238
1.2.3 Promoter Effect over MDA Catalyst
Besides controlling Mo concentration and Brønsted acidity of MDA catalyst (Mo/
Zeolite), attempts of promoter metal addition to Mo/Zeolite catalyst have also been
performed in view of improving activity. In these attempts, different metals such as
Ga, Zn, Cr, Cu, Ni, W, Pd, Pt, etc. have been studied as promoters for MDA catalyst
and results in the positive effect on activity and stability [5, 37, 38]. Nevertheless,
different observations have been seen in previous studies showing inconsistencies in
results of metal-promoted Mo/Zeolite catalysts. As negative effect of promoter metals over Mo/Zeolite catalyst have also been claimed in previous reports [39]. This
may be due to the different treatments used for catalyst preparation and different
reaction conditions. In addition, due to complex insights of MDA catalyst activity,
mechanistic understanding of various promoters’ effect over Mo/Zeolite catalyst
has not been clearly stated. Promoter metals such as Ru improves the stability of
Mo/HZSM-5 catalyst [40], Ni and Co have also been reported for improving the
stability of Mo/HZSM-5 catalyst via preventing aggregation of Mo species as per
recent studies of Sridhar et al. (2020) [41]. Ahmed K. Aboul-Gheit et al. in 2011
have reported Zn as promoter over Mo/HZSM-5 for its lower carbon deposition
which is advantageous in MDA reaction [42]. Victor Abdelsayed and co-workers in
2015 have claimed increased benzene formation rate for Fe-Mo/ZSM-5 catalysts as
compared to unpromoted Mo/ZSM-5 catalyst [43]. Cr incorporation over Mo/
ZSM-5 catalyst tunes the acidity which promotes the catalytic activity as per previous reports [38]; however, it remains contradictory whether Cr and Mo species
simultaneously interact with acid sites or separately. Thus additive metals along
with molybdenum significantly control the catalytic activity and stability; however,
their interaction and incorporation over Mo/Zeolite catalyst is still debatable and
thus needs more scientific investigations to study the bimetallic MDA catalyst.
1.2.4 Induction Effect: Carburization
In MDA reaction, Mo/Zeolite catalyst is firstly reduced or carburized to achieve its
active phase for reaction progress. After calcination, oxide form of catalyst MoO 3 /
Zeolite is generally converted into its carbide or oxycarbide form (Mo 2 C/Zeolite)
which is the active site for methane activation and C–C coupling reactions in MDA
process [35]. During the transformation of MoO 3 /Zeolite to Mo 2 C/Zeolite phase,
formation of oxygenates such as CO, CO 2 and H 2 O occurs in the initial period which
is known as induction or carburization period before aromatization reaction.
Molybdenum carbide species (Mo 2 C) generated in the induction period has been
confirmed using different techniques (XPS, XNASE) [44]. A mixture of H 2 and CH 4
(3:1 or 4:1) or pure methane is used for carburization/reduction of MoO 3 /Zeolite
which results in carbide or oxycarbide form of molybdenum [45]. As per reports of
Lee et al. in 2000, during the transformation of MoO 3 into Mo 2 C, mainly two steps
occur in which the first is the reduction of initial MoO 3 to MoO 2 whereas the second
step is the carburization of MoO 2 resulting into molybdenum carbide (Mo 2 C) [33].
S. Mishra et al.
1.2.3 Promoter Effect over MDA Catalyst
Besides controlling Mo concentration and Brønsted acidity of MDA catalyst (Mo/
Zeolite), attempts of promoter metal addition to Mo/Zeolite catalyst have also been
performed in view of improving activity. In these attempts, different metals such as
Ga, Zn, Cr, Cu, Ni, W, Pd, Pt, etc. have been studied as promoters for MDA catalyst
and results in the positive effect on activity and stability [5, 37, 38]. Nevertheless,
different observations have been seen in previous studies showing inconsistencies in
results of metal-promoted Mo/Zeolite catalysts. As negative effect of promoter metals over Mo/Zeolite catalyst have also been claimed in previous reports [39]. This
may be due to the different treatments used for catalyst preparation and different
reaction conditions. In addition, due to complex insights of MDA catalyst activity,
mechanistic understanding of various promoters’ effect over Mo/Zeolite catalyst
has not been clearly stated. Promoter metals such as Ru improves the stability of
Mo/HZSM-5 catalyst [40], Ni and Co have also been reported for improving the
stability of Mo/HZSM-5 catalyst via preventing aggregation of Mo species as per
recent studies of Sridhar et al. (2020) [41]. Ahmed K. Aboul-Gheit et al. in 2011
have reported Zn as promoter over Mo/HZSM-5 for its lower carbon deposition
which is advantageous in MDA reaction [42]. Victor Abdelsayed and co-workers in
2015 have claimed increased benzene formation rate for Fe-Mo/ZSM-5 catalysts as
compared to unpromoted Mo/ZSM-5 catalyst [43]. Cr incorporation over Mo/
ZSM-5 catalyst tunes the acidity which promotes the catalytic activity as per previous reports [38]; however, it remains contradictory whether Cr and Mo species
simultaneously interact with acid sites or separately. Thus additive metals along
with molybdenum significantly control the catalytic activity and stability; however,
their interaction and incorporation over Mo/Zeolite catalyst is still debatable and
thus needs more scientific investigations to study the bimetallic MDA catalyst.
1.2.4 Induction Effect: Carburization
In MDA reaction, Mo/Zeolite catalyst is firstly reduced or carburized to achieve its
active phase for reaction progress. After calcination, oxide form of catalyst MoO 3 /
Zeolite is generally converted into its carbide or oxycarbide form (Mo 2 C/Zeolite)
which is the active site for methane activation and C–C coupling reactions in MDA
process [35]. During the transformation of MoO 3 /Zeolite to Mo 2 C/Zeolite phase,
formation of oxygenates such as CO, CO 2 and H 2 O occurs in the initial period which
is known as induction or carburization period before aromatization reaction.
Molybdenum carbide species (Mo 2 C) generated in the induction period has been
confirmed using different techniques (XPS, XNASE) [44]. A mixture of H 2 and CH 4
(3:1 or 4:1) or pure methane is used for carburization/reduction of MoO 3 /Zeolite
which results in carbide or oxycarbide form of molybdenum [45]. As per reports of
Lee et al. in 2000, during the transformation of MoO 3 into Mo 2 C, mainly two steps
occur in which the first is the reduction of initial MoO 3 to MoO 2 whereas the second
step is the carburization of MoO 2 resulting into molybdenum carbide (Mo 2 C) [33].
S. Mishra et al.