Chemistry Reference
In-Depth Information
Scheme 8 Competing
catalytic cycles for
oxidation of methane
catalysed by Au
2
+
: Cycle
1 involves dehydrocoupling
of methane to yield ethane
(Eq. (
78
)) [
326
,
327
]; Cycle
2 involves oxidation of
methane to formaldehyde
(Eq. (
79
)) [
327
]
reaction with a second molecule of methane (step 2 of Scheme
8
). The subsequent
reactions of Au
2
(CH
4
)
2
+
represent the branch points for the competing cycles.
Either in the absence of O
2
or at higher temperatures that disfavour the adsorption
of O
2
, cycle 1 proceeds via elimination of H
2
(step 3) [
326
,
327
]. Desorption of ethene
from Au
2
(C
2
H
4
)
+
does not occur spontaneously, but rather is triggered by reaction
with a third molecule of methane (step 5), which closes catalytic cycle 1 via the
regeneration of Au
2
(CH
4
)
+
. This cycle corresponds to the dehydrocoupling of two
molecules of methane to form ethene (Eq. (
78
)). By studying the relative abundances
of the various product ions as a function of temperature, information was gleaned on
the relative barriers associated with key steps. Thus the barrier for dehydrogenation
(step 3) can only be surmounted at 250 K and above, while loss of ethene from
Au
2
(C
2
H
4
)(CH
4
)
+
only occurs at temperatures above 270K. At the lowest temperature
studied (200 K), a minor product formulated as Au
2
(C
4
H
12
)
+
was observed, while the
ion Au(C
2
H
4
)(CH
4
)
+
, which corresponds to loss of an Au atom from Au
2
(C
2
H
4
)
(CH
4
)
+
, appears at 250 K. The DFT calculations reveal that the mechanistic sequence
for dehydrogenation of Au
2
(CH
4
)
2
+
involves stepwise losses of H
2
and involves
11 intermediates and 9 transition states. The first loss of H
2
gives rise to the organo-
metallic cluster ion, (CH
3
Au)
2
+
as a key intermediate. A related catalytic cycle was
recently reported for the reaction of mixed metal cluster Pd
2
Au
+
with CD
4
[
319
]:
2CH
4
!
CH
2
¼
CH
2
þ
2H
2
ð
78
Þ
Cycle 2, which corresponds to the oxidation of methane to formaldehyde
(Eq. (
79
)), dominates at 210 K, while cycle 2 becomes competitive at 240 K and
then dominates at temperatures of 250 K and above [
327
]. The cooperative action
of multiple substrate and oxidant molecules is vital for the successful progress of
cycle 2. Only Au
2
(CH
4
)
2
+
can react with a molecule of O
2
to form Au
2
(CH
4
)
2
O
2
+
(step 3), which reacts with a third molecule methane and a second molecule of O
2
to
form Au
2
(C
3
H
8
O
2
)
+
and 2 equiv. of water (step 4) [
327
]. The final step involves loss
of two molecules of formaldehyde from Au
2
(C
3
H
8
O
2
)
+
to regenerate the catalyst
Au
2
(CH
4
)
+
. A multistep process for oxidation of both adsorbed methane molecules
