6.4 Zeolite-Based Catalysts for the Conversion …
141
The values in parentheses are the rates of methane consumption in units of methane
molecules reacted per metal atom per hour. As shown in Table 6.5, the Mo-loaded
H-ZSM-5 showed the highest overall catalytic performance for the production of
aromatic hydrocarbons among the different metals tested.
The catalytic performances Re (5 wt%)/H
+ -exchanged zeolites are also shown
in Table 6.3. The trend in the catalytic performances of the Re-impregnated H
+ -
exchanged zeolites was the same as those for their Mo-impregnated counterparts in
all respects, such as the methane conversion and the benzene formation rate.
6.5 Interaction of Mo Species with Brønsted Acid Sites
on H + -Exchanged Zeolites
As discussed in Sect. 6.4, Mo species are believed to interact with H
+ (acidic protons)
because of the presence of Brønsted acid sites on and/or in the zeolite. The catalytic
activity of Mo/H
+ -exchanged zeolite catalysts in the formation of aromatic hydrocarbons has been demonstrated to depend on the ratio of Mo to Brønsted acid sites.
To understand the role of the Mo species generated on H
+ -exchanged zeolite, first,
the Mo species present during the preparation and pretreatment of the Mo-modified
zeolite catalyst, as well as during the MDA reaction, should be investigated. In the
following sections, the formation and generation of Mo species on and/or in H
+ -
exchanged zeolites during these three steps is discussed, mainly in terms of the
zeolite H-ZSM-5.
6.5.1 Methods for the Preparation of Mo-Modified H-ZSM-5
Catalysts
Mo-modified H-ZSM-5 (Mo/H-ZSM-5) catalysts for the MDA reaction are usually
prepared by one of the following two methods [21, 48, 65–67].
Method 1: A zeolite is impregnated using a slurry or the incipient wetness method
using a basic solution of a molybdenum compound (ammonium heptamolybdate
((NH 4 ) 6 Mo 7 O 24 )) or ammonium paramolybdate), followed by treatment in air at a
temperature of 700–1000 K.
Method 2: A molybdenum compound such as MoO 3 is mechanically mixed with
an H
+ -exchanged zeolite (H-ZSM-5) and then heated at the reaction temperature. The
molybdenum species are thus anchored on the zeolite as the result of a solid-phase
reaction.
In both Method 1 and Method 2, an H
+ -exchanged zeolite or NH 4
+ -exchanged
zeolite must be used. Furthermore, in Method 1, changing the pH of the molybdenum
solution affects the properties of the resulting catalysts [68].
141
The values in parentheses are the rates of methane consumption in units of methane
molecules reacted per metal atom per hour. As shown in Table 6.5, the Mo-loaded
H-ZSM-5 showed the highest overall catalytic performance for the production of
aromatic hydrocarbons among the different metals tested.
The catalytic performances Re (5 wt%)/H
+ -exchanged zeolites are also shown
in Table 6.3. The trend in the catalytic performances of the Re-impregnated H
+ -
exchanged zeolites was the same as those for their Mo-impregnated counterparts in
all respects, such as the methane conversion and the benzene formation rate.
6.5 Interaction of Mo Species with Brønsted Acid Sites
on H + -Exchanged Zeolites
As discussed in Sect. 6.4, Mo species are believed to interact with H
+ (acidic protons)
because of the presence of Brønsted acid sites on and/or in the zeolite. The catalytic
activity of Mo/H
+ -exchanged zeolite catalysts in the formation of aromatic hydrocarbons has been demonstrated to depend on the ratio of Mo to Brønsted acid sites.
To understand the role of the Mo species generated on H
+ -exchanged zeolite, first,
the Mo species present during the preparation and pretreatment of the Mo-modified
zeolite catalyst, as well as during the MDA reaction, should be investigated. In the
following sections, the formation and generation of Mo species on and/or in H
+ -
exchanged zeolites during these three steps is discussed, mainly in terms of the
zeolite H-ZSM-5.
6.5.1 Methods for the Preparation of Mo-Modified H-ZSM-5
Catalysts
Mo-modified H-ZSM-5 (Mo/H-ZSM-5) catalysts for the MDA reaction are usually
prepared by one of the following two methods [21, 48, 65–67].
Method 1: A zeolite is impregnated using a slurry or the incipient wetness method
using a basic solution of a molybdenum compound (ammonium heptamolybdate
((NH 4 ) 6 Mo 7 O 24 )) or ammonium paramolybdate), followed by treatment in air at a
temperature of 700–1000 K.
Method 2: A molybdenum compound such as MoO 3 is mechanically mixed with
an H
+ -exchanged zeolite (H-ZSM-5) and then heated at the reaction temperature. The
molybdenum species are thus anchored on the zeolite as the result of a solid-phase
reaction.
In both Method 1 and Method 2, an H
+ -exchanged zeolite or NH 4
+ -exchanged
zeolite must be used. Furthermore, in Method 1, changing the pH of the molybdenum
solution affects the properties of the resulting catalysts [68].