Intramolecular selective hydrogenation of cinnamaldehyde over CeO 2-ZrO2-supported Pt catalysts

Article abstract of DOI:10.1016/j.jcat.2011.09.006

Selective liquid phase hydrogenation of cinnamaldehyde is reported, for the first time, over CeO₂, ZrO₂, and CeO₂-ZrO ₂-supported Pt catalysts. Cinnamyl alcohol is the selective product. These catalysts are highly active and selective even at 25 °C and found to be superior to most of the hitherto known supported Pt catalysts. Alkali addition (NaOH) has enhanced the performance of these catalysts. At an optimized reaction condition, 95.8% conversion of cinnamaldehyde and 93.4% selectivity of cinnamyl alcohol have been obtained. Acidity of the support (due to the presence of ZrO₂ component) and higher electron density at Pt (due to CeO₂ component) are attributed to be responsible for the superior catalytic activity of Pt supported on CeO₂-ZrO₂ composite material.

Full text of DOI:10.1016/j.jcat.2011.09.006

Journal of Catalysis 285 (2012) 31–40
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Intramolecular selective hydrogenation of cinnamaldehyde
over CeO2–ZrO2-supported Pt catalysts
⇑
S. Bhogeswararao, D. Srinivas
Catalysis Division, National Chemical Laboratory, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective liquid phase hydrogenation of cinnamaldehyde is reported, for the first time, over CeO2, ZrO2,
and CeO2–ZrO2-supported Pt catalysts. Cinnamyl alcohol is the selective product. These catalysts are
highly active and selective even at 25 °C and found to be superior to most of the hitherto known sup-
ported Pt catalysts. Alkali addition (NaOH) has enhanced the performance of these catalysts. At an opti-
mized reaction condition, 95.8% conversion of cinnamaldehyde and 93.4% selectivity of cinnamyl alcohol
have been obtained. Acidity of the support (due to the presence of ZrO2 component) and higher electron
density at Pt (due to CeO2 component) are attributed to be responsible for the superior catalytic activity
of Pt supported on CeO2–ZrO2 composite material.
Received 12 May 2011
Revised 12 August 2011
Accepted 6 September 2011
Available online 19 October 2011
Keywords:
Hydrogenation
Cinnamaldehyde
Ó 2011 Elsevier Inc. All rights reserved.
a,b-Unsaturated aldehyde
Ceria–zirconia
Supported Pt catalyst
Promotion by alkali ions
1. Introduction
issues to be resolved. This is a highly demanding task due
to the complexity of the system involving a rather complicated
Selective hydrogenation of
a
,b-unsaturated aldehydes is of both
reaction mechanism, including competitive/non-competitive and
dissociative/non-dissociative adsorption, side reaction, adsorption
of solvent, etc. [2,4,12]. In order to prevent consecutive hydrogena-
tions to the saturated alcohol and the isomerization of the allylic
alcohol (Scheme 1), the catalyst has to suppress these reactions.
The selectivity to unsaturated alcohol must be controlled by
changes in the rate constants of both competitive reactions and
in adsorption constants of the components [1]. However, the key
factors that control the activity and selectivity in the hydrogena-
tion of cinnamaldehyde are still unclear. Support material and par-
ticle size of the metals, selective poisoning, presence of a second
metal, preparation method, catalyst precursor, temperature of
reduction, pressure, etc., have been proposed to influence the
intramolecular hydrogenation selectivity [1].
The selectivity for the unsaturated alcohol can be enhanced by
polarization of the C@O group or by hindering the adsorption of
the substrate through the C@C group. This can be achieved by a right
choice of the support, which can interact with the supported metal
[13–15], addition of a more electropositive metal such as iron, tin,
or zinc [16–19], or by steric and electronic effects provided by the
support [1,19]. Recently, Ramos-Fernández et al. [20] have reported
the effect of the support, Al2O3 and SiO2, on the catalytic behavior of
Cr–ZnO promoted Pt catalysts. The SiO2 support enabled high dis-
persion of the promoter, which, in turn, facilitated high selectivity
(>95%) toward the unsaturated alcohols in the hydrogenation of cin-
namaldehyde. However, high temperature (110 °C), high pressure
fundamental and industrial importance [1–3]. In such compounds,
hydrogenation can occur either at the olefinic (C@C) or carbonyl
(C@O) groups (Scheme 1). While the hydrogenation of olefinic
group leads to saturated aldehydes, that of carbonyl group yields
unsaturated alcohols. Both these hydrogenation products of cinna-
maldehyde—a representative a,b-unsaturated aldehyde, are widely
used in pharmaceuticals, perfumes, and flavors [4,5]. Recently,
hydrocinnamaldehyde was found to be an important intermediate
in the preparation of a drug used in the treatment of HIV [6].
Although extensively studied, the selective hydrogenation of
a
,b-unsaturated aldehydes remains as a challenging task [7–11].
Thermodynamics favor the reduction in C@C group. Selective
hydrogenation of the carbonyl group to achieve the corresponding
unsaturated alcohol with high yield is quite demanding [1–3].
Stoichiometric reducing agents like metal hydrides, aluminum
isopropoxide, etc., can accomplish this selective reduction to
unsaturated alcohols [1,4]. But such processes involve costly chem-
icals and co-produce large amount of waste. Heterogeneous cata-
lysts offer several advantages. A large number of supported Pd,
Pt, Rh, Ru, Au, and Co catalysts have been reported for this transfor-
mation [7–11]. However, high selectivity at high conversion levels
particularly at ambient conditions and catalyst reusability are still
⇑
Corresponding author. Fax: +91 20 2590 2633.
E-mail address: d.srinivas@ncl.res.in (D. Srinivas).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.09.006

32
S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
Scheme 1. Hydrogenation products of cinnamaldehyde.
(70 bar), and long reaction times (>10 h) were needed to obtain high
conversion of cinnamaldehyde over those catalysts. Teddy et al. [21]
studied the influence of alloying in carbon nanotube-supported Pt–
Ru catalysts for the selective hydrogenation. High selectivity (94%)
toward cinnamyl alcohol together with high activity (cinnamalde-
hyde conversion = 70%) was obtained through high temperature
treatment (1000 °C) of the catalyst. Several years ago, Englisch
et al. [22] investigated the effect of polar sites generated on the Pt
surface by addition of metals and oxide promoters (Ga, Sn, and Ge)
that decorated part of the Pt surface and improved the selectivity to-
ward unsaturated alcohols in the liquid phase hydrogenation of cro-
tonaldehyde. However, this addition of metal and oxide promoters
decreased the activity of the catalysts. Coq et al. [23] studied zirco-
nia-supported Ru catalysts and proposed the formation of Ru–ZrOx
sites at the periphery of the Ru particles to explain the enhanced
selectivity for cinnamyl alcohol. Although many studies have been
addition, the temperature of the flask was maintained at 80 °C
and pH at 10. The cations were precipitated in the form of their
hydroxides. The mixture was digested at 80 °C for another 3 h
and then cooled to 25 °C. The precipitate was separated and
washed with distilled water until all the Na+ ions were removed.
It was, then, air-dried, powdered, and calcined at 450 °C for 2.5 h
to get the final product.
Pure ceria and pure zirconia supports were also prepared by the
precipitation method.
2.1.2. Pt/CeO2–ZrO2
Pt (5 wt%) supported on CeO2–ZrO2 was prepared by impregna-
tion method. Tetraaminoplatinum(II) nitrate precursor (1.767 g)
[Pt(NH3)4(NO3)2, 3 wt% aqueous solution, Aldrich] taken in 10 ml
of distilled water was added to 1 g of CeO2–ZrO2 contained in a
glass round bottom flask. It was mixed thoroughly and dried at
80 °C using a rotary evaporator. The solid obtained was recovered
and dried at 100 °C for 6 h in an electric oven. Then, the material
was calcined at 400 °C for 2 h.
conducted on this or other a,b-unsaturated aldehydes using Pt sup-
ported on different oxides, a more efficient and selective catalyst
that is active even at room temperature and low pressures of H2 is
still not known.
Catalyst samples with different Pt loadings (0.5, 1, and 2 wt%)
were prepared in a similar manner taking appropriate amounts
of platinum precursor.
We report here the application of Pt/CeO2–ZrO2 catalysts for
this reaction. Ceria is a reducible oxide. It facilitates dispersion of
Pt and provides good support–metal interactions. Ceria–zirconia
solid solutions provide thermal stability and prevent sintering of
the supported metals. Presence of zirconia enhances the oxygen
storage capacity of ceria. All these superior characteristics of
CeO2–ZrO2 support are expected to impart higher activity and
selectivity in hydrogenation reactions. The ceria-based catalysts
are in use in automobile exhaust emission control, fuel cell re-
search, natural gas combustion, total oxidation of hydrocarbons,
methanol decomposition, and several other reactions [25–30]. In
this study, we have found that the support, electronic structure
of the metal, and alkali ion addition influence the conversion and
product selectivity. Cinnamyl alcohol (formed via C@O hydrogena-
tion) is the major product. A comparison of these results with those
obtained over the Pd catalysts [24] is made. The differences in
activity/selectivity are discussed in terms of electronic and redox
properties of these catalyst systems.
2.2. Characterization procedures
X-ray diffractograms (XRD) of the catalysts were recorded in the
2h range of 10–80° (scan rate of 2.3°/min) on a Xpert Pro PANalyt-
ical X-ray diffractometer with Ni-filtered Cu K
a radiation (40 kV,
30 mA). The specific surface area (BET) of the samples was deter-
mined using a NOVA 1200 Quanta Chrome instrument. The micro-
pore volume was estimated from the t-plot, and the pore diameter
was determined using the Barret–Joyner–Halenda (BJH) model.
Morphological characteristics of the samples were determined
using a high resolution transmission electron microscope (HRTEM,
JEOL, model 1200 EX) operating at 100 kV. X-ray photoelectron
spectra (XPS) of the samples were acquired on a VG Microtech
Multilab ESCA 3000 with Mg
Ka radiation (hm = 1253.6 eV).
Base pressure in the analysis chamber was maintained at 3–
6 Â 10À10 mbar. The peak corresponding to carbon 1s (at
284.2 eV) was taken as a reference in estimating binding energy
(BE) values of various elements in the catalyst.
2. Experimental
2.1. Catalyst preparation
Temperature-programmed desorption of ammonia (NH3-TPD;
Micromeritics Auto Chem 2910 instrument) quantified the density
of acid sites. Diffuse-reflectance infrared Fourier transform (DRIFT;
Shimadzu SSU 8000) spectroscopy of adsorbed pyridine provided
information on the type of acid sites present in the catalyst [24].
Temperature-programmed reduction experiments were carried
on a Micromeritics Auto Chem 2920 instrument: 0.05 g of the cat-
alyst was placed in a quartz tube and treated with He gas (20 cc/
min) at 200 °C for 1 h. A gas mixture of H2(5%)–Ar(95%) was then
passed (20 cc/min) through the quartz reactor at 40 °C for 1 h.
2.1.1. CeO2–ZrO2
CeO2–ZrO2 composites (Ce/Zr molar ratio = 1:1) were prepared
by co-precipitation method [7]. Ce(NO3)3Á6H2O (7.35 g, Semco)
and ZrO(NO3)2ÁxH2O (2.8 g, Loba Chemie) were dissolved sepa-
rately in distilled water (168 and 120 ml, respectively). These solu-
tions were mixed together and added dropwise to a continuously
stirred 0.1 M aq. solution (1 L) of NaOH taken in a 2 L round bottom
flask, placed in an oil bath. Stirring was continued for 3 h. During

S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
33
The temperature was raised to 950 °C at a heating rate of 10°/min
1500
1000
500
0
and held at 950 °C for 10 min. The amount of hydrogen consumed
was estimated, and the reduction capacity of the catalyst was
determined. A standard CuO powder was used to calibrate the H2
consumption.
*
Pt/CeO2-ZrO2
Pt/CeO2
*
*
*
2500
2000
1500
1000
500
Fourier transform infrared (FTIR) spectra of adsorbed CO were
recorded on a Bruker IFS 88 spectrometer with DTGS detector
using OPUS software at a resolution of 4 cmÀ1 over 64 scans. Sam-
ples were made in the form of self-supporting wafers (ca. 12 mg/
cm2) of thickness suitable for transmission IR experiments. Sam-
ples were directly treated in a specially designed quartz cell
equipped with KBr windows, suitable for activation and in situ IR
studies. The samples were degassed while increasing the tempera-
ture to 450 °C at a slow heating rate (ca. 1–2°C/min), treated with
133 mbar of oxygen at 450 °C for 1 h, and then followed by degas-
sing at that temperature, in order to clean any pre-adsorbed mol-
ecules at the active sites. Reduction was carried out under
hydrogen (80 mbar) for 1 h after bring down the temperature to
250 °C. Subsequently, the hydrogen was degassed at this tempera-
ture and cooled slowly to 25 °C 298 K. Spectra were recorded by
gradually increasing the CO equilibrium pressure in the range of
0.001–5 mbar. Spectra were recorded by injecting increasing dos-
0
7500
6000
4500
3000
1500
0
Pt (111)
Pt (220)
Pt (200)
Pt/ZrO2
20
30
40
50
60
70
80
2θ (degree)
Fig. 1. XRD patterns of the reduced supported Pt (5 wt%) catalysts. Asterisks
represent the peaks corresponding to the tetragonal phase of the support. Arrows
denote the peaks due to metallic Pt.
(appearing as shoulders) are marked in the figure by asterisks.
Recently, Li et al. [34] have also observed a mixed crystallite
phase for CeO2–ZrO2 prepared at lower temperatures (<500 °C).
Neat CeO2 showed a cubic fluorite structure [35]. The unit cell
parameter (acubic) of ‘‘neat’’ CeO2–ZrO2 was smaller (0.537 nm)
than ‘‘neat’’ CeO2 (0.542 nm), suggesting formation of a CeO2–
ZrO2 solid solution. The ionic radius of Zr4+ (0.084 nm) is smaller
than that of Ce4+ (0.097 nm)/Ce3+ (0.112 nm), and hence, a de-
crease in unit cell parameter of CeO2–ZrO2 was observed [36].
The unit cell parameter decreased (from 0.537 to 0.512 nm) also
when Pt was impregnated (Table 1). Samples reduced in a flow
of hydrogen showed three additional peaks of low intensity at
39.7, 46.1, and 67.7° (Fig. 1) corresponding to reflections from
(111), (200), and (220) planes of metallic platinum, respec-
tively. The intensity of (111) reflection for all these supported
Pt samples of the present study is higher than that of the other
two, indicating that these planes are preferentially oriented to
the others. Jung et al. [37] found a preferential orientation of
(111) plane when Pt was supported on herringbone carbon
nanofibers and of (220) plane when it was supported on alu-
mina. Support and the method of preparation can influence the
orientation of metal crystallites. Using Debye–Scherrer formula
and the XRD peak corresponding to (111) reflection, the crystal-
lite size of X-ray detectable metallic Pt on CeO2, CeO2–ZrO2, and
ZrO2 was estimated to be 57.2, 50.5, and 34.2 nm, respectively.
Representative HRTEM images of Pt supported on CeO2 and
CeO2–ZrO2 are shown in Fig. 2. From these micrographs, the parti-
cle size of Pt was found to be in the range of 5–12 nm. There are a
few particles that are of 40 nm size. The values of Pt crystallite
sizes determined from XRD measurements are higher than those
evaluated from HRTEM. The Pt peak profile in XRD can be sepa-
rated into two contributions: that corresponding to large crystal-
lites in the thin part of the Pt peaks and to small crystallites in
the tails of the diffraction line. The values reported from XRD are
those for the large crystallites. From the lattice fringes, we have
found that the (111) plane of Pt is preferentially exposed, which
agrees well with the inference derived from the XRD results.
age of known small amounts of CO starting from 0.07
lmol till as
high as 50 mol using a pirani vacuum gauge (Oerlikon Leybold
l
Vakuum Thermovac) after calibrating the glass linings. The catalyst
spectrum collected before CO dosage was used as a reference for
obtaining the reported background-subtracted spectra. Spectra
were normalized with respect to pellet thickness. The particle size
of Pt metal was calculated from the dispersion values (CO-FTIR) by
considering the surface density of Pt atoms.
2.3. Reaction procedure
Prior to reactions, the catalysts were freshly reduced at 300 °C,
for 2 h, in a flow of hydrogen (30 cc/min). Upon reduction, the color
of the catalyst changed from dark brown to black. The temperature
was brought down to 25 °C. Without exposing to atmosphere,
0.05 g of reduced catalyst was transferred into a Parr reactor
(300 ml) containing cinnamaldehyde (4 g; Aldrich Co.) and solvent
(40 ml; s.d. fine Chem Ltd., India). The reactor was initially flushed
and then pressurized with hydrogen to a desired value (1–30 bar).
The reaction was conducted for 2 or 8 h at 25 °C while stirring at a
speed of 500 rpm. In kinetic studies, samples were collected at reg-
ular intervals of time, diluted with the solvent, and then analyzed
and quantified by gas chromatographic technique (Varian 3400;
CP-SIL8CB column; 30 m-long and 0.53 mm i.d.). The products
were identified by GC–MS (Shimadzu GCMS-QP5050A; HP-5 col-
umn; 30 m-long  0.25 mm i.d.  0.25
lm thickness). Some exper-
iments were carried out at higher temperatures (50, 75, and
100 °C).
3. Results and discussion
3.1. Structural properties
The ceria–zirconia mixed oxides are formed in different crystal-
line structures depending on their chemical composition. According
to the Ce1–xZrxO2 phase diagram, for x 6 0.15, a cubic fluorite-type
phase is formed and for x P 0.85, a monoclinic phase is obtained
[31]. At intermediate compositions, various phases have been iden-
tified [31–33]. In other words, the ceria–zirconia material exhibits
complex structural characteristics. The mixed oxide composite of
the present study showed XRD pattern typical of a mixture of cubic
and tetragonal phases with the former being more predominant
than the latter (Fig. 1). The peaks due to the tetragonal phase
3.2. Temperature-programmed reduction and CO adsorption
While XRD and HRTEM, respectively, enabled information on
crystallite and particle sizes of supported metallic Pt, tempera-
ture-programmed reduction with hydrogen (H2-TPR) and in situ
DRIFTS measurements of adsorbed CO provided knowledge on

34
S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
Table 1
Physicochemical properties of CeO2–ZrO2-supported Pt catalysts.
Sample
XRD
N2 adsorption–desorption
Acidity (NH3-
TPD, mmol/g)
H2 uptake – Pt
(TPR, mmol/g)
Support
Average crystallite
size of Pt (nm)
SBET
Pore volume Average pore
(m2/g) (cm3/g)
diameter (nm)
Lattice parameter
(acubic, nm)
Average crystallite
size (nm)
CeO2–ZrO2
0.537
0.512
0.509
0.512
7.72 (cubic)
7.314 (tetra)
14.07
–
115
91
0.17
0.16
0.16
0.14
6.1
6.9
6.0
5.9
0.179
0.295
0.440
0.367
–
Pt (5 wt%)/
CeO2
Pt (5 wt%)/
ZrO2
Pt (5 wt%)/
CeO2–
57.2
34.2
50.5
0.270
0.236
0.376
11.9
107
94
10.97 (cubic), 6.5
(tetragonal)
ZrO2
Fig. 2. HRTEM images of: (a) Pt(5 wt%)/CeO2 and (b) Pt(5 wt%)/CeO2–ZrO2 reduced samples.
metal dispersion and support–metal interactions. Bulk platinum
shows a hydrogen uptake peak in TPR at 130–160 °C. This peak
for Pt supported on CeO2 (70 and 106 °C) and CeO2–ZrO2 (80 and
149 °C) occurred at lower temperatures possibly due to support–
metal interactions and variation in crystallite/particle sizes
(Fig. 3). CeO2 is a reducible oxide and keeps the metal in lower oxi-
dation state. While the peak at 80 °C in Pt/CeO2–ZrO2 is attributed
to Pt strongly interacting with the support, that at 149 °C is attrib-
uted to relatively larger size Pt crystallites. These hydrogen uptake
peaks in the case of Pt supported on ZrO2 were observed at 98° and
184 °C, respectively. Neat CeO2 showed a broad reduction peak
centered at 850 °C. In CeO2–ZrO2, this peak due to CeO2 reduction
had shifted to lower temperatures (376 and 494 °C) due formation
of a CeO2–ZrO2 solid solution. These experiments, thus, provide
definite evidence that the reduction peaks observed below 184 °C
in the case of supported Pt catalysts are due to metal only and
not because of the support oxide. The amount of hydrogen uptake
by these catalysts was found to decrease in the order:
Pt/CeO2–ZrO2 (0.376 mmol/g) > Pt/CeO2 (0.270 mmol/g) > Pt/ZrO2
(0.236 mmol/g) (Table 1). This observation indicates that the ex-
tent of dispersion and the amount of exposed Pt are higher on
CeO2–ZrO2 than on the other supports.
Metal dispersion was estimated also from the DRIFT spectra of
the adsorbed CO of the catalyst samples. Representative CO
adsorption profiles for Pt/CeO2–ZrO2 at different CO dosages are
shown in Fig. 4. It was assumed that one mole of surface Pt adsorbs
one mole of CO. It was also presumed that the surface Pt was very
sensitive to gas molecules at ambient temperatures and the
adsorption is irreversible. On the basis of these assumptions, Pt
will adsorb CO irrespective of the CO partial pressure, i.e. as long
as free surface Pt sites are available, it will keep on adsorbing CO
until all CO present is consumed. Based on this, small doses of cal-
149oC
513oC
854oC
378oC
322oC
80oC
Pt/CeO2-ZrO2
184oC
98oC
Pt/ZrO2
820oC
106oC
70oC
362oC
Pt/CeO2
100 200 300 400 500 600 700 800 900
Temperature (oC)
Fig. 3. Temperature-programmed reduction (H2) profiles of Pt (5 wt%) supported
on ZrO2, CeO2 and CeO2–ZrO2.
culated amount of CO were injected into the vacuum line and the
spectra were recorded. Unless all the surface of Pt is consumed
upon each injection, the resultant partial pressure in the line shows
zero reading. Once each surface Pt atom is consumed in PtACO
bond formation, further injection of CO leads to no further increase
in CO adsorption as indicated by no (or negligible) drop of pressure
and thus no significant increase in integrated intensity of the
absorption band (bands). Thus, a plot of integrated intensity verses
CO dosage (Fig. 4, right panel) shows a straight line passing
through origin (i.e. until no surface Pt is left unsatisfied) and then
an almost horizontal line to X-axis. The intersection of these two
lines gives the point i.e. amount of CO required for consumption

S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
35
0.015 mbar
0.025 mbar
0.035 mbar
0.045 mbar
7
0.01 a.u.
2064
6
5
4
3
2
1
0
0.06 mbar
0.08 mbar
0.1 mbar
0.2 mbar
0.3 mbar
0.4 mbar
0.5 mbar
0.7 mbar
0.8 mbar
2 mbar
5 mbar
0
5
10
15
20
2300 2250 2200 2150 2100 2050 2000 1950 1900 1850 1800
Wavenumber (cm-1)
CO introduced (μmol)
Fig. 4. FTIR spectra of CO adsorbed on Pt (5 wt%)/CeO2–ZrO2 catalyst.
Table 2
indicating that approximately the half of platinum atoms is located
at the surface. However, the crystallite size, as measured from
Scherrer equation (XRD), indicated quite large crystal size
(50.5 nm). These large crystal sizes strongly indicate that the dis-
persion of platinum is low, much lower than 40.5%. HRTEM
(Fig. 2), on the other hand, showed the presence of a large number
of platinum particles of dimension 5–12 nm along with some par-
ticles of much larger size. Croy et al. [40] and Dömök et al. [41]
demonstrated that the binding energy (BE) values of photoelec-
trons ejected from metal particles depend on the metal particle
size. The smaller the species, the higher would be the binding en-
ergy. The BE values of Pt supported on CeO2–ZrO2 are higher than
those of CeO2 and ZrO2 (Table 2). This experimental observation
also points out the presence of a large number of smaller size Pt
particles on CeO2–ZrO2 support. The observed metal dispersion
from CO adsorption studies (40.5% for Pt (5%)/CeO2–ZrO2) is due
to a stoichiometric average of crystals of smaller as well as larger
dimensions.
Binding energy values (in eVs) of supported Pt catalysts.
Sample
Pt0
Pt2+
4f7/2
4f5/2
4f7/2
4f5/2
Pt (5 wt%)/CeO2
Pt (5 wt%)ZrO2
Pt (5 wt%)/CeO2–ZrO2
70.7
70.9
71.3
74.1
74.6
74.6
72.4
72.2
72.5
75.5
75.7
75.8
by surface Pt atoms. Thus, by counting the amount of CO, the
amount of surface Pt atoms was calculated.
Two overlapping IR bands at 2058 and 2078 cmÀ1 were ob-
served (Fig. 4). On the basis of earlier reports [38,39], the former
is attributed to CO linearly bonded to low-coordinated, open Pt
atoms, and the latter to Pt adsorbed at terrace sites. The position
of the former shifted to 2064 cmÀ1 as the dosing of CO was in-
creased. Also, a weak band was observed at 1854 cmÀ1 attributable
to bridge bonded CO. Observation of this peak indicates that there
are also a few number of small size metallic Pt particle on the sur-
face of CeO2–ZrO2. In other words, the CO adsorption studies, in
concurrence with HRTEM, reveal the presence of different types
of Pt particles whose concentration determines the hydrogenation
selectivity. On the basis of these CO adsorption studies, metal dis-
persion on different supports was estimated to increase in the or-
der: Pt(5%)/ZrO2 (16.2%) < Pt(5%)/CeO2 (20.3%) < Pt(5%)/CeO2–ZrO2
(40.5%). Incorporation of ZrO2 in CeO2 promoted the dispersion of
Pt by nearly two times. This is a quite high dispersion value,
3.3. X-ray photoelectron spectroscopy
The electronic structure of Pt was determined using X-ray pho-
toelectron spectroscopy. Fig. 5 shows the binding energy (B.E.)
peaks arising from the 4f core levels of reduced Pt (5 wt%) sup-
ported on CeO2, ZrO2 and CeO2–ZrO2. The peaks at 70.7 and
74.1 eV for Pt/CeO2 are corresponded to 4f7/2 and 4f5/2 levels of
Pt0. The positions of these peaks are in agreement with the values
Pt/CeO
70.7
Pt/ZrO
Pt/CeO -ZrO
4f
74.6
2
4f
74.6
4f
2
2
2
5/2
7/2
4f
5/2
74.1
71.25
4f
7/2
70.9
4f
7/2
5/2
66
68
70
72
74
76
78
66
68
70
72
74
76
78
80
68
70
72
74
76
78
80
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Fig. 5. XPS of 5 wt% Pt supported on CeO2, ZrO2, and CeO2–ZrO2. Prior to experiments, the catalysts were reduced at 300 °C for 2 h. 4f7/2 and 4f5/2 peaks of Pt0 are indexed. The
additional two unresolved peaks in small intensity are due to PtO.

36
S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
100
80
60
40
20
0
80
100
80
Pt/ZrO
2
60
60
(a) Without alkali
(b) With alkali
100
80
60
40
20
0
100
80
60
40
20
Pt/CeO2
40
CA conv.
HCA sel.
PPL sel.
CAL sel.
Others
40
20
0
8
6
4
2
0
Pt/CeO2-ZrO2
2
4
6
8
2
4
6
8
Reaction time (h)
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 8. Influence of alkali addition on the hydrogenation of cinnamaldehyde over Pt
(5 wt%)/CeO2–ZrO2. Reaction conditions: Catalyst (reduced at 300 °C for 2 h) = 0.05 g,
cinnamaldehyde (CA) = 4 g, solvent (iso-propanol) = 40 ml, alkali solution = 0.02 g
of NaOH in 10 ml water (pH = 13), H2 pressure = 20 bar, reaction tempera-
ture = 25 °C, stirring speed = 500 rpm. HCA = hydrocinnamaldehyde, PPL = 3-phen-
ylpropanol, CAL = cinnamyl alcohol, other products include acetals.
Relative pressure (P/P0)
Fig. 6. N2 adsorption–desorption isotherms of 5 wt% Pt supported on ZrO2, CeO2,
and CeO2–ZrO2 (prior to experiments, catalyst samples were reduced at 300 °C for
2 h).
reported by others [42]. Deconvolution of the peaks revealed the
presence of oxidized form of platinum (Pt2+) with peaks at 72.4
and 75.5 eV, the concentration of which is small (Table 2). Pt
(5 wt%)/ZrO2 showed the peaks due to Pt0 at 70.9 and 74.6 eV.
These for Pt (5 wt%)/CeO2–ZrO2 occurred at 71.3 and 74.6 eV. On
the basis of these peak positions, it can be concluded that Pt on
CeO2 is relatively rich in electron density than that supported on
ZrO2 and CeO2–ZrO2. It is also likely that the crystallites of Pt are
smaller on CeO2–ZrO2 than on neat ZrO2 and CeO2 [40,41].
temperature range 100–400 °C. While the peak at 150–175 °C is
attributed to NH3 desorbed from the surface hydroxyl groups, that
at 250–275 °C is corresponded to weak Lewis acid sites [13]. Metal
impregnation enhanced the acidity (Table 1). The acidity (NH3
desorption) of the supported Pt catalysts increased in the order:
Pt/CeO2 (0.295 mmol/g) < Pt/CeO2–ZrO2 (0.367 mmol/g) < Pt/ZrO2
(0.440 mmol/g).
The nature and type of acid sites on CeO2–ZrO2 were deter-
mined by DRIFT spectroscopy using pyridine as probe molecule
(Fig. 7 (right panel)). The sample showed IR peaks of adsorbed pyr-
idine at 1608, 1597, 1485, and 1440 cmÀ1 (Fig. 7 (right panel)). The
peaks at 1608 and 1440 cmÀ1 are attributed to H-bonded pyridine,
and those at 1597 and 1485 cmÀ1 are assigned to pyridine coordi-
nated to weak Lewis acid sites [43]. Strong Lewis (IR peaks at 1623
3.4. Textural properties
All these materials showed typical type IV isotherms with H3
hysterisis loops (Fig. 6). The specific surface area (SBET) of different
catalysts decreased in the order: Pt/ZrO2 (107 m2/g) > Pt/CeO2–
ZrO2 (94 m2/g) > Pt/CeO2 (91 m2/g) (Table 1). Upon Pt impregna-
tion, SBET of CeO2–ZrO2 had decreased from 115 to 107 m2/g.
and 1455 cmÀ1) and Brönsted acid sites (1639 and 1546 cmÀ1
)
were absent. Upon impregnation of Pt, the relative concentration
of these acidic sites has changed (note the change in the intensity
of peak at 1485 cmÀ1 in Fig. 7 (right panel)).
3.5. Acidic properties: NH3-TPD and DRIFT spectroscopy of adsorbed
pyridine
3.6. Catalytic activity
The density of acid sites on supported Pt catalysts was quanti-
fied by NH3-TPD measurements (Fig. 7 (left panel) and Table 1).
Two overlapping, broad desorption peaks were observed in the
Supported Pt catalysts of the present study are highly efficient
even at room temperature (25 °C) for the hydrogenation of cinna-
maldehyde (CA). Fig. 8 shows the variation of CA conversion and
Pt/CeO2-ZrO2
CeO -ZrO
2
2
Pt/CeO
Pt/ZrO2
2
Pt/CeO -ZrO
2
2
Pt/CeO2
1650
1600
1550
1500
1450
1400
100
200
300
400
500
600
Temperature (oC)
Wavenumber (cm-1
)
Fig. 7. Left: NH3-TPD profiles of 5 wt% Pt loaded on CeO2–ZrO2, ZrO2 and CeO2 (prior to experiment, catalyst samples were reduced at 300 °C for 2 h). Right: DRIFT spectra of
adsorbed pyridine.

S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
37
Table 3
Influence of base on hydrogenation of cinnamaldehyde over Pt (5 wt%)/CeO2–ZrO2.
3.6.2. Effect of solvent
Further, the nature of solvent was found to have a profound
influence on CA conversion and CAL selectivity (Table 4). The reac-
tions were conducted in non-polar (toluene) and polar–aprotic
(dioxane and acetonitrile) and protic (ethanol, iso-propanol, and
tert-butanol) solvents. CA conversion is much higher in ethanol
and iso-propanol than in other solvents (TOF = 843 and 586,
respectively). However, formation of undesired acetal products
was detected in the presence of alcohol solvents. Extent of this is
lower in iso-propanol than in ethanol (Table 4). As noted from Ta-
ble 3, when base (NaOH) was added to the reaction mixture, the
side reactions were suppressed and CAL formed selectively. In tol-
uene and acetonitrile solvents, hydrogenation of olefinic bond was
preferred to the carbonyl bond of CA. In view of higher apparent
activity and CAL selectivity, iso-propanol was chosen as the solvent
of choice in further studies.
Base
CA conversion (wt%)
TOF (hÀ1
)
Product selectivity (%)
HCA
PPL
CAL
NaOH
42.3
36.3
36.5
40. 6
39.7
36.0
27.9
1233
1058
1064
1184
1158
1050
813
0.6
2.1
12.0
8.9
8.9
9.7
97.3
61.6
72.3
76.5
74.3
74.0
72.8
Na2CO3
NaHCO3
(COONa)2
Na2SO4
KOH
26.4
18.8
14.6
16.0
17.2
16.2
8.8
11.0
Cs(OH)2
Reaction conditions: Catalyst (Pt (5 wt%)/CeO2–ZrO2 reduced at 300 °C for 2 h) = 0.05 g,
cinnamaldehyde (CA) = 4 g, solvent (iso-propanol) = 40 ml, base = 0.02 g (in 10 ml
water), H2 pressure = 20 bar, reaction temperature = 25 °C, reaction time = 2 h,
stirring speed = 500 rpm. Turnover frequency (TOF) = moles of CA converted per
mole of exposed Pt (derived from CO adsorption studies) per hour. HCA = hydro-
cinnamaldehyde, PPL = 3-phenylpropanol, CAL = cinnamyl alcohol. No other prod-
ucts (acetals) formed in these reactions.
3.6.3. Effect of Pt loading
Metal loading is essential to get high yields of CAL. As seen from
Table 5, CA conversion increased with increasing amount of Pt
loading. Conversion of CA increased from 70.4% to 95.8% when
the Pt loading on CeO2–ZrO2 support was increased from 0.5 to
5 wt%. Only a marginal variation in CAL selectivity (89–93.4%)
was observed. This suggests a uniform dispersion of Pt on CeO2–
ZrO2 support up to a metal loading of at least 5 wt%.
product selectivity as a function of reaction time for Pt (5 wt%)/
CeO2–ZrO2 catalyst. This reaction usually yields different hydroge-
nated products as shown in Scheme 1. Hydrogenation of the
carbonyl group (1,2-addition) results in unsaturated alcohol–
cinnamyl alcohol (CAL), and hydrogenation of the olefinic bond
(3,4-addition) provides the saturated aldehyde–hydrocinnamalde-
hyde (HCA). The 1,4-addition gives enol, which isomerizes into
HCA. Further hydrogenation leads to the formation of 3-phenylpro-
panol (PPL) and subsequently, propylbenzene (PB). Also, CAL can
get reduced to PPL. In the present study, over supported Pt cata-
lysts, 1,2-addition is more selective than 3,4 and 1,4-additions. This
is in contrast to that observed over supported Pd catalysts [24]
where 3,4, and 1,4-addition products were predominant at all
conversion levels. Over Pt(5 wt%)/CeO2–ZrO2, CAL formed with a
selectivity of 60–70%, while HCA + PPL together formed with a
selectivity of 23–33% (Fig. 8). Formation of a minor quantity of ace-
tal side product (selectivity = 4–6%) was also detected. At the end
of 8 h, CA conversion was 47.1%. Formation of PB was not detected.
3.6.4. Effect of reaction temperature
The effect of reaction temperature was studied in the absence of
alkali promoters as the conversions were lower without alkali
addition and the effect of temperature on the reaction can be mon-
itored better. The reactions were conducted at 25, 50, 75, and
100 °C. As seen from Table 6, CA conversion increased from 20.1%
to 63.4% for 2 h as the temperature was raised from 25 to 100 °C.
Increase in CAL selectivity from 60% to 71% at the expense of
HCA + PPL product selectivity was also noted. This study thus re-
veals that although Pt is effective even at 25 °C for the selective
hydrogenation of carbonyl group of CA, temperature further facil-
itates this selective transformation.
3.6.5. Effect of hydrogen pressure
3.6.1. Effect of alkali addition
Hydrogen pressure was varied from 1 to 30 bar while keeping
the other parameters unchanged. CA conversion and CAL selectiv-
ity have increased with increasing hydrogen pressure (Table 7). A
pressure of 20 bar was found optimum to yield CA conversion of
95.8% and CAL selectivity of 93.4% at end of 8 h at 25 °C.
When alkali (0.02 g of NaOH in 10 ml of water) was added to the
reaction medium, a significant enhancement in catalytic activity
was observed (Fig. 8). At 8 h, CA conversion increased from 47.1%
to 95.8%, turnover frequency (TOF; moles of CA converted per mole
of exposed Pt per hour) increased from 343 to 698, and CAL selec-
tivity increased from 70% to 93.4%. Side reactions (acetal forma-
tion) got completely suppressed. Several bases including NaOH,
KOH, Cs(OH)2, NaHCO3, Na2CO3, (COONa)2, and Na2SO4 were exam-
ined. Among all these, NaOH was found to provide high catalytic
activity and CAL selectivity (Table 3).
3.6.6. Effect of catalyst amount
CA conversion increased (Table 8) with increasing amount of
catalyst from 0.025 to 0.15 g. However, the selectivity for CAL
had decreased. At higher amounts of the catalyst, acidity of the
Table 4
Influence of solvent on hydrogenation of cinnamaldehyde over Pt (5 wt%)/CeO2–ZrO2.
Solvent
Solvent properties
CA conversion
(wt%)
TOF
(hÀ1
Product selectivity (%)
)
Polarity
index
Dielectric constant
(Debye)
Miscibility in water
(%)
Gutmann acceptor
number
HCA PPL
CAL
Others
Isopropanol 3.9
Acetonitrile 5.8
17.9
36.6
2.38
2.25
24.3
18
100
100
0.051
100
100
33.8
18.9
20.1
4.9
3.0
10.6
28.9
10.0
586
143
87
309
843
292
18.3 15.9 59.7
42.6 11.8 45.5
47.1 27.2 25.7
24.1 12.5 63.4
6.2
0
0
Toluene
Dioxane
Ethanol
Tert-
2.4
4.8
5.2
4.0
10.3
37.9
36.8
0
9.9
8.1 47.3 34.7
0.43
19.9 10.9 69.1
0
butanol
Reaction conditions: Catalyst (Pt (5 wt%)/CeO2–ZrO2 reduced at 300 °C for 2 h) = 0.05 g, cinnamaldehyde (CA) = 4 g, solvent = 40 ml, promoter alkali = nil, H2 pressure = 20 bar,
reaction temperature = 25 °C, reaction time = 2 h, stirring speed = 500 rpm. Turnover frequency (TOF) = moles of CA converted per mole of exposed Pt (derived from CO
adsorption studies) per hour. HCA = hydrocinnamaldehyde, PPL = 3-phenylpropanol, CAL = cinnamyl alcohol, other products include acetals.

38
S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
Table 5
was noted. This variation in product selectivity with conversion
follows the trend proposed in the reaction Scheme 1.
Influence of percentage metal loading on hydrogenation of cinnamaldehyde.
Pt loading (wt%)
CA conversion (wt%)
Product selectivity (%)
HCA
PPL
CAL
Others
3.6.7. Effect of support
Fig. 9 demonstrates the influence of support on the catalytic
activity and product selectivity of the Pt catalysts. CeO2–ZrO2
was found to be a better support than neat CeO2 and neat ZrO2.
While CA conversion was 95.8% with CAL selectivity of 93.4% at
the end of 8 h, by using Pt (5%)/CeO2–ZrO2, the conversion was only
88.9 and 81.8% on Pt (5 wt%)/CeO2 and Pt (5 wt%)/ZrO2, respec-
tively. In the presence of zirconia, the activity and selectivity (for
CAL) of ceria has increased.
Table 9 compares the catalytic activity of Pt/CeO2–ZrO2 with
that of other reported catalysts. High amount of catalyst and tem-
perature are the requirements with most of the known catalysts.
They are either poorly active or less selective. The catalyst of the
present study shows efficient activity and selectivity even at ambi-
ent temperature (25 °C).
0.5
1
2
70.4
78.2
85.0
95.8
1.9
3.4
1.1
1.3
2.0
2.8
2.5
5.3
89.1
82.5
90.5
93.4
7.0
11.3
5.9
0
5
Reaction conditions: Catalyst (Pt/CeO2–ZrO2 reduced at 300 °C for 2 h) = 0.05 g,
cinnamaldehyde (CA) = 4 g, solvent (iso-propanol) = 40 ml, alkali solution = 0.02 g
of NaOH in 10 ml water (pH = 13), H2 pressure = 20 bar, reaction tempera-
ture = 25 °C, reaction time = 8 h, stirring speed = 500 rpm. HCA = hydrocinnamal-
dehyde, PPL = 3-phenylpropanol, CAL = cinnamyl alcohol, other products include
acetals.
medium increases, which facilitates activation of olefinic bonds
and formation of acetal products. An amount of 0.05 g of catalyst
was found ideal at our reaction conditions. The selectivity to CAL
verses conversion plots, as varied by changing the amount of cata-
lyst in the reactor (Table 8) and by changing the reaction time
(Fig. 8), showed a similar trend. The selectivity to CAL decreased
with increasing conversion. At the same time, some increase in
the selectivity to PPL (formed via further hydrogenation of CAL)
3.7. Structure–activity correlation
The catalysts of the present work are highly active and selective
for the hydrogenation of CA even at 25 °C and H2 pressures as low
Table 6
Effect of reaction temperature on the hydrogenation of cinnamaldehyde over Pt (5 wt%)/CeO2–ZrO2.
Temperature (°C)
CA conversion (wt%)
TOF (hÀ1
)
Product selectivity (%)
HCA
PPL
CAL
Other products
25
50
75
20.1
39.3
43.5
63.4
586
1146
1268
1849
18.3
14.3
15.3
10.8
15.9
12.7
11.3
13.1
59.7
64.9
66.9
71.1
6.2
8.1
6.5
5.0
100
Reaction conditions: Catalyst (reduced at 300 °C for 2 h) = 0.05 g, cinnamaldehyde (CA) = 4 g, solvent (iso-propanol) = 40 ml, alkali addition = nil, H2 pressure = 20 bar, reaction
time = 2 h, stirring speed = 500 rpm. Turnover frequency (TOF) = moles of CA converted per mole of exposed Pt (derived from CO adsorption studies) per hour.
HCA = hydrocinnamaldehyde, PPL = 3-phenylpropanol, CAL = cinnamyl alcohol, other products include acetals.
Table 7
Effect of H2 pressure:hydrogenation of cinnamaldehyde over Pt (5 wt%)/CeO2–ZrO2.
H2 pressure (bar)
CA conversion (wt%)
TOF (hÀ1
)
Product selectivity (%)
HCA
PPL
CAL
Others
1
5
10
20
30
17.0
40.0
80.7
95.8
99.2
124
292
588
698
723
28.5
11.6
3.2
1.3
0.1
4.8
6.4
3.2
5.3
10.2
66.8
82.0
89.6
93.4
89.6
0
0
4.1
0
0
Reaction conditions: Catalyst (Pt/CeO2–ZrO2 reduced at 300 °C for 2 h) = 0.05 g, cinnamaldehyde (CA) = 4 g, solvent (iso-propanol) = 40 ml, alkali solution = 0.02 g of NaOH in
10 ml water (pH = 13), reaction temperature = 25 °C, reaction time = 8 h, stirring speed = 500 rpm. Turnover frequency (TOF) = moles of CA converted per mole of exposed Pt
(derived from CO adsorption studies) per hour. HCA = hydrocinnamaldehyde, PPL = 3-phenylpropanol, CAL = cinnamyl alcohol, other products include acetals.
Table 8
Influence of amount of catalyst on hydrogenation of cinnamaldehyde over Pt (5 wt%)/CeO2–ZrO2.
Catalyst amount (g)
CA conversion (wt%)
TOF (hÀ1
)
Product selectivity (%)
HCA
PPL
CAL
Others
0.025
0.050
0.075
0.1
33.7
40.0
42.2
75.0
86.4
491
292
205
273
210
11.8
11.7
17.8
10.6
20.5
5.9
6.4
7.7
6.1
9.3
82.3
82.0
74.5
75.9
68.8
0
0
0
7.5
1.4
0.15
Reaction conditions: Catalyst = Pt (5 wt%)/CeO2–ZrO2 reduced at 300 °C for 2 h, cinnamaldehyde (CA) = 4 g, solvent (iso-propanol) = 40 ml, alkali solution = 0.02 g of NaOH in
10 ml water (pH = 13), H2 pressure = 5 bar, reaction temperature = 25 °C, reaction time = 8 h, stirring speed = 500 rpm. Turnover frequency (TOF) = moles of CA converted per
mole of exposed Pt (derived from CO adsorption studies) per hour. HCA = hydrocinnamaldehyde, PPL = 3-phenylpropanol, CAL = cinnamyl alcohol, other products include
acetals.

S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
39
CeO2–ZrO2 (40.5%) than on CeO2 (20.3%) and ZrO2 (16.2%). It is
interesting to note that variation in catalytic activity (CA conver-
sion; Fig. 9) is in line with the variation in metal dispersion.
The acidity of different catalysts increased in the order: Pt/CeO2
(0.295 mmol/g) < Pt/CeO2–ZrO2 (0.367 mmol/g) < Pt/ZrO2 (0.440
mmol/g). This led to unwanted side reactions (acetal formation).
Lewis acidity facilitated adsorption of CA through its carbonyl
group and enabled selective formation of CAL product. As seen
from the XPS data (Table 2), CeO2 (a reducible metal oxide) pro-
moted electron density at the site of Pt. Higher electron density
on metal facilitates pÃCO back bonding, and thereby, an enhance-
ment in the selectivity to CAL during the hydrogenation of CA is
observed. The factors of acidity of the support (due to the presence
of ZrO2 component) and higher electron density at Pt (due to CeO2
component) are responsible for the higher catalytic activity and
selectivity of Pt supported on CeO2–ZrO2 composite material.
Addition of base to the reaction medium suppressed the side
reactions. Apart from altering the electronic structure of metal
[24], the alkali ions (Na+) have also facilitated the adsorption of
CA through their carbonyl groups and thereby enhanced the cata-
lytic activity and CAL selectivity.
(a)
(b)
(c)
CAL sel.
100
80
60
40
20
0
100
80
100
80
60
60
CA conv.
40
40
8
6
4
2
0
4
2
0
PPL sel.
HCA sel.
2
4
6
8
0 2 4 6 8
Reaction time (h)
0 2 4 6 8
Fig. 9. Influence of support on hydrogenation of cinnamaldehyde: (a) Pt(5 wt%)/
ZrO2, (b) Pt (5 wt%)/CeO2, and (c) Pt(5 wt%)/CeO2–ZrO2. Reaction conditions: Catalyst
reduced at 300 °C for 2 h = 0.05 g, cinnamaldehyde (CA)
propanol) = 40 ml, alkali solution = 0.02 g of NaOH in 10 ml water (pH = 13), H2
pressure = 20 bar, reaction temperature = 25 °C, stirring speed = 500 rpm. HCA =
hydrocinnamaldehyde, PPL = 3-phenylpropanol, CAL = cinnamyl alcohol, other
products include acetals.
= 4 g, solvent (iso-
The physical properties of solvents used in the present study are
listed in Table 4 along with the catalytic activity data. Solvents
showed a marked effect on catalytic activity. Dielectric constant
and miscibility with water are a measure of the polarity of the sol-
vent. Gutmann’s acceptor number (AN) determines the Lewis acid-
ity of the solvent. A comparison of these physical parameters and
catalytic activity data reveals that the polarity of solvent alone is
not sufficient but a higher value of AN is also essential to get good
activity and selectivity. Ethanol and iso-propanol have higher val-
ues of AN associated with high miscibility in water. Hence, they are
the right choice of solvents to get high catalytic activity. Acetoni-
trile, in spite of being more polar and miscible with water, has low-
er value of AN and hence is not a good choice of solvent. Madon
et al. [55], in hydrogenation of cyclohexene, found a dependence
of turnover rates (at a given H2 pressure) on the chemical entity
of the solvent used. The concentration of dissolved H2 is the crucial
parameter that determined the reaction rate. The rate increased
with the increasing liquid phase H2 concentration, which depends
on the nature of the solvent. Later, Madon and Iglesia [56] ex-
plained this solvent-induced non-ideal behavior effect in terms of
transition state theory. They reported that the rates of catalytic
reactions in the presence of solvents depend only when it influ-
ences the activated complexes involved in the kinetically signifi-
cant steps in the catalytic cycle. Possibly, the same explanation
holds good even in the present study.
as 20 bar. Cinnamyl alcohol is the major product. CA conversion at
the end of 8 h over these catalysts increased in the order: Pt/ZrO2
(81.8%) < Pt/CeO2 (88.9%) < Pt/CeO2–ZrO2 (95.8%), and the selectiv-
ity for CAL increased in the order: Pt/ZrO2 (73.6%) < Pt/CeO2
(85.6%) < Pt/CeO2–ZrO2 (93.4%). The selectivity in the hydrogena-
tion of cinnamaldehyde is affected by the metal particle size. An in-
crease in metal particle size (>3 nm) favors selectivity toward the
unsaturated alcohols (CAL). Gallezot and Richard [1] have ex-
plained this on the basis of steric effects, wherein with large metal
particles, the aromatic ring restrains a close approach of the C@C
bond to the metal surface. But on small metal particles, the aro-
matic ring can lie aside of the metal particle and the C@C bond
can be hydrogenated. Originally, Loffreda et al. [52,53] and
Serrano-Ruiz et al. [54] explained based on theoretical calculations
that the
different crystal planes and steps. While on Pt(111) plane, CA pre-
fers to adsorb through
1(C@O) coordination; on Pt(100), the
adsorption is through both
2(C@C) and 1(C@O) modes. With
a,b-unsaturated aldehydes adsorb in a different way on
g
g
g
increasing crystallite size, the fraction of steps and corners de-
creases and (111) planes are preferentially oriented. The particle
size of Pt in the catalysts of present work is 5–12 nm. A few parti-
cles of 40 nm were also observed (HRTEM). Hence, as expected
(111), plane is more exposed (XRD and HRTEM), and thereby,
the catalysts of this study are more selective for the CAL product.
As seen from CO adsorption studies, support influenced to a
great extent the Pt dispersion. Dispersion of Pt is more on
Fig. 10 depicts the comparative catalytic activity of CeO2–ZrO2-
supported Pt and Pd catalysts. The study on Pd catalysts was
reported by us earlier [24]. While both these catalysts are equally
active, they showed marked differences in the product selectivity.
Table 9
Catalytic activity of different supported Pt catalysts for hydrogenation of cinnamaldehyde.
Catalyst
Reaction conditions (amount of catalyst,
temperature, H2 pressure, reaction time)
CA conversion (%)
CAL selectivity (%)
Reference
Pt (14%)/zeolite-Y
0.4 g, 60 °C, 40 bar, 2 h
0.02 g, 25 °C, 4 bar, 17 h
0.3 g, 100 °C, 5 bar, 5 h
0.05 g, 40 °C, 10 bar, 4 h
0.4 g, 60 °C, 40 bar, 2 h
0.1 g, 100 °C, 50 bar
0.3 g, 100 °C, 16 bar, 12 h
0.4 g, 80 °C, 20 bar, 9 h
0.3 g, 110 °C, 70 bar, 2 h
0.2 g, 90 °C, 10 bar, 4 h
0.4 g, 60 °C, 40 bar, 2 h
0.05 g, 25 °C, 20 bar, 8 h
75
70
89
70
75
90
96
100
100
60
90
95.8
96
99
80
83
87
68
86
88
96
80
64
93.4
[44]
[45]
[46]
[47]
[48]
[37]
[48]
[42]
Pt(5%)/K-10 montmorillonite
Pt(2%)/modified montmorillonite
Pt(3%)/potassium titanate nanotube
Pt(4%)-FeCl2(0.2%)/charcoal
Pt(2.2%)carbon nanofiber
Pt(5%)Fe(0.8%)Zn(0.4%)/mesoporous carbon
Pt(3%)/multi-walled carbon nanotubes
Pt(5%)/Cr–ZnO
[20,49]
[50]
[51]
PtSn0.8/SiO2
Pt(1%)/Al2O3
Pt(5%)/CeO2–ZrO2
This work

40
S. Bhogeswararao, D. Srinivas / Journal of Catalysis 285 (2012) 31–40
[5] K. Weissermel, H.J. Arpe, Industrial Organic Chemistry, Verlag Chemie,
100
80
60
40
20
Pd(2%)/CeO -ZrO
2
HCA
PPL
2
Weinheim, 1978.
[6] S. Mahmoud, A. Hammoudeh, S. Gharaibeh, J. Melsheimer, J. Mol. Catal. A:
Chem. 178 (2002) 161.
[7] J. C Serrano-Ruiz, J. Luettich, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso, J.
Catal. 241 (2006) 45.
Others
c
4
2
0
100
90
80
[8] P. Kluson, L. Cerveny, Appl. Catal. 128 (1995) 13.
[9] X.F. Chen, H.X. Li, Y.P. Xu, M.H. Wang, Chin. Chem. Lett. 13 (2002) 107.
[10] P. Claus, A. Brülckner, C. Mohr, H. Hofmeister, J. Am. Chem. Soc. 122 (2000)
11430.
[11] F. Delbecq, P. Sautet, J. Catal. 152 (1995) 217.
[12] T. Vergunst, F. Kapteijn, J.A. Moulijn, Catal. Today 66 (2001) 381.
[13] A. Dandekar, M.A. Vannice, J. Catal. 183 (1999) 344.
[14] B. Campo, M. Volpe, S. Ivanova, R. Touroude, J. Catal. 242 (2006) 162.
[15] K. Liberkova, R. Touroude, D.Y. Murzin, Chem. Eng. Sci. 57 (2002) 2519.
[16] S. Galvagne, A. Donato, G. Neri, R. Pietrepaele, C. Capannolli, J. Mol. Catal. 78
(1993) 227.
a
b
d
CAL
d
b
c
a
Pt(5%)/CeO -ZrO
2
2
PPL
4
2
0
HCA
90
30
40
50
60
70
80
100
[17] S. Galvagne, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 17 (1993)
55.
CA conversion (wt%)
[18] J.Q. Wang, Y.Z. Wang, S.H. Zhe, M.H. Qiao, H.X. Li, K.N. Fan, Appl. Catal. A: Gen.
272 (2004) 29.
[19] D. Richard, J. Ockelford, A. Giroir-Fendler, P. Gallezot, Catal. Lett. 3 (1989) 53.
[20] E.V. Romos-Fernández, J. Ruiz-Martínez, J.C. Serrano-Ruiz, J. Silvestre-Albero,
A. Sepúlveda-Escribano, F. Rodriguez-Reinoso, Appl. Catal. A: Gen. 402 (2011)
50.
[21] J. Teddy, A. Falqui, A. Corrias, D. Carta, P. Lecante, I. Gerber, P. Serp, J. Catal. 278
(2011) 59.
[22] M. Englisch, V.S. Ranade, J.A. Lercher, J. Mol. Catal. A: Chem. 121 (1997) 62.
[23] B. Coq, P.S. Kubhar, C. Moreau, P. Moreau, F. Figueras, J. Phys. Chem. 98 (1994)
10180.
Fig. 10. Comparative catalytic activity of CeO2–ZrO2-supported Pd and Pt catalysts.
Reaction conditions: Catalyst (0.05 g), cinnamaldehyde (CA, 4 g), solvent – ethanol
(40 ml) for Pd and iso-propanol for Pt catalysts, alkali additive = 0.02 g of NaOH in
10 ml water, H2 pressure = 20 bar, reaction temperature = 50 °C (for Pd) and 25 °C
(for Pt). HCA = hydrocinnamaldehyde, PPL = 3-phenylpropanol, CAL = cinnamyl
alcohol, others include acetals; a = 1 (for Pd) and 2 h (for Pt), b = 4 h, c = 6 h, and
d = 8 h.
While the Pd catalyst is selective mainly for HCA at all conversion
levels, Pt catalysts are selective for CAL. In other words, the nature
of the metal influences the product selectivity. Delbecq and Sautet
[11] reported that metal selectivities can be rationalized in terms
of the different radial expansion of their d bands; the larger the
band, the stronger the four-electron repulsive interactions with
the C@C bond and the lower the probability of its adsorption. In-
deed, the d band width increases in the series Pd < Pt < Ir, Os,
which accounts well for the selectivities observed over Pd and Pt
catalysts. The crystallite size (5–12 nm), preferential exposure of
[111] plane, high dispersion (40.5%), high electron density, strong
metal–support interactions, and facile low-temperature reducibil-
ity of Pt are the unique features that made Pt/CeO2–ZrO2 as a
highly efficient and selective hydrogenation catalyst.
[24] S. Bhogeswararao, D. Srinivas, Catal. Lett. 140 (2010) 55.
[25] M. Shen, J. Wang, J. Shang, Y. An, J. Wang, W. Wang, J. Phys. Chem. C 113 (2009)
1543.
[26] M. Shen, M. Yang, J. Wang, J. Wen, M. Zhao, W. Wang, J. Phys. Chem. C 113
(2009) 3212.
[27] M. Zhao, S. Chen, X. Zhang, M. Gong, Y. Chen, J. Rare Earths 27 (2009) 728.
[28] P. Bera, K.C. Patil, V. Jayaram, G.N. Subbanna, M.S. Hegde, J. Catal. 186 (2000)
293.
[29] H. Wang, Y. Chen, Q. Zhang, Q. Zhu, M. Gong, M. Zhao, J. Nat. Gas Chem. 18
(2009) 211.
[30] L. Vivier, D. Duprez, ChemSusChem 3 (2010) 654.
[31] M. Yashima, T. Hirose, S. Katano, Y. Suzuki, M. Kakihana, M. Yoshimura, Phys.
Rev. B: Condens. Matter 51 (1995) 8018.
[32] S. Otsuka-Yao-Matsuo, T. Omata, N. Izu, H. Kishimoto, J. Solid State Chem. 138
(1998) 47.
[33] T. Omata, H. Kishimoto, S. Otsuka-Yao-Matsuo, N. Ohtori, N. Umesaki, J. Solid
State Chem. 147 (1999) 573.
[34] G. Li, B. Zhao, Q. Wang, R. Zhou, Appl. Catal. B: Environ. 97 (2010) 41.
[35] A. Trovarelli, Catal. Rev. 38 (1996) 439.
[36] A. Martínez-Arias, M. Fernández-García, V. Ballesteros, L.N. Salamanca, J.C.
Conesa, C. Otero, J. Soria, Langmuir 15 (1999) 4796.
[37] A. Jung, A. Jess, T. Schubert, W. Schütz, Appl. Catal. A: Gen. 362 (2009) 95.
[38] P. Hollins, Surf. Sci. Rep. 16 (1992) 51.
4. Conclusions
Intramolecular liquid phase hydrogenation of cinnamaldehyde
was investigated over Pt (5 wt%) supported on CeO2, ZrO2, and
CeO2–ZrO2 catalysts. Cinnamyl alcohol was obtained as the selec-
tive product. Structural and electronic factors, alkali promoters,
and solvent influenced the catalytic properties. Acidity of the sup-
port (due to the presence of ZrO2 component) and higher electron
density at Pt (due to CeO2 component) are responsible for the high-
er catalytic activity and selectivity of Pt supported on CeO2–ZrO2
composite material. These catalysts were highly active and selec-
tive even at 25 °C and found to be superior to most of the hitherto
known supported Pt catalysts.
ˇ
[39] A. Martínez-Arias, J.M. Coronado, R. Cataluna, J.C. Conesa, J. Soria, J. Phys.
Chem. B 102 (1998) 4357.
[40] J.R. Croy, S. Mostafa, L. Hickman, H. Heinrich, B. Roldan Cuenya, Appl. Catal. A:
Gen. 350 (2008) 207.
[41] M. Dömök, A. Oszkó, K. Baán, I. Sarusi, A. Erdöhelyi, Appl. Catal. A: Gen. 383
(2010) 33.
[42] Z.-T. Liu, C.-X. Wang, Z.-W. Liu, J. Lu, Appl. Catal. A: Gen. 344 (2008) 114.
[43] E. Bekyarova, P. Fornasiero, J. Kašpar, M. Graziani, Catal. Today 45 (1998) 79.
[44] P. Gallezot, A. Giroir-Fendler, D. Richard, Catal. Lett. 5 (1990) 169.
[45] G. Szöllösi, B. Török, L. Baranyi, M. Bartók, J. Catal. 179 (1998) 619.
[46] D. Manikandan, D. Divakar, T. Sivakumar, Catal. Commun. 8 (2007) 1781.
[47] C.-Y. Hsu, T.-C. Chiu, M.-H. Shih, W.-J. Tsai, W.-Y. Chen, C.-H. Lin, J. Phys. Chem.
C 114 (2010) 4502.
[48] N. Mahata, F. Conçalves, M. Fernando, R. Pereira, J.L. Figueiredo, Appl.Catal. A:
Gen. 339 (2008) 159.
[49] E.V. Ramos-Fernández, A.F.P. Ferreira, A. Sepúlveda-Escribano, F. Kapteijn, F.
Rodríguez-Reinoso, J. Catal. 258 (2008) 52.
Acknowledgment
[50] A.J. Plomp, D.M.P. van Asten, A.M.J. van der Eerden, P. Mäki-Arvela, D.Y.
Murzin, K.P. de Jong, J.H. Bitter, J. Catal. 263 (2009) 145.
[51] A. Giroir-Fendler, D. Richard, P. Gallezot, Catal. Lett. 5 (1990) 175.
[52] D. Loffreda, F. Delbecq, F. Vigné, P. Sautet, Angew. Chem. Ind. Ed. 44 (2005)
5279.
[53] D. Loffreda, F. Delbecq, F. Vigné, P. Sautet, J. Am. Chem. Soc. 128 (2006) 1316.
[54] J.C. Serrano-Ruiz, A. López-Cudero, J. Solla-Gullón, A. Sepúlveda-Escribano, A.
Aldaz, F. Rodríguez-Reinoso, J. Catal. 253 (2008) 159.
S.B. acknowledges the Council of Scientific and Industrial
Research, New Delhi for the award of Senior Research Fellowship.
References
[1] P. Gallezot, D. Richard, Catal. Rev. Sci. Eng. 40 (1998) 81.
[2] P. Claus, Topic Catal. 5 (1998) 51.
[3] U.K. Singh, M.A. Vannice, Appl. Catal. A: Gen. 213 (2001) 1.
[4] P. Mäki-Arvela, J. Hájek, T. Salmi, D.Y. Murzin, Appl. Catal. A: Gen. 292 (2005) 1.
[55] R.J. Madon, J.P. O-Connell, M. Boudart, AIChE J. 24 (1978) 904.
[56] R.J. Madon, E. Iglesia, J. Mol. Catal. A: Chem. 163 (2000) 189.