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Aziridine-functionalized Mesoporous Silica Membranes on Polymeric Hollow Fibers: Synthesis and Single-Component CO2 and N2 Permeation Properties Hyung-Ju Kim, Watcharop Chaikittisilp, Kwang-Suk Jang, Stephanie A. Didas, Justin R. Johnson, William J. Koros, Sankar Nair, and Christopher W Jones Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503781u • Publication Date (Web): 11 Nov 2014 Downloaded from http://pubs.acs.org on November 15, 2014

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Industrial & Engineering Chemistry Research

Aziridine-functionalized Mesoporous Silica Membranes on Polymeric Hollow Fibers: Synthesis and Single-Component CO2 and N2 Permeation Properties

Hyung-Ju Kim, Watcharop Chaikittisilp, Kwang-Suk Jang, Stephanie A. Didas, Justin R. Johnson, William J. Koros, Sankar Nair,* and Christopher W. Jones,*

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia, 30332-0100 USA

Keywords: mesoporous silica, membrane, gas separation, hollow fiber, CO2 capture, aminefunctionalization

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ABSTRACT

The synthesis of aziridine-functionalized mesoporous silica membranes on polymeric hollow fibers is described, and their single-component CO2 and N2 permeation properties are investigated under both dry and humid conditions to elucidate the unusual permeation mechanisms observed in these membranes. Hollow fiber-supported mesoporous silica membranes are amine-functionalized with aziridine to yield hyperbranched aminopolymers within the membrane pores. The effects of the hyperbranched polymers in the mesopores on gas transport properties are investigated by single-component gas permeation measurements. The hyperbranched aminosilica membrane shows counter-intuitive N2 selective (over CO2) permeation during operation under dry conditions. Further characterization of the permeation behavior reveals the effects of strong adsorption of CO2 under dry permeation conditions, leading to reduced CO2 diffusivity because of CO2-induced amine crosslinking in the mesopores. On the other hand, the hyperbranched aminosilica membrane shows CO2 selective properties under humid conditions. Water molecules cause a lower degree of amine crosslinking and thus allow facilitated transport of CO2. This first study of hollow fiber-supported hyperbranched aminosilica membranes indicates that they can be tuned to have N2 or CO2 selective permeation properties under the conditions employed.


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1. INTRODUCTION A number of CO2 capture, sequestration, and utilization technologies are currently under investigation.1-3 Commercially available technologies for CO2 capture from flue gas are based on absorption using liquid amines.4, 5 One of their major drawbacks is the large amount of energy required for liquid amine regeneration. Polymeric membranes have been widely studied as an alternative for CO2 separation from the flue gas.6,

7

For polymeric membranes to be

economically attractive, they should provide high gas permeability and sufficient selectivity.8-10 Numerous studies have established the permeability-selectivity tradeoff in polymer membranes.11-14 Mesoporous silica membranes and thin films have the unique advantage that their pore size can be varied from 2-50 nm,15-19 and that their pores can be functionalized with a wide variety of organic or inorganic moieties. In the case of mesoporous silica powders, different types of organic molecules have been grafted to the mesopore walls to create catalysis or adsorption sites,20-24 including

monomeric or polymeric amine containing species via

impregnation,25, 26 grafting,27-29 or in-situ polymerization30,

31

to create CO2 selective binding

sites, thus yielding selective CO2 adsorbents that are effective in dry or humid environments. Amine modification of polymeric membranes has been used to enhance CO2 adsorption and permeate CO2 selectively by facilitated transport.32-34 We hypothesized that worm-like mesoporous silica membranes, reported in our recent work,35 could yield interesting properties for CO2 separation from N2 in the presence or absence of water vapor if functionalized with amine moieties. Poly(ethyleneimine) (PEI) is the most utilized polymer for CO2 adsorption.36 We have also previously developed an in-situ aziridine polymerization technique to graft hyperbranched amine polymers from the walls of mesoporous



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silica powders, creating hyperbranched aminosilica (HAS) materials.30,

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31, 37

It has been

reported38, 39 that ‘reverse’ selectivities (i.e., favoring the other component over CO2) could be obtained when there are extremely strong interactions between amine-containing polymer chains and CO2 molecules. Freeman et al39 showed that PEI functionalization of poly(sulfone) membranes led to strong interactions between PEI and CO2, thereby significantly retarding the diffusion of CO2 and leading to H2/CO2 diffusivity selectivity. Kumar et al40 reported that PEI was successfully introduced into the mesopore channels of an MCM-48 membrane on a αalumina support. Their PEI-modified membranes showed high (>80) N2/CO2 selectivity in mixed-gas permeation at room temperature in the presence of water vapor. This finding is surprising, because the theoretical basis for N2 selective permeation over CO2 in such membrane materials is unclear.41 This also stimulates the need for further studies to better understand the mechanism of unusual N2/CO2 selectivity in PEI-modified mesoporous membranes.42 In this work, we first describe the successful modification (by in-situ polymerization of aziridine) of worm-like mesoporous silica membranes grown on macroporous Torlon® poly(amide-imide) hollow fibers, to fabricate defect-free HAS membranes for CO2 separation applications. We demonstrate that the HAS membranes display tunable permeation properties, being CO2 selective in the presence of water vapor, and N2 selective under dry conditions.


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2. EXPERIMENTAL SECTION 2.1. Materials The following reagents were used as received without further purification: 2chloroethylamine hydrochloride (Aldrich, 99%), sodium hydroxide (EMD, ACS grade, 97%), tetraethylorthosilicate (TEOS, 98% Sigma-Aldrich), cetyl trimethylammonium bromide (CTAB, Sigma-Aldrich), 1 N aqueous hydrochloric acid (HCl) solution (Sigma-Aldrich), ethanol (EtOH, BDH), and Torlon® 4000T-LV (Solvay Advanced Polymers).

2.2. Membrane Synthesis Macroporous Torlon® poly(amide-imide) hollow fiber supports were fabricated by a dryjet/wet-quench method as described in detail elsewhere.43 The outer and inner diameters of the support fibers were ca. 380 µm and 230 µm, respectively. The fibers did not possess skin layers and had open pores of ~100 nm size at the outer surface. The mesoporous silica membrane was fabricated as described in detail in our previous report.35 Before the membrane coating, both ends of the fiber were sealed with epoxy to prevent the membrane growth in the interior of the fiber support. The support Torlon® hollow fibers were immersed in the coating solution for 5 hours at room temperature. The mixture had a molar composition of 1 TEOS : 0.425 CTAB : 0.00560 HCl : 62.2 H2O. After membrane growth, the membranes were aged with saturated TEOS vapor prior to use. In this step, a 22 cm-long fiber membrane was placed with 25 µL of TEOS in a closed vessel at 373 K for 24 hours. For surfactant extraction, the fiber membranes were washed with 0.05 N HCl/ethanol with stirring for 24 hours at room temperature. 5 ACS Paragon Plus Environment


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2.3. Aziridine Functionalization Aziridine was synthesized through a procedure reported by us elsewhere.30 Aziridine is a carcinogen and reproductive hazard. Extreme caution must be taken when dealing with this chemical, including use of proper personal protection equipment.31 Polymerization of aziridine on the mesoporous membrane supports via vapor phase transport30 in the absence of added homogeneous acid was carried out in a 290 ml glass pressure tube. Prior to aziridine functionalization, the surfactant-extracted mesoporous silica membranes were evacuated in a vacuum oven at 423 K under 0.07 atm, to remove physically adsorbed moisture and residual surfactant. A small glass test tube (12 × 75 mm, VWR) containing 3 g of pure aziridine was then placed inside of the pressure tube. The pressure tube was closed tightly and maintained at a room temperature for 7 days of reaction time. After functionalization, the membranes were washed with ethanol for 30 min in a separate container under stirring to remove physisorbed aziridine and soluble polymer from the surface. Then, the mesoporous silica membrane coated hollow fibers were dried at room temperature before preparing the gas permeation module.

2.4. Characterization Scanning electron microscopy (SEM) was performed with a LEO 1530 instrument to examine the membranes. The membrane samples were prepared on carbon tape and coated with gold to prevent image distortion due to surface charging. Single-gas permeation was measured using a hollow fiber permeation measurement apparatus (Figure S1, Supporting Information) constructed in-house as described earlier.44, 45 As shown in Figure S2 (Supporting Information), a


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hollow fiber module (~15 cm in length) was prepared with a single fiber. N2 and CO2 gases were fed directly from gas cylinders to the module through stainless steel tubing (Swagelok®) for measurements at dry conditions, whereas in the case of humid conditions the gases passed through a water bubbler before entering the permeation module. The gases were fed into the bore side of the fiber interior at one end of the module, and the other end of the module was deadended. The flux through the walls of the fiber was measured on the shell side using a soapbubble flow meter. Atmospheric pressure was maintained on the downstream side. The temperature of the system was maintained at 308 K. The permeance of any component i is expressed as its flux normalized by its transmembrane partial pressure:

=

∆

(1)

A commonly used unit for permeance is the GPU (Gas Permeation Unit), where 1 GPU = 10-6 cm3 (STP) cm-2 s-1 cmHg-1. For single gases A and B, the ideal selectivity is described by:

α ⁄ =

(2)

3. RESULTS AND DISCUSSION 3.1. Membrane Characterization Figure 1a shows a cross-section SEM image of an aziridine-functionalized mesoporous silica/Torlon® hollow fiber membrane, after evacuation/degassing to remove physically adsorbed moisture and residual organic species. A continuous and uniform silica layer is observed on the outer surface of the hollow fiber support with a thickness of about 2 µm, which is consistent with



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our previous report on the synthesis of mesoporous silica membranes on hollow fibers.35 Figure 1b shows a top-view SEM image of the membranes. The continuous and crack-free silica layer indicates that the membrane is not damaged by the functionalization process. Also, there is no apparent change in the membrane morphology after amine-functionalization.

Figure 1. SEM images of the aziridine-functionalized membrane: (a) cross-sectional view, and (b) top-view.

3.2. Gas Permeation Characteristics The mesoporous silica membranes after pore modification with aziridine were characterized by pure component N2 and CO2 gas permeation measurements to understand their permeation mechanisms. Figure 2 shows cyclic dry gas permeation data for CO2 and N2 at 308 K. Each gas was contacted with the membrane for 1500 min before switching to the other gas. Overall, permeances of both gases were significantly decreased (from the bare mesoporous silica membrane) after aziridine functionalization, suggesting decreased pore size and pore volume due 8 ACS Paragon Plus Environment

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to the modification, as intended. For example, the N2 permeance decreased (from 20,000 GPU in the bare mesoporous silica membrane) to 12 GPU, and CO2 permeance decreased (from 18,000 GPU in the bare mesoporous silica membrane) to 2 GPU. Based upon these large reductions, it is inferred that aziridine forms polymeric amine species in the mesopores of membrane. These significant reductions cannot be achieved by grafting of a single layer of aziridine molecules along the pore walls of the mesopores,35 and is known to be associated with the polymerization of aziridine to create hyperbranched PEI polymers, leading to the formation of a hyperbranched aminosilica31, 37 membrane. More interestingly, the CO2 permeance of the HAS membrane was observed to decrease over time within each cycle, suggesting that CO2 molecules were strongly adsorbed to the amine groups and stagnant. The data suggest that the adsorbed CO2 molecules progressively block the pores by reducing the effective pore size, stabilizing the permeance at a steady-state value of about 2 GPU. When the gas feed was then switched to N2, the gas permeance progressively recovered to the initial value of 12 GPU (observed at the start of CO2 permeation) in a short period of time. This suggests that the adsorbed CO2 molecules were desorbed by the N2 flow and a larger effective pore size was recovered. The N2 permeance returned to a value similar to that obtained with the fresh HAS membrane. Using the observed steady-state single gas permeance values, we obtain an unusual ‘reverse’ selectivity (N2/CO2) of 6.8 ± 0.2. Figure 3 shows the same information as Figure 2, but in an expanded manner to better identify the points at which steady states were achieved.



15 CO2 N2 12

Permeance (GPU)

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9

6

3

0

N2

CO2 0

1000

CO2 2000

3000

N2 4000

5000

6000

Time (min)

Figure 2. Cyclic CO2 N2 CO2 N2 dry gas permeation at 308 K through an aziridinefunctionalized mesoporous silica/Torlon® hollow fiber membrane.


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Figure 3. Cyclic CO2 N2 CO2 N2 dry gas permeation at 308 K through an aziridinefunctionalized mesoporous silica/Torlon® hollow fiber membrane. 11 ACS Paragon Plus Environment


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(a) NH2

O C O

NH2

NH3 H N O O

(b) NH2 H N

O C O

NH3 N

O O

O

C

O

H N

O

H2O N

H2 N

(c) NH H N

O C O N

O O

Figure 4. Mechanisms of CO2 reaction with pairs of (a) primary-primary, (b) primary secondary, and (c) secondary-secondary amines.

CO2 adsorption under dry conditions on amine-modified oxide materials (such as the HAS membrane studied here) is well known to occur on amine pairs, with two amines required on average to bind one CO2 molecule. A substantial set of available IR and NMR spectroscopy data suggests that the adsorbed CO2 exists primarily in the form of ammonium carbamates.46-48 Thus, adsorption of CO2 in the HAS membrane is hypothesized to occur at primary-primary, primary-secondary, and secondary-secondary amine pairs, as shown in Figure 4, and likely occurs mainly on pairs of primary-tertiary, and on secondary-tertiary amines to a more limited extent due to steric hindrance. Because CO2 adsorption involves coordination with two amines, 12 ACS Paragon Plus Environment

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substantial organic chain crosslinking occurs, increasing the polymer rigidity and effectively decreasing the membrane permeability. Figure 5 is a schematic depiction of a pore channel of the HAS membrane, and the likely mechanism of dry adsorption of CO2 molecules.

O

OH N

OH H N

OH

OH

NH2

H2N

OH H N

OH N

N

N

N

OH

NH2

N

H2N

OH

OH

OH

O C O

NH2

NH2

O NH3 O

NH N H

OH

NH O

O

OH H2 N

OH

O

N

NH2 N

NH O

H 3N O

N

HN OH

OH H N

H3N O

NH2

H2N N H

O

N

H 2N NH2

O

N

N

N H

O

OH

OH

HN O

OH

OH

OH

OH

OH

OH

OH

O

OH

OH

Figure 5. Schematic mode of CO2 adsorption and polymer chain crosslinking under dry conditions in the aziridine functionalized HAS membrane, which is responsible for triggering reverse selectivity in single gas permeation.

Figure 6 shows single-gas permeation data for humidified CO2 and N2 at 308 K. In the presence of water vapor, the N2 permeances were much lower than those measured during dry gas permeation (Figure 2 & Figure 3), but the CO2 permeances were slightly higher than under dry conditions. The significant change in the N2 permeation behavior results in a wet-gas N2/CO2 ideal selectivity (0.4 ± 0.02) that is much lower than under dry gas permeation (6.8 ± 0.2). In the presence of water vapor, CO2 adsorption37,

52

in the HAS membrane increases considerably,



leading to higher CO2 solubility in the membrane.36 For example, the CO2 adsorption capacity of PEI-modified MCM-41 increases from about 90 mL(STP)/g PEI under dry CO2 exposure conditions to 130 ml(STP)/g PEI under simulated flue gas flow (10 mL/min) containing 14% CO2 and 10% H2O at 348 K.49 At steady state, the HAS membrane becomes CO2-selective, showing a CO2/N2 ideal selectivity of 2.7 ± 0.2 (i.e., N2/CO2 selectivity of 0.4 ± 0.02 as mentioned earlier). The significant decrease in the gas permeance of N2 is a result of competitive water vapor adsorption in the hydrophilic HAS membranes, which is therefore expected to lower the free volume and effective pore size available for permeation of gases such as N2 which interact weakly with the HAS surfaces. In contrast, the ability of CO2 to adsorb at a single amine site under humid conditions is suggested to lessen the degree of cross-linking, giving enhanced CO2 permeation rates.

12 CO2 N2

10

Permeance (GPU)

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8

6 4 2 0 0

300

600

900

1200

1500

Time (min)

Figure 6. Humid gas permeation data of N2 and CO2 for an aziridine functionalized mesoporous silica/Torlon® hollow fiber membrane at 308 K, as a function of time-on-stream. 14 ACS Paragon Plus Environment

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O

NH2

C

O

NH3 H N

NH2

O O

O O

O H

C C O O

H O

H

H

NH3HCO3 NH3HCO3

Figure 7. Hypothesized surface reactions of tethered amine groups with CO2 under dry or humid conditions.

The humid gas permeation results can be understood in terms of the nature of the chemical interactions between the HAS membrane and CO2 in the presence of water in the mesopores. Figure 7 and Equations 3-5 show the hypothesized CO2 adsorption mechanism to the primary amine groups in the presence and absence of water molecules.40 The interaction between the basic surface and acidic CO2 molecules is thought to result in the formation of surface ammonium carbamates under anhydrous conditions (Eq. 4), The CO2 molecule is covalently bonded to the one of the amines, and the other amine is converted to an alkylammonium ion,36 leading to crosslinking between the two reaction products of Eq. 4. In the presence of water, Eqs 3 and 5 lead to the formation of alkylammonium ions and bicarbonate species, and there is no cross-linking effect.50, 51 Thus, in our dry CO2 permeation measurements, the solubility of CO2 in the HAS membrane is limited to one mole of CO2 for every two moles of surface-bound amine 15 ACS Paragon Plus Environment


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groups, whereas in the presence of water the solubility may increase up to two moles of CO2 for every two moles of surface-bound amine groups.49,

52

However, the solubility increase alone

cannot explain the increase in CO2 selectivity. There is an additional enhancement from the fact that under wet conditions, the amine groups are not cross-linked. Therefore, the diffusion of CO2 molecules would be enhanced because of the less rigid and more porous nature of the HAS. Overall, increased CO2 permeance under humid conditions can be explained by both enhanced solubility and diffusivity.

R-NH3+ + HCO3-

(3)

R-NH3+ + R-NH-COO-

(4)

CO2 + R-NH2 + H2O CO2 + 2R-NH2 CO2 + CO32- + H2O

2HCO3-

(5)

To demonstrate the reproducibility of the HAS membrane fabrication and characterization, Table 1 summarizes steady state N2 and CO2 permeation results for nonfunctionalized and the three independent HAS membranes at 308 K. As noted above, the dry gas permeation data are consistent with CO2 molecules being tightly adsorbed to the amine sites, reducing effective pore size by amine cross-linking and causing slow diffusion of CO2. Under these conditions, the membranes are consistently ideal N2 selectivity. In the presence of water vapor, all the membranes consistently show CO2-selective single-component permeation, due to high CO2 solubility and lack of amine cross-linking. In this kind of facilitated transport membrane, the CO2 molecules react with the carrier amine groups through a selective reversible 16 ACS Paragon Plus Environment

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chemical reaction by which its solubility is greatly enhanced. In contrast, the non-reactive gas (N2 here) transports purely by conventional solution-diffusion with a low solubility. Hence, under humid conditions, CO2 selectivity for the CO2/N2 gas pair is obtained.

Table 1. Steady-state single gas permeation data from HAS membranes and a control nonfunctionalized membrane at 308 K. *Permeation properties of the HAS-1 membrane are described in detail in the figures. N2 permeance (GPU)

CO2 permeance (GPU)

N2/CO2 Ideal Selectivity

Dry

20,000

18,000

1.11

Dry

12

1.8

6.7

Wet

3.6

9.7

0.37

Dry

13

1.9

6.8

Wet

3.4

10

0.34

Dry

12

1.7

7.1

Wet

3.3

8.6

0.38

Dry

12.3 ± 0.6

1.8 ± 0.1

6.8 ± 0.2

Wet

3.4 ± 0.2

9.4 ± .0.7

0.4 ± 0.02

Membrane Non-functionalized HAS #1*

HAS #2

HAS #3

HAS Average

4. CONCLUSIONS This work demonstrates that an aziridine functionalization of worm-like mesoporous silica membranes on polymeric hollow fiber supports yields HAS membranes, and that the HAS membranes can yield tunable CO2 and N2 permeation properties in the presence or absence of



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co-fed water vapor. The HAS membranes show selectivity for N2 over CO2 at 308 K in anhydrous single-gas permeation experiments due to the cross-linking of the hyperbranched amines under dry CO2 flow, which reduced the effective permeation rate of the CO2 gas through the pores. However, the same membranes are selective to CO2 over N2 in the presence of water vapor, since water suppresses the cross-linking reaction between CO2 and amine groups and also increases the solubility of CO2 in the HAS membranes. Thus, the gas permeation characteristics of the aziridine-functionalized mesoporous silica membrane can be tuned by the absence or presence of water vapor. This first work on HAS membranes leads to new understanding of the behavior of amine-modified mesoporous membranes that have previously been suggested to have non-intuitive permeation properties. Future work using CO2/H2O/N2 mixtures is warranted. AUTHOR INFORMATION Corresponding Authors *C.W. Jones. E-mail: [email protected] *S. Nair. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Phillips 66 Company.


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ASSOCIATED CONTENT Supporting Information Available Diagram of permeation measurement setup, and photograph of membrane module. This information is available free of charge via the Internet at http://pubs.acs.org/.

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