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Y. Hori and T. Abe
same spin state and the ferromagnetic coupled or antiferromagnetic coupled state.
A broken-symmetry approach using DFT can treat different electronic configurations, although spin contamination becomes a problem. The solvent, which plays an
important role in catalytic reactions, can be accounted for by using an implicit solvent
model (where the solvent is treated as a dielectric continuum) or an explicit solvent
model (where solvent molecules are treated at a lower level of quantum mechanics
or using molecular mechanics). Enzymatic systems are often modeled by a mixed
quantum mechanical/molecular mechanical (QM/MM) model, in which active sites
are treated quantum mechanically, and the remainder of the system is treated molecular mechanically. The choice of computational methods and models depends on the
level of accuracy required and the scale of the system [6].
3 Organometallic Approaches
Methane activation at low temperatures has been investigated by using homogeneous
catalysis. The C–H bond activation of methane under low temperatures does not
generate radical species and may lead to more selective reactions than those promoted
by heterogeneous catalysts operating at high temperatures.
Shilov reported some of the earliest evidence that the inert C–H bond of several
alkanes, including methane, were activated in a catalytic fashion [8]. Shilov showed
that H/D exchange would occur between alkanes and deuterium acid by the Pt(II)
and Pt(IV) complexes. Although this reaction is impractical owing to the use of
the Pt(IV) complex as the oxidant with high cost, these results gave an indication
that methane functionalization could be achieved by organometallic approaches,
and the transformation, known as the Shilov reaction, was developed in the period
1970–1985.
Based on the Shilov reaction, Periana reported two different systems for the oxidation of methane in sulfuric acid containing SO 3 . One of the catalysts was a simple
Hg(II) halide, and the Hg(II)-catalyzed reactions generated methyl sulfate with a
turnover frequency (TOF) of 10
−3 s
−1 [9]. The second system was more reactive
and was based on a complex of Pt(II) with a bipyrimidine (bpym) derivative [10].
In this case, methane was converted to methyl bisulfate (CH 3 OSO 3 H) [Eq. (4)] with
81% selectivity, >500 turnovers, and a TOF of 10
−2 s
−1 . The thus obtained methyl
bisulfate was then readily hydrolyzed to produce methanol [Eq. (5)].
CH 4 + 2H 2 SO 4 → CH 3 OSO 3 H + SO 2 + 2H 2 O
( 4 )
CH 3 OSO 3 H + H 2 O → CH 3 OH + H 2 SO 4
(5)
In this system, methane selectively converts to methanol because the electronwithdrawing group attached to the oxygen protects the overoxidation of methanol.
However, the major disadvantages are difficulty in separating the methanol product
Y. Hori and T. Abe
same spin state and the ferromagnetic coupled or antiferromagnetic coupled state.
A broken-symmetry approach using DFT can treat different electronic configurations, although spin contamination becomes a problem. The solvent, which plays an
important role in catalytic reactions, can be accounted for by using an implicit solvent
model (where the solvent is treated as a dielectric continuum) or an explicit solvent
model (where solvent molecules are treated at a lower level of quantum mechanics
or using molecular mechanics). Enzymatic systems are often modeled by a mixed
quantum mechanical/molecular mechanical (QM/MM) model, in which active sites
are treated quantum mechanically, and the remainder of the system is treated molecular mechanically. The choice of computational methods and models depends on the
level of accuracy required and the scale of the system [6].
3 Organometallic Approaches
Methane activation at low temperatures has been investigated by using homogeneous
catalysis. The C–H bond activation of methane under low temperatures does not
generate radical species and may lead to more selective reactions than those promoted
by heterogeneous catalysts operating at high temperatures.
Shilov reported some of the earliest evidence that the inert C–H bond of several
alkanes, including methane, were activated in a catalytic fashion [8]. Shilov showed
that H/D exchange would occur between alkanes and deuterium acid by the Pt(II)
and Pt(IV) complexes. Although this reaction is impractical owing to the use of
the Pt(IV) complex as the oxidant with high cost, these results gave an indication
that methane functionalization could be achieved by organometallic approaches,
and the transformation, known as the Shilov reaction, was developed in the period
1970–1985.
Based on the Shilov reaction, Periana reported two different systems for the oxidation of methane in sulfuric acid containing SO 3 . One of the catalysts was a simple
Hg(II) halide, and the Hg(II)-catalyzed reactions generated methyl sulfate with a
turnover frequency (TOF) of 10
−3 s
−1 [9]. The second system was more reactive
and was based on a complex of Pt(II) with a bipyrimidine (bpym) derivative [10].
In this case, methane was converted to methyl bisulfate (CH 3 OSO 3 H) [Eq. (4)] with
81% selectivity, >500 turnovers, and a TOF of 10
−2 s
−1 . The thus obtained methyl
bisulfate was then readily hydrolyzed to produce methanol [Eq. (5)].
CH 4 + 2H 2 SO 4 → CH 3 OSO 3 H + SO 2 + 2H 2 O
( 4 )
CH 3 OSO 3 H + H 2 O → CH 3 OH + H 2 SO 4
(5)
In this system, methane selectively converts to methanol because the electronwithdrawing group attached to the oxygen protects the overoxidation of methanol.
However, the major disadvantages are difficulty in separating the methanol product