J. Yuan et al.
methane by H2O2 or K2S2O8 [6–8]. With the catalysis of
H4PVMo11O40, 33.0 % conversion of methane could be
obtained at 80 °C in (CF3CO)2O using H2O2 as the oxidant
[6, 7].
magnetic stirring bar coated with Teflon. The reactor was
three times purged with 1.0 MPa of methane and then
pressurized with methane and oxygen. It was heated to
80 °C in an oil bath and kept under stirring. After the
reaction, the reactor was cooled in ice/water mixture to
3 °C, thus the pressure was slowly reduced.
The products were analyzed by gas chromatography-
mass spectroscopy (GC–MS) and 1H-NMR (Brucker
AV600, Swiss). The gas phase was analyzed by a gas
chromatograph (GC 4000A, East & West Analytical, PRC)
with TDX-01 column, and the liquid was qualified by a gas
chromatograph with a Porapak QS column.
Besides catalysts, the oxidants are crucial to the partial
oxidation process. Strong oxidants, such as concentrated
sulfuric acid, H2O2 and K2S2O8, are necessary in transition
metal catalytic systems or in HPA catalytic systems.
Molecular oxygen is an environmentally friendly, inex-
pensive oxidant; however, due to its relatively poor oxi-
dation capacity, there are only a few reports on partial
oxidation of methane using molecular oxygen as the oxi-
dant. In addition to the above mentioned catalytic system
suggested by Bell et al., An et al. developed a catalytic
system for the one-pot aerobic oxidation of methane with
the combination of the three redox couples Pd(II)/Pd(0),
p-benzoquinone (BQ)/hydrobenzoquinone (H2Q), NO2/NO
in CF3COOH [9]. However, methane conversion is too
low in this catalytic system [9].
2.3 Preparation of the Nascent Pd(0)
The nascent Pd(0) powders were prepared by the reduction
of Pd(OAc)2. pH value of Pd(OAc)2 aqueous solution was
adjusted to about ten by adding ammonia solution, then
hydrazine hydrate was added into the solution, thus the
nascent Pd(0) sediment formed. The black sediment was
cleaned by distilled water for times in order to remove the
residual Pd2? ions.
Pd(II)/HPA/O2 system is very important in homoge-
neous catalysis of organic synthesis [10]. In this system,
HPA is a reversible oxidant that allows to retain Pd(II)
in the solution and thus to complete a two-step catalytic
cycle of oxidation of the substrates by molecular oxygen
[10]. In order to develop an efficient method for partial
oxidation of methane using molecular oxygen as the oxi-
dant, K2PdCl4/H5PMo10V2O40 catalytic system is sug-
gested in this study.
2.4 Oxidation Reaction of the Nascent Pd(0)
by H5PMo10V2O40 in CF3COOH
0.0044 g of H5PMo10V2O40 powders was firstly dissolved
in 25 ml of CF3COOH, then 20 ml of H5PMo10V2O40
solution were used to oxidize 0.0022 g of the nascent
Pd(0). At the same time, 1 ml acetic acid was also added to
provide acetate ions. Concentration of Pd2? in the samples
at various intervals was analyzed by colorimetric method
using 5-Cl-PADAB as the developer at wavelength of
570 nm.
2 Experimental
2.1 Materials
K2PdCl4 and CF3COOH were purchased from Shanghai
Jingchun Reagent Co. Ltd., P. R. China. BQ was purchased
from Sinopharm Chemical Reagent Co. Ltd., P. R. China.
Methane, oxygen and nitrogen were obtained from Beijing
Huayuan Gas Co. Ltd., P. R. China. H5PMo10V2O40 was
prepared from H3PO4, MoO3, and V2O5 [11]. H3PO4,
MoO3 and V2O5 were from Beijing Chemical Factory,
P. R. China. All of the chemical reagents are at analytic
reagent grade.
3 Results and Discussion
3.1 Partial Oxidation of Methane
The results of Gretz et al. [12] implied that Pd(II) could
oxidize methane selectively to CF3COOCH3 in CF3COOH
at 80 °C. In that process, Pd(II) did not act as a catalyst
because it was stoichiometrically converted to Pd(0) [13].
In order to regenerate Pd(II), we chose H5PMo10V2O40 to
oxidize Pd(0), and molecular oxygen as the terminal
oxidant to regenerate H5PMo10V2O40.
2.2 Partial Oxidation of Methane
K2PdCl4 and H5PMo10V2O40 were dissolved in CF3COOH
before methane oxidation reaction. In addition, NaCl was
also introduced into CF3COOH, and the molar ratio of
NaCl to K2PdCl4 is 50.
Table 1 lists a series of typical reaction results for par-
tial oxidation of methane in CF3COOH catalyzed by
H5PMo10V2O40 or K2PdCl4/H5PMo10V2O40. It shows in
Table 1 that Pd2? can stoichiometrically oxidize methane
into CF3COOCH3 in CF3COOH, which is consistent with
the previous results [9, 13]. Using molecular oxygen as the
Methane oxidation in 10 ml of CF3COOH was con-
ducted in a 50 ml cylindrical stainless steel autoclave with
a Teflon liner inside. Mild stirring was provided by a
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