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Synthesis of porous titanium dioxide membranes
Desalination 206 (2007) 531–537
Synthesis of porous titanium dioxide membranes N. Agoudjil*, T. Benkacem Laboratoire de physico-chimie des matériaux et application à l’environnement, Faculte de chimie USTHB, Bab Ezzouar, Alger, Algerie Tel. +213 (21) 536171; email: [email protected] Received 22 January 2006; Accepted 15 March 2006
Abstract Until recently, the industrial membranes used for filtration, distillation, separation or extraction were manufactured from polymers. However, inorganic membranes are emerging as an alternative to polymeric membranes for applications due to their thermal, mechanical and chemical characteristics. The sol-gel process is the way to obtain an inorganic structure that becomes a porous ceramic layer after thermal treatment with controllable porosity. The sol-gel process involves low-temperature synthesis of an inorganic network by a chemical reaction in solution. The synthesis of this material was accomplished using a titanium isopropoxide, Ti(OC3H7)4, as a precursor followed by peptization with nitric acid. The synthesis procedure strongly influenced the formation of the mesophase. The control of sol-gel transition and thermal decomposition allowed the synthesis of two forms of titanium dioxide — anatase and rutile. TiO2 dioxide was chosen for study for its interesting properties and the interest in which it may bring to the domain of chemical and mechanical high resistance of inorganic membranes. Deposit thin layers were created with a sol prepared with the destabilization of a colloidal solution process. These results show the great potentiality of inorganic membranes and new applications to be taken into account, particulary wastewater treatement and gas separation. Crossflow microfiltration and ultrafiltration are becoming of increasing interest to produce drinking water. X-ray diffraction, thermogravimetric analysis, and infrared spectroscopy have been used to characterize the mesostructure formed at room temperature as well as calcined at different temperatures. The pore diameters, porous volume and surface area of the materials measured from N2 adsorption–desorption isotherms indicate the variation according to temperature. Scanning electron microscopy is used to characterize membrane morphology, that is, the absence of cracks, the thickness and its homogeneity along the support. Some experimental results on the synthesis and characterization of inorganic membranes are presented. Keywords: Membrane; Porous; Sol-gel
*Corresponding author. Presented at the EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.580
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N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
1. Introduction A common element in many industrial processes is the necessity to separate the products from reactants and waste components. Membranes are becoming attractive separators because they offer several advantages relative to other processes. One of the most widely used methods for the preparation of porous membranes with different structures in the last decades is sol-gel processing [1–7]. With this method a network of partially hydrolyzed and polycondensed monomers diluted in the solvent is formed. A porous structure is obtained upon layer application (coating), drying and calcination. While the sol-gel process may appear to be a simple operation, many variables can influence the quality of the final product [1]. These variables include the choice of solvent, whether acid or base catalysis is employed, and use of stabilizing agents. In this study a procedure for the preparation of titania membranes by sol-gel technique is reported. The influence of several processing variables on particle size and sol stability was investigated. Additionally, the thermal evolution, structural characteristics of calcined powders and films were studied.
2. Experimental 2.1. Synthesis Titanium dioxide was prepared [2,4] by the hydrolysis and condensation of titanium isopropoxide Ti(OC3H7)4 (Aldrich) and was used as precursor; nitric acid (solution 65%, Merck) was employed as an acid catalyst to promote the formation of polymeric sols and ethanol as solvent. A specific amount of Ti(OC3H7)4 was dissolved in an equal volume of ethanol and mixed with deionized water. The synthesis difficulty is to eliminate the effects of phase separation and precipitation.
Fig. 1. Preparation steps of TiO2 oxide powder. Ti(OiPr)4: titanium isopropoxide = Ti(OC3H7)4.
Since membranes are manufactured from gels, it is extremely important to define the conditions required to form a gel. For a given titanium concentration, the hydrolysis condensation reactions were mainly governed by two parameters, namely the initial hydrolysis ratio, [H2O]/[Ti], and the inhibitor ratio, [H+]/[Ti]. Controlled hydrolysis of alkoxide precursors is the key to making thin films using the sol-gel process. The scheme of preparation is illustrated in Fig. 1. The synthesis mode was chosen as the maximum of connection formation: Ti-O-Ti. Thus, in a first step the idea is to use hydrolysis water by forming a maximum of connection of Ti-OH groupings. The overall hydrolysis and condensation reactions are illustrated below. C Hydrolysis
C Condensation
C Further condensation
N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
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C Overall reaction
The hydrolysis reaction of titanium isopropoxide. 2.2. Characterization Powder XRD data were carried out with a Philips PW 1830 diffractometer with CuKα radiation (λ =1.5406 Å). TGA curves were obtained in flowing nitrogen on a TGA 2050 with a temperature increasing rate of 10EC/min. N2 adsorption–desorption isotherms were recorded on a Micromeritics ASAP 2010 automated sorption analyzer. The samples were outgassed at 150EC before the analysis. The FTIR spectra of samples were obtained using the KBr wafer technique. The Barret-Joyner-Halende (BJH) method was used to determine pore size. Scanning electron microscopy was used to characterize membrane morphology, that is, the thickness and homogeneity along the support.
Fig. 2. Infra-red spectra of powders treated at different temperatures.
3. Results 3.1. Structural characterization Using absorption infra-red spectroscopy, infra-red spectra were recorded for every powder treated at different temperatures (Fig. 2). Infrared studies show that some OH remains in the membrane. The Ti-OH groups may be characterized by an absorption band situated at 500 cm!1. Desydroxylation starts in the early stages of the thermal treatment and continues up to 400EC. These OH groups are responsible for the membrane reactivity. Above a temperature of 450EC, we notice only the bands characterized by a metallic connection — Ti-O-Ti and O-Ti-O or Ti-OH between 500 and 1200 cm!1.
Fig. 3. Simultaneous TG/DTA curves of the powder obtained.
3.2. Thermogravimetric analysis The TGA of the as-synthesized samples under N2 showed the loss of water below 120EC and was completely removed at about 350EC (Fig. 3). The analysis of the as-synthesized sample revealed 30.52% total weight loss on heating to 400EC. The first effect is attributed to the release of adsorbed water, the second to dehydroxylation of the surface and removal of little residual. Fig. 3 also shows the differential thermal analysis curve. The endothermic peaks observed
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N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
Fig. 4. Temperature programmed XRD spectra of powders at different temperatures.
at 120EC and 375EC can be attributed, respectively, to water loss and decomposition of the organic groups. The exothermic peaks appearing at 410EC and 503EC are assigned to a progressive transformation of the anatase to rutile forms of TiO2. 3.3. By X-ray diffraction Temperature programmed XRD spectra are shown in Fig .4. It can be seen that the material retains its amorphous structure until 60EC, while at 200EC, a phase transformation into the crystalline anatase phase occurred. Above 200°C we obtained a mixture of two forms — anatase and rutile. Anatase and rutile are the two forms of titanium dioxide produced in the laboratory at atmospheric pressure. It is a transformation of anatase to rutile after thermal treatment at 600EC for 3 h, resulting in a loss of surface area and porosity. X-ray diffraction measurements are summarized in Table 1. We notice structural evolution from anatase to rutile with the increasing of the calcined temperature.
Table 1 Structural evolution from anatase to rutile Temp., EC
Anatase, %
60 9 200 350 400 500 600
Progressive crystallization 99.16 68.75 45.65 30.12 3.76
Rutile, % 0 0.83 31.25 54.34 69.87 96.24
4. Textural characterization by nitrogen adsorption–desorption N2-sorption isotherms were recorded for 350, 400, 500 and 600EC calcined mesoporous structured samples and are shown in Fig. 5. The shape of these curves is typical of mesoporous materials. Figs. 6–8 show the specific surface area, the pore volume and the pore size data measured from N2 adsorption–desorption isotherms for the mesostructured titanium oxide samples assynthesized and heat-treated at various temperatures. With the increasing of the calcined
N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
Fig. 5. Adsorption–desorption isotherm of sample using N2 at 77 K.
Fig. 6. Evolution of specific surface as a function of the temperature.
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Fig. 8. Evolution of pore diameters as a function of the temperature.
7. The mesostructured produced after calcination at 350EC has a Brunuer–Emmett–Teller surface area of 101 m2/g. Between 350EC and 600EC, the specific surface area decreased from 101 m2/g to 2 m2/g. At the same time, the pore size increased from 24 Å to more than 36 Å, respectively. These results indicated that as the calcination temperature increased, the number of pores decreased as a result of sintering while at the same time the pore size increased. This is a very interesting fact because with one sol, pore diameters are adjusted as a function of temperature, and the whole UF range can be screened.
5. Deposits of thin porous layers
Fig. 7. Evolution of porous volume as a function of the temperature.
temperature, the specific surface area and the pore volume began to decrease as shown in Figs. 6 and
Titania layers were deposited by a coating technique on mesoporous α-alumina substrate. The dried membranes were calcined at 400EC and 600EC, respectively, during 3 h. Scanning electron micoscopy observation showed homogeneous layers without cracking (Figs. 9–12. It was found that the layer thickness could be varied in the range 1.85 µm to 1.25 µm, respectively, at 400EC and 600EC. The physicochemical phenomena have to be well controlled to obtain porous layers with good characteristics.
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Fig. 9. Micrography of the layer obtained at 400EC.
Fig. 10. Cross section of the layer obtained at 400EC.
Fig. 11. Micrography of the layer obtained at 600EC.
Fig. 12. Cross section of the layer obtained at 600EC.
6. Conclusions
increase of the pore diameters according to the temperature while porous volume and specific surface decrease. The control of a sol-gel transition as well as a thin layer deposit on a porous α-alumina support contributes to the realization of UF inorganic membranes.
The synthesis of mesoporous titania using titanium isopropoxide as precursor under acidic conditions was investigated. The procedure for titania sol synthesis was optimized. It was found that polymeric sols could be obtained only when the hydrolysis conditions were strictly controlled. A crystallization of the anatase form begins at 60EC. Above this temperature, there is a coexistence of two forms — anatase and rutile — and a transformation of TiO2 anatase to the rutile at 600EC. We note a slight
References [1] C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-cryst. Solids, 100 (1988) 65–76.
N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537 [2] M.A. Anderson, M.J. Gieselmann and Q. Xu, J. Membr. Sci., 39(3) (1988) 243–258. [3] J. Livage, M. Henry and C. Sanchez, Prog. Solid St. Chem., 18 (1988) 259. [4] Q. Xu and M.A. Anderson, J. Mat. Res., 6(5) (1991) 1073–1081. [5] K.C. Chen, T. Tsuchiya and J.D. Mackenzie, J. Noncryst. Solids, 81 (1986) 227. [6] C.J. Brinker and G.W. Scherer, Sol-gel Science, Academic Press, New York, 1990.
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[7] J. Kims and Y.S. Lin, J. Mat. Sci., 75 (1998) 39. [8] J. Blanchard, S. Barboux-Doeuff, J. Maquet and C. Sanchez, New J. Chem., 19 (1995) 929. [9] C. Guizard, A. Julbe and A. Ayral, J. Mat. Sci., 55 (1999) 9. [10] P.M. Biesheuvel and H. Verweij, J. Membr. Sci., 156 (1999) 141. [11] J. Sekulic, A. Magrasso, J.E. Ten Elshof and D.H.A. Blank, Micropor. Mesopor. Mat., 72 (2004) 49.
Synthesis of porous titanium dioxide membranes N. Agoudjil*, T. Benkacem Laboratoire de physico-chimie des matériaux et application à l’environnement, Faculte de chimie USTHB, Bab Ezzouar, Alger, Algerie Tel. +213 (21) 536171; email: [email protected] Received 22 January 2006; Accepted 15 March 2006
Abstract Until recently, the industrial membranes used for filtration, distillation, separation or extraction were manufactured from polymers. However, inorganic membranes are emerging as an alternative to polymeric membranes for applications due to their thermal, mechanical and chemical characteristics. The sol-gel process is the way to obtain an inorganic structure that becomes a porous ceramic layer after thermal treatment with controllable porosity. The sol-gel process involves low-temperature synthesis of an inorganic network by a chemical reaction in solution. The synthesis of this material was accomplished using a titanium isopropoxide, Ti(OC3H7)4, as a precursor followed by peptization with nitric acid. The synthesis procedure strongly influenced the formation of the mesophase. The control of sol-gel transition and thermal decomposition allowed the synthesis of two forms of titanium dioxide — anatase and rutile. TiO2 dioxide was chosen for study for its interesting properties and the interest in which it may bring to the domain of chemical and mechanical high resistance of inorganic membranes. Deposit thin layers were created with a sol prepared with the destabilization of a colloidal solution process. These results show the great potentiality of inorganic membranes and new applications to be taken into account, particulary wastewater treatement and gas separation. Crossflow microfiltration and ultrafiltration are becoming of increasing interest to produce drinking water. X-ray diffraction, thermogravimetric analysis, and infrared spectroscopy have been used to characterize the mesostructure formed at room temperature as well as calcined at different temperatures. The pore diameters, porous volume and surface area of the materials measured from N2 adsorption–desorption isotherms indicate the variation according to temperature. Scanning electron microscopy is used to characterize membrane morphology, that is, the absence of cracks, the thickness and its homogeneity along the support. Some experimental results on the synthesis and characterization of inorganic membranes are presented. Keywords: Membrane; Porous; Sol-gel
*Corresponding author. Presented at the EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.580
532
N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
1. Introduction A common element in many industrial processes is the necessity to separate the products from reactants and waste components. Membranes are becoming attractive separators because they offer several advantages relative to other processes. One of the most widely used methods for the preparation of porous membranes with different structures in the last decades is sol-gel processing [1–7]. With this method a network of partially hydrolyzed and polycondensed monomers diluted in the solvent is formed. A porous structure is obtained upon layer application (coating), drying and calcination. While the sol-gel process may appear to be a simple operation, many variables can influence the quality of the final product [1]. These variables include the choice of solvent, whether acid or base catalysis is employed, and use of stabilizing agents. In this study a procedure for the preparation of titania membranes by sol-gel technique is reported. The influence of several processing variables on particle size and sol stability was investigated. Additionally, the thermal evolution, structural characteristics of calcined powders and films were studied.
2. Experimental 2.1. Synthesis Titanium dioxide was prepared [2,4] by the hydrolysis and condensation of titanium isopropoxide Ti(OC3H7)4 (Aldrich) and was used as precursor; nitric acid (solution 65%, Merck) was employed as an acid catalyst to promote the formation of polymeric sols and ethanol as solvent. A specific amount of Ti(OC3H7)4 was dissolved in an equal volume of ethanol and mixed with deionized water. The synthesis difficulty is to eliminate the effects of phase separation and precipitation.
Fig. 1. Preparation steps of TiO2 oxide powder. Ti(OiPr)4: titanium isopropoxide = Ti(OC3H7)4.
Since membranes are manufactured from gels, it is extremely important to define the conditions required to form a gel. For a given titanium concentration, the hydrolysis condensation reactions were mainly governed by two parameters, namely the initial hydrolysis ratio, [H2O]/[Ti], and the inhibitor ratio, [H+]/[Ti]. Controlled hydrolysis of alkoxide precursors is the key to making thin films using the sol-gel process. The scheme of preparation is illustrated in Fig. 1. The synthesis mode was chosen as the maximum of connection formation: Ti-O-Ti. Thus, in a first step the idea is to use hydrolysis water by forming a maximum of connection of Ti-OH groupings. The overall hydrolysis and condensation reactions are illustrated below. C Hydrolysis
C Condensation
C Further condensation
N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
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C Overall reaction
The hydrolysis reaction of titanium isopropoxide. 2.2. Characterization Powder XRD data were carried out with a Philips PW 1830 diffractometer with CuKα radiation (λ =1.5406 Å). TGA curves were obtained in flowing nitrogen on a TGA 2050 with a temperature increasing rate of 10EC/min. N2 adsorption–desorption isotherms were recorded on a Micromeritics ASAP 2010 automated sorption analyzer. The samples were outgassed at 150EC before the analysis. The FTIR spectra of samples were obtained using the KBr wafer technique. The Barret-Joyner-Halende (BJH) method was used to determine pore size. Scanning electron microscopy was used to characterize membrane morphology, that is, the thickness and homogeneity along the support.
Fig. 2. Infra-red spectra of powders treated at different temperatures.
3. Results 3.1. Structural characterization Using absorption infra-red spectroscopy, infra-red spectra were recorded for every powder treated at different temperatures (Fig. 2). Infrared studies show that some OH remains in the membrane. The Ti-OH groups may be characterized by an absorption band situated at 500 cm!1. Desydroxylation starts in the early stages of the thermal treatment and continues up to 400EC. These OH groups are responsible for the membrane reactivity. Above a temperature of 450EC, we notice only the bands characterized by a metallic connection — Ti-O-Ti and O-Ti-O or Ti-OH between 500 and 1200 cm!1.
Fig. 3. Simultaneous TG/DTA curves of the powder obtained.
3.2. Thermogravimetric analysis The TGA of the as-synthesized samples under N2 showed the loss of water below 120EC and was completely removed at about 350EC (Fig. 3). The analysis of the as-synthesized sample revealed 30.52% total weight loss on heating to 400EC. The first effect is attributed to the release of adsorbed water, the second to dehydroxylation of the surface and removal of little residual. Fig. 3 also shows the differential thermal analysis curve. The endothermic peaks observed
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N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
Fig. 4. Temperature programmed XRD spectra of powders at different temperatures.
at 120EC and 375EC can be attributed, respectively, to water loss and decomposition of the organic groups. The exothermic peaks appearing at 410EC and 503EC are assigned to a progressive transformation of the anatase to rutile forms of TiO2. 3.3. By X-ray diffraction Temperature programmed XRD spectra are shown in Fig .4. It can be seen that the material retains its amorphous structure until 60EC, while at 200EC, a phase transformation into the crystalline anatase phase occurred. Above 200°C we obtained a mixture of two forms — anatase and rutile. Anatase and rutile are the two forms of titanium dioxide produced in the laboratory at atmospheric pressure. It is a transformation of anatase to rutile after thermal treatment at 600EC for 3 h, resulting in a loss of surface area and porosity. X-ray diffraction measurements are summarized in Table 1. We notice structural evolution from anatase to rutile with the increasing of the calcined temperature.
Table 1 Structural evolution from anatase to rutile Temp., EC
Anatase, %
60 9 200 350 400 500 600
Progressive crystallization 99.16 68.75 45.65 30.12 3.76
Rutile, % 0 0.83 31.25 54.34 69.87 96.24
4. Textural characterization by nitrogen adsorption–desorption N2-sorption isotherms were recorded for 350, 400, 500 and 600EC calcined mesoporous structured samples and are shown in Fig. 5. The shape of these curves is typical of mesoporous materials. Figs. 6–8 show the specific surface area, the pore volume and the pore size data measured from N2 adsorption–desorption isotherms for the mesostructured titanium oxide samples assynthesized and heat-treated at various temperatures. With the increasing of the calcined
N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537
Fig. 5. Adsorption–desorption isotherm of sample using N2 at 77 K.
Fig. 6. Evolution of specific surface as a function of the temperature.
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Fig. 8. Evolution of pore diameters as a function of the temperature.
7. The mesostructured produced after calcination at 350EC has a Brunuer–Emmett–Teller surface area of 101 m2/g. Between 350EC and 600EC, the specific surface area decreased from 101 m2/g to 2 m2/g. At the same time, the pore size increased from 24 Å to more than 36 Å, respectively. These results indicated that as the calcination temperature increased, the number of pores decreased as a result of sintering while at the same time the pore size increased. This is a very interesting fact because with one sol, pore diameters are adjusted as a function of temperature, and the whole UF range can be screened.
5. Deposits of thin porous layers
Fig. 7. Evolution of porous volume as a function of the temperature.
temperature, the specific surface area and the pore volume began to decrease as shown in Figs. 6 and
Titania layers were deposited by a coating technique on mesoporous α-alumina substrate. The dried membranes were calcined at 400EC and 600EC, respectively, during 3 h. Scanning electron micoscopy observation showed homogeneous layers without cracking (Figs. 9–12. It was found that the layer thickness could be varied in the range 1.85 µm to 1.25 µm, respectively, at 400EC and 600EC. The physicochemical phenomena have to be well controlled to obtain porous layers with good characteristics.
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Fig. 9. Micrography of the layer obtained at 400EC.
Fig. 10. Cross section of the layer obtained at 400EC.
Fig. 11. Micrography of the layer obtained at 600EC.
Fig. 12. Cross section of the layer obtained at 600EC.
6. Conclusions
increase of the pore diameters according to the temperature while porous volume and specific surface decrease. The control of a sol-gel transition as well as a thin layer deposit on a porous α-alumina support contributes to the realization of UF inorganic membranes.
The synthesis of mesoporous titania using titanium isopropoxide as precursor under acidic conditions was investigated. The procedure for titania sol synthesis was optimized. It was found that polymeric sols could be obtained only when the hydrolysis conditions were strictly controlled. A crystallization of the anatase form begins at 60EC. Above this temperature, there is a coexistence of two forms — anatase and rutile — and a transformation of TiO2 anatase to the rutile at 600EC. We note a slight
References [1] C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-cryst. Solids, 100 (1988) 65–76.
N. Agoudjil, T. Benkacem / Desalination 206 (2007) 531–537 [2] M.A. Anderson, M.J. Gieselmann and Q. Xu, J. Membr. Sci., 39(3) (1988) 243–258. [3] J. Livage, M. Henry and C. Sanchez, Prog. Solid St. Chem., 18 (1988) 259. [4] Q. Xu and M.A. Anderson, J. Mat. Res., 6(5) (1991) 1073–1081. [5] K.C. Chen, T. Tsuchiya and J.D. Mackenzie, J. Noncryst. Solids, 81 (1986) 227. [6] C.J. Brinker and G.W. Scherer, Sol-gel Science, Academic Press, New York, 1990.
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[7] J. Kims and Y.S. Lin, J. Mat. Sci., 75 (1998) 39. [8] J. Blanchard, S. Barboux-Doeuff, J. Maquet and C. Sanchez, New J. Chem., 19 (1995) 929. [9] C. Guizard, A. Julbe and A. Ayral, J. Mat. Sci., 55 (1999) 9. [10] P.M. Biesheuvel and H. Verweij, J. Membr. Sci., 156 (1999) 141. [11] J. Sekulic, A. Magrasso, J.E. Ten Elshof and D.H.A. Blank, Micropor. Mesopor. Mat., 72 (2004) 49.