Electrochemical preparation of polypyrrole membranes and their application in ethanol-cyclohexane separation by pervaporation

j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 108 (1995) 89-96

Electrochemical preparation of polypyrrole membranes and their application in ethanol-cyclohexane separation by pervaporation Ming Zhou a, Michel Persin a, Wojciech Kujawski b, Jean Sarrazin a,. "Laboratoire des Mat~riaux et Proc~dds Membranaires, UMR CNRS 9987, ENSCM, 8, rue de l'Ecole Normale, 34053 Montpellier Cedex 1, France b Faculty of Chemistry, iV. Copernicus University, 7, Gagarin Street, 87-100 Torun, Poland

Received 25 October 1994; revised 30 May 1995; accepted 31 May 1995

Abstract Polypyrrole films were deposited on stainless steel meshes by anodic electropolymerization of pyrrole dissolved in acetonitrile. Established on the electrochemical and morphological studies on the growth of polypyrrole film, both the oxidized, with PF~ as counter-ion, and neutral polypyrrole membranes were obtained. The performances of these membranes towards ethanolcyclohexane separation by pervaporation were investigated. Results indicate preferential permeation of ethanol and clearly show a feasibility of exploiting conducting polymers in the pervaporation process. Keywords: Conducting polymer; Polypyrrole; Pervaporation

1. I n t r o d u c t i o n Polyheterocycles form an interesting class of electrosynthesized conducting polymers, among which polypyrrole (PPy), polythiophene (PTh) and their derivatives have been widely and deeply investigated [ 1-3 ]. The wide spectra of their applications proposed [4] relate to the electrical conductivity, switching property (insulator-conductor transition), electrochromic property, etc. Membrane-relatedapplications, although few, have also been paid attention for over a decade [ 5 ]. Recently, research interests are being increasingly directed towards the application of a conducting polymer as a permselective membrane for ion transport [6-9], neutral solution species transport [10,11] , and even for gas separation [12-15] andpervaporation [ 15,16]. * Corresponding author, 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0376-7388 (95)00145-X

Two approaches are generally used in the preparation of conducting polymers, i.e. the chemical and electrochemical syntheses. The electrodeposition of conducting polymer has been proved to be an easy way to prepare an adherent film on metal supports or a freestanding film after it has been peeled off. Polymers obtained by anodic oxidation show an oxidized cationic state. Electrical neutrality is maintained by the incorpotation of anions from the supporting electrolyte during the electropolymerization and deposition. The doping anions greatly influence the conductivity, morphology, and no doubt, the other properties of the polymeric deposit. The choice of the anions available both in aqueous and in aprotic solvents is large, and substitution of anion by soaking conducting polymer in salt solution with other anion turned out to be possible [ 17]. Therefore, besides the easiness in film fabrication, there is also a possibility of changing properties over a great range, after the film has been fabricated or

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M. Zhou et al. / Journal of Membrane Science 108 (1995) 89-96

even as it is used in situ. In view of that, we propose to use an electrodeposited polymeric film as a membrane in pervaporation. However, because of the intractable mechanical properties of conducting polymers, a large area free-standing film is not easy to obtain by peelingoff after it is deposited on a polished surface. So it would be advisable to deposit it directly on a suitable porous material that serves both as the anode during the electrosynthesis and as a structural support mechanically acceptable for pervaporation, the result being a composite membrane. Using dense non-porous polymeric films, pervaporation is a membrane technology where the liquid feed mixture is brought into contact with one side of membrane and the permeate, in the form of vapor, is removed either by a sweep gas or creating vacuum at the other side [ 8 ]. The driving force in pervaporation is the partial pressure difference between the upstream and downstream sides. Selectivity is, according to the sorption-~:tiffusion concept [18,19], based on the favorable chemical affinity of the membrane to one of the components in the liquid mixture. Pervaporation is of special interest to separate liquids with an azeotropic composition and mixtures with close boiling points, which are difficult to separate by distillation [20].Until now, most of the research has been directed towards the aqueous-organic system, with highlight on the water removal from ethanol. Recently the organicorganic separation has also been receiving more and more attention. A lot of polymers were tested for pervaporation use [ 21,22]. Electrodeposited conducting polymers are known to be insoluble in many organic solvents and are then good candidates as membrane materials for pervaporation of purely organic mixtures, Anodically deposited polypyrrole film was firstly reported by Diaz et al [ 23 ]. After that, much work has been concentrated on this film with respect to the preparation, properties, characterisation, mechanism, electrochemistry and application. By comparison with the electrosynthesis of another promising conducting polymer polythiophene [2], polypyrrole presents the advantage that its oxidation potential is much lower. It can be deposited on oxidizable substrates, such as stainless steel, without any anodic corrosion. Successful depositing of polypyrrole on aluminium and mild steel has already been reported [24]. The present paper is devoted to the electrochemical deposition of PPy films on a stainless steel mesh and

the investigation of the resulting composite membrane towards the separation of ethanol-cyclohexane mixtures by pervaporation.

2. Experimental The present work is divided into three parts in which different equipment and/or procedures were utilised as described below. (1) For the purpose of investigating the electrochemical behavior and the influence of experimental conditions on the morphology of deposit, electropolymerization was performed, unless otherwise noted, in a 4-necked mini-cell (applied volume 10 ml) with three electrode compartments separated by sintered glass. The slip of stainless steel mesh (Gantois), which was woven using 25 and 18/zm stainless steel wires, was directly used as working electrode (apparent depositingarea0.25cm2) without any pretreatment. Platinum gauze and Ag ÷ (0.1 M AgNO3 in CH3CN)/Ag were used as the auxiliary and reference electrodes, respectively. Typical solutions were 0.1 M monomer in the presence of 0.05 moldm-3Me4NBF4or0.1 m o l d m -3 Bu4NBF4 or 0.1 mol dm -3 Bu4NPF 6 in acetonitrile without or with 1 wt% water. Acetonitrile (SDS, purex. >99.5%, water content _<0.05%), pyrrole (Fluka, purum, >96%, ref. no. 83220), Me4NBF4 (Fluka, purum, ref. no. 87747), Bu4NBFa (Fluka, purum, p.a., ref. no. 86873) and Bu4NPF6 (Fluka, puriss, p.a., ref. no. 86879) were used as received without further purification. (2) Using larger sheets (disks of Q52 mm) of the above-mentioned mesh, preparative electrodeposition of polypyrrole films designed for the pervaporation test was then performed in a beaker containing 0.1 M pyrrole + 0.1 M Bu4NPF6 + 1 wt% H20 in acetonitrile at constant potential of 1.0 V with depositing period of time 20 min. All experiments mentioned above were accomplished in the laboratory atmosphere at room temperature. The apparatus for electrochemical control and measurement were EG&G Princeton Applied Research model 362 scanning potentiostat and Kipp & Zonen XY recorder with a time-Y recording capability. Two modes, namely, cyclic voltammetric and potentiostatic techniques, were applied in our experiments.

M. Zhou et a l . / Journal of Membrane Science 108 (1995) 89-96

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3. Results and discussion

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After the deposits were obtained, they were rinsed thoroughly with the acetonitrile solvent in order to remove residues of the electrolyte. ForSEM(scanning electron micrography) surface morphology observation, samples were dried by directly exposing to air. The instrument used was a Leica Steroscan 260. Fractography was obtained by fracturing only the polymer layer in liquid nitrogen. (3) Vacuum pervaporation experiments were performed using a GFT laboratory equipment schematically presented in Fig. 1. A stainless steel crossflow cell with an effective membrane area of 10 cm 2 was used. The membrane was clamped in the cell in such a way that the polymer layer of the composite membrane faced the liquid feed mixture, The feed mixture at temperature of 40°C was kept continuously circulated by a feed pump. The vacuum at the permeate side was maintained at about 1 mbar during the experiments. The permeate was collected once a stationary state of permeation was attained, which took about 1.5 h. The permeate flux and the permeate composition were determined for each feed concentration. The composition of the permeate was analysed by gas chromatography. Two types of PPy membranes, namely, the oxidized membrane with PF6 counter-ions (denoted as PPy-PF) and the reduced membrane (denoted as PPy-R), were tested under the same conditions, Ethanol-cyclohexane mixtures in the concentration range of 0 to 100 wt% of ethanol were used as feed. The ethanol-cyclohexane mixture is an azeotropic systern that forms one phase over the entire concentration range. The distillation azeotropic composition for this system is 30.5 wt% of ethanol and the corresponding boiling point is 64.9°C at atmospheric pressure [ 25 ].

PPN m e m b r a n e s For having a basic understanding of the electropolymerization of pyrrole on the stainless steel meshes, and in order to determine the experimental conditions leading to the most suitable material for our purpose, a preliminary electrochemical study was performed with small anodes. Different parameters were investigated by means of cyclic voltammetry [26], such as the nature of supporting electrolytes and the effect of water in the acetronitrile solvent. Cyclic voltammograms (CVs) shown in Fig. 2 are related to the use of Bu4NPF6 as supporting electrolyte with a stainless steel mesh as the anode. Those illustrate that all the potential scans lead to the formation of polymer deposit on the fibre surface. During scanning, "nucleation loops" [27,28] were observed, particularly in the first scan, when the scan was reversed at the end of the forward sweep. The oxidation of pyrrole on the mesh surface begins at about 0.8 V and subsequent polymerization and deposition on the polypyrrole-covered stainless steel mesh occurs at a potential a little lower than 0.8 V. However, the supporting electrolytes and the addition of water influence the reactions on the electrode. In Fig. 2, the effect of water, which leads to higher current intensities and thicker films, is also clearly displayed. Aiming at obtaining a dense film of good adhesion to the substrate, preparation of PPy in these solutions were then carried out at constant potential of 1.0 V. Morphological investigation repeatedly proved that all the systems are not suitable for a compact deposition meeting our requirements. Some conditions will generate loosely agglomerated, porous deposition, and some will give rise to cracks on the deposit. In the case of adding water, more deposit can be obtained, which is consistent with the current-time transients (Fig. 3), since larger currents can be found in the case of the addition of water. Besides, samples prepared in the presence of water exhibit smoother surface morphology. So, as found by other researchers [ 29 ] in the case of Pt electrode, a certain amount of water has a favorable effect on the electropolymerization and the growth of deposit also on the present electrode. Despite the relatively rough surface of PPy, the pyrrole-Bu4NPF6-

92

M, Zhou et al. / Journal of Membrane Science 108 (1995) 89-96

mA

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H 2 0 s y s t e m s e e m s to g i v e a d e n s e d e p o s i t w i t h less b u m p y s u r f a c e . E v e n t u a l l y , this s y s t e m w a s s e l e c t e d f o r t h e p r e p a r a t i o n o f P P y m e m b r a n e s , as it w a s d e s c r i b e d in t h e e x p e r i m e n t a l s e c t i o n . F r a c t o g r a p h y ( F i g . 4 ) i n d i c a t e s t h e t h i c k n e s s o f ca. 8 /xm a n d n o e v i d e n c e o f p o r e s in t h e b u l k . R e s u l t s o f e l e m e n t a l analysis of the deposit demonstrate a carbon and hydrog e n - r i c h f o r m u l a o f C4.2H3.67 N 1.o(PF6) o.21. T h i s is c o n s i s t e n t w i t h t h e o t h e r c o n d u c t i n g p o l y m e r s s u c h as polypyrene, polyfluorene, polyfluoranthene and polytriphenylene [ 30], and the PPy deposition on different s u p p o r t s [ 2 4 ] as well. A s it w a s a l w a y s f o u n d , this does not match the theoretical stoichiometric formula

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Fig. 4. SEM fracture observation of polypyrrole membrane electrodeposited on the stainless steel mesh (0.1 M pyrrole +0.1 M Bu4NPF 6 + 1 wt.% H20 in acetonitrile at constant potential of 1.0 V with a deposition period of 20 min).

M. Zhou et al./Journal of Membrane Science 108 (1995) 89-96

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93

separation diagram, permeation flux as well as the fO1lowing parameters [ 32-34]. The overall separation factor a:

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The enrichment factor/3: of CaH3N l ( P F 6 ) x according to the generally accepted mechanism [31]. The slight excess of C and H are considered as a result of possible reaction between the polymer in the oxidized form and the alkyl radicals produced at the cathode, resulting in the formation of alkyl terminating groups. On the other hand, it is also possible that the supporting electrolyte can be trapped in the matrix of polymer during its growth. In this case, the experimental formula canbe rewritten in three parts as follows, (CnH3N)0.775 q_ (CaH3NPF6)o.188 + ( ( C 4 H 9 ) 4NPF6)0.022.

This formula corresponds to a stoichiometry of ca. 0.2 anions per repeating pyrrole unit. After electropolymerization at 1.0 V, the deposit is obtained in its oxidized state with PF6 as counter-ion in it for electroneutrality of the material. The chemical structure of the oxidized polypyrrole is presented in Fig. 5. By applying a negative potential of - 0 . 6 V (see Fig. 2(B) ) on the anodically deposited material, the oxidized PPy can be reduced to, or more precisely saying, nearly to its neutral state. Elemental analysis indicates that less counter-ions exist in the reduced polymer [24], whereas SEM observation does not reveal any perceptible morphological differences between the two states. The detailed investigation and discussion on the material and its preparation are beyond the scope of the present paper. The more comprehensive results about the electrochemical and morphological studies of PPy and some other conducting polymer systems electrosynthesized on the stainless steel meshes will be published separately,

3.2. Performance ofpolypyrrole membranes in pervaporation The performance in pervaporation of investigated membranes was characterized by the McCabe-Thiele

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0A = (3) flA' CA where C is the weight fraction of component A or B in the feed mixture; C' the weight fraction of component A or B in the permeate; JA the permeation flux of component A in a binary mixture; J0A the permeation flUX of the pure component A. Both the separation factor a and the enrichment factor/3 represent the membrane selectivity in pervaporation process, c~is always chosen in such a way that its value is greater than unity. Thus if component B preferentially permeates the membrane, the separation factor is then denoted as CtB/A. If no separation happens the separation factor is equal to unity. The permeation ratio 0A describes the coupling effects occurring during the transport of a binary mixture through the membrane [34,35]. When 0A > 1 the permeation of component A is enhanced by the presence of the other component, whereas when 0A < 1, the permeation of component A is retarded by the other one. When the system exhibits ideal permeation behavior, 0A is equal to unity. The separation diagram for ethanol-cyclohexane mixtures with the two membranes used is presented in Fig. 6. It shows preferential permeation of ethanol over the entire concentration range. For high concentrations of ethanol in feed mixture both membranes show almost the same selectivity. But when the feed mixture is getting poorer in ethanol, e.g. lower than 20%, a higher selectivity is observed with the oxidised PPyPF membrane: the enrichment factor/3EtOH for PPy-PF membrane is then in the range 3-16 whereas flEtOHfor PPy-R membrane does not exceed 6 in the same concentration range (Fig. 7). Comparing the pervaporation to separation based on the liquid-vapor equilibrium [36] (Fig. 6), one can state that pervaporation with such membranes is an efficient technique to separate the ethanol-cyclohexane mixtures.

94

M. Zhou et al. / Journal of Membrane Science 108 (1995) 89-96

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Interactions between polypyrrole and organic molecules such as methanol has been recently investigated, which showed that electron exchange between the organic molecule and the polymer can occur [37,38], but no correlation with the transport properties has been established until now. Nevertheless, in order to rationalise our results, one hypothesis can be proposed: ethanol molecules due to their higher polarity and smaller molar volume (Table 1) could be transported prefer-

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20 40 60 80 100 Ethanol content in feed mixture Fig. 6. Separation diagram of ethanol-cyclohexanemixtures by pervaporation using polypyrrole membranes: PPy-PF oxidized state; PPy-R reduced state, 20

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entially through PPy-PF membrane and their transport would occur most likely along the charged sites probably by a push-pull mechanism [ 34,39 ]. On the other hand the cyclohexane molecules would be transported through the amorphous regions, away from the charged sites, of PPy-PF membrane, according to the solutiondiffusion mechanism [40,41 ]. The reduced PPy-R membrane, at low concentration of ethanol, shows much higher affinity to cyclohexane than PPy-PF membrane although the former is still selective to ethanol (Fig. 6 and Fig. 8). This suggests that in the PPy-R membrane transport of cyclohexane and ethanol molecules could occur through the entire 100

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Ethanol content in feed mixture (wt.%) Fig. 7. Enrichmentfactors of the membranes for ethanol-cyclohexane mixtures: PPy-PFoxidized state; PPy-Rreduced state. The total permeation fluxes shown in Fig. 8 indicate that the PPy-R membrane is much less permeable than the PPy-PF. The difference in membranes behaviours can be related to several possible factors such as the oxidation state of polypyrrole chain, the presence of the counter-anion, different void volumes or even bulk morphologies though our SEM observations did not reveal any change after the membrane was reduced. Moreover, the influence of the presence of the permeant species on the membrane structure in our experiments was not established.

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Ethanol content in feed mixture (wt.%) Fig. 8. Pervaporationflux of ethanol-cyclohexanemixtures through the oxidized (PPy-PF) and reduced (PPy-R) membranes. Table 1 Molar volumeand polar properties of ethanol and cyclohexane(data cited from [25] ) Molar volume Dielectric (cm3/mol) constant(-) Ethanol 58.4 Cyclohexane 108.1

24.3 2.0

Dipolar moment (D) 1.69 0

M. Zhou et al. / Journal of Membrane Science 108 (1995) 89-96

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wider explored. Other systems involving electrochemical preparation of conducting membranes and other organic-organic mixtures separations are under investigation at our laboratory.

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Ethanol content in feed mixture (wt.%) Fig. 9. Permeation ratios of ethanol and cyclohexane through the oxidized (PPy-PF) membrane,

amorphous matrix by the solution-diffusion mechanism. The concentration dependencies of the permeation ratios [Eq. (3) ] of ethanol and cyclohexane for PPyPF (Fig. 9) describe the range of the flux coupling. During the transport of ethanol-cyclohexane mixtures through PPy-PF (Fig. 9) 0EtOHis close to unity within the whole concentration range, which suggests that transport of ethanol is not influenced by the cyclohexane molecules, but on the contrary, retards the transport of cyclohexane within the whole concentration range ( 0 c y c l o h e x a n e < 1 ). Such results are consistent with the above proposed mode of cyclohexane-ethanol t r a n s p o r t through oxidized PPy membranes.

4. Conclusions The present study shows the feasibility of depositing

nonporous films of the oxidized and the reduced polypyrrole on oxidizable stainless steel meshes by electropolymerization, T h e performance of pervaporation of ethanol-cyclohexane mixtures revealed a selectivity of polypyrrole membranes for ethanol. The oxidation state of the polypyrrole influences the membrane performance: w i t h respect to the permeation rate, the oxidized polypyrrole membrane appears more promising; as for the selectivity, differences between these t w o s t a t e s are observed at l o w a l c o h o l f e e d .

Attempts of applying the nonporous polypyrrole films to the pervaporation process proved that conductive polymer membranes are worth being deeper and

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