- Email: [email protected]
Microporous silica and doped silica membrane for alcohol dehydration by pervaporation
DESALINATION Desalination 148 (2002) 19-23
ELSEVIER
www.elsevier.com/locate/desal
Microporous silica and doped silica membrane for alcohol dehydration by pervaporation J. SekuliC*, M.W.J. Luiten, J.E. ten Elshof, N.E. Benes, K. Keizer Laboratory of Inorganic Materials Science, MESA’ Research institute, University of Twente, PO Box 2 17, 7500 AE, Enschede, The Netherlands Tel. +31 (53) 489-2695; Fax +31 (53) 489-4683; email: [email protected] Received 1 February 2002; accepted 15 February 2002
Abstract The aim of this work is the development of inorganic membranes that will enable broad application of pervaporation/vapour permeation technology in the chemical industry. This can be achieved by improvement of the existing microporous membranes and the development of new types with enhanced thermochemical stability and separation characteristics. The materials in the system, SiO,-Al,O,-TiO,-ZrO-MgO, were investigated with respect to their chemical stability and pervaporation performance in alcohol dehydration processes. It was found that, depending on the nature and amount of dopant, composite membranes with improved pervaporation characteristics and chemical stability were obtained. Keywords:
Ceramic membranes; Pervaporation; Chemical stability; Titania; Doped silica
1. Introduction Pervaporation and vapour permeation are separation technologies in which one of the components of a liquid mixture (pervaporation) or of a vapour phase (vapour permeation) is separated from the feed mixture by selective evaporation (pervaporation) or gas transport (vapour permeation) through a membrane. In principle, these technologies have better separa-
*Corresponding
author.
Presented at the International July 7-12, 2002.
Congress on Membranes
tion capacity and energy efficiency than competing distillation, adsorption and extraction technologies and their application may lead to energy reductions of 40-60% [l]. However, the application of pervaporation/vapour permeation in the chemical industry has been, up till now, restricted due to severe limitations of the current generation of commercially available membranes, i.e., their low chemical and thermal stability, insufficient selectivity and low flux [2-51. The pervaporation characteristics of a membrane are usually expressed in terms of flux and and Membrane
Processes
001 l-9164/02/!& See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO0 1 1-9 164(02)00647-
I
(ICON),
Toulouse, France,
20
J. Sekulid et al. / Desalination 148 (2002) 19-23
separation factor. The separation factor is defined as: c1 =
Yi”i
YjI’j
in which y and x are the fractions of components i andj in the permeate and feed, respectively. At present, the most mature membrane system is the asymmetric composite membrane aalumina-y-alumina-silica [6], but their operation window is limited, particularly at a combination of high temperature and high pH [8]. Both mesoporous y-alumina and microporous silica show a limited chemical stability at extreme pH values. A promising material to replace y-alumina is mesoporous titania. Amorphous microporous silica has a limited chemical stability in alkaline media. Incorporation of a second component such as A&O,, TiO, or ZrO, has shown to improve the chemical stability in glass [9]. The incorporation of these components in sol-gel-derived silica is expected to have the same effect [lo]. In the present paper, various composite membrane systems were investigated with respect to their chemical stability and pervaporation performance in alcohol dehydration processes.
2. Membrane development
and performance
2.1. Membranepreparation andcharacterisation
A particulate titania sol was prepared from titanium-isopropoxide, as is described in more detail by Kumar et al. [ 1l] A mesoporous titania layer was applied by dip-coating the titania sol onto the a-alumina support (disc-shaped, diameter 39 mm, thickness 2 mm, mean pore radius 100 nm, porosity -50%) [12]. As binders, both hydroxy-propyl cellulose and polyvinyl-alcohol were used. It was found to be difficult to obtain a crack-free titania layer. This is caused by the fact that a partial anatase-to-rutile phase
transformation occurs upon calcination ofthe layer at 450°C [ 11,131. The transformation process is accompanied by considerable grain and pore growth. As suggested by Kumar [8], there are two basic methods to avoid phase transformation: l retarding phase transformation and grain growth to a temperature above the calcination temperature, l avoiding phase transformation by direct synthesis of rutile-phase titania. Based on the first approach, Kumar [l l] observed that the presence of alumina in a membrane retarded the phase transformation temperature from 400°C for pure unsupported porous titania to 700°C for the titania phase of a titania-alumina unsupported composite membrane. The same approach is applied in this work. Titania-alumina composite membranes were synthesised by dip-coating two-component sols that were made by mixing boehmite [14] and titania sols in the appropriate ratio. All membranes were dried for 3 h at 40°C and a relative humidity of 60%, and calcined for 3 h at 450°C. It was found that defect-free layers are obtained when 30-50% of alumina is present in the layer. If necessary, the coating, drying and calcination steps were repeated in order to repair defects present in the first layer. Double-coated, mesoporous composite titania/alumina layers with an average thickness of 2 urn were obtained (Fig. 1). The mean pore diameters measured with the permporometry technique [ 151were 2.0-2.5 nm. Fig. 2 shows the pore size distributions in mesoporous layers of g-alumina and titania/30% alumina. Polymeric silica and doped silica sols were prepared by acid-catalysed hydrolysis and condensation of alkoxides in alcohol. It has been well established that silicon alkoxides react much slower with water than the corresponding titanium-, zirconium- or aluminum alkoxides [ 161.In order to circumvent inhomogeneities in the sol upon hydrolysis of a mixture of precursors with
21
J. Sekulid et al. /Desalination 148 (2002) 19-23
0
2
4
6
II
10
Kelvinm&us,nm
Fig. 1. SEM micrograph of the cross section of the composite titania/alumina layer.
largely different reactivities, the non-silicon metal alkoxide was added only after the silica precursor has already been partially hydrolysed. Following this procedure, stable polymeric sols were obtained that resulted in amorphous microporous materials. Microporous membranes were prepared from the sols by dip-coating, followed by calcination under static air for 3 h at 400°C. Unsupported microporous material was prepared by pouring the dip solution in the petri dish, followed by overnight drying and a calcination step similar to the procedure for supported membranes. XRD examination of the material showed that the material remained amorphous when the dopant level did not exceed 30 mol% of the total silica content. The unsupported doped silica material did not adsorb nitrogen (at 77”C), indicating that nitrogen is too large to enter a microporous matrix [17]. This indicates a truly micro-porous structure with pores smaller than the kinetic diameter of nitrogen, 3.65 A. 2.2. Chemical stability The chemical stability of amorphous and the doped silicas were investigated by bility tests with the unsupported material at temperature [IS]. The pH of the solution
silica soluroom was
Fig. 2. Pore size distribution of the y-alumina titania/alumina mesoporous layer.
and
adjusted with nitric acid and/or ammonium hydroxide. The concentration of dissolved silica in the solution after 120 h immersion was measured using atomic adsorption spectroscopy (AAS), and from this value the total weight loss was calculated. The material was defined to be stable when the amount of dissolved silica in the solution was lower than or close to the AAS detection limit, which is 0.3 mg/l and corresponds to the weight loss of the 0.07%. The results are presented in Fig. 3. These show that the addition of alumina has a negative effect on the chemical stability, while zirconia and titania give clear improvements at high pH. It was found that while pure silica is stable (negligibly soluble) in the pH range 2-8, addition of 10 mol% of zirconia or titania leads to a stable material in the pH range 2-10. It should be noted that no experiments were performed at a pH below 2. The addition of 30 mol% of the same titania or zirconia dopants led to severe destabilisation of the material. Although the material remains amorphous, the polymeric structure is obviously destroyed at these dopant levels. 2.3. Pervaporation
characteristics
The pervaporation performances of composite membrane systems with various compositions
22
J. SekuliC et al. /Desalination
148 (2002) 19-23
Table 1 Pervaporation fluxes and separation factors at 80°C Total flux, Water kg m-’ h“ cont. in permeate, %
Si/ 1O%Ti Si/lO%Zr
Si/30%Ti Sil3O%Zr Si/lO%Al Si/l O%Al, Mg
Fig. 3. Schematic diagram showing the chemical stability range of several microporous structures.
were measured in a dead-end pervaporation unit. The permeate composition was analysed with refractive index measurements. Pervaporation measurements were conducted with several alcohol/water feed mixtures, at a temperature of 75-SOY!, and a feed pressure of 2.5 bar. A selection of pervaporation data is listed in Table 1. Obviously, both the nature and the amount of dopant have a significant influence on the membrane microstructure, and consequently also on its pervaporation performance. It can be noticed that addition of the 10% zirconia and alumina to silica leads to a membrane with reasonably high separation properties. On the other hand, 30% of dopant significantly decreased selectivity, most likely because no microporous structure exists. The flux values are similar for all compositions, except for the case of alumina where the dopant is thought to lead to significant densilication of the microporous structure.
a
90% i-propanoll 10% water: Si Si/Zr 10 mol% Si/Zr 30 mol% Si/Ti 10 mol% Si/AI 10 mol% Si/(Al, Mg) 10 mol%
0.65 0.86 0.67 0.78 0.08 0.3 1
89 97 75 98 96 91
73 3.0x102 27 4.0x102 2.1x102 90
95% 5% Si Si/Zr Si/Ti
0.76 0.61 1.16
9.5 95 91
3.6~ lo2 3.6~ lo2 1.9x102 __
2-butanoll water: 10 mol% 10 mol%
3. Conclusions Mesoporous composite titanialy-alumina layers with mean pore diameters of about 2 nm were prepared. As separative microporous top layer, doped silica layers were deposited on the mesoporous composite layers. Doping silica with zirconium or titanium was seen to lead to a moderate improvement of the chemical stability, while cations with a lower valency led to a reduction of the chemical stability of the silica matrix. Similarly, microporous silica doped with small amounts of zirconium showed improved pervaporation performance compared with undoped silica material. The flux was significantly reduced when Al is present, while other systems showed fluxes similar to silica itself. Further investigations are necessary for better understanding ofthe effect of doping on microstructure and pervaporation performance. Acknowledgements Financial support of the Commission
of the
J. SekuliC et al. /Desalination
European Communities in the framework of the Growth Programme, INMEMPERV (High-performance microporous inorganic membranes for pervaporation and vapour permeation technology) contract no. G 1RD-CT- 1999-00076, is gratefully acknowledged.
References VI
PI
[31
[41
[51
El
[71
PI
X. Feng and R.Y.M. Huang, Liquid separation by membrane pervaporation; a review, Ind. Engineering Chem. Res., 36(4) (1997) 1048-1066. G.P. Fotou, Y.S. Lin and S.E. Pratsinis, Hydrothermal stability of pure and modified microporous silica membranes, J. Mater. Sci., 30 (1995) 2803-2808. H. Imai, H. Morimoto, A. Tominaga and H. Hirashima, Structural changes in sol-derived SiO, and TiO, films by exposure to water vapour, J. Sol-Gel Sci. Tech., 10 (1997) 45-54. C.-H. Chang, R. Gopalan and Y.S. Lin, A comparative study on thermal and hydrothermal stability of alumina, titania and zirconia membranes, J. Membr. Sci., 91 (1994) 27-45. R. W. can Gemert and F.P. Cuperus, Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation, J. Membr. Sci., 105 (1995) 287-291. H.M. van Veen, Y.C. vanDelft, C.W.R. Engelen and P.P.A.C. Pex, Dewatering of organics by pervaporation with silica membranes, Sep. Purification Technol., 22-23 (2001) 361-366. N. Benes, A. Nijmeijer and H. Verweij, Microporous silica membranes, in: N.K. Kanellopoulos, ed., Recent Advanced in Gas Separation by Microporous Ceramic Membranes, Elsevier, Amsterdam, 2000. R.K. Iler, The Chemistry of Silica; Solubility, Polymerisation, Colloid and Surface Properties and Biochemistry, Wiley, New York, 1979.
I48 (2002) 19-23
23
[91 T. Yazawa, H. Tanaka, H. Naakmichi and T. Yokoyama, Preparation ofwater and alkali durable porous glass membrane coated on porous alumina tubing by sol-gel method, J. Membr. Sci., 60 (1991) 307-3 17. [lOI R.S.A. de Lange, Microporous sol-gel derived ceramic membranes for gas separation, PhD Thesis, University of Twente, 1993. Pll K-N.P. Kumar, Nanostructured ceramic membranes; layer and texture formation, PhD Thesis, University of Twente, 1993. WI P.M. Biesheuvel and H. Verweij, Design of ceramic membrane supports: permeability, tensile strength, and stress, J. Membr. Sci., 156 (1996) 250-260. I?31 Y.S. Lin, C.H. Chang and R. Gopahm, Improvement of thermal stability of porous nanostructured ceramic membranes, Ind. Eng.Chem. Res., 33 (1994) 860870. P41 R.J. van Vuren, B.C. Bonekamp, K. Keizer, R.J.R. Ulhom, H.J. Veringa and A.J. Burggraaf, foramtion of ceramic alumina membranes for gas separation, High Tech. Ceram. (1987) 2235-2245. P51 F.P. Cuperus, D. Bargeman and C.A. Smolders, Permporometry. The determination of the size distribution of active pores in UF membranes, J. Membr. Sci., 71 (1992) 57-67. [I61 R.S.A. de Lange, K.-N.P. Kumar, J.H.A. Hekkink, G.M.H. Van der Velde, K. Keizer, A.J. Burggraaf, W.H. Dokter, H.F. Van Garderen and T.P.M. Beelen, Microporous SiOz and SiO,/MO, (M=Ti, Zr, Al) for ceramic membrane applications, J. Sol-Gel. Sci., 2 (1994) 489-495. J.H.A.HekkinkandA.J. P71 K.Keizer,R.S.A.DeLange, Burggraaf, Microstructural properties of nonsupported microporous ceramic membrane top-layers obtained by the sol-gel process, J. Non-Crystalline Solids, 195(3) (1996) 203-217. [I81 F. Shojao and T.A. M&ttyla, Structural stability of yttria doped zirconia membranes in acid and basic aqueous solution, J. Eur. Ceram. Sot.. 21(2001) 3744.
ELSEVIER
www.elsevier.com/locate/desal
Microporous silica and doped silica membrane for alcohol dehydration by pervaporation J. SekuliC*, M.W.J. Luiten, J.E. ten Elshof, N.E. Benes, K. Keizer Laboratory of Inorganic Materials Science, MESA’ Research institute, University of Twente, PO Box 2 17, 7500 AE, Enschede, The Netherlands Tel. +31 (53) 489-2695; Fax +31 (53) 489-4683; email: [email protected] Received 1 February 2002; accepted 15 February 2002
Abstract The aim of this work is the development of inorganic membranes that will enable broad application of pervaporation/vapour permeation technology in the chemical industry. This can be achieved by improvement of the existing microporous membranes and the development of new types with enhanced thermochemical stability and separation characteristics. The materials in the system, SiO,-Al,O,-TiO,-ZrO-MgO, were investigated with respect to their chemical stability and pervaporation performance in alcohol dehydration processes. It was found that, depending on the nature and amount of dopant, composite membranes with improved pervaporation characteristics and chemical stability were obtained. Keywords:
Ceramic membranes; Pervaporation; Chemical stability; Titania; Doped silica
1. Introduction Pervaporation and vapour permeation are separation technologies in which one of the components of a liquid mixture (pervaporation) or of a vapour phase (vapour permeation) is separated from the feed mixture by selective evaporation (pervaporation) or gas transport (vapour permeation) through a membrane. In principle, these technologies have better separa-
*Corresponding
author.
Presented at the International July 7-12, 2002.
Congress on Membranes
tion capacity and energy efficiency than competing distillation, adsorption and extraction technologies and their application may lead to energy reductions of 40-60% [l]. However, the application of pervaporation/vapour permeation in the chemical industry has been, up till now, restricted due to severe limitations of the current generation of commercially available membranes, i.e., their low chemical and thermal stability, insufficient selectivity and low flux [2-51. The pervaporation characteristics of a membrane are usually expressed in terms of flux and and Membrane
Processes
001 l-9164/02/!& See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO0 1 1-9 164(02)00647-
I
(ICON),
Toulouse, France,
20
J. Sekulid et al. / Desalination 148 (2002) 19-23
separation factor. The separation factor is defined as: c1 =
Yi”i
YjI’j
in which y and x are the fractions of components i andj in the permeate and feed, respectively. At present, the most mature membrane system is the asymmetric composite membrane aalumina-y-alumina-silica [6], but their operation window is limited, particularly at a combination of high temperature and high pH [8]. Both mesoporous y-alumina and microporous silica show a limited chemical stability at extreme pH values. A promising material to replace y-alumina is mesoporous titania. Amorphous microporous silica has a limited chemical stability in alkaline media. Incorporation of a second component such as A&O,, TiO, or ZrO, has shown to improve the chemical stability in glass [9]. The incorporation of these components in sol-gel-derived silica is expected to have the same effect [lo]. In the present paper, various composite membrane systems were investigated with respect to their chemical stability and pervaporation performance in alcohol dehydration processes.
2. Membrane development
and performance
2.1. Membranepreparation andcharacterisation
A particulate titania sol was prepared from titanium-isopropoxide, as is described in more detail by Kumar et al. [ 1l] A mesoporous titania layer was applied by dip-coating the titania sol onto the a-alumina support (disc-shaped, diameter 39 mm, thickness 2 mm, mean pore radius 100 nm, porosity -50%) [12]. As binders, both hydroxy-propyl cellulose and polyvinyl-alcohol were used. It was found to be difficult to obtain a crack-free titania layer. This is caused by the fact that a partial anatase-to-rutile phase
transformation occurs upon calcination ofthe layer at 450°C [ 11,131. The transformation process is accompanied by considerable grain and pore growth. As suggested by Kumar [8], there are two basic methods to avoid phase transformation: l retarding phase transformation and grain growth to a temperature above the calcination temperature, l avoiding phase transformation by direct synthesis of rutile-phase titania. Based on the first approach, Kumar [l l] observed that the presence of alumina in a membrane retarded the phase transformation temperature from 400°C for pure unsupported porous titania to 700°C for the titania phase of a titania-alumina unsupported composite membrane. The same approach is applied in this work. Titania-alumina composite membranes were synthesised by dip-coating two-component sols that were made by mixing boehmite [14] and titania sols in the appropriate ratio. All membranes were dried for 3 h at 40°C and a relative humidity of 60%, and calcined for 3 h at 450°C. It was found that defect-free layers are obtained when 30-50% of alumina is present in the layer. If necessary, the coating, drying and calcination steps were repeated in order to repair defects present in the first layer. Double-coated, mesoporous composite titania/alumina layers with an average thickness of 2 urn were obtained (Fig. 1). The mean pore diameters measured with the permporometry technique [ 151were 2.0-2.5 nm. Fig. 2 shows the pore size distributions in mesoporous layers of g-alumina and titania/30% alumina. Polymeric silica and doped silica sols were prepared by acid-catalysed hydrolysis and condensation of alkoxides in alcohol. It has been well established that silicon alkoxides react much slower with water than the corresponding titanium-, zirconium- or aluminum alkoxides [ 161.In order to circumvent inhomogeneities in the sol upon hydrolysis of a mixture of precursors with
21
J. Sekulid et al. /Desalination 148 (2002) 19-23
0
2
4
6
II
10
Kelvinm&us,nm
Fig. 1. SEM micrograph of the cross section of the composite titania/alumina layer.
largely different reactivities, the non-silicon metal alkoxide was added only after the silica precursor has already been partially hydrolysed. Following this procedure, stable polymeric sols were obtained that resulted in amorphous microporous materials. Microporous membranes were prepared from the sols by dip-coating, followed by calcination under static air for 3 h at 400°C. Unsupported microporous material was prepared by pouring the dip solution in the petri dish, followed by overnight drying and a calcination step similar to the procedure for supported membranes. XRD examination of the material showed that the material remained amorphous when the dopant level did not exceed 30 mol% of the total silica content. The unsupported doped silica material did not adsorb nitrogen (at 77”C), indicating that nitrogen is too large to enter a microporous matrix [17]. This indicates a truly micro-porous structure with pores smaller than the kinetic diameter of nitrogen, 3.65 A. 2.2. Chemical stability The chemical stability of amorphous and the doped silicas were investigated by bility tests with the unsupported material at temperature [IS]. The pH of the solution
silica soluroom was
Fig. 2. Pore size distribution of the y-alumina titania/alumina mesoporous layer.
and
adjusted with nitric acid and/or ammonium hydroxide. The concentration of dissolved silica in the solution after 120 h immersion was measured using atomic adsorption spectroscopy (AAS), and from this value the total weight loss was calculated. The material was defined to be stable when the amount of dissolved silica in the solution was lower than or close to the AAS detection limit, which is 0.3 mg/l and corresponds to the weight loss of the 0.07%. The results are presented in Fig. 3. These show that the addition of alumina has a negative effect on the chemical stability, while zirconia and titania give clear improvements at high pH. It was found that while pure silica is stable (negligibly soluble) in the pH range 2-8, addition of 10 mol% of zirconia or titania leads to a stable material in the pH range 2-10. It should be noted that no experiments were performed at a pH below 2. The addition of 30 mol% of the same titania or zirconia dopants led to severe destabilisation of the material. Although the material remains amorphous, the polymeric structure is obviously destroyed at these dopant levels. 2.3. Pervaporation
characteristics
The pervaporation performances of composite membrane systems with various compositions
22
J. SekuliC et al. /Desalination
148 (2002) 19-23
Table 1 Pervaporation fluxes and separation factors at 80°C Total flux, Water kg m-’ h“ cont. in permeate, %
Si/ 1O%Ti Si/lO%Zr
Si/30%Ti Sil3O%Zr Si/lO%Al Si/l O%Al, Mg
Fig. 3. Schematic diagram showing the chemical stability range of several microporous structures.
were measured in a dead-end pervaporation unit. The permeate composition was analysed with refractive index measurements. Pervaporation measurements were conducted with several alcohol/water feed mixtures, at a temperature of 75-SOY!, and a feed pressure of 2.5 bar. A selection of pervaporation data is listed in Table 1. Obviously, both the nature and the amount of dopant have a significant influence on the membrane microstructure, and consequently also on its pervaporation performance. It can be noticed that addition of the 10% zirconia and alumina to silica leads to a membrane with reasonably high separation properties. On the other hand, 30% of dopant significantly decreased selectivity, most likely because no microporous structure exists. The flux values are similar for all compositions, except for the case of alumina where the dopant is thought to lead to significant densilication of the microporous structure.
a
90% i-propanoll 10% water: Si Si/Zr 10 mol% Si/Zr 30 mol% Si/Ti 10 mol% Si/AI 10 mol% Si/(Al, Mg) 10 mol%
0.65 0.86 0.67 0.78 0.08 0.3 1
89 97 75 98 96 91
73 3.0x102 27 4.0x102 2.1x102 90
95% 5% Si Si/Zr Si/Ti
0.76 0.61 1.16
9.5 95 91
3.6~ lo2 3.6~ lo2 1.9x102 __
2-butanoll water: 10 mol% 10 mol%
3. Conclusions Mesoporous composite titanialy-alumina layers with mean pore diameters of about 2 nm were prepared. As separative microporous top layer, doped silica layers were deposited on the mesoporous composite layers. Doping silica with zirconium or titanium was seen to lead to a moderate improvement of the chemical stability, while cations with a lower valency led to a reduction of the chemical stability of the silica matrix. Similarly, microporous silica doped with small amounts of zirconium showed improved pervaporation performance compared with undoped silica material. The flux was significantly reduced when Al is present, while other systems showed fluxes similar to silica itself. Further investigations are necessary for better understanding ofthe effect of doping on microstructure and pervaporation performance. Acknowledgements Financial support of the Commission
of the
J. SekuliC et al. /Desalination
European Communities in the framework of the Growth Programme, INMEMPERV (High-performance microporous inorganic membranes for pervaporation and vapour permeation technology) contract no. G 1RD-CT- 1999-00076, is gratefully acknowledged.
References VI
PI
[31
[41
[51
El
[71
PI
X. Feng and R.Y.M. Huang, Liquid separation by membrane pervaporation; a review, Ind. Engineering Chem. Res., 36(4) (1997) 1048-1066. G.P. Fotou, Y.S. Lin and S.E. Pratsinis, Hydrothermal stability of pure and modified microporous silica membranes, J. Mater. Sci., 30 (1995) 2803-2808. H. Imai, H. Morimoto, A. Tominaga and H. Hirashima, Structural changes in sol-derived SiO, and TiO, films by exposure to water vapour, J. Sol-Gel Sci. Tech., 10 (1997) 45-54. C.-H. Chang, R. Gopalan and Y.S. Lin, A comparative study on thermal and hydrothermal stability of alumina, titania and zirconia membranes, J. Membr. Sci., 91 (1994) 27-45. R. W. can Gemert and F.P. Cuperus, Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation, J. Membr. Sci., 105 (1995) 287-291. H.M. van Veen, Y.C. vanDelft, C.W.R. Engelen and P.P.A.C. Pex, Dewatering of organics by pervaporation with silica membranes, Sep. Purification Technol., 22-23 (2001) 361-366. N. Benes, A. Nijmeijer and H. Verweij, Microporous silica membranes, in: N.K. Kanellopoulos, ed., Recent Advanced in Gas Separation by Microporous Ceramic Membranes, Elsevier, Amsterdam, 2000. R.K. Iler, The Chemistry of Silica; Solubility, Polymerisation, Colloid and Surface Properties and Biochemistry, Wiley, New York, 1979.
I48 (2002) 19-23
23
[91 T. Yazawa, H. Tanaka, H. Naakmichi and T. Yokoyama, Preparation ofwater and alkali durable porous glass membrane coated on porous alumina tubing by sol-gel method, J. Membr. Sci., 60 (1991) 307-3 17. [lOI R.S.A. de Lange, Microporous sol-gel derived ceramic membranes for gas separation, PhD Thesis, University of Twente, 1993. Pll K-N.P. Kumar, Nanostructured ceramic membranes; layer and texture formation, PhD Thesis, University of Twente, 1993. WI P.M. Biesheuvel and H. Verweij, Design of ceramic membrane supports: permeability, tensile strength, and stress, J. Membr. Sci., 156 (1996) 250-260. I?31 Y.S. Lin, C.H. Chang and R. Gopahm, Improvement of thermal stability of porous nanostructured ceramic membranes, Ind. Eng.Chem. Res., 33 (1994) 860870. P41 R.J. van Vuren, B.C. Bonekamp, K. Keizer, R.J.R. Ulhom, H.J. Veringa and A.J. Burggraaf, foramtion of ceramic alumina membranes for gas separation, High Tech. Ceram. (1987) 2235-2245. P51 F.P. Cuperus, D. Bargeman and C.A. Smolders, Permporometry. The determination of the size distribution of active pores in UF membranes, J. Membr. Sci., 71 (1992) 57-67. [I61 R.S.A. de Lange, K.-N.P. Kumar, J.H.A. Hekkink, G.M.H. Van der Velde, K. Keizer, A.J. Burggraaf, W.H. Dokter, H.F. Van Garderen and T.P.M. Beelen, Microporous SiOz and SiO,/MO, (M=Ti, Zr, Al) for ceramic membrane applications, J. Sol-Gel. Sci., 2 (1994) 489-495. J.H.A.HekkinkandA.J. P71 K.Keizer,R.S.A.DeLange, Burggraaf, Microstructural properties of nonsupported microporous ceramic membrane top-layers obtained by the sol-gel process, J. Non-Crystalline Solids, 195(3) (1996) 203-217. [I81 F. Shojao and T.A. M&ttyla, Structural stability of yttria doped zirconia membranes in acid and basic aqueous solution, J. Eur. Ceram. Sot.. 21(2001) 3744.