Chemistry Reference
In-Depth Information
2.1.1 Redox functionality: oxidative dehydrogenation of alkanes. The
conversion of alkanes to alkenes or to oxygenated molecules is an im-
portant research area very relevant considering the great demand of more
than 50 millions of tons of monomer per year.
30
Despite the large in-
vestigation effort, the selectivity to akene in the current process is very
limited. The catalyst used for the oxidative dehydrogenations are nor-
mally based on vanadium and molybdenum oxides. The supports use to
be doped with Pt, which favours coke deposition due to the complete
dehydrogenation.
Metal-free carbon materials have been reported to catalyze the oxida-
tive dehydrogenation (ODH) of an aromatic molecule, ethylbenzene.
However, conventional carbons, in particular activated carbon, undergo
unavoidable deactivations due to coking or combustion.
31,32
Recently, it
was shown that only well-nanostructured carbons are stable and coke-
free catalysts for styrene synthesis.
20,29,33,34
Activation of C-H bonds in
the ethyl group is considered to be coordinated by the ketonic carbonyl
(C
=
O) groups on carbon.
Ethylbenzene has an aromatic moiety that enables relatively facile
activation. The activation of short alkanes is more challenging. A
breakthrough point was the discovery by Su et al. that carbon nanotubes
with modified surface functionality eciently catalyze the oxidative de-
hydrogenation of propane to propene
35
and n-butane to butenes,
24
es-
pecially butadiene. For low O
2
/butane ratios, high selectivity to alkenes
was achieved for periods as long as 100 hours. This process is mildly
catalyzed by nucleophylic oxygen atoms, such as ketonic C
=
O groups,
located at the prismatic edges of stacked graphene sheets or at the sur-
face defects in the (0001) graphitic surface.
35
It occurs via a combination
of parallel and sequential oxidation steps. A small amount of phosphorus
greatly improved the selectivity by suppressing the combustion of
hydrocarbons. The authors demonstrated that it is possible to selectively
inhibit a particular reaction path by precise modification of the active
site. The selective pathway to dehydrogenated product takes place fol-
lowing a Mars-van Krevelen mechanism (Fig. 1). According to this, the
Fig. 1 The oxidative dehydrogenation of alkanes over functionalized CNTs. Reproduced
from Ref. 35 page 6913 with permission of Wiley-VCH.
