78
M. H. Mahyuddin et al.
as the active species abstracting the H atom of methane. For further discussion on
methane hydroxylation by metal-exchanged zeolites, interested readers are invited
to read recent excellent reviews [33–39].
Although most of theoretical works predict that methanol is formed through a
direct HO–CH 3 recombination, several experimental works suggested that a surface
methoxy is first formed as an intermediate state and then followed by water hydrolysis to form methanol [40, 41]. However, the fact that water acts only as a solvent
and the catalysts need to be reactivated after each cycle of the reaction strongly indicates that the CH 3 species recombine with the OH originated from the active site,
not the OH originated from water splitting. Despite this, theoretical understanding
would be worthful to evaluate all possible reaction routes for methanol formation. In
this chapter, we overview recent advances in the mechanistic understanding of the
homolytic C–H bond cleavage of methane by the [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+
active species in zeolites and deliver our analysis on the mechanism of methanol
formation by performing density functional theory (DFT) calculations. We present
energy diagrams for the formation of methanol through two possible reaction routes,
where the formation of a surface methoxy and the adsorption of a water molecule
are involved.
2 Methane Hydroxylation by [Cu 2 (µ-O)] 2+
and [Cu 3 (µ-O) 3 ] 2+ in Zeolites
2.1 Mechanism of C–H Activation
The C–H activation of methane by [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+ active species in
zeolites mostly reported in theoretical works follows the homolytic HAA mechanism
(Scheme 1, pathway 2) [17, 19, 42–44]. Here, the Cu
2+ ions provide electrons to the
bridging O atoms(s), resulting in a radical O
·– that is highly reactive toward CH 4 .
The ground states of [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+ active species have been an
intensive discussion between theoretical chemists because the triplet and open-shell
singlet states in the [Cu–O–Cu]
2+ as well as the quartet and doublet states in the
[Cu 3 (μ-O) 3 ]
2+ are energetically competitive. This is reasonable since the difference
between the two competitive spin states (i.e., high and low spin states) is only the
spin direction of the Cu
2+ ions. As shown in Table 1, the triplet state of [Cu–O–
Cu]
2+ is formed when the unpaired electron of each Cu
2+ is an α electron, whereas
the open-shell singlet state is formed when one of the Cu
2+ ions has an unpaired α
electron and the other one has an unpaired β electron. Similarly, the quartet state
of [Cu 3 (μ-O) 3 ]
2+ is formed when all of the three Cu
2+ ions have an unpaired α
electron while the doublet state is formed when one of the Cu
2+ ions has an unpaired
β electron. Alternatively, the three unpaired electrons might originate from two Cu
2+
ions and an O
·– radical [44]. The data showing spin densities of each Cu
2+ ion in the
M. H. Mahyuddin et al.
as the active species abstracting the H atom of methane. For further discussion on
methane hydroxylation by metal-exchanged zeolites, interested readers are invited
to read recent excellent reviews [33–39].
Although most of theoretical works predict that methanol is formed through a
direct HO–CH 3 recombination, several experimental works suggested that a surface
methoxy is first formed as an intermediate state and then followed by water hydrolysis to form methanol [40, 41]. However, the fact that water acts only as a solvent
and the catalysts need to be reactivated after each cycle of the reaction strongly indicates that the CH 3 species recombine with the OH originated from the active site,
not the OH originated from water splitting. Despite this, theoretical understanding
would be worthful to evaluate all possible reaction routes for methanol formation. In
this chapter, we overview recent advances in the mechanistic understanding of the
homolytic C–H bond cleavage of methane by the [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+
active species in zeolites and deliver our analysis on the mechanism of methanol
formation by performing density functional theory (DFT) calculations. We present
energy diagrams for the formation of methanol through two possible reaction routes,
where the formation of a surface methoxy and the adsorption of a water molecule
are involved.
2 Methane Hydroxylation by [Cu 2 (µ-O)] 2+
and [Cu 3 (µ-O) 3 ] 2+ in Zeolites
2.1 Mechanism of C–H Activation
The C–H activation of methane by [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+ active species in
zeolites mostly reported in theoretical works follows the homolytic HAA mechanism
(Scheme 1, pathway 2) [17, 19, 42–44]. Here, the Cu
2+ ions provide electrons to the
bridging O atoms(s), resulting in a radical O
·– that is highly reactive toward CH 4 .
The ground states of [Cu–O–Cu]
2+ and [Cu 3 (μ-O) 3 ]
2+ active species have been an
intensive discussion between theoretical chemists because the triplet and open-shell
singlet states in the [Cu–O–Cu]
2+ as well as the quartet and doublet states in the
[Cu 3 (μ-O) 3 ]
2+ are energetically competitive. This is reasonable since the difference
between the two competitive spin states (i.e., high and low spin states) is only the
spin direction of the Cu
2+ ions. As shown in Table 1, the triplet state of [Cu–O–
Cu]
2+ is formed when the unpaired electron of each Cu
2+ is an α electron, whereas
the open-shell singlet state is formed when one of the Cu
2+ ions has an unpaired α
electron and the other one has an unpaired β electron. Similarly, the quartet state
of [Cu 3 (μ-O) 3 ]
2+ is formed when all of the three Cu
2+ ions have an unpaired α
electron while the doublet state is formed when one of the Cu
2+ ions has an unpaired
β electron. Alternatively, the three unpaired electrons might originate from two Cu
2+
ions and an O
·– radical [44]. The data showing spin densities of each Cu
2+ ion in the