water mixtures through silicalite filled polydimethylsiloxane membranes

Journal of Membrane Science 176 (2000) 159–167

Pervaporation of acetic acid/water mixtures through silicalite filled polydimethylsiloxane membranes Shih-Yuan Lu∗ , Chung-Ping Chiu, Hsiang-Yuan Huang Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan 30043, ROC Received 1 October 1999; accepted 31 March 2000

Abstract The preferential pervaporation of acetic acid over water is achieved with silicalite filled polydimethylsiloxane (PDMS) membranes. The effect of silicalite addition is not positive at the feed temperature of 25◦ C, but improves with increasing feed temperature. At a feed temperature of 45◦ C, silicalite addition enhances not only the separation factor but also the permeation flux of the pervaporation. This improvement may be attributed to the reduction in kinetic limitation on sorption/desorption processes and the enlargement of sorption difference between acetic acid and water towards silicalite. At 25◦ C, the sorption ratio of acetic acid to water is 3.9, but 4.9 at 45◦ C. It is further found that at a silicalite loading of 49.9 wt.%, the separation factor versus feed acetic acid concentration curve exhibits a maximum and this maximum shifts to lower feed acetic acid concentrations with increasing feed temperature. Further increasing the silicalite loading to 69.2 wt.%, results in the formation of connected pores in the membrane and thus failure of the membrane in providing a separative pervaporation. The addition of silicalite is also found to enhance the thermal stability of the membrane. The pervaporation behavior of the silicalite filled PDMS membrane seems to fall in between those of pure PDMS and pure silicalite membranes. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Acetic acid; Silicalite; Composite membrane; Absorbent

1. Introduction Membrane pervaporation has become one of the most promising candidates for low-cost separation processes, especially for mixtures of close volatility, and mixtures of thermal or chemical sensitivity [1]. Its applications can be found in (a) dehydration of organic/water (azeotropic) mixtures; (b) removal/ recovery of organic compounds from water and (c) separation of organic/organic mixtures. Among the three major applications, most of the past work in this ∗ Corresponding author. Tel.: +886-3-561-4364; fax: +886-3-571-5408. E-mail address: [email protected] (S.-Y. Lu)

area has been devoted to the dehydration of organic/ water (azeotropic) mixtures. The main reason is probably for the relative ease in developing pervaporation membranes of high water selectivity. It is, however, necessary to develop organophilic membranes for more efficient separation of water-rich organic/water mixtures. In the area of organophilic pervaporation membranes, there has been quite a lot of work done for the ethanol/water system but much less effort for the acetic acid/water system. Bai et al. [2] attempted the pervaporation separation of acetic acid/water mixtures by silicone rubber-coated polyetherimide membranes. They found that the laminated membrane could become either water or acetic acid selective depending

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on the pore size of the supporting polyetherimide membrane and the coating condition of the silicon rubber (polydimethylsiloxane, PDMS). Basically, polyetherimide is water selective while PDMS is acetic acid selective. The laminated membrane becomes water selective when polyetherimide is dominant, acetic acid selective when the effect of PDMS is more prevailing, and non-selective if the two components are comparable. Yoshikawa et al. [3] used crosslinked polybutadiene membranes for preferential pervaporation of acetic acid. The preferential pervaporation of acetic acid was attributed to hydrophobic nature of the crosslinked polybutadiene. Deng et al. [4] investigated the pervaporation behaviors of PDMS, aromatic polyamide (PA), and laminated PDMS/PA membranes. They found that PDMS was hydrophobic and acetic acid selective while PA was hydrophilic and water selective. The lamination of PDMS with PA tended to impair the acetic acid selectivity of PDMS. Liu et al. [5] used silicalite membranes supported on the inner surface of a porous stainless steel cylindrical tube to preferentially separate organics from corresponding aqueous solutions. They found that silicalite membrane was unable to selectively remove acetic acid from acetic acid/water mixtures at low acetic acid concentrations. However, Sano et al. [6,7] also prepared silicalite membranes on sintered stainless and porous alumina supports for separation of acetic acid/water mixtures and found that silicalite membrane coated on the sintered stainless steel support was able to selectively permeate acetic acid in the acetic acid feed concentration range of 5–40 vol.%, while that coated on the porous alumina support showed no separation effects. Silicalite membranes, although show better acetic acid selectivity, are lack of necessary mechanical strength for stand-alone usage and normally need a support for applications. A recent development for pervaporation using adsorbent-filled membranes [8] has fostered a great number of possibilities in creating new, improved composite separation membranes. It may prove useful to incorporate microporous adsorbents of high acetic acid sorption capacity (such as silicalite) into a hydrophobic polymeric matrix such as PDMS for preferential pervaporation of acetic acid from the acetic acid/ water mixtures. In fact, silicalite filled polymer membranes have been used in pervaporation studies of various organic/water and organic/organic mixtures,

such as methanol/toluene [9], propanol/water [10], and alcohol/water [11–13]. In this article, we study the pervaporation characteristics of silicalite filled PDMS membranes for preferential pervaporation of acetic acid from acetic acid/water mixtures. The important operation conditions investigated include silicalite loading, feed acetic acid concentration, and feed temperature. All three parameters are found to play an important role in determining the pervaporation behavior of the membrane. The filled membrane shows improvement on both the separation factor and permeation flux over the unfilled membrane at high feed temperatures, and this improvement becomes more pronounced with increasing silicalite loading. At a silicalite loading of 49.9 wt.%, there exists an optimal feed acetic acid concentration at which the separation factor is a maximum.

2. Experimental 2.1. Preparation and characterization of silicalite A gel for synthesis of silicalite is prepared following the procedure of Grose and Flanigrn [14]. A mixture of 0.95 g NaOH (Wako, Japan), 2.1 g TPABr (tetra-n-propylammonium bromide) (Lancaster, UK), and 10 g 30 wt.% SiO2 sol (Aldrich, USA) is mixed thoroughly with 125 g deionized water in a flask by stirring for 12 h. The solution is then poured into a clean teflon cup of the autoclave. The autoclave is then tightened and placed in a vacuum oven. The crystallization proceeds in the autoclave for 3–12 h at a temperature of 180◦ C. The resulting suspension of silicalite is centrifuged and washed with deionized water. This procedure is repeated (usually three to four times) until the suspension is tested neutral. The washed collection is dried in a vacuum oven set at 50◦ C for 1–2 h before transferring to a furnace to be calcined at 550◦ C for 16 h. The calcination drives out the residual reactants and solvents trapped inside silicalite to make room for later sorption performance. Roughly 13 wt.% loss of silicalite accompanies the calcination. The XRD (Shimadzu, XD-5) spectrum of the obtained silicalite shows major adsorption peaks in 2θ of 6–9◦ and 23–24◦ , complying with that obtained by Zecchina et al. [15] who studied the structure, adsorptive capacity, and IR spectroscopic characteristics

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of silicalite. The specific surface area of silicalite is measured with BET (Micromeritics, ASAP2000), and ranges from 360 to 400 m2 /g, which is typical for zeolite materials. The size distribution of silicalite particles is measured with both dynamic light scattering (for particles smaller than 1 ␮m) and natural sedimentation (for particles larger than 1 ␮m) by a particle size analyzer (Otsuka Electronic, LPA-3000). The size distribution is bimodal in terms of weight average with two average diameters of 0.7 and 13.3 ␮m. We also study the sorption behavior of silicalite with respect to acetic acid and water, since it is one of the major contributing factors to the preferential pervaporation of acetic acid. Two grams of silicalite particles, after being dried in a vacuum oven at 250◦ C for 2.5 h, are soaked with 10 ml of acetic acid or deionized water for 24 h. The soaked silicalite particles are placed in the hood for mild drying, and the weight loss of silicalite particles is weighed every 0.5 or 1 h until the weight loss becomes negligibly small. The specific sorption amount versus drying time curves are plotted in Fig. 1. It is evident from the plot that the specific sorption amount levels off after certain period of drying, and there is a sharp turn before leveling off. At this turning point, the excessive surface acetic acid or deionized water is dried up and the further supplied energy is used to break up the physical association between silicalite and acetic acid/deionized water, resulting in a dramatic slow down in silicalite weight loss. The sorption measurement is performed

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at two different temperatures, 25 and 45◦ C, to investigate the effect of temperature on silicalite sorption. It is evident that silicalite is hydrophobic since it adsorbs much more acetic acid than water, and this hydrophobicity increases with increasing temperature, as can be judged from the increase of sorption ratio (acetic acid versus water) from 3.9 at 25◦ C to 4.9 at 45◦ C. It is worth mentioning that if silicalite particles are not calcined, the residues trapped inside silicalite will block the passage for following sorption, and silicalite shows no sorption capacity for both acetic acid and water. It is to be stressed that the present sorption experiments are performed with respect to pure water or acetic acid, while the situation encountered in the pervaporation process is sorption in an acetic acid/water mixture. 2.2. Preparation and characterization of silicalite filled PDMS membranes A desired amount of silicalite is dried at 180◦ C for 1 h in a vacuum oven, and then mixed with a suitable amount of toluene (Union Chemical Works, Taiwan). The resulting suspension is sonicated for 1 h to help disperse the silicalite powder. The sonicated suspension is then mixed with a desired amount of PDMS (Shin-Etsu, Japan), and stirred for 1 h. The silicalite filled PDMS paste is cast with a casting knife on a teflon plate, and dried and cured the formed membrane at 120◦ C for 4 h in a vacuum oven.

Fig. 1. Sorption curves of silicalite particles with respect to acetic acid and deionized water at 25 and 45◦ C.

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Fig. 2. The TGA graph of PDMS and 49.9 wt.% silicalite filled PDMS membranes.

TG analysis is performed for pure PDMS and silicalite filled PDMS membranes. It is evident from Fig. 2 that the addition of silicalite improves the thermal stability of PDMS. The decomposition temperature raises from about 350◦ C for pure PDMS to about 410◦ C for 49.9 wt.% silicalite filled PDMS membranes. Interestingly, the weight loss at thermal decomposition is much larger for the silicalite filled PDMS than that for the pure PDMS. One possible explanation is as follows. The presence of silicalite particles helps to adsorb the thermal energy and thus protects PDMS from thermal attack. On the other hand, the presence of silicalite particles also interferes with the cross-linking of PDMS at the curing stage and makes PDMS less cross-linked. The former effect helps to raise the decomposition temperature, while the latter effect makes PDMS easier to decompose into smaller and thus more volatile products, leading to a more rapid weight loss. The top and cross-sectional views of the resulting membranes are observed with SEM. Fig. 3 shows the top and cross-sectional view SEM graphs for (a) pure PDMS; (b) 49.9 wt.% silicalite filled PDMS and (c) 69.2 wt.% silicalite filled PDMS membranes. The pure PDMS and 49.9 wt.% silicalite filled PDMS membranes are both dense with no connected pores, while the 69.2 wt.% silicalite filled membrane is quite porous with many connected pores. 2.3. Pervaporation The membrane is mounted on the opening of a jacketed stainless steel cylinder through which a circulat-

ing water stream is flowing to maintain a constant feed temperature. Thirty to forty grams of acetic acid of desired concentration are poured into the jacketed cylinder. The jacketed cylinder is then screw tightened with a second stainless steel cylinder that is connected to the vacuum pump. Collecting tubes immersed in liquid nitrogen and located downstream of the membrane holder are used to collect the permeation product. The pervaporation is run for at least 1 h before we mount the liquid-nitrogen containing tube into the collecting tube to start the condensing collection. We have checked that 1 h of pre-collection pervaporation run is enough to get the system up to a steady state. The composition of the permeation product is determined with GC (Shimadzu, GC-14A). The upstream pressure of the pervaporator is atmospheric pressure, while the downstream pressure is maintained at 110–140 mTorr. From the collection amount and permeation product composition, the permeation flux and separation factor can be obtained. The separation factor is defined as α=

yAA /yw , xAA /xw

where xAA (xw ) is the weight fraction of acetic acid (water) in the feed, and yAA (yw ) is the weight fraction of acetic acid (water) in the permeation product.

3. Results and discussion 3.1. Effect of feed acetic acid concentration at 25◦ C Fig. 4 shows the separation factor and permeation flux versus feed acetic acid concentration plots for the feed temperature of 25◦ C for three silicalite loadings: 0, 17.8, and 49.9 wt.%. The membrane thickness is determined by cross-sectional view SEM graphs of the membranes and is controlled to be around 95 ␮m. In the figure, we include a set of scattered data for the pure PDMS result to ensure the data reliability. The data reliability appears to be reasonably good. Evident from the figure, these membranes are all preferential to acetic acid pervaporation. For the pure PDMS membrane, both the separation factor and permeation flux increase with increasing feed acetic acid concentration. As for the 17.8 wt.% silicalite filled PDMS membrane, the separation factor seems to reach a saturation state, and the permeation flux

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Fig. 3. Top view SEM graphs of (a) PDMS; (b) 49.9 wt.% silicalite filled PDMS and (c) 69.2 wt.% silicalite filled PDMS membranes. Cross-sectional view SEM graphs of (d) PDMS; (e) 49.9 wt.% silicalite filled PDMS and (f) 69.2 wt.% silicalite filled PDMS membranes.

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Fig. 4. Plots of (a) separation factor and (b) permeation flux vs. feed acetic acid concentration for PDMS, 17.8 wt.% silicalite filled PDMS, and 49.9 wt.% silicalite filled PDMS membranes at feed temperature of 25◦ C.

increases with increasing feed acetic acid concentration. A maximum and a minimum appear in the separation factor and permeation flux curves, respectively, for the 49.9 wt.% silicalite filled PDMS membrane. Deng et al. [4] studied the pervaporation of PDMS for the mixture of acetic acid and water, and found that the permeation flux increases with increasing feed acetic acid concentration and the separation factor also increases with increasing feed acetic acid concentration for feed acetic acid concentration less than 59 wt.%. Their finding is the same with ours. In addition, Sano et al. [7] studied the pervaporation of silicalite membranes (on a stainless steel support) for the mixture of acetic acid and water, and found that the permeation flux drops with increasing feed acetic acid concentration and levels off at the feed acetic acid concentration of about 15 wt.%, while the separation

factor goes through a maximum before dropping to almost 1 at the feed acetic acid concentration of 45 wt.%. Our results for the silicalite filled PDMS membranes seem to be a composite of those of Deng et al. [4] and Sano et al. [7]. For the 17.8 wt.% silicalite filled PDMS membrane, it behaves more like the PDMS membrane in permeation flux, but silicalite characteristics start to influence its performance in separation factor at higher feed acetic acid concentrations. As for the 49.9 wt.% silicalite filled PDMS membrane, with much more silicalite present in the membrane, silicalite characteristics play a more important role in determining its pervaporation behavior. As a result of competition between silicalite and PDMS characteristics, the permeation flux of the 49.9 wt.% silicalite filled PDMS membrane drops initially, but rebounds to increase with increasing feed acetic acid concentration due to the effect of PDMS characteristics. For the separation factor, it goes through a maximum with increasing feed acetic acid concentration, apparently showing the silicalite characteristics. It is also evident from Fig. 4 that at 25◦ C, the addition of silicalite does not enhance the preferential performance of PDMS membranes towards acetic acid, despite the fact that silicalite shows a high preferential sorption for acetic acid over water. This could be due to the kinetic barrier that exists in the sorption and desorption processes of acetic acid. If this is the case, then raising the feed temperature should improve the situation by providing more energy to overcome the activation energy associated with the sorption and desorption processes of acetic acid. Liu et al. [5] prepared silicalite membranes on stainless steel support which gave a separation factor of 0.3 and a permeation flux of 200 g/m2 h for a feed acetic acid concentration of 5 wt.%. On the other hand, the silicalite membranes on stainless steel prepared by Sano et al. [7] gave a separation factor of 2.6 and a permeation flux of 38 g/m2 h for a feed acetic acid concentration of about 15 wt.% and a feed temperature of 30◦ C. Our silicalite-filled PDMS membranes, at roughly the same pervaporation condition, gives a lower separation factor but a higher permeation flux than the membranes of Sano et al. [7]. However, the separation factor and permeation flux of membranes made by Sano et al. [7] drop to about 1.2 and remain at 38 g/m2 h, respectively, when the feed acetic acid

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concentration goes up to 40 wt.%, while those of the silicalite-filled PDMS membranes of this work stay at above 1.5 and increase to about 80 g/m2 h, respectively. The incorporation of silicalite into a polymeric matrix seems to offer certain advantages. 3.2. Effect of feed temperature In Figs. 5 and 6, we examine the effect of feed temperature on the pervaporation performance of PDMS membranes of silicalite loadings of 0, 17.8, and 49.9 wt.%, with the feed acetic acid concentration set at 10 and 15 wt.%, respectively. The figures show clearly that the advantage of silicalite addition becomes more pronounced with increasing feed temperature. The improvement is achieved not only for the separation factor, but also for the permeation flux.

Fig. 6. Plots of (a) separation factor and (b) permeation flux vs. feed temperature for PDMS, 49.9 wt.% silicalite filled PDMS membranes with feed acetic acid concentration set at 15 wt.%.

Although it has been shown in Fig. 1 that the equilibrium sorption amounts of acetic acid and water decrease with increasing temperature, the sorption and desorption processes proceed more rapidly with increasing temperature, resulting in an overall increase in permeation flux. Basically, raising temperature lessens the relevant kinetic limitation, and thus reflects the advantage of silicalite addition in preferential sorption to that in preferential pervaporation. Without enough thermal energy, the advantage of silicalite addition in preferential sorption will not have the chance to show in the pervaporation process. 3.3. Effect of feed acetic acid concentration at 45◦ C Fig. 5. Plots of (a) separation factor and (b) permeation flux vs. feed temperature for PDMS, 17.8 wt.% silicalite filled PDMS membranes with feed acetic acid concentration set at 10 wt.%.

It is also interesting to examine the effect of feed acetic acid concentration on pervaporation improve-

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Fig. 7. Plots of (a) separation factor and (b) permeation flux vs. feed acetic acid concentration for PDMS, 17.8 wt.% silicalite filled PDMS, and 49.9 wt.% silicalite filled PDMS membranes at feed temperature of 45◦ C.

ment achieved at a higher feed temperature (45◦ C). Fig. 7 shows that at 45◦ C, the silicalite filled PDMS membranes consistently have better performance over the unfilled PDMS membrane on both separation factor and permeation flux. The improvement becomes more pronounced with increasing silicalite loading in the PDMS membrane. The separation factor curve of the 49.9 wt.% silicalite filled PDMS membrane again shows a maximum as that occurs at the feed temperature of 25◦ C, only that the peak occurs at a lower feed acetic acid concentration.

4. Conclusions The preferential pervaporation of acetic acid over water by using silicalite filled PDMS membranes is studied. The pervaporation behavior of the silicalite filled PDMS membrane seems to fall in between those of pure PDMS and pure silicalite membranes. As the feed temperature is increased, the kinetic limitation on sorption and desorption processes of silicalite with acetic acid becomes negligible and the effect of equilibrium sorption preference for acetic acid is

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materialized to improve both the separation factor and permeation flux of the pervaporation. Acknowledgements The authors gratefully acknowledge the support of the National Science Council of the Republic of China under grant NSC 89-2214-E-007-003. The authors also thank Profs. Ken-Sen Chou and Yu-Der Lee of our department for their generosity in providing relevant analytic instruments (BET, TGA, PSDA). References [1] R.Y.M. Huang, Pervaporation Membrane Separation Processes, Elsevier, New York, 1991. [2] J. Bai, A.E. Fouda, T. Matsuura, J.D. Hazlett, A study on the preparation and performance of polydimethylsiloxane-coated polyetherimide membranes in pervaporation, J. Appl. Polym. Sci. 48 (1993) 999. [3] M. Yoshikawa, S. Kuno, T. Wano, T. Kitao, Specialty polymeric membranes. 4. Pervaporation separation of aceticacid water mixtures through modified polybutadiene membranes, Polym. Bull. 31 (1993) 607. [4] S. Deng, S. Sourirajan, T. Matsuura, A study of polydimethylsiloxane/aromatic polyamide laminated membranes for separation of acetic acid/water mixtures by pervaporation process, Sep. Sci. Technol. 29 (1994) 1209. [5] Q. Liu, R.D. Noble, J.L. Falconer, H.H. Funke, Organics/water separation by pervaporation with a zeolite membrane, J. Membr. Sci. 117 (1996) 163.

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