ceramic hybrid membranes for ethanol dehydration

Separation and Purification Technology 206 (2018) 218–225

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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Enhanced pervaporation performance of SA-PFSA/ceramic hybrid membranes for ethanol dehydration Hao-Ran Xie, Chen-Hao Ji, Shuang-Mei Xue, Zhen-Liang Xu, Hu Yang, Xiao-Hua Ma

T ⁎

Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Sodium alginate Perfluorinated sulfonic acid Contorted rigid structure Dehydration Pervaporation

In this study, sodium alginate (SA)-perfluorinated sulfonic acid (PFSA)/ceramic hybrid membranes were successfully prepared by dip-coating method. The contorted rigid structure of PFSA immobilized the loose structure of SA as well as improved the interconnectivity so as to demonstrated high separation factors. Meanwhile, the ion clusters formed by eSO3H groups of PFSA possessed high affinity for water, resulting in high water flux. The viscosity of blend solution, structures and properties of the hybrid membranes were investigated by viscometry, Fourier transform infrared (FTIR), scanning electron microscopy (SEM), water contact angle meter and pervaporation dehydration. The effects of SA-PFSA ratio, PFSA content, feed composition and temperature on the separation performances of the hybrid membranes were investigated. The hybrid membrane fabricated by 2.0 wt % SA and 2.0 wt% PFSA demonstrated a high flux of 1155 g m−2 h−1 coupled with separation factor of 1149 by dehydration of 15 wt% water content ethanol-water mixture at 75 °C, reflecting superior pervaporation processing capacity.

1. Introduction Compared with conventional separation processes, pervaporation (PV) is considered as an efficient and environmental friendly membrane separation process that provides several advantages, such as low energy consumption, high selectivity, and uncomplicated process design [1]. In addition, PV demonstrates tremendous potential for separating azeotrope and close-boiling mixtures [2]. Usually, ethanol-water separation is used as a kind of standard to test the effectiveness of a PV membrane due to it is difficult to remove one of these solutions from each other. As for the dehydration of alcohol-water solution, highly hydrophilic membranes which yield high permeability, good selectivity and sufficient mechanical strength are preferred [3–5]. Therefore, various hydrophilic materials [2,3] have been reported as PV membrane materials, including poly (vinyl alcohol) [6], polyimides [7], polyamides [8,9], polysaccharides [10], and polyelectrolytes [11–13]. Sodium alginate (SA) is a natural polysaccharide that possesses excellent performances (e.g., hydrophilicity) as a PV membrane material for the dehydration of alcohol-water mixture [14]. Its carbohydrate chains consist of sugar moieties which contain a large number of carboxyl groups and hydroxyl groups, endowing SA membrane with outstanding hydrophilicity and excellent permselectivity nanochannels for water [15]. However, high hydrophilicity of SA leads to instability in

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Corresponding author. E-mail address: [email protected] (X.-H. Ma).

https://doi.org/10.1016/j.seppur.2018.05.060 Received 26 March 2018; Received in revised form 29 May 2018; Accepted 29 May 2018 Available online 30 May 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.

aqueous solution during PV. Apart from its water solubility, mechanical weakness of SA membrane has also been a weak point in its possible use in PV [16]. Therefore, great efforts have been devoted to improving the performance of SA PV membranes. A popular modification method is crosslinking [17,18]. It is widely reported that cross-linking with different organic cross-linkers successfully enhances the chemical structure and solves the polymeric chains relaxation of SA membrane [19]. However, a serious drawback for cross-linking is that this chemical reaction is difficult to control (e.g., extent of reaction), which greatly influences the degree of cross-linking [20,21] and thus impacts the performance of the SA membrane. Another frequently used modification method is incorporating nanomaterials with well-defined pore structures, such as metal-organic frameworks (MOFs) [10,22–24] , carbon nanotubes [25], graphenes [26] and nanoparticles [27–29]. However, dispersion of nanomaterials in the polymeric matrix is a great challenge because the nanomaterials trend to agglomerate to create defects in the membranes, resulting in a decrease in selectivity [30]. Polymer blends are attractive alternative materials for modification of SA membranes due to ease of processing and synergetic properties of polymer blends. Moreover, blend method can avoid the drawbacks of cross-linking (e.g., control of cross-linking degree) and incorporating nanomaterials (e.g., dispersion). Perfluorinated sulfonic acid (PFSA) is considered as a good blend polymer due to its excellent chemical,

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Fig. 1. Schematic diagram of blending SA with PFSA.

solutions at 20 °C) and ethanol were purchased from Sinopharm (China). Perfluorinated sulfonic acid (PFSA, MW = 1130) resin was purchased from Shanghai Baichun Chemical Materials Co., Ltd. (China). The α-Al2O3 ceramic tube M20 with a mean pore size of 20 nm was supplied by Hyflux SIP PTE Ltd. Deionized water was used throughout the work.

Table 1 Compositions of SA-PFSA blend solutions. Blend

SA (wt.%)

PFSA (wt.%)

Ethanol (wt.%)

Water (wt.%)

SA-1.5/PFSA-2.5 SA-2.0/PFSA-2.0 SA-2.5/PFSA-1.5 SA-3.0/PFSA-1.0

1.5 2.0 2.5 3.0

2.5 2.0 1.5 1.0

30.8 24.0 16.3 9.5

65.2 72.0 79.7 86.5

2.2. Membrane preparation Table 2 Variation of PFSA content in SA-PFSA blend solutions and nomenclature. Blend

SA (wt.%)

PFSA (wt.%)

Ethanol (wt.%)

Water (wt.%)

SA-2.0/PFSA-1.0 SA-2.0/PFSA-1.5 SA-2.0/PFSA-2.0 SA-2.0/PFSA-2.5

2.0 2.0 2.0 2.0

1.0 1.5 2.0 2.5

24.5 24.3 24.0 23.8

72.5 72.2 72.0 71.7

PFSA was dissolved in ethanol-water (1:1, wt/wt) solution to prepare a 5.0 wt% solution at ambient temperature. A certain amount of SA was dissolved in water under continuously stirring to prepare an aqueous solution. After filtration to remove insoluble materials, a desired amount of PFSA solution was added in SA aqueous solution under continuously stirring to prepare SA-PFSA blend solutions. The compositions of SA-PFSA blend solutions as well as nomenclature were shown in Table 1. SA-PFSA/ceramic hybrid membrane was prepared by coating the blend solution on the ceramic hollow fiber membrane. Specifically, a 50 mm in length ceramic hollow fiber membrane was connected to a specially made stainless steel tube. The joint between the ceramic membrane and stainless steel tube as well as the other end of the ceramic membrane were sealed by epoxy sealant. After the epoxy sealant dried up, the ceramic membrane was dip-coated with the prepared SA-PFSA blend solution. Every membrane was coated twice and placed upside down during the second drying process to ensure uniform distribution of the active layer along the hollow fiber membrane axis [36]. For convenience, these hybrid membranes share the same names with blend solutions (Table 1). The PV performance of the obtained membranes made above was investigated and it was found that the one with 2 wt% SA showed the best performance. In order to investigate the effect of PFSA content, another four hybrid membranes were prepared as shown in Table 2, in which SA concentration and ethanol content were remained the same. The hybrid membrane preparation process was the same as above.

thermal and mechanical properties [31]. Its fluorocarbon main chain forms a rigid main body (polytetrafluoroethylene backbone) which can effectively improve the mechanical properties of the blend membrane [32]. The terminated hydrophilic sulphonate ionic groups are considered as a cluster-type structure [33] containing aqueous phase ions which can facilitate the transport of water [34]. In the current work, PFSA was blended with SA to prepare PV membranes. A schematic diagram of blending SA with PFSA was shown in Fig. 1. The contorted rigid structure of PFSA can effectively immobilize the loose structure of SA. On the other hand, the blending process enabled PFSA ion clusters to physically cross-link with SA carbohydrate chains (Fig. 1), thus enhancing interconnectivity of intermolecular voids [35]. The physic-co-chemical properties of the obtained PV hybrid membranes were investigated by viscometry, Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and water contact angle meter. The effects of SA-PFSA ratio, PFSA content, feed composition and temperature on the separation performances of the hybrid membranes were investigated by ethanol dehydration.

2.3. Pervaporation test 2. Experimental In order to systematically investigate the separation performance, the obtained SA-PFSA hybrid hollow fiber membranes were used to dehydrate ethanol-water solution. The effective area of the membrane was 5.0 cm2. The operating temperature varied from 60 to 75 °C and the

2.1. Materials Sodium alginate (SA, viscosity above 0.02 Pa·s in 1 wt% aqueous 219

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Fig. 2. SEM images of the cross sections of the hybrid membranes: SA-1.5/PFSA-2.5 (a), SA-2.0/PFSA-2.0 (b), SA-2.5/PFSA-1.5 (c), and SA-3.0/PFSA-1.0 (d).

Fig. 3. (a) Shear viscosity of SA-PFSA blend solutions of different SA-PFSA ratios, and (b) FTIR spectra of SA, PFSA and hybrid membranes of different SA-PFSA ratios.

feed solution was 500 mL of ethanol − water solution with the water content varying from 10 to 20 wt%. The downstream pressure was 1.1 kPa controlled by a vacuum pump to provide the chemical potential difference for permeation. The calculation of the flux (J) and separation factor (α) refers to the following formula:

J=

m A×t

α=

Yw / Xe Xw / Xe

(2)

where m represents the total weight of the permeate, A and t represent the effective area of SA-PFSA hybrid hollow fiber membrane and duration time of the PV process, respectively. Y and X stand for the mass fraction of a component in the permeate and in the feed, respectively. And subscripts of w and e represent water and ethanol, respectively. The concentration of feed and permeate solution were

(1)

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the prepared hybrid membranes were scanned in the range from 4000 to 750 cm−1 by Fourier Transform Infrared Spectroscopy (FTIR, Nicolet-6700). Hydrophilicity of the hybrid membrane was characterized by the contact angle meter (JC2000A, Shanghai Zhong Cheng Digital Equipment Co., Ltd, China) at 25 ± 0.2 °C.

Table 3 Pervaporation performance of the SA-PFSA/ceramic hybrid membranes in the feed containing 10–20 wt% water at 75 °C. Blend

Water in the Water flux feed (wt.%) (g m−2 h−1)

SA-1.5/PFSA-2.5 10 15 20

1782 2812 4547

Separation factor

PSI (g m−2 h−1)

742 261 132

1.32 × 106 7.33 × 105 5.98 × 105

3. Results and discussion

6

SA-2.0/PFSA-2.0 10 15 20

834 1155 3641

5052 1149 290

4.21 × 10 1.33 × 106 1.06 × 106

SA-2.5/PFSA-1.5 10 15 20

514 709 2585

7688 1700 458

3.95 × 106 1.21 × 106 1.01 × 106

SA-3.0/PFSA-1.0 10 15 20

312 345 1380

∞ 3915 1062

1.35 × 106 1.47 × 106

3.1. Effect of SA-PFSA ratio Fig. 2 displayed the cross-sectional morphologies of the hybrid membranes observed by SEM. It was clear that the thickness of the blend polymer layer increased with SA-PFSA ratio. A possible reason for the thickness variation was the effect of viscosity of the SA-PFSA blend solution [37]. The curve of the viscosity of the SA-PFSA blend solution was displayed in Fig. 3a. With the increase of SA-PFSA ratio, the shear viscosity of the blend solution increased and this uptrend led to an increase of film thickness (Fig. 2). SA is a polysaccharide which existed in the form of random coils in solution and the rotation of such molecules occupied a lot of space. Therefore, there were high probability of collision and strong friction between molecules in SA solution. Consequently, the increasing amount of SA resulted in an obvious rise in viscosity. Obvious cracks (especially in Fig. 2d) were observed between the blend polymer layer and the ceramic support layer. It was because SEM samples were prepared using liquid nitrogen and the overly dense organic layer brought a great tensile force when it suffered the thermal contraction [38]. As a result, this stress concentrated on the most fragile region where the crack generated. FTIR spectra of SA, PFSA and hybrid membranes were displayed in Fig. 3b. Concerning the spectrum of SA, a broad and deep band around

determined by gas chromatography (Techcomp GC7890T, China). Samples were taken after the PV system reached a steady state, approximately 1 h. 2.4. Characterizations Field-emission scanning electron microscopy (Nova NanoSEM 450, USA) was used to observe the outer surface and cross-sectional morphology of the hybrid membrane. The kinetic viscosity of the SA-PFSA blend solutions were measured by rheometer (senior rotary rheometer, MCR302). The rate of shear was kept at 100 s−1 and the test temperature is controlled at 25 ± 0.2 °C. All FTIR spectra of SA, PFSA and

Fig. 4. SEM images of the cross sections of the hybrid membranes: SA-2.0/PFSA-1.0 (a), SA-2.0/PFSA-1.5 (b), SA-2.0/PFSA-2.0 (c) and SA-2.0/PFSA-2.5 (d). 221

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Fig. 5. (a) Shear viscosity of SA-PFSA blend solutions containing varying PFSA content, and (b) dynamic water contact angle of the corresponding SA-PFSA hybrid membranes and pristine polymers.

of SA and PFSA, the spectra of SA-PFSA hybrid membranes possessed all the characteristic peaks mentioned above. The characteristic absorption peak assigned to O]S]O bond of PFSA was almost overlaid by the peak at 1025 cm−1 of SA when the PFSA content was low (e.g., 1 wt% and 1.5 wt%), but the peak intensity increased gradually with the increase in PFSA content which contributed to show a shoulder. Table 3 depicted PV performance of blend hollow fiber membranes with different SA-PFSA ratios at 75 °C. As can be observed from the PV separation characteristics in Table 3, water flux showed a downtrend with the increase of SA-PFSA ratio while the corresponding separation factor improved. When the water content of the feed increased, both the flux and separation factor changed evidently. The flux increased while the separation factor decreased. It was because water influenced the ion cluster size and ion channel interconnection of the PFSAs [39]. The transport capacity of water improved with the increase of water content in outer aqueous solution while the separation ability was correspondingly reduced. In order to define the separation ability of a membrane, a PV separation index (PSI, g m−2 h−1) which was defined as J × (α − 1), was widely used. This factor could be used to compare similar membranes, but it cannot distinguish the overall performance of the membrane, because the membrane with low separation factor and the high flux had the same PSI as the one with high separation factor and the low flux [40]. As shown in Table 3, SA-2.0/PFSA-2.0 had high values of PSI and simultaneously possessed relatively high flux and relatively high separation factor. Subsequently, we kept SA content at 2.0 wt% and varied PFSA content in the range of 1.0–2.5 wt% to investigate the effect of PFSA on the structure-performance of the hybrid membrane.

Fig. 6. FTIR spectra of hybrid membranes with PFSA contents.

3.2. Effect of PFSA content Fig. 4 displayed the cross-section of the hybrid membranes observed by SEM. When the PFSA content increased from 1.0 to 2.5 wt%, the film thickness tended to increase firstly and then decrease. Accordingly, the shear viscosity of the blend solution increased firstly, reaching at a maximum of near 1500 mPa·s, and then decreased with the increase of PFSA content (Fig. 5a). As for the variety of shear viscosity, it was believed that when the blending ratio changed, the viscosity of the blend may appear maximum or minimum value due to the interaction between two different polymers [41]. When PFSA content was 1.5 wt%, the viscosity of blend solution reached the maximum which meant the interaction of SA and PFSA came to strongest. The effect of PFSA content of blend polymer layer on the contact angle was presented in Fig. 5b. It could be observed that both the

Fig. 7. Influence of PFSA contents on PV performance at 60 °C.

3340 cm−1 assigned to the stretching vibration of eOH bond and a strong absorption band around 1020–1095 cm−1 was due to the stretching vibration of CeO bond. As for PFSA, the peaks at 1236 and 1155 cm−1 were attributed to the asymmetric and symmetrical stretching vibration of CF2 bond of PFSA, respectively. The characteristic absorption at 1063 cm−1 attributed to the symmetric stretching vibration of O]S]O bond and the broad peak at 1632 cm−1 was related to the eOH bond on sulfonic groups. Compared with the spectra 222

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Fig. 8. The Arrhenius fitting curve of ln J vs. 1000/T (a) and the curve of ɑ vs. T (b) for SA-2.0/PFSA-2.0 at different feed temperatures. Table 4 Dehydration of alcohols using alginate-based PV membranes in the literature. Ethanol/H2O (wt.%)

Membrane support

Active layer

Modifier (cross-linker)

Separation factor

Flux (g m−2 h−1)

PSI (g m−2 h−1)

Temperature (◦C)

Reference

90/10

PAN

SA

zwitterionic grapheme oxides attapulgite nanorods zeolite 4A glycogen maleic acid phosphoric acid glutaraldehyde

1370

2140

2.93× 106

77

[45]

1356 106 1250 630 240 220 1149 821

6

76 25 75 45 30 30 75 75

[46] [47] [38] [48] [14] [16] this work this work

90/10 90/10 75/25 90/10 95/5 95/5 85/15 85/15

PAN ceramic membrane

ceramic membrane ceramic membrane

SA SA SA PVA-SA SA CS/SA SA-2.0/PFSA-2.0 SA-2.0/PFSA-1.5

2030 396 187 110 2182 436 1155 3799

2.75× 10 4.20× 105 2.34× 105 6.93× 104 5.24× 105 9.59× 104 1.33× 106 3.12× 106

well as improved the interconnectivity so as to demonstrate high separation factors. The hydrophilic ions clusters formed by PFSA led to preferential sorption and diffusion of water through the membrane. As shown in Fig. 7, water flux of the hybrid membrane decreased firstly and then increased with PFSA content, agreeing well with the contact angle results. While the separation factor changed in the opposite trend simultaneously as a result of the impact of membrane thicknesses (Fig. 4). The relationship between water flux of PV membrane and feed temperature obeys the Arrhenius law [44]. In the Eq. (3), Ji represents the permeation flux of the membrane, Ai is the pre-exponential factor, R is the gas constant, T is the absolute temperature of the feed liquid, and Ep,i stands for the apparent activation energy.

dynamic and the initial water contact angles increased dramatically when PFSA content increased from 1.0 wt% to 1.5 wt%, indicating a decrease in the hydrophilicity of the hybrid membrane. Next, the dynamic and the initial water contact angles presented a slightly decrease as PFSA content increased to 2.5 wt%. With regard to this, we supposed when the PFSA content was 1.5 wt%, it nearly came to a maximum point where the ion clusters fully interlocked with SA chains (Fig. 1). Sulfonate ion groups interconnected to form ion clusters and then interlocked with SA chains. This interlock structure could immobilize the long chain of SA and improve the interconnectivity. However, a part of ion clusters were unavoidably blocked by SA chains [42]. Therefore, SA chains limited the exposure of hydrophilic sulfonate ion groups, which greatly depended on the blending ratio of SA and PFSA. The FTIR spectra of the hybrid membranes with different PFSA contents were shown in Fig. 6. Apparently, all the characteristic peaks of SA and PFSA were present in the spectra of the hybrid membranes and there was no new functional group forming. It was worth noting that the peaks which assigned to sulfonate ion group of PFSA did not increase notably from SA-2.0/PFSA-1.0 to SA-2.0/PFSA-2.0 which was supposed to result from the cover of SA chains. When the PFSA content increased to 2.5 wt%, the characteristic absorption peaks of sulfonate ion group finally demonstrated a marked rise. As for the PV performance, it is well known that separation characteristics are evidently affected by the interaction between solvent to be separated and the membrane matrix [43]. The physical crosslinking of SA and PFSA was supposed to have a significant influence on the ethanol dehydration ability of hybrid membranes. The contorted structure of employed PFSA immobilized the loose structure of SA as

Ep, i ⎞ Ji = Ai exp ⎛− ⎝ RT ⎠ ⎜

⎟

(3)

Fig. 8 displayed the Arrhenius fitting curve of ln J vs. 1000/T and the curve of ɑ vs. T for SA-2.0/PFSA-2.0 at different feed temperatures. With the increase of feed temperature, the vapor partial pressure of water molecules on the feed side increased, while that on the vacuum side remained almost the same. Therefore, the differential pressure increased, which increased the mass transfer efficiency significantly. In addition, the increase of temperature made more water molecules adsorbed on the membrane surface, and the diffusion rate in the membrane was accelerated, which also promoted the increase of mass transfer efficiency. Moreover, the free volume between the carbohydrate chains expanded mildly [38] with the increasing temperature. As

223

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a result, the PV data of hybrid membranes showed reasonable changes: flux increased while separation factor decreased with the increase of feed temperature. Table 4 demonstrated PV performances of alginate-based membranes in the literature. Comparing the PV performance of SA-PFSA/ ceramic hybrid membranes fabricated in this work (Table 3) with those in the representative literature in Table 4, it can be found that SAPFSA/ceramic hybrid membrane has competitive PV performance, even demonstrating better PV performance.

[13]

[14]

[15] [16]

4. Conclusions

[17] [18]

In conclusion, SA-PFSA hybrid membranes were successfully prepared on the ceramic supporting layer by dip-coating method. Better pervaporation performance could be obtained by regulating the ratio of SA and PFSA. An optimized SA-PFSA blend ratio was 2.0 wt% SA and 2.0 wt% PFSA. The corresponding hybrid membrane had the highest PSI value with flux of 1155 g m−2 h−1 and separation factor of 1149 at the operating temperature of 75 °C. The high water flux resulted from the hydrophilic ions clusters formed by PFSA, while the relatively high separation factor was caused by the interlock structure of SA and PFSA. Subsequently, the effect of PFSA content on the structure-performance of the prepared hybrid membrane was investigated. It was found that the hybrid membrane SA-2.0/PFSA-1.5 appeared maximum value in shear viscosity, contact angle and separation factor which reflected the influence of interlock structure between SA and PFSA reached the peak at this blending ratio. This hybrid membrane had a flux of 821 g m−2 h−1 coupled with a high separation factor of 3799 at the operating temperature of 75 °C.

[19]

[20]

[21]

[22]

[23]

[24]

[25]

Acknowledgments

[26]

The authors gratefully acknowledge the research funding provided by Hong Kong Scholars Program (No. XJ2015015), National Natural Science Foundation of China (21406060), Fundamental Research Funds for the Central Universities (WA1514305) and China Postdoctoral Science Foundation (2016M601527).

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