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Y. Hori and T. Abe
such as dimethyl ether, formaldehyde, and propylene, in place of petroleum. Therefore, chemical conversion from methane to valuable chemicals has captivated the
interest of not only the academic community for more than a century but also of
industry.
The conversion of methane into methanol has been implemented in industry
as follows: (i) methane is first converted into synthesis gas (a mixture of carbon
monoxide and hydrogen) by steam reforming [Eq. (1)], and (ii) the synthesis gas is
then converted into methanol [Eq. (2)].
CH 4 + H 2 O → CO + 3H 2 H
0
298 K = + 206.2 kJ mol
−1
(1)
CO + 2H 2 → CH 3 OH H
0
298K = −90.7 kJ mol
−1
(2)
Steam reforming, catalyzed by alumina-supported nickel, proceeds at temperatures as high as 1100 K and results in significant operating and maintenance costs. On
the other hand, the direct hydroxylation of methane is more energy efficient [Eq. (3)]
than the indirect process via steam reforming, because methane hydroxylation is an
exothermic process [4].
CH 4 + 0.5O 2 → CH 3 OH H
0
298 K = −126.2 kJ mol
−1
(3)
Therefore, efficient catalyst design for the partial oxidation of methane to
methanol has been one of the most important goals of industry.
Direct conversion of methane to methanol is a major challenge in catalysis because
there are two hurdles to overcome in the oxidation process—the difficulty in activating the C–H bond of methane and the challenge thermodynamically to preserve
the desired methanol product without producing overoxidized products, such as CO 2
or coke. Methane is a stable and inert hydrocarbon owing to its perfectly symmetrical
tetrahedral structure, non-polar nature, and its C–H bond dissociation energy (BDE)
of about 104 kcal mol
−1 . Thus, C–H bond activation may require harsh conditions.
In contrast, because the BDE of the C–H bonds of methanol is 11 kcal mol
−1 lower
than that of methane [5], methanol is highly susceptible to overoxidation to CO 2
under the reaction conditions in which the C–H bond of methane is activated. Thus,
an effective catalyst would have to advance methane activation (conversion) and
simultaneously prevent methanol oxidation (selectivity) [2].
The partial oxidation of methane to methanol has been carried out in four ways,
namely high-temperature non-catalytic gas-phase oxidation, heterogeneous catalytic
oxidation, low-temperature homogeneous catalytic oxidation in solution, and enzymatic or biological catalytic oxidation [1]. In this chapter, we focus on homogeneous
catalysis in solution, based on the organometallic and biomimetic approaches. An
advantage of methane hydroxylation using homogeneous catalysts is that methanol
is produced more selectively by homogeneous catalysts than by heterogeneous catalysts operating at high temperatures, because in the systems using homogeneous
Y. Hori and T. Abe
such as dimethyl ether, formaldehyde, and propylene, in place of petroleum. Therefore, chemical conversion from methane to valuable chemicals has captivated the
interest of not only the academic community for more than a century but also of
industry.
The conversion of methane into methanol has been implemented in industry
as follows: (i) methane is first converted into synthesis gas (a mixture of carbon
monoxide and hydrogen) by steam reforming [Eq. (1)], and (ii) the synthesis gas is
then converted into methanol [Eq. (2)].
CH 4 + H 2 O → CO + 3H 2 H
0
298 K = + 206.2 kJ mol
−1
(1)
CO + 2H 2 → CH 3 OH H
0
298K = −90.7 kJ mol
−1
(2)
Steam reforming, catalyzed by alumina-supported nickel, proceeds at temperatures as high as 1100 K and results in significant operating and maintenance costs. On
the other hand, the direct hydroxylation of methane is more energy efficient [Eq. (3)]
than the indirect process via steam reforming, because methane hydroxylation is an
exothermic process [4].
CH 4 + 0.5O 2 → CH 3 OH H
0
298 K = −126.2 kJ mol
−1
(3)
Therefore, efficient catalyst design for the partial oxidation of methane to
methanol has been one of the most important goals of industry.
Direct conversion of methane to methanol is a major challenge in catalysis because
there are two hurdles to overcome in the oxidation process—the difficulty in activating the C–H bond of methane and the challenge thermodynamically to preserve
the desired methanol product without producing overoxidized products, such as CO 2
or coke. Methane is a stable and inert hydrocarbon owing to its perfectly symmetrical
tetrahedral structure, non-polar nature, and its C–H bond dissociation energy (BDE)
of about 104 kcal mol
−1 . Thus, C–H bond activation may require harsh conditions.
In contrast, because the BDE of the C–H bonds of methanol is 11 kcal mol
−1 lower
than that of methane [5], methanol is highly susceptible to overoxidation to CO 2
under the reaction conditions in which the C–H bond of methane is activated. Thus,
an effective catalyst would have to advance methane activation (conversion) and
simultaneously prevent methanol oxidation (selectivity) [2].
The partial oxidation of methane to methanol has been carried out in four ways,
namely high-temperature non-catalytic gas-phase oxidation, heterogeneous catalytic
oxidation, low-temperature homogeneous catalytic oxidation in solution, and enzymatic or biological catalytic oxidation [1]. In this chapter, we focus on homogeneous
catalysis in solution, based on the organometallic and biomimetic approaches. An
advantage of methane hydroxylation using homogeneous catalysts is that methanol
is produced more selectively by homogeneous catalysts than by heterogeneous catalysts operating at high temperatures, because in the systems using homogeneous