(178a) Solvatochromic Studies On The CO2 Antisolvent Ability In Ionic Liquid/organic Mixtures | AIChE

(178a) Solvatochromic Studies On The CO2 Antisolvent Ability In Ionic Liquid/organic Mixtures

Authors 

Mellein, B. R. - Presenter, University of Notre Dame
Brennecke, D. J. - Presenter, University of Notre Dame


Solvatochromic Studies on the CO2 Antisolvent Ability
in Ionic Liquid/Organic Mixtures

            Ionic Liquids (ILs) have proven to be apt
solvents for a variety of reactions.  However, separations of products and
impurities, such as spent catalyst, can be difficult to remove.  Easy
separations are needed to ensure the ability to recycle the IL, an important
aspect of economic feasibility.  CO2 can be used as a separation
aid, such as for supercritical CO2.  However, this method only works
for solutes that are soluble in CO2.  In this paper, we discuss the
use of CO2 to remove solutes that are not soluble in CO2
by using CO2 as an antisolvent.  This behavior can be explained in
terms of solvent strength using solvatochromic probes.

            CO2 can be used as a separation aid
by acting as an antisolvent.  As CO2 is added to a liquid with a
dissolved solute, the pressure increases and CO2 dissolves into the
liquid mixture.  This alters the solvation ability of the solvent, and
essentially creates a supersaturated solution.  At this point, for a specific
temperature and pressure, a phase separation occurs.  For a liquid solute, this
is the Lower Critical End Point (LCEP).  For a solid solute, it is the
nucleation pressure.  This behavior has been documented for a variety of organic-based
systems.  The accepted explanation for this behavior  is that the CO2
causes the organic to expand greatly, which lowers the solvent strength and
thus induces a phase split.  This behavior has also been seen in IL systems, as
well as for liquid organic solvents in ILs[1-3] and solid solutes in ILs[4, 5].  However, ILs do not expand greatly.  This behavior is not well understood in IL systems

            In order to understand the mechanisms behind
this behavior, a variety of ILs and organics were studied.  The main IL studied
was [hmim][Tf2N], the IUPAC standard.  The organics chosen for this
study were acetonitrile, which is polar and aprotic, 2-butanone, a hydrogen
bond acceptor, and 2,2,2-trifluoroethanol, a strong hydrogen bond donor.  In
order to study the effect of the acidic hydrogen on the C2 position of the
imidazolium ring, we studied [hmmim][Tf2N]/2-butanone mixtures.  For
the anion, we studied [hmim][TfO], where [TfO] is a stronger hydrogen bond
acceptor, with 2,2,2-trifluoroethanol.  As another type of IL, we looked at [N2113][Tf2N]
with each of the organics.  The solubility of CO2 in the IL/organic
mixtures and the volume expansion were also measured.  It was found that less CO2
was required with mixtures that have less interactions between the IL and
organic.  The ease of separation from [hmim][Tf2N] is as follows:
2-butanone > acetonitrile > 2,2,2-trifluoroethanol.  In addition, it was
found that the ease of separation depends on the CO2 solubility
rather than volume expansion.  The higher the CO2 solubility in the
IL/organic mixture, the easier it is to induce a phase split.

            CO2 can also be used to induce a
phase split for solid solutes in ILs.  A copper compound, which acts as a model
catalyst, can be removed from IL/organic mixtures using CO2
pressure.  We did not see a nucleation pressure for the copper compound
dissolved in pure IL.  In general, as the concentration of the copper compound
increases, the nucleation pressure lowers.

            We will explain the ability of CO2 to
act as an antisolvent by determining the solvent strength of these mixtures:
IL/organic, IL/CO2 and organic/CO2 binary mixtures and
IL/organic/CO2 ternary mixtures.  Solvatochromic probes can be used
as a measure of solvent strength.  We use the Kamlet-Taft parameters, which
divide solvent strength into three categories: dipolarity/polarizability π*,
hydrogen bond donating α, and hydrogen bond accepting β.  The
organics do not have a strong effect on the solvent strength of the ILs until
large mole fractions are present.  The exceptions are the interaction between
the ILs and 2,2,2-trifluorothanol, which dominates the hydrogen bond donating
interactions.  The CO2 does not have a significant effect on the
solvent strength of the ILs, but does have a large effect on the solvent
strength, especially the general polarity/polarizability π*, for the
organic solvents.  The IL/organic mixtures with CO2 behave somewhat
between the IL/CO2 and organic/CO2 binaries, although the
CO2 seems to alter the solvent strength of the mixture less, it may
be because the IL preferentially solvates the probe in this situation.

            The CO2 definitely affects the
solvation ability of the IL/organic mixtures enough to induce a phase split. 
The degree to which either the organic or CO2 effects the solvent
strength of the IL can be used to predict this behavior.

 

1.         Aki, S.N.V.K., A.M. Scurto, and J.F. Brennecke, Ternary Phase Behavior of Ionic
Liquid (IL)-Organic-CO2 Systems.
Industrial & Engineering
Chemistry Research, 2006. 45(16): p. 5574-5585.

2.         Scurto,
A.M., S.N.V.K. Aki, and J.F. Brennecke, CO2 as a Separation 
Switch for Ionic Liquid/Organic Mixtures.
Journal of the American Chemical
Society, 2002. 124(35): p. 10276-10277.

3.         Mellein,
B.R. and J.F. Brennecke, Characterization of the Ability of CO2
to Act as an Antisolvent for Ionic Liquid/Organic Mixtures.
Journal of
Physical Chemistry B, 2007. 108(52): p. 20355-20365.

4.         Saurer,
E.M., S.N.V.K. Aki, and J.F. Brennecke, Removal of Ammonium Bromide,
Ammonium Chloride, and Zinc Acetate from Ionic Liquid/Organic Mixtures Using
Carbon Dioxide.
Green Chemistry, 2006. 2: p. 141-143.

5.         Kroon,
M.C., et al., Recovery of pure products from ionic liquids using
supercritical carbon dioxide as a co-solvent in extractions or as an
anti-solvent in precipitations.
Green Chemistry, 2006(8): p. 246 - 249.