Solvent-resistant nanofiltration with filled polydimethylsiloxane (PDMS) membranes

Journal of Membrane Science 278 (2006) 199–204

Solvent-resistant nanofiltration with filled polydimethylsiloxane (PDMS) membranes Lieven E.M. Gevers, Ivo F.J. Vankelecom ∗ , Pierre A. Jacobs Centre for Surface Chemistry and Catalysis, Faculty of Bioengineering Sciences, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium Received 5 September 2005; received in revised form 17 October 2005; accepted 28 October 2005 Available online 7 December 2005

Abstract The separation of solutions containing non-polar solvents, like toluene, was found to be limited for PDMS membranes, which is due to the extensive swelling of PDMS in those solvents. To improve the performance of silicone-based membranes for separations in non-polar solvents, extra cross-linking was introduced via the introduction of fillers. Of all fillers tested, zeolites were most efficient in decreasing the swelling of the PDMS network. Zeolites induce an increased cross-linking density, which is explained by the interactions between the silanol-groups of the filler and the PDMS-chains. The performance in SRNF of filled and unfilled PDMS membranes were compared, demonstrating increased solute rejection in non-polar solvents and at higher temperatures upon filler addition. © 2005 Elsevier B.V. All rights reserved. Keywords: Fillers; Zeolites; PDMS; Swelling; Solvent-resistant nanofiltration

1. Introduction A challenge in nanofiltration and reverse osmosis is to broaden the range of applications for water treatment towards applications in the chemical industry. Among others, this requires solvent-resistant membranes that preserve their separation characteristics in chemically more aggressive conditions. Silicone rubber is chemically and thermally stable, but, like most rubbers, tends to swell in solvents, especially in non-polar solvents. The use of strongly swelling solvents will result in the increase of the free space between the polymer chains. This promotes the less selective convective solute transport over the diffusive transport, which is slower [1]. This convective transport can be considered as the solute being ‘dragged’ by the moving solvent front. To overcome this problem, swelling must be reduced. Several solutions have been proposed in literature. One is the use of halogen-substituted silicone rubber [2] and another is the extra cross-linking of the silicone rubber, for instance via plasma treatment [3].

∗

Corresponding author. Tel.: +32 16 321594; fax: +32 16 321998. E-mail address: [email protected] (I.F.J. Vankelecom).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.10.056

This work will report on the addition of fillers in the silicone rubber, which in general leads to the reinforcement of polymers. For silica and zeolites in PDMS, such reinforcement has been explained by adsorption of the polymer chains on the silanolgroups of the filler surface [4,5]. When the filler interacts well with the polymer, the effective cross-linking density increases and the swelling degree of the polymer network decreases [6]. Montmorillonite [7], mica [8], zinc oxide [9], zeolite A [10], ZSM-5, zeolite Y and silicalite [5,11] have thus all been used already to decrease swelling in membranes. The zeolite-filled PDMS membranes have already been used in gas separation and pervaporation. Filled membranes have so far not yet been applied in SRNF. In first instance, a screening was done of different filler types and their suitability for incorporation in PDMS was tested via swelling experiments on unsupported filled films. A good interaction at the filler–polymer interface was considered crucial in this respect. Three types of sillers were tested in this work, namely silica, carbon fillers and zeolites. Besides reinforcement of the PDMS network, the use of zeolites could offer an additional advantage in SRNF: their pores are wide enough (0.55 nm in the case of ZSM-5 zeolites) to allow transport most of the solvents, while solutes of a certain size (>0.55 nm) cannot pass them. This is a similar principle as described

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for the use of zeolite-filled membranes in pervaporation [11]. Finally, a number of filtration experiments have been carried out to investigate how the filler affects the separation performance of PDMS in strongly swelling solvents. 2. Experimental 2.1. Materials The PDMS (RTV-615 A and B) and the adhesion promotor (SS 4155) were obtained from General Electric Corp. (USA). Component A contains a prepolymer with vinyl groups. Component B has hydrosilyl groups and acts as cross-linker. Hexane was found to be the most suitable solvent for use in the solventcoating procedure. ZSM-5 (CBV-3002, CBV-2802) and USY (CBV-780) were supplied by PQ-corporation and dried at 110 ◦ C before use. Printex G, SB 100 and Aerosil 380 were provided by Degussa. Hi-Sil 233D was provided by PPG Industries. Hi-Sil 233D was silylated by reaction with N-methyl-N(trimethylsilyl)trifluoroacetamide (MSTFA) in toluene at room temperature for 24 h. The PAN support was kindly provided by VITO, Belgium. MPF-50 was supplied by KOCH Membrane Systems. 2.2. Membrane preparation 2.2.1. PDMS composite membrane A PDMS solution (RTV A/B = 10/1) was prepolymerised for 1 h at 60 ◦ C. The PAN support was saturated with water and taped to a glass or stainless steel plate. The excess of water was wiped away with a tissue and the support was treated with the adhesion promotor, before coating the PDMS solution on the impregnated support. The plate was tilted to a defined angle and the polymer solution was poured over the PAN support. After evaporation of the hexane, cross-linking was completed in a vacuum oven at 110 ◦ C. 2.2.2. Filled PDMS composite membrane The filler was dispersed in hexane. To improve the dispersion, a treatment of 1 h in an ultrasonic bath was applied to break crystal aggregates. The cross-linker was added to the zeolite dispersion and this mixture was stirred for 2 h at 40 ◦ C. Finally, the prepolymer (RTV A) was added and the mixture was stirred for another hour at 60 ◦ C. The filler fraction is expressed in weight percent. The filler content in the membrane is kept constant at 30 wt%. The PDMS/filler solutions were coated the same way as described above.

2.3. Swelling experiments Dried pieces of PDMS slabs were weighed and immersed in the solvent. After equilibrium, the membranes were quickly wiped with a tissue to remove the solvent from the external surface before weighing. The additional weight of the membrane was recalculated to the amount of solvent sorbed (ml) per gram membrane. The obtained swelling values had a deviation of less than 10% on an average of three samples measurements per membrane. S (cm /g) = 3

meq −m0 m0

ρs

2.4. Filtration experiments Filtrations were done in a stainless steel dead-end pressure cell with 15.2 cm2 membrane area. The feed solution was poured in the cell, the cell was heated to the desired temperature and subsequently pressurised with nitrogen to 15 × 105 Pa (15 bar). During filtration, the feed solution was stirred at 11.66 Hz (700 rpm) to avoid concentration polarisation. Permeate samples were collected in cooled flasks as a function of time, weighed and analysed. The retention values were calculated with the permeate concentration and the concentration of the original feed solution (70 ␮M). The permeation was stopped when the retention reached a constant value. In the case of unfilled PDMS and MPF-50, no constant values were reached and the average of the last three measurements is given. Typically, less than 10 ml of an initial total feed volume of 50 ml had thus permeated before obtaining the retention. Wilkinson catalyst (925 Da) was chosen as test solute. To determine the concentration, the samples were analysed for Rh with an atomic absorption spectrometer of Varian Techtron (type IL 651 AA/AE). 2.5. SEM-measurements Membrane cross-sections were obtained by breaking the membranes under liquid nitrogen. All SEM-samples were coated with a 1.5–2 nm Pt/Pd coating in a Cressington HR208 high-resolution sputter coater, to reduce sample charging under the electron beam. SEM-images of both the surface and the cross-section of the membranes were obtained with a Jeol JSM-6340F. This is a semi-in-lens type SEM with a cold field-emission electron source (FEG-SEM). 3. Results and discussion

2.2.3. Dense membranes for swelling experiments A PDMS solution or the different PDMS/filler dispersions were prepared like mentioned above and poured in a petridish. The solvent was allowed to evaporate and the resulting film was cured at 100 ◦ C. The filler fraction in the membrane ranged from 0 to 30 wt%. The unfilled samples are referred to as reference samples.

3.1. Selection of fillers It can be expected that fillers enhance the stability and the performance of PDMS in SRNF. However, in order to achieve this enhancement the filler should be very well dispersed. Nondispersed filler-aggregates have to be avoided because they cre-

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ate voids in the polymer network, which weaken the structure and increase the swelling. An increased swelling of a polymer film upon addition of fillers is thus an indication for a bad dispersion [5,6]. Fillers with high polarity are expected to disperse poorly in the PDMS matrix and it has been indicated that aggregates of small particles are more difficult to separate [5]. The surface properties, like functional groups and specific surface, determine the filler–polymer adhesion. 3.1.1. Silica In principle, silica, that contains silanol-groups at the surface, should exhibit good cross-linking properties. These silanolgroups (proton donors) adsorb the PDMS-chains through hydrogen bonds with the oxygen atoms of the PDMS backbone, which are proton acceptors [12]. Considering the reactivity of the silanol-groups toward the SiH groups of the RTV-615B cross-linker, also chemical bonds between silica and the PDMS network can be present to some extent [5]. This is also the reason for the order of mixing used: the filler is first contacted with the cross-linker, and the pre-polymer is added after 2 h of stirring at 40 ◦ C. Two types of silica were tested in this work: precipitated silica (S1) and fumed silica (S2). PDMS films comprising precipitated silica showed a swelling increase, indicating the formation of voids. This is due to the bad dispersability of this type of silica in hexane creating a lot of voids and thus masking a possible cross-linking. Besides the small size, the high polarity of this type of silica explains this observation. The silylation of the silica is expected to lower the polarity of the silica and hence improve the dispersion in the apolar PDMS matrix. Indeed, a slight swelling decrease resulted upon addition. The trimethyl silane groups only allow van der Waals bonds to exist in the polymer network and hinder interactions of polymer chains with the residual silanol. Because of its less polar character, addition of fumed silica (S2) seemed more appropriate. However, the maximal filler content was only 10 wt%, with higher filler contents resulting in brittle films. Despite the limited filler content, a strong swelling reduction was already achieved with the fumed silica. 3.1.2. Carbon The two types carbon filler were selected because of the functional groups on their surfaces. C1 has typically carboxyl groups and phenolic hydroxides on the surface and C2 carbonyl moieties, typically lactones and quinones. This difference in surface properties was expected to have significant influence on the filler–polymer interaction. Taken into account the presence of 25 wt% rigid matter, the solvent uptake for the 75 wt% PDMS left in the SB 100-filled membrane is higher than for the reference PDMS, indicating void formation. The carboxylic moieties at the surface of this filler increase the polarity of the filler, thus decreasing the dispersability in non-polar media. A moderate swelling decrease was observed by adding Printex G (C2), which has a better affinity for hexane and is thus easier to disperse. The carbonyls on the carbon filler are proton acceptor groups, thus not enabling hydrogen bonds with the PDMS-chains. Only van

Fig. 1. Swelling of PDMS filled with different fillers (toluene, RT). S1: 15 wt%, S2: 10 wt%; C1 and C2: 25 wt%; Z1, Z2 and Z3: 30 wt%.

der Waals bonds are possible, which allow a modest swelling reduction. 3.1.3. Zeolite The selected zeolite types have a low charge density, given the high Si/Al ratio, hence a good dispersion in PDMS/hexane solution is expected. Also, the aggregates of the micron-sized zeolite crystals are easier to break. This is confirmed by the swelling decrease of the filled PDMS films in toluene (Fig. 1). The swelling decrease of the zeolite-filled PDMS is much stronger than that of the PDMS films filled with the silica and carbon materials. This is explained by a better dispersion and the higher cross-linking effect of the zeolites. The latter can be linked to the high specific surface of these zeolites, as indicated by the BET value (Table 1): the higher the specific surface, the more surface groups are available for the polymer chains to adsorb on. The surface groups at the zeolite surface are also silanol functionalities, evidently allowing similar physical and chemical interactions as in the case of silica. USY and to a lesser extent ZSM-5 thus proved to be the “efficient” filler to realise the aimed swelling decrease. The efficiency of these fillers to decrease swelling was tested for three other solvents with high PDMS-affinity (Fig. 2). In all these four solvents similar trends could be observed. 3.2. SEM-analysis of filled PDMS membranes The decreased swelling is expected to enhance the solute retention of silicone membranes in these strong-swelling solvents. To prove the increased performance, several filled PDMS membranes were prepared. SEM-analysis of the cross-section of these composite membranes gave information about the exact film thickness and the dispersion of the filler in the polymer matrix (Fig. 3). The ZSM-5 and USY-filled PDMS membrane have both relatively thick top-layers. This explains the low permeability in the solvents tested (Table 4). In later optimisation work, the preparation of defect free top-layers with a thickness of 5 ± 2 ␮m was successful. These thicker membranes are very useful for the preliminary study of the separation performance of filled PDMS membranes.

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Table 1 Main characteristics of the fillers used in this work Filler code

Type

Si/Al

Crystal size (nm)

BET (m2 /g)

Z1a Z2b Z3b S1c S2d C1d C2d

USY ZSM-5 (CBV-2802) ZSM-5 (CBV-3002) Hi-Sil® Aerosil 380 SB-100 Printex G

80 275 240 ∞ ∞ n.a. n.a.

400–800 400–800 1000–1500 7 13 50 51

738 (micropores) + 174 (meso + macropores) 410 405 150 380 30 30

a b c d

E. Feijen, Ph.D. Thesis, KU Leuven (1991). Technical information provided by PQ-corporation. Technical information provided by PPG Industries. Technical information provided by Degussa.

Fig. 2. Swelling reduction caused by addition of zeolites (RT).

The SEM pictures show an acceptable zeolite dispersion, not creating any defects in the top-layer. 3.3. Separation properties of zeolite-filled PDMS membranes in organic solvents To have a good view on the general utility of the zeolitefilled PDMS membranes in organic processes, the retention of the Wilkinson catalyst was measured in several solvents. In the strongly swelling solvents, namely toluene, ethylacetate (EA)

and dichloromethane (DCM) the Wilkinson catalyst retentions were considerably lower for the unfilled PDMS, confirming the limited applicability of PDMS as an SRNF-membrane in these solvents (Table 2). Addition of the fillers substantially improved the separation performance of PDMS in strongly swelling solvents. The higher cross-linking density and the reduced swelling tendency of the filled PDMS films in the non-polar solvents explain these observations. Compared with unfilled PDMS, the free volume is strongly decreased because of this reduced swelling, which consequently reduces the diffusivity of molecules. The diffusivity for high-molecular weight solutes is reduced more drastically than for the low-molecular weight solvents, which explains the higher retention values. The low permeances of the filled PDMS membranes are directly linked with the top-layer thickness. In order to make a good comparison, the permeances were recalculated to permeability values. The calculation is done assuming a generally accepted reciprocal proportionality between SEM-measured top-layer thickness and flux, which seems appropriate under these experimental conditions. Fig. 4 shows the toluene and ethylacetate permeability for the three types of membranes tested. The permeability of the ZSM-5 filled membrane is higher than for the unfilled membrane, indicating an additional selective solvent transport through the zeolite pores. The effect is less pronounced with USY incorporated, where a lower solvent permeation in the zeolite pores is due to the physical intrusion by PDMS-chains in the pores [5].

Fig. 3. SEM picture of the cross-section of the composite zeolite-filled PDMS membranes. (A) USY-filled PDMS membrane and (B) ZSM-5 filled PDMS membrane.

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Table 2 Filtrations with solutions of the Wilkinson catalyst in different solvents (feed concentration: 0.225 ␮M, 30 ◦ C, 15 × 105 Pa (15 bar)) Permeance (l/(m2 h × 105 Pa) (l/(m2 h bar)))

Toluene EA DCM THF a b c

Retention (%)

PDMSa

ZSM-5b

USYc

MPF-50

PDMS

ZSM-5

USY

MPF-50

1.15 1.14 1.71

0.58 0.55 0.71 0.85

0.22 0.20

0.47 0.50

78 62 81

98.5 97 93 80

98 94

81 76

0.50

55

Top-layer thickness: 6 ± 1 ␮m. Top-layer thickness: 19 ± 2 ␮m. Top-layer thickness: 31 ± 1 ␮m.

3.4. Comparison with MPF-50

Table 3 Influence of preconditioning procedure on permeance

Table 2 also compares the PDMS membranes and the commercially available MPF-50. This SRNF-membrane is used here as the commercial benchmark, since it is also based on silicone rubber but without any filler. As a consequence, the retentions obtained are clearly in line with those of the unfilled PDMS-membrane. The retentions measured for MPF-50 in toluene, EA and tetrahydrofuran (THF) are all lower than for the two laboratory-made filled PDMS membranes, again indicating increased utility of silicone rubber in non-aqueous membrane separations upon incorporation of filler. Despite the assumed ultrathin top-layer (<200 nm) [13], the MPF-50 membrane only shows a similar permeance. The permeance of MPF-50 was found to be strongly dependent on the pre-treatment given. Table 4 shows the permeance of MPF-50, when used as such after being taken from the EtOH/H2 O storage solution (preconditioning solvent A). It can be anticipated that this polar solvent mixture is difficult to be replaced by the nonpolar solvents during the filtrations applied in this work. One way to solve this problem was the pre-treatment of a MPF-50 coupon in the same solvent as used in the filtration afterwards (preconditioning solvent B). In the case of toluene, this resulted in a decrease of the permeance (Table 3). A solvent exchange with a solvent of intermediate polarity, like acetone, followed by a solvent exchange with ethyl acetate, applied in the filtration, renders the expected high permeance. This pre-treatment, however, was found to create defects that lowered the selectivity significantly.

Pre-conditioning solvent

Permeance (l/(m2 h × 105 Pa) (l/(m2 h bar))) Toluene

Ethyl acetate

A B C

0.47 0.02 2.8

0.5 – 2.3

Wilkinson catalyst feed concentration: 0.225 ␮M, 30 ◦ C, (15 × 105 Pa) 15 bar. (A) Ethanol/water (as delivered), (B) immersed in filtration solvent, (C) first immersed in acetone, second exchanged with ethyl acetate.

The fact that no pretreatment is needed with the laboratoryprepared zeolite-filled PDMS membranes to show a good performance in the non-polar solvent, is considered as another important advantage over the MPF-50 membrane. 3.5. Performance at higher temperature Many potential SRNF applications, e.g. the removal of homogeneous catalysts from reaction mixtures, require membranes with good separation properties at higher temperatures. According to the technical information [14], the MPF-50 membrane cannot be used above 40 ◦ C. When used at 50 ◦ C, the unfilled PDMS membranes also failed to retain the Wilkinson catalyst out of toluene. The zeolite-filled PDMS membranes on the other hand, successfully maintained their separation characteristics at higher temperatures (Table 4). Like already indicated, the decreased chain mobility caused by a higher cross-linking density keeps the diffusion of molecules through the network hindered and hence more selective. Even at increased temperatures, the diffusivity of the Rh-catalyst stays low enough to be almost fully retained.

Table 4 Filtrations with solutions of the Wilkinson catalyst at different temperatures (feed concentration: 0.225 ␮M, toluene, 15 × 105 Pa (15 bar)) Toluene

Fig. 4. Recalculated permeabilities from data of Table 2 (feed concentration: 0.225 ␮M, 30 ◦ C, 15 × 105 Pa (15 bar)).

PDMS ZSM-5 USY

Permeance (l/(m2 h × 105 Pa) (l/(m2 h bar)))

Retention (%)

30 ◦ C

50 ◦ C

30 ◦ C

50 ◦ C

80 ◦ C

1.15 0.58 0.22

1.50 0.72 0.27

78 98.5 98

0 98 98

88

80 ◦ C 0.84

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4. Conclusion The filler–polymer adhesion and thus the efficiency of the filler in reducing membrane swelling, proved to be depending on the character and the availability of functional groups at the surface. The zeolites used in this work were the most efficient fillers, due to their good dispersion and their high specific surface thus making many silanol functionalities available. Addition of zeolites turned PDMS into a very useful material for SRNFmembranes, even at temperatures up to 80 ◦ C and in solvents that induce high swelling, such as toluene and ethylacetate.

[2]

[3]

Acknowledgements

[4]

L.G. acknowledges the IWT for a grant as doctoral research fellow. This research was done in the frame of an IAP-PAI grant on Supramolecular Catalysis sponsored by the Belgian Federal Government and of a grant from the Concerted Research Action (GOA) from the Flemish Government.

[5]

Nomenclature meq m0 S Sfiller Sref

weight of membrane at swelling equilibrium (g) weight of the air-dried sample (g) degree of swelling (cm3 /g) degree of swelling of membrane with filler (cm3 /g) degree of swelling of membrane without filler (cm3 /g)

Greek letter ρs density of the solvent (g/cm3 )

[6] [7]

[8]

[9]

[10]

[11]

[12]

[13]

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