3.4 Homogeneous Catalysts for the Production of Methanol …
65
[77]. The turnover number (TON) of the catalytic reaction was 5.3, and the yield was
very low. However, the TON was increased by using a bridged bis-N-heterocyclic
carbene (NHC)–Pd(II) complex as the catalyst and K 2 S 2 O 8 as the oxidant for the
trifluoroacetoxylation of methane in TFAA/TFA at 363 K [78]. The TON for the
methane (3 MPa) conversion reached 30 in 14 h at 363 K. According to a computational study [79], this reaction proceeds via a Pd(II)/Pd(IV)/Pd(II) catalytic cycle
and involves C–H bond activation by Pd(II).
The use of environmentally friendly and inexpensive molecular oxygen (O 2 ) as
the terminal oxidant is always desirable in oxidation reactions. However, only a
few reports have been published on the partial oxidation of methane to methanol
derivatives under homogeneous catalysts by using molecular oxygen as the oxidant.
The poor solubility of molecular oxygen in solvents for liquid phase reaction is
one of the main limitations for this type of reaction in homogeneous catalysis.
For example, the Co(III)-catalyzed trifluoroacetoxylation of methane to methyl
trifluoroacetate was observed with O 2 in TFA at 453 K [80]. This reaction has been
improved to obtain a methyl trifluoroacetate yield of 50% (based on methane) via
the oxidation of methane (1 MPa) with O 2 (0.5 MPa) and 3.8 mol% Co(OAC) 2
4H 2 O as the catalyst in TFAA/TFA (6:1) at 453 K [81]. An et al. achieved the
one-pot aerobic oxidation of methane with a Pd(II) catalyst at 353 K using a
p-benzoquinone/hydrobenzoquinone redox shuttle, NO 2 /NO couple, and O 2 [82].
The TON (expressed in terms of moles of CF 3 COOCH 3 per mole of Pd(II)) of the
Pd catalyst was only 7, but the yield of methyl trifluoroacetate was 0.06% based
on methane. A Pd(OAc) 2 /p-benzoquinone/H 5 PMo 10 V 2 O 40 system for the direct
oxidation of methane to the methanol derivative CF 3 COOCH 3 in CF 3 COOH at
low temperature was demonstrated. The catalysts can be regenerated by molecular
oxygen [83]. Perfluorocarbons exhibit very high oxygen solubility and transport
properties [84–86]. Therefore, the perfluorocarbon C 8 F 18 , which is inert in catalytic
oxidation, was introduced to improve oxygen solubility and transport in CF 3 COOH.
In the reaction, the methanol derivative CF 3 COOCH 3 was the only liquid product,
and CO 2 was the only gaseous product. Higher partial pressures of O 2 led to higher
yields of CF 3 COOCH 3 by the promotion of H 5 PMo 10 V 2 O 40 regeneration [87].
3.5 Heterogeneous Catalysts for the Production
of Methanol Derivatives from Methane
A series of methane conversion reactions originating from the Hg catalyst developed
by Periana and co-workers have been found to proceed at relatively low temperatures
(273–493 K). The product of these reactions is the methyl ester, which is hydrolyzed
to give the final target compound, methanol. These catalytic systems have inspired
the development of heterogeneous catalysts for the oxidation of methane to methanol
derivatives. For example, the conversion of methane via an M–CH 3 intermediate has
65
[77]. The turnover number (TON) of the catalytic reaction was 5.3, and the yield was
very low. However, the TON was increased by using a bridged bis-N-heterocyclic
carbene (NHC)–Pd(II) complex as the catalyst and K 2 S 2 O 8 as the oxidant for the
trifluoroacetoxylation of methane in TFAA/TFA at 363 K [78]. The TON for the
methane (3 MPa) conversion reached 30 in 14 h at 363 K. According to a computational study [79], this reaction proceeds via a Pd(II)/Pd(IV)/Pd(II) catalytic cycle
and involves C–H bond activation by Pd(II).
The use of environmentally friendly and inexpensive molecular oxygen (O 2 ) as
the terminal oxidant is always desirable in oxidation reactions. However, only a
few reports have been published on the partial oxidation of methane to methanol
derivatives under homogeneous catalysts by using molecular oxygen as the oxidant.
The poor solubility of molecular oxygen in solvents for liquid phase reaction is
one of the main limitations for this type of reaction in homogeneous catalysis.
For example, the Co(III)-catalyzed trifluoroacetoxylation of methane to methyl
trifluoroacetate was observed with O 2 in TFA at 453 K [80]. This reaction has been
improved to obtain a methyl trifluoroacetate yield of 50% (based on methane) via
the oxidation of methane (1 MPa) with O 2 (0.5 MPa) and 3.8 mol% Co(OAC) 2
4H 2 O as the catalyst in TFAA/TFA (6:1) at 453 K [81]. An et al. achieved the
one-pot aerobic oxidation of methane with a Pd(II) catalyst at 353 K using a
p-benzoquinone/hydrobenzoquinone redox shuttle, NO 2 /NO couple, and O 2 [82].
The TON (expressed in terms of moles of CF 3 COOCH 3 per mole of Pd(II)) of the
Pd catalyst was only 7, but the yield of methyl trifluoroacetate was 0.06% based
on methane. A Pd(OAc) 2 /p-benzoquinone/H 5 PMo 10 V 2 O 40 system for the direct
oxidation of methane to the methanol derivative CF 3 COOCH 3 in CF 3 COOH at
low temperature was demonstrated. The catalysts can be regenerated by molecular
oxygen [83]. Perfluorocarbons exhibit very high oxygen solubility and transport
properties [84–86]. Therefore, the perfluorocarbon C 8 F 18 , which is inert in catalytic
oxidation, was introduced to improve oxygen solubility and transport in CF 3 COOH.
In the reaction, the methanol derivative CF 3 COOCH 3 was the only liquid product,
and CO 2 was the only gaseous product. Higher partial pressures of O 2 led to higher
yields of CF 3 COOCH 3 by the promotion of H 5 PMo 10 V 2 O 40 regeneration [87].
3.5 Heterogeneous Catalysts for the Production
of Methanol Derivatives from Methane
A series of methane conversion reactions originating from the Hg catalyst developed
by Periana and co-workers have been found to proceed at relatively low temperatures
(273–493 K). The product of these reactions is the methyl ester, which is hydrolyzed
to give the final target compound, methanol. These catalytic systems have inspired
the development of heterogeneous catalysts for the oxidation of methane to methanol
derivatives. For example, the conversion of methane via an M–CH 3 intermediate has