Abstract
Periodic
density functional theory (DFT) calculations were carried
out to investigate the mechanism of methane oxidation with H
2
O
2
over the defined Fe sites in Fe/ZSM-5 zeolite. The
initial Fe site is modeled as a [(H
2
O)
2
–Fe(III)–(μO)
2
–Fe(III)–(H
2
O)
2
]
2+
extraframework cluster deposited in the zeolite pore and charge-compensated
by two anionic lattice sites. The activation of this cluster with
H
2
O
2
gives rise to the formation of a variety
of Fe(III)-oxo and Fe(IV)-oxo complexes potentially reactive toward
methane dissociation. These sites are all able to promote the first
C–H bond cleavage in methane by following three possible reaction
mechanisms: namely, (a) heterolytic and (b) homolytic methane dissociation
as well as (c) Fenton-type reaction involving free OH radicals as
the catalytic species. The C–H activation step is followed
by formation of MeOH and MeOOH and regeneration of the active site.
The Fenton-type path is found to proceed with the lowest activation
barrier. Although the barriers for the alternative heterolytic and
homolytic pathways are found to be somewhat higher, they are still
quite favorable and are expected to be feasible under reaction conditions,
resulting ultimately in MeOH and MeOOH products. H
2
O
2
oxidant competes with CH
4
substrate for the same
sites. Since the oxidation of H
2
O
2
to O
2
and two [H
+
] is energetically more favorable than
the C–H oxofunctionalization, the overall efficiency of the
latter target process remains low.