COMMUNICATION
Selective partial hydrogenation of hydroxy aromatic derivatives with
palladium nanoparticles supported on hydrophilic carbonw
Philippe Makowski,a Rezan Demir Cakan,a Markus Antonietti,a
ab
Frederic Goettmann* and Maria-Magdalena Titirici*a
´
´
Received (in Cambridge, UK) 20th November 2007, Accepted 21st December 2007
First published as an Advance Article on the web 11th January 2008
DOI: 10.1039/b717928f
Selective hydrogenation of phenol to cyclohexanol in the
aqueous phase was achieved using a new catalytic system based
on palladium particles supported on hydrophilic carbon prepared
by one-pot hydrothermal carbonisation.
hydroxymethylfurfural are the main products of the first,
dehydration, step. We have already shown that, using furfural
as a carbon precursor, we could produce monodisperse hydro-
thermal carbon particles, which were identical in terms of
chemical composition and structural properties to those pro-
duced from glucose.13
Cyclohexanone is one of the main intermediates in the pre-
paration of caprolactam and adipic acid which are for instance
used in the manufacturing of nylon-6, nylon-6,6 and poly-
amide resins.1 Cyclohexanone is usually prepared from phenol
in one or two steps (Fig. 1). The one-step reaction is becoming
the preferred industrial process, because it is more advanta-
geous with regard to energy efficiency and processing than the
two-step reaction proceeding via cyclohexanol.2 The hydro-
genation of phenol to cyclohexanone is run in the gas phase,
and supported palladium catalysts have been reported as the
best catalysts for this reaction.3–10 Only a few studies report
such selective hydrogenation of phenol in the liquid phase, at
the best in water.11,12 Even if the required reaction tempera-
ture in a batch process is lower than for gas phase reactions,
the selectivity of the phenol hydrogenation reaction remains
poor, which imposes further separation steps. Such a separa-
tion is rather difficult due to the formation of phenol–cyclo-
hexanol and phenol–cyclohexanone azeotropes.11
In the present approach, furfural was used as the carbon
source and reducing agent and Pd(acac)2 as the metal source.
Thus, a 10 wt% aqueous solution of furfural was mixed with
1 mmol Pd(acac)2 in an autoclave with a Teflon inlet. This
mixture was then hydrothermally treated at 190 1C for 14 h.
As can be seen from scanning electron micrographs (Fig. 2a),
well-defined, spherical carbon particlesw of about 200–400 nm
were obtained.
As a result of the mild hydrothermal treatment, the carbon
framework keeps oxygen rich functional groups at its surface,
which increase both the hydrophilicity and colloidal stability
of the carbon spheres. This was confirmed by the FT-IR
measurements (Fig. 3). The carbon nanocomposite produced
after hydrothermal carbonisation, denoted by Pd@hydrophi-
lic-C, features absorption bands at 3000, 1700 and 1590 cmꢀ1
,
typical for –OH bending, CQO and CQC vibration, respec-
tively. In contrast, almost no such functional polar groups
could be detected on commercial palladium supported on
activated charcoal (used as a reference catalyst).
Here, we report a simple and environmentally friendly route
for the synthesis of a novel metal–carbon nanocomposite
catalyst, which is highly selective for the hydrogenation of
phenol to cyclohexanone. The catalyst is made by a one-pot
and mild hydrothermal carbonisation using furfural and
palladium acetylacetonate.
Recently, we reported on the synthesis of metal oxide
hollow spheres using hydrothermal carbonisation in the pre-
sence of water soluble metal precursors. During the carbonisa-
tion, metal oxide nanoparticles were formed in situ and were
bound and positioned in the hydrophilic shell of the carbon
particles.16 In contrast, when noble metal salts are used, they
are effectively reduced by furfural or in situ formed aldehydes
yielding metal(0) nanoparticles (analogously to the Tollens
reaction) in the early states of particle formation. This was
evidenced by X-ray diffractionw showing the typical fcc pat-
tern of Pd(0) and transmission electron microscopy showing
Hydrothermal carbonisation of water soluble carbohydrates
is a well known process to produce monodisperse carbon
spheres featuring a hydrophilic surface.13–15 A simplified
reaction mechanism for the formation of the carbon spheres
involves the dehydration of the carbohydrate in the first step
and subsequent polymerisation and carbonisation of the so-
formed organic compounds in the second step. Furfural and
a Max-Planck Institute of Colloids and Interfaces, Colloid Chemistry
Department, Scientific campus Golm, 14424 Potsdam, Germany.
E-mail: magdalena.titirici@mpikg.mpg.de; Fax: +49 331 567 9502;
Tel: +49 331 567 9508
b Institut de Chimie Se´parative de Marcoule, UMR 5257, ICSM Site
de Marcoule, BP 17171 30207 Bagnols Sur Ceze Cedex, France.
E-mail: frederic.goettmann@icsm.fr
w Electronic supplementary information (ESI) available: Character-
isation of the Pd@hydrophilic-C catalyst, including X-ray diffracto-
gram, thermogravimetric analysis and N2 adsorption–desorption
isotherm. See DOI: 10.1039/b717928f
Fig. 1 Synthesis of cyclohexanone.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 999–1001 | 999