H/D isotope exchange between methane and magic acid (HSO 3F-SbF5): An in situ NMR study

Article abstract of DOI:10.1039/b309362j

The kinetics of hydron exchange between methane and a series of DSO ₃F-SbF₅ superacids were measured by in situ ²H decoupled ¹H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation ΔG^# (30°C) decreasing from 97 to 84 kJ mol ^-1 over the range of concentration 19 to 49 mol % SbF₅. The constant free enthalpy of activation ΔH^# (ca. 65 kJ mol^-1) and the decreasing entropy of activation ΔS^# seem to indicate that an increase in acidity of the superacid system does not substantially change the nature of the transition state but rather acts on its solvation.

Full text of DOI:10.1039/b309362j

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P A P E R
H/D isotope exchange between methane and magic acid
(HSO3F–SbF5): an in situ NMR study
´
Stephane Walspurger, Alain Goeppert, Mohamed Haouas and Jean Sommer*
´
Laboratoire de Physico-chimie des Hydrocarbures (CNRS UMR 7513), Faculte de Chimie,
Universite Louis Pasteur, 4, rue Blaise Pascal, 67070, Strasbourg cedex, France.
´
E-mail: sommer@chimie.u-strasbg.fr; Fax: þ33 390241487; Tel: þ33 390241486
Received (in Montpellier, France) 5th August 2003, Accepted 9th October 2003
First published as an Advance Article on the web 6th January 2004
The kinetics of hydron exchange between methane and a series of DSO3F–SbF5 superacids were measured by
2
1
in situ H decoupled H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony
pentafluoride concentration, with the free energy of activation DG# (30 ꢀC) decreasing from 97 to 84 kJ molꢁ1
over the range of concentration 19 to 49 mol % SbF5 . The constant free enthalpy of activation DH# (ca. 65 kJ
molꢁ1) and the decreasing entropy of activation DS# seem to indicate that an increase in acidity of the superacid
system does not substantially change the nature of the transition state but rather acts on its solvation.
part in the exchange mechanism in superacidic media.
Introduction
Recently, we reinvestigated the behavior of methane in
superacid and strong liquid acid media. H/D exchange was
observed when methane was recirculated in D2SO4 at tempera-
tures of 250 ꢀC and above and an apparent activation energy of
176 kJ molꢁ1 was measured.18 In the strongest superacid, HF–
SbF5 , we experimentally determined that the secondary kinetic
isotopic effect for the H/D exchange, k(CH4)/k(CH3D), is very
close to unity;19 moreover, high level computational studies
using H2Fþ, more or less solvateþd by excess HF as the super-
acid species, suggested that CH5 was not an intermediate in
the exchange reaction but acted rather as a part of the
activated complex strongly solvated by (HF)x.19 The apparent
activation energy of 75 kJ molꢁ1 measured by Hogeveen and
Gaasbeek9 shows the large difference of stability of this acti-
vated complex between strong acidic media and superacid
media. In another paper, the same group reported that
exchange in HF–SbF5 was 100 times faster than in HSO3
F–SbF5 .20 Thus, we found it of interest to re-examine the
H/D exchange between methane and DSO3F–SbF5 in a more
quantitative way by in situ NMR measurements and to deter-
mine the relation between exchange rates and acidity of the
superacid medium.
Methane is the major component of natural gas and one of the
largest organic carbon reserves.1 Unfortunately, methane is
almost inert and for this reason mainly used as a clean and
inexpensive burning fuel. The large number of papers dealing
with the conversion of methane testify to the great economical
efforts to transform it into more valuable chemicals such as
methanol, formaldehyde, etc. Indirect conversion of methane
to heavier hydrocarbons via syn-gas, such as Fischer–Tropsch
synthesis,2 has been well-developed, especially during the oil
crisis. Recent reviews summarize progress in direct transforma-
tions such as oxidative coupling and conversion under nonoxi-
dative conditions in heterogeneous catalysis.3,4 Periana et al.
reported excellent yields in methane to methyl bisulfate cataly-
tic conversion in sulfuric acid media using a Pt catalyst5 and
more recently with iodine.6 Carbonylation of methane under
superacidic conditions has also been reported recently.7 These
promising studies, among others, make direct catalytic
transformation of methane into more reactive molecules an
exciting challenge for chemists. Although conversion of
methane is more thermodynamically favorable under oxidative
conditions, it is interesting to understand the mechanisms
involved in the presence of electrophilic species and
particularly strong acids and superacids.8
The activation of methane requires a C–H bond cleavage
that can be monitored by H/D exchange between CH4 and a
source of deuterium such as D2 gas, deuteride (Dꢁ) or
deuteron (Dþ) in the presence of neutral, basic or acidic
catalysts. For this reason, numerous studies have been carried
out using the isotope exchange technique to clarify mecha-
nisms. Hogeveen and Gaasbeek9 have observed protium/
deuterium exchange between CH3D and HF–SbF5 (8.4%
SbF5) at moderate pressure (7 atm) in the ꢁ10 to 25 ꢀC
temperature range. Olah and Schlosberg10 have made similar
observations by reacting CH4 with DSO3F–SbF5 (1:1) (Magic
Acid1) at room temperature. These authors suggested that
methane can be considered as a s-base11 in superacidic media
and proposed a methonium ion as the intermediate or transi-
tion state for the reversible protonation to explain the
exchange process. This penta-coordinated ion, first discovered
by mass spectrometry,12 has been widely studied by theoretical
approaches.13–17 Nowadays, it is well-admitted that it takes
Experimental
Synthesis of deuterated fluorosulfonic acid (DSO3F)
DSO3F was prepared by reaction of sulfur trioxide with a 5
mol % excess of DF at ꢁ30 ꢀC and then distilled three times
through a short Vigreux column under vacuum [(bp(15
mmHg) ¼ 85 ꢀC]; the yield was 79% and the isotopic purity
determined by NMR using a CDCl3–CHCl3 external reference
was 96%.
DF was obtained by hydrolysis of benzoylfluoride with D2O
(99.90% D, SDS). Benzoylfluoride was synthesized by reaction
of benzoylchloride with HF (Aldrich Chemicals). Methods of
synthesis were already described elsewhere.21 Methane as
N35 grade was purchased from Messer and used without
further purification.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
266
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 6 6 – 2 6 9

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k3
NMR experiments
CH2D2 þ DSO3F=SbF5
CHD3 þ DSO3F=SbF5
CHD þ HSO F=SbF
ƒƒƒ!
3
3
5
Mixtures of DSO3F–SbF5 were prepared in a Teflon tube
flushed with argon. After transfer of the superacid into the
NMR tube under argon, 10 mL of methane were bubbled
through the superacid mixture using a syringe equipped with
a capillary Teflon tube. NMR spectra were recorded after
4–8 min of temperature equilibration in the NMR probe.
NMR experiments were carried out with a Bruker Avance
400 spectrometer operating at 400 MHz for 1H. 1H NMR
experiments were performed with deuterium decoupling in
order to suppress overlapping of the J(1H–2H) coupling
pattern of the different signals of isotopologues.
k4
CD þ HSO F=SbF
ƒƒƒ!
4
3
5
Investigation of the effect of the molar ratio of antimony
pentafluoride on the H/D exchange was carried out at 25 ꢀC
with different DSO3F–SbF5 mixtures containing 20.7, 33.4,
36.0 and 49.0 mol % SbF5 . Fig. 2 shows a typical evolution
of the isotopologue distribution plot versus reaction time
obtained in a DSO3F–SbF5 mixture of 36 mol % SbF5 . Simu-
lation of the experimental points with the following theoretical
curves based on first-order kinetic law allows a determination
of the successive rate constants k1 to k4 :
½CH4ꢂ ¼ ½CH4ꢂ0 eꢁk t
ð1Þ
ð2Þ
1
Results and discussion
Â
Ã
k1½CH4ꢂ0
½CH3Dꢂ ¼
eꢁk t ꢁ eꢁk t
1
2
Methane is only slightly soluble in superacid solution and
preliminary NMR experiments showed that its solubility
decreases with increasing temperature as expected for a gas.
However, it was found to be soluble enough (0.0007 mole
per mole of HSO3F in 1:1 HSO3F–SbF5 at 25 ꢀC) for time-
resolved NMR observation. This allows us to use the same
in situ NMR technique used earlier for H/D exchange studies
k2 ꢁ k1
ꢀ
eꢁk t
eꢁk t
1
2
½CH2D2ꢂ ¼ k1k2½CH4ꢂ0
þ
ðk2 ꢁ k1Þðk3 ꢁ k1Þ ðk1 ꢁ k2Þðk3 ꢁ k2Þ
ꢁ
eꢁk t
3
þ
ð3Þ
ðk1 ꢁ k3Þðk2 ꢁ k3Þ
ꢀ
eꢁk t
1
in the HF–SbF5 system.19 Separate H NMR signals of CH4 ,
1
½CHD3ꢂ ¼ k1k2k3½CH4ꢂ0
CH3D, CH2D2 and CHD3 isotopologues were obtained thanks
to the deuterium decoupling technique. Real time observation
of the evolution of each species could then be monitored.
In situ NMR analysis provides a real advantage by measur-
ing the catalytic system without perturbation, whereas ex situ
analysis like GC/MS requires preliminary separation of the
products from the reactive medium. No by-products were
detected during these in situ experiments.
ðk2 ꢁ k1Þðk3 ꢁ k1Þðk4 ꢁ k1Þ
eꢁk t
2
þ
ðk1 ꢁ k2Þðk3 ꢁ k2Þðk4 ꢁ k2Þ
eꢁk t
3
þ
ðk1 ꢁ k3Þðk2 ꢁ k3Þðk4 ꢁ k3Þ
ꢁ
eꢁk t
4
þ
ð4Þ
ðk1 ꢁ k4Þðk2 ꢁ k4Þðk3 ꢁ k4Þ
1
Fig. 1 shows a typical evolution of the H{2H} decoupled
½CD4ꢂ ¼ ½CH4ꢂ0 ꢁ ½CH4ꢂ ꢁ ½CH3Dꢂ ꢁ ½CH2D2ꢂ ꢁ ½CHD3ꢂ
NMR spectrum during the H/D exchange reaction. Signals
of the different isotopologues CH4 , CH3D, CH2D2 and
CHD3 can be easily identified by their order of appearance
with reaction time; therefore, each substitution of protium
by deuterium leads to a high-field shift of the signals.
The spectra were deconvoluted using a modified version of
the Bruker Winfit program.22 The distribution of isotopolo-
gues as a function of reaction time verified a pseudo-first-order
kinetic model, since superacid is in large excess over methane.
The following equations describe the consecutive H/D
exchange reactions:
ð5Þ
Experimental points for [CD4] were deduced by subtracting
the integrated spectra from the integration value of the first
spectrum recorded in the early stages of the reaction. All our
results verified the following statistical relationship (k1 ¼ 4/
3k2 ¼ 2k3 ¼ 4k4) within 7%. The probability of exchange (1,
0.75, 0.5 and 0.25 for CH4 , CH3D, CH2D2 and CHD3 , respec-
tively) is only dependant on the number of exchangeable
protons in the isotopologues and there is no perturbation
due to the presence of deuterium in the molecule. This points
out clearly that secondary isotopic effects can be neglected.
This is in agreement with our previous study in HF–SbF5
where experimental and calculated secondary kinetic isotopic
effects were very close to one for each isotopologue.19
k1
CH4 þ DSO3F=SbF5
CH3D þ DSO3F=SbF5
CH D þ HSO F=SbF
ƒƒƒ!
3
3
5
k2
CH D þ HSO F=SbF
ƒƒƒ!
2
2
3
5
Fig. 1 Deuterium-decoupled proton (1H{2H}) NMR spectra during
Fig. 2 Distribution percentages of methane isotopologues plotted
versus exchange reaction time between methane and DSO3F–SbF5
(36 mol % SbF5) at 25 ꢀC.
the reaction between methane and
(41 mol % SbF5) at 30 ꢀC.
a mixture of DSO3F–SbF5
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 6 6 – 2 6 9
267

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Fig. 5 Eyring plot for the H/D exchange between CH4 and DSO3
F–SbF5 (49 mol % SbF5).
Fig. 5 presents a typical plot for the determination of the
activation parameters. The numerical results for various
concentrations are collected in Table 1.
Fig. 3 Determination of the rate of incorporation of deuterium in
CH4 and correlation with SbF5 concentration at 25 ꢀC.
It appears that an increase in concentration of SbF5 from 19
to 49 mol % in HSO3F–SbF5 leads to a decrease of the free
energy of activation from 97 to 84 kJ molꢁ1, respectively,
whereas the free enthalpy of activation DH # remains constant
64.5 ꢅ 1 kJ molꢁ1. Correspondingly, the importance of the free
The rate of the first H/D exchange reaction step (eqn. 1) at
25 ꢀC is reported in Fig. 3 as a function of the SbF5
concentration in the DSO3F–SbF5 mixture. Its intrinsic acidity
depends on the molar ratio between the Brønsted acid and the
Lewis acid and it is well-known that the acidity increases with
increasing Lewis acid concentration.23 Thus, it appears clearly
that the rate of H/D exchange increases with the acidity in
agreement with an acid-base type mechanism.
entropy of activation decreases from ꢁ25 to ꢁ15 J molꢁ1 Kꢁ1
.
In comparison with the value found by Hogeveen et al.9
(DH# ¼ 75 kJ molꢁ1) for the H/D exchange between mono-
deuterated methane and HF–SbF5 , the DH # value found here
is surprisingly lower despite the higher acidity of HF based
superacids. It is, however, difficult to compare the reactivity
of both systems, in which the ionic and cationic compositions
differ substantially. While in the HF based superacid only 4
types of anions were detected,25 more than 7 different anionic
species were observable by 19F NMR in HSO3F superacids.26
Comparison of the anionic part of both systems shows that
the addition of SbF5 to HF leads preferentially to the genera-
tion of larger ions instead of ionization of HF, whereas addi-
tion of SbF5 to HSO3F leads preferentially to ionization of
HSO3F. As a consequence HSO3F is fully ionized in the pre-
sence of 50 mol % SbF58 whereas HF can still be observed until
the antimony pentafluoride concentration reaches 80 mol %.25
Several theoretical studies on alkane protonation and reactiv-
ity in HF–SbF5 superacid are available in the literature,19,27–29
but no comparable studies have been published on HSO3F–
SbF5 superacids. Increasing the concentration of the Lewis
acid shifts the autoprotonation equilibrium towards the
production of more acidic species:
The rate constant k1 at ꢁ20 ꢀC in a DSO3F–SbF5 (49%
SbF5) was found to be 8.14 ꢃ 10ꢁ5
s
ꢁ1, much lower than was
found previously in DF–SbF5 (15% SbF5) at ꢁ20 ꢀC
(1.28 ꢃ 10ꢁ3
s
ꢁ1).19 Rates of H/D exchange are faster in
HF–SbF5 than in HSO3F–SbF5 as earlier claimed by
Hogeveen and Bickel.20 Further investigations on the reaction
kinetics as a function of temperature and composition of the
acid system have been carried out, but limited as follows: (1)
at lower concentration of SbF5 , temperature has to be
increased for observable exchange and solubility of methane
at more than 50 ꢀC was the limiting factor of the method; (2)
at higher concentration it was the viscosity of the media at
low temperatures. Rates of exchange were measured for sev-
eral temperatures for 5 mixtures containing from 49% to
19% molar SbF5 . An illustration of the effect of temperature
on the exchange rate is given in Fig. 4. The corresponding
Gibbs free energy of activation DG #, enthalpy of activation
DH # and entropy of activation DS # were estimated from the
plot based on the Eyring equation:24
SbF5 þ 2AH Ð SbF5Aꢁ þ H2Aþ
Due to the large excess of acidic species in comparison with
the amount of methane present in the system (molar ratio ca.
103:1) we cannot assign the increase in rate to an increase of
concentration of the superacid when going from 19 to 49
mol % SbF5 . For this reason we suggest that the entropy fac-
tor due to a solvation effect is most probably at the origin of
ꢁDH#
ꢁDS#
kB
h
RT
R
k ¼
ꢄ T ꢄ e
ꢄ e
ð6Þ
Table 1 Activation parameters for H/D exchange between CH4 and
different mixtures of DSO3F–SbF5
DSO3F–SbF5/ Texpt
ꢀC
/
DG # (30 ꢀC)a / DH # a
/
ꢁDS # a
/
mol % SbF5
kJ molꢁ1 kJ molꢁ1 J molꢁ1 Kꢁ1
19
25
31
41
49
25–75 97
35–55 93b
25–45 93
65.4
65.5
63.5
64.5
65.2
25
23
23
20
15
ꢁ10–30 90
ꢁ20–25 84b
a
Estimated errors within the limit of 10% of the corresponding values.
Extrapolated
b
Fig. 4 Determination of the rate of incorporation of deuterium in
CH4 in DSO3F–SbF5 (49 mol % SbF5) at several temperatures.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
268
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 6 6 – 2 6 9

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5
6
7
8
9
R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh and
H. Fujii, Science, 1998, 280, 560.
R. A. Periana, O. Mirinov, D. J. Taube and S. Gamble, Chem.
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P. J. E. de Rege, J. A. Gladysz and I. T. Horvath, Adv. Synth.
Catal., 2002, 344, 1059.
G. A. Olah, G. K. S. Prakash and J. Sommer, Superacids, Wiley,
New York, 1985.
the observed variation. The hydrogen bonding of the metho-
nium-ion-like transition state with the counter anion decreases
as the acidity increases. The importance of solvation has been
demonstrated by theoretical studies for HF-based systems.
A
computational study on methane protonation by
[H2SO3Fþ]-like species would be helpful to understand the nat-
ure of solvation of methonium ion and then to compare the
activated complex involved in both systems. In line with these
results it seems even more risky to relate the rate of exchange
between methane and various deuterated solid acids to the
acidity of such solid acidic catalysts as has been suggested
recently.30,31
H. Hogeveen and C. J. Gaasbeek, Rec. Trav. Chim. (Pays Bas),
1968, 87, 319.
10 G. A. Olah and R. H. Schlosberg, J. Am. Chem. Soc., 1968,
90, 2726.
11 G. A. Olah, Y. Halpern, J. Shen and Y. K. Mo, J. Am. Chem.
Soc., 1973, 95, 4960.
12 V. L. Tal’roze and A. K. Ljubimova, J. Mass Spectrom., 1998,
33, 502.
13 P. R. Schreiner, Angew. Chem., Int. Ed., 2000, 39, 3239.
14 D. Marx and M. Parrinello, Science, 1999, 284, 59.
15 G. A. Olah and G. Rasul, Acc. Chem. Res., 1997, 30, 245.
16 H. Muller, W. Kutzelnigg, J. Noga and W. Klopper, J. Chem.
Phys., 1997, 106, 1863.
17 P. R. Schreiner, S.-J. Kim, H. F. Schaefer III and P. R. Schleyer,
J. Chem. Phys., 1993, 99, 3716.
18 A. Goeppert, P. Diner, P. Ahlberg and J. Sommer, Chem.-Eur. J.,
2002, 8, 3277.
19 P. Ahlberg, A. Karlsson, A. Goeppert, S. O. N. Lill, P. Diner and
J. Sommer, Chem.-Eur. J., 2001, 7, 1936.
20 H. Hogeveen and A. F. Bickel, Rec. Trav. Chim. (Pays Bas),
1969, 88, 371.
21 J. Sommer, J. Bukala and M. Hachoumy, Res. Chem. Intermed.,
1996, 22, 753.
22 D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calve,
B. Alonso, J. O. Durand, B. Bujoli, Z. H. Gan and G. Hoatson,
Magn. Reson. Chem., 2002, 40, 70.
Conclusion
Kinetic studies of the H/D exchange between methane and
DSO3F–SbF5 in the concentration range of 19 to 49 mol %
SbF5 were performed by means of in situ NMR spectroscopy.
A relationship between the rates of the isotopic exchange and
the acidity of the superacid was established. The free enthalpy
of activation of the exchange process appears to be indepen-
dent of the system’s acidity, which seems to indicate only
minor changes in the transition state in the studied acidity
range. However, a substantial change in the entropy of activa-
tion is observed, related to solvation by the acid system. This is
in line with results reported for the HF–SbF5 system in which
solvation of the methonium ion plays an important role.27
23 R. Jost and J. Sommer, Rev. Chem. Intermed., 1988, 9, 171.
24 T. H. Lowry and A. Schueller-Richardson, Mechanism and
Theory in Organic Chemistry, Harper & Rowe, New York,
1987, 3rd edn.
25 J. C. Culmann, M. Fauconet, R. Jost and J. Sommer, New J.
Chem., 1999, 23, 863.
Acknowledgements
Financial support of the Loker Hydrocarbon Institute of USC
(Los Angeles, CA) is gratefully acknowledged.
26 J. Kuhn-Velten, M. Bodenbinder, R. Brochler, G. Hagele and
F. Aubke, Can. J. Chem.-Rev. Can. Chim., 2002, 80, 1265.
27 S. Raugei and M. L. Klein, J. Phys. Chem. B, 2002, 106, 11 596.
28 P. M. Esteves, A. Ramirez-Solis and C. J. A. Mota, J. Am. Chem.
Soc., 2002, 124, 2672.
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