Functionalization of platinum nanoparticles with l -proline: Simultaneous enhancements of catalytic activity and selectivity

Article abstract of DOI:10.1021/ja511349p

In this work we present the successful application of functionalizing Pt nanoparticles (NPs) with hydrophilic organic ligands as a strategy for enhancing their catalytic activity and selectivity. In the first step, Pt NPs were prepared by a colloidal approach and subsequently functionalized in a separate synthesis step with l-proline (PRO). The functionalized NPs were supported onto Al₂O₃ and investigated as heterogeneous catalysts for the selective hydrogenation of acetophenone. Whereas significant amounts of side products are formed by supported, unprotected (ligand-free) NPs, the PRO-functionalized Pt NPs are highly chemoselective even at 100% conversion. Experiments under kinetically controlled conditions reveal that this high chemoselectivity is not accompanied by a loss of catalytic activity. In contrast, an enhanced rate toward the desired product was found for PRO-Pt in comparison to the unprotected Pt NPs. This finding demonstrates that the use of ligands in heterogeneous catalysis allows for simultaneous enhancements of activity and selectivity.

Full text of DOI:10.1021/ja511349p

Article
pubs.acs.org/JACS
Functionalization of Platinum Nanoparticles with L‑Proline:
Simultaneous Enhancements of Catalytic Activity and Selectivity
Imke Schrader,† Jonas Warneke,† Jana Backenkohler,‡ and Sebastian Kunz*,†
̈
‡
†Institute for Applied and Physical Chemistry and Institute for Organic and Analytical Chemistry, University of Bremen,
Leobenerstraße, 28359 Bremen, Germany
S
* Supporting Information
ABSTRACT: In this work we present the successful application of
functionalizing Pt nanoparticles (NPs) with hydrophilic organic
ligands as a strategy for enhancing their catalytic activity and
selectivity. In the first step, Pt NPs were prepared by a colloidal
approach and subsequently functionalized in a separate synthesis
step with L-proline (PRO). The functionalized NPs were supported
onto Al2O3 and investigated as heterogeneous catalysts for the
selective hydrogenation of acetophenone. Whereas significant
amounts of side products are formed by supported, “unprotected”
(ligand-free) NPs, the PRO-functionalized Pt NPs are highly
chemoselective even at 100% conversion. Experiments under
kinetically controlled conditions reveal that this high chemo-
selectivity is not accompanied by a loss of catalytic activity. In contrast, an enhanced rate toward the desired product was
found for PRO-Pt in comparison to the “unprotected” Pt NPs. This finding demonstrates that the use of ligands in
heterogeneous catalysis allows for simultaneous enhancements of activity and selectivity.
the catalytic properties.6 As a result, the use of ligands has
1. INTRODUCTION
recently started to attract significant attention within the field of
In organometallic, homogeneous catalysis, the use of ligands is
heterogeneous catalysis and their ability for the control of
essential for the stabilization of the catalytically active metal in
solution. If a coordinating species desorbs from the metal
center a free adsorption site is generated, which may become
occupied by a reactant to undergo a reaction catalyzed by the
metal. Alternatively a reactant may also become adsorbed by
insertion. From homogeneous catalysis research it is known
that the ligand can positively contribute to catalytic reactions
and enable for the control of selectivity by interacting with the
adsorbed reactant.1 Furthermore, examples are known in which
ligands enhance reaction rates by changing and participating
within the catalytic cycle.2
chemo- and stereoselectivity was demonstrated.7−9
So far most studies in heterogeneous catalysis that focus on
selectivity were performed with thiol ligands.7,8,10,11 Sulfur is
known to bind strongly to catalytically active metals so that the
ligand can be tightly anchored to the particle surface. As a result
partial blocking of the surface occurs that leads to the above-
mentioned decrease of the catalytic activity.8,11 This seems to
be a general issue when functionalizing NPs with ligands that
may not be avoidable.
For nanoparticles (NPs) applied in heterogeneous catalysis,
only the surface atoms contribute to the reaction. Most of the
coordination sites of the surface atoms are however blocked by
adjacent metal atoms. If ligands are bound to the remaining free
coordination sites of the surface atoms, their ability to adsorb
and catalytically convert reactants may eventually become
inhibited. In heterogeneous catalysis ligands are hence
considered as catalyst poisons. They are usually only applied
for the preparation of model catalysts by means of colloidal
methods (to control, e.g., size, shape, or composition) and
removed after particle deposition. This can be achieved by, e.g.,
ex situ or in situ O3 treatments under ambient conditions3,4 or
using specific heating cycles under reductive and oxidative
conditions.5 The relevance of controlling selectivity as one of
the major research goals in catalysis for the next decades
however requires new and sophisticated strategies to modify
In the present study colloidally prepared Pt NPs function-
alized with L-proline (PRO) were investigated as heterogeneous
catalysts for the selective hydrogenation of acetophenone. It
was found that in contrast to bare Pt particles the PRO-Pt NPs
are highly chemoselective. This enhanced selectivity is notas
observed in previous casesaccompanied by inhibition of the
catalytic activity. Instead we show that the reaction rate toward
the desired product on PRO-Pt is enhanced in comparison to
the bare Pt NPs. Our results therefore demonstrate that PRO
cannot be considered as a catalyst poison but to act as a
promoter with positive effects on the activity and selectivity.
Received: November 4, 2014
© XXXX American Chemical Society
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2. EXPERIMENTAL SECTION
Article
cleaned as described in 2.1.3 and then redispersed in D2O (99.9%,
Deutero). Afterward the solvent was removed at a rotary evaporator
(T = 50 °C; P = 20 mbar) until the sample appeared to be dry. The
dried particles were redispersed in D2O for a second time followed by
solvent removal at a rotary evaporator (T = 50 °C; P = 20 mbar). After
these purification steps the particles were again redispersed in alkaline
D2O and subsequently investigated by means of NMR spectroscopy
2.1. Material Synthesis. 2.1.1. Syntheses of “Unprotected” (EG-
Stabilized) Pt Nanoparticles. “Unprotected” Pt NPs were prepared by
a slightly modified protocol of the original recipe established by Wang
et al.12 First, 0.25 g of H2PtCl6·H2O (40% metal, ChemPur) was
dissolved in 25 mL of ethylene glycol (EG) (99.8%, Sigma-Aldrich) in
a 250 mL glass flask. A solution of 0.50 g of NaOH (98.9%, Fisher
Chemical) dissolved in 25 mL of EG was added and the mixture
vigorously stirred at 500 rpm (stir bar length = 2.5 cm) to ensure
proper mixing. The flask was equipped with a reflux condenser, and
the precursor solution was heated to 150 °C using a preheated oil bath
while the stirring rate was maintained at 500 rpm. The yellow solution
turned black after about 5 min, indicating the formation of Pt NPs.
The reaction mixture was kept at 150 °C for 1.5 h to ensure complete
reduction of the Pt precursor followed by cooling to ambient
temperature. The particles were precipitated by adding 50 mL of 1 M
HCl (VWR) and separated from the supernatant solvent by
centrifugation. The precipitated particles were washed once with 1
M HCl and then redispersed in 100 mL of cyclohexanone (≥99.0%,
Sigma-Aldrich) for all further preparation steps.
2.1.2. Synthesis of PRO-Functionalized NPs. The preparation of
PRO-functionalized NPs followed a previously established route for
the functionalization of “unprotected” Pt NPs with hydrophilic
ligands8 and was the same for all other amine ligands investigated
within the present study. First a ligand solution was prepared with
deionized water, resulting in a L-proline (≥99%, Sigma-Aldrich)
concentration of 8.3 mM, and 12.5 mM NaOH. In order to
functionalize the NPs four aliquots of this ligand solution were
added to the previously prepared dispersion of Pt NPs in
cyclohexanone (see section 2.1.1). This corresponds to a ligand-to-
Pt ratio of 6.4 to achieve saturation of the particle surface with ligands.
The resulting emulsion was vigorously stirred for 30 min. During this
mixing the particles are transferred from the organic phase into the
aqueous phase, indicated by a color change of both phases. The
organic phase turned clear, while the aqueous phase became black
indicating the successful functionalization of the particles. The PRO-Pt
NPs were then filled in a separation funnel and separated from the
supernatant organic solvent after proper phase separation was
achieved.
2.1.3. Purification and Cleaning of PRO-Functionalized NPs. For
the characterization of functionalized NPs it is essential to clean them
from residual nonbinding ligands. Therefore, the aqueous dispersion of
PRO-functionalized NPs was concentrated by removing the solvent at
a rotary evaporator (P = 20 mbar; T = 50 °C) until a very thick, tarry
looking dispersion is obtained. Next, an excess of acetone (99.9%,
VWR) was added that initiates precipitation of the functionalized NPs.
After centrifugation the supernatant solvent is removed and the
precipitate washed twice with ethanol (99.9%, VWR) and once with
acetone (99.9%, VWR).
2.2. Characterization. 2.2.1. Transmission Electron Microscopic
Investigations of “Unprotected” and Ligand-Functionalized Pt NPs.
Samples were prepared by drop-casting of the particle dispersion onto
the transmission electron microscopy (TEM) grid (ultrathin carbon
film, Quantifoil, Cu 200 mesh). The grids were then dried in an oven
for 30 min at 80 °C. For “unprotected” NPs the cyclohexanone Pt
stock solution was used for TEM grid preparation (see section 2.1.1).
PRO-functionalized NPs prepared and purified as described in sections
2.1.2 and 2.1.3 were redispersed in an alkaline solution with a
concentration of 0.01 M NaOH (98.9%, Fisher Chemical), followed by
drop-casting onto the TEM grid. A Tecnai F20 S-TWIN microscope
(FEI) was used at an acceleration voltage of 200 kV and a
magnification of 150k. Particle sizes were determined using ImageJ
and counting at least 200 particles. From the average size, the
dispersion (ratio of surface atoms to total number of atoms within the
particle) was estimated according to a model calculation previously
described in detail.8
1
(Bruker AVANCE NB-360). H NMR spectra were recorded with
standard methods; see reference for applied pulse sequence of HH-
COSY (with gradients, not phase sensitive).13
2.2.3. Determination of Ligand Coverage. In order to estimate the
ligand coverage, the nitrogen-to-Pt ratio was determined. Therefore,
elemental analysis (EA) was applied to measure the nitrogen content
of PRO-functionalized NPs and atomic absorption spectroscopy
(AAS) to determine the Pt content. Preparation and cleaning of PRO-
functionalized NPs was performed as described in section 2.1 followed
by drying for 12 h in a desiccator under reduced pressure. For AAS
measurements the samples were digested in freshly prepared aqua
regia. EA measurements were performed using a Euro ES elemental
analyzer with chromatographic separation and a TCD. AAS was
conducted on a Varian AA 280 FS spectrometer. To obtain the ligand
coverage the N:Pt ratio was corrected by taking the dispersion of the
particle (ratio of surface atoms to total number of atoms of a particle;
see section 2.2.1 for determination) into account.
2.3. Deposition of “Unprotected” and Functionalized
Particles. For catalytic investigations the “unprotected” and function-
alized NPs were deposited onto Al2O3 (PURALOX SCCa 150/200;
Sasol, grain size = 200−500 μm) to give nominal metal loadings of 2
wt% with respect to the initially used amount of Pt precursor. The
support material was added to the particle dispersions and the solvent
removed using a rotary evaporator (P = 20 mbar; T = 50 °C). In order
to clean the supported, functionalized Pt NPs from residual ligands
that do not bind to the particle surface, the samples were twice rinsed
with ethanol (99.9%, VWR). Supported “unprotected” Pt NPs were
rinsed twice with acetone (99.9%, VWR). The cleaned catalysts were
kept under vacuum in a desiccator for 30 min prior to their application
in catalytic experiments. Accurate determination of catalytic activities
requires the actual metal loading of the supported particle catalysts.
Therefore, the metal loading of every catalyst was measured by AAS
(Carl Zeiss Technology AAS 5 FL) and used for normalization of the
reaction rates. Digestion of the particles was achieved with freshly
prepared aqua regia. The typical loadings for supported “unprotected”
and functionalized NPs are around 1.6 and 1.3 wt%, respectively.
2.4. Catalytic Investigations. 2.4.1. Catalytic Hydrogenation of
Acetophenone. Two custom-designed autoclaves (Parr Instrument
Company) were used for catalytic studies, both connected to the same
H2 gas line and the same thermostat in order to perform two catalysis
experiments in parallel under identical experimental conditions. In a
typical experiment each autoclave was loaded with 1 mL of
acetophenone (99%, Sigma-Aldrich), 9 mL of cyclohexane (99.99%,
Acros), and 200 mg of catalyst. After purging with H2 (Linde 5.0), the
reaction pressure in the autoclaves was set to 20 bar H2, and the
experiments were performed at a temperature of 293 K. In order to
determine reaction rates, the conversion was kept below 10% to
achieve differential operation conditions. The conversion was tested to
scale linearly with the amount of catalyst used within the experiment.
Furthermore, the stirring rate was varied as well as the catalyst pellet
size by grinding the support particles, but no effect on the conversion
was obtained. The presence of diffusion limitations can hence be
excluded and the requirements to determine turnover rates from
conversions below 10% are fulfilled.14 The experimental errors of the
selectivities and activities were determined from the standard deviation
of five catalysis experiments, each performed with a separately
prepared catalyst of individually synthesized particles. The presented
errors do thus not merely reflect the error of the catalytic experiments,
but also deviations that may appear from the catalyst preparation.
2.4.2. Product Analysis of Catalytic Experiments. The reaction
mixtures of the catalytic experiments were analyzed by gas
chromatography (Shimdazu GC-2010plus AF IVD) using a Lipodex
E (Macherey-Nagel, 27 m length, 0.25 mm inner diameter, 0.25 μm
2.2.2. NMR Spectroscopic Characterization of PRO-Function-
alized NPs. Residual H2O is a significant issue for NMR spectroscopic
studies, as water protons lead to a huge background in the spectrum.
In order to remove residual H2O from PRO-Pt NPs, the samples were
B
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film thickness) column and a flame ionization detector (FID). A
column flow of 0.86 mL min−1 was applied with He (Linde, 5.0) as
carrier gas and a linear velocity of 25.1 cm s−1. Prior to analysis the
samples were diluted with acetone (VWR, 99.9%) to a solvent-to-
sample ratio of 5:1. Injection was performed at 200 °C and a split ratio
of 50:1. The oven temperature was held at 90 °C for 1 min, then
heated to 120 °C at a rate of 2 °C min−1 and kept for 1 min. After
further heating to 180 °C at a rate of 5 °C min−1 the temperature
program was stopped. The activity and selectivity were determined
from the total amount of detected products and reactant and by taking
the corresponding response factors into account.15 Representative
chromatograms of catalysis experiments performed with “unprotected”
and PRO-functionalized NPs are shown in Figure S1.
and Pt-bound PRO (blue, see also Figure 1). Complete
removal of organic impurities from functionalized NPs for
NMR spectroscopic investigations is a hardly achievable
preparation task.20,21 In the present case some organic residues
are visible (black). The most significant residues that appear
reproducibly but to varying extent were identified as ethanol
and acetic acid. Addition of both of these molecules led to
increase of the assigned signals demonstrating they are not
bound to the particle surface. All other residues were not found
to appear reproducibly.
Only carbon-bound hydrogen was detected, because H/D
exchange of oxygen- and nitrogen-bound protons in D2O
occurs too fast. Due to the stereogenic carbon atom of PRO (1
in Figure 1, labeled with *), the 1H NMR signals of CH2 groups
are not chemically equivalent. This effect is most pronounced
for the CH2 group (2 in Figure 1) that is adjacent to the chiral
GC-MS (Agilent Technologies 5975 C) and Agilent ChemStation
Software were applied in order to achieve assignment of the different
side products. Samples were injected at 200 °C with a split ratio of
20:1 and separated under isothermal conditions (120 °C for 30 min)
using a Innopeg 1000 column (50 m).
1
carbon atom (1). The H signals of the two inequivalent
3. RESULTS AND DISCUSSION
protons are labeled with 2a and 2b in Figure 2. Comparison of
the same protons in free and Pt-bound PRO shows that the
signals of 1 and 4 are considerably more shifted than the 2 and
3 protons (numbering according to Figure 1). This effect may
be related to the fact that these protons are in closer proximity
to the nitrogen atom which is chemically altered due to the
binding to Pt. A further suitable effect may be the closer
proximity of 1 and 4 to the particle surface compared to 2 and
3.
It has been demonstrated that “unprotected” Pt NP cannot
be functionalized with carboxyl ligands.22 We hence conclude
that PRO binds only via the amine group to the particle surface.
This assumption is further supported by correlations in HMBC
spectra of the hydrogen of 1 (label according to Figure 1) with
the carboxyl carbon in free and Pt-bound PRO (Figure S5).
The 13C chemical shifts are very similar suggesting that the
carboxyl group in the Pt-bound state does not differ
considerably from the free PRO. We therefore use the
abbreviation PRO-N-Pt NPs for Pt NPs functionalized with
PRO to indicate the binding mode.
A unique approach for investigating the effect of ligands on the
properties of NPs is the use of so-called “unprotected” NPs.12,16
Such particles are only weakly stabilized by ethylene glycol and
CO, which are removed upon functionalization with, e.g., amine
or thiol ligands, while the particle size is maintained.12,17 As this
allows the separation of particle preparation and their
functionalization into two independent steps, changes in the
material properties can be related exclusively to the influence of
the ligand.18 Based on this strategy, we recently presented a
synthesis route for the functionalization of Pt NPs with
hydrophilic ligands.8 This recipe was applied in the present
study to prepare Pt NPs functionalized with L-proline (PRO,
see Figure 1 for structure). Particle size analysis performed for
Hydrogenation of acetophenone was performed in order to
investigate and compare the catalytic properties of PRO-N-Pt
NPs to “unprotected” Pt. This reaction represents a typical
chemoselectivity challenge in heterogeneous catalysis. Chemo-
selective hydrogenation of the carbonyl group to phenylethanol
(see structure 3B in Figure 3, green) is the desired reaction but
simultaneously hydrogenation of the aromatic moiety to form
methylcyclohexylketone (3C, red) can occur. This is a
particular problem for practical hydrogenation catalysts like
supported Ni, Pd, or Pt particles, which usually exhibit
selectivities for phenylethanol (3B) around 60−80%.23−26
Both these products (3B and 3C) may serve as intermediates
that become eventually fully hydrogenated to yield the
saturated alcohol cyclohexylethanol (3D, gray). Besides, a
stereogenic center is generated by formation of the desired
product (3B, the stereogenic C atom is labeled with *). The
reaction thus enables to investigate the ability to control
chemoselectivity with ligands and furthermore to probe the
presence of stereoselectivity that may appear when using chiral
ligands such as PRO.
Figure 1. Structures of unbound (magenta) and Pt-bound PRO (blue)
under alkaline conditions.
“unprotected” and PRO-functionalized NPs confirms that the
particle size is maintained during functionalization (see Figure
S2).
NMR spectroscopic studies were performed on PRO-
functionalized Pt NPs dispersed in alkaline D2O to demonstrate
that PRO is successfully bound to the particle surface and
cleaned from residual nonbinding ligands. The cleaned particles
1
give only one set of H signals that can be related to the
structure of PRO (see Figure S4). In order to demonstrate that
these signals originate from Pt-bound PRO and not from
desorbed ligand molecules, a small amount of the pure ligand
was added (see Figure 2). This leads to the appearance of a
1
Table 1 shows the selectivities of supported, “unprotected”
Pt NPs and PRO-N-Pt NPs at 100% conversion. A total
amount of 35% of the undesired saturated alcohol (cyclo-
hexylethanol, 3D) was obtained for the “unprotected” Pt NP
catalyst demonstrating the above-mentioned limitations of Pt
for this reaction.25,26 In contrast, PRO-N-Pt NPs are highly
second set of H signals which can be assigned to free PRO in
alkaline D2O.19 Identification of the signals that belong to the
same molecule can be verified by correlations in the HH-COSY
spectrum (Figure 2). The correlations are identical confirming
that the Pt-bound ligand is indeed PRO. It can thus be
distinguished between unbound (magenta, see also Figure 1)
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1
Figure 2. H and HH-COSY spectrum (360 MHz, 8.4 T, D2O) of a mixture of free PRO (magenta) and Pt bound PRO (blue). Signals of
contaminations are black.
catalytic activity (see reference for synthesis and character-
ization of NAC-S-Pt)27
A selectivity enhancement by using additives like ligands or
adsorbates is usually accomplished at the expense of activity.28
The additive binds to the particle surface which may lead to
poisoning of unselective sites.29 Simultaneously the catalytically
active surface becomes at least partially blocked which causes a
decrease in activity.30,31 In order to determine if the activity of
the Pt NPs is reduced due to the functionalization with PRO,
the formation rates on “unprotected” Pt NPs and PRO-N-Pt
NPs for the individual products (3B, 3C, and 3D) were
determined (see Figure 4, color code for reaction products is
the same as in Figure 3).
Even though the ligands bind to surface atoms we
Figure 3. Reaction scheme for the catalytic hydrogenation of
acetophenone (3A, black). The desired reaction product is the
aromatic alcohol (3B, green), but hydrogenation of the aromatic
surprisingly found that the catalytic activity normalized to the
total number of surface atoms (Figure 4a) is not negatively
affected by the presence of PRO. Instead the formation rate
toward the desired product phenylethanol (3B) is enhanced.
This finding seems to be inconsistent with the explanation that
the ligands merely poison the unselective sites. For this scenario
the rate of formation for phenylethanol (3B) over PRO-N-Pt
should be either the same or lower than that of “unprotected”
Pt NPs.32 It has been shown that the “unprotected” Pt NPs
used by us are in the metallic state after preparation and that
binding amines does not significantly alter the electronic
properties of Pt.16 Furthermore, if electronic effects are of
significance for the catalytic properties of amine-functionalized
Pt NPs, the presence of ligands should lead to changes of
activation energies.33 However, it has been previously shown
for various amines on Pt that they do not alter activation
energies of reactions that strongly depend on the electronic
properties of the catalytic metals like CO oxidation.30 If an
electronic effect would be the determining factor for the
enhanced rate, one should be able to achieve the same effect
with other amines. However, the use of, e.g., N-methyl-proline
moiety occurs as a competitive reaction path leading to the saturated
carbonyl (3C, red). 3B and 3C may both further react to give the
saturated alcohol (3D, gray).
Table 1. Catalytic Experiments Performed in Cyclohexane at
T = 293 K, PH = 20 bar, t = 24 h
2
ligand
conversion
(%)
coverage
3B
3C 3D
ee (%)
0
Pt NPs
0
100
100
65
0
0
35
0
PRO-N-Pt
NPs
0.85
≥99
14
1
chemoselective toward the desired aromatic alcohol with some
modest stereoselectivitiy (14% enantiomeric excess (ee)). This
exclusive chemoselectivity at full conversion is not specific for
PRO but was also obtained for other hydrophilic primary and
secondary amine ligands (see Figure S6a,b for further ligand
structures tested). In contrast, a thiol-bound ligand (N-acetyl-
cysteine (NAC), Figure S6c) led to complete inhibition of the
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Figure 4. Formation rates normalized to the total number of surface atoms for each product (see panel a) on “unprotected”, PRO-N-Pt, and N-
methyl-PRO Pt NPs. The color code reflects the structures shown in Figure 3. A significantly enhanced rate toward the desired product
phenylethanol (3B) is obtained for PRO-N-Pt in comparison to the “unprotected” Pt NPs. In contrast N-methyl-PRO inhibits the activity while
showing a selectivity improvement. All hydrogenation rates normalized to the total number of free surface Pt atoms for “unprotected”, PRO-N-Pt,
and N-methyl-PRO-Pt NPs are shown in panel b). The phenylethanol (3B) formation rate on PRO-Pt NPs is significantly higher than that on
“unprotected” Pt NPs, whereas it is inhibited on N-methyl-PRO-functionalized Pt NPs. A differently scaled form of panel b is shown in Figure S7,
that allows for a better comparison of “unprotected” and N-methyl-PRO-functionalized particles.
(N-Me-PRO), as discussed in further detail below, led to a
significant inhibition of the activity (see Figure 4). Therefore,
we can exclude that electronic effects induced by the amine
ligands are the reason for the enhanced catalytic activity. So far
we only discussed rates normalized to the total number of
surface atom. It should however be taken into account that only
ligand free surface sites can contribute to the reaction. With
respect to the ligand coverage of PRO-N-Pt NPs (see Table 1)
only 15% of all surface atoms are free of ligands. If this value is
applied in order to normalize the rate to the total number of
free surface atoms (Figure 4b), the formation rate for the
desired product on PRO-N-Pt NPs exceeds that of the
“unprotected” NPs by a factor of 10. Based on this finding
and the fact that an electronic effect can be excluded, we
conclude that the reaction on PRO-N-Pt NPs does not proceed
via the same mechanism as on “unprotected” Pt NPs. Thus,
different mechanistic scenarios are discussed in the following
that may account for simultaneous enhancements of activity
and selectivity.
First it is considered if homogeneous catalytic species are
formed by metal leaching that may contribute to the reaction.
Catalytic reactions with supported metal particles in liquid
media can cause formation of homogeneous species through
metal leaching. This is a typical problem for heterogeneous
catalysis under strongly oxidizing conditions.34 In hydro-
genation reactions the opposite reaction is favored (particle
formation from homogeneous catalysts), as reduced metallic
species nucleate to minimize their free surface energy.35 The
formation of homogeneous species thus seems unlikely to
account for the obtained rate enhancement. To demonstrate
that in the present case no homogeneous species are formed
that lead to the obtained activity and selectivity enhancements,
a filtration test was performed. For this purpose a catalysis
experiment was run to about 50% conversion and then stopped.
The heterogeneous catalyst removed by filtration and the
reaction mixture again exposed to reaction conditions. No
further conversion was obtained. We hence conclude that the
obtained activity and selectivity enhancements are not related
to leaching of the catalytically active metal.
A further suitable explanation may be the presence of a
ligand acceleration effect, which describes the ability of a ligand
not to just merely act as a spectator but to contribute to the
reaction.2 The participation of the ligand leads to alternative
activation and reaction mechanisms that may exhibit enhanced
reaction rates in comparison to the purely metal-catalyzed
reaction path. The appearance of acceleration effects are known
from homogeneous as well as heterogeneous catalysis.2 In
heterogeneous catalysis the use of cinchona alkaloids as so-
called modifiers has been reported to accelerate hydrogenation
rates for ketones.36 The modifier is dissolved in the reaction
medium and can coordinate CO groups to form a highly
activated complex.37 If such a reactant−modifier complex
adsorbs on a Pt particle that activates H2 via dissociative
adsorption, chemo- and stereoselective hydrogenation of the
carbonyl occurs at rates that are enhanced in comparison to
catalytic hydrogenation of the carbonyls by Pt. The acceleration
effect of cinchona alkaloids is, however, limited to specific
ketones. For the reaction investigated in the present study
(Figure 3) modifiers were actually found to reduce the reaction
rate toward the desired product by 84%.38 The difference
between the modifier approach and the concept of function-
alizing particles with ligands for applications in heterogeneous
catalysis is that the latter is based on the strategy that the ligand
is not soluble in the reaction medium and strongly bound to
the particle.8 Otherwise the particles would desorb from the
support and simultaneously the ligands from the particle surface
leading to a loss of the functionalization. To demonstrate that
ligands indeed do not act like modifiers two tests were
performed. First, an experiment was performed in exactly the
same way modifiers are applied. The heterogeneous catalyst
(supported “unprotected” Pt NPs) was added to the reaction
mixture together with the ligand and then the reaction was
started. As expected, the selectivity of this experiment reflects
exactly the results obtained for “unprotected” Pt NPs, because
the ligand is not soluble in the reaction medium. Second, a
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supported PRO-N-Pt sample was applied as catalyst under the
same conditions used for all other experiments (see section
2.4), but using ethanol as solvent instead of cyclohexane. PRO
is slightly soluble in ethanol. It can hence desorb from the
particle surface to act like a modifier, which is soluble in the
reaction medium.39 In addition, the functionalized NPs can also
desorb from the support and act as “quasi-homogeneous”
catalysts when the ligand is soluble in the reaction medium.
This effect was indeed observed. After catalytic experiments the
reaction mixture showed a pale dark color, which indicates the
presence of colloidal particles. The result of this catalytic
experiment shows that neither full conversion nor full
chemoselectivity was obtained (see Table S1). Furthermore,
the stereoselectivity was found to decrease from 14% to 3% in
comparison to the experiments performed under the same
conditions but in cyclohexane (see Table 1). We can thus
conclude that the selectivity controlling ability of PRO is not
fully utilized when being applied as a modifier. Instead it has to
be kept in the surface-bound state similar to a ligand in
homogeneous catalysis which can only enhance the selectivity
when being coordinated to the metal center.
In homogeneous catalysis the use of primary and secondary
amines as ligands to accelerate the hydrogenation of ketones is
a well-established strategy.40 The key for this effect is that
amine-bound hydrogen atoms become acidic upon coordina-
tion of the amine to a late transition D-metal like Ru or Pt.40 In
comparison to aromatic moieties, the electronic structure of
carbonyl groups makes them suitable for attractive interactions
with protons. The acidic amine protons can coordinate to the
oxygen of the CO group and activate it (see step 1 in
Scheme 1). The amine hydrogen protonates the CO group
oxygen while a hydride is transferred from the metal to the
carbon atom (Scheme 1, step 2).41 Afterward, the catalyst
adsorbs H2 (Scheme 1, step 3), which is dissociated (Scheme 1,
step 4) to re-form the catalytically active species.
A catalytic experiment to test for the presence of a N−H
effect can be performed by using tertiary amines as ligands. As
illustrated in Scheme 1 the N−H guided reaction path can only
proceed when amine protons are available.40 If however tertiary
amines are applied as ligands no amine proton is present and
hence no N−H effect can occur (compare Scheme 1).41 In
order to perform this test, a catalyst was prepared by using Pt
NPs that were functionalized with N-Me-PRO (see Figure S6d
for structure), the simplest tertiary amine derivative of PRO. An
increasing steric demand of ligands leads to a decreasing ligand
coverage.8,42 As a result a decrease of the ligand coverage from
0.85, as determined for PRO-N-Pt NPs, to 0.4 was obtained
when using a N-methylated derivative, which was taken into
account when normalizing the rates to the total number of free
surface atoms (Figure 4b). Comparison of the N-Me-PRO NPs
with the “unprotected” Pt NPs shows that the formation rates
of all three products are reduced by the presence of this ligand.
However, as the rates for the side products (3C and 3D) are
even stronger inhibited than that of the desired product (3B) a
selectivity enhancement (from 70% to 81%) is obtained by
functionalizing with N-Me-PRO (Figure S7 shows a zoom into
the part of Figure 4b being relevant for this discussion). This
finding can be explained by a dilution effect that is also known
from the field of bimetallic catalysis.43 In order to react an
aromatic ring has to adsorb in a “laying down” geometry, which
requires larger ensembles of adjacent free surface sites than the
adsorption and hydrogenation of the carbonyl group.32,44,45
Ligands, however, dilute adjacent surface atoms and hence
decrease the size of Pt ensembles on the particle surface.10,42 As
a result the probability of reactions of aromatic moieties
decreases with an increasing ligand density on the surface.32
Because hydrogenation of the carbonyl group requires less
adjacent surface atoms than adsorption and reactions of the
phenyl moiety, one may question if for this reaction the
dilution effect could also lead to a simultaneous increase in the
number of sites available for carbonyl hydrogenation. If this
assumption holds the formation rate of phenylethanol (3B) on
N-Me-PRO-Pt NPs, normalized by the number of free surface
atom, should be enhanced compared that on “unprotected” Pt
NPs, which is however not the case (see Figure 4b and Figure
S7). Instead the rate is reduced by almost a factor of 2. We thus
conclude that N-Me-PRO leads to a selective poisoning of
unselective sites by diluting larger surface ensembles, which is
not accompanied by the formation of additional sites for
carbonyl hydrogenation. Comparison of N-Me-PRO- with
PRO-functionalized NPs shows that for the latter no
considerable reaction rates for the undesired hydrogenation
of the aromatic moiety are obtained. This effect could be
explained by the higher ligand coverage of PRO that may
ultimately lead to complete inhibition of phenyl hydrogenation
on PRO-N-Pt NPs. While the presence of a dilution effect can
be concluded to occur on PRO-N-Pt NPs, it does not, however,
account for the fact that the rate for the desired product (3B) is
about 18 times higher on PRO-N-Pt NPs (see Figure 4b).
Instead a second effect must be considered in order to explain
the enhanced catalytic activity. With respect to the homoge-
neous catalysis literature and the above-described testing
strategy we conclude that our findings evidence that for PRO
the amine proton plays an essential role for obtaining high
catalytic activity, by participating within the reaction.40,41 The
enhanced selectivity and catalytic activity may thus be described
Scheme 1. Ligand-Accelerated Hydrogenation of Carbonyls
a
via N−H Effect
a
Amine-bound hydrogen atoms become acidic through coordination
to late transition D-metals (gray sphere). Such protons can interact
with carbonyl oxygen (step 1) and participate within the reaction,
leading to an enhanced hydrogenation rate (step 2). After reaction the
catalyst adsorbs (step 3) and dissociates H2 (step 4) by which the
initial catalyst is re-formed.
F
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Journal of the American Chemical Society
Article
as an interplay of two effects: (i) the high coverage of PRO
leads to inhibition of undesired reactions of the phenyl group
(ii) the presence of a ligand acceleration effect leads to an
enhanced activity toward the desired carbonyl hydrogenation.
Finally, we investigated the stability of the PRO-N Pt NPs
under different conditions as the stability of the ligand shell is a
crucial aspect of such materials. In order to avoid suitable
changes or degradation during catalysis, very mild reaction
conditions are chosen (T = 293 K; P = 20 bar H2). No change
in the catalytic performance was obtained after 850 catalytic
turnovers per ligand. Doubling of the number of turnovers to
1700 still led to more than 99% chemoselectivity. However, the
stereoselectivity decreased to an 2% ee. We thus conclude that
after 1700 turnovers changes have started to occur within the
ligand shell. For thiol-functionalized Pd NPs it has been shown
that storage at air led to altering of the catalytic properties.46
We thus tested the stability of the PRO-functionalized Pt NPs
at ambient conditions. Therefore, supported, PRO-function-
alized Pt NPs were stored at ambient conditions and then
applied as catalysts. After exposure for 1 week in air at room
temperature the enhanced selectivity was completely lost and
the catalytic properties revealed those of the “unprotected” Pt
NPs. We thus conclude that similar as thiol-functionalized Pd
NPs also PRO-N-Pt NPs are not stable over long periods under
ambient conditions. Finally, we tested the ability of recycle the
catalyst. Therefore, a supported PRO-N-Pt NP catalyst with full
chemoselectivity and a stereoselectivity of 14% ee was
separated from the reaction mixture after the catalysis
experiment. The sample was then washed and dried under
vacuum using the same procedure as for the freshly prepared
catalysts (see section 2.3). While the chemoselectivity after
recycling was still higher than 99%, the stereoselectivity
decreased to an ee of 3%, which shows that recycling leads to
changes of the ligand shell.
inhibition of undesired reactions of the phenyl group, by
diluting Pt ensembles. In addition PRO exhibits a ligand
acceleration effect that can be related to a N−H effect, which
leads to an enhanced activity toward the desired carbonyl
hydrogenation.
The presented results demonstrate that ligands can at the
same time positively affect the catalytic activity and selectivity
of NPs. If this finding is indeed related to a N−H effect it
should be feasible to mimic ligand acceleration effects, known
from homogeneous catalysis, on supported NPs thus making
them applicable to heterogeneous catalysts. As next step, we
will aim at using our strategy of functionalizing NPs with
hydrophilic ligands in order to develop heterogeneous catalysts
that are not just very active and chemoselective, but also highly
stereoselective.
ASSOCIATED CONTENT
■
S
* Supporting Information
Representative chromatograms of catalytic experiments,
representative TEM images and particle size analysis, NMR
spectra of PRO-N-functionalized Pt NPs, structures of
additionally tested ligands, results of catalysis experiments
with PRO-Pt NPs in ethanol, and a differently scaled version of
the graph in Figure 4b. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
sebkunz@uni-bremen.de
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The current view on the use of ligands in heterogeneous
catalysis is that they enable for the control of selectivity but at
the expense of activity. The presented results however
demonstrate that this conclusion is not general. Instead both
activity and selectivity can also be enhanced simultaneously,
revealing a so far unknown potential of ligands for their
application in heterogeneous catalysis that needs to be
explored.
The authors gratefully acknowledge Eva Morsbach for TEM
investigations and careful proofreading of the manuscript. Dr.
Normen Szesni (Clariant) and Dr. Jan Deska (University of
Cologne) are thankfully acknowledged for fruitful discussions.
The authors thank Dr. Arne Wittstock for GC-MS measure-
ments. I.S. and S.K. thank the “Fonds der Chemischen
Industrie” (FCI) for financial support.
REFERENCES
4. CONCLUSION
■
(1) Sawamura, M.; Ito, Y. Chem. Rev. 1992, 92, 857.
(2) Berrisford, D. J.; Bolm, C.; Sharpless, K. B. Angew. Chem., Int. Ed.
1995, 34, 1059.
(3) Aliaga, C.; Park, J. Y.; Yamada, Y.; Lee, H. S.; Tsung, C. K.; Yang,
P. D.; Somorjai, G. A. J. Phys. Chem. C 2009, 113, 6150.
(4) Kunz, S.; Iglesia, E. J. Phys. Chem. C 2014, 118, 7468.
(5) Lang, H. F.; May, R. A.; Iversen, B. L.; Chandler, B. D. J. Am.
Chem. Soc. 2003, 125, 14832.
(6) Somorjai, G. A.; McCrea, K. Appl. Catal., A 2001, 222, 3.
(7) Marshall, S. T.; O’Brien, M.; Oetter, B.; Corpuz, A.; Richards, R.
M.; Schwartz, D. K.; Medlin, J. W. Nat. Mater. 2010, 9, 853.
(8) Kunz, S.; Schreiber, P.; Ludwig, M.; Maturi, M. M.; Ackermann,
O.; Tschurl, M.; Heiz, U. Phys. Chem. Chem. Phys. 2013, 15, 19253.
(9) Witte, P. T.; Berben, P. H.; Boland, S.; Boymans, E. H.; Vogt, D.;
Geus, J. W.; Donkervoort, J. G. Top. Catal. 2012, 55, 505.
A recently introduced preparation route for the functionaliza-
tion of “unprotected” Pt nanoparticles with hydrophilic ligands
was applied to synthesize Pt NPs functionalized with L-proline.
Characterization by NMR spectroscopy was performed, which
confirms binding of PRO to the particles and the successful
removal of residual nonbinding ligands from the samples. PRO-
functionalized NPs were supported on Al2O3, investigated as
heterogeneous catalysts for the hydrogenation of acetophenone
and compared to “unprotected” and N-methyl-PRO function-
alized Pt NPs. While a considerable amount of side products is
formed by the “unprotected” Pt NPs, the PRO-functionalized
particles were found to be highly chemoselective. Surprisingly
this enhanced selectivity induced by PRO was not achieved at
the expense of activity, as in the case of N-methyl-PRO
functionalized Pt NPs, but led to an increase of the formation
rate toward the desired product. This finding evidence that the
amine proton of PRO plays an essential role for these activity
enhancements. Based on our results we conclude that the effect
of PRO is twofold. The high coverage of PRO leads to
(10) Altmann, L.; Kunz, S.; Baumer, M. J. Phys. Chem. C 2014, 118,
8925.
(11) Kahsar, K. R.; Schwartz, D. K.; Medlin, J. W. J. Am. Chem. Soc.
2014, 136, 520.
(12) Wang, Y.; Ren, J. W.; Deng, K.; Gui, L. L.; Tang, Y. Q. Chem.
Mater. 2000, 12, 1622.
̈
G
DOI: 10.1021/ja511349p
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(13) Brereton, I. M.; Crozier, S.; Field, J.; Doddrell, D. M. J. Magn.
Reson. 1991, 93, 54.
(14) Perego, C.; Peratello, S. Catal. Today 1999, 52, 133.
(15) Scanlon, J. T.; Willis, D. E. J. Chromatogr. Sci. 1985, 23, 333.
(16) Fu, X. Y.; Wang, Y.; Wu, N. Z.; Gui, L. L.; Tang, Y. Q. J. Colloid
Interface Sci. 2001, 243, 326.
(17) Morsbach, E.; Sped
́
er, J.; Arenz, M.; Brauns, E.; Lang, W.; Kunz,
S.; Baumer, M. Langmuir 2014, 30, 5564.
̈
(18) Wang, X.; Sonstrom, P.; Arndt, D.; Stover, J.; Zielasek, V.;
Borchert, H.; Thiel, K.; Al-Shamery, K.; Baumer, M. J. Catal. 2011,
278, 143.
(19) Ellenber, M.; Pogliani, L. Biochem. Biophys. Res. Commun. 1974,
58, 613.
(20) Kohlmann, O.; Steinmetz, W. E.; Mao, X. A.; Wuelfing, W. P.;
Templeton, A. C.; Murray, R. W.; Johnson, C. S. J. Phys. Chem. B
2001, 105, 8801.
(21) Hens, Z.; Martins, J. C. Chem. Mater. 2013, 25, 1211.
(22) Wang, X. D.; Stover, J.; Zielasek, V.; Altmann, L.; Thiel, K.; Al-
Shamery, K.; Baumer, M.; Borchert, H.; Parisi, J.; Kolny-Olesiak, J.
Langmuir 2011, 27, 11052.
(23) Chen, C. S.; Chen, H. W.; Cheng, W. H. Appl. Catal., A 2003,
248, 117.
(24) Lenarda, M.; Casagrande, M.; Moretti, E.; Storaro, L.; Frattini,
R.; Polizzi, S. Catal. Lett. 2007, 114, 79.
(25) Lin, S. D.; Sanders, D. K.; Vannice, M. A. Appl. Catal., A 1994,
113, 59.
(26) Lin, S. D.; Sanders, D. K.; Vannice, M. A. J. Catal. 1994, 147,
370.
(27) Kunz, S.; Maturi, M. M.; Schrader, I.; Backenkohler, J.; Tschurl,
̈
M.; Heiz, U. J. Colloid Interface Sci. 2014, 426, 264.
(28) Borodzinski, A.; Bond, G. C. Catal. Rev. Sci. Eng. 2008, 50, 379.
(29) Claus, P. Top. Catal. 1998, 5, 51.
(30) Park, J. Y.; Aliaga, C.; Renzas, J. R.; Lee, H.; Somorjai, G. A.
Catal. Lett. 2009, 129, 1.
(31) Hutchings, G. J.; King, F.; Okoye, I. P.; Rochester, C. H. Catal.
Lett. 1994, 23, 127.
(32) Pang, S. H.; Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W.
Nat. Commun. 2013, 4, No. 2448.
(33) Lam, Y. L.; Criado, J.; Boudart, M. Nouv. J. Chim. 1977, 1, 461.
(34) Maki-Arvela, P.; Hajek, J.; Salmi, T.; Murzin, D. Y. Appl. Catal.,
A 2005, 292, 1.
(35) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198,
317.
(36) Garland, M.; Blaser, H. U. J. Am. Chem. Soc. 1990, 112, 7048.
(37) Hess, R.; Vargas, A.; Mallat, T.; Burgi, T.; Baiker, A. J. Catal.
̈
2004, 222, 117.
(38) Hess, R.; Mallat, T.; Baiker, A. J. Catal. 2003, 218, 453.
(39) Baiker, A. J. Mol. Catal. A: Chem. 1997, 115, 473.
(40) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev.
2004, 248, 2201.
(41) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40.
(42) Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W. Acc. Chem.
Res. 2014, 47, 1438.
(43) Sinfelt, J. H.; Carter, J. L.; Yates, D. J. C. J. Catal. 1972, 24, 283.
(44) Campbell, J. M.; Seimanides, S.; Campbell, C. T. J. Phys. Chem.
1989, 93, 815.
(45) Vargas, A.; Reimann, S.; Diezi, S.; Mallat, T.; Baiker, A. J. Mol.
Catal. A: Chem. 2008, 282, 1.
(46) Kahsar, K. R.; Schwartz, D. K.; Medlin, J. W. Appl. Catal., A
2012, 445, 102.
H
DOI: 10.1021/ja511349p
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX