Journal of Membrane Science 447 (2013) 134–143

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Journal of Membrane Science

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Study on ageing/fouling phenomena and the effect of upstreamnanofiltration on in-situ product recovery of n-butanol throughpoly[1-(trimethylsilyl)-1-propyne] pervaporation membranes

Marjorie F.S. Dubreuil n, Pieter Vandezande, Wouter H.S. Van Hecke, Wim J. Porto-Carrero,Chris T.E. DotremontVITO (Flemish Institute for Technological Research), Boeretang 200, 2400 Mol, Belgium

a r t i c l e i n f o

Article history:Received 10 April 2013Received in revised form9 July 2013Accepted 11 July 2013Available online 18 July 2013

Keywords:PTMSPPervaporationin-situ n-butanol recoveryFoulingNanofiltration

88/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.memsci.2013.07.032

esponding author. Tel.: +32 14 33 56 86; fax:ail address: Marjorie.dubreuil@vito.be (M.F.S.

a b s t r a c t

Thin film composite poly[1-(trimethylsilyl)-1-propyne] – PTMSP – pervaporation membranes have beeninvestigated for in-situ product recovery of n-butanol from a fermentation broth using a Clostridiumacetobutylicum strain. For this specific application, a strong flux decline is observed which can beattributed to ageing and fouling phenomena in the membrane. In order to have a better understanding ofthese complex phenomena, X-ray photoelectron spectroscopy and infra-red spectroscopy have been usedto monitor the ageing of the PTMSP membranes under different steady state conditions. The foulingeffect of different components in the fermentation broth has been systematically investigated throughoff-line pervaporation tests on model mixtures with stepwise addition of fermentation by-products,revealing the negative impact of butyric acid and long chain fatty acids on the permeate flux.Additionally, long chain fatty acids, such as stearic acid, impact negatively the butanol/water separationfactor. In order to remedy or at least decrease the impact of this fouling issue, the integration of anupstream nanofiltration step has been evaluated. This pre-treatment step has led to a drasticimprovement of the flux through the filled PTMSP membrane (factor 4), while the butanol–waterseparation factor (27.5) remained much higher than the separation factor obtained with a commercialPDMS membrane (14.7).

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The increasing interest in the production of bio-alcohols asalternative for or additive to fossil fuels and as platform moleculesfor the synthesis of bio-chemicals has led to a revival of fermenta-tion processes. n-Butanol, for instance, is an important commoditychemical used as a solvent and as a precursor for chemicalsynthesis. In addition, it holds tremendous promise as a second-generation biofuel as it combines a considerably higher combus-tion value than ethanol with chemical properties that allowstraightforward blending with conventional, fossil-based fuels.The default technology for recovery of n-butanol is distillation,the most energy intensive step in the entire production process[1]. Even though multi-component distillation has well merited itsstatus as a workhorse of the chemical industry, energetic gains areexpected in this case by complementing it with a sufficientlyselective/efficient primary recovery step. Integration of the fer-mentation with the first step of the downstream process by using

ll rights reserved.

+32 14 32 11 86.Dubreuil).

a selective in situ product recovery (ISPR) technique is an inter-esting strategy to improve the water balance and decrease theenergy consumption of the production process. Several literaturestudies emphasize the potential of pervaporation as one of themost promising alternative technologies for bio-alcohol capture[2–4] and also for the separation of organic–organic mixtures [5].

Indeed, these last decades have demonstrated the stakes of thedevelopment of robust and selective hydrophobic membranes forthe pervaporative removal of alcohols from dilute aqueous mix-tures. While, the use of polydimethylsiloxane (PDMS) as selectivemembrane layer has been significantly emphasized, the develop-ment of polyacetylenes-based membranes, particularly consistingof poly[1-(trimethylsilyl)-1-propyne] (PTMSP), has attracted con-siderable attention due to the intrinsic properties of this polymer(especially its extremely high free volume fraction) and itspotential for increased flux and selectivity compared to PDMSmembranes. By incorporating hydrophobic silica particles,PTMSP's intrinsic free volume can be further increased, resultingin enhanced fluxes, as demonstrated in the separation of propane/hydrogen [6] and ethanol/water mixtures [7,8].

However, despite the intrinsic properties of PTMSP, and the poten-tial of PTMSP-based pervaporation membranes in the separation of

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143 135

alcohol/water mixtures, in particular for alcohol removal of acetone–butanol–ethanol (ABE) fermentation broths, a major problem has notyet been resolved, preventing the further application of this polymer.Indeed, the PTMSP suffers from fouling during pervaporative in-situ n-butanol recovery, leading to a significant flux decrease and a partialloss of the selectivity [9–12]. Similar problems have been encounteredwith PDMS for the recovery of ethanol and butanol [13–15].

Earlier publications by VITO have demonstrated the progressachieved in the development of thin film PTMSP membranes [6–8,16]. In this new study, besides testing on model alcohol/watermixtures [8,16], long term trials on the most optimal supportedPTMSP membrane (as identified in [8]) have been conducted usingan in-house developed fermentation–pervaporation set-up wherethe membranes were directly coupled to the second stage of acontinuous two-stage ABE fermentation using a Clostridium acet-obutylicum strain as described by Van Hecke et al. [17,18]. In thisstudy, while the butanol/water separation factor was significantlylarger than for PDMS membranes, the tests revealed a steady fluxloss, tentatively attributed to fouling of the membrane by fermen-tation by-products [9–12]. To get insight into the mechanism ofthis performance decline and evaluate the effect of these by-products, pervaporation tests on cell-free ABE model mixtureswith progressively increasing complexity have been conducted.In parallel, the ageing of the PTMSP membranes under differentconditions, as well as the reversibility of this phenomenon, hasbeen studied through angle-resolved X-ray photoelectron spectro-scopy (AR-XPS) and infra-red spectroscopy (IR). Finally, in anattempt to remove fouling inducing molecules and thus enablingpervaporation under more favorable conditions, the integration ofan upstream nanofiltration step has been investigated.

2. Experimental part

2.1. Materials

PTMSP was purchased from Gelest, Inc. (USA). Cab-O-Sil TS 530hydrophobic silica was obtained from Cabot Aerogel (Germany).Toluene and acetone (A) were purchased from VWR (Germany).Ethanol (E), n-butanol (B) and acetic acid were purchased fromMerck (Belgium). Butyric acid, palmitic acid, stearic acid andL-asparagine were obtained from Sigma Aldrich (Germany). Allreagents were of analytical grade and used as received.

Fig. 1. Ageing conditions of the PTMSP membrane.

2.2. Synthesis of supported PTMSP membranes

Supported PTMSP–silica membranes were prepared asreported elsewhere [8,16]. Casting solutions of toluene, silica andPTMSP were prepared according to De Sitter et al. [6]. Toluene-based silica dispersions with a concentration of 1.7 wt% silica wereprepared by ultrasonic treatment (Branson, MT-5210, 40 kHz,185 W) for 30 min. To these dispersions, PTMSP was added so asto obtain suspensions with total solid concentrations of 5 wt%.Casting solutions containing 25 wt% of silica on total solids basiswere thus prepared. The polymer–silica suspensions were magne-tically stirred until complete dissolution of PTMSP. The solutionswere then coated as thin layers on top of a NADIRs MV020T PVDFsupport membrane (Microdyn-Nadir GmbH, Germany), using adoctor blade and an automatic film applicator (Braive Instruments,Belgium) set at a coating gap of 100 mm and a coating velocity of3.4�10�2 m s�1. After 24 h of evaporation in ambient air, theresulting composite membranes were thermally treated for 1 h at70 1C to remove the residual toluene. The final membrane sheets(30�42 cm2) having a top-layer thickness of approximately 5 mmwere stored under dry conditions. For the ageing study, supported

PTMSP membranes without silica fillers and with a top-layer ofapproximately 2.5 mm were prepared in a similar way.

Commercially available polydimethylsiloxane (PDMS) membranesfrom Pervatech (The Netherlands) were chosen as benchmark andevaluated under similar conditions. This membrane consists of a1 mm PDMS separating layer on top of a porous polyimide support(200 mm).

2.3. Ageing of the PTMSP membranes

To study the influence of the surrounding atmosphere/solutionon the PTMSP structure, unfilled PTMSP membranes wereimmersed in different model solutions at 50 1C (Heratherm oven,Weiss Technik, Germany):

(1)

ethanol (5 vol%)–water (95 vol%), (2) acetone (4.5 g l�1)–n-butanol (9 g l�1)–ethanol (1.5 g l�1) in

water, and

(3) a cell-free fermentation broth.

In parallel, a PTMSP sample was allowed to age under atmo-spheric conditions, i.e. at room temperature and humidity andunder natural light irradiation (Fig. 1). The structure and elementalcompositionwere monitored as a function of time during 6 months.

2.4. Off-line pervaporation tests on model mixtures with stepwiseaddition of potential fouling compounds

Different components from the complex fermentation brothmight play a role in the fouling of PTMSP membranes. In order toget more insight into the role of some potential fouling agents, off-line pervaporation tests at 50 1C have been carried out with modelmixtures of different compositions.

For this purpose, a filled PTMSP membrane was subsequentlysubjected to different, freshly prepared model mixtures withoutchanging the membrane intermittently. The pervaporation testwas started with a mixture of ABE (4.5–9–1.5 g l�1 respectively)that was used as reference mixture. After flux equilibration, themembrane was screened on a new ABE mixture containing 5 g l�1

of acetic acid, followed by ABE mixtures containing butyric acid(5 g l�1), L-asparagine (0.25 g l�1), stearic sodium salt (0.25 g l�1)and palmitic acid sodium salt (0,25 g l�1). Finally, the membranewas again tested on the ABE reference mixture.

The longer term pervaporation test was carried out on a bench-scale cross-flow unit, type LTU-054, designed by Pervatech BV (TheNetherlands). This stainless steel unit consists of an isolated feed tank(5 l) with integrated heat exchanger, a magnetically driven circula-tion pump (type CY-4281-MK, Speck Pumpen GmbH, Germany), atest cell for flat sheet membranes (type FSTC-049, Pervatech BV, TheNetherlands), a tubular heat exchanger with condensing surface of25 cm² (type Exergy 256-5, Exergy Miniature Heat Exchangers, LLC,

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143136

USA) connected to an external chiller (type Huber Unichiller UC022Advanced, Peter Huber Kältemaschinenbau GmbH, Germany) filledwith glycol, and a vacuum pumpwith integrated heat exchanger (LabSC 920, KNF Neuberger GmbH, Germany). The rectangular test cell(external dimensions of 280�130 mm) provides an effective mem-brane area of 0.0076 m2 (dimensions of 250�7 mm2) and is fore-seen with a flow channel (dimensions of 8.4�2 mm2, length ofapproximately 90 cm) and two Kalrez o-rings. The feed mixture(approximately 4 l) was kept at 5072 1C and circulated at approxi-mately 150 l h�1; the permeate pressure was maintained below20 mbar and the setpoint of the external chiller was set at �2 1C.During the test, fresh feed solution with the same composition as theinitial feed mixture was pumped into the feed tank using an externalvolumetric dosing pump (type Delta Optodrive DLTA 2508, ProMi-nent S.A., Belgium) with addition being controlled by a level sensoron the feed tank (Deltabar M PMD55, Endress+Hauser, USA), thuskeeping the feed volume constant at approximately 4 l throughoutthe entire test. The permeate was collected in a glass bottle on abalance (Extend Series, type ED2202S-0CE, Sartorius, Germany)connected to a computer with installed MelPerva v3.010 processcontrol and data storage software, thus enabling in-line flux mon-itoring. In between the individual sub-tests on different feed mix-tures, both the feed and permeate circuits were thoroughly rinsedwith hot RO water and acetone, and subsequently dried, while themembrane was rinsed with ethanol and stored in an ABE solutionupon changing the feed mixture.

2.5. Pervaporation tests on integrated fermentation–pervaporationunit using real fermentation broth

The protocols for preparation of stock culture using Clostridiumacetobutylicum strain ATCC 824, seed culture and inoculation ofthe fermentors as well as VITO's integrated fermentation–perva-poration set-up (Fig. 2) have been described previously by VanHecke et al. [17,18]. A continuous ABE fermentation was carriedout, using an acidogenic fermentor of 3 l (working volume 1.2–1.3 l) and a solventogenic fermentor of 7 l (working volume 2.4 l).The former was run at 35 1C; the latter was run at a slightly highertemperature of 37 1C in order to obtain increased fluxes for thepervaporation process. The in-house developed and assembledpervaporation unit consisted of three serially connected mem-brane modules (Pervatech BV, The Netherlands) with a totalmembrane exchange area of approximately 0.027 m2. Both thethin film composite PTMSP membranes prepared as described inSection 2.2 and the commercial PDMS membrane were tested.An average permeate pressure of 12.3 mbar was established usinga membrane vacuum pump (Lab SC920, KNF Neuberger GmbH,Germany).

feed

pump 1

pump 2

puacidogenic

fermentation solventogenic fermentation

QE effluen

acidogenic fermentation

solventogenic fermentation

effluen

Fig. 2. Schematic representation of VITO's ISPR bench-top unit integrating a two-stage ABE

2.6. Nanofiltration pre-treatment

The tests were carried out off-line on a cross-flow filtration unitequipped with a feed vessel (30 l) and a magnetically drivencirculation pump (type D-10, Hydra-Cell, USA). For the screeningtrials, an Amafilter test cell (MAHLE Industry, Germany) with anactive surface area of approximately 0.0044 m2 was used. Themembrane coupons were sealed with Kalrez o-rings. The MEFIASsoftware was used for process monitoring.

Two polyamide-based membranes were selected for this study,based on their properties and low molecular weight cut-off(MWCO): the charged MPF-34 from Koch Membrane Systemswith a MWCO of 200 Da, and the Desal-5 DK form GE Osmonicswith a MWCO comprised between 150 and 350 Da. The feedsolution (4.5 l) was a cell-free fermentation broth issued fromthe solventogenic fermentor.

The test was carried at a flow rate of 270 l h�1, correspondingto a crossflow velocity of 2 m s�1 across the membrane, and atrans-membrane pressure of 20 bar. The initial feed after circula-tion was sampled (a), as well as the permeate (b) and retentate atsteady-state (c).

For up-scaling purposes, additional trials were carried out on aspiral-wound Desal-5 DK element with a membrane area of2.6 m2. 20 l of cell-free fermentation broth was filtrated at atrans-membrane pressure of 15 bar.

Membrane fluxes J (kg m�2 h�1) were determined by weighingthe permeate samples and calculated according to

J ¼m=At ð1Þwhere m denotes the weight of the permeate per unit membranearea A and time t.

The permeate issued from the nanofiltration pre-treatment wasused afterwards as feed for the pervaporation trials. The permea-tion flux was calculated according to Eq. (1). The alcohol/waterseparation factor, αROH=H2O, was calculated according to

αROH=H2O ¼ ðYROH=YH2OÞðXH2O=XROHÞ ð2Þ

in which X and Y represent the weight fractions of the alcohol andwater in the feed and permeate, respectively.

The retention of the component “i” was calculated according to

Ri ¼ ð1�Cpi=Cf iÞ100 ð3Þwith Cpi and Cfi the concentrations of component “i” in thepermeate and in the feed respectively.

2.7. Determination of solvents, carbohydrates and volatile fatty acids

The determination of acetone, ethanol, n-butanol, acetic acidand butyric acid was performed as described elsewhere [17]. Theconcentration of glucose (retention time 11.8 min) was determined

mp 3 PV

PV

PV

vacuum pump

condenser

t

QP products pervaporation modules

pervaporation modules

products

t

condenser

fermentation with a pervaporation unit (arrows show the direction of liquid flow) [17].

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143 137

by high performance liquid chromatography (1200 Series, AgilentTechnologies, USA) using a Prevail Carbohydrate ES 5u column(250 mm�4.6 mm) with evaporative light scattering detector(Alltech 3300 ELSD, Grace, USA) for peak detection.

Both stearic acid and palmitic acid were analyzed with liquidchromatography tandem mass spectrometry (LC–MS).

An ultra performance liquid chromatography (UPLC) instru-ment (Acquity Binairy UPLC, Waters, USA) was coupled to aMicromass Quattro Premier XE tandem mass spectrometer(Waters). The mass spectrometer was operated in negative elec-trospray ionization mode (ESI) and for each fatty acid a specificparent–daughter mass transition (MRM) was registered. Liquidchromatography separation of the compounds was performedusing an Acquity BEH C18 column (100�2.1 mm2, 1.7 mm) fromWaters with both 2 mM ammonium acetate in water (A) andethylacetate/methanol (1/1) (B) as mobile phases. The columntemperature was kept at 40 1C during the analysis, and theinjection volume on column was 10 ml. Calibration standards ofpalmitic acid and stearic acid were prepared in methanol in aconcentration range going from 0.01 to 50 mg l�1.

All samples were filtrated with Chromafil AO-20/25 0.20 mmpolyamide disposable syringe filters (Machery-Nagel GmbH, Ger-many) as the compounds precipitate in the matrix of the samplesolutions. 5 ml of sample was filtrated and the resulting filtratewas marked as ‘solution’ subsample. The precipitate on the filterwas then washed off with 5 ml of ethyl acetate/methanol (1/1) andmarked as ‘precipitate’ subsample. Both ‘solution’- and ‘precipi-tate’-subsamples of each sample were analyzed using the instru-mental method described above. Since the fatty acids precipitatein the sample matrix, the highest concentrations were found inthe ‘precipitate’-subsample. For each series of samples, a proce-dure blank was also filtrated and measured. All sample resultswere corrected for the concentrations found in this blank sample.

2.8. Membrane characterization

The XPS measurements were carried out with a Thermo Scien-tific K-Alpha, equipped with a monochromatic small-spot X-raysource and a 1801 double focusing hemispherical analyzer with a128-channel detector. Spectra were obtained using an aluminumanode (Al Kα¼1486.6 eV) operating at 72 W and a spot size of400 mm. Survey scans were measured at a constant pass energy of200 eV and region scans at 50 eV. The background pressure was2�10�9 mbar, and during measurement, 3�10�7 mbar argonbecause of the charge compensation dual beam source. Sputteringwas done with a beam energy of 2000 eV at high current.

The aged membranes were analyzed by attenuated totalreflection infra-red spectroscopy (ATR-IR) using a Thermo NicoletNexus instrument. An OMNIC software was used.

Prior to XPS and IR analyses, the samples were dried 4 h at50 1C.

3. Results and discussion

3.1. Membrane synthesis

Two types of PTMSP membranes have been synthesized:unfilled and silica filled PTMSP membranes. Unfilled PTMSPmembranes have been used to study the ageing of the polymermaterial under various conditions. Indeed, the presence of silicananoparticles in the polymer matrix could have interfered withthe membrane characterization in terms of peak overlappingduring IR measurements, and/or increased the total siliciumconcentration in XPS, masking the real behavior of the PTMSP.

However, as it has been demonstrated in previous studies thatsilica-based PTMSP membranes present enhanced permeationproperties, silica filled PTMSP membranes have been selected forpervaporation trials. The dispersion of silica nano-fillers in PTMSPmembranes has been discussed by De Sitter et al. [19]. It wasnoticed that the τ3 component of the polymer free volume is notchanging by the incorporation of silica, but that interstitial cavitiesare formed through formation of silica aggregates, enhancing thepermeation performances. A detailed description of membraneprocessing has been described elsewhere [7,8]. The optimaldeveloped supported PTMSP–silica membrane was used in thepresent study.

3.2. Ageing of unfilled PTMSP membrane under steady stateconditions

PTMSP is a glassy polymer with the highest known free volumefraction (up to 34%) [20]. This is an advantage in terms of flux ingas separation and pervaporation, but it might also lead tounstable properties due to compaction of the polymer structureand/or blocking of free volume upon permeation. Masuda et al.studied the thermal stability of PTMSP through heating of thepolymer in air up to 100 1C for 20 h. They did not observe anyoxidation through IR spectroscopy [21]. However, upon furtherheating, PTMSP can undergo thermal degradation in air, wherecarbonyl groups are produced through degradation and oxidationof the acetylene groups [22]. Nagai et al. studied the physicalageing of PTMSP and its influence on the gas separation propertiesof PTMSP membranes through positron annihilation lifetimespectroscopy (PALS) [23]. They observed time-dependent collap-sing of the free volume in the polymer matrix, leading to adecrease of the methane gas permeation. However, little is knownon the ageing of PTMSP under atmospheric conditions, and inparticular in the presence of components such as ethanol, acetoneor n-butanol and by-products present in ABE fermentation broths.Indeed, during pervaporation of an ABE fermentation broth, themembrane comes into contact with a complex mixture of compo-nents including solvents (alcohols), cells, sugars, acids and lipids,and the decline in membrane performance is usually attributed tothe impact of these fermentation by-products on the membrane.In order to determine the fouling tendency of these individualcomponents during pervaporation of an ABE fermentation brothand their impact on the separation properties of PTMSP mem-branes, the behavior of the membrane in direct contact with asynthetic mixture has to be fully understood. For this purpose,supported unfilled PTMSP membranes were immersed in differentmodel solutions (Fig. 1): in an aqueous ethanol (5 vol%) solution, ina model ABE mixture composed of acetone (4.5 g l�1)–butanol(9 g l�1)–ethanol (1.5 g l�1), and finally in a cell-free ABE fermen-tation broth.

In parallel, a PTMSP membrane has been exposed to the ambientenvironment (air, dust particles, UV irradiation and temperatureand humidity fluctuations) for 6 months. The changes in thestructure and the elemental composition of the membrane havebeen monitored as a function of time. AR-XPS has been used todetermine the elemental composition of the polymer near thesurface (angle of 80.51) and in the bulk of the membrane (angleof 25.51). When the PTMSP membrane is stored under atmosphericair conditions (Fig. 3), the membrane composition remains rela-tively constant during the first 8 weeks, while a strong increase ofthe oxygen content is observed afterwards, leading to a decrease ofthe total atomic carbon concentration. This is explained by theoxidation of the membrane. It is believed that ageing of themembrane has been accelerated in the last months of the studydue to increased exposure of the material to UV radiations andtemperature increase (summer period). Similar results are observed

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

At.%

Ageing time (weeks)

C

O

Si

Fig. 3. Carbon, oxygen and silicium atomic concentration in the unfilled PTMSPmembrane as a function of the membrane ageing time under atmospheric conditions;AR-XPS measurements at the membrane surface.

2904

,41

3228

,90

86

88

90

92

94

96

98

100

%T

30003500

Wavenumbers (cm-1)

to 1 week 2 weeks

8 weeks

24 weeks

4 weeks

OH, NHx, COOH

1431

,33

1408

,74

84

86

88

90

92

94

96

98

100

%T

140016001800

Wavenumbers (cm-1)

to

1 week 2 weeks

8 weeks

24 weeks

4 weeks

C=O

amide

Fig. 4. IR spectroscopy of an unfilled PTMSP membrane as a function of the ageingtime under atmospheric conditions: (a) region 3800–2800 cm�1 and (b) region1900–1400 cm�1.

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

At.%

Ageing time (weeks)

C

O

Si

Fig. 5. Carbon, oxygen and silicium atomic concentration in the unfilled PTMSPmembrane as a function of membrane ageing time in a solution water (95 vol%)–ethanol (5 vol%) at 50 1C; AR-XPS measurements at the membrane surface.

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143138

through IR spectroscopy (Fig. 4). After 6 months, a significantstructural modification is present in the area's 2800–3800 cm�1

and 1400–1900 cm�1. The appearance of a broad peak between3100 and 3700 cm�1 is assigned to the stretching vibrations of analcohol, amine or acid function present in the membrane. Inparallel, an intense peak is observed around 1721 cm�1 due to thepresence of a carbonyl functionality, which can be explained by thepartial oxidation of the CQC double bond. After 6 months a smallshoulder to this peak (�1660 cm�1) also appears, which could be

related to an amide functionality. On the contrary, the evolution ofthe peak assigned to the stretch of PTMSP's CQC double bond(�1550 cm�1) is not clearly observable due to peaks overlapping.The correlation between the XPS and IR measurements seems tosupport the structural modification of the PTMSP through oxidationof the CQC double bond on the main chain, accelerated by UVradiations. This is in agreement with the observations of Gallo et al.,who observed the appearance of a carbonyl group after storage of aPTMSP membrane under outdoor exposition for several months[24]. They attributed these oxidative changes to the high oxygensolubility of PTMSP. However, the degradation of the PTMSP back-bone is only observed under severe radiation conditions, while thematerial remains chemically stable under normal storage conditionsin terms of temperature, humidity and light exposure.

During the ageing of the unfilled PTMSP membrane in a diluteethanol solution, the carbon concentration remains relativelystable (Fig. 5). A slight decrease in the silicium concentrationlinked to a slight increase of the oxygen concentration is takingplace after 6 months. However, this cannot be attributed to areaction of the alkene group which would need a catalyst to reactwith water and/or an alcohol. The slight modification of theelemental composition of the PTMSP membrane after 6 monthsmight be explained by the residual presence of ethanol/water inthe analyzed sample. In a similar way, the results obtained for theageing of a PTMSP membrane in a synthetic ABE solution shows noevolution of the PTMSP elemental composition (Fig. 6).

Being aware of the absence of detrimental interaction between theABE solution and the PTMSP membrane, a final study has been carriedout with the membrane immersed in a cell-free broth. Besidesacetone, n-butanol, ethanol, water and some starting components,additional products are being formed or released during the fermen-tation process and are present in the broth potentially leading tomembrane fouling: fatty acids (stearic acid, palmitic acid); eventuallynucleic acids or proteins through cell lysis; fermentation by-productssuch as lactate and acetate derivatives; highly sorbing and lowvolatility by-products of fermentation, such as diols or glycerol [9–12].

Already in the first ageing weeks a drastic change of theelemental composition is observed (Fig. 7): a strong decrease ofthe carbon and silicium concentrations, a significant increase ofthe oxygen concentration, and the apparition of nitrogen up toabout 10 at%. Nitrogen is not present in the membrane formingmaterial; it definitely comes from the adsorption/absorption ofnitrogen containing molecules, e.g. proteins on/in the PTMSPmembrane. Similar findings emerge from the IR-spectra (Fig. 8),namely the appearance of a broad band in the 3000–3600 cm�1

area due to the presence of hydroxyl, carboxylic acid, or/and amino

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

At.%

Ageing time (weeks)

C

O

Si

Fig. 6. Carbon, oxygen and silicium atomic concentration in the unfilled PTMSPmembrane as a function of the membrane ageing time in a synthetic ABE solutionat 50 1C; AR-XPS measurements at the membrane surface.

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

At.%

Ageing time (weeks)

O

C

N Si

Fig. 7. Carbon, oxygen, nitrogen and silicium atomic concentration in the unfilledPTMSP membrane as a function of the membrane ageing time in a cell-free ABEbroth at 50 1C; AR-XPS measurements at the membrane surface.

80

85

90

95

100

%T

30003500

Wavenumbers (cm-1)

to 1 week 2 weeks 4 weeks

24 weeks

OH, NHx, COOH

1408

,60

1431

,35

1535

,72

80

85

90

95

100

105

%T

16001800

Wavenumbers (cm-1)

to

24 weeks

4 weeks 2 weeks

1 week

C=O (ester)

C=O (amide)

Fig. 8. IR spectroscopy of an unfilled PTMSP membrane as a function of the ageingtime in a cell-free ABE broth: (a) region 3800–2800 cm�1 and (b) region 1900–1400 cm�1.

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143 139

groups in the membrane. Peaks at 1745 and 1655 cm�1 are alsoobserved. The peak at 1750 cm�1 can be assigned to the carbonylvibration of an ester group (O–CQO). Ester groups can be found infermentation by-products such as lactate, acetate and butyrateesters. An increase of the peak intensity can be observed when theageing time increases. After 6 months, a sharp and intense peak ispresent.

The surface modification of the PTMSP membrane throughexposure to the surrounding atmosphere is mainly due to astructural modification of the membrane bulk through oxidationof the alkene groups. On the contrary, when the PTMSP membraneis stored in a fermentation broth, the changes observed for theelemental composition and for the IR spectra are attributed to theabsorption/adsorption of fouling components, and much less to amodification of the membrane bulk.

In order to get some insight whether these fouling componentsare adsorbed on the surface or absorbed in the membrane,membranes immersed for 6 months in a cell-free ABE broth havebeen either gently washed with RO water, or immersed for 72 h inethanol (Table 1). In case of absorption, the immersion in ethanol isexpected to allow a better membrane regeneration since the PTMSPmembrane is organophilic. Through washing with RO water, the

surface elemental composition is barely affected. On the other hand,the bulk of the membrane, which had undergone a strongermodification after ageing than the surface, is slightly recovered.The oxygen concentration decreases while the carbon and siliciumconcentrations increase. However, a more important regenerationof the membrane composition is obtained after immersion of theaged membrane in ethanol for 72 h. This is explained by the highethanol permeability and selectivity of PTMSP over water. A moresignificant decrease of the oxygen concentration is observed forboth the membrane surface and the bulk, while the carbonconcentration and the silicium concentration rise more drastically.A simultaneous absorption and adsorption phenomena is high-lighted through the immersion of the PTMSP membrane in afermentation broth. Unfortunately, under the tested conditions,the membrane fouling was not fully reversible.

3.3. Identification of the fouling components in the n-butanol ISPR

The role of different broth components such as carbohydrates,short and long chain fatty acids, microorganisms on flux declineand alcohol/water selectivity of pervaporation membranes hasbeen partially reported by different groups, especially towards ABEfermentation. Most of the studies have been carried out on PDMS-based pervaporation membranes. However, no consensus could befound. Aroujalian et al. observed an ethanol flux decrease of aPDMS pervaporation membrane due to the presence of glucose[13]. Vane et al. explained the flux decline of a MFI zeolite-filledPDMS membrane by the competitive adsorption between aceticacid and ethanol in a model study [25]. They further mentionedthe possible blocking of the zeolite pores by adsorption of minor

Table 1Atomic concentration (at%) in carbon, oxygen, silicium and nitrogen for an unfilledPTMSP membrane, after immersion for 6 months in a cell-free ABE broth at 50 1C,and after washing with water or immersing the membrane in ethanol; AR-XPSmeasurements at the surface (angle of 80.51) and in the bulk (25.51); t0 is theelemental composition of the PTMSP membrane before ageing.

t0 Ageing 6 monthsat 50 1C

After ageing andH2O washing

After ageing and 72 hethanol immersion

SurfaceC 77.6 61.1 60.4 63.3O 10.1 26.7 25.2 21.0Si 12.4 1.7 2.3 4.8N – 7.0 9.0 7.4

BulkC 79.5 52.4 60.7 65.2O 6.4 33.0 25.5 19.3Si 14.1 1.3 3.2 6.1N – 8.9 8.7 7.7

0

10

20

30

40

50

60

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120 140 160 180 200

Butanol/w

ater separation factor

Flux

(kg.

m-2

.h-1

)

Time (h)

Fig. 9. Total permeate flux of supported silica-filled PTMSP membrane as a functionof the pervaporation time and of the feed composition (◊), and butanol/waterseparation factors (■).

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200

Flux

(kg.

m-2

.h-1

)

Time (h)

Fig. 10. Total permeate flux of commercial PDMS membrane and supportedPTMSP–silica membrane with and without nanofiltration pre-treatment withDesal-5 DK spiral wound membrane.

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143140

constituents in the broth including organic acids, esters andalcohols [26]. Similar conclusions were addressed by Chovauet al. who studied the influence of diols, acids and sugars on theethanol transport through PDMS membranes [27]. As mentionedin the introduction, the group of Fadeev and Volkov has exten-sively worked on the development and application of PTMSPmembranes in pervaporative solvent removal from ABE broths,and on the elucidation of the observed performance decline of themembrane. They described the adsorption of lipids on the mem-brane surface [11], the contamination of the PTMSP surface byhigh boiling fermentation by-products [9] and attributed addi-tionally the decrease of the performances to the occurrence ofvolume collapse of the PTMSP membrane [10].

Unfortunately, no concrete and durable solution has emergedfrom these different studies. In order to get more insight into theimpact of some potential fouling agents, known to be present inABE broths, off-line pervaporation tests have been carried out withmodel mixtures of different potential fouling components (seeSection 2.4). The choice of the components and their concentrationhas been dictated by the knowledge of the fermentation brothcomposition. Stearic and palmitic acids are the major buildingblocks of lipids and are present in the growth medium as well as L-asparagine. By-products such as acetic acid and butyric acid havealso been reported as fouling components and have been includedin this study. A longer term pervaporation test was set up in whichthe same membrane couponwas subsequently contacted with ABEmodel mixtures with or without extra components. The test wasinitiated with a feed solution containing only acetone, n-butanoland ethanol at respective concentrations of 4.5, 9.0 and 1.5 g l�1 inwater (Fig. 9). After stabilization of the membrane flux, a plateau isreached around 3.6 kg m�2 h�1. This initial flux decline is attrib-uted to the reorganization of the free volume of the PTMSPmembrane in the swollen state (relaxation), leading to a partialloss of this free volume or eventually a partial loss of the freevolume interconnectivity. After redistribution, an equilibrium isreached. This initial flux decrease can additionally be assigned to apartial adsorption of n-butanol on the free volume cavity walls,leading to a decrease of the available free volume and a slowingdown of the permeants diffusion. It has been reported that thisearly flux decrease might be avoided through pre-conditioning ofthe membrane in a feed mixture, allowing the membrane to reachits equilibrium prior to the start of the measurements. Fadeev et al.reported the influence of such a conditioning on the pervaporationperformances of a PTMSP membrane [11]. They mentioned thatconditioning the PTMSP membrane in an alcohol/water mixturewith a composition similar to the pervaporation feed could help inachieving better membrane performance.

The exchange of the feed for an ABE mixture additionallycontaining acetic acid at a concentration of 5 g l�1 has no significantimpact on the flux. After approximately 50 h of testing, an ABEsolution containing 5 g l�1 of butyric acid was applied, leading to adecline in flux of 25% down to approximately 2.4 kg m�2 h�1. Thisflux is maintained further on when the membrane is exposed to anABE mixture containing 0.25 g l�1 of L-asparagine. A strongdecrease (about 45%) of the flux is further observed through theaddition of 0.25 g l�1 of stearic acid salt, down to 1.25 kg m�2 h�1.Upon subsequent testing on an ABE mixture containing 0.25 g l�1 ofpalmitic acid salt, the flux remains at about the same level. Finally,the membrane was again contacted with the initial synthetic ABEreference mixture. No flux increase is observed; on the contrary, thefinal flux, after 200 h of testing is approximately 0.8 kg m�2 h�1.This test indicates that the most severe fouling of the preparedsupported PTMSP–silica membranes, in terms of flux, occurs when-ever butyric acid and stearic acid are present in the feed mixtureamong the studied foulants.

The butanol/water separation factors were assessed duringthese experiments. The results presented in Fig. 9 correspond tothe separation factors observed when the total flux has stabilized.At the start of the experiment, a butanol/water separation factor of40.0 is achieved. This remains above 20 during approximately140 h. A decrease is then observed, reaching a value of 10.6 after200 h of pervaporation and after having subjected the membraneto different feed mixtures. Probably, the adsorption of foulants onthe membrane surface leads to a modification of the interface

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143 141

organophilicity and has an influence on the affinity of the feedconstituents for the membrane. Internal fouling might also takeplace, increasing the impact on the diffusivity of the penetrants.While both butyric acid and stearic acid impact negatively the fluxthrough the PTMSP membrane, the presence of stearic acid is evenmore detrimental. Indeed, the butanol/water separation factor isdrastically decreased with this long chain fatty acid, what is notthe case for butyric acid, where an increase of the butanol/waterseparation factor is observed.

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140 160 180

But

anol

/wat

er se

para

tion

fact

or

Time (h)

Fig. 11. Butanol/water separation factor of commercial PDMS membrane andsupported PTMSP–silica membrane with and without nanofiltration pre-treatmentwith Desal-5 DK spiral wound membrane; the average separation factor is given.

3.4. Nanofiltration pre-treatment

A commercial PDMS membrane and a supported PTMSP–silicamembrane have been tested on a real fermentation broth using anin-house developed fermentation–pervaporation set-up where themembranes are directly coupled to a continuous two-stage ABEfermentation. While the pervaporation flux of the PDMS mem-brane remains constant around 0.37 kg m�2 h�1, the flux obtainedwith the filled PTMSP decreases drastically from 0.39 to approxi-mately 0.10 kg m�2 h�1 (Fig. 10).

Flux decline for filled or unfilled PDMS and PTMSP membraneshas been reported by different research groups. To remedy thislack of stability, solutions such as pH modification for aciddissociation [26,27] for model mixtures or off-line cleaning ofthe membrane [11,14] have been reported. But these approachesare not the most viable solutions for the ISPR of n-butanol in afermentation process from an economical point of view.

An alternative solution could be the selective removal of thefouling components prior to the pervaporation step to reduce anycontamination of the PTMSP membrane. For this purpose, theinsertion of an upstream nanofiltration (NF) step has been inves-tigated. To validate this hypothesis off-line nanofiltration trialshave been carried out, first with a flat-sheet membrane and a feedvolume of 4.5 l, and subsequently with a spiral-wound NF mem-brane and a feed of 20 l as described in the experimental part. Inboth cases a cell-free fermentation broth was used as feedmixture. The NF permeate was afterwards used as feed for thepervaporation tests. The flux and separation factors of the PTMSPhave been monitored as a function of time on the pervaporationunit of the ISPR set-up.

Two commercial polyamide flat-sheet NF membranes wereselected: MPF-34 (charged membrane) with a MWCO of200 g mol�1, and Desal-5 DK with a MWCO comprised between150 and 350 g mol�1. In both cases, the retention of stearic acidand palmitic acid is successful (Table 2). Indeed, a retention forstearic acid above 70% is achieved, while palmitic acid is retainedup to almost 90%. In parallel, glucose is also highly retained by

Table 2Concentrations in mg l�1 before and after the nanofiltration of a cell-free fermentatioannotations, Section 2.6); SW: spiral wound.

Initial flux (l m�2 h�1) Acetic acid Butyric acid Glucose

Desal-5DK(a) 32 1687 809 887(b) 1042 556 273(c) 1278 750 1131

MPF-34(a) 28 1468 762 829(b) 445 177 52(c) 1382 808 929

SW Desal-5DK(a) 65 1126 776 6295(b) 1438 855 780(c) 1302 835 59,003

n High incertitude on the measurement.

both membranes (69% for the Desal-5 DK, and 94% for the MPF-34). The charged membrane, MPF-34, retains to a higher extentacetic acid (70%) and butyric acid (77%) than Desal-5DK (38% and31% respectively), but unfortunately, retains also the ABE compo-nents. For this reason, Desal-5 DK has been selected for up-scalingwith a spiral-wound module. Similar results were observed(Table 2), namely the retention of the long chain fatty acids(higher than 70% retention) and the glucose (88%) in the feed,leading to a transparent, colorless permeate (yellow coloration inthe feed). In parallel, enrichment of the permeate in acetic acidand butyric acid is observed (negative retention), at the oppositeof the flat-sheet membrane. No explanation has been found yet forthis phenomenon. This permeate has been used afterwards as feedfor a pervaporation test with a filled PTMSP membrane using thepervaporation unit of the integrated ISPR set-up. It is important tonotice, as the trials were carried out off-line, that the solventconcentration in the pervaporation feed was not maintainedconstant. A depletion in acetone, n-butanol and ethanol was takingplace during the course of the experiment through the permeationof the ABE components, which had an influence on the flux(Fig. 10). Indeed, as the PTMSP membrane is hydrophobic, a fluxdecline is expected as the concentration of organics decreases as afunction of time. However, the implementation of a nanofiltrationstep enables a significant increase of the flux through the PTMSPmembrane. For a similar pervaporation time, a flux increase by afactor 4 is observed. Furthermore, in case of a continuous set-upwhereby the nanofiltration and pervaporation units are coupled inseries to the fermentation reactor vessel, it is believed that afurther improvement can be achieved.

Since the aim of this research is the in-situ n-butanol recovery,attention has been paid on the n-butanol/water separation factor(Fig. 11). Fluctuations are observed, which can be explained, especially

n broth through polyamides NF membranes at 25 1C (see experimental part for

Acetone Butanol Ethanol Stearic acid Palmitic acid

4826 9561 1118 0.048 0.0993112 7386 1007 o0.01 o0.014204 9072 1140 0.053 0.093

5474 10,176 1200 0.035 0.0721065 2165 668 o0.01 o0.013957 9832 1256 0.037 0.094

5623 8707 5214 n 0.1055587 9007 5239 o0.03 o0.035344 9074 4980 n 0.159

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143142

in the case of the off-line trials, by the non-steady-state conditions ofthe feed formulation. In this case also, no particular correlation ispresent between the flux and the separation factor. The separationfactors have been calculated as an average over the whole pervapora-tion period. The average n-butanol/water separation factors of thePTMSP membranes before and after a pre-nanofiltration step aresimilar, and lie respectively around 24.0 and 27.5, much higher thanfor the commercial PDMS membrane (14.7) studied under the sameconditions. No comparison with literature data can be made forPTMSP membranes since most of the research published in thescientific literature has been focused on model ethanol/water mix-tures, and/or on the study of PDMS membranes. Fadeev et al. [10]have reported a butanol/water separation factor of 40–70 but lowerfluxes (0.075–0.16 kg m�2 h�1) for model solutions with a butanolconcentration up to 6 wt%. Chen et al. studied the ABE fermentationin a continuous and closed-circulating fermentation system withcomposite PDMS membrane [28]. Fluxes of 0.78 (0.024 m2 surfacemembrane area) and 0.57 kg m�2 h�1 (0.08 m2 surface membranearea) were observed, with butanol/water separation factors of 10.03and 7.02 respectively. No data concerning the continuous long-termpervaporation of a real fermentation broth with a PTMSP membranehas been published to our knowledge.

4. Conclusions

The ageing of supported PTMSP membranes through structuralchanges and elemental composition has been studied as a functionof time. While no ageing is observed when the membrane isimmersed in an aqueous ethanol solution or a synthetic acetone–butanol–ethanol mixture, oxidation is taking place when themembrane is stored under atmospheric temperature and increasedexposure to UV radiations leading to the formation of carbonylgroups through interactions with the CQC double bond of thePTSMP backbone. When the PTMSP membrane is stored in a realfermentation broth, a modification of the membrane is observedalready in the first weeks. Adsorption, as well as absorption offouling components, is taking place at the membrane surface and inthe membrane bulk. Washing of the membrane or immersion in analcohol solution does not allow the full regeneration of the PTMSPmembrane.

In order to get more insights into the role of some potentialfouling agents, known to be present in ABE broth, off-line pervapora-tion tests have been carried out with model mixtures of increasingcomplexity. These tests revealed the predominant role of butyric acidand long chain fatty acids on the flux decline of supported PTMSP–silica membranes. While stearic acid additionally leads to a drop ofthe butanol/water separation factor from 24.7 down to 14.4, thepresence of butyric acid seems to favor the pervaporation of butanolwith an increase of its separation factor up to 46.8.

To remedy the loss of performance of the prepared PTMSPmembranes through fouling during the in-situ n-butanol recoveryfrom an ABE fermentation broth, an off-line pre-nanofiltrationstep using commercial polyamide-based membranes has beenimplemented prior to the pervaporation step. A significantincrease of the membrane flux has been obtained (factor 4), withan n-butanol/water separation factor of 27.5. Implementation of ananofiltration pre-treatment step prior to the pervaporation pro-cess results in comparable fluxes for the PTMSP and the commer-cial PDMS membranes; however the selectivity of the PTMSPmembrane is markedly superior to the PDMS membrane (14.7).

The next step will be the direct coupling of a nanofiltration unitin-line with the ISPR system, which is believed to further enhancethe overall performance.

Acknowledgments

The Flemish Government is acknowledged for funding thiswork in the framework of the Membrane based Product Recovery(MemProRec) project.

References

[1] Y. Ni, Z. Sun, Recent progress on industrial fermentative production ofacetone–butanol–ethanol by Clostridium acetobutylicum in China, AppliedMicrobiology and Biotechnology 83 (2009) 415–423.

[2] L. Vane, A review of pervaporation for product recovery from biomassfermentation processes, Journal of Chemical Technology and Biotechnology80 (2005) 603–629.

[3] A. Oudshoorn, L. van der Wielen, A. Straathof, Assessment of options forselective 1-butanol recovery from aqueous solutions, Industrial and Engineer-ing Chemistry Research 48 (2009) 7325–7336.

[4] P. Shao, R. Huang, Polymeric membrane pervaporation, Journal of MembraneScience 287 (2007) 162–179.

[5] B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organic-organic mixtures by pervaporation – a review, Journal of Membrane Science241 (2004) 1–21.

[6] K. De Sitter, P. Winberg, J. D'Haen, C. Dotremont, R. Leysen, J.A. Martens,S. Mullens, F. Maurer, I. Vankelecom, Silica-filled poly(1-trimethylsilyl-1-propyne) nanocomposite membranes: relation between the transport of gasesand structural characteristics, Journal of Membrane Science 278 (2006) 83–91.

[7] S. Claes, P. Vandezande, S. Mullens, R. Leysen, K. de Sitter, A. Andersson,F. Maurer, H. Van den Rul, R. Peeters, M. Van Bael, High flux composite PTMSP-silica nanohybrid membrane for pervaporation of ethanol/water mixtures,Journal of Membrane Science 351 (2010) 160–167.

[8] S. Claes, P. Vandezande, S. Mullens, K. De Sitter, R. Peeters, M. Van Bael,Preparation and benchmarking of thin film supported PTMSP-silica perva-poration membranes, Journal of Membrane Science 389 (2012) 265–271.

[9] A. Fadeev, S. Kelley, J. McMillan, Y. Selinskaya, V. Khotimsky, V. Volkov, Effectof yeast fermentation by-products on poly[1-(trimethylsilyl)-1-propyne] per-vaporative performance, Journal of Membrane Science 214 (2003) 229–238.

[10] A. Fadeev, Y. Selinskaya, S. Kelley, M. Meagher, E. Litvinova, V. Khotimsky,V. Volkov, Extraction of butanol from aqueous solutions by pervaporationthrough poly(1-trimethylsilyl-1-propyne), Journal of Membrane Science 186(2001) 205–217.

[11] A. Fadeev, M. Meagher, S. Kelley, V. Volkov, Fouling of poly[-1-(trimethylsilyl)-1-propyne] membranes in pervaporative recovery of butanol from aqueoussolutions and ABE fermentation broth, Journal of Membrane Science 173(2000) 133–144.

[12] P. Peng, B. Shi, Y. Lan, A review of membrane materials for ethanol recovery bypervaporation, Separation Science and Technology 46 (2011) 234–246.

[13] A. Aroujalian, K. Belkacemi, S. Davids, G. Turcotte, Y. Pouliot, Effect of residualsugars in fermentation broth on pervaporation flux and selectivity for ethanol,Desalination 193 (2006) 103–108.

[14] G. Liu, W. Wei, H. Wu, X. Dong, M. Jiang, W. Jin, Pervaporation performance ofPDMS/ceramic composite membrane in acetone butanol ethanol (ABE)fermentation-PV coupled process, Journal of Membrane Science 373 (2011)121–129.

[15] R. Offeman, C. Ludvik, Poisoning of mixed matrix membranes by fermentationcomponents in pervaporation of ethanol, Journal of Membrane Science 367(2011) 288–295.

[16] S. Claes, P. Vandezande, S. Mullens, P. Adriaensens, R. Peeters, F. Maurer,M. Van Bael, Crosslinked poly[1-(trimethylsilyl)-1-propyne] membranes:characterization and pervaporation of aqueous tetrahydrofuran mixtures,Journal of Membrane Science 389 (2012) 459–469.

[17] W. Van Hecke, P. Vandezande, S. Claes, S. Vangeel, H. Beckers, L. Diels, H. DeWever, Integrated bioprocess for long-term continuous cultivation of Clostri-dium acetobutylicum coupled to pervaporation with PDMS composite mem-branes, Bioresource Technology 111 (2012) 368–377.

[18] W. Van Hecke, T. Hofmann, H. De Wever, Pervaporative recovery of ABE duringcontinuous cultivation: enhancement of performance, Bioresource Technology129 (2013) 421–429.

[19] K. De Sitter, A. Andersson, J. D’Haen, R. Leysen, S. Mullens, F. Maurer,I. Vankelecom, Silica filled poly(4-methyl-2-pentyne) nanocomposite mem-branes: similarities and differences with poly(1-trimethylsilyl-1-propyne)–silica systems, Journal of Membrane Science 321 (2008) 284–292.

[20] S. Tsarkov, V. Khotimsky, P. Budd, V. Volkov, J. Kukushkina, A. Volkov, Solventnanofiltration through high permeability glassy polymers: effect of polymerand solute nature, Journal of Membrane Science 423–424 (2012) 65–72.

[21] T. Masuda, E. Isobe, T. Higashimura, Polymerization of TMSP by halides ofniobium V and tantalum V and polymer properties, Macromolecules 18 (1985)841–845.

[22] K. Nagai, T. Masuda, T. Nakagawa, B. Freeman, I. Pinnau, PTMSP and relatedpolymers: synthesis, properties and functions, Progress in Polymer Science 26(2001) 721–798.

[23] K. Nagai, B. Freeman, A. Hill, Effect of physical aging of poly(1-trimethylsilyl-1-propyne) films synthesized with TaCl5 and NbCl5 on gas permeability, fractional

M.F.S. Dubreuil et al. / Journal of Membrane Science 447 (2013) 134–143 143

free volume, and positron annihilation lifetime spectroscopy parameters,Journal of Polymer Science Part B: Polymer Physics 38 (2000) 1222–1239.

[24] R. Gallo, M. Pegoraro, F. Severini, S. Ipsale, E. Nisoli, Degradation of outdoorexposed poly(1-trimethylsilyl)-1-propyne, Polymer Degradation and Stability58 (1997) 247–250.

[25] T. Bowen, R. Meier, L. Vane, Stability of MFI zeolite-filled PDMS membranesduring pervaporative ethanol recovery from aqueous mixtures containingacetic acid, Journal of Membrane Science 298 (2007) 117–125.

[26] L. Vane, V. Namboodiri, R. Meier, Factors affecting alcohol–water pervapora-tion performance of hydrophobic zeolite-silicone rubber mixed matrix mem-branes, Journal of Membrane Science 364 (2010) 102–110.

[27] S. Chovau, S. Gaykawad, A. Straathof, B. Van der Bruggen, Influence offermentation by-products on the purification of ethanol from water usingpervaporation, Bioresource Technology 102 (2011) 1169–1674.

[28] C. Chen, Z. Xiao, X. Tang, H. Cui, J. Zhang, W. Li, C. Ying, Acetone–butanol–ethanol fermentation in a continuous and closed-circulating fermentationsystem with PDMS membrane bioreactor, Bioresource Technology 128 (2013)246–251.