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Preliminary studies on the potential for gas separation by mesoporous ceramic oxide membranes surface modified by alkyl phosphonic acids
rnalo! BRANE
SCIENCE ELSEVIER
Journal of Membrane Science 134 (1997) 219-223
Preliminary studies on the potential for gas separation by mesoporous ceramic oxide membranes surface modified by alkyl phosphonic acids a*
J~rrme Randon ' , Russell Paterson b a Laboratoire des Sciences Analytiques, Universit( Claude Bernard, Bat 308, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France b Colloid and Membrane Science Research Group, Chemistry Department, University of Glasgow, Glasgow GI2 8QQ, UK Received 19 November 1996; received in revised form 28 April 1997; accepted 30 April 1997
Abstract
A composite ceramic-organic membrane has been prepared by chemical grafting of organo-phosphate molecules to the surface of an aluminium-oxide membrane. Gas-transport mechanism through the initial mesoporous membrane with pore size of 5 nm is essentially based on Knudsen diffusion and so does not give significant separation factors between gases of similar molecular weights. Modification of membrane surface properties allows control of the relative contribution of differing transport mechanisms. Modified membranes have been tested for various gas permeations (methane, ethane, propane, hydrogen, nitrogen and carbon dioxide) at room temperature. The modified membranes display high permeability and high selectivity coefficient for propane/nitrogen separation. The chemical, physical and geometrical properties of the modifying molecules can be chosen in order to improve the performances of any specific application.
Keywords: Gas separations; Surface modification; Oxide membranes; Alkyl phosphonic acids
I. Introduction
Gas separation can be achieved by either nonporous or porous membranes. Non porous membranes usually exhibit high selectivity but low flow rate while high flow rates can be obtained using porous membranes but with low selectivity factors. Alternative membrane structures giving both high flow and selectivity are needed. *Corresponding author. Tel.: +33 4 72431079; fax: +33 4 72431078; e-mail: [email protected]. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00110-5
Starting from commercially available porous ceramic membranes, surface-pore modifications of oxide material have been performed by bonding organo-phosphonic compounds to the surface hydroxyl groups of the ceramic membrane. A new composite organic-inorganic membrane was formed which has both high flow rate and selectivity factor. By a proper choice of pore size and surface properties of the membrane, the contribution of each transport mechanism can be controlled leading to an improved efficiency of the separation process.
220
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223
2. Experimental The mesoporous "v-alumina membrane used in this study was provided by Soci6t6 des C6ramiques Techniques (Bazet-France). The membrane had a 3/amthick active layer with a pore diameter of 5 nm with a porosity of 40%. The active layer was deposited on the inner surface of an a-alumina tube with a much larger pore size which had no significant resistance to gas permeation compared to the membrane. The tubular "v-alumina membrane was modified by n-butyl phosphonic acid n - C a H 9 P O 3 H 2 and n-dodecylphosphate n-CI2H25OPOaH2. The n-butane phosphonic acid modification is described here (the same process was used for n-dodecylphosphate modifications). A saturated toluene solution of n-butane phosphonic acid (0.1 g of n-CaH9PO3H2 in 20 ml of toluene), was poured into the tubular alumina membrane. The solution was kept in contact with the membrane for 2 h. The membrane was then washed with pure toluene, rinsed with 2/1 ethanol/water mixture and then dried at 60°C before use. The permeabilities of pure gases were measured by monitoring pressure in both high- and low-pressure sides of a gas-permeation cell (Fig. 1). The highpressure side was maintained around 2 bar. The low-pressure side was initially a 1 bar reservoir, filled with the same gas in which the pressure increased as the gas was permeated the membrane. The pressure at both sides were recorded by computer
High
I
3. Transport mechanisms The transport of gas through porous membrane can occur by several mechanisms [1]. When the mean free path of the gas is much smaller than the membrane pore diameter, gas transport will occur predominantly by Poiseuille flow, which is essentially non-separative. Knudsen diffusion occurs when the mean free path of the gas is much larger than the pore diameter. Since porous membranes have a pore-size distribution, gas transport may simultaneously occur by both Knudsen and Poiseuille flow mechanisms. Capillary condensation occurs when gas condenses within the pore and flows as a condensed phase under a capillary pressure gradient across the membrane. Selective surface flow is characterised by a selective adsorption of gas within the pores and the surface flow of these molecules in the adsorbed phase through the pore. Molecular sieving occurs when the pores are smaller than some component of the gas mixture. Polymeric non-porous membranes exhibit a different mechanism [2] based on solubilisation of gas molecules at the high-pressure interface, diffusion through the membrane, and desorption at the lowpressure interface.
4. Alumina membrane characteristics
Gas input Low Pressure
and the rate of transmission per unit area was calculated as a function of the pressure drop across the membrane. The permeability is defined as the amount of permeate per unit time per unit membrane area and per unit pressure drop across the membrane (mol s 1 m-2 pa-1)
~--
>
Fig. 1. Diagrammatic representation of the gas-permeation cell.
At 20°C, the nitrogen permeability of the original membrane was constant at 0.8× l0 -5 tool s -1 m -2 Pa -1 in the pressure range A p equal to 0.05 × 105-1 x 105 Pa (Fig. 2). This constant value is in agreement with a Knudsen flow mechanism for a defect-free membrane. The permeability, Fk, can be estimated theoretically Ill: 2e/Zk~ Fk - - - -
3RTL
(1)
where e is the porosity, #k the shape factor, P the modal
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223
0.4
221
5.1. n-Butyl phosphonic acid
0.3 cq ~
E
-~ 0.2
_'o IJ -
0.1
0
0.2 0.4 0.6 0.8 Trans Membrane Pressure (Pa x 10 5)
Fig. 2. Nitrogen permeation through n-butyl phosphonic acid modified alumina membrane: (IZ]) original alumina membrane; (O) after treatment with n-butyl phosphonic acid solution; (0) after cleaning with EtOH/H20 mixture. Filled and open symbols represent two sets of experiments.
pore radius,
~ the average molecular velocity,
= (8RT/TrM) 1/2, M the molecular weight of the molecule, R the gas constant, T thz absolute temperature and L the thickness of the active layer of the membrane. The shape factor, estimated for a pore diameter of 5 n m and a porosity of e - - 0 . 4 , is #k = 0.25, which is a 'reasonable' value for such membrane [ 1].
5. Membrane modification The same membrane modification was used recently to link organo-phosphate molecules to the rigid backbone of oxide membranes [3] for application in protein ultrafiltration. It was shown that the three oxygen atoms of the phosphate head are linked to the oxide surface and the organic chain of the molecule extends perpendicular to the surface. In this study, the alumina membrane was modified by n-butane phosphonic acid and n-dodecylphosphate.
After n-C4H9PO3H2 treatment (Fig. 2), the nitrogen permeability decreased by a factor of 2.6 to 0.31 × l0 -5 mol s - t m -2 Pa -~. In this situation, the pore size cannot be estimated using Eq. (1) because both the porosity and the shape factor have changed. These two parameters are up to now a difficult problem in sol-gel membrane morphology. The separation factors of pure gas, ethane and propane, against nitrogen have been measured. For n-C4HgPO3H2 modified membrane, they are respectively OLC2H6/N2 = PC2H6/PN2= 1.3 and aC3H6/N2 = 1.66. On the basis of Knudsen mechanism, separation factors less than one were expected, but the creation of a hydrophobic layer covering the entire pore surface enhances the permeation of low molecular weight hydrocarbon molecules. This improved transport can be explained by an increased surface-flow mechanism. Ethane and propane molecules adsorb on the pore-wall surface and diffuse throughout the membrane on the pore-wall surface. Propane is more hydrophobic than ethane, so the propane adsorption on hydrophobic surface is higher than ethane and the corresponding flow rate is higher too. The n-butyl phosphonic layer can be removed by washing with an alkaline solution. The initial alumina membrane is then restored and can be further modified (Fig. 2).
5.2. n-Dodecylphosphate Fig. 3 shows the selectivity factor of propane against nitrogen as a function of the propane permeability for several n-dodecylphosphate membrane treatments. From stages 1-5, successive treatments by low concentration n-dodecylphosphate solution in benzene were performed. At the first stage of treatment (right-hand side of the graph), the propane permeability is high (0.6 × 10 -5 mol s t m-2 p a - t ) but the propane/nitrogen selectivity is low (a = 1.3). Thereafter, the Knudsen diffusion part of the permeability decreases due to lower value of e/ZkF geometric factors but this reduction is not balanced by the surface-flow enhancement. The global permeability decreases and, by a symmetric effect, the selectivity increases.
222
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223 20
z
olyethylene 3 I.tm
n
Fille~
0
,
I ii,llli
Ikli,,I
i
l i~ll,ll
,
Propane Permeability mol.sl.m'2.Pa "1 x 10 5 Fig. 3. Propane/nitrogen selectivity through n-dodecylphosphate modified alumina membrane. Initial membrane pore size 5 nm; thickness 3 Ixm; and temperature 20°C.
At stage 5, the pore necks can be blocked by the alkyl chains of the n-dodecylphosphate molecules. In this situation, the gas transport through the pore necks is controlled by a solubilisation-diffusion mechanism as in dense polymer membranes. Ageing can also improve membrane selectivity. Stage 6 corresponds to a 60°C thermal treatment for 3 months. This composite membrane can be compared to the polyethylene membrane (PE) which has the same chemical structure as the alkyl chain of the n-dodecylphosphate molecules. Low-density polyethylene, d = 0.92, has a permeability coefficient of 0.73 × 10 -13 cm3(STP) cm cm -2 s - t Pa - t for nitrogen and 7.3 × 10 -13 cm3(STP) cm cm -2 s -1 Pa -x for propane [4]. On this basis, a polyethylene membrane with the same thickness as the active layer of the ceramic membrane (3/am) should have a propane permeability of 10 - 9 mol s -1 m 2 pa-1 and a propane/nitrogen selectivity coefficient of 9.7. For the n-dodecylphosphate (nDP)-modified alumina membrane, the corresponding permeability and selectivity are respectively 10 -7 mol s -1 m -2 Pa -1 and 16 for the highest selectivity factor obtained. Hence, compared to PE membrane, the selectivity is slightly improved and the permeability 100 times faster. For an equal selectivity, the composite membrane has a permeability coeffi-
Fig. 4. Diagrammatic representation of the composite structure of the n-dodecylphosphate-modified alumina membrane.
cient 300 times higher than a 3 ~tm-thick PE membrane. So the thickness of the nDP-modified membrane is certainly lower than the thickness of the ceramic membrane support. Conversely, the nDP modified alumina membrane has fewer available paths than PE for gas molecules because of the initial porous alumina structure with a porosity of 40% (i.e. 60% of the membrane volume is lost due to the impermeable character of alumina). But even so, higher gas flow is obtained because of the particular composite structure (Fig. 4). First, the voids between the pore necks which control the gas-flow rate have no resistance to the flow. Secondly, the number of ends of chain in a PE membrane is much smaller than in the nDP-modified membrane. The diffusion step is improved by the higher mobility of the alkyl chains.
6. Selectivity factors for other gases The permeabilities of various gases have been measured for the nDP-modified alumina membrane (stage 5) and the results are summarised in Fig. 5. The low molecular weight alkane series shows similar behaviour than for permeation through lowdensity polyethylene membrane. For PE, an increase in the molecular weight gives lower diffusivity factors but higher solubility factors leading to an overall
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223
Propane
Ethane
Methane
Carbon dioxide m
0
I
2 4 6 8 10 12 14 Selectivity factor / Nitrogen
nDP modified 1 A1203 membrane
Polyethylene I d=0.914 I
Polyethylene I d--0.964
Fig. 5. Selectivity factors of various gases compared to nitrogen for n-dodecylphosphate-modified alumina membrane (stage 5). Selectivity factors for low- and high-density polyethylene membranes are given for comparison.
permeability which increases from methane to propane. The same rules seems to be followed for the nDP-modified membrane. Carbon dioxide selectivity is lower than expected from PE characteristics, but selectivity depends on PE density (selectivity varies from 12.6 to 2.5 for, respectively, low- and high-density polyethylene). Hydrogen selectivity is of the same order for high as for low density PE.
7. Conclusion
The modification of the mesoporous membranesurface properties allows the control of the relative
223
contribution of different transport mechanisms. Mesoporous alumina membrane has been modified by chemical bonding of n-butane phosphonic acid and n-dodecylphosphate. The membrane has been tested in gas-separation process of low molecular weight hydrocarbons. This composite structure improves permeability and selectivity coefficient of propane/ nitrogen separation. Such type of structure can be created using any kind of organic molecules which are able to bind the oxide surface [5,6]. To improve separation for any specific gas application, the modifying molecules can be chosen, based on the knowledge of organic membrane, to obtain both high selectivity and permeability.
References [1] K. Keizer, R.J.R. Uhlhorn, V.T. Zaspalis and A.J. Burggraf, Transport and related gas and vapour separation in ceramic membranes, Key Engineering Materials, 61/62 (1991) 143. [2] T. Nakagawa, Gas separation and pervaporation, in: Y. Osada and T. Nakagawa (Eds.), Membrane Science and Technology, Marcel Dekker, 1992, p. 239. [3] J. Randon and R. Paterson, Modification of ceramic membrane surfaces using phosphoric acid and alkyl phosphonic acids and its effects on ultrafiltration of BSA protein, J. Membr. Sci., 98 (1995) 119. [4] S. Pauly, Permeability and diffusion data, in: J. Brandup and E.H. Immergut (Eds.), Polymer Handbook, 3rd edn., John Wiley and Sons, 1989. [5] C. Leger, H.L. Lira and R. Paterson, Preparation and properties of surface modified ceramic membranes. Part II: Gas and liquid permeability of 5 nm alumina membranes modified by a monolayer of bound polydimethyl siloxane silicone oil, J. Membr. Sci., 120 (1996) 135. [6] C. Leger, H.L. Lira and R. Paterson, Preparation and properties of surface modified ceramic membranes. Part III: Gas permeation of 5 nm alumina membranes modified by trichloro-octadecyl silane, J. Membr. Sci., 120 (1996) 187.
SCIENCE ELSEVIER
Journal of Membrane Science 134 (1997) 219-223
Preliminary studies on the potential for gas separation by mesoporous ceramic oxide membranes surface modified by alkyl phosphonic acids a*
J~rrme Randon ' , Russell Paterson b a Laboratoire des Sciences Analytiques, Universit( Claude Bernard, Bat 308, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France b Colloid and Membrane Science Research Group, Chemistry Department, University of Glasgow, Glasgow GI2 8QQ, UK Received 19 November 1996; received in revised form 28 April 1997; accepted 30 April 1997
Abstract
A composite ceramic-organic membrane has been prepared by chemical grafting of organo-phosphate molecules to the surface of an aluminium-oxide membrane. Gas-transport mechanism through the initial mesoporous membrane with pore size of 5 nm is essentially based on Knudsen diffusion and so does not give significant separation factors between gases of similar molecular weights. Modification of membrane surface properties allows control of the relative contribution of differing transport mechanisms. Modified membranes have been tested for various gas permeations (methane, ethane, propane, hydrogen, nitrogen and carbon dioxide) at room temperature. The modified membranes display high permeability and high selectivity coefficient for propane/nitrogen separation. The chemical, physical and geometrical properties of the modifying molecules can be chosen in order to improve the performances of any specific application.
Keywords: Gas separations; Surface modification; Oxide membranes; Alkyl phosphonic acids
I. Introduction
Gas separation can be achieved by either nonporous or porous membranes. Non porous membranes usually exhibit high selectivity but low flow rate while high flow rates can be obtained using porous membranes but with low selectivity factors. Alternative membrane structures giving both high flow and selectivity are needed. *Corresponding author. Tel.: +33 4 72431079; fax: +33 4 72431078; e-mail: [email protected]. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00110-5
Starting from commercially available porous ceramic membranes, surface-pore modifications of oxide material have been performed by bonding organo-phosphonic compounds to the surface hydroxyl groups of the ceramic membrane. A new composite organic-inorganic membrane was formed which has both high flow rate and selectivity factor. By a proper choice of pore size and surface properties of the membrane, the contribution of each transport mechanism can be controlled leading to an improved efficiency of the separation process.
220
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223
2. Experimental The mesoporous "v-alumina membrane used in this study was provided by Soci6t6 des C6ramiques Techniques (Bazet-France). The membrane had a 3/amthick active layer with a pore diameter of 5 nm with a porosity of 40%. The active layer was deposited on the inner surface of an a-alumina tube with a much larger pore size which had no significant resistance to gas permeation compared to the membrane. The tubular "v-alumina membrane was modified by n-butyl phosphonic acid n - C a H 9 P O 3 H 2 and n-dodecylphosphate n-CI2H25OPOaH2. The n-butane phosphonic acid modification is described here (the same process was used for n-dodecylphosphate modifications). A saturated toluene solution of n-butane phosphonic acid (0.1 g of n-CaH9PO3H2 in 20 ml of toluene), was poured into the tubular alumina membrane. The solution was kept in contact with the membrane for 2 h. The membrane was then washed with pure toluene, rinsed with 2/1 ethanol/water mixture and then dried at 60°C before use. The permeabilities of pure gases were measured by monitoring pressure in both high- and low-pressure sides of a gas-permeation cell (Fig. 1). The highpressure side was maintained around 2 bar. The low-pressure side was initially a 1 bar reservoir, filled with the same gas in which the pressure increased as the gas was permeated the membrane. The pressure at both sides were recorded by computer
High
I
3. Transport mechanisms The transport of gas through porous membrane can occur by several mechanisms [1]. When the mean free path of the gas is much smaller than the membrane pore diameter, gas transport will occur predominantly by Poiseuille flow, which is essentially non-separative. Knudsen diffusion occurs when the mean free path of the gas is much larger than the pore diameter. Since porous membranes have a pore-size distribution, gas transport may simultaneously occur by both Knudsen and Poiseuille flow mechanisms. Capillary condensation occurs when gas condenses within the pore and flows as a condensed phase under a capillary pressure gradient across the membrane. Selective surface flow is characterised by a selective adsorption of gas within the pores and the surface flow of these molecules in the adsorbed phase through the pore. Molecular sieving occurs when the pores are smaller than some component of the gas mixture. Polymeric non-porous membranes exhibit a different mechanism [2] based on solubilisation of gas molecules at the high-pressure interface, diffusion through the membrane, and desorption at the lowpressure interface.
4. Alumina membrane characteristics
Gas input Low Pressure
and the rate of transmission per unit area was calculated as a function of the pressure drop across the membrane. The permeability is defined as the amount of permeate per unit time per unit membrane area and per unit pressure drop across the membrane (mol s 1 m-2 pa-1)
~--
>
Fig. 1. Diagrammatic representation of the gas-permeation cell.
At 20°C, the nitrogen permeability of the original membrane was constant at 0.8× l0 -5 tool s -1 m -2 Pa -1 in the pressure range A p equal to 0.05 × 105-1 x 105 Pa (Fig. 2). This constant value is in agreement with a Knudsen flow mechanism for a defect-free membrane. The permeability, Fk, can be estimated theoretically Ill: 2e/Zk~ Fk - - - -
3RTL
(1)
where e is the porosity, #k the shape factor, P the modal
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223
0.4
221
5.1. n-Butyl phosphonic acid
0.3 cq ~
E
-~ 0.2
_'o IJ -
0.1
0
0.2 0.4 0.6 0.8 Trans Membrane Pressure (Pa x 10 5)
Fig. 2. Nitrogen permeation through n-butyl phosphonic acid modified alumina membrane: (IZ]) original alumina membrane; (O) after treatment with n-butyl phosphonic acid solution; (0) after cleaning with EtOH/H20 mixture. Filled and open symbols represent two sets of experiments.
pore radius,
~ the average molecular velocity,
= (8RT/TrM) 1/2, M the molecular weight of the molecule, R the gas constant, T thz absolute temperature and L the thickness of the active layer of the membrane. The shape factor, estimated for a pore diameter of 5 n m and a porosity of e - - 0 . 4 , is #k = 0.25, which is a 'reasonable' value for such membrane [ 1].
5. Membrane modification The same membrane modification was used recently to link organo-phosphate molecules to the rigid backbone of oxide membranes [3] for application in protein ultrafiltration. It was shown that the three oxygen atoms of the phosphate head are linked to the oxide surface and the organic chain of the molecule extends perpendicular to the surface. In this study, the alumina membrane was modified by n-butane phosphonic acid and n-dodecylphosphate.
After n-C4H9PO3H2 treatment (Fig. 2), the nitrogen permeability decreased by a factor of 2.6 to 0.31 × l0 -5 mol s - t m -2 Pa -~. In this situation, the pore size cannot be estimated using Eq. (1) because both the porosity and the shape factor have changed. These two parameters are up to now a difficult problem in sol-gel membrane morphology. The separation factors of pure gas, ethane and propane, against nitrogen have been measured. For n-C4HgPO3H2 modified membrane, they are respectively OLC2H6/N2 = PC2H6/PN2= 1.3 and aC3H6/N2 = 1.66. On the basis of Knudsen mechanism, separation factors less than one were expected, but the creation of a hydrophobic layer covering the entire pore surface enhances the permeation of low molecular weight hydrocarbon molecules. This improved transport can be explained by an increased surface-flow mechanism. Ethane and propane molecules adsorb on the pore-wall surface and diffuse throughout the membrane on the pore-wall surface. Propane is more hydrophobic than ethane, so the propane adsorption on hydrophobic surface is higher than ethane and the corresponding flow rate is higher too. The n-butyl phosphonic layer can be removed by washing with an alkaline solution. The initial alumina membrane is then restored and can be further modified (Fig. 2).
5.2. n-Dodecylphosphate Fig. 3 shows the selectivity factor of propane against nitrogen as a function of the propane permeability for several n-dodecylphosphate membrane treatments. From stages 1-5, successive treatments by low concentration n-dodecylphosphate solution in benzene were performed. At the first stage of treatment (right-hand side of the graph), the propane permeability is high (0.6 × 10 -5 mol s t m-2 p a - t ) but the propane/nitrogen selectivity is low (a = 1.3). Thereafter, the Knudsen diffusion part of the permeability decreases due to lower value of e/ZkF geometric factors but this reduction is not balanced by the surface-flow enhancement. The global permeability decreases and, by a symmetric effect, the selectivity increases.
222
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223 20
z
olyethylene 3 I.tm
n
Fille~
0
,
I ii,llli
Ikli,,I
i
l i~ll,ll
,
Propane Permeability mol.sl.m'2.Pa "1 x 10 5 Fig. 3. Propane/nitrogen selectivity through n-dodecylphosphate modified alumina membrane. Initial membrane pore size 5 nm; thickness 3 Ixm; and temperature 20°C.
At stage 5, the pore necks can be blocked by the alkyl chains of the n-dodecylphosphate molecules. In this situation, the gas transport through the pore necks is controlled by a solubilisation-diffusion mechanism as in dense polymer membranes. Ageing can also improve membrane selectivity. Stage 6 corresponds to a 60°C thermal treatment for 3 months. This composite membrane can be compared to the polyethylene membrane (PE) which has the same chemical structure as the alkyl chain of the n-dodecylphosphate molecules. Low-density polyethylene, d = 0.92, has a permeability coefficient of 0.73 × 10 -13 cm3(STP) cm cm -2 s - t Pa - t for nitrogen and 7.3 × 10 -13 cm3(STP) cm cm -2 s -1 Pa -x for propane [4]. On this basis, a polyethylene membrane with the same thickness as the active layer of the ceramic membrane (3/am) should have a propane permeability of 10 - 9 mol s -1 m 2 pa-1 and a propane/nitrogen selectivity coefficient of 9.7. For the n-dodecylphosphate (nDP)-modified alumina membrane, the corresponding permeability and selectivity are respectively 10 -7 mol s -1 m -2 Pa -1 and 16 for the highest selectivity factor obtained. Hence, compared to PE membrane, the selectivity is slightly improved and the permeability 100 times faster. For an equal selectivity, the composite membrane has a permeability coeffi-
Fig. 4. Diagrammatic representation of the composite structure of the n-dodecylphosphate-modified alumina membrane.
cient 300 times higher than a 3 ~tm-thick PE membrane. So the thickness of the nDP-modified membrane is certainly lower than the thickness of the ceramic membrane support. Conversely, the nDP modified alumina membrane has fewer available paths than PE for gas molecules because of the initial porous alumina structure with a porosity of 40% (i.e. 60% of the membrane volume is lost due to the impermeable character of alumina). But even so, higher gas flow is obtained because of the particular composite structure (Fig. 4). First, the voids between the pore necks which control the gas-flow rate have no resistance to the flow. Secondly, the number of ends of chain in a PE membrane is much smaller than in the nDP-modified membrane. The diffusion step is improved by the higher mobility of the alkyl chains.
6. Selectivity factors for other gases The permeabilities of various gases have been measured for the nDP-modified alumina membrane (stage 5) and the results are summarised in Fig. 5. The low molecular weight alkane series shows similar behaviour than for permeation through lowdensity polyethylene membrane. For PE, an increase in the molecular weight gives lower diffusivity factors but higher solubility factors leading to an overall
J. Randon, R. Paterson~Journal of Membrane Science 134 (1997) 219-223
Propane
Ethane
Methane
Carbon dioxide m
0
I
2 4 6 8 10 12 14 Selectivity factor / Nitrogen
nDP modified 1 A1203 membrane
Polyethylene I d=0.914 I
Polyethylene I d--0.964
Fig. 5. Selectivity factors of various gases compared to nitrogen for n-dodecylphosphate-modified alumina membrane (stage 5). Selectivity factors for low- and high-density polyethylene membranes are given for comparison.
permeability which increases from methane to propane. The same rules seems to be followed for the nDP-modified membrane. Carbon dioxide selectivity is lower than expected from PE characteristics, but selectivity depends on PE density (selectivity varies from 12.6 to 2.5 for, respectively, low- and high-density polyethylene). Hydrogen selectivity is of the same order for high as for low density PE.
7. Conclusion
The modification of the mesoporous membranesurface properties allows the control of the relative
223
contribution of different transport mechanisms. Mesoporous alumina membrane has been modified by chemical bonding of n-butane phosphonic acid and n-dodecylphosphate. The membrane has been tested in gas-separation process of low molecular weight hydrocarbons. This composite structure improves permeability and selectivity coefficient of propane/ nitrogen separation. Such type of structure can be created using any kind of organic molecules which are able to bind the oxide surface [5,6]. To improve separation for any specific gas application, the modifying molecules can be chosen, based on the knowledge of organic membrane, to obtain both high selectivity and permeability.
References [1] K. Keizer, R.J.R. Uhlhorn, V.T. Zaspalis and A.J. Burggraf, Transport and related gas and vapour separation in ceramic membranes, Key Engineering Materials, 61/62 (1991) 143. [2] T. Nakagawa, Gas separation and pervaporation, in: Y. Osada and T. Nakagawa (Eds.), Membrane Science and Technology, Marcel Dekker, 1992, p. 239. [3] J. Randon and R. Paterson, Modification of ceramic membrane surfaces using phosphoric acid and alkyl phosphonic acids and its effects on ultrafiltration of BSA protein, J. Membr. Sci., 98 (1995) 119. [4] S. Pauly, Permeability and diffusion data, in: J. Brandup and E.H. Immergut (Eds.), Polymer Handbook, 3rd edn., John Wiley and Sons, 1989. [5] C. Leger, H.L. Lira and R. Paterson, Preparation and properties of surface modified ceramic membranes. Part II: Gas and liquid permeability of 5 nm alumina membranes modified by a monolayer of bound polydimethyl siloxane silicone oil, J. Membr. Sci., 120 (1996) 135. [6] C. Leger, H.L. Lira and R. Paterson, Preparation and properties of surface modified ceramic membranes. Part III: Gas permeation of 5 nm alumina membranes modified by trichloro-octadecyl silane, J. Membr. Sci., 120 (1996) 187.