6.8 Various Attempts to Improve the Production …
155
activity than Mo(3 wt%)/H-ZSM-5, with the maximum methane conversion increasing from 7.4 to 10.1% [52]. The selectivity towards benzene also increased from
92.7 to 94.8%. Furthermore, the deactivation of the catalyst was suppressed because
of the resulting change in the character of the carbonaceous deposits. Zeolite dealumination was also suppressed in the presence of the Cu
2+ ions, which increased the
concentration of Mo
5+ in Mo/CuH-ZSM-5 compared to that in Mo/H-ZSM-5.
The addition of Ru [61], Co [47], W [62], and Zr [62] to H-ZSM-5 has also
been found to result in improved catalytic performance. For example, the improved
catalytic properties of Ru-modified Mo/H-ZSM-5 were attributed to its decreased
concentration of Brønsted acid sites with strong acid strength and increased concentration of sites with weak or medium acid strength, as well as the easier reduction
of the initially formed molybdenum oxides. Pt-modified Mo/H-ZSM-5 showed
increased catalyst stability due to the suppression of carbonaceous deposits during
the MDA reaction [106]. Furthermore, Zr and La modification also decreased the
carbonization rate of W/H-ZSM-5 catalysts [38]. W/H-ZSM-5 was prepared by
dissolving (NH 4 ) 2 WO 4 in water and adding a small amount of H 2 SO 4 to adjust the
pH value of the solution to 2–3, followed by drying at 383 K for 2 h and calcination at
773 K for 4 h. Subsequently, Zn (ZnSO 4 ) or La (La(NO 3 ) 3 ) was impregnated into the
W-H 2 SO 4 /H-ZSM-5 in NH 3 solution, and the samples were dried at 383 K for 2 h and
finally calcined at 673 K for 4 h. When W (2.5 wt%)–Zn (1.5 wt%)–H 2 SO 4 /ZSM-5
was used as a catalyst at 1123 K, 0.1 MPa, and a gas hourly space velocity (GHSV) of
1500 mL g
−1 h
−1 , the methane conversion was 23% with ~96% selectivity towards
benzene, while only 0.02% coke relative to the catalyst weight was formed after 3 h.
To reduce the formation of carbonaceous residues, dealumination of the parent
zeolite has been examined [107–109]. For example, the selectivity of coke formation
decreased from 37.9% over Mo/H-ZSM-5 to 18.8% over dealuminated Mo/H-ZSM5; the methane conversion was maintained at ~11% while the selectivity towards
benzene increased from 46.1 to 63.5% [107].
6.8.2 Suppression of Brønsted Acid Sites on the External
Surface of H + -Exchanged Zeolites
To suppress coke formation, silanation treatment was used to decrease the amount of
Brønsted acid sites on the outer (external) surface of the parent zeolite H-ZSM-5; this
method had almost no effect on the acidic O–H groups (Brønsted acid sites) located in
the zeolite channels [110]. For example, 3-aminopropyl triethoxysilane was used to
silanate the parent H-ZSM-5 zeolite. The addition of the optimum amount (0.5 wt% as
SiO 2 ) of the silanizing agent to H-ZSM-5 relative to the final weight of Mo/H-ZSM-5
(0.5 wt% SiO 2 /6 wt% Mo/H-ZSM-5) tuned the apertures of the external micropores
of H-ZSM-5 to a diameter of ~5 Å, which was similar to that of MCM-22. The
SiO 2 (0.5 wt%)/Mo(6 wt%)/H-ZSM-5 exhibited an improved benzene selectivity of
~90% in the MDA reaction because of the suppression of both naphthalene and coke
formation.
155
activity than Mo(3 wt%)/H-ZSM-5, with the maximum methane conversion increasing from 7.4 to 10.1% [52]. The selectivity towards benzene also increased from
92.7 to 94.8%. Furthermore, the deactivation of the catalyst was suppressed because
of the resulting change in the character of the carbonaceous deposits. Zeolite dealumination was also suppressed in the presence of the Cu
2+ ions, which increased the
concentration of Mo
5+ in Mo/CuH-ZSM-5 compared to that in Mo/H-ZSM-5.
The addition of Ru [61], Co [47], W [62], and Zr [62] to H-ZSM-5 has also
been found to result in improved catalytic performance. For example, the improved
catalytic properties of Ru-modified Mo/H-ZSM-5 were attributed to its decreased
concentration of Brønsted acid sites with strong acid strength and increased concentration of sites with weak or medium acid strength, as well as the easier reduction
of the initially formed molybdenum oxides. Pt-modified Mo/H-ZSM-5 showed
increased catalyst stability due to the suppression of carbonaceous deposits during
the MDA reaction [106]. Furthermore, Zr and La modification also decreased the
carbonization rate of W/H-ZSM-5 catalysts [38]. W/H-ZSM-5 was prepared by
dissolving (NH 4 ) 2 WO 4 in water and adding a small amount of H 2 SO 4 to adjust the
pH value of the solution to 2–3, followed by drying at 383 K for 2 h and calcination at
773 K for 4 h. Subsequently, Zn (ZnSO 4 ) or La (La(NO 3 ) 3 ) was impregnated into the
W-H 2 SO 4 /H-ZSM-5 in NH 3 solution, and the samples were dried at 383 K for 2 h and
finally calcined at 673 K for 4 h. When W (2.5 wt%)–Zn (1.5 wt%)–H 2 SO 4 /ZSM-5
was used as a catalyst at 1123 K, 0.1 MPa, and a gas hourly space velocity (GHSV) of
1500 mL g
−1 h
−1 , the methane conversion was 23% with ~96% selectivity towards
benzene, while only 0.02% coke relative to the catalyst weight was formed after 3 h.
To reduce the formation of carbonaceous residues, dealumination of the parent
zeolite has been examined [107–109]. For example, the selectivity of coke formation
decreased from 37.9% over Mo/H-ZSM-5 to 18.8% over dealuminated Mo/H-ZSM5; the methane conversion was maintained at ~11% while the selectivity towards
benzene increased from 46.1 to 63.5% [107].
6.8.2 Suppression of Brønsted Acid Sites on the External
Surface of H + -Exchanged Zeolites
To suppress coke formation, silanation treatment was used to decrease the amount of
Brønsted acid sites on the outer (external) surface of the parent zeolite H-ZSM-5; this
method had almost no effect on the acidic O–H groups (Brønsted acid sites) located in
the zeolite channels [110]. For example, 3-aminopropyl triethoxysilane was used to
silanate the parent H-ZSM-5 zeolite. The addition of the optimum amount (0.5 wt% as
SiO 2 ) of the silanizing agent to H-ZSM-5 relative to the final weight of Mo/H-ZSM-5
(0.5 wt% SiO 2 /6 wt% Mo/H-ZSM-5) tuned the apertures of the external micropores
of H-ZSM-5 to a diameter of ~5 Å, which was similar to that of MCM-22. The
SiO 2 (0.5 wt%)/Mo(6 wt%)/H-ZSM-5 exhibited an improved benzene selectivity of
~90% in the MDA reaction because of the suppression of both naphthalene and coke
formation.