Direct sulfonation of methane to methanesulfonic acid with SO2 using Ca salts as promoters

Article abstract of DOI:10.1021/ja0281737

Direct liquid-phase sulfonation of methane to methanesulfonic acid (MSA) with SO₂ has been achieved in triflic acid using K₂S₂O₈ as the oxidant and a small amount of a Ca salt as the promoter. The effects of reaction conditions on the conversion of SO₂ to MSA were examined. Included were the influence of solvent acidity, reaction duration, reaction temperature, amount of K₂S₂O₈, and composition and amount of promoters. Copyright

Full text of DOI:10.1021/ja0281737

Published on Web 03/21/2003
Direct Sulfonation of Methane to Methanesulfonic Acid with SO
2
Using Ca
Salts as Promoters
Sudip Mukhopadhyay and Alexis T. Bell*
Department of Chemical Engineering, UniVersity of California, Berkeley, California 94720
Received August 16, 2002; Revised Manuscript Received February 18, 2003; E-mail: bell@cchem.berkeley.edu
Selective catalytic functionalization of methane to value-added
products is a subject of considerable contemporary interest. While
SO
2
and defined as the ratio of the moles of SO
2
converted to MSA
1
to total moles of SO
detected.
Table 1 shows the effect of different promoters on the SO
2
conversion to MSA after 10 h. Almost no MSA is detected in the
absence of a promoter. The chloride salts of K, Mg, Ba, and Pd
are only minimally effective in promoting the formation of MSA,
2
taken initially. No other byproducts were
many authors have investigated the oxidation and oxidative
2
carbonylation of methane, the sulfonation of methane has not
3
received as much attention despite its commercial importance. The
current commercial process for the synthesis of methanesulfonic
acid (MSA) occurs via the chlorine-oxidation of methylmercaptan.4
as compared to CaCl
with Ca also affects the conversion of SO
-14). CaF is completely inactive, and CaBr
significantly less effective than CaCl . Ca(OCl) and Ca(CF
are moderately effective, but Ca(CH COO) and CaSO
completely inactive. While CaO
of the SO is converted to MSA.
The use of HCl together with CaO
SO to MSA. A maximum SO conversion to MSA of 14% was
achieved when the ratio of HCl to CaO was increased to 2.5. When
KCl was used in combination with CaO , only a 7% conversion of
SO to MSA was observed. On the other hand, addition of
approximately 2.5 psig Cl gas into the system together with CaO
2
(Table 1, entries 1-6). The anion associated
to MSA (Table 1, entries
and CaI are
COO)
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. There, therefore, is an
incentive to identify a direct route for methane sulfonation with
2
7
2
2
2
2
2
3
2
5
6
3
2
4
are
SO
shown that K
methane with SO
3 2
or SO . Sen and co-workers and more, recently, we have
2
is an active promoter, only 8%
S O
2 2 8
can be used as a free radical initiator to sulfonate
in fuming sulfuric acid. The same approach,
2
3
2
increased the conversion of
2 2 3
however, does not work if SO and O are used instead of SO .
While Ishii and co-workers have reported success in the vanadium-
catalyzed sulfonation of adamantane to the corresponding sulfonic
2
2
2
2
acids using SO
MSA. The question therefore arises as to whether K
2
and O
2
, methane did not undergo sulfonation to
might
2
7
2 2
S
O
8
2
2
not be used as an effective source of oxygen for the sulfonation of
methane with SO , since Fujiwara and co-workers have shown that
CaCl can be employed as an initiator for the carbonylation of
methane in trifluoroacetic acid in the presence of K
.8 In this
communication, we show that methane will undergo liquid-phase
sulfonation to MSA with SO in triflic acid, with K serving
resulted in a maximum 15% SO
entry 19).
2
conversion to MSA (Table 1,
2
2
Table 2 shows the effects of reaction conditions on the conversion
of SO to MSA after 10 h of reaction time. The conversion increased
from 0 to 14% when the amount of CaCl in the reaction mixture
was raised from 0 to 0.6 mmol. However, a further increase in the
amount of CaCl resulted in a decrease in the conversion to MSA.
With 0.6 mmol CaCl when the reaction was performed for 26 h,
2% SO conversion to MSA was observed. On the basis of K
2 2 8
S O
2
2
2
2 2 8
S O
as the oxidant and Ca salts as the promoter. To the best of our
knowledge, this is the first example of the liquid-phase sulfonation
2
2
2
of methane utilizing SO as the sulfating agent.
2
2
2 2 8
S O ,
In a typical reaction,CH
a high-pressure, glass-lined autoclave. K
4
and SO
2
were reacted in triflic acid in
2
the conversion of SO to MSA was 58% (Table 2, entries
1-6).
The reaction rate also depends on the amount of K S O in the
2 2 8
reaction mixture (Table 2, entries 7-10). Increasing the amount
9
2
S O
2 8
and a catalytic
amount of a Ca salt (or salts of other metals) were added to the
liquid phase. Reactions were carried out for 10 h, and the MSA
1
6
thus formed was identified and quantified by H NMR. Since SO
2
of K S O from 0 to 5 mmol, increased the conversion of SO to
2
2
8
2
is the limiting reagent, conversions are reported on the basis of
MSA from 0 to 14%, but increasing the amount of oxidant further
Table 1. Effect of different promoters on the rate of sulfonation of methanea
promoter,
mmol
MSA,
mmol
% conv. of
promoter,
mmol
MSA,
mmol
% conv. of
entry
promoter
None
KCl
MgCl2
BaCl2
PdCl2
CaCl2
Ca(OCl)2
Ca(CH3COO)2
CaF2
SO
2
to MSA
entry
promoter
SO
2
to MSA
1
2
3
4
5
6
7
8
9
0
tr
tr.
1
3
2
2
14
4
0
10
11
12
13
14
15
16
17
18
Ca(CF3COO)2
CaI2
CaBr2
0.6
0.6
0.6
0.7
0.6
0.6
0.6
0.6
0.6
0.6
0.26
0.4
0.66
0
1.05
1.44
1.71
1.84
0.72
1.97
2
3
5
0
8
11
13
14
7
0.6
0.7
0.7
0.5
0.6
0.6
0.7
0.6
0.13
0.4
0.26
0.26
1.84
0.52
0
CaSO4
CaO2
b
CaO2/HCl
CaO2/HClc
d
CaO2/HCl
0
0
CaO2/KCle
CaO2/Cl2
19
15
a
Reaction conditions: methane, 1000 psig (268 mmol); SO2, 35 psig (13.14 mmol); molar ratio of methane to SO2, 20.4; K2S2O8, 5 mmol; triflic acid,
b
5
mL; time, 10 h; temperature, 65 °C. Molar ratio of HCl to CaO2, 1. c Molar ratio of HCl to CaO2, 2. d Molar ratio of HCl to CaO2, 2.5. e Molar ratio of
KCl to CaO2, 1. Molar ratio of Cl2 to CaO2, 3.
f
4406
9
J. AM. CHEM. SOC. 2003, 125, 4406-4407
10.1021/ja0281737 CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Table 2. Effect of Reaction Conditions on the Sulfonation of Methane to MSAa
CH
psig
4
SO
psig
2
K
mmol
2
S
2
O
8
CaCl
mmol
2
MSA
mmol
% conv. of
CH
psig
4
SO
psig
2
K
mmol
2
S
2
O
8
CaCl
mmol
2
MSA
mmol
% conv. of
entry
T °C
SO
2
to MSA
entry
T °C
SO
2
to MSA
1
2
3
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
35
35
35
35
35
35
35
35
35
35
35
35
5
5
5
5
5
5
0
1.9
3.7
6.3
5
0
65
65
65
65
65
65
65
65
65
65
45
55
tr
tr.
6
14
22
12
9
0
3
10
7
tr.
13
14
15
1000
1000
200
600
800
1200
1200
1000
1000
1000
1000
35
35
35
35
35
35
0
10
20
25
30
5
5
5
5
5
5
5
5
5
5
5
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
75
85
65
65
65
65
65
65
65
65
65
1.57
1.18
0.92
0.26
0.78
2.1
12
9
7
2
6
16
0
4
9
12
14
0.4
0.6
0.6
0.9
1.8
0.6
0.6
0.6
0.6
0.6
0.6
0.79
1.84
2.89
1.57
1.18
0
0.39
1.314
0.91
tr
c
b
4
16
17
18
19
20
21
22
23
5
6
7
8
9
1
1
1
0
0.53
1.18
1.57
1.84
0
1
2
5
0.79
6
a
Reaction conditions: time, 10 h; solvent, triflic acid, 5 mL. b Time, 26 h. c Time, 72 h.
resulted in a decrease in MSA formation and the detection of a
small amount of methytriflate. The observed decrease in the yield
more than 6.3 mmol of K
2
S
2
O
8
is used can be attributed to the
high rate of formation of O
2
, which can act as a free radical
5
of MSA above an initial K
S O
2 2 8
loading of 5 mmol is possibly
scavenger, thereby inhibiting the formation of MSA. This inter-
pretation is consistent with the failure to observe any MSA when
2
due to the release of O , which, as discussed below, would inhibit
the reaction.
the reaction was carried out with 2-atm O
In conclusion, we have demonstrated a synthetic approach for
the direct, liquid-phase sulfonation of methane with SO . Under
the best reaction conditions, 22% conversion of SO to MSA was
achieved after 26 h of reaction. For this case, 58% of the K
was consumed. Efforts are now in progress to develop a catalytic
process scheme to sulfonate methane with SO in which molecular
is used as the oxygen source instead of K
Acknowledgment. This work was supported by a grant from
2
pressure in the autoclave.
2
The extent of SO conversion observed in 10 h increased with
increasing temperature up to 65 °C. However, a decrease in the
conversion to MSA was observed for temperatures above 65 °C
2
2
(Table 2, entries 11-14).
2 2 8
S O
The conversion of SO
2
to MSA increased from 2 to 14% after
1
6
0 h of reaction when the methane pressure was increased from
00 to 1000 psig, but reached a plateau at 1000 psig. The reaction
2
O
2
2 2 8
S O .
proceeds well even with 200-psig methane pressure. Thus, 7%
conversion of SO to MSA was observed after 72 h (Table 2, entries
5-18).
The reaction rate is also a function of SO
MSA was detected in the absence of SO . This confirms that the
source of SO is neither K nor the solvent triflic acid. At a
SO pressure of 10 psig, only 4% conversion to MSA was achieved.
However, at 25 psig, a 12% conversion was achieved and at 30-
2
Atofina Chemicals, Inc.
1
2
partial pressure. No
References
2
(
1) (a) Hill, C. L. ActiVation and Functionalization of Alkanes; Wiley: New
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2
2 2 8
S O
2
35 psig, a 14% conversion was obtained (Table 2, entries 19-23).
Solvent composition had a marked influence on the rate of MSA
formation. For the typical reaction conditions, a 2% conversion of
SO to MSA was found as well, using H SO
as the solvent.9
However, in trifluoroacetic acid, 4% conversion of SO to MSA
was attained, and in triflic acid the conversion rose to 14%.
The mechanism of MSA formation from CH and SO is not
1
995, 95, 987. (j) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97,
2
2
4
2879. (k) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698. (l) Gesser,
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(
2) (a) Chepaikin, E. G.; Bezruchenko, A. P.; Leshcheva, A. A.; Boyko, G.
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Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H.
Science 1998, 280, 560.
4
2
known; however, it is reasonable to propose that the reaction
involves free radical processes, inasmuch as it was observed that
(
3) (a) Ullmann’s Encyclopedia of Industrial Chemistry; VCH: Weinheim,
O
2
inhibits MSA formation. At least three possible initiators can
be identified, each of which could react with CH to produce CH
radical anions, formed via the
, OH• radicals, produced by the decom-
produced in situ via the reaction of K and
1
994; Vol. A25, pp 503-506. (b) Beringer, F. M.; Falk, R. A. J. Am.
4
3
•
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.
-
•
radicals. These include SO
decomposition of K
position of H
4
(
4) (a) Kroschwitz, J. I.; Howe-Grant, M. Kirk Othmer Encyclopedia of
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2 2 8
S O
O
2 2
2 2 8
S O
(
(
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the acid solvent, and Cl• radicals, produced via the oxidation of
-
Cl anions. Only OH• and Cl• radicals are thought to play a
significant role in the initiation process, since Table 1 shows that,
(7) Ishii, Y.; Matsunaka, K.; Sakaguchi, S. J. Am. Chem. Soc. 2000, 122,
7390.
in the absence of CaCl
formed. The results presented in Table 1 also suggest that Ca
cations are more effective than other divalent cations in promoting
in situ formation of H and its subsequent decomposition to form
OH• radicals. The CH • radicals once formed could react subse-
quently with SO , K , and methane to form MSA as suggested
2 2
or a mixture of CaO and HCl, no MSA is
(
8) Asadullah, M.; Kitamura, T.; Fujiwara, Y. Angew. Chem., Int. Ed. 2000,
2+
39, 2475.
(
9) In a 100-mL glass-lined Parr autoclave, 5 mmol (1.4 g) K
0.07 g) CaCl , and 5 mL of triflic acid were charged together with a
small Teflon coated magnetic stir bar. The reactor was then purged with
to expel the air out of the system. It was then pressurized first with 35
psig SO (13.14 mmol) and then finally with 1000 psig methane (268
2 2 8
S O , 0.6 mmol
(
2
2 2
O
N
2
3
2
2
2 2 8
S O
mmol) from the adjacent connecting cylinders. The reactor was then heated
to 65 °C under stirring and kept at that temperature for 10 h. After the
stipulated period of time, the reactor was quenched with ice and opened
7
by Ishii and co-workers in the sulfonation of adamantane.
The reason for the observed decrease in the conversion of SO
2
to collect the reaction mixture. The mixture then added slowly to 0.6 g of
1
to MSA when more than 0.9 mmol of CaCl
mixture is not understood. Since the solubility of CaCl
2
is used in the synthesis
in triflic
water and then taken for H NMR analysis. D
2
O and methanol was used
in a capillary as the lock references. The corresponding chemical shifts
for MSA were 2.78-3.02 ppm, depending on the concentration of MSA
in the mixture.
2
acid is limited, a plateau in the conversion level should have been
observed. The observed lowering in the conversion to MSA when
JA0281737
J. AM. CHEM. SOC.
9
VOL. 125, NO. 15, 2003 4407