128
6 Conversion of Methane to Aromatic Hydrocarbons
In this chapter, the catalytic properties of various zeolite catalysts, the roles of the
catalysts, including their active sites, and the reaction mechanisms of the formation
of benzene in the MDA reaction will be the main focus.
6.2 Initial Reports of the Methane Dehydroaromatization
Reaction
The methane dehydroaromatization (MDA) reaction was first carried out by Bragin
et al. in 1982 using zeolite catalysts [2]. They reported that the high-silica zeolite
TsVM (Si/Al ratio of 33.3), which was modified with 0.1–1.0 wt% of group VIII
metal. The MDA reaction was carried out at 873 K in a pulsed micro-reactor. TsVM
gave C 6 –C 10 hydrocarbons, and a benzene yield of 6.4% was achieved.
Shortly after, in 1983, zeolite catalysts such as Fe 2 O 3 -modified H
+ -exchanged
ZSM-5 (H-ZSM-5) were found to produce aromatic hydrocarbons using N 2 O as
the oxidant [3]. The aromatic hydrocarbon yields of these reactions were very low.
Anderson et al. reported that when the reaction was carried out at a CH 4 /N 2 O ratio
of 98:2 at 734 K using H-ZSM-5 as the catalyst, the methane conversion was 2.7%,
the aromatic selectivity was 6.7%, and the major product was CO 2 (87.1%) [4].
In 1993, Wang et al. reported that Mo- or Zn-impregnated H-ZSM-5 zeolites
catalyzed the conversion of methane to aromatic hydrocarbons (mainly benzene)
without an oxidant [5]. When the methane conversion was carried out at 973 K,
using Mo (2 wt%)/H-ZSM-5 or Zn (2 wt%)/H-ZSM-5 in a flow reactor, the methane
conversions were 7.2 and 2.3%, respectively. In both cases, the selectivity towards
benzene was 100%, although the material balance including coke formation was not
reported. In a temperature-programmed experiment, the starting temperature of the
MDA reaction was found to be about 50 K lower for Mo/H-ZSM-5 than for Zn/HZSM-5, with ethane and H 2 being detected at the same time that benzene began to
evolve.
Since then, many researchers have invested great effort into the investigation of
the MDA process, and promising catalysts have been developed [6–11]. In addition to
the development of improved catalysts, optimization of the reactor design is another
general way to improve the MDA reaction, and various designs such as fluidized bed
reactors [12], chemical looping processes [13], and membrane reactors [14–16] have
been reported. However, a detailed discussion of MDA reactors is outside the scope
of this book.
6 Conversion of Methane to Aromatic Hydrocarbons
In this chapter, the catalytic properties of various zeolite catalysts, the roles of the
catalysts, including their active sites, and the reaction mechanisms of the formation
of benzene in the MDA reaction will be the main focus.
6.2 Initial Reports of the Methane Dehydroaromatization
Reaction
The methane dehydroaromatization (MDA) reaction was first carried out by Bragin
et al. in 1982 using zeolite catalysts [2]. They reported that the high-silica zeolite
TsVM (Si/Al ratio of 33.3), which was modified with 0.1–1.0 wt% of group VIII
metal. The MDA reaction was carried out at 873 K in a pulsed micro-reactor. TsVM
gave C 6 –C 10 hydrocarbons, and a benzene yield of 6.4% was achieved.
Shortly after, in 1983, zeolite catalysts such as Fe 2 O 3 -modified H
+ -exchanged
ZSM-5 (H-ZSM-5) were found to produce aromatic hydrocarbons using N 2 O as
the oxidant [3]. The aromatic hydrocarbon yields of these reactions were very low.
Anderson et al. reported that when the reaction was carried out at a CH 4 /N 2 O ratio
of 98:2 at 734 K using H-ZSM-5 as the catalyst, the methane conversion was 2.7%,
the aromatic selectivity was 6.7%, and the major product was CO 2 (87.1%) [4].
In 1993, Wang et al. reported that Mo- or Zn-impregnated H-ZSM-5 zeolites
catalyzed the conversion of methane to aromatic hydrocarbons (mainly benzene)
without an oxidant [5]. When the methane conversion was carried out at 973 K,
using Mo (2 wt%)/H-ZSM-5 or Zn (2 wt%)/H-ZSM-5 in a flow reactor, the methane
conversions were 7.2 and 2.3%, respectively. In both cases, the selectivity towards
benzene was 100%, although the material balance including coke formation was not
reported. In a temperature-programmed experiment, the starting temperature of the
MDA reaction was found to be about 50 K lower for Mo/H-ZSM-5 than for Zn/HZSM-5, with ethane and H 2 being detected at the same time that benzene began to
evolve.
Since then, many researchers have invested great effort into the investigation of
the MDA process, and promising catalysts have been developed [6–11]. In addition to
the development of improved catalysts, optimization of the reactor design is another
general way to improve the MDA reaction, and various designs such as fluidized bed
reactors [12], chemical looping processes [13], and membrane reactors [14–16] have
been reported. However, a detailed discussion of MDA reactors is outside the scope
of this book.