PAMAM dendrimer composite membrane for CO2 separation: Formation of a chitosan gutter layer

Journal of Membrane Science 287 (2007) 51–59

PAMAM dendrimer composite membrane for CO2 separation: Formation of a chitosan gutter layer Takayuki Kouketsu, Shuhong Duan, Teruhiko Kai, Shingo Kazama ∗ , Koichi Yamada Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawa-dai, Kizu-Cho, Soraku-Gun, Kyoto 619-0292, Japan Received 16 June 2006; received in revised form 3 October 2006; accepted 4 October 2006 Available online 13 October 2006

Abstract A poly(amidoamine) (PAMAM) dendrimer composite membrane with an excellent CO2 /N2 separation factor was developed in-situ. The In-situ Modification (IM) method was used to modify the surface of commercial porous membranes, such as ultrafiltration membranes, to produce a gas selective layer by controlling the interface precipitation of the membrane materials in the state of a received membrane module. Using the IM method, a chitosan layer was prepared on the inner surface of a commercially available ultrafiltration membrane as a gutter layer, in order to affix PAMAM dendrimer molecules on the porous substrate. After chitosan treatment, the PAMAM dendrimer was impregnated into the gutter layer to form a PAMAM/chitosan hybrid layer. The CO2 separation performance of the resulting composite membrane was tested at a pressure difference of 100 kPa and a temperature of 40 ◦ C, using a mixed CO2 (5 vol%)/N2 (95 vol%) feed gas. The PAMAM dendrimer composite membrane, with a gutter layer prepared from ethylene glycol diglycidyl ether and a 0.5 wt% chitosan solution of two different molecular weight chitosans, revealed an excellent CO2 /N2 separation factor and a CO2 permeance of 400 and 1.6 × 10−7 m3 (STP) m−2 s−1 kPa−1 , respectively. SEM observations revealed a defect-free chitosan layer (thickness 200 nm) positioned directly beneath the top surface of the UF membrane substrate. After PAMAM dendrimer treatment, the hybrid chitosan/PAMAM dendrimer layer was observed with a thickness of 300 nm. XPS analysis indicated that the hybrid layer contained about 20–40% PAMAM dendrimer. © 2006 Elsevier B.V. All rights reserved. Keywords: PAMAM dendrimer; Composite membrane; CO2 separation; Chitosan gutter layer

1. Introduction Carbon dioxide (CO2 ) gas is widely considered as one of the major contributors to the greenhouse effect. Efforts to reduce the amount of CO2 emission to the atmosphere have been investigated, and have largely concentrated on establishing methods for (i) separating CO2 from the combustion gases of fossil fuels, and (ii) for storing CO2 in geological formations or oceans [1–3]. However, the energy and cost penalties are far too large to accept CO2 sequestration easily with current technologies. The best-known method for separating and recovering CO2 to date is chemical absorption, however this method consumes more than 70% of the overall cost of carbon sequestration [1].

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Corresponding author. Tel.: +81 774 75 2305; fax: +81 774 75 2318. E-mail address: [email protected] (S. Kazama).

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.10.014

One promising means of lowering the cost of CO2 separation involves the development of new high-performance CO2 separation membranes. To date, many kinds of CO2 separation membrane have been reported, including polymeric membranes [4], inorganic membranes [5–7], facilitated transported membranes [8,9], and so on. However, further improvements in membrane performance are necessary to reduce the costs for realizing effective CO2 capture. Sirkar and co-workers have recently reported excellent CO2 /N2 selectivity results using the viscous and nonvolatile poly(amidoamine) (PAMAM) dendrimer as an immobilized liquid membrane under isobaric and saturated water vapor test conditions [10,11]. Here, a flue gas is usually rinsed with water to remove the acidic gases, such that it becomes saturated with water vapor. As such, the separation material is generally preferred to have good performance in saturated water vapor conditions, and the separation results produced using PAMAM dendrimers are encouraging [10,11].

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Since the PAMAM dendrimer is liquid materials, it is necessary to affix PAMAM dendrimer to appropriate support materials. The excellent CO2 selectivity of this liquid dendrimer is encouraging for effective CO2 separation from fossil fuel emissions, providing that the dendrimer can be successfully fabricated with a stable membrane configuration, having sufficient tolerance for practical use (e.g., pressure difference). In this regard, a composite membrane would be considered an appropriate configuration for preparing stable, selective dendrimer layers. Composite membranes typically possess a selective layer affixed to the top of a mesoporous support, where the selective layer and support are fabricated from different materials. The composite membrane requires the selective layer to be as thin as possible in order to get economical fluxes. Composite membranes provide a more flexible approach due to their costeffectiveness, ease of fabrication, and the many combinations in which various supports and selective layers can be prepared. To date, various composite membranes have been examined and reported [12–17]. In the preparation of a composite membrane, a gutter layer is often formed prior to formation of the selective layer. The gutter layer [12–14] is a permeable material which serves as both a channeling mechanism and an adhesive medium between the selective layer and the support substrate. The gutter layer is required to have an affinity for both the selective layer and the support substrate. In our previous paper, we successfully prepared PAMAM dendrimer composite membranes using chitosan as the gutter layer [18], demonstrating the possibility of fabricating solid stable membranes from liquid viscous PAMAM dendrimers. However, the details concerning the preparation of the chitosan gutter layer were not discussed. In this paper, the preparation and role of the chitosan gutter layer in the PAMAM dendrimer composite membrane have been investigated. Here, the effects of the chitosan molecular weight and the introduction of cross-linkages between the chitosan molecules upon obtaining a uniform chitosan layer were investigated. The influences of both the chitosan solution concentration and the ratio of cross-linking reagent to chitosans, on the CO2 separation properties, permeance and selectivity, were also discussed. In addition, the membrane structure was observed and discussed.

2. Experimental 2.1. Materials Industrial grade chitosans were obtained as colorless ramentum from Kimika Co. Ltd., and used as received. The molecular weights of chitosan H and LL are indicated in the corresponding Kimika technical data sheets as 500,000 and 50,000, respectively. The deacetylation ratios of chitosan H and LL were 84% and 85%, respectively. Reagent grade poly(amidoamine) dendrimers (Generation 0, G = 0) with ethylenediamine cores were obtained as 20 wt% methanol solutions from Aldrich Co. and used without further purification. Reagent grade ethylene glycol diglycidyl ether (EGDGE) was purchased from Tokyo Chemical Industry Co., Ltd. (the epoxy equivalent weight of EGDGE was 87 g/eq.). Methanol, ethanol, and acetic acid were obtained from Wako Co. Ltd. Ultrafiltration (UF) hollow fiber membranes (NTU-3250-C1R, Nitto Denko Corp.), constructed from polysulfone (PSF) were used as the support substrates of the composite membranes. The outer and inner diameters of the hollow fiber membranes were 1900 and 1100 ␮m, respectively, while the molecular weight cutoff of the membrane is 6000, as indicated on the corresponding data sheet. The ultrafiltration membrane obtained in the wet state was dried under ambient temperature and atmosphere, and further dried at 60 ◦ C for 8 h. SEM images of the inner surface of the dry PSF hollow fiber membrane revealed oval pores with a longitudinal length of 5–100 nm (Fig. 1). The size of the pores is far larger than those of the PAMAM dendrimer (G = 0), 1.5 nm [19] formed on the surface of the UF membrane substrate. To affix the smaller PAMAM dendrimer molecules to the porous surface of the PSF substrate, the pores should be filled or covered with a gutter layer that has an affinity for both the PSF substrate and the PAMAM dendrimer. The chemical structures of polysulfone, the PAMAM dendrimer (G = 0), and chitosan are shown in Fig. 2. From the figure, it can be seen that the hydrophobic PSF has no polar moiety, whereas the PAMAM dendrimer comprises hydrophilic moieties consisting of amino groups. Therefore, the formation of an interaction between the hydrophilic PAMAM dendrimer and the hydrophobic PSF substrate that is strong enough to withstand the operation conditions for membrane separation, requires a suitable amphiphilic gutter material. From Fig. 2, it can be seen that chitosan has both a hydrophobic main chain and hydrophilic

Fig. 1. SEM images of PSF ultrafiltration membrane: (a) cross-section and (b) inner surface.

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Fig. 2. Chemical structure of polysulfone, chitosan, and PAMAM dendrimer (Generation 0).

moieties comprising both amino and hydroxyl groups. As such, amphiphilic chitosan was selected as the gutter layer material for the composite membrane described herein. 2.2. Preparation of composite membrane Fig. 3 shows a schematic diagram of the adopted In-situ Modification (IM) method, which utilizes the interfacial precipitation of the membrane materials. The IM method is for the preparation of ultrathin functional layers of membrane materials, and is applicable to any size of module. In the figure, a solution containing the membrane materials is directly circulated in the lumen side of the hollow fiber membranes within the membrane module, while the shell side of the hollow fiber membranes was evacuated. As shown in Fig. 3(c), if a hydrophobic porous substrate and hydrophilic solution are applied, the hydrophilic solution is unable to penetrate into the hydrophobic porous substrate, but instead generates a gas–liquid interface at the substrate surface. Due to the reduced pressure on the opposite side of the membrane, the solvent undergoes evaporation at the interface, resulting in a dense solute concentration in the vicinity of the interface. Finally, the membrane material precipitates to form an ultrathin layer on or beneath the surface. One of the main advantages of this IM method utilizing the interface precipitation is to cover the membrane pores at the surface with molecules smaller than the pore size. In this study, a PSF-UF membrane

and an aqueous chitosan solution were selected as hydrophobic and hydrophilic media, respectively. Even though the average pore size of the PSF substrate is far larger than those of chitosan, the interfacial precipitation is likely to create an effective uniform chitosan layer. Three UF hollow fiber membrane samples were inserted into a module housing (length = 20 cm, diameter = 0.9525 × 10−2 m (3/8 in.)), such that the effective membrane area of the module was about 18 cm2 . Various chitosan solutions for investigating the performance of the resulting gutter layers were prepared by dissolving chitosan in 2 wt% aqueous acetic acid. Chitosan layers were prepared from either a 0.1 wt% chitosan H solution, or from a 0.1 wt% mixed solution of chitosan H (Mw : 500,000) and chitosan LL (Mw : 50,000), with a H/LL wt. ratio: 10. A small amount of addition of chitosan H, a H/LL ratio of 10, was chosen in order to keep a viscosity similar to chitosan H solution. All chitosan solutions were filtered prior to use. When EGDGE was used as the crosslinker, the molar ratio of the epoxy moiety of EGDGE to the amine moiety of chitosan ranged from 0.1 to 0.5. When an epoxy/amine molar ratio exceeded 0.5, the chitosan solution was not stable to form small gelatinous particles. The chitosan solution was circulated through the lumen side of the hollow fiber membranes in the membrane module at a linear velocity of 8 m/min for 30 min, while the shell side was evacuated with a vacuum pump at 3 kPa. After coating the

Fig. 3. Diagrammatic illustration of In-situ Modification (IM) method. (a) Flow diagram, (b) solution flow in hollow fiber, and (c) concept of interfacial precipitation of membrane materials.

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membrane, the excess chitosan solution was removed from the bore of the fiber by air purge, followed by heating at 60 ◦ C for 2 h to form a nascent chitosan layer in an ammonium ion state. The chitosan layer was first treated with 0.1 M NaOH aqueous solution to neutralize the amino moiety, and then rinsed with 50 wt% aqueous ethanol, followed by drying at 60 ◦ C for 8 h to produce the desired chitosan layer on the porous surface of the substrate. A 20 wt% PAMAM dendrimer (G = 0) methanol solution was diluted with ionized water to 10 wt%, and then circulated through the lumen side of the chitosan-treated hollow fiber membrane in the module at a linear velocity of 8 m/min for 30 min, without first evacuating the shell side. After removing the remaining PAMAM dendrimer solution by air purge, the resulting product was dried at 60 ◦ C for 8 h to produce a PAMAM dendrimer composite membrane.

fiber membrane was swept with He gas at a rate of 24 mL/min at atmospheric pressure. The flow rate of the feed stream was at least 100 times larger than that of the permeate to keep gas concentration constant in the axial direction in the lumen side of the fiber. The permeate flow rate was measured using a soap bubble meter. The composition of the permeate was measured by gas chromatography (GC: GL Science Co. Ltd., GC-380) equipped with a thermal conductivity detector (TCD). For the pressure difference experiments, the permeance (m3 (STP) m−2 s−1 kPa−1 ), Q, was obtained by: Q=

Fp · Xp A · p

where Fp is the total permeate flow rate (m3 (STP) s−1 ), Xp the gas molar fraction in the permeate, A the membrane area (m2 ), and p is the partial pressure difference (kPa). The separation factor of CO2 over N2 is given by:

3. Characterization of the composite membrane αCO2 /N2 = 3.1. Gas permeation The gas permeation rate of the composite membranes was measured using an apparatus equipped with a humidifier, as shown in Fig. 4. The temperatures of the humidifier, membrane module, and the piping were set to 40, 40, and 60 ◦ C, respectively. The CO2 (5%)/N2 (95%) feed gas mixture was saturated with water vapor produced by the humidifier, and was supplied to the lumen side of the hollow fiber membrane at atmospheric pressure. For the pressure difference experiments, the shell side of the hollow fiber membrane was evacuated with a vacuum pump to produce a pressure difference of about 100 kPa. For the isobaric experiments, the permeate shell side of the hollow

(1)

pCO2 -p /pN2 -p pCO2 -f /pN2 -f

(2)

where pCO2 -p , pCO2 -f , pN2 -p , and pN2 -f represent the gas molar fractions of CO2 or N2 for the permeate and feed, respectively. Water vapor pressure was subtracted from the total pressure in the feed and the permeate to obtain the exact partial pressures of CO2 and N2 . Under these experimental conditions, the separation factor is almost equivalent to the ideal separation factor (i.e., the ratio of CO2 permeance over N2 ), due to the sufficiently large pressure ratio of feed to permeate such as 20 or more. For the isobaric experiments, the permeance (Q) was obtained by: Q=

F · Xg A · p

(3)

where F is the total flow rate of the He carrier and permeate (m3 (STP) s−1 ), Xg the gas molar fraction in the He carrier and permeate, A the membrane area (m2 ), and p is the partial pressure difference (kPa). Permeance and selectivity became stable 360 and 120 min after the start of the test in the pressure difference and the isobaric experiments, respectively. A variation of membrane performance stayed within 10%. 3.2. Scanning electron microscopy The morphologic analysis of the hollow fiber membrane was performed using scanning electron microscopy (SEM, Hitachi S-5000), at an accelerating voltage of 7 kV. Cross-sections of the hollow fiber membrane were obtained by fracturing the membrane in liquid N2 , and these sections were made conductive by coating with Pt/Pd. 3.3. X-ray photoelectron spectrometry Fig. 4. Schematic diagram of experimental setup for CO2 /N2 separation testing of a hollow fiber membrane module. (a) A pressure difference experiment and (b) an isobaric experiment. (1) CO2 /N2 (v/v, 5/95) mixed gas, (2) humidifier, (3) thermostat, (4) membrane module, (5) gas chromatograph, (6) soap bubble meter, (7) vacuum pump, (8) He sweep gas, and (9) manometer.

X-ray photoelectron spectrometry (XPS, Surface Science Instruments S-Probe ESCA Model 2803) equipped with an Al K␣. X-ray source, was used to investigate the surface of the composite membrane.

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4. Results and discussion Commercial ultrafiltration hollow fiber membranes prepared from polysulfone were selected as substrates for use as PAMAM dendrimer composite membranes. As mentioned in Fig. 1(b), the UF-substrate pore size is typically at least 30 times greater than the molecular size of the PAMAM dendrimer. Amphiphilic chitosan was selected as a gutter layer to reduce the PAMAM dendrimer pore size by covering the openings. 4.1. Formation of chitosan gutter layer Chitosan layers were prepared from either a 0.1 wt% chitosan H solution, or from a 0.1 wt% mixed solution of chitosan H (Mw : 500,000) and chitosan LL (Mw : 50,000), with a H/LL wt. ratio: 10. SEM images of the inner surface of the chitosan-treated PSF substrates are shown in Fig. 5. After chitosan H treatment, the pores on the surface were either diminished or even removed, as compared to those in Fig. 1. In particular, the nodular structure observed on the surface in Fig. 1 became obscured after chitosan H treatment. Treatment with chitosan H/LL on the other hand, filled the pore. These results suggest that IM treatment has possibility of preparing chitosan layers on the PSF substrate pores. Here, both treatments were effective in removing the pores in the UF membrane. However, neither treatment was sufficient to completely remove the pores. Immediately after chitosan H/LL treatment of the UF membrane surface using the IM method, the resulting wet nascent chitosan layer should cover the whole membrane surface, including the existing pores. However, shrinkage of the swollen chitosan layer during the drying process resulted in defects in the chitosan layer over the PSF substrate pores. Self-standing chitosan films prepared from a mixture of chitosan H and LL, absorbed about 100 wt% of ionized water against its dry weight at 25 ◦ C. That is, the volume of the swollen chitosan film was approximately twice as large as that of the dry film. This result implies that the volume of the swollen chitosan layer just after preparation by the IM method is halved on drying. As such, the pores observed after chitosan treatment might be due to the fracture of the existing chitosan layer on

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the pores caused by shrinkage. Consequently, preventing the formation of fractures in the chitosan layer during shrinkage may result in uniform, defect-free chitosan layers on the porous surface. 4.2. Cross-linking of the chitosan layer Here, it was decided that a chitosan network structure should be adopted to avoid fracturing of the chitosan layer during shrinkage. The frequently used epoxy compound ethylene glycol diglycidyl ether was employed as a cross-linking agent, to react with chitosan to form a network structure, as shown in Fig. 6. EGDGE was added to the 0.1 wt% chitosan H/LL solution (H/LL wt. ratio: 10) such that the epoxy moiety in EGDGE is in a 0.1 molar ratio with the amine moiety in the chitosan mixture. Fig. 7 shows the inner surface of the chitosan-treated substrate. As shown in the SEM image, almost all of the pores on the surface were covered with a chitosan layer. The addition of the cross-linking agent was effective in preparing a uniform defect-free chitosan layer. Fig. 8 shows XPS spectra of the inner surface of the PSF substrate, before and after treatment with the chitosan H/LL solution containing EGDGE. In the figure, the N(1s) signal derived from chitosan was observed for the chitosan-treated substrate, while no N(1s) signal was observed for the original PSF substrate. On the other hand, the S(2p) signal derived from the PSF substrate was also observed for the chitosan-treated substrate. This suggests that a chitosan layer was formed on the surface of the PSF substrate, and that the thickness of this layer is sufficiently thin (<10 nm) to enable photoelectrons to escape. For obtaining an insight into the cross-linkage in the chitosan layer, the reaction between chitosan and EGDGE in the corresponding film was analyzed by FT-IR. A chitosan solution containing EGDGE was deposited onto a silicon wafer and dried at room temperature for 1 h in vacuo to give a soft, wet residue, typical of a nascent chitosan gutter layer formed just after interfacial precipitation. FT-IR was then used to trace the amount of chitosan amino groups remaining in the wet residue over time. Here, the FT-IR results indicated that 10% of the amino groups had reacted with the EGDGE epoxy moieties after 1 h at room temperature, and 50% had reacted after 3 h. This result implies

Fig. 5. SEM images of the inner surface of substrate treated with, (a) 0.1 wt% chitosan H and (b) 0.1 wt% chitosan H/LL (H/LL wt. ratio: 10).

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Fig. 6. Conceptual diagram of cross-linked structure of chitosan and EGDGE.

that cross-linkages arise in the chitosan gutter layer during preparation. Considering the large molecular weight associated with chitosan H (Mw = 500,000), it would take only a relatively small number of cross-linkages to obtain a uniform chitosan gutter layer that is stable during drying.

Fig. 9 shows the relationship between the CO2 /N2 separation properties and the chitosan concentration used for preparing gutter layers in PAMAM dendrimer composite membranes. From Fig. 9, a higher chitosan concentration gives a larger CO2 /N2

4.3. Influence of chitosan concentration on CO2 separation properties To obtain an optimum chitosan gutter layer, various chitosan solution concentrations (0.1–0.5 wt%) were examined. Here, the weight ratio of chitosan H to LL was 10, and the ratio of EGDGE epoxy groups to chitosan amine groups (epoxy/amine molar ratio) was 0.1. After forming a chitosan layer using the IM method, the chitosan-treated PSF substrate was then treated with a 10 wt% PAMAM dendrimer (G = 0) aqueous methanol solution in order to produce a PAMAM dendrimer composite membrane. The gas permeation data for the PAMAM dendrimer composite membrane was obtained under isobaric conditions, using the apparatus shown in Fig. 4. Chitosan-treated substrates prior to treatment with PAMAM dendrimers did not show CO2 selectivity with a large CO2 permeance which is comparable to that exhibited by the UF membrane.

Fig. 7. SEM image of the inner surface of substrate treated with 0.1 wt% chitosan H/LL solution (H/LL wt. ratio: 10) containing EGDGE (epoxy/amine molar ratio: 0.1).

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Fig. 8. XPS spectra of the inner surface of original PSF substrate and that treated with 0.1 wt% chitosan H/LL solution (H/LL wt ratio: 10) containing EGDGE (epoxy/amine molar ratio: 0.1).

separation factor. A PAMAM dendrimer composite membrane prepared from a 0.5 wt% chitosan solution showed a CO2 /N2 separation factor of over 100 under isobaric measurement conditions. On the other hand, the CO2 permeance (QCO2 ) was almost the same regardless of the chitosan concentration (0.1–0.5 wt%). Based on this result, a higher chitosan solution concentration is preferable for achieving a larger CO2 /N2 selectivity. However, high chitosan concentrations lead to higher viscosities, making it difficult to handle the solution. As a result, the optimum chitosan concentration is determined to be 0.5 wt%. For practical usage, a PAMAM dendrimer composite membrane should be able to endure an applied pressure difference. When a pressure difference of about 100 kPa was applied to a composite membrane having a CO2 /N2 separation factor of 100 in isobaric conditions, no selectivity was obtained, indicating the destruction of the active layer for CO2 separation. In other words, the mechanical strength of the chitosan gutter layer was not strong enough to withstand an applied pressure of 100 kPa. For a mechanically stable gutter layer, the optimum ratio of EGDGE epoxy groups to chitosan amine groups was investigated in the range from 0.1 to 0.5. 4.4. Influence of epoxy/amine ratio on CO2 separation performance A chitosan gutter layer was prepared from a 0.5 wt% chitosan H/LL solution (H/LL wt. ratio: 10) with various epoxy/amine

Fig. 9. Influence of chitosan concentration on QCO2 and αCO2 /N2 of PAMAM dendrimer composite membrane at isobaric experimental condition. () QCO2 (m3 (STP) m−2 s−1 kPa−1 ), (䊉) αCO2 /N2 , chitosan H/LL wt. ratio: 10, epoxy/amine molar ratio: 0.1.

molar ratios (ranging from 0.1 to 0.5), and then treated with PAMAM dendrimer. Fig. 10 shows the CO2 permeances and CO2 /N2 separation factors for the PAMAM dendrimer composite membranes prepared from these different epoxy/amine ratios. During the tests, a pressure of about 100 kPa was applied. As shown in the figure, the CO2 /N2 separation factor increased with increasing epoxy/amine ratios up to 0.5. In particular, an epoxy/amine ratio of 0.5 afforded a large CO2 /N2 separation factor of 90 under a total pressure difference of 100 kPa. On the other hand, QCO2 was observed to decrease slightly on increasing the epoxy/amine ratio. As a result, the optimum chitosan gutter layer was prepared from the following chitosan solution: • Chitosan concentration: 0.5 wt%. • Ratio of chitosan H to LL: 10 (=chitosan H/LL (10/1)). • Ratio of epoxy to amine: 0.5. Table 1 summarizes the gas permeation results for the original and chitosan-treated PSF substrate, and for the resulting PAMAM dendrimer composite membrane. Chitosan treatment was performed under the optimized conditions mentioned above. The gas permeation values for the PAMAM dendrimer composite membrane were obtained 360 min after the start of the operation. The PAMAM dendrimer membrane showed an excellent CO2 separation factor of 400 under practical operating conditions (100 kPa pressure difference at 40 ◦ C). CO2 /N2 sep-

Fig. 10. Influence of epoxy/amine ratio on QCO2 and αCO2 /N2 of PAMAM dendrimer composite membrane at a pressure difference of 100 kPa. () QCO2 (m3 (STP) m−2 s−1 kPa−1 ), (䊉) αCO2 /N2 , chitosan solution concentration: 0.5 wt%, chitosan H/LL wt. ratio: 10.

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Fig. 11. SEM images of PAMAM dendrimer composite membrane and substrates. Surfaces image: (1a) PSF substrate, (2a) chitosan-treated substrate, (3a) PAMAM dendrimer composite membrane, cross-sectional image: (1b) PSF substrate, (2b) chitosan-treated substrate, (3b) PAMAM dendrimer composite membrane. Schematic diagram of cross-section: (1c) PSF substrate, (2c) chitosan-treated substrate, (3c) PAMAM dendrimer composite membrane.

aration factor of 400 at 360 min was larger than that at 120 min, while QCO2 was constant during the operation. A decline of N2 permeance between at 120 and 360 min leaded to increase of CO2 /N2 selectivity even though CO2 permeance was constant. SEM images of the surface and cross-sections of the PSF substrate, chitosan-treated substrate and PAMAM dendrimer composite membrane, are shown in Fig. 11. The same SEM image of the PSF substrate surface in Fig. 1 was used again here for the convenience of comparison. In Fig. 11(1a), the inner surface of the PSF substrate revealed many pores of varying size (from 5 to 50 nm). After chitosan treatment, no observable pores were found to exist in Fig. 11(2a), and the surface of the PAMAM dendrimer composite membrane was observed to be uniform and flat, as shown in Fig. 11(3a). Table 1 CO2 permeance and CO2 /N2 separation factor of PAMAM dendrimer composite membrane and its substrate

1. PSF substrate 2. (1) + chitosan treatment 3. (2) + PAMAM dendrimer treatment

QCO2 (m3 (STP) m−2 s−1 kPa−1 )

αCO2 /N2

1.5 × 10−4 4.3 × 10−5 1.6 × 10−7

1 1 400

Experimental condition for PAMAM dendrimer composite membrane: feed, a mixture of 5 vol% of CO2 and 95 vol% of N2 saturated with water vapor at atmospheric pressure. Pressure difference: 100 kPa, temperature: 40 ◦ C.

Regarding the cross sectional SEM image of the PSF substrate, the top surface (of thickness 200 nm) was composed of tightly packed polymer nodules, while the inside of the substrate revealed a porous morphology. In the chitosan-treated substrate, the chitosan layer was observed to be formed about 200 nm from the top surface in Fig. 11(2b). For a better understanding, a schematic diagram of the cross sectional SEM image is shown in Fig. 11(2c). The thin uniform chitosan layer observed in the SEM image seems to suggest that a gas–liquid interface is formed between the hydrophilic chitosan solution and the porous surface of the hydrophobic PSF substrate. The formation of the gas–liquid interface could be useful for preparing ultrathin chitosan gutter layers directly beneath the surface. In the PAMAM dendrimer composite membrane, a single uniform dense layer of thickness 300 nm was observed. Further observation revealed that part of the layer (200 nm thickness) existed beneath the surface of the substrate, while the remainder (100 nm thick) existed on the surface. The XPS spectrum of the inner surface of the PAMAM dendrimer composite membrane is shown in Fig. 12. The observed XPS signal intensity fractions (0.68, 0.28, and 0.12) for C(1s), O(1s), and N(1s), are intermediate between those observed for the chitosan and PAMAM dendrimer components. From the XPS analysis, the ratio of PAMAM dendrimer to chitosan was calculated in the range from 20 to 40%. This result suggests that the top layer of 300 nm observed in Fig. 11(3b) is a hybrid of

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Okuda for conducting the SEM measurements and Ms. Natsumi Baba for performing the gas permeation tests. References

Fig. 12. XPS spectrum of the inner surface of PAMAM dendrimer composite membrane. Value in the parenthesis means a peak intensity.

chitosan and PAMAM dendrimer molecules. Here, we can confirm that in the PAMAM dendrimer composite membrane, the chitosan gutter layer plays the role of medium for fixing liquid PAMAM dendrimers. The hybridization of PAMAM dendrimer and chitosan also seems to overcome the minute defects of the chitosan layer to exhibit an excellent CO2 /N2 selectivity of 400. 5. Conclusions Novel composite hollow fiber membranes prepared from PAMAM dendrimers were developed using an In-situ Modification method that utilizes the interfacial precipitation of the membrane material. PAMAM dendrimer composite membranes suitable for CO2 separation were fabricated on porous PSF hollow fiber substrates, comprising a chitosan gutter layer and PAMAM dendrimers as the active material. The PAMAM dendrimer composite membrane fabricated with a chitosan gutter layer prepared from a 0.5 wt% solution of two different molecular weight chitosans (H/LL wt ratio: 10) and an ethylene glycol diglycidyl ether cross-linking agent (epoxy/amine molar ratio: 0.5), afforded an excellent CO2 /N2 separation factor (400) and CO2 permeance (1.6 × 10−7 m3 (STP) m−2 s−1 kPa−1 ) under practical operating conditions (pressure difference of 100 kPa and a temperature of 40 ◦ C). The resulting PAMAM dendrimer composite membrane of thickness 300 nm comprised a hybrid layer of chitosan and PAMAM dendrimer molecules. Using the IM method, ultra-thin active layers for CO2 separation were successfully fabricated on porous substrates. Acknowledgements This work was supported by the Ministry of Economy, Trade, and Industry. The authors would like to thank Ms. Masayo

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