Highly Active Supported Pt Nanocatalysts Synthesized by Alcohol Reduction towards Hydrogenation of Cinnamaldehyde: Synergy of Metal Valence and Hydroxyl Groups

Article abstract of DOI:10.1002/asia.201500188

The hydrogenation of α,β-unsaturated aldehydes to allylic alcohols or saturated aldehydes provides a typical example to study the catalytic effect on structure-sensitive reactions. In this work, supported platinum nanocatalysts over hydrotalcite were synthesized by an alcohol reduction method. The Pt catalyst prepared by the reduction with a polyol (ethylene glycol) outperforms those prepared with ethanol and methanol in the hydrogenation of cinnamaldehyde. The selectivity towards the C=O bond is the highest over the former, although its mean size of Pt particles is the smallest. The hydroxyl groups on hydrotalcite could act as an internally accessible promoter to enhance the selectivity towards the C=O bond. The optimal Pt catalyst showed a high activity with an initial turnover frequency (TOF) of 2.314s^-1. This work unveils the synergic effect of metal valence and in situ promoter on the chemoselective hydrogenation, which could open up a new direction in designing hydrogenation catalysts. Support your local catalysts: Supported platinum nanocatalysts were synthesized by an alcohol reduction method towards cinnamaldehyde hydrogenation. The findings showed that high activity and selectivity towards the C=O bond were achieved on the small-sized Pt nanoparticles. The synergy of Pt valence and hydroxyl moieties contribute to the high selectivity.

Full text of DOI:10.1002/asia.201500188

DOI: 10.1002/asia.201500188
Full Paper
Heterogeneous catalysis
Highly Active Supported Pt Nanocatalysts Synthesized by Alcohol
Reduction towards Hydrogenation of Cinnamaldehyde: Synergy
of Metal Valence and Hydroxyl Groups
[a, b]
[a]
[a]
[a]
[a]
[b]
Yanyan Wang,
and Feng Li
Wanhong He, Liren Wang, Junjiao Yang, Xu Xiang,* Bing Zhang,*
[a]
Abstract: The hydrogenation of a,b-unsaturated aldehydes
to allylic alcohols or saturated aldehydes provides a typical
example to study the catalytic effect on structure-sensitive
reactions. In this work, supported platinum nanocatalysts
over hydrotalcite were synthesized by an alcohol reduction
method. The Pt catalyst prepared by the reduction with
a polyol (ethylene glycol) outperforms those prepared with
ethanol and methanol in the hydrogenation of cinnamalde-
hyde. The selectivity towards the C=O bond is the highest
over the former, although its mean size of Pt particles is the
smallest. The hydroxyl groups on hydrotalcite could act as
an internally accessible promoter to enhance the selectivity
towards the C=O bond. The optimal Pt catalyst showed
a high activity with an initial turnover frequency (TOF) of
À1
2.314 s . This work unveils the synergic effect of metal va-
lence and in situ promoter on the chemoselective hydroge-
nation, which could open up a new direction in designing
hydrogenation catalysts.
Introduction
of the metals and supports, their high chemical and structural
stability, and excellent recyclability, although their activity may
[2]
The selective hydrogenation of a,b-unsaturated aldehydes to
allylic alcohols and/or saturated aldehydes is of scientific im-
portance and industrial relevance. For instance, the unsaturat-
ed alcohol products are highly sought for in syntheses of nu-
merous fine chemicals such as fragrances, pharmaceuticals, fla-
vorings and so on. From the academic viewpoint, this reaction
provides a good example of catalytic impact on chemoselec-
be inferior compared to their homogeneous counterpart. The
[3]
[4]
[5]
supported noble metals like Pt, Ru, and Ir have been
proved to be excellent heterogeneous catalysts. Many factors
[
3c,6]
affect the catalytic activity and selectivity including the size
[7]
[8]
and shape of metal particles, a second metal, the types of
[9]
[10]
supports, and even the preparation methods. Until now, it
is challenging to achieve high selectivity towards C=O bond
hydrogenation under a high substrate conversion, although
numerous catalysts have been explored.
[
1]
tive reactions. Among the competing attack on either the
conjugated C=C double bond or the carbonyl group (C=O),
the former is thermodynamically favored over most of metals
Recent studies reported on the synthesis of Pt-based mono-
metallic or bimetallic catalysts over carbon materials by
À1
(Pd, Rh, Pt) due to a lower free reaction enthalpy of 35 kJmol
[9a,10b]
than the latter. To overcome the thermodynamic limitation,
great efforts have been made to develop catalysts that are
able to selectively hydrogenate the C=O bond.
a polyol reduction for cinnamaldehyde hydrogenation.
The relatively mild reduction process is favorable in controlling
the sizes and exposed planes of Pt particles. Furthermore, it
d+
Supported metal catalysts have attracted much attention in
the selective hydrogenation owing to the flexible combination
was reported that the cationic Pt species could exist after
d+
the reduction. However, the role of Pt has not been associat-
[9a]
ed with the hydrogenation performance of catalysts. Xu et
3
+
al. found Au species in the Au/ZrO catalyst and confirmed
2
+
+
3+
[
a] Y. Wang, W. He, L. Wang, J. Yang, Prof. Dr. X. Xiang, Prof. Dr. F. Li
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
that isolated surface Au ions were active sites for the selec-
[11]
tive hydrogenation of 1, 3-butadiene. This suggests that cat-
ionic species in the supported catalyst could play a unique
role in selective hydrogenation. Furthermore, it was found that
an alkali promoter like KOH or NaOH can facilitate C=O hydro-
Beijing 100029 (PR China)
E-mail: xiangxu@mail.buct.edu.cn
+
[
b] Y. Wang, Prof. Dr. B. Zhang
School of Chemical Engineering
Zhengzhou University
[3c,12]
genation.
Whether the support can act as a native promot-
er instead of a externally added one is a question that still
needs to be addressed. The remaining issues inspired us to
design the structure and examine the characteristics of cata-
lysts by controlling the metal valence and surface properties of
the support, and finally tune the catalytic performance.
Zhengzhou 450001 (PR China)
E-mail: zhangb@mail.buct.edu.cn
+
[
] These authors contribute equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500188.
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The hydrotalcites, known as MgAl-layered double hydroxides
LDHs), are a class of anionic clays with a layered structure that
with metallic Pt or other Pt species cannot be detected within
the XRD detection limit in all Pt catalysts. This could be due to
the relatively low Pt loading (~2.4 wt%) and/or high dispersion
of the tiny Pt nanoparticles anchored onto the support. The
characteristics of high dispersion and small size of metal parti-
cles are critical to the enhancement in the catalytic activity. In
addition, the specific surface areas were measured. Before the
loading of Pt, the hydrotalcite has a specific surface area of
(
involve positively charged brucite-like layers together with
charge-balancing anions and water molecules in the interlayer
galleries. The cations are surrounded octahedrally by hydroxide
ions. These octahedral units form infinite layers by edge-shar-
[
13]
ing, with the hydroxide ions perpendicular to the layers.
LDHs have been employed as supports to load a variety of cat-
[
14]
2
À1
alytically active metals. The outward hydroxyl groups on the
layers could be ideal as “internally accessible promoter”,
analog to a foreign alkali, to tune the selectivity of hydrogena-
tion products.
91.9 m g , and the supported catalysts Pt/LDH-EG, Pt/LDH-EA,
2
À1
and Pt/LDH-MA have values of 87.8, 88.2, and 87.4 m g , re-
spectively. Accordingly, there is little difference among the Pt-
supported catalysts.
Herein, we explored a mild reduction method to load Pt
nanoparticles on a pre-synthesized LDH support. The findings
Tetradecyl trimethyl ammonium bromide (TTAB) has been re-
ported to act as a stabilizing and structure-directing agent
2
+
[16]
showed that the Pt species exist in the catalysts after reduc-
tion. High activity and selectivity of hydrogenation towards the
C=O bond were achieved on the small-sized Pt nanoparticles.
Thus, we prove, for the first time, that the synergy of Pt va-
lence and in situ alkali promoter contributes to the high selec-
tivity towards the C=O bond.
during the synthesis of metal nanocrystals. The addition of
TTAB can mediate the reduction and growth rate of the crystal-
lographic planes with distinct lattice energy, leading to a con-
trolled morphology of the reduced metal nanocrystals. The
sizes and morphology of Pt nanoparticles were observed by
transmission electron microscopy (TEM) in order to determine
the effects of TTAB and various reducing agents. Firstly, the
effect of TTAB was studied using ethylene glycol (EG) as a re-
ducing agent. Pt/LDH-EG shows a slightly smaller Pt mean size
Results and Discussion
(
2.1 nm, Figure 2b) than that of Pt/LDH (2.5 nm, Figure 2a).
Characterization of the catalysts
However, the former has a narrower size distribution. More
than 30% of Pt particles are at or above 3 nm in the latter. No
obvious aggregates were observed in the Pt/LDH-EG, thus sug-
gesting excellent dispersion of reduced Pt nanoparticles on
the LDH support. It is known that EG, as a mild reducing
agent, could mediate the reduction rate and growth kinetics of
The phase structure of the catalysts was studied by XRD. The
XRD patterns of the as-prepared samples are shown in
Figure 1. The patterns display characteristic (00l) diffractions
and (110), (113) ones, which can be well indexed to hydrotal-
cite-like LDH materials. The d003 basal spacing is 7.68 Š, in
good agreement with the carbonate-intercalated LDH reported
[17]
metal nanocrystals, thus resulting in narrow size distribution.
Besides, TTAB can also tune the reduction kinetics of Pt parti-
cles, mainly by the stabilizing effect of bromine ions on the ex-
[
13]
in the literature. The diffractions and the d003 basal spacing
of the LDH support (Figure 1a) are almost the same with those
of the loaded Pt nanoparticles (Figure 1b–e). This result sug-
[18]
posed crystal planes of metal. Consequently, the difference
between Pt/LDH and Pt/LDH-EG in the size distribution could
be ascribed to the cooperation effect of EG and TTAB.
2À
gested that the CO3 anions in the interlayer galleries of LDH
are more stable and could not be easily replaced by the reac-
Secondly, we examined the Pt catalysts obtained using vari-
ous alcohols under the same reduction conditions. Compared
to Pt/LDH-EG, Pt/LDH-EA and Pt/LDH-MA reduced by ethanol
and methanol, respectively, have a larger Pt mean size of 3.6
and 2.8 nm (Figure 2c and 2d), and broader size distribution
as well. A small amount of aggregates was observed in both
catalysts. The findings reveal that EG, as a polyol, is superior to
the mono-hydroxyl alcohols for mono-dispersed Pt particles.
Furthermore, the EDS mappings show that the Pt nanoparti-
cles are highly dispersed on the support in Pt/LDH-EG (Fig-
ure S1, Supporting Information). The size and monodispersity
of metal particles are critical factors for the catalytic per-
formance. The polyol (EG) reduction is a more favorable way
to achieve Pt nanoparticles with a smaller size and narrower
size distribution.
2À
tant PtCl6 anions. Thus, after the reaction, the reduced Pt
nanoparticles are mainly located on the surface of two-dimen-
sional layers or the edge sites of LDH rather than within the in-
[
15]
terlayer space.
It is noted that the diffractions associated
To further reveal the microstructure of the Pt NPs, HRTEM
observations were carried out. The Pt NPs in Pt/LDH have
a quasi-cuboctahedral shape (Figure 2e). The interplanar spac-
ing was measured to be 0.227 nm, suggesting that (111) is the
plane perpendicular to the surface. As to Pt/LDH-EG, the Pt
NPs present an ellipsoidal appearance with smaller sizes (Fig-
Figure 1. XRD patterns of the LDH support and supported Pt catalysts:
a) LDH, (b) Pt/LDH, (c) Pt/LDH-EG, (d) Pt/LDH-EA, (e) Pt/LDH-MA. The Miller
indices are indicated in the patterns.
(
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Figure 3. XPS spectra of the Pt 4f core level of the supported catalysts:
(
A) Pt/LDH, (B) Pt/LDH-EG, (C) Pt/LDH-EA, (D) Pt/LDH-MA.
has a value of about 3.4 eV, which is in agreement with the re-
[21]
ported value. The other two components with maxima at
2
+
7
3.1Æ0.2 eV and 76.5Æ0.2 eV are assigned to divalent Pt
species. The binding energy of Pt 4f7 and Pt 4f has almost
/2
5/2
the same values, indicating that the electronic structures of
Figure 2. TEM images of the supported Pt catalysts: (a) Pt/LDH, (b) Pt/LDH-
EG, (c) Pt/LDH-EA, (d) Pt/LDH-MA. The corresponding size distribution of Pt
nanoparticles is shown in the middle column. HRTEM lattice images of Pt
catalysts: (e) Pt/LDH, (f) Pt/LDH-EG, (g) Pt/LDH-EA, (h) Pt/LDH-MA. The inter-
planar spacings are indicated.
2
+
0
both Pt and Pt are less affected by other species in all Pt
2
+
0
catalysts. Furthermore, the ratio of Pt /Pt was estimated by
2+
0
the integral peak area of fitted Pt and Pt components. The
Pt samples reduced by EG (Pt/LDH and Pt/LDH-EG) have
2
+
0
higher Pt /Pt ratios (1.21 and 1.28, respectively, Table 1) than
ure 2 f). Besides the (111) observed, a fraction of particles ex-
hibited a d-spacing of 0.195 nm, which is indicative of the
presence of (200). A previous work demonstrated the shape-
controlled synthesis of free-standing Pt NPs via a bromine-di-
those reduced by ethanol and methanol (Pt/LDH-EA=1.01, Pt/
2
+
LDH-MA=0.99). This interesting finding reveals that more Pt
species exist in the Pt catalysts synthesized by a polyol reduc-
4
+
tion. Moreover, the reduction of initial Pt ions can be mediat-
ed by changing the types of alcohols. It has been reported
that cationic species of active metals exhibit a unique activity
[
19]
rected way; however, no significant differences in the mor-
phology between Pt NPs synthesized with and without addi-
tion of TTAB were observed. These data indicate that the mor-
phology evolution of the supported Pt NPs differs from that of
the free-standing ones in the presence of a directing or stabi-
lizing agent. The Pt NPs reduced by ethanol and methanol
show a d-spacing of about 0.225 nm (Figure 2g and 2h). It is
known that the catalytic hydrogenation is a reaction that usu-
[
11]
or selectivity in hydrogenation reactions.
Therefore, we
2
+
expect that the Pt species in the Pt catalysts may have a cat-
ionic effect on the cinnamaldehyde hydrogenation, which will
be discussed in the following subsection.
The O 1s spectra were further investigated to gain insight
into the oxygen-associated species. For all four Pt catalysts, the
[
20]
ally depends on the size and/or shape of catalyst. One can
therefore assume that the differences in the sizes and size dis-
tributions of the catalysts obtained in our study may lead to
a distinct activity and selectivity.
Table 1. XPS binding energy of Pt 4f core levels in the supported cata-
lysts.
The chemical valence of catalytically active species is a critical
factor for the catalytic performance. Therefore, we carried out
XPS characterization to study the valence of Pt. For all cata-
lysts, a constant contribution with a maximum at 74.8Æ0.1 eV
is due to the Al 2p core level, coming from LDH support. The
Pt 4f core level spectra consist of the spin-orbit components
2
+
0 [a]
Catalysts
4f7/2 [eV]
Pt
4f5/2 [eV]
Pt
Ratio (Pt /Pt )
0
2+
0
2+
Pt
Pt
Pt/LDH
70.7
70.9
70.7
70.9
73.1
73.1
73.1
73.3
74.2
74.4
74.1
74.1
76.6
76.6
76.3
76.6
1.21
1.28
1.01
0.99
Pt/LDH-EG
Pt/LDH-EA
Pt/LDH-MA
Pt 4f7/2 and Pt 4f , which can be fitted with four components
5/2
2
+
[
a] The ratio was calculated by the integral peak area of fitted Pt and
(
Figure 3). The peaks at 70.7Æ0.2 eV and 74.1Æ0.2 eV can be
0
Pt spectra.
0 [8b,12]
assigned to metallic Pt species.
The spin-orbital splitting
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spectra were deconvoluted into two components with a maxi-
mum at a binding energy (BE) of 531.9Æ0.2 eV and 532.8Æ
0
.2 eV (Figure 4 and Table 2), which is assigned to a hydroxyl
À
[22]
group (OH ) and H O absorbed on the surface. Little differ-
2
Figure 4. XPS spectra of the O 1s core level of the supported catalysts:
(A) Pt/LDH, (B) Pt/LDH-EG, (C) Pt/LDH-EA, (D) Pt/LDH-MA.
Table 2. XPS binding energy of the O 1s core level in the supported cata-
lysts.
[a]
Catalysts
O 1s
OH
Ratio
Figure 5. FTIR spectra of CO molecules adsorbed on the supported Pt cata-
lysts (A) Pt/LDH-EG, (B) Pt/LDH-EA, (C) Pt/LDH-MA. The time refers to the
purging time using a nitrogen gas flow in the stage of gas purge after the
CO adsorption.
2
H O
Pt/LDH
531.9
531.9
531.9
531.8
532.8
532.9
532.9
532.5
1.31
1.27
1.06
1.07
Pt/LDH-EG
Pt/LDH-EA
Pt/LDH-MA
[
a] The ratio of integral peak areas of oxygen species from OH and H
2
O.
provide powerful surface-related information. The FTIR spectra
of catalysts reduced by alcohols are shown and compared in
Figure 5. For the three Pt catalysts, the spectra display three
ence in the binding energies of both components was ob-
served for all samples. The XPS O 1s analyses verify the hydrox-
ide nature of the LDH support and the presence of a considera-
main vibration absorption bands of CO molecules (uC
=
O). The
À1
band ranging from 2010–2050 cm can be assigned to the
[24]
linear adsorption mode of CO onto an individual Pt site. This
À1
ble amount of absorbed water. The ratio of hydroxyl to H O
band consists of a main peak at about 2020 cm and a should-
2
À1
was estimated by the integral peak area. It was found that the
Pt catalysts (Pt/LDH and Pt/LDH-EG) synthesized by EG reduc-
tion have higher ratios (1.31 and 1.27) than ethanol- and meth-
anol-reduced Pt catalysts (Pt/LDH-EA and Pt/LDH-MA) with
a ratio about 1.07 (Table 2). This reveals that EG-reduced Pt cat-
alysts possess more surface hydroxyl species. Previous studies
reported that alkali (KOH or NaOH) could act as a promoter to
er at about 2040 cm . It was reported that different Pt species
[25]
=
may cause a shift in the linearly adsorbed vibration uC O.
Compared to Pt/LDH-EG (Figure 5A) and Pt/LDH-EA (Fig-
ure 5B), the shoulder in the FTIR spectrum of Pt/LDH-MA is too
weak to be observed (Figure 5C). The other two absorptions at
À1
approximately 1861 and 1918 cm are attributable to the
[
26]
bridging CO on two or more Pt sites.
Also, the band at
[23]
À1
tune the selectivity of cinnamaldehyde hydrogenation. Con-
sequently, it is expected that the native hydroxyl-rich charac-
teristic of the LDH support could enhance the selectivity to-
wards C=O hydrogenation without the need of external alkali
addition.
1918 cm could be due to multiple CO adsorption at low-co-
ordinated sites of small-sized Pt particles (e.g., corners, steps
[27]
or defect sites). It is observed that the peak positions hardly
shift with the purging time of nitrogen, thus indicating stable
adsorption of CO in both linear and bridging modes. Further-
more, it is found that the bridging adsorption and multiple CO
adsorption are very similar among the catalysts. Pt/LDH-EG
Supported metal catalysts are usually studied using a probe
molecule, for example, CO by IR spectroscopy, which is able to
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shows a relatively stronger linearly adsorbed vibration band
the highest dispersion (53.8%) due to the smallest metal parti-
cle size. It is seen that Pt/LDH-EG results in the highest conver-
sion value of 93.6% under the same catalytic conditions and in
a relatively high C=O selectivity towards CMO (78.8%), which
is slightly lower than that over Pt/LDH catalyst (81.5%). The
high conversion and C=O selectivity lead to a comparably high
yield of CMO (73.7%), a desired and thermodynamically unfav-
orable product. In previous studies, it was usually believed that
the selectivity towards CMO decreases with a reduction in Pt
À1
(2010–2050 cm ). This indicates that the linear adsorption of
CO on Pt species of this catalyst may be the dominant mode.
Catalytic evaluation in cinnamaldehyde hydrogenation
It is known that the competing reactions of C=C and C=O hy-
drogenation in cinnamaldehyde (CMA) lead to selective prod-
ucts, that is, hydrocinnamaldehyde (HCMA) and cinnamyl alco-
hol (CMO), respectively (Scheme 1), in which the former is
[
6a,27]
particle sizes, but results in higher activity;
that is, small-
sized Pt facilitates the adsorption of the C=C bond owing to
a large amount of low-coordinated surface sites. However, in
this work, it is found that Pt/LDH-EG (Pt size of 2.1 nm) and Pt/
LDH (Pt size of 2.5 nm) have a higher selectivity towards C=O
hydrogenation (78.8% and 81.5%, respectively), although they
have smaller Pt particle sizes than Pt/LDH-EA (Pt size of
3.6 nm) and Pt/LDH-MA (Pt size of 2.8 nm). This finding is in-
consistent with the usual claim of a metal size effect on the se-
lectivity. It could be explained by both the valence of Pt and
the surface hydroxyl species on the LDH support. The Pt could
form stable species with the surface oxygenated groups and
Scheme 1. The reaction pathway and main products for hydrogenation of
cinnamaldehyde. Path A shows selective hydrogenation of the C=C bond to
afford HCMA (hydrocinnamaldehyde). Path B shows selective hydrogenation
of the C=O bond to afford CMO (cinnamyl alcohol). The complete hydroge-
nation yields HCMO (phenyl propanol).
[8b,12]
remain partially reduced species in the final catalysts.
It
2
+
was convincingly shown herein that the Pt species exist in all
2
+
catalysts and that Pt/LDH-EG and Pt/LDH have a higher Pt
0
/
Pt ratio (1.28 and 1.21) than the other two catalysts (Table 1).
a thermodynamically favorable one. Furthermore, it is favora-
ble over the complete hydrogenation product, that is, HCMO.
Consequently, it is a great challenge to improve the selectivity
towards C=O hydrogenation to obtain the desired CMO prod-
uct while keeping a high conversion. It was reported that the
selectivity of Pt catalysts towards either the C=C or C=O bond
is dependent on many factors such as the preparation
method, the types of supports, and the metal/support interac-
On the other hand, Pt/LDH-EG and Pt/LDH possess higher
ratios of OH/H O species (1.27 and 1.31) than Pt/LDH-EA and
2
Pt/LDH-MA (~1.07) according to the XPS results (Table 2). It
suggests that the former two have a larger fraction of hydroxyl
groups among the oxygen-containing species. It was reported
that the addition of alkali promoters could change the adsorp-
tion mode of cinnamaldehyde. For instance, the addition of
KOH elevates the selectivity to CMO product in the cinnamal-
[9a,10b,28]
[23a]
tion.
Here, the cinnamaldehyde hydrogenation was ach-
dehyde hydrogenation.
Concerning the Pt nanocatalysts de-
ieved over the Pt catalysts under mild conditions (1 MPa H2,
scribed in this work, the hydroxyl groups bonded onto LDH
layers may act as in situ alkali promoters. The hydroxyl groups
on the support could reduce or suppress the adsorption of the
C=C bond by electrostatic repulsion of the phenyl ring in cin-
608C).
For all catalysts, the Pt loading was close to about 2.4 wt%
(
Table 3). This verifies that the different reducing agents have
2
+
little impact on the metal loading. The metal dispersion was
estimated based on the mean size of Pt particles assuming
namaldehyde. As a result, the synergy of cationic Pt and the
hydroxyl species outperforms the size effect, leading to a supe-
rior CMO selectivity over the small-sized Pt particles. In com-
parison, Pt/LDH-EA and Pt/LDH-MA, despite having
larger Pt sizes, possess a moderate selectivity toward
CMO (63.5% and 57%) and a considerable selectivity
[
29]
a cuboctahedral shape of the metal particles. Pt/LDH-EG has
Table 3. The characteristics of different catalysts and their catalytic performance for
hydrogenation of cinnamaldehyde.
toward the undesired HCMA (23.2% and 29.8%, re-
spectively). These results further support the assump-
tion that the synergic effect of metal valence and hy-
droxyl of the LDH plays a dominant role rather than
the size effect.
[
c]
[e]
Catalysts
Pt loading Pt size Dispersion Conv. TOF Yield Selectivity [%]
[a]
[b]
À1
[d]
[
wt%]
[nm] [%]
[%]
[s
]
[%]
CMO HCMA HCMO
Pt/LDH
2.39
2.5
2.1
3.6
2.8
45.2
72.5 0.894 59.1 81.5 10.5
93.6 2.314 73.7 78.8 9.1
62.5 0.947 39.7 63.5 23.2
50.0 0.603 28.5 57.0 29.8
6.7
The initial turnover frequency (TOF), calculated at
a CMA conversion of about 10%, was utilized to eval-
uate the intrinsic kinetics of hydrogenation reac-
Pt/LDH-EG 2.41
Pt/LDH-EA 2.43
Pt/LDH-MA 2.37
53.8
31.4
40.4
11.2
12.2
12.1
[30]
tions. Among all Pt catalysts, Pt/LDH-EG has the
[
a] As determined by ICP-AES. [b] Estimated from the mean particle size of Pt. [c] Cal-
À1
highest TOF value of 2.314 s , indicative of its high
culated at CMA conversion of approximately 10%. [d] Yield of CMO. [e] Selectivity is
reported at the level of conversion in the table. Reaction conditions: catalyst: 0.05 g;
initial activity. It is interesting that the TOF of Pt/LDH-
substrate: 5 mmol CMA; solvent: 12.5 mL ethanol; H
08C; reaction time: 30 min.
2
pressure: 1.0 MPa; temperature:
À1
EG is twice higher than that of Pt/LDH (0.894 s ), al-
6
though they were synthesized by using the same re-
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ducing agent. The only difference is that TTAB was added
while preparing Pt/LDH-EG. From the TEM analyses one can
see that Pt/LDH-EG has a narrower size distribution and a small-
er mean size than Pt/LDH (Figure 2). Besides, it has been re-
ported that the reactivity of monovalent or mixed-valence
lyst prepared by a polyol method possesses a high selectivity
to the CMO product while maintaining a high activity that is
even comparable to that of Pd catalysts in the cinnamaldehyde
hydrogenation.
Next, the evolution of CMA conversion and product selectivi-
ty with the reaction time was explored over the Pt/LDH-EG cat-
alyst (Figure 6). The CMA conversion reaches approximately
90% after a short period of 10 min and continues to increase
to 93% at 30 min. The CMO selectivity remains at about 80%
until a reaction time of 30 min, showing a preferable C=O
bond hydrogenation. After a reaction time of 60 min, CMA is
almost completely converted (>99%). The CMO selectivity rap-
idly drops to 67.8% and the HCMO increases to 25.6%. This
clearly indicates that CMO is a thermodynamically unstable
product and tends to convert into the undesired HCMO via
deep hydrogenation, which increases the difficulty in structure-
d+
metastable Pd species (Pd ) is higher than that of conven-
0
[31]
2+
tional Pd . In this study, Pt species exist in the supported
catalysts synthesized by a mild alcohol reduction method. In
2
+
analogy to Pd catalysis, the metastable Pt could be partly re-
sponsible for the high TOF value of Pt/LDH-EG because of the
2
+
largest ratio of Pt /Pt and the smallest size of Pt in this cata-
lyst.
The TOF values of the synthesized Pt catalysts were com-
pared with those of previously reported Pt-based and Pd-
based ones for cinnamaldehyde hydrogenation (Table 4). For
a better comparison, the reaction conditions were also listed. It
is clearly seen that Pt/LDH-EG has a higher TOF value
À1
(
2.314 s ) than almost all Pt catalysts over various supports
À1 [32]
except Pt(1%)/CA-LiS973 (4.4 s ). Nevertheless, the CMO se-
lectivity on Pt(1%)/CA-LiS973 is only 21%, much lower than
that on Pt/LDH-EG (78.8%). The Pt(10.7%)/UiO-66-NH catalyst
2
[
10c]
has a very high CMO selectivity of 91.7%.
However, the TOF
À1
value is as low as 0.069 s and the Pt loading is relatively high
(
10.7%). In addition, the TOF value of Pt/LDH-EG is almost
times higher than that of Pt(1.96%)/MA with the same LDH
5
[
3c]
support. In this work the Pt nanoparticles were reduced and
loaded on pre-synthesized LDH, different from the “one-pot”
[
3c]
synthetic method used previously. As a result, the present
approach allows sufficient exposure of active Pt sites, leading
to higher activity.
It is known that Pd is a highly active catalyst for hydrogena-
tion of unsaturated aldehydes, with a preference toward the
Figure 6. Variation of conversion and selectivity as a function of reaction
time over Pt/LDH-EG. CMA conversion (squares), CMO selectivity (circles),
HCMA selectivity (triangles), HCMO selectivity (upside down triangles). Reac-
tion conditions: catalyst: 0.05 g, substrate: 5 mmol CMA, solvent: 12.5 mL
[
8e,33]
C=C bond with regard to the selectivity.
We found that
the Pt/LDH-EG catalyst has a comparable activity with those Pd
[
33,34]
catalysts over carbon supports.
Consequently, the Pt cata-
2
ethanol, H pressure: 1.0 MPa, temperature: 608C.
Table 4. Comparison of the catalytic performance of supported Pt and Pd catalysts for cinnamaldehyde hydro-
genation.
sensitive hydrogenation reac-
tions.
[
a]
It is usually believed that the
reaction temperature has a criti-
cal effect on the activity as well
as the selectivity. It was found
that the CMA conversion in-
creases from 50.7% to 93.6%
when the temperature was in-
creased from 30 to 608C
(Table 5). This is in agreement
with previous data on the tem-
Catalyst
Reaction conditions
TOF
CMA
CMO
Reference
À1
[
s
]
Conversion Selectivity
[
%]
[%]
Pt(1%)/SiO
2
-NH
3
0.05 g, 1508C, 1 MPa, toluene
0.05 g, 608C, 1 MPa, 2 h, water
0.03
15
12
[2a]
[3c]
[9a]
[9a]
[10b]
[10c]
[12]
[32]
[35]
[36]
[37]
[38]
[33]
[34]
This work
Pt(1.96%)/MA
Pt(4%)/CNTs
Pt(2.9%)/RGO
Pt(5%)/CNT-973 K
Pt(10.7%)/UiO-66-NH
Pt(5%)/CeO
Pt(1%)/CA-LiS973
Pt(0.18%)/ZrO
Pt(0.5%)/Graphite
Pd(0.5 mol%)/BP
Pd(0.2%)/OMC
Pd(4.8%)/AC-E
Pd(10%)/C
0.488 79.7
0.089 87.2
0.127 89.6
0.819 64.9
0.069 98.7
0.352 43.5
85.4
48.3
69.6
47.6
91.7
66.9
21
0.02 g, 408C, 2 MPa, 2.5 h, isopropanol
0.02 g, 408C, 2 MPa, 2.5 h, isopropanol
0.01 g, 608C, 1 MPa, 1 h, ethanol
0.005 g, 258C, 4 MPa, 44 h, methanol
0.05 g, 758C, 2 MPa, 2 h, isopropanol
0.2 g, 908C, 1 MPa, 19.5 min, heptane
0.6 g, 708C, 2 MPa, 6 h, ethanol
0.36 g, 708C, 2 MPa, 1.5 h, ethanol
0.0244 g, 258C, 0.1 MPa, 4 h, methanol
0.1 g, 408C, 0.1 MPa, 2 h, isopropyl alcohol 0.0125 4.5
2
2 2
-ZrO
4.4
50
2
0.018 49.9
0.030 83.6
0.014 90
42.2
28
(92)
[3c]
perature impact. Furthermore,
[b]
the effect of pressure was stud-
ied with the reaction pressure
of H2 ranging from 0.5 to
(60.2)
(91)
(71)
0.05 g, 608C, 1 MPa, cyclohexane
0.01 g, 508C, 4 MPa, 1 h, 1-propanol
0.05 g, 608C, 1 MPa, 30 min, ethanol
4.1
2.3
90
75
1.5 MPa (Table S1, Supporting
Pt(2.41%)/LDH-EG
2.314 93.6
78.8
Information). The conversion of
CMA increases (from 81.7% to
[
2
a]Amount of catalyst, temperature, H pressure, time, solvent. [b] The values in parentheses refer to the selec-
tivity towards HCMA.
98.3%) with increased pressure
Chem. Asian J. 2015, 10, 1561 – 1570
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cules can interact with two types of basic sites on the surface
of catalysts. The basic hydroxyl groups form hydrogen carbon-
Table 5. The effect of reaction temperature on the catalytic performan-
ce.
[a]
ate species with CO and basic oxygen atoms form surface car-
2
[
b]
T [8C]
Conversion [%]
Selectivity [%]
CMO
[39,40]
bonate species.
It is found that the FTIR spectra of catalyst
HCMA
HCMO
before and after reactions show two absorption bands. One
À1
6
4
3
0
0
0
93.6
71.1
50.7
78.8
80.4
82.9
9.1
10.6
12.3
11.2
8.2
4.1
has a maximum at about 1205 cm , which is assigned to the
d(OH) of hydrogen carbonate species from the reaction of CO
2
À1
with basic OH groups. The other at about 1698 cm is as-
[
a] Catalyst: Pt/LDH-EG. [b] Selectivity is reported at the level of conver-
sion in the table. Reaction conditions: catalyst: 0.05 g; substrate: 5 mmol
CMA; solvent: 12.5 mL ethanol; H pressure: 1.0 MPa; reaction time:
0 min.
signed to bridged carbonate species or to a bent form of
[41]
strongly adsorbed carbon dioxide. The intensity of the band
2
À1
3
at 1205 cm decreases after catalytic reactions based on the
comparison of peak areas. It suggests that the amount of hy-
drogen carbonate species diminishes, possibly due to the
leaching of OH groups on the surface, that is, the decreasing
basicity.
whereas the selectivity to CMO (C=O hydrogenation) decreases
(
from 79.9% to 69.4%). The yield of CMO reaches a maximum
73.7%) at the pressure of 1 MPa.
(
In addition, we analyzed the O 1s XPS spectrum of the cata-
lyst (Pt/LDH-EG) after the fourth use (Figure S3, Supporting In-
formation). The ratio of hydroxyl species to water is 1.19,
a value smaller than that of the fresh catalyst (1.27). It is an in-
dication that the decrease of conversion and selectivity could
be caused by the leaching of basic hydroxyl species.
The CMO selectivity decreases from 82.9% to 78.8% with in-
creasing temperature. Correspondingly, the selectivity of
HCMO product increases from 4.1% to 11.2%. This finding veri-
fies that a higher temperature is favorable for complete hydro-
genation. Consequently, the development of active catalysts
that work at lower temperatures is beneficial to improve the
selectivity towards CMO.
The structure of the Pt/LDH-EG catalyst after use was charac-
terized by XRD, TEM and XPS. The XRD patterns show no struc-
tural change compared to that of the fresh catalyst (Figure S4,
Supporting Information). TEM imaging reveals highly dispersed
particles and a narrow size distribution, indicating that no ob-
vious agglomeration occurs during the catalytic reactions (Fig-
ure S5, Supporting Information). The XPS spectra show the
The recyclability of the Pt/LDH-EG catalyst was also investi-
gated (Figure 7). Elemental analysis by inductively coupled
plasma–atomic emission spectroscopy (ICP–AES) shows that
the Pt loading is 2.25 wt% after four cycles, a value slightly
lower than that of the catalyst used initially (2.41 wt%). The
CMA conversion was decreased from 93.6% to 86% upon the
fourth run, possibly due to small leaching of the Pt species.
The CMO selectivity remains still at 73% after the third cycle.
The HCMA selectivity remained almost unchanged. However,
the selectivity of HCMO product increases with each cycle. This
indicates that the main obstacle to achieve highly selective C=
O hydrogenation is the thermodynamically favorable conver-
sion from CMO into HCMO.
2
+
0
same peaks assigned to Pt and Pt species (Figure S6, Sup-
2
+
0
porting Information) with a ratio of Pt /Pt =1.23. The recy-
cling data and structural studies confirm that the Pt catalyst is
considerably stable with no significant activity and selectivity
loss.
Conclusions
The basicity of the catalyst (Pt/LDH-EG) before and after use
was studied by FTIR measurements of adsorbed CO2 probe
Hydrotalcite-supported Pt nanocatalysts were prepared by
a mild alcohol reduction approach. The types of alcohols used
and the addition of stabilizing agent have a major effect on
the sizes and size distribution of reduced Pt particles. The Pt
catalyst prepared by reduction with EG possesses the smallest
molecules (Figure S2, Supporting Information). The CO mole-
2
2
+
mean size (2.1 nm) and the narrowest size distribution. Pt
0
2+
and Pt species co-exist in the catalysts, and the ratio of Pt
0
/
Pt varies with the types of alcohols. The catalyst prepared by
reduction with EG has a higher ratio than those reduced by
ethanol and methanol. Besides, the former possesses a larger
amount of hydroxyl species than the latter two. The small-
sized Pt catalyst (Pt/LDH-EG) shows an unusual selectivity to-
wards the C=O bond and the highest activity (TOF of
À1
2
.314 s ), which is higher than that of most Pt catalysts and
comparable to Pd catalysts reported previously. It is attributed
to the synergy of cationic species and the hydroxyl moieties
on the support. Analogous to alkali (KOH or NaOH), the sup-
port provides an in situ promoter (hydroxyl) for improving the
selectivity towards C=O hydrogenation. The present study pro-
vides new insights to further understand the synergic contribu-
Figure 7. Recycling of Pt/LDH-EG for cinnamaldehyde hydrogenation. Reac-
tion conditions: catalyst: 0.05 g; substrate: 5 mmol CMA; solvent: 12.5 mL
ethanol; H
2
pressure: 1.0 MPa; temperature: 608C; reaction time: 30 min.
Chem. Asian J. 2015, 10, 1561 – 1570
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tions of metal valence and native promoter to the catalytic ac-
tivity and selectivity of hydrogenation reactions.
X-ray photoelectron spectra (XPS) measurements. The X-ray pho-
toelectron spectroscopy (XPS) spectra of catalysts were acquired
using a Thermo VG ESCALAB250 X-ray photoelectron spectrometer
using Al X-ray source (1486.6 eV) at a pressure of about 2”
Ka
À9
1
0
Pa. The shifts of all binding energies were calibrated using the
Experimental Section
C 1s core level at 284.6 eV.
Elemental analyses. Elemental analyses for compositions were
measured by inductively coupled plasma–atomic emission spec-
troscopy (ICP-AES, Shimadzu ICPS-7500).
Materials
Hexachloroplatinic acid (H PtCl ·6H O, 99.9%) was purchased from
2
6
2
Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Trans cin-
namaldehyde (98.0%), received from J&K Chemical Co. Ltd., was
used without further purification. Tetradecyl trimethyl ammonium
bromide (TTAB) was obtained from Aladdin reagent Co. Ltd. Ethyl-
CO-adsorption Fourier-transform infrared (FTIR) measurements.
FTIR spectra of CO adsorption were obtained on a PC-controlled
FTIR spectrophotometer (PWP 110-40). A sample of 30 mg was
pressed into a self-standing pellet and placed in a sealed chamber.
ene glycol and NaBH were purchased from Kermel Chemical Re-
4
The chamber was firstly purged with a flow of high-purity N and
2
agent Co. Ltd (Tianjin, China). Other chemicals and regents were all
analytical reagent grade and used as received without further pu-
rification. The purity of hydrogen was 99.999%. Deionized water
was used throughout the process.
heated to 708C, and maintained for 1 h. After the pretreatment,
the chamber was cooled down to room temperature under a N2
atmosphere. Subsequently, high-purity CO gas was introduced to
carry out the adsorption of CO molecules onto the samples. After-
wards, the chamber was re-purged with N to drive the CO into
2
the gas phase and the weakly adsorbed CO away. The IR spectra
Preparation of the support (LDH)
À1
were recorded with a resolution of 2 cm
.
The synthesis of hydrotalcite (LDH) was carried out in a procedure
FTIR measurements of adsorbed CO2 probe molecules. FTIR
[
42]
similar to that reported previously.
An aqueous solution of
spectra of CO adsorption on the catalysts were obtained on a FTIR
2
2
+
3+
Mg(NO ) ·6H O and Al(NO ) ·9H O ([Mg ]=0.3m, [Al ]=0.1m)
3
2
2
3 3
2
spectrophotometer (PWP 110-40) to study the basicity of the cata-
lysts. The catalyst (~30 mg) was pressed into a self-standing pellet
and placed in a sealed chamber. The chamber was purged with
À
3+
and a mixed alkali solution of NaOH and Na CO ([OH ]=1.6(Al
2
3
2
+
2À
3+
+
Mg ), CO3 =2[Al ]) were rapidly mixed under high-speed stir-
ring within several tens of seconds. Subsequently, the resulting
suspension was centrifuged once (4000 rpm, 10 min) and then the
wet solid was transferred to a round-bottom flask with 50 mL dis-
tilled water. The suspension was then heated to 808C for 6 h
under vigorous stirring. After cooling to room temperature, the
products were centrifuged, washed with deionized water three
times until the pH reached 7, and then dried at 808C for 12 h to
obtain a white solid powder.
high-purity N and heated to 1008C, and kept for 1 h. Subsequent-
2
ly, the chamber was cooled to room temperature under a flow of
N . High-purity CO (99.99%) was introduced to achieve the ad-
2
2
sorption of CO onto the catalysts. Afterwards, the chamber was
2
purged by N again for 1 h to drive the CO into the gas phase
2
2
and the weakly adsorbed CO2 away. The spectral resolution is
À1
2
cm
.
Catalytic Evaluation
Loading of Pt onto the support
The cinnamaldehyde hydrogenation was carried out in a pressure-
controlled, stainless steel autoclave with a Teflon liner (100 mL)
under magnetic stirring. The autoclave was put in a water bath of
constant temperature. For each run, 50 mg catalyst, 5 mmol CMA
and 12.5 mL ethanol were added into the autoclave. The autoclave
The Pt nanoparticles supported on LDH were produced by an alco-
hol reduction approach. In a typical run, 15 mL ethylene glycol (or
ethanol or methanol) was mixed with 60 mL deionized water. Then
0
.2 g LDH powder, 0.8 mL H PtCl (0.0382m) and TTAB (tetradecyl
2 6
trimethyl ammonium bromide, TTAB:Pt=50:1 in molar equiv.)
were added to the mixed solution above. The solution was stirred
vigorously for 2 h at room temperature. Afterwards, the solution
was transferred into a 100 mL stainless steel autoclave with
a Teflon liner, which was kept at 1208C for 4 h. The precipitates
were separated by centrifugation (8000 rpm, 5 min) and washed
with ethanol and deionized water three times, respectively. Finally,
the solid was dried at 508C in air for 14 h. The catalysts were de-
noted as Pt/LDH-EG, Pt/LDH-EA and Pt/LDH-MA, respectively. The
catalyst prepared without the addition of TTAB during the EG re-
duction was labeled Pt/LDH.
was sealed and flushed with 2 MPa H
/vacuum to thoroughly remove any residue air. The autoclave
was preheated to 608C. Then the reaction was started at 608C
under 1 MPa H for 30 min. In order to maximize the external mass
transfer, the stirring speed was sufficiently high during the reac-
tion. After the reaction, the reactor was cooled to room tempera-
ture and depressurized to atmospheric pressure slowly and careful-
ly. The products were separated from the catalyst powder by cen-
trifugation. The liquid substances were analyzed by using a gas
chromatograph (GC-9800) equipped with a flame ionization detec-
tor (FID) and an Agilent DB-624 capillary column.
2
for 10 consecutive cycles of
H
2
2
For recycling use, the catalyst was collected by centrifugation and
washed with ethanol and distilled water several times after each
run. The catalyst was dried at 508C for more than 10 h before
being used in the next run.
Sample Characterization
X-ray diffraction (XRD) measurements. The X-ray diffraction (XRD)
analyses of the catalysts were done on a D8ADVANCE instrument
with a scan range between 38 and 708.
Kinetic studies during the initial hydrogenation of CMA were car-
ried out over all Pt catalysts. Since the initial H amount was high
2
enough and the reaction rate was independent of H , the hydroge-
nation at the initial stage can be assumed to follow pseudo-first-
order kinetics, that is
High-resolution transmission electron microscopy (HRTEM) anal-
yses. TEM measurements were taken on a JEOL JEM-2100 micro-
scope. The catalyst samples were ultrasonically dispersed in etha-
nol and a drop of the suspension was added onto a carbon-coated
Cu grid followed by the evaporation of solvent in air.
2
¼
lnðC =CÞ ¼ kt
0
Chem. Asian J. 2015, 10, 1561 – 1570
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1568
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
where C and C are the concentrations of CMA at the beginning
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nCMA Â conv:%
TOF ¼
Pt
ð1Þ
ðmcaty =M Þ Â D Â t
Pt
Pt
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cat
Pt
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Acknowledgements
This work was supported by the 973 Program (Grant no.
2011CBA00506), the National Natural Science Foundation of
China (Grant no. 21376020, 21271158), Beijing Natural Science
Foundation (Grant no. 2152022), the Program for Changjiang
Scholars and Innovative Research Team in University (Grant no.
IRT1205) and the Fundamental Research Funds for the Central
Universities (YS1406). X. Xiang thanks the support from Beijing
Engineering Center for Hierarchical Catalysts. We thank the
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Keywords: heterogeneous catalysis
platinum · support · supported catalysts
·
hydrogenation
·
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Manuscript received: February 26, 2015
Revised: April 3, 2015
Accepted article published: April 16, 2015
Final article published: May 12, 2015
[
Chem. Asian J. 2015, 10, 1561 – 1570
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim