148
6 Conversion of Methane to Aromatic Hydrocarbons
between two Al atoms at Si/Al ratios lower than 25. Goodman et al. reported that in
the straight channels of H-ZSM-5, all T sites were within 4.75 Å of the axis running
down the center of the zeolite channel (Si/Al ratio < 25) [85].
On the basis of the above estimations of the distance between two Al atoms in
next-next-nearest neighbor T sites in the zeolite ZSM-5, the formation of Species
(II) and Species (III) was concluded to occur in H-ZSM-5 zeolites with a Si/Al ratio
lower than 95 if dealumination did not occur.
Borry et al. examined the ratio of the number of acidic protons to the number of
Mo atoms introduced into H-ZSM-5 (H
+ /Mo) at a Si/Al ratio of 14.3. They experimentally demonstrated that the H
+ /Mo ratio was constant at approximately 1.24
when the Mo content was lower than 3.6 wt% [71]. This indicated that Species (II)
was mainly produced as the Mo species on and/or in the H-ZSM-5. However, when
the Mo content was increased to 6.3 wt%, the H
+ /Mo ratio decreased to 0.64. This
means that the formation of Species (III) predominates over that of Species (II).
Tessonnier et al. also examined the H
+ /Mo ratios in Mo-modified H-ZSM-5 zeolites [86]. When 2 wt% Mo was supported on H-ZSM-5 with a Si/Al ratio of 40, the
H
+ /Mo ratio was almost unity, indicating the formation of Species (II). On the other
hand, when Mo (2 wt% or 4 wt%) was supported on H-ZSM-5 with a Si/Al ratio
of 15, the H
+ /Mo ratio was almost two in both cases. This result indicated that the
monomeric bidentate Mo species (Species (III)) was formed. As mentioned previously, a species similar to Species (III) was formed in the H-Y zeolite, which has a
low Si/Al ratio of 2.65 [74]. At higher Si/Al ratios (low Al contents), the distance
between two Brønsted acid sites becomes too long for this bidentate Mo complex to
be formed. Therefore, the dimeric Species (II) is formed via the condensation of two
monomeric Mo species to overcome the distance between two Brønsted acid sites.
6.6 Mo Species of Mo/H-ZSM-5 Zeolite Catalysts During
the Working Stage
Section 6.5 discussed the anchored Mo species (II–VI) that can potentially form via
the interaction of MoO 3 with the surface of the H
+ -exchanged zeolites. However,
these Mo species are not necessarily the catalytically active species in the MDA
reaction. When the Mo-modified H
+ -exchanged zeolites such as Mo/H-ZSM-5 are
used as catalysts in the MDA reaction, the production of hydrocarbons, such as
ethylene and benzene, is usually observed in the induction period and does not
depend on the type of Mo-modified catalyst [66, 87–93]. This induction period has
also been observed when unsupported Mo 2 O 3 is used as a catalyst.
In both cases, the formation of CO 2 has been observed upon contact of the catalyst with methane, followed by the production of CO and H 2 O. After the formation
of CO 2 , CO, and H 2 O, the hydrocarbons C 2 H 6 , C 2 H 4 , and benzene were produced.
These phenomena indicate that the catalytic active species (active sites) were generated during the induction period. The details of the generation of the catalytically
6 Conversion of Methane to Aromatic Hydrocarbons
between two Al atoms at Si/Al ratios lower than 25. Goodman et al. reported that in
the straight channels of H-ZSM-5, all T sites were within 4.75 Å of the axis running
down the center of the zeolite channel (Si/Al ratio < 25) [85].
On the basis of the above estimations of the distance between two Al atoms in
next-next-nearest neighbor T sites in the zeolite ZSM-5, the formation of Species
(II) and Species (III) was concluded to occur in H-ZSM-5 zeolites with a Si/Al ratio
lower than 95 if dealumination did not occur.
Borry et al. examined the ratio of the number of acidic protons to the number of
Mo atoms introduced into H-ZSM-5 (H
+ /Mo) at a Si/Al ratio of 14.3. They experimentally demonstrated that the H
+ /Mo ratio was constant at approximately 1.24
when the Mo content was lower than 3.6 wt% [71]. This indicated that Species (II)
was mainly produced as the Mo species on and/or in the H-ZSM-5. However, when
the Mo content was increased to 6.3 wt%, the H
+ /Mo ratio decreased to 0.64. This
means that the formation of Species (III) predominates over that of Species (II).
Tessonnier et al. also examined the H
+ /Mo ratios in Mo-modified H-ZSM-5 zeolites [86]. When 2 wt% Mo was supported on H-ZSM-5 with a Si/Al ratio of 40, the
H
+ /Mo ratio was almost unity, indicating the formation of Species (II). On the other
hand, when Mo (2 wt% or 4 wt%) was supported on H-ZSM-5 with a Si/Al ratio
of 15, the H
+ /Mo ratio was almost two in both cases. This result indicated that the
monomeric bidentate Mo species (Species (III)) was formed. As mentioned previously, a species similar to Species (III) was formed in the H-Y zeolite, which has a
low Si/Al ratio of 2.65 [74]. At higher Si/Al ratios (low Al contents), the distance
between two Brønsted acid sites becomes too long for this bidentate Mo complex to
be formed. Therefore, the dimeric Species (II) is formed via the condensation of two
monomeric Mo species to overcome the distance between two Brønsted acid sites.
6.6 Mo Species of Mo/H-ZSM-5 Zeolite Catalysts During
the Working Stage
Section 6.5 discussed the anchored Mo species (II–VI) that can potentially form via
the interaction of MoO 3 with the surface of the H
+ -exchanged zeolites. However,
these Mo species are not necessarily the catalytically active species in the MDA
reaction. When the Mo-modified H
+ -exchanged zeolites such as Mo/H-ZSM-5 are
used as catalysts in the MDA reaction, the production of hydrocarbons, such as
ethylene and benzene, is usually observed in the induction period and does not
depend on the type of Mo-modified catalyst [66, 87–93]. This induction period has
also been observed when unsupported Mo 2 O 3 is used as a catalyst.
In both cases, the formation of CO 2 has been observed upon contact of the catalyst with methane, followed by the production of CO and H 2 O. After the formation
of CO 2 , CO, and H 2 O, the hydrocarbons C 2 H 6 , C 2 H 4 , and benzene were produced.
These phenomena indicate that the catalytic active species (active sites) were generated during the induction period. The details of the generation of the catalytically