Early-Transition-Metal Carbide and Nitride Nanoparticles
FULL PAPER
Table 4. Friedel–Crafts reactions of various alcohols with benzene, toluene and anisole catalysed by Mo2C, WC and W2N nanoparticles prepared by the
urea route.[a]
Mo2C
WC
W2N
Anisole
Toluene
Conv.
[%]
Anisole
Conv.
[%]
Toluene
Conv.
[%]
Anisole
Conv.
[%]
Toluene
Conv.
[%]
Conv.
p/o[c]
p/o
p/o
p/o
p/o
p/o
[%][b]
benzyl alcohol
4-methylbenzyl alcohol
cinnamyl alcohol
hexanol
95
100
91
95
45
1/1
3/2
4/1
4/1
1/1
70
100
0
0
0
1/1
3/2
–
–
–
100
95
80
95
52
1/1
3/2
4/1
4/1
1/1
82
91
0
0
0
1/1
3/2
–
–
–
100
100
100
100
100
1/1
3/2
4/1
4/1
1/1
95
75
0
0
0
4/3
4/3
–
–
–
cyclohexanol
[a] In a typical reaction the catalyst (25 mg), alcohol (100 mg) and aromatic compound (5 mL) were heated to 1508C for 40 h. [b] Conv.=conversion; the
conversions were determined by GC–FID with mesitylene as a reference and correspond to the molar ratio between the obtained alkylation product and
the initial amount of alcohol. [c] p/o Corresponds to the molar ratio between para- and ortho-substituted alkylation products.
The elemental analysis was carried out for carbon, hydrogen and nitrogen
by using a Vario EL Elementar. The TEM images were taken by using a
Zeiss EM 912 W operated at an acceleration voltage of 120 kV. Samples
were ground and then suspended in ethanol. One drop of this suspension
reaction (which is rather poor), but that each alcohol/aro-
matic pair yielded similar para- to ortho- ratios regardless of
the catalyst. This suggests that the reaction mechanisms in-
volved are roughly the same and that the difference be-
tween these catalysts is only marginal.
was placed on a 400 mesh carbon-coated copper grid and left in air to
dry. To prevent agglomeration of the nanoparticles, the copper grid was
placed on filter paper. SEM was performed on a LEO 1550 Gemini in-
strument. The samples were loaded on carbon coated stubs and coated
by sputtering an Au/Pd alloy prior to imaging.
Conclusion
Synthesis of the nanoparticles: A concentrated solution of the metal pre-
cursor in ethanol was prepared (usually 1 g of metal precursor in 2 g of
ethanol). In particular, we used TiCl4, MoCl5, WCl4, NbCl5, TiN, Mo2N
and Mo2C, W2N and WC, NbN and NbC(N). The metal chloride reacted,
partly vigorously, with the alcohol to release major parts of the chlorine
as HCl and forming the corresponding metal orthoesters. In every case, a
clear solution was obtained. Then, a varying amount of solid urea was
added to the alcoholic solution to give the wanted urea/metal precursor
molar ratio (see Table 1) and was stirred until the urea was completely
dissolved (dissolution time depended on the ratio but was usually is less
than 1 h) to form a gel phase. The use of an ultrasonic bath or heating
was not necessary to obtain homogeneity. The gels were placed in an
oven and treated under a flow of N2 at 8008C for 3 h (plus 4 h to reach
the final temperature).
In this contribution we have shown that a broad variety of
early-transition-metal carbides and nitrides obtained under
relatively soft conditions were able to promote the use of al-
cohols as alkylating agents. In particular, it has been shown
that depending on the metal used, high selectivity could be
achieved for the alkylation of ketones in one direction or
the alkylation of aromatics in the other. Also note that in
this study nanoparticles were employed because they were
easy to produce through the urea route and also because
they naturally feature relatively high surface areas. Never-
theless, we do think that carbides and nitrides produced
through other synthetic routes should feature similar catalyt-
ic properties, which in turn should promote a renewed inter-
est for their use as catalysts for the fine chemical industry.
Catalytic tests: Because our aim was to access cheap and easy to handle
catalysts, the obtained powders were neither stored nor handled under
inert gas. The chemicals were used as received and the solvents were not
further purified. As preliminary tests, the activities of our catalysts were
À
tested in C C bond coupling reactions between acetophenone and
benzyl alcohol. In a typical reaction, the metal carbide or nitride (25 mg)
were placed in a SCHOTT screw-capped glass tube (160 mm length,
about 10 mm inside diameter). Benzyl alcohol (1 mmol) and acetophe-
none (1 mmol) in xylene (2 mL) were then added and the solution was
heated to 1508C for 48 h. The products were analysed and quantified by
GC–MS.
Experimental Section
Methods and materials: GC–MS injections were carried out by using an
Agilent autosampler 7683B. Gas-phase chromatography was performed
on an Agilent GC 6890N equipped with a HP-5MS phenyl methyl silox-
ane capillary column (5%, 30 mꢂ250 mmꢂ0.25 mm). The routine analysis
method used was a splitless injection and the classical oven program was
1 min at 508C then a ramp from 50 to 3008C at 208CminÀ1, followed by
a 5 min final plateau at 3008C. The GC column was coupled to an Agi-
lent 5975 mass spectrometer equipped with an electron ionisation source
and a quadrupole. XRD measurements were performed on a D8 diffrac-
tometer from Bruker instruments (CuKa radiation, l=0.154 nm)
equipped with a scintillation counter. An Enraf–Nonius PDS-120 powder
diffractometer in reflection mode, equipped with an FR-590 generator as
the source of CuKa radiation was also used. For the gas adsorption meas-
urements, all of the samples were degassed at 1508C for 20 h before the
measurements were taken. Nitrogen sorption experiments were done
with a Quantachrome Autosorb-1 or Quadrasorb at liquid nitrogen tem-
perature, and data analysis were performed by Quantachrome software.
Alkylation reactions: In a typical reaction, the catalyst (25 mg) was
placed in a SCHOTT screw-capped glass tube (160 mm length, about
10 mm inside diameter). The desired amount of the pure ketone, or alter-
natively, a solution of ketone and aldehyde in xylene (2 mL) was added.
The tube was closed and heated to 1508C for 20 h. The products were an-
alysed and quantified by GC–MS.
Acknowledgements
The Max-Planck Society, the BASF company and the CEA (French
atomic energy council) are gratefully acknowledged for financially sup-
porting this work.
Chem. Eur. J. 2009, 15, 11999 – 12004
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12003