Selective liquid phase hydrogenation of citral on Au/Fe O catalysts
2
3
C. Milone, M. L. Tropeano, G. Gulino, G. Neri, R. Ingoglia and S. Galvagno*
Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Salita Sperone 31, 98166 Messina, Italia.
E-mail: galvagno@ingegneria.unime.it
Received (in Cambridge, UK) 11th February 2002, Accepted 11th March 2002
First published as an Advance Article on the web 21st March 2002
Gold supported on iron oxide hydrogenates citral (an a,b-
unsaturated aldehyde) to the corresponding a,b-unsat-
urated alcohols (geraniol and nerol) with a selectivity higher
than 95%.
wt% = 16.6) gold loading were prepared by coprecipitation
using HAuCl (Fluka) and Fe(NO ·9H O (Fluka) as starting
materials. An aqueous mixture of HAuCl
was poured into an aqueous solution of 1 M Na
1.9) kept at 80 °C under vigorous stirring. The solid was
digested overnight at room temperature and then washed with
4
3
)
3
2
.
4
and Fe(NO
3 3 2
) H O
2
CO (pH =
3
1
Beside CO oxidation, several studies have demonstrated that
gold supported catalysts show a remarkable catalytic activity for
a wide number of reactions including hydrogenations.1
Of great interest are the few literature data concerning the
activity showed by gold supported catalysts in the hydro-
genation of complex systems such as a,b-unsaturated alde-
hydes.3
3
water until free of chloride ions (AgNO test).
,2
22
Catalysts were dried under vacuum (p = 10 mbar) at 80 °C
for one day. Gold content has been determined by a spectrofoto-
9
metric method. Pt supported on Fe
2
O
3
catalysts were prepared
by either metal vapor deposition (code: PtMVS) or wetness
impregnation of iron oxide with a solution of H PtCl (code:
–6
2
6
3
Shibata et al. have reported that an oxidized amorphous Au–
PtI). The support was prepared by precipitation of iron
hydroxide following the same procedure used for the prepara-
Zr alloy (30 atom% Au) catalyses the hydrogenation of but-
2
-enal and 2-methylprop-2-enal with a maximum selectivity
tion of the coprecipitated gold catalysts. Pt/Fe
wetness impregnation was reduced under H flow at 300 °C.
Catalytic experiments were carried out at atmospheric
pressure under H flow, at 60 °C, in a four-necked flask fitted
2 3
O prepared by
towards the formation of unsaturated alcohols of 59.4 and
7.3%, respectively. Moreover it was found that Au/ZrO
2
4
2
prepared by impregnation shows a much lower activity with
respect to Au–Zr alloy. The influence of the preparation method
on the hydrogenation activity of gold supported catalysts has
2
with a reflux condenser, dropping funnel, thermocouple and a
stirrer head. Citral (Aldrich) and solvent (ethanol) were
commercial analytical grade products and were used without
further purification. The catalyst was added to 25 ml of solvent
and reduced in situ at 70 °C for 1 h. After cooling at reaction
temperature the substrate (0.5 ml) was injected through one arm
of the flask. The reaction mixture was stirred at 700 rpm. The
progress of the reaction was followed by sampling a sufficient
number of microsamples. Chemical analyses were performed
by means of GC-FID. The gas chromatographic column was a
EC-WAX wide-bore capillary column (60 m, 0.53 mm i.d.).
Preliminary runs carried out at different stirring conditions,
loading and catalyst grain size have demonstrated the absence
of external and internal diffusion limitations.
4
been pointed out by Hutchings et al. in their study on the
2
hydrogenation of crotonaldehyde on Au/ZnO and Au/ZrO .
They found that catalysts prepared by coprecipitation are more
active than analogous catalysts prepared by impregnation. The
maximum selectivity towards the formation of crotyl alcohol
was almost 50% on both catalytic systems, however, the
conversion is lower than 20%.
The selective hydrogenation of a,b-unsaturated aldehydes to
the corresponding unsaturated alcohols is a reaction of great
importance in the synthesis of fine chemicals. The selectivity of
this reaction presents some difficult challenges, since one is
attempting to enhance the rate of hydrogenation of the CNO
bond while suppressing that of hydrogenation of the CNC bond.
It is well known that classical hydrogenation metal catalysts
show no or low selectivity towards the hydrogenation of the
CNO bond and that the selectivity of the metal catalysts can be
improved by addition of promoters such as Fe(III), Sn(II),
Ge(IV). Also effective are electron donating ligand effects from
the support, steric constraints in the metal environment, strong
metal–support interaction, selective poisoning, presence of
On all catalysts, hydrogenation of citral leads mainly to the
formation of unsaturated alcohols (geraniol and nerol, the E and
Z forms of 3,7-dimethyl-2,6-octadienol, respectively), derived
from the hydrogenation of the E and Z isomers of citral.
Citronellal (3,7-dimethyl-6-octenal) is the other main reaction
product. A further addition of hydrogen to citronellal gives
citronellol (3,7-dimethyl-6-octenol). Up to 100% of citral
conversion, the further hydrogenation of the unsaturated
alcohols to citronellol was not observed. Products of hydro-
genation of the isolated CNC double bond as well as isopulegol,
7
substituents on the CNC bond, solvent and pressure effects.
Taking into account the hydrogenation activity showed by
gold supported catalysts, we thought that Au/Fe
2
O
3
prepared by
a cyclic isomer of citronellal, previously reported in the
hydrogenation of citral on Ru catalysts,1
0,11
were not detected.
coprecipitation should be in principle a suitable catalyst for the
selective hydrogenation of a,b-unsaturated aldehydes to a,b-
unsaturated alcohols. Iron on the surface of the support would
act as a promoter activating the CNO bond thus increasing its
hydrogenation activity. Moreover dispersed gold shows a
weaker interaction with the CNC bond with respect to noble
metals such as Pt or Pd.2 Therefore this could contribute to a
decrease of the rate of formation of saturated aldehyde during
the hydrogenation of an a,b-unsaturated aldehyde.
The results obtained for the hydrogenation of citral on gold
supported catalysts are reported in Table 1. The catalytic
activity increases with the gold loading while the products
distribution is similar regardless of the amount of the noble
metal. It is noteworthy that the unsaturated alcohols, geraniol
and nerol, are the main reaction products thus indicating the
high activity of these catalysts towards the hydrogenation of the
CNO bond with respect to the conjugated CNC bond. Selectivity
to unsaturated alcohols, measured at 90% of conversion of
citral, is higher than 95% regardless of the gold loading.
Recycle tests of gold supported catalysts have shown a loss of
activity of 20% with respect to the fresh catalysts with no
changes in the products distribution.
,8
Liquid phase hydrogenation of citral (3,7-dimethyl-2,6
octadienal) was taken as a reaction model to investigate the
catalytic activity of gold supported on Fe
catalytic activity of gold supported catalysts was also compared
with that of Pt supported on Fe catalysts.
Au/Fe catalysts with low (Au wt% = 3.1) and high (Au
2 3
O catalysts. The
2 3
O
2
O
3
For comparison, the hydrogenation of citral has been carried
8
68
CHEM. COMMUN., 2002, 868–869
This journal is © The Royal Society of Chemistry 2002
Milone
