6.8 Various Attempts to Improve the Production …
157
6.8.4 Effect of the Addition of Small Amounts of O 2 , H 2 ,
and H 2 O
The presence of very small amounts of O 2 [116], H 2 [117], and H 2 O [118] have
been found to have a positive effect on the MDA reaction. The critical oxygen concentration in the feed, which should not be exceeded to avoid the deep oxidation of
methane, has been found to increase with reaction temperature. At 973 K and 1073 K,
the O 2 /CH 4 molar ratio should not exceed 6.5 × 10
−3 and 2.4 × 10
−2 , respectively.
The addition of small amounts of H 2 O (1.7–2.2%) to the reactant improved the
catalytic activity and stability of Mo (6 wt%)/H-ZSM-5 catalysts.
6.9 Summary of the Catalytic Features
of Mo/H + -Exchanged Zeolite Catalysts for MDA
Reaction
The MDA reaction can potentially afford both high-value aromatic compounds,
as well as pure hydrogen simultaneously. The most important features of Mo/H
+ -
exchanged zeolite catalysts for the MDA reaction can be summarized as follows:
(1) Mo/H
+ -exchanged zeolites are bifunctional catalysts. Thus, the Mo species
should be modified onto an H
+ -exchanged zeolite to achieve good catalytic
activity.
(2) There is an induction period in the early stage of the reaction, during which the
active Mo species, such as MoO x , are reduced into molybdenum carbonaceous
species with CH 4 .
(3) Various interactions, such as the interaction between the Mo species and the
Brønsted acid sites on H
+ -exchanged zeolite, may be involved in the catalysis
and have a crucial influence on the catalytic performance.
(4) Carbonaceous compounds are formed during the MDA reaction and lead to
severe deactivation of the Mo/H
+ -exchanged zeolite catalysts.
However, catalyst deactivation remains a serious problem, and the reaction
inevitably produces carbon deposits on the catalyst, which precludes industrial applications. To increase catalyst lifetime and optimize the condition of continuous catalyst regeneration, one can resort to computational studies [84] and perform further
catalyst characterization [119]. A few studies also employ DFT methods to follow the
reaction mechanism and carbon deposition. Developments of these studies possibly
bring proper technological implementation that will make the synthesis of benzene
and hydrogen by MDA economically profitable.
157
6.8.4 Effect of the Addition of Small Amounts of O 2 , H 2 ,
and H 2 O
The presence of very small amounts of O 2 [116], H 2 [117], and H 2 O [118] have
been found to have a positive effect on the MDA reaction. The critical oxygen concentration in the feed, which should not be exceeded to avoid the deep oxidation of
methane, has been found to increase with reaction temperature. At 973 K and 1073 K,
the O 2 /CH 4 molar ratio should not exceed 6.5 × 10
−3 and 2.4 × 10
−2 , respectively.
The addition of small amounts of H 2 O (1.7–2.2%) to the reactant improved the
catalytic activity and stability of Mo (6 wt%)/H-ZSM-5 catalysts.
6.9 Summary of the Catalytic Features
of Mo/H + -Exchanged Zeolite Catalysts for MDA
Reaction
The MDA reaction can potentially afford both high-value aromatic compounds,
as well as pure hydrogen simultaneously. The most important features of Mo/H
+ -
exchanged zeolite catalysts for the MDA reaction can be summarized as follows:
(1) Mo/H
+ -exchanged zeolites are bifunctional catalysts. Thus, the Mo species
should be modified onto an H
+ -exchanged zeolite to achieve good catalytic
activity.
(2) There is an induction period in the early stage of the reaction, during which the
active Mo species, such as MoO x , are reduced into molybdenum carbonaceous
species with CH 4 .
(3) Various interactions, such as the interaction between the Mo species and the
Brønsted acid sites on H
+ -exchanged zeolite, may be involved in the catalysis
and have a crucial influence on the catalytic performance.
(4) Carbonaceous compounds are formed during the MDA reaction and lead to
severe deactivation of the Mo/H
+ -exchanged zeolite catalysts.
However, catalyst deactivation remains a serious problem, and the reaction
inevitably produces carbon deposits on the catalyst, which precludes industrial applications. To increase catalyst lifetime and optimize the condition of continuous catalyst regeneration, one can resort to computational studies [84] and perform further
catalyst characterization [119]. A few studies also employ DFT methods to follow the
reaction mechanism and carbon deposition. Developments of these studies possibly
bring proper technological implementation that will make the synthesis of benzene
and hydrogen by MDA economically profitable.