16
1 Overview of Direct Methane Conversion to Chemicals …
1.6.5 Formation of Metal Carbene Complexes (M–CH 2 )
The remaining category of intermediates is metal carbene species (M–CH 2 , where
M is a metal ion, metal atom, or metal cluster), which are generated by the dehydrogenation of CH 4 by the M species, which is typically Ta, W, Ir, Pt, or Os [87, 88].
The intermediate M–CH 2 then takes part in C–C bond formation.
1.7 Why This Book Was Written and How It Is Structured
Many reports of methods for the conversion of methane to produce oxygenates, especially methanol and methanol derivatives, and higher hydrocarbons (C 2
+ ), such as
C 2 H 4 , by controlling the generation of the reaction intermediates have been published
over the past century; most of these methods involve the use of catalysts and careful
control of reaction conditions. However, very few commercial processes for the direct
conversion of methane have been developed. This is due to the fact that methane is
the most inert of the hydrocarbons. CH 4 lacks low-energy empty orbitals and highenergy filled orbitals; thus, it does not readily participate in chemical reactions. Its
negligible electron affinity, large ionization energy (
+ CH 3 /CH 4 = 13.5 ± 0.1 eV),
and high pK a (40–46) also impede the reaction of CH 4 . Furthermore, the C–H bond
in CH 4 is stronger than those in the products; that is, products such as methanol and
ethylene are more reactive than methane. Based on these chemical properties, the
selectivity toward the reaction products would seem to be more important than the
reactivity of CH 4 , especially in oxidative reactions of methane.
As discussed at the beginning of this chapter, the development of processes based
on the direct conversion of methane is essential from both an economic development
and an energy security point of view. However, for scientists and engineers, the
development of direct methane conversion as a next-generation process represents
a “grand challenge” in chemistry independent of trends in the larger world, such as
the discovery of shale gas and the drive to reduce carbon dioxide emissions.
In this book, the authors have consciously focused on providing various examples of the direct conversion of methane using heterogeneous, homogeneous, and
biological catalysts that illustrate their mechanistic and functional aspects. The main
aim is to provide an understanding of the way in which these catalysts and their
reaction environments control the formation of reaction intermediates from methane
and contribute to the formation of C–O or C–C bonds to produce methanol, methanol
derivatives, or hydrocarbons via the direct conversion of methane.
To achieve the objectives mentioned above, this book consists of the following
chapters.
Chapter 1: Overview of direct methane conversion to chemicals with C–O and C–C
bonds
Chapter 2: Selective production of methanol from methane and molecular oxygen
at atmospheric temperature and pressure using methane monooxygenases
1 Overview of Direct Methane Conversion to Chemicals …
1.6.5 Formation of Metal Carbene Complexes (M–CH 2 )
The remaining category of intermediates is metal carbene species (M–CH 2 , where
M is a metal ion, metal atom, or metal cluster), which are generated by the dehydrogenation of CH 4 by the M species, which is typically Ta, W, Ir, Pt, or Os [87, 88].
The intermediate M–CH 2 then takes part in C–C bond formation.
1.7 Why This Book Was Written and How It Is Structured
Many reports of methods for the conversion of methane to produce oxygenates, especially methanol and methanol derivatives, and higher hydrocarbons (C 2
+ ), such as
C 2 H 4 , by controlling the generation of the reaction intermediates have been published
over the past century; most of these methods involve the use of catalysts and careful
control of reaction conditions. However, very few commercial processes for the direct
conversion of methane have been developed. This is due to the fact that methane is
the most inert of the hydrocarbons. CH 4 lacks low-energy empty orbitals and highenergy filled orbitals; thus, it does not readily participate in chemical reactions. Its
negligible electron affinity, large ionization energy (
+ CH 3 /CH 4 = 13.5 ± 0.1 eV),
and high pK a (40–46) also impede the reaction of CH 4 . Furthermore, the C–H bond
in CH 4 is stronger than those in the products; that is, products such as methanol and
ethylene are more reactive than methane. Based on these chemical properties, the
selectivity toward the reaction products would seem to be more important than the
reactivity of CH 4 , especially in oxidative reactions of methane.
As discussed at the beginning of this chapter, the development of processes based
on the direct conversion of methane is essential from both an economic development
and an energy security point of view. However, for scientists and engineers, the
development of direct methane conversion as a next-generation process represents
a “grand challenge” in chemistry independent of trends in the larger world, such as
the discovery of shale gas and the drive to reduce carbon dioxide emissions.
In this book, the authors have consciously focused on providing various examples of the direct conversion of methane using heterogeneous, homogeneous, and
biological catalysts that illustrate their mechanistic and functional aspects. The main
aim is to provide an understanding of the way in which these catalysts and their
reaction environments control the formation of reaction intermediates from methane
and contribute to the formation of C–O or C–C bonds to produce methanol, methanol
derivatives, or hydrocarbons via the direct conversion of methane.
To achieve the objectives mentioned above, this book consists of the following
chapters.
Chapter 1: Overview of direct methane conversion to chemicals with C–O and C–C
bonds
Chapter 2: Selective production of methanol from methane and molecular oxygen
at atmospheric temperature and pressure using methane monooxygenases