Solid catalysts for the selective low-temperature oxidation of methane to methanol

Full text of DOI:10.1002/anie.200902009

Angewandte
Chemie
DOI: 10.1002/anie.200902009
Methane Oxidation
Solid Catalysts for the Selective Low-Temperature Oxidation of
Methane to Methanol**
Regina Palkovits, Markus Antonietti, Pierre Kuhn, Arne Thomas, and Ferdi Schꢀth*
The development of catalyst systems for the direct low-
temperature oxidation of methane to methanol has been one
which made them appear promising as a solid matrix for
methane oxidation along the lines of Perianaꢀs work for
liquid-phase conditions. Utilizing 2,6-dicyanopyridine as
monomer, a covalent triazine-based framework (CTF) with
numerous bipyridyl structure units is accessible, which should
allow coordination of platinum and resemble the coordina-
tion sites for platinum coordination in the molecular Periana
catalyst (Scheme 1).
[1–8]
of the major challenges in catalysis over the last decades.
ꢀ
1
The high binding energy of the CH ꢀH bond (435 kJmol )
3
together with the ease of overoxidation to form CO require
2
not only a highly active but also a highly selective catalyst
[
9]
system to tackle this reaction. In the past, various inves-
[
10–16]
tigations addressed this challenge.
However, the catalysts
mostly suffered from irreversible reduction and bulk metal
formation, together with consequently poor selectivity.
The CFT material was characterized with physicochem-
ical techniques. Nitrogen sorption analysis of CTF reveals a
type I isotherm corresponding to a microporous material with
[
5,6,13]
Some palladium, gold, and mercury complexes with superior
stability initially appeared to be promising but still suffer from
2
ꢀ1
a specific surface area of 1061 m g , a pore volume of
ꢀ
1
3
ꢀ1
turnover frequencies (TOFs) below 1 h . In the field of
heterogeneous catalysis, nearly all reported investigations
0.934 cm g , and an average micropore diameter of 1.4 nm as
determined by nonlocal DFT analysis. Pore volume and
specific surface area are somewhat higher than reported in the
[
7,17]
involve temperatures far above 2508C over basic oxides,
[
13]
[23,24]
transition-metal oxides, and iron complexes encapsulated
in zeolites. All these catalysts showed poor selectivity
owing to overoxidation, and maximum methanol yields were
initial publications on this material by Kuhn et al.
[
16]
Although CTF materials based on 1,4-dicyanobenzene
exhibit some regularity, X-ray diffraction measurements of
the material based on 2,6-dicyanopyridine indicate a pre-
dominantly amorphous structure, and the material has at most
short-range ordering. In line with this finding, TEM micro-
graphs support the amorphous nature of the CTF, with pores
in the micropore range and neither long-range nor short-
range order.
[7,18]
around 5%.
Promising progress in molecular catalysis, however, has
recently been made by Periana et al., who demonstrated the
selective low-temperature oxidation of methane at temper-
atures around 2008C over platinum bipyrimidine complexes
[
19–22]
in concentrated sulfuric acid.
Methane conversions
above 90% at 81% selectivity to methylbisulfate were
reached. However, despite these promising results, commer-
cial application seems to be hampered by difficult separation
and recycling of the molecular catalyst.
For modification with platinum, two different routes were
chosen, either an in situ pathway by simply combining CTF
and the platinum precursor in the reaction mixture for the
methane oxidation reaction (K [PtCl ]-CTF), or by pre-
2
4
We report herein on the development of solid catalysts for
the direct low-temperature oxidation of methane to methanol
reaching high activity at high selectivity and stability over
several recycling steps, which could provide a breakthrough
for this reaction. The development is based on the recent
discovery of a new class of high-performance polymer
frameworks that are formed by the trimerization of aromatic
coordination of platinum (Pt-CTF) in a separate step.
The platinum-modified material was tested in the direct
methane oxidation in concentrated sulfuric acid according to
[
19]
the conditions described by Periana et al.
In principle,
utilization of sulfuric acid and sulfur trioxide as oxidants, as
schematically described in Equations (a)–(d), would allow
design of a continuous process. All process steps, including
methane oxidation to methyl bisulfate (a), hydrolysis to form
free methanol (b), and reoxidation of SO2 (c) could be
integrated in such a system. A solid catalyst, with its
advantages of easy separation and recyclability, would
facilitate the implementation of such processes to allow
efficient conversion of natural gas on-site.
[23,24]
nitriles in molten ZnCl2.
The materials are thermally
stable up to 4008C and resist strongly oxidizing conditions,
[
*] Dr. R. Palkovits, Prof. Dr. 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
CH
4
þ H
2
SO
4
þ SO
3
! CH
3
OSO
3
H þ H
2
O þ SO ðaÞ
2
Prof. Dr. M. Antonietti, Dr. P. Kuhn, Dr. A. Thomas
Max-Planck-Institut fꢀr Kolloid- und Grenzflꢁchenforschung
Am Mꢀhlenberg 1, 14476 Potsdam-Golm (Germany)
CH OSO H þ H O ! CH OH þ H SO ðbÞ
3
3
2
3
2
4
[
**] This work was supported by the Project House “ENERCHEM” of the
Max Planck Society. We thank B. Spliethoff (MPI fꢀr Kohlenfor-
schung) for TEM measurements, S. Palm for SEM measurements,
and Dr. C. Weidenthaler for XRD and XPS measurements and for
helpful discussions.
SO þ 1=2 O ! SO ðcÞ
2
2
3
SCH þ 1=2 O ! CH OH ðdÞ
4
2
3
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Scheme 1. a) Trimerization of 2,6-dicyanopyridine (DCP) in molten ZnCl , conversion to a covalent triazine-based framework (CTF), and
2
subsequent platinum coordination (Pt-CTF); b) Periana’s platinum bipyrimidine complex.
In a typical reaction, the catalyst is mixed with the oleum
and the reactor is then sealed and pressurized with methane.
The pressure in the vessel gradually decreases, suggesting
consumption of methane. After workup of the reaction
mixture, the formed methanol is analyzed to quantify the
activity. Interestingly, the achieved methanol concentrations
in the final reaction mixtures and the turnover numbers
(
TON) proved to be rather similar for all three systems (Pt-
Figure 1. Catalytic activity of a) Pt-CTF and b) K [PtCl ]-CFT in the
direct oxidation of methane to methanol over several recycling steps n.
2
4
CTF, K [PtCl ]-CTF, and the molecular Periana catalyst;
2
4
Table 1). For all samples, selectivities for methanol formation
above 75% could be reached. The major by-product,
determined by FTIR analysis of the gas phase, is CO2.
with some intensity for Zn and Cl from material synthesis.
After platinum coordination, XPS measurements of Pt-CTF
reveal N/Pt ratios of 4.7:1 prior to catalysis, 4:1 after the first
run, and 4.25:1 after the fifth run, indicating stable Pt
coordination within the material even after several recycling
steps.
Table 1: Catalytic activity of the molecular Periana catalyst and the
heterogeneous Pt-CFT and K [PtCl ]-CFT catalysts in methane oxidation.
2
4
[
a]
ꢀ1
[b]
Catalyst
Final methanol conc. [molL
]
TON
[
[
c]
Periana catalyst
Periana catalyst
1.65
1.49
1.54
1.80
158
355
201
246
Combined TEM/EDX analysis (EDX = energy-dispersive
X-ray spectroscopy) also supports a very homogenous
platinum loading without detectable platinum clusters or
nanoparticles within the Pt-CTF catalyst (Figure 2). Corre-
d]
[
e]
K [PtCl ]-CTF
2
4
[
f]
Pt-CTF
II
[
a] Reaction conditions: 15 mL H SO (30% SO ), 40 bar CH pressure
2 4 3 4
sponding to atomically coordinated platinum, Pt is the
(
258C), 2.5 h at 2158C. [b] TON based on the platinum content
determined from SEM/EDX. [c] 65 mg Periana catalyst. [d] 26 mg
Periana catalyst. [e] 48 mg CTF with 92 mg K [PtCl ]. [f] Data from the
2
4
second run with 62 mg Pt-CTF.
Although these results are promising, single-run activity is
obviously not the decisive point for a heterogeneous catalytic
system, but rather recyclability and stable catalytic activity of
the material. Pre-coordination of platinum in the CTF
material prior to catalysis leads to the Pt-CTF catalyst,
which shows very stable activity over several runs with TONs
above 250 (Figure 1a). Surprisingly, the material exhibits very
low activity in the first catalytic cycle, reaching a TON of only
2
6, which increases to stable values above 250 for subsequent
cycles. The nature of the activation process is not yet fully
clear but may be related to rearrangement of platinum species
within the samples, resulting in formation of the active species
under reaction conditions.
X-ray photoelectron spectroscopy (XPS) analysis of the
CFT material reveals an average C/N ratio of 3.2:1 together
Figure 2. TEM micrographs of a) pure CTF, b) Pt-CTF from pre-coordi-
nation of platinum prior to catalysis, c) Pt-CTF after the first catalytic
run, and d) Pt-CTF after utilization of the material in five catalytic runs.
6
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predominant species detected by XPS with above 95% of the
total intensity. XRD and TEM, however, additionally suggest
the presence of a low concentration of extraframework Pt in
the form of agglomerated Pt nanoparticles outside the
polymeric material. These particles have diameters of 2–
Notably, for both types of catalyst, after several runs the
chlorine concentration in the samples is reduced to below the
detection limit of the EDX system, suggesting that chlorido
ligands may not be necessary in the catalytic cycle.
0
In summary, we have shown that highly active solid
catalysts for methane oxidation by SO3 in concentrated
sulfuric acid can be produced, and that these systems are
stable over at least five recycling steps. This finding could
reinvigorate research into such a process for commercial
exploitation, which had already been scaled up to pilot scale
for the molecular Pt bipyrimidine system, and thus make a
small-scale methane activation process viable.
3
nm and are already present in the sample after Pt
coordination prior to catalysis.
Interestingly, simple addition of CTF and K [PtCl ] as
2
4
platinum precursor in the catalytic reaction leads to formation
of a very stable K [PtCl ]-CTF catalyst as well. The material
2
4
exhibits not only high activity from the very first catalytic run
but also little deactivation, with TONs around 300 even after
five recycling steps (Figure 1b). XPS analysis indicates that
the material possesses a somewhat lower amount of incorpo-
rated platinum than Pt-CTF, reaching a N/Pt ratio of about Experimental Section
.6:1 after six catalytic runs, which is due to the overall lower
K [PtCl ] concentration utilized for the catalyst formed
6
Platinum bipyrimidine complexes and triazine-based materials were
[
4,7]
synthesized as described elsewhere.
(170 mg) and K [PtCl ] (340 mg) were reacted in water for 4 h at
08C, filtered, washed with water, and dried overnight at 908C.
For Pt coordination, CTF
2
4
II
2
4
in situ. In this catalyst, exclusively Pt species were detected,
corresponding to platinum coordinated in the fashion sug-
gested in Scheme 1. TEM and SEM micrographs, together
with EDX analysis, support the notion of homogeneously
distributed platinum within the polymer matrix throughout
the whole sample (Figure 3).
6
Catalytic tests were carried out in a 50 mL stainless steel autoclave
with a Teflon insert. The autoclave was filled with oleum (30% SO3,
15 mL) and catalyst (50–70 mg), closed, and flushed with argon. In the
case of K
and K [PtCl ] (48 mg). The reactor was pressurized with 40 bar CH
4
and heated to 2158C, kept there for 2.5 h, and cooled down to room
temperature. The pressure has to be released slowly to prevent foam
formation. The reaction solution was filtered using a glass frit. The
[PtCl ]-CTF, the autoclave was charged with CTF (92 mg)
2
4
2
4
catalyst was rinsed with water to remove most of the remaining H SO4
2
and dried at 908C prior to recycling. The reaction solution was
hydrolyzed by heating at reflux in water for 4 h and analyzed by
HPLC. Selectivity for methanol was estimated on the basis of
methanol formation and pressure drop during the reaction, while
FTIR analysis of the released gas phase was utilized to determine
byproducts. In each recycling step, about 5–10 wt% of the catalytic
material could not be recovered, which has been considered in
calculation of the catalytic activity. Turn over numbers (TONs) were
calculated on the basis of the ratio of produced methanol and the
platinum content of the catalyst (molMeOH/mol ). Platinum contents
Pt
determined by SEM-EDX were used (Pt-CTF and K [PtCl ]-CTF
2
4
run 2–6), or the content was based on the amount of platinum utilized
K [PtCl ]-CTF, first run). The CTF material has been characterized
(
2
4
using nitrogen sorption measurements, XRD, TEM, and XPS
analysis. The catalysts have been characterized using TEM, SEM/
EDX, XPS, and XRD.
Received: April 15, 2009
Revised: July 1, 2009
Published online: August 4, 2009
Keywords: methane · methanol · oxidation · Periana catalyst ·
.
triazines
[
[
1] R. Pitchai, K. Klier, Catal. Rev. Sci. Eng. 1986, 28, 13 – 88.
2] H. D. Gesser, N. R. Hunter in Methane Conversion by Oxidative
Processes (Ed.: E. E. Wolf), Van Nostrand Reinhold, New York,
1992, p. 403.
[
3] M. G. Axelrod, A. M. Gaffney, R. Pitchai, J. A. Sofranko in
Natural Gas Conversion II (Eds.: H. E. Curry-Hyde, R. F.
Howe), Elsevier, Amsterdam, 1994, p. 93.
[
4] J. H. Lunsford, Catal. Today 2000, 63, 165 – 174.
Figure 3. Top: SEM micrograph of the K [PtCl ]-CFT material after five
recycling steps; bottom: corresponding EDX Pt mapping.
[5] P. M. Henry, Palladium Catalyzed Oxidation of Hydrocarbons,
Reidel, Dordrecht, 1980.
2
4
Angew. Chem. Int. Ed. 2009, 48, 6909 –6912
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6911

Communications
[
[
6] A. E. Shilov, Activation of Saturated Hydrocarbons by Transition
[16] R. Raja, P. Ratnasamy, Appl. Catal. A 1997, 158, L7L15.
[17] O. Forlani, S. Rossini, Mater. Chem. Phys. 1992, 31, 155 – 158.
[18] D. Wolf, Angew. Chem. 1998, 110, 24; Angew. Chem. Int. Ed.
1998, 37, 24.
[19] R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh, H.
Fujii, Science 1998, 280, 560 – 564.
[20] C. J. Jones, D. Taube, V. R. Ziatdinov, R. A. Periana, R. J.
Nielsen, J. Oxgaard, W. A. Goddard III, Angew. Chem. 2004,
116, 4726 – 4729; Angew. Chem. Int. Ed. 2004, 43, 4626 – 4629.
[21] R. A. Periana, G. Bhalla, W. J. Tenn, K. J. H. Young, X.
Yang Liu, O. Mironov, C. J. Jones, V. R. Ziatdinov, J. Mol.
Catal. A 2004, 220, 7 – 25.
Metal Complexes (Reidel, Dordrecht, Netherlands, 1984).
7] M. Baerns, J. R. H. Ross in Perspectives in Catalysis (Ed.: J. A.
Thomas, K. I. Zamaraev), Blackwell, Oxford, 1992.
8] H. Crabtree, Chem. Rev. 1995, 95, 987 – 1007.
9] CRC Handbook of Chemistry and Physics, 89th ed., (Internet
Version 2009; Ed. D. R. Lide), CRC Press/Taylor & Francis,
Boca Raton.
[
[
[
10] G.-J. ten Brink, I. W. C. E. Arends, R. A. Sheldon, Science 2000,
87, 1636 – 1639.
11] M. Muehlhofer, T. Strassner, W. A. Herrmann, Angew. Chem.
2
[
2002, 114, 1817 – 1819; Angew. Chem. Int. Ed. 2002, 41, 1745 –
1
747.
[22] B. L. Conley, W. J. Tenn, K. J. H. Young, S. K. Ganesh, S. K.
Meier, V. R. Ziatdinov, O. Mironov, J. Oxgaard, J. Gonzales,
W. A. Goddard, R. A. Periana, J. Mol. Catal. A 2006, 251, 8 – 23.
[23] P. Kuhn, M. Antonietti, A. Thomas, Angew. Chem. 2008, 120,
3499 – 3502; Angew. Chem. Int. Ed. 2008, 47, 3450 – 3453.
[24] P. Kuhn, A. Thomas, M. Antonietti, Macromolecules 2009, 42,
319- 326.
[
12] D. de Vos, B. F. Sels, Angew. Chem. 2005, 117, 30 – 32; Angew.
Chem. Int. Ed. 2005, 44, 30 – 32.
13] R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh, H.
Fujii, Science 1993, 259, 340 – 343; U.S. Patent 5233113 and U.S.
Patent 5306855.
[
[
[
14] A. M. Maitra, Appl. Catal. A 1993, 104, 11 – 59.
15] O. V. Krylov, Catal. Today 1993, 18, 209 – 302.
6
912
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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