Quinone tailored selective oxidation of methane over palladium catalyst with molecular oxygen as an oxidant

Article abstract of DOI:10.1039/b915412d

With the in situ generated H₂O₂ tailored by the addition of p-tetrachlorobenzoquinone, the product can be effectively steered towards either HCOOH or the methanol derivative CF₃COOCH₃ during the direct oxidation of methane with molecular oxygen over palladium catalyst.

Full text of DOI:10.1039/b915412d

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Quinone tailored selective oxidation of methane over palladium catalyst
with molecular oxygen as an oxidantw
Yafang Fan,z Zengjian An,z Xiulian Pan,* Xiumei Liu and Xinhe Bao*
Received (in Cambridge, UK) 29th July 2009, Accepted 22nd October 2009
First published as an Advance Article on the web 9th November 2009
DOI: 10.1039/b915412d
With the in situ generated H2O2 tailored by the addition of
p-tetrachlorobenzoquinone, the product can be effectively
steered towards either HCOOH or the methanol derivative
CF3COOCH3 during the direct oxidation of methane with
molecular oxygen over palladium catalyst.
efficiency is significantly improved with a TON as high as
120 hꢀ1 compared to our former catalyst system based on the
electron transfer chain.6 The roles of TCQ, in situ generated
H2O2 and Pd2+/Pd0 are discussed based on reaction tests and
characterizations.
The 1H and 13C NMR spectra of the liquid products of the
reaction11 under 1.8 MPa CH4 are shown in Fig. 1. The peaks
at 4.7 and 7.5 ppm in the 1H NMR spectrum were assigned to
DHO and CF3COOH, respectively. The one at about 4.1 ppm
was attributed to the methyl resonance of CF3COOCH3 while
8.2 ppm was attributed to *HCOOH. In the 13C NMR
spectrum, the peaks at 166.4 and 54.5 ppm corresponded to
Efforts have never ceased in the development of processes
to achieve the direct selective oxidation of methane to
oxygenates, although it remains a great challenge. Recent
research has shown that liquid phase methane conversion
catalyzed by metal cations such as Hg2+, Pt2+ and Pd2+ in
concentrated sulfuric acid can be a promising route in
comparison to high temperature gas–solid reaction.1–3
Palladium has also been widely used with an electron-transfer
mediator (ETM) for aerobic oxidations of C–H bonds in
the carboxyl peak of HCOOH and the methyl resonance of
1
CF3COOCH3, respectively. Those at 115.1 ppm (q. JCF
=
285.4 Hz) and 160.8 ppm (q. 2JCF = 41.8 Hz) were attributed
to the two carbon peaks of CF3COOH. The above peaks
indicated that HCOOH and CF3COOCH3 had been generated
as the oxidation products. In addition to CO2 at 125.1 ppm, no
additional products were observed by NMR.
olefins and dienes.4,5 We have previously developed
a
catalyst based on three redox couples Pd2+/Pd0, quinone/
hydroquinone and NO2/NO, which form an electron transfer
chain and catalyze selective oxidation of methane to
CF3COOCH3 by molecular oxygen in CF3COOH at 80 1C.6
Sen and his co-workers discovered that activated carbon
supported palladium could catalyze methane oxidation to
CF3COOCH3 by O2 in the presence of CO and CuCl or
CuCl2.7 Following that, Park et al. carried out elaborate
characterization of the catalysts and proposed that the
H2O2 generated in situ from a water gas shift reaction
(WGS, reaction between CO and H2O) combined with the
continuous reoxidation of metallic Pd0 to Pd2+ by Cu2+ were
essential for CH4 oxidation to form CF3COOCH3 by O2.8
Inspired by those earlier studies, we intend to replace the
heavy metal copper with quinone for CH4 oxidation because
quinones such as 2-alkyl anthraquinone, together with Pd
catalyst, are used for industrial production of H2O2.9 H2 was
provided by a WGS reaction to avoid the danger of direct
mixing of H2 and O2. However, quinones containing more
than one hydrogen atom on the benzene ring are easily
polymerized by H2O2 in an acidic medium.10 Therefore, we
used p-tetrachlorobenzoquinone (TCQ) here in combination
with carbon supported palladium (Pd/C). The results show
that the addition of TCQ can tune the oxidation product from
HCOOH to methanol derivative CF3COOCH3. The catalytic
The origin of CF3COOCH3 and HCOOH was deter-
mined by isotope experiments by mixing 1.4 MPa 13CH4 and
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian 116023, China.
E-mail: panxl@dicp.ac.cn, xhbao@dicp.ac.cn;
Fax: +86 411 8469 4447; Tel: +86 411 8437 9128
Fig. 1 1H (a) and 13C (b) NMR spectra of liquid products obtained
under the following reaction conditions: 0.01 g 1% Pd/C, 100 mmol
TCQ, 3 ml CF3COOH, 1 ml H2O, 1.8 MPa CH4, 1.0 MPa CO,
0.5 MPa O2, 140 1C and 3 h.
w Electronic supplementary information (ESI) available: Details of
catalyst preparation and characterization, and experimental procedure.
See DOI: 10.1039/b915412d
z Y. Fan and Z. An made equal contributions to this communication.
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0.6 MPa 12CH4. GC-MS analysis showed that the ratio of
COO13CH3 (m/z = 60) to COO12CH3 (m/z = 59) fragments
of CF3COOCH3 in the product equalled 2.4, practically the
same as that of 13CH4/12CH4 in the feeding gas. This implies
that all CF3COOCH3 originates from CH4. However, the ratio
of fragments H13CO (m/z = 30) to H12CO (m/z = 29) of
HCOOH is 0.44, indicating that only about 45% HCOOH
comes from CH4. The rest of the HCOOH might be
derived from the WGS reaction between CO and H2O, where
formate species are believed to be the intermediates.12
In addition, the ratio of 13CO2 (m/z = 45) to 12CO2
(m/z = 44) in the product is 0.033 in the isotope experiment,
which is higher than that of natural abundance CO2 (0.013).
This implies that there is over-oxidation of CH4 or
CF3COOCH3 and HCOOH to give CO2. However, it is
difficult to estimate the extent of over-oxidation since the
WGS reaction and thermal decomposition of CF3COOH also
lead to formation of CO2.13
3.1 ꢂ 0.5 nm and it grows to 15.0 ꢂ 4.4 nm after reaction. This
is likely due to the redox cycles of Pd2+/Pd0. Park et al. also
observed the agglomeration of Pd particles involved in the
reaction with CuCl or CuCl2, as evidenced by XRD and
XAFS results.8 Here, in addition to TCQ, the in situ generated
H2O2, which has a high redox potential (E0
=
H2O2/H2O
1.766 V15), could also oxidize metallic Pd0 to Pd2+ 16
.
This was corroborated by experiments involving mixing
Pd/C with H2O2 at different concentrations, which was
added continuously via a syringe pump at a flow rate of
0.02 ml minꢀ1 in CF3COOH at 80 1C under N2. After 30 min,
the solid was removed by centrifugation. Inductively coupled
plasma atomic emission spectrometry (ICP-AES) analysis of
the resulting liquid showed that 46.2% and 100% of the
palladium had dissolved in the presence of 1% and 15%
H2O2 solution, respectively. The resulting Pd2+ can react with
CH4 stoichiometrically, forming CF3COOCH3 in CF3COOH.17
Thus, through cycles of dissolution of Pd0 and re-deposition of
Pd2+, Pd particles grow.
Fig. 2 shows the yields of CF3COOCH3 and HCOOH as a
function of the amount of TCQ. In the absence of TCQ,
HCOOH is the predominant product and its average yield is
194 mmol while only about 57 mmol CF3COOCH3 forms. At
the same time, merely a trace of H2O2 has been detected after
reaction. A small amount of H2O2 was also detected in a
similar Pd/C catalyzed alkane oxidation system in the
presence of CO, O2 and water.14 Interestingly, the yield of
CF3COOCH3 jumps to 255 mmol while that of HCOOH drops
to 82 mmol upon the addition of 100 mmol TCQ to the reaction
system (Fig. 2). It demonstrates that TCQ promotes the
formation of CF3COOCH3 and suppresses that of HCOOH.
The turnover number (TON, defined as moles of products per
mole Pd per hour) is 120 hꢀ1. On increasing the dosage of
TCQ from 100 to 700 mmol, the yield of CF3COOCH3
decreases from 255 to 60 mmol and that of HCOOH reduces
from 82 to 24 mmol.
Titration shows that the consumption of KMnO4 increases
almost linearly with the amount of TCQ, indicating that more
H2O2 is generated, although KMnO4 in an acidic solution can
also oxidize p-tetrachlorohydroquinone (TCHQ) generated
from the hydrogenation of TCQ. In order to further check
the role of H2O2 in the reaction we carried out experi-
ments replacing CO/H2O/O2 and TCQ with excess H2O2
(250–1000 mmol) in the presence of 0.01 g Pd/C catalyst.
The yield of CF3COOCH3 was found to vary in a range of
13–20 mmol and HCOOH was not detectable, probably due to
too low concentration. This shows that H2O2 can oxidize CH4
to CF3COOCH3, although the added H2O2 decomposes
rapidly under the reaction conditions.
This appears to be a complicated process. Based on earlier
studies,7,8,14 we suggest a mechanism as shown in Scheme 1. In
the absence of TCQ, only a trace of H2O2 is produced and thus
palladium exists mainly as Pd0. Therefore, CH4 is mainly
oxidized to HCOOH, as proposed by Sen and Lin.14 In
contrast, upon addition of TCQ, hydrogen produced from a
WGS reaction could readily react with an intermediate
complex Pd0–TCQ, leading to TCHQ.18 TCHQ is then
oxidized by O2, forming TCQ and releasing H2O2. The
in situ generated H2O2 oxidizes surface Pd0 to Pd2+, which
reacts with CH4 stoichiometrically forming CF3COOCH3.17
Since H2O2 is more efficient than quinone in oxidizing Pd0
to Pd2+, this is likely the reason that the TON here is
significantly higher than that of our previous system.6 In
addition, peroxytrifluoroacetic acid resulting from the reaction
between H2O2 and CF3COOH could also oxidize CH4 to
CF3COOCH3.19 CH3OH thus can be easily obtained through
hydrolysis of CF3COOCH3 (Scheme 1).
Reactions under the standard conditions11 but in the
absence of Pd/C did not give any products, indicating the
necessity for palladium. We characterized the morphology of
Pd/C before and after the reaction by TEM (see Fig. S1 of the
ESIw). The average size of Pd particles in the fresh catalyst is
Thus, Pd/C appears to play multi-functions here. It does not
only catalyze a WGS reaction and the production of H2O2, but
it is also involved in a direct reaction with CH4. Therefore, we
assume that there might be a balance between Pd0 and
Pd2+, which could be the reason for a maximum yield of
CF3COOCH3 and a declining yield of HCOOH in Fig. 2.
However, excess TCQ could competitively coordinate with
Fig. 2 Effect of the amount of TCQ on the yields of CF3COOCH3
and HCOOH. (squares: 1st run; up-triangles: 2nd run; down-triangles:
3rd run; and circles: 4th run; open symbols for CF3COOCH3 and filled
ones for HCOOH).
Pd2+ 20
, which inhibits its reaction with CH4. This is confirmed
by the oxidation of p-xylene over Pd/C catalyst using 400 mmol
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We acknowledge the financial support of the National Basic
Research Program of China (G2005CB221405) and the Clean
Energy Facing the Future program between the Chinese
Academy of Sciences and BP.
Notes and references
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benzoquinone as the main product and the yield is 9.4%.
Upon the addition of 100 mmol TCQ, the yield lowers to 5.2%.
Furthermore, the over-oxidation of CF3COOCH3 to HCOOH
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TCQ, 3 ml CF3COOH, 1 ml H2O, 140 1C and 3 h.
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In summary, with the in situ generated H2O2 tailored by the
addition of TCQ, methane can be selectively oxidized to
HCOOH or the methanol derivative CF3COOCH3, catalyzed
by Pd/C. The process appears to involve a complicated
mechanism with several reactions. In the absence of TCQ,
CH4 is mainly converted to HCOOH. A small amount of TCQ
promotes the in situ generation of H2O2, which facilitates the
oxidation of methane to CF3COOCH3. For example, in the
presence of 100 mmol TCQ, the TON is as high as 120 hꢀ1
.
However, excess TCQ results in reduced yields of
CF3COOCH3 and HCOOH. Since Pd is a widely used catalyst
in the oxidation of organics, the catalyst of Pd in combination
with quinone developed here could be useful for aerobic
oxidations of other hydrocarbons.
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7490 | Chem. Commun., 2009, 7488–7490