PPEPG Membranes for CO2

Sep 1, 2010 - Hashem Ahmadizadegan , Mahdi Ranjbar , Sheida Esmaielzadeh. Polymer ... Hashem Ahmadizadegan , Ruhollah Khajavian. Journal of the ...
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Ind. Eng. Chem. Res. 2010, 49, 9363–9369

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Aminosilane Cross-linked PEG/PEPEG/PPEPG Membranes for CO2/N2 and CO2/H2 Separation Hailin Cong* and Bing Yu College of Chemical and EnVironmental Engineering, Qingdao UniVersity, Qingdao 266071, China

Aminosilane-modified poly(ethylene glycol) (PEG), PEG-block-poly(propylene glycol) (PPG)-block-PEG (PEPEG), and PPG-block-PEG-block-PPG (PPEPG) diacrylate prepolymers are synthesized as new membrane materials for gas separation. By combining the in situ sol-gel process and cross-linking process together, the silane-modified prepolymers form flexible nanocomposite membranes with a maximum CO2 permeability of 274 barrer and CO2/N2 selectivity of about 30. The CO2 permeabilities of silane cross-linked PEPEG and PPEPG membranes are higher than those of silane cross-linked PEG membranes, whereas the CO2/N2 and CO2/H2 selectivities increase as the PEG content increases in the membrane. A proper aminosilane/prepolymer reaction ratio benefits the gas separation performance of the membrane due to the formation of different cross-linking structures and composites. 1. Introduction Significant improvements in the performance of polymeric gas separation membranes have come to light in the past two decades,1-3 and our understanding of the relationships among the structure, permeability, and selectivity of polymeric membranes has been greatly advanced.4,5 Newer polymeric membrane materials such as polyimide (PI) and cross-linked polyethylene glycol (PEG) have been developed continuously.6-10 It is well-known that PEG has very strong affinity to CO2 molecules,11-14 and its derivatives have been studied for potential applications in CO2 separation membranes.15-18 The studies were focused on copolymerization, composite, and crosslinking of the membrane, because of the poor film formation ability and strong crystallization tendency of the PEG-containing materials.19-24 Okamoto et al.25 investigated gas permeation properties of poly(ether imide) segmented copolymer films prepared from PEG derivatives. The copolymer films with a PEG content of 70 wt % displayed high performance; for instance, the CO2 permeability (PCO2) was 140 barrer and CO2/ N2 selectivity (RCO2/N2) was 70 at 25 °C. Polymer-inorganic nanocomposite membranes of poly(amide-6-block-PEG) (PEBAX)/silica were prepared by Lee et al.26 via in situ polymerization of TEOS using the sol-gel process. The membrane had a CO2 permeability of 277 barrer and CO2/N2 selectivity of 79. Patel et al.27 prepared cross-linked membranes of PEG/silica by dispersing silica nanoparticles in diacrylate-terminated PEG and then adding 2,2′-azobis(isobutyronitrile) to initiate the polymerization. The membrane had a CO2 permeability of 83 barrer and CO2/H2 selectivity of 11. Kim et al.28 reported that by using the sol-gel method, the nanocomposite membrane with PEG as the organic phase and hydrolysate of TEOS as the inorganic phase showed good performance in CO2/N2 separation. The CO2 permeability was 94.2 barrer with a CO2/N2 selectivity of 38.3. By simply changing the PEG matrix to PPEPG, Sforca et al.29 prepared nanocomposite membranes by using the same method and reported that the CO2 permeability increased to 125 barrer with a CO2/N2 selectivity of 89. Freeman et al.30 reported cross-linked membranes of highly branched PEG derivatives can be used as plasticization-enhanced hydrogen purification materials, which exhibited outstanding separation performance * To whom correspondence should be addressed. Tel: +86-5328508-3445. Fax: +86-532-8595-5529. E-mail: [email protected].

for H2 purification by removing acid gases such as CO2 and H2S from feed streams of practical interest. Recently, Chung et al.31 prepared cross-linked PPEPG/silica membranes for hydrogen purification by using the in situ sol-gel reaction of amino-terminal PPEPG oligomers with epoxy-functional silanes. The membrane demonstrated an appealing CO2 permeability of 367 barrer with an attractive CO2/H2 selectivity of 8.95 at 3.5 atm and 35 °C, and the cross-linked composite membranes with enhancements in mechanical and thermal properties were promising for industrial-scale hydrogen purification. In this paper, by combining the in situ sol-gel process and cross-linking process together, we report aminosilane-modified PEG, PPEGP, and PEPEG diacrylate prepolymers as new membrane materials for gas separation. The effects of prepolymer molecular weight, struture, PEG content, and silane/ prepolymer reaction ratio to the CO2/N2, CO2/H2 separation performance of the silane cross-linked membranes were studied and are discussed preliminarily. 2. Experimental Section 2.1. Chemicals. PEG (PEG2000, Mn ) 2000), PPEPG (PPEPG2000, Mn ) 2000 containing 50 wt % PEG; PPEPG2700, Mn ) 2700 containing 40 wt % PEG), PEPEG (PEPEG1900, Mn ) 1900 containing 50 wt % PEG; PEPEG2000, Mn ) 2000 containing 10 wt % PEG; PEPEG2800, Mn ) 2800 containing 15 wt % PEG; PEPEG2900, Mn ) 2900 containing 40 wt % PEG; PEPEG4400, Mn ) 4400 containing 30 wt % PEG), 3-aminopropyltrimethoxysilane (97%), chloroform (CHCl3, 99.8%), and dichloromethane (CH2Cl2, 99.8%) were purchased from Aldrich. Poly(ethylene glycol) (PEG1000, Mn ) 1000; PEG3000, Mn ) 3000) and ethyl ether (99%, anhydrous) were purchased from Fluka. Triethylamine (99.5%) and tetrahydrofuran (THF, 99.99%) were purchased from EMD. Acryloyl chloride (96%) was purchased from Lancaster. All of the chemicals were used as received. 2.2. Synthesis of PEG/PEPEG/PPEPG Diacrylate. PEG2000 diacrylate was synthesized according to a method reported eleswhere.32 To a 100 mL flask equipped with a magnetic stirrer were added 10 g of PEG2000, 8.14 g of triethylamine, and 40 mL of THF and mixed into a homogeneous solution at 25 °C. After the flask had been cooled to 2 °C in an ice-water bath, 3.77 g of acryloyl chloride was added dropwise into the flask

10.1021/ie1012568  2010 American Chemical Society Published on Web 09/01/2010

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Scheme 1. Reaction Mechanism during the Fabrication of Aminosilane Cross-linked PEG/PEPEG/PPEPG Membranes

in 30 min. The reaction was carried out at 2 °C for 2 h and then at 25 °C for 6 h. The byproduct of triethylamine hydrochloride salt was removed by filtration. The polymer product was precipitated out from the filtrate by 300 mL of cold ethyl ether and dried under vacuum at 25 °C for 1 h. The obtained polymer was dissolved in 2 mL of water to further remove the salt byproduct and extracted out by using 80 mL of CH2Cl2 from its water solution. After removal of the CH2Cl2 by a rotavapor at 35 °C and drying under vacuum at 25 °C for 5 h, 5.3 g of PEG2000 diacrylate was obtained, which was a light yellow powder, and the yield was 50%. The other PEG, PPEPG, and PEPEG diacrylate prepolymers with different molecular weights were synthesized by using the same method as the aforementioned PEG2000 diacrylate. 2.3. Membrane Preparation. The 3-aminopropyltrimethoxysilane and PEG (PEPEG, PPEPG) diacrylate were mixed at 2:1 or 1:1 mol ratio in 50 wt % chloroform and sealed by N2 for 24 h at 25 °C to form the casting solutions, following the Michael reaction depicted in Scheme 1.33 The casting solution was dropped into Teflon Petri dishes, and the solvent was slowly evaporated at 25 °C, opening to moisture air for about 2 days.

During the process, cross-linked membranes were formed by the in situ sol-gel reaction of silane groups.31 A final drying step was carried out in a vacuum oven at 40 °C for 1 day. The resulting membranes were peeled off and stored in a desiccator for testing. The thickness of the membrane was about 800 µm. All of the membranes were transparent except the silane crosslinked PEG3000. 2.4. Characterizations. PEG (PPEPG, PEPEG) diacrylate and casting solution samples were dissolved in deuterated chloroform at a concentration of ∼2 wt % for 1H NMR analyses on a Bruker Advance DRX-400 spectrometer. The glass transition temperature (Tg) was determined by a differential scanning calorimeter (DSC, TA Instruments QP10) with a heating rate of 20 °C/min. All tests were repeated at least twice to ensure reproducibility. Transmission electron microscope (TEM, Philips CM120) was used to observe the membrane morphologies. The operation voltage of TEM was 75 kV. 2.5. Gas Separation Performance Test. Pure-gas permeation tests were performed at 25 °C and 10 psig feed upstream pressureusingtheconstant-volume,variable-pressuretechnique,34-37

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Figure 1. 1H NMR spectra of (a) PEG1000, (b) PEG1000 diacrylate, and (c) 2:1 silane/PEG1000 prepolymer casting solution.

in which a specimen was held under vacuum until it was exposed to a gas at a specific pressure. The value of gas permeability measured was determined from P)

dp VL ART∆P dt

[( ) - ( dpdt ) ] ss

leak

(1)

where P is the permeability (cm3(STP) cm/cm2 s cmHg), V is the downstream volume (cm3), L is the membrane thickness (cm), A is the membrane area (cm2), R is the gas constant () 0.278 cmHg cm3/cm3(STP) K), T is the absolute temperature (K), ∆P is the transmembrane pressure difference () p2 - p1, where p2 and p1 are the upstream and downstream pressures (cmHg), respectively), and (dp/dt)ss and (dp/dt)leak are the steadystate rates of pressure rise (cmHg/s) in the downstream volume at a fixed upstream pressure and under vacuum, respectively.38 The permselectivity (R) was determined from R ) PA/PB

(2)

where PA and PB are the permeabilities of pure gases A and B, respectively. 3. Results and Discussion 3.1. 1H NMR. 1H NMR spectroscopy (Figures 1 and 2) is used to montior each reaction step of the prepolymer synthesis and modification. For the PEG raw material (Figure 1a), the peak at δ 3.60 (4H, C2H4O) is the characteristic signal of the protons on the ethylene oxide units of the PEG polymer. After the PEG reacted with acryloyl chloride, three characteristic acrylate peaks at δ 6.33 (1H, CH2), 6.05 (1H, CH), and 5.75 (1H, CH2) appear in the spectrum of the PEG diacrylate (Figure 1b). After mixing of 3-aminopropyltrimethoxysilane with the PEG diacrylate at the ratio of 2:1 (Figure 1c) for 24 h, the three characteristic acrylate peaks disappear in the casting solution and change to δ 2.85 (2H, CH2N) and 2.50 (2H, CH2) after addition with the amino groups, and new peaks of the silane groups appear at δ 3.55 (3H, CH3OSi), 2.60 (2H, CH2N), 1.58 (2H, CH2), and 0.62 (2H, CH2Si), which means the Michael

addition reaction of amino groups and acrylate groups occurred successfully as depicted in Scheme 1. For the PPEPG raw material (Figure 2a), the peaks at δ 3.50 (2H, CH2), 3.34 (1H, CHO), and 1.10 (3H, CH3) are the characteristic signals of the protons on the propylene oxide units of the poly(propylene glycol) (PPG) segments. The peak at δ 3.60 (4H, C2H4O) is the characteristic signal of the protons on the ethylene oxide units of the PEG segments. After the PPEPG reacted with acryloyl chloride, three characteristic acrylate peaks at δ 6.33 (1H, CH2), 6.05 (1H, CH), and 5.75 (1H, CH2) appear in the spectrum of the PPEPG diacrylate (Figure 2b). After mixing of 3-aminopropyltrimethoxysilane with the PPEPG diacrylate at the ratio of 1:1 (Figure 2c) or 2:1 (Figure 2d) for 24 h, the three characteristic acrylate peaks disappear in the casting solutions and change to δ 2.85 (2H, CH2N) and 2.50 (2H, CH2) after the addition with the amino groups, and new peaks of the silane groups appear at δ 3.55 (3H, CH3OSi), 2.60 (2H, CH2N), 1.58 (2H, CH2), and 0.62 (2H, CH2Si), which means the Michael addition reactions of aminosilane and PPEPG diacrylate occurred successfully as depicted in Scheme 1. Comparing Figure 2c with Figure 2d, we can see that when the ratio of silane/PPEPG2700 prepolymer is smaller, the peaks of the silane groups are weaker. Because PEPEG has characteristic peaks very similar to those of PPEPG, the 1H NMR spectra of PEPEG products of different reaction steps are not given to avoid repetition. 3.2. Silane Cross-linked PEG Membranes. The DSC curves of the silane cross-linked PEG membranes are shown in Figure 3. The Tg value s of silane cross-linked PEG2000 (Figure 3a) and PEG1000 (Figure 3b) are -52.7 and -42.4 °C, respectively. The Tm values of silane cross-linked PEG3000 (Figure 3c) and pure PEG2000 diacrylate (Figure 3d) are 51.2 and 54.9 °C, respectively. Comparing Figure 3a with Figure 3b, we can see the silane cross-linked PEG1000 has a higher Tg than the silane cross-linked PEG2000, because the PEG1000 membrane is heavily cross-linked with a shorter chain length between the cross-linking points. Comparing Figure 3a,b with Figure 3c, we can see the silane cross-linked PEG3000 is

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Figure 2. 1H NMR spectra of (a) PPEPG2700, (b) PPEPG2700 diacrylate, (c) 1:1, and (d) 2:1 silane/PPEPG2700 prepolymer casting solution.

Figure 3. DSC spectra of (a) 2:1 silane cross-linked PEG2000, (b) 2:1 silane cross-linked PEG1000, (c) 2:1 silane cross-linked PEG3000, and (d) pure PEG2000 diacrylate.

crystallized, whereas the silane cross-linked PEG1000 and PEG2000 are amorphous. Comparing Figure 3a with Figure 3d, we can see the pure PEG2000 diacrylate is highly crystallized, whereas the silane cross-linked one is amorphous, which means cross-linking can destroy the crystallization of PEG2000. Due to the effect of crystallization, the Tg values of the silane crosslinked PEG3000 (Figure 3c) and pure PEG 2000 diacrylate (Figure 3d) do not appear in our measurement. The gas separation performance of silane cross-linked PEG is shown in Table 1. Due to the crystallization, the silane crosslinked PEG3000 membrane has the worst gas separation performance among the three membranes. The silane crosslinked PEG2000 membrane performs better than the silane

cross-linked PEG1000 membrane due to their difference in cross-linking degrees. The PEG2000 membrane is relatively lightly cross-linked with a longer chain length between the crosslinking points and, thus, has a lower Tg and better morphology for the movement of polymer segments. 3.3. Silane Cross-linked PEPEG/PPEPG Membranes. From the gas separation performance of silane cross-linked PEG membranes, we can see that the noncrystalline state and crosslinking degree are the two important effect factors. The crosslinking degree can be controlled by the chain length (molecular weight of the prepolymers) between cross-linked points, and the crystallization of PEG can be eliminated by blocking the amorphous PPG segment into the PEG. Seven noncrystallized triblock polymers of PEG and PPG, which are the PPEPG and PEPEG polymers with different molecular weights and PEG contents, are studied in this section. As shown in Table 2, comparing the silane cross-linked PEG2000 membrane with silane cross-linked PEPEG2000 and PPEPG2000 membranes, we can see that when the prepolymer molecular weight is the same, the PEPEG2000 and PPEPG2000 membranes have much higher CO2 permeabilities than the PEG2000 membranes, because the ordered packing of PEG segments is alternated by the PPG segments in the copolymers, which subsequently improves the gas permeability of the membrane. Because the PEG segment is highly CO2-philic, we can see that the silane cross-linked PEG2000 membrane (100% PEG content) has much better CO2/N2 and CO2/H2 selectivities than the PPEPG2000 (50% PEG content) and PEPEG2000 (10% PEG content) membranes. Therefore, the more PEG content in the polymers, the higher the selectivity of CO2 over other gases is obtained.

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Table 1. Gas Separation Performance of Silane Cross-linked PEG sample silane cross-linked

silane/PEG

PCO2 (barrer)

PN2 (barrer)

PH2 (barrer)

R CO2/N2

R CO2/H2

PEG1000 (2:1) PEG2000 (2:1) PEG3000 (2:1)

2:1 2:1 2:1

14.7 66.0 3.8

1.1 2.1 3.0

7.9 7.3 3.9

13.4 31.3 1.3

1.9 9.1 1.0

Table 2. Gas Separation Performance of Silane Cross-linked PEPEG and PPEPG sample silane cross-linked

silane/polymer

PEG content (%)

PCO2 (barrer)

PN2 (barrer)

PH2 (barrer)

R CO2/N2

R CO2/H2

PEG2000 (2:1) PPEPG2000 (2:1) PEPEG2000 (2:1) PEPEG1900 (2:1) PEPEG2800 (2:1) PEPEG2900 (2:1) PEPEG4400 (2:1) PPEPG2700 (2:1)

2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1

100 50 10 50 15 40 30 40

66.0 128.3 151.7 173.6 219.1 197.1 286.1 163.7

2.1 6.3 11.0 7.2 16.7 8.2 22.6 7.8

7.3 26.6 47.5 30.1 52.0 45.0 62.0 38.3

31.3 20.3 13.8 24.1 13.1 24.1 12.7 21.1

9.1 4.8 3.2 5.8 4.2 4.4 4.6 4.3

From Table 2, we can see that the silane cross-linked PEPEG4400 has the highest CO2 permeability, and the PEPEG2900 has the best all-round gas separation performance in the seven copolymer membranes. Like the other six PEPEG and PPEPG membranes, the silane cross-linked PEPEG2900 membrane is transparent and homogeneous in microscale as shown in Figure 4. 3.4. Effect of Silane/Polymer Ratio. As shown in Table 3, the silane/polymer ratios will affect the gas separation performance of the cross-linked membranes. Compared with the 0:1 ratio cross-linked PEPEG2900 membrane, the 1:1 ratio crosslinked PEPEG2900 membrane has a higher gas permeability, whereas the gas selectivity decreases very little. Compared with the 2:1 ratio membranes, the 1:1 ratio cross-linked PEPEG and PPEPG membranes have better gas separation performances especially in the gas permeabilities. One possible reason for this phenomenon is that in the cross-linking process of the membranes, the trimethoxylsilane groups will turn into impermeable silica nanoparticles after hydrolysis under moisture, and the small amount of silica nanoparticles generated at a 1:1 silane/ polymer ratio will disrupt the polymer chain packing and improve the separation performance of the membrane; however, the presence of too many impermeable silica nanoparticles

generated at a 2:1 silane/polymer ratio will increase the penetrant diffusion pathway length and thus slow gas diffusion. The other possible reason is that when the silane/polymer reaction ratios are different, the formed silane-modified prepolymers have different structures for cross-linking (Scheme 1), which will affect the gas separation performance of the cross-linked membranes. The DSC curves of the cross-linked PEPEG2900 membranes at different silane/polymer ratios are shown in Figure 5. The Tg values of 0:1, 1:1, and 2:1 silane cross-linked PEPEG2900 membranes are -59.7, -63.1, and -62.6 °C, respectively. Both the 1:1 and 2:1 silane cross-linked PEPEG2900 membranes have lower Tg values than the 0:1 cross-linked one, which shows that the silane cross-linked membrane has a better morphology for the movement of polymer segments. 3.5. Gas Separation Performance of Silane Cross-linked PEG/PPEPG/PEPEG Membranes. As shown in Figure 6, the CO2 permeability and CO2/N2 selectivity points of 2:1 silane cross-linked PEG2000, 1:1 silane cross-linked PPEPG2700, and 2:1 and 1:1 silane cross-linked PEPEG2900 membranes are above a reference line referred to as Robeson’s line.39 The CO2/ H2 separation performances also lie close to the upper bound of Robeson’s line. These results indicate that the aminosilane

Figure 4. Photo and TEM images of the 2:1 silane cross-linked PEPEG2900 membrane. Table 3. Effect of Silane/Polymer Ratio on Gas Separation Performance of the Membrane sample silane cross-linked a

PEPEG2900 (0:1) PEPEG2900 (1:1) PEPEG2900 (2:1) PPEPG2700 (1:1) PPEPG2700 (2:1) a

silane/polymer

PEG content (%)

PCO2 (barrer)

PN2 (barrer)

PH2 (barrer)

R CO2/N2

R CO2/H2

0:1 1:1 2:1 1:1 2:1

40 40 40 40 40

259.2 273.8 197.1 240.1 163.7

8.1 9.3 8.2 9.5 7.8

34.9 40.4 45.0 45.9 38.3

32.0 29.3 24.1 25.3 21.1

7.4 6.8 4.4 5.2 4.3

Was cross-linked from PEPEG2900 diacrylate by 1 wt % BPO at 60 °C for 3 days.

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the gas permeability of the membrane, whereas the CO2/N2 and CO2/H2 selectivities increase as the CO2-philic PEG content increases in the membrane. A proper aminosilane/prepolymer reaction ratio benefits the gas separation performance of the membrane due to the formation of different cross-linking structures and composites. The aminosilane cross-linked PEG, PEPEG, and PPEPG membranes are promising for CO2/N2 and CO2/H2 separation. Acknowledgment We greatly acknowledge Prof. M. Radosz and Y. Shen at University of Wyoming for the help in finishing the paper. Figure 5. DSC spectra of (a) 0:1, (b) 1:1, and (c) 2:1 silane cross-linked PEPEG2900.

Figure 6. Gas separation performance of the silane cross-linked PEG, PEPEG, and PPEPG membranes.

cross-linked PEG, PEPEG, and PPEPG membranes are promising for CO2/N2 and CO2/H2 separation. 4. Conclusions Aminosilane-modified PEG, PPEPG, and PEPEG diacrylate prepolymers were synthesized successfully as new membrane materials for gas separation. After the in situ sol-gel process and cross-linking in moist air, the silane-modified prepolymers form flexible nanocomposite membranes with higher CO2 permeability than the unmodified ones. The gas permeabilities of silane cross-linked PEPEG and PPEPG membranes are much higher than those of silane cross-linked PEG membranes, because the ordered packing of PEG segments is altered by the PPG segments in the copolymers, which subsequently improves

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ReceiVed for reView June 9, 2010 ReVised manuscript receiVed August 9, 2010 Accepted August 20, 2010 IE1012568