N2 mixture gas separation

Accepted Manuscript Title: Development of thin film nanocomposite membranes incorporated with sulfated ␤-cyclodextrin for water vapor/N2 mixture gas separation Authors: Xinghai An, Pravin G. Ingole, Won-Kil Choi, Hyung-Keun Lee, Seong Uk Hong, Jae-Deok Jeon PII: DOI: Reference:

S1226-086X(17)30571-3 https://doi.org/10.1016/j.jiec.2017.10.031 JIEC 3687

To appear in: Received date: Revised date: Accepted date:

2-8-2017 17-10-2017 19-10-2017

Please cite this article as: Xinghai An, Pravin G.Ingole, Won-Kil Choi, Hyung-Keun Lee, Seong Uk Hong, Jae-Deok Jeon, Development of thin film nanocomposite membranes incorporated with sulfated ␤-cyclodextrin for water vapor/N2 mixture gas separation, Journal of Industrial and Engineering Chemistry https://doi.org/10.1016/j.jiec.2017.10.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Development of thin film nanocomposite membranes incorporated with sulfated β-cyclodextrin for water vapor/N2 mixture gas separation

Xinghai Ana,c, Pravin G. Ingoleb, Won-Kil Choia, Hyung-Keun Leea,c, Seong Uk Hongd, and Jae-Deok Jeona,*

a

Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129,

Republic of Korea b

Chemical Engineering Group, Engineering Science and Technology Division, CSIR- North

East Institute of Science and Technology, Jorhat, Assam 785006, India c

Department of Advanced Energy and Technology, Korea University of Science and

Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea d

Department of Chemical and Biological Engineering, Hanbat National University, 125

Dongseodero, Yuseong-gu, Daejeon 34158, Republic of Korea *Corresponding author: Tel.: + 82 42 860 3023; Fax: + 82 42 860 3133. E-mail address: [email protected] (J.-D. Jeon)

Graphical abstract

1

Highlights 

Microporous sulfated β-cyclodextrin (sb-CD) was used for fabricating TFN membranes.



TFN membranes incorporated with sb-CD exhibited improved permeance and selectivity.



Diethylene triamine demonstrated a structural advantage over m-phenylenediamine.



Water vapor separation behavior using TFN membranes highly depends on structural characteristics.

Abstract In this work, thin film nanocomposite (TFN) membranes incorporated with sulfated β-cyclodextrin (sb-CD) were fabricated by interfacial polymerization using aliphatic diethylene triamine (DETA) and trimesoyl chloride (TMC) for water vapor separation. Aromatic m-phenylenediamine (MPD) was used for performance comparison with DETA. The intrinsic properties of fabricated TFN membranes were investigated by Attenuated total reflectance-Fourier transformed infrared (ATR-FTIR), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM) and water contact angle (WCA). The contact angle (35o) of the TFC membranes with DETA was very lower than that (59o) of The TFC with MPD. Moreover, the TFN membrane with sb-CD loading of 0.15 wt% showed the lowest contact angle of 14o, implying that many hydrophilic sulfonic acid groups of sb-CD nanoparticles contributed to decreased contact angle. The effect of sb-CD loading on water vapor permeance and selectivity was studied. Increase in sb-CD loading caused synergistic increase in both permeance and selectivity below the agglomeration point due to its hydrophilic and packable nature. The maximal

2

selectivity of 503 along with 1597 GPU in permeance was obtained with the sb-CD loading of 0.15 wt%. In addition, the structural characteristics were found to have a bigger impact on TFN membrane performance than intrinsic properties.

Keywords: Sulfated β-cyclodextrin; Thin film nanocomposite membrane; Interfacial polymerization; Hollow fiber membrane; Water vapor separation

1. Introduction Membrane separation has been developed and applied in various industries over many decades owing to its simplicity, scalability and high energy efficiency [1]. Regarding water treatment, thin film composite (TFC) membranes prepared by interfacial polymerization were pervasively studied such as desalination [2], flue gas dehydration [3], etc. Recently, nanoparticle incorporated thin film nanocomposite (TFN) membranes are being emphasized as they can significantly improve membrane performance [4-7]. Initiated by Jeong et al. [8] who first to fabricate zeolite-A incorporated TFN membranes to obtain higher water flux while maintaining salt rejection, widespread researches have been carried out using various hydrophilic nanoparticles for water treatment, such as carbon nanotubes [9], titania [10], silica [11]. It is based on a concept that the nanoparticles act as active centers via functional groups and structural characteristics in terms of facilitated sorption and sieving mechanisms, respectively. In our previous work, Engelhard titanosilicate-4 (ETS-4), a new zeolite-type microporous nanoparticle, was used to fabricate TFN membranes and gained enhanced 3

selectivity by surrendering little permeance [12]. However, most of nanoparticles that have been studied were inorganic, and the compatibility between inorganic and organic phases is one of challenging issues as their interactions are only by hydrogen bonding. Therefore, the nanoparticles that can be chemically introduced via functional groups are more desirable for TFN membranes. The stable dispersions of β-cyclodextrin particles are carried out in polymer matrices when the β-cyclodextrin particles are decorated with dense layers of polymer. Moreover, the nanoparticles containing functional groups also take part in polymerization reaction. Regarding these points, the organic β-cyclodextrin with abundant hydroxyl groups was found to be the perfect candidate to synthesize the thin selective polymer layer on the PSf substrate. It is well known that β-cyclodextrin is extensively used in industries such as food, pharmaceutical, etc. owing to its availability and cheapness [13]. It has a toroid-like shape and tends to allow water molecules to pass through prior to others [14]. Although its pore size is not small enough (5.6 Å) to sieve water molecule from air, it has an effect of molecular discrimination to some extent [15–19]. Additionally, its hydroxyl groups can be substituted by more hydrophilic sulfonic acid groups, and the substituted one is termed as sulfated β-cyclodextrin (sb-CD). The sb-CD possesses excellent hydrophilicity and packability owing to functional groups and small particle size, which is promising in membrane separation. Jeon et al. [20] incorporated sb-CD to Nafion membranes to gain improved performance of direct methanol fuel cell. Wu et al. [21] utilized sb-CD incorporated polyester composite membranes to enhance water flux without sacrificing rejection in nanofitration. 4

In this work, sb-CD was utilized to fabricate TFN membranes by interfacial polymerization of diethylene triamine (DETA) with trimesoyl chloride (TMC) on the surface polysulfone (PSf) hollow fiber membranes. DETA with linear structure has one secondary amine and two primary amine groups. Aromatic m-phenylenediamine (MPD) was also used for performance comparison with DETA. The intrinsic properties of TFN membranes were characterized by various modern techniques. The performance of TFN membranes was evaluated and the effect of sb-CD loading was investigated. To our best knowledge, sb-CD incorporated TFN hollow fiber membranes for water vapor separation have not yet been reported.

2. Experimental 2.1 Materials PSf hollow fiber membranes purchased from Guiyang Shidaihuitong Film Technology (China) with 1510 μm in outer diameter and 1040 μm in inner diameter were used as the substrate. MPD from Sigma-Aldrich as well as DETA was used as aqueous phase monomer while TMC as organic phase monomer. sb-CD was purchased from Sigma-Aldrich. Deionized water (Millipore) and n-hexane (Duksan, Korea) were used as aqueous and organic solvents, respectively. The hollow fiber membrane modules containing 5 fibers with an effective area of 62 cm2 in each were used in this study. All the chemicals were used as received.

2.2 Fabrication of TFN membranes by interfacial polymerization In this work, DETA was selected as the aqueous monomer since its linear structure was 5

reported to have a positive effect on membrane performance [22]. Hence, the TFC membrane fabricated with most widely used MPD was also prepared to compare with the one with DETA. The interfacial polymerization reactions with different aqueous monomer are illustrated in Fig. 1. DETA is multi-amine monomer when compared to MPD with two amine groups. Thus, the network of crosslinked structure can be formed for DETA/TMC system, leading to the formation of polyamide layer with more compact structure. Particularly, the sb-CD can react with DETA to form new amine bonds and may participate in interfacial polymerization to form polyester through its unreacted hydroxyl groups. As shown in Table 1, only sb-CD loading within 0.05-0.20 wt% (with respect to DETA) was changed while other variables were kept constant in this study. The aqueous and organic monomer solutions were kept 2 wt% (8 g monomer in 400 g water) and 0.2 wt%, respectively. The reaction time was kept 3 min. The detailed procedure for preparation of TFN membranes was described elsewhere [12]. Briefly, the PSf substrates were firstly immersed in DETA/sb-CD mixture solution for 10 min in a coating bath (40cm5cm5cm) then immersed in TMC solution for 3 min (reaction time), and they were dried in ambient air after each step. Finally, the membranes were transferred to heat treatment at an oven at 70 oC for 10 min (stabilization) followed by a washing process for 30 min by water before use.

2.3 Characterization Attenuated total reflectance-Fourier transformed infrared (ATR-FTIR) spectra were utilized to 6

recognize characteristic functional groups on the membrane surface via an ALPHA-P spectrometer (Bruker Optik GmbH) with a diamond crystal in the range of 600–4000 cm-1.

The morphologies of fabricated membranes were observed by a field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi). To observe the cross-section images, the hollow fiber membranes were fractured by liquid nitrogen. All samples were coated with a thin gold layer via a sputter coater (Balzers Union SCD 040) before scanning. Besides, energy dispersive X-ray spectroscopy (EDX) was also used to detect composition in membranes.

The surface roughness of membranes was examined by an atomic force microscope (AFM, XE-100, Veeco) with a scanning area of 3 μm3 μm in non-contact tapping mode and topography was visualized by mapping. The surface roughness parameters along with surface area were calculated by the in-built software.

The hydrophilicities of membranes were quantified in terms of water contact angle (WCA) via a water contact angle analyzer (Phoenix 300 Plus, SEO). The measurements were carried out at room temperature by sessile-drop method and 3 μL of deionized water was used as probe liquid. The water was extruded from a syringe and contacted with membranes meanwhile being captured by a camera, and the resulting contact angles were calculated by the in-built software. At least five different positions were measured for each sample and average values were reported. 7

2.4 Membrane performance test The details of the equipment of water vapor separation was described elsewhere [12]. Concisely, the water vapor was generated by a steam generator and carried by nitrogen gas. The relative humidity was changed via dilution in a controlled manner. The driving force was maintained by feed and vacuum pressures up to 3 bars. The water vapor flow rates at feed and retentate stream were calculated by the following equation, (1) Where QH2O (cm3 (STP)/min) refers to water vapor flow rate, QN2 (cm3/min) to nitrogen flow rate, and ϕ (g/cm3) to absolute humidity at the corresponding stream. Vm is gas molar volume at standard temperature and pressure (22.4 L/mol) and MH2O is molecular weight of water (18 g/mol). The permeance was thus determined by the follows, (2) Where PH2O represents water vapor permeance (GPU=10-6 cm3 (STP)/(cm2.cmHg.s)). QH2OP refers to water vapor flow rate at permeate side and A to effective membrane area. PH2OR and pH2OP are denoted for partial pressures of water vapor at retentate and permeate sides determined from pressure fraction. The selectivity (α) as follows is the permeance ratio of water vapor to nitrogen, (3) 3. Results and discussion 8

3.1 Intrinsic properties of sb-CD incorporated TFN membranes The ATR-FTIR spectra of sb-CD incorporated TFN membranes are shown in Fig. 2. The peak corresponding to polyamide appeared at 1662 cm-1 in all membranes except for pristine PSf substrate. The peaks accounting for stretching vibration of SO2 and SO appeared at 1243 cm-1 and 1049 cm-1, respectively. For latter, there were little changes in all spectra. In fact, the spectra for SO2 groups were overlaid since they exist in both PSf and sb-CD, nonetheless, there was an apparent trend. This peak slightly decreased upon interfacial polymerization owing to the cover of the polyamide layer and tended to increase upon incorporation of sb-CD due to introduced SO2 groups. The EDX analyses for the TFC membrane without sb-CD and the TFN membrane with sb-CD are displayed in Fig. 3. It can be seen from the result that the sulfur content was not obviously changed given that it is mainly present in PSf substrate as well. Instead, there was an evident change in nitrogen composition. As aforementioned, sb-CD also takes part in interfacial polymerization by competing with aqueous monomer. Thus, the decrease in the unique element, nitrogen, from polyamide formation indirectly confirmed the incorporation of sb-CD. The cross-sectional FE-SEM images of TFC and TFN membranes are shown in Fig. 4. It can be seen from the images that the thickness of TFC membrane using MPD was 268 nm while the one using DETA was 157 nm. Such a difference in thickness was attributed to structural properties of aqueous monomers. Compared with bulk aromatic MPD, DETA has a linear structure, which potentially provides more crosslinking sites in relatively smaller dimensions 9

hence forming a thinner coating layer. On the other hand, the thickness of sb-CD incorporated TFN membrane gradually grew as sb-CD loading increased up to 255 nm since nanoparticle incorporation is accompanied with thickness increase. The surface morphologies of those membranes are presented in Fig. 5. According to the images, the TFC membrane fabricated with MPD had a piece-like structure with many interstitial cavities while the one with DETA demonstrated a homogenous surface, which confirmed the different structural properties of monomers. With the addition of sb-CD nanoparticles, a sphere-like surface appeared and it got obvious by increasing the loading as shown in Fig. 5(d)-(f). It was similar with the reported literature that increase in sb-CD loading resulted in enlarging circular matter on the surface [15]. As sb-CD is able to participate in interfacial polymerization, this property can facilitate distribution and stabilization of nanoparticles on the membrane surface. Another speculation on sb-CD incorporation is that it could be threaded by polymer chains to be further immobilized. Similar with other nanoparticles, the sb-CD got agglomerated beyond 0.2 wt% and showed an overlapped surface as shown in Fig. 5(f).

The topographic roughness profiles of membranes are displayed in Fig. 6 and roughness parameters are summarized in Table 2. The roughness parameters are termed of root mean square roughness (Rq), average roughness (Ra), and maximum roughness (Rmax). It can be seen from the result that there was little difference in average roughness between the TFC membranes fabricated with MPD (47.9 nm) and DETA (45.7 nm). As the sb-CD loading increased, the roughness increased up to 50.3 nm till 0.15 wt%. Notably, the corresponding 10

relative surface area increased 28% (from 1.23 to 1.57) owing to the porous and highly packable nature of sb-CD nanoparticles. Moreover, the roughness decreased beyond 0.15 wt% due to agglomeration of sb-CD nanoparticles.

The water contact angles of membranes are listed in Table 3. According to the result, the contact angle of the TFC membrane fabricated with MPD was 59o while the one with DETA was 35o. Such a difference was accredited to linear structure of DETA since it provides more crosslinking sites to form more hydrophilic amide groups. Remarkably, the contact angle decreased even up to 14o (0.15 wt%) by increasing sb-CD loading, which substantiated the hydrophilic nature of sb-CD nanoparticles contributed from their hydroxyl/sulfonic acid groups. Moreover, the contact angle slightly increased at 0.2 wt% due to agglomeration of sb-CD nanoparticles.

3.2 Performance of sb-CD incorporated TFN membranes The water vapor separation performance of fabricated TFN membranes is presented in Fig. 7. In this topic, the effect of sb-CD loading on water vapor permeance and water vapor/N2

11

selectivity was studied. According to the result, both permeance and selectivity increased as sb-CD loading increased up to 0.15 wt% and declined thereafter. A maximum selectivity of 503 along with 1597 GPU (1 GPU=10-6 cm3 (STP)/(cm2.cmHg.s)) in permeance was obtained for the TFN membrane with the sb-CD loading of 0.15 wt% (DETS-0.15).

Firstly, the initial increase in permeance was ascribed to increased hydrophilicity and adsorption sites. On one hand, sb-CD contributes many hydroxyl/sulfonic groups to essentially enhance hydrophilicity as presented in WCA measurements. On the other hand, porous sb-CD provides larger surface area and enriches adsorption sites for water vapor as demonstrated in AFM results. However, the increase extent in permeance was not so eminent, which manifested that water vapor separation behavior is more correlated to structural characteristics than sorption affinity for TFN membranes. Affinity improvement is prerequisite for enhancing permeance though it is not proportionally reflected on permeance. Secondly, the increase in selectivity was attributed to the characteristics of sb-CD. For one thing, porous and packable nature of sb-CD offers tortuous pathways to slower the permeation of bigger molecules. For another, sb-CD also participates in interfacial polymerization and the competitive reaction leads to more crosslinked structure thereby increasing selectivity. The subsequent decrease in permeance and selectivity beyond the certain point was due to agglomeration of sb-CD nanoparticles. Similar with other nanoparticles, agglomeration results in less crosslinked structure and has a negative effect on membrane performance. Comparatively, the performance of the TFC membrane prepared with MPD was found to be 1398 GPU in permeance and 213 in selectivity, which was lower than the one with DETA. As aforementioned, this is due to the fact that linear DETA provides more crosslinking sites of

12

amide groups and higher crosslinking density. Thus, it is indicated that the linear DETA has structural advantages over the bulk aromatic MPD towards membrane performance.

Moreover, the effect of concentration polarization was investigated for the membrane with optimal sb-CD loading (DETS-0.15). The procedure for resistance deconvolution was described elsewhere [12,23-25]. As a result, the thin selective layer provided the major resistance of 60% while the feed side boundary layer resistance was found to be 35%, which demonstrated the unnegligible effect of concentration polarization. The apparent permeability of water vapor through the selective film was estimated to be 364 Barrer (1 Barrer=10-10 cm3 (STP).cm/(cm2. s.cmHg)) and the intrinsic permeability was found to be 602 Barrer after correcting for boundary layer resistance.

4. Conclusion The sb-CD incorporated TFN membranes were successfully fabricated by interfacial polymerization method. Incorporated sb-CD was confirmed by ATR-FTIR and EDX. The effect of sb-CD loading on water vapor permeance and selectivity was investigated. As sb-CD loading increased below the agglomerating point, both permeance and selectivity increased due to its hydrophilic and packable nature. The maximal selectivity of 503 along with 1597 GPU in permeance was obtained under the optimal sb-CD loading of 0.15 wt%. Moreover, the linear DETA demonstrated structural advantages over the bulk aromatic MPD towards membrane performance. It was manifested that the structural characteristic is more important than sorption affinity on water vapor separation behavior for TFN membranes. The sb-CD 13

incorporated TFN membranes not only enhanced membrane performance, but also provided a new perspective in TFN membrane design.

Acknowledgement This work was conducted under framework of the research and development program of the Korea Institute of Energy Research (B7-2431).

14

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18

Transmittance (%)

Fig. 1 Polymerization reaction between (a) MPD and TMC, (b) DETA and TMC

PSf DET DETS-0.05 DETS-0.10 DETS-0.15 DETS-0.20

1049 (-S-O-)

1662 (-CO-NH-) 1243 (-O-S-O-)

1800 1700 1600 1500 1400 1300 1200 1100 1000

900

800

700

600

-1

Wavelength (cm )

Fig. 2 ATR-FTIR spectra of sb-CD incorporated TFN membranes

19

a

b

Fig. 3 EDX analysis for DET (a) and DETS-0.15 (b)

20

Element

Weight%

Atomic%

CK

63.79

71.42

NK

7.78

7.47

OK

21.82

18.34

SK

6.62

2.78

Totals

100.00

Element

Weight%

Atomic%

CK

66.77

74.16

NK

5.91

5.63

OK

21.16

17.64

SK

6.16

2.56

Totals

100.00

Element

Weight%

Atomic%

CK

66.77

74.16

NK

5.91

5.63

OK

21.16

17.64

SK

6.16

2.56

Fig. 4 Cross-sections of sb-CD incorporated TFN membranes by FE-SEM: (a) MT, (b) DET, (c) DETS-0.05, (d) DETS-0.10, (e) DETS-0.15, and (f) DETS-0.20

Fig. 5 Surface morphologies of sb-CD incorporated TFN membranes by FE-SEM: (a) MT, (b) DET, (c) DETS-0.05, (d) DETS-0.10, (e) DETS-0.15, and (f) DETS-0.20

21

Fig. 6 Topographic roughness profiles of sb-CD incorporated TFN membranes: (a) MT, (b) DET, (c) DETS-0.05, (d) DETS-0.10, (e) DETS-0.15, and (f) DETS-0.20

22

1800

550

500

1400 1200

450 1000 800 400 600 400

Water vapor permeance (GPU) Water vapor/N2 Selectivity (-)

200 0

350

Water vapor/N2 Selectivity (-)

Water vapor permeance (GPU)

1600

300 0.0

0.1

0.2

sb-cd concentration with respect to DETA (wt%)

Fig. 7 Effect of sb-CD loading on water vapor permeance and selectivity within the feed side absolute humidity range of 26-29 g/cm3

23

Table 1 Synthetic conditions for preparation of TFN membranes. MPD

DETA

TMC

sb-CD loading

Reaction

loading

loading

loading

wrt.

time

(wt%)

(wt%)

(wt%)

(wt%)

(min)

MT

2.0

-

0.2

-

3

DET

-

2.0

0.2

-

3

DETS-0.05 -

2.0

0.2

0.05

3

DETS-0.10 -

2.0

0.2

0.10

3

DETS-0.15 -

2.0

0.2

0.15

3

DETS-0.20 -

2.0

0.2

0.20

3

Membrane DETA

code

Note: MT refers to the membrane prepared by the reaction between MPD and TMC whereas DET refers to DETA and TMC.

24

Table 2 Surface roughness parameters of sb-CD incorporated TFN membranes

Membrane code

Rq (nm)

Ra (nm)

Rmax (nm)

Relative surface area* (-)

MT

59.8

47.9

548.3

1.27

DET

58.4

45.7

389.6

1.23

DETS-0.05

59.8

46.6

384.8

1.40

DETS-0.10

52.9

47.4

394.4

1.48

DETS-0.15

67.2

50.3

476.8

1.57

DETS-0.20

44.4

35.1

316.6

1.35

* Relative surface area=actual surface area/planar area

25

Table 3 Water contact angles of sb-CD incorporated TFN membranes

Membrane code

Contact angle (o)

MT

59±2

DET

35±3

DETS-0.05

25±2

DETS-0.10

17±2

DETS-0.15

14±2

DETS-0.20

15±1

26