104
5 C–C Bond Formation via the Condensation …
methane is also often referred to as the non-oxidative coupling of methane. In this
chapter, the oxidative and dehydrogenative methane coupling reactions are discussed.
The discussion focuses mainly on the active sites of the catalysts, their reaction
mechanisms, and their catalytic performance under various reaction conditions. In
particular, the OCM reaction network, including the role of the methyl radical (•CH 3 ),
the catalytically active sites for its formation, and the following coupling reactions
of •CH 3 in the gas phase are discussed, while for the dehydrogenative coupling of
methane, the catalytic performance and catalytic properties are emphasized.
5.2 Oxidative Coupling of Methane
The oxidative coupling of methane (OCM) can theoretically produce C 2
+ hydrocarbons. However, the products are generally limited to C 2 H 6 and C 2 H 4 , even though
C 2 H 2 is thermodynamically more favorable than C 2 H 6 and C 2 H 4 . The formation of
C 2 H 6 and C 2 H 4 together with other higher hydrocarbons was first reported in a US
patent [1], while OCM was reported by Fang et al. in 1981 [2]. Furthermore, Keller
et al. reported the conversion of methane to C 2
+ hydrocarbons using various metal
oxides [3]. They examined 26 metal oxides supported on alumina at 773–1273 K at
atmospheric pressure and explored both the simultaneous and sequential feeding of
CH 4 , air, and nitrogen onto a fixed bed of the catalyst.
Much of the research into the energetics of the OCM up to the early 1990s has
been reviewed in refs. [4] and [5]. The typical reaction temperatures in these studies
ranged from nearly 950 to 1200 K. Hundreds of materials have been prepared and
tested as catalysts for this reaction, principally during the 1990s and in recent years.
Zavyalova et al. published a statistical analysis of the catalytic data published before
2011 [6], while Kondratenko et al. also reviewed the literature before 2015 [7].
The extensive studies of the OCM reaction have improved its conversion of
methane and the selectivity for C 2
+ hydrocarbons. A summary of the important
features of this reaction is provided below.
(1) At relatively low methane conversion, the selectivity towards C 2
+ hydrocarbons
increases with increasing reaction temperature.
(2) The C 2 H 4 /C 2 H 6 ratio increases with increasing methane conversion. Thus, C 2 H 6
is the primary product of the OCM reaction.
(3) C 2
+ hydrocarbons can be formed by supplying methane over a pre-oxidized
catalyst in the absence of oxygen in the gas phase, although the rate of methane
conversion decreases with increasing time on stream.
(4) The •CH 3 radical is the intermediate of the OCM reaction, which occurs via a
combination of heterogeneous and homogeneous steps [8–11]. Thus, the initial
abstraction of a hydrogen atom from CH 4 (cleavage of a C–H bond) takes place
on the surface of the catalyst to produce the •CH 3 radical, which is subsequently
desorbed from the catalyst surface and undergoes recombination to produce
C 2 H 6 in the gas phase, not on the catalyst surface.
5 C–C Bond Formation via the Condensation …
methane is also often referred to as the non-oxidative coupling of methane. In this
chapter, the oxidative and dehydrogenative methane coupling reactions are discussed.
The discussion focuses mainly on the active sites of the catalysts, their reaction
mechanisms, and their catalytic performance under various reaction conditions. In
particular, the OCM reaction network, including the role of the methyl radical (•CH 3 ),
the catalytically active sites for its formation, and the following coupling reactions
of •CH 3 in the gas phase are discussed, while for the dehydrogenative coupling of
methane, the catalytic performance and catalytic properties are emphasized.
5.2 Oxidative Coupling of Methane
The oxidative coupling of methane (OCM) can theoretically produce C 2
+ hydrocarbons. However, the products are generally limited to C 2 H 6 and C 2 H 4 , even though
C 2 H 2 is thermodynamically more favorable than C 2 H 6 and C 2 H 4 . The formation of
C 2 H 6 and C 2 H 4 together with other higher hydrocarbons was first reported in a US
patent [1], while OCM was reported by Fang et al. in 1981 [2]. Furthermore, Keller
et al. reported the conversion of methane to C 2
+ hydrocarbons using various metal
oxides [3]. They examined 26 metal oxides supported on alumina at 773–1273 K at
atmospheric pressure and explored both the simultaneous and sequential feeding of
CH 4 , air, and nitrogen onto a fixed bed of the catalyst.
Much of the research into the energetics of the OCM up to the early 1990s has
been reviewed in refs. [4] and [5]. The typical reaction temperatures in these studies
ranged from nearly 950 to 1200 K. Hundreds of materials have been prepared and
tested as catalysts for this reaction, principally during the 1990s and in recent years.
Zavyalova et al. published a statistical analysis of the catalytic data published before
2011 [6], while Kondratenko et al. also reviewed the literature before 2015 [7].
The extensive studies of the OCM reaction have improved its conversion of
methane and the selectivity for C 2
+ hydrocarbons. A summary of the important
features of this reaction is provided below.
(1) At relatively low methane conversion, the selectivity towards C 2
+ hydrocarbons
increases with increasing reaction temperature.
(2) The C 2 H 4 /C 2 H 6 ratio increases with increasing methane conversion. Thus, C 2 H 6
is the primary product of the OCM reaction.
(3) C 2
+ hydrocarbons can be formed by supplying methane over a pre-oxidized
catalyst in the absence of oxygen in the gas phase, although the rate of methane
conversion decreases with increasing time on stream.
(4) The •CH 3 radical is the intermediate of the OCM reaction, which occurs via a
combination of heterogeneous and homogeneous steps [8–11]. Thus, the initial
abstraction of a hydrogen atom from CH 4 (cleavage of a C–H bond) takes place
on the surface of the catalyst to produce the •CH 3 radical, which is subsequently
desorbed from the catalyst surface and undergoes recombination to produce
C 2 H 6 in the gas phase, not on the catalyst surface.