Communications
Table 2: Catalytic activity of mp-C3N4/1 in the Friedel–Crafts acylation of
benzene with hexanoyl chloride in different solvents.
to improve the catalytic activity,and the best results were
obtained for mpg-C3N4/0.5 (90% conversion).
To conclude,we have synthesized a mesoporous,high-
surface-area graphitic carbon nitride by using colloidal silica
nanoparticles as a template,and its catalytic properties were
tested in Friedel–Crafts reactions. Remarkable activity
enhancement was found for reactions with benzene as the
coupling partner. We are currently trying to prove exper-
imentally that this catalysis arises from back-donation of
electron density from molecular orbitals of the catalyst to
unoccupied orbitals of the arene. As defects in the graphitic
layers appear to be responsible for the catalytic activity,in
future work we will focus on determining the structure of the
catalyst more precisely to gain better insight into the under-
lying activation mechanism.
Benzene
Heptane
Anisole
T [8C]
Conversion [%]
80
16
90
80
150
60
spectroscopy. The choice of anisole as a solvent might be
surprising as it is well known to be much more reactive than
benzene as a Friedel–Crafts substrate. Indeed,in mixtures of
benzene and anisole the only product formed is n-hexano-
phenone. This finding indicates that anisole does not react as
long as benzene is present. As the catalysis is supposed to
originate from electron donation from melem units to the
aromatic substrate,substrates having lower-lying molecular
orbitals should interact more strongly with the C3N4 surface.
On the other hand,substitutions on the aromatic ring alter the
symmetry of the molecular orbitals,resulting in an inferior
overlap with the HOMO of localized melem units. These two
considerations together with steric effects make it difficult to
predict the reactivity of individual arenes.
Since heptane was found to be a suitable solvent for the
Friedel–Crafts acylation (Table 2),it was used in subsequent
experiments. We set out to analyze the effect of the surface
and crystallinity of the catalysts. The acylations were per-
formed with hexanoyl chloride (50 mg) and benzene (150 mg)
in heptane (5 g) for 20 h at 908C using 25 mg of the
corresponding carbon nitride as the catalyst. The products
of the reactions were analyzed by GC with an internal
standard for quantification. Some results are summarized in
Table 3. Most of the mesoporous carbon nitrides used are
Most of the solids used in heterogeneous catalysis are
Lewis acids.[21] We hope that mpg-C3N4 described here will
pave the way for metal-free heterogenous catalysts with
Lewis-base character. This would be an important step
towards metal-free coordination chemistry and catalysis.
À
Moreover,this system is likely to be active for other C
C
À
coupling and C H activation reactions. It is remarkable that
this was achieved with a material first described by Berzelius
and Liebig,one of the first organic solid-state materials made.
Experimental Section
TEM images were recorded on a Zeiss EM91 microscope. WAXS
spectra were recorded on a Bruker D8 Advance diffractometer and
the SAXS spectra on a Enraf Nonius FR590. FTIR spectra were
recorded on a BioRad FTS 6000 spectrometer.
Synthesis of g-C3N4: Molten cyanamide (1 g,24 mmol; Aldrich)
was heated and stirred at 708C,and different amounts of a 40%
dispersion of 12-nm SiO2 particles (Ludox HS40,Aldrich) in water
were added dropwise (1.25,2.0,4.0 g Ludox for the r= 0.5,1,1.6). The
Table 3: Catalytic activity of mpg-C3N4 in the Friedel–Crafts acylation of
benzene.[a]
Catalyst
Conversion [%]
TOF100 [hÀ1 [b]
]
resulting transparent mixtures were then heated at
a rate of
mp-C3N4/0.5
mp-C3N4/1
mp-C3N4/1.6
mpg-C3N4/0.5
bulk g-C3N4
graphite
52
80
52
90
0
3.6
5.5
3.6
6.2
–
4.5 KminÀ1 over 2 h to reach a temperature of 5508C and then
tempered at this temperature for another 4 h. To prepare mp-C3N4,
the resulting brown–yellow powder was treated with a 4m NH4HF2
for two days to remove the silica template. The powders were then
centrifuged and washed three times with distilled water and twice
with ethanol. Finally the powders were dried at 708C under vacuum
for several hours. To prepare mpg-C3N4,samples resulting from the
first heat treatment (without silica extraction) were heated to 6008C
for 10 h under static vacuum in a sealed quartz ampule. To remove the
silica templates,the obtained light-brown powders were treated with
NH4HF2 as described above.
1
–
[a] Reaction conditions: A mixture of benzene (150 mg), hexanoyl
chloride (50 mg), and (C3N4) in heptane (5 g) was stirred at 908C for
20 h. The products were analyzed by gas chromatography. [b] Turnover
frequency: n(hexanoyl chloride) per n(melem units) per hour.
All computations were performed using the Gaussian03 suite of
programs[22] and gradient-corrected density functional theory by using
the B3LYP functional.[23] Optimizations were carried out using the 6-
21G basis set.
Catalysis experiments: A sample of mesoporous C3N4 (25 mg,
0.5 mmol melem units) was suspended in a solution of benzene
(2 mmol) and hexanoyl chloride (50 mg,0.75 mmol) in the respective
solvent (5 g). The mixture was heated to reflux. After 20 h reaction
time,an aliquot of the mixture was poured into ethanol and analyzed
by GC. The structure of the product was confirmed by 1H NMR
spectroscopy.
indeed active catalysts for the Friedel–Crafts acylation,
whereas the bulk carbon nitride was inactive. As a reference,
graphite particles (1 mm in diameter) were used under the
same conditions and were found to be far less active.
No clear correlation can be found between the specific
surface area and the catalytic activity of samples that were
submitted to the same heat treatment,as the activity appears
to go through a maximum. This can also indicate that only the
pore walls of a certain minimal thickness have a suitable
electronic structure. The second heat treatment at 6008C and
the resulting increase in organization and condensation seem
Received: January 31,2006
Published online: June 13,2006
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 4467 –4471