Highly cis-selective and lead-free hydrogenation of 2-hexyne by a supported Pd catalyst with an ionic-liquid layer

Article abstract of DOI:10.1039/c4cc04183f

A simple Pd/SiO₂ catalyst which was modified with the ionic liquid [BMPL][DCA] gave an excellent yield of 88% towards cis-2-hexene in the stereoselective hydrogenation of 2-hexyne. The catalyst outperforms, even at full conversion, the commonly used lead-poisoned, toxic Lindlar catalyst and supported colloidal-based Pd as well. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04183f

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Highly cis-selective and lead-free hydrogenation
of 2-hexyne by a supported Pd catalyst with an
ionic-liquid layer†
Cite this: Chem. Commun., 2014,
0, 10406
5
Received 1st June 2014,
Accepted 22nd July 2014
Frederick Schwab, Natascha Weidler, Martin Lucas and Peter Claus*
DOI: 10.1039/c4cc04183f
www.rsc.org/chemcomm
A simple Pd/SiO
BMPL][DCA] gave an excellent yield of 88% towards cis-2-hexene in was the main advantage. The influence of the preparation para-
the stereoselective hydrogenation of 2-hexyne. The catalyst outper- meters of immobilized PdO H colloids on catalytic activity was
2
catalyst which was modified with the ionic liquid of hexynes. An increased activity compared to the Lindlar system
3
[
x
y
4
forms, even at full conversion, the commonly used lead-poisoned, investigated by Klasovsky et al. The optimized catalyst showed
toxic Lindlar catalyst and supported colloidal-based Pd as well.
both a superior activity and selectivity to cis-2-hexene.
The usage of Pd nanoparticles in ionic liquids (IL‡) as
The stereoselective hydrogenation of the triple bond of internal catalysts showed a great impact on the hydrogenation of
alkynes is of great industrial interest for the production of fine hexynes. With these new catalysts the same performance as
chemicals or pharmaceuticals. Beside the challenge to control the the Lindlar system can be obtained, in some cases with even
stereoselectivity, also over-hydrogenation of the desired cis-alkene higher selectivities. However, nanoparticles stabilized with
and its isomerization must be avoided. Therefore, in the hydro- ionic liquids often require biphasic operation mode for alkyne
5
genation of 2-hexyne a suitable catalyst has to (i) produce cis-2- semi-hydrogenation, additional solvents and modifiers.
hexene with high yield, (ii) prevent the consecutive hydrogenation
Another promising modification of heterogeneous catalysts
to hexane and the isomerization to trans-2-hexene (Scheme 1). The with ionic liquids is the so-called SCILL‡ (Solid Catalyst with an
favored intermediate cis-2-hexene could also isomerize to 1- or Ionic Liquid Layer) concept successfully applied for hydrogena-
6,7
3-hexene where also cis- or trans-isomers are possible.
tions. The concept takes into account the variability of their
The state-of-the-art catalyst system for the stereoselective physico-chemical properties, and herein, the impact of the IL can
hydrogenation of alkynes to cis-alkenes is the Lindlar catalyst be considered two-fold: (i) the surface concentration of the educt/
containing a quinoline treated, CaCO supported Pd catalyst intermediate/product of a consecutive reaction is dependent on
3
1
which is poisoned by toxic Pb.
their solubility in the IL which was described as a ‘‘physical
Several approaches have been made to avoid the toxic lead solvent effect’’, (ii) the catalytic properties of supported metal
2
compound in accordance to the 12 principles of Green Chemistry.
particles can be influenced directly by the IL acting as ligand in a
The modification of catalysts with other second metals like Ni or Bi similar way how a second metal may modify the active sites of a
8
reached a similar performance of the catalyst in the hydrogenation parent metal in bimetallic catalysts (‘‘ligand effect’’).
One example is the SCILL catalyzed citral hydrogenation to
7
citronellal investigated by Arras et al. With the SCILL system
5
0 wt% [BMIM][DCA]-Pd/C a quantitative yield of citronellal was
achieved while the neat Pd/C catalyst only reached a selectivity of
1% at full conversion. The over-hydrogenation of citronellal to
4
dihydrocitronellal was completely suppressed by the SCILL system.
The influence of the IL was described as a new ligand effect due to the
8
DCA (dicyanamide) anion which modified the active sites directly.
Scheme 1 Reaction network for the selective hydrogenation of 2-hexyne.
9
In a recent study, Herrmann et al. used the SCILL concept in
the gas-phase selective hydrogenation of acetylene to ethylene
and could enhance the selectivity towards ethylene.
In this work, the SCILL concept was applied in the stereoselective
hydrogenation of 2-hexyne to cis-2-hexene for the first time. The
influence of three ionic liquids as well as the effect of the amount of
Technische Universit a¨ t Darmstadt, Ernst-Berl-Institut, Technische Chemie II,
Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany. E-mail:
claus@ct.chemie.tu-darmstadt.de; Fax: +49 6151 16 4788; Tel: +49 6151 16 5369
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc04183f
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Fig. 1 Effect of different ILs in the SCILL catalyzed hydrogenation of
Fig. 2 Effect of the amount of [BMPL][DCA] (conditions: T = 25 1C, p(H
2
) =
2
-hexyne (conditions: T = 25 1C, p(H
mL 2-hexyne, 5 mL n-octane, 80 mL n-heptane).
2
) = 1.04 bar, m = 350 mg Pd/SiO
2
-1,
2
1.04 bar, m = 350 mg Pd/SiO -2, 5 mL 2-hexyne, 5 mL n-octane, 80 mL
n-heptane).
5
the IL were investigated. The best SCILL catalyst was compared to
the commercial Lindlar catalyst and the NanoSelectt system from desired intermediate is consumed by over-hydrogenation and iso-
10
BASF. Note, that the selectivity produced by different catalysts must merization. Consequently, the corresponding S–X plot, typical for
be compared at equal conversion so that selectivity-conversion-plots consecutive reactions, was obtained in this case (Fig. 2). Further
are shown.
The hydrogenation of 2-hexyne catalyzed by the neat 1Pd/SiO
increase of the IL loading resulting in the SCILL system 30[BMPL]-
[DCA]-1Pd/SiO increases tremendously the catalyst performance
2
2
followed a typical consecutive reaction because the intermediate exhibiting an excellent yield of 88% at full 2-hexyne conversion.
cis-2-hexene was isomerized and further hydrogenated to the Surprisingly, even when the reaction approaches full conversion the
by-products shown in Scheme 1. These side reactions led to a selectivity towards cis-2-hexene is further increased (Fig. 2), and the
maximum yield of cis-2-hexene of only 61% and a complete corresponding concentration-time curves showed that the formation
consumption at full conversion (Fig. 1). Then, this catalyst was of all side-products of isomerization and over-hydrogenation is
modified with three different ionic liquids which are N-butyl-N- negligible (Fig. S1, ESI†). Also the catalyst with the highest amount
methyl-pyrrolidinium dicyanamide ([BMPL][DCA]), 1-butyl-3- of [BMPL][DCA] (50 wt%) is suitable for the stereoselective semi-
methyl-imidazolium hexafluorophosphate ([BMIM][PF
-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide not quantitative (X2-hexyne = 70%). These results show that an opti-
[BMIM][NTf ]). Their formulas are shown in Scheme S1 (ESI†). mum of IL loading exists where the SCILL system is beneficial with
6
]) and hydrogenation (Scis-2-hexene = 80%), however, the conversion level was
1
(
2
The impregnation of the catalyst with one of these ionic liquids respect to high selectivity and conversion: if the IL amount is too
influenced the activity of the catalyst as well as the selectivity to small, the formed cis-2-hexene is consumed at very high conversion
cis-2-hexene. [BMIM][NTf ] hardly influenced the catalyst resulting level, i.e. when the catalyst surface exhibits low alkyne concentration.
2
in similar hydrogenation behavior and a maximum intermediate In the opposite, if the IL loading is too high, tough the high selectivity
yield of 66% because of the solubility of this IL in the organic is retained, the reaction does not progress at a certain degree of
phase leading to a rinse off of the ionic liquid during the reaction. conversion (i.e., X2-hexyne { 100%). This dependence is consistent
6
The [BMIM][PF ]-SCILL catalyzed hydrogenation of 2-hexyne with our observation of an optimum Pd/N ratio (nitrogen from
reached an almost constant cis-2-hexene selectivity of 70% and PVP (poly(vinylpyrrolidone))) in supported (Hydr-)oxidic PdOxHy
4
achieved a yield of 70% at full conversion. The largest improve- colloids used for 2-hexyne hydrogenation. Based on these results
ment of the selectivity to cis-2-hexene was achieved by modification and taking into account our in-depth characterization of such
8
of the Pd catalyst with 15 wt% [BMPL][DCA]. The selectivity to cis-2- SCILL systems we conclude that over-hydrogenation is sup-
hexene reached more than 80% over the whole conversion range pressed by a ligand side-blocking effect due to the N-containing
À
and resulted in an excellent yield of 82% at 100% conversion. anion, (CN)
2
N , of the ionic liquid. Moreover, in such a SCILL
Furthermore, this SCILL system largely prevented the isomeriza- system also an electronic influence of the DCA anion on palla-
8
tion and over-hydrogenation of the formed cis-2-hexene because dium was observed leading to a partial oxidation of the latter
no intermediate reacted anymore, even at full conversion. In and, thus, influencing the adsorption strength of 2-hexyne/cis-2-
consequence of this positive impact of the ionic liquid on the hexene and hydrogen as well. Indeed, DCA based SCILL decreases
semi-hydrogenation of 2-hexyne, the IL [BMPL][DCA] was used for (i) the amount of chemisorbed hydrogen suppressing the further
8
further systematic optimization of this catalyst–reaction system.
hydrogenation and (ii) the adsorption enthalpy of hydrogen
The influence of the IL amount was investigated in the range of which indicated, compared to the non-modified Pd catalyst, a
0
wt% o w(IL) o 50 wt% (Fig. 2). In comparison to the experiment weaker hydrogen bonding making the SCILL more selective.
without the IL [BMPL][DCA] the selectivity as well as the yield of cis-2-
The superior performance of this innovative SCILL system could
hexene was generally increased. An ionic liquid loading up to 10 wt% be shown by the comparison to Pd nanoparticles in methanol
5
could not suppress the further reaction at full conversion and the described by Lee et al. To reach a yield of cis-2-hexene of 82%,
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conversion, is ascribed to alkene re-adsorption followed by hydro-
genation. This is prevented with the SCILL system because the
alkene is less soluble in the IL than the alkyne enabling faster
alkene desorption and preventing its re-adsorption. It follows that,
beside the finding that the IL directly modifies the Pd sites by
ligand-effects, also reactant solubility in the IL film is involved in
controlling the selectivity. Taking further into account the poor
solubility and slow diffusion of hydrogen in the IL we propose that
also the formation of Pd bulk-dissolved (subsurface) hydrogen can
be avoided by the IL layer. The presence of such kind of hydrogen
11
would shift the selectivity towards the alkane, i.e. then over-
hydrogenation occurs.
In summary, for the first time the SCILL concept is successfully
Fig. 3 Comparison of the SCILL with industrial catalysts used as reference
(
conditions: T = 25 1C, p(H
0 mL n-heptane).
2
) = 1.04 bar, 5 mL 2-hexyne, 5 mL n-octane, applied in the stereoselective hydrogenation of 2-hexyne to cis-2-
hexene which is produced with an outstanding yield of 88%. The
8
catalytic properties of this innovative and environmentally benign
SCILL system outperform the industrially used Lindlar and Nano-
the 8-fold higher amount of palladium was required and even the Selectt catalyst even when the reaction approaches full conversion.
used amount of 2-hexyne was much smaller. Taking into account In the next step, the SCILL system will be transformed in a shaped
the different amounts of active metal and educt used in these product for continuous hydrogenation in order to study catalyst
experiments a modified space-time-yield (STY) was estimated (Table stability with time-on-stream.
S1, ESI†). The catalyst 30[BMPL][DCA]-1Pd/SiO
2
reached an excellent
À1
À1
STY of 3.8 gprod.
gPd min which was more, at least, than one
Notes and references
order of magnitude higher than the Pd nanoparticles of Lee et al.
‡
Abbreviations: IL = ionic liquid, SCILL = solid catalyst with an ionic liquid
The catalyst was also compared to a commercial Lindlar and
6
layer, [BMIM][PF ] = 1-butyl-3-methyl-imidazolium hexafluorophosphate,
NanoSelectt catalyst from BASF. The Lindlar catalyst is the state- [BMIM][NTf ] = 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)-
2
amide, [BMPL][DCA] = N-butyl-N-methyl-pyrrolidinium dicyanamide, 30[IL]-
of-the-art catalyst for stereoselective hydrogenation of internal
alkynes as described above. The NanoSelectt catalyst is a Lead-
1
3
Pd/SiO
0 wt% of the ionic liquid.
free nano-colloid which consists of supported Pd nanoparticles. Catalyst preparation: the SCILL catalysts were prepared via incipient-wetness
impregnation to give a 1 wt% Pd/SiO . After drying the support material (2 h,
200 1C, charge 1, or 130 1C, charge 2), a certain amount of Pd(AcO) was
dissolved in acetone and impregnated on the silica. The charges of the parent
2
= the catalyst 1 wt% Pd supported by silica was impregnated with
The Pd content is 10-fold lower than that of the Lindlar system
2
1
0
2
producing cis-products with high selectivity.
The commercial Lindlar and NanoSelectt catalysts showed a catalyst, denoted by Pd/SiO -1 and Pd/SiO -2, were dried (25 1C, 15 h) and
2
2
À1
reduced (100 1C, 100 mL min
2
H , 1 h). Then, the catalysts were modified by
similar selectivity to cis-2-hexene almost over the whole conversion
range as the SCILL 30[BMPL][DCA]-1Pd/SiO (Fig. 3). However, it
different ILs via incipient-wetness impregnation as well (solvent: acetone). The
resulting SCILL catalysts were dried (25 1C, 15 h). Catalyst testing: the
2
must be noted that there is a main difference between these hydrogenation experiments were performed in a batch reactor (300 mL, Parr
À1
Instruments) at 1.04 bar H
2
, 25 1C and a stirring rate of 1000 min . An educt
commercial catalysts and our SCILL system, and this most striking
effect is the performance near and at 100% conversion. The
modification of the Pd catalyst with the ionic liquid completely
suppressed the further reaction of the intermediate, whereas the
commercial systems catalyzed the hydrogenation and isomeriza-
tion of cis-2-hexene which led to a decrease of the yield of the
desired product for the Lindlar and NanoSelectt catalyst of 74%
and 71%, respectively (see also yield vs. time plot, Fig. S2, ESI†). In
other words, at these conditions the SCILL system outperformed
the commercially available catalysts for the stereoselective hydro-
genation of internal alkynes reaching a yield of cis-2-hexene of 88%
at full conversion in the hydrogenation of 2-hexyne.
mixture of 5 mL 2-hexyne, 5 mL n-octane (internal GC standard) and 80 mL
n-heptane as well as 350 mg of the catalyst were used in each experiment.
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1
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10408 | Chem. Commun., 2014, 50, 10406--10408
This journal is ©The Royal Society of Chemistry 2014