Expanding the useful range of ionic liquids: Melting point depression of organic salts with carbon dioxide for biphasic catalytic reactions

Article abstract of DOI:10.1039/b606130c

Large and previously unreported melting point depressions (even exceeding ΔT_m of 100°C) were observed for simple ammonium and phosphonium salts in the presence of compressed CO₂, bringing them well within the range of typical ionic liquids; possible applications include biphasic catalysis in IL/scCO₂ systems as demonstrated for rhodium complex catalyzed hydrogenation, hydroformylation, and hydroboration of 2-vinyl-naphthalene using a CO₂-induced molten sample of [NBu ₄][BF₄] as a catalyst phase at temperatures in the range of 55-75°C, i.e. 100°C below the normal melting point of the organic salt. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b606130c

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COMMUNICATION
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Expanding the useful range of ionic liquids: melting point depression of
organic salts with carbon dioxide for biphasic catalytic reactions
Aaron M. Scurtoac and Walter Leitner*ab
Received (in Cambridge, UK) 2nd May 2006, Accepted 28th June 2006
First published as an Advance Article on the web 18th July 2006
DOI: 10.1039/b606130c
presence of compressed gases.6 At a particular pressure of a
Large and previously unreported melting point depressions
gaseous component, the first temperature at which a solid partially
(even exceeding DTm of 100 uC) were observed for simple
melts represents a point on the solid–liquid–vapor equilibrium
(SLV) line. The difference between this SLV temperature and the
normal melting point (or the triple point) is called the melting
point depression (DTm). The melting point depression of organic
molecular solids by application of compressed CO2 is a well
established phenomenon.7–9 CO2-induced melting has been used to
carry out reactions in neutral organic substances that would
otherwise be solid at the reaction temperature.10,11 The CO2-
induced melting of high molecular weight poly(ethyleneglycol) has
been exploited to utilize this medium for liquid/supercritical
biphasic catalysis.12–14 Melting point depressions were typically
in the range of 20–25 uC in these systems. There have also been
isolated reports on melting point depressions in the IL/CO2 system.
In our earlier study on Ni-catalyzed hydrovinylation in this system,
we noticed that some of the low-melting salts showed a reduced
viscosity or transition from a waxy solid to a liquid under CO2-
pressure.15a,16 In an IR spectroscopic study attempting to under-
stand the molecular interaction between ILs and scCO2, Kazarian
et al.17 observed a reduction of the liquid-crystal transition
temperature of long alkyl-chain imidazolium salts. However, so
far no attempts have been made to explore this effect more
systematically and to apply it to reactive systems. We therefore
initially set out to study the melting point depression for a variety
of organic salts with melting points above room temperature under
operating conditions typical of the IL/scCO2 system using 150 bar
of CO2 pressure (Table 1).
ammonium and phosphonium salts in the presence of
compressed CO2, bringing them well within the range of typical
ionic liquids; possible applications include biphasic catalysis in
IL/scCO2 systems as demonstrated for rhodium complex
catalyzed hydrogenation, hydroformylation, and hydrobora-
tion of 2-vinyl-naphthalene using a CO2-induced molten sample
of [NBu4][BF4] as a catalyst phase at temperatures in the range
of 55–75 uC, i.e. 100 uC below the normal melting point of the
organic salt.
Room temperature ionic liquids (ILs) are currently the subject of
intense research focus due to their demonstrated potential as green
solvents and advanced materials in a broad range of technical
processes.1–5 In particular, this includes their application as
solvents for modern chemical synthesis and catalysis. In fact, the
typical operating temperatures of organic synthesis have played an
important role in defining ILs as ‘‘organic salts having melting
points below 100 uC’’.1–3,5 Consequently, reaching a low melting
temperature has been the primary design principle for the
molecular structure of ionic liquids. In practice, however, designing
organic salts with particular properties and functions that are at
the same time liquid below 100 uC or even near room-temperature
is not a trivial task. The effective combinations of cations and
anions that produce liquids under the present definition are small
compared to the combinatorial possibilities. Therefore, a metho-
dology or process to increase the range of possible ionic
compounds for use as solvents for reactions and extractions would
be highly useful. In the present communication, we demonstrate
that pressurizing simple organic salts with CO2 can lead to
remarkably high melting point depressions (up to DTm of 120 uC)
allowing use of them as liquid phases in catalytic processes at
temperatures far below their regular melting points (Fig. 1). This
opens the possibility to consider typical ‘‘ionic liquid applications’’
for structures that had to be discarded previously owing to their
high melting points.
All solid samples were dried in vacuo (y0.1 mbar) for
approximately 8 hours and stored under argon in Schlenk tubes
prior to use. The melting points of the solids in the absence and
presence of CO2 were visually determined in a high-pressure view
It has long been known that organic compounds can undergo
solid/liquid transitions below their normal melting points in the
aInstitut fu¨r Technische und Makromolekulare Chemie, RWTH Aachen,
Worringerweg 1, D-52074 Aachen, Germany.
E-mail: leitner@itmc.rwth-aachen.de; Fax: +49-(0)241-80 22177
bMax-Planck-Institut fu¨r Kohlenforschung, Mu¨lheim an der Ruhr,
Germany
Fig. 1 CO2-induced melting point depression to generate ionic liquids:
schematic representation of catalytic processes carried out in an ‘‘ionic
liquid’’/scCO2 biphasic mixture using CO2-induced molten [NBu4][BF4] as
catalyst phase at reaction temperatures 100 uC below its regular melting
point.
cDepartment of Chemical & Petroleum Engineering and NSF-ERC
Center for Environmentally Beneficial Catalysis, University of Kansas,
Lawrence, KS 66045, USA. E-mail: ascurto@ku.edu;
Fax: +1 (785) 864-4947
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Table 1 Summary of melting point depression (DTm) of organic ionic solids in CO2
Entry
1
Organic salt
Cation
Anion
PSLV [bar]
TSLV [uC]
Tm [uC] (lit.)
67 (50.1)a
MPD (DTm)
[BMIM][OTos]
150
40–42
26
2
2
3
[BMIM][CH3SO3]
[EMEPy][EtSO4]
CH3SO3
150
150
52
48
72 (71)a
20
20
2
CH3CH2SO3
68
156 (159–162)b
99 (97–100)b
120
72
2
4
5
[NBu4][BF4]
[NHex4][Br]
BF4
150
150
36
27
Br2
6
7
[NEt4][BTA]
[AB][OTf]
35
40
20
17
102
48
82
31
2
CF3SO3
2
8
[TBMP][OTf]
CF3SO3
150
40
119
79
a
b
Reference 18. Reference 19.
cell of 10 mL volume with a maximum operating pressure of 400
bar.{ The so-called ‘‘first melting’’ method was used, where the
solid sample is slowly heated at constant pressure and visually
observed for the first signs of melting, i.e. loss of crystallinity and
flow due to gravity. The method was verified by detection of the
normal melting point of several of the compounds with the same
technique.
accessible region also for these compounds. The simple tetra-
ethylammonium based salt exhibited a remarkable DTm 5 82 uC
at a CO2 pressure of only 35 bar. This demonstrates that even
gaseous CO2 at low to moderate pressures can be used to generate
ILs from organic salts and that it is possible to control the liquid
range by adjusting the system pressure.
The ability to melt organic ionic compounds under CO2
pressure opens the door to a larger number of applications and
increases the number of structures and functional groups to be
considered for use in ionic liquids. To demonstrate the potential of
this approach, we have performed a series of catalytic reactions in
a biphasic ionic liquid/scCO2 system using [NBu4][BF4] to form a
liquid catalyst phase (cf. Fig. 1). [NBu4][BF4] was chosen as a
prototypical catalyst phase with a weakly coordinating anion.
Scheme 1 lists the results of batch reactions of this biphasic system
with 2-vinyl-naphthalene as the common substrate for hydrogena-
tion, hydroformylation, and hydroboration. The reaction tem-
peratures for some of the reaction were kept a full 100 uC below
the normal melting point of the ionic solid. Visual inspection of the
reaction mixtures in window-equipped high pressure reactors
confirmed the presence of an ionic liquid/scCO2 with the catalyst in
the liquid phase as observed for related systems with standard
ionic liquids.15
First, we tested some imidazolium and pyridinium salts that
would qualify as ionic liquids but with melting points significantly
above room temperature. As seen in Table 1, entries 1–3, they
experienced moderate melting point depressions in the range of
20–26 uC at 150 bar. In contrast, salts based on quaternary
ammonium and phosphonium cations displayed much larger
melting point depression with CO2, reaching and surpassing DTm
of 80 uC in several cases (entries 4–8). Notably, the simple
quaternary ammonium salt tetra-n-butyl-ammonium tetrafluoro-
borate [NBu4][BF4] with a normal melting point of 156 uC first
encounters solid–liquid–vapor equilibrium at 36 uC at 150 bar of
CO2. This corresponds to
a melting point depression of
DTm 5 120 uC. After higher pressure, the temperature of melting
decreases further, but less pronounced than in the lower pressure
regime (DTm 5 129 uC at 335 bar). To our knowledge, such a
dramatic decrease in the melting point of a simple solid organic
compound with CO2 is unprecedented in the literature
The well-known cationic rhodium phosphine complex
[(cod)Rh(dppe)][BF4] (cod 5 1,5-cyclooctadiene, dppe 5 1,2-
bis(diphenylphosphino)ethane) was employed for the hydrogena-
tion and hydroboration reactions. Quantitative hydrogenation was
achieved at 55 uC and 125 bar with 25 bar of H2 using 0.2 mol-%
catalyst. The hydroboration had a somewhat lower conversion at
79% with a chemoselectivity to the borate of 100% and ratio of the
linear (n) to the branched (iso) product of 30 : 70. The in situ
For [NEt4][BTA] and the chiral IL [AB][OTf], the melting points
in the presence of 150 bar CO2 were too low to be determined with
the setup used here (entries 6 and 7, respectively). In the phase
equilibrium of a binary solid/gas mixture, the melting point
depression is associated with the solid–liquid–vapor equilibrium
line and is therefore pressure dependent. Indeed, reducing the CO2
pressure brought the melting points into the experimentally
3682 | Chem. Commun., 2006, 3681–3683
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prototypical examples. This methodology has the potential to
greatly expand the numbers of compounds and structures that can
be employed as ionic liquids in reactive systems and other
applications.
Financial support of this work by the National Science
Foundation (USA, post-doctoral fellowship NSF-IRFP for
AMS), the Federal Ministry of Research and Education (bmbf,
ConNeCat lighthouse project ‘‘Regulated Systems for Multiphase
Catalysis’’) and the Fonds der Chemischen Industrie is gratefully
acknowledged. We thank Solvent Innovation (Cologne) and Prof.
Peter Wasserscheid for generous donations of chemicals, Dr
Maurizio Solinas for help with the catalytic reaction systems and
Andreas Falkenstein for experimental assistance.
Notes and references
Scheme 1 Prototypical catalytic reactions carried out in molten
{ Experimental note: A small sample of the organic salt (approx. 200 mg)
was transferred under argon to a small vial and put inside the autoclave
(V 5 10 mL). The solid was pressurized with CO2 to induce melting and
was purified by dynamic extraction with supercritical CO2 (40 uC, 250 bar)
to remove any trace volatile impurities. The sample was vented and re-
solidified on the bottom of the vial to form a thin crystalline film. The
autoclave was pressurized with CO2 to the desired pressure and the
temperature was raised slowly (0.5 uC/minute) until the film both became
clear and began to flow with gravity. This was repeated until better than
1 uC reproducibility was achieved.
For the catalytic reactions, approximately 2 grams of the ionic salt,
[NBu4][BF4], 100 mg of 1-vinyl-naphthalene (1VN), and a 500 : 1 ratio of
1VN to Rh were added to a 10 mL autoclave. CO2 was added until the
total pressure was constant at 150 bar. For the hydroformylation reaction,
CO2 was used to induce melting of the ionic salt, and allow complexation
of the Rh precursor and ligand for 4 hours. The mixture was vented and
the reactants, synthesis gas, and CO2 were added to initiate the reaction. All
reactions were extracted with either with scCO2 or a 5-fold excess of hexane
and analyzed with GC-MS. Product recovery was found to be . 97% in
selected examples.
[NBu4][BF4]/scCO2 using the setup depicted in Fig. 1.
generated neutral hydroformylation catalyst of type [HRh(CO)
(PAr3)3] was solubilized in the liquid salt by using the ionic ligand
P(m-C6H4SO3Na)3 (TPPTS), leading to a 98% yield of aldehyde
with the typical n/iso ratio of ca. 20 : 80. All these reactions were
carried out under a standard set of benchmark conditions and
remain un-optimized as to temperature, pressure, and catalytic
system. Nevertheless, they adequately illustrate the range of
catalytic transformations possible in this biphasic system using
what would normally be an overlooked salt as an ionic liquid.
Finally, the possibility of catalyst recycling was briefly examined
using the hydrogenation of 2-vinyl-naphthalene as test reaction.
After a standard reaction time of 1 hour (two hours in the first
run), the reactor content was extracted for 2.5 hours with CO2 at
55 uC and 200 bar with approximate flow rates of 200 liter per
hour (standard temperature and pressure). The CO2 stream was
passed through two cold traps held in dry ice/acetone to condense
out the products. After the extraction step the reactor was
depressurized leaving the re-solidified salt with the catalyst trapped
within the matrix in the reactor. Conversions of 93–98% were
achieved in the first three runs (0.25 mol-% catalyst at 55 uC and
125 bar with 25 bar of H2) corresponding to an average turnover
frequency of ca. 400 h21 in each run. The conversion dropped
markedly in the next two runs (run 4: 80%, run 5: 62%), indicating
a certain loss of catalyst activity which may be attributed to
accidental introduction of traces of oxygen or catalyst loss during
this non-optimized procedure.
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In conclusion, we have shown here that compressed carbon
dioxide has the potential to control the transition between organic
ionic solids and ‘‘ionic liquids’’ by inducing significant melting
point depressions, some of which are the largest known to us in the
literature. Lowering of melting points reaching DTm 5 120 uC
were observed for simple ammonium and phosphonium salts,
bringing them well within the definition of ionic liquids as
characterized by melting points below 100 uC and even below
room temperature. Rhodium complex catalyzed hydrogenation,
hydroformylation, and hydroboration of 2-vinyl-naphthalene were
performed using a CO2-induced molten sample of [NBu4][BF4] as
liquid catalyst phase at temperatures in the range of 50–70 uC, i.e.
100 uC below its normal melting point. High rates and selectivities
were observed and catalyst recycling was demonstrated for
18 Solvent Innovations, GmbH, http://www.solvent-innovation.com/.
19 Sigma-Aldrich Catalog 2002–2003, Sigma-Aldrich, Inc.
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