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γ Alumina membranes grafting by organosilanes and its application to the separation of solvent mixtures by pervaporation
Separation and Purification Technology 32 (2003) 175 /179 www.elsevier.com/locate/seppur
g Alumina membranes grafting by organosilanes and its application to the separation of solvent mixtures by pervaporation S. Alami Younssi a, A. Iraqi a, M. Rafiq a, M. Persin b,*, A. Larbot b, J. Sarrazin b b
a Laboratoire des mate´riaux et protection de l’environnement, Faculte´ des Sciences, Dhar Mehraz, Fes, Morocco Institut Europe´en des Membranes (IEM), UMR 5635 CNRS ENSCM UMII, 1919 Route de Mende, 34293 Montpellier, Cedex 5, France
Abstract The selectivity and the membrane flux depend on the textural and also on the chemical composition of the materiel used to prepare the membrane. The surface properties of g alumina membranes prepared by sol /gel route can be modified by grafting mono-, di- and tri-functional organosilanes ( /OCH3, /OC2H5, /Cl) in the goal to change the selectivity of the membrane towards the chemical solutes. The characterization of the different membranes showed that the grafting depends on the nature of the silanes used and is achieved with the multifunctional silanes. The modification of the surface properties of the membrane prepared after grafting has been confirmed by pervaporation tests on different binary mixtures of solvents (water/ethanol, methylethylketone/cyclohexane, cyclohexane/ethanol). In these conditions, the flux of the different solvents and the selectivity of the membranes depends clearly on the grafting conditions. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Membrane; g Alumina; Grafting; Silanes
1. Introduction Nowadays the mineral membranes are in constant development and they present interesting potentialities for separations due to their mechanical and chemical resistance [1,2]. In the goal to extend their use to new applications the authors
attempted to modify the selectivity of the filtering layer by grafting chemical moieties [3,4]. The surface treatment consist to modify the hydrophilic/hydrophobic balance of the ceramic layer, it is generally obtained by the reaction of a functionalized silane X /Si /R (R / /OCH3, /OC2H5, /Cl) with the residual hydroxyl groups present on the surface of the oxide.
* Corresponding author. Tel.: /33-4-67-613402; fax: /33-467-613385. E-mail address: [email protected] (M. Persin). 1383-5866/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1383-5866(03)00031-5
176
S. Alami Younssi et al. / Separation and Purification Technology 32 (2003) 175 /179
The main parameters of the grafting which should well controlled are the temperature, the nature of the solvent and the water amount adsorbed on the surface of the oxide [5]. In this work, the membranes properties of modified g alumina membrane are compared before and after grafting using different organosilanes. The modified membranes were tested in the pervaporation of different organic solvent mixtures.
pure silane after corking the bottom of the tube by means of an epoxy resin. After reflux in toluene for 2 h at 110 8C; washing and drying, the membrane was obtained. In both cases the membrane was then treated at 150 8C in air. The method permits the preparation of stable grafted membranes with a good reproducibility.
3. Results 2. Experimental 2.1. Preparation of alumina sol The sol was prepared under stirring by the complete hydrolysis in water of Al(OBu)3, followed by the peptisation of the formed in situ boehmite precipitate with a concentrated nitric acid solution [6]. The evaporation of the solvent leads to the ceramic g alumina powder after grinding and calcination at 450 8C. 2.2. Preparation of g alumina membrane The membrane was prepared by slipcasting at the surface of the inner part of a commercial tubular support tube in a alumina made of a 0.2 mm microfiltration tube with a 5 nm g alumina UF layer deposited inside. The final g alumina layer was dried for 15 min and fired at 450 8C for 1 h [6].
Membrane and powder grafting characterization. 3.1. Reflecting IR spectroscopy The grafting of the membrane surface (Fig. 1)was analysed by reflecting IR spectroscopy. The comparison of the response of the different grafted or ungrafted membranes (Fig. 2) shows that neither signal was observed for the ungrafted membrane nor for the membrane grafted by means of the monofunctional Me3SiCl compound. On the contrary the presence of the C /H band at 3000 cm 1, Si /C band at 1300 cm 1 and Si /O band at 950 cm 1 attested that the functionalization was obtained by means of the di- or tri-functional chloro, ethoxy or methoxy silanes. These results may be explained by the possibility for the multifunctional silane to form a polymerised network by the formation of Si /O /Si bonds.
2.3. Preparation of grafting g alumina powder The grafting method has been already described in former work [5]. The g alumina powder reacts with the organosilanes in dry toluene for 48 h at 110 8C, after filtration, washing with toluene and drying at 150 8C, the grafted powder is obtained. 2.4. Preparation of membrane The grafted membranes were prepared by two methods, the first consisted to immerse the g alumina membrane in a mixture of toluene and silane under reflux at 110 8C for 48 h. The second method consisted to fill the membrane tube with
Fig. 1. SEM of a g alumina membrane (thickness of the nanofiltration layer: 200 nm).
S. Alami Younssi et al. / Separation and Purification Technology 32 (2003) 175 /179
177
3.2. N2 adsorption /desorption The microporous volume and the specific area of the different membranes determined from the N2 adsorption/desorption isotherms are gathered in Table 1. The grafting is attested by the decrease of the specific area and the microporous volume for the grafted membrane by means of the Me2Si(OEt)2 or MeSi(OEt)3 silanes and this effect may be explained by the covering of the pore wall with the grafted moities. 3.3. Application to pervaporation of organic solvent mixtures
Fig. 2. Reflection IR spectra on ungrafted and grafted g alumina powders by different silanes: (a) ungrafted membrane, (b) Me3SiCl, (c) Me2SiCl2, (d) MeSiCl3, (e) MeSi(OMe)3, (f) MeSi(OEt)3, (g) Me2Si(OEt)2.
The separations of different binary organic solvent mixtures were performed by pervaporation process using four membranes: ungrafted membrane M, grafted membrane by MeSi(OMe)3 (M1 (MeSi(OMe)3) according to the method 1, grafted membrane by Me2Si(OEt)2, (M.1 Me2Si(OEt)2) according to the method 1, grafted membrane by Me2Si(OEt)2 (M.2 Me2Si(OEt)2) according to the method 2. Table 1 Specific area and microporous volume of ungrafted and grafted g alumina membranes by organosilanes
The comparison of the thermal differential analysis (Fig. 3) obtained for the different powders (grafted or ungrafted) confirms the results obtained for the membranes. The presence of an exothermic peak at 500 8C for the grafted powders is only observed for the grafting by means of the bi- or tri-functional silanes.
Membrane
Specific area (m2 g 1)
g Alumine 329 g Alumine g grafted by 178 Me2Si(OEt)2 g Alumine g grafted by 219 MeSi(OEt)3
Microporous volume (cm3 g 1) 0.22 0.10 0.11
Fig. 3. Differential thermogravimetric analysis of ungrafted and grafted g alumina powder by different silanes: (a) ungrafted, (b) Me2Si(OEt)2 (c) MeSi(OMe)3.
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Table 2 Pervaporation of water /ethanol mixture through ungrafted M and grafted M.1 (MeSi(OMe)3) membranes M (ungrafted) % EtOH in feed % EtOH in permeate Flux (g h 1 m2)
10.5 1.8 4150
M.1 (MeSi(OMe)3) 53 6.4 2830
87.5 70 2300
10.5 0.7 78
49 4.7 60
93 29 29
Table 3 Pervaporation of cyclohexane /e´thanol mixtures through M.1 (Me2Si(OEt)2), and M.2 (Me2Si(OEt)2) grafted membranes M.1 (Me2Si(OEt)2) % EtOH in feed % EtOH in permeate Flux (g h 1 m2)
29.5 37.5 196
For the water /methanol mixtures (Table 2), the water concentration in the permeate is higher than in the feed whatever the membranes used whereas the fluxes decrease for the grafted membranes and the selectivity does not change for both membranes. For the ethanol /cyclohexane mixtures (Table 3), the fluxes depend on the ethanol composition and the selectivity is better for the grafted M.2 Me2Si(OEt)2 membrane according to condition 2, which is a probe that the grafting operation is more complete by the method 2. In the case of the cyclohexane/methylethylketone (MEK) mixtures (Table 4), the MEK concentration in the permeate is higher than in the feed for both membranes but the selectivity of the grafted M.2 Me2Si(OEt)2 membrane is better, nevertheless the corresponding fluxes decrease significantly. The results in the whole show the influence of the grafting on the membrane performances, the decrease of the fluxes observed for the grafted
67 54 112
M.2 (Me2Si(OEt)2) 89.5 83 95
21.3 99 14
47 96.2 30
90 98.1 66
membranes might be caused by the clogging of the pore that is in agreement with the decrease of the specific area. We can also observe that the selectivity of the prepared membranes towards the different solvents depends on the adsorption energy of the solvent molecule on the surface of the material which is in relation with the Snyder number [7].
4. Conclusion The hydrophobic/hydrophilic character of g alumina membrane is well modified by grafting operation by means of organosilanes; nevertheless the grafting is achieved only with bi- or trifunctional silanes. The grafting operation leads to the decrease of the specific area and of the microporous volume of the material due to the clogging of the pores. The separation of solvent mixtures by pervaporation is possible with high selectivity but low flux for industrial applications.
Table 4 Pervaporation of cyclohexane /me´thyle´thylce´tone (MEC) mixture through ungrafted M and grafted M.2 Me2Si(OEt)2 membranes M ungrafted % MEC in feed % MEC in permeate Flux (g h 1 m2)
11.3 35 170
M.2 Me2Si(OEt)2 55.3 81 630
86.2 93.5 1300
12.4 99 10
50.2 97 29
80 98 66
S. Alami Younssi et al. / Separation and Purification Technology 32 (2003) 175 /179
Further, investigations should be considered for grafting membrane with larger pores to limit this drawback.
References [1] R.R. Bhave (Ed.), Inorganic Membranes, Van Nostrand Reinhold, 1991.
179
[2] A.J. Burgraaf, L. Cot (Eds.), Fundamentals of Inorganic Membranes Science and Technology, Elsevier, Amsterdam, 1996. [3] R.K. Gilpin, M.F. Burke, Anal. Chem. 45 (1973) 1383. [4] J.J. Pesek, Jr., E. Sandoval, Su. Minggong, J. Chromatogr. 630 (1993) 95. [5] S. Alami younssi, C. Kiefer, A. Larbot, M. Persin, J. Sarrazin, J. Membr. Sci. 143 (1998) 27. [6] A. Larbot, S. Alami younssi, M. Persin, J. Sarrazin, L. Cot, J. Membr. Sci. 97 (1994) 167. [7] L.R. Snyder, Principles of Adsorption Chromatography, Dekker, New York, 1968.
g Alumina membranes grafting by organosilanes and its application to the separation of solvent mixtures by pervaporation S. Alami Younssi a, A. Iraqi a, M. Rafiq a, M. Persin b,*, A. Larbot b, J. Sarrazin b b
a Laboratoire des mate´riaux et protection de l’environnement, Faculte´ des Sciences, Dhar Mehraz, Fes, Morocco Institut Europe´en des Membranes (IEM), UMR 5635 CNRS ENSCM UMII, 1919 Route de Mende, 34293 Montpellier, Cedex 5, France
Abstract The selectivity and the membrane flux depend on the textural and also on the chemical composition of the materiel used to prepare the membrane. The surface properties of g alumina membranes prepared by sol /gel route can be modified by grafting mono-, di- and tri-functional organosilanes ( /OCH3, /OC2H5, /Cl) in the goal to change the selectivity of the membrane towards the chemical solutes. The characterization of the different membranes showed that the grafting depends on the nature of the silanes used and is achieved with the multifunctional silanes. The modification of the surface properties of the membrane prepared after grafting has been confirmed by pervaporation tests on different binary mixtures of solvents (water/ethanol, methylethylketone/cyclohexane, cyclohexane/ethanol). In these conditions, the flux of the different solvents and the selectivity of the membranes depends clearly on the grafting conditions. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Membrane; g Alumina; Grafting; Silanes
1. Introduction Nowadays the mineral membranes are in constant development and they present interesting potentialities for separations due to their mechanical and chemical resistance [1,2]. In the goal to extend their use to new applications the authors
attempted to modify the selectivity of the filtering layer by grafting chemical moieties [3,4]. The surface treatment consist to modify the hydrophilic/hydrophobic balance of the ceramic layer, it is generally obtained by the reaction of a functionalized silane X /Si /R (R / /OCH3, /OC2H5, /Cl) with the residual hydroxyl groups present on the surface of the oxide.
* Corresponding author. Tel.: /33-4-67-613402; fax: /33-467-613385. E-mail address: [email protected] (M. Persin). 1383-5866/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1383-5866(03)00031-5
176
S. Alami Younssi et al. / Separation and Purification Technology 32 (2003) 175 /179
The main parameters of the grafting which should well controlled are the temperature, the nature of the solvent and the water amount adsorbed on the surface of the oxide [5]. In this work, the membranes properties of modified g alumina membrane are compared before and after grafting using different organosilanes. The modified membranes were tested in the pervaporation of different organic solvent mixtures.
pure silane after corking the bottom of the tube by means of an epoxy resin. After reflux in toluene for 2 h at 110 8C; washing and drying, the membrane was obtained. In both cases the membrane was then treated at 150 8C in air. The method permits the preparation of stable grafted membranes with a good reproducibility.
3. Results 2. Experimental 2.1. Preparation of alumina sol The sol was prepared under stirring by the complete hydrolysis in water of Al(OBu)3, followed by the peptisation of the formed in situ boehmite precipitate with a concentrated nitric acid solution [6]. The evaporation of the solvent leads to the ceramic g alumina powder after grinding and calcination at 450 8C. 2.2. Preparation of g alumina membrane The membrane was prepared by slipcasting at the surface of the inner part of a commercial tubular support tube in a alumina made of a 0.2 mm microfiltration tube with a 5 nm g alumina UF layer deposited inside. The final g alumina layer was dried for 15 min and fired at 450 8C for 1 h [6].
Membrane and powder grafting characterization. 3.1. Reflecting IR spectroscopy The grafting of the membrane surface (Fig. 1)was analysed by reflecting IR spectroscopy. The comparison of the response of the different grafted or ungrafted membranes (Fig. 2) shows that neither signal was observed for the ungrafted membrane nor for the membrane grafted by means of the monofunctional Me3SiCl compound. On the contrary the presence of the C /H band at 3000 cm 1, Si /C band at 1300 cm 1 and Si /O band at 950 cm 1 attested that the functionalization was obtained by means of the di- or tri-functional chloro, ethoxy or methoxy silanes. These results may be explained by the possibility for the multifunctional silane to form a polymerised network by the formation of Si /O /Si bonds.
2.3. Preparation of grafting g alumina powder The grafting method has been already described in former work [5]. The g alumina powder reacts with the organosilanes in dry toluene for 48 h at 110 8C, after filtration, washing with toluene and drying at 150 8C, the grafted powder is obtained. 2.4. Preparation of membrane The grafted membranes were prepared by two methods, the first consisted to immerse the g alumina membrane in a mixture of toluene and silane under reflux at 110 8C for 48 h. The second method consisted to fill the membrane tube with
Fig. 1. SEM of a g alumina membrane (thickness of the nanofiltration layer: 200 nm).
S. Alami Younssi et al. / Separation and Purification Technology 32 (2003) 175 /179
177
3.2. N2 adsorption /desorption The microporous volume and the specific area of the different membranes determined from the N2 adsorption/desorption isotherms are gathered in Table 1. The grafting is attested by the decrease of the specific area and the microporous volume for the grafted membrane by means of the Me2Si(OEt)2 or MeSi(OEt)3 silanes and this effect may be explained by the covering of the pore wall with the grafted moities. 3.3. Application to pervaporation of organic solvent mixtures
Fig. 2. Reflection IR spectra on ungrafted and grafted g alumina powders by different silanes: (a) ungrafted membrane, (b) Me3SiCl, (c) Me2SiCl2, (d) MeSiCl3, (e) MeSi(OMe)3, (f) MeSi(OEt)3, (g) Me2Si(OEt)2.
The separations of different binary organic solvent mixtures were performed by pervaporation process using four membranes: ungrafted membrane M, grafted membrane by MeSi(OMe)3 (M1 (MeSi(OMe)3) according to the method 1, grafted membrane by Me2Si(OEt)2, (M.1 Me2Si(OEt)2) according to the method 1, grafted membrane by Me2Si(OEt)2 (M.2 Me2Si(OEt)2) according to the method 2. Table 1 Specific area and microporous volume of ungrafted and grafted g alumina membranes by organosilanes
The comparison of the thermal differential analysis (Fig. 3) obtained for the different powders (grafted or ungrafted) confirms the results obtained for the membranes. The presence of an exothermic peak at 500 8C for the grafted powders is only observed for the grafting by means of the bi- or tri-functional silanes.
Membrane
Specific area (m2 g 1)
g Alumine 329 g Alumine g grafted by 178 Me2Si(OEt)2 g Alumine g grafted by 219 MeSi(OEt)3
Microporous volume (cm3 g 1) 0.22 0.10 0.11
Fig. 3. Differential thermogravimetric analysis of ungrafted and grafted g alumina powder by different silanes: (a) ungrafted, (b) Me2Si(OEt)2 (c) MeSi(OMe)3.
178
S. Alami Younssi et al. / Separation and Purification Technology 32 (2003) 175 /179
Table 2 Pervaporation of water /ethanol mixture through ungrafted M and grafted M.1 (MeSi(OMe)3) membranes M (ungrafted) % EtOH in feed % EtOH in permeate Flux (g h 1 m2)
10.5 1.8 4150
M.1 (MeSi(OMe)3) 53 6.4 2830
87.5 70 2300
10.5 0.7 78
49 4.7 60
93 29 29
Table 3 Pervaporation of cyclohexane /e´thanol mixtures through M.1 (Me2Si(OEt)2), and M.2 (Me2Si(OEt)2) grafted membranes M.1 (Me2Si(OEt)2) % EtOH in feed % EtOH in permeate Flux (g h 1 m2)
29.5 37.5 196
For the water /methanol mixtures (Table 2), the water concentration in the permeate is higher than in the feed whatever the membranes used whereas the fluxes decrease for the grafted membranes and the selectivity does not change for both membranes. For the ethanol /cyclohexane mixtures (Table 3), the fluxes depend on the ethanol composition and the selectivity is better for the grafted M.2 Me2Si(OEt)2 membrane according to condition 2, which is a probe that the grafting operation is more complete by the method 2. In the case of the cyclohexane/methylethylketone (MEK) mixtures (Table 4), the MEK concentration in the permeate is higher than in the feed for both membranes but the selectivity of the grafted M.2 Me2Si(OEt)2 membrane is better, nevertheless the corresponding fluxes decrease significantly. The results in the whole show the influence of the grafting on the membrane performances, the decrease of the fluxes observed for the grafted
67 54 112
M.2 (Me2Si(OEt)2) 89.5 83 95
21.3 99 14
47 96.2 30
90 98.1 66
membranes might be caused by the clogging of the pore that is in agreement with the decrease of the specific area. We can also observe that the selectivity of the prepared membranes towards the different solvents depends on the adsorption energy of the solvent molecule on the surface of the material which is in relation with the Snyder number [7].
4. Conclusion The hydrophobic/hydrophilic character of g alumina membrane is well modified by grafting operation by means of organosilanes; nevertheless the grafting is achieved only with bi- or trifunctional silanes. The grafting operation leads to the decrease of the specific area and of the microporous volume of the material due to the clogging of the pores. The separation of solvent mixtures by pervaporation is possible with high selectivity but low flux for industrial applications.
Table 4 Pervaporation of cyclohexane /me´thyle´thylce´tone (MEC) mixture through ungrafted M and grafted M.2 Me2Si(OEt)2 membranes M ungrafted % MEC in feed % MEC in permeate Flux (g h 1 m2)
11.3 35 170
M.2 Me2Si(OEt)2 55.3 81 630
86.2 93.5 1300
12.4 99 10
50.2 97 29
80 98 66
S. Alami Younssi et al. / Separation and Purification Technology 32 (2003) 175 /179
Further, investigations should be considered for grafting membrane with larger pores to limit this drawback.
References [1] R.R. Bhave (Ed.), Inorganic Membranes, Van Nostrand Reinhold, 1991.
179
[2] A.J. Burgraaf, L. Cot (Eds.), Fundamentals of Inorganic Membranes Science and Technology, Elsevier, Amsterdam, 1996. [3] R.K. Gilpin, M.F. Burke, Anal. Chem. 45 (1973) 1383. [4] J.J. Pesek, Jr., E. Sandoval, Su. Minggong, J. Chromatogr. 630 (1993) 95. [5] S. Alami younssi, C. Kiefer, A. Larbot, M. Persin, J. Sarrazin, J. Membr. Sci. 143 (1998) 27. [6] A. Larbot, S. Alami younssi, M. Persin, J. Sarrazin, L. Cot, J. Membr. Sci. 97 (1994) 167. [7] L.R. Snyder, Principles of Adsorption Chromatography, Dekker, New York, 1968.