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Determination of hydrogen concentration in ionic liquids and the
effect (or lack of) on rates of hydrogenation
Paul J. Dyson,a Gábor Laurenczy,a C. André Ohlin, a James Vallance,a,b and Thomas Weltonb
a
Institut de Chimie Moléculaire et Biologique, Ecole Polytechnique Fédérale de Lausanne, EPFLBCH, CH-1015 Lausanne, Switzerland. E-mail paul.dyson@epfl.ch
b
Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington,
London, SW7 2AY, UK.
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The solubility of hydrogen and the corresponding Henry
coefficients for 12 ionic liquids have been determined in situ
at 100 atm H2 pressure and is much lower than expected;
attempts to correlate the solubility of hydrogen in the ionic
liquids with the rate of reaction for the hydrogenation of
benzene to cyclohexane in these solvents have been made.
pressure to 10.1 MPa (100 atm) that the peak corresponding to
hydrogen could be clearly resolved and integrated relative to the
solvent peaks. The proton NMR spectra were fitted with
WINNMR and NMRICMA2.8/MATLAB programmes (nonlinear least squares fit, minimising the difference between the
measured and calculated spectra to determine the spectral
parameters and integrals). Figure 1. illustrates a typical spectrum
fit showing the dissolved dihydrogen peak (δ= 4.63 ppm,
external TMS reference) next to the two methylene protons of
the ionic liquid [omim][BF4] ([omim]+ = 1-octyl-3methylimidazolium). The measured peak (in blue) is
superimposed on the calculated peak (in red).
The concentration of hydrogen that dissolves in these ionic
liquids is very low, much lower than for molecular organic
solvents, and is in the same range as for water.7 For comparison
purposes, the solubility and Henry’s constants for a range of
solvents including ionic liquids at 1 atm are listed in Table 1.
The solubility and Henry’s constants for the ionic liquids were
not determined at atmospheric pressure, therefore the estimates
were made by extrapolation, since we established a linear
relationship at higher pressures.
Not unreasonably, it has been proposed that the low
concentration of gases in ionic liquid could lead to mass transfer
problems in catalysed reactions,4 therefore we decided to
conduct the same hydrogenation reaction under identical
conditions in the 12 different ionic liquids to determine whether
there is an effect on the reaction rate. The system chosen for this
study was the biphasic hydrogenation of benzene to cyclohexane
that has previously been demonstrated in ionic liquids.3e The
main reason for selecting this reaction is because the benzene
substrate is highly soluble in the ionic liquid under the reaction
conditions employed (although only determined quantitatively at
ambient temperature for the tetrafluoroborate ionic liquids)9 and
therefore the mass transport of benzene into the ionic liquidcatalyst medium will not be a limiting factor. Furthermore, the
hydrogenation of arenes is an important reaction in synthetic
chemistry 10 and for large scale industrial processes.11 The
results from the hydrogenation studies, and the solubility of
hydrogen in ionic liquid/benzene mixtures, are listed in Table 2.
Based on the solubility one would predict that the lowest
turnover would be observed in [omim][BF4] and the highest in
[P(C6H13)3(C14H29)][PF3(C2F5)3].
However,
to
within
experimental error, there is essentially no difference between the
rate of catalysed reaction in any of the ionic liquids. This
conclusion would indicate that the rate of mass transfer of
hydrogen into the ionic liquids is very high, possibly due to
intensive mixing. The similarities in the turnover frequencies
could also be due, in part, to the higher solubility of hydrogen in
the substrate compared to the ionic liquids. However, in the ideal
biphasic process the ratio of substrate to catalyst immobilisation
solvent should be high.
Ionic liquids are currently under intensive investigation as
alternative solvents for biphasic catalysis and a number of
reviews on this subject are available.1 Catalytic applications
involving gaseous substrates, in particular hydroformylation 2 and
hydrogenation,3 have been particularly well developed. The main
advantages of the ionic liquid to arise from these studies are that
catalyst lifetimes can be extended, often considerably, and rates
of reaction are often accelerated. Other benefits such as good
product separation and catalyst reuse have also been noted.
Clearly, the solubility of gases in ionic liquids is an important
factor in catalysed reactions where the gas is used as a substrate
and it has been suggested that increased reaction rates in
biphasic hydrogenation reactions in ionic liquids could be due to
high solubility of hydrogen in the ionic liquid. Recently, the
solubility of a number of gases in [bmim][PF6] ([bmim]+ = 1butyl-3-methylimidazolium) was determined using a gravimetric
microbalance, but H2 could not be detected using this method.4
Using an electronic flow mass controller the Henry coefficient
solubility constant of hydrogen in [bmim][BF4] and [bmim][PF6]
were reported to be 1.65x102 and 5.43x102 MPa, respectively
(calculated in same units – MPa – and with the equation kH = PH2
/ XH2, for comparison).5 We decided to determine the
concentration of H2 spectroscopically in ionic liquids most
commonly used in hydroformylation and hydrogenation
reactions in order to ascertain the effect on reactivity.
Fig. 1 Determination of H2 solubility. The measured and the calculated
1
H NMR spectra of dissolved dihydrogen and the methylene protons of
the [omim][BF4]. The relative concentrations are given in Table 1.
The solubility of H2 in ionic liquids was determined using
high pressure 1H NMR spectroscopy.6 At atmospheric pressure
the dissolved hydrogen concentration was lower than the
detection limit of the method and it was only on increasing the
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Table 1 Solubility of H2 in water, organic solvents and ionic liquids, at 0.101 MPa (1 atm).
Solvent
Watera
Methanola
Ethanola
Toluenea
Benzeneb
Cyclohexane
[bmim][BF4]b
[bmim][PF6]b,c
[bmim]Tf2N]c
[bm2im][Tf2N]c, e
[bupy][Tf2N] c, f
[bmpy][Tf2N]c, g
[bmim][SbF6]c
[bmim][CF3COO]c
[hmim][BF4]c, h
[omim][BF4]c
[bmim][CF3SO3]c
[P(C6H13)3(C14H29)][PF3(C2F5)3]c
Henry’s constant, kH /MPa*
6.8x103
6.6x102
5.9x102
2.69x102
4.47x102 (4.39x102)
3.97x102
5.8x102 (1.63x102)
6.6x102 (5.38x102)
4.5x102
3.8x102
3.9x102
3.7x102
4.9x102
4.9x102
5.7x102
6.4x102
4.6x102
0.7x102
103[H2]/M
0.81
3.75
2.98
3.50
2.54 (2.57)
3.77
0.86d (3.0)
0.73d (0.88)
0.77d
0.86d
0.89d
0.90d
0.93d
0.98d
0.79d
0.62d
0.97d
1.84d
Density (g/ml)
0.998213a
0.791413a
0.789313a
1.496113a
0.87813a
0.77713a
1.1213b
1.36313b
1.433
1.421
1.449
1.387
1.699
1.198
1.1413d
1.106
1.29013c
1.196
reference
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* kH = PH2 / XH2, where the partial pressure of hydrogen expressed in MPa, a293 K, broom temperature, c298 K, dcalculated from the solubility under 10.1
MPa, supposing that it changes linearly with the partial pressure. e[bm2im]+ = 1,2-methyl-3-butylimidazolium. f[bupy]+ = N-butyl-pyridinium. g[bmpy]+ =
N-Butyl-N-methylpyrrolidinium. h[hmim]+ = 1-hexyl-3-methylimidazolium.
Table 2 Hydrogenation of benzene to cyclohexane using [H4Ru4(6C6H6)4][BF4]2 12 as the catalyst and solubility of H2 in Ionic
Liquid/Benzene mixtures (3:1 v/v) at 0.101 MPa (1 atm). #
Solvent
Turnover
frequency
(molmol-1hr-1)
232
4
103[H2]/
M
5
6
[bmim][PF6]
1.3
[bmim]Tf2N]
1.4
[bm2im][Tf2N]
1.5
[bupy][Tf2N]
1.2
[bmpy][Tf2N]
1.6
[bmim][SbF6]
1.7
[bmim][CF3COO]
1.7
[bmim][BF4]
254
1.3
[hmim][BF4]
246
1.4
[omim][BF4]
263
1.3
[bmim][CF3SO3]
216
[P(C6H13)3(C14H29)][PF3(C2F5)3]
2.2
#
Reactions were carried out under identical conditions using a
Baskerville multicell high pressure autoclave. Reagents and
conditions: ionic liquid (1 ml), benzene (1 ml), catalyst (5 mg), H 2
(45 atm), 100 ºC, 2 h. Turnover frequencies are quoted in number of
moles of substrate converted per mole of catalyst per hour.
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8
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10
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12
13
We would like to thank to thank the Engineering and Physical
Sciences Research Council and the Swiss National Science
Foundation for financial support.
Notes and references
1
2
3
Ellis, T. Welton, Canadian J. Chem., 2001, 79, 705. S. Steines, P.
Wasserscheid, B. Drieβen-Hölscher, J. Prakt. Chem., 2000, 342,
348.
J. L. Anthony, E. J. Maginn, J. F. Brennecke, J. Phys. Chem. B,
2002, 106, 7315.
A. Berger, R. F. de Souza, M. R. Delgado, J. Dupont, Tetrahedron
Asymmetry, 2001, 12, 1825.
All measurements carried out in a sapphire NMR tube at a pressure
of 100 atm. after shaking for 8 hours (it was checked by double
measurements that the [H2] has reached its maximum value). The
measurements were carried out on a 360 MHz Bruker NMR. The
integral of the dihydrogen peak in relation to the peak corresponding
to two methylene protons of the ionic liquid was determined using
the program NMRICMA2.8 for MATLAB.
C. L. Young, Ed. IUPAC Solubility Data Series; Pergamon Press:
Oxford, U.K., 1981; Vol 5-6
a) W. F. Linke, A. Seidell, Solubilities of Inorganic and MetalOrganic Compounds, American Chemical Society, Washington,
D.C., USA, 1958, Vol. I. p. 1075. b) E. Wilhem and R. Battino,
Chem. Rev. 1973, 73, 1
P. J. Dyson, D. J. Ellis, W. Henderson,G. Laurenczy, Adv. Synth.
Catal., 2003, 345, 216.
T. J. Donohoe, R. Garg, C. A. Stevenson, Tetrahedron: Asymmetry,
1996, 7, 317.
A. Corma, A. Martínez, V. Martínez-Soria, J. Catal., 1997, 169,
480.
L. Plasseraud, G. Süss-Fink, J. Organomet. Chem., 1997, 539, 163.
a) R. C. Weast (ed.), CRC Handbook of Chemistry and Physics, 53rd
Ed., CRC, Ohio, U.S.A, 1972-1973. b) J. G. Huddleston, A. E.
Visser, W. M. Reichert, H. D. Willauer, G. A. Broker, R. D. Rogers,
Green Chemistry, 2001, 3, 156-164. c) P. Bonhôte, A. P. Dias, N.
Papageorgiou, K. Kalyanasumdaram, M. Grätzel, Inorg. Chem.,
1996, 35, 1168-1178. d) Merck KGaA.
P. Wasserschields, W. Keim, Angew. Chem. Int. Ed. Engl., 2000, 39,
3773. C. M. Gordon, App. Catal. A: General, 2001, 222, 101. D.
Zhao, M. Wu, Y. Kou, E. Min, Catal. Today, 2002, 74, 157. H.
Olivier-Bourbigou, L. Magna, J. Mol. Catal. A, 2002, 182-183, 419.
J. Dupont, R. F. de Souza, P. A. Z. Suarez, Chem. Rev., 2002, 102,
3667.
For example see: K. W. Kottsieper, O. Stelzer, P. Wasserscheid, J.
Mol. Catal. A: Chem. 2001, 175, 285. F. Favre, H. OlivierBourbigou, D. Commereuc, L. Saussine, Chem. Commun. 2001,
1360. C. C. Brasse, U. Englert, A. Salzer, H. Waffenschmidt, P.
Wasserscheid, Organometallics 2000, 19, 3818. H. Waffenschimidt,
P. Wasserscheid, J. Mol. Catal. A: Chem. 2000, 164, 61. W. Keim,
D. Vogt, H. Waffenschmidt, P. Wasserscheid, J. Catal. 1999, 186,
481. M. F. Sellin, P. B. Webb, D. J. Cole-Hamilton, Chem.
Commun. 2001, 781. O. Stenzel, H. G. Raubenheimer, C.
Esterhuysen, J. Chem. Soc., Dalton Trans. 2002, 1132.
For example see: Y. Chauvin, H. Olivier-Bourbigou, CHEMTECH,
1995, 25, 26. P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de
Souza, J. Dupont, Polyhedron, 1996, 15, 1217. P. A. Z. Suarez, J. E.
L. Dullius, S. Einloft, R. F. de Souza, J. Dupont, Inorg. Chim. Acta,
1997, 255, 207. L. A. Müller, J. Dupont, R. F. de Souza, Macromol.
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Table 3 Solubility of H2(g) at 0.101 MPa (1 atm) in
[P(C6H13)3(C14H29)][PF3(C2F5)3] as a function of
temperature.
103[H2]/M
1.85
2.10
2.38
2.57
2.89
Temperature
20
40
60
80
100
Hydrogen solubility as a function of temperature in
[P(C6H13)3(C14H29)][PF3(C2F5)3]
3
10 [H2]/M
3.00
2.50
2.00
1.50
10.00
30.00
50.00
70.00
90.00
110.00
T (°C)
[PHex3(C14H29)][PF3(C2F5)3]
20
Henry’s
Constant,
kH/MPa
70
40
62
60
55
80
51
100
45
T (°C)
48
 (mole fraction)
.
100
12.92
76
10.20
90
5.78
P (bar)
y = 0.3044x - 4.2862
R2 = 0.9971
Pdec - mol% vs P
mol% H2
15.00
10.00
5.00
0.00
40.0
50.0
60.0
70.0
80.0
90.0
100.0
P (bar)
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