Pd nanoparticles in hydrotalcite: Mild and highly selective catalysts for alkyne semihydrogenation

Article abstract of DOI:10.1016/S0021-9517(03)00269-0

Pd nanoparticles incorporated in a hydrotalcite (HT) host were prepared via anion exchange between a dilute suspension of HT-nitrate and a Pd hydrosol stabilized by the anionic surfactant sodium dodecyl sulfate. Samples of two of the resulting Pd organoclays, Pd-HT1 and Pd-HT2, with metal contents of 0.1 and 0.42%, respectively, were subjected to further investigations. Characterization via ICP-AES, XRD, and TEM measurements indicated the deposition of fairly monodispersed Pd particles, predominantly on the external surface of the HT layers. The Pd-HT samples proved to be efficient catalysts for the liquid-phase semihydrogenations of both terminal and internal alkynes under mild conditions. For the transformation of phenylacetylene to styrene, a bond selectivity of 100% was obtained, and the cis stereoselectivities for the hydrogenations of the internal alkynes 4-octyne and 1-phenyl-1-pentyne reasonably approached 100%. The catalytic activity of Pd-HT increased with the Pd dispersion, whereas the alkene selectivity remained essentially unaffected.

Full text of DOI:10.1016/S0021-9517(03)00269-0

Journal of Catalysis 220 (2003) 372–381
www.elsevier.com/locate/jcat
Pd nanoparticles in hydrotalcite: mild and highly selective catalysts
for alkyne semihydrogenation
Ágnes Mastalir a,∗ and Zoltán Király b
a
Department of Organic Chemistry, University of Szeged, Dóm tér 8, H-6720 Szeged, Hungary
Department of Colloid Chemistry, University of Szeged, Aradi vt. 1, H-6720 Szeged, Hungary
b
Received 17 March 2003; revised 3 June 2003; accepted 3 June 2003
Abstract
Pd nanoparticles incorporated in a hydrotalcite (HT) host were prepared via anion exchange between a dilute suspension of HT–nitrate and
a Pd hydrosol stabilized by the anionic surfactant sodium dodecyl sulfate. Samples of two of the resulting Pd organoclays, Pd–HT1 and Pd–
HT2, with metal contents of 0.1 and 0.42%, respectively, were subjected to further investigations. Characterization via ICP-AES, XRD, and
TEM measurements indicated the deposition of fairly monodispersed Pd particles, predominantly on the external surface of the HT layers.
The Pd–HT samples proved to be efficient catalysts for the liquid-phase semihydrogenations of both terminal and internal alkynes under mild
conditions. For the transformation of phenylacetylene to styrene, a bond selectivity of 100% was obtained, and the cis stereoselectivities for
the hydrogenations of the internal alkynes 4-octyne and 1-phenyl-1-pentyne reasonably approached 100%. The catalytic activity of Pd–HT
increased with the Pd dispersion, whereas the alkene selectivity remained essentially unaffected.
2003 Elsevier Inc. All rights reserved.
Keywords: Palladium; Hydrotalcite; Anionic surfactant; Hydrogenation; Phenylacetylene; 4-Octyne; 1-Phenyl-1-butyne; Selectivity
1. Introduction
most important properties of HT-like compounds is the high
anion-exchange capacity, related to their lamellar structure,
which allows replacement of the original interlayer anions
by others, including anionic surfactants [1,2,5,8–11].
Hydrotalcite-type (HT) anionic clays (also known as lay-
ered double hydroxides) are homogeneous, basic, mixed
metal hydroxides with a lamellar structure. HT materials can
be characterized by their chemical composition, basal spac-
In recent years, HT compounds have been extensively
studied as anion exchangers [2,5,10,11], catalyst precursors,
and catalysts [1,3,12]. Calcination of HTs at temperatures
over 723 K results in thermal decomposition, leading to the
formation of basic mixed oxides that have proved to be ef-
ficient catalysts for a variety of organic reactions, includ-
ing aldol condensations [13–16], nitrile hydrogenation [17],
phenol oxidation [18], and alkoxylation [19]. The applica-
tion of HT as a support material for transition and noble
metal catalysts has also been investigated [20–22]. Rh, Ru,
Ir, Pd, and Pt-based catalysts have been prepared from HT-
type anionic clays by coprecipitation, with a view to methane
activation studies [23,24]. Pd/HT materials synthesized by a
similar coprecipitation method have been examined as cata-
lysts for selective hydrodechlorination reactions [25]. Fur-
ther, HT-supported Pd catalysts prepared by impregnation
ing, and stacking sequence [1–3]. The general formula of
2+
HT is M M3x+(OH)2(An−
)
· yH2O, where M2+ and
x/n
1−x
M3+ are di- and trivalent metals, respectively, and An− is an
interlayer anion (CO32−, Cl−, or NO3−) having a valence
of n, which influences the basal spacing. The value of x is
in the range 0.17–0.33 [2,4,5]. The structure of the HT com-
pounds consists of brucite-like Mg(OH)2 layers, in which
the Mg2+ ions are surrounded by hydroxyl ions in an oc-
tahedral arrangement [1,6–8]. Isomorphous substitution of
Mg2+ by Al3+ renders the brucite-type sheets positively
charged; electric neutrality is maintained by inorganic an-
ions of relatively high mobility, located in disordered inter-
layer regions containing water molecules [1–8]. One of the
have been employed for phenol hydrogenation [26]. Another
2−
*
synthetic method, based on the reduction of PdCl4
af-
Corresponding author.
E-mail address: mastalir@chem.u-szeged.hu (Á. Mastalir).
ter anion exchange, leading to the formation of a Pd/HT
0021-9517/$ – see front matter
2003 Elsevier Inc. All rights reserved.
doi:10.1016/S0021-9517(03)00269-0

Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
373
catalyst applied for C–C coupling reactions, was recently re-
ported by Choudary et al. [27].
ticles released were deposited on the clay lamellae. After
removal of the excess surfactants in ethanol by centrifuga-
tion/redispersion sequences, the Pd–HT material was dried
in vacuum at 353 K. Variation of the Pd hydrosol:HT suspen-
sion volume ratio afforded Pd–HT materials with different
Pd contents.
For comparative investigations, 5% Pd on γ -Al2O3 (Pd/
Al2O3, a Merck product) and 5% Pd on activated carbon
(Pd/C, a Janssen product) were utilized. The Pd dispersions
D of the samples were determined from TEM measurements
as D = 0.885/d, where d is the mean particle diameter
(nm) [38]. The dispersion values obtained were 0.47 and
0.17 for Pd/Al2O3 and Pd/C, respectively.
The synthetic utility of selective alkyne hydrogenations
has long been established for the preparation of fine chem-
icals and biologically active compounds [28–30]. Although
a number of catalytic applications for transition and noble
metal–HT systems have been described, no published in-
formation is currently available on their potential use for
alkyne semihydrogenations. The main objective in alkyne
semihydrogenation is to achieve the highest alkene selec-
tivity possible. As confirmed by previous studies, Pd is
the most selective metal catalyst for the semihydrogena-
tion of monosubstituted acetylenes, and also for the trans-
formations of nonterminal alkynes to cis-alkenes [31–34].
The pronounced selectivity of Pd is attributed to stronger
chemisorption of the alkyne species on the active center than
that of the alkene, which is related to the high electron den-
sity and the restricted rotation of the C≡C bond [30,32,34].
In the present work, Pd–HT samples were prepared via a
surfactant-mediated synthetic route in a micellar system.
The hydrophobic Pd–HT materials were tested as catalysts
in the liquid-phase semihydrogenations of phenylacetylene,
4-octyne, and 1-phenyl-1-pentyne.
2.2. Methods
X-ray diffraction (XRD) measurements were made in
the scattering angle range 1◦ ꢀ 2ꢀ ꢀ 20◦ with a Philips
PW diffractometer at 40 kV and 35 mA (Cu-Kα radiation,
λ = 0.154 nm). Wet samples were taken from the sediments
of the suspensions formed in different solvents. In order to
prevent solvent evaporation, the sample holder was covered
with a Mylar film. The dL values were calculated from the
first-order (001) Bragg reflections by using the PW 1877
powder diffraction software.
2. Experimental
The Pd contents of the samples were determined by
inductively coupled plasma atomic emission spectroscopy
(ICP-AES), with a Jobin Yvon 24 sequential ICP-AES spec-
trometer at 229.7 and 324.3 nm. Before measurement, the
Pd–HT samples were dissolved in aqua regia. The Pd con-
tents were obtained from the emission intensities by means
of a calibration curve.
2.1. Materials
−
A HT compound with exchangeable NO3 gallery ions
(HTNO3) was synthesized by a standard method described
earlier [35]. Aqueous solutions of Al(NO3)3 · 6H2O and
Mg(NO3)2 · 9H2O (Mg:Al = 2:1) were added dropwise to
a NaOH solution under a N2 atmosphere during stirring,
which was maintained at 333 K for 5 h. The pH of the
reaction system was adjusted to between 9 and 10. When
the reaction was completed, the white precipitate that had
formed was aged in an aqueous solution at 333 K for 7 days.
The product was purified by washing with distilled water in
several centrifugation/redispersion cycles (Sorwall RC cen-
trifuge, 3500 rpm). XRD analysis revealed the formation of
a crystalline material HTNO3 (the basal spacing dL for the
dry clay was 0.78 nm).
The particle-size distributions were obtained from trans-
mission electron microscopy (TEM) images, taken with a
Philips C-10 electron microscope, with a LaB6 cathode at
100 kV. The instrument was equipped with a Megaview II
digital camera. The samples were suspended in hexane and
mounted on a plastic film supported on a Cu grid. The
ꢀ
ꢀ
mean particle diameters d were calculated as nidi/ ni
ꢀ
(
ni > 250), by using the UTHSCSA Image Tool program.
The total organic carbon (TOC) contents of the samples
were determined with a Euroglass TOC 1200 apparatus at
309 K.
Pd hydrosols were prepared via reduction of the pre-
cursor palladium acetylacetonate (Pd(acac)2, 99%, Aldrich)
in the presence of the anionic surfactant sodium dodecyl
sulfate (NaDS, 99%, Sigma). The precursor was first dis-
solved in chloroform (1% w/w) and then introduced into
the aqueous surfactant solution (8 mM) so as to produce
a normal micellar system (0.1 mM Pd) [36,37]. Reduction
by a 50-fold excess of aqueous hydrazine (55% w/w) af-
forded surfactant-stabilized Pd nanoparticles. On addition
of the NaDS-stabilized Pd hydrosol to an aqueous suspen-
Na+ analysis was carried out by using a Jenway PFP 7
flame photometer. Prior to measurement, the samples were
dissolved in 5 cm3 of cc HCl, followed by addition of the
same amount of distilled water. Na+ contents were gained
by means of a calibration curve constructed from the results
on NaCl solutions of known concentrations (0–10 ppm). The
intensities observed for the solutions of the Pd–HT samples
were corrected for the intensity of a blank solution contain-
ing a 1:1 mixture of cc HCl and distilled water.
The catalytic test reactions were performed in an au-
tomated liquid-phase reactor, illustrated in Fig. 1. The re-
actants phenylacetylene, 4-octyne, and 1-phenyl-1-pentyne,
all Aldrich products of 99% purity, were used as received.
−
sion of HTNO3 (2% w/w), anion exchange of NO3 by
DS− occurred, resulting in the formation of a hydropho-
bic clay organocomplex. Simultaneously, the bare Pd par-

374
Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
imental error in the GC analysis ( 0.1–0.28%). It follows
that no Pd leaching of the samples occurred during catalytic
applications.
3. Results and discussion
The synthesis of Pd particles is based on the application
of the anionic surfactant NaDS, which increases the solubil-
ity of Pd(acac)2 and chloroform in water through a solubi-
lization effect [39], thereby producing an aqueous micellar
solution of Pd(acac)2, previously dissolved in chloroform.
The hydrophobic alkyl chains of the surfactant molecules
are directed to the inner pool containing the precursor mole-
cules, whereas the hydrophilic head groups are situated on
the outer shell of the swollen micelles, in contact with the
aqueous phase. Addition of an aqueous hydrazine solution to
this micellar system resulted in the formation of Pd nanopar-
ticles in the form of a highly stable hydrosol, stabilized both
sterically and electrostatically by the surfactant molecules:
the hydrophobicparts of the NaDS molecules were adsorbed
in a perpendicular orientation on the surface of the Pd nano-
clusters and further aggregation was therefore prevented by
a dense, protective surfactant layer around the metal parti-
cles [40]. The proposed mechanism for the interaction of the
Pd hydrosol with the anionic clay lattice is outlined in Fig. 2.
On addition of an aqueous HTNO3 suspension to the Pd
hydrosol stabilized by NaDS, an ion-exchange reaction took
place between the DS− and the NO3−, which released the Pd
nanoparticles and made the HT surface hydrophobic. After
complete decomposition of the surrounding protective sur-
factant layer, the Pd particles were adsorbed on the surface of
the clay lamellae. In this way, a hydrophobic Pd organoclay
was formed, which, unlike hydrophilic clays, is capable of
Fig. 1. Experimental apparatus for catalytic hydrogenation reactions:
(1) vacuum piping, (2) hydrogen reservoir, (3) two-way valve, (4) meter-
ing valve, (5) pressure transmitter, (6) high-pressure hydrogen chamber,
(7) pressure regulator, (8) pressure gauge, (9) three-way valve, (10) reac-
tion vessel, (11) septum, (12) thermostat, (13) rubber cup, (14) DC motor,
(15) variable-speed motor controller, (16) pressure transmitter interface, and
(17) PC.
Before measurements, the solvents toluene, tetrahydrofuran
(THF), ethanol, and hexane were dried and distilled under a
N2 atmosphere.
Measurements were carried out with 10 mg of Pd–HT
in a glass reaction vessel at 298 K. After evacuation and
flushing with H2, the catalyst was pretreated in 105 Pa of
static H2 for 1 h. Next, 1 cm3 of solvent was added, and
vigorous stirring was applied for 45 min. The reaction was
subsequently initiated by injecting the alkyne substrate into
the reaction vessel. Hydrogenation was conducted under ef-
ficient stirring (1400 rpm) to eliminate diffusion control. The
H2 consumption was balanced by a pressure regulator at the
expense of the high-pressure side; accordingly, a constant
pressure of 105 Pa was maintained throughout the reaction.
The H2 chamber was connected to a pressure transmitter, in-
terfaced to a PC. The H2 uptake was recorded continuously
at a data collection frequency of 0.5 s−1. After the reaction
was completed, the catalyst was removed by gravity filtra-
tion. Quantitative analysis of the products was performed
with an HP 5890 gas chromatograph equipped with a DB-5
capillary column and a flame ionization detector (FID). The
S:Pd values were expressed as [mole reactant]:[mole Pd].
The catalytic activity was characterized by the turnover
frequency (TOF), calculated from the initial reaction rate
R (cm3 H2 min−1 g Pd−1) as TOF = 7.253 × 10−5 R/D,
where D is the dispersion of Pd, obtained from TEM mea-
surements, as noted above [38].
Reusability of the samples was investigated in repeated
catalytic reactions. It was found that neither the catalytic ac-
tivities nor the selectivities decreased appreciably in up to
3 runs. The conversion curves could be reproduced within
2.5% in repeated runs, and the changes in the selectivities
of the main reaction products were within the range of exper-
Fig. 2. Schematic representation of Pd–HT formation via anion exchange.

Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
375
Table 1
was found, whereas the frequency of crystallites 3–6 nm
in size was increased considerably. Nevertheless, aggregates
larger than 6 nm were not detected and therefore no appre-
ciable increase in the mean particle diameter could be estab-
lished, as shown in Table 1. It follows that the Pd particles
may be considered fairly monodispersed for both samples,
which confirms the stabilizing effect of the surfactant mole-
cules in the hydrosol and the absence of surface aggregation
after Pd deposition.
The pronounced d001 reflections observed for the Pd–
HTs by means of XRD measurements confirmed that the
lamellar structure of the HT host was preserved after the in-
corporation of the Pd particles. The basal spacings of the
Pd–HTs (see Table 1) were somewhat higher than that of
the corresponding organoclay in the absence of Pd particles
(2.52 nm), which points to the presence of Pd nanoparticles
in the interlayer region of the HT host. This is supported by
a slight broadening of the characteristic d001 reflections, in-
dicating a less ordered structure for the Pd–HTs than that of
the pristine HT material. However, regular intercalation of
the Pd particles between the clay sheets is rather unlikely. It
follows that, in accordance with the TEM images, a substan-
tial proportion of the Pd particles are situated on the external
surface sites of the HT lamellae.
The semihydrogenation of phenylacetylene has been in-
vestigated on various supported Pd catalysts [43–46] and on
clay-intercalated Pd crystallites [47]. For Pd supported on
pumice, high turnover frequencies and a selectivity of 80%
for the formation of styrene have been reported [45]. For Pd
on vitreous supports, a 100% selectivity for styrene produc-
tion and a linear trend for the percentage of styrene vs time
have been described [46].
The transformation of phenylacetylene on Pd–HT1 was
studied in toluene, at S:Pd = 2500. The results are displayed
in Fig. 4.
The marked initial activity of Pd–HT1 gradually dimin-
ished as the hydrogenation proceeded, and complete con-
Characterization of organophilic Pd–hydrotalcite catalysts
Sample
Pd (%)
d
(nm)
d (nm)
D
L
ICP-AES
0.10
0.42
XRD
2.73
2.77
TEM
2.4
3.5
Pd–HT1
Pd–HT2
0.37
0.25
swelling in the organic solvents usually applied as disper-
sion media in liquid-phase catalytic reactions [41,42].
For further investigations, two representative Pd–HT
samples with low Pd loadings were selected, Pd–HT1 and
Pd–HT2, for which the Pd contents determined from ICP-
AES were 0.10 and 0.42%, respectively.
TOC analysis revealed that the anion-exchange capacity
of the pristine HT material treated with the same surfactant
solution (1.77 meq g−1) did not exhibit any significant dif-
ference from those of the Pd organoclays. It follows that the
Pd–HT samples contain surfactant molecules fixed by elec-
trostatic interactions only in the anion-exchange positions of
the clay lattice, and thus the presence of residual NaDS on
the surface of the Pd particles may be excluded. This finding
complies with the results of Na+ analysis: the Na+ contents
of the samples proved to be 0.0062 and 0.0084% for Pd–HT1
and Pd–HT2, respectively. As Na+ ions originate only from
NaDS, it may safely be assumed that the effect of residual
surfactant molecules on the Pd particles is negligible. Fur-
ther, Na+ is unlikely to have an electronic effect on the Pd
particles.
Characteristic data on the Pd–HT samples are collected
in Table 1.
The TEM micrographs of the Pd–HTs revealed the ap-
pearance of quasispherical Pd particles with relatively nar-
row size distributions (Fig. 3). For Pd–HT1, 74% of the Pd
crystallites had diameters less than 3 nm and only few par-
ticles measuring over 4 nm were observed. For Pd–HT2,
the size distribution was broader and the overall crystallite
dimensions were larger. As compared with Pd–HT1, a sig-
nificant decline in the number of particles measuring 1–2 nm
−3
Fig. 4. Transformation of phenylacetylene on Pd–HT1: m = 10 g, T =
5
Fig. 3. Particle-size distributions of Pd–HT1 and Pd–HT2.
298 K, p = 10 Pa, S:Pd = 2500.

376
Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
version was attained at a reaction time of 170 min. The
selectivity of styrene formation proved to be 100% up to
90 min; overhydrogenation, leading to the appearance of 6%
of ethylbenzene in the product mixture, was first observed
at 124 min. Hence, in order to produce only one semihydro-
genated product with a high conversion (87%), the reaction
should be terminated at a reaction time of 90 min.
the clay lamellae. Access to such internal active sites can
be achieved through diffusion of the reactant molecules be-
tween the layers, which tends to have a limiting effect on the
reaction rate. On the incorporationof the Pd particles into the
organophilic HT host, the interlayer distance increased from
1.74 to 1.97 0.2 nm (see Table 1). This implies that parti-
cles smaller than 2 nm are most likely to be intercalated be-
tween the clay lamellae. As revealed by Fig. 3, such particles
are more abundant in Pd–HT1, which may therefore contain
a larger amount of interlamellar Pd particles than Pd–HT2.
Accessibility for the reactants to these species is suggested
by the pronounced catalytic activity of Pd–HT1, which may
also account for the moderate overhydrogenation observed
at high conversions. Nevertheless, direct alkane production
may be ruled out for both samples [32,50]. In fact, both
catalysts displayed an excellent selectivity for styrene for-
mation, which considerably exceeded that reported for the
same reaction conducted on Pd nanoparticles intercalated in
a hydrophilic clay host [47].
In liquid-phase reactions in organic solvents, the hy-
drophobic character of the Pd–HT samples is a distinct
advantage as compared with catalysts prepared from hy-
drophilic clay hosts, which tend to be ill-dispersed in most
organic media. Moreover, depending on the solvent em-
ployed for the reaction, the catalytic properties of the Pd–
HTs may be influenced by the swelling ability of the HT
host, especially when interlamellar Pd particles are regarded
as potential active centers. In order to gain more information
on the role of the dispersion medium for the above reac-
tion, the catalytic performance of Pd–HT1, the sample with
higher activity, was studied in different solvents. The results
are listed in Table 2.
For the above reaction investigated on Pd–HT2 under the
same conditions (Fig. 5), the conversion was substantially
lower than that observed on Pd–HT1, which is consistent
with the difference in the Pd dispersions (0.37 and 0.25 for
Pd–HT1 and Pd–HT2, respectively). This is indicative of a
decrease in the number of active sites for Pd–HT2, which
may be related to the appearance of Pd particles larger than
4 nm. In fact, the latter particles mainly consist of crys-
tal planes (terrace atoms with high coordination number),
whereas small particles, such as those predominant in Pd–
HT1 (see Fig. 3) mostly contain Pd atoms with low coordi-
nation numbers (edge or corner atoms) that are more active
in hydrogenation. It follows that a site modification effect
may account for the substantial activity difference observed
for the Pd–HTs, which is in accordance with previous studies
disclosing that alkyne hydrogenation is a structure-sensitive
reaction at medium to high Pd dispersions [34,48,49].
On decrease of the S:Pd ratio from 2500 to 700, an ap-
preciable increase in the conversion of phenylacetylene on
Pd–HT2 could be achieved (Fig. 5). The 100% selectivity of
styrene formation throughout the reactions in both cases is
related to the conversions being substantially lower than the
87% obtained for Pd–HT1, above which ethylbenzene for-
mation was observed.
The evidence that the conversion plots were saturation
curves for both Pd–HTs indicated that these samples differed
from the conventional supported Pd catalysts cited above.
This points to the existence of Pd crystallites in the inter-
lamellar regions of HT, which may participate in the reac-
tion, in addition to those situated on the external surface of
For the solvents toluene, THF, and ethanol, no important
difference in the turnover frequency was observed, and the
conversions did not vary to any appreciable extent. On the
other hand, when hexane was applied, significant decreases
in both the initial rate and the conversion were experienced.
The question arises whether the change in the catalytic activ-
ity of Pd–HT1 is related to the variation in H2 solubility in
the above dispersion media. The corresponding data in Ta-
ble 2, taken from the literature, reveal that the solubilities
of H2 in both toluene and THF are very close and higher
Table 2
Effect of the solvent on the hydrogenation of phenylacetylene on Pd–HT1
a
b
b
c
d
d
Solvent
R
TOF Conversion
S
δ
styrene
(%)
L
3
−1
−1
(cm H min
2
(s
)
(%)
(nm)
−1
g
Pd)
Toluene
THF
Ethanol
Hexane
8264
1.62
79.2
68.9
70.2
56.7
100
100
100
100
3.4 3.48
3.5 3.62
2.9 3.12
4.7 2.80
7817
7623
5612
1.53
1.49
1.10
a
−3
5
m = 10 g, T = 298 K, p = 10 Pa, S:Pd = 2500.
b
c
d
Reaction time: 55 min.
Solubility of H (10 mol dm ), taken from Ref. [52].
Basal spacing, obtained from XRD measurements.
−3
3
−3
Fig. 5. Transformation of phenylacetylene on Pd–HT2: m = 10
g,
2
5
T = 298 K, p = 10 Pa.

Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
377
than that in ethanol [51–53]. Further, the highest H2 solubil-
ity (4.7 mmol dm−3) has been reported for hexane [51–53].
This trend, however, does not conform with our results, indi-
cating the lowest activity in hexane. Accordingly, no system-
atic correlation can be discerned between the H2 solubility
and the catalytic performance of Pd–HT1. This finding is
consistent with the results of earlier studies on the liquid-
phase hydrogenation of cyclohexene: whereas the TOF val-
ues over supported Pt and Rh particles varied considerably
with the H2 solubility, no such effect was found for Pd crys-
tallites [54]. The absence of a solvent effect, based on H2
solubility, may be regarded as an intrinsic feature of Pd, and,
as suggested by Boudart, may be attributed to gas phase, liq-
uid phase, and adsorbed H2 being quasi-equilibrated in the
reaction system [52,55]. Under such conditions, the surface
coverage of hydrogen is claimed to be independent of the
liquid-phase H2 concentration [52].
−3
Fig. 6. Transformation of 4-octyne on Pd–HT1: m = 10 g, T = 298 K,
5
p = 10 Pa, S:Pd = 2500.
On the other hand, the variation in dL (Table 2) implies
that the swelling ability of the catalyst depends on the sol-
vent. In hexane, practically no swelling took place, as the
value of dL hardly surpassed that for the dry sample. In con-
trast, a similar substantial swelling was observed for toluene
and THF, and a moderate swelling for ethanol. It is therefore
reasonable to suggest that the catalytic activity of Pd–HT1
correlates with the swelling of the HT host, accompanied
by disaggregation [56]; in a suitable solvent, this makes
the interlayer active sites more readily accessible for reac-
tant molecules. This may occur in toluene, THF, and even
ethanol, for which the catalytic performance of Pd–HT1 was
comparable with those in the former solvents. Thus, it may
be stated that dL = 3.12 nm, observed in ethanol, is suffi-
ciently high to provide access for the reactant molecules to
interlamellar Pd particles that may participate in hydrogena-
tion, in addition to those available on the external surface
sites of the HT host. As may be seen in Table 2, a further
increase in the basal spacing has no significant effect on
the TOF values. On the other hand, the lack of swelling in
hexane suggests that the HT lamella packages are situated
close together and hence access for the reactant molecules
to the interlayer Pd crystallites tends to be restricted. This
may account for the decreased activity of Pd–HT1, as related
to diffusion limitations [52,56]. Nevertheless, the occurrence
of interlamellar hydrogenation in hexane cannot be excluded
with certainty.
−3
Fig. 7. Transformation of 4-octyne on Pd–HT2: m = 10 g, T = 298 K,
5
p = 10 Pa, S:Pd = 700.
actant (or that of the activated complex) may be favored in
toluene, which tends to contribute to an improved catalytic
performance.
It should be stressed that the moderate solvent effect ob-
served for Pd–HT1 is merely reflected in the catalytic activ-
ities, as the selectivity of styrene formation proved to be the
same in all solvents investigated.
The evidence that the TOF value observed in toluene
slightly surpassed that in THF, despite the value of dL be-
ing somewhat higher in the latter solvent, may be related to
the reaction proceeding in a liquid mixture. It may be as-
sumed that the adsorption of phenylacetylene from toluene
is preferred to that from THF, thereby leading to an increased
reactant concentration on the active sites, which may ac-
count for a higher catalytic activity. Moreover, the effect of
solvent polarity on the reaction mechanism can also be taken
into consideration. Since the polarity of phenylacetylene is
similar to that of toluene (both being substituted aromatics),
whereas THF is a more polar solvent, solvation of the re-
The second reactant studied on the Pd–HTs was the inter-
nal alkyne 4-octyne, for which semihydrogenation resulted
in the predominant formation of cis-4-octene (Figs. 6 and 7).
It is generally accepted that the cis product is formed by
the consecutive addition of two adsorbed hydrogens from
below the axis of the triple bond [31,32]. For the transfor-
mation of 4-octyne on Pd–HT1, a conversion of 93% was
obtained in a reaction time of 90 min. It was noteworthy
that the initial stereoselectivity for cis-alkene formation was
97.5%, which had decreased merely to 95.6% by 90 min.
Fig. 6 reveals that the overhydrogenationproduct octane was
formed via simultaneous hydrogenation of both the reac-

378
Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
−3
−3
Fig. 9. Transformation of 1-phenyl-1-pentyne on Pd–HT2: m = 10
g,
Fig. 8. Transformation of 1-phenyl-1-pentyne on Pd–HT1: m = 10
g,
5
5
T = 298 K, p = 10 Pa, S:Pd = 700.
T = 298 K, p = 10 Pa, S:Pd = 2500.
Table 3
tant and the cis-alkene molecules [32]. It is important to
stress that no trans-4-octene formation was detected at all,
which is rather exceptional for the transformations of dis-
ubstituted acetylenes [32,46,57]. For the same reaction on
Pd–HT2, similar observations were made: at a lower S:Pd
ratio of 700 (see Figs. 6 and 7), both the conversion and the
selectivities were comparable to those obtained on Pd–HT1.
Full conversion of 4-octyne was achieved by a reaction time
of 140 min. Likewise, only a minor decrease in the initial
stereoselectivity of formation of the main product (97%) was
experienced, in parallel with a similar increase in the selec-
tivity of octane production. The stereoselectivity of trans-4-
octene formation was negligible, as it never surpassed 0.3%.
The unusually low extent of overhydrogenationobserved for
both catalysts can clearly be attributed to the triple bond of
the reactant being more strongly adsorbed (due to its more
electron-rich character) than the corresponding double bond.
Accordingly, as long as the reactant 4-octyne is present, it
displaces the cis-alkene from the active site, thereby prevent-
ing its readsorption, which would lead to overhydrogenation.
In view of this “poisoning” effect of 4-octyne, the exception-
ally high bond selectivity of the Pd–HTs is mainly thermo-
dynamically controlled. However, the lack of trans-alkene
formation may be related to a kinetic factor. As reported ear-
lier, the two effects are frequently combined and cannot be
readily distinguished [58].
For the sake of comparison, and with consideration of our
previous experiments on higher aromatic alkynes [37,41],
the catalytic performances of the Pd–HTs were further ex-
amined for the semihydrogenation of 1-phenyl-1-pentyne
(Figs. 8 and 9).
As demonstrated by Figs. 8 and 9, the catalytic activi-
ties of the Pd–HTs in the case of 1-phenyl-1-pentyne were
inferior to those observed for the hydrogenations of pheny-
lacetylene and 4-octyne. By a reaction time of 120 min, the
reaction rate had decreased considerably: the conversions
obtained for Pd–HT1 and Pd–HT2 were 69.2 and 57.6%,
Hydrogenations of alkynes on Pd–HT catalysts
c
d
Catalyst
Reactant
R
TOF
S
Y
3
−1
−1
(cm H min
2
(s
)
(%)
−1
g
Pd)
a
b
a
b
a
b
Pd–HT1
Pd–HT2
Pd–HT1
Pd–HT2
Pd–HT1
Pd–HT2
Phenylacetylene
Phenylacetylene
4-Octyne
4-Octyne
1-Phenyl-1-pentyne
1-Phenyl-1-pentyne
8264
4722
10559
5791
2296
1241
1.62 100
1.37 100
–
–
2.07
1.68
0.45
0.36
96.8
1
96.6 0.997
89.2 0.969
91.3 0.971
a
5
3
m = 10 mg, T = 298 K, p = 10 Pa, solvent: 1 cm of toluene, S:Pd =
2500.
b
5
3
m = 10 mg, T = 298 K, p = 10 Pa, solvent: 1 cm of toluene, S:Pd =
700.
c
Selectivity of the main reaction product (alkene or cis-alkene), deter-
mined at a conversion of 50%.
d
Y = S /S
, calculated from the selectivities at a conversion
cis
(cis+trans)
of 50%.
respectively. Further, loss of the initial stereoselectivity for
cis-alkene formation occurred to a lesser extent for Pd–HT2,
for which the cis stereoselectivity at 10 min was higher
(92.8%) than that for Pd–HT1 (89.9%). Both the overhy-
drogenation and the trans isomerization were found to be
more pronounced on Pd–HT1. For the latter, overhydrogena-
tion of both the reactant and the cis-alkene stereoisomer took
place. The finding that the stereoselectivity of trans-alkene
formation remained constant (2.9%) throughout the entire
reaction time interval indicated that the trans stereoisomer
was an initial product in the reaction [32,50,59]. On the other
hand, on Pd–HT2, 1-phenyl-trans-1-pentenewas formed via
isomerization of the cis-alkene stereoisomer [50,60,61] and
1-phenylpentane was produced through overhydrogenation
of the reactant.
Comparative data on the transformations of all reactants
investigated on these Pd–HTs are presented in Table 3.
It may be seen that the initial rates determined for Pd–
HT1 were considerably higher than those for Pd–HT2,

Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
379
which confirmed that, despite the marked difference in the
Table 4
Hydrogenations of alkynes on supported Pd catalysts
S:Pd ratios, the catalytic activity of Pd–HT1 surpassed that
of Pd–HT2 for all reactants. This activity difference is also
reflected in the TOF values. On the other hand, little or no
variation was experienced in the selectivities of the Pd–HTs
at the same conversion level, and thus the Y values for the
internal alkynes were nearly the same. It follows that, in
contrast with previous observations in the literature, the se-
lectivity of Pd–HT was not decreased by increasing the Pd
dispersion [34,46,49]. This finding, however, is not unusual,
as the lack of any particle-size effect on the selectivity has
also been reported [49,62]. In our case, increase of the Pd
dispersion mainly resulted in an enhanced catalytic activity,
as found for Pd–HT1, which is therefore regarded as a more
efficient catalyst than Pd–HT2.
a
b
c
Y
Catalyst
Reactant
R
TOF
S
3
−1
−1
(cm H min
2
(s
)
(%)
−1
g
Pd)
Pd/Al O Phenylacetylene
5486
5258
11172
9996
7272
10257
0.85 100
2.24 100
–
–
2
3
Pd/C
Phenylacetylene
Pd/Al O 4-Octyne
1.72
4.26
1.12
4.37
91.3 0.985
90.7 0.983
83.2 0.958
80.5 0.957
2
3
Pd/C
4-Octyne
Pd/Al O 1-Phenyl-1-pentyne
2
3
Pd/C
1-Phenyl-1-pentyne
a
5
3
m = 10 mg, T = 298 K, p = 10 Pa, solvent: 1 cm of toluene, S:Pd =
250.
b
Selectivity of the main reaction product (alkene or cis-alkene), deter-
mined at a conversion of 50%.
c
Y = S /S
, calculated from the selectivities at a conversion
cis
(cis+trans)
of 50%.
As revealed by Table 3, the catalytic performances of
the Pd–HT samples were also affected by the reactivity and
size of the substrate molecules. It is generally suggested
that terminal alkynes hydrogenate faster than internal ones,
although, depending on the nature of the substituents, excep-
tions may occur [30,63]. In the current study, the activities
for the hydrogenationof the terminal aromatic alkyne pheny-
lacetylene proved to be lower than those for the internal
aliphatic alkyne 4-octyne, although there was no substan-
tial difference in the TOF values. It may be assumed that the
hindering effect of the bulky aromatic ring on the chemisorp-
tion of the alkyne species was comparable to that of the
long carbon chain of 4-octyne. On the other hand, a pro-
nounced fall in the catalytic activities of both Pd–HTs, in-
dicated by a severalfold decrease in the TOF values, was
established for the hydrogenation of the bulkiest reactant,
the internal aromatic alkyne 1-phenyl-1-pentyne,which sug-
gests rather limited access of this reactant to the active sites.
Nevertheless, a molecular sieving effect of the Pd–HTs may
be excluded, in part because of the presence of Pd parti-
cles on the external surface of the HT host. Furthermore,
each substrate is sufficiently small, at least in one dimen-
sion, to enter the solvated toluene interlayers. (The critical
dimension of phenylacetylene, essentially determined by the
thickness of the benzene ring, is 0.3 nm [64] and the extent
of swelling for Pd–HT1 in toluene, relative to the dry state,
is ∆ = 0.75 nm, obtained from the corresponding dL val-
ues (see Tables 1 and 2).) It is therefore suggested that the
apparent substrate size dependence arises from spatial re-
quirements for reactant chemisorption both on the external
surface sites and in the restricted interlamellar region. At any
rate, the organophilic character of the HT material ensures
that the interlayer spacings of the solvated Pd–HTs are large
enough to accommodate each substrate, except when hexane
was applied as a solvent, in which the extent of swelling was
merely ∆ = 0.07 nm. Accordingly, no catalytic discrimina-
tion can be made between our reactants on the basis of their
size and shape [65].
tigated for the same reactions. The results are displayed in
Table 4.
The evidence that the TOF values obtained for Pd/C
were significantly higher for each reaction than those for
Pd/Al2O3 may be related in part to the organophilic char-
acter of the carbon support, which ensured a more facile dis-
persion of this sample in toluene than that of Pd/Al2O3. For
the transformation of phenylacetylene, the catalytic activi-
ties of the supported Pd samples proved to be similar to those
of the Pd–HTs. Although the TOF value obtained for Pd/C
exceeded those for the Pd–HT catalysts, none of the differ-
ences were substantial. The selectivity of styrene formation
was 100% for each sample. For the semihydrogenation of
4-octyne, the catalytic activities of the Pd–HTs were of a
similar order to that of Pd/Al2O3, whereas the TOF value for
Pd/C was considerably higher. On the other hand, the stere-
oselectivities of cis-alkene formation observed for the sup-
ported catalysts proved to be lower than those for the Pd–HT
samples. For the transformation of 1-phenyl-1-pentyne, sim-
ilar observations were made, the differences between the cis
stereoselectivities of the Pd–HTs and those of the supported
samples being even more pronounced. The evidence that the
catalytic activities of both supported Pd samples surpassed
those of the Pd–HTs is in accordance with our previous find-
ings concerning a limited access for 1-phenyl-1-pentyne to
the active sites of the Pd–HTs, including those in the in-
terlamellar region. It may be concluded that, even at a low
S:Pd ratio of 250, the catalytic activities of the supported Pd
catalysts were not substantially higher than those of the Pd–
HT samples, except for the reaction of 1-phenyl-1-pentyne.
Moreover, for the transformations of internal alkynes, the
Pd–HTs proved to be more stereoselective catalysts.
It is worth mentioning that, for the semihydrogenation
of 1-phenyl-1-pentyne, the catalytic activities of the Pd–
HT samples proved to be somewhat lower than those of
low-loaded Pd-alkylammonium montmorillonites [41]. This
may be attributed to several factors, including minor dif-
ferences in the size distributions of the surface Pd crystal-
lites. More importantly, the disaggregation of solvated HT
For further comparison, the catalytic performances of
supported Pd catalysts (Pd/Al2O3 and Pd/C) were inves-

380
Á. Mastalir, Z. Király / Journal of Catalysis 220 (2003) 372–381
materials occurs less readily than that of montmorillonite.
Further, a higher alkyl chain packing density between the
HT layers must be considered, which implies that, as com-
pared with organophilic montmorillonite, only a limited in-
terlamellar space in the Pd–HTs is available for the guest
molecules, even if the alkyl chains are in a perpendicular
orientation [66].
It is important to note that the cis stereoselectivities for
the semihydrogenations of disubstituted acetylenes on Pd–
HTs were exceptionally high, approaching 100%, which was
actually experienced for the bond selectivity of styrene for-
mation. The evidence that overhydrogenations of the reac-
tants on both catalysts were either suppressed or nonexistent
may be regarded as indirect evidence of the absence of Pd
β-hydride, a potential hydrogen supply source that tends to
promote oversaturation [46,67,68].
for the transformation of 1-phenyl-1-pentyne, the bulkiest
substrate, may be attributed to steric restrictions of reac-
tant chemisorption, rather than to a molecular sieving effect
of the catalysts. For this reaction, cis-alkene stereoselectiv-
ities of at least 86% were obtained. For the transformations
of internal alkynes, the Pd–HT samples proved to be more
stereoselective than the conventional supported Pd catalysts.
Increase of the Pd dispersion of Pd–HT resulted in an
enhanced catalytic activity for all reactants, due to a site
modification effect, but it had no appreciable influence on
the alkene selectivity.
The exceptionally high bond selectivities and cis-alkene
stereoselectivities of the Pd–HT catalysts may be attributed
to their low metal loading and controlled Pd particle size.
Further studies to extend their applications in the field of
organic catalysis are currently in progress.
As recently suggested by Somorjai, a major challenge in
future catalysis research is the achievement of 100% selec-
tivity for the desired product, i.e., the absence of wasteful
or polluting by-products. It is claimed that this goal can be
attained by controlled catalyst fabrication [69,70]. In view
of this perspective, our Pd–HT samples with controlled Pd
particle size can be regarded as promising catalysts with im-
proved selectivities under mild conditions, which may find
further applications in the synthesis of fine chemicals.
Acknowledgment
Financial support of the National Scientific Foundation
through OTKA Grant T 042521 is gratefully acknowledged.
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