Chemistry Reference
In-Depth Information
carbonyl group in carbon abstracts a hydrogen from the alkane. Carbonyl
group is reduced to hydroxyl group, which is subsequently reoxidised by
gas O
2
closing the cycle. These authors have compared the catalytic ac-
tivity and selectivity of multiwalled carbon nanotubes (MWCNT) in three
different forms: parent CNT, oxidised and modified with phosphorous.
In the parent CNT and in the presence of oxygen, butane is totally oxi-
dised to CO
2
and water and only a small fraction is converted to alkenes.
However, when the nanotubes contain oxygenated functional groups, the
selectivity to alkene increases significantly and it increases even further
when phosphorous is impregnated. Other notable attributes of these
CNT as catalyst include the remarkable stability in reaction (it is not
deactivated in 100 h) and the ability to withstand low O
2
/hydrocarbon
ratios without deactivation.
2.1.2 Oxygen activation: Selective oxidation reactions. Very recently,
it has been reported the selective oxidation of acrolein to acrylic acid by
several types of oxygen-functionalised carbons, such as activated carbon,
nanodiamond or MWCNT.
36
The high activity/selectivity of carbon allo-
tropes is strictly correlated to the exposition of the (0001) basal plane to
the outer surface. It is suggested a mechanism in which a hydrogen is
abstracted from acrolein by nucleophilic oxygen species followed by
oxygen insertion from the mobile epoxide C-O-C species generated by O
2
disociation on the carbon basal planes (Fig. 2). The acrylic acid prod-
uctivity was of 26.5mmol g
1
h
1
, which is almost half of that achieved
with the industrial doped MoV mixed oxide.
Fig. 2 Suggested reaction pathway for the oxidation of acrolein at the graphitic carbon
surface. The active domain is illustrated as a rectangular section of a planar graphene
sheet with a hole defect, which is terminated by arbitrarily positioned oxygen function-
alities. O
2
adsorbs dissociatively at the (0001) surface to form mobile epoxy groups, which
migrate to the prismatic edge sites. The adsorption of acrolein at the nucleophilic oxygen
sites, i.e., the ketones/quinones, initiates its oxygenation by epoxy oxygen atoms to form
acrylic acid. Reproduced from Ref. 36 page 10229 with permission of Wiley-VCH.
