Reports: UR453212-UR4: Synthesis and Thermophysical Behaviors of Mesothermal Liquid Salts
James H. (Jr.) Davis, PhD, University of South Alabama
The objective of this project is
to design, prepare, and characterize new ionic liquids that are stable for
extended periods, in air, at high (250 oC+)
temperatures. While ILs have long been touted as having high thermal
stabilities, the studies which have led to these claims are often flawed or
misleading, their conclusions having been based upon TGA experiments conducted
at high T/time ramp rates.
Preliminary work led us to
believe that ILs based upon tetraarylphosphonium
cations (TPP+) and the Tf2N- anion [(CF3SO2)2N-]
offered the most promising platform on which to develop new ionic liquids of
genuinely high thermal stability. Consequently, we began an intensive synthetic
effort to prepare structural and compositional variants of the ‘parent’ system by
making modifications to one or more of the phosphonium
cation rings. At this point, we have
prepared 31 new salts of this type, characterized them by multinuclear NMR and
ESI-MS, and performed preliminary screenings for thermal stability. The results of the latter were
remarkable. Without exception, we
observed that cations containing substituents with any aliphatic character at
all were markedly less thermally stable than those having only aromatic
substituents. Careful ESI-MS work showed
that upon thermal stressing, cations having alkyl substituents on their
aromatic rings oxidized to leave –COOH groups on the ring in place of their
original substituents. Remarkably, these materials do not appear to further
decompose below 350 oC. In turn, cations in which one or more of the
Ph4P+ rings was further linked to an extended aromatic
moiety exhibited much greater stabilities than their aliphatic-modified
counterparts. However, a trend is
emerging in the screening that suggests that cations having aryl ether, aryl
sulfone, or p-bromo
substituents have the best stabilities of all of the variants prepared thus
far. In fact, Tf2N-
salts of the three cations shown below have been heated to 300 oC for 90 days in air (muffle furnace) with no
changes in their spectroscopic characteristics, their MS, and < 5% mass
loss. To our knowledge, these
characteristics make them the most thermally stable ionic liquids reported to
date.
Having begun to establish the
structural boundaries of thermally stable TPP salts, we are now analyzing (in
conjunction with a collaborator in our Department of Chemical & Biomolecular Engineering) the DSC and TGA characteristics
of mixes of stable salts. Our objectives
in doing so are two-fold. First, we want
to determine if we can drive down melting points by eutectic formation, while
retaining high-T stability. Second, we
want to determine the enthalpy of fusion and heat capacity of the new salts,
since one envisioned application of these materials is as heat transfer fluids.
In addition to the synthetic and
stability screening work, we have pursued preliminary qualitative studies on
our new salts. In one set of
experiments, we were able to form ‘bio-oil’ by the pyrolysis of cellulose upon
its direct addition to molten [TPP]Tf2N. Likewise, we have been able to use the diphenyl-ether functionalized salt (center, above) to
dissolve sulfur and melt/disperse Sn and Pb, and by
doing so to make very fine crystallites believed to be SnS
and PbS (analyses in process). We plan to follow-up this experiment to
determine if de-sulfurization of benzo- and dibenzothiophene
can be accomplished by thermolysis in our ILs having
in them molten Sn. If so, this could
provide a means to de-sulfurize a hydrocarbon stream while producing a material
of potential value in the process.
Finally, in a most unexpected twist, we were able to make available to a
young Assistant Professor at Washington University, Prof. Katie Henzler-Wildman, samples of our structurally unique TPP
variants. Prof. Henzler-Wildman
uses the TPP cation as a probe of the workings of drug-resistance protein ‘efflux
pumps,’ and came to be aware of our work in making structural variants of this
cation. Already, using one of our new cations as a novel probe, she has begun
to collect information that is providing heretofore unavailable insights on a
protein of interest.
For the coming year, our work to
prepare further stable structural variants will continue – not because the
stabilities achieved thus far are unsatisfactory, but in an effort to make
salts that are still thermally stable but which have lower melting points; none
of the salts characterized thus far are liquids at ambient temperature. Too, in the coming year we will begin to
examine the thermal stability of salts with different anions – especially the saccharinate anion.
This anion – a well-known non-nutritive sweetener – was first used by
our group to formulate ILs in 2005.
Since then, work by Wishart at Brookhaven National Laboratories has
shown it to be exceedingly stable to ionizing radiation. We think that this augers
well for it being highly stable from a thermal standpoint as well. We also will expand the synthetic element of
our program into the realm of preparing in-house new triaryphosphines,
having (thus far) used only those structural variants available to us from
commercial vendors.
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