Adsorption of ethyl, iso-propyl, n-butyl and iso-butyl mercaptans on AgX zeolite: Equilibrium and...

1

3

4

5

6

7 Q1

89

10

111213

1 51617181920212223242526272829

3 03132

34

35

3737

3 9

4041424344

454647484950

5 1

64

Q2

Fuel xxx (2014) xxx–xxx

JFUE 8007 No. of Pages 8, Model 5G

20 April 2014

Contents lists available at ScienceDirect

Fuel

journal homepage: www.elsevier .com/locate / fuel

Adsorption of ethyl, iso-propyl, n-butyl and iso-butyl mercaptans on AgXzeolite: Equilibrium and kinetic study

http://dx.doi.org/10.1016/j.fuel.2014.04.0130016-2361/� 2014 Published by Elsevier Ltd.

⇑ Corresponding author at: Chemical Engineering Department, AmirkabirUniversity of Technology, P.O. Box 15875-4413, Tehran, Iran. Tel.: +982164543160; fax: +98 2166405847.

E-mail address: [email protected] (C. Falamaki).

Please cite this article in press as: Barzamini R et al. Adsorption of ethyl, iso-propyl, n-butyl and iso-butyl mercaptans on AgX zeolite: Equilibriukinetic study. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.04.013

Reza Barzamini a, Cavus Falamaki b,c,⇑, Rahman Mahmoudi a

a Chem. Eng. Dept., Mahshahr Campus, Amirkabir University of Technology, 415 Mahshahr, Iranb Chemical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iranc Petrochemical Center of Excellence, Amirkabir University of Technology, P.O. Box 15875-4413,Tehran, Iran

h i g h l i g h t s

� AgX zeolite performance for deepmercaptan removal from liquid fuelwas studied.� Ethyl, iso-propyl, n-butyl and iso-

butyl mercaptans were studied.� The isotherms for each mercaptan

molecule was obtained for 26, 40 and60 �C.� The anomalous increase in adsorption

capacity with increasing temperaturehas been discussed.� The kinetics of adsorption at 26 �C for

each mercaptan is studied.

g r a p h i c a l a b s t r a c t

AgX zeolite shows a substantially higher adsorption capacity for ethyl mercaptan compared to NaX, NiXand NiAgX zeolites.

5253545556575859606162

a r t i c l e i n f o

Article history:Received 21 December 2013Received in revised form 3 April 2014Accepted 6 April 2014Available online xxxx

Keywords:AdsorptionZeoliteMercaptanAgX

a b s t r a c t

It has been shown that AgX zeolite has a distinctly higher adsorption capacity compared to NaX, NiX andNiAgX zeolites for ethyl mercaptan. Chosen as the best adsorbent, the adsorption isotherms of ethyl,iso-propyl, n-butyl and iso butyl mercaptans on AgX zeolite at three different temperatures of 26, 40and 60 �C had been studied. The AgX zeolite showed the highest adsorption capacity for the ethyl mer-captan adsorbate molecule. On the other hand, this mercaptan molecule was the easiest to be removed bythermal treatment in the range of 150–300 �C. Except ethyl mercaptan, all the other mercaptan moleculesshowed increased adsorption on AgX zeolite with increasing the temperature in the range of 26–60 �C.The kinetic analysis of the adsorption process showed that there exists a rational correlation betweenthe size and configuration (linear or branched) of the molecules and the effective diffusion coefficient.

� 2014 Published by Elsevier Ltd.

63

65

66

67

68

69

1. Introduction

The last two decades have been accompanied with an intenseresearch on the development of new adsorbents for the removalof mercaptan contaminants from liquid hydrocarbon fuels. Aimingat a high selectivity, high adsorption capacity, regenerability and

m and

http://dx.doi.org/10.1016/j.fuel.2014.04.013

mailto:[email protected]


http://www.sciencedirect.com/science/journal/00162361

http://www.elsevier.com/locate/fuel


70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

2 R. Barzamini et al. / Fuel xxx (2014) xxx–xxx


20 April 2014

safe operations, zeolites may be considered as potential competi-tors to metallic oxides and sulfides, reduced metals and activatedcarbon. Published reports on deep mercaptan removal using zeo-lites are relatively limited [1–6]. Weber et al. [4] conducted aninteresting study on the equilibrium adsorption of ethyl mercap-tan/toluene and ethyl mercaptan/n-heptane gas phase mixtureson NaX zeolite at 25 �C. They concluded that the adsorption ofethyl mercaptan is reversible and that the adsorption selectivitymay be explained based on entropic effects due to steric hindranceof molecules confined in a-cages. Faujasite structure type zeolitesin different cationic forms (Na, Cs, CsNa, Ca, MgNa, Ba, ZnNa, Ni,NiNa forms) have been investigated by Ryzhikov et al. [6] for theremoval of methyl mercaptan from CH4 gas streams concludingthat the Ni form possesses the maximum adsorption capacity.Although not explicitly stated in the latter text, it goes without say-ing that a p-complexation type bond between the nickel atoms andthe d electron pair of the sulfur atom of the mercaptan moleculehas been the main responsible for the high adsorption capacityreported. Accordingly, it is expected that the latter bond is irrevers-ible and the regeneration of the mercaptan loaded zeolite after theseparation process needs a rather high temperature treatment. Theapplication of p-complexation reactions for the development ofhigh capacity/selective zeolitic adsorbents for the removal of thio-phenic sulfur compounds had been first introduced by workinggroups under the leading of Yang et al. [7,8].

The present study comprehends a systematic study on theadsorption behavior of cation-exhanged X zeolites for ethyl, iso-propyl, n-butyl and iso-butyl mercaptan molecules in the liquidstate using iso-octane as fuel simulating liquid. As a first step,the adsorption behavior of NaX, NiX, NiAgX and AgX zeolites forethyl mercaptan at 26 �C are compared. This is merely a shortintroduction to show qualitatively the distinctly higher adsorptioncharacteristic of AgX zeolite compared to the others. A small sec-tion is dedicated to the XRD and FTIR characterization of the ion-exchanged zeolites. Based on the obtained results and the discus-sion provided, AgX zeolite has been selected for a thorough andsystematic analysis of ethyl, iso-propyl, n-butyl and iso-butyl mer-captan molecules for three different temperatures of 26, 40 and60 �C. It should be reminded that mercaptan removal using AgXzeolite has not been considered so far in open literature. Theadsorption isotherms will be discussed in terms of adsorbate mol-ecule carbon number and normal or iso configuration as a functionof adsorption temperature. The final section concerns the adsorp-tion kinetics of the different mercaptan molecules at 26 �C on theAgX zeolite. The estimated diffusion coefficients will be discussedin terms of adsorbate molecule carbon number and normal or isoconfiguration.

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

2. Experimental

NaX zeolite, in powder form, with an Si/Al molar ratio of 1.31and average particle size of 3 lm was purchased from SPAG Co.(Iran). Reagent grade Ni(NO3)2�6H2O and AgNO3 were purchasedfrom Merck and Aldrich, respectively.MX form zeolites (whereM = Ni or Ag), were obtained by contacting twice the zeolite withthe aqueous solution of the metal nitrate for a period of 24 h atRT. For the ion exchange process, 7.5 g zeolite was put into contactwith 750 cm3 0.1 M nickel nitrate (or silver nitrate) solution for24 h under agitation at RT. The resulting solid was thoroughlywashed with de-ionized water and afterwards dried at 110 �C for8 h. In the case of Ag+ ion-exchange, the treatment was performedin the dark to avoid precipitation of metallic silver. For the synthe-sis of AgNiX zeolite, the NaX parent zeolite was first ion-exchangedwith Ni (one step, 24 h treatment period at RT), washed to separateany residual Ni salt and further ion-exchanged with Ag (one step,

Please cite this article in press as: Barzamini R et al. Adsorption of ethyl, iso-pkinetic study. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.04.013

24 h treatment at RT). The final powder samples were dried at110 �C for 24 h.

Ethyl, iso-propyl, n-butyl and iso-butyl mercaptans with a pur-ity of 97%, 97%, 98% and 92%, respectively, were purchased fromAldrich. Iso-octane (Merck), was used as liquid fuel simulating sol-vent. The procedure for sulfur compounds adsorption has been asfollows:

Determined amounts of the zeolite were heated in an electricalfurnace (Lenton) with a heating rate of 2.5 �C min�1 up to 350 �C.Afterwards, the zeolite was maintained for 2 h at 350 �C and fur-ther cooled to 100 �C in a period of 2 h. The hot zeolite powderwas then immediately transferred to special 50 cm3 glass contain-ers. The latter were closed with rubber caps and then punchedwith aluminum rings to completely seal them from the surround-ings. In the next step, 10 cm3 of the mercaptan/iso-octane solutionof pre-determined concentration was injected to the containerusing a syringe. Then, the bottle was transferred to a shaker-incu-bator (Orbital SL 50), where the temperature was set (27–60 �C)and the adsorption process was continued for 2 h. Actually thevalue of 2 h is based on our previous experiments for understand-ing the time for an acceptable constancy in the concentration of thesimulated liquid fuel during the adsorption process. The concentra-tion of sulfur in the final solutions was determined using a TotalSulfur Analysis instrument (TS-100) with a detection limit of0.5 ppm. Kinetic experiments were run isothermally at 26 �C undershaking, using an average initial sulfur concentration of357 mg S L�1.

The reversibility of the adsorption process of the AgX zeolite, forethyl mercaptan as adsorbate molecule, has been investigated asfollows: (a) The routine batch adsorption experiment was per-formed as explained previously but using an initial mercaptan con-centration of 748 mg S L�1 (b) After performing the adsorptionexperiment, the zeolite was regenerated by isothermal heating inambient atmosphere at 240 �C for 12 h (c) The regenerated zeolitewas activated at 300 �C by the same procedure explained for theroutine adsorption experiments.

The metal content of the ion-exchanged and parent zeolite wasdetermined by ICP analysis (ARL 3410). An Inel Equinox 3000 appa-ratus was used for taking the XRD patterns of the different zeolitesamples. The FTIR spectra of the parent, ion-exchanged and, insome cases, mercaptan loaded zeolite samples were taken usinga Spectrum GX apparatus. For this means the solid sample wasmixed with KBr powder and analyzed in pellet form. The FTIR spec-tra of the liquid mercaptan samples have also been taken (ethylmercaptan excluded due to high volatility). Thermogravimetricanalysis of the mercaptan containing zeolite samples was per-formed using a Diamond TG/DTA instrument. The moisture of thesimulated solutions was determined by the Karl Fisher methodusing a Mettler DL 350 apparatus.

3. Results and discussion

According to ICP analysis results, the degree of substitution ofNa+ in NiX, NiAgX and AgX zeolites has been 66.4%, 58.0% and80.6%, respectively. In the case of NiAgX zeolite, the weight percentof Ni and Ag have been 6.0 and 6.7, respectively. As mentioned inthe introduction, total cation substitution of Na+ was not intendedas we aimed at doing a screening test for the type of cation to beexchanged. Fig. 1 shows the XRD patterns of the NaX, NiX, andNiAgX and AgX zeolites. It is remarkable that the pattern of NaXparent zeolite is retained only in the case of NiX zeolite. Introduc-tion of Ag+ cations changes abruptly the XRD pattern. The effect isless pronounced in the case of NiAgX zeolite and is extreme in thecase of AgX zeolite.

ropyl, n-butyl and iso-butyl mercaptans on AgX zeolite: Equilibrium and


195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

229229

230

231

232

233

234

235

236

237

238

239

240

241

Fig. 1. XRD patterns of NaX, NiX, NiAgX and AgX zeolites.

Fig. 3. The adsorption isotherms of ethyl mercaptan on NaX, NiX, NiAgX and AgX at26 �C.

R. Barzamini et al. / Fuel xxx (2014) xxx–xxx 3


20 April 2014

Fig. 2 shows the FTIR spectra of the NaX, NiX, NiAgX and AgXzeolites. Comparing the peaks belonging to the NaX and NiXzeolites it is observed that introduction of Ni as extra-frameworkcation exhibits itself by the appearance of a new distinct peak at920 cm�1, due to internal tetrahedral lattice asymmetrical stretch-ing vibrations. Comparing the peaks due to the NaX and AgXzeolites, it is observed that introduction of Ag results in the crea-tion of a new peak at 483 cm�1, due to T–O bending vibrations.In addition, the peak at 1051 cm�1 belonging to NaX zeolite under-goes a meaningful shift to 1102 cm�1. Accordingly, the peaksobserved at 483, 907 and 1102 cm�1 in the FTIR pattern of theNiAgX zeolite testify the presence of Ni and Ag extra-frameworkcations in the crystalline structure, respectively.

Fig. 3 shows the adsorption isotherms of NaX, NiX, NiAgX andAgX for ethyl mercaptan at 26 �C. It should be reminded that theKarl Fisher test showed no detectable water in the simulated liquidfuel solutions. This series of adsorption experiments were run as ascreening test for selecting the best kind of adsorbent. It isobserved that NaX shows the smallest adsorption capacity. Majorsubstitution of Na with Ni enhances the mercaptan uptake of thezeolite. However, a slight substitution of Ni with Ag in the NiXzeolite substantially increases the adsorption characteristic. The

Fig. 2. FTIR spectra of (a) NaX, AgX and (b) NiAgX and NiX


zeolite in its Ag form exhibits a distinctly favorable isotherm withan adsorption capacity substantially higher than that of the NiAgXzeolite. Based on these results, AgX was selected as the best adsor-bent and all the other experiments were performed using thiszeolite.

Fig. 4a–d shows the adsorption behavior of AgX for each of theadsorbates understudy and as a function of temperature. Theuncertainty of the experimental data is estimated to be less than2 mg S g�1. All the pertaining isotherms follow a Langmuir typecorrelation:

q ¼ qmbCs

1þ bCsð1Þ

where q is the adsorption capacity (mg S g�1), qm is the saturationadsorption capacity (mg S g�1), b is a constant and Cs is the equilib-rium sulfur concentration in the liquid phase (mg S L�1). Table 1lists the corresponding Langmuir parameters and the pertainingcorrelation function for each adsorbate at each temperature. TheLangmuir isotherms have been drawn in Fig. 5a–c as bold lines(to be discussed later).

According to Fig. 4a, the ethyl mercaptan adsorption isothermhas generally a weak dependence on temperature in the range of26–60 �C. The saturation adsorption capacity lies between 102and 105 mg S g�1 over the whole temperature range and may beassumed approximately unaffected by the adsorption temperature.

zeolites. The vertical axis is the transmittance (a.u.).



Fig. 4. Adsorption isotherms of (a) ethyl (b) iso-propyl (c) n-butyl and (d) iso-butylmercaptan on AgX zeolite at 26, 40 and 60 �C .

Table 1Langmuir parameters as a function of adsorbate molecule kind and temperature.

Adsorbate molecule Temperature (�C) qm (mg S g�1) b (mg S�1 L)

ethyl mercaptan 26 105.26 0.025ethyl mercaptan 40 102.04 0.050ethyl mercaptan 60 102.04 0.070

iso-propyl mercaptan 26 76.92 0.039iso-propyl mercaptan 40 76.92 0.151iso-propyl mercaptan 60 84.03 0.300

n-butyl mercaptan 26 59.85 0.080n-butyl mercaptan 40 58.82 0.197n-butyl mercaptan 60 59.88 0.710

iso-butyl mercaptan 26 47.5 0.079iso-butyl mercaptan 40 58.48 0.154iso-butyl mercaptan 60 59.88 0.582

Fig. 5. Adsorption isotherms of all mercaptan molecule types on AgX zeolite at (a)26 �C (b) 40 �C and (c) 60 �C (symbols: experimental data; solid lines: fitted data byLangmuir model).



20 April 2014

Please cite this article in press as: Barzamini R et al. Adsorption of ethyl, iso-propyl, n-butyl and iso-butyl mercaptans on AgX zeolite: Equilibrium andkinetic study. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.04.013


242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269



20 April 2014

Fig. 4b shows the isotherms due to iso-propyl mercaptan as afunction of temperature. In this case, the temperature dependenceof the adsorption behavior is worth consideration. From 26 to40 �C, an increase in the adsorption capacity at low concentrations(Cs < 125 mg S L�1 is observed, while the saturation adsorptioncapacity is approximately the same. In terms of Langmuirparameters (Table 1), qm is 76.92 mg S g�1 in each case but bincreases from 0.039 (20 �C) to 0.151 (40 �C). From 40 to 60 �C,the adsorption capacity undergoes a clear increase in all theconcentration range understudy. This fact manifests itself in termsof a substantially higher value of qm (84.03 mg S g�1). The latterenhancement in adsorption capacity as a function of temperatureincrease is an ‘abnormal’ behavior in adsorption processes andcannot be explained by the conventional thermodynamic rules.

Fig. 6. TGA diagrams of AgX zeolite pre-loaded (saturated) with (a


This phenomenon will be addressed later, after having consideredthe other adsorbates.

Fig. 4c shows the isotherms pertaining to n-butyl mercaptan asa function of temperature. The thermal behavior is highly similarto that of iso-propyl mercaptan. From 26 to 40 �C, an increase inthe adsorption capacity at low liquid concentrations (Cs < 50mg S L�1 is observed. The change in saturation sulfur removalcapacity is less pronounced. From 40 to 60 �C, the adsorptioncapacity increases over the whole concentration range but the sat-uration capacity remains quite the same (qm = 59.88 mg S g�1) .

Fig. 4d shows the isotherms due to iso-butyl mercaptan as afunction of temperature. Increasing the temperature from 26 to60 �C, results in an increase of the adsorption capacity over thewhole sulfur concentration understudy. The saturation capacity

) ethyl (b) iso-propyl (c) n-butyl and (d) iso-butyl mercaptan.



270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

308

309

310

311

312

313

314

315

316

318318

319

320

321

322

323

324

325

326

327

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359

360

Fig. 7. The first and second peak adsorbate release temperatures in diagrams forethyl, iso-propyl, n-butyl and iso-butyl mercaptans.

Fig. 8. The ratio of the area of the second (higher temperature) peak to the first(lower temperature) peak in the TGA diagram for the different mercaptanmolecules.



20 April 2014

increases from 47.50 to 58.48 mg S g�1 from 26 to 40 �C, i.e.10.98 mg S g�1. The saturation capacity increases from 58.48 to59.88 mg S g�1 from 40 to 60 �C, i.e. only 1.40 mg S g�1.

Regarding the adsorption/temperature dependence, anenhancement is clearly observed in the temperature range 26–60 �C for branched mercaptans with increasing temperature. Ethylmercaptan shows the least temperature dependence, while iso-propyl mercaptan shows the highest.

The authors of the present work postulate that the increase inthe adsorptive capacity for branched mercaptans, is mainly dueto the enhanced diffusion rate at higher temperatures accompa-nied with a slight expansion of the zeolite framework. The ethylmercaptan molecule possesses the most appropriate moleculargeometric configuration with respect to the other adsorbatemolecules (smallest carbon chain, no branches). Accordingly, ethylmercaptan molecules are supposed to be able to enter and diffusethrough the pores much easier than iso-butyl mercaptan mole-cules. Hence, increasing the temperature from 26 to 60 �C doesnot result in a substantial increase in the adsorption capacity ofthe zeolite for this type of adsorbate molecules. Instead, highertemperatures favor the adsorption process of the zeolite for themore complex molecules like iso-butyl mercaptan (refer toFig. 5). What has been said is also in accordance with our kineticexperiments discussed later.

At this stage, we aim at comparing the adsorption behavior ofAgX zeolite as a function of kind of mercaptan at each temperature.Fig. 5a shows the isotherms at 26 �C for all the adsorbate kinds.Normal butyl and iso-butyl mercaptans show a similar isothermand have the least adsorption capacity throughout the whole rangeof sulfur concentration understudy. It is observed that theLangmuir model is not able to fit well the iso-butyl mercaptanisotherm. Iso-propyl mercaptan is adsorbed more favorably withrespect to n-butyl and iso-butyl mercaptan. Ethyl mercaptanshows a distinct affinity versus AgX zeolite and is adsorbed signif-icantly better than all the other mercaptan molecules investigated.The same trend holds for the adsorption temperature of 40 �C(Fig. 5b). Again, n-butyl and iso-butyl mercaptans show a similarisotherm and have the least adsorption capacity.

According to Fig. 5c, increasing the temperature from 40 to60 �C has a slight effect on the n-butyl and iso-propyl mercaptanisotherms. Instead, the adsorption behavior of iso-propyl mercap-tan molecules assimilates that of ethyl mercaptan at thistemperature.

Summing up, it may be stated that the capacity for mercaptanadsorption in the temperature range of 26–60 �C obeys the follow-ing order:

ethyl mercaptan > iso-propyl mercaptan > n-butyl mercaptan

ffi iso-butyl mercaptan

Fig. 6a–d shows the TGA diagrams of the AgX zeolite pre-loaded(saturated) with ethyl, iso-propyl, n-butyl and iso-butyl mercap-tan. Generally, the curves show an initial weight loss in the tem-perature range of 25–150 �C, which is attributed to adsorbedimpurities (mostly water). It should be reminded that the zeolitewas handled in contact with ambient atmosphere prior to theThermogravimetric analyses and therefore was prone to adsorbwater vapor from the air. All the zeolites show a double peak inthe temperature range of 150–300 �C, which is accompanied witha substantial weight loss (ca. 20 wt.%). These two peaks do not cor-respond to the egress of two different components. Instead, theydemonstrate the existence of two distinct types of adsorbate/adsorbent energetic interaction (in other words, two differentadsorption sites). In the case of ethyl and n-butyl mercaptans, avery weak third peak is observed centered on 275 �C. No distinctpeak in the range 300–900 �C is observed for each case. The first


observation is that the mercaptan molecules, regardless of theirtype, can be desorbed at temperatures lower than 300 �C and thatno chemically very strong interaction between the adsorbate andadsorbent exists (no peak in the range of 300–900 �C). Stronginteractions leading to desorption temperatures in the range of300–900 �C have been reported in the case of benzothiopheneadsorption on faujasite zeolites and are attributed to S–M bonds(r bonds) [9]. The absence of S–M type bonds in our case is attrib-uted to the absence of conjugated aryl groups in the adsorbatemolecules under study. The second observation is based on aquantitative analysis of the peak areas and the corresponding tem-peratures of the peaks maxima. Fig. 7 shows the first and secondpeak temperatures for the different mercaptan molecules. The firstand second peaks of ethyl mercaptan occur at significantly lowervalues with respect to iso-propyl, n-butyl and iso-butyl mercaptan.Fig. 8 shows the ratio of the area of the second (higher tempera-ture) peak to the first (lower temperature) peaks. The area of eachpeak was evaluated with respect to the background. In addition,the area of the two overlapping peaks was determined after de-convolution of the total diagram. Interestingly the correspondingvalues for the iso-butyl and n-butyl mercaptan molecules are sim-ilar. However, the latter value is significantly smaller for iso-propylmercaptan. Recall that the area of each peak is proportional to thecontent of the pertaining adsorbate/adsorbent interaction. It maybe deduced that, generally, two types of zeolite/mercaptan interac-tions with different but close energetic levels exist. The nature of



361

362

363

364

365

366

367

368

369

370

371

372

373

374

375

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

396

397

398

399

400

401

402

403

404



20 April 2014

the bonds is presumably of a semi-covalent nature as in the case ofmost thiol-metal bonds. A third kind of interaction was observedfor ethyl and n-butyl mercaptan, but, as explained, it is not signif-icant. This may be due to the major involvement of two differentadsorption sites within the zeolite framework. In the case of iso-propyl mercaptan, most adsorption takes place on the sites withthe lowest level of energy. The case is reversed for ethyl, n-butyland iso-butyl mercaptan molecules. Generally speaking, iso-propylmercaptan molecules have a distinct different energetic interactionwith the zeolite framework. It is presumed that these differentinteractions are due to different cationic sites (denominated as I,II and III) present in the zeolite channels. These sites usually com-prehend the exchangeable cation sites in the 6-ring between thealpha and beta cages, recessed into the beta cages, recessed intothe alpha cages and in the hexagonal prism connecting two betacages in the X zeolite [12].

The AgX zeolites saturated with the different mercaptan mole-cules have been subjected to FTIR analysis. For the sake of place,only the spectra due to iso-propyl and iso-butyl mercaptan havebeen shown in Fig. 9a and b. A band around 1630 cm�1 is presentin both figures and is attributed to adsorbed water. The spectrumof the parent liquid phase mercaptan has also been included for

Fig. 9. FTIR spectra of AgX before and after saturation with (a) iso-propyl and (b) iso-bucomparison.


a better analysis of the results. It is observed that the mercaptanloaded zeolites show distinct peaks in the range of 2800–3000 cm�1 (C–H stretching vibrations) and 1200–1600 cm�1 (CH2

and CH3 bending vibrations). However, compared to the pertainingFTIR spectrum of the parent liquid mercaptan, a clear shift of theabove peaks toward lower wave-numbers is observed in the caseof iso-propyl mercaptan. Iso-butyl, n-butyl and ethyl mercaptansdid not show such a significant shift in wave-number uponadsorption. It is recalled that ethyl mercaptan showed the highestadsorption capacity over the whole sulfur concentration rangeunderstudy compared to the other mercaptan molecules. Accord-ingly, it may be stated that, as far as the molecule deformation isconcerned, the interaction between the zeolite cavity and poresand the mercaptan molecule is highest in the case of iso-propylmercaptan.

As a final remark, it would be interesting to consider the kinet-ics of adsorption at 26 �C for the 4 type of mercaptan molecules onthe AgX zeolite. Fig. 10 shows the liquid phase normalized sulfurconcentration (actual concentration/initial concentration) as afunction of adsorption time for each of the mercaptan moleculetype at 26 �C. The results are interesting. The adsorption ratesfollow the following order:

tyl mercaptan. Spectra of iso-propyl and iso-butyl mercaptan have been added for



405

407407

408

409

410

411

412

413

414

415

416

417

418

419

420

421

422

423

424

425

426

427

428

429

430

431

432

433

434

435

436Q3

437

438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474

475

Fig. 10. The liquid phase normalized sulfur concentration (actual concentration/initial concentration) as a function of adsorption time for each mercaptan at 26 �C.



20 April 2014

ethyl mercaptan > n-butyl mercaptan > iso-propyl mercaptan

> iso-butyl mercaptan

It is observed that ethyl mercaptan and n-butyl mercaptan pos-sess the fastest kinetics. On the other hand, the isomers (iso-propyland iso-butyl) show the slowest kinetics. These results show thatfor a fast kinetics, it is essential that the molecule has a quasi-linearconfiguration. The number of carbon atoms of the carbon backboneis less determining in the carbon number range 2–4. Based on ashrinking core model [13], a preliminary estimate of the diffusioncoefficients has been calculated and is 28.0 � 10�5, 11.5 � 10�5,5.9 � 10�5 and 4.6 � 10�5 cm2 s�1 for ethyl, n-butyl, iso-propyland iso-butyl mercaptans, respectively.

4. Conclusions

It has been shown that AgX zeolite has a distinctly higheradsorption capacity compared to NaX, NiX and NiAgX zeolites forethyl mercaptan. Chosen as the best adsorbent, the adsorptionisotherms of ethyl, iso-propyl, n-butyl and iso butyl mercaptanson AgX zeolite at three different temperatures of 26, 40 and60 �C had been studied. The AgX zeolite showed the highestadsorption capacity for the ethyl mercaptan adsorbate molecule.Except ethyl mercaptan, all the other mercaptan molecules showed


increased adsorption on AgX zeolite with increasing the tempera-ture in the range of 26–60 �C. Ethyl mercaptan molecule was theeasiest to be removed by thermal treatment in the range of 150–300 �C without excessive loss in the zeolite adsorption capacity.The kinetic analysis of the adsorption process showed that thereexists a rational correlation between the size and configuration(linear or branched) of the molecules and the effective diffusioncoefficient.

5. Uncited references

[10,11].

References

[1] Scherbina YI, Yakubovich VA, Mikhal’skaya LI. Kinetics of mercaptanadsorption from oil fractions using X type zeolites. Neftekhymiya1977;17:151–5.

[2] Sarbak Z. Desulfurization of ethanethiol over cadmium and mercury modifiedzeolite NaX. Appl Catal A: General 1996;147:47–54.

[3] Benoit F, Weber G, Bellat J-P, Paulin C, Limburg-Noetinger S, Thomas M, et al.Adsorption equilibria of binary ethylmercaptan/hydrocarbon mixtures on NaXzeolite. Stud Surf Sci Catal 2005;158:1185–92.

[4] Weber G, Benoit F, Bellat J-P, Paulin C, Mougin P, Thomas M. Selectiveadsorption of ethyl mercaptan on NaX zeolite. Micropor Mesopor Mater2008;109:184–92.

[5] Seyedeyn-Azad F, Ghandi AH, Aghamiri SF, Khaleghian-Moghadam R. Removalof mercaptans from light oil cuts using Cu(II)-Y type zeolite. Fuel Proc Technol2009;90:1459–63.

[6] Ryzhikov A, Hulea V, Tichit D, Leroi C, Anglerot D, Coq B, et al. Methylmercaptan and carbonyl sulfide traces removal through adsorption andcatalysis on zeolites and layered double hydroxides. Appl Catal A: General2011;397:218–24.

[7] Yang RT, Hernandez-Maldonado AJ, Yang FH. Desulfurization of transportationfuels with zeolites under ambient conditions. Science 2003;301:79–81.

[8] Hernandez-Maldonado AJ, Yang RT. New sorbents for desulfurization of dieselfuels via p-complexation. AIChE J 2004;50:791–801.

[9] Zhang ZY, Shi TB, Jia CZ, Ji WJ, Chen Y, He MY. Adsorptive removal of aromaticorganosulfur compounds over the modified Na-Y zeolites. Appl Catal B:Environ 2008;82:1–10.

[10] Jiang M, Ng FTT, Rahman A, Patel V. Flow calorimetric and thermalgravimetricstudy of adsorption of thiophenic sulfurcompounds on NaYzeolite. Thermochim Acta 2005;434:27–36.

[11] Bezverkhyy I, Bouguessa K, Geantet C, Vriant M. Adsorption oftetrahydrothiophene on Faujasite type zeolites: breakthrough curves andFTIR spectroscopy study. Appl Catal B: Environ 2006;62:299–305.

[12] Kevan L. Electron spin resonance characterization of microporous andmesoporous oxide materials. In: Auerbach SM, Carrado KA, Dutta PK, editors.Handbook of zeolite science and technology. New York Basel: Marcel Dekker;2003. p. 257.

[13] Levenspiel O. Chemical reaction engineering. New York: John Wiley & Sons;1972. p. 360.



Adsorption of ethyl, iso-propyl, n-butyl and iso-butyl mercaptans on AgX zeolite: Equilibrium and...

Documents

Transcript of Adsorption of ethyl, iso-propyl, n-butyl and iso-butyl mercaptans on AgX zeolite: Equilibrium and...