282
P. Veerakumar et al.
The RGO catalyst was demonstrated to be capable of effectively removing a
broad range of sulfur-containing compounds from fuels with excellent reusability.
The studies on X-ray photoelectron spectroscopy (XPS), chemical titration method,
and a series of comparative experiments revealed that carbonyl groups played
a crucial role during the oxidation process. The RGO with defects in such as
vacancies is beneficial to the catalytic performance because carbonyl groups could
be generated in situ on these defects under the reaction conditions. Although
the carbonyl groups were not directly involved in the generation of ROS, their
electron-withdrawing properties reduce the electron population of the carbon atoms
at their adjacent positions, which facilitates absorption and activation of molecular
oxygen. The strongly adsorbed oxygen molecules may then convert to super-oxygen
anion radicals (RGO–OO–•). Meanwhile, the sulfur-containing substrates turn into
sulfur-centered cation radicals, which react with the negative charged radicals of
RGO–OO–• to generate sulfones as the final products.
7.3.6 C–H Bond Activation
Transition metals and organometallic complexes were employed traditionally to
catalyze the C−H bond activation [81]. Recently, cheap metal-free catalysts have
emerged as promising candidates for this transformation. Gao and coworkers
reported that N-doped Gr plays a major role in the work on sp 2 C−H activation
that covers the selectivity for the oxidation of arylalkanes in aqueous phase,
affording high value-added products for biomedical applications [82]. The Ndoped sp 2 hybridized carbon was prepared through a chemical vapor deposition
(CVD) process with acetonitrile vapor as the N source with 8.9% N content. DFT
calculation suggested that the nitrogen atoms are not able to host the peroxide
species because of the high negative charge of nitrogen. Both the electronic charge
and spin density on the o-carbons are superior positions for the adsorption of
reactive oxygen species such as peroxide. Both C K-edge and N K-edge X-ray
absorption spectroscopy (XAS) were used to study N-doped carbon catalysts before
and after TBHP (t-BuOOH) and ethylbenzene treatment, clearly revealing that the
graphitic nitrogen dopant modulated the electronic structure of sp 2 carbon material.
The intensities of density of states near the Fermi level for the adjacent ortho-carbon
are much stronger than those of undoped graphene carbon, which gives the nitrogenneighboring carbon a metal-like d-band electronic structure.
In 2016, an inexpensive, metal-free GO catalyst used for the C–H bond arylation
of benzene enables the formation of bi-aryl compounds in the presence of aryl
iodides [83]. The oxygen functional groups in these GO sheets and the addition
of KOtBu are essential for the observed catalytic activity. The DFT calculations on
reactions with various model compounds confirmed that these negatively charged
oxygen atoms promote the overall transformation by stabilizing and activating K +
ions, which in turn facilitate the activation of the C-I bond, [as benzyl alcohol was a
particularly active model system]. Based on DFT calculations, a GO nanopore was
P. Veerakumar et al.
The RGO catalyst was demonstrated to be capable of effectively removing a
broad range of sulfur-containing compounds from fuels with excellent reusability.
The studies on X-ray photoelectron spectroscopy (XPS), chemical titration method,
and a series of comparative experiments revealed that carbonyl groups played
a crucial role during the oxidation process. The RGO with defects in such as
vacancies is beneficial to the catalytic performance because carbonyl groups could
be generated in situ on these defects under the reaction conditions. Although
the carbonyl groups were not directly involved in the generation of ROS, their
electron-withdrawing properties reduce the electron population of the carbon atoms
at their adjacent positions, which facilitates absorption and activation of molecular
oxygen. The strongly adsorbed oxygen molecules may then convert to super-oxygen
anion radicals (RGO–OO–•). Meanwhile, the sulfur-containing substrates turn into
sulfur-centered cation radicals, which react with the negative charged radicals of
RGO–OO–• to generate sulfones as the final products.
7.3.6 C–H Bond Activation
Transition metals and organometallic complexes were employed traditionally to
catalyze the C−H bond activation [81]. Recently, cheap metal-free catalysts have
emerged as promising candidates for this transformation. Gao and coworkers
reported that N-doped Gr plays a major role in the work on sp 2 C−H activation
that covers the selectivity for the oxidation of arylalkanes in aqueous phase,
affording high value-added products for biomedical applications [82]. The Ndoped sp 2 hybridized carbon was prepared through a chemical vapor deposition
(CVD) process with acetonitrile vapor as the N source with 8.9% N content. DFT
calculation suggested that the nitrogen atoms are not able to host the peroxide
species because of the high negative charge of nitrogen. Both the electronic charge
and spin density on the o-carbons are superior positions for the adsorption of
reactive oxygen species such as peroxide. Both C K-edge and N K-edge X-ray
absorption spectroscopy (XAS) were used to study N-doped carbon catalysts before
and after TBHP (t-BuOOH) and ethylbenzene treatment, clearly revealing that the
graphitic nitrogen dopant modulated the electronic structure of sp 2 carbon material.
The intensities of density of states near the Fermi level for the adjacent ortho-carbon
are much stronger than those of undoped graphene carbon, which gives the nitrogenneighboring carbon a metal-like d-band electronic structure.
In 2016, an inexpensive, metal-free GO catalyst used for the C–H bond arylation
of benzene enables the formation of bi-aryl compounds in the presence of aryl
iodides [83]. The oxygen functional groups in these GO sheets and the addition
of KOtBu are essential for the observed catalytic activity. The DFT calculations on
reactions with various model compounds confirmed that these negatively charged
oxygen atoms promote the overall transformation by stabilizing and activating K +
ions, which in turn facilitate the activation of the C-I bond, [as benzyl alcohol was a
particularly active model system]. Based on DFT calculations, a GO nanopore was