Enhancement of water vapor separation using ETS-4 incorporated thin film nanocomposite membranes prepared by interfacial polymerization

Author’s Accepted Manuscript Enhancement of water vapor separation using ETS4 incorporated thin film nanocomposite membranes prepared by interfacial polymerization Xinghai An, Pravin G. Ingole, Won-Kil Choi, Hyung-Keun Lee, Seong Uk Hong, Jae-Deok Jeon www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)30082-0 http://dx.doi.org/10.1016/j.memsci.2017.02.045 MEMSCI15104

To appear in: Journal of Membrane Science Received date: 10 January 2017 Revised date: 19 February 2017 Accepted date: 27 February 2017 Cite this article as: Xinghai An, Pravin G. Ingole, Won-Kil Choi, Hyung-Keun Lee, Seong Uk Hong and Jae-Deok Jeon, Enhancement of water vapor separation using ETS-4 incorporated thin film nanocomposite membranes prepared by interfacial polymerization, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.02.045 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 galley proof before it is published in its final citable 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.

Enhancement of water vapor separation using ETS-4 incorporated thin film nanocomposite membranes prepared by interfacial polymerization

Xinghai Ana,b, Pravin G. Ingolea, Won-Kil Choia, Hyung-Keun Leea,b, Seong Uk Hongc, and Jae-Deok Jeona,* a

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

Republic of Korea b

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

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

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

Dongseodero, Yuseong-gu, Daejeon 34158, Korea *

Corresponding author: Tel.: + 82 42 860 3023; Fax: + 82 42 860 3133. [email protected] (J.-D. Jeon)

Abstract In this work, microporous Engelhard titanosilicate-4 (ETS-4) incorporated thin film nanocomposite (TFN) membranes for water vapor separation were fabricated via interfacial polymerization. ETS-4 particles were synthesized and ground by an attrition mill to reduce particle size. The ground ETS-4 nanoparticles were well dispersed in 3,5-diaminobenzoic acid (DABA) solution followed by reacting with trimesoyl chloride (TMC) to get embedded in the polyamide selective layer on the outer surface of polysulfone (PSf) hollow fibers. The intrinsic properties of ETS-4 as well as TFN membranes were investigated by field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), X-ray 1

diffraction (XRD), atomic force microscopy (AFM) and water contact angle (WCA). The effect of ETS-4 concentration on water vapor permeance and selectivity was investigated. Increase in ETS-4 concentration during interfacial polymerization causes significant increase in selectivity due to its molecular sieving effect by sacrificing little permeance till 0.5 wt%, above which the selectivity deteriorates due to the agglomeration of ETS-4 nanoparticles. The maximum selectivity of 346 was obtained with 1377 GPU in permeance for the TFN membrane (DTE-0.5) with ETS-4 concentration of 0.5 wt%. Additionally, the effect of concentration polarization was examined and individual resistances from the overall resistance were calculated.

Graphical Abstract

Permeate Water vapor Nitrogen

ETS-4 sieve

Feed

TFN hollow fiber membrane

Polyamide layer

Keywords: Engelhard titanosilicate-4; Thin film nanocomposite membrane; Interfacial polymerization; Hollow fiber membrane; Water vapor separation

2

1. Introduction Membrane separation has been developed for several decades and many applications have been found in various industrial fields due to its simplicity, scalability and energy efficient nature [1]. As water treatment is demanded, thin film composite (TFC) polyamide membranes prepared by interfacial polymerization were extensively investigated including reverse osmosis [2], desalination [3], flue gas dehydration [4], etc. However, these membranes are still limited to meet target performance and economic expectation for commercial use.

In recent years, nanoparticle incorporated thin film nanocomposite (TFN) membranes are brought to great attention owing to their potentiality for enhancing membrane performance by taking advantages of inorganic materials with excellent properties. Normally, hydrophilic nanoparticles are preferred for water or water vapor separation and they are under widespread investigation. Jeong et al. firstly utilized zeolite-A to fabricate TFN membranes to gain high water flux without sacrificing salt rejection [5]. Kim et al. used carbon nanotubes to prepare TFN membranes and achieved high water flux while maintaining salt rejection as well [6]. Currently, our group is also being dedicated to exploiting TFN membranes aiming at the application in flue gas dehydration. It is well known to researchers that the nanoparticles are prone to be agglomerated, to resolve this problem the surface functionalization was found to be one of effective solutions [7]. In our foregoing work, functionalized titania and silica incorporated TFN membranes were studied to obtain improved water vapor permeance as well as selectivity since nanoparticles provide additional water vapor permeation pathways and more compact selective layer [8,9]. Additionally, it was found that the addition of porous 3

nanoparticles in membranes could be a good strategy to improve selectivity without surrendering permeance.

Zeolite is well-known water adsorbent and widely applied in industry. Initiated by Jeong et al [5], many researches on zeolite based TFN membranes have been carried out [10–18]. Engelhard titanosilicate (ETS) is a new class of zeolite-type titanosilicate as a microporous ionic framework [19]. Compared with zeolite, ETS is a size-tunable molecular sieve and consists of ETS-4 and ETS-10 depending on pore size. ETS comprises orthogonal channels of 12-membered and 8-membered rings based on its mixed tetrahedral/octahedral structure, and the effective molecular transport takes place in 8-membered ring due to the presence of faulting in the plane of 12-membered ring [20]. ETS-4 is reported to essentially block molecules larger than its pore size (4 Å) [21]. As a result, ETS-4 possesses both water affinity and size-selective properties. Moreover, ETS-4 has a larger adsorption area and tortuous pathways compared with non-framework nanoparticles, from which high membrane performance can be expected. Jeon et al. used Nafion/ETS-4 composite membranes to improve the performance of direct methanol fuel cell [22]. Braunbarth et al. used ETS-4/TiO2 composite membranes to get comparable selectivity in water/ethanol pervaporation experiment [23].

In this work, ETS-4 was synthesized and utilized to fabricate TFN membranes by interfacial polymerization. In order to get better dispersion, ETS-4 was ground in solution state by an attrition mill after synthesis. During the interfacial polymerization, ETS-4 nanoparticles with 4

good water affinity were dispersed in the aqueous monomer solution and followed by reacting with organic monomer, thereby forming the thin selective layer on the outer surface of hollow fiber membranes. The intrinsic properties of synthesized ETS-4 particles as well as ETS-4 incorporated TFN membranes were characterized by various modern techniques. The performance of TFN membranes was evaluated by the lab-scale experiments and the effect of ETS-4 concentration was examined. To our best knowledge, ETS-4 incorporated TFN hollow fiber membranes for water vapor separation have not yet been reported.

Additionally, water vapor permeation through highly permeable and selective polymeric membranes can easily cause concentration polarization, which makes higher fluxes through membranes and lower fluxes through boundary layers [24]. Hence, the overall mass transfer resistance was deconvoluted to account for the contribution of boundary layer resistance. 2. Experimental 2.1 Materials Polysulfone (PSf) hollow fiber membranes (Guiyang Shidaihuitong Film Technology, China) with 1510 μm in outer diameter and 1040 μm in inner diameter was used as the raw material. Sodium hydroxide (98%), titanium trichloride (ca.12 wt% in 17–22 wt% HCl), hydrogen peroxide (30 wt% in water), and sodium silicate (27% SiO2, 10% NaOH) from Sigma-Aldrich were used for ETS-4 synthesis. 3,5-diaminobenzoic acid (DABA) and trimesoyl chloride (TMC) from Sigma-Aldrich were used as aqueous and organic phase monomer, respectively. Deionized water (DI, Millipore) and n-hexane (Fisher Scientific, NJ) were used as aqueous and organic solvents, respectively. The cylinder-type hollow fiber membrane modules containing 5 5

fibers with an effective area of 62 cm2 were produced from a commercial centrifugal modulation system and used in the lab-scale experiment system. Polyurethane epoxy resin (Hepce Chem, Korea) was used as potting material during the modulation. All the chemicals were used as received.

2.2 Synthesis of ETS-4 The ETS-4 particles were synthesized as previously reported elsewhere [22]. Firstly, sodium hydroxide was dissolved in DI water. Secondly, titanium trichloride solution was added dropwise thus forming white precipitate. After that hydrogen peroxide solution was added dropwise immediately and stirred for 30 min, turning the solution from colorless to bright yellow. Subsequently, sodium silicate solution was added and stirred further for 30 min. The molar composition of final solution was 18NaOH:675H2O:0.5TiO2:5H2O2:10SiO2 and all the above-mentioned operation was performed under stirring condition (800 rpm). Resulting solution was transferred to a 500 mL of homemade Teflon-coated autoclave and crystallized at 210 oC in an oven for 2 days. As-synthesized product was filtered and washed several times with DI water. Finally, the product was dissolved in DI water and ground by an attrition mill (KMC-1B, KoreaKiyon, Korea) to reduce particle size and gain better dispersion.

2.3 Fabrication of TFN membranes by interfacial polymerization The overall procedure for preparation of nanocomposite membranes is illustrated in Fig. 1. Firstly, aqueous and organic solutions were prepared by dissolving aqueous monomer (DABA) and organic monomer (TMC) into corresponding solvents (DI water and n-hexane, 6

respectively) and stirred sufficiently. After that, DABA solution was filtered by vacuum filtration. Subsequently, ETS-4 solution was added to DABA solution followed by ultrasonication for 2 h in order to be fully dispersed. The main step of interfacial polymerization was done by dip coating method. The reaction conditions are summarized in Table 1. In this study, only ETS-4 concentration (with respect to DABA) was changed while keeping other variables constant. The PSf hollow fiber membranes were immersed in aqueous DABA solution in a coating bath for 10 min followed by drying in ambient air. After that, membranes were immersed in organic TMC solution for 3 min (reaction time) followed by drying in ambient air as well. The reaction occurs instantaneously and forms the thin cross-linked polyamide/polyester layer on the outer surface of hollow fiber membranes. The ETS-4 was embedded in this layer and connected with both monomers by hydrogen bonding interaction as shown in Fig. 1. Finally, the nanocomposite membranes were transferred to an oven at 70 oC for 10 min to further densify the polyamide film.

7

Fig. 1 Overall procedure for preparation of TFN membranes

Table 1 Reaction conditions of interfacial polymerization for preparation of TFN membranes. ETS-4 concentration

DABA concentration

TMC concentration Reaction time

wrt. DABA (wt%)

(wt%)

(wt%)

(min)

-

0.5

0.2

3

Membrane code

DT

8

DTE-0.05

0.05

0.5

0.2

3

DTE-0.1

0.1

0.5

0.2

3

DTE-0.2

0.2

0.5

0.2

3

DTE-0.5

0.5

0.5

0.2

3

DTE-1.0

1.0

0.5

0.2

3

2.4 Characterization

The morphologies of ETS-4 particles and membranes were investigated by a field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi). To obtain cross-section images, hollow fiber membranes were moisturized with DI water and fractured by liquid nitrogen to get clear cross-section stubs. After that membrane samples were covered with the thin gold layer by a sputter coater (Balzers Union SCD 040) before scanning. Thus, thickness of thin layer as well as surface morphologies can be visualized. Besides, energy dispersive X-ray spectroscopy (EDX) analysis was also performed to verify the identity of ETS-4 and its presence in TFN membranes.

The crystal structure of synthesized ETS-4 was examined by X-ray diffraction (XRD) patterns using an X-ray diffractometer (Rigaku D/Max-III C, CuKa radiation) in the range of 5o≤2θ≤40o. The pristine and ground ETS-4 particles were analyzed to confirm its stability after grinding. 9

The surface roughness and topography of membranes were investigated by atomic force microscope (AFM, XE-100, Veeco). An area of 3 μm3 μm was scanned in a non-contact tapping mode and topography was visualized by mapping. The surface roughness parameters including root mean square roughness (Rq), average surface roughness (Ra) as well as surface area were calculated by the in-built software.

The hydrophilicities of membranes were quantified by water contact angle (WCA) analysis using a water contact angle analyzer (Phoenix 300 Plus, SEO). The samples were measured by sessile-drop method at room temperature and 3 μL of DI water was used as probe liquid. The water was extruded from a syringe and the sample was elevated to contact with water droplet followed by returning back to original position. The water droplet was adsorbed on the surface of the sample and captured by a camera, and contact angles were calculated by the in-built software. Each sample was measured at more than five different positions and average values were reported.

2.5 Performance of water vapor separation The lab-scale experiment system for water vapor separation performance is presented in Fig. 2. The water vapor was produced from a steam generator by feeding DI water via a HPLC pump at a flow rate of 0.03 mL/min. The temperature of steam generator was controlled at 300 oC. As-generated water vapor was carried by nitrogen gas towards a demister in order to remove mist. After that the mixture gas was diluted by nitrogen gas before feeding. The relative 10

humidity (RH) of feed gas, as the experimental variable, was controlled by changing the composition of carrier gas and dilution gas via mass flow controllers (MFC) while maintaining the total flow rate as 1000 mL/min. The humid gas was fed to shell side of hollow fiber membrane module in an oven under a counter-current flow mode. The temperature of oven was kept at 30 oC. Humidity sensors (probe type 344, Vaisala, Finland) were placed at feed and retentate sides in the oven. The feed side pressure was controlled by a back pressure regulator on the retentate side at 2 bar (gauge pressure) whereas permeate side pressure was controlled by a vacuum pump at 1 bar (gauge pressure), thus total pressure difference of 3 bar was applied. The water vapor in retentate and permeate streams were captured by an ice cold trap (CTB-10, JEIO Tech, Korea) before measuring corresponding nitrogen gas flow rates by bubble flow meters (Gilibrator, USA). All data were recorded when the system reached steady state.

Fig. 2 Schematic diagram of lab-scale unit of water vapor separation system

11

The water vapor flow rate at each stream was calculated by the following equation, 𝑄𝑤 =

𝑄𝑁2 (𝐴𝐻)𝑉𝑚 𝑀𝑤

(1)

Where Qw (cm3 (STP)/min) is water vapor flow rate, QN2 (cm3/min) and AH (g/cm3) are nitrogen gas flow rate (measured from the bubble flow meter) and absolute humidity at the corresponding stream, respectively. Vm is gas molar volume at standard temperature and pressure (22.4 L/mol) and Mw is molecular weight of water (18 g/mol). The permeance was calculated by the following equation, 𝑄𝑃

𝑃𝑖 = 𝐴(𝑝𝑅𝑖−𝑝𝑃 ) 𝑖

𝑖

(2)

Where Pi is permeance (GPU=10-6 cm3 (STP)/(cm2.cmHg.s)) and QiP permeate side flow rate for the corresponding species. A is effective membrane area; piR and piP are partial pressures at retentate and permeate sides calculated from pressure fraction. The selectivity (αi,j) is the permeance ratio of two species as follows, 𝑃

𝛼𝑖,𝑗 = 𝑃 𝑖

𝑗

(3)

2.6 Resistance deconvolution According to Fick’s law, the water flux is expressed in terms of overall mass transfer coefficient and driving force as follows. 𝐽𝐻2 𝑂 = 𝑘𝑜𝑣 ∆𝑝

(4)

Where JH2O (cm3/cm2.s) is water flux through membrane, kov (cm/s) indicates overall mass transfer coefficient and p difference of water vapor partial pressure between feed and permeate sides. The overall mass transfer coefficient (kov) is apparent 12

permeance in physical meaning and can be experimentally determined. Due to the effect of concentration polarization, the water vapor transports additionally through stagnant boundary layers placed on feed and permeate sides. Thus, the overall mass transfer resistance (reciprocal of coefficient) can be divided into several individual resistances. In case of homogenous support, the resistance is expressed as 1

1

𝑘𝑜𝑣

=𝑘 +𝑘 𝑓

1 𝑠𝑢𝑝

1

+𝑘

(5)

𝑝

Where 1/ksup is support, 1/kf feed and 1/kp permeate side boundary layer resistance, respectively. It is reported that the boundary layer resistance is function of cross flow velocity and fluid properties [24]. By applying sweep gas additionally with the same flow rate of feed (sweep ratio=1) to ensure channel geometry, it simplifies to 1 𝑘𝑜𝑣

=𝑘

1 𝑠𝑢𝑝

2

+𝑘

(6)

𝑏𝑙

Where 1/kbl is boundary layer resistance lumping feed and permeate sides and it can be determined by the method from Metz et al [25]. The support resistance can be subsequently determined. These two values are assumed to hold for composite membranes. In case of composite membrane, the resistance is expressed as 1 𝑘𝑜𝑣

1

=𝑘 +𝑘 𝑓

1 𝑓𝑖𝑙𝑚

+𝑘

1 𝑠𝑢𝑝

1

+𝑘

𝑝

(7)

Where 1/kfilm represents the resistance of selective film and it can be calculated by subtracting other individual resistances (with same method) from the overall resistance. The detailed procedure is mentioned in supplementary information.

3. Results and discussion 13

3.1 Intrinsic properties of ETS-4 particles The FE-SEM images of synthesized ETS-4 are shown in Fig. 3. The pristine ETS-4 particles had a dumbbell-like shape (ca. 20 μm) with bilateral needle-like crystallites (inset), which was same as reported publications [22,26]. In fact, the morphology and particle size of primitive ETS-4 depends on synthetic conditions, specifically, initial pH, Si/Ti ratio, crystallization temperature and crystallization time. However, as ETS-4 nanoparticles were used as the coating material in the present study, all synthetic conditions were fixed. The image of ground ETS-4 was also shown to check grinding effect. It can be seen from the picture that the particle size was significantly reduced to nano-scale (tens to hundreds of nanometers), from which grinding with an attrition mill was proved to be a feasible way to reduce particle size for improving applicability of nanoparticles.

The result of EDX analysis of ETS-4 and TFN membrane are reflected in Fig. 4. It has reported that the Si/Ti ratios of ETS-4 and ETS-10 are 2.7 and 5.0, respectively [27]. From the result, the Si/Ti ratio of synthesized ETS-4 was 2.9, which was placed in the consistent range and proved the identity of ETS-4. By exploring the surface of TFN membranes, the signals of Ti and Si with the ratio of 2.9 were also detected, from which the presence of ETS-4 in TFN membranes was confirmed.

14

a

b

Fig. 3 FE-SEM images of ETS-4 before (a) and after (b) grinding

a

b

Element

Weight%

Atomic%

Si K

62.82

74.24

Ti K

37.18

25.76

Totals

100.00

Element

Weight%

Atomic%

Si K

62.94

74.34

Ti K

37.06

25.66

Totals

100.00

Fig. 4 EDX analysis for ETS-4 (a) and TFN membrane (b) from DTE-0.5

The XRD patterns of ETS-4 particles are shown in Fig. 5. The characteristic peaks appeared at diffraction angles (2θ) of 7.5, 12.7, 30o, which met good agreement with the reported 15

literatures [22,26,28,29] and further verified its identity. Moreover, the pattern of ground ETS-4 was depicted as well to examine the stability of ETS-4 after grinding. As a result, the characteristic peaks still existed, which manifested that the crystalline structure was maintained hence its stability could be assured.

Intensity (a.u.)

ETS-4 (before grinding) ETS-4 (after grinding)

5

10

15

20

25

30

35

40

2 (degree)

Fig. 5 XRD patterns of ETS-4 before and after grinding

3.2 Intrinsic properties of TFN membranes The FE-SEM cross-sectional images of prepared membranes are exhibited in Fig. 6. The PSf substrate in Fig. 6(a) appeared to have a quite porous structure with the average pore size of 0.3 μm measured from a capillary flow porometer (CFP-1200-AEX, Porous Material Inc.). After interfacial polymerization, it can be observed that the pores are fully blocked and the compact dense layer forms on the outer surface of hollow fiber membranes as in Fig. 6(b)-(f). During the interfacial polymerization, DABA along with ETS-4 particles diffuse into 16

membranes thereby filling and wetting the pores near to the substrate surface. Subsequently, the DABA crosslinks with TMC to form polyamide network containing ETS-4. The thickness of TFC membrane in Fig. 6(b) was found to be 162 nm and it drastically increased to 215 nm upon adding ETS-4 particles as shown in Fig. 6(c) due to the incorporation of larger nanoparticles as well as their interaction with polyamide layer via hydrogen bonding. With further increase in ETS-4 concentration, the thickness of polyamide layer gradually grew up to 254 nm. The thickness was in the reasonable range of 100-500 nm for TFN membranes [30]. The surface morphology of prepared membranes is presented in Fig. 7. The PSf substrate had the smoothest surface whereas the TFC membrane effectively blocked the pores on the surface causing a little rough surface. By adding ETS-4, the membrane got rougher and the particles became gradually visible. Especially, some big particles can be observed in the TFN membrane with 1.0 wt% of ETS-4 concentration in Fig. 7(f), which might be agglomerated nanoparticles.

Fig. 6 FE-SEM cross-sectional images of membranes showing thickness changes: (a) PSf substrate, (b) DT, (c)

17

DTE-0.1, (d) DTE-0.2, (e) DTE-0.5, (f) DTE-1.0.

Fig. 7 FE-SEM images of membranes showing surface morphology: (a) PSf substrate, (b) DT, (c) DTE-0.1, (d) DTE-0.2, (e) DTE-0.5, (f) DTE-1.0.

The topographic roughness profiles of membranes are illustrated in Fig. 8 and surface roughness parameters including root mean square roughness (Rq), average roughness (Ra), and maximum roughness (Rmax) are summarized in Table 2. The average roughness of the TFC membrane was found to be 13.6 nm, and there was little change upon adding 0.05 wt% of ETS-4 (15.2 nm). As ETS-4 concentration further increased to 0.5 wt%, the roughness was kept increasing due to the incorporation of nanoparticles followed by a decline afterwards because of agglomeration of ETS-4 nanoparticles, which coincides with SEM results. Agglomeration of ETS-4 causes less extent of polymerization and leads to smoother surface, resulting from the drastically decreased roughness from 32.8 nm to 18.5 nm. Similar trend also happened to the relative surface area which indicates the extent of adsorption sites. 18

Generally, higher surface area provides more adsorption sites for water vapor and contributes to increasing permeance to some extent; however, it is not the only factor for determining permeance. This is explained in experimental result in details.

Fig. 8 Topographic roughness profiles of membranes: (a) DT, (e) DTE-0.05, (c) DTE-0.1, (d) DTE-0.2, (e) DTE-0.5, (f) DTE-1.0.

Table 2 Surface roughness parameters of membranes. Membrane code

Rq (nm)

Ra (nm)

Rmax (nm)

Relative surface area* (-)

DT

18.4

13.6

143.5

1.03

DTE-0.05

19.8

15.2

129.7

1.03

DTE-0.1

21.6

17.8

167.7

1.06

DTE-0.2

29.0

21.6

277.3

1.09

DTE-0.5

40.5

32.8

242.8

1.15

19

DTE-1.0

24.5

18.5

176.7

1.08

* Relative surface area=actual surface area/planar area

The water contact angle is considered as a measure of hydrophilicity. The result of water contact angle measurements is listed in Table 3. According to the result, the contact angle of the TFC membrane (DT) was 75o and it suddenly decreased to 59o upon adding ETS-4 nanoparticles, which reflects water uptake characteristic of ETS-4. With further increase in ETS-4 concentration, the contact angles had very a slight trend to decrease. This phenomenon insinuates that ETS-4 does not necessarily contribute hydrophilicity to membranes but acting as a water sieve. The water affinity of ETS-4 might be more likely defined in the point of water capturing ability by molecular sieving effect rather than hydrophilicity itself. Furthermore, given that ETS-4 loading was extremely small (based on the weight of the aqueous monomer) in order to reduce interference during interfacial polymerization, the concentration-dependent changes in contact angle is usually not significant.

Table 3 Water contact angles of membranes Membrane code

Contact angle (o)

DT

75±3

DTE-0.05

59±2

DTE-0.1

59±2

DTE-0.2

58±2

20

DTE-0.5

57±2

DTE-1.0

56±2

3.3 Performance of water vapor separation using TFN membranes The result of water vapor separation experiment is plotted in Fig. 9. In this experiment, the effect of ETS-4 concentration on membrane performance in terms of water vapor permeance and water vapor/N2 selectivity was examined. As shown in the graph, the permeance sharply increased by adding ETS-4 particles at the initial point and started to decrease to a small extent with further increase in ETS-4 concentration, whereas the selectivity initially increased till 0.5 wt% and decreased thereafter. The maximum selectivity of 346 was obtained along with 1377 GPU in permeance for the TFN membrane with the ETS-4 concentration of 0.5 wt% (DTE-0.5).

In the first place, the reason for initial increase in permeance from 788 GPU (DT) to 1434 GPU (0.05 wt%) is attributed to water uptake nature of ETS-4 since it provides additional water vapor permeation channels. The subsequent slight decrease in permeance to 1321 GPU (1.0 wt%) is due to the growing thickness as observed from SEM images, which is the typical paradigm for polyamide membranes. Although the increased surface area from AFM result coupled with slightly increased hydrophilicity from WCA result should cause permeance to increase, the thickness turned out to be dominant in determining permeance in this study. Membrane permeance depends on many factors, such as surface area, hydrophilicity, 21

thickness, crosslinking density, permeation pathway, etc. For water vapor separation, the solution-diffusion model is applied, which describes that permeability is product of solubility and diffusivity. Solubility is more associated with surface area and hydrophilicity whereas diffusivity with thickness and crosslinking density. Therefore, permeance is determined by taking both solubilization and diffusion into consideration. This result had a different trend compared with other TFN membranes using titania or silica reported in our previous works. The reason for such a difference is more likely to be the nanoparticle size. For smaller sized titania or silica, surface area and hydrophilicity play important roles in determining permeance even the thickness increases due to the fact that they have better distribution in the polyamide layer and more hydrophilic compared with zeolite. However, in the case of ETS-4, thickness is more sensitive to permeance since less hydrophilic and larger particles provide more resistance. An attempt of using further size-reduced ETS-4 might be made in the future work. Additionally, as the thickness growth of TFN membranes was very small (39 nm), hence the decrease in permeance was also small (82 GPU) accordingly. The little change in water contact angles supported this phenomenon.

Secondly, the reason for initial increase in selectivity from 127 (DT) to 346 (0.5 wt%) is accredited to sieving effect of ETS-4. As explained earlier, ETS-4 mainly acts as a molecular sieve relied on its water selective pore size while distinguishing other species via its tortuous pathways. Membrane selectivity depends on relative permeation rates of water vapor and nitrogen, and simultaneous increase or decrease in permeance of both species to same extent will not affect selectivity, viz., selectivity increases when overall water vapor permeation rate 22

increases much than that of nitrogen or decreases less than that of nitrogen. Hence, increase in ETS-4 concentration lead to evident increase in selectivity since ETS-4 (4 Å) preferentially allow water vapor (2.6 Å) to permeate through while effectively limiting permeation of nitrogen (3.6 Å). Admittedly, as the pore size of ETS-4 is a little bigger than that of nitrogen, unprecedented enhancement of selectivity is restricted. Furthermore, the subsequent decrease in selectivity to 166 (1.0 wt%) is owing to the agglomeration of ETS-4, as observed in the SEM image. It is acknowledged that nanoparticles tend to be aggregated beyond a certain agglomerating point (0.5 wt% in this study), and interfere with polymerization thus resulting in separation deficiency. The result was similar with other researches [16,31] that high loading of nanoparticles leads to agglomeration and causes lower crosslinking density and reduced performance.

To sum up, the main merit of this kind of TFN membrane has turned out to be selectivity enhancement by only compromising little permeance, which was our primary target. Particularly in flue gas dehydration, selectivity is more important than permeance in terms of humidity control since it is the key to reduce humidity and avoid the white fume that has the negative effect in most industries. Therefore, selectivity enhancement of water vapor in this work is meaningful.

23

1600

400

350 1200 300 1000 800

250

600 200 400

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

200 0

Water vapor/N2 Selectivity (-)

Water vapor permeance (GPU)

1400

150

100 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

ETS-4 concentration with respect to DABA (wt%)

Fig. 9 Effect of ETS-4 concentrations on water vapor permeance and selectivity from lab-scale experiments within the feed side absolute humidity range of 26-29 g/cm3

3.4 Effect of concentration polarization The contribution of boundary layer resistance due to concentration polarization is reflected in Fig. 10. Each resistance profile according to feed gas flow rate and their contribution to overall resistance in percentage for DTE-0.5 with normal operating flow rate are presented. It can be seen from the graph that as feed gas flow increased, overall resistance and both boundary layer resistances decreased while the selective film resistance remained constant, which is in good agreement with the reported literature [24]. The support resistance slightly increased with feed gas flow rate which might be caused from competitive permeation. For resistance composition, as expected, the dense selective film provides the major resistance (62.2%) for water vapor transport and support resistance and permeate side resistance are 24

negligibly low (totally 3.6%). Remarkably, the feed side boundary layer resistance was approximately half of the selective film resistance (34.2%). It was confirmed that the effect of concentration polarization is evident and the boundary layer resistance cannot be neglected. The apparent permeability of water vapor through selective film was estimated to be 326 Barrer (1 Barrer=10-10 cm3 (STP).cm/(cm2. s.cmHg)) and the intrinsic permeability was found to be 527 Barrer after correcting for boundary layer resistance. Furthermore, the result insinuates that increasing feed flow rate can effectively minimize boundary layer resistance.

1200

1/kov 1/kf 1/kp 1/ksup 1/kfilm

Resistance (s/cm)

1000

100%

800

600

62.2% 400

34.2% 200

3.4% 0

0.2% 400

600

800

1000

1200

1400

1600

1800

2000

2200

Feed gas flow rate (mL/min)

Fig. 10 Resistance deconvolution for DTE-0.5

4. Conclusion ETS-4 particles were successfully synthesized by typical hydrothermal method and ground by an attrition mill to reduce their size. Ground ETS-4 particles were dispersed in DABA solution and accompanied to interfacial polymerization with TMC thus fabricating TFN membranes. The identity as well as the stability of pristine and ground ETS-4 was confirmed by XRD. The successful incorporation of ETS-4 nanoparticles on the hollow fiber membranes 25

was confirmed by FE-SEM and EDX. The effect of ETS-4 concentration on water vapor permeance and selectivity was studied in the lab-scale unit. As ETS-4 concentration increased, the permeance slightly decreased whereas the selectivity significantly increased due to molecular sieving effect followed by the decrease at the agglomerating point. The maximum selectivity of 346 was obtained along with 1377 GPU in permeance under the optimum ETS-4 concentration of 0.5 wt% (DTE-0.5). Moreover, the resistance of feed side boundary layer was found to be significant. To conclude, the ETS-4 incorporated TFN membrane would not only make itself as a promising candidate for water vapor separation but also provide a new approach to design TFN membranes for various applications.

Acknowledgement This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B6-2468).

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Highlights    

Microporous Engelhard titanosilicate-4 (ETS-4) is used for fabricating thin film nanocomposite (TFN) membranes. ETS-4 particle size is reduced by applying an attrition mill. ETS-4 incorporated TFN membranes exhibit increased selectivity owing to molecular sieving effect of ETS-4. The selective film provides major resistance and the feed side boundary layer has unnegligibly high resistance due to concentration polarization.

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