[HMIM][TfO] > [3C1C4N] [Tf2N] > [MOIM][PF6] > NMP
> [HMIM] [Tf2N] > [MOIM][BF4] > [MOIM][Tf2N] > [3C8C1N][Tf2N] >
[3C6C14P][BF4] > [3C6C14P][(C2F5)3PF3] > [C16MIM][BF4] >
[3C6C14P][PF6] >[3C6C14P] [Tf2N]
In terms of good compromise between selectivity and capacity,
FILs are arranged in decreasing order of limiting performance index values, as
follows:
[BMPy][BF4] > [Et3S][Tf2N] > [MMIM][Tf2N] >
[Epy][Tf2N] > [BMPyrr][Tf2N] > [HMIM][PF6] > [EMIM][Tf2N] >
[BMIM][SbF6] > [BMIM][Tf2N] > [HMIM][Tf2N] > [3C6C14P][(C2F5)3PF3]
> [EMIM][BF4] > [BMIM][TfO] > [EMIM][TfO] > [MOIM][Tf2N] >
[HMIM][BF4] > [MOIM][PF6] > NMP > [3C1C4N][Tf2N] > [BMIM][BF4] >
[EMIM][TFA] > [HMIM][TfO] > [3C8C1N] [Tf2N] > [MOIM][BF4] >
[3C6C14P][BF4] > Sulfolane > [3C6C14P][Tf2N] > [3C6C14P][PF6] >
[C16MIM][BF4]
The best selectivity is obtained with [EMIM] [BF4]. The ionic
liquid [BMPy] [BF4] presents the best compromise between selectivity and
capacity at infinite dilution. It is a remarkable fact that a great deal of
FILs would potentially perform better than industrial molecular solvents NMP
and Sulfolane, as separation agents. At 313.15 K [EMIM] [BF4] is twice as
selective as Sulfolane. The infinite dilution performance index of [BMPy] [BF4]
is three times higher than the one obtained with the same industrial molecular
solvent.
6.3.1.1. Imidazolium-based fluorinated ionic
liquids
According to Figures H-1 and H-2 in Appendix H, favorable
structures of FILs for separating aromatic/aliphatic compounds mixtures are
that the alkyl chain of the cation is short and the anion is small with
sterical shielding around its charge centre. For the same anion, selectivity at
infinite dilution decreases with alkyl chain extension. For the same cation,
the smaller the anion,
the higher the limiting selectivity. In all cases, limiting
capacity and limiting selectivity for the n-hexane (1)/benzene (2) system vary
in inverse orders.
Extending the alkyl chain of the IL cation accentuates the
steric hindrance on the imidazolium ring. This results in poorer ability to
polarize the benzene molecule and thus, lower selectivity whereas interactions
with n-hexane do not vary notably. Capacity increases with increasing alkyl
chain length because greater van der Waals volume weakens the intrinsic
hydrogen bond within the IL.
Under the same anion, it is observed that selectivities of
pyrrolidinium and pyridinium-based FILs which have six-membered rings exhibit
higher selectivities of n-hexane to benzene in comparison with the
corresponding imidazolium-based ones. The similarity of their rings to benzene
explains this. It is one of the reasons why investigated phosphonium and
ammonium-based FILs manifest poor potential for separating aliphatic/aromatic
compounds mixtures. Kumar and Banerjee (2009) came up with a similar
interpretation when studying thiophene separation from diesel. They pointed out
that thiophene with its five-membered ring preferentially attaches more to an
imidazolium-based cation provided with a five-membered ring, in comparison with
a pyridinium or quinolinium (six-membered) cation.
In the absence of sterical shielding effect, the anion tends
to approach the positive center of the cation leading to a closed shape for the
molecule. Interactions between the positive charge centers of the IL cation and
the delocalized electrons of benzene are hindered since the polarity of the IL
is reduced. This is why anions with sterical shielding effect around the charge
center lead to higher selectivities. Moreover, the increase in the volume of
the cation results in a greater solvent ability to remove benzene.
These explanations are consistent with the recent publication
by Kumar and Banerjee (2009), as well as, the findings related to
olefin/aliphatic separation reported by Lei et al. (2006, 2007b) and hold for
non-polar systems discussed in the following sections as well. Concerning polar
systems, the overall effect of structure on selectivity and capacity is a
result of the relative magnitude of polarity and molecular volume of entities
composing the ionic liquid.
To understand the trends described hereafter, one should bear
in mind that, depending on anions comprising FILs , polarity together with
intrinsic strength of hydrogen bonding varies in the order:
[TFA]- [TfO]- > [BF4]- >
[PF6]- > [Tf2N] - . Volume increases as follows: [BF4] - <
[TFA] -
< [PF6] - < [TfO] - < [Tf2N] - .
