Mesostructured silicas as supports for palladium-catalyzed hydrogenation of phenyl acetylene and 1-phenyl-1-hexyne to alkenes

Article abstract of DOI:10.1016/j.molcata.2005.11.031

The stereoselective hydrogenation of phenyl acetylene and 1-phenyl-1-hexyne at 298 K and atmospheric pressure of H₂ over Pd catalysts supported on mesostructured silica was studied. The catalysts were prepared by the impregnation of HMS and MSU

Full text of DOI:10.1016/j.molcata.2005.11.031

Journal of Molecular Catalysis A: Chemical 247 (2006) 145–152
Mesostructured silicas as supports for palladium-catalyzed hydrogenation
of phenyl acetylene and 1-phenyl-1-hexyne to alkenes
a,∗
a
b
Norman Mar ´ı n-Astorga , Gina Pecchi , Thomas J. Pinnavaia ,
c
a,∗
Gabriela Alvez-Manoli , Patricio Reyes
a
Departamento de Fisicoqu ´ı mica, Facultad de Ciencias, Universidad de Concepci o´ n, Casilla 160-C, Concepci o´ n, Chile
b
Department of Chemistry, Michigan State University, East Lansing, MI 48824-1322, USA
c
Departamento de An a´ lisis Instrumental, Facultad de Farmacia, Universidad de Concepci o´ n, Casilla 237, Concepci o´ n, Chile
Received 30 September 2005; received in revised form 4 October 2005; accepted 16 November 2005
Available online 28 December 2005
Abstract
The stereoselective hydrogenation of phenyl acetylene and 1-phenyl-1-hexyne at 298 K and atmospheric pressure of H
2
over Pd catalysts
supported on mesostructured silica was studied. The catalysts were prepared by the impregnation of HMS and MSU-X silicas with 3-D wormhole
framework structures and MCM-41 silica with a 1-D hexagonal framework using a toluene solution of Pd(acac) to obtain a metal content close
to 1 wt.%. All the supports were characterised by nitrogen adsorption–desorption isotherms at 77 K and XRD. The catalysts were characterized
by H chemisorption and TEM measurements. The reactions were found to be zero order with respect to the phenyl acetylene and 1-phenyl-1-
2
2
hexyne concentration. Each catalyst presented a different catalytic performance. The 1%Pd/HMS catalyst was the most active in comparison with
the 1%Pd/MSU-X and 1%Pd/MCM-41 catalysts. This superior performance in the case of the HMS support was attributed to the presence of
interconnected framework channels and textural mesoporosity that can increase the accessibility of the Pd centers to a greater extent than the more
monolithic MSU-X and MCM-41 supports. All catalysts displayed high selectivity to styrene and cis-1-phenyl-1-hexene compounds.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Mesostructured; Pd; MSU-X; HMS; MCM-41; Silica; Stereo-selectivity
1
. Introduction
and pillared clays for stereo-specific hydrogenation reactions
partly due to their large pore size, which offers the possibil-
ity of minimizing diffusion limitations. Indeed, 1%Pd/MCM-41
has been shown to be an effective catalyst for the reduction of
phenyl alkyl acetylene to cis-alkene [8]. To overcome diffusion
resistance, large pore supports are usually preferred and recent
interest in heterogeneous catalysis has focused on the develop-
ment of a new family of mesoporous HMS and MSU-X silicas
with wormhole framework structures [14–18]. HMS silica has
been reported to exhibit a higher olefin adsorption capacity than
exhibited by the corresponding alkanes. The higher affinity of
the HMS framework for propylene over propane was verified by
heats of adsorption for propylene that are significantly higher
than for propane [19].
Semi-hydrogenation of alkynes is a versatile reaction widely
used in organic synthesis [1] and often performed by noble metal
supported catalysts [2]. In order to obtain higher selectivity and
activity different supports matrices such as silica [3,4], carbon
[
5,6], and mesostructured MCM-41 silica [7,8] have been used.
Although zeolites and pillared clays have been successfully used
as supports of palladium catalysts for the production of fine
organic chemicals [9–13], they have limited utility for the trans-
formations of large molecules in the liquid state due to diffusion
limitations caused by the restricted pore sizes.
Mesostructured silica with pores in the 2–5 nm range have
been recognised as potentially superior supports over zeolites
It was of interest in the present work to verify the cat-
alytic properties of these mesostructured silica supports for the
selective hydrogenation of alkynes, a reaction, which also is of
industrial interest in fine chemical synthesis. Indeed, the com-
plete elimination of alkynes from alkene feed stocks via selective
∗
Corresponding authors.
E-mail addresses: nmarin@udec.cl (N. Mar ´ı n-Astorga),
preyes@udec.cl (P. Reyes).
1
381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.11.031

1
46
N. Mar ´ı n-Astorga et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 145–152
hydrogenation is an important process in the polymer industry
20]. Pd catalysts supported on low surface area alumina are
and the selectivity of Pd-catalysts for the liquid phase stere-
oselective hydrogenation of phenyl acetylene and 1-phenyl-1-
hexyne (Fig. 1). In addition to hexagonal MCM-41 with one-
dimensional framework pores, we have included in the study
HMS and MSU-X mesostructures, which have wormhole and
hexagonal framework structures, respectively. Specific surface
areas were determined from the nitrogen adsorption isotherms,
and metal dispersion from H2 chemisorption. TEM and XRD
studies were also performed. The liquid phase hydrogenation
phase of phenyl acetylene and 1-phenyl-1-hexyne was per-
formed at 298 K, 1 bar using tetrahydrofuran (THF) as solvent.
[
usually employed in the industrial process, but there has been
renewed interest in supported palladium catalysts [21,22] in an
effort to find more selective processes [23,24]. Several workers
have studied the selective hydrogenation of alkynes to alkenes
not only from the theoretical point of view, but also for indus-
trial utilization, particularly in the selective elimination of low
amounts of alkynes from alkenic fractions. Phenyl acetylene is
the commonly studied substrate for hydrogenation in the liquid
phase, and is often used as a probe molecule to compare catalytic
activity.
The catalytic hydrogenation of alkynes is the easiest of all
hydrogenation reactions, generally taking place in the presence
of all other functional groups including conjugated dienes and
aromatic nitro groups. Alkyne hydrogenation is enhanced if the
triple bond is terminal. The complete hydrogenation of alkynes
to saturated compounds occurs on all common hydrogenation
catalysts at room temperature and atmospheric pressure. This
reaction, and the related semi-hydrogenation to produce alkenes,
is important in synthetic procedures because the acetylenic
group can participate in a wide variety of substitution reac-
tions, and thus can join two segments of a carbon chain to
produce a variety of aliphatic species. The selective modifica-
tion of the triple bond to form either a double or single bond
further enhances the synthetic utility of acetylenic materials.
The selective hydrogenation of a triple bond to give an alkene
without concomitant positional or geometric isomerisation is
particularly important in synthetic procedures and many indus-
trial processes. In the absence of any isomerisation, selective
partial hydrogenation of a di-substituted alkyne produces the
cis-alkene. Small amounts of the trans-alkene are sometimes
formed in these reactions, but catalytic processes do not lead to
the production of the trans-olefin as the primary product.
The present work was undertaken with the aim to study
the influence of mesostructured silicas supports on the activity
2. Experimental
2.1. Materials
Phenyl acetylene and 1-phenyl-1-hexyne were obtained form
Aldrich and used without further purification. The solvent
tetrahydrofurane (THF) was obtained from Aldrich. THF was
dried with sodium wires and distilled under a nitrogen atmo-
sphere before use. Fumed silica (Sigma), cetyltrimethylam-
monium bromide (CTAB) (Aldrich), tetramethylammonium
hydroxide (TMAOH) at 25% solution in water (MERCK),
tetraethylorthosilicate (TEOS) (Aldrich), dodecylamine (DDA)
(Aldrich), and decaoxyethylene cetyl ether (Brij 56) (Aldrich)
were used for support synthesis, and hydrogen (99.995% purity,
an AGA product) was used.
2.2. MCM-41 silica
The synthesis of hexagonal MCM-41 was performed using
a standard procedure [25]. Basically, MCM-41 was prepared
with gel of the composition SiO :CTABr:TMAOH:H O =
1:0.15:0.26:24.3. A mixture of fumed silica, cetyltrimethylam-
monium bromide, trimethylammonium hydroxide, and water
was stirred at room temperature for 1 h. The synthesis gel was
2
2
Fig. 1. Reaction diagram for the stereoselective hydrogenation of 1-phenyl-1-hexyne.

N. Mar ´ı n-Astorga et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 145–152
147
◦
heated at 100 C for 24 h in an autoclave to obtain hexagonal
Powder X-ray diffraction patterns were measured using Cu-K␣
radiation (λ = 1.542 A) and a Rigaku diffractometer equipped
˚
MCM-41 silica. The final product was filtered, washed, dried at
room temperature, and calcined in nitrogen and air at 540 C for
h to remove the surfactant.
◦
with a rotating anode operated at 45 kV and 100 mA. Counts
◦ ◦
were accumulated every 0.02 (2θ) at a scan speed of 1
6
−
1
2θ min
.
2
.3. MSU-X silica
Metal dispersion was determined by H2 chemisorption in an
automatic Micromeritics apparatus Model ASAP 2010. Hydro-
gen chemisorption was carried out at 343 K in the pressure range
1–100 mmHg. The hydrogen uptake was evaluated from the irre-
versible amount of adsorbed H2 as determined by the difference
between the first (total) and the second (reversible) isotherms.
TEM micrographs were obtained in a Jeol Model JEM-1200
EXII System. The specimen was loaded onto a holey carbon
film that was supported on a copper grid by dipping the grid
into a sample suspension in ethanol. The catalysts samples were
prepared by the extractive replica procedure.
This mesostructured silica with a wormhole framework struc-
ture was prepared as follows. A 10 wt.% of a micellar solution
waspreparedbydissolvingdecaoxyethylenecetylether(Brij56)
in distilled water. Sulphuric acid was then added to adjust the pH
◦
to a value of 2. After homogenization at 70 C, TEOS was added
drop-by-drop, and stirred further for 1 h. Hydrothermal treat-
◦
ment was performed at 40 C for 3 days. The surfactant/silica
molar ratio was 0.50. The recovered gel was then extracted with
ethanol using a Soxhlet apparatus to eliminate the remaining sur-
factant molecules in the porous structure, and then dried under
air at room temperature [26,27].
2.7. Catalytic experiments
2
.4. HMS silica
Semi-hydrogenation in liquid-phase of phenyl acetylene and
1-phenyl-1-hexyne was performed in a batch reactor at 298 K
A second silica mesostructure with a wormhole framework
and 1 bar of H2 pressure. In each measurement, 10 mg of the
powdered catalyst was placed in the reaction reactor. Hydro-
genation was conducted under efficient stirring (1400 rpm) to
eliminate diffusion control. The total volume used was 15 ml.
A molar ratio substrate:Pd of 2500 was used. Prior to the reac-
tion, the sample was reduced at 573 K during 1 h in H2 flow
and cooled to the reaction temperature. Then, the solvent (dried
THF) was injected and the substrates were fed under constant
stirring. Samples were taken periodically and analysed by a Star
VARIAN 3400-CX gas chromatograph with a capillary col-
umn DB-Wax (0.53 mm, 30 m) and flame ionisation detector
(FID). The reaction products were identified with a Shimadzu
QP5050A GC-MS apparatus.
structure, denoted HMS, was prepared according to previously
reported methods [18]. Dodecylamine (DDA, 4.9 mmol) was
dissolved in 5 ml of ethanol, and 45 ml of H2O was then added
to obtain a 90:10 (v/v) H2O/EtOH solution of the surfactant. The
surfactant solution was heated to the desired reaction tempera-
ture, and tetraethyl orthosilicate (TEOS, 19.6 mmol) was added
◦
◦
to give a reaction mixture with an I /S ratio of 4. The reaction
flask was sealed and shaken at 220 rpm in a heated water bath
◦
at the desired synthesis temperature, 60 C for 20 h. The result-
ing white precipitates were filtered out, washed with copious
amounts of H2O, and allowed to air dry at room temperature for
◦
2
4 h. The surfactant was removed by calcination in air at 600 C
for 4 h.
3. Results and discussion
2
.5. Catalysts
3.1. Structural and textural properties of the supports
Calcined forms of mesostructured HMS, MSU-X and MCM-
1 silicas were impregnated using a slight excess of solution
4
The XRD patterns for HMS, MSU-X and MCM-41 silica
required to fill the pore volume of the supports with a solution
of Pd(acac)2 in toluene in the amount required to obtain 1 wt.%
of Pd. The catalysts were dried at 373 K and calcined at 673 K
underairflow for4 h. The catalysts werelabelled as 1%Pd/HMS,
mesostructures are presented in Fig. 2. The MCM-41 mesophase
exhibits four hkl diffraction lines consistent with the expected
hexagonal framework structure. The diffraction peak related to
the (1 0 0) plane exhibits the highest intensity among all of the
synthesised samples, which can be taken as an indication of the
higher long range ordering of the mesoporous channels [25].
The wormhole framework of HMS exhibits a single diffraction
line. For the MSU-X sample, the picture is different. In addition
to the main reflection peak at low angle 2θ, the secondary reflec-
tions cannot be well distinguished, reflecting less ordering and
homogeneity in the sample (Fig. 2) in comparison to MCM-41
and tending to a wormhole structure. TEM images (Fig. 3) ver-
ify the disordered channel arrangement and wormhole structure
assignments for the MSU-X and HMS mesophases, as well as
the hexagonal framework structure for MCM-41 material. The
XRD patterns of the Pd catalysts indicated the retention of the
framework structure after Pd incorporation.
1
%Pd/MSU-X and 1%Pd/MCM-41. The catalysts were reduced
in situ in hydrogen at 573 K for 1 h prior to their characterisation
or catalytic evaluation.
2
.6. Characterisation
Specific area and porosity were obtained from nitrogen
adsorption–desorption isotherms at 77 K performed in an auto-
matic Micromeritics apparatus Model ASAP 2010 in the
0
.05–0.995 relative pressure range. BET surface areas were cal-
culated from a linear part of the BET plot according to IUPAC
recommendations. Pore-size distributions were calculated from
the N2 adsorption branch using the Horvath–Kawazoe model.

1
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N. Mar ´ı n-Astorga et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 145–152
Table 1
Textural properties of the silicate mesophases
Sample
d1 0 0
BET
(m /g)
Pore diameter
HK (nm)
Wall thicknessa
(nm)
VTb
(cc/g)
2
(
nm)
HMS
MSU-X
MCM-41 4.1
4.5
5.8
910
1086
1469
3.2
5.1
3.3
1.3
0.7
0.8
0.9
1.4
2.4
a
Wall thickness calculated by subtracting the HK pore diameter from the
correlation distance as determined by X-ray diffraction.
b
VT, total volume obtained from the volume of N2 adsorbed at 0.99 P/P0.
The textural properties of the silicate mesophases are pro-
vided in Table 1. All the isotherms are type IV, characteristic of
mesoporous compounds, with a type H1 hysteresis loop [28].
0
The capillary condensation occurs at different P/P values for
the MSU-X sample, indicative of variable pore size. The sur-
face areas, as determined by fitting the BET equation to nitrogen
adsorption isotherms in the partial pressure region below 0.30,
decreased in the order MCM-41 > HMS > MSU-X. The BET
surface areas of the Pd catalysts show that only small changes
occur in the textural characteristics. It can be concluded that Pd
Fig. 2. XRD patterns of the silicate mesophases.
Fig. 3. Transmission electron micrograph of mesostructured supports (a) HMS, (b) MSU-X and (c) MCM-41.

N. Mar ´ı n-Astorga et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 145–152
149
Table 2
Table 3
H/Pd ratio obtained from chemisorption data and metal particle size of Pd-
supported catalysts
Specific rate constant (k), turnover frequency (TOF) and selectivity to the styrene
in the hydrogenation of phenyl acetylene at 298 K on Pd-supported catalysts
−
1
−1
a
−1
Catalysts
H/Pd
dH2 (nm)
dTEM (nm)
Catalysts
k (mol l min
TOF (s
)
Selectivity to
styrene (%)
−
1 a
b
gcat
)
1
1
1
%Pd/HMS
%Pd/MSU-X
%Pd/MCM-41
0.14
0.25
0.21
7.2
3.9
4.5
0.7
1.1
3.2
1%Pd/HMS
1%Pd/MSU-X
0.18
0.16
0.14
4
2
2
96
96
96
1%Pd/MCM-41
a
Conversion level 40%.
Conversion level 80%.
b
impregnation does not significantly affect the original structure,
in agreement with the XRD results. Note that mesostructured
catalysts have framework pore sizes in the range 3–5 nm, which
are large enough to accommodate the starting reagent and the
reaction products. Crocker et al. [29] reports that the critical
dimension of phenyl acetylene, essentially determined by the
thickness of the benzene ring, is 0.3 nm. It is therefore suggested
that the apparent substrate size dependence (see below) arises
from spatial requirements for reactant chemisorption both on the
external surface sites and in the restricted internal region.
provides the Pd particles with more resistance to sinterisation
under high temperature reduction and H flow.
2
3.3. Phenyl acetylene semi-hydrogenation
The semi-hydrogenation of phenyl acetylene on Pd-
supported catalysts was studied in tetrahydrofurane (THF) at
a substrate:Pd ratio of 2500. To minimize mass transfer limita-
tion, the catalysts were used as powdered samples with a small
catalyst grain size (<200 ␮m) and the stirring was 1400 rpm. Pd
catalyst activity and selectivity are related to high electron den-
sity and the rigidity of triple bond. This results in considerably
strongerchemisorptionthanthatofthecorrespondingofthedou-
ble bond [24]. During the activity measurements performed in
hydrogen, the alkyne concentration changed linearly over time.
The calculated reaction rate was denoted k. The turnover fre-
quencies (TOF), reported in Table 3, were determined from the
rate constant normalised by the number of palladium sites, and
the selectivity to styrene, calculated as the ratio of the styrene
concentration over total products concentration at a total conver-
sion of 80%, are reported in Table 3 for the palladium catalysts.
Fig. 4 compares the zero-order plots for different catalysts
under identical reaction conditions. The linear decrease in the
alkyne concentration over time was maintained up to a reaction
timeof90 minforallofthePdcatalystsstudiedandaalkynecon-
version of >90%, confirming that zero order kinetics applies (the
initialrateofsubstratehydrogenationcorrespondstothereaction
3
.2. Characterisation of Pd catalysts
Table 2 provides the H2 chemisorption data for the cata-
lysts used in the semi-hydrogenation of phenyl acetylene and
-phenyl-1-hexyne. The H/Pd ratio of 1%Pd/HMS catalyst is
lower than the ratio found for the 1%Pd/MSU and 1%Pd/MCM-
1 catalysts. The H/Pd ratio of 0.14, obtained for 1%Pd/HMS,
1
4
suggests the presence of surfactant residues on the metallic sites
of palladium. The surfactant coating of the Pd ensembles results
in a significant decrease of the ability to chemisorb hydrogen.
H2 chemisorption generally takes place dissociatively, resulting
in the formation of two non-stoichiometric bulk hydride phases
◦
coexisting below the critical temperature (300 C). At low H2
concentrations, the ␣-hydride phase is formed [30], which is a
disordered solid solution almost identical in structure to the pure
Pd lattice. At a high H2 concentration, hydrogen-rich ␤-hydride
phase is formed, which is an ordered interstitial solid solution
exhibiting non-ideal behaviour [30].
TEM results for the Pd on HMS, MSU-X or MCM-41 cata-
lysts revealed a very narrow palladium particle size distribution.
The average values of the metal particle size obtained by TEM
presented a lower value for 1%Pd/MSU-X and 1%Pd/HMS cat-
alysts, indicating higher Pd dispersion for both catalysts. When
TEM results are compared with chemisorption values, signifi-
cant differences were observed for 1%Pd/HMS catalyst. This
behaviour supports the presence of surface organic residues,
resulting in a decreased ability to chemisorb hydrogen. The
narrowest size distribution was observed for 1%Pd/HMS, with
8
5% of the particles ranging in size between 0.6 and 0.8 nm.
Accordingly, the average particle diameters of 1%Pd/HMS and
%Pd/MSU-X proved to be 0.7 and 1.1 nm, respectively (see
Table 2).
On the other hand, a metal particle size in the range of 2–4 nm
1
was observed for 1%Pd/MCM-41, including a large number of
crystallites with mean particle diameter of 3.2 nm. One possible
explanation for this behaviour could be that during the precur-
sor reduction, the confinement inside the lower size channel
Fig. 4. Kinetic curve obtained for the phenyl acetylene hydrogenation on Pd-
supported catalysts, T = 298 K, PH = 1 bar, R:P = 2500 and THF as solvent.
2

1
50
N. Mar ´ı n-Astorga et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 145–152
rate). This behaviour is in agreement with previously reported
results for phenyl acetylene hydrogenation over number of Pd-
supported catalysts [7]. For the 1%Pd/HMS, the activity was
slightly higher than that for 1%Pd/MSU-X and 1%Pd/MCM-41
catalysts (see Fig. 4), respectively. This result is not consistent
with the difference in the Pd dispersions (see Table 2). This
is indicative of a wormhole mesostructure silica effect because
the interconnected channels can enhance the activity compared
to catalysts having straight channels as possessed by the other
studied supports. It can be readily seen that an increase in the
Pd particle size of 1%Pd/MCM-41 and 1%Pd/MSU-X resulted
in a decrease in the initial rate when comparing 1%Pd/HMS
indicating a predominant formation of styrene. Fig. 5 reveals
that the over-hydrogenation product, ethyl benzene, was formed
by simultaneous hydrogenation of the styrene molecule at high
conversions. The trend was similar for all the studied systems.
The mode of adsorption over different mesostructured solids
as supports is the only defining factor in the phenyl acety-
lene reactivity. Alkynes are known to have strong complexing
abilities, and this is one of the reasons for the high semi-
hydrogenation selectivity over Pd, the adsorbing alkyne prevents
the re-adsorption of the product alkene and hence the consecu-
tive hydrogen addition cannot take place [31–33]. A comparison
of the selected data for styrene selectivity is also presented in
Table 3. It should be mentioned that styrene selectivity is fairly
constant, in the range 96–98%, and up to approximately 80%
of conversion for all studied catalysts. At higher conversion, a
decrease in the selectivity and a simultaneous increase in the
saturated compound derivative are observed (see Fig. 5).
(
see TEM values in Table 2), the most active sample, with a
decline in the reaction rate that was observed for 1%Pd/MCM-
1. Under the above conditions, 1%Pd/HMS, the catalyst with
4
the smallest particle size, was the most efficient catalyst for the
semi-hydrogenation of phenyl acetylene.
The results for the Pd-supported catalysts on mesostructured
solids are presented in Table 3. The results indicate that the
TOF for the 1%Pd/HMS catalyst is twice the values obtained
for the other two Pd-supported catalysts under the same con-
ditions. 1%Pd/HMS, the sample with the smallest particle size
3.4. 1-Phenyl-1-hexyne semi-hydrogenation
For 1-phenyl-1-hexyne hydrogenation, the reaction rate (k),
the turnover frequency (TOF), and the selectivity to cis-1-
phenyl-1-hexene (defined as the ratio of cis-1-phenyl-1-hexene
to all of the hydrogenated products) are compiled in Table 4 for
the palladium catalysts. In contrast with the reaction of phenyl
acetylene, marked differences in activity were observed between
1%Pd/HMS and 1%Pd/MSU-X, 1%Pd/MCM-41 catalysts. This
implies that the pronounced initial activity of 1%Pd/HMS is
characteristic of the small Pd particles and the mesoporous
interconnected channels. The differences mainly reflected in
the activity was even more drastic than that for the reaction of
phenyl acetylene, and may be associated with the differences in
the chemical structure and size of the phenyl acetylene and 1-
phenyl-1-hexyne, while no appreciable difference was observed
for selectivity.
(
see TEMresults, Table 2)and mesoporous interconnected chan-
nels, was shown to be the most efficient catalysts for the semi-
hydrogenation of phenyl acetylene. On other hand, the catalytic
performances indicate that certain types of mesostructured solid
can enhance catalytic activity.
On palladium-supported catalysts, the semi-hydrogenation
of phenyl acetylene basically results in the predominant forma-
tion of styrene, which is attributed to the associative adsorption
of the reactant and the consecutive addition of two adsorbed
hydrogen atoms from below the axis of the triple bond [24].
All the catalysts studied demonstrated that high selectivity to
styrene (96%) was achieved and remained constant even for
the highest conversion degrees (see Fig. 5). Fig. 5 displays the
evolution of the selectivity with respect to conversion level in
the phenyl acetylene hydrogenation for a representative catalyst,
The hydrogenation of 1-phenyl-1-hexyne was carried out at
298 K and 1 bar in THF. No catalyst deactivation during the
catalytic run was observed.
The semi-hydrogenation of alkynes has been investigated
on some Pd-supported catalysts [7,11–13,34]. Michalska et al.
[35] reported 77 and 80% cis-alkene selectivity for hydrogena-
tion of 1-phenyl-1-propyne and 1-phenyl-1-butyne, respectively,
over palladium catalysts supported on heterocyclic polyamides.
In previous experiments on phenyl alkyl acetylenes, a ∼95%
cis-alkenes selectivity was reported for the hydrogenation of
different phenyl alkyl acetylenes on palladium catalysts sup-
Table 4
Specific rate constant (k), turnover frequency (TOF) and selectivity to the cis-
isomer in the hydrogenation of 1-phenyl-1-hexyne at 298 K on Pd-supported
catalyst
−
1
−1
3
−1
)
Catalysts
k (mol l min
TOF (×10 ) (s
Selectivity to
cis-isomer (%)
−
1
a
gcat
)
1
1
1
%Pd/HMS
%Pd/MSU-X
%Pd/MCM-41 0.11
0.27
0.10
60
11
6
79
79
83
Fig. 5. Selectivity to hydrogenated products in the hydrogenation of phenyl
acetylene over 1%Pd/HMS catalyst at 298 K, PH = 1 bar and R:P = 2500.
a
2
Conversion level 50%.

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151
Fig. 6. Kinetic curve obtained for 1-phenyl-1-hexyne hydrogenation over Pd
Fig. 7. Selectivity as a function of conversion for 1-phenyl-1-hexyne hydro-
supported catalysts at 298 K, PH = 1 bar and R:P = 2500.
2
genation on 1%Pd/HMS catalyst at 298 K, PH = 1 bar and R:P = 2500.
2
to several factors, including differences in the size particles.
More importantly, the wormhole framework effect occurs when
the structure contains interconnected channels. Furthermore, a
larger alkyl chain limits the available space inside the chan-
nels, which may be affect molecule mobility. Therefore, the
probability for reaction with hydrogen is higher for the larger
molecules, as is the case for 1-phenyl-1-hexyne. Additionally,
it is worth mentioning that this effect is even more important in
the mesostructured solids.
ported on mesoporous and pillared clays materials with a linear
trend for the conversion percentage versus reaction time [8,36].
Mesostructured solids, such as HMS and MSU-X, display a new
advantage for the hydrogenation of phenyl alkynes with long
alkyl chains as supports of palladium catalysts.
Fig. 6 displays the evolution of the alkyne concentration over
time in the 1-phenyl-1-hexyne hydrogenation for all the studied
catalysts. It can be seen that the conversion increases linearly
with time indicating a zero order dependence on the concentra-
tion of the 1-phenyl-1-hexyne molecule. Even though the trend
was similar for all the studied catalysts, significant differences in
the catalytic activity were observed, where the 1%Pd/HMS cata-
lyst was the most active. A summary of the results, expressed as
zero-order specific rate constant and turnover frequency (TOF),
for the three studied catalysts are provided in Table 4.
It is remarkable that the 1%Pd/HMS catalyst exhibits much
higher activity in comparison with the other Pd-supported cat-
alysts. The observed behaviour may be attributed to the worm-
hole framework effect and the high textural porosity since the
presence of interconnected channels can enhance the activity
compared with straight channels as observed in the others stud-
ied supports. This result is due to an increased possibility of
reactant entrance in multiple locations. This trend is similar
to the one observed in the phenyl acetylene hydrogenation,
where Pd ensembles were partially destroyed by the presence of
chemisorbed nitrogen compounds that had not been completely
eliminated in the synthesis process, producing a significant drop
in the H/Pd ratio. For 1%Pd/MSU-X and 1%Pd/MCM-41 cata-
lysts, the turnover frequencies (TOF) and specific rate constant
Fig. 7 shows the evolution of the selectivity during 1-phenyl-
1-hexyne hydrogenation for a representative Pd-supported cat-
alyst. The main product is the cis-1-phenyl-1-hexene with a
lowproportionoftrans-1-phenyl-1-hexeneand1-phenylhexane,
respectively. The selectivity to the cis-1-phenyl-1-hexene is
fairly constant up to approximately 70% of conversion, when
it starts to decrease. Simultaneously, an increase in both the
trans-1-phenyl-1-hexene and the 1-phenylhexane derivative is
observed. Similar behaviour was exhibited by the other catalysts
with some differences in the selectivity levels. Table 4 sum-
marises the selectivity for the different studied catalysts during
thehydrogenationofthe1-phenyl-1-hexyneobtainedatthesame
conversion level for the studied catalysts. The selectivity to the
cis-isomers is in the range 80–83% for conversion levels 10 and
50% for all studied catalysts. As reported in the literature, trans-
isomer is always formed in the alkyne hydrogenation, either as
initial products or as a result of isomerisation of the cis-isomers
[24]. It is generally accepted that direct trans-isomer formation
may occur via the addition of molecular hydrogenation to the
adsorbed alkyne molecule [37]. The problem of stereoselectivity
has been rarely addressed in recent years and only a few papers
have disclosed results on the hydrogenations of internal alkynes
(
k) (Table 4)do notdisplay many differences, andtheyfall within
the experimental error. The higher TOF values obtained for
%Pd/HMS (see Table 4) were significantly higher in compar-
[7,35,38].
1
ison to those exhibited by 1%Pd/MSU-X and 1%Pd/MCM-41
catalysts may also be partially related to the differences in the
mesostructure support, which ensured an easier solvatation of
the reactant with THF due to interconnected channels, which
is not observed in the other catalysts. This may be attributed
4. Conclusions
The obtained results show that, in general, the effect of the
support on the catalytic performance of palladium-supported

1
52
N. Mar ´ı n-Astorga et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 145–152
[12] A. Mastalir, Z. Kir a´ ly, J. Catal. 220 (2003) 372.
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solids are effective catalysts in the stereoselective hydrogenation
of phenyl acetylene and 1-phenyl-1-hexyne in the liquid phase.
The catalytic performance was different for each catalyst stud-
ied. The most efficient catalyst was 1%Pd/HMS in comparison
to 1%Pd/MSU-X and 1%Pd/MCM-41 catalysts. This behaviour
is attributed to the presence of interconnected wormhole chan-
nels in 1%Pd/HMS, which permits an increase in accessibility
in comparison with the other studied supports. The catalytic
activity is modified due to a mesostructured material effect and
the preferential adsorption of phenyl alkynes molecules. The
main hydrogenation product was the cis-1-phenyl-1-hexene and
styrene, and over-hydrogenation to alkanes only appears at high
conversion levels.
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