Catalytic performance in selective hydrogenation of citral of bimetallic Pt-Sn catalysts supported on MgAl2O4 and γ-Al 2O3

Article abstract of DOI:10.1016/j.apcata.2010.11.017

The liquid phase citral hydrogenation, using Pt and PtSn catalysts supported on MgAl₂O₄ and γ-Al₂O ₃, was studied in a stirred reactor at 70 °C and atmospheric pressure. It was found that the addition of Sn to the Pt catalysts increases the selectivity to double unsaturated alcohols for both catalyst series. Besides, monometallic catalysts hydrogenate the α,β-unsaturation with a high selectivity in absence of cyclization secondary products. The performance of these catalysts in the citral hydrogenation was related with the characteristics of the metallic phase. Results from test reactions - cyclohexane dehydrogenation (CHD) and cyclopentane hydrogenolysis (CPH) - H₂ chemisorption, 2-propanol dehydration, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS), were used to explain the influence of the support and the Sn loading and postulate the models of the catalytic metallic surface. Results indicated that, a fraction of ionic Sn would be deposited near Pt, thus increasing the polarization of the carbonyl group, and a fraction of metallic Sn could form Pt-Sn alloy phases that would hinder the hydrogenation of the olefinic bonds and would be active to the hydrogenation of the carbonyl group. Both effects contribute to a higher selectivity to unsaturated alcohols in bimetallic PtSn/Al₂O₃ catalysts than PtSn/MgAl₂O₄ ones, which display a low alloy formation.

Full text of DOI:10.1016/j.apcata.2010.11.017

Applied Catalysis A: General 392 (2011) 208–217
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic performance in selective hydrogenation of citral of bimetallic Pt–Sn
catalysts supported on MgAl O and ␥-Al O
2
4
2
3
∗
Patricia D. Zgolicz , Virginia I. Rodríguez, Irene M.J. Vilella, Sergio R. de Miguel, Osvaldo A. Scelza
Instituto de Investigaciones en Catálisis y Petroquímica (INCAPE), Facultad de Ingeniería Química, Universidad Nacional del Litoral – CONICET,
Santiago del Estero 2654, 3000 Santa Fe, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2010
Received in revised form 3 November 2010
Accepted 5 November 2010
Available online 16 November 2010
The liquid phase citral hydrogenation, using Pt and PtSn catalysts supported on MgAl O and ␥-Al O ,
2
4
2
3
◦
was studied in a stirred reactor at 70 C and atmospheric pressure. It was found that the addition of
Sn to the Pt catalysts increases the selectivity to double unsaturated alcohols for both catalyst series.
Besides, monometallic catalysts hydrogenate the ␣,␤-unsaturation with a high selectivity in absence of
cyclization secondary products. The performance of these catalysts in the citral hydrogenation was related
with the characteristics of the metallic phase. Results from test reactions – cyclohexane dehydrogenation
Keywords:
(CHD) and cyclopentane hydrogenolysis (CPH) – H2 chemisorption, 2-propanol dehydration, temperature
Citral selective hydrogenation
Supported PtSn catalysts
MgAl2O4
programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS), were used to explain the
influence of the support and the Sn loading and postulate the models of the catalytic metallic surface.
Results indicated that, a fraction of ionic Sn would be deposited near Pt, thus increasing the polarization
of the carbonyl group, and a fraction of metallic Sn could form Pt–Sn alloy phases that would hinder the
hydrogenation of the olefinic bonds and would be active to the hydrogenation of the carbonyl group.
Both effects contribute to a higher selectivity to unsaturated alcohols in bimetallic PtSn/Al2O3 catalysts
than PtSn/MgAl2O4 ones, which display a low alloy formation.
␥
-Al2O3
Characterization of the metallic phase
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
be responsible for the increment of the selectivity to unsaturated
alcohols. In this sense, the behaviour of the PtSn systems has been
explained as caused by the promotion effect of oxidized species of
Sn (placed near to Pt atoms) that promote the carbonyl hydrogena-
tion [17–22]. In this case the interaction of the oxidized Sn species
with the oxygen of the carbonyl group could weaken the C O bond
and hence enhance the carbonyl hydrogenation rate. However, the
presence of metallic Sn was found by XPS, XRD and TEM in several
studies on selective hydrogenation of unsaturated ␣,␤ aldehydes
[17,20,23–30] and it was considered as an important parameter
of the catalytic activity in PtSn catalysts. The metallic Sn could be
forming aggregates which produce a geometric effect on the metal-
lic phase [21,25,29]. Besides, this metallic Sn can be alloyed with
Pt, thus avoiding the hydrogenation of the olefinic double bond or
showing activity to the hydrogenation of the carbonyl group, and
in this way it would increase the selectivity to unsaturated alcohols
[17,23,26,28,30–32].
The production of ␣,␤-unsaturated alcohols is a very interesting
process since it leads to the obtention of very important intermedi-
ariesoforganic synthesisused in the manufactureofallylethers and
allyl esters, and final products for Fine Chemistry used in perfumery
and pharmacy [1–4]. The selective hydrogenation of unsaturated
␣
,␤ aldehydes to produce the corresponding unsaturated alcohols
is a difficult reaction due to the double olefinic bonds are prefer-
ably hydrogenated, both by thermodynamic and kinetic reasons.
The use of monometallic catalysts based on Pt, Pd, Ru or Ni sup-
ported on non reducible oxides [5–8] leads mainly to the formation
of saturated aldehydes. In consequence a modification of the metal-
lic phase is necessary to obtain the desirable products. In order to
increase the selectivity of monometallic catalysts in the hydrogena-
tion of carbonyl group, several studies [5,9–14] have been carried
out to study the effect of the support, promoter addition, metallic
particle size and solvents used in the reaction. In particular, it was
found that the addition of Sn, Ge, Fe, and Ga to Pt catalysts [10,15,16]
can strongly modify the catalytic properties of the metal active by
the presence of geometric and/or electronic effects, which would
Even though there are several papers on the catalytic behaviour
of mono and bimetallic catalyts based on Pt supported on Al O3
2
[5,23,29,30,33–43] in selective hydrogenation of ␣,␤-unsaturated
aldehydes, there is practically no information on mono and
bimetallic catalysts based on Pt supported on spinels [44]. Besides,
it must be noted that none of these papers report the selective
hydrogenation of citral (3,7-dimethyl-2,6-octadienal) to unsatu-
rated alcohols.
∗ Corresponding author. Tel.: +54 342 4555279.
E-mail address: pzgolicz@fiq.unl.edu.ar (P.D. Zgolicz).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.11.017

P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
209
Fig. 1. Main reaction routes in citral hydrogenation.
The objective of this paper is to study the catalytic behaviour
in the citral hydrogenation of Pt and PtSn catalysts supported on
MgAl O and Al O . In this sense the effect of the Sn amount
nitrates with a Al/Mg molar ratio = 2 were used for the prepa-
ration of the precursor by slow addition of aqueous solution of
ammonia under stirring, until reaching pH = 11 at 40 C. After its for-
◦
2
4
2
3
added to a given Pt loading for both supports was investigated. The
bimetallic catalysts were prepared by successive impregnation of
the corresponding support. The catalysts were submitted to differ-
ent characterization techniques: test reactions of the metallic phase
mation, the precursor gel was aged for 1 h at room temperature and
then filtered. The gel was washed with an excess of distilled water
(volume ratio water/gel = 4) under stirring and filtered again. Addi-
tionally, the gel was washed in the same filter paper with distilled
◦
(
cyclohexane dehydrogenation and cyclopentane hydrogenolysis),
water. Then it was dried at 120 C overnight, and finally calcined
◦
H2 chemisorption, temperature programmed reduction (TPR) and
X-ray photoelectron spectroscopy (XPS), and the support acidity
was examined by 2-propanol dehydration. For this study the cit-
ral hydrogenation was selected, because citral is an abundant raw
material from the lemongrass oil. Besides, citral is a very interest-
ing molecule model since it has three different unsaturated bonds:
a carbonyl group, a double C C bond conjugated with the carbonyl
group and one isolated –C C– bond. It also exists as cis and trans
isomers and it can also be cyclized. Fig. 1 shows the main products
which can be obtained by hydrogenation and cyclization.
at 800 C for 4 h. Monometallic Pt catalysts (5 wt%) were prepared
◦
by impregnation (25 C for 6 h) of the corresponding support with
−
1
an aqueous solution of H PtCl6 (12.6 g L ), by using an impregna-
tion volume/mass of support ratio = 4 mL g . After impregnation
the samples were dried at 120 C overnight and then calcined at
500 C for 3 h. The bimetallic catalysts of Pt(5 wt%)Sn(1, 2, 3 or
2
−
1
◦
◦
4 wt%)/MgAl O4 and Pt(5 wt%)Sn(1, 2 or 3 wt%)/Al O3 were pre-
2
2
pared by successive impregnation of the monometallic ones. Thus,
the precursor of the monometallic catalyst was impregnated for
6 h at room temperature with a hydrochloric solution of SnCl2 by
using a Sn concentration such as to obtain 1, 2, 3 or 4 wt% Sn when
the support was MgAl O4 and 1, 2 or 3 wt% when the support was
2
2
. Experimental
Al O . It was used a volume of impregnation/mass of support ratio
2
3
−
1
of 1.4 mL g for catalysts with 1 and 2% of Sn. The impregnating
volume/support weight ratios were 2 and 3 mL g for 3 and 4 wt%
Sn loading, respectively. Finally the samples were dried at 120 C
overnight and then calcined at 500 C for 3 h.
2.1. Preparation of the catalysts
−
1
◦
Two supports were used: (i) MgAl O4 (obtained in the
2
◦
2
−1
laboratory, SBET = 108 m g ) with a spinel structure and equi-
librium pH = 8.6, and (ii) commercial ␥-Al O (CK 300 from
2
3
2
−1
◦
Cyanamid Ketjen, SBET = 180 m g ) previously calcined at 650 C.
The first support was prepared by using the co-precipitation
method [45] from solutions of Mg(NO ) ·6H O (Merck, purity
2.2. Hydrogenation of citral
The different catalysts were tested in the citral hydrogenation
3
2
2
◦
9
9%), Al(NO ) ·9H O (Merck, purity 98.5%) and ammonia solu-
at 70 C and atmospheric pressure in a discontinuous reactor with
3
3
2
tion (Merck, 28%, analytical grade). 0.5 M solutions of Al and Mg
a device for sampling the reaction products. In each experiments

2
10
P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
0.3 mL of citral (Aldrich, 61% cis and 36% trans) were hydrogenated
100
by using 0.300 g of catalyst and 30 mL of solvent (2-propanol). The
reaction mixture was stirred at 1400 rpm (in order to minimize the
limitations of the mass transfer steps). In fact, it is worth notic-
ing that from previous experiments, diffusional limitations were
found to be absent under these conditions. Prior to the reaction,
8
6
4
2
0
0
0
0
0
◦
catalysts were reduced “in-situ” under flowing H2 at 500 C for
3
h, and then they were cooled down to the reaction tempera-
ture. The reaction products were analyzed in a GC system coupled
to a capillary column (Supelcowax 10 M) with a FID detector. All
products were identified by using gas chromatography standards
except for 3,7-dimethyloctanal. This product was identified by gas
chromatography–mass spectrometry (GC–MS) technique. The cal-
culation of the activity and selectivity was made by using the
internal normalization methods. The catalytic activity was defined
as the sum of percentages of citral converted into different prod-
ucts. The selectivity to a given product (i) was calculated as the ratio
between the amount of product i and the total amounts of products.
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 2. Citral conversion as a function of the reaction time for the following catalysts:
-
ꢀ- Pt(5 wt%)/Al2O3, -ꢀ- Pt(5 wt%)Sn(1 wt%)/Al2O3, -ꢁ- Pt(5 wt%)Sn(2 wt%)/Al2O3
and -ꢂ- Pt(5 wt%)Sn(3 wt%)/Al2O3.
2.3. Characterization of catalysts
◦
carried out at 210 C in a continuous flow reactor. Samples were
The test reactions of the metallic phase (cyclohexane dehydro-
◦
previously treated “in situ” under flowing H2 at 500 C for 3 h.
The alcohol was vaporized in a H2 stream (H /2-propanol molar
ratio = 19) and then it was fed to the reactor with a spacial velocity
of 0.52 mol alcohol h g cat , the sample weight being 0.100 g in
all cases.
genation – CHD and cyclopentane hydrogenolysis – CPH) were
◦
performed in a differential continuous flow reactor at 300 C. In
2
both reactions the catalysts were previously reduced “in situ” in
−
1
−1
◦
flowing H at 500 C for 3 h and the experiments were performed by
2
using a H /CH (or CP) molar ratio equal to 26 and a volumetric flow
2
−
1
of 6 mL m . The catalyst weight used in these test reaction experi-
ments was such as to obtain a conversion lower than 5% (differential
flow reactor model). The reaction products and the remaining reac-
tants were analyzed by using a gas chromatographic system. The
activation energy values for CHD (EaCH) were calculated by linear
regression using the reaction rates measured at three temperatures
3. Results and discussion
3.1. Citral hydrogenation
Fig. 2 shows the total conversion of citral for the Pt/Al O3
and PtSn/Al O3 catalyst series as a function of the reaction time.
2
◦
0
(
(
270, 285 and 300 C) and the Arrhenius plot (ln R
versus 1/T
CH
2
−
1
K
)).
The H2 chemisorption measurements were made in a volumet-
It can be observed that the activities of the bimetallic catalysts
are higher than that of the monometallic one after 1.5 h reaction
time. This behaviour is in agreement with those reported by other
authors [20,25,28,34,47]. Besides, for the bimetallic series, when
the Sn loading increases, the activity decreases. In order to clar-
ify the results, ÂPt and ÂPtSn parameters were defined, these being
the required times to reach a same citral conversion (95%) for
monometallic and bimetallic catalysts, respectively. Thus, the rel-
ative activity defined as the ratio ÂPt/ÂPtSn shows that when the Sn
amount added to Pt increases from 1 to 3 wt%, it decreases follow-
ing the sequence: ÂPt/ÂPtSn = 6 for Sn = 1 wt%, 3 for Sn = 2 wt% and 2
for Sn = 3 wt%.
ric equipment at room temperature. The sample weight used on
the experiments was 0.200 g. In these experiments the catalysts
◦
were previously reduced in H at 500 C for 3 h, then outgassed
2
−5
under high vacuum (10 Torr) at the same temperature for 1 h
and finally cooled down to room temperature. The H2 adsorption
isotherms were performed at room temperature between 0 and
1
00 Torr. The isotherms were linear in the range of used pressures
and the H chemisorption capacity was calculated by extrapolation
2
of the isotherms to zero pressure [46].
The catalysts were also characterized by temperature pro-
grammed reduction (TPR) by using a reductive mixture of H (5%,
Fig. 3 shows the results of the citral conversion to all prod-
ucts obtained by using Pt/MgAl O4 and PtSn/MgAl O4 catalysts as
a function of the reaction time. In this case, the catalytic activity is
slightly affected by the Sn loading except for the catalyst with the
highest Sn content. It must be noted when compared the results of
2
−1
v/v)–N2 (10 mL min ) in a flow reactor. Samples were heated at
2
2
◦
−1
◦
6
“
C min from 25 to 750 C. Before TPR, samples were calcined
◦
in situ” at 500 C for 3 h.
XPS determinations were carried out in a Multitechnic Specs
photoemission electron spectrometer equipped with an X-ray
source Mg/Al and a hemispherical analyzer PHOIBOS 150 in the
fixed analyzer transmission (FAT) mode. The spectrometer oper-
ates with an energy power of 100 eV and the spectra were obtained
with a pass energy of 30 eV and a Mg anode operated at 100 W.
The pressure of the analysis chamber was kept a pressure lower
activity of both catalyst series that catalysts supported on MgAl O4
2
are far more active than those supported on Al O .
2
3
Figs. 4 and 5 show the selectivity values to different reaction
products measured at three citral conversion levels (35, 65 and
95%) for Pt/Al O , Pt/MgAl O , PtSn/Al O and PtSn/MgAl O4 cat-
2
3
2
4
2
3
2
alyst series. The names used in these figures are: UA: unsaturated
alcohol (geraniol and nerol), CAL: citronellal, ISOP: isopulegol, COL:
citronellol, MEN: menthol, DA: dimethyloctanal, DO: dimethyloc-
tanol. It can be seen that the monometallic Pt catalysts on both
supports (Fig. 4) are selective for unsaturated aldehydes and they
display a low formation of unsaturated alcohols.
It can be observed in Fig. 5 that the bimetallic catalysts on
both supports display a notorious higher selectivity to unsaturated
alcohols than the corresponding monometallic ones. Moreover, it
must be noted that the higher the Sn loading added to Pt, the
−8
than 5 × 10 mbar. Samples were previously reduced under H2 at
◦
5
00 C for 3 h in a flow reactor and then they were introduced in
◦
the equipment and reduced “in situ” with H2 at 400 C for 1 h. The
binding energies (BEs) of the signals were referred to the C1s peak
at 284 eV. Peak areas and BE values were estimated by fitting the
curves with combination of Lorentzian–Gaussian curves of variable
proportion using the CasaXPS Peak-fit software version 1.2.
In order to characterize the acidic properties of the supports,
the 2-propanol dehydration reaction was used. The reaction was

P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
211
1
00
Table 1
Selectivity values to different reaction products measured at different reaction time
after total conversion of citral for Pt–Sn/Al2O3 and Pt–Sn/MgAl2O4 catalyst series.
8
6
4
2
0
0
0
0
0
Catalyst
Reaction time (h)
Selectivity (%)
UA
CAL
COL
DO
7.0
0
0.25
100
100
100
PtSn(1 wt%)/Al2O3
PtSn(2 wt%)/Al2O3
PtSn(3 wt%)/Al2O3
PtSn(1 wt%)/MgAl2O4
PtSn(3 wt%)/MgAl2O4
PtSn(4 wt%)/MgAl2O4
2
2
2
4
4
4
28.5
77.2
83.9
–
0.5
0
0
54.5
21.6
14.6
–
–
–
–
–
–
–
–
0
0,15
0,3
0,45
0,6
0,75
since the selectivity value for Pt(5 wt%)Sn(4 wt%)/MgAl O4 cata-
lyst is of about 73% this being lower than the one showed by
2
Time (h)
Pt(5 wt%)Sn(3 wt%)/Al O . Then, it is the nature of support that
Fig. 3. Citral conversion as
a
function of the reaction time for the follow-
2
3
ing catalysts: -ꢀ- Pt(5 wt%)/MgAl2O4, -ꢀ- Pt(5 wt%)Sn(1 wt%)/MgAl2O4,
produces noticeable differences both on the catalytic activity and
selectivity.
-
ꢁ- Pt(5 wt%)Sn(2 wt%)/MgAl2O4 and -ꢂ- Pt(5 wt%)Sn(3 wt%)/MgAl2O4, -᭹-
Pt(5 wt%)Sn(4 wt%)/MgAl2O4.
On the other hand, it was observed during the reaction that
the formation of citronellol is mainly produced by the citronellal
hydrogenation in the monometallic catalysts, while for bimetallic
catalysts the citronellol formation is due to not only the citronellal
hydrogenation but also the hydrogenation of unsaturated alcohols.
This effect is more important for the bimetallic catalysts with low
Sn loading supported on Al2O3 and in all the PtSn/MgAl2O4 cat-
alytic series. In these cases, the amount of citronellol, when all
citronellal was consumed, increases because the route of hydro-
genation of unsaturated alcohols to citronellol remains active.
Besides, the experiments showed that this hydrogenation step con-
tinues towards the formation of dimethyloctanol for these catalysts
after the total citral conversion. In fact, Table 1 shows the selectivity
values to different reaction products measured at different reaction
time after citral was completely consumed in the reaction medium
for PtSn/Al2O3 and PtSn/MgAl2O4 catalyst series. It is observed
higher the selectivity to unsaturated alcohols for both catalyst
series, while the selectivities to CAL and COL decrease. These facts
indicate that the hydrogenation of the carbonyl group is pref-
erentially favoured with respect to the hydrogenation of double
–
C C– bonds in bimetallic catalysts. Besides, the carbonyl group
hydrogenation would probably require a particular site with a
given surface structure [17,28]. However, it should also be noted
that the effect of the addition of the same Sn loading to the
Pt catalysts supported on Al O3 and MgAl O4 does not lead to
2
2
obtain the same selectivity. Thus, the selectivity to UA for the
Pt(5 wt%)Sn(3 wt%)/Al O3 catalyst reaches a value of about 88%
2
while for the Pt(5 wt%)Sn(3 wt%)/MgAl O4 catalyst is lower (65%).
2
Moreover, the addition of higher Sn amounts to Pt catalysts based
on MgAl O does not enhance significantly the selectivity to UA
2
4
for PtSn/MgAl O4 catalysts that after 4 h reaction time (after the
2
total citral conversion), the different compounds are completely
hydrogenated and converted into dimethyloctanol. For bimetal-
lic catalysts supported on alumina, at 2 h reaction time (after the
total citral conversion), the selectivity to unsaturated alcohols of
PtSn(1 wt%)/Al O catalyst markedly decreases with respect to that
2
3
at 95% citral conversion, while the selectivities to COL and DO
increase. Additionally, it must be noted in Fig. 5 that, when the
citral conversion increases from 65 to 95%, the selectivity to unsat-
urated alcohols in bimetallic catalysts supported on Al O with low
2
3
Sn content (1 wt%) decreases too. In the case of bimetallic catalysts
supported on Al O3 with Sn amount >1 wt%, it can be observed
2
in Table 1 that the selectivity to UA at 2 h reaction time (after
the total citral conversion) slightly decreases with respect to the
values corresponding to PtSn(2 wt%)/Al O3 and PtSn(3 wt%)/Al O3
2
2
catalysts, respectively, at 95% of citral conversion (see Fig. 5B and
C). In this sense, for these catalysts, results show that both the
transformation rate of unsaturated alcohols into citronellol and the
dimethyloctanol formation appear to be slower than those cor-
responding to the other catalysts. These facts indicate that the
selective hydrogenation of the carbonyl group and the inhibition
of the hydrogenation of the C C bonds of citral would require a
given surface structure of the reaction sites [17,20].
From the above mentioned results related to the catalytic activ-
ity in the citral hydrogenation, it was concluded that the metallic
phases and probably the density of the reaction sites for the hydro-
genation of the C O and C C bonds in both catalytic series are
very different. Besides, these results would indicate that the differ-
ent catalytic behaviours of the metallic phase of monometallic and
bimetallic catalysts of both series could also be due to the support
effect.
Fig. 4. Selectivities to different products measured at different citral conversion lev-
els: 35, 65 and 95% for monometallic catalysts Pt(5 wt%) supported on Al2O3 (A) and
MgAl2O4 (B). UA: unsaturated alcohol (geraniol and nerol), CAL: citronellal, ISOP:
isopulegol, COL: citronellol, MEN: menthol, DA: dimethyloctanal, DO: dimethyloc-
tanol.

2
12
P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
Fig. 5. Selectivities to different products measured at different citral conversion levels, 35, 65 and 95%, for different bimetallic catalysts: (A) PtSn(1 wt%)/Al2O3, (B)
PtSn(2 wt%)/Al2O3, (C) PtSn(3 wt%)/Al2O3, (D) PtSn(1 wt%)/MgAl2O4, (E) PtSn(3 wt%)/MgAl2O4 and (F) PtSn(4 wt%)/MgAl2O4. UA: unsaturated alcohol (geraniol and nerol),
CAL: citronellal, ISOP: isopulegol, COL: citronellol, MEN: menthol, DA: dimethyloctanal, DO: dimethyloctanol.
Furthermore, it is very important to note the absence of cycliza-
tion products (isopulegol and menthol) for both catalyst series [48].
on Al O , where the support has an acidic character [49,50]. Fig. 6
2 3
shows the 2-propanol dehydration conversion (as a measurement
of the total acid character of the support [45]) as a function of the
reaction time for Al O3 and MgAl O4 supports. It can be observed
This fact is expected for catalysts supported on MgAl O due to the
2
4
neutral character of this material, but not for catalysts supported
2
2

P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
213
4
3
2
1
0
0
0
0
0
Taking into account the structure-insensitive character of the CHD,
it can be inferred that there is not practically electronic effect in
PtSn/MgAl O4 catalysts.
2
The CPH is a structure-sensitive reaction [53], which is carried
out on ensembles of a certain number of active atoms. This means
that the modification of the reaction rate by addition of a second
metal to the active metal can break the metallic ensembles nec-
essary for the reaction. Table 2 shows that the addition of 1 wt%
γ
-Al2O3
of Sn to Pt supported on Al O3 produces a significative decrease
2
of the reaction rate, but the addition of higher Sn amounts com-
pletely inhibits the reaction. In consequence, this means that Sn
MgAl O
2
4
addition to Pt/Al O produces not only an electronic effect but also
2
3
a geometric one [34] since there is a decrease of the concentration
of metallic ensembles necessary to carry out the hydrogenolysis
reaction, probably due the deposition of Sn in the vicinity of Pt.
Table 2 also shows the initial reaction rates in CPH for the cat-
alysts supported on MgAl O . It is observed that the reaction rate
0
15
30
45
60
75
90
Time (min)
2
4
Fig. 6. 2-Propanol conversion versus time for Al2O3 and MgAl2O4.
decreases in an important way in bimetallic catalysts with respect
to the monometallic one, though this decrease is lower than that
corresponding to bimetallic catalysts supported on Al O3 when
increasing Sn amounts are added to Pt.
2
that alumina has a higher acidic character than MgAl O . How-
2
4
ever, the acidic strength of the alumina is not enough to catalyze
the cyclization reaction. It should be remarked that Sn produces an
additional poisoning effect on the acidic centers of the support [51].
It is also observed a marked diminution of the H chemisorption
2
values (Table 2) when increasing Sn amounts are added to Pt in both
catalytic series. These results are in agreement with the results of
the test reactions. For catalysts based on Al O , this behaviour can
2
3
3
3
.2. Catalyst characterization
probably be due to a modification of the electronic structure of the
metallic phase [54,55], and also the existence of geometric effects
(dilution or blocking of Pt by Sn). However considering the CHD
and CPH results, for catalysts based on MgAl2O4, it is mainly the
presence of geometric effects that would produce the diminution
of the H2 chemisorption values in these bimetallic catalysts.
The Pt dispersion values, calculated from the chemisorption data
shown in Table 1, are 33.4% for Pt/Al2O3 and 25.3% for Pt/MgAl2O4.
Then, as expected, the TON value (defined as R0CH/Pt dispersion) in
CHD for the monometallic catalyst supported on MgAl2O4 (39.8 mol
.2.1. Test reactions of the metallic phase and H2 chemisorption
Table 2 shows the values of the initial rates of cyclohexane
0
) and cyclopentane hydrogenolysis (R0CP)
dehydrogenation (R
for both catalyst series. The values of activation energy in CHD
CH
(
EaCH) and the H2 chemisorption capacity (H) are also included.
The CHD is a structure-insensitive reaction [52], which is carried
out on the surface active atoms of the catalyst. In this reaction the
activation energy will not be modified if the nature of the metallic
site is not changed by any other effect (for example the addition of
a second metal). Hence any modification in the activation energy
values is caused by an electronic modification in the metal–metal
interaction or a change of the metallic phase structure. The results
of the CHD show that the Sn addition to Pt/Al O3 catalyst leads
to an important increment of the activation energy (from 22 to
4
decrease is higher than one order of magnitude with respect to
the value of the monometallic catalyst). These results clearly indi-
cate an important electronic modification of the active phase by Sn
with probable alloy formation in bimetallic catalysts supported on
Al O .
−
1
−1
of exposed Pt) is lower than the one corresponding
CHD h
g
to the monometallic catalyst supported on Al2O3 (52.7 mol CHD
−
1
−1
g of exposed Pt). However, in spite of the lower dispersion
h
found for Pt/MgAl2O4 catalyst, the specific activity in the citral
hydrogenation reaction was clearly higher for this catalyst than for
Pt/Al2O3 one. These results would indicate that the different cat-
alytic behaviours in CHD and citral hydrogenation of the metallic
phase of both monometallic catalysts could be due not only to the
support effect but also to the existence of a different accessibility
of the reactants (citral and CH) towards the active sites.
In conclusion the above mentioned results clearly show the
differences in the structure of the metallic phase of monometal-
lic and bimetallic catalysts according to the used support (Al2O3
or MgAl2O4). The different catalytic behaviours when increasing
amounts of Sn are added to Pt could be related to the presence
of different species of Sn (ionic, zerovalent), which could be inter-
calated among Pt atoms and could be partially blocking them in
2
−1
5 kcal mol ) and to a decrease of the initial reaction rate (this
2
3
In the case of bimetallic catalysts supported on MgAl O4 it can
2
be observed that the initial reaction rate of CHD slightly decreases
when increasing Sn amounts are added to Pt (Table 2). The acti-
vation energy values increase in lower proportion (with respect
to the monometallic sample) than those for PtSn/Al O3 catalysts.
2
Table 2
Initial reaction rates of CHD (R0CH) and CPH (R0CP), activation energy in CDH (EaCH) and H2 chemisorption capacity (H) for different catalysts.
R0CH (mol h−1
g
−1
Pt)
EaCH (kcal mol
−1
)
R0CP (mol
−1 −1 −1
h
g
Pt)
H (␮mol/g catal)
Catalyst
Pt/Al2O3
17.6
5.4
0.5
0.1
9.9
5.9
6.4
5.1
4.6
22
25
36
45
21
22
23
25
26
2.14
0.02
n.m.
n.m.
2.25
0.12
0.08
0.06
0.04
42.6
14.8
7.7
PtSn(1 wt%)/Al2O3
PtSn(2 wt%)/Al2O3
PtSn(3 wt%)/Al2O3
Pt/MgAl2O4
PtSn(1 wt%)/MgAl2O4
PtSn(2 wt%)/MgAl2O4
PtSn(3 wt%)/MgAl2O4
PtSn(4 wt%)/MgAl2O4
3.1
32.3
13.4
6.6
3.5
3.3
n.m.: not measurable.

2
14
P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
Sn(3wt%)/Al O
2
3
PtSn(3wt%)/Al O
2
3
PtSn(4wt%)/MgAl O4
2
PtSn(3wt%)/MgAl O4
2
PtSn(2wt%)/Al O
2
3
PtSn(2wt%)/MgAl O4
2
PtSn(1wt%)/Al O
2
PtSn(1wt%)/MgAl O4
2
3
Pt/MgAl O
2
4
Sn(4wt%)/MgAl O4
2
Pt/Al O
2
3
0
100
200
300
400
500
600
700
800
0
100
200
300
400
500
600
700
800
Temperature (ºC)
Temperature (ºC)
Fig.
8. TPR
profiles
of
Pt(5 wt%)/MgAl2O4,
Sn(4 wt%)/MgAl2O4
and
Fig. 7. TPR profiles for Sn(3 wt%)/Al2O3, Pt(5 wt%)/Al2O3 and Pt(5 w%)Sn/Al2O3 with
Sn contents of 1, 2 and 3 wt%.
Pt(5 wt%)Sn/MgAl2O4 catalysts with different Sn contents.
values observed in CHD. The broadening of the reduction peaks in
bimetallic catalysts could be due to the existence of different metal-
lic species and the presence of Sn species in the vicinity of Pt. These
effects appear to be more important as the Sn loading increases.
a different proportion. Besides, the formation of PtSn alloys could
produce an electronic modification of the metallic phase. According
to the catalyst type, the structure of the metallic phase produced by
Sn addition could favour the hydrogenation of the carbonyl group.
In this sense, the presence of electronic or/and geometric effects
of Sn species on the metallic phase appears to be not only more
Fig. 8 displays the TPR profiles for the PtSn/MgAl O4 catalyst
2
series. The TPR profiles of the monometallic Pt(5 wt%)/MgAl O and
2
4
Sn(4 wt%)/MgAl O catalysts are also included. Sn(4 wt%)/MgAl O
2
4
2
4
pronounced for PtSn/Al O3 catalysts (which display a significant
2
catalyst shows a very small and broad reduction zone between 400
change of the EaCH, and a higher decrease both of H2 chemisorp-
◦
and 700 C. In contrast with the bimetallic catalysts supported on
tion and CPH reaction rates than the PtSn/MgAl O4 ones) but also
2
Al O , PtSn/MgAl O catalysts show only one important reduction
2
3
2
4
more suitable to conform a better structure of the metallic phase
for the hydrogenation of the carbonyl group.
◦
peak at about 275 C, this temperature being similar to that found
in the Pt(5 wt%)/MgAl O . In all series of bimetallic catalysts sup-
2
4
ported on MgAl O4 it is observed that there are no shifts in the
2
3
.2.2. TPR profiles
Other evidence of a better Pt–Sn interaction in catalysts sup-
temperature of this reduction peak and the H2 consumption val-
ues up to 400 C are similar, which indicates that the first peak
◦
ported on Al O3 were observed from TPR results. Fig. 7 shows
corresponds to the reduction of Pt species in all bimetallic cata-
lysts and a small Pt–Sn coreduction. Moreover, it must be noted
that all bimetallic catalysts, mainly those with higher Sn loadings
2
the TPR profiles for the PtSn/Al O3 catalyst series. For the sake
2
of comparison the TPR profiles of the monometallic catalysts
◦
(
Pt(5 wt%)/Al O and Sn(3 wt%)/Al O ) are also included. Pt/Al O
show a small reduction zone at temperatures higher than 400 C.
2
3
2
3
2
3
catalyst shows a reduction peak with a maximum at about
All these results would indicate a co-reduction of Pt and Sn, but
with low alloy formation (in agreement with the activation energy
values found for CHD), and probably with a certain fraction of free
Sn species stabilized on the support.
◦
2
00–210 C, while Sn(3 wt%)/Al O presents a broad reduction zone
2
3
◦
between 240 and 600 C. Bimetallic PtSn/Al O catalysts show only
2
3
one reduction peak similar to monometallic catalyst. These peaks
in bimetallic catalysts are broader than that of Pt/Al O catalyst and
2
3
they are shifted to higher temperatures as the Sn loading increases
3.2.3. XPS measurements
◦
(
maximums at about 250, 260 and 270 C for catalysts with 1, 2 and
In order to determinate the oxidation state of the different
metals in the monometallic and bimetallic catalysts after the
3
wt% of Sn, respectively) with respect to that of the monometallic
◦
Pt catalyst. It is observed a certain reduction zone at temperatures
reduction at 500 C, XPS experiments were carried out. From
◦
higher than 400 C (temperature zone where the reduction of free
Pt4d spectra of these samples, the presence of only one peak
(at about 313.6–314.8 eV) in all catalysts was found. This peak
can be assigned to Pt0 [56,57]. Fig. 9 shows the Sn3d5/2 XPS
spectra for PtSn(1 wt%)/Al O3 and PtSn(3 wt%)/Al O3 catalysts and
Sn stabilized on the support takes place), this showing the presence
of Sn species on the support. These findings can be understood as a
co-reduction of both metals (Pt and Sn) with probable alloy forma-
tion, which is in agreement with the increment of activation energy
2
2
Fig. 10 shows the Sn3d5/2 XPS spectra for PtSn(1 wt%)/MgAl O4
2

P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
215
A
B
II, IV
II,IV
0
Sn
Sn
0
Sn
Sn
all
all
0
0
Sn
Sn
4
75
480
485
490
495
475
480
485
490
495
Binding Energy (eV)
Binding Energy (eV)
Fig. 9. Sn3d5/2 XPS profiles for (a) Pt(5 wt%)Sn(1 wt%)/Al2O3 and (b) Pt(5 wt%)Sn(3 wt%)/Al2O3 samples.
and PtSn(4 wt%)/MgAl O4 ones. In the case of bimetallic catalysts
sidering the results reported by Homs et al. [58,59] who found
2
0
II,IV
supported on Al O , the Sn3d
XPS profiles are more complex
Sn , Sn and oxidized Sn (Sn ) in bimetallic PtSn/Al O3 and
2
3
5/2
all
2
than those corresponding to PtSn/MgAl O4 catalysts. Taking into
PtSn/SiO catalysts by using XPS, EXAFS, TEM and XRD. In this sense,
2
2
account the shape and the width of the profiles (for catalysts sup-
ported on Al O ), they were deconvoluted in three peaks: the first
one at about 482.7–483.3 eV which can be assigned to Sn , the
peak at about 485.1–485.3 eV that can be due to the presence of
alloyed Sn (Snall) [23,58,59], and the third peak located at about
our results of CHD support the XPS results since they indicate an
electronic modification of the metallic phase due to the activation
energy values clearly increase when Sn is added to Pt.
2
3
0
For the PtSn/MgAl O4 catalysts, the deconvolution of the
2
Sn3d5/2XPS profiles was carried out in two peaks: one at about
482.3–483.0 eV and the other one at about 485.4–485.7 eV. The
first peak was assigned to Sn0 [20,29], while the second one was
4
86.7–486.8 eV, which is the major one and it can be attributed to
oxidized Sn species [20,29]. The peak assignation was made con-
A
B
II, IV
Sn
II, IV
Sn
0
Sn
0
Sn
4
75
480
485
490
495
475
480
485
490
495
Binding Energy (eV)
Binding Energy (eV)
Fig. 10. Sn3d5/2 XPS profiles for (a) Pt(5 wt%)Sn(1 wt%)/MgAl2O4 and (b) Pt(5 wt%)Sn(4 wt%)/MgAl2O4 samples.

2
16
P.D. Zgolicz et al. / Applied Catalysis A: General 392 (2011) 208–217
Table 3
XPS results of different catalysts corresponding to Pt4d5/2 and Sn3d5/2 levels.
Catalyst
BE (eV) Pt4d5/2
level; species (%)
BE (eV) Sn3d5/2 level; species (%)
Bulk SnT/Pt atomic
ratio
Surface SnT/Pt
atomic ratio
Pt/Al2O3
314.5; Pt(0); 100%
314.8; Pt(0); 100%
314.4; Pt(0); 100%
313.8; Pt(0); 100%
313.6; Pt(0); 100%
314.1; Pt(0); 100%
–
–
–
PtSn(1 wt%)/Al2O3
PtSn(3 wt%)/Al2O3
Pt/MgAl2O4
PtSn(1 wt%)/MgAl2O4
PtSn(4 wt%)/MgAl2O4
483.3; Sn(0); 24.4%485.3; Sn(alloy); 11.0%486.8; Sn(II,IV); 64.7%
482.7; Sn(0); 5.9% 485.1; Sn(alloy); 16.9% 486.7; Sn(II,IV); 77.2%
–
483.0; Sn(0); 15.8%
482.3; Sn(0); 7.7%
0.33
0.99
–
0.33
1.33
1.66
4.43
–
1.26
1.88
485.4; Sn(II,IV); 84.2%
485.7; Sn(II,IV); 92.3%
attributed to oxidized species of SnII,IV, since according to the liter-
UA in both catalyst series (Fig. 5). However, not only this effect but
also the electronic one must be taken into account to explain the
higher selectivity of PtSn supported on Al2O3 than those supported
on MgAl2O4. In fact, in PtSn/Al2O3 catalysts, PtSn alloyed parti-
cles would inhibit the hydrogenation of the olefinic bond probably
due to the additional dilution effect and on the other hand would
they enhance the hydrogenation of the carbonyl group because this
group could be preferentially adsorbed over the alloyed surface. It
must be noted that the presence of alloys in these catalysts was
observed by XPS and inferred from test reaction (CHD). Besides, the
additional dilution effect produced by the alloy particles is in agree-
ment with the lower hydrogenolytic capacity found for PtSn/Al2O3
catalysts with respect to PtSn/MgAl2O4 ones.
II
IV
ature Sn and Sn cannot be discriminated because their binding
energies are very similar [60].
Table 3 displays the BE and the percentage of Sn0 and oxidized
Sn, the SnT/Pt molar ratio in the bulk (SnT: total moles of Sn) and the
(
Sn/Pt)surface (surface molar ratio) obtained from XPS experiments
0
on the different catalysts. The presence of small quantities of Sn
in bimetallic catalysts supported on MgAl O4 indicates the low Sn
2
reducibility degree in these catalysts. It should be noted that these
results are in agreement with those found by TPR measurements.
On the other hand, taking into account the TPR and XPS results for
bimetallic catalysts supported on Al O , it can be concluded that a
2
0
3
certain amount of Sn is present as Sn , other Sn amount as Sn and
all
the remaining Sn fraction as oxidized species. Besides, the presence
of Sn in the zerovalent state decreases as its content increases for
both catalyst series (see Table 3). Moreover, in the case of bimetallic
4. Conclusions
catalysts based on Al O the fraction of alloyed Sn increases when
2
3
The citral hydrogenation was studied by using Pt and PtSn cat-
the Sn loading increases. By comparing the values of (Sn/Pt)surface
with those of (SnT/Pt)bulk it can be concluded that there is a surface
enrichment of the metallic surface by Sn in both catalyst series, this
enrichment being higher in catalysts based on Al O .
alysts supported on Al O3 and MgAl O4 and using mild reaction
2
2
conditions. It has been found that for both catalyst series when
the Sn loading added to Pt increases, the selectivity to unsatu-
rated alcohols it also enhanced, the alumina support giving the
best selectivity to UA. Thus, PtSn(3 wt%)/Al O catalyst displays the
2
3
In conclusion, XPS results indicate that the structure of metallic
phase is modified by the nature of the support and the pro-
moter (Sn) loading, in agreement with those results obtained from
other characterization techniques. However, XPS measurements
also allow to determine with more details the nature of the metal-
lic surface. Then, the differences found in the catalytic behaviour
for bimetallic catalysts supported on Al O3 and MgAl O4 can be
2
3
highest selectivity value to UA (88%) while PtSn(3 wt%)/MgAl O4
2
and PtSn(4 wt%)/MgAl O4 catalysts show a lower selectivity val-
2
ues (65 and 73%, respectively). According to the characterization
results, it can be inferred that the metallic phase in bimetallic
catalysts is strongly affected by the tin addition and the nature
of the support. In this sense, for the catalysts supported on alu-
mina, both dilution and electronic effects were found, while for
2
2
better explained on the basis of the different metallic surface
structures. According to the literature, the oxidized Sn species in
the catalytic surface are responsible for the polarization of the
carbonyl group [17,18,20] and the consequent increment of the
hydrogenation rate of this group leading to unsaturated alcohols.
These effects together with the partial dilution of Pt by Sn species
could increase the selectivity to unsaturated alcohols of bimetal-
lic catalysts with respect to monometallic Pt ones. It should be
samples supported on MgAl O4 mainly geometric effects appear
2
to take place. The presence of oxidized Sn species in both catalyst
series would favour the polarization of the carbonyl group thus
increasing the selectivity to UA. The presence of alloyed particles
in catalysts supported on Al O3 would increase the selectivity val-
2
ues to unsaturated alcohols probably due to an additional dilution
effect of Pt by Sn0 species and they could also increase the hydro-
genation of carbonyl group. In this sense, the high Pt–Sn interaction
0
noted that Sn can be present in the metallic phase as Sn or form-
0
ing PtSn alloys in some cases. In both cases (Sn alone or PtSn
in PtSn/Al O catalysts seems to have a positive effect for obtaining
2
3
alloys) the Sn would also contribute to the dilution effect [17,23].
Besides, the PtSn alloys could be active for the hydrogenation of
a high selectivity to the unsaturated alcohols.
–
C C– groups and for the polarization of the carbonyl group. In this
sense, several authors studied the adsorption of ␣,␤-unsaturated
aldehydes by means of density functional calculations and high-
resolution photoelectron spectroscopy on well-defined PtSn alloys
and Pt(1 1 1) surfaces [31,32,61,62]. They reported that for the most
stable possible adsorption structures of ␣,␤-unsaturated aldehy-
des, the adsorption energy is smaller on the alloys than on platinum,
but they also observed that on the alloys there is an enhancement
to the relative stability of adsorption structures which leads to the
hydrogenation of carbonyl group. According to our characteriza-
tion results, the state of the metallic phase in bimetallic catalysts
of both series is strongly influenced by both the nature of the sup-
port and the Sn addition (geometric and electronic effects). In this
sense, XPS results show that an important fraction of Sn is found as
oxidized Sn, which explains the important values of selectivity to
Acknowledgements
Authors thank Miguel A. Torres and Julieta P. Stassi for the exper-
imental assistance. This work was made with the financial support
of ANPCYT and Universidad Nacional del Litoral (Project CAI+D) –
Argentina.
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