106
5 C–C Bond Formation via the Condensation …
Dubois et al. concluded that most, if not all, effective catalysts are strongly basic
oxides that act as p-type semiconductors at elevated temperature and exhibit oxygen
anion mobility [14]. Moreover, these catalysts contained free reducible surface ions,
such as Na 2 WO 4 /SiO 2 . The Na 2 WO 4 /SiO 2 catalyst was reported for the first time
by Yan et al. in 1992 [15], and is discussed in detail in Sect. 5.4.6.
5.4 Role of Oxygen Species in Methyl Radical Formation
Over Various Metal Oxide Catalysts
5.4.1 Evidence of Methyl Radical Formation During OCM
The formation of the •CH 3 radical in the OCM reaction, which was long considered
to be a well-established fact, was experimentally confirmed by Driscol et al. [8, 16].
Evidence for this heterogeneous–homogeneous reaction process was also provided
by the detection of the •CH 3 radical by using multiphoton ionization mass spectroscopy [17, 18]. Luo et al. also confirmed the formation of the •CH 3 radical over
Li/MgO in the OCM process using synchrotron vacuum ultraviolet photoionization
spectroscopy [9].
The methyl radical has also been detected in the gas phase using a matrix isolation
electron spin resonance (MIESR) system when methane was brought into contact
with Na/CaO [19], Na/CeO 2 [20], Sr/La 2 O 3 [21], Li/NiO 2 [22], Li/ZnO [23], La 2 O 3
[24], other lanthanide oxides [25], and Li–MgO–Cl [26].
5.4.2 O − Oxygen Species for Methyl Radical Formation
In addition to the •CH 3 radical, [Li
+ O
− ] centers were also detected in lithium-doped
MgO (Li/MgO) via electron spin resonance (ESR) spectroscopy when Li/MgO was
heated to 773 K or higher in air and quenched in liquid nitrogen [27]. Here, [Li
+ O
− ] is
a trapped hole at one of the six O
− ion sites surrounding a Li
+ ion. In Li/MgO, the Li
+
ions are located at Mg
2+ cation sites and trap holes. They have a linear configuration
that can be expressed as Mg
2+ –O
2− –Li
+ –O
− –Mg
2+ [28, 29]. Such [Li
+ O
− ] centers
were found to be formed in the alkali metal-doped alkaline earth oxides MgO, CaO,
and SrO [30].
Driscoll et al. also detected [Li
+ O
− ] centers in Li/MgO via ESR spectroscopy.
The amount of the •CH 3 radical that was generated from CH 4 over Li/MgO varied
according to the amount of [Li
+ O
− ] centers, which, in turn, was a function of the
amount of lithium doped into MgO [8]. Ito et al. reported that the amount of [Li
+ O
− ]
centers (the intensity of ESR spectra due to [Li
+ O
− ] centers) and the total amount
of converted CH 4 both showed a very similar dependence on the oxygen pressure
[27]. On the basis of these results, the •CH 3 is formed on [Li
+ O
− ] centers via the
5 C–C Bond Formation via the Condensation …
Dubois et al. concluded that most, if not all, effective catalysts are strongly basic
oxides that act as p-type semiconductors at elevated temperature and exhibit oxygen
anion mobility [14]. Moreover, these catalysts contained free reducible surface ions,
such as Na 2 WO 4 /SiO 2 . The Na 2 WO 4 /SiO 2 catalyst was reported for the first time
by Yan et al. in 1992 [15], and is discussed in detail in Sect. 5.4.6.
5.4 Role of Oxygen Species in Methyl Radical Formation
Over Various Metal Oxide Catalysts
5.4.1 Evidence of Methyl Radical Formation During OCM
The formation of the •CH 3 radical in the OCM reaction, which was long considered
to be a well-established fact, was experimentally confirmed by Driscol et al. [8, 16].
Evidence for this heterogeneous–homogeneous reaction process was also provided
by the detection of the •CH 3 radical by using multiphoton ionization mass spectroscopy [17, 18]. Luo et al. also confirmed the formation of the •CH 3 radical over
Li/MgO in the OCM process using synchrotron vacuum ultraviolet photoionization
spectroscopy [9].
The methyl radical has also been detected in the gas phase using a matrix isolation
electron spin resonance (MIESR) system when methane was brought into contact
with Na/CaO [19], Na/CeO 2 [20], Sr/La 2 O 3 [21], Li/NiO 2 [22], Li/ZnO [23], La 2 O 3
[24], other lanthanide oxides [25], and Li–MgO–Cl [26].
5.4.2 O − Oxygen Species for Methyl Radical Formation
In addition to the •CH 3 radical, [Li
+ O
− ] centers were also detected in lithium-doped
MgO (Li/MgO) via electron spin resonance (ESR) spectroscopy when Li/MgO was
heated to 773 K or higher in air and quenched in liquid nitrogen [27]. Here, [Li
+ O
− ] is
a trapped hole at one of the six O
− ion sites surrounding a Li
+ ion. In Li/MgO, the Li
+
ions are located at Mg
2+ cation sites and trap holes. They have a linear configuration
that can be expressed as Mg
2+ –O
2− –Li
+ –O
− –Mg
2+ [28, 29]. Such [Li
+ O
− ] centers
were found to be formed in the alkali metal-doped alkaline earth oxides MgO, CaO,
and SrO [30].
Driscoll et al. also detected [Li
+ O
− ] centers in Li/MgO via ESR spectroscopy.
The amount of the •CH 3 radical that was generated from CH 4 over Li/MgO varied
according to the amount of [Li
+ O
− ] centers, which, in turn, was a function of the
amount of lithium doped into MgO [8]. Ito et al. reported that the amount of [Li
+ O
− ]
centers (the intensity of ESR spectra due to [Li
+ O
− ] centers) and the total amount
of converted CH 4 both showed a very similar dependence on the oxygen pressure
[27]. On the basis of these results, the •CH 3 is formed on [Li
+ O
− ] centers via the