Alumina ultrafiltration membranes derived from carboxylate–alumoxane nanoparticles

Journal of Membrane Science 193 (2001) 175–184

Alumina ultrafiltration membranes derived from carboxylate–alumoxane nanoparticles Christopher D. Jones a , Maria Fidalgo b , Mark R. Wiesner b , Andrew R. Barron a,c,∗ a

Department of Chemistry and Center for Nanoscale Science and Technology, Rice University, Houston, TX 77005, USA b Department of Environmental Science and Engineering, Rice University, Houston, TX 77005, USA c Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, USA Received 2 January 2001; received in revised form 3 May 2001; accepted 4 May 2001

Abstract The fabrication of asymmetric alumina ultrafiltration membranes using acetic acid surface stabilized alumina nanoparticles (A-alumoxanes) has been investigated. Contacting ␣-alumina supports with an aqueous solution of A-alumoxane (after firing to 1000◦ C) yields a defect free alumina membrane with a thickness of ca. 2 ␮m. The alumoxane-derived membranes have a molecular weight cut-off in the range of 35,000–44,000 g mol−1 , high porosity, and a permeability that is comparable to or greater than that of commercially available alumina membranes. SEM and AFM show that the surface of the alumoxane-derived membranes is quite smooth and contact angles show that the membrane is hydrophillic. A comparison with commercial alumina and polymer membranes, as well as those derived from sol–gel methods is presented. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ceramic; Alumoxane; Alumina; Membrane; Nanoparticle; Ultrafiltration

1. Introduction Membrane technologies play an increasingly important role in pollution prevention, resource recovery and waste treatment activities [1]. Due in large part to cost considerations, polymeric membranes dominate these applications [2], however, the use of polymeric membranes in separations involving aggressive materials such as many organic solvents, acids, bases and oxidants is often limited by the tolerance of the polymeric material to extreme conditions [3,4]. Ceramic membranes are noted for their excellent mechanical strength and tolerance to solvents, as well as pH, oxidation, and temperature extremes [5]. ∗

Corresponding author. Tel.: +1-713-348-5610; fax: +1-713-348-5619; URL: www.rice.edu/barron. E-mail address: [email protected] (A.R. Barron).

An ideal ceramic membrane must be highly selective, permeable and durable [6,7]. For aqueous applications, or aqueous/organic separations it is desirable for the ceramic to be hydrophilic to maximize flow and minimize fouling. The membrane selectivity is primarily dependent upon the pore size distribution; the narrower the pores size distribution, the more selective the membrane. Mechanical integrity is enhanced in such applications by slip-casting a relatively thin selective membrane onto a thicker, more permeable support yielding an asymmetric membrane. The sintering of ceramic particles is perhaps the simplest approach to forming a porous ceramic filter, however, the sintering of bulk ceramics is a very energy expensive process due to the high temperatures required. The pore size is controlled by the starting particle size, sintering time and temperature [8–10]. Due to the large size of the starting particles, it is

0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 4 9 0 - 2

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extremely difficult, and not very efficient, to make ultrafiltration filters by this method because of grain growth during sintering, resulting in low porosity and large pores [11]. This method is generally used to produce alumina microfiltration filters, which contain larger pores and supports for ultrafiltration membranes, which contain smaller pores [12,13]. Of the present technologies, sol–gel is the best method for making ceramic ultrafiltration membranes. However, the pore size is generally limited to the sizes of the ceramic precursor particles prior to sintering. For sol–gels, the particle size distribution is difficult to control, and they must be used immediately after preparation to avoid aggregation or precipitation. We have previously reported that aluminum-oxide nanoparticles may be prepared by the reaction of the mineral boehmite with carboxylic acids [14,15]. These nanoparticles can be used as precursors for ceramic membranes as an alternative to the conventional sol– gel techniques. The identity of the carboxylic acid appears to control the size of the nanoparticles (5–80 nm). These nanoparticles, “carboxylate–alumoxanes”, are readily processed to alumina bodies and coatings [16,15]. The thermolysis of carboxylate–alumoxanes yield alumina membranes that exhibit narrow pore size distributions with pore sizes in the ultrafiltration range [17]. The pore size and porosity are dependent on the choice of carboxylate periphery and the sintering temperature [18]. In this paper, we present results from experiments in which alumoxane nanoparticles were cast on porous ceramic supports to yield asymmetric alumina membranes.

2. Experimental procedure 2.1. Materials Acetate-alumoxane (A-alumoxane) was prepared by previously published methods [15]. Aqueous solutions of A-alumoxane were degassed prior to use. RefractronTM ␣-alumina supports were obtained from the Refractron Technologies Corp. (Newark, NJ) and were heated to 600◦ C prior to use to remove surface grease. For comparison, WhatmanTM (AnodiscTM ) and CorningTM (NucleoporeTM ) filters were used as received.

2.2. Fabrication of asymmetric membranes One face of a RefractronTM ␣-alumina support was brought into contact with an A-alumoxane solution (0.5–10 wt.%) so that only the surface touched the solution for approximately 2–5 s. Penetration of the A-alumoxane solution into the voids of the support occurs by capillary action. The support was then shaken to remove any excess solution, and dried at room temperature for 2 h. The coated support was heated to 600◦ C for 4 h, held for 3 h and then, heated to 1000◦ C for 3 h. Multiple coatings were obtained by treating a previously coated filter (1 wt.% A-alumoxane) fired to 1000◦ C, with a 1 wt.% aqueous solution of A-alumoxane followed by firing to 1000◦ C. X-ray diffraction analysis revealed the resultant membrane to be ␥-Al2 O3 (after heating to 600◦ C) or ␪-Al2 O3 (with traces of ␣-Al2 O3 after sintering to 1000◦ C). Porosity, surface area, pore volume, and pore size distribution measurements were made on a symmetric membranes prepared by pouring a 1 wt.% solution of A-alumoxane into a TeflonTM drying mold. The solution was dried at room temperature, yielding a green body of ca. 1 mm in thickness. The green body was then heated under identical conditions to that used for the asymmetric membranes. 2.3. Characterization methods Scanning electron microscopy (SEM) studies were performed on a Phillips XL-30 ESEM scanning microscope. The samples were attached to a metal mount using carbon tape. Due to the insulating nature of the materials, a thin layer of gold was applied as a coating to provide a conducting surface. AFM images of samples were obtained using a Nanoscope IIIa Scanning Probe Microscope (Digital Instruments, Santa Barbara, CA) in tapping mode. FESP tips were used with a pyramidal shape and end radius of 5–10 nm (Digital Instruments). Samples were attached to 15 mm magnetic specimen disks with carbon tape. Roughness and cross-section analysis were determined by the accompanying Nanoscope IIIa software. Contact angles were measured using a Goniometer. Surface charge was determined by measuring electrophoretic mobility with a Zeta Meter 3.0 (ZetaMeter Inc.).

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Porosity, surface area, pore volume, and pore size distributions were obtained using nitrogen adsorption/desorption techniques using a CoulterTM SA3100TM . Helium was used to determine the free space in the sample tube and nitrogen as the absorbate gas. All samples were outgassed at 300◦ C for 2 h under a stream of dry nitrogen using a CoulterTM SAPrepTM . Calculations were based on the cross-sectional area of nitrogen using the value of 0.162 nm2 . Surface area was calculated using the BET (Brunauer, Emmett and Teller) equation with five data points. Pore size distributions were determined using the BJH (Barrett, Joyner, and Halenda) technique using 65 data points from the nitrogen adsorption isotherm [19]. Pore volume calculation was performed at a relative pressure of 0.9814. Pure water flux was measured on both the carboxylate–alumoxane-derived filters and commercial samples (AnodiscTM and NucleoporeTM ). Samples were glued using silicone adhesive to plastic rings to adapt their size to the filtration cell. The effective filtration area was 1.61 × 10−3 m2 for all samples. The experiment was conducted using a dead end filtration cell (Amicon, Model 8200). A zero air tank was connected to the cell for pressure, and a regulator was used to maintain a constant pressure of 10 psi. Permeate was collected at atmospheric pressure, so that the pressure at the end of the regulator was equal to the transmembrane pressure. Permeate volume was measured over time to calculate flux. Molecular weight cut-off (MWCO) experiment were performed following the American Society for Testing and Materials (ASTM) Publication E 1343-90. The concentration, molecular weight characteristics of the dextran used in this study and the solute diameters are summarized in Table 1. Sodium azide (99%, Aldrich) was added at a concentration of 0.05 wt.% to prevent bacterial growth and NaCl (0.05 M) was Table 1 Molecular weight (Mw ) and solute (ds ) diameters of dextran in aqueous solution used to calculate the MWCO Dextran

Concentration (wt.%)

Average Mw (g mol−1 )

ds (nm)

T-10 T-40 T-70 T-500

0.25 0.10 0.10 0.20

10500 37500 69800 413000

1.8–3 4–6 6–9 >15

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added to control the ionic strength. Ultra-pure water was used in all the experiments (10 M). Samples were prepared identical to those for the permeability measurements and placed in a stirred ultrafiltration cell (Amicon, Model 8200). The cell was modified to allow for re-circulation of the feed. A hole was drilled in the top of the cell to insert a 1/8 in. stainless steel tube, used as inlet for the feed. The existing fitting was used as exit of flow. The feed was pumped at a rate of 100–110 ml min−1 . A valve and pressure gage at the flow exit of the cell allowed for transmembrane pressure control. The pressure in the regulator was set at 7 ± 0.5 psi. The permeate samples were collected after allowing the system to run for 30 min. The velocity of the flow across the membrane was kept below 0.15 ml min−1 to avoid deformation effects of the macromolecules with a peristaltic pump at the permeate line. A sample of the feed was taken at the end of each filtration. Feed and permeate samples were analyzed by gel permeation chromatography (GPC). A HPLC system (Waters 717+ Autosampler, Waters 600E System Controller) was used with a GPC column (TosoHaas G4000PWXL). Calibration curves (molecular weight versus elution time) were obtained running each dextran fraction separately. The peak was assumed to correspond to the average molecular weight given by the manufacturer. The elute was collected every milliliter with a fraction collector (Waters). The concentration of solute in each fraction was determined measuring organic carbon concentration, using a Total Organic Carbon Analyzer (Shimatzu, TOC 5050A). Three HPLC runs were performed with each sample and each fraction was analyzed three times by the TOC analyzer.

3. Results and discussion Commercially available ␣-alumina (Refractron Technologies Corp.) porous disks were used as supports for casting the alumoxane-derived membranes. The surface of the RefractronTM ␣-alumina support is quite rough and there appears to be a significant number of void spaces on the top surface of the support (Fig. 1a). A closer view of the support (Fig. 1b) shows that the support is composed of alumina particles sintered together with particle sizes ranging from

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Fig. 2. Nitrogen adsorption BJH pore volume distribution of the RefractronTM ␣-alumina support.

Fig. 1. SEM image of the top surface of a commercial ␣-alumina support.

1 to 0.05 ␮m, and that pore channels exist between sintered particles. Many surface defects are observed, however, these do not appear to continue through the support. The surface area (4.1 m2 g−1 ) and pore volume (0.0244 ml g−1 ) are quite low on the support indicating that the particles composing the support are not porous. Nitrogen absorption revealed a broad distribution of pores with a maximum pore size over 180 nm, see Fig. 2. A wide distribution of pores is to be expected, considering that the support itself is composed of particles with a large distribution of sizes. One side of the RefractronTM ␣-alumina supports was dip-coated with an aqueous solution of A-alumoxane. The alumoxane solution is drawn into the voids on the support through capillary action. After drying in at room temperature, the filter was heated, in stages to 1000◦ C. Solutions of various solutions concentrations ranging from 0.5 to 10 wt.% were investigated. Membranes formed using 10 wt.% were observed to have a high fraction of defects;

cracks, agglomerates, bare patches, curling, etc. Although the membranes prepared from the 5 to 3 wt.% solutions were greatly improved over the membranes formed from 10 wt.% solutions, surface cracking of the A-alumoxane-derived membrane were observed upon sintering. In contrast, the membranes formed from the 2 and 1 wt.% solutions appeared to form membranes with a uniform surface. SEM images reveal no cracks, surface defects, or pin holes found within the membranes, see Fig. 3a. Closer magnification (Fig. 3b) reveals that the surface is composed of tightly packed spherical particles, consistent with alumina derived from A-alumoxane [20]. The thickness of membranes formed by using a 1 wt.% aqueous solution of A-alumoxane were approximately 2 ␮m, see Fig. 4. Doubling the concentration of the A-alumoxane solution resulted in membranes approximately 3–4 ␮m thick. SEM cross section images (Fig. 4) show that the alumoxane membranes are very homogeneous throughout with no obvious defects. Attempts to physically remove the A-alumoxane-derived alumina membrane from the support suggest that the bond between the membrane and support is adequate. Microscopic examination of the membrane and support (Fig. 4a) indicated that the membrane conforms to the curves of the support without any sign of breaking away from the support. Overall, the membranes derived from dilute solutions used in the dip-coating technique appear to

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Fig. 3. SEM images of the A-alumoxane (1 wt.%)-derived alumina membrane cast on top of a RefractronTM ␣-alumina support heated to 1000◦ C.

be largely defect free. However, use of a very dilute A-alumoxane solution (<1 wt.%) was found to result in incomplete coverage of the support. Surface scratches (presumably due to handling) were observed by SEM. However, scratches were not as deep as thickness of the membrane (Fig. 5), and thus, do not impact membrane integrity. Pinholes in the final membrane caused by air bubbles in solution, or by air trapped between the support and the solution during dip-coating were observed (Fig. 6). This type of defect can be corrected by ensuring that the surface of the support is clean (removal of any grease) by heating to 600◦ C before dip-coating it and by removing any air bubbles trapped in the A-alumoxane solution. Resolution of occasional problems with air bubbles is important to the implementation of the alumoxane method as a single-cast process. Multiple dip-fire sequences are often required using the sol–gel process to produce membranes of high degree of integrity [21].

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Fig. 4. SEM images of cross-sections of an A-alumoxane (1 wt.%)-derived alumina membrane (top layer) cast on a RefractronTM ␣-alumina support (lower layer) after firing to 1000◦ C.

Fig. 5. SEM image of a surface defect in an A-alumoxane-derived alumina membrane after firing to 1000◦ C, showing that it does not continue to the support.

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Fig. 6. SEM image of a large pin hole defect in an A-alumoxane-derived alumina membrane caused by contaminated surface of the substrate and/or air bubbles in the A-alumoxane solution.

The alumoxane process may also be adapted to the fabrication of catalytic membranes in which a structurally homogeneous membrane is prepared using mixtures of different doped and non-doped carboxylate–alumoxanes, to yield a multi-layer catalytically active membrane [22]. In this regard we have investigated the possibility of multiple dip-coats on the support of the membrane material. As expected, each time the membrane is dip-coated and fired it increases the thickness of the membrane. For example, using a 1 wt.% A-alumoxane solution, the first dip-coat forms a membrane of ca. 2 ␮m thickness (Fig. 7a). A second dip-coat, and subsequent firing to 1000◦ C, yields a membrane that is now approximately 4 ␮m in thickness (Fig. 7b). It is important to note that distinct boundaries between layers were not observed in contrast with membranes produced in some sol–gel systems [23,24]. Given the asymmetric nature of the membranes the pore size distributions for the A-alumoxane-derived membrane layer were determined on samples of the A-alumoxane treated under comparable conditions to the fire step in the membrane preparation. The surface area of alumina derived from A-alumoxane was determined to be between 170 and 225 m2 g−1 , dependent on the sintering temperature, i.e. 1000◦ C versus 600◦ C. However, the total pore volume (0.35–0.42 ml g−1 ) was found to show only a small variation with sintering temperature and equates to a porosity of 54–69%.

Fig. 7. SEM images of cross-sections from A-alumoxane-derived alumina membranes supported on a RefractronTM ␣-alumina support after (a) one dip-coat and subsequent heating to 1000◦ C; (b) a second dip-coat and subsequent heating to 1000◦ C.

Pore size distributions determined from the adsorption branch of the isotherm tend to be broader than those derived from the desorption branch [25], due to the contribution of the narrow regions of the pore structure. An example of the difference between pore size distributions calculated from both branches of the isotherm is shown in Fig. 8. An average pore size of ca. 8 nm is obtained for alumina membranes derived from A-alumoxane, which agrees well with TEM measurements [18]. It should also be noted that the pore size distributions is very narrow with essentially no pores over 10 nm being observed. The relationship between the flow of a liquid through a membrane and the applied pressure drop across the membrane typically follows a Darcy’s law form under conditions of clean water permeation. The Darcy constant of permeability, k (units of length

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Fig. 8. BJH pore size distributions of A-alumoxane heated to 1000◦ C from both the adsorption (䊐) and desorption (䉬) branches of the isotherm.

squared) is independent of the membrane thickness in the case of a symmetric membrane. For an asymmetric membrane in which the resistance of the support material is negligible, the permeability must be normalized in terms of the thickness of the membrane skin for comparison with a symmetric membrane. The permeability of the RefractronTM ␣-alumina supports was found to vary between samples but in all cases was high compared with the permeability of the membrane skin. A total of 11 supports were measured and the mean permeability was found to be 32 ± 3 nm2 . The supports were coated with A-alumoxane and were fired to 1000◦ C for 3 h. The permeabilities of the coated filters were once again measured. A mean permeability of 20.01 ± 2 nm2 was determined for the ensemble of membrane skin and support corresponding to an average permeability of 0.32 nm2 for the membrane alone. The MWCO of the supports were measured and concentrations of dextran in the permeates were found to be the same as those in the feed solutions for all molecular weights, indicating that the MWCO of the support was greater than 500,000 g mol−1 . The molecular weight cut-off for the A-alumoxane coated filters fired to 1000◦ C for 3 h was in the range of 35,000–44,000 g mol−1 . This roughly corresponds to average pore diameter of 7.0–8.4 nm, which is in good agreement with nitrogen adsorption/desorption and TEM measurements. However, it should be stressed that the MWCO and its relationship to pore size, are both operationally defined by the type of molecule

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used to perform the MWCO test. MWCO depends greatly on the structure and conformation of the test molecules. Measurement of the electrophoetic mobility for alumina derived from A-alumoxane, as a function of the pH, allows for the calculation of membrane zeta potential. The iso-electric point, or zero point of charge (zpc), of symmetric membranes was determined to exist at pH = 8.5 [26]. This value is consistent with the known zpc for alumina. Contact angle measurements (pH = 7) on the A-alumoxane-derived membranes (10◦ ) confirmed the hydrophilic nature of the surface. This is important since hydrophilic membranes tend to foul less than hydrophobic membranes. AFM analyses of A-alumoxane-derived membranes shows an average roughness over a 1 mm scan length that is relatively low (ca. 4 nm). Membrane roughness is an important consideration in membrane fouling and in the case where a membrane is used as support for a tighter membrane skin as may be required to produce reverse osmosis or nanofiltration membrane [27]. It is also possible to perform line section analyses on AFM images. Line analysis for A-alumoxane yields surface pores between 13 and 20 nm in size [28]. This would suggest that the pores are much larger than indicated by nitrogen adsorption and MWCO experiments. However, it should be noted that AFM measurements overestimate pore size due to the geometry of the pore entrance. When pores are made by spherical or near spherical particles, the entrances to pores are larger than the pore necks. AFM measurements are limited to the entrances to pores. Nitrogen adsorption yields a surface-area average for pore size while pore sizes from the MWCO determinations are a function of molecular shape and size relative to the pore neck.

4. Comparison to commercially available filters Laboratory scale filters with pore diameters reported in the same range of our A-alumoxane-derived filters were obtained and tested in the same manner described above. These were the Whatman AnodiscTM filter (alumina), and the Millipore NucleoporeTM filter (polycarbonate), with reported pore diameters of 20, 15 nm, respectively. Comparable sol–gel-derived filters were not available, however, a comparison with literature values is given.

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Table 2 Comparison of commercially available ultrafiltration filters

Surface area (m2 g−1 ) Porosity (%) Pore size (nm) Permeability (nm2 ) MWCO (103 Da) AFM estimated pore size (nm) Contact angle (◦ ) Roughness (nm)e Maximum operating temperature (◦ C)

Alumoxane

Whatman AnodiscTM (20 nm)a

Millipore NucleoporeTM (15 nm)a

100–225 54–69 8–20 20 (0.32)b 11–44c (100%) 16–23 10 4–6 1000f

n/a 25–30 20 3 20d (0%) 30 10 6.5 <400

1.5 <15 15 0.05 20d (44%) 27 30 2.8 140

a

Nominal pore size reported by manufacturer. Value for ensemble of 2 ␮m membrane and porous support. Intrinsic permeability of alumoxane-derived membrane given in parentheses. c Lower values obtained for membranes fired to 600◦ C. d Based on PEG, percent retention (R) in parenthesis. e 1 ␮m scan length. f Pore size decreases due to sintering and grain growth. b

The AnodiscTM filter is formed from the anodic oxidation of aluminum metal, resulting in a filter made of amorphous alumina. The pores grow so that their diameter is proportional to the applied voltage, while current governs film thickness [4]. The bulk porous layer contains pores approximately 100 nm, while the membrane contains pores approximately 20 nm in diameter. The NucleoporeTM filter is made from dense polycarbonate that has been exposed to high energy particle radiation applied perpendicular to the film [29]. A summary of characteristics for the A-alumoxane alumina membranes, the AnodiscTM filter and NucleoporeTM filter is given in Table 2. Despite a smaller or comparable pore size, the A-alumoxane-derived alumina membranes have a much higher surface area and porosity than the AnodiscTM and the NucleoporeTM membranes (Table 2). The A-alumoxane-derived filters have smaller pore sizes than either of the other filters tested. All three filters exhibited a narrow pore size distribution and similar dimension of roughness and contact angle. An important advantage of the A-alumoxane-derived alumina membranes can be operated at temperatures higher than the alumina AnodiscTM (maximum temperature 400◦ C). Alumina sol–gel-derived membranes are presently the most accepted route to making alumina ultrafiltration filters. Lennears et al. [30], Lennears and Burggraaf [31–33], first developed the technique of using sol–gel processes to make alumina ultrafiltration

membranes. These filters, along with the vast majority of those reported in the literature [34–37], were made by the controlled hydrolysis of aluminum alkoxides to form alumina. Mixed metal membranes have also been reported by similar methods [38,39]. The preparation techniques used by various researchers vary the drying or sintering conditions which results to small changes in porosity or pore size. The pore sizes for sol–gel membranes are dependent on the preparation and firing temperature. The pore sizes at temperatures below 900◦ C all had modal diameters below 9 nm, while those fired to 1000◦ C showed an increase in the average pore diameter to 78 nm. The pore size below 900◦ C is comparable to A-alumoxane-derived alumina membranes, however at 1000◦ C, all A-alumoxane-derived membranes exhibited a much lower average pore size. In contrast, Lindqvist and Lidén reported sol–gel alumina membranes having a pore size distributions with a maximum pore diameter of 23 nm at 800◦ C, and 30 nm at 1000◦ C [36], both of which are larger than the A-alumoxane-derived membranes. Leenaars and Burggraaf have reported that sol–gel membranes fired to 400 and 800◦ C were prepared with an average pore diameter of 2.7 and 4.0 nm, respectively, by nitrogen absorption [33]. The MWCO’s (based on R (%) of 90) for the membranes reported by Leenaars and Burggraaf fired to 400 and 800◦ C were determined to be 2000 and 20,000 g mol−1 , respectively [34]. It is

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interesting to note that the membrane fired to 800◦ C was slightly smaller than the A-alumoxane-derived membrane which had a MWCO between 30,000 and 40,000 g mol−1 , and which was fired to 1000◦ C. The A-alumoxane-derived membrane fired to 600◦ C showed a MWCO of 11,000 g mol−1 which falls between the 400◦ C and 800◦ C sol–gel-derived filters. Another group testing membranes made by the same sol–gel method reports a MWCO of 44,000 g mol−1 for membranes fired to 900◦ C [37], which is similar to the A-alumoxane-derived membranes fired to 1000◦ C. The A-alumoxane process for creating ceramic membranes appears to be capable of yielding pore sizes comparable to those obtained using the sol–gel method. Possible advantages of the alumoxane process include a reduction in the number of slip-casting steps required to obtain a membrane of high integrity, a lower surface roughness, and better temperature stability towards increases in pore size.

5. Conclusions A new process for the formation of alumina ultrafiltration membranes from alumina nanoparticles (alumoxanes) has been investigated. The new method is an alternative method to the sol–gel technique. The alumoxanes-derived membranes have a molecular weight cut-off in the range of 35,000–44,000 g mol−1 and permeability that comparable to or greater than that of alumina membranes that are currently available commercially. Contact angles show that the membrane is hydrophillic and AFM imagery shows that the surface of the alumoxane-derived membranes is quite smooth, which may reduce fouling. Alumoxane-derived alumina membranes have similar pore characteristics to sol–gel-derived membranes, however the alumoxane nanoparticle method appears to be simpler and more environmentally benign.

Acknowledgements Financial support for this work was provided jointly by the Environmental Protection Agency (EPA) and the National Science Foundation (NSF) under the Technology for a Sustainable Environment Program.

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