Grafting cellulose acetate with ionic liquids for biofuel purification membranes : Influence of the anion

Accepted Manuscript Title: Grafting cellulose acetate with ionic liquids for biofuel purification membranes: Influence of the anion Authors: Faten Hassan Hassan Abdellatif, J´erˆome Babin, Carole Arnal-Herault, Laurent David, Anne Jonquieres PII: DOI: Reference:

S0144-8617(18)30530-7 https://doi.org/10.1016/j.carbpol.2018.05.008 CARP 13589

To appear in: Received date: Revised date: Accepted date:

15-2-2018 20-4-2018 3-5-2018

Please cite this article as: Hassan Hassan Abdellatif, Faten., Babin, J´erˆome., ArnalHerault, Carole., David, Laurent., & Jonquieres, Anne., Grafting cellulose acetate with ionic liquids for biofuel purification membranes: Influence of the anion.Carbohydrate Polymers https://doi.org/10.1016/j.carbpol.2018.05.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

REVISED MANUSCRIPT Grafting cellulose acetate with ionic liquids for biofuel purification membranes : Influence of the anion. Faten HASSAN HASSAN ABDELLATIFa),b), Jérôme BABINa), Carole ARNAL-HERAULTa), Laurent DAVIDc), Anne JONQUIERESa)* a)

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Laboratoire de Chimie Physique Macromoléculaire, Université de Lorraine, CNRS, LCPM, ENSIC, 1 rue Grandville, BP 20451, F-54 000 Nancy, France. b)

Current affiliation : Textile Research Division, National Research Centre, 33 EL Buhouth Street, Dokki, Giza, 12622, Egypt. c)

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Laboratoire IMP@Lyon1, Université Claude Bernard Lyon 1, Univ Lyon, CNRS UMR 5223, 15 Bd. André Latarjet, 69622 Villeurbanne Cedex, France.

Corresponding author. Email address: [email protected], tel: +33 3 83 17 50

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29, fax: +33 3 83 37 99 77.

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Graphical abstract

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Highlights     

Cellulose acetate was grafted with ionic liquids (ILs). The anion of the grafted ILs was systematically varied by anion exchange. The corresponding cellulosic membranes were used for ETBE biofuel purification. The IL grafting avoided IL dissolution in the feed mixture during separation. The influence of the IL structure/features on membrane properties is discussed.

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Abstract (< 150 words) Membranes made from cellulose acetate grafted with imidazolium or ammonium ionic liquids (ILs) containing different anions were considered for ethyl tert-butyl ether biofuel purification

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by pervaporation. The new cellulosic materials were obtained after bromide (Br) exchange by different anions (Tf2N, BF4, AcO). IL structure-membrane property relationships revealed that the membrane properties were strongly improved by varying the anion structure, molecular size and hydrogen bonding acceptor ability β in the Kamlet-Taft polarity

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scale. The grafted ammonium IL with AcO anion combined the highest parameter β with big

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cation/anion sizes and finally led to the best membrane properties with a normalized pervaporation flux of 0.41 kg/h m2 (almost 20 times that of virgin cellulose acetate) for a

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reference thickness of 5 µm and a permeate ethanol content of 100%. Such properties thus

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corresponded to an outstanding separation factor at 50 °C.

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1. Introduction

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Keywords: polysaccharide modification; ionic liquids; membranes; biofuel; ethyl tert-butyl ether; structure-property relationships.

Ethyl tert-butyl ether (ETBE) is an attractive biofuel widely used in the European Union.

It is generally blended with gasoline fuels to reduce toxic hydrocarbon emissions by improving fuel combustion (Noureddini, 2002; Yee, Mohamed, & Tan, 2013). ETBE is usually

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synthesized from bio-ethanol and isobutene by a catalytic process. During its industrial production, ETBE forms an azeotropic mixture containing 20 wt% of ethanol. This mixture is usually separated by a highly energy intensive ternary distillation process. However, former works have shown that ETBE could be purified by the pervaporation (PV) membrane process or related hybrid processes, which offer important energy savings, modularity and lower environmental impact compared to ternary distillation (Drioli & Giorno, 2010; Hoek & Tarabara, 2013; Norkobilov, Gorri, & Ortiz, 2017). 2

In membrane separation processes, designing of the membrane material is always critical for the efficiency of the separation process. Ideally, the membrane should be both highly permeable and highly selective. However, in most cases, permeability and selectivity vary in opposite ways thus leading to a permeability/selectivity trade-off (Baker, 2004; Drioli & Giorno, 2010; Hoek & Tarabara, 2013; Robeson, 2008). Polymeric membranes are generally preferred to inorganic membranes for extracting ethanol from the azeotropic mixture EtOH/ETBE due to their low cost and higher fluxes. Nevertheless, the design of

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these polymer membranes still limited by the permeability/selectivity trade-off (Chen et al., 2014; Jonquieres, Arnal-Herault, & Babin, 2013; Jonquieres, Clement, Lochon, Dresch, & Chrétien, 2002; Weber de Menezes & Cataluna, 2008; Yee et al., 2013).

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Cellulose acetate (CA) is an efficient membrane material widely used for industrial

water purification and gas separation. This polysaccharide derivative was also reported for ETBE purification by PV. Its selectivity was impressive with a permeate ethanol weight fraction of 100% but its flux of 0.08 kg/m2 h for a reference thickness of 5 µm was too low at

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40°C (Nguyen, Leger, Billard, & Lochon, 1997). Cellulosic esters blends led to higher PV

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fluxes (0.6 kg/m2 h -3 kg/m2 h) with permeate ethanol contents still in the high range (0.967-

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0.907) at 40°C (Luo, Niang, & Schaetzel, 1997). Cellulosic semi-interpenetrated networks and cellulosic copolymers with polymethacrylate or polylactide grafts also displayed

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improved properties (fluxes up to 0.87 kg/m2 h with a very high permeate ethanol content (94%) at 50°C) compared to virgin CA for ETBE purification (Billy et al., 2010; Hassan

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Hassan Abdellatif et al., 2017; Nguyen, Clement, Noezar, & Lochon, 1998; Nguyen et al., 1997).

On the other hand, ionic liquids (ILs) have had a very strong impact in membrane

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science over the past decade due to their unique properties, which can be specifically tailored by selecting the nature of their cation and anion. By overcoming the

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permeability/selectivity trade-off for CO2 capture, IL-containing membranes have led to outstanding features in gas permeation (Close, Farmer, Moganty, & Baltus, 2012; Deng et al., 2016; Gomez-Coma et al., 2016; Hanioka et al., 2008; Hojniak et al., 2014; Lam et al., 2016; Lozano et al., 2011; Noble & Gin, 2011; Scovazzo, 2009). Comparatively, IL-

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containing membranes have been rarely reported for liquid separations so far (Cascon & Choudhari, 2013; Dong, Guo, Su, Wei, & Wu, 2015; Fadeev & Meagher, 2001; Hassan Hassan Abdellatif, Babin, Arnal-Herault, David, & Jonquieres, 2015; Heitmann et al., 2012; Izak, Friess, & Sipek, 2010; Izak, Ruth, Fei, Dyson, & Kragl, 2008; Marszalek, Rdzanek, & Kaminski, 2015; Matsumoto, Murakami, & Kondo, 2011; Plaza et al., 2013; Rdzanek, Heitmann, Gorak, & Kaminski, 2015; Uragami, Matsuoka, & Miyata, 2016; Yahaya, Hamad, Bahamdan, Tammana, & Hamad, 2013). The former related works mainly focused on the 3

recovery of biofuels from dilute fermentation broths or that of volatile organic compounds (VOCs) from water with organophilic membranes. Hydrophilic membranes containing ILs impregnated in buckypaper have also been reported for removing water from hydro-organic propan-2-ol mixtures (Ong & Tan, 2015). In addition, the PV separation of purely organic mixtures by IL-containing organoselective membranes has been very scarcely reported so far mainly because of the IL dissolution in many organic solvents (Dong et al., 2015; Hassan Hassan Abdellatif et al.,

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2015). We have reported the first IL-containing organoselective membranes for ETBE purification by PV and extended the scope of IL-containing membranes to the challenging

separation of purely organic mixtures, in which these ILs were soluble. (Hassan Hassan

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Abdellatif et al., 2015). These membranes were obtained by grafting CA with ILs containing the same bromide anion and different cations (imidazolium, pyridinium and ammonium) with increasing hydrophilic features. This previous work revealed the strong influence of the IL

cation on the membrane properties. For the highest IL content (ca. 17 wt%), CA grafted with

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the ammonium bromide IL having the best hydrogen bonding acceptor ability led to a very

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strong improvement in flux (7.9 compared to virgin CA) and a permeate ethanol content of

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100% corresponding to an infinite membrane separation factor at 50°C. According to the best of our knowledge, very little is known about the influence of the IL

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anion on the membrane properties for liquid separations due to the lack of systematic studies involving homologous series of ILs with the same cation and several anions (Dong et al.,

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2015; Heitmann et al., 2012; Izak et al., 2010; Yuan & Antonietti, 2011). However, a few authors have already mentioned that this influence is particularly complex for gas permeation membranes (Carvalho & Coutinho, 2011). In this new work, we report the results of the first

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systematic study of a series of homologous IL-containing membranes with different IL anions

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(Tf2N, BF4, Br and AcO) for liquid separation. In a first section, we describe the variation of the anion chemical structure by anion

exchange from two CA derivatives with very close amounts (ca. 17 wt%) of grafted IL (i.e. . either imidazolium bromide or ammonium bromide). These particular CA derivatives were

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selected as starting materials for anion exchange because they had displayed the lowest and highest membrane flux, respectively, in our previous work. In the present study, the anion exchange rate and morphology of the new cellulosic materials are characterized by complementary techniques (1H NMR, 19F NMR, DSC, SAXS). In the second part, the sorption and PV properties of the new cellulosic membranes are analyzed for the separation of the azeotropic mixture EtOH/ETBE, revealing the key role played by the IL anion on the membrane properties for the purification of a major biofuel. 4

2. Experimental 2.1. Materials and reagents Virgin CA (acetyl content 39.7 wt%, degree of substitution for the acetyl groups DSAc = 2.46 (1H NMR), MW = 50,000 g/mol) was purchased from Sigma Aldrich. The influence of the CA molecular weight was not investigated in this work but it is usually admitted that the influence of the polymer Mw is fairly low on membrane properties as soon as this molecular

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weight is sufficiently high. This was confirmed by the later comparison of the membrane properties of the virgin CA used in this work with those formerly reported for another CA with

much lower molecular weight (see sub-section 3.2.2. Influence of the IL anion on the PV

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properties of CA grafted with different ILs). The two starting cellulosic materials for anion exchange were obtained in our former work (Hassan Hassan Abdellatif et al., 2015). These cellulosic derivatives were grafted with ca. 17 wt% of 1-pentyl-3-methyl-imidazolium bromide

[C5C1im][Br] or N,N,N,N-pentyldiethylmethyl-ammonium bromide [C5C2C2C1N][Br] and

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corresponded to the highest obtained grafting rates. The corresponding degree of substitution of the grafted ILs (DSLI) was ca. 0.42 for both cellulosic materials (1H NMR). The

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different salts used for anion exchange (sodium acetate (NaAcO ≥ 99%), sodium

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tetrafluoroborate (NaBF4, 98%), lithium bis(trifluoromethanesulfonyl)imide (LiTf2N, ≥ 99%)) and the Amberlite® resin A-26 (OH form) were purchased from Sigma-Aldrich and used

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without further purification. The solvent N,N-dimethylformamide (DMF, ≥ 99.8 %, Sigma

nitrogen atmosphere.

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Aldrich) was purified by fractional distillation over calcium hydride and stored under dry

2.2. Synthesis and characterization of CA grafted with ILs containing different anions

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2.2.1. Synthesis of CA grafted with ILs containing acetate anion As way of example, the bromide to acetate anion exchange is described for CA

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grafted with the imidazolium bromide IL. In 100 mL round bottom reactor, 2 g of CA-g[C5C1im][Br] (corresponding to 2.05 mmol of bromide anions) were dissolved in 60 mL of purified DMF before adding a solution of 1.68 g (10 eq.) of sodium acetate in 5 mL of distilled

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water. The reaction mixture was left under magnetic stirring for 2 days at room temperature. The solution was filtered through a Whatman cellulosic filter paper Grade 1 (Sigma-Aldrich) to remove the precipitated excess of sodium acetate and the bromide salt. The cellulosic polymer was collected after precipitation in diethyl ether followed by extensive washing with distilled water to remove residual sodium bromide. The resulting polymer CA-g-[C5C1im][Ac] was dried in vacuum oven at 60°C. The anion exchange rate was determined by 1H NMR (see main text).

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2.2.2. Synthesis of CA grafted with different ILs containing Tf2N or BF4 anions As way of example, the bromide to fluorinated anion exchange is described for CA grafted with the imidazolium bromide ionic liquid. In 100 mL round bottom reactor, 2 g of CAg-[C5C1im][Br] (corresponding to 2.05 mmol of bromide anions) were dissolved in 60 mL of purified DMF before adding 3.53 g (6 eq) of LiTf2N or 1.35 g (6 eq) of NaBF4 in powder form. After dissolution of the fluorinated inorganic salt, the reaction mixture was left under magnetic stirring for 2 days at room temperature. The resulting polymers were recovered by

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precipitation in distilled water or in ethanol (96%) for the cellulosic polymers containing the Tf2N or BF4 anions, respectively. These products were dried in vacuum oven at 60°C for one night. The anion exchange rates were determined by quantitative

2.2.3. Polymer characterization H NMR and

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F NMR spectra were recorded on a Bruker Avance 300 spectrometer

at 300 MHz and 376 MHz, respectively.

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H NMR for CA-g-[C5C1im][Br] and CA-g-

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1

F NMR (see main

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text).

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[C5C2C2C1N][Br] and their derivatives with acetate anions were obtained from DMSO-d6 solution at 300 K except for CA grafted with the ammonium bromide IL. In the latter case, the

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temperature for 1H NMR analysis had to be increased to 353 K to strongly improve spectral

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resolution (Hassan Hassan Abdellatif et al., 2015). The chemical shifts referenced to TMS were calculated using the residual isotopic impurities of the deuterated solvent. 1H NMR

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analysis was not suitable for assessing the anion exchange for the fluorinated anions since they did not contain any protons. Quantitative

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F NMR was used to characterize these

particular cellulosic derivatives. As the four cellulosic materials with fluorinated anions did not

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contain any other fluorinated groups, pentafluorophenol was used as an internal standard for determining the anion exchange rate. In

F NMR, the chemical shifts were referenced to

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F NMR samples were prepared by dissolving ca. 15 mg of each

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trifluoroacetic acid. The

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grafted cellulosic polymer in 0.5 mL of DMSO-d6 and 0.1 mL of a fresh pentafluorophenol stock solution (26.1 mg/1 mL of DMSO-d6) was then added for quantitative analysis. Differential Scanning Calorimetry (DSC) analysis was performed with a TA

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Instruments DSC Q2000 with ca. 10 mg of each polymer sample under a continuous flow of nitrogen. Two cycles of measurements were recorded between 20 and 210 °C with heating and cooling rates of 10 °C/min. Following a common practice to erase the influence of past thermal history for better comparison, the glass transition temperatures reported in this work correspond to the second heating scan. The nano-scale morphology of virgin CA and CA grafted with different ILs was determined by means of small X-ray scattering (SAXS) using polymer membranes with 6

thicknesses of ca. 200 µm, which were obtained in the same way as for sorption experiments (see below). SAXS experiments were carried out at room temperature, without additional thermal treatment, at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) following the procedure reported in our former work (Hassan Hassan Abdellatif et al., 2015). 2.3. Membrane preparation for pervaporation and sorption experiments Membranes for pervaporation were obtained by a simple casting procedure of polymer

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solutions (polymer concentration of 3.5 % w/v ) in DMF (pure for synthesis) on PTFE molds. After solvent evaporation at 45 °C first at ambient pressure and then under vacuum,

membrane thicknesses of 85 to 100 µm were obtained and the difference in thickness within

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a given membrane did not exceed 10 µm. This relatively slow procedure enabled a good reproducibility of the membrane casting. The procedure for sorption membrane casting was similar except for polymer concentration, which was increased to5% w/v, and the resulting membranes had thicknesses of ca. 200 µm. The sorption membranes were carefully dried at

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60 °C for 24 hours before sorption experiments.

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2.4. Sorption experiments

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The sorption experiments were determined using the azeotropic mixture EtOH/ETBE (20/80 wt%) at 30 °C and a gravimetric procedure formerly reported (Hassan Hassan

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Abdellatif et al., 2015). This sorption temperature was chosen to avoid handling hot samples for safety reasons. The swelling, S (wt%), was determined from wS and wD, which are the

(equation (1)).

wS  wD  100 wD

(1)

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S

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weights of the dry and swollen membranes, respectively, with an average error of 0.1 wt%

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The ethanol content in the liquid mixture absorbed by the swollen membrane, CSEtOH, was determined by gas chromatograph (GC) analysis with an average experimental error of 0.005 after desorption in diethyl ether following our former procedure (Hassan Hassan

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Abdellatif et al., 2015). The sorption separation factor was then estimated from equation (2) where CSEtOH and

C are the ethanol weight fractions in the absorbed mixture and in the azeotropic mixture EtOH/ETBE, which was used in very large excess, respectively. S C  S  C EtOH / S

1 C

EtOH

1 C

(2)

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2.5. Pervaporation experiments Pervaporation experiments were performed for the azeotropic mixture EtOH/ETBE at 50°C and a downstream pressure of less than 0.04 kPa using a pervaporation set-up described elsewhere (Billy et al., 2010). The permeate flux was calculated using equation (3) and the corresponding experimental error was less than 5% :

w

p

(3)

t  A

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Permeate flux 

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where wp is the permeate weight obtained during a time Δt and A is the membrane area for mass transfer. To allow comparison between different membranes with close but non-equal

thicknesses, normalized fluxes, J, are reported for a reference thickness of 5 µm (i.e. a thickness which can be easily reached for polymer layers on top of asymmetric membranes)

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Membrane thickness Permeate flux 5

(4)

A

J

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(equation (4)).

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The permeate ethanol weight fraction, C'EtOH, was determined by GC analysis with an average experimental error of 0.005. The pervaporation separation factor, βPV , was

C  C EtOH / ' 1  C EtOH 1  C

(5)

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 PV

'

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calculated by analogy with the sorption separation factor from equation (5)

The detailed procedure for permeability calculation has already been reported (Hassan

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Hassan Abdellatif et al., 2015) and the new permeability data are presented in Supplementary material - Appendix A - Table A.1.

3. Results and discussion

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3.1. Synthesis and chemical characterization of CA grafted with ILs containing different anions Figure 1 shows the general procedure used for exchanging the bromide anions in CA grafted with either imidazolium or ammonium ILs. The anion exchange was performed in solution in DMF by using a large excess in different inorganic salts containing the targeted anions and removing the resulting bromide salt. After anion exchange, eight cellulosic materials with imidazolium or ammonium cations and one of the following anions Tf2N, BF4, 8

Br and AcO were available for membrane preparation and characterization. The corresponding IL chemical structures and abbreviations are given in Supplementary material - Appendix B.

H OR

H

H

H O O RO

OR

H RO O

H O

OR

H

H

H

n

OR

O

O

O

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H

-

+

Br Or

Or acetate

Or 7-Bromo-heptan-2-oate

+

= Imidazolium or ammonium cation

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R= H

N

+ N

= Bromide anion (Br-)

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-

+ N

-

A

+

+

-

+

-

+

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-

N

Inorganic salt/DMF 2 days at room temp

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-

= Bis(trifluoromethanesulfonyl)imide (Tf2N-) or Tetrafluoroborate (BF4- ) or Acetate (AcO-)

Figure 1. Synthesis of CA with grafted ILs with either imidazolium or ammonium cations and

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different anions by anion exchange of cellulosic precursors with bromide anions 3.1.1. CA grafted with imidazolium or ammonium ILs with acetate anions

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One of the best reported methods for exchanging halide anions by the acetate anion in

ILs takes advantage of the Amberlite® resin A-26 with hydroxide anions (Dinares, Garcia de Miguel, Ibanez, Mesquida, & Alcalde, 2009). After a first exchange of the bromide anions by the Amberlite® hydroxide anions, these hydroxide anions are then converted to acetate

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anions by treating the corresponding ILs with acetic acid. This method is highly efficient for molecular ILs with nearly 100% exchange rates. Nevertheless, when it was applied to the CA grafted with imidazolium/ammonium bromide ILs, this strategy led to the polymer degradation. In any case with this first anion exchange method, a quantitative 1H NMR spectroscopic analysis of the cellulosic materials, before and after anion exchange, revealed a strong decrease ( 30%) in their esterification degree, which was ascribed to the cleavage of the ester groups by the hydroxide anions ( strong bases). 9

Therefore, the anion exchange was achieved directly with acetate salts. Sodium acetate was preferred to other acetate salts due to its slightly better solubility in DMF, its lower cost and good stability. Nevertheless, the solubility of sodium acetate was still very limited in DMF and using a DMF/water solution of this salt improved its solubility and the anion exchange rate. 1

H NMR analysis of both cellulosic materials grafted with either imidazolium or

ammonium ILs before and after bromide anion exchange confirmed the anion exchange with

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the appearing of a new peak in the range of 1.4 ppm to 1.7 ppm, which was characteristic for the new acetate anions (Figure 2). The bromide to acetate exchange rate was calculated

from the new peak integration. Even with a very large excess (10 eq.) in sodium acetate, the

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exchange rates obtained for CA-g-[C5C1im][AcO] and CA-g-[C5C2C2C1N][AcO] were limited to 64% and 66%, respectively. This limitation was most likely due to the very low solubility of

sodium acetate, which remained partially insoluble in the reaction medium (heterogeneous conditions). A quantitative analysis by 1H NMR spectroscopy showed that the esterification

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A

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degree remained constant after anion exchange with sodium acetate (a weak base).

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a) H OR 6

4 O RO

H 5 H O

OR

H

c

RO O

2

H

3 H

R= H

H O

OR 1 H

H

O

O Or a

j

Bri

H

Or

e

b

a, e

N h

+ N

d

g

f

n

OR

DMSO H2O

1,2,3,4,5,6

g, h

f, j

i

c

H

H

5 H O

O RO

H

OR

RO 2

H

3

c

O

R= H

H O

OR 1

H

H

O

O

H

Or a

n

OR

Or

e

b

h

+ N

d

j

g

f

DMSO

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H OR 6

k CH3COON i

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a, e

4

d

b

H2O

g, h

1,2,3,4,5,6

c, k

f, j

d

b

i

b) O

H

c

OR

H

R= H

RO 3

2

H

a

O

Or

H

n

OR

ED

H

d

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O RO

H

5 H O

H

R= H

CC E 3

A

H

H

2

O

H

H

OR

c, e f

b

i j CH3COO- h e + g N

c a

H O

OR 1

O Or

DMSO

g

O

Or

b

d

f i

n

d, i

h

i

h

OR

H

RO

f

H2O

1,2,3,4,5,6

H OR 6 4

e

b

H O

OR 1

H

Or

Br- h + g N a

A

H 5 H O

O

M

H OR 6 4 O RO

N

U

i

H2O, h

d, i a

h

DMSO

c, e, j 1,2,3,4,5,6

f

g b

Figure 2. Comparison of the 1H NMR spectra in DMSO-d6 solution at 300 MHz before and after anion exchange with sodium acetate for : a) CA-g-[C5C1im][X] and b) CA-g[C5C2C2C1N][X]

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3.1.2. CA grafted with imidazolium or ammonium ILs with fluorinated anions The bromide anion exchange by fluorinated anions was much more efficient compared to that by the acetate anion due to the higher solubility of both fluorinated salts (LiTf2N and NaBF4) in DMF. Thus, the anion exchange was carried out in homogeneous condition with a large excess in fluorinated salts and ensured almost complete anion exchange. 1

H NMR analysis showed that the esterification degree of the cellulosic materials did

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not change after anion exchange in these conditions. However, 1H NMR analysis was not suitable for assessing the anion exchange for the fluorinated anions since they did not contain any protons. Therefore, quantitative

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F NMR analysis with an internal fluorinated

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standard was performed to assess the anion exchange rates (Fig. 3).

For CA grafted with imidazolium IL (Figure 3a), using 6 equivalents of the fluorinated salts and a very long exchange time (2 days) led to 100% exchange rates for both Tf2N and

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BF4 anions. In the same conditions, the anion exchange for CA grafted with the ammonium IL resulted in significantly lower exchange rates of 89% and 80% for Tf2N and BF4,

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respectively. The anion exchange was thus less efficient with the ammonium IL most likely

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due to its bigger steric hindrance. Nevertheless, further increasing the amount of both fluorinated salts up to 10 equivalents enabled 100% anion exchange even for the ammonium

H OR

H H O

O RO

H

O OR

R= H

H O

H

OR

Or

N

+ N Or

n

X =Tf2N- or BF4OH

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H

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X-

O

O

OR

RO

H

TFA

H H

ED

a)

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IL (Figure 3b).

F

Tf2N-

F

F

F F

BF4 OH F

F

F

F F

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H OR

b)

H H O

O RO

H H

RO H H

R= H

O H

Or

Or

H O

OR H

OR

X+ N

O

O

OR

n

X = Tf2N- or BF4 OH F

Tf2N-

TFA

F

F

F

OH F

F

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BF4

TFA

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F

F

F

N

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F

M

A

Figure 3. Quantitative 19F NMR characterization in DMSO-d6 solution at 376 MHz after anion exchange with two fluorinated salts LiTf2N or NaBF4 for : a) CA-g-[C5C1im][X] and b) CA-g[C5C2C2C1N][X] 3.1.3. Physical characterization of CA grafted with ILs containing different anions

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In our former work, CA grafting with nearly identical amounts of ILs containing different cations was responsible for a strong decrease in the polymer glass transition temperature

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(Tg) compared to that of virgin CA. The corresponding increase of molecular mobility strongly improved membrane flux (Hassan Hassan Abdellatif et al., 2015). More specifically, the polymer Tg increased with the IL melting temperature (Tm) in the following order : imidazolium

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bromide < pyridinium bromide < ammonium bromide. This new work focuses on the influence of the IL anion for grafted imidazolium/ammonium ILs. The IL Tg and Tm are known to depend strongly on their anion structure (Dinares et al., 2009). Therefore, it was also expected that

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the anion exchange would have a strong influence on the resulting polymer Tg. For both series of CA derivatives grafted with imidazolium or ammonium ILs with

different monovalent anions X, the DSC analysis did not reveal any endothermic melting peaks and the corresponding cellulosic polymers were amorphous. However, DSC analysis showed the strong influence of the IL anion on the polymer Tg, which increased in the following order : Tf2N < AcO < BF4 < Br (Supplementary material - Figs. A.1 and A.2) The lowest Tg corresponded to Tf2N with the grafted imidazolium IL. This anion was loosely 13

interacting with the imidazolium cation due to its large size resulting in lower Coulomb interaction force between the IL cation and anion (Izgorodina, 2011). In these conditions, the imidazolium IL with the Tf2N anion interacted better with the polymer chains than the other grafted ILs. Therefore, it induced the best membrane plasticization and strongly decreased the polymer Tg from 184°C for virgin CA to 111°C. However, unlike what had been observed for the grafted ILs with different cations in our previous work, the influence of the IL anion on the polymer Tg was not directly correlated to the IL Tm of a series of very close imidazolium

IP T

ILs [C4C1im][X] (Supplementary material - Table A.2). For the second series of CA derivatives with grafted ammonium ILs, the influence of

the IL anion on the polymer Tg followed the same general trend as for the first series with

SC R

grafted imidazolium ILs but the polymer Tg decrease was systematically lower, as expected owing to the higher stiffness of the ammonium ILs.

In addition, the short scale morphology of CA grafted with imidazolium/ammonium ILs

U

containing different anions was analyzed by synchrotron SAXS to assess the IL dispersion state (Supplementary materials - Fig. A.3). The SAXS patterns showed that the cellulosic

N

materials grafted with the different ILs were more homogeneous than virgin semicrystalline

A

CA and that all the grafted ILs were homogeneously dispersed in the polymer membranes (no microphase separation). This is an important point because former studies on grafted

M

cellulosic membranes have shown that the membrane properties depended on the dispersion state of their grafts (Billy et al., 2010). In this work, the SAXS studies showed that

ED

the dispersion of the IL grafts and the membrane morphology were identical for all the new grafted cellulosic membranes.

PT

3.2. Membrane properties for ETBE purification by pervaporation The sorption-diffusion model considers that the permeation of a liquid mixture through

CC E

a dense membrane during PV is mainly governed by two successive steps (Wijmans & Baker, 1995). The first step corresponds to some liquid absorption at the upstream side of the membrane. This sorption step is usually selective and contributes to PV selectivity. The second step is the diffusion of the absorbed molecules through the membrane. In the

A

following part, the sorption and pervaporation properties of CA grafted with ILs containing different anions will be analyzed in terms of IL structure-membrane property relationships to reveal the influence of the IL anion on the membrane properties for ETBE purification. 3.2.1. Influence of the IL anion on the sorption properties of CA grafted with different ILs

14

Sorption experiments were carried out in the azeotropic mixture EtOH/ETBE (20/80 wt %) at 30 °C. Both series of CA derivatives grafted with imidazolium or ammonium ILs containing different anions were investigated in comparison with virgin CA (Table 1). Virgin CA led to fairly low swelling S (4.2 wt%) and very high sorption separation factor (129) due to its chain stiffness and its hydroxyl side groups interacting strongly with ethanol by hydrogen bonding.

IP T

After CA grafting with imidazolium ILs containing different anions, swelling strongly depended on the anion structure and on the anion hydrophilic or hydrophobic feature. The hydrophilic/hydrophobic features of cations and anions in ionic liquids depend on the cation

SC R

and anion structure and these features have been well documented in the literature (Anthony et al., 2003; Yuan & Antonietti, 2011; Zhang, Sun, He, Lu, & Zhang, 2006). In particular, it has already been reported that the fluorinated anions (i.e. Tf2N, BF4) are hydrophobic

anions and that the bromide and acetate anions (i.e. Br, AcO) are hydrophilic anions. The

U

hydrophobic fluorinated anions led to slightly lower swelling of 4 wt% and 3 wt% and

N

decreased sorption separation factors S of 62 and 76 for Tf2N and BF4, respectively. It was initially surprising that the most hydrophobic fluorinated anion (Tf2N) led to a slightly higher

A

swelling compared to the less hydrophobic fluorinated one (BF4). This particular behavior

M

will be better understood on the basis of the Kamlet-Taft analysis of the membrane properties reported in the last section. In contrast and as expected, the hydrophilic anions

ED

(Br and AcO) improved the swelling in the azeotropic mixture EtOH/ETBE compared to virgin CA by +45% and +114%, respectively, with very high ethanol sorption separation

PT

factors S of 396 and 196.

For CA grafted with ammonium ILs containing different anions, the anion influence followed the same trends as for the first series based on imidazolium ILs. Nevertheless, for a

CC E

given anion, the swelling was systematically slightly higher but S was significantly decreased.

Similarly as what had been observed in our former work (Hassan Hassan

Abdellatif et al., 2015), an improved swelling of the cellulosic membranes induced better membrane plasticization. The induced material softening facilitated the absorption of ETBE

A

along with that of ethanol, by a phenomenon known as sorption synergy. This sorption synergy eventually resulted in significantly decreased sorption separation factor S.To our knowledge, the sorption separation factors of both series of IL-containing cellulosic materials with hydrophilic anions (Br and AcO) were the best ever reported for ETBE purification membranes.

15

Table 1 Influence of the IL anion X on the membrane sorption and pervaporation properties for the separation of the azeotropic mixture EtOH/ETBE with CA-g-[C5C1im][X] and CA-g[C5C2C2C1N][X] with nearly identical IL amounts (W IL  17 wt%) in comparison with virgin CA. Sorption properties

Anion

Ammonium cation

CSEtOH

S (wt%)

(wt. fraction)

βs

S (wt%)

CSEtOH (wt. fraction)

4.2

0.97

129

4.2

0.97

Tf2N

4

0.94

62

5.3

0.91

BF4

3

0.95

76

4

Br

6.1

0.99

396

8.4

AcO

9

0.98

196

11

62

0.975

156

0.96

96

U

0.94

C'EtOH

for 5 µm

(wt. fraction)

for 5 µm

(wt. fraction)

Virgin CA

0.023

1

∞

0.023

1

∞

Tf2N

0.078

1

∞

0.095

1

∞

BF4

0.038

1

∞

0.026

1

∞

0.05

1

∞

0.1815

1

∞

0.13

1

∞

0.41

1

∞

βPV

βPV

CC E

M

J (kg/h m2)

ED

C'EtOH

PT

J (kg/h m2)

AcO

40

Ammonium cation

A

Imidazolium cation

Br

129

N

Pervaporation properties

Anion

SC R

Virgin CA

βs

IP T

Imidazolium cation

W IL : ionic liquid weight fraction in the grafted cellulose acetate; S : swelling at sorption equilibrium; CsEtOH : ethanol content in the absorbed mixture; βs: sorption separation factor. J: total flux normalized for a reference thickness of 5 µm; C’EtOH: permeate ethanol content; βpv: pervaporation separation factor.

A

3.2.2. Influence of the IL anion on the PV properties of CA grafted with different ILs A PV temperature of 50°C (easily obtained in industrial PV modules) instead of 30 °C used for sorption analysis was chosen to improve the membrane fluxes and facilitate their comparison. However, this change in temperature had a negligible effect on the sorption properties because the ethanol and ETBE activity coefficients are almost constant between 30 and 50 °C (Billy et al., 2010). 16

During the permeation experiments, the cellulose acetate-g-IL membranes have generally been used for several days at 50 °C with steady-state constant membrane fluxes, which was characteristic for a stable IL content in the different membranes. We have indeed shown in our former work that the membrane flux strongly depends on the grafted IL content in the membrane (Hassan Hassan Abdellatif et al., 2015). Therefore, the grafting of the ILs prevented their dissolution in the feed mixture and interestingly extended the scope of ILcontaining membranes to the challenging separation of purely organic mixtures, in which

IP T

these ILs were soluble. IL grafting systematically improved the membrane flux but, again, its influence strongly depended on the IL cation and anion (Table 1). Furthermore, all of the cellulosic membranes

SC R

with grafted ILs showed outstanding PV selectivity with permeates containing 100 wt% of ethanol corresponding to infinite separation factors, whatever the type of the IL anion.

The PV properties of virgin CA were in good agreement with the data reported by Nguyen et al. for this separation (J = 0.08 kg/m²/h and ethanol weight fraction in permeate

U

C’EtOH = 1 at 40°C) (Nguyen et al., 1997) although the flux obtained in this work (J = 0.023

N

kg/m²/h) was slightly inferior despite the higher temperature. This slight flux difference may

A

be related to different CA molecular weights (50,000 g/mol compared to 29,000 g/mol in the former work) and to different casting procedures.

M

For the CA grafted with imidazolium ILs with different anions, the PV flux was increased nearly 3.4-fold, compared to virgin CA with the most hydrophobic fluorinated Tf2N

ED

anion whereas it increased 1.6-fold only with the less hydrophobic fluorinated BF4 anion. For the fluorinated anions, the Tf2N anion led to the best flux due to better membrane swelling

PT

and plasticization. The hydrophilic bromide anion improved the flux nearly 2-fold, which still remained lower than that obtained with Tf2N, although the hydrophilic bromide anion had led to higher membrane swelling (+152%). This particular behavior was ascribed to the weaker

CC E

plasticizing effect of the grafted imidazolium IL with the bromide Br anion, as shown by the highest polymer Tg obtained in that case. The hydrophilic AcO anion led to a further strong flux improvement (5.7-fold that of virgin CA) due to the high membrane plasticization and

A

swelling. Interestingly, this strong flux improvement was not accompanied by a decrease in membrane selectivity. By overcoming the common permeability-selectivity trade-off, this behavior was outstanding

because the membrane still permeated ethanol only,

corresponding to infinite PV value. For the CA grafted with ammonium ILs with different anions, the membrane fluxes were further improved with all anions except for BF4 compared to the first imidazolium series. Compared to virgin CA, the flux was increased 4-fold for the hydrophobic Tf2N anion. The 17

hydrophilic anions (Br and AcO) led to the best flux improvement by nearly 8 and 18-fold, respectively, due to better membrane plasticization and swelling. The best PV properties (J = 0.41 kg/h m2 for 5 µm and infinite separation factor) were thus achieved with CA grafted with the ammonium IL containing the acetate anion. 3.3. Kamlet-Taft analysis of the membrane properties based on IL polarity parameters The Kamlet-Taft (KT) parameters α, β and * were initially defined for correlating the

IP T

chemical physical properties of organic solvents. These parameters are characteristic for the solvent hydrogen bonding (HB) donor abilitythe solvent HB acceptor ability and thesolventpolarizability, respectively (Kamlet & Taft, 1976). More recently, the KT

SC R

parameters have also been used to correlate the chemical physical features of ILs (Ab Rani et al., 2011; Lee, Ruckes, & Prausnitz, 2008; Reichardt, 2005; Spange, Lungwitz, & Schade, 2014). In our former study on the influence of the IL cation on the membrane properties of CA-g-IL for the targeted application (Hassan Hassan Abdellatif et al., 2015), we have shown

U

that the swelling and flux varied in opposite way with the IL HB donor ability α, which is known to mainly depend on the IL cation. It turned out that the swelling and flux both

N

increased with the IL HB acceptor ability β, which was thus mainly governed by the IL anion.

A

This revealed the key role played by the IL anion for the membrane properties of the first

M

series of CA-g-IL. In this new work, the systematic variation of the IL anion is investigated to provide a better understanding of the IL anion influence on the basis of KT parameters.

ED

3.3.1. Choice of the KT parameters used for the physical-chemical analysis The different KT parameters of the ILs of interest ([C5C1im][X] and [C5C2C2C1N][X])

PT

have not been reported yet and their experimental determination at a common temperature (because KT parameters are known to vary slightly with temperature) would be very difficult

CC E

due to the very high melting temperature (Tm = 142°C) of one of them ([C5C2C2C1N][Br]). Therefore, to allow the ranking of the new ILs of interest as a function of their KT

parameters α and β, we have extended the approach described in our former work (Hassan Hassan Abdellatif et al., 2015) to the new ILs of interest, by considering the KT parameters

A

for a homologous series of imidazolium ILs [C4C1im][X] with the same cation and the different anions of interest X (Table 2). The KT parameters depend on the molecular probes used for their experimental determination. Therefore, the KT parameters reported in Table 2 were all determined with the same "classical" molecular probe set (N,N-diethyl-4-nitroaniline/4nitroaniline) for allowing comparison. Furthermore, the KT parameters of the IL [C4C1im][Br] could not be determined with this molecular probe set due to its high Tm of ca. 65 °C (Ramenskaya, Grishina, Pimenova, & Gruzdev, 2008). Consequently, the KT parameters of 18

a closely related IL ([C6C1im][Br]) having a much lower Tm were used to assess the influence of the bromide anion. Nevertheless, Spange et al. have shown that [C4C1im][Br] and [C6C1im][Br] have very close KT parameters by spectroscopic measurements using another molecular probe set (Spange et al., 2014). Therefore, the different KT parameters reported in Table 2 can be properly compared to reveal the influence of the anion on the IL physical chemical features. In the same way, for the ammonium ILs with the different anions of interest, the KT

IP T

parameters have not been reported yet. However, according to former works (Ab Rani et al., 2011; Reichardt, 2005; Spange et al., 2014), their KT parameters are expected to vary with the anion in the same way as that reported for the imidazolium ILs.

SC R

Consequently, for both imidazolium and ammonium ILs, the HB donor ability  decreases and the HB acceptor ability  increases in the following order of their anion : TF2N, BF4, Br, AcO, allowing the similar ranking of the imidazolium and ammonium ILs with

U

respect to the anion in the KT parameter scales.

N

Table 2 Kamlet-Taft parameters of closely related imidazolium ILs for the structure-property

A

relationship analysis of the membrane sorption and pervaporation properties.

HB donor ability α

HB acceptor ability

ED

Anion

M

Imidazolium cationa)

Polarizability

Reference

Β

*

0.23

0.984

(Ab Rani et al., 2011)

0.61

BF4

0.63

0.37

1.05

(Ab Rani et al., 2011)

Br

0.45

0.74

1.09

(Jelicic, Garcia, Lohmannsroben, & Beuermann, 2009)

AcO

0.57

1.18

0.89

(Doherty, Mora-Pale, Foley, Linhardt, & Dordick, 2010)

A

CC E

PT

TF2N

Ethanolb)

a)

(Kamlet & Taft, 1976) 0.83

0.75

0.51

(Kamlet, Abraham, 1983)

Abboud, & Taft,

The imidazolium cation is [C4C1im] except for the ionic liquid with the bromide counterion for which it is

[C6C1im] (see main text). b) The KT parameters of ethanol are included in this table for comparison.

19

3.3.2. Kamlet-Taft analysis for the interpretation of the sorption and pervaporation properties Figure 4 displays a comparison of the sorption properties for both series of CA grafted with imidazolium or ammonium ILs combined with different anions X. In this Figure, the anions were ranked in the order of increasing HB acceptor ability β of the grafted IL, showing that the swelling in the azeotropic mixture EtOH/ETBE usually increased with this parameter. A better HB acceptor ability β favored strong interactions with ethanol, which is a strong HB

IP T

donor (α = 0.83 (Kamlet et al., 1983)), and contributed to improve membrane performance.

Interestingly, the most hydrophobic anion (Tf2N) led to an interesting significant

SC R

swelling increase compared to BF4 for both membrane series, although the corresponding

parameter β was lower (Table 2). These results evidenced that at least another parameter was contributing to the sorption behavior. In particular, the anion volume (Br << BF4  AcO << Tf2N Supplementary material - Table A.2) also played a role in membrane swelling.

U

Increasing the anion volume was responsible for weakening the interactions between the

N

cation and anion, finally allowing better interactions with the azeotropic mixture EtOH/ETBE and higher swelling. This influence was particularly obvious for the biggest hydrophobic

M

the biggest hydrophilic anion (AcO).

A

fluorinated anion (Tf2N) but it certainly also participated to the best swelling obtained with

For both membranes series, the ethanol weight fraction in the absorbed mixture during

ED

sorption (CSEtOH) followed the same general trend (Fig. 4). As expected for these highly selective membranes, it increased with the HB acceptor ability β in the following order of the

PT

anions : Tf2N < BF4 < Br. The AcO anion led to a slightly reduced CSEtOH compared to the former trend. For both membrane series, the highest swelling obtained with the AcO anion induced the strongest membrane plasticization, which facilitated simultaneous ETBE

A

CC E

absorption andwas responsible for this slight CSEtOH reduction.

20

Cellulose acetate-g-[C C im][X]

Cellulose Acetate-g-[C C im][X] 5

5

1

12

1

1

Total swelling S (wt%)

10

C

S

4

0.9

IP T

6

EtOH

(wt. fraction)

0.95

8

0.85

0

0.8

CA

Tf2N

Br

BF4

CA

AcO

[Cellulose acetate-g-C C C C N][X] 5

2

2

BF4

Br

AcO

[Cellulose acetate-g-C C C C N][X]

1

5

1

2

2

1

N

10

(wt. fraction)

0.95

A

8

C

S

4 2

Tf2N

BF4

PT

CA

Br

AcO

0.85

0.8

CA

Tf2N

BF4

Br

AcO

CC E

0

0.9

EtOH

M

6

ED

Total swelling S (wt%)

Tf2N

U

12

SC R

2

A

Figure 4. Influence of the IL anion on the sorption properties for the separation of the azeotropic mixture EtOH/ETBE with CA-g-[C5C1im][X] and CA-g-[C5C2C2C1N][X] with nearly identical IL amounts (W IL  17 wt%) in comparison with virgin CA. In pervaporation, all the membranes for both series led to permeate samples

containing ethanol only, meaning that the total flux was equal to the ethanol flux. The PV flux followed the same general trend as a function of the IL HB acceptor ability β (Fig. 5) as that observed for the swelling (Fig. 4) but this influence was much stronger on the flux. Here again, the anion size obviously played a role on membrane flux. The biggest hydrophobic and hydrophilic anions (Tf2N and AcO) favored permeation by improving membrane 21

swelling, solvent-induced plasticization and, consequently, also solvent diffusion to a great extent. Consequently, the membrane CA-g-[C5C2C2C1N][AcO] with the IL combining the best HB acceptor ability β with big cation (C5C2C2C1N+) and anion (AcO) sizes led to the best separation performance for ETBE purification by multiplying the flux by 18 and maintaining the initial outstanding selectivity of virgin CA. Cellulose Acetate-g-[C C im][X]

SC R

0.4

0.3

2

J (kg/h m for 5 microns)

1

IP T

5

0.5

U

0.2

N

0.1

Tf2N

Br

BF4

AcO

M

CA

A

0

[Cellulose acetate-g-C C C C N][X] 5

2

1

PT

0.3

CC E

0.2

0.1

0

A

2

ED

0.4

2

J (kg/h m for 5 microns)

0.5

CA

Tf2N

BF4

Br

AcO

Figure 5. Influence of the IL anion on the pervaporation flux for the separation of the azeotropic mixture EtOH/ETBE with CA-g-[C5C1im][X] and CA-g-[C5C2C2C1N][X] with nearly identical IL amounts (W IL  17 wt%) in comparison with virgin CA. 22

4. Conclusion Two CA derivatives grafted with bromide imidazolium or ammonium ILs were used as precursors for anion exchange with various salts. Complete exchange was achieved for the fluorinated anions while the exchange rate for the AcO anion remained limited to 66% because of the poor solubility of the acetate salts in CA solvents. Nevertheless, eight CA derivatives with grafted ILs with different anions (Tf2N, BF4, Br and AcO) enabled the first physical-chemical analysis of the anion influence in homologous membrane series for liquid

IP T

separation.

For ETBE purification from the azeotropic mixture EtOH/ETBE, the membrane swelling

SC R

increased with the IL HB acceptor ability , which is mainly governed by the IL anion,. The

very high sorption selectivity also increased with this parameter but this increase was slightly compensated by sorption synergy for the highest swelling obtained with the best anion (AcO). Therefore, during sorption, an optimum of IL HB acceptor ability was observed for

U

limiting swelling below a limit inducing a slight reduction in sorption selectivity due to sorption

N

synergy.

A

During the PV separation, all of these membranes permeated ethanol only, which remains exceptional for ETBE purification by PV. The PV flux strongly increased with the IL

M

HB acceptor ability  and for a given cation, increasing the anion size also contributed to improve the membrane performance by loosening the IL cation/anion interactions and by

ED

favoring those with ethanol. Therefore, the ammonium IL with the acetate anion combined the highest HB acceptor ability with big cation and anion sizes and led to the best membrane

PT

properties.

Finally, even if the membrane properties resulted from the complex combination of

CC E

different parameters, the KT analysis showed the key role played by the anion ability to interact with ethanol and will provide the basis for developing other high performance cellulosic membrane materials.

A

5. Acknowledgements The authors would like to thank the ELEMENT Erasmus Mundus Programme for the

funding of the PhD scholarship and that of the corresponding extension offered to Mrs Faten HASSAN HASSAN ABDELLATIF, and Dr Guillaume SUDRE for the beam time at the ESRF.

23

6. References

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A

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U

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List of figures Figure 1 Synthesis of CA with grafted ILs with either imidazolium or ammonium cations and different anions by anion exchange of cellulosic precursors with bromide anions Figure 2

Comparison of the 1H NMR spectra in DMSO-d6 solution at 300 MHz before and after anion exchange with sodium acetate for : a) CA-g-[C5C1im][X] and b) CA-g[C5C2C2C1N][X] Quantitative

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F NMR characterization in DMSO-d6 solution at 376 MHz after

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Figure 3

anion exchange with two fluorinated salts LiTf 2N or NaBF4 for : a) CA-g-

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[C5C1im][X] and b) CA-g-[C5C2C2C1N][X]

Influence of the IL anion on the sorption properties for the separation of the azeotropic mixture EtOH/ETBE with CA-g-[C5C1im][X] and CA-g-[C5C2C2C1N][X]

Figure 5

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with nearly identical IL amounts (W IL  17 wt%) in comparison with virgin CA. Influence of the IL anion on the pervaporation flux for the separation of the

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azeotropic mixture EtOH/ETBE with CA-g-[C5C1im][X] and CA-g-[C5C2C2C1N][X]

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with nearly identical IL amounts (W IL  17 wt%) in comparison with virgin CA.

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List of tables Table 1

Influence of the IL anion X on the membrane sorption and pervaporation properties for the separation of the azeotropic mixture EtOH/ETBE with CA-g[C5C1im][X] and CA-g-[C5C2C2C1N][X] with nearly identical IL amounts (WIL  17 wt%) in comparison with virgin CA.

Table 2

Kamlet-Taft parameters of closely related imidazolium ILs for the structure-

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property relationship analysis of the membrane sorption and pervaporation

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properties.

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