136
6 Conversion of Methane to Aromatic Hydrocarbons
oxygen ring cages with dimensions of 18 × 7.1 × 7.1 Å. The cages are connected
through 10-membered ring windows. The other system is a two-dimensional 10membered ring pore system that does not contain any cages. There is no direct
connection between the two pore systems. Thus, MCM-22 consists of an interconnected building unit forming two independent pore systems: Two-dimensional, sinusoidal 10-membered interlayer channels of 4.0 × 5.9 Å and 12-membered interlayer
supercages of 7.1 × 18 Å with 4.0 × 5.9 Å entrance apertures.
ZSM-11 has a tetragonal crystal lattice and a two-dimensional pore structure
consisting of straight, perpendicularly intersecting channels; the pores consist
of 10-membered oxygen rings and have dimensions of 5.1 × 5.5 Å. This pore
structure is almost the same as that of the ZSM-5 zeolite. ZSM-8, which also
consists of 10-membered oxygen rings, has an orthorhombic crystal lattice. It has
a two-dimensional system of channels with an average entrance diameter of ~5 Å.
SAPO-34 has 8-membered oxygen rings and narrow pores (pore entrance diameter
of 4.3 Å), which impedes the diffusion of aromatic compounds. In β-zeolite, three
crystal systems (tetragonal, anorthic, and monoclinic) exist; they are highly disordered and have many defects. The pore structure of β-zeolite is two-dimensional,
with pore entrance dimensions of 6.6 × 8.1 Å and 5.5 × 6.5 Å. Empirically curved
channels with dimensions of 5.5 × 6.5 Å are formed by 12-membered oxygen rings.
X, Y, SAPO-5, and SAPO-11 are large-pore zeolite materials, whose pore systems
are three-dimensional straight channels with supercages and large pore entrance
diameters of ~7.5 Å, which is larger than the kinetic diameter of benzene molecule.
Additionally, although the selectivity towards coke and amount of coke formed
during the MDA reaction were not measured in some cases, the catalytic performances of many Mo-modified H
+ -exchanged zeolites have been examined [28, 29].
For example, Zhang et al. reported that the catalytic activities for the MDA reaction followed the order Mo/H-ZSM-11 > Mo/H-ZSM-5 > Mo/H-ZSM-8 > Mo/Hβ > Mo/MCM-41 > Mo/H-SAPO-34 > Mo/H-mordenite > Mo/H-X > Mo/H-Y >
Mo/H-SAPO-5 > Mo/H-SAPO-11 [28].
Thus, based on the results summarized in Tables 6.1 and 6.3 and the discussion
above, the zeolite pore structure can be seen to play an important role in achieving
high catalytic performance in the MDA reaction. Mo-modified porous materials
containing pores with diameters of 5.3–5.6 Å (ZSM-5, MCM-22, ZSM-11, and
ZRP-1), which is close to the kinetic diameter of a benzene molecule, and twoor three-dimensional pore systems are superior as catalysts for the MDA reaction.
Mo-modified zeolites with pore diameters greater than ~6 Å exhibited low catalytic
activity, with coke formation predominating.
6 Conversion of Methane to Aromatic Hydrocarbons
oxygen ring cages with dimensions of 18 × 7.1 × 7.1 Å. The cages are connected
through 10-membered ring windows. The other system is a two-dimensional 10membered ring pore system that does not contain any cages. There is no direct
connection between the two pore systems. Thus, MCM-22 consists of an interconnected building unit forming two independent pore systems: Two-dimensional, sinusoidal 10-membered interlayer channels of 4.0 × 5.9 Å and 12-membered interlayer
supercages of 7.1 × 18 Å with 4.0 × 5.9 Å entrance apertures.
ZSM-11 has a tetragonal crystal lattice and a two-dimensional pore structure
consisting of straight, perpendicularly intersecting channels; the pores consist
of 10-membered oxygen rings and have dimensions of 5.1 × 5.5 Å. This pore
structure is almost the same as that of the ZSM-5 zeolite. ZSM-8, which also
consists of 10-membered oxygen rings, has an orthorhombic crystal lattice. It has
a two-dimensional system of channels with an average entrance diameter of ~5 Å.
SAPO-34 has 8-membered oxygen rings and narrow pores (pore entrance diameter
of 4.3 Å), which impedes the diffusion of aromatic compounds. In β-zeolite, three
crystal systems (tetragonal, anorthic, and monoclinic) exist; they are highly disordered and have many defects. The pore structure of β-zeolite is two-dimensional,
with pore entrance dimensions of 6.6 × 8.1 Å and 5.5 × 6.5 Å. Empirically curved
channels with dimensions of 5.5 × 6.5 Å are formed by 12-membered oxygen rings.
X, Y, SAPO-5, and SAPO-11 are large-pore zeolite materials, whose pore systems
are three-dimensional straight channels with supercages and large pore entrance
diameters of ~7.5 Å, which is larger than the kinetic diameter of benzene molecule.
Additionally, although the selectivity towards coke and amount of coke formed
during the MDA reaction were not measured in some cases, the catalytic performances of many Mo-modified H
+ -exchanged zeolites have been examined [28, 29].
For example, Zhang et al. reported that the catalytic activities for the MDA reaction followed the order Mo/H-ZSM-11 > Mo/H-ZSM-5 > Mo/H-ZSM-8 > Mo/Hβ > Mo/MCM-41 > Mo/H-SAPO-34 > Mo/H-mordenite > Mo/H-X > Mo/H-Y >
Mo/H-SAPO-5 > Mo/H-SAPO-11 [28].
Thus, based on the results summarized in Tables 6.1 and 6.3 and the discussion
above, the zeolite pore structure can be seen to play an important role in achieving
high catalytic performance in the MDA reaction. Mo-modified porous materials
containing pores with diameters of 5.3–5.6 Å (ZSM-5, MCM-22, ZSM-11, and
ZRP-1), which is close to the kinetic diameter of a benzene molecule, and twoor three-dimensional pore systems are superior as catalysts for the MDA reaction.
Mo-modified zeolites with pore diameters greater than ~6 Å exhibited low catalytic
activity, with coke formation predominating.