PtSn/SiO2 catalysts prepared by surface controlled reactions for the selective hydrogenation of cinnamaldehyde

Article abstract of DOI:10.1016/j.apcata.2010.05.020

Tin-modified platinum catalysts were prepared by means of a controlled surface reaction, varying the Sn:Pt atomic ratio between 0 and 0.8. The addition of tin caused a noticeable change both in product distribution and the reaction rate. The selectivity to cinnamyl alcohol increases as a function of Sn content, reaching a value of 80% for a Sn:Pt ratio of 0.8. Reaction rate is also higher for bimetallic catalysts than for Pt/SiO₂, with the highest value obtained for systems with Sn:Pt = 0.2-0.4. These results are indicative of the formation of a new type of active sites, compared to those present in the monometallic catalyst. Bimetallic catalysts using a water-soluble tin precursor Bu₃SnOH, were also prepared. An enhanced performance regarding the monometallic catalyst was also obtained with these materials proving to be an environmentally friendlier alternative to obtain PtSn catalysts.

Full text of DOI:10.1016/j.apcata.2010.05.020

Applied Catalysis A: General 383 (2010) 43–49
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
PtSn/SiO2 catalysts prepared by surface controlled reactions for the selective
hydrogenation of cinnamaldehyde
Andrea B. Merloa, Bruno F. Machadob, Virginia Veterea, Joaquim L. Fariab,∗, Mónica L. Casellaa,∗∗
a Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA) (Facultad de Ciencias Exactas,
Universidad Nacional de La Plata and CCT La Plata, CONICET), Calle 47 No. 257, C.C. 59, 1900 La Plata, Argentina
b Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Departamento de Engenharia Química,
Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Tin-modified platinum catalysts were prepared by means of a controlled surface reaction, varying the
Sn:Pt atomic ratio between 0 and 0.8. The addition of tin caused a noticeable change both in product dis-
tribution and the reaction rate. The selectivity to cinnamyl alcohol increases as a function of Sn content,
reaching a value of 80% for a Sn:Pt ratio of 0.8. Reaction rate is also higher for bimetallic catalysts than
for Pt/SiO2, with the highest value obtained for systems with Sn:Pt = 0.2–0.4. These results are indicative
of the formation of a new type of active sites, compared to those present in the monometallic catalyst.
Bimetallic catalysts using a water-soluble tin precursor Bu3SnOH, were also prepared. An enhanced per-
formance regarding the monometallic catalyst was also obtained with these materials proving to be an
environmentally friendlier alternative to obtain PtSn catalysts.
Received 22 December 2009
Received in revised form 13 May 2010
Accepted 15 May 2010
Available online 24 May 2010
Keywords:
Surface organometallic chemistry
Bimetallic catalysts
PtSn
Selective hydrogenation
Cinnamaldehyde
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
order to search for a catalytic system able to actively and selec-
tively carry out the preferential hydrogenation of the C O bond
The selective hydrogenation of ␣,␤-unsaturated aldehydes to
the corresponding allylic alcohols is still a great challenge in het-
erogeneous catalysis, both from the academic and the industrial
point of view [1]. The application of these alcohols often includes
processes of the pharmaceutical and cosmetic industries [2], along
with several uses as intermediary products in preparative organic
synthesis [3]. A typical example is cinnamyl alcohol, which can be
obtained from the selective hydrogenation of cinnamaldehyde, and
finds applications in the perfumery industry due to its aroma and
its fixative properties. It is also an important intermediate in the
synthesis of the antibiotic chloromycetin [4].
From a thermodynamic point of view, it is well known that the
formation of the saturated aldehyde is favored over that of the
unsaturated alcohol. Moreover, it is easier to hydrogenate an iso-
lated C C bond rather than the C O from an aldehyde or ketone
[5]. As a consequence, considerable efforts have been made in
in the presence of a conjugated C C bond [6,7]. In spite of these
difficulties, selectivity towards the unsaturated alcohol can be
significantly improved using carefully designed heterogeneous cat-
alysts. The selectivity of the hydrogenation reactions depends on
several parameters such as particle size, metal precursor employed,
nature of the support, presence of promoters and reaction sol-
vent. The solvent effects on the hydrogenation of ␣,␤-unsaturated
aldehydes has been extensively reviewed by Singh and Vannice
[8], analyzing factors such as polarity, solubility of hydrogen and
substrate–solvent interactions. So far it seems difficult to have a
complete description of the so-called “solvent effect”. As an exam-
ple, a variety of results with respect to solvent polarity has been
discussed in the literature. Furthermore, in cinnamaldehyde hydro-
genation using polar solvents is known to favor hydrogenation of
the C O bond, whilst with non-polar solvents hydrogenation of the
C
C double bond is favored over Pd/C, Pt/C and Co/Al2O3 catalysts
[9].
When using metal oxides as catalytic supports, there are two
main mechanisms concurring to produce high amounts of the
unsaturated alcohol: one includes the use of supports with high
reducibility (e.g. TiO2) [10,11], whilst the other involves the addi-
tion of metal promoters (e.g. Sn) to yield multi-metallic catalysts
[12,13]. Whereas in the first case the mechanism is based on the
sub-oxide species migrated from the support to decorate the metal
∗
Corresponding author.
Corresponding author at: Facultad de Ciencias Exactas, Universidad Nacional de
∗∗
La Plata, Department of Chemistry, Calle 47 No. 257, 1900 La Plata, Buenos Aires,
Argentina.
E-mail addresses: jlfaria@fe.up.pt (J.L. Faria), casella@quimica.unlp.edu.ar
(M.L. Casella).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.05.020

44
A.B. Merlo et al. / Applied Catalysis A: General 383 (2010) 43–49
surface, the latter takes advantage of an increased electronic inter-
action between the metals. Among the different combinations, PtSn
bimetallic catalysts have been widely studied due to the significant
improvement in selectivity results [14–16].
In previous works we studied the hydrogenation of unsaturated
carbonyl compounds using various bimetallic PtSn supported cat-
alysts. A significant increase in selectivity towards the unsaturated
alcohol was found when the Pt catalysts were modified with Sn
introduced using techniques derived from Surface Organometal-
lic Chemistry on Metals (SOMC/M) [17]. This technique involves
2.2. Catalyst characterization
The platinum and tin contents were determined by atomic
absorption spectroscopy (Varian Spectra AA55). The tin content of
bimetallic catalysts was also analyzed using a Varian CP-3800 gas
chromatograph equipped with a flame ionization detector and a
capillary column FactorFour CP8907 (VF-1 ms, 15 m × 0.25 mm ID,
DF = 0.25).
The Fourier transform infrared spectra (FT-IR) of the
Bu3SnCOOCH3 (solid) was recorded using
a Thermo Nicolet
the reaction between
a
supported transition metal and an
Avatar 370DTGS apparatus, using pressed disks of the solid dilutes
in KBr. The liquid sample containing the Bu3SnOH, was examined
as a film and both of them were obtained in the range between
organometallic compound in a H2 atmosphere allowing control of
the various stages of preparation of the catalyst leading to well-
defined solid structures with very good catalytic performances
[13,18]. In order to extend this methodology for the preparation
of catalysts in a more environmentally friendly way, we could
examine the use of a water-soluble organometallic tin precursor
Bu3SnOH. This work, reports the preparation of PtSn bimetallic cat-
alysts using SOMC/M techniques. The effect of both, Sn precursor
(SnBu4 and Bu3SnOH) and its load (0 < Sn:Pt < 1.0), is investigated
in the liquid-phase selective hydrogenation of cinnamaldehyde to
cinnamyl alcohol.
400 and 4000 cm−1
.
Temperature-programmed reduction (TPR) tests were carried
out in a conventional reactor equipped with a thermal conductivity
detector with a feeding flow of 25 cm3 min−1 (5% H2 in N2) at a
heating rate of 10 ◦C min−1
.
H2 and CO chemisorption measurements were performed in
a static volumetric apparatus at ambient temperature (Advanced
Scientific Designs Inc., USA).
The size distribution of metallic particles was determined by
transmission electron microscopy (TEM) using a JEOL 100 CX
instrument. The samples were ground and ultrasonically dispersed
in distilled water. To estimate the mean particle size, the particles
were considered spherical and the second moment of the distribu-
tion was employed.
2. Experimental
2.1. Catalyst preparation
X-ray photoelectron spectra (XPS) were acquired with a mul-
titechnique system (SPECS) equipped with a Al-K␣ 100 W X-ray
source and a hemispherical electron analyzer PHOIBOS 150, oper-
ated in fixed analyzer transmission (FAT) mode. The spectra were
collected at an energy pass of 30 eV. The powder samples were
pressed to form a disc and mounted onto a manipulator that
allowed the transfer from the pretreatment chamber to the analysis
chamber. In the pretreatment chamber, the samples were reduced
for 1 h at 400 ◦C in flowing H2. The spectra were recorded once the
pressure in the analysis chamber reached a residual pressure of less
than 5 × 10−9 mbar. The binding energies were referenced to the C
1s line at 284.6 eV. The intensities were estimated by calculating
the integral of each peak after subtracting the S-shaped background
and fitting the experimental peak to a Lorentzian/Gaussian mix of
variable proportion, using the Casa XPS program (Casa Software
Ltd., UK).
An Evonik (previously Degussa) silica (Aerosil 200, 180 m2 g−1
)
was used as support to prepare Pt and PtSn catalysts. The pI (iso-
electric point) of the SiO2 is 2, indicating that at pH > 2 the solid
surface has mainly negative charges. However, this becomes signif-
icant at pH values above 5. Moreover, at pH > 11 occurs an important
dissolution of the SiO2. Therefore, the solid is an excellent cation
exchanger at pH between 9 and 11 [19,20].
The catalyst Pt/SiO2, was obtained by cationic exchange
between the support (SiO2) and the metallic precursor
([Pt(NH3)4]Cl2 Aldrich 99%) in water solution. The support
was previously treated with amoniacal solution (pH = 10.6) for
about 30 min. The concentration of the [Pt(NH3)4]2+ solution was
set as to obtain 1 wt.% Pt exchanged on the support. The solid was
stirred for 24 h at 25 ◦C and then it was recovered by filtration
from the solution using a vacuum pump. The material was then
repeatedly washed with water, dried at 105 ◦C and reduced in H2
flow at 500 ◦C, yielding the monometallic Pt/SiO2 catalyst.
The bimetallic PtSn/SiO2 catalysts were prepared according to
SOMC/M techniques. A portion of reduced Pt/SiO2 catalyst was
allowed to react with tetrabutyltin (SnBu4) in a paraffinic solvent.
The reaction was carried out in H2 atmosphere at either 90 ◦C in n-
heptane or 120 ◦C in n-decane, depending on the amount of tin to
be loaded. After 4 h of reaction, the liquid-phase was separated and
the solid was repeatedly washed with n-heptane. The material was
then dried in N2 at 90 ◦C and finally reduced in flowing H2 at 500 ◦C
for 2 h. The bimetallic catalysts were designated PtSny, y being the
Sn:Pt atomic ratio.
Alternatively, tributyltin hydroxide (Bu3SnOH) was used as
tin precursor in the aqueous phase preparation of the PtSn/SiO2
bimetallic catalyst. This organometallic precursor was obtained
through hydrolysis of tributyltin acetate (Bu3SnCOOCH3) at reflux
temperature in KOH 0.001 M (pH = 10) for 6 h. Afterwards, the
Bu3SnOH solution reacted with a portion of reduced monometallic
catalyst, in flowing H2 at 90 ◦C for 4 h. The liquid-phase was sepa-
rated and the solid was repeatedly washed with water and finally
reduced in H2 at 500 ◦C for 2 h. These catalysts were designated
PtSnA and PtSnB and they have different nominal amounts of tin
(Sn/Pt = 1.0 and 0.8 respectively).
2.3. Cinnamaldehyde hydrogenation
The experimental conditions employed were chosen after
checking that the hydrogenations were measured under a kinetic
regime, i.e., in the absence of internal and external mass transfer
limitations. To do so, several tests with different catalyst masses
and stirring rates were investigated.
In a typical test, the liquid-phase hydrogenation of cinnamalde-
hyde was carried out in a well-stirred stainless steel batch reactor
(1000 rpm), the reaction mixture contained n-heptane (Aldrich,
99%), 0.14 g cinnamaldehyde (Fluka, 98%), decane (Fluka, 98%, as
internal standard for gas chromatography) and 0.20 g of cata-
lyst. Before this mixture was heated to the reaction temperature
(90 ◦C), the solution was first bubbled with nitrogen (under
3 bar pressure) to remove traces of dissolved oxygen. Once the
final temperature was reached, hydrogen (also under 3 bar) was
used to purge the nitrogen and to pressurize the system to
the desired 10 bar. The reaction was then allowed to proceed,
and samples were withdrawn to monitor product distribution.
The analysis was performed with a DANI GC-1000 gas chro-
matograph, equipped with a split/splitless injector, a capillary
column (WCOT Fused Silica 30 m, 0.32 mm i.d., coated with CP-Sil

A.B. Merlo et al. / Applied Catalysis A: General 383 (2010) 43–49
45
Fig. 1. TEM micrographs and particle size distribution (inset) for (a) Pt/SiO2 and (b)
PtSn0.8/SiO2.
Fig. 2. Variation of the concentration of tetra n-butyl tin (SnBu4) as a function of
time for various initial concentrations of the organotin compound.
8 CB low bleed/MS 1 ␮m film) and a flame ionization detec-
tor.
The results of the reaction runs were analyzed in terms of reac-
tant conversion, product selectivity and initial reaction rate as
described elsewhere [13,21]. To compare the activity exhibited
by the catalysts, the turn-over frequency (TOF) was also calcu-
lated.
particles and is homogeneously dispersed through the support sur-
face [23].
Tin-modified catalysts were obtained by a surface controlled
reaction between the reduced monometallic catalyst and a SnBu4
solution, using a paraffinic solvent like n-heptane or n-decane.
This reaction involves a two-step anchoring process: in the first
step, the organic groups are anchored to the surface producing an
organobimetallic system (temperatures between 90 and 150 ◦C);
the second step corresponds to the formation of the bimetallic
phase after the release of all the organic groups (temperatures
between 150 and 500 ◦C). During these steps, the following reac-
tions take place:
3. Results and discussion
3.1. Preparation and characterization of catalysts
The TPR spectrum for Pt/SiO2 catalyst shows two H2 consump-
tion peaks, one about 254 ◦C and another at 456 ◦C. According to
the literature, the peak at low temperature is assigned to Pt (IV)
species, generated during the calcination pretreatment. On the
other hand, the peak at a higher temperature could be assigned
xy
Pt/SiO2 + ySnBu4
+
H2 → Pt(SnBu4−x)y/SiO2 + xyBuH
(1)
2
(4 − x)y
n−y
(n = 2+ or 4+) species, formed from the metallic
at Pt–(O–Si
)
y
Pt(SnBu4−x)y/SiO2
+
H2 → PtSny/SiO2 + (4 − x)yBuH (2)
2
precursor-support interaction [22]. The peaks are wide and have
shoulders, which could be due to the environment of the ions on
the support surface.
Data obtained from the TPR test allowed us to establish the
conditions for activation of the Pt/SiO2 monometallic catalyst:
reduction in H2 flow at 500 ◦C for 2 h.
The monometallic Pt/SiO2 catalyst was prepared using an ionic
exchange approach and a material with a relatively high metal dis-
persion (H/Pt = 0.65, CO/Pt = 0.55) was obtained. Analysis by TEM
confirmed the presence of uniform particles with a mean particle
size centered at around 2 nm (Fig. 1). In order to prepare bimetallic
catalysts by means of SOMC/M techniques it is essential that the
noble metal present in the parent material possesses both small
Reaction (1) was carried out at two different temperatures: 90 and
120 ◦C. In Fig. 2a is the variation of concentration of tetra n-butyl tin
as function of time for various initial concentrations of the organ-
otin compound. Blank experiments carried out on silica support,
without metallic platinum, did not yield any detectable reaction of
tetra n-butyl tin with the support.
In addition to Fig. 1, Table 1 shows the influence of increasing
amounts of tin in the platinum particle size (measured by TEM) for
the studied materials. The metal particle size distribution was in
all cases very narrow, with only slight oscillations in dTEM being
observed for the bimetallic systems regarding the original Pt/SiO2
catalyst. These results are in good agreement with the selective

46
A.B. Merlo et al. / Applied Catalysis A: General 383 (2010) 43–49
Table 1
Pt:Sn atomic ratio and Pt particle size (measured by TEM) for SiO2 supported Pt and
PtSn catalysts.
Catalyst
Sn:Pt (atomic ratio)
dPt (nm)
Pt
–
2.2
2.2
2.2
2.2
2.1
PtSn0.2
PtSn0.4
PtSn0.6
PtSn0.8
0.2
0.4
0.6
0.8
addition of Sn over Pt, already reported with other bimetallic sys-
tems prepared by SOMC/M techniques [24–26].
The reaction between SnBu4 and Pt/SiO2 was followed by GC
analysis, measuring the variation of SnBu4 concentration in the
impregnating solution. The total amount of tetra n-butyl tin fixed
on the catalysts surface was measured by two different and inde-
pendent methods: difference between the initial and the final
concentration of tetra n-butyl tin and by chemical analysis of
the samples. A good agreement was found between both meth-
ods.
The results reported in Fig. 2b, show that after a certain time of
reaction, the amount of tin fixed per superficial Pt (Sn:Pt) reaches
a plateau, except for the highest Sn:Pt ratio. Once the reaction is
finished, washing the catalyst with n-heptane did not remove any
significant amount of SnBu4. Fig. 2b also shows that an increase
in the reaction temperature has a beneficial influence on both the
SnBu4 reacted with Pt/SiO2 and the rate of reaction (1). At 90 ◦C it
was possible to fix tin selectively onto platinum up to a Sn:Pt ratio
of approximately 0.40, whereas this quantity increased to 0.8 when
the reaction temperature was 150 ◦C.
Fig. 3. XPS spectra for the Sn region (Sn 3d5/2 level): (a) PtSn = 0.2 and (b) PtSn = 0.8.
The binding energies (BE) of the Pt 4f7/2 and Sn 3d5/2 levels for
Pt/SiO2, PtSn0.2 and PtSn0.8 catalysts are reported in Table 2. Addi-
tionally, the surface Sn(0)/Pt atomic ratios and the relative amounts
of reduced tin (Sn(0)/Sntotal), estimated from the integral of the XPS
peaks, are also given.
(probably alloys), ionic tin located at the platinum–support inter-
face and some segregated metallic platinum.
In order to develop an environmentally friendlier process for
Sn addition, it was tested the preparation of bimetallic catalysis in
aqueous phase using a water-soluble precursor. Tributyltin hydrox-
ide (Bu3SnOH) was selected as Sn precursor and was obtained from
Bu3SnCOOCH3 according to the following reaction:
For all the catalysts, the region corresponding to Pt 4f7/2 showed
a single peak, which is characteristic of platinum in the metal-
lic state [27]. Both bimetallic samples presented a shift of this
peak toward lower BE. This fact is indicative of the existence of an
electronic effect of tin over platinum, in agreement with previous
results obtained in studies performed with PtSn catalysts supported
on SiO2 and Al2O3, prepared by SOMC/M techniques [28,29]. The Sn
3d5/2 spectra for the bimetallics PtSn = 0.2 and PtSn = 0.8 are plot-
ted in Fig. 3. They present two bands, one centered at ca. 484 eV
which can be ascribed to reduced tin [30], and another one at
higher BE (486.3 and 487.3 eV for PtSn = 0.2 and PtSn = 0.8, respec-
tively), which is attributed to oxidized tin (Sn(II) and Sn(IV)) [27].
The presence of metallic tin in bimetallic catalysts is an indication
for the existence of bimetallic PtSn phases (even alloys), as it has
been readily assessed by EXAFS analysis performed on analogous
systems as the ones here employed [28].
Although both bimetallic samples have more or less the same
Sn(0)/Sntotal ratio (Table 2), the Sn(0)/Pt ratio increases when the
amount of tin increases, which can be assigned to the following
effects: (i) dilution of platinum by metallic tin, due to the forma-
tion of SnPt phases and (ii) covering of metal particles surface by
oxidized tin species. Taking into account all the XPS results, the
catalytic surface maybe depicted by the coexistence of SnPt phases
Bu3SnCOOCH3 → OH− → Bu3SnOH + CH3COO−
(3)
This reaction was carried out in the presence of KOH (pH = 10)
according to a reported procedure [31]. The FT-IR spectrum of the
as-synthesized product (Bu3SnOH) was obtained, and compared
to that observed for the precursor (Bu3SnCOOCH3). Strong absorp-
tion bands at wavenumbers between 2857 and 2957 cm−1 were
observed for both compounds, corresponding to C–H stretching
modes of butyl groups. For the product, the bands at 1076 cm−1
(assigned to –Sn–O–CO) and 1581 cm−1 (assigned to –O–CO) have
almost disappeared, confirming the formation of Bu3SnOH; the
bands in the range 612–409 cm−1 due to Sn–O and Sn–C vibrations,
further confirms its formation.
Once synthesized, the Bu3SnOH was used to prepare bimetallic
catalysts with different Sn:Pt ratios (PtSnA and PtSnB) according
to SOMC/M methodologies. In this case, the amount of Sn incorpo-
rated in the Pt surface was always lower than the nominal value
expected.
Table 2
Binding energies (eV) of the Pt 4f7/2 and Sn 3d5/2 levels for SiO2 supported Pt and PtSn catalysts.
Catalyst
BE Pt 4f7/2 (eV)
BE Sn 3d5/2 (eV)
Sn(0):Sntotal (atomic ratio)
Sn(0):Pt (atomic ratio)
Pt/SiO2
PtSn0.2
PtSn0.8
71.6
71.1
70.8
–
–
0.55
0.59
–
0.11
0.47
484.1, 486.3
484.2, 487.3

A.B. Merlo et al. / Applied Catalysis A: General 383 (2010) 43–49
47
Fig. 4. Reaction scheme proposed for the selective hydrogenation of cinnamaldehyde using SiO2 supported Pt and PtSn catalysts.
3.2. Selective hydrogenation of cinnamaldehyde
The Pt/SiO2 catalyst presents a lower TOF value when compared
to the bimetallic ones. In addition, selectivity to the desired cin-
namyl alcohol is rather low (<20% throughout the reaction). These
selectivity results are similar to those reported for other catalytic
systems where metal oxides do not exhibit the SMSI effect after
reduction at high temperatures [33,34].
Tin addition to the monometallic catalyst produces a signifi-
cant change to both product distribution (Fig. 5) and reaction rate
(Table 3). With Pt/SiO2 catalyst, concentration of COL remains very
low, whereas that of HCAL is much higher and increases through-
out the reaction. After Sn addition there is a total inversion on
the preferential pathway between C C and C O, regarding the
monometallic catalyst. Furthermore, the maximum COL concen-
tration observed is much higher than that observed for HCAL with
the Pt catalyst. Selectivity to COL increases as a function of the
Sn load, reaching 80% when the catalyst having the highest Sn:Pt
ratio is used. Simultaneously, selectivity to HCAL decreased with
increasing Sn:Pt ratio.
Selectivity results often depend on the nature of the support
where the metal is dispersed. Several examples of how the support
can effectively improve selectivity towards the unsaturated alcohol
can be found using, for example, zeolites [35–37] or reducible metal
oxides [38–40]. In these cases, the enhanced selectivity results are
explained in terms of shape selectivity and steric effects due to the
presence of small channels and by sub-oxide species decorating
the surface of the metal phase, respectively. In the work here pre-
sented, the use of SiO2 discards any promoting effect attributed to
the support, being the results entirely ascribed to the nature of the
prepared active metal phase.
Besides the improvement in selectivity, tin addition also
enhanced the TOF obtained comparing with the monometallic
Pt/SiO2 catalyst. The highest reaction rates and TOF values were
obtained using a Sn:Pt ratio of 0.2 and 0.4. This result is in perfect
agreement with those previously reported for the hydrogenation
of carbonyl compounds using PtSn bimetallic catalysts, and is
assigned to the creation of a new type of active site, as a conse-
quence of the specific interaction between SnBu4 and the supported
The prefered solvent is n-heptane due to high H2 solubility when
compared to other commonly used solvents [8], and because the
corresponding acetal and ether cannot be formed during the reac-
tion as opposed to what occurs when alcohol-based solvents are
used [6,32].
The reaction pathway proposed for the selective hydrogena-
tion of cinnamaldehyde (CAL) with SiO2 supported catalysts is
shown in Fig. 4. The hydrogenation of CAL involves the paral-
lel and consecutive reduction of different functional groups i.e.,
C
C and C O bonds. Typically, a mixture of the desired cinnamyl
alcohol (COL), hydrocinnamaldehyde (HCAL) and hydrocinnamyl
alcohol (HCOL) is obtained. A number of side reactions involving
the hydrogenation of aromatic ring (3-cyclohexyl-1-propanol, CHP)
and hydrogenolysis can also be detected. In this case, the presence
of ˇ-methylstyrene (MS) and 1-propylbenzene (PB) could indicate
a strong adsorption of the hydroxyl groups over the metal sites and,
thus, a possible poisoning effect [13].
In Table 3 it is presented the turn-over frequency (TOF) results
and selectivities to COL, HCAL and HCOL, measured at 60% conver-
sion of cinnamaldehyde, as a function of tin content over the SiO2
supported catalysts (SnBu4 was used as Sn precursor for bimetallic
catalysts).
Table 3
TOF, initial reaction rate and selectivity results obtained for the hydrogenation of
cinnamaldehyde using SiO2 supported Pt and PtSn catalysts (selectivities measured
at 60% conversion).
−1
Catalyst TOF
ri* (␮mol s−1 gPtsuperficial
)
Selectivity (%)
COL HCAL HCOL Othersa
Pt
0.067 106
0.620 3180
0.504 2582
0.146 750
0.029 147
6
55
8
16
19
7
31
21
13
11
10
PtSn0.2
PtSn0.4
PtSn0.6
PtSn0.8
33 30
42 26
73
80
9
6
4
a
PB, MS and CHP.

48
A.B. Merlo et al. / Applied Catalysis A: General 383 (2010) 43–49
Scheme 1. Adsorption modes of cinnamaldehyde on Pt and PtSn modified surfaces.
rical modification introduced by tin is more pronounced for the
samples with higher tin content, in this way PtSn = 0.8 presents the
highest dilution of platinum sites and the highest concentration of
species ␩1-(O), correspondingly, the selectivity to COL reaches the
highest level (Scheme 1b and c).
Fig. 5. Product distribution for the selective hydrogenation of cinnamaldehyde
using (a) Pt and (b) PtSn = 0.8 catalysts.
Electronic effects must be also important in the chemisorption
of the reaction intermediates. The existence of “Lewis acid sites”,
due to the presence of ionic tin, tends to promote the H2 attack
to the C O group and in this way, leads to higher selectivity to
COL. This type of electronic modifications seems to play a funda-
mental role for molecules with phenyl groups, as in the case of the
cinnamaldehyde [13].
and reduced Pt [41]. A higher addition of tin reduces the initial
hydrogenation rate and the TOF value, which may be due to the low-
ered number of active sites for the formation of hydrogen atoms.
This way, the activity of the bimetallic systems depends on the con-
centration of tin present in different oxidation status, leading to an
optimum in the Sn/Pt atomic ratio.
Catalytic results suggest that cinnamaldehyde hydrogenation
on Pt-based supported catalyst occurs through different interme-
diates depending on the surface properties of the catalyst. Pt/SiO2
catalyst showed low activity for converting CAL and also a low
selectivity towards COL. Delbecq and Sautet have modeled the
adsorption of several unsaturated ␣,␤-aldehydes (acrolein, croton-
aldehyde, prenal and cinnamaldehyde) using the extended Hückel
approach on platinum and palladium crystals [42]. These authors
found that the adsorption mode of the molecule is strongly depen-
dent on the nature of the metal and on the type of exposed face.
For the case of high dispersions (which is the case of the catalysts
here employed), the percentage of Pt (1 1 0) and Pt (1 0 0) faces is
higher and for this reason adsorption of CAL through a ␩2-␲(C,C)
type would be favored leading to the preferential hydrogenation of
the C C group (Scheme 1a).
The hydrogenation of the C O group, leading to COL produc-
tion, shows a noticeable increase in catalysts modified with tin,
with respect to the monometallic catalyst. The higher the tin con-
tent, the higher the selectivity towards COL. In these catalysts, the
dilution of platinum sites would favor the presence of species of
the type ␩1-(O) for the adsorption of cinnamaldehyde, inhibiting
other forms of chemisorption such as ␩2-(C,C) and ␩4-(C,C,C,O),
favorable for the hydrogenation of the C C group. These geomet-
Fig. 6. Effect of the Sn precursor over the selectivity results: Pt (no tin), PtSn = 0.2
(SnBu4) and PtSnA and PtSnB (nominal amount of tin SnPt = 1.0 and 0.8 respectively)
(Bu3SnOH).

A.B. Merlo et al. / Applied Catalysis A: General 383 (2010) 43–49
49
The selective hydrogenation of cinnamaldehyde was also car-
ried out with bimetallic catalysts prepared using a water-soluble
Sn precursor (Bu3SnOH), in order to assess if these materials were
as active and selective as those prepared using SnBu4. The catalytic
results using these materials are depicted in Fig. 6.
Both catalysts provided an enhanced selectivity to COL regard-
ing the Pt/SiO2 catalyst, as the catalytic behavior was similar to that
exhibited by PtSn = 0.2 (prepared using SnBu4). This result is not
surprising since the amount of Sn that was successfully introduced
using the Bu3SnOH precursor was rather lower than the nominal
load intended, as already indicated in the experimental section. In
spite of this, the improvement in selectivity to COL allows us to
conclude that SOMC/M techniques can also be performed in the
aqueous phase. Thus, this methodology could provide an alterna-
tive, environmentally friendlier, pathway to the production of new
bimetallic catalysts.
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Acknowledgements
The authors would like to thank the following institutions for
funding this work: Consejo Nacional de Investigaciones Científicas
y Técnicas (CONICET) (PIP 6527), Agencia Nacional de Promoción
Científica y Tecnológica (PICT 14-11243), Universidad Nacional de
La Plata (Project X 487) and Ministerio de Ciencia, Tecnología e
Innovación Productiva (Bilateral Cooperation between Argentina
and Portugal). Bilateral Cooperation FCT/Argentina, 2008–2009.
This research was carried out under the project FEDER/POCI/2010
approved by the Fundac¸ ão para a Ciência e a Tecnologia (FCT) and
co-supported by FEDER.
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