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Journal of Membrane Science 302 (2007) 265–270

Novel synthesis of a porous stainless steel-supported Knudsenmembrane with remarkably high permeability

Dong-Wook Lee a, Sang-Jun Park a, Chang-Yeol Yu a, Son-Ki Ihm b, Kew-Ho Lee a,∗a National Research Laboratory for Functional Membranes, Environment and Energy Research Center, Korea Research Institute of Chemical Technology,

P.O. Box 107, Yuseong, Daejeon 305-606, South Koreab Department of Chemical and Biomolecular Engineering, National Research Laboratory for Environmental Catalysis,Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea

Received 13 February 2007; received in revised form 21 June 2007; accepted 22 June 2007Available online 1 July 2007

bstract

A stainless steel-supported Knudsen membrane (SKM) with remarkably high permeability was successfully synthesized using only 100 nm-ized colloidal silica sol by means of a dipping–rolling–freezing–fast drying (DRFF) and soaking–rolling–freezing–fast drying (SRFF) method.ydrogen and nitrogen permeances of the SKM were (6.7–8.2) × 10−6 and (1.8–2.3) × 10−6 mol m−2 s−1 Pa−1 with a H2/N2 permselectivity of.5–3.7, which approaches to a theoretical H2/N2 selectivity for the Knudsen diffusion mechanism (3.74). In comparison with a typical mesoporous-alumina membrane supported on a porous stainless steel or �-alumina substrate, the SKM had 20 and 5.5 times permeance with almost same2/N2 permselectivity, respectively. Generally, mesoporous materials such as �-alumina and the M41S family with several nm-sized pores are

mployed to obtain the Knudsen-dominated permeation characteristics. In this case, a decrease in the gas permeance is inevitable due to depositionf a mesoporous skin layer. However, in the case of the SKM, the Knudsen-dominated permselectivity was extraordinarily obtained through

odification of porous stainless steel substrates with the colloidal silica particles having relatively large particle size, because a well-densified

ayer of the 100 nm-sized colloidal silica could be obtained without formation of defects via the freezing procedure. In addition, the large porosityf the 100 nm-sized colloidal silica layer gave rise to the remarkably high gas permeance.

2007 Elsevier B.V. All rights reserved.

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eywords: Porous stainless steel; Colloidal silica; Knudsen membrane

. Introduction

Mesoporous inorganic membranes are promising for ultra-ltration and membrane reactor process requiring highermeability. From a viewpoint of industrial application of thoserocesses, high permeability of the membrane is a preferentialactor compared to high permselectivity particularly needed forhe high-purity gas production, because membrane permeabilitys directly associated with capacity of the membrane processes.lthough dense and microporous inorganic membranes showery high permselectivity, their permeability is extremely low

hat it is difficult to utilize them in the industrial field [1–3].owever, mesoporous membranes give significantly high per-eability with the Knudsen-dominated permeation behavior

∗ Corresponding author. Tel.: +82 42 860 7240; fax: +82 42 861 4151.E-mail address: khlee@krict.re.kr (K.-H. Lee).

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376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2007.06.054

4,5]. Therefore, except a few special applications requiringigh permselectivity, the mesoporous inorganic membranes arexpected to be more suitable for industrial application of theembrane process.Mesoporous membranes usually have an asymmetric struc-

ure, consisting of a mesoporous skin layer and a porousubstrate layer with large pores and a low pressure drop. The typ-cal mesoporous materials for a separation layer are �-aluminand ordered mesoporous silica such as the M41S family andBA-15. Important characteristics of ordered mesoporous mem-rane films on porous supports include pore size control, poreonnectivity, porosity and the control of thickness and defects.f defects and pore size can be controlled, then crystallographicrrangement of pores may be important in the overall perme-

nce characteristics of the membranes [6]. In the last few years,considerable amount of work on the application of orderedesoporous silica as membranes was reported [7–14]. Recently,umar et al. prepared ordered mesoporous MCM-48 mem-

2 mbrane Science 302 (2007) 265–270

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Fig. 1. Preparation procedures of porous stainless steel-supported compositemembranes. M1: prepared via dipping–rolling–freezing–fast drying (DRFF)method, M2: prepared via DRFF and soaking–rolling–freezing–fast drying(M

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66 D.-W. Lee et al. / Journal of Me

ranes with cubic pore structure by solution growth method15]. Higgins et al. reported on the preparation and characteri-ation of mesoporous silica membranes via micellar templatingsing non-ionic ethylene oxide–propylene oxide–ethylene oxideri-block copolymer surfactant as a template [6]. In theseases, however, after the mesoporous skin layer is coatedn the porous substrate, a significant falloff in permeabilitys inevitable. Moreover, the time-demanding process such asydrothermal treatment is needed to obtain surfactant-organizedesostructure.In this study, we prepared Knudsen membranes without a

onsiderable decrease in permeance by simple modificationf porous stainless steel (PSS) substrates with only colloidalilica sol of 100 nm in particle diameter. According to ourrevious paper [16,17], the PSS substrates, modified by infil-rating submicron Ni powder or pressing the colloidal silicaarticles (100 nm in particle diameter) into the substrate macro-ores, showed viscous-flow permeation behavior. In this case,o obtain Knudsen-dominated membranes, mesoporous mate-ials such as �-alumina must be coated on the modified PSSubstrates. However, in current report, we successfully synthe-ized the stainless-steel-supported Knudsen membrane (SKM)hrough the novel synthetic procedure using only 100 nm-sizedilica particles. The SKM with remarkably high permeance andnudsen permselectivity is considered to be very promising for

n inorganic membrane reactor, ultrafiltration and support foricroporous layers.

. Experimental

.1. Preparation of colloidal silica sol

Colloidal silica sol was synthesized under base-catalyzedondition via hydrolysis of tetraethyl orthosilicate (TEOS)nd condensation reaction. A TEOS:ammonia: water:ethanololar ratio was 1:0.64:53.6:40.1. Prior to addition of an

mmonia/water mixture, a TEOS/ethanol mixture was stirredigorously at 50 ◦C. The addition of the ammonia/water mix-ure was carried out dropwise, followed by refluxing the final

ixture for 3 h, resulting in stable colloidal silica sol includingilica spheres with about 100 nm in diameter.

.2. Preparation and characterization of compositeembranes

Disks of 316L porous stainless steel used as a sub-trate were purchased from Matt Metallurgical. The PSSisk has a thickness of 1 mm and surface area of 5 cm2.e synthesized stainless steel-supported Knudsen mem-

ranes using the colloidal silica sol (100 nm in diameter)y means of a dipping–rolling–freezing–fast drying (DRFF)nd soaking–rolling–freezing–fast drying (SRFF) method. Thehole synthesis procedures of the stainless steel-supported com-

osite membranes are shown in Fig. 1.

The PSS substrate was dipped into the colloidal silica solncluding 100 nm-sized silica spheres for 30 s. The colloidal sil-ca sol on the substrate surface was rolled out with a urethane

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SRFF) method, M3: prepared via dipping–rolling–fast drying (DRF) method,4: prepared via DRF and soaking–rolling–fast drying (SRF) method.

olling pin in order to eliminate a silica cake layer formed onhe substrate. Subsequently, the substrate was frozen in liquiditrogen for 10 s, followed by fast drying of the substrate for 10 sn a hot plate heated to 250 ◦C. The DRFF process was repeatedeven times. After calcination of the substrate at 650 ◦C for 2 h,e obtained a membrane M1.To minimize defects on the membrane surface, we conducted

dditional infiltration of the colloidal silica spheres into theembrane M1 by means of the SRFF process. A back side of theembrane M1 loaded on the o-ring sealed-cell was vacuumed

y a rotary vacuum pump. The colloidal silica sol was pourednto a front side of the M1, and then the vacuum was maintainedor 3 min so that the silica sol penetrates into defects of the mem-rane M1. After the soaking process, the colloidal silica sol onhe substrate surface was rolled out with a urethane rolling pin.ubsequently, the substrate was frozen in liquid nitrogen for 10 s,ollowed by fast drying of the substrate for 10 s on a hot plateeated to 250 ◦C. The SRFF process was repeated three times.fter calcination of the substrate at 650 ◦C for 2 h, a Knudsen-ominated membrane with high permeability was successfullyynthesized. The membrane prepared by the DRFF and SRFFethod is designated as a membrane M2, which is the stainless

teel-supported Knudsen membrane.To investigate the effect of the freezing step, we fabricated

membrane M3 by repeating the dipping–rolling–fast dry-

D.-W. Lee et al. / Journal of Membrane Science 302 (2007) 265–270 267

Table 1Composite membranes prepared by various methods

Membrane code Coating materials Coating method Composition of the layers

M1 SiO2 sol (100 nm) DRFF SiO2 (100 nm)/PSSM2 SiO2 sol (100 nm) DRFF and SRFF SiO2 (100 nm)/PSSM3 SiO2 sol (100 nm) DRF SiO2 (100 nm)/PSSM4 SiO2 sol (100 nm) DRF and SRF SiO2 (100 nm)/PSSMMM

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wtSmdasssrtmembrane M1 and M2. The membrane M1 shows hydrogenpermeance of (2.8–5.4) × 10−5 mol m−2 s−1 Pa−1 and nitrogenpermeance of (1.0–1.9) × 10−5 mol m−2 s−1 Pa−1 with a H2/N2permselectivity of 2.8 in a permeation temperature range from

5 SiO2 xerogel (100 nm)6 Boehmite sol7 Boehmite sol

ng (DRF) process seven times without the freezing step, andlso prepared a membrane M4 through additional infiltrationf the colloidal silica spheres into the membrane M3 via theoaking–rolling–fast drying (SRF) process. The SRF processas repeated three times to fabricate the M4. The compositeembranes prepared via various synthetic routes are listed inable 1.

In order to confirm the excellent performance of the mem-rane M2, other Knudsen composite membranes were alsoynthesized via different synthetic methods. A membrane M5as fabricated by infiltrating the colloidal silica xerogel (100 nm

n particle size), obtained by drying the colloidal silica sol withrotary evaporator, into the PSS substrates by a press under

0 MPa [17]. A membrane M6 was prepared through coating�-alumina layer on the membrane M5 three times by means

f the soaking–rolling method, previously reported in our pub-ication [17]. A membrane M7 was synthesized by coating the-alumina on a porous �-alumina disk three times by meansf the dip-coating method. The porous �-alumina support has ahickness of 1 mm, surface area of 5 cm2 and average pore diam-ter of 100 nm. The calcination of the membrane M5, M6 and7 was carried out at 650 ◦C for 2 h. The morphology of the

omposite membranes was observed using a scanning electronicroscopy (SEM; XL 30S FEG, Philips).

.3. Permeation measurement

Permeation measurements for a single gas were made withure nitrogen and hydrogen between 25 and 450 ◦C. The per-eation area was 4.52 cm2. A single gas test was conducted byeans of the pressure drop method. A feed side of the membraneas pressurized by pure hydrogen or nitrogen while a permeate

ide of the membrane was under atmospheric pressure withoutsweeping gas. The transmembrane pressure was 0.042 MPa

nd the flux of permeated gases was measured by a soap filmow meter. The permselectivity for the single gas permeation

est is defined as a ratio of hydrogen permeance to nitrogenermeance measured at the same transmembrane pressure andemperature.

. Results and discussion

We have successfully synthesized the stainless steel-upported Knudsen membrane (membrane M2) with remarkablyigh permeability through modification of the PSS substrate

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Press [17] SiO2 (100 nm)/PSSSoaking–rolling [17] �-Al2O3/M5Dip-coating �-Al2O3/�-Al2O3

ith only colloidal silica sol of 100 nm in particle diame-er by means of the DRFF and SRFF method. Fig. 2 showsEM images of the membrane M1 and M2. Compared to theembrane M1, surface morphology of the membrane M2 was

enser and surface defects considerably decreased. That is, thedditional modification via the SRFF significantly diminishedurface defects, which could not be avoided during the DRFFtep. This is because, in comparison to the dipping process, theoaking process led to forced penetration of the coating mate-ials into pores or defects via a vacuum in the opposite side ofhe membrane [17]. Fig. 3 exhibits permeation results of the

ig. 2. Surface SEM images of (a) M1 and (b) M2. The inset of the part b is aigh magnification image.

268 D.-W. Lee et al. / Journal of Membrane Science 302 (2007) 265–270

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ig. 3. Permeation results of (a) M1 and (b) M2 with different permeationemperature.

5 to 450 ◦C. After the additional modification of the membrane1 by the SRFF process, the hydrogen and nitrogen perme-

nce of the membrane M2 decreased to (6.7–8.2) × 10−6 and1.8–2.3) × 10−6 mol m−2 s−1 Pa−1. The H2/N2 permselectivityncreased to 3.5–3.7, which approaches to a theoretical H2/N2electivity for the Knudsen diffusion mechanism (3.74). Thencrease in the H2/N2 permselectivity implies that defects andinholes of the membrane M1 were successfully eliminated viahe SRFF process. In general, mesoporous materials such as �-lumina, the M41S family and SBA-15 with several nm-sizedores are employed to obtain the Knudsen-dominated perme-tion characteristics. However, in the case of the membrane2, the Knudsen-dominated permselectivity was extraordi-

arily obtained only with the colloidal silica particles havingelatively large particle size.

As shown in Fig. 1, to confirm the importance of the freezingrocedure, we also prepared stainless steel-supported com-osite membranes via the dipping–rolling–fast drying and theoaking–rolling–fast drying method without the freezing step.ig. 4 exhibits surface morphology of the membrane M3 and

4. Compared to the membrane M1 (Fig. 2a), the membrane3 had much more defects and cracks. The membrane M4

howed considerable cracks on the layer of the 100 nm-sized sil-ca particles, while the well-densified layer of the colloidal silica

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ig. 4. Surface SEM images of (a) M3 and (b) M4. The inset of the part b is aigh magnification image.

articles was successfully formed on the surface of the mem-rane M2 (Fig. 2b). Fig. 5 also revealed that the SKM (membrane2) gave much denser morphology of the cross-section in

omparison to the membrane M4. The membrane M3 showsydrogen permeance of (4.3–11.0) × 10−5 mol m−2 s−1 Pa−1

nd nitrogen permeance of (2.1–5.0) × 10−5 mol m−2 s−1 Pa−1

ith H2/N2 permselectivity of 2.0–2.3 in the permeation tem-erature range from 25 to 450 ◦C (Fig. 6a). The hydrogen anditrogen permeance of the membrane M4 were (3.3–7.6) × 10−5

nd (1.4–3.2) × 10−5 mol m−2 s−1 Pa−1 in the permeation tem-erature range from 25 to 450 ◦C, and its H2/N2 permselectivityas 2.3–2.4 (Fig. 6b). The membrane M2 synthesized by theRFF and SRFF method showed the H2/N2 permselectivity

pproaching to the theoretical Knudsen selectivity, whereas theembrane M4 prepared by the DRF and SRF method gave

ower H2/N2 permselectivity due to the viscous flow-dominatedermeation behavior, which is derived from a great number ofracks shown in Fig. 4b. In general, defects such as cracks orinholes, leading to a significant falloff of permselectivity, aresually formed through evaporation of solvent during dryingrocedure. However, in the case of the DRFF and SRFF method,he freezing procedure was carried out before the fast drying, so

hat formation of cracks was avoided through the confinement ofolloidal silica particles at extremely low temperature before theolvent removal. Therefore, it can be concluded that densifica-ion of the colloidal silica particles via the freezing procedure led

D.-W. Lee et al. / Journal of Membrane Science 302 (2007) 265–270 269

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sgmc1ctfdeduce, therefore, the remarkably high permeance and Knudsen

Fig. 5. Cross-sectional SEM images of (a) M2 and (b) M4.

o the crack-free surface morphology and the Knudsen H2/N2ermselectivity of the SKM (membrane M2).

As reported in our previous publication [17], the PSS sub-trates can be also modified by pressing the 100 nm-sized silicaerogel into the front side of the substrate under 10 MPa. Ashown in Table 1, such a substrate modified by a press is des-gnated as a membrane M5. The membrane M5 has hydrogenermeance of (4.1–8.1) × 10−6 mol m−2 s−1 Pa−1 and nitrogenermeance of (1.6–3.0) × 10−6 mol m−2 s−1 Pa−1 with H2/N2ermselectivity of 2.6–2.7 (Fig. 7a). Compared to the mem-rane M2, the permselectivity of the membrane M5 was lower,lthough the gas permeance was almost same as the membrane2. To obtain the H2/N2 permselectivity approaching to a theo-

etical Knudsen selectivity, a membrane M6 was synthesizedy coating a boehmite sol on the surface of the membrane5. Fig. 7b exhibits permeation results of the membrane M6

n the permeation temperature range from 25 to 450 ◦C. The2/N2 permselectivity of the membrane M6 increased to 3.3–3.4ue to textural properties of mesoporous �-alumina skin layerith 4–5 nm in an average pore size. However, after formationf the �-alumina skin layer, the gas permeance of the mem-rane M6 was one order of magnitude smaller than that ofhe membrane M5. In contrast, the membrane M2 prepared

y the DRFF and SRFF method showed the Knudsen perms-lectivity and 20 times permeance as the membrane M6. Thiss because the Knudsen permselectivity of the membrane M2

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ig. 6. Permeation results of (a) M3 and (b) M4 with different permeationemperature.

as obtained through the modification with only colloidal silicaol.

For comparison, a porous �-alumina-supported Knudsenembrane (membrane M7) was also synthesized by coatingboehmite sol on a porous �-alumina disk by means of the

ip-coating method. The hydrogen and nitrogen permeancef the membrane M7 were (9.0–15.0) × 10−7 and (2.8–4.3) ×0−7 mol m−2 s−1 Pa−1, and its H2/N2 permselectivity was.5–3.6 (Fig. 8). The H2/N2 permselectivities of the membrane2 and M7 were almost consistent, whereas the membrane M2

ave 5.5 times gas permeance as the membrane M7.Although mesoporous materials with several nm in pore

ize, such as �-alumina, M41S family and SBA-15, should beenerally employed for synthesis of the Knudsen compositeembranes, we successfully synthesized the SKM by using only

olloidal silica particles with the large average particle size of00 nm. In addition, as shown in Fig. 7a, even though the sameolloidal silica particle was used to modify the PSS substrates,he Knudsen permselectivity could not be obtained via the dif-erent synthetic route, not the DRFF and SRFF method. We

ermselectivity of the SKM are attributed to the densification ofhe colloidal silica particles without the crack formation via theRFF and SRFF method.

270 D.-W. Lee et al. / Journal of Membran

Fig. 7. Permeation results of (a) M5 and (b) M6 with different permeationtemperature.

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. Conclusions

We successfully fabricated a stainless steel-supported Knud-

en membrane with remarkably high permeability by using only00 nm-sized colloidal silica sol as a coating material. It wasonfirmed that the freezing step of the DRFF and SRFF methodas most important to obtain a crack-free silica layer. In gen-

[

e Science 302 (2007) 265–270

ral, defects such as cracks or pinholes, leading to a significantecrease in permselectivity, are usually formed by solvent evap-ration during drying procedure. However, in the case of theRFF and SRFF, a well-densified colloidal silica layer was suc-

essfully formed on the PSS substrate, as the freezing procedureas carried out before the fast drying. That is, formation of

racks was avoided through the confinement of colloidal silicaarticles at extremely low temperature before the evaporation ofolvent. The densified colloidal silica layer without defect for-ation led to the remarkably high gas permeance as well as thenudsen permselectivity.

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