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: