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Supported ionic liquid membranes in nanopore structure for gas separation and transport studies
Desalination 199 (2006) 535–537
Supported ionic liquid membranes in nanopore structure for gas separation and transport studies Quan Gan*, David Rooney, Yiran Zou School of Chemistry & Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, UK email: [email protected] Received 21 October 2005; accepted 2 March 2006
1. Introduction Research in room temperature ionic liquids (RTILs) has currently its main focus on exploring the unique and specific solvent and catalytic properties in homogeneous or heterogeneous catalysis aiming for greater selectivity, specificity and yield at reduced environmental liabilities. Many ionic liquids also exhibit unique gas solubility, transport and separation properties [1,2], providing opportunities for developing new gas separation/gas enrichment technologies using a thin layer ionic liquids as the separation barrier, a largely undeveloped field for potential commercial application of ionic liquids. Supported ionic liquid membranes have distinctive advantages over conventional SLMs because the non-volatile viscous nature of ionic liquids means they cannot evaporate or easily displace from supporting media to cause contamination of the gas streams. 2. Materials and experimental methods The ionic liquids used in this study contain a common anion [NTf2], bis-(trifluoromethylsul*Corresponding author.
fonyl)-imide, but differ in cations. The molecular structures of the ionic liquids are shown in Table 1. We used for the first time nanofiltration membranes as the support media.
3. Results and discussion 3.1. SILM stability The new SILM system based on nanofiltration membranes exhibited an incredible stability at high gas phase pressure, which was not reported in the past for SILMs using microporous membranes as support. The stabilising mechanism based on the interactions between cations/anions with the charged NF membrane materials in a nano-spatial scale is not entirely clear. It is speculated that strong cationic/anionic interactions with functional groups present at the nanopore walls within a radial dimension of 1–10 Å may form structured matrix arrangement of the cations/anions within the nanopore networks. Such a formation could give rise to unique gas transport and separation properties different to ionic liquids in bulk liquid condition. The system stability and gas transport/separation properties were also found dependent on the type of nanofiltration membranes, membrane materials and pore sizes.
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.122
Q. Gan et al. / Desalination 199 (2006) 535–537
Table 1 Molecular structure of [C4-mim][NTF2], [C10-mim] [NTf2], [N8881][NTf2], and [C8Py][NTf2]
N2/CO2 selectivity
536
100 90 80 70 60 50 40 30 20 10 0 0
2
4
6
8
10
Gas phase pressure
Fig. 2. Calculated N2/CO2 selectivity in Bmim[NTf2] based on single gas feed permeability.
3.2. Gas permeation and separation
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
optima (Fig. 2) obtained at gas phase pressure of 4.0 bar based on the calculation of permeability coefficient in single gas feed experiments. It was rather revealing that, contrary to the findings of Quinn [1] and Scovazzo et al. [2] which were obtained at much lower pressure range, our unexpected result was attained despite CO2 solubility in [Bmim][BF4] is 28 times higher than that of N2 at 303 K and 76.5 kPa [3]. Fig. 3 shows H2/CO selectivity in four different ionic liquids supported using the SILM system. Selectivity generally decreased with increasing pressure.
5.50 5.00
N8881 C8Py
C4mim C10mim
4.50 H2/CO selectivity
Permeability × 102 (cm3cm/cm2/bar/s)
Fig. 1 presents typical fluxes of N2 and CO2 through [Bmim]NTf2 supported on a nanofiltration membrane. The gas fluxes through the pristine membrane without ionic liquids are 3 orders of magnitude higher than that through the SILM system, vindicating that gas transport is conducted through solution–diffusion mechanism, rather than convective flow or Knudsen diffusive flux through naked nanopores. Selectivity and permeability of N2 and CO2 were found nonlinearly dependent on gas phase pressure (Fig. 2). Another interesting finding is that N2/CO2 selectivity is also highly pressure dependent and there exists a selectivity N2/CO2
4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50
0
2
4 6 Pressure (bar)
8
10
Fig. 1. Permeability of N2 & CO2 Bmim[NTf2] supported on nanomembranes.
2
3
4 5 6 Gas phase pressure (bar)
7
8
Fig. 3. Dependence of H2/CO selectivity on gas pressure in ionic liquids.
Q. Gan et al. / Desalination 199 (2006) 535–537
4. Conclusions
References
The high stability of the SILM system supported in nanopore structure enables a new proposition for future application of supported liquid membrane technology in gas separation and enrichment. The new SILM platform also offers a new approach for studying fundamental gas transport and selectivity in ionic liquids which are still largely incomplete and in need of independent verification for existing reported data. The observed long run stability and gas separation capability of the bench scale SILM system indicates that the SILM based on nanomembrane support has promising potential to be scaled up for commercial application.
[1]
[2]
[3]
537
R. Quinn, J.B. Appleby and G.P. Pez, Hydrogen sulfide separation from gas streams using salt hydrate chemical absorbents and immobilized liquid membranes, Sep. Sci. Technol., 37 (2002). P. Scovazzo, J. Kieft, D.A. Finan, C. Koval, D. DuBois and R. Noble, Gas separations using non-hexafluorophosphate [PF6]– anion supported ionic liquid membranes, J. Membr. Sci., 238 (2004) 57–63. J. Jacquemin, P. Husson, V. Majer and M.F. Costa Gomes, Low-pressure solubilities and thermodynamics of solvation of eight gases in 1-butyl3-methylimidazolium hexafluorophosphate, Fluid Phase Equilib., 240 (2006) 87–95.
Supported ionic liquid membranes in nanopore structure for gas separation and transport studies Quan Gan*, David Rooney, Yiran Zou School of Chemistry & Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, UK email: [email protected] Received 21 October 2005; accepted 2 March 2006
1. Introduction Research in room temperature ionic liquids (RTILs) has currently its main focus on exploring the unique and specific solvent and catalytic properties in homogeneous or heterogeneous catalysis aiming for greater selectivity, specificity and yield at reduced environmental liabilities. Many ionic liquids also exhibit unique gas solubility, transport and separation properties [1,2], providing opportunities for developing new gas separation/gas enrichment technologies using a thin layer ionic liquids as the separation barrier, a largely undeveloped field for potential commercial application of ionic liquids. Supported ionic liquid membranes have distinctive advantages over conventional SLMs because the non-volatile viscous nature of ionic liquids means they cannot evaporate or easily displace from supporting media to cause contamination of the gas streams. 2. Materials and experimental methods The ionic liquids used in this study contain a common anion [NTf2], bis-(trifluoromethylsul*Corresponding author.
fonyl)-imide, but differ in cations. The molecular structures of the ionic liquids are shown in Table 1. We used for the first time nanofiltration membranes as the support media.
3. Results and discussion 3.1. SILM stability The new SILM system based on nanofiltration membranes exhibited an incredible stability at high gas phase pressure, which was not reported in the past for SILMs using microporous membranes as support. The stabilising mechanism based on the interactions between cations/anions with the charged NF membrane materials in a nano-spatial scale is not entirely clear. It is speculated that strong cationic/anionic interactions with functional groups present at the nanopore walls within a radial dimension of 1–10 Å may form structured matrix arrangement of the cations/anions within the nanopore networks. Such a formation could give rise to unique gas transport and separation properties different to ionic liquids in bulk liquid condition. The system stability and gas transport/separation properties were also found dependent on the type of nanofiltration membranes, membrane materials and pore sizes.
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.122
Q. Gan et al. / Desalination 199 (2006) 535–537
Table 1 Molecular structure of [C4-mim][NTF2], [C10-mim] [NTf2], [N8881][NTf2], and [C8Py][NTf2]
N2/CO2 selectivity
536
100 90 80 70 60 50 40 30 20 10 0 0
2
4
6
8
10
Gas phase pressure
Fig. 2. Calculated N2/CO2 selectivity in Bmim[NTf2] based on single gas feed permeability.
3.2. Gas permeation and separation
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
optima (Fig. 2) obtained at gas phase pressure of 4.0 bar based on the calculation of permeability coefficient in single gas feed experiments. It was rather revealing that, contrary to the findings of Quinn [1] and Scovazzo et al. [2] which were obtained at much lower pressure range, our unexpected result was attained despite CO2 solubility in [Bmim][BF4] is 28 times higher than that of N2 at 303 K and 76.5 kPa [3]. Fig. 3 shows H2/CO selectivity in four different ionic liquids supported using the SILM system. Selectivity generally decreased with increasing pressure.
5.50 5.00
N8881 C8Py
C4mim C10mim
4.50 H2/CO selectivity
Permeability × 102 (cm3cm/cm2/bar/s)
Fig. 1 presents typical fluxes of N2 and CO2 through [Bmim]NTf2 supported on a nanofiltration membrane. The gas fluxes through the pristine membrane without ionic liquids are 3 orders of magnitude higher than that through the SILM system, vindicating that gas transport is conducted through solution–diffusion mechanism, rather than convective flow or Knudsen diffusive flux through naked nanopores. Selectivity and permeability of N2 and CO2 were found nonlinearly dependent on gas phase pressure (Fig. 2). Another interesting finding is that N2/CO2 selectivity is also highly pressure dependent and there exists a selectivity N2/CO2
4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50
0
2
4 6 Pressure (bar)
8
10
Fig. 1. Permeability of N2 & CO2 Bmim[NTf2] supported on nanomembranes.
2
3
4 5 6 Gas phase pressure (bar)
7
8
Fig. 3. Dependence of H2/CO selectivity on gas pressure in ionic liquids.
Q. Gan et al. / Desalination 199 (2006) 535–537
4. Conclusions
References
The high stability of the SILM system supported in nanopore structure enables a new proposition for future application of supported liquid membrane technology in gas separation and enrichment. The new SILM platform also offers a new approach for studying fundamental gas transport and selectivity in ionic liquids which are still largely incomplete and in need of independent verification for existing reported data. The observed long run stability and gas separation capability of the bench scale SILM system indicates that the SILM based on nanomembrane support has promising potential to be scaled up for commercial application.
[1]
[2]
[3]
537
R. Quinn, J.B. Appleby and G.P. Pez, Hydrogen sulfide separation from gas streams using salt hydrate chemical absorbents and immobilized liquid membranes, Sep. Sci. Technol., 37 (2002). P. Scovazzo, J. Kieft, D.A. Finan, C. Koval, D. DuBois and R. Noble, Gas separations using non-hexafluorophosphate [PF6]– anion supported ionic liquid membranes, J. Membr. Sci., 238 (2004) 57–63. J. Jacquemin, P. Husson, V. Majer and M.F. Costa Gomes, Low-pressure solubilities and thermodynamics of solvation of eight gases in 1-butyl3-methylimidazolium hexafluorophosphate, Fluid Phase Equilib., 240 (2006) 87–95.