dimethyl carbonate mixtures by pervaporation

Journal Pre-proof Supported ionic liquid membranes for the separation of methanol/dimethyl carbonate mixtures by pervaporation Wenqi Li, Cristhian Molina-Fernández, Julien Estager, Jean-Christophe M. Monbaliu, Damien P. Debecker, Patricia Luis PII:

S0376-7388(19)33095-9

DOI:

https://doi.org/10.1016/j.memsci.2019.117790

Reference:

MEMSCI 117790

To appear in:

Journal of Membrane Science

Received Date: 5 October 2019 Revised Date:

21 December 2019

Accepted Date: 25 December 2019

Please cite this article as: W. Li, C. Molina-Fernández, J. Estager, J.-C.M. Monbaliu, D.P. Debecker, P. Luis, Supported ionic liquid membranes for the separation of methanol/dimethyl carbonate mixtures by pervaporation, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117790. 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 B.V.

CRediT author statement The following authors’ contributions took place during the research: Wenqi Li: conceptualization, methodology, validation, formal analysis, investigation, data curation, writing the original draft, writing-review-editing, visualization. Cristhian Molina-Fernández: conceptualization, methodology, validation, formal analysis, data, investigation, data curation, writing-review-editing. Julien Estager : writing-review-editing, funding acquisition. Jean-Christophe M. Monbaliu : writing-review-editing, funding acquisition. Damien P. Debecker : conceptualization, writing-review-editing, funding acquisition. Patricia Luis : conceptualization, methodology, formal analysis, resources, writing-review-editing, supervision, project administration, funding acquisition.

1

Supported ionic liquid membranes for the separation of

2

methanol/dimethyl carbonate mixtures by pervaporation

3 4

Wenqi Lia, Cristhian Molina-Fernándeza, Julien Estagerc, Jean-Christophe M. Monbaliud, Damien P.

5

Debeckerb, Patricia Luisa* a

6

Materials & Process Engineering (iMMC-IMAP), UCLouvain, Place Sainte Barbe 2, 1348

7 8

Louvain-la-Neuve, Belgium b

Institute of Condensed Matter and Nanosciences (IMCN), UCLouvain, Place Louis Pasteur, 1, box

9 10

L4.01.09, 1348 Louvain la-Neuve, Belgium c

Certech, Centre de ressources technologiques en chimie, Rue Jules Bordet, Zone Industrielle C,

11 12

7180 Seneffe, Belgium d

Center for Integrated Technology and Organic Synthesis, MolSys Research Unit, University of

13

Liège, B-4000 Liège (Sart Tilman), Belgium

14

* Tel: +32 16 322348; Fax: +32 16 322991; Email: [email protected]

15 16

Highlights:

17

•

[C8MIM][NTf2] and [C8C1Pyrr][NTf2] have been studied to design a new kind of SILMs

18

•

The membranes achieved high flux and high selectivity towards DMC vs. methanol

19

•

A separation factor of 21 was achieved for 0.8 molar fraction of DMC at 30 °C

20 21

Abstract

22

Two supported ionic liquid membranes (SILM) based on 1-octyl-3-methylimidazolium

23

bis(trifluoromethanesulfonyl)imide

24

bis(triuoromethanesulfonyl)imide ([C8C1Pyrr][NTf2]) were prepared and studied for the

25

pervaporation separation of binary mixtures of dimethyl carbonate (DMC)/methanol. Scanning

26

electron microscope (SEM) analyses were carried out to evaluate the cross section morphology of

27

the porous membranes before and after incorporating the ionic liquids. The pervaporation

28

performance of SILMs was found to be highly concentration dependent. At low methanol

29

concentration (0.2 molar fraction), both SILMs tend to preferentially permeate DMC. In general,

([C8MIM][NTf2])

and

N-octyl-N-methylpyrrolidinium

30

the SILM based on [C8MIM][NTf2] exhibited a better performance than the one with

31

[C8C1Pyrr][NTf2]. Under optimal conditions, the SILM composed of [C8MIM][NTf2] enabled a

32

transmembrane flux of 0.739 kg/m2h, a DMC/methanol selectivity of 67 and separation factor of

33

21 at 30 °C at 0.8 molar fraction of DMC. However, at high concentration of methanol, the

34

permeance of methanol increased due to coupling effects therefore decreasing the membrane

35

selectivity to around 2.

36 37

Keywords: Ionic liquids; Supported ionic liquid membrane; Methanol; Dimethyl carbonate;

38

Transesterification reaction;

39 40

1. Introduction

41

Membrane technology has been recognized as an environmentally friendly technology thanks to

42

its low energy consumption and low waste generation.1 Pervaporation is generally used to

43

separate challenging mixtures which separation with conventional methods, such as distillation,

44

requires high energy consumption. These complicated cases typically include azeotropic mixtures

45

or close-boiling point compounds. In the present work, a binary mixture of dimethyl carbonate

46

(DMC) and methanol has been studied. It is of interest as dimethyl carbonate is an important

47

biodegradable “green chemical” with low toxicity with an increasing number of applications.2,3,4

48

Methanol appears in different dimethyl carbonate synthesis routes,5–7 as for example DMC can

49

be produced by the reaction of CO2 and methanol, by the transesterification of ethylene

50

carbonate and methanol

51

methanol is also a by-product in different syntheses involving DMC, such as the production of

52

glycerol carbonate via the transesterification reaction between glycerol and DMC.14,15 In both

53

cases, an efficient separation of DMC and methanol is needed to get a sustainable and

54

economically favorable process. However, methanol forms an azeotrope with dimethyl carbonate

55

at 30/70 wt% DMC/methanol concentration16, making the separation process energetically

56

intensive by conventional distillation.17 Hence, the development of energy-efficient processes for

57

the separation of DMC and methanol is an important challenge to be addressed.

8–11

or by the reaction of urea again with methanol.12,13 In addition,

58 59

The application of pervaporation is a very attractive approach. Commercial pervaporation

60

membranes based on polyvinyl alcohol from Sulzer have been previously studied by our group for

61

the separation of a quaternary mixture including DMC and methanol,18 showing that methanol is

62

concentrated in the permeate at 44 mol% concentration of methanol in the feed, with a

63

separation factor of 14 (methanol relative to DMC), a selectivity of 5.7 (methanol relative to DMC)

64

and a permeance of methanol of 723 GPU. The pervaporation separation of a DMC/methanol

65

binary mixture was also studied by using self-made PEEK membranes.19 It was shown that good

66

separation could be achieved at low concentration of methanol (0.1 molar fraction): separation

67

factor of 13.4 (methanol relative to DMC), selectivity of 3.5 (methanol relative to DMC) and

68

permeance of methanol 293 GPU.

69

Supported liquid membranes (SLMs) have been introduced in pervaporation as potential

70

solutions to increase the selectivity and transmembrane flux by tuning the affinity of the liquid to

71

the target compound and the higher diffusivity through the liquid phase immobilized inside the

72

membrane pores.20 The mass transport mechanism in SLM involves three stages: 1) the

73

molecules are sorbed from the feed solution into the solvent in the SLM; 2) the sorbed molecules

74

diffuse through the liquid membrane to the permeate side; 3) the molecules are desorbed into

75

the permeate side.21 The solubility and diffusion coefficients of different solutes in a liquid leads

76

to high flux if compared to dense membranes since diffusion coefficients in liquids are much

77

higher than in polymers22. However, the stability of SLMs remains the major limitation for a large

78

scale commercial application.23,24 Low stability of supported liquid membranes has been

79

observed in the literature, with a loss of immobilized solvent after relatively short application

80

time, leading to a dramatic increase of flux and decrease of selectivity.25 Solvent evaporation,

81

dissolution into contiguous phases and pressure gradient are the major factors leading to the loss

82

of solvent.26 In order to solve this issue, ionic liquids have been used as the active separation

83

medium in SLMs, leading to the so-called supported ionic liquid membranes (SILMs).27

84 85

Ionic liquids are generally defined as organic salts containing an organic cation and an inorganic

86

or organic anion that have a melting temperature below 100 °C.28 They can be designed by

87

combining different cation and anion therefore modifying both their chemical and physical

88

properties, such as their solubility properties. Such tunability gives these solvents a very good

89

potential to achieve a good selectivity toward target component.29 In addition, ionic liquids have

90

high chemical and thermal stabilities and negligible vapor pressure.30 Therefore, they are often

91

considered as “green solvents” to replace volatile organic solvents in the chemical industry. Ionic

92

liquids have wide applications in chemistry for instance as catalysts or additives,30–33 for

93

extraction,32–35 as electrolytes,38–41 or in gas purification.42–44 In fluid-fluid separation processes,

94

ionic liquids are good media for extraction. However, the high price of most of ionic liquids and

95

the high energy consumption needed to purify ionic liquids for reuse are important factors for

96

the limitation of their application in separation processes.45 These shortcomings can be solved by

97

using SILMs since only a small amount of IL is required to fill the membrane pores and the

98

recycling of ionic liquid for further reuse is not necessary. Due to their negligible vapor pressure

99

and high viscosity, ionic liquids in SILMs can be more stable than organic solvents.

100 101

In the literature, SILMs have been extensively used for gas separation, such as SO2/CO2,

102

CO2/H2/N2, H2S/CO2/CH4 and natural gas purification.27,46–52 While their application in

103

pervaporation is not as widespread, SILMs have received increasing attention in recent years, for

104

example for the separation of transesterification reaction mixtures containing alcohols, organic

105

acids, hydrocarbons and amines.53–61

106 107

The use of SILMs for the separation of transesterification mixtures has been studied based on

108

ionic liquids such as [C4MIM][BF4], [C8MIM][BF4], [C4MIM][PF6] or [C8MIM][PF6].55,62 In addition,

109

the ionic liquids [C2MIM][Cl] and [C4MIM][Cl] have been investigated to be used as carriers for

110

breaking the azeotrope of methanol and DMC.63 These two ionic liquids showed their capability

111

to separate the azeotrope when the molar fraction of ionic liquids in the methanol, DMC and

112

ionic liquid ternary system increased up to certain level, such as 0.1168 molar fraction of

113

[C4MIM][Cl]. However, the application of SILMs for the separation methanol/DMC mixtures has

114

not been reported yet.

115 116

In this work, two ionic liquids were synthesized and impregnated in a porous polyacrylonitrile

117

(PAN) support membrane to prepare the corresponding SILMs. These materials have been tested

118

for the separation of DMC/methanol mixtures by pervaporation. The ionic liquids,

119

1-octyl-3-methylimidazolium

bis(triuoromethanesulfonyl)imide

[C8MIM][NTf2]

and

120

N-octyl-N-methylpyrrolidinium bis(triuoromethanesulfonyl)imide [C8C1Pyrr][NTf2] characterized

121

as hydrophobic ionic liquids,64 were used. Their molecular structures are presented in Figure 1.

122

The ionic liquid [C8MIM][NTf2] was selected taken as reference the works by Hernández-Fernández

123

and de los Ríos,56–58 which showed the interest of this ionic liquid for organic-organic separations.

124

In addition, in order to investigate the impact of the structure of the cation on the separation

125

performance, the ionic liquid [C8C1Pyrr][NTf2], containing the pyrrolidinium cation and the same

126

anion and alkyl chain, was selected.65 The performance of the SILMs prepared with those ionic

127

liquids was evaluated in terms of flux, separation factor, permeance and selectivity.

128 129

Figure 1. The molecular structure of the cations and anion forming the ionic liquids studied here, together with

130

dimethyl carbonate (DMC)

131

2. Materials and methods

132

2.1 Materials

133

The support membrane used for the preparation of the SILMs is a PAN flat ultrafiltration

134

hydrophilic membrane (Type: PX), which was purchased from Synder Filtration (USA).

135

Polypropylene (PP) flat sheet membrane (hydrophobic) model ACCUREL PP 1E (R/P) was

136

purchased from 3M GmbH (Germany). Dimethyl carbonate (purity >99%) and methanol

137

(purity >99.8%) were purchased from VWR International and Alfa Aesar, respectively.

138

Lithium bis(triuoromethanesulfonyl)imide (purity>99%) was purchased from Abcr GmbH,

139

Germany.

140

N-methylpyrrolidine (purity >98%) was purchased from Acros Organics. These chemicals were

141

used for the synthesis of the ionic liquids without further purification.

142

3-methylimidazole

(purity

99%)

was

purchased

from

Alfa

Aesar

and

143

2.2 Ionic liquid synthesis

144

The ionic liquids have been synthesized in a two-step process based on known procedure from

145

the literature, namely a quaternarization of a tertiary amine66 followed an anion metathesis using

146

lithium bis(trifluoromethanesulfonyl)imide67. The first step was the quaternization of

147

N-methylimidazole or N-methylpyrrolidine using 1-chlorooctane in acetonitrile at 80°C. The

148

second step consists in an anion metathesis using lithium bis(triuoromethanesulfonyl)imide at

149

room temperature. The purity of the different ionic liquids was assessed based on 1H and

150

Nuclear magnetic resonance (NMR) analyses. No signal for starting materials or eventual

151

by-products were observed.

13

C

152 153

2.3 Membrane preparation

154

First, hydrophobic and hydrophilic porous membranes were tested as supports. On one hand, the

155

hydrophobic membrane (polypropylene) could not hold the ionic liquid inside the membrane

156

pores. The high vacuum applied in the permeate side during the immobilization procedure and

157

its larger pore size could explain why the polypropylene membrane was not able to hold the ionic

158

liquids. On the other hand, the hydrophilic (PAN) membrane was able to hold the ionic liquids

159

inside its pores thanks to intramolecular interactions of the sulfoxide group (S=O) from [NTf2]-

160

anion and the cyano groups (C≡N)68,69. Therefore, the hydrophilic PAN flat sheet membrane was

161

used as a supporting membrane.

162

All the SILMs used through this study were prepared by the following immobilization procedure:

163

a commercial circular flat sheet ultrafiltration membrane (PAN) was placed inside the membrane

164

cell. The ionic liquid was added on top of the membrane using a pipette. The quantity of the IL

165

added was sufficient to cover entirely the surface of the porous membrane. An O-ring was

166

installed on the circular membrane and pressed gently on it. Then, the cell was fixed and

167

tightened by closing the bolts. The structure of the membrane cell is shown in Figure 2. Vacuum

168

was applied for 2 hours using a rotatory pump (50 mbar) on the permeate side to remove the air

169

from the pores of the membrane and suck the ionic liquids into the pores. When the

170

immobilization was completed, the excess of IL on the membrane surface was removed carefully

171

using a tissue. To determine the amount of ionic liquid immobilized in the supported membrane,

172

all the membranes were weighted before and after impregnation with an analytical balance (AE

173

260 METTLER TOLEDO, Belgium) with precision +/- 0.0001 g.

174 175 176

Figure 2. The cell for preparing supported ionic liquid membrane

2.4 Scanning electron microscopy (SEM) analysis

177

In order to evaluate the quality of the immobilization of the ionic liquid inside the membrane

178

pores, the morphology of the cross section before (raw PAN membrane) and after adding the ILs

179

was analyzed by SEM (Zeiss, ULTRA). The membranes were cut in small rectangular pieces and

180

immersed into liquid nitrogen. As the polymeric material from which they are made is very brittle

181

at such low temperatures, samples were broken without deforming the cross section. Before

182

analysis, all the samples were sputter coated with a thin layer of gold (BALZERS UNION FL 9460

183

BALZERS SCD 030) to make them conductive.

184 185

2.5 Gas chromatography analysis

186

The composition of feed and permeates was analyzed by gas chromatography (Interscience

187

TRACE 1300) equipped with a flame ionization detector (FID), split/splitless injection (SSL) unit,

188

thermal conductivity detector (TCD) and a capillary column (Stabilwax, 30 m, 0.32 mm, 1 μm).

189

The carrier gas was Helium and the injection was performed in split mode with a split ratio of 100.

190

Initially, the oven temperature was set at 50°C and it was increased at the rate of 20°C /min until

191

it reached 150 °C. Then, it was maintained at this temperature for 1 min. The FID and injection

192

temperatures were 250°C and 300°C, respectively. A calibration curve was obtained by

193

performing GC analysis of samples of known concentrations. Three trials were done for each of

194

the data points.

195 196

2.6 Pervaporation experiments

197

The pervaporation experiments were performed in a 3’’ round cell unit (Sulzer Chemtech GmbH,

198

Switzerland), the same unit used to prepare the SILMs (Figure 3). The scheme of the

199

pervaporation system is shown in Figure 3.

200 201

Figure 3. The scheme of pervaporation separation experimental equipment

202

The experimental temperature inside the membrane cell was kept at 30 °C (+/- 0.3 °C) using a

203

heating circulator (Julabo, Germany). A vacuum pump was used at the permeate side giving a

204

vacuum pressure of 1-2 mbar. The surface area of installed SILM was 38.48 cm2 (diameter 7.0 cm).

205

Sampling of the permeate was started after running the system for two hours to reach stable

206

conditions. The permeate was collected and weighed every 30 or 60 minutes depending on the

207

amount of permeate. The composition of the permeate samples was analyzed every 120 minutes

208

by means of gas chromatography as indicated in section 2.5. The membranes prepared were

209

tested with different compositions of binary mixtures methanol/DMC. The feed compositions

210

were 0.2, 0.5, or 0.8 mole fraction of methanol. In this work, each experiment was carried out

211

twice in order to check the reproducibility of experimental results.

212

The performance of SILMs was evaluated in terms of transmembrane flux

(kg/m2∙h),

213

separation factor

/

and selectivity

, permeance

/

, expressed as follows:

214 = /

=

(

× /



(1)

∆ × / = / ×

=

−

/ = /

(2)

×



(3)

)

(4)

215

where A is the membrane effective area (m2), ∆ is the permeate collecting time (h) and

216

the weight of permeate (kg).

217

the components i and j in the permeate (yi, yj) and feed (xi, xj) solutions, respectively. Ji is the

218

partial flux of component i (kg/m2∙h) and Pp is the pressure at permeate side. Aspen Plus 11

219

was used to calculate the vapor pressure P0i (atm) and activity coefficients γi of component i at

220

different concentrations. The NRTL method was employed to estimate the thermodynamic

221

parameters since it shows good approach for DMC/methanol mixtures.70,71

222

thickness. The unit of permeance is expressed in GPU 1 GPU=1×10-6 cm3 (STP)/(cm2 s cmHg) and

223

1 m3 /m2 s kPa=1.33×108 GPU; the unit conversion can be found in Baker et al..72 The selectivity

224

(αi/j ) is the ratio of permeance of component i and j. If the value of selectivity (αi/j ) is larger than 1,

225

this indicates that component i is more permeable to the membrane than component j.

226

The SILMs performance results are interpreted by analyzing the Kamlet-Taft solvatochromic

227

parameters, the Hildebrand solubility parameters and the chemical structure of the molecules.

and

is

are molar fraction of components, the subscript indicates

is the membrane

228 229

2.7. Kamlet-Taft solvatochromic parameters and Hildebrand solubility parameters

230

The Kamlet-Taft solvatochromic parameters were used to provide a comprehensive insight into

231

the solvent-space structure regarding to the similarity of solute and solvent interactions.73

232

Kamlet–Taft solvatochromic parameters are the most comprehensive and frequently used

233

quantitative measure of solvent properties, such as polarity and hydrogen-bonding ability. Three

234

Kamlet–Taft parameters include:

235

(acidity), hydrogen-bond accepting ability (basicity) and polarity/polarizability, respectively.

,

and

∗

, which quantify hydrogen-bond donating ability

236 237 238

The Hildebrand solubility parameter is derived from the square root of the cohesive energy

239

density of the solvent, in terms of the heat of vaporization divided by the molar volume, a more

240

detailed explanation can refer to Barton et al..74 Table 2 shows the Hildebrand solubility

241

parameters of ionic liquids, pure methanol and DMC, and their mixtures at different molar

242

fraction. The solubility parameter of a mixture is estimated by the following equation (5):74 = ! "# #

is Hildebrand solubility parameter and

$#

(5)

243

the

is the Hildebrand solubility parameter of

244

pure component i. " is the volume fraction of the pure component i in the mixture. A shorter

245

distance of

246

them.

between component A and component B indicates a stronger affinity between

247 248

3

249

3.1 SEM analysis

250

The cross section morphologies of the raw PAN membrane and the prepared supported ionic

251

liquid membranes are shown in Figure 4. Before immobilization, regular empty pores can be

252

clearly observed in the raw PAN porous membrane (Figure 4a). After immobilization, the

253

membranes with the ionic liquids [C8MIM][NTf2] and [C8C1Pyrr][NTf2] are shown in Figures 4b and

254

4c, respectively. It shows that the PAN porous membrane can hold the ionic liquids inside

255

membrane pores, being present in all the membrane thickness.

Results and discussion

256 257

(a)

258 259

(b)

(c)

260 261

Figure 4. The PAN membrane before immobilization (a); immobilization of [C8MIM][NTf2] (b); and immobilization

262

3.2 Pervaporation separation performance

263

The separation performances of SILMs prepared with [C8C1Pyrr][NTf2] and [C8MIM][NTf2]

264

immobilized in PAN membranes were determined for binary mixtures at different concentrations

265

of methanol/DMC. The transmembrane flux, separation factor, permeance and selectivity of

266

these two SILMs are shown in Figure 5.

267

Figure 5a, e and f shows that the total transmembrane flux and partial flux of both SILMs are

268

strongly dependent on the concentration in methanol. The raw flux value does not reflect the

269

real interaction between the feed components and ionic liquids due to the presence of driving

270

force.72 Therefore, the permeance is discussed instead because permeance removes the effect of

271

the driving force. Figure 5b shows the permeance of DMC and methanol of both SILMs. It is clear

272

that the permeance, selectivity and separation factor are strongly dependent on the feed

273

composition, which indicates the presence of strong coupling effects (the presence of one

274

compound changes the permeability properties of the other). A phenomenon of coupled

275

transport happens in pervaporation resulting from strong interaction among membrane and

276

penetrants.75

277 278 279 280 281 282

of [C8C1Pyrr][NTf2] (c)

4500

[C8MIM][NTf2]

2.2

[C8C1Pyrr][NTf2]

2.0

4000

[C8MIM][NTf2] DMC [C8MIM][NTf2] MeOH

3500

[C8C1Pyrr][NTf2] DMC

1.8

Permeance (GPU)

Total Transmemrbane Flux (kg/h⋅m2)

2.4

1.6 1.4 1.2 1.0 0.8 0.6

[C8C1Pyrr][NTf2] MeOH

3000 2500 2000 1500 1000

0.4

500

0.2 0.0

0 0.0

0.1

0.2

283

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

Molar fraction of methanol

284

0.3

0.4

0.5

0.6

(a)

0.8

0.9

1.0

(b)

25

80

[C8MIM][NTf2]

[C8MIM][NTf2] [C8C1Pyrr][NTf2]

70

[C8C1Pyrr][NTf2] 20

60

Selectivity αDMC/MeOH

Separation Factor βDMC/MeOH

0.7

Molar fraction of methanol

15

10

50 40 30 20

5 10

0

0

0.0

0.1

0.2

285

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

Molar fraction of methanol

286

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Molar fraction of methanol

(c)

(d)

1.2

2.0

MeOH DMC

1.0

MeOH DMC

Partial Flux (kg/h⋅m )

2

Partial Flux (kg/h⋅m2)

1.5

0.8

0.6

0.4

1.0

0.5

0.2

0.0

0.0 0.0

287 288

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Molar fraction of methanol

(e)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Molar fraction of methanol

(f)

289 290 291

Figure 5. Performance of SILMs based on ionic liquid [C8C1Pyrr][NTf2] and [C8MIM][NTf2] at 30 °C, (a) Total

292

Regarding the separation factor and selectivity, both of them are larger than 1. The high

293

selectivity means that both membranes are favorable to permeate DMC rather than methanol,

294

which can be also observed when comparing the permeance values. However, as indicated

295

before, the permeation behavior is highly concentration dependent. At high concentration of

296

methanol (0.8 molar fraction), the separation factor and selectivity remain only between 1.77

transmembrane flux, (b) Permeance of DMC and methanol, (c) Separation factor, (d) Selectivity, (e) the partial flux of DMC and methanol for [C8MIM][NTf2] and (f) [C8C1Pyrr][NTf2]

297

and 3, respectively. With an increase of the concentration of DMC in the feed solution, the

298

separation factor and selectivity increase. Both SILMs can achieve a separation factor around 21,

299

and selectivity of 67 for [C8MIM][NTf2] (0.2 molar fraction of methanol) and 48 for

300

[C8C1Pyrr][NTf2] (0.5 molar fraction of methanol).

301 302

DMC concentration in the permeate (mol%)

1.0

0.8

0.6

0.4

0.2

Vapor-Liquid Equilibrium

[C8MIM][NTf2] DMC [C8C1Pyrr][NTf2] DMC 0.0 0.0

303

0.2

0.4

0.6

0.8

1.0

DMC concentration in the feed (mol%)

304

Figure 6. Relationship of DMC concentrations in feed side and in permeate side.

305

Figure 6 shows the vapor liquid equilibrium behavior of DMC/methanol mixture along with the

306

pervaporation DMC permeate concentrations vs. its feed concentration. The separation

307

performance of the permeation selectivities of the two types of SILMs [C8MIM][NTf2] and

308

[C8C1Pyrr][NTf2] is compared with distillation separation based on vapor-liquid equilibrium. It

309

illustrated that both SILMs exhibit excellent separation behavior when DMC concentration is

310

higher (80 mol%). The selectivities of both SILMs are slightly poor at low concentration of DMC

311

(<20 mol%), but the SILMs can still break the azeotropic balance. Therefore, the SILMs prepared

312

in this work may be used to separate DMC/MeOH mixtures by pervaporation.

313 314

3.3 Kamlet-Taft solvatochromic parameters analysis

315

In Table 1, solvatochromic parameters of solvents and ionic liquids were given. In Table 1, by

316

comparing solvatochromic parameters of solvents and ionic liquids, it appears that methanol is

317

not only a hydrogen bond acceptor but also a stronger hydrogen bond donor than DMC and ILs.

318

Therefore, methanol could preferably form hydrogen bonds with DMC rather than ILs in the feed

319

solution because DMC has a higher hydrogen bond acceptor ( ) value than that of ionic liquids.

320

At low concentration of methanol in the feed solution, most methanol molecules tend to form

321

hydrogen bonds with DMC. As a result, methanol molecules have less opportunity to contact

322

ionic liquids and to pass through the membrane. In this case, the major interaction takes place

323

between the DMC molecules and ionic liquids. In addition, the permeance of DMC at low

324

methanol concentration is [C8C1Pyrr][NTf2]> [C8MIM][NTf2]. This is consistent with the

325

of ILs. [C8C1Pyrr][NTf2] has higher hydrogen bond donor capacity than [C8MIM][NTf2]. Hence,

326

DMC/[C8C1Pyrr][NTf2] have stronger affinity than DMC/[C8MIM][NTf2] and [C8C1Pyrr][NTf2] is

327

prone to permeate DMC. When the concentration of methanol increases, in this case, methanol

328

molecules have more opportunity to contact ionic liquids then permeating through the

329

membranes because methanol is not only hydrogen bond donor but also hydrogen bond

330

acceptor.

331

Table 1. Solvatochromic parameters (Kamlet-Taft solvation parameters),

,

and

∗

value

, which

332

quantify hydrogen-bond donating ability (acidity), hydrogen-bond accepting ability (basicity) and

333

polarity/polarizability, respectively Component

Hydrogen bond

Hydrogen bond

Dipolarity

Ref.

donor（ （%） ）

acceptor (&)

/polarizability ('∗ )

[C8MIM][NTf2]

0.60

0.29

0.96

76

[C8C1Pyrr][NTf2]

0.80

0.08

0.73

77

Methanol

1.05

0.61

0.73

77, 78, 79

DMC

0

0.38

0.47

78, 80, 81

334 335

3.4 Solubility parameter analysis

336

Two distinct values of the Hildebrand solubility parameter have been found in the literature for

337

DMC; thus, the calculation of Hildebrand solubility parameter of the mixture methanol/DMC

338

includes two values. The data concerning the Hildebrand solubility parameter of ionic liquids are

339

limited and different sources can provide different values. From Table 2, it can be seen that the

340

Hildebrand solubility parameters of both ionic liquids are around 20-25. On the other hand, the

341

Hildebrand solubility parameter of mixture increases with increasing methanol concentration.

342

When the concentration of methanol increases up to 0.5 to 0.8 molar fraction, the Hildebrand

343

solubility parameter of the mixture is closer to the ones of ionic liquids. This indicates that the

344

compatibility of the mixture and ionic liquids is higher at a higher concentration of methanol.

345

Therefore, it can be deduced that the coupling effect is more likely to appear at high

346

concentration of methanol due to the variation of the solubility parameters of a mixture.82

347

Table 2. Hildebrand solubility parameter of ionic liquids, pure methanol, pure DMC and their mixtures

Material

()

[C8MIM][NTf2]

20, 22, 25

83, 84, 85

[C8C1Pyrr][NTf2]

20

85

Methanol

29.7

86

Dimethyl carbonate

12.7/15.9

86

0.2 MeOH + 0.8 DMC

14.5/17.4

This work*

0.5 MeOH + 0.5 DMC

18.2/20.4

This work*

0.8 MeOH + 0.2 DMC

23.9/25.0

This work*

Ref.

Mixture

* Calculated using equation 5.

348 349

From the analysis of solubility parameter, it can be observed that the Hildebrand solubility

350

parameter (

351

the value for the ionic liquids. This implies a stronger affinity between them due to a shorter

352

distance of

353

behavior follows this prediction. Thus, the selectivity and separation factor become lower at

354

higher concentration of methanol. This is consistent with an increase of permeance of both

355

compound as methanol concentration in the feed solution increases.

in Table 2) increases with the increase of methanol concentration, being closer to

between ionic liquids and solution. The concentration effect on the permeation

356 357

3.5 Impact of the molecular structure of DMC and methanol on coupling effects

358

The molecular structures of all components studied in this work were shown in Figure 1. In the

359

DMC-methanol-ionic liquid system, different molecular interactions occur. The hydroxy group

360

(-OH) of methanol can generate a hydrogen bound and dipole-dipole attraction to the group (C=O)

361

in the DMC molecule. DMC has a stronger interaction with both ionic liquids than methanol,

362

because both ionic liquids are hydrophobic.

363 364

Thus, at low concentration of methanol in the feed solution, most methanol could form hydrogen

365

bonding with DMC due to this interaction. Therefore, the formation of intermolecular attraction

366

between DMC and methanol makes them difficult to permeate through the membrane. However,

367

the other DMC molecules which do not interact with methanol can permeate through the

368

membrane much easier. In addition, because of the existing interaction between DMC and

369

methanol, methanol has less opportunity to contact ionic liquids. As a result, most of methanol

370

remains in the feed solution and the permeance of methanol through both supported ionic liquid

371

membranes is very low at low concentration of methanol.

372 373

When the concentration of methanol increases, methanol has more opportunity to contact ionic

374

liquids leading to methanol permeation. On the other hand, the ionic liquids prefer to permeate

375

DMC. As a result, coupling effect takes place during the permeation, methanol also permeates

376

through the membrane with DMC due to hydrogen bonding interaction between them. From

377

Figure 5 (b), it can be seen that the permeance of methanol in both supported ionic liquid

378

membranes increased dramatically but it is still lower than permeance of DMC for each SILM. The

379

selectivity and separation factor showed very low value at high methanol concentration.

380 381

In addition, the structure of the cation has an impact on the permeation behavior. The only

382

difference between the structure of the two ionic liquids lays into their cationic heterocycles

383

(pyrrolidine vs. imidazole). As mentioned before, at low concentration of methanol, the

384

interaction between methanol and ionic liquids are weak due to the presence of large amount of

385

DMC. When the concentration of methanol increases, the methanol can interact more easily with

386

[C8MIM]+ than [C8C1Pyrr]+. It may be ascribed to the presence of a tertiary amine in the

387

imidazolium cation that does not exist in the pyrrolidinium one therefore leading to hydrogen

388

bonds between hydroxyl groups of methanol and the pair of non-bounding electrons borne by

389

the nitrogen atom of the tertiary amine.87

390 391

3.6 Membrane stability

392

A frequent major issue with SILMs is their low stability due to ionic liquid loss during operation.

393

Hence, a long-term stability test was carried out for both SILMs. The prepared membrane was

394

tested for 120 h under the concentration of 0.2 molar fraction of methanol. The stability test is

395

shown in Figure 7, showing stable fluxes during the experimental time. The test confirms that

396

both ionic liquids were kept in the pores of supported PAN membrane and gives stable

397

transmembrane flux and separation factor. 35

3.5 [C8MIM][NTf2] Flux [C8C1Pyrr][NTf2] Flux [C8MIM][NTf2] Separation Factor [C8C1Pyrr][NTf2] Separation Factor

2.5

25

2.0

20

1.5

15

1.0

10

0.5

5

0.0

0 0

398 399 400

30

Separation Factor

2

Transmembrane Flux (kg/m ⋅h)

3.0

20

40

60

80

100

120

Time (h) Figure 7. Operational stability of SILMs based on PAN membrane with supported [C8MIM][NTf2] and [C8C1Pyrr][NTf2] under 0.2 molar fraction of methanol at 30 °C

401

3.7 Comparison with DMC/methanol pervaporation separation in the literature

402

A comparison of pervaporation separation of methanol DMC mixtures is shown in Table 3. In the

403

literature, most of study of separation methanol/DMC mixture is to maximize to permeation of

404

methanol. Therefore, the separation factor and selectivity are reported by means of methanol

405

relative to DMC as the methanol is usually concentrated in the permeate in the literature. In this

406

work, the ILs were in favor of permeating DMC.

407

Comparing with other studies, the SILMs evaluated in this work have an outstanding separation

408

performance at low temperature (30°C) with high selectivity (DMC towards methanol) of 67 and

409

48 for [C8MIM][NTf2] (0.2 molar fraction of methanol) and [C8C1Pyrr][NTf2] (0.2 molar fraction of

410

methanol) based SILMs, respectively. [C8C1Pyrr][NTf2]-SILM can also keep good separation

411

performances at 0.5 molar fraction of methanol in the feed.

412 413

414

Table 3. Comparison of pervaporation performances for the separation of methanol/DMC mixtures reported in the literature (only optimal conditions are shown in this table).

Membrane material

Feed

Temperature

Total

concentration (wt%)

Methanol

(°C)

(g/m2.h)

Flux

Separation Factor

Permeance

Selectivity

Ref.

Chitosan/silica

70

50

1265

30.1

-

-

88

Poly(vinyl alcohol)

50

70

248

37

-

-

89

Nano-Silica/polydimethylsiloxane

70

40

702

3.97

-

-

90

Silicotungstic acid hydrate/Chitosan

10

50

1163

67.3

-

-

91

Chitosan hollow fiber membrane

20

50

150

23

~30 DMC; ~800 MeOH

~15

92

Poly(acrylic acid)/poly(vinyl alcohol)

70

60

577

13

~20 DMC; ~80 MeOH

~4

93

Crosslinked chitosan

70

55

480

60

-

-

94

PDMS/PVDF

72

40

487.2

3.95

-

-

95

[C8MIM][NTf2] SILM

13 (0.2 mol%)

30

739.8

21.2

947 DMC; 14 MeOH

67

This

Methanol-selective membranesa

DMC-selective membranesb

work [C8C1Pyrr][NTf2] SILM

26 (0.5 mol%)

30

241

21.0

395 DMC; 8.2 MeOH

48.41

This work

415 416

a

Separation factor and selectivity are methanol relative to DMC.

b

Separation factor and selectivity are DMC relative to methanol.

417

418

3

419

In this work, two SILMs containing the ionic liquids [C8MIM][NTf2] and [C8C1Pyrr][NTf2] were

420

studied for the separation of a binary mixture of DMC and methanol. It was found that at high

421

concentration of DMC (0.8 molar fraction), the SILMs show good separation performances with

422

high selectivity. However, the membrane performance is highly dependent on concentration. At

423

high concentration of methanol, the separation performance decreases due to strong coupling

424

effects as the coupled transport of DMC and methanol through the membrane because of their

425

hydrogen bonding. The ionic liquid structure has an impact on the permeation behavior. For both

426

of the studied cation structures there are quite significant differences in permeation behavior of

427

DMC and methanol.

Conclusions

428

429

Acknowledgements

430

This research project was supported by the European Regional Development Fund (ERDF) and

431

Wallonia within the framework of the program operational "Wallonie-2020.EU". The authors

432

acknowledge the “Fonds européen de développement régional“ (FEDER) as well as the Wallonia

433

region (Belgium) for their financial supports via the “INTENSE4CHEM” projects (projects N°

434

699993-152208).

435 436

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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: