Journal of Membrane Science 331 (2009) 21–30
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Enhancement of the gas separation properties of polybenzimidazole (PBI) membrane by incorporation of silica nano particles Morteza Sadeghi, Mohammad Ali Semsarzadeh ∗ , Homayoon Moadel Polymer Group, Chemical Engineering Department, Tarbiat Modares University, Jalal Al-Ahmad Highway, P.O. Box 14155-143, Tehran, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 10 February 2008 Received in revised form 20 October 2008 Accepted 29 December 2008 Available online 20 January 2009 Keywords: Polybenzimidazole Membrane Nanocomposite Silica Gas separation
a b s t r a c t In the present work, the effect of silica nano particles on the permeability of CO2 , CH4 and N2 gases in polybenzimidazole (PBI) membranes has been studied. Silica particles were prepared via the sol–gel method through the hydrolysis of tetraethoxysilane (TEOS). PBI and PBI–silica hybrid membranes were prepared by thermal phase inversion method. Scanning electron microscopy (SEM), X-ray diffraction and FTIR analyses were employed in order to characterize the PBI and PBI–silica composite membranes. The obtained SEM micrographs confirmed the nano-scale distribution of silica particles in the polymer matrix. Gas permeation experiments showed an increase in the solubility and a corresponding reduction in the diffusivity of the gases through the membranes by increasing the silica content in the polymer matrix; consequently, the permeability of the condensable CO2 and CH4 gases were enhanced whereas that of non-condensable N2 gas significantly decreased upon increasing the silica content of the nano-composite membranes. The permeability of CO2 and its selectivity over N2 was increased from 0.025 Barrer and 3.5 in pure PBI to 0.11 Barrer and 71.3 in the nano-composite containing 20 wt% of the silica particles. There was a strong correlation between the solubility coefficients and condensability of the gases, as well as the diffusion coefficient of the penetrants and their kinetic diameter. Higuchi model fitted fairly well with the experimental results concerning the permeability of nitrogen gas in nanocomposite membranes, supposing the KH = 3.8. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Increasing the efficiency of polymeric membranes for instance through the development of modern membranes with enhanced permeability and selectivity is one of the most predominant concerns of the researchers specialized in the field of membranes science and technology [1–3]. One of the most interesting applications of the polymer membranes is the separation of CO2 gas which is of significant importance in natural gas sweetening [4,5], in the food packaging industries [6] and industrial waste gas streams in view of global warming prevention [7]. One of the most important and practical methods for improving the performance of polymer membranes used in the gas separation especially in the CO2 separation in gas streams is the incorporation of inorganic materials like silica particles into the polymer membrane [8–16]. The presence of silica particles in the polymer matrix with a desirable distribution leads to the enhancement of the mechanical strengths as well as thermal stability of the polymer. Controlling the morphology and the phase separation phenomenon is a key factor to achieve a
∗ Corresponding author. Tel.: +98 21 82883339; fax: +98 21 88005040. E-mail addresses:
[email protected] (M. Sadeghi),
[email protected] (M.A. Semsarzadeh). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.12.073
homogeneous structure in the composite membranes. The required homogeneity is most often provided by the physical connectivity between the organic and inorganic phases for instance through the employment of some compatibilizers. The addition of silica particles into the polymer matrix may bring about some possible effects on the polymer–silica composite as follows: • The presence of silica particles can serve to induce an increase in the mean distance between polymer chains. The alterations made to the polymer structure by limiting the chain packing, leads to an increased permeability and also may result in the enhancement of permselectivity (the ratio of permeablities of pure gases) provided that it causes an increase in the polymer structure stiffness due to the restricted segmental motion which in comparison with the solubility factor, plays a more important role in the modification of the performance of the polymer [8–10]. • The interactions between the extra OH groups of the inorganic materials and gases containing polar bonds such as CO2 and SO2 , as well as the consequent morphological changes, results in the enhancement of solubility due to the increase of the Henry’s law coefficient [11]. • The weak interaction between the silica and polymer leads to the formation of some voids at the interface of the two phases (polymer and silica) which in turn results in a major boost to the
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gas permeability [26]. By controlling the interaction between the silica and polymer, the amount of the voids at the interface can be controlled as well. • The presence of silica particles brings about some sort of physical barriers in the polymer membrane which changes the gas permeation pathway to an indirect path, hence resulting in decreased polymer permeability [16]. Kusakabe et al. studied the effect of silica particles on a polyimide type polymer [10]. They showed that the incorporation of silica in the polymer matrix results in a 10 times increase in the permeability of CO2 . Joly et al. investigated the effect of silica on another type of polyimide [11]. They observed 1.5–2 times increase in the gas sorption and 50% increase in the gas permeability as a result of incorporating silica particles in polyimide membranes. Observation of the morphological changes in the polyimide–silica composite membrane confirmed a significant reduction in the polymer chain packing density at the interface between the two phases that causes an increase in the Henry’s law coefficient and the Langmuir sorption constants in the composite membrane [11]. Moaddeb and Koros studied the gas transportation properties of thin polyimide membranes in the presence of silica particles [9]. The presence of silica improved the gas separation properties of the polyimide, particularly for O2 and N2 . Cornelius et al. studied the effects of alkoxysilanes on the morphology and gas permeation properties in a group of polyimide–silica nanocomposite membranes [17–20]. After annealing, an increase of about 200–500% in the gas permeation of the nanocomposite membranes was detected, while the permselectivity dropped slightly. The authors attributed the increase in gas permeation to the changes in the free volume distribution which leads to the enhanced local segmental mobility of the chain ends. Suzuki and Yamada [21] reported the physical and gas transport properties of a 6FDA-based hyperbranched polyimide–silica nanocomposite membrane. The gas permeability coefficients of the membrane increased as the silica content increased. It was pointed out that the increased gas permeabilities were mainly attributable to the increase in the gas solubilities. Pinnau and He [22,23] and Merkel et al. [24,25] suggested that the addition and increasing the amount of nano-sized impermeable particles of commercial fumed silica to poly(4-methyl-2-pentyne) (PMP) increases the gas and vapor permeabilities. Pinnau et al. also investigated the effect of silica particles on polysulfone membranes [26]. They observed that the addition of silica significantly enhances the gas permeability of polysulfone with increasing silica content. They concluded that such a behavior results from an increase in the free volumes because of the inefficient chain packing as well as the presence of extra void volume at the interface between polymer and silica [24–26]. Kim et al. investigated the effect of the incorporation of silica particles in poly(amide-6-b-ethylene oxide) (PEBAX) [16]. By the study of the filled membrane, they realized that the presence of silica has a significant influence on the membrane morphology hence, the gas permeability and solubility increase 2–3 times in composite membranes. Kim et al. suggested that the crystalline phase of the PA is considerably reduced upon the incorporation of silica domains into the polymer matrix [16]. In our previous study we have investigated the effect of silica nano particles on the gas separation properties of ethylene vinyl acetate (EVA) [41]. The presence of silica particles was proved to increase the gas permeation of the nanocomposite membranes by a significant amount of about 200%, as well as a 60–80% increase in the permselectivity. Polybenzimidazole, represents excellent thermal, mechanical and chemical stabilities which makes it potentially a suitable choice for the gas separation purposes. However, due to the relatively high chain packing density, it exhibits some shortcomings in the gas separation applications [27–29]. Recently, a study has been conducted
in order to improve the gas separation properties of this polymer type via systematic structure architecture in polybenzimidazole. Kumbharkar et al. [29], suggested that the incorporation of hexafluoroisopropylidene and butyl groups led to amorphous polymers with slightly lowered thermal stability and decreased chain packing. PBI based on 4,4-(hexafluoroisopropylidene)bis(benzoic acid) and 5-butyl isophthalic acid exhibited 10–40 times higher permeability, while changes in selectivities for industrially important gas pairs except O2 /N2 ranged from decrease of 75% or less than that of PBI based on isophthalic acid. The O2 /N2 selectivity was almost doubled in case of PBI based on 5-butyl isophthalic acid [29]. In the present work, we have tried to enhance the gas separation properties of polybenzimidazole membranes via incorporation of silica nano particles in membrane. The silica particles were prepared by the hydrolysis of TEOS using GOTMS compatibilizer through the sol–gel method. The permeabilities of methane, nitrogen and carbon dioxide gases were measured under 20 bar pressure and at a temperature of 25 ◦ C. Eventually, the influence of the particles on the structure and separation properties of the gas was explored. 2. Theory Proposing the models to predict the gas permeability of composite membranes is one of the research aspects in mixed matrix membrane area. There are different equations to predict gas permeability behavior of composite membranes based on the permeability of composing phases in the mixed matrix membranes. Maxwell [30], using the potential theory for electrical conduction through a heterogeneous media, obtained the exact solution for the conductivity of random distributed and non-interacting homogeneous solid spheres in a continuous matrix. Maxwell equation applied to the effective permeability of a dispersion of spheres in a continuous phase with the permeabilities of Pd and Pc , respectively, is as follows [30]: Peff = Pc
Pd + 2Pc − 2(Pc − Pd ) 2(1 − ) + ˛(1 + 2) = Pc Pd + 2Pc + (Pc − Pd ) (2 + ) + ˛(1 − )
(1a)
where ˛=
pd Pc
(2)
where Pc and Pd are the permeabilities of a given penetrant in the continuous and disperse phases, respectively. It is useful to define the “reduced permeation polarizability”, ˇ, as ˇ=
P − Pc ˛−1 = d ˛+2 Pd + 2Pc
(3)
ˇ is a convenient measure of penetrant permeability difference between the two phases; it is bounded by −0.5 ≤ ˇ ≤ 1, where the lower and upper limits correspond to totally nonpermeable and to perfectly permeable filler particle (disperse phase), while, ˇ = 0, implies ˛ = 1 (equal permeability in both phases). Eq. (1a), as a function of ˇ is then reduced to: Peff = Pc
1 + 2ˇ 1 − ˇ
(1b)
Other expressions have been proposed to predict composite membrane permeability. The analogue dielectric model has been extensively studied [31]. The more important models developed for two-phase mixed matrix membranes are those of Bruggeman [32], Higuchi [33], Maxwell–Wagner–Sillar [34] and the simple power law (percolation theory model) [35]. The formulas of Higuchi [33], considered to be applicable to random dispersions of spherical par-
M. Sadeghi et al. / Journal of Membrane Science 331 (2009) 21–30 Table 1 Sample name of prepared membranes and their silica contents.
ticles, is Peff Pc
=1+
3ˇ
1 − ˇ − KH (1 − )ˇ2
23
(4)
Parameter KH in Eq. (4) is treated as an empirical constant assigned a value of 0.78 on the basis of experimental data. Just like the Maxwell model, the Higuchi model was originally suggested to predict dielectric constant of two-phase mixtures. Higuchi suggested that some factors like particle size distribution for a system of spheres would influence the effective dielectric constant. He also suggested that other phenomena such as preferential agglomeration, sedimentation and surface effects may also affect the KH value [33]. 3. Experimental 3.1. Material PBI polymer (I.V. = 1 dl/g, inherent viscosity measured in dimethylsulfoxide) was provided by Hoechst Celanese. Poly(2,2(m-phenylene)-5,5-bibenzimidazole) or polybenzimidazole is a thermally stable polymer which is typically condensed from aromatic bis-o-diamines and dicarboxylates. PBI is a thermoplastic glassy polymer with a Tg of 435 ◦ C. The measured and reported density of PBI is 1.3 g/cm3 [36,37]. Its repeat unit is shown in Fig. 1. Dimethylacetamide (DMAc) was purchased from Merck. Tetraethoxysilane (TEOS), 3-glycidyloxypropylterimethoxy silane (GOTMS), hydrochloric acid (HCl) and ethanol required for preparation of silica particles were also provided from Merck. The CO2 and N2 gases (purity 99.99) were purchased from Roham Gas Co., Tehran, Iran and also CH4 (purity 99.95) was purchased from Air Products Co. 3.2. Preparation of PBI membrane The 7 wt% PBI solution in DMAc was prepared by dissolving PBI powder in DMAc at 200 psig and 100 ◦ C in a high pressure Parr reactor for 2 h. Then, the solution was agitated for 24 h at 40 ◦ C and 20 psig to dissolve PBI in DMAc, completely. The prepared solution was filtered via a 20 m ceramic filter. Before casting the solution, the PBI solution was concentrated to 20 wt% in a rotary evaporator. The PBI films were casted by doctor blade on the clean glass sheet from the 20 wt% solution and were heated at 70 ◦ C for 2 h without any cover at the oven. Then, the prepared film remained at 100 ◦ C for 24 h for complete removal of the solvent. The thicknesses of prepared membranes measured by micrometer were 40 m. 3.3. Preparation of silica nano particles and PBI–silica composite membrane Silica nano particles were prepared via the hydrolysis of tetraethoxysilane (TEOS) in ethanol at the presence of hydrochloric acid as a catalyst. Initially, tetraethoxysilane (25 g) and a coupling agent 3-glycidyloxypropylterimethoxy silane (GOTMS) (4 g) were mixed together in dry ethanol (30 ml) at a temperature of 70 ◦ C for 1 h. Consequently, a mixture of ethanol (30 ml), deionized water
Fig. 1. Repeat unit of poly(2,2-(m-phenylene)-5,5-bibenzimidazole) (PBI).
Membrane sample
Weight fraction of silica (wt%)
PBI PBI-S2 PBI-S5 PBI-S9 PBI-S16 PBI-S20
0 2.8 5.5 9.4 16 20
(7.5 g) and hydrochloric acid (0.83 g) was added drop-wise to the reacting mixture. TEOS was hydrolyzed under continuous agitation at 80 ◦ C for 1 h leading to the formation of a clear and transparent silica-sol. The density of silica was measured to be 2.2 g/cm3 . PBI–silica nanocomposite membranes were prepared by the same method following the addition of the silica-sol in different weight fractions to the polymer solution. The volume fraction (˚f ) of silica in hybrid membranes was calculated by the following equation: ˚f =
wf /f (wp /p ) + (wf /f )
(5)
where wf and wp refer to the weight of filler and polymer, and f and p are the density of filler and polymer, respectively. Table 1 shows the amount of silica which is incorporated in hybrid PBI–silica membranes in terms of volume and weight fraction of silica in polymer. 3.4. Characterization The presence of silica particles on the PBI–silica hybrid membranes were investigated by FT-IR BioRad spectrometer (USA) in the range of 4000–500 cm−1 . X-ray diffraction patterns were recorded by monitoring the diffraction angle 2 from 5◦ to 60◦ on a Philips X’Pert (Netherlands) using Cu radiation under a voltage of 40 kV and a current of 40 mA. The morphological aspects of the produced membranes in terms of the distribution state of silica particles in the polymer matrix were investigated using scanning electron microscope (SEM) (Cam Scan MV2300, Cambridge Co., England). The SEM analyzer is from Oxford Instruments Company model No. 7538. The membrane samples were fractured in liquid nitrogen in order to expose the cross sectional area and subsequently where sputtered with Au under vacuum. 3.5. Gas permeability measurement The permeability of nitrogen, methane and carbon dioxide were determined using constant pressure/variable volume method at 20 bar pressure and at 25 ◦ C [38]. The typical membrane area which located at test cell was 12.56 cm2 . The gas permeability of membranes was determined using the following equation: P=
q A(p1 − p2 )
(6)
where P is permeability expressed in Barrer (1 Barrer = 10−10 cm3 (STP) cm/(cm2 s cmHg), q the flow rate of the permeate gas passing through the membrane (cm3 (STP)/s), the membrane thickness (cm), p1 and p2 the absolute pressures of feed side and permeate side, respectively (cmHg), and A is the effective membrane area (cm2 ). The ideal selectivity (or permselectivity), ˛A/B , of membranes was calculated from pure gas permeation experiments: ˛A/B =
PA PB
(7)
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Following each test, the remaining gases from the previous experiment were removed from the permeation system by purging the stream lines with fresh gas which was considered for test. The diffusion coefficient (D) was determined by the time lag method, represented by Eq. (3): D=
2 6
(8)
where is the time lag (s), i.e. the intercept obtained from extrapolating the linear region of the P versus the time plot to the time axis, the membrane thickness (cm) and D is the diffusion coefficient (cm2 /s). The solubility coefficient (S) was then calculated as S=
P D
(9)
4. Results and discussion 4.1. Characterization of membranes Structural characterization of pure silica, PBI and hybrids were carried out using the FTIR analysis. The results are shown in Figs. 2 and 3. The most intensive peak at 1077 cm−1 representing Si O Si asymmetric stretching and Si O Si Rocking stretch peak at 453 cm−1 in pure silica are observed in the spectra of hybrids. In addition, the Si O Si symmetric stretching (vibrational mode of ring stretching) at 800 cm−1 suggested that the sol–gel reaction was successfully carried out. The broad peak at around 3500 cm−1 in silica indicated that there was a significant amount of OH groups. These non-condensed OH groups provide the connection (hydrogen bonding) between PBI and partially hydrated silica particles, resulting in the homogeneous films. For more investigation of the effect of silica nano particles on PBI, the N H stretching regions of PBI–silica hybrids are shown in Fig. 3. As shown in this figure the pure PBI shows a broad intensive peak in the range of 3400 to 3066 cm−1 and a weak peak at 3640 cm−1 . By increasing, the silica particles in hybrid membranes the intensity of the peak that appeared at 3640 cm−1 decreased significantly and also the peak that appeared for N H stretching broadened in the wider range from 3400 to 2920 cm−1 . These phenomena indicate the hydrogen bonding between N H groups in PBI repeat unit and OH groups existed in silica particles, which lead to broadening of N H absorbance peak to lower frequencies. This observation indicates
Fig. 2. The FTIR spectra of silica, PBI and PBI–silica hybrid membranes.
Fig. 3. The FTIR spectra at N–H stretching region.
a good distribution and dispersion of silica particles in the polymer matrix [16]. It should be noted that overlaps of small peak at 3640 cm−1 and OH groups in silica particles may affect the reduction of the intensity of this peak. The morphological changes caused by the presence of silica nano particles in the polymer matrix were investigated using wide angle X ray diffraction (WAXD). Fig. 4 illustrates the XRD pattern of pure PBI in comparison with PBI–silica composite samples. As can be observed in this figure, a broad peak can be identified at 2 = 22.5 in the pure sample. The presence of silica particles has not a significant influence on the crystalline structure of the polymer except for a mild reduction in the intensity of the broad peak, as confirmed by the X ray patterns. So it can be concluded that from the morphological point of view, the prepared membranes are all amorphous and no crystallinity is formed in the pure polymer and the PBI–silica composite membrane. The morphology of the prepared pure and composite membranes was investigated using scanning electron microscopy (SEM).
Fig. 4. WAXD patterns obtained for PBI and PBI–silica hybrid membranes.
M. Sadeghi et al. / Journal of Membrane Science 331 (2009) 21–30
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sivity of the gases was measured according to the time lag method using Eq. (8). The solubility (S) of the gases for each sample was then calculated according to Eq. (9) using the permeability and diffusivity values of the gases. Table 2, summarizes the diffusivity and solubility values of the studied gases for the PBI and PBI–silica composite membranes at different weight fractions of silica particles. Fig. 8 represents the permeability results of the gases in pure and composite membranes where in the latter case; the permeabilities were measured with respect to the silica content of the polymer. As can be elucidated from Fig. 8, the permeability of gases in pure PBI membrane varies in the following order: CO2 > N2 > CH4 The difference observed in the permeability results of the membranes can be explained through the solution–diffusion mechanism [39]. As mentioned earlier, PBI is an amorphous glassy polymer and as a result the diffusion is the dominant mechanism in permeation of gases through the polymer matrix in this case. Moreover, the data reported in Table 2 suggests that, the order of the diffusivity of the gases in PBI and PBI–silica nano-composite membranes is the same as the order observed in the permeability. The diffusivity is dependent on the kinetic diameter of the permeant gases through the membranes so that the diffusivity of a gas is increased upon the reduction of the kinetic diameter of the gas. So CO2 , with the smallest molecular size compared to other gases, has the highest and CH4 with the largest molecular size has the lowest permeability. Comparison of the permeability of CO2 , CH4 and N2 gases in hybrid membranes leads to the following order: CO2 > CH4 > N2
Fig. 5. SEM micrographs of cross section of hybrid membranes: (a) the membrane contains 5% silica nano particles (PBI-S5), (b) the membrane contains 20% silica nano particles (PBI-S20) and (c) the cross-section of hybrid membrane contain 16% silica (PBI-S16).
As can be seen in Fig. 5, the prepared membranes are dense and symmetric. A 50,000–100,000 magnification of the composite membranes micrographs, demonstrates a nano-scale distribution of silica particles in the polymer matrix. This confirms the desirable mixing of the silica in polymer and also a good compatibility between the two phases. For more consideration the distribution of silica nano particles have been evaluated by mapping the particles in the cross section of the membranes. As shown in Fig. 6 the silica particles are distributed homogenously in all of the membrane cross section. The SEM-EDX analysis also has been done to show the presence of silica particles in the membrane (Fig. 7). 4.2. Gas permeation The permeabilities of nitrogen, methane and carbon dioxide in PBI and PBI–silica nano-composite membranes under a pressure of 20 bar and at a temperature of 25 ◦ C were investigated. The diffu-
As can be seen in Table 2, the order of the gas permeabilities in the composite membranes is similar to the order of solubility coefficient of gases in membranes. Increasing the gas condensability, leads to the enhancement of the interactions and solubility of the gas in the polymer matrix and ultimately results in an increase in the gas solubility factor of the polymer. This confirms the dominancy of the solution mechanism in the permeation of gases through the composite membranes. So it can be concluded that the incorporation of silica nano particles into the polymer matrix, enhances the solubility of the gases in the membrane and also increases the permeation of the condensable gases in the PBI–silica composite membranes, hence, changing the dominant gas permeation mechanism from the diffusion mechanism to the solution one. As can be observed in Fig. 8, the permeability of CO2 gas through the composite PBI–silica membrane, significantly increases in comparison with the pure polymer. The permeability of CO2 increases from 0.025 Barrer in pure PBI to 0.11 Barrer in the composite membrane containing 20 wt% silica particles. The results of the permeability of the CH4 gas in pure and composite membranes indicate a slight increase in the permeability of methane from 0.005 Barrer to 0.010 Barrer. However, Nitrogen gas, exhibits an opposite trend compared to the previously mentioned results. Incorporation of silica particles in the polymer matrix, results in a significant decrease in the permeability of N2 from 0.007 Barrer in PBI to 0.002 Barrer in PBI-S20. This is due to the reduction of gas diffusivity in the polymer matrix. Investigation of the obtained results (Table 2) reveals that increasing the silica content in the membranes, leads to the reduction of the diffusivity for all of the gases applied in this study. This reduction is actually related to the restricted motion of the gas molecules in the polymer matrix and formation of pathways with more tortuosity in the polymer due to the presence of silica particles. As can be inferred from the results of the WAXD studies, the presence of silica domains in the polymer matrix, has not any noticeable influence on the crystallinity of
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M. Sadeghi et al. / Journal of Membrane Science 331 (2009) 21–30
Fig. 6. Distribution of silica nano particles in membrane cross-section obtained by SEM-mapping.
the polymer since the broad peak which may belong to the small crystallinities, does not change significantly by increasing the silica content. This implies that the silica domain is distributed in the amorphous region hence, lengthening the gas diffusion pathways in the membrane. On the other hand, the increased permeability of the condensable CO2 and CH4 gases is mainly related to the enhancement of gas solubility due to increasing of the active sites for gas solution in polymer matrix. The comparison of the solubility factors of the gases under study in PBI and PBI–silica composite membranes indicates an increase in the solubility of CO2 and CH4 gases of 17 and 15 times, respectively. The solubility factor of the Nitrogen gas shows an increase of only 2.7 times. Incorporation of silica particles in the polymer membrane increases the density of polar OH groups in the matrix, forming polar sites and cavities at the polymer–silica interface and also the morphological changes occurred at the silica–polymer interface which aids the solution of the condensable gases in the membrane. Kim et al., suggested that the increased permeability of the condensable gases in comparison with other gases is due to the morphological changes occurring at the interface between silica and polymer which ultimately results
in an enhancement in the amorphous region in the composite membrane [9]. According to the obtained diffusivity and solubility results of the gases in the PBI–silica composite membranes, it can be concluded that silica particles do not play any special role on the reduction of packing density of the polymer chains even if they are able to restrict the gas diffusion paths. However, the presence of silica domains in the polymer matrix can result in the formation of more sites for the solution of condensable gases so as to increase the solubility of the condensable gases in the membrane. Therefore, the effect of silica particles on the PBI membrane can be summarized as follows: • The presence of silica particles in the polymer matrix results in a decrease in the permeability of the gases since they restrain the diffusion of the gases through the polymer matrix due to increasing the tortuosity of the matrix. • Incorporation of the silica particles into the polymer matrix enhances the solubility of the condensable and polar gases in the polymer by increasing the number of polar OH groups and also by modifying the morphology at the interface between the two phases.
Table 2 Diffusion and solubility coefficient of the gases in PBI and PBI–silica hybrid membranes. Membrane sample
PBI PBI-S2 PBI-S5 PBI-S9 PBI-S16 PBI-S20
Diffusivity coefficient, D (×10−10 cm2 /s)
Solubility coefficient, S (×10−3 cm3 (STP)/cm3 of polymer cmHg)
CO2
N2
CH4
CO2
N2
CH4
4.90 3.60 3.10 2.50 1.73 1.30
3.05 2.10 1.50 0.80 0.34 0.25
1.83 1.10 0.69 0.48 0.30 0.23
5.06 18.80 24.20 34.00 56.10 85.10
2.31 2.10 2.00 3.13 5.88 6.20
2.73 7.09 12.30 18.50 31.30 42.20
M. Sadeghi et al. / Journal of Membrane Science 331 (2009) 21–30
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Fig. 7. The analytical point of the membranes by SEM-EDX.
Fig. 9 illustrates the variation of the selectivities in the gas pairs of CO2 /CH4 and CO2 /N2 with increasing the silica content in the polymer. The comparison of the selectivities of CO2 /CH4 indicates an increase in this quantity from 4.96 in pure polymer to 11.4 in the composite membrane containing 20 wt% silica particles. As mentioned earlier, the polar OH groups are introduced to the polymer together with the inorganic materials. Carbon dioxide, as a polar gas, has a great tendency to the OH groups and therefore is much more soluble in the composite membrane compared to the non-
polar methane gas and as a result exhibits a significant increase in its permeability value. Consequently, the selectivity of the CO2 /CH4 is slightly increased upon increasing the silica content while for CO2 /N2 , the increase in the selectivity is much more significant and increases from 3.5 in the pure polymer to 71.3 in the composite membrane containing 20 wt% silica particles. As stated before, the permeability of nitrogen through the PBI–silica nano-composite membrane is reduced upon increasing the silica content whereas that of CO2 gas demonstrates a considerable increase due to the high interaction of the gas molecules with the polar OH groups
Fig. 8. Permeability of CO2 , CH4 and N2 through PBI and PBI–silica hybrid membranes versus weight fraction of silica in polymer.
Fig. 9. Permselectivity of CO2 /CH4 and CO2 /N2 gases in PBI and PBI–silica hybrid membranes versus weight fraction of silica in polymer.
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the curves vary in following order: PBI −1.7
< PBI-S2 < −2.4
< PBI-S5 < −2.59
< PBI-S9 < −2.92
< PBI-S16 ≈ PBI-S20 < −3.2 ≈ −3.18
From the obtained results, it can be concluded that the diffusivity of the gases with bigger size molecules has been more restricted by increasing the silica nano particle content of the hybrid membranes. 4.4. Modeling of gas permeability in hybrid membranes
Fig. 10. Correlation of solubility factors to gas condensability in PBI and hybrid PBI–silica membranes.
As mentioned earlier in the theory section, various models can be utilized in order to predict the gas permeation in hybrid membranes. One of the models that is used for the prediction of the permeability in membranes wherein spherical particles are randomly distributed in a polymer matrix, is Higuchi model. In the case of PBI–silica hybrid membranes, since the silica particles are impermeable, the ˇ parameter turns to −0.5. By incorporation of ˇ parameter the Higuchi model turns to: Peff
as well as increasing the solubility of the gas in the composite membrane. 4.3. Correlation between the gas diffusivity and solubility coefficients The correlation of solubility coefficients for PBI and PBI–silica hybrid membranes has been shown in Fig. 10. As shown in this figure the solubility coefficients correlated well (R2 > 0.985) with condensability of the gases. In case of pure PBI, this correlation was weaker (R2 = 0.79). Fig. 10 also shows that the slope of ln(S) versus Tc in hybrid membranes is 3–3.6 times higher than pure PBI membrane. This observation confirms the increasing of solubility of gases in the membranes by increasing the amount of silica particles in membrane. The variations of the diffusion coefficient (ln D) of penetrants as a function of their kinetic diameter (as plotted in Fig. 11), showed a good correlation for membranes (R2 > 0.95) except PBIS16 (R2 = 0.92) and PBI-S20 (R2 = 0.93) specimens which show weaker correlation. The weaker correlation of the diffusivity coefficient for these two membranes may be due to the results obtained for nitrogen gas. Since the permeability of nitrogen in these two membranes is at the minimum level, the test procedure may contain some inaccuracies; hence the calculated diffusion coefficient would be more erroneous. Comparison and further investigation of the ln(D) versus kinetic diameter curves reveals that the slope of
Fig. 11. Correlation of diffusion coefficients to kinetic diameter of gases in PBI and hybrid PBI–silica membranes.
Pc
=1−
6˚ 4 + 2˚ − KH (1 − ˚)
(10)
As shown by Higuchi and other similar models, incorporation of impermeable silica particles in a polymer membrane reduces the permeability of gases compared to the pure polymer. However, the experimental results show higher permeabilities in the case of CO2 and CH4 gases upon incorporation of silica nano particles as a result of the changes occurring in the polymer–silica interface and also due to the enhancement of the gas solubility. The results obtained by other researchers also verifies the same trend, i.e. an increase in the gas permeability in hybrid membranes due to the morphology changes occured at the polymer–silica interfaces [16,26]. While the permeability of nitrogen is reduced upon increasing the silica content, the Higuchi model was employed for modeling the permeability behavior of this gas in hybrid membranes. By incorporation of nitrogen permeability instead of Pc , Eq. (10) turns to: Peff = 0.00705 −
0.0423˚ 4 + 2˚ − KH (1 − ˚)
(11)
As mentioned earlier, the term KH in Eq. (11) is considered as an empirical constant with an assigned value of 0.78 on the basis of experimental data [33]. There is no significant correlation between model and experimental results supposing KH = 0.78 in the model. However, using a least square method, the model fitted very well to the experimental data and the best value for KH was calculated. The obtained results suggest the values of 3.8 and 0.97 for KH and R2 respectively in order to make an acceptable correlation between the experimental data and Higuchi model. Fig. 12 shows the correlation of Higuchi model to the experimental data based on KH = 3.8.
Fig. 12. Correlation of nitrogen permeability coefficient to Higuchi model by considering new constant (KH = 3.8).
M. Sadeghi et al. / Journal of Membrane Science 331 (2009) 21–30
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results in a 4.5 times increase in permeability of CO2 and 20 times increase in selectivity of CO2 /N2 . The solubility coefficients correlated well with condensability of gases and the slope of ln(S) versus Tc in hybrid membranes were 3–3.6 times higher than pure PBI membrane. The diffusion coefficient (ln D) of penetrants showed a good correlation to their kinetic diameter. The higher value of slope of ln(D) versus kinetic diameter obtained in higher silica content hybrid membranes. The Higuchi model showed good correlation to experimental results for the permeability of nitrogen gas in nanocomposite membranes, by using the KH = 3.8. References Fig. 13. CO2 /N2 separation performance of PBI and PBI–silica hybrid membranes in comparison to Robeson’s upper line.
Such an increase in the value of KH is related to the nano size distribution of silica particles in hybrid membranes. By decreasing the size of particles to the nano scale, the effect of impermeability of silica particles is more highlighted, due to the enhancement of the specific surface area of the particles. Thus the KH constant which is closely related to the amount of permeability, is increased. As a result, it can be concluded that the Higuchi model can make a more realistic prediction about the permeability of gases in nanocomposite polymer–silica membranes, if the KH constant in this model is increased to higher values between 3 and 4. The new general value of KH for these types of hybrid membranes could be extracted by fitting more experimental data to the model. The completion of this model would be pursued in future works. 4.5. Evaluation of gas permeation performance Fig. 13 compares the relationship between the permeability of Carbon dioxide and the selectivity of CO2 /N2 with Robson’s upper line [40]. As can be clearly seen, the incorporation of silica in PBI polymer increases the gas separation ability of hybrid membranes. In addition, it could be observed that the PBI–silica composite membrane offers desirable gas separation characteristics in comparison to the pure PBI membrane. The performance of the pure PBI membrane in the separation of CO2 gas, is located under the desirable performance line of the membranes; however, as can be seen, incorporation of silica particles, significantly improves the performance of this membrane type and sets its position in the efficient and desirable area. According the above-mentioned results, incorporation of silica particles into the PBI–silica polymer, significantly increases the efficiency of this membrane type in the separation of CO2 gas respect to N2 . This, along with the unique physical, mechanical and chemical properties of PBI, makes it an appropriate choice as far as the industrial applications are concerned. 5. Conclusion In this study, the effect of incorporation of silica particles on the permeability of CO2 , CH4 and N2 in polybenzimidazole (PBI) membranes has been investigated. The results of the gas permeability experiments indicated an increase in the permeability of CO2 and CH4 and a decrease in the permeability of N2 upon increasing the silica content of the polymer membrane. Furthermore, the obtained results revealed the fact that incorporation of silica particles in the polymer matrix, i.e. increasing the number of polar OH groups as well as morphological changes induced at the silica–PBI interface, leads to the enhancement of the gas solubility in such membranes, while the diffusivity of the gases in the nano-composite membrane is reduced due to the obstructions made by the particles. According to the results of the permeability and selectivity experiments, a 20 wt% increase in the silica content of the polymer membrane
[1] P. Pandey, R.S. Chauhan, Membranes for gas separation, Prog. Polym. Sci. 26 (2001) 853. [2] S.A. Stern, Polymers for gas separations: the next decade, J. Membr. Sci. 94 (1994) 1. [3] W.J. Koros, G.K. Fleming, Membrane-based gas separation, J. Membr. Sci. 83 (1993) 1. [4] W.J. Schell, Commercial applications for gas permeation membrane systems, J. Membr. Sci. 22 (1985) 217. [5] R. Rautenbach, K. Welsch, Treatment of landfill gas by gas permeation—pilot plant results and comparison to alternatives, Desalination 90 (1993) 193. [6] M.L. Rooney, Active Food Packaging, 1st ed., Chapman & Hall, London, 1995, pp. 111–135. [7] T. Suda, M. Fujii, K. Yoshida, M. Iijima, T. Seto, S. Mitsuoka, Development of flue gas carbon dioxide recovery technology, Energy Convers. Manage. 33 (1992) 317. [8] M. Moaddeb, W.J. Koros, Effects of colloidal silica incorporation on oxygen/nitrogen separation properties of ceramic-supported 6FDA-IPDA thin films, J. Membr. Sci. 111 (1996) 283. [9] M. Moaddeb, W.J. Koros, Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles, J. Membr. Sci. 125 (1997) 143–163. [10] K. Kusakabe, K. Ichiki, J. Hayashi, H. Maeda, S. Morooka, Preparation and characterization of silica–polyimide composite membranes coated on porous tubes for CO2 separation, J. Membr. Sci. 115 (1996) 65. [11] C. Joly, S. Goizet, J.C. Schrotter, J. Sanchez, M. Escoubes, Sol–gel polyimide–silica composite membrane: gas transport properties, J. Membr. Sci. 130 (1997) 63. [12] C. Joly, M. Smaihi, L. Porcar, R.D. Noble, Polyimide–silica composite materials: how does silica influence their microstructure and gas permeation properties? Chem. Mater. 11 (1999) 2331. [13] S.P. Nunes, K.V. Peinemann, K. Ohlrogge, A. Alpers, M. Keller, A.T.N. Pires, Membranes of poly(ether imide) and nanodispersed silica, J. Membr. Sci. 157 (1999) 219. [14] M. Smaihi, J.C. Schrotter, C. Lesimple, I. Prevost, C. Guizard, Gas separation properties of hybrid imide–siloxane copolymers with various silica contents, J. Membr. Sci. 161 (1999) 157. [15] Q. Hu, E. Marand, S. Dhingra, D. Fritsch, J. Wen, G. Wilkes, Poly(amideimide)/TiO2 nano-composite gas separation membranes; fabrication and characterization, J. Membr. Sci. 135 (1997) 65. [16] J.H. Kim, Y.M. Lee, Gas permeation properties of poly(amide-6-bethyleneoxide)–silica hybrid membranes, J. Membr. Sci. 193 (2001) 209. [17] C. Hibshman, C.J. Cornelius, E. Marand, The gas separation effects of annealing polyimide-organosilicate hybrid membranes, J. Membr. Sci. 211 (2003) 25. [18] C.J. Cornelius, C. Hibshman, E. Marand, Hybrid organic–inorganic membranes, Sep. Purif. Technol. 25 (2001) 181. [19] C.J. Cornelius, E. Marand, Hybrid silica–polyimide composite membranes: gas transport properties, J. Membr. Sci. 202 (2002) 97. [20] C. Hibshman, M. Mager, E. Marand, Effects of feed pressure on fluorinated polyimide-organosilicate hybrid membranes, J. Membr. Sci. 229 (2004) 73. [21] T. Suzuki, Y. Yamada, Physical and gas transport properties of novel hyperbranched polyimide–silica hybrid membranes, Polym. Bull. 53 (2005) 139. [22] I. Pinnau, Z. He, Filled superglassy membrane, U.S. Patent 6,316,684 (2001). [23] Z. He, I. Pinnau, A. Morisato, Nanostructured poly(4-methy-2-pentyne)-silica hybrid membranes for gas separation, Desalination 146 (2002) 11. [24] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, A.J. Hill, Ultrapermeable, reverse-selective nanocomposite membranes, Science 296 (2002) 519. [25] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, A.J. Hill, Sorption, transport and structural evidence for enhanced free volume in poly(4methyl-2-pentyne)/fumed silica nanocomposite membranes, Chem. Mater. 15 (2003) 109. [26] J. Ahn, W.J. Chung, I. Pinnau, M.D. Guiver, Polysulfone-silica nanoparticle mixed matrix membranes for gas separation, J. Membr. Sci. 314 (2008) 123. [27] D.R. Pesiri, B. Jorgensen, R.C. Dye, Thermal optimization of polybenzimidazole meniscus membranes for the separation of hydrogen, methane, and carbon dioxide, J. Membr. Sci. 218 (2003) 11. [28] J.R. Klaehn, T.A. Luther, C.J. Orme, M.G. Jones, A.K. Wertsching, E.S. Peterson, New soluble N-substituted polybenzimidazoles by postpolymerization modification, Polym. Preprints 46 (2005) 708.
30
M. Sadeghi et al. / Journal of Membrane Science 331 (2009) 21–30
[29] S.C. Kumbharkar, P.B. Karadkar, U.K. Kharul, Enhancement of gas permeation properties of polybenzimidazoles by systematic structure architecture, J. Membr. Sci. 286 (2006) 161. [30] C. Maxwell, Treatise on Electricity and Magnetism, vol. 1, Oxford University Press, London, 1973. [31] E.E. Gonzo, M.L. Parentis, J.C. Gottifredi, Estimating models for predicting effective permeability of mixed matrix membranes, J. Membr. Sci. 277 (2006) 46. [32] G. Banhegy, Comparison of electrical mixture rules for composites, Colloid Polym. Sci. 264 (1986) 1030. [33] W.I. Higuchi, A new relationship for the dielectric properties of two phase mixtures, J. Phys. Chem. 62 (1958) 649. [34] R.H.B. Buma, A. Checchetti, G. Chidichimo, E. Drioli, Permeation through a heterogeneous membrane: the effect of the dispersed phase, J. Membr. Sci. 128 (1997) 141. [35] G. Chen, W. Weng, D. Wu, C. Wu, PMMA/graphite nanosheets composite and its conducting properties, Eur. Polym. J. 39 (2003) 2329.
[36] D. Mecerreyes, H. Grande, O. Miguel, E. Ochoteco, R. Marcilla, I. Cantero, Porous polybenzimidazole membranes doped with phosphoric acid: highly protonconducting solid electrolytes, Chem. Mater. 16 (2004) 604. [37] E. Foldes, E. Fekete, F.E. Karasz, B. Pukanszky, Interaction, miscibility and phase inversion in PBI/PI blends, Polymer 41 (2000) 975. [38] M.A. Semsarzadeh, M. Sadeghi, M. Barikani, H. Moadel, The effect of hard segments on the gas separation properties of polyurethane membranes, Iran. Polym. J. 16 (2007) 819. [39] R. Kesting, A.K. Fritzsche, Polymeric Gas Separation Membranes, John Wiley & Sons, New York, 1993. [40] L.M. Robeson, Correlation of separation factor versus permeability of polymeric membranes, J. Membr. Sci. 62 (1991) 165. [41] M. Sadeghi, G. Khanbabaei, A.H.S. Dehaghani, M. Sadeghi, M.A. Aravand, M. Akbarzade, S. Khatti, Gas permeation properties of ethylene vinyl acetate–silica nanocomposite membranes, J. Membr. Sci. 322 (2008) 423.
