98
M. H. Mahyuddin
at moderate temperature (up to 300 °C) to form a [Cu
II –O–Cu
II ]
2+ species. Raising
the temperature to more than 450 °C reduces the two Cu centers to Cu
I and desorbs
the bridging O atom to form a [Cu
I ··· Cu
I ]
2+ species [36] and an O 2 molecule after
rebound with another desorbed O atom from the neighboring Cu site [37]. The high
temperature seems to be reasonable since the formation energy for [Cu
II –O–Cu
II ]
2+
→ [Cu
I ··· Cu
I ]
2+
+ ½ O 2 reaction in MOR was calculated to be quite high (about
55 kcal/mol) [33]. At the same temperature, some of the reduced Cu
I species migrate
to the Cu
I pairs located nearby the Al pairs (i.e., the active site), where O 2 and
· OH
then oxidize them at low temperature to form the [Cu 3 (μ-O) 3 ]
2+ active site [36]. This
therefore confirms the above discussion on DFT work showing that the O 2 activation
forming [Cu
II –O–Cu
II ]
2+ active site requires low activation barrier and clarifies that
the high temperature required for the Cu-MOR activation by O 2 is mainly due to
dehydration, reduction, and migration of the Cu centers.
4 Conclusion
In this chapter, I have overviewed recent topics in the mechanistic understanding
of oxidative activation of Fe- and Cu-exchanged zeolites from a theoretical point
of view. While the N 2 O and H 2 O 2 decompositions are straightforward two-electron
cleavage processes, the O 2 activation is a four-electron cleavage process involving
more complicated mechanisms. Despite such a difference, we have shown that the
activation energies required for N 2 O, H 2 O 2 , and O 2 activations on the reduced Fe
and Cu centers discussed here (except on the [Cu 3 O]
2+ ) are lower than 20 kcal/mol,
implying that the activation can in principle be achieved at low temperatures. This
is actually consistent with the recent experimental work on XAS and UV–vis spectroscopies [36] although it is known that a high-temperature activation of Cu-MOR
by O 2 is more favored for higher yield of methanol. Such a thermal pre-treatment
is necessary for dehydration, reduction, and migration of the Cu centers to form a
precursor that is favorable for the O=O bond cleavage forming the [Cu 3 (μ-O) 3 ]
2+
active site [36]. Despite this interesting mechanistic understanding, further spectroscopic and DFT investigations are still needed to probe other possible mechanistic
reasons, since the same mechanism might not be transferable to the formation of
[FeO]
2+ and [Cu 2 (μ-O)]
2+ active sites. There are at least two other reasons that are
possibly true: (1) different Fe- and Cu-oxo active site structures are formed at high
temperature, and (2) a high concentration of the Fe and Cu active sites is achieved
only at high temperature.
Until today, the best performance of Fe- and Cu-exchanged zeolites for methane
hydroxylation is achieved with the stepwise process. Thus, it is not exaggerating to
say that this process will still be the main topic of discussion in methane hydroxylation
by metal-exchanged zeolite catalysts. However, the oxidative activation step involved
in this process will be an endless bottleneck if massive investigations are not directed
to address this challenge. Beside the O 2 activation, the H 2 O decomposition to form
the Cu active sites [38] is also highly desirable, as water is abundant and the reaction
M. H. Mahyuddin
at moderate temperature (up to 300 °C) to form a [Cu
II –O–Cu
II ]
2+ species. Raising
the temperature to more than 450 °C reduces the two Cu centers to Cu
I and desorbs
the bridging O atom to form a [Cu
I ··· Cu
I ]
2+ species [36] and an O 2 molecule after
rebound with another desorbed O atom from the neighboring Cu site [37]. The high
temperature seems to be reasonable since the formation energy for [Cu
II –O–Cu
II ]
2+
→ [Cu
I ··· Cu
I ]
2+
+ ½ O 2 reaction in MOR was calculated to be quite high (about
55 kcal/mol) [33]. At the same temperature, some of the reduced Cu
I species migrate
to the Cu
I pairs located nearby the Al pairs (i.e., the active site), where O 2 and
· OH
then oxidize them at low temperature to form the [Cu 3 (μ-O) 3 ]
2+ active site [36]. This
therefore confirms the above discussion on DFT work showing that the O 2 activation
forming [Cu
II –O–Cu
II ]
2+ active site requires low activation barrier and clarifies that
the high temperature required for the Cu-MOR activation by O 2 is mainly due to
dehydration, reduction, and migration of the Cu centers.
4 Conclusion
In this chapter, I have overviewed recent topics in the mechanistic understanding
of oxidative activation of Fe- and Cu-exchanged zeolites from a theoretical point
of view. While the N 2 O and H 2 O 2 decompositions are straightforward two-electron
cleavage processes, the O 2 activation is a four-electron cleavage process involving
more complicated mechanisms. Despite such a difference, we have shown that the
activation energies required for N 2 O, H 2 O 2 , and O 2 activations on the reduced Fe
and Cu centers discussed here (except on the [Cu 3 O]
2+ ) are lower than 20 kcal/mol,
implying that the activation can in principle be achieved at low temperatures. This
is actually consistent with the recent experimental work on XAS and UV–vis spectroscopies [36] although it is known that a high-temperature activation of Cu-MOR
by O 2 is more favored for higher yield of methanol. Such a thermal pre-treatment
is necessary for dehydration, reduction, and migration of the Cu centers to form a
precursor that is favorable for the O=O bond cleavage forming the [Cu 3 (μ-O) 3 ]
2+
active site [36]. Despite this interesting mechanistic understanding, further spectroscopic and DFT investigations are still needed to probe other possible mechanistic
reasons, since the same mechanism might not be transferable to the formation of
[FeO]
2+ and [Cu 2 (μ-O)]
2+ active sites. There are at least two other reasons that are
possibly true: (1) different Fe- and Cu-oxo active site structures are formed at high
temperature, and (2) a high concentration of the Fe and Cu active sites is achieved
only at high temperature.
Until today, the best performance of Fe- and Cu-exchanged zeolites for methane
hydroxylation is achieved with the stepwise process. Thus, it is not exaggerating to
say that this process will still be the main topic of discussion in methane hydroxylation
by metal-exchanged zeolite catalysts. However, the oxidative activation step involved
in this process will be an endless bottleneck if massive investigations are not directed
to address this challenge. Beside the O 2 activation, the H 2 O decomposition to form
the Cu active sites [38] is also highly desirable, as water is abundant and the reaction