Chemistry Reference
In-Depth Information
oxidation of alcohols.
163
CNFs and CNTs were functionalized by oxi-
dation with nitric acid and further aminated with gaseous NH
3
. Sub-
sequently, Au and Au-Pd nanoparticles were prepared by PVA/NaBH
4
system and anchored on the surface of pristine and N-doped carbon
nanostructures (CNS). TEM analysis revealed that the introduction of
nitrogen functionalities improves the dispersion of the metal nano-
particles on the surface of the support. This fact leads to an improved
activity of N-CNS based catalysts with respect to pristine CNS when tested
in the liquid phase oxidation of alcohols.
N-containing CNT have been used as gold support for preferential CO
oxidation
164
and compared with gold supported on N-free CNT prepared
by different methods. Using nitrogen-doped CNTs as a support entailed
much higher activity and selectivity at low temperature than the other
CNT supports. This catalyst also showed a good stability under reaction
conditions without detectable sintering.
N-CNT have been used as support for Pd and tested in the direct
synthesis of H
2
O
2
from H
2
and O
2
.
165
The Pd on N-CNT-like gave high
productivities to H
2
O
2
, comparable to the best literature results. Intro-
ducing nitrogen in the CNT-like material favours not only the dispersion
of Pd (with a consequent improvement of the activity), but also the spe-
cific turnover. However, this was detrimental for selectivity to H
2
O
2
be-
cause also the rate of H
2
O
2
consecutive conversion to water increased.
3.2.2 Selective hydrogenations. N-free CNTs and two N-CNTs with
different types of nitrogen incorporated species have been used as pal-
ladium supports for the liquid-phase hydrogenation of cinnamalde-
hyde.
166
The introduction of nitrogen atoms into the carbon matrix
significantly modified the chemical properties of the support compared
to the N-free carbon nanotube resulting in a higher metal dispersion. N-
CNTs exhibited much higher activity in the hydrogenation reaction
compare to the undoped counterpart. Nitrogen incorporation improved
also strongly the selectivity towards the C
=
C bond hydrogenation. The
results show that the type of nitrogen species incorporated in CNTs
structure can influence the catalytic activity. Recycling test confirmed the
high stability of the catalyst because neither palladium leaching nor
deactivation was observed.
Grafitic carbon nitride (g-C
3
N
4
) has been used as support for Pd
catalyst. It exhibited a high selectivity under mild conditions in the hy-
drogenation of phenol to cyclohexanone, an important intermediate in
the synthesis of polyamides.
167
Conversion of 99% and a selectivity
higher than 99% were achieved within 2 h at 65 1C. The proposed
mechanism is depicted in Fig. 15. According to this, phenol adopts a flat
configuration upon adsorption via the hydroxyl group on C
3
N
4
support.
Motoyama et al.
168
showed that Pt supported on N-doped CNF is a
chemoselective catalyst for hydrogenation of nitroarenes to the corres-
ponding anilines. The high selectivity is attributed to the effect of ni-
trogen that inhibits side reactions.
Murzin et al.
169
used a bifunctional catalyst of Pd
þ
lipase supported
on N-CNF for a sequential reaction of hydrogenation
þ
acylation to
