6.6 Mo Species of Mo/H-ZSM-5 Zeolite Catalysts …
149
active species, the mechanisms of the C–C bond formation reaction, i.e., the formation of hydrocarbons such as C 2 H 4 and benzene, and the roles of the catalysts will
be discussed in the following sections.
6.6.1 Changes in the Reaction Products During
the Induction Period
As discussed above, the types of Mo species on H
+ -exchanged zeolites such as HZSM-5 may change during the induction period. The change in the Mo species would
be expected to affect the types or amounts of reaction products produced during
this induction period. Ding et al. examined the changes in the products during the
initial reaction stage [93]. They observed changes in the concentrations of unreacted
CH 4 and the reaction products (CO, CO 2 , H 2 O, and hydrocarbons) as the reaction
progressed at 950 K using a Mo (4 wt%)/H-ZSM-5 (Si/Al = 25) catalyst and a
1:1:1.3 CH 4 /Ar/He mixtures at a flow rate of 100 ml min
−1 . The results are shown
in Fig. 6.2 as plots of the conversion of CH 4 and the formation rates of CO, CO 2 ,
H 2 O, and hydrocarbons (C 2 H 6 , C 2 H 4 , benzene, and naphthalene) against the reaction
time. After an induction period of about 200 s, the consumption rate of CH 4 increased
sharply and reached its maximum at 310 s. During this ~200 s induction period, the
conversion of CH 4 led predominantly to CO 2 , H 2 O, and CO, along with H 2 , without
the concurrent formation of hydrocarbons. After ~200 s, the formation of C 2 H 4
and benzene began; the rate of C 2 H 6 formation increased gradually and reached
maximum at 310 s. The formation of naphthalene began later than that of benzene
at about 300 s.
6.6.2 Changes in the Mo Species During Induction Period
As discussed in Sect. 6.6.1, during the initial stage of the reaction, CO 2 , H 2 O, and CO
were formed when Mo/H-ZSM-5 was brought into contact with CH 4 . The formation
rates of these compounds varied strongly with the reaction time until the formation
of C 2 H 6 , C 2 H 4 , and benzene began [93]. The formation of these initial products was
attributed to the partial removal of oxygen from the Mo species on H-ZSM-5. Thus,
the reduction of Mo species such as Species (II) occurred, with CO and H 2 as the
predominant products. The amount of H 2 produced was approximately twice that
of CO. In ref. [93], the authors showed that carbon species were retained on the
catalyst surface as Mo carbidic carbon, MoC x . Other groups have also reported the
reduction of Mo species to lower-valent molybdenum species including Mo
0 metal
and the formation of Mo carbidic carbon (MoC x and Mo 2 C) and oxycarbidic carbon
(MoC x O y ) [21, 88–90, 94–99].
149
active species, the mechanisms of the C–C bond formation reaction, i.e., the formation of hydrocarbons such as C 2 H 4 and benzene, and the roles of the catalysts will
be discussed in the following sections.
6.6.1 Changes in the Reaction Products During
the Induction Period
As discussed above, the types of Mo species on H
+ -exchanged zeolites such as HZSM-5 may change during the induction period. The change in the Mo species would
be expected to affect the types or amounts of reaction products produced during
this induction period. Ding et al. examined the changes in the products during the
initial reaction stage [93]. They observed changes in the concentrations of unreacted
CH 4 and the reaction products (CO, CO 2 , H 2 O, and hydrocarbons) as the reaction
progressed at 950 K using a Mo (4 wt%)/H-ZSM-5 (Si/Al = 25) catalyst and a
1:1:1.3 CH 4 /Ar/He mixtures at a flow rate of 100 ml min
−1 . The results are shown
in Fig. 6.2 as plots of the conversion of CH 4 and the formation rates of CO, CO 2 ,
H 2 O, and hydrocarbons (C 2 H 6 , C 2 H 4 , benzene, and naphthalene) against the reaction
time. After an induction period of about 200 s, the consumption rate of CH 4 increased
sharply and reached its maximum at 310 s. During this ~200 s induction period, the
conversion of CH 4 led predominantly to CO 2 , H 2 O, and CO, along with H 2 , without
the concurrent formation of hydrocarbons. After ~200 s, the formation of C 2 H 4
and benzene began; the rate of C 2 H 6 formation increased gradually and reached
maximum at 310 s. The formation of naphthalene began later than that of benzene
at about 300 s.
6.6.2 Changes in the Mo Species During Induction Period
As discussed in Sect. 6.6.1, during the initial stage of the reaction, CO 2 , H 2 O, and CO
were formed when Mo/H-ZSM-5 was brought into contact with CH 4 . The formation
rates of these compounds varied strongly with the reaction time until the formation
of C 2 H 6 , C 2 H 4 , and benzene began [93]. The formation of these initial products was
attributed to the partial removal of oxygen from the Mo species on H-ZSM-5. Thus,
the reduction of Mo species such as Species (II) occurred, with CO and H 2 as the
predominant products. The amount of H 2 produced was approximately twice that
of CO. In ref. [93], the authors showed that carbon species were retained on the
catalyst surface as Mo carbidic carbon, MoC x . Other groups have also reported the
reduction of Mo species to lower-valent molybdenum species including Mo
0 metal
and the formation of Mo carbidic carbon (MoC x and Mo 2 C) and oxycarbidic carbon
(MoC x O y ) [21, 88–90, 94–99].