Table 2 Effects of water concentration in different catalyst systems on methane C–H activation and oxidation to methanol
Activation at 150 uCa
Oxidation at 200 uCb
Entry
System
[CHxD42x]/mmol
TON
[CH3OH]/mmol
TON
1
2
3
4
a
(bpym)PtCl2 + 2% water
(bpym)PtCl2 + 6% water
PtCl2 + [1-mim][HSO4] + 2% water
PtCl2 + [1-mim][HSO4] + 6% water
0.367
0.276
0.480
0.420
7.35
5.51
9.60
8.39
0.047
0.016
0.065
0.037
0.94
0.31
1.31
0.73
H/D exchange occurred between CH4 and deuterated liquid media D2SO4, D2O and [1-mim-d4][DSO4]. TONs were determined from the gas
analysis of methane isotopomers by GC-MS. Reaction conditions: 0.05 mmol Pt(II) + 0.3 mmol IL + 1 mL D2SO4 + 3.4 MPa CH4 in a 15 mL
b
reactor; 150 uC, 2.0 h. Tests were performed using non-deuterated liquid media. The reaction conditions were the same as those in Table 1.
which is similar to the Catalytica system.3 The data in Table 1
show that all Pt species (PtCl2, PtCl4, PtO2, K2PtCl4 and H2PtCl6)
could exhibit significant catalytic activity depending on the nature
of the IL used. In the case of PtCl2 + [1-mim][Cl] in 96% H2SO4
(Table 1, entry 3) a methanol concentration of 0.17 M was
demonstrated, which was about 5 times higher than that for
(bpym)PtCl2 (Table 1, entry 1). As a sharp contrast to the
imidazolium- and pyrazolium-based IL systems, the pyridinium-
based (Table 1, entry 11) and triazolium-based (Table 1, entry 15)
IL systems exhibited either negligible or no measurable activity.
The reason for this striking difference is not as yet well understood.
Furthermore, comparison between Table 1, entries 8 and 9
suggests that having at least one chlorine coordinated to Pt might
be essential in the catalysis. This is consistent with a previous
theoretical study of the Catalytica system,4b which revealed that
chlorine participated in the most active catalysis state.
Pt/IL/H2SO4 ternary systems was studied. Methane oxidation to
methanol in concentrated H2SO4 by the Catalytica catalyst can be
written as: CH4 + 2H2SO4 A CH3OSO3H + 2H2O + SO2 in
which water is generated in situ during the reaction. The reactivity
of (bpym)PtCl2 was found to be extremely sensitive to even small
amounts of water. Quantum chemistry computations suggest that
the water complex [(Hbpym)PtCl(H2O)]2+ is about 30 kJ mol21
more thermodynamically stable than the starting active Pt complex
[(Hbpym)PtCl(HSO4)]2+, which is the so-called ground state effect
for C–H activation.4 Experimentally demonstrated in Fig. 1, the
catalytic activity dropped sharply when the sulfuric acid was
diluted from the oleum to below 100%. This leads to uneconomical
catalysis rates and high separation costs for the methanol.4a As a
comparison, the Pt/IL/H2SO4 systems at lower H2SO4 concentra-
tion (90 to 100%) exhibited higher methanol yields than the
Catalytica system and hence were more water-tolerant.
This trend was further examined through methane C–H bond
activation tests via H/D exchange between the regular CH4 and
deuterated reaction media at 150 uC. The data are summarized in
Table 2. At this temperature, the Catalytica system catalyzed
extensive H/D exchange while producing no measurable amount
of methanol.3 Table 2, entries 1–4 indicate that ternary systems of
Pt/IL/H2SO4 are more active than the Catalytica reaction, both in
the C–H activation and oxidation steps, prompting a different
mechanistic interpretation.
In many other applications, the most widely used ILs are those
with long alkyl chains, e.g., 1-butyl-3-methylimidazolium
([bmim]).5 However, in methane oxidation, it has been found that
alkyl chains longer than –CH3 would be oxidized quickly by Pt
catalysis in concentrated H2SO4. This is understandable as
methane is more inert than any longer alkyl group. We have
actually observed intermediate oxygenated products due to partial
oxidation of an ethyl group in an IL, 1-ethyl-3-methylimidazolium
chloride ([emim][Cl]). Thus, only no-methyl, 1-methyl or dimethyl,
imidazolium or pyrazolium ILs were used in the methane
oxidation tests (Table 1). To eliminate the suspicion that the
products might be coming from the methyl substituent on the IL
ring, control experiments using 13C-enriched methane for a system
of PtCl2 + [1-mim][HSO4] were carried out under similar
conditions. Subsequent analyses by GC-MS and 1H NMR
confirmed that the produced methylbisulfate was indeed
13CH3OSO3H.
In summary, the ILs in the homogeneous catalysis of methane
to methanol conversion not only acted as a dissolution media for
those otherwise insoluble Pt salts/oxide, but also played a key role
in promoting Pt reactivity, possibly through coordination and/or
intermolecular interactions. The versatile method described here
could also be used in other chemical reactions.
Notes and references
The stability of an IL’s ring structure could itself be a challenge
under the harsh catalytic conditions, since they are also
hydrocarbons. Slight decomposition of the imidazolium ring was
{ Methane oxidation in 1 mL of these ternary solutions was conducted in a
69 mL cylindrical stainless steel reactor (outside diameter = 1 inch) with a
glass liner inside. Mild stirring was provided via a magnetic stirring bar.
observed after the reaction had been run for a few hours, as
+
1 (a) R. H. Crabtree, J. Organomet. Chem., 2004, 689, 4083; (b) S. S. Stahl,
J. A. Labinger and J. E. Bercaw, Angew. Chem., Int. Ed., 1998, 37, 2180;
(c) J. A. Labinger and J. E. Bercaw, Nature, 2002, 417, 507.
2 (a) A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879; (b)
A. E. Shilov and G. B. Shul’pin, Activation and Catalytic Reactions of
Saturated Hydrocarbons in the Presence of Metal Complexes, Kluwer
Academic Publishers, Dordrecht, 2000.
3 R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh and H. Fujii,
Science, 1998, 280, 560.
4 (a) R. A. Periana, G. Bhalla, W. J. Tenn, III, K. J. H. Young, X. Y. Liu,
O. Mironov, C. J. Jones and V. R. Ziatdinov, J. Mol. Catal. A: Chem.,
2004, 220, 7; (b) J. Kua, X. Xu, R. A. Periana and W. A. Goddard, III,
Organometallics, 2002, 21, 511.
monitored by the products due to deep oxidation, CO2 and NH4
.
The formation of extra CO2 complicated the calculation of the
selectivity of methane-to-methanol conversion. We subsequently
developed the pyrazolium-based ILs (Table 1), in which the ring
structure proved to be stable against oxidation in the presence of
Pt catalysts in concentrated sulfuric acid within the duration of our
tests. Selectivity in these ternary systems was then found to be
comparable to the Catalytica reaction under similar conditions.
Because of the desired use of a dilute sulfuric acid solution in
practice, the effect of water concentration on the reactivity of
4618 | Chem. Commun., 2006, 4617–4619
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