54
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
representative examples for the conversion of methane to methanol derivatives in
heterogeneous and homogeneous systems.
3.2 Heterogeneous Reactions for the Production
of Methanol from Methane
Many heterogeneous catalysts have been investigated for the direct and selective
oxidation of methane to methanol using molecular oxygen as the oxidant. Metal
ions with valences greater than 3, such as Mo, V, Ga, Fe, Co, Mn, and Pd, are active
in methane oxidation reactions [1, 2]. However, in most reported reactions, carbon
dioxide and carbon monoxide have been the major products, and the yield of partially
oxidized methane products has been low. In addition, higher reaction temperatures
have typically been required when molecular oxygen is used as the oxidant rather
than N 2 O [3, 4]. However, some materials based on the aforementioned metals have
been found to be capable of catalytically producing methanol from methane.
3.2.1 Iron
Fe(III) species impregnated on fumed silica [5], iron-exchanged zeolite (Fe/ZSM-5)
[6], mesoporous silica SBA-15-supported FeO x [7], FeO x /SiO 2 prepared by adsorption–precipitation [8] and sol–gel methods [9], and supported crystalline FePO 4
[10–15] have been reported to display various degrees of activity and selectivity in
the direct oxidation of methane to formaldehyde and methanol. These studies have
highlighted iron-based catalysts as promising candidates for the selective oxidation
of methane to methanol and formaldehyde.
Silica-supported Fe 2 O 3 catalyzes the conversion of methane under relatively mild
conditions [2, 16]. A Fe/SiO 2 catalyst afforded reasonable methane conversion and
methanol selectivity (13.2% and 78.8%, respectively) compared to those achieved
with molybdenum and vanadium oxides (mentioned in Sects. 3.2.2 and 3.2.3) [17].
Mössbauer spectroscopic analysis of the Fe/SiO 2 catalyst revealed that 81% of the
iron in the catalyst existed as supported hematite (Fe 2 O 3 ), whereas the remaining
19% was embedded into the silica matrix as tetrahedral Fe
3+ sites [17]. This result
suggests the promising potential of suitable silica-supported iron catalysts for the
partial oxidation of methane to methanol.
On the other hand, several iron-based catalysts have been investigated for the
molecular oxygen-mediated oxidation of methane to methanol. However, their
catalytic performances are not superior to those seen in the silica-supported Fe 2 O 3
systems discussed above.
FePO 4 catalysts supported on SiO 2 with iron loadings of 2–16 wt% were
examined in the selective oxidation of methane using molecular oxygen in a
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
representative examples for the conversion of methane to methanol derivatives in
heterogeneous and homogeneous systems.
3.2 Heterogeneous Reactions for the Production
of Methanol from Methane
Many heterogeneous catalysts have been investigated for the direct and selective
oxidation of methane to methanol using molecular oxygen as the oxidant. Metal
ions with valences greater than 3, such as Mo, V, Ga, Fe, Co, Mn, and Pd, are active
in methane oxidation reactions [1, 2]. However, in most reported reactions, carbon
dioxide and carbon monoxide have been the major products, and the yield of partially
oxidized methane products has been low. In addition, higher reaction temperatures
have typically been required when molecular oxygen is used as the oxidant rather
than N 2 O [3, 4]. However, some materials based on the aforementioned metals have
been found to be capable of catalytically producing methanol from methane.
3.2.1 Iron
Fe(III) species impregnated on fumed silica [5], iron-exchanged zeolite (Fe/ZSM-5)
[6], mesoporous silica SBA-15-supported FeO x [7], FeO x /SiO 2 prepared by adsorption–precipitation [8] and sol–gel methods [9], and supported crystalline FePO 4
[10–15] have been reported to display various degrees of activity and selectivity in
the direct oxidation of methane to formaldehyde and methanol. These studies have
highlighted iron-based catalysts as promising candidates for the selective oxidation
of methane to methanol and formaldehyde.
Silica-supported Fe 2 O 3 catalyzes the conversion of methane under relatively mild
conditions [2, 16]. A Fe/SiO 2 catalyst afforded reasonable methane conversion and
methanol selectivity (13.2% and 78.8%, respectively) compared to those achieved
with molybdenum and vanadium oxides (mentioned in Sects. 3.2.2 and 3.2.3) [17].
Mössbauer spectroscopic analysis of the Fe/SiO 2 catalyst revealed that 81% of the
iron in the catalyst existed as supported hematite (Fe 2 O 3 ), whereas the remaining
19% was embedded into the silica matrix as tetrahedral Fe
3+ sites [17]. This result
suggests the promising potential of suitable silica-supported iron catalysts for the
partial oxidation of methane to methanol.
On the other hand, several iron-based catalysts have been investigated for the
molecular oxygen-mediated oxidation of methane to methanol. However, their
catalytic performances are not superior to those seen in the silica-supported Fe 2 O 3
systems discussed above.
FePO 4 catalysts supported on SiO 2 with iron loadings of 2–16 wt% were
examined in the selective oxidation of methane using molecular oxygen in a