Investigation of the Potential Use of (IILs) Immobilized Ionic Liquids in Shale Gas Sweetening

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Investigation of the Potential Use of
(IILs) Immobilized Ionic Liquids in
Shale Gas Sweetening
Case Study: #01521017GB/H
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The Potential Use of (IILs) Immobilized Ionic
Liquids in Shale Gas Sweetening
BACKGROUND:
Ionic liquids are salts of nitrogen- or phosphorus-containing organic cations
coupled with inorganic anions. Because of the asymmetry of the cations and size
differences between them and the anions, they do not pack readily into a crystal,
and consequently are liquids at room temperature. Typical ionic liquids are
shown in the following Figure.
Due to the unique chemical physical properties of ionic liquids, they have been
called, green solvents.
Especially room temperature ionic liquids (RTILs), such as those based on
N,Ndialkylimidazolium ions, are interesting solvents for catalytic reactions, for
example:

Other examples include:
Ionic liquids are non-volatile and non-flammable, eliminating the hazards
associated with volatile organic compounds (VOCs). In addition, the properties of
ionic liquids may be tuned by varying the identities of the cations and anions,
thereby tailoring the solvent to a specific application.
The cations and anions can be varied to give different degrees of lipophilicity,
and hence different solvating properties. They are usually air and water stable.
They consist mainly of discrete dissociated ions or as strongly structured liquids,
with the electric conductivity of salts. They have practically zero vapor pressures,
which means that in terms of emissions they are ideal replacements for volatile
organic solvents. The downside is that they themselves cannot be purified by
distillation.
The reaction products in ionic liquids can either be distilled off or extracted with
water or alkanes. There appear to be no immediate industrial applications but BP
is said in 2001 to be on the point of using ionic-liquid Friedel–Crafts alkylation
technology to upgrade an existing process. The technology apparently applies to
the Heck reaction:

The ionic liquids show excellent extraction capabilities and allow catalysts to be
used in a biphasic system for convenient recycling. For example, the
hydrovinylation of styrene with ethene can be carried out successfully using an
ionic liquid and supercritical CO2 as solvent (Eq. 10-15).
The ionic liquid dissolves the metal organic complex catalyst and sc-CO2
facilitates mass transfer and continuous processing.
IFP France has developed dimerization, hydrogenation, isomerization, and
hydroformylation reactions without conventional solvents. For butene
dimerization a commercial process exists. There is formed a biphasic system
with the catalyst in the IL phase, which is immiscible with the reactants and
products. This system can be extended to a number of organometallic catalysts.
A variety of other reactions such as acylation of toluene, anisole, and
chlorobenzene to give selectively p-isomer, alkylations, etc. have been
conducted with ionic liquids.
Immobilized Ionic Liquids in Shale Gas
Sweetening
The oxidation of thiols to disulfides with cobalt II phthalocyanine catalyst in an
ionic liquid at room temperature. The ionic liquid is there to solubilize the Co
catalyst ( which is insoluble in all other solvents ) plus it enhances the rate of
reaction. The reaction mechanism is said to go via the dimerization of RS
radicals, formed via reaction of RS- with Co(I)-02.
The question is what sort of benefit an immobilized ionic liquid could bring in this
type of reaction ?

CONCEPT PROPOSAL:
One possible approach would be to solubilize a Co(II) catalyst in the surface ionic
liquid layer. This would create a "homogeneous catalyst" complex dissolved in a
solid, supported ionic liquid.
The Co(II) catalyst would be considered as bound within the ionic liquid layer.
This overall concept would therefore be, a solid IIL catalyst containing the Co ( or
other ) oxidation catalyst in a fixed bed process using air as the primary oxidant.
In any sweetening reaction involving IILs there would be competing reactions to
consider, such as oxidation of any sulfides to sulfones and strong adsorption of S
species onto the IIL.
RELATED:
ExxonMobil and Lyngby University ( Topsoe ) have used this approach in
hydroformylation where the active rhodium catalyst is dissolved in an immobilized
ionic liquid and is reported to behave like a homogeneous system.
A copy of the ExxonMobil paper is attached to explain this concept more clearly.
For completeness I have summarized other uses of IILs in oxidation reactions
below, and attached several papers for your convenience:
Recent work done by a European researcher on the use of IILs in selective
oxidation reactions has concentrated on "difficult" alkane activation reactions
such as the oxidation of cyclohexane to adipic acid. For this work he has
developed a new class of water stable phosphonium based IILs using BF4-,
PF6- and Cr as anions. The contribution of the IIL is its superacidity which
activates the oxidant ( such as H2O2 or O2) and enables the reaction to
proceed under relatively mild conditions of 80'C in a single phase containing
cyclohexane/acetonitrile/30%aq. H202.
Another European technology licensor is looking at the use of IILs in the
oxidation of sulfur species in ultra low sulfur diesel. In this reaction they are
again activating H202 and oxidizing, for example, sulfides to
sulfones/sulfoxides.

REFERENCES:
1. Industrial Catalysis A Practical Approach Second, Completely Revised
and Extended Edition, Prof. Dr. Jens Hagen University of Applied
Sciences Mannheim Windeckstrasse , WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim, Germany
2. INDUSTRIAL ORGANIC CHEMICALS, Second Edition; HAROLD A.
WITTCOFF Scientific Adviser, Nexant ChemSystems Inc.; Vice President
of Corporate, Research, General Mills, Inc. (ret.) BRYAN G. REUBEN
Professor Emeritus of Chemical Technology London South Bank
University JEFFREY S. PLOTKIN Director, Process Evaluation and
Research Planning Program, Nexant ChemSystems Inc. Published by
John Wiley & Sons, Inc., Hoboken, New Jersey
ATTACHMENTS
1. Supported Ionic Liquid Catalysis – A New Concept for Homogeneous
Catalysis Christian P. Mehnert,† Raymond A. Cook,† Nicholas C.
Dispenziere,‡ Edmund J. Mozelski, ‡ and Mobae Afeworki† †ExxonMobil
Research and Engineering Company, Corporate Strategic Research, and
‡ExxonMobil Chemical Company, Basic Chemicals & Intermediates
Technology, 1545 Route 22 East, Annandale, NJ 08801, USA,
christian.p.mehnert@exxonmobil.com.
2. Supported ionic liquid phase (SILP) catalysts for hydroformylation
A. Riisager1, K.M. Eriksen1, R. Fehrmann1, P. Wasserscheid2
1 Interdisciplinary Research Center for Catalysis (ICAT) and Department
of Chemistry, Technical University of Denmark, DK-2800 Lyngby,
Denmark. 2 Institut für Technische Chemie und Makromolekulare Chemie,
RWTH, D-52074 Aachen, Germany.
3. Extractive Desulfurization and Denitrogenation of Fuels Using Ionic
Liquids Shuguang Zhang, Qinglin Zhang,† and Z. Conrad Zhang*
Akzo Nobel Chemicals Inc., 1 Livingstone Avenue, Dobbs Ferry, New
York 10522

4. Hydroformylation of 1-hexene with rhodium in non-aqueous ionic
liquids : how to design the solvent and the ligand to the reaction.
Frédéric Favre, Hélène Olivier-Bourbigou,* Dominique Commereuc and
Lucien Saussine Institut Français du pétrole, 1-4 Avenue de Bois-Préau,
92852 Rueil- Malmaison, France. E-mail: helene.olivier-bourbigou@ifp.fr
Received (in Cambridge, UK) 10th May 2001, Accepted 7th June 2001
First published as an Advance Article on the web 6th July 2001
5. Catalytic reactions in ionic liquids, Roger Sheldon
Laboratory of Organic Chemistry and Catalysis, Delft University of
Technology, Julianalaan 136, Delft. BL-2628, The Netherlands. E-mail:
secretariat-ock@tnw.tudelft.nl. Received (in Cambridge, UK) 10th August
2001, Accepted 11th September 2001. First published as an Advance
Article on the web 18th October 2001

Supported Ionic Liquid Catalysis –
A New Concept for Homogeneous Catalysis
Christian P. Mehnert,†
Raymond A. Cook,†
Nicholas C. Dispenziere,‡
Edmund J. Mozelski, ‡
and Mobae Afeworki†
†
ExxonMobil Research and Engineering Company, Corporate Strategic Research, and ‡
ExxonMobil
Chemical Company, Basic Chemicals & Intermediates Technology, 1545 Route 22 East, Annandale, NJ
08801, USA, christian.p.mehnert@exxonmobil.com.
Recently ionic liquids have attracted significant attention in the scientific literature. These ionic
phases are salts that have melting points at ambient temperatures and can be utilized as liquid
solvent media for a wide variety of chemical processes. Unlike conventional solvent systems
these liquids exhibit low vapor pressure, tunable polarity, and high thermal stability. Depending
on the choice of the ionic fragments a reaction environment can be designed to accommodate the
catalysis and the separation of a chemical process in the most efficient way. By combining solid
support materials with the advantages of ionic liquids we were successful in the preparation of
surface-immobilized ionic liquid phases.
In the new concept of supported ionic liquid catalysis (SILC) a homogeneous catalyst complex is
dissolved in a multiple layer of an ionic liquid on a heterogeneous support (Fig. 1). This layer
serves as the reaction phase in which a homogeneous catalyst is dissolved. Although the
resulting material is a solid, the active species is dissolved in the immobilized ionic liquid phase
and performs like a homogeneous catalyst.
The presentation will focus on the investigation and characterization of supported ionic liquid
catalysis for hydroformylation reactions. A comparison study will evaluate the differences of
biphasic hydroformylation catalysis in ionic liquid media and conventional solvent systems. A
high-pressure NMR study of hydroformylation reactions in ionic liquid systems will be presented
to complement the catalyst evaluations.
Figure 1. Complex HRh(CO)(tppts)3 is immobilized in a multiple layer of ionic liquid on the
surface of a heterogeneous support material and performs like a homogeneous catalyst.
P
SO3Na
HRh(CO)
R R
C
O
H
RhRh Rh
NN
Me Bu
BF4
3 3
Support Material
Surface Immobilized
Ionic Liquid Complex
Supported
Ionic Liquid PhaseCatalyst:
Organic Phase
CO/H2

Supported ionic liquid phase (SILP) catalysts for hydroformylation
A. Riisager1
, K.M. Eriksen1
, R. Fehrmann1
, P. Wasserscheid2
1
Interdisciplinary Research Center for Catalysis (ICAT) and Department of
Chemistry, Technical University of Denmark, DK-2800 Lyngby, Denmark.
2
Institut für Technische Chemie und Makromolekulare Chemie, RWTH, D-52074
Aachen, Germany.
Introduction
Room-temperature ionic liquids (IL's) receive significant interest due to their
potential application as solvents in catalytic processes. So far, work has mainly
involved improvement of current two-phase aqueous-organic catalytic systems by
substituting water with IL as the solvent for immobilization of organometallic
catalysts. This approach has been particular successful for Rh-catalyzed, liquid
biphasic hydroformylation where several groups have reported good to excellent
results for the conversion of C5-C8 and longer-chained olefins to the corresponding
aldehydes. In most of these applications the IL [bmim][PF6] (bmim: 1-butyl-3-
methyl-imidazolium) was used generating an effective medium for catalyst separation
and recycling, provided the Rh-catalysts were modified with ligands containing
charged groups. These charged ligands ensured a high relative catalyst affinity for the
polar IL-phase compared to the lipophilic product phase.
Only minor attention has been given to catalysts made by "heterogenization" of ionic
liquids on solid supports despite their potential for fixed bed gas phase reactions
leading to advantageous industrial process design. To date only a few applications
have been reported [1] where acidic chlorometallate ionic liquids on silica or coal
were used as Lewis acid catalysts for Friedel-Craft (F-C) reactions. Moreover,
tethered catalysts made of e.g. Lewis-acidic [bmim]-chloroaluminate complexes
immobilized on supports via covalent anchoring have been used as catalytic active
sites for F-C [2] and paraffin alkylations [3]. Only very recently a short
communication has appeared on liquid biphasic hydroformylation [4] using an
immobilized catalyst system composed of a supported ionic liquid phase.
Results and Discussion
Preliminary kinetic results have been
obtained from continuous biphasic gas-liquid
and liquid-liquid phase hydroformylation
using novel [bmim][X]/silica (X = e.g. PF6
or octyl-SO4) supported ionic liquid phase
(SILP) catalysts containing Rh-complexes of
the charged ligands 1, 2 and 3. As an
example the applicability of the SILP
concept for continuous liquid biphasic
hydroformylation of oct-1-ene using Rh-2
catalyst is here illustrated.

A monotonic increase in the activity was observed until steady-state was reached
after 4 hours (Fig. 1) remaining unchanged for at least 3 additional hours (end of
experiment). At steady-state a final n/iso ratio of 2.6 was reached which is in the
anticipated range using a mono-dentate phosphine ligand. However, fluctuations were
observed initially most likely due to a pre-activation reaction. ICP-AES analysis of
outlet samples taken at steady-state demonstrated that Rh metal leaching was
negligible (≤ 0.7%, detection limit).
In comparison to the conventional liquid-liquid biphasic catalysis in organic/IL
mixtures the new supported ionic liquid phase (SILP) catalyst systems offer the
significant advantage of very efficient IL use and short diffusion distances due to the
highly dispersed ionic liquid catalyst solution. Due to these advantages we believe
that other applications of organometallic catalysts using ILs as solvents could be
reconsidered in form of SILP catalyst systems.
References
[1] M.H. Valkenberg, C. deCastro and W.F. Hölderich, Appl. Catal. A, 215 (2001) 185.
[2] M.H. Valkenberg, C. deCastro and W.F. Hölderich, Green Chem., 4 (2002) 88 and cited
references; M.H. Valkenberg, E. Sauvage, C.P. deCastro-Moreira and W.F. Hölderich, WO
0132308 (2000) to ICI, UK.
[3] E. Benazzi, H. Olivier, Y. Chauvin, J.F. Joly and A. Hirschauer, Abst. Pap. Am. Chem. Soc.,
212 (1996) 45; E. Benazzi, A. Hirschauer, J.F. Joly, H. Olivier and J.Y. Bernhard, EP
0553009 (1993) to IFP, France.
[4] C.P Mehnert, R.A. Cook, N.C. Dispenziere and M. Afeworki, J. Am. Chem. Soc., 124 (2002)
12932.
Fig. 1 Hydroformylation of oct-1-ene pre-saturated with CO/H2 gas using SILP Rh-2 catalyst.
(CO:H2) = 1; p(CO/H2) = 14 atm; T = 115 °C; LHSV = 16 h-1
. ●: TOF, ∆: n/iso ratio.

SEPARATIONS
Extractive Desulfurization and Denitrogenation of Fuels Using Ionic
Liquids
Shuguang Zhang, Qinglin Zhang,†
and Z. Conrad Zhang*
Akzo Nobel Chemicals Inc., 1 Livingstone Avenue, Dobbs Ferry, New York 10522
Two types of ionic liquids, 1-alkyl-3-methylimidazolium [AMIM] tetrafluoroborate and hexafluo-
rophosphate and trimethylamine hydrochloride (AlCl3-TMAC), were demonstrated to be
potentially applicable for sulfur removal from transportation fuels. EMIMBF4 (E ) ethyl),
BMIMPF6 (B ) butyl), BMIMBF4, and heavier AMIMPF6 showed high selectivity, particularly
toward aromatic sulfur and nitrogen compounds, for extractive desulfurization and denitroge-
nation. The used ionic liquids were readily regenerated either by distillation or by water
displacement of absorbed molecules. The absorbed aromatic S-containing compounds were
quantitatively recovered. Organic compounds with higher aromatic π-electron density were
favorably absorbed. Alkyl substitution on the aromatic rings was found to significantly reduce
the absorption capacity, as a result of a steric effect. The cation and anion structure and size in
the ionic liquids are important parameters affecting the absorption capacity for aromatic
compounds. At low concentrations, the N- and S-containing compounds were extracted from
fuels without mutual hindrance. AlCl3-TMAC ionic liquids were found to have remarkably high
absorption capacities for aromatics.
Introduction
Sulfur present in transportation fuels leads to SOx
emission to air and inhibits the performance of pollution
control equipment on vehicles. To minimize the negative
health and environmental effects from automobile ex-
hausts, increasing regulatory pressures are imposed on
oil refineries to reduce the sulfur levels of the fuels,1-4
with the ultimate goal of zero emissions. While conven-
tional hydroprocessing catalysts have been highly ef-
fective for the reduction of sulfur levels, further im-
provement of the hydrodesulfurization (HDS) efficiency
is limited to increasingly severe operating conditions at
escalated cost. Not only does the energy consumption
become intensive, but also more severe conditions
required result in an increased hydrogen consumption,
which causes undesired side reactions. When gasoline
is desulfurized at higher pressure, many olefins are
saturated, resulting in lowered octane numbers. Higher
temperature processing also leads to increased coke
formation and subsequent catalyst deactivation.5
The reactivity of organosulfur compounds over HDS
catalysts depends on the molecular structures of S-
containing compounds.6,7 The aliphatic organosulfur
compounds are very reactive in conventional hydrotreat-
ing processes, and they can be completely removed from
the fuels without much difficulty. The aromatic sulfur
compounds including thiophenes, benzothiophenes, and
their alkylated derivatives, however, are generally more
difficult to convert over HDS catalysts. Therefore, the
aromatic sulfur compounds present the most difficult
challenges to the HDS processes.
Alternative technologies are of particular interest in
providing potential solutions for sulfur-free clean fuels.
For example, reactive adsorption8 and extraction with
organic solvents have been studied.9 The extractive
desulfurization (EDS) is an attractive alternative be-
cause the process is applicable at ambient conditions
without special equipment requirements. Besides the
low energy consumption, hydrogen consumption and
handling are also eliminated. In addition, the process
does not change the chemical structure of the fuel
components. The organosulfur components can be re-
covered at higher concentration following the extraction
process if the solvents chosen for such a process can be
regenerated. Therefore, the extractive solvents should
be sufficiently selective for absorption of sulfur com-
pounds at high capacity without affecting the olefin
contents. In addition, the solvents must be readily
regenerated following the extraction step.
Ionic liquids have been studied for applications re-
lated to green chemical processes, such as liquid/liquid
extractions, gas separations, electrochemistry, and
catalysis.10-18 Ionic liquids are typically nonvolatile,
nonflammable, and thermally stable.19 In general, ionic
liquids have higher density than organic liquids and
water. Therefore, many ionic liquids exist as a separate
phase when in contact with organic and aqueous phases.
These features make it possible to readily recycle the
ionic liquids for multiple extractions without additional
environmental concern. The ionic liquids based on
tetrafluoroborate and hexafluorophosphate are known
to be moisture-insensitive. With short-chain 1-alkyl, the
* To whom correspondence should be addressed. Tel.:
1 (914) 674 5034. Fax: 1 (914) 693 1782. E-mail:
zongchao.zhang@akzonobel.com.
†
Current address: Millennium Cell Inc., One Industrial
Way West, Eatontown, NJ 07724.
614 Ind. Eng. Chem. Res. 2004, 43, 614-622
10.1021/ie030561+ CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/18/2003

former is water miscible and the latter is water im-
miscible, even though a small amount of water (∼1%)
can be dissolved in the latter. The melting points of
EMIMBF4 (1-ethyl-3-methylimidazolium tetrafluorobo-
rate) and BMIMPF6 (1-butyl-3-methylimidazolium hexa-
fluorophosphate) are both close to 5 °C. The BMIMBF4
has a melting point of about -80 °C.19 As liquids at room
temperature, these compounds are thermally stable up
to about 300 °C, in the absence of strong acid. For
example, Holbrey and Seddon’s thermogravimetric study
showed that BMIMBF4 had a small weight loss of 3.5
wt % between 280 and 320 °C when heated at 10 °C/
min under nitrogen, and no further degradation was
observed until 360 °C.20 Trimethylamine hydrochloride
(TMAC) and AlCl3 based ionic liquids are easy to
prepare and have low cost and low melting points.
Although basic chloroaluminate molten salts at a nar-
row AlCl3 percentage are in a liquid state at room
temperature,12 they are less attractive to serve as
extractive solvents because of their high viscosity. Thus,
our focus is mainly on the EDS efficiency of acidic
AlCl3-TMAC ionic liquids for transportation fuels.
In this study, we studied in detail the molecular
absorption properties of several water-sensitive ionic
liquids (AlCl3-TMAC) and water-insensitive ionic liq-
uids (AMIMBF4 and AMIMPF6) for various fuel com-
ponents as well as for desulfurization efficiencies from
commercial fuel samples. The absorption capacities of
water-insensitive ionic liquids for N-containing com-
pounds were also evaluated because these compounds
severely inhibit the conversion efficiency of HDS cata-
lysts, even at N compound concentrations below <30
ppm.21-24 The possible effects of N-containing aromatic
compounds on the extractive removal of sulfur com-
pounds were determined.
Experimental Section
Preparation of EMIMBF4. EMIMBF4 ionic liquid
was prepared by mixing equal moles of 1-ethyl-3-
methyl-1H-imidazolium chloride (Aldrich) and lithium
tetrafluoroborate (Aldrich). Details of the preparation
were described elsewhere.25 The general structures of
the ionic liquids used in this work with 1-alkyl-3-
methylimidazolium (AMIM) cations are shown in Chart
1.
Preparation of BMIMPF6. A BMIMPF6 ionic liquid
was prepared by mixing 1-butyl-3-methylimidazolium
chloride, [BMIM]Cl, and LiPF6 (Aldrich) in acetonitrile
followed by filtration to remove a LiCl precipitate and
distillation to remove acetonitrile. [BMIM]Cl was ob-
tained by refluxing equal molar amounts of 1-meth-
ylimidazole and 1-chlorobutane in a flask while heating
and stirring at about 70 °C for 48 h.26 Another ionic
liquid sample, prepared by the reaction of 1-butyl-3-
methylimidazolium chloride and HPF6, was obtained
from University of Alabama.27
Preparation of BMIMBF4. A BMIMBF4 ionic liquid
was prepared by mixing 1-butyl-3-methylimidazolium
chloride and LiBF4 (Aldrich) in acetonitrile followed by
filtration to remove a LiCl precipitate and distillation
to remove acetonitrile. One sample of BMIMPF6 and two
other ionic liquids, 1-methyl-3-octylimidazolium tet-
rafluoroborate (MOIMBF4), and 1-hexyl-3-methylimi-
dazolium hexafluorophosphate (HMIMPF6), were ob-
tained from Aldrich Chemicals.
Preparation of Trimethylammonium Chloroalu-
minate Ionic Liquids. Two acidic trimethylammo-
nium chloroaluminate ionic liquids were prepared with
Al-TMAC ratios of 1.5 and 2.0, respectively. The ratio
was carefully chosen at two levels so room temperature
ionic liquid can be obtained with different acid strengths
for the study.
(a) 2.0:1.0 AlCl3-TMAC Ionic Liquid. Aluminum
trichloride (2 mol) was added slowly to trimethyl-
ammonium chloride salt (TMAC, 1 mol), both from
Aldrich, in a glovebox under dry nitrogen. The reaction
between the two solids was exothermic. A light-brown-
ish liquid was formed. This liquid was stirred for 5 h.
It has a density of 1.4-1.5 g/cm-3 at room temperature.
The product was stable as a liquid at room temperature
under a dry atmosphere. The formation of this type of
ionic liquid is expressed in reaction (1).
(b) 1.5:1.0 AlCl3-TMAC Ionic Liquid. The same
procedure as that described above for the preparation
of 2.0:1.0 AlCl3-TMAC ionic liquid was followed, except
that a molar ratio of 1.5 for AlCl3-TMAC was used. A
light-yellow liquid was formed. This particular ionic
liquid can be viewed as an equal mixture of (CH3)3NH+-
Al2Cl7
- and (CH3)3NH+AlCl4
-.
Measurement of the Absorption Capacity and
S-Removal Efficiency. The model compounds 2-me-
thylpentane, 1-hexene, methylcyclopentane, benzene,
toluene, trimethylbenzene, thiophene, 2-methylthiophene,
isobutyl mercaptan, dibenzothiophene (DBT), and 4,6-
dimethyldibenzothiophene (DMDBT) were selected to
represent typical types of molecules in gasoline and
diesel fuels. A few model fuels with about 1000 ppm
sulfur were prepared by dissolving a certain amount of
DBT or DMDBT in n-dodecane (n-C12). Model fuels
containing nitrogen compounds were also made by
adding pyridine or piperidine to n-C12 with or without
sulfur-containing compounds.
The absorption capacity of an ionic liquid for a specific
model compound was measured at room temperature
by adding an excess amount of the model compound
dropwise to a glass vial containing the ionic liquid to
form a two-phase system. After absorption equilibrium
was established, excess model compound in the upper
phase was carefully separated with a pipet from the
ionic liquid phase. The amount of absorbed model
compound in the ionic liquid phase was measured by
the weight gain. No detectable ionic liquid was found
in the upper phase.
The absorption selectivity for thiophene and toluene
was measured from a mixture containing both at nearly
equal concentration by weight. The concentrations of
absorbed toluene and thiophene in the ionic liquid phase
were measured by NMR spectroscopy after the equilib-
rium absorption.
In the present work, ionic liquids AlCl3-TMAC,
EMIMBF4, BMIMBF4, MOIMBF4, BMIMPF6, and
HMIMPF6 were also applied for sulfur removal from
transportation fuels. Gasoline and diesel fuel samples
with different S contents shown in Table 1 were
obtained from a commercial source. To make sure an
Chart 1. AMIMBF4 and AMIMPF6 Ionic Liquids
(CH3)3NHCl + 2AlCl3 f (CH3)3NH+
Al2Cl7
-
(1)
Ind. Eng. Chem. Res., Vol. 43, No. 2, 2004 615

extraction equilibrium was reached, all single extrac-
tions were conducted for 30 min with a weight ratio
(organic phase/ionic liquid) of 5:1, except where other-
wise indicated at room temperature. It has been verified
that shaking the absorption vial for 30 min is more than
sufficient to establish the equilibrium. The organic
phase was then separated from the ionic liquid phase
for analysis.
Analytical Procedures. Quantitative elemental
analysis of sulfur was conducted on a Bruker S4
Explorer wavelength-dispersive X-ray fluorescence spec-
trometer. Hydrocarbon and aromatics analyses were
carried out on an HP 6890 gas chromatograph (GC)/
mass spectrometer (MS) using an OV-1 column (30 m
× 0.32 mm i.d. × 5 µm). The GC/MS was also used to
obtain S-compound and N-compound contents in the
model fuels containing both.
A Varian Inova 500 MHz NMR spectrometer and a
JEOL GSX 270 MHz NMR spectrometer were used to
verify the structures of the ionic liquids. NMR analyses
(13C quantitative and attached proton test) were con-
ducted to obtain the sulfur content in the ionic liquid
phase after absorption. Some ionic liquids with absorbed
compound(s) were characterized using Fourier trans-
form infrared (FTIR) and X-ray photoelectron spectro-
scopic analyses.
Results
Sulfur-Compound Absorption Equilibrium. To
establish the time needed to reach absorption equilib-
rium, a model fuel containing 990 ppm S in the form of
DBT was treated with BMIMPF6. Sulfur analysis was
conducted on the fuels treated for 5, 10, 20, and 30 min.
As shown in Figure 1, the equilibrium was reached
after 10 min of contact between the model fuel phase
and the ionic liquid phase.
Absorption Capacities for Model Organosulfur
Compounds. A previous work has established that the
AMIMBF4 and AMIMPF6 ionic liquids, with the 1-alkyl
being ethyl and butyl, have negligible absorption for
alkanes and very low absorption for olefins, in compari-
son to aromatic compounds with and without sulfur.28
The absorption capacities of typical BF4
-- and PF6
--
based ionic liquids for the simple sulfur compounds are
compared in Figure 2.
As shown in Figure 2, for thiophene absorption,
BMIMPF6 has the highest absorption capacity, followed
by BMIMBF4. EMIMBF4 has the lowest absorption
capacity among the three. NMR analysis of the absorbed
thiophene in these ionic liquids indicated that the
structure and size of both anion and cation in the ionic
liquids have a strong effect on their absorption capac-
ity,29 consistent with the results of measurement based
Table 1. Sulfur Concentrations of Commercial Fuels
sample
total sulfur
(ppm) sample
total sulfur
(ppm)
low-sulfur gasoline 250 low-sulfur diesel 220
high-sulfur gasoline 820 high-sulfur diesel 12122
Figure 1. Sulfur content versus contact time.
Figure 2. Absorption capacity for thiophenes and alkylthiol.
616 Ind. Eng. Chem. Res., Vol. 43, No. 2, 2004

on mass balance. Methyl substitution of thiophene
decreased the absorption capacity, similar to the alky-
lated benzene molecules.28 The absorption for isobut-
ylthiol is much lower because of the lack of aromaticity.
The absorption capacities of the AlCl3-TMAC ionic
liquids are shown in Figure 3. The absorption of the
ionic liquids for the saturated hydrocarbons is negli-
gible, and that for the olefin is very low. It is evident
that these ionic liquids have remarkably high absorption
capacity for the aromatic compounds and the absorption
capacity is clearly influenced by the nature and steric
effects of the absorbed molecules. The observed steric
hindrance is more significant for the alkylated thiophene
than the alkylated benzene. The absorption capacity of
1.5:1.0 AlCl3-TMAC ionic liquid for toluene is about
58% of that for benzene, and that for 2-methylthiophene
is only about 15% of that for thiophene. It appears that
1.5:1.0 AlCl3-TMAC ionic liquid is more sensitive to
steric hindrance than 2.0:1.0 AlCl3-TMAC ionic liquid.
It was further observed that, upon addition of
thiophene or 2-methylthiophene into the AlCl3-TMAC
ionic liquids, the color of the mixture changed im-
mediately to brown and then gradually to dark brown.
The ionic liquids interacted with 2-methyl-1-propaneth-
iol mildly and led to the formation of a solid phase in
the ionic liquid phase and a colorless organic phase.
The mixing of aromatics with an ionic liquid resulted
in the formation of a light-yellowish solution as an
organic phase. The saturated hydrocarbons did not
interact with the ionic liquid. Upon mixing, a clear two-
phase solution was formed. The absorption capacity of
2.0:1.0 AlCl3-TMAC ionic liquids for aromatics was
slightly higher than that of 1.5:1.0 AlCl3-TMAC (Figure
3).
Absorption Efficiency for DBT and DMDBT
from Model Fuels. The absorption efficiencies of the
three ionic liquids BMIMPF6, EMIMBF4, and BMIMBF4
for DBT or DMDBT from two model fuels are compared
in Figure 4.
Like the case for thiophene absorption, EMIMBF4
having the smallest cation among the three showed
again the least extractive removal of DBT and DMDBT
from dodecane. On a weight basis, BMIMBF4 appeared
to be slightly more effective than BMIMPF6 for extract-
ing DBT or DMDBT from n-C12. However, on a molar
basis, the order is reversed, with a slight difference
between the two. Interestingly, DMDBT was more
difficult to remove than DBT because of methyl substi-
tution.
Competitive Absorption of Thiophene and Tolu-
ene in the Ionic Liquids. The competitive absorption
selectivity of two model compounds, toluene and thio-
phene, from their model mixture, was measured using
BMIMPF6 and BMIMBF4 ionic liquids. As shown in
Figure 5, the ionic liquid BMIMPF6 showed remarkably
higher absorption for both thiophene and toluene, as
compared to the BMIMBF4 ionic liquid. A total of about
1.7 mol of thiophene and toluene was absorbed per mole
Figure 3. Absorption capacity of AlCl3-TMAC ionic liquids for model compounds.
Figure 4. Extraction of DBT/DMDBT from n-C12 using different
ionic liquids.

of BMIMPF6 ionic liquid. The total absorbed thiophene
and toluene per mole of BMIMBF4 was about 0.7 mol.
The thiophene-toluene ratio in BMIMPF6 was 1.6, and
that in BMIMBF4 was 2.1. Even though the total
absorption capacity of BMIMBF4 was lower than that
of BMIMPF6, the former was more selective for the
thiophene absorption than the latter.
Absorption of N-Containing Compounds. Several
model N compounds were studied to assess the ap-
plicability of ionic liquids for the removal of organoni-
trogen compounds. The model aromatic N compounds,
pyridine and 2-methylpyridine (2-picoline), were found
to be completely miscible with BMIMBF4. The corre-
sponding saturated compounds, piperidine and 2-me-
thylpiperidine, showed limited absorption in the ionic
liquid, as shown in Table 2. It is clear that absorption
of aromatic N compounds is favored.
Extractive Removal of Both Organonitrogen
and Organosulfur Compounds. The absorption re-
sults by the BMIMBF4 ionic liquid for the model
compounds, DBT with either pyridine or piperidine from
dodecane, are given in Table 3.
When a model fuel consisted of n-C12 and another
compound, either DBT, pyridine, or piperidine, the ionic
liquid removed 12% S (DBT) in n-C12, 45% N (pyridine)
in n-C12, and 9% N (piperidine) in n-C12. The amount
removed from each model fuel is much less than the
absorption capacity for the corresponding pure model
compound by the ionic liquid, reflecting a partitioning
of the model compounds in both the ionic liquid and the
dodecane phases. The most effective extraction was the
removal of pyridine from dodecane. This is not surpris-
ing because pure pyridine is fully miscible with the ionic
liquid.
For the two model fuels, which contained either
pyridine or piperidine and DBT, the most remarkable
observation is that the extraction of DBT and of the
N-containing compounds by the ionic liquid was not
mutually affected.
Single Extraction of Sulfur Compounds from
Gasoline and Diesel Fuels. Table 4 shows experi-
mental results of two AlCl3-TMAC ionic liquids for S
removal from the high-sulfur gasoline sample.
About 20% sulfur removal from the high-sulfur gaso-
line was achieved in a single contact with the 1.5:1.0
AlCl3-TMAC ionic liquid at a gasoline to ionic liquid
ratio of 180 by weight. The 2.0:1.0 AlCl3-TMAC ionic
liquid, which is more acidic, appears to be inferior to
the 1.5:1.0 AlCl3-TMAC ionic liquid for sulfur removal
on the basis of equal weight of the ionic liquids.
The effect of the ratio of ionic liquid to fuel on the
extraction efficiency is shown in Figure 6. The 1.5:1.0
AlCl3-TMAC ionic liquid is effective for sulfur removal
from diesels and high-sulfur gasoline at a low ionic
liquid usage. An increase in the ratio of ionic liquid to
fuel did not result in a proportional increase in S
removal. Water and sulfur in diesel samples were all
removed by the AlCl3-TMAC ionic liquids. However,
because chloroaluminate-based ionic liquid is moisture-
sensitive, irreversible loss of ionic liquid was also
observed as a result of the formation of solid materials
upon absorption.
Ionic liquid treatments were found to remove almost
completely the color of the gasoline samples, even at low
sulfur removal efficiency. On the other hand, the color
of the diesel samples after the treatments became more
intense.
Multicycle Extractions of Sulfur Compounds
from Gasoline. It has been established that the chain
length of the alkyl group in the 1-alkyl-3-methylimmi-
dazolium cation has a pronounced effect on the absorp-
tion of aromatic sulfur compounds,28 with increased
Figure 5. Competitive absorption of thiophene and toluene in
BMIMPF6 and BMIMBF4.
Table 2. Absorption Capacities of BMIMBF4 for
N-Containing Compoundsa
absorption capacity mol/mol of BMIMBF4
pyridine fully miscible
piperidine 0.72
2-picoline fully miscible
2-methylpiperidine 0.23
a Based on 2 g of model compound in 1 g of ionic liquid.
Table 3. Absorption of S- and N-Containing Compounds by BMIMBF4
before treatment (ppm) after treatment (ppm) S or N removal (%)
model fuel S N S N S N
DBT in n-C12 747 660 12
pyridine in n-C12 779 425 45
DBT and pyridine in n-C12 764 779 660 460 14 41
piperidine in n-C12 661 601 9
DBT and piperidine in n-C12 764 723 677 658 11 9
Figure 6. Sulfur removal efficiency with 1.5:1.0 AlCl3-TMAC
ionic liquid at room temperature.

absorption capacity at increased chain length. There-
fore, MOIMBF4 and HMIMPF6, two ionic liquids with
long alkyl chains, were used for the extractive removal
of sulfur compounds from a commercial gasoline sample
containing about 820 ppm sulfur. Figure 7 shows the
contents of sulfur and aromatics after various cycles of
treatments. Using MOIMBF4, the first extraction re-
duced the sulfur level to 700 ppm. Because the ionic
liquid can be regenerated after equilibrium absorption
(see below), multiple extraction cycles were applied for
this fuel sample. After 10 treatment cycles, a sample
with 320 ppm sulfur was obtained with a lighter color
than that of the original fuel sample. At the same time,
the aromatics concentration in the sample dropped from
14.7 to 7.9 wt % after the 10 extraction cycles. The
HMIMPF6 liquid was slightly less effective on extrac-
tion.
Multiple Extractions for Sulfur Removal from
Diesel Fuel. The two ionic liquids, MOIMBF4 and
HMIMPF6, were also tested for sulfur removal from a
commercial diesel fuel that contained 1.2 wt % sulfur
and 7.9 wt % aromatics. With MOIMBF4, the sulfur
level was lowered to 0.9 wt % and the aromatics content
to 3.8 wt % after 10 treatments. The ionic liquid became
brown after use. The sulfur content in the diesel sample
treated with HMIMPF6 for 10 cycles was 1.0 wt %. The
used HMIMPF6 turned dark.
FTIR and XPS Characterization of Absorbed
Model Compounds. FTIR spectra (not shown) of the
1.5:1.0 AlCl3-TMAC ionic liquid after benzene absorp-
tion indicated a simple addition of the spectral features
of benzene and ionic liquid that shows that benzene was
simply dissolved in the ionic liquid phase without a
strong chemical interaction.
The absorption of thiophene and 2-methylthiophene
in the ionic liquid produced a dark solid in the ionic
liquid phase. FTIR study of the liquid products showed
the spectral feature of thiophene, with additional char-
acteristic IR bands of alkylation products. The absorbed
2-methylthiophene in the ionic liquid also showed
alkylation relative to the starting material. Detailed
characterization of the alkylation product is beyond the
scope of this paper.
Elemental analysis by XPS of the dark solids formed
through the thiophene-ionic liquid reaction revealed
the presence of sulfur in the solid phase and that
elements of N and Al are at a ratio corresponding to
the original ionic liquid formulation.
Regeneration of Used Ionic Liquid and Recov-
ery of Absorbed Compounds. For a thiophene-
saturated EMIMBF4 phase, the absorbed thiophene was
released into a separated phase upon addition of water
because of the fact that EMIMBF4 is miscible with
water. Water was then vaporized from the ionic liquid
phase under a nitrogen flow at 110 °C for about 3 h.
The EMIMBF4 ionic liquid was nearly quantitatively
recovered.
Because BMIMPF6 has little miscibility with water,
its regeneration was carried out by direct distillation
after saturated absorption of thiophene. The ionic liquid
was fully regenerated after heating at 110 °C for 3 h
under nitrogen. The absorbed thiophene recovered from
distillation corresponds to the amount absorbed. NMR
analyses indicated that the ionic liquids, EMIMBF4 and
BMIMPF6, maintained their original structures after
the regenerations.
Regeneration of the used AlCl3-TMAC ionic liquids
by distillation was found to be infeasible because of the
formation of the dark solids during their contact with
gasoline and diesel. The sulfur-containing compounds
cannot be removed from the used ionic liquid under
vacuum at 80-110 °C for about 6 h. However, it was
found that the solid material was soluble in methanol
and acetone but only slightly soluble in carbon tetra-
chloride.
Discussions
[AMIM]BF4 and [AMIM]PF6 Ionic Liquids. The
rapid establishment of absorption equilibrium for the
organosulfur compounds between the fuel phase and the
ionic liquid phase, as shown in Figure 1, suggests that
such a process is rather simple to carry out. Because
the AMIMBF4 and AMIMPF6 ionic liquids used in this
work are insensitive to water and are liquid at rather
low temperatures, the extraction is widely applicable
at ambient conditions.
In general, the AMIMBF4 and AMIMPF6 ionic liquids
have a strong propensity for absorbing compounds with
aromatic nature from the main aliphatic fuels. Their
absorption capacities decrease as the aromatics become
hindered by the alkyl groups in the aromatic rings. For
example, the absorption capacity decreases in the order
of benzene > toluene > xylene > cumene, as reported
in an earlier work.28 As shown in Figure 2, the methyl
group in 2-methylthiophene significantly lowered the
absorption capacity with respect to thiophene.
The strong affinity of the ionic liquids for the aromat-
ics, organosulfur and organonitrogen compounds, is
conceivably related to the high polarity of the ionic
liquids. The aromatic molecules with highly delocalized
electron density can be readily polarized through their
interaction with the ionic liquids. It follows then that
linear alkanes, cyclic alkanes, and olefins are barely
absorbed. Similarly, alkylthiols and saturated alky-
lamines are slightly absorbed, as observed through this
work (Figures 2 and 3 and Table 2).
The structural features of ionic liquids play an
important role in the absorption of organic molecules,
particularly for the aromatic compounds. For example,
as shown in Figure 2, EMIMBF4 has the lowest absorp-
Figure 7. Sulfur and aromatics content in gasoline treated for
different cycles (aromatics: toluene, ethylbenzene, and xylene).
Table 4. Sulfur Removal from a High-Sulfur Gasoline
Sample Using AlCl3-TMAC
sulfur after
treatment (ppm)
sulfur
removal (%)
2.0:1.0 AlCl3-TMAC 700 15
1.5:1.0 AlCl3-TMAC 660 20

tion capacity for thiophene, while BMIMBF4 has a
largely increased absorption capacity for it. With the
same BF4
- anion, BMIM is a larger cation than EMIM
by extending the alkyl group chain length. The increase
of the cation size by the substitution of a longer alkyl
group to the imidazolium ring was responsible for the
nearly 2-fold increase of the absorption capacity for
thiophene. A similar phenomenon also holds with
increasing anion size. In this case, the effect of PF6
-
(with a diameter of 2.4 Å) as a larger anion is compared
to that of the BF4
- anion (with a diameter of 2.2 Å). An
increase in the anion size led to an additional increase
in the absorption capacity, as evidenced in Figure 2, by
comparing the thiophene absorption by BMIMBF4 and
BMIMPF6 ionic liquids. The effect of the anion size
became most pronounced when BMIMPF6 and BMIMBF4
were applied to a mixture of toluene and thiophene, as
shown in Figure 5. The total absorbed toluene and
thiophene in BMIMPF6 is about 2.4 times that in
BMIMBF4. A similar trend is observed for the absorp-
tion of 2-methylthiophene. The effects of cation and
anion sizes on the interaction of absorbed thiophene and
the ionic liquids were further confirmed by NMR
study.29
These results clearly point to the nature of the
interaction between absorbed aromatic compounds and
the ionic liquids. Molecules with highly polarizable
π-electron density preferably insert into the dynamic
molecular structure of the ionic liquids. The driving
force for the molecular insertion is the favorable elec-
tronic interaction of polarized aromatic molecules with
the charged ion pairs of ionic liquids. On the other hand,
the insertion of aliphatic molecules of little polarizable
electronic structure would only weaken the Columbic
interaction of the ion pairs of the ionic liquids. There-
fore, such an insertion is not favored. Alkyl substitution
on the aromatic ring may effectively disturb the molec-
ular interaction in preferred orientation.
The higher thiophene-toluene ratio in BMIMBF4
than in BMIMPF6 (as shown in Figure 5) could be
rationalized by the larger anion size in BMIMPF6.
Because toluene is a larger molecule than thiophene,
in addition to the steric hindrance from the methyl
group, it is relatively easier to be accommodated in
BMIMPF6 than in BMIMBF4.
The effect of cations and anions of the ionic liquids
and the steric effect of absorbed compounds on the
absorption capacity were further displayed when the
three ionic liquids, EMIMBF4, BMIMPF6, and BMIMBF4,
were applied to extract DBT and DMDBT from the
mixture with n-C12, as shown in Figure 4.
The absorption capacity measurements of BMIMPF6
and BMIMBF4 for thiophene from a model mixture with
toluene (Figure 5) showed that the presence of toluene
significantly reduced the absorbed amount of thiophene
as compared to a pure model thiophene compound
(Figure 2). However, it is remarkable to note that the
absorption of toluene by the ionic liquids was only
slightly reduced by the absorption of thiophene. For
example, the absorption capacities for pure toluene by
BMIMPF6 and BMIMBF4 are 0.78 and 0.3 mol/mol of
ion liquid, respectively.25 In equilibrium with a model
mixture containing a nearly equal amount of toluene
and thiophene, the amount of absorbed toluene is 0.65
mol/mol of BMIMPF6 and 0.23 mol/mol of BMIMBF4.
It is known that the methyl group on the aromatic
benzene ring is an electron-donating group. Therefore,
the interaction of toluene and the ionic liquid would be
expected to be strong. It is likely that the absorbed
toluene inhibits the absorption of thiophene as a result
of steric hindrance of toluene. The relative strength of
the interaction of toluene and thiophene with the ionic
liquids remains to be determined.
When BMIMBF4 was applied for the extractive re-
moval of DBT, pyridine, and piperidine from model
fuels, the results showed that DBT and the N com-
pounds were independently absorbed into the ionic
liquid without noticeable mutual hindrance. It is likely,
in this case, that the ionic liquid was not saturated by
the S and N compounds because of the low concentration
of the model compounds in dodecane. Again, the absorp-
tion by the ionic liquid for saturated alkylamine from
dodecane was very low. However, the extractive removal
efficiency for aromatic nitrogen compounds, in this case
pyridine, was remarkably high. The results indicated
that the ionic liquids were particularly selective for
aromatic N-containing compounds from fuels.
The water-insensitive ionic liquids were readily re-
generated by two methods. One was by wetting the
saturated ionic liquids with water. Water as a small
molecule with strong polarity has stronger interaction
with ionic liquids than polarized aromatic compounds.
As a result, aromatic sulfur compounds were quantita-
tively repelled from the ionic liquids. The absorbed
water can be removed to regenerate ionic liquids. Water
or other small polar molecules in fuels were shown to
favorably compete with organosulfur compounds for
absorption, leading to reduced absorption efficiency for
the organosulfur compounds. Another method of ionic
liquid regeneration is by direct distillation. This method
is applicable for the removal of absorbed molecules at
temperatures within the ionic liquid stability range.
Even though the absorptive removal of S compounds
is not very high in a single extraction because of the
extremely low concentration of S compounds in the
fuels, the feasibility for ionic liquid regeneration and
reuse makes water-insensitive ionic liquids attractive
for processes involving multiple cycles. As shown in
Figure 7, multiple-cycle extractive removal was dem-
onstrated to be an effective process.
The results obtained on sulfur removal from gasoline
and diesel samples suggest that the S removal from
diesel is more difficult than that from gasoline. This
observation is likely related to a less favored equilibrium
absorption by ionic liquid in contact with the diesel
phase of heavier molecules. The partitioning of aromatic
compounds in ionic liquids was reduced in the presence
of a heavy organic solvent, such as diesels.
The removal of saturated S-containing compounds
from fuels by ionic liquids is much less effective than
the removal of aromatic S-containing compounds. The
residual S compounds in the fuels after extractive
removal by ionic liquids may mainly consist of saturated
ones. As stated in the Introduction, conventional HDS
catalysts are highly effective for the reduction of satu-
rated organosulfur compounds. Therefore, the EDS
could be a complementary process to the HDS.
AlCl3-TMAC Ionic Liquids. In the chloroaluminate
ionic liquids with a ratio of AlCl3-TMAC between 1 and
2, the predominant Lewis acidic species present is well
established and known to be Al2Cl7
-. The Al3Cl10
- is a
minor species,30 and AlCl4
- as a neutral partner species
coexists through equilibrium (2) where Al2Cl7
- is the

Lewis acid and Cl- is the Lewis base. Indeed, Al2Cl7
-
was reported to catalyze alkylation and acylation reac-
tions.18
In this study, [(CH3)3NH]+ is the cation, which is not
expected to largely change the nature of the anion. In
fact, AlCl3-TMAC ionic liquids were found to effectively
catalyze the alkylation of aromatics with olefins,12 which
could explain our observation of the formation of alky-
lation products when thiophene was in contact with
AlCl3-TMAC ionic liquids.
Both 1.5:1.0 and 2.0:1.0 AlCl3-TMAC ionic liquids
have nearly equal (and the highest) absorption capacity
for thiophene. This indicates that the acidity change
does not have a significant effect on the sulfur removal
efficiency with these ionic liquids. The difference in
amount absorbed by 2.0:1.0 and 1.5:1.0 AlCl3-TMAC
for benzene is not as pronounced as that for alkylated
aromatics. Therefore, the more pronounced steric hin-
drance for 1.5:1.0 AlCl3-TMAC ionic liquid could be
attributed to the smaller AlCl4
- anion, which accounted
for half of the total chloroaluminate anions. The larger
Al2Cl7
- anion appeared to be the cause of the reduced
steric effect for 2.0:1.0 AlCl3-TMAC ionic liquids.
The highest absorption capacity observed with PF6
--
based ionic liquids for thiophene was 3.5, about half of
the absorption capacity of AlCl3-TMAC ionic liquids
observed in the present study.
A high capacity for sulfur removal from the diesel and
high-sulfur gasoline (Figure 6) was achieved at a low
ratio of ionic liquid to fuels. Therefore, multiple extrac-
tions at a low ionic liquids-to-fuels ratio might be a
viable approach. The low S-removal efficiency observed
for the low-S gasoline could be related to distribution
of a low concentration of aromatic S compounds in the
sample. For example, the ratio of sulfur concentration
to aromatics in low-S gasoline is about 5.8 au compared
to 424 au for high-S gasoline. Although AlCl3-based ionic
liquids are effective for the removal of S-containing
compounds, contact between AlCl3-based ionic liquids
and thiol-containing compounds resulted in the forma-
tion of dark precipitates. Thus, the application of AlCl3-
based ionic liquids is limited to the absorption of certain
aromatic compounds such as DBT.
Conclusions
Ionic liquids EMIMBF4, BMIMPF6, and BMIMBF4
and other heavier AMIMPF6 ones showed remarkable
selectivity for the absorption of aromatics and aromatic
S- and N-containing molecules from transportation
fuels. These ionic liquids are moisture-insensitive,
thermally stable under the distillation conditions, and
readily regenerated for reuse. The absorbed aromatic
S-containing compounds were quantitatively recovered
during the regeneration. The Lewis acidic AlCl3-TMAC
ionic liquids were found to have remarkably high
absorption capacities for aromatics, particularly sulfur-
containing aromatic compounds, but their regeneration
is problematic. The results suggest that compounds with
higher aromatic π-electron density are favorably ab-
sorbed. A methyl group on the aromatic rings was found
to significantly reduce the absorption capacity, possibly
because of a steric effect. The cation and anion structure
and size in the ionic liquids are important parameters
affecting the absorption capacity for aromatic com-
pounds. At low concentrations, the N- and S-containing
compounds were extracted from fuels without mutual
hindrance.
Acknowledgment
We thank the Akzo Nobel Multi-BU Program for
providing financial support. Analytical support given by
Dr. Gary Darsey and Evan Chen for elemental sulfur
analysis and by Dr. Biing-Ming Su for NMR analysis is
greatly appreciated.
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(28) Zhang, S.; Zhang, Z. C. Novel Properties of Ionic Liquids
in Selective Sulfur Removal from Fuels at Room Temperature.
Green Chem. 2002, 4, 376-379.
(29) Su, B.; Zhang, S.; Zhang, Z. C. Structural Elucidation of
Thiophene Interaction with Ionic Liquids by Multinuclear NMR
Spectroscopy. J. Phys. Chem. 2003, submitted for publication.
(30) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L.
Dialkylimidazolium Chloroaluminate Melts: A New Class
of Room-Temperature Ionic Liquids for Electrochemistry,
Spectroscopy, and Synthesis. Inorg. Chem. 1982, 21, 1263-
1264.
Received for review July 7, 2003
Revised manuscript received November 3, 2003
Accepted November 5, 2003
IE030561+

Communication
www.rsc.org/chemcomm
CHEMCOMM
Hydroformylation of 1-hexene with rhodium in non-aqueous ionic
liquids : how to design the solvent and the ligand to the reaction
Frédéric Favre, Hélène Olivier-Bourbigou,* Dominique Commereuc and Lucien Saussine
Institut Français du pétrole, 1-4 Avenue de Bois-Préau, 92852 Rueil-Malmaison, France.
E-mail: helene.olivier-bourbigou@ifp.fr
Received (in Cambridge, UK) 10th May 2001, Accepted 7th June 2001
First published as an Advance Article on the web 6th July 2001
A wide range of ionic liquids based on imidazolium and
pyrrolidinium cations and weakly coordinating anions
proved to be efficient solvents for the biphasic rhodium
catalyzed hydroformylation of 1-hexene; the reaction rate
and regioselectivity, and the retention of the rhodium can be
optimized by fitting the nature of the anions and cations of
the ionic liquid and the modified phosphite or phosphine
ligands.
Ionic liquids are good solvents for transition-metal complexes
in many homogeneously catalyzed reactions, e.g. olefin hydro-
genation, hydroformylation, oligomerization and Pd mediated
carbon–carbon coupling reactions.1 In many cases the reaction
products are very weakly soluble in the ionic phase so that the
catalyst can be separated by simple decantation and recycled.
The aqueous two-phase catalysis concept, which has been
already applied industrially for propene hydroformylation,2 can
then be extended to substrates and ligands that are poorly
soluble or non stable in water. Higher olefin Rh-hydro-
formylation has been performed using different 1-butyl-
3-methylimidazolium room-temperature liquid salts as sol-
vents, in the presence of phosphine ligands. The main difficulty
is to immobilize the rhodium catalyst in the ionic liquid phase
while maintaining its activity and selectivity. A solution is to
modify the neutral phosphine ligands with ionic groups.3
Thanks to their chemical and physical versatility,4 ionic liquids
can be specially designed to fit with the ligand and the operating
conditions that provide the best performances in catalysis.
In this communication, for the first time we report the effect
of the nature of the cations and anions of the ionic liquids on the
Rh-catalyzed hydroformylation of 1-hexene. We also provide
our preliminary study on the performances of different
phosphorus ligand–ionic liquid systems.
We have prepared a wide range of ionic liquids by varying the
nature of the cation e.g. 1,3-dialkylimidazolium, 1,2,3-trialk-
ylimidazolium and N,N-dialkylpyrrolidinium and the nature of
the anion e.g. BF4
2, PF6
2, CF3CO2
2, CF3SO3
2 (OTf2) and
N(CF3SO2)2
2 (NTf2
2). The BF4
2, NTf2
2 and PF6
2 ionic
liquids were prepared by anion exchange starting from
imidazolium or pyrrolidinium chloride. The CF3SO3
2 and
CF3CO2
2 salts were prepared by direct methylation of 1-alkyli-
midazole or 1-alkylpyrrolidine with the corresponding methyl
esters.5 We have measured the solubility of 1-hexene in these
ionic liquids (Fig. 1). For a given anion, e.g. CF3CO2
2, the
solubility of 1-hexene increases upon increasing the length of
the alkyl chain of the 1,3-dialkylimidazolium e.g. 1-butyl-
3-methylimidazolium (BMI+) vs. 1-hexyl-3-methylimidazo-
lium (HMI+). Methylation of the C(2) atom of the imidazolium
ring tends to decrease the solubility of 1-hexene e.g. 1-butyl-
2,3-dimethylimidazolium BDMI+NTf2
2 vs. BMI+NTf2
2. No
significant differences are observed by changing the 1-butyl-
3-methylimidazolium cation for N,N-butylmethylpyrrolidinium
(BMP+). For a same cation, e.g. BMI+, the solubility of
1-hexene increases as follows: BF4
2 < PF6
2 < OTf2 <
CF3CO2
2 < NTf2
2.
In a first series of experiments, we performed 1-hexene
hydroformylation using these different ionic liquids as solvents
for the Rh(CO)2(acac) precursor associated with the sodium salt
of monosulfonated triphenylphosphine (TPPMS) (Fig. 1). The
results reveal that there is a correlation between the reaction
rates (TOF min21) and the solubility of 1-hexene in ionic
Fig. 1 Turnover frequency as a function of 1-hexene solubility in the ionic
liquids. Reaction conditions: Rh(CO)2(acac) 0.075 mmol, 1-hexene/Rh =
800, TPPMS/Rh = 4 , heptane was used as internal standard, CO/H2 (molar
ratio) = 1, P(CO/H2) = 2 MPa, T = 80 °C, TOF determined at 25%
conversion of 1-hexene.
Scheme 1
Scheme 2 Synthesis of ligand 2. Reagents and conditions: i, KOBut, reflux
for 3 h in THF; ii, Me3O+BF4
+ in CH2Cl2, 278 °C.
This journal is © The Royal Society of Chemistry 2001
1360 Chem. Commun., 2001, 1360–1361 DOI: 10.1039/b104155j

liquids. In all cases, the selectivity in aldehydes is > 97%, the
remainder being isomerized hexenes. Surprisingly, in the
NTf2
2 based ionic liquids, lower TOF are obtained despite the
relatively good solubility of 1-hexene in these media. As
suggested by the determination of ionic liquid relative polarity,6
the NTf2
2 based salts could be more coordinating than the PF6
2
salts. In all cases, the n/i ratio is not affected by the nature of the
solvent.
In a second series of experiments, we have synthesized the
monosubstituted guanidinium triphenylphosphine ligands 1a
and 1b,7 and the pyridinium diphenylethylphosphine ligand 2
(Scheme 1). Ligand 2 was prepared in two steps according to
Scheme 2. Ligands 1a and 1b show good solubility in the ionic
liquid BMI+BF4
2 . They give good selectivities towards the
linear aldehydes. Similar catalytic performances were obtained
by using 1a or 1b (Table 1, entries 1 and 2). However, the
retention of the Rh in the BMI+BF4
2 phase was more efficient
with 1a (the Rh content in the organic phase was lower than the
detection limit according to ICP analysis for 1a, while the level
was 0.8% of the initial Rh for 1b). Ligand 2 (entry 3) presents
higher reaction rates and higher selectivity towards aldehydes
than 1a and 1b. However, the leaching of the Rh in the organic
phase was found to be higher for 2 (2% of the initial Rh).
Phosphites and bisphosphites are well known ligands for Rh-
hydroformylation to afford higher reaction rates.8 Because of
their instability toward hydrolysis, examples of their use in
aqueous two-phase hydroformylations are rare.9 Ionic liquids
offer suitable alternative solvents. We describe here the first use
of phosphite based ligands for the biphasic hydroformylation of
1-hexene in ionic liquids. Ligand 3, a mixture of tetra-
butylammonium salt of the mono- di- and tri-sulfonated
triphenylphosphites, has been prepared by transesterification of
triphenylphosphite with the tetrabutylammonium salt of p-
hydroxyphenylsulfonic acid.9 In the reaction with the ligand 3,
using BMI+PF6
2 as the solvent, good catalytic activity is
observed (entry 4). The selectivity for the linear aldehyde is
much higher than the selectivity obtained with phosphine
ligands (enties 1–3). The use of the modified phosphite 3 limits
the loss of the Rh in the organic phase (leaching is 2% of the
initial Rh used). At the end of the run, the organic phase is
decanted and separated from the ionic liquid which is reused
(entry 5 and 6). Despite a loss of activity which could be
ascribed to a partial degradation of the Rh active catalyst during
the separation, the n/i ratio remains high after two recyclings.
In conclusion, it is shown that thanks to the great versatility
of ionic liquids, it is possible to optimize Rh-hydroformylation
performances by adjusting the nature of the anions and cations
present in the solvent and the nature of the ligands. Phosphite
ligands, which are unstable in an aqueous two-phase system,
can be used. The problem of Rh leaching can be minimized by
the modification of phosphorus ligands with cationic (guanidin-
ium or pyridinium) or anionic (sufonate) groups. By adjusting
the ligand and the ions of the solvent, excellent Rh retention has
been achieved.
Notes and references
1 For a review see: H. Olivier, in Aqueous-Phase Organometallic
Catalysis, ed. B. Cornils and W.A. Herrmann, Wiley-VCH, Weinheim,
1998, p. 553. T. Welton, Chem. Rev., 1999, 99, 2071; P. Wasserscheid
and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3772; J. Dupont, C. S.
Consorti and J. Spencer, J. Braz. Chem., 2000, 11, 337; J. D. Holbrey and
K. R. Seddon, Clean Prod. Processes, 1999, 1, 223.
2 E. G. Kuntz, Fr. Pat., 2314910, 1975 (to Rhône-Poulenc); B. Cornils and
E. Wiebus, CHEMTECH, 1995, 25listlistr, 33.
3 Y. Chauvin, L. Mussmann and H. Olivier, Angew. Chem., Int. Ed., 1995,
34, 2698; C. C. Brasse, U. Englert, A. Salzer, H. Waffenschmidt and P.
Wasserscheid, Organometallics, 2000, 19, 3818; P. Wasserscheid, H.
Waffenschmidt, P. Machnitzki, K. W. Kottsieper and O. Stelzer, Chem.
Commun., 2001, 451.
4 A. J. Carmichael, C. Hardacre, J. D. Holbrey, K. R. Seddon and M.
Nieuwenhuyzen, Electrochem. Soc. Proceedings, Molten Salts XII, ed.
P. C. Trulove, H. C. De Long, G. R. Stafford and S. Deki, The
Electrochemical Society, Pennington, NJ, 2000, vol. 91-41, p. 209.
5 P. Bonhôte, A. Dias, N. Papageorgiou, K. Kalyanasundaram and M.
Grätzel, Inorg. Chem., 1996, 35, 1168.
6 A. J. Carmichael and K. Seddon, J. Phys. Org. Chem., 2000, 13, 591.
7 A. Hessler, O. Stelzer, H. Dibowski, K. Worm and F. P. Schmidtchen,
J. Org. Chem., 1997, 62, 2362; P. Machnitzki, M. Tepper, K. Wenz, O.
Stelzer and E. Herdtweck, J. Organomet. Chem., 2000, 602, 158.
8 P. C. J. Kamer, J. N. H. Reek and P. W. N. M. van Leeuwen, in Rhodium
Catalyzed Hydroformylation, ed. P.W.N.M. van Leeuwen and C. Claver,
Kluwer Academic Publishers, Netherlands, 2000, p. 35.
9 B. Fell, G. Papadogianakis, W. Konkol, J. Weber and H. Bahrmann,
J. Prakt. Chem., 1993, 335, 75.
Table 1 Hydroformylation with different ligand–ionic liquid systemsa
Entry Ligand L L/Rh Ionic liquid
Reaction
time/min
Conversionb
(%)
Aldehydesc
(mol %) n/id TOFe/min21
1 1a 10 BMI+BF4
2 180 77 74 3.7 3
2 1b 7 210 83 78 4 3
3 2 4 180 87 96 2.6 4
4 3 9.5 BMI+PF6
2 180 96 88 12.6 4
5f 3 240 85 89 11.2 2
6g 3 330 42 88 11.7 1
a Reaction conditions: Rh(CO)2(acac) 0.075 mmol, 1-hexene/Rh = 800, CO/H2 (molar ratio) = 1 ; P(CO/H2) = 2 MPa, T = 80 °C, heptane (internal
standard) = 2 mL, 1-hexene = 7.5 mL, ionic liquid = 4 mL. b Conversion = [(initial 1-hexene) 2 (1-hexene after reaction)]/(initial 1-hexene). c The other
products are 2- and 3-hexenes. d Linear to branched aldehyde ratio. e Mol of aldehydes per mol Rh per minute at 25% conversion. f Recycling of 4. g Recycling
of 5.
Chem. Commun., 2001, 1360–1361 1361

FeatureArticle
www.rsc.org/chemcomm
CHEMCOMM
Catalytic reactions in ionic liquids
Roger Sheldon
Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, Delft
BL-2628, The Netherlands. E-mail: secretariat-ock@tnw.tudelft.nl
Received (in Cambridge, UK) 10th August 2001, Accepted 11th September 2001
First published as an Advance Article on the web 18th October 2001
The chemical industry is under considerable pressure to replace
many of the volatile organic compounds (VOCs) that are
currently used as solvents in organic synthesis. The toxic and/or
hazardous properties of many solvents, notably chlorinated
hydrocarbons, combined with serious environmental issues,
such as atmospheric emissions and contamination of aqueous
effluents is making their use prohibitive. This is an important
driving force in the quest for novel reaction media. Curzons and
coworkers,1 for example, recently noted that rigorous manage-
ment of solvent use is likely to result in the greatest improve-
ment towards greener processes for the manufacture of
pharmaceutical intermediates. The current emphasis on novel
reaction media is also motivated by the need for efficient
methods for recycling homogeneous catalysts. The key to waste
minimisation in chemicals manufacture is the widespread
substitution of classical ‘stoichiometric’ syntheses by atom
efficient, catalytic alternatives.2 In the context of homogeneous
catalysis, efficient recycling of the catalyst is a conditio sine qua
non for economically and environmentally attractive processes.
Motivated by one or both of the above issues much attention has
been devoted to homogeneous catalysis in aqueous biphasic3,4
and fluorous biphasic5 systems as well as in supercritical carbon
dioxide.6 Similarly, the use of ionic liquids as novel reaction
media may offer a convenient solution to both the solvent
emission and the catalyst recycling problem.7,8
Historical development
What are ionic liquids? Quite simply, they are liquids that are
composed entirely of ions. Molten sodium chloride, for
example, is an ionic liquid but a solution of sodium chloride in
water is an ionic solution. The term molten salts, which was
previously used to describe such materials, evokes an image of
high-temperature, viscous and highly corrosive media. The term
ionic liquid, in contrast, implies a material that is fluid at (or
close to) ambient temperature, is colourless, has a low viscosity
and is easily handled, i.e. a material with attractive properties
for a solvent. Room temperature ionic liquids are generally salts
of organic cations, e.g. tetraalkylammonium, tetraalkylphos-
phonium, N-alkylpyridinium, 1,3-dialkylimidazolium and
trialkylsulfonium cations (Fig. 1).
In order to be liquid at room temperature, the cation should
preferably be unsymmetrical, e.g. R1 and R2 should be different
alkyl groups in the dialkylimidazolium cation. The melting
point is also influenced by the nature of the anion (see
Table 1).
Room temperature ionic liquids are not new. Ethylammon-
ium nitrate, which is liquid at room temperature (but usually
contains 200–600 ppm water) was first described in 1914.9 In
the late 1940s, N-alkylpyridinium chloroaluminates were
studied as electrolytes for electroplating aluminium. These
systems were reanimated by the groups of Hussey,10 Oster-
young11 and Wilkes12 in the late 1970s. The first examples of
ionic liquids based on dialkylimidazolium cations were reported
in the early 1980s by Wilkes and coworkers.12 They contained
Roger Sheldon was born in Nottingham (UK) in 1942. After
receiving a PhD in Organic Chemistry from Leicester Uni-
versity (1967) he spent two years as a postdoc with Professor
Jay Kochi in the USA. From 1969–1980 he was with Shell
Research in Amsterdam and from 1980–1990 he was R&D
Director of DSM Andeno. In 1991 he moved to his preent
position as Professor of Organic Chemistry and Catalysis at the
Delft University of Technology. His research interests are
focused on the application of catalytic methodologies—homo-
geneous, heterogeneous and enzymatic—in organic synthesis,
particularly in relation to fine chemicals production. He has
widely promoted the concepts of E factors and atom efficiency
for assessing the environmental impact of chemical processes.
He is the author of ca. 300 scientific publications, numerous
patents and three books on the subject of catalysis and
chirotechnology. He is the Editor-in-Chief of Journal of
Molecular Catalysis B: Enzymatic and Chairman of the
Editorial Board of Green Chemistry. Among other distinctions
he was recently awarded a Doctor Honoris Causa from the
Russian Academy of Sciences.
Fig. 1 Structures of ionic liquids.
Table 1 Melting points of some dialkylimidazolium salts
R X mp/°C
Me Cl 125
Et Cl 87
n-Bu Cl 65
Et NO3 38
Et AlCl4 7
Et BF4 6
Et CF3SO3 29
Et (CF3SO3)2N 23
Et CF3CO2 214
n-Bu CF3SO3 16
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b107270f Chem. Commun., 2001, 2399–2407 2399

chloroaluminate anions (AlCl4
2 or Al2Cl7
2) and proved to be
useful catalysts/solvents for Friedel–Crafts acylations.13 How-
ever, a serious obstacle for widespread use of these ionic liquids
is the high reactivity of the chloroaluminate anion towards
water.
The first example of the new ionic liquids, that currently are
receiving so much attention as novel media for homogeneous
catalysis, ethylmethylimidazolium tetrafluoroborate
(emimBF4)† was reported by Wilkes et al. in 1992.14 The
synthesis of the corresponding hexafluorophosphate followed
shortly thereafter.15 In contrast to the chloroaluminate salts the
fluoroborates and hexafluorophosphates are stable towards
hydrolysis. Subsequently, 1,3-dialkylimidazolium salts con-
taining a wide variety of anions, e.g. CF3SO3
2, [CF3SO2]2N2,
CF3CO2
2, CH3CO2
2, PhSO3
2 and many more have been
prepared.16
Ionic liquids can be prepared by direct quaternisation of the
appropriate amine or phosphine. Different anions can subse-
quently be introduced by anion exchange. It is beyond the scope
of this review to discuss in detail the synthesis of ionic liquids
and the reader is referred to excellent reviews for many
details.17–20 It is important to note, however, that ionic liquids,
owing to their non-volatile nature, cannot be purified by
distillation. Consequently, they should be produced in high
purity. For example, if synthesis involves exchange of chloride
ions it is important that no chloride ions remain in the product
as they may seriously impede catalysis by strongly coordinating
to low valent transition metal complexes.
The hydrophilicity/lipophilicity of an ionic liquid can be
modified by a suitable choice of anion, e.g. bmimBF4 is
completely miscible with water while the PF6 salt is largely
immiscible with water. The lipophilicity of dialkylimidazolium
salts, or other ionic liquids, can also be increased by increasing
the chain length of the alkyl groups.21
Ionic liquids containing ‘fluorous ponytails’ have even been
described.22 When these are added to conventional ionic liquids
they facilitate emulsification with perfluorocarbons. This
provides the possibility of performing (catalytic) reactions in
ionic liquids/perfluorocarbon biphasic systems.
Catalysis in ionic liquids: general considerations
Room temperature ionic liquids exhibit many properties which
make them potentially attractive media for homogeneous
catalysis:
4 They have essentially no vapour pressure, i.e. they do not
evaporate and are easy to contain.
4 They generally have reasonable thermal stability. While
tetraalkylammonium salts have limited thermal stability,
owing to decomposition via the Hoffmann elimination,
emimBF4 is reportedly stable up to 300 °C and emim-
(CF3SO2)2N up to 400 °C.16a In other words many ionic
liquids have liquid ranges of more than 300 °C, compared to
the 100 °C liquid range of water.
4 They are able to dissolve a wide range of organic, inorganic
and organometallic compounds.
4 The solubility of gases, e.g. H2, CO and O2, is generally good
which makes them attractive solvents for catalytic hydro-
genations, carbonylations, hydroformylations, and aerobic
oxidations.
4 They are immiscible with some organic solvents, e.g.
alkanes, and, hence, can be used in two-phase systems.
Similarly, lipophilic ionic liquids can be used in aqueous
biphasic systems.
4 Polarity and hydrophilicity/lipophilicity can be readily
adjusted by a suitable choice of cation/anion (see earlier) and
ionic liquids have been referred to as ‘designer solvents’.7
4 They are often composed of weakly coordinating anions, e.g.
BF4
2 and PF6
2 and, hence, have the potential to be highly
polar yet non-coordinating solvents. They can be expected,
therefore, to have a strong rate-enhancing effect on reactions
involving cationic intermediates.
4 Ionic liquids containing chloroaluminate ions are strong
Lewis, Franklin and Brønsted acids. Protons present in
emimAlCl4 have been shown to be superacidic with
Hammett acidities up to 218.23 Such highly acidic ionic
liquids are, nonetheless, easily handled and offer potential as
non-volatile replacements for hazardous acids such as HF in
several acid-catalysed reactions.
Faced with these numerous potential benefits one may
wonder if ionic liquids have any problems associated with their
use. Atmospheric emissions may not be an issue but, when used
on an industrial scale, small amounts of ionic liquids will
inevitably find their way into the environment via the proverbial
‘mechanical losses’. So, what is known about their potential
environmental impact? A cursory examination of the literature
reveals a dearth of information regarding the biodegradability
and toxicity of ionic liquids. A prerequisite for industrial use is,
therefore, the generation of appropriate data to enable the
assessment of the potential environmental impact of ionic
liquids.
Another question which arises in any discussion of ionic
liquids as reaction media pertains to the isolation of soluble
reaction products. Volatile products can be separated by
distillation. Non-volatile products, on the other hand, can be
separated by solvent extraction. Although this seems para-
doxical—using an ionic liquid to avoid atmospheric emissions
and subsequently using a volatile organic solvent to extract the
product—it could have environmental benefits. For example,
substituting an environmentally unacceptable solvent by an
ionic liquid as the reaction medium, followed by extraction with
a more benign organic solvent would constitute an environ-
mental benefit. In this context it is worth noting the use of
supercritical carbon dioxide to extract products from ionic
liquids, which is currently the focus of attention.24 Quite
remarkably, scCO2 is highly soluble (up to 60 mol%) in
bmimPF6 while the latter is insoluble in scCO2. Naphthalene,
for example, was recovered quantitatively from bmimPF6 by
scCO2 extraction, without any contamination of the extract by
the ionic liquid.
One can envisage various scenarios for catalysis in and/or by
ionic liquids:
4 Monophasic systems in which the catalyst and substrate are
dissolved in the ionic liquid.
4 Monophasic systems in which the ionic liquid acts as both
the solvent and the catalyst, e.g. dialkylimidazolium chloro-
aluminates as Friedel–Crafts catalysts (see later).
4 Biphasic systems in which the catalyst resides in the ionic
liquid and the substrate/product in the second phase or vice
versa.
4 Mono- or biphasic systems in which the anion of the ionic
liquid acts as a ligand for the homogeneous catalyst, e.g. a
sulfonated phosphine ligand (see later).
4 Triphasic systems comprising, for example, an ionic liquid,
water and an organic phase in which the catalyst resides in
the ionic liquid, the substrate and product in the organic
phase and salts formed in the reaction are extracted into the
aqueous phase, e.g. in Heck reactions (see later).
The first example of homogeneous transition metal catalysis
in an ionic liquid is the platinum catalysed hydroformylation of
ethene in tetraethylammonium trichlorostannate, described by
Parshall in 1972.25 This ionic liquid (referred to as a molten salt
back in those days) has a melting point of 78 °C. These results
were largely ignored for two decades. The potential of ionic
liquids as novel media for homogeneous catalysis became more
widely appreciated largely due to the pioneering studies and
extensive promotion of the groups of Seddon17 and Chauvin and
Olivier-Bourbigou18 and, more recently, the groups of Welton19
2400 Chem. Commun., 2001, 2399–2407

and Keim and Wasserscheid.20 In the last five years their use as
novel media for, inter alia, catalytic hydrogenations, hydro-
formylations, isomerisations, olefin dimerisations, oligomerisa-
tions and polymerisations and Heck couplings, has been rapidly
expanding. The salient features of these studies will be reviewed
in the ensuing discussion, with emphasis on their potential as
clean synthetic methodologies.
Hydrogenation
The first example of catalytic hydrogenation in an ionic liquid
was reported by Chauvin et al. in 1995.26 A solution of the
cationic [Rh nbd(Ph3P)2]PF6 complex [nbd = norbornadiene
(bicyclo[2.2.1]hepta-2,5-diene)] in bmimPF6 or bmimSbF6 was
shown to be an effective catalyst for the biphasic hydrogenation
of pent-1-ene. Reaction rates were up to five times higher than
in acetone as solvent which was attributed to the formation of an
unsolvated cationic rhodium(III) dihydride complex with two
free coordination sites in the nonsolvating ionic liquid. In
contrast, poor results were obtained with bmimBF4 which was
ascribed to the presence of trace amounts of strongly coordinat-
ing chloride ions in their sample of this ionic liquid. The catalyst
solution in the ionic liquid could be reused with rhodium losses
below the detection limit of 0.02%.
Similarly, advantage was taken of the biphasic system to
perform the selective hydrogenation of cyclohexadiene. The
solubility of cyclohexadiene in bmimSbF6 is about five times
that of cyclohexene and, hence, the latter was obtained in 98%
selectivity at 96% conversion.
Dupont and coworkers27 performed the biphasic hydro-
genation of cyclohexene with Rh(cod)2BF4 (cod = cycloocta-
1,5-diene) in ionic liquids. They observed roughly equal rates
(turnover frequencies of ca. 50 h21) in bmimBF4 and bmimPF6
(presumably their bmimBF4 was chloride-free).
The same group showed that RuCl2(Ph3P)3 in bmimBF4 is an
effective catalyst for the biphasic hydrogenation of olefins, with
turnover frequencies up to 540 h21.28 Similarly, (bmim)3-
Co(CN)5 dissolved in bmimBF4 catalysed the hydrogenation of
butadiene to but-1-ene, in 100% selectivity at complete
conversion.28
More recently, the ruthenium-catalysed hydrogenation of
sorbic acid to cis-hex-3-enoic acid (Reaction 1) was achieved in
a biphasic bmimPF6–methyl tert-butyl ether system.29
(1)
The ruthenium cluster, [H4Ru(h6-C6H6)4] [BF4]2 in
bmimBF4 was shown to be an effective catalyst for the
hydrogenation of arenes, to the corresponding cycloalkanes, at
90 °C and 60 bar.30 The cycloalkane product formed a separate
phase which was decanted and the ionic liquid phase, containing
the catalyst, could be repeatedly recycled.
Enantioselective hydrogenation in ionic liquids is of partic-
ular interest as it could provide a means for facile recycling of
metal complexes of expensive chiral ligands. In their original
study Chauvin et al.26 reported that [Rh(cod)(2)-(diop)]PF6
catalysed the enantioselective hydrogenation of a-acetamido-
cinnamic acid to (S)-phenylalanine, in 64% ee, in a biphasic
bmimSbF6–isopropyl alcohol (Reaction 2). The observed
(2)
enantioselectivity is what one would expect with diop which is
not a particularly good ligand for this reaction. The product,
contained in the isopropyl alcohol, could be separated quantita-
tively and the recovered ionic liquid, containing the catalyst,
reused.
Similarly, Dupont and coworkers31 extended their studies of
ruthenium-catalysed hydrogenations in ionic liquids to enantio-
selective reactions. The chiral [RuCl2(S)-BINAP]2NEt3 com-
plex was shown to catalyse the asymmetric hydrogenation of
2-phenylacrylic acid and 2-(6-methoxy-2-naphthyl)acrylic acid
in bmimBF4–isopropyl alcohol. The latter afforded the anti-
inflammatory drug, (S)-naproxen, in 80% ee (Reaction 3). The
(3)
product could be quantitatively separated and the recovered
ionic liquid catalyst solution recycled several times without any
significant change in activity or selectivity.
An interesting recent development is the use of a biphasic
ionic liquid–supercritical CO2 for catalytic hydrogenation32,33
and other processes (see later). Tumas and coworkers32 showed
that the catalytic hydrogenation of olefins could be conducted in
a biphasic bmimPF6–scCO2 system. The ionic liquid phase
containing the catalyst was separated by decantation and reused
in up to four consecutive batches.
Jessop and coworkers33 extended this concept to the
asymmetric hydrogenation of tiglic acid (Reaction 4) and the
precursor of the antiinflammatory drug ibuprofen (Reaction 5)
using Ru(OAc)2(tolBINAP) as the catalyst.
(4)
(5)
They found that Reaction 4 was more selective in a
bmimPF6–water biphasic mixture while Reaction 5 gave poor
enantioselectivities in the wet ionic liquid. In this case the best
result (85% ee) was obtained using methanol as cosolvent at 100
bar H2 pressure. In both cases the product was separated by
scCO2 extraction when the reaction was complete. The different
solvent effects observed with the two substrates was assumed to
be due to the solubility of H2 in the reaction mixture. The
hydrogen concentration dependence of asymmetric catalytic
hydrogenation with ruthenium BINAP complexes is known to
be dependent on the substrate.34 Class I substrates such as the
ibuprofen precursor give higher enantioselectivities at high H2
concentration while class II substrates, exemplified by tiglic
acid, give higher enantioselectivities at low H2 concentra-
tions.
Hydroformylation
Hydroformylation of propene in an aqueous biphasic system,
using a water-soluble rhodium complex of the sodium salt of
trisulfonated triphenylphosphine (tppts) forms the basis of the
Ruhr Chemie Rhone Poulenc process for the manufacture of
butanal.35 Unfortunately this process is limited to C2 to C5
olefins owing to the very low solubility of higher olefins in
water. Hence, one can envisage that the use of an appropriate
ionic liquid could provide the basis for biphasic hydro-
formylation of higher olefins.
Chem. Commun., 2001, 2399–2407 2401

As noted earlier, Parshall showed, in 1972, that platinum-
catalysed hydroformylations could be performed in tetra-
ethylammonium trichlorostannate melts.25 More recently, Waf-
fenschmidt and Wasserscheid36 studied the platinum-catalysed
hydroformylation of oct-1-ene in bmimSnCl3 (Reaction 6)
(6)
which is liquid at room temperature. Despite the limited
solubility of oct-1-ene in the ionic liquid, high activities (TOF =
126 h21) were observed together with a remarkably high
regioselectivity (n/iso = 19). The product was recovered by
phase separation and no leaching of platinum was observed.
The ruthenium- and cobalt-catalysed hydroformylation of
internal and terminal olefins in molten tetra-n-butylphosphon-
ium bromide was reported by Knifton in 1987.37 More recently,
the rhodium-catalysed hydroformylation of hex-1-ene was
conducted in molten phosphonium tosylates, e.g. Bu3PEt+TsO2
and Ph3PEt+TsO2 having melting points of 81–83 °C and
94–95 °C, respectively, at 120 °C and 40 bar.38 Advantage was
taken of the higher melting points of these 'ionic liquids' to
decant the product from the solid catalyst medium at room
temperature.
Chauvin and coworkers26 investigated the rhodium-catalysed
biphasic hydroformylation of pen-1-tene in bmimPF6. High
activities (TOF = 333 h21 compared with 297 h21 in toluene)
were observed with the neutral Rh(CO)2(acac)–Ph3P as the
catalyst precursor but some leaching of the catalyst into the
organic phase occurred. This could be avoided by using
Rh(CO)2acac with tppts or tppms (monosulfonated triphenyl-
phosphine) as the catalyst precursor, albeit at the expense of rate
(TOF = 59 h21 with tppms). Higher activities (TOF = 810
h21) and high regioselectivity (n/iso = 16) were observed in the
biphasic hydroformylation of oct-1-ene in bmimPF6 using
cationic cobaltocenium diphosphine ligands but some catalyst
leaching ( < 0.5%) was observed.39
Better results were obtained with cationic guanidine-mod-
ified diphosphine ligands containing a xanthene backbone.40
Xanthene-based diphosphine ligands with large bite angles (P–
metal–P ~ 110°) are known to give high selectivities (!98%)
towards the linear aldehyde.41 Biphasic hydroformylation of
oct-1-ene, using rhodium complexes of these ligands in
bmimPF6 (Reaction 7), afforded high regioselectivities (ca. 20)
(7)
and the catalyst could be recycled ten times (resulting in an
overall turnover number of 3500) without detectable ( < 0.07%)
leaching of Rh to the organic phase.
The group of Olivier-Bourbignou42 has recently explored the
use of a wide range of ionic liquids, based on imidazolium and
pyrrolidinium cations and weakly coordinating anions, for the
biphasic hydroformylation of hex-1-ene catalysed by rhodium
complexes of modified phosphine and phosphite ligands. The
latter are, in contrast, unstable in aqueous biphasic media. The
rate and regioselectivity could be optimized by choosing a
suitable combination of cation, anion and phosphine or
phosphite ligand. Rhodium leaching was minimised by mod-
ification of the ligands with cationic (guanidinium or pyr-
idinium) or anionic (sulfonate) groups.
Another interesting recent development is the rhodium-
catalysd biphasic hydroformylation of oct-1-ene in bmimPF6–
scCO2 in a continuous flow process.43 Because of the low
solubility of Rh–tppms and Rh–tppts complexes in the ionic
liquid, [pmim]+Ph2PC6H4SO3
2 (pmim = 1-propyl-3-methyl-
imidazolium) was synthesised and used together with
Rh2(OAc)4 as the catalyst precursor. Aldehydes were produced
at a constant rate for 72 h albeit with moderate regioselectivity
(n/iso = 3.8). Analysis of recovered products revealed that < 1
ppm Rh is leached into the organic phase.
The monophasic hydroformylation of methylpent-3-enoate in
bmimPF6 has been reported.44 The linear aldehyde product
(Reaction 8) is a precursor of adipic acid in an alternative
(8)
butadiene-based route. The product was removed by distillation
(0.2 mbar/110 °C) and the ionic liquid recycled ten times
without significant loss in activity.
Alkoxycarbonylation
Much less attention has been focused on carbonylation reactions
in ionic liquids. The biphasic palladium-catalysed alkoxy-
carbonylation of styrene (Reaction 9) in bmimBF4–cyclohex-
(9)
ane has been reported.45 Very high regioselectivities (!99.5%
iso) were obtained, using PdCl2(PhCN)2 in combination with
(+)-neomenthyldiphenylphosphine and toluene-p-sulfonic acid,
under mild conditions (70 °C and 10 bar).
More recently, the palladium-catalysed alkoxycarbonylation
and amidocarbonylation of aryl bromides and iodides in
bmimBF4 and bmimPF6 has been described.46 Enhanced
reactivities were observed compared to conventional media and
the ionic liquid–catalyst could be recycled.
Olefin dimerisation and oligomerisation
The nickel-catalysed dimerisation of lower olefins in ionic
liquids containing chloroaluminate anions is probably the most
investigated reaction in ionic liquids.18,26,47–49 As early as 1990
the group of Chauvin at the Institut Francais du Petrole (IFP)
reported the nickel-catalysed dimerisation of propene in
bmimAlCl4.47 The catalyst precursor consisted of L2NiCl2 (L =
Ph3P or pyridine) in combination with EtAlCl2 (bmimCl–
AlCl3–EtAlCl2 = 1+1.2+0.25). The active catalyst is a cationic
nickel(II) complex, [LNiCH2CH3]+AlCl4
2, formed by reaction
of L2NiCl2 with EtAlCl2. Since ionic liquids promote the
dissociation of ionic metal complexes it was envisaged that they
would have a beneficial effect on this reaction.18
This proved to be the case: at 5 °C and atmospheric pressure
productivities ( > 250 kg dimers per g Ni) much higher than
those observed in organic solvents were achieved.18,47,48 The
mixture of dimers obtained, containing 2,3-dimethylbutene as
the major component (83%), has commercial importance as the
precursor of octane boosters for reformulated gasoline. It has
been produced since the mid-seventies by the IFP ‘Dimersol’
process (25 units worldwide with a production of 3.4 3 106 tons
per annum) in a single-phase solvent-free medium.
The methodology was subsequently extended to the dimer-
isation of butenes. The isooctene product constitutes the
feedstock for the manufacture of isononanols (plasticizers) by
2402 Chem. Commun., 2001, 2399–2407

Investigation of the Potential Use of (IILs) Immobilized Ionic Liquids in Shale Gas Sweetening

Investigation of the Potential Use of (IILs) Immobilized Ionic Liquids in Shale Gas Sweetening

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