8
1 Overview of Direct Methane Conversion to Chemicals …
The formation of CH 3 OSO 3 H via the reaction of methane with H 2 SO 4 is shown
below.
CH 4 + 4 H 2 SO 4
CH 3 OSO 3 H + 2 H 2 O·H 2 SO 4 + SO 2
(1.8)
Above, H 2 O·H 2 SO 4 represents H 2 SO 4 hydrated with H 2 O. The ΔG value of the reaction was estimated to be –25 kJ mol
−1 . Therefore, the reaction is thermodynamically
downhill.
On the other hand, the direct oxidation of methane to methanol with H 2 SO 4 shown
in reaction (1.9) is thermodynamically unfavorable.
CH 4 + H 2 SO 4
CH 3 OH + H 2 O + SO 2
(1.9)
The ΔG of reaction (1.9) was estimated to be nearly 79 kJ mol
−1 at 298 K based on the
standard electrode potential [10]. That is, SO 4
2− is too weak of an oxidant for reaction
(1.9) to proceed. Thus, the direct production of methanol by methane conversion via
the intermediate M–CH 3 is considered to be difficult under mild conditions, while
the strategy of converting methane to a methanol derivative is useful for avoiding the
further oxidation of the product.
In this book, the direct formation of methanol from methane with oxygen (reaction
(1.4)) is clearly distinguished from the formation of methanol derivatives such as
methyl sulfate. The latter topic is discussed in further detail in Chapters 2–4.
1.4.2 C–C Bond Formation: Production of Higher
Hydrocarbons
The formation of C–C bonds to produce higher hydrocarbons (C 2
+ ) can be classified
into two further categories based on whether the C 2
+ hydrocarbons are produced
in the presence or the absence of oxygen. In this book, the former is referred to as
the oxidative coupling of methane, while reactions of the latter type are referred to
as non-oxidative reactions of methane, and are further classified into three kinds of
reactions as described below.
1.4.2.1 C–C Bond Formation via the Oxidative Coupling of Methane
The oxidative coupling of methane produces mainly ethane and ethylene, accompanied by the simultaneous production of H 2 O. Reaction temperatures higher than
800 K and heterogeneous catalysts are typically used in such processes. The key
reaction intermediate may be •CH 3 , which is often generated on the surface of heterogeneous catalysts. For example, an EPR signal corresponding to •CH 3 was observed
1 Overview of Direct Methane Conversion to Chemicals …
The formation of CH 3 OSO 3 H via the reaction of methane with H 2 SO 4 is shown
below.
CH 4 + 4 H 2 SO 4
CH 3 OSO 3 H + 2 H 2 O·H 2 SO 4 + SO 2
(1.8)
Above, H 2 O·H 2 SO 4 represents H 2 SO 4 hydrated with H 2 O. The ΔG value of the reaction was estimated to be –25 kJ mol
−1 . Therefore, the reaction is thermodynamically
downhill.
On the other hand, the direct oxidation of methane to methanol with H 2 SO 4 shown
in reaction (1.9) is thermodynamically unfavorable.
CH 4 + H 2 SO 4
CH 3 OH + H 2 O + SO 2
(1.9)
The ΔG of reaction (1.9) was estimated to be nearly 79 kJ mol
−1 at 298 K based on the
standard electrode potential [10]. That is, SO 4
2− is too weak of an oxidant for reaction
(1.9) to proceed. Thus, the direct production of methanol by methane conversion via
the intermediate M–CH 3 is considered to be difficult under mild conditions, while
the strategy of converting methane to a methanol derivative is useful for avoiding the
further oxidation of the product.
In this book, the direct formation of methanol from methane with oxygen (reaction
(1.4)) is clearly distinguished from the formation of methanol derivatives such as
methyl sulfate. The latter topic is discussed in further detail in Chapters 2–4.
1.4.2 C–C Bond Formation: Production of Higher
Hydrocarbons
The formation of C–C bonds to produce higher hydrocarbons (C 2
+ ) can be classified
into two further categories based on whether the C 2
+ hydrocarbons are produced
in the presence or the absence of oxygen. In this book, the former is referred to as
the oxidative coupling of methane, while reactions of the latter type are referred to
as non-oxidative reactions of methane, and are further classified into three kinds of
reactions as described below.
1.4.2.1 C–C Bond Formation via the Oxidative Coupling of Methane
The oxidative coupling of methane produces mainly ethane and ethylene, accompanied by the simultaneous production of H 2 O. Reaction temperatures higher than
800 K and heterogeneous catalysts are typically used in such processes. The key
reaction intermediate may be •CH 3 , which is often generated on the surface of heterogeneous catalysts. For example, an EPR signal corresponding to •CH 3 was observed