Atomically precise Au25(SR)18 nanoparticles as catalysts for the selective hydrogenation of α,β-unsaturated ketones and aldehydes

Article abstract of DOI:10.1002/anie.200906249

(Chemical equation presented) A golden opportunity: A mechanism has been proposed to account for the chemoselective hydrogenation of α,β- unsaturated ketones (or aldehydes) to unsaturated alcohols catalyzed by monodisperse Au₂₅(SR)₁₈ particles (see picture). Now that the structure of these nanoparticles is known, structure-activity correlations can be drawn.

Full text of DOI:10.1002/anie.200906249

Angewandte
Chemie
DOI: 10.1002/anie.200906249
Gold Nanocatalysts
Atomically Precise Au25(SR)18 Nanoparticles as Catalysts for the
Selective Hydrogenation of a,b-Unsaturated Ketones and Aldehydes**
Yan Zhu, Huifeng Qian, Bethany A. Drake, and Rongchao Jin*
=
Gold nanoparticles have been found to be capable of
catalyzing a variety of reactions, such as selective oxidation
and hydrogenation;[1–3] intense efforts have been made in
recent years in hope of unravelling the origin of the catalytic
properties of gold nanoparticles.[4–7] However, in most studies
the nanoparticles are polydispersed; even in the best case the
particle dispersity is still about 5%. Therefore, the observed
catalytic properties of Au nanoparticles reflect only an
ensemble average. Hitherto, there has been no success in
preparing atomically monodisperse gold nanoparticle cata-
lysts. The polydispersity of Au nanoparticles and their
unknown surface structure preclude the precise correlation
of particle structure and electronic properties with their
catalytic properties. Thus, in order to understand the origin of
their catalytic properties, it is critical to first obtain atomically
precise Au nanoparticles.
We have recently succeeded in preparing atomically
precise, thiolate-stabilized gold nanoparticles (referred to as
Aun(SR)m, where n and m represent the number of gold atoms
and ligands, respectively).[8–11] These ultrasmall nanoparticles
constitute a well-defined system and may be utilized for
catalysis. On the basis of their crystal structures, these Au
particles will permit a correlation of particle structure with
catalytic properties and an identification of catalytically
active sites on the particle; the latter has long been pursued
in nanocatalysis, but active sites are difficult to determine with
conventional polydisperse nanoparticles since the particle
surface structure is unknown.
activation of the C O bond based on literature work,[15,16] and
owing to the low coordination, the exterior-shell Au atoms
may provide a favorable environment for the adsorption and
dissociation of H2.[17] This implies that Au25 particles may be
=
new catalysts for the hydrogenation of the C O bond in
ketones or aldehydes. Our results indeed demonstrate that
thiolate-stabilized Au25 particles can catalyze the hydrogena-
=
tion of the C O bond. More importantly, this work gives
insight into the fundamental aspects of gold nanoparticle
catalysis since we have already solved the crystal structure of
Au25.[12]
Herein, we report on the selective hydrogenation of a,b-
unsaturated ketones (e.g. benzalacetone) as a target reaction
to investigate the catalytic performance of Au25 particles. The
resulting unsaturated alcohol products are valuable inter-
mediates for the production of perfumes and flavors. Pre-
viously, conventional supported gold nanoparticle catalysts
have been demonstrated to be capable of the selective
hydrogenation of a,b-unsaturated ketones; predominantly
a,b-unsaturated alcohols form, but also side products—
=
saturated ketones from C C hydrogenation as well as
saturated alcohols from further hydrogenation—have been
reported.[18–22] Although conventional gold nanoparticles can
achieve high conversion and selectivity (> 90% for the
unsaturated alcohol) in the hydrogenation of a,b-unsaturated
ketones, the polydispersity of the Au nanoparticles precludes
further studies on the effects of the electronic properties of
the nanoparticles on the catalytic performance as well as the
nature of the gold atoms at the metal–support interface.
Moreover, conventional gold nanoparticles cannot achieve
100% chemoselectivity for the unsaturated alcohol.[22] In our
work, we demonstrate that the ultrasmall Au25(SR)18 particles
(0.97 nm metal-core diameter) can indeed catalyze the
Among the series of Aun(SR)m nanoparticles, we have
solved the crystal structure of Au25 nanoparticles stabilized by
thiolate ligands (Au25(SR)18, where SR denotes thiolate) and
also have studied their electronic structure.[12–14] As these
particles are well defined and their crystal structure is known,
we should be able to study some fundamental aspects of gold
nanoparticle catalysis. The Au25 structure[12] can be viewed as
a Au13 icosahedral core (which is electron-rich) encapsulated
by an incomplete shell consisting of the exterior 12 gold atoms
(which are electron-deficient).[12–14] We speculate that the
electron-rich Au13 core may facilitate the adsorption and
=
hydrogenation of the C O bond in a,b-unsaturated ketones
or aldehydes with 100% chemoselectivity for a,b-unsaturated
alcohols. Interestingly, the Au25(SR)18 particles are catalyti-
cally active for hydrogenation reactions even at low temper-
atures (e.g. 08C), which is not possible with conventional Au
nanoparticles.
The catalytic reaction was carried out at 08C (or room
temperature) in the solution phase and initiated by introduc-
ing H2 at atmospheric pressure. Both free (unsupported) and
oxide-supported Au25(SR)18 catalysts were evaluated. The
reaction product was analyzed by NMR spectroscopy and
GC-MS (Figure 1). NMR analysis identified two components:
the unsaturated alcohol product, which shows signals at d =
6.55 (b-CH), 6.25 (a-CH), 4.46 (CH-OH), and 1.35 ppm
(CH3), and unconverted benzalacetone, which shows peaks at
d = 7.51 ppm (b-CH), 6.70 (a-CH), and 2.32 ppm (CH3). The
integrated peak areas and the coupling constants confirm the
[*] Dr. Y. Zhu, H. Qian, B. A. Drake, Prof. R. Jin
Department of Chemistry, Carnegie Mellon University
4400 Fifth Avenue, Pittsburgh, PA 15213 (USA)
Fax: (+1)412-268-1061
E-mail: rongchao@andrew.cmu.edu
Homepage: http://www.chem.cmu.edu/groups/jin/
[**] We are grateful for financial support from CMU, AFOSR, and
NIOSH.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906249.
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their atomic packing structure and electronic properties (see
the discussion below). We also investigated the effect of the
type of thiolate on the Au25(SR)18 particles (e.g. aromatic
thiolate vs. long-chain-alkane thiolate); no significant differ-
ences were found in terms of the conversion yield and product
selectivity (compare entries 1 and 2, Table 1).
The complete selectivity for the unsaturated alcohol
product obtained with unsupported Au25(SR)18 particles is
remarkable. For real-world applications, it would be desirable
to use supported catalysts for the ease of recycling and reuse
in catalytic reactions. Therefore, we prepared oxide-sup-
ported catalysts simply by soaking the oxide powder in a
CH2Cl2 solution of Au25(SR)18 particles, followed by drying
(no calcination). The as-obtained Au25(SR)18/oxide catalysts
were used directly in the reactions. Interestingly, we found
that the metal oxide (e.g. Fe2O3, TiO2) supported Au25(SR)18
catalysts show significantly higher conversion yields (Table 1,
entries 3 and 4) yet the same complete selectivity for the
unsaturated alcohol. In contrast, the catalyst on the inert
oxide support (SiO2) showed no enhancement in activity
(conversion ꢀ 19%, entry 5, Table 1). In all cases, no satu-
rated ketone or saturated alcohol was identified by NMR
analysis (Figure S1 in the Supporting Information) and GC-
MS.
Kinetic studies show that all the three supported catalysts
provide maximum conversion at a 3 hour reaction time
(Figure S2 in the Supporting Information); the order of the
reaction rate, Au25/Fe2O3 > Au25/TiO2 > Au25/SiO2, is consis-
tent with the order of the activity of the catalysts. In both free
and supported catalyst systems, longer reaction times were
found not to lead to any increase in the conversion (Figure S2
in the Supporting Information), indicating that an equilibrium
is reached. Indeed, when an additional benzalacetone was
added to the mixture, a similar conversion (about 20% for
unsupported catalysts and roughly 40% for supported
catalysts) of the newly added reactants was obtained,
indicating that the catalysts are not deactivated. We also
investigated the recyclability of the catalysts (Figure S2 in the
Supporting Information). After six cycles, the supported
Au25(SR)18 catalysts only show a slight decrease in activity;
importantly, the selectivity for unsaturated alcohol remains at
100% throughout the six cycles. Similar results were obtained
with unsupported Au25(SR)18 catalysts. The robustness of the
Au25(SR)18 nanoparticle catalysts is of particular importance
for their real-world use.
Figure 1. 1H NMR spectrum, GC trace (inset), and mass spectra
(smaller insets) of the crude product arising from the hydrogenation
of benzalacetone with the Au25 catalyst. Two species (unsaturated
alcohol and residual starting material) are identified.
assignment. No saturated ketone or saturated alcohol product
was detected by NMR spectroscopy. This is also consistent
with GC-MS analysis. Only two species were detected by GC,
and MS analysis shows that the first eluted species (13.9 min)
has a mass signal at m/z 148 (assigned to the unsaturated
alcohol), and that the residual benzalacetone (eluting at
14.4 min) shows a signal at m/z 145. The conversion and
selectivity are quantified on the basis of NMR and GC
analysis. For the unsupported Au25(SC2H4Ph)18 catalyst, the
conversion yield (3 h reaction time) is 22% with 100%
selectivity for the unsaturated alcohol product (Table 1,
entry 1).
Table 1: The catalytic performance of Au25(SR)18 catalysts for the
selective hydrogenation of benzalacetone with H2.
Entry
Catalyst
Conv. [%]
Selectivity [%]
UA[a]
SK[b]
SA[c]
1
2
3
4
5
6
Au25(SCH2CH2Ph)18
Au25(SC12H25)18
Au25(SCH2CH2Ph)18/Fe2O3
Au25(SCH2CH2Ph)18/TiO2
Au25(SCH2CH2Ph)18/SiO2
Au nanocrystals[d]
22
20
43
40
19
0
100
100
100
100
100
–
0
0
0
0
0
–
0
0
0
0
0
–
Reaction conditions: 1 mg Au25 nanoparticle catalyst (or 100 mg oxide
powder loaded with 1% wt Au25(SR)18 particles), and 0.1 mmol benza-
lacetone in a mixture of toluene (5 mL) and ethanol (5 mL) stirred at 08C
for 3 h under an atmosphere of H2. [a] UA: unsaturated alcohol. [b] SK:
saturated ketone. [c] SA: saturated alcohol. [d] Nanocrystals are roughly
3 nm in diameter.
To confirm that the Au25(SR)18 nanoparticles do not
fragment during the catalytic reaction, we compared the
starting catalysts to those after reaction by employing UV/Vis
spectroscopy and mass spectrometry. The Au25(SR)18 nano-
particles show multiple absorption bands at 670, 450, and
400 nm (Figure 2A), and these bands can serve as spectro-
scopic fingerprints for comparison purposes; no spectral
changes were found after the catalytic reaction. MS analysis
(Figure 2B) also confirms that the particles before and after
reaction show identical signals at 7394 Da (theoretical mass of
Au25(SC2H4Ph)18: 7394 Da). Taken together, these results
confirm that the Au25(SR)18 particles remain intact during the
reaction; this finding is crucial in understanding the origin of
catalysis by Au25(SR)18 particles.
Interestingly, thiolate-capped Au nanoparticles roughly
3 nm in diameter (note that these large particles are not
atomically monodisperse) are not catalytically active under
our experimental conditions (Table 1, entry 6, also see NMR
results in Figure S1 in the Supporting Information). These
results demonstrate the extraordinary catalytic performance
of ultrasmall Au25(SR)18 particles, which should be related to
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discrete, molecule-like electronic structure, which is in strik-
ing contrast to the quasi-continuous electronic band structure
of Au nanocrystals (e.g. the roughly 3 nm diameter particles
used in this work). The multiband UV/Vis spectrum of
Au25(SR)18 (Figure 2A) is indeed a manifestation of the
discrete electronic structure of the particles. DFT calculations
assigned the 670 nm absorption band to the HOMO–LUMO
transition; note that the HOMO is nearly triply degenerate
(excluding spin degeneracy). DFT calculations[14,23] also show
a major difference in charge distribution on the Au13 core and
the Au12 shell: the superatomic Au13 core possess 8 valence
electrons (1s21p6) and is electron rich, while the exterior Au12
shell is electron deficient as a result of electron transfer to
sulfur of the thiolate ligand. In contrast to molecule-like
Au25(SR)18 nanoparticles, the roughly 3 nm Au particles used
in our study are crystalline with fcc packing and show a
surface plasmon absorption band at 520 nm, indicating that
the 3 nm gold core is already metallic, as opposed to the
semiconducting Au25 particles. We propose that the discrete
electronic structure of Au25(SR)18 and its incomplete Au12
exterior shell are responsible for the extraordinary selectivity
and activity observed. The eight uncapped Au3 faces of the
Figure 2. A) UV/Vis spectra of Au25(SCH2CH2Ph)18 nanoparticles
before (a) and after (b) the hydrogenation reaction, and b) after
reaction; the inset shows the crystal structure of Au25(SCH2CH2Ph)18.
B) ESI mass spectra of Au25(SCH2CH2Ph)18 before reaction (upper
panel) and after reaction (lower panel), respectively; The inset shows
the isotope pattern of Au25(SCH2CH2Ph)18.
=
The catalytic properties of intact Au25(SR)18 nanoparticles
are inspiring and correlate with the particleꢀs atomic packing
structure and electronic properties, as we envisioned. On the
basis of the crystal structure of Au25(SR)18 particles,[12] we
propose a mechanism for the Au25(SR)18-catalyzed selective
hydrogenation of a,b-unsaturated ketones to unsaturated
alcohols. The Au25 particle possesses a core–shell structure
(Figure 2A inset and Scheme 1). Note that there are 20
triangular faces on the Au13 icosahedral core. Only 12 facets
are face-capped by the exterior 12 Au atoms; thus, eight facets
are left open (Scheme 1). These “hole” sites may act as active
icosahedron should favor adsorption of the C O group by
=
interaction with the O atom of the C O group in the a,b-
unsaturated ketone; electron transfer between Au13 and the O
atom would activate the C O bond. Subsequently, the weakly
nucleophilic hydrogen attacks the activated C O group,
leading to the unsaturated alcohol product. Overall, the
presence of the electron-rich gold core (Au13) favors the
selective hydrogenation of the C O bond over the C C bond
in a,b-unsaturated ketone.
=
=
=
=
As for H2 activation, it is thought that the hydrogenation
activity of gold catalysts is determined by the step of H2
dissociation.[24] Theoretical studies have suggested that gold
atoms in corner or edge positions can readily activate
molecular hydrogen.[25,26] The necessary condition for H2
dissociation is the existence of low-coordinate Au atoms.
Corma et al.[17] presented evidence that H2 adsorbs and
dissociates with small activation barriers on low-coordinate
Au atoms situated in the corner positions of Au nanoparticles.
Their study supports the idea that a single low-coordinate Au
atom can simultaneously interact with the two hydrogen
atoms of H2, and after dissociation, the two H atoms are in
bridge positions sharing the low-coordinate gold atom. In the
case of Au25 particles, the surface Au atomsꢀ low-coordination
character (coordination number N = 3, c.f. N = 12 in bulk
gold) should provide a favorable environment for the
adsorption and dissociation of H2, and H2 dissociation
should occur on the Au atoms of the exterior shell
(Scheme 1). The two H atoms form two nearly symmetrical
Au-H-Au bridges (Scheme 1) based on the study by Corma
et al.[17] It is important to note that during the activation/
reaction process catalyzed by Au25(SR)18, H2 dissociation does
not induce any significant deformation of the Au25(SR)18
catalyst surface (Figure 2); in other words, the Au25(SR)18
particle retains its integrity (otherwise one would observe
changes in the UV/Vis spectrum).
=
sites for C O activation (Scheme 1). A recent theoretical
=
study suggests that the C O group can coordinate to the
surface atoms of Au13 clusters, and charge-transfer results in a
negatively charged Au13 core.[15] Experimental work on the
Aun-catalyzed oxidation of CO with O2 demonstrates that Aun
particles donate charge to CO and hence, the Aun core
exhibits high catalytic activity.[16] We believe that a similar
=
charge-transfer effect in our system leads to C O activation.
With respect to the electronic structure of the Au25(SR)18
nanoparticles, previous DFT calculations[12–14,23] revealed a
Scheme 1. The proposed mechanism of the chemoselective hydrogena-
tion of a,b-unsaturated ketone to unsaturated alcohol catalyzed by
Au25(SR)18 nanoparticles. For clarity, the thiolate ligands are not
shown. Dark grey: Au atoms of the core; light gray: Au atoms of the
shell.
To further support our attribution of the catalytic proper-
ties of Au25(SR)18 nanoparticles to the unique geometrical and
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ature (no difference in catalytic activity or selectivity was found). The
reaction was typically allowed to proceed under continuous H2 flow
for roughly 3 h (unless otherwise noted). The crude products were
1
analyzed by H NMR spectroscopy and GC-MS. NMR spectroscopy
was conducted on a Bruker Avance 300 MHz instrument. GC mass
spectra were obtained using a Thermo Finnigan Trace GC 2000 and
an Rxi-XLB chromatographic column.
Scheme 2. Au25(SR)18-catalyzed hydrogenation of a range of substituted
a,b-unsaturated ketones and aldehydes. X=H or CH3, Y= H, CH3, or
Ph, and Z=H or CH3.
Received: November 6, 2009
Published online: January 14, 2010
electronic structures of these particles, we have further
investigated a number of representative substituted a,b-
unsaturated ketones and aldehydes (Scheme 2, Table S1 in
the Supporting Information). Complete (100%) selectivity
was observed in all cases except crotonadehyde (91%
selectivity for the allylic alcohol). For future work, we plan
to perform full density functional theory (DFT) calculations
incorporating a,b-unsaturated ketone (aldehyde) and H2 to
Keywords: gold · heterogeneous catalysis · hydrogenation ·
nanoparticles · structure–activity relationships
.
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Experimental Section
Au25(SR)18 catalysts: Au25(SR)18 nanoparticles were synthesized
following a kinetically controlled approach.[8] UV/Vis absorption
spectra (190–1100 nm) were recorded using a Hewlett-Packard (HP)
8453 diode-array spectrophotometer. Electrospray ionization mass
spectra were acquired using a Waters Q-TOF mass spectrometer
equipped with Z-spray source. The as-prepared nanoparticles were
used as homogeneous catalysts, and the supported catalysts were
prepared as follows: 100 mg of the oxide support (in powder form)
was impregnated by soaking in a solution of 1 mg Au25(SR)18
nanoparticles in CH2Cl2 (ca. 10 mL) in a sealed vial for one day,
followed by drying; no calcination was done.
Catalyst testing: In a 50 mL three-neck glass flask benzalacetone
(0.1 mmol) was dissolved in a mixed solvent (5 mL toluene and 5 mL
ethanol). The gold catalyst, either 1 mg Au25(SR)18 (unsupported), or
100 mg catalyst on an oxide support (1% wt loading of Au25(SR)18)
was added. The reaction was initiated by introducing a H2 flow (under
atmospheric pressure). Reaction temperature: 08C or room temper-
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