Direct sulfonation of methane at low pressure to methanesulfonic acid in the presence of potassium peroxydiphosphate as the initiator

Article abstract of DOI:10.1021/op020079n

A high-yield, direct sulfonation of methane with SO₃ to methanesulfonic acid (MSA) is effected in sulfuric acid using potassium peroxydiphosphate (K₄P₂O₈) as the initiator. The influences of initiator concentration, temperature, CH₄ pressure, the initial concentration of SO₃, and solvent acidity were investigated. A mechanism is proposed to explain the observed effects of reaction conditions on the conversion of SO₃ to MSA.

Full text of DOI:10.1021/op020079n

Organic Process Research & Development 2003, 7, 161−163
Direct Sulfonation of Methane at Low Pressure to Methanesulfonic Acid in the
Presence of Potassium Peroxydiphosphate as the Initiator
Sudip Mukhopadhyay and Alexis T. Bell*
Department of Chemical Engineering, UniVersity of California, Berkeley, California 94720-1462, U.S.A.
Table 1. Direct free radical initiated sulfonation of methane
with SO3
Abstract:
a
A high-yield, direct sulfonation of methane with SO3 to
methanesulfonic acid (MSA) is effected in sulfuric acid using
potassium peroxydiphosphate (K4P2O8) as the initiator. The
influences of initiator concentration, temperature, CH4 pressure,
the initial concentration of SO3, and solvent acidity were
investigated. A mechanism is proposed to explain the observed
effects of reaction conditions on the conversion of SO3 to MSA.
initiator
time, h
% conversion of SO3 to MSA
K4P2O8
K2S2O8
Oxoneb
H2O2
12
14
14
16
21
11
3
5
a Reaction conditions unless otherwise stated: methane, 200 psig (58 mmol);
SO3, 30 wt % (21.4 mmol); initiator, 0.104 mmol; temperature, 95 °C; H2SO4
(SO3), 5.7 g. b Potassium peroxymonosulfate
Introduction
The selective catalytic functionalization of methane to
value-added products is a subject of considerable contem-
porary interest.1 Because of favorable thermodynamics, many
researchers have investigated the oxidation and oxidative
carbonylation of methane.2By contrast, the sulfonation of
methane has not yet received as much attention despite of
its commercial importance.3 The current commercial process
for the synthesis of methanesulfonic acid (MSA) involves
the chlorine oxidation of methylmercaptan.4 While this
process is highly 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. Thus, there is an
incentive to find a method for the direct methane sulfonation
of methane, which avoids the coproduction of HCl. Sen and
co-workers5 and, more recently, we6,7 have shown that
K2S2O8 can be used as a free radical initiator to sulfonate
methane with SO3 in fuming sulfuric acid under 1000 psig
methane pressure. In the latter work, it was shown that, with
increasing reaction temperature, the conversion of SO3 to
MSA, the limiting reagent, reaches a maximum at 65 °C.
Above this temperature, O2 released due to initiator decom-
position inhibits the free-radical processes involved in the
formation of MSA. In view of this, the question arises as to
whether one might be able to use potassium peroxydiphos-
phate (K4P2O8) as the initiator, since this material is more
stable to decomposition and release of O2 than K2S2O8.8 In
this work, we show that methane will undergo liquid-phase
sulfonation to MSA with SO3 in sulfuric acid, using a small
amount of K4P2O8 as a free radical initiator and that this
initiator is more effective than K2S2O8 at elevated temper-
ature.
* To whom correspondence should be addressed. 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, 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, 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. (m) Lunsford, J. H. Catal. Today 2000,
63, 165.
(2) (a) 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. (b) Periana, R. A.; Taube, D. J.; Evitt, E. R.; Loffer, D. G.;
Wentrcek, P. R.; Voss, G.; Masuda, T. Science 1993, 259, 340. (c) Periana,
R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science
1998, 280, 560.
Results and Discussion
In a typical reaction (see Experimental Section) methane
was reacted with SO3 in H2SO4 to form MSA in the presence
of K4P2O8 in a high-pressure, glass-lined Parr autoclave.
Reactions were carried out for 12 h, and the MSA thus
formed was identified and quantified by H NMR.6,7
1
Table 1 shows the effect of different promoters on the
rate conversion of SO3, the limiting reagent, to MSA. Using
K4P2O8 as the initiator, a 21% conversion of SO3 to MSA
was achieved in 12 h. Under identical reaction conditions,
the conversion of SO3 to MSA decreased to 11% when
K2S2O8 was used as the initiator. When Oxone (KHSO5) was
used as the initiator, only a 3% conversion of SO3 to MSA
was achieved in 14 h. Similarly, a conversion of 5% was
obtained using 30 wt % H2O2 as the initiator. In this last
case, additional SO3 was added to the reaction mixture to
convert the water in the peroxide solution to H2SO4.
(3) 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.
(4) (a) Kroschwitz, J. I.; Howe-Grant, M. Kirk Othmer Encyclopedia of
Chemical Technology; Wiley: New York, 1991; (b) Guertin, R. U.S. Patent
3,626,004, 1971.
(7) Mukhopadhyay, S.; Bell, A. T. Ind. Eng. Chem. Res. 2002, 41, 5901.
(8) Vol’nov, I.; Chamova, V. N.; Kolotova, E. I. Zh. Neorg. Khim. 1961, 6,
268.
(5) Basickes, N.; Hogan, T. E.; Sen, A. J. Am. Chem. Soc. 1996, 118, 13111.
(6) Lobree, L. J.; Bell, A. T. Ind. Eng. Chem. Res. 2001, 40, 736.
10.1021/op020079n CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/30/2003
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Table 2. Direct free radical initiated methane sulfonation
with SO3
Scheme 1. Proposed reaction mechanism
a
entry
parameters
time /h % conv. of SO3 to MSA
1
2
3
4
5
6
7
8
K4P2O8, 0 mmol
0.052 mmol
0.104 mmol
0.104 mmol
0.208 mmol
Temperature, 85 °C
95 °C
105 °C
CH4, 50 psig
80 psig
120 psig
200 psig
SO3, 15 wt %
30 wt %
38 wt %
12
12
12
6
0
14
21
12
16
7
21
14
2
6
9
21
11
21
26
19
the conversion of SO3 to MSA increased from 21% without
SbF5 to 30% upon SbF5 addition.
12
12
12
12
14
12
12
12
12
12
12
12
The experimental results reported here can be interpreted
by a mechanism that is analogous to that previously proposed
for the K2S2O8-initiated sulfonation of methane.6,7 As shown
in Scheme 1, reaction begins with the decomposition of the
initiator breaks in to two radical anions (reaction 1). The
radical anions thus formed then abstract a hydrogen atom
from methane and generate a methyl radical (reaction 2).
The methyl radical in turn reacts with sulfur trioxide to
generate methanesulfonyl radicals (reaction 3), which sub-
sequently abstract hydrogen from another methane molecule
to form the product, MSA (reaction 4). In support of the
proposed mechanism, we note that at the end of the reaction,
31P NMR reveals that all of the initiator has been converted
to K2HPO4, consistent with what would be expected from
reaction 1.
The observed lowering in the conversion of SO3 to MSA
when more than 0.104 mmol of initiator is used can be
attributed to the high rate of formation of free O2 by the
thermal dissociation of K4P2O8. Oxygen released in this
manner can act as a free-radical scavenger, thereby inhibiting
the formation of MSA.5 This interpretation is consistent with
the failure to observe any MSA when the reaction was carried
out in the presence of 30 psig of O2. The decrease in SO3
conversion for temperatures above 95 °C is attributed to an
increasing release of O2, which offsets the favorable effect
of increasing temperature on the reaction kinetics.
9
10
11
12
13
14
15
16
52 wt %
a Reaction conditions unless otherwise stated: methane, 200 psig (58 mmol);
SO3, 30 wt % (21.4 mmol); initiator, 0.104 mmol; temperature, 95 °C; H2SO4
(SO3), 5.7 g.
Table 2 shows the effects of reaction conditions on the
conversion of SO3 to MSA, using K4P2O8 as the initiator.
Entries 1-5 show that the conversion of SO3 to MSA
increases with the amount of K4P2O8 added to the reaction
mixture up to a value of 21%. Addition of more than 0.104
mmol of K4P2O8 reduced the conversion of SO3 to MSA. In
contrast to K2S2O8, which is most effective in the temperature
range of 65-70 °C, K4P2O8 is most effective at 95 °C (Table
2, entries 6-8). At higher temperatures, the conversion of
SO3 to MSA decreases with increasing temperature. It should
be noted, though, that when the effectiveness of K2S2O8 and
K4P2O8 is compared at 65 °C with all other reactions
conditions as shown for entries 6-8, then the conversion of
SO3 to MSA is 4% using K2S2O8 as the initiator and 2%
using K4P2O8 as the initiator. Entries 9-12 show that the
conversion of SO3 to MSA increases monotonically from 2
to 21% as the pressure of CH4 is increased from 50 to 200
psig. The conversion of SO3 to MSA is also a fucntion of
the initial SO3 concentration (Table 2, entries 13-16). With
an increase in concentration from 15 to 38 wt %, the SO3
conversion to MSA increased from 11 to 26%. However,
increasing the intial SO3 concentration up to 52% caused a
decrease in the conversion of SO3 to MSA.
Since SO3 is an essential component of MSA, the rise in
SO3 conversion to MSA with increased initial concentration
of SO3 is not surprising. The decrease in SO3 conversion to
MSA once the initial SO3 concentration rises to 52 wt % is
possibly due to the removal of dissolved SO3 through the
formation of products such as H2S2O7, H2S3O10, H2S4O13,
and so forth.10 The other contributors to the decrease in the
conversion of SO3 to MSA is the formation of methanedi-
sulfonic acid (0.5-1%), and CH3(SO3)2H (0.3-0.4%), which
are observed at SO3 concentrations above 38 wt %.7
Previous studies in which K2S2O8 was used as the initiator
have shown that for a given set of reaction conditions, the
conversion of SO3 to MSA increases with the acidity of the
Conclusions
solvent. This effect was attributed to an increase in the rate
We have demonstrated that the direct sulfonation of
methane to MSA can be achieved using K4P2O8 as the
initiator. The highest conversion of SO3 to MSA is achieved
at 95 °C. Comparison of the relative performance of K2S2O8
and K4P2O8 reveals that while the former initiator is more
active, the latter can be used at higher temperatures because
-• 7
of initiator decomposition to produce radical anions, SO4
.
As discussed below, radical anions are believed to be
responsible for initiating a sequence of elementary processes
that lead ultimately to MSA. A similar influence of solvent
acidity was observed using K4P2O8 as the initiator. As
supplied by Aldrich, K4P2O8 contains 5 wt % KF. This
compound reacts with H2SO4 to produce HF in situ. To
achieve a higher acidity, the HF was reacted with SbF5 (0.008
mmol) to form magic acid, HSbF6.9 For all other reaction
conditions being the same as those for entry 7 in Table 2,
(9) (a) Olah, G. A.; Schlosberg, R. H. J. Am. Chem. Soc. 1968, 90, 2726. (b)
Commeyras, A.; Olah, G. A. J. Am. Chem. Soc. 1969, 91, 2929.
(10) (a) Gillespie, R. J.; Robinson, E. A. Non-Aqueous SolVent Systems;
Waddington, T. C., Ed.; Academic Press: London, 1965; p 162. (b)
Gillespie, R. J. J. Chem. Soc. 1950, 2516.
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of its higher thermal stability to decomposition and release
of O2, a reaction inhibitor. The net result is that a higher
conversion of SO3 to MSA can be attained at 95 °C using
K4P2O8 as an initiator, all other reaction conditions being
equal. The reaction mechanism proposed in Scheme 1 offers
an effective basis for explaining the observed effects of
reaction conditions.
was then maintained for up to 12 h, unless stated otherwise.
Following reaction, the reactor was cooled in an ice bath to
room temperature (∼0.5 h), the system was then purged with
N2 purified by passage through oxysorb, ascarite, and
molecular sieve traps. The gases exiting the reactor were
passed through scrubbers containing NaOH and Carbusorb
to remove sulfur (II) compounds. The liquid product was
removed from the glass liner and added slowly to 0.5 g of
H2O to convert any unreacted SO3 to H2SO4. Reaction
Experimental Section
1
Reactions were carried out in a 100-mL 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. K4P2O8
(0.104 mmol) (Aldrich, tech.) and 5.7 g of fuming sulfuric
acid (Aldrich Chemical Co., 27-33% SO3) were added to a
glass liner containing a Teflon-encased stirring bar. 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 with N2 (Matheson, ultrahigh purity) and CH4
(Matheson, ultrahigh purity) and then pressurized to the
desired level with 200 psig CH4. The reactor was heated to
95 °C, a process which took 8-10 min, and this temperature
products were characterized by H NMR. All spectra were
acquired using a Bruker AMX-400 MHz FT-NMR spec-
trometer. A capillary containing D2O and CH3OH, immersed
within the NMR tube containing the sample, was used as a
lock, reference, and integration standard. The corresponding
chemical shifts for MSA was 2.81-3.04 ppm, depending
on the concentration of MSA in the mixture.
Acknowledgment
This work was supported by a grant from ATOFINA
Chemicals, Inc.
Received for review September 13, 2002.
OP020079N
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