High yield conversion of methane to methyl bisulfate catalyzed by iodine cations

Article abstract of DOI:10.1039/b205366g

Iodine in 2% oleum is an efficient catalyst for the selective, high yield oxidation of methane to methyl bisulfate.

Full text of DOI:10.1039/b205366g

High yield conversion of methane to methyl bisulfate catalyzed by iodine
cations
Roy A. Periana,*a Oleg Mirinov,a Douglas J. Taubeb and Scott Gambleb
a University of Southern California, Department of Chemistry, Loker Hydrocarbon Institute, Los Angeles,
CA 90089, USA. E-mail: rperiana@usc.edu; Fax: 213-821-2656; Tel: 213-821-2035
b Catalytica Advanced Technologies, Inc, 430 Ferguson Drive, Mountain View, CA 94043, USA.
Received (in Purdue, IN, USA) 6th June 2002, Accepted 22nd August 2002
First published as an Advance Article on the web 18th September 2002
Iodine in 2% oleum is an efficient catalyst for the selective,
high yield oxidation of methane to methyl bisulfate.
1, # 4 and 8) and iodine (Table 1, # 4–7) are observed. The
reaction rate is also strongly dependent on the concentration of
SO3 in the sulfuric acid solvent and does not proceed in < 98%
H2SO4 where free SO3 is not present, Table 1, # 1–4.4 Reactions
of higher alkanes with oleum catalyzed by iodine were briefly
examined but while oxidation to esters was observed, the
reactions are much less selective than with methane. Evidently,
the increased rates of proton catalyzed side reactions with these
higher alkanes make them unsuitable substrates in oleum.
Recently, several reports of alkane CH activation by homoge-
neous, cationic metal complexes with poorly coordinating
anions in poorly coordinating solvents have been reported.1 The
success of this strategy relies on the ease of substitution of the
weakly coordinating anion or solvent by the poorly coordinating
alkanes. A related strategy that lends itself well to net CH
functionalization via the CH activation reaction is to generate
such poorly coordinated, redox-active, cationic species by use
of strongly acidic, oxidizing solvents.
(1)
Attesting to the utility of this strategy, to date, the highest
reported yields and selectivities for the low temperature
oxidation of methane to methanol are with cationic catalysts in
sulfuric acid solvent.2 A key requirement for working in such
strongly acidic, poorly coordinating, oxidizing media is that the
catalysts be stable.
Other researchers have reported on the use of sulfuric acid
and other strong acid solvents for conversion of methane but in
these cases, the product yields, selectivities and volumetric
productivities were significantly lower and relatively expensive
oxidants, such as persulfate were utilized.4,5 Given the sim-
plicity of the catalyst and high reaction efficiency and
selectivity, it is of interest to provide some information on the
reaction mechanism.
Herein we report that 1–10 mM elemental iodine dissolved in
sulfuric acid containing 2–3% SO3 (oleum) generates a stable,
active species that at 165–220 °C catalyzes the functionalization
of methane (500 psig) to methyl bisulfate, eqn. (1).3 Concentra-
tions of methyl bisulfate of up to 1M, 45% yields (based on
methane) at > 90% selectivity, volumetric productivities of
~ 1027 mol per cc.s, turn-over numbers of 300 and turn-over-
frequencies of 3 3 1022 s21 have been observed. Iodine is
required for the reaction as no methyl bisulfate is formed in the
absence of added iodine, Table 1, # 7. Use of enriched 13C
Two key considerations of our work were to determine the
nature of the species that reacts with methane and whether free
radicals are involved. It is well known that methane can react
with super-acids such as HF+SbF5 via the formation of CH5 . 6
+
Consequently, it is possible that added iodine catalyses the
oxidation of CH5+ species by SO3 to generate methyl bisulfate.
To investigate this we examined the extent of deuterium
incorporation into methane as well as methyl bisulfate in the
presence and absence of added iodine when the reaction was
carried out in D2SO4/SO3. Consistent with earlier reports,6 we
find extensive, ~ 50%, deuterium exchange into methane even
at short reaction times (30 min) in the absence of iodine.
Interestingly, the extent of deuterium exchange between the
D2SO4/SO3 solvent and gas-phase methane is suppressed when
iodine is added presumably due to a net decrease in solvent
acidity on addition of iodine and the known6 strong dependence
of the exchange on solvent acidity. More significantly, the
methyl bisulfate produced (which as noted above is only formed
in the presence of added iodine) shows only low levels ( < 5%)
of deuterium incorporation under the same conditions. This
observation makes it unlikely that the methyl bisulfate is formed
1
methane and H, 13C NMR and HPLC analyses of the crude
reaction mixtures confirmed that methyl bisulfate is the only
liquid-phase product generated from methane; no methanol,
methyl iodide, methane sulfonic acid (CH3SO3H) or other
liquid phase produced were observed. Only SO2 and very low
levels ( < 1% based on added CH4) of CO2 were observed in the
gas phase. Carbon mass-balances of > 95% based on unreacted
methane and methyl bisulfate confirmed the high reaction
selectivity and yield. The reaction rates and selectivities are
reproducible and first order dependence on both methane (Table
Table 1 Yield of CH3OSO3H in different reaction conditionsa
% CH4
SO3 Wt X2 (mM) CH4 psig Conv
[CH3Xb]
TOF s21 mM, % sel
+
from iodine catalyzed oxidation of a putative CDH4 inter-
#
mediate as statistically, (ignoring isotope effects) this should
lead to the formation of a minimum of ~ 20% CDH2OSO3H.7
1
2
3
4
5
6
7
8
9
0c
0.5
1
2.5
2.5
2.5
2.5
2.5
2.5
I2 (5)
I2 (5)
I2 (5)
I2 (5)
I2 (10)
I2 (1)
none
I2 (5)
Br2 (10) 500
Cl2 (10) 500
500
500
500
500
500
500
500
200
0
—
10
30
53
5
0
0, 0
54, 95
218, 95
600, 95
1050, 95
95, 95
2+
It has been reported that S8 cations reacts slowly with
0.003
0.012
0.033
0.03
0.035
0
0.01
—
—
methane at room temperature in liquid SO2 to generate low
yields of methane thiol although no catalysis or mechanistic
results were reported.8 Gillespie has shown that the character-
istic bright blue color formed on dissolution of iodine in oleum
+ 9
0
0, 0
is due to the formation of the paramagnetic species I2 . The
10
20
25
31
28
205,95
240. 50
150, 30
610, 95
575, 94
+
reported disproportionation of I2 in oleum with decreasing
9
+
+
SO3 to form the more stable, poly-iodo cations, I3 or I4 ,
coupled with our observations of the strong dependence of the
methane oxidation reaction on SO3 concentration is consistent
10 2.5
11 2.5
12 2.5
I2 (5)
I2 (5)
500d
500e
0.033
0.025
with either I2 or possibly I+ (which could be formed at the
+
a Reaction time and temperature: 60 min at 195 °C. b 96% H2SO4. c X =
OSO3H. d 50 psig O2 added. e 10 mM K2S2O8 added.
elevated reaction temperatures although it has never been
synthesized) being the reactive species. Other possible reactive
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CHEM. COMMUN., 2002, 2376–2377
This journal is © The Royal Society of Chemistry 2002

species are iodyl sulfate (IO2 HSO42),10 iodosyl sulfate
(IO+HSO42 10 and I(HSO4)311 as these are also reported to be
+
)
formed on oxidation of iodine in sulfuric acid.
To determine which of these species could be the active
catalyst we explicitly examined the stoichiometric reactions of
+
methane at 50 °C with independently synthesized samples of I2
[Sb2F11]2, IOHSO4, IO2HSO4 and I(HSO4)3 in oleum and SO2
solvents. Significantly, only the I2 [Sb2F11]2 shows stoichio-
+
metric reactions under these mild conditions. Thus, reaction of
+
15 ml of 100 mM I2 Sb2F112 (1.5 mmols) with excess methane
(30 mmols) at 50 °C for 1 h in 2.5 wt% oleum led to 30% yield
+
of methyl bisulfate based on the added I2 Sb2F112. Importantly,
no reaction is observed at these mild conditions if the
+
2
I2 Sb2F11 is replaced with 100 mmol of either elemental
iodine, IOHSO4, IO2HSO4 or I(HSO4)3. Repeating the reaction
with 100 mmols of I2 [Sb2F11]2 in 96% sulfuric acid also gave
+
no methyl bisulfate and is consistent with reported instability of
I2 species in SO3-free sulfuric acid solutions.9 We also
+
+
2
examined the reaction of SO2 solutions of I2 Sb2F11 with
methane at 35 °C as, unlike oleum, this solvent is not oxidizing
Fig. 1 Proposed electrophilic CH activation mechanism for the iodine
catalyzed oxidation of methane in oleum.
+
and is unlikely to oxidize the I2 to higher oxidation state
species. As was reported for the case of S82+,8 there is a clear
not catalyze H/D exchange between D2SO4/SO3 and CH4, no
methane is produced in this reaction. The proposed oxidation
step in Fig. 1 has been reported.9,11 Elemental sulfur, selenium
and tellurium are also reported to generate cationic species on
dissolution in oleum and we have found that these species also
catalyze reaction between methane and oleum, but at much
lower efficiencies as compared to iodine.
+
2
reaction with methane as the blue color of the I2 Sb2F11 is
bleached on exposure to 500 psig of methane after 10 h.
However, the reaction is not as clean as in oleum and NMR
analysis shows that several methyl products, among them
(CH3)2I+ and CH3F, are formed.
These observations are consistent with I2 (or I+) species as
+
the active catalyst but do not indicate whether free radical
mechanisms are involved. While the free radical reaction of
atomic iodine with methane can be discounted,12 free radical
Notes and references
1 P. Burger and R. G. Bergman, J. Am. Chem. Soc., 1993, 115, 10462; M.
W. Holtcamp, J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 1997,
119, 848; L. Johansson, D. D. Wick and K. I. Goldberg, J. Am. Chem.
Soc., 1997, 119, 10235; O. B. Ryan and M. Tilset, J. Am. Chem. Soc.,
1999, 121, 1974; J. T. Golden, R. A. Andersenand and R. G. Bergman,
J. Am. Chem. Soc., 2001, 123, 5837.
2 R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh and H. Fuji,
Science, 1998, 280, 560; R. A. Periana, D. J. Taube, E. R. Evitt, D. G.
Loffler, P. R. Wentrcek, G. Voss and T. Masuda, Science, 1993, 259,
340.
3 In a typical reaction, a 50 ml glass-lined, stirred, high-pressure reactor
containing 5 mM iodine and 2.5 wt% sulfur trioxide dissolved in 15 ml
of concentrated sulfuric acid was pressurized with methane to a final
pressure of 500 psig. This reaction mixture was vigorously stirred and
maintained at 195 °C for 2 h before analysis by NMR and HPLC. The
reaction must be properly mixed to avoid mass-diffusion control with
respect to CH4.
4 We did not examine oleum concentrations > 5 wt% SO3 as many
species are catalysts for the methane oxidation under those conditions.
A patent (N. J Bjerrum, C. Radhusvej, G. Xiao, L. Bauneporten, H. A.
Hjuler, R. K. Dreyervej, WO 99/24383) claiming the use of many
materials including iodine as catalysts for conversion of methane to
methyl bisufate in 65 wt% oleum was reported after we carried out our
initial work. No mechanistic work was reported on this patent.
5 A. Sen, Acc. Chem. Res., 1998, 31, 550 and references therein.
6 G. A. Olah, G. K. S. Prakash and J. Sommer, Superacids, Wiley-
Interscience, New York, 1985.
+
reactions are plausible with the stronger oxidants I2 , IOHSO4,
IO2HSO4 or I(HSO4)3. However, given the high reaction yield
and selectivity as well as reproducible, first order kinetics with
respect to both methane and iodine, we are biased toward a non-
free radical pathway. This bias is strengthened by the observa-
tion that added oxygen gas or K2S2O8 (Table 1, #11) has no
affect on the reaction rate or selectivity. Oxygen and persulfate
are known radical scavengers and initiators, respectively, and
these species could be expected to lead to changes in rate or
selectivity. We also investigated elemental bromine and
chlorine as methane oxidation catalysts in 2–3% oleum. In both
of these cases, both the reaction rates and selectivities to methyl
bisulfate were lower (Table 1, # 9 and 10), than with iodine and
extensive poly-halogenated methanes typical of free-radical
reactions were observed. This marked difference in reactivity
between chlorine, bromine and iodine is consistent with the
proposed involvement of iodo cations such as I2+ as it has been
reported that the related cations of bromine and chlorine are not
stable in oleum.9
The unusually high reaction efficiency and product selectiv-
ity for methane conversion, along with the strong dependence
on solvent acidity is very similar to the reported, high yield
oxidation of methane in sulfuric acid solvent to methyl bisulfate
catalyzed by the soft, stable, redox-active electrophile, Hg(II).2
Based on the proposal that Hg(II) catalyzes methane oxidation
via CH activation by electrophilic substitution,2 it is possible
7 The formation of low levels of CH2DOSO3H is consistent with the
oxidation of the low levels of CH3D that is formed.
8 A. M. Rosan, J. Chem. Soc., Chem. Commun., 1985, 7, 377.
9 R. J. Gillespie and M. J. Morton, Quart. Rev., 1971, 25, 553; T. A.
O’Donnel, Super Acid and Acidic Melts as Inorganic Chemical
Reaction Media, VCH Publishers Inc, New York, 1992, Chapter 4 and
references therein.
10 R. J. Gillespi and J. B. Senio, Inorg. Chem., 1964, 3, 972.
11 F. Aubke, H. A. Carter and S. P. Jones, Inorg. Chem., 1970, 9, 2485; A.
Bali and K. C. Malhotra, J. Inorg. Nucl. Chem., 1976, 38, 411.
12 The reaction of iodine radicals with methane is highly endothermic and
not a viable pathway for catalytic methane oxidation: D. M. Golden and
S. W. Benson, Chem. Rev., 1969, 69, 125.
13 Such reaction may involve inner-sphere proton-coupled electron
transfer: A. A. Fokin, T. E. Shubina, P. A. Gunchenko, S. D. Isaev, A.
G. Yurchenko and P. R. Schreiner, J. Am. Chem. Soc., 2002, 124,
10718.
14 P. J. Stang and V. V. Zhdankin, Chem. Rev., 1996, 96, 1123; V. A.
Grushin, Chem. Soc. Rev., 2000, 29, 315.
+
that the poorly coordinated I2 H2S2O72, in spite of its known
radical character,9 is sufficiently electrophilic, soft and stable to
also react with methane by a predominantly electrophilic
substitution pathway as shown in Fig. 1, that does not involve
the formation of free radicals. The proposed electrophilic
+
substitution by I2 shown in Fig. 1, is not without precedent.
Similar electrophilic substitutions of CH bonds have been
proposed for the reaction of alkanes with H+, O3 , NO2 , and
other electrophiles.6,13 The electrophilic substitution of arenes
by aryl iodo cations is also known.14 Consistent with the
proposed functionalization step in Fig. 1 and the observation
that no free methyl iodide is formed, we find that addition of
methyl iodide to 2.5% oleum at 150 °C leads to the immediate
and quantitative formation of methyl bisulfate and blue colored
+
+
+
species due to I2 . Consistent with the observation that I2 does
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