Published: November 23, 2011

r 2011 American Chemical Society 147 dx.doi.org/10.1021/ie2024068 | Ind. Eng. Chem. Res. 2012, 51, 147–157

ARTICLE

pubs.acs.org/IECR

Kinetic Study and Optimization of Oxidative Desulfurization ofBenzothiophene Using Mesoporous Titanium Silicate-1 CatalystAryav Sengupta, Prashant D. Kamble, Jayanta Kumar Basu, and Sonali Sengupta*

Department of Chemical Engineering, Indian Institute of Technology, Kharagpur India 721302

ABSTRACT: The oxidative desulfurization (ODS) of benzothiophene (BT) in isooctane as a model fuel with 30% aqueous H2O2

was studied using three different titanium silicate (TS) zeolites, synthesized mesoporous TS-1, synthesized mesoporous titaniumbeta, and commercial TS-1 catalyst, which were found to give 85.6, 45.74, and 25.31% conversions, respectively. Therefore,mesoporous TS-1 was selected as the catalyst for ODS of BT. Reaction time, temperature, catalyst loading, andmolar ratio of H2O2:Swere selected as the pertinent parameters for the optimization of conversion based on the Box�Behnken design. The predictedmaximum conversion was observed to be 89.9% at a temperature of 60 �C, catalyst loading of 0.064 g, andmole ratio of BT andH2O2

of 0.209. An empirical kinetic model was used to fit the rate data. The activation energy was found to be 25.20 kJ/mol.

1. INTRODUCTION

The presence of organosulfur compounds such as thiophene(Th), benzothiophene (BT), dibenzothiophene (DBT), and theiralkyl derivatives are the major unwanted species present in crudeoil fractions. These lead to corrosion in refinery equipment andengines of automobiles, to poisoning of catalysts used in sec-ondary treatment in refineries, and to formation of sulfur oxidesafter combustion of fuel that cause severe environmental pollu-tion such as acid rain, depletion of the ozone layer, and smoggeneration.1 Hence it is very essential to reduce the sulfurcontent in sulfur-bearing petroleum fractions by suitable tech-niques which are both technologically and economically feasible.Legislation in Japan and Europe have limited the sulfur content inlight oil to a maximum of 50 ppm. The U.S. EnvironmentalProtection Agency (EPA) issued new sulfur standards of 30ppm by 2004 and of 15 ppm by 2006 in diesel fuels and gasoline.2

The Indian government also issued a notification to introduce theEURO IV standards and a sulfur level of 50 ppm in fuel.3

At present, there are several methods which are available forthe removal of sulfur compounds from hydrocarbon fuels such asselective adsorption, extractive separation, biodegradation, hy-drodesulfurization (HDS), and oxidative desulfurization (ODS).Currently, catalytic HDS is the most popular method for reducingsulfur content in petroleum fractions. HDS is a high severity processwhich accompanies large operating and capital costs. Moreover, it isdifficult to remove polyaromatic sulfur compounds such as BT,DBT, and their derivatives because of very low reactivity.4

By contrast, oxidative desulfurization (ODS) was consideredas one of the most effective and alternative methods to producefuels with very low sulfur content.2,5�7 ODS is advantageous overHDS because the former process can be carried out at near-ambientconditions such as 50 �C and atmospheric pressure in the liquidphase. The mechanism of the ODS reaction is the electrophilicaddition of oxygen atom to divalent sulfur with the formation ofunstable sulfoxides (1-oxides) and then sulfones (1,1-dioxides) in theheterocyclic thiophene ring.8 The difference between the physico-chemical properties of the sulfones and those of the hydrocarbonspresent in fuel oil makes easy separation of sulfones from the fuel oilby solvent extraction, distillation, adsorption, etc.

The selective oxidation of thiophene and its alkyl derivatives inn-octane solvent using titanium silicate-1 (TS-1) as catalyst,H2O2 as oxidant, and tert-butyl alcohol or water as solvent wasreported.9 Also, ODS of Th, BT, DBT, and their derivatives withtert-butyl hydroperoxide (t-BuOOH) as oxidant were studiedusing catalysts such as titanium zeolites, Mo�Al2O3, and cobaltaluminumphosphate.10,11 Improved reactivities of aromatic sulfurcompounds were observed at mild conditions using V2O5�Al2O3 and V2O5�TiO2 catalysts, when the oxidant H2O2 wasadded slowly.12 The use of peroxy acids such as performic acid,pertrifluoroacetic acid, and a mixture of formic acid and H2O2 asefficient oxidants was reported for selectively oxidizing sulfurcompounds in fuel oil.10 The oxidation of tetrahydrothiophene,diphenyl sulfide, 2-acetylthiophene, and 2,5-dimethylthiophenewith H2O2 as oxidant and molecular sieve catalysts such as TS-1,titanium-beta, and Ti�HMS has been studied.1The use of H2O2

as oxidant over catalysts such as formic acid, polyoxometalate,and molecular sieve zeolites for selective oxidation of thiophene-based compounds has also been reported.13 Among thesecatalysts, titanium silicates were found to give the maximumconversions under mild conditions. The use of complexes ascatalysts, containing transition metals such as Ti, Mo, Fe, Ru, andRe, with H2O2, O2, and O3 as oxidants was investigated foroxidizing sulfur compounds in fuel oil.11

The use of an ultrasound-assisted ODS process was found toenhance the reaction rate of sulfur compounds greatly comparedto that of ODS without ultrasound.14 The two-phase ODSreactions faced decreased reaction rate and low conversionsbecause the reaction occurs only at the interface. The use ofspecific phase-transfer catalysts (PTC) in ODS reactions wasproved to be of great advantage compared to ordinary catalysts asPTC transport reactants from one phase to another and henceimprove the reaction rate. Hence the application of PTC is ofgreat importance to the industrial use of ODS processes. The useof quaternary ammonium salts such as tetramethylammonium

Received: June 24, 2011Accepted: November 23, 2011

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bromide, tetraethylammonium bromide, tetrapropylammoniumbromide, and tetrabutylammonium bromide as PTC with formicacid as the oxidizing agent under ambient conditions has beenreported, mentioning high conversions and faster reaction rateswithout the use of expensive solvents.14 The use of coordinatedionic liquid (CIL) catalysts formed by the reaction of urea andtetraethylammonium chloride as PTC was also reported for theODS of fuel oil in the presence of a mixture of acetic acid andH2O2.

15 The use of phosphotungstic acid as catalyst, tetraoctyl-ammonium bromide (TOAB) as PTC, and H2O2 as oxidant wasreported for ultradeep desulfurization of diesel fuel.16

Titanium silicate zeolites such as TS-1, TS-2, and titanium-beta with Ti ions in various locations were found to give highconversions in the catalytic oxidation of alkenes, alcohols, andaromatics using H2O2 as oxidant under mild conditions.17 TS-1was used for removing sulfur compounds such as thiophene in n-octane solvent using H2O/tert-butyl alcohol as solvent.

9,18 TheAg loaded TS-1 catalyst was used to remove sulfur compoundsfrom FCC gasoline usingH2O2 as an oxidizing agent and water asa polar solvent to achieve 86% conversion after 4 h.19 Commer-cial microporous TS-1 catalyst and 30% aqueous H2O2 solutionwere used as catalyst and oxidizing agent respectively for oxida-tion and simultaneous extraction of thiophene from n-dodecaneusing a CSTR with methanol as the solvent. The oxidationactivity was found to increase with the increase in solvent/oilratio in the mixture.20 In our present study a large pore titanium-beta zeolite was synthesized by the DGC (dry gel conversion)method using tetraethylammonium hydroxide (TEAOH) as anorganic template/structure-directing agent. The catalyst gavehigh conversions in reactions using either H2O2 or tert-butylhydroperoxide as oxidant.21 Because of the larger pore size of theDGC synthesized Ti-beta catalyst compared to commercialmicroporous TS-1, the former gave higher conversions in thecase of oxidation of organic sulfur compounds with largemolecular structures such as BT, DBT, 2,5-dimethylthiophene,and 4,6-dimethyl-BT. Therefore, to overcome the difficultiesassociated with microporous TS-1 for the reaction of largestructured sulfur compounds, mesoporous TS-1 was synthesizedby the DGC method using activated carbon black as thestructure-directing agent.22 In our study, we have chosen carbonblack as template instead of other better templates such asCMK-3, triethanolamine, and organic silane surfactants such as[3-(trimethoxysilylpropyl)]octadecyldimethylammonium chlo-ride (TPOAC). This is because activated charcoal is cheap andreadily available.22 The mesoporous TS-1 catalyst synthesized bythe DGCmethod was used to desulfurize sulfur compounds suchas benzothiophene dissolved in isooctane as a synthetic mixtureconsidered as a model fuel. A comparative study for oxidativedesulfurization of different sulfur compounds such as thiophene,benzothiophene, and dibenzothiophene over mesoporous TS-1catalyst is also done. A set of experiments were conducted tostudy the effects of agitation speed, reaction time, temperature,catalyst loading, and mole ratio of benzothiophene and H2O2 onthe conversion of the given sulfur compound. Response surface

methodology (RSM) technique was applied to determine theoptimum conversion and the corresponding values of threemajor process parameters, namely, reaction temperature, molarratio of benzothiophene and H2O2 in the reaction mixture, andcatalyst loading. A kinetic rate equation was also proposed.

2. MATERIALS AND METHODS

2.1. Materials. Benzothiophene was procured from Himedia,India. Dibenzothiophene, hydrogen peroxide solution (30%,v/v), isooctane, tetrapropylammonium hydroxide (40% solutionin water), tetra-n-butyl orthotitanate, tetraethyl orthosilicate,sodium hydroxide, and 1-butanol were procured from MerckSpecialties Pvt. Ltd. Thiophene and tetraethylammonium hydro-xide (25% aqueous solution) was procured from SpectrochemPvt. Ltd., India. Commercial titanium silicate (TS-1) (1.5 mmextrudate) was supplied by Sud Chemie, India. Sodium alumi-nate was obtained from Standard Products Pvt. Ltd., India, as agift chemical. Activated charcoal was obtained from Ranbaxy,India. Concentrated sulfuric acid (assay 97�99%) was procuredfrom Qualigens, India. Fumed silica was procured from Sigma-Aldrich, Laborchemikalien.2.2. Methods. 2.2.1. Preparation of Titanium-beta and Tita-

nium Silicate-1 byDGCMethod. In order to carry out the oxidativedesulfurization (ODS) reaction of various thiophene compoundsin isooctane, two catalysts in the mesoporous range weresynthesized: titanium-beta, prepared by the dry gel conversion(DGC) method using TEAOH as the structure-directingagent,21 and titanium silicate-1, prepared by the DGC methodby using a different template, activated carbon.22 TS-1 catalyst

Table 1. Properties of Catalytic Materials

catalyst Ti/(Ti + Si) (mole ratio) surface area (m2/g) average pore width (Å) pore volume (cm3/g)

Ti-beta (mesoporous) 0.059 134.88 128.5 0.418

TS-1 (commercial) (microporous) 0.022 442.64 14.16 0.378

TS-1 (mesoporous) 0.034 514.16 29.91 0.3708

Figure 1. (a) Reaction mechanism of ODS of benzothiophene. (b)Reaction stoichiometry of ODS of BT.

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with pore width in the microporous range was commerciallyobtained. The catalysts were characterized by BET apparatus(Quantachrome Autosorb-1, Model AS 1 MP/Chemi-LP) todetermine surface area, pore volume, and average pore diameter,and the particle size analysis was done by a Mastersizer 2000EVer 5.20 (Malvern Instruments Ltd., U.K.). A comparative studyof the composition and physical properties of the three differentcatalysts used in the ODS of BT is shown in Table 1. The particlesize analysis of those three catalysts is shown in Figure 4.2.2.2. Catalytic Experiments. The ODS reaction was per-

formed in a three-necked glass batch reactor (150 cm3) with5.5 cm i.d. equipped with a glass stirrer, a thermometer formeasuring the exact reaction temperature, and a reflux conden-ser, kept in a water bath whose temperature was maintained at aconstant value with an accuracy of (1 �C by using a digitalcontroller cum indicator. The catalytic experiments were carriedout in a laboratory scale batch reactor using the three differenttitanium silicate zeolites, namely, commercial TS-1, laboratory-synthesized Ti-beta, and the TS-1. The model fuel was preparedby dissolving the required amount of sulfur compound (Th/BT/DBT) in 40 mL of isooctane. It was then added to the batchreactor followed by the addition of specific amounts of differenttitanium silicate catalysts. The reactor was then heated to adesired reaction temperature. A typical experimental run wascarried out at optimum conditions such as maintaining of thetemperature at 60 �C, catalyst loading of 0.064 g, and H2O2 to Smole ratio of 4.77 in the reactor. The mixture was then stirred at adesired revolutions per minute up to a reaction time of 3 h, duringwhich samples were withdrawn and analyzed periodically using highperformance liquid chromatography (HPLC). The experiment wasrepeated by varying different process parameters, namely, catalystloading, reaction temperature, stirrer speed, and mole ratio of H2O2

and sulfur compound, and also the catalyst type and the type of sulfurcompound. The analyzer was equipped with an Agilent SB C-18column (length, 250 mm; diameter, 4.6 mm; packing size, 5 μm).The pump and detector used were a Perkin-Elmer Series 200 pumpand a Perkin-Elmer Series 200 UV/vis detector. The mobile phaseused was a mixture of methanol and water (9:1 v/v) with a flow rateof 1 mL/min, and detection of the organic sulfur compounds wasperformed at a wavelength of 254 nm.Parts a and b of Figure 1 show the detailed mechanism and

stoichiometry of the BT oxidation reaction byH2O2, respectively.The reaction is a series reaction where BT is first oxidized to

benzothiophene sulfoxide (BTO) and water which is subse-quently oxidized by another H2O2 molecule to form benzothio-phene sulfone (BTS) and water. The mass balance of the ODS ofBT under optimized conditions is described in Table 2.2.2.3. Experimental Design. Response surface methodology

(RSM) was applied to optimize the process parameters forcatalytic oxidative desulfurization of benzothiophene with TS-1as a catalyst and hydrogen peroxide as an oxidant. The reactiontemperature, X1 (20�60 �C), moles of benzothiophene per moleof hydrogen peroxide, X2 (1:2.5 to 1:15), and amount of catalystloading, X3 (0�0.075 g) were used as independent variables tooptimize the benzothiophene conversion. The coded and un-coded levels of the independent parameters are shown in Table 3.RSM can be used to define the relationships between the

dependent variable called response(s) and the independentprocess variables by statistical analysis along with the contribu-tion of the effect of the independent variables, alone or incombination, in the processes. In addition to analyzing the effectsof the independent variables, this experimental methodology alsogenerates a mathematical model to predict the response. Therange of the process variables is predetermined on the basis ofsome selective trial experimental runs depending on the specialcriteria and the selection of experimental points. Working with astatistical experimental design not only randomizes the experi-mental error to each experimental point but also equals thedistribution of experimental points in the investigated range ofindependent variables. These increase the accuracy of the modelequation. The main objective of the RSM is to determine theoptimum operational conditions for the system and to determinethe region that satisfies the operating specifications. Response surfacemethodology (RSM) is a very efficient tool for optimizing differentsignificant process parameters, and the prediction of a model

Table 3. Range and Levels of Process Parameters

range and levels

independent variables �1 0 1

temperature, X1 (�C) 20 40 60

mole ratio, X2 (S:H2O2) 0.06 0.23 0.4

catalyst loading, X3 (g) 0 0.0375 0.075

Table 2. Mass Balance of the ODS of BT under OptimizedConditions

basis 1 mol of H2O2

BT + 2H2O2 f BTS + 2H2O

initial mass (before reaction)

BT 28.1283 g

H2O2 34 g

H2O 42 g (from 30% H2O2)

total 104.1283 g

final mass (after reaction)

BT 2.8409 g

BTS 31.3171 g

H2O2 21.1867 g

H2O 42 + 6.7835 = 48.7835 g

total 104.1282 g

Figure 2. Influence of catalyst type on BT conversion. Stirring speed,900 rpm; reaction time, 180 min; reaction temperature, 50 �C; H2O2:S,10:1; catalyst loading, 0.05 g; volume of isooctane, 40 mL, with 2780ppm BT; oxidizing agent, 30% aqueous H2O2.

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generated by them can yield three-dimensional (3D) surface plotsand contour plots. Design of experiment (DOE) is a structured,organized optimization technique that is used to determine therelationship between the different factors (X) affecting a process(inputs) and the output of that process termed as response (Y).The response surfacemethodology coupledwith factorial design

is an empirical optimization technique used to evaluate therelationship between a set of controllable experimental factorsand observed results, and it involves three major steps: (i)performing statistically designed experiments, (ii) estimation ofthe coefficients in a mathematical model, and (iii) predicting theresponse and checking the adequacy of the model. Design Expert7.0 was used as the statistical analysis software for the design. Thefollowing quadratic equationwas used for the optimization process.

YðxÞ ¼ a0 þ ∑ aixi þ ∑ aiixi2 þ ∑ aijxixj

whereY(x) is the response (substrate conversion) and a0, ai, aii, andaij are the coefficients of the intercept, linear, square, and interaction

effects, respectively. The optimum response (Yopt) and also thecorresponding process parameters were also determined. The statis-tical significance of the model and the coefficients were analyzed bymeans of the F-test and t test, respectively.23

3. RESULTS AND DISCUSSION

3.1. Selection of Catalyst.The ODS reaction of BT is carriedout with three different titanium silicate catalysts, namely, com-mercial TS-1, Ti-beta, and mesoporous TS-1, under the samereaction conditions. The variation of substrate conversion withtime as a function of the catalyst type is illustrated in Figure 2.Figure 2 shows that, after a reaction time of 180min, mesoporousTS-1 gives the highest conversion among the three differenttitanium silicate catalysts. Mesoporous TS-1 is found to give aconversion of 85.6%, while Ti-beta and commercial microporousTS-1 give conversions of 45.74 and 25.31%, respectively, underthe same reaction conditions.

Figure 3. (a) Pore size distribution curves for (a) synthesized Ti-beta catalyst, (b) commercial TS-1 catalyst, and (c) synthesized TS-1 catalyst.

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The low conversion of BT in the case of commercial TS-1 isbecause of the large size of bulky BT molecules that could hardlypenetrate the small pores of the catalyst1 as the average catalystpore width lies in the microporous range (14.16 Å). Moreover,the total pore volume of TS-1 commercial is 0.378 cm3/g; of that,themicropore volume is 0.2544 cm3/g, and themicropore surfacearea is 319.7 m2/g. However, in the cases of Ti-beta andmesoporous TS-1, the BT conversion is sufficiently higher,probably due to their large pore size and mesopore volume.Mesopore volumes of Ti-beta and TS-1 (mesoporous) are 0.4185and 0.1978 cm3/g and mesopore surface areas are 134.3 and329.3 m2/g, respectively. The titanium silicate catalysts (Ti-betaand TS-1) prepared by the DGC method have the averagecatalyst pore width lying in the mesoporous range. Figure 3shows the pore size distribution curves for the three catalysts.Figure 3a, showing the pore size distribution curve for Ti-beta,

indicates that the majority of the pores present in the catalysthave a pore width/diameter greater than 20 Å but less than 500 Å,suggesting the sample is of highly mesoporous type. Figure 3b,showing the pore size distribution curve for commercial TS-1catalyst, proves that the majority of the pores present in thecatalyst have a pore width/diameter less than 20 Å, suggestingthe commercial variety of TS-1 catalyst is of highly microporoustype. Figure 3c, the pore size distribution curve for DGCsynthesized TS-1 catalyst, shows an appreciable number of poreswith the pore diameter lying within the range from 20 to 500 Å,although some of the pores have a pore width below 20 Å. Thereason behind the higher conversion of BT for mesoporous TS-1compared to synthesized Ti-beta is the higher intrinsic catalyticactivity of Ti atoms present in the former catalyst.1 Therefore, inour experimental study we have chosen DGC synthesized meso-porous TS-1 as a catalyst for the ODS of BT. The effects ofvarious process parameters affecting the substrate conversionhave also been illustrated. Prior to optimization of the process, allexternal mass transfer effects were minimized to a sufficientlypossible extent through stirrer speed variation. The internal masstransfer effect is assumed to be negligible for all size ranges ofcatalyst particles because of their very small sizes. The particlesize distributions of the three catalysts determined with the helpof a particle size analyzer are shown in Figure 4, where it is provedthat 90% of the particle sizes of the catalysts, commercial TS-1,mesoporous TS-1, and Ti-beta are 170, 31.2, and 69 μm,respectively. In our earlier work24 the internal resistance to masstransfer for the Ti catalysts was found to be negligible in the sizerange 845�215 μm. Hence, further experiments to determinethe internal diffusional effect for this system were not done.3.2. Effect of Variation of Different Process Parameters on

Reactant Conversion/Kinetic Study. The stirrer speed, tem-perature, catalyst loading, and mole ratio of H2O2 to S are themain process parameters that affect the substrate conversion.The effects of these parameters on reactant conversion areillustrated in Figures 5�8.3.2.1. Elimination of External Mass Transfer. The effect of

stirrer speed on oxidation of BTwas studied by varying the stirrerspeed from 300 to 1100 rpm. This was done in order to observethe optimum stirrer speed to overcome the influence of externalmass transfer resistance on conversion. Figure 5 illustrates theeffect of different stirrer speeds, namely, 300, 500, 700, 900, and1100 rpm, on reactant conversion. All other process parameters

Figure 4. (a) Particle size distribution for commercial TS-1 catalystpowder: d(0.1) = 4.777 μm; d(0.5) = 38.805 μm; d(0.9) = 170.075 μm.(b) Particle size distribution for mesoporous TS-1 catalyst powder:d(0.1) = 5.191μm; d(0.5) = 13.288μm; d(0.9) = 31.233μm. (c) Particlesize distribution for Ti-beta catalyst powder: d(0.1) = 5.977 μm; d(0.5)= 30.812 μm; d(0.9) = 69.049 μm.

Figure 5. Influence of stirrer speed on BT conversion. Reaction time,180 min; reaction temperature, 50 �C; H2O2:S, 10:1; catalyst loading,0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent,30% aqueous H2O2; catalyst, mesoporous TS-1.

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such as reaction temperature, catalyst loading, and H2O2 to Smole ratio were kept fixed. It was found that the substrateconversion was highly influenced by the external mass transferat or below 900 rpm and above that value of revolutions perminute all external mass transfer effects are negligible. Hence, allthe successive experiments were carried out at 900 rpm in orderto save power consumption for agitating the reactant mixture.3.2.2. Temperature Dependency of ODS Reaction. The effect

of temperature on reactant conversion was studied by varying thetemperature from 20 to 70 �C at a stirrer speed of 900 rpm,catalyst loading of 0.05 g, andmole ratio of H2O2 to S at 10:1, andthe results are illustrated in Figure 6. The temperature has a directeffect on the kinetic rate constant (k), thereby increasing the rateof reaction and hence the substrate conversion. It was found thatthe conversions at 60 and 70 �C are 89 and 90.1%, respectively, at3 h. Hence 60 �C was chosen as the optimum temperature.3.2.3. Variation of Catalyst Loading. The effect of catalyst

loading on reactant conversion was studied by varying thecatalyst amount from 0.01 to 0.075 g and by keeping the stirrerspeed constant at 900 rpm, the reaction temperature at 60 �C,and theH2O2:Smole ratio at 10:1. The same reaction was carriedout without catalyst, also. Figure 7 illustrates the effect of catalyst

loading on the reactant conversion, with or without catalyst. Theabsence of any catalyst in the reaction medium shows a very lowconversion of BT. It is noted that the substrate conversionincreases by increasing the catalyst amount from no catalyst to0.075 g of catalyst from 0.69 to 95% at 3 h point of time. Therefore,this result reflects a very prominent catalyst effect on conversion.With increase in catalyst loading from 0.01 to 0.075 g, theincrease in conversion is 82%. The bulk preparation of DGCsynthesized mesoporous TS-1 is costly. Therefore, in order toeconomize the production of TS-1 catalyst, we consider 0.05 g asthe catalyst amount in most of our experiments because theconversions for 0.05 and 0.075 g are close.3.2.4. Influence of H2O2 and BT (S) Mole Ratio on the

Reaction. The effect of the mole ratio of H2O2:S on reactantconversion was studied by varying it from 2.5:1 to 15:1 and bykeeping the stirrer speed constant at 900 rpm, the reactiontemperature at 60 �C, and the catalyst loading at 0.05 g. Figure 8illustrates the effect of mole ratio on reactant conversion. It isnoted that with an increase in the molar ratio of H2O2 and BT,the reactant conversion increases markedly until the ratio reaches10:1. Beyond this ratio there is no appreciable increase in theconversion of BT. Hence the mole ratio is considered to be 10:1for all subsequent reactions.3.2.5. Effect of Reaction Time on the Conversion of BT. In

order to choose the optimum process time, BT conversion wasobserved by varying the reaction time from 15 to 240 min, in trialexperiments, keeping all other process parameters constant. Thisis represented in Figure 9. The BT conversion increases from53.2% to about 89.07% from 15 to 240 min time at 60 �C, stirrerspeed of 900 rpm, and catalyst loading of 0.05 g. However, it isfound that at 180 min the conversion is 88.79%; hence, there isvery little increase in conversion from 180 to 240 min. Therefore,further increase in the reaction time is not significant in increas-ing the conversion. However, H2O2 is highly unstable and withan increase in reaction time more and more H2O2 decomposesinto water and oxygen, which is not desirable for the reaction.13

Therefore, 180 min was chosen as the optimum reaction time.3.3. Effect of TS-1 Catalyst on the Conversion of Various

Thiophene Derivatives. Thiophene (Th), benzothiophene(BT), and dibenzothiophene (DBT) are the major thiophenicsulfur compounds present in gasoline. Three separate reactions

Figure 6. Influence of reaction temperature on BT conversion. Stirringspeed, 900 rpm; reaction time, 180 min; H2O2:S, 10:1; catalyst loading,0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent,30% aqueous H2O2; catalyst, mesoporous TS-1.

Figure 7. Influence of catalyst loading onBT conversion. Stirring speed,900 rpm; reaction time, 180 min; reaction temperature, 60 �C; H2O2:S,10:1; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent,30% aqueous H2O2; catalyst, mesoporous TS-1.

Figure 8. Influence of mole ratio of hydrogen peroxide to S on BTconversion. Stirring speed, 900 rpm; reaction time, 180 min; reactiontemperature, 60 �C; catalyst loading, 0.05 g; volume of isooctane, 40mL,with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2; catalyst,mesoporous TS-1.

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were carried out taking those sulfur compounds singly, undersimilar reaction conditions, which include a stirrer speed of900 rpm, a reaction temperature of 50 �C, a catalyst loading of0.05 g, and a H2O2:S mole ratio of 10:1.With the use of mesoporous TS-1 as the catalyst, it was found

that thiophene and its derivatives were oxidized at different rates,thereby leading to different conversions at the end of 180 min.The more condensed aromatic thiophenes, namely, DBT andBT, were oxidized much faster than the thiophene.1 The effect ofDGC synthesized mesoporous TS-1 on the conversion ofdifferent thiophenic sulfur components is illustrated in Figure 10.The reaction was carried out at a temperature of 50 �C, a stirrerspeed of 900 rpm, a catalyst loading of 0.05 g, and H2O2:S at10:1. Mesoporous TS-1 catalyst was found to give 51.01%conversion in the case of thiophene, 85.6% conversion in thecase of BT, and a maximum of 89.64% conversion in the case ofa more condensed DBT.3.4. Optimization by Box�Behnken Design. The Box�

Behnken design was chosen in this study for optimization of theconversion of BT substrate (Y) in the ODS reaction. In our study15 experimental observations were taken at random for theoptimization process. Table 4 shows the data resulting fromthe 15 random experiments which involve the coded values of the

parameters (+1, 0, �1), their actual values, and the correspond-ing responses (predicted values). The residual conversion isdetermined by the difference between the experimental andpredicted conversions. The plot of actual versus predictedconversion of BT is shown in Figure 11. The best fit curve forthe comparative study represents a 45� line passing through theorigin and having a regression coefficient (R2) value of 0.994.The following quadratic model was obtained to describe themathematical relationship between the three independent pro-cess parameters (X1, X2, X3) and the dependent response (Y):

YðXÞ ¼ 58:36 þ 13:85X1 � 13:28X2 þ 30:54X3

� 4:08X12 � 5:07X2

2 � 20:72X32 � 9:84X1X2

þ 9:68X1X3 � 11:18X2X3 ð1Þ

The results of the analysis of variance (ANOVA) shown inTable 5 present the successful validation of the experimental datato the predicted model. The F-value for the model was found tobe 106.38, which is much higher than the tabulated F-value(F95,0.05) of 4.77. Such a large value of F for all models indicatesthat the predicted second-order polynomial is highly significantto a 95% confidence level. The corresponding P-values in Table 5

Figure 10. Conversions of different thiophenic compounds on meso-porous TS-1 (180 min of reaction time). Stirring speed, 900 rpm;reaction temperature, 50 �C; H2O2:S, 10:1; catalyst loading, 0.05 g;volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30%aqueous H2O2.

Table 4. Box�Behnken Design Matrix

coded values actual values conversion

observation X1 X2 X3 X1 X2 X3 Yexpt Ypred residual

1 1 0 1 60 0.23 0.075 85.62 87.63 �2.01

2 1 �1 0 60 0.06 0.038 89.37 86.18 3.19

3 �1 0 1 20 0.23 0.075 44.19 40.57 3.62

4 0 0 0 40 0.23 0.0375 58.36 58.36 0.0

5 0 �1 1 40 0.06 0.075 86.37 86.57 �1.2

6 �1 0 �1 20 0.23 0.000 0.850 �1.15 2.0

7 0 1 1 40 0.4 0.075 38.23 38.65 �0.42

8 1 0 �1 60 0.23 0.000 3.570 7.19 �3.62

9 �1 1 0 20 0.4 0.0375 28.72 31.92 �3.2

10 0 0 0 40 0.23 0.038 58.36 58.36 0.0

11 0 0 0 40 0.23 0.0375 58.36 58.36 0.0

12 �1 �1 0 20 0.06 0.0375 36.39 38.80 �2.41

13 1 1 0 60 0.4 0.0375 42.35 39.94 2.41

14 0 1 �1 40 0.4 0.0000 1.13 �0.07 1.2

15 0 �1 �1 40 0.06 0.0000 4.57 4.13 0.44

Figure 11. Comparisonbetweenexperimental andpredicted responses (Y).

Figure 9. Effect of reaction time on conversion of BT. Stirring speed,900 rpm; reaction temperature, 60 �C; n(H2O2):n(S),10:1; catalystamount, 0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizingagent, 30% aqueous H2O2; reaction time, 240 min.

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are used to analyze the F-statistics to explain the statisticalsignificance between the predicted response and actual response.A P-value lower than 0.05 signifies that the model is statisticallysignificant. Therefore, in this case the terms X1, X2, X3, X1X2,

X2X3, X1X3, X22, and X3

2 are significant model parameters. TheP-value for the term X1

2 was found to be slightly higher than 0.05.This signifies that the term is of relatively less significance. Otherterms such as X1

3, X23, and X3

3 with very large P-values areneglected from the predicted equation to improve the accuracy,and the correlation coefficient was found to be 0.9974. Thissignifies that the predicted response tallies well with the experi-mental data. A very high R2 (R-Sq) value of 0.9948 indicates thatthe predicted polynomial model is reasonably well fitted with thedata. The predicted R2 (Pred R-Sq) value of 0.9169 is inreasonable agreement with the adjusted R2 (Adj R-Sq) value of0.9855.Response surfaces can be visualized as 3D plots that represent

the variation of the response with two parameters, keeping theother parameters fixed. The resulting 3D surface response plotsfor the BT conversion as a function of (a) temperature and molesof BT per mole of H2O2, (b) temperature and catalyst loading,and (c) catalyst loading and moles of BT per mole of H2O2 areshown in Figure 12, respectively. The response surfaces ofmutual interactions between the parameters were found to beelliptical in nature. The coordinates of the central point withinthemaximum contour levels in each of the given figures representthe optimum values of the respective parameters. The maximum

Table 5. Analysis of Variance (ANOVA) for Conversion

source DF Seq SS Adj SS Adj MS F P

model 9 3314.48 3314.48 1479.39 106.38 <0.0001

X1 1 1533.47 1533.47 1533.47 110.27 0.0001

X2 1 1411.66 1411.66 1411.66 101.51 0.0002

X3 1 7459.70 7459.70 7459.70 536.43 <0.0001

X12 1 61.61 61.61 61.61 4.43 0.0892

X22 1 94.82 94.82 94.82 6.82 0.0476

X32 1 1584.79 1584.79 1584.79 113.96 0.0001

X1X2 1 387.11 387.11 387.11 27.84 0.0033

X1X3 1 374.62 374.62 374.62 26.94 0.0035

X2X3 1 499.52 499.52 499.52 35.92 0.0019

res err 5 69.53 69.53 13.91

lack of fit 3 69.53 69.53 23.18

pure error 2

R-Sq = 99.48%, Adj R-Sq = 98.55%, Pred R-Sq = 91.69%

Figure 12. Surface plots of conversion as a function of (a) mole ratio and temperature, (b) catalyst loading and temperature, and (c) mole ratio andcatalyst loading.

155 dx.doi.org/10.1021/ie2024068 |Ind. Eng. Chem. Res. 2012, 51, 147–157

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predicted conversion lies in the surface confined in the smallestcurve of the 3D contour diagram.Response optimization is a technique which produces a

combination of input parameters that jointly optimizes theresponse. The respective desirability of both the variance andseal strength is 1.0, which indicates that the composite desir-ability of these two variables is also 1.0. In order to obtain thisdesirability limit, it has been decided to set the factor levels at thestarting point for optimization. The values of the processparameters giving the highest conversion are shown in Table 6.The optimum values of the conversion (response) are obtainedby setting the starting point of temperature, mole ratio of H2O2

to S, and catalyst loading at 52.36 �C, 10.64, and 0.0521 g,respectively. The optimum (Yopt) or maximum conversion(Ymax) was determined as 89.92% with optimum temperature,mole ratio, and catalyst loading of 60 �C, 4.77, and 0.0641 g,respectively.For the starting point of optimization, the values of the

independent parameters are as follows: temperature (X1) =52.36 �C (coded value of X1, +0.618); mole ratio (X2) = 0.094(coded value of X2, �0.8); catalyst loading (X3) = 0.0521 g(coded value of X3, +0.389).Thus the RSM technique was usefully applied to optimize the

conversion of BT for the oxidative desulfurization reaction usingH2O2 as an oxidizing agent.3.5. Kinetics of ODS Reaction. The design of experiment

(DOE) does not throw any light on the kinetics of the ODSreaction. The kinetics of the oxidation of benzothiophene wasstudied under the condition of different temperatures: 20, 40,and 60 �C. In our experiment the reaction is a three-phaseheterogeneous type, with the organic phase containing BTdissolved in isooctane, the aqueous phase containing H2O2

oxidant, and mesoporous TS-1 catalyst as the solid phase. Theeffects of the external mass transfer resistances are eliminated byconducting the ODS reaction at a reasonably high stirrer speed tomake the reaction kinetically controlled.Figure 1 shows the probable reaction mechanism scheme.

The H2O2 first oxidizes benzothiophene in the presence of TS-1to benzothiophene 1-dioxide, which is further oxidized byanother H2O2 molecule to 2-phenylethenesulfonic acid andsulfuric acid.25 In general, for a solid catalytic fluid phaseheterogeneous reaction, the rate law obeys the Langmuir�Hinshelwood mechanism. In the present reaction, this mechan-ism is applied to develop a rate equation based on a similar dualsite mechanism.26 The solvent isooctane is assumed to be notadsorbed on the catalyst surface. The overall surface reactionmay be written as

A þ B f C þ D

where A = BT, B = H2O2, C = benzothiophene oxide (BTO),and D = H2O.

adsorption step:

A þ S S A 3 S ðS is the active site of the catalystÞ

B þ S S B 3 S

surface reaction:

A 3 S þ B 3 S f C 3 S þ D 3 S ðrate-limiting stepÞ

desorption step:

C 3 S þ S C þ S

D 3 S S D þ S

It should be noted that the adsorption and desorption steps areassumed to be reversible in nature whereas the surface reaction isthe rate-controlling one. This is possible only during the initialstages of the reaction when reactant formation from product isnegligible.Under these conditions the reaction rate for species A (�rA)

may be written as

�rA ¼ �dCA

dt¼ kKAKBCACB

ð1 þ KACA þ KBCBÞ2ð2Þ

where k = kinetic rate constant, KA = equilibrium adsorptionconstant for species A, KB = equilibrium adsorption constant forspecies B, CA = concentration of species A in reactant mixture(mol/cm3), and CB = concentration of species B in reactantmixture (mol/cm3). The concentration of a species in a biphasicsystem written in the paper is according to their moles in theirrespective phases.Equation 2 can be written in terms of conversion (XA) as

dXdt

¼ CA0kKAKBð1� XAÞðM� XAÞ½1 þ KACA0ð1� XAÞ þ KBCA0ðM� XAÞ�2

dXdt

¼ k0ð1� XAÞðM� XAÞ½1 þ k2ð1� XAÞ þ k3ðM� XAÞ�2

ð2aÞ

where k0 = CA0kKAKB, k2 = CA0KA, k3 = CA0KB, and M = CB0/CA0 = 10.Equation 2a was fitted with a nonlinear method of analysis by

usingMatlab 7.1. The values of the parameter k2 were found to benegative for 20, 30, 40, and 50 �C and also with largemean squareerrors. Table 7 shows the results of the nonlinear estimation ofparameters of eq 2a. As the k2 values at all temperatures are foundto be negative, the model represented by eq 2a is rejected.Therefore, a trial has been attempted by neglecting the denomi-nator of either eq 2 or eq 2a to get an empirical rate equation ofthe following form:

�rA ¼ �dCA

dt¼ k1CACB ð3Þ

where k1= kKAKB (cm3/mol 3min).

Now, the amount of H2O2 was taken in excess and the changein concentration of H2O2 in comparison to benzothiophene(BT) is negligible, so CB can be considered as constant. Hence,

Table 6. Values of Process Parameters for OptimumConversiona

parameter coded value actual value

dependent parameter

Y (conversion %) 89.9179

independent parameters

X1 (temperature, �C) +1 60

X2 (mole ratio, S:H2O2) �0.12 0.2096

X3 (catalyst loading, g) +0.71 0.0641aComposite desirability = 1.000 000.

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Industrial & Engineering Chemistry Research ARTICLE

the reaction may be considered as a pseudo-first-order reaction,where the rate is apparently dependent on the concentration ofthe limiting reactant (BT).Therefore, eq 3 may be simplified as

�rA ¼ � dCA

dt¼ kappCA ð4Þ

where kapp = apparent rate constant = k1CB0 min�1.Equation 3 is integrated with limits for t = 0, CA = CA0, to t = t,

CA = CA:

�lnCA

CA0¼ kappt

or

ln1

1� XA

� �¼ kappt ð5Þ

whereXA = fractional conversion of reactant species A at time t= t.

XA ¼ 1� CA

CA0

ln(1/1� XA), calculated from the conversion data, are plotted asa function of time (t) at 20, 40, and 60 �C, as shown in Figure 13.The straight lines fitted at different temperatures with goodcorrelation coefficients confirm the kinetics of pseudo-first-orderreaction. The apparent rate constant kapp is determined from theslope of the straight line.Now from the Arrhenius equation we can write

kapp ¼ kapp0 exp�EaRT

� �ð6Þ

where Ea = activation energy for the reaction (kJ/mol), T =reaction temperature (K), and kapp0 = frequency factor or pre-exponential factor (min�1). Taking the logarithm on both sidesof eq 6, it may be expressed by

ln kapp ¼ ln kapp0 � EaRT

ð7Þ

The plot of ln kapp versus 1/T gives a linear plot as shown inFigure 14. The activation energy for the system and thefrequency factor are determined from the slope and interceptof eq 7, respectively, and are found to be Ea = 25.2 kJ/mol andfrequency factor kapp0 = 8450.7 min�1.

4. CONCLUSION

The oxidative desulfurization of a syntheticmixture of benzothio-phene and isooctanewas carried out withH2O2 as an oxidizing agentusing DGC synthesized mesoporous TS-1 catalyst. A kinetic studywas performed for the above system to study the effect of variousprocess parameters. Apseudo-first-order kineticmodelwas proposedas a rate equation for the reaction system. The activation energy (Ea)was found to be 25.20 kJ/mol. The process was optimized withrespect to three important parameters, temperature, mole ratio of SandH2O2, and catalyst loading, using response surface methodologywith the Box�Behnken design. A second-order quadraticmodel wasfitted with the experimental data with a good correlation coefficientof 0.9974. The optimum conversion was found to be 89.92% for anoptimum temperature of 60 �C, a mole ratio of H2O2 to S of 4.77,and a catalyst loading of 0.0641 g.

’AUTHOR INFORMATION

Corresponding Author*Tel.: +91-3222-283954. Fax: +91-3222-282250. E-mail: sonalis@che.iitkgp.ernet.in.

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Figure 14. Arrhenius plot.

157 dx.doi.org/10.1021/ie2024068 |Ind. Eng. Chem. Res. 2012, 51, 147–157

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