Preparation and pervaporation performance of vinyl-functionalized silica membranes

Author’s Accepted Manuscript Preparation and pervaporation performance of vinyl-functionalized silica membranes Sadao Araki, Ami Okabe, Akira Ogawa, Daisuke Gondo, Satoshi Imasaka, Yasuhisa Hasegawa, Koichi Sato, Kang Li, Hideki Yamamoto www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)31471-0 https://doi.org/10.1016/j.memsci.2017.10.066 MEMSCI15693

To appear in: Journal of Membrane Science Received date: 29 May 2017 Revised date: 28 October 2017 Accepted date: 31 October 2017 Cite this article as: Sadao Araki, Ami Okabe, Akira Ogawa, Daisuke Gondo, Satoshi Imasaka, Yasuhisa Hasegawa, Koichi Sato, Kang Li and Hideki Yamamoto, Preparation and pervaporation performance of vinyl-functionalized silica membranes, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2017.10.066 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.

Preparation and pervaporation performance of vinyl-functionalized silica

membranes

Sadao Araki 1 * , Ami Okabe 1 , Akira Ogawa 1 , Daisuke Gondo 1 , Satoshi Imasaka 1 ,

Yasuhisa Hasegawa 2 , Koichi Sato 2 , Kang Li 3 , Hideki Yamamoto 1

1. Department of Chemical Engineering, Kansai University, 3 -35 Yamatecho

3-chome, Suita 564-8680, Japan

2. National Institute of Advanced Industrial Science and Technology (AIST),

Research Institute for Chemical Process Techno logy, Nigatake 4-2-1, Sendai

983-8551, Japan

3. Department of Chemical Engineering, Imperial College London, London

SW7 2AZ, UK

*[email protected]

Research highlights

࣭ A vinyl-functionalized silica (VFS) membrane was prepared on an alumina

hollow fiber.

࣭ The ethyl acetate flux of this membrane is three times higher than that of a

membrane prepared on a tubular substrate.

࣭ This membrane exhibited a separation factor of 85 and an EA flux of 3.5 kgm –

2 –1

h for EA removal.

Abstract

Pervaporation using hydrophobic membranes to separate azeotropic and

thermally degradable mixtures, and organics from aqueous solutions with low

concentrations has attracted attention. On the other hand, distillati on, which is

the conventional and commercialized separation me thod, is unsuitable for the

separation of these mixtures. In this study, an D-alumina substrate coated with a

J-alumina layer in both hollow fibe r and tubular geometries was used for

preparation of vinyl-functionalized silica (VFS) membrane s by the sol-gel

method using vinyltrimethoxysilane as the silica precursor. Silica functionalized

with vinyl groups shows excellent affinity for ethyl acetate (EA) based on

Hansen solubility parameters [1]. The VF S membrane prepared on a tubular

alumina substrate at 453 K showed the EA flux (0.85 kg m – 2 h – 1 ) and separation

factor (77.3) for pervaporation of a 5 % (mass fraction) EA aqueous solution at

295 K. An D-alumina hollow fiber was further used instead of the conventional

tubular substrate to improve the EA flux. The hollow fiber-supported VFS

membrane has a flux of 2.53 kg m – 2 h – 1 , which is three times higher than that

prepared on the tubular-substrate, and a separation factor of 81.5 for the

pervaporation of a 5 % (mass fraction) EA from aqueous solution at 295 K . The

fluxes of methyl acetate and methyl ethyl ketone for the hollow fiber -supported

VFS membrane are also about three times higher than th ose obtained for

tubular-supported VFS membranes.

Keywords: Pervaporation; Hydrophobic; Vinyl; Silica; Hollow fiber

1. Introduction

Separation by pervaporation (PV) uses hydrophobic membranes to separate

azeotropic or thermally degradable mixtures, and has attracted attention [2].

Recovery or removal of volatile organic compounds (VOCs) with low

concentration

in

aqueous

solutions, such

as

wastewater,

by

PV using

hydrophobic membranes has also been reported [3, 4]. VOCs are used in the

manufacture of various products, including paints, pharmaceuticals, plasticizers,

varnishes, and adhesives [3]. Factory wastewater containing VOCs can be

harmful to the environment and to human health; it can also lead to air pollution

without proper treatment [4]. PV can be used to remove or recover VOCs from

water with high efficiency, and in eco -friendly manner compared to conventioanl

distillation [5].

Recently,

Zhou

et

al.

reported

using

an

octyl -functionalized

polydimethylsiloxane membrane to remove VOCs from water, and achieved a

separation factor of 592 and total flux of 0.19 kg m – 2 h – 1 for PV of 1 % (mass

fraction) ethyl acetate (EA) in aqueous sol ution at 313 K [4]. Penkova et al.

reported

the

physicochemical

poly(2,6-dimethyl-1,4-phenylene

features

oxide)

and

of

nanocomposites

fullerene

(C 6 0 )

of

composite

membranes for EA production [6]. This membrane concentrated EA from 17.9 %

to 67.4% in a solution containing ethanol, water, and acetic acid. In their PV

process, the EA flux was about 0.06 kg m – 2 h – 1 at 293 K. Kujawa et al. reported

the use of a hydrophobic ceramic membrane to remove VOCs from a waste

solution [7]. They modified the surfaces of a lumina (pore size 5 nm), titania

(pore size 2–4 nm), and zirconia (pore size 2 –4 nm) substrates with

1H,1H,2H,2H-perfluorooctyltriethoxysilane (C6). The C6 -modifed titania gave a

separation factor of about 30 and EA flux of about 1.8 kg m – 2 h – 1 in PV

experiments for 2 % (mass fraction) EA aqueous solution at 308 K. In our

previous study, we separated EA and methyl ethyl ketone (MEK) from water

using PV with hydrophobic silica membranes modified with methyl, propyl,

isobutyl, n-hexyl, and phenyl groups [1]. In addition, the effects of the

concentrations of organic compounds in the feed solution and temperature on PV

performance for the propyl-functionalized silica membrane were also evaluated.

A separation factor of 78 and EA flux of 1.75 kg m – 2 h – 1 were achieved by PV

using the membrane for a 5 % (mass fraction) EA aqueous solution at 315 K. Our

results suggested that the organic flux improved when the Hansen solubility

parameters (HSPs) of the membrane were close to that of the organic compound

[1]. The HSP consists of the dispersion force (δ d MPa 1 /2 ), the dipole interaction

force factor (δ p MPa 1 /2 ), and the hydrogen-bonding force factor (δ h MPa 1 / 2 ). The

compatibility between substances A and B can generally be evaluated using the

HSP distance between A and B, conventionally called R a , as follows: R a = [4(δ d A − δ d B ) 2 + (δ p A – δ p B ) 2 + (δ h A – δ h B ) 2 ] 1 /2 (1)

Based on the HSP values [1], the vinyl group for modified silica was selected for

the separation EA, which is widely used for the production of perfumes,

plasticizers, varnishes, synthetic resins and adhesive agents, from water [ 8].

Compared with silica functionalized in previous wo rk [1], the HSP of

vinyl-functionalized silica (VFS) is more similar to that of EA. An earlier study

found that for a hydrophobic silica membrane prepared by the sol -gel method

using vinyltrimethoxysilane (VTMS) and t e t r a e t h y l o r t h o s i l i c a t e ( T E O S ) ,

the contact angle of water droplets on the membrane increased with the ratio of

VTMS/TEOS, and reached 144° at a VTMS/TEOS ratio of 0.97 [9]. Therefore,

the VFS membrane is expected to show good performance for the separation of

EA from water by PV.

In addition to the type of membrane used in PV, the properties of the substrate

also affect the permeation flux [10]. Since hollow fibers generally show a lower

transport resistance than conventional tubular substrates, the use of hollow

fibers could improve the permeation flux for the same membrane [11, 12]. Wan g

et al. reported the water flux of a zeolite LTA membrane prepared on a hollow

fiber substrate is 6.2 kg m – 2 h – 1 and the separation factor (D H 2 O / e th an o l ) is 125,000.

This separation factor is about three times higher than that of the membrane

prepared on a tubular substrate for a 90 % (mass fraction) eth anol aqueous

solution at 348 K [13]. Shao et al. found the water flux increased linearly with

increasing porosity of the hollow fiber substrate [14]. The effect of changes in

the substrate properties should be investigated further to improve the flux.

Although many researchers have reported hollow fiber supported membranes,

there are little studies of the impact of support properties and feed concentration

on PV performance [15-20].

In this study, the effect of the calcination temperature on PV performance for

a 5 % (mass fraction) EA aqueous solution at 295 K a of a VFS membrane

prepared on a conventional tubular alumina substrate (outer diameter = 10mm)

was investigated for the first time. An alumina hollow fiber (outer diameter =

2mm) with porosity and average pore diameter similar to that of the tubular

substrate was used to improve the EA flux. In addition, the effect of support

resistance was investigated. The thickness of the J-alumina layer was controlled by the withdrawal rate during dip-coating. The influence of the J-alumina layer

and type of substrate on the PV properties of the VFS membrane for separation

of EA from water were investigated in detail. In addition, we evaluat e the effect

of the concentration of EA in the feed solution on the EA flux and se paration

factor for VFS membranes prepared on the conventional tubular substrates as

well as the hollow fiber substrates.

2. Experimental

2.1 D-alumina supports Porous D-alumina tubes (outer diameter = 10 mm, inner diameter = 6 mm, porosity = 60 %, average pore size = 150 nm) and porous D-alumina hollow

fibers (outer diameter = 2 mm, inner diameter = 1.6 mm, porosity = 60 %,

average pore size = 150 nm) were used as the support for the silica membrane

(Fig. 1). One end of the supports was capped with a dense D-alumina plate and the other end was connected to a dense D-alumina tube, in both cases using a

glass sealant.

2.2 Preparation of J-alumina coating

A boehmite sol was prepared by the hydrolysis of aluminum sec-butoxide

(TCI, Japan) using the following procedure. Aluminum sec-butoxide was added

to approximately 200 mL of water at 353 K and then 10 mL of 1 mol/L HNO 3

was

㻔㼍㻕

㻔㼎㻕

Fig. 1 Photograph of the D-alumina (a) tubular support and (b) hollow fiber. 㻌

added to this mixture under stirring. The mixture was heated to 363 K for 3 h,

and then refluxed for 24 h. The concentration of the boehmite sol was adjusted

to 1.0 mol/L by evaporation or by dilution with deionized water. A polyvinyl

alcohol (PVA, MW = 2,000, Wako Pure Chemical Industries, Japan) solution was

prepared by adding 1.75 g of PVA to 50 mL of 0.05 mol/L HNO 3 aqueous

solution at 363 K. The boehmite solutions used for dip -coating were prepared by

mixing 25 mL of boehmite sol and 10 mL of the PVA solution, and mixing

ultrasonically for 20 min. The α-alumina substrate was dip-coated in the

boehmite solution for 20 s. The thickness of the J-alumina layer was controlled

by the withdrawal rate of the alumina support from the solution (0.5, 1, 3 5 mm

s – 1 ). The J-alumina layer which was prepared using a withdrawal rate of 1 mm min – 1 was used to confirm the optimal calcination temperature of the VTMS

membrane. The support was dried at room temperature for 3 h, and then calcined

at 873 K for 24 h.

2.3 Preparation of VFS membranes

The VFS layer was formed on the D-alumina supports coated with J-alumina

layer. Cetyl trimethylammonium bromide (0.008 mol), VTMS (0.1 mol), and 7.5

mL of 1 mol L – 3 HNO 3 were dissolved into 25 mL of ethanol, and then the

mixture was stirred at room temperature for 3 h. The J-alumina coated support

was immersed in the sol for 60 s and then removed at a rate of 1 mm s – 1 . The

membrane was dried at room temperature and then calcined in air at 453 K.

Finally, the membrane was washed with a large volume of ethanol to remove

cetyl trimethylammonium bromide.

2.4 Characterization

The membrane morphology was observed by a scanning electron microscopy

(-4800,

Hitachi

High-Technologies

Corp).

The

hydrophobicity

of

VFS

membranes was evaluated by measuring the contact angle of water (DMs-400,

Kyowa Interface Science Co., Ltd.), and the effective pore diameter was

determined using permporometry with EA [1]. In addition, the thermal properties

of the VTMS was analyzed by thermogravimetric analysis (Thermo plus EV O2,

Rigaku, Japan).

2.5 Gas permeation and pervaporation

Single gas permeation of helium, nitrogen and methane were determined from

a permeation pressure drop across the membrane at room temperature. The pure

gas was fed at the outer surface of membrane (feed side), and the permeated gas

flow rate at the inside of membrane (permeate side) was measured using a

bubble flowmeter. The pressure difference between feed and permeate side s was

kept at 100kPa.

The PV properties of the VFS membrane for EA, methyl acetate (MA) and

MEK in aqueous solutions were measured at 295 K. The membrane was

immersed in the aqueous solution, and vacuum was applied at the inner surface

of the membrane using a vacuum pump. The permeated vapors were collected

using a condenser with liquid nitrogen for 1 h, and the mass and composition of

the liquid after condensation for 1 h on liquid nitrogen were measured using a

scale and a Karl Fischer water meter (AQV-2100, Hiranuma Industry),

respectively. When the permeated mixture had separa ted into two phases,

ethanol was added to obtain a homogeneous mixture. The mass fraction of EA in

the feed was changed from 1 % to 7 % to confirm the effect of feed concentration

on the PV performance of the VFS membrane. The flux and separation factor of

the organic compounds with respect to water were calculated as follows:

J=W/At D=(y o rg an i c /x o rg an i c )/(y w a t e r /x w at e r )

(2)

(3)

where W is the permeate mass (kg), A is membrane area (m 2 ), t is the

measurement time (h), and x and y are the r mass fractions of water and an

organic compound in the feed ( x) and permeate (y).

3. Results and Discussion

3.1 Effect of calcination temperature

First, the VFS membranes were prepared on a conventional alumina tube

to optimize the calcination temperature. Figure 2 shows the effect of the

calcination temperature for the VFS membrane on the PV properties for 5 %

(mass fraction) EA aqueous solution at 290 K. The EA flux gradually increased

to

100

2

Flux / kg m-2 h-1

80 60 1 40

Separation factor / -

EA Water

20 0

0 380

430

480

530

Calcination temperature / K

Fig. 2 Effect of calcination temperature on the EA and water fluxes and

separation factor.

0.85 kg m – 2 h – 1 from 0.64 kg m – 2 h – 1 for calcination temperatures of up to 453 K,

and then drastically decreased at higher calcination temperatures. On the other

hand, the water flux was almost constant for calcination temperatures of up to

473 K, and became much higher for higher calcination temperatures . As a result,

the separation factor of the membrane calcined at 453 K was the highest among

all the membranes.

Previously, we suggested that the organic fl ux can be predicted using the

following equation [1]. J = a u e (b

u Ra)

™M

(4) 㻌

where J is the flux of the organic compound [kg m – 2 h – 1 ], M is molecular weight [g mol - 1 ] of organic compound, and a (0.0193 kmol m - 2 h -1 ) and b (– 0.0245 MPa - 1 /2 ) are constants. Since R a [MPa 1 /2 ] between EA and VFS is 2.67, the EA flux of the VFS membrane is calculated to be 0.87 kg m - 2 h -1 . The EA

flux at 453 K agreed with the predicted value. This suggests that the calcination

temperature influences details of the membrane structure such as the surface

modification group, pore diameter. In addition, the EA flux of the VFS

membrane calcined at 453 K was higher than that of the hydrophobic silica

membrane with n-butyl or n-propyl groups [1].

Fig. 3 shows the contact angles

of the membranes calcined at temperatures between 383 K and 493 K. The

contact angle increased slightly to 86 r for the membranes calcined at

temperatures up to 453 K and was much less for membranes made with a higher

calcination temperatures. This behavior with the variation of the EA flux and

separation factor with the calcination temperature, shown in Fig. 2. The

thermogravimetric analysis curve of VTMS is shown in Fig. 4. A gradual

decrease was observed at around 430 K, and then a sharp decr ease occurred at

around 470 K. This behavior was similar to that observed for the effect of

calcination temperatures of VFS membranes on their contact angle .

It is

expected that the vinyl groups started to decompose at around 470 K and are

almost completely absent at around 530 K. These results also support the

postulate that the vinyl group plays an important role in the permeation of the

organic solvent.

The results of permporometry for VFS membranes calcined at 393 K, 453 K,

and 493 K are shown in Fig. 5. The He permeances of the VFS membrane

prepared at 393 K, at 453 K and 493 K were 7. 1 u 10 – 8 mol m – 2 s – 1 Pa – 1 8.2 u 10 – 8

mol m – 2 s – 1 Pa – 1 2.4 u 10 – 7 mol m – 2 s – 1 Pa – 1 at 293 K, respectively. The

dimensionless permeability of He of the VFS membrane calcined at 393 K was

broad in the Kelvin diameter range of 0 -4.5 nm, and the apparent average pore

diameter, which is the value at half the dimensionless permeability of He [21],

was 2.67 nm. By comparison, the VFS membrane calcined at 453 K has an

apparent average pore diameter of 0.96 nm and pores

Fig. 3 Effect of calcination temperature on the contact angles of water droplets

on the VFS membranes

1

TG / %

0

-1

-2 380

430

480

530

Temperature / K

Dimensionless Permieability of He / -

Fig. 4 Thermogravimetric analysis curve of VTMS

1 393 K 453 K 493 K

0.8

0.6

0.4

0.2

0 0

1

2

3

4

5

Kelvin diameter / nm

Fig. 5 Dimensionless permeability of He as a function of the Kelvin diameter for

VFS membranes prepared at 393 K, 453 K, and 493 K.

over 5.0 nm were not observed. The apparent average diameter of the VFS

membrane calcined at 493 K (1.12 nm) was slightly bigger than that at 453 K,

and some pores were larger than 4 nm. The affinity between the membrane and

condensable gas is very important to measure the pore size accurately by

permporometry. Tsuru et al. reported that the N 2 permeation of the hydrophobic

methylated SiO 2 was decreased by only 10% when water was used as

condensable vapor [22] even at Kelvin diameter of 10 nm. Since the VFS

membranes calcined at 495 K are expected to be hydrophilic nature, the pore

distribution derived from permporometry may not match the actual pore

distribution.

The EA flux of the VFS membrane calcined at 393 K was slightly lower than

that of the membrane calcined at 453 K (Fig. 2). There are two possible causes

for this lower flux, even with the pore size of the membrane at 393 K being

bigger than that of the membrane at 453 K. One is based on the lower contact

angle of the VFS membrane calcined at 393 K compared with that calcined at

453 K. A temperature of 393 K is lik ely to be insufficient for the condensation

reaction, and hydroxyl groups will remain in the pores. The other reason is based

on a lower porosity. The He permeance of the VFS membrane prepared at 393 K

was slightly lower than that of membrane prepared at 4 53 K. Even though the

VFS membrane prepared at 393 K has big pores, its porosity was similar to or

less than that of the membrane prepared at 453 K. Therefore, the difference in

hydrophobicity or porosity might affect the EA flux. The VFS membrane

calcined at 493 K has a low contact angle, and it showed separation factor (D=

5.5) due to the decomposition of vinyl groups. We concluded that the drastic

decrease in EA flux at around 480 K was caused by a decrease in the affinity

between EA and the VFS membrane.

3.2 Influence of the support

The effect of the substrate on the PV properties of VFS membrane was

investigated using an D-alumina hollow fiber instead of a tubular substrate, and

(a)

(b)

(c)

(d)

(e)

Fig. 6 Field emission scanning electron microscopy images of cro ss-sections of

VFS membranes on D-alumina hollow fibers with a J-alumina interlayer prepared with withdrawal rates of 0.5 (a), 1 (b), 3 (c), or 5 (d) mm s – 1 , and on the

conventional alumina substrate (f).

by changing the thickness of the J-alumina layer. The thickness of the J-alumina

layer was controlled by the withdrawal rate from the boehmite sol during

dip-coating. Cross-sectional images were obtained for these VFS membranes

(Fig. 6 (a)–(b)). In addition, the cross-sectional image of the VFS membrane

prepared on the D-alumina tube is shown in Fig .6 (e). The withdrawal rate of the substrate during dip-coating was used to control the thickness of the J-alumina layer between 1.0 Pm and 1.7 Pm. In addition, the J-alumina layer on the hollow

fiber was thinner than that on the tube despite the fact that the same withdrawal

rate was used. The thickness of the VFS layer on the tubular support was about

0.8 Pm. By comparison, the thickness of the VFS layer on the hollo w fiber coated with J-alumina (1.0–1.5 Pm) was about 0.1 Pm, and on that with a

J-alumina layer of 1.7 Pm was about 0.2 Pm. It is not clear why the J-alumina

layer thickness affected the VFS layer because the procedure for VSF layer

preparation was consistent. This difference might be within the normal

variability of VFS membrane thickness

120

5 EA

100

Water

80 3 60 2 40 1

Separation factor / -

Flux / kg m-2 h-1

4

20 0

0 0.9

1.1

1.3

1.5

1.7

Intermediate layer thickness / vm Fig. 7 Effect of intermediate layer thickness on the fluxes and separation factors.

Table 1 Comparison of fluxes and separation factors for VFS membranes on

tubular substrate and hollow fiber.

for this process. For 5 % (mass fraction) EA aqueous solution at 290 K, the PV

performances of VSF membranes on hollow fibers with different J-alumina layer

thicknesses are shown in (Fig. 7). It was found that the PV performance was not

affected by the thickness of the J-alumina layer. However, the EA and water

fluxes for the hollow fiber-supported VFS membrane were three times higher

than those obtained for the tube-supported VFS membrane. The separation facto r

of the hollow fiber-supported VFS membrane was almost the same as that of the

tube-supported VFS membrane.

Table 1 compares the PV performances for the VFS membranes prepared

using the hollow fiber and tubular support for EA, MA, and MEK solutions ( 5 %

mass fraction) at 295 K. The J-alumina layer was prepared with a withdrawal rate of 1 mm s – 1 . MA and MEK fluxes for the tube -supported VFS membrane

were estimated using eq. 4 [1]. The estimated fluxes of MA and MEK were 0.61

kg m – 2 h – 1 and 0.44 kg m – 2 h – 1 . Although the estimated and measured values for

the MA flux were slightly different, the estimated MEK flux was very close to

the measured value. From these results , it was concluded that eq. 4 can be used

to determine the most suitable membrane struc ture and the flux for the

membrane on a conventional tubular support. However, modification of eq. 4 is

needed to estimate fluxes for hollow fiber-supported membranes. To do this, the

contribution of the substrate to the flux needs to be investigated through further

experiments. The EA and MEK fluxes for the hollow fiber -supported VFS

membrane

were

also

three

times

higher

than

th ose

obtained

for

the

tube-supported VFS membrane, and the separation factors were similar. These

results show that the affinity properties of the hollow fiber-supported VSF

membrane were similar to those of the tube -supported VFS membrane under the

same conditions.

Table 2 Comparison of gas permeance properties for tubular and hollow fiber

substrates

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Table 2 shows the single gas permeation results for helium, nitrogen, and

methane for the hollow fiber and tubular support. The helium, nitrogen and

methane permeances of the hollow fiber was about 9 times higher than those of a

tubular support. On the other hand, the helium permea nce of the hollow fiber

with the J-alumina layer decreased slightly with the thickness of the J-alumina layer increased as shown in Fig.8. For a J-alumina thickness of 1.0 Pm and 1.7 Pm, the helium permeance was 2.3 u 10 – 5 mol m – 2 s – 1 Pa – 1 and 2.0 u 10 – 5 mol m – 2

s – 1 Pa – 1 ,respectively. The relationship between the permeance and the

reciprocal of square root of the quantity of the absolute temperature times

molecular weight is shown in Fig.9. The permeance vs conditions of all supports

follows closely predictions based on Knudsen mechanism [23].

The He permeance results for the hollow fiber-supported VFS membrane

with J-alumina layers of different thicknesses are also shown in Fig. 8. There was only a small decrease in He permeance with the thickness of J-alumina layer.

Interestingly, the He permeance of VFS membranes on the hollow fibre was

similar to that obtained for the tube -supported membrane, despite the presence

of a thicker VFS layer. This result also suggested that the variation in

thicknesses of the D-alumina, J-alumina, and VFS layers is too small to affect

the He permeance. In any case, for gas permeation of the VFS membrane,

differences in the support did not affect the He permeance. This means that mass

transfer through the D-alumina substrate would not be the rate determining

process. However, the EA flux of the VSF membrane did improve when the

hollow fiber substrate was used instead of the tubular substrate (Table 1).

Therefore, mass transfer through the D-alumina substrate greatly affects the EA

flux. The transfer of EA molecules

Fig. 8 Effect of J-alumina thickness on He permeance of the alumina substrate

and VFS membrane

Fig.9 Relation between the reciprocal of square root of the quantity of the

temperature times molecular weight and gas permeances.

through the VFS membrane involves adsorption of EA molecules on the VFS

layer, diffusion of EA as a liquid, evaporation of the EA molecule, and then

diffusion as a vapor. Because the thicknesses of the J-alumina and VFS layers on

the hollow fiber did not affect the EA flux, EA molecules might adsorb on the

VFS layer and occupy pores of the J-alumina. Therefore, diffusibilit y of organic compounds through D-alumina is improved using the hollow fiber compared with

the tubular support.

The effect of the EA mass fraction in the feed solution on the PV is shown

in Fig. 10. Is was found that the EA fluxes with a 1 % mass fraction of EA in t he

feed solution were similar for VFS membranes prepared on hollow fiber and

tubular support. Although the EA fluxes increased with the EA mass fraction in

the feed solution, the rate of increase of the EA flux for the hollow

fiber-supported VFS membrane was larger than that for the tube -supported

membranes. The EA flux of the hollow fiber-supported VFS membrane was

about 3.5 kg m – 2 h – 1 , which was three times that of the tube-supported membrane,

and the separation factor was 85. These values are higher than the upper bound

of the relationship between flux and separation factor of hydrophobic ceramic

membranes reported in the past three years (Fig. 11). By contrast, the water

fluxes of the VFS membranes on both the hollow fiber and tubular support

6

120 EA Water Open: Tube Close: Hollow fiber

100

4

80

3

60

2

40

1

20

0

Separation factor / -

Flux / kg m-2 h-1

5

0 0

2

4

6

8

EA concentration / wt%

Fig. 10 Effect of EA mass fraction on the PV performance of VFS membranes

prepared on tubular and hollow fiber substrates.

Separation factor / -

100

Tube Hollow fiber

80 60 40 20 0 0

2

4 Flux / kg m-2 h-1

6

8

Fig 11 Relationship flux and separation factor of ceramic hydrophobic

membranes for EA separation from aqueous solution reported from 2015 to 2017.

Open: Temp.= 308 K, EA conc. 1-2 % (mass fraction) [7, 23-25], Close: Temp.=

295 K, EA conc. =3-7 % (mass fraction) [1].

were constant at about 0.6 kg m – 2 h – 1 and 0.2 kg m – 2 h – 1 , respectively. The

hydrophobic silica membranes prepared on tubular support s have water fluxes

independent of the nature of the functional groups, and the type and

concentration of organic compound in the feed solut ion [1]. The water flux for

the hollow fiber-supported VFS membrane was higher than that of the

tube-supported VFS membrane because of the reduced thickness of the substrate

or VFS layer. If silica and J-alumina layers does not depend on EA flux

regardless of types of support as shown in Fig.7, the ratio of fluxes equals the

ratio of reciprocals of support resistances. Although we expected that the EA

flux for the hollow fiber-supported VFS membrane would increase in proportion

to the EA flux for the tube-supported VFS membrane, this did not occur.

Therefore, the EA flux must be affected by different processes. In other words,

EA flux was determined by the adsorption process with an EA mass fraction of

1 % and 3% in the feed solution, and then the influence of the substrate

increased as the EA mass fraction increased. For EA mass fraction of 5 % and

7% in the feed solution, the EA fluxes of hollow fiber-supported VFS membrane

were three times higher than the EA fluxes of tube -supported VFS membrane,

which was approximately same with the ratio of water fluxes of VFS membranes

on hollow fiber to tube-supported. Therefore, it is suggested that the resistance

of hollow fiber is three times lower than that of tube support.

Conclusions

The PV properties of VFS membranes for the separation of EA from water were

investigated. A VFS membrane prepared on a tubular alumina support at 453 K

gave the highest EA flux and separation factor. The vinyl group decomposes at

temperatures above 453 K, and this greatly increases the water flux and

decreases the EA flux. A VFS membrane prepared on an alumina hollow fiber at

the optimal temperature was used for PV tests of EA/water, MA/water, and

MEK/water mixtures with organic mass fractions of 5 %. The fluxes with

obtained with this VFS membrane are three times those obtained with a VFS

membrane prepared on a conventional tubular support. However, for PV of a 1 %

(mass fraction) EA solution, the EA fluxes of VFS membranes on hollow fiber

and tubular support are similar. The EA flux of a hollow fiber-supported VFS

membrane (3.5 kg m – 2 h – 1 ) was three times that of a tube-supported VFS

membrane, and the separation factor was 85 for PV of a 7 % (mass fraction) EA

solution. These results suggest that the adsorption rate of EA on the VFS has a

major influence on the EA flux in the 1 -3% (mass fraction) solution, and the

diffusivity of the EA molecule through the alumina support has a major

influence on the EA flux in the 5-7% (mass fraction) solution.

Acknowledgments

This work was partially supported by MEXT-Supported Program for the

Strategic Research Foundation at Private Universities, 2012 –2016 and 㻌 Kansai

University Overseas Research Program, 2016.

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Table and figure legends

Fig.1 Photograph of D-alumina (a) tubular support and (b) hollow fiber

Fig.2 Effect of calcination temperature on the EA and water fluxes and

separation factor

Fig. 3 Effect of calcination temperature on contact angle of water droplets on the

VFS membranes

Fig.4 TG curve of VTMS

Fig.5 Dimensionless permeability of He as a function of the Kelvin diameter for

VFS membranes prepared at 393 K, 453 K and 493 K.

Fig.6 FE-SEM images of cross-sections of VFS membranes on a-alumina hollow

fibre with J-alumina interlayer prepared by pulling rate of 0.5 (a), 1 (b), 3 (c)

and 5 (d) mm s-1, and on the conventional alumina substrate (f).

Fig.7 Effect of intermediate layer thickness on the fluxes and separation factor.

Fig.8 Effect of intermediate layer thickness on He permeance of alumina

substrates and VFS membranes.

Fig.9 Relationship between the reciprocal of square root of the quantity of the

temperature times molecular weight and gas permeances.

Fig. 10 Effect of EA mass fraction on the PV performance of VFS membranes

prepared on tubular and hollow fiber substrates.

Fig 11 Relationship flux and separation factor of ceramic hydrophobic

membranes for EA separation from aqueous solution reported from 2015 to 2017.

Open: Temp.= 308 K, EA conc. 1-2 % (mass fraction) [7, 23-25], Close: Temp.=

295 K, EA conc. =3-7 % (mass fraction) [1].

Table 1 Comparison of fluxes and separation factors for VFS membranes on

tubular substrate and hollow fiber.

Table 2 Comparison of gas permeances for tubular substrate and hollow fibre



Tubular support

Hollow fiber

*UDSKLFDODEVWUDFW







EA concentration / wt%

0.0

8

0

6

20.0

1

4

40.0

2

2

60.0

3

100.0

120.0

80.0

0

Substrate type Open: Tube Close: hollow fiber

4

5

6

Separation factor / -

EA flux / kg m-2 h-1