5.2 Oxidative Coupling of Methane
105
Details of the processes described in points (1)–(4) are given in the subsequent
sections of this chapter.
5.3 Classification of Catalysts for the Oxidative Coupling
of Methane
OCM, which involves methyl radical dimerization to produce C 2 H 6 , is usually carried
out at high temperature. Methyl radicals (•CH 3 ) produced on the catalyst surface
diffuse to the gas phase and then engage in coupling reactions to afford C 2 H 6 and C 2
+
hydrocarbons, i.e., it is rather difficult to control the selectivity of these reactions by
using a suitable catalyst. Moreover, •CH 3 are preferentially oxidized to carboxylates
and ultimately to CO 2 rather than undergo homocoupling in the gas phase and/or on
the catalyst surface [12]. Thus, OCM catalysts should not only initiate the formation
of methyl radicals but also suppress the non-selective surface oxidation of methane
and C 2
+ hydrocarbons to CO 2 . Therefore, the yields (selectivities) of C 2 H 6 and C 2
+
hydrocarbons do not exceed ~30%, as mentioned in Sect. 5.4.6.
Various systems for the classification of OCM catalysts have been developed.
Zavyalova et al. classified OCM catalysts into four groups based on the type of
catalyst material, as follows [6]:
(1) Non-reducible metal oxides,
(2) Reducible metal oxides
(3) Halogen-containing oxide materials
(4) Solid electrolytes.
Amenomiya et al. classified the catalysts into three major groups based on their
location on the periodic table [5]:
Group 1: Alkali and alkaline earth metal compounds
Group 2: Lanthanide and actinide compounds
Group 3: Other metal compounds.
Group 3 can be further divided into transition metal and post-transition metal
compounds (Group 11–15 elements). This classification scheme was used to classify
both one- and two/multi-component catalysts.
Furthermore, Lunsford classified the catalysts more concretely into the following
five groups [13]:
(1) Highly basic pure oxides, of which the early members of the lanthanide oxide
series excluding CeO 2 are the best catalysts
(2) Group IA or IIA ions supported on basic oxides, such as Li/MgO and Sr/La 2 O 3
(3) Monophasic oxides
(4) Transition metal oxides that contain Group IA ions
(5) Any of these materials that are promoted with chloride ions, such as
LiCa 2 B 3 O 4 Cl 6
105
Details of the processes described in points (1)–(4) are given in the subsequent
sections of this chapter.
5.3 Classification of Catalysts for the Oxidative Coupling
of Methane
OCM, which involves methyl radical dimerization to produce C 2 H 6 , is usually carried
out at high temperature. Methyl radicals (•CH 3 ) produced on the catalyst surface
diffuse to the gas phase and then engage in coupling reactions to afford C 2 H 6 and C 2
+
hydrocarbons, i.e., it is rather difficult to control the selectivity of these reactions by
using a suitable catalyst. Moreover, •CH 3 are preferentially oxidized to carboxylates
and ultimately to CO 2 rather than undergo homocoupling in the gas phase and/or on
the catalyst surface [12]. Thus, OCM catalysts should not only initiate the formation
of methyl radicals but also suppress the non-selective surface oxidation of methane
and C 2
+ hydrocarbons to CO 2 . Therefore, the yields (selectivities) of C 2 H 6 and C 2
+
hydrocarbons do not exceed ~30%, as mentioned in Sect. 5.4.6.
Various systems for the classification of OCM catalysts have been developed.
Zavyalova et al. classified OCM catalysts into four groups based on the type of
catalyst material, as follows [6]:
(1) Non-reducible metal oxides,
(2) Reducible metal oxides
(3) Halogen-containing oxide materials
(4) Solid electrolytes.
Amenomiya et al. classified the catalysts into three major groups based on their
location on the periodic table [5]:
Group 1: Alkali and alkaline earth metal compounds
Group 2: Lanthanide and actinide compounds
Group 3: Other metal compounds.
Group 3 can be further divided into transition metal and post-transition metal
compounds (Group 11–15 elements). This classification scheme was used to classify
both one- and two/multi-component catalysts.
Furthermore, Lunsford classified the catalysts more concretely into the following
five groups [13]:
(1) Highly basic pure oxides, of which the early members of the lanthanide oxide
series excluding CeO 2 are the best catalysts
(2) Group IA or IIA ions supported on basic oxides, such as Li/MgO and Sr/La 2 O 3
(3) Monophasic oxides
(4) Transition metal oxides that contain Group IA ions
(5) Any of these materials that are promoted with chloride ions, such as
LiCa 2 B 3 O 4 Cl 6