Zeolite-based composite PEEK-WC membranes: Gas transport and surface properties

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Microporous and Mesoporous Materials 115 (2008) 67–74 www.elsevier.com/locate/micromeso

Zeolite-based composite PEEK-WC membranes: Gas transport and surface properties G. Clarizia a, C. Algieri a,*, A. Regina b, E. Drioli a,b b

a Institute on Membrane Technology, ITM-CNR, Via P. Bucci, I-87030 Rende, Italy Department of Chemical and Materials Engineering, University of Calabria, Via P. Bucci, I-87030 Rende, Italy

Received 19 July 2007; accepted 15 January 2008 Available online 19 February 2008

Abstract Mixed matrix membranes (MMMs) have been prepared by using different protocols in order to improve the affinity between NaA (LTA) zeolite and a modified polyetheretherketone (PEEK-WC). A terpenic resin has been added to the polymer–zeolite slurry for enhancing the polymer chain flexibility and, consequently, its adhesion with the crystals. Coupling agents such as c-aminopropyl-diethoxymethyl silane (APDEMS) and diethanolamine (DEA) have been used to modify the zeolite surface. The instrumental analyses showed a relative efficiency of these treatments with respect to the simple dispersion of the inorganic fillers within the polymer matrix. At high zeolite concentration (30 wt%), MMMs are characterised by gas transport properties lower than neat polymer-based membrane. DEA-based treatment seems the more encouraging approach also taking into account that as the filler concentration decreases, the occurrence of defects at the interface becomes less significant. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Mixed matrix membranes; Zeolite; PEEK-WC; Coupling agents; Gas permeation

1. Introduction Nowadays membrane operations are increasing their importance on industrial scale in gas and liquid separation due to operation simplicity, easy scale-up and efficient energy use. Polymers are the materials more widely used for membrane manufacturing in virtue of their simple processability, intrinsic transport properties and low cost. On the other hand, inorganic materials present a strong capability to discriminate gas species also in severe temperature and pressure conditions and aggressive environments. However, their use is still limited for reproducibility problems in the preparation step, as well as life time and high cost [1–3]. In this last decade, different research groups have studied the effect of inorganic fillers on transport performance *

Corresponding author. Tel.: +39 0984 492030; fax: +39 0984 402103. E-mail address: [email protected] (C. Algieri).

1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.01.048

of polymeric membranes. These systems, called mixed matrix membranes (MMMs), are designed to improve the properties of a host polymeric matrix taking advantage of peculiar properties of specific inorganic fillers such as carbon molecular sieves and zeolites [4]. In the separation of gas mixtures, zeolite crystals result more selective than commercial polymer membranes for different applications. ˚, For example NaA zeolite, with pore size of about 4 A exhibits a O2/N2 selectivity of 37 at 35 °C [5] whereas the best polymer-based membranes do not overcome 10 in air separation. In the preparation of MMMs, a key factor is represented by the affinity between the two phases involved in the final structure. The compatibility between an inorganic filler and a rubbery polymer is good due to the high mobility of the polymeric chains capable to embed entirely the particles [6–8]. However, these polymers are not interesting for industrial applications because of low gas separation factors. Amorphous glassy polymers are widely used in gas separations due to their attractive transport properties.

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Unfortunately, the dispersion of particles in these materials causes defects at the organic–inorganic interface as a consequence of the rigidity of the polymer chains [9]. In fact, a few positive results by simple dispersion of the crystals into polyethersulfone have been reported in the literature [10]. The authors explained the permeability decrease and permselectivity increase for different gas pairs with zeolite loading as a consequence of the rigidification of the polymer chains near zeolite particle and partial blockage of the crystal pores by the polymer. Some methods have been developed in order to further improve the adhesion between the two phases and, consequently, the gas transport performance of these composite membranes. The use of different silane coupling agents such as c-aminopropyl-triethoxy silane, N-b-(aminoethyl)-c-aminopropyltrimethoxy silane, c-glycidyloxy-propyltrimethoxy silane and c-aminopropyl-dimethylethoxy silane, allows to modify the zeolite surface in order to compatibilise the inorganic filler with the host matrix [11]. The results obtained by Duval et al. by using these coupling agents put in evidence that the permeability decreases and the permselectivity decreases or remains constant with respect to the neat polymer [12]. Mahajan and Koros also found, at fixed zeolite concentration (20 vol%), a decrease of permeability and permselectivity of the MMMs based on modified zeolites with respect to the unmodified ones. These results indicate that the defects have been cured but not completely [13]. Other researchers, using a new silane coupling agent (c-aminopropyl-diethoxymethyl silane, APDEMS), improved both permeability and permselectivity of the polyethersulfone (PES), avoiding the pore zeolite blockage [14]. Mahajan and Koros performed a polymer ‘‘priming” treatment on the modified zeolite crystals in order to improve the performance of the samples. The O2/N2 permselectivity at 35 °C enhances, while the permeability decreases for the investigated polyimide types [15]. Since the results are not extremely attractive for industrial applications, other strategies have been developed. Working above the glass transition temperature (Tg) of a polymer allows to have more flexible chains during membrane formation. However, the solvents usually used for the membrane preparation have a boiling point below the Tg of the most widely used polymers. For this reason Mahajan et al. [16] used different plasticizers for lowering the Tg of a polyimide. Different permeability and permselectivity trends have been achieved as a function of the plasticizer type. The contact between zeolite particles and polyimide chains has been enhanced by using 2,4,6-triaminopirymidine, capable to form hydrogen bonding between them. The membranes showed outstanding permselectivity values for O2/N2, CO2/CH4 and CO2/N2 combined with very low permeability values, especially when NaA zeolite has been used [17]. Another way to prepare mixed matrix membranes which does not require the modification of zeolite surface is the

employment of block copolymer containing both rigid and flexible chains. Pechar et al., adding the polydimethylsiloxane to the polyimide backbone, obtained an increase of the permeability and a decrease of the permselectivity for different gas pairs. Nevertheless, the addition of the zeolite to the copolymer resulted in a loss of both permeability and permselectivity [18]. In this work a modified polyetheretherketone (PEEKWC) has been selected as host matrix for MMMs preparation. This material, in contrast to the traditional crystalline PEEK, is an amorphous polymer soluble in a wide range of organic solvents. Due to its good thermal, chemical and mechanical stability, this polymer is suitable for the preparation of membranes according to phase inversion method in applications ranging from microfiltration and ultrafiltration to gas separation. PEEK-WC has interesting permselectivities but not exceptional permeability values, according to the Robeson trade-off relation [19,20]. NaA zeolite has been selected because of its capability to discriminate gas species having small kinetic diameters. Different protocols for preparing MMMs by dispersing zeolite crystals in PEEK-WC have been compared. 2. Experimental 2.1. Materials The polymer used in this study is PEEK-WC received from ChanChung Institute of Applied Chemistry (Academia Sinica, China). The structure of this material is displayed in Fig. 1. NaA zeolite has been purchased by Sigma–Aldrich. Zeolite crystals before using were activated at 500 °C for at least 5 h and stored into a dryer to avoid water sorption. The products used for membrane preparation are dichloromethane by Carlo Erba Reagenti and dimethylacetamide (DMA) purchased from Lab-Scan Analytical Sciences Ltd., Ireland. Toluene, n-hexane, methanol, (caminopropyl)-diethoxymethyl silane (APDEMS), diethanolamine (DEA) and Poly-a-pinene (PaP) have been purchased from Sigma–Aldrich. For all products used the purity is higher than 99.5%. The chemical structures of both additive and coupling agents are showed in Fig. 1. 2.2. Zeolite surface modification The physical ‘‘priming” treatment consists of the polymer precipitation, promoted by a non-solvent, on the crystal surface. In this way the particles are coated with a thin polymer layer that should favour the compatibility between the two phases during the membrane preparation. A ratio 1/5 between polymer and zeolite was used, dichloromethane and n-hexane were used as solvent and non-solvent, respectively. The surface of NaA zeolite has been modified with APDEMS and DEA in order to increase the affinity with the polymer. The APDEMS has been chosen as a silane

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69

Fig. 1. Chemical structure of PEEK-WC (a), PaP (b), DEA (c) and APDEMS (d).

coupling agent because it has a lower number of coupling points on the zeolite surface with respect to the c-aminopropyl-triethoxy silane. In this way the blocking of the zeolite pores is reduced. The chemical modification by using APDEMS has been performed according to Li et al. [14]. In a flask containing 200 mL of toluene, 20 mL of APDEMS and 2.5 g of NaA have been added. The suspension has been kept under magnetic stirring for 24 h at 40 °C in N2 atmosphere. The modified zeolite has been filtered and washed by using 500 mL of toluene and then 250 mL of methanol to remove the unreacted silane. Finally, the modified zeolite has been dried for 3 h at 110 °C under vacuum. A similar protocol has been adopted to modify the NaA surface with DEA. In a flask containing 150 mL of DEA, have been dispersed 10.5 g of NaA zeolite at 50 °C under N2 flow. The suspension has been kept under magnetic stirring for 24 h before filtering and washing of the NaA crystals with fresh DMA for removing the unreacted DEA. Afterwards, the zeolite has been dried for 6 h at 100 °C, and finally for 4 h at 150 °C under vacuum. 2.3. Membrane preparation MMMs have been prepared by dry phase inversion method. The slurry, obtained dissolving the polymer and modified NaA in the solvent dichloromethane, has been stirred at room temperature for 24 h. The polymer concentration is equal to 15 wt%, while the ratio zeolite/(zeolite + polymer) is equal to 30 wt%. The films have been evaporated before in air for 24 h and after for 6 h at 60 °C under vacuum. Some composite membranes have been prepared by dissolving a terpenic resin (PaP) in the polymer–zeolite slurry. A ratio of zeolite/solute mass and PaP/solute mass of 30 wt% and 10 wt% have been selected, respectively. 2.4. Membrane Characterization The membrane thickness has been measured by using a digital micrometer (Carl Mahr D7300 Esslingen a.N.) aver-

aging 15 measurements, the standard deviation calculated on the sample has been always lower than 5%. Si/Al ratio of the zeolite crystals has been evaluated by means of the Energy Dispersive X-ray (EDX) technique. This analysis has been carried out using an EDAX-Phoenix instrument (detector Super Ultra Thin Window, Si/Li crystal analyser). Filler size and its dispersion in the host matrix has been analysed by using a Phoenix instrument scanning electron microscope (SEM). Elemental analysis has been carried out by using a Perkin-Elmer PE 2400 CHNS/O analyzer. The composition of the samples is expressed in weight percentage (wt%) of the each element with respect to the total mass of the sample. The surface zeolite modification has been investigated by Fourier Transform Infrared (FT-IR) analysis in the transmission mode from 4000 cm1 to 400 cm1 wavenumber, with a resolution of 4 cm1 and 10 scans (FT-IR Paragon 1330, Perkin-Elmer). Thermogravimetric analyses (TGA) were carried out with a Netzsch STA 409 instrument between 20 °C and 850 °C at a ramp of 10 °C/min in dry air with a flow rate of 10 cm3/min. The hydrophobic/hydrophilic character of the upper and bottom sides of the membrane samples has been evaluated by contact angle technique. Water contact angles have been measured using the sessile drop method at room temperature by a CAM 200 contact angle meter (KSV Instruments Ltd., Helsinki, Finland), depositing a drop of water on the membrane surface by an automatic microsyringe. At least ten measurements have been performed for each membrane sample on upper (exposed to air) and bottom (exposed to support) sides. The standard deviation calculated is always lower than 3%. The experimental semi-automatic apparatus used for carrying out the permeation rate tests, purchased by GKSS Forschungszentrum GmbH (Germany), is described elsewhere [8]. Gas permeation tests were performed in a temperature range of 25–65 °C, feeding the single gases (purity level 99.998%) at atmospheric pressure and keeping the permeate side under vacuum (102 mbar). The same protocol has been followed for pure polymer and all com-

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posite samples. Before each test the system has been evacuated on both feed and permeate sides and, between two cycles of measures at different temperatures, it has been swept with an inert gas. 3. Results and discussion 3.1. Microscopy results The mean value of the filler size is about 3.0 lm and Si/Al ratio is equal to 1.0 as determined by SEM and EDX, respectively.

20.0kV x 2400

20.0kV x 6000

The cross-section images of mixed matrix membranes are shown in Fig. 2. In absence of pretreatment of zeolite crystals, a cave-like structure is evident, due to a weak interaction of the polymer with zeolite framework (Fig. 2a). When the zeolite surface is coated by a thin film of polymer, the defects at the polymer–zeolite interface are reduced (Fig. 2b). A better adhesion of the polymer chains on the crystal as well as a better crystal distribution are observed as the terpenic resin is added to the polymeric matrix, as shown in Fig. 2c. Thus, the additive tends to enhance the flexibility of the polymer matrix as confirmed by a macroscopic evidence and also by a lowering of Tg

5μm

20.0kV x 1600

10 μm

10μm

20.0kV x 2400

20μm

20.0kV x 6000

10μm

Fig. 2. Cross-section of different composite samples loaded with NaA zeolite at 30 wt% ((a) unmodified zeolite, (b) priming, (c) PaP addition, (d) APDEMS treatment and (e) DEA treatment).

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of the polymer [20]. This favours a better interaction between the heterogeneous phases. The chemical treatment of the zeolite that produces a thin layer around the crystals further improves the adhesion between organic and inorganic phases (see Figs 2d and e). In addition, a layer containing a high crystal concentration has been observed close to the bottom side of the samples. 3.2. Elemental, spectroscopic and thermal analyses Table 1 shows the results of elemental analysis before and after the chemical modification of the zeolite surface. In particular, nitrogen and carbon elements are detected after the chemical treatment with respect to the pure crystals, confirming the presence of coupling agents on the zeolite surface. Fig. 3 shows the FT-IR spectra of zeolite and zeolite modified by means of the chemical treatments. The bands at 1000 and 667 cm1 correspond to the SiAO asymmetric and symmetric stretching vibrations in the spectrum of NaA [21]. The peak at 3467 cm1 corresponds to the inner-hydroxyl group (see Fig. 3, a spectrum). In the modified zeolite spectra additional bands are present. In fact, the typical bands of aliphatic CAH stretching (2960 cm1 and 2896 cm1 in the b spectrum, 2961 cm1 and 2893 cm1 in the c spectrum) are evident. The NAH bending vibrations, typical of amine species, are visible at 1596 cm1. A peak at 1770 cm1, present Table 1 Comparison of elemental analysis data of zeolites before and after chemical modification Sample

N (wt%)

C (wt%)

H (wt%)

NaA NaA-DEA NaA-APDEMS

0 2.23 3.01

0 7.29 8.57

1.10 2.41 2.24

Aliphatic C-H stretching

with a different intensity in both b and c spectra, can be attributed to the ammonium ion formation for interaction between amino group of the coupling agent and SiAOH group of the zeolite. The amine may enter into a hydrogen bonding interaction with a surface hydroxyl group [11]. Furthermore, the 1240 cm1peak, characteristic of the SiACH2 group, can be observed in the spectrum of the zeolite modified with APDEMS. In the reference spectrum, can be observed the SiAOH and SiAO bands, respectively. In addition, the bands characteristic of the pure zeolite (3467 cm1 and 1000 cm1) are shifted by 67 and 25 wavenumbers when DEA is used and by 57 and 25 wavenumbers as APDEMS treatment is performed. The shift of the absorbance peak to lower wavenumbers indicates qualitatively an interaction between coupling agent and zeolite [18]. The weight loss and DSC curves of NaA-DEA and NaA-APDEMS particles are shown in Fig. 4. The TGA curves of both samples reveal that thermal events occur in two temperature ranges: 25–200 °C and 200–600 °C. In the first zone of Fig. 4a, the decrease of weight is 12.5 wt% which can be mainly attributed to the desorption of water and residue solvent. The second weight loss of about 14 wt% corresponds to the coupling agent decomposition. The presence of an exothermal peak at 338 °C on DSC curve is an experimental evidence of the combustion of the DEA chemically bound to the zeolite surface. The amount of DEA on the dry zeolite is equal to 0.16 (gDEA/gzeolite). Similar result was found for NaA-APDEMS sample with a weight loss of the silane coupling agent of about 17 wt% in the range 200–600 °C. In this case an exothermal peak is observed at 354 °C. The amount of APDEMS on the dry zeolite is equal to 0.18 (gAPDEMS/ gzeolite).

Aliphatic C-H stretching

1596

1596 1770

c

3400

b

3400

N-H bending

2961-2893

N-H bending

2960-2896

975

975

Transmittance

Transmittance

71

a

a

4000

3200

2400

1800

1400

1000

600

350

4000

3200

2 400

1 800

1400

Wavenumber cm-1 3467

Wavenumber cm-1

1000

1000

1240

667

Fig. 3. IR spectra of unmodified NaA zeolite (a), NaA-DEA zeolite (b) and NaA-APDEMS (c).

600

360

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105

4

338 °C

3.5 100 3 2.5

-12.5 %

2

90

1.5 85

1 -13.84 %

DSC, uV/mg

Weight loss, %

95

0.5

80

0 75 -0.5 70

-1 0

100

200

300

400

500

600

700

800

Temperature, °C

105

4

354 ºC 3.5 100

3 2.5

95

2 1.5

90

1

-17.47 % 85

DSC, uV/mg

Weight loss, %

-4.30 %

0.5 0

80

-0.5 75 0

100

200

300

400

500

600

700

800

-1 900

Temperature, ºC

Fig. 4. TGA and DSC analyses for: NaA-DEA (a) and NaA-APDEMS (b) modified crystals.

Thus, thermal analysis confirms the chemical interaction between zeolite and coupling agents supporting the qualita120

UPPER BOTTOM

100

CAM (°)

80

60

40

20

0

PEE

PEE

K-W

C

PEE PEE PEE K-W K-W K-W C-P C-D C-N C-A aPEA aA PD -Na EM NaA S-N A aA

K-W

Fig. 5. Comparison between the contact angles measured on upper and bottom side of different membrane samples.

tive information obtained by FT-IR and elemental measurements. 3.3. CAM analysis The water contact angle measurements have been carried out in order to evaluate how the surface properties of the pure polymer change as a consequence of filler presence. These results give a direct information about the anisotropy and the hydrophilic/hydrophobic character of the membrane specimens. In addition, these tests provide useful indications for a possible application in the separation of wet gas streams. Water contact angle values measured for pure polymer are in agreement with literature data [20]. The addition of unmodified and modified zeolite in the polymer matrix determines a modification of the contact angle value. The significant change of the surface properties observed in the static contact angle measurements mainly depends on the modality of the crystal treatment. In fact, the samples prepared by treating the filler with DEA result more hydrophilic than the original polymer. It is possible to explain these results considering the hydrophilic nature of the zeo-

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lite and the polarity of the coupling agents. On the other hand, the presence of poly-a-pinene confers a hydrophobic character to the membrane surface. As confirmed by SEM observations, the not homogeneous distribution of the additive along the membrane cross-section causes a different behaviour of the upper and bottom sides (see Fig. 5).

-10

3.4. Permeation tests

-40

PEEK-WCPaP-NaA

PEEK-WCAPDEMS-NaA

PEEK-WCDEA-NaA

0

-20

-30

(%)

The permeability and permselectivity results at two different temperatures are reported in Table 2. The permeability in the pure PEEK-WC increases with the temperature for all species considered and the permeation rate order is the following: H2 ffi He > CO2 > O2 > CH4 > N2. However, it is necessary to put in evidence that the less permeable species increase their permeation rate as temperature rises in a more significantly way with respect to the more permeable gases. As a consequence of this behaviour, a decrease of the permselectivity for the gas pairs occurs at high temperature values. The same trend has been observed also for the mixed matrix membranes. However, the ideal permselectivity values, measured trough the MMMs, result lower than those calculated in pure material. The composite samples, prepared dispersing the unmodified NaA zeolite crystals in the polymer matrix, present some defects that drastically abate the intrinsic permselectivity of the polymer. In fact, a viscous flow has been observed for all gases permeating in these membrane specimens. The ‘‘priming” treatment of the crystals improves the transport properties of the MMMs. In this case, the gas permeation rate trough these samples is controlled by the Knudsen mechanism (see Table 2). For this reason, a further treatment of the crystalline surface is necessary in order to minimize the presence of defects. The addition of PaP to the suspension determines a significant increase of the permeability (three and four times at 25 °C and 45 °C, respectively) with respect to neat polymer combined to a remarkable decrease of the permselectivity. Membrane prepared by using zeolite, modified with a coupling agent, showed higher permeability and lower ideal permselectivity values than neat polymer. However, the permselectivity values for these samples are better than

73

-50

-60

-70

O2/N2 (25 °C) O2/N2 (45°C)

-80

CO2/N2 (25 °C) CO2/N2 (45 °C)

-90

Fig. 6. Permselectivity reduction for different MMMs loaded at 30 wt% of zeolite with respect to neat polymer-based samples.

those measured trough the membranes containing unmodified zeolite (Fig. 6). In this figure it is possible to observe a lower decrease of the PEEK-WC-DEA-NaA permselectivity for the two gas pairs at the two temperatures reported. This result put in evidence a further improvement of the zeolite–polymer affinity obtained by using DEA as coupling agent. It is possible to explain the better results obtained when DEA is used with respect to the APDEMS considering the smaller size of the former molecule. The species used in this work (APDEMS and DEA) introduce ˚ and 3.7–6 A ˚ between polymer a distance of about 5–9 A chains and zeolite surface, respectively. These distances, determined minimizing the steric hindering of each molecular structure by means of a molecular mechanics model, are in agreement with experimental data reported in the literature for similar species [22]. A tightening between polymer and sieve is obtained when smaller linkage is used according to the literature [15]. On the basis of these results, although no strategy followed in this work allowed to obtain MMMs having gas transport performance better than PEEK-WC membranes, the DEA-treatment of the zeolites seems the more encouraging approach. However, it is necessary to consider that the high zeolite concentration investigated in this research

Table 2 Permeability and permselectivity values for: PEEK-WC and NaA filled PEEK-WC membranes prepared by: (a) ‘‘priming”, (b) PaP addition, (c) APDEMS coupling agent, (d) DEA coupling agent

PN2 (25 °C), (barrer) O2/N2 (25 °C), (–) CO2/N2 (25 °C), (–) PN2 (45 °C), (barrer) O2/N2 (45 °C), (–) CO2/N2 (45 °C), (–)

PEEK-WC

PEEK-WC-NaA(a)

PEEK-WC-PaP-NaA(b)

PEEK-WC-APDEMS-NaA(c)

PEEK-WC-DEA-NaA(d)

0.12 5.80 33 0.25 4.4 20

90 0.94 0.87 – – –

0.35 3.1 6.6 0.94 2.0 3.4

0.22 3.6 7.5 0.88 1.8 3.3

0.16 4.2 21 0.60 2.7 9.0

Zeolite concentration is 30 wt%.

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(30 wt%) increases the risk of defects presence, as demonstrated in some literature studies [16,23]. At confirmation of this fact, best performance have been achieved when a lower zeolite concentration (10 wt%) is considered in a MMM containing PaP as additive. Nitrogen permeability values similar to those measured through the pure polymer have been observed combined to less important permselectivity decreases. At 25 °C, O2/N2 permselectivity increases from 3.1 to 4.8, while CO2/N2 rises from 6.6 to 22 as zeolite content changes from 30 wt% to 10 wt%. This trend is further confirmed at 45 °C, where the gas transport performance of this composite sample equals pure PEEK-WCbased membranes. These results show the possibility to improve more the permselectivity values if a coupling agent protocol will be used at low zeolite content. 4. Conclusions Different protocols for preparing PEEK-WC-based mixed matrix membranes have been compared. All mixed matrix membranes show lower permselectivity values with respect to the pure polymer. Coupling agent protocol is better than priming one since it causes an improved adhesion between polymer and zeolite phases. DEA, being a coupling agent smaller than APDEMS, is more effective to reduce the voids between the two phases. These results are confirmed by SEM observation and gas permeation tests. The effective chemical interaction between external surface of zeolite and coupling agents is quantitatively proved by thermal analyses and qualitatively by FT-IR and elemental measurements. On the basis of gas permeation tests, although no protocol developed in this work allowed to prepare mixed matrix membranes with performance better than PEEK-WC membranes, the DEA-treatment of the zeolites seems the more encouraging at 30 wt% NaA zeolite content. Presumably better results could be achieved at lower zeolite content as demonstrated by preliminary measurements carried out with the PaP protocol at 10 wt%.

Acknowledgments The authors would like to thank Mr. S. Armentano from Department of Chemistry of University of Calabria for elemental analysis and Dr. L. Pasqua from Department of Chemical and Materials Engineering for TGA measurements. References [1] P. Pandey, R.S. Chauhan, Prog. Polym. Sci. 26 (6) (2001) 853. [2] H. Lin, B.D. Freeman, J. Mol. Struct. 739 (1–3) (2005) 57. [3] E.F. McLeary, J.C. Jansen, F. Kapteijn, Micropor. Mesopor. Mater. 90 (2006) 198. [4] T.S. Chung, L.Y. Jiang, Y. Li, S. Kulprathipanja, J. Membr. Sci. 32 (2007) 483. [5] D.M. Ruthven, R.I. Derrah, J. Chem. Soc. Faraday Trans. 71 (1975) 2031. [6] M. Jia, K.V. Peinemann, R.D. Behling, J. Membr. Sci. 57 (2005) 289. [7] J.M. Duval, Adsorbent filled polymeric membranes, Ph.D thesis. The Netherlands, University of Twente, 1995. [8] G. Clarizia, C. Algieri, E. Drioli, Polymer 45 (2004) 5671. [9] M.G. Suer, N. Bacß, L. Yilmaz, J. Membr. Sci. 91 (1994) 77. [10] Y. Li, T.S. Chung, C. Cao, S. Kulprathipanja, J. Membr. Sci. 260 (2005) 45. [11] K.C. Vrancken, K. Possemiers, P. Van der Vroot, E.F. Vansant, Colloids Surf. A: Physchem. Eng. Aspects 98 (1995) 2354. [12] J.M. Duval, A.J.B. Kemperman, B. Folkers, M.H.V. Mulder, J. Appl. Polym. Sci. 54 (1994) 409. [13] R. Mahajan, W. Koros, Polym. Eng. Sci. 42 (7) (2002) 1420. [14] Y. Li, H.-M. Guan, T.S. Chung, S. Kulprathipanja, J. Membr. Sci. 275 (2006) 17. [15] R. Mahajan, W. Koros, Polym. Eng. Sci. 42 (7) (2002) 1432. [16] R. Mahajan, R. Burns, M. Scheffer, W.J. Koros, J. Appl. Polym. Sci. 86 (2002) 881. [17] H.H. Yong, H.C. Park, Y.S. Kang, J. Won, W.N. Kim, J. Membr. Sci 188 (2001) 51. [18] T.W. Pechar, S. Kim, B. Vaughan, E. Marand, V. Baranauskas, J. Riffle, H.K. Yeong, M. Tsapatsis, J. Membr. Sci 277 (2006) 210. [19] J.C. Jansen, M. Macchione, E. Drioli, J. Membr. Sci. 255 (2005) 167. [20] A.M. Torchia, G. Clarizia, A. Figoli, E. Drioli, Ital. J. Food Sci. 26 (2004) 173 (special issue). [21] Y. Huang, Z. Jiang, Micropor. Mater. 12 (1997) 341. [22] V.V. Tsukruk, V.N. Bliznyuk, Langmuir 14 (1998) 446. [23] H. Wang, B.A. Holberg, Y. Yan, J. Mater. Sci. 12 (2002) 3640.