water mixtures by pervaporation through zeolite-filled polysulfone membrane containing 3-aminopropyltrimethoxysilane

water mixtures by pervaporation through zeolite-filled polysulfone membrane containing 3-aminopropyltrimethoxysilane

Desalination 193 (2006) 119–128 Separation of ethanol/water mixtures by pervaporation through zeolite-filled polysulfone membrane containing 3-aminop...

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Desalination 193 (2006) 119–128

Separation of ethanol/water mixtures by pervaporation through zeolite-filled polysulfone membrane containing 3-aminopropyltrimethoxysilane Ywu-Jang Fua, Chien-Chieh Hub*, Kueir-Rarn Leeb, Juin-Yih Laic a

Department of Polymer Materials, Vanung University, Chung-Li 32023, Taiwan Tel. +886 (3) 453521; Fax +886 (3) 4652040; email: [email protected] b Department of Chemical Engineering, Nanya Institute of Technology, Chung Li, 32034, Taiwan c R&D Center for Membr. Technol. and Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan

Received 15 March 2005; accepted 13 July 2005

Abstract Ethanol/water mixture is separated by pervaporation through a polysulfone (PSf) membrane filled with zeolite. The coupling agent effect of 3-aminopropyltrimethoxysilane (APTMS) on the preparation of PSf/zeolite membrane was also studied. APTMS enhanced the contact of zeolite particles with PSf chains and suppressed the formation of microvoids in polymer-zeolite interface. The effects of zeolite type, zeolite loading, feed composition, coupling agent concentration, and treatment time of zeolite surface modification on the pervaporation performance were investigated. The pervaporation performance of the zeolite-filled PSf membranes was strongly affected by the zeolite type. In addition, the molecular sieving effect of zeolite seemed to take place when coupling agent concentration and treatment time increased. Compared with the PSf membrane, the PSf/zeolite 13X/ APTMS membrane effectively improved the pervaporation performances. Keywords: Zeolite-filled polysulfone membrane; Pervaporation; 3-aminopropyltrimethoxysilane

1. Introduction Pervaporation is recognized as an effective process for separating azeotropic mixtures, close*Corresponding author.

boiling point compounds, and mixtures consisting of heat sensitive compounds. In recent years, numerous investigations have been carried out regarding the pervaporation of water–ethanol mixtures through polymeric membranes [1–3]. A good

Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

doi:10.1016/j.desal.2005.07.049

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pervaporation membrane material should have high permeation flux and separation factor for the pervaporation dehydration of alcohol. Research efforts have been directed to the selection of proper membrane materials [4,5]. It has been recognized that hydrophilic polymers are good dehydration membrane materials due to their strong affinity to water molecules. Major drawbacks of hydrophilic materials are its excessive swelling behavior and water-soluble properties. Hydrophobic polymers may be an accepted strategy to suppress the swelling and insolubilize of the water-soluble membrane in water. However, the selective extraction of water is rather more difficult since it is necessary to find a polymer material which hydrophobic as well as highly selective and permeable towards water. Zeolite has been focused on as one of the materials for the inorganic membranes because of its molecular sieving property, thermal resistance and chemical stability [6–8], but this material is expensive and difficult to process as membranes. Deficiencies in both polymeric and purely molecular sieving media suggest the need for a hybrid approach to membrane materials development and material processing. Organic–inorganic hybrid membranes have received much attention as potential membrane materials for pervaporation [9,10]. Such hybrid membranes are typically composed of porous inorganic zeolite particles dispersed in a polymeric matrix. These so-called “mixed matrix composite membranes” can combine the excellent size-sieving capacity of zeolites with the desirable mechanical and processing attributes of polymers. The transport properties of membranes for the separation of alcohol/water mixtures can be improved by the addition of zeolites to the polymeric matrix [11–13]. In a series of papers covering the subject of zeolitefilled elastomeric membranes [14–17], various aspects of these composite membranes have been investigated by varying membrane composition with respect to the type of zeolite and zeolite content, type of polymer and the effect of several

process parameters on flux and selectivity. In contrast to elastomers, the incorporation of zeolites in glassy polymers is not much described in the literature for membrane purposes. Formation of a mixed matrix material using glassy polymers presents special challenge. Mixed matrix membranes fail to exhibit their theoretical separation performance due to the formation of relatively nonselective defects at the interface between the zeolite particles and the polymer medium [18]. Mahajan and Koros [19] incorporated 4A zeolite into polyimide. They found that two factors seem to be critical to the formation of the nonselective defects at the interface: the nature of the polymersieve interaction, and the stress encountered during material preparation. Yong et al. studied the interfacial void-free Matrimid polyimide membranes filled with zeolites [20]. 2,4,6-triaminopyrimidine (TAP) was introduced as a kind of compatibilizer to eliminate the interfacial voids. They concluded that the void-free PI/zeolite 13X/ TAP membrane showed the higher gas permeability with little expense of selectivity compared to the PI/TAP membrane having the same PI/PAT ratio. It is well known that a silane coupling reagent is very effective for the modification of the surfaces of organic and inorganic materials. The effect of the silane coupling reagent on pervaporation performance of silicalite membranes were investigated by Sano et al. [21]. It is indicated that the modification with the silane coupling reagent is very effective for the improvement of the separation performance of the zeolite membrane. Polysulfones have been studied as suitable materials because of their high thermal stability, excellent mechanical strength, and high resistance to organic solvents. Polysulfones show high selectivity in dehydration of alcohol at wide range of water concentration [22–24]. Their high selectivity stems from high diffusion-selectivity due to the small size of water relative alcohol. However, they showed low permeation flux because they have very low free volume and low

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solubility in water. It is expected that the incorporation of zeolites in polysulfone would preserve or even improved the selectivity of the polymer and simultaneously increase the low fluxes characteristic for polysulfone membranes. In this study, 3-aminopropyltrimethoxysilane (APTMS) was introduced as a kind of compatibilizer to eliminate the interfacial voids between zeolite and poysulfone. If the interfacial voids could be eliminated completely, the molecular sieving effect of zeolite could be clearly observed.

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feed solution and the permeates were measured by gas chromatography (G.C. Chain Chromatography 8700T). The degree of swelling of the membrane was defined by the following equation: Degree of swelling = (Ww − Wd ) / Wd × 100% (1)

where Wd and Ww denote the weight of dry and swollen membrane, respectively. 3. Results and discussion

2. Experimental Polysulfone (PSf) used in this study was supplied from AMOCO performance products Inc. (Ridgefield, CT, USA) under the trade name of (Udle-P-3500). Zeolite 4A and zeolite 13X (Aldrich) were chosen as the filler. 3-aminopropyltrimethoxysilane (APTMS, Aldrich) and trichloromethane were used as received. The APTMS is mixed with distilled water and then the zeolite is added. The reacting mixture is then stirred for adequate time. After filtration and thoroughly washing with distilled water to remove unreacted silane, the zeolite is dried at 80°C in air for 2 days. PSf solution at 10 wt% was first prepared by dissolving PSf in trichloromethane, and then zeolite were added in turn with desired percentages. The solutions were cast on a glass plate with a casting knife of 600 µm, and then dried under ambient condition for 24 h. After membranes were detached from the glass plate, the residual solvent was removed by drying for 3 days in a vacuum oven at room temperature. The crosssection morphology of membranes was observed with scanning electron microscope (SEM, Hitachi S-4100). A traditional pervaporation process was used [25]. The effective area of the membrane in contact with the feed stream was 10.2 cm2. Pervaporation experiments were carried out at 25°C. The permeation rate was determined by measuring the weight of the permeate. The compositions of the

3.1. Zeolite type effect on pervaporation performance of zeolite-filled PSf membrane Composite membranes consisting of zeolites with different characteristics, such as 4A, and 13X, and PSf polymer were prepared. The pervaporation separation of ethanol–water mixtures was performed to investigate the characteristics of zeolite effect on the pervaporation performance of zeolite-filled PSf membranes. Fig. 1 shows the effect of zeolite 4A loading on the permeation rate and water concentration in the permeate. The permeation rate slowly decreased as the zeolite 4A loading increased and then increased as the zeolite 4A loading increased over 44.4 wt%. However, the trend of water concentration in the permeate was an opposite of the permeation rate. These phenomena might be due to the fact that zeolite particles suppress the swelling of PSf in the ethanol–water mixtures. The degree of swelling decreases while the zeolite loading increases (Fig. 5), the presence of zeolite on the other hand suppresses the swelling of PSf, therefore results in a decline in the permeation rate. In addition, the permeation rate increases while the zeolite loading larger than 44.4 wt% may be due to the formation of more interfacial void in the composite membrane, which has also decreased the water concentration in the permeate. The effect of composition of feed mixtures on the pervaporation performances of PSf/zeolite 4A membrane is shown in Fig. 2. The permeation rate increased as the feed

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Fig. 1. Effect of zeolite 4A loading on the pervaporation performance of PSF/zeolite membranes.

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Fig. 2. Effect of ethanol concentration on the pervaporation performance of PSF/zeolite 4A (44 wt%) membrane.

ethanol concentration increased. This result can be explained by the plasticizing effect of ethanol. Generally, the PSf has a strong interaction with ethanol. When the ethanol concentration in the feed solution is higher, the amorphous regions of the PSf are more swelling, which results in per-

meation rate increase with the ethanol concentration in the feed solution. Moreover, the water concentration in permeates decreased with the swelling of PSf (more ethanol concentration in the feed solution). With the alteration of the type of zeolite in the composite membrane Fig. 3

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reports the permeation rate as well as the water concentration in the permeate of PSf/zeolite 13X membranes with different zeolite loading. The permeation rate increases and the water concentration in the permeate decreases obviously while the zeolite 13X loading increases. These results can be illustrated as the presence of interfacial

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void within PSf/zeolite 13X membrane that may result in permeation rate increase and water concentration in the permeate decrease. The effect of ethanol concentration in the feed on the permeation rate and water concentration in the permeate through the PSf/zeolite 13X membrane is shown in Fig. 4. The permeation rate first decreased and

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Fig. 3. Effect of zeolite 13X loading on the pervaporation performance of PSF/zeolte membrane.

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Fig. 4. Effect of ethanol concentration on the pervaporation performance of PSF/zeolite 13X (44 wt%) membrane.

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then increased with an increase in the feed ethanol concentration (50 wt% feed ethanol concentration). The water concentration in the permeate decreased with the feed ethanol concentration. Zeolite 13X shows selectivity towards ethanol, while being transported through the membrane the ethanol molecule is preferentially adsorbed but uneasily desorbed. If enough ethanol is present inside the membrane, ethanol molecules, prohibiting the water molecules from entering the pores, will largely occupy the pores of zeolite. Thus on its way through the membrane the water molecule has to travel around the zeolite particles [11]. These assumptions may possibly be the reason why the permeation rate and the water concentration in permeate decreased as the feed ethanol con-

centration increased. Depending on the plasticizing effect of ethanol, permeation rate increases and the water concentration in permeate decrease with more ethanol in the feed mixtures (Fig. 6). These results agree well with the ones mentioned above. Previous study shows that the pore size and Si/Al ratio of zeolite 13X are higher than that of the zeolite 4A [20]. The PSf/zeolite 13X membrane in this work exhibits relatively high permeation rate and lower water concentration in permeate compared to PSf/zeolite 4A membrane (Figs. 1 and 3). Based on these results of permeation rate and water concentration in permeate, it is likely that pore size and hydrophilicity of zeolite play a key role in determining pevaporation performance of zeolite-filled PSf membranes. In this

Fig. 5. Degree of swelling of the PSF/zeolite 13X membrane in aqueous ethanol (90 wt%) solution as a function of zeolite loading.

Fig. 6. Degree of swelling of the PSF/zeolite 13X (44 wt%) membrane as a function of ethanol concentration of aqueous solution.

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study, we hope to increase the permeation rate without sacrificing the water concentration in the permeate. PSf/zeolite 13X membranes have been studied as suitable membranes because of their relatively high permeation rate compared to PSf membranes so the follow-up study focuses on the PSf/zeolite 13X membrane system. Fig. 5 shows the degree of swelling of PSf/zeolite 13X membrane as a function of zeolite loading. Increasing the zeolite loading gives a significantly declined degree of swelling, indicating that the addition of zeolite suppresses the swelling of PSf. The degree of swelling of the PSf/zeolite 13X membrane increased with increasing ethanol concentration as shown in Fig. 6. These results might have been expected because of the strong interaction of ethanol with the zeolite and PSf.

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particles. It was thought feasible to make interfacial void-free zeolite-filled PSf membranes by modifying the zeolite surface with APTMS, which could interact simultaneously with zeolite and polymer. By comparing two photographs in Fig. 7 it becomes very clear that the presence of APTMS significantly improves the interfacial adhesion of zeolite and polymer, but the interfacial voids do not disappear completely. The improved interaction of zeolite and polymer inhibited interfacial void was also found in the study of Yong et al. [20]. 3.3. Transport properties of PSF/zeolite 13X/ APTMS membranes

The SEM images are shown in Fig. 7 for the cross-sectional morphology of PSf membranes filled with zeolite 13X. It can be seen from Fig. 7 that the zeolite particles are distributed uniformly throughout the polymer matrix. As shown in the Fig. 7a, PSf membrane filled with zeolite 13X contains many interfacial voids around zeolite

It can be seen from Fig. 3 that the incorporation of zeolites into a polymer matrix results in an improvement of the permeation rate of the membrane. However, the membrane contains a considerable amount of voids due to the poor adhesion of the polymer chains to the external zeolite surface, which results in a decrease of the water concentration in the permeate. In this article we use APTMS to modify the surface property of the zeolite 13X to make it more compatible with PSf. Various modification conditions were investigated to increase the water concentration in the permeate

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3.2. Morphological change of zeolite-filled PSf membranes

Fig. 7. Cross-section morphologies of the PSf membranes filled with zeolite: (a) PSf/zeolite 13X (44 wt%) (b) PSf/zeolite 13X (44 wt%)/ APTMS.

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rate and water concentration in the permeate increased with the feed ethanol concentration increase. This means that the modification of the zeolite only resulted in an increase of the PSf/ zeolite 13X/APTMS membrane permeation rate without affecting the water concentration in the permeate. This phenomenon is quite different from the result of Fig. 4. Next, the influence of the APTMS concentration on the pervaporation performance was studied using the PSf/zeolite 13X/APTMS membrane (Fig. 10). The permeation rate reached a minimum value at an APTMS concentration of 2 wt% and then increased with the APTMS concentration. In addition, the water concentrations in the permeate increased rapidly with the APTMS concentration and then remained the same. The water concentration in the permeate of PSf/zeolite 13X/APTMS(10wt%) membrane is 98.9%, which is much higher than the water concentration in the permeate of PSf/zeolite 13X membrane (89%). Zeolite addition and coupling agent treatment resulted in a significant increase in the permeation rate and a slight increase in water

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without affecting or decreasing the permeation rate. The permeation rate and the water concentration in permeate of the PSf/zeolite 13X/ APTMS membranes are plotted in Fig. 8 as a function of the APTMS treatment time. It can be observed that the modification of zeolite improves the water concentration in permeate but decreases the permeation rate. Furthermore, compared with the pure PSf membrane, the permeation rate of the PSf/zeolite 13X/APTMS membrane effectively improved. The improvement of the interfacial adhesion as demonstrated in Fig. 7 accounts for the decrease in the permeation rate of PSf/ zeolite 13X/APTMS membranes. The previous paragraph gave strong evidence that modification of zeolite improves adhesion of zeolite 13X and PSf. The next step of the study was then to investigate whether this surface modification had any effect on the effect of ethanol concentration on the pervaporation properties of PSf/zeolite 13X/APTMS. Fig. 9 illustrates the relationship between the permeation rate and water concentrations in the permeate as a function of the feed ethanol concentration. The permeation

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Fig. 8. Effect of coupling agent treatment time on the pervaporation performance of PSF/zeolite 13X (44wt%)/ APTMS membrane.

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concentration in the permeate of the PSf membrane. 4. Conclusion The incorporation of zeolites into PSf induces important changes in the pervaporation separation

performance of the composite membrane. In all cases, the incorporation of the hydrophilic zeolite caused an increase in the permeation rate but decrease in water concentration in the permeate. Adhesion between PSf and zeolite was found to be bad that led to decreasing of selectivity of the PSf/zeolite membrane. Surface modification of the

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zeolite’s external surface by means of a coupling agent (APTMS) resulted in a considerable improvement of the internal structure of PSf/zeolite membranes. It can be observed that the most promising approach for improving pervaporation properties of the PSf/zeolite membrane is the modification of zeolite by a higher coupling agent concentration combined with longer treatment time. Addition of zeolite 13X with appropriately surface modification to the PSf membrane appears to improve both permeation rate and water concentration in the permeate. Acknowledgement The authors wish to sincerely thank the National Science Council of Taiwan, ROC (NSC 91-2626-E-238-001), for the financial support. Reference [1] X. Feng and R.Y.M. Huang, J. Membr. Sci., 116 (1996) 67–76. [2] K.R. Lee, M.Y. Teng, H.H. Lee and J.Y. Lai, J. Membr. Sci., 164 (2000) 13–23. [3] L. Liang and E. Ruckenstein, J. Membr. Sci., 106 (1995) 167–182. [4] J. Hung, Y.C. Wang, C.L. Li, K.R. Lee, S.C. Fan, T.T. Wu and J.Y. Lai, Eur. Polym. J., 38 (2002) 179– 186. [5] R.Y. M. Huang, R. Pal and G.Y. Moon, J. Membr. Sci., 160 (1999) 101–113. [6] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami and K. Haraya, J. Membr. Sci., 95 (1994) 221–228. [7] Q. Liu, D.R. Noble, J.L. Falconer and H.H. Funke, J. Membr. Sci., 117 (1996) 163–174. [8] T. Sano, S. Ejiri, K. Yamada, Y. Kawakami and H. Yanagishita, J. Membr. Sci., 123 (1997) 225–233. [9] C. Bartels-Caspers, E. Tsuel-Langer and R.N.

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