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Multiple sprayed composite membranes with high flux for alcohol permselective pervaporation
Journal of Membrane Science 432 (2013) 33–41
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Multiple sprayed composite membranes with high flux for alcohol permselective pervaporation Ren Wang, Linglong Shan, Guojun Zhang n, Shulan Ji Center for Membrane Technology, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
abstract
Article history: Received 24 September 2012 Received in revised form 21 December 2012 Accepted 4 January 2013 Available online 14 January 2013
Alcohol permselective pervaporation is a promising technology for bio-alcohol production. In this study, a multiple spray technique has been developed to prepare hydrophobic composite membranes for alcohol permselective pervaporation. A defect-free permselective layer was successfully obtained by repeated alternate spraying of dihydroxypolydimethylsiloxane (PDMS) and tetraethylorthosilicate with the addition of dibutyltin dilaurate on to a polysulfone ultrafiltration membrane using an automatic spray system. The effects of PDMS molecular weight and concentration, number of spraying cycles, spraying time and standing interval on the membrane performance were intensively investigated. Experiments proved that, compared with the dip-coated membrane, the membrane flux obtained in this way showed a dramatically increase while the selectivity remained at a comparable level. It was found that the multiple spray technique can ensure that there are no defects in the separation layer, by repeated adjustment of the number of spraying cycles, and can also greatly reduce the thickness of a selective layer. In addition, the automatic spray technology removes the inherent unreliability of manual techniques, so is beneficial for large-scale rapid preparation of composite membranes. & 2013 Elsevier B.V. All rights reserved.
Keywords: Multiple spray technique Alcohol permselective membranes Pervaporation High flux Dihydroxypolydimethylsiloxane
1. Introduction In recent years, fuel prepared from renewable bio-alcohol has been promoted as an environmental benign source of energy for the next generation. The integration of pervaporation (PV) and fermentation has been attracting increasing attention [1] because pervaporation can rapidly recover bio-alcohol from biomass fermentation in-situ and, in turn, reduce the inhibitory effect of high alcohol concentration and thereafter promote greater fermentation efficiency [2]. For this combined process, the most important task at present is to develop an alcohol permselective pervaporation membrane with an adequate separation factor and flux [3,4]. A number of research programs have set out to develop a suitable hydrophobic pervaporation composite membrane. Dihydroxypolydimethylsiloxane (PDMS) is a well studied representative alcohol-permselective pervaporation membrane material [5–8]. For example, Ishihara et al. [9] have prepared crosslinked PDMS membranes to recover ethanol from the ethanol/water system. In this example, the separation factor and total flux reached 10.8 and 25.1 g/m2 h, respectively, at 30 1C for 8% ethanol/water feed solution. Li et al. [10] reported the construction of a PDMS selective layer on a cellulose acetate(CA)
n
Corresponding author. Tel./fax:þ 86 10 67392393. E-mail address: [email protected] (G. Zhang).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.01.006
supporting membrane which produced a separation factor of 8 and 1300 g/m2 h total flux. They put a matrix membrane into water so that the surfaces of the membrane matrix were covered by water molecules, which hinder PDMS molecules passing into the holes of the matrix membrane. This, therefore, reduces the mass transfer resistance of the composite membrane and enhances the total flux. More recently, we also used PDMS and tetraethylorthosilicate (TEOS) as the crosslinker to prepare ethanol permselective hollow fiber pervaporation membranes [11]. Generally, the reported separation factors of polymer-supported PDMS membranes for the ethanol/water system have ranged from 4.4 to 10.8 with an average of about 7–8 [1]. For such dense composite membranes, the mass transport across the membranes is governed by a solution-diffusion mechanism. Therefore, one key feature is that the ultrathin selective layer on the porous supporting membrane must be defect-free. In addition, it is wellknown that the membrane flux is inversely proportional to the selective layer thickness. Conventionally, a dip-coating method has been used to prepare alcohol permselective pervaporation membranes. This involves the submersion of a matrix membrane into the PDMS and crosslinker solution, then withdrawing the matrix membrane from the solution to allow the solution on the surface to dry out. Most of the top selective layer thicknesses obtained from this conventional dip-coating method are usually dozens of micrometers, even reaching 82 mm [12]. Therefore, the flux of polymer-supported PDMS membranes obtained using the dip-coating method is
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relatively low, in the range 19–1300 g/m2 h. In addition, the manual operation involved in dip-coating brings about many uncontrollable human factors, leading to instability and low reproducibility of the membrane performance. Therefore, it is necessary to look for a simpler and more reproducible process if this technology is to extend into industrial applications. Schlenoff et al. [13] were the first to report the construction of a poly(diallyldimethylammonium)/poly(styrenesulfonate) multilayer film by spraying the respective solutions on to the substrate. Decher et al. [14] have reported the fabrication of a multilayer by successive spraying of poly(styrenesulfonate) and poly(allylamine hydrochloride) solutions on to a poly(ethyleneimine) precursor layer. This can dramatically reduce preparation time to 2 min, compared with 50 min using the classical dipping method. The increasing development of the spray-LbL assembly process has demonstrated that spray technology has great potential for the construction of various films. However, most previous studies have focused on the control of film growth, thickness and morphology. The spray assembly of multilayer membranes for separation applications still remains largely unexplored. More recently, we have developed a sprayed polyelectrolyte multilayer membrane for pervaporation dehydration using an automatic spray system [15], which has been proved to be a highly efficient process for assembling films of different composition on to substrate membranes. However, developing the sprayed membrane for separation is only the first step in the pervaporation dehydration process, which has been mostly conducted using aqueous hydrophilic polymer solutions. The preparation of a hydrophobic membrane for separation applications using automatic spray technology is yet to be achieved. It was expected that an ultrathin selective layer could be obtained by alternately spraying non-aqueous hydrophobic polymer and crosslinker solutions on to a substrate so that the defects could be remedied by multiple spraying cycles. The resulting membrane could thereafter be used for alcohol permselective pervaporation. In order to accomplish this work, a new automatic spray apparatus was constructed in our laboratory to produce a defect-free ultrathin selective layer on a porous supporting membrane. Dihydroxypolydimethylsiloxane (PDMS) was chosen as the ethanol permselective membrane material. Tetraethylorthosilicate (TEOS) and dibutyltin dilaurate (DBTDL) was selected as the crosslinker and the catalyst, respectively, for the spray assembly process. The PDMS and TEOS/DBTDL solutions were sprayed horizontally on to the vertical PS substrate. The growth of the multiple layers of polymer was tracked using a scanning electron microscope (SEM), an atomic force microscope (AFM) and a contact angle analyzer. The effects of PDMS molecular weight and concentration, spraying cycles, spraying time and standing interval on membrane performance were intensively investigated.
2. Experimental 2.1. Materials Dihydroxypolydimethylsiloxane (PDMS), with the molecular weight of 80,000 g/mol, 40,000 g/mol and 4200 g/mol, was purchased from China Bulestar Chengrand Chemical Co., Ltd. Tetraethylorthosilicate (TEOS), n-heptane, dibutyltin dilaurate and ethanol were purchased from Beijing Chemical Factory. All chemicals were of analytical grade and used without further purification. Flat sheet polysulfone (PS) ultrafiltration (UF) membranes, with a nominal molecular weight cutoff of 20,000, were supplied by Sepro Membranes.
Scheme 1. Automated spray system for the preparation of flat sheet polyelectrolyte multilayer membranes.
2.2. Automated spray apparatus The automated spray system was constructed in our laboratory and is represented schematically in Scheme 1. The equipment consists of two identical atomizing nozzles with a nozzle diameter of 0.5 mm, each connected to a pressure tank and a solenoid valve that controls the opening and closing of the nozzles. A high volume air compressor was utilized to maintain a constant pressure of 2 bars inside the two pressure tanks while spraying. A metal substrate was fixed perpendicularly and rotated at 100 rpm, solely to accelerate the drainage of the solution from the surface of the films, as described in our previous studies [15]. A burner was erected beside Nozzle 1 and directed towards the substrate to accelerate the drainage of the solution from the surface of the films in the spraying time. The operating parameters, such as spraying time, standing interval, spray cycles, and on-off bunker, were controlled by a programmable logic controller (PLC) system. PLC also controlled sprayer and burner start and stop commands and allowed for fully automatic operation. Both nozzles, the burner and the substrate were placed inside a Constant Temperature and Low Humidity Box (CTLH Box) which can be controlled at a constant temperature and humidity. 2.3. Preparation of PDMS/PS composite membrane by multiple spraying process The PS UF membrane was immersed in a 30% ethanol solution for 2 h at room temperature. Then, the residual ethanol on the surface of the membranes was rinsed using ultrapure water. A vacuum pump was used to form a negative pressure for extracting air from the membrane pores over a period of 4 h. The membrane was cut into 10 cm squares, which were then fixed perpendicularly on to the substrate. PDMS was dissolved in n-heptane to form a PDMS solution and the polymer solutions were stirred at room temperature for 1 h. Meanwhile, 1 wt% of the crosslinking agent TEOS and 0.5 wt% of catalyst dibutyltin dilaurate (DBTDL) was also dissolved in n-heptane and stirred at room temperature for 1 h. For the preparation of the defect-free pervaporation composite membrane, the PDMS and TEOS/DBTDL solutions were repeatedly and alternately sprayed on to the flat porous sheet of the PS substrate. Both PDMS and TEOS/DBTDL solutions were sprayed at a flow rate of 2–3 mL/s. The distance between the substrate and spray nozzles was fixed at 45 cm, and the sprayed area was a circle of diameter over 10 cm. The substrate was perpendicularly spun at 100 rpm. The spray process was performed as shown in Scheme 2. To remove the remaining solvent, the finished samples were allowed to stand for 1 day. After that, the samples were placed in a convection oven set at 80 1C for 8 h to fully crosslink the PDMS solution.
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vacuum and fractured in liquid nitrogen. Surface topography was also observed in tapping mode by an atomic force microscope (AFM) (Pico ScanTM 2500, USA). Water contact angle measurements were also performed using a contact angle analyzer (DSA 100, Germany).
3. Results and discussion Scheme 2. Schematic illustration of multiple spray process. (t1, t2 are the process time of polymer and crosslinker solutions, respectively. td represents the required interval between two spraying process. ts is the standing interval for drying the solution on the surface. n is the spraying cycles.).
Scheme 3. Experimental apparatus for pervaporation evaluation.
2.4. Pervaporation experiments The pervaporation apparatus used in the experiments has been described previously [16]. As shown in Scheme 3, the system used for pervaporation contains pervaporation cell, cold trap, feed tank, constant temperature tank, digital vacuum gauge, air valve, circulation and vacuum pump. Membranes were loaded into pervaporation cell for experiments. The effective area of the membrane in the permeation equipment was 24.6 cm2. The feed aqueous solution had an ethanol concentration of 5.0 wt%, and the feed temperature was 60 1C. The down-stream pressure was about 200 Pa and monitored by a digital vacuum gauge. Each experimental run for the pervaporation process was 30 min. The permeate vapor was trapped with liquid nitrogen and measured by gas chromatography (GC-14C, SHIMADZU) using a thermal conductivity detector. The parallel membranes were prepared in exactly the same way. Each membrane was measured three times using the same pervaporation conditions. Fluxes were determined by measuring the weight of liquid collected in the cold traps over a certain time under steady-state conditions. The permeation total flux (J) and separation factor (a) represent the permeability and selectivity, respectively. The separation factor (a) was calculated according to the following equation, with Yw and Xw representing the weight fractions of water in permeate and feed.
a¼
Y w ð1=X w Þ X w ð1=Y w Þ
2.5. Characterization The membrane surface and cross-section before and after spraying was observed with a scanning electron microscope (SEM) (Hitachi S-4300, Japan) attached to an energy dispersive spectrometer (EDX). All membrane samples were dried under
3.1. SEM-EDX, AFM and contact angle analyses The changes of the surface and cross-sectional morphology revealed by SEM before and after spraying are shown in Fig. 1. From the high magnification image (100k ), a large number of pores can clearly be observed on the PS supporting membrane (Fig. 1(a-1)). In contrast, after spraying with 5 cycles of PDMS and TEOS/DBTDL, all the surface pores have been covered and no defects were found in a randomly selected area. Clearly, a new defect-free selective layer has been formed (Fig. 1(a-2)). The incorporation of PDMS endowed the membrane with remarkably high density and compactness. From the cross-sectional SEM images (Fig. 1(b-1) and (b-2)), it is difficult to obtain the exact thicknesses of the selective layer on top of the supporting PS membrane because the boundary between the selective layer and supporting membrane is not clear. Further, it was observed from the higher magnification of SEM Fig. (5k ) that the selective layer merged well with the supporting membrane (Fig. 1(b-3)). These observations suggest that the selective layer obtained by spraying relatively diluted polymer solution is ultrathin, which is beneficial for a high total flux. In order to analyze the thickness of the top selective layer, the changes of elemental composition through the cross-section were also analyzed using EDX. As shown in Fig. 2, the concentrations of carbon and oxygen remain almost unchanged across the membrane. However, just before the 1 mm point, the silicon content decreased while the sulfur content increased. The concentrations of silicon and sulfur therefore show opposite trends. Since the PS supporting membrane does not contain silicon while the PDMS polymer does not contain sulfur, all the silicon must arise from the selective PDMS layer and all the sulfur from the PS supporting membrane. From this analysis, we deduce that some PDMS has intruded into the PS membrane pores because some silicon was found there. These observations led us to conclude that the thickness of the PDMS selective layer is far less than 1 mm. To characterize the changes of morphology, an inspection of the top surface by AFM before and after spraying is shown in Fig. 3 using two different enlargements. In comparison with Fig. 3(a-1) and (a-2), significant changes were observed on the surface of the membrane. Before being sprayed with PDMS, the surface of the PS supporting membrane was relatively flat, with a Ra value of 4.4 nm. It was observed that after being sprayed with PDMS 5 times, the membrane surface was much rougher and the Ra increased to 60.4 nm. This increased roughness might be attributed to the surface enrichment of crosslinked PDMS. Observations over a much larger area (80 80 mm) (Fig. 3(b-1) and (b-2)), show that we also obtain a rougher surface after the spray process, with the Ra increasing from 18.2 nm to 80.4 nm. Interestingly, we found that the highest bump on the surface is less than 1.0 mm. This further confirmed that the selective layer after spraying is much less than 1 mm thick. The contact angle of the PS supporting and PDMS/PS composite membranes are shown in Fig. 4. It was observed that after spraying with PDMS, the contact angle increased from 62.71 to 106.11 (Fig. 4(a) and (b)). Meanwhile, the contact angles of the PDMS/PS composite membrane varied over a very narrow band no matter how many spray cycles were carried out (Fig. 4(b) and (c)).
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Fig. 1. SEM images of sprayed PDMS/PS composite membrane. (a-1) Surface image of PS supporting membrane before spray (100.0k ). (a-2) Surface image of PDMS/PS composite membrane sprayed with 5 cycles (100.0k ). (b-1) Cross-sectional image of PS supporting membrane before spray (1.0k ). (b-2) Cross-sectional image of PDMS/PS composite membrane sprayed with 5 cycles (1.0k ). (b-3) Cross-sectional image of PDMS/PS composite membrane sprayed with 5 cycles (5.0k ). (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼ 3 s, td ¼ 1 s, and ts ¼ 120 s; 25 1C; 20% humidity).
Clearly, after being sprayed with PDMS, the surface became more hydrophobic compared with the uncoated PS membrane. This characteristic is beneficial for ethanol permselective permeability. Together, the above experiments confirm that a dense, rough, ultrathin and hydrophobic selective layer was successfully formed on the PS supporting membrane after spraying with crosslinked PDMS. 3.2. Effects of PDMS molecular weight and concentration For the spraying process, both the molecular weight and concentration of the polymer are crucial parameters for the formation of a defect-free ultrathin selective layer. As expected, Table 1 shows that the selectivity increased while the total flux decreased with the increase of PDMS molecular weight. It was
noted that the low molecular weight PDMS resulted in a higher flux but compensated with a decrease of selectivity. Therefore, the PDMS with relatively high molecular weight of 80,000 g/mol was selected for subsequent studies. The effects of PDMS concentration were investigated by keeping the TEOS concentration at a constant value of 1 wt%. As shown in Fig. 5, the selectivity first increased and then decreased with increasing PDMS concentration. The separation factor increased from 6.9 to 8.4 when the PDMS concentration varied from 0.1 wt% to 10 wt% and then decreased to 3.8 wt% at 20 wt%. The total flux was significantly reduced, from 4000 g/m2 h to 301 g/m2 h, as the PDMS concentration increased from 0.1 wt% to 20 wt%. Clearly, in addition to the PDMS molecular weight, the PDMS concentration during the spray process has a strong impact on the pervaporation performance. Fig. 6 shows that a big lump was formed on the
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Fig. 2. Cross-sectional EDX analyses of PDMS/PS composite membrane. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼3 s, td ¼ 1 s and ts ¼ 120 s, five spraying cycles; 25 1C; 20% humidity).
selective layer at 20 wt% PDMS concentration. This suggests that the polymer begins to shrink so allowing a big lump to be easily formed on the surface, which will negatively affect the total flux. In subsequent experiments, 1 wt% was selected as an appropriate PDMS concentration for the spray assembly process. 3.3. Effects of spraying cycles Pervaporation is a very strict process, which requires a nonporous membrane. Any minute defects will cause a decline of selectivity. Considering the flexibility and rapidity of the spray assembly, multiple spraying has been proposed to obtain a defectfree selective layer. In this way, each added cycle is used to cover the defects in previous layers. Therefore, it was expected that the pervaporation performance of the sprayed membrane could be easily tuned by adjusting the spraying cycles. As shown in Fig. 7, the separation factor increased while the total flux decreased with increasing number of spraying cycles. For example, the separation factor was 4.7 while flux was as high as 8981 g/m2 h for one crosslinked PDMS layer. After spraying 5 times, the separation factor increased to 7.5 while the flux decreased to 3275 g/m2 h. Clearly, later spraying cycles are beneficial for remedy the defects
in previous layers, which, in turn, improves the separation factor of multiple sprayed membranes. In addition, it was noted that, as the number of spray cycles continuously increased from 5 to 12, the separation factor remained almost unchanged while the flux dropped continuously to 1034 g/m2 h. This is because the repeated cycles lead to an increase of the thickness of the top selective layer. Since the thickness is inversely proportional to the total transport resistance, the total permeate flux decreases [17,18]. Moreover, once the defects are completely covered with multiple crosslinked PDMS layers, the selectivity was independent of any further increase of sprayed layers. Therefore, there is generally a compromise between pervaporation efficiency and spraying cycles. For a reasonable separation factor and suitable flux, 5 cycles were selected for subsequent experiments. 3.4. Effects of spraying time and standing interval Usually, the polymer spraying time is thought to be directly linked to the film quality and structure. Therefore, the effects of PDMS spraying time on pervaporation performance were investigated, with the results shown in Fig. 8. It was noted that the selectivity showed a trend of first increasing and then decreasing
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Fig. 3. Three-dimensional AFM images of PS supporting and PDMS/PS composite membrane. (a-1) PS supporting membrane before spray (10 10 mm, Ra¼ 4.4 nm). (a-2) PDMS/PS composite membrane sprayed with 5 cycles (10 10 mm, Ra ¼60.4 nm). (b-1) PS supporting membrane before spray (80 80 mm, Ra ¼18.2 nm). (b-2) PDMS/PS composite membrane sprayed with 5 cycles (80 80 mm, Ra ¼80.4 nm). (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ 0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼ 3 s, td ¼ 1 s, and ts ¼ 120 s; 25 1C; 20% humidity).
Fig. 4. Contact angles of PS supporting and PDMS/PS composite membrane. (a) Uncoated PS supportinging membrane before spray (62.7 7 0.131). (b) PDMS/PS composite membrane sprayed with 1 cycle (105.4 70.251). (c) PDMS/PS composite membrane sprayed with 10 cycles (106.1 70.091). (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOSþ 0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼ 3 s, td ¼ 1 s, and ts ¼ 120 s; 25 1C; 20% humidity).
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Table 1 Effects of PDMS molecular weight on pervaporation performance. PDMS molecular weight (g/mol)
Ethanol content in feed solution (wt%)
Ethanol content in permeate (wt%)
Total flux J (g/m2 h)
Separation factor a
80,000 40,000 4,200
5 5 5
28.3 21.1 21.1
3275 3230 3666
7.5 5.1 5.1
Fig. 7. Effects of spraying cycles on pervaporation performance. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼5 s, t2 ¼ 3 s, td ¼1 s, and ts ¼120 s; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
Fig. 5. Effects of PDMS concentration on pervaporation performance. (Spray preparative conditions: 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼3 s, td ¼ 1 s, and ts ¼120 s; five spraying cycles; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
Fig. 8. Effects of PDMS spraying time on pervaporation performance. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOSþ 0.5 wt% dibutyltin dilaurate n-heptane solution; t2 ¼3 s, td ¼ 1 s, and ts ¼ 120 s; five spraying cycles; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
Fig. 6. SEM images of PDMS/PS composite membrane obtained from 20 wt%. (Spray preparative conditions: 20 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼5 s, t2 ¼3 s, td ¼ 1 s, and ts ¼120 s, five spraying cycles; 25 1C; 20% humidity; 5.0k ).
while the total flux continuously decreased. For instance, the separation factor obtained from spraying times of 1 s, 3 s and 5 s were 6.4, 6.5 and 7.5 while the total fluxes were 5395 g/m2 h, 3374 g/m2 h and 3275 g/m2 h, respectively. When the spraying time was prolonged to 7 s, both the separation factor and total flux appeared to decline. If the spraying time is too short, it is difficult to completely cover all the surface defects of the PS supporting membrane. However, excessive spraying time does not benefit the pervaporation performance because superabundant PDMS cannot be completely crosslinked, which then causes a reduction of both the separation factor and total flux. Therefore, 5 s PDMS spraying time was concluded to be a suitable spraying time.
The standing interval between two cycles (ts) is also important to the formation of the selective layer and membrane performance because the pre-crosslinking and rearrangement of PDMS take place during this period. Fig. 9 shows the variation of the pervaporation performance with standing interval. When standing interval was less than 50 s, the performance is not steady because the pre-crosslink process was not completed while the rearrangement was ongoing. When the standing interval was 120 s, the separation factor reached a relatively high level. By comparison, the separation factor declined to 4.3 when the standing interval increased to 300 s. This suggested that prolonging the standing interval might result in excessive crosslinking, which was not helpful for the formation of highly selective separation layers. Based on the above investigations, the appropriate conditions for multiple sprayed PDMS/PS membranes were selected as follows: PDMS concentration 1 wt%, TEOS concentration 1 wt%, t1 ¼ 5 s, t2 ¼3 s, td ¼ 1 s, ts ¼120 s and five spraying cycles. Using this set of conditions, the membrane obtained had a separation factor of 7.5 and a total flux of 3275 g/m2 h for pervaporation separation of 5 wt% ethanol/water system (60 1C).
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3.5. Comparison with dip-coated composite membrane and reference data in the literature In order to understand clearly the difference between dip-coated and spayed composite membranes, control experiments were carried out by dip-coating PDMS on to the same supporting membrane. The comparative pervaporation experiments between dip-coated and sprayed membranes were performed under the same pervaporation conditions. For the dip-coating process, it was noted that the PDMS
Fig. 9. Effects of standing interval on pervaporation performance. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOSþ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼5 s, t2 ¼ 3 s, and td ¼ 1 s; five spraying cycles; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
concentration plays a very important role in the formation of high performance composite membranes. For example, the composite membrane dip-coated with 1 wt% PDMS (80,000 g/mol) solution has almost no selectivity, even after prolonging the coating time to 1 h. By comparison, after dip-coating in a 10 wt% PDMS solution added with 1 wt% TEOS and 0.5 wt% DBDTL for 1 min, the separation factor obtained from dip-coated membrane reached 7.9. Clearly, a high concentration polymer solution was necessary for the formation of a defect-free selective layer for the dip-coating method. However, when using such a high concentration, the selective layer obtained was relatively thick. As shown in the SEM picture (Fig. 10), the thickness of the selective layer obtained from the dip-coating method could reach more than 3 mm. Although the selectivity of the dip-coated membrane was comparable with that of the sprayed membrane (7.5), the flux of the dip-coated membrane was only 786 g/m2 h. Clearly, the membrane flux obtained from the spray assembly process was dramatically increased while the selectivity remained at a comparable level. This is because the spray method could form the defect-free selective layer by repeatedly spraying relatively dilute polymer solutions, which result in a thinner selective layer. Moreover, very highly concentrated PDMS solution had negative effects on the spraying assembly process because the high concentration was not helpful for the spraying itself and dispersing of polymer solutions. As shown in Table 2, the pervaporation performance of the sprayed membrane was also compared with reference data from the literature. Table 2 also shows that, although it was difficult to directly compare the pervaporation performance of these membranes, due to some extent to the different preparation and testing conditions, the results demonstrate that the sprayed PDMS/PS composite membrane exhibited the distinguishing features of much higher flux compared with other polymer-supported PDMS membranes, commonly ranging from 19 g/m2 h to 1300 g/m2 h. The thickness of the reported composite membrane usually ranged from several mm to 82 mm, whereas the thickness of the sprayed membrane was far less than 1 mm. Obviously, the much thinner PDMS separation layer obtained from the spraying method leads to lower transport resistance and results in much higher flux. Therefore, the spraying strategy provides a new way to significantly reduce the thickness of the selective layer while keeping the defect-free feature, by using dilute polymer solution and simply repeating the number of spraying cycles. Moreover, the PLC automatic system can effectively avoid many uncontrollable human factors, greatly improving the stability and reproducibility of the membrane performance. Therefore, this method has great potential for industrial technology.
4. Conclusions Fig. 10. Cross-sectional image of dip-coated PDMS/PS composite membrane. (Preparative conditions: 10 wt% PDMS þ1 wt% TEOSþ 0.5 wt% dibutyltin dilaurate n-heptane solution; 25 1C; 20% humidity).
In this study, a multiple sprayed PDMS/PS composite membrane was successfully prepared using an automatic spraying
Table 2 Prevaporation performance of different PDMS membranes. Membrane
Ethanol feed concentration (wt%)
Temperature (1C)
Membrane thickness (lm)
J (g/m2 h)
a
Down-stream pressure (Pa)
Refs.
crosslinked PDMS containing vinyl groups crosslinked PDMS containing acetoxysilyl groups PDMS–PS block copolymer PDMS–PPP graft copolymer PDMS–PS IPN supported on PES PDMS–PI copolymer PDMS–PS IPN supported on PTFE PDMS supported on a CA crosslinked PDMS supported on PS
16.5 8 10 7 10 10 10 5 5
40 30 25 30 60 60 30 40 60
82 – 39 30 15 20 34 8 Less than 1
85 25.1 27 19 160 560 85 1300 3275
8.6 10.8 6.2 40 5.5 10.6 6 8.5 7.5
13.3–66.5 133 Below 130 67 4007 133 – 7000 266 200
[12] [9] [19] [20] [21] [22] [23] [24] This study
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system. SEM-EDX and AFM analyses confirmed that the thickness of the selective layer obtained from five spraying cycles was still far less than 1 mm. Therefore, the resulting composite membrane shows extremely high flux compared with those from classical dip-coating methods for the pervaporation separation of ethanol/ water mixture system. It was demonstrated that the performance of the sprayed membrane was strongly dependent on the PDMS molecular weight and concentration, spraying cycles, spraying time and standing interval. A set of appropriate conditions was selected as follows: PDMS concentration 1 wt%, t1 ¼5 s, t2 ¼3 s, td ¼1 s, ts ¼120 s, five spraying cycles. With this set of given conditions, the membrane obtained had a separation factor of 7.5 and a total flux of 3275 g/m2 h for the pervaporation of a 5 wt% ethanol/water mixture system (60 1C). To the best of our knowledge, this is the first study that extends the multiple spray technology to prepare an ethanol permselective membrane with an ultrathin defect-free selective layer. It is reasonable to expect that the multiple spray technique will contribute to the preparation of various composite membranes by a suitable choice of the spraying conditions and organic species such as functional polymer, surfactants and dye molecules.
Acknowledgments This work was financially supported by the National High Technology Research and Development Program of China (No. 2012AA03A607), Natural Science Foundation of Beijing (No. 8122010), the National Basic Research Program of China (No.2009CB623404) and Fok Ying Tung Education Foundation (No.131068). References [1] L.M. Vane, A review of pervaporation for product recovery from biomass fermentation processes, J. Chem. Technol. Biotechnol. 80 (2005) 603–629. [2] G.P. Liu, W. Wei, H. Wu, X.L. Dong, M. Jiang, W.Q. Jin, Pervaporation performance of PDMS/ceramic composite membrane in acetone butanol ethanol (ABE) fermentation-PV coupled process, J. Membr. Sci. 373 (2011) 121–129. [3] D.J. O’Brien, L.H. Roth, A.J. McAloon, Ethanol production by continuous fermentation–pervaporation: a preliminary economic analysis, J. Membr. Sci. 166 (2000) 105–111. [4] T.C. Bowen, R.G. Meier, L.M. Vane, Stability of MFI zeolite-filled PDMS membranes during pervaporative ethanol recovery from aqueous mixtures containing acetic acid, J. Membr. Sci. 298 (2007) 117–125. [5] T. Mohammadi, A. Aroujalian, A. Bakhshi, Pervaporation of dilute alcoholic mixtures using PDMS membrane, Chem. Eng. Sci. 60 (2005) 1875–1880.
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[6] A. Aroujalian, K. Belkacemi, S.J. Davids, G. Turcotte, Y. Pouliot, Effect of residual sugars in fermentation broth on pervaporation flux and selectivity for ethanol, Desalination 193 (2006) 103–108. [7] D.J. O’Brien, G.E. Senske, M.J. Kurantz, J.C. Craig, Ethanol recovery from corn fiber hydrolysate fermentations by pervaporation, Bioresour. Technol. 92 (2004) 15–19. [8] L.M. Vane, V.V. Namboodiri, T.C. Bowen, Hydrophobic zeolite–silicone rubber mixed matrix membranes for ethanol–water separation: effect of zeolite and silicone component selection on pervaporation performancer, J. Memb. Sci. 308 (2008) 230–241. [9] K. Ishihara, K. Matsui, Pervaporation of ethanol–water mixture through composite membranes composed of styrene-fluoroalkyl acrylate graft copolymers and cross-linked polydimethylsiloxane membrane, J. Appl. Polym. Sci. 34 (1987) 437–440. [10] L. Li, Z. Xiao, S. Tan, L. Pu, Z. Zhang, Composite PDMS membrane with high flux for the separation of organics from water by pervaporation, J. Membr. Sci. 243 (2004) 177–187. [11] J. Guo, G. Zhang, W. Wu, S. Ji, Z. Qin, Z. Liu, Dynamically formed inner skin hollow fiber polydimethylsiloxane/polysulfone composite membrane for alcohol permselective pervaporation, Chem Eng J. 158 (2010) 558–565. [12] S. Takegami, H. Yamada, S. Tsuijii, Pervaporation of ethanol/water mixtures using novel hydrophobic membranes containing polydimethylsiloxane, J Membr. Sci. 75 (1992) 93–105. [13] J.B. Schlenoff, S.T. Dubas, T. Farhat, Sprayed polyelectrolyte multilayers, Langmuir 16 (2000) 9968–9969. [14] A. Izquierdo, S.S. Ono, J.C. Voegel, P. Schaaf, G. Decher, Dipping versus spraying: exploring the deposition conditions for speeding up layer-bylayer assembly, Langmuir 21 (2005) 7558–7567. [15] H. Tang, G. Zhang, S. Ji, Rapid assembly of polyelectrolyte multilayer membranes, AlChE J. 59 (2013) 250–257. [16] G. Zhang, W. Gu, S. Ji, Z. Liu, Y. Peng, Z. Wang, Preparation of polyelectrolyte multilayer membranes by dynamic layer-by-layer process for pervaporation separation of alcohol/water mixtures, J. Membr Sci. 280 (2006) 727–733. [17] F. Xiangli, W. Wei, Y. Chen, W. Jin, N. Xu, Optimization of preparation conditions for polydimethylsiloxane (PDMS)/ceramic composite pervaporation membranes using response surface methodology, J. Membr. Sci. 311 (2008) 23–33. [18] F. Xiangli, Y. Chen, W. Jin, N. Xu, Polydimethylsiloxane (PDMS)/ceramic composite membrane with high flux for pervaporation of ethanol–water mixtures, Ind. Eng. Chem. Res. 46 (2007) 2224–2230. [19] K. Okamoto, A. Butsuen, S. Nishioka, K. Tanaka, H. Kita, S. Asakawa, Pervaporation of water–ethanol mixtures through polydimethylsiloxane blockcopolymer membranes, Polym. J. 19 (1987) 747–756. [20] Y. Nagase, S. Mori, K. Matsui, Chemical modification of poly (substitutedacetylene) IV. Pervaporation of organic liquid–water mixture through poly (1-phenyl-1-propyne)/polydimethylsiloxane graft copolymer membrane, J. Appl. Polym. Sci. 37 (1989) 1259–1267. [21] L. Liang, E. Ruckenstein, Pervaporation of ethanol–water mixtures through polydimethylsiloxane–polystyrene interpenetrating polymer network supportinged membranes, J. Membr. Sci. 114 (1996) 227–234. [22] M. Krea, D. Roizard, N. Moulai-Mostefa, D. Sacco, New copolyimide membranes with high siloxane content designed to remove polar organics from water by pervaporation, J. Membr. Sci. 241 (2004) 55–64. [23] I. Ahmed, N.F.C. Pa, M.G.M. Nawawi, W.A.W.A. Rahman, Modified polydimethylsiloxane/polystyrene blended IPN pervaporation membrane for ethanol/water separation, J. Appl. Polym. Sci. 122 (2011) 2666–2679. [24] L. Li, Z. Xiao, S. Tan, L. Pu, Z. Zhang, Composite PDMS membrane with high flux for the separation of organics from water by pervaporation, J. Membr. Sci. 243 (2004) 177–187.
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Multiple sprayed composite membranes with high flux for alcohol permselective pervaporation Ren Wang, Linglong Shan, Guojun Zhang n, Shulan Ji Center for Membrane Technology, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
abstract
Article history: Received 24 September 2012 Received in revised form 21 December 2012 Accepted 4 January 2013 Available online 14 January 2013
Alcohol permselective pervaporation is a promising technology for bio-alcohol production. In this study, a multiple spray technique has been developed to prepare hydrophobic composite membranes for alcohol permselective pervaporation. A defect-free permselective layer was successfully obtained by repeated alternate spraying of dihydroxypolydimethylsiloxane (PDMS) and tetraethylorthosilicate with the addition of dibutyltin dilaurate on to a polysulfone ultrafiltration membrane using an automatic spray system. The effects of PDMS molecular weight and concentration, number of spraying cycles, spraying time and standing interval on the membrane performance were intensively investigated. Experiments proved that, compared with the dip-coated membrane, the membrane flux obtained in this way showed a dramatically increase while the selectivity remained at a comparable level. It was found that the multiple spray technique can ensure that there are no defects in the separation layer, by repeated adjustment of the number of spraying cycles, and can also greatly reduce the thickness of a selective layer. In addition, the automatic spray technology removes the inherent unreliability of manual techniques, so is beneficial for large-scale rapid preparation of composite membranes. & 2013 Elsevier B.V. All rights reserved.
Keywords: Multiple spray technique Alcohol permselective membranes Pervaporation High flux Dihydroxypolydimethylsiloxane
1. Introduction In recent years, fuel prepared from renewable bio-alcohol has been promoted as an environmental benign source of energy for the next generation. The integration of pervaporation (PV) and fermentation has been attracting increasing attention [1] because pervaporation can rapidly recover bio-alcohol from biomass fermentation in-situ and, in turn, reduce the inhibitory effect of high alcohol concentration and thereafter promote greater fermentation efficiency [2]. For this combined process, the most important task at present is to develop an alcohol permselective pervaporation membrane with an adequate separation factor and flux [3,4]. A number of research programs have set out to develop a suitable hydrophobic pervaporation composite membrane. Dihydroxypolydimethylsiloxane (PDMS) is a well studied representative alcohol-permselective pervaporation membrane material [5–8]. For example, Ishihara et al. [9] have prepared crosslinked PDMS membranes to recover ethanol from the ethanol/water system. In this example, the separation factor and total flux reached 10.8 and 25.1 g/m2 h, respectively, at 30 1C for 8% ethanol/water feed solution. Li et al. [10] reported the construction of a PDMS selective layer on a cellulose acetate(CA)
n
Corresponding author. Tel./fax:þ 86 10 67392393. E-mail address: [email protected] (G. Zhang).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.01.006
supporting membrane which produced a separation factor of 8 and 1300 g/m2 h total flux. They put a matrix membrane into water so that the surfaces of the membrane matrix were covered by water molecules, which hinder PDMS molecules passing into the holes of the matrix membrane. This, therefore, reduces the mass transfer resistance of the composite membrane and enhances the total flux. More recently, we also used PDMS and tetraethylorthosilicate (TEOS) as the crosslinker to prepare ethanol permselective hollow fiber pervaporation membranes [11]. Generally, the reported separation factors of polymer-supported PDMS membranes for the ethanol/water system have ranged from 4.4 to 10.8 with an average of about 7–8 [1]. For such dense composite membranes, the mass transport across the membranes is governed by a solution-diffusion mechanism. Therefore, one key feature is that the ultrathin selective layer on the porous supporting membrane must be defect-free. In addition, it is wellknown that the membrane flux is inversely proportional to the selective layer thickness. Conventionally, a dip-coating method has been used to prepare alcohol permselective pervaporation membranes. This involves the submersion of a matrix membrane into the PDMS and crosslinker solution, then withdrawing the matrix membrane from the solution to allow the solution on the surface to dry out. Most of the top selective layer thicknesses obtained from this conventional dip-coating method are usually dozens of micrometers, even reaching 82 mm [12]. Therefore, the flux of polymer-supported PDMS membranes obtained using the dip-coating method is
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relatively low, in the range 19–1300 g/m2 h. In addition, the manual operation involved in dip-coating brings about many uncontrollable human factors, leading to instability and low reproducibility of the membrane performance. Therefore, it is necessary to look for a simpler and more reproducible process if this technology is to extend into industrial applications. Schlenoff et al. [13] were the first to report the construction of a poly(diallyldimethylammonium)/poly(styrenesulfonate) multilayer film by spraying the respective solutions on to the substrate. Decher et al. [14] have reported the fabrication of a multilayer by successive spraying of poly(styrenesulfonate) and poly(allylamine hydrochloride) solutions on to a poly(ethyleneimine) precursor layer. This can dramatically reduce preparation time to 2 min, compared with 50 min using the classical dipping method. The increasing development of the spray-LbL assembly process has demonstrated that spray technology has great potential for the construction of various films. However, most previous studies have focused on the control of film growth, thickness and morphology. The spray assembly of multilayer membranes for separation applications still remains largely unexplored. More recently, we have developed a sprayed polyelectrolyte multilayer membrane for pervaporation dehydration using an automatic spray system [15], which has been proved to be a highly efficient process for assembling films of different composition on to substrate membranes. However, developing the sprayed membrane for separation is only the first step in the pervaporation dehydration process, which has been mostly conducted using aqueous hydrophilic polymer solutions. The preparation of a hydrophobic membrane for separation applications using automatic spray technology is yet to be achieved. It was expected that an ultrathin selective layer could be obtained by alternately spraying non-aqueous hydrophobic polymer and crosslinker solutions on to a substrate so that the defects could be remedied by multiple spraying cycles. The resulting membrane could thereafter be used for alcohol permselective pervaporation. In order to accomplish this work, a new automatic spray apparatus was constructed in our laboratory to produce a defect-free ultrathin selective layer on a porous supporting membrane. Dihydroxypolydimethylsiloxane (PDMS) was chosen as the ethanol permselective membrane material. Tetraethylorthosilicate (TEOS) and dibutyltin dilaurate (DBTDL) was selected as the crosslinker and the catalyst, respectively, for the spray assembly process. The PDMS and TEOS/DBTDL solutions were sprayed horizontally on to the vertical PS substrate. The growth of the multiple layers of polymer was tracked using a scanning electron microscope (SEM), an atomic force microscope (AFM) and a contact angle analyzer. The effects of PDMS molecular weight and concentration, spraying cycles, spraying time and standing interval on membrane performance were intensively investigated.
2. Experimental 2.1. Materials Dihydroxypolydimethylsiloxane (PDMS), with the molecular weight of 80,000 g/mol, 40,000 g/mol and 4200 g/mol, was purchased from China Bulestar Chengrand Chemical Co., Ltd. Tetraethylorthosilicate (TEOS), n-heptane, dibutyltin dilaurate and ethanol were purchased from Beijing Chemical Factory. All chemicals were of analytical grade and used without further purification. Flat sheet polysulfone (PS) ultrafiltration (UF) membranes, with a nominal molecular weight cutoff of 20,000, were supplied by Sepro Membranes.
Scheme 1. Automated spray system for the preparation of flat sheet polyelectrolyte multilayer membranes.
2.2. Automated spray apparatus The automated spray system was constructed in our laboratory and is represented schematically in Scheme 1. The equipment consists of two identical atomizing nozzles with a nozzle diameter of 0.5 mm, each connected to a pressure tank and a solenoid valve that controls the opening and closing of the nozzles. A high volume air compressor was utilized to maintain a constant pressure of 2 bars inside the two pressure tanks while spraying. A metal substrate was fixed perpendicularly and rotated at 100 rpm, solely to accelerate the drainage of the solution from the surface of the films, as described in our previous studies [15]. A burner was erected beside Nozzle 1 and directed towards the substrate to accelerate the drainage of the solution from the surface of the films in the spraying time. The operating parameters, such as spraying time, standing interval, spray cycles, and on-off bunker, were controlled by a programmable logic controller (PLC) system. PLC also controlled sprayer and burner start and stop commands and allowed for fully automatic operation. Both nozzles, the burner and the substrate were placed inside a Constant Temperature and Low Humidity Box (CTLH Box) which can be controlled at a constant temperature and humidity. 2.3. Preparation of PDMS/PS composite membrane by multiple spraying process The PS UF membrane was immersed in a 30% ethanol solution for 2 h at room temperature. Then, the residual ethanol on the surface of the membranes was rinsed using ultrapure water. A vacuum pump was used to form a negative pressure for extracting air from the membrane pores over a period of 4 h. The membrane was cut into 10 cm squares, which were then fixed perpendicularly on to the substrate. PDMS was dissolved in n-heptane to form a PDMS solution and the polymer solutions were stirred at room temperature for 1 h. Meanwhile, 1 wt% of the crosslinking agent TEOS and 0.5 wt% of catalyst dibutyltin dilaurate (DBTDL) was also dissolved in n-heptane and stirred at room temperature for 1 h. For the preparation of the defect-free pervaporation composite membrane, the PDMS and TEOS/DBTDL solutions were repeatedly and alternately sprayed on to the flat porous sheet of the PS substrate. Both PDMS and TEOS/DBTDL solutions were sprayed at a flow rate of 2–3 mL/s. The distance between the substrate and spray nozzles was fixed at 45 cm, and the sprayed area was a circle of diameter over 10 cm. The substrate was perpendicularly spun at 100 rpm. The spray process was performed as shown in Scheme 2. To remove the remaining solvent, the finished samples were allowed to stand for 1 day. After that, the samples were placed in a convection oven set at 80 1C for 8 h to fully crosslink the PDMS solution.
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vacuum and fractured in liquid nitrogen. Surface topography was also observed in tapping mode by an atomic force microscope (AFM) (Pico ScanTM 2500, USA). Water contact angle measurements were also performed using a contact angle analyzer (DSA 100, Germany).
3. Results and discussion Scheme 2. Schematic illustration of multiple spray process. (t1, t2 are the process time of polymer and crosslinker solutions, respectively. td represents the required interval between two spraying process. ts is the standing interval for drying the solution on the surface. n is the spraying cycles.).
Scheme 3. Experimental apparatus for pervaporation evaluation.
2.4. Pervaporation experiments The pervaporation apparatus used in the experiments has been described previously [16]. As shown in Scheme 3, the system used for pervaporation contains pervaporation cell, cold trap, feed tank, constant temperature tank, digital vacuum gauge, air valve, circulation and vacuum pump. Membranes were loaded into pervaporation cell for experiments. The effective area of the membrane in the permeation equipment was 24.6 cm2. The feed aqueous solution had an ethanol concentration of 5.0 wt%, and the feed temperature was 60 1C. The down-stream pressure was about 200 Pa and monitored by a digital vacuum gauge. Each experimental run for the pervaporation process was 30 min. The permeate vapor was trapped with liquid nitrogen and measured by gas chromatography (GC-14C, SHIMADZU) using a thermal conductivity detector. The parallel membranes were prepared in exactly the same way. Each membrane was measured three times using the same pervaporation conditions. Fluxes were determined by measuring the weight of liquid collected in the cold traps over a certain time under steady-state conditions. The permeation total flux (J) and separation factor (a) represent the permeability and selectivity, respectively. The separation factor (a) was calculated according to the following equation, with Yw and Xw representing the weight fractions of water in permeate and feed.
a¼
Y w ð1=X w Þ X w ð1=Y w Þ
2.5. Characterization The membrane surface and cross-section before and after spraying was observed with a scanning electron microscope (SEM) (Hitachi S-4300, Japan) attached to an energy dispersive spectrometer (EDX). All membrane samples were dried under
3.1. SEM-EDX, AFM and contact angle analyses The changes of the surface and cross-sectional morphology revealed by SEM before and after spraying are shown in Fig. 1. From the high magnification image (100k ), a large number of pores can clearly be observed on the PS supporting membrane (Fig. 1(a-1)). In contrast, after spraying with 5 cycles of PDMS and TEOS/DBTDL, all the surface pores have been covered and no defects were found in a randomly selected area. Clearly, a new defect-free selective layer has been formed (Fig. 1(a-2)). The incorporation of PDMS endowed the membrane with remarkably high density and compactness. From the cross-sectional SEM images (Fig. 1(b-1) and (b-2)), it is difficult to obtain the exact thicknesses of the selective layer on top of the supporting PS membrane because the boundary between the selective layer and supporting membrane is not clear. Further, it was observed from the higher magnification of SEM Fig. (5k ) that the selective layer merged well with the supporting membrane (Fig. 1(b-3)). These observations suggest that the selective layer obtained by spraying relatively diluted polymer solution is ultrathin, which is beneficial for a high total flux. In order to analyze the thickness of the top selective layer, the changes of elemental composition through the cross-section were also analyzed using EDX. As shown in Fig. 2, the concentrations of carbon and oxygen remain almost unchanged across the membrane. However, just before the 1 mm point, the silicon content decreased while the sulfur content increased. The concentrations of silicon and sulfur therefore show opposite trends. Since the PS supporting membrane does not contain silicon while the PDMS polymer does not contain sulfur, all the silicon must arise from the selective PDMS layer and all the sulfur from the PS supporting membrane. From this analysis, we deduce that some PDMS has intruded into the PS membrane pores because some silicon was found there. These observations led us to conclude that the thickness of the PDMS selective layer is far less than 1 mm. To characterize the changes of morphology, an inspection of the top surface by AFM before and after spraying is shown in Fig. 3 using two different enlargements. In comparison with Fig. 3(a-1) and (a-2), significant changes were observed on the surface of the membrane. Before being sprayed with PDMS, the surface of the PS supporting membrane was relatively flat, with a Ra value of 4.4 nm. It was observed that after being sprayed with PDMS 5 times, the membrane surface was much rougher and the Ra increased to 60.4 nm. This increased roughness might be attributed to the surface enrichment of crosslinked PDMS. Observations over a much larger area (80 80 mm) (Fig. 3(b-1) and (b-2)), show that we also obtain a rougher surface after the spray process, with the Ra increasing from 18.2 nm to 80.4 nm. Interestingly, we found that the highest bump on the surface is less than 1.0 mm. This further confirmed that the selective layer after spraying is much less than 1 mm thick. The contact angle of the PS supporting and PDMS/PS composite membranes are shown in Fig. 4. It was observed that after spraying with PDMS, the contact angle increased from 62.71 to 106.11 (Fig. 4(a) and (b)). Meanwhile, the contact angles of the PDMS/PS composite membrane varied over a very narrow band no matter how many spray cycles were carried out (Fig. 4(b) and (c)).
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Fig. 1. SEM images of sprayed PDMS/PS composite membrane. (a-1) Surface image of PS supporting membrane before spray (100.0k ). (a-2) Surface image of PDMS/PS composite membrane sprayed with 5 cycles (100.0k ). (b-1) Cross-sectional image of PS supporting membrane before spray (1.0k ). (b-2) Cross-sectional image of PDMS/PS composite membrane sprayed with 5 cycles (1.0k ). (b-3) Cross-sectional image of PDMS/PS composite membrane sprayed with 5 cycles (5.0k ). (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼ 3 s, td ¼ 1 s, and ts ¼ 120 s; 25 1C; 20% humidity).
Clearly, after being sprayed with PDMS, the surface became more hydrophobic compared with the uncoated PS membrane. This characteristic is beneficial for ethanol permselective permeability. Together, the above experiments confirm that a dense, rough, ultrathin and hydrophobic selective layer was successfully formed on the PS supporting membrane after spraying with crosslinked PDMS. 3.2. Effects of PDMS molecular weight and concentration For the spraying process, both the molecular weight and concentration of the polymer are crucial parameters for the formation of a defect-free ultrathin selective layer. As expected, Table 1 shows that the selectivity increased while the total flux decreased with the increase of PDMS molecular weight. It was
noted that the low molecular weight PDMS resulted in a higher flux but compensated with a decrease of selectivity. Therefore, the PDMS with relatively high molecular weight of 80,000 g/mol was selected for subsequent studies. The effects of PDMS concentration were investigated by keeping the TEOS concentration at a constant value of 1 wt%. As shown in Fig. 5, the selectivity first increased and then decreased with increasing PDMS concentration. The separation factor increased from 6.9 to 8.4 when the PDMS concentration varied from 0.1 wt% to 10 wt% and then decreased to 3.8 wt% at 20 wt%. The total flux was significantly reduced, from 4000 g/m2 h to 301 g/m2 h, as the PDMS concentration increased from 0.1 wt% to 20 wt%. Clearly, in addition to the PDMS molecular weight, the PDMS concentration during the spray process has a strong impact on the pervaporation performance. Fig. 6 shows that a big lump was formed on the
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Fig. 2. Cross-sectional EDX analyses of PDMS/PS composite membrane. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼3 s, td ¼ 1 s and ts ¼ 120 s, five spraying cycles; 25 1C; 20% humidity).
selective layer at 20 wt% PDMS concentration. This suggests that the polymer begins to shrink so allowing a big lump to be easily formed on the surface, which will negatively affect the total flux. In subsequent experiments, 1 wt% was selected as an appropriate PDMS concentration for the spray assembly process. 3.3. Effects of spraying cycles Pervaporation is a very strict process, which requires a nonporous membrane. Any minute defects will cause a decline of selectivity. Considering the flexibility and rapidity of the spray assembly, multiple spraying has been proposed to obtain a defectfree selective layer. In this way, each added cycle is used to cover the defects in previous layers. Therefore, it was expected that the pervaporation performance of the sprayed membrane could be easily tuned by adjusting the spraying cycles. As shown in Fig. 7, the separation factor increased while the total flux decreased with increasing number of spraying cycles. For example, the separation factor was 4.7 while flux was as high as 8981 g/m2 h for one crosslinked PDMS layer. After spraying 5 times, the separation factor increased to 7.5 while the flux decreased to 3275 g/m2 h. Clearly, later spraying cycles are beneficial for remedy the defects
in previous layers, which, in turn, improves the separation factor of multiple sprayed membranes. In addition, it was noted that, as the number of spray cycles continuously increased from 5 to 12, the separation factor remained almost unchanged while the flux dropped continuously to 1034 g/m2 h. This is because the repeated cycles lead to an increase of the thickness of the top selective layer. Since the thickness is inversely proportional to the total transport resistance, the total permeate flux decreases [17,18]. Moreover, once the defects are completely covered with multiple crosslinked PDMS layers, the selectivity was independent of any further increase of sprayed layers. Therefore, there is generally a compromise between pervaporation efficiency and spraying cycles. For a reasonable separation factor and suitable flux, 5 cycles were selected for subsequent experiments. 3.4. Effects of spraying time and standing interval Usually, the polymer spraying time is thought to be directly linked to the film quality and structure. Therefore, the effects of PDMS spraying time on pervaporation performance were investigated, with the results shown in Fig. 8. It was noted that the selectivity showed a trend of first increasing and then decreasing
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Fig. 3. Three-dimensional AFM images of PS supporting and PDMS/PS composite membrane. (a-1) PS supporting membrane before spray (10 10 mm, Ra¼ 4.4 nm). (a-2) PDMS/PS composite membrane sprayed with 5 cycles (10 10 mm, Ra ¼60.4 nm). (b-1) PS supporting membrane before spray (80 80 mm, Ra ¼18.2 nm). (b-2) PDMS/PS composite membrane sprayed with 5 cycles (80 80 mm, Ra ¼80.4 nm). (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ 0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼ 3 s, td ¼ 1 s, and ts ¼ 120 s; 25 1C; 20% humidity).
Fig. 4. Contact angles of PS supporting and PDMS/PS composite membrane. (a) Uncoated PS supportinging membrane before spray (62.7 7 0.131). (b) PDMS/PS composite membrane sprayed with 1 cycle (105.4 70.251). (c) PDMS/PS composite membrane sprayed with 10 cycles (106.1 70.091). (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOSþ 0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼ 3 s, td ¼ 1 s, and ts ¼ 120 s; 25 1C; 20% humidity).
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Table 1 Effects of PDMS molecular weight on pervaporation performance. PDMS molecular weight (g/mol)
Ethanol content in feed solution (wt%)
Ethanol content in permeate (wt%)
Total flux J (g/m2 h)
Separation factor a
80,000 40,000 4,200
5 5 5
28.3 21.1 21.1
3275 3230 3666
7.5 5.1 5.1
Fig. 7. Effects of spraying cycles on pervaporation performance. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼5 s, t2 ¼ 3 s, td ¼1 s, and ts ¼120 s; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
Fig. 5. Effects of PDMS concentration on pervaporation performance. (Spray preparative conditions: 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼ 5 s, t2 ¼3 s, td ¼ 1 s, and ts ¼120 s; five spraying cycles; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
Fig. 8. Effects of PDMS spraying time on pervaporation performance. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOSþ 0.5 wt% dibutyltin dilaurate n-heptane solution; t2 ¼3 s, td ¼ 1 s, and ts ¼ 120 s; five spraying cycles; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
Fig. 6. SEM images of PDMS/PS composite membrane obtained from 20 wt%. (Spray preparative conditions: 20 wt% PDMS n-heptane solution; 1 wt% TEOS þ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼5 s, t2 ¼3 s, td ¼ 1 s, and ts ¼120 s, five spraying cycles; 25 1C; 20% humidity; 5.0k ).
while the total flux continuously decreased. For instance, the separation factor obtained from spraying times of 1 s, 3 s and 5 s were 6.4, 6.5 and 7.5 while the total fluxes were 5395 g/m2 h, 3374 g/m2 h and 3275 g/m2 h, respectively. When the spraying time was prolonged to 7 s, both the separation factor and total flux appeared to decline. If the spraying time is too short, it is difficult to completely cover all the surface defects of the PS supporting membrane. However, excessive spraying time does not benefit the pervaporation performance because superabundant PDMS cannot be completely crosslinked, which then causes a reduction of both the separation factor and total flux. Therefore, 5 s PDMS spraying time was concluded to be a suitable spraying time.
The standing interval between two cycles (ts) is also important to the formation of the selective layer and membrane performance because the pre-crosslinking and rearrangement of PDMS take place during this period. Fig. 9 shows the variation of the pervaporation performance with standing interval. When standing interval was less than 50 s, the performance is not steady because the pre-crosslink process was not completed while the rearrangement was ongoing. When the standing interval was 120 s, the separation factor reached a relatively high level. By comparison, the separation factor declined to 4.3 when the standing interval increased to 300 s. This suggested that prolonging the standing interval might result in excessive crosslinking, which was not helpful for the formation of highly selective separation layers. Based on the above investigations, the appropriate conditions for multiple sprayed PDMS/PS membranes were selected as follows: PDMS concentration 1 wt%, TEOS concentration 1 wt%, t1 ¼ 5 s, t2 ¼3 s, td ¼ 1 s, ts ¼120 s and five spraying cycles. Using this set of conditions, the membrane obtained had a separation factor of 7.5 and a total flux of 3275 g/m2 h for pervaporation separation of 5 wt% ethanol/water system (60 1C).
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3.5. Comparison with dip-coated composite membrane and reference data in the literature In order to understand clearly the difference between dip-coated and spayed composite membranes, control experiments were carried out by dip-coating PDMS on to the same supporting membrane. The comparative pervaporation experiments between dip-coated and sprayed membranes were performed under the same pervaporation conditions. For the dip-coating process, it was noted that the PDMS
Fig. 9. Effects of standing interval on pervaporation performance. (Spray preparative conditions: 1 wt% PDMS n-heptane solution; 1 wt% TEOSþ0.5 wt% dibutyltin dilaurate n-heptane solution; t1 ¼5 s, t2 ¼ 3 s, and td ¼ 1 s; five spraying cycles; 25 1C; 20% humidity. Pervaporation conditions: feed solution temperature 60 1C, pervaporation time 30 min, vacuum pressure 200 Pa, ethanol content in feed solution 5.0 wt%).
concentration plays a very important role in the formation of high performance composite membranes. For example, the composite membrane dip-coated with 1 wt% PDMS (80,000 g/mol) solution has almost no selectivity, even after prolonging the coating time to 1 h. By comparison, after dip-coating in a 10 wt% PDMS solution added with 1 wt% TEOS and 0.5 wt% DBDTL for 1 min, the separation factor obtained from dip-coated membrane reached 7.9. Clearly, a high concentration polymer solution was necessary for the formation of a defect-free selective layer for the dip-coating method. However, when using such a high concentration, the selective layer obtained was relatively thick. As shown in the SEM picture (Fig. 10), the thickness of the selective layer obtained from the dip-coating method could reach more than 3 mm. Although the selectivity of the dip-coated membrane was comparable with that of the sprayed membrane (7.5), the flux of the dip-coated membrane was only 786 g/m2 h. Clearly, the membrane flux obtained from the spray assembly process was dramatically increased while the selectivity remained at a comparable level. This is because the spray method could form the defect-free selective layer by repeatedly spraying relatively dilute polymer solutions, which result in a thinner selective layer. Moreover, very highly concentrated PDMS solution had negative effects on the spraying assembly process because the high concentration was not helpful for the spraying itself and dispersing of polymer solutions. As shown in Table 2, the pervaporation performance of the sprayed membrane was also compared with reference data from the literature. Table 2 also shows that, although it was difficult to directly compare the pervaporation performance of these membranes, due to some extent to the different preparation and testing conditions, the results demonstrate that the sprayed PDMS/PS composite membrane exhibited the distinguishing features of much higher flux compared with other polymer-supported PDMS membranes, commonly ranging from 19 g/m2 h to 1300 g/m2 h. The thickness of the reported composite membrane usually ranged from several mm to 82 mm, whereas the thickness of the sprayed membrane was far less than 1 mm. Obviously, the much thinner PDMS separation layer obtained from the spraying method leads to lower transport resistance and results in much higher flux. Therefore, the spraying strategy provides a new way to significantly reduce the thickness of the selective layer while keeping the defect-free feature, by using dilute polymer solution and simply repeating the number of spraying cycles. Moreover, the PLC automatic system can effectively avoid many uncontrollable human factors, greatly improving the stability and reproducibility of the membrane performance. Therefore, this method has great potential for industrial technology.
4. Conclusions Fig. 10. Cross-sectional image of dip-coated PDMS/PS composite membrane. (Preparative conditions: 10 wt% PDMS þ1 wt% TEOSþ 0.5 wt% dibutyltin dilaurate n-heptane solution; 25 1C; 20% humidity).
In this study, a multiple sprayed PDMS/PS composite membrane was successfully prepared using an automatic spraying
Table 2 Prevaporation performance of different PDMS membranes. Membrane
Ethanol feed concentration (wt%)
Temperature (1C)
Membrane thickness (lm)
J (g/m2 h)
a
Down-stream pressure (Pa)
Refs.
crosslinked PDMS containing vinyl groups crosslinked PDMS containing acetoxysilyl groups PDMS–PS block copolymer PDMS–PPP graft copolymer PDMS–PS IPN supported on PES PDMS–PI copolymer PDMS–PS IPN supported on PTFE PDMS supported on a CA crosslinked PDMS supported on PS
16.5 8 10 7 10 10 10 5 5
40 30 25 30 60 60 30 40 60
82 – 39 30 15 20 34 8 Less than 1
85 25.1 27 19 160 560 85 1300 3275
8.6 10.8 6.2 40 5.5 10.6 6 8.5 7.5
13.3–66.5 133 Below 130 67 4007 133 – 7000 266 200
[12] [9] [19] [20] [21] [22] [23] [24] This study
R. Wang et al. / Journal of Membrane Science 432 (2013) 33–41
system. SEM-EDX and AFM analyses confirmed that the thickness of the selective layer obtained from five spraying cycles was still far less than 1 mm. Therefore, the resulting composite membrane shows extremely high flux compared with those from classical dip-coating methods for the pervaporation separation of ethanol/ water mixture system. It was demonstrated that the performance of the sprayed membrane was strongly dependent on the PDMS molecular weight and concentration, spraying cycles, spraying time and standing interval. A set of appropriate conditions was selected as follows: PDMS concentration 1 wt%, t1 ¼5 s, t2 ¼3 s, td ¼1 s, ts ¼120 s, five spraying cycles. With this set of given conditions, the membrane obtained had a separation factor of 7.5 and a total flux of 3275 g/m2 h for the pervaporation of a 5 wt% ethanol/water mixture system (60 1C). To the best of our knowledge, this is the first study that extends the multiple spray technology to prepare an ethanol permselective membrane with an ultrathin defect-free selective layer. It is reasonable to expect that the multiple spray technique will contribute to the preparation of various composite membranes by a suitable choice of the spraying conditions and organic species such as functional polymer, surfactants and dye molecules.
Acknowledgments This work was financially supported by the National High Technology Research and Development Program of China (No. 2012AA03A607), Natural Science Foundation of Beijing (No. 8122010), the National Basic Research Program of China (No.2009CB623404) and Fok Ying Tung Education Foundation (No.131068). References [1] L.M. Vane, A review of pervaporation for product recovery from biomass fermentation processes, J. Chem. Technol. Biotechnol. 80 (2005) 603–629. [2] G.P. Liu, W. Wei, H. Wu, X.L. Dong, M. Jiang, W.Q. Jin, Pervaporation performance of PDMS/ceramic composite membrane in acetone butanol ethanol (ABE) fermentation-PV coupled process, J. Membr. Sci. 373 (2011) 121–129. [3] D.J. O’Brien, L.H. Roth, A.J. McAloon, Ethanol production by continuous fermentation–pervaporation: a preliminary economic analysis, J. Membr. Sci. 166 (2000) 105–111. [4] T.C. Bowen, R.G. Meier, L.M. Vane, Stability of MFI zeolite-filled PDMS membranes during pervaporative ethanol recovery from aqueous mixtures containing acetic acid, J. Membr. Sci. 298 (2007) 117–125. [5] T. Mohammadi, A. Aroujalian, A. Bakhshi, Pervaporation of dilute alcoholic mixtures using PDMS membrane, Chem. Eng. Sci. 60 (2005) 1875–1880.
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