Selective hydrogenation of α,β-unsaturated aldehydes catalyzed by amine-capped platinum-cobalt nanocrystals

Article abstract of DOI:10.1002/anie.201108593

More Greasy, More Selective: Amine-capped Pt₃Co nanocatalysts were synthesized and used for the hydrogenation of cinnamaldehyde (CAL). Capping the catalysts with amines that contain long carbon chains results in an ordered surface "array" (see scheme), in which high selectivity towards Ci-O hydrogenation can be achieved because the Ci-C bond in CAL does not interact with the surface. The longer the carbon chains in the amine, the higher the selectivity. Copyright

Full text of DOI:10.1002/anie.201108593

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Angewandte
Communications
DOI: 10.1002/anie.201108593
Heterogeneous Catalysis
Selective Hydrogenation of a,b-Unsaturated Aldehydes Catalyzed by
Amine-Capped Platinum-Cobalt Nanocrystals**
Binghui Wu, Huaqi Huang, Jing Yang, Nanfeng Zheng,* and Gang Fu*
Nanoparticle-based metal catalysts, particularly those that
have well-defined size/shape, have attracted increasing
research attention for catalytic applications.[1–4] By means of
colloidal chemistry, monodisperse nanoparticles with con-
trolled size, morphology, and composition can be synthesized
through the use of stabilizing ligands.[5–11] These shell ligands
were formerly considered to hinder the catalytic/electro-
catalytic activity, and were typically removed (for example, by
calcination, ligand exchange, UV-ozone treatment) to eval-
uate the size/shape effect or metal-support interaction in
catalysis/electrocatalysis.[6–11] However, it should be noted
that many studies have demonstrated that capping ligands on
the surface of nanoparticles can effectively steer the chemo-
selectivity and enantioselectivity of nanoparticle-catalyzed
liquid-phase reactions.[12–15] For instance, self-assembled mon-
olayer coatings of n-alkane thiols improve the selectivity of 1-
epoxybutane formation from 1-epoxy-3-butene on Pd nano-
catalysts from 11% to 94% under equivalent reaction
conditions and with equivalent conversions.[16] Enantioselec-
tive allylic alkylation reactions were achieved with 97% ee by
using Pd nanoparticles stabilized by a chiral xylofuranoside
diphosphite.[17]
In this study, the chemoselective hydrogenation of a,b-
unsaturated aldehydes (that is, cinnamaldehyde (CAL) and
citral) was chosen as the model reaction for studies on the
effect of capping ligands on the selectivity of nanocrystal
catalysts. As the formation of the saturated aldehydes is
favored over the unsaturated alcohol because of thermody-
namics,[18] the chemoselective hydrogenation of a,b-unsatu-
rated aldehydes to the corresponding unsaturated alcohols is
a scientific challenge.[19–26] Significant efforts have been made
to search for catalytic systems that are able to efficiently and
and achieve high selectivity because of electronic and
structural effects.[26,28,29,31–36] In contrast, the effect of the
surface ligands on the catalytic selectivity is less well-studied
and is usually ignored. Herein, we demonstrate how the
amine capping layer of Pt3Co nanocatalysts influences the
chemoselective hydrogenation of a,b-unsaturated aldehydes.
Our studies reveal that amines that have longer chain lengths
offer enhanced hydrogenation selectivity to a,b-unsaturated
alcohols and also stability against over-hydrogenation. A
steric effect from the long-chain amine is proposed to explain
the enhanced selectivity.
Monodisperse, alkylamine-capped Pt3Co alloy nanocrys-
tals were prepared by using carbon monoxide as reducing
agent according to a modified method that was described
previously (see the Experimental Section and the Supporting
Information for details).[37] As detected by energy-dispersive
X-ray spectroscopy (EDX) and inductively coupled plasma
mass spectrometry (ICP-MS), the Pt/Co ratio in the as-
synthesized nanoparticles was approximately 3:1. As shown
in the typical TEM images (Figure 1; see also Figure S1 in the
Supporting Information), the as-synthesized oleylamine
(OAm)-capped Pt3Co nanoparticles were uniform in size
and nearly all have a truncated octahedral shape (Figure S2 in
=
selectively hydrogenate a C O bond that is conjugated with
[23,26–30]
=
a C C bond.
Among these, Pt nanoparticles modified
with Fe, Sn, or Co were demonstrated as effective catalysts
[*] B. H. Wu, H. Q. Huang, Dr. J. Yang, Prof. N. F. Zheng, Prof. G. Fu
State Key Laboratory for Physical Chemistry of Solid Surfaces and
Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005 (China)
E-mail: nfzheng@xmu.edu.cn
gfu@xmu.edu.cn
Homepage: http://chem.xmu.edu.cn/person/nfzheng/index.html
[**] We thank the MOST of China (2011CB932403, 2009CB930703), the
NSFC (21131005, 21133004, 21021061, 20925103, 20973139), the
Fok Ying Tung Education Foundation (121011), the NSF of Fujian
Province (Distinguished Young Investigator Grant 2009J06005), and
the Fundamental Research Funds for the Central Universities for
financial support.
Figure 1. TEM images of 8.2 nm Pt3Co nanoparticles. a) A monolayer
assembly. b) A multilayer assembly. c) HRTEM image of a single Pt3Co
nanoparticle along the [110] zone axis. Inset in (b) is a model of
truncated octahedron along the [110] zone axis with two vertices in the
h001i direction cutting off from the octahedron.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108593.
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the Supporting Information). The average distance between
the two opposite faces of the as-prepared nanoparticles was
8.2 nm. The single-crystal nature of the nanoparticles was
clearly revealed by the well-resolved lattice fringes in the
high-resolution TEM (HRTEM) images (Figure 1c). The
average crystalline size that was estimated from Scherrerꢀs
formula by using the X-ray diffraction (111) peak (Figure S3)
was approximately 8 nm, which is consistent with the
dimensions shown in the TEM images.
The use of an amine as the capping agent is important to
obtain the monodisperse Pt3Co nanocrystals and to stabilize
the particles against aggregation. Even after three careful
purification cycles, the amine capping makes the Pt3Co still
highly dispersible in nonpolar solvents. Based on a simple
truncated octahedral model, an 8 nm Pt3Co particle has
approximately 3000 metal atoms on its surface. According to
the data from CNH elemental analysis and thermogravimet-
ric analysis,[38–39] the number of OAm molecules that are
coated on each Pt3Co nanoparticle is approximately 490.
Therefore, the coverage of amine molecules on the Pt3Co
particles is estimated to be approximately 16%. By using
density functional calculation (DFT) calculations (see the
Supporting Information for computational details), we found
that OAm can form an ordered 2 2 ꢀ 2 2 “array” on the Pt
surfaces at q = 0.125 (Figure S4 in the Supporting Informa-
tion). Two possible adsorption modes have been considered,
and the differences between them are the conformation of
OAm, as well as the tilt angle a (Figure S5 in the Supporting
Information). Although the eclipsed conformation of OAm is
less stable than the staggered conformation in the gas phase,
our calculations reveal that on the Pt surface, the eclipsed
conformation is favored over the staggered conformation by
0.3–0.4 eV/OAm (see Table S1 in the Supporting Informa-
tion). This is reasonable because the eclipsed conformation
benefits more from the van der Waals interactions among the
adsorbates. In this case, the long carbon chains that are
capped onto the Pt3Co nanocatalysts impart steric hindrance
so that CAL molecules do not lie flat on the nanoparticle
surface. Computationally, we found that CAL molecules can
only enter into the array of OAm molecules edge on with
their aldehyde groups interacting with the Pt3Co (100) surface
Figure 2. a) Optimized structure of CAL adsorption (ball-and-stick) on
the Pt3Co(100) surface capped by OAm (line). Key bond distances:
pffiffi
pffiffi
*
RCꢁO =1.370 ꢀ; RCꢁC =1.356 ꢀ. b) Curves of conversion of CAL ( ) and
~
&
selectivity for COL (N), HCAL ( ), and HCOL ( ) from the hydro-
genation of CAL with OAm-capped Pt3Co nanocatalysts. Conditions:
Pt3Co nanocatalysts (11 mg), n-butanol (10 mL), CAL (3 mmol),
nonane (1 mmol), H2 (1.5 atm), 258C.
What was even more surprising was that the selectivity for
COL was not reduced much by lengthening the reaction time.
When the reaction time was extended to 24 h, the selectivity
for COL was still above 90%. In contrast, the small amount of
HCAL that was produced was converted to HCOL by further
reduction of the carbonyl group. These results suggest that the
=
=
hydrogenation of C C and C O bonds does not take place in
parallel on the OAm-capped Pt3Co nanoparticles, which is
different from previous reports.[27,33] Being able to prevent the
=
C C bonds of COL from further hydrogenation over a long
period suggests that the steric effect that is created by the
surface OAm molecules cannot be easily destroyed under the
reaction conditions. Even when high temperature (808C) and
high H2 pressure (4 atm) was applied to the reactions
(Figure S6 in the Supporting Information), the selectivity
for COL was over 95%, which confirms that the steric effect
created by the OAm molecules on the surface is fairly stable.
It should be noted that the steric effect of OAm on the
catalytic selectivity was also effective in the hydrogenation of
other a,b-unsaturated aldehydes. For example, as a natural
product, citral is a mixture of two isomeric forms: 50% neral
(cis-form) and 50% geranial (trans-form, Figure 3). When
OAm-capped Pt3Co nanoparticles were applied as the
=
and the C C bonds directed away from the catalytically active
surface (Figure 2a). The predicted adsorption energy is
=
ꢁ0.63 eV relative to the OAm-capped surface, and the C O
=
bond is elongated to 1.370 ꢁ, which indicates that the C O
bonds in CAL molecules can be hydrogenated more easily
=
than the C C bond. To demonstrate such chemoselectivity,
we carried out the hydrogenation under H2 (1.5 atm) at 258C
catalyzed by 8.2 nm OAm-capped Pt3Co nanoparticles. The
conditions were much milder than many previous reports, in
which high temperature (808C or more) and high pressure
(for example, 20 bars) were used.[28,29] As illustrated in
Figure 2b, our OAm capped Pt3Co nanoparticles catalyzed
the hydrogenation of CAL. Within 9 h, the hydrogenation of
CAL was completed with a selectivity for the cinnamyl
alcohol (COL) of up to 95%. The selectivity for the undesired
saturated aldehyde hydrocinnamaldehyde (HCAL) and the
saturated alcohol hydrocinnamyl alcohol (HCOL) remained
low, which supports our theoretical prediction.
=
catalysts under the mild reaction conditions, the C O bonds
of both isomeric forms were easily hydrogenated with
a selectivity of above 90%. However, the OAm-capped
Pt3Co particles were more active in the hydrogenation of
geranial than neral, which can be explained by a larger steric
hindrance for neral in the OAm capping layer. The results
from the hydrogenation of citral reinforce our hypothesis on
the steric effect of the amine capping agents on the catalytic
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&
*
Figure 3. Curves of conversion of neral ( ) and geranial ( ) and total
=
C O selectivity (N) for products from the hydrogenation of citral with
OAm-capped Pt3Co nanocatalysts. Conditions: Pt3Co nanocatalysts
(11 mg), n-butanol (10 mL), citral (3 mmol, 1:1 neral and geranial),
nonane (1 mmol), H2 (1.5 atm), 258C.
performance of OAm-capped Pt3Co nanoparticles (Fig-
ure 2a).
OAm can help to prevent CAL from lying flat on the
metal surface, which leads to high selectivity towards COL.
Now the question emerges as to how CAL interacts with the
catalytically active sites when a shorter-chain amine is used. In
this case, a C4NH2/Pt3Co system was used as an instructive
case. DFT (PBE/PAW) calculations demonstrate that the
adsorption energies for C4NH2 onto different Pt3Co surfaces
are nearly equal to those for OAm (see Table S1 in the
Supporting Information). However, when dispersion inter-
actions are taken into account, that is, DFT-D, C4NH2
becomes 0.3–1.0 eV per amine molecule less favorable than
OAm, as the latter has a longer carbon chain. This indicates
that, unlike OAm, C4NH2 is liable to diffuse on the surface or
to be liberated into the solvent, which might, in turn, destroy
the ordered “array”. We also estimated the rotation barriers
Figure 4. Histograms of a) Selectivity for COL (white), HCAL (cross-
hatch), and HCOL (gray) at nearly 100% conversion in the hydro-
genation of CAL with different amine-capped Pt3Co catalysts. b) Rates
of HCOL production after complete conversion of CAL with different
amine-capped Pt3Co nanocatalysts. Conditions: Pt3Co nanocatalysts
(11 mg), n-butanol (10 mL), CAL (3 mmol), nonane (1 mmol), H2
(1.5 atm), 258C.
Figure S9 in the Supporting Information; see also the
Supporting Information for details of the synthesis).
Figure 4a shows the ligand-dependent selectivity in the
catalytic hydrogenation of CAL. The results reveal that the
selectivity for COL gradually decreases with the shortening of
the chain length of the amines that are coated onto the Pt3Co
nanocrystals. The Pt3Co nanocatalysts that are coated with
C18NH2, OAm, C16NH2, and C12NH2 catalyzed the hydro-
genation with a high selectivity of over 90% for COL at
100% conversion of CAL. In comparison, the Pt3Co particles
that are capped by C8NH2 gave a medium selectivity for COL
at approximately 78%. The C4NH2-capped nanocatalysts led
to the lowest selectivity at approximately 23% at 100%
conversion of CAL. The relatively low selectivity for COL by
ꢁ
of both OAm and C4NH2 around the C N bond on the Pt3Co
(100) surface (Figure S7 in the Supporting Information).
Interestingly, we found that converting from the eclipsed
conformation into the staggered conformation that is,
increasing f from approximately 808 to 1608, has to overcome
a barrier of 1.4 eV per amine molecule, which means that the
conformation of the OAm array is rather rigid once formed.
In contrast, the conformation of C4NH2 on the surface is
rather fluxional, as the predicted barrier for such a rotation is
only approximately 0.1 eV per amine molecule. All these
results indicate that when shorter-chain amines are used as
capping agents, the a,b-unsaturated aldehyde are still able to
lie flat on the surface of the metal. Indeed, we did locate
a structure in which CAL can adsorb onto C4NH2/Pt3Co (100)
=
=
C4NH2-capped nanoparticles implies that both C O and C C
bonds can competitively access the metal surface during the
reaction. When shorter-chain amines (that is, C8NH2 or
C4NH2) were used as the capping ligands, the catalysts
easily hydrogenated COL further into HCOL over time
(Figure 4b; see also Figures S10 and S11 in the Supporting
Information). The shorter the chain, the higher the yield of
over-hydrogenated products. Whereas the shorter chain
length of the capping amines is deleterious to the selectivity
for COL, it is beneficial to the diffusion of CAL molecules
onto the catalytic metal surface and, thus, enhances the
hydrogenation activity. Indeed, our studies reveal that the
catalytic activity by Pt3Co nanoparticles capped by different
amines is highly dependent on the chain length of the capping
amines (Figure S12 in the Supporting Information). A
shorter-chain capping amine offers higher activity in the
hydrogenation.
=
at q = 0.125 through the C C bond of CAL (Figure S8 in the
=
Supporting Information). In this case, the C C bond is
significantly activated (RCꢁC = 1.444 ꢁ), and the calculated
adsorption energy is ꢁ1.30 eV. To prove this experimentally,
Pt3Co nanocrystals with a similar size, shape, and composition
but capped by various amines (C18NH2, C16NH2, C12NH2)
were synthesized under conditions similar to OAm-capped
Pt3Co nanoparticles, or by ligand exchange with OAm-capped
nanoparticles with the target amines (C8NH2 or C4NH2;
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Pt3Co-alloy nanoparticles can be used as efficient catalysts for
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under mild conditions. Detailed studies and periodic DFT
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selectivity of the hydrogenation towards a,b-unsaturated
alcohols. A steric effect in long-chain amines prevents a,b-
unsaturated aldehydes lying flat on the surface and, therefore,
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avoids direct contact of the C C bonds with the catalytic
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Experimental Section
8.2 nm OAm-capped Pt3Co nanocatalysts were synthesized as fol-
lows: [Pt(acac)2] (20 mg) and [Co(acac)2] (10 mg) were dissolved in
OAm (10 mL) in a flask that was immersed in a water bath at 508C,
and stirred for 10 min (simultaneously a CO flow was applied into the
flask to remove air). The outside of the flask was dried and the flask
was directly immersed into an oil bath that was preheated to 2408C.
The mixture was kept stirring at 2408C with a CO flow (ca. 50 mL
minꢁ1) for 40 min. The black dispersion was then cooled to room
temperature, precipitated out with ethanol, and washed twice with
cyclohexane/ethanol. Other amine-capped Pt3Co nanoparticles were
synthesized by ligand exchange, or under the same conditions
described above, but changing the type of amine.
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Catalytic hydrogenation of CAL was carried out in a well-stirred
glass pressure vessel (48 mL) at room temperature. The Pt3Co
nanocatalysts were dispersed in butanol (5 mL) by ultrasound, and
then transferred into the pressure vessel. A mixture of butanol
(5 mL), nonane (1 mmol, used as an internal standard), and CAL
(3 mmol) was added. H2 flow was applied into the vessel for several
minutes to remove any traces of oxygen. The vessel was pressurized
with H2 (1.5 atm). The reaction was then allowed to proceed and
samples were withdrawn at regular intervals, filtered, and analyzed by
gas chromatography (GC) and gas chromatography-mass spectrom-
etry (GC-MS).
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Received: December 5, 2011
Published online: February 28, 2012
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Keywords: aldehydes · alkenes · hydrogenation · ligand effects ·
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