146
6 Conversion of Methane to Aromatic Hydrocarbons
Therefore, the impregnation of Mo compounds, such as ammonium heptamolybdate ((NH 4 ) 6 Mo 7 O 24 ) initially leads to the formation of extra MoO 3 crystals during
calcination. These MoO 3 crystals may react with surface OH groups, such as the
acidic protons of Brønsted acid sites, to form Species (II–VI). The structural details
of Species (II) and Species (III) will be further discussed in Sect. 6.5.4, which deals
with the relationship between the Si/Al ratios in H-ZSM-5 zeolites and the formation
of Species (II) and Species (III).
6.5.3 Chemical Properties of the Mo Species on H-ZSM-5
After Calcination at 773 K
As mentioned above, a variety of Mo species (Species II–VI) may be formed on
H-ZSM-5 zeolites after calcination at around 700–1000 K. Xu et al. attempted to
use an ammoniacal solution to extract the Mo species on Mo-impregnated H-ZSM-5
with a Si/Al ratio of 25 that had been calcined for 773 K for 6 h and then dried at
383 K overnight [77]. They reported that two kinds of Mo species were present on the
Mo/H-ZSM-5 catalysts: a soluble species (Mo extracted in NH 3 solution) and species
that were not dissolved into the NH 3 solution (residual Mo). They also showed that
the ratio of soluble Mo species to residual Mo species depended on the original Mo
loading. For example, for Mo loadings of less than 3.0 wt%, the amount of residual
Mo was almost constant. However, at Mo loadings greater than 3.0 wt%, the amount
of residual Mo increased with the Mo loading. The non-linear dependence of the
amount of residual Mo on the original Mo loading implied that some Mo species
were anchored on the surface and/or inside the micropores of the H-ZSM-5.
6.5.4 Influence of the Si/Al Ratio on the Structure of Mo
Species on H + -Exchanged Zeolites After Calcination
As described in Sect. 6.3, after the calcination of Mo-modified H
+ -exchanged zeolites, Mo may possibly exist as Species (II), Mo 2 O 5
2+ , and Species (III), MoO 2
2+ ,
while dealuminated Al species may react with MoO 3 to produce the aluminummolybdic complexes Species (IV) and Species (V). Thus, a variety of Mo oxide
species are formed on H
+ -exchanged zeolites such as H-ZSM-5. For example, Gao
et al. reported the formation of nanostructured Mo oxide on and/or in H-ZSM-5 [78].
According to ref. [79], the formation of the anchored structures of Species (II) and
Species (III) depends on the distance between an Al atom and the first neighboring
Al atom (next-nearest neighbor Al atom) in the zeolite lattice, and the distances
between two oxygen atoms anchored to Al atoms on the zeolite in Species (II) and
Species (III) were estimated.
6 Conversion of Methane to Aromatic Hydrocarbons
Therefore, the impregnation of Mo compounds, such as ammonium heptamolybdate ((NH 4 ) 6 Mo 7 O 24 ) initially leads to the formation of extra MoO 3 crystals during
calcination. These MoO 3 crystals may react with surface OH groups, such as the
acidic protons of Brønsted acid sites, to form Species (II–VI). The structural details
of Species (II) and Species (III) will be further discussed in Sect. 6.5.4, which deals
with the relationship between the Si/Al ratios in H-ZSM-5 zeolites and the formation
of Species (II) and Species (III).
6.5.3 Chemical Properties of the Mo Species on H-ZSM-5
After Calcination at 773 K
As mentioned above, a variety of Mo species (Species II–VI) may be formed on
H-ZSM-5 zeolites after calcination at around 700–1000 K. Xu et al. attempted to
use an ammoniacal solution to extract the Mo species on Mo-impregnated H-ZSM-5
with a Si/Al ratio of 25 that had been calcined for 773 K for 6 h and then dried at
383 K overnight [77]. They reported that two kinds of Mo species were present on the
Mo/H-ZSM-5 catalysts: a soluble species (Mo extracted in NH 3 solution) and species
that were not dissolved into the NH 3 solution (residual Mo). They also showed that
the ratio of soluble Mo species to residual Mo species depended on the original Mo
loading. For example, for Mo loadings of less than 3.0 wt%, the amount of residual
Mo was almost constant. However, at Mo loadings greater than 3.0 wt%, the amount
of residual Mo increased with the Mo loading. The non-linear dependence of the
amount of residual Mo on the original Mo loading implied that some Mo species
were anchored on the surface and/or inside the micropores of the H-ZSM-5.
6.5.4 Influence of the Si/Al Ratio on the Structure of Mo
Species on H + -Exchanged Zeolites After Calcination
As described in Sect. 6.3, after the calcination of Mo-modified H
+ -exchanged zeolites, Mo may possibly exist as Species (II), Mo 2 O 5
2+ , and Species (III), MoO 2
2+ ,
while dealuminated Al species may react with MoO 3 to produce the aluminummolybdic complexes Species (IV) and Species (V). Thus, a variety of Mo oxide
species are formed on H
+ -exchanged zeolites such as H-ZSM-5. For example, Gao
et al. reported the formation of nanostructured Mo oxide on and/or in H-ZSM-5 [78].
According to ref. [79], the formation of the anchored structures of Species (II) and
Species (III) depends on the distance between an Al atom and the first neighboring
Al atom (next-nearest neighbor Al atom) in the zeolite lattice, and the distances
between two oxygen atoms anchored to Al atoms on the zeolite in Species (II) and
Species (III) were estimated.