COMMUNICATION
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Highly efficient catalysts for the hydrogenation of nitro-substituted
aromatics{
a
a
bc
Robert Raja,* Vladimir B. Golovko, John M. Thomas, Angel Berenguer-Murcia, Wuzong Zhou,
d
e
e
Songhai Xie and Brian F. G. Johnson
a
Received (in Cambridge, UK) 7th December 2004, Accepted 7th February 2005
First published as an Advance Article on the web 28th February 2005
DOI: 10.1039/b418273a
Nanoparticles of Co and NiPd, derived from colloidal
precursors and supported on commercially available non-
ordered mesoporous silica, are highly effective, cheap, recycl-
able and industrially viable catalysts for the hydrogenation of a
range of nitro-substituted aromatics under mild conditions.
in Table 1}) is comparable to those of their of Pd-containing
analogues, and, they are also relatively inexpensive to synthesize.
The silica-immobilized catalysts are also easily recyclable. A NiPd
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colloid, known to be an active catalyst for the hydrogenation of
nitrobenzene, is used here as a model Pd-containing catalyst.
The nanoparticles are supported on thermally robust, attrition
resistant, cheap, commercially available non-ordered mesoporous
Aminoaromatics, produced by the selective catalytic hydrogena-
tion of corresponding nitro precursors, are important inter-
mediates for agrochemicals, pharmaceuticals, dyestuffs, urethanes
˚
silica of sharply-defined mean pore diameter 60 ¡ 8 A. (Both the
monometallic and bimetallic nanoparticles and the mesoporous
13
silica support are distinctly different from those used by us in
1–3
and other industrially important products.
Raney nickel is
widely used as catalyst but suffers the twin disadvantages of being
4
moisture sensitive and pyrophoric. Palladium on carbon is also
previous selective hydrogenations.) Moreover, it is possible to
study the silica-immobilized nanoparticles by means of transmis-
sion electron microscopy and tomography to get a clearer picture
known to catalyse this reaction but it is more expensive and also
5
quite sensitive to trace impurities. A number of homogeneous
14
of catalyst composition and structure before and after catalysis.
catalysts encompassing macromolecule–metal complexes as well as
mono- and bi-metallic platinum- or palladium-based hetero-
geneous catalysts and polymer-anchored palladium anthranilic
acid complexes have also been reported as viable catalysts for
Finally, soluble metal nanoparticle-based hydrogenation catalysts
15a
often suffer from modest activity and decreased life-times, due
to numerous catalyst decomposition processes, such as agglomera-
15b
tion of nanoparticles, which take place in solution. The merits of
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the mesoporous silica support, which we have also used for
6–9
this hydrogenation. However low turnover numbers and the
concomitant use of external sources of hydrogen (such as triethyl
ammonium or methyl formate), hydrogen-transfer media (cyclo-
hexene in ethanol, for example), involvement of a suitable base
enantioselective hydrogenations via anchored asymmetric organo-
metallic catalysts, are that it is much more thermally stable than
the highly ordered (micelle-templated) mesoporous silicas typified
5
(tri-n-butylamine or pyridine) in the catalytic cycle, reflux
by the M41S and SBA families. Moreover, the silica support of
1
7
temperatures and high pressures (800 psi) coupled with longer
reaction times, and diffusion limitations (where microporous
solids are used) preclude the wider commercial applicability of
these catalysts.
our choice is produced by controlled hydrolysis and, thus, does
not require the use of expensive structure-directing agents for
growth and de-templation before use.
Detailed descriptions of the characterization of these catalysts
are not yet available, but high resolution transmission electron
microscopy (HRTEM) studies{ show (Fig. 1) that the nano-
particles are well-distributed over the support and the particle size
distribution is reasonably sharp. The average particle size of Co is
about 4.3 nm, while that of NiPd is marginally smaller (3.0 nm).
The compositions of both samples were examined and confirmed
by energy dispersive X-ray microanalysis (EDX). The catalytic
tests were performed in a PTFE-lined, high-pressure reactor (Parr)
using high purity hydrogen at ca. 25 bar initial pressure. The
products were analysed (using mesitylene as the internal standard)
by gas chromatograpgy (GC, Varian, Model 3400 CX) employing
a HP-1 capillary column (25 m 6 0.32 mm) and flame ionisation
detector. The catalysts were re-used thrice (to test their
recyclability) and the results show (Table 1) that reproducibility
is high. Hot filtration experiments and ICP measurements were
independently carried out to rule out the possibility of leaching.
In summary the performance of Co and NiPd nanoparticles,
immobilized on non-ordered mesoporous silica, is reported for
the hydrogenation of nitro-substituted aromatics under mild
Compared to the wealth of information available on the
hydrogenation of nitro- and dinitro-benzenes, halonitro-benzenes,
nitroanilines and nitrophenols, there are comparatively few reports
dealing with the hydrogenation of nitrocresols. A convenient
reliable catalytic method for the ready production of aminocresols
is therefore required. In this communication we report the direct
hydrogenation of 3-nitro-o-cresol, as a representative of the nitro-
cresol family, under mild conditions using a simple strategy that
is also applicable for the reduction of nitrobenzene and other
nitrophenols. The catalyst consists of monometallic cobalt or
bimetallic nickel–palladium (Fig. 1) nanoparticles prepared by
10,11
well-known inverse-micelle (Co)
or by polyol reduction
NiPd) methods with some modifications.{ The performance of
the cobalt-based catalysts (high selectivity and reactivity {see TOF
12
(
{
Electronic supplementary information (ESI) available: experimental
details for the synthesis of Co and NiPd nanoparticles, their purification
and preparation of the supported catalysts. See http://www.rsc.org/
suppdata/cc/b4/b418273a/
*robert@ri.ac.uk
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026 | Chem. Commun., 2005, 2026–2028
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