Cyclic CVD Modification of Straight Pore Alumina Membranes

Alumina oxide was deposited in the straight nanosize pores of Anopore alumina membrane by the alternative introduction of trimethylaluminum (TMA) and ...
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Langmuir 2003, 19, 7307-7314

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Cyclic CVD Modification of Straight Pore Alumina Membranes Hatem M. Alsyouri,† C. Langheinrich,† Y. S. Lin,*,† Zhibin Ye,‡ and Shiping Zhu‡ Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221, and Department of Chemical Engineering, McMaster University, Hamilton, Canada L8S 4L7 Received January 13, 2003. In Final Form: June 23, 2003 Alumina oxide was deposited in the straight nanosize pores of Anopore alumina membrane by the alternative introduction of trimethylaluminum (TMA) and water vapor to the membrane pores. The chemical vapor deposition (CVD) modification was conducted under two different schemes: (1) cyclic CVD with residual pressure and (2) cyclic CVD with purge. The membranes were characterized by single gas helium permeance and water vapor/oxygen multiple gas permeation. Helium gas permeation results show that the modification by the residual pressure scheme caused greater reduction in the pore size than porosity, suggesting the deposition of alumina in a fractal structure in the Anopore membrane pores. Such a structure is caused by combined homogeneous and heterogeneous reactions taking place respectively on the pore wall and randomly in the pores. The membranes modified with CVD under the purge scheme exhibited greater reduction in porosity than the pore size, indicating that the alumina in the latter case was deposited on the pore wall in an atomic layer fashion. The structure in the latter case is caused by homogeneous reactions taking place on the pore wall. Membranes modified under the purge scheme exhibited improved water vapor/oxygen separation properties.

1. Introduction Nanoporous inorganic membranes with pore sizes in the range of about 2-6 nm were studied extensively in the past decade.1,2 These membranes include alumina, titania, and zirconia3 and their binary oxide4 membranes. Most of the membranes have a microstructure derived from compacting the ceramic particles. The membrane pores, defined by the interparticle spaces, are interconnected and highly tortuous. Nanoporous inorganic membranes with hierarchical architectures have attracted the attention of researchers with respect to their structure and applications.5 Among several ordered mesoporous structures, the hexagonal structure, of MCM-41 type, has gained a particular interest for preparation in the form of thin film membranes. This structure has straight, uniform, and hexagonally packed pores with sizes ranging between 2 and 10 nm. Preparation of hexagonally ordered membranes with pores oriented vertical to the membrane surface represents a major challenge for membrane scientists. Such architecture can be of particular interest for potential applications in separation, catalysis, optical devices, and nanomembrane reactors for production of materials with properties unattainable by traditional technologies. Much work has been devoted to preparation of hexagonally ordered nanoporous thin films of different materials, most notably silica, on various types of supports utilizing colloidal sols containing the surfactants and silica source.5 Films were supported using different techniques * To whom correspondence should be addressed. Telephone: (513) 556-2769. Fax: (513) 556-3473. E-mail: [email protected]. † University of Cincinnati. ‡ McMaster University. (1) Burggraaf, A. J.; Cot, L. Fundamentals of Inorganic Membrane Science and Technology; Elsevier: Amsterdam, 1996; Chapter 7. (2) Hseis, H. P. Inorganic Membranes for Separation and Reactions; Elsevier: Amsterdam, 1996; Chapters 8-10. (3) Chang, C. H.; Gopalan, R.; Lin, Y. S. J. Membr. Sci. 1994, 91, 27. (4) Tsuru, T.; Wada, S.; Izumi, S.; Asaeda, M. J. Membr. Sci. 1998, 149, 127. (5) Brinker, C. J. Curr. Opin. Colloid Interface Sci. 1998, 3, 166.

including spin and dip coating6-10 and growth from solution directly on the support.11,12 In all cases the hexagonal structure was achieved but the pores were aligned parallel to the support surface. Other trials applied an external field such as continuous shear flow13 to affect the pore alignment. The applied flow oriented domains of the meso structure and resulted in pores aligned in the direction of the applied flow. Moreover, these methods cause local alignment of the pores and not alignment over the whole support. They require special apparatus and restricted support shapes and, above all, cannot align the pores vertical to the support surface. Since it is difficult to achieve vertical pore alignment by coating the surfactant-precursor colloidal sol on the support, it is important to find a simple and reliable approach to get the desired structure. Here we report the modification of straight pore alumina membranes as an alternative approach. Anopore membranes, also called Anodisc, are alumina films with well-defined cylindrical, straight, and hexagonally packed pores running in the direction normal to the membrane surface.13,14 They are made by electrochemical anodic oxidation of aluminum and are available in 60 µm thickness.14 Anopore membranes with the smallest pore size available commercially have a pore diameter of 20 nm. For proper performance (6) Lu, Y.; Ganguli, R.; Drewien, C.; Anderson, M.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M.; Zink, J. Nature 1997, 389, 364. (7) Martin, J.; Anderson, M.; Odinek, J.; Newcomer, P. Langmuir 1997, 13, 4133. (8) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941. (9) Ogawa, M.; Masukawa, N. Microporous Mesoporous Mater. 2000, 38, 35. (10) Nishiyama, N.; Park, D.; Koide, A.; Egashira, Y.; Ueyama, K. J. Membr. Sci. 2001, 182, 235. (11) Tolbert, S.; Schaffer, T.; Feng, J.; Hansma, P.; Stucky, G. D. Chem. Mater. 1997, 9, 1962. (12) Yang, H.; Kuperman, A.; Coombs, N.; Afara, S.; Ozin, G. Nature 1996, 379, 703. (13) Furneaux, R. C.; Rigby, W. R.; Davidson, A. P. Nature 1989, 337, 147. (14) Crawford, G. P.; Steele, L. M.; Ondris, R.; Iannacchione, G. S.; Yeager, C. J.; Doane, J. W.; Finotello, D. J. Chem. Phys. 1992, 96, 7788.

10.1021/la034056t CCC: $25.00 © 2003 American Chemical Society Published on Web 08/01/2003

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in membrane reactor applications, the pore size of Anopore membranes is required to be further narrowed to the low limit of mesoporous size (2-4 nm). The modified 2-4 nm pore Anopore membrane may also offer applications for gas/vapor separation. Several membrane modification techniques have been studied to reduce membrane pore size to enhance membrane application performance.15 Atomic layer chemical vapor deposition (ALCVD) is a useful method for the modification of porous inorganic membranes.16 Compared to conventional CVD,17 ALCVD can precisely tailor the membrane pores by the controlled growth of the oxide deposition layer inside the pores. For the ALCVD of alumina (Al2O3), the binary reaction 2Al(CH3)3 + 3H2O f Al2O3 + 6CH4 was separated into two half-reactions: AlOH* + Al(CH3)3 f Al-O-Al (CH3)2* + CH4 and AlCH3* + H2O f AlOH* + CH4, where asterisks indicate surface species. The reagents, Al(CH3)3 vapor and H2O vapor, are introduced separately in a stepwise manner. The thickness of the deposit layer can be precisely controlled by the number of subsequent additions of the reagents causing one atomic layer to be formed at a time. Ott et al.18 have used the ALCVD method to reduce the pore size of a 22 nm Anopore alumina membrane by the alternative introduction of trimethylaluminum (TMA) and water vapor using variable reaction cycles. The nitrogen permeance value for the unmodified Anopore consistently decreased from 6.7 × 10-4 mol/s‚m2‚Pa (at 1.5 atm feed pressure and differential pressure about 0.7 atm) to 1.0 × 10-4 mol/s‚m2‚Pa after many cycles of ALCVD. They reported that the pore size was progressively reduced from 22 to 14 nm after 120 reaction cycles, with Al2O3 reaction growth rate of 0.37 Å/cycle. This study, which was focused on the chemistry of ALCVD on Anopore membrane pores, shows that the ALCVD method can precisely control pore size. However, reducing the pore size of a membrane from about 20 nm to 3-4 nm by this ALCVD method would require several hundred CVD cycles. Our research group recently reported a modified ALCVD method to effectively narrow the pore size of a sol-gel derived γ-alumina membrane for condensable vapor separation purposes.19,20 In the modified ALCVD some residual reactant molecules were allowed to present in the gas phase between half-reaction cycles. This may have caused the deposition to take place both on the pore wall by heterogeneous reaction and in the pore gas phase by homogeneous reaction of residual reactants. It was proposed that such heterogeneous/homogeneous CVD might result in deposition of alumina of a fractal structure desirable for separation applications. In this work, we studied modification of a 20 nm straight pore Anopore membrane by the simple, modified ALCVD method using two schemes adopted from the homogeneous/ heterogeneous deposition mechanism proposed above. In one scheme some precursor residual will be allowed to present inside the pore, therefore allowing the deposition to take place on the pore wall as well as inside the pore gas phase in a fractal-like microstructure. The other scheme is based on minimizing the presence of any residual precursor from the pore gas phase. This would cause a (15) Lin, Y. S.; Kumakiri, I.; Nair, B. N.; Alsyouri, H. Sep. Purif. Methods 2002, 32 (2), 229. (16) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121. (17) Lin, Y. S.; Burggraaf, A. J. J. Membr. Sci. 1993, 79, 65. (18) Ott, A. W.; Klaus, J. W.; Johnson, J. M.; George, S. M. Chem. Mater. 1997, 9, 707. (19) Pan, M.; Cooper, C.; Lin, Y. S.; Meng, G. Y. J. Membr. Sci. 1999, 158, 235. (20) Cooper, C. A.; Lin, Y. S. J. Membr. Sci. 2002, 195, 35.

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heterogeneous deposition to take place only on the pore wall, giving rise to pores with a narrowed cylindrical microstructure.The cylindrical structure represents an alternative approach for obtaining a mesoporous ordered film with pores vertical to the support. The fractal structure would give a membrane with reduced pore size but appreciable porosity desirable for membrane separation application. Gas permeation and separation data were measured for the original and modified membranes and used to interpret the CVD modification mechanisms and the resulting deposit microstructure. The objective of the present work is to compare the CVD modification of the straight pore alumina membranes by these two schemes. 2. Experimental Section 2.1. Anopore Membrane Modification. The membranes used for modification were Anopore alumina membranes obtained commercially (Whatman Co., Maidstone, England). These membranes are of 60 µm thickness and have an asymmetric structure. The majority of the membrane is comprised of straight, cylindrical, and nonconnected pores of 200-250 nm diameter lying over 58 µm of the membrane thickness. The top layer of the membrane consists of 20 nm straight pores and has a thickness of 2 µm. A schematic of the Anopore membrane structure and SEM image of the composite pores, measured in our laboratory (HitachiS4000), are shown in Figure 1. The Anopore membrane has a diameter of 25 mm and is supported with a polypropylene ring heat glued to the outer edge of the membrane. Before conducting CVD, Anopore membranes were heat treated in oven at 480 °C to remove the polypropylene support ring. The CVD modifications were conducted in a simple hot-wall CVD reactor used for CVD of metallic/ceramic composite membranes.21 The modified CVD for the present work is schematically shown in Figure 2. The CVD system consisted of a reactor chamber made of quartz and a central tube made of dense R-Al2O3. The two sides of the CVD reactor were respectively connected to a vacuum pump and reactant containers. Prior to deposition, the Anopore membrane was carefully placed on one end of the dense alumina tube with the 20 nm pore side facing outward since we needed to modify the small pore side. The other end of the R-Al2O3 tube was connected to the vacuum pump. Liquid trimethylaluminum (TMA) (97% purity, Aldrich) in a stainless steel cylinder and distilled water in a Pyrex flask were used as precursors for the deposition of Al2O3 in the CVD process. These precursors and a He gas cylinder were connected directly into the CVD reaction chamber as shown in Figure 2. In this work, CVD modification of Anopore membranes was conducted under two different schemes: cyclic CVD with residual pressure and cyclic CVD with purge. The specific experimental conditions in this work were adapted from conditions used to narrow the pore size of mesoporous γ-alumina membranes by CVD modification conducted in our laboratory.19,20 In the first scheme (with residual pressure), the CVD system was evacuated to 1 mbar and slowly heated to the desired deposition temperature (180 °C). Then the system was isolated from the vacuum pump and water vapor was introduced into the reaction chamber. The pressure in the reaction chamber quickly reached about 32 mbar. After the Anopore membrane was exposed to water vapor for about 5 min, the system was evacuated to 1 mbar for at least 10 min. TMA vapor was introduced into the reaction chamber, whose pressure reached 27 mbar. After 5 min exposure to TMA, system was evacuated to 1 mbar again. This completed one cycle of CVD of alumina. The cycle was repeated to grow additional layers of alumina. In this scheme, evacuation of the system to 1 mbar after introduction of each precursor allowed the presence of the precursor molecules in an amount equivalent to 1 mbar in the membrane pores when the second precursor is introduced into the CVD system. The second scheme (with purge) followed the same procedure as the first scheme except for the use of a purge gas to lower or eliminate the presence of the precursor in the CVD system prior to introduction of the second precursor. The reaction chamber (21) Xomeritakis, G.; Lin, Y. S. AIChE J. 1998, 44, 174.

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Figure 1. Images of straight pore alumina membrane. (A) Schematic of the straight pore structure and dimensions; (B) SEM top view showing the top layer of 20 nm pore size; (C) and (D) SEM cross-sectional view showing the support side with 200 nm pore size.

Figure 2. Schematic illustration of the CVD system. was evacuated to 1 mbar for 10 min after the introduction of each precursor. Helium, as a diluting gas, was introduced to the chamber to 1500 mbar and was left for 2 min. Then the system was evacuated to 1 mbar for at least 10 min before introducing the second precursor. The diluting step was done after the introduction of each precursor to the reaction chamber. The main difference between the two schemes is the controlled removal of residual precursor between the half-reaction cycles. 2.2. Characterization. Helium and nitrogen permeance at different average pressures were measured by a steady-state single gas permeation system17 to examine the microstructure of the Anopore membranes before and after CVD modification. The permeation system is normally used for strong 2 cm thick ceramic membranes. As the Anopore membranes were thin and very fragile, extra care was taken during the permeation measurements in order to ensure the integrity of the membrane.

In experiments, a 60 µm thick Anopore membrane was carefully fixed in a stainless steel permeation cell, with a stack of two O-rings (Viton) put on each side of the membrane and the permeation cell tightened softly. Permeation data were measured at small transmembrane pressure (6.3-11.2 kPa) by a careful control of the gas flow rate (240-360 cm3/min) using a downstream needle valve. The separation of water vapor from air by the membrane was conducted using the apparatus shown in Figure 3. Unmodified or CVD-modified Anopore membranes were carefully fixed in a stainless steel permeation cell. A 100 cm3/min mixture of dry air and wet air were introduced to the upstream side of the sample. Dry nitrogen sweep gas from a cylinder was passed, at a flow rate of 75 cm3/min, in a cross-flow fashion over the downstream side of the sample. Different upstream humidity was obtained by changing the flow rate of dry air and wet air, with a total flow

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Figure 3. Water vapor separation system. rate kept about 100 cm3/min. The temperature was maintained at about 25 °C. The relative humidity and oxygen content of the streams were respectively detected by a humidity sensor (Thermohygrometer, Cole-Parmer 37950-10) and an oxygen sensor (6000 Oxygen Analyzer, Illinois Instruments). The permeance of each species was calculated from the permeation flux of that species divided by its transmembrane partial pressure. Then the water to oxygen separation factor was calculated from the ratio of water to oxygen molar fraction in the outlet of downstream to that in the outlet of the upstream. The microstructure of the CVD-modified Anopore membranes was also studied by environmental scanning electron microscopy (FEI XL-30 ESEM-FEG). Membrane samples were mounted on carbon double-sided adhesive tape with the 20 nm modified membrane surface facing upward. To improve specimen conductivity and reduce image shift, samples were painted with silver adhesive paste along the perimeter while retaining an uncoated part of the sample in the center for imaging in the gaseous mode. Images with magnifications up to 400K times were taken at 10 kV in the pressure range 2-20 Torr.

3. Results and Discussion 3.1. Single Gas Permeation Properties of CVDModified Anopore Membranes. CVD modification was conducted on the 20 nm pore side of the asymmetric Anopore membrane. The membranes were modified by different numbers of cycles (two and six) under the two different schemes as described in the Experimental Section. Unmodified Anopore samples are referred to as 0 time CVD-modified membranes and samples modified with two and six CVD cycles are referred to as 2 and 6 time CVD modified membranes. Samples with CVD modification under the residual pressure scheme are referred to as CVD residual, and those modified under the He purge scheme are referred to as CVD purge. Helium gas permeation results of unmodified Anopore membrane and 6 time modified under residual pressure and gas purge schemes are shown in Figure 4. The permeation data presented in Figure 4 exhibit a linear relationship. Unmodified Anopore membrane has a He permeance of about 1.6 × 10-4 mol/s‚m2‚Pa at 1 atm transmembrane average pressure. This value is almost 2 orders of magnitude higher than the He permeance values of 0.2 µm pore R-alumina and supported 4 nm pore γ-alumina membranes.17 This is attributed mainly to the low mass transfer resistance to He permeance exhibited by the thin (60 µm) asymmetric Anopore composite membrane and also to its ordered structure with low

Figure 4. Single gas He permeance of unmodified and 6 time CVD modified Anopore membrane under the two different schemes of CVD modification.

tortuosity factor. The resistances of the Anopore membrane to gas permeance due to the 59 µm of 250 nm and the 1 µm top layer of 20 nm diameter pores are respectively much less than those exhibited by the 2 mm thick 200 nm pore diameter of R-alumina and the 5 µm thick 4 nm pore diameter of the γ-alumina top layer. As presented in Figure 4, the permeance values for unmodified, 6 time CVD modified with residual pressure, and 6 time CVD modified with purge are respectively 1.92 × 10-4, 1.79 × 10-4, and 1.70 × 10-4 mol/s‚m2‚Pa at ∼2.5 atm average transmembrane pressure. The He gas permeance decreases as a result of modification under the two schemes of CVD modification. This is obviously due to reduction of pore size of the membrane by the deposition of solid alumina inside the pores. As shown in Figure 4, the CVD modification under the purge scheme resulted in a greater reduction of He permeance (11%) than that by the residual pressure scheme (7%). The same trend has been observed by the 2 time CVD modified samples. However, in the 2 time CVD modified membrane, the He permeance trend lines are closer to each other and the reduction in He permeance values after modification by the purge and residual pressure schemes are respectively 7% and 5% at 2.5 atm average pressure. By

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Table 1. Helium Permeation Properties for Anopore Membranes after CVD Modification residual pressure 2 CVD

with purge 6 CVD

2 CVD

6 CVD

before

after

before

after

before

after

before

after

1.54 1.45 9.40 248

1.53 1.21 7.88 208

1.55 1.45 9.35 246

1.54 1.17 7.59 200

1.64 1.43 8.73 230

1.49 1.21 8.14 215

1.62 1.25 7.76 205

1.47 0.97 6.59 174

R (10-4 mol/m2 s Pa) β (10-10 mol/m2 s Pa2) β/R (10-7 Pa-1) dp (nm)

comparing the reduction in permeance for both 2 and 6 time CVD modification under the same scheme, it can be generally concluded that the reduction in the pore size by the six CVD cycles is more than that by the two CVD cycles. The difference in reduction of permeance between the two CVD modification schemes for each sample reveals that the two schemes end up with deposits of different microstructure, as shown next with further analysis of the permeation data. Helium permeance data (F/L) at different average pressures (Pav) shown in Figure 4 can be regressed by a straight line (F/L) ) R + βPav. The coefficients R and β represent contributions from Knudsen and viscous flows, respectively, as17

rp

(τ)LxM RT

R ) 1.06

(1)

w

β ) 0.125

2  rp τ LηRT

()

(2)

where , τ, rp, and L are the porosity, tortuosity (about 1 for the straight pore Anopore membrane), average pore radius, and thickness of the membrane and η and Mw are the viscosity and molecular weight of the permeating gas. The change in R and β due to CVD modification can be theoretically used to examine the change in the average pore radius (rp) (from the ratio of β/R) and porosity () (from the value of R). It should be noted that R and β values obtained from the permeance data represent the average values of the whole Anopore membrane (thick large pore support and thin 20 nm pore top layer). For the unmodified Anopore membrane, the resistance of permeation in the top layer is around 3 times higher than that of the support assuming a viscous flow mechanism (resistance ∼ dp2/thickness). After CVD modification, which causes deposition in the top layer, the ratio of mass transfer resistance for the top layer increases due to reduction in pore size of the top layer. Therefore, the reduction in R and β after CVD modification is mainly due to the change in the pore structure of the membrane top layer. Due to the effects of the support, a small reduction in the average pore size or porosity for the whole membrane represents a much larger reduction in the pore size and porosity in the top layer of the Anopore membrane. The presence of pinholes in the tested membrane can be easily checked from the value of the slope (β) and the calculated average pore size. Pinholes are normally macroporous and have gas permeance governed by viscous flow mechanism. Therefore, the presence of pinholes would increase the slope value (β) and normally end up with an average pore size larger than 1 µm. In this work, the absence of pinholes in the tested unmodified and modified membranes was ensured by the trend line of gas permeance and the obtained average pore size values, which were all less than 250 nm. The permeance of the unmodified sample shown in Figure 4 was regressed by the straight line (F/L) ) R +

Table 2. Percent Reduction in Average Microstructural Values due to CVD Modification residual pressure

 dp

with purge

2 CVD

6 CVD

2 CVD

6 CVD

0.62 16

0.98 19

9.23 7

8.51 15

βPav, where R ) 1.62 × 10-4 mol/s‚m2‚Pa and β ) 1.25 × 10-10 mol/s‚m2‚Pa2. From the ratio of β/R ) 7.76 × 10-7 Pa-1, the average pore size of the unmodified membrane was estimated to be 205 nm. Table 1 summarizes R, β, (β/R), and average pore size (dp) of the Anopore membranes before and after modification under the two schemes. As shown in Table 1, the average pore sizes of the starting unmodified membranes obtained from the (β/R) ratio are slightly different. For membranes modified under the same scheme the increase in the number of CVD cycles causes more reduction in the (β/R) and hence more reduction in the average pore size (dp). This demonstrates that the cyclic CVD is narrowing the pores in the membrane 20 nm top layer by deposition of solid alumina oxide inside the pores. The percentage reductions in the porosity and average pore size by cyclic CVD modification under the two schemes are listed in Table 2. For 2 time CVD modified membranes under the residual pressure scheme, the percentage reduction in porosity (0.62%) is much smaller than reduction in the average pore size (16%). This indicates that the presence of some residual pressure between the successive additions of precursor causes high reduction in the average pore size of the membrane top layer while almost maintaining its porosity. A higher number of CVD cycles under the residual scheme causes the same trend but with slightly more reduction in porosity (0.98%) and further more reduction in the average pore size (19%). Modification under the purge scheme, on the other hand, resulted in a different microstructure. As shown in Table 2, two cycles of CVD with purge resulted in higher reduction in the porosity (9.23%) compared to the residual scheme (0.62%) as well as less reduction in the average pore size (7%) compared to that under the residual scheme (16%). Similarly, six cycles of CVD cause additional reduction in the pore size (15%) and a porosity value (8.5%) that are respectively lower and higher than the corresponding values obtained under the residual pressure scheme. This reveals that purging the pore opening after each addition of precursors using helium allows the alumina to deposit in such a way that reduces the porosity more effectively than the pore size. 3.2. Multiple Gas Permeance and Separation Properties of CVD-Modified Anopore Membranes. The data of permeation and separation of water vapor from oxygen (or air) were measured for two purposes. First, these data are needed for selection of industrially important inorganic membranes with high permeance and good selectivity for removal of water from air.19 For example, inorganic membranes with controlled pore microstructure could provide unprecedented water vapor separation properties with high selectivity at significantly

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Figure 5. Permeance of oxygen and water vapor through unmodified Anopore and γ-alumina membranes.

lower cost compared to conventional energy-intensive technologies.19 Second, these data resemble those from a permporosimeter useful for characterization of the pore structure of the top layer of composite membranes.20 Figure 5 shows the oxygen and water permeance values for the unmodified Anopore membrane and the well-known sol-gel derived 4 nm pore γ-alumina membrane prepared in a previous work.19 For the γ-alumina membrane the water vapor permeance decreases slightly with increasing relative humidity (RH). Oxygen permeance, on the other hand, remains almost constant at 3 × 10-7 mol/s‚m2‚Pa with RH values less than 60%, and then it decreases sharply to 0.7 × 10-7 mol/s‚m2‚Pa at 90% RH. This sharp decrease in oxygen permeance is typically due to capillary condensation of water vapor inside the mesoporous 4 nm pores of γ-alumina.20,22 For a mixture of condensable vapor and noncondensable gas permeating inside mesopores, vapor may condense inside the mesopores at pressures lower than the saturated vapor pressure. As the RH of condensable vapor increases, its condensate starts to block the mesopore and hence reduces the flow of the noncondensable gas. Here water vapor was used as the condensable vapor and oxygen (in the air) as the noncondensable gas. Utilizing the Kelvin equation, the gradual decrease of oxygen permeance from 60 to 90% RH indicates the presence of a distribution in the pore size of γ-alumina membrane ranging between almost 2 and 5 nm. The oxygen and water vapor permeances for unmodified Anopore membrane are respectively 7 and 4 times higher than the corresponding values for the γ-alumina membrane. The water permeance value decreases from 6.9 × 10-6 to 2 × 10-6 mol/s‚m2‚Pa from 10 to 90% RH. Unlike the case of γ-alumina, oxygen permeance for the Anopore remained constant at almost 1.7 × 10-6 mol/s‚m2‚Pa over the studied range of RH. This obviously is due to the large pores (20 nm) of the Anopore membranes where the amount of water vapor even at 90% RH was not sufficient to block the pores for the passage of oxygen. This indicates that capillary condensation using water vapor is effective for pores in the low range of mesoporous size (2-4 nm). The oxygen permeance value obtained here (1.7 × 10-6 mol/s‚m2‚Pa) is 30 times smaller than the pure oxygen permeance (56 × 10-6 mol/s‚m2‚Pa) estimated from the (22) Ulhorn, R. J. R.; Keizer, K.; Burggraaf, A. J. J. Membr. Sci. 1992, 66, 259.

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Figure 6. Oxygen and water permeance for Anopore membranes modified under the residual pressure scheme.

pure He permeance values for the unmodified Anopore membrane given in Table 1 using the Knudsen permeation relation (eq 1). Helium permeance was obtained through single gas permeation, while oxygen permeance was measured under multiple gas permeation. The driving forces for the single gas (total pressure difference) and multiple gas (partial pressure difference) permeation were almost equal (0.11 atm). Therefore, the driving force has no effect on the low value of oxygen permeance. The presence of other permeating components, such as water vapor, is expected to interact with oxygen and thus lowers its permeance. Moreover, the presence of back flow of N2 from the N2 sweep gas flow at the membrane downstream is also expected to lower the oxygen permeance.23,24 The binary oxygen and water permeance values for the Anopore modified by two and six CVD cycles under the residual pressure scheme are presented in Figure 6. Water permeance for both samples decreased slightly due to the modification, which means that some water is being adsorbed on the pore walls. Oxygen permeance for each modified membrane was constant [(1.5-1.6) × 10-6 mol/ s‚m2‚Pa] over the studied range of RH. The percentage reduction in this value for the two and six CVD cycles were respectively 8 and 10% compared to the unmodified oxygen permeance. These observations reveal that the pore size reduction and pore structure properties for CVD modification under residual pressure are not favorable for capillary condensation of water. For that reason the permeance curves for the unmodified and modified membranes almost coincide. A possible pore structure for which such observations can be obtained is a pore channel with high porosity. Figure 7 shows the oxygen and water permeance for the Anopore membranes modified by two and six cycles of CVD under the He purge scheme. Modification by two CVD cycles with purge slightly reduced the oxygen and water permeance values compared to the unmodified membrane. On the contrary, the 6 time CVD modified membrane exhibits a significant improvement in the water/oxygen separation properties. Modification by six cycles under purge reduced the oxygen permeance by 40% and increased the water permeance by at least 7 times. This means that the final pore structure after six CVD (23) Burggraaf, A. J.; Vroon, Z. A. E. P.; Keizer, K.; Verweij, H. J. Membr. Sci. 1998, 144, 77. (24) van de Graaf, J. M.; Kapteijn, F.; Moulijn, J. A. J. Membr. Sci. 1998, 144, 87.

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Figure 7. Oxygen and water permeance for Anopore membranes modified under the purge scheme.

Figure 8. Water/oxygen separation factor for unmodified and CVD-modified Anopore membranes under the two schemes.

cycles under purge is favorable to water permeance. A possible explanation is that the pore size has been narrowed such that the condensation of water vapor is enhanced but not to the limit of blocking the pores. This is confirmed by the increase in the water permeance and the decrease of oxygen permeance to a lower constant value without being dropped sharply at high RH. These observations reveal that the structure of the modified pores of the 20 nm pore Anopore membrane has a cylindrical shape with low porosity in which capillary condensation is favorable. Separation factor of water/oxygen for the unmodified and modified samples under both schemes are shown in Figure 8. The unmodified Anopore membrane has a separation factor of 2-4 that is constant over the studied range of RH. Modification by two and six CVD cycles under the residual pressure had no effect on the separation factor of the membrane. As mentioned previously, this may be attributed to the final highly porous structure, which is not favorable for condensation of water vapor. Another explanation is that the CVD modification under the residual pressure did not take place uniformly on the whole membrane surface. There may still some large pores (20 nm) that give separation properties close to those of the unmodified membrane. Two time CVD modification under the purge scheme gives a separation factor in the vicinity

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Figure 9. Schematic representation of radial and sectional axial views of different pore microstructures obtained under CVD modification: (A) Fractal microstructure obtained via residual pressure scheme; (B) atomic layer deposition cylindrical microstructure obtained via purge scheme.

of that for the unmodified sample. The separation factor for the six CVD cycles under the purge scheme was 20-25 in the range of 20-70% RH, which is almost 1-fold higher than that for the unmodified samples. The separation factor for six CVD modified sample is about 5 times higher than that for the 4 nm γ-alumina membrane in the same range of RH.20 3.3. Pore Structure and Mechanisms of CVD Modification. The above permeation and separation data have indicated two different microstructures of Anopore membrane modified by CVD with the residual pressure and purge schemes. The residual pressure scheme is more likely to narrow the pore diameter of the membrane top layer while maintaining its high porosity. This suggests that alumina was deposited inside the pores of the modified layer in a cluster-like or fractal structure. For such a structure, the porosity is high and the average pore size measured by single gas permeation is reduced due to random deposition of alumina inside the pore. Modification under the purge scheme exhibited a greater reduction in the porosity than the pore size. This can be explained by a cylindrical pore narrowed by deposition of alumina on the pore wall in an atomic layer fashion. Schematic representation of the final pore microstructures obtained via the residual pressure and purge schemes are shown in Figure 9. The fractal structure obtained by the residual pressure scheme would be caused by combined homogeneous and heterogeneous reactions. Allowing a residual pressure (1 mbar) of precursor to exist in the gas phase of the pore after each addition of precursors causes the precursors to exist both on the pore wall and in the pore gas phase. This would cause the deposition of alumina on the pore wall due to heterogeneous solid/gas phase reactions and randomly inside the pore volume due to homogeneous vapor phase reactions. This microstructure is consistent with CVD modification on γ-alumina membranes under the same conditions.20 The fractal structure is desired in separation applications such as separation of gas molecules based on size and shape selectivities. Purging the CVD reactor after each addition of precursors causes the removal of excess precursors from the gas phase in the pores. Therefore, only heterogeneous reactions take place on the pore wall, which cause narrowing of the cylindrically shaped pores by deposition of alumina oxide in an atomic layer fashion. The cyclic CVD modi-

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Figure 10. ESEM images of Anopore modified by two CVD cycles under (a) residual pressure scheme (left) and (b) purge scheme (right) at 400K magnification.

fication under purge gas represents a simple and costeffective alternative for modification of pore size in a fashion similar to atomic layer CVD. This simple CVD method does not require very low vacuum pressure, thus greatly simplifying the CVD system. Modification of Anopore membrane by the purge scheme has resulted in a narrowing of the 20 nm cylindrically shaped pores. Such a structure represents an alternative for obtaining a supported hexagonally ordered mesoporous film with pores vertical to the support that has been unattainable up to this moment. The narrowed cylindrical pore structure obtained by the purge scheme is desirable and may find new applications in membrane reactor applications as molecular extruders. Cylindrical pore structure was more effective in separation of water vapor from oxygen compared to the fractal structure, which is more desirable for separation of gas mixtures rather than vapor/gas mixtures. This is because the separation of water vapor is based on capillary condensation, which is favored in cylindrical pores such as those obtained by the purge scheme. It should be noted that direct verification of the pore structures of the Anopore membrane top layers discussed above is very difficult, if not impossible. Nitrogen or mercury porosimetry is not suitable for characterization of the top layer pore structure of the Anopore membranes for the following reasons. Anopore membranes have a low surface area (