Ks

GC

a

ARRAA

KDLHK

1

waicHctagisft

ssiwimer

h1

Journal of Molecular Catalysis A: Chemical 411 (2016) 78–86

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

jou rna l h om epa ge: www.elsev ier .com/ locate /molcata

inetic investigation on liquid–liquid–solid phase transfer catalyzedynthesis of dibenzyl disulfide with H2S-laden monoethanolamine

aurav Singh, Priya G. Nakade, Pratik Mishra, Preeti Jha, Sujit Sen ∗, Ujjal Mondalatalysis Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela 769008, India

r t i c l e i n f o

rticle history:eceived 6 September 2015eceived in revised form 11 October 2015

a b s t r a c t

An investigation has been done on the utilization of H2S for the synthesis of dibenzyl disulfide(DBDS) using Amberlite IR-400 as a phase transfer catalyst. This involves absorption of H2S in aque-ous monoethanolamine (MEA) followed by reaction of this H2S-laden MEA with organic reactant benzyl

ccepted 12 October 2015vailable online 17 October 2015

eywords:ibenzyl disulfideiquid–liquid–solid phase transfer catalyst

chloride (BC) to yield DBDS under liquid–liquid–solid (L–L–S) phase transfer catalysis condition. Theeffect of various parameters on the conversion of BC was studied and the selectivity of desired productwas 100% at some level of process parameters. A suitable reaction mechanism has been proposed and amathematical model has been developed to explain the kinetics of the reaction. Waste minimization wastherefore affected with the utilization of H2S-laden gas for production of a value-added fine chemical.

ydrogen sulphideinetics

. Introduction

Due to decline in light-to-process crude, refineries around theorld are forced to process heavy crude that contains substantial

mount of organo-sulfur and organo-nitrogen compounds. Typ-cally hydro-desulfurization process converts those organosulfurompounds to form hydrogen sulfide (H2S) and ammonia (NH3).2S is a toxic gas, poisonous even in a very small concentration,

orrosive in presence of water and classified as a hazardous indus-rial waste [1,2]. Lots of work has already been done on removalnd recovery of hydrogen sulfide so far. Vilmain removed hydro-en sulfide using aqueous chlorine solution by reactive absorptionn a mechanically agitated gas- liquid reactor [3]. Copper–zinc oxideupported on mesoporous silica reported good absorption capacityor hydrogen sulfide [4]. Ferrous and alum water were also used forhe removal of hydrogen sulfide [5].

The removal of H2S from various gas streams using aqueousolution of alkanolamines is a commercialized industrial processince early thirties [6]. In refineries and natural gas processingndustries, removal of NH3 and H2S are done by scrubbing with

ater and amine respectively. Once H2S is absorbed, it is converted

nto element sulfur using Claus process, which is a conventional

ethod to produce elemental sulfur from H2S but it is very energyxpensive. Another disadvantage of this method is high productionate of elemental sulfur in comparison to the rate of consumption,

∗ Corresponding author. Fax: +91 661 2462999.E-mail addresses: sensujit@nitrkl.ac.in, sensujit@gmail.com (S. Sen).

ttp://dx.doi.org/10.1016/j.molcata.2015.10.013381-1169/© 2015 Elsevier B.V. All rights reserved.

© 2015 Elsevier B.V. All rights reserved.

so the disposal of elemental sulfur is a severe problem for refiner-ies. Thus, an alternative process to Claus process has been in highdemand for better utilization of hydrogen sulfide. The present workwas undertaken to develop substitute of Claus process. This workdealt with the synthesis of dibenzyl disulfide from hydrogen sulfidein the presence Amberlite IR- 400 as phase transfer catalyst.

Dibenzyl disulfide is very important compound and havingvery diversified applications in the field of organic synthesis. Sev-eral methods have been described for the preparation of organicdisulfides using different reagents and catalysts. Dhar synthesizeddisulfide by alkylation of alkyl halides with tetrathiotungstate andtetrathiomolybdates [7]. Benzyltriethylammonium tetracosathio-heptamolybdate [(C6H5CH2N(Et)3) 6Mo7S24] was found to be agood reagent for the synthesis of disulfide from alkyl halides [8].Disulfides were synthesized by oxidative coupling of thiols in pres-ence of air using Fe (III)/NaI as a catalyst [9]. Thiols were oxidizedinto their corresponding disulfides through oxidative coupling inpresence of potassium phosphate as a catalyst [10]. Disulfides werealso synthesized by oxidative cleavage of aryl or alkyl tert-butylsulfide [11]. Landini and Rolla synthesized primary and secondarydialkyl and aryl alkyl sulfides by using sodium sulfide in presence ofPhase transfer catalyst (PTC) [12]. In the same way Wang and Tsangprepared symmetrical thioethers using sodium sulphide (Na2S) andn-bromobutane [13]. Sen reported synthesis of dibenzyl sulfide

from benzyl chloride using H2S-rich aqueous monoethanolamine[14] . A few literatures are available on the preparation of sym-metrical thioethers from H2S-laden alkanolamine in presence ofPTC. Na2S was also used for preparation of aliphatic polysulfides inpresence of quaternary onium salt and for reduction of nitroarenes

G. Singh et al. / Journal of Molecular Catal

Nomenclature

MEA MonoethanolamineBC Benzyl chloridePTC Phase transfer catalystH2S Hydrogen sulfideDBS Dibenzyl sulfideDBDS Dibenzyl disulfideS2− Sulfide anionS2−

2 Disulfide anionQSQ Catalyst active intermediateQ2S2 Catalyst active intermediate[

S2−2

]concentration of disulfide anion (kmol/m3)

[Cl−] concentration of chloride anion (kmol/m3)[RCl]org,0 initial concentration of organic substrate in organic

phase (kmol/m3)[RCl]org concentration of organic substrate in organic phase

at any time t (kmol/m3)[Q+]

tottotal concentration of catalyst per total system vol-

ume (kmol/m3)kapp apparent reaction rate constant [min−1]korg overall reaction rate constant [m3/(kmol of catalyst

.min)]KCl equilibrium attachment/detachment constant for

S2−2 ions [m3/(kmol of catalyst min)]

KS equilibrium attachment/detachment constant forCl− ions [m3/(kmol of catalyst min)]

�S fraction of total number of catalyst cation attachedto S2−

2�Cl fraction of total number of catalyst cation attached

to Cl−

�ClS fraction of total number of catalyst cation attachedto both S2−

2 and Cl−

tsr[

toipMemsmfoPeiosasrscs

shows that rate of reaction is practically same in all stirring speeds.

XRCl fractional conversion of reactantt time (min)

o anilines under PTC condition [15,16]. Bandgar reported synthe-is of variety of symmetrical disulfides from aryl alkyl halides byeduction of sulfur with borohydride exchange resin in methanol17].

Phase transfer catalysis is a technique to lead a reaction fromhe reactants present in two mutually insoluble phases under mildperating conditions. It is now commercially mature discipline hav-

ng wide range of industrial applications such as intermediates,erfumes, agrochemicals, pharmaceuticals and polymers [18,19].any organic compounds have been synthesized under the pres-

nce of different PTC [20–24]. Sonavane also developed a one-potethod for synthesis of disulfides from the reaction of sulfur with

odium sulfide in the presence of didecyldimethylamonium bro-ide (DDAB) as a PTC [25,26] . Disulfides were also synthesized

rom alkyl halides using thiourea and elemental sulfur in presencef polyethylene glycol (PEG 200) [27]. The conventional solubleTCs are having one disadvantage of separation. Distillation orxtraction can be used for the separation of the catalyst but it mayncrease complexity of the process and may affect purity and costf the product. These drawbacks can be overcome with the use ofolid PTC such as polymeric resins and the phenomenon is knowns liquid–liquid–solid (L–L–S) Tri-phase catalysis. Tri-phase PTC haseveral other advantages over soluble PTC in terms of recovery and

eusability of the catalyst. Amberlite IR-400 resin has been used forometimes as a heterogeneous catalyst for the synthesis of organicompounds [28]. The aim of the work is to utilize H2S to synthe-ize fine chemical dibenzyl disulfide selectively. This work has a

ysis A: Chemical 411 (2016) 78–86 79

great industrial relevance as it could replace the conventional Clausmethod followed in most refineries.

2. Experimental

2.1. Chemicals

Toluene (≥99%), Monoethanolamine (MEA) (≥99%) and BenzylChloride of analytical grade were procured from Rankem (India),New Delhi, India. Amberlite IR-400 (Cl− form) was obtained fromMerck (India) Ltd., Mumbai, India.

2.2. Experimental setup

All the reactions were carried out in a 6.5 cm internal diame-ter, fully baffled mechanically agitated batch reactor of capacity250 cm3. 2 cm diameter six-bladed glass disk turbine impeller withdigital speed regulation system, kept at a height of 1.5 cm from thebottom of the reactor, was used for stirring the reaction mixture.The reactor was kept in an isothermal water bath (±1 ◦C) havingPID controller.

2.3. Preparation of H2S-laden aqueous MEA

30–35 wt.% aqueous MEA solution was prepared by adding ade-quate quantity of raw MEA in measured amount of distilled water.Then H2S gas was bubbled through the aqueous MEA solution in agas bubbler, placed in an ice bath. This gas bubbling process wascontinued till desired sulfide concentration was attained. Sulfideconcentration was estimated using iodometric titration

2.4. Experimental procedure

In each experimental run, 50 cm3 of aqueous alkanolamine solu-tion containing a known concentration of sulfide was taken into the250 cm3 reactor and kept well agitated by stirrer until the steadystate temperature was reached. Then 50 cm3 of the organic phasecontaining measured quantity of organic reactant (benzyl chloride)was dissolved in the solvent (toluene) and the catalyst (AmberliteIR-400) was added into the reactor. The reaction mixture was thenagitated at a constant speed of stirring. Minimal amount of samplefrom the organic phase was withdrawn by micropipette at regulartime intervals after stopping the agitation and allowing the phasesto separate (Scheme 1).

2.5. Organic phase analysis

In the present work, analysis of organic phase was performedon GC (Agilent GC 7890B) by using a capillary column DB-5MS,2 m × 3 mm, coupled with flame ionization detector. The productwas further confirmed by GC–MS (Agilent 5977A).

3. Result and discussion

3.1. Parametric study.

3.1.1. Effect of stirring speed.The speed of agitation was varied from 1000 to 2500 rpm in

both the conditions, with and without catalyst to determine effectof mass transfer resistance of reactants on the reaction rate. Fig. 1

Therefore, it can be assumed that beyond 1500 rpm, increase instirring speed has no influence on reaction rate, so reaction can besafely considered as a kinetically controlled reaction. Thus, the fur-ther synthesis was carried out at 1500 rpm to remove mass transfer

80 G. Singh et al. / Journal of Molecular Catalysis A: Chemical 411 (2016) 78–86

Scheme 1. Overall reaction system.

Fig. 1. Effect of stirring speed on the rate of reaction. Operating conditions: volumeoioM

rto

3

s[Dffa

attllgcDoew

3

mrvrci

a

Fig. 2. Effect of sulfur powder loading on DBDS Selectivity. Operating condi-tions: volume of aqueous and organic phase = 5.0×10−5 m3 each; concentrationof BC = 2.6 kmol/m3 in org. phase; concentration of toluene = 6.6 kmol/m3 in org.

In present work, 100% selectivity of DBDS was observed due toadequate quantity of sulfur powder in aqueous phase giving onlydisulfide S2−

2 anions.

f aqueous and organic phase = 5.0×10−5 m3each; concentration of BC = 2.6 kmol/m3

n org. phase; concentration of toluene = 6.6 kmol/m3 in org. phase; concentrationf catalyst = 0.29 kmol/m3 in org. phase; concentration of sulfide = 2.53 kmol/m3,EA/H2S mole ratio = 2.28; temperature = 323 K; sulfur loading = 1.875 kmol/m3.

esistance. In absence of catalyst the rate of reaction is very low,herefore, further experiments have been performed in presencef catalyst only.

.1.2. Effect of sulfur loadingDisulfide and polysulfide anions can easily be obtained by dis-

olving elemental sulfur powder in S2− anion rich aqueous phase25]. The effect of this elemental sulfur loading on synthesis ofBDS was investigated by adding different concentration of sul-

ur powder in H2S-laden MEA. The color of aqueous MEA changedrom greenish to reddish brown indicating formation of polysulfidenions S2−

x (where x = 1–4) depending on sulfur loading.At low sulfur loading, hydrosulfide (HS−) and sulfide (S2−) ions

re the dominating sulfur species giving BM and DBS respectively ashe product [29]. So the selectivity of DBDS at 0.25 and 0.49 of sulfuro sulfide mole ratio was found to be low and it was almost neg-igible in absence of sulfur loading (Fig. 2). Whereas at high sulfuroading, polysulfide anions S2−

3,4,5 are the dominating sulfur species,iving unwanted products like trisulfidse and polysulfides, whichonsequently gives again low DBDS selectivity. 100% selectivity ofBDS was observed at 1.875 kmol/m3 of sulfur due to the presencef only disulfide anions S2−

2 . So 1.875 kmol/m3 of sulfur was consid-red as an optimum reaction parameter and further experimentsere performed at this condition.

.1.3. Effect of catalyst concentrationThe influence of catalyst loading on BC conversion was deter-

ined both in presence and absence of catalyst keeping othereaction condition constant (Fig. 3). It was noticed that BC con-ersion was only 61% in absence of catalyst even after 480 min of

eaction. A drastic increase in BC conversion was observed withatalyst loading and conversion was observed to increase withncrease in catalyst loading.

In the presence of catalyst, sulfide (S2−) and disulfide(

S2−2

)

nions of aqueous phase react with quaternary ammonium halide

phase; concentration of catalyst = 0.29 kmol/m3 in org phase; concentration ofsulfide = 2.53 kmol/m3; MEA/H2S mole ratio = 2.28; temperature = 323 K; agitationspeed = 1500 rpm.

(Q+) to give catalyst active intermediates QSQ and Q2S2 respec-tively. These catalyst intermediates then migrate from aqueous toorganic phase to react with BC to give DBS and DBDS respectively.With increase in catalyst concentration, more catalyst interme-diates are formed and travel to organic phase giving increasedconversion of BC.

Fig. 3. Effect of catalyst loading on BC conversion. Operating conditions: volume ofaqueous and organic phase = 5.0×10−5 m3each; concentration of BC = 2.6 kmol/m3

in org. phase; concentration of toluene = 6.6 kmol/m3 in org. phase; concentrationof sulfide = 2.53 kmol/m3; MEA/H2S mole ratio = 2.28; temperature = 323 K; sulfurloading = 1.875 kmol/m3; agitation speed = 1500 rpm.

G. Singh et al. / Journal of Molecular Catalysis A: Chemical 411 (2016) 78–86 81

Table 1Effect of catalyst loading on initial reaction ratea.

Concentration of Amberlite IR-400 (kmol/m3 org phase) Initial reaction rate (kmol/m3s) at 5% conversion Enhancement factor

0.00 0.00222 1.00.15 0.00350 1.60.29 0.00485 2.20.44 0.00617

0.58 0.00755

a All other conditions are same as Fig. 3.

Fa

caI

Ieofw

3

otonw

Aafo

3

ocrrpwlt

3.1.6. Effect of sulfide concentrationSulfide concentration of aqueous phase was varied from 1.75 to

2.5 kmol/m3 keeping constant MEA concentration (35 wt.%). Therewas slight decrease in conversion noticed with decreasing sul-

Fig. 5. Effect of Temperature on BC conversion. Operating conditions: vol-ume of aqueous and organic phase = 5.0 ×10−5 m3 each; concentration ofBC = 2.6 kmol/m3 in org. phase; concentration of toluene = 6.6 kmol/m3 in org.phase; concentration of catalyst = 0.29 kmol/m3 in org phase; concentration ofsulfide = 2.53 kmol/m3; MEA/H2S mole ratio = 2.28; sulfur loading = 1.875 kmol/m3;agitation speed = 1500 rpm.

ig. 4. ln (Initial Reaction Rate) vs ln (Catalyst Concentration). All other conditionsre same as Fig. 3.

Table 1 shows enhancement of initial reaction rate at differentatalyst loading as compared to that in absence of Amberlite cat-lyst. Enhancement factor indicates the importance of AmberliteR-400 as a PTC in DBDS synthesis.

In order to calculate order of reaction with respect to AmberliteR-400 catalyst, natural logarithm of initial reaction rate of differ-nt catalyst concentration was plotted against natural logarithmf Amberlite concentration (Fig. 4). The order of reaction, obtainedrom the slope of the plot, was 0.55 which is considered as 1st order

ith respect to catalyst concentration.

.1.4. Effect of temperatureThe effect of temperature on conversion of BC was studied under

therwise similar conditions. The temperature was varied from 303o 333 K keeping other experimental conditions constant. The effectf temperature on conversion of BC is shown in Fig. 5. The expectedature of increased BC conversion with increase in temperatureas noticed according to transition theory.

Initial reaction rate was calculated at different temperature forrrhenius plot. Natural logarithm of initial reaction rate was plottedgainst 1/T (K−1) and apparent activation energy was calculatedrom the slope of the plot (Fig. 6). The apparent activation energyf kinetically controlled reaction was obtained as 56.03 kJ/mol.

.1.5. Effect of benzyl chloride concentrationThe effect of reactant concentration was investigated by varying

rganic reactant concentration (BC) keeping constant sulfide con-entration in aqueous phase i.e., different initial BC/sulfide moleatio. Decrease in BC conversion was noticed with increase in the

atio due to the limitation of sulfide anions present in aqueoushase (Fig. 7). At 1.04 BC/sulfide mole ratio, optimum conversionas obtained but reduced to 94% for 1.39. So it is concluded that

ow initial BC/sulfide mole ratio is preferable for the present worko get maximum conversion.

2.83.4

From the plot of ln(initial rate) vs. ln (concentration of BC)(Fig. 8), the order of reaction with respect to BC concentration wasobtained as 2.44, which is close to 2. Hence the order of reaction issecond order with respect to the concentration of reactant.

Fig. 6. Arrhenius plot of ln (Initial Reaction Rate) vs 1/T. All other conditions aresame as Fig. 5.

82 G. Singh et al. / Journal of Molecular Catalysis A: Chemical 411 (2016) 78–86

Fig. 7. Effect of BC concentration on reactant conversion. Operating conditions:vlm

fi2(

ctc

3

aapaawwwsi

l

Fs

Fig. 9. Effect of sulfide concentration on BC conversion. Operating condi-tions: volume of aqueous and organic phase = 5.0 ×10−5 m3 each; concentrationof BC = 2.6 kmol/m3 in org. phase; concentration of toluene = 6.6 kmol/m3 inorg. phase; concentration of catalyst = 0.29 kmol/m3 in org phase; tempera-ture = 323 K; MEA concentration = 5.77 kmol/m3; sulfur loading = 1.875 kmol/m3;agitation speed = 1500 rpm.

olume of aqueous and organic phase = 5.0 ×10−5 m3 each; concentration of cata-yst = 0.29 kmol/m3 in org phase; concentration of sulfide = 2.53 kmol/m3; MEA/H2S

ole ratio = 2.28; sulfur loading = 1.875 kmol/m3; agitation speed = 1500 rpm.

de concentration but almost 98% conversion was achieved at.50 kmol/m3 sulfide concentration at 480 min of reaction timeFig. 9).

From the plot of ln (initial rate) against ln (initial sulfide con-entration) (Fig. 10), the slope of the linear fit line was found outo be to be 2.14. Since this value is closer to 2, the reaction wasonsidered second order with respect to the sulfide concentration.

.2. Catalyst recovery and reuseAfter the completion of synthesis process, agitation was stopped

nd aqueous and organic phases were allowed to settle down in separating funnel. Organic phase, present on top of aqueoushase, containing product was removed and Amberlite catalyst wasllowed to settle down in aqueous phase. The catalyst present inqueous phase was first recovered using filter paper then it wasashed with acetone first to remove organic traces, then withater and 50% aqueous sodium chloride (NaCl) solution. Afterashing, catalyst was dried at 50 ◦C in oven to remove any adsorbed

ubstance present on it. For each run, recovered catalyst was reusedn kinetic run four times and data obtained were shown in Fig. 11.

From the Fig. 11, it is seen that reusability of Amberlite is excel-ent up to four uses. But in every run, decrease in conversion was

ig. 8. Plot of ln (initial rate) vs. ln (reactant concentration). All other conditions areame as Fig. 7.

Fig. 10. Plot of ln(initial rate) vs. ln(conc. of sulfide). All other conditions are sameas Fig. 9.

Fig. 11. Conversion of BC with the cycle number. Volume of aqueous and organicphase = 5.0 × 10−5 m3 each; concentration of BC = 2.61 kmol/m3; concentration ofcatalyst = 0.29 kmol/m3 org phase; MEA/H2S mole ratio = 2.28; temperature = 323 K;sulfur loading = 1.875 kmol/m3, agitation speed = 1500 rpm

Catal

od

3L

Maalwns(aiTc

3

(raD2faeo

3

p

o

cs

t

a(

a

3

ad(

iQtfei

2

R

It

G. Singh et al. / Journal of Molecular

bserved due to loss of catalyst in aqueous and organic phase anduring catalyst washing process.

.3. Proposed mechanism of synthesis of dibenzyl disulfide under–L–S PTC

The mechanism of the reaction between BC with H2S- ladenEA and sulfur powder in aqueous phase using Amberlite IR-400

s a PTC is shown in Scheme 2. Generally aqueous phase reactionsre faster as compare to organic phase reactions, so an ionic equi-ibrium exists in aqueous phase between RNH2 (MEA)-H2O–H2S

hich results in formation of three active inorganic nucleophilesamely hydroxide (OH)−, sulfide (S2−) and disulfide (S2−

2 ) as repre-ented in aqueous phase. In the present system, hydrosulfide anionHS−) is not expected to stay due to the presence of sulfur powder inqueous phase. Sulfur shifts the ionic equilibrium to the right giv-ng only sulfide (S2−) and disulfide (S2−

2 ) anions in aqueous phase.he product was obtained from both catalytic and non-catalyticontribution.

.3.1. Non-catalytic contributionEthanolamine sulfide ((RNH3)2S) and ethanolamine disulfide

(RNH3)2S2), formed in aqueous phase via Reaction (2) and (3)espectively in Scheme 2, are insoluble in organic phase. So theyre expected to react with BC in aqueous–organic interface to giveBS and DBDS respectively as shown in Reaction (7)–(9). Due tond order reaction rate of nucleophilic reaction, DBS is expected toorm via formation of intermediate product C6H5CH2SNH3R whichgain reacts with BC to give desired DBS. The formed product trav-ls from interface to organic phase from where selective separationf products takes place.

.3.2. Catalytic contributionMass transfer and surface reaction are two important steps in

resence of solid catalyst in L–L–S PTC [30] . Synthesis steps involve:Step 1: Diffusion of aqueous anions S2−and S2−

2 from bulk aque-us phase to PTC.

Step 2: Ion exchange reaction of these nucleophiles with catalystation (Q+) to form catalyst active intermediates QSQ and Q2S2 ashown in Reaction (4)–(6) and leaving Cl− anion.

Step 3: Diffusion of organic reactant BC from bulk organic phaseo PTC.

Step 4: Synthesis reaction of BC at catalyst active site presentt aqueous organic interphase to give desired products (Reaction10)–(12).

Step 5: Diffusion of anion Cl− and product from interface toqueous and organic phase respectively as shown in Scheme 2.

.4. Kinetic modeling

The GC analysis shows that the formation of DBS is very lesss compared to DBDS. This observation can be attributed to lessiffusion of aqueous sulfide (S2−) anion in comparison to disulfideS2−

2

)anion in the aqueous phase and the corresponding insignif-

cant formation of catalyst active intermediate QSQ as compare to2S2 in the ion exchange step. The overall rate of reaction between

he organic substrate RCl and a di-ionic inorganic nucleophile S2−2 to

orm organic product RS2R in the presence of PTC, Q+Cl− then can bexpressed as a function of the concentrations of RCl andQ+S2−

2 Q+,.e.,

RCl + Q+S2−2 Q+ → RS2R + 2Q+Cl− (13)

ate, −rorg = − d[RCl]org

dt= korg[RCl]2

org

[Q+S2−

2 Q+]s

(14)

n the present work, a rigorous model based on the modification ofhe Langmuir–Hinshelwood/Eley–Rideal mechanism will be devel-

ysis A: Chemical 411 (2016) 78–86 83

oped and experimental data from the reaction between benzylchloride in the organic phase and disulfide in the aqueous phaseto yield dibenzyl disulfide will be used to verify it. This reactionsystem belongs to a general class of esterification reactions wherethe nucleophile is extracted from the aqueous phase using a phasetransfer catalyst. The reaction is mediated by polymer-supportedquaternary ammonium chloride and conducted in the batch slurrymode.

The whole reaction can be compared to the Eley–Rideal reac-tion mechanism [30], which includes reaction between an adsorbedreactant with an un-adsorbed reactant from the bulk phase. Here,the ion-exchange step can be considered as the adsorption of firstreactant to convert inactive sites into active sites, and in organicphase reaction step, second reactant reacts with adsorbed reactantto yield desired product RS2R.

We assume that the reaction mechanism consist of an ion-exchange reaction step between S2−

2 and Q+X− to form an activesite, Q+S2−

2 Q+ followed by reaction of RCl at this site to form afinal product, RS2R, and an inactive site, Q+Cl−. These steps maybe described as follows:

Ion Exchange step:

2(

Q+Cl−)s+

(S2−

2

)aq

↔(

Q+S2−2 Q+)

s+ 2

(Cl−

)aq

(15)

Organic phase reaction step:(

Q+S2−2 Q+)

s+ 2(RCl)org ↔ 2

(Q+Cl−

)s+ (RS2R)org (16)

The reversible ion-exchange step may be compared to theLangmuir–Hinshelwood adsorption/desorption mechanism. Wecan express the Eq. (15) using traditional notations of heteroge-neous catalysis as,(S2−

2

)+ 2

(Q+Cl−

)↔ 2(Cl−) +

(Q+S2−

2 Q+)(17)

where Q+ is catalyst’s cation.Assuming the formation of transitional site Cl−Q+S2−

2 Q+Cl−

between the forward and backward reaction steps, the whole reac-tion can be written as,(S2−

2

)+ 2

(Q+Cl−

)↔ Cl−Q+S2−

2 Q+Cl− ↔ 2(

Cl−)

+(

Q+S2−2 Q+)

(18)

We can split the ion exchange step as two separate equilib-rium attachment/detachment steps. The attachment/detachmentof S2−

2 anion on the inactive site of Q+Cl− can be seen in the forwardreaction step as,

2(

Q+Cl−)s+

(S2−

2

)aq

↔ Cl−Q+S2−2 Q+Cl− (19)

Similarly, attachment/detachment of Cl− anion on an active site ofQ+S2−

2 Q+ can be seen in the backward reaction step as,(

Q+S2−2 Q+)

s+ 2

(Cl−

)aq

↔ Cl−Q+S2−2 Q−Cl− (20)

Assuming the rates of attachment/detachment are in equilib-rium, Eqs. (21) and (22) can be obtained from Eqs. (19) and (20),respectively as,

�ClS = KS[

S2−2

]aq

(1 − �S − �ClS

)(21)

�ClS = KCl

[Cl−

]2

aq

(1 − �Cl − �ClS

)(22)

where, KS and KCl are the equilibrium attachment/detachment con-2− − [

2−] [ −]

stants for S2 and Cl anions, respectively; S2 and Cl are the

concentrations of S2−2 and Cl− anions in the aqueous phase, respec-

tively; and �S, �Cl, �ClS are the fractions of total number of triphasecatalyst cations attached to S2−

2 , Cl− and both S2−2 and Cl− anions,

respectively.

84 G. Singh et al. / Journal of Molecular Catalysis A: Chemical 411 (2016) 78–86

sis of

sttnst

ts

�

Scheme 2. Proposed mechanism of synthe

Mathematically, this can be written as: �Cl + �ClS + �S = 1, whichignifies that the sum total of all the fractions of active and inac-ive sites of the catalyst equals to unity. It is assumed that once,ransition sites Cl−Q+S2−

2 Q+Cl− are formed, they are instanta-eously transformed either into active sites Q+S2−

2 Q+ or inactiveitesQ+Cl−. At any instant of time there is very less fraction ofransition site

(�ClS

)present in the reaction mixture as compare

o active and inactive sites(� and �

)but cannot be neglected

Cl S

o, �Cl + �S ≈ 1. Eqs. (21) and (22) can be re-written as

S = KS[

S2−2

]aq

(1 − �S − �Cl

)(23)

DBDS by H2S-laden MEA under L–L–S PTC.

�Cl = KCl

[Cl−

]2

aq

(1 − �S − �Cl

)(24)

We have obtained a hyperbolic equation for the fraction of activePTC sites by combining the expressions for �S and �Cl as

�S =KS

[S2−

2

]aq

1 + KCl

[Cl−

]2

aq+ KS

[S2−

2

]aq

(25)

Catalysis A: Chemical 411 (2016) 78–86 85

Wt

[

w

ar

b

−q)

−

Lb

X

wbw

−

=

=

w

io

o

3

vtkccat

Fig. 12. Validation of the kinetic model with experimental data at differ-ent temperature. Volume of organic phase = 5.0 × 10−5 m3, volume of aqueousphase = 5.0 × 10−5 m3, concentration of toluene = 6.6 kmol/m3 in org. phase, concen-tration of BC = 2.6 kmol/m3 in org. phase, concentration of catalyst = 0.29 kmol/m3

org. phase; sulfide conc. = 2.53 kmol/m3, MEA/H2S mole ratio = 2.28, sulfur load-ing = 1.875 kmol/m3, stirring speed = 1500 rpm.

Table 2Apparent rate constants (kapp) at different temperaturesb.

Temperature (◦C) 30 40 50 60kapp (min−1) 0.01673 0.03276 0.05033 0.10171b All the considerations are same as mentioned in Fig. 12.

G. Singh et al. / Journal of Molecular

e can also write above equation in terms of catalyst concentra-ion,

Q+S2−2 Q+]

=[Q+]

tot

KS[S2−

2

]aq

1 + KCl[Cl−]2aq + KS

[S2−

2

]aq

(26)

here,[

Q+]tot

and[

Q+S2−2 Q+]

are the total concentrations of cat-

lyst and the concentration of catalyst attached to S2−2 anions,

espectively.We obtain Eq. (27) for the rate of the organic reactions by com-

ining Eqs. (14) and (26) as,

rorg = − d[RCl]org

dt= korg[RCl]2

org

[Q+]

tot

KS[

S2−2

]aq

1 + KCl

[Cl−

]2

aq+ KS

[S2−

2

]a

(27

d[RCl]org

dt= kogr[RCl]2

org

[Q+]

tot

Ks[

S2−2

]aq

1 + KCl

[Cl−

]2

aq+ Ks

[S2−

2

]aq

(28)

et us introduce fractional conversion of reactant (XRCl), which cane calculated as

RCl = [RCl]org,0 − [RCl]org

[RCl]org,0(29)

here, [RCl]org,0and [RCl]org represents initial concentration ofenzyl chloride and concentration at any time respectively. Nowe can write Eq. (28) in the form of fractional conversion as

rorg = dXRCl

dt= korg[RCl]org,0

[Q+]

tot

KS[

S2−2

]aq

(1 − XRCl)2

1 + KCl

[Cl−

]2

aq+ KS

[S2−

2

]aq(30)

dXRCl

(1 − XRCl)2

= korg[RCl]org,0

[Q+]

tot

KS[

S2−2

]aq

1 + KCl

[Cl−

]2

aq+ KS

[S2−

2

]aq

dt

(31)

dXRCl

(1 − XRCl)2

= kappdt (32)

here kapp = korg[RCl]org,0

[Q+]

tot

KS

[S2−

2

]aq

1+KCl[Cl−]2aq+KS

[S2−

2

]aq

.The terms

n the kapp can be calculated experimentally. Thus, after integrationf Eq. (32) we get,

XRCL

1 − XRCL= kappt (33)

From the Eq. (33) it is clear that the reaction follows secondrder kinetics.

.5. Validation of kinetic model

The kinetic model was validated by considering Eq. (33) wasalid at different temperatures by plotting of XRCl/(1−XRCl) againstime (Fig. 12). The slope of each line gives the apparent rate constant

app at different temperatures as shown in Table 2. Fig. 13 showsomparison of calculated conversions of BC based on app. rateonstants and experimentally obtained conversions of BC. Goodgreement has been observed between calculated and experimen-al conversions.

Fig. 13. Comparison of calculated and experimental BC conversions at 480 min dif-ferent temperatures and all conditions are as same as Fig. 12.

4. Conclusion

A detailed study has been carried out on the synthesis of DBDSfrom BC using Amberlite IR-400 as a solid phase transfer catalyst.It is seen that at different speed of agitation there was no signifi-cant change on the rate of reaction so the reaction was found to bekinetically controlled and all experiments were done at a speed of1500 rpm. The reaction was found to be approximately first order

with respect the concentration of the catalyst, second order withrespect to the reactant concentration and second order with respectto the sulfide concentration. 100% selectivity of DBDS was observedat 1.875 kmol/m3 of sulfur loading. The obtained activation energy,for DBDS synthesis, from Arrhenius plot was 56.03 kJ/mol. Catalyst

8 Catal

wtold

A

ou

A

t0

R

[

[

[[[[[[[

[

[[[[[[

[

[201.

[28] H. Gurusamy, P. Parkunan, K. Madasamy, S. Ponnusamy, Tetrahedron Lett. 56

6 G. Singh et al. / Journal of Molecular

as recovered and reused four times with successive decrease inhe conversion of BC. A mathematical model was developed, basedn kinetic study and proposed mechanism, to account for the calcu-

ation of the rate constant and it was validated using experimentalata.

cknowledgment

We appreciate the technical support provided by Departmentf Chemical Engineering of the institute in identifying the productsing NMR.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.molcata.2015.10.13.

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