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Beta-cyclodextrin functionalized MWCNT: A potential nano-membrane material for mixed matrix gas separation membranes development
Accepted Manuscript Beta-cyclodextrin functionalized MWCNT: A potential nano membrane mate‐ rial for mixed matrix gas separation membranes development M.A. Aroon, A.F. Ismail, T. Matsuura PII: DOI: Reference:
S1383-5866(13)00249-9 http://dx.doi.org/10.1016/j.seppur.2013.04.025 SEPPUR 11161
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
3 August 2012 15 March 2013 18 April 2013
Please cite this article as: M.A. Aroon, A.F. Ismail, T. Matsuura, Beta-cyclodextrin functionalized MWCNT: A potential nano membrane material for mixed matrix gas separation membranes development, Separation and Purification Technology (2013), doi: http://dx.doi.org/10.1016/j.seppur.2013.04.025
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ACCEPTED MANUSCRIPT Beta-cyclodextrin functionalized MWCNT: A potential nano membrane material for mixed matrix gas separation membranes development
M. A. Aroona*, A.F. Ismailb, T. Matsuurac
a
Membrane Research Laboratory, School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran, Tel: +981824652604, Fax: +981824652600, Email: [email protected] b
Advanced Membrane Technology Center, Universiti Teknologi Malaysia , 81310 Skudai, Johor, Malaysia c
Industrial Membrane Research Laboratory, Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
Abstract
In this paper, synthesized (raw) multi-walled carbon nanotubes (rMWCNTs) were functionalized with beta-cyclodextrin (bC) using Chen’s soft cutting technique. Raw multiwalled carbon nanotubes (rMWCNTs) and beta-cyclodextrin functionalized multi-walled carbon nanotubes (bCfMWCNTs) were characterized by transmission electron microscope (TEM). Neat polyimide (PI) and polyimide/bCfMWCNTs mixed matrix membranes were fabricated by immersion precipitation method. Pure methane and carbon dioxide were used as
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ACCEPTED MANUSCRIPT membranes and MWCNTs were characterized by differential scanning calorimetery (DSC) and thermogravimetric analysis (TGA), respectively. The TEM results showed that betacyclodextrin can not only attach to the inner and outer surface of the MWCNT walls but also fill the open-ended tips of the MWCNTs. TGA thermograms showed that bCfMWCNTs are thermally less stable than rMWCNTs due to the decomposition of beta-cyclodextrin layer, which occurred before the decomposition of rMWCNTs. According to the DSC experiment, the glass transition temperature (Tg) increased as bCfMWCNTs increases in the polymer matrix. The increase in glass transition was attributed to good dispersion of the bCfMWCNTs in the polymer matrix as a result of improved attachment of bCfMWCNTs to the polymer segments. Separation properties and morphology (and distribution of bCfMWCNTs within the polymer matrix) of the fabricated membranes were analyzed by using permeation test and cross sectional FESEM micrographs respectively. Permeation test results revealed that the ideal CO2/CH4 selectivity increased considerably from neat polyimide membrane to polyimide/bCfMWCNTs mixed matrix membranes. Cross sectional FESEM results showed that well dispersed bCfMWCNTs within the polymer matrix are observable especially in case of PI/6 wt.% BFBM MMM.
Key words: MWCNTs; Cyclodextrin; Functionalization; MMMs; Gas Separation Membranes
1. Introduction
Because inorganic membranes are difficult to process, expensive to fabricate, easily plagued and fragile, membrane researchers usually prefer to use these nano-materials as fillers embedded in the polymer matrix and fabricate mixed matrix membranes [1-3]. In mixed matrix membranes (MMMs) the superior gas separation properties of inorganic particles are
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ACCEPTED MANUSCRIPT combined with flexibility and processability of polymeric membranes and one can fabricate a low price high performance membrane [1-3]. As reviewed in Table 1, various inorganic particles including zeolites, carbon molecular sieves (CMS), activated carbons (AC), silica, silicon dioxide, metal organic frameworks (MOF) and carbon nanotubes (CNTs) have been used by researchers to fabricate the MMMs. Each MMM has its own advantages and disadvantages.
Table 1: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected MMMs
As shown in Table 1, among different inorganic particles, zeolites and carbon molecular sieves are the most frequently used fillers for the preparation of dense MMMs. In contrast, metal organic frameworks and carbon nanotubes (CNTs) have been used only rarely. Especially, CNTs have emerged as a new potential nano-membrane material (NMM) for gas separation applications due to their very smooth internal surfaces, their nano-sized structure and also their high surface area [24-29]. In our recent work, we have shown that raw MWCNTs, when embedded in polyimide, act as a barrier in the polymer matrix and increase the CO2 /CH4 selectivity [30]. Similar to other inorganic fillers, although carbon nanotubes can improve separation and mechanical properties of polymeric membranes, fabrication of defect free MMMs is difficult due to their weak interaction with organic polymer phase. To overcome this problem, the particle surface is usually modified by using the silane coupling agents (e.g. TMS, TPS, APTES, TMCS), priming by polymeric materials (e.g. PDMC and the other matrix polymers) or functionalizing by soft organic materials (chitosan, cyclodextrin, surfactants) [28-29].
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ACCEPTED MANUSCRIPT Regarding CNTs, their tendency to agglomerate is another very serious problem. Furthermore, the pristine CNTs are normally impure [31-32], generally long and closed ended [30] and their dispersion in polymer matrix is difficult [33-34]. Functionalization of CNTs by covalent or non-covalent treatment is an effective way to improve their dispersion inside the polymer matrix since functionalization increases the interfacial interactions between CNTs and polymer chains, thereby reducing their agglomeration inside the polymer matrix [35]. For example, MWCNTs were functionalized by chitosan, a biodegradable and hydrophilic low molecular weight polymer from a linear polysaccharide family, by applying Chen’s soft cutting technique [36].They were found to be well dispersed in the polymer matrix. Permeation tests also revealed that adding them into polyimide dope could increase CO2 and CH4 permeabilities [20]. In this paper, as an extension of our earlier works, raw MWCNTs were functionalized with beta-cyclodextrin, a bottomless bowl shaped macro-rings from a cyclic oligosaccharides family [37-39], by applying Chen’s soft cutting technique [36]. Both non-functionalized and functionalized MWCNTs were characterized by transmission electron microscopy (TEM) and thermogravimetric analysis. Neat polyimide and asymmetric PI/bCfMWCNTs MMMs were fabricated by the phase inversion technique using immersion precipitation method and were characterized by gas permeation and differential scanning calorimetery (DSC). It is necessary to note that, in contrast to the MMMs most commonly fabricated in dense film form (see 3rd column in Table 1), MMMs were fabricated in asymmetric flat sheet form in this research, viewing their commercial applications.
2. Experimental
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ACCEPTED MANUSCRIPT 2.1. Materials
Ethanol (EtOH) and tetrahydrofuran (THF) were purchased from Merck and used as dope additives. Ethanol (EtOH) was also used as the diluent in MWCNTs functionalization. 1-Methyl-2-pyrrolidone (NMP) was supplied by Merck and used as the solvent in polymer dope preparation. Beta-cyclodextrin (bC) was purchased from Sigma-Aldrich and used as the functionalization agent. Multi-walled carbon nanotubes (MWCNTs) were synthesized at AMTEC (Malaysia) by the CCVD method as reported elsewhere [40-41]. Tap water was used as coagulant in the asymmetric membrane fabrication. Polyimide (PI) was purchased from Alfa-Aesar; A Johnson Matthey Company [20, 30] and used as the polymer matrix. Table 2 shows polyimide properties.
Table 2: Polyimide properties
2.2. Functionalization of multi-walled carbon nanotubes
Synthesized raw MWCNTs (rMWCNTs) were functionalized using beta-cyclodextrin as soft cutting agents or functionalization agents by applying Chen’s soft cutting technique [36]. Raw MWCNTs and cyclodextrin were dried in an oven at 80 ◦C overnight to remove any adsorbed water. Then, 30 g of cyclodextrin and 1 g of rMWCNTs were mixed and ground in a mortar and pestle system for 10 min. During the 10 minutes of grinding, ethanol was added gradually to form a sticky and grayish mixture. This sticky mixture was further ground without addition of ethanol for 2 h to obtain a homogeneous black powder (bCfMWCNTs/bC
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ACCEPTED MANUSCRIPT mixture; BFBM), which was dried in an oven at 80 ◦C overnight. It should be noted that BFBM contains both unreacted bC and functionalized MWCNTs.
2.3. Characterization of raw MWCNTs and functionalized MWCNTs
Raw MWCNTs (rMWCNTs) and bCfMWCNTs were observed by a transmission electron microscope (TEM: JEOL JEM-2100). The method of TEM sample preparation is as follows. A small amount of rMWCNTs was added to chloroform and sonicated for 15 minutes in an ultrasonic bath (Delta 1100: DC150H). The rMWCNTs particles were then picked up from the dispersion onto the sample stub surface for observation by TEM. Similarly, BFBM was dispersed in chloroform. Beta-cyclodextrin in BFBM was dissolved to chloroform in the dispersion process, leaving bCfMWCNTs as solid particles. The bCfMWCNTs particles were then picked up onto the sample stub surface for TEM observation. Thermogravimetric analysis (TGA: METTLER TOLEDO SDTA 851e) was used to measure the rate of weight change of beta-cyclodextrin and bCfMWCNTs.
2.4. Dope preparation and fabrication of membrane samples
The casting dopes for MMMs fabrication were prepared by the solution-blending technique [20] as follows. A predetermined amount of the BFBM was added to the solvent and mixed by a stirrer for 2 h. Beta-cyclodextrin (bC) in BFBM was dissolved in the solvent and a homogeneous suspension of the bCfMWCNTs particles in the solvent was obtained. Polyimide (PI) was then added gradually to the solvent/bC/bCfMWCNTs suspension, followed by stirring for more 24 h. The resulting black viscose mixture (solvent/bC/PI
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ACCEPTED MANUSCRIPT homogeneous mixture containing bCfMWCNTs inorganic nano-particles) was degassed by using an ultrasonic bath (Branson Ultrasonic Corporation; 3510 E-MTH) and cast immediately to prevent the bCfMWCNTs settling. The dope compositions are listed in Table 3. Table 3: The dope composition for the fabrication of PI and PI/bCfMWCNTs membranes
It should be noted that the membranes are coded as PI/x wt. % BFBM mixed matrix membrane where x represents the percent of BFBM in BFBM and PI mixture (see 3rd column in Table 3).
Flat sheet asymmetric membranes (neat PI and PI/BFBM) were fabricated by the wet phase inversion method. The details of the fabrication procedure can be found elsewhere [42].
2.5. Membrane samples characterization
Neat polyimide (PI) membranes and PI/BFBM mixed matrix membranes were characterized by gas permeation tests using pure CO2 and pure CH4 as test gases at 15 bar gauge and 25 ◦C. A constant pressure permeation cell with a circular flat plate was used. The effective membrane area was 13.5 cm2. It is necessary to note that the membrane samples were coated with a layer of silicon rubber before permeation test. The silicon rubber coating procedure has been reported in details elsewhere [30].
Equation 1 was used to calculate the permeance (Pri= Pi/l) of gas i.
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ACCEPTED MANUSCRIPT Pri (GPU)=106×[Qi (cm3 (STP)]/[A(cm2)×Δp(cmHg)]
(1)
where Qi is the measured volumetric flow rate of gas i (at standard temperature and pressure), A is the effective membrane area and Δp is the pressure difference across the membrane. The ideal separation factor or ideal selectivity between gas i and j was calculated by Eq. 2.
Ideal Selectivity=αij=(Pri/Prj)
(2)
A differential scanning calorimeter (DSC: METTLER TOLEDO DSC 822e) was used to measure the glass transition temperature (Tg) of the membranes. Morphology of membrane samples and also distribution of bCfMWCNTs inside the polymer matrix of the MMMs were studied using high magnification field emission scanning electron microscope (FESEM Model: Hitachi S4160). The sample preparation procedure for FESEM analysis has been described elsewhere [20].
3. Results and discussions
3.1. MWCNTs properties and TEM Results
TEM micrographs of the raw MWCNTs (rMWCNTs) reported in our recent work [30] is reproduced in Fig. 1. From the figure, the average inner and the outer diameters were about 3.5 nm and 15nm, respectively [30]. In addition, rMWCNTs were long and closed ended. Impurities and surface defects were detected inside the carbon nanotubes and on their outer surface [Fig.1].
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ACCEPTED MANUSCRIPT
Figure 1: TEM micrographs of the long, closed ended and defective rMWCNTs; impurities and closed tips of rMWCNTs are shown by arrows [20, 30]
Beta-cyclodextrin functionalized MWCNTs (bCfMWCNTs) were observed by transmission electron microscope (TEM). As shown in Fig. 2, cyclodextrin is attached to the outer surface of the MWCNT walls and wrapped around the axis of the MWCNTs.
Figure 2: TEM micrographs of the bCfMWCNTs
Figure 3 compares the structure of the MWCNT before (Fig. 3a) and after functionalization (Fig. 3b). As shown in this figure parallel alignment observed at the walls, both inside and outside, of the raw MWCNT has become irregular after functionalization with betacyclodextrin. This finding is in accordance with Raman spectroscopy analysis of the beta cyclodextrin functionalized MWCNTs by Sanip et al [21].
Figure 3: TEM micrographs of a) the rMWCNTs and b) the bCfMWCNTs (arrows show irregular alignment at walls of the bCfMWCNTs)
Figure 4 show that the bCfMWCNTs are open-ended. It is likely that during the soft cutting and functionalization procedures, the MWCNTs are cut to become open-ended [20, 36]. It is interesting to note that not only the outer surfaces of the MWCNTs are covered by cyclodextrin (see Fig. 4a) but also their open-ended tips are filled with beta-cyclodextrin molecules (see Fig. 4b). This can be attributed to the molecular size of the beta-cyclodextrin,
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ACCEPTED MANUSCRIPT i.e. the outer diameter of the beta-cyclodextrin (1.66 nm) [37], which is less than the average inner diameter of the synthesized rMWCNTs (nearly 3.5 nm) [30].
Figure 4: TEM images of a) open-ended tips and outer surface of the bCfMWCNTs and b) outer surface and open-ended tips of the bCfMWCNTs (arrows show open-ended tips of the bCfMWCNTs are filled by betacyclodextrin)
Figure 5d (high magnification TEM micrograph of the bCfMWCNTs) shows that the distance between MWCNTs has increased. As shown in Fig. 5e (low magnification TEM micrograph of the bCfMWCNTs) even at a far lower magnification this distance increase between the MWCNTs is observable and one can conclude that bCfMWCNTs on the TEM stub surface are shortened and less agglomerated than rMWCNTs (see Figs. 5a-5c which were selected from different points on the stub sample).
Figure 5: a-c) high magnification TEM micrographs of the generally endless and agglomerated rMWCNTs (images 5a-5c were selected from different points on the stub sample), d) high magnification TEM micrograph of the relatively well dispersed bCfMWCNTs and e) low magnification TEM micrograph of the relatively well dispersed bCfMWCNTs (images 5d-5e were also selected from different points on the stub sample)
As shown in Fig. 6, the bCfMWCNTs (Figs. 6d-6f) are shorter than the rMWCNTs (Figs. 6a6c). It is necessary to note that TEM Micrographs were selected from different points on the TEM stub sample,
Figure 6: a-c) TEM micrographs of the long, generally endless and impure rMWCNTs, d-f) TEM micrographs of the open-ended and relatively shortened bCfMWCNTs (images 6a-6c and 6d-6f were selected from different points on the stub sample)
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ACCEPTED MANUSCRIPT
3.2. TGA Results
Figures 7, 8 and 9 show the TGA thermograms of rMWCNTs, beta-cyclodextrin and bCfMWCNTs, respectively. According to Fig. 7, the largest weight loss of rMWCNTs occurs between 400-640◦C, while Figs. 8 and 9 show that the largest weight loss of beta-cyclodextrin and bCfMWCNTs occurs at around 330 ◦C, due to the decomposition of beta-cyclodextrin, which occurs before the decomposition of rMWCNTs. Figure 7: rMWCNTs TGA thermogram
Figure 8: Pure beta-cyclodextrin powder TGA thermogram
Figure 9: BFBM TGA thermogram
3.3. DSC Results
Figure 10 shows the effect of BFBM content on the glass transition temperature of the membranes. Tg increased as the BFBM content increased. Considering Figs. 2-4, it is likely that the bC layer wrapped around the bCfMWCNTs increases polymer/bCfMWCNTs interactions, and the segmental mobility of polymer chains decreases. As a result, one can expect an increase in the membrane’s glass transition temperature. However, more than one glass transition temperature were not observed, which implies that decrease in chain mobility extended beyond the bCfMWCNTs/polymer interphase and reached deep into the polymer matrix. This can be attributed to good dispersion of the bCfMWCNTs in the polymer
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ACCEPTED MANUSCRIPT continuous phase. In other words, strong attachment of the well dispersed bCfMWCNTs in the polymer matrix decreases polymer chain mobility and results in an increase in the glass transition temperature [3, 20, 43]. These results have been proved by gas permeation test in section 3.4.
Figure 10: Glass transition temperature of the membranes
3.4. Gas Permeation Test Results
Figure 11 shows the effect of BFBM content on CO2permeance. It is clear that the permeance decreases as BFBM content increases.
Figure 11: Gas permeation results of neat PI and PI/BFBM mixed matrix membranes
This effect can be attributed to the beta-cyclodextrin functionalized MWCNTs (as inorganic dispersed phase embedded inside the polymer matrix) and the unreacted betacyclodextrin (as a soluble additive), both present in the MMMs. Regarding the bCfMWCNTs, as was observed in TEM micrographs earlier, beta-cyclodextrin molecules have closed the MWCNTs entrance, which probably hindered gas molecules from entering into the MWCNTs inner surface. It should be noted that some rMWCNTs were left without functionalization, and thus maintained their original length and remained also closed ended. As well, the ends of MWCNTs may have been plugged not necessarily by beta cyclodextrin but by CCVD catalysts, impurities or by polymer chains. Therefore, it is likely that the bCfMWCNTs acted as impermeable inorganic particles, around which the gas molecules were forced to flow, thus enhancing the length of the tortuous diffusion pathway [30, 44-45]. With an increase in the
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ACCEPTED MANUSCRIPT tortuous pattern of polymer matrix, the diffusion coefficient of penetrant molecules decreases and the penetrant permeability also decreases. It is necessary to note that increase in BFBM content leads to a decrease in permeability due to increase in matrix tortuosity (see Fig. 11). Regarding the presence of beta-cyclodextrin and its effect on the membrane separation properties, one may consider the following interpretation. Since beta-cyclodextrin is soluble in solvent (NMP), addition of BFBM increases the solid soluble contents (e.g. PI and bC) in the casting dope, which results in the formation of a thicker and denser skin layer, reducing the permeance of both CO2 and CH4 [42]. Figure 12 shows the CO2/CH4 ideal selectivity of fabricated membranes.
Figure 12: CO2/CH4 ideal Selectivity of neat PI and PI/BFBM mixed matrix membranes
Figure 12 shows the effect of BFBM content on the ideal CO2/CH4 selectivity. The selectivity increased from neat PI membrane to PI/2 wt. % BFBM MMM by 61.1 % (from 24.12 to 38.88).The selectivity further increased by 61.7 % (from 38.88 to 62.86) from PI/2 wt. % BFBM to PI/6 wt. % BFBM MMM. The ideal selectivity increase may be interpreted as follows: 1- As a result of the increase in polymer diffusion pathway (due to the presence of bCfMWCNTs) the difference in the diffusivity of the larger penetrant molecule (i.e. CH4) and the smaller molecule (i.e. CO2) is enhanced and as a result CO2/CH4 selectivity increases. It is obvious that the increase in BFBM content results in an increase in diffusion pathway length and so one can expect that CO2/CH4 selectivity increases as the BFBM content increases (as shown in Fig. 12).
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ACCEPTED MANUSCRIPT 2- Since the mobility of the polymer chains is reduced by the addition of bCfMWCNTs, the diffusivity difference between larger (CH4) and smaller (CO2) gas molecules increases and a higher CO2/CH4 selectivity is expected. 3- Beta-cyclodextrin increases the coalescence of polymer chains in the skin layer and as a result, a less defective and more selective skin layer can be formed. 4- Increasing the beta-cyclodextrin concentration (which occurs with increasing BFBM content) causes a thicker and denser skin layer to be formed and a more selective membrane is produced.
Table 4 compare separation properties of the mixed matrix polyimide membranes fabricated in this research with the some selected mixed matrix polyimide (or co-polyimide) membranes that contain inorganic fillers other than bCfMWCNTs. Table 4: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected mixed matrix polyimide membranes in comparison with the fabricated MMMs in this research work
As shown in Table 4, the order of magnitude of CO2 permeance of the membranes fabricated in this work is in accordance with the other works. Moreover, CO2/CH4 selectivity of MMMs in this work is generally higher than polyimide MMMs containing other inorganic fillers except for H-PI containing TMOS-m-silica. It should be noted that the intrinsic selectivity of the H-PI is very high, even without addition of the fillers. It is also necessary to note that addition of BFBM increased the CO2/CH4selectivity by 26.8 % for each 1 wt.% increase of BFBM while addition of TMOS-m-silica into H-PI matrix could increase CO2/CH4 selectivity by 7.24 % for each 1 wt. % increase of TMOS-m-silica.
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ACCEPTED MANUSCRIPT 3.5. FESEM Results
Cross sectional FESEM micrographs of the PI/BFBM mixed matrix membranes are shown in Fig. 13. As shown in Fig. 13, both PI/2wt.%BFBM and PI/6wt.%BFBM fabricated mixed matrix membranes are asymmetric containing a skin layer supported by a porous sub-layer. The sub-layer structure of both PI/2wt.%BFBM and PI/6wt.%BFBM mixed matrix membranes seems nodular near the skin layer and cellular with closed cell far from the skin layer. Figure 13: Cross sectional FESEM micrographs of a) the PI/2 wt. % BFBM MMM and b) the PI/6 wt. % BFBM MMM
Distribution of bCfMWCNTs inside the polymer matrix for PI/2wt.%BFBM and PI/6wt.%BFBM MMMs was also studied by using high magnification FESEM. Usually since a clear view of the skin layer cannot be achieved during sample preparation for FESEM analysis (using liquid nitrogen) and because MWCNTs hide beyond the dense structure of the dense layer, finding MWCNTs in the skin layer is difficult [20]. Hence, we tried to find them in the nodular (near the skin layer) or cellular (far from the skin layer) regions of the membrane cross section. Fig.14 shows the high magnification FESEM micrographs of PI/2wt.%BFBM. Figure 14: High magnification FESEM micrographs of PI/2 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
As shown in Fig. 14a, fining bCfMWCNTs within the polymer matrix in sub-layer near the skin layer is so difficult most likely because they hide beyond the nodular or dens structure of polymer in this region. Nevertheless, some well-dispersed bCfMWCNTs within the polyimide matrix in sub-layer are observable in Fig. 14b.
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ACCEPTED MANUSCRIPT Figure 15 shows high magnification FESEM images of PI/6wt.%BFBM MMM.
Figure 15: High magnification FESEM micrographs of PI/6 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
As shown in Fig. 15, well-dispersed bCfMWCNTs are distinguishable within the polymer matrix (near the skin layer, Fig. 15a, and far from the skin layer, Fig. 15b).
4. Conclusions
MWCNTs were functionalized with beta-cyclodextrin and both non-functionalized (rMWCNTs) and functionalized MWCNTs (bCfMWCNTs) were characterized by TEM and TGA. The TEM micrographs revealed that beta-cyclodextrin can not only attach to the outer surfaces of the MWCNT walls but also can fill their open-ended tips. TGA thermograms showed that bCfMWCNTs are thermally less stable than rMWCNTs. The glass transition temperature increases as BFBM loading increases in the polymer matrix and this effect was attributed to the well dispersed bCfMWCNTs in the continuous polymer phase (which was supported with high magnification FESEM micrographs) and also their strong attachment which decreases the segmental mobility of polymer chains. The CO2/CH4ideal selectivity increased as the amount of BFBM (and bCfMWCNTs as well) in the membrane increased, indicating that the permeant gas flows through the tortuous pathway of higher rigidity formed around the bCfMWCNTs that are strongly attached to the PI matrix.
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ACCEPTED MANUSCRIPT Acknowledgment
The authors gratefully acknowledge the financial support from the Iran National Science Foundation under the Grant number 16963.
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ACCEPTED MANUSCRIPT [18] S. Kim, E. Marand, High permeability nano-composite membranes based on mesoporous MCM-41 nanoparticles in a polysulfone matrix, Microporous Mesoporous Mater. 114 (2008) 129–136. [19] J. Ahn, W.-J. Chung, I. Pinnau, M.D. Guiver, Polysulfone/silica nanoparticle mixedmatrix membranes for gas separation, J. Membr. Sci. 314 (2008) 123-133. [20] M.A. Aroon, A.F. Ismail, M.M. Montazer-Rahmati, T. Matsuura, Effect of chitosan as a functionalization agent on the performance and separation properties of polyimide/multiwalled carbon nanotubes mixed matrix flat sheet membranes, J. Membr. Sci. 364 (2010) 309– 317. [21] S.M. Sanip, A.F. Ismail, P.S. Goh, T. Soga, M. Tanemura, H. Yasuhiko, Gas separation properties of functionalized carbon nanotubes mixed matrix membranes, Sep. Purif. Technol. 78(2011) 208-213. [22] A.F. Ismail, N.H. Rahim, A.Mustafa, T. Matsuura, B.C.Ng, S. Abdullah, S.A. Hashemifard, Gas separation performance of polysulfone/multi-walled carbon nanotubes mixed matrix membranes, Sep. Purif. Technol. 80 (2011) 20-31. [23] O. Bakhtiari, S. Mosleh, T. Khosravi, T. Mohammadi, Synthesis and characterization of polyimide mixed matrix membranes, Sep. Purif. Technol. 46 (2011) 2138–2147. [24] A.I. Skoulidas, D.M. Ackerman, J.K. Johnson, D.S. Sholl, Rapid transport of gases in carbon nanotubes, Phys. Rev. Lett. 89 (2002) 185901/1–185901/4. [25] D.M. Ackerman, A.I. Skoulidas, D.S. Sholl, J.K. Johnson, Diffusivities of Ar and Ne in carbon nanotubes, Mol. Simul. 29 (2003) 677–684. [26] B.J. Hinds, N. Chopra, T. Rantell, R. Andrews, V. Gavalas, L.G. Bachas, Aligned multi walled carbon nanotube membranes, Science 303 (2004) 62–65.
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ACCEPTED MANUSCRIPT [27] H. Chen, D.S. Sholl, Predictions of selectivity and flux for CH4/H2 separations using single walled carbon nanotubes as membranes, J. Membr. Sci. 269 (2006) 152–160. [28] M.A. Aroon, A.F. Ismail, T. Matsuura, M.M. Montazer-Rahmati, Performance studies of mixed matrix membranes for gas separation: A review, Sep. Purif. Technol.75 (2010) 229– 242. [29] A.F. Ismail, P.S. Goh, S.M. Sanip, M. Aziz, Transport and separation properties of carbon nanotube-mixed matrix membrane, Sep. Purif. Technol.70 (2009) 12–26. [30] M. A. Aroon, A. F. Ismail, M. M. Montazer-Rahmati, T. Matsuura, Effect of raw multiwall carbon nanotubes on morphology and separation properties of polyimide membranes, Sep. Sci.Tech. 45 (2010) 2387–2397. [31] E. Raymundo-Pinero, T. Cacciaguerra, P. Simon, F. Be´guin, A single step process for the simultaneous purification and opening of multi-walled carbon nanotubes, Chem. Phys. Lett. 412 (2005) 184–189. [32] S.E. Baker, W. Cai, T.L. Lasseter, K.P. Weidkamp, R.J. Hamers, Covalently bonded adducts of deoxyribonucleic acid (DNA) oligonucleotides with single-wall carbon nanotubes: synthesis and hybridization, Nano Lett. 2 (2002) 1413–1417. [33] A. Eitan, K. Jiang, D. Dukes, R. Andrews, L.S. Schadler, Surface modification of multiwalled carbon nanotubes: toward the tailoring of the interface in polymer composites, Chem. Mater. 15 (2003) 3198–3201. [34] S. Banerjee, T. Hemraj-Benny, S.S. Wong, Covalent surface chemistry of single-walled carbon nanotubes, Adv. Mater. 17 (2005) 17–29. [35] N.G. Sahoo, S. Rana, J.W. Cho, L. Li, S.H. Chan, Polymer nanocomposites based on functionalized carbon nanotubes, Prog. Polym. Sci. 35 (2010) 837–867.
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ACCEPTED MANUSCRIPT [36] J. Chen, M.J. Dyer, M.F. Yu, Cyclodextrin-mediated soft cutting of single walled carbon nanotubes, J. Am. Chem. Soc. 123 (2001) 6201–6202. [37] M. Chaplin, Cyclodextrins, www.lsbu.ac.uk/water/cyclodextrin.html. [38] J. Szejtli, Introduction and general overview of cyclodextrin chemistry, Chem. Rev. 98 (1998) 1743-1754. [39] E. Schneiderman, A. M. Stalcup, Review, Cyclodextrins: a versatile tool in separation science, J.Chrom.B 745 (2000) 83–102. [40] J.C. Tee, M. Aziz, A.F. Ismail, Effect of reaction temperature and flow rate of precursor on formation of multi-walled carbon nanotubes, AIP Conf. Proc. 1136 (2009) 214–218. [41] J.C. Tee, A.F. Ismail, M. Aziz, T. Soga, Influence of catalyst preparation on synthesis of multi-walled carbon nanotubes, IEICE Trans. Electron. E92-C (2009) 1421–1426. [42] M.A. Aroon, A.F. Ismail, M.M. Montazer-Rahmati, T. Matsuura, Morphology and permeation properties of polysulfone membranes for gas separation: Effects of non-solvent additives and co-solvent, Sep. Purif. Technol. 72 (2010) 194–202. [43] T.T. Moore, W.J. Koros, Non-ideal effects in organic–inorganic materials for gas separation membranes, J. Mol. Struct. 739 (2005) 87–98. [44] 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(4-methyl-2pentyne)=fumed silica nanocomposite membranes. Chem. Mater. 15 (2003) 109. [45] J.P. DeRocher, B. T. Gettelfinger, J. Wang, E.E. Nuxoll, E.L. Cussler, Barrier membranes with different sizes of aligned flakes. J. Membr. Sci. 254 (2005) 21.
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Figure 1: TEM micrographs of the long, closed ended and defective rMWCNTs; impurities and closed tips of rMWCNTs are shown by arrows [20, 30]
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Figure 2: TEM micrographs of the bCfMWCNTs
(a)
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(b) Figure 3: TEM micrographs of a) the rMWCNTs and b) the bCfMWCNTs (arrows show irregular alignment at walls of the bCfMWCNTs)
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(b) Figure 4: TEM images of a) open-ended tips and outer surface of the bCfMWCNTs and b) outer surface and open-ended tips of the bCfMWCNTs (arrows show open ended tips of the bCfMWCNTs are filled by betacyclodextrin)
(a)
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(b)
(c)
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(d)
(e) Figure 5: a-c) high magnification TEM micrographs of the generally endless and agglomerated rMWCNTs (images 5a-5c were selected from different points on the stub sample), d) high magnification TEM micrograph of the relatively well dispersed bCfMWCNTs and e) low magnification TEM micrograph of the relatively well dispersed bCfMWCNTs (images 5d-5e were also selected from different points on the stub sample)
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(f) Figure 6: a-c) TEM micrographs of the long, generally endless and impure rMWCNTs, d-f) TEM micrographs of the open-ended and relatively shortened bCfMWCNTs (images 6a-6c and 6d-6f were selected from different points on the stub sample)
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ACCEPTED MANUSCRIPT mg
%
RAW CNT ASAN, 31.07.2009 18:11:24 RAW CNT ASAN, 5.3300 mg ? Content
88.2774 % 4.7052 mg
100
5 90
80 4 70
60 3 50
40
2
30
20
1
100
200 10
0
300 20
400 30
500
600
40
50
700 60
800 70
10 °C
800 80
90
100
min
STARe SW 9.00
Lab: METTLER Figure 7: rMWCNTs TGA thermogram mg 6
% B-CYCLODEXTRIN , 29.07.2009 17:24:43 B-CYCLODEXTRIN , 5.6800 mg
100
90
5
Content
79.0662 % 4.4910 mg 80
4
70
60 3 50
40 2 30
20 1 10 50 0
100 5
150 10
200 15
250 20
300 25
350 30
400 35
450 40
500 45
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650 60
700 65
750 70
°C min
STARe SW 9.00
Lab: METTLER
Figure 8: Pure beta-cyclodextrin powder TGA thermogram
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ACCEPTED MANUSCRIPT mg 6
% CYCLODEXTRIN RAW ASW, 29.07.2009 15:09:47 CYCLODEXTRIN RAW ASW, 5.6300 mg
100
Content 83.5496 % 4.7038 mg
5
80
4
60 3
40 2
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0 0
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Figure 9: BFBM TGA thermogram
Figure 10: Glass transition temperature of the membranes
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Figure 11: Gas permeation results of neat PI and PI/BFBM mixed matrix membranes
Figure 12: CO2/CH4 ideal Selectivity of neat PI and PI/BFBM mixed matrix membranes
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(a)
(b) Figure 13: Cross sectional FESEM micrographs of a) the PI/2 wt. % BFBM MMM and b) the PI/6 wt. % BFBM MMM
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(a)
(b) Figure 14: High magnification FESEM micrographs of PI/2 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
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(a)
(b) Figure 15: High magnification FESEM micrographs of PI/6 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
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ACCEPTED MANUSCRIPT Table 1: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected MMMs Membra Polymer Inorganic IP. CO2-P α Conditions Ref. Year ne (Barrer) Matrix Phase(IP) wt. % Type 0 8.34 1.22 [4] 2001 SDF M5218 35◦C, 1 atm TAP-m-4A 43 0.185 617 TAP-m-13X 43 0.640 133 0 21.97 30.23 [5] 2002 SDF D-PI APTES-m-ZSM-2 20 15.96 24.18 0 2.87 24.12 20 7.49 28.37 T=293 K, 28 bar [6] 2004 SDF ABS AC 25 9.82 32.52 33 13.16 40.10 40 20.50 50.49 0 7.4 75 10 10 114 76 cmHg,25 [7] 2005 SDF H-PI TMOS-m-silica 20 12 150 ºC. 30 19 238 0 8.80 23.5 room [8] 2006 SDF PC zeolite 4A 20 7.80 32.5 temperature 30 7.00 37.6 0 166.02 5.89 [9] 2006 SDF PDMS SWCNTs 2 190.67 5.58 4 atm,35°C, 10 191.30 5.21 0 78 15.6 10 psig, silica 9 177.0 15.3 room [10] 2007 SDF BPPOdp TMS-m-silica 9 104.0 13.8 temperature TPS-m-silica 9 112.7 14.3 0 8.80 23.6 5 6.03 41.9 25 ◦C, 3.7 10 4.89 44.1 [11] 2007 SDF PC pNA-m-zeolite 4A bar 20 4.61 51.8 30 3.64 44.9 114.7 psia 10.3 for CH4, 23 6.23 GPU 43.9 [12] 2007 AHF PEI(U) HSSZ-13 zeolites psia for vol.% CO2, 35 ◦C. 0 ˜105 ˜15.2 10 psig,22 [13] 2007 SDF BPPO silicon dioxide 30 ˜220 ˜15.2 ◦C [14]
2007
SDF
PSF(U3)
LCAA-f-SWCNTs
[15]
2008
SDF
PDMC
[16]
2008
AHF
PES
DA-m-zeolite 4A
[17]
2008
SDF
M-PI
Cu–BPY–HFS
[18]
2008
SDF
PSF(U3)
MCM-41
PDMC- p-SSZ-13 SSZ-13 APDMES-m-SSZ-13.
37
0 5 10 15 0 ˜15 ˜15 ˜15 20 0 10 20 30 40 0
3.9 5.12 5.19 4.52 ˜57.5 ˜66 ˜56.5 ˜88.6 4.73 GPU 1.62 GPU 7.29 7.81 9.88 10.36 15.06 4.5
23.55 18.82 18.41 16.09 ˜37.1 ˜41 ˜43.8 ˜41.9 28.75 46.28 34.71 31.93 27.62 27.45 25.55 23
4 atm,308 K
˜65 psia, 35 ◦C 10 bar, room temperature, 1500 Torr, 35 ◦C 4 atm,308 K
ACCEPTED MANUSCRIPT
TMCS-m-MCM-41 APTES- m-MCM-41
10 20 20 wt.% 20 wt.% 40 wt.% 0 vol. % 5 10 15 20 0 1 1 0 0.7
6.6 7.8
23 23
7.8
23
7.3
28
14.8
15
6.3
29
50 psig, 35 27 ◦C (delta p 25 of 4.4 atm) 21 18 10.9 15 bar [20] 2010 AFS A-PI gauge,room raw MWCNTs 17.5 temperature Ch-f-MWCNTs 16.5 ˜2 3-10 bar, [21] 2011 AFS A-PI bC- f-MWCNTs ˜ 7-8 35◦C 30.90 APTES-f-MWCNTs 1 2.794 GPU 6 19.57 4 bar, room [22] 2011 AFS PES(R) Purified-MWCNTs 1 2.742 GPU temperature 1 APTES-f-MWCNTs 1 2.588 GPU 7.567 pristine MWCNTs 1 2.990 GPU 5.299 0 4.45 37 M5218 Zeolite 4A 10 5.89 43 [23] 2011 SDF 10 bar,30 oC 0 0.63 5 P84 (CNTs) (41) 10 1.08 11 AFS=Asymmetric Flat Sheet, SDF=Symmetric Dense Film, AHF=Asymmetric Hollow Fiber; M5218= Matrimid 5218 Polyimide; A-PI=1,3-Isobenzofurandione, 5,5’-carbonylbis-, polymer with 1 (or 3)-(4-aminophenyl)-2,3dihydro-1,3,3 or (1,1,3)-trimethyl-1H-inden-5-amine Polyimide); M-PI= Matrimid® polymer; PI=Polyimide; DPI=6FDA-6FpDA-DABA polyimide; H-PI=6FDA-TAPOB Hyperbranched Polyimide; ABS= Acrylonitrile– Butadiene–Styrene; PC= Polycarbonate; PDMS=6FDA-6FpDA-PDMS Copolymer; BPPOdp =Brominated Poly(2,6-diphenyl-1,4-Phenylene Oxide) (BPPOdp); PEI(U)=Ultem® 1000 Polyetherimide; BPPO= Brominated Poly(Phenylene Oxide); PSF(U3)=Polysulfone (UDEL P-3500); PSF(U1)=Polysulfone (Udel P1700);PDMC=Polyimide 3:2 6FDA-DAM:DABA with a propyl monoester chain attached to the DABA monomer from reaction with 1,3-propanediol; PES=polyethersulfone; PES(R)=Radel A Polyethersulfone; TAPm-4A=2,4,6- triaminopyrimidine modified zeolite 4A; TAP-m-13X=2,4,6- triaminopyrimidine modified zeolite 13X; APTES-m-ZSM-2= 3-aminopropyltriethoxysiliane modified ZSM-2 zeolite; AC=Activated Carbon; TMOS-m-silica =tetramethoxysilane modified silica; TMS-m-silica= Trimethylsilyl modified silica;TPS-msilica= triphenylsilyl modified silica; pNA-m-zeolite 4A =p-nitroaniline modified zelite 4A;LCAA-f-SWCNTs= Long chain alkyl amines functionalized Single Walled Carbon Nanotubes; PDMC p-SSZ-13= PDMC polymer rprimed SSZ-13;APDMES-m-SSZ-13.=Aminopropyldimethylethoxysilane modified SSZ-13; DA-m-zeolite 4A= DynasylanAmeosilane modified zeolite 4A;TMCS-m-MCM-41=Trimethylchlorosilane modified MCM-41; APTES- m-MCM-41=Aminopropyltriethoxysilane modified MCM-41;Ch-f-MWCNTs=Chitosan functionalized multi-walled carbon nanotubes;bC- f-MWCNTs=beta-cyclodextrin functionalized multi-walled carbon nanotubes; APTES-f-MWCNTs=3-aminopropyltriethoxysilane functionalized multi-walled carbon nanotubes [19]
2008
SDF
Trade Name Chemical Composition
PSF(U1)
TMS-m-silica
7.7 9.3 12.9 19.7 16.83 10.47 37.31 ˜0.5 GPU ˜ 4-10 GPU
Table 2: Polyimide properties Polyimide resin 1,3-Isobenzofurandione, 5,5’-carbonylbis-, polymer with 1 (or 3)-(4aminophenyl)-2,3-dihydro-1,3,3 or (1,1,3)-trimethyl-1H-inden-5-amine
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ACCEPTED MANUSCRIPT Form Color Density at 20oC
Powder Yellow 1.2 gr/cm3
Table 3: The dope composition for the fabrication of PI and PI/bCfMWCNTs membranes Sample
PI (wt. %)
(BFBM)/(BFBM+ PI)
(wt. %)
bC (wt. %)
bCfMWCNTs a (wt. %)
Neat polyimide (PI)
25
0
0
0
PI/2 wt. % BFBM MMM
25
2
0.495
0.0165
PI/6 wt. % BFBM MMM
25
6
1.545
0.0515
Solvent (wt. %) NMP 48.215, THF 16.071, EtOH 10.714 NMP 47.8855, THF 15.962, EtOH 10.641 NMP 47.1885, THF 15.729, EtOH 10.486
a
The amount of bCfMWCNTs in BFBM is assumed nearly equal to that of rMWCNTs.
Table 4: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected mixed matrix polyimide membranes in comparison with the fabricated MMMs in this research work Membrane Polymer Inorganic IP. CO2-P Ref. Year α Conditions Type Matrix Phase(IP) wt. % (Barrer) 0 21.97 30.23 5 2002 SDF D-PI APTES-m-ZSM-2 20 15.96 24.18 0 7.4 75 76 cmHg,25 10 10 114 7 2005 SDF H-PI TMOS-m-silica ºC. 20 12 150 30 19 238 0 7.29 34.71 10 7.81 31.93 1500 Torr, 17 2008 SDF M-PI Cu–BPY–HFS 20 9.88 27.62 35 ◦C 30 10.36 27.45 40 15.06 25.55 0 16.83 10.9 15 bar 20 2010 AFS A-PI raw MWCNTs 1 10.47 17.5 gauge, room Ch-f-MWCNTs 1 37.31 16.5 temperature 0 ˜0.5 GPU ˜2 3-10 bar, 21 2011 AFS A-PI bC- f-MWCNTs 0.7 ˜ 4-10 GPU ˜ 7-8 35◦C 0 9.41GPU 24.12 15 bar This AFS A-PI bCfMWCNTs 2 3.11GPU 38.88 gauge, 25◦C Work 6 2.2GPU 62.86
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ACCEPTED MANUSCRIPT Graphical abstract In this paper, synthesized (raw) multi-walled carbon nanotubes (rMWCNTs) were functionalized with beta-cyclodextrin (bC) using Chen’s soft cutting technique. Neat polyimide (PI) and polyimide/bC functionalized MWCNTs (bCfMWCNTs) mixed matrix membranes (MMMs) were fabricated by immersion precipitation method. The TEM results showed that beta-cyclodextrin can not only attach to the outer surface of the MWCNT walls but also can fill the open-ended tips of the MWCNTs.
DSC experiment showed that the glass transition temperature increased as bCfMWCNTs content increases in the polymer matrix.
Permeation test results revealed that the ideal CO2/CH4 selectivity increased considerably as bCfMWCNTs content increases in the polymer matrix.
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S1383-5866(13)00249-9 http://dx.doi.org/10.1016/j.seppur.2013.04.025 SEPPUR 11161
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
3 August 2012 15 March 2013 18 April 2013
Please cite this article as: M.A. Aroon, A.F. Ismail, T. Matsuura, Beta-cyclodextrin functionalized MWCNT: A potential nano membrane material for mixed matrix gas separation membranes development, Separation and Purification Technology (2013), doi: http://dx.doi.org/10.1016/j.seppur.2013.04.025
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ACCEPTED MANUSCRIPT Beta-cyclodextrin functionalized MWCNT: A potential nano membrane material for mixed matrix gas separation membranes development
M. A. Aroona*, A.F. Ismailb, T. Matsuurac
a
Membrane Research Laboratory, School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran, Tel: +981824652604, Fax: +981824652600, Email: [email protected] b
Advanced Membrane Technology Center, Universiti Teknologi Malaysia , 81310 Skudai, Johor, Malaysia c
Industrial Membrane Research Laboratory, Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
Abstract
In this paper, synthesized (raw) multi-walled carbon nanotubes (rMWCNTs) were functionalized with beta-cyclodextrin (bC) using Chen’s soft cutting technique. Raw multiwalled carbon nanotubes (rMWCNTs) and beta-cyclodextrin functionalized multi-walled carbon nanotubes (bCfMWCNTs) were characterized by transmission electron microscope (TEM). Neat polyimide (PI) and polyimide/bCfMWCNTs mixed matrix membranes were fabricated by immersion precipitation method. Pure methane and carbon dioxide were used as
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ACCEPTED MANUSCRIPT membranes and MWCNTs were characterized by differential scanning calorimetery (DSC) and thermogravimetric analysis (TGA), respectively. The TEM results showed that betacyclodextrin can not only attach to the inner and outer surface of the MWCNT walls but also fill the open-ended tips of the MWCNTs. TGA thermograms showed that bCfMWCNTs are thermally less stable than rMWCNTs due to the decomposition of beta-cyclodextrin layer, which occurred before the decomposition of rMWCNTs. According to the DSC experiment, the glass transition temperature (Tg) increased as bCfMWCNTs increases in the polymer matrix. The increase in glass transition was attributed to good dispersion of the bCfMWCNTs in the polymer matrix as a result of improved attachment of bCfMWCNTs to the polymer segments. Separation properties and morphology (and distribution of bCfMWCNTs within the polymer matrix) of the fabricated membranes were analyzed by using permeation test and cross sectional FESEM micrographs respectively. Permeation test results revealed that the ideal CO2/CH4 selectivity increased considerably from neat polyimide membrane to polyimide/bCfMWCNTs mixed matrix membranes. Cross sectional FESEM results showed that well dispersed bCfMWCNTs within the polymer matrix are observable especially in case of PI/6 wt.% BFBM MMM.
Key words: MWCNTs; Cyclodextrin; Functionalization; MMMs; Gas Separation Membranes
1. Introduction
Because inorganic membranes are difficult to process, expensive to fabricate, easily plagued and fragile, membrane researchers usually prefer to use these nano-materials as fillers embedded in the polymer matrix and fabricate mixed matrix membranes [1-3]. In mixed matrix membranes (MMMs) the superior gas separation properties of inorganic particles are
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ACCEPTED MANUSCRIPT combined with flexibility and processability of polymeric membranes and one can fabricate a low price high performance membrane [1-3]. As reviewed in Table 1, various inorganic particles including zeolites, carbon molecular sieves (CMS), activated carbons (AC), silica, silicon dioxide, metal organic frameworks (MOF) and carbon nanotubes (CNTs) have been used by researchers to fabricate the MMMs. Each MMM has its own advantages and disadvantages.
Table 1: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected MMMs
As shown in Table 1, among different inorganic particles, zeolites and carbon molecular sieves are the most frequently used fillers for the preparation of dense MMMs. In contrast, metal organic frameworks and carbon nanotubes (CNTs) have been used only rarely. Especially, CNTs have emerged as a new potential nano-membrane material (NMM) for gas separation applications due to their very smooth internal surfaces, their nano-sized structure and also their high surface area [24-29]. In our recent work, we have shown that raw MWCNTs, when embedded in polyimide, act as a barrier in the polymer matrix and increase the CO2 /CH4 selectivity [30]. Similar to other inorganic fillers, although carbon nanotubes can improve separation and mechanical properties of polymeric membranes, fabrication of defect free MMMs is difficult due to their weak interaction with organic polymer phase. To overcome this problem, the particle surface is usually modified by using the silane coupling agents (e.g. TMS, TPS, APTES, TMCS), priming by polymeric materials (e.g. PDMC and the other matrix polymers) or functionalizing by soft organic materials (chitosan, cyclodextrin, surfactants) [28-29].
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ACCEPTED MANUSCRIPT Regarding CNTs, their tendency to agglomerate is another very serious problem. Furthermore, the pristine CNTs are normally impure [31-32], generally long and closed ended [30] and their dispersion in polymer matrix is difficult [33-34]. Functionalization of CNTs by covalent or non-covalent treatment is an effective way to improve their dispersion inside the polymer matrix since functionalization increases the interfacial interactions between CNTs and polymer chains, thereby reducing their agglomeration inside the polymer matrix [35]. For example, MWCNTs were functionalized by chitosan, a biodegradable and hydrophilic low molecular weight polymer from a linear polysaccharide family, by applying Chen’s soft cutting technique [36].They were found to be well dispersed in the polymer matrix. Permeation tests also revealed that adding them into polyimide dope could increase CO2 and CH4 permeabilities [20]. In this paper, as an extension of our earlier works, raw MWCNTs were functionalized with beta-cyclodextrin, a bottomless bowl shaped macro-rings from a cyclic oligosaccharides family [37-39], by applying Chen’s soft cutting technique [36]. Both non-functionalized and functionalized MWCNTs were characterized by transmission electron microscopy (TEM) and thermogravimetric analysis. Neat polyimide and asymmetric PI/bCfMWCNTs MMMs were fabricated by the phase inversion technique using immersion precipitation method and were characterized by gas permeation and differential scanning calorimetery (DSC). It is necessary to note that, in contrast to the MMMs most commonly fabricated in dense film form (see 3rd column in Table 1), MMMs were fabricated in asymmetric flat sheet form in this research, viewing their commercial applications.
2. Experimental
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ACCEPTED MANUSCRIPT 2.1. Materials
Ethanol (EtOH) and tetrahydrofuran (THF) were purchased from Merck and used as dope additives. Ethanol (EtOH) was also used as the diluent in MWCNTs functionalization. 1-Methyl-2-pyrrolidone (NMP) was supplied by Merck and used as the solvent in polymer dope preparation. Beta-cyclodextrin (bC) was purchased from Sigma-Aldrich and used as the functionalization agent. Multi-walled carbon nanotubes (MWCNTs) were synthesized at AMTEC (Malaysia) by the CCVD method as reported elsewhere [40-41]. Tap water was used as coagulant in the asymmetric membrane fabrication. Polyimide (PI) was purchased from Alfa-Aesar; A Johnson Matthey Company [20, 30] and used as the polymer matrix. Table 2 shows polyimide properties.
Table 2: Polyimide properties
2.2. Functionalization of multi-walled carbon nanotubes
Synthesized raw MWCNTs (rMWCNTs) were functionalized using beta-cyclodextrin as soft cutting agents or functionalization agents by applying Chen’s soft cutting technique [36]. Raw MWCNTs and cyclodextrin were dried in an oven at 80 ◦C overnight to remove any adsorbed water. Then, 30 g of cyclodextrin and 1 g of rMWCNTs were mixed and ground in a mortar and pestle system for 10 min. During the 10 minutes of grinding, ethanol was added gradually to form a sticky and grayish mixture. This sticky mixture was further ground without addition of ethanol for 2 h to obtain a homogeneous black powder (bCfMWCNTs/bC
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ACCEPTED MANUSCRIPT mixture; BFBM), which was dried in an oven at 80 ◦C overnight. It should be noted that BFBM contains both unreacted bC and functionalized MWCNTs.
2.3. Characterization of raw MWCNTs and functionalized MWCNTs
Raw MWCNTs (rMWCNTs) and bCfMWCNTs were observed by a transmission electron microscope (TEM: JEOL JEM-2100). The method of TEM sample preparation is as follows. A small amount of rMWCNTs was added to chloroform and sonicated for 15 minutes in an ultrasonic bath (Delta 1100: DC150H). The rMWCNTs particles were then picked up from the dispersion onto the sample stub surface for observation by TEM. Similarly, BFBM was dispersed in chloroform. Beta-cyclodextrin in BFBM was dissolved to chloroform in the dispersion process, leaving bCfMWCNTs as solid particles. The bCfMWCNTs particles were then picked up onto the sample stub surface for TEM observation. Thermogravimetric analysis (TGA: METTLER TOLEDO SDTA 851e) was used to measure the rate of weight change of beta-cyclodextrin and bCfMWCNTs.
2.4. Dope preparation and fabrication of membrane samples
The casting dopes for MMMs fabrication were prepared by the solution-blending technique [20] as follows. A predetermined amount of the BFBM was added to the solvent and mixed by a stirrer for 2 h. Beta-cyclodextrin (bC) in BFBM was dissolved in the solvent and a homogeneous suspension of the bCfMWCNTs particles in the solvent was obtained. Polyimide (PI) was then added gradually to the solvent/bC/bCfMWCNTs suspension, followed by stirring for more 24 h. The resulting black viscose mixture (solvent/bC/PI
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ACCEPTED MANUSCRIPT homogeneous mixture containing bCfMWCNTs inorganic nano-particles) was degassed by using an ultrasonic bath (Branson Ultrasonic Corporation; 3510 E-MTH) and cast immediately to prevent the bCfMWCNTs settling. The dope compositions are listed in Table 3. Table 3: The dope composition for the fabrication of PI and PI/bCfMWCNTs membranes
It should be noted that the membranes are coded as PI/x wt. % BFBM mixed matrix membrane where x represents the percent of BFBM in BFBM and PI mixture (see 3rd column in Table 3).
Flat sheet asymmetric membranes (neat PI and PI/BFBM) were fabricated by the wet phase inversion method. The details of the fabrication procedure can be found elsewhere [42].
2.5. Membrane samples characterization
Neat polyimide (PI) membranes and PI/BFBM mixed matrix membranes were characterized by gas permeation tests using pure CO2 and pure CH4 as test gases at 15 bar gauge and 25 ◦C. A constant pressure permeation cell with a circular flat plate was used. The effective membrane area was 13.5 cm2. It is necessary to note that the membrane samples were coated with a layer of silicon rubber before permeation test. The silicon rubber coating procedure has been reported in details elsewhere [30].
Equation 1 was used to calculate the permeance (Pri= Pi/l) of gas i.
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ACCEPTED MANUSCRIPT Pri (GPU)=106×[Qi (cm3 (STP)]/[A(cm2)×Δp(cmHg)]
(1)
where Qi is the measured volumetric flow rate of gas i (at standard temperature and pressure), A is the effective membrane area and Δp is the pressure difference across the membrane. The ideal separation factor or ideal selectivity between gas i and j was calculated by Eq. 2.
Ideal Selectivity=αij=(Pri/Prj)
(2)
A differential scanning calorimeter (DSC: METTLER TOLEDO DSC 822e) was used to measure the glass transition temperature (Tg) of the membranes. Morphology of membrane samples and also distribution of bCfMWCNTs inside the polymer matrix of the MMMs were studied using high magnification field emission scanning electron microscope (FESEM Model: Hitachi S4160). The sample preparation procedure for FESEM analysis has been described elsewhere [20].
3. Results and discussions
3.1. MWCNTs properties and TEM Results
TEM micrographs of the raw MWCNTs (rMWCNTs) reported in our recent work [30] is reproduced in Fig. 1. From the figure, the average inner and the outer diameters were about 3.5 nm and 15nm, respectively [30]. In addition, rMWCNTs were long and closed ended. Impurities and surface defects were detected inside the carbon nanotubes and on their outer surface [Fig.1].
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ACCEPTED MANUSCRIPT
Figure 1: TEM micrographs of the long, closed ended and defective rMWCNTs; impurities and closed tips of rMWCNTs are shown by arrows [20, 30]
Beta-cyclodextrin functionalized MWCNTs (bCfMWCNTs) were observed by transmission electron microscope (TEM). As shown in Fig. 2, cyclodextrin is attached to the outer surface of the MWCNT walls and wrapped around the axis of the MWCNTs.
Figure 2: TEM micrographs of the bCfMWCNTs
Figure 3 compares the structure of the MWCNT before (Fig. 3a) and after functionalization (Fig. 3b). As shown in this figure parallel alignment observed at the walls, both inside and outside, of the raw MWCNT has become irregular after functionalization with betacyclodextrin. This finding is in accordance with Raman spectroscopy analysis of the beta cyclodextrin functionalized MWCNTs by Sanip et al [21].
Figure 3: TEM micrographs of a) the rMWCNTs and b) the bCfMWCNTs (arrows show irregular alignment at walls of the bCfMWCNTs)
Figure 4 show that the bCfMWCNTs are open-ended. It is likely that during the soft cutting and functionalization procedures, the MWCNTs are cut to become open-ended [20, 36]. It is interesting to note that not only the outer surfaces of the MWCNTs are covered by cyclodextrin (see Fig. 4a) but also their open-ended tips are filled with beta-cyclodextrin molecules (see Fig. 4b). This can be attributed to the molecular size of the beta-cyclodextrin,
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ACCEPTED MANUSCRIPT i.e. the outer diameter of the beta-cyclodextrin (1.66 nm) [37], which is less than the average inner diameter of the synthesized rMWCNTs (nearly 3.5 nm) [30].
Figure 4: TEM images of a) open-ended tips and outer surface of the bCfMWCNTs and b) outer surface and open-ended tips of the bCfMWCNTs (arrows show open-ended tips of the bCfMWCNTs are filled by betacyclodextrin)
Figure 5d (high magnification TEM micrograph of the bCfMWCNTs) shows that the distance between MWCNTs has increased. As shown in Fig. 5e (low magnification TEM micrograph of the bCfMWCNTs) even at a far lower magnification this distance increase between the MWCNTs is observable and one can conclude that bCfMWCNTs on the TEM stub surface are shortened and less agglomerated than rMWCNTs (see Figs. 5a-5c which were selected from different points on the stub sample).
Figure 5: a-c) high magnification TEM micrographs of the generally endless and agglomerated rMWCNTs (images 5a-5c were selected from different points on the stub sample), d) high magnification TEM micrograph of the relatively well dispersed bCfMWCNTs and e) low magnification TEM micrograph of the relatively well dispersed bCfMWCNTs (images 5d-5e were also selected from different points on the stub sample)
As shown in Fig. 6, the bCfMWCNTs (Figs. 6d-6f) are shorter than the rMWCNTs (Figs. 6a6c). It is necessary to note that TEM Micrographs were selected from different points on the TEM stub sample,
Figure 6: a-c) TEM micrographs of the long, generally endless and impure rMWCNTs, d-f) TEM micrographs of the open-ended and relatively shortened bCfMWCNTs (images 6a-6c and 6d-6f were selected from different points on the stub sample)
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ACCEPTED MANUSCRIPT
3.2. TGA Results
Figures 7, 8 and 9 show the TGA thermograms of rMWCNTs, beta-cyclodextrin and bCfMWCNTs, respectively. According to Fig. 7, the largest weight loss of rMWCNTs occurs between 400-640◦C, while Figs. 8 and 9 show that the largest weight loss of beta-cyclodextrin and bCfMWCNTs occurs at around 330 ◦C, due to the decomposition of beta-cyclodextrin, which occurs before the decomposition of rMWCNTs. Figure 7: rMWCNTs TGA thermogram
Figure 8: Pure beta-cyclodextrin powder TGA thermogram
Figure 9: BFBM TGA thermogram
3.3. DSC Results
Figure 10 shows the effect of BFBM content on the glass transition temperature of the membranes. Tg increased as the BFBM content increased. Considering Figs. 2-4, it is likely that the bC layer wrapped around the bCfMWCNTs increases polymer/bCfMWCNTs interactions, and the segmental mobility of polymer chains decreases. As a result, one can expect an increase in the membrane’s glass transition temperature. However, more than one glass transition temperature were not observed, which implies that decrease in chain mobility extended beyond the bCfMWCNTs/polymer interphase and reached deep into the polymer matrix. This can be attributed to good dispersion of the bCfMWCNTs in the polymer
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ACCEPTED MANUSCRIPT continuous phase. In other words, strong attachment of the well dispersed bCfMWCNTs in the polymer matrix decreases polymer chain mobility and results in an increase in the glass transition temperature [3, 20, 43]. These results have been proved by gas permeation test in section 3.4.
Figure 10: Glass transition temperature of the membranes
3.4. Gas Permeation Test Results
Figure 11 shows the effect of BFBM content on CO2permeance. It is clear that the permeance decreases as BFBM content increases.
Figure 11: Gas permeation results of neat PI and PI/BFBM mixed matrix membranes
This effect can be attributed to the beta-cyclodextrin functionalized MWCNTs (as inorganic dispersed phase embedded inside the polymer matrix) and the unreacted betacyclodextrin (as a soluble additive), both present in the MMMs. Regarding the bCfMWCNTs, as was observed in TEM micrographs earlier, beta-cyclodextrin molecules have closed the MWCNTs entrance, which probably hindered gas molecules from entering into the MWCNTs inner surface. It should be noted that some rMWCNTs were left without functionalization, and thus maintained their original length and remained also closed ended. As well, the ends of MWCNTs may have been plugged not necessarily by beta cyclodextrin but by CCVD catalysts, impurities or by polymer chains. Therefore, it is likely that the bCfMWCNTs acted as impermeable inorganic particles, around which the gas molecules were forced to flow, thus enhancing the length of the tortuous diffusion pathway [30, 44-45]. With an increase in the
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ACCEPTED MANUSCRIPT tortuous pattern of polymer matrix, the diffusion coefficient of penetrant molecules decreases and the penetrant permeability also decreases. It is necessary to note that increase in BFBM content leads to a decrease in permeability due to increase in matrix tortuosity (see Fig. 11). Regarding the presence of beta-cyclodextrin and its effect on the membrane separation properties, one may consider the following interpretation. Since beta-cyclodextrin is soluble in solvent (NMP), addition of BFBM increases the solid soluble contents (e.g. PI and bC) in the casting dope, which results in the formation of a thicker and denser skin layer, reducing the permeance of both CO2 and CH4 [42]. Figure 12 shows the CO2/CH4 ideal selectivity of fabricated membranes.
Figure 12: CO2/CH4 ideal Selectivity of neat PI and PI/BFBM mixed matrix membranes
Figure 12 shows the effect of BFBM content on the ideal CO2/CH4 selectivity. The selectivity increased from neat PI membrane to PI/2 wt. % BFBM MMM by 61.1 % (from 24.12 to 38.88).The selectivity further increased by 61.7 % (from 38.88 to 62.86) from PI/2 wt. % BFBM to PI/6 wt. % BFBM MMM. The ideal selectivity increase may be interpreted as follows: 1- As a result of the increase in polymer diffusion pathway (due to the presence of bCfMWCNTs) the difference in the diffusivity of the larger penetrant molecule (i.e. CH4) and the smaller molecule (i.e. CO2) is enhanced and as a result CO2/CH4 selectivity increases. It is obvious that the increase in BFBM content results in an increase in diffusion pathway length and so one can expect that CO2/CH4 selectivity increases as the BFBM content increases (as shown in Fig. 12).
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ACCEPTED MANUSCRIPT 2- Since the mobility of the polymer chains is reduced by the addition of bCfMWCNTs, the diffusivity difference between larger (CH4) and smaller (CO2) gas molecules increases and a higher CO2/CH4 selectivity is expected. 3- Beta-cyclodextrin increases the coalescence of polymer chains in the skin layer and as a result, a less defective and more selective skin layer can be formed. 4- Increasing the beta-cyclodextrin concentration (which occurs with increasing BFBM content) causes a thicker and denser skin layer to be formed and a more selective membrane is produced.
Table 4 compare separation properties of the mixed matrix polyimide membranes fabricated in this research with the some selected mixed matrix polyimide (or co-polyimide) membranes that contain inorganic fillers other than bCfMWCNTs. Table 4: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected mixed matrix polyimide membranes in comparison with the fabricated MMMs in this research work
As shown in Table 4, the order of magnitude of CO2 permeance of the membranes fabricated in this work is in accordance with the other works. Moreover, CO2/CH4 selectivity of MMMs in this work is generally higher than polyimide MMMs containing other inorganic fillers except for H-PI containing TMOS-m-silica. It should be noted that the intrinsic selectivity of the H-PI is very high, even without addition of the fillers. It is also necessary to note that addition of BFBM increased the CO2/CH4selectivity by 26.8 % for each 1 wt.% increase of BFBM while addition of TMOS-m-silica into H-PI matrix could increase CO2/CH4 selectivity by 7.24 % for each 1 wt. % increase of TMOS-m-silica.
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ACCEPTED MANUSCRIPT 3.5. FESEM Results
Cross sectional FESEM micrographs of the PI/BFBM mixed matrix membranes are shown in Fig. 13. As shown in Fig. 13, both PI/2wt.%BFBM and PI/6wt.%BFBM fabricated mixed matrix membranes are asymmetric containing a skin layer supported by a porous sub-layer. The sub-layer structure of both PI/2wt.%BFBM and PI/6wt.%BFBM mixed matrix membranes seems nodular near the skin layer and cellular with closed cell far from the skin layer. Figure 13: Cross sectional FESEM micrographs of a) the PI/2 wt. % BFBM MMM and b) the PI/6 wt. % BFBM MMM
Distribution of bCfMWCNTs inside the polymer matrix for PI/2wt.%BFBM and PI/6wt.%BFBM MMMs was also studied by using high magnification FESEM. Usually since a clear view of the skin layer cannot be achieved during sample preparation for FESEM analysis (using liquid nitrogen) and because MWCNTs hide beyond the dense structure of the dense layer, finding MWCNTs in the skin layer is difficult [20]. Hence, we tried to find them in the nodular (near the skin layer) or cellular (far from the skin layer) regions of the membrane cross section. Fig.14 shows the high magnification FESEM micrographs of PI/2wt.%BFBM. Figure 14: High magnification FESEM micrographs of PI/2 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
As shown in Fig. 14a, fining bCfMWCNTs within the polymer matrix in sub-layer near the skin layer is so difficult most likely because they hide beyond the nodular or dens structure of polymer in this region. Nevertheless, some well-dispersed bCfMWCNTs within the polyimide matrix in sub-layer are observable in Fig. 14b.
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ACCEPTED MANUSCRIPT Figure 15 shows high magnification FESEM images of PI/6wt.%BFBM MMM.
Figure 15: High magnification FESEM micrographs of PI/6 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
As shown in Fig. 15, well-dispersed bCfMWCNTs are distinguishable within the polymer matrix (near the skin layer, Fig. 15a, and far from the skin layer, Fig. 15b).
4. Conclusions
MWCNTs were functionalized with beta-cyclodextrin and both non-functionalized (rMWCNTs) and functionalized MWCNTs (bCfMWCNTs) were characterized by TEM and TGA. The TEM micrographs revealed that beta-cyclodextrin can not only attach to the outer surfaces of the MWCNT walls but also can fill their open-ended tips. TGA thermograms showed that bCfMWCNTs are thermally less stable than rMWCNTs. The glass transition temperature increases as BFBM loading increases in the polymer matrix and this effect was attributed to the well dispersed bCfMWCNTs in the continuous polymer phase (which was supported with high magnification FESEM micrographs) and also their strong attachment which decreases the segmental mobility of polymer chains. The CO2/CH4ideal selectivity increased as the amount of BFBM (and bCfMWCNTs as well) in the membrane increased, indicating that the permeant gas flows through the tortuous pathway of higher rigidity formed around the bCfMWCNTs that are strongly attached to the PI matrix.
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ACCEPTED MANUSCRIPT Acknowledgment
The authors gratefully acknowledge the financial support from the Iran National Science Foundation under the Grant number 16963.
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ACCEPTED MANUSCRIPT [36] J. Chen, M.J. Dyer, M.F. Yu, Cyclodextrin-mediated soft cutting of single walled carbon nanotubes, J. Am. Chem. Soc. 123 (2001) 6201–6202. [37] M. Chaplin, Cyclodextrins, www.lsbu.ac.uk/water/cyclodextrin.html. [38] J. Szejtli, Introduction and general overview of cyclodextrin chemistry, Chem. Rev. 98 (1998) 1743-1754. [39] E. Schneiderman, A. M. Stalcup, Review, Cyclodextrins: a versatile tool in separation science, J.Chrom.B 745 (2000) 83–102. [40] J.C. Tee, M. Aziz, A.F. Ismail, Effect of reaction temperature and flow rate of precursor on formation of multi-walled carbon nanotubes, AIP Conf. Proc. 1136 (2009) 214–218. [41] J.C. Tee, A.F. Ismail, M. Aziz, T. Soga, Influence of catalyst preparation on synthesis of multi-walled carbon nanotubes, IEICE Trans. Electron. E92-C (2009) 1421–1426. [42] M.A. Aroon, A.F. Ismail, M.M. Montazer-Rahmati, T. Matsuura, Morphology and permeation properties of polysulfone membranes for gas separation: Effects of non-solvent additives and co-solvent, Sep. Purif. Technol. 72 (2010) 194–202. [43] T.T. Moore, W.J. Koros, Non-ideal effects in organic–inorganic materials for gas separation membranes, J. Mol. Struct. 739 (2005) 87–98. [44] 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(4-methyl-2pentyne)=fumed silica nanocomposite membranes. Chem. Mater. 15 (2003) 109. [45] J.P. DeRocher, B. T. Gettelfinger, J. Wang, E.E. Nuxoll, E.L. Cussler, Barrier membranes with different sizes of aligned flakes. J. Membr. Sci. 254 (2005) 21.
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Figure 1: TEM micrographs of the long, closed ended and defective rMWCNTs; impurities and closed tips of rMWCNTs are shown by arrows [20, 30]
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Figure 2: TEM micrographs of the bCfMWCNTs
(a)
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(b) Figure 3: TEM micrographs of a) the rMWCNTs and b) the bCfMWCNTs (arrows show irregular alignment at walls of the bCfMWCNTs)
(a)
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(b) Figure 4: TEM images of a) open-ended tips and outer surface of the bCfMWCNTs and b) outer surface and open-ended tips of the bCfMWCNTs (arrows show open ended tips of the bCfMWCNTs are filled by betacyclodextrin)
(a)
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(b)
(c)
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(d)
(e) Figure 5: a-c) high magnification TEM micrographs of the generally endless and agglomerated rMWCNTs (images 5a-5c were selected from different points on the stub sample), d) high magnification TEM micrograph of the relatively well dispersed bCfMWCNTs and e) low magnification TEM micrograph of the relatively well dispersed bCfMWCNTs (images 5d-5e were also selected from different points on the stub sample)
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(a)
(b)
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(d)
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(e)
(f) Figure 6: a-c) TEM micrographs of the long, generally endless and impure rMWCNTs, d-f) TEM micrographs of the open-ended and relatively shortened bCfMWCNTs (images 6a-6c and 6d-6f were selected from different points on the stub sample)
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ACCEPTED MANUSCRIPT mg
%
RAW CNT ASAN, 31.07.2009 18:11:24 RAW CNT ASAN, 5.3300 mg ? Content
88.2774 % 4.7052 mg
100
5 90
80 4 70
60 3 50
40
2
30
20
1
100
200 10
0
300 20
400 30
500
600
40
50
700 60
800 70
10 °C
800 80
90
100
min
STARe SW 9.00
Lab: METTLER Figure 7: rMWCNTs TGA thermogram mg 6
% B-CYCLODEXTRIN , 29.07.2009 17:24:43 B-CYCLODEXTRIN , 5.6800 mg
100
90
5
Content
79.0662 % 4.4910 mg 80
4
70
60 3 50
40 2 30
20 1 10 50 0
100 5
150 10
200 15
250 20
300 25
350 30
400 35
450 40
500 45
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600 55
650 60
700 65
750 70
°C min
STARe SW 9.00
Lab: METTLER
Figure 8: Pure beta-cyclodextrin powder TGA thermogram
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ACCEPTED MANUSCRIPT mg 6
% CYCLODEXTRIN RAW ASW, 29.07.2009 15:09:47 CYCLODEXTRIN RAW ASW, 5.6300 mg
100
Content 83.5496 % 4.7038 mg
5
80
4
60 3
40 2
20
1
0 0
100
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500 45
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STARe SW 9.00
Lab: METTLER
Figure 9: BFBM TGA thermogram
Figure 10: Glass transition temperature of the membranes
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Figure 11: Gas permeation results of neat PI and PI/BFBM mixed matrix membranes
Figure 12: CO2/CH4 ideal Selectivity of neat PI and PI/BFBM mixed matrix membranes
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(a)
(b) Figure 13: Cross sectional FESEM micrographs of a) the PI/2 wt. % BFBM MMM and b) the PI/6 wt. % BFBM MMM
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(a)
(b) Figure 14: High magnification FESEM micrographs of PI/2 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
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ACCEPTED MANUSCRIPT
(a)
(b) Figure 15: High magnification FESEM micrographs of PI/6 wt. % BFBM MMM a) in sub-layer near the skin layer and b) in sub-layer far from the skin layer (arrows show some well-distributed bCfMWCNTs within the polymer matrix)
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ACCEPTED MANUSCRIPT Table 1: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected MMMs Membra Polymer Inorganic IP. CO2-P α Conditions Ref. Year ne (Barrer) Matrix Phase(IP) wt. % Type 0 8.34 1.22 [4] 2001 SDF M5218 35◦C, 1 atm TAP-m-4A 43 0.185 617 TAP-m-13X 43 0.640 133 0 21.97 30.23 [5] 2002 SDF D-PI APTES-m-ZSM-2 20 15.96 24.18 0 2.87 24.12 20 7.49 28.37 T=293 K, 28 bar [6] 2004 SDF ABS AC 25 9.82 32.52 33 13.16 40.10 40 20.50 50.49 0 7.4 75 10 10 114 76 cmHg,25 [7] 2005 SDF H-PI TMOS-m-silica 20 12 150 ºC. 30 19 238 0 8.80 23.5 room [8] 2006 SDF PC zeolite 4A 20 7.80 32.5 temperature 30 7.00 37.6 0 166.02 5.89 [9] 2006 SDF PDMS SWCNTs 2 190.67 5.58 4 atm,35°C, 10 191.30 5.21 0 78 15.6 10 psig, silica 9 177.0 15.3 room [10] 2007 SDF BPPOdp TMS-m-silica 9 104.0 13.8 temperature TPS-m-silica 9 112.7 14.3 0 8.80 23.6 5 6.03 41.9 25 ◦C, 3.7 10 4.89 44.1 [11] 2007 SDF PC pNA-m-zeolite 4A bar 20 4.61 51.8 30 3.64 44.9 114.7 psia 10.3 for CH4, 23 6.23 GPU 43.9 [12] 2007 AHF PEI(U) HSSZ-13 zeolites psia for vol.% CO2, 35 ◦C. 0 ˜105 ˜15.2 10 psig,22 [13] 2007 SDF BPPO silicon dioxide 30 ˜220 ˜15.2 ◦C [14]
2007
SDF
PSF(U3)
LCAA-f-SWCNTs
[15]
2008
SDF
PDMC
[16]
2008
AHF
PES
DA-m-zeolite 4A
[17]
2008
SDF
M-PI
Cu–BPY–HFS
[18]
2008
SDF
PSF(U3)
MCM-41
PDMC- p-SSZ-13 SSZ-13 APDMES-m-SSZ-13.
37
0 5 10 15 0 ˜15 ˜15 ˜15 20 0 10 20 30 40 0
3.9 5.12 5.19 4.52 ˜57.5 ˜66 ˜56.5 ˜88.6 4.73 GPU 1.62 GPU 7.29 7.81 9.88 10.36 15.06 4.5
23.55 18.82 18.41 16.09 ˜37.1 ˜41 ˜43.8 ˜41.9 28.75 46.28 34.71 31.93 27.62 27.45 25.55 23
4 atm,308 K
˜65 psia, 35 ◦C 10 bar, room temperature, 1500 Torr, 35 ◦C 4 atm,308 K
ACCEPTED MANUSCRIPT
TMCS-m-MCM-41 APTES- m-MCM-41
10 20 20 wt.% 20 wt.% 40 wt.% 0 vol. % 5 10 15 20 0 1 1 0 0.7
6.6 7.8
23 23
7.8
23
7.3
28
14.8
15
6.3
29
50 psig, 35 27 ◦C (delta p 25 of 4.4 atm) 21 18 10.9 15 bar [20] 2010 AFS A-PI gauge,room raw MWCNTs 17.5 temperature Ch-f-MWCNTs 16.5 ˜2 3-10 bar, [21] 2011 AFS A-PI bC- f-MWCNTs ˜ 7-8 35◦C 30.90 APTES-f-MWCNTs 1 2.794 GPU 6 19.57 4 bar, room [22] 2011 AFS PES(R) Purified-MWCNTs 1 2.742 GPU temperature 1 APTES-f-MWCNTs 1 2.588 GPU 7.567 pristine MWCNTs 1 2.990 GPU 5.299 0 4.45 37 M5218 Zeolite 4A 10 5.89 43 [23] 2011 SDF 10 bar,30 oC 0 0.63 5 P84 (CNTs) (41) 10 1.08 11 AFS=Asymmetric Flat Sheet, SDF=Symmetric Dense Film, AHF=Asymmetric Hollow Fiber; M5218= Matrimid 5218 Polyimide; A-PI=1,3-Isobenzofurandione, 5,5’-carbonylbis-, polymer with 1 (or 3)-(4-aminophenyl)-2,3dihydro-1,3,3 or (1,1,3)-trimethyl-1H-inden-5-amine Polyimide); M-PI= Matrimid® polymer; PI=Polyimide; DPI=6FDA-6FpDA-DABA polyimide; H-PI=6FDA-TAPOB Hyperbranched Polyimide; ABS= Acrylonitrile– Butadiene–Styrene; PC= Polycarbonate; PDMS=6FDA-6FpDA-PDMS Copolymer; BPPOdp =Brominated Poly(2,6-diphenyl-1,4-Phenylene Oxide) (BPPOdp); PEI(U)=Ultem® 1000 Polyetherimide; BPPO= Brominated Poly(Phenylene Oxide); PSF(U3)=Polysulfone (UDEL P-3500); PSF(U1)=Polysulfone (Udel P1700);PDMC=Polyimide 3:2 6FDA-DAM:DABA with a propyl monoester chain attached to the DABA monomer from reaction with 1,3-propanediol; PES=polyethersulfone; PES(R)=Radel A Polyethersulfone; TAPm-4A=2,4,6- triaminopyrimidine modified zeolite 4A; TAP-m-13X=2,4,6- triaminopyrimidine modified zeolite 13X; APTES-m-ZSM-2= 3-aminopropyltriethoxysiliane modified ZSM-2 zeolite; AC=Activated Carbon; TMOS-m-silica =tetramethoxysilane modified silica; TMS-m-silica= Trimethylsilyl modified silica;TPS-msilica= triphenylsilyl modified silica; pNA-m-zeolite 4A =p-nitroaniline modified zelite 4A;LCAA-f-SWCNTs= Long chain alkyl amines functionalized Single Walled Carbon Nanotubes; PDMC p-SSZ-13= PDMC polymer rprimed SSZ-13;APDMES-m-SSZ-13.=Aminopropyldimethylethoxysilane modified SSZ-13; DA-m-zeolite 4A= DynasylanAmeosilane modified zeolite 4A;TMCS-m-MCM-41=Trimethylchlorosilane modified MCM-41; APTES- m-MCM-41=Aminopropyltriethoxysilane modified MCM-41;Ch-f-MWCNTs=Chitosan functionalized multi-walled carbon nanotubes;bC- f-MWCNTs=beta-cyclodextrin functionalized multi-walled carbon nanotubes; APTES-f-MWCNTs=3-aminopropyltriethoxysilane functionalized multi-walled carbon nanotubes [19]
2008
SDF
Trade Name Chemical Composition
PSF(U1)
TMS-m-silica
7.7 9.3 12.9 19.7 16.83 10.47 37.31 ˜0.5 GPU ˜ 4-10 GPU
Table 2: Polyimide properties Polyimide resin 1,3-Isobenzofurandione, 5,5’-carbonylbis-, polymer with 1 (or 3)-(4aminophenyl)-2,3-dihydro-1,3,3 or (1,1,3)-trimethyl-1H-inden-5-amine
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ACCEPTED MANUSCRIPT Form Color Density at 20oC
Powder Yellow 1.2 gr/cm3
Table 3: The dope composition for the fabrication of PI and PI/bCfMWCNTs membranes Sample
PI (wt. %)
(BFBM)/(BFBM+ PI)
(wt. %)
bC (wt. %)
bCfMWCNTs a (wt. %)
Neat polyimide (PI)
25
0
0
0
PI/2 wt. % BFBM MMM
25
2
0.495
0.0165
PI/6 wt. % BFBM MMM
25
6
1.545
0.0515
Solvent (wt. %) NMP 48.215, THF 16.071, EtOH 10.714 NMP 47.8855, THF 15.962, EtOH 10.641 NMP 47.1885, THF 15.729, EtOH 10.486
a
The amount of bCfMWCNTs in BFBM is assumed nearly equal to that of rMWCNTs.
Table 4: CO2/CH4ideal selectivity (α) and carbon dioxide permeability/permeance (CO2-P) of some selected mixed matrix polyimide membranes in comparison with the fabricated MMMs in this research work Membrane Polymer Inorganic IP. CO2-P Ref. Year α Conditions Type Matrix Phase(IP) wt. % (Barrer) 0 21.97 30.23 5 2002 SDF D-PI APTES-m-ZSM-2 20 15.96 24.18 0 7.4 75 76 cmHg,25 10 10 114 7 2005 SDF H-PI TMOS-m-silica ºC. 20 12 150 30 19 238 0 7.29 34.71 10 7.81 31.93 1500 Torr, 17 2008 SDF M-PI Cu–BPY–HFS 20 9.88 27.62 35 ◦C 30 10.36 27.45 40 15.06 25.55 0 16.83 10.9 15 bar 20 2010 AFS A-PI raw MWCNTs 1 10.47 17.5 gauge, room Ch-f-MWCNTs 1 37.31 16.5 temperature 0 ˜0.5 GPU ˜2 3-10 bar, 21 2011 AFS A-PI bC- f-MWCNTs 0.7 ˜ 4-10 GPU ˜ 7-8 35◦C 0 9.41GPU 24.12 15 bar This AFS A-PI bCfMWCNTs 2 3.11GPU 38.88 gauge, 25◦C Work 6 2.2GPU 62.86
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ACCEPTED MANUSCRIPT Graphical abstract In this paper, synthesized (raw) multi-walled carbon nanotubes (rMWCNTs) were functionalized with beta-cyclodextrin (bC) using Chen’s soft cutting technique. Neat polyimide (PI) and polyimide/bC functionalized MWCNTs (bCfMWCNTs) mixed matrix membranes (MMMs) were fabricated by immersion precipitation method. The TEM results showed that beta-cyclodextrin can not only attach to the outer surface of the MWCNT walls but also can fill the open-ended tips of the MWCNTs.
DSC experiment showed that the glass transition temperature increased as bCfMWCNTs content increases in the polymer matrix.
Permeation test results revealed that the ideal CO2/CH4 selectivity increased considerably as bCfMWCNTs content increases in the polymer matrix.
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