56
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
extracts hydrogen from methane at 298 K, forming a methyl radical that binds with
oxygen via a rebound mechanism to give a CH 3 O–Fe complex. The CH 3 O group
can be extracted as methanol with water [22, 23]. The selectivity for methanol after
aqueous extraction was shown to be 75%. This stoichiometric methanol production
suggests that the catalytic oxidation of methane to methanol using molecular oxygen
to methanol might be achieved over iron-exchanged ZSM-5, but the catalytic reaction
has not been established yet.
3.2.2 Molybdenum
Molybdenum oxide (MoO 3 ) has been investigated as a catalyst for the partial
oxidation of methane to methanol using molecular oxygen as the oxidant. Although
the major products of MoO 3 -catalyzed methane conversion are formaldehyde,
carbon dioxide, and carbon monoxide [2, 24, 25], supported molybdenum oxide
catalysts were recognized as the most active catalysts for the partial oxidation of
methane to methanol until the beginning of this century [1, 26, 27].
Various materials have been investigated as supports for the dispersion of
molybdenum oxide. SiO 2 -supported MoO 3 exhibits particularly good performance;
other materials, such as Al 2 O 3 , Al 2 O 3 –SiO 2 , MgO, TiO 2 , TiO 2 –SiO 2 , SnO, ZnO 2 ,
H-ZSM-5, and US-Y, have shown quite poor performance compared to SiO 2 . The
Mo(V) loading has significant effects on properties of the catalysts that determine
their catalytic performance, such as their structure and reducibility [28]. Generally,
catalysts with low Mo(V) loadings (high dispersion) show better activity and
selectivity than those with high Mo(V) loadings. It remains unclear whether the
participation of lattice oxygen species (O
2− ) or adsorbed oxygen species (O
− ) plays
an important role in the formation of C 1 -oxygenates.
Enhancement of methane conversion and its selectivity toward methanol was
observed in Mo catalysts in the presence of water vapor. In the case of the partial
oxidation of methane to methanol and formaldehyde on highly dispersed MoO 3 /SiO 2
catalysts at 873 K using a gas mixture consisting of oxygen, methane, and water
[29], the selectivity toward the oxygenates (methanol and formaldehyde) was found
to depend strongly on the water vapor fraction; the distribution of the oxygenates in
the products increased with increasing content of water in the feed gas. Inversely,
the distribution of CO and CO 2 in the products decreased with increasing water
content. In the case of the partial oxidation of methane over silica-supported MoO 3
catalysts in the presence of excess water vapor, silicomolybdic acid (H 4 SiMo 12 O 40 )
was determined to be the active species [30]; the formation of this species during
the reaction was partially evidenced by infrared (IR) spectroscopy measurements.
Accordingly, silica-supported silicomolybdic acid catalysts were prepared and tested
under the conditions described above to determine whether silicomolybdic acid could
effectively catalyze the partial oxidation of methane. The results showed that silicomolybdic acid was active for the production of oxygenates at water vapor fractions
of more than 50% in the feed gas. The oxygenate yield increased to 20% of methane
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
extracts hydrogen from methane at 298 K, forming a methyl radical that binds with
oxygen via a rebound mechanism to give a CH 3 O–Fe complex. The CH 3 O group
can be extracted as methanol with water [22, 23]. The selectivity for methanol after
aqueous extraction was shown to be 75%. This stoichiometric methanol production
suggests that the catalytic oxidation of methane to methanol using molecular oxygen
to methanol might be achieved over iron-exchanged ZSM-5, but the catalytic reaction
has not been established yet.
3.2.2 Molybdenum
Molybdenum oxide (MoO 3 ) has been investigated as a catalyst for the partial
oxidation of methane to methanol using molecular oxygen as the oxidant. Although
the major products of MoO 3 -catalyzed methane conversion are formaldehyde,
carbon dioxide, and carbon monoxide [2, 24, 25], supported molybdenum oxide
catalysts were recognized as the most active catalysts for the partial oxidation of
methane to methanol until the beginning of this century [1, 26, 27].
Various materials have been investigated as supports for the dispersion of
molybdenum oxide. SiO 2 -supported MoO 3 exhibits particularly good performance;
other materials, such as Al 2 O 3 , Al 2 O 3 –SiO 2 , MgO, TiO 2 , TiO 2 –SiO 2 , SnO, ZnO 2 ,
H-ZSM-5, and US-Y, have shown quite poor performance compared to SiO 2 . The
Mo(V) loading has significant effects on properties of the catalysts that determine
their catalytic performance, such as their structure and reducibility [28]. Generally,
catalysts with low Mo(V) loadings (high dispersion) show better activity and
selectivity than those with high Mo(V) loadings. It remains unclear whether the
participation of lattice oxygen species (O
2− ) or adsorbed oxygen species (O
− ) plays
an important role in the formation of C 1 -oxygenates.
Enhancement of methane conversion and its selectivity toward methanol was
observed in Mo catalysts in the presence of water vapor. In the case of the partial
oxidation of methane to methanol and formaldehyde on highly dispersed MoO 3 /SiO 2
catalysts at 873 K using a gas mixture consisting of oxygen, methane, and water
[29], the selectivity toward the oxygenates (methanol and formaldehyde) was found
to depend strongly on the water vapor fraction; the distribution of the oxygenates in
the products increased with increasing content of water in the feed gas. Inversely,
the distribution of CO and CO 2 in the products decreased with increasing water
content. In the case of the partial oxidation of methane over silica-supported MoO 3
catalysts in the presence of excess water vapor, silicomolybdic acid (H 4 SiMo 12 O 40 )
was determined to be the active species [30]; the formation of this species during
the reaction was partially evidenced by infrared (IR) spectroscopy measurements.
Accordingly, silica-supported silicomolybdic acid catalysts were prepared and tested
under the conditions described above to determine whether silicomolybdic acid could
effectively catalyze the partial oxidation of methane. The results showed that silicomolybdic acid was active for the production of oxygenates at water vapor fractions
of more than 50% in the feed gas. The oxygenate yield increased to 20% of methane