5.6 Relationship Between the Conversion of Methane …
117
performance of this system was attributed to the presence of chlorine, which, unfortunately, was removed from the catalysts with increasing time on stream, resulting in
decreasing activity. The selectivity and yield of C 2
+ hydrocarbons achieved using a
membrane reactor were more formidable than those obtained using a continuous-flow
reactor.
5.7 Membrane Reactor for OCM
As discussed above, fluidized bed reactors come close to achieving the desired
isothermal performance, although the thus obtained C 2
+ hydrocarbon yields do
not exceed 30% because of poor selectivity. Catalytic membrane reactors, which
properly implement the oxygen dosing policy, offer higher C 2
+ hydrocarbon selectivity than fluidized bed reactors [114–119]. For example, Othman et al. developed a catalytic hollow fiber membrane micro-reactor using micro-structured
La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−δ hollow fiber as an oxygen separation membrane, as well
as a functional substrate for in situ deposition of a Bi 1.5 Y 0.3 Sm 0.2 O 3−δ catalyst by a
sol-gel method [114]. A maximum C 2
+ yield of 39% and a selectivity of 79% were
achieved at 1173 K.
The further objectives for designing an efficient OCM membrane reactor are (1)
establishment of fine oxygen distribution to obtain a high level of methane conversion with high selectivity toward ethylene, (2) minimization of the contribution of
the undesired gas-phase reaction, and (3) prevention of the formation of hot spots,
which decrease reaction performance.
5.8 Non-oxidative Coupling of Methane
Direct methane conversion via methane coupling can be accomplished not only in the
presence of oxygen (OCM reaction), but also in the absence of oxygen (non-oxidative
coupling of methane reaction, non-OCM reaction) [120]. Non-oxidative methane
coupling reactions are classified as low-temperature two-step methane coupling
reactions or one-step high-temperature coupling reactions.
5.8.1 Two-Step Methane Coupling
As mentioned in Sect. 5.1, the non-oxidative coupling (dehydrogenative coupling) of
methane is thermodynamically unfavorable. Belgued et al. attempted to circumvent
this thermodynamic limitation for the conversion of methane into C 2
+ hydrocarbons
[121]. In this early investigation into the low-temperature activation of methane,
a 6 wt% Pt/SiO 2 catalyst (EUROPT) was first exposed to methane at 523 K and
117
performance of this system was attributed to the presence of chlorine, which, unfortunately, was removed from the catalysts with increasing time on stream, resulting in
decreasing activity. The selectivity and yield of C 2
+ hydrocarbons achieved using a
membrane reactor were more formidable than those obtained using a continuous-flow
reactor.
5.7 Membrane Reactor for OCM
As discussed above, fluidized bed reactors come close to achieving the desired
isothermal performance, although the thus obtained C 2
+ hydrocarbon yields do
not exceed 30% because of poor selectivity. Catalytic membrane reactors, which
properly implement the oxygen dosing policy, offer higher C 2
+ hydrocarbon selectivity than fluidized bed reactors [114–119]. For example, Othman et al. developed a catalytic hollow fiber membrane micro-reactor using micro-structured
La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−δ hollow fiber as an oxygen separation membrane, as well
as a functional substrate for in situ deposition of a Bi 1.5 Y 0.3 Sm 0.2 O 3−δ catalyst by a
sol-gel method [114]. A maximum C 2
+ yield of 39% and a selectivity of 79% were
achieved at 1173 K.
The further objectives for designing an efficient OCM membrane reactor are (1)
establishment of fine oxygen distribution to obtain a high level of methane conversion with high selectivity toward ethylene, (2) minimization of the contribution of
the undesired gas-phase reaction, and (3) prevention of the formation of hot spots,
which decrease reaction performance.
5.8 Non-oxidative Coupling of Methane
Direct methane conversion via methane coupling can be accomplished not only in the
presence of oxygen (OCM reaction), but also in the absence of oxygen (non-oxidative
coupling of methane reaction, non-OCM reaction) [120]. Non-oxidative methane
coupling reactions are classified as low-temperature two-step methane coupling
reactions or one-step high-temperature coupling reactions.
5.8.1 Two-Step Methane Coupling
As mentioned in Sect. 5.1, the non-oxidative coupling (dehydrogenative coupling) of
methane is thermodynamically unfavorable. Belgued et al. attempted to circumvent
this thermodynamic limitation for the conversion of methane into C 2
+ hydrocarbons
[121]. In this early investigation into the low-temperature activation of methane,
a 6 wt% Pt/SiO 2 catalyst (EUROPT) was first exposed to methane at 523 K and