Chemistry Reference
In-Depth Information
Fig. 9 Multiple functionalities of C
3
N
4
as a catalyst. Reproduced from Ref. 94 page 4907.
nanotubes have been used for Knoevenagel Condensation.
73
The activity
was comparable to activated carbons doped with nitrogen or hydro-
talcites and it was determined by the amount of pyridinic groups. Un-
fortunately, deactivation by reaction products was observed.
In a recent work, CNTs functionalised with ammine groups have
shown to be active for biomass conversion
91
and for the transester-
ificaton ot tryglicerides with methanol,
92,93
which is considered a test
reaction for biodiesel production. This catalyst has the advantage that it
does not contain high amounts of acidic oxygenated surface groups
which are inevitably present in metal oxides. The anity of carbon for
organic liquids and the minimization of mass transport limitations are
other of its potential advantages over other solid basic catalysts.
Graphitic carbon nitride (g-C
3
N
4
) is a polymer consisting of nitrogen
and carbon atoms which have a structure analogous of graphite. It can
behave as a semiconductor and as a base depending on the preparation.
Figure 9 shows the structure and functionalities of g-C
3
N
4
.Moreinfor-
mation about graphitic carbon nitride materials can be found elsewhere.
94
These properties make it useful as a metal-free base catalyst in a variety
of reactions such as the activation of benzene in Friedel-Crafts acyla-
tions,
95
trimerization of triple bonds
96
and splitting of carbon dioxide
assisting the formation of phenol from benzene
97
or its fixation as
carbamates.
94
For the use of carbon nitride in organic reactions, usually meso-
porosity must be created. For instance, mesoporous carbon nitride has
been prepared using silica nanoparticles
98
or SBA-15
99
as template and it
was tested in base-catalysed transesterification reactions. In the trans-
esterification of b-keto ester of different alcohols, the catalyst exhibited
high activity and 100% selectivity to the corresponding ester in short
reaction times.
98
Furthermore, the catalyst was stable and could be re-
cycled several times.
2.2.3 Electrocatalysis: Oxygen reduction reaction. The research on
N-doped CNTs used as electrocatalyst was spurred by the report that
vertically aligned nitrogen-containing carbon nanotubes can act as a
metal-free electrode with a much better electrocatalytic activity, long-
term operation stability, and tolerance to crossover effect compared to
