Steric and Electronic Effects of Phosphane Additives on the Catalytic Performance of Colloidal Palladium Nanoparticles in the Semi-Hydrogenation of Alkynes

Article abstract of DOI:10.1002/cctc.202001121

We report on the influence of phosphanes on the catalytic activity and selectivity of colloidal, tetraoctylammonium bromide (TOAB) stabilised palladium nanoparticles (NPs) in the semi-hydrogenation of alkynes to olefins. Full characterisation of the catalytic system (HRTEM, EDX, XPS, IR, NMR) confirmed the formation of spherical particles with a narrow size distribution (1.9±0.5 nm). The catalytic performance of the Pd NPs in the semi-hydrogenation of 1-octyne, 2-octyne and phenylacetylene to the respective olefins and the influences on the selectivity was investigated. The system shows high activities and selectivities at mild conditions (0 °C and 1.0 bar H₂ pressure). It was shown that generally, phosphanes lead to an increase of both the reaction rate and selectivity towards the olefin where both steric and electronic effects of the ligand play a crucial role for the catalyst performance. A moderate steric demand of the ligand with a rather weak σ-donating ability turned out to give the highest catalytic performance.

Full text of DOI:10.1002/cctc.202001121

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Steric and Electronic Effects of Phosphane Additives on the
Catalytic Performance of Colloidal Palladium Nanoparticles in the
Semi-Hydrogenation of Alkynes
Authors: Lena Staiger, Tim Kratky, Sebastian Günther, Ondrej
Tomanek, Radek Zbořil, Richard W. Fischer, Roland A.
Fischer, and Mirza Cokoja
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10.1002/cctc.202001121
ChemCatChem
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Steric and Electronic Effects of Phosphane Additives on the
Catalytic Performance of Colloidal Palladium Nanoparticles in the
Semi-Hydrogenation of Alkynes
Lena Staiger,[a] Tim Kratky,[b] Sebastian Günther,[b] Ondrej Tomanek,[c] Radek Zbořil,[c] Richard W.
Fischer,[d] Roland A. Fischer,[a] and Mirza Cokoja*[a]
[a]
L. Staiger, Prof. R. A. Fischer, Dr. M. Cokoja
Chair of Inorganic and Metal-Organic Chemistry
Department of Chemistry and Catalysis Research Center
Technical University of Munich
Ernst-Otto-Fischer-Straße 1
D-85747 Garching bei München, Germany
E-mail: mirza.cokoja@tum.de
[b]
T. Kratky, Prof. S. Günther
Chair of Physical Chemistry with Focus on Catalysis
Department of Chemistry and Catalysis Research Center
Technical University of Munich
Ernst-Otto-Fischer-Straße 1
D-85747 Garching bei München, Germany
O. Tomanek, Prof. R. Zbořil
Regional Center of Advanced Technologies and Materials RCPTM
Šlechtitelů 27
78371 Olomouc, Czech Republic.
Prof. R. W. Fischer
[c]
[d]
Clariant Produkte (Deutschland) GmbH
Waldheimer Straße 15
D-83052 Bruckmühl, Germany.
Supporting information for this article is given via a link at the end of the document.
Abstract: We report on the influence of phosphanes on the catalytic
activity and selectivity of colloidal, tetraoctylammonium bromide
(TOAB) stabilized palladium nanoparticles (NPs) in the semi-
hydrogenation of alkynes to olefins. Full characterization of the
catalytic system (HRTEM, EDX, XPS, IR, NMR) confirmed the
formation of spherical particles with a narrow size distribution
(1.9 ± 0.5 nm). The catalytic performance of the Pd NPs in the semi-
hydrogenation of 1-octyne, 2-octyne and phenyl acetylene to the
respective olefins and the influences on the selectivity was
investigated. The system shows high activities and selectivities at mild
conditions (0 °C and 1.0 bar H2 pressure). It was shown that generally,
phosphanes lead to an increase of both the reaction rate and
selectivity towards the olefin where both sterics and electronics of the
ligand play a crucial role for the catalyst performance. A moderate
steric demand of the ligand with a rather weak σ-donoring ability
turned out to give the highest catalytic performance.
nickel[5] with a second metal. The alloying results in a reduction of
the adsorption energy of the olefin, leading to its desorption from
the metal surface and hence to an increased reaction
selectivity.[6,7] In addition, the isolation of Pd atoms leads to an
increased catalyst stability as undesired oligomerization (so-
called ‘green oil’ formation) and carbon deposition side reactions
causing catalyst deactivation are suppressed.[8,9] The selective
hydrogenation of alkynes to olefins with monometallic NPs would
be an attractive alternative; however, it is, on the other hand, very
difficult. For this, modifications on the catalyst surface have to be
undertaken, usually by using surfactants.[10] An elegant approach
to study the effect of additives on the selectivity is to use colloidal,
monometallic NPs.[11,12] Studying the catalytic behavior of such
model particles allows for a fundamental understanding of
structure-performance relationships. The colloid surfactant
molecules, particularly σ-donor type stabilizing agents, exert
spatial control of the accessibility and the electronic properties of
the surface atoms of the colloids, and therefore influence the
catalytic activity/selectivity as well.[10,13] The interaction of the
additive with the particle surface can lead to a preferred alkyne
adsorption instead of the competitive olefin coordination to the
surface and therefore increase the selectivity towards the semi-
hydrogenation product.[14] A systematic study of the effect of
additives on the chemoselectivity of colloidal metal NPs has,
however, so far not been carried out for this reaction type. Dupont
et al. reported on the formation of stable and highly
monodispersed Pd NPs stabilized in a nitrile-functionalized ionic
liquid (IL), which are highly active in the semi-hydrogenation of
alkynes in a multiphase system.[15] Phosphane-functionalized IL
surfactants showed an enhanced catalyst performance.[16]
Another study shows the effect of the nature of thiolate capping
Introduction
The selective semi-hydrogenation of alkynes to olefins is of high
relevance for the production of polyolefins and fine chemicals.[1]
The undesired full hydrogenation to alkanes is thermodynamically
favored, as the hydrogenation of the olefin is energetically
preferred over desorption from the catalyst surface,[2,3] and
therefore, the isolation of the olefin is difficult. The best known
concept to convert alkynes to olefins – particularly acetylene to
ethylene, which is of high industrial interest[1]
– without
(significant) over-hydrogenation is the doping of a catalytically
active metal, e.g. palladium,[4] as the most prominent example,
1
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agents and influence of the chemical and structural composition
of the surface monolayers on the catalytic activity and selectivity
of colloidal Pd[17] and Pt[18] NPs. Beller et al. reported on a change
of stereoselectivity in the semi-hydrogenation of alkynes to the
respective E- and Z alkenes upon addition of multidentate
phosphane ligands using a monometallic Ni catalyst.[19] Yang, Li
et al. have studied the electronic influence of triphenylphosphane
on the catalytic performance of Pd NPs in selective
hydrogenations.[20] A deeper investigation on a series of similar
additives bearing subtle differences regarding spatial and
electronic features could give rise to new insights of the
underlying mechanisms responsible for catalyst activity and
selectivity.
Figure 1. HRTEM images showing a small size distribution (above and bottom
Therefore, in this study, we report on the influence of the addition
of triorganylphosphanes PR3 bearing different substituents R
(alkyl, aryl) on the catalytic activity and selectivity of colloidal Pd
NPs for the semi-hydrogenation of alkynes. We investigated the
steric and electronic effects of the phosphanes in order to learn
more on the crucial parameters for controlling the selectivity of the
reaction. Furthermore, we have studied the influence of different
phosphanes on the stability of the colloids in solution and the
interactions of the ligands with the particle surface. For the
synthesis of the Pd colloids, we chose a simple literature-known
route via reduction of K2[PdCl4] with NaBH4 in presence of the
surfactant tetraoctylammonium bromide (TOAB),[21] which can
easily be substituted by stronger (σ-donor) stabilizing agents,
such as phosphanes.
left) and the lattice planes of the Pd@TOAB NPs (bottom right).
No agglomerates were detected (SI, Figure S3). EDX spectra
confirm the sample composition of Pd, N, C and Br (from TOAB).
Chlorine residues from the Pd precursor were not detected.
Elemental mapping shows that the capping agent is distributed
homogeneously over the entire sample (SI, Figure S4). The
crystallite size of the NPs is also confirmed by PXRD
measurements, as the Pd reflections are very broadened (SI,
Figure S7). The Fourier-transform-attenuated-total-reflection-
infrared spectrum (FT-ATR-IR; SI, Figure S2) of a sample of
Pd@TOAB NPs exhibits bands which are in accordance with
those of neat TOAB, namely characteristic ν (
bands in a range from 2953 to 2850 cm-1 as well as a very weak
(C-H,sp2) absorption band at 3004 cm-1. Furthermore,
symmetric and antisymmetric ν (C-C/C-Me) vibrations are
̃
C-H,sp3) absorption
ν
̃
̃
Results and Discussion
observed at 1466 and 1353 cm-1, and at 753 and 720 cm-1,
respectively, which is in accordance to a previously reported IR
spectrum of neat TOAB.[21] 1H-NMR spectroscopy confirms an
intact structure of the capping agent without any impurities, as the
recorded spectrum matches with the recorded reference spectra
of neat TOAB (SI, Figure S5). X-ray photoelectron spectroscopy
(XPS) shows the main signal corresponding to metallic Pd(0) (SI,
Figure S8).
Synthesis and characterization of TOAB-stabilized Pd NPs
Highly monodispersed Pd NPs were prepared in toluene on the
basis of a modified literature-known procedure.[21] Reduction of
K2[PdCl4] with an excess of aqueous NaBH4 in toluene in the
presence of an excess of TOAB (10 equiv.) as capping agent
yielded colloidal Pd NPs as a black solution (Scheme 1). The NPs
were analyzed by high-resolution transmission electron
microscopy (HRTEM; Figure 1) and dynamic light scattering
(DLS; see the SI, Figures S1 and S3) confirming NPs with a
narrow size distribution (1.5 ± 0.9 nm). Noteworthy, variation of
the literature-reported work-up procedure led to an undesired
surface modification resulting in a color change of the colloidal
solution from dark brown to amber and no catalytic activity.
Therefore, the synthesis procedure was optimized in terms of
avoiding acidic or basic treatment leading to a reproducible and
simple synthesis protocol. The particles were stable for weeks at
low temperatures (ca. 4 °C). Decreasing the amount of the
capping agent TOAB to 5.0 equiv. with respect to palladium
results in less stable particles, which agglomerate and precipitate
in a shorter amount of time (i.e. after one week). Figure 1 shows
representative HRTEM images of the spherical, well-defined
Pd@TOAB NPs in toluene with lattice planes separated by 0.8 Å.
Influence of temperature and H2 pressure on the catalytic
activity and selectivity of Pd@TOAB NPs
For a systematic investigation of the catalytic activity of
Pd@TOAB NPs in the semi-hydrogenation reaction of alkynes,
1-octyne was used as model substrate (Scheme 2). Error
calculations assuming an instrumental GC inaccuracy of 1 %
showed that all calculated error bars on both conversion and
selectivity are too small to be considered (detailed calculation in
the SI, Table S1) and have no impact on the following
interpretation of the catalytic results. We have investigated the
effect of temperature and H2 pressure on substrate conversion
and product selectivity (Table 1 and SI, Figure S10).
Scheme 2. Semi-hydrogenation catalysis using 1 mol% catalyst Pd@TOAB in
a H2 atmosphere.
Scheme 1. Synthesis of colloidal TOAB-stabilized Pd NPs.
2
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Table 1. Parameter optimization in the semi-hydrogenation of 1-octyne using
1 mol% Pd@TOAB NPs after 3 h reaction time.
Entry
T
p(H2) Conv.
Yield
1-octene n-octane
[%][b]
Yield
Oligom.
[%][c]
TOF
[h-1][d]
[°C] [bar]
[%][a]
[%]
1
2
3
4
25
0
0.5
0.5
1.0
2.0
100
45
15
84
80
11
54
11
8
31
5
460
210
325
340
0
80
12
24
0
100
65
Figure 2. DLS (left) and HRTEM measurements (right) of Pd@TOAB exposed
to PPh3 under inert conditions with and without 1-octyne (DLS) and without
1-octyne (HRTEM).
[a] Conversion of 1-octyne. [b] Selectivity towards 1-octene.
[c] Oligomerization to higher hydrocarbons. [d] for details see SI, chapter 2.11.
Influence of phosphane additives on the catalytic
performance of Pd@TOAB NPs
Table 1 shows that a variation of either reaction temperature or
hydrogen pressure significantly influences both activity and
selectivity. Noteworthy, oligomerization of 1-octene to dimers and
higher products has been detected in all experiments of this study
by LIFDI-MS measurements (SI, Figures S16-S17 and Table S3).
The oligomerization to dimers and trimers catalyzed by Group 10
metals is well-known since the early works of Wilke, Keim, Jolly
and others.[22] Regarding heterogeneous acetylene semi-
hydrogenation catalysis, oligomerization of the product and
subsequent carbon deposits on the catalyst surface was reported
in literature earlier.[3] In contrast, to the best of our knowledge, for
this reaction oligomerization as side-reaction has not been
reported so far for colloidal metal NPs. Low temperatures and a
relatively low hydrogen partial pressure have a positive influence
on the selectivity. Starting at room temperature and a H2 pressure
of 0.5 bar, expectedly, a highly active but unselective system was
observed (Table 1, Entry 1).
Consequently, carrying out the reaction at lower temperature
(0 °C) and 0.5 bar H2 leads to an improved selectivity but
decreased activity (Table 1, Entry 2). Increasing the H2 pressure
to 1.0 bar at 0 °C results in a high conversion of 80 % and an
enhanced selectivity towards 1-octene of 80 % after 3 h (Table 1,
Entry 3). Here, an oligomerization of 1 % and full-hydrogenation
to n-octane of 8 % was observed with a TOF of 325 h-1. Further
increase of the hydrogen pressure at low temperatures does not
result in increased activity, as a similar TOF value of 340 h-1 is
observed, but in decreased selectivity as olefin hydrogenation
and oligomerization are observed (Table 1, Entry 4). Expectedly,
at elevated temperatures and hydrogen pressure, alkane
formation and oligomerization are increased. Noteworthy, no
particle precipitation is observed independent on the applied
catalytic conditions. A linear correlation of the catalyst amount and
the reaction rate is monitored (SI, Figure S11), suggesting no
influence of the catalyst concentration on its performance.
Structural analysis of the catalyst after catalysis runs by DLS and
HRTEM shows a slight particle agglomeration (SI, Figures S20
and S21), which is not uncommon for colloids stabilized by weak
capping agents.[13] Nevertheless, no precipitation of the catalyst is
observed. These experiments show a highly active catalytic
system at mild conditions (low temperature, low H2 partial
pressure). Yet, without any additives the selectivity towards
1-octene is rather low as hydrogenation of alkynes is not stopped
at the olefin but reacts further to alkanes. Most likely, the favored
olefin hydrogenation over olefin desorption is responsible for the
observed overreduction.[2,3]
For further improvement of the catalytic performance of the
Pd@TOAB colloids and a comprehensive understanding of the
major factors influencing the catalytic system, we have added
phosphanes with different steric and electronic features in co-
catalytic amounts. Upon addition of 1.0 equiv. PPh3 with respect
to Pd to the colloidal solution, a shift in the 31P-NMR from -5.3 ppm
to -5.0 ppm with a signal broadening is observed, which points to
phosphane coordination to the Pd surface (SI, Figure S26). The
coordination of PPh3 results in particle agglomeration as shown
by DLS and HRTEM in Figure 2. Taking an equilibrium of
coordination and dissociation into account, the addition of only
1.0 equiv. PPh3 might be to insufficient for long-term stable
colloids. Conversely, this implements that TOAB can be
considered as a stronger capping agent than PPh3. Notably,
particle agglomeration is not observed when 1-octyne is present
in the Pd@TOAB/PPh3 solution, indicating an additional
stabilizing effect by alkyne coordination to the NPs surface. For
catalysis experiments, variation of the phosphane amount from
30 equiv. to 0.5 equiv. with respect to Pd showed that selectivity
remains nearly constant, whereas the activity increased with
decreasing PPh3 amount, with an optimum at 1.0 equiv. PPh3
based on the amount of Pd (Table 2 and Figure 3). Here, at full
conversion a 1-octene selectivity of 83 % is observed (6 %
oligomerization and 10 % n-octane) with a TOF of 840 h-1, which
compared to the catalysis without any additive is increased by a
factor of 2.5.
Table 2. Additive approach in the semi-hydrogenation of 1-octyne using 1 mol%
Pd@TOAB after 2 h reaction time (0 °C, 1.0 bar H2).
[a]
Equiv. PPh3
Conversion [%]
Selectivity [%]
0.5
1.0
2.0
5.0
10
38
99
74
62
54
23
81
83
98
100
100
100
20
[a] Equivalents based on the amount of Pd.
3
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splitting in the 31P-NMR spectra of the phosphane signal using
1.0 equiv. PPh3 is observed. Here, an optimum of coordinated
and non-coordinated phosphane is achieved. With higher
amounts of PPh3, only one broadened signal is detected (SI,
Figure S27). In addition, using higher amounts of PPh3 (5.0 to
30 equiv. phosphane), crystals of the complex [Pd(PPh3)4] were
1
isolated and identified via H-/31P-NMR and IR spectroscopy (SI,
Figures S6, S23 and S32), which clearly point to a leaching of Pd
atoms from the NPs by using an excess of phosphane. Control
experiments using various amounts of [Pd(PPh3)4] as catalyst
show no activity in the semi-hydrogenation of 1-octyne under the
described reaction conditions (SI, Figure S13), also explaining
why the activity is decreased with higher PPh3 amounts.
Therefore, the following catalysis studies were carried out at 0 °C
using 1.0 bar H2 and 1.0 equiv. phosphane per Pd (here, no Pd
leaching was observed). In detail, addition of 1.0 equiv. PPh3
results in almost full substrate conversion after 2 h with an
increased selectivity of 83 % towards 1-octene. The particle size
does not change during catalysis (SI, Figure S21). Substitution of
a phenyl group with a methyl group in the phosphane, namely
MePPh2, shows even higher selectivity after 2 h reaction time
(92 % conv. and 92 % sel.). Nevertheless, consecutive
substitution of the phenyl groups by sterically less demanding
methyl groups, namely Me2PPh and Me3P, leads to an increased
selectivity but also strongly decreased catalytic activity (SI,
Figures S14 and S15 and Table S2). The stronger interaction of
these smaller phosphanes with the Pd NPs results in particle
precipitation, as the stabilization effect of phosphanes is weaker
than that of TOAB. In consequence, catalyst deactivation by
particle agglomeration and precipitation is observed. By testing a
series of sterically different demanding phosphanes, a relation
between the cone angle[24] and the catalytic activity was found with
an optimum using PPh3 with a cone angle of 145° (Figure 4 and
SI, Table S2). Phosphanes with a high steric demand, e.g.
P(o-tolyl)3, PiPr3 and PCy3, do not destabilize the particles but also
show a decreased activity compared to PPh3. The decreased
activity can be explained by the steric inhibition of the phosphane
of the particle surface impeding substrate coordination to the
catalyst.
Figure 3. Kinetic study of the additive approach using 1.0 equiv. PPh3 based on
the amount of Pd leading to an enhanced selectivity and conversion (1 mol%
Pd@TOAB, 0 °C, 1.0 bar H2).
Comparison of our system with literature-known monometallic Pd
NPs for the semi-hydrogenation of 1-octyne show a rather high
activity for the herein reported catalyst (see the SI, Table S9).
Similar systems, such as Pd@PVP NPs provide a TOF of
1320 h-1,[22] while the TOF of Pd NPs stabilized by an ionic liquid
was only estimated about 23 h-1 for the semi-hydrogenation of
3-hexyne.[11] Nevertheless, for both examples the catalysis was
carried out at higher temperatures and the authors do not state
whether their catalyst is colloidal or heterogeneous. A further
decrease of the PPh3 concentration to 0.5 equiv. leads to a
decreased selectivity and activity including 1 % oligomerization
and 6 % n-octane. Based on the (simplified) concept of the model
of magic number clusters, for a perfect Pd NP with icosahedral
structure and a diameter of ca. 1.5 nm, it can roughly be estimated
that ca. 70-75 % of all Pd atoms are located at the surface of the
NPs. Therefore, by adding 1.0 equiv. of phosphane, it can be
deduced that the additive is always in small excess and that the
catalyst surface can be fully coordinated with phosphane. The
decrease in activity by increased PPh3 amount from 1.0 to
30 equiv. might occur from a competitive coordination of the
phosphane and the alkyne. Under catalytic conditions, a signal
Figure 4. Substrate conversion after 2 h as a function of the phosphane cone angle (left) and the electronic properties (right). Catalysis tests were performed under
standard catalytic conditions (1 mol% Pd@TOAB, 1.0 equiv. phosphane based on the amount of Pd, 0 °C, 1.0 bar H2).
4
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Table 3. Comparison of catalytic tests with 1-octyne as substrate using 1.0 equiv. phosphanes bearing the same cone angle of 145° under standard conditions
(1 mol% Pd@TOAB NPs, 0 °C, 1.0 bar H2).
Entry
Additive
-
Conversion after 2 h [%]
Sel. 1-octene at full conv. [%]
Full hydrogenation [%]
35 (5 h)
Oligomerization [%]
43(5 h)
1
2
3
4
56
99
46
34
22 (5 h)
83 (2 h)
61 (5 h)
50 (6 h)
PPh3
9 (2 h)
6(2 h)
P(p-tolyl)3
P(p-F-phenyl)3
32 (5 h)
10(5 h)
36 (6 h)
14(6 h)
constraints at the NP surface.[25] The addition of PPh3 further
leads to an increase of the TOF value for all substrates.
In contrast to the arylphosphane P(o-tolyl)3, which shows a similar
selectivity at full conversion compared to PPh3, the
alkylphosphanes PiPr3 and PCy3 show a decreased selectivity at
full conversion. Hence, it can be concluded that the influence of
the phosphane on the selectivity does not underly steric reasons
but rather electronic ones, as an increased σ-donor ability seems
to be undesirable for a high selectivity. Further investigation on
the influence of the phosphane on the catalytic activity indicated
not only steric reasons but also electronic differences as a
comparison of PPh3, P(p-tolyl)3 and P(p-fluorophenyl)3 shows
(Figure 4, Table 3 and SI, Figures S14, S15 and Table S2).
These phosphanes all exhibit a Tolman cone angle of 145° but
show a drastic difference in catalytic activity. In detail, catalysis
tests with both the electron donating P(p-tolyl)3 (Table 3, Entry 2)
and the more electron-withdrawing P(p-fluorophenyl)3 (Table 3,
Entry 4) result in a decreased activity. In addition, the best
selectivity at full conversion is achieved using PPh3 as additive
(Table 3, Entry 2). This illustrates that both steric and electronic
properties play a crucial role in influencing the colloidal catalyst
system and that PPh3 combines these requirements in an optimal
way. Hydrogenation experiments using the olefin, namely
1-octene, as substrate under standard catalytic conditions (0 °C,
1.0 bar H2 pressure) using Pd@TOAB NPs and 1.0 equiv. PPh3
show that the formation of n-octane was suppressed efficiently,
as after 5 h only 5 % of 1-octene was hydrogenated (SI,
Table S6). In contrast, without PPh3, full conversion of 1-octene
was reached at 3 h. For a more comprehensive understanding of
the increased selectivity, the hydrogenation of cis-2-octene was
tested with and without PPh3 as additive. By adding 1.0 equiv.
PPh3, olefin hydrogenation is not observed, while without PPh3
already after 1 h cis-2-octene is hydrogenated to n-octane in 87 %
conversion (SI, Table S7). This experiment gives evidence that
the semi-hydrogenation of alkynes by addition of phosphanes can
be extended to other substrates and not exclusively for 1-octyne.
The catalysis tests using phenylacetylene and 2-octyne with PPh3
and MePPh2 as additives exhibit similar results with respect to
conversion and selectivity compared to 1-octyne as substrate
(Table 4 and SI, Tables S4 and S5). A comparison of the
conversions of 1-octyne, 2-octyne and phenyl acetylene shows
that without any additives, the TOF values of 2-octyne and
phenylacetylene are higher than for 1-octyne, which was also
reported in literature before.[23] By the addition of PPh3, the
conversion of the terminal alkyne is slightly faster than that of the
substrates with an internal C≡C bond (Table 4, Entry 1 vs.
Entries 4 and 6). This can be explained by the influence of the
PPh3 coordination to favor the hydrogenation of terminal alkynes
on the metal surfaces than that for internal ones or terminal alkyne
with a higher steric demand, mainly due to coordination
Noteworthy, in the semi-hydrogenation of 2-octyne, phosphane
addition leads not only to a suppressed alkane formation, but
selectively to the cis-isomer which is typical for catalytic surface
reactions pointing to a syn-hydrogen addition.[26] Interestingly,
MePPh2 as additive solely enhances the conversion and
selectivity using 1-octyne and phenylacetylene, whereas a
deactivation is recorded with 2-octyne as substrate. This finding
also strengthens the assumption that not only steric but also
electronic considerations have to be considered.
For
a deeper understanding of the positive influence of
phosphane additives on the catalytic activity of the Pd NPs,
mechanistic investigations were carried out by means of in-situ
NMR, IR and HRTEM measurements. For this, colloidal
Pd@TOAB/PPh3 NPs were selected as catalyst, since they
exhibit an overall high activity, conversion and selectivity for all
examined substrates. Furthermore, PPh3 is generally stable
towards oxidation under an oxygen containing environment. In the
31P-NMR spectrum (recorded in toluene-d8), a resonance shift
from -5.3 to -5.0 ppm and signal broadening indicate a Pd-PPh3
coordination (SI, Figures S27 and S28) explaining the increased
selectivity by changing the electronic properties of the metal
surface. Nevertheless, surprisingly, the 31P-NMR spectrum also
shows about 30 % phosphane oxidation directly after addition of
1.0 equiv. PPh3 (referred to Pd) to a solution of 1-octyne and
1 mol% Pd@TOAB NPs, which is supported by IR measurements
(Figure 11 and SI, Figure S20). Since the synthesis is being
carried out under air, it is very likely that after formation of Pd
metal atoms and subsequent Ostwald ripening, Pd surface atoms
are oxidized by air oxygen.[26] Decreasing the PPh3 amount from
1.0 equiv. to 0.5 equiv. results in almost full phosphane oxidation
and no phosphane coordination, illustrated in the 31P-NMR
spectra by the presence of only one signal for PPh3O at 24.9 ppm,
and the absence of the signal for coordinated PPh3 at -5.0 pm,
respectively (SI, Figure S29). The absence of PPh3 results, as
already stated before, in a lower selectivity suggesting that
phosphane coordination is crucial for an enhanced selectivity. To
verify whether the phosphane or the phosphane oxide is
responsible for the increased activity, catalytic experiments using
1.0 equiv. PPh3O or MePPh2O were performed. Almost the exact
results as without additive were obtained, indicating that
phosphane oxides do not bind to the Pd surface, and that they are
not capable of reducing the surface Pd–O species (SI, Table S1
and Figure S14). Therefore, it can be assumed that the
phosphane is responsible for (a) the reduction of Pd–O, (b) the
coordination to the Pd(0) surface, and (c) the enhanced selectivity
in semi-hydrogenation catalysis.
5
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Table 4. Substrate variation using 1 mol% Pd@TOAB NPs and 1.0 equiv. PPh3 (catalysis conditions: 0 °C, 1.0 bar H2). PPh3 amount is based on Pd.
Entry
Substrate
1-octyne
Additive
Conversion [%]
56 (5 h)
Selectivity with full conv. [%]
Oligomerization with full conv. [%]
TOF [h-1]
330
1
2
3
4
5
6
-
22 (5 h)
83 (2 h)
66 (4 h)
92 (4 h)
41 (4 h)
87 (4 h)
8 (5 h)
4 (2 h)
4 (4 h)
0 (4 h)
9 (4 h)
6 (4 h)
1-octyne
PPh3
-
99 (2 h)
840
2-octyne
59 (4 h)
480
2-octyne
PPh3
-
72 (4 h)
520
phenylacetylene
phenylacetylene
66 (4 h)
760
PPh3
91 (4 h)
900
(SI, Figures S28 and S31). It can be assumed that for an
improvement of the catalytic system, a distinct amount of
phosphane is required to remove oxygen species from the
palladium surface, resulting in an increased amount of catalytic
active sites. An additional phosphane coordination to the
palladium surface leads to increased selectivity.
To answer the question whether the oxygen is originating from the
palladium surface or is physically dissolved in the solvent,
31P-NMR in situ studies were performed both under air and under
argon. These experiments show that phosphane oxidation is
enhanced by using moist solvents but also is present under inert
conditions (SI, Figures S29 and S30). In addition, experiments on
catalyst deactivation were performed (SI, Figure S18 and S19).
After full conversion of 1-octyne to 1-octene under standard
conditions (T = 0 °C, p(H2) = 1 bar, 1 mol% Pd@TOAB NPs,
1.0 equiv. PPh3, 2 h reaction time), solvent and (by)product(s)
were removed in vacuo. Accordingly, fresh solvent and substrate
were added, and a new catalysis run was carried out in order to
investigate the principal recyclability and activity of the catalyst
after product separation. Herein, it must be noted that due to
particle agglomeration, a lower activity for the second catalytic
test is expected. Using 1.0 equiv. PPh3 as additive, comparable
activity and selectivity was observed in the second catalysis test,
suggesting that this amount of additive is sufficient for an
enhanced performance (SI, Figures S18 and S19). Nevertheless,
by adding additional 1.0 equiv. PPh3 after the first catalytic test a
higher activity was observed. The results lead to the assumption
that both solvent and Pd surface contain a small amount of
oxygen, which can react with the phosphane. The latter is
confirmed by XPS as the Pd 3d core level spectrum of freshly
synthesized Pd@TOAB NPs (SI, Figure S8) proves a small
amount of oxidized Pd species (19 %) which was also previously
reported in literature, where XPS spectra of freshly synthesized
Pd@TOAB NPs clearly indicate a partial palladium oxidation on
the particle surface.[21]
When any possible oxygen source during particle synthesis is
avoided, no particle stabilization is achieved, suggesting that a
certain amount of surface oxygen is crucial for the formation of a
stable colloidal system. This hypothesis is supported by the
reactivity of the colloidal system under reductive conditions.
Adding PPh3 and hydrogen pressure to the system, without any
alkyne, Pd(0) precipitation is observed accompanied by the
formation of phosphane oxide confirmed by 31P-NMR, IR and
HRTEM measurements (Figure 2 and SI, Figures S22-S25). The
groups of Dupont and Dyson reported on Pd NPs stabilized by an
ionic liquid, which exhibit oxidized surface Pd atoms, as shown by
XPS experiments,[11,27] assuming that a thin layer of metal oxide
is beneficial for particle stabilization as the polarity of the oxygen
atoms facilitates a better interaction with the ionic capping agent.
In contrast, upon addition of 1-octyne to the system, no
precipitation of Pd(0) is observed; 31P-NMR spectra confirm that
PPh3O formation is still present, suggesting that the alkyne has a
stabilizing effect on the colloidal system under catalytic conditions
Conclusion
In this work, we demonstrate the enormous influence of
ligands on (a) the catalytic performance of (weakly stabilized)
colloidal metal nanoparticles in the semi-hydrogenation of alkynes,
(b) the selectivity towards the olefin product, and (c) the stability
of the colloids in solution. The addition of the optimal amount of
phosphane ligands to the surface of Pd colloids is an effective
site-blocking strategy for the semi-hydrogenation. Studies with a
set of electron-donating and withdrawing phosphanes show that
optimal catalytic performance requires a balance of electron
density at the NP. The activity as well as selectivity is strongly
influenced by the electron-donating or -withdrawing nature of the
added phosphane. We found that both the more electron-
donating P(p-tolyl)3 as well as the more electron-withdrawing
P(p-F-phenyl)3 do not lead to an ideal performance compared to
PPh3. The catalytic activity of Pd NPs depends on the additive as
follows: PPh3 > P(p-tolyl)3 > P(p-F-phenyl)3, suggesting that
strongly electron-withdrawing phosphanes decrease the activity
of the surface catalyst atoms. On the other hand, stronger donors,
such as P(p-tolyl)3 bind more strongly to the catalyst surface, thus
making it less accessible for substrate molecules, hence the
decreased activity. Comparison of the selectivity at full conversion
shows minor differences; in all cases, the selectivity was strongly
increased compared to catalysis without any additive. Therefore,
we conclude that the substitution of the phenyl groups does not
lead to a change in selectivity and only influences the activity of
our system. This points to the steric bulk of the additive as origin
of selectivity, while the binding strength of the phosphane appears
to influence the activity; the stronger the phosphane-Pd bond is,
the lower the activity is. Studies with phosphanes having different
cone angles showed that phosphanes with smaller cone angles
lead to particle precipitation. Phosphanes with a high steric
demand e.g. P(o-tolyl)3, PiPr3 and PCy3 stabilize the particles as
colloids, but they lead to a decreased activity compared to PPh3.
This is most likely a consequence of the shielding of the surface
by the additive, so that the substrate cannot access the catalyst.
Therefore, PPh3 seems to have a perfect balance between steric
demand for stabilizing the particle in solution and to allow the
6
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10.1002/cctc.202001121
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FULL PAPER
atmosphere and placed on a silicon wafer cut. The detection was carried
out with a Pixel3D detector. All measurements were performed with Cu-
Kα (λ = 1.54 Å) radiation and at 298 K.
substrate to access the surface, and a certain lability of the bond
to Pd.
Mechanistic investigations (in-situ NMR and HRTEM) during
catalysis show the presence of both phosphane and phosphane
oxide species on the metal surface, suggesting that oxygen plays
a role in stabilizing the colloids in solution, but also for the
Pd NP synthesis
The Pd NPs were obtained by reduction of K2[PdCl4] with NaBH4 and
subsequent stabilization with the capping agent tetraoctylammonium
bromide (TOAB). The method is a variation of those proposed by the
Coronado et al.20 In a typical synthesis, 5 mL of a 30 mM aqueous solution
of K2[PdCl4] (37 mg, 0.11 mmol, 1.0 equiv.) was mixed with 20 mL of a
50 mM TOAB solution (602 mg, 1.1 mmol, 10 equiv.) in toluene. After
30 min, the transfer of the [PdCl4]2- from the aqueous solution to the
organic phase was completed as revealed by the red color of the organic
phase containing the [PdCl4]2-/TOAB mixture. The aqueous phase was
disposed. Then, 6 mL of a freshly prepared 0.1 M NaBH4 (11 mg,
0.285 mmol, 2.5 equiv.) aqueous solution was added at 60 °C, keeping the
mixture under vigorous stirring (1400 rpm). Consequently, the solution
darkened immediately, indicating the formation of NPs. The mixture was
stirred for 30 min at 25 °C and after removing the aqueous phase, the
organic phase was washed with deionized water (3x50 mL) and finally
dried over anhydrous Na2SO4. The samples were degassed under an
atmosphere of purified argon.
selectivity. As
a perspective, avoiding bromide-containing
surfactants is likely to lead to an enhanced catalyst performance,
since halides are known catalyst poisons. Gaining more
knowledge on the interplay of the ligand and the catalyst surface
will lead to extending our studies to less expensive, i.e. ignoble
and more abundant metals for semi-hydrogenation catalysis,
especially for industrially relevant substrates.
Experimental Section
Materials and analytical methods
All chemicals and solvents were purchased from different suppliers or pre-
synthesized in the Fischer group (e.g. Sigma Aldrich, VWR). Catalysis
experiments were performed under an atmosphere of purified argon using
conventional Schlenk and glovebox techniques. Solvents were dried using
an MBRAUN Solvent Purification System, final H2O contents were
checked by Karl Fischer titration and did not exceed 5 ppm.
1H-NMR (400 MHz, C6D6): 3.5 (m, 2 H, NCH2), 1.7 (dt, 2 H, NCH2CH2), 1.4
(m, 10 H, CH2), 1.0 (t, 3 H, CH3).
NMR spectra (1H, 13C, 31P) were measured in C6D6 and toluene-d8 at 298 K
using a Bruker Avance DPX-250, Bruker DRX 400, Bruker Avance 300 or
a Bruker AV400 US spectrometer operating at the appropriate frequencies.
Chemical shifts are given relative to TMS (1H, 13C) or 85 % H3PO4 (31P),
and spectra were referenced relative to the residual solvent signal.
FT-IR spectra were recorded on a Bruker Alpha FT-IR spectrometer with
ATR (attenuated total reflexion) geometry, using a diamond ATR unit
under argon atmosphere. Signal intensities are described with the
following abbreviations: s = strong, m = medium, w = weak. The spectra
were processed with OPUS (Version 7.5, Bruker Optik GmbH 2014).
LIFDI-MS measurements were conducted on a Thermo Scientific Exactive
Plus instrument, equipped with an Orbitrap detector and a “LIFDI 700”
ionization source, located at the University of Würzburg. All samples were
prepared with dry toluene and filtered under argon atmosphere.
DLS measurements were performed on a Malvern Zetasizer Nano using
quartz cuvettes in 173° backscattering mode. A general-purpose method
was used to calculate the mean particle sizes and dispersities.
HRTEM samples were prepared by dropping a solution of Pd@TOAB in
toluene on carbon-coated Cu400 TEM grids. Samples were imaged by
HR-TEM “FEI TITAN Themis 60 300” equipped with X-FEG type emission
gun operated at 300 kV, spherical aberration Cs image corrector improving
resolution limit in TEM mode below 0.7 nm, high angle annular dark field
detector (HAADF) for high atomic number (Z)-contrast in scanning
IR (ATR, neat, cm-1): 720 (s), 753 (m), 1353 (m), 1466 (s), 2850 (s), 2871
(s), 2953 (m), 3004 (w).
Catalysis experiments
All catalysis experiments were carried out under inert atmosphere and n-
dodecane was chosen as internal standard. For semi-hydrogenation
experiments, 0.9 mL of the NP solution (1 mol% catalyst in toluene),
0.1 mL substrate (1-octyne, 2-octyne, phenyl acetylene) and 0.1 mL
dodecane were placed in a 90 mL Fischer Porter bottle and cooled to 0 °C.
This solution was purged with 1.0 bar H2 and continuously stirred for 5 h.
For GC measurements, aliquots were taken under a continuous argon flow.
After taking of aliquots, the Fischer Porter bottle was again purged with 1.0
bar H2.
For recycling experiments, all liquids were removed in high vacuum and
fresh solvent and substrate was added. For quantification of the
oligomerization products, all low-boiling components were evaporated at
reduced pressure and Pd@TOAB was removed by filtration of the residue
over SiO2 in hot toluene. LIFDI-MS measurements were carried out for an
exact determination of the residue components.
transmission electron microscopy (STEM) and SUPER
X energy
Acknowledgements
dispersive X-ray (EDX) spectrometer with 4x30 mm2 windowless silicon
drift detectors. Particle size determination was carried out using ImageJ
(version 2019).
The authors thank Mr. Christoph Mahler from Julius-Maximilians-
Universität Würzburg for the LIFDI-MS measurements.
X-ray photoelectron spectra were recorded on a Leybold-Heraeus LHS 10
spectrometer using a non-monochromatised Al Kα source (1486.7 eV).
Samples involved in catalytic reactions were dissolved in toluene and spin-
coated on a Si-wafer. Sample preparation and transfer into the XPS
spectrometer were carried out under argon atmosphere. The analyser was
operated at a constant pass energy of 100 eV leading to an energy
resolution with a full width at half-maximum (fwhm) of ~1.1 eV. The energy
scale of the spectra was corrected for sample charging by using the C 1s
main signal (284.5 eV). All spectra were recorded in an ultra-high vacuum
chamber at a pressure below 5×10-8 mbar. Core level spectra were
deconvoluted by using Voigt functions and linear background subtraction.
Due to the charging correction by an intrinsic signal of the sample, which
might be erroneous comparison to reference samples with known
oxidation states reveals the oxidation states of the new materials.
Powder X-ray diffraction measurements were performed on a Panalytical
Empyrean instrument. The sample was put in a capillary under argon
Keywords: colloidal nanoparticles • additive approach • semi-
hydrogenation reaction • phosphane ligands • homogeneous
catalysis
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Entry for the Table of Contents
Colloidal Pd nanoparticles, stabilized by the ionic capping agent N(Octyl)4Br were tested as quasi-homogeneous catalysts for the semi-
hydrogenation of liquid alkynes. The addition of phosphanes to the catalyst surface leads to a boost of both activity and product
selectivity, owing to the steric and electronic properties of the additives.
9
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