Influence of ionic association, transport properties, and solvation on the catalytic hydrogenation of 1,3-cyclohexadiene in ionic liquids

Article abstract of DOI:10.1021/jp206619c

The influence of the nature of two different ionic liquids, namely 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C ₁C₄Im][NTf₂], and 1-butyl-2,3- dimethylimidazolium bis(trifluoromethylsulfonyl)imide, [C₁C ₁C₄Im][NTf₂], on the catalytic hydrogenation of 1,3-cyclohexadiene with [Rh(COD)(PPh₃)₂][NTf₂] (COD=1,5-cyclooctadiene) was studied. Initially, the effect of different concentrations of 1,3-cyclohexadiene on the molecular interactions and on the structure in two ionic liquids was investigated by NMR and by molecular dynamic simulations. It was found that in both ionic liquids 1,3-cyclohexadiene is solvated preferentially in the lipophilic regions. Furthermore, the higher solubility of 1,3-cyclohexadiene in [C₁C₄Im][NTf ₂] and the smaller positive values of the excess molar enthalpy of mixing for the 1,3-cyclohexadiene + [C₁C₄Im][NTf ₂] system in comparison with 1,3-cyclohexadiene + [C ₁C₁C₄Im][NTf₂] indicate more favorable interactions between 1,3-cyclohexadiene and the C₁C ₄Im⁺ cation than with the C₁C₁C ₄Im⁺ cation. Subsequently, diffusivity and conductivity measurements of the 1,3-cyclohexadiene + ionic liquid mixtures at different compositions allowed a characterization of mass and charge transport in the media and access to the ionicity of ionic liquids in the mixture. From the dependence of the ratio between molar conductivity and the conductivity inferred from NMR diffusion measurements, &_imp/&_NMR, on concentration of 1,3-cyclohexadiene in the ionic liquid mixture, it was found that increasing the amount of 1,3-cyclohexadiene leads to a decrease in the ionicity of the medium. Finally, the reactivity of the catalytic hydrogenation of 1,3-cyclohexadiene using [Rh(COD)(PPh₃)₂][NTf ₂] performed in [C₁C₄Im][NTf₂] at different compositions of 1,3-cyclohexadiene and in [C₁C ₁C₄Im][NTf₂] at one composition was related linearly to the viscosity, hence the reaction rate is determined by the mass transport properties of the media.

Full text of DOI:10.1021/jp206619c

ARTICLE
pubs.acs.org/JPCB
Influence of Ionic Association, Transport Properties, and Solvation on
the Catalytic Hydrogenation of 1,3-Cyclohexadiene in Ionic Liquids
Ajda Podgorꢀsek,† Gorka Salas,‡ Paul S. Campbell,‡ Catherine C. Santini,*,‡ Agílio A. H. Pꢁadua,†,§
Margarida F. Costa Gomes,*,†,§ Bernard Fenet,|| and Yves Chauvin‡
†CNRS, Laboratoire de Thermodynamique et Interactions Molꢁeculaires, UMR 6272, 24 avenue des Landais, BP 80026, 63171 Aubiꢂere,
France
‡Universitꢁe de Lyon, Institut de Chimie de Lyon, UMR 5265 CNRS, Universitꢁe de Lyon 1-ESCPE Lyon, C2P2, Equipe Chimie
Organomꢁetallique de Surface, ESCPE 43 Boulevard du 11 Novembre 1918, F-69616 Villeurbanne, France
§Clermont Universitꢁe, Universitꢁe Blaise Pascal, Clermont-Ferrand, France
Centre Commun de RMN, UCB Lyon 1 - ESCPE Lyon, 43 Boulevard du 11 Novembre 1918, F-69616 Villeurbanne Cedex, France
S Supporting Information
b
ABSTRACT: The influence of the nature of two different ionic
liquids, namely 1-butyl-3-methylimidazolium bis(trifluoromethyl-
sulfonyl)imide, [C1C4Im][NTf2], and 1-butyl-2,3-dimethylimi-
dazolium bis(trifluoromethylsulfonyl)imide, [C1C1C4Im][NTf2],
on the catalytic hydrogenation of 1,3-cyclohexadiene with [Rh-
(COD)(PPh3)2][NTf2] (COD=1,5-cyclooctadiene) was stud-
ied. Initially, the effect of different concentrations of 1,3-cyclo-
hexadiene on the molecular interactions and on the structure in
two ionic liquids was investigated by NMR and by molecular
dynamic simulations. It was found that in both ionic liquids 1,3-
cyclohexadiene is solvated preferentially in the lipophilic regions. Furthermore, the higher solubility of 1,3-cyclohexadiene in
[C1C4Im][NTf2] and the smaller positive values of the excess molar enthalpy of mixing for the 1,3-cyclohexadiene +
[C1C4Im][NTf2] system in comparison with 1,3-cyclohexadiene + [C1C1C4Im][NTf2] indicate more favorable interactions
between 1,3-cyclohexadiene and the C1C4Im+ cation than with the C1C1C4Im+ cation. Subsequently, diffusivity and conductivity
measurements of the 1,3-cyclohexadiene + ionic liquid mixtures at different compositions allowed a characterization of mass and
charge transport in the media and access to the ionicity of ionic liquids in the mixture. From the dependence of the ratio between
molar conductivity and the conductivity inferred from NMR diffusion measurements, Λimp/ΛNMR, on concentration of 1,3-
cyclohexadiene in the ionic liquid mixture, it was found that increasing the amount of 1,3-cyclohexadiene leads to a decrease in the
ionicity of the medium. Finally, the reactivity of the catalytic hydrogenation of 1,3-cyclohexadiene using [Rh(COD)(PPh3)2]-
[NTf2] performed in [C1C4Im][NTf2] at different compositions of 1,3-cyclohexadiene and in [C1C1C4Im][NTf2] at one
composition was related linearly to the viscosity, hence the reaction rate is determined by the mass transport properties of the media.
Numerous experimental studies, ranging from NMR spec-
troscopy focusing on through-space NOE interactions,20
through X-ray diffraction,21À23 to neutron scattering,24,25
provided strong evidence for a local structuring of pure ILs.
Neutron scattering experiments found significant charge
ordering in the [C1C2Im][NTf2] ionic liquid,20,26 even in
the liquid state, whereas X-ray diffraction patterns of super-
cooled and liquid imidazolium ionic liquids with different chain
lengths18 indicated structural heterogeneities that varied in
size with the length of the alkyl chains in the imidazolium
cations. A high degree of self-organization of ionic liquids due
to the coexistence of charged moieties and hydrophobic alkyl
1. INTRODUCTION
Ionic liquids (ILs) are up-and-coming solvents for catalysis, with
the number of publications in this field growing at an exponential
rate.1À3 ILs have specific properties that, when they are used as
reaction media, may have more important consequences on catalytic
processes than traditional solvents.4À9 In some catalytic reactions,
the difference between ILs and traditional solvents has been related
to their chemical role, interfering with the reaction.2,10 For example,
imidazolium moieties of the cation may be deprotonated in situ
and therefore coordinate to metal centers as imidazolylidene-N-
heterocyclic carbene ligands.11À16 Certain ILs themselves exhibit
catalytic activity, for instance FriedelÀCrafts alkylations and
acylations occur readily in these media.17,18 In other cases,
differences in reactivity have an origin in physical chemistry,2,19
resulting from peculiar solvation phenomena in ILs, which are
more structured solvents than molecular fluids.16,18,23
Received: July 12, 2011
Revised:
September 2, 2011
Published: September 12, 2011
r
2011 American Chemical Society
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chains was also predicted by molecular dynamics simula-
tions.27,28
Scheme 1. Catalytic Hydrogenation of CYD
The presence of persistent microscopic structures such as
seggregated nonpolar domains and a polar network influences
the way solutes are solvated in ionic liquids.29,30 Indeed, radial
distribution functions from molecular simulation show that
nonpolar solutes dissolve in nonpolar domains, polar solvents
interact strongly with polar regions, and amphiphilic molecules
such as alcohols have more complex behavior.31 However, polar-
ity is not the only relevant property since ionic liquids can exhibit
specific interactions, i.e., cationÀπ interactions between the
ionic moieties and the solute, as evidenced by X-ray diffraction
of benzene + [C1C2Im][NTf2] mixtures at various composi-
tions.32 Similarly, by combining ROESY NMR and molecular
simulation such interactions were shown to be present in the
mixtures of toluene with the trialkylimidazolium IL - [C1C1C4Im]-
[NTf2].26 Contrarily, in the dialkylimidazolium IL, [C1C4Im]-
[NTf2], πÀcation interactions are inhibited by the strong
H-bond between anion and cation due to the relatively acidic
hydrogen on the C2 position of the imidazolium cation.32
Another NMR study provided supporting evidence that the
nature of the IL and its interactions are crucial in determining the
solvation environment of thiophene in mixtures with ILs.33
All these specific interactions may have an effect on reactions
depending on the substrate involved in a chemical process.
Finally, properties such as density and viscosity have higher
values for ionic liquids than for most traditional solvents and thus
play more important roles in reaction kinetics by affecting the
diffusion of molecules.34,35 Viscosity varies greatly with the nature of
the IL, and although related to the strength of cationÀanion
association, it is a physical property that is difficult to predict.36,37
Recently, we have reported how the difference of the activity
and the selectivity of the hydrogenation of 1,3-cyclohexadiene,
CYD (Scheme 1), in [C1C4Im][NTf2] and [C1C1C4Im][NTf2]
depends on the differences in the physicalÀchemical properties
of the catalytic medium and also solvation of the substrate.38
Following these results, the aim of the present work is to extend
and deepen the study of the dynamics of different species
involved in the catalytic system of hydrogenation of CYD. We
have investigated the transport of species in pure [C1C4Im]-
[NTf2] and [C1C1C4Im][NTf2] and in their mixtures with CYD
at different compositions corresponding to mole fractions of
xCYD = 0.09, 0.17, 0.20, 0.23, 0.29, and 0.33 (CYD/IL mole ratios
R = 0.1, 0.2, 0.25, 0.3, 0.4, and 0.5). Furthermore, ion association
in the medium (ionicity, strength of cationÀanion association)
and its effect on the catalytic activity and on the rate of the
reaction have been evaluated. This information has been assessed
through measurement of the density, viscosity, conductivity, and
diffusion coefficients (by 2D DOSY NMR) of CYD + IL
mixtures.
All other reagents and solvents were commercially available and
were used as received. Ionic liquids, synthesized as previously
reported,14 were dried overnight under high vacuum and stored
in a glovebox (Jacomex) in order to guarantee anhydrous products.
Density and Viscosity. The mixtures of CYD and ionic liquids
at different compositions, expressed in mole fraction xCYD
=
nCYD/(nCYD + nIL) or in the molar ratio R = nCYD/nIL, with
xCYD=R/(R+1), were prepared gravimetrically following a pro-
cedure already described, including the precautions to minimize
vapor headspace.38 The uncertainty of the mole fraction is
estimated as (0.0001. The viscosity of the mixtures was mea-
sured using a rolling-ball viscometer from Anton Paar, model
AMVn equipped with capillary tubes of 3.0 and 1.8 mm in
diameter, at 298.15 ( 0.05 K (the uncertainty is the total
combined accuracy and precision; the temperature is controlled
to within (0.005 K). Before starting the measurements, the
3.0 mm diameter capillary tube had been calibrated as a function
of temperature and angle of measurement using a standard
viscosity oil from Cannon (N35). The capillary with 1.8 mm
diameter had been calibrated with water by the manufacturer.
The overall uncertainty in viscosity is estimated to be (2%.
The density of the mixtures, necessary to determine the
dynamic viscosities, was measured with an Anton Paar vibrating
tube densimeter model 512 P at 298.15 ( 0.02 K. The
densimeter was calibrated on n-heptane, bromobenzene, and
2,4-dichlorotoluene. The overall uncertainty of the density is
estimated as (0.03%.
Conductivity. Electrolytic conductivities were measured by
the AC impedance bridge technique at 298.15 K using a
conductivity cell, as described previously.39 The cell, made of
borosilicate glass with two platinum electrodes, was used in a
sealed vessel to avoid atmospheric moisture while measuring.
The cell was immersed in a thermostatic bath and the tempera-
ture was measured using a four-wire platinum resistance thermo-
meter leading to an estimated uncertainty of (0.1 K. The cell had
been previously calibrated at 298.15 K using a commercial
conductivity standard consisting of an aqueous potassium chlor-
ide solution (HI 7031, Hanna Instruments, with conductivity
1413 μS/cm) from which the constant of the cell was determined
to be Kcell = 104.98 ( 0.01 mÀ1. The conductivity cell was
carefully cleaned using deionized water and ethanol and dried
using a flow of dry nitrogen before the measurements. The
mixtures of CYD and ionic liquid at different compositions were
prepared gravimetrically. The conductivity cell was then filled
with approximately 2.5 mL of the mixture, sealed, and immersed
in the thermostatic bath. The electrical resistance, R, was
measured at several frequencies ranging from f = 0.8 to 5 kHz
using a Wayne Kerr Precision Component Analyzer model
6430B. Afterward the resistance at infinite frequency R∞ was
determined by plotting R against fÀ1/2 and extrapolating to 0,
leading to the electrical conductivity, k = Kcell/R∞. The un-
certainty in the measurements of k is estimated to be (0.05%.
The molar conductivity Λ = k/c was then calculated using the
molar concentration of ionic liquid.
2. EXPERIMENTAL SECTION
Materials and Methods. 1-Methylimidazole (>99%, Aldrich)
and 1,2-dimethylimidazole (>98%, Aldrich) were distilled prior
to use. Anhydrous 1,3-cyclohexadiene (99.8%, Aldrich) and 1,3-
cyclohexadiene (96%, Acros Organics, stabilized with 50 ppm
of 2,6-di-tert-butyl-4-methylphenol, BHT), distilled over NaK
alloy and stored on zeolites, were used for the reactions and for
the physicalÀchemical measurements, respectively. Bis(trifluoro-
methylsulfonyl)imide lithium salt (>99%, Solvionic) and [Rh-
(COD)Cl]2 (>99%, Strem) were used without further purification.
NMR Spectroscopy. 1H and 13C solution NMR spectra were
recorded on a Bruker DRX 500 instrument at 298 K (nominal)
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with a resonance frequency at 500.130 MHz. The solvent used
was CD2Cl2, distilled and kept in a rotaflo with molecular sieves.
Chemical shifts are reported in ppm (singlet = s, doublet = d,
doublet of doublet = dd, and multiplet = m) and were measured
relative to residual proton of the solvent to CHDCl2 for 1H, and
to CD2Cl2 for 13C.
Samples with different compositions were prepared by stirring
the mixtures of CYD and the corresponding IL for a few hours,
as already described.38 The sample was then introduced into a
5 mm NMR tube, in which a capillary tube with CD2Cl2 was
loaded to avoid any contact between the deuterated solvent and
the mixture analyzed. The deuterium in CD2Cl2 was used for the
external lock of the NMR magnetic field. The composition was
further verified by 1H NMR spectroscopy.
For the pulsed-field-gradient spin echo NMR DOSY (DOSY,
Diffusion Order SpectroscopY) experiments, a Bruker sequence
ledbpgp2s was implemented for shorter pulse gradients to
improve line shape and trapezoidal gradients (the pulse se-
quences are shown in Figure S1 in the Supporting Information).
The diffusion evolution time was 100 ms, the constant amplitude
part of the gradient was 3 ms, and the cosine raising and falling
parts of gradient were 150 μs. The diffusion space was sampled
by 32 linearly spaced gradients.
Figure 1. Density of the mixtures CYD + [C1C4Im][NTf2] (() and
CYD + [C1C1C4Im][NTf2] (b) as a function of composition. The
lines represent functions: F(g/mL) = À0.2217xCYD + 1.4206 (b) and
F(g/mL) = À0.2359xCYD + 1.4397 ((); xCYD is the mole fraction of
CYD in the mixture.
Molecular Simulation. The microscopic structures of the
CYD + IL mixtures studied experimentally were investigated by
molecular dynamic simulations using an atomistic force field that
describes interactions and conformations.40À42 CYD was repre-
sented by the Optimized Potential for Liquid Simulations
force field in its all-atom explicit version (OPLS-AA).43 ILs were
represented by a specifically parametrized force field, compatible
with OPLS-AA, in which particular attention was paid to the
description of electrostatic charge distributions and torsion
energy profiles. Details were given in previous work.38
Molecular dynamics simulations of condensed-phase CYD +
[C1C4Im][NTf2] and CYD + [C1C1C4Im][NTf2] mixtures were
performed using the DL_POLY program.44 Numbers of cations,
anions and CYD molecules varied according to composition
(e.g., 182 ion pairs and 18 molecules of CYD for xCYD = 0.09, 192
ion pairs and 64 molecules of CYD for xCYD = 0.25, 128 ion pairs
and 64 molecules of CYD for xCYD = 0.33, 111 ion pairs and 89
molecules of CYD for xCYD = 0.45). Initial low-density config-
urations, with ions and molecules placed at random in periodic
cubic boxes, were equilibrated to attain liquid-like densities and
structures at 400 K and 1 bar. The complete conditions under
which molecular simulations were performed were described in a
previous publication.38
Figure 2. Logarithm of the viscosity of CYD + [C1C4Im][NTf2] (()
and CYD + [C1C1C4Im][NTf2] (b) mixtures as a function of composi-
tion. The lines represent functions log(η/mPas) = À1.2481xCYD
+
2.030 (b) and log(η/mPas) = À0.8281xCYD + 1.705 (().
detector temperatures were both set to 230 °C. Samples were
injected in volumes of 1 μL. The temperature of the column was
fixed at 70 °C for 13.5 min, followed by a temperature ramp of
40 °C minÀ1 up to 250 °C.
The experiments to study the effect of stirring and of hydrogen
pressure on the reaction were carried out in a multireactor device
Thermo Fisher Scientific ATS20000, Integrity 10, allowing simulta-
neous reactions with well-controlled and different stirring speeds.
Synthesis of the Catalyst. The whole procedure for the
synthesis of [Rh(COD)(PPh3)2]NTf2, including characteriza-
1
tion by H, 31P, and 103Rh NMR and mass spectrometry, was
described previously.38
Hydrogenation of 1,3-Cyclohexadiene. The hydrogenation
of CYD was carried out at 1.2 atm of H2 and 303 K. CYD was
dissolved under argon in a solution of [Rh(COD)(PPh3)2]-
[NTf2] in one of the ionic liquids, [C1C4Im][NTf2] or [C1C1-
C4Im][NTf2], resulting in red homogeneous solutions. The
reaction mixture was kept under hydrogen atmosphere (1.2
bar, constant pressure) until 4 mL of acetonitrile were added.
The distribution of products in the reaction mixture and the
conversion were determined by GC analyses using a HP6890
chromatograph equipped with a FID detector and a cross-linked
methyl siloxane column (L = 30 m, ϕint = 0.32 mm, film thickness
=0.25 μm) and toluene as an internal standard. The injector and
3. RESULTS AND DISCUSSION
Density and Viscosity. The density and viscosity of the
mixtures of CYD+[C1C4Im][NTf2] and CYD+[C1C1C4Im]-
[NTf2] as a function of mole fraction were measured at 298.15 K
and atmospheric pressure. The results are presented in Table S1
(Supporting Information) and Figures 1 and 2. As already observed,
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Scheme 2. Labeling of the Atoms in the Imidazolium Cation
and in 1,3-Cyclohexadiene
Figure 4. Walden plot, logarithm of molar conductivity against loga-
rithm of reciprocal viscosity, for the mixtures CYD + [C1C4Im][NTf2]
(() and CYD + [C1C1C4Im][NTf2] (b) as a function of composition.
The solid line indicates the ideal line for a completely dissociated strong
electrolyte aqueous KCl solution (open symbols for pure ILs, full
symbols for mixtures).
increase in electrolytic conductivity, indicating increasing charge
mobility.
Figure 3. Electrolytic conductivity, k of the mixtures CYD + [C1C4Im]-
[NTf2] (() andCYD + [C1C1C4Im][NTf2] (b) as a function of
composition. The lines represent functions: k (S/m) = 0.3653xCYD
0.2053 (b) and k (S/m) = 0.388xCYD + 0.3982 (().
+
The observed increase in conductivity with increasing CYD
content in the mixtures with ionic liquids can be attributed either
to the decreased viscosity and hence increased mobility of
charges, or to changes in the structure of the system leading to
increasing numbers of charged and mobile species. This informa-
tion can be accessed through the relationship between conduc-
tivity and viscosity in a Walden plot, log(Λimp) vs log(1/η),
where Λimp is the molar conductivity measured by an impedance
method.47À49 Plotting the molar conductivity (Λimp, Figure S2
in the Supporting Information) instead of the absolute conduc-
tivity (k) normalizes to an extent the effects of molar concentra-
tion on the conductivity and thus gives a better indication of the
number of mobile charge carriers.46 The diagonal 1:1 line
represents the ideal Walden line, obeyed by 0.01 M aq. KCl,
which yields fully dissociated ions with essentially equal mobility.
Most ionic liquids fall slightly below the 1:1 line suggesting that
ionization (or proton transfer in protic ILs) is not complete in
those cases. According to Angell et al.48 the vertical distance ΔW
of the data points to the reference KCl line is a measure of the
deviation of ionic liquid media from electrolytic ideality, i.e., of its
(loss of) ionicity. The magnitude of the deviation below the ideal
line depends on the structure of ions, since their interactions can
strongly influence iconicity, a strong cationÀanion association
causing poor IL ionicity.50 Recently, MacFarlane et al. argued that
to compare ionicity between different ionic liquids, one should take
into account the radii of the conductive species (ions). For this,
an adjusted form of the Walden plot was proposed.51
the additional methyl group on the imidazolium carbon C2 in
[C1C1C4Im][NTf2] (Scheme 2 for atom labeling) increases the
molar volume2 and hence lowers molar density of the liquid in
comparison with [C1C4Im][NTf2]. The same effect remains in
the mixtures with CYD. On the other hand, the viscosity of pure
[C1C1C4Im][NTf2] and also of its mixtures with CYD is roughly
twice that of [C1C4Im][NTf2] and of [C1C4Im][NTf2] + CYD
(Figure 2), as already reported.38 However, the decrease in
viscosity with concentration for the two systems is slightly
different. These results are consistent with the observation that
reduction in hydrogen bonding leads to an increase in viscosity
for the C1C1C4Im+ based ionic liquids.36
Conductivity. The electrolytic conductivity of a medium is a
measure of the number and mobility of available charge carriers
and is of critical importance for electrochemical applications.2,45
Although ionic liquids are composed entirely of ions, they are in
general significantly less conductive than concentrated aqueous
electrolytes.46 The lower-than-expected conductivity of ionic
liquids can be attributed to the reduction of available charge
carriers due to ion pairing or other forms of aggregation.
Table S2 (Supporting Information) and Figure 3 show the
electrolytic conductivity, k, for CYD + [C1C4Im][NTf2] and
CYD + [C1C1C4Im][NTf2] as a function of composition. Pure
[C1C4Im][NTf2] and its mixtures with CYD have higher con-
ductivity than pure [C1C1C4Im][NTf2] and the respective mixtures
with CYD. Increasing the cation size leads to lower conductivity,
most likely due to the lower mobility of the larger cations, as
already observed.46 For both mixtures, addition of CYD leads to
Figure 4 shows the Walden plot for the mixtures CYD +
[C1C4Im][NTf2] and CYD + [C1C1C4Im][NTf2] at different
compositions, with pure ionic liquids represented by open symbols.
From the difference of the deviation of the pure ILs from the
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Figure 5. Diffusion coefficients of the cations (+), the anions (À) and
CYD (() in CYD + [C1C4Im][NTf2]; diffusion coefficients of the
cations (Â) and CYD32 (b) in CYD + [C1C1C4Im][NTf2], as a
function of CYD mole fraction. Measured by PFG NMR at 298 K.
Figure 6. Diffusion coefficients of the 1,3-cyclohexadiene in CYD +
[C1C4Im][NTf2] (() and CYD + [C1C1C4Im][NTf2] (b) mixtures as
a function reciprocal viscosity of the medium.
According to the StokesÀEinstein equation56 the diffusion
coefficient of the sphere-like moving species is inversely propor-
tional to the viscosity η of the medium: D = kT/(fπηa),
where k is the Boltzmann constant, f is a constant parameter
(equals 6), and a is the radius of the diffusing species. The diffusion
coefficient of 1,3-cyclohexadiene in CYD + [C1C4Im][NTf2] and
CYD + [C1C1C4Im][NTf2] mixtures does not follow a linear
relationship with reciprocal viscosity (Figure 6), as has been
reported by other groups for the diffusion of solutes in imidazo-
lium ionic liquids.35 In some studies, a fractional StokesÀ
Einstein equation, D µ (T/η)β with β ≈ 0.8À0.9 has been used
to describe the relationship between the diffusion and the
viscosity.57À59
1:1 line it can be seen that [C1C4Im][NTf2] is less ionic than
[C1C1C4Im][NTf2], indicating that ion pairs in the former case
are more associated due to the presence of hydrogen bonds.36,52
For both ionic liquids it was found that addition of CYD causes a
decrease in ionicity, meaning an alteration in the structure through
increased ionic association. In spite of this stronger ionic association,
conductivity does increase upon dilution of the IL in CYD, meaning
that the effect of diminishing viscosity is dominant, as seen from
the Λimp vs xCYD plot (Figure S2 in the Supporting Information).
DOSY NMR. The behavior of ionic liquids as electrolytes is
strongly influenced by the transport properties of their ionic con-
stituents.45 These transport properties are related to the mobility
of ions and to the degree of ion association (individual ions, ion-
pairs, or larger aggregates). The quantity most often used to evaluate
the transport properties of electrolytes is the ion diffusion coeffi-
cient. In this work the diffusion coefficients were determined by
pulse-field-gradient spinÀecho (PFGSE) DOSY NMR. Table S3
(Supporting Information) and Figure 5 represent diffusion co-
efficients of the cation, the anion and the 1,3-cyclohexadiene
in CYD + [C1C4Im][NTf2] and CYD + [C1C1C4Im][NTf2]
mixtures at different compositions, with the exception of the
In continuation of this work, we established the relationship
between the diffusion coefficients and the conductivity. From
PGSE-NMR self-diffusion coefficients of cations and anions, D+
and D , in [C1C4Im][NTf2] and CYD + [C1C4Im][NTf2]
À
mixtures, conductivity was calculated according to the NernstÀ
Einstein equation, ΛNMR = NAe(D+ + D_)/kT, where NA is
Avogadro’s number and e is the electrostatic charge on each
ionic carrier. This equation assumes that every diffusing
species detected by the NMR measurements contribute to
the molar conductivity. In contrast, Λimp, as described pre-
viously, relies on migration of charged species in an electric
field.50,60 The ratio Λimp/ΛNMR indicates the proportion of
charged species that contribute to the ionic conduction. Thus,
Λimp/ΛNMR is correlated with the ionic nature of ILs and is
used as a physicochemical parameter for representing ionicity
or dissociation of ILs.
À
diffusion coefficients for NTf2 in [C1C1C4Im][NTf2], which
were Ànot measured. Diffusion coefficients of C1C4Im+ and
NTf2 in the pure [C1C4Im][NTf2] are 3.03 Â 10À11 and
2.22 Â 10À11 m2 sÀ1, respectively (obtained from extrapolation
of linearly fitted data to xCYD = 0), which is in a good agreement
with the data reported in the literature:53 3.4 Â 10À11 and 2.6 Â
10À11 m2 sÀ1, respectively. On the other hand, the diffusion of the
cation in pure [C1C1C4Im][NTf2], 1.38 Â 10À11 m2 sÀ1, is
slower than that of the cation in pure [C1C4Im][NTf2], which can
be ascribed to higher viscosity of the former IL. In general, the
C1C4Im+ cation has higher diffusion coefficient than the anion, even
though the cationic radius is larger.54 This is mainly attributed to the
planar structure of C1C4Im+ that can improve diffusion in certain
directions, making C1CnIm+ (with short alkyl chains, n e 4)
good charge carriers in view of their molecular size.55 The
diffusion of the molecules of 1,3-cyclohexadiene is more than
three times faster than the diffusion of the anions in the mixtures
and these values increase with increasing concentration of CYD.
Figure 7 depicts the dependence of the ratio Λimp/ΛNMR for
the CYD+[C1C4Im][NTf2] mixtures including the pure IL. In
agreement with other IL systems reported in the literature, it was
found that Λimp/ΛNMR is less than unity, showing the presence
of association in ion pairs or larger aggregates, even in the pure
IL. The ratio Λimp/ΛNMR decreases with increasing amount of
CYD showing that, although a higher content in organic molecule
leads to an increase in diffusion coefficients, it also causes a decrease
in the ionicity of the medium, i.e., dilution of the IL in CYD promotes
stronger ionic association. This confirms what was already
established from the Walden plot (Figure 4). Similar behavior
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Figure 9. Average deviation of proton chemical shifts in CYD + IL
mixtures with respect to pure IL, as a function of composition: CYD +
[C1C4Im][NTf2] (() and CYD + [C1C1C4Im][NTf2] (b). Note that
the first point, corresponding to R = 0.1, lies beneath the linear
regression in both ILs. So, if the lines exclude the first point of each
series, the functions are described as: Δ(ppm) = 0.1694R + 0.0346 (()
and Δ(ppm) = 0.1773R + 0.0179 (b).
Figure 7. Λimp/ΛNMR values for the CYD + [C1C4Im][NTf2] (()
mixtures as a function of composition.
and S5 in the Supporting Information). The change in the mean
chemical shift (relatively to pure ionic liquids), Δδ, is plotted
with respect to the molar ratio CYD/IL (R) in Figure 9. Atom
labeling is depicted in Scheme 2.
All of the chemical shifts of the IL and CYD protons are shifted
downfield (Tables S4ÀS6 in the Supporting Information) contrarily
to what we have reported for mixtures of toluene + [C1C4Im]-
[NTf2] and toluene + [C1C1C4Im][NTf2],32 where all of the
chemical shifts of the toluene protons are shifted downfield,
whereas the IL protons are shifted upfield when the concentra-
tion of organic compound increases. These observations as well
as the MD results clearly indicated that the imidazolium cation is
found with high probability close to toluene.32 Herein, the
evolution of chemical shifts of the IL and CYD protons as well
as MD results38 reveal that the preferential solvation sites of the
substrate in the ILs are less distinct for CYD than for toluene.
Concerning the proton chemical shifts of ILs (and excluding
the first point of each series) Δδvs mixture composition given by
the molar ratio R follows a straight line, as shown in Figure 9. The
similar linear variation of the deshielding effect for both ILs was
not attributed to a significant structural reorganization, but to the
slight changes in the dynamic properties of the system, with
increasing CYD concentration, as observed by conductivity
measurements.
Catalytic Hydrogenation. We have previously studied the
hydrogenation of CYD in [C1C4Im][NTf2] and [C1C1C4Im]-
[NTf2], at R = nCYD/nIL = 0.5 using [Rh(COD)(PPh3)2][NTf2]
(COD = 1,5-cyclooctadiene) as catalyst.38 Hydrogenation of
CYD in both ILs led selectively to cyclohexene (CYE). The fact
that the reaction was twice as fast in [C1C4Im][NTf2] than in
[C1C1C4Im][NTf2] could not only be explained in terms of the
thermodynamic factors such as CYD solubility or mixing enthalpies
of CYD and IL but was related to the ratio of measured viscosities
of both ionic liquids studied: in factη([C1C1C4Im][NTf2])/
η[C1C4Im][NTf2]) = 1.9.38
Figure 8. Comparison of the cationÀanion interaction energy in the
mixtures CYD
+ [C1C4Im][NTf2] (dark gray) and CYD +
[C1C1C4Im][NTf2] (light gray) at different compositions obtained by
molecular simulations. xCYD is molar fraction of 1,3-cyclohexadiene.
was also observed in mixtures of N,N-diethyl-N-methoxyethynyl-
N-methylammonium BF4À and NTf2À with organic solvent such
as propylene carbonate or dichloroethane.50,60
Molecular Simulations. Conductivity, viscosity, and diffusion
coefficients indicate that addition of CYD to the ionic liquid
affects its structure through stronger ionic association. This
observation was confirmed also by molecular dynamics simula-
tion, where the overall system configuration energy of the
mixtures CYD + IL was decomposed to the energy between
each pair of species as represented in Figure 8. The overall
cationÀanion interactions are stronger for the pair [C1C4Im] +
[NTf2] than for [C1C1C4Im] + [NTf2], which is consistent with
the lower ionicity of the former ionic liquid system proved by
conductivity and viscosity measurements. It was also observed
that, when increasing the amount of CYD present in the mixture,
the interaction becomes more negative, confirming again that the
addition of molecular solute increases the ion pair association.
NMR. The structural effect of adding CYD to [C1C4Im][NTf2]
and [C1C1C4Im][NTf2] was examined through the change in
the chemical shifts of all hydrogen atoms of the ILs (Tables S4
At this point the following question arose: is the relationship
between viscosity and reaction rate general for hydrogenation of
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Figure 10. CYE production (mol/L) vs. time (s) in [C1C4Im][NTf2]
for R = 0.2 ((), 0.3 (À), 0.4 (9), and 0.5 (b).
Figure 12. Initial rate (mol LÀ1
hydrogenation of CYD in [C1C4Im][NTf2] (() and [C1C1C4Im]-
[NTf2] (b). A linear fit of [C1C4Im][NTf2] data (() gives a straight
line with r2 > 0.99.
s
À1) vs inverse of the viscosity for
substrate. Interestingly, when representing initial rates vs inverse
of the viscosity, a linear dependence is observed (Figure 12).
The results obtained with [C1C1C4Im][NTf2] (Figures 11
and 12) are even more interesting. The initial rate observed for
[C1C1C4Im][NTf2] is far from the values obtained for [C1C4Im]-
[NTf2] at the same concentration in CYD (Figure 11), but it is
very close to the expected value when viscosity is taken in account
(Figure 12). In fact, the initial rate with [C1C1C4Im][NTf2]
represented in Figure 12 is from the mixture at R = 0.5 but lies
close to the initial rate value obtained with [C1C4Im][NTf2] at
R = 0.2. Both samples have very different compositions, but quite
similar viscosities and H2 concentrations, meaning that in these
systems viscosity is crucial for the catalytic performance. This
suggests that the reaction kinetics is controlled by diffusion. In
order to see if mass transfer is playing the main role in catalytic
hydrogenation of CYD in ILs, hydrogenation experiments with
different stirring rates, catalyst loading, and hydrogen pressure
were performed.
Figure 11. Initial rate (mol LÀ1 À1) vs ratio R = nCYD/nIL for
s
[C1C4Im][NTf2] (() and for [C1C1C4Im][NTf2] (b).
CYD in IL media at all compositions of the reaction mixture?
Hence, in this work we report a more in-depth study of the effect
of viscosity on the hydrogenation reaction by varying different
parameters, such as composition, stirring rate, hydrogen pressure
and catalyst loading.
Effect of Stirring. Two compositions, R = 0.2 and 0.5, were
chosen for this experiment (Figure 13a and Table S8 in the
Supporting Information). From the results it is clearly seen that
increasing the stirring speed leads to a faster reaction regardless
of the value R. This shows that mass transfer should be taken into
account in the hydrogenation reactions studied here, and that it is
important to explain the previous results obtained for R = 0.2, 0.3,
0.4, and 0.5. Note that it has been previously reported that the
stirring speed induces also a very strong effect H2ÀILs mass
transfer coefficients.61,62
Effect of H2 Pressure. Hydrogen is not very soluble in ILs, some
authors suggest that the mass transfer of H2 may limit the rate of
olefin hydrogenation processes,63,64 whereas others proposed
that the high diffusivity of hydrogen results in high hydrogen
transfer rates to the catalyst so that, during a hydrogenation
reaction, the consumed hydrogen can be replenished rapidly.65
In order to determine if this applies to the present system, the
reaction was performed under 1.2 and 4 bar of H2 pressure, at
compositions R= 0.2 and 0.5 and nCYD/ncatalyst = 1000 (Figure 13b).
The production of CYE is dramatically increased under higher
pressures of hydrogen, indicating that the pressure under which the
reaction is carried out limits the amount of hydrogen in the
Influence of Composition. All the catalytic reactions were
performed at 30 °C, under 1.2 bar of hydrogen, and with a molar
ratio nCYD/ncatalyst = 1000. Molar ratios nCYD/nIL, R = 0.1, 0.2,
0.3, 0.4, and 0.5 were studied. For all of the systems only CYE
was obtained, with the CYE production (mol/L) vs time (s) in
[C1C4Im][NTf2] depicted in Figure 10. Reactions with R > 0.5
were not performed in order to avoid the formation of a
biphasic system due to the low solubility of CYE in IL. In
[C1C4Im][NTf2] hydrogenation of CYD at R = 0.1 afforded
no measurable amount of CYE detected by gas chromatography.
With [C1C1C4Im][NTf2], little to no detectable amount of the
hydrogenation product was observed after 1 h of reaction, for all
mixtures with R e 0.4. Data for these experiments are therefore
not collected in the Supporting Information in Table S7.
As shown in the Figure 11, the initial rate of hydrogenation in
[C1C4Im][NTf2] increases exponentially with the nCYD/nIL ratio
R, and the rate is not directly proportional to the concentration of
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Figure 13. Conversion of CYD into CYE (%, vertical axe) under different stirring rates for compositions: R = 0.2 (light gray) and 0.5 (dark gray), left
CYE production (mol/L) after 30 min of reaction at compositions R = 0.2 (light gray) and R = 0.5 (dark gray), under 1.2 and 4 bar of hydrogen, right.
The stirring rate was 700 rpm.
of diffusion with catalyst concentration, the bulk concentration of
hydrogen in solution continuously decreases faster than can be
compensated by diffusion. The hydrogenation is slower than
would be expected for the kinetically controlled range. These
results again support the importance of mass transport.71
In summary, in the experiments with different values of R
described above, it was shown that the initial rate of the catalytic
hydrogenation reaction exhibits a linear dependence with the
inverse of viscosity. This relationship between the reactivity and
the viscosity should be understood in terms of mass transfer
issues. In more viscous systems the mobility of molecules is lower
and a lower reactivity is observed. The mass transfer issues have
been highlighted by investigations of the effect of stirring, catalyst
loading and hydrogen pressure.
Mass transport effects in reactions in ILs has been often
proposed for catalytic hydrogenation4,65,72 and for organic
reactions.73 However, to our knowledge, no work before this
one has been published concerning the study of all catalytic
reaction compositions, carefully calculated in order to keep all
parameters constant (including the concentration of hydrogen)
except the concentration ratio CYD/IL, allowing us to evidence
the prevailing effects of the solvation and mass transfer (viscosity
and diffusivity) on the chemical reaction.
Figure 14. Initial rate (mol LÀ1 sÀ1) vs catalyst concentration (mmol LÀ1
for the hydrogenation of CYD in [C1C4Im][NTf2] at R = 0.2 (2) and
0.5 ([).
)
medium. Therefore, H2 concentration in the medium has to be
taken into account as an important rate-determining factor.
Effect of Catalyst Loading. The effect of changing the con-
centration of catalyst was also studied, at compositions R = 0.2
and 0.5, with nCYD/ncatalyst ratios of 100, 500, and 1000. The plot
of the measured rates as a function of the initial catalyst con-
centration is shown in Figure 14. Generally it is accepted that if
the reaction is kinetically controlled, i.e., if mass transfer has no
effect, then the initial rate should change linearly with catalyst
concentration. In particular in the catalytic hydrogenation of
dienes as COD (cycloocta-1,5-diene) or NBD (norborna-2,
5-diene) with cationic rhodium(I) chelates the pseudorate con-
stant is a linear function of the initial catalyst concentration
providing information about the limitations of the regime, which
is exclusively controlled by kinetics.66À70 In other words, the
hydrogen concentration in solution (gas solubility) for the
mentioned range of rates is constant, and it is measured in the
kinetically controlled range.
’ CONCLUSIONS
In this work we studied the influence of the nature of two ionic
liquids, namely 1-butyl-3-methylimidazoliumbis(trifluoromethyl-
sulfonyl)imide, [C1C4Im][NTf2], and 1-butyl-2,3-dimethylimi-
dazolium bis(trifluoromethylsulfonyl)imide, [C1C1C4Im][NTf2],
on the kinetics of the catalytic hydrogenation of 1,3-cyclohex-
adiene with [Rh(COD)(PPh3)2]NTf2 (COD = 1,5-cyclooctadiene).
For this purpose, the effect of the concentration of 1,3-cyclohex-
adiene on the molecular interactions, the structure, and the
physicochemical properties in mixtures with the two ionic liquids
was investigated. Measurements of the viscosity of 1,3-cyclohex-
adiene + ionic liquid mixtures at different compositions showed
that mixtures with [C1C1C4Im][NTf2] are more viscous than
with [C1C4Im][NTf2], reflecting the order of viscosities of the
pure ionic liquids. Furthermore, determination of mass and
charge transport properties of the mixtures 1,3-cyclohexadiene+
ionic liquid, by DOSY NMR and conductivity measurements, allowed
access to the ionicity of the mixtures. It was established that pure
This is not the case in the present system as seen in Figure 14,
where the reaction rate under the given experimental conditions
is determined by mass transport. Because of the rising influence
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ARTICLE
[C1C1C4Im][NTf2] is more ionic than pure [C1C4Im][NTf2],
in agreement with the presence of hydrogen bonds in the latter.
From the dependence of the ratio of conductivities obtained by
impedance method and NMR (Λimp/ΛNMR) on the concentra-
tion of 1,3-cyclohexadiene, it was found that increasing the
amount of 1,3-cyclohexadiene leads to a decrease in the ionicity
of the media through stronger ionic association. This was
confirmed by calculations of the cationÀanion pair association
energy by molecular dynamics simulations. In spite of this
stronger association, conductivity increases when the concentra-
tion of 1,3-cyclohexadiene increases, meaning that the effect of
the decrease in viscosity, and hence the increase in mobility of
charged species, is dominant.
The catalytic hydrogenation of 1,3-cyclohexadiene in
[C1C4Im][NTf2] and [C1C1C4Im][NTf2] yields cyclohexene
selectively. The reaction is strongly affected by the viscosity of the
medium, and a linear relationship was found between the initial
rate of the reaction and the inverse of viscosity. In less viscous
systems there is a higher mobility of reactants, so higher
conversions and reaction rates are observed. The mass transfer
issues were also highlighted by experiments with different
catalyst loadings and higher hydrogen pressure.
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Supporting Information. Density, viscosity, and conduc-
b
tivity data for the mixtures CYD + [C1C4Im][NTf2] and CYD +
[C1C1C4Im][NTf2]; DOSY NMR pulse sequence; diffusion
coefficients of cations, anions, and CYD; 1H NMR shifts; initial
rates and conversions for the hydrogenation of CYD in
[C1C4Im][NTf2] and [C1C1C4Im][NTf2]. This material is
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