- Email: [email protected]
CH4 separation
international journal of hydrogen energy 34 (2009) 8707–8715
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Preparation and characterization of multi-walled carbon nanotube/PBNPI nanocomposite membrane for H2/CH4 separation Tzu-Hsiang Weng a, Hui-Hsin Tseng b,c, Ming-Yen Wey a,* a
Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC School of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan, ROC c Department of Occupational Medicine, Chung Shan Medical University Hospital, Taichung 402, Taiwan, ROC b
article info
abstract
Article history:
By combining organic polymers normally used to make membrane filters with inorganic
Received 24 April 2009
substances, multi-walled carbon nanotube (MWCNTs), an extraordinary ability to separate
Received in revised form
H2 from CH4 was developed in this study. A series of MWCNTs/PBNPI nanocomposite
6 August 2009
membrane with a nominal MWCNTs content between 1 and 15 wt% were prepared by
Accepted 9 August 2009
solution casting method, in which the very fine MWCNTs were embedded into glassy
Available online 27 August 2009
polymer membrane. Detailed characterizations, such as morphology, thermal stability and crystalline structure have been conducted to understand the structures, composition and
Keywords:
properties of nanocomposite membranes. The results found that this new class of
Multi-walled carbon nanotube
membrane had increased permeability and enhanced selectivity, and a useful ability to
Gas separation
filter gases and organic vapours at the molecular level.
Nanocomposite membrane
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
H2 CH4
1.
Introduction
In recent years, there has been a dramatic increase in the number of research studies carried out on the separation of gases by using polymer membranes. In membrane-based gas separation, the desired compounds are separated from mixtures by passing them through tiny holes. Membranes are attractive as filters because they are a low-cost, energyefficient and green technology [1]. However, membrane use for separating gases has so far been limited by the lack of the right sort of membranes to yield pure products with the desired permeability, low operating cost, while remaining stable. Therefore, many researches on membrane-based gas separation have focused on the use of different type of
polymer membranes with the aim of improving gas permeability and selectivity [2–5]. For example, Kulshrestha et al. [1], Yi et al. [6] and Acharya et al. [7] have carried out on the use of organic-organic blend polymer membranes; in addition, inorganic-organic nanocomposite membrane also drew much attention. Nano-sized particle or nano-pore materials, such as MgO [8], zeolite [9,10] and activated carbon [10,11] are also used as nanofillers to prepared nanocomposite membranes. However, the materials are difficult to synthesize and the permselectivity of them are still far from the Robison line. Thus, it needs to develop some other methods for modifying polymer membrane in order to improve the permselectivity of H2 from CH4 mixture.
* Corresponding author. Tel.: þ886 4 22852455; fax: þ886 4 22862587. E-mail address: [email protected] (M.-Y. Wey). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.027
8708
international journal of hydrogen energy 34 (2009) 8707–8715
Carbon nanotubes (CNTs) have attracted considerable attention since their discovery [12]. With the improvement of the fabrication technology, the price of CNTs has reduced significantly and it is possible for them to be one of the most remarkable embedding materials. Taking advantage of the unique electronic, adsorption, mechanical and thermal properties of the CNTs, nanocomposite membrane containing CNTs and polymer are believed to provide many applications and exhibit cooperative and synergetic effects between the polymer and carbon phases. Some recent works have emphasized on the prediction [13] or experimentation [14–19] of the permeability and selectivity for gas separations which used CNTs as nanofillers in nanocomposite polymer membrane and a few examples are worth noticing. Kim et al. [17] reported novel nanocomposite membranes containing single-walled carbon nanotube (SWCNTs) in a polysulfone matrix can improve the selectivity and permeability of small molecules; they also observed a slight increase in permeability when the SWCNT loading weight in the polymer matrix was increased from 5 to 10 wt%. Cong et al. [16] fabricated nanocomposite membranes containing SWCNTs and MWCNTs in poly(2,6-diphenyl-1,4-phenylene oxide). They found that the CO2 permeability and selectivity decreased at high concentration of CNTs and that better gas permeability could be achieved with MWCNTs than with SWCNTs. Chio et al. [14] prepared MWCNTs/polysulfone (PSf) blend membranes by the phase-inversion process; their research results indicated that after acid treatment, the MWCNTs are well dispersed in organic solvents for the preparation of homogeneous MWCNTs/PSf blend solutions. Their results also showed that the flux of a PSf membrane with 4 wt% MWCNTs was higher than that of the PSf membrane without MWCNTs. Furthermore, MWCNTs/polyetherimide nanocomposite membranes were prepared by solution casting and fine dispersion of MWCNTs in the polyetherimide (PEI) matrix [19]. In the foregoing research to understand added carbon nanotube into polymer matrix cloud improved gas permeability. However, the reported study with a low CNTs content (1–5 wt%) failed to confirm the predicted synergetic effect in improving the permeability. Furthermore, only a few papers have reported on the different MWCNTs loading in the polymer matrix and on the relationship between gas permeability and the structural characteristics of nanocomposite membranes. The aim of this present work study is to develop and characterize a novel nanocomposite membrane obtained by dispersing MWCNTs in a polymer matrix. In this study, inexpensive commercially available MWCNTs were used to fabricate highly permeable and selective nanocomposite membranes. MWCNTs (diameter: 10–20 nm) were acid treatment to facilitate their dispersion in the membrane and were added to the matrix in different concentrations. In our previous study [20] we have reported the good H2/CH4 selectivity of a pure poly(bisphenol A-co-4-nitrophthalic anhydride-co-1,3-phenylene diamine) (PBNPI) membrane. In this study, we attempted to improve the PBNPI membrane with high H2 permeability and selectivity by embedding the MWCNT as nanofiller. The morphology, crystalline structure, functional group, and thermal properties of the MWCNTs/ PBNPI nanocomposite membrane were investigated by filed
emission-scanning electron microscopy (FESEM), X-ray diffraction (XRD), fourier transform infrared spertroscopy, attenuated total reflectance (FTIR-ATR), and thermogravimetric analysis (TGA), respectively. The changes in the permeability and selectivity of the membrane caused by different weight ratio of MWCNTs, and the predominately mechanism for gas transport were also investigated by diffusivity and solubility coefficients.
2. Experimental and characterization methods 2.1.
Materials and membrane preparation
PBNPI powder was purchased from Sigma–Aldrich. N-methyl2-pyrrolidone (NMP) purchased from Mallinckrodt Chemicals Co. USA was used as the co-solvent. High-purity MWCNTs (average diameter: w10–20 nm; length: 5–15 mm; surface area: 116 m2/g) were obtained from Desunnano Co., Ltd. The raw CNT materials were cut into small pieces and purified using a 3:1 mixture of concentrated H2SO4 (98 vol%) and HNO3 (70 vol%) to obtain open-ended CNTs. The mixture was stirred thoroughly for 4 h at 80 C and then washed with deionized water. The details of the purification method are described elsewhere [21]. Acid-treated and purified MWCNTs are known to have carboxyl groups on their surfaces, and can be easily dispersed in polar organic solvents [22,23]. H2 (99.9999%), CO2 (99.9999%) and CH4 (99.9999%) were obtained from Toyo Gas Company, Taiwan.
2.2. Fabrication of MWCNTs/PBNPI nanocomposite membrane Pure PBNPI and MWCNTs/PBNPI nanocomposite membranes were prepared by the solution casting method. The PBNPI membrane was prepared as follows 15 g of PBNPI was dissolved in 85 ml of NMP by stirring for 24 h (polymer concentration: 15 wt%). The solution was cast onto a glass plate at room temperature for 24 h. Slow evaporation of the solvent resulted in a smooth film with a uniform thickness. Once dry, the membrane was heated to 60 C and maintained at this temperature for 24 h. This was to ensure evaporation of any residual agent (solvent) in the polymer membrane. Subsequently, the resulting membrane was peeled off in deionized water and stored in a desiccator until before use. The preparation procedure for the nanocomposite membrane was identical to that of the pure polymer membrane preparation except for the additional step in which MWCNTs were incorporated in the membrane. Appropriate predetermined amounts of the purified MWCNTs were added slowly to 15 wt% PBNPI solution to obtain MWCNTs/PBNPI polymer membranes containing 1, 2.5, 5, 10 and 15 wt% MWCNTs. The mixture was stirred vigorously at 5000 rpm for 1 h; then, the rotation speed was increased to 10,000 rpm and stirred for 15 min so that the CNTs were uniformly dispersed in the polymer matrix. Subsequent to stirring, the MWCNTs/ PBNPI solution was cast onto a clean and dry glass plate using a typical hand-casting knife at ambient temperature. The evaporation and heat treatments carried out for the
international journal of hydrogen energy 34 (2009) 8707–8715
nanocomposite membranes were identical to those carried out for the pure polymer membrane. The final thickness of the membranes was approximately 50–90 mm. In this study, all the pure and nanocomposite membrane were prepared for three times by the abovementioned procedure for ensuring the reproducible of the membrane.
2.3. Characterization of pure and nanocomposite membrane FESEM (JSM 5600) was used to observer morphology, surface structure, cross-sectional images and dispersion of MWCNTs in the membranes. The FESEM images were support to explain gas transport behavior through nanocomposite membrane. The XRD measurements of nanocomposite membranes were recorded on a SIEMENS D5000 thin-film X-ray diffrac˚ ) were generated tometer system, and the X-rays (l ¼ 1.5418 A by a Cu Ka source. The XRD measurements were carried out within the range of 5 2q 35 with a step increment ratio of 0.4 /sec. This was done to identify the changes in crystal structure and intermolecular distances between the polymer matrix’s intersegmental chains [24,25]. The d-spacing are calculated from the scattering angle (2q) according to Bragg’s equation: d ¼ nl/2sinq. FTIR spectra of the MWCNTs/PBNPI nanocomposite membrane was measured on a JASCO-4100 FTIR-ATR spectrometer over the wavenumber range of 4000–400 cm1 to identify the changes in the membrane’s polymer chemical function group after adding CNTs. The TGA and differential scanning calorimeter (DSC) of the polymer membrane were measured on a Perkin Elmer STA 6000 control system. The pure polymer and MWCNTs/PBNPI nanocomposite membranes were heated up to 800 C at a scanning rate of 10 C/min under nitrogen atmosphere in order to diminish oxidation.
2.4.
Gas permeation measurement
For non-porous membranes, the permeation process based on the solution diffusion model occurs in three stages. These are sorption, diffusion, and desorption under a pressure gradient [21]. The gas permeation performance of both the PBNPI membrane and nanocomposite membranes were investigated by using a standard vacuum time lag method at room temperature (26 C) and a feed pressure of 2 kg/cm2 [26]. The self-built vacuum time-lag apparatus is shown elsewhere [20,27]. A circular stainless permeability cell (47-mm disc filters, Millipore, USA), with an effective area 4.7 cm2, was used for measure transport properties of the membrane. The gases used in this research were H2, CO2 and CH4 with kinetic ˚ , 3.3 A ˚ and 3.8 A ˚ , respectively. Prior to all diameters of 2.8 A experiments, the leak detection tests were performed in all valves. Before feeding and permeating, the upstream and downstream sides of the membrane cell were evacuated (< 105 Torr). The pressure at upstream and downstream side was measured by using pressure sensors and data was recorded with the help of Visidaq Builder computer software, which ensures automated measurement with an automatically adapting data-sampling rate (1 data sec1) to yield at downstream pressure data points and to describe time-lag
8709
and steady-state gas transport completely. The permeability coefficient is permeation rate (P) value, determined by the following equation: P¼
dp VT0 L ; dt ADP TP0
(1)
where P is the permeability coefficient expressed in Barrer (1 Barrer ¼ 1 1010 cm3 cm (STP) cm2 s1 cm Hg1), (dp/dt) is the steady rate at which pressure increases on the downstream side, V (cm3) is the calibrated downstream volume, DP (cmHg) is the transmembrane pressure difference between the two sides, P1 and P2 are the upstream and downstream pressures, respectively of the membrane cell, A (cm2) is the effective area of the membrane, L (cm) is the effective thickness of the separating layer, and T0 and P0 are the standard temperature and pressure, respectively. The ideal separation factor of pure gas A over B (aAB) is defined as the ratio of permeation rate of A to B, which can be expressed by aAB ¼
PA PB
(2)
Besides permeability and selectivity, the gas membrane separation study evaluated the diffusivity and solubility of gas through the membrane. Diffusivity (D) was determined from: D¼
L2 6q
(3)
where q is the time lag when a steady dp/dt rate is obtained from the downstream side in gas permeation tests [26]. The solubility coefficient (S ) of each gas was taken as the ratio of the permeability to diffusivity coefficient determined from: S¼
P D
(4)
All pure and nanocomposite polymer membrane which prepared for three times was performed with each gas one time and the average results and the standard deviations were recorded.
3.
Results and discussion
3.1. Characterizations of pure PBNPI and MWCNTs/ PBNPI nanocomposite membranes The FESEM image shown in Fig. 1 illustrates the cross-section images of the MWCNTs/PBNPI nanocomposite membranes prepared with different weight ratio of MWCNTs. As shown in Fig. 1(a), the MWCNTs/PBNPI nanocomposite membrane containing 1 wt% MWCNTs shows only a few MWCNTs are distributed throughout the PBNPI matrix. With the MWCNTs’ concentration increased, from the FESEM observations shown in Fig. 1(b–e), it is apparent that the degree of distribution of MWCNTs in the MWCNTs/PBNPI membranes initially increased with the MWCNTs concentrations. Further, when 10–15% of MWCNTs is added to the PBNPI matrix, no aggregation occurred in the MWCNTs/PBNPI membrane. It can be seen that at higher MWCNTs’ concentrations, the MWCNTs
8710
international journal of hydrogen energy 34 (2009) 8707–8715
Fig. 1 – FESEM image showing an overall morphology of a cross-section for 15 wt% MWCNTs/PBNPI nanocomposite with different MWCNT weight ratios: (a) 1 wt%, (b) 2.5 wt%, (c) 5 wt%, (d) 10 wt% and (e) 15 wt%.
are more randomly distributed and loosely entangled in the PBNPI matrix. Table 1 shows the glass transition temperature (Tg) and final decomposition temperature values of the pure PBNPI and MWCNTs/PBNPI nanocomposite membrane, which obtained from the The DSC heat flow curves and TGA curves. The Tg of the MWCNTs/PBNPI membranes became slightly lower than that of the pure PBNPI membrane (122.9 C) upon the addition of 1 wt% and 2.5 wt% MWCNTs to the polymer matrix. However, Tg increased by approximately 5 C when 5 wt% MWCNTs were added to the polymer matrix; after the 10 wt% and 15 wt% of MWCNTs were added to the polymer matrix, the Tg of the MWCNTs/PBNPI membrane decreased significantly. These results show that addition of 5 wt% MWCNTs to the polymer matrix resulted in an increase in the crystallinity
Table 1 – The glass transition and final decomposition temperatures of MWCNTs/PBNPI fabricated from different MWCNT concentrations. MWCNT/PBNPI nanocomposite membrane
Glass transition Final decomposition temperature (Tg C) temperature ( C)
MWCNT wt% PBNPI PBNPI PBNPI PBNPI PBNPI PBNPI
15 wt% 0 15 wt% 1 15 wt% 2.5 15 wt% 5 15 wt% 10 15 wt% 15
122.9 120.3 122.3 127.3 122.7 118.6
529.7 536.3 535.4 536.7 532.7 544.1
8711
international journal of hydrogen energy 34 (2009) 8707–8715
1679.69 cm-1
(g)
Absorbance
(f)
(e)
(d) 0.1 0.08
(c)
0.06 0.04
1355.71 cm-1C-N 1717.3 cm-1 Symmetric C=O 1773.23 cm-1 Asymmetric C=O
(b)
0.02 0 4000
3000
2000
1000
650
Wavenumber (cm-1) 0.2
Absorbance
of the nanocomposite membranes, whereas there is a slight decreased in the crystallinity when high MWCNT concentrations were added to the polymer matrix. Liu et al. [19] reported that Tg increased after MWCNTs were added to the polymer matrix; according to them, this increase is due to the decreased mobility of the polymer chains, which in turn can be attributed to the constraint effect of the MWCNTs. In addition, the thermal stability of PBNPI is improved by the incorporation of MWCNTs (Table 1). The final decomposition temperature of the MWCNTs/PBNPI membrane was show slight increase with the MWCNT concentration; the final decomposition temperature of all the nanocomposite membranes fabricated in this study increased after the addition of MWCNTs into the PBNPI matrix. The addition of 15 wt% MWCNTs to PBNPI resulted in a 14 C increase in the final decomposition temperature. These results show that addition of MWCNTs into polymer matrix helps improve thermal stability of the polymer membrane. Fig. 2 shows the FTIR spectra of the purified MWCNTs, pure PBNPI membrane, and MWCNTs/PBNPI membranes with 1, 2.5, 5, 10 and 15 wt% MWCNTs. As can be seen from the FTIR spectrum of purified MWCNTs shown in Fig. 2(a), the peaks at 3418.21 cm1 and 1641.13 cm1 can be attributed to the –OH group and the C]O carboxylate group, respectively. This peak pattern is similar to that observed in a previous research, indicating the existence of carboxyl C]O and –OH groups in the modified MWCNTs [14]. Fig. 2(b) shows the FTIR spectrum of pure PBNPI membrane, which shows characteristic imide group absorptions at around 1773.23 cm1 (asymmetric C]O stretching), 1717.3 cm1(symmetric C]O stretching) and 1355.71 cm1 (C–N stretching). Fig. 2(c–g) shows the FTIR spectra of the MWCNTs/PBNPI membranes with different MWCNTs concentrations. The FTIR spectra of MWCNTs/PBNPI membranes are similar to those of the pure PBNPI membrane; the peak observed at 1641.13 cm1 corresponding to the MWCNTs did not significantly increase with the MWCNT concentrations. However, the new bands that appeared at 1679.69 cm1 were presumably due to the C]O bond of carboxyl groups present on the surface of the added MWCNTs. The cause for the new bands appeared at 1679.69 cm1 can be explained in that the presence of MWCNTs in the nanocomposite membrane could affect the concentration of MWCNTs surface functional groups via chemical adsorption between carboxylic acid groups of MWCNTs and surface C]O groups on PBNPI. The XRD patterns of the purified MWCNTs and MWCNTs/ PBNPI nanocomposite membranes with different MWCNT loading: (a) 0 wt% (b) 1 wt% (c) 2.5 wt%, (d) 5 wt%, (e) 10 wt% and (f) 15 wt% were showing in Fig. 3(1) and (2), respectively. As shown in Fig. 3(1), the XRD pattern obtained from the purified MWCNTs illustrates three crystalline peaks at 2q ¼ 26 , 43 and 54 , in which intensity of the first one is more significant. For the pure PBNPI polymer membrane, three XRD pattern crystalline peak at 2q of 13.8 , 30 and 40.5 were also found (Fig. 3(2)a). Comparing with the pure PBNPI polymer membrane, the crystalline peak at 30 of MWCNTs/PBNPI nanocomposite membrane was shifted to lower angles with increasing the MWCNTs concentration. Previous studies have reported that the shift of the characteristic diffraction peak to the low-angle region suggests an increase in interlayer
0.1
0
-0.08 4000
3418.21 cm-1 -OH
(a)
2850.27 cm-1 C-H
1641.13 cm-1 C=O Carboxylic acids
2919.7 cm-1C-H
3000
2000
1000
650
Wavenumber (cm-1) Fig. 2 – The FTIR spectra of the acid-treated MWCNTs and the MWCNTs/PBNPI nanocomposite membrane with different contents of MWCNTs: (a) acid-treated MWCNTs; (b) just PBNPI membrane (c–g) the nanocomposite membrane with different content of MWCNTs 1, 2.5, 5, 10 and 15 wt%, respectively.
spacing [28]. Furthermore, the degree of dispersion measured ˚ ) from Bragg’s equation is by calculating the d-spacing (A depicted in Table 2. The results shows that the d-spacing of the MWCNTs/PBNPI nanocomposite membranes in the 2q range of 28–30 was increased with an increase in the MWCNT concentrations from 1 to 15 wt%. The maximum d-spacing in the case of MWCNTs/PBNPI membrane is observed at an MWCNT concentration of 15 wt%. Furthermore, since the d-spacing value increases in the 2q range of 28–30 with an increase in MWCNTs concentrations, it is suggested that the significant peak of MWCNT at 30 could be because of the increase in polymer chain spacing. This result suggests the relationship between d-spacing and the gas permeability of the MWCNTs/PBNPI membranes. Meanwhile, d-spacing value slightly decreased in main polymer crystalline regions in the 2q range of 13–14 and 40–43 with an increase in the
8712
international journal of hydrogen energy 34 (2009) 8707–8715
(1)
10-20nm CNT
Table 2 – X-ray diffraction parameters. ˚) d-Spacing (A Polymer
Intensity
PBNPI PBNPI PBNPI PBNPI PBNPI PBNPI
0
10
20
30
40
50
60
2 Theta (2)
(f)
Intensity
(e)
(d)
(c)
(b)
(a) 0
10
20
30
40
50
60
2 Theta Fig. 3 – X-ray diffraction patterns of the acid treatment MWCNT (1) and MWCNTs/PBNPI nanocomposite membranes (2) with different MWCNT weight ratios: (a) 0 wt% (b) 1 wt%, (c) 2.5 wt%, (d) 5 wt%, (e) 10 wt% and (f) 15 wt%.
MWCNTs concentrations. Further, it can be seen that polymer crystallinity resulted in a slight decrease in the d-spacing after addition of MWCNTs.
3.2. Gas permeation performance of the MWCNTs/ PBNPI membranes The permeabilities of H2, CO2 and CH4 of pure PBNPI and MWCNTs/PBNPI membranes are summarized in Table 3 as a function of MWCNTs concentrations. The results show that
Carbon 2q ¼ 13–14 2q ¼ 28–30 2q ¼ 40–43 nanotube wt% 0 1 2.5 5 10 15
6.65 6.33 6.65 6.20 6.33 6.33
3.11 3.13 3.17 3.20 3.18 3.21
2.37 2.31 2.32 2.31 2.35 2.27
the gas permeation rates observed for the nanocomposite membranes are higher than those of pure PBNPI polymer membrane. The permeation rates of H2, CO2 and CH4 in the case of the pure PBNPI membrane are 4.7, 2.6 and 0.7 Barrer, respectively. The H2 permeability of the MWCNTs/PBNPI nanocomposite membranes initially increased with MWCNT concentrations. The H2 permeability of the MWCNTs/PBNPI membrane reached 12.06 Barrer at an MWCNT loading of 10 wt%. When the MWCNT concentration was increased to 15 wt%, the H2 permeability increased remarkably to 14.31 Barrer, which was thrice that observed for the pure PBNPI membrane. It is also evident from the data shown in Table 3 that the CO2 and CH4 permeabilities of the MWCNTs/ PBNPI membranes (with different MWCNTs concentrations) were higher than those of the pure PBNPI membrane. When the MWCNTs’ loading weight increased to 15 wt% in the nanocomposite membrane, it resulted in a 131% increase in the CO2 permeability (from 2.61 to 6.03 Barrer). The CH4 permeability was also increased when the MWCNTs’ loading weight in the polymer matrix was increased from 2.5 wt% to 15 wt%, which resulted in a 154% increase in the CH4 permeability. From the FESEM images, it can be seen that MWCNTs are well dispersed in the polymer matrix after MWCNT loading of 5 wt%; this suggests that acid treatment causes the nanotubes to disintegrate into small fragments. These fragments are well dispersed in the polymer matrix at high MWCNT loading, thus improving gas permeability. Furthermore, this study suggests that the increase in gas permeability is not only due to gas transportation in the nanotubes [16], because the inside diameters of the MWCNTs (10–20 nm) are significantly larger than the kinetic diameters of H2, CO2 and CH4 molecules. The vertical dispersion of the MWCNTs in the nanocomposite membrane allows various gas molecules to pass through the membrane rapidly, but could not obtained good selectivity. Therefore, the content in the nano-gap would increase at high MWCNT concentrations, leading to even higher gas permeability. This inference is similar to that drawn for the permeability coefficient in this research. Furthermore, improved gas permeability of the MWCNTs/ PBNPI membranes can be further interpreted using the Tg value; the Tg value decreases when MWCNT loading of 15 wt% [17]. Maximum permeability is observed when MWCNT loading of 15 wt%. It can be seen main reason of low Tg, MWCNT loading effect on polymer crystalline region result in polymer chain moved, therefore, increased the gas
8713
international journal of hydrogen energy 34 (2009) 8707–8715
Table 3 – Effects of MWCNT concentration on permeability of MWCNTs/PBNPI nanocomposite membranes. Nanocomposite membrane Polymer membrane PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt%
Permeability (Barrer)
MWCNT wt% 0 1 2.5 5 10 15
P(H2) 4.710.77 4.950.31 6.491.94 6.421.62 12.063.16 14.313.07
P(CO2) 2.610.33 2.180.36 2.700.10 1.980.15 4.930.68 6.01.64
(5.12%)a (37.83%) (36.41%) (156.02%) (203.87%)
P(CH4) 0.700.03 0.740.04 0.950.48 1.170.65 1.550.54 1.780.26
d (3.54%) d (89.02%) (131.18%)
(5.26%) (35.83%) (66.84%) (120.96%) (154.09%)
a ( ) increment from pure polymer.
permeability of the MWCNTs/PBNPI membranes. Furthermore, some reports have suggested that the enhanced gas permeability of polymer nanocomposite membranes is due to the disturbed polymer chain packing caused by the addition of nanofillers [29]. Furthermore, the d-spacing value of the MWCNTs/PBNPI nanocomposite membranes increases in the 2q range of 28–30 with an increase in MWCNTs concentrations, the maximum in 15 wt% loading of MWCNT. This result suggests after adding MWCNTs in polymer matrix the d-spacing value was increased, meanwhile the high d-spacing could improve gas permeability.
understood by analyzing the contributions of diffusivity and solubility coefficients to the overall permeability. The gas diffusivity and solubility of the pure PBNPI and MWCNTs/ PBNPI membranes are shown in Tables 4and 5, respectively,
H2/CH4 selectivity 12
A
MWCNT/PBNPI membranes Pure PBNPI membrane
10
8
3.3.
Effect of MWCNT loading on gas selectivity 6
The selectivity of the pure PBNPI and MWCNTs/PBNPI membranes are summarized in Fig. 4. Fig. 4(a) comparison between H2/CH4 selectivities for the pure PBNPI and MWCNTs/ PBNPI membranes shows an increase in H2/CH4 selectivities after MWCNT loading in the membranes. In the case of the MWCNTs/PBNPI nanocomposite membrane, the H2/CH4 selectivities slightly increased with the MWCNTs’ loading weight increasing from 1 to 15 wt%, and reached a maximum value when MWCNT loading of 15 wt%. Therefore, increasing the MWCNT loading to 10 wt% can reached high value of H2/ CH4 selectivities. At 10 and 15 wt% of MWCNT loading, the d-spacing value at 13–14 and 40–43 of MWCNTs/PBNPI membrane was less than that at 14 and 40 of the pure PBNPI membrane. Thus, it can be concluded that addition of MWCNTs in the polymer matrix have little impact on the d-spacing of the polymer membrane and improved H2/CH4 selectivities. The CO2/CH4 selectivities observed for the pure PBNPI and MWCNTs/PBNPI nanocomposite membranes are shown in Fig. 4(b). The CO2/CH4 selectivity of pure PBNPI membrane was high then MWCNTs/PBNPI nanocomposite membranes as MWCNTs loading in polymer matrix increased from 1 wt% to 5 wt%, while the MWCNT loading increased from 10 wt% to 15 wt%, the CO2/CH4 selectivities increase further. These results suggest that low MWCNT concentrations did not significantly affect the CO2/CH4 selectivities in the membranes.
4
2
0
1%
The permeation mechanism of the pure gases permeating through the nanocomposite membrane can be better
loa
%
2.5
g
loa
g
5%
T CN
loa
%
10
g
din
din
din
T CN
T CN
loa
%
15
g
din
T CN
loa
CO2/CH4 selectivity 6
B
MWCNT/PBNPI membranes Pure PBNPI membrane
5
4
3
2
1
0 ing
oad
1%
3.4. Diffusivity and solubility of MWCNTs/PBNPI membrane
g
din
T CN
C
l NT
%
2.5
ing
C
5%
ing
C
l NT
%
10
ing
oad
oad
oad
l NT
C
l NT
%
15
ing
oad
l NT
C
Fig. 4 – Comparisons of (A) H2/CH4 and (B) CO2/CH4 selectivity of pure PBNPI membrane before and after MWCNTs/PBNPI nanocomposite membranes fabricated from different carbon nanotube concentrations.
8714
international journal of hydrogen energy 34 (2009) 8707–8715
Table 4 – Diffusivity (10L8, cm2/s) of various gases in the pure PBNPI and MWCNTs/PBNPI nanocomposite membranes. Membrane
PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI
MWCNT wt%
0 1 2.5 5 10 15
Diffusivity coefficient H2
CO2
CH4
24.07 6.93 7.58 1.20 8.06 0.44 14.94 1.96 56.69 19.46 81.35 36.89
0.69 0.06 0.67 0.17 0.86 0.22 1.03 0.32 2.93 0.72 5.44 1.91
0.39 0.13 0.85 0.03 0.45 0.18 1.13 0.20 1.88 0.39 4.28 1.19
Table 5 – Solubility (cm (STP)/cm3) of various gases in the pure PBNPI and MWCNTs/PBNPI nanocomposite membranes. Membrane
PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI
MWCNT wt%
0 1 2.5 5 10 15
as a function of MWCNTs concentrations. The diffusivity coefficient of H2 in the case of the pure PBNPI membrane was high then MWCNTs/PBNPI membranes when MWCNT loading was increased from 1 to 5 wt%, after 5 wt% the diffusivity coefficient was higher then pure PBNPI. This result suggested that the major influence factor for the diffusivity coefficient is the high MWCNT loading; at low loadings, the MWCNTs agglomerate within the domain limits, and the permeability not increased remarkably. However, after addition of 10 wt% MWCNTs to the polymer, the diffusivity coefficients of H2 increased remarkably. The diffusivity of CO2 and CH4 gases at low MWCNT loading were also high then pure PBNPI. Apparently, the diffusivity of CO2 and CH4 through the MWCNTs/PBNPI membranes increased when MWCNTs concentration was increased from 1 to 15 wt%. A maximum increase in the diffusivities of CO2 and CH4 gases was observed at 15 wt% MWCNT loading in the polymer matrix. In a previous research, a behavior similar to that observed for diffusivity was observed for permeability for changes in MWCNT concentrations; the increase in diffusivity could be a consequence of the presence of high diffusivity tunnels in the CNTs within the polymer matrix [15]. Table 5 shows the solubility coefficients for H2, CO2 and CH4. The solubility coefficients decreased with an increase in the MWCNT concentrations. In the case of H2, there was no significant change in the solubility even after the addition of MWCNTs. The solubilities of CO2 and CH4 for the pure PBNPI membrane were higher than those of all the other kinds of nanocomposite membranes. These results suggested that the addition of MWCNTs to a polymer matrix significantly decreased the solubility of H2, CO2 and CH4. From this series of experiments, the increase in gas permeation (H2, CO2 and CH4) of MWCNTs/PBNPI membranes in which MWCNT loading is
Solubility coefficient H2
CO2
CH4
0.2 0.03 0.66 0.06 0.81 0.26 0.44 0.14 0.24 0.12 0.14 0.02
3.84 0.79 3.34 0.36 3.30 0.87 2.06 0.61 1.88 0.51 0.91 0.20
2.19 0.67 0.87 0.08 2.67 1.59 0.99 0.40 0.84 0.27 0.39 0.08
higher than 10 wt% was successfully demonstrated by the diffusion mechanism.
4.
Conclusions
Nanocomposite membranes were prepared using MWCNTs with PBNPI as the polymer matrix. MWCNT/PBNPI membranes with different MWCNT loadings were analyzed to decide the optimum loading at which the permeability of the membranes was maximum. The FESEM images obtained for the cross-sectional area of the MWCNTs/PBNPI membranes indicated that after acid treatment, CNTs were well dispersed in the PBNPI matrix at 5, 10 and 15 wt% MWCNT loading. The XRD patterns obtained for crystalline of MWCNTs/PBNPI nanocomposite membrane indicate that the crystalline peak at 30 shifts to the low-angle region with an increase in the MWCNT concentrations; this suggests that a high MWCNT loading increases the polymer interlayer spacing. At low MWCNT concentrations (1–5 wt%) in the MWCNTs/PBNPI membranes the H2, CO2 and CH4 permeabilities did not show any notable increase with the MWCNT concentrations. At high MWCNTs concentrations, the permeabilities and selectivities of H2, CO2 and CH4 improved significantly. It was concluded that high MWCNTs loading in the polymer matrix affected the gas permeability and selectivity of nanocomposite membranes. Furthermore, the gas transport in the nanocomposite membranes through the interstice between of MWCNTs and polymer chain is the major reason for the enhanced gas permeability achieved because of high MWCT loading. Therefore, the diffusion coefficient can also be determined using the present experiments by high MWCNT loading, and the gas transport mechanism can be determined using the diffusion mechanism. Finally, it can be concluded
international journal of hydrogen energy 34 (2009) 8707–8715
that the gas permeability and selectivity (especially of H2 and CH4) of MWCNTs/PBNPI membranes can be improved by acid treatment of 15 wt% MWCNTs. It can also be concluded that the Tg and d-spacing of the polymer would change with the addition of MWCNTs into the polymer matrix, at the same time improving gas permeability and selectivity.
references
[1] Kulshrestha V, Acharya NK, Awasthi K, Singh M, Avasthi DK, Vijay YK. Study of gas permeation for asymmetric tracketched polymer blends. Int J Hydrogen Energy 2006;31(10): 1266–70. [2] Vijay YK, Wate S, Acharya NK, Garg JC. The titanium-coated polymeric membranes for hydrogen recovery. Int J Hydrogen Energy 2002;27(9):905–8. [3] Vijay YK, Acharya NK, Wate S, Avasthi DK. Nanofilter for hydrogen purification. Int J Hydrogen Energy 2003;28(9): 1015–8. [4] Jain AK, Acharya NK, Kulshreshtha V, Awasthi K, Singh M, Vijay YK. Study of hydrogen transport through porous aluminum and composite membranes. Int J Hydrogen Energy 2008;33(1):346–9. [5] Sharma A, Kumar S, Tripathi B, Singh M, Vijay YK. Aligned CNT/Polymer nanocomposite membranes for hydrogen separation. Int J Hydrogen Energy 2009;34(9):3977–82. [6] Yi C, Wang Z, Li M, Wang J, Wang S. Facilitated transport of CO2 through polyvinylamine/polyethlene glycol blend membranes. Desalination 2006;193(1–3):90–6. [7] Acharya NK, Kulshrestha V, Awasthi K, Jain AK, Singh M, Vijay YK. Hydrogen separation in doped and blend polymer membranes. Int J Hydrogen Energy 2008;33(1):327–31. [8] Hosseini SS, Li Y, Chung T-S, Liu Y. Enhanced gas separation performance of nanocomposite membranes using MgO nanoparticles. J Memb Sci 2007;302(1–2):207–17. [9] Ciobanu G, Carja G, Ciobanu O. Preparation and characterization of polymer-zeolite nanocomposite membranes. Mater Sci Eng C 2007;27(5–8):1138–40. [10] Vital J, Ramos AM, Silva IF, Valente H, Castanheiro JE. Hydration of [alpha]-pinene over zeolites and activated carbons dispersed in polymeric membranes. Catal Today 2000;56(1–3):167–72. [11] Ballinas L, Torras C, Fierro V, Garcia-Valls R. Factors influencing activated carbon-polymeric composite membrane structure and performance. J Phys Chem Solids 2004;65(2–3):633–7. [12] Iijima S. Helical microtubeles of graphitic carbon. Nature 1991;354:56–8. [13] Chen H, Sholl DS. Predictions of selectivity and flux for CH4/ H2 separations using single walled carbon nanotubes as membranes. J Memb Sci 2006;269(1–2):152–60.
8715
[14] Choi J-H, Jegal J, Kim W-N. Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes. J Memb Sci 2006;284(1–2):406–15. [15] Kim S, Pechar TW, Marand E. Poly(imide siloxane) and carbon nanotube mixed matrix membranes for gas separation. Desalination 2006;192(1–3):330–9. [16] Cong H, Zhang J, Radosz M, Shen Y. Carbon nanotube composite membranes of brominated poly(2,6-diphenyl-1,4phenylene oxide) for gas separation. J Memb Sci 2007;294 (1–2):178–85. [17] Kim S, Chen L, Johnson JK, Marand E. Polysulfone and functionalized carbon nanotube mixed matrix membranes for gas separation: theory and experiment. J Memb Sci 2007; 294(1–2):147–58. [18] Peng F, Hu C, Jiang Z. Novel ploy(vinyl alcohol)/carbon nanotube hybrid membranes for pervaporation separation of benzene/cyclohexane mixtures. J Memb Sci 2007;297(1–2): 236–42. [19] Liu T, Tong Y, Zhang W-D. Preparation and characterization of carbon nanotube/polyetherimide nanocomposite films. Compos Sci Technol 2007;67(3–4):406–12. [20] Weng T-H, Tseng H-H, Wey M-Y. Preparation and characterization of PPSU/PBNPI blend membrane for hydrogen separation. Int J Hydrogen Energy 2008;33(15): 4178–82. [21] Rao PS, Wey M-Y, Tseng H-H, Kumar IA, Weng T-H. A comparison of carbon/nanotube molecular sieve membranes with polymer blend carbon molecular sieve membranes for the gas permeation application. Micropor Mesopor Mat 2008; 113(1–3):499–510. [22] Balasubramanian K, Burghard M. Chemically functionalized carbon nanotubes. Small 2005;1(2):180–92. [23] Banerijee S, Hemraj-Benny T, Wong SS. Covalent surface chemistry of single-walled carbon nanotubes. Adv Mater 2005;17:17–29. [24] Sridhar S, Suryamurali R, Smitha B, Aminabhavi TM. Development of crosslinked poly(ether-block-amide) membrane for CO2/CH4 separation. Colloid Surf A 2007; 297(1–3):267–74. [25] Sridhar S, Smitha B, Ramakrishna M, Aminabhavi TM. Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane. J Memb Sci 2006; 280(1–2):202–9. [26] Lin WH, Vora RH, Chung TS. Gas transport properties of 6FDA-durene/1,4-phenylendiamine (pPDA) copolyimides. J Polym Sci B: Polym Phys 2000;38:2703–13. [27] Rutherford SW, Do DD. Review of time lag permeation technique as a method for characterisation of porous media and membranes. Adsorption 1997;3:283–312. [28] Frounchi M, Dadbin S, Salehpour Z, Noferesti M. Gas barrier properties of PP/EPDM blend nanocomposites. J Memb Sci 2006;282(1–2):142–8. [29] Merkel TC, Freeman BD, Spontak RJ, He Z, Pinnau I, Meakin P, et al. Ultrapermeable, reverse-selective nanocomposite membranes. Science 2002;296:519–22.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Preparation and characterization of multi-walled carbon nanotube/PBNPI nanocomposite membrane for H2/CH4 separation Tzu-Hsiang Weng a, Hui-Hsin Tseng b,c, Ming-Yen Wey a,* a
Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC School of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan, ROC c Department of Occupational Medicine, Chung Shan Medical University Hospital, Taichung 402, Taiwan, ROC b
article info
abstract
Article history:
By combining organic polymers normally used to make membrane filters with inorganic
Received 24 April 2009
substances, multi-walled carbon nanotube (MWCNTs), an extraordinary ability to separate
Received in revised form
H2 from CH4 was developed in this study. A series of MWCNTs/PBNPI nanocomposite
6 August 2009
membrane with a nominal MWCNTs content between 1 and 15 wt% were prepared by
Accepted 9 August 2009
solution casting method, in which the very fine MWCNTs were embedded into glassy
Available online 27 August 2009
polymer membrane. Detailed characterizations, such as morphology, thermal stability and crystalline structure have been conducted to understand the structures, composition and
Keywords:
properties of nanocomposite membranes. The results found that this new class of
Multi-walled carbon nanotube
membrane had increased permeability and enhanced selectivity, and a useful ability to
Gas separation
filter gases and organic vapours at the molecular level.
Nanocomposite membrane
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
H2 CH4
1.
Introduction
In recent years, there has been a dramatic increase in the number of research studies carried out on the separation of gases by using polymer membranes. In membrane-based gas separation, the desired compounds are separated from mixtures by passing them through tiny holes. Membranes are attractive as filters because they are a low-cost, energyefficient and green technology [1]. However, membrane use for separating gases has so far been limited by the lack of the right sort of membranes to yield pure products with the desired permeability, low operating cost, while remaining stable. Therefore, many researches on membrane-based gas separation have focused on the use of different type of
polymer membranes with the aim of improving gas permeability and selectivity [2–5]. For example, Kulshrestha et al. [1], Yi et al. [6] and Acharya et al. [7] have carried out on the use of organic-organic blend polymer membranes; in addition, inorganic-organic nanocomposite membrane also drew much attention. Nano-sized particle or nano-pore materials, such as MgO [8], zeolite [9,10] and activated carbon [10,11] are also used as nanofillers to prepared nanocomposite membranes. However, the materials are difficult to synthesize and the permselectivity of them are still far from the Robison line. Thus, it needs to develop some other methods for modifying polymer membrane in order to improve the permselectivity of H2 from CH4 mixture.
* Corresponding author. Tel.: þ886 4 22852455; fax: þ886 4 22862587. E-mail address: [email protected] (M.-Y. Wey). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.027
8708
international journal of hydrogen energy 34 (2009) 8707–8715
Carbon nanotubes (CNTs) have attracted considerable attention since their discovery [12]. With the improvement of the fabrication technology, the price of CNTs has reduced significantly and it is possible for them to be one of the most remarkable embedding materials. Taking advantage of the unique electronic, adsorption, mechanical and thermal properties of the CNTs, nanocomposite membrane containing CNTs and polymer are believed to provide many applications and exhibit cooperative and synergetic effects between the polymer and carbon phases. Some recent works have emphasized on the prediction [13] or experimentation [14–19] of the permeability and selectivity for gas separations which used CNTs as nanofillers in nanocomposite polymer membrane and a few examples are worth noticing. Kim et al. [17] reported novel nanocomposite membranes containing single-walled carbon nanotube (SWCNTs) in a polysulfone matrix can improve the selectivity and permeability of small molecules; they also observed a slight increase in permeability when the SWCNT loading weight in the polymer matrix was increased from 5 to 10 wt%. Cong et al. [16] fabricated nanocomposite membranes containing SWCNTs and MWCNTs in poly(2,6-diphenyl-1,4-phenylene oxide). They found that the CO2 permeability and selectivity decreased at high concentration of CNTs and that better gas permeability could be achieved with MWCNTs than with SWCNTs. Chio et al. [14] prepared MWCNTs/polysulfone (PSf) blend membranes by the phase-inversion process; their research results indicated that after acid treatment, the MWCNTs are well dispersed in organic solvents for the preparation of homogeneous MWCNTs/PSf blend solutions. Their results also showed that the flux of a PSf membrane with 4 wt% MWCNTs was higher than that of the PSf membrane without MWCNTs. Furthermore, MWCNTs/polyetherimide nanocomposite membranes were prepared by solution casting and fine dispersion of MWCNTs in the polyetherimide (PEI) matrix [19]. In the foregoing research to understand added carbon nanotube into polymer matrix cloud improved gas permeability. However, the reported study with a low CNTs content (1–5 wt%) failed to confirm the predicted synergetic effect in improving the permeability. Furthermore, only a few papers have reported on the different MWCNTs loading in the polymer matrix and on the relationship between gas permeability and the structural characteristics of nanocomposite membranes. The aim of this present work study is to develop and characterize a novel nanocomposite membrane obtained by dispersing MWCNTs in a polymer matrix. In this study, inexpensive commercially available MWCNTs were used to fabricate highly permeable and selective nanocomposite membranes. MWCNTs (diameter: 10–20 nm) were acid treatment to facilitate their dispersion in the membrane and were added to the matrix in different concentrations. In our previous study [20] we have reported the good H2/CH4 selectivity of a pure poly(bisphenol A-co-4-nitrophthalic anhydride-co-1,3-phenylene diamine) (PBNPI) membrane. In this study, we attempted to improve the PBNPI membrane with high H2 permeability and selectivity by embedding the MWCNT as nanofiller. The morphology, crystalline structure, functional group, and thermal properties of the MWCNTs/ PBNPI nanocomposite membrane were investigated by filed
emission-scanning electron microscopy (FESEM), X-ray diffraction (XRD), fourier transform infrared spertroscopy, attenuated total reflectance (FTIR-ATR), and thermogravimetric analysis (TGA), respectively. The changes in the permeability and selectivity of the membrane caused by different weight ratio of MWCNTs, and the predominately mechanism for gas transport were also investigated by diffusivity and solubility coefficients.
2. Experimental and characterization methods 2.1.
Materials and membrane preparation
PBNPI powder was purchased from Sigma–Aldrich. N-methyl2-pyrrolidone (NMP) purchased from Mallinckrodt Chemicals Co. USA was used as the co-solvent. High-purity MWCNTs (average diameter: w10–20 nm; length: 5–15 mm; surface area: 116 m2/g) were obtained from Desunnano Co., Ltd. The raw CNT materials were cut into small pieces and purified using a 3:1 mixture of concentrated H2SO4 (98 vol%) and HNO3 (70 vol%) to obtain open-ended CNTs. The mixture was stirred thoroughly for 4 h at 80 C and then washed with deionized water. The details of the purification method are described elsewhere [21]. Acid-treated and purified MWCNTs are known to have carboxyl groups on their surfaces, and can be easily dispersed in polar organic solvents [22,23]. H2 (99.9999%), CO2 (99.9999%) and CH4 (99.9999%) were obtained from Toyo Gas Company, Taiwan.
2.2. Fabrication of MWCNTs/PBNPI nanocomposite membrane Pure PBNPI and MWCNTs/PBNPI nanocomposite membranes were prepared by the solution casting method. The PBNPI membrane was prepared as follows 15 g of PBNPI was dissolved in 85 ml of NMP by stirring for 24 h (polymer concentration: 15 wt%). The solution was cast onto a glass plate at room temperature for 24 h. Slow evaporation of the solvent resulted in a smooth film with a uniform thickness. Once dry, the membrane was heated to 60 C and maintained at this temperature for 24 h. This was to ensure evaporation of any residual agent (solvent) in the polymer membrane. Subsequently, the resulting membrane was peeled off in deionized water and stored in a desiccator until before use. The preparation procedure for the nanocomposite membrane was identical to that of the pure polymer membrane preparation except for the additional step in which MWCNTs were incorporated in the membrane. Appropriate predetermined amounts of the purified MWCNTs were added slowly to 15 wt% PBNPI solution to obtain MWCNTs/PBNPI polymer membranes containing 1, 2.5, 5, 10 and 15 wt% MWCNTs. The mixture was stirred vigorously at 5000 rpm for 1 h; then, the rotation speed was increased to 10,000 rpm and stirred for 15 min so that the CNTs were uniformly dispersed in the polymer matrix. Subsequent to stirring, the MWCNTs/ PBNPI solution was cast onto a clean and dry glass plate using a typical hand-casting knife at ambient temperature. The evaporation and heat treatments carried out for the
international journal of hydrogen energy 34 (2009) 8707–8715
nanocomposite membranes were identical to those carried out for the pure polymer membrane. The final thickness of the membranes was approximately 50–90 mm. In this study, all the pure and nanocomposite membrane were prepared for three times by the abovementioned procedure for ensuring the reproducible of the membrane.
2.3. Characterization of pure and nanocomposite membrane FESEM (JSM 5600) was used to observer morphology, surface structure, cross-sectional images and dispersion of MWCNTs in the membranes. The FESEM images were support to explain gas transport behavior through nanocomposite membrane. The XRD measurements of nanocomposite membranes were recorded on a SIEMENS D5000 thin-film X-ray diffrac˚ ) were generated tometer system, and the X-rays (l ¼ 1.5418 A by a Cu Ka source. The XRD measurements were carried out within the range of 5 2q 35 with a step increment ratio of 0.4 /sec. This was done to identify the changes in crystal structure and intermolecular distances between the polymer matrix’s intersegmental chains [24,25]. The d-spacing are calculated from the scattering angle (2q) according to Bragg’s equation: d ¼ nl/2sinq. FTIR spectra of the MWCNTs/PBNPI nanocomposite membrane was measured on a JASCO-4100 FTIR-ATR spectrometer over the wavenumber range of 4000–400 cm1 to identify the changes in the membrane’s polymer chemical function group after adding CNTs. The TGA and differential scanning calorimeter (DSC) of the polymer membrane were measured on a Perkin Elmer STA 6000 control system. The pure polymer and MWCNTs/PBNPI nanocomposite membranes were heated up to 800 C at a scanning rate of 10 C/min under nitrogen atmosphere in order to diminish oxidation.
2.4.
Gas permeation measurement
For non-porous membranes, the permeation process based on the solution diffusion model occurs in three stages. These are sorption, diffusion, and desorption under a pressure gradient [21]. The gas permeation performance of both the PBNPI membrane and nanocomposite membranes were investigated by using a standard vacuum time lag method at room temperature (26 C) and a feed pressure of 2 kg/cm2 [26]. The self-built vacuum time-lag apparatus is shown elsewhere [20,27]. A circular stainless permeability cell (47-mm disc filters, Millipore, USA), with an effective area 4.7 cm2, was used for measure transport properties of the membrane. The gases used in this research were H2, CO2 and CH4 with kinetic ˚ , 3.3 A ˚ and 3.8 A ˚ , respectively. Prior to all diameters of 2.8 A experiments, the leak detection tests were performed in all valves. Before feeding and permeating, the upstream and downstream sides of the membrane cell were evacuated (< 105 Torr). The pressure at upstream and downstream side was measured by using pressure sensors and data was recorded with the help of Visidaq Builder computer software, which ensures automated measurement with an automatically adapting data-sampling rate (1 data sec1) to yield at downstream pressure data points and to describe time-lag
8709
and steady-state gas transport completely. The permeability coefficient is permeation rate (P) value, determined by the following equation: P¼
dp VT0 L ; dt ADP TP0
(1)
where P is the permeability coefficient expressed in Barrer (1 Barrer ¼ 1 1010 cm3 cm (STP) cm2 s1 cm Hg1), (dp/dt) is the steady rate at which pressure increases on the downstream side, V (cm3) is the calibrated downstream volume, DP (cmHg) is the transmembrane pressure difference between the two sides, P1 and P2 are the upstream and downstream pressures, respectively of the membrane cell, A (cm2) is the effective area of the membrane, L (cm) is the effective thickness of the separating layer, and T0 and P0 are the standard temperature and pressure, respectively. The ideal separation factor of pure gas A over B (aAB) is defined as the ratio of permeation rate of A to B, which can be expressed by aAB ¼
PA PB
(2)
Besides permeability and selectivity, the gas membrane separation study evaluated the diffusivity and solubility of gas through the membrane. Diffusivity (D) was determined from: D¼
L2 6q
(3)
where q is the time lag when a steady dp/dt rate is obtained from the downstream side in gas permeation tests [26]. The solubility coefficient (S ) of each gas was taken as the ratio of the permeability to diffusivity coefficient determined from: S¼
P D
(4)
All pure and nanocomposite polymer membrane which prepared for three times was performed with each gas one time and the average results and the standard deviations were recorded.
3.
Results and discussion
3.1. Characterizations of pure PBNPI and MWCNTs/ PBNPI nanocomposite membranes The FESEM image shown in Fig. 1 illustrates the cross-section images of the MWCNTs/PBNPI nanocomposite membranes prepared with different weight ratio of MWCNTs. As shown in Fig. 1(a), the MWCNTs/PBNPI nanocomposite membrane containing 1 wt% MWCNTs shows only a few MWCNTs are distributed throughout the PBNPI matrix. With the MWCNTs’ concentration increased, from the FESEM observations shown in Fig. 1(b–e), it is apparent that the degree of distribution of MWCNTs in the MWCNTs/PBNPI membranes initially increased with the MWCNTs concentrations. Further, when 10–15% of MWCNTs is added to the PBNPI matrix, no aggregation occurred in the MWCNTs/PBNPI membrane. It can be seen that at higher MWCNTs’ concentrations, the MWCNTs
8710
international journal of hydrogen energy 34 (2009) 8707–8715
Fig. 1 – FESEM image showing an overall morphology of a cross-section for 15 wt% MWCNTs/PBNPI nanocomposite with different MWCNT weight ratios: (a) 1 wt%, (b) 2.5 wt%, (c) 5 wt%, (d) 10 wt% and (e) 15 wt%.
are more randomly distributed and loosely entangled in the PBNPI matrix. Table 1 shows the glass transition temperature (Tg) and final decomposition temperature values of the pure PBNPI and MWCNTs/PBNPI nanocomposite membrane, which obtained from the The DSC heat flow curves and TGA curves. The Tg of the MWCNTs/PBNPI membranes became slightly lower than that of the pure PBNPI membrane (122.9 C) upon the addition of 1 wt% and 2.5 wt% MWCNTs to the polymer matrix. However, Tg increased by approximately 5 C when 5 wt% MWCNTs were added to the polymer matrix; after the 10 wt% and 15 wt% of MWCNTs were added to the polymer matrix, the Tg of the MWCNTs/PBNPI membrane decreased significantly. These results show that addition of 5 wt% MWCNTs to the polymer matrix resulted in an increase in the crystallinity
Table 1 – The glass transition and final decomposition temperatures of MWCNTs/PBNPI fabricated from different MWCNT concentrations. MWCNT/PBNPI nanocomposite membrane
Glass transition Final decomposition temperature (Tg C) temperature ( C)
MWCNT wt% PBNPI PBNPI PBNPI PBNPI PBNPI PBNPI
15 wt% 0 15 wt% 1 15 wt% 2.5 15 wt% 5 15 wt% 10 15 wt% 15
122.9 120.3 122.3 127.3 122.7 118.6
529.7 536.3 535.4 536.7 532.7 544.1
8711
international journal of hydrogen energy 34 (2009) 8707–8715
1679.69 cm-1
(g)
Absorbance
(f)
(e)
(d) 0.1 0.08
(c)
0.06 0.04
1355.71 cm-1C-N 1717.3 cm-1 Symmetric C=O 1773.23 cm-1 Asymmetric C=O
(b)
0.02 0 4000
3000
2000
1000
650
Wavenumber (cm-1) 0.2
Absorbance
of the nanocomposite membranes, whereas there is a slight decreased in the crystallinity when high MWCNT concentrations were added to the polymer matrix. Liu et al. [19] reported that Tg increased after MWCNTs were added to the polymer matrix; according to them, this increase is due to the decreased mobility of the polymer chains, which in turn can be attributed to the constraint effect of the MWCNTs. In addition, the thermal stability of PBNPI is improved by the incorporation of MWCNTs (Table 1). The final decomposition temperature of the MWCNTs/PBNPI membrane was show slight increase with the MWCNT concentration; the final decomposition temperature of all the nanocomposite membranes fabricated in this study increased after the addition of MWCNTs into the PBNPI matrix. The addition of 15 wt% MWCNTs to PBNPI resulted in a 14 C increase in the final decomposition temperature. These results show that addition of MWCNTs into polymer matrix helps improve thermal stability of the polymer membrane. Fig. 2 shows the FTIR spectra of the purified MWCNTs, pure PBNPI membrane, and MWCNTs/PBNPI membranes with 1, 2.5, 5, 10 and 15 wt% MWCNTs. As can be seen from the FTIR spectrum of purified MWCNTs shown in Fig. 2(a), the peaks at 3418.21 cm1 and 1641.13 cm1 can be attributed to the –OH group and the C]O carboxylate group, respectively. This peak pattern is similar to that observed in a previous research, indicating the existence of carboxyl C]O and –OH groups in the modified MWCNTs [14]. Fig. 2(b) shows the FTIR spectrum of pure PBNPI membrane, which shows characteristic imide group absorptions at around 1773.23 cm1 (asymmetric C]O stretching), 1717.3 cm1(symmetric C]O stretching) and 1355.71 cm1 (C–N stretching). Fig. 2(c–g) shows the FTIR spectra of the MWCNTs/PBNPI membranes with different MWCNTs concentrations. The FTIR spectra of MWCNTs/PBNPI membranes are similar to those of the pure PBNPI membrane; the peak observed at 1641.13 cm1 corresponding to the MWCNTs did not significantly increase with the MWCNT concentrations. However, the new bands that appeared at 1679.69 cm1 were presumably due to the C]O bond of carboxyl groups present on the surface of the added MWCNTs. The cause for the new bands appeared at 1679.69 cm1 can be explained in that the presence of MWCNTs in the nanocomposite membrane could affect the concentration of MWCNTs surface functional groups via chemical adsorption between carboxylic acid groups of MWCNTs and surface C]O groups on PBNPI. The XRD patterns of the purified MWCNTs and MWCNTs/ PBNPI nanocomposite membranes with different MWCNT loading: (a) 0 wt% (b) 1 wt% (c) 2.5 wt%, (d) 5 wt%, (e) 10 wt% and (f) 15 wt% were showing in Fig. 3(1) and (2), respectively. As shown in Fig. 3(1), the XRD pattern obtained from the purified MWCNTs illustrates three crystalline peaks at 2q ¼ 26 , 43 and 54 , in which intensity of the first one is more significant. For the pure PBNPI polymer membrane, three XRD pattern crystalline peak at 2q of 13.8 , 30 and 40.5 were also found (Fig. 3(2)a). Comparing with the pure PBNPI polymer membrane, the crystalline peak at 30 of MWCNTs/PBNPI nanocomposite membrane was shifted to lower angles with increasing the MWCNTs concentration. Previous studies have reported that the shift of the characteristic diffraction peak to the low-angle region suggests an increase in interlayer
0.1
0
-0.08 4000
3418.21 cm-1 -OH
(a)
2850.27 cm-1 C-H
1641.13 cm-1 C=O Carboxylic acids
2919.7 cm-1C-H
3000
2000
1000
650
Wavenumber (cm-1) Fig. 2 – The FTIR spectra of the acid-treated MWCNTs and the MWCNTs/PBNPI nanocomposite membrane with different contents of MWCNTs: (a) acid-treated MWCNTs; (b) just PBNPI membrane (c–g) the nanocomposite membrane with different content of MWCNTs 1, 2.5, 5, 10 and 15 wt%, respectively.
spacing [28]. Furthermore, the degree of dispersion measured ˚ ) from Bragg’s equation is by calculating the d-spacing (A depicted in Table 2. The results shows that the d-spacing of the MWCNTs/PBNPI nanocomposite membranes in the 2q range of 28–30 was increased with an increase in the MWCNT concentrations from 1 to 15 wt%. The maximum d-spacing in the case of MWCNTs/PBNPI membrane is observed at an MWCNT concentration of 15 wt%. Furthermore, since the d-spacing value increases in the 2q range of 28–30 with an increase in MWCNTs concentrations, it is suggested that the significant peak of MWCNT at 30 could be because of the increase in polymer chain spacing. This result suggests the relationship between d-spacing and the gas permeability of the MWCNTs/PBNPI membranes. Meanwhile, d-spacing value slightly decreased in main polymer crystalline regions in the 2q range of 13–14 and 40–43 with an increase in the
8712
international journal of hydrogen energy 34 (2009) 8707–8715
(1)
10-20nm CNT
Table 2 – X-ray diffraction parameters. ˚) d-Spacing (A Polymer
Intensity
PBNPI PBNPI PBNPI PBNPI PBNPI PBNPI
0
10
20
30
40
50
60
2 Theta (2)
(f)
Intensity
(e)
(d)
(c)
(b)
(a) 0
10
20
30
40
50
60
2 Theta Fig. 3 – X-ray diffraction patterns of the acid treatment MWCNT (1) and MWCNTs/PBNPI nanocomposite membranes (2) with different MWCNT weight ratios: (a) 0 wt% (b) 1 wt%, (c) 2.5 wt%, (d) 5 wt%, (e) 10 wt% and (f) 15 wt%.
MWCNTs concentrations. Further, it can be seen that polymer crystallinity resulted in a slight decrease in the d-spacing after addition of MWCNTs.
3.2. Gas permeation performance of the MWCNTs/ PBNPI membranes The permeabilities of H2, CO2 and CH4 of pure PBNPI and MWCNTs/PBNPI membranes are summarized in Table 3 as a function of MWCNTs concentrations. The results show that
Carbon 2q ¼ 13–14 2q ¼ 28–30 2q ¼ 40–43 nanotube wt% 0 1 2.5 5 10 15
6.65 6.33 6.65 6.20 6.33 6.33
3.11 3.13 3.17 3.20 3.18 3.21
2.37 2.31 2.32 2.31 2.35 2.27
the gas permeation rates observed for the nanocomposite membranes are higher than those of pure PBNPI polymer membrane. The permeation rates of H2, CO2 and CH4 in the case of the pure PBNPI membrane are 4.7, 2.6 and 0.7 Barrer, respectively. The H2 permeability of the MWCNTs/PBNPI nanocomposite membranes initially increased with MWCNT concentrations. The H2 permeability of the MWCNTs/PBNPI membrane reached 12.06 Barrer at an MWCNT loading of 10 wt%. When the MWCNT concentration was increased to 15 wt%, the H2 permeability increased remarkably to 14.31 Barrer, which was thrice that observed for the pure PBNPI membrane. It is also evident from the data shown in Table 3 that the CO2 and CH4 permeabilities of the MWCNTs/ PBNPI membranes (with different MWCNTs concentrations) were higher than those of the pure PBNPI membrane. When the MWCNTs’ loading weight increased to 15 wt% in the nanocomposite membrane, it resulted in a 131% increase in the CO2 permeability (from 2.61 to 6.03 Barrer). The CH4 permeability was also increased when the MWCNTs’ loading weight in the polymer matrix was increased from 2.5 wt% to 15 wt%, which resulted in a 154% increase in the CH4 permeability. From the FESEM images, it can be seen that MWCNTs are well dispersed in the polymer matrix after MWCNT loading of 5 wt%; this suggests that acid treatment causes the nanotubes to disintegrate into small fragments. These fragments are well dispersed in the polymer matrix at high MWCNT loading, thus improving gas permeability. Furthermore, this study suggests that the increase in gas permeability is not only due to gas transportation in the nanotubes [16], because the inside diameters of the MWCNTs (10–20 nm) are significantly larger than the kinetic diameters of H2, CO2 and CH4 molecules. The vertical dispersion of the MWCNTs in the nanocomposite membrane allows various gas molecules to pass through the membrane rapidly, but could not obtained good selectivity. Therefore, the content in the nano-gap would increase at high MWCNT concentrations, leading to even higher gas permeability. This inference is similar to that drawn for the permeability coefficient in this research. Furthermore, improved gas permeability of the MWCNTs/ PBNPI membranes can be further interpreted using the Tg value; the Tg value decreases when MWCNT loading of 15 wt% [17]. Maximum permeability is observed when MWCNT loading of 15 wt%. It can be seen main reason of low Tg, MWCNT loading effect on polymer crystalline region result in polymer chain moved, therefore, increased the gas
8713
international journal of hydrogen energy 34 (2009) 8707–8715
Table 3 – Effects of MWCNT concentration on permeability of MWCNTs/PBNPI nanocomposite membranes. Nanocomposite membrane Polymer membrane PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt% PBNPI 15 wt%
Permeability (Barrer)
MWCNT wt% 0 1 2.5 5 10 15
P(H2) 4.710.77 4.950.31 6.491.94 6.421.62 12.063.16 14.313.07
P(CO2) 2.610.33 2.180.36 2.700.10 1.980.15 4.930.68 6.01.64
(5.12%)a (37.83%) (36.41%) (156.02%) (203.87%)
P(CH4) 0.700.03 0.740.04 0.950.48 1.170.65 1.550.54 1.780.26
d (3.54%) d (89.02%) (131.18%)
(5.26%) (35.83%) (66.84%) (120.96%) (154.09%)
a ( ) increment from pure polymer.
permeability of the MWCNTs/PBNPI membranes. Furthermore, some reports have suggested that the enhanced gas permeability of polymer nanocomposite membranes is due to the disturbed polymer chain packing caused by the addition of nanofillers [29]. Furthermore, the d-spacing value of the MWCNTs/PBNPI nanocomposite membranes increases in the 2q range of 28–30 with an increase in MWCNTs concentrations, the maximum in 15 wt% loading of MWCNT. This result suggests after adding MWCNTs in polymer matrix the d-spacing value was increased, meanwhile the high d-spacing could improve gas permeability.
understood by analyzing the contributions of diffusivity and solubility coefficients to the overall permeability. The gas diffusivity and solubility of the pure PBNPI and MWCNTs/ PBNPI membranes are shown in Tables 4and 5, respectively,
H2/CH4 selectivity 12
A
MWCNT/PBNPI membranes Pure PBNPI membrane
10
8
3.3.
Effect of MWCNT loading on gas selectivity 6
The selectivity of the pure PBNPI and MWCNTs/PBNPI membranes are summarized in Fig. 4. Fig. 4(a) comparison between H2/CH4 selectivities for the pure PBNPI and MWCNTs/ PBNPI membranes shows an increase in H2/CH4 selectivities after MWCNT loading in the membranes. In the case of the MWCNTs/PBNPI nanocomposite membrane, the H2/CH4 selectivities slightly increased with the MWCNTs’ loading weight increasing from 1 to 15 wt%, and reached a maximum value when MWCNT loading of 15 wt%. Therefore, increasing the MWCNT loading to 10 wt% can reached high value of H2/ CH4 selectivities. At 10 and 15 wt% of MWCNT loading, the d-spacing value at 13–14 and 40–43 of MWCNTs/PBNPI membrane was less than that at 14 and 40 of the pure PBNPI membrane. Thus, it can be concluded that addition of MWCNTs in the polymer matrix have little impact on the d-spacing of the polymer membrane and improved H2/CH4 selectivities. The CO2/CH4 selectivities observed for the pure PBNPI and MWCNTs/PBNPI nanocomposite membranes are shown in Fig. 4(b). The CO2/CH4 selectivity of pure PBNPI membrane was high then MWCNTs/PBNPI nanocomposite membranes as MWCNTs loading in polymer matrix increased from 1 wt% to 5 wt%, while the MWCNT loading increased from 10 wt% to 15 wt%, the CO2/CH4 selectivities increase further. These results suggest that low MWCNT concentrations did not significantly affect the CO2/CH4 selectivities in the membranes.
4
2
0
1%
The permeation mechanism of the pure gases permeating through the nanocomposite membrane can be better
loa
%
2.5
g
loa
g
5%
T CN
loa
%
10
g
din
din
din
T CN
T CN
loa
%
15
g
din
T CN
loa
CO2/CH4 selectivity 6
B
MWCNT/PBNPI membranes Pure PBNPI membrane
5
4
3
2
1
0 ing
oad
1%
3.4. Diffusivity and solubility of MWCNTs/PBNPI membrane
g
din
T CN
C
l NT
%
2.5
ing
C
5%
ing
C
l NT
%
10
ing
oad
oad
oad
l NT
C
l NT
%
15
ing
oad
l NT
C
Fig. 4 – Comparisons of (A) H2/CH4 and (B) CO2/CH4 selectivity of pure PBNPI membrane before and after MWCNTs/PBNPI nanocomposite membranes fabricated from different carbon nanotube concentrations.
8714
international journal of hydrogen energy 34 (2009) 8707–8715
Table 4 – Diffusivity (10L8, cm2/s) of various gases in the pure PBNPI and MWCNTs/PBNPI nanocomposite membranes. Membrane
PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI
MWCNT wt%
0 1 2.5 5 10 15
Diffusivity coefficient H2
CO2
CH4
24.07 6.93 7.58 1.20 8.06 0.44 14.94 1.96 56.69 19.46 81.35 36.89
0.69 0.06 0.67 0.17 0.86 0.22 1.03 0.32 2.93 0.72 5.44 1.91
0.39 0.13 0.85 0.03 0.45 0.18 1.13 0.20 1.88 0.39 4.28 1.19
Table 5 – Solubility (cm (STP)/cm3) of various gases in the pure PBNPI and MWCNTs/PBNPI nanocomposite membranes. Membrane
PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI MWCNT/PBNPI
MWCNT wt%
0 1 2.5 5 10 15
as a function of MWCNTs concentrations. The diffusivity coefficient of H2 in the case of the pure PBNPI membrane was high then MWCNTs/PBNPI membranes when MWCNT loading was increased from 1 to 5 wt%, after 5 wt% the diffusivity coefficient was higher then pure PBNPI. This result suggested that the major influence factor for the diffusivity coefficient is the high MWCNT loading; at low loadings, the MWCNTs agglomerate within the domain limits, and the permeability not increased remarkably. However, after addition of 10 wt% MWCNTs to the polymer, the diffusivity coefficients of H2 increased remarkably. The diffusivity of CO2 and CH4 gases at low MWCNT loading were also high then pure PBNPI. Apparently, the diffusivity of CO2 and CH4 through the MWCNTs/PBNPI membranes increased when MWCNTs concentration was increased from 1 to 15 wt%. A maximum increase in the diffusivities of CO2 and CH4 gases was observed at 15 wt% MWCNT loading in the polymer matrix. In a previous research, a behavior similar to that observed for diffusivity was observed for permeability for changes in MWCNT concentrations; the increase in diffusivity could be a consequence of the presence of high diffusivity tunnels in the CNTs within the polymer matrix [15]. Table 5 shows the solubility coefficients for H2, CO2 and CH4. The solubility coefficients decreased with an increase in the MWCNT concentrations. In the case of H2, there was no significant change in the solubility even after the addition of MWCNTs. The solubilities of CO2 and CH4 for the pure PBNPI membrane were higher than those of all the other kinds of nanocomposite membranes. These results suggested that the addition of MWCNTs to a polymer matrix significantly decreased the solubility of H2, CO2 and CH4. From this series of experiments, the increase in gas permeation (H2, CO2 and CH4) of MWCNTs/PBNPI membranes in which MWCNT loading is
Solubility coefficient H2
CO2
CH4
0.2 0.03 0.66 0.06 0.81 0.26 0.44 0.14 0.24 0.12 0.14 0.02
3.84 0.79 3.34 0.36 3.30 0.87 2.06 0.61 1.88 0.51 0.91 0.20
2.19 0.67 0.87 0.08 2.67 1.59 0.99 0.40 0.84 0.27 0.39 0.08
higher than 10 wt% was successfully demonstrated by the diffusion mechanism.
4.
Conclusions
Nanocomposite membranes were prepared using MWCNTs with PBNPI as the polymer matrix. MWCNT/PBNPI membranes with different MWCNT loadings were analyzed to decide the optimum loading at which the permeability of the membranes was maximum. The FESEM images obtained for the cross-sectional area of the MWCNTs/PBNPI membranes indicated that after acid treatment, CNTs were well dispersed in the PBNPI matrix at 5, 10 and 15 wt% MWCNT loading. The XRD patterns obtained for crystalline of MWCNTs/PBNPI nanocomposite membrane indicate that the crystalline peak at 30 shifts to the low-angle region with an increase in the MWCNT concentrations; this suggests that a high MWCNT loading increases the polymer interlayer spacing. At low MWCNT concentrations (1–5 wt%) in the MWCNTs/PBNPI membranes the H2, CO2 and CH4 permeabilities did not show any notable increase with the MWCNT concentrations. At high MWCNTs concentrations, the permeabilities and selectivities of H2, CO2 and CH4 improved significantly. It was concluded that high MWCNTs loading in the polymer matrix affected the gas permeability and selectivity of nanocomposite membranes. Furthermore, the gas transport in the nanocomposite membranes through the interstice between of MWCNTs and polymer chain is the major reason for the enhanced gas permeability achieved because of high MWCT loading. Therefore, the diffusion coefficient can also be determined using the present experiments by high MWCNT loading, and the gas transport mechanism can be determined using the diffusion mechanism. Finally, it can be concluded
international journal of hydrogen energy 34 (2009) 8707–8715
that the gas permeability and selectivity (especially of H2 and CH4) of MWCNTs/PBNPI membranes can be improved by acid treatment of 15 wt% MWCNTs. It can also be concluded that the Tg and d-spacing of the polymer would change with the addition of MWCNTs into the polymer matrix, at the same time improving gas permeability and selectivity.
references
[1] Kulshrestha V, Acharya NK, Awasthi K, Singh M, Avasthi DK, Vijay YK. Study of gas permeation for asymmetric tracketched polymer blends. Int J Hydrogen Energy 2006;31(10): 1266–70. [2] Vijay YK, Wate S, Acharya NK, Garg JC. The titanium-coated polymeric membranes for hydrogen recovery. Int J Hydrogen Energy 2002;27(9):905–8. [3] Vijay YK, Acharya NK, Wate S, Avasthi DK. Nanofilter for hydrogen purification. Int J Hydrogen Energy 2003;28(9): 1015–8. [4] Jain AK, Acharya NK, Kulshreshtha V, Awasthi K, Singh M, Vijay YK. Study of hydrogen transport through porous aluminum and composite membranes. Int J Hydrogen Energy 2008;33(1):346–9. [5] Sharma A, Kumar S, Tripathi B, Singh M, Vijay YK. Aligned CNT/Polymer nanocomposite membranes for hydrogen separation. Int J Hydrogen Energy 2009;34(9):3977–82. [6] Yi C, Wang Z, Li M, Wang J, Wang S. Facilitated transport of CO2 through polyvinylamine/polyethlene glycol blend membranes. Desalination 2006;193(1–3):90–6. [7] Acharya NK, Kulshrestha V, Awasthi K, Jain AK, Singh M, Vijay YK. Hydrogen separation in doped and blend polymer membranes. Int J Hydrogen Energy 2008;33(1):327–31. [8] Hosseini SS, Li Y, Chung T-S, Liu Y. Enhanced gas separation performance of nanocomposite membranes using MgO nanoparticles. J Memb Sci 2007;302(1–2):207–17. [9] Ciobanu G, Carja G, Ciobanu O. Preparation and characterization of polymer-zeolite nanocomposite membranes. Mater Sci Eng C 2007;27(5–8):1138–40. [10] Vital J, Ramos AM, Silva IF, Valente H, Castanheiro JE. Hydration of [alpha]-pinene over zeolites and activated carbons dispersed in polymeric membranes. Catal Today 2000;56(1–3):167–72. [11] Ballinas L, Torras C, Fierro V, Garcia-Valls R. Factors influencing activated carbon-polymeric composite membrane structure and performance. J Phys Chem Solids 2004;65(2–3):633–7. [12] Iijima S. Helical microtubeles of graphitic carbon. Nature 1991;354:56–8. [13] Chen H, Sholl DS. Predictions of selectivity and flux for CH4/ H2 separations using single walled carbon nanotubes as membranes. J Memb Sci 2006;269(1–2):152–60.
8715
[14] Choi J-H, Jegal J, Kim W-N. Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes. J Memb Sci 2006;284(1–2):406–15. [15] Kim S, Pechar TW, Marand E. Poly(imide siloxane) and carbon nanotube mixed matrix membranes for gas separation. Desalination 2006;192(1–3):330–9. [16] Cong H, Zhang J, Radosz M, Shen Y. Carbon nanotube composite membranes of brominated poly(2,6-diphenyl-1,4phenylene oxide) for gas separation. J Memb Sci 2007;294 (1–2):178–85. [17] Kim S, Chen L, Johnson JK, Marand E. Polysulfone and functionalized carbon nanotube mixed matrix membranes for gas separation: theory and experiment. J Memb Sci 2007; 294(1–2):147–58. [18] Peng F, Hu C, Jiang Z. Novel ploy(vinyl alcohol)/carbon nanotube hybrid membranes for pervaporation separation of benzene/cyclohexane mixtures. J Memb Sci 2007;297(1–2): 236–42. [19] Liu T, Tong Y, Zhang W-D. Preparation and characterization of carbon nanotube/polyetherimide nanocomposite films. Compos Sci Technol 2007;67(3–4):406–12. [20] Weng T-H, Tseng H-H, Wey M-Y. Preparation and characterization of PPSU/PBNPI blend membrane for hydrogen separation. Int J Hydrogen Energy 2008;33(15): 4178–82. [21] Rao PS, Wey M-Y, Tseng H-H, Kumar IA, Weng T-H. A comparison of carbon/nanotube molecular sieve membranes with polymer blend carbon molecular sieve membranes for the gas permeation application. Micropor Mesopor Mat 2008; 113(1–3):499–510. [22] Balasubramanian K, Burghard M. Chemically functionalized carbon nanotubes. Small 2005;1(2):180–92. [23] Banerijee S, Hemraj-Benny T, Wong SS. Covalent surface chemistry of single-walled carbon nanotubes. Adv Mater 2005;17:17–29. [24] Sridhar S, Suryamurali R, Smitha B, Aminabhavi TM. Development of crosslinked poly(ether-block-amide) membrane for CO2/CH4 separation. Colloid Surf A 2007; 297(1–3):267–74. [25] Sridhar S, Smitha B, Ramakrishna M, Aminabhavi TM. Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane. J Memb Sci 2006; 280(1–2):202–9. [26] Lin WH, Vora RH, Chung TS. Gas transport properties of 6FDA-durene/1,4-phenylendiamine (pPDA) copolyimides. J Polym Sci B: Polym Phys 2000;38:2703–13. [27] Rutherford SW, Do DD. Review of time lag permeation technique as a method for characterisation of porous media and membranes. Adsorption 1997;3:283–312. [28] Frounchi M, Dadbin S, Salehpour Z, Noferesti M. Gas barrier properties of PP/EPDM blend nanocomposites. J Memb Sci 2006;282(1–2):142–8. [29] Merkel TC, Freeman BD, Spontak RJ, He Z, Pinnau I, Meakin P, et al. Ultrapermeable, reverse-selective nanocomposite membranes. Science 2002;296:519–22.