cyclohexane mixtures on micro-mesoporous silica SBA-2

Journal Pre-proof Dynamic adsorption separation of benzene/cyclohexane mixtures on micromesoporous silica SBA-2 Maria Jose Emparan-Legaspi, Jorge Gonzalez, Gabino Gonzalez-Carrillo, Silvia G. Ceballos-Magaña, Jesus Canales-Vazquez, Ismael Alejandro Aguayo-Villarreal, Roberto Muñiz-Valencia PII:

S1387-1811(19)30801-7

DOI:

https://doi.org/10.1016/j.micromeso.2019.109942

Reference:

MICMAT 109942

To appear in:

Microporous and Mesoporous Materials

Received Date: 12 September 2019 Revised Date:

3 December 2019

Accepted Date: 6 December 2019

Please cite this article as: M.J. Emparan-Legaspi, J. Gonzalez, G. Gonzalez-Carrillo, S.G. CeballosMagaña, J. Canales-Vazquez, I.A. Aguayo-Villarreal, R. Muñiz-Valencia, Dynamic adsorption separation of benzene/cyclohexane mixtures on micro-mesoporous silica SBA-2, Microporous and Mesoporous Materials (2020), doi: https://doi.org/10.1016/j.micromeso.2019.109942. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.

CRediT author statement Maria Jose Emparan-Legaspi: Investigation, Methodology, Jorge Gonzalez: Supervision, Methodology, Gabino Gonzalez-Carrillo: Investigation, Silvia G. Ceballos-Magaña: Conceptualization, Validation, Jesus Canales-Vazquez: Investigation, Ismael Alejandro Aguayo-Villarreal: Conceptualization, Visualization, Roberto Muñiz-Valencia: Methodology, Supervision, Writing - Original Draft

1

Dynamic adsorption separation of benzene/cyclohexane mixtures on

2

micro-mesoporous silica SBA-2

3

Maria Jose Emparan-Legaspia, Jorge Gonzaleza, Gabino Gonzalez-Carrilloa, Silvia G.

4

Ceballos-Magañab, Jesus Canales-Vazquezc, Ismael Alejandro Aguayo-Villarreala,

5

Roberto Muñiz-Valenciaa *

6

a

7

km. 9, 28400 Coquimatlán, Colima, México

8

b

9

Colima, México

Facultad de Ciencias Químicas, Universidad de Colima, Carretera Colima-Coquimatlán

Facultad de Ciencias, Universidad de Colima, Bernal Díaz del Castillo 340, 28045,

10

c

11

Albacete 02071, Spain

Instituto de Energías Renovables, University of Castilla-La Mancha, Campus Universitario,

12 13

Abstract

14

Due to benzene industrial applications, the development of selective separation methods had

15

been taken importance. In this sense, benzene and cyclohexane separation is considered one

16

of the most challenging isolation processes in petrochemical industry. This is due to the

17

similarity of their structures and physicochemical characteristics, as well as the formation of

18

an azeotrope. Recently, separation methods have been developed using porous materials as

19

sorptive materials such as activated carbons, zeolites or ordered mesoporous silicas. In this

20

work, SBA-2 heated at 240, 550 and 800 °C was tested as separation material using dynamic

21

adsorption to separate benzene and cyclohexane. SBA-2 materials were characterized using

22

powder XRD, nitrogen adsorption, TEM and 13C NMR. They were pelletized and introduced

23

into a column to address the dynamic adsorption experiments. A dynamic adsorption system

24

coupled to a GC-FID was built in which dynamic adsorption experiments were carried out at

25

50°C using nitrogen as carrier gas at a flow-rate of 5 mL/min. The evaluation of

26

hydrocarbons adsorption was done as single and bi-component mixture. The adsorption

27

capacities of the three SBA-2 variants for benzene and cyclohexane using bi-component

28

mixtures were in the range of 44-1190 and 1-20 µmol/gadsorbent, respectively. SBA-2 heated at

29

240°C shows the largest adsorption capacity (1190 µmol/gadsorbent). The greatest selectivity

30

toward benzene with SBA-2 was achieved when it was heated at 550°C obtaining a 799.9

31

value. Thus, this separation can be used for the industrial separation of cyclohexane and

32

benzene, or for the removal of benzene from gasoline.

33

Keywords: SBA-2; Benzene; Cyclohexane; Dynamic adsorption; Selectivity. 1

34

1. Introduction

35

According

to the World Health Organization (WHO), chronic exposure to benzene could

36

cause leukemia, anemia and aplastic anemia [1]. The main sources of benzene contamination

37

in air are petrochemical industries and combustion processes. Therefore, the amount of

38

benzene in gasoline should be controlled to reduce the human exposure of it. In Mexico, the

39

CRE (Energy Regulatory Commission) established in 2016 that maximum concentration for

40

benzene in gasoline is 1 vol% for metropolitan zones and 2 vol% for the rest of the country

41

[2]. In 2011, USA government established 0.6 vol% as limit of benzene in gasoline, but in

42

2012, the maximum allowed concentration was risen to 1.3 vol% [3]. Because of this,

43

benzene removal from gasolines and gas emissions from industries have taken importance in

44

recent years. In addition, separation of benzene from cyclohexane is an indispensable process

45

due to economic aspects because it is used as raw material for fabricating synthetic fibers like

46

nylon. Cyclohexane is produced by catalytic hydrogenation of benzene, nevertheless the

47

reaction is not complete, and the unreacted benzene must be removed. Cyclohexane and

48

benzene have close boiling points, 81 and 80 °C, respectively; thereby forming an azeotropic

49

mixture, that make it one of the most difficult separations in the petrochemical industry [4].

50

Separation processes represent 10-15% of the total energy consumption in the worldwide.

51

From these, the hydrocarbons separation is one of the most challenging separations for

52

chemical engineering industry [5]. There are many methods that had been developed to

53

separate hydrocarbons that cannot be separated by traditional methods. Some of them are

54

liquid-liquid extraction, extractive distillation, azeotropic distillation, crystallization,

55

pervaporation and adsorption on solids [6]. The most common methods to isolate

56

cyclohexane from benzene are azeotropic distillation and extractive distillation. However,

57

these processes are expensive and complicated [3].

58

To overcome those problems, selective adsorption employing porous materials has arisen as a

59

solution due to it is a flexible and simple method with low energy consumption and cheap

60

operational costs [7]. The necessary characteristics for an adsorbent is to have high

61

adsorption capacity and physical, chemical and thermal stability [8]. Nowadays, activated

62

carbons are the most common materials for the adsorption of volatile organic compounds

63

(VOC’s) such as benzene and cyclohexane. However, pore obstruction, hygroscopicity, lack

64

of regenerative capacity and fire risk are the principal disadvantages of activated carbons [9].

65

Other kind of materials that had been used as sorptive materials are metal-organic

66

frameworks (MOF’s), like MIL-101 as adsorbent material for linear alkanes [10] or MOF-

2

67

199 for benzene, cyclohexane and n-hexane adsorption [11]. Nevertheless, the biggest

68

disadvantage of MOF’s materials is the lack of chemical stability.

69

Because of their excellent sorptive characteristics and chemical stability, ordered mesoporous

70

silicas and zeolites are used for selective adsorption. The most important characteristics of

71

these materials include high porosity, large pore volume, high surface area, narrow and

72

controllable pore size distribution, open pore structure and reliable desorption performance

73

[9,12,13]. Since the discovery of ordered mesoporous silicas in the earlies 1990’s, they have

74

been used in different applications such as environmentally hazardous materials studies,

75

reaction catalysis, catalysis supports, chemical sensors, electrical and optical devices, drug

76

control delivery and membrane separations [9,14]. One of these applications is VOC’s

77

adsorption. Hu et al. reported the use of organofunctionalized SBA-15 materials for dynamic

78

adsorption of volatile organic compounds [12,13]

79

Dou et al. evaluated SBA-15, MCM-41, MCM-48 and KIT-6 and their functionalization with

80

phenyl for dynamic adsorption of benzene/cyclohexane mixture [12]. They concluded that the

81

largest adsorption was obtained on KIT-6 functionalized with phenyl, attributing the increase

82

in the adsorption capacity to the phenyl group in a cubic structure and large pore size. Zhang

83

et al. studied adsorption of toluene on SBA-15 and NaY zeolite [7].

84

There are many types of SBA materials. However, one of the most common and studied

85

mesoporous material is SBA-15 [15,16]. It is a bi-dimensional hexagonal mesoporous silica

86

(spatial group P6mm) published in 1998 [17,18]. Since the publication of this material, it has

87

been evaluated in a plethora of applications, as well as its variations (modifications and

88

functionalizations) [19]. The acid synthesis of this material uses P123 (triblock copolymer) as

89

structure director agent. Therefore, SBA-15 is a large pore type material and the morphology

90

is influenced by the acid concentration [20]. In 2001, SBA-15 was evaluated as separation

91

material for short hydrocarbons (C1-C3) by volumetric adsorption. They concluded that

92

SBA-15 is selective to light alkanes and they attribute this to the presence of micropores on

93

the material walls [21]. A similar three-dimensional structure material, SBA-16, was

94

published in 1998 with a pore diameter of around 61 Å and a spatial group Im3̅m [22]. SBA-

95

16 have been used to separate different types of molecules including hydrocarbons [22,23].

96

Also, within the family of SBA silicas, there is a material with a similar pore size to a zeolite,

97

SBA-2.

98

SBA-2 is a tri-dimensional mesoporous material published in 1995 with a pore diameter of

99

around 35 Å [24] and a spatial group Fm3̅m [25]. SBA-2 has been scarcely explored,

100

probably because the entrance to the mesopore is too small to allow the access of big 3

101

molecules, as Zapiko and Anwander report [26]. It has spherical and hexagonal cavities

102

interconnected by small windows that can be tuned by calcination temperature [27] and the

103

size of the cavities can be manipulated by the gemini surfactant [28]. The difference between

104

the geometry of the cavities into the SBA-2 is due to the micelles layers arrangement during

105

its synthesis [27].

106

In this work, SBA-2 heated at three temperatures was tested as adsorptive material for the

107

separation of benzene from cyclohexane by dynamic adsorption. Powder XRD, TEM,

108

MAS-NMR and nitrogen adsorption were use as characterization techniques. 13C MAS-NMR

109

was used to ensure surfactant removal. Powder XRD and TEM were used to determinate

110

spatial group and unit cell parameters. Nitrogen adsorption was used to determinate

111

adsorption capacity, surface area, pore size and pore volume. A labmade dynamic adsorption

112

system coupled to a GC-FID was built. The evaluated materials were pelletized before it was

113

introduced into the adsorption columns for its evaluation into the dynamic adsorption system

114

using a benzene/cyclohexane mixture.

13

C

115 116 117 118

2. Experimental 2.1. Synthesis of SBA-2

119

All reagents were purchased from Merck (Guadalajara, Mexico).

120

SBA-2 synthesis procedure was based on Hunter and Wright [28]. An aqueous solution was

121

prepared adding 2.5 g of tetramethylammonium hydroxide (25 wt %TMAOH in water) and

122

0.53 g of gemini surfactant (CH3(CH2)15N(CH3)2(CH2)3N(CH3)3Br2) in 30 mL of distilled

123

water. When the dissolution got homogeneous, 5 g of TEOS was added. That mixture was

124

maintained stirred at room temperature for 24 h. The resultant precipitate was washed,

125

filtered and dried. To remove the surfactant, the product was put on reflux with 200 mL of

126

ethyl alcohol and 10 mL of HCl for 8 h. The reflux step was repeated three times. Hunter et al

127

[29] report a range of molar composition for this synthesis of 0.025–0.05 surfactant∶ 0.5

128

TMAOH∶ 1 TEOS∶ 45–150 H2O. The final mixture for this work had molar composition of

129

0.05 gemini surfactant: 0.29 TMAOH: 1 TEOS: 71 H2O. The resulting material, SBA-2, was

130

heated for 6 h at three different temperatures, 240, 550 and 800°C. The lowest temperature,

131

240 °C, was chosen taking into account that it was high enough for the material to have

132

structural stability; 550 and 800 °C were chosen based on previously reported studies [25,28].

133 134 4

135 136

2.2. Characterization 2.2.1. X-ray diffraction

137

Powder XRD pattern was obtained at ambient temperature on a Panalitical X’Pert Pro

138

diffractometer in the low-angle region range of 0.6-7.0° 2θ value with Cu-Kα radiation

139

operating at 45 kV and 40 mA. For the analysis, a fixed 10 mm mask, antiscatter slit 1/8° and

140

divergence slit 1/16° were used at primary beam.

141 142

2.2.2. Nitrogen adsorption

143

Nitrogen adsorption isotherms were determined on a Micromeritics ASAP 2020 at liquid

144

nitrogen temperature (77 K) at a relative pressure (P/P0) range of 0.000009-0.95. Samples

145

were pelletized in order to evaluate the same type of physical configuration as the one used

146

for dynamic adsorption experiments. Before measurement, all samples were degassed under

147

vacuum at 200 °C for 6 h. Brunauer-Emmett-Teller (BET) model was used to calculate

148

specific surface area. The pore size, pore volume and adsorption volume were evaluated with

149

the Barrett–Joyner–Halenda (BJH) method with a thickness curve type of Broekhoff-De Boer

150

and Fass correction.

151 152

2.2.3. Transmission electron microscopy

153

Transmission electron microscopy analysis was obtained on a Jeol JEM 2100 operated at 200

154

V and equipped with an energy dispersive spectroscopy detector. Samples were prepared

155

dispersing precursors on acetone and depositing some drops of the suspension on a cooper

156

grating carbon coating. The images were analyzed on Gatan Digital Micrograph TM software.

157 158

2.2.4. Nuclear magnetic resonance

159

Solid-state 13C magic angle spinning nuclear magnetic resonance (MAS NMR) analyses were

160

used to corroborate the surfactant removal from the material pores.

161

experiments were carried out on a Bruker Avance III 400 MHz spectrometer (main magnetic

162

field, B0 = 9.4 T, 13C at 100.62 MHz) equipped with a BRUKER 4.0 mm double resonance

163

probe (H/X) 400 MHz CP-MAS SB VTN. The cross-polarization technique was used with a

164

rotation speed of 8 kHz, a pulse of 4.7 µs (π/2), a contact time of 2 ms and a 6 sec recycling

165

delay. The spectra were acquired with the accumulation of 4000 to 6000 pulses.

13

C MAS NMR

166 167

5

168

2.3. Chromatographic analyses

169

Sample analyses were carried out using a Perkin Elmer Gas chromatograph (GC), model

170

Clarus 500, coupled to a flame ionization detector (FID). A capillary column SPB-624 (60 m

171

x 0.25 mm internal diameter (i.d.) x 0.14 µm film thickness) from Supelco (Merck,

172

Guadalajara, Mexico) was used. Nitrogen was employed as carrier gas set at 30 psi. The GC

173

oven temperature was isothermally programmed at 100 °C. The temperatures of the injector

174

and detector ports were kept at 150 and 220 °C, respectively. The injector was operated in a

175

split mode using a nitrogen flow-rate of 8 mL/min.

176 177

2.4. Calibration curves

178

The purities of benzene and cyclohexane were higher than 99.8%. A fresh standard

179

containing both hydrocarbons 500 mM was prepared daily in hexane. The calibration curves

180

for benzene and cyclohexane were obtained using standard mixtures of both compounds at

181

10, 20, 40, 60, 80, 100, 200, 300, 400 and 500 mM. One microliter of these working solutions

182

was analyzed as indicated in Chromatographic analyses section.

183

The results were analyzed by linear regression. By plotting each hydrocarbons peak-area (y

184

axis) versus the concentration of each hydrocarbon (x axis), calibration equations y = mx + b

185

were obtained. In all cases the intercepts were not significantly different from zero. The

186

regression coefficient for cyclohexane and benzene was 0.997 and 0.996, respectively.

187 188

2.5. Dynamic adsorption measurements

189

SBA-2 was pelletized on Chemplex SpectroPress table press with 0.5 ton and cracked to a

190

particle size (2 - 3 mm) that fit in a stain steel adsorption column (250 x 0.5 mm i.d.). The

191

adsorption column was heated at 200 °C overnight with N2 flow to remove any adsorbed

192

molecule. The dynamic adsorption experiments were carried out using a labmade system

193

shown in Fig. 1. Nitrogen was used as carrier gas at a total flow-rate of 5 mL/min. The

194

nitrogen flows through a thermostated saturator maintained at 40 °C (when valves A and B

195

are open) containing liquid hydrocarbons and then flows through the adsorption column

196

placed in a tubular furnace maintained at 50 °C. The adsorption column gas effluent at the

197

exit was connected to the automatic injection valve to inject 20 µL to the GC-FID. The data

198

acquisition was programmed with an interval of two minutes. The concentrations of the

199

hydrocarbons were obtained by using the lineal equation (describe above).

200

The dynamic adsorption capacity (q) of the adsorbents was calculated using Eq. 1 as

201

described by Serna-Guerrero and Sayari [30]. 6

FA tq

202

q=

203

where, FA is molar flow rate of hydrocarbon, W is the mass of adsorbent load in the column,

204

and tq is the stoichiometric adsorption saturation time.

205

In this sense tq is determined using Eq. 2.

206

tq =

207

where, Cout and Cin are the outlet and inlet adsorbate molar concentration in the flow rate,

208

respectively. In the first term of the equation, the integral limits from t0 to t1 refer to the start

209

time of the experiment until the time where the molar ratio Cout/Cin equals 1. The second

210

term, tD, refers to the dead time of the system. In the third term of the equation, the integral

211

limits from t1 to tf refer to the time where the molar ratio Cout/Cin gets over 1 (t1) and after a

212

roll up effect ends, and the molar ratio returns to 1 (tf). The third term of this equation is only

213

used when a roll up effect is observed.

214

The data acquisition was stopped when adsorbates concentration remained constant for at

215

least 30 minutes. The experiments were carried out by triplicate as single component and as

216

bi-component mixture.

W

(Eq.1)

1−

dt − tD −

−1

(Eq. 2)

217 218

2.6. Selectivity

219

The selectivity factor (α) for a gas mixture provides a numerical value for selectivity. The

220

selectivity for benzene relative to cyclohexane was calculated using the equation

221

#$/ = q B/x B ,

222

where xC and xB are the mole fractions of cyclohexane and benzene and qC and qB are the

223

adsorption capacities of cyclohexane and benzene.

224 225

3. Results and discussion

226 227

q /x C

*

3.1. Characterization 3.1.1. X-ray diffraction

228

Diffraction patterns in the low-angle region (1 < 2θ < 7) are used to analyze amorphous

229

material where only pores have ordered arrangements. Low-angle powder XRD patterns of

230

SBA-2 heated at 240, 550 and 800 °C temperatures were acquired (see Characterization in

231

Experimental section). In Fig. 2 are shown the diffractograms of the 3 materials. The peaks

232

indexed as (111), (220) and (311) indicate SBA-2 as a cubic structure (Fm3̅m), according to

233

Pérez-Mendoza, et al. [25]. As can be seen in Fig. 2, as the temperature increases the indexed

7

234

peaks shift to higher 2θ values indicating that the unit cell size decreases but still cubic

235

structured. To determine the unit cell size reduction the parameter a was calculated for the

236

SBA-2 heated at 240, 550 and 800 °C obtaining 73.10, 71.05 and 68.45 Å, respectively.

237 238

3.1.2. Nitrogen adsorption

239

The effect of temperature leads to the reduction of the nitrogen adsorbed volume which is

240

clearly shown in the experimental isotherms of nitrogen adsorption at 77 K (Fig. 3).

241

Nevertheless, the filling of the cavities takes place at the same relative pressure. This can

242

denote that the number of accessible cavities decreases when the calcination temperature

243

increases, but the size of the cavities and interconnecting channels remains at almost the same

244

size. In Table 1 are shown the porous properties of the variants of SBA-2. The SBA-2 heated

245

at 240 °C presents the greatest values for BJH cavities pore size (24.62 Å), BJH

246

interconnecting channels pore size (6.22 Å), BJH pore volume (0.54 cm3/g), and adsorption

247

volume (386.60 cm3/g) but its surface area (314.81 m2/g) is not the greatest. In this sense, the

248

material heated at 550 °C presents the greatest BET surface area (337.92 m2/g) and a BJH

249

interconnecting channels pore size of 5.93 Å. The material heated at 800 °C presents an

250

adsorption capacity (119.45 cm3/g), BJH interconnecting channels pore size of 4.78 Å, pore

251

volume of 0.15 cm3/g and BET surface area (139.83 m2/g) of an approximately a third of the

252

other two materials. The materials calcined at 240, 550 and 800 °C present a trend for BJH

253

cavities and interconnecting channels pore size, which is that increasing calcination

254

temperature decreases the pore size of cavities and interconnecting channels. In Fig. 4 are

255

shown the pore size distribution of the three SBA materials.

256 257

3.1.3. Transmission electron microscopy

258

Each white spot in the TEM image, presented in Fig. 5 (a), is a cavity in the SBA-2 material.

259

As it is known, the pores are distributed in cubic arrangement centered on faces. In Fig 5 (b)

260

the unit cell is indicated with red lines according to its spatial group. The found type of the

261

unit cell is in accordance with the findings by XRD analysis. In Fig. 5 (c) the representation

262

of the cubic structure on the same position as it is on the TEM image.

263 264

3.1.4. Nuclear magnetic resonance

265

Due to SBA-2 morphology, solvent extraction was not efficient enough to expel surfactant

266

from the pores even do the procedure was repeated for three times. 13C MAS-NMR analysis

267

was done to SBA-2 before and after solvent extraction to corroborate that surfactant was 8

268

completely removed from the material. In Fig. 6 are shown the experiments before and after

269

solvent extraction; from this information can be concluded that the surfactant was removed

270

with a solvent extraction using 4.75% of concentrated hydrochloric acid in ethanol.

271 272

3.2. Dynamic adsorption of single component on adsorbent

273

The dynamic adsorption behavior of benzene and cyclohexane were evaluated on SBA-2

274

heated at 240 and 550 °C. In general, the longer the breakthrough time, the largest adsorption

275

capacity. A breakthrough curve presents the evolution of Cout/Cin molar ratio versus time,

276

where Cout is the concentration of the hydrocarbon at the outlet of the adsorption column and

277

Cin is the concentration at the inlet. In Fig. 7 are shown the cyclohexane and benzene

278

breakthroughs curves on SBA-2 heated at 240 (a) and 550 °C (b). The shape of the benzene

279

breakthrough curve on the material heated at 550 °C shows and inflection point at 100

280

minutes approximately (Fig. 7b) which corresponds to the capillary condensation inside the

281

material. Breakthrough time in the material heated at 240 °C for cyclohexane is about 20 min

282

and for benzene about 60 min. Breakthrough time in the material heated at 550 °C, for

283

cyclohexane is around 2 min and for benzene is longer than 20 min. For both components, the

284

breakthrough time for the material heated at 240 °C it is longer. Also, the adsorption

285

capacities were calculated for single component as described in 2.5 section. Adsorption

286

capacities for the three materials are shown in Table 2. The adsorption capacity (q) of SBA-2

287

heated at 240, 550 and 800 °C for cyclohexane was 164, 65 and 39 µmol/gadsorbent and for

288

benzene was 1743, 713 and 220 µmol/gadsorbent. From these results, a trend can be stablished

289

which is that increasing calcination temperature decreases adsorption capacities for both

290

compounds. In this sense, the q values reflect the highest adsorption capacity for the SBA-2

291

heated at 240 °C, whilst the lowest for the SBA-2 heated at 800 °C. This trend agrees with

292

the obtained data of the nitrogen adsorption capacity and BJH pore size and volume.

293 294

3.3. Dynamic adsorption of bi-component on adsorbent

295

SBA-2 heated at different temperatures was used to evaluate the change of selectivity for

296

benzene/cyclohexane mixture. Adsorption capacities and selectivities for the three materials

297

are shown in Table 2. As can be seen, SBA-2 heated at 240 °C has the highest benzene

298

adsorption capacity for bi-component mixture, 1190 µmol/gadsorbent, behavior that could be

299

due to the porous and structural properties as BJH pore size, BJH pore volume, and

300

adsorption volume, sufficiently higher values that allow a better diffusion into the material.

301

SBA-2 heated at 550 °C has an adsorption capacity for bi-component mixture of 443 9

302

µmol/gadsorbent, On the other hand, SBA-2 heated at 800 °C has the lowest benzene adsorption

303

capacity for bi-component mixture, 44 µmol/gadsorbent.

304

The cyclohexane adsorption capacity for bi-component mixture of SBA-2 heated at 240 °C

305

and 550 °C decreases from 8.0 to 0.6 µmol/gadsorbent (Table 2). However, the amount of

306

cyclohexane adsorbed by the material heated to 800 °C is greater than in the rest of the

307

materials. This is because in the breakthrough curves, materials heated at lower temperatures

308

have a roll-up effect. As explained by equation 2 in Section 2.5, the molar amount of

309

cyclohexane retained in the adsorbent is calculated taking into account the integrated area

310

until Cout/Cin equals 1 minus the roll-up area [31], therefore, a higher q value is obtained

311

when there is no roll-up effect. As it was mentioned before, breakthrough curve for

312

hydrocarbons adsorption on SBA-2 calcined at 800 °C does not have roll up effect.

313

Therefore, there is not a subtraction of the area of the roll up. Consequently, the area under

314

the curve for the SBA-2 calcined at 800 °C is greater than SBA-2 calcined at 240 and 550 °C.

315 316

In Table 3, the volume in milliliters accessible for liquid nitrogen and for liquid benzene and

317

cyclohexane in each material is shown. The higher calcination temperature, the lower

318

accessible volume for nitrogen and hydrocarbons. However, the volume accessible for

319

hydrocarbons decrease even more compared with liquid nitrogen this is due to nitrogen

320

molecules are small enough to pass through the interconnecting channels and thus, enter the

321

cavities.

322 323

In Fig. 8 are shown the breakthrough curves of the mixture separation using the three

324

variations of SBA-2, heated at 240°C (a), 550°C (b) and 800°C (c). All of the SBA-2

325

materials evaluated in this study for cyclohexane (unsaturated hydrocarbon) presented short

326

breakthrough time and low adsorption capacity. Nevertheless, for benzene (saturated

327

hydrocarbon) presented greater breakthrough time and higher adsorption capacity; and thus,

328

higher selectivity for benzene. This is in accordance with the fact that in single component

329

experiments benzene was also the most retained (Fig. 7).

330

The shape of the breakthroughs in the SBA-2 heated at 240 and 550 °C show an overshoot. In

331

Fig 8 (a) and (b) can be seen that cyclohexane is the first breakthrough component and shows

332

an increment of Cout/Cin greater than 1. This effect is due to a roll-up effect. In the roll-up

333

effect during the adsorption process, the adsorbate with lower affinity for the adsorbent is

334

displaced by the adsorbate with higher affinity and promote the increase Cout/Cin > 1 of the

335

lower affinity by the adsorbate [32,33]. In this context, the presence of roll-up in Fig 8 (a) and 10

336

(b) is an indicative of the competitive adsorption. However, it is important to note that the

337

roll-up effect is more pronounced in Fig 8 (a) than (b), behavior that could be due to the

338

porous properties and their higher adsorption capacity.

339

In Fig 8 (c) can be seen that the material heated at 800 °C presents poor adsorption of

340

cyclohexane and benzene. According with the nitrogen adsorption analysis, the adsorption

341

capacity is significantly lower than the other variants of SBA-2. This becomes evident with

342

the breakthrough curve. Both components have the breakthrough point at the same time and

343

the time of permanence in the material is very short (4 min). The results of powder XRD,

344

nitrogen adsorption and breakthrough curves agree that the unit cell and pore size (cavities

345

and channels) are smaller than the other variants of SBA-2. Although the material has low

346

nitrogen adsorption capacity, maybe some small molecules could be adsorbed. In this sense,

347

breakthrough curves in Fig 8 (c) show that none of these hydrocarbons can be absorbed by

348

the material because they are too big to pass through the interconnecting channels to access

349

the cavities.

350 351

Pérez-Mendoza et al. found that a model based on single-sized spherical cavities was

352

inadequate and that it is necessary to explicitly account for the interconnecting channels; and

353

that despite the basic regularity of the SBA-2 structure, it is necessary to allow for a

354

distribution of the sizes of both the cavities and the channels [34]. For understanding the

355

behavior in the adsorption of cyclohexane and benzene Fig. 9 shows the proposed

356

mechanism. In the first stage of the process, benzene (red circles) and cyclohexane (orange

357

circles) begin to adsorb on the surface of the material. In the second stage (Fig. 9a), benzene

358

gradually displace cyclohexane from the material surface. This replacement is the reason for

359

the roll-up effect (cyclohexane molecules that have been substituted leave the material)

360

indicative of selectivity toward benzene. In the third stage (Fig 9b), because of adsorption,

361

the material surface is covered with benzene reducing the pore size of the interconnecting

362

channels.

363 364

11

365 366

From the obtained results is demonstrated that when the calcination temperature is higher, the

367

cavities pore size decreases as well as the pore size of the interconnecting channels. In our

368

study, the spherical cavities were found to be 23-25 Å in diameter, while the interconnecting

369

channels were found to be 4.7-6.3 Å in diameter (Table 1). Although the BJH pore size, BJH

370

pore volume, BET surface area and N2 adsorption volume are similar for materials heated at

371

240 and 550 °C, their adsorption capacities and selectivities for benzene over cyclohexane are

372

widely distant. The interconnecting channels pore size of the SBA-2 material calcined at 240

373

and 550 °C is 6.22 Å and 5.93 Å, respectively. The kinetic radius for benzene is 5.85 Å and

374

for cyclohexane is 6.0 Å. Also, because of benzene adsorption on material surface, the width

375

of the channels is also reduced causing worse diffusion for cyclohexane on SBA-2 material

376

calcined at 550 °C thus it cannot easily enter to the pores. In this sense, the channel pore size

377

leads to a percolation effect whereby the pore structure is not equally accessible to Benzene

378

and cyclohexane. Considering this information, the interconnecting microporous channels of

379

SBA-2 are mainly responsible for the selectivity toward benzene because cyclohexane kinetic

380

diameter is greater than the channels pore size of SBA-2 calcined at 550 °C.

381 382

3.4. Comparison with other separation methods and mesoporous silicas

383

Benzene and cyclohexane separation had been achieved by different methods. In 2009, Shiau

384

and Yu performed stripping crystallization [35], concluding that is possible to obtain crystals

385

of pure benzene but not of pure cyclohexane. In 2015, Dong et al [36] carried out this

386

separation by pervaporation using AAOM-ionic liquids/polyurethane membranes, concluding

387

that the separation was successful, but they achieved a selectivity factor of 34.4, a value much

388

smaller than what is achieved in this work. In 2017, Saini and Pires [11] synthesized a zeolite

389

composite ZSM-5 and MOF-199 and evaluated for this separation, nevertheless, the

390

selectivity values obtained were 2.5.

391

Due to the great characteristics that mesoporous silicas have, they had been tested as dynamic

392

adsorption materials for benzene and cyclohexane. On Table 4 there is a summary of

393

previous studies [9,12,13,37] where mesoporous silicas, zeolites and active carbons are used

394

as adsorption materials for dynamic adsorption for single component and bi-component

395

mixtures. For the experiments for single component, cyclohexane had not been evaluated in

396

those works.

397

However, the benzene adsorption capacity in SBA-2 heated at 240 °C is higher than the most

398

adsorption capacities of previously reported mesoporous silicas. Adsorption capacity on 12

399

SBA-2 heated at 550 °C is comparable with the reported results, although adsorption is

400

higher.

401

In order to carry out a comparative analysis of the results obtained with results reported in the

402

literature, the adsorption capacities reported by Dou et al. [12] and Hu et al. [13] were

403

processed to calculate selectivity (α). Benzene adsorption capacity in a bi-component mixture

404

on SBA-2 heated at 550 °C (443 µmol/gadsorbent) is comparable with pure SBA-15 reported by

405

Dou et al. [12] (540 µmol/gadsorbent) and Hu et al. [13] (340 µmol/gadsorbent). Kousage [9] made

406

a comparison of benzene adsorption capacity by dynamic adsorption using SBA-15

407

synthetized with different molar proportions, MCM-41, active carbon, zeolite HY and Q3

408

(silica-gel) as sorptive materials; they concluded that microporous materials, like HY and

409

active carbons, have better adsorption capacity than materials with bigger pore size. The

410

results in the presented work match with their conclusion, since SBA-2, with micropores,

411

have greater adsorption capacity than SBA-15 (which has mesopores). Dou et al. [37] used

412

carbon silica aerogel (CSA) and activated carbon for adsorption of benzene and toluene by

413

dynamic adsorption. They concluded that activated carbon has the best adsorption capacity.

414

Due to the low adsorption capacities that were obtained for cyclohexane and high for benzene

415

on SBA-2 heated at 240 and 550 °C, selectivity values are higher (161.2 and 799.9,

416

respectively) compared with the selectivity obtained by Dou et al. [12] and Hu et al. [13].

417 418

4. Conclusions

419

Mesoporous silica SBA-2 was prepared and heated at different temperatures to be evaluated

420

as separation material in a dynamic adsorption system. Pore size of SBA-2 was tuned by

421

heating the material at three different temperatures. Therefore, the low adsorption capacities

422

of cyclohexane (0.6-19.7 µmol/gadsorbent) in the material variants, as well as the high benzene

423

adsorption capacities (44-1190 µmol/gadsorbent), are reflected in high selectivity factors for

424

benzene over cyclohexane. SBA-2 heated at 240°C presented the highest adsorption capacity,

425

nevertheless SBA-2 heated at 550 °C presented the highest selectivity factor (799.9 value).

426

However, the material heated to 800 °C does not have a high adsorption capacity or a very

427

high selectivity factor, so it could be concluded that the thermal treatment at that temperature

428

reduces the pore size that remains inaccessible for the evaluated hydrocarbons.

429

As it was mentioned, recently porous materials have been used as separation materials for

430

methods like pervaporation and selective adsorption. According to this, it can be concluded

431

that SBA-2 is an option for the industrial separation of benzene and cyclohexane, as well as

13

432

for the removal of benzene from gasolines to compliance with international regulations due to

433

the toxicity of this compound.

434 435 436

Acknowledgments Emparan-Legaspi thanks Consejo-Nacional-de-Ciencia-y-Tecnología Mexico for the research

437

grant provided.

438

14

439

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553 554

17

555 556 557

Figure captions

558

Figure 2. XRD patterns of SBA-2 heated at 240, 550 and 800 °C.

559

Figure 3. Nitrogen adsorption isotherms of SBA-2 calcined at 240, 550 and 800 °C.

560 561 562

Figure 4. Pore size distribution of the three SBA materials. Figure 5. TEM imagen of SBA-2 (a) close up to a region of the TEM image (b) unit cell position of the according to the TEM image (c).

563

Figure 6. 13C NMR of SBA-2 before (above) and after (below) solvent extraction.

564 565

Figure 7. Breakthrough curves for cyclohexane (a) and benzene (b) in SBA-2 calcined at 240 and 550 °C.

566 567

Figure 8. Breakthrough curves of the mixture cyclohexane/benzene in SBA-2 calcined at 240 °C (a), 550 °C (b), and 800 °C (c).

568

Figure 9. Benzene and cyclohexane adsorption mechanism in SBA-2 material.

Figure 1. Schematic diagram of experimental set-up.

18

Table 1. Porous properties of mesoporous silica SBA-2. Sample BET BJH Cavities surface area pore size (Å) (m2/g) SBA-2 calcined 240 °C SBA-2 calcined 550 °C SBA-2 calcined 800 °C

314.81 337.92 139.83

24.62 24.13 22.94

BJH Interconnecting channels pore size (Å) 6.22 5.93 4.78

BJH pore volume (cm3/g)

Adsorption volume (cm3/g)

0.54 0.45 0.15

386.60 335.52 119.45

Table 2. Dynamic adsorption capacity q (µmol/gadsorbent) Material

Single component Cyclohexane

Benzene

SBA-2 240 °C 164.0 1743.0 SBA-2 550 °C 65.0 713.0 SBA-2 800 °C 39.0 220.0 * Selectivity for benzene relative to cyclohexane

Mixture Cyclohexane

Benzene

Selectivity ( / )*

8.0 0.6 19.7

1190.0 443.0 44.0

161.2 799.9 2.4

Table 3.Accessible volume for liquid nitrogen, benzene and cyclohexane in each evaluated material (mL/100 g adsorbent). Material SBA-2 240 °C SBA-2 550 °C SBA-2 800 °C

Nitrogen 54.89 47.64 16.96

Cyclohexane 0.09 0.01 1.12

Benzene 10.68 3.92 2.07

Table 4. Adsorption capacity of benzene and cyclohexane reported by other authors (µmol/gadsorbent). Material SBA-2 240 °C SBA-2 550 °C SBA-2 800 °C Pure SBA-15 (1:20)MTES (1:10)MTES (1:5)MTES (1:20)PTES (1:15)PTES (1:10)PTES (1:5)PTES SBA-15 pSBA-15 MCM-41 pMCM-41 MCM-48 pMCM-48 KIT-6 pKIT-6 Activated carbon CSA-0 CSA-2 Fiber A

Rod B

Rod C

MCM-41

Q3

HY Activated carbon

Single component Benzene 1743.0 713.0 292 360 418 283 478 632 650 626 910 650 1020 900 800 780 1260 1170

Selectivity ( / )*

Cyclohexane 8.0 0.6 105.0 160 189 330 285 193 320 240 360 200 370 210 320 340

Benzene 1190.0 443.0 234.0 340 419 524 442 433 540 400 570 540 580 550 670 690

161.15 799.86 2.41 2.30 2.40 1.72 1.68 2.43 1.83 1.81 1.72 2.93 1.70 2.84 2.27 2.20

880

-

-

-

1580 3990 247 (0.022 mL/g) 112 (0.010 mL/g) 45 (0.004 mL/g) 67 (0.006 mL/g) 157 (0.014 mL/g) 752 (0.067 mL/g) 865 (0.077 mL/g)

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Mixture

Reference

This study

[13]

[12]

[25]

[9]

Highlights • • • •

SBA-2 can be used as separation material for cyclohexane/benzene mixtures SBA-2 calcination is a vital factor for improving hydrocarbons adsorption capacity SBA-2 calcined at 240 °C have the greatest adsorption capacity SBA-2 calcined at 550 °C have the greatest selectivity factor

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Declaration of interest: None