3.2 Heterogeneous Reactions for the Production of Methanol …
57
at a water vapor fraction of around 60%, with a methane conversion of about 25%
and an oxygenate selectivity of 90%. However, in the absence of water vapor, the
silicomolybdic acid easily decomposed into SiO 2 and MoO 3 at 873 K.
The properties of the oxygen species generated on the surface of MoO x by different oxidants have been investigated [31]. Barbaux et al. found that O
− was generated
from O 2 and led to the formation of CO x , while N 2 O generated O
2− led to increased
selectivity toward formaldehyde. With an emphasis on the surface species formed
on the supports and their effects on reactivity and selectivity, the partial oxidation
of methane to formaldehyde over MoO 3 /SiO 2 was examined [32]. Smith and Ozkan
found that low MoO 3 loadings led to the generation of silicomolybdic species with
terminal Mo=O sites, while higher MoO 3 loadings led to the generation of polymolybdate species with Mo–O–Mo bridges; the former showed better selectivity
toward formaldehyde than the latter. Interestingly, another study seemed to support
the opposite view; bridging oxygens [–O–] were concluded to be responsible for
selective oxidation, while terminal oxygens [=O] were associated with full oxidation
[33].
Different C–H activation pathways over various terminal [=O] and bridging [–O–]
active sites have been investigated by Fu et al. using density functional theory (DFT)
calculations of the Mo 3 O 9 model system [34]. Their theoretical calculations indicated
that H abstraction is a viable pathway for the activation of C–H bonds on molybdenum
oxide. The terminal sites [=O] were calculated to be more reactive in H abstraction
than the bridging sites [–O–]. The H abstraction produced radical pairs, which have
also been detected in electron paramagnetic resonance (EPR) experiments. These
radical pairs underwent a fast rebound process resulting in the generation of stable
hydroxyl and alkoxy species, which have been detected in IR experiments. These
experimental observations can be explained via the mechanism below:
1. The calculated activation energy for CH 4 conversion is 188 kJ mol
−1 , which was
consistent with the values derived from experimental data (172 [35], 176 [3], and
189 [36] kJ mol
−1 ).
2. A one-electron oxidation process results in the generation of a Mo
5+ species
and an alkyl radical, which is consistent with electron paramagnetic resonance
measurements [37].
3. H atom abstraction followed by fast oxygen atom insertion results in the generation of stable hydroxyl and alkoxy species, consistent with IR observations
[38].
The mechanism shown in Fig. 3.1 is more favorable than the alternative (5 + 2)
cycloaddition reaction mechanism. H abstraction/O rebound mechanisms are ubiquitous in various areas of chemistry, including biological reactions of metalloenzymes,
heterogeneous reactions on solid surfaces, homogeneous reactions with organic metal
complexes, and gas-phase and matrix-isolated species [39]. The calculation results
also imply that the (2 + 2) cycloaddition reaction mechanism (Mo-O reacts with
H-CH 3 ) is most feasible when the Mo=O bond is highly polar, while the (5 + 2)
mechanism can provide another effective pathway if the two Mo=O bonds are in
close proximity.
57
at a water vapor fraction of around 60%, with a methane conversion of about 25%
and an oxygenate selectivity of 90%. However, in the absence of water vapor, the
silicomolybdic acid easily decomposed into SiO 2 and MoO 3 at 873 K.
The properties of the oxygen species generated on the surface of MoO x by different oxidants have been investigated [31]. Barbaux et al. found that O
− was generated
from O 2 and led to the formation of CO x , while N 2 O generated O
2− led to increased
selectivity toward formaldehyde. With an emphasis on the surface species formed
on the supports and their effects on reactivity and selectivity, the partial oxidation
of methane to formaldehyde over MoO 3 /SiO 2 was examined [32]. Smith and Ozkan
found that low MoO 3 loadings led to the generation of silicomolybdic species with
terminal Mo=O sites, while higher MoO 3 loadings led to the generation of polymolybdate species with Mo–O–Mo bridges; the former showed better selectivity
toward formaldehyde than the latter. Interestingly, another study seemed to support
the opposite view; bridging oxygens [–O–] were concluded to be responsible for
selective oxidation, while terminal oxygens [=O] were associated with full oxidation
[33].
Different C–H activation pathways over various terminal [=O] and bridging [–O–]
active sites have been investigated by Fu et al. using density functional theory (DFT)
calculations of the Mo 3 O 9 model system [34]. Their theoretical calculations indicated
that H abstraction is a viable pathway for the activation of C–H bonds on molybdenum
oxide. The terminal sites [=O] were calculated to be more reactive in H abstraction
than the bridging sites [–O–]. The H abstraction produced radical pairs, which have
also been detected in electron paramagnetic resonance (EPR) experiments. These
radical pairs underwent a fast rebound process resulting in the generation of stable
hydroxyl and alkoxy species, which have been detected in IR experiments. These
experimental observations can be explained via the mechanism below:
1. The calculated activation energy for CH 4 conversion is 188 kJ mol
−1 , which was
consistent with the values derived from experimental data (172 [35], 176 [3], and
189 [36] kJ mol
−1 ).
2. A one-electron oxidation process results in the generation of a Mo
5+ species
and an alkyl radical, which is consistent with electron paramagnetic resonance
measurements [37].
3. H atom abstraction followed by fast oxygen atom insertion results in the generation of stable hydroxyl and alkoxy species, consistent with IR observations
[38].
The mechanism shown in Fig. 3.1 is more favorable than the alternative (5 + 2)
cycloaddition reaction mechanism. H abstraction/O rebound mechanisms are ubiquitous in various areas of chemistry, including biological reactions of metalloenzymes,
heterogeneous reactions on solid surfaces, homogeneous reactions with organic metal
complexes, and gas-phase and matrix-isolated species [39]. The calculation results
also imply that the (2 + 2) cycloaddition reaction mechanism (Mo-O reacts with
H-CH 3 ) is most feasible when the Mo=O bond is highly polar, while the (5 + 2)
mechanism can provide another effective pathway if the two Mo=O bonds are in
close proximity.