Kinetics of reaction of benzyl chloride with H2S-rich aqueous monoethanolamine: selective synthesis of dibenzyl sulfide under liquid–liquid phase-transfer catalysis

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2011; 6: 257–265Published online 19 March 2010 in Wiley Online Library(wileyonlinelibrary.com) DOI:10.1002/apj.430

Research ArticleKinetics of reaction of benzyl chloride with H2S-richaqueous monoethanolamine: selective synthesis ofdibenzyl 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 (H2S) to producecommercially important chemicals is important in process industries, particularly in refineries handling large quantityof sour crude. This work was undertaken to synthesize value-added chemicals such as dibenzyl sulfide (DBS) andbenzyl mercaptan (BM) utilizing H2S from various by-product gas streams. This process is a viable alternative tothe expensive Claus process, which produces only the less valuable elemental sulfur product from H2S. The reactionbetween benzyl chloride (BC) and H2S-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 identifiedin 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, concentrationof BC, concentration of MEA, concentration of sulfide, and temperature. The highest selectivity of DBS obtainedwas about 99% after 480 min of reaction with excess BC at 60 ◦C. The apparent activation energy for the kineticallycontrolled reaction was found to be 51.3 kJ/mol. The MEA/H2S mole ratio was found to have a significant effect onthe 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 producehydrogen sulfide (H2S) in one or more gaseous streams.As H2S is corrosive to process equipment and apotential environmental pollutant, it is separated fromthe gaseous streams and then converted to harmlessforms. Generally, H2S from the gaseous streams isremoved through an amine treating unit and thenprocessed in the Claus unit to produce elementalsulfur.[1] However, there are several disadvantages ofair oxidation of H2S to elemental sulfur such as loss ofa valuable hydrogen source, the requirement of preciseair rate control, the removal of trace sulfur compoundsfrom spent air, and a limit on the concentration of H2Sin the feed gas stream. Therefore, the development ofa viable alternative process for the conversion of H2Sto produce commercially important chemicals, is verymuch welcome in the process industry, particularly in

*Correspondence to: Narayan C. Pradhan, Department of ChemicalEngineering, Indian Institute of Technology, Kharagpur 721 302,India. E-mail: [email protected]

the refineries handling large quantities of sour crude.The present work was undertaken to synthesize value-added chemicals such as dibenzyl sulfide (DBS) andbenzyl mercaptan (BM) utilizing H2S of various by-product gas streams.

The DBS finds many applications as additives forextreme pressure lubricants, anti-wear additives formotor oils, stabilizers for photographic emulsions, inrefining and recovery of precious metals, and in differ-ent anti-corrosive formulations.[2] BM is useful as a rawmaterial for the synthesis of herbicides in the thiocar-bamate family.[3] It is mainly used for the synthesis ofherbicides like esprocarb, prosulfocarb, tiocarbazil, etc.

The preparation of DBS and BM using various typesof reagents and starting materials are well documented.For example, the kinetics of synthesis of DBS by thereaction of benzyl chloride (BC) with sodium sulfidewas reported using phase-transfer catalysts (PTCs) inliquid–liquid and solid–liquid modes[2] and unimpreg-nated inorganic solid catalyst like basic alumina andamberlyst A27 (Cl− form) anion exchange resins undersolid–liquid mode.[4] The preparation of DBS fromBC was also reported using polymer-supported sulfide

2010 Curtin University of Technology and John Wiley & Sons, Ltd.Curtin University is a trademark of Curtin University of Technology

258 S. SEN, N. C. PRADHAN AND A. V. PATWARDHAN Asia-Pacific Journal of Chemical Engineering

anions.[5] There are reports in the literature on the prepa-rations of DBS by the reduction of disulfide using zincpowder in the presence of AlCl3

[6–8] and deoxygenationof sulfoxide using various reducing agents[9–11]. How-ever, the reduction of sulfoxides suffers from seriousdisadvantages, such as use of expensive reagents, diffi-cult workup of the reaction mixture, harsh acidic condi-tions, very high reaction temperatures and long reactiontimes. In addition, the preparation of DBS by the reduc-tion of the corresponding sulfoxide is impractical assulfoxide is usually prepared by the oxidation of thesulfide. DBS was also reported to be prepared under liq-uid–liquid–liquid phase transfer catalysis from BC andaqueous sodium sulfide using tetra-n-hexylammoniumbromide as PTC.[12]

The preparation of BM from BC was also reportedin the literature using various types of reagents suchas methanolic ammonium hydrosulfide (NH4SH),[3]

aqueous ammonium hydrosulfide,[13] sodium hydrosul-fide salt under hydrogen sulfide atmosphere,[14] andpolymer-supported hydrosulfide.[15] BM was also pre-pared by Pd-catalyzed methanolysis of thioacetateswith borohydride exchange resin.[16] However, useof industrially relevant reagent, H2S-rich aqueousmonoethanolamine (MEA), for preparation of DBS andBM was not reported earlier.

Although both ammonia- and alkanolamine-basedprocesses are used for the removal of acid constituents(H2S and CO2) from gas streams, alkanolamine-basedprocess has received widespread commercial acceptanceas the preferred gas treatment method, because of itsadvantages of low vapor pressure (high boiling point)and ease of reclamation.[1] The low vapor pressure ofalkanolamines can make the operation more flexible,in terms of operating pressure, temperature, and con-centration of alkanolamine, in addition to negligiblevaporization loses. Among the various alkanolamines,MEA has been used widely because of its high reactiv-ity, low solvent cost, ease of reclamation, low absorp-tion of hydrocarbons, and low molecular weight (whichresults in high solution capacity at moderate concen-trations). The H2S-rich aqueous MEA, which could beobtained from the conventional scrubbing step of theamine treatment unit, was therefore used in the presentstudy. Moreover, in this process, the costly regenerationof the H2S-rich amine solution can also be avoided.

The applications of PTCs have been discussed inmany reports, mostly from the point of view ofthe scientific features and the potential of the catal-ysis in the field of synthesis of chemicals.[17–19]

These catalysts are highly valuable in most hetero-geneous chemical processes, including liquid–liquid,solid–liquid, gas–liquid, solid–liquid–liquid, and liq-uid–liquid–liquid types of reaction, where more thanone phase are involved. The reaction between two mutu-ally insoluble phases can be promoted by use of PTCsunder mild operating condition to give products of high

yield or selectivity. This catalyst is capable of dissolv-ing or extracting the reagent into the organic phase, inthe form of an ion pair, where the reaction with thesubstrate takes place.

Organic soluble quaternary ammonium or phospho-nium cations were found to be excellent agents for thetransport of anions from aqueous phase to an organicphase.[20] However, quaternary ammonium salts aremost preferred, for their better activity and ease of avail-ability. Tetra-n-butylammonium bromide (TBAB) hasbeen reported to be the most active PTC among sixdifferent catalysts used to intensify the reaction of BCwith solid sodium sulfide.[2] The same catalyst, TBAB,was therefore used in the present study.

Recently, authors of the present work reportedthe preparation of DBS and BM from BC usingaqueous ammonium sulfide under liquid–liquid PTCconditions.[21] The use of aqueous ammonium sulfideand H2S-rich aqueous alkanolamines for liquid–liquidPTC-catalyzed reduction of aromatic nitro compoundsto produce value-added aromatic amines were also doc-umented in the literature by co-authors of the presentwork.[22–26] Considering the industrial importance ofH2S capture and making it harmless, the present workwas undertaken to synthesize DBS in high selectivity byreacting BC with industrially relevant H2S-rich aque-ous MEA in the presence of a PTC, TBAB. Moreover,a suitable mechanism has been formulated based onthe experimental findings to explain the course of thereaction.

MATERIALS AND METHODS

Materials

Toluene (≥99%), MEA (≥98%), and BC (≥99%) ofsynthesis grade were procured from Merck (India)Ltd., Mumbai, India. TBAB (≥99%) was obtainedfrom SISCO Research Laboratories Private Limited,Mumbai, India. All chemicals were used as such withoutfurther purification.

Experimental set-up

The reactions of BC with H2S-rich aqueous MEA wereperformed batch-wise in a fully baffled mechanicallyagitated three necked glass reactor with a capacityof 250 cm3 (6.5 cm I.D.). The reactor was equippedwith a four-leg vertical baffle and a vertical refluxcondenser. A 2.0-cm diameter six-bladed glass-diskturbine impeller with the provision of speed regulation,located at a height of 1.5 cm from the bottom ofthe reactor, was used to stir the reaction mixture.Throughout the course of the reaction, the reactor was

2010 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2011; 6: 257–265DOI: 10.1002/apj

Asia-Pacific Journal of Chemical Engineering KINETICS OF REACTION OF BC WITH H2S-RICH AQUEOUS MEA 259

kept immersed in a constant-temperature water bath, thetemperature of which could be controlled within ±1 ◦C.

Preparation of H2S-rich aqueous MEA solution

For the preparation of H2S-rich aqueous MEA, ∼20wt% MEA was prepared first by adding a suitablequantity of MEA in distilled water. H2S gas wasthen bubbled through this aqueous MEA in a 250 cm3

standard gas bubbler. Liquid samples were withdrawnfrom time to time after stopping the gas bubbling andthe samples were then analyzed for sulfide content.The gas bubbling was continued until the desiredsulfide concentration was obtained in the aqueous MEAsolution.

Experimental procedure

In a typical run, 50 cm3 of the aqueous phase containinga known concentration of sulfide was introduced intothe reactor and kept well agitated until the constantreaction temperature was attained. The organic phasecontaining a measured amount of BC, catalyst TBAB,and solvent-toluene, kept separately at the reactiontemperature, was then charged into the reactor at zerotime. The reaction mixture was then agitated at aconstant speed. Approximately 0.3 cm3 of the organiclayer was withdrawn at a regular interval after stoppingthe agitation and allowing the phases to separate.

Analytical technique

All the samples from the organic phase were ana-lyzed by gas–liquid chromatography (GLC) using a2 m × 3 mm stainless steel column packed with 10%OV-17 on Chromosorb W(80/100). A gas chromato-graph (Chemito Model 8610 GC) interfaced with adata processor (Shimadzu C-R6A Chromatopac) wasused for the analysis. The column temperature wasprogrammed with an initial temperature of 150 ◦C for1 min, increased at a rate of 20 ◦C/min up to 300 ◦C,and maintained at 300 ◦C for 4 min. Nitrogen was usedas carrier gas with a flow rate of 20 cm3/ min. An injec-tor temperature of 250 ◦C was used during the analysis.An flame ionization detector was used at a temperatureof 320 ◦C. The products were characterized by GLCand infrared spectra. The composition of the samplesbeing analyzed was calculated by direct comparison ofthe peak areas against a calibration curve. The initialsulfide concentrations were determined by the standardiodometric titration method. The aqueous phase sulfideconcentrations during the reaction were obtained fromthe overall mass balance. The term selectivity of the two

products, DBS and BM, used in this study are definedas the fraction of BC converted to a particular productdivided by the total conversion of BC.

RESULTS AND DISCUSSION

The reactions of BC with aqueous H2S-rich MEA werecarried out in batch mode both in the absence and inthe presence of PTC. DBS and BM were detected asthe products from the reaction mixture by GLC. Nobenzyl alcohol or dibenzyl disulphide was detected inthe reaction mixture even after a batch time of 8 h.

Effect of speed of agitation

For any kinetic study, elimination of mass-transfer resis-tance during the reaction is very important to obtain truereaction kinetics. To determine the role of mass-transferresistance, the effect of stirring speed on the conversionof BC was studied in the range 1000–2000 rpm underotherwise identical experimental conditions in the pres-ence of PTC as shown in Fig. 1. As it is evident fromthe figure, the variation of conversion of BC with speedof agitation in the range studied is so small that the reac-tions may be considered to be free from mass-transferresistance. All other experiments were performed at1500 rpm with negligible effect of mass-transfer resis-tance on the reaction kinetics.

Figure 1. Effect of stirring speed on conversion of BC.Volume of organic phase = 5.0 × 10−5 m3; concentrationof BC = 2.6 kmol/m3 organic phase; concentration oftoluene = 6.6 kmol/m3 organic phase; volume of aqueousphase = 5.0 × 10−5 m3; concentration of catalyst = 1.0 ×10−1 kmol/m3 organic phase; concentration of sulfide =1.88 kmol/m3; MEA/H2S mole ratio = 1.74; temperature= 333 K.



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 organicphase; concentration of sulfide = 1.88 kmol/m3; MEA/H2Smole ratio = 1.74.

Effect of temperature

The effect of temperature was studied at five differenttemperatures in the range 313–353 K. Initial rate ofreaction of BC was calculated at different temperaturesand an Arrhenius plot of logarithm of initial rate versus1/T was made as shown in Fig. 2. As observed form thefigure, the rate of reaction of BC increases with increas-ing temperature as expected. The apparent activationenergy for the reaction of BC was calculated from theslope of the best fitted straight line as 51.3 kJ/mol. Theobserved high apparent activation energy confirms thatthe reaction is kinetically controlled.

Effect of catalyst loading

The effect of catalyst (TBAB) loading on the conversionof BC was studied in the concentration range of0–0.15 kmol/m3 of organic phase, as shown in Fig. 3.With increase in catalyst concentration, the conversionof BC as well as reaction rate increases. Only byincreasing the catalyst concentration, BC conversion ofmore than 98% was achieved, whereas it was about92% without catalyst even after 480 min of reactionunder otherwise identical conditions. Fig. 3 also showsthat over certain concentration of the catalyst, ca0.10 kmol/m3 of organic phase, the conversion of BCbecomes constant. This could be attributed to interfacesaturation, which means that mass-transfer of the activespecies into organic phase reaches a maximum value.The selectivity of DBS increases with increase in

Figure 3. Effect of catalyst loading on conversion of BC.Volume of organic phase = 5.0 × 10−5 m3; concentration ofBC = 2.6 kmol/m3 organic phase; volume of aqueous phase= 5.0 × 10−5 m3; concentration of sulfide = 1.88 kmol/m3;MEA/H2S mole ratio = 1.74; temperature = 333 K; stirringspeed = 1500 rpm.

Figure 4. Effect of catalyst loading on selectivity of DBS.Volume of organic phase = 5.0 × 10−5 m3; concentration ofBC = 2.6 kmol/m3 organic phase; volume of aqueous phase= 5.0 × 10−5 m3; concentration of sulfide = 1.88 kmol/m3;MEA/H2S mole ratio = 1.74; temperature = 333 K; stirringspeed = 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 rateof reaction is governed by rate of transportation ofanions from aqueous phase to organic phase. In thepresence of PTC, the transportation of anions (in thepresent case S2− and HS−) is facilitated and the reactionbecomes organic-phase limited. The hydrosulfide (HS−)and sulfide (S2−) ions present in the aqueous phasereadily form ion pairs [Q+ HS− and Q+ S2− Q+]



with quaternary cation [Q+], and are transferred to theorganic phase and then react with BC. With increasedcatalyst concentration, more amount of [Q+]2S2− ionpair is formed and transferred to the organic phaseto react with BC to form DBS. The selectivity ofDBS, therefore, increases with increase in catalystconcentration.

Table 1 shows the enhancement factor, which is theratio of rate of reaction in the presence of TBAB tothat in the absence of TBAB for a fixed conversionof BC (40%), for various catalyst concentrations. Theenhancement factor increases with increasing catalystconcentration as observed from the table. A maximumrate enhancement factor of 2.68 was obtained with cat-alyst concentration of 0.15 kmol/m3 of organic phase.However, a marginal change in enhancement factor wasobserved at conversion levels >80% (2.95 at 90% con-version of BC).

Effect of sulfide concentration

Figure 5 shows the effect of sulfide concentration inthe aqueous phase on the conversion of BC at MEAconcentration of 3.274 kmol/m3. As evident from thefigure, the conversion of BC increases with increasein the concentration of sulfides. Keeping all otherconditions fixed, a conversion of more than 98% wasachieved after 480 min of run. However, an oppositetrend was observed (Fig. 6) for selectivity of DBS. Aselectivity of DBS of more than 98% has been obtainedwith a sulfide concentration of 0.474 kmol/m3 after480 min of run under otherwise identical conditions.Further increase of sulfide concentration results indecrease of selectivity of DBS as observed from thefigure.

With increase in sulfide concentration, selectivity ofDBS decreases. This can be explained by consideringthe fact that although MEA as such does not participatein the reaction with BC, it does affect the equilibrium

Table 1. Effect of catalyst TBAB loading on the reactionrate of benzyl chloridea.

Concentrationof TBAB(×102 kmol/m3 oforganic phase)

Reaction rate(×104 kmol/m3 × s)

Enhancementfactor

0 7.004 –5 14.095 2.0110 15.292 2.1815 18.739 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; volumeof aqueous phase = 5.0 × 10−5 m3; concentration of sulfide =1.884 kmol/m3; MEA/H2S mole ratio = 1.737.

Figure 5. Effect of sulfide concentration on conversionof BC. Volume of organic phase = 5.0 × 10−5 m3;concentration of BC = 2.6 kmol/m3 organic phase; volumeof aqueous phase = 5.0 × 10−5 m3; concentration ofMEA = 3.27 kmol/m3; concentration of catalyst = 9.98 ×10−2 kmol/m3 organic phase; temperature = 333 K; stirringspeed = 1500 rpm.

Figure 6. Effect of sulfide concentration on selectivityof DBS. Volume of organic phase = 5.0 × 10−5 m3;concentration of BC = 2.6 kmol/m3 organic phase; volumeof aqueous phase = 5.0 × 10−5 m3; concentration ofMEA = 3.27 kmol/m3; concentration of catalyst = 9.98 ×10−2 kmol/m3 organic phase; temperature = 333 K; stirringspeed = 1500 rpm.

among MEA, H2S, and water, which results in twoactive anions, sulfide (S2−) and hydrosulfide (HS−), inthe aqueous phase. These two active anions participatein two different reactions. In the presence of a base(MEA), the dissociation equilibrium shifts toward moreionization and the concentration of sulfide ions, relativeto hydrosulfide ions in the aqueous phase, increases asthe MEA concentration increases. Therefore, only bychanging the MEA concentration with constant sulfide



concentration in the aqueous phase, it would be easy toprove the existence of two different reactions.

Effect of MEA concentration

Although MEA, as such, does not participate in thereaction with BC, it does affect the equilibrium amongMEA, H2S, and water, which results in two activeanions, sulfide (S2−) and hydrosulfide (HS−), in theaqueous phase. These two active anions participate intwo different reactions. With increase in the concentra-tion of a base, MEA, the dissociation equilibrium shiftstoward more ionization and the concentration of sulfideions relative to hydrosulfide ions in the aqueous phaseincreases.

To study the effect of MEA concentration, H2S-rich aqueous MEA of different MEA concentrations(but constant sulfide concentration) was prepared bytaking 30 cm3 of H2S-rich aqueous MEA (with knownsulfide and MEA concentration) and then adding variousproportions of pure MEA and distilled water to it insuch a way that the total volume became 50 cm3 in allthe cases.

As seen from the Figs 7 and 8, both the conversionof BC and selectivity of DBS increase with increasein MEA concentration under otherwise identical exper-imental conditions. The concentration of sulfide ions(S2−) relative to hydrosulfide ions (HS−) increases asthe concentration of MEA increases for a fixed sulfideconcentration. Thus, with increase in MEA concentra-tion, there is an increase in the conversion of BC viathe transfer of sulfide ions resulting in higher selectivityof DBS at higher MEA concentration.

Figure 7. Effect of MEA concentration on conversion of BC.Volume of organic phase = 5.0 × 10−5 m3; concentration ofBC = 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 organicphase; temperature = 333 K; stirring speed = 1500 rpm.

Figure 8. Effect of MEA concentration on selectivity of DBS.Volume of organic phase = 5.0 × 10−5 m3; concentration ofBC = 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 organicphase; 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; concentrationof catalyst = 1.0 × 10−1 kmol/m3 organic phase; volume ofaqueous phase = 5.0 × 10−5 m3; concentration of sulfide= 1.24 kmol/m3; MEA/H2S mole ratio = 2.64; temperature= 333 K; stirring speed = 1500 rpm.

Effect of concentration of benzyl chloride

The effect of concentration of BC on its conversionand selectivity of DBS was studied at three differ-ent concentrations as shown in Figs 9–11. The selec-tivity of DBS increases with increase in the concen-tration of BC as shown in Fig. 10. Therefore, theselectivity of BM decreases with the concentration of



Figure 10. Effect of BC concentration on selectivity of DBS;Volume of organic phase = 5.0 × 10−5 m3; concentrationof catalyst = 1.0 × 10−1 kmol/m3 organic phase; volume ofaqueous phase = 5.0 × 10−5 m3; concentration of sulfide= 1.24 kmol/m3; MEA/H2S mole ratio = 2.64; temperature= 333 K; stirring speed = 1500 rpm.

Figure 11. Relationship between conversion of BC andselectivity of DBS under different BC concentration. All theconditions are same as in Figure 9 and 10.

BC. From the plot of selectivity of DBS versus con-version of BC (Fig. 11), it is seen that there is asharp increase of slope of the curve with increase inthe concentration of BC. Because the reaction leadingto the formation of BM is very fast compared withthat of DBS, at low BC concentration there will beinsufficient quantity of BC present to produce DBS,which results in low selectivity of DBS. It is alsoseen from Fig. 9 that with increase in the concentra-tion of BC, the conversion of BC decreases becauseof limited quantity of sulfide present in the aqueousphase.

With low BC concentration in the organic phase,almost complete conversion of BC was achieved. Thisresulted in very low selectivity of DBS, i.e. high selec-tivity of BM. With excess BC, higher DBS selec-tivity was achieved with efficient utilization of sul-fide in the aqueous phase although the BC conversionremained low.

MECHANISM

Two different mechanisms, extraction and interfacial,are generally used to explain the liquid–liquid phasetransfer catalysis based on the lipophilicity of PTCused. The extraction mechanism is useful to explainthe course of the reaction when the PTC is not highlylipophilic one so that it can distribute themselvesbetween the organic and the aqueous phase.[17–18]

According to this mechanism, the PTC exchangesanions with hydrophilic reactant in the aqueous phaseand forms a lipophilic ion pair. The active catalysts thusformed are then transferred to the organic phase andreact with lipophilic reactants there. In the interfacialmechanism, catalysts remain entirely in the organicphase because of their high lipophilicity and exchangeanions across the liquid–liquid interface. The reactionof BC with H2S-rich aqueous MEA was studied inpresence of TBAB, which is not so highly lipophilicone and, therefore, the reaction can be represented byextraction mechanism as shown by Scheme 1.

In the aqueous phase, there exist ionic equilibriaamong MEA, H2S, and water, which result three activeanions: hydroxide (HO−), hydrosulfide (HS−), and sul-fide (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 ammo-nium cation, Q+ [(n-C4H9)4N+]. However, no benzylalcohol, C6H5CH2OH (substitution product of BC withQOH), was identified in the GC analysis from thetwo-phase reaction in the presence of TBAB. This is

RNH2 + H2O RNH3+ + OH− … (1)

H+ X− + RNH3+OH−

Q+ + X−

Q+ + HS−QSH

QSH + ΦX QSQ + ΦX

ΦSQ + ΦXΦSH + ΦX

ΦSH + QX QX + ΦSQ

QX + ΦSΦΦSΦ + HX

Q+ + S2− QSQ

H2S H+ + HS− … (3) HS− H+ + S2− … (4)

H2O H+ + HO− … (2)

Interface

OrganicPhase

AqueousPhaseRNH3

+ X− + H2O

R=HOCH2CH2−X = Br/Cl; Φ = C6H5CH2−; Q+= (n-C4H9)4N

+

Scheme 1. Mechanism of liquid–liquid phase transfercatalyzed reaction of BC with H2S-rich aqueous MEA.



because of the fact that the active catalyst, QOH, ismore hydrophilic in nature and not easily transferredto the organic phase[27] and therefore the hydrolysis ofBC under weak alkaline medium of aqueous MEA isslow.[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 reportedthat the sulfide ions present in the aqueous phaseare first converted to the hydrosulfide ions (S2− +H2O ↔ HS− + OH−) and are then transferred tothe organic phase via phase transfer catalysis.[29] Ifonly hydrosulfide ions are transferred, the presentreaction system becomes a case of series reactionsand the DBS should form only by the reaction ofBM with BC. However, it was observed from anindependent experiment with sodium sulfide that no BMwas formed. Also it was observed that with increase inMEA concentration, the selectivity of DBS increases asdiscussed earlier. These observations confirm that bothsulfide and hydrosulfide ions present in the aqueousphase are simultaneously transferred to the organicphase 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 typi-cal batch. It is seen from the figure that concentration ofBM reaches a maximum and then falls gradually withtime. Therefore, BM is converted to DBS whose con-centration increases with time. Probably, BC reacts withBM to produce DBS and hydrochloric acid. Because,the hydrochloric acid (strong acid) is formed from aweak acid, BM, this reaction is expected to be slow andis favored only because of the presence of MEA, which

Figure 12. Concentration profile for a typical run.Volume of organic phase = 5.0 × 10−5 m3; concentrationof BC = 2.6 kmol/m3 organic phase; concentration ofcatalyst = 9.98 × 10−2 kmol/m3 organic phase; volume ofaqueous 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.

reacts with hydrochloric acid irreversibly to producemethanolamine hydrochloride in the aqueous phase.

CONCLUSIONS

The reaction of BC with H2S-rich aqueous MEA is ofgreat industrial relevance, which could lead to differentproducts, DBS and BM, of high commercial value. Thisreaction has been studied in detail under liquid–liquidphase transfer catalysis conditions. The various processparameters (stirring speed, catalyst concentration, reac-tants concentration, and temperature) have been opti-mized. The observed variations of conversion of BCand selectivity of products (DBS and BM) with theprocess parameters were used to establish a mecha-nism with cyclic phase transfer initiation step in theheterogeneous liquid–liquid system. The reaction hasbeen found to be kinetically controlled with an apparentactivation energy value of 51.3 kJ/mol. The MEA/H2Smole ratio has been found to have enormous effect onthe selectivity of DBS and BM. The higher ratio favorsDBS while the lower ratio favors BM. The change inthe temperature and the catalyst concentration (beyonda certain value) only changes the reaction rate withoutsignificantly affecting the selectivity. The selectivity ofDBS increases with excess BC in the organic phasealthough the conversion of BC remains low. However,the opposite trend was observed for BM.

The process involves a complex mechanism. Theexistence of ionic equilibriums among MEA, hydrogensulfide, and water producing sulfide (S2−) and hydrosul-fide (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 theorganic phase and then react with BC to produce DBSand BM, respectively. The DBS is also formed by thereaction of BM with BC.

Acknowledgement

Financial support for this work from the Council ofScientific and Industrial Research (CSIR), New Delhi,India is gratefully acknowledged.

REFERENCES

[1] A.L. Kohl, R.B. Nielsen. Gas Purification, Gulf PublishingCompany: Houston, TX, 1997.

[2] N.C. Pradhan, M.M. Sharma. Ind. Eng. Chem. Res., 1990; 29,1103–1108.

[3] J.E. Bittel, J.L. Speier. J. Org. Chem., 1978; 43, 1687–1689.[4] N.C. Pradhan, M.M. Sharma. Ind. Eng. Chem. Res., 1992; 31,

1610–1614.[5] B.P. Banger, V.S. Sadavarte, S.G. Pawar, V.T Kamble. Indian

J. Heterocycl. Chem., 2000; 10, 159–160.



[6] M.M. Lakouraj, B. Movassagh, Z. Fadaei. Synth. Commun.,2002; 32, 1237–1242.

[7] B. Movassagh, A. Movassagh. Synth. Commun., 2004; 34,2337–2343.

[8] B. Movassagh, A. Movassagh. Synth. Commun., 2004; 34,1685–1690.

[9] B.R. Raju, G. Devi, Y.S. Nongpluh, A.K.A. Saikia. Synlett,2005; 2, 358–360.

[10] N. Iranpoor, H. Firouzabadi, H.R. Shaterian. J. Org. Chem.,2002; 67, 2826–2830.

[11] S.J. Miller, T.R. Collier, W. Wu. Tetrahedron Lett., 2000; 41,3781–3783.

[12] T. Ido, T. Susaki, G. Jin, S. Goto. Appl. Catal. A Gen., 2000;201, 139–143.

[13] Y. Labat. Patent No. EP0337838, 1989.[14] J.B. Heather. U.S. Patent No. 4740623, 1988.[15] B.P. Banger, S.B. Pawar. J. Chem. Res. Synop., 1998; 4,

212–213.[16] J. Choi, N.M. Yoon. Synth. Commun., 1995; 25, 2655–2663.[17] C.M. Starks, C.L. Liotta, M. Halpern. Phase Transfer Catal-

ysis: Fundamentals, Applications and Industrial perspective.Chapman and Hall publications: New York, 1994; pp.668.

[18] E.V. Dehmlow, S.S. Dehmlow. Phase Transfer Catalysis,VCH: New York, 1993.

[19] Y. Sasson, R. Neumann, (Eds). Handbook of Phase TransferCatalysis, Blackie/Chapman and Hall: London, UK, 1997;pp.576.

[20] C.M. Starks, C.L. Liotta. Phase Transfer Catalysis Principlesand Techniques, Academic Press: New York; 1978.

[21] S. Sen, S.K. Maity, N.C. Pradhan, A.V. Patwardhan. Int. J.Chem. Sci., 2007; 5, 1569–1578.

[22] S.K. Maity, N.C. Pradhan, A.V. Patwardhan. Appl. Catal. AGenl., 2006; 301, 251–258.

[23] S.K. Maity, N.C. Pradhan, A.V. Patwardhan. Ind. Eng. Chem.Res., 2006; 45, 7767–7774.

[24] S.K. Maity, N.C, Pradhan, A.V. Patwardhan. Chem. Eng. Sci.,2007; 62, 805–813.

[25] S.K. Maity, N.C. Pradhan, A.V. Patwardhan. Appl. Catal. BEnviron., 2008; 77, 418–426.

[26] S.K. Maity, N.C. Pradhan, A.V. Patwardhan. Chem. Eng. J.,2008; 140, 187–193.

[27] M. Wang, Y. Tseng. J. Mol. Catal. A Chem., 2003; 203,79–93.

[28] G.D. Yadav, Y.B. Jadhav, S. Sengupta. J. Mol. Catal. AChem., 2003; 200, 117–129.

[29] G.D. Yadav, Y.B. Jadhav, S. Sengupta. Chem. Eng. Sci., 2003;58, 2681–2689.


Documents

Kinetics of reaction of benzyl chloride with H2S-rich aqueous monoethanolamine: selective synthesis of dibenzyl sulfide under liquid–liquid phase-transfer catalysis