60
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
species ([Cu 3 (µ-O) 3 ]
2+ ) has also been reported [55, 56]. Although the oxygen
molecules are activated by the same binuclear copper ion species as in the enzymecatalyzed reaction, the subsequent oxo species differs from that produced in the
enzyme; namely, a CH 3 O-group is formed in the methane conversion reaction on the
copper ion-exchanged zeolite. Methanol is then obtained by extracting this species
using water. Additionally, in most copper-exchanged zeolites, expensive oxidants
such as tert-butyl hydroperoxide [57], H 2 O 2 [58], and N 2 O [59] have been required
to generate reactive oxygen species.
Interestingly, methanol can be obtained using Cu-exchanged zeolites, even in the
absence of oxygen using water as the oxidizing agent [60]. A quantum chemical
calculation of this process indicated that after the copper ion bonds with the carbon
of methane, a zeolite lattice oxygen bonds with hydrogen, giving Cu–CH 3 and a
Brønsted acid. The CH 3 group then reacts with the oxygen atom bridging the Cu(II)
centers to form a CH 3 O group. At this time, both Cu(II) centers are reduced to Cu(I).
Subsequently, a water molecule reacts with the CH 3 O group to dissociate methanol,
a binuclear copper complex in which water molecules are crosslinked is formed, and
finally, hydrogen dissociates. The reaction of water differentiates this mechanism
from the formation of methanol via the reaction of CH 3 O-groups formed on the
surface of the zeolite with water [61]. In this reaction mechanism, the oxygen of the
zeolite lattice also contributes to the reaction, in addition to the copper ions and the
oxygen atoms bridging them. This is a function of the zeolite lattice, which creates
an environment more similar to the reaction field of the enzyme than to a molecular
catalyst involving only a metal ion and its ligands.
3.2.5 Heteropoly Compounds
Heteropoly compounds have been investigated as catalysts for the partial oxidation
of methane to formaldehyde and methanol at atmospheric pressure in the temperature
range 973–1023 K. (NH 4 ) 6 HSiMo 11 FeO 40 , (NH 4 ) 4 PMo 11 FeO 39 , and H 4 PMo 11 VO 40
showed activity toward methane oxidation when O 2 or N 2 O was used as the oxidizing agent; the products of these reactions were CH 3 OH, HCHO, CO, CO 2 ,
and water [62]. The catalytic conversion of methane using solid CsPW 12-x M x O 40
(M=Fe, Co, or Ni) and the corresponding reaction mechanism were also investigated
[63]. According to the reported mechanism, methanol and carbon monoxide are
produced stoichiometrically, while formaldehyde and carbon dioxide are produced
catalytically.
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
species ([Cu 3 (µ-O) 3 ]
2+ ) has also been reported [55, 56]. Although the oxygen
molecules are activated by the same binuclear copper ion species as in the enzymecatalyzed reaction, the subsequent oxo species differs from that produced in the
enzyme; namely, a CH 3 O-group is formed in the methane conversion reaction on the
copper ion-exchanged zeolite. Methanol is then obtained by extracting this species
using water. Additionally, in most copper-exchanged zeolites, expensive oxidants
such as tert-butyl hydroperoxide [57], H 2 O 2 [58], and N 2 O [59] have been required
to generate reactive oxygen species.
Interestingly, methanol can be obtained using Cu-exchanged zeolites, even in the
absence of oxygen using water as the oxidizing agent [60]. A quantum chemical
calculation of this process indicated that after the copper ion bonds with the carbon
of methane, a zeolite lattice oxygen bonds with hydrogen, giving Cu–CH 3 and a
Brønsted acid. The CH 3 group then reacts with the oxygen atom bridging the Cu(II)
centers to form a CH 3 O group. At this time, both Cu(II) centers are reduced to Cu(I).
Subsequently, a water molecule reacts with the CH 3 O group to dissociate methanol,
a binuclear copper complex in which water molecules are crosslinked is formed, and
finally, hydrogen dissociates. The reaction of water differentiates this mechanism
from the formation of methanol via the reaction of CH 3 O-groups formed on the
surface of the zeolite with water [61]. In this reaction mechanism, the oxygen of the
zeolite lattice also contributes to the reaction, in addition to the copper ions and the
oxygen atoms bridging them. This is a function of the zeolite lattice, which creates
an environment more similar to the reaction field of the enzyme than to a molecular
catalyst involving only a metal ion and its ligands.
3.2.5 Heteropoly Compounds
Heteropoly compounds have been investigated as catalysts for the partial oxidation
of methane to formaldehyde and methanol at atmospheric pressure in the temperature
range 973–1023 K. (NH 4 ) 6 HSiMo 11 FeO 40 , (NH 4 ) 4 PMo 11 FeO 39 , and H 4 PMo 11 VO 40
showed activity toward methane oxidation when O 2 or N 2 O was used as the oxidizing agent; the products of these reactions were CH 3 OH, HCHO, CO, CO 2 ,
and water [62]. The catalytic conversion of methane using solid CsPW 12-x M x O 40
(M=Fe, Co, or Ni) and the corresponding reaction mechanism were also investigated
[63]. According to the reported mechanism, methanol and carbon monoxide are
produced stoichiometrically, while formaldehyde and carbon dioxide are produced
catalytically.