after 4 h reaction time was increased for all IL coated
was determined under the assumption that the GC standard
n-tetradecane is insoluble in ionic liquids. As a general trend,
the solubility of citronellal in dicyanamide based ionic liquids
catalysts compared to the IL-free reaction (S = 45%), whereby
[
BMPL][NTf
2
] showed the best performance (S = 62%).
6
Minimum amounts of fluoride present in [BMIM][PF
] may
was lower than for citral. In comparison to [BMIM][NTf ], di-
2
also decrease the conversion level. It is known that citral
hydrogenation is sensitive towards halides as contaminants,
which was shown for chloride contamination by Hardacre
cyanamide based ionic liquids gave lower solubilities indicating
better desorption of citronellal from the ionic liquid layer into
the organic phase.
To understand in detail the interaction of DCA based ionic
liquids with active metal palladium and the reactants citral and
hydrogen, respectively, further examinations are in progress.
25
et al. The higher average pore diameter of [BMIM][PF
Pd/SiO determined by N physisorption (167 nm instead of
14 nm for Pd/SiO ) could be interpreted by partially etching
of silica by the fluoride possibly forming SiF
6
] coated
2
2
1
2
4
.
In contrast to the dicyanamide based ionic liquids, the
selectivity towards citronellal was enhanced to a lesser extent.
4
. Conclusion
Dicyanamide containing ionic liquids, independent of the
cation, act very well as selectivity modifiers for the hetero-
geneously catalysed citral hydrogenation on Pd supported
catalysts. Regardless of the nature and origin of the prepared Pd
catalysts, excellent selectivities towards citronellal were observed
by using these ILs as catalyst additive or coating, whereby con-
secutive and side reactions are strongly inhibited. This concept
offers a greener opportunity to a solvent-less hydrogenation of
citral, as experiments on neat citral at high citronellal selectivity
in considerable space-time-yield have shown.
3.3.2. Dicyanamide containing ionic liquids. By applying
dicyanamide based ionic liquids as catalyst coating, citral con-
version and citronellal selectivity could be improved indepen-
+
+
+
dent of which cation (either [BMIM] , [BMPL] or [B3MPYR] )
was used (Table 6, entries 5–9). Moreover, nearly quantitative
citronellal yield was obtained if the catalyst was loaded with
2
9wt% [B3MPYR][DCA]. In comparison to previous findings,
the citronellal selectivity is moderately lowered due to a de-
creased amount of ionic liquid. With decreased IL loading, the
citronellal selectivity at 80% conversion was diminished either
with [BMIM][DCA] (S = 81%) or [B3MPYR][DCA] (S = 82%)
as catalyst coating (entries 5 and 6); dihydrocitronellal was then
observed as the consecutive product (S ~ 10%).
In comparison with [BMIM][DCA] and [B3MPYR][DCA]
at equal IL loading, experiments with 10wt% [BMPL][DCA]
gave the highest selectivity towards citronellal, supporting the
Acknowledgements
The authors want to thank Dipl.-Ing. K. Lehnert, Dipl.-Ing.
F. Klasovsky and Dipl.-Ing. A. Pachulski for catalyst character-
ization, and Dipl.-Ing. N. W o¨ rz for catalyst preparation. P. C.
thanks the Fonds der Chemischen Industrie for financial support.
findings with [NTf ]-based ILs, whereas the [BMPL]-cation
showed increased selectivities compared to the [BMIM]-cation.
Quantitative citronellal yield was obtained with a 24wt%
2
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3
329.
30
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40
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22 | Green Chem., 2009, 11, 716–723
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