methyl cation (CH3+).11a,b The resulting CH3 carbocation
radical-enabling reaction with excess methane to give ethane
(eqn (4)). The overall reaction (eqn (4)) is endothermic by
+47.7 kJ/mol. This model offers an alternative to the more
endothermic carbocationic condensation of CH4 to form C2H6,
as shown in Scheme 1 (steps-3, 4, and 5) (DH = +64.5 kJ/mol).
+
+
reacts with methane, forming the initial C–C bond in a C2H7
carbocation (step-4). This then either deprotonates, forming
+
ethane (step-5), or dehydrogenates to form C2H5 (step-6),
+
which can then react with methane to form a C3H9 carbo-
cation, continuing the chain growth process.
The general chain growth process leading to the formation of
high molecular weight carbonium ions is shown in step-10. The
overall reaction sequence proposed here is more complicated
since the intermediate carbonium ions themselves can undergo
various di-, tri- and polymerization processes, and the higher
molecular weight hydrocarbon ions may undergo fragmentation
or hydrocracking. Olefins may be formed by the elimination of a
proton from an alkyl cation (CnH2n+1+) (steps-12 and 13).
Rasul et al. use ab initio computational studies to show that
ð3Þ
ð4Þ
Fig. 2 shows the effect of reaction time and temperature on the
yields of H2 and C2+ hydrocarbons. As the reaction continues,
the H2 yield increases at all temperatures. The hydrogen yield with
time on stream shows a trend similar to CH4 conversion.
Comparing Fig. 1 and 2, as the CH4 conversion increases over
time on stream, the H2 yield also increases. Our results show that
formation of higher hydrocarbons ranging from C2–C26 at 673 K
in presence of superacid AlBr3–HBr which can be hydroprocessed
further to produce fuel range hydrocarbons. Another potential
advantage of the process is that the H2 produced can be separated
and used elsewhere. However, efficient recovery and recycling of
AlBr3 and HBr is needed to make the production of hydrocarbons
economically viable.
+
the exothermic protonation of CH4 to form CH5 with
subsequent and more facile homolytic cleavage results in the
+
exothermic formation of CH4
(DH
=
ꢀ100.9 kJ/mol
(eqn (3)).12 Subsequent reaction with CH4 gives the methyl
In conclusion, our results show the essentially complete con-
ꢀ
version of methane to higher hydrocarbons using H+AlBr4
superacid catalyst in a continuous reactor at r673 K by
protolytic condensation of CH4. This non-oxidative reaction
may pave the way for essentially complete CH4 conversion to
hydrocarbons, an alternative to F-T and MTG processes.
We gratefully acknowledge the financial support from the
Louisiana Board of Regents and Albemarle Corporation.
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time on stream.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 785–787 787