Infinite dilution selectivities of the experimentally
investigated fluorinated ionic liquids for the ethanol (1)/butan-2-one (2)
system decrease in this order:
Chlorobenzene > [3C6C14P] [Tf2N] > [BMIM][SbF6] >
[EMIM] [Tf2N] > [HMIM] [Tf2N] > [3C8C1N] [Tf2N] > [BMPyrr] [Tf2N] >
[3C6C14P][PF6] > [BMIM][PF6] > [Epy] [Tf2N] > [BMPy][BF4] >
[MOIM][BF4] > [C16MIM][BF4] > [HMIM][BF4] > [3C6C14P][BF4] >
[BMIM][BF4] > [EMIM][BF4].
Performance indices follow the sequence below:
[3C6C14P] [Tf2N] > [3C8C1N] [Tf2N] > [HMIM] [Tf2N] >
[EMIM] [Tf2N] > [BMPyrr] [Tf2N] > [BMIM][SbF6] > [3C6C14P][PF6] >
[Epy] [Tf2N] > Chlorobenzene > [BMIM][PF6] > [3C6C14P][BF4] >
[BMPy][BF4] > [MOIM][BF4] > [C16MIM][BF4] > [HMIM][BF4] >
[BMIM][BF4] > [EMIM][BF4].
On the basis of selectivity and performance index at infinite
dilution, the best potential solvent for this separation problem is
[3C6C14P] [Tf2N]. Chlorobenzene, a commonly used industrial
molecular solvent is twice as selective as [3C6C14P] [Tf2N]. However
its limiting performance index is 3.25 times smaller than the one achieved with
the same ionic liquid.
6.3.6.1. Imidazolium-based fluorinated ionic
liquids
According to Figure H-20 and H-18 (Since the capacity value
at a given temperature for the ethanol/ butan-2-one is the same as for the
benzene/butan-2-one system), high limiting selectivity and capacity are
achieved with imidazolium-based fluorinated ionic liquids consisting of
long-chained cations and big-sized anions.
6.3.6.2. Phosphonium-based fluorinated ionic
liquids
It can be seen in Figure H-21 that the limiting selectivity
of phosphonium-based fluorinated ionic liquids for the ethanol (1)/ butan-2-one
(2) system increases with increasing anion volume. It is impossible to derive a
reliable trend for capacity variation.
6.4. Correlation of limiting activity coefficient and
selectivity with the FIL alkyl chain
It is the outcome of this study that a simple model can
correlate with the alkyl chain length
of the pyrrolidinium and
imidazolium-based FILs at constant temperature. As shown in the plots
presented in Figures I-1 through I-4 in Appendix I, obtained
from experimental data published
by different groups under different
conditions, the natural logarithm of of n-hexane, hex-1-
ene, cyclohexane, ethanol, benzene and acetone is a linear
function of the carbon number of the alkyl chain attached to the imidazolium or
pyrrolidinium moiety. The quality of correlation is good except for ketones and
alkan-1-ols in [BF4]--containing FILs. Available data allowed to
examine imidazolium-based FILs containing [BF4]- (Figure I-1),
[Tf2N]- .
(Figure I-2) and [TfO]- (Figure I-3) anions as well
as a few systems involving pyrrolidinium- based fluorinated ionic liquids
(Figure I-4) The correlating equation is:
(6.1)
where a and b are parameters related to the
nature of the anion, as well as, the solute under consideration. is obviously
always greater than zero. Consequently, it can be stated that the
natural logarithm of both limiting selectivity and capacity of
imidazolium-based FILs for different systems are also linear functions of the
carbon number of the alkyl chain attached to the methylimidazolium as well as
the methylpyrrolidinium. A similar equation to (6-1) can be used to relate
selectivity to the alkyl chain length as follows:
(6.2)
Figures I-5 and I-6 in Appendix D show a good quality of the
linear regression used to approximate experimental selectivity data. Systems
referred to include n-hexane/benzene (Figure I-5) and n-hexane/hex-1-ene
(Figure I-6).
