As indicated by Figure H-3 in Appendix H, for the n-hexane
(1)/benzene (2) system
decreases with the increasing anion volume. Selectivity values
remain very small as compared
to Sulfolane, a commonly used industrial
solvent for aliphatic/aromatic compounds mixtures.
Smaller anions lead to higher selectivities. The value for
[3C6C14P] [(C2F5)3PF3] is an
outlier, probably due to differences in experimental
conditions. Limiting capacity tends to increase with increasing anion volume.
In the absence of data related to the ionic liquid [3C6C14P] [PF6] which were
provided by this thesis, it would be difficult to reliably derive these
hierarchies of limiting capacity and selectivity values with only the
previously published experimental results. This emphasizes the contribution of
the present study to the expansion of knowledge in relation of FILs.
6.3.1.3. Ammonium-based fluorinated ionic
liquids
Only the effect of the cation can be discussed in the light
of available data. Extending the alkyl chain of the ionic liquid leads to lower
selectivities and higher capacities. This is observed in Figure H-4 in Appendix
H.
6.3.2. Methanol (1)/benzene (2) separation
problem
Infinite dilution selectivities of the experimentally
investigated fluorinated ionic liquids for the Methanol (1)/benzene (2) system
decrease in this order:
[3C6C14P][(C2F5)3PF3] > [3C6C14P][PF6] > [3C6C14P] [Tf2N]
> [3C8C1N] [Tf2N] >
[MOIM][PF6] > [MOIM] [Tf2N] > [HMIM] [Tf2N] >
[HMIM][PF6] > [C16MIM][BF4] > [BMIM][SbF6] > [3C6C14P][BF4] >
[Et3S] [Tf2N] > [BMIM] [Tf2N] > [EMIM] [Tf2N] > [3C1C4N] [Tf2N] >
[Epy] [Tf2N] > [MOIM][BF4] > [BMPy][BF4] > [BMIM][BF4] >
[BMIM][TfO] > [HMIM][BF4] > [EMIM][TfO] > [DMPIM][BF4] >
[EMIM][BF4] > [EMIM][TFA]
Performance indices follow the sequence below:
[3C6C14P][(C2F5)3PF3] > [3C6C14P] [Tf2N] > [3C8C1N] [Tf2N]
> [3C6C14P][PF6] >
[3C6C14P][BF4] > [MOIM] [Tf2N] > [HMIM] [Tf2N] >
[MOIM][PF6] > [C16MIM][BF4] > [HMIM][PF6] > [BMIM] [Tf2N] > [EDMIM]
[Tf2N] > [Et3S] [Tf2N] > [BMIM][SbF6] > [EMIM] [Tf2N] > [3C1C4N]
[Tf2N] > [Epy] [Tf2N] > [MOIM][BF4] > [BMPy][BF4] > [BMIM][TfO]
> [HMIM][BF4] > [BMIM][BF4] > [EMIM][TfO] > [EMIM][BF4] >
[EMIM][TFA]
Depending on whether methanol is collected as part of the
bottom or overhead stream, [EMIM] [TFA] and [3C6C14P] [(C2F5)3PF3] are found to
be the best solvents for this separation problem. Due to the absence of the
required data, no comparison with industrial molecular solvent is attempted.
6.3.2.1. Imidazolium-based fluorinated ionic
liquids
For the same anion, it is observed in Figures H-2 and H-5
that infinite dilution selectivity and capacity for the methanol (1)/benzene
(2) decrease when the alkyl chain is shortened. Figure H-2 is applicable to
both methanol/benzene and hexane/benzene capacity values. Experimental data do
not totally agree on a common pattern as far as the effect of the anion is
concerned. However, according to Figure H-5, it seems that imidazolium-based
fluorinated ionic liquids with low selectivity are the ones containing anions
such as [BF4]-, [TFA] - and [TfO]-, characterized by
small volume and a sterical shielding effect around the anion charge centre. As
far as limiting capacity is concerned, it is found to increase with increasing
anion volume.
6.3.2.2. Phosphonium-based fluorinated ionic
liquids
Limiting selectivity values of phosphonium-based FILs containing
the common cation
[3C6C14P] + are observed to decrease when the anion is changed
in this order (Figure H-6): [(C2F5)3PF3]- > [PF6] - > [Tf2N] -
> [BF4]-
The general trend is that limiting selectivity and limiting
capacity increase with increasing anion volume and decreasing polarity of the
ionic liquid, [3C6C14P] [PF6] being an outlier.
6.3.2.3. Ammonium-based fluorinated ionic
liquids
Figure H-7 in Appendix H suggests that lengthening the ionic
liquid alkyl chain leads to higher
values of infinite dilution selectivity and capacity for the
methanol (1)/benzene (2) system.
6.3.3. Methanol (1)/acetone (2) separation
problem
For the methanol (1)/acetone (2) system, the hierarchy of
selectivity values is:
[BMIM][SbF6] > [3C6C14P][Tf2N] > [HMIM][Tf2N] >
[3C1C4N][Tf2N] > [EMIM][Tf2N] > [3C6C14P][PF6] > [3C8C1N][Tf2N] >
[BMIM][PF6] > [BMIM][Tf2N] > [Epy][Tf2N] > [BMPy][BF4] >
[C16MIM][BF4] > [MOIM][BF4] > [3C6C14P][BF4] > [BMIM][BF4] >
[HMIM][BF4] > [BMIM][TfO] > [EMIM][BF4] > Dimethylsulfoxide
Infinite dilution performance indices for the methanol
(1)/acetone (2) system can be arranged in this order:
Dimethylsulfoxide > [3C6C14P] [Tf2N] > [HMIM] [Tf2N]
> [BMIM] [Tf2N] > [3C8C1N] [Tf2N] > [EMIM] [Tf2N] > [BMIM][SbF6]
> [3C1C4N] [Tf2N] > [3C6C14P][BF4] > [Epy] [Tf2N] > [3C6C14P][PF6]
> [BMPy][BF4] > [BMIM][PF6] > [MOIM][BF4] > [HMIM][BF4] >
[BMIM][TfO] > [C16MIM][BF4] > [BMIM][BF4] > [EMIM][BF4].
[BMIM][SbF6] and [3C6C14P] [Tf2N] lead to the
highest selectivity and performance index respectively. No FIL is potentially
better than dimethylsulfoxide, one of the most suitable molecular solvents for
this separation problem. The limiting selectivity of [BMIM][SbF6] is 19 %
smaller than the one given by dimethylsulfoxide. Compared with
[3C6C14P] [Tf2N] , the infinite dilution performance index of DMSO
is three times higher.
6.3.3.1. Imidazolium-based fluorinated ionic
liquids
Data represented in Figure H-8 show that under the same
anion, selectivity and capacity tend to increase with increasing alkyl chain
length of the FIL cation. Both properties increase with increasing anion
volume. It can be noted that [BMIM] +-containing ionic liquids do
not rigorously comply with trends possibly due to experimental uncertainties.
More data are required for a more accurate description of the influence of
structure on the separation performance of imidazolium-based FILs for the
methanol (1)/acetone (2) system.
6.3.3.2. Phosphonium-based fluorinated ionic
liquids
An examination of Figure H-10 in Appendix H reveals that
infinite dilution selectivity for the methanol (1)/acetone (2) system increases
with increasing anion volume. No regular variation trend regarding capacity
emerges.
6.3.3.3. Ammonium-based fluorinated ionic
liquids
Infinite dilution selectivity of ammonium-based FILs for the
methanol (1)/acetone (2) system reduces when the alkyl chain is extended as
shown by Figure H-11. The reverse is true for limiting capacity.
6.3.4. n-hexane (1)/ hex-1-ene (2) separation
problem
Selectivity of n-Hexane to Hex-1-ene decreases according to this
order:
[BMPyrr][TfO] > [EMIM][TFA] > [Epy] [Tf2N] >
[EMIM][TfO] > [MMIM][Tf2N]> [BMIM][TfO] > [Et3S] [Tf2N] >
[HMIM][PF6] > [BMIM][SbF6] > [EMIM] [Tf2N] > [EDMIM] [Tf2N] >
[EMIM][BF4] > [HMIM][BF4] > [BMPyrr] [Tf2N] > [BMIM] [Tf2N] >
[3C1C4N] [Tf2N] > [MOIM][PF6] > [MOIM][BF4] > [BMIM][BF4] >
[HMPyrr] [Tf2N] >
[HMIM] [Tf2N] > [MOIM] [Tf2N] > [OMPyrr] [Tf2N] >
[C16MIM][BF4] > [3C8C1N] [Tf2N]
> [3C6C14P][BF4] > [3C6C14P][(C2F5)3PF3] > [3C6C14P]
[Tf2N] > [3C6C14P][PF6] > NMP.
Performance indices follow this trend:
[3C6C14P][(C2F5)3PF3] > [3C6C14P] [Tf2N] > [3C6C14P][BF4]
> [3C8C1N] [Tf2N] >
[C16MIM][BF4] > [3C6C14P][PF6] > [MOIM] [Tf2N] >
[OMPyrr] [Tf2N] > [HMIM] [Tf2N] > [HMPyrr] [Tf2N] > [BMPyrr] [Tf2N]
> [MOIM][PF6] > [BMIM] [Tf2N] > [MOIM][BF4] > [HMIM][PF6] >
[HMIM][BF4] > [3C1C4N] [Tf2N] > [Et3S] [Tf2N] > [Epy] [Tf2N] >
[EMIM] [Tf2N] > [EDMIM] [Tf2N] > [BMIM][SbF6] > [BMIM][TfO] >
[MMIM] [Tf2N] > [BMPyrr][TfO] > NMP > [EMIM][TfO] > [EMIM][TFA]
> [BMIM][BF4] > EMIM][BF4].
On the basis of experimental data, the best selectivity is
obtained with the ionic liquid [BMPyrr] [TfO]. It amounts to an increase of 30
% when compared to NMP. It can be seen that the ionic liquid [3C6C14P]
[(C2F5)3PF3] leads to the best compromise between selectivity and capacity. The
calculated value of its limiting performance index is 29 times higher than NMP.
Where comparison is possible, the above hierarchies are consistent with the
findings of Lei and coworkers (Lei et al. 2006, 2007) in their study related to
n-hexane (1)! hex-1-ene (2) separation using the quantum approach. In relation
to the effect of the anion, their finding is that higher selectivities are
obtained with smaller anions with sterical shielding effect around the anion
charge centre. Additionally, they predicted that the best ionic liquid for this
separation problem is 1-octylquinolinium bis (trifluoromethylsulfonyl) imide.
However, the present study could not confirm this result due to the lack of
experimental infinite dilution activity coefficient data in the ionic liquid
1-octylquinolinium bis (trifluoromethylsulfonyl) imide.
6.3.4.1. Imidazolium-based Fluorinated ionic
liquids
For the same anion, selectivity at infinite dilution
decreases with increasing alkyl chain length (Figure H-12). A look at Figure
H-12, as well as the hierarchy of selectivities provided in the previous
section reveals that small anions with sterical shielding effect around the
charge center are essential to achieve high selectivities. Capacity increases
with increasing alkyl chain length, as well as, increasing anion volume, as
illustrated by Figure H-13.
6.3.4.2. Phosphonium-based fluorinated ionic
liquids
For phosphonium-based FILs containing the [3C6C14P] + cation,
selectivity at infinite dilution for the n-hexane (1)! hex-1-ene (2) system
decreases slightly with increasing anion size (Figure H-14) whereas capacity at
infinite dilution for this system and this class of FILs seems to be
constant.
6.3.4.3. Ammonium-based Fluorinated ionic
liquids
With increasing alkyl chain length the limiting selectivity of
ammonium-based FILs decreases and limiting capacity increases, as can be seen
in Figure H-15.
6.3.4.4. Pyrrolidinium-based Fluorinated ionic
liquids
The available experimental data used in Figure H-16, Appendix
H, suggest that for the same cation, limiting selectivity of
pyrrolidinium-based FILs reduces when the anion size increases, whereas
limiting capacity follows the opposite trend.; and for the same anion, the
longer the alkyl chain of the ionic liquid cation, the smaller the selectivity
and the higher the capacity (Figure H-16).
6.3.5. Benzene (1)/ butan-2-one (2) separation
problem
Limiting selectivities of the experimentally investigated
fluorinated ionic liquids follow this pattern:
[EMIM] [Tf2N] > [Epy][Tf2N] > [BMIM][SbF6] >
[BMPy][BF4] > [HMIM][Tf2N] > [BMPyrr][Tf2N]> [EMIM][BF4] >
[BMIM][BF4] > [HMIM][BF4] > [3C6C14P][Tf2N] > [3C8C1N][Tf2N] >
[MOIM][BF4] > [3C6C14P][PF6] > [3C6C14P][BF4] > [C16MIM][BF4]
In terms of good compromise between selectivity and capacity,
FILs are arranged in decreasing order of limiting performance index values as
follows:
[EMIM][Tf2N] > [HMIM][Tf2N] > [3C6C14P][Tf2N] >
[3C8C1N][Tf2N] > [BMIM][SbF6] > [Epy][Tf2N] > [BMPyrr][Tf2N]>
[3C6C14P][BF4] > [BMPy][BF4] > [3C6C14P][PF6] > [HMIM][BF4] >
[MOIM][BF4] > [EMIM][BF4] > [BMIM][BF4] > [C16MIM][BF4].
The best selectivity and the best performance index are obtained
with [EMIM] [Tf2N]. No
comparison with industrial solvents is possible due to lack of
required experimental data.
6.3.5.1. Imidazolium-based fluorinated ionic
liquids
Figures H-17 and H-18 show that with increasing anion volume,
the limiting selectivity and the limiting capacity of imidazolium-based FILs
for the benzene (1)/butan-2-one (2) system increase. And the longer the alkyl
chain of the FIL cation, the smaller the selectivity and the higher the
capacity at infinite dilution.
6.3.5.2. Phosphonium-based fluorinated ionic
liquids
Due to a very limited database, it is not possible to reliably
derive the effect of structure on
selectivity or capacity in this case using
Figure H-19. Only three ionic liquids having a common
cation are considered and the obtained selectivity and capacity
plots do not allow any derivation of the effect of the anion on these two
properties.
