Continuous selective hydrogenation of citral in a trickle-bed reactor using ionic liquid modified catalysts

Article abstract of DOI:10.1016/j.apcata.2010.06.025

The influence of the ionic liquid [BMIM][N(CN)₂] on the palladium catalyzed hydrogenation of citral in a trickle-bed reactor has been investigated. Applying the SCILL concept (solid catalyst with ionic liquid layer), it was possible to attain citronellal selectivities close to 100% at the cost of catalyst activity. However, the yield of this intermediate was approximately four times higher compared to the neat palladium catalyst. The latter and its SCILL counterpart both seem to have long-term stability, which is relevant for any future industrial application. This is the first time that SCILL systems have been compared directly to their IL-free equivalents in continuous mode.

Full text of DOI:10.1016/j.apcata.2010.06.025

Applied Catalysis A: General 391 (2011) 319–324
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Continuous selective hydrogenation of citral in a trickle-bed reactor using ionic
liquid modified catalysts
∗
Nicolai Wörz, Jürgen Arras, Peter Claus
Technische Universität Darmstadt, Ernst-Berl-Institut, Technische Chemie II, Darmstadt, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The influence of the ionic liquid [BMIM][N(CN) ] on the palladium catalyzed hydrogenation of citral in
a trickle-bed reactor has been investigated. Applying the SCILL concept (solid catalyst with ionic liquid
2
Received 26 February 2010
Received in revised form 1 June 2010
Accepted 15 June 2010
layer), it was possible to attain citronellal selectivities close to 100% at the cost of catalyst activity. How-
ever, the yield of this intermediate was approximately four times higher compared to the neat palladium
catalyst. The latter and its SCILL counterpart both seem to have long-term stability, which is relevant for
any future industrial application. This is the first time that SCILL systems have been compared directly
to their IL-free equivalents in continuous mode.
Available online 30 June 2010
Keywords:
Trickle-bed reactor
Liquid phase hydrogenation
Citral
©
2010 Elsevier B.V. All rights reserved.
Ionic liquid
SCILL catalyst
1
. Introduction
utilized conventional Pd/C or Pd/SiO2 catalysts were impregnated
via incipient wetness with the ionic liquid, e.g. [BMIM][N(CN)2]
(1-butyl-3-methylimidazolium dicyanamide). The so-called SCILL
concept was applied (solid catalyst with ionic liquid layer), which
was first introduced by Kernchen et al. in the nickel catalyzed
hydrogenation of cyclooctadiene [17]. The ionic liquid is assumed
to cover the catalytically active metallic nanoparticles on its porous
support causing solvent- as well as co-catalytic effects.
The selective hydrogenation of ␣,␤-unsaturated aldehydes is
still a challenging task in heterogeneous catalysis [1–4]. Citral (CIT;
,7-dimethyl-2,6-octadienal) belongs to this class of compounds
3
and may be used as model substance. Its hydrogenation leads to a
highly complex reaction network depending on the catalyst. Apply-
ing palladium as active metal, hydrogenation of the carbonyl group
may be hindered and the conjugated C C bond is hydrogenated
prior to the terminal isolated one [5–8]. This results in a simpli-
fied reaction network with citronellal (CAL) and dihydrocitronellal
Trickle-bed reactors (TBR) are part of packed-bed reactors char-
acterized by a two-phase flow of gas and liquid reactants. The liquid
is in downflow mode and the gaseous components can either be
cocurrent or countercurrent. TBR are utilized in numerous indus-
trial hydrogenations [18] and criteria to ensure their ideal behavior
have recently been reviewed by Mederos et al. [19]. Recchia and
co-workers have studied the hydrogenation of citral on Pt–Sn/MgO
in a micropilot trickle-bed reactor [20]. The authors reported 97%
selectivity to the unsaturated alcohols (geraniol and nerol) at full
conversion as well as long-term stability of the applied catalyst.
Recently, Virtanen et al. [21] examined the hydrogenation of citral
in a tubular reactor applying palladium as active metal and N-butyl-
4-methylpyridiniumtetrafluoroborate [NB MPy][BF ] as IL layer on
(
DHC) as main products. The unsaturated alcohols geraniol and
nerol as well as citronellol are usually not produced via supported
palladium catalysts. Furthermore, dimethyloctanol, isocitral and
the two isomers of 3,7-dimethyl-2-octenal are usually produced
in minor amounts (Fig. 1).
The application of ionic liquids (IL) in catalysis [9,10] is of
growing interest enabling novel concepts of catalysts immobiliza-
tion, e.g. the supported ionic liquid phase (SILP) catalysis [11,12].
Furthermore, ionic liquids were introduced in the selective hydro-
genation of citral, whereby they were used as solvents [13] or
reaction modifiers [14–16]. With the aid of dicyanamide based ionic
liquids, we managed to completely inhibit the consecutive hydro-
genation of citronellal realizing its one-pot production [14,15]. The
4
4
active carbon cloths. The catalyst cloths were packed in layer by
layer on a stainless steel net in order to immobilize the catalyst.
The authors mentioned a maximum dihydrocitronellal selectivity
of ∼58%. Unfortunately, a comparison of the results to the equiva-
lent catalyst without IL layer was not provided. To the best of our
knowledge this is the only work dealing with the hydrogenation of
citral in continuous mode making use of modifiers based on ionic
liquids.
∗ Corresponding author at: Petersenstraße 20, 64287 Darmstadt, Germany.
Tel.: +49 6151 165369; fax: +49 6151 164788.
E-mail address: claus@ct.chemie.tu-darmstadt.de (P. Claus).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.06.025

3
20
N. Wörz et al. / Applied Catalysis A: General 391 (2011) 319–324
Fig. 1. Simplified reaction network of the palladium catalyzed citral hydrogenation.
The aim of this study was to transfer the SCILL concept from
been proven that they equal their corresponding experimental
amounts.
Pulse H2 chemisorption was performed on a TPDRO1100
(Thermo-Porotec) to obtain hydrogen uptakes of the studied cata-
lysts. Prior to the investigation, the catalysts were reduced under
batch-mode to the continuous hydrogenation of citral in a trickle-
bed reactor and to investigate its future industrial application,
especially for the production of citronellal. To date, the latter may
be produced in a bubble column with feedback and the aid of an
organic base, e.g. triethylamine, over palladium or ruthenium cat-
alysts [22]. We wish to demonstrate that ionic liquids as modifiers,
prepared onto the catalyst surface, may be an alternative to classical
additives such as organic bases.
−
1
◦
−1
hydrogen flow (Linde 5.0, 3 K min , 100 C, 20 mL min , 1 h)
◦
−1
,
and desorbed under helium flow (Linde 5.0, 100 C, 20 mL min
2 h). Then, pulses of hydrogen (Vloop = 450 ␮L, ꢀtPulse = 12 min,
= 10 times) under argon flow (Linde 5.0, 20 mL min ) at
1 C were exposed on the investigated catalysts. The SCILL cat-
−
1
N
Pulse
◦
2
alysts were investigated the same way as their IL-free counterparts
except that they have been coated with IL prior to the experiment
2
. Experimental
(Table 1).
2.1. Catalyst preparation and characterization
For a detailed catalyst characterization (N2 physisorption,
pulse CO chemisorption, infrared spectroscopy, transmission elec-
tron microscopy, ICP-OES), especially of the SCILL system, we
refer to our previous publications [14–16]. Recently, we studied
the adsorption of hydrogen on palladium based SCILL catalysts,
whereby XANES/EXAFS, XPS, hydrogen chemisorption and adsorp-
tion microcalorimetry were applied [23]. On the basis of these
experiments we could conclude that the catalytically active pal-
ladium interacts with the ionic liquid enabling a complexation of
1
Pd/SiO2 (1 wt% Pd) and 5Pd/Al O3 (5 wt% Pd) have been
2
provided from KataLeuna GmbH Catalysts/CRI or supplied by
Sigma–Aldrich, respectively. The catalysts have been pretreated
with hydrogen (20 mL min gCat ) for 1 h at 100 C. For the
SCILL preparation, the ionic liquid [BMIM][N(CN) ] (1-butyl-3-
methylimidazolium dicyanamide, Merck) was dissolved in acetone
−1
−1
◦
2
(
Merck) and impregnated over the corresponding catalyst. To
obtain a SCILL system containing 32 wt% IL, typically 12 g of the IL
was dissolved in 25 mL of acetone and impregnated over 25 g of the
pure palladium catalyst. In this example addition of the IL reduced
the mass fraction of palladium based on the total mass of the SCILL
system (37 g) from 1 wt% to 0.7 wt%. The IL content will be dis-
played as index, whereas the content of active metal will be stated
as prefix in the following (e.g. 0.7Pd/SiO @[BMIM][N(CN) ]32). To
Pd by [N(CN) ]. The adsorption characteristics, hydrogen uptake
2
and respective heat of adsorption indicated a ligand effect known
from bimetallic catalysis.
2.2. Hydrogenation experiments
2
2
accomplish uniform distribution of the IL layer, it was important
that the amount of liquid solution was sufficient to cover the solid
catalyst completely. The impregnated catalyst was then dried over
night at room temperature in the laboratory hood to remove excess
organic solvent. It is to mention that all IL contents refer to their
theoretical amounts after preparation but applying ICP-OES it has
All reactions were conducted in a thermostated trickle-bed reac-
tor (Vinci Technologies). Two thermocouples were located inside
the reactor and two were placed at its outer wall. The reactor was
equipped with a two zone heating jacket, which could either use
the inside or outside temperature as reference (inside tempera-
ture in this work). Supply of liquid and gaseous reactants was
Table 1
◦
Hydrogen uptakes of various supported palladium SCILL catalysts at 21 C after reductive pre-treatment.
nH2,ads (␮mol g 1
−
)
Entry
Catalyst
IL
Pd content (wt%)
IL content (wt%)
1
2
3
4
Pd/SiO2
Pd/SiO2
Pd/SiO2
Pd/Al2O3
–
1
–
20
32
32
13.5
3.1
1.6
[BMIM][N(CN)2]
[BMIM][N(CN)2]
[BMIM][N(CN)2]
0.8
0.7
3.4
6.3

N. Wörz et al. / Applied Catalysis A: General 391 (2011) 319–324
321
Fig. 2. Conversion and selectivities vs. time on stream of the 1Pd/SiO2 catalyzed citral hydrogenation (VCat = 40 mL, mCat = 15.9 g, dParticle = 3–3.5 mm (spherical),
−1
−1
◦
−1
◦
−1
◦
−1
◦
cCIT,0 = 1.2 mol L , VH2 = 27 L h ). I: T = 50 C, p = 20 bar, L = 2 mL min . II: T = 70 C, p = 20 bar, L = 2 mL min . III: T = 70 C, p = 20 bar, L = 1 mL min . IV: T = 70 C, p = 10 bar,
−
1
L = 1 mL min
.
accomplished in cocurrent downflow using HPLC pumps and mass
flow controllers, respectively. Data recording and system control
were managed online. The cylindrical reactor with an inner diam-
eter of 16 mm had a total volume of 67 mL, but usually only 40 mL
was used for catalyst loading. The top and bottom part of the
tube were filled with quartz wool. Typically, 1 L citral (Merck),
3. Results and discussion
3.1. Citral hydrogenation with pure 1Pd/SiO2
To elucidate the influence of the ionic liquid on the catalyst prop-
erties, it was necessary to investigate the neat catalyst as reference
first. So, the trickle-bed reactor was charged with 40 mL (15.9 g) of
the spherical catalyst with particle diameters of 3–3.5 mm. The cor-
responding citral conversion and product selectivities vs. time on
stream (TOS) are shown in Fig. 2. Marked fields indicate changes
in reaction conditions, whereas the parameters hydrogen flow
0.2 L n-tetradecane (Merck) and 3.8 L n-hexane (Roth, ROTIPU-
RAN > 99%) were premixed to obtain the starting liquid solution,
−1
hence, containing 1.2 mol L
citral. Quantitative analyses of the
reaction mixtures were performed off-line by gas chromatography
(
Hewlett-Packard 6890 with flame ionization detector operating
◦
−1
−1
at 250 C) with helium as mobile phase. The capillary column in
the system was an Agilent DB-Wax column with a length of 30 m,
an inner diameter of 0.25 mm and a film thickness of 0.25 ␮m.
The samples were analyzed with the following temperature pro-
gram: temperature was at first set to 85 C, and then increased with
5
(27 L h ) and citral concentration (1.2 mol L ) remained constant
in all cases. Linear approximations with prefixed slopes of zero
were conducted to obtain mean values (y-intercept) of each con-
version and the corresponding selectivities, which are summarized
in Table 2.
◦
◦
◦
C/min to 175 C.
Obviously, dihydrocitronellal was the main product followed by
citronellal. As already mentioned, dimethyloctanol, isocitral, both
isomers of 3,7-dimethyl-2-octenal as well as unknown compounds
(<5%) were combined as “others” for simplification purposes. As
expected, geraniol and nerol could not be found within the product
spectrum. This product distribution is quite typical for palladium
catalysts and is similar to previously conducted batch experiments
[15]. With rising conversion, the selectivity to dihydrocitronellal
increased while the selectivity to citronellal decreased, which is a
common feature of consecutive reactions. Elevated temperature as
Selectivities and citral conversion were calculated assuming
negligible changes in the liquid volume and concentrations were
determined using n-tetradecane as internal standard. It is to
mention that most compounds exhibit stereochemistry, but their
discrimination is beyond the scope of this article. Therefore, given
concentrations and selectivities are summations over all stereoiso-
mers. For further simplification, dimethyloctanol, isocitral, both
isomers of 3,7-dimethyl-2-octenal as well as unknown compounds
are combined as “others”.
Table 2
Influence of the ionic liquid [BMIM][N(CN)2] on the Pd/SiO2 catalyzed continuous hydrogenation of citral in a trickle-bed reactor. VCat = 40 mL, dCat = 3–3.5 mm (spherical),
−1
−1
cCIT,0 = 1.2 mol L , VH2 = 27 L h
.
◦
−1
)
Entry
Pd content (wt%)
IL content (wt%)
T ( C)
p (bar)
L (mL min
XCIT (%)
SCAL (%)
SDHC (%)
SOthers (%)
YCAL (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
0.7
0.7
0.7
0.7
0.8
0.8
0.8
–
–
–
50
70
70
70
70
20
20
20
10
20
40
40
40
20
20
20
2
2
1
1
1
1
1
0.75
2
77
88
96
89
52
52
67
83
45
57
70
36
26
15
51
61
76
61
–
–
2
4
–
13
13
9
14
<1
<1
1
2
1
4
9
28
23
14
22
52
52
65
78
45
54
62
–
25
32
32
32
32
20
20
20
>99
>99
97
94
99
70
100
100
70
70
70
1
1
0
1
1
0.5
95
89
1
2

3
22
N. Wörz et al. / Applied Catalysis A: General 391 (2011) 319–324
Fig. 3. Conversion and selectivities vs. time on stream of the citral hydrogenation applying the SCILL system 0.7Pd/SiO2@[BMIM][N(CN)2]32 (VCat = 40 mL, mCat = 23.4 g,
−
1
−1
◦
−1
◦
−1
◦
dParticle = 3–3.5 mm (spherical), cCIT,0 = 1.2 mol L , VH2 = 27 L h ). I: T = 70 C, p = 20 bar, L = 1 mL min . II: T = 70 C, p = 40 bar, L = 1 mL min . III: T = 100 C, p = 40 bar,
−1
◦
−1
L = 1 mL min . IV: T = 100 C, p = 40 bar, L = 0.75 mL min .
well as reduced liquid flow rates (L) resulted in higher conversions
the selectivity remained constant, whereas a slight increase of con-
version could be observed after resetting the initial conditions. In
view of analytical errors, the utilized catalyst was stable within the
total duration of the experiment (TOS = 78 h). However, at temper-
◦
−1
of citral. At 70 C and L = 1 mL min almost complete citral conver-
sion was achieved. Lowering the total pressure from 20 bar to 10 bar
caused a decrease in conversion of about 7%. This was associated
with diminished dihydrocitronellal selectivity and increased selec-
tivity to the less hydrogenated product citronellal. After 97 h TOS,
the initial conditions have been reset to examine catalyst deactiva-
tion. Approximately identical conversion and selectivities could be
observed in comparison to the initial state. Apparently, the utilized
catalyst exhibits long-term stability.
◦
atures higher than 100 C desorption of the ionic liquid could be
observed. This was indicated by the emergence of a second phase
at the reactor outlet. Below this threshold value desorption did
not occur and steady-state conditions regarding conversion and
selectivities remained unchanged. Virtanen et al. observed strong
deactivation in case of a palladium based SCILL system contain-
ing [NB4MPy][BF ] as IL [21]. Unfortunately, the authors neither
4
compared their results to the IL-free catalyst nor did they run the
experiments sufficiently long (TOS = 100 min) to disentangle the
startup phase from deactivation.
Experiments with lower IL content (20 wt%) showed the same
behavior in respect of the operating conditions (Table 2). However,
the citral conversion has slightly increased, whereas the selectiv-
ity to citronellal has decreased compared to the experiments with
3
.2. Influence of [BMIM][N(CN)2]
To obtain comparable results, it was necessary not to change
the amount of active metal and the hydrodynamic behavior of the
trickle-bed reactor. Hence, again 40 mL of the catalyst 1Pd/SiO was
used and coated with [BMIM][N(CN) ], which resulted in a higher
catalyst mass, but led to a matching absolute amount of palladium.
Assuming that the IL layer did not affect the bulk porosity, 40 mL of
the SCILL system should equal its neat counterpart and hydrody-
namics stayed the same. Fig. 3 refers to a SCILL system with 32 wt%
IL. Results of other modifications are summarized in Table 2.
As shown in Fig. 3, the product spectrum has completely
2
2
3
2 wt% IL. Fig. 4 demonstrates the influence of the IL content on
the catalyst performance under same conditions in other respects.
Obviously, presence of the ionic liquid was associated with higher
changed compared to the catalyst without [BMIM][N(CN) ]. In
2
all cases, almost no production of dihydrocitronellal could be
observed and citronellal was the main product with nearly quan-
titative selectivity. It is to mention that besides the consecutive
hydrogenation of citronellal to dihydrocitronellal, all other reaction
pathways seem to be inhibited as well. Only at conversions higher
than 80%, selectivities to dihydrocitronellal became significant. An
◦
◦
increase in temperature from 70 C to 100 C was associated with
5% gain of conversion. Further reduction of the liquid flow rate
(
1
−1
L = 0.75 mL min ) led to the highest measured citronellal yield of
8% at 83% citral conversion (SCAL = 94%). In contrast to the neat cat-
7
alyst, total pressure did not affect the performance at all. Because
the partial pressure of hydrogen definitely influences its solubility
within the IL [24], this characteristic may be attributed to the fact
that in presence of the IL layer, hydrogen does not appear in the
rate-determining step.
Fig. 4. Influence of [BMIM][N(CN)2] on the palladium catalyzed citral hydrogena-
tion. Variation of the IL content under same conditions in other respects: VCat = 40 mL,
To test for catalyst deactivation in the presence of the IL, the
same procedure has been chosen as already described. Apparently,
T = 70 ◦C, p = 20 bar, L = 1 mL min−1, d
= 3–3.5 mm (spherical), cCIT,0 = 1.2 mol L−1,
Particle
−
1
VH2 = 27 L h
.

N. Wörz et al. / Applied Catalysis A: General 391 (2011) 319–324
323
Table 3
Hydrogenation of citral in a trickle-bed reactor using 3.4Pd/Al2O3@[BMIM][N(CN)2]32 (3.4 wt% Pd and 32 wt% IL content) as catalyst. VCat = 35 mL, dCat = 2–6 mm (grit),
−1
−1
cCIT,0 = 1.2 mol L , VH2 = 27 L h in all cases.
◦
−1
)
Entry
T ( C)
p (bar)
L (mL min
XCIT (%)
SCAL (%)
SDHC (%)
SOthers (%)
YCAL (%)
1
2
3
4
70
70
50
50
20
20
20
10
2
3
3
4
99
92
80
72
36
45
61
63
55
43
23
20
9
12
16
17
36
41
49
45
citronellal selectivity but lower conversion in both cases. Never-
theless, the citronellal yield could be considerably enhanced due
to the IL from 14% (no IL) to 52% (32 wt% IL) and 54% (20 wt% IL),
respectively.
There are plenty of ways to heighten the conversion of citral,
but several of them exhibit some drawbacks. Elevated temper-
atures should be avoided because the IL could possibly desorb
or even decompose. Further reduction of the liquid flow rate
would decrease the output of citronellal and may lead to dry
zones in the trickle-bed reactor, which consequently comes along
with diminished catalyst efficiency. Therefore, another approach
to enhance conversion was attempted. A commercial powder of
H2 chemisorption experiments conducted in this work con-
firm the already stated trend [15,23] that presence of an ionic
liquid strongly reduces the hydrogen uptake and that this is even
more pronounced with higher IL contents. The determined hydro-
gen uptakes of the investigated catalysts are shown in Table 1.
Considering a coverage of the palladium nanoparticles with an
ionic liquid layer, the lower hydrogen uptakes in the case of
[BMIM][N(CN) ] coated Pd/SiO2 can be interpreted by the low
2
hydrogen solubility in ionic liquids. In particular, Dyson et al. have
applied 1H NMR to measure the Henry-coefficient of hydrogen in
ionic liquids. The authors figured out that the concentration of
dissolved hydrogen in ionic liquids is much lower than in typ-
ical organic solvents and in the same range as for water [25].
A small local concentration of hydrogen in vicinity to the active
sites may cause the reduced activity observed in case of SCILL
catalysts. It is a common feature of consecutive reactions that
the selectivity to intermediates will be increased with decreasing
activity. Consequently, this might be one reason for the selectiv-
ity enhancement. On the other hand this may not explain the
fact that SCILL systems exhibit much higher selectivities to cit-
ronellal even at similar conversions compared to their IL-free
equivalents.
It has been shown that an ionic liquid tremendously influ-
ences the adsorption of hydrogen. Potentially, the adsorption
of citral and citronellal is also affected. A rapid desorption
associated with an inhibited re-adsorption of the interme-
diate citronellal could explain the enhanced selectivity even
at high citral conversion. First experiments indicate that cit-
5
Pd/Al O was applied to synthesize a SCILL system containing
2 3
3
2 wt% [BMIM][N(CN) ] and 3.4 wt% palladium. The preparation
2
method was similar to the description in Section 2. In order to avoid
a severe pressure drop, it was necessary to press the powder of the
SCILL system into pellets (pressing procedure: 7–10 t for 2 min),
which were crushed into 2–6 mm grit subsequently. 35 mL of the
catalyst was packed into the reactor. Again quartz wool was placed
on top and bottom to fill the reactor and to distribute the flows
evenly over the catalyst.
As shown in Table 3, it was possible to enhance the catalyst activ-
◦
−1
,
ity and to achieve 99% conversion at T = 70 C and L = 2 mL min
with a selectivity towards citronellal of 36%. Increasing the liquid
flow rate and reducing the temperature, both resulted in higher
citronellal selectivities and lower dihydrocitronellal selectivities.
It is important to compare the selectivity of citronellal at simi-
lar conversions between the SCILL system and the neat catalyst
(
1Pd/SiO2 as reference). At 96% citral conversion, the latter exhib-
ited a citronellal selectivity of 15% (Table 2). Hence, in case of
.4Pd/Al O @[BMIM][N(CN) ] the citronellal selectivity could be
ral dissolves better within [BMIM][N(CN) ] than citronellal but
2
further studies still need to be performed [26]. Kernchen et
al. have already proposed this way of explanation in case of
the hydrogenation of cyclooctadiene applying SCILL catalysts
[17].
Besides the reduced hydrogen uptake, a modification with IL
results in some changes in respect of the catalyst itself [23]. In
particular, XPS analysis revealed that Pd is transformed into Pd2+
3
2
3
2 32
more than doubled at almost full conversion. Nevertheless, it is still
necessary to optimize this system regarding its IL content. The ratio
of palladium to IL seems to be a crucial variable in respect of the
citronellal yield. Apparently, higher IL amounts increase the selec-
tivity of the desired intermediate at the cost of activity, but it is to
mention that in all cases its yield could be enhanced.
due to its coating with [BMIM][N(CN) ]. Even a complexation of
2
Pd may be possible. This was also indicated by a blue shift in
the nitrile vibrations when comparing IR spectrometric results of
neat Pd/SiO2 with its corresponding SCILL system [15]. The results
have recently been published and should not be repeated here.
Nevertheless, they have not been interpreted in view of the hydro-
genation reaction and the selectivity enhancement. Although there
is no incontrovertible proof it is quite obvious that this drastic
electronic change may alter adsorption coefficients as well as rate
constants. Hence, this will modify selectivities within the reaction
network.
To summarize the effects investigated so far, the observed
benefit of dicyanamide based ionic liquids has two main causes.
First, there is a transport limitation reducing the local concen-
tration of hydrogen in vicinity to the active sites which leads
to a reduced activity of the catalytic system which inherently
comes along with an increased selectivity to citronellal. Second,
we were able to detect a strong electronic interaction between
palladium and the dicyanamide anion. This may affect adsorp-
tion coefficients as well as rate constants in a way that the
3
.3. Interpretation of the selectivity enhancement
Within this section we try to elucidate the observed beneficial
effect of [BMIM][N(CN) ]. Besides some new results also findings
of our earlier studies will be considered and discussed in respect to
this work.
2
First of all, the texture of a SCILL system should be illus-
trated. It has been assumed that the ionic liquid forms a thin layer
onto the porous support covering the active metal sites. This has
been deduced from N2 physisorption experiments which indicate
reduced pore volumes and BET-surfaces of SCILL catalysts com-
pared to their neat counterparts [15,16]. The thickness of the IL
layer is supposed to depend on the loading of IL [17] and conse-
quently there may be a threshold for a complete coverage. Whether
the latter is uniform or maldistributed still needs to be clarified.
To explain the effect of the modification ex situ characterizations
have been applied. The results may then be related to the observed
changes in the catalyst performance.

3
24
N. Wörz et al. / Applied Catalysis A: General 391 (2011) 319–324
citronellal consumption tends to almost zero maximizing its
yield.
investigated catalysts exhibited long-term stability, which is quite
important for any industrial application.
4
. Conclusion
References
[
[
1] P. Claus, Top. Catal. 5 (1998) 51–62.
2] P. Gallezot, D. Richard, Catal. Rev. Sci. Eng. 40 (1998) 81–126.
In this work, the so-called SCILL concept (solid catalyst with
ionic liquid layer) could successfully be applied in the contin-
uous hydrogenation of citral in a trickle-bed reactor. In good
accordance with previous findings on palladium based SCILL sys-
tems in discontinuous batch reactors [14,15], the advantages of
the SCILL concept could be demonstrated once again. This means
that despite lower citral conversions, highly improved citronel-
lal yields due to selectivities close upon 100% could be achieved
in continuous mode as well. To our knowledge, this is the first
time that SCILL systems have been investigated in continuous
experiments and compared to their neat counterparts. Besides,
this is even the only study dealing with the hydrogenation of
citral in a trickle-bed reactor applying neat palladium based cat-
alysts at all. The selectivity enhancement may not be ascribed to
the reduced activity of the catalyst exclusively because the SCILL
system exhibited higher citronellal yields even at comparable con-
versions.
[3] P. Mäki-Arvela, J. Hájek, T. Salmi, D. Yu. Murzin, Appl. Catal. A 292 (2005) 1–49.
[
4] P. Claus, Y. Önal, in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.), Hand-
book of Heterogeneous Catalysis, 2nd ed., Wiley–VCH, Weinheim, 2008, pp.
3308–3329.
[5] M.A. Aramendía, V. Borau, C. Jiménez, J.M. Marinas, A. Porras, F.J. Urbano, J.
Catal. 172 (1997) 46–54.
[
6] R. Liu, Y. Yu, K. Yoshida, G. Li, H. Jiang, M. Zhang, F. Zhao, S. Fujita, M. Arai, J.
Catal. 269 (2010) 191–200.
[7] V. Satagopan, S.B. Chandalia, J. Chem. Technol. Biotechnol. 59 (1994) 257–263.
[8] J. Hao, C. Xi, H. Cheng, R. Liu, S. Cai, M. Arai, F. Zhao, Ind. Eng. Chem. Res. 47
(
2008) 6796–6800.
9] V.I. Parvulescu, C. Hardacre, Chem. Rev. 107 (2007) 2615–2665.
10] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal. A 373 (2010) 1–56.
[11] C.P. Mehnert, E.J. Mozeleski, R.A. Cook, Chem. Commun. (2002) 3010–3011.
[
[
[
[
12] M. Haumann, R. Fehrmann, Chem. Rev. 108 (2008) 1474–1497.
13] K. Anderson, P. Goodrich, C. Hardacre, D.W. Rooney, Green Chem. 5 (2003)
448–453.
[14] J. Arras, M. Steffan, Y. Shayeghi, P. Claus, Chem. Commun. (2008) 4058–4060.
[
[
15] J. Arras, M. Steffan, Y. Shayeghi, D. Ruppert, P. Claus, Green Chem. 11 (2009)
16–723.
16] J. Arras, D. Ruppert, P. Claus, Appl. Catal. A 371 (2009) 73–77.
7
In case of 1Pd/SiO , the performance of the catalyst could be
[17] U. Kernchen, B. Etzold, W. Korth, A. Jess, Chem. Eng. Technol. 30 (2007) 985–994.
2
[18] M.P. Dudukovic, F. Larachi, P.L. Mills, Catal. Rev. 44 (2002) 123–246.
[19] F.S. Mederos, J. Ancheyta, J. Chen, Appl. Catal. A 355 (2009) 1–19.
[20] S. Recchia, C. Dossi, N. Poli, A. Fusi, L. Sordelli, R. Psaro, J. Catal. 184 (1999) 1–4.
significantly enhanced using [BMIM][N(CN) ] as ionic liquid layer.
2
The citronellal selectivity was found to be close to 100% in almost
all conducted experiments (Table 2), which may be ascribed to the
inhibited consecutive reaction of citronellal to dihydrocitronellal.
The yield of citronellal was approximately four times higher com-
pared to the neat palladium catalyst under same conditions in other
respects. However, this was associated with a reduced conversion
of citral (Fig. 4). To circumvent this feature, a SCILL system with
higher palladium content has been investigated. It was possible
to achieve full conversion, but at lower citronellal selectivities. All
[21] P. Virtanen, J.P. Mikkola, E. Toukoniitty, H. Karhu, K. Kordas, K. Eränen, J. Wärna,
T. Salmi, Catal. Today 147S (2009) S144–S148.
[
22] F.J. Bröcker, G. Kaibel, W. Aquila, H. Fuchs, G. Wegner, M. Stroezel, EP 0 947 493
A1 (1999).
[23] J. Arras, E. Paki, C. Roth, J. Radnik, M. Lucas, P. Claus, J. Phys. Chem. C, in press.
[24] J.P. Mikkola, J. Wärna, P. Virtanen, T. Salmi, Ind. Eng. Chem. Res. 46 (2007)
3
932–3940.
25] P.J. Dyson, G. Laurenczy, C.A. Ohlin, J. Vallance, T. Welton, Chem. Commun.
2003) 2418–2419.
[26] D. Ruppert, Diploma Thesis, TU Darmstadt, 2009.
[
(