235
In summary, thermodynamic analysis corroborates that methane conversion into
benzene and hydrogen in MDA process is thermodynamically favourable only at
limited conditions, and is expected to be kinetically driven to undesired solid carbonaceous species. Addition of other oxygenates (O 2 , CO, CO 2 and CH 3 OH) and light
hydrocarbons with methane also controls thermodynamics and experimentally have
been shown to drive the MDA reaction with certain limitations.
1.2 Catalysts for MDA Reaction
Basically, molybdenum-supported ZSM-5 is the well-known catalyst for methane
dehydroaromatization reaction as firstly reported by Wang et al. in 1993 [9]. Being
an important reaction, MDA was widely investigated for long time and different
metals besides molybdenum (Mo) were tested with ZSM-5. Cr, W, Mn, Fe, Ni, Zn,
Cu, Re, V and various other transition metals were analysed for MDA reaction and
it was concluded that Mo supported over zeolite is the most active metal for methane aromatization reaction [5]. However transition metals such as Re, W and Cu
supported over ZSM-5 have been reported with 7, 5.7 and 2.4 methane conversion
having higher benzene selectivity (48% for Re, 94% for W and 70% for Cu) respectively [10]. Zeolite support also controls the activity of Mo/Zeolite catalyst as it
imparts bi-functionality for methane to aromatic reaction. Microporous and mesoporous zeolites with unique framework structure, shape selectivity and strong acidity tune the metal interaction and dispersion and thus affect the activity. Different
types of zeolites along with molybdenum have been investigated showing activity
towards MDA reaction [20]. ZSM-5 contains two-dimensional pore structures with
10-membered rings and effective pore diameter (0.6 nm) equivalent to the kinetic
diameter of benzene molecule which provides shape selectivity to aromatized products. Due to these characteristics and high thermal stability of ZSM-5, it has been
frequently used for MDA reaction along with molybdenum to date [21]. In addition
to ZSM-5, other zeolite supports such as HMCM-22, ZSM-11, H-β, H-Y, ZSM-8,
HMCM-41, HMCM-36 and ITQ type zeolites have also been tested for MDA reaction as per previous reports [5, 22]. Different outcomes and suggestions were made
with these supports in reference to MDA activity and it was concluded that
HMCM-22 and HMCM-49 besides ZSM-5 are also effective supports for the synthesis of molybdenum-based catalyst for MDA activity. Basically, Mo/HZSM-5 and
Mo/HMCM-22 are known as well-established catalysts showing similar MDA
activity. However, reports for higher stability of Mo/HMCM-22 with high benzene
selectivity in comparison to Mo/HZSM-5 are also claimed [23]. HMCM-22 has
unique framework structure as compared to HZSM-5 zeolite support, due to which
a better tolerance of coke deposits occurs in this zeolite and results in higher benzene selectivity. HMCM-22 possesses a unique pore structural design with two
independent pore systems: two-dimensional 10-ring sinusoidal pore system
(4.1 × 5.1 Å), and a larger, three-dimensional 12-ring super-cage system interconnected via 10-ring windows (4.0 × 5.5 Å). This type of pore structure and presence
Thermocatalytic Conversion of Natural Gas to Petrochemical Feedstocks…
In summary, thermodynamic analysis corroborates that methane conversion into
benzene and hydrogen in MDA process is thermodynamically favourable only at
limited conditions, and is expected to be kinetically driven to undesired solid carbonaceous species. Addition of other oxygenates (O 2 , CO, CO 2 and CH 3 OH) and light
hydrocarbons with methane also controls thermodynamics and experimentally have
been shown to drive the MDA reaction with certain limitations.
1.2 Catalysts for MDA Reaction
Basically, molybdenum-supported ZSM-5 is the well-known catalyst for methane
dehydroaromatization reaction as firstly reported by Wang et al. in 1993 [9]. Being
an important reaction, MDA was widely investigated for long time and different
metals besides molybdenum (Mo) were tested with ZSM-5. Cr, W, Mn, Fe, Ni, Zn,
Cu, Re, V and various other transition metals were analysed for MDA reaction and
it was concluded that Mo supported over zeolite is the most active metal for methane aromatization reaction [5]. However transition metals such as Re, W and Cu
supported over ZSM-5 have been reported with 7, 5.7 and 2.4 methane conversion
having higher benzene selectivity (48% for Re, 94% for W and 70% for Cu) respectively [10]. Zeolite support also controls the activity of Mo/Zeolite catalyst as it
imparts bi-functionality for methane to aromatic reaction. Microporous and mesoporous zeolites with unique framework structure, shape selectivity and strong acidity tune the metal interaction and dispersion and thus affect the activity. Different
types of zeolites along with molybdenum have been investigated showing activity
towards MDA reaction [20]. ZSM-5 contains two-dimensional pore structures with
10-membered rings and effective pore diameter (0.6 nm) equivalent to the kinetic
diameter of benzene molecule which provides shape selectivity to aromatized products. Due to these characteristics and high thermal stability of ZSM-5, it has been
frequently used for MDA reaction along with molybdenum to date [21]. In addition
to ZSM-5, other zeolite supports such as HMCM-22, ZSM-11, H-β, H-Y, ZSM-8,
HMCM-41, HMCM-36 and ITQ type zeolites have also been tested for MDA reaction as per previous reports [5, 22]. Different outcomes and suggestions were made
with these supports in reference to MDA activity and it was concluded that
HMCM-22 and HMCM-49 besides ZSM-5 are also effective supports for the synthesis of molybdenum-based catalyst for MDA activity. Basically, Mo/HZSM-5 and
Mo/HMCM-22 are known as well-established catalysts showing similar MDA
activity. However, reports for higher stability of Mo/HMCM-22 with high benzene
selectivity in comparison to Mo/HZSM-5 are also claimed [23]. HMCM-22 has
unique framework structure as compared to HZSM-5 zeolite support, due to which
a better tolerance of coke deposits occurs in this zeolite and results in higher benzene selectivity. HMCM-22 possesses a unique pore structural design with two
independent pore systems: two-dimensional 10-ring sinusoidal pore system
(4.1 × 5.1 Å), and a larger, three-dimensional 12-ring super-cage system interconnected via 10-ring windows (4.0 × 5.5 Å). This type of pore structure and presence
Thermocatalytic Conversion of Natural Gas to Petrochemical Feedstocks…