Desorption and pervaporation properties of zeolite-filled Poly(dimethylsiloxane) membranes

Transcription

1 Mat Res Innovat (2001) 5: Springer-Verlag 2001 ORIGINAL ARTICLE H.Yang Q.T. Nguyen Z. Ping Y. Long Y. Hirata Desorption and pervaporation properties of zeolite-filled Poly(dimethylsiloxane) membranes Received: 11 June 2001 / Accepted: 11 June 2001 Abstract The role played by zeolite fillers in PDMS membranes was studied by desorption and pervaporation. The data on the desorption of different solvents from the membranes made of poly(dimethylsiloxane) (PDMS) filled with silica and different zeolites show that the solvent molecules are extracted in two stages. The first stage, which occurs at a low temperature (ca.50 C), would correspond to the extraction of the solvent molecules in the bulk-like state in PDMS, while the last stage, at high temperature, would correspond to the extraction of the solvent molecules in the bound state. There is a clear contribution of the zeolites to the membrane performances in pervaporation, even at a low zeolite content in PDMS (ca. 20 wt. %). The hydrophobic zeolites enhance significantly the organic flux, leading to an improvement of both the flux and the selectivity of PDMS. The ZSM-5 zeolite imparts to PDMS a higher selectivity but lower flux compared with SY-2 zeolite. The silica filler exhibits a crosslinking effect on PDMS, i.e. a slightly higher selectivity and lower flux than the pure PDMS membrane. Polydimethylsiloxane (PDMS) is the most known membrane material for the extraction of volatile organic components (VOC) from aqueous waste stream by pervaporation 1 4. Although it is quite permeable and selective to many VOC in water, its selectivity can be improved further with appropriate zeolite fillers. Such improvement may be needed for polar solutes such as aroma, fermentation products, whose high value makes the pervaporation process attractive. ZSM-5, a hydrophobic zeolite, is the most popular filler for the PDMS membranes used in the extraction of such VOC 5 8. However, FAU-type zeolites could be more interesting than ZSM-5(pore size Å), as the cavity of the hydrophobic FAU zeolites (pore size Å, see Fig. 1) 9 can accommodate large-size solutes, thus make it possible to extract solutes of higher molecular weight. In a previous paper, we studied the influence of the filler on the PDMS crystallinity and the states of different sorbed organic solvents 10. At low solvent contents, the PDMS crystallinity decreases with the increase in the solvent content due to a decrease in the zeolite-induced crystallization effect of the solvent adsorbed on the zeo- Keywords zeolite PDMS desorption pervaporation Introduction H. Yang ( ) Q.T. Nguyen Rouen University, UFR Sciences, UMR 6522, Mont St Aignan, France polymer2000@mailcity.com Z. Ping Y. Long Department of Macromolecular Science, Fudan University, Shanghai, , People s Republic of China Y. Hirata Department of Industrial Chemistry, Meiji University, Higashi-mita, Tama-ku, Kawasaki , Japan Fig. 1 The structure of Y zeolite

2 102 lite surface. At higher solvent contents, the solvent influence on the PDMS crystallite prevails. A medium solvent (ethyl acetate) induces the polymer crystallization, while a good solvent (cyclohexane and hexane) disrupts PDMS crystallites. On the other hand, the absorbed solvent molecules may exist in the materials under a bound state or bound and bulk-like states, depending on the solvent content in the materials. The present work aims at studying the behavior of zeolite-filled PDMS membranes in the extraction of organic solvents by desorption and by pervaporation. In fact, pervaporation study of zeolite filled membrane is not enough to give a complete protocol of such complex membrane, as the system is multiphase. Desorption is the last step in the pervaporation transport mechanism. A study of the process of VOC desorption from different membranes may lead to a better understanding the role played by zeolite fillers in the membrane materials, give a better understanding of the pervaporation behavior of such complex membrane. The fillers chosen for the present work, silica, ZSM-5 silicalite, and three FAU-type zeolites, allow us to study the influence of the filler nature on the behavior of filled PDMS membranes. Experimental Materials Fumed silica was purchased from Shanghai Chemicals. Ultem P3500 polyimide was obtained from General Electrics. The raw material for the preparation of two types of FAU zeolite, named SY-1 and SY-2, was the hydrophilic NaY zeolite provided by Wenzhou Chem. Corp (China). SY-1 was prepared by treating NaY with SiCl 4 for 4 hours. SY-2 was prepared by hydrothermal treatment of SY-1 at 800 C for 12 hours. All the zeolites were fully characterized in a previous work 11. Among these zeolites, SY-2 is highly hydrophobic, with only trace of hydrophilic Al and surface OH in the framework. It has also the most perfect crystalline structure. The 20% filled PDMS membranes were prepared according to the method reported elsewhere 11. Briefly, the filler was dispersed in a cyclohexane solution of PDMS and its crosslinker until a homogeneous suspension was obtained, the suspension was then cast into a liquid film, which is subsequently dried and allowed to crosslink for 48 hours at room temperature. Films of ca. 300-micrometer thickness were used in the tests. The zeolite-filled microporous polyimide membranes were prepared by dispersing the filler in a 12 wt.% polyimide solution in N-methyl pyrrolidone. The dispersion medium was next cast into a film, then rapidly coagulated in water. Before its equilibration with a liquid medium, the sample was activated at 50 C under vacuum (<2.7 Pa) complete desorption of solvent contaminants. The sorbed amount is expressed as the weight of sorbed solvent per 100 g of dry material. Desorption experiments were performed by thermogravimetric analysis (TGA) (with a NETZSCH DSC 200) under a dry nitrogen stream on the samples previously equilibrated with the studied solvent at room temperature. The scanning rate was 10 C per minute. The desorption data are shown as the proportions of solvent desorbed at different temperatures. The weight of the completely dry membranes after thorough desorption was used for the calculation of those proportions. Pervaporation measurements The pervaporation separation of aqueous solutions of VOC was measured at 30 C under 100 Pa pressure on the downstream side, using a stainless steel cell with an effective membrane area of 28 cm 2. The permeate collected in a trap cooled in liquid nitrogen for given time intervals was analyzed by gas chromatography. The permeation flux J and selectivity α were calculated as follows: J=Wp/(A*t) α=(csolvent/cw) p /(Csolvent/Cw) f where W is the weight of the permeate, A is the membrane area, t is the pervaporation time; Csolvent and Cw are respectively the solvent and the water weight fractions, and p, f subscripts refer to the permeate and the feed liquids, respectively. Discussion Desorption from free zeolites The desorption behavior of zeolites has been extensively studied in the literature 12,13. Normally the solvent desorption temperature is used to characterize the interaction power between the zeolite and the solvent molecules. High desorption temperatures mean that more energy is needed to extract the solvent out of the zeolite pore channel. The zeolite used as a filler for membranes should have a desorption temperature as low as possible, since a high solvent desorption energy makes larger the contribution of the desorption step to the total transport resistance in the pervaporation process. The desorption data for solvents from three zeolites are listed in Table 1. The highest desorption temperatures were obtained with the NaY zeolite, whatever the solvent. This can be explained by the presence of Na + ions in its framework which makes possible strong solvent-nay zeolite interactions (of electrostatic or charge transfer type). After de-alunimation, Na + and Al ions are removed from the zeolite framework, the solvent-nay zeolite interactions change to Van der Waals forces. The lower solvent desorption temperature reflects the weak Van der Waals with the de-aluminated zeolites (SY-1 and SY-2, Table 1). The SY-2 zeolite shows higher desorption temperature for ethyl acetate than SY-1 one, while the contrary was found for ethanol. This means SY-2 has Desorption measurements Table 1 The desorption results of ester and ethanol from FAU zeolites Desorption temperature ( C) Desorption amount (g/100 g dry material) Ethanol Ethyl acetate Ethanol Ethyl acetate NaY SY SY

3 103 Fig. 2 Desorption curves of ethyl acetate from filled and unfilled PDMS membrane Fig. 3 Desorption curves of ethanol from filled and unfilled PDMS membrane stronger affinity with non polar solvent. It is quite normal as SY-2 zeolite does not have any framework-al atoms, neither surface-oh groups, as does SY-1 one 11. In other words, the less polar the solvent, the stronger its affinity with the zeolites having less ionic and silanol groups. However, a polar VOC like ethanol still develops strong enough interactions with SY-2, the most hydrophobic zeolite, to lead to a desorption temperature higher than 50 C. The desorption amount is related to the pore volume of zeolites. The amounts of ethanol and ethyl acetate desorbed from zeolites decrease according to the order: NaY>SY-1>SY-2 (Table 1), in agreement with the surface area and pore volume orders of the zeolites 11. It can also be noticed that NaY zeolite does not show clear differences in affinity between organic molecules of different polarities. Desorption from zeolite-filled silicone rubber The desorption curves of ethanol and ethyl acetate on PDMS and zeolite-filled membranes are shown in Figs. 2 and 3. The curves were drawn with the raw TGA data, so that they start at 100% sorbed solvent on the Y-axis. The desorption curves of ethyl acetate for both the filled and unfilled PDMS exhibit two distinct segments: a steep segment which is almost perpendicular to the X axis at below about 50 C, and a leveling-off segment at temperatures above 50 C. In the first part, the solvent loss accounts for more than half of the total desorbed amount. The fast desorption reflects the weak interactions between the solvents and the materials. Solvent having strong interactions with the polymer desorbs at a much slower rate in the second part. We showed in a former DSC study the existence of two states of solvent molecules sorbed in PDMS, namely the bound state and bulk-like state 10,14. The solvent molecules in the bound state should be in contact with the polymer chains, thereby having strong interactions with the polymer. The solvent molecules in the bulklike state stay at larger distance from the polymer chains, thus having weaker interactions with the polymer. Due to the weaker interactions, and also to the tendency of the stretched PDMS chains (in the swollen matrix) to recover their relaxed conformation, the extraction of the latter molecules would be much easier. Therefore, less steep segment in the desorption curve would correspond to the extraction of solvent molecules in the bulklike state, and the low-slope segment to the solvent molecules in the bound state. Ethanol which has a very low sorption level in PDMS, should exist in PDMS in the bound state, as reflected by a unique low-slope segment in its desorption curve (Fig. 3). On the contrary, PDMS has much higher affinity towards ethyl acetate ester, and the more swollen PDMS has the ester molecules in both the bound and bulk-like states in its matrix 10,14. The similar two-segment curves observed for the pure PDMS as well as the zeolite-filled PDMS (Fig. 2) is consistent with the above explanation. They further indicate that the PDMS phase controls the ester desorption from all membranes. The larger fraction of ester desorbed from the zeolite-filled membranes (Fig. 2, Table 2) reflects the contribution of the zeolite phase as ester reservoir. Virtually, from the view of thermodynamics, the system of crosslinked polymer swollen in solvent is an equilibrium system, after it leaves the solvent phase, the entropy effect forces the phases of solvent and polymer separation, or desorption of solvent from polymer. For the desorption in the high swelling degree, mainly for good solvent, the entropy effect play a mainly role, for the polymer chain was stretched, fixed by solvent molecular. Such morphology needs very high energy, is unstable, since we know the chain of silicone rubber is quite

4 104 Table 2 The desorption amount (g per g of dry membrane) of zeolite phase in filled PDMS membrane in the range of C (left) and its amount in the free state at the same temperature range. (assuming that the desorption amount from the PDMS phase in zeolite-filled membranes is the same as that in SiO 2 filled membrane) Membrane Ethanol Ethyl acetate NaY 3.2%/14.8% 5.2%/21.2% SY-2 1.5%/6.3% 3.6%/13.2% SY-1 1.1%/4.3% 3.5%/12.7% PDMS in SiO 2 filled membrane 0.3% 0.7% Pure PDMS 0.3% 1.2% Fig. 4 Desorption curves of ethyl acetate from filled and unfilled Polyimide membrane flexible. Thus the desorption in high swelling degree is much quick. When the content of solvent in membrane becomes lower, the effect of entropy decreases too, then enthalpy effect becomes more important, it will increase of decrease such process, depending on its property. and the incorporation of zeolites only have some influence on such process, but can t change it. Furthermore, the desorption amount of zeolite was calculated to examine the role of zeolite in filled membrane(table 2). Over the same temperature range, the amounts of solvent desorbed from zeolites in the filled PDMS membrane reached only about 1/4 of those in the free state (Table 2). Such a large decrease in the zeolite desorption amount cannot be only explained by the loss in the channel volume due to the partial channel occlusion by PDMS chain segment. In order to explain such a loss, let us describe the microscopic structure of Y zeolite. Zeolite particle is not a real single crystal cell, composed of Na 56 [Al 56 Si 136 O 384 ] 264H 2 O (NaY) 9, but a gathering of many single crystal cells. In our case, a zeolite particle is multi-crystals, volume about 10,000 Å 10,000 Å 10,000 Å, with a large number of pores entrance about Å on its surfaces. The diameter of the PDMS unit is about 6.4 Å. As the PDMS chains are very flexible, provided that it is not highly crosslinked, they will occupy some part of pore channel, but mainly the entrance of zeolite channels. It is impossible to occupy the majority part of pore volume of zeolite, as the diffusion of a very long flexible chain molecules into a narrow pore channel is very difficult. In fact, the desorption experimental data for filled membrane are not simple to analyze due to the interplay of the kinetic processes and the thermodynamic process. In the view of thermaldynamic, the swollen polymer is unstable. While on the countrary, the desorption of zeolite is non spontaneous, as the zeolite has very large surface area, it is unstable in thermodynamic. Due to the strong interaction between PDMS and zeolite, there exists an equilibrium distribution of solvent between zeolite and polymer phases. As the solvent leaves the outer part of the PDMS matrix, the created chemical potential gradient induces the diffusion of solvent first from the zeolite channels towards the zeolite-pdms interface, then towards the outer part of the PDMS matrix. During this process, the difference between diffusion rates become very important, which is a kinetic view. The diffusion coefficient of common organic solvents in zeolite is much smaller than that in the rubbery PDMS, i.e instead of 10 6 cm 2 /s 15,16. On the other hand, the diffusion coefficient of organic vapors in a gas phase is several order of magnitude larger than that in PDMS 17. This leads to that when solvent molecules diffuse outside the pore channel of zeolite, it becomes much easier for them to jump into gas atmosphere through PDMS phase than reenter into the pore channel of zeolite. This means the larger desorption amount at relatively lower temperature for the zeolite particles embedded in a PDMS matrix. In this way, we can see that the zeolite is more like a reservoir of solvent in filled membrane. Moreover it should be pointed out, although in phenomena, the lower desorption amount of zeolite phase in filled membrane at high desortpion temperature is consistent with its lower adsorption amount at low partial pressure, the reason differs. The later is due to the less pore entrances of zeolites available for solvent molecules in filled membrane 11. After solvent molecules occupied the remained pore channel, the later solvent molecules would meet greater difficulty to penetrate into the inner part of the pore channel of zeolite, that is, the higher solvent vapor pressure or activity. Influence of the nature of the polymer matrix on the desorption behavior We further investigate the desorption behavior of zeolitefilled polyimides. Figs. 4 and 5 show that the desorption curves for unfilled and filled microporous polyimide membranes are different from those for PDMS (Figs. 2 3). For both filled and unfilled polyimide membranes, when the temperature increases, the amount of desorbed solvent gradually increases with temperature. In spite of the presence of micropores in the polymer matrix, the

5 Fig. 5 Desorption curves of ethanol from filled and unfilled Polyimide membrane 105 desorption is more progressive (without the steep desorption part). In the absence of data on the states of solvents in polyimide, we speculate that there is no bulklike solvent, the one which can be easily extracted, in the polyimide-based membranes. Indeed, unlike PDMS which is a crosslinked elastomer ( 124 C glass transition temperature), polyimide is an uncrosslinked glassy polymer (Tg=250 C). The sorption of these poor solvents in polyimide is very low and the solvent cannot exist in the bulk-like state in polyimide; if the solvent power was high, the uncrosslinked polyimide would not swell to a large extent as PDMS, but dissolve in the solvent. Furthermore, the diffusivity of solvent molecules in the glassy polyimide would be ca. four orders of magnitude smaller than that in PDMS. The extraction of dispersed solvent molecules through polyimide would be limited by the diffusion towards the surface of the polymer in contact with the gas phase (i.e. the potential well for the desorption). Therefore, the solvent molecules are gradually desorbed from the polymer, as well as from the zeolite phase via the polyimide phase. The behavior of VOC in the desorption from the filled membranes with different polymer matrices is schematically shown in Fig. 6. Extraction of VOC from aqueous solution by pervaporation Fig. 6 Schematic representation of the desorption behavior of zeolite-filled PDMS (left) and zeolite-filled polyimide (right). In PDMS, the solvent in the bulk-like state and that in the bound state have different extraction rates. In polyimide, the desorption is controlled by the very slow diffusion of the solvent in the glassy polymer Tables 3, 4 and 5 list the pervaporation flux and selectivity of different membranes in the extraction of ethanol, 2-propanol and ethyl acetate from aqueous solution. For all feed systems, the silica filled membrane has the lowest flux with a selectivity slightly higher than that of the unfilled membrane. As silica has negligible sorption capacity, it acts mainly as a physical crosslinker for PDMS 18. An increase in the PDMS crosslinking is known to lead to an increased selectivity and a decreased flux due to the increase in its resistance to the swelling by solvent sorption 19. Table 3 Flux and selectivity of filled and unfilled PDMS membrane in the pervaporation of 4.78% ethanol/water mixture at 50 C Selectivity Flux(total) Flux(water) Flux(ethanol) α (g/m 2.h.100 µ) (g/m 2.h.100 µ) (g/m 2.h.100 µ) PDMS %SiO %SY %ZSM Table 4 Flux and selectivity of filled and unfilled PDMS membrane in the pervaporation of 4.84% 2-propanol/water mixture at 50 C Selectivity Flux(total) Flux(water) Flux(2-propanol) α (g/m 2.h.100 µ) (g/m 2.h.100 µ) (g/m 2.h.100 µ) PDMS %SiO %SY %ZSM

6 106 Table 5 Flux and selectivity of filled and unfilled PDMS membrane in the pervaporation of 1.23% ethyl acetate /water mixture at 50 C Selectivity Flux(total) Flux(water) Flux(ester) α (g/m 2.h.100 µ) (g/m 2.h.100 µ) (g/m 2.h.100 µ) PDMS %SiO %SY %ZSM The selectivity of the zeolite-filled membranes is higher than that of the silica-filled membrane. This means that the increase of their selectivity is not only caused by the crosslinking effect of the incorporated fillers, but also by their higher sorption ability at low activity. The zeolite filled membranes have higher solvent fluxes, but lower water fluxes than the unfilled membranes (Tables 3, 4 and 5). The higher solvent flux would come from the contribution of the zeolite phase to the sorption, which leads to a higher diffusion rate of solvent in the PDMS phase, by maintaining a higher solvent content in the phase surrounding the solvent reservoirs. The low water flux would be caused by the lower amount of sorbed water in the hydrophobic zeolites, and the more tortuous route for diffusion (as the diffusion through the zeolite particles is very slow compared with that through PDMS). ZSM-5-filled membranes shows higher selectivity but smaller flux than the SY-2 one (Tabs. 3 5). This behavior can be explained by the smaller volume of the ZSM-5 cavity compared with that of SY-2; thus ZSM-5 sorbs less but more selectively VOC molecules than SY-2 does. It is understandable that the larger pores of SY-2 would also let water go through more easily, leading to a lower selectivity for the SY-2 filled membrane, in comparison with the ZSM-5 one. The higher sorption extent in the filled membranes, which is shown in our former study, compared with the unfilled one supports the explanation 11,20. Among the studied VOCs, ethyl acetate is extracted with the highest selectivity and also with the highest flux: the flux of ethyl acetate is much larger than the flux of the two alcohols (Tables 3 5). The high selectivity towards the ester is consistent with the increasing VOC sorption when the VOC hydrophobicity increases. If we consider the activity of water in three solutions, the water activity is the highest in the ester solution. Normally, if there is no influence of the solvent on the sorption and diffusion of water in membrane, the water flux should be the lowest. Such abnormal high water flux for the ester solution should be the result of a larger ethyl acetate sorption in membrane, which leads a better water sorption in the membrane, and faster diffusion of water due to the membrane plasticization. The highest flux value for pure water can be explained by the highest chemical potential for pure water(which is the driving force for water transport). Conclusion The incorporation of hydrophobic zeolites into PDMS enhances the permeation selectivity towards the VOCs, but decreases the permeation rate in the corresponding membranes. Such a behavior can be explained by the contribution of the zeolite phase to the VOC sorption. The higher the VOC hydrophobicity, the larger the VOC sorption, and the larger the permeation selectivity. The decrease in the permeation rate results from the crosslinking effect of the zeolite particles, and also from the increase of the diffusion path: the zeolite particles act as solvent reservoirs in the sorption, but as obstacles for the permeant diffusion. Acknowledgements We are indebted to the Ministry of Education and Research of France for the fellowship granted to Y.H. and we would like to thank the Key Laboratory of Polymer Molecular Engineering of the Ministry of Eduction of China for their financial support. References 1. 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