(387i) Design of Ionic Liquids for Aliphatic/Aromatic Separations

In a supported liquid membrane the given solvent is immobilized inside the porous structure of the membrane which separates the feed phase from the receiving phase. The obvious advantage of such a system is that, it requires minimal amount of solvent and the solvent is continuously regenerated as the solute moves to the receiving phase. Ionic liquids are particularly suited for this application as they have very low vapor pressure and hence the solvent loss is negligible. The fact that the solubility of ionic liquids in the surrounding phases can be tailored, allows us to develop very stable supported liquid membranes.

Ionic liquids are molten salts that exist as liquids at a relatively low temperature. Ionic liquids consist of a large organic cation and a charge-delocalized inorganic or organic anion. These liquids are entirely composed of ions and are characterized by weak interactions between the cation and anion through hydrogen bonds and van der Walls forces. ILs are considered as ‘green’ solvents because of their extremely low vapor pressure due to the columbic attraction of the ions (< 10^-9 bar). In addition ionic liquids have high thermal stability (thermal decomposition temperature of ~ 400 K).

The ionic liquids considered for this application needs to have the following characteristics a) should have high selectivity for aromatics; b) should have high aromatic distribution coefficient; c) moderate viscosity. In addition the ionic liquid should have relatively high long term thermal stability at higher temperatures so that the feed stream temperature/pressure need not be decreased significantly.

In this work we present a computer-aided ionic liquid design (CAILD) approach for designing task specific ionic liquids for separation of aromatics from aliphatics. This algorithm helps design optimal ionic liquid structure by appropriate selection/modification of the cation, the anion and/or the alky chains attached to the cation. The ionic liquids are tailored to have high selectivity, distribution coefficient and solubility for aromatics, wide liquid range, higher thermal stability and optimal physico-chemical properties (e.g. viscosity). The key criterion for ionic liquid selection for this application is aromatic selectivity.