239
In 1997, Wang et al. have claimed that during the induction period complete transformation of MoO x into Mo 2 C does not occur and some oxide or oxycarbide forms
of molybdenum remain over the catalyst which shows that nature of molybdenum
species during the induction may be different [46]. In addition, different forms of
molybdenum carbide such as β-Mo 2 C and α-MoC 1−x have been also claimed in the
induction period in which α-MoC 1−x form is reported effective for higher activity
and stability for MDA catalyst [47, 48]. Duration of induction period also affect the
carburization depending on the carburizing agent (CO, light hydrocarbons) but is
observed independent of temperature [49]. Catalytic constituent such as Mo loading
and Si/Al ratio of Mo/Zeolite catalyst significantly affect the carburization process.
Higher molybdenum loading results in higher reducibility due to formation of
agglomerated MoO x species over zeolite surface. Variation in Si/Al ratio of zeolite
not only tunes the acidity of Mo/Zeolite catalyst but also control the anchoring of
initial MoO x species over zeolite framework as discussed in Sect. 1.2.2. Change in
anchoring mode of MoO x species via Si/Al ratio variation controls the MoO x and
zeolite interaction and thus affects the reducibility of MoO x species as also confirmed by recent studies of Zhao et al. [50].
1.3 Mechanistic Insights of MDA Reaction
Methane dehydroaromatization reaction occurs over bifunctional Mo/Zeolite catalyst at which methane C–H activation and C–C coupling takes place on molybdenum site whereas aromatization of intermediates occurs at Brønsted acid sites of the
zeolite. Thus basically methane activation, ethylene formation via C–C coupling
and aromatization by acidic sites control the methane conversion and benzene yield.
Aromatization of intermediates is significantly controlled by zeolite shape selectivity and its acidity as discussed in Sect. 1.2.
In MDA reaction, methane C–H activation and C–C coupling are key steps and
occurs over active phase of molybdenum species. Initial MoO x species are reduced
or carburized to form molybdenum oxycarbide or carbide in the induction period as
discussed in Sect. 1.2.4. Lunsford et al. in 1996 have firstly reported the induction
period required in MDA reaction for Mo/HZSM-5 catalyst and claimed transformation of MoO x species into molybdenum carbide in the induction period using characterization tools such as XPS and XRD [51]. In the induction period, no hydrocarbon
with negligible coke deposits is formed except CO and water vapours (H 2 O) as
major products. The active phase of molybdenum is rigorously debated in previous
reports and is considered to be a carbidic or oxycarbidic [52]. As per experimental
investigations of Weckhuysen et al. in 2016, methane activation starts over the oxycarbide phase of molybdenum [53]. Nevertheless, benzene formation starts only
when MoO x is completely carburized/reduced into molybdenum carbide (Mo x C y )
species. Identification of molybdenum carbide structure has been a debatable discussion in reported theoretical and experimental studies. Experimentally, Mo 2 C
clusters have been characterized as molybdenum carbide structure by XANES/
Thermocatalytic Conversion of Natural Gas to Petrochemical Feedstocks…
In 1997, Wang et al. have claimed that during the induction period complete transformation of MoO x into Mo 2 C does not occur and some oxide or oxycarbide forms
of molybdenum remain over the catalyst which shows that nature of molybdenum
species during the induction may be different [46]. In addition, different forms of
molybdenum carbide such as β-Mo 2 C and α-MoC 1−x have been also claimed in the
induction period in which α-MoC 1−x form is reported effective for higher activity
and stability for MDA catalyst [47, 48]. Duration of induction period also affect the
carburization depending on the carburizing agent (CO, light hydrocarbons) but is
observed independent of temperature [49]. Catalytic constituent such as Mo loading
and Si/Al ratio of Mo/Zeolite catalyst significantly affect the carburization process.
Higher molybdenum loading results in higher reducibility due to formation of
agglomerated MoO x species over zeolite surface. Variation in Si/Al ratio of zeolite
not only tunes the acidity of Mo/Zeolite catalyst but also control the anchoring of
initial MoO x species over zeolite framework as discussed in Sect. 1.2.2. Change in
anchoring mode of MoO x species via Si/Al ratio variation controls the MoO x and
zeolite interaction and thus affects the reducibility of MoO x species as also confirmed by recent studies of Zhao et al. [50].
1.3 Mechanistic Insights of MDA Reaction
Methane dehydroaromatization reaction occurs over bifunctional Mo/Zeolite catalyst at which methane C–H activation and C–C coupling takes place on molybdenum site whereas aromatization of intermediates occurs at Brønsted acid sites of the
zeolite. Thus basically methane activation, ethylene formation via C–C coupling
and aromatization by acidic sites control the methane conversion and benzene yield.
Aromatization of intermediates is significantly controlled by zeolite shape selectivity and its acidity as discussed in Sect. 1.2.
In MDA reaction, methane C–H activation and C–C coupling are key steps and
occurs over active phase of molybdenum species. Initial MoO x species are reduced
or carburized to form molybdenum oxycarbide or carbide in the induction period as
discussed in Sect. 1.2.4. Lunsford et al. in 1996 have firstly reported the induction
period required in MDA reaction for Mo/HZSM-5 catalyst and claimed transformation of MoO x species into molybdenum carbide in the induction period using characterization tools such as XPS and XRD [51]. In the induction period, no hydrocarbon
with negligible coke deposits is formed except CO and water vapours (H 2 O) as
major products. The active phase of molybdenum is rigorously debated in previous
reports and is considered to be a carbidic or oxycarbidic [52]. As per experimental
investigations of Weckhuysen et al. in 2016, methane activation starts over the oxycarbide phase of molybdenum [53]. Nevertheless, benzene formation starts only
when MoO x is completely carburized/reduced into molybdenum carbide (Mo x C y )
species. Identification of molybdenum carbide structure has been a debatable discussion in reported theoretical and experimental studies. Experimentally, Mo 2 C
clusters have been characterized as molybdenum carbide structure by XANES/
Thermocatalytic Conversion of Natural Gas to Petrochemical Feedstocks…