244
After methane activation and C–C coupling reactions over active molybdenum
carbide or oxycarbide nanoclusters producing ethylene intermediate, in the second
step of the MDA reaction, ethylene oligomerization occurs at BAS of zeolite support which results in final aromatic products (BTX). This is controlled by Si/Al ratio
of zeolite support in Mo/Zeolite catalyst as discussed in Sect. 1.2.2. Major challenge with the reaction is to control over-aromatization of benzene and toluene
products. Further aromatization leads to coke formation in the form of polyaromatic- hydrocarbons (PAH) as per previous reports [15]. However, other forms of
coke such as CH x type or molybdenum-associated carbide coke are also responsible
for catalyst deactivation which can be controlled via optimizing the composition
and size of active molybdenum carbide or oxycarbide nanocluster [15]. Contrary to
this, there are various contradictions in mechanistic insights of chain elongation
reactions producing aromatic product and coke formation after C–C coupling reactions at Mo sites. Polyaromatic hydrocarbons can be controlled by optimizing Si/Al
ratio and BAS strength inside the zeolite channels which triggers the oligomerization reaction.
1.4 Reaction Parameters
1.4.1 Effect of Temperature
Methane dehydroaromatization is highly endothermic reaction limited by thermodynamic equilibrium (Eq. (1)) as discussed in Sect. 1.1. It requires high temperatures (973 K) to achieve remarkable methane conversion into aromatic hydrocarbons
performed in a fixed-bed type reactor [57, 58]. As per previous reports, the methane
conversion rate in MDA reaction is a function of temperature as supported by thermodynamics calculations which approve the need of high temperature in this case.
However, high-temperature range causes fast catalyst deactivation by severe coke
deposition and promote agglomeration of metallic particles reducing the active
phases of MDA catalyst [58]. Different observations have been made in the previous
experimental reports on optimization of temperature for MDA reaction and it has
been concluded that 973–1073 K temperature range is the optimum range for methane conversion into aromatic hydrocarbons. Above these range of temperatures,
decrease in benzene yield has been reported probably due to loss in the active phase
of Mo/HZSM-5 catalyst resulting in severe coke formation. In addition, lower temperatures range has also been claimed for MDA reaction as an alternative aspect of
lower energy requirement which directly relates to the economy of the process
[59, 60].
S. Mishra et al.
After methane activation and C–C coupling reactions over active molybdenum
carbide or oxycarbide nanoclusters producing ethylene intermediate, in the second
step of the MDA reaction, ethylene oligomerization occurs at BAS of zeolite support which results in final aromatic products (BTX). This is controlled by Si/Al ratio
of zeolite support in Mo/Zeolite catalyst as discussed in Sect. 1.2.2. Major challenge with the reaction is to control over-aromatization of benzene and toluene
products. Further aromatization leads to coke formation in the form of polyaromatic- hydrocarbons (PAH) as per previous reports [15]. However, other forms of
coke such as CH x type or molybdenum-associated carbide coke are also responsible
for catalyst deactivation which can be controlled via optimizing the composition
and size of active molybdenum carbide or oxycarbide nanocluster [15]. Contrary to
this, there are various contradictions in mechanistic insights of chain elongation
reactions producing aromatic product and coke formation after C–C coupling reactions at Mo sites. Polyaromatic hydrocarbons can be controlled by optimizing Si/Al
ratio and BAS strength inside the zeolite channels which triggers the oligomerization reaction.
1.4 Reaction Parameters
1.4.1 Effect of Temperature
Methane dehydroaromatization is highly endothermic reaction limited by thermodynamic equilibrium (Eq. (1)) as discussed in Sect. 1.1. It requires high temperatures (973 K) to achieve remarkable methane conversion into aromatic hydrocarbons
performed in a fixed-bed type reactor [57, 58]. As per previous reports, the methane
conversion rate in MDA reaction is a function of temperature as supported by thermodynamics calculations which approve the need of high temperature in this case.
However, high-temperature range causes fast catalyst deactivation by severe coke
deposition and promote agglomeration of metallic particles reducing the active
phases of MDA catalyst [58]. Different observations have been made in the previous
experimental reports on optimization of temperature for MDA reaction and it has
been concluded that 973–1073 K temperature range is the optimum range for methane conversion into aromatic hydrocarbons. Above these range of temperatures,
decrease in benzene yield has been reported probably due to loss in the active phase
of Mo/HZSM-5 catalyst resulting in severe coke formation. In addition, lower temperatures range has also been claimed for MDA reaction as an alternative aspect of
lower energy requirement which directly relates to the economy of the process
[59, 60].
S. Mishra et al.