236
of supercages are supposed to be responsible for high coke accumulation ability of
HMCM-22 zeolite, while retaining shape selectivity is the necessity for aromatic
formation. This combination promotes the selectivity of aromatic compounds and
stabilizes the Mo/HMCM-22 catalyst under MDA reaction conditions [24]. Due to
unique framework characteristics, zeolite support known as TNU-9 has been also
used for Mo-based catalyst in MDA reaction and results in higher activity than conventional ZSM-5 support [25]. In view of catalytic constituents of Mo/Zeolite catalyst, both molybdenum and zeolite supports have their own characteristics in the
MDA activity. In MDA reaction, methane C–H bond activation and C–C coupling
occur at metallic (Mo) sites which provides ethylene intermediate. Ethylene is further aromatized at acidic sites of zeolite support of the catalyst. Acidity of the catalyst is tuned by varying Si/Al ratio zeolite support. Thus molybdenum concentration
and zeolite acidity of the catalyst must be in trade off for effective MDA activity.
1.2.1 Effect of Mo Dispersion
Molybdenum loading over zeolite (HZSM-5/HMCM-22) support significantly
affects the Mo dispersion which controls the MDA activity as studied in detail in the
previous reports [26, 27]. In the earlier work, 2–6 wt% molybdenum loading has
been optimized for MDA activity and it is found that higher Mo loading (>6 wt%)
is not effective and results in lower benzene yield [28]. Lower Mo loading maintains
the integrity of zeolite framework such as crystallinity and porosity and also prevents extraction of framework Al species which results in Al 2 (MoO 4 ) 3 species [29].
Higher Mo loading shows large clusters of in situ generated molybdenum carbide
(the active site for C–H activation and C–C coupling) which retards the catalytic
activity as nano-cluster of molybdenum carbide have been reported as highly active
centres [10]. In addition, Mo species interacts with the zeolite framework via its
acidic sites, thus exchange of acidic protons with molybdenum occurs which
reduces the acidity of zeolite support and hence reduces the total acidity of MDA
catalyst. Reduction in acidity also affects the yield and conversion. Thus a trade off
between molybdenum concentration and zeolite acidity must be maintained to
achieve higher activity.
1.2.2 Effect of Zeolite Acidity
As per zeolite chemistry, negative charge at aluminium atoms in the framework is
compensated by cations which are further exchanged with other required cations.
The proton cation is incorporated via well-known ion exchange with NH 4
+
ions followed by calcinations at higher temperatures (450–550 °C) and provides Brønsted
acid sites (BAS) [30]. This directs that Brønsted acid sites directly depend on Al
concentration of zeolite framework and hence variation in Al content would change
the Brønsted acidity over the framework. As per the mechanism of MDA reaction,
firstly initial MoO x species grafted over zeolite framework is converted into active
S. Mishra et al.
of supercages are supposed to be responsible for high coke accumulation ability of
HMCM-22 zeolite, while retaining shape selectivity is the necessity for aromatic
formation. This combination promotes the selectivity of aromatic compounds and
stabilizes the Mo/HMCM-22 catalyst under MDA reaction conditions [24]. Due to
unique framework characteristics, zeolite support known as TNU-9 has been also
used for Mo-based catalyst in MDA reaction and results in higher activity than conventional ZSM-5 support [25]. In view of catalytic constituents of Mo/Zeolite catalyst, both molybdenum and zeolite supports have their own characteristics in the
MDA activity. In MDA reaction, methane C–H bond activation and C–C coupling
occur at metallic (Mo) sites which provides ethylene intermediate. Ethylene is further aromatized at acidic sites of zeolite support of the catalyst. Acidity of the catalyst is tuned by varying Si/Al ratio zeolite support. Thus molybdenum concentration
and zeolite acidity of the catalyst must be in trade off for effective MDA activity.
1.2.1 Effect of Mo Dispersion
Molybdenum loading over zeolite (HZSM-5/HMCM-22) support significantly
affects the Mo dispersion which controls the MDA activity as studied in detail in the
previous reports [26, 27]. In the earlier work, 2–6 wt% molybdenum loading has
been optimized for MDA activity and it is found that higher Mo loading (>6 wt%)
is not effective and results in lower benzene yield [28]. Lower Mo loading maintains
the integrity of zeolite framework such as crystallinity and porosity and also prevents extraction of framework Al species which results in Al 2 (MoO 4 ) 3 species [29].
Higher Mo loading shows large clusters of in situ generated molybdenum carbide
(the active site for C–H activation and C–C coupling) which retards the catalytic
activity as nano-cluster of molybdenum carbide have been reported as highly active
centres [10]. In addition, Mo species interacts with the zeolite framework via its
acidic sites, thus exchange of acidic protons with molybdenum occurs which
reduces the acidity of zeolite support and hence reduces the total acidity of MDA
catalyst. Reduction in acidity also affects the yield and conversion. Thus a trade off
between molybdenum concentration and zeolite acidity must be maintained to
achieve higher activity.
1.2.2 Effect of Zeolite Acidity
As per zeolite chemistry, negative charge at aluminium atoms in the framework is
compensated by cations which are further exchanged with other required cations.
The proton cation is incorporated via well-known ion exchange with NH 4
+
ions followed by calcinations at higher temperatures (450–550 °C) and provides Brønsted
acid sites (BAS) [30]. This directs that Brønsted acid sites directly depend on Al
concentration of zeolite framework and hence variation in Al content would change
the Brønsted acidity over the framework. As per the mechanism of MDA reaction,
firstly initial MoO x species grafted over zeolite framework is converted into active
S. Mishra et al.