6.4 Zeolite-Based Catalysts for the Conversion …
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coke in greater than 80% selectivity. Furthermore, as shown in Table 6.2, Mo species
supported on Al 2 O 3 and SiO 2 catalysts also showed high selectivities towards coke,
even at methane conversions of 7.4 and 5.3%, respectively. Al 2 O 3 and SiO 2 have no
ordered pore structures, whereas zeolites do. Therefore, the catalytic performances of
Mo-based zeolites seem to depend on their pore structure, which plays an important
role in the selective production of aromatic hydrocarbons, as discussed below.
Neither H-ZSM-5 without Mo nor Mo/Na
+ -exchanged ZSM-5 (Na-ZSM-5) produced aromatic hydrocarbons such as benzene. Furthermore, MoO 3 showed almost
no catalytic activity in the MDA reaction, while Mo/Al 2 O 3 and Mo/SiO 2 showed very
low selectivities towards aromatic hydrocarbons (Table 6.2). Therefore, both the Mo
species and the H
+ -exchanged zeolite are essential for the conversion of methane to
aromatic hydrocarbons with high selectivity, as discussed in the following sections.
6.4.2 Effect of the Zeolite Pore Structure on the Catalytic
Production of Aromatic Hydrocarbons
As discussed in Sect. 6.4, the structure of zeolites used as Mo-functionalized catalysts seems to influence their catalytic properties, especially their selectivity towards
benzene. Mo/H-MCM-22 exhibited higher selectivity towards benzene than the other
Mo-modified zeolites in Table 6.1, such as Mo/H-ZSM-5. The catalytic performances
of various Mo-modified H
+ -exchanged zeolites, including their benzene formation
rates and methane conversions, are summarized in Table 6.3 [26].
ZSM-5 has a three-dimensional pore structure that consists of networks of straight
and zigzagging pores and their intersections. Both pore structures are formed by 10membered oxygen rings with entrance diameters of ~5.5 Å. One is straight with
an elliptical cross-section, while the second type has a circular cross-section. The
straight pores are connected by the zigzag channels. The effective pore diameter is
5.9 Å, which is roughly equal to the kinetic diameter of benzene (5.8 Å).
As shown in Table 6.3, Mo/H-ZRP-1 showed the highest rate of benzene formation
among the Mo-modified H
+ -exchanged zeolite catalysts; Mo/H-ZSM-5 also showed
performance comparable to that of Mo/H-ZRP-1. ZRP-1 zeolites are prepared by
modification of high Si/Al ratio H-ZSM-5 with phosphorous and rare earth (RE)
oxides. The chemical composition of ZRP-1 is represented by the formula x RE 2 O 3 :
y Na 2 O · Al 2 O 3 : z SiO 2 , where x = 0.01–0.03, y = 0.4–1.0, and z = 20–60. ZRP-1
was prepared based on the concept of blocking the acid sites on the exterior surface
of ZSM-5 using phosphorous and rare earth oxides. The high catalytic performance
may have resulted from the resulting dealumination (loss of aluminum from the
ZSM-5 zeolite) and the formation of various P- and Al-containing compounds inside
the pores, which decreased the channel size [27].
The zeolite MCM-22 has larger pores than ZSM-5, and its pore structure consists
of two independent pore systems. One of these systems is composed of 12-membered
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coke in greater than 80% selectivity. Furthermore, as shown in Table 6.2, Mo species
supported on Al 2 O 3 and SiO 2 catalysts also showed high selectivities towards coke,
even at methane conversions of 7.4 and 5.3%, respectively. Al 2 O 3 and SiO 2 have no
ordered pore structures, whereas zeolites do. Therefore, the catalytic performances of
Mo-based zeolites seem to depend on their pore structure, which plays an important
role in the selective production of aromatic hydrocarbons, as discussed below.
Neither H-ZSM-5 without Mo nor Mo/Na
+ -exchanged ZSM-5 (Na-ZSM-5) produced aromatic hydrocarbons such as benzene. Furthermore, MoO 3 showed almost
no catalytic activity in the MDA reaction, while Mo/Al 2 O 3 and Mo/SiO 2 showed very
low selectivities towards aromatic hydrocarbons (Table 6.2). Therefore, both the Mo
species and the H
+ -exchanged zeolite are essential for the conversion of methane to
aromatic hydrocarbons with high selectivity, as discussed in the following sections.
6.4.2 Effect of the Zeolite Pore Structure on the Catalytic
Production of Aromatic Hydrocarbons
As discussed in Sect. 6.4, the structure of zeolites used as Mo-functionalized catalysts seems to influence their catalytic properties, especially their selectivity towards
benzene. Mo/H-MCM-22 exhibited higher selectivity towards benzene than the other
Mo-modified zeolites in Table 6.1, such as Mo/H-ZSM-5. The catalytic performances
of various Mo-modified H
+ -exchanged zeolites, including their benzene formation
rates and methane conversions, are summarized in Table 6.3 [26].
ZSM-5 has a three-dimensional pore structure that consists of networks of straight
and zigzagging pores and their intersections. Both pore structures are formed by 10membered oxygen rings with entrance diameters of ~5.5 Å. One is straight with
an elliptical cross-section, while the second type has a circular cross-section. The
straight pores are connected by the zigzag channels. The effective pore diameter is
5.9 Å, which is roughly equal to the kinetic diameter of benzene (5.8 Å).
As shown in Table 6.3, Mo/H-ZRP-1 showed the highest rate of benzene formation
among the Mo-modified H
+ -exchanged zeolite catalysts; Mo/H-ZSM-5 also showed
performance comparable to that of Mo/H-ZRP-1. ZRP-1 zeolites are prepared by
modification of high Si/Al ratio H-ZSM-5 with phosphorous and rare earth (RE)
oxides. The chemical composition of ZRP-1 is represented by the formula x RE 2 O 3 :
y Na 2 O · Al 2 O 3 : z SiO 2 , where x = 0.01–0.03, y = 0.4–1.0, and z = 20–60. ZRP-1
was prepared based on the concept of blocking the acid sites on the exterior surface
of ZSM-5 using phosphorous and rare earth oxides. The high catalytic performance
may have resulted from the resulting dealumination (loss of aluminum from the
ZSM-5 zeolite) and the formation of various P- and Al-containing compounds inside
the pores, which decreased the channel size [27].
The zeolite MCM-22 has larger pores than ZSM-5, and its pore structure consists
of two independent pore systems. One of these systems is composed of 12-membered