A mild solution chemistry method to synthesize hydrotalcite-supported platinum nanocrystals for selective hydrogenation of cinnamaldehyde in neat water

Article abstract of DOI:10.1039/c3cy00437f

Hydrotalcite-supported platinum nanocrystals (Pt NCs) were synthesized by a facile solution chemistry method, and then applied as an efficient catalyst for the selective hydrogenation of cinnamaldehyde (CMA) in neat water. The reduction of metal precursor ions was achieved in an aqueous solution at a low temperature (323 K), simultaneously accompanied by the crystallization of the hydrotalcite support. The size of the Pt NCs can be delicately tuned by the relative ratio of surfactant to metal precursor ions, and characterized by HRTEM and CO-adsorption infrared spectroscopy. The Pt particle sizes are closely associated with the hydrogenation selectivity toward cinnamyl alcohol (CMO), with a higher selectivity up to 85% over the larger-sized Pt in an aqueous medium. The effects of alkali (NaOH) on the catalytic performance were explored. The findings indicated that the addition of alkali enhances the selectivity toward CMO (to 90%). The catalysts showed high stability with a marginal decrease in activity and selectivity after repeated use. The hydrogenation products could be easily separated from the solvent by simple extraction, which is a greener and more convenient process than those using organic solvents.

Full text of DOI:10.1039/c3cy00437f

Catalysis
Science & Technology
View Article Online
View Journal | View Issue
PAPER
A mild solution chemistry method to synthesize
hydrotalcite-supported platinum nanocrystals for
selective hydrogenation of cinnamaldehyde in neat
water†
Cite this: Catal. Sci. Technol., 2013,
3, 2819
Xu Xiang,*a Wanhong He,ab Lisha Xiea and Feng Lia
Hydrotalcite-supported platinum nanocrystals (Pt NCs) were synthesized by a facile solution chemistry
method, and then applied as an efficient catalyst for the selective hydrogenation of cinnamaldehyde
(CMA) in neat water. The reduction of metal precursor ions was achieved in an aqueous solution at a
low temperature (323 K), simultaneously accompanied by the crystallization of the hydrotalcite support.
The size of the Pt NCs can be delicately tuned by the relative ratio of surfactant to metal precursor ions,
and characterized by HRTEM and CO-adsorption infrared spectroscopy. The Pt particle sizes are closely
associated with the hydrogenation selectivity toward cinnamyl alcohol (CMO), with a higher selectivity
up to 85% over the larger-sized Pt in an aqueous medium. The effects of alkali (NaOH) on the catalytic
performance were explored. The findings indicated that the addition of alkali enhances the selectivity
toward CMO (to 90%). The catalysts showed high stability with a marginal decrease in activity and
selectivity after repeated use. The hydrogenation products could be easily separated from the solvent
by simple extraction, which is a greener and more convenient process than those using organic
solvents.
Received 25th June 2013,
Accepted 31st July 2013
DOI: 10.1039/c3cy00437f
www.rsc.org/catalysis
of the CQO bond to the desired unsaturated alcohol without
reacting the CQC bond.6 Common noble metals like Pt,7,8 Pd9–11
Introduction
and Ru12 presented an intrinsically higher selectivity toward the
CQC bond to produce a saturated aldehyde. Hence, great efforts
have been made to enhance their selectivity either by fine control
of the sizes13 and/or the exposed facets of the active metal
nanoparticles,14 or by decorating the primary metal with a
second metal component.15–20 Also, modification of the sup-
ports21–24 or the addition of a promoter (Lewis acids or bases)8
are alternative ways to tune the selectivity. Biocatalytic routes
have also been explored as alternatives for the selective reduction
in recent years.25 Although most of these strategies are proven
effective, the preparation of catalysts is usually tedious and
complicated or less environmentally friendly. A facile and mild
approach for obtaining supported metal catalysts capable of
high activity and selectivity is highly desirable.
Chemoselective hydrogenation of a,b-unsaturated aldehydes
(UALs) or ketones (UKEs) to the corresponding allylic alcohols
is of great importance to the synthesis of fine chemicals, such
as pharmaceuticals, foods and cosmetics.1,2 Selective hydro-
genation of cinnamaldehyde (CMA) to cinnamyl alcohol (CMO)
is a representative of such a class of reactions. Two competitive
products of selective hydrogenation can be produced through
the hydrogenation of the conjugated CQC or CQO bond over
the supported metal catalysts.3,4 The hydrogenation of CQC
versus CQO in the conjugated aldehydes and ketones is thermo-
dynamically favorable of the former because of the significantly
higher activation barriers to hydrogenate the CQO bond com-
pared with those required to hydrogenate the CQC bond.5
Generally, the challenge is associated with the hydrogenation
In addition to the catalyst preparation, the increasing con-
cerns around environmental issues in the past decade, have
promoted the progress of metal-catalyzed liquid-phase reac-
tions achieved in an aqueous medium or ‘‘on water’’.26,27 It is
generally accepted that one of the key principles of green
chemistry is the elimination of solvents in chemical processes
or the replacement of hazardous solvents with environmentally
benign solvents like water.28 Regarding the liquid-phase
a State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing, 100029, China. E-mail: xiangxu@mail.buct.edu.cn;
Fax: +86-10-64425385; Tel: +86-10-64412109
b Key Laboratory of Fine Petrochemical Engineering of Jiangsu Province, Changzhou,
213164, P.R. China
† Electronic supplementary information (ESI) available: Effect of solvents; TEM
photographs; FTIR spectra, XRD pattern and XPS spectra of the catalyst after use.
See DOI: 10.1039/c3cy00437f
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 2819--2827 2819

View Article Online
Paper
Catalysis Science & Technology
hydrogenation of UALs, the catalytic reactions were often
carried out in organic media (alcohol, cyclohexane or toluene
et al.),29 increasing the risk of environmental hazards and the
complexity of product separation. There are few reports on the
hydrogenation of UALs over supported catalysts in an aqueous
medium,5,30 even though there are potential advantages in its green
and environmental friendly approach. Very recently, Cao et al.
demonstrated that gold supported on mesostructured ceria can
catalyze the reduction of UALs and UKEs to the corresponding
allylic alcohols in high yields in an aqueous medium,31 which
established a green protocol for chemoselective conversion of UALs
to a desired unsaturated alcohol product.
Fig. 1 Powder XRD patterns of hydrotalcite-supported Pt catalysts: (a) MA, (b)
Inspired by this pioneering work, herein, we synthesized
hydrotalcite-supported platinum nanocrystals (Pt NCs) by a
mild and facile method, which were used as catalysts for
selective hydrogenation of cinnamaldehyde in neat water. The
Pt/MA, (c) Pt/MA-1, (d) Pt/MA-2, (e) Pt/MA-3.
preparation process integrates the reduction of the metal reducing agent NaBH4, simultaneously accompanied by the
precursor, the deposition of the Pt NCs and the crystallization aging of the hydrotalcite support. The phase structure and
of the support into one process (all-in-one). The hydrotalcite, composition of the catalysts were confirmed by XRD. The
also known as MgAl-layered double hydroxide (MgAl-LDH) with patterns of MA with no loading of Pt showed characteristic
a formula of Mg6Al2(OH)16CO3ÁnH2O, consists of a positively- diffractions of hydrotalcite (Fig. 1a), which agrees well with the
charged brucite-like layer and charge-balanced anions within the reported results (JCPDS Card No. 89-0460). The (00l) diffractions in
interlayer gallery. MgAl-LDH has extensively been used to support low angles indicate its layered structure. The d003 basal spacing is
noble metals and been applied in a variety of catalytic reactions.32–34 B7.6 Å, suggestive of the intercalation of carbonate ions as
The hydroxyl group in the structure of hydrotalcite may inhibit the compensating anions. The Pt NCs supported on hydrotalcite
adsorption of the CQC bond by electrostatically repelling the exhibit the same diffractions to that of hydrotalcite (Fig. 1b–e).
phenyl ring of cinnamaldehyde without the need of a foreign The diffraction of d003 shows little shift compared to the support
promoter (e.g. an alkali). Also, the hydrophilicity of hydrotalcite itself. This indicates that the layered hydrotalcite is not intercalated
could be beneficial to the orientation of the hydrophilic CQO by PtCl4À anions during the reaction. It is reasonable as carbonate
moiety on the surface active sites of the catalyst.
anions are prone to intercalate and stable in the interlayer galleries
The supported Pt catalysts exhibited high selectivity toward in the carbonate-containing solution.
the CQO bond (85% selectivity), which is closely related to the
Besides the characteristic diffractions corresponding to the
size of the Pt NCs. The larger Pt NCs resulted in higher layered structure of hydrotalcite, the diffractions associated
selectivity toward the cinnamyl alcohol (CMO). The high activity with metal Pt or other Pt species are not observed. This can
and selectivity could be retained in the third recycle. In particular, be attributed to the high dispersion of the Pt NCs of small sizes
the products could be easily separated from the aqueous medium and/or the low loading of Pt (B1.96 wt%). It has been reported
after the reaction by extraction since water was used as a green that supported Pt or Pd nanoparticles are located mostly on the
solvent. No organic solvents or toxic agents were used throughout surface or the edge-on sites of the hydrotalcite rather than in
either the preparation of the catalysts or the catalytic reactions. This the interlayer space.32,34
current study, for the first time, explores a green process for the
The reduction of the metal precursor was mediated by the
hydrogenation of cinnamaldehyde over highly efficient hydrotalcite- surfactant TTAB during the synthesis. It was reported that
supported Pt catalysts using neat water as a reaction medium.
surfactant residues in catalysts may poison the active sites.35
Therefore, FTIR analyses were carried out to detect possible
residues of surfactant in the as-synthesized catalysts. The IR
spectra of Pt/MA (with no addition of TTAB) and Pt/MA-3 (with
a maximal quantity of TTAB added) were compared (Fig. 2).
Results and discussion
Characterization of the catalysts
The catalysts were prepared with an initial ratio of TTAB A broad absorption band in the range of 3600–3200 cmÀ1
(tetradecyl trimethyl ammonium bromide) to Pt4+ of 25 : 1, centered at 3468 cmÀ1 corresponds to the O–H stretching
50 : 1 and 100 : 1 denoted as Pt/MA-1, Pt/MA-2 and Pt/MA-3, vibrations (nOH) of the hydroxyl groups in the brucite-like layers
respectively. The same procedure was carried out without the and interlayer water molecules. The absorption bands peaked
addition of TTAB and the catalyst was denoted as Pt/MA (Table S1 at 1376 cmÀ1 and 855 cmÀ1 are attributable to the symmetric
in the ESI†). Also, the MgAl-LDH without the loading of Pt was stretching vibrations (n3) and out-of-plane deformation (n2) of
synthesized under the same conditions and denoted as MA.
the carbonate ions, suggestive of the presence of interlayer
The supported Pt catalysts were synthesized by an aqueous carbonate anions.36 The weak absorption at 1643 cmÀ1 is
nanochemistry method under mild conditions (323 K). The Pt4+ assigned to the bending vibration of interlayer water (dOH). The
ions were reduced to metallic Pt nanocrystals using the bands in the lower frequency region, around 660–580 cmÀ1, are
c
2820 Catal. Sci. Technol., 2013, 3, 2819--2827
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
Fig. 2 FTIR spectra of the supported Pt catalysts: (a) Pt/MA, (b) Pt/MA-3.
attributed to metal–oxygen (M–O) and metal–hydroxyl (M–OH)
vibrations in the lattice of the hydrotalcite. There is no detectable
difference between the two spectra, indicative of the absence of
any residues associated with TTAB. It was reported that the
bromide-mediated strategy is more favorable catalytically
because the bromide species are only on the nanocrystal surface
and are easily removed after synthesis by washing.37 The results
of the IR analyses clearly suggest that the TTAB species have
been efficiently removed from the surface of the catalysts by
repeated washing.
The reduction of the metal precursors was achieved by a
surfactant-mediated solution synthesis. It was expected that the
sizes or morphologies of the Pt NCs could be tuned by the
kinetics-controlled reduction by the surfactant. The particle
sizes and morphology of the reduced Pt NCs were studied by
HRTEM. Fig. 3 shows TEM photographs of a series of Pt
catalysts. It is observed that Pt NCs are highly dispersed on
Fig. 3 TEM photographs of the supported Pt catalysts: (a) Pt/MA, (b) Pt/MA-1,
(c) Pt/MA-2, (d) Pt/MA-3. Pt size distribution histograms were shown in the right
column.
the support regardless of the addition of the surfactant. How- growth kinetics. It is interesting that the size of the Pt NCs
ever, the Pt sizes present an obvious difference for these decreases with increasing quantity of TTAB. We are not
catalysts. The Pt/MA catalyst has smaller Pt NCs with a mean sure whether more TTAB promotes an accelerated nucleation,
size of 2.3 nm and a wider size distribution (Fig. 3a). While the leading to a decrease in Pt size at present. Nevertheless, it is
metal precursor was reduced in the presence of the surfactant clear that the supported Pt NCs have tunable sizes, suitable for
(TTAB), the Pt NCs show larger sizes and a narrower size a size-dependent study in catalysis.
distribution (Fig. 3b–d). Furthermore, the particle sizes is
HRTEM observations were carried out to further reveal the
dependent on the quantity of TTAB. The higher the ratio of microstructure of the Pt NCs. As to Pt/MA (Fig. 4a), the lattice
TTAB to metal precursor, the smaller the size of the Pt NCs image displays an interplanar spacing of 0.226 nm, corres-
(varying from 5.0, 4.0 to 3.4 nm).
ponding to the plane Pt{111}. The Pt NCs show a poor contrast
This finding could be explained by the role of TTAB as a with the support, suggestive of a low degree of crystallization.
stabilizing agent in controlling the reduction kinetics of the Such a structure could cause a high occupancy of defective and
metal precursor. The reducing agent NaBH4, has robust low-coordinated sites on the surface of the Pt. The Pt NCs for
reduction ability. The Pt4+ ions can be instantaneously reduced Pt/MA-1 and Pt/MA-2 exhibit a quasi-cuboctahedral shape with
upon the addition of NaBH4, which was witnessed by the rapid clear boundaries (Fig. 4b and c). The previous study reported
color change of the solution from bright-yellow to black. The the control of the shape of free-standing Pt nanoparticles by a
reduction of Pt4+ ions slightly slowed down in the presence of bromine-mediated method.38 In our case, both the size and
TTAB and the color changed to light black, resulting from the shape of supported Pt NCs can be tuned by a stabilizing agent
change of local micro-environment around Pt4+ ions and the TTAB during the synthesis. The cuboctahedral shape results in
stabilizing role of TTAB. It has been reported that the sizes of Pt a predominance of plane {111} in the exposed facets of Pt. With
NCs could be tuned by controlling the reducing rate of the further increasing the quantity of TTAB, the Pt NCs appear
metal precursor ions in a polyol synthesis.37 The faster nuclea- spherical or ellipsoidal in shape with a smaller size (Fig. 4d). As
tion rate leads to the smaller Pt nanoparticles, while the slower a result, the shape evolution causes an average of exposed
nucleation and growth gives rise to bigger ones. In this work, facets and no specific facets are predominant in this catalyst.
the size of the Pt NCs could be tuned by reduction and/or The catalytic hydrogenation of unsaturated aldehyde could be
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 2819--2827 2821

View Article Online
Paper
Catalysis Science & Technology
Table 1 XPS binding energy of Pt4f core levels in the supported catalysts
Catalysts
Pt4f7/2
Pt4f5/2
Ratiob
Pt/MA
71.0(2.2)a
70.9(2.4)
70.8(2.2)
71.0(2.2)
74.2(2.0)
74.1(2.2)
74.0(2.0)
74.2(2.0)
1.40
1.35
1.39
1.36
Pt/MA-1
Pt/MA-2
Pt/MA-3
a
The Full-Width Half-Maximum (FWHM) values are listed in parenth-
b
eses. The ratio of integral peak area of Pt4f7/2 to Pt4f5/2, close to a
value of 4 : 3.
remarkable effect on the size of the Pt NCs. In addition, the
XPS results confirm that the metal–support interaction hardly
affects the electronic levels of the Pt species as there is no
observable B.E. shift among these catalysts (Table 1).
To further address the size or surface effect of the Pt
catalysts, the catalysts were examined using a probe molecule
i.e. CO, which was adsorbed onto the Pt surface and was
subsequently studied by infrared spectroscopy. Fig. 6 shows
the FTIR spectra of CO adsorbed onto a series of Pt catalysts.
The IR spectra recorded display two distinctive uCQO vibration
bands. The first in the range of 2060–2030 cmÀ1 is assigned to
linearly adsorbed CO molecules on a single Pt atom site. The
second between 1830–1860 cmÀ1 displays a broad and weak
peak, attributable to bridging CO on two or more Pt atoms.41
The results indicate that Pt adsorbs CO predominantly in a
linear way, with a low occupancy of bridged sites at high CO
coverage. It is found that the uCQO linear mode shows a red
shift with decreasing size of the Pt NCs. Similar findings have
also been observed in mono- and bi-metallic Pt catalysts.39,41
The maximal size (5.0 nm) of Pt gives rise to a peak appearing at
2051 cmÀ1, while the Pt NCs with a mean size of 2.3 nm make
Fig. 4 HRTEM images of the supported Pt catalysts: (a) Pt/MA, (b) Pt/MA-1, (c)
Pt/MA-2, (d) Pt/MA-3. The Pt NCs were highlighted by white circles in (a); the
interplanar spacing of Pt{111} labelled in (b), (c) and (d).
size- or shape-sensitive to the metal nanoparticles.14,39 Conse-
quently, it is expected that these catalysts may demonstrate different
activity and selectivity in cinnamaldehyde hydrogenation.
To study the chemical valence of elements in the Pt catalysts,
XPS characterization was carried out. For all four catalysts, the
core level spectra can be fitted with three contributions (Fig. 5).
The component at 74.7 Æ 0.1 eV is assigned to the Al2p level,
resulting from the hydrotalcite (Table 1). The other two con-
tributions come from the spin–orbit components Pt4f7/2 and
Pt4f5/2, respectively, peaked at 70.9 Æ 0.1 eV and 74.1 Æ 0.1 eV,
which is assigned to zerovalent Pt0 in the metallic state.24,40 The
spin orbital splitting shows the same value B3.2 eV, which
agrees well with reports in the literature.40 No contributions
associated with Pt2+ or Pt4+ species are observed in the XPS
spectra. The results suggest that the amount of TTAB added
has little effect on the reduction of Pt4+ although it has a
the linear mode redshift to 2031 cmÀ1
.
It is known that smaller Pt particles contain more low-
coordinated surface sites and the larger particles have increasing
occupancy of flat surfaces. The CO-adsorption results indicate
that the exposed Pt atom bearing a CO ligand experiences an
increasing degree of back-bonding of its d-electrons (increasing
dÀ on the Pt surface) to the p* of the CO molecules, when the size
of Pt is larger.39 The increasing dÀ of the Pt atoms destabilizes the
adsorption of the CQC bond and thus enhances the selectivity
Fig. 5 XPS spectra of Pt4f core level in the supported catalysts: (a) Pt/MA,
Fig. 6 FTIR spectra of CO adsorbed on the Pt catalysts at room temperature: (a)
(b) Pt/MA-1, (c) Pt/MA-2, (d) Pt/MA-3.
Pt/MA, (b) Pt/MA-1, (c) Pt/MA-2, (d) Pt/MA-3.
c
2822 Catal. Sci. Technol., 2013, 3, 2819--2827
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
similar reaction conditions.14,18,24 The catalysts with smaller
sizes of Pt NCs present a decrease in TOF owing to a higher
dispersion (Pt/MA and Pt/MA-3). It is noted that the selectivity
toward CMO is closely associated with the Pt particle size. The
selectivity reaches 85% (CMO yield of 68%) under a high con-
version of B80% over the Pt/MA-1 catalyst (5.0 nm Pt). Besides,
the complete hydrogenation toward HCMO is almost negligible
(1%). With a decrease in Pt size, the selectivity toward CMO
decreases and increases towards HCMA and HCMO. This sug-
gests that the small-sized Pt favors the adsorption of the CQC
bond due to the abundance of low-coordinated surface sites,
which is consistent with the results of the CO-adsorption IR. The
size-dependence of hydrogenation selectivity on the Pt catalysts
has also been found in the previous studies.13,39
In addition, the morphology effect cannot be neglected. The
Pt/MA catalyst showed a remarkably lower selectivity toward
CMO compared to the other catalysts. This could partly be
explained by its high occupancy of defective sites on the
surface, resulting from the low degree of crystallinity. The
abundance of exposed flat facets in the Pt/MA-2 and Pt/MA-3
(see HRTEM in Fig. 4) increases the repulsive interaction with
the phenyl ring in cinnamaldehyde, leading to the carbonyl
group (CQO) aligning with the surface as a head-on attack.
However, it is quite difficult to further enhance the selectivity
toward the CQO bond as the dynamic adsorption of the
unsaturated aldehyde on the remaining terrace sites of the
larger Pt NCs cannot be completely inhibited.
The nanochemistry synthesis in the mild reaction condi-
tions makes the smaller Pt NCs contain higher proportions
of defective and low-coordinated metal surface sites. The
increasing occupancy of the low-coordination surface sites by
smaller-sized Pt could explain the decrease in hydrogenation
selectivity. In comparison, the larger-sized Pt NCs contain a
higher proportion of flat surfaces with a smaller occupancy of
defective surface sites. This leads to an enhancement in the
selectivity. Besides, the current work could be more advanta-
geous over previous studies because of the two following
merits. Firstly, the synthesis of Pt catalysts was performed
by a mild solution chemistry method, which avoids tedious
or unfriendly involvement of organic solvents and/or high
temperature post-treatments. Secondly, the catalytic reactions
were carried out using neat water as a medium, representative
of a greener process and providing a facile separation after the
reaction by simple extraction.31
Scheme 1 The reaction pathway and main products for hydrogenation of
cinnamaldehyde (CMA).
toward CMO (CQO hydrogenation). The bridging absorption
of CO in the 1830–1860 cmÀ1 shows little difference for the Pt
catalysts synthesized in the presence of the surfactant (Fig. 6b–d).
In contrast, this bridging mode presents a weaker band on the
Pt/MA (in the absence of TTAB during the synthesis). It may be
attributable to more bridged sites being accessible to larger and
better-crystallized Pt NCs.
Hydrogenation of cinnamaldehyde over the Pt catalysts
Scheme 1 shows the reaction pathway and main products for
the hydrogenation of cinnamaldehyde. The hydrogenation of
CQC versus CQO in conjugated aldehydes is thermodyn-
amically in favor of the former. Recently, Corma et al. found
that H2 dissociation on Pt nanoparticles is considerably faster
than that on Au, because H2 adsorption on Pt atoms is
dissociative and no activation barriers are involved. The high
chemoselectivity of Au/TiO2 catalysts does not exist in Pt/TiO2
catalysts no matter how small the Pt particles are.42 Hence, it is
more challenging to enhance the selectivity toward CQO bond
to produce the desired unsaturated alcohol (CMO) over the
supported Pt catalysts. Here, the catalytic hydrogenation of
cinnamaldehyde was achieved using water as a medium under
relatively mild reaction conditions (1 MPa H2, 333 K).
The catalytic performance of cinnamaldehyde hydrogena-
tion over the supported Pt catalysts are listed in Table 2. The
dispersion was estimated from the mean Pt particle size
assuming cuboctahedrally shaped metal particles.10 The initial
turnover frequency (TOF) was applied to estimate the kinetics
of the catalytic reactions. They are calculated at CAL conversion
of approximately 10% in order to get the intrinsic kinetics.30
The TOF reach 0.488 sÀ1 over the Pt/MA-1 catalyst, higher than
such Pt catalysts on carbon or common oxide supports under
Table 2 The size of the Pt nanocrystals and their catalytic performance for hydrogenation of cinnamaldehyde
Selectivityd (%)
Dispersiona (%)
TOFb (sÀ1
)
Conversion (%)
Yieldc (%)
CMO
HCMA
HCMO
Catalysts
Pt size (nm)
Pt/MA
2.3
5.0
4.0
3.4
49.1
22.6
28.2
33.2
0.270
0.488
0.410
0.363
91.1
79.7
84.3
87.2
45.6
68.0
64.5
61.4
50.1
85.4
76.6
70.5
35.6
13.3
17.3
20.4
13.9
1.0
5.6
8.7
Pt/MA-1
Pt/MA-2
Pt/MA-3
a
b
c
Estimated from TEM results (the mean Pt particle size). TOF was calculated at CMA conversion of approximately 10%. Yield of CMO.
Selectivity was reported at the level of conversion in the table. Reaction conditions: 0.05 g catalyst, substrate: 4 mmol CMA, solvent: 15 mL water,
d
H2 pressure: 1.0 MPa, temperature: 333 K, reaction time: 2 h.
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 2819--2827 2823

View Article Online
Paper
Catalysis Science & Technology
In contrast, the catalytic reactions were carried out using to transform into HCMO (complete hydrogenation) under high
common organic solvents to reveal the effects of the solvent on conversion (490%). This is a strong indication that CMO is less
catalytic performance. It is found that the activity and CMO thermodynamically stable compared to its competitive product
selectivity in ethanol and isopropanol are slightly higher than HCMA.44 Consequently, it is more challenging to obtain CMO
those in water, while the activity and selectivity dramatically than HCMA by a selectivity-controlled hydrogenation.
decreases in cyclohexane (Table S2 in the ESI†). However, other
It has been reported that the alkali promoter was able to
products increase to B5% when using ethanol or isopropanol modify the adsorption mode of cinnamaldehyde and thus
as a solvent compared to a negligible value (0.3%) using water change the selectivity to a desired product. Lin et al. found
as a solvent. Other products mainly include acetals, resulting that the addition of KOH enhanced the selectivity to CMO
from the effect of solvent. Therefore, using water as a green (77–83% at a CMA conversion of 49–74%).8 The electropositive
solvent can effectively inhibit the formation of other products alkali metal ions such as K+ or Na+ can activate the CQO bond
like acetals.
by a robust interaction with the lone pair of electrons in the
The slightly higher activity and selectivity in ethanol and oxygen of carbonyl group.45,46 Nearby OHÀ ions could inhibit
isopropanol could be attributable to the polarity of the solvents the adsorption of CQC bond by electrostatically repelling the
and the solubility of H2. There is a comparable polarity in phenyl ring of cinnamaldehyde. The hydrogen atom adsorbed
ethanol and isopropanol, while water has a stronger polarity on the Pt atom is favorable to attack and hydrogenate the CQO
(Table S3 in the ESI†). It has been reported that the water- bond. In our work, indeed, we found that the addition of alkali
assisted orientation of hydrophilic CQO moiety on the active (NaOH) promoted the selectivity toward CMO (91% at 2 h, CMO
sites favored chemoselective hydrogenation.7,43 Hence, water yield of 82%). Besides this, the selectivity reduces with increasing
could be a beneficial solvent than organic ones. However, the conversion after a reaction time of 2 h (Fig. 7). This trend is
solubility of H2 in ethanol and isopropanol is four times higher consistent with that without addition of alkali.
than that in water. It could be a decisive factor to affect the
The effects of the reaction temperature was also explored.
activity of hydrogenation due to the dissociative characteristic The CMA conversion shows an increasing trend with elevating
of H2-adsorption on Pt atoms. The present findings with Pt reaction temperature (Table 3). The selectivity toward CMO
catalysts differ from those with Au/CeO2 catalysts, in which presents a reverse tendency. The higher selectivity is achieved
higher activity was obtained when using water as a medium (B90%) at a lower temperature (323 K). Elevating the tempera-
than those using organic solvents.31
ture to 333 K, the selectivity to CMO slightly decreases and
The time course of CMA conversion and selectivity over the HCMA selectivity increases. To further elevate the temperature
catalyst Pt/MA-1 was investigated (Fig. 7). In neat water, the to 353 K, the CMO selectivity dramatically decreases to 61%,
CMA conversion reaches B80% after 2 h and the selectivity to while the quantity of HCMO remarkably increases to 34%.
CMO keeps almost unchanged (B85%). To further extend the Again, this implies that the CMO is a less thermodynamically
reaction time to 2.5 h, the conversion increases to B90%, while stable product although a desired one and it is more difficult to
the selectivity toward CMO decreases to 80% and the amount of achieve high selectivity to CMO. Besides this, we studied
HCMO obviously increases. It is concluded that CMO is prone another typical a,b-unsaturated aldehyde, citral hydrogenation
over the Pt catalyst. The results indicated that the selectivity has
a more remarkable dependence on the reaction temperature
(Table S4 in the ESI†). Higher temperature leads to a decrease in
the selectivity toward the unsaturated alcohol (CQO bond) and
deep hydrogenation to citronellol (the conjugated CQC–CQO
bonds). Further study is required to understand the dependence
of the catalytic properties on the molecular structure of the
substrates.
Regarding the Pt loading, it is found that the CMA conver-
sion decreases with decreasing amount of Pt loading, as shown
in Table 4. This is ascribed to the fewer active sites accessible
Table 3 The effect of reaction temperature on the catalytic performancea
Selectivityb (%)
Fig. 7 Evolution of CMA conversion and selectivity in neat water over the
CMO
HCMA
HCMO
Temp. (K)
Conversion (%)
catalyst Pt/MA-1. CMA conversion (solid square), CMA conversion with the
addition of alkali (empty square); CMO selectivity (solid star), HCMA selectivity
(solid up triangle), HCMO selectivity (solid down triangle); with the addition of
alkali: CMO selectivity (empty star), HCMA selectivity (half-empty up triangle),
HCMO selectivity (empty down triangle). Reaction conditions: 0.05 g catalyst,
substrate: 4 mmol CMA, solvent: 15 mL water, H2 pressure: 1.0 MPa, temperature:
333 K.
353
333
323
97.8
79.7
61.6
61.3
85.4
90.5
4.5
13.3
8.2
34.1
1.0
0.8
a
b
Catalyst: Pt/MA-1. Selectivity was reported at the level of conversion
in the table. Reaction conditions: 0.05 g catalyst, substrate: 4 mmol
CMA, solvent: 15 mL water, H2 pressure: 1.0 MPa, reaction time: 2 h.
c
2824 Catal. Sci. Technol., 2013, 3, 2819--2827
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
Table 4 The effect of Pt loading on the catalytic performancea
increases, accompanied by a small decrease in the CMO
selectivity after recycling. This proves once again that the
transformation from CMO to HCMO could be a main obstacle
to be overcome in order to achieve highly selective CQO
hydrogenation.
Selectivityc (%)
Pt loadingb (wt%)
Conversion (%)
CMO
HCMA
HCMO
1.96
0.96
0.47
79.7
63.7
39.6
85.4
77.2
70.8
13.3
15.4
17.6
1.0
6.8
10.5
Conclusions
a
b
c
Catalyst: Pt/MA-1. As determined by ICP-AES. Selectivity was
reported at the level of conversion in the table. Reaction conditions:
0.05 g catalyst, substrate: 4 mmol CMA, solvent: 15 mL water, H2
pressure: 1.0 MPa, temperature: 333 K, reaction time: 2 h.
We have synthesized hydrotalcite-supported Pt catalysts by a
solution chemistry method under mild and environmentally
benign conditions. The reduction of the metal precursor, the
deposition of Pt NCs and the crystallization of the support were
achieved in a one-step green process with no involvement of
any toxic reagents. The as-synthesized catalysts were applied for
hydrogenation of cinnamaldehyde in neat water and showed high
selectivity toward CMO (85%) and high activity (TOF 0.488 sÀ1). The
selectivity showed dependence on the size and/or morphology of
the Pt NCs. The catalysts can be reused for 4 runs without
appreciable loss of activity and decrease of selectivity, indicative
of the excellent stability. Particularly, the hydrogenation products
could be easily separated from the reaction medium by simple
extraction. The organic solvents or toxic agents were completely
avoided throughout the preparation and catalytic processes. In
addition, the findings suggest that the conversion from CMO to
HCMO is a major issue obstructing the achievement of high
selectivity toward the CQO bond. This study implies that hydro-
talcite-supported Pt NCs are highly selective and stable catalysts for
hydrogenation of cinnamaldehyde to a desired unsaturated alcohol
by an overall green strategy.
for CMA hydrogenation. The CMO selectivity shows an observable
decrease from 85% to 71% when the loading ranges from 1.96% to
0.47%. This is not consistent with the results recently reported,
where only a marginal variation in CMO selectivity was observed
with varied Pt loading.24 Hence, we observed the Pt particle sizes by
TEM. From the TEM results, the decrease in Pt loading leads to a
reduction in Pt particle size (Fig. S1 in the ESI†). The catalysts with a
Pt loading of 0.96% and 0.47% present a mean Pt particle size of
4.2 and 3.5 nm, respectively. The decrease in selectivity could be
explained by the reduced Pt size.
The stability of the supported Pt catalyst was evaluated by
recycling in neat water. It is noted that the CMO selectivity still
remains at 80% after the fourth use (Fig. 8). The CMA conver-
sion has a marginal decrease from 80% (1st use) to 72% (4th
use) mostly owing to a slight leaching of active Pt on the
catalysts. The ICP measurement confirmed the amount of Pt
loading is 1.86 wt% after the third cycle, slightly lower than that
of the initial loading (B1.96 wt%).
In addition, the FTIR and XRD characterization verified that
the catalyst is not subject to the structural change after
repeated use and no adsorbed species are left over to poison
the active Pt (Fig. S2 and S3 in the ESI†). Also, XPS analysis
indicated that the Pt species exist in zerovalent form (Pt0) and is
unchanged compared to the catalyst before use (Fig. S4 in the
ESI†). Hence, the activity and selectivity remain almost
unchanged after repeated use, indicative of their high stability.
It is interesting that the HCMA selectivity is little changed even
after repeated use. In contrast, the amount of HCMO slightly
Experimental
Materials
Hexachloroplatinic acid (H2PtCl6Á6H2O, 99.9%) was purchased
from Sinopharm Chemical Reagent (Beijing Co. Ltd.). Tetra-
decyl trimethyl ammonium bromide (TTAB) was obtained from
J&K Chemical Co. Ltd. The cinnamaldehyde was purchased
from Aldrich Co. Ltd. and used without further purification.
The other reagents are of analytical grade. The purity of hydro-
gen is 99.99%.
Catalyst preparation
Mg(NO3)2Á6H2O and Al(NO3)3Á9H2O were dissolved in 30 mL
deionized water at room temperature to form a clear salt
solution ([Mg2+] = 0.3 M and [Al3+] = 0.1 M, the ratio of
Mg/Al = 3 in molar). NaOH and Na2CO3 were dissolved in
30 mL deionized water at room temperature to form an alkali
2À
solution ([OHÀ] = 1.6(Al3+ + Mg2+) and CO3 = 2[Al3+]). The
above two solutions were rapidly mixed in a high-speed nuclea-
tion reactor for several minutes. Afterwards, the resulting
suspension was centrifuged once (4000 rpm, 10 min). The wet
precipitate was transferred to a flask, in which 3 mL H2PtCl6
(0.0193 M) aqueous solution was diluted to 40 mL and TTAB
was added to the solution with an adjustable ratio to Pt metal
precursor. The flask was firstly ultrasonicated for 10 min, and then
transferred to a temperature-controlled oil bath. After a rapid
Fig. 8 Recycling use of the supported Pt catalyst in neat water. Reaction
conditions: 0.05 g catalyst (Pt/MA-1), substrate: 4 mmol CMA, solvent: 15 mL
water, H2 pressure: 1.0 MPa, temperature: 333 K, reaction time: 2 h.
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 2819--2827 2825

View Article Online
Paper
Catalysis Science & Technology
addition of ice-cold NaBH4 into the solution (NaBH4 : Pt4+
=
autoclave (100 mL, Parr-5000). The stirring rate was sufficiently
15 : 1 in molar), the flask was capped with a rubber septum and high to minimize the mass transfer effect. The catalyst (0.05 g)
H2 generated inside was released via a needle in the septum was firstly added to 15 mL of solvent. The cinnamaldehyde (4 Â
for 10 min. Afterwards, the needle was removed and the flask 10À3 mol) was injected into the solvent. The autoclave was
was kept at a constant temperature of 323 K for 6 h. Sub- repeatedly flushed with H2 for 10 times before reaction. The
sequently, the suspension was centrifuged (4000 rpm, 10 min), autoclave was placed in a heating jacket and preheated to a
washed, and re-dispersed in deionized water for 3 cycles to reaction temperature of 333 K. Then it was pressurized to total
remove the residues. The solid obtained was dried at 343 K H2 pressure of 1.0 MPa. Zero reaction time was recorded while
for 12 h before use. The Pt catalysts with reduced loadings the autoclave was pressurized to the desired H2 pressure and the
(0.96% Pt and 0.47% Pt) were prepared under the same condi- reactions typically lasted for 2 h. The products were analyzed
tions only reducing the amount of H2PtCl6 by half or three using a gas chromatograph (Agilent 7890A) with a HP-5 capillary
quarters. The ratio of TTAB to H2PtCl6 was kept constant column attached to a flame ionization detector (FID). In order to
(25 : 1 in molar).
reveal the role of alkali as a promoter, NaOH of 0.01g (0.25 mmol)
was added to the reaction solvent (water of 15 mL). The catalytic
reactions were carried out in the presence of alkali.
Catalyst characterization
Powder X-ray diffraction (PXRD) measurements. The PXRD
Recovery and reuse of the supported Pt catalysts. The
patterns of the samples were collected using a Shimadzu XRD- catalyst was collected after filtration washed with acetone three
6000 diffractometer (40 kV, 30 mA, graphite-filtered CuKa times and then with distilled water several times. The catalyst
radiation, l = 0.15418 nm).
was then dried at 343 K for 12 h before being used for the
Elemental analyses. Elemental analyses for metal loadings next run.
were performed using an inductively coupled plasma atomic
emission spectroscopy (ICP-AES, Shimadzu ICPS-7500).
Abbreviations
Fourier-transform infrared spectroscopy measurements.
The FTIR spectra of the catalysts were recorded at room
temperature using the KBr dilution and pelleting technique
on a Bruker Vector 22 spectrometer in the range 4000–400 cmÀ1
at a 4 cmÀ1 resolution.
UALs
UKEs
CMA
CMO
a,b-Unsaturated aldehydes
a,b-Unsaturated ketones
Cinnamaldehyde
Cinnamyl alcohol
Transmission electron microscopy (TEM) analyses. TEM
photographs were taken using a JEOL JEM-2100 microscope. For
TEM observations, the samples were ultrasonically dispersed in
ethanol and a drop of the suspension was deposited on a carbon-
coated Cu grid followed by the evaporation of solvent in air.
X-ray photoelectron spectra (XPS) measurements. The XPS spec-
tra of catalysts were recorded on a Thermo VG ESCALAB250 X-ray
photoelectron spectrometer at a pressure of about 2 Â 10À9 Pa
using Al Ka X-ray as the excitation source (1486.6 eV). The shifts of
all binding energies were calibrated using C1s core level at 284.6 eV.
Nitrogen adsorption–desorption measurements. The specific
surface areas were measured from N2-adsorption at 77 K using a
static volumetric Quantachrome Autosorb-1C-VP Analyzer. The
specific surface area was calculated using a BET method.
CO-adsorption infrared analyses. The FTIR of CO-adsorption
was achieved on a PC-controlled PWP 110-40 FTIR spectrophot-
ometer. A catalyst of B30 mg was pressed into a self-supporting
pellet and placed in a sealed chamber. The chamber was purged
with a flow of highly-pure N2 and heated to 343 K and maintained
for 1 h. After the pretreatment, the chamber was cooled down to
298 K under a N2 atmosphere. Subsequently, highly pure CO gas
was introduced to achieve the adsorption of CO molecules onto the
catalysts. Afterwards, the chamber was re-purged by N2 to drive the
CO in the gas phase away. The IR spectra were recorded by
HCMA Hydrocinnamaldehyde
HCMO Phenyl propanol
Acknowledgements
The authors acknowledge the financial support from the
National Natural Science Foundation of China (Grant no.
21076016), the Program for Changjiang Scholars and Innovative
Research Team in University (Grant no. IRT1205), the 973
Program (Grant no. 2011CBA00506) and the fund of the Key
Laboratory of Fine Petrochemical Engineering of Jiangsu
Province.
Notes and references
1 R. L. Augustine, in Heterogeneous Catalysts in Organic Synthesis,
ed. Dekker, New York, 1995.
2 B. Coq and F. Figueras, Coord. Chem. Rev., 1998, 1753, 178.
3 D. Y. Murzin, P. Maki-Arvela, J. Hajek and T. Salmi, Appl.
Catal., A, 2005, 292, 1.
4 D. Loffreda, F. Delbecq, F. Vigne and P. Sautet, J. Am. Chem.
Soc., 2006, 128, 1316.
5 M. S. Ide, B. Hao, M. Neurock and R. J. Davis, ACS Catal.,
2012, 2, 671.
6 X. Zhang, Y. C. Guo, Z. C. Zhang, J. S. Gao and C. M. Xu,
J. Catal., 2012, 292, 213.
´
averaging 64 scans with a resolution of 2 cmÀ1
.
Catalytic tests
´
7 J. Hidalgo-Carrillo, M. A. Aramendıa, A. Marinas,
General procedure for the hydrogenation of cinnamaldehyde.
The catalytic tests were carried out in a magnetically stirred
J. M. Marinas and F. J. Urbano, Appl. Catal., A, 2010,
385, 190.
c
2826 Catal. Sci. Technol., 2013, 3, 2819--2827
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
8 C.-Y. Hsu, T.-C. Chiu, M.-H. Shih, W.-J. Tsai, W.-Y. Chen and 28 I. T. Horvaeth, Green Chem., 2008, 10, 1024.
C.-H. Lin, J. Phys. Chem. C, 2010, 114, 4502.
9 J. P. Tessonnier, L. Pesant, G. Ehret, M. J. Ledoux and
C. Pham-Huu, Appl. Catal., A, 2005, 288, 203.
10 B. C. Campo, M. A. Volpe and C. E. Gigola, Ind. Eng. Chem.
Res., 2009, 48, 10234.
29 B. H. Zhao, J. G. Chen, X. Liu, Z. W. Liu, Z. Hao, J. Xiao and
Z. T. Liu, Ind. Eng. Chem. Res., 2012, 51, 11112.
30 J. Zhu, Y. Jia, M. Li, M. Lu and J. Zhu, Ind. Eng. Chem. Res.,
2013, 52, 1224.
31 M. M. Wang, L. He, Y. M. Liu, Y. Cao, H. Y. He and
K. N. Fan, Green Chem., 2011, 13, 602.
32 D. Tongsakul, S. Nishimura, C. Thammacharoen, S. Ekgasit
and K. Ebitani, Ind. Eng. Chem. Res., 2012, 51, 16182.
33 F. Zhang, X. Zhao, C. Feng, B. Li, T. Chen, W. Lu, X. Lei and
S. Xu, ACS Catal., 2011, 1, 232.
34 B. M. Choudary, S. Madhi, N. S. Chowdari, M. L. Kantamn
and B. Sreedhar, J. Am. Chem. Soc., 2002, 124, 14127.
11 J. Zhu, M. Li, M. Lu and J. Zhu, Catal. Sci. Technol., 2013,
3, 737.
´
´
´
12 J. Alvarez-Rodrıguez, I. Rodrıguez-Ramos, A. Guerrero-Ruiz
and A. Arcoya, Appl. Catal., A, 2009, 366, 114.
13 A. K. Prashar, S. Mayadevi and R. Nandini Devi, Catal.
Commun., 2012, 28, 42.
´
´
14 J. C. Serrano-Ruiz, A. Lopez-Cudero, J. Solla-Gullon,
´
A. Sepu´lveda-Escribano, A. Aldazb and F. Rodrıguez-Reinoso, 35 D. G. Li, C. Wang, D. Tripkovic, S. Sun, N. M. Markovic and
J. Catal., 2008, 253, 159. V. R. Stamenkovic, ACS Catal., 2012, 2, 1358.
15 X. Han, R. Zhou, B. Yue and X. Zheng, Catal. Lett., 2006, 36 M. Ogawa and H. Kaiho, Langmuir, 2002, 18, 4240.
109, 157.
37 C. K. Tsung, J. N. Kuhn, W. Huang, C. Aliaga, L.-I. Hung,
G. A. Somorjai and P. Yang, J. Am. Chem. Soc., 2009,
131, 5816.
38 K. M. Bratlie, H. Lee, K. Komvopoulos, P. Yang and
G. A. Somorjai, Nano Lett., 2007, 7, 3097.
39 S. C. Tsang, N. Cailuo, W. Oduro, A. T. S. Kong, L. Clifton, K.
M. K. Yu, B. Thiebaut, J. Cookson and P. Bishop, ACS Nano,
2008, 2, 2547.
16 Y. Li, P. F. Zhu and R. X. Zhou, Appl. Surf. Sci., 2008, 254, 2609.
17 Y. Li, Z. G. Li and R. X. Zhou, J. Mol. Catal. A: Chem., 2008,
279, 140.
18 Z. Guo, Y. Chen, L. Li, X. Wang, G. L. Haller and Y. Yang,
J. Catal., 2010, 276, 314.
19 K. Q. Sun, Y. C. Hong, G. R. Zhang and B. Q. Xu, ACS Catal.,
2011, 1, 1336.
20 Rodiansono, S. Khairi, T. Hara, N. Ichikuni and S. Shimazu, 40 P. Bera, K. R. Priolkar and A. Gayen, Chem. Mater., 2003,
Catal. Sci. Technol., 2012, 2, 2139.
15, 2049.
21 M. L. Toebes and Y. H. Zhang, J. Catal., 2004, 226, 215.
22 A. Sepulveda-Escribano, E. V. Ramos-Fernandez and
F. Rodriguez-Reinoso, Catal. Commun., 2008, 9, 1243.
41 L. Charles de Menorval, A. Chaqroune, B. Coq and
F. Figueras, J. Chem. Soc., Faraday Trans., 1997, 93, 3715.
42 M. Boronat and A. Corma, Langmuir, 2010, 26, 16607.
´
´
23 E. V. Ramos-Fernandez, J. Ruiz-Martınez, J. C. Serrano-Ruiz, 43 H. G. Manyar, D. Weber, H. Daly, J. M. Thompson,
´
J. Silvestre-Albero, A. Sepu´lveda-Escribano and F. Rodrıguez-
Reinoso, Appl. Catal., A, 2011, 402, 50.
D. W. Rooney, L. F. Gladden, E. Hugh Stitt, J. Jose Delgado,
S. Bernal and C. Hardacre, J. Catal., 2009, 265, 80.
44 Z. Tian, X. Xiang, L. Xie and F. Li, Ind. Eng. Chem. Res., 2013,
52, 288.
24 S. Bhogeswararao and D. Srinivas, J. Catal., 2012, 285, 31.
25 A. Rajagopalan and W. Kroutil, Mater. Today, 2011, 14, 144.
26 P. Barbaro, L. Gonsalvi, A. Guerriero and F. Liguori, Green 45 P. Gallezot and D. Richard, Catal. Rev. Sci. Eng., 1998, 40,
Chem., 2012, 14, 3211. 81.
27 X. Wu, J. Liu, X. Li, A. Zanotti-Gerosa, F. Hancock, D. Vinci, 46 W. Yu, H. Liu, M. Liu and Q. Tao, J. Mol. Catal. A: Chem.,
J. Ruan and J. Xiao, Angew. Chem., Int. Ed., 2006, 45, 6718.
1999, 138, 273.
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 2819--2827 2827