156
6 Conversion of Methane to Aromatic Hydrocarbons
6.8.3 Effect of the Addition of Small Amounts of CO
and CO 2 to the Methane Feed
Adding small amounts of CO and CO 2 to the methane reactant feed is another
approach that can be used to improve the catalytic performance of modified H-ZSM5. These compounds have been found to promote the formation of benzene and
improve the stability of Mo/H-ZSM-5 catalysts [111–114]. For example, Ohnishi
et al. examined the effect of the addition of CO and CO 2 on the catalytic performances of H-ZSM-5 zeolite catalysts modified with various metal species (Mo, Re,
and Fe) [30, 111, 112]. When Fe-modified Mo (3 wt%)/H-ZSM-5 (Fe/Mo (3 wt%)/HZSM-5) and Co-modified Mo (3 wt%)/H-ZSM-5 (Co/Mo (3 wt%)/H-ZSM-5) were
used as catalysts for the conversion of methane containing 1.8% (v/v) CO at 973 K,
benzene was produced at a rate of ~1000 mmol g
−1 s
−1 on a carbon basis. Catalytic
stability was observed for >100 h over both catalysts because of the minimization of
the coke formation (<20%). Similarly, the catalytic stability of 3 wt% Mo/H-ZSM-5
was also improved by adding CO 2 to the methane feed gas instead of CO at the same
reaction temperature [30].
The CO 2 reforming reaction and reverse Boudouard reaction are shown below as
reaction (6.19) and (6.20).
CO 2 + CH 4
2 CO + 2 H 2
(6.19)
CO 2 + C
2 CO
(6.20)
As shown in reaction (6.19), when CO 2 is added to methane, it is converted to
CO, therefore, the effect of CO 2 is analogous to that of CO. Furthermore, Ohnishi
et al. carried out a temperature-programed oxidation (TPO) reaction to measure
the amount of coke deposited on Mo/H-ZSM-5 and Fe/Co-modified Mo/H-ZSM-5
catalysts [111]. This TPO experiment showed that in contrast to CO, the addition of
CO 2 to the methane feed reduced not only the inert coke, but also the reactive coke,
which was converted to aromatics such as benzene and naphthalene.
13 CO isotopic
labeling tracer studies also showed that the
13 C from
13 CO was easily incorporated
into the methane and products such as benzene and ethylene. On the basis of these
results, the authors speculated that the role of CO addition to methane feed was
based on the formation of minute amounts of CO 2 and C by the Boudouard reaction,
in which C represents hydrogenated carbon species (CH x ), resulting in improved
catalyst stability due to the suppression of coke formation on the catalyst surface.
However, Liu et al. reported that the CH 4 conversion rates were not influenced by
the addition of CO and CO 2 as co-reactants [115]. They explained that CO 2 led to the
oxidation of the MoC x on H-ZSM-5, but the CH x reaction intermediates formed in
the rate-determining C–H bond activation step increasingly formed CO rather than
hydrocarbons.
6 Conversion of Methane to Aromatic Hydrocarbons
6.8.3 Effect of the Addition of Small Amounts of CO
and CO 2 to the Methane Feed
Adding small amounts of CO and CO 2 to the methane reactant feed is another
approach that can be used to improve the catalytic performance of modified H-ZSM5. These compounds have been found to promote the formation of benzene and
improve the stability of Mo/H-ZSM-5 catalysts [111–114]. For example, Ohnishi
et al. examined the effect of the addition of CO and CO 2 on the catalytic performances of H-ZSM-5 zeolite catalysts modified with various metal species (Mo, Re,
and Fe) [30, 111, 112]. When Fe-modified Mo (3 wt%)/H-ZSM-5 (Fe/Mo (3 wt%)/HZSM-5) and Co-modified Mo (3 wt%)/H-ZSM-5 (Co/Mo (3 wt%)/H-ZSM-5) were
used as catalysts for the conversion of methane containing 1.8% (v/v) CO at 973 K,
benzene was produced at a rate of ~1000 mmol g
−1 s
−1 on a carbon basis. Catalytic
stability was observed for >100 h over both catalysts because of the minimization of
the coke formation (<20%). Similarly, the catalytic stability of 3 wt% Mo/H-ZSM-5
was also improved by adding CO 2 to the methane feed gas instead of CO at the same
reaction temperature [30].
The CO 2 reforming reaction and reverse Boudouard reaction are shown below as
reaction (6.19) and (6.20).
CO 2 + CH 4
2 CO + 2 H 2
(6.19)
CO 2 + C
2 CO
(6.20)
As shown in reaction (6.19), when CO 2 is added to methane, it is converted to
CO, therefore, the effect of CO 2 is analogous to that of CO. Furthermore, Ohnishi
et al. carried out a temperature-programed oxidation (TPO) reaction to measure
the amount of coke deposited on Mo/H-ZSM-5 and Fe/Co-modified Mo/H-ZSM-5
catalysts [111]. This TPO experiment showed that in contrast to CO, the addition of
CO 2 to the methane feed reduced not only the inert coke, but also the reactive coke,
which was converted to aromatics such as benzene and naphthalene.
13 CO isotopic
labeling tracer studies also showed that the
13 C from
13 CO was easily incorporated
into the methane and products such as benzene and ethylene. On the basis of these
results, the authors speculated that the role of CO addition to methane feed was
based on the formation of minute amounts of CO 2 and C by the Boudouard reaction,
in which C represents hydrogenated carbon species (CH x ), resulting in improved
catalyst stability due to the suppression of coke formation on the catalyst surface.
However, Liu et al. reported that the CH 4 conversion rates were not influenced by
the addition of CO and CO 2 as co-reactants [115]. They explained that CO 2 led to the
oxidation of the MoC x on H-ZSM-5, but the CH x reaction intermediates formed in
the rate-determining C–H bond activation step increasingly formed CO rather than
hydrocarbons.