Partial hydrogenation of 3-hexyne over low-loaded palladium mono and bimetallic catalysts

Article abstract of DOI:10.1016/j.apcata.2012.07.016

Partial hydrogenation of alkynes has industrial and academic relevance on a large scale, especially those with high selectivities. To increase the activity, selectivity and lifetime of low-loaded Pd monometallic catalyst, the development of bimetallic systems has been investigated. In this work Pd mono (Pd/A) and bimetallic catalysts (PdNi/A and WPd/A) supported on γ-alumina with low metal content are prepared and evaluated during the partial hydrogenation of 3-hexyne at mild conditions. XPS, XRD, TPR and hydrogen chemisorption techniques are used for the characterization, suggesting different kind of Pd species on the mono and bimetallic catalysts. Furthermore, XPS results indicate the presence of electron rich and electron-deficient palladium species (Pd^δ- and Pdⁿ⁺, with δ close to 0 and 0 < n < 2, respectively), electron-deficient species of nickel (Ni ⁿ⁺, with n close to 2) and of W^ρ+ (with ρ < 6) on the surface of the bimetallic systems. The Lindlar commercial catalyst, commonly used in these types of reactions is used for comparative purposes. Bimetallic catalysts showed higher activities and very similar selectivities (>93%) than the monometallic system and further than the Lindlar catalyst. The rank of activity order is: WPd/A > PdNi/A > Pd/A Lindlar.

Full text of DOI:10.1016/j.apcata.2012.07.016

Applied Catalysis A: General 441–442 (2012) 90–98
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Partial hydrogenation of 3-hexyne over low-loaded palladium mono and
bimetallic catalysts
M. Juliana Maccarrone , Cecilia R. Lederhos , Gerardo Torres , Carolina Bettia,
a
a
a,b
c
a,b,∗
a,b
Fernando Coloma-Pascual , Mónica E. Quiroga
INCAPE, Instituto de Investigaciones en Catálisis y Petroquímica (FIQ-UNL, CONICET), Santiago del Estero 2654, S3000AOJ Santa Fe, Argentina
Facultad de Ingeniería Química – Universidad Nacional del Litoral, Santiago del Estero 2829, S3000AOJ Santa Fe, Argentina
Facultad de Ciencias, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain
, Juan C. Yori
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2012
Received in revised form 12 July 2012
Accepted 14 July 2012
Available online 20 July 2012
Partial hydrogenation of alkynes has industrial and academic relevance on a large scale, especially those
with high selectivities. To increase the activity, selectivity and lifetime of low-loaded Pd monometallic
catalyst, the development of bimetallic systems has been investigated. In this work Pd mono (Pd/A) and
bimetallic catalysts (PdNi/A and WPd/A) supported on ␥-alumina with low metal content are prepared
and evaluated during the partial hydrogenation of 3-hexyne at mild conditions. XPS, XRD, TPR and hydro-
gen chemisorption techniques are used for the characterization, suggesting different kind of Pd species
on the mono and bimetallic catalysts. Furthermore, XPS results indicate the presence of electron rich
Keywords:
Bimetallic catalysts
Alkyne
Selective hydrogenation
Lindlar
ı−
n+
and electron-deficient palladium species (Pd and Pd , with ı close to 0 and 0 < n < 2, respectively),
n+
ꢀ+
electron-deficient species of nickel (Ni , with n close to 2) and of W (with ꢀ < 6) on the surface of
the bimetallic systems. The Lindlar commercial catalyst, commonly used in these types of reactions is
used for comparative purposes. Bimetallic catalysts showed higher activities and very similar selectivities
(>93%) than the monometallic system and further than the Lindlar catalyst. The rank of activity order is:
WPd/A > PdNi/A > Pd/A ꢀ Lindlar.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
Up to now, practically all the studies were concentrated on the
reaction of short chain terminal alkynes. There is however only
scarce reports that deal with the selective hydrogenation of long
chain non terminal alkynes [2–4]. In particular, partial hydrogena-
tion reaction of 3-hexyne has been relatively little studied in the
open literature and generally using supported monometallic or
complexes as catalysts [2,5–7]. Moreover, information available in
the literature is conclusive regarding the use of high loaded Pd cat-
alyst and low reaction temperatures to obtain the (Z)-stereoisomer
alkene as the main product. For example, for the hydrogenation of
3-hexyne over Pd/C (1 wt% of Pd) at 298 K Mastalir et al. [2] found a
decrease of the selectivity to the (Z)-isomer with an increase of the
substrate/Pd molar ratio, and reported values of selectivity to the
(Z)-isomer > 98%, to (E)-isomer < 1.4% and to n-hexane < 0.5%. Sim-
ilar product distribution was informed for Pd/SBA-15, Pd/MCM-48
The design of highly selective catalytic systems for which there
is no need to remove undesirable products clearly falls within the
postulates of the so-called “Green Chemistry”. Thus, in the produc-
tion of fine chemicals where obtaining a product involves a series
of complex reactions steps, is essential to achieve greater selectiv-
ity in each step. An interesting case is the use of alkynes as raw
material for several processes of fine chemicals. The main attrac-
tion of the use of alkynes is attributed to its ability to form new C C
bonds via alkylation. In particular, the stereoselective hydrogena-
tion of non terminal alkynes to give alkenes without the occurrence
of geometric isomerization is very useful in these processes. When
the isomerization reaction is not carried out, the (Z)-alkene (desired
product) is mainly formed [1]. Furthermore, as in the case of termi-
nal alkynes, it is also necessary that the catalyst does not promote
an over hydrogenation forming the corresponding alkane.
and Pd/MSU-Al O catalysts (1 wt% of Pd) for different substrate/Pd
2
3
molar ratios by Alvez-Manoli et al. [5]. Furthermore, Marín-Astorga
et al. [8] reported high selectivity to (Z)-alkene isomer during
hydrogenation of aromatic alkynes over 1 wt% Pd catalysts. In turn,
Papp et al. [9] also reported high selectivity to (Z)-alkene form dur-
ing partial hydrogenation reactions of alkynes of high molar mass
∗ Corresponding author at: INCAPE, Instituto de Investigaciones en Catálisis y
Petroquímica (FIQ-UNL, CONICET), Santiago del Estero 2654, S3000AOJ Santa Fe,
Argentina. Fax: +54 342 453 1068.
(3-butyn-1-ol, phenylacetylene, 4-octyne and 1-phenyl-1-butyne)
E-mail address: mquiroga@fiq.unl.edu.ar (M.E. Quiroga).
over Pd/MCM-41 (1.4 and 5.8 wt% of Pd) catalysts.
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.07.016

M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
91
The literature is consistent in that if the (Z)-isomer of the alkene
is the desired product, it is convenient to use a supported Pd catalyst
in order to obtain high activity and selectivity. However, consid-
ering that palladium is an expensive metal and high loads are
apparently necessary, the final cost of the catalyst for alkyne partial
hydrogenation processes is very high. So it is a challenge to explore
the feasibility of using lower Pd loadings or cheaper metals as cat-
alysts for the hydrogenation of non terminal alkyne of high molar
mass.
Before its catalytic evaluation, the catalysts were reduced in a
−
1
hydrogen stream (105 mL min ) during 1 h at the optimum reduc-
tion temperature found earlier: Pd/A at 573 K, PdNi/A at 673 K and
WPd/A at 393 K [14,15]. All the catalysts were immediatelly used
in the tests reaction after the reduction treatment.
The commercial Lindlar catalyst, 5 wt% of Pd on calcium car-
bonate and poisoned with lead acetate, was provided by Aldrich
(Cat. No. 20,573-7) and was used without any pretreatment for
comparative purposes as suggested by other authors for alkyne
hydrogenation reactions [16].
Another exciting possibility is to analyze the effect of the addi-
tion of another metal to Pd, studying its influence on activity
and product selectivity. The addition of a second metal to Pd
monometallic catalysts has also been found to improve the catalytic
properties in several reactions. These so-called bimetallic catalysts
have improved activity, selectivity and stability. Many authors have
reported the effect of this second metal as Ni, Ag, Cu, Au, Ge, Sn, Pb,
etc., using several supports like ␥-Al O , zeolites, silica, etc. on the
2.2. Catalyst characterization
Palladium, tungsten and nickel contents were determined by
inductively coupled plasma analysis (ICP) in an OPTIMA 2100
Perkin Elmer equipment.
The hydrogen chemisorption of the metal particles on the cata-
lysts surface was measured at 303 K in a Micromeritics AutoChem
II 2920 instrument equipped with a thermal conductivity detec-
tor (TCD). 0.2 g of the samples was reduced in situ in a 5% (v/v)
hydrogen/argon stream during 1 h at the same reduction temper-
ature used before each catalytic test. This methodology ensures
that the results of chemisorption are not masked by the formation
of ␤-PdH phase. The samples were degassed in situ in an argon
flow (AGA, 99.99%) and cooled up to 303 K. Then the hydrogen
uptake was measured by sending calibrated pulses. The metal dis-
persion was calculated assuming that: (i) in bimetallic catalysts
only Pd chemisorbs hydrogen at room temperature, nor W neither
Ni [14,15] and (ii) a chemisorption stoichiometry H:Pd = 1 [17].
The electronic states of surface species were determined by X-
ray photoelectron spectroscopy (XPS) in a VG-Microtech Multilab
equipment, equipped with a Mg K␣ (hꢁ: 1253.6 eV) radiation and a
pass energy of 50 eV. The analysis pressure during data acquisition
2
3
performance of palladium-based bimetallic catalysts [9–12]. The
most clear example studied since 1954 is the well-known Lindlar
catalyst, Pd(5%)/CaCO3 modified with lead, used for the selective
hydrogenation of alkynes [13].
In previous contributions of the research group, different low-
loaded Pd (W–Pd and Pd–Ni) bimetallic catalysts supported on
␥-alumina, were tested for the partial hydrogenation of 1-heptyne,
a high molecular weight terminal alkyne [14,15]. It was found that
both bimetallic catalysts were more active than the Pd monometal-
lic catalyst presenting high selectivity to 1-heptene (>95%). Pd–Ni
catalysts were however less active than the commercial Lindlar
catalyst, while bimetallic PdW/Al O3 or WPd/Al O3 were more
2
2
active.
Based on the above considerations the objectives of this work
are: (a) to prepare and characterize catalysts of low Pd content,
both monometallic and bimetallic (W–Pd and Pd–Ni) catalysts; (b)
to evaluate the activity and selectivity of catalysts during 3-hexyne
partial hydrogenation at mild reaction conditions; (c) to evaluate
the effect of reaction temperature; and (d) to compare the catalytic
performance of these catalysts with commercial Lindlar catalyst.
−
7
was 5 × 10 Pa. Samples were treated in situ in the presence of
a H stream following the same pretreatment conditions for each
2
catalyst. A careful deconvolution of the spectra was made and the
areas under the peaks were estimated by calculating the integral
of each peak after subtracting a Shirley background and fitting the
experimental peak to a combination of Lorentzian/Gaussian lines
of 30–70% proportions. The reference binding energy (BE) was C 1 s
peak at 284.6 eV.
2
. Experimental
2
.1. Catalyst preparation
X-ray diffraction (XRD) measurements of powdered samples
were obtained using Shimadzu XD-D1 instrument with a Cu K␣
◦
␥
-Al O Ketjen CK 300 previously calcined in air at 823 K to
radiation (ꢂ = 1.5405 A˚ ) in the 21 < 2ꢃ < 49 at a scan speed of
2
3
2
−1
−1
stabilize its texture (SBET: 180 m g ) was used as support. Mono
and bimetallic palladium catalysts supported over alumina were
prepared by the incipient wetness technique. Volume and concen-
tration of the impregnating solutions were adjusted to get 0.4 wt%
of Pd, 2.4 wt% of W and 0.9 wt% of Ni, on the final mono and bimetal-
lic catalysts.
0.25 min . Samples were powdered and reduced under a hydro-
gen flow, the samples were put into the chamber of the equipment
and then data acquisition was made.
The reducibility of the surface species in all the catalysts were
determined by temperature programmed reduction (TPR), using a
Micromeritics AutoChem II instrument equipped with a TCD. Sam-
ples were pretreated under an oxygen flow stream at 673 K during
30 min, and then they were cooled down up to room temperature
under an argon flow. The TPR profiles were obtained increasing
An aqueous acid solution of Pd(NO ) (Fluka, Cat. No. 76070) at
3
2
pH = 1 (with HNO ) was used to prepare the palladium monometal-
3
lic catalyst (sample Pd/A). After impregnation catalyst was dried
overnight at 373 K and calcined in an air flow at 823 K during 3 h.
On the other hand, part of the Pd/A catalyst batch was impreg-
nated with an aqueous acid solution of Ni(NO ) ·6H O (Fluka,
−
1
the temperature up to 1223 K at 10 K min in a 5% (v/v) hydro-
gen/argon stream.
3
2
2
Cat. No. 72253) to prepare the bimetallic catalyst, labeled PdNi/A,
and then was dried and calcined in the same way and conditions
described for the monometallic Pd/A catalyst.
2.3. Catalytic evaluations
Catalytic reaction tests were carried out in a stainless steel
stirred tank batch reactor equipped with a magnetically coupled
stirrer with two blades in counter-rotation that was operated at
800 rpm. The inner wall of the reactor was completely coated with
PTFE in order to prevent the catalytic action of steel reported by
other authors [18]. The reactant 3-hexyne (Fluka, Cat. No. 51950,
purity > 98%) was dissolved to a 2% (v/v) in toluene (Merck, Cat.
No. TX0735-44, purity > 99%). The tests were performed using the
WPd/A bimetallic catalyst was prepared by successive impreg-
nations using an acid solution of H PO ·12WO (Fluka, Cat. No.
3
4
3
7
9690), dried overnight at 373 K and calcined in an air flow at 823 K
during 3 h. This calcination temperature used assures the complete
elimination of phosphorus from the catalysts [15]. Then, sample
was impregnated with an acid aqueous solution of Pd(NO ) (pH = 1
3
2
with HNO ), dried overnight and calcined.
3

9
2
M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
◦
presented the typical signals of the gamma phase of alumina: 37.7 ,
following conditions: 1.4 bar of hydrogen pressure, between 273
and 323 K reaction temperatures, 60–120 mesh particle size, during
◦
◦
45.98 and 66.98 [21,22]. Because of the low amount of Pd on the
catalysts (ca. 0.4 wt%), well below the detection limit of this tech-
nique, the presence of Pd crystallites is undetectable. For bimetallic
0–200 min. The 3-hexyne/Pd molar ratio used was constant for all
the prepared catalysts: S/Pd = 14,360. The commercial Lindlar cat-
alyst was used as a reference at identical operational conditions of
temperature, pressure and initial concentration of 3-hexyne, with
S/Pd = 1115.
◦
◦
WPd/A catalyst, the absence of peaks in the region 20 < 2ꢃ < 30
indicates that there is no tungsten oxide present as a crystalline
segregated phase. Segregated monoclinic WO3 crystallites can be
detected by XRD for W surface concentrations higher than the
With the object to eliminate external diffusional limitations,
experiences were carried out using different stirring speeds in the
range of 180–1400 rpm. It was found that at stirring rates higher
than 500 rpm, 3-hexyne conversion values remained constant, indi-
cating that external gas–liquid limitations were absent. A stirring
rate of 800 rpm was therefore chosen for all the catalytic evalua-
tions. On the other hand and in order to ensure that the catalytic
results were not influenced by external and intraparticle mass
transfer limitations, the catalyst pellets were milled to samples of
different particle size: a fraction bigger than 100 mesh (<150 ␮m),
a fraction of 60–100 mesh (250–150 ␮m) and pellets of 1500 ␮m
−
2
corresponding surface saturation value (4.3 W atoms nm ) [23].
◦
◦
◦
◦
Similarly, the absence of peaks at 2ꢃ = 30 , 43.3 , 63.0 , 75.5 and
79.5 in bimetallic PdNi/A catalyst indicates that there is no nickel
◦
oxide present as a crystalline segregated phase [24].
Fig. 2 contains the TPR traces of the Pd/A, PdNi/A and WPd/A
catalysts. The TPR trace of the Pd monometallic catalyst has a main
hydrogen consumption peak at a low temperature (287 K), which
can be assigned to the presence of bulk PdO particles that are easily
reduced. These particles are agglomerated over the support during
the calcination of the catalyst as it has been extensively reported in
the related literatures [25–27]. The reduction profile of Pd/A also
shows a negative peak at about 339 K that is supposedly due to the
(
not milled). The obtained values of 3-hexyne conversion were the
same for the two milled fractions indicating the absence of inter-
nal diffusional limitations. Then particles with sizes smaller than
issue of H during the decomposition of the ␤-phase of palladium
2
2
50 ␮m were used in all the tests.
Catalytic evaluations were carried out in duplicates within an
hydrides (␤-HPd) that is formed during the reduction of the PdO
particles at low temperatures [17,25–27]. These species interact
weakly with the support and therefore Pd can be easily reduced.
Reduction can indeed proceed completely at room temperature in
a hydrogen atmosphere [28].
experimental error <5%. Reactants and products were analyzed by
gas chromatography in a Shimadzu GC-2014 equipment using a
flame ionization detector and a GS-GAS PRO capillary column (60 m
long, 0.32 mm ID). In order to identify each possible reaction prod-
uct, pure chromatographic patrons of: 3-hexyne, (Z)-3-hexene,
Furthermore, up to 500 K the reduction profiles bimetallic
of PdNi/A and WPd/A shown in Fig. 2, are very similar to the
monometallic Pd/A one, presenting a main peak at 296 and 305 K,
respectively, corresponding to PdO reduction and to the forma-
tion of the ␤-PdH phase. The decomposition peak for the ␤-PdH
phase is also present at 307 and 332 K, respectively for PdNi/A and
WPd/A. Comparing these profiles with that obtained for Pd/A, in
the bimetallics PdNi/A and WPd/A there is a shift of PdO reduction
peak to higher temperatures (9 and 18 K, respectively), indicat-
ing that the presence of Ni or W modifies the reducibility of Pd.
A shift to higher temperatures in general may be related to an
interaction or electron transfer between Pd and Ni or Pd and W.
Besides, there is shift to lower temperatures of the ␤-PdH phase
decomposition peak (−32 and −7 K respectively for PdNi/A and
WPd/A) suggesting that the decomposition of this phase is more
easily accomplished when nickel or tungsten are interacting with
palladium. These shifts to lower temperatures are indicative of dif-
ferent active sites on the catalysts, suggesting an electronic effect
(
E)-3-hexene, (Z)-2-hexene, (E)-2-hexene and n-hexane, were pre-
viously injected according with the reaction network presented in
Fig. 1 proposed by Alvez-Manoli et al. [5].
3
. Results and discussion
3.1. Catalyst characterization
ICP chemical analysis confirmed the metal loading of each cata-
lyst; these values are shown in Table 1. Dispersions of the prepared
catalysts, calculated from the hydrogen chemisorption values, are
also included in Table 1. As it was said earlier for the bimetallic
catalysts we considered that only the palladium sites were respon-
sible for the hydrogen chemisorption, because W and Ni do not
chemisorb H at room temperature. As expected the dispersion of
2
the bimetallic catalysts are lower than that of the monometallic
catalyst Pd/A. Also, as a consequence of the order of impregnation
of the precursors, PdNi/A catalyst presents the lowest dispersion,
indicating the biggest particle sizes The lowest dispersion of PdNi/A
could be due to an effect of decoration of Pd particles by Ni species
between Pd–Ni–Al O3 or Pd–W–Al O . Furthermore, as shown in
2
2
3
Fig. 2 the bimetallic catalyst PdNi/A has a peak at 621 K, due to
the reduction of NiO species with weak metal-support interaction
[
29–33], and a broad peak with a maximum at 1000 K which is
attributed to the reduction of nickel aluminates, NiAl O , showing
(
geometrical effect) [19,20]. The possible decoration of Pd particles
2
4
a strong metal-support interaction [24,30,34]. On the other hand,
the high temperature peak (>950 K) of the WPd/A trace is similar
to that found in the case of the tungsten monometallic catalyst at
high temperatures [20,35] and would therefore be related to the
by WOx and NiOx species is an example of metal-support interac-
tion related to the mobility of support surface species during heat
treatments, especially at high temperatures. The difractograms of
all the catalysts obtained by XRD measurements (not shown), only
Table 1
Metal loading, dispersion (D) XPS data of BE and atomic ratios, degree of reduction of palladium (Pdred/Pdtotal).
Catalysts Metal loading
wt%)
D (%)
XPS
Pdred/Pdtotal (%)
(
Pd
W
Ni
Pd 3d5/2
W 4f7/2
35.0
Ni 2p3/2
856.6
Pd/Al W/Pd Ni/Pd Pb/Pd
ı−
Pdı+
Pdn+
Pd
Pd
Pd/A
0.35
0.37
0.37
5
60
334.9
0.015
99.0
75.0
52.0
–
PdNi/A
WPd/A
Lindlar
0.89 18
33
334.2(54%)
334.5(92%)
335.3 (46%)
335.2 (69%)
0.003
1.3
2.37
336.2 (8%)
336.9 (31%)
0.007
1.2
a
0.7
a
Pd/Ca = 0.243.

M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
93
CH3
H C
3
(
Z)-2-hexene
H C
H3C
H3C
3
CH3
CH3
CH3
3-hexyne
(Z)-3-hexene
n-hexane
H C
3
CH3
(
E)-3-hexene
CH3
H C
3
(
E)-2-hexeno
Fig. 1. Scheme of the 3-hexyne reversible hydrogenation reactions.
reduction of amorphous WOx species. Furthermore, the reduction
temperature of oxidized W species is not altered by the presence
of Pd, indicating that the reducibility of these superficial species
remains very low.
The reduction temperature adopted for palladium mono and
bimetallic catalysts (>350 K) during the synthesis of the catalysts
ensures the elimination of the ␤-PdH phase. It must be noted that
according to the TPR tests, Pd on the mono and bimetallic catalysts
(54%, at/at) and 335.3 eV (46%, at/at). According to the literature,
ı−
these values can be assigned to Pd
electron-rich species and
ı+
slightly electron-deficient palladium species Pd (with ı close to
0), respectively [36]. The former could be attributed to the forma-
tion of metallic bonds or alloy, occurring at low temperatures [37].
Additionally in Fig. 4, for PdNi/A catalyst, the BE of Ni 2p3/2 peaks
appear at 856.6 eV and at 862.9 eV, which are attributed to electron-
n+
deficient species (Ni , with n close to 2) and to the shake-up
(
Pd/A, PdNi/A or WPd/A) is, at least in part, in the Pd metal state
after the reduction treatment used before the catalytic tests.
The degrees of reduction of the mono and bimetallic catalysts
(
Pdred/Pdtotal, reduced metal amount to total metal amount ratio)
were determined by integration of the TPR trace, using a sto-
Pd/A
2+
ichiometry of reduction of one (H /Pd = 1), and are shown in
2
Table 1. These values indicate that Pd is mostly reduced on the
monometallic Pd/A catalyst (99%) while in PdNi/A or in WPd/A are
only considerably and moderately reduced: 74% and 52%, respec-
tively, suggesting the presence of quite strong Pd–Ni or Pd–W
intermetallic interaction in the catalysts; however, the interaction
of Pd and Ni with the support or Pd and W with alumina cannot be
neglected.
WPd/A
PdNi/A
Figs. 3–5 show the XPS spectra of: Pd/A, PdNi/A and WPd/A cata-
lysts. The maximum binding energy (BE) of Pd 3d5/2, W 4f7/2 and Ni
2
p3/2 peaks and Pd/Al, W/Pd and Ni/Pd atomic ratios are also listed
in Table 1.
As shown in Fig. 3, the Pd 3d5/2 spectrum of Pd/A catalyst has
200
400
600
800
1000
a peak at 334.9 eV that corresponds to Pd [36]. In the case of the
PdNi/A bimetallic catalyst after the deconvolution of the Pd 3d5/2
spectrum shown in Fig. 4, two signals can be seen at 334.2 eV
Temperature (K)
Fig. 2. TPR traces of Pd/A, WPd/A and NiPd/A samples.

9
4
M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
3
3
2
2
1
1
5000
0000
5000
0000
5000
0000
6
5
4
3
2
1
000
000
000
000
000
000
0
WPd/A
Pd
5000
-
-
1000
2000
0
3
22 324 326 328 330 332 334 336 338 340 342 344 346
325
330
335
340
345
Binding Energy (eV)
Binding Energy (eV)
2
2
1
1
5000
0000
5000
0000
WPd/A
W
Fig. 3. XPS spectra of the Pd 3d572 region for Pd/A catalyst.
structure of Ni(II). Nin+ electron-deficient species probably corre-
sponds to different interactions between Ni–Pd or Ni–Al (from the
support) [38], or to the formation of intermediate Pd–Ni–Al O sur-
face species [14]. Figs. 3–5 are also shown for Pd, the 3d3/2 peak
doublet shifted 5.2 eV respect the 3d5/2 peak.
2
3
Fig. 5 shows the XPS spectra of Pd 3d5/2 and W 4f7/2 for bimetallic
WPd/A catalyst. The deconvolution of the Pd 3d5/2 spectrum shown
in Fig. 5 displayed two peaks at 334.5 (92%, at/at) and 336.2 eV (8%,
5
000
0
1
1
1
4000
2000
0000
1
8
20 22 24 26 28 30 32 34 36 38 40 42
Pd
PdNi/A
Binding Energy (eV)
Fig. 5. XPS spectra of WPd/A catalyst for: Pd 3d5/2 and W 4f7/2 regions.
8
6
4
2
000
000
000
000
0
ı−
at/at) that can be attributed to slightly electron-rich Pd species
(
n+
with ı close to 0) and to electron-deficient Pd species (with n
close to 2), respectively [36]. The spectrum of W 4f7/2 in Fig. 5
presents a peak at 35.0 eV suggesting the presence of electron-
deficient species, W (with ꢀ < 6) [36]. According to a previous
work, this value of BE indicates that tungsten is present on the alu-
mina surface of WPd/A catalyst, with a lower value of BE than on
the tungsten monometallic catalyst [15,39]. This pattern and the
ꢀ
+
ı−
presence of Pd electron-rich species can be explained due to the
3
24 326 328 330 332 334 336 338 340 342 344 346
formation of metallic Pd-W bonds or alloys; however this transfer
might be due to the formation of intermediate Pd–W–Al O surface
Binding Energy (eV)
2
3
species [19].
2
1
1
1
1
0000
For the Lindlar catalyst, in Fig. 6 are presented the Pd 3d5/2 and
Pb 4f7/2 spectra. The deconvolution of the Pd 3d5/2 signal indicates
two peaks at 335.2 eV (69%, at/at) and 336.9 eV (31%, at/at), assigned
PdNi/A
Ni
7500
5000
2500
0000
ı±
n+
to Pd and electro-deficient Pd species (with ı close to 0 and n
close to 2), respectively [36]. On the other hand, the deconvolution
of Pb 4f7/2 spectrum shows two peaks at 136.8 eV (20%, at/at) and
1
38.6 (80%, at/at), attributed to Pb and Pb(OAc) , respectively.
2
7
5
2
500
000
500
0
3
.2. Catalytic evaluations
During the catalytic tests the only reaction products observed
were (Z)-3-hexene, (E)-3-hexene and n-hexane, the presence of iso-
mers (Z)-2-hexene and (E)-2-hexene were not detected. The results
obtained for the partial hydrogenation of the internal aliphatic
alkyne, 3-hexyne, over Pd/A, PdNi/A, WPd/A and Lindlar catalysts
at 303 K are depicted in Fig. 7. In the Figure are plotted the evolu-
tion of the concentration of 3-hexyne, (Z)-3-hexene, (E)-3-hexene
and n-hexane as a function of reaction time.
-
2500
8
50 852 854 856 858 860 862 864 866 868 870 872 874 876
Binding Energy (eV)
Fig. 4. XPS spectra of PdNi/A catalyst for: Pd 3d5/2 and Ni 2p3/2 regions.

M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
95
2
2
1
1
50000
00000
50000
00000
temperature is 323 K because higher total conversions of 3-hexyne
are obtained with high selectivity to (Z)-3-hexene (≥93%).
For the purpose of comparing the performances of the catalysts,
in Table 2 are presented TOF values obtained for the prepared cata-
lysts at 303 K. The same were calculated using initial reaction rates
of 3-hexyne calculated from the data plotted in Fig. 8 and disper-
sion values indicated in Table 1. Initial reaction rate of 3-hexyne
was calculated using the following formula:
Lindlar
Pd
ꢀ
ꢁ
∂ XA
0
A
V × CA0
Wcat
5
0000
0
r
=
×
(1)
∂ t
t=0
0
A
−1
Pd
−1
min ];
where r is the initial reaction rate of 3-hexyne [mol g
(
∂ X /∂ t)
the tangent value of the 3-hexyne total conversion
A
t=0
3
26 328 330 332 334 336 338 340 342 344 346 348 350
0
vs. time curve at t = 0; C the initial concentration of 3-hexyne
[mol L 1]; Wcat the mass of catalyst [g]; V the reaction volume [L]
and t is the reaction time [min].
A
−
Binding Energy (eV)
3
2
2
1
1
00000
50000
00000
50000
00000
TOF values are useful in order to predict possible changes in the
nature of the active sites, in this case due to Ni or W interactions
with Pd. The calculated TOF values shown in Table 2, indicate the
following activity order: WPd/A ∼ PdNi/A ≥ Pd/A. So, the obtained
TOF values confirm that different active sites are present on the
prepared catalysts. Table 2 also shows product selectivity values
for the catalysts at different reaction temperatures. The selectiv-
ities are the average values obtained in each experiment while
there is 3-hexyne in the reacting media. In all the cases the (Z)-
alkene stereoisomer is obtained as the main product with a very
high selectivity (>93%) and remains approximately constant with
temperature, in contrast with results obtained with nickel boride
catalyst or homogeneous Pd(II) and Rh(I) complexes [6,7,40]. These
values are comparable to those obtained with the commercial Lind-
lar catalyst. Apparently, this pattern of product selectivities shown
in Table 2 is independent of the type of support used (low acidic
Al O for the prepared catalysts, or basic characteristics of CaCO3
Pb
Lindlar
5
0000
0
1
30 132 134 136 138 140 142 144 146 148 150 152 154
Binding Energy (eV)
2
3
Fig. 6. XPS spectra of Lindlar catalyst for: Pd 3d5/2 and Pb 4f7/2 regions.
for the Lindlar catalyst) and even of the S/Pd (substrate molecules
that react per Pd atom in the catalyst) used the prepared or the
Lindlar catalysts during the catalytic tests, in total accordance with
Alvez-Manoli et al. [5]. It is well known that in the case of alkyne
hydrogenation reactions over palladium supported catalysts, the
␤-hydride phase may act as a hydrogen source that promotes over
hydrogenation, with a consequent decrease in the selectivity to
alkene formation [41]. Additionally, Coq and Figueras [42], after
analyzing a wide number of papers on the matter of palladium
bimetallic catalysts, concluded that the disappearance of the ␤-
PdH phase plays an important role on the activity and selectivity,
in the sense of decreasing the alkyne hydrogenation rate to alka-
nes, and thus increasing the selectivity to alkene formation. For the
prepared mono and bimetallic catalysts the palladium ␤-hydride
phase is not present as it is proved by the TPR profiles at the pre-
treatment reduction temperature adopted. However the existence
of steric constraints might not be discarded. Ulan et al. [43] reported
that morphological differences in the Pd crystals, that make up
the catalyst, affect selectivity in the partial hydrogenation reaction
of 3-hexyne. In this regard the authors obtained high selectivi-
ties to the (Z)-stereoisomer when the alkyne is mainly adsorbed
on the (1 1 1) plane of Pd crystals, while the adsorption onto the
(1 1 0) plane gave low selectivities to this isomer. The analysis of
the activities, expressed as initial reaction rate of 3-hexyne per
mass of Pd, shown in Fig. 9, indicates for each catalytic system an
Fig. 7 shows that at 303 K, all the prepared catalysts present
varying degrees of activity for the partial hydrogenation of 3-
hexyne. In all the catalytic systems, the concentration profiles
obtained experimentally are indicative of irreversible reactions
rather than reversible ones shown in Fig. 1. Besides, the partial
hydrogenation of 3-hexyne over the Pd-based catalysts results in
the predominant formation (Z)-3-hexene stereo-isomer accompa-
nied by the production of n-hexane (through over hydrogenation)
and of (E)-3-hexenes, which may be formed as initial product rather
than (Z) → (E) isomerization as it can be deduced from the concen-
tration profiles shown in Fig. 7. Also it can be observed that the
bimetallic catalysts are more active regarding the monometallic
Pd/A at 303 K. Total conversion of 3-hexyne of ca. 100% is reached
with PdNi/A and WPd/A, and with the commercial Lindlar cata-
lysts. After the total consumption of the alkyne the isomerization
(
Z) → (E) reaction begin to be predominant rather than the over
hydrogenation of the stereo-isomer (not shown for sake of clar-
ity). Furthermore, while 3-hexyne is present in the reaction media,
(
Z)-3-hexene concentration profiles always present an increasing
tendency, while the concentration of (E)-3-hexene and n-hexane
are almost constant all over the reaction tests.
On the other hand, in Fig. 8 are presented the 3-hexyne total
conversion vs. time obtained at different reaction temperatures
0
increase of r with temperature. It can be observed that all the pre-
A
(
between 273 and 323 K) for the mono and bimetallic catalysts,
pared low-loaded mono and bimetallic catalysts, present higher
initial reaction rate of 3-hexyne than the high-loaded Lindlar cat-
alyst. Besides, it can be remarked that the addition of W or Ni to
Pd catalyst considerably promotes the activity of the monometal-
lic catalyst, without modifying the very high selectivity (typical of
Pd). At each reaction temperature evaluated, the following order
and for the Lindlar catalyst. In this figure it is observed that all the
catalysts are active in the range of temperatures studied, even at
the lowest temperature, 273 K. Experimental results show that, as
expected from a kinetic point of view, a beneficial effect of tem-
perature on reaction rate is observed, and the optimum reaction

9
6
M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
1
1
1
1
600
400
200
000
140
Pd/A
Lindlar
1
1
20
00
8
6
4
2
0
0
0
0
0
8
6
4
2
00
00
00
00
0
0
20 40 60 80 100 120 140 160 180 200
0
20
40
60
80
100
Time (min)
Time (min)
1
1
1
1
600
400
200
000
1600
1400
1200
1000
800
600
400
200
0
PdNi/A
W-Pd
8
6
4
2
00
00
00
00
0
0
20 40 60 80 100 120 140 160 180 200
0
20 40 60 80 100 120 140 160 180 200
Time (Min)
Time (min)
Fig. 7. Evolution of the concentrations of reactant, 3-hexyne (᭹), and products: (Z)-3-hexene (ꢀ), (E)-3-hexene (ꢁ) and n-hexane(ꢂ) as a function of time during the partial
hydrogenation of 3-hexyne over Pd/A, Lindlar, PdNi/A and WPd/A. Reaction conditions: 1.5 bar of H2, 303 K, 800 rpm.
of initial reaction rate of 3-hexyne per mass of Pd, was found:
WPd/A > PdNi/A ꢀ Pd/A ꢀ Lindlar. Besides, at the highest evaluated
temperature, WPd/A and PdNi/A present initial reaction rate of
selective hydrogenation reaction of 1-heptyne, we have found that
on W or Ni monometallic catalysts, that no chemisorbs H at room
2
temperature, the controlling reaction rate step is the dissociative
adsorption of H2 [44,45]; however, on metals with high capacity
of H2 chemisorption, such as Pd, the controlling step is the sur-
face chemical reaction. In the case of Pd it was also verified that
an increase in the partial pressure of hydrogen leads to lower val-
ues of the reaction rate [45]. As stated in the previous Section, the
3
-hexyne, 3.5 and 1.4 times higher than that of Pd/A, while the
monometallic initial reaction rate is 2.5 times higher than that of
Lindlar catalyst. Furthermore, as for all the catalysts the selectivi-
ties are maintained nearly constant with temperature, the yield to
the (Z)-stereoisomer is maximal at the higher reaction tempera-
ture studied (323 K). This means that the optimum is verified at the
highest temperature contradicting the opinion of other researchers
ı−
existence of electron-rich Pd species on the bimetallic Pd–M cat-
alysts, with M = Ni or W, can be explained due to the formation
of metallic bonds or alloys occurring at low temperatures [37,46];
however this transfer might be due to the formation of intermedi-
ate Pd–M–Al O surface species [14,19]. The causes of the influence
[
6,7,40] who warn about the inconvenience of overcoming 303 K
of reaction temperature since that worsens the selectivity to the
desired product. The results indicates that higher throughput of
product per mass of Pd are obtained for WPd/A (firstly), PdNi/A
2
3
of a second metal on the performance of monometallic palladium
catalysts are not as straightforward. Alkynes partial hydrogenation
reactions are more or less sensitive to geometrical and electronic
effects, being the latter the most important ones. The properties of
palladium based bimetallic catalysts are related to the preparation
method, which affects the electronic and chemical state of Pd and of
the second metal, as well as their spaial distribution. As observed by
XPS, the species situated on the surface of the prepared catalysts are
(
secondly) and Pd/A (thirdly) than with commercial Lindlar cata-
lyst.
The dispersion of the prepared catalysts showed that Pd/A cata-
lyst presented the highest D value, but as observed this catalyst
presented the lowest reaction rate, indicating in this case that
a greater capacity for H2 chemisorption not necessarily means a
higher reaction rate. In recent works on kinetic modeling of the
Table 2
TOF values estimated at 303 K and products selectivities obtained at different temperatures, 1.4 bar and 800 rpm.
TOF (s 1
−
)
Products selectivity (%)
Z)-3-hexene
73 K
Catalyst
S/Pd
(
(E)-3-hexene
n-hexane
273 K
2
303 K
323 K
273 K
303 K
323 K
303 K
323 K
Pd/A
14,360
14,360
14,360
1115
4.73
19.40
19.30
95.0
94.0
97.0
98.0
94.0
94.0
96.0
98.0
93.0
93.0
95.0
97.0
4.0
5.0
3.0
1.7
4.0
5.0
3.0
1.4
5.0
6.0
4.0
2.0
1.0
1.0
1.0
0.3
2.0
1.0
1.0
0.6
2.0
1.0
1.0
1.0
PdNi/A
WPd/A
Lindlar

M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
97
T= 323 K
T= 303 K
Lindlar
1
00 Pd/A
100
80
60
40
20
0
T=323 K
T=303 K
8
6
4
2
0
0
0
0
0
T= 273 K
T=273 K
0
50
100
150
200
0
50
100
150
200
Time (Min)
Time (Min)
T=323 K
T=323 K
T=303 K
PdNi/A
WPd/A
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
T=303 K
T=273 K
T=273 K
0
50
100
150
200
0
50
100
150
200
Time (Min)
Time (Min)
Fig. 8. 3-Hexyne total conversion (%) as a function of time for Pd/A, Lindlar, PdNi/A and WPd/A. catalysts, measured at 1.5 bar and different temperatures: T1 = 273 K (ꢀ),
T2 = 303 K (ꢃ) and T3 = 323 K (᭹).
more electron-rich than those present on the surface of the Lindlar
catalyst. Even more, the most active catalyst, WPd, presents 92%
of electron-rich Pd species, while the secondly active, PdNi/A,
interaction of a filled d metal orbital with the empty sigma anti-
bonding molecular orbital of H [47]. The rupture of the hydrogen
2
ı−
bond is more easily done on metals with a high amount of available
presents a 54% of these species. On the other hand Pd/A presents
electrons in the external d orbital, as it is the case of catalysts with
ı±
ı−
1
00% of palladium totally reduced while there is a 69% of Pd on the
electron-rich Pd species as WPd/A or PdNi/A catalysts (with 92 or
ı−
Lindlar catalyst. So the characterization results should indicate that,
at least in part, the modification of the electronic state of palladium
could be responsible for the better catalytic behavior.
56% of Pd electron-rich species, respectively), or totally reduced
Pd species as Pd/A. This rupture should be less likely on metals
with fewer d electrons, as in the case of the commercial Lindlar
ı±
It is well known that during hydrogenation reactions metallic
catalyst (with 69% of Pd species, with ı close to 0). Therefore,
centers rich in electrons can cleave the bond in H by means of the
the differences in activity between the reported catalysts could be
attributed, at least in part, to differences in the electronic density of
the external d orbital of palladium (electronic factor). However, the
influence of geometrical effects and/or mixed sites on the activity
of bimetallic catalysts cannot be discarded. According to the order
of impregnation of Pd during the preparation of each bimetallic
catalyst, and because of the low values of hydrogen chemisorption,
geometrical effects should be more important on PdNi/A than on
WPd/A catalyst, due to an effect of decoration of Pd active sites
with NiOx species [14,15,48]. More work is necessary to continue
elucidating the activities enhancement in order to improve the
preparation technique of the bimetallic catalysts.
2
4. Conclusions
Supported low-loaded mono and bimetallic palladium catalysts
PdW and WPd were prepared by incipient wetness impregnation
of gamma alumina.
The XRD, XPS, TPR and hydrogen chemisorption results reveals
that the palladium active sites present in the monometallic Pd/A
Fig. 9. Activities expressed as initial reaction rates of 3-hexyne per mass of palla-
dium for Pd/A, PdNi/A, WPd/A and Lindlar at 273, 303 and 323 K.

9
8
M.J. Maccarrone et al. / Applied Catalysis A: General 441–442 (2012) 90–98
catalyst, are quite different respect to those located on the bimetal-
lic PdNi/A or WPd/A catalysts: palladium is totally reduced in
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