Journal of Membrane Science 256 (2005) 1–6

Preparation and characterization of Al2O3 hollow fiber membranes

Li Jiansheng, Wang Lianjun∗, Hao Yanxia, Liu Xiaodong, Sun XiuyunChemical Engineering School, Nanjing University of Science and Technology, Nanjing 210094, PR China

Received 8 April 2004; received in revised form 25 July 2004; accepted 26 July 2004Available online 3 May 2005

Abstract

�-Al 2O3 hollow fibers were prepared by a phase-inversion method combined with the reaction bonded aluminum oxide (RBAO) process.Next,�-Al 2O3 membranes, which were fabricated by a sol–gel process, were cast on the outer surface of the hollow fiber supports. Scanningelectron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), nitrogenisothermal absorption measurements and gas permeability testing were used to characterize the supports and�-Al 2O3 membranes. The resultsshowed that the mechanical strength of hollow fiber supports is better than that of non-RBAO hollow fiber, and their porosity and meanpore size were 50.1% and 0.88�m, respectively. SEM images of�-Al 2O3 membranes showed that the membranes were defect-free. Them 2 sp aa©

K

1

egpdoiahmsgapa

f

h asortsfiber

ce-ys-g

eth-heat

s isram-in thessingrialsgrain

many

hese

0d

ost concentrative pore diameter and the specific surface area of�-Al 2O3 membranes were about 4.5 nm and 228.9 m/g, respectively. Gaermeability tests suggested that the�-Al 2O3 membranes possessed gas selectivity, and the separation factor for N2/Ar are 1.133 at 0.3 MPnd 1.139 at 0.4 MPa, respectively, which is slightly smaller than the value expected from the ideal Kundsen diffusion (α = 1.194).2004 Elsevier B.V. All rights reserved.

eywords:RBAO; �-Al2O3 hollow fiber;�-Al2O3 membrane; Preparation; Characterization

. Introduction

The importance of membrane technology is increasing invery industry that needs separation processes for liquids orases. Various polymeric hollow fiber membranes have beenrepared and been used widely due to their high packingensity and high permselectivity. However, at temperaturesf 200◦C and above, the stability of polymeric membranes

s insufficient. Ceramic materials are both more thermallynd chemically stable than polymeric materials and thereforeave a great potential advantage. For this reason, membranesade from ceramic materials have been of great interest for

ituation requiring severe separation conditions. In case ofas separations and membrane reactors, both high selectivitynd high packing density must be achieved. In general, theacking densities for tubes and multi-channel monolithics arebout 30–250 m2/m3 and 130–400 m2/m3, respectively. For

∗ Corresponding author. Tel.: +86 25 8431 5522/5941;ax: +86 25 8431 5518.E-mail address:wanglj@mail.njust.edu.cn (W. Lianjun).

ceramic hollow fiber supports, packing density as hig1000 m2/m3 can be easily obtained. Therefore, many effhave been made in the preparation of ceramic hollowsupports[1–6].

There are many different methods used for preparingramic hollow fiber supports, including dry spinning a stem of inorganic material and binder[3,7], and wet spinnina binder solution containing ceramic powders[1,5]. How-ever, ceramic hollow fiber supports prepared by these mods have the drawbacks of large shrinkage during thetreatment process and relatively poor strength.

The reaction-bonded aluminum oxide (RBAO) procesan alternative for the manufacture of alumina-based ceics and composites and has been of general interestpast ten years. Various advantages, such as low procetemperature, good mechanical behavior, low raw matecost, near-net-shape tailorability, and glass-phase-freeboundaries, are some of the most attractive attributes fortechnical and high-performance applications[8–11]. How-ever, information on RBAO process used to prepare tceramic supports is generally lacking.

376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2004.07.014

2 L. Jiansheng et al. / Journal of Membrane Science 256 (2005) 1–6

In this study, we have attempted to use the phase-inversionmethod combined with the RBAO process to prepare theAl2O3 hollow fiber supports with relatively small shrink-age and strong strength. The preparation process, formingmechanism, and resulting microstructure and properties ofAl2O3 hollow fiber supports were investigated. After that,�-Al2O3 membranes were cast on the outer surface of thehollow fiber supports using sol–gel methods, the resultingsuggested membranes are particularly suitable for ultrafiltra-tion or gas separation process.

2. Experimental

2.1. Preparation of hollow fiber support

Commercially available�-Al2O3 powders with an aver-age particle diameter of 0.5�m (HFF-5, Shanghai Wusongfertilizer factory) and aluminum powders with an average par-ticle diameter of 5–50�m were used as the support materi-als. A mixture of Al2O3 (70 wt.%) and Al (30 wt.%) powdersto which suitable acetone was added was milled 10 h. Then,500 g of this mixture was added to 300 ml dimethylfomamide(DMF) to disperse the suspension about 30 min in ultrasonicvibrator and a stable slurry was obtained. Polysulfone (PSF)of 70 g was dissolved in 200 ml DMF at 60◦C over a period of2 asf mers on.F ture.

nlesss 60f zed to0 eretw /0.8( airg ternalc wasu d was5 wasd ufflefa en-t

ing5 asp tifieda

2

eth-o xide( wa-t ,f

H+/Al3+ mol) to form a stable colloidal sol. The sol was keptat about 80◦C for about 12 h under reflux conditions, duringwhich most of the alcohol was evaporated. A boehmite solhaving a pH value of about 3.7 was obtained.

A solution of polyvinyl alcohol (PVA) with an averagemolecular weight of 72,000 (3 g/100 ml H2O) was then addedto the boehmite sol at room temperature. The PVA was used asa drying control chemical additive (DCCA) to prevent mem-brane cracking in the drying process.

The supported�-Al2O3 membranes were prepared by dip-ping the RBAO hollow fiber supports for 10 s in the boehmitesol and using withdrawal speed of 20 cm/min in air. Thecoated fibers were dried in the climate chamber with rel-ative humidity of 60% and at a temperature of 40◦C for12 h. The unsupported membranes were prepared by pour-ing the boehmite sol into glass-dishes. The unsupported andsupported membranes were then calcined in muffle oven at700◦C for 2 h at a heating and cooling rate of 1.5◦C/min.

2.3. Characterization

The reaction-bonding process was studied by thermo-gravimetric analysis (TGA, SHIMADZU TGA-50). Crys-tal size was measured by transmission electron microscopy(TEM, H-800HITACHI). The 3-point-bending strength wasc U,A em-p 8,BT sup-p scan-n ores uredb 100,C

3

3s

dya ofF berg -t n-t resa f thefi ined.T r-l cts5 -n ofF

h to form polymer solution. After the polymer solution wormed, the obtained slurry was then added into the polyolution slowly with stirring to form a uniform suspensiinally, the suspension was degassed at room tempera

The resulting suspension was transferred to a staiteel reservoir and degassed at a temperature of◦Cor 4 h. Then, the degassed suspension was pressuri.15–0.2 MPa using nitrogen. A tube-in-orifice spinnith orifice diameter/inner diameter of the tube of 2.0

mm) was used to obtain hollow fiber green body. Theap was kept at 15 cm. Deionwater was used as the inoagulant and its injection rate was 2 ml/min. Tap watersed as the external coagulant. Linear extrusion speem/min. Finally, the prepared hollow fiber green bodyried at room temperature, and then sintered in the m

urnace (Model SSX-12–16, Shanghai) at 1450◦C for 2 h atheating rate of 3–5◦C/min. The obtained sample was id

ified as RBAO hollow fiber.For comparison, the hollow fiber green body contain

00 g Al2O3 powders (without Al powders) and 70 g PSF wrepared by above-mentioned process, which was idens non-RBAO hollow fiber.

.2. Preparation ofγ-Al2O3 membrane

The�-Al2O3 membranes were prepared by sol–gel mds. To prepare boehmite sols, aluminum iso-propoAl(C3H7O)8) was hydrolyzed in an excess amount ofer (113:1 H2O/Al3+ mol) at 80◦C with vigorous stirringollowed by the peptization with appropriate HNO3 (0.2:1

arried out on a material testing machine (SHIMADZGS-10KND). Crystal phases of supports at different teratures were identified by X-ray diffraction (XRD, DRUKER) with Cu K� radiation in the range of 10–70◦.he surface morphology and thickness of hollow fiberorts and supported membranes were observed using aing electron microscopy (SEM, JSM-6300, JEOL). The pize distribution of unsupported membrane was measy the nitrogen isothermal absorption technique SA3OULTER.

. Results and discussions

.1. Microstructure and properties of hollow fiberupport

SEM micrographs of the RBAO hollow fiber green bore shown inFig. 1. It can be seen from the micrographig. 1(a) that the o.d. and i.d. of the prepared hollow fireen body are measured to be 1260 and 840�m, respec

ively. The micrograph ofFig. 1(a) illustrates that at the ceer of the hollow fiber green body, long finger-like structure present and that near the outer and inner walls ober green body, relatively compact structures are obtahe micrograph ofFig. 1(b) shows that the length of finge

ike structures is about 100�m and the thickness of compatructure near the outer walls and inner walls is 100�m and0�m, respectively.Fig. 1(c) is the magnified picture of iner wall inFig. 1(b). It can be seen from the micrographig. 1(c) that sheet aluminum with the length less than 10�m

L. Jiansheng et al. / Journal of Membrane Science 256 (2005) 1–6 3

Fig. 1. SEM images of RBAO hollow fiber green body: (a) cross-section; (b) membrane wall; (c) inside membrane wall; (d) outer surface.

and Al2O3 particles with the size about 0.5�m are bondedby PSF. The micrograph of outer surface view of the greenbody is presented inFig. 1(d). It shows more clearly thatthe alumina particles and sheet aluminum are buried in thecontinuous PSF phase.

Fig. 2shows the micrographs of the RBAO hollow fiber.The sintering process was carried out in air at temperatureof 1450◦C. It can be seen from the micrograph ofFig. 2(a)that the o.d. and i.d. of the sintered fiber shrink from 1260and 840 to 1117 and 630�m, respectively, but that wall thick-ness expands from 210 to 244�m. These results are due to theoxidation of aluminum particles in the green body. The cross-sectional structure of the sintered fiber as shown inFig. 2(a)is the same as that of the green body, and the long finger-likestructures are located at the center of the sintered fiber. ASEM micrograph of the outer surface of the sintered hollowfiber is shown inFig. 2(b). It can be seen from the micro-graph ofFig. 2(b) that the binders (PSF) have completelybeen removed and alumina particles are partly connected af-ter sintering at 1450◦C for 2 h.

The properties of RBAO hollow fibers and non-RBAOhollow fibers before and after sintering are shown inTable 1.It can be seen fromTable 1that the RBAO hollow fibers haverelatively strong strength and small shrinkage as comparedto that of non-RBAO hollow fiber samples. Furthermore, thebending strength of RBAO hollow fibers is more than twiceas it is reported by Smid et al.[3] under similar conditionsof porosity. This improved mechanical property is due to theformation of “new” Al2O3 crystals (see Section3.2).

3.2. The reaction behavior during sintering process

The TG-curve showing the weight changes of RBAO hol-low fiber green body during heating in an air atmosphere isshown inFig. 3. Before 430.5◦C, there is no clear weightloss because the organic solvent (DMF) had been removedfrom the green body after drying. At temperatures between430.5◦C and 540.7◦C, the decomposition of PSF takes place,demonstrated by the strong weight loss. It is interesting tonote that from 540.7◦C to 660◦C, the TG curve shows weight

r after

Fig. 2. SEM images of RBAO hollow fibe sintering: (a) cross-section; (b) outer surface.

4 L. Jiansheng et al. / Journal of Membrane Science 256 (2005) 1–6

Table 1Properties of RBAO hollow fiber and non-RBAO hollow fiber

Properties RBAO hollow fiber Non-RBAO hollow fiber

Before sintering After sintering Before sintering After sintering

Inner diameter (�m) 840 630 856 638Outer diameter (�m) 1260 1117 1308 1026Wall thickness (�m) 210 244 226 194Porosity (%) – 50.1 – 63.2Max pore diameter (�m) – 0.93 – 1.42Mean pore diameter (�m) – 0.88 – 1.04Compress strength (MPa) – 89.6 – 26.6Bending strength (MPa) – 41.1 – 17.9

gain; this is attributed to the oxidation of small aluminum par-ticles. The small aluminum particles are oxidized in gas/solidreaction[8]. The melting of aluminum is marked at 660◦C bya flat curve. Above the melting point of Al (660◦C), oxidationof Al is accelerated. Because of the 11.5% and 45% volumeexpansion associated with the melting and oxidation of alu-minum, respectively, the pressure in the molten aluminumpool increases until aluminum permeates into the microc-racks, puncturing the scale and spilling into the void space ofneighboring particles. Because of the bad wetting of Al2O3by liquid aluminum, droplets that are readily coated by an ox-ide skin form. This process continues until all aluminum isoxidized[8]. In fact, there are “new” Al2O3 nanometer crys-tallites produced after green body sintering at 700◦C. Fig. 4shows a TEM photograph of these Al2O3 nanometer crystal-lites whose particle size is about 10–40 nm. When sinteringtemperature increases, “old” Al2O3 particles are bonded bythe “new” crystals and grain growth takes place. In the finalfully reaction bonded body, “new” and “old” particles can nolonger be distinguished and the microstructure is more ho-mogeneous; so the mechanical property of sintered fiber isgreatly improved.

3.3. The phase transform during sintering process

a-t

the sample exists in two phases of Al and�-Al2O3. At roomtemperature, the intensity of the Al peak is stronger. With arise in temperature, this peak decreases gradually. After sin-tering at 700◦C, the characteristic peak of�-Al2O3 can be ob-served. This fact indicates that the aluminum in RBAO greenbody has been oxidized partly, which corresponds well withthe TG curve (Fig. 3). Up to 1450◦C, the diffraction peaks ofAl and�-Al2O3 disappear completely, which suggest that the

Fig. 4. TEM photograph of Al2O3 nanometer crystalline produced duringRBAO process (700◦C).

The XRD patterns (Fig. 5) indicate the phase transformion during the sintering process. Before sintering at 950◦C,

Fig. 3. TG curve of RBAO hollow fiber green body.

L. Jiansheng et al. / Journal of Membrane Science 256 (2005) 1–6 5

Fig. 5. XRD patterns of RBAO hollow fiber green body sintered at: (a) roomtemperature; (b) 700◦C; (c) 800◦C; (d) 950◦C; (e) 1450◦C.

oxidization of aluminum is complete and�-Al2O3 formed byoxidization of aluminum is fully transformed to�-Al2O3.

3.4. The microstructure of top layer

The micrograph of the typical top surface of�-Al2O3membranes supported by RBAO hollow fiber is presentedin Fig. 6. It can be seen that the membrane surface is smoothand no microcracks or pinholes are observed (Fig. 6(a)). Thebig white spots are the particles of�-Al2O3 particles in thehollow fiber support beneath top�-Al2O3 membranes. Thisindicates that the top�-Al2O3 membranes are thin.Fig. 6(b)shows that the thickness of top�-Al2O3 membranes is about1.0�m and the top membranes exhibit good adhesion to thehollow fiber support.

3.5. Pore size distribution of composite membrane

The pore morphology of the unsupported�-Al2O3 mem-branes was studied from its nitrogen adsorption–desorptionisotherm. The pore size distribution computed from the ad-

Fig. 7. Pore size distribution of�-Al2O3 unsupported membrane.

sorption isotherm by BJH method[12] is shown inFig. 7.After calcining at 700◦C for 2 h, the pore size distributioncurve has one peak, and its most concentrative pore diameteris about 4.5 nm. Moreover, the portion of pore size smallerthan 10 nm is 81.23% in unsupported�-Al2O3 membranes.Therefore, the pore size distribution is rather narrow. Specificsurface area of unsupported�-Al2O3 membranes is calcu-lated to be 228.9 m2/g.

3.6. Gas permeation measurements

Gas permeability testing is usually applied to character-ize the integrity of membrane. The gas permeation has beentested with single gases, i.e. N2 (99.999%) and Ar (99.999%).Dead-end measurements are carried out on one fiber (effec-tive membrane area around 6.0 cm2) situated in the centerof a polypropylene tube. The end of the fiber is sealed witha commercial epoxy resin (E-51, Wuxi resin factory). Thefluxes are measured by changing the mean pressure whilemaintaining the pressure difference across the membranes.The separation factorα (N2/Ar) is defined as the ratio of thepermeances of pure N2 to pure Ar.

Fig. 8 indicates the relation of nitrogen permeabilityversus pressure difference of the RBAO hollow fiber supportand the supported�-Al2O3 membranes. The nitrogen

section

Fig. 6. SEM images of (a) top surface and (b) cross- of supported�-Al2O3 membranes calcined at 700◦C for 2 h.

6 L. Jiansheng et al. / Journal of Membrane Science 256 (2005) 1–6

Fig. 8. The relation of nitrogen permeability and pressure difference of�-Al2O3 membrane.

Fig. 9. Gas separation property of�-Al2O3 membrane.

permeability increases linearly with the rise in pressuredifference. The intercept and slope of line is the constantresponding to Knudsen’s flow and laminar flow, respectively.The permeability data of supported�-Al2O3 membranes aremuch lower than that of the hollow fiber support. The slopeof hollow fiber support is 17.7 times higher than that ofthe �-Al2O3 membranes. This indicates that the Knudsen’sflow is dominant mechanism of transport in the�-Al2O3membranes. Therefore, the pore size of�-Al2O3 membranesis far smaller than hollow fiber support and the surfaceof�-Al2O3 membrane is defect-free.

Gas separation measurement (Fig. 9) reveals clearly thatthe �-Al2O3 membranes show gas selectivity, and the sep-aration factor for N2/Ar are 1.133 at 0.3 MPa and 1.139at 0.4 MPa, respectively, which is slightly smaller than thevalue expected from the ideal Kundsen diffusion (α = 1.194).This shows that�-Al2O3 membranes is governed by Knudsenflow.

4. Conclusion

�-Al2O3 hollow fiber supports with relatively smallshrinkage and strong strength have been successfullyprepared by phase-inversion method combined with RBAOprocess. The prepared hollow fiber supports have longfinger-like structures at the center and relatively compactstructures near their outer and inner walls. Its porosity andmean pore size were 50.1% and 0.88�m, respectively.Therefore, the prepared hollow fibers are appropriate for ap-plication in microfiltration processes under severe separationconditions.

Defect-free�-Al2O3 membranes have been fabricated bysol–gel process on the outer surface of prepared supports.The separation factor of the�-Al2O3 membranes for N2/Aris 1.133 at 0.3 MPa and 1.139 at 0.4 MPa, respectively. Thesemembranes can be used as support materials for catalyticmembrane reactors at high temperature.

Acknowledgement

The authors gratefully acknowledge the research fund-ing provided by Ministry of Education of China (Grant No.20020288015).

R

es,

gas

arac-em.

tra,Bull.

n of.rac-

m.

pro-

mul-994)

kage

[ rane

[ f re-c. 76

[ ina

eferences

[1] K.H. Lee, Kim.F Y.M., Asymmetric hollow inorganic membranKey Eng. Mater. 61 (1991) 17.

[2] J.D. Way, D.L. Roberts, Hollow fiber inorganic membranes forseparation, Sep. Sci. Technol. 27 (1992) 29.

[3] J. Smid, C.G. Avci, V. Gunay, R.A. Terpstra, Preparation and chterization of microporous ceramic hollow fiber membranes, J. MSci. 112 (1996) 85.

[4] H.W. Brinkman, J.P.G.M. van Eijk, H.A. Meinema, R.A. TerpsInnovative hollow fiber ceramic membranes, Am. Ceram. Soc.78 (1999) 51.

[5] T. Xiaoyao, L. Shaomin, K. Li, Preparation and characterizatioinorganic hollow fiber membranes, J. Mem. Sci. 188 (2001) 87

[6] L. Shaomin, T. Xiaoyao, K. Li, R. Hughes, Preparation and chaterisation of SrCe0.95Yb0.05O2.975 hollow fiber membranes, J. MeSci. 193 (2001) 249.

[7] R.A. Terpstra, J.P.G.M. van Eijk, F.K. Feenstra, Method for theduction of ceramic hollow fibers. US Patent 5,707,584 (1998).

[8] W. Suxing, Reaction bonding and mechanical properties oflite/silicon carbide composites, J. Am. Ceram. Soc. 77 (12898.

[9] Wu Suxing, N. Claussen, Fabrication and properties of low-shrinreaction-bonded mullite, J. Am. Ceram. Soc. 74 (1991) 2460.

10] J. Luyten, J. Cooymans, R. Lrysen, Shaping of a RBAO-membsupport, Key Eng. Mater. 132 (1997) 1691.

11] D. Suxing Wu, N. Holz, Claussen, Mechanisms and kinetics oaction bonded aluminum oxide ceramics, J. Am. Ceram. So(1993) 970.

12] M.A. Anderson, M.J. Gieselmann, Q.Y. Xu, Titania and alumceramic membranes, J. Mem. Sci. 39 (1988) 243.