A hybrid polyketone-SiO2 support for palladium catalysts and their applications in cinnamaldehyde hydrogenation and in 1-phenylethanol oxidation

Article abstract of DOI:10.1016/j.apcata.2015.01.045

An organic-inorganic hybrid material, PK-SiO₂ (PK = polyketone), was employed as support for Pd catalysts. Their synthesis was carried out by MW irradiation of an ethanol solution of Pd(OAc)₂ in the presence of the support. The obtai

Full text of DOI:10.1016/j.apcata.2015.01.045

Applied Catalysis A: General 496 (2015) 40–50
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A hybrid polyketone–SiO2 support for palladium catalysts and their
applications in cinnamaldehyde hydrogenation and in
1-phenylethanol oxidation
Claudia Antonettia, Luigi Toniolob, Gianni Cavinatoc, Claudia Forted, Chiara Ghignolia,
Randa Ishake, Fabrizio Cavanif, Anna Maria Raspolli Gallettia,∗
a Department of Chemistry and Industrial Chemistry, University of Pisa, Via G. Moruzzi 3, 56124 Pisa, Italy
b Department of Molecular Sciences and Nanosystems, University Ca’ Foscari of Venice, Dorsoduro 2137, 30123 Venice, Italy
c Department of Chemical Sciences, University of Padua, Via Marzolo 1, 35131 Padua, Italy
d ICCOM CNR, via G. Moruzzi 1, 56124 Pisa, Italy
e Department of Civil and Industrial Engineering, Largo Lucio Lazzarino 2, 56126 Pisa, Italy
f
Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Viale Risorgimento 4, 41036 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2014
Received in revised form 22 January 2015
Accepted 28 January 2015
Available online 28 February 2015
An organic–inorganic hybrid material, PK–SiO2 (PK = polyketone), was employed as support for Pd cata-
lysts. Their synthesis was carried out by MW irradiation of an ethanol solution of Pd(OAc)2 in the presence
of the support. The obtained systems were characterized by solid state NMR, SEM, TEM, ICP, XPS, BET,
gas porosimetry and tested in two probe reactions: the selective hydrogenation of cinnamaldehyde to
hydrocinnamaldehyde carried out in decalin and the oxidation of 1-phenylethanol to acetophenone in
water. For comparison, Pd/PK and Pd/SiO2 catalysts were also prepared by using the same procedure,
characterized and tested in the same reactions. On the hybrid support the Pd nanoparticles resulted sig-
nificantly smaller and with a more homogeneous size distribution compared to those on bare SiO2 or PK.
The catalysts on the hybrid support allowed a higher performance and could be recycled up to five times
without loss of activity and metal leaching. These results were related to the improved surface area of the
hybrid material (compared to the low surface area of bare PK) ascribable to silica introduction, combined
with the stabilizing effect of the polymeric counterpart against Pd nanoparticles agglomeration.
© 2015 Published by Elsevier B.V.
Keywords:
Hybrid materials
Polyketone–SiO2
Pd catalysts
Hydrogenation
Oxidation
1. Introduction
such as decrease in thermal, mechanical and chemical stability,
shrinking phenomena, changes of volume and porosity that can
Palladium nanostructured catalysts find many catalytic applica-
tions and their performances depend on their size, shape and ability
to avoid agglomeration [1,2]. Polymeric stabilizers, such as poly-
(1-vinylpirrolidone), polypyrrole, nafion and also natural polymers,
are generally employed in order to prevent the agglomeration of Pd
colloids towards the bulk metal [3,4]. The obtained “quasi” homo-
geneous metal nanoparticles offer the same high catalytic activity
as the homogeneous ones, however they are not as easily separated
and recycled. To overcome these drawbacks, they are generally
supported on inorganic carriers.
adversely influence their overall performances [5–8]. Recently, PKs,
being high temperature melting materials, insoluble in the most
common solvents, have been successfully proposed by us as sup-
port for palladium catalysts [9]. PK can act not only as mere support,
but also as stabilizing agent through interactions of its keto groups
with the Pd nanoparticles without suppressing the catalysis, differ-
ently from other polymeric supports. Unfortunately, PKs present
a low surface area and scarce compatibility with hydroxylated
media, such as water, the eco-friendly medium par excellence. The
incorporation of high surface area hydrophilic SiO2 into PK could
overcome these drawbacks and also improve the mechanical prop-
erties and stability of the resulting composites [10–13]. So far, a
hybrid PK–SiO2 material has been employed for the preparation
of new fuel cell membranes [14], but has never been tested as
a catalyst support. These considerations prompted us to prepare
some Pd nanostructured catalysts supported on the hybrid PK–SiO2
On the contrary, the application of polymers as supports has
been investigated to a lower extent, due to many disadvantages,
∗
Corresponding author. Tel.: +39 050 2219290; fax: +39 050 2219260.
E-mail address: anna.maria.raspolli.galletti@unipi.it (A.M. Raspolli Galletti).
http://dx.doi.org/10.1016/j.apcata.2015.01.045
0926-860X/© 2015 Published by Elsevier B.V.

C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
41
material, obtained by carrying out the synthesis of PK in the pres-
ence of high surface area Aerosil 380 SiO2, aiming also at their future
applications in the field of catalytic membranes [15].
to 300 W (with 1 W-step increment) with a fine and automated
control of the sample temperature based on the CEM PowerMax
technology. The oven allows two different operating ways: at con-
stant temperature (by varying the irradiation power in order to
maintain the desired temperature value) or at constant power. The
80 mL glass reactor (Sealed Vessel accessory) was equipped with
a magnetic drive stirrer and a vertically-focused IR temperature
sensor which provided an accurate temperature control during
the process (maximum operating temperature 250 ◦C). The reac-
tor was also equipped with a pressure line and a pressure probe,
the latter allowing the control of the internal pressure through-
out each run (maximum operating pressure 1.4 MPa). The selected
amount of Pd(OAc)2 was dissolved in 40 mL EtOH-support suspen-
sion under stirring at room temperature. The resulting mixture was
MW-irradiated for 10 min at 120 ◦C under nitrogen atmosphere.
The reactor was then rapidly cooled to room temperature, the sol-
vent was removed by filtration under nitrogen and the supported
catalysts were washed several times with acetone and ether, dried
under vacuum and stored under dry argon.
The synthesis of Pd catalysts was carried out in ethanol
employing microwave (MW) irradiation, a promising less energy-
demanding approach compared to conventional heating [16–21].
The Pd/PK–SiO2 catalysts were tested in two probe reactions: the
selective hydrogenation of cinnamaldehyde (CAL) to hydrocin-
namaldehyde (HCAL) carried out in decalin and, as a preliminary
investigation, in the oxidation of 1-phenylethanol (1-PE) to ace-
tophenone (AcPh) performed in water. The first reaction is a model
reaction to investigate the effect of the catalyst morphology on
the selectivity [7,9,19,22–28] and, in addition, HCAL has recently
been found as an important intermediate for the preparation of
pharmaceuticals [25].
The other reaction is important for synthetic organic chemistry
[29–36] and water represents the ideal green solvent for industrial
applications. In the last decade, several supported metal catalysts
were found to be active in the aerobic oxidation of alcohols in
water [29,30,32,33]. Among these catalysts, palladium-based ones
show promising performances even under mild reaction condi-
tions [35–46], although the number of highly recyclable catalysts
working in pure water is still rather limited.
2.3. Characterization of the catalysts
TEM images of all samples were taken using a Philips CM12
microscope working at 120 kV. Histograms of the particles size
distribution were obtained by counting at least 500 particles
onto the micrographs. The mean diameter (dm) was calculated
as dm = ꢀdini/ꢀni where ni was the number of particles of diam-
eter di. The counting was carried out on electron micrographs
obtained starting from 50,000 to 100,000 magnifications, where
well-contrasted Pd particles were clearly detected. The graduation
of the particle size scale was 0.5 nm.
Hereafter, we report the synthesis and characterization of pal-
ladium nanoparticle catalysts supported on Aerosil 380, PK and
hybrid organic–inorganic PK–SiO2, in order to investigate the
potential of the hybrid systems for catalytic applications.
2. Experimental
2.1. Chemicals
SEM analyses were carried out using a JEOL JSM 5600 LV working
at 200 kV.
NMR experiments were carried out on a Bruker AMX-300WB
spectrometer working at 300.13 MHz for proton and at 75.47 MHz
for carbon-13 using a 4 mm CP/MAS probe head for solid-state mea-
surements. All experiments were performed at 25 ◦C.
(Pd(OAc)2), 1,3-bis(diphenylphospino)propane (dppp) and p-
toluenesulfonic acid (TsOH) were purchased from Aldrich and
employed as received. [Pd(TsO)(H2O)(dppp)](TsO) was synthesized
as described in the literature [37]. SiO2 (AEREOSIL 380) was pur-
chased from Evonik Industries and employed after treatment under
vacuum at 100 ◦C for 3 h.
The PK support (poly-3-oxotrimethylene) was prepared
through CO-ethene copolymerization as described in [38],
except for the concentration of the copolymerization catalyst
[Pd(TsO)(H2O)(dppp)](TsO) in MeOH and in the presence of H2O
and TsOH ([Pd] = 0.125 mmol L−1, Pd:H2O:TsOH = 1:350:10, 85 ◦C, P
4.5 MPa, CO:ethene = 1:1, 1 h). The PK yield was 6.2 g with an aver-
age molecular weight of about 12 kDa. The PK–SiO2 support was
prepared under the same conditions, but in the presence of SiO2.
In the presence of 2 g or 4 g of SiO2, 8.3 g or 10.1 g of PK–SiO2 were
obtained, respectively. Thus, the presence of SiO2 does not alter
the productivity of the catalytic system. These supports contain
76% and 60% PK and are indicated as 76%PK–SiO2 and 60%PK–SiO2,
respectively.
13C cross polarization with magic angle spinning (CP–MAS) NMR
spectra were recorded using a contact pulse (tCP) of 1.5 ms with
a spin-lock field of 75 kHz, a recycle delay of 10 s and acquiring
6400 scans. The MAS frequency was 9 kHz. Variable contact time CP
experiments were performed with tCP values ranging from 100 ␮s
to 80 ms; 400 scans were acquired for each experiment. 1H–13
C
cross-polarization times (TCH) and 1H spin-lattice relaxation times
in the rotating frame (1H T1ꢁ) were determined by fitting the peak
intensities obtained from variable contact time experiments to the
equation [40]:
ꢀ
ꢁ
ꢀ
ꢁ
exp −t/T1ꢁ(H) − exp t/TCH
1/TCH − 1/T1ꢁ(H)
I(0)
TCH
I(t) =
.
All other reagents and standard products were also obtained
from Aldrich, purified according to standard procedures [39] and
stored under dry argon.
The 1H spin–spin relaxation times (T2) were measured from
transverse relaxation curves built using the FID following a solid
echo sequence (ꢂ/2x–t–ꢂ/2y–t) [41] in order to avoid dead time
effects, with a 90◦ pulse of 3.5 ␮s and a delay ꢃ of 16 ␮s. The exper-
iments were performed at resonance on static samples. The FIDs
recorded clearly showed two components which could be fitted by
a Pake function [42,43] and an exponential one, respectively.
The BET surface area was determined by nitrogen adsorption,
using a single point ThermoQuest Surface Area Analizer Qsurf S1.
The content of Pd was determined by inductively coupled
plasma-optical emission spectrometers (ICP–OES) with a Spectro-
Genesis instrument equipped with a software Smart Analyzer
Vision. For ICP–OES of the solid catalysts, each sample was heated
over a heating plate in a porcelain crucible in the presence of aqua
2.2. Preparation of the Pd nanoparticles supported on PK
Table 1 reports the reaction conditions adopted for the prepa-
ration of the catalysts and their morphological properties.
The MW-assisted synthesis of Pd catalysts was performed using
a CEM Discover S-class oven with a PC control. The apparatus
consists of a single-mode, self-tuning cavity where the correct
position of the reaction vial is automatically tuned to ensure repro-
ducible conditions at each run. The oven operates with a continuous
power generator capable of supplying an irradiation power from 0

42
C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
Table 1
Preparation conditions and characterization of different supported Pd catalysts. The syntheses were carried out under MW irradiation at 120 ◦C for 10 min in 40 mL of EtOH.
%
Sample
Pd
precursor
(mg Pd)
Support (g)
Pd (wt%)a
Pd: average
diameter [Std.
dev.] (nm)
Support: average diameter
microstructure [Std. dev.]
(␮m)
BET (m2/g)b
PMEc
XPS binding
energies Pd 3d5/2
(eV)
PD 1
PD 2
PD 3
PD 4
PD 5
Pd(OAc)2
(9.5)
Pd(OAc)2
(24.3)
Pd(OAc)2
(9.9)
Pd(OAc)2
(21.7)
Aerosil
0.70
Aerosil
0.70
PK
0.70
PK
0.71
60%PK–SiO2
0.70
0.9
2.6
1.0
2.6
1.0
17.8
[3.4]
19.2
[3.5]
8.7
[3.3]
10.1
[3.5]
5.2
12.2
[4.1]
14.7
[5.1]
20.8
[5.2]
22.4
[5.3]
18.7
[4.0]
281
228
14
6.3
5.8
335.6 (99%)337.6 (1%)
335.6 (98%)337.6 (2%)
335.6 (99%)337.6 (1%)
335.6 (98%)337.6 (2%)
335.6 (99%)337.6 (1%)
12.9
11.1
21.5
11
Pd(OAc)2
(9.8)
128
[2.2]
PD 6
Pd(OAc)2
(24.9)
Pd(OAc)2
(9.8)
Pd(OAc)2
(9.7)
Pd(OAc)2
(24.0)
60%PK–SiO2
0.71
76%PK–SiO2
0.70
76%PK–SiO2
0.70
76%PK–SiO2
0.70
2.8
1.0
1.0
2.5
6.3
[2.4]
5.1
[2.1]
5.0
[2.1]
6.1
19.1
[5.0]
22.0
[7.0]
22.6
[6.9]
24.3
[7.2]
113
90
17.8
21.9
22.4
18.4
335.6 (98%)337.6 (2%)
335.6 (99%)337.6 (1%)
335.6 (99%)337.6 (1%)
335.6 (98%)337.6 (2%)
PD 7
PD 7 bis
PD 8
90
74
[2.5]
a
Determined by inductively coupled plasma analysis (ICP).
Obtained by single point BET.
Percentage of metal exposed determined by TEM analysis.
b
c
regia (2 mL) for four times, dissolving the solid residue in 0.5 M
aqueous HCl. The limit of detection calculated for Pd was 2 ppb.
The X-ray photoelectron spectroscopy analyses were performed
with a VGMicrotech ESCA 3000 Multilab spectrometer, using the
unmonochromatised Al K␣ source (1486.6 eV) run at 14 kV and
15 mA. Survey spectra were collected at constant pass energy of
50 eV. Individual peak energy regions were collected at constant
pass energy of 20 eV. Samples were mounted on a stub holder
using double-sided adhesive tape. In order to avoid the contact with
air, the sample mounting was performed inside a glove bag filled
with N2 and then the holder was loaded inside the XPS machine
under a N2 atmosphere. The C 1s peak, set at 285.1 eV, arising from
adventitious carbon, was used as reference for the binding energy
values. Differential surface charging was ruled out by checking the
reproducibility of XPS results in repeated scans under different
X-ray exposures. Analyses of the peaks were performed with the
software provided by VG, based on non-linear least-square fitting
routine using a weighted sum of Lorentzian and Gaussian compo-
nent curves after background subtraction according to Sherwood
[44].
the liquid through a valve and analyzing it by gas-chromatography.
GC analyses were performed with a HP 5890 gas-chromatograph
equipped with a HP 3396 integrator, a flame ionization detector and
a PONA capillary column (50 m × 0.2 mm × 0.5 ␮m) with a station-
ary phase 100% dimethylpolysiloxane (carrier gas nitrogen, flow
1 mL/min).
In every test, it was found that when the stirring speed was
greater than 450 rpm further increase of the stirring speed did not
affect the catalytic performances, ensuring the absence of external
diffusion limitations. Therefore, a stirring speed of 500 rpm was
employed.
In order to ascertain the extent of l/s mass-transfer and internal
resistances, the following criteria reported by Ramachandran and
Chaudhari [45,46] were applied:
Liquid–solid hydrogen-transfer resistance negligible if:
Rꢁpdp
6k1/sCwH∗2
˛
1
=
< 0.1,
internal hydrogen resistance negligible if:
Infrared spectra were collected using a Nicolet FT-IR Nexus spec-
trometer equipped with a Smart Diffuse Reflectance Accessory.
dp
˛
2
==
< 0.2
6 × [ꢁpR/CwDeH H∗2]0.5
2
2.4. Hydrogenation of cinnamaldehyde to hydrocinnamaldehyde
where
R = reaction
rate,
mol/(m3 s);
ꢁp
is
density
of the particle; dp = average particle diameter, m; k1/sH
=
2
Hydrogenation reactions were carried out in a stainless steel
300 mL mechanically stirred Parr 4560 autoclave equipped with a
P.I.D. controller 4843. In each experiment, the proper amount of cat-
alyst, always containing 2.2 mg of palladium, was introduced into
the reactor under inert atmosphere. The autoclave was then closed,
evacuated up to 65 Pa and a solution of 3.8 mL of cinnamalde-
hyde in 100 mL of decalin was introduced by suction. The reactor
was then pressurized with hydrogen to 2 MPa and heated up to
100 ◦C, maintaining a stirring speed of 500 rpm, which assured the
absence of mass transfer limitations. In order to ascertain the role
of mass transfer, the stirring speed was varied from 400 rpm to
1000 rpm adopting the chosen reaction conditions (100 ◦C, 2 MPa
of H2, 3.8 mL of cinnamaldehyde in 100 mL of decalin) using the
catalysts PD 3 and PD 7 (2.2 mg of Pd for every run. The pressure
value was held constant at 2 MPa throughout the reaction time.
The course of the reaction was monitored by sampling periodically
transfer coefficient of hydrogen from liquid to catalyst,
m/s;
Cw = catalyst
concentration,
kg/m3;
H2∗
=
hydrogen solubility in decaline; De (m2/s) is the effective dif-
fusivity of hydrogen in the pores. R is taken as the average reaction
rate in the first 30 min for catalyst PD7 (1%Pd/76%PK–SiO2), which
has the biggest particle diameter (22 × 10−6 m) and is the most
active, so that the mass transfer resistances are the highest. From
the data reported in Table 3, run 4, it is R = 4.52 × 10−2 mol/(m3 s).
ꢁp is calculated as (76%ꢁPK + 24%ꢁSiO2); bulk density of PK
1240 kg/m3, 2200 kg/m3 for SiO2 [47]; it is ꢁp = 1470 kg/m3.
Due to the small dimensions of the particles, k1/sH
is calculated by using the relation: k1/sH = 2DH /dp2,
2
2
where DH is the diffusivity of hydrogen in decalin. DH
2
2
is calculated by using the relation reported in [45–47]:
0.6
DH2 (cm2/s) = 7.4 × 10−8 · T(MWdecaline
)
0.5/(ꢄdecaline · VH
)
where MWdecalin = molecular weight of decalin; ꢄdecalin =2viscosity

C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
43
of decalin, taken as 0.1 cP, by extrapolating the data
reported in [48] at 373 K, the reaction temperature;
VH = molar volume of hydrogen at the normal boiling point, 7.03
2
cm3/mol [47]. It is DH = 1.1 × 10−7 m2/s and k1/sH
=
2
2
1.0 × 10−2 m/s. DeH can be safely taken as DH2 /5 = 0.22 m2/s
2
[51]. It is Cw = 2.2 kg/m3, H∗2 = 66 mol/m3, calculated at 373 K,
under 2 MPa from [49]. Using these data, it is ˛1 = 1.67 × 10−4 ꢀ 0.1
and ˛2 = 1.66 × 10−2 < 0.2, therefore the external and internal
resistances are negligible. These parameters are even smaller
because the actual ꢁp is <1470 kg/m3, due to the porosity of the
particles. The same conclusions can be drawn with the other
catalysts, which are less active and have smaller particle size, or
also if one takes into consideration the resistances of CAL transfer.
In the above relations it is enough to substitute DH and DeH with
2
DCAL and DeCAL, H∗2 with the concentration of CAL2in mol/m3 and
VH with VCAL at the normal boiling point.
2The reproducibility of repeated catalytic runs was within 5%.
Recycling experiments were carried out by removing the liquid
reaction mixture through the sample valve, evacuating the auto-
clave and then charging again with the fresh CAL solution for the
subsequent catalytic run. Leaching tests were carried out as fol-
lows: at the end of the first hydrogenation run the solid catalyst
was filtered off under inert atmosphere and the recovered solution
was reused for a second hydrogenation.
Fig. 1. Infrared spectra of pristine PK, SiO2 and 60%PK–SiO2 in the region from 4000
to 400 cm−1
.
Activity was calculated as converted moles of CAL per mole of Pd
in the whole catalyst per second, measured after 30 min of reaction
and at 100 mol% CAL conversion. TOF numbers (turnover frequen-
cies) after 30 min of reaction were calculated as moles of converted
cinnamaldehyde per mole of exposed Pd per second. The amount
of exposed Pd was calculated on the basis of percentage of metal
exposed (PME) estimated from the expression PME = 112/d (nm)
assuming spherical particles for Pd [50].
2.5. Oxidation of 1-phenylethanol to acetophenone
Oxidation reactions and recycling and leaching experiments
were carried out using the same apparatus and procedures adopted
for the hydrogenation reaction. Reaction conditions are reported
in Table 5. GC analyses were performed using the same instrument
and conditions reported in the previous section. The reproducibility
of the catalytic runs was within 5%.
Fig. 2. 13C CPMAS NMR spectrum of PK (asterisks indicate spinning side bands).
209.9 ppm ascribable to methylene and carbonyl carbons, respec-
tively. Variable contact time CP experiments reveal that the only
difference between the different samples was in the 1H T1ꢁ val-
ues, which were 42 2 ms for PK and 50 2 for 76%PK–SiO2 and
60%PK–SiO2, thus showing a slightly higher rigidity for PK. In order
to better highlight differences in the degree of crystallinity, 1H FID
analysis was performed. The FIDs were well fitted with a Pake
and an exponential contribution, the former ascribable to a crys-
talline component, the latter to an amorphous one. The results of
the fitting indicated that the crystalline contribution is higher for
PK, i.e. 84% of the 1H signal compared to 80% and 78% in the case
of 76%PK–SiO2 and 60%PK–SiO2, respectively (see Table 2). Fur-
thermore, the amorphous component in PK is characterized by a
T2 value of 410 10 ␮s compared to a value of 122 20 ␮s deter-
mined for 60%PK–SiO2 indicating a more rigid amorphous phase in
the latter sample. These results suggest that the presence of SiO2
induces a change in polymer morphology, causing a decrease in
crystalline component accompanied by a decrease in mobility of
the amorphous one.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Characterization of the supports
The features of the supports used for the preparation of the Pd
catalysts are reported in Table 2.
The infrared spectra of Aerosil 380, of the hybrid 60%PK–SiO2
and of the PK powders are shown in Fig. 1.
The infrared spectrum of the hybrid 60%PK–SiO2 is essentially
the superposition of SiO2 and PK spectra. The presence of the OH
stretching modes of hydroxyl groups in the spectra of both pristine
SiO2 and the PK–SiO2 material between 3800 and 3000 cm−1 indi-
cates that there are not chemical bonds between the PK and the OH
groups present on the surface of silica nanoparticles.
The surface areas of the different supports (SiO2, 60%PK–SiO2,
76%PK–SiO2 and bare PK) were measured using a single point BET
equation and resulted 306, 140, 117 and 16 m2/g, respectively. The
decreasing values of the surface area of the hybrid materials with
increasing % of PK are in line with the much lower contribution of
the PK to the surface area.
Fig. 2 shows the 13C CPMAS NMR spectrum of PK, whereas Fig. 3
reports 1H FID of PK (solid line) and of 60%PK–SiO2 (dashed line).
SEM analyses show their macro-structures where aggregated
particles are evidenced. The average diameters of macro-structures
of bare silica aggregates, bare PK, 60%PK–SiO2, 76%PK–SiO2 are 20.7,
6.9, 7.1 and 7.0 ␮m, respectively. The structure of Aerosil was also
13C CP–MAS NMR spectra of PK with and without SiO2 do not
show differences, all spectra displaying only the signals at 35.5 and

44
C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
Table 2
Characterization of the employed supports.
Support
% PK
BET (m2/g)a
% PK crystallinityb
Average diameter of macrostructure
[Std. dev.]
(␮m)
Aerosil
0
100
60
306
16
–
84
78
80
20.7
[9.2]
6.9
[1.5]
7.1
PK
60%PK–SiO2
76%PK–SiO2
140
117
[2.1]
7.0
76
[2.2]
a
Obtained by single point BET.
Crystallinity of the PK component determined by NMR.
b
investigated using TEM technique, which allowed us to detect its
primary nanoparticles having an average diameter of 13.3 nm with
a standard deviation of 4.2. The pore size characterization (see Sup-
plementary Information) evidenced that the deposition of PK over
silica support leads to a total disappearance of the original silica
microporosity.
PD 1 and PD 2 samples show an almost complete Pd reduc-
tion, as evidenced by the 99% and 98% abundance of the peak at
335.6 eV.
On the other hand, PD 3 and PD 4 were synthesized employ-
ing the bare PK as support, with analogous Pd loading, 1 and
2.6 wt%, respectively. The introduction of palladium causes a low-
ering of surface area compared to the bare support, as expected.
TEM analysis shows a Pd particle size distribution with average
diameters of 8.7 and 10.1 nm and standard deviations of 3.3 and
3.5 respectively. In Fig. 4B the TEM image and the particle size
distribution of sample PD 3 are reported, where it is also possi-
ble to observe the presence of some Pd aggregates. On the PK Pd
particles are smaller than those supported on silica. The higher
stabilizing ability of PK may be due not only to steric but also to
electronic effects through the interactions of its keto groups with Pd
nanoparticles.
3.1.2. Characterization of the catalysts
All these supports were used for the synthesis of Pd catalysts,
whose features are reported in Table 1. The Pd average particle
diameter of samples supported on silica PD 1 and PD 2, with 0.9 and
2.6 wt% Pd, are 17.8 and 19.2 nm, with standard deviations of 3.4
and 3.5, respectively. Fig. 4A shows the TEM image and the particle
size distribution for PD 1.
The surface areas of PD 1 and PD 2 are 281 and 228 m2/g,
respectively (Table 1), smaller if compared to that of the pristine
SiO2, which suggests that standard pre-treatments carried out on
SiO2 before its employment as support, the successive Pd depo-
sition and the final catalyst work-up partially aggregate the SiO2
particles. These samples were also characterized by XPS analysis.
In Table 1 the binding energies relative to Pd 3d5/2 photoelec-
tron transition characterized by two components, respectively at
335.6 and 337.6 eV, are reported. The low binding energy com-
ponent at 335.6 eV is attributed to metallic Pd(0), whereas the
high energy component at 337.6 eV is due to oxidized Pd2+ [51].
SEM and TEM analyses show that the texture of the PK before
and after the deposition of palladium is similar (Fig. 5).
Again, the complete reduction of Pd was again ascertained by
XPS spectroscopy (Table 1).
Samples PD 5 and PD 6 were synthesized in similar manner as
the previous ones, with a 1 and 2.8 wt% Pd loading, respectively, but
using the hybrid 60%PK–SiO2 material as support. Both catalysts
are characterized by much smaller palladium nanoparticles with a
more homogeneous particle size distribution, average diameters of
5.2 and 6.3 nm and standard deviations of 2.2 and 2.4, respectively.
Fig. 6A shows the TEM image and the particle size distribution for
sample PD 5. On the PK–SiO2 supports the Pd nanoparticles result
significantly smaller with a more homogeneous size distribution
compared to those on bare SiO2 or PK. This suggests that for the
hybrid supports the stabilizing role of the prevailing PK component
is preserved and rather enhanced by the increase of surface area
resulting in lower probability of metal particles to be closed to other
ones and aggregate.
Also in this case, XPS analysis evidenced that both catalysts
present an almost complete Pd reduction. Samples PD 7 and
PD 8, prepared using the hybrid support with 76% of PK, show
Pd nanoparticles with dimensions similar to those of the previ-
ous hybrid catalysts, characterized by average diameters of 5.1
and 6.1 nm and standard deviations of 2.1 and 2.5 respectively.
Fig. 6B shows TEM image and particle size distribution for sam-
ple PD 7. BET surface areas for these samples resulted of 90 and
74 m2/g, lower than those of the corresponding catalysts supported
on 60%PK–SiO2. Again, XPS analysis confirms the complete Pd
reduction.
When both hybrid supports were used, the pore size characteri-
zation (see Supplementary Information) reveals that the deposition
of palladium increases the disappearance of the original silica
microporosity, as already observed before the introduction of Pd
in the composite material respect to the pristine SiO2.
Fig. 3. 1H FID of PK (solid line) and of 60%PK–SiO2 (dashed line).

C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
45
Fig. 4. TEM images and particle size distributions of samples PD 1 (A) and PD 3 (B).
3.2.1. Hydrogenation of CAL
The synthesis of sample PD 7 was duplicated (sample PD 7 bis,
Table 1): TEM, XPS and BET analysis show that the preparation
under MW irradiation is highly reproducible (Table 1).
The supported Pd catalysts were tested in the hydrogenation of
CAL to HCAL adopting the reaction conditions reported in Table 3.
Scheme 1 shows the possible hydrogenations of the different func-
tional groups, C C and C O double bonds.
It is remarkable that in each case the only ascertained by-
product was 3-phenyl-1-propanol (HCOL), whereas cinnamyl
alcohol (COL) was detected only in trace amounts.
The result obtained testing the sample PD 3 and PD 4 supported
on pure PK is in line with our previous results where Pd was sup-
ported on a PK of higher molecular weight (>50 kDa) [9]. In the
present study, we preferred to use a PK of 12 kDa prepared accord-
ing to the conditions reported in the experimental section, since the
yield in the hybrid PK–SiO2 was more easily reproducible when
the synthesis was repeated in the presence of SiO2, then when
3.2. Catalytic tests
As reported in the experimental section, the employed PK was
prepared by CO–ethene copolymerization catalyzed by a Pd(II)
complex, with the risk of contamination by trace amounts of pal-
ladium. Therefore, preliminary blank hydrogenation of CAL and
oxidation of 1-PE tests were carried out using the bare PK under
typical reaction conditions reported in Tables 3 and 5: no reaction
occurred, thus any contribution to the catalytic activity from traces
of palladium employed in the copolymerization can be excluded.
Fig. 5. SEM image comparison between PK and PD 3.

46
C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
Fig. 6. TEM images and particle size distributions of samples PD 5 (A) and PD 7 (B).
and the metal nanoparticle. Consequently, in the second case, the
distance between C C bond and the catalyst is increased, causing
a lower selectivity towards the C C bond hydrogenation, as well
evidenced by Chizari et al. [54]. In effect, in our study, the high-
est selectivity to HCAL was obtained in run 5 carried out with PD
7 (1.0% Pd/76%PK–SiO2), characterized by the smallest Pd particle
size and narrowest size distribution.
A further inspection of the results reported in Table 3 shows
that the activity based on Pd in the whole catalyst is in the order
Pd/PK–SiO2 > Pd/SiO2 > Pd/PK for both Pd loadings, even though the
surface area follows the order Pd/SiO2 > Pd/PK–SiO2 ꢁ Pd/PK.
The recovery and reusability of the catalysts was investigated
by recycling samples PD 7 and PD 1 five times. The results shown
in Fig. 7 and Table 3 highlight that no loss of activity and selectivity
occurs with sample PD 7, whereas for sample PD 1 there is a marked
decrease both in activity and selectivity.
Scheme 1. Reaction pathway of the hydrogenation of cinnamadehyde.
These results can be ascribed to the aggregation of Pd particles
ascertained for PD 1 after the catalytic runs, as revealed by TEM
analyses (Table 4) of the sample recovered after five catalytic cycles
(run 10, Table 3).
On the contrary, after the catalytic run 9, sample PD 7 reveals
unchanged morphology at TEM analysis. The progressive decrease
of activity shown by PD 1 can be also ascribed to the leaching of Pd
from the solid catalyst. In fact, the Pd loading of PD 1 after five cycles
was decreased to 0.7 wt%, whereas for PD 7 remained unchanged at
using the conditions described in [9]. For each couple of catalysts
on the same support, the most active and selective to HCAL are
those with lower Pd loading, characterized by smaller palladium
particle sizes, as found for other palladium catalysts employed in
the same reaction [9,52,53]. The particle size effect was attributed
to the position of cinnamaldehyde molecule respect to the metal
surface. The surface of small particles was assimilated to a curved
surface, whereas the surface of big particles was seen as a flat sur-
face which leads to an enhanced repulsion between the phenyl ring

C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
47
Table 3
Catalytic performances of supported Pd catalysts in the hydrogenation of cinnamaldehyde to hydrocinnamaldehyde.
b
c
Runa
Sample description
Conv. (mol%)
after 30 min
A30 min (mol (mol
A100% (mol (mol Pd s)−1
)
Sel. to HCAL at
100 mol%
TOFd (s−1
)
Pd s)−1
)
conversion (mol%)
Blank test
1
PK
–
56.1
–
0.46
–
0.10
–
72.8
–
7.2
Sample PD 1
0.9 wt% Pd/Aerosil
Sample PD 2
2.6 wt% Pd/Aerosil
Sample PD 5
1.0 wt%
2
3
18.2
93.3
0.15
0.76
0.05
0.41
71.6
80.2
2.5
3.5
Pd/60%PK–SiO2
Sample PD 6
2.8 wt%
Pd/60%PK–SiO2
Sample PD 7
1.0 wt%
Pd/76%PK–SiO2
Sample PD 8
2.5 wt%
4
5
6
51.4
98.0
22.2
0.42
0.79
0.18
0.14
0.41
0.08
78.4
82.4
78.3
2.3
3.6
1.0
Pd/76%PK–
-SiO2
7
8
9
Sample PD 3
1.0 wt% Pd/PK
Sample PD 4
2.6 wt% Pd/PK
Sample PD 7
recovered after 5
catalytic cycles
Sample PD 1
recovered after 5
catalytic cycles
30.6
23.0
94.4
0.25
0.19
0.76
0.04
0.03
0.32
76.8
75.9
82.0
1.9
1.7
3.5
10
35.0
0.28
0.07
60.5
5.0
a
Reaction conditions: 100 ◦C, 2 MPa of H2, 2.2 mg of Pd, 3.8 mL of cinnamaldehyde in 100 mL of decalin as solvent.
Activity measured at 30 min of reaction as converted moles of CAL per mole of Pd in the whole catalyst per second.
Activity measured at 100 mol% CAL conversion.
b
c
d
TOF (turnover frequency) after 30 min of reaction, calculated as moles of converted cinnamaldehyde per mole of exposed Pd per second.
1.0 wt%. In order to exclude any metal leaching from the supported
Pd systems (which could lead to highly active soluble Pd species),
the solution recovered by filtration after the catalytic run 5 of sam-
ple PD 7 was tested in the hydrogenation of CAL. No further catalytic
activity was ascertained. Moreover, the presence of palladium in
the reaction mixture was excluded by ICP–OES, thus confirming
the stability of the Pd/PK–SiO2 heterogeneous catalysts. In our pre-
vious paper we had yet underlined the stability and recyclability of
Pd/PK systems [9]. The above results evidence that PK present on
the hybrid support is still able to play a significant stabilizing effect
for Pd species also during catalysis, not exerted by bare silica. The Pd
catalysts supported on the hybrid systems generally show higher
metal dispersions and TOF values than the corresponding systems
supported on bare PK (see Table 3), the increase of surface area of
the hybrid supports resulting in lower probability of Pd particles
to aggregate. The high TOF values determined for Pd/SiO2 could
be scarcely reliable due to the ascertained contribution of leached
palladium species to the catalytic performance.
In general, it is difficult to compare the obtained results (in terms
of activity and TOF values) with those reported in the literature, due
to the different adopted conditions (pressure, temperature, dura-
tion, solvent) and lacking of information on Pd particles dimensions
for many systems. In a previous work of our group, Pd catalysts
anchored on a cross-linked styrene/divinylbenzene resin function-
alized with bis(diphenylphosphino)methane ligands were used for
the hydrogenation of CAL in decalin adopting reaction conditions
similar to those of the present paper. After 30 min of reaction, an
activity of about 0.43 converted moles of CAL per mole of Pd in
the whole catalyst per second was achieved [7]. An activity value
up to 0.79 after 30 min has been reached using the catalyst PD7.
Fig. 7. Activity30 min and selectivity to HCAL at 100 mol% CAL conversion in the hydrogenation of CAL in the presence of samples PD 1 and PD 7 carried out at 100 ◦C, 2 MPa
of H2 using 2.2 mg of Pd, 3.8 mL of cinnamaldehyde in 100 mL of decalin (first hydrogenation runs, runs 1 and 5 respectively and successive recycles of the solid catalysts).

48
C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
Table 4
Averages diameters, standard deviations and percentage of metal exposed (PME) of
samples PD 7 and PD 1 after their employment in catalytic runs 9 and 15 (PD 7) and
10 and 16 (PD 1).
Sample
Average diameter [Std. dev.] (nm)
PMEa
Catalysts used for 5 cycles in the hydrogenation of CAL
Recovered sample PD 7
1.0 wt% Pd/76%PK–SiO2
Recovered sample PD 1
0.9 wt% Pd/Aerosil
5.1
[2.2]
[4.2]
22.0
5.7
19.8
Catalysts used for 5 cycles in the oxidation of 1-PE
Recovered sample PD 7
1.0 wt% Pd/76%PK–SiO2
Recovered sample PD 1
0.9 wt% Pd/Aerosil
5.2
[2.2]
21.5
5.8
19.4
[4.1]
a
Percentage of metal exposed determined by TEM analysis.
On the other hand, when a 5% Pd/C catalyst was adopted for the
hydrogenation of CAL in decalin under mild conditions (30 ◦C and
0.1 MPa of hydrogen), hydrocinnamaldehyde was produced with a
very low selectivity, 33.6%, at complete CAL conversion. Moreover,
no mention of the recyclability of the catalysts was made [22].
A very recent paper reports an interesting comparison of TOF
data for CAL hydrogenation employing different Pd supported
catalysts (Pd/C, Pd/carbon nanofiber, Pd/carbon nanofiber/TiO2,
Pd/Al2O3, Pd/SiO2) in the temperature range of 60–80 ◦C and pres-
sure range of 0.2–4 MPa of hydrogen, in different solvents [23].
The reported TOF values are within the range 0.16–6.7 s−1 and for
Pd/SiO2 up to 3.4 s−1, but in this last case the selectivity to HCAL at
complete conversion of the substrate is always lower than 60%.
Finally, in a very recent paper of Fujiwara, Pd/SiO2 in water
at 25 ◦C and 0.1 MPa of hydrogen allowed to reach a conversion
value after 4 h of reaction of only 23% with a selectivity to HCAL of
87%, corresponding to a TOF of 0.004 s−1. When novel Pd nanopar-
ticles supported on boronate polymers were adopted, 90% yield
was ascertained at complete conversion, with a TOF of 0.019 s−1, a
value significantly low. Moreover, the preparation of this catalyst
required a complex multi-step synthetic procedure [24].
Fig. 8. Yield of acetophenone (mol%) in the oxidation of 1-PE after 6 of reaction in
the presence of samples PD 1 and PD 7 carried out at 100 ◦C, 1 MPa of O2 using 1.0 mg
of Pd, 300 mg of 1-phenylethanol in 5 mL of water (first hydrogenation runs, runs
11 and 12 respectively and successive recycles of the solid catalysts).
apolar solvent. In particular, they studied the oxidation of benzyl
alcohol in cyclohexane and in water at 100 ◦C under 0.2 MPa of
oxygen in the presence of Pd catalysts supported on functional-
ized carbon nanotubes (CNTs). In cyclohexane the performances
of modified Pd/CNTs catalysts followed their hydrophobic proper-
ties, being the most hydrophilic support worst, while in water the
behaviour of the most hydrophilic catalyst considerably increased.
The beneficial effect of water was explained taking into account
that in this medium the catalyst with the most hydrophilic surface
could take the highest advantage for the surface–solvent contact,
due to their high affinity. This effect was also ascertained in the
aerobic oxidation of several alcohols (benzylic, 1-phenylethanol
etc.) in water under atmospheric pressure of O2 employing pal-
ladium nanoparticles supported on a new ordered mesoporous
silica with cage-like structure. In particular, in the oxidation of 1-
phenylethanol, a conversion of 96.9% with selectivity more than
99% to acetophenone was reached, working at 50 ◦C for 10 h. The
catalysts showed good recyclability and it is possible to address
this behaviour to the particular ordered structure of the support
which, however, requires a complex multi-step preparation [34].
Our results are in line with those just mentioned: the presence of
hydrophilic SiO2 in the hybrid support favours the activation of 1-
PE on catalyst surface. This effect may explain the higher yields
achieved when employing catalysts PD 7 and PD 5 compared to
that obtained with PD 3. The yields in the case of PD 7 and PD 5 are
even better than that of PD 1 and this may be ascribed to the better
stabilizing effect of PK towards Pd NPs in comparison with the pris-
tine SiO2. This was also evidenced by recycle tests, an aspect that
represents one of the main criticisms for Pd catalyzed oxidation of
alcohols in water. Samples PD 7 and PD 1 were recycled five times
under the same reaction conditions (Fig. 8 and Table 5).
On the base of the above discussion, the reached results, in terms
of TOF values and recyclability, are comparable, in terms of TOF val-
ues and recyclability, with the leading results up to now reported.
3.2.2. Oxidation of 1-PE
Pd catalysts which showed the most promising performances in
the hydrogenation of CAL (i.e. samples PD 1, 3, 5 and 7) were also
tested in a preliminary study on the oxidation of 1-PE to AcPh in
water (Scheme 2).
The catalytic performances are reported in Table 5. All the reac-
tions are completely selective to ketone, no other by-products were
detected in the reaction mixture also when complete conversion
was reached (run 12, Table 5), in particular benzaldehyde and ben-
zoic acid, well known by-products of this reaction, were absent
[55].
In the case of PD 1 supported on SiO2, the yields in acetophenone
are 44.9 and 75.1 mol% after 6 and 12 h of reaction, respectively.
Sample PD 3, where Pd is supported on the bare PK, gives the poo-
rest results. More promising outcomes are achieved with samples
PD 7 and PD 5 supported on the hybrid material SiO2–PK, reach-
ing yields of 58.3 and 62.2 mol% after 6 h of reaction and 94.8 and
100 mol% after 12 h respectively. These results can be related to
the different affinity among the catalyst surface, the substrate, the
oxidation product and the solvent. In fact, according to Prati et al.
[33,56,57], the performance in the oxidation of hydrophobic and
hydrophilic alcohols in the presence of Pd catalysts supported on
carbon nanotubes, whose surface was modified in order to change
the hydrophobic/hydrophilic properties, was different in polar or
Sample PD 7 maintained its initial performance, whereas with
sample PD 1 a marked decrease was observed. This last result is
ascribable to the aggregation of Pd particle after the catalytic run,
as shown by TEM analysis of samples PD 7 and PD 1 after use (see
Table 4), which again confirms the capacity of the PK component
to act as a stabilizer. The significant aggregation on bare silica has
been already observed for other Pd nanocatalysts [34].
In order to exclude metal leaching from the supported Pd sys-
tems leading to highly active soluble Pd species, the solution
recovered by filtration after the catalytic run 13 of sample PD 7

C. Antonetti et al. / Applied Catalysis A: General 496 (2015) 40–50
49
Scheme 2. Reaction pathway for the oxidation of 1-phenylethanol.
Table 5
Catalytic performances of supported Pd catalysts in the oxidation of 1-phenylethanol to acetophenone after 6 and 12 h of reaction.
Runa
Sample description
Yield of acetophenone
Selectivity to
acetophenone
Yield of
acetophenone
Selectivity to
acetophenone
(mol%) after 6 h
(mol%) after 12 h
Blank test
11
PK
–
44.9
–
100
–
75.1
–
100
Sample PD 1
0.9 wt% Pd/Aerosil
Sample PD 5
1.0 wt% Pd/60%PK–SiO2
Sample PD 7
1.0 wt% Pd/76%PK–SiO2
Sample PD 3
1.0 wt% Pd/PK
Sample PD 7 recovered after 5
catalytic cycles
Sample PD 1 recovered after 5
catalytic cycles
12
13
14
15
16
62.2
58.3
30.1
57.8
24.7
100
100
100
100
100
100
100
100
100
100
100
94.8
61.6
93.6
41.3
a
Reaction conditions: 100 ◦C, 1 MPa of O2, 1.0 mg of Pd, 300 mg of 1-phenylethanol in 5 mL of water as solvent.
was tested in the oxidation of 1-PE. Neither further catalytic activ-
ity of the liquid phase nor the formation of palladium black were
ascertained, thus confirming the effective stabilizing capacity of the
PK in the hybrid PK–SiO2.
conversion of the substrate, and the recyclability of the catalyst for
five cycles without metal leaching was again ascertained.
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.
2015.01.045.
4. Conclusions
The organic–inorganic hybrid material PK–SiO2 has been
employed for the first time as support and stabilizer for Pd nanopar-
ticles. The prepared Pd catalysts, synthesized in ethanol adopting
MW irradiation, were morphologically characterized and tested in
the hydrogenation of CAL to HCAL and in the oxidation of 1-PE
to AcPh. In both reactions they revealed higher activity in com-
parison with the corresponding systems supported on the bare PK
or SiO2 and excellent recyclability without metal leaching. These
results, due to the combined properties of the hybrid support, were
ascertained in very different polarity media. In the hydrogenation
reaction carried out in decalin, the beneficial effect of SiO2 on the
increase of the surface area combines with the capacity of PK to
stabilize the palladium nanoparticles, preventing not only their
growth during the catalytic reactions but also their leaching into
solution. On the other side, in polar media, such as water, the sta-
bilizing effect of PK is combined with the presence of hydrophilic
SiO2, which induces a better compatibility among the catalyst sur-
face, the reaction medium and the substrate. The oxidation of
1-PE to AcPh evidenced total selectivity to ketone also at complete
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