F. Schꢀth et al.
Despite the potential of such a continuous sulfuric acid-
based process for methane oxidation, some downsides should
be mentioned: First of all, formation of one mole of methyl bi-
Results and Discussion
Homogeneous molecular catalyst for direct methane
AHCTUNGTRENNGaUN ctivation
sulfate requires not one, but two moles of SO , as the resulting
3
water reacts further to form sulfuric acid with the second mole
of SO . Additionally, oxidation of methane to CO requires four
We report the development of the platinum complex N-(2-
methylpropyl)-4,5-diazacarbazolyl-dichloro-platinum(II), DACbz-
Pt, which shows significantly improved activity in the direct
low-temperature oxidation of methane to methanol, compared
3
2
moles of SO per mole of methane. Consequently, when evalu-
3
ating the economics of the process, reoxidation of SO and rec-
2
oncentration of the diluted sulfuric acid should be considered.
In an earlier review, Conley et al. summarized the benchmark
for this process to be a 1 mm catalyst concentration that gen-
erates ca. 2m solution of methanol after about 1.5 h reaction
to the conventional Periana system dichloro
dyl))platinum(II), [Pt(bpym)] (Figure 1).
ACHTUNGERTNNUNG( h-2-(2,2’-bipyrimi-
AHCTUNGTRENNUNG
[
9]
time.
For this process, the Shilov system, [PtCl2 AHCTUNGTRNEUNG( H O) ] appeared to
2 2
be a very promising system showing catalytic activity, but suf-
fered from instability due to the irreversible decomposition to
[
10]
Pt metal or insoluble polymeric Pt salts. In contrast, Periana
et al. reported on a homogeneous system for the selective, cat-
alytic oxidation of methane to methanol via methyl bisulfate,
II
catalyzed by mercuric ions, Hg , where the reduced metal can
readily be reoxidized under the reaction conditions previously
[
5]
described. The system reached a methane conversion of 50%
Figure 1. a) Dichloro ACHTUNGTERUNNN(G h-2-(2,2’-bipyrimidyl))platinum(II) and b) N-(2-methyl-
propyl)-4,5-diazacarbazolyl-dichloro-platinum(II).
III
II
and a selectivity to methyl bisulfate of 85%. Tl , Pd , and the
cations of Pt and Au also oxidized methane to methyl bisul-
fate, but suffered from irreversible reduction or bulk metal for-
[
5,11]
mation.
In further investigations, Au proved to be a stable
The complex has been synthesized based on a Buchwald–
Hartwig cross coupling reaction of 2-chloro-3-iodopyridine and
3-amino-2-chloropyridine, followed by an alkylation with 2-
methylpropylbromide, an intramolecular Yamamoto coupling,
catalyst system when combined with H SeO , which allowed
2
4
reoxidation of the reduced species; however, turn over num-
[
12]
bers (TONs) remained low (i.e., 32).
Muelhofer et al. developed an alternative catalytic system in
trifluoroacetic acid with trifluoroacetic acid methyl ester as the
and complexation with K ACHTUNGTNERGUN[ PtCl ]. This development is based on
2 4
our search for p-acidic, chelating N-based ligands, with the ex-
[
13]
resulting species. Under such reaction conditions, platinum
complexes decomposed, producing platinum black. Palladium-
based N-heterocyclic carbene (NHC) complexes proved to be
stable under these reaction conditions, with sufficient activity
to form up to 980% triflouroacetic acid methyl ester based on
the amount of Pd in the system. Recently, Meyer et al. reported
improved Pd complexes based on pyrimidine-functionalized
pectation of having lower proton affinity and higher affinity for
II
[15]
Pt , a p-donor metal. The electronic properties of the ligand
are different compared to those of 2,2’-bipyrimidine.
As a derivative of bipyridine, the electron density is higher
than for the electron-deficient bipyrimidine. The tertiary nitro-
gen of pyrrole has an additional electron-donating effect on
the aromatic system (Figure 1). Overall, the electron density on
the platinum is higher and could be increased, for example by
para-amino-substitution towards the chelating nitrogens (see
the Supporting Information, Scheme S1).
[14]
NHC ligands, reaching TONs of 41 within 17 h reaction time.
Systematic complex and ligand design appear to open the
way for further improvements. A superior system for methane
[15]
oxidation in sulfuric acid has been reported by Periana et al.
They identified a platinum-bipyrimidine complex, [Pt(bpym)],
The increased electron density on the platinum appeared to
have a positive effect on the catalytic activity of the system. A
significantly increased catalytic activity of DACbz-Pt was de-
tected, with both superior TONs and turn over frequencies
(TOFs) compared to the conventional Periana system. Conse-
quently, these results gave insight into structure–activity rela-
tionships of this reaction and point towards the need for
higher electron density on the platinum to further optimize
the catalyst systems. Table 1 summarizes the catalytic activity
of the molecular catalyst in comparison to the conventional
AHCTUNGTRENNUNG
reaching TONs of around 300 at 81% selectivity to methyl bi-
sulfate as a highly promising system with the highest activity
described thus far. Further investigations focused on explora-
tion of alternative catalytic systems for the described process,
including iridium–hydroxo complexes and rhenium complexes,
and development of improved ligands for platinum complexes,
[
16–18]
such as cyclometalated systems.
The described efforts,
[9,19]
however, did not result in any improved catalytic systems.
Our investigation aimed to develop an improved catalyst
system and to obtain insights regarding structure–activity rela-
tionships. We also demonstrated that the concept can be
transferred to polymeric materials, which allows for the prepa-
ration of highly active solid catalysts for the direct oxidation of
methane to methanol.
Periana system. Both, [Pt AHCTUNGTERUNNN(G bpym)] and DACbz-Pt exhibited com-
parable methanol selectivities of >75% (after hydrolysis) and
CO as the main byproduct, determined by FTIR analysis of the
2
gas phase. From a mechanistic point of view, additional investi-
gations are necessary, but it is very likely that the catalytic
cycle is similar to the one described by Periana for the original
2
78
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ChemSusChem 2010, 3, 277 – 282