Selective Adsorption of Dimethyl Disulfide on Acid-Treated

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Selective Adsorption of Dimethyl Disulfide on AcidTreated CuYH@silicalite-1 Core-Shell Structure: Methyl tert-Butyl Ether as Competition Components Chao Yang, Xuan Meng, Dezhi Yi, Zhiming Ma, Naiwang Liu, and Li Shi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03079 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Industrial & Engineering Chemistry Research

1

Selective Adsorption of Dimethyl Disulfide on Acid-Treated

2

CuYH@silicalite-1 Core-Shell Structure: Methyl tert-Butyl Ether as

3

Competition Components

4

Chao Yang, Xuan Meng, Dezhi Yi, Zhiming Ma, Naiwang Liu, Li Shi*

5

State Key Laboratory of Chemical Engineering,

6

East China University of Science and Technology, Shanghai 200237, China

7

E-mail: [email protected]

8

Abstract: To selectively adsorb dimethyl disulfide (DMDS) from its competition

9

components, such as methyl tert-butyl ether (MTBE), the shell layer of silicalite-1

10

crystals was synthesized on commercial Y zeolite crystal surface (CuYH@silicalite-1

11

core@shell structured composites) with a mass composition of tetraethyl orthosilicate/

12

tetrapropylammonium hydroxide/ ethanol/ H2O/ CuYH=20g (0.096 mol):19g (0.0234

13

mol):17g (0.369 mol):87g (4.829 mol):5g. CuYH zeolites were acquired by ion

14

exchanging Cu2+ with HCl treated NaY (noted as NaYH). Results showed that by the

15

dealuminzation of HCl, the ratio of Si/Al for NaY zeolites had been dramatically

16

increased, providing NaYH a favourable condition for the growing of silicalite-1

17

coatings; and many vacancies left by the dissolved Al also enlarged the out surface

18

area of NaYH to provide more load locations for silicalite-1 crystals. Among all these

19

synthesized adsorbents, the core@shell YH-CuCl2 displayed a best DMDS adsorption

20

of about 34.585 mgs/gadsorbent in MTBE solution and a 100% desulfurization rate for

21

about 3 h. Also, the process of synthesizing CuYH@silicalite-1 core@shell composites

22

and their shape-selective adsorption were detailed.

23

Key words: Dimethyl disulfide; Shape-selective adsorption desulfurization; Methyl

24

tert-butyl ether; CuYH@silicalite-1 core@shell composites; Dealuminzation.

25

1. Introduction

26

The environmental pollution derived from sulfur containing fuel due to the sulfur

27

oxide emissions has focused more and more attentions. To this end, since the year

28

2009, sulfur content in gasoline has been limited to less than 10 ppmw.1-3 Many

29

effective measures have been taken to reduce the sulfur in gasoline and diesel fuels as

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30

well as their modifying additives, such as Methyl tert-butyl ether (MTBE), which

31

possesses considerable octane-enhancing property. However, during the production

32

process (reaction of isobutene and methanol), the C4 stream (the main source of

33

isobutene) contains many sulfur compounds, such as DMS (dimethyl sulfide) and

34

DMDS (dimethyl disulfide; about 45 ppm in MTBE)4, 5. The molecular structures of

35

DMDS as well as MTBE are illustrated in Scheme. 1. Recently, the desulfurization for

36

MTBE are primarily distillation desulfurization, hydrodesulfurization, and adsorption

37

desulfurization.6-9 By comparison, distillation is always accompanied by huge energy

38

consumption and hydrodesulfurization always requires a harsh operating condition

39

under high temperature and pressure. For deep desulfurization, the adsorption

40

desulfurization has been considered the promising technology due to its mild

41

condition of atmospheric temperature, low pressure, low energy consumption and

42

simple operation.10, 11

43 44

Scheme. 1. Molecular structural formula of DMDS and MTBE

45

Adsorption desulfurization had been extensively studied on numerous kinds of

46

adsorbents such as modified composite oxide, activated carbons (ACs), mesoporous

47

silica, metal-organic frameworks (MOF) and zeolite,12-14 among which zeolites had

48

displayed wide application in chemical industry based on their high porosity of

49

microporous or mesoporous silica-alumina crystalline frameworks, large surface areas,

50

thermal stability, and form selectivity.15,

51

well organized nanopores and nanochannels had exhibited tremendous supports for

52

desulfurizing organic sulfur compounds especially DMDS.17 By NaY, NaX, Hβ

53

zeolites, Wakita et al.18 discussed removing DMS from the city gas and found the sites

54

for adsorption on NaY were relied on Na+ with a adsorption capacity of sulfur for 1.1

55

mmols/gadsorbents. Lv et al.19 discussed the adsorption desulfurization (DMDS) in

56

liquefied petroleum gas on modified NaY adsorbents, and revealed with 5 wt% Ag2O,

57

NaY displayed the best sulfur capacity 87.86 gs/gadsorbents by the adsorption ability and

58

selectivity of S-Ag(I) interaction. Yi et al.20 also studied exchanging NaY with Ni2+,

16

Actually, different kinds of zeolites with


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59

Cu2+etc. on desulfurizing DMDS in n-octane and found that S-M bonds were the keys

60

in desulfurization. Lee et al.4 investigated desulfurizing DMDS in C4 hydrocarbon

61

mixtures with β zeolites and claimed that Cu(I) ion-exchanged zeolites showed high

62

DMDS capacity of 8.70 mgs/gadsorbents. Nevertheless, when adsorbing DMDS in MTBE,

63

there would be a fierce competition between them on π-complexation adsorbents,

64

which had been detailedly described in the previous studies;21-23 and Yang10,

65

claimed the adsorption capacity of sulfur on π-complexation adsorbents decreased

66

sharply in the present of MTBE. To solve the competitive adsorption, by using

67

modified ZSM-5 zeolites, Zhao et al.25 had tried to adsorb DMDS from MTBE; and

68

acknowledged the pore structure and acidity both played key roles in adsorption

69

process and reached a not too optimistic adsorption capacity of 8.24 mgs/gadsorbents.

70

Therefore, new strategies on selectively adsorbing DMDS from MTBE are attractive.

24

had

71 72

Scheme. 2. Structure of (a) silicalite-1 and (b) NaY

73

In consideration of the fact that Y zeolite had been proved a promising material

74

in adsorption extensively owing to its high solid acidity, crystalline microporous

75

properties, and 3D pores (tetrahedral framework structures);26-28 in addition, their

76

activity and stability could be further improved by steaming, ion-exchange, and

77

treatment with acid or base.15,

78

structured composite with NaY, whose structure were shown in Scheme. 2, and CuY

79

as the core, silicalite-1 as the shell to remove DMDS in MTBE solution and obtain an

80

adsorption capacity of sulfur on NaY@silicalite-1 and CuY@silicalite-1 to be 20.711

81

and 32.882 mgs/gadsorbent, respectively.21, 22 In present study, we are trying a new way in

82

synthesizing core@shell composites by the dealuminization of HCl on Y zeolite to

83

obtain a better growth environment for silicalite-1 shell. Dealuminization is an

16, 29

Previously, we have applied a core@shell



Page 4 of 25

84

efficient method to increase the ratio of Si/Al by transferring framework Al into

85

extra-framework position and then being washed away.15,

86

USY zeolite with citric acid and obtained improved Si/Al ratio Y zeolite with smaller

87

unit cell parameters. Qiao et al.35, by malic acid and nitric acid treatment, studied the

88

modified USY zeolites; and found the aluminum removed by coordination reaction of

89

MA could create extensive secondary pores while NA could only remove alumina of

90

extra-framework. In most of these studies, the nature of framework and

91

extra-framework aluminum was characterized on a

92

determining the Al coordination with high resolution.15 Besides, the adsorbents

93

synthesized in this study were also investigated by SEM, TEM, BET, XRD, FT-IR,

94

ICP-OES, and EDS measurements. Also, the selective adsorption on these core@shell

95

structured composites was also analyzed according to these characterization.

96

2. Experimental

97

2.1. Materials.

30-33

Liu et al.34 modified

Al MAS solid-state NMR by

27

98

NaY zeolites (Si/Al, 2.4) were obtained from Wenzhou Catalyst Plant. 12 mol/L

99

hydrochloric acid (HCl) and analytically pure CuCl2, Cu(NO3)2, CuSO4, EtOH

100

(ethanol), TEOS (tetra ethyl orthosilicate), and 25% TPAOH (tetrapropylammonium

101

hydroxide) solution were purchased from Tansoole (Shanghai). All water used in the

102

experiments was deionized.

103

2.2. Acid treatment on NaY.

104

With a solid to liquid of 1g NaY : 30 mL HCl solution, NaY were put into 0.5

105

mol/L HCl(aq) prepared by diluting 12 mol/L HCl solution in a volumetric flask; after

106

a full mix with stirring for half an hour at 25 °C, then the mixtures were separated by

107

distinct methods to collect NaYH: filtration (filter paper, aperture: 15-20 μm), and

108

centrifugation (rotation speed: 6000 r/min); NaYH were finally collected after a

109

thorough washing and drying.

110

2.3. Preparation of core (CuYH), shell (silicalite-1) and core@shell structures

111

2.3.1. Core zeolites (CuYH).

112

CuYH were obtained by ion exchanging NaYH with Cu2+ (CuCl2 , Cu(NO3)2, as

113

well as CuSO4 ): by solid to liquid ratio of 1 g: 20 ml, NaYH (in powder) were added


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114

into 0.5 mol/L Cu2+ solution in water bath (90 °C); after a day and night stirring, the

115

mixtures were separated by filtration, washed, dried, and finally calcined at 450°C for

116

6h. These modified NaYH were marked as YH-CuCl2, YH-Cu(NO3)2 and YH-CuSO4.

117

2.3.2. core@shell structures (CuYH as the core).

118 119

In this research, by sol-gel coating process, (TPAOH as the template)36, the CuYH@silicalite-1 were prepared as follow:

120

Firstly, the precursor solution was prepared, including TEOS, TPAOH, EtOH,

121

and water at a mass ratio of TEOS/ TPAOH/ EtOH/ H2O=20 g:19 g:17 g:87 g (0.096:

122

0.0234: 0.369: 4.829 mol). To sufficiently hydrolyze TEOS into water, TEOS was

123

dropwise added and kept stirring for 2 h; then 5 g CuYH (power) was added; by

124

continuous stirring for another 0.5 h, a homogeneous suspension was formed. Then it

125

was transferred into an hydrothermal autoclave to synthesize the core@shell

126

structures at 180 °C for 24 h; the products were obtained by filtration, then washed,

127

dried, and calcined at 550 °C for 6 h. The synthesized core@shell structures were

128

marked as CSYH-CuCl2, CSYH-Cu(NO3)2 and CSYH-CuSO4, respectively.

129

2.3.3 Silicalite-1 and core@shell structures (NaYH as the core).

130 131 132

Silicalite-1 crystals were synthesized at the same mass ratio of TEOS, TPAOH, EtOH, and H2O (20:19:17:87 g) only without adding CuYH. NaYH core@shell structures were synthesized in the same method by adding

133

NaYH instead of CuYH, marked as CSNaYH.

134

2.4. DMDS adsorption (shape selective adsorption) experiments

135

2.4.1. adsorption test in static state.

136

The adsorption capacity of sulfur on each adsorbent was obtained by static

137

adsorption, performed in a 30 ml airtight container by putting 0.2 g adsorbents into 20

138

mL MTBE solution (sulfur content: 748.52 mg/L). After standing for 24 h at room

139

temperature, the sulfur concentration after the static tests was analyzed by a TS-3000

140

fluorescence sulfur tester. The adsorption capacity of sulfur on each adsorbent was

141

calculated by the following equation:

142

Sulfur adsorption capacity (mgs/gadsorbent) = (748.52- Ct)×0.02/0.2;

143

(Ct (mg/L) was the sulfur concentration after static tests)



144

2.4.2. Adsorption test in dynamic state.

145

To acquire the adsorption desulfurization rate, each sample was loaded in a bed

146

flow reactor (operating condition: weight hourly space velocity: 5 h-1, 0.1 MPa and

147

25 °C). In a quartz column (length, 250 mm; internal diameter, 6 mm), about 0.89 g

148

adsorbents (size, 20-40 mesh) were loaded intermediately; and the spaces up and

149

down were filled with quartz sands. MTBE solution (DMDS sulfur content: 286.13

150

mg/L) was pumped into the quartz column at a flow rate of 6 mL/h by a double

151

plunger microscale pump. The sulfur content of export solution was analyzed in real

152

time every half an hour.

153

2.5. Characterization for adsorbent samples.

154 155

SEM and TEM photographs were achieved on a Hitachi S-3400 microscope (15 KV; magnification,1-20 K) and a JEM-2100 (200 KV; magnification, 2-1500 K).

156

N2 adsorption-desorption was performed via a JW-BK200C instrument. At

157

-196 °C, 150 mg of samples were analyzed to acquire surface area, mesoporous and

158

microporous volume.

159 160 161 162 163

XRD was used to analyze the crystal structures of different samples via a D8 Advance polycrystalline diffractometer (40 KV, 100 mA; 10° to 75°; step, 0.02°). By a Magna-IR550 spectrometer (Nicolet Company), the FT-IR spectra were collected by mixing the powder sample with KBr (mass ratio, 1:100). The solid-state

Al magic-angle spinning (MAS) nuclear magnetic resonance

27

164

(NMR) spectra (27Al MAS NMR) were obtained on a Bruker MSL-300 spectrometer

165

(frequency, 104.34 MHz; delay time,10 ms).

166

EDS and ICP-OES data were collected on a TEAMEDS and an Agilent 725

167

ICP-OES, respectively, to analyze the element content of superficial and total Si, Al

168

Na and Cu in different adsorbents after HCl treatment and core@shell experiments

169

(mainly the variety of Si/Al and Al/Na).

170

3. Results and discussion

171

3.1. 27Al MAS NMR spectra.


Page 6 of 25

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NaY NaYH

100

172 173

80

60

40

20

Chemical shift (ppm)

0

-20

Figure. 1. 27Al MAS NMR spectra of NaY and NaYH.

174

Before and after acid treatment, the changes of Al environments (the chemical

175

state of Al atoms) in NaY zeolite was presented via a 27Al MAS NMR in Figure. 1.

176

Two types of Al chemical state in NaY zeolites were observed: tetrahedral coordinated

177

framework Al (65 ppm) and octahedral coordinated extra-framework Al (3 ppm).37 As

178

could be seen, for NaY zeolite, the most signal occurred at around 65 ppm; and almost

179

no obvious signal at 3 ppm was detected, indicating that in NaY zeolite, almost all the

180

Al species were located in framework, providing NaY zeolite a good crystallinity as

181

shown in XRD patterns (Figure. 2). However, after acid treatment, the signal intensity

182

at 65 ppm had dropped dramatically, which suggested that most of the framework Al

183

(about 90%) had disappeared, attributing to the dealuminization effect of HCl on

184

framework Al. In addition, the signal observed at 3 ppm stated clearly that the

185

disappeared framework Al was converted into extra-framework Al; nevertheless,

186

owing to the adequate washing process, only a little extra-framework Al remained,

187

leading to a weak signal at 3 ppm.

188

there was a strong dealuminization effect on NaY zeolite; and, of course, the

189

crystallinity of NaY zeolite after acidic treatment would be severely declined, which

190

was also consistent with XRD patterns. Meanwhile, the disappeared framework Al

191

would also provide the the NaY zeolite more vacancies especially the surface portion.

Al MAS NMR spectra showed that, for HCl,

27



192

3.2. XRD spectra of adsorbents. 20.3° 15.6° 18.6°

(6) (5)

MFI 26.3° (11)

MFI (10)

(4)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

MFI

(3)

(9)

MFI

(2)

(8)

18.6° * 15.6° *

10

15

* 22.1° 26.3° * 20.3° * 20

25

30

MFI (1)

(7) 35 10

15

20

25

30

35

193

2θ (degrees)

194 195 196

Figure. 2. XRD analysis of adsorbents: (1) NaY, (2) NaYH, (3) YH-CuCl2, (4) YH-Cu(NO3)2, (5) YH-CuSO4, (6) NaYH after 2h acid treatment (7) silicalite-1, (8) CSNaYH, (9) CSYH-CuCl2, (10) CSYH-Cu(NO3)2, (11) CSYH-CuSO4.

197

XRD analysis displayed the variations on the mineralogical structure of NaY

198

zeolites after acid treatment, Cu2+ modification, and core@shell experiment. The

199

peaks at 2θ=15.6°, 18.6°, 20.3°, 22.1°, 23.6°, 26.3° and 27.0° were confirmed as the

200

feature peaks of NaY zeolites;20 dipping in 0.5 mol/L HCl for 30 min and Cu2+ ion

201

exchange made the most characteristic peaks of NaY zeolites unchanged. However,

202

Figure. 2 (2)-(5) also suggested that the crystallinity of NaY decreased drastically;

203

besides the framework defects caused by Cu2+ modifying process, the dealuminization

204

effect of HCl played the most important role as depicted on 27Al MAS NMR spectra

205

(about 90% framework Al disappeared). To demonstrate this dealuminization effect of

206

HCl, we further dipped NaY zeolites in 0.5 mol/L HCl for 2 h. As was shown in

207

Figure. 2 (6), all the characteristic peaks on NaY zeolite had vanished, indicating that,

208

at this point, NaY owned no obvious crystal structures with only some amorphous

209

SiO2 and little amorphous Al2O3.

210

After these acid treated zeolites being coated by silicalite-1 crystals, the same

211

peaks of NaY zeolites at 2θ=15.6°, 18.6°, 20.3°, and 26.3° were also presented on

212

CSNaYH and CSCuYH. However, the intensity of their peaks were decreased, due to


Page 9 of 25

213

the coating process and the weak shielding effects of mesoporous shells on X-rays.26

214

Combined with the XRD spectra of NaYH and silicalite-1 crystals (Figure. 2 (7)), both

215

the peaks of NaY and silicalite-1 zeolites were observed on the core@shell structured

216

NaYH and CuYH (Figure. 2 (8)-(11)), especially the MFI-type peaks at 2θ=22-25°,

217

indicating silicalite-1 had been successfully coated on NaYH and CuYH.38

218

3.3. Mass increasement of NaYH and NaYH/CuYH after core@shell experiments.

14 12

in theory after filtration after centrifugation

in theory after filtration after centrifugation

mass gain (%)

mass gain (g)

140 120

10

100

8

80

6 4

5g NaY

60

4.33 4.41

40

2

20

0

219 220 221

160

Mass gain (%)

16

Mass gain (g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NaYH

CSNaYH

CSYH CuCl2

CSYH Cu(NO3)2

CSYH CuSO4

0

Figure. 3. Mass increasement of NaY after HCl treatment and NaYH/CuYH after core@shell experiments (in theory: assuming that silicalite-1 was entirely coated around the core zeolites).

222

We have previously confirmed that HCl had a strong dealuminization effect on

223

NaY zeolite; and in order to determine this dealuminization effect more intuitively,

224

the mass variation of NaY before and after acid treatment were measured and shown

225

in Figure. 3. NaYH and the core@shell composites were collected from the mixtures

226

by two different methods in this experiment: filtration and centrifugation. Obviously,

227

there was a slight difference between filtration and centrifugation on NaYH; that was

228

because during the dealuminization process, partial NaY particle was broken into

229

smaller one ( 0.4, no

258

clear hysteresis loop or notable plateau was observed, guaranteeing NaY a

259

microporous property.39 After acid treatment, owing to the dealuminization effect of

260

HCl, some micropores in NaY zeolites were enlarged and finally presented the

261

mesoporous properties. As could be seen, NaYH and CuYH owned microporous as

262

well as mesoporous.

263


Page 10 of 25

Page 11 of 25

250

3

200 175

266

320 280 240

125

200

100 75

160

50

120

25

80

0.00

0.25

0.50

0.75

1.00

0.00

0.25

0.50

0.75

1.00

P/P0 Figure. 4. Adsorption and desorption isotherms of N2 on adsorbents (shifted along y-axis). Table. 1. BET analysis of NaY, NaYH, CuYH, silicalite-1, as well as core@shell structures. Adsorbents NaY Silicalite-1 NaYH YH-CuCl2 YH-Cu(NO3)2 YH-CuSO4 CSNaYH CSYH-CuCl2 CSYH-Cu(NO3)2 CSYH-CuSO4

267 268

Silicalite-1 CSNaYH CSYH-CuCl2 CSYH-Cu(NO3)2 CSYH-CuSO4

360

150

0

264 265

400

NaY NaYH YH-CuCl2 YH-Cu(NO3)2 YH-CuSO4

225

Volume adsorbed (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1S

t

2S

micro

3S

meso

4V

t

5V mic

6D

a

7D

mic

(m2/g)

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

(nm)

(nm)

786.27 489.21 263.32 233.65 225.67 214.53 386.54 368.88 342.49 322.85

754.24 471.68 173.95 149.44 144.26 142.85 347.86 339.37 313.72 294.31

32.02 17.54 89.37 84.21 81.41 71.68 38.68 29.51 28.77 28.54

0.38 0.35 0.19 0.19 0.18 0.17 0.29 0.29 0.27 0.27

0.29 0.20 0.15 0.14 0.12 0.12 0.23 0.22 0.19 0.18

1.81 2.06 3.13 3.16 3.23 3.22 2.31 2.03 2.14 2.19

0.84 0.65 0.84 0.84 0.84 0.85 0.82 0.82 0.83 0.84

1: total surface area; 2: microporous surface area; 3: mesoporous surface area; 4: total pore volume; 5: pore volume of micropores; 6: average aperture; 7: pore size of micropores.

269

Table. 1 and Figure. 5 made further efforts to represent the structural properties

270

of these zeolites, such as St (the total surface area), Smicro, Smeso (micropores and

271

mesopores surface area), Vt (total pore volume), as well as the aperture distributions.

272

Compared with the parent NaY zeolites, the St of NaYH had sharply decreased by as

273

more as 66.5%. Meanwhile, the Smicro decreased by more than 77%; and in contrast,

274

the Smeso increased by 1.8 times, indicating that the dealuminization of HCl played a

275

great effect on the structure of NaY zeolites by removing framework Al. Besides,

276

owing to the collapsed structure, the Vt and Vmic of NaYH decreased, and the Da was



277

enlarged. The BJH pore size distributions showed in Figure. 5 also demonstrated the

278

increased mesopores. After a further modification by Cu2+, compared to NaYH, the

279

structural properties of CuYH did not put up a much difference with only a slight

280

decrease in St, Smicro, Smeso Vt, Vmic and a minor increase in Da. Silicalite-1 shell

281

owned microporous as well as mesoporous according to Figure. 4. The surface area of

282

micropores in silicalite-1 occupied a leading position of total surface area (about 96%);

283

and what made silicalite-1 chosen as the shell basically depended on its microporous

284

aperture (about 0.65 nm), compared to NaY, NaYH, and CuYH (0.84, 0.84, and ～0.85

285

nm), which guaranteed silicalite-1 a shape selective capacity. When adsorbents were

286

coated with silicalite-1 shell, the adsorbates with a molecular size more than 0.65 nm

287

would be held back outside; thereby, the shape selective adsorption separation of

288

substances with different molecular sizes could be realized. NaY Silicalite-1 NaYH YH-CuCl2 YH-Cu(NO3)2 YH-CuSO4 CSNaYH CSYH-CuCl2 CSYH-Cu(NO3)2 CSYH-CuSO4

0.035

1.00

0.030

3 -1 -1 dV/dD (cm g nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

290

0.020 0.015

0.50

0.010 0.005

0.25

0.00

289

0.025

0.75

0.000 2

0

2

4

4 6 Aperture (nm)

6

8

8

10

Figure. 5. Distributions for aperture of different adsorbents (BJH method).

291

However, owing to the dealuminization effect of HCl, on the one hand, the total

292

pore volume of NaYH decreased from 0.38 to 0.19 cm3/g, which was bound to affect

293

the sulfur (DMDS) storage capacity. On the other hand, large amount of disappeared

294

framework Al made more Si elements exposure to the outside surface, and these

295

exposed Si would played an key role in growing silicalite-1 shell. Generally, at the

296

cost of a decreased pore volume, a more uniform and compact shell was achieved

297

(Figure. 6 and 7), providing NaYH and CuYH a higher degree on shape selective


Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


298

property to realize the removal of DMDS from MTBE.

299

3.5. SEM and TEM pictures for NaY, NaYH, CuYH and core@shell structures. (b)

(a)

2 um

(c)

1 um

(d)

0.5 um

um

0.5 um

300 301

Figure. 6. SEM pictures for (a) NaY, (b) NaYH, (c) CSNaYH, (d) CSYH-CuCl2.

(a)

(b)

100 nm

(c)

100 nm

～100 nm

(d) ～30 nm

302 303

100 nm

～30 nm

50 nm

Figure. 7. TEM pictures for (a) NaY, (b) YH-CuCl2, (c) CSNaYH, (d) CSYH-CuCl2.

304

After HCl treatment, Figure. 6 (b) exhibited a slightly damaged morphology of

305

NaYH zeolites, suggesting that the dealuminization effect could keep the original

306

skeleton of NaY basically unchanged compared with the regular shape of parent NaY

307

zeolites (Figure. 6 (a) and Figure. 7 (a)); in addition, after dealuminization, the surface

308

of NaY zeolite was no longer flat, leading to an improved outer surface area and more

309

silicon being exposed. After being coated, as shown in Figure. 6 (c and d), a clear

310

cladding structure could be observed that NaYH was enclosed by many fine

311

silicalite-1 crystalline grains which could be more intuitively showed up by TEM

312

pictures (Figure. 7 (c and d)); the thickness of the shell was of about 30-100 nm. Even

313

being homogeneously ion exchanged by Cu2+ (CuCl2) (Figure. 7 (b)), a clear and

314

smooth silicalite-1 shell could be still formed around the CuYH zeolite (Figure. 7 (d)).



315

Page 14 of 25

3.6. Analysis of element distribution on different adsorbents.

316

Table. 2 and Figure. S2 revealed the dealuminization effect of HCl more

317

intuitively by displaying the element content and distribution (mainly Si and Al ) of

318

NaY and the acid treated adsorbents. According to the ICP data, the molar ratio of

319

Si/Al was 2.41; while EDS data displayed a relatively larger deviation to be 2.48; that

320

was because the EDS detected only the surface element distribution of the sample,

321

while the ICP-OES analyzed the Si/Al ratio in the whole adsorbent by completely

322

dissolving the sample into aqueous solution.

323

Table. 2. Variety of Si, Al and Na element after acid treatment and core@shell experiment (ICP). Samples NaY NaYH YH-CuCl2 CSY-CuCl2 CSYH-CuCl2

324

Si

Al

★

★

28.3 35.7 36.8 30.8 42.9

11.3 4.6 4.5 8.5 2.1

Na

Si/Al

Al/Na

9.5 4 1.2 2.9 0.6

2.41 7.48 7.89 3.49 19.70

1.01 0.98 3.2 2.5 3

★

The unit of Si, Al, Na was mg/g.

★

325

After being treated by HCl, the Si/Al of NaYH was obviously increased by as

326

much as about twice from 2.41 to 7.48, indicating that abundant Al in NaY zeolites

327

were dissolved and removed after acid treatment. And massive loss of Al would

328

inevitably, on one hand, leave many vacancies around the surface enlarging the out

329

surface area of NaYH zeolites and providing more load locations for silicalite-1 shell;

330

on the other hand, expose more Si elements on the surface, which was the crystal

331

cores of silicalite-1 shell. After being ion-exchanged by CuCl2, the Si/Al of CuYH

332

remained almost the same (7.89, ICP; 8.12, EDS) compared to NaYH (7.48, ICP; 7.94,

333

EDS). while after the coating process, the Si/Al ratio of the corresponding core@shell

334

structures increased from 2.41 (2.48, EDS) to 3.49 (3.56, EDS) and 7.48 (7.94, EDS)

335

to 19.70 (21.48, EDS), respectively. Compared CSY-CuCl2 with CSYH-CuCl2, more

336

Si were detected in CSYH-CuCl2, indicating that HCl treatment could improve the

337

coating by providing more loading positions and crystal nucleus for silicalite-1.

338

In addition, the ratio of Al/Na was also detected and calculated in Table. 2. Na+

339

as the equilibrium charge, it was basically equivalent to the negative charge of


Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


340

framework Al in NaY and NaYH zeolite (Al/Na=1.01 and 0.98). As the Na+ being ion

341

exchanged by Cu2+, the loss of Na+ in YH-CuCl2 led to an increased Al/Na ratio to be

342

3.2. After been coated with silicalite-1 shell, the ratio of Al/Na was slightly reduced,

343

which may be attribute to the continuous ion exchange between Na+ and superfluous

344

Cu2+ adsorbed by NaY during core@shell experiment.

345

3.7. Cu2+ loading amount in different adsorbents.

346

Table. 3. Cu2+ amount (ICP) loaded in CuYH and corresponding core@shell structures. Cu2+

Concentration (aq, mol/L)

CuYH zeolite (mg/g)

core@shell composites (mg/g)

CuCl2 Cu(NO3)2 CuSO4

0.5 0.5 0.5

4.3 3.6 1.8

4.1 3.1 1.6

347

Many researches had proved that, as the adsorptive center, Cu2+ could adsorb

348

DMDS by π-complexation.40, 41 Thus, the loading amount of Cu2+ on zeolites would

349

directly affect the quantity of sulfur adsorbed. Via an Agilent 725 ICP-OES, the Cu2+

350

loading amount in NaYH zeolites were displayed in Table. 3. After an ion exchange

351

process in different Cu2+ solution (1 g: 20 ml, 0.5 mol/L Cu2+, 90 °C, 24 h), the Cu2+

352

amount loaded from CuCl2 showed the maximum in both CuYH and its core@shell

353

composites to be 4.3 and 4.1 mg Cu2+/g adsorbents. Actually, in order to complete ion

354

exchange between Cu2+ and Na+, Cu2+ should enter into the channel of NaYH zeolites

355

and reach the point where Na+ stood. However, to maintain the electroneutrality of the

356

solution, the anions (Cl-, NO3-, SO42-) were bound to follow and keep close to Cu2+ to

357

enter into the channel at the same time; thus, the molecular size of anions would play

358

a decisive role by limiting whether Cu2+ could enter the channels; in other words, if

359

the molecular size of anions were larger than some channels which were smaller than

360

the size of Cu2+, at this time, it would still be difficult for Cu2+ to get into smoothly.

361

From this point of view, owing to the fact that the molecular size of Cl-, NO3-, SO42-

362

increased in turn, the Cu2+ loading amount in NaYH zeolites would decrease

363

successively, which was in according with the results shown in Table. 3. In addition,

364

during the coating process (180 °C, 24 h) of synthesizing the CuYH@silicalite-1, a

365

fraction of the loaded Cu2+ in CuYH would be leached out, leading to a slight



Page 16 of 25

366

decreased Cu2+ loading amount in the corresponding core@shell composites. After

367

Cu2+ being loaded in NaYH zeolites, and used as adsorbents to remove DMDS from

368

MTBE, the Cu-O bonds could also be formed, which was easier to form than Cu-S,

369

lead to the fierce competition between DMDS and MTBE on NaYH and CuYH.

370

3.8. Analysis of FT-IR spectra on adsorbents. 1108

(a) (b)

1008

C-O-C

(c) (d) (e)

1230 C-S

(f) (g) (h) (i) (j)

2000

1619 H-OH

1800

1600

1400

1200

1000

800

600

-1

wave numbers (cm )

371 372 373 374

Figure. 8. The FT-IR spectra of different adsorbents after adsorbing DMDS in MTBE: (a) NaY, (b) NaYH, (c) YH-CuCl2, (d) YH-Cu(NO3)2, (e) YH-CuSO4, (f) silicalite-1, (g) CSNaYH, (h) CSYH-CuCl2, (i) CSYH-Cu(NO3)2, (j) CSYH-CuSO4.

375

After adsorbing DMDS, the FT-IR spectra of adsorbents were displayed in

376

Figure. 8. The bonds at 1619 cm-1 were confirmed as the -OH vibration of water

377

adsorbed in adsorbents;42 1230 cm-1 was the C-S stretching vibration; 1108 and 1008

378

cm-1 were the C-O-C stretching vibration. Owing to the fierce competition between

379

DMDS and MTBE, in MTBE solution, MTBE was much more easier to be adsorbed

380

than DMDS; Figure. 8 (a)-(e) exactly proved that no bonds were located at 1230 cm-1,

381

indicating little DMDS being adsorbed on NaY, NaYH or CuYH. As to silicalite-1

382

crystals, both C-S and C-O-C were recorded, suggesting that both DMDS and MTBE

383

were absorbed. After NaYH or CuYH being enclosed and coated by the shell, the C-S

384

bond could always be recorded (Figure. 8 (g-j)), suggesting that these synthesized

385

core@shell composites could adsorb DMDS in MTBE solution. However, these

386

detected C-S bonds was not enough to further confirm whether the core@shell

387

composites could selectively adsorb DMDS from MTBE, because there was difficulty


Page 17 of 25

388

in distinguishing if the C-S bonds came from the DMDS adsorbed on silicalite-1 shell

389

or the core zeolites. To test and verify the shape-selection characteristic of the

390

synthesized core@shell structured composites, the active adsorption data was

391

collected by both static and dynamic activity tests.

392

3.9. Desulfurization performances on core@shell zeolites.

393

3.9.1. Analysis on static active data in static state. As shown in Figure. 9, in the MTBE solution with a raw DMDS concentration of

395

748.52 mg/L, by static adsorption tests, the adsorption capacities of DMDS on

396

different adsorbents were collected. Obviously, a great difference could be observed

397

between adsorbents with and without silicalite-1 shell. After adsorption, as could be

398

seen, those adsorbents with no silicalite-1 coatings (NaY, NaYH and CuYH) showed a

399

negative desulfurization by adsorbing the solvent (MTBE) instead of the solute

400

(DMDS); because of the intense competition effect between DMDS and MTBE on

401

these adsorbents32; owing to a much larger quantity of MTBE than DMDS, on these

402

adsorbents, MTBE was preferentially adsorbed, leading to an increased sulfur content,

403

which was higher than 748.52 mg/L (Figure. 9 a, c-f).

Raw material sulfur concentration：748.52

1000

800 789.92 742.34 779.72

831.83 823.02

34.585

26.921

517.52 402.67

400

435.81

0

0.618 -4.14

a

20

479.31 10

0

0

30

23.127

785.22

600

40

31.271

748.52

200

404 405 406 407

mgs/gadsorbent

mg/L

1200

b

-3.12

c

-8.331 -7.45

d

e

-3.67

f

Sulfur adsorbtion capacity (mgs/gadsorbent)

394

After adsorbtion (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-10

g

h

i

j

Figure. 9. Adsorption capacity of sulfur on different adsorbents in desulfurizing DMDS from MTBE: (a) NaY, (b) NaYH, (c) silicalite-1, (d) YH-CuCl2, (e) YH-Cu(NO3)2, (f) YH-CuSO4, (g) CSNaYH, (h) CSYH-CuCl2, (i) CSYH-Cu(NO3)2, (j) CSYH-CuSO4.

408

The adsorption capacities of sulfur on CSNaYH, CSYH-CuCl2, CSYH-Cu(NO3)2,

409

and CSYH-CuSO4 were also calculated and displayed in Figure. 9 g-j. After coating



410

silicalite-1, a significant capacity (23.127, 34.585, 31.271, and 26.921mgs/gadsorbents) in

411

desulfurization of DMDS from MTBE was obtained, which was far greater than that

412

of silicalite-1 alone as the adsorbent (0.618 mgs/gadsorbents), proving the remarkable role

413

of silicalite-1 shell in selective adsorbing DMDS from MTBE. And compared Figure.

414

9 d (YH-CuCl2) with h (CSYH-CuCl2), based on silicalite-1 shell, the adsorption

415

capacity of sulfur demonstrated a wide range of improvements from -8.331 to 34.585

416

mgs/gadsorbents, which further expounded and proved that after being coated by

417

silicalite-1 crystals, the synthesized core@shell composites owned the capacity to

418

selectively adsorb DMDS from MTBE by preventing MTBE from migrating into the

419

adsorbents (shape selective function of silicalite-1 shell), while letting through only

420

DMDS and adsorbing DMDS inside the adsorbents (adsorption and storage function

421

of the core zeolites).

422

Also, considering the effect the raw material sulfur (DMDS) concentration on the

423

sulfur adsorption capacities of the synthesized core@shell composites, the sulfur

424

adsorption capacity and desulfurization rate of CSYH-CuCl2 in different DMDS

425

concentration of MTBE solution was conducted. As shown in Table S1, the DMDS

426

concentration in MTBE solution was quantified from 268.71 to 856.47 mg/L. With

427

the increase of DMDS concentration, the sulfur adsorption capacity of CSYH-CuCl2

428

was in a uptrend. However, when the concentration of DMDS was more than 451.3

429

mg/L, the sulfur adsorption capacity of CSYH-CuCl2 was kept at 34.162 or so. And as

430

the concentration of DMDS was 268.71 mg/L, the DMDS was adsorbed of only

431

97.2%, owing to the diffusion limitation of static test itself.

432

3.9.2. Analysis on active data in dynamic state.

433

To further evaluate the practical application potential of these core@shell

434

structured adsorbents in desulfurizing DMDS from MTBE, they were pressed and

435

broken into particles, then loaded in a quartz column reactor to collect the dynamic

436

active data every half an hour as shown in Figure. 10. As could be seen, the

437

desulfurization curve of silicalite-1 also confirmed it could selectively adsorb DMDS

438

from MTBE in a small degree. Overall, as the time going on, the desulfurization of all

439

the adsorbents displayed a downside; however, bounded by the desulfurization curve


Page 18 of 25

Page 19 of 25

440

of silicalite-1, adsorbents with and without core@shell structures showed a significant

441

distinction in desulfurization rate. 100

Desulfurization rate of DMDS(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


442 443 444 445

80

a b c d e f g h i j

60 40 20 0 -20 0

1

2

3

4

Time (h) Figure. 10. Desulfurization rate of adsorbents: (a) NaY, (b) NaYH, (c) silicalite-1, (d) YH-CuCl2, (e) YH-Cu(NO3)2, (f) YH-CuSO4, (g) CSNaYH,(h) CSYH-CuCl2, (i) CSYH-Cu(NO3)2, (j) CSYH-CuSO4.

446

Due to the competitive adsorption of DMDS and MTBE, NaY, NaYH as well as

447

CuYH zeolites displayed very poor desulfurization rate throughout the tests, indicating

448

there was no desulfurization capacity in adsorbing DMDS from MTBE on these

449

adsorbents, which was consistent with the static active data analysis. On the contrary,

450

the adsorbents with core@shell structure, such as CSYH-CuCl2, CSYH-Cu(NO3)2, and

451

CSYH-CuSO4, showed a remarkably improved desulfurization rate by 100%

452

desulfurization for about 2.5 h; CSNaYH could also keep this desulfurization level for

453

about 1.5 h. among these core@shell composites, CSYH-CuCl2 showed the best

454

performance by desulfurizing DMDS at 80% level for at least 4.5 hours. And the

455

active life of these core@shell adsorbents would quickly diminish after about 3-3.5 h.

456

3.10. Analysis on desulfurization mechanism.

457

Figure. 11 intuitively presented the dealuminization effect, the silicalite-1 shell

458

growth and the selective adsorption process of DMDS. The NaY core owned 18

459

four-membered rings, 4 six-membered rings and 4 twelve-membered rings in a unit

460

cell with an aperture of about 0.9 nm in main channel. As depicted in Figure. 11, there



461

were three positions (NaⅠ, NaⅡ, NaⅢ) for Na+ to balance the skeleton negative

462

charge in Y zeolite; when ion exchange Na+ with Cu2+, NaⅡand NaⅢ would be

463

exchanged preferentially than NaⅠwhich was in the center of hexagonal prism cage.

464

After being ion exchanged by Cu2+, owing to its divalent valence state, the adsorbed

465

or bound water in Y zeolite would form hydrated ions with Cu2+ (Cu(H2O)2+) or

466

dissociate into H+ and Cu(H2O)+ and then exist steadily as CuY.

: Si

NaY:

: Al

Si/Al = 2.4

silicalite-1 crystal

467 468 469 470

Figure. 11. The dealuminization of HCl and the silicalite-1 shell growth on CuYH as well as the selective desulfurization mechanism of adsorbing DMDS from MTBE on CuYH@silicalite-1.

471

The Si/Al (molar ratio) of NaY zeolite used in this study was 2.4. After being

472

acid treated the Si/Al had been significantly increased as discussed in element

473

distribution analysis (section 3.6), manifesting as the framework Al being transferred

474

into the extra-framework Al and then removed, leaving many hydroxyl nests (or

475

cavities). when this dealuminized NaY or CuY was coated with silicalite-1 by

476

hydrothermal methods, new silicon sources (from TEOS) would be admitted by the


Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


477

hydroxyl nests and extensively grow along the surface to finally form a complete shell,

478

whose maximum aperture was about 0.65 nm.

479

Except for the competition, the different size of MTBE (0.74 nm) and DMDS

480

(0.37 nm) had made it possible in shape selective desulfurization.43,

481

synthesized core@shell composites were used to adsorb DMDS in MTBE solution,

482

both DMDS and MTBE would contact the silicalite-1 shell preferentially; by virtue of

483

small size, DMDS could diffuse and pass rapidly through silicalite-1 shell, leaving the

484

larger sized and branched structured MTBE being shut out. By metal-S bonds DMDS

485

could be fixed and stored inside the core zeolites; with the continuation of adsorption,

486

the concentration of DMDS would constantly decline and eventually disappeared to

487

achieve the purpose of shape selective desulfurization.

488

4. Conclusion

44

When these

489

At the cost of a decreased surface area, acid treatment had been proved a

490

significant mean to increase the Si/Al ratio of NaY zeolite by the dealuminization

491

effect of HCl from 2.48 to 7.94 (EDS data), which was quite beneficial for the

492

growing of silicalite-1 shell for the sake of more Si elements being exposed on NaY

493

surface to act as the crystal cores of silicalite-1 shell; in addition, the loss of Al after

494

acid treatment would inevitably leave many vacancies around the surface enlarging

495

the out surface area of NaYH zeolites and providing more load locations for

496

silicalite-1 shell as well. After core@shell process, the notable increased Si/Al in

497

corresponding core@shell composites further demonstrated the remarkable role of

498

acid treatment in coating process. As the core, depending on forming the Cu-S bonds

499

within DMDS and Cu2+, CuYH displayed a larger adsorption capacity of sulfur than

500

both NaY and NaYH in storing DMDS. Relying on silicalite-1 shell (shape selectivity)

501

and CuYH core (adsorption and storage centers), CuYH@silicalite-1 displayed a great

502

advantage on desulfurization of DMDS at the present of MTBE; CSYH-CuCl2 had

503

possessed the best adsorption capacity of sulfur to be 34.585 mgs/gadsorbent and 100%

504

desulfurization could be realized and kept for as long as 3 h. After 4.5 hours, more

505

than 80% desulfurization rate was still achieved.

506



507

Associated Content

508

The X-ray diffraction patterns of silicalite-1 and small particles that filtered out

509

after filtration (Figure S1); EDS analysis of (a) NaY, (b) NaYH, (c) YH-CuCl2, (d)

510

CSY-CuCl2, and (e) CSYH-CuCl2 (Figure S2); as well as the sulfur adsorption

511

capacity and desulfurization rate of CSYH-CuCl2 in different DMDS concentration of

512

MTBE solution (Table S1) were provided in the Supporting Information file.

513

Acknowledgments

514

Project financially supported by the National Science Foundation for Young

515

Scientists of China (No. 21706065); the Open Project of State Key Laboratory of

516

Chemical Engineering (SKL-ChE-18C02); China Postdoctoral Science Foundation

517

(NO.2017M621389), Shanghai Sailing Program (NO.18YF1406300), and Explore

518

and Research Foundation for Youth Scholars of Ministry of Education of China (NO.

519

222201814011).

520

Reference

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TOC graphic

: Si

NaY:

: Al

Si/Al = 2.4

silicalite-1 crystal

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Selective Adsorption of Dimethyl Disulfide on Acid-Treated

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