The promoting effect of a dicyanamide based ionic liquid in the selective hydrogenation of citral

Article abstract of DOI:10.1039/b810291k

Conventional supported heterogeneous palladium catalysts in combination with a dicyanamide based ionic liquid are highly active with excellent selectivity in enabling the one-pot synthesis of citronellal through citral hydrogenation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b810291k

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The promoting effect of a dicyanamide based ionic liquid in the selective
hydrogenation of citralw
Jurgen Arras, Martin Steffan, Yalda Shayeghi and Peter Claus*
¨
Received (in Cambridge, UK) 17th June 2008, Accepted 18th July 2008
First published as an Advance Article on the web 31st July 2008
DOI: 10.1039/b810291k
Conventional supported heterogeneous palladium catalysts in
combination with a dicyanamide based ionic liquid are highly
active with excellent selectivity in enabling the one-pot synthesis
of citronellal through citral hydrogenation.
obtained. The heterogeneous Pd/C catalyst could be reused
several times, but product extraction is still necessary and
restricts the utilisation of ILs on a larger scale.
On applying monometallic Pd/Al2O3 or bimetallic Ni–
Sn/Al2O3 catalysts in either n-hexane and [BMIM][NTf2], a
modification of the selectivity pattern could not be observed.9
Detailed studies concerning solid ionic liquid coated cata-
lysts were published recently,10 where fluorous based ILs
served to stabilise palladium nanoparticles on an activated
carbon cloth (ACC). The structure of the applied Pd/ACC
catalyst is comparable to the SCILL (solid catalyst with ionic
liquid layer) system, which was used in the nickel catalysed
hydrogenation of cyclooctadiene. The IL-coated catalysts
revealed higher selectivities towards the intermediate cyclo-
octene than under IL-free conditions.11
Heterogeneously catalysed selective hydrogenations play an
evident role in the production of fine chemicals on a multi-ton
scale.1 The determination of factors influencing activity and
selectivity of solid catalysts is still challenging, whereby a,b-
unsaturated aldehydes, e.g. acrolein, crotonaldehyde or citral,
are widely used as model substrates.2
Citral—nowadays a valuable intermediate for the production
of vitamins and perfumes—consists of three unsaturated bonds
(a carbonyl group, and a conjugated as well as an isolated double
bond) and its hydrogenation on supported palladium catalysts
results often in a consecutive reaction network (Scheme 1). On
the industrial scale, citronellal is produced through the hydro-
genation of citral over palladium/carbon catalysts in a contin-
uous slurry-phase, with methanol as the solvent and
trimethylamine as an additive.3
The aim of this study was to investigate systematically the
role of ionic liquids in the selective hydrogenation of citral,
whereby conventional heterogeneous palladium catalysts were
applied. Among commonly used ILs, we introduce—for the
first time in selective hydrogenations—a hydrophilic, low-
viscosity ionic liquid based on the dicyanamide anion ([DCA]ꢀ
= [N(CN)2]ꢀ),12 which allows the selective production of
citronellal in quantitative yields. Furthermore, we wish to
present that ionic liquids, even in small amounts, could affect
the selectivity pattern for this kind of reaction.
Over the past decade, the application of ionic liquids (ILs) in
catalysis has been of growing interest due to their unique
properties.4 Principally, in catalysed reactions, ILs were widely
used as solvents or as stabilising agents for transition metal
complexes.4 Due to their non-volatility, supported ionic liquid
phase catalysis (SILPC) was established for olefin hydroformy-
lation,5 methanol carbonylation,6 and olefin hydrogenation.7
In the selective hydrogenation of citral on supported metal
catalysts, ILs were applied at first as bulk organic solvents
with good selectivities towards citronellal.8 Compared to
organic solvents such as toluene, the reaction rate was lower
in [EMIM][NTf2]z and quantitative yields of citronellal were
Hydrogenation experiments were performed using different
catalyst types: IL-free catalyst, IL coated catalyst or IL as
additive. The catalysts were characterised by ICP-OES, TEM
and nitrogen physisorption.z
Physisorption characteristics for Pd/SiO2 revealed a de-
crease in pore volume and BET surface area after treating
with ionic liquid (Table S1w). Post-reaction BET surface
analysis showed that the pore texture remained unaltered;
the catalysts can be assumed as mechanically stable.
Scheme 1 Reaction network of palladium catalysed citral
hydrogenation.
Technische Universitat Darmstadt, Ernst-Berl-Institut, Technische
¨
Chemie II, Petersenstr. 20, 64287 Darmstadt, Germany.
E-mail: claus@ct.chemie.tu-darmstadt.de; Fax: +49 6151 16 4788;
Tel: +49 6151 16 5369
w Electronic supplementary information (ESI) available: TEM images,
physisorption data and selectivity vs. conversion plots (recycling,
DHC and other products, respectively). See DOI: 10.1039/b810291k
Scheme 2 Supposed structure of an IL coated solid catalyst.
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Table 1 Citral hydrogenation using different reaction conditions with
Pd/C as catalyst and [BMIM][DCA] as ILa
Selectivity (%)
Reaction
type
Conversion
(%)
CAL
DHC
Otherse
IL free
100
100
42
41
49
10
IL coatedb
Additivec
Bulk solventd
a
499
499
97
o1
o1
1
o1
o1
2
100
Conditions: 323 K, 200 mg catalyst, cCitral,0 = 1.1 mol Lꢀ1, 1.0 MPa
b
H2, 360 min. Catalyst with 50 wt% IL loading. 200 mg [BMIM]
c
d
[DCA] added to the citral–n-hexane mixture. n-Hexane was replaced
e
by [BMIM][DCA]. 3,7-Dimethylocta-2,7-dienal, isopulegol and non-
identified products.
Fig. 1 Selectivity vs. conversion plot for different [BMIM][DCA]
quantities during citral hydrogenation (conditions: 323 K, 250 mg
Pd/SiO2, cCitral,0 = 1.1 mol Lꢀ1, 2.0 MPa H2).
An exemplified structure for this kind of catalyst is given in
Scheme 2, whereby the ionic liquid covers the catalytically
active metallic nanoparticles.
In a first approach, the influence of ILs was investigated
under different reaction modes with a conventional Pd/C
(10 wt%, Aldrich) catalyst (Table 1). Under IL-free conditions,
a product mixture was obtained containing principally citro-
nellal (CAL) and dihydrocitronellal (DHC). Generally, in the
presence of [BMIM][DCA], citronellal was the exclusive pro-
duct (S 4 99%), even at high conversion levels with IL coated
catalyst. A high citral conversion with excellent selectivity was
revealed in a reference experiment with [BMIM][DCA] as bulk
solvent.
A variety of reactions with different quantities between 0
and 210 mg of [BMIM][DCA] as additive were carried out to
study the influence on product selectivity. Fig. 1 shows that the
selectivity towards citronellal is dependent on the quantity of
the added [BMIM][DCA].
Except for the highest amount of [BMIM][DCA] (210 mg),
the conversion levels of the performed reactions were nearly
quantitative. Moreover, coating of catalysts prior to reaction
gave slightly higher conversion levels.
Post reaction physisorption of an ‘‘IL-as-additive-catalyst’’
indicated that the ionic liquid was deposited on Pd/SiO2,
because the BET surface area and pore volume were in the
same range as for an IL coated catalyst prepared prior to
reaction (Table S1w).
Validating the selectivity enhancement towards the primary
product, citronellal was used as starting material and the
hydrogenation was performed under similar conditions.
Hereby, the course of hydrogenation was strongly inhibited
if the [BMIM][DCA] coated catalyst was applied and a very
low conversion (B1%) was detected. In contrast, the IL-free
Pd/C catalyst showed high conversion (X) and an excellent
yield (Y) towards dihydrocitronellal (X = 89%, Y = 85%).
In order to exclude influences of the support material,
palladium was also deposited on silica. The results with
[BMIM][DCA] coated Pd/SiO2 catalysts are given in Table 2
and support the findings with Pd/C. The catalyst activity was
lowered if the IL content of the catalyst was about 50 wt%,
without affecting the high citronellal selectivity. Leaching of
[BMIM][DCA] into the substrate solution can be excluded,
because a recycling experiment of the IL coated catalyst gave
congruent selectivity–conversion profiles (Fig. S2w).
It is important to note that similar selectivity enhancement
could also be achieved with other ionic liquids, but to a much
lower extent (SCAL,max = 62% with [BMPL][NTf2]). Palla-
dium catalysed citral hydrogenations with basic promoters
such as sodium hydroxide lead to higher selectivities towards
citronellal.13 We propose that the dicyanamide based ionic
liquid takes also the role of a basic promoter, which was
assumed previously in the acetylation of sugars and alcohols.14
In conclusion, ionic liquids—even in small amounts as coated
catalyst or additive—can manipulate tremendously the selectiv-
ity pattern of conventional solid catalysts in the regioselective
hydrogenation of citral. By minimising diffusion limitations
through the thin film of ionic liquid, the conversion levels were
similar to IL-free catalysts. In addition, product extraction could
be circumvented compared to the use of ionic liquids as bulk
solvents. As it seems, the role of the anion is more crucial than
that of the cation. Although some other ILs such as
[BMPL][NTf2] could increase the selectivity and hence the yield
to citronellal, one could not allude to a more ‘‘greener’’ process
or even of an intensification of the latter. Up to now, only the
DCA-based ionic liquid could improve the hydrogenation of
citral to citronellal in a way which has not been observed with
other ones yet, because consecutive hydrogenation to dihydroci-
tronellal is strongly inhibited leading to a selectivity of 499% at
full conversion, i.e. the desired product is exclusively formed.
Therefore, we propose that this function is not only suitable for
Table 2 Citral hydrogenation with [BMIM][DCA] coated Pd/SiO2
catalystsa
Selectivity (%)
[BMIM][DCA]
content (wt%)
Conversion
(%)
CAL
37
DHC
43
Othersb
20
Without
[BMIM][DCA]
95
33
50
a
89
49
499
499
o1
o1
o1
o1
Conditions: mcitral/mPd = const., 323 K, cCitral,0 = 1.1 mol Lꢀ1
,
b
2.0 MPa H2, 360 min. 3,7-Dimethylocta-2,7-dienal, isopulegol and
non-identified products.
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¨
palladium catalysed citral hydrogenation, but also for other
regioselective hydrogenations.
2 P. Claus and Y. Onal, Regioselective Hydrogenations, in Hand-
book of Heterogeneous Catalysis, ed. G. Ertl, H. Knozinger, F.
¨
Schuth and J. Weitkamp, Wiley-VCH, Weinheim, 2008, p. 3308; P.
¨
Claus, Top. Catal., 1998, 5, 51; C. Mohr, H. Hofmeister, J. Radnik
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¨
J. Hajek, T. Salmi and D. Y. Murzin, Appl. Catal., A, 2005, 292, 1.
The authors are grateful to K. Lehnert for physisorption
and F. Klasovsky for TEM examinations, P. C. thanks the
Fonds der Chemischen Industrie.
3 H. G. Gobbel, G. Wegner, H. Fuchs, S. Unverricht and A. Salden,
¨
WO2004007414A1, 2004.
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z Abbreviations: [EMIM][NTf2] = 1-ethyl-3-methylimidazolium-bis-
(trifluoromethanesulfonyl)amide, [BMIM][NTf2] = 1-butyl-3-methyl-
imidazolium-bis(trifluoromethanesulfonyl)amide, [BMIM][DCA]
1-butyl-3-methylimidazolium-dicyanamide, [BMPL][NTf2]
=
=
5 M. Haumann and A. Riisager, Chem. Rev., 2008, 108, 1474; C. P.
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Catalyst preparation: silica was treated with palladium acetate as metal
precursor and acetone as solvent, applying an incipient-wetness tech-
nique. After drying at room temperature overnight, the orange powder
was reduced at 373 K in a flow of hydrogen to obtain palladium in
metallic state.
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and Pd/SiO2) via incipient-wetness. The catalyst was then dried at
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was 4.6 wt% as revealed by ICP-OES. TEM examinations (JEOL
JEM, 300 kV) showed palladium particles in diameters between 5 and
10 nm (Fig. S1w). Textural properties were determined by nitrogen
physisorption (Quantachrome Autosorb) at 77 K; the samples were
dried in situ at 423 K prior to investigation.
Hydrogenation experiments: in a batch reaction vessel (Parr Co.), citral
(c = 1.1 mol Lꢀ1 in n-hexane) was hydrogenated with Pd catalysts at
323 K and a hydrogen pressure of 2.0 MPa. Samples were taken
periodically and analysed by gas chromatography (HP 6890, DB-Wax)
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4060 | Chem. Commun., 2008, 4058–4060