1.5 Types of Key Reaction Intermediates
11
CH x (0 < x < 3) including the carbene methylene (:CH 2 ),
+ CH 3 , M–CH 3 , M–CH 2 ,
and M–OCH 3 . The key reaction intermediate depends on both the type of methane
conversion reaction and the type of used catalyst.
On the other hand, the key reaction intermediates shown in Fig. 1.3 can be generated by methods other than the use of catalysts to accomplish the direct conversion
of methane to chemicals. In particular, methane conversion can be achieved using
an electric field [65–67] or plasma [68, 69]; the combination of these methods with
catalysts [67–75] including photocatalysts [74, 75] is also effective.
It should be noted that methyl carbanions (
− CH 3 ) do not take part in the formation
of C–O and C–H bonds to produce methanol or higher hydrocarbons in methane
conversion. In other words,
− CH 3 is not an intermediate in the formation of C–O and
C–H bonds in the methane conversion, even though in deuterium exchange reactions,
the C–H bond of CH 4 is cleaved and H–D exchange occurs between D 2 and CH 4 , as
shown in reaction (1.14).
D 2 + CH 4
CH 3 D + HD
(1.14)
For example, deuterium exchange proceeds over strong base catalysts, such as
KNH 2 supported on Al 2 O 3 (KNH 2 /Al 2 O 3 ), and the reaction between C 2 H 6 and D 2
over KNH 2 /Al 2 O 3 exhibits a faster exchange rate than that of CH 4 and D 2 [76].
These results indicate that the H–D exchange reaction between C 2 H 6 with D 2 in
the presence of a catalyst proceeds via
− CH 3 reaction intermediates. Prior to this
exchange reaction, the cleavage of a C–H bond of CH 4 should occur.
However,
− CH 3 does not proceed to form C–O and C–H bonds in methane conversion. Thus, methanol and C 2
+ hydrocarbons are not produced via
− CH 3 reaction
intermediates, and C–H bond cleavage does not always result in the formation of
a new C–H bond. This means that the rate of the H–D exchange reaction does not
always correspond to the activity of the catalyst in the methane conversion reaction.
1.6 Generation of Key Reaction Intermediates
As shown in Fig. 1.3, in order for the direct conversion of methane to proceed,
key reaction intermediates must be generated from methane using various catalysts.
However, a given catalyst may generate more than one kind of reaction intermediate.
For example, CH x (0 < x < 3) intermediates were generated over Mo-zeolites in
dehydroaromatization [13, 16, 17], while some reaction intermediates with metal–
carbon bonds have been observed in the reaction of metal ions with CH 4 [43, 77, 78].
These results demonstrate the importance of investigating the reaction intermediates
responsible for the production of C–O and C–H bonds in the direct conversion of
methane. Therefore, in this section, the methods by which various reaction intermediates are generated are briefly illustrated via some main examples; the individual
reactions are further discussed in their respective chapters.
11
CH x (0 < x < 3) including the carbene methylene (:CH 2 ),
+ CH 3 , M–CH 3 , M–CH 2 ,
and M–OCH 3 . The key reaction intermediate depends on both the type of methane
conversion reaction and the type of used catalyst.
On the other hand, the key reaction intermediates shown in Fig. 1.3 can be generated by methods other than the use of catalysts to accomplish the direct conversion
of methane to chemicals. In particular, methane conversion can be achieved using
an electric field [65–67] or plasma [68, 69]; the combination of these methods with
catalysts [67–75] including photocatalysts [74, 75] is also effective.
It should be noted that methyl carbanions (
− CH 3 ) do not take part in the formation
of C–O and C–H bonds to produce methanol or higher hydrocarbons in methane
conversion. In other words,
− CH 3 is not an intermediate in the formation of C–O and
C–H bonds in the methane conversion, even though in deuterium exchange reactions,
the C–H bond of CH 4 is cleaved and H–D exchange occurs between D 2 and CH 4 , as
shown in reaction (1.14).
D 2 + CH 4
CH 3 D + HD
(1.14)
For example, deuterium exchange proceeds over strong base catalysts, such as
KNH 2 supported on Al 2 O 3 (KNH 2 /Al 2 O 3 ), and the reaction between C 2 H 6 and D 2
over KNH 2 /Al 2 O 3 exhibits a faster exchange rate than that of CH 4 and D 2 [76].
These results indicate that the H–D exchange reaction between C 2 H 6 with D 2 in
the presence of a catalyst proceeds via
− CH 3 reaction intermediates. Prior to this
exchange reaction, the cleavage of a C–H bond of CH 4 should occur.
However,
− CH 3 does not proceed to form C–O and C–H bonds in methane conversion. Thus, methanol and C 2
+ hydrocarbons are not produced via
− CH 3 reaction
intermediates, and C–H bond cleavage does not always result in the formation of
a new C–H bond. This means that the rate of the H–D exchange reaction does not
always correspond to the activity of the catalyst in the methane conversion reaction.
1.6 Generation of Key Reaction Intermediates
As shown in Fig. 1.3, in order for the direct conversion of methane to proceed,
key reaction intermediates must be generated from methane using various catalysts.
However, a given catalyst may generate more than one kind of reaction intermediate.
For example, CH x (0 < x < 3) intermediates were generated over Mo-zeolites in
dehydroaromatization [13, 16, 17], while some reaction intermediates with metal–
carbon bonds have been observed in the reaction of metal ions with CH 4 [43, 77, 78].
These results demonstrate the importance of investigating the reaction intermediates
responsible for the production of C–O and C–H bonds in the direct conversion of
methane. Therefore, in this section, the methods by which various reaction intermediates are generated are briefly illustrated via some main examples; the individual
reactions are further discussed in their respective chapters.