Orbital Concept for Methane Activation
3
the benzene ring, respectively. The synthesis of aniline from benzene is a simple
chemistry experiment for undergraduate students. In contrast, such selective chemical processes for methane under mild conditions have not yet been established even
in laboratory.
The selective oxidation of methane [1–5] has attracted increased attention for
a long time because of its scientific interest and industrial importance. The direct
conversion of methane to methanol (Eq. 1), which is an exothermic process,
is catalyzed under physiological conditions by soluble methane monooxygenase
(sMMO) [6], which has a diiron active center, and particulate methane monooxygenase (pMMO) [7], which has mono- and dicopper sites. This reaction is also catalyzed
by the bare transition-metal-oxide ions such as FeO
+ [8] in the gas-phase and metalexchanged zeolites such as Fe-ZSM-5 zeolite [9]. The enzymatic reactions of the
MMOs use molecular dioxygen as an oxidant, while the latter two reactions are very
similar in that nitrous oxide (N 2 O) is used as an oxidant. Iron-, nickel-, and copperoxo (or -oxyl) species are involved in these difficult chemical processes. To develop
a man-made catalytic system for this attractive reaction, it is important to reveal the
mechanism of the direct hydroxylation processes by these catalytic systems.
CH 4 + 1/2 O 2 → CH 3 OH (H 0 = −30.7 kcal/mol)
(1)
Quantum chemical calculations and orbital interaction thinking at various levels
of theory play a key role in understanding the mechanism of the C–H activation of
methane, which is an essential, initial process in the direct methane hydroxylation.
However, the mechanism has not been well understood because of its extremely
rapid reaction. Figure 1 shows computed C–H BDEs of various alkanes and related
hydrocarbons from DFT calculations at the B3LYP/6-311++G** level of theory. In
general, the C–H BDEs at the primary (1°), secondary (2°), and tertiary (3°) carbon
atoms of alkanes are approximately 97, 94, and 90 kcal/mol, while those at the
benzylic positions are less than 85 kcal/mol. In particular, dihydroanthracene and
allylbenzne have very weak benzylic C–H bonds of less than 75 kcal/mol. Only
methane has a C–H BDE of over 100 kcal/mol among these alkanes.
The C–H BDEs also have a good correlation with computed HOMO–LUMO
energy gaps, as shown in Fig. 2. Methane’s HOMO–LUMO gap is calculated to be
10.6 eV at the same level of theory, where HOMO is highest occupied molecular
orbital and LUMO is lowest unoccupied molecular orbital. Since the gap between
HOMO and LUMO is a good measure of molecular hardness, methane is a very hard
molecule, which means that its ionization energy is very high and its electron affinity
is very poor. As a consequence, it is very difficult to perform the selective activation
of methane.
We have carried out mechanistic studies of the direct hydroxylation of methane by
various enzymatic and catalytic systems at various levels of theory. In this chapter,
the C–H bond activation and the subsequent hydroxylation of methane are considered
on the basis of orbital interaction analyses and DFT calculations.
3
the benzene ring, respectively. The synthesis of aniline from benzene is a simple
chemistry experiment for undergraduate students. In contrast, such selective chemical processes for methane under mild conditions have not yet been established even
in laboratory.
The selective oxidation of methane [1–5] has attracted increased attention for
a long time because of its scientific interest and industrial importance. The direct
conversion of methane to methanol (Eq. 1), which is an exothermic process,
is catalyzed under physiological conditions by soluble methane monooxygenase
(sMMO) [6], which has a diiron active center, and particulate methane monooxygenase (pMMO) [7], which has mono- and dicopper sites. This reaction is also catalyzed
by the bare transition-metal-oxide ions such as FeO
+ [8] in the gas-phase and metalexchanged zeolites such as Fe-ZSM-5 zeolite [9]. The enzymatic reactions of the
MMOs use molecular dioxygen as an oxidant, while the latter two reactions are very
similar in that nitrous oxide (N 2 O) is used as an oxidant. Iron-, nickel-, and copperoxo (or -oxyl) species are involved in these difficult chemical processes. To develop
a man-made catalytic system for this attractive reaction, it is important to reveal the
mechanism of the direct hydroxylation processes by these catalytic systems.
CH 4 + 1/2 O 2 → CH 3 OH (H 0 = −30.7 kcal/mol)
(1)
Quantum chemical calculations and orbital interaction thinking at various levels
of theory play a key role in understanding the mechanism of the C–H activation of
methane, which is an essential, initial process in the direct methane hydroxylation.
However, the mechanism has not been well understood because of its extremely
rapid reaction. Figure 1 shows computed C–H BDEs of various alkanes and related
hydrocarbons from DFT calculations at the B3LYP/6-311++G** level of theory. In
general, the C–H BDEs at the primary (1°), secondary (2°), and tertiary (3°) carbon
atoms of alkanes are approximately 97, 94, and 90 kcal/mol, while those at the
benzylic positions are less than 85 kcal/mol. In particular, dihydroanthracene and
allylbenzne have very weak benzylic C–H bonds of less than 75 kcal/mol. Only
methane has a C–H BDE of over 100 kcal/mol among these alkanes.
The C–H BDEs also have a good correlation with computed HOMO–LUMO
energy gaps, as shown in Fig. 2. Methane’s HOMO–LUMO gap is calculated to be
10.6 eV at the same level of theory, where HOMO is highest occupied molecular
orbital and LUMO is lowest unoccupied molecular orbital. Since the gap between
HOMO and LUMO is a good measure of molecular hardness, methane is a very hard
molecule, which means that its ionization energy is very high and its electron affinity
is very poor. As a consequence, it is very difficult to perform the selective activation
of methane.
We have carried out mechanistic studies of the direct hydroxylation of methane by
various enzymatic and catalytic systems at various levels of theory. In this chapter,
the C–H bond activation and the subsequent hydroxylation of methane are considered
on the basis of orbital interaction analyses and DFT calculations.