Multilayer mixed matrix membranes containing modified-MWCNTs for dehydration of alcohol by pervaporation process

Desalination 355 (2015) 45–55

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Multilayer mixed matrix membranes containing modified-MWCNTs for dehydration of alcohol by pervaporation process Saeid Panahian a, Ahmadreza Raisi a,b,⁎, Abdolreza Aroujalian a,b a b

Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran Food Process Engineering and Biotechnology Research Centre, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran

H I G H L I G H T S • • • • •

MWCNTs with high purity were prepared using T-CVD technique. MWCNTs were modified by adding carboxyl groups and synthesizing nano-TiO2 on them. Multilayer mixed matrix membranes were prepared by incorporating MWCNTs into PVA. Modification of MWCNTs led to better dispersion of them into the polymeric matrix. Functionalized MWCNTs improved the pervaporative separation performance of membrane.

a r t i c l e

i n f o

Article history: Received 23 August 2014 Received in revised form 14 October 2014 Accepted 19 October 2014 Available online xxxx Keywords: Multilayer mixed matrix membranes Multi-walled carbon nanotubes (MWCNTs) Functionalized CNTs Pervaporation Poly(vinyl alcohol) (PVA)

a b s t r a c t In this work, multilayer mixed matrix membranes containing carbon nanotubes (CNTs), poly(vinyl alcohol) (PVA), polyethersulfone (PES) and polyester as inorganic filler and selective top, intermediate and support layers, respectively, were prepared for dehydration of ethanol/water mixtures by the pervaporation process. For this purpose, the MWCNTs were synthesized by the thermal-chemical vapor deposition (T-CVD) technique and then the CNTs were modified by incorporating carboxyl functional groups and synthesizing titanium oxide (TiO2) nanocrystals on them. The results indicated that the MWCNTs with high purity were prepared and carboxyl functional groups were formed on the CNTs. The membranes containing modified MWCNTs had lower swelling degree and higher crosslinking density which resulted in lower total permeation flux in comparison to the neat membrane and the membranes containing pure MWCNTs. Furthermore, the hydrophilicity, surface roughness and crosslinking of the membranes were improved by increasing the concentration of modified CNTs. All mixed matrix membranes had lower total flux than the neat membranes due to an increase of the membrane top layer resistance by incorporating the MWCNT filler. Finally, it was observed that the modification of CNTs led to better dispersion of MWCNTs into the polymeric matrix and improved the separation performance of the multilayer mixed matrix membranes. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Pervaporation (PV) has created an attractive approach to conventional energy intensive technologies such as extractive or azeotropic distillation in the liquid mixture separation [1]. The properties of the membrane and the target substance determine the separation performance of the pervaporation process. Various polymeric membranes have been used in pervaporation for dehydration of solvents [2], recovery of organics from dilute aqueous solutions [3] and separation of organic–

⁎ Corresponding author at: Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran. E-mail address: [email protected] (A. Raisi).

http://dx.doi.org/10.1016/j.desal.2014.10.027 0011-9164/© 2014 Elsevier B.V. All rights reserved.

organic mixtures [4]. In recent years, advanced pervaporation membranes with improved separation performance have been developed for various applications such as dehydration of alcohols. Composite and nanocomposite membranes as well as mixed matrix membranes have been utilized for the pervaporative dehydration purposes due to their high separation performance. Composite membranes basically consist of one or more polymeric layers as a selective layer coated over a porous substrate that may be either an inorganic or polymeric membrane. In these membranes, the selective layer plays a basic role in controlling the mass transport of species through the membrane and the support layer provides the mechanical strength for the selective layer [5]. In hydrophilic mixed matrix membranes, addition of the hydrophilic inorganic materials into the polymeric matrix increases the permeation flux and selectivity for the dehydration of aqueous alcoholic solutions [6,7]. The multilayer mixed matrix membrane is another type of pervaporation membrane consisting of

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S. Panahian et al. / Desalination 355 (2015) 45–55

different layers, generally, a thin selective dense layer and a porous support layer in which the hydrophilic inorganic materials are added to the selective top layer in order to improve the separation performance and surface properties [8,9]. Previous researches have shown that the multilayer mixed matrix membranes would have better separation performance as well as improved chemical and mechanical resistance in the pervaporation process. Huang et al. [8] and Guan et al. [9] fabricated zeolite-incorporated three-layer poly(vinyl alcohol) (PVA) membranes for the pervaporative dehydration of ethanol/ water mixtures and observed that adding zeolites into the PVA matrix led to higher permeation flux and selectivity. Amnuaypanich et al. [10] prepared a two-layer mixed matrix membrane from natural rubber (NR) and crosslinked PVA on a nylon membrane containing NaA zeolite as inorganic filler for the pervaporative dehydration of water/ethanol mixtures. Various polymeric materials including poly(vinyl alcohol) (PVA) [8, 11–13], chitosan [14,15], polyamide (PA) [16,17], polyimide (PI) [18, 19], polyacrylonitrile (PAN) [20], poly(tetrafluoroethylene) (PTFE) [21], polyethersulfone (PES) [7] and polysulfone (PSf) [22] have been used for preparation of the different types of pervaporation membranes for dehydration purposes. Among them, the PVA membrane was given more attention due to its strong affinity to water molecules, good membrane forming properties and mechanical strength [11]. Furthermore, several inorganic compounds have been used as filler in the synthesis of the mixed matrix membranes for solvent dehydration by the pervaporation process. For example, zeolite [5–7,17,18,23], titanium oxide (TiO2) [24,25], iron [22], sodium montmorillonite clay [26], carbon nanotubes (CNTs) [27–29] and silica [30]. There are some drawbacks in the preparation of mixed matrix membranes such as poor distribution of the filler in the polymer matrix and weak contact of inorganic filler particles in the polymer matrix. Several methods have been proposed in the literature to overcome these drawbacks [31]. Functionalization of the inorganic filler in order to modify the particle surface chemistry and covalently bind the filler and polymer phases is one of the techniques employed to improve the surface interaction between the filler particles and the polymer matrix [29,32–36]. Qiu et al. [29] functionalized multi-walled carbon nanotubes (MWCNTs) by diisobutyryl peroxide and incorporated them into chitosan to prepare hydrophilic mixed matrix membranes for separation of water/ethanol mixtures by the pervaporation process. Beltran et al. [32] fabricated hydrophobic mixed matrix membranes by incorporating surface-functionalized fumed silica in polydimethylsiloxane (PDMS) for pervaporative recovery of 1-butanol from its aqueous solutions. They reported that functionalized fillers improved 1-butanol permeability through the membrane and resulted in higher separation efficiency. Li et al. [34] improved both gas permeability and selectivity of NaA zeolite/PES mixed matrix membranes by adding (3-aminopropyl)-diethoxymethyl silane (APDEMS) as silane agent to the zeolitic fillers. Also, Ghaffari Nik et al. [36] synthesized gas separation mixed matrix membranes by mixing several amine-grafted zeolitic fillers with a glassy polyimide. They observed that the presence of APDEMS coupling agents improved the performance of the mixed matrix membranes for separation of CO2/CH4 mixtures. The main goal of this work is to prepare multilayer mixed matrix membranes from PVA as an active layer and CNTs or modified CNTs as inorganic filler for the pervaporation process. For this purpose, three types of MWCNTs including pure CNTs, functionalized CNTs and TiO2–CNTs were synthesized by the thermal-chemical vapor deposition (T-CVD) technique. The three-layer mixed matrix membranes were fabricated by incorporation of different amounts of CNTs into the PVA as an active layer of the membrane using the phase inversion method. The effects of modification treatment on the CNT and filler content on the separation performance of the prepared membranes were investigated for the dehydration of ethanol/ water mixtures. Also, the prepared CNTs and membranes were characterized by XRD, TEM, FESEM, SEM, TGA, AFM, ATR-FTIR and contact angle analysis.

2. Experimental 2.1. Materials Analytical grade N,N-dimethyl-formamide (DMF), ethanol, sulfuric acid, nitric acid, citric acid, hydrochloric acid, cyclohexane, tetrahydrofuran (THF), ammonia, calcium carbonate (CaCO3), cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and titanium tetrachloride (TiCl4) were supplied from Merck Co. Ltd. (Darmstadt, Germany). The commercial polyethersulfone (PES) with molecular weight of 58,000 g/mol (Ultrason E 6020P), polyvinylpyrrolidone (PVP-K90) with molecular weight of 360,000 g/mol and polyvinyl alcohol (PVA) with molecular weight of 145,000 g/mol were purchased from BASF (Ludwigshafen, Germany), Sigma-Aldrich (MO, USA) and Merck Co. Ltd. (Darmstadt, Germany), respectively. Acetylene (C2H2) as carbon precursor and nitrogen (N2) and hydrogen (H2) with high purity (≥99%) as precursor gases were used to synthesize the CNTs. Laboratory de-ionized water was also utilized in the experiments. 2.2. Synthesis of MWCNTs The MWCNTs were synthesized by the T-CVD method using a fixedbed reactor consisting of a furnace with a quartz tube which was 60 cm long and had a diameter of 4.2 cm. A certain amount of Co(NO3)2·6H2O was dissolved into the de-ionized water to make a 5 wt.%. homogeneous solution. After 15 min stirring at room temperature, a specific amount of CaCO3 powder as catalyst support was added into the stirring solution to make a suspension by weight ratio of Ca/Co = 19. Then the suspension was heated at 80 °C to evaporate most of the water. The desired catalyst was obtained by a freeze drying process according to the procedure proposed by Mionic et al. [37]. In this procedure, the concentrate suspension was frozen by dropping into a liquid nitrogen bath. Once collected, it is subsequently placed in a freeze drying chamber. The resulting product was placed in an oven at 130 °C for 12 h, and finally ground into fine powder which was denoted as Co/CaCO3 catalyst. For synthesis of CNTs, the prepared catalyst was spread on ceramic boats and placed in the reactor where N2 and H2 gases with flow ratio of 10 to 1 were passed over the catalyst for 20 min at 750 °C. Subsequently, H2 flow was stopped and C2H2 and N2 with a flow ratio of 2 to 10 were entered into the reactor for 40 min at the same temperature. After formation of CNTs, the reactor was cooled down to room temperature by sweeping N2. Finally, a four step purification method was used to separate the synthesized CNTs from the catalyst [38]: i) Acid washing by soaking the particles into a mixture of HNO3/H2SO4 (v/v = 1:3) for 1 h at room temperature under stirring; ii) Filtration using PTFE membranes with average pore size of 0.5 μm; iii) Oxidization using N2/O2 mixtures at 500 °C for 30 min for selective removal of amorphous carbons; and iv) Washing by de-ionized water and drying at 100 °C for 1 h. 2.3. Modification of MWCNTs The functionalization of MWCNTs was performed by attaching carboxylic acid groups on the CNTs according to the method presented by Tsang et al. [39]. Typically, the CNTs were dispersed in a HNO3/ H2SO4 mixture (v/v = 1:3) and the mixture was refluxed for 8 h at 100 °C to attach OH and COOH groups. Then the functionalized CNTs were blended with monohydrate citric acid and heated up to 150 °C for 3 h under stirring and vacuum condition in order to replace the OH groups by COOH ones. Finally, the obtained functionalized CNTs were rinsed by THF and cyclohexane. To prepare TiO2 coated MWCNTs based on the procedure reported by Vatanpour et al. [40], a part of the functionalized MWCNTs and TiCl4 were dispersed into a 1 mol/L HCl solution at room temperature and sonicated for 20 min. Afterwards, a 5 mol/L ammonia solution was added dropwise to the mixture while stirring. The products were washed with de-ionized water and dried under vacuum at 60 °C for

S. Panahian et al. / Desalination 355 (2015) 45–55

47

24 h. Finally, the nanoparticles were heated at 500 °C in N2 medium for 100 min to obtain TiO2–MWCNTs.

The total flux (J) and water separation factor (α) were calculated using the following equations, respectively:

2.4. Membrane preparation

J¼

The three-layer mixed matrix membranes were prepared by casting an active top layer of PVA on the PES/polyester support. The PES layer was prepared using the phase inversion method by the immersion precipitation technique according to the procedure presented by Sadeghi et al. [41]. For this purpose, PES and PVP as a pore former which were previously dried in an oven at 80 °C for 5 h were dissolved in DMF as solvent. The polymeric solution underwent vigorous stirring until clear homogeneous solutions were obtained. Afterwards, the solution was vacuumed to remove any air bubbles. The bubble-free solution was cast onto polyester nonwoven fabric supported by a glass plate with a cast knife. Then, the glass plate was immediately immersed into a de-ionized water bath at room temperature. After coagulation and when the formed PES/polyester membranes were separated from the glass plate, the membranes were stored in a new de-ionized water bath for three days for complete removal of the residual solvent. The membranes were then dried at room temperature for at least one day. The PVA–CNT selective top layer was synthesized on the PES/polyester support by solution casting and the solvent evaporation technique. PVA was dissolved in de-ionized water as solvent at 90 °C then stirred for 2 h to form a homogeneous solution. Then a given amount of citric acid as a cross-linking agent was added to the PVA solution and further stirring. Simultaneously, a certain amount of the CNTs samples, i.e. nonfunctionalized MWCNTs, functionalized MWCNTs and TiO2–MWCNTs were dispersed in de-ionized water and sonicated for 4 h. The CNTs solutions were mixed with the PVA solution, stirred for 2 h at 60 °C and again sonicated for 2 h. Afterwards, the PVA–CNT solution was vacuumed to remove any bubbles. The resulting solutions were cast on the porous PES/polyester support membranes which were placed on a glass plate with a cast knife. The composite membrane was allowed to dry in an oven at 80 °C for 1 day, then further cross-linked by heating at 150 °C for 1 h and finally the obtained membrane was peeled off from the glass plate. The multilayer mixed matrix membranes were synthesized at different concentrations of the CNT samples according to Table 1. 2.5. Pervaporation experiments The pervaporation performance of the prepared mixed matrix membranes was investigated with 90 wt.% aqueous solutions of ethanol by a cross flow flat sheet membrane module at room temperature. The pervaporation apparatus has been previously described in detail [17].

Table 1 The composition of the active top layer of the multilayer mixed matrix membranes. Membrane PVA CA Pure Functionalized TiO2–MWCNTs Water (wt.%) (wt.%) (wt.%) (wt.%) MWCNTs MWCNTs (wt.%) (wt.%) NM S1M1 S1M2 S1M3 S1M4 S2M1 S2M2 S2M3 S2M4 S3M1 S3M2 S3M3 S3M4

10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

0.0 0.5 1.0 2.0 4.0 – – – – – – – –

– – – – – 0.5 1.0 2.0 4.0 – – – –

– – – – – – – – – 0.5 1.0 2.0 4.0

80.0 79.5 79.0 78.0 76.0 79.5 79.0 78.0 76.0 79.5 79.0 78.0 76.0

W S:t yH

2O

ð1Þ 

α ¼ xH O  2

yEtOH

ð2Þ

xEtOH

where W, S and t are the weight of collected permeate, area of membrane and time duration of the experiment, respectively and y and x are the weight fraction of components in the permeate and feed, respectively. The pervaporation experiments were conducted at a feed temperature of 30 °C and permeate-side pressure of 1 mm Hg. 2.6. Characterization tests The nanostructure of the CNT samples and their morphologies were characterized by a field-emission scanning electron microscope (FESEM) (Hitachi S4160, NJ, USA) operating at 30 kV and a transmission electron micrograph (TEM) (Philips CM200, FEI Company, Oregon, USA) operating at 200 kV. Before the FESEM analysis, the CNT samples were prepared by mixing of the CNTs into a given amount of de-ionized water to obtain a 0.5 wt.% mixture and sonicated for 3 h. Then one drop of the CNT/water mixture was poured on a glass slide and dried at 60 °C. The prepared samples were coated under vacuum with a thin layer of gold by the sputtering system. For the TEM analysis, the samples were prepared by placing a drop of the colloidal CNT solution onto a small carbon film coated copper grid, followed by water evaporation under vacuum. Thermal gravimetric analysis (TGA) (STA 409 PC, Netzsch-Geratebau GmbH, Germany) was used to determine the purity of the synthesized CNTs. For the TGA analysis, the samples were heated from 25 to 1000 °C at a ramp rate of 20 °C/min with an air flow rate of 100 mL/min. The powder's X-ray diffraction (XRD) patterns were also recorded at 25 °C on a Philips instrument (X'pert diffractometer using CuKα radiation) with a scanning speed of 0.03° (2θ) min−1 to confirm the structure of MWCNTs and TiO2–MWCNTs. The ratio of Ti/C in the TiO2 coated MWCNTs was determined by energy dispersive X-ray (EDX) elemental analysis using the SEM instrument. The attenuated total reflection-Fourier transforms infrared spectroscopy (ATR-FTIR) analysis was used for detecting the presence of functional groups on the surface of the prepared membranes. The ATRFTIR instrument used consisted of a Nicolet Nexus 670 spectrometer (Nicolet Instrument Co., Madison, WI, USA) with 4 cm− 1 resolution over a wave number range of 4000–600 cm−1. The surface and cross section morphologies of the fabricated membranes were characterized by SEM analysis using a Hitachi SEM model S-4160 (Hitachi, NJ, USA). The membrane samples were coated under vacuum with a ~ 10– 20 nm thin layer of gold by the sputtering system. Furthermore, the water contact angle (CA) of the various membrane samples was measured using an optical contact angle measurement system (OCA-20, DataPhysics GmbH, Filderstadt, Germany) at room temperature. Measurements were made immediately after 5 μL of de-ionized water was dropped on the surface by a micro-syringe and the needle tip was removed from the surface. At least six readings were made on different parts of the membranes and the results were averaged. Surface roughness of the prepared membranes was also measured using atomic force microscopy (AFM). The AFM analysis was performed on a Hysitron TriboScope® nanomechanical test instrument with 2D transducer (Hysitron Inc., Eden Prairie, MN, USA). Samples with a size of 0.2 cm × 0.2 cm were fixed on a metal substrate and 10 μm × 10 μm areas were scanned by semi-contact mode in the air. Three different locations were tested and the average values of roughness were reported. The roughness was expressed as RMS (root mean square) and RA (average

S. Panahian et al. / Desalination 355 (2015) 45–55

roughness) values. Finally, the swelling degree of the membrane which is defined as the sorption uptake per unit mass of the dry membrane was measured using a well-known gravimetric procedure [42]. The swelling degree (SD) can be calculated as follows: WW −WD  100 WD

ð3Þ

where WW and WD are the wet and dry weights of the membrane, respectively.

(a)

(b)

3.1. Characterization of synthesized nanoparticles The FESEM and TEM images of the non-functionalized (unmodified) CNTs synthesized by the T-CVD method after purifications are shown in Fig. 1. The FESEM image (Fig. 1a) illustrates that the sinuous and entangled CNTs are formed with a length up to several micrometers. The TEM images (Fig. 1b and c) demonstrate that the synthesized CNTs are multiwall and without amorphous carbons and deactivated catalysts or other impurities. The inner and outer diameters of the MWCNTs grown on the Co/CaCO 3 catalyst are about 20–30 and 5–10 nm, respectively. The TGA analysis was utilized to determine purity and thermal stability of the prepared CNTs. The results of TGA analysis as weight loss and differential weight loss versus temperature for the prepared CNT samples before and after purification are presented in Fig. 2a and b, respectively. As shown in Fig. 2a, three peaks can be observed in the curve of differential weight loss, the peak at 50–150 °C corresponds to evaporation of the residual moisture [43] and peaks at 403 and 624 °C are due to oxidation of the amorphous carbon and MWCNTs, respectively. The amorphous carbons oxidize at temperatures lower than the crystallized carbons in the nanotubes [44]. The differential weight loss profile of the purified CNTs has a peak at 619 °C as shown in Fig. 2b. It is implied that the impurities such as metal oxides and amorphous carbons were removed from the synthesized CNTs by the proposed purification method and the purity of the MWCNTs was about 95%. The XRD pattern of the synthesized CNTs and TiO2 coated CNTs is presented in Fig. 3. The XRD pattern of the CNTs (Fig. 3a) has a peak at 2θ of 26.3° (002) which is assigned to the non-functionalized MWCNTs. Also, the observed pattern for the TiO2–CNTs indicated that the TiO2 nanocrystals were formed on the MWCNTs. The peaks at 2θ of 28.3°

(a)

100

0.50 0.45

Weight (%)

80

0.40 0.35

60

0.30 0.25

40

0.20 0.15

20

0.10 0.05

0

0

200

400

600

800

0.00 1000

Temperature (°C)

(c) (b) 100

0.45 0.40

Weight (%)

80

0.35 0.30

60

0.25 0.20

40

0.15 0.10

20

Differential weight (%/°C)

SDð%Þ ¼

3. Results and discussion

Differential weight (%/°C)

48

0.05 0

0

200

400

600

800

0.00 1000

Temperature (°C) Fig. 1. The FESEM (a) and TEM (b & c) images of the pure CNTs.

Fig. 2. The TGA results of the synthesized MWCNTs before (a) and after (b) purification.

S. Panahian et al. / Desalination 355 (2015) 45–55

49

Intensity

(a)

10

20

30

10

20

30

40

50

60

70

40

50

60

70

2Ө (degree)

Intensity

(b)

2Ө (degree) (c)

Fig. 3. The XRD pattern of the pure MWCNTs (a) and TiO2–MWCNTs (b) as well as the EDX analysis of the TiO2 coated MWCNTs (c).

50

S. Panahian et al. / Desalination 355 (2015) 45–55

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 4. The SEM images from the surface of the NM (a), S1M1 (b), S1M2 (c), S1M3 (d), S1M4 (e), S2M4 (f) and S3M4 (g).

S. Panahian et al. / Desalination 355 (2015) 45–55

51

(a) Transmittance (a.u.)

d c b a

4000

3500

3000

2500

2000

1500

1000

500

Wave number (cm-1) Fig. 6. The ATR-FTIR spectra for the fabricated membranes: NM (a), S1M4 (b), S2M4 (c) and S3M4 (d).

(b)

the TiO2 coated MWCNTs, the EDX analysis was performed and the result is presented in Fig. 3c. Based on the EDX results, the ratio of Ti/C was about 0.313. 3.2. Characterization of multilayer mixed matrix membranes

Fig. 5. The SEM images from cross section of the NM (a), S1M4 (b) and S2M4 (c).

Transmittance (a.u.)

(c)

The SEM images from the surface and cross section of the prepared multilayer mixed matrix membranes with different CNT samples as filler are presented in Figs. 4 and 5, respectively. Some bright spots can be observed on the surface of the membrane samples containing CNTs. These spots represent the CNTs on the membrane surface. As shown in Fig. 4a to e, the amount of CNTs on the membrane surface increases with an enhancement in the CNT content of the membrane. The SEM image on the membrane containing 4 wt.% CNTs (Fig. 4e) indicates that the MWCNTs are agglomerated. Also, a comparison between the membrane samples with 4 wt.% of various MWCNTs reveals that the modification of CNTs prevents their agglomeration in the membrane matrix. Moreover, the SEM images from the cross section of the NM, S1M4 and S2M4 membrane samples (Fig. 5) indicate that a dense PVA layer was formed on the porous PES layer with finger-like macrovoids. Fig. 6 shows the ATR-FTIR spectra of the neat membrane and mixed matrix membranes containing 4 wt.% of different CNT samples. In four ATR-FTIR spectra, the peaks at wave numbers of 3500–3000 cm−1 and 1098 cm− 1 are assigned to OH groups of the PVA and C–O groups of the PES in the membrane intermediate layer, respectively. The spectra of the neat membrane (Curve a) and the mixed matrix membrane containing the non-functionalized MWCNT sample (Curve b) are similar.

S2M

(110), 37.2° (101), 42.8° (111) and 55.9° (211) are assigned to the rutile crystalline structure, so the prepared TiO2 nanoparticles are pure rutile phase [45]. The crystal size of TiO2 particles that was estimated using Scherrer's equation [45] from the width of the peak with 2θ of 28° is found to be 9 nm. Furthermore, to determine the amount of TiO2 in

S3M 4000

3500

3000

2500

2000

1500

1000

Wave number (cm-1) Fig. 7. The ATR-FTIR spectra for the S2M4 and S3M4 membrane samples.

500

52

S. Panahian et al. / Desalination 355 (2015) 45–55

(a)

(c)

(b)

(d)

Fig. 8. The AFM images of the fabricated membranes: NM (a), S1M4 (b), S2M4 (c) and S3M4 (d).

This implies that adding non-functionalized MWCNTs to the PVA layer of the membrane had no effect on the chemical functional group of the membrane. A peak at a wave number of 1725 cm−1 corresponding to C_O of carboxyl acid groups is observed in the spectrum of the S2M4 membrane sample (Curve c). A comparison between the spectra of the NM membrane sample and the membrane containing functionalized CNTs shows that the carboxyl groups were created on the MWCNTs after functionalization. The peaks at a wave number of 1300–1000 cm−1 which interfered with the peaks of ether and hydroxyl indicate that the esterification reaction occurred between the PVA and functionalized MWCNTs. The reduction in the peak intensity of the OH groups also confirmed the esterification reaction. Furthermore, a comparison between the ATR-FTIR spectra of the S2M4 (Curve c) and S3M4 (Curve d) membrane samples is presented in Fig. 7. It can be seen that the intensity of the carboxyl peak is similar for both membrane samples. This implies that the carboxyl groups of the functionalized CNTs were not replaced by the TiO2 crystals, therefore the modification of MWCNTs by TiO2 led to a blend of CNTs and TiO2 nanoparticles. The change in the peak of hydroxyl groups can be related to the bond between Ti4 + from TiO2 molecules and oxygen from OH groups of the PVA [46]. Also, the differences between the spectra at a wave number of 3000–2500 cm− 1 may be attributed to the created hydrogen bonds which lead to further crosslinking of the membrane. The AFM analysis was also employed to evaluate changes in surface morphology and roughness of the prepared membranes. The AFM surface topography of the neat and CNT loaded mixed matrix membranes is shown in Fig. 8. Also, the values of average roughness (Ra) and root mean square of the Z data (RMS) of different membranes are presented

in Table 2. It is observed that the surface roughness of the CNT loaded membranes is higher than the neat membrane and the roughness slightly increases with an enhancement in the CNT content of the membranes. This means that as the filler content of the PVA layer varied from 0 to 4 wt.%, the amount of MWCNTs on the membrane surface enhances and results in a rougher membrane surface. For the S1M membrane samples, the AFM data agree with the SEM analysis which indicates that non-functionalized MWCNTs at high concentration (4 wt.%) tend to aggregate on the membrane surface and increase the roughness because of high electrostatic interactions among the MWCNTs [47]. Moreover, a comparison between the surface roughness of the S1M membranes and the S2M and S3M membranes reveals that the roughness of Table 2 The roughness parameters of the prepared membranes. Membrane sample

Ra (nm)

RMS (nm)

NM S1M1 S1M2 S1M3 S1M4 S2M1 S2M2 S2M3 S2M4 S3M1 S3M2 S3M3 S3M4

0.93 2.30 2.58 4.76 7.97 1.79 1.92 2.49 3.14 1.38 1.94 1.90 2.11

1.17 3.78 2.63 4.82 11.66 2.66 2.81 3.78 4.69 1.82 2.55 2.75 3.12

S. Panahian et al. / Desalination 355 (2015) 45–55

decreases. Therefore, the results of the swelling test are in good agreement with the results of ATR-FTIR and SEM analysis. Similar results were reported in literature [24,28]. For example, Shirazi et al. [28] observed that the presence of CNTs in the PVA membranes leads to higher crosslinking density and reduces the degree of swelling. Also, Sairam et al. [24] reported that incorporation of TiO2 nanoparticles into the PVA membrane results in lower swelling of the membrane.

Table 3 The contact angles of the prepared membranes. Membrane sample

Contact angle (°)

NM S1M1 S1M2 S1M3 S1M4 S2M1 S2M2 S2M3 S2M4 S3M1 S3M2 S3M3 S3M4

51 52 56 58 64 49 48 47 43 48 46 45 41

3.3. Pervaporation performance The pervaporation experiments with 90 wt.% aqueous ethanol feed solutions were conducted in order to determine the separation performance of the fabricated multilayer mixed matrix membranes. The total permeation flux and separation factor versus the CNT content of membranes are depicted in Fig. 10. It is observed that the total flux of the S1M membranes is higher than that of the S2M and S3M membrane samples. These results follow similar trends with the swelling test results. The lower permeation flux of the mixed matrix membranes containing functionalized MWCNTs and TiO2–MWCNT samples can be attributed to the crosslinking density of these membranes. As discussed previously, the membranes containing modified CNTs are more crosslinked due to the esterification reaction between hydroxyl groups of the PVA molecules and carboxyl groups of the modified MWCNTs. The crosslinking of the PVA layer decreases the chain mobility of the membrane top layer, thus the diffusion of permeating molecules through the membrane is hindered and the permeation flux decreases. Furthermore, for all membrane samples, the permeation flux slightly decreases when the CNT content of the membranes varies from 0 to 2 wt.% and then with enhancement of filler content from 2

70

71.47

NM 35.82

30

22.92

2

3

4

S3M

200 100 0

1

2

3

4

0

1.2

Water flux ratio ...................

1.0

Ethanol flux ratio

S1M 0.6

0.0

0 1

S2M

S2M S3M

0.2

18.87 12.35

14.82

10 0

300

0.4

28.11 22.63

31.97

20

200

S1M

0.8

S2M

50 40

400

S3M

67.53

60

500

300

CNTs content (%wt.)

Flux ratio

Swelling degree (%)

70.25

76.89

600

100

S1M 68.17

Separation factor

700

400

0

Total flux

800

500

(b) 90

900

600

Separation factor

(a)

Total flux (g/m2.h)

the membranes containing modified MWCNTs is lower than that of the membranes containing non-functionalized MWCNTs due to dispersion of the functionalized CNTs in the polymeric matrix. These results are in good agreement with the results of the SEM and ATR-FTIR analysis. The contact angle tests for the neat and MWCNT loaded membranes were also measured to determine the changes in membrane hydrophilicity and the results are given in Table 3. According to Table 3, adding non-functionalized MWCNTs to the PVA layer of the membrane results in a higher contact angle and the membrane hydrophilicity decreases, while the S2M and S3M membranes have lower contact angles and higher hydrophilicity than the neat membrane due to the hydroxyl and carboxyl groups of the modified CNTs as filler. Finally, the swelling degree of the fabricated multilayer mixed matrix membranes was measured by the gravimetric technique. The swelling degree of various membrane samples is indicated in Fig. 9. As shown in this figure, the swelling degree of the S1M membrane samples is higher than the NM membrane. On the other hand, addition of nonfunctionalized MWCNTs into the PVA layer of the membrane increases the membrane swelling and an enhancement in the CNT content of the membrane leads to a higher swelling degree. This behavior can be related to the agglomeration of CNTs in the membrane matrix and increasing the membrane free volumes. Due to high electrostatic interactions among the MWCNTs, the CNTs agglomerate and produce larger strains and consequently the free volume between fillers and polymeric chains increases and sorption uptake of the membrane enhances and results in a higher swelling degree. Also, the swelling degree of the S2M and S3M membrane samples is lower than that of S1M membranes, as indicated in Fig. 9. This implies that the mixed matrix membranes containing the modified MWCNTs are more crosslinked. Increasing crosslinking of the PVA top layer leads to reduction of the free space for the water molecules and consequently the degree of swelling

80

53

5

0

1

2

3

4

CNTs content (%wt.)

CNTs content (%wt.) Fig. 9. The swelling degrees of the prepared membranes.

Fig. 10. The total flux and water separation factor (a) as well as the partial water and ethanol flux ratio (b) of various membranes.

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S. Panahian et al. / Desalination 355 (2015) 45–55

Table 4 The pervaporation performance of different PVA mixed matrix membranes containing CNTs. Membrane

Alcohol

Alcohol concentration in feed (wt.%)

Feed temperature (°C)

Total flux (g/m2 h)

Selectivity

Reference

PVA/CNT PVA/oxidized CNT PVA/CNT PVAm–PVA/CNT Chitosan/CNT-SA PVA/CNT–PAH PVA/CNT (S1M3) PVA/modified CNT (S2M4) PVA/TiO2–CNT (S3M4)

Ethanol Isopropanol Ethanol Ethylene glycol Isopropanol Isopropanol Ethanol Ethanol Ethanol

90 90 90 90 90 90 90 90 90

40 30 40 38 30 30 30 30 30

82 79 50 90 218 207 471 395 388

460 1794 780 400 6419 984 78 662 805

[27] [28] [48] [49] [50] [51] This study This study This study

to 4 wt.%, the total flux increases. The total permeation flux of the S1M, S2M and S3M membranes is also lower than that of the NM membrane sample. Generally, adding CNT samples as filler into the membrane selective layer reduces the free spaces in the polymeric matrix because the CNT filler acts as reinforcing bridge elements, increases rigidity of the polymeric chains and thus the permeation flux decreases. For better explanation of the permeation results, the ratio of the partial fluxes of the mixed matrix membranes to the partial fluxes of the neat membrane is plotted in Fig. 10b. For the S1M membranes, in can be seen from Fig. 10a that the water separation factor slightly enhances from 56 to 78 as the CNT content changes from 0 to 2 wt.% and by further increase in the membrane CNT content to 4 wt.%, the separation factor reduces to 63. The reduction in the water separation factor at a filler content of 4 wt.% may be related to the agglomeration of CNTs and formation of larger free volumes around the MWCNTs in the membrane matrix which causes a convective flow around the agglomerated CNTs. Therefore, both water and ethanol molecules can easily transport through the PVA layer of the membrane and consequently the permeation flux increases and the water separation factor decreases. For the S2M and S3M membrane samples, adding more amounts of the modified MWCNTs to the selective layer of the membrane leads to higher water separation factor as far as the maximum separation factor is observed at the CNT content of 4 wt.%, the values of 662 and 805 for the S2M and S3M membranes, respectively. The water separation factor can be related to the ratio of water partial flux to ethanol partial flux. As shown in Fig. 10b, both water and ethanol fluxes decrease with increasing the CNT content up to 2 wt.%, while the decreasing rate in the ethanol flux is more significant than the water flux. Therefore, the ratio of the two fluxes which is proportional to the water separation factor increases when the CNT content goes to a higher level. For the filler content of 2 to 4 wt.%, the water permeation flux increases, while the ethanol flux is almost constant. Thus the ratio of water to ethanol partial fluxes and the separation factor enhance. Similar results have been reported in the literature [17,27,28]. For example, Shirazi et al. [28] prepared PVA–CNT nanocomposite membranes for dehydration of isopropanol aqueous solutions and observed that incorporating CNTs in the PVA matrix significantly increases water separation factor due to rigidification of the polymer chains. Finally, for comparison purposes, the pervaporation performance of different mixed matrix membranes containing CNTs as filler reported by other research groups for alcohol dehydration is listed in Table 4. It can be seen that the total permeation flux of the PVA/CNT membranes prepared in the present study is higher than those of most mixed matrix membranes containing CNTs reported in the literature for the ethanol dehydration and the S2M4 and S3M4 membranes have relatively high water separation factor. In the case of the ethanol separation from 90 wt.% ethanol solutions at a feed temperature of 30 °C under permeate side pressure of 1 mm Hg, the S2M4 and S3M4 (PVA/modified MWCNT membranes with the CNT content of 4 wt.%) have a total flux of 395 and 388 g/m2 h and a water separation factor of 662 and 805, respectively.

4. Conclusion Multilayer mixed matrix membranes were prepared by incorporating multi-walled carbon nanotubes in the poly(vinyl alcohol) layer as a selective top layer of the membrane. The CNTs were synthesized by the T-CVD method and modified by adding carboxyl functional groups and synthesizing TiO2 nanocrystals. The characterization tests revealed that the MWCNTs with a purity of 95% which were formed on the Co/CaCO3 catalyst had inner and outer diameters of 20–30 and 5–10 nm, respectively. It was observed from the ATR-FTIR results that the carboxyl functional groups were attached to the MWCNTs. The modified MWCNTs were well dispersed into the membrane top layer which led to lower swelling degree and higher crosslinking density while the pure MWCNTs agglomerated on the membrane surface. The surface hydrophilicity enhanced and the swelling degree decreased by increasing the CNT concentration in the membrane selective top layer for the S2M and S3M membranes, while a reverse trend was observed for the S1M membrane samples. The pervaporation experiments showed that the total flux of membranes containing CNTs was lower than that of the neat membrane sample and the permeation flux of the S1M, S2M and S3M membranes had a minimum CNT content of 2 wt.%. For the S2M and S3M membranes, an enhancement in the membrane filler content resulted in higher separation factors while the water separation factor slightly varied by increasing the CNT content from 0 to 4 wt.%. The water separation factors of 662 and 805 were observed for the pervaporative dehydration of 90 wt.% ethanol aqueous solutions with the S2M and S3M membranes, respectively. Finally, the swelling degree and water separation factor of various fabricated membrane samples were in the order of S3M N S2M N NM N S1M and S3M N S2M N S1M N NM, respectively.

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