solvent toluene to enhance the reaction rate and product selectivity. Full conversion of p-CBC was obtained with 100% selectivity towards the desired product BPCBS at optimized specific level of process parameters. The catalyst has shown substantial activity even after three times of reuse, which leads to waste minimization and economic benefits. A generalized empirical kinetic model was developed and successfully validated against the experimental results. The triphasic reaction thus leads to process intensification, waste minimization and selectivity enhancement. The process can be utilized as an alternative to many other HSCU processes to utilize the sour gas in synthesizing value-added chemicals.

Full text of DOI:10.1016/j.molcata.2016.03.030

Accepted Manuscript
Title: Novelties of selective triphasic synthesis of
bis-(p-chlorobenzyl) sulfide using hydrogen sulfide and
reusable phase transfer catalyst
Author: Preeti Jha Ujjal Mondal Devipriya Gogoi Gaurav
Singh Sujit Sen
PII:
S1381-1169(16)30097-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.03.030
MOLCAA 9829
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
11-2-2016
13-3-2016
20-3-2016
Please cite this article as: Preeti Jha, Ujjal Mondal, Devipriya Gogoi, Gaurav Singh,
Sujit Sen, Novelties of selective triphasic synthesis of bis-(p-chlorobenzyl) sulfide using
hydrogen sulfide and reusable phase transfer catalyst, Journal of Molecular Catalysis
A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.03.030
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Novelties of selective triphasic synthesis of bis-(p-chlorobenzyl) sulfide using
hydrogen sulfide and reusable phase transfer catalyst
Preeti Jha, Ujjal Mondal, Devipriya Gogoi, Gaurav Singh, Sujit Sen*
Catalysis Research Laboratory, Department of Chemical Engineering, National Institute of
Technology Rourkela, Odisha-769008, India
*Corresponding author: Tel: +91-9938246590; Fax: +91-661-2462999; Email: sensujit@nitrkl.ac.in
1

GRAPHICAL ABSTRACT
Highlights





Methyldiethanolamine was used for selective absorption of toxic H2S gas
The product bis-(para-chlorobenzyl) sulfide was synthesized with 100% selectivity.
The green PTC catalyst was regenerated and reused up to 2 cycles.
The catalyst used was found to perform better than other catalysts tested.
A generalized empirical kinetic model was developed and validated.
2

ABSTRACT
The present investigation is based on the advancement in hydrogen sulfide (H2S) capture and utilization
(HSCU). Commercially H2S is absorbed efficiently using alkanolamines such as methyldiethanolamine
(MDEA). Aqueous H2S-rich MDEA is proposed to act as a sulfiding agent for aromatic halides such as p-
chlorobenzyl chloride (p-CBC) to synthesize value-added thioethers. The objective of the present
investigation is to synthesize bis-(p-chlorobenzyl) sulfide (BPCBS), a value-added thioether, selectively
using p-CBC and H2S-laden aqueous MDEA. For the immiscible bi-phasic system, reusable solid phase
transfer catalyst, polymer-bound tributylmethylammonium chloride (PBTBMAC) was employed under
liquid-liquid-solid (L-L-S) mode in the presence of solvent toluene to enhance the reaction rate and product
selectivity. Full conversion of p-CBC was obtained with 100% selectivity towards the desired product
BPCBS at optimized specific level of process parameters. The catalyst has shown substantial activity even
after three times of reuse, which leads to waste minimization and economic benefits. A generalized
empirical kinetic model was developed and successfully validated against the experimental results. The
triphasic reaction thus leads to process intensification, waste minimization and selectivity enhancement.
The process can be utilized as an alternative to many other HSCU processes to utilize the sour gas in
synthesizing value-added chemicals.
Keywords: bis-(p-chlorobenzyl) sulfide, liquid-liquid-solid phase transfer catalyst, hydrogen sulfide,
kinetics, modeling
Nomenclature
MDEA
MEA
TEA
N-Methyldiethanolamine
monoethanolamine
triethanolamine
DEA
diethanolamine
DIPA
diisopropanolamine
3

BC
benzyl chloride
DBS
dibenzyl sulfide
p-CBC
p-CBM
BPCBS
PTC
para- chlorobenzyl chloride
para- chlorobenzyl mercaptan
bis-(para-chlorobenzyl) sulfide
phase transfer catalyst
TBMAC
tributylmethyammonium chloride
1. Introduction
Hydrogen sulfide (H2S) is a very poisonous and corrosive colorless gas. Most of the H2S emission in the
earth’s atmosphere is from the natural sources like volcanoes, sulfur spring, lakes, and saline marsh, etc.
but 10% of the total H2S emission has an anthropogenic source. Petroleum refining and Kraft pulping
industries are the primary sources of H2S emission whereas other industries like natural gas and heavy crude
oil industry, viscose rayon production plant, wastewater treatment plants, and wood pulp industry contribute
some. In the era of human health effects, the concentration of H2S at the level greater or equal to 10.76 ppm
causes conjunctival irritation, a concentration greater than 161.42 ppm or 150 ppm has a paralyzing effect
on olfactory perception and concentration around 286 ppm can cause pulmonary edema [1]. In the
industries, the presence of H2S gas in pipelines may lead to severe corrosion as it reacts directly with the
metals contained in the pipe. Therefore, the prime objective of the investigation is to utilize H2S gas in
synthesizing fine chemicals so that H2S emission can be minimized.
Due to stringent environmental pollution regulations as well as above mentioned reasons, the level
of H2S in the by-product streams, needs to be brought down to the safer limit before releasing it into the
atmosphere. There are various laboratory methods as well as industrial scale based processes available for
H2S recovery and utilization. Claus process is an industrially important process that produces elemental
sulfur from H2S gas. But the process is accompanied by many disadvantages [2,3]. There are some
4

processes corroborated in the literature for the removal of H2S gas via absorption. H2S is soluble in various
liquids including water, ammonia, various alkanolamines like monoethanolamine, diethanolamine,
diglycolamine, and N-methyldiethanolamine (MDEA). The removal and recovery of hydrogen sulfide
(H2S) from the streams by aqueous ammonium hydroxide solution are well documented [4,5]. Ammonium
hydroxide was used by Hamblin in 1973 for the removal for removal of H2S from the gas stream.
Triethanolamine (TEA), monoethanolamine (MEA) [6], diethanolamine (DEA) [7], Diisopropanolamine
(DIPA) [8] and N-methyldiethanolamine (MDEA) [9] are the alkanolamines that have been used for H2S
absorption in industry. Now a days MDEA is replacing the other absorbent because of its better selectivity
towards the H2S in presence of CO2 [9]. Therefore, the present work has been carried out using MDEA for
the absorption of H2S gas, and H2S-laden MDEA has been used for the synthesis of value-added fine
chemicals like bis-(p-chlorobenzyl) Sulfide.
Thioethers like dibenzyl sulfide (DBS) and bis-(p-chlorobenzyl) sulfide (BPCBS) are some of the
fine chemicals that can be synthesized using H2S-laden amines. These fine chemicals have found a wide
range of applications like use 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
anticorrosive formulations [10]. DBS can be oxidized to prepare some useful synthetic intermediates like
dibenzyl sulfoxide and dibenzyl sulfone formulations [11]. Synthesis of DBS and BPCBS in L-L and S-
LPTC mode using sodium sulfide has been done by using benzyl chloride and p-chlorobenzyl chloride (p-
CBC) respectively [12]. Kinetic study of DBS has also been done using solid sodium sulfide in S-LPTC in
the presence of solid catalysts, basic alumina and Amberlyst A27 (Cl− form) anion exchange resin [10].
DBS has been synthesized from benzyl chloride using tetrahexylammonium bromide as phase transfer
catalyst (PTC) [13]. DBS has been successfully synthesized by reducing corresponding disulfides in the
presence of Zn powder and AlCl3 in aqueous media [14-15]. Reduction of sulfoxide using Al-NiCl2-6H2O
as reducing agents was also performed for the synthesis of DBS [16]. But due to long reaction time,
5

expensive reagents and extreme reaction conditions, the reduction is not found to be an economical method
[16].
All of the catalysts used so far for the synthesis of thioethers like DBS are soluble in nature, and it
is, therefore, tough to reclaim back and reuse. In the case of L-L PTC, disposal of the used catalyst is also
an issue because of its hazardous nature. So, in the field of utilization of hydrogen sulfide for the synthesis
of thioethers like BPCBS, the use of a reusable polymer-supported catalyst is still awaited. In this case,
polymer-supported PTC is a ‘green’ option because it is insoluble and therefore easy to reclaim and
environment-friendly. In this type of PTC (also known as triphase L-L-S catalyst), the catalyst is
impregnated with some polymers like polystyrene and polyacrylamide. The support can either be soluble
or insoluble as well [17]. Ambelite IRA-400 has been used as L-L-S PTC for the synthesis of dibenzyl
disulfide by Singh et al. [18].
Biphasic reactions have always been very difficult to carry out because of their immiscible nature.
The reaction between two immiscible phases can be enhanced with the aid of a phase transfer catalyst (PTC)
that can act as a carrier of ions between both the phases. PTC has got worldwide acceptance because of its
effectiveness, mild operating conditions, and versatility in the field chemical reaction engineering. PTC is
now being utilized for the synthesis of many value-added chemicals. Ido and Susaki studied the effect of
the third phase in a phase transfer catalytic system where tetrahexylammonium bromide acts as a PTC and
BC as a reactant [19]. Wang used tetrabutylammonium bromide as a PTC in higher concentration to form
a third phase and its effect on the rate of the reaction was studied [20].
The synthesis of thioethers like BPCBS demands two reactants - H2S-laden aqueous alkanolamine
and p-chlorobenzyl chloride (organic). This reaction system is a biphasic one that needs a good solvent and
a suitable phase transfer catalyst. According to Stark’s mechanism, a quaternary onium halide dissolved in
the aqueous phase (Q+X-), undergoes anion exchange with the anion (Y-) of the organic phase reactant
(MY). The ion-pair formed (Q+Y-) crosses the liquid-liquid interface due to its lipophilic nature, diffuses
from the interface into the organic phase (phase transfer step). Now in the organic phase, the anion (Y-) of
6

the ion-pair, being nucleophilic in nature, undergoes a nucleophilic substitution reaction with the organic
reagent (RX) forming the desired product (RY). The catalyst subsequently recycles back to the aqueous
phase and the cycle continues [21]. The reaction rate is a function of various parameters like organic group
structure, the activity of the leaving group X-, nucleophilicity of the displacing group Y-, relative ease of
transfer of X- and Y- between the phases, solvent types, reagent concentrations, agitation intensity,
temperature, and catalyst structure. For triphase catalysis (TPC), the catalyst structure is more accountable
for the determination of the rate of the reaction than in any other phase transfer catalysis.
The mechanism for polymer-supported catalyst was firstly developed and explained by Desikan
and Doraiswamy [22]. Contrary to the assumption made in Stark’s mechanism (the catalyst moves freely
in both the phases), in this mechanism, the movement of polymer-supported catalyst was stated to be
confined in the interphase. Therefore, organic and aqueous phase need to be brought into contact with the
catalyst in a sequential manner. The reaction takes place through the attachment and detachment of ions of
aqueous and organic phase on the active sites of the catalyst that is better explained in the diagram of the
reaction mechanism of TPC system (Fig.1).
The reaction occurs in different steps which comprise of diffusion, attachment, detachment and again
diffusion of ions [23].
1. Nucleophile from the aqueous phase (HS- and S2-) travels from bulk to the vicinity of the inactive site of
the catalyst through diffusion.
2. Attachment followed by ion-exchange reaction between the nucleophile and the inactive site of the
catalyst leaving the catalyst site active (Q+HS-) for further ion exchange. (The catalyst is marked as
inactive when attached to its original anion).
3. Diffusion of the organic substrate (RX) from bulk to the active site of the catalyst, followed by ion
exchange to form the product (RSH). During this ion-exchange, the inactive form of the catalyst also
gets regenerated to Q+X-.
7

2. Experimental
2.1.Chemicals
Toluene (>99%), MDEA (>99%) and p-CBC (>99%), all of analytical reagent grade, were procured
from Sigma-Aldrich, Mumbai, India. The catalyst tributylmethylammonium chloride polymer-bound is
procured from Fluka, Switzerland. Ferrous sulfide (FeS) stick, used for H2S production, has been purchased
from Rankem, India.
2.2.Experimental set-up
All the experiments have been performed in fully baffled three-necked 150 cm3 glass reactor, which
was connected to a direct-drive digital stirrer with a six-bladed glass turbine impeller and a reflux
condenser. The whole reaction set-up was assembled in an isothermal water bath (±1oC) having PID
temperature controller.
2.3.Preparation of H2S-laden MDEA
35 wt. % of aqueous MDEA was prepared as an optimum concentration for MDEA by dissolving
sufficient raw MDEA in Millipore ultrapure water in an ice bath. H2S gas was bubbled through the aqueous
MDEA solution kept in a gas bubbler. The gas bubbler was kept immersed in an ice bath to prevent
oxidation of sulfide ion. The process of chemisorption was continued until the desired concentration of
sulfide or desired sulfide/MDEA mole ratio, ideal for sulfidation reaction, has been achieved. The
concentration of sulfide was estimated by iodometric titration method [21].
2.4.Experimental procedure
The measured quantity of catalyst was first introduced into the reactor, followed by 25 cm3 of
aqueous sulfide solution of known sulfide concentration. The organic phase, prepared by dissolving p-CBC
8

in the solvent toluene in a definite proportion, was then poured into the three-necked batch reactor, and the
stirrer was turned on at a definite rpm to start the reaction provided the temperature of the water bath reaches
its set point. The samples were collected after certain time interval using micropipette from both the phases
after turning the stirrer off and allowing the phases to separate to stop the reaction between the phases.
2.5.Sample analysis
Samples from the organic phase were analyzed in Agilent GC-MS, (Model 7890B/5977A) to
identify the products using a capillary column DB-5MS, 2m × 3m. GC with FID detector was used for the
quantification of all the reactants and products.
3. Result and discussion
Two products were identified namely BPCBS and p-chlorobenzyl mercaptan (p-CBM) by GC-MS.
The following sensitivity study was performed by varying the range and levels of different process
parameters as shown in Table 1 and the effect of variations in those parameters on conversion of p-CBC
and selectivity of BPCBS and PCBM was investigated. The optimum values of range and levels were
chosen based on some preliminary studies [10,13,25]. The process parameters that can affect the kinetics
of reactions are reaction time, stirring speed, p-CBC concentration, sulfide concentration, MDEA
concentration, catalyst concentration, different PTC catalysts, and temperature.
3.1 Proposed Mechanism
In this investigation, polystyrene supported tributylmethylammonium chloride catalyst has been
used which can be regenerated using NaCl solution. H2S-laden MDEA solution with the known
concentration of sulfide and p-CBC were reacted.
The whole mechanism of L-L-S was found to be very complex. The reaction among H2S, MDEA
and H2O forms ionic complexes like RNH+HS- and (RNH)2S that react with the organic reactant p-CBC to
form p-CBM as an intermediate product and BPCBS as shown in Scheme 1. The intermediate p-CBM also
9

combines with the excess organic reactant p-CBC to form the final product BPCBS as shown in
stoichiometric Eq. (12).
3.1.1. Reaction in aqueous phase
The aqueous phase contains, MDEA and H2S absorbed in it. The reaction among H2S, MDEA and
H2O forms ions like HS-, S2- and OH- as shown in the Eq. (1), (2), (3)and (4) [18]. These nucleophiles
migrate from bulk of the aqueous phase to the catalytic interface, then attachment of nucleophile occurs at
the inactive site of the catalyst followed by the ion exchange reaction as shown in the reaction Eq. (5)- (7).
This ion exchange reaction is very fast and always considered to be in equilibrium.
3.1.2. Non-catalytic contribution
The presence of the solvent and the rigorous stirring of both phases trigger the onset of the reaction.
The reaction between the organic and inorganic phase in the absence of the catalyst was also found to be
enough to be taken into the consideration (Fig.3). The ionic complex RNH+HS- reacts with the organic
reactant p-CBC to form p-CBM (Eq. (11)), which can again react with the organic reactant p-CBC to form
the final product BPCBS. The S2- plays a significant role in the synthesis of product BPCBS as it forms the
complex (RNH)2S with MDEA that combines with two molecules of p-CBC to form BPCBS. This reaction
is a two steps process as shown in the Eq. (13) and (14). The products p-CBM and BPCBS diffuse back to
the organic phase when reaction stops.
3.1.3 .Catalytic contribution
These nucleophilic ions migrate from bulk of the aqueous phase to the catalytic interface where
attachment of nucleophile to the inactive site of the catalyst occurs, followed by the ion exchange reaction
as shown in the reaction Eq. (5) - (7). This ion exchange reaction step is very fast and so always considered
to be in equilibrium. As the active sites are immobilized on the catalyst itself, reactants sequentially come
in the contact of the catalyst, as shown in the Eq. (8)-(10). The reactant, p-CBC travels to the active site
and ion exchange takes place between to form p-CBM and BPCBS. The whole mechanism is shown in
10

Scheme 1. The overall stoichiometric equation of the reaction between p-CBC and H2S-laden MDEA is
shown as Eq. (15).
(15)
3.2. Sensitivity study
3.2.1 Effect of catalyst concentration
The catalyst concentration was varied from 0.0 to 2.4 x 10-2 kmol/m3 and its effect on p-CBC
conversion, and BPCBS selectivity was plotted against time as shown in Fig.2 (a) and (b). The selectivity
of BPCBS and conversion of p-CBC were found to be 30% and 40% respectively without catalyst and both
of them shoot up to 100% for the catalyst concentration of 2.4 x 10-2 kmol/m3. The S2- and HS- ions form
catalyst active intermediates QSQ and QSH that combine with p-CBC to form BPCBS and p-CBM
respectively. Upon increasing the catalyst concentration, the concentrations of these intermediates increase
and hence the conversion of p-CBC and selectivity of BPCBS also increase [6]. The enhancement of
reaction rate with catalytic loading can also be observed in terms of enhancement factor as shown in Table
1. For minimizing the catalyst consumption for each run, the optimum catalytic concentration was chosen
as 6.0 × 10-3 kmol/m3 for rest of the reactions.
The order of the reaction with respect to the catalyst concentration was calculated by plotting a
graph of ln (initial rate) vs. ln (catalyst concentration). From Fig. 3, the slope of the graph was found to be
0.63 which is the order of the reaction with respect to catalyst concentration.
3.2.2. Comparison with different catalysts
11

For the establishment of the superiority of the catalytic activity of the catalyst, polymer-bound
tributylmethylammonium chloride (PBTBMAC) over the other catalysts, insoluble PTC like Amberlite IR-
400 and soluble PTC like Tetrabutylphosphonium bromide (TBPB) and Ethyltriphenylphosphonium
bromide (ETPB) were chosen. All catalysts were used at same concentration i.e. 6.0 x 10-3 kmol/m3 keeping
all the parameters at optimized values. The best result was observed in the case of PBTBMAC (Fig. 4).
From the past literature, we have found that the activity of the polymer-supported catalyst is greater than
the soluble catalyst due to reasons like increase in its lipophilicity due to its polymer backbone, the
microphase effect of catalyst, and substrate selectivity [23, 26]. For both the soluble catalysts, their
distribution in both the phases is necessary for which again more amount of catalyst is required. But
polymer-supported catalyst PBTBMAC doesn’t get distributed in both of the phases, its active sites which
get activated and deactivated time and again throughout the reaction, are responsible for the reaction. So, it
is effective at very low concentration. Thus, the results imply that the catalyst PBTBMAC is not only
reusable but also gives an excellent conversion of the reactant at remarkably low concentration.
3.2.3. Effect of reactant concentration
The effect of reactant concentration on conversion of p-CBC and selectivity of BPCBS was
observed by varying the p-CBC concentration from 1.6 to 3.9 kmol/m3 as shown in Fig. 5. It is clearly
visible from Fig.5. (a), that conversion of p-CBC decreases with increase in p-CBC concentration. Decrease
in consumption of the reactant p-CBC with more p-CBC is due to the presence of a limited amount of S2-
and HS- ions in the aqueous phase. On the other hand, the selectivity of BPCBS increases with increasing
reactant concentration as the undesired product i.e. p-CBM, recombines with the excess unreacted p-CBC
to form more BPCBS (Fig.5. (b)). For simultaneous optimization of conversion of reactant along with the
selectivity of the product, an intermediate value of p-CBC concentration i.e. 2.4 kmol/m3 was taken for all
other reactions.
12

The order of the reaction with respect to the p-CBC concentration was calculated by plotting a
graph of ln (initial rate) vs. ln (reactant concentration). From the slope of the graph of Fig. 6, the order of
the reaction was found to be 1.58 with respect to catalyst concentration.
3.2.4. Effect of sulfide concentration
The sulfide concentration was varied between 2.0 kmol/m3 to 1.25 kmol/m3 keeping other
parameters at an optimum value. The effect of the sulfide concentration on conversion of p-CBC and
selectivity of BPCBS are shown in the Fig. 7 (a) and (b) respectively. From the graph, it can be interpreted
that conversion of p-CBC decreases with a decrease in sulfide concentration as S2- and HS- ions present in
limiting quantity. Selectivity of the BPCBS was found to decrease with increasing sulfide concentration
because an increase in total sulfide concentration, results in an increase in HS- ion concentration in
comparison to that of S2- ions, that leads to higher rate of formation of p-CBM and not BPCBS [25]. As per
the stoichiometry of Eq. (6)-(9) and (13)-(14) in Scheme 1, S2- is more responsible for the formation of
BPCBS than HS-.
The order of the reaction with respect to the sulfide concentration was calculated by plotting a
graph of ln (initial rate) vs. ln (sulfide concentration). The slope of the straight line in Fig. 8 was calculated
as 1.72, which is the order of the reaction with respect to sulfide concentration.
3.2.5. Effect of MDEA concentration
To study the effect of MDEA concentration on the kinetics of the reaction, MDEA concentration was varied
keeping the sulfide concentration same for each case Fig.9. For this experiment, a fixed H2S-laden MDEA
was taken, and the concentration of MDEA was managed by adding the proper quantity of raw MDEA.
Distilled water was used to make up the volume of aqueous phase if needed.
13

Even though MDEA doesn’t take part in the reaction, the change in concentration of MDEA affects
the conversion of p-CBC as well as the selectivity of BPCBS. By changing the MDEA concentration, the
equilibrium between the MDEA, H2O, S2- and HS- gets affected. When MDEA concentration is increased
keeping H2S concentration fixed, the equilibrium shifts towards the product side according to Le-
Chatelier’s principle, and that results in more RNH+HS- and (RNH)2S. Therefore, the conversion of p-CBC
as well as the selectivity of BPCBS increase due to a rise in S2- ion in comparison to HS- with an increase
in MDEA concentration [13].
3.2.6. Effect of temperature
To study the effect of temperature, four set of experiments have been performed at different
temperatures like 313, 323, 333 and 343 K, keeping the other parameters constant. The graphical
representation of p-CBC conversion and BPCBS selectivity vs. time at different temperatures are shown in
the Fig. 10. From the graph, it is clearly understandable that the conversion (and rate of reaction) increases
with increasing temperature as the number of particles colliding with sufficient energy (i.e. more than the
activation energy) increases by increasing the temperature. But no significant effect on the selectivity of
product BPCBS due to temperature change was observed.
The effect of temperature on chemical reactions is explained in the terms of activation energy
present in the Arrhenius equation. The colliding molecules should have sufficient energy to overcome
the threshold energy i.e. activation energy for a successful reaction. The apparent activation energy was
calculated from the slope of the Arrhenius plot (Fig. 9) as 34.86 kJ/mol.
3.2.7. Effect of stirring speed.
Stirring speed was varied in the range 500 - 2000 rpm keeping other parameters fixed at the
optimized condition and the impact of the variation in the conversion of p-CBC and the selectivity of
BPCBS is shown in Fig. 12. The conversion of p-CBC and the selectivity of BPCBS after 30, 60 and 120
min were plotted against different stirring speed. From the Fig. 12, it is visually clear that role of mass
14

transfer on conversion of p-CBC is negligible in the stirring speed ranges from 1000-2000 rpm. The
conversion of p-CBC as well as the selectivity of BPCBS is comparatively less in 500 rpm than in 1000,
1500 and 2000 rpm. The relation between ln (initial rate of the reaction) and stirring speed is also plotted
in Fig. 12(c) that shows, there is no effect of agitation intensity on the initial rate of reaction. Therefore, to
avoid mass transfer resistance, 1500 rpm was chosen as stirring speed for all above reactions conditions.
3.2.8. Reusability of the catalyst
The major advantages of polymer-supported insoluble catalyst over the other soluble L-L PTC
catalysts are manifold. It is reusable in nature whereas the other L-L PTC are not, so the polymer-bound
catalyst can be used multiple times without much loss of its catalytic activity. It is also environment-friendly
in nature. In the present study, after each run, the catalyst was separated using filtration and regenerated by
washing it with NaCl solution. The reusability of the catalyst was investigated up to two cycles and a little
decrement in the conversion of p-CBC was observed i.e. from 100% for a fresh catalyst to 95% for 1st reuse
and 91% for 2nd reuse. The conversion of p-CBC for 1st and 2nd cycle of reuse is as shown in the Fig.13.
The reduction in the conversion can be attributed to the loss of little catalyst during filtration and
regeneration.
3.3. Kinetic modelling
3.3.1. Development of kinetic model
The model for the synthesis of DBS in L-L mode of reaction is also not found to be applicable in L-
L-S mode of reaction because solid catalyst does not get distributed in both the phases [25]. There is various
literature available for different the type of reaction of L-L [27,28], L-L-L [29,30], L-L-S PTC-driven
reactions [18] where authors have developed and applied their kinetic model. But the model for L-L-S mode
of synthesis of any thioethers or ethers is not available in the literature.
From Fig.2, it can easily be interpreted that non-catalytic contribution in this reaction rate is
unavoidable. The rate equation (Eq. (16)) thus comprises of two terms - first one is accountable for catalytic
reaction and dependents on concentration of reactant (ꢀꢁꢂ), sulfide (ꢀꢃꢂ) and catalyst (ꢀꢄ) and second term
15

is for non-catalytic reaction and dependents on concentration of reactant (ꢀꢁꢅ) and sulfide (ꢀꢃꢅ) only [Eq.
(16)]. Non-catalytic reactions at different temperatures - 313, 323, 333 and 343 K were carried out to
estimate ꢆꢅ from Eq. (16). The effect of temperature change in the catalytic as well as non-catalytic part of
the reaction is shown in terms of Arrhenius plot in the Fig. 15(a) and (b) respectively. Here it was assumed
that the contribution of reactant concentration and sulfide concentration to rate of reaction for non-catalytic
reaction is same as catalytic reaction. Thus, order of reaction with respect to the reactant and sulfide for
catalytic and non-catalytic reaction are assumed to be same.
ꢂ.ꢌꢅ
ꢂ.ꢌꢅ
ꢇꢈ ꢉ ꢆꢂ ꢀꢁꢂꢂ.ꢊꢋ
ꢀ
ꢀꢄꢍ.ꢎꢅ ꢏ ꢆꢅꢀꢁꢂꢅ.ꢊꢋ
ꢀ
(16)
ꢃꢂ
ꢃꢅ
The overall mass balance for sulfide can be obtained from the stoichiometric equations Eq. (8), (9),
(10) and (12) and is given by Eq. (17).
ꢀꢃ ꢉ ꢀꢃꢍ ꢇ 1/2ꢐꢑꢀꢁꢍ-ꢀꢁꢒ
(17)
Where, ꢀꢁꢍ and ꢀꢃꢍ are initial concentration of p-CBC and sulfide and f is the ratio of volume of
organic phase to that of aqueous phase.
A nonlinear regression algorithm is used for the estimation of parameters. The optimum values of
the rate constants, ꢆꢂ and ꢆꢅ was estimated by minimizing the objective function (E) as given by the
following equation:
ꢡ
ꢓ ꢉ ꢛꢢꢂꢔꢕ ꢇꢈꢁ ꢖꢗꢘꢙꢚꢛ ꢇ ꢜꢑꢇꢈꢁꢒꢘꢝꢖꢞꢟꢛꢠꢅ
(18)
∑
ꢑ
ꢒ
The optimum values of rate constant ꢆꢂ and ꢆꢅ of the reaction for different temperatures were
listed the table below:
From its slope of Fig. 15 (a) and (b), the activation energies for catalytic and non-catalytic reaction were
found to be 28.89 kJ/mol and 49.96 kJ/mol respectively, which also shows the effectiveness of the catalyst.
16

3.3.2. Validation of kinetic model
From the regression of the model, the predicted values for reaction rate were found. Using those
values, the predicted conversions are compared with experimentally calculated values of conversion.
Excellent agreement between the predicted and experimental conversion was obtained as shown in the
Fig.16.
4.
Conclusion
Toxic H2S gas has been utilized for the preparation of BPCBS with a final selectivity of 100%
against p-Chlorobenzyl Mercaptan. Polymer-supported tributylmethylammonium chloride was found to be
better than other catalysts like TBPB, Amberlite IRA-400, and ETPB in terms of enhancement of reaction
rate and product selectivity. Also, the catalyst is green and can be reused several times with a little
compromise in its activity. The reaction is found to be more optimum in the sense of selectivity when
agitation speed is 1500 rpm, but the conversion was found to be undisturbed through the range of variation
of agitation speed. The synthesis of BPCBS is a success with minimal side product formation i.e. p-CBM
which was identified by mass spectroscopy. Based on the detailed kinetic study, a general empirical kinetic
model was developed that was found to predict conversions reasonably well. The present process can be
used as a possible alternative to energy-expensive Claus Process for better utilization of downstream toxic
by-product H2S.
Acknowledgement
The financial support from Science and Engineering Research Board (SERB), Department of Science and
Technology (DST) (Grand no. SB/FTP/ETA-202/2012), India is greatly acknowledged. PJ is also grateful
for the fellowship from SERB for her assistantship.
Associated content
Supplementary Information
Condition for GC-FID and GC-MS, Gas Chromatographs and MS spectra.
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17

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19

Fig. 1: The mechanism of L-L-S catalysis
20

(a)
(b)
Fig. 2. The effect of catalyst concentration on (a) conversion of p-CBC and (b) selectivity of BPCBS.
Thermochemical condition: Volume of aqueous and organic phase 2.5 ×10-5 m3 each, temperature: 323
K, MDEA concentration: 3.0 kmol/m3, stirring speed- 1500 rpm, reactant concentration: 2.4 kmol/m3,
sulfide concentration of aqueous phase: 2.0 kmol/m3.
21

Fig. 3. Plot between ln (initial rate) and ln (catalyst conc.) (All conditions are same as mentioned under Fig.
2.)
Fig. 4. Effect of the different catalysts on conversion of p-CBC
Thermochemical condition: Volume of aqueous and organic phase: 2.5 × 10-5 m3 each, MDEA
concentration: 3.0 kmol/m3, temperature: 323 K, stirring speed: 1500 rpm, all catalyst concentration: 6.0 ×
10-3 kmol/m3, reactant concentration: 2.4 kmol/m3, sulfide concentration: 1.29 kmol/m3.
22

(a)
(b)
Fig. 5. The effect of reactant concentration on (a) conversion of p-CBC and (b) selectivity of BPCBS.
Thermochemical condition: Volume of aqueous and organic phase: 2.5 × 10-5 m3 each, temperature: 323 K,
MDEA concentration: 3.0 kmol/m3, stirring speed: 1500 rpm, catalyst concentration: 6.0 × 10-3 kmol/m3,
sulfide concentration: 2.0 kmol/m3.
23

Fig.6. Plot of ln (initial rate) vs. ln (reactant conc.) All conditions are same as mentioned under Fig. 4.
24

(a)
(b)
Fig. 7. Effect of sulfide concentration on (a) conversion of p-CBC and (b) selectivity of BPCBS.
Thermochemical condition: Volume of aqueous and organic phase: 2.5 ×10-5 m3, temperature: 323 K,
MDEA concentration: 3.0 kmol/m3, stirring speed: 1500 rpm, catalyst concentration: 6.0 x 10-3 kmol/m3,
reactant concentration: 2.4 kmol/m3.
25

Fig.8. Plot between ln (initial rate) and ln (sulfide conc.) All conditions are same as mentioned under Fig.
6.
26

(a)
(b)
Fig. 9.The effect of MDEA concentration on (a) conversion of reactant and (b) selectivity of the
product. Thermochemical condition: Volume of aqueous and organic phase: 2.5 ×10-5 m3, temperature:
323 K, stirring speed: 1500 rpm, catalyst concentration: 6.0 × 10-3 kmol/m3, reactant concentration: 2.4
kmol/m3.sulfide concentration: 1.29 kmol/m3.
27

(a)
(b)
Fig. 10. The effect of Temperature on (a) conversion of reactant and (b) selectivity of the product.
Thermochemical condition: Volume of aqueous and organic phase: 2.5 ×10-5 m3 each, MDEA
concentration: 3.0 kmol/m3, stirring speed- 1500 rpm, catalyst concentration: 6.0 x 10-6 kmol/m3,
reactant concentration: 2.4 kmol/m3, sulfide concentration: 1.29 kmol/m3.
28

Fig. 11. Arrhenius plot of ln (rate of reaction) and 1/Temperature. All conditions are same as mentioned
under Fig. 9.
29