Ultrathin, hydrogen-selective silica membranes deposited on alumina-graded structures prepared from size-controlled boehmite sols

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Journal of Membrane Science 306 (2007) 216–227

Ultrathin, hydrogen-selective silica membranes deposited on alumina-graded structures prepared from size-controlled boehmite sols Yunfeng Gu 1 , S. Ted Oyama ∗ Environmental Catalysis and Nanomaterials Lab, Department of Chemical Engineering, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, United States Received 8 June 2007; received in revised form 30 July 2007; accepted 28 August 2007 Available online 2 September 2007

Abstract A novel membrane is described consisting of an ultrathin 20–30 nm permselective layer of silica deposited on an intermediate multilayer ␥alumina substrate with a graded structure. The alumina substrate is formed on top of a macroporous alumina support by sequentially dipping and calcining a series of dilute solutions containing boehmite (AlOOH) sols of different particle sizes. The size of the sol particles is tuned by precisely controlling synthesis parameters including acid type, acid concentration and hydrolysis time. The topmost silica layer is deposited on top of the intermediate alumina layer by chemical vapor deposition of a silica precursor, tetraethylorthosilicate, in an inert atmosphere. Cross-sectional images of the membranes obtained from scanning electron microscopy (SEM) show that the intermediate alumina layers are formed from particles of progressively smaller size so as to form a smooth interface between the rough support and the topmost amorphous silica layer. The resulting silica-on-alumina composite membrane had high permeance of 5.0 × 10−7 mol m−2 s−1 Pa−1 and good selectivities for hydrogen over CH4 , CO and CO2 of over 1500 at 873 K. © 2007 Elsevier B.V. All rights reserved. Keywords: Boehmite sol; Alumina membrane; Multilayer; Sol–gel; Graded structure; CVD; Hydrogen

1. Introduction In this paper we describe a novel composite inorganic membrane for hydrogen separation consisting of an ultrathin permselective silica layer of thickness 20–30 nm deposited on the outer surface of an intermediate ␥-alumina substrate with a graded structure. This substrate provides an intermediate transition region between the silica layer and a coarse macroporous ␣-alumina support. The graded intermediate layer is obtained using dilute boehmite sols with different particle sizes. The concentration of the boehmite sols used in this work was 0.15 M, which was 4–6 times lower than those used previously. This results in an intermediate layer of around 1 ␮m (1000 nm) thickness, which is 5–10 times thinner than previously obtained. The resulting substrate is substantially defect-free and allows the deposition of a thin hydrogen-selective silica layer by CVD, giv∗

Corresponding author. Tel.: +1 540 231 5309; fax: +1 540 231 5022. E-mail address: [email protected] (S. Ted Oyama). 1 Current address: Corning Inc., Science and Technology Division, SP-FR-03, Corning, NY 14831, United States. 0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.08.045

ing rise to composite silica/alumina membranes with superior permeation properties. The membrane is unique in displaying unusual selectivity for small gaseous species in the order He > H2 > Ne, which does not follow mass or molecular size, and which is inconsistent with a porous material. The unusual order can be explained by a theory which involves the jumping of the permeating molecules between adjacent solubility sites [1]. Silica-based membranes prepared by chemical vapor deposition (CVD) or sol–gel methods on mesoporous supports have been shown to be effective for selective hydrogen permeation [2–12]. In the first membranes Okubo and Inoue [2,3] and the group of Gavalas deposited silica within the pores of Vycor glass with 4 nm mean pore diameter by tetraethylorthosilicate (TEOS) hydrolysis [2,3], SiH4 oxidation [4], or SiCl4 hydrolysis [5–7]. Similarly Wu et al. [8] modified the pore structure of tubular ␥-alumina membranes by CVD of TEOS using O2 as a co-reagent. Our group also carried out CVD of TEOS on both porous Vycor glass and ␥-alumina supports with pore diameters of 4 nm [9,10] and obtained thin silica membranes with very high selectivities for H2 (>20,000). Our preparation conditions (high temperature, inert gas) resulted in the deposition of the

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silica on the exterior surfaces of the supports, and this membrane was denoted Nanosil to distinguish it from previous materials. The initial membranes which used mesoporous supports were found difficult to improve with macroporous supports of pore sizes substantially larger than 50 nm. Hwang et al. [13] attempted CVD of TEOS on a porous alumina tube with pore size of 100 nm and obtained only a selectivity of 5.2 for the separation of H2 from N2 at 873 K after 32 h of deposition. Such low selectivities are indicative of the presence of large pore defects. A solution to the problem of using macroporous supports is the deposition of an intermediate layer before addition of the silica component. Considerable work has been done with ␥-alumina intermediate layers [14–19]. The group of Morooka covered macroporous ␣-alumina tubes with 110–180 nm pore diameter with three identical ␥-alumina layers with pore diameters of 6–9 nm to obtain a composite support [15,16]. Addition of a silica layer by CVD gave a membrane with a selectivity of H2 to N2 of 100–1000 and a H2 permeance of 10−8 to 10−7 mol m−2 s−1 Pa−1 at 873 K. The addition of an intermediate ␥-alumina layer was also used by de Lange et al. [17,18], de Vos and Verweij [19] on macroporous alumina discs before the placement of a selective layer of silica by a sol–gel method. The alumina layers were deposited on the surface of the macroporous support (pore size 160 nm) using concentrated dipping solutions with boehmite sol concentrations of 0.5–1.2 M. Sometimes polymeric binders such as polyvinyl alcohol (PVA) were used with the sols [20]. The thickness of a ␥-alumina supported layer, made with a 0.6 M dipping solution containing PVA, was typically 5–6 ␮m after three consecutive dipping steps [20]. A thick layer was needed to eliminate defects. The subject of microporous silica membranes for hydrogen purification has been recently reviewed [21]. Barbieri and coworkers [22] have prepared a silica membrane supported on porous stainless steel with three coatings of a boehmite sol to give an intermediate ␥-Al2 O3 layer of thickness 5 ␮m. The membrane had a permeance of H2 at 564 K of 4.3 × 10−9 mol m−2 s−1 Pa−1 and a selectivity of H2 /N2 of 22. The ␥-Al2 O3 layer likely reduced permeance, but did not allow the formation of a defect-free silica layer. Kanezashi and Asaeda [23] report the preparation of a hydrogen-selective membrane consisting of a Ni-doped silica sol deposited on an intermediate layer of ␣-Al2 O3 formed from particles of size 0.2–1.9 ␮m placed on a porous ␣-Al2 O3 of average pore size 1 ␮m. The intermediate layer is about 2.5 ␮m thick and

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is rough, requiring a Ni–SiO2 separation layer 0.3 ␮m thick. The H2 permeance at 773 K was 4.5 × 10−7 mol m−2 s−1 Pa−1 with a selectivity of H2 /N2 of 100. Araki et al. [24] used high-pressure CVD to deposit silica inside the pores and as a separate 100 nm layer on a ␥-Al2 O3 intermediate layer 3.5–4.0 ␮m thick based on SEM images deposited on an ␣-Al2 O3 support of average pore size 0.15 ␮m. The permeance at 573 K was 5.0 × 10−8 mol m−2 s−1 Pa−1 with a selectivity of H2 /CO2 of 800. Zivkovic et al. have prepared a composite silica on alumina membrane by sol–gel techniques incorporating a thin (20 nm) intermediate surfactant-templated silica (STS) layer over which a sol–gel silica layer (<50 nm) was placed [25]. The STS layer had larger pore size (1 nm) than the toplayer of silica (0.3–0.5 nm), and therefore, this constitutes a graded membrane, but the particle size of the STS was uncontrolled and too small, and therefore penetrated (70 nm) into the ␥Al2 O3 support. The H2 permeance of the membrane at 473 K was high 1.1 × 10−6 mol m−2 s−1 Pa, but the H2 /CH4 selectivity was low, ∼15, indicating that defects were present. Recently the group of Nakao and coworkers [26] reported a hybrid processing method for a high-permeance silica membrane consisting of depositing a silica layer by reactive CVD on top of an ␣Al2 O3 support with pore size reduced from 100 nm to the 5 nm range by using a sol–gel deposited ␥-Al2 O3 layer. The thickness of the layers is not reported. The permeance at 873 K was 6.4 × 10−7 mol m−2 s−1 Pa−1 with a selectivity of H2 /N2 of 2300, confirming that good results can be obtained when the pore size of the substrate is reduced. A summary of these results is provided in Table 1. Current commercial products utilize intermediate layers with a graded structure, but these have thickness of 10–50 ␮m [27], and the method of fabrication is not revealed for proprietary reasons [27]. Thus, although the use of an intermediate layer has been used in the past, work with graded structures has not been reported in detail. In this work we have prepared an intermediate substrate with graded layers of thickness ∼1 ␮m which gives rise to a membrane with superior permeability and selectivity properties. This work required preparation of boehmite sols of controlled particles sizes. Unlike SiO2 , TiO2 and ZrO2 sols, however, boehmite sols still remain poorly understood [28]. The published data on the relationship between the particle size of boehmite sols and synthesis parameters are very limited and contradictory to some extent, even when the same measuring techniques

Table 1 Summary of recently reported hydrogen-selective silica membranes System

Permeance mol m−2 s−1 Pa−1

Selectivity

Active and intermediate layer width

Comments

Ref.

SiO2 /␥-Al2 O3 /␣-Al2 O3 Oyama, Gu

5 × 10−7 (873 K)

5900 (H2 /CH4 )

20–30 nm, 1 ␮m

This work

SiO2 /␥-Al2 O3 /SS Barbieri Ni–SiO2 /␣-Al2 O3 /␣-Al2 O3 Asaeda SiO2 /␥-Al2 O3 /␣-Al2 O3 Araki SiO2 /␥-Al2 O3 /␣-Al2 O3 Zivkovic SiO2 /␥-Al2 O3 Nakao

4.3 × 10−9 4.5 × 10−7 5.0 × 10−8 1.1 × 10−6 6.4 × 10−7

22 (H2 /N2 ) 100 (H2 /N2 ) 800 (H2 /CO2 ) 15 (H2 /CH4 ) 2300 (H2 /N2 )

1 ␮m, 5 ␮m 0.3 ␮m, 2.5 ␮m 100 nm, 3.5–4.0 ␮m <50 nm, 1.3 ␮m Unreported

Graded intermediate layer with size selected boehmite sol precursors Ungraded intermediate layer Ungraded intermediate layer Ungraded intermediate layer Graded two-layer Ungraded intermediate layer

(564 K) (773 K) (573 K) (473 K)

22 23 24 25 26

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are used. For example, using the same aluminum precursor and peptization agent for synthesis, and the same quasi-elastic light scattering technique for analysis, Larbot et al. [29] reported that the particle size of a boehmite sol increased from 100 to 250 nm when the pH value was raised from 3.4 to 4.2, while Xia et al. [30] found that the sol particle size decreased considerably from 470 to 140 nm and then slightly increased to 230 nm when the pH was increased from 3.22 to 4.42. Lijenga et al. [31] obtained a boehmite sol with particle size in the range of 40–60 nm at a pH between 4 and 5. Zakharchenya [32] reported that the average particle sizes of boehmite sols were always in the interval of 20 ± 15 nm even when different aluminum precursors, peptization acids and molar ratios of H+ /alkoxide in the range of 0.01–0.3 were employed. Yoldas [33] prepared aluminum sols by the hydrolysis of aluminum alkoxide precursors in various acid solutions and found that, using HCl, particle size increased from 5 to 35 nm to 10–100 nm with increasing acid concentration. As will be seen, the basis for the thin, graded membranes reported here, size control of the sols, required the development of reliable methods of preparation of these sols. 2. Experimental Boehmite (AlOOH) sols with different particle size were prepared by careful control of the hydrolysis of aluminum alkoxides and the subsequent acid peptization of the boehmite precipitate obtained. The boehmite sols were prepared using a sequence of hydrolysis and peptization steps. A quantity of 0.2 mol of aluminum isopropoxide (Aldrich, 98+%) was added to 300 ml of distilled water at room temperature. The mixture was quickly heated to 353 K within 0.5 h with high-speed stirring and was maintained at this temperature for various times (0.5–72 h) for the hydrolysis of the isopropoxide and the formation of a boehmite precipitate. The precipitate was then heated to 365 K and was peptized using a quantity of acetic acid (GR, 99.7%), nitric acid (VWR, 68.0–70.0%) or hydrochloric acid (GR, 36.5–38.0%) with a molar ratio of H+ /alkoxide in the range of 0.03–0.25. Peptization refers to acid treatment to reform large oxide precipitates into colloidal sized particles by hydrolysis and condensation reactions. The solution was refluxed at 365 K for 20 h to get a clear or slightly translucent sol. The concentration of the resulting boehmite sols was calculated from the volume of the liquid and the known quantity of isopropoxide used. A dynamic light scattering analyzer (Horiba Model LB-500) was used to measure the particle size of the boehmite sols. The analyzer was calibrated using a standard polystyrene latex microsphere solution with mean diameter of 102 ± 3 nm (Duke Scientific Co.), and a value of 1.65 was used as the refractive index of boehmite for the internal calculation of particle size using a Fourier transform procedure. In this work, four boehmite sols with median particle sizes of 40, 55, 200 and 630 nm were used to prepare the ␥-alumina multilayer substrate. The boehmite sols are denoted as BS40, BS55, BS200 and BS630, respectively. Multilayer substrates of ␥-alumina were prepared on a macroporous ceramic support by a dipping–calcining method. A commercial porous alumina tube (PALL Corpo-

Fig. 1. Schematic of motor-driven dip-coating apparatus.

ration, Membralox® TI-70-25Z Membrane Tube, i.d. = 7 mm, o.d. = 10 mm) with a nominal pore size of 100 nm was used as the support. The manufacture has not published details of this membrane, but it appears to be a three-layer asymmetric material. The preparation involved several steps. First, the alumina tube was cut to a length of 3–4 cm with a diamond saw and was connected to non-porous alumina tubes at both ends with ceramic joints. The ceramic joints were made with a glaze (Duncan IN 1001) fired at 1153 K for 0.5 h. Second, dilute dipping solutions were prepared by mixing the boehmite sols with a polyvinyl alcohol (PVA, M.W. = 72,000) solution and diluting with distilled water to obtain a 0.15 M concentration of the sol and a 0.35 wt.% concentration of the PVA. Four dipping solutions, DS40, DS55, DS200 and DS630, were prepared from the sols, BS40, BS55, BS200 and BS630, respectively. Third, the alumina support was dipped into the dipping solution and was withdrawn after 10 s at a rate of 0.01 m s−1 using a motor-driven dip-coating apparatus. The tubular alumina support was wrapped in polytetrafluoroethylene (PTFE) sealing wrap and the coating was applied on the inside. The apparatus was built in-house and used a stepping motor drive (Fig. 1). Fourth, the dip-coated alumina was dried in ambient air for 24 h, and then was heated to 923 K in air at a rate of 1 K min−1 and calcined at 923 K for 2 h. The dipping–calcining process was repeated 4–5 times using the same or different dipping solutions. Two ␥-alumina multilayer substrates with an ungraded structure denoted as UGA-55 and UGA-630 were obtained by dipping–calcining four times using the same dipping solution DS55 or DS630, respectively. The other two ␥-alumina multilayer substrates with a graded three-layer structure denoted as GA4 and GA5 were prepared by dipping–calcining four and five times using the dipping solutions

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Fig. 2. Schematic of CVD apparatus for use in the deposition of the silica layer.

in the order of DS630-DS630-DS200-DS40 and DS630-DS630DS200-DS40-DS40, respectively. A silica/alumina composite membrane was prepared using the as-prepared ␥-alumina multilayer substrate as the support for the deposition of a thin silica layer by a chemical vapor deposition (CVD) method [34]. This process places a silica layer on the substrate by the thermal decomposition of tetraethylorthosilicate (TEOS) at high temperature. The setup is shown in Fig. 2, and the CVD process parameters are listed in Table 2. Briefly, the CVD procedure was carried out in a concentric tubular apparatus with an outer quartz tube and an inner alumina support consisting of the ␥-alumina multilayer. After heating the apparatus to 873 K at a rate of 0.017 K s−1 , an argon gas flow (17.1 ␮mol s−1 ) was introduced on the outer shell side and a dilute argon gas flow (13.4 ␮mol s−1 ) on the inner tube side. After 0.5 h a carrier gas flow (3.7 ␮mol s−1 ) was passed through a bubbler filled with TEOS at 296 K and was premixed with the dilute Ar flow before introduction to the inside of the support. The TEOS concentration was 0.019 mol m−3 , and the deposition time was varied from 3 to 6 h. The gas permeation measurements on H2 , CH4 , CO and CO2 were conducted at 873 K before and after CVD by admitting the pure gases at a pressure close to 2 atm (p = 1 atm) into the inner tube side, one end of which was closed, and measuring the quantity of gas flowing from the outer tube with a bubble flow meter at atmospheric pressure. For low permeances a sweep gas at a known flow rate was used and the composition of the permeate stream was determined with a gas chromatograph (GC). Table 2 CVD process parameters for the preparation of silica/alumina composite membranes Carrier gas flow rate (␮mol s−1 )a Dilute gas flow rate (␮mol s−1 ) Balance gas flow rate (␮mol s−1 ) TEOS concentration (mol m−3 ) CVD temperature (K)

3.7 13.4 17.1 0.0193 873

a Flow rates in ␮mol s−1 can be converted to cm3 (NTP) min−1 by multiplication by 1.5.

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The selectivity was calculated as the ratio of the permeances of H2 to CH4 , N2 , CO or CO2 . Hydrothermal stability tests were carried out at 873 K up to 157 h under an Ar flow containing 75 mol% water vapor. First, an Ar flow at 15 ␮mol s−1 (23 cm3 (NTP) min−1 ) was passed through a heated bubbler containing distilled water and was then introduced on the inner membrane tube side to directly contact the fresh composite membranes, while another Ar flow also at 15 ␮mol s−1 was maintained on the outer shell side. The Ar flow through the bubbler with a temperature of 365 K gave an Ar stream with a partial pressure of water vapor of 76.3 kPa (75 mol%). The H2 and CH4 permeation rates were measured periodically during the hydrothermal stability test to monitor the changes in the permeance and selectivity. To make the measurements the water vapor was shut off for about 20 min to dry the membranes under a dry Ar flow. The wet Ar flow was resumed immediately after the permeance measurements. The cross-sectional microstructures of the intermediate ␥alumina substrate and topmost silica layer were characterized using a field emission scanning electron microscope (FESEM, Leo 1550). Samples were obtained by mechanically fracturing the membranes after cooling them to liquid nitrogen temperature. The samples were then coated with a layer of gold by sputtering before examination in the electron microscope. 3. Results and discussion A general discussion of the morphological and transport properties of ceramic membranes has been given by Verweij [35], who outlined the concept of membranes with multiple layers of thin dimensions composed of different particle sizes, and without particle interpenetration. However, only examples of a support and a single intermediate layer are given. Indeed, a survey of the literature [14,25,36–39] indicates, to the best of our knowledge, no attempts to systematically change particle size to form graded layers and no effort to simultaneously reduce layer thickness. This results in membranes that are inferior in combined permeance and selectivity. In this paper we describe a research strategy to improve hydrogen-selective membranes through the use of thin, graded intermediate layers to support a permselective layer of silica so as to give superior properties for hydrogen permeation. The formation of an ultrathin, defect-free topmost layer of silica can be achieved with substrates of uniform structure with pores smaller than 5 nm. This has been demonstrated in the past with supports such as Vycor glass with 4 nm pores [9] or commercial alumina tubes with 5 nm pores [40]. The key characteristics for the intermediate layer to serve as an effective substrate on top of a rough macroporous support are possession of (a) a smooth surface, (b) small and uniform pore sizes, (c) a defect-free and continuous structure, and (d) a small thickness. Thin intermediate layers with small and uniform pore size can be obtained by the use of dilute dipping solutions containing sol particles of small size. However, when the sol particles of the dipping solution are too small compared with the pore size of the supports, these small particles do not easily form “bridges” over the large features of the supports because

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Fig. 3. Schematic of supports formed from (a) small sol particles, (b) large sol particles, and (c) graded sol particles.

of infiltration during dip-coating [41]. Even if such “bridges” are formed, they are not strong and are easily broken or cracked (Fig. 3(a)). This problem becomes more serious for supports with a wide pore size distribution. That is the reason why a concentrated dipping solution with a boehmite sol concentration of 0.5–1.2 M was generally used in the past to place ␥-alumina layers on the surface of a macroporous support. The group of Morooka [15,16] used a 0.6 M boehmite sol for coating macroporous alumina tubes with 110–180 nm pore diameter, and the group of Burggraaf [20,41,42] used 0.6–1.2 M sols for macroporous alumina discs with 120–340 nm pore size. The use of large sol particles can overcome the problem of infiltration [43]. But sol particles that are too large give rise to other deficiencies. Such particles will necessarily have large interstitial spaces, and, because of site exclusion, will not cover the surface uniformly leaving patches of exposed untreated surface (Fig. 3(b)). A simple solution is to use large particles and then sequentially deposit particles of smaller size on top. This gives rise to a graded structure (Fig. 3(c)) with better filling of voids to avoid defects and to smooth the surface. 3.1. Effect of synthesis parameters on particle size of boehmite sols The synthesis of colloidal boehmite sols from alkoxide precursors involves a number of steps. First, the precursors are hydrolyzed to remove the alkoxide groups, while simultaneously forming aluminum oxy hydroxide precipitates. The following types of reactions are believed to occur. Hydrolysis: Al(OR)3 + H2 O → Al(OR)2 (OH) + R(OH), etc.

(1)

Hydrolysis–polymerization:

(2) The precipitates that are formed by hydrolysis are then heated in acid in a step known as peptization in order to break up the larger precipitates and form smaller particles through hydrolysis and condensation reactions. The acid also charges the surface of the particles causing them to repel each other and allowing the formation of stable suspensions.

The formation of stable colloidal boehmite sols depends on many synthesis parameters such as alkoxide–water ratio, hydrolysis temperature and time, acid type, acid concentration and aluminum source. As previously reported, aluminum alkoxides must be added into hot water above 353 K or the mixture of aluminum alkoxides and water should be heated quickly above 353 K to prevent formation of substantial amounts of bayerite [beta Al(OH)3 ]. This is because the product bayerite does not form stable sols [44]. Also, very limited kinds of acids can be used to peptize boehmite precipitates. These acids must meet two general requirements: their anion must be noncomplexing with aluminum and they must have sufficient strength to produce the required charge effect at low concentrations [33]. Among these parameters, however, only acid type and acid concentration were reported to have an influence on the particle size of the sols obtained [29–31,33]. Nitric acid or hydrochloric acid was used with an H+ /alkoxide in the range of 0.03–0.20, and the particle size of the resulting sols varied only within 100 nm [29,32,33,45,46]. However, the results have been inconsistent. For example, Yoldas [33] found an increase of boehmite sol particle size from 10–35 nm to 10–100 nm with increasing molar ratio of hydrochloric acid to aluminum secondary butoxide from 0.020 to 0.21. Shi and Wong [45] reported a decrease in the mean particle size of the alumina sols from 88 to 45 nm for nitric acid and 90–25 nm for hydrochloric acid when the H+ /Al2 O3 molar ratio was increased from 0.035 to 0.15. Burggraaf et al. [46] reported that particles prepared by a colloidal route ranged from 5 to 100 nm, depending on the system and the processing. Because of the contradictory results reported in the literature, the conditions leading to the formation of sols of controlled particle size were re-investigated in this study. First, the effect of acid type was studied. Fig. 4 shows the particle size distributions of the boehmite sols peptized with HNO3 , HCl and acetic acid, respectively at the same molar ratio of H+ /alkoxide (R = 0.10). It was found that inorganic acids, HNO3 and HCl, give smaller particle size (54 and 47 nm) compared to acetic acid (176 nm). Since the pore size of the bare alumina support was 100 nm, a large particle size was needed, and acetic acid was chosen as the peptizing agent. Acetic acid with different molar ratios of H+ /alkoxide (R = 0.03, 0.04, 0.055, 0.07, 0.10, 0.15 and 0.25) was used to systemically investigate the dependence of particle size on acid concentration, and considerable differences in the particle sizes were found, as shown in Fig. 5. As the molar ratio of H+ /alkoxide

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Fig. 4. Particle size distributions of boehmite sols peptized with various acids at the same molar ratio of H+ /alkoxide (R = 0.10).

decreased from R = 0.25 to R = 0.03, the median particle size of the resulting boehmite sol increased from 65 to 950 nm. Thus, lower acetic acid concentration favors the formation of larger particles. The influence of hydrolysis time on sol particle size has not been reported. One previous study based on electron microscopy showed no observable changes in size of the resulting boehmite during aging when the aluminum secondary butoxide was hydrolyzed in hot water [44]. In this work, the aluminum alkoxides were hydrolyzed at 353 K or above and were aged for 1.5, 3, 6, 24 and 72 h, respectively before peptization with acetic acid at R = 0.15. The particle size distributions of the resulting sols are shown in Fig. 6. As the aging time increased from 1.5 to 72 h, the median particle size of the resulting boehmite sols increased from 32 to 120 nm. The longer aging time produces larger sol particles either by a polymerization process or by particle aggregation. Peptization temperature did not have much influence on sol particle size, as shown in Fig. 7. The lower peptization temperature of 353 K resulted in a slightly larger sol particle (208 nm) compared to the higher peptization temperature of 365 K (176 nm), while a peptization temperature lower than 353 K did not give a clear sol. Table 3 summarizes the effect of synthesis parameters on the particle size of the boehmite sols.

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Fig. 6. Particle size distributions of boehmite sols hydrolyzed for various times.

Fig. 7. Particle size distributions of boehmite sols peptized with acetic acid at 353 and 365 K, respectively.

The boehmite sols peptized by acetic acid are very stable. Stored at room temperature in a closed container, all the CH3 COOH-peptized sol samples (Table 3) remained in a suspended colloidal suspension state for more than 6 months, while the HNO3 -peptized sol and HCl-peptized sol became gels after 3 months and 3 weeks, respectively. Fig. 8(a) shows the changes

Fig. 5. Effect of acid concentration on boehmite particle size. (a) Particle size distributions of boehmite sols peptized with acetic acid at different molar ratios of H+ /alkoxide. (b) Mean particle size vs. H+ /alkoxide.

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Table 3 Effect of synthesis parameters on particle size of boehmite sols Sample

Hydrolysis time (h)

Peptization agent

Peptization temperature (K)

Molar ratio of H+ /alkoxide

Median particle size (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

24 24 24 24 24 24 24 24 24 24 72 6 3 1.5

HNO3 HCl CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH CH3 COOH

365 365 365 353 365 365 365 365 365 365 365 365 365 365

0.10 0.10 0.10 0.10 0.25 0.15 0.07 0.055 0.04 0.03 0.15 0.15 0.15 0.15

54.1 46.8 176.4 208.2 66.2 113.2 204.8 314.9 631.4 952.1 100.4 60.2 40.7 31.9

of particle size as a function of the number of days in storage for the CH3 COOH-peptized boehmite sols prepared using different molar ratios of H+ /alkoxide. It was found that for all sols the particle size decreased gradually in the first week, and then stabilized. The sols prepared with a molar ratio of H+ /alkoxide lower than 0.07 show smaller changes in particle size of around 10%, in comparison with those prepared using a molar ratio of H+ /alkoxide higher than 0.07 which show a size reduction of 20–40%. Aging for a further 60 days indicates a change in particle size of less than 15%. The reduction in particle size is probably due to the slow dissolution of particles driven by the interaction with the electrolyte and tending towards thermodynamic equilibrium. Fig. 8(b) shows the changes of the particle size of the CH3 COOH-peptized boehmite sols prepared with different hydrolysis time but the same molar ratio of H+ /alkoxide of 0.15. As observed in Fig. 8(a), the particle size of all the sols decreased especially in the first day. Comparatively, however, the sol prepared with a hydrolysis time of 24 h had slightly better stability. In summary, sol suspensions could be prepared which after 24 h were stable in particle size for prolonged periods. This is important from a practical standpoint as it simplifies their use in the preparation of membranes.

3.2. Microstructure and stability of the intermediate γ-alumina multilayer It is known that the formation of gel layers on porous supports is caused by a capillary pressure drop between the support pores and the liquid, which drives sol particles to concentrate at the support-solution boundary if the particles cannot enter into the pores. The membrane growth rate increases with increasing concentration of the dipping solution [41,47]. Once the sol solutions are prepared, they are used for dip-coating the tubular membrane supports. As mentioned earlier, this is carried out using a mechanical instrument to obtain reproducible results. After each dipping step the deposited sols are dried and calcined. The drying process increases the concentration of the sols, increasing the contact between particles, and gives rise to gelling [43]. Fig. 9(a) and (b) shows the cross-sectional low (5000×) and high (50,000×) resolution images of the commercial asymmetric ␣-alumina support obtained by field emission scanning electron microscopy (FESEM). The ␣-alumina support has a pore size of around 100 nm, which is in agreement with the nominal value reported by the supplier. However, some extra large pores with a diameter of around 400 nm are still observed.

Fig. 8. Stability of particle size of the CH3 COOH-peptized boehmite sols as a number of days during storage. (a) Sol samples prepared with different molar ratio of H+ /alkoxide but with the same hydrolysis time of 24 h and (b) sol samples prepared with different hydrolysis time but the same molar ratio of H+ /alkoxide of 0.15.

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Fig. 9. Scanning electron micrographs of fractured sections of the ␣-alumina support and ␥-alumina multilayer substrate with a graded three-layer structure. ␣Alumina support with low resolution 5000× (a) and high resolution 50,000× (b); graded ␥-alumina multilayer substrate with low resolution 50,000× (c) and high resolution 100,000× (d).

Fig. 9(c) and (d) shows the cross-sectional low (50,000×) and high (100,000×) resolution images of the graded intermediate ␥-alumina membrane GA5. As described in Section 2, the macroporous support was dip-coated five times, but the first and second coatings, and the fourth and fifth coatings were made with the same sol solution, respectively, and so only three ␥alumina layers with different textures are observed in Fig. 9(c) and (d). The top ␥-alumina layer had a finer structure than the other two layers below, because of the use of a solution with smaller sol particles. Fig. 9(c) and (d) also demonstrate a clear but uniform interphase between the ␥-alumina multilayers and the macroporous ␣-alumina support, suggesting that the multilayer membrane has good adhesion to the support. Furthermore, it was found that the pores of the support were unclogged, indicating that no infiltration of sol particles occurred during the dip-coating process. This structure resulted from the use of first large sol particles of size 630 nm, which were larger than the 100 nm pore size of the support, followed by particles of smaller size. This sequence prevents the small sol particles penetrating into the large pores of the support, which would block pores and increase the resistance for permeation of gases. If the sol concentration and/or the particle size is large enough, pore clogging hardly occurs and a gel layer is immediately formed [43]. This is the reason why at a given pore size of the support a certain minimum concentration (0.5–0.7 M) is required to obtain a gel layer [20,27,41]. But concentrated sol solutions will lead to larger thickness of the resulting membranes. Significantly in this work, the use of dilute solutions (0.15 M) containing large boehmite sol particles in the first coating did not only decrease the thickness of the multilayer considerably, but also prevented pore clogging of the substrate. As indicated in Fig. 9(c) and (d), the total thickness of the ␥-alumina multilayer was around 1200–1400 nm (1.2–1.4 ␮m), much thinner than that of typical

intermediate alumina membranes. For example, de Lange et al. placed a ␥-alumina layer with a thickness of 7–10 ␮m using a sol solution of 0.6 M on the support before depositing a silica layer by sol–gel [17,18], while Sea et al. inserted an intermediate ␥alumina layer with the thickness of 6–9 ␮m by dip-coating three times with a 0.6 M sol before carrying out CVD of TEOS [16]. The use of a lower sol concentration of 0.15 M in this work leads to a thin multilayer membrane. Leenaars and Burggraaf [41] reported that the thickness of a ␥-alumina layer was decreased from 5.8 to 4.0 nm when the sol concentration was reduced from 1.2 to 0.7 M with the same dipping time of 3 s. Additionally, it can be seen that the second and third ␥-alumina layers are much thinner than the first layer. This is because the layer growth rate is much smaller in the second layer compared to that in the first due to the additional flow resistance offered by the first applied mesoporous layer. Fig. 10 shows the changes on the H2 permeance and selectivity of the graded intermediate ␥-alumina membrane GA5 with the exposure time when exposed to a humid Ar flow containing 75 mol% water. It was found that the H2 permeance increased by 7% after exposure for 2 h, 18% for 64 h, and then remain unchanged for another 93 h. The selectivity of H2 over CH4 almost did not vary during the exposure. The result indicates that the ␥-alumina membrane in this work is more hydrothermally stable, compared to those reported in the literature [48,49]. We speculate that the graded structure results in greater interactions among the constituent particles in the region where particle size changes, resulting in greater stability. Gallaher and Liu [48] studied the hydrothermal stabilities of commercial Membralox® ␥-alumina membranes with a pore size of 4 nm. Their results showed a dramatic increase by 36% in N2 permeance after exposure to 5 mol% steam at 913 K for 95 h due to pore enlargement. Zahir et al. [49] also found an increase by 24% in H2 permeance

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Fig. 10. Changes of H2 permeance and selectivity of graded ␥-alumina multilayer substrate GA5 during the exposure to a humid Ar flow containing 75 mol% water at 873 K.

for a ␥-alumina membrane during the first 2 h of exposure to 75 mol% water at 783 K. 3.3. Permeation properties of silica/alumina multilayer composite membranes The quality of the mesoporous ␥-alumina multilayer substrate can be assessed experimentally by measuring the permeation properties of the silica layer on the top of this multilayer substrate, since the integrity of the selective layer of silica depends significantly on the microstructure of the substrate, including pore size and pore size distribution, surface roughness, and structural continuity. Substrates with small and uniform pores, smooth surface and continuous structure are expected to produce thin and defect-free layers, which in turn gives rise to high-gas permeance and selectivity. Fig. 11 compares the changes of permeation properties of four composite membranes deposited on graded and ungraded ␥-alumina substrates at 873 K before and after CVD. Before CVD, these four freshly prepared substrates show little differ-

Fig. 11. Hydrogen permeance vs. selectivity at 873 K for the composite membranes composed of a silica layer deposited on different ␥-alumina multilayer substrates. Ungraded substrates: UGA-55 and UGA-630; graded substrates: GA4 and GA5.

ence in H2 permeance and almost the same selectivity for H2 over other gases (2.7–2.8 and 4.4–4.9 over CH4 and CO2 ), which is close to the values predicated by the Knudsen diffusion mechanism (2.8 and 4.7 over CH4 and CO2 ). Considerable differences were revealed with silica deposition between the permeation properties of the ungraded ␥-alumina substrates (UGA-55 and UGA-630) and the graded ␥-alumina substrates (GA4 and GA5). For the ungraded substrates the H2 permeance through the composite membranes declined slightly and the H2 selectivity improved a little. After 6 h of silica deposition on the ungraded substrate UGA-55, the H2 /CH4 selectivity through the resulting composite membrane S/UGA-55 was almost the same as that of the substrate, indicating that a continuous selective silica layer could not be formed on the substrate. The ungraded substrate UGA-55 was prepared by four applications of the same dilute sol solution of the boehmite sol BS55. Here it is noted that the sol BS55 was synthesized by a general route (peptization by nitric acid with a molar ratio of H+ /alkoxide of 0.10). The median and mean particle sizes were 55 and 62 nm, which are consistent with those reported in the literature [45]. As shown in Fig. 3(b), the use of small sol particles and low sol concentrations probably resulted in ineffective coverage over the extra large pores of the macroporous ␣-alumina support, and consequently large defects were left in the substrate after calcination. As shown in Fig. 3(b) the use of large particles did not give rise to a good membrane. The composite membrane with the ungraded substrate UGA630 did not show much improvement in the selectivity after 6 h of deposition, although it performed better than the composite membrane formed from UGA-55 because of the use of larger sol particles of size 630 nm. It is concluded that composite membranes formed from ungraded substrates had high permeance but poor selectivity. The two composite membrane with the graded substrates GA4 and GA5 displayed much better permeation properties than their ungraded substrate counterparts. Fig. 12(a) and (b) compares the changes of the gas permeance and selectivity on the composite membranes at 873 K with the silica deposition time. The substrates GA4 and GA5 were prepared by dipping–calcining four and five times using different dipping solutions of gradually decreasing particle size. Both freshly prepared substrates showed high-gas permeance in the order of 10−5 mol m−2 s−1 Pa−1 but very low H2 selectivity. As the CVD deposition proceeded for 3 h, the permeance of CH4 , CO and CO2 decreased considerably, but that of H2 decreased more slowly, leading to an increase in the H2 selectivity. Compared with the membrane S/GA4, however, the membrane S/GA5 shows a deeper drop in the permeance of CH4 , CO and CO2 (10−10 versus 10−9 ) and thus one order of magnitude higher selectivity, as shown in Fig. 12(b). It is likely that for the membrane S/GA4 a continuous silica layer was completely formed but that some defects or uncovered pores remained through which CH4 , CO and CO2 could pass through. As the CVD deposition continued, these defects and pores were repaired and, the permeances of CH4 , CO and CO2 continued to drop fast while the permeance of H2 was almost unchanged, as shown in Fig. 12(a). This behavior indicates that the multilayer substrate GA5 had a more uniform and finer microstructure. As a

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Fig. 12. Changes of gas permeance with deposition time of silica/alumina composite membranes with different graded ␥-alumina multilayer substrates. (a) Substrate GA4 and (b) substrate GA5.

Fig. 13. High-resolution SEM imagines (magnification = 300,000×) of fracture sections of the graded ␥-alumina multilayer substrate and the silica/alumina composite membrane: (a) graded ␥-alumina multilayer substrate GA5 obtained by sequentially dipping–calcining sols with particle size of 630, 630, 200, 40 and 40 nm; (b) silica/alumina membrane deposited on the substrate GA5.

result, the selectivities of H2 over CH4 , CO and CO2 through the composite membrane S/GA5 reached to 5900, 5100 and 1500, respectively while the H2 permeance was as high as 5 × 10−7 mol m−2 s−1 Pa−1 at 873 K. Fig. 13 shows the high-resolution (300,000×) images of the cross-sectional structures of the graded substrate GA5 and the resulting silica/alumina composite membrane. It can be seen that a very thin and smooth layer of silica was formed on top of the ␥-alumina layer after 3 h of silica deposition. The thickness of the silica layer was 20–30 nm. As we previously reported, the same highly selective silica membrane prepared on a commercial ␥-alumina support with 5 nm pore size showed a similar thickness of 30 nm after 12 h-deposition under the same CVD process parameters [40]. This implies that much less silica was deposited within the pores of supports when a graded multilayer alumina substrate was used in this work, resulting in the improvement of the permeance. As a result, the present silica/alumina multilayer membrane showed both excellent H2 selectivity and good H2 permeance in comparison to other silica membranes reported in the literature (Fig. 14) [5–7,9,11,13,16,23,25,26,40,50–52]. Clearly, the ␥-alumina multilayer with a graded structure, formed from the sequential use of boehmite sols with decreasing particle size, displayed excellent performance in its role as an intermediate layer connecting the macroporous support with the hydrogenselective Nanosil silica layer. The concept of a thin, graded layer presented and demonstrated here is likely to have broad utility for other types of membranes.

Fig. 14. Comparison of H2 permeability and selectivity at 873 K of the silica/␥alumina multilayer membrane with other silica membranes reported in the literature. (()Yan et al. [15], Sea et al. [16]; () Hwang et al. [13]; () Tsapatsis et al. [5,6]; () Kim and Gavalas [7]; (♦) Ha et al. [11,50]; () Akiyama, et al. [51]; (×) Nomura et al. [52]; (*) our group [9,40]), () Zivkovic et al. [25], ( ) Asaeda [23], ( ) Nakao [26], () This work.

4. Conclusions Boehmite sols of controlled particle size in the range of 30–950 nm were successfully obtained from aluminum isopropoxide by using acetic acid as a peptization agent. Higher acid concentration and shorter hydrolysis time gave smaller par-

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ticle size. Acetic acid was more effective than the inorganic acids, HNO3 and HCl, in producing sols of greater stability and in the desired size range. A thin and defect-free ␥-alumina-graded substrate with a three-layer structure was formed on a macroporous alumina support by sequential dipping and calcining with boehmite sols of decreasing particle sizes. The use of dilute dipping solutions made the layers and the overall membrane thin. SEM microphotographs of the fracture surface of the multilayer indicated a total thickness of the order of 1000 nm, about 4–6 times thinner than that of a conventional intermediate ␥-alumina layer. After 3 h of silica deposition, an ultrathin silica layer of 20–30 nm thickness was formed on this membrane substrate. The resulting composite membrane showed high-H2 selectivity (>1500) over CH4 , CO and CO2 while maintaining high-H2 permeance of 5 × 10−7 mol m−2 s−1 Pa−1 at 873 K.

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