Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective hydrogenation of succinic acid in aqueous-phase

Article abstract of DOI:10.1016/j.cattod.2014.02.018

Bimetallic Re-Pd catalysts were prepared by addition of Re onto a Pd/TiO₂ monometallic catalyst, either by a catalytic reduction method or by successive impregnation. The physical and chemical properties of the bimetallic catalysts were evaluated by several characterization techniques, including analysis by transmission electronic microscopy, hydrogen chemisorption, temperature programmed reduction and X-ray photoelectron spectroscopy to explain their different catalytic performances in the selective hydrogenation of succinic acid to 1,4-butanediol in aqueous solution. The localization of the Re deposit on the parent Pd catalyst surface varied according to the Re introduction mode. The catalytic reduction method induced a selective Re deposit at the Pd-TiO₂ interface compared to a random distribution obtained by successive impregnation. The oxidation state of the Re species (Re³⁺, Re⁰) on the reduced bimetallic samples was also linked to their preparation mode.

Full text of DOI:10.1016/j.cattod.2014.02.018

G Model
CATTOD-8909; No. of Pages7
ARTICLE IN PRESS
Catalysis Today xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Influence of the Re introduction method onto Pd/TiO2 catalysts
for the selective hydrogenation of succinic acid in aqueous-phase
Benoit Tapina, Florence Eprona, Catherine Especela,∗, Bao Khanh Lyb,
Catherine Pinelb, Michèle Bessonb,∗∗
a Université de Poitiers, CNRS UMR 7285 IC2MP, Institut de Chimie des Milieux et des Matériaux de Poitiers, 4 rue Michel Brunet, 86022 Poitiers Cedex,
France
b IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, UMR 5256 CNRS-Université Lyon1, 2 avenue Albert Einstein, 69626
Villeurbanne, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Bimetallic Re-Pd catalysts were prepared by addition of Re onto a Pd/TiO2 monometallic catalyst, either
by a catalytic reduction method or by successive impregnation. The physical and chemical properties
of the bimetallic catalysts were evaluated by several characterization techniques, including analysis by
transmission electronic microscopy, hydrogen chemisorption, temperature programmed reduction and
X-ray photoelectron spectroscopy to explain their different catalytic performances in the selective hydro-
genation of succinic acid to 1,4-butanediol in aqueous solution. The localization of the Re deposit on the
parent Pd catalyst surface varied according to the Re introduction mode. The catalytic reduction method
induced a selective Re deposit at the Pd–TiO2 interface compared to a random distribution obtained
by successive impregnation. The oxidation state of the Re species (Re3+, Re0) on the reduced bimetallic
samples was also linked to their preparation mode.
Received 18 November 2013
Received in revised form 27 January 2014
Accepted 13 February 2014
Available online xxx
Keywords:
Re-Pd catalysts
Bimetallic catalyst preparation
Succinic acid hydrogenation
1,4-butanediol
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Re-Pd bimetallic catalysts which have been demonstrated as effi-
cient systems for the succinic acid hydrogenation to 1,4-butanediol.
Many bimetallic catalysts were shown to have catalytic prop-
erties that differ distinctly from those of their parent metals [1].
In many cases, these unique properties result in activities and/or
selectivities that exceed those achievable with a monometallic cat-
alyst. A lot of experimental and fundamental studies were reported
with the aim to investigate the origin of this interesting effect
using notably model surfaces [2–4]. It is generally recognized that a
combination of electronic and/or geometric effects can explain the
catalytic behavior (selectivity and activity) of bimetallic systems.
The electronic (or ligand) effect is the result of a change in the
electron structure of the catalyst, while by geometric (or ensem-
ble) effect the second metal may block sites on the surface of an
active metal and by that means, the average size and composition
of the ensemble of active sites. In this context, the present study
is focused on the preparation and characterization of supported
Succinic acid (SUC) is a “building block” involved in various
reactions and leading to molecules with diverse applications [5,6].
Succinic acid is traditionally produced from petroleum, but thanks
to the new developments in biotechnology, it can be now efficiently
produced from glucose [7,8]. According to the USDA (United States
Department of Agriculture), world production of 200,000 tons per
year is planned in 2015, and succinic acid market could easily rep-
resent more than 1 billion Euros per year [9,10]. The succinic acid
derivatives are numerous with uses as diverse as heat transfer flu-
ids, solvents, pigments, polyester, polyurethane, intermediate for
the chemical industry, plasticizers, etc. [11]. Among these products,
␥-butyrolactone (GBL), 1,4-butanediol (BDO) and tetrahydrofuran
(THF) are obtained by hydrogenation according to Scheme 1. In the
present work, the intended product is BDO used industrially as sol-
vent, in the fabrication of plastics (polybutylene terephthalate PBT),
elastic fibers (such as spandex) and polyurethane.
Different types of catalysts have been described for SUC
hydrogenation. This reaction is catalyzed either by homogeneous
catalysts such as RuCl2(PPh3)3, RuH2(PPh3)3 complexes [12,13], or
by heterogeneous catalysts in gas or liquid phase with solids con-
stituted of Cu, Ni, Co or supported noble metals such as Ru or Pd,
as mostly described in patents [14–20]. With SUC coming from a
∗
Corresponding author. Tel.: +33 5 49 45 39 94; fax: +33 5 49 45 37 41.
Corresponding author. Tel.: +33 4 72 44 53 58; fax: +33 4 72 44 53 99.
E-mail addresses: catherine.especel@univ-poitiers.fr (C. Especel),
∗∗
michele.besson@ircelyon.univ-lyon1.fr (M. Besson).
http://dx.doi.org/10.1016/j.cattod.2014.02.018
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: B. Tapin, et al., Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective
hydrogenation of succinic acid in aqueous-phase, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.018

G Model
CATTOD-8909; No. of Pages7
ARTICLE IN PRESS
2
B. Tapin et al. / Catalysis Today xxx (2014) xxx–xxx
Scheme 1. Catalytic hydrogenation of succinic acid (SUC) to ␥-butyrolactone (GBL), 1,4-butanediol (BDO) and tetrahydrofuran (THF).
reconstituted air flow (80% N2 + 20% O2, 3.6 L h−1) at 300 ◦C for 4 h,
and finally by reduction for 4 h in flowing H2 (3.6 L h−1) at 300 ◦C.
Bimetallic xwt.%Re-2 wt.%Pd catalysts supported on TiO2 were
prepared by two different methods using ammonium perrhenate
(NH4ReO4): (i) successive impregnation (SI) and (ii) catalytic reduc-
tion (CR). The percentage of Re introduced by both method was
different and chosen as a function of the best results obtained for
succinic acid conversion [32]. In the SI method, the parent Pd/TiO2
catalyst was added to distilled water and a predetermined amount
of NH4ReO4 was introduced in this suspension which was main-
tained under stirring at room temperature for 5 h. Then, the solution
was evaporated and the catalyst precursor was dried in vacuum at
50 ◦C during 20 h, before reduction at 450 ◦C for 3 h, followed by
passivation in 1 vol.%O2/N2. The CR preparation method involved a
surface redox reaction between hydrogen activated on the parent
Pd/TiO2 catalyst and the Re species introduced by the ammonium
perrhenate (NH4ReO4) solution according to the overall redox reac-
tion: 7 Pd-Hads + ReO4− + H+ → Re0(Pd)7 + 4H2O. The parent Pd/TiO2
catalyst was placed in a fixed bed reactor, reactivated under H2 flow
(3.6 L h−1) at 300 ◦C for 1 h and then cooled down to room temper-
ature while maintaining the H2 flow. Subsequently, the rhenium
solution (acidified with HCl, pH 1), previously degassed under N2
bubbling, was introduced onto the catalyst at room temperature.
After 1 h reaction time under H2 bubbling, the solution was filtered
out, and the catalyst was dried overnight at 100 ◦C under H2 flow
(3.6 L h−1). Finally, the CR bimetallic catalyst was reduced under
hydrogen flow (3.6 L h−1) at 450 ◦C for 3 h before storage in ambient
air.
fermentation process in aqueous phase, its further hydrogenation
is especially interesting if performed in this medium. The hetero-
geneous catalysts mentioned in the literature as the most efficient
for the selective transformation of SUC in aqueous solution con-
tain two or three metals, i.e. M1-M2/support or M1-M2-M3/support
systems with M1 = Pd, Ru, Pt; M2 = Re, Sn, Mn; M3 = Ag, Sn, Mn; sup-
port = C, TiO2, ZrO2 [21–24]. Nevertheless in many studies, little
information is given on the metal–metal interactions and the effect
of promoters.
Rhenium was largely used in the formulation of the patented
catalysts for the hydrogenation of SUC or derivatives in aqueous
medium. High loadings are usually used (wt.%Re ≥4) [21,23,25–28].
In a first paper devoted to the SUC hydrogenation in a batch reactor
(160 ◦C, 150 bar H2) in the presence of various bimetallic systems
constituted of Re-Pd, Re-Ru and Re-Pt supported on C or TiO2 [24],
we observed that 2 wt.%Pd catalysts modified by 4 or 6 wt.%Re were
particularly active and selective for the BDO formation (maximum
BDO selectivity >70%). Monometallic supported Pd catalysts were
active for the SUC conversion but they led almost exclusively to
GBL [29]. Concerning the two studied supports, TiO2 was observed
to less adsorb impurities present in bio-sourced succinic acid com-
pared to activated carbon which possesses a large microporous
surface, while presenting a good hydrothermal stability [30,31]. At
this stage, the first characterizations performed on the studied sam-
ples seemed to indicate a non-homogeneous distribution of Re on
the catalysts surface.
Furthermore, we explored the effect of Re addition mode in
bimetallic Re-Pd/TiO2 catalysts upon their performances for the
selective SUC hydrogenation to BDO by comparing the primary
preparation method (by successive impregnation) with the cat-
alytic reduction one (implying a surface redox reaction) [32].
Catalytic results indicated that the Pd modification by both Re
deposit techniques induced a synergy phenomenon, with optimal
Re loadings differing according to the addition mode. From catalytic
reduction, lower Re contents (<1 wt.%) were necessary to initiate
the synergy compared to the important Re contents (>3.4 wt.%)
required from successive impregnation. Nevertheless, these pre-
vious papers did not focus on the characterization of the Pd–Re
interaction as well as on the study of the precise localization of the
Re deposit on the catalyst surface.
2.2. Catalysts characterization
The actual Pd and Re contents in the catalysts were determined
by ICP-OES (inductively coupled plasma optical-emission spec-
trometer, Perkin) with an accuracy of 0.1 wt.%. The different metal
loadings in the catalyst references given in the text are directly
derived from the ICP analysis results.
The metallic accessibility was determined by H2 chemisorption
using a pulsed technique. The catalysts were reduced in H2 flow
(1.8 L h−1) at 300 ◦C for 1 h, then flushed by Ar flow (1.8 L h−1) at
the same temperature for 2 h, and finally cooled to 70 ◦C before
H2 pulses. Chemisorption of H2 was performed at 70 ◦C in order
to avoid formation of ␤-Pd hydride phase. The value of metallic
accessibility (H/M) was estimated from the ratio of irreversibly
adsorbed hydrogen on the Pd total number, considering that one
hydrogen chemisorbs on one accessible Pd atom (Re atoms does
not chemisorb hydrogen under the experimental conditions).
Catalysts morphology was studied by transmission electron
microscopy (TEM) coupled with energy dispersive X-ray spec-
troscopy (EDXS) to ensure accurate localization of metallic
particles. Micrographs were collected on a JEOL 2100 instrument
(operated at 200 kV with a LaB6 source and equipped with a Gatan
Ultra scan camera).
The present study is thus dedicated to the effect of the prepa-
ration method on the physical and chemical characteristics of the
bimetallic Re-Pd/TiO2 catalysts and on the catalytic behavior for
the selective hydrogenation of SUC in aqueous phase to BDO.
2. Experimental
2.1. Catalysts preparation
A
commercial titania (TiO2 Degussa P25, specific
area = 50 m2 g−1
)
was used as support. monometallic
A
2.0 wt.%Pd/TiO2 catalyst was prepared via an impregnation
method performed in an acidic aqueous medium (pH 1, HCl
32 wt.%) and using palladium chloride (PdCl2) as precursor salt.
After the impregnation step, the solvent was evaporated and
the catalyst was further dried overnight in oven at 120 ◦C. The
supported catalyst was treated by calcination under artificial
Temperature programmed reduction (TPR) was performed on
catalysts pretreated under pure O2 for 1 h at 300 ◦C, and cooled
down to room temperature before the reduction under a 1.0 vol.%
H2/Ar gas mixture. The temperature range was 25–700 ◦C with a
ramp of 5 ◦C min−1. The measurements of the H2 consumption were
Please cite this article in press as: B. Tapin, et al., Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective
hydrogenation of succinic acid in aqueous-phase, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.018

G Model
CATTOD-8909; No. of Pages7
ARTICLE IN PRESS
B. Tapin et al. / Catalysis Today xxx (2014) xxx–xxx
3
Table 1
Characteristics of the Pd/TiO2, Re/TiO2 and Re-Pd/TiO2 catalysts (Pd content = 2 wt.%).
Catalyst/TiO2
Pd
3.5 wt.%Re-Pd SIa
0.9 wt.%Re-Pd CRa
1.9 wt.%Re
H/Mb (%)
33
6
14
0
Pd 3d5/2 (eV)
Pd oxidation state
Re 4f7/2 (eV)
335.0
335.0
335.2
100% Pd0
41.1
-
-
100% Pd0
100% Pd0
40.1–41.2
30% Re0, 70% Re3+
0.027
-
-
40.5–41.3
Re oxidation state
100% Re3+
0.036
0.78
15% Re0, 85% Re3+
c
(Pd/Ti)atom
0.036
-
-
-
-
c
(Re/Pd)atom
2.18
0.059
(Re/Ti)atom
0.028
0.049
a
SI, successive impregnation method; CR, catalytic reduction method.
Metallic accessibility determined by H2 chemisorption at 70 ◦C.
Atomic ratio determined by XPS analysis.
b
c
made in an AutoChem II/Micromeritics apparatus, using a thermal
conductivity detector.
part of the Pd surface by Re or/and from a sintering of Pd particles
occurring during the preparation of the bimetallic samples. Fig. 1
displays representative TEM images of the monometallic and
bimetallic SI and CR catalysts, with the histograms gathering
the distribution of the particles size on each sample. On the
monometallic catalyst (Fig. 1a), a homogeneous distribution of the
Pd particles can be observed, confirmed by the histogram showing
the presence of particles with diameter comprised mainly between
1 and 2 nm. The average Pd particles size estimated from the his-
togram (1.8 nm) is lower than the one that could be inferred from
hydrogen chemisorption considering Hugues et al. hypothesis [33]
(2.8 nm). This difference may be due to the presence of a few Pd
particles with much more important size but not observed during
TEM analysis. From Fig. 1b and c, it can be seen that particles are
larger for the bimetallic catalyst prepared by CR compared to the SI
one, with a wide size distribution, from 0.5 to 10.5 nm, which was
not predicted by the hydrogen chemisorption results. Two addi-
tional experiments were performed on the monometallic catalyst
in order to estimate the impact of the SI and CR preparation pro-
tocols on the Pd particle size. For that purpose, the parent catalyst
was treated in the same way as for the preparation of the bimetallic
samples but the Re precursor salt was replaced by an aqueous
solution of same pH, in neutral medium under ambient air for the
SI protocol and in acidic medium under hydrogen flow for the CR
protocol. The as-obtained samples (blank samples) display H/M
values equal to 26% and 17% for SI and CR protocols, respectively.
Consequently, these results indicate that: (i) the synthesis of the SI
catalyst induces a slight sintering of the Pd particles of the parent
catalyst as observed by TEM analysis, and the Re deposit covers
partly the Pd particles surface significantly limiting the hydrogen
chemisorption; (ii) for the CR catalyst, a noticeable sintering of
the Pd particles of the parent catalyst occurs during the course of
the preparation in acidic medium under hydrogen in accordance
with literature [34,35], and the Re deposit seems to less cover
the surface of Pd compared to the SI catalyst. Indeed, taking into
account the Pd accessibility of the blank CR catalyst, an amount
of 0.6 wt.% of Re is enough to completely cover Pd particles with
as a monolayer. Even for a Re content deposited by CR similar to
the one of SI sample, i.e. 3.5 wt.% (not shown in Table 1), the H/M
value remains superior to 11%, compared to a value of 17% without
Re. This suggests that Re is not only deposited on the Pd surface
by the CR method, but also on the Pd-support interface and/or on
the support. However, the deposition of 4 wt.%Re on the lone TiO2
support (without Pd) conducted under the CR conditions led to
only a 0.3 wt.%Re load on the solid. Therefore, on the bimetallic
CR catalyst, it can be considered that the deposition of Re on the
support without interaction with Pd remains extremely limited.
Consequently, on the CR bimetallic catalyst, Re is mostly deposited
at the Pd-support interface.
X-ray photoelectron spectroscopy (XPS) measurements were
collected using a hemispherical analyzer on a Kratos Axis Ultra
DLD XPS with a monochromated Al K␣ X-ray source. Catalysts were
pretreated in H2 atmosphere under temperature in a pretreatment
cell directly connected to the XPS chamber. After pretreatment, the
samples were then cooled to room temperature and moved without
exposure to air into the ultra-high-vacuum chamber (10−9 mbar).
Peak fitting was achieved using CASA XPS software. Elemental sur-
face stoichiometries were obtained from peak area ratios corrected
by appropriate sensitivity.
2.3. Catalytic test
Succinic acid hydrogenation was performed in a Hastelloy Parr
4560 high pressure reactor of 300 mL equipped with an electrically
heated jacket, a turbine agitator with a magnetic driver, and a liq-
uid sample line. The reactor was loaded with 100 mL of a 5 wt.%
SUC aqueous solution (420 mmol L−1) and 1 g catalyst (molar ratio
SUC/Pd = 225). After purging with Ar, the reactor was heated to
160 ◦C. At 160 ◦C, hydrogen is added at a pressure of 150 bar to
start the reaction. The aqueous samples taken from the reactor at
regular intervals were analyzed using both gas chromatography
(HP-5 column, 30 m × 0.25 mm column, thickness 0.25 ␮m) and
a high performance liquid chromatography instrument equipped
with UV and RI detection (ICSep Coregel 107H column at 40 ◦C,
0.005 N H2SO4 as mobile phase at a flow rate of 0.5 mL min−1). The
main reaction products consisted of GBL, THF and BDO. By-products
analyzed in the liquid phase were n-butanol, n-propanol, butyric
acid, and propionic acid (Scheme 1). The mass balance was checked
by measuring Total Organic Carbon (TOC) in the liquid phase using
a Shimadzu TOC-VCSH analyzer.
3. Results and discussion
Table 1 gives the characteristics of the parent monometallic
2 wt.% Pd/TiO2 catalyst and of the two bimetallic Re-Pd/TiO2
samples prepared either by successive impregnation (SI) or by
catalytic reduction (CR). A preliminary study (not presented here)
was performed in order to determine the optimal Re content for
each bimetallic preparation method leading to the best catalytic
performances (i.e. the highest 1,4-butanediol yield) [32]. The
monometallic catalyst displayed a metal accessibility (determined
by hydrogen chemisorption) equal to 33%. On the bimetallic cata-
lysts, the hydrogen chemisorption occured only on the accessible
Pd atoms, the Re atoms not chemisorbing hydrogen under the
experimental conditions. After impregnation of 3.5 wt.%Re by SI,
the accessibility value decreased notably and reached a value of
6%, this fall was less drastic when 0.9 wt.%Re was deposited by CR
(H/M = 14%), which may be explained by the lower amount of Re.
The Pd accessibility decrease can result either from a coverage of
XPS analysis performed after a reduction treatment shows
that each Pd-based catalyst is composed exclusively of Pd0 in the
explored surface (depth around 3 nm), corresponding to a binding
Please cite this article in press as: B. Tapin, et al., Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective
hydrogenation of succinic acid in aqueous-phase, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.018

G Model
CATTOD-8909; No. of Pages7
ARTICLE IN PRESS
4
B. Tapin et al. / Catalysis Today xxx (2014) xxx–xxx
Fig. 1. Representative TEM images and distribution of the particle sizes for the catalysts: (a) Pd/TiO2, (b) 3.5 wt.%Re-Pd/TiO2 SI and (c) 0.9 wt.%Re-Pd/TiO2 CR. [SI: successive
impregnation method; CR: catalytic reduction method].
energy of 335 eV for the Pd 3d5/2 core level (Table 1). On bimetallic
samples, Re is present in Re0 and Re3+ states in the case of the SI
sample and exclusively in the Re3+ state for the CR one [36,37],
indicating that the reduction of the Re species is never complete.
XPS analysis of a monometallic 1.9 wt.%Re/TiO2 sample (prepared
by impregnation of the NH4ReO4 precursor salt in acidic medium
followed by a direct reduction under hydrogen at 450 ◦C) reveals
also that the reduction of the Re species initially introduced in a 7+
oxidation state is not complete (presence of 15% Re0 and 85% Re3+).
After a reduction step, different oxidation states for Re species were
already observed on various catalysts in the literature, according
notably to the nature of the support and then to the generated
metal-support interactions [38–41]. Here, the same TiO2 support
being used for all samples, the different reduction states for the
Re entities in the bimetallic systems can be linked to their specific
localization on the Pd/TiO2 parent catalyst. In the case of the CR
catalyst, the Pd/Ti atomic ratio remains equal to the value obtained
with the parent catalyst (Table 1, 0.036) suggesting that the Pd
surface is slightly modified by the Re addition by CR, in agreement
with a Re deposition on the Pd-support interface as proposed
Please cite this article in press as: B. Tapin, et al., Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective
hydrogenation of succinic acid in aqueous-phase, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.018

G Model
CATTOD-8909; No. of Pages7
ARTICLE IN PRESS
B. Tapin et al. / Catalysis Today xxx (2014) xxx–xxx
5
Scheme 2. Schematic representation of the Re localization on the Pd/TiO2 parti-
cles according to the preparation method of the bimetallic catalysts: SI, successive
impregnation method; CR, catalytic reduction method.
Fig. 2. TPR profiles of catalysts after oxidation at 300 ◦C: (a) 1.9 wt.%Re/TiO2, (b)
2 wt.%Pd/TiO2, (c) 0.9 wt.%Re-Pd/TiO2 CR and (d) 3.5 wt.%Re-Pd/TiO2 SI. [SI: succes-
sive impregnation method; CR: catalytic reduction method].
observed in Fig. 2d: one at ambient temperature corresponding to
the reduction of both Pd and Re species according to the hydrogen
consumption values (Table 2), and a second around 300 ◦C associ-
ated to the reduction of Re species isolated on the support as on the
monometallic Re sample. By correlation of the XPS and TPR results
obtained on the SI sample, it can be suggested that the reduction of
the Re species catalyzed by Pd (at Tamb) leads to Re3+ entities and
that of the isolated Re species is certainly complete until Re0 state.
To summarize, as illustrated in Scheme 2, the Re deposit seems
exclusively located in contact with Pd particles, at the Pd-support
interface, by CR method, and both in contact with Pd and on the sup-
port by SI protocol, confirming the existence of a controlled surface
redox reaction between the Pd parent catalyst and the Re precursor
salt in the first case. In both cases, Re species in interaction with Pd
are easily reduced, at nearly room temperature, but this reduction
is not complete, leading to Re3+ species. Only Re species deposited
on the support, only observed in the case of SI catalysts, can be
completely reduced to the metallic state, between 300 and 400 ◦C.
Fig. 3 compares the catalytic results concerning the suc-
cinic hydrogenation (SUC) in aqueous phase performed with the
different bimetallic catalysts prepared by SI and CR methods.
As described in a previous paper [29], the monometallic Pd-
based sample catalyses the SUC hydrogenation mainly towards
␥-butyrolactone (GBL) (Fig. 3a and b), the further hydrogena-
tion of GBL to 1,4-butanediol (BDO, intended product) remaining
extremely limited (Fig. 3c). With deposition of Re, compared to
the monometallic Pd sample, the formed GBL was further hydro-
genated to BDO and THF [32]. Introducing 0.9 wt.%Re by the CR
method involves an increase of the SUC conversion, since a total
conversion is observed before 25 h reaction time. A 53% BDO yield
was obtained after 48 h. The 3.5 wt.%Re-Pd/TiO2 catalyst prepared
by SI method was less active than the previous bimetallic sample
for the SUC conversion, however an improvement of the consec-
utive hydrogenation of the GBL in BDO was observed and a 67%
BDO yield was obtained after 48 h. All these catalysts led to less
10% of THF at the end of the reaction time (Fig. 3d), and very few
cracking products in solution (less than 5% of n-butanol and butyric
acid, traces of n-propanol and propionic acid). The formation of THF
in the course of diacid hydrogenation was previously observed in
studies involving for example alumina-silica support or Cu-Al-Zn-
O catalysts, and was attributed to acid sites on the catalyst surface
[48,49]. Compared to the other bimetallic catalyst prepared by SI,
the Re deposition by CR method occurs in an acid medium (HCl, pH
1), the surface of the as-obtained sample is certainly more acidic,
explaining the highest THF formation observed in this case. More-
over, the TOC analysis performed in the course of the reaction
showed a carbon balance almost total in aqueous solution for all
samples, indicating negligible formation of gaseous products.
above from H2 chemisorption results. On the contrary, a lower
Pd/Ti atomic ratio is observed on the SI sample accompanied by a
higher Re/Pd atomic ratio, in accordance with the more important
coverage of the Pd surface by Re on the SI bimetallic catalyst shown
by H2 chemisorption.
Fig. 2 compares the TPR profiles of the three catalysts (Pd/TiO2,
0.9 wt.%Re-Pd/TiO2 CR and 3.5 wt.%Re-Pd/TiO2 SI), with that of
the monometallic 1.9 wt.%Re/TiO2 sample. Re species deposited on
TiO2 support are reduced between 250 and 350 ◦C with a peak max-
imum at 300 ◦C (Fig. 2a), in agreement with the literature [42,43].
−1
This peak corresponds to a consumption of 2.3 × 10−4 mol gcata
(Table 2), whereas a complete reduction of Re7+ to Re0 needs
3.6 × 10−4 mol gcata−1, which proves that a complete reduction of
rhenium oxide is not possible during the TPR experiment per-
formed up to 700 ◦C, in accordance with the XPS results. On the
monometallic 2 wt.%Pd/TiO2 catalyst, the reduction of the palla-
dium oxide occurs from ambient temperature and is complete
around 40 ◦C (Fig. 2b). Pd oxides are known to be easily reduced
at ambient temperature on various kinds of supports (Al2O3, SiO2,
TiO2 and ZrO2) [44–46]. With Pd-based systems, hydrides can be
formed under hydrogen atmosphere, which are decomposed at
70–80 ◦C, leading to the desorption of hydrogen. However their for-
mation resulting from a mass phenomenon is limited in the case of
small metallic particles [47], which can explain the absence of neg-
ative peak (H2 desorption) on the TPR profile of the monometallic
catalyst. The bimetallic catalyst prepared by CR method (Fig. 2c)
displays a single reduction peak at ambient temperature as the
monometallic Pd sample, but with higher hydrogen consumption
−1
(Table 2: 3.8 × 10−4 mol × gcata against 2.7 × 10−4 mol × gcata−1).
Consequently, Re species are also reduced in this temperature range
via a Pd catalyzed process, highlighting the existence of an inter-
action between Pd and Re entities on this sample. According to
XPS results, this reduction is not complete, Re7+ yielding Re3+
.
In the case of the bimetallic SI catalyst, two reduction peaks are
Table 2
H2 consumption determined from TPR profiles for the monometallic Pd/TiO2 and
Re/TiO2, and bimetallic Re-Pd/TiO2 catalysts (Pd content = 2 wt.%).
−1
Catalyst/TiO2
nH2 (mol gcata
)
Temperature range
(◦C) 20–40
Temperature range
(◦C) 250–400
2 wt.%Pd
2.7 × 10−4
-
-
1.9 wt.%Re
2.3 × 10−4
-
0.9 wt.%Re-Pd CRa
3.5 wt.%Re-Pd SIa
3.8 × 10−4
4.1 × 10−4
1.5 × 10−4
a
SI, successive impregnation method; CR, catalytic reduction method.
Please cite this article in press as: B. Tapin, et al., Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective
hydrogenation of succinic acid in aqueous-phase, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.018

G Model
CATTOD-8909; No. of Pages7
ARTICLE IN PRESS
6
B. Tapin et al. / Catalysis Today xxx (2014) xxx–xxx
500
500
(b)
(a)
400
300
200
100
0
400
300
200
100
0
0
10
20
Time (h)
30
40
50
0
10
20
Time (h)
30
40
50
500
400
300
200
100
0
100
80
60
40
20
0
(c)
(d)
0
10
20
30
40
50
0
10
20
30
40
50
Time (h)
Time (h)
Fig. 3. (a) SUC conversion, (b) GBL, (c) BDO and (d) THF formation vs. time on catalysts: (ꢀ) 2 wt.%Pd/TiO2, (ꢀ) 3.5 wt.%Re-Pd/TiO2 SI, (ꢁ) 0.9 wt.%Re-Pd/TiO2 CR. [SI: successive
impregnation method; CR: catalytic reduction method].
In term of BDO yield obtained after 48 h reaction time, the one
obtained by CR (53%) is lower than that obtained by SI (67%),
remembering that the Re loading was optimized for each prepa-
ration technique. Thus, the method used for the Re deposition has
an impact on the catalytic performances since the behaviors of the
bimetallic samples (SUC conversion, products distribution) are not
strictly the same according to the preparation procedure. Lowest
deposited Re quantities are needed when the CR method is used,
which can be interesting to limit the cost of the material. This low-
est content of additive can be explained by the selective deposit
of Re species in contact with the Pd particles via the controlled
surface reaction, as revealed previously by characterization tech-
niques. Nevertheless, this preparation method implies a higher
acidity on the catalyst surface, involving a more important for-
mation of THF by-product and then a lowest BDO yield. The SI
preparation involving a non-controlled Re deposition, which occurs
on the Pd particles as well as on the TiO2 support as showed by TPR
analysis, requires consequently a higher Re content (3.5 wt.%) to
generate optimal catalytic performances, which are finally the best
ones in the present study in term of BDO yield at 48 h reaction time.
These results can be linked to the respective localization of the Re
and Pd species on the bimetallic catalysts surface together with the
Re oxidation states (various Re3+ and Re0 proportions, as observed
by XPS analysis according to the nature of the bimetallic sample).
introduced was different since it was previously shown that a high
Re content is necessary for obtaining a synergy between Pd and
Re for SI, necessary for the reaction chosen (SUC conversion to
BDO in aqueous phase at 160 ◦C under 150 bar hydrogen pressure),
whereas the optimum Re content for the CR method is 3–3.5 times
less important. Characterization techniques clearly demonstrated
the existence of interaction between the two metals, notably TPR
displayed for all bimetallic systems a hydrogen consumption peak
at room temperature corresponding to the simultaneous reduc-
tion of both metals, and then highlighting the presence of Re in
contact with Pd. Furthermore a decrease in hydrogen accessibility
was observed between the monometallic sample and the bimetal-
lic ones, due to the Re introduction and function of the deposit
method. On the SI bimetallic catalyst, Re was also observed in an
isolated form on the support. The Re oxidation state was demon-
strated to differ according to the localization of these species: in
contact with Pd, Re species were not completely reduced (Re3+
state), but isolated on the support, the reduction was complete (Re0
state).
Acknowledgements
This work was supported by the French Agence Nationale de
la Recherche within the Programme Chimie et Procédés pour le
Développement Durable CP2D 2009 (HCHAIB) and AXELERA.
4. Conclusions
References
Two different methods were used to add Re onto a parent
monometallic Pd/TiO2 catalyst, namely a classical impregnation
technique (SI method), leading to a random deposition, and a cat-
alytic reduction method (CR method), occurring between hydrogen
preadsorbed on Pd particles and Re precursor. The amount of Re
[1] J.H. Sinfelt, Bimetallic Catalysts: Discoveries, Concepts, and Applications, John
Wiley & Sons, New York, 1983.
[2] J. Massardier, J.C. Bertolini, J. Catal. 90 (1984) 358–361.
[3] M.P. Humbert, J.G. Chen, J. Catal. 257 (2008) 297–306.
[4] W. Yu, M.D. Porosoff, J.G. Chen, Chem. Rev. 112 (2012) 5780–5817.
Please cite this article in press as: B. Tapin, et al., Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective
hydrogenation of succinic acid in aqueous-phase, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.018

G Model
CATTOD-8909; No. of Pages7
ARTICLE IN PRESS
B. Tapin et al. / Catalysis Today xxx (2014) xxx–xxx
7
[5] T. Werpy, G. Petersen, Top value added chemicals from biomass volume I:
Results of screening for potential candidates from sugars and synthesis gas,
U.S. Department of Energy (DOE), 2004.
[30] A. Pintar, M. Besson, P. Gallezot, Appl. Catal. B 31 (2001) 275–290.
[31] M. Balaraju, V. Rekha, B.L.A. Prabhavathi Devi, R.B.N. Prasad, P.S. Sai Prasad, N.
Lingaiah, Appl. Catal. A 384 (2010) 107–114.
[6] J.J. Bozell, G.R. Petersen, Green Chem. 12 (2010) 539–554.
[7] H. Song, S.Y. Lee, Enzyme Microb. Technol. 39 (2006) 352–361.
[8] C.S.K. Lin, R. Luque, J.H. Clark, C. Webb, C. Du, Biofuels Bioprod. Biorg. 6 (2012)
88–104.
[32] B.K. Ly, D.P. Minh, C. Pinel, M. Besson, B. Tapin, F. Epron, C. Especel, Top. Catal.
55 (2012) 466–473.
[33] T.R. Hugues, R.J. Houston, R.P. Sieg, Ind. Eng. Chem. Proc. Des. Dev. 1 (2) (1962)
96–102.
[9] U.S. biobased products market potential and projections through 2025, United
States Department of Agriculture (USDA), 2005.
[34] A. Douidah, P. Marecot, S. Labruquere, J. Barbier, Appl. Catal. A 210 (2001)
111–120.
[10] Strategic Analysis of the Worldwide Market for Biorenewable Chemicals, Frost
& Sullivan Research Service, 2008.
[35] M.T. Schaal, J. Rebelli, H.M. McKerrow, C.T. Williams, J.R. Monnier, Appl. Catal.
A 382 (2010) 49–57.
[11] C. Delhomme, D. Weuster-Botz, F.E. Kuhn, Green Chem. 11 (2009) 13–26.
[12] L. Pu, L. Ye, Y. Yuanqi, J. Mol. Catal. A: Chem. 138 (1999) 129–133.
[13] Y. Hara, H. Kusaka, H. Inagaki, K. Takahashi, K. Wada, J. Catal. 194 (2000)
188–197.
[14] US 6958404 (2005), BASF Aktiengesellschaft (Germany).
[15] DE 2005058853 (2004), BASF Aktiengesellschaft (Germany).
[16] US 5969164 (1998), The Standard Oil Company (US).
[17] US 5347021 (1994), ISP Investments Inc. (US).
[36] G. Beamson, A.J. Papworth, C. Philipps, A.M. Smith, R. Whyman, J. Catal. 278
(2011) 228–238.
[37] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photo-
electron Spectroscopy, Perkin-Elmen Corp., 1992, p. 205.
[38] E.S. Shpiro, V.I. Avaev, G.V. Antoshin, M.A. Ryashentseva, K.M. Minachev, J. Catal.
55 (1978) 402–406.
[39] X. Yide, W. Xinguang, S. Yingzhen, Z. Yihua, G. Xiexian, J. Mol. Catal. 36 (1986)
79–89.
[18] US 5037996 (1991), Tonen Corporation (Japan).
[40] P. Arnoldy, E.M. van Oers, O.S.L. Bruinsma, V.H.J. de Beer, J.A. Moulijn, J. Catal.
93 (1985) 231–245.
[41] M.A. Vuurman, D.J. Stufkens, A. Oskam, I.E. Wachs, J. Mol. Catal. 76 (1992)
263–285.
[19] U. Herrmann, G. Emig, Ind. Eng. Chem. Res. 36 (1997) 2885–2896.
[20] S.M. Jung, E. Godard, S.Y. Jung, K. Park, J.U. Choi, J. Mol. Catal. A: Chem. 198
(2003) 297–302.
[21] US 5478952 (1995), E.I. Dupont de Nemours and Company (US).
[22] R.V. Chaudhari, C.V. Rode, R.M. Deshpande, R. Jaganathan, T.M. Leib, P.L. Mills,
Chem. Eng. Sci. 58 (2003) 627–632.
[23] US 6008384 (1999), E.I.Dupont de Nemours and Company (US).
[24] D.P. Minh, M. Besson, C. Pinel, P. Fuertes, C. Petitjean, Top. Catal. 53 (2010)
1270–1273.
[42] S.R. Bare, S.D. Kelly, F.D. Vila, E. Boldingh, E. Karapetrova, J. Kas, G.E. Mickelson,
F.S. Modica, N. Yang, J.J. Rehr, J. Phys. Chem. C 115 (2011) 5740–5755.
[43] B. Mitra, X. Gao, I.E. Wachs, A.M. Hirt, G. Deo, Phys. Chem. Chem. Phys. 3 (2001)
1144–1152.
[44] W.J. Shen, M. Okumura, Y. Matsumura, M. Haruta, Appl. Catal. A: Gen. 213
(2001) 225–232.
[25] US 5698749 (1997), The Standard Oil Company (US).
[26] EP 1232789 (2000), Asahi Kasei Kabushiki Kaisha (Japan).
[27] US 2003/0050516 (2001), Keil & Weinkauf (US).
[45] T. Garcia, B. Solsona, S.H. Taylor, Catal. Lett. 97 (2004) 99–103.
[46] J. Batista, A. Pintar, D. Mandrino, M. Jenko, V. Martin, Appl. Catal. A: Gen. 206
(2001) 113–124.
[28] WO 2011/123269 A1 (2011), Bioamber S.A.S (Canada).
[29] B. Tapin, F. Epron, C. Especel, B.K. Ly, C. Pinel, M. Besson, ACS Catal. 3 (2013)
2327–2335.
[47] M. Boudart, H.S. Hwang, J. Catal. 39 (1975) 44–52.
[48] S. Varadarajan, D.J. Miller, Biotechnol. Prog. 15 (1999) 845–854.
[49] Q. Zhang, Z. Wu, L. Xu, Ind. Eng. Chem. Res. 37 (1998) 3525–3532.
Please cite this article in press as: B. Tapin, et al., Influence of the Re introduction method onto Pd/TiO2 catalysts for the selective
hydrogenation of succinic acid in aqueous-phase, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.018