Low-temperature oxygenation of methane into formic acid with molecular oxygen in the presence of hydrogen catalysed by Pd0.08Cs2.5H1.34PVMo11O40

Article abstract of DOI:10.1039/a702514i

The title reaction is catalysed by Pd_0.08Cs_2.5H_1.34PVMo₁₁O₄₀ at temperatures as low as 423-593 K, and the catalytic rate, which reached a maximum at 573 K, is 1.2 × 10^-4 mol h^-1 g^-1, ca. 3 × 10² times higher than FePO₄, the most active catalyst previously reported.

Full text of DOI:10.1039/a702514i

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Low-temperature oxygenation of methane into formic acid with molecular
oxygen in the presence of hydrogen catalysed by Pd0.08Cs2.5H1.34PVMo11O40
Noritaka Mizuno,*a Hiromi Ishige,a Yasuhiro Seki,a Makoto Misono,a Don-Jin Suh,b Wonchull Hanb and
Tetsuichi Kudob
a Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113,
Japan
b Institute of Industrial Science, The University of Tokyo, Minato-ku, Roppongi, Tokyo 106, Japan
The title reaction is catalysed by Pd0.08Cs2.5H1.34PVMo11O40
at temperatures as low as 423–593 K, and the catalytic
rate, which reached a maximum at 573 K, is 1.2 3 1024
mol h21 g21, ca. 3 3 102 times higher than FePO4, the
most active catalyst previously reported.
vol% methane, 14 vol% O2 with the balance N2, and the total
flow rate was 22 cm3 min21. The gases at the outlet of the
reactor were analysed by FID (equipped with methanizer) and a
TCD gas chromatograph. Selectivity was calculated on the C1
(methane)-basis. The products were formic acid, methanol and
COx. Similar products were observed for the other heteropoly
catalysts used. The conversion and the selectivity data were
collected after 2–5 h of reaction, when nearly steady-state
conversion and selectivity were obtained for each catalyst.
Fig. 1 shows the temperature dependency of the conversion
and selectivity for Pd0.08Cs2.5H1.34PVMo11O40. It is noted that
the reaction proceeded even at 423 K. The conversion of
methane monotonously increased with increase in the reaction
temperature and reached a maximum at 573 K. The rate for the
methane conversion at 573 K was 1.2 3 1024, ca. 3 3 102
greater than that of the most active heterogeneous catalyst
(FePO4) reported previously.15 The selectivity to formic acid
was > 67%. Such high selectivity towards formic acid has not
been reported previously in heterogeneous oxidation of meth-
ane.1–8 The highest yield of formic acid was obtained at 573 K.
Pd0.08Cs2.5H1.34PVMo11O40 also catalysed the oxidation of
ethane into acetic acid under similar conditions. Similar
conversion and selectivity with reaction temperature were
observed for Pd0.08Cs2.5H3.34PV3Mo9O40.
The activation and functionalization of methane has attracted
much attention owing to its abundance in natural gas and low
reactivity.1–8 Recently, there has been a renewed interest in the
direct conversion of methane into oxygenates.6–19 Various
catalysts have been tested with a variety of oxidants in
heterogeneous9–17 and homogeneous systems.18–20
The utilization of molecular oxygen for the oxidation of
methane is a rewarding goal since it has the highest content of
active oxygen and forms no by-products. However, direct
oxidation with molecular oxygen cannot be achieved using
heterogeneous catalysts under atmospheric pressure because the
high temperatures and pressures required, which induce radical
formation, needed for the activation of methane and/or oxygen
leads to overoxidation of the oxygenated products. Therefore, a
lowering of the reaction temperature is a key requirement for
selective oxidation. We have reported the direct oxidation of
lower alkanes with molecular oxygen catalysed by heteropoly
compounds, but oxidation of methane was unsuccessful.21
On the other hand, reductants such as aldehyde,22 hydrogen23
and zinc24 promoted the oxidation of alkanes and alkenes with
molecular oxygen and significantly lowered the reaction
temperature in homogeneous systems. However, concerning
catalytic oxygenation of methane with dioxygen in the presence
of reductants in heterogeneous systems, only the FePO4–O2–H2
system has been studied;15 Even in this system, a temperature as
high as 623 K was required which is much higher than for
homogeneous systems. Here we report the selective oxy-
genation of methane into formic acid with a O2–H2 mixture at
temperatures as low as 423–593 K.
An increase in the yield of formic acid was achieved by
further
V5+-substitution.
The
conversions
for
Pd0.08Cs2.5H0.34 + xPVxMo12 2 xO40 (x = 0, 1, 2 and 3) were
0.08, 0.08, 0.10 and 0.14%, respectively and increased with V5+
substitution. The selectivities to formic acid were 46, 70, 76 and
60% for x = 0, 1, 2 and 3, respectively, and the highest
selectivity to formic acid was observed at x ca. 2. It follows that
the substitution of V5+ for Mo6+ in Pd0.08Cs2.5H0.34PMo12O40
resulted in the enhancement of formic acid production and the
yield reached a maximum at x = 2–3.
100
10
H3+xPVxMo12 2 xO40 heteropolyacids were obtained com-
mercially from Nippon Inorganic Colour and Chemical Co.,
Ltd. The other reagents used were of analytical grade and used
without further purification.
8
80
60
40
20
0
The catalysts were prepared as follows: 3.6 cm3 of an
aqueous solution of palladium nitrate (0.08 mol dm23) was
added dropwise to 60.0 cm3 of an aqueous solution of
H4PVMo11O40 (0.06 mol dm23), followed by the addition of
56.3 cm3 of an aqueous solution of caesium carbonate (0.08 mol
dm23) at 323 K and the powder samples were carefully
collected and the resulting suspension evaporated to dryness at
323 K. The actual composition of the powder is
6
4
2
Pd0.08Cs2.5HyPVMo11Oz,
but
are
designated
Pd0.08Cs2.5H1.34PVMo11O40 according to the stoichiometry of
the starting materials.
Catalytic reactions were performed in a flow reactor. Prior to
the reaction, 120 mg of catalyst was treated in an O2 stream (60
cm3 min21) for 1 h at 573 K. The reactions were performed at
atmospheric pressure and the feed gas typically consisted of 28
0
373
473
573
673
T / K
Fig. 1 Temperature dependence of conversion and selectivity; catalyst,
Pd0.08Cs2.5H1.34PVMo11O40. Conversion of methane (5) and selectivities
towards formic acid (-), methanol (8), CO (Ω) and CO2 (2).
Chem. Commun., 1997
1295

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8 R. D. Srivastava, S. V. Gollakota, G. J. Stiegel and A. C. Bose, Methane
and Alkane Conversion Chemistry, Plenum, New York, 1995, p. 291.
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No selective oxidation proceeded in the absence of Pd or
when carbon monoxide was used instead of hydrogen. The
addition of 1.9 vol% hydrogen peroxide instead of a gaseous
mixture of H2 and O2 led to similar conversion and selectivity to
oxygenates. This suggests that the active oxygen species arises
from hydrogen peroxide formed by an H2–O2 reaction catalysed
by Pd. Reports that Pd in acidic medium catalyses the reaction
of dioxygen with dihydrogen to form hydrogen peroxide,25 and
the fact that heterpolyacids and their caesium salts are strongly
acidic26 supports this hypothesis. Formic acid is formed by the
liquid-phase oxidation of methane with hydrogen peroxide
catalysed by H4PVMo11O40;27 this suggests that successive
oxidation of methane via hydrogen peroxide can be catalysed by
heteropolyacids.
11 M. Faraldos, M. A. Banares, J. A. Anderson, H. Hu, I. E. Wachs and
J. L. G. Fierro, J. Catal., 1996, 160, 214.
12 C. T. Au, H. He, S. Y. Lai and C. F. Ng, J. Catal., 1996, 163, 399.
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15 Y. Wang and K. Otsuka, J. Catal., 1995, 155, 256.
16 W. Li and S. T. Oyama, Heterogeneous Hydrocarbon Oxidation, ACS,
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17 M. A. Banares, L. A. Alemany, F. M.-Jimenez, J. M. Blasco,
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18 N. Basickes, T. E. Hogan and A. Sen, J. Am. Chem. Soc., 1996, 118,
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This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science,
Sports and Culture of Japan.
19 Y. Fujiwara, K. Takai and Y. Taniguchi, Chem. Lett., 1996, 591.
20 A. Bagno, H. Bukala and G. A. Olah, J. Org. Chem., 1990, 55, 4284.
21 N. Mizuno, D.-J. Suh, W. Han and T. Kudo, J. Mol. Catal. A: Chem.,
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M. Iwamoto, Stud. Surf. Sci. Catal. B, 1996, 101, 1001.
22 S. Murahashi, Y. Oda and T. Naota, J. Am. Chem. Soc., 1992, 114,
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23 M. T. Atlay, M. Preece, G. Strukal and B. R. James, J. Chem. Soc.,
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24 I. Yamanaka, M. Soma and K. Otsuka, J. Chem. Soc., Chem. Commun.,
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25 Y. Izumi, H. Miyazaki and S. Kawahara, Jpn. Pat., 47121, 1981;
G. L. Wayne, Eur. Pat., 132294, 1983.
26 T. Okuhara, N. Mizuno and M. Misono, Adv. Catal., 1996, 41, 113.
27 Y. Seki, N. Mizuno and M. Misono, 72th Symposium of the Chemical
Society of Japan, Tokyo, March, 1997, 1D237.
Footnote
* E-mail: tmizuno@hongo.ecc.u-tokyo.ac.jp
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Received in Cambridge, UK, 14th April 1997; Com. 7/02514I
1296
Chem. Commun., 1997