Development of molecular and solid catalysts for the direct low-temperature oxidation of methane to methanol

Article abstract of DOI:10.1002/cssc.200900123

The direct low-temperature oxidation of methane to methanol is demonstrated on a highly active homogeneous molecular catalyst system and on heterogeneous molecular catalysts based on polymeric materials possessing ligand motifs within the material structure. The N-(2-methylpropyl)-4,5-diazacarbazolyl-dichloro-platinum(II) complex reaches significantly higher activity compared to the well-known Periana system and allows first conclusions on electronic and structural requirements for high catalytic activity in this reaction. Interestingly, comparable activities could be achieved utilizing a platinum modified poly(benzimidazole) material, which demonstrates for the first time a solid catalyst with superior activity compared to the Periana system. Although the material shows platinum leaching, improved activity and altered electronic properties, compared to the conventional Periana system, support the proposed conclusions on structure-activity relationships. In comparison, platinum modified triazine-based catalysts show lower catalytic activity, but rather stable platinum coordination even after several catalytic cycles. Based on these systems, further development of improved solid catalysts for the direct low-temperature oxidation of methane to methanol is feasible.

Full text of DOI:10.1002/cssc.200900123

DOI: 10.1002/cssc.200900123
Development of Molecular and Solid Catalysts for the
Direct Low-Temperature Oxidation of Methane to
Methanol
[
a]
[b]
[b]
[b]
Regina Palkovits, Christian von Malotki, Martin Baumgarten, Klaus Mꢀllen,
[
a]
[c]
[c]
[c]
[c]
Christian Baltes, Markus Antonietti, Pierre Kuhn, Jens Weber, Arne Thomas, and
[a]
Ferdi Schꢀth*
The direct low-temperature oxidation of methane to methanol
is demonstrated on a highly active homogeneous molecular
catalyst system and on heterogeneous molecular catalysts
based on polymeric materials possessing ligand motifs within
the material structure. The N-(2-methylpropyl)-4,5-diazacarba-
zolyl-dichloro-platinum(II) complex reaches significantly higher
activity compared to the well-known Periana system and
allows first conclusions on electronic and structural require-
ments for high catalytic activity in this reaction. Interestingly,
comparable activities could be achieved utilizing a platinum
modified poly(benzimidazole) material, which demonstrates for
the first time a solid catalyst with superior activity compared
to the Periana system. Although the material shows platinum
leaching, improved activity and altered electronic properties,
compared to the conventional Periana system, support the
proposed conclusions on structure–activity relationships. In
comparison, platinum modified triazine-based catalysts show
lower catalytic activity, but rather stable platinum coordination
even after several catalytic cycles. Based on these systems, fur-
ther development of improved solid catalysts for the direct
low-temperature oxidation of methane to methanol is feasible.
Introduction
The direct oxidation of methane to methanol has already chal-
lenged more than a generation of chemists. The high binding
tection of methanol against overoxidation. Therefore, research
efforts have focused on systems in which methanol is not pro-
duced directly but in the form of methyl esters (i.e., methyl bi-
sulfate) for a reaction in concentrated sulfuric acid [Equa-
ꢀ
1
energy of the CH ꢀH bond (435 kJmol ) and the ease of over-
3
oxidation to form CO place high demands on improving the
2
[2–4]
activity and selectivity of potential catalytic systems. In scope
of a changing raw material base, the efficient and sustainable
utilization of remaining resources becomes imperative. Further-
more, development and optimization of technologies to elimi-
nate greenhouse gas emissions are equally important.
tions (1)–(4)].
Indeed, such a process utilizing sulfuric acid
and sulfur trioxide as oxidants would allow for a continuous
process design combining (1) oxidation of methane to methyl
bisulfate, (2) hydrolysis of methyl bisulfate to form methanol,
and (3) reoxidation of SO2.
The efficient use of the remaining natural gas reservoirs,
which are estimated to hold for another 60 years, is a major
CH
þ H
SO
þ SO
! CH
OSO
H þ H
O þ SO
ð1Þ
ð2Þ
ð3Þ
ð4Þ
4
2
4
3
3
3
2
2
[1]
challenge from a scientific and a technological point of view.
CH OSO H þ H O ! CH OH þ H SO
3
3
2
3
2
4
Due to its low density, transportation and storage of natural
gas is complicated and often non-economical, resulting in
SO þ 1=2 O ! SO
2
2
3
“
stranded gas”, which is the burning of significant amounts in
remote locations.
S : CH þ 1=2 O ! CH OH
4
2
3
Consequently, efficient technology to convert natural gas
onsite into liquid, such as methanol, would be highly desirable.
Current technologies include synthesis gas formation in combi-
nation with Fischer–Tropsch technology. A local application is
hampered by high energy demand and also by high capital costs
of such plants due to the tremendous economy of scale. There-
fore, a technology to convert methane onsite into liquid is attrac-
tive from both an ecological and an economical point of view.
The direct oxidation of methane to methanol is promising.
[
a] Dr. R. Palkovits, Dr. C. Baltes, Prof. F. Schꢀth
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mꢀlheim (Germany)
Fax: (+49)208 306 2995
E-mail: schueth@mpi-muelheim.mpg.de
[b] C. von Malotki, Dr. M. Baumgarten, Prof. K. Mꢀllen
Max-Planck-Institut fꢀr Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
[
c] Prof. M. Antonietti, Dr. P. Kuhn, Dr. J. Weber, Prof. A. Thomas
Max-Planck-Institute fꢀr Kolloid- und Grenzflꢁchenforschung
Am Mꢀhlenberg, 14476 Potsdam-Golm (Germany)
[2–8]
In the past, numerous groups have investigated this process.
Key factors in the development of suitable catalyst systems are
the selection of weakly coordinating solvent systems and pro-
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.200900123.
ChemSusChem 2010, 3, 277 – 282
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
277

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
www.chemsuschem.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2010, 3, 277 – 282

Direct Low-Temperature Oxidation of Methane to Methanol
methane to methanol and the low level of success concerning
the development of selective solid catalysts for this reaction
prompted us to combine the properties of both catalyst types.
Such a combination is possible based on polymeric materials
resembling potential ligand motifs within the structure, which
allow platinum coordination at a molecular level. Additionally,
such a material should withstand the harsh reaction conditions
Table 1. Catalytic activity of the molecular Periana catalyst and DACbz-
Pt
[
a]
[
b]
ꢀ1
[c]
ꢀ1 [d]
Catalyst
Pt conc. [mmolL
]
TON
TOF [s ]
ꢀ
2
[
[
[
Pt
Pt
Pt
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpym)]
10
3.33
4
2.67
3.33
158
290
355
473
570
1.8ꢂ10
8.1ꢂ10
3.9ꢂ10
5.3ꢂ10
[
e]
ꢀ2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpym)]
ꢀ
ꢀ
2
ACHTUNGTRENNUNG( bpym)]
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[DACbz-Pt]
[
e]
ꢀ2
of the Periana system (i.e., oleum, 30% SO ). However, not
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[DACbz-Pt]
16ꢂ10
3
many polymeric materials fulfill these requirements. For exam-
ple, melamine-based structures were investigated, but they
[
[
(
a] Methane conversions are summarized in the Supporting Information.
b] Reaction conditions: 15 mL H SO (30% SO ), 40 bar CH pressure
258C), at 2158C for 2.5 h. [c] TONs were calculated based on the ratio of
2
4
3
4
[24,25]
simply decomposed under the reaction conditions.
Recent-
ꢀ
1
produced methanol and Pt content of the catalyst (molMeOHmol ). [d] Cal-
culated as average rate based on TON and reaction time. [e] Reaction
time of 1 h.
ly however, reports on two promising polymer types appeared,
which could allow the envisaged platinum coordination and
could prove to be stable under the required reaction condi-
tions. Weber et al. described the synthesis of a mesoporous
Pt
(
poly)benzimidazole (PBI) network through polycondensation
system. A difference occurs upon protonation (Scheme 1). [Pt-
of 3,3’-diaminobenzidine, diphenyl isophthalate, and benzene-
1,3,5-tricarboxylic acid triphenyl ester, together with silica
spheres as a hard template to introduce porosity when re-
ACHTUNGTRENNUNG( bpym)] has two accessible nitrogen atoms for protonation
and DACbz-Pt has a pyrrole nitrogen. Deprotonation results for
both systems but a ring-opening reaction degrades DACbz-Pt
and causes a drop in catalytic activity. This degradation reac-
tion would lead to bipyridyl-derivatives of the Pt catalyst. Nev-
ertheless, investigations suggest the stability of the system do
not point towards a significant fraction of decomposed cata-
lyst, although this cannot fully be excluded.
[26,28]
moved.
Synthesis and structure of the PBI material and the
potential platinum coordinations are shown in Scheme 2.
The second material reported by Kuhn et al. belongs to a
new class of high-performance polymer frameworks which are
thermally stable up to 4008C and resist strong oxidizing condi-
[28,29]
tions.
Recently, we introduced this material as a promising
polymer for application in the direct oxidation of methane to
[30]
methanol. The material was formed by trimerization of aro-
[28,29]
matic nitriles in molten ZnCl2.
For application in direct
methane oxidation, 2,6-dicyanopyridine was used as a mono-
mer resulting in a covalent triazine-based framework (CTF)
with numerous bipyridyl motifs. Synthesis and structure of the
material with possible platinum coordination sites resembling
those of the molecular [Pt ACHUTNGRNEGUN( bpym)] catalyst are described else-
[30]
where.
The materials were characterized by physicochemical tech-
niques. Nitrogen sorption analysis of PBI reveals a type IV iso-
therm, corresponding to a mainly mesoporous material with a
3
ꢀ1
total pore volume of 0.19 cm g and little porosity in the mi-
3
ꢀ1
cropore region of about 0.01 cm g . Brunauer–Emmett–Teller
BET) analysis reveals a specific surface area of around
(
2
ꢀ1
1
12 m g and Barrett–Joyner–Halenda (BJH) analysis of the
Scheme 1. Possible deactivation and degradation pathway.
adsorption branch shows an average pore size of 9 nm, both
in agreement with the structural properties reported earli-
[
26,28]
er.
here since the isotherm shape suggested critical instability at
The desorption branch, which is normally used, was not
Heterogeneous molecular catalysts for direct methane acti-
vation
[30]
p/p0 close to 0.4. As reported previously,
CTF revealed a
Limited progress has been made concerning solid catalysts for
the direct low-temperature oxidation of methane to methanol.
Heterogeneously catalyzed reactions were almost exclusively
type I isotherm corresponding to a microporous material with
2
ꢀ1
a specific surface area of 1061 m g , a pore volume of
3
ꢀ1
0.93 cm g , and an average micropore diameter of 1.4 nm. X-
ray diffraction measurements (XRD) of both materials indicated
an amorphous structure, although, in the case of CTF, short-
range ordering cannot be excluded. X-ray photoelectron spec-
troscopy (XPS) measurements revealed a significantly lower ni-
trogen content of the PBI network compared to the CTF mate-
rial, with C/N ratios of 6.1 and 3.2, respectively.
[
20,21]
studied at temperatures >2508C. Basic oxides,
transition
or encapsulated Fe-phthalocyanine com-
plexes were reported, showing poor selectivity due to over-
[
7,22]
metal oxides,
[
8]
[23]
oxidation, and maximum methanol yields of around 5%.
Consequently, the main challenges for this reaction appear to
be increasing selectivity and minimizing overoxidation.
Knowledge gained from the investigation of homogeneous
molecular catalysts for the direct low-temperature oxidation of
Platinum modification of the materials was carried out using
two approaches: through precoordination of platinum in aque-
ChemSusChem 2010, 3, 277 – 282
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
279

F. Schꢀth et al.
centrations obtained for the
three consecutive cycles with Pt-
PBI as the catalyst. Although Pt-
PBI reached superior TONs, after
the first run, methanol produc-
tivity decreased likely due to sig-
nificant Pt leaching under the re-
action conditions.
Energy dispersive X-ray spec-
troscopic analysis conducted by
means of scanning electron mi-
croscopy (i.e., SEM/EDX), sup-
ports the notion of a homogene-
ous platinum distribution within
the material (Figure 3). Before
the application of Pt-PBI, the Pt
content amounts to 8.3 wt.%;
however, the material loses
almost all of the Pt within three
catalytic runs (i.e., <0.4 wt.%).
Accordingly, XPS measurements
showed a N/Pt ratio of 3.4 prior
to catalysis with binding ener-
gies of 72.12 and 72.7 eV for the
Scheme 2. Polycondensation of 3,3’-diaminobenzidine, diphenyl isophthalate, and benzene-1,3,5-tricarboxylic acid
triphenyl ester to form a polybenzimidazole network (PBI) and possible platinum coordinations and in the materi-
al.
II
IV
Pt and Pt species, respectively,
ous solution prior to catalysis; and through an in situ pathway,
[
a]
Table 2. Catalytic activity of Pt-modified PBI and CTF catalysts.
which involved mixing the material and the platinum precursor
together in the reaction mixture. For both pathways, K [PtCl ]
[
a]
[b]
AHCTUNGTRENNUNG
2
4
Catalyst
Pt conc.
Final MeOH conc.
TON
ꢀ
1
ꢀ1
served as the platinum precursor. Precoordinated materials
were denoted Pt-CTF and Pt-PBI, while in situ formed catalysts
A[ mmolL
H
U
G
R
N
N
]
A[ molL
H
U
G
E
N
N
]
[
Pt
[Pt
Pt-PBI
A
T
N
T
N
U
G
4
10
1.7
3.23
7.25
6.93
1.49
1.65
0.95
1.13
1.54
1.80
355
158
559
338
201
246
were termed K
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[PtCl ]-CTF and K
A
H
U
G
R
N
N
[PtCl ]-PBI. The platinum-
A
H
U
G
R
N
U
2
4
2
4
[
c]
modified materials were tested as catalysts in the direct low-
temperature oxidation of methane to methanol in concentrat-
ed sulfuric acid according to the conditions described earlier.
[
d]
K
K
2
A
H
U
G
R
N
U
G
4
4
]-PBI
[e]
2
A
H
U
G
R
N
U
G
]-CTF
[f]
Pt-CTF
[
30]
As reported recently, we identified Pt-CTF and K2 ACHTUNGTERNNUN[G PtCl ]-
4
[
a] Reaction conditions: 15 mL H
2
SO
4
3 4
(30% SO ), 40 bar CH pressure
CTF as highly promising systems, showing on the one hand
comparable activity to the molecular Periana catalyst, and on
the other hand to each other, independent of the platinum co-
ordination pathway (Table 2). Pt-CTF benefited from an activa-
tion effect, which results in low catalytic activity for the very
first run, but significantly increased activity for all subsequent
runs. Pt-PBI, however, appeared to be superior in terms of cat-
alytic activity, reaching TONs of up to 340 for in situ coordinat-
(
258C), at 2158C for 2.5 h. [b] TONs based on the platinum content deter-
[PtCl ] and
mined from SEM-EDX. [c] 60.5 mg Pt-PBI. [d] 20.1 mgK2
0.2 mg PBI. [e] 92 mg CTF with 47.5 mgK [PtCl ]. [f] Data from the 2
run with 62 mg Pt-CTF.
ACHTUNGTRENNUNG
4
nd
4
2
A
H
U
G
R
N
U
G
4
which corresponds to atomically coordinated platinum. After
three runs however, the platinum content went below the de-
tection limit. Also, TEM/EDX measurements suggested a homo-
geneous distribution of atomically distributed platinum within
the material. However, platinum nanoparticles appeared to be
present within the material (Figure 4), although they can only
account for a minor fraction of the total platinum content.
ed K2 ACHTUNGTRENNUNG[ PtCl ]-PBI and even up to 550 for precoordinated Pt-PBI.
4
Thus, comparable TONs for the newly developed molecular
DACbz-Pt complex could be reached. Methanol (after hydroly-
sis) was found to be the main product, with selectivites >75%
for all the materials. CO was identified as the major byproduct
2
0
of the gas phase by FTIR analysis. Methane conversions are in-
cluded in the Supporting Information. Table 2 summarizes the
catalytic activity of the solid catalysts in comparison to the
conventional Periana system.
Hardly any indications of Pt species or platinum crystallites
were found in the XPS or XRD measurements, respectively.
The origin of the strong leaching of this material is not fully
clear, but may be related to an instable Pt coordination within
the material under the reaction conditions. Possible explana-
tions may be related to the relatively low nitrogen content of
the PBI material. XPS measurements gave a C/N ratio of only
High single-run activity is not the deciding factor concerning
a solid catalyst; recyclability and stability over several cycles
are more important. Figure 2 illustrates the final methanol con-
2
80
www.chemsuschem.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2010, 3, 277 – 282

Direct Low-Temperature Oxidation of Methane to Methanol
Figure 2. Methanol productivity of Pt-PBI in three consecutive catalytic runs
(Reaction conditions: 15 mL H
2 4 3 4
SO (30% SO ), 60.5 mg Pt-PBI, 40 bar CH
pressure (258C), at 2158C for 2.5 h).
Figure 4. TEM images of Pt-PBI prior to catalysis.
2
). However, the lack of a chelating ligand may also result in
low stability of the platinum coordination. In agreement with
the latter assumption, in situ coordination to form K2[PtCl ]-PBI
did not result in stable coordination of Pt. In contrast, both Pt-
CTF and K [PtCl ]-CTF exhibited stable catalytic activity over
AHCTUNGTRENNUNG
4
AHCTUNGTRENNUNG
2
4
several runs, showing more than 250 turnovers even after six
[30]
cycles, and only little decrease in platinum loading. Concern-
ing the relationship between electron density and catalytic ac-
tivity, the platinum coordination in Pt-CTF is suggested to be
comparable to that of [Pt
However Pt-PBI exhibits higher electron density compared to
the electron deficient [Pt(bpym)] complex, and presumably
ACHTUNGTRENNUNG( bpym)] resulting in similar activities.
Figure 3. SEM image of Pt-PBI material and corresponding EDX Pt mapping.
AHCTUNGTRENNUNG
even compared to DACbz-Pt; the carbon in the aromatic ring
that replaced the chlorine ligand, leads to a higher electron
density on the metal centre compared to a complex with two
chlorine ligands. Nevertheless, DFT calculations should be car-
ried out for further investigations.
6
.1, whereas CTF exhibited a value of 3.2. In contrast to the
stable bidentate complex formed for [Pt(bpym)], a comparable
AHCTUNGTRENNUNG
platinum coordination within the PBI structure would most
probably exhibit low stability due to the unfavorable bite
angle between the two coordinating nitrogens. This could also
be reflected by the observed platinum nanoparticle formation
within the material structure which would be facilitated by a
less stable Pt coordination. Besides, the formation of a pincer-
type coordination as described by Fossey et al., 1 is likely to
Current investigations are aiming to develop and optimize
the homogeneous and heterogeneous catalytic systems to-
wards superior activity and stability, which may open up po-
tential for industrial applications. Three main points are being
addressed: 1) tuning of the electron density on the Pt through
ligand modification of homogeneous catalysts to further sub-
stantiate conclusions on structure–activity relationship; 2) de-
velopment of a wider range of solid catalysts on the basis of
polymeric materials with sufficient stability and platinum coor-
dination; 3) combination of both approaches by preparation of
solid catalysts with optimized electron density and stability of
the incorporated platinum.
occur, resulting in
a
very stable platinum coordination
[
31]
(Scheme 2). Under the reaction conditions, however, the co-
ordinating carbon of the aromatic ring system is protonated,
which leads to cleavage of the carbon–platinum bond, and
thus causing an unstable bidentate ligand with an unfavorably
wide bite angle. Alternatively, a platinum coordination be-
tween two different polymer branches may occur (Scheme 2,
ChemSusChem 2010, 3, 277 – 282
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
281

F. Schꢀth et al.
[
3] M. Muehlhofer, T. Strassner, W. A. Herrmann, Angew. Chem. 2002, 114,
Experimental Section
1817–1819; Angew. Chem. Int. Ed. 2002, 41, 1745–1747.
The synthesis of N-(2-methylpropyl)-4,5-diazacarbazolyl-dichloro-
platinum(II) was based on a Buchwald–Hartwig cross coupling re-
action of 2-chloro-3-iodpyridine and 3-amino-2-chlorpyridine, fol-
lowed by an alkylation with 2-methylpropylbromide, an intramo-
[4] D. de Vos, B. F. Sels, Angew. Chem. 2005, 117, 30–32; Angew. Chem. Int.
Ed. 2005, 44, 30–32.
[
5] a) R. A. Periana, D. J. Taube, E. R. Evitt, D. G. Loffler, P. R. Wentrcek, G.
Voss, T. Masuda, Science 1993, 259, 340–343; b) U.S. Patents 5233113
and 5306855.
lecular Yamamoto coupling, and complexation with K ACHTUNGRTNEUNG[ PtCl ]. A de-
2 4
[
[
[
6] A. M. Maitra, Appl. Catal. A: Gen. 1993, 104, 11–59.
7] O. V. Krylov, Catal. Today 1993, 18, 209–302.
8] R. Raja, P. Ratnasamy, Appl. Catal. A: Gen. 1997, 158, 7–15.
tailed synthesis is described in the Supporting Information. The [Pt-
ACHTUNGTRENNUNG( bpym)] complex, PBI, and CTF-based materials were synthesized
[15,27–30]
as described elsewhere.
For platinum coordination, PBI
[PtCl ] (64 mg), or CTF (170 mg) and K2 A[ PtCl ]
340 mg), respectively, were reacted in water for 4 h at 608C, fil-
[9] B. L. Conley, W. J. Tenn, K. J. H. Young, S. K. Ganesh, S. K. Meier, V. R. Ziat-
dinov, O. Mironov, J. Oxgaard, J. Gonzales, W. A. Goddard, R. A. Periana,
J. Mol. Catal. A: Chem. 2006, 251, 8–23.
(
126 mg) and K
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
CHTUNGTRENNUNG
2
4
4
(
[
[
[
10] L. A. Kushch, V. V. Lavrushko, Yu. S. Misharin, A. P. Moravsky, A. E. Shilov,
Nouv. J. Chim. 1983, 7, 729.
11] A. E. Shilov, Activation of Saturated Hydrocarbons by Transition Metal
Complexes, Reidel, Dordrecht, Netherlands, 1984.
12] C. J. Jones, D. Taube, V. R. Ziatdinov, R. A. Periana, R. J. Nielsen, J. Ox-
gaard, W. A. Goddard, Angew. Chem. 2004, 116, 4726–4729; Angew.
Chem. Int. Ed. 2004, 43, 4626–4629.
tered, washed with water, and dried overnight at 908C. Catalytic
tests were carried out in a 50 mL stainless steel autoclave with a
Teflon insert. The autoclave was filled with 15 mL (28.88 g;
2
95 mmol) oleum (30% SO ) and catalyst (50–70 mg ), closed, and
3
flushed with argon. In the case of K ACHTUNGTRENNUG[N PtCl ]-PBI, K ACHTUNGTRENNUN[G PtCl ] (20.1 mg)
2 4 2 4
and PBI (40.2 mg), or in the case of K ACTHUNGTRNEUNG[ PtCl ]-CTF, CTF (92 mg ) and
2 4
K2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[PtCl ] (48 mg), were placed in the autoclave. The autoclave was
4
[13] M. Muehlhofer, T. Strassner, W. A. Hermann, Angew. Chem. 2002, 114,
1817–1819; Angew. Chem. Int. Ed. 2002, 41, 1745–1747.
[14] D. Meyer, M. A. Taige, A. Zeller, K. Hohlfeld, S. Ahrens, T. Strassner, Orga-
nometallics 2009, 28, 2142–2149.
pressurized with CH (40 bar), heated to 2158C, left for 2.5 h, and
4
cooled to room temperature. The pressure was released slowly to
prevent foam formation. The reaction solution was filtered through
a glass frit to remove the solid catalyst, hydrolyzed by refluxing in
[
[
[
[
15] R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh, H. Fujii, Science
998, 280, 560–564.
16] W. J. Tenn, K. J. H. Young, J. Oxgaard, R. J. Nielsen, W. A. Goddard, R. A.
Periana, Organometallics 2006, 25, 5173–5175.
1
3
0 mL water for 4 h, and analyzed by HPLC. Methanol selectivity
was estimated based on methanol formation and pressure drop
during the reaction, while FTIR analysis of the released gas phase
was used to determine the byproducts. In every recycling step,
about 5–10 wt% of the catalyst material was lost in the frit during
filtration, which was considered in the calculation of the catalytic
activity. The TONs were calculated based on the ratio of produced
methanol to the platinum content of the catalyst (molMeOH/molPt).
Platinum contents were determined by SEM-EDX (Pt-CTF and K2-
ACHTUNGTRENNUNG[ PtCl ]-CTF, runs 2–6) or was based on the used platinum amount
4
(K ACHTUNGTRENNUNG[ PtCl ]-CTF, first run). The CTF material was characterized using
2 4
nitrogen sorption measurements, XRD, TEM, and XPS. The plati-
num-containing catalysts were characterized using TEM, SEM/EDX,
XPS, and XRD.
17] J. M. Gonzales, J. Oxgaard, W. A. Goddard, R. A. Periana, Organometallics
2
007, 26, 1505–1511.
18] K. J. H. Young, S. K. Meier, J. M. Gonzales, J. Oxgaard, W. A. Goddard,
R. A. Periana, Organometallics 2006, 25, 4734–4737.
[19] R. A. Periana, G. Bhalla, W. J. Tenn, K. J. H. Young, X. Yang Liu, O. Miro-
nov, C. J. Jones, V. R. Ziatdinov, J. Mol. Catal. A: Chem. 2004, 220, 7–25.
[
20] M. Baerns, J. R. H. Ross, in Perspectives in Catalysis (Eds.: J. A. Thomas,
K. I. Zamaraev), Blackwell, Oxford, 1992.
[
[
21] O. Forlani, S. Rossini, Mater. Chem. Phys. 1992, 31, 155–158.
22] H. D. Gesser, N. R. Hunter, in Direct Methane Conversion by Oxidative Pro-
cesses (Ed.: E. E. Wolf), Van Nostrand Reinhold, New York, 1993.
23] D. Wolf, Angew. Chem. 1998, 110, 3545–3547; Angew. Chem. Int. Ed.
[
[
1
998, 37, 3351–3353.
24] A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.-O. Mꢀller, R.
Schlçgl, J. M. Carlsson, J. Mater. Chem. 2008, 18, 4893–4908.
Acknowledgements
[25] X. Wang, X. Chen, A. Thomas, X. Fu, M. Antonietti, Adv. Mater. 2009, 21,
609–1612.
26] J. Weber, M. Antonietti, A. Thomas, Macromolecules 2007, 40, 1299–
304.
27] A. Thomas, F. Goettmann, M. Antonietti, Chem. Mater. 2008, 20, 738–
755.
1
[
[
[
This work was supported by the Project House “ENERCHEM” of
the Max Planck Society. We thank Mr. B. Spliethoff for the TEM
measurements, Ms. S. Palm for the SEM measurements, and Dr.
C. Weidenthaler for the XRD and XPS measurements and the
helpful discussions.
1
28] P. Kuhn, M. Antonietti, A. Thomas, Angew. Chem. 2008, 120, 3499–3502;
Angew. Chem. Int. Ed. 2008, 47, 3450–3453.
[
[
29] P. Kuhn, A. Thomas, M. Antonietti, Macromolecules 2009, 42, 319–326.
30] R. Palkovits, M. Antonietti, P. Kuhn, A. Thomas, F. Schꢀth, Angew. Chem.
2
009, 121, 7042–7045; Angew. Chem. Int. Ed. 2009, 48, 6909–6912.
Keywords: heterogeneous catalysis · homogeneous catalysis ·
oxidation · platinum · polymers
[
31] J. S. Fossey, C. J. Richards, Organometallics 2002, 21, 5259–5264.
[1] Statistical Review of World Energy 2008, BP.
Received: May 28, 2009
[2] G.-J. ten Brink, I. W. C. E. Arends, R. A. Sheldon, Science 2000, 287, 1636–
Revised: August 7, 2009
1639.
Published online on September 24, 2009
2
82
www.chemsuschem.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2010, 3, 277 – 282