Liquid phase selective hydrogenation of cinnamaldehyde over copper supported catalysts

Article abstract of DOI:10.1016/j.apcata.2011.11.028

The selective hydrogenation of cinnamaldehyde is carried out in a batch reactor, at 100 °C and 1 MPa of H₂ using isopropyl alcohol as the solvent, over a series of copper supported catalysts: Cu/Al₂O ₃, Cu/SiO₂ Cu/MCM-48, Cu/CeO₂ and Cu/α-Fe₂O₃. The selectivity of the samples is compared with that corresponding to Pti/SiO₂. Reduced Cu/Al ₂O₃ and Cu/SiO₂ showed lower selectivity to the cinnamyl alcohol (16-22%) than Pt/SiO₂ (35%), at 15% of conversion. Following a calcination at 300 °C both, activity and selectivity of copper catalysts were increased. The calcined surface would hydrogenate CO bond by hydrogen transfer from the solvent. TPR, XRD and FTIR of adsorbed CO showed that Cu (I) species are stabilized on the mesoporous structure of MCM-48. This particular feature renders Cu/MCM-48 a selective catalyst, reaching high selectivity values (51%, at 15% of conversion). Cu/CeO₂ and Cu/α-Fe₂O₃ showed higher selectivity than Pt based catalyst due to a promotion of the catalytic properties of copper by reduced support species.

Full text of DOI:10.1016/j.apcata.2011.11.028

Applied Catalysis A: General 413–414 (2012) 358–365
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Liquid phase selective hydrogenation of cinnamaldehyde over copper supported
catalysts
Victoria Gutierreza, Mariana Alvarezb, María A. Volpea,∗
a Planta Piloto de Ingeniería Quimica, PLAPIQUI (UNS-CONICET) Camino La Carrindanga km 7, CC 717, 8000 Bahía Blanca, Argentina
b Departamento de Química, Instituto de Química del Sur (INQUISUR-CONICET), Universidad Nacional del Sur, Avenida Alem 1253, 8000 Bahía Blanca, Argentina
a r t i c l e i n f o
a b s t r a c t
The selective hydrogenation of cinnamaldehyde is carried out in a batch reactor, at 100 ◦C and 1 MPa of
H2 using isopropyl alcohol as the solvent, over a series of copper supported catalysts: Cu/Al2O3, Cu/SiO2
Cu/MCM-48, Cu/CeO2 and Cu/␣-Fe2O3. The selectivity of the samples is compared with that corresponding
to Pti/SiO2. Reduced Cu/Al2O3 and Cu/SiO2 showed lower selectivity to the cinnamyl alcohol (16–22%)
than Pt/SiO2 (35%), at 15% of conversion. Following a calcination at 300 ◦C both, activity and selectivity
of copper catalysts were increased. The calcined surface would hydrogenate C O bond by hydrogen
transfer from the solvent. TPR, XRD and FTIR of adsorbed CO showed that Cu (I) species are stabilized on
the mesoporous structure of MCM-48. This particular feature renders Cu/MCM-48 a selective catalyst,
reaching high selectivity values (51%, at 15% of conversion). Cu/CeO2 and Cu/␣-Fe2O3 showed higher
selectivity than Pt based catalyst due to a promotion of the catalytic properties of copper by reduced
support species.
Article history:
Received 17 July 2011
Received in revised form
19 November 2011
Accepted 25 November 2011
Available online 4 December 2011
Keywords:
Supported copper
Selective hydrogenation
Conjugated carbonyl
Ceria
Copper oxide
␣,␤-Unsaturated aldehydes
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
producing predominantly hydro-cinnamaldehyde during the liquid
phase hydrogenation of cinnamaldehyde on Cu-based catalysts. In
The selective hydrogenation of ␣,␤-unsaturated compounds is a
reaction of paramount importance in the field of organic synthesis.
The most important product is the unsaturated alcohol which is
employed in the synthesis of cosmetic, pharmaceutical and fine
chemicals. Metal hydrides have been employed to carry out this
reaction for industrial purpose [1]. However due to environmental,
economical and technical considerations, heterogeneous catalysts
should be preferred. Traditionally, supported noble metal catalysts
have been employed due to their relative high selectivity [1–6].
Supported copper catalysts are an interesting option to per-
form selective hydrogenation reactions due to the low cost of the
metal combined with advantageous catalytic properties of copper
for selective hydrogenation reactions [7–12]. From kinetic studies
performed over Pt, Cu and Pd catalysts supported on silica, Pham
et al. clearly observed that the hydrogenation of C O bond is faster
than that of C C bond over Cu/SiO2 catalysts than over Pd/SiO2 or
Pt/SiO2 for the hydrogenation of 2-methyl-2-pentenal, though this
effect is masked for high conversion level [11]. On the other hand,
Marchi et al. [13] concluded that Cu/SiO2 and binary Cu-Al cata-
lysts were unselective toward the hydrogenation of carbonyl bonds,
the same work the author observed that ternary Cu–Zn–Al and qua-
ternary Cu–Ni(Co)–Zn catalysts were more selective than Cu/SiO2.
Cationic species of the promoter (Zn, Ni, Co) were considered to be
the responsible for the creation of selective sites for the hydrogena-
tion of C O. In the same sense, the group of Hutchings concluded
that Cu/Al2O3 preferentially hydrogenates carbon–carbon double
bond, though the modification by sulfur compounds leads to a
selective catalyst. The creation of selective Cu◦-S and Cu+ sites is
the origin of the high selectivity [7,8].
The present study investigates a series of copper supported cata-
lysts for the selective hydrogenation of ␣,␤-unsaturated aldehydes.
Inert support (SiO2, ␥-Al2O3 and MCM-48) as well as supports
possessing redox properties (CeO2 and ␣-Fe2O3) was selected for
supporting copper. It is speculated that the very different nature of
the support would lead to different copper species, from morpho-
logical and from chemical aspects.
The most part of the chemical catalytic reactions that are par-
ticularly important in the preparation of pharmaceutical or fine
chemicals are performed in the liquid phase, under constant H2
pressure. For this reason the catalysts are tested for the hydro-
genation of cinnamaldehyde and crotonaldehyde, in a batch reactor
using H2 as the reductant and isoproanol as the solvent. The selec-
tivity to the unsaturated alcohol (the desired product) as well as the
activity of the different samples is measured. The catalytic results
are discussed in the light of the characterization results, obtained
∗
Corresponding author. Tel.: +54 2914861700; fax: +54 2914861600.
E-mail address: mvolpe@plapiqui.edu.ar (M.A. Volpe).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.11.028

V. Gutierrez et al. / Applied Catalysis A: General 413–414 (2012) 358–365
359
from TEM, XRD, BET, FTIR and TPR studies. Finally the catalytic prop-
erties of supported copper are compared with those corresponding
to platinum and gold catalysts tested under the same experimental
conditions in previous works.
X-ray diffraction (XRD) spectra were obtained ex situ using a
Philips PW1710 BASED diffractometer equipped with a Cu-K˛ radi-
ation source. Cu crystallite sizes were calculated from the linewidth
at half height of the primary CuO peak using the Scherrer equation
with Warren’s correction for instrumental line broadening.
2. Experimental
2.3. Catalytic testing
2.1. Materials
Liquid phase hydrogenation of cinnamaldehyde and crotonalde-
hyde was conducted in a batch reactor, at 373 K, and 1 MPa of
H2 using isopropyl alcohol (Sintorgan, 99+%). The reactive liquid
mixture was 0.1 M. Before testing 0.150 g of the copper based cata-
lyst were reduced under flowing H2 at 300 ◦C for 1 h. The catalysts
were handled without exposing to the air. Care was taken to avoid
trace of oxygen in the reactor. For some samples, another pre-
treatment was carried out: a calcination under chromatographic air
at the same temperature and for the same period of time as for the
reduction pretreatment. The stirring rate was varied in the range
300 rpm. The progress of the reaction was followed by sampling a
sufficient number of microsamples. The composition of the sam-
ples was analyzed by means of a Perkin Elmer gas chromatograph,
equipped with a 30 m DB-WAX capillary column and a flame ioniza-
tion detector. The conversion and selectivity toward the different
products were measured.
Silica (SiO2), Ceria (CeO2), and alumina (␥-Al2O3) all from Rhone
Poulenc were employed as-received for supporting for copper.
MCM-48 was prepared by conventional hydrothermal synthesis,
following the technique reported by Xu et al. [14]. For further
details, see Ref. [15].
Hematite (␣-Fe2O3) was prepared by hydrothermal synthesis
from a Fe(NO3)3 solution [16].
Copper was fixed to the different supports following a wet
impregnation with solutions of Cu(AcAc)2 (Aldrich, 99.99%) in THF.
The supports ceria, hematite, alumina and silica were calcined at
300 ◦C, while goethite was calcined at 120 ◦C. Then the oxides were
put in contact with the Cu(AcAc)2 solution for 48 h, at 60 ◦C, under
constant stirring. The solids were filtered and heated at 70 ◦C for
24 h. Finally, all samples, but goethite supported one, were calcined
at 400 ◦C for 4 h under air flow. For goethite the calcination was car-
ried out at 150 ◦C. In this way copper supported on alumina (CuA),
hematite (CuH), ceira (CuC) and silica (CuS) was obtained. For the
case of MCM-48 the Cu(AcAc)2 solution was put in contact with the
mesoporous solid before eliminating the structure directing agent.
Upon copper fixation, the mesoporous system was heated under
nitrogen flow, increasing the temperature from room temperature
up to 400 ◦C by 1◦/min. Nitrogen was switched to chromatographic
air, and the sample was calcined at 400 ◦C for 4 h. This procedure
was followed for preventing the destruction of the mesoporous
structure. This sample was named as CuM.
3. Results
3.1. Catalysts characterization
The catalysts, the metal loading and BET surface area are listed
in Table 1.
Considering that the target copper concentration was 5 wt.%, a
relative high metal loading was achieved for all the samples.
The BET results show that impregnating the supports with the
solution of Cu(AcAc)2 and decomposition of the supported copper
precursor did not result in a loss of surface area of the carriers.
The catalysts were examined by XRD to identify the phases
present. In all cases, CuO (tenorite phase) diffractions peaks corre-
sponding to 2ꢀ = 35.37, 38.58 and 43.14 can be observed, indicating
that the cupric oxide agglomerates are present.
An extra copper supported alumina sample was prepared by
impregnation of ␥-Al2O3 with aqueous solution of Cu(NO3)2.
For all the samples, the target copper loading was 5 wt.%, while
the actual metal concentration was measured by AAS.
2.2. Sample characterization
The tenorite (0 0 2) diffraction peak was selected for determin-
ing copper particle size from Scherrer equation. It was assumed that
no lattice distortions take place. The crystallite sizes, as measured
from XRD, are reported in Table 1 for all the catalysts. Since an over-
lapping of some of the diffraction of peaks of alumina with those
corresponding to CuO exists, certain uncertainty in the determina-
tion of crystal size of the CuA sample should be considered. TEM
analysis of this sample showed a low contrast between copper par-
ticles and the support, turning the measurement of particle size by
this technique unreliable. In order to further investigate on cop-
per supported on alumina by XRD analysis, and extra sample was
prepared from Cu(NO3)2 (the CuAN sample). The XRD pattern of
this catalyst showed that CuO is more crystalline and gives more
intense pattern, compared with CuA (see Fig. 1). In this way the
determination of copper particle size for CuAN was reliable, with
a value of 19 nm. From the comparison between CuA and CuAN it
could be concluded that copper crystals are, at least, smaller than
19 m for the former sample.
Adsorption/desorption isotherms of the supports and the corre-
sponding catalysts were measured in a Nova 1200e Quantachrom
equipment. From these measurements, BET surface area was
obtained.
Some catalysts were analyzed by FTIR of adsorbed CO with a
Nicolet 20 DXB FTIR spectrometer at 4 cm−1 resolution. Catalyst
samples of approximately 30–40 mg were pressed to form trans-
parent disks of 13 mm in diameter that were mounted in a metal
holder. The holder was placed in the beam path of a stainless-steel
cell, sealed with CaF2 windows, and coupled to a vacuum system
for evacuation to 10−6 Torr. It was possible to perform heat treat-
ments up to 300 ◦C in air and H2 and to dose small amounts of
CO. CO adsorption spectra were obtained 77 K under a gas phase
pressure of 5 Torr.
The reducibility of the catalysts was studied by Temperature
Programmed Reduction (TPR) using a home made apparatus. The
temperature of the sample was increased linearly at 10 K min−1 in
5% H2/Ar at a flow rate of 20 cm3 min−1, from ambient to 773 K. A
thermal conductivity detector was used to measure variation in H2
concentration in the gas stream. The apparatus was calibrated by
performing the reduction of known amounts of CuO.
For copper supported on silica and on hematite (CuS and CuH
respectively) the XRD analysis of crystal size (reported in Table 1)
indicated that the copper particle size values are 18.5 for the sil-
ica supported sample and 21.3 nm for the hematite supported one.
These results are in quite good agreement with the size obtained
from TEM which indicated that the mean particle size is 20 and
23 nm for CuS and CuH, respectively.
Copper particle size was determined in some cases by TEM mea-
surements, on a Jeol 100 CX2 (Tokyo, Japan) apparatus.

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V. Gutierrez et al. / Applied Catalysis A: General 413–414 (2012) 358–365
Table 1
Physicochemical properties of Cu/support catalysts.
Sample
Support
Cu (wt.%)
BET area (m2/g)
174
174
210
942
240
26
Particle size (nm)b
Copper dispersion (%)
16
5
5
5
10
5
Tmax
H2/Cu
CuA
␥-Al2O3
␥-Al2O3
SiO2
MCM-48
CeO2
3.0
5.2
8.1
4.1
3.8
4.4
6.2
19.3
18.5
20.1
9.5
230
–
240
220
180
175
0.98
–
1.1
0.6
1.8
2.2
CuANa
CuS
CuM
CuC
CuH
␣-Fe2O3
21.3
a
Sample prepared from Cu(NO3)2.
Calculated from Scherrer approximation from XRD diffraction peaks.
b
The crystal size of CuO corresponding to CuM was 20 nm, as
measured by XRD. Few particles were observed by TEM for CuM.
The number of particles is too low to obtain a histogram and the
corresponding mean particle size. Due to the high specific surface
area of the mesoporous oxide, the surface concentration of copper
particles is low, the examination of this sample by TEM being diffi-
cult. On the contrary, the mesoporous channels network is clearly
observed in TEM images (see Fig. 2).
It is interesting to note that diffraction peaks due to Cu2O were
not observed in any sample, but for CuM. In Fig. 3 the XRD pat-
tern of this sample is shown. Both CuO and Cu2O are detected.
Another interesting feature found for CuM is that the diffraction
peaks corresponding to the (2 1 1) plane of MCM-48 are kept upon
copper introduction, showing that the mesoporous cubic order is
not destroyed in CuM. However, the fine structure of the minor
peaks of the (4 2 0), (3 3 2), etc. planes (shown in the inset of Fig. 3),
disappears due to copper being incorporated in the network. Fol-
lowing XRD analysis of CuM, it can be concluded that three copper
phases exists: crystalline oxidic species of Cu(I) and Cu(II) as well as
highly dispersed copper incorporated in the mesoporous structure.
CuA, CuS and CuM samples were analyzed by FTIR of adsorbed
CO. The characterization by means of FTIR employing CO as the
probe molecule was previously reported for studying the differ-
ent oxidation state of supported copper [17,18]. The catalysts were
previously submitted to a reduction pre-treatment or to calcination
pre-treatment. The infrared spectrum of CO adsorption on follow-
ing an in situ reduction at 300 ◦C is shown in Fig. 4(a). The CuA
Fig. 2. TEM image corresponding to CuM (Cu/MCM-48).
catalyst presents an intense band at 2106 cm−1 due to CO adsorbed
on Cu0. The spectrum corresponding to the same sample, but after
an in situ oxidizing pre-treatment at 300 ◦C (see Fig. 4(b)), shows a
low intensity band at 2149 cm−1 that could be assigned to CO on
Fig. 1. XRD pattern for: (a) CuA (Cu/Al2O3 prepared from Cu(AcAc)2), (b) CuAN
(Cu/Al2O3 prepared from Cu(NO3)2), and (c) CuS (Cu/SiO2 prepared from Cu(AcAc)2)
samples.
Fig. 3. XRD patterns: (a) CuM (Cu/MCM-48) and (b) MCM-48.

V. Gutierrez et al. / Applied Catalysis A: General 413–414 (2012) 358–365
361
Fig. 4. FTIR spectra of CO adsorbed at 77 K on CuA (Cu/Al2O3). Equilibrium CO pres-
sure of 5 Torr. (a) Sample reduced in situ at 300 ◦C, and (b) sample calcined in situ at
300 ◦C.
Cu(II). It is generally accepted that Cu(II) species does not form sta-
ble carbonyls species, even at low temperatures. The low intensity
of this band is due to the fact that the interaction between Cu2+
and CO is of electrostatic nature [19,20]. The FTIR results of CuS
are similar to that observed for CuA: the reduced sample shows a
band assigned to metallic copper, while the pre-calcined showed
only a minor band. It is important to note that for both cases, the
CO on cupric species easily disappears after evacuating the anal-
ysis chamber. On the other hand, for CuM the spectra of calcined
or reduced samples (see Fig. 5(a and b)) are both different from
those of CuS and CuA. Two bands are observed for the sample
reduced in situ: one at 2124 cm−1 due to CO on metallic copper
and another at 2156 cm−1 assigned to CO on the silanol groups
of the mesoporous support. Following an evacuation of the cham-
ber up to 10−6 Torr, both bands disappear, showing the lability of
these species. The spectrum corresponding to pre-calcined CuM
shows an intense band at 2132 cm−1, besides the signal due to the
silanol groups. This band is assigned to CO on Cu1+ [18–20]. Fol-
lowing an evacuation of the chamber, the signal remains, showing
that the CO–Cu1+ interaction is particularly strong. The low charge
and the high d density of cuprous species is the reason for the high
interaction between the probe molecule and Cu1+ [18].
Fig. 6. TRP profiles for Cu/support catalysts. (a) CuA, (b) CuM, (c) CuS, (d) CuH, (e)
CuC.
The TPR profiles of the copper supported catalysts, as well as
the corresponding to some of the supports are shown in Fig. 6.
The temperature at each peak maximum and the specific hydro-
gen consumption (expressed as the ratio between the moles of H2
consumed and the moles of copper) for each catalyst are given in
Table 1.
The TPR profiles for CuA and CuS consist of a single reduction
peak at 230–240 ◦C corresponding to the complete reduction of CuO
to metallic copper. CuM is also reduced in one step, at 220 ◦C, but
the reduction processes is incomplete, and the measured hydro-
gen consumption is lower than the one corresponding to the total
reduction to CuO. This diminished reducibility of copper on MCM-
48 was previously observed for a series of Cu/MCM-48 samples [15].
In line with XRD pattern, showing the presence of Cu2O, the lower
uptake of hydrogen could be attributed to a stabilization of Cu(I)
species in the mesoporous oxide.
The hydrogen consumption of copper supported on the
reducible oxides, ceria and hematite, was relatively high. The
enhanced consumption would be due to support species (Ce4+
,
Fe3+) engaged in the reduction process at temperatures lower than
the ones observed for the bare supports. The higher reducibility
of support species observed over metal supported catalysts was
previously observed for Pt/CeO2 [21], Au/CeO2 [22], Au/FexOy [23].
Besides, for the supports with redox properties, the reduction
of copper species is shifted to lower temperatures, as compared to
species supported on inert oxide (see Tmax in Table 2). Summing up,
from TPR results it could be concluded that ceria and iron oxides
supports notably enhances the reducibility of copper species.
3.2. Catalytic test
Catalyst weights in the 0.1–0.6 g were used to check that the
reaction was not diffusion limited under the experimental con-
ditions used. The conversion measured after 3 h of reaction time
was found to increase linearly with the amount of catalyst used,
indicating the absence of external diffusion control under the
experimental conditions used in the present work. Weisz–Prater
Fig. 5. FTIR spectra of CO adsorbed at 77 K on CuM (Cu/MCM-48). Equilibrium CO
pressure of 5 Torr. (a) Sample reduced in situ at 300 ◦C, and (b) sample calcined in situ
at 300 ◦C.

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V. Gutierrez et al. / Applied Catalysis A: General 413–414 (2012) 358–365
Table 2
ferent copper catalysts indicates that hematite, alumina and silica
supported ones show similar values. On the other hand, Cu/MCM-
48 was less active, while Cu/CeO2 shows a relatively high TOF. For
the latter sample a promotion of the support species on copper
activity could be postulated.
Table 2 also lists the percentage of selectivity to cinnamyl alco-
hol at 15 and 30% of conversion levels. For CuA, it was found that
the selectivity to the desired product is low (25–15% range) for all
the reaction time period, as could be observed in Fig. 7. For CuS the
trend was similar, though the selectivity to cinnamyl alcohol was
slightly higher (see Table 2). This difference could be assigned to
the higher copper particle size found for silica support (18.5 nm)
than for alumina (6.2 nm).
In Fig. 11 the catalytic pattern corresponding to Pt/SiO2
is shown. The reaction yields other products than the pri-
mary hydrogenation ones, as styrene, condensation products,
etc., thought that their concentration is relatively low. For
the sake of simplicity in Fig. 11, only the products of
the hydrogenation of cinnamaldehyde are shown, and the
selectivities were calculated on the basis of only these prod-
ucts.
Catalytic properties of Cu/support catalysts for the liquid phase hydrogenation of
cinnamaldehyde.
Sample
CuA
CuAcal
CuS
Activity (␮mol s−1 gCu−1/Pt−1
)
TOF (s−1) × 103
S%b
S%c
6.3
12.7
2.7
4.8
1.1
16.5
3.2
51.6
2.5
5.0
2.7
4.5
1.3
7.4
4.0
22.3
16
73
22
75
51
45
38
35
15
51
15
48
46
35
36
37
a
a
CuScal
CuM
CuC
CuH
PtSd
a
Samples previously calcined at 300 ◦C.
Selectivity toward cinnamyl alcohol measured at 15% of conversion.
Selectivity toward cinnamyl alcohol measured at 30% of conversion.
Pt (2%)/SiO2, 45% of dispersion.
b
c
d
criterion was considered in order to evaluate the absence of pore
diffusion limitations. A value of WP less than 0.15 can be considered
a sufficient condition for the absence of significant pore diffusion
limitation [24]. The WP number corresponding to the catalysts sup-
ported on MCM-48, Al2O3 and SiO2 were lower than 0.15, both
for H2 and for cinnamaldehyde. These values verify the absence of
transport limitation for both the gas and the aldehyde. Though the
WP number corresponding to the transport of H2 in CeO2 indicated
that diffusion limitation can be neglected, the cinnamaldehyde dif-
fusion within the ceria pore would be restricted.
The product of the decarbonylation of cinnamaldehyde
(styrene) was not observed for the reaction performed over the cop-
per supported catalysts, but for CuC. This is an important difference
between some of the copper catalysts and gold or platinum ones:
the cinnamaldehyde hydrogenation over Pt/Al2O3 or Au/Al2O3 cat-
alysts (performed under the same reaction conditions as the one of
the present work) produced styrene [23].
The percentage of selectivity to the three products of hydrogena-
tion (cinnamyl alcohol, hydrocinnamaldehyde, phenyl propanol)
as a function of reaction time is plotted for some of the cop-
per catalysts and for Pt/SiO2 previously reduced at 300 ◦C in
Figs. 7–11.
For all the cases the aldehyde concentration decreased linearly
with reaction time, indicating that the reaction is of zero order
overall. The gradients of conversion against time were used to
determine rate constants, k, for each of the catalysts. These val-
ues are listed in Table 2 as ␮mol of cinnamaldehyde converted per
second and per g of copper. The copper dispersion (see Table 1)
was estimated considering both, the surface density of copper
(1.35 × 1019 Cu/m2 [25]) and the particle size as determined from
XRD, following the relation described by Scholten et al. [26].
The Pt/SiO2 catalyst shows a much higher TOF than the copper
based catalyst due to the high intrinsic activity of Pt against Cu for
hydrogenation reactions. The comparison of TOF number of the dif-
It could be observed that the noble metal based sample showed a
higher selectivity to cinnamyl alcohol than the ones corresponding
to copper catalysts supported on SiO2 or Al2O3.
MCM-48 should be considered as an inert support, like silica
or alumina. However the catalytic pattern of the CuM catalyst
(see Fig. 8) is completely different from those of the CuS or CuA
samples. The selectivity toward cinnamyl alcohol measured over
CuM is much higher than the ones corresponding to the lat-
ter samples, at the same level of conversion. For low conversion
level, at initial reaction time, the catalyst is highly selective, but
it turns rather unselective with time, due to a higher production
of hydrocinnamaldehyde than that of cinnamyl alcohol. Anyway
CuM is more selective than CuS or CuA during the entire reac-
tion time. The existence of Cu1+, stabilized on MCM-48 would be
related with the high selectivity of copper species on the meso-
porous support. Previous works proposed that the fraction of
cuprous ions plays an important role in the catalytic activity of
copper catalysts employed for selective hydrogenation reactions
[8,9,27].
The variation of the selectivity toward cinnamyl alcohol with
time, for CuM, would be related with a decrease in the concentra-
tion of Cu1+ species. The characterization of the spent CuM catalyst
by XRD showed that the intensity of the diffraction peaks due to
Cu2O is lower than the one corresponding to peaks of the fresh
sample.
Considering that copper species in the +1 oxidation state would
notably influence on the catalytic properties of copper based cat-
alysts, it was decided to perform a calcination treatment to the
samples previously to the catalytic test. It could be speculated
that such a treatment would modify the relative concentration of
Cu0, Cu1+ and Cu2+. The dependence of the conversion and the
selectivity to the different products for CuA following calcination
pre-treatment is plotted in Fig. 7. When a comparison of the cat-
alytic patter of CuA submitted to a calcination treatment at 300 ◦C
with that of the same catalyst following a reduction is performed,
remarkable differences arise. Firstly, the conversion attained by the
previously calcined sample is higher than the one obtained over the
reduced one. Concomitantly, the TOF number of CuA previously
calcined is higher than the TOF of the sample previously reduced.
Besides the selectivity to cinnamyl alcohol notably increases fol-
lowing the calcination pre-treatment. Thus a much higher yield of
the desired product is obtained for the pre-calcined CuA sample
than for the reduced CuA catalyst. The same trends were observed
for CuS previously calcined: a notable improvement of the selectiv-
ity and an increase in the activity (see Table 2). It is interesting to
100
80
60
40
20
0
0
50
100
150
200
250
Time (min)
Fig. 7. Dependence of the conversion of cinnamaldehyde, (ꢀ) and of the selectivities
to cinnamyl alcohol (ꢁ), to hydrocinnamaldehyde (᭹), and to phenyl propanol (ꢂ)
on time for CuA (Cu/Al2O3). Samples previously reduced (—) and previously calcined
(- - -).

V. Gutierrez et al. / Applied Catalysis A: General 413–414 (2012) 358–365
363
100
80
60
40
20
0
0
50
100
150
200
250
Time (min)
Fig. 8. Dependence of the conversion of cinnamaldehyde (ꢀ) and of the selectivities to cinnamyl alcohol (ꢁ), to hydrocinnamaldehyde (᭹), and to phenyl propanol (ꢂ) on
time for CuM (Cu/MCM-48).
60
On the other hand the calcination performed over CuM does
not modify both the activity and the selectivity of this sample. This
result is rather difficult to explain. Probably the calcination pre-
treatment carried out over CuM does not deeply change the relative
40
concentration of the different copper species.
Turning the attention to the catalysts in which copper is sup-
ported on reducible oxide, i.e. the CuH and CuC samples, the
20
selectivity is relatively high in these cases. For CuH the selectivity
to cinnamyl alcohol is nearly constant with conversion (see Fig. 9).
0
This catalyst is more selective than the catalysts supported on inert
oxides, CuA and CuS. The existence of Fe2+/Fe3+ species would pro-
mote the selectivity of copper for the selective hydrogenation of
0
50
100
150
200
250
300
350
Time(min)
Fig. 9. Dependence of the conversion of cinnamaldehyde (ꢀ) and of the selectivities
to cinnamyl alcohol (ꢁ), to hydrocinnamaldehyde (᭹), and to phenyl propanol (ꢂ)
on time for CuH (Cu/␣Fe2O3).
C
O against C C.
On CuC (see Fig. 10) a high selectivity level is measured at low
conversion, which decreases with time mainly due to an increase
in the production of hydrocinnamaldehyde. The origin of the high
initial selectivity would be related with support sites. In an addi-
tional test, the hydrogenation of cinnamaldehyde was carried out
over the bare support. A low conversion (5%) was attained at ini-
tial reaction time, but the activity was completely lost after a short
period of reaction time. The selectivity to the cinnamyl alcohol was
extremely high (90%), showing that ceria posses sites for the selec-
tive hydrogenation of C O, though they become deactivated. Thus,
the variation of the selectivity toward cinnamyl alcohol with time
observed for CuC could be explained: highly selective ceria sites
become deactivated.
The comparison of the activities achieved by the different
Cu/support catalysts showed that CuC is the most active sample.
As commented above the high activity would be related with sites
other than copper ones, probably in the metal/support interphase.
For the samples supported on reducible oxides, CuH and CuC, the
calcination pretreatment also originates an improvement of both
selectivity and activity, though in a much lesser extent than for
note that over the calcined samples, CuS or CuA, no phenyl propanol
was detected.
The higher activity of the calcined CuA and CuS would be
related with a higher availability of hydrogen species. To investigate
on the origin of such species, the hydrogenation of cinnamalde-
hyde was carried out in the absence of H2, under N2 pressure.
The other experimental parameters (mass of the catalysts, stirring
speed, temperature, etc.) were kept constant. Under the inert gas
atmosphere, the catalyst remained active, though the conversion
attained was lower than when H2 is employed. Considering this
result it is postulated that the solvent isopropanol is responsible for
the hydrogenation of cinnamaldehyde. Isopropanol would transfer
hydrogen to C O group in the presence of calcined copper cata-
lyst. Finally it is important to note that neither in the absence of
isopropanol (using THF as the solvent) nor in the absence of the
catalyst, hydrogenation reactions were observed.
80
60
40
20
0
60
50
40
30
20
10
0
0
50
100
150
200
250
0
50
100
150
200
250
Time(min)
Time (min)
Fig. 10. Dependence of the conversion of cinnamaldehyde (ꢀ) and of the selectivities
to cinnamyl alcohol (ꢁ), to hydrocinnamaldehyde (᭹), and to phenyl propanol (ꢂ)
on time for CuC (Cu/CeO2).
Fig. 11. Dependence of the conversion of cinnamaldehyde (ꢀ) and of the selectivities
to cinnamyl alcohol (ꢁ), to hydrocinnamaldehyde (᭹), and to hydrocinnamyl alcohol
(ꢂ) on time for Pt/SiO2.

364
V. Gutierrez et al. / Applied Catalysis A: General 413–414 (2012) 358–365
CuA or CuS. As for the case of CuM, it could be supposed that the
calcination cannot significantly alter the relative concentration of
the different copper species.
after a short period of reaction time. On the contrary for CuC the
deactivation of support sites is not complete, some of them (prob-
ably located at the Cu/ceria interphase) remain active giving rise
to a higher selectivity to cinnamyl alcohol than when copper is
supported on inert oxides.
4. Discussion
The most important feature of copper catalysts supported on
alumina or silica, following a calcination pre-treatment, is that they
show higher selectivity as well as higher activity than the corre-
sponding pre-reduced samples. This a surprising result, considering
that copper oxides surface do not dissociate dihydrogen and that it
was previously reported that the activity of copper supported on sil-
ica was completely lost, if the samples were calcined at 623 K [28].
In order to explain the activity of calcined copper catalyst it is postu-
lated that the hydrogenation of cinnamaldehyde is carried out by a
catalytic hydrogen transfer from the solvent, isopropanol. The sam-
ples previously calcined are active for hydrogenating the aldehyde
under inert gas atmosphere (N2), though in a lesser extent than
under H2 atmosphere. This result confirms that catalytic hydrogen
transfer from the solvent takes place over calcined copper surface
for CuA and CuS.
A decrease of the selectivity was observed for previously cal-
cined samples: the selectivity was higher than 80% at early reaction
times and decreased to approximately 50%. A morphological mod-
ification of the catalysts under reaction conditions cannot be
invoked to explain the loss of selectivity, since XRD performed over
the spent catalysts showed that copper particle size was not modi-
fied. It is likely that the oxidation state of copper species varies due
to the hydrogenating conditions and this modification leads to a
decrease in the hydrogen transfer.
At this point it is interesting to note that the production of the
total hydrogenation product, phenyl propanol was nil for calcined
samples. It could be postulated that copper surface after the calcina-
tion pre-treatment is inert for the adsorption of the desired product,
cinnamyl alcohol as well as for the adsorption of hydrocinnamalde-
hyde, preventing the formation of the total hydrogenation product,
the hydrocinnamaldehyde alcohol. In this aspect the improvement
of the selectivity after the calcination pre-treatment is originated
in a different way to that corresponding to the CuM, CuC or CuH
catalysts.
Reduced CuS and CuA catalysts showed a low selectivity for the
hydrogenation of C O against C C bond, in the hydrogenation of
cinnamaldehyde, showing that supported metallic copper is intrin-
sically unselective toward the hydrogenation of ␣,␤-unsaturated
aldehydes.
However, copper could be modified in order to increase its
selectivity. Supporting copper on MCM-48 results in a selective
catalyst for the hydrogenation of cinnamaldehyde. For CuM three
different species coexist: highly dispersed copper in the meso-
porous framework, Cu2O particles and CuO particles. It could be
suggested that the selective sites in Cu/MCM-48 are highly dis-
persed copper and/or to Cu2O. Both sites are related with Cu(I)
species, in which beneficial effect on selectivity has been previously
established [28,29]. Still another interpretation should not be disre-
garded: highly dispersed copper presents an increase in the number
of d electrons due to the loss of bulky properties should be expected.
This effect would favor C O hydrogenation. This argument was
invoked previously for explaining the high selectivity achieved by
gold supported catalysts for the hydrogenation of ␣,␤-unsaturated
compounds [30]. Whatever the nature of the modification of cop-
per which turns it selective for C O bond hydrogenation, it arises
upon supporting copper on MCM-48. Another interesting feature
of copper on MCM-48 (CuM) is its low specific activity (moles of
cinnamaldehyde converted per second and per g of copper) by
comparison with copper supported on silica or alumina. Since the
activity of the samples is due to adsorbed hydrogen species, it is
likely that certain fraction of copper is inactive for activating H2.
Considering the scenario of the different copper species on MCM-
48, H2 is not dissociated on it.
On the contrary, CuO particles would be related with unselec-
tive sites. CuO is the only copper phase in for silica or alumina (as
concluded from FTIR, TPR and XRD analysis), for this reason Cu/SiO2
and Cu/Al2O3 are unselective catalysts.
When comparisons of the present results are carried out with
the literature ones (mainly with those corresponding to noble met-
als based catalysts), it can be considered that for some of our
copper samples the selectivity is relatively high. Gold based sam-
ples, tested under similar experimental conditions as the ones
of the present work, presented selectivities to cinnamyl alco-
hol in the 26–88% range, for conversion levels lower than 20%
[23]. Bus et al. tested a series of gold supported on alumina
catalysts for the high-pressure liquid phase hydrogenation of
cinnamaldehyde [33]. The authors reported selectivities in the
38–89% range. Plomp et al. studied the selective hydrogenation
of cinnamaldehyde over carbon nanofibers supported platinum
and ruthenium catalysts, and reported selectivities approximately
of 60% and of 45% for Pt and Ru respectively [34]. Merlo and
coworkers prepared tin-modified platinum catalysts by means
of controlled surface reaction. For a certain a Pt/Sn ratio a high
selectivity level (80%) was achieved in the hydrogenation of cin-
namaldehyde. The general trend is that for reaching selectivities
higher than 80%, sophisticated preparation method of the cat-
alysts [23,35], or rigorous reaction conditions were employed
[33].
The catalysts based on supports possessing redox properties,
hematite and ceria (CuH and CuC) should also be considered as
selective ones. It is likely that the promotional effect is due to
support species (Ce3+/Ce+ and Fe2+/Fe3+). Metal-support interfacial
sites would be responsible for the increase of selectivity toward
C
O hydrogenation, as observed previously for noble metal based
catalysts [5,6,31,32]. It is important to note that the mean size
of copper crystals supported on ceria or hematite is in the same
range as in the case of copper supported on inert oxides (CuS
and CuA). Thus, a geometric effect for explaining the relative high
selectivity of copper when supported on reducible oxides should
be disregarded. The reduction of the supports, and concomitantly
the creation of reactive sites in the metal-support interface are
easily achieved for ceria and hematite due to the presence of cop-
per crystals. TPR characterization clearly demonstrates that the
reducibility of the supports is increased by the presence of the
metal.
For CuC a very high selectivity to cinnamyl alcohol was mea-
sured for the first reaction times (60% following 20 min of reaction
time). Undoubtedly, ceria sites are responsible for the high selec-
tivity to hydrogenate C O, since bare ceria showed initial activity
for hydrogenating cinnamaldehyde to cinnamyl alcohol. Although
the yield to cinnamyl alcohol over bare ceria was rather low, the
selectivity was nearly 90%. However, for both, the bare support and
the CuC catalyst, the support sites became deactivated. The deac-
tivation is complete for bare ceria, for which the activity is null
Some of our copper samples (CuM, CuC, previously calcined CuA
and CuS) showed selectivities in the 50–75% range. Thus it can be
concluded that the selectivities attained over the present copper
catalysts are in some cases higher than the values corresponding
to noble metal based samples. This is an important result, mainly
due to the low cost of copper. Even more, the relatively simple

V. Gutierrez et al. / Applied Catalysis A: General 413–414 (2012) 358–365
365
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Copper is intrinsically unselective toward the hydrogenation
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C
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Besides, supporting copper on reducible oxides also modifies
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Selective hydrogenation could also be achieved in liquid phase
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