Communications
The reaction was studied with different amounts of RhCl3
logic, Table 2shows that the maximum conversion of SO to
3
(Table 2, entries 9–12). With an increase in the amount of
RhCl3, the conversion of SO3 to MSA increased marginally. A
maximum SO3 conversion of 95% was achieved with
0.14 mmol RhCl3 for a methane pressure of 650 psig. The
conversion of methane to MSA under this reaction condition
was 11%.
The conversion of SO3 to MSA increased from 86 to 91%
(3 h reaction time) when the temperature was increased from
65 to 758C. However, with a further increase in temperature
to 858C, the conversion decreased to 54% (Table 2, entries 4,
13, and 14) and a small amount of CH3OSO3H appeared as a
product. When the reaction temperature was increased to
1308C, CH3OSO3H became the major product. At 1608C, the
total conversion of methane was 24%, of which 0.86% was to
MSA, 1.26% to CO2, 1.2% to methanedisulfonic acid, and
the balance to CH3OSO3H (entries 15 and 16).
MSA (86–95%) is obtained when the molar ratio of urea/
H2O2 to RhCl3 is between 2.24 and 5.28. Furthermore, when a
fresh amount of urea/H2O2, methane, and SO3 was added to
the reaction mixture after 3 h, the reaction proceeded in the
same way as the initial reaction with fresh reaction mixture.
This strongly suggests that the promoter, RhCl3, is recycled in
situ in the acidic solution. The importance of RhCl3 is further
supported by an experiment in which a fresh batch of urea/
H2O2, methane, and SO3 was added to a reaction mixture after
3 h, but one to which RhCl3 had not been added. In this case,
no additional reaction was observed.
When the reaction mixture is cooled to 08C, most of the
Rh salt can be separated from the reaction mixture by
precipitation. The separation of MSA from the reaction
mixture is straightforward, as MSA can be used as a solvent
instead of H2SO4.
The highest conversion of methane to MSA, 36%
(Table 2, entry 17), was obtained after 72 h of reaction. In
this case the initial methane pressure was 100 psig, the
temperature was 858C, the amount of urea/H2O2 was
0.65 mmol, and the amount of RhCl3 was 0.3 mmol. Since
the selectivity to MSA was 99.99%, the overall yield of MSA
was 36% based on methane. Comparison of entries 12and 18
in Table 2shows that the conversion of methane to MSA can
be raised by increasing the initial amount of fuming sulfuric
acid. When the amount was doubled, the methane conversion
increased from 10% to 18%.
The effectiveness of K2S2O8, K4P2O8, CaO2, Br2, Cl2, and I2
as initiators was examined in the presence of RhCl3. It was
found that the combination of RhCl3 and urea/H2O2 was the
most effective combination of promoter and initiator under
the reaction conditions used (Table 2, entries 19–23). The
order of reactivity of the initiators in the presence of RhCl3 as
the promoter was urea/H2O2 > CaO2 > K2S2O8 > K4P2O8 >
Br2 > Cl2. With iodine as the initiator, 3% of the SO3 in the
reactor was converted to CH3OSO3H but no MSA was
detected (Table 2, entry 24), which is consistent with recent
reports.[8]
In conclusion, we have developed a highly effective, low-
temperature reaction protocol for sulfonating methane to
give methanesulfonic acid at low methane pressures. For a
methane pressure of 100 psig, the conversion of methane to
MSA is 29–36% and the selectivity to MSA is 99.9%. RhCl3
enhances the initiation of the reaction by urea/H2O2, pre-
sumably through the formation of a metal–peroxo or metal–
hydroperoxo species in situ. Most of the RhCl3 can be
recovered from the reaction mixture by precipitation at 08C.
The separation of MSA from the reaction mixture is
straightforward as MSA can be used as a solvent instead of
H2SO4. At higher temperatures (160–1708C) the same
reaction scheme can be extended to synthesize CH3OSO3H,
which can be easily hydrolyzed to methanol.
Received: January 20, 2003 [Z50976]
Keywords: C–H activation · homogeneous catalysis · methane ·
.
sulfonation
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The mechanism by which H2O2 either alone or in
combination with RhCl3 promotes the sulfonation of methane
is not understood. In previous studies[5,6] of methane sulfona-
tion utilizing inorganic peroxide initiators, e.g., K2S2O8,
K4P2O8, CaO2, it was suggested that decomposition of the
initiator produces methyl radicals, which then react with SO3
to form CH3SO3C radicals and then CH3SO3H. Inhibition of
the reaction by O2 together with the appearance of small
amounts of ethane in the gas phase suggest that a free-radical
mechanism may be operative in the systems investigated in
this study. The initiating species in the present case could be
OHC radicals formed by decomposition of H2O2, or RhClO2 or
RhCl2OOH formed from the reaction of H2O2 with RhCl3.
The increase in the conversion of SO3 from 23% to 86%
(after 3 h reaction time) when a small amount of RhCl3 is
added to the synthesis mixture together with urea/H2O2
strongly suggests that the activation of methane is initiated
not only by H2O2 but also by a metal–peroxo or metal–
hydroperoxo species[9] produced in situ by the reversible
reaction between urea/H2O2 and RhCl3. Consistent with this
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Angew. Chem. Int. Ed. 2003, 42, 2990 – 2993