A novel method for the direct sulfonation of CH4 with SO 3 in the presence of KO2 and a promoter

Article abstract of DOI:10.1021/op0300140

Direct sulfonation of methane with SO₃ to methanesulfonic acid (MSA) is accomplished in sulfuric acid in the presence of a small amount of KO₂ as the free radical initiator and a metal chloride. Of the several metal chlorides examined, RhCl₃ was found to be the most effective promoter. While KO₂ alone can activate methane, the conversion of SO₃ to MSA increases 2.3-fold when KO₂ and RhCl₃ are both present in the reaction mixture. The effects of different process parameters such as temperature, SO₃ concentration, methane pressure, KO₂ concentration, and RhCl₃ concentration have been examined on the rate of reaction. The reaction is optimized at a KO₂-to-RhCl₃ molar ratio of 3.16. Strongly acidic solvents such as H₂SO₄ or CF₃SO ₃H are necessary for the reaction. No MSA was formed when the reaction was carried out in DMSO. A mechanism is proposed to explain the activation of CH4 to form MSA. A critical part of the sequence is in situ formation of a metal-peroxo species via the reaction of KO₂, acid solvent, and RhCl₃.

Full text of DOI:10.1021/op0300140

Organic Process Research & Development 2003, 7, 754−757
A Novel Method for the Direct Sulfonation of CH with SO in the Presence of
4
3
KO and a Promoter
2
Sudip Mukhopadhyay and Alexis T. Bell*
Department of Chemical Engineering, UniVersity of California, Berkeley, California 94720, U.S.A.
Abstract:
tion of methylmercaptan.5,6 While this process is highly
Direct sulfonation of methane with SO3 to methanesulfonic acid
(MSA) is accomplished in sulfuric acid in the presence of a small
amount of KO2 as the free radical initiator and a metal chloride.
Of the several metal chlorides examined, RhCl3 was found to
be the most effective promoter. While KO2 alone can activate
methane, the conversion of SO3 to MSA increases 2.3-fold when
KO2 and RhCl3 are both present in the reaction mixture. The
effects of different process parameters such as temperature, SO3
concentration, methane pressure, KO2 concentration, and RhCl3
concentration have been examined on the rate of reaction. The
reaction is optimized at a KO2-to-RhCl3 molar ratio of 3.16.
Strongly acidic solvents such as H2SO4 or CF3SO3H are
necessary for the reaction. No MSA was formed when the
reaction was carried out in DMSO. A mechanism is proposed
to explain the activation of CH4 to form MSA. A critical part
of the sequence is in situ formation of a metal-peroxo species
via the reaction of KO2, acid solvent, and RhCl3.
productive, it produces six moles of HCl per mole of MSA,
resulting in a coupling of the demand for the primary product
and the byproduct. As an alternative it is interesting to
consider the direct sulfonation of methane using SO3 or SO2
and O2 as the sulfonating agent.7 It has been shown8 that
9
methane can be sulfonated with SO3 in strong acids using
a free radical initiator.10 The free radical initiators used in
this reaction are often highly expensive potassium persulfate
or phosphate salts or highly reactive metal peroxides which
are difficult to handle in case of a bulk production scale.
Since aqueous H2O2 is only minimally active, it is worth
considering alkali superoxides. In this communication, we
show that methane will undergo liquid-phase sulfonation with
SO3 in sulfuric acid to form MSA, using KO2 as free radical
initiators in conjunction with RhCl3. To the best of our
knowledge, this is the first example of using potassium
superoxide in the liquid phase to activate methane.
Results and Discussion
In a typical reaction (see Experimental Section), CH4 and
SO3 were reacted in fuming sulfuric acid in a high-pressure
autoclave. Small amounts of KO2 and a metal chloride were
added to the liquid phase. Reactions were carried out for 18
h at 95 °C, and the MSA thus formed was identified and
Introduction
Selective functionalization of methane to value-added
products is a subject of considerable contemporary interest.1,2
Because of favorable thermodynamics, considerable effort
has been devoted to the oxidation and oxidative carbonylation
of methane.3 By contrast, the sulfonation of methane has not
received as much attention despite its commercial impor-
tance.4 The current commercial process for the synthesis of
methanesulfonic acid (MSA) occurs via the chlorine oxida-
1
quantified by H NMR.8a
Table 1 shows the effect of different promoters on the
rate of methane sulfonation. Notably, using KO2 as the free
radical initiator in the absence of any promoters leads to a
6% conversion of SO3 to MSA. The presence of promoters
such as PtCl2, PdCl2, and RuCl3 has a negative effect on
MSA synthesis. On the other hand, CaCl2, FeCl3, and HgCl2
are moderately active as promoters, and RhCl3 is particularly
effective under the reaction conditions investigated.
* Author for correspondence. E-mail: bell@cchem.berkeley.edu. Fax: (510)
642-4778.
(1) (a) Hill, C. L. ActiVation and Functionalization of Alkanes; Wiley: New
York, 1989. (b) Axelrod, M. G.; Gaffney, A. M.; Pitchai, R.; Sofranko, J.
A. Natural Gas ConVersion II; Elsevier: Amsterdam, The Netherlands,
1994; p 93. (c) Starr, C.; Searl, M. F.; Alpert, S. Science 1992, 256, 981.
(d) Shilov, A. E. ActiVation of Saturated Hydrocarbons by Transition Metal
Complexes; D. Reidel: Dordrecht, The Netherlands, 1984. (e) Olah, G.
A.; Molnar, A. Hydrocarbon Chemistry; Wiley: New York, 1995. (f) Lin,
M.; Sen, A. Nature 1994, 368, 613. (g) Sen, A. Acc. Chem. Res 1998, 31,
550. (h) Labinger, J. A. Fuel Process. Technol. 1995, 42, 325. (i) Crabtree,
R. H. Chem. ReV. 1995, 95, 987. (j) Shilov, A. E.; Shul’pin, G. B. Chem.
ReV. 1997, 97, 2879. (k) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698.
(l) Gesser, H. D.; Hunter, N. R. Catal. Today 1998, 42, 183.
(2) (a) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970. (b) Bromberg,
S. E.; Yang, W.; Asplund, M. C.; Lian, T.; McNamara, B. K.; Kotz, K. T.;
Yeston, J. S.; Wilkens, M.; Frei, H.; Bergman, R. G.; Harris, C. B. Science
1997, 278, 260.
(3) (a) Asadullah, M.; Kitamura, T.; Fujiwara, Y. Angew. Chem., Int. Ed. 2000,
39, 2475. (b) Chepaikin, E. G.; Bezruchenko, A. P.; Leshcheva, A. A.;
Boyko, G. N.; Kuzmenkov, I. V.; Grigoryan, E. H.; Shilov, A. E. J. Mol.
Catal. A: Chem. 2001, 169, 89. (c) Periana, R. A.; Taube, D. J.; Evitt, E.
R.; Loffer, D. G.; Wentrcek, P. R.; Voss, G.; Masuda, T. Science 1993,
259, 340. (d) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh,
T.; Fujii, H. Science 1998, 280, 560.
(4) (a) Ullmann’s Encyclopedia of Industrial Chemistry; VCH: Weinheim,
1994; Vol. A25, pp 503-506. (b) Beringer, F. M.; Falk, R. A. J. Am. Chem.
Soc. 1959, 81, 2997. (c) Young, H. A. J. Am. Chem. Soc. 1937, 59, 811.
(d) Murray, R. C. J. Chem. Soc. 1933, 739.
(5) Kroschwitz, J. I.; Howe-Grant, M. Kirk Othmer Encyclopedia of Chemical
Technology; Wiley: New York, 1991.
(6) Guertin, R. U.S. Patent 3,626, 004, 1971.
(7) (a) Basickes, N.; Hogan, T. E.; Sen, A. J. Am. Chem. Soc. 1996, 118, 13111.
(b) Mukhopadhyay S.; Bell, A. T. J. Am. Chem. Soc. 2003, 125, 4406. (c)
Mukhopadhyay, S.; Bell, A. T. Chem. Commun. 2003, 1590.
(8) (a) Lobree, L. J.; Bell, A. T. Ind. Eng. Chem. Res. 2001, 40, 736. (b)
Mukhopadhyay, S.; Bell, A. T. Ind. Eng. Chem. Res. 2002, 41, 5901. (c)
Mukhopadhyay, S.; Bell, A. T. Org. Process Res. DeV. 2003, 7, 161. (d)
Mukhopadhyay, S.; Bell, A. T. Angew. Chem., Int. Ed. 2003, 42, 1019. (e)
Mukhopadhyay, S.; Bell, A. T. Angew. Chem., Int. Ed. 2003, 42, 2993.
(9) (a) Sulfur Trioxide and Oleum: Storage and Handling; Dupont Corpora-
tion: Wilmington, DE. (b) Royle, A. T. Ind. Chem. 1956, 32, 421.
(10) For a representative review on the selective alkane transformation via
radicals and radical cations, see: Fokin, A. A.; Schreiner, P. R. Chem.
ReV. 2002, 102, 1551.
754
•
Vol. 7, No. 5, 2003 / Organic Process Research & Development
10.1021/op0300140 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/07/2003

Table 1. Effect of different promoters on the sulfonation of
methane to MSAa
% conversion of
SO3 to MSA
entry
promoters
t, h
1
2
3
4
5
6
7
8
none
18
20
19
21
20
18
18
18
5
tr
0
2
8
9
10
14
PtCl2
PdCl2
RuCl3
CaCl2
FeCl3
HgCl2
RhCl3
a
Reaction conditions unless otherwise stated: solvent, fuming sulfuric acid,
5.67 g; SO3, 1.7 g (21.25 mmol) methane, 300 psig (84.9 mmol); KO2, 0.98
mmol; promoters, 0.31 mmol; time, 18 h; temperature, 95 °C
Figure 2. Effect of RhCl3 amount on the conversion of SO3 to
MSA. Reaction conditions: solvent, fuming sulfuric acid, 5.67
g; SO3, 1.7 g (21.25 mmol) methane, 300 psig (84.9 mmol); KO2,
0.98 mmol; time, 18 h; temperature, 95 °C.
Figure 1. Effect of KO2 concentration on the conversion of
SO3 to MSA. Reaction conditions: solvent, fuming sulfuric acid,
5.67 g; SO3, 1.7 g (21.25 mmol) methane, 300 psig (84.9 mmol);
KO2, 0.98 mmol; RhCl3, 0.31 mmol; time, 18 h; temperature,
95 °C.
Figure 3. Effect of temperature on the conversion of SO3 to
MSA formation. Reaction conditions: solvent, fuming sulfuric
acid, 5.67 g; SO3, 1.7 g (21.25 mmol) methane, 300 psig (84.9
mmol); KO2, 0.98 mmol; RhCl3, 0.31 mmol; time, 18 h.
The reaction was studied with different amounts of KO2
but a fixed amount of RhCl3. It is observed in Figure 1 that
the conversion of SO3 to MSA increases when the amount
of KO2 in the reaction mixture is raised from 0 to 0.98 mmol.
However, a further increase in the amount of KO2 results in
a decrease in the SO3 conversion to MSA.
The effect of the amount of RhCl3 added to the reaction
mixture is shown in Figure 2. For a constant amount of KO2,
the conversion of SO3 to MSA increases from 4 to 14% after
18 h as the amount of RhCl3 is increased from 0.08 to 0.31
mmol. However, a further increase in the ratio of KO2 to
RhCl3 is deleterious to the MSA yield. The highest conver-
sion of SO3 to MSA is obtained with 0.31 mmol RhCl3 in
the presence of 0.98 mmol KO2. Thus, the effective ratio of
KO2 to RhCl3 is 3.16.
Figure 3 shows that the conversion of SO3 observed after
18 h increases with increasing temperature up to 95 °C.
However, a decrease in the conversion to MSA is observed
for temperatures higher than 95 °C.
Figure 4 shows the effects of methane pressures on the
conversion of SO3 to MSA. The conversion of SO3 to MSA
increases from 1 to 24% when the methane pressure is
increased from 50 to 1000 psig.
concentration, the MSA conversion increases initially;
however, above an initial concentration of 39%, the conver-
sion of SO3 to MSA decreases due to the formation of
methylbisulfate as a byproduct.
The sulfonation of CH4 requires an acidic solvent. H2SO4
is the most effective solvent (14% conversion in 18 h), triflic
acid is moderately active (7% conversion in 18 h), and
trifluoroacetic acid is the least effective (3% conversion in
20 h). No MSA was formed when DMSO was used as the
solvent.
The experimental results can be interpreted in terms of a
free radical mechanism. The appearance of C2H6 in the
autoclave headspace in the absence of SO3 and the termina-
tion of the reaction in the presence of molecular O2 support
our hypothesis of a free radical mechanism. KO2 readily
reacts with H2SO4 to form H2O2 (2KO2 + H2SO4 f K2SO4
+ H2O2 + O2).11 The H2O2 thus formed could react with
the promoter, RhCl3 to generate the active radical species,
I•, which may consist of a rhodium-peroxo or hydroperoxo
The reaction rate also depends on the initial concentration
of SO3. As shown in Figure 5, with increasing SO3
(11) Vol’nov, I. I. In Peroxides, Superoxides, and Ozonides of Alkali and Alkaline
Earth Metals; Petrocelli, A. W., Ed.; Plenum Press: New York, 1966.
Vol. 7, No. 5, 2003 / Organic Process Research & Development
•
755

The observed lowering in the conversion of SO3 to MSA
when more than 0.98 mmol of metal superoxide is used can
be attributed to an increase in the rate of O2 formation. Since
O2 is known to act as a free radical scavenger,7a it would be
expected to inhibit the formation of MSA. This interpretation
is consistent with the failure to observe any MSA when the
reaction was carried out in the presence of 40 psig of O2.
It is noted that KO2 can initiate the reaction in the absence
of a promoter (Table 1, entry 1), suggesting that H2O2
generated by the reaction of KO2 with the acid solvent can
activate methane. Consistent with this observation, no
reaction was observed in a nonacidic solvent such as DMSO.
When RhCl3 is added to the reaction mixture, the conversion
of SO3 to MSA increases up to a maximum value and then
decreases, as shown in Figure 2. The rising portion of this
plot can be attributed to an increase in the amount of the
metal-peroxo species formed via the reaction of RhCl3 with
H2O2. The decrease in the conversion to MSA for a KO2/
RhCl3 ratio of less than 3 might be associated with the
catalyzed decomposition of H2O2 by excess RhCl3. Two other
observations support the in situ formation of a metal-peroxo
species. The first is that after reaction for 18 h in the presence
of RhCl3 and KO2 (KO2/RhCl3 ) 3), the addition of more
KO2 resulted in an additional 11% conversion of SO3 to MSA
after an additional 18 h of reaction. On the other hand, when
the same experiment was carried out in the absence of RhCl3,
no additional MSA was observed after an additional 18 h of
reaction. These experiments suggest that a part of the RhCl3
may be recycled during the reaction. To examine whether
Rh2(SO4)3 might have formed by the reaction of RhCl3 and
H2SO4, we performed a reaction with Rh2(SO4)3 as a
promoter instead of RhCl3. In this case, hardly any MSA
formation was observed. This suggests that during the
reaction RhCl3 does not dissociate completely in fuming H2-
SO4 and that the chloride anion is not replaced fully by
sulfate or bisulfate anions.
The decrease in SO3 conversion above 95 °C seen in
Figure 3 might be due to rapid decomposition of H2O2 and
the release of O2. As noted above, the presence of O2 in the
reaction mixture inhibits the formation of MSA. This is
attributed to the formation of alkylperoxide species via the
reaction of methyl radicals with O2, which are less active
for reaction with SO3 than methyl radicals.
The effect of SO3 concentration on the conversion of SO3
to MSA seen in Figure 5 can be explained as follows. Below
40 wt % SO3, the increase in conversion is attributed to the
increase in the driving force for reaction 2. However, for
higher SO3 levels, MSA formation is reduced by the
formation of CH3OSO3H and CH3(SO3)2H. A similar effect
has been observed in several other reaction systems utilizing
a free radical initiator to produce MSA from CH4 and SO3.8
Figure 4. Effect of CH4 partial pressure on the conversion of
SO3 to MSA. Reaction conditions: solvent, fuming sulfuric acid,
5.67 g; SO3, 1.7 g (21.25 mmol); KO2, 0.98 mmol; RhCl3, 0.31
mmol; temperature, 95 °C; time, 18 h.
Figure 5. Effect of SO3 initial concentration on the conversion
of SO3 to MSA. Reaction conditions: solvent, fuming sulfuric
acid; methane, 300 psig (84.9 mmol); KO2, 0.98 mmol; RhCl3,
0.31 mmol; temperature, 95 °C; time, 18 h.
species.12,13 The I• abstracts hydrogen from CH4 to form CH3•
radicals (reaction 1) which react with SO3 (reaction 2) to
form CH3SO3• radicals which in turn abstract hydrogen from
methane to form MSA according to reaction 3.
CH4 + I• f CH3• + IH
CH3• + SO3 f CH3SO3•
(1)
(2)
(3)
CH3SO3• + CH4 f CH3SO3H + CH3•
While the nature of the composition of I• has not been
confirmed, we have recently demonstrated that RhCl3 in
combination with urea-H2O2 is a particularly effective free
radical initiator for the direct sulfonation of CH4 by SO3 to
form MSA.8e
(12) (a) Springborg, J.; Zehnder, M. HelV. Chim. Acta 1984, 67, 2218; cf. Chem.
Abstr. 1985, 102, 105050. (b) Komozin, P. N.; Zakharchenko, E. A. Zh.
Neorg. Khim. 1997, 42, 1297; cf. Chem. Abstr. 1997, 127, 338510. (c)
Selke, M.; Foote, C. S.; Karney, W. L. Inorg. Chem. 1993, 32, 5425. (d)
Conte, V.; Di Furia, F.; Modena, G. In Transition-Metal-Catalyzed
Oxidation: The Role of Peroxometal Complexes. Organic Peroxides; Ando,
W., Ed.; Wiley: Chichester, New York, 1992; pp 559-598. (e) Meunier,
B., Ed. Metal-Oxo and Metal-Peroxo Species in Catalytic Oxidations;
Springer: Berlin, 2000.
(13) (a) For methane activation with various V-, Nb-, and Ta-based metal-
peroxo species, see: Hoehn, A.; Suess-Fink, G.; Shul’pin, G. B.; Nizova,
G. V. German Patent DE 19720344, 1997; cf. Chem. Abstr. 128, 49802,
1997. (b) For oxidation of organic compounds with peroxo-metal
complexes, see: Conte, V.; Di-Furia, F.; Moro, S. J. Phys. Org. Chem.
1996, 9, 329.
756
•
Vol. 7, No. 5, 2003 / Organic Process Research & Development

Conclusions
otherwise. Following reaction, the reactor was cooled in an
ice bath to room temperature (∼0.35 h), and the reactor was
vented. The gases exiting the reactor were passed through
scrubbers containing NaOH and Carbusorb to remove sulfur
(II) compounds. The system was then purged with N2 purified
by passage through oxysorb, ascarite, and molecular sieve
traps. The liquid product was removed from the glass liner
and added slowly to 0.5 g of H2O to convert any unreacted
We have demonstrated a synthetic approach for the direct,
liquid-phase sulfonation of methane with SO3. Under the best
suitable reaction conditions, 24% conversion of SO3 to MSA
was achieved. RhCl3 emerged as an effective promoter when
used with KO2 serving as the radical initiator. It is proposed
that a rhodium-peroxo or hydroperoxo species is formed
in situ, which is a more effective free radical initiator than
KO2 in activating methane.
1
SO3 to H2SO4. Reaction products were characterized by H
NMR. All spectra were acquired using a Bruker AMX-400
MHz FT-NMR spectrometer. A capillary containing D2O and
CH3OH, immersed within the NMR tube containing the
sample, was used as a lock, reference, and integration
standard. All other chemicals were purchased from Aldrich
Chemical Co. and used without further purification. Products
were identified by H NMR. The chemical shift for methyl
group of MSA was observed to be 2.9-3.07 ppm, depending
on the concentration of MSA in the reaction mixture.
Experimental Section
Reactions were carried out in a 100 cm3 high-pressure
autoclave (Parr Instruments: 3000 psig maximum) con-
structed of Hastelloy B. Unless otherwise stated, the fol-
lowing procedure was used for all experiments. Typically,
0.98 mmol of KO2 (Aldrich Chemical Co.) and 0.31 mmol
of RhCl3 (Aldrich Chemical Co., 99.98%) were added to
5.7 g of fuming sulfuric acid (Aldrich Chemical Co., 27-
33% SO3) taken in the glass liner containing a Teflon-encased
stirring bar at 0-5 °C. For some experiments, additional SO3
(Aldrich Chemical Co., 99%) was added to the liquid in the
liner. The glass liner was then transferred to the reactor, after
which the reactor was sealed and attached to a gas handling
system. The reactor was purged twice with N2 (Matheson)
and then twice with CH4 (Matheson, ultrahigh purity) and
then pressurized with CH4 to 300 psig. The reactor was
heated to 95 °C, a process which took 8-10 min, and this
temperature was then maintained for up to 18 h, unless stated
1
Acknowledgment
This work was supported by a grant from ATOFINA
Chemicals, Inc. We acknowledge Dr. R. Vijay Srinivas and
Dr. Gary S. Smith of ATOFINA Chemicals, Inc. for their
valuable suggestions on different aspects of the research.
Received for review March 29, 2003.
OP0300140
Vol. 7, No. 5, 2003 / Organic Process Research & Development
•
757