(174ap) Supported Liquid Membranes for ? Electron Induced Fractionation and Separation of Aromatics and Stereoisomers

Kamaz, M., University of Arkansas
Sengupta, A., University of Arkansas
Qian, X., University of Arkansas
Wickramasinghe, R., University of Arkansas
Supported ionic liquid membranes (SILM) using imidazolium based ionic liquids and hydrophobic polypropylene (PP) base membrane were used for unconventional non-aqueous medium separation. Ionic liquid based supported liquid membranes have been utilized for the unconventional π electron mediated fractionation of aromatics and separation of stereoisomers. The FTIR of SILMs showed the signature of the functionalities for both base membrane and the imidazolium moiety. The spectrum for the base membrane showed peaks corresponding to C-H and C=C bond stretching. For ionic liquid membranes, extra peaks were observed in the range 1500 cm−1 to 1700 cm−1, which was attributed to the stretching frequency of imidazolium ring. The SEM image confirmed the pore filling of base membrane with the selected ionic liquids. SEM images exhibited that the ILs were homogeneously distributed in the membrane substrate filling the pores of the membranes. The stability of the SILMs was studied using hexane, tetradecane and water solvents. The effect of alkyl chain length on imidazolium moieties was found to influence significantly on the separation behavior as it influences the π electron density on the imidazolium ring of ionic liquid. The fractionations followed the trend phenanthrene > naphthalene > benzene, which was attributed to the extent of the π electron cloud density due to the extensive conjugation on multiple aromatic rings. The nature of substituents on the aromatic ring was also found to influence the separation as seen from the fractionation trend of nitrobenzene < benzene < toluene.

The ionic liquids were having affinity for the π electron cloud. This π electron cloud density for phenanthrene is higher than naphthalene followed by benzene, due to the presence of extended conjugations of benzyl rings and hence the aromatic π electron cloud. Due to this extended conjugation, phenanthrene interacted strongly with the ionic liquid sitting inside pores. This might be responsible for bringing the selectivity in the separation for the SILMs. The lower transport in case of allyl ionic liquid might be attributed to the presence of electron withdrawing sp2 hybridized carbon of the ally group resulting reduction in the electron density over imidazolium ring. The (+) inductive effect of the alkyl group containing sp3 hybridized carbon atom pushed the electron density towards imidazolium ring. More electron cloud on the imidazolium ring, it will be more favorable for the π electron interaction resulting enhancement in the separation efficiency.

The electron withdrawing effect of nitro (-NO2) group and the electron donating inductive effect of methyl group modifies the π electron cloud density on aromatic system and was the basis for their mutual fractionation. The cis conformation of stilbene has the stereo-chemical configuration of phenyl group existing in the same side resulting loss of planarity due to stereo-chemical crowding. This led to loss of extended conjugation, while such stereo-chemical constrains was not present in the trans isomer and hence the extensive conjugation brings higher density of π electron cloud favoring π stacking interaction and its preferential separation compared to cis isomer. These ionic liquid supported membranes were having significant stability over nonpolar nonprotic solvent. However, polar protic solvent like water brings instability to these supported liquid membranes for electrostatic interaction.

Since the stability of the SLIMs could be a serious concern despite the extraordinary membrane performance, membranes were submerged to an equilibration with the feed solutions encountered for separation and the conductivity of the supernatant solutions was measured. When leaching off ionic liquids occurs, ILs would modify the medium conductivity. All the organic solvents used did not show any change in conductivity over one week, on the other side in the presence of water the conductivity was found to increase very rapidly with time. This test revealed that the SILM membranes are highly stable in organic non-polar solvent solution, while not stable in aqueous media. The high degree of stability of the SILM membranes might be due to high capillary force between ionic liquids and the membrane substrate and their high viscosity as well. The capillarity force could be defined by the ratio between the viscosity and the surface tension acting across the interface layer. Apparently, the more viscous fluids, the higher is the capillary force and thus the capillarity effect is stronger.

Membranes with higher viscosity ionic liquids, shorter alkyl side chain length, have higher stability. Ionic liquids used in this study are highly soluble in water and a small amount of water impurity in the solvent can be problematic for the stability of SILM membranes. The stability test results for SILM membranes are shown in figure 3.9. Another technique to investigate SILM membranes stability is using ILLP which is simply the minimal required applied pressure to push ionic liquid out of the membrane pores. The DI water filled membrane exhibited ILLP of 225 kPa while its for SILMs went up to 541 kPa. The ILLP values detoriated for the ionic liquids with less viscosity showing the lowest value of 332 kPa. Thus, the immobilization of ionic liquids helped improve the stability of the membrane over pressures larger than of DI water as a carrier. The higher ILLP is correlated to the enhancement of the stability of ionic liquids inside the membranes pores as more pressure requires to be applied to push them out of the membrane.