catalytic investigation of pd particles supported on mcm-41 for the selective hydrogenations of...

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Applied Catalysis A: General 289 (2005) 256–266

Catalytic investigation of Pd particles supported on MCM-41 for

the selective hydrogenations of terminal and internal alkynes

Attila Papp, Arpad Molnar, Agnes Mastalir *

Department of Organic Chemistry, University of Szeged, H-6720 Szeged, Dom ter 8, Hungary

Received 4 March 2005; received in revised form 3 May 2005; accepted 10 May 2005

Available online 22 June 2005

Abstract

Pd-MCM-41 samples with different Pd contents were synthetized by using PdCl2, tetraethyl orthosilicate (TEOS) and the cationic

surfactant cetyltrimethylammonium bromide (CTAB). The template-free materials were characterized by inductively coupled plasma atomic

emission spectroscopy (ICP-AES), X-ray diffraction (XRD), N2 sorption, H2 chemisorption and TEM measurements. The representative

samples Pd-MCM(1.39) and Pd-MCM(5.85) had Pd contents 1.39 and 5.85%, respectively. Although the formation of the Pd particles was

found to decrease the crystalline character of the host material, the structure of the MCM-41 framework was retained for both samples. The

evidence that the Pd crystallite sizes exceeded 5 nm suggested that the Pd particles were mainly situated on the surface of MCM-41, although

some of the Pd content of Pd-MCM(5.85) may also be embedded in the mesopores. Accordingly, the Pd-MCMs can essentially be regarded as

MCM-41-supported Pd materials. Both samples proved to be active and selective catalysts for the liquid-phase semihydrogenations of

phenylacetylene, 3-butyn-1-ol, 4-octyne and 1-phenyl-1-butyne. The initial activity of Pd-MCM(1.39) surpassed that of Pd-MCM(5.85) for

each reaction, indicating that the catalytic activity was dependent on the Pd crystallite size. In contrast, the selectivity of alkene formation was

irrespective of the particle diameter for most reactants. For the semihydrogenation of 4-octyne, Pd-MCM(1.39) proved to be an extremely

efficient catalyst. The pronounced (Z)-alkene stereoselectivities obtained for the hydrogenations of internal alkynes over both Pd-MCMs may

be attributed to the participation of high-coordination terrace atoms as active sites. The catalytic activities of the Pd-MCMs were considerably

higher than those of silica-supported Pd catalysts.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Palladium; MCM-41; X-ray diffraction; N2 sorption; Transmission electron microscopy; Alkyne hydrogenation; (Z)-alkene; Stereoselectivity;

Silica-supported palladium catalyst

1. Introduction

The MCM-41 material, a member of the M41S ordered

mesoporous silicate and aluminosilicate family discovered

in 1992 by Kresge et al. [1], has been the subject of an

extended research since, mainly in terms of its physical and

chemical engineering applications [2–4]. The most char-

acteristic feature of MCM-41 is its regular pore system

consisting of an hexagonal array of uniformly sized,

hexagonally shaped mesopores [2–4]. The pore diameter

of MCM-41 can be systematically varied between 2 and

10 nm. Due to its high specific surface area (700–

1500 m2 g�1), large specific pore volume (0.8–1.3 cm3 g�1)

* Corresponding author. Tel.: +36 62 544207; fax: +36 62 544200.

E-mail address: [email protected] (A. Mastalir).

0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.05.007

1) and enhanced thermal stability, MCM-41 has been found

to be a suitable model substance for the adsorption of various

gases and vapours involving large molecules [2–6]. The

surface properties of MCM-41 can be modified by

incorporating metal atoms into the framework [5–7].

Moreover, MCM-41 can be employed as a support material

for the preparation of various metal catalysts, some of which

represent a significant improvement as compared with

conventional and commercial catalysts [4,8].

As a rule, the ordered mesoporous solids are synthetized

by using surfactants as structure directing agents and the

procedure can be rationalized by means of a liquid

templating mechanism where the surfactant molecules act

as templates [2,9]. The synthesis of MCM-41 is typically

carried out with cationic surfactants at a variety of pH [10]

and above the critical micelle concentration of the

A. Papp et al. / Applied Catalysis A: General 289 (2005) 256–266 257

surfactant, but far below the concentration at which the

binary surfactant/water mixture forms a lyotropic liquid

crystalline phase [7]. MCM-41 can also be prepared by using

anionic and neutral surfactants [2,10–12]. The versatility of

the synthesis conditions has also been confirmed [13,14].

Several methods have been described in the literature for

the synthesis and the characterization of noble metal

containing mesoporous materials [15–18]. MCM-41-sup-

ported Pt samples obtained in situ and via ion-exchange have

been reported to be efficient catalysts for the asymmetric

hydrogenation of ethyl pyruvate [19]. Pd nanoparticles on

MCM-41 have been obtained via deposition by photo-

catalytic reduction [20] and by gamma-irradiation of Pd

colloids. The latter method has been reported to result in the

formation of Pd particles embedded in the mesoporous host

[21]. Similarly as for zeolites, the mesoporous framework

tends to control the metal crystallite size by limiting the

particle growth in the confined space of the channels [17]. As

reported by Koh et al., small Pd particles, located in the

mesopores of MCM-41, have been tested as catalysts in

hydrogenation reactions [9]. Alternatively, particle size

control has been achieved by Niederer et al. [22] via the

synthesis of MCM-41 in the presence of surfactant-

stabilized colloidal Pd nanoparticles [23,24]. Nevertheless,

metal nanoparticles supported on mesoporous materials are

regarded as more convenient for catalytic applications [15].

Recently, Marın-Astorga et al. has reported on the

application of supported Pd-MCM-41 catalysts for the

stereoselective hydrogenation of aromatic alkynes [25].

The subject of the present study was the preparation of Pd

particles supported on MCM-41, by using a cationic

surfactant as a template. Pd was selected as an active

species as it has been regarded as the most efficient metal for

the selective hydrogenation of alkynes [26]. The Pd-MCM-

41 samples were tested as catalysts for the liquid-phase

hydrogenations of terminal and internal alkynes. The

catalytic performances of the samples were compared with

those of other Pd-containing catalysts obtained by different

preparation methods.

2. Experimental

2.1. Materials

The precursor PdCl2 (>99.9%), tetraethyl orthosilicate

(TEOS, 98%), the cationic surfactant cetyltrimethylammo-

nium bromide (CTAB, 98%) and the reactants phenylacety-

lene, 4-octyne, 3-butyn-1-ol and 1-phenyl-1-butyne, all of

99% purity, were Aldrich products. All chemicals were emp-

loyed without further purification. The solvent toluene applied

for the catalytic reaction was freshly distilled before use.

2.2. Preparation of the catalysts

Pd-MCM-41 samples with different Pd contents were

prepared via simultaneous self-assembling and Pd incor-

poration, according to the modified synthesis method

reported by Grun et al. [5]. In a stoppered 250-ml

Erlenmeyer flask, 0.0975 g (0.55 mmol) of PdCl2 was

suspended in 120 cm3 of distilled water and then 9.5 g of

aqueous ammonia (25 wt.%, 0.14 mol) was added. After

complete dissolution of the PdCl2, indicated by the orange

colour of the suspension turning colourless, 2.4054 g CTAB

was added to the mixture, leading to the formation of a

0.055 mol dm�3 solution. Afterwards, 10 g of TEOS

(0.05 mol) was added dropwise over a period of 10 min.

The resulting gel had the molar composition TEOS:C-

TAB:NH3:H2O:PdCl2 = 1:0.152:2.8:141.2:0.011. The mix-

ture was subjected to magnetic stirring for 1 h and then the

white precipitate was filtered and washed with 100 cm3 of

distilled water three times. After drying at room temperature

for 12 h, the sample was heated up to 813 K in a N2

atmosphere (heating rate: 2 K min�1) and calcined in air for

5 h in order to remove the template. Reduction of the PdCl2precursor was accomplished in static hydrogen at 300 K for

2 h and could be observed by the original yellow colour of

the sample turning grey. For an enhanced Pd content, the

amount of the precursor was increased, while maintaining

the amounts of the other components given above. Of the

materials synthetized by the above procedure, two repre-

sentative samples with nominal Pd contents 1.5 and 6% were

selected for further applications. Structural characterization

was performed for the template free materials.

Two other samples were employed for comparative

investigations in the catalytic test reactions, including an

MCM-41-supported Pd sample prepared by a different

method and a SiO2-supported Pd catalyst. The first sample

was synthetized by suspending 1 g of as-prepared, dried

MCM-41 framework [5] in 10 cm3 of an aqueous solution,

obtained by dissolving 1.1349 g (0.03 mol) of NaBH4 in

distilled water (3 mol dm�3). After stirring at room

temperature for 15 min, a solution containing 0.1489 g

(0.84 mmol) PdCl2 and 10 cm3 of 5.25% NH4OH was added

and the mixture was stirred at room temperature until the

formation of hydrogen gas was no longer observed. The

black powder obtained was subsequently filtered and

washed with distilled water three times. The sample was

dried first under an IR lamp for 6 h and then in vacuo at

393 K for overnight. The Pd content of the sample was

8.64% from ICP-AES analysis and the dispersion of 0.017

was calculated from the mean particle diameter of 51 nm,

obtained from TEM measurements [27]. The XRD pattern of

the sample revealed a structural disorder and the absence of

the characteristic reflections of the MCM-41 structure,

which gave evidence that the original MCM-41 framework

was destroyed under the preparation conditions. It follows

that the sample can be regarded as a silica-supported Pd

catalyst.

The second sample was prepared by a conventional

impregnation method, by using an amorphous silica support

(Cabosil, a Fluka product) and PdCl2 as a precursor.

Reduction was performed at 773 K in a H2 stream for 16 h to

A. Papp et al. / Applied Catalysis A: General 289 (2005) 256–266258

Fig. 1. Powder X-ray diffraction patterns of MCM-41 and the Pd-MCM

samples.

eliminate the chlorine content [28]. The Pd loading of the

sample was 3% and the dispersion was found to be 0.187

from H2 titration. The above samples, prepared by using

MCM-41 and Cabosil as support materials, are referred to as

Pd/SiO2 and Pd/Cabosil, respectively.

2.3. Characterization

The Pd contents of the Pd-MCM-41 samples were

determined by inductively coupled plasma atomic emission

spectroscopy (ICP-AES), by using a Jobin Yvon 24

sequential ICP-AES spectrometer at 229.7 and 324.3 nm.

Before measurement, the samples were dissolved in a 1:2

mixture of aqua regia and 0.5 M (NH4)HF2 at 333 K for

24 h. The Pd loadings were obtained from the emission

intensities by means of a calibration curve. According to the

actual Pd contents 1.39 and 5.85%, the samples are denoted

Pd-MCM(1.39) and Pd-MCM(5.85), respectively.

N2 sorption measurements were performed at the

temperature of liquid N2 (77 K), by using a Micromeritics

2375 BET apparatus, after degassing the samples at 13 Pa

and 393 K for overnight. The specific surface areas were

calculated from the BET equation and the average pore

diameters were determined from the desorption branches of

the isotherms by using the BJH method [29,30].

X-ray diffraction (XRD) patterns were obtained with a

Philips PW 1820 diffractometer operated at 40 kV and

35 mA (Cu Ka radiation, l = 0.154 nm). Diffraction data

were collected in the range 18 < 2Q < 108 at an interval of

0.01 2Q. The interplanar spacing of the MCM-41 structure

(d1 0 0) was calculated from the first order Bragg reflections

by using a PW 1877 automated powder diffraction software.

The lattice parameter a, regarded as the average distance

between two neighbouring pore centres in the MCM-41

structure, was determined as a = d1 0 0 � 2/H3 [3,17,31].

TEM images were recorded with a Philips C 10

transmission electron microscope (LaB6 cathode, 100 kV)

equipped with a Megaview II digital camera. The powdered

samples were suspended in ethanol and deposited on

Formvar-coated copper grids. The mean particle diameters d

were calculated as number average values (Snidi/Sni,

Sni > 250), by means of the UTHSCSA Image Tool

program.

The samples subjected to H2 chemisorption measure-

ments were pretreated first in 13.3 kPa of O2 at 473 K for

30 min and then in a H2 stream of 50 cm3 min�1 at 573 K for

2 h, followed by evacuation and cooling to room tempera-

ture.

2.4. Catalytic test reaction

The catalytic test reactions were carried out in an

automated hydrogenation reactor [32]. For each measure-

ment, 5 � 10�3 g of catalyst was used. The reactant:Pd ratio

(S:Pd), expressed as [mol reactant]:[mol Pd], was typically

2000, except for the hydrogenation of 1-phenyl-1-butyne,

for which a lower S:Pd ratio of 1000 was employed. After

several cycles of flushing with H2 and evacuation, the

sample was pretreated in 105 Pa of static H2 for 60 min.

After evacuation, H2 was reintroduced, 1 cm3 of solvent was

added and the pretreatment was completed by applying an

efficient stirring for 45 min. The hydrogenation reaction was

initiated by injecting the substrate into the glass reactor.

Each reaction was conducted at 298 K under a constant H2

pressure of 105 Pa and vigorous stirring (1400 rpm) to

eliminate transport limitations. The reaction temperature

was controlled by means of a Julabo thermostate. The H2

consumption was monitored by a PC, at a data collection

frequency of 0.5 s�1. After reaction, the catalyst was

removed from the product mixture by gravity filtration. The

reaction products were analysed with a HP 5890 gas

chromatograph equipped with a flame ionization detector

(FID), by using either a HP-1 or a HP-5 capillary column,

depending on the reactant. The catalytic activities were

characterized by the turnover frequencies (TOF), deter-

mined from the initial rates R (cm3 H2 min�1 g Pd�1) as

TOF = 7.253 � 10�5 R/D, where D is the dispersion of the

catalyst, calculated from H2 chemisorption measurements.

3. Results and discussion

The X-ray diffraction patterns of as-synthetized MCM-41

and the Pd-MCM samples are shown in Fig. 1. At small

angles (2Q = 1.58–58), three characteristic Bragg reflections

were observed, which can be indexed by assuming a

hexagonal symmetry [5]. The pronounced d1 0 0 reflections

and the less intense d1 1 0 and d2 0 0 peaks correspond well to

the hexagonally arranged pore structure of the MCM-41

framework, which confirms the long-range order of these

materials.

At higher angles, no further peaks were detected,

indicating that the samples are not crystalline at an atomic

level [3]. The characteristic data obtained for MCM-41 and

the Pd-MCMs are listed in Table 1.


Table 1

Characteristic data of as-synthetized MCM-41 and the Pd-MCM materials

Sample Pd (%) As BET (m2 g�1) Dp (nm)a Vp (cm3 g�1)b d1 0 0 (nm)c a (nm)d dTEM (nm)e Df

MCM-41 – 1239 2.39 0.82 3.76 4.34 – –

Pd-MCM(1.39) 1.39 1099 2.38 0.67 3.60 4.16 23 0.082

Pd-MCM(5.85) 5.85 806 2.83 0.60 4.63 5.35 10 0.136a Average pore diameter, obtained from N2 desorption by the BJH method.b Total pore volume, calculated from N2 desorption by the BJH method.c Periodicity of the MCM-41 host, determined from XRD data.d Unit cell parameter of the MCM-41 structure, calculated as a = d1 0 0 � 2/H3.e Average particle diameter, determined from TEM measurements.f Pd dispersion, obtained from H2 chemisorption.

It may be observed in Fig. 1 that the position of the d1 0 0

peak for Pd-MCM(1.39) was slightly shifted as compared

with that for the pristine MCM-41, which resulted in a minor

decrease of the interplanar spacing. More importantly, the

d1 0 0 reflection for Pd-MCM(5.85) was shifted towards

smaller angles, and thus the lattice parameter increased from

3.76 to 4.63 nm. The losses in the peak intensities for the Pd-

MCM materials are being indicative of less ordered

structures than that of the pristine MCM-41. Further,

enhancement of the Pd content was found to decrease the

crystalline character of Pd-MCM, which was also indicated

by a peak broadening for Pd-MCM(5.85). Although the

formation of the Pd particles was found to reduce the

crystallinity of the MCM-41 framework, the original

structure was maintained for both samples.

The N2 sorption isotherms obtained for the Pd-MCM

samples (Fig. 2) correspond to type IV according to the

IUPAC classification [3,33]. The fairly linear increment of

the adsorbed volume at low pressures is followed by a steep

growth in the nitrogen uptake at p/p0 = 0.2–0.35, which is

due to capillary condensation inside the mesopores [5].

Both isotherms exhibit narrow hysteresis loops, being

indicative of reversible filling and emptying of the pores

[30], over the relative pressure range 0.2–0.55. The steep

adsorption and desorption branches of the hysteresis loops

refer to a narrow mesopore size distribution [15]. For Pd-

MCM(5.85), the shift in the position of the capillary

condensation (adsorption branch) and the evaporation

Fig. 2. N2 sorption isotherms of Pd-MCM(1.39) and Pd-MCM(5.85).

(desorption branch) steps towards higher pressures reveals

an increase in the pore size. Further, the long plateau at

higher relative pressures indicates that secondary meso-

pores are not present [31] and that pore filling occurs in a

rather small pressure range [5]. The specific surface area

obtained for pure MCM-41 was of a similar order as those

reported in the literature [5,31]. The values for the Pd-

MCMs were found to be lower and decreased with the

increase in the Pd content [15]. A similar trend could be

observed for the pore volumes.

As shown in Fig. 3, the pore size distributions displayed

sharp peaks in the mesoporous region, with maxima

corresponding to the diameters 2.38 and 2.83 nm for Pd-

MCM(1.39) and Pd-MCM(5.85), respectively (see

Table 1). In accordance with the results of XRD meas-

urements, the pore size of Pd-MCM(1.39) was nearly the

same as that of pure MCM-41. It follows for Pd-

MCM(1.39) that the formation of the Pd particles did

not exert an appreciable effect on the structure of the host

material. On the other hand, the enhanced pore diameter

obtained for Pd-MCM(5.85) suggests a moderate expan-

sion of the mesopores of MCM-41 following the

generation of the Pd particles, which is in line with the

increased interplanar distance of the MCM-41 structure,

observed by XRD.

TEM images revealed the presence of Pd nanoparticles

on the surface of both Pd-MCMs, although the particle

diameters and the size distributions proved to be signifi-

Fig. 3. Pore size distributions of MCM-41 and the Pd-MCM samples.


Fig. 4. Particle size distribution of Pd-MCM(1.39) and Pd-MCM(5.85).

Fig. 5. Hydrogenation of phenylacetylene over Pd-MCM(1.39). m = 5 �10�3 g, T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: toluene.

cantly different (see Fig. 4). For Pd-MCM(1.39), a relatively

broad distribution was obtained, with 60% of the particles

ranging in size between 23–30 nm, whereas crystallites

smaller than 10 nm and aggregates larger than 31 nm were

uncommon. On the other hand, the size range for Pd-

MCM(5.85) was considerably narrower, as 77% of the

particles had diameters less than 11 nm. The mean crystallite

diameters were found to be 23 and 10 nm for Pd-MCM(1.39)

and Pd-MCM(5.85), respectively. It follows that the

preparation procedure did not ensure size control of the

Pd particles. The finding that the particles detected for the

sample with higher Pd loading were smaller than those for

Pd-MCM(1.39) is in contradiction with our expectations

based on previous results [34,35] and may be related to a

substantial difference in the nucleation and growth of the

crystallites for the two samples. The initial nucleation of the

Pd particles has been reported to be considerably affected by

the surface structure of the support material [36].

According to the above considerations, it may be

concluded that the Pd-MCM samples retained their

mesoporous structures, as revealed by their high specific

surface areas and narrow pore size distributions. Meanwhile,

the reduced XRD peak intensities indicated that the

introduction of Pd particles had a decreasing effect on both

the local symmetry of the channels and the crystallinity of

the MCM-41 framework. The evidence that particles smaller

than 5 nm were not detected for any of the samples

suggested that incorporation of the Pd crystallites in the

mesopores of the host material was rather unlikely.

Nevertheless, as reported by Zhu et al., metal nanoparticles

of 2–10 nm have been found to be incorporated into the

mesoporous structure of SBA-15, leading to expansion of

the mesopore channels [37]. Accordingly, it is reasonable to

assume for Pd-MCM(5.85) that a proportion of the smallest

Pd particles are embedded in the MCM-41 framework,

which may be accounted for the reduced BET surface area,

the lattice expansion detected by XRD and the increased

pore diameter observed for Pd-MCM(5.85). In contrast, no

indication of Pd incorporation was found for Pd-

MCM(1.39). Given the predominance of relatively large

particles for both samples, as seen in Fig. 4, the Pd-MCMs

may essentially be regarded as MCM-41-supported Pd

materials.

The Pd-MCM samples were tested as catalysts for the

selective hydrogenations of terminal and internal alkynes

including phenylacetylene, 3-butyn-1-ol, 4-octyne and 1-

phenyl-1-butyne. The selective semihydrogenation of

alkynes is a particularly important reaction in the synthesis

of fine chemicals and biologically active compounds. For

such reactions, Pd offers the best combination of activity and

selectivity, and therefore has become the most widely

applied commercial hydrogenation catalyst [38]. The

selectivity of Pd is based on the triple bond being more

strongly adsorbed on the active centres than the correspond-

ing double bond, which is due to its high electron density and

restricted rotation [26,38–40]. As a result, the acetylenic

compound displaces the alkene from the surface and

prevents its readsorption, thereby exerting a poisoning

effect for the subsequent reactions, which holds as long as

the alkyne species is present [40,41].

Several studies have been reported in the literature on the

semihydrogenation of phenylacetylene on supported Pd

catalysts [42–46]. For this reaction, a linear trend for the

reaction rate versus time has been demonstrated [43,45,47],

indicating that the reaction rate is independent of the alkyne

concentration.

The hydrogenation of phenylacetylene over Pd-

MCM(1.39) was investigated at S:Pd = 2000. The results

are displayed in Fig. 5.

The conversion of the reactant, leading to the formation

of styrene as the main reaction product, was found to be 82%

at a reaction time of 45 min, suggesting a pronounced

hydrogenation activity, although the linear trend observed

initially was not maintained throughout the reaction. The

high selectivity of styrene formation (97%) decreased

slightly by 45 min, with a parallel increase in the select-

ivity of ethylbenzene formation from 3 to 5.6%, which

implies that the substantial hydrogenation activity of Pd-


Fig. 6. Hydrogenation of phenylacetylene over Pd-MCM(5.85). m = 5 �10�3 g, T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: toluene.

Fig. 7. Hydrogenation of 3-butyn-1-ol over Pd-MCM(1.39) and Pd-

MCM(5.85). m = 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 2000, solvent:

toluene.

MCM(1.39) also manifested itself in the overhydrogenation

of phenylacetylene, even at low conversions.

As demonstrated in Fig. 6, the catalytic activity of Pd-

MCM(5.85) for the semihydrogenation of phenylacetylene

under the same conditions was found to be lower than that of

Pd-MCM(1.39), especially at reaction times exceeding

25 min. On the other hand, a more pronounced over-

hydrogenation was observed, which considerably decreased

the selectivity of styrene formation after 35 min. The finding

that ethylbenzene formation was detected at rather low

conversions, similarly as for Pd-MCM(1.39), indicated that

alkane production is likely to occur via both direct and

indirect hydrogenation [48,49]. It follows that Pd-

MCM(1.39), which exhibited both a higher activity and

selectivity, is a more efficient catalyst for the above reaction

than Pd-MCM(5.85).

The second reaction investigated over the Pd-MCMs was

the semihydrogenation of the terminal aliphatic alkynol 3-

butyn-1-ol. As reported by Bailey and King [38], the selective

hydrogenation of acetylenic alcohols to the respective allylic

alcohols is a rather difficult procedure, as it tends to be

accompanied by hydrogenolysis of the hydroxyl group, which

is characteristic of Pd-based catalysts. Recently, a (Z)-alkenol

stereoselectivity of 91% has been demonstrated for the

hydrogenation of 3-hexyn-1-ol over hydrotalcite-supported

Pd nanoclusters by Roelofs and Berben [50].

Table 2

Hydrogenation of terminal alkynes over Pd-MCM catalysts

Catalysta Reactant R (cm3 H2 min�1 g Pd�1)

Pd-MCM(1.39)b Phenylacetylene 14566

Pd-MCM(5.85)c Phenylacetylene 9568

Pd-MCM(1.39)d 3-Butyn-1-ol 6838

Pd-MCM(5.85)d 3-Butyn-1-ol 4729a Mass of catalyst: 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: tob Reaction time: 30 min.c Reaction time: 45 min.d Reaction time: 80 min.

The conversions and the selectivities determined for the

semihydrogenation of 3-butyn-1-ol are demonstrated in

Fig. 7.

It may be observed for both samples that the

hydrogenation rate is irrespective of the reactant concentra-

tion. Further, the conversions obtained for the individual

samples could be fitted with the same straight line,

indicating that catalytic activities were very similar: at a

reaction time of 80 min, conversions exceeding 90% were

obtained for both catalysts. The selectivity of 3-butene-1-ol

formation was 100% for both Pd-MCMs throughout the

entire reaction time range, suggesting that the half-

hydrogenated alkenol species was desorbed from the Pd

surface before either overhydrogenation or hydrogenolysis

could occur. It may therefore established that the hydro-

genation activities of the Pd-MCM samples by far outweigh

their hydrogenolysis activities, which contributes to their

outstanding selectivities. The initial rates and the turnover

frequencies obtained for the semihydrogenations of terminal

alkynes are listed in Table 2.

Both the initial rates and the turnover frequencies

obtained for the the hydrogenation of phenylacetylene

proved to be about twice as high as those for the

transformation of 3-butyn-1-ol for each sample. The

enhanced catalytic activity for the former reaction is

probably due to the adsorption of the aromatic alkyne

TOF (s�1) Conversion (%) Salkene (%) Salkane (%)

12.9 58.1 95.5 4.5

5.1 56.4 84.1 15.9

6.0 91.8 100 0

2.5 93.1 100 0

luene.


Fig. 8. Hydrogenation of 4-octyne over Pd-MCM(1.39). m = 5 � 10�3 g,

T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: toluene.

being more favourable on the active Pd sites than that of the

aliphatic compound containing an additional hydroxyl

group. It may also be observed for both reactions that the

initial rates and the TOF values obtained for Pd-MCM(1.39)

are considerably higher than those for Pd-MCM(5.85). The

variation in the initial activities refers to a moderate particle

size dependence. Nevertheless, the conversions obtained for

the individual reactants over the Pd-MCMs at reaction times

exceeding 45 min are not significantly different. At a similar

conversion level, the selectivity of styrene formation over

Pd-MCM(5.85) was considerably lower than that for Pd-

MCM(1.39). On the other hand, the 100% selectivity of 3-

butene-1-ol formation revealed the complete absence of

overhydrogenation for both catalysts, even at high conver-

sions. It may therefore be established for the semihydro-

genations of terminal alkynes that the Pd loading of the

samples has no crucial effect on the catalytic behaviour, and

therefore both Pd-MCMs may be regarded as active and

selective catalysts.

Considering that the initial rates for Pd/SiO2 were found

to be significantly lower than those obtained for Pd/Cabosil

(see Table 3), the relatively high turnover frequencies for Pd/

SiO2 can be attributed to its extremely low dispersion, as

related to the presence of large Pd aggregates. It may also be

noticed that the initial activities of both silica-supported

samples were clearly inferior to those of the Pd-MCM

catalysts. Accordingly, the high selectivities of styrene

formation listed in Table 3 may be ascribed in part to the

conversion levels at 45 min being considerably lower than

those obtained for the Pd-MCMs. On the other hand, the

selectivity of 3-butene-1-ol formation over Pd/SiO2 proved

to be less than the 100% determined for the other catalysts.

For the hydrogenation of 3-butyn-1-ol, the conversions for

the reference samples at 60 min proved to be one order of

magnitude lower than those for the Pd-MCMs, indicating

poor activities. It may therefore be ascertained that the

catalytic activities of the Pd-MCM samples for the above

reactions are significantly higher than those of the silica-

supported samples, whereas the selectivities are of a similar

order.

Over Pd-containing catalysts, the semihydrogenations of

internal alkynes typically result in the predominant

formation of (Z)-alkene stereoisomers, as related to the

associative adsorption of the reactant on the active sites and

the consecutive addition of two adsorbed hydrogens from

Table 3

Hydrogenation of terminal alkynes over supported Pd catalysts

Catalysta Reactant R (cm3 H2 min�1 g Pd�1)

Pd/SiO2b Phenylacetylene 849

Pd/Cabosilb Phenylacetylene 2818

Pd/SiO2c 3-Butyn-1-ol 446

Pd/Cabosilc 3-Butyn-1-ol 1384a Mass of catalyst: 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: tob Reaction time: 45 min.c Reaction time: 60 min.

below the axis of unsaturation [48,51]. In contrast, the

formation of (E)-alkenes has been attributed to the addition

of molecular hydrogen from above the axis of the triple bond

[48,51]. As reported in the literature, the formation of (Z)-

alkenes is accompanied by the production of alkanes

through overhydrogenation [43] and that of (E)-alkenes,

which may be formed either as initial products or via Z ! E

isomerization [48,52]. The results obtained for the

semihydrogenation of the internal aliphatic alkyne 4-octyne

are depicted in Fig. 8.

The finding that a conversion of 92.6% was achieved by a

reaction time of 20 min indicated a pronounced catalytic

activity. Moreover, the stereoselectivity of (Z)-alkene

formation was remarkably high (>95%) throughout the

reaction, which implies that the amount of by-products was

negligible. This applies for the formation of (E)-alkene in

particular, which was first detected at a conversion of 52.8%.

The maximum value of the E stereoselectivity was 0.4% at

20 min. The selectivity of octane production via over-

hydrogenation proved to be somewhat higher (3–3.5%), but

lower than those obtained for other Pd catalysts under

similar conditions [47]. It may be deduced from Fig. 11 that

octane was an initial product, unlike the (E)-alkene, which

was more likely to proceed via isomerization of the (Z)-4-

octene stereoisomer. Although this finding seems to be in

contradiction with some of the previous results imparted in

the literature [50,53], evidences for the occurrence of Z ! E

isomerism have also been published [52,54]. The conversion

TOF (s�1) Conversion (%) Salkene (%) Salkane (%)

3.7 15.4 99.8 0.2

1.1 27.1 99.6 0.4

1.9 4.1 97.1 2.9

0.5 4.2 100 0

luene.


Fig. 9. Hydrogenation of 4-octyne over Pd-MCM(5.85). m = 5 � 10�3 g,

T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: toluene.Fig. 10. Hydrogenation of 1-phenyl-1-butyne over Pd-MCM(1.39).

m = 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 1000, solvent: toluene.

Fig. 11. Hydrogenation of 1-phenyl-1-butyne over Pd-MCM(5.85).

m = 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 1000, solvent: toluene.

and the selectivity plots for Pd-MCM(5.85) are shown in

Fig. 9.

The overall catalytic performance of Pd-MCM(5.85)

proved to be very similar to that of Pd-MCM(1.39),

including high conversions and pronounced (Z)-alkene

stereoselectivities. Since the conversions were somewhat

lower than those for Pd-MCM(1.39), the linear trend was

maintained throughout the reaction. The stereoselectivity of

(E)-alkene formation was not significantly different from

that for Pd-MCM(1.39), and hence the moderate decrease in

the Z stereoselectivity (93.5–95%) can be attributed to an

enhanced overhydrogenation. Nevertheless, the selectivity

of alkane formation did not surpass 5.5% and may be

regarded as constant during the reaction. The selectivities of

all the reaction products had the same trends as discussed

above, which may be assigned to the same reasons.

The stereoselective hydrogenations of internal aromatic

alkynes have been the subject of several studies, including

our previous investigations over low-loaded Pd-montmor-

illonite and Pd-hydrotalcite samples, for which the Z

stereoselectivities proved to be comparable with that of the

Lindlar catalyst [32,35,55]. For the competitive hydrogena-

tion of 1-phenyl-1-propyne in the presence of 1-pentyne and

2-pentyne, a reduction of the hydrogenation rate has been

reported by Hamilton et al. [46]. Recently, Marın-Astorga

et al. have disclosed that the stereoselectivities of (Z)-

alkenes produced from internal aromatic alkynes over a

pillared clay-supported Pd catalyst displayed no variation

with the S:Pd ratio up to a conversion of 80% [56]. For the

stereoselective hydrogenation of 1-phenyl-1-butyne over the

Pd-MCM samples, preliminary investigations indicated a

reduced catalytic activity, and therefore the S:Pd ratio was

decreased to 1000. The results are illustrated in Figs. 10

and 11.

At a reaction time of 75 min, the conversions for Pd-

MCM(1.39) and Pd-MCM(5.85) were 49.8 and 71.1%,

respectively, indicating an enhanced activity for Pd-

MCM(5.85). Nevertheless, the Z stereoselectivities proved

to be very similar, as the values obtained for Pd-MCM(1.39)

and Pd-MCM(5.85) varied between 88–90% and 86–92%,

respectively. It may be observed in Figs. 10 and 11 that the

formation of the by-products took place in simultaneous

reactions. The selectivities for the production of (E)-alkene

and alkane were of a similar order for the two samples,

although at prolonged reaction times, minor differences

could be observed. For Pd-MCM(1.39), the selectivities

hardly varied up to 120 min, whereas for Pd-MCM(5.85),

the Z stereoselectivity slightly dropped after 75 min, with a

parallel increase in the stereoselectivity of (E)-alkene

formation, which suggested the occurrence of Z ! E

isomerization. The initial rates and the turnover frequencies

for the hydrogenations of internal alkynes are summarized in

Table 4.

Similarly to that observed for the hydrogenations of

terminal alkynes, a significant variation in the initial rates

and the TOF values can be observed. The initial activity of

Pd-MCM(1.39) proved to be considerably higher for both

reactants. For 4-octyne, this activity difference was

maintained throughout the reaction, as revealed by the

reaction times corresponding to similar conversions. In


Table 4

Hydrogenation of internal alkynes over Pd-MCM catalysts

Catalyst Reactant R (cm3 H2 min�1 g Pd�1) TOF (s�1) Conversion (%) S(Z) (%) S(E) (%) Salkane (%) Yg

Pd-MCM(1.39)a,b 4-Octyne 20818 18.4 92.6 95.1 0.4 4.3 0.996

Pd-MCM(5.85)a,c 4-Octyne 12919 6.9 91.3 93.1 1.4 5.5 0.985

Pd-MCM(1.39)d,e 1-Phenyl-1-butyne 7742 6.9 54.1 88.3 3.6 8.1 0.961

Pd-MCM(5.85)d,f 1-Phenyl-1-butyne 5194 2.8 56.4 91.2 2.9 5.9 0.969a Mass of catalyst: 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: toluene.b Reaction time: 20 min.c Reaction time: 30 min.d Mass of catalyst: 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 1000, solvent: toluene.e Reaction time: 120 min.f Reaction time: 50 min.g Y = S(Z)/S(Z + E).

contrast, for the hydrogenation of 1-phenyl-1-butyne, the

catalytic activity of Pd-MCM(5.85) was found to surpass

that of Pd-MCM(1.39) as the reaction progressed. It follows

that no unambigous correlation can be established between

the catalytic activity and the Pd particle size. Furthermore,

the (Z)-alkene stereoselectivities and the Y-values, obtained

for the individual reactions at the same conversion level,

were remarkably similar, suggesting that the selectivity was

irrespective of the particle diameter. This finding is in

consistence with our previous observations for low-loaded

Pd-montmorillonite catalysts [47] and also with other results

reported in the literature [57,58]. It may be concluded from

Table 4 that both Pd-MCMs are active and stereoselective

catalysts for the hydrogenations of internal alkynes, the

activities and the (Z)-alkene stereoselectivities for the

aliphatic species exceeding those for the aromatic one. The

reduced activities observed for 1-phenyl-1-butyne may be

attributed to steric factors restricting the chemisorption of

the reactant on the active Pd surface [32]. It should be

stressed that Pd-MCM(1.39) exhibited an outstanding

catalytic performance for the hydrogenation of 4-octyne,

as confirmed by the remarkably high turnover rate of 18.4,

the marked (Z)-alkene stereoselectivity of 95.1% and the Y-

value of 0.996.

The comparative data obtained for the reactions of

internal alkynes over silica-supported catalysts are listed in

Table 5.

It may be readily seen from Table 5 that the initial rates

for the silica-supported samples were subtantially lower than

those for the MCM-41-supported catalysts. The difference

Table 5

Hydrogenation of internal alkynes over supported Pd catalysts

Catalyst Reactant R (cm3 H2 min�1 g Pd�1) TOF (s�1)

Pd/SiO2a 4-Octyne 1539 6.6

Pd/Cabosila 4-Octyne 2906 1.1

Pd/SiO2b 1-Phenyl-1-butyne 614 2.6

Pd/Cabosilb 1-Phenyl-1-butyne 2632 1.0a Mass of catalyst: 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 2000, solvent: tob Mass of catalyst: 5 � 10�3 g, T = 298 K, p = 105 Pa, S:Pd = 1000, solvent: toc Selectivity at a conversion of 20%.d Y = S(Z)/S(Z + E), calculated from the selectivities at a conversion of 20%.

between any initial rate obtained for Pd/SiO2 and that for

either Pd-MCM was one order of magnitude, whereas for

Pd/Cabosil, a severalfold decline was experienced. The

finding that the decrease of the turnover rates was less

significant than that can be attributed to the variation in the

Pd dispersions. The differences in the initial rates also

manifested themselves in the reduced conversions (see

Table 5), as compared with those obtained for the Pd-MCMs

(see Table 4), Pd/SiO2 being clearly less active than Pd/

Cabosil for both reactions. On account of the reduced

catalytic activities of the silica-supported samples, the

product selectivities are referred to a significantly lower

conversion (20%) than those listed in Table 4. Accordingly,

the finding that the Z stereoselectivities of the silica-

supported samples exceed those of the Pd-MCMs is not

surprising. Nevertheless, for the hydrogenation of 4-octyne,

no appreciable difference can be observed. The above results

reinforce our previous statement that the Z stereoselectivity

is irrespective of the Pd particle diameter. In contrast, the

catalytic activity was dependent on the Pd crystallite size

[26,47,59]. The specific activity of Pd-MCM was found to

decrease on increasing the Pd dispersion, which is in

consistence with the results of previous studies [26,60,61].

Nevertheless, all samples considered, no systematic varia-

tion could be pointed out.

As shown in Fig. 4, the mean Pd particle diameters for

both Pd-MCMs considerably surpassed 5 nm, and hence

these particles predominantly consist of high-coordination

terrace atoms [26,62]. The latter atoms are regarded to be

less active in overhydrogenation than low-coordination sites,

Conversion (%) S(Z)c (%) S(E)

c (%) Salkanec (%) Yd

29.1 97.0 0 2.9 1.000

55.0 96.7 0.1 3.2 0.999

24.1 96.6 1.3 2.1 0.987

48.4 95.4 1.6 3.0 0.983

luene, reaction time: 20 min.

luene, reaction time: 75 min.


for which the residence time of alkyne tends to be longer and

the concentration of surface hydrogen is higher [63]. Terrace

atoms have also been reported to be particularly selective for

the formation of (Z)-alkenes, as related to a decline in the

rate of (Z)-alkene hydrogenation on decreasing the

frequency of surface imperfections, rather than that of

alkyne hydrogenation, for which the rate has been claimed to

be unaffected by the different surface orientations [64,65].

The sequence of the specific activities for the reactants

proved to be the same for both Pd-MCMs: internal aliphatic

alkyne > terminal aromatic alkyne > internal aromatic

alkyne > terminal aliphatic alkynol. For Pd-MCM(1.39),

more pronounced differences between the turnover rates

were experienced. The trend for the aromatic alkynes is in

agreement with the results of previous studies [38] and the

relatively low initial activity for the alkynol may be

attributed to the presence of the hydroxyl group affecting

reactant chemisorption. It may be concluded that, notwith-

standing the rather large Pd crystallite diameters, the Pd-

MCM samples were highly efficient catalysts for the

semihydrogenation of both terminal and internal alkynes.

Considering that the initial activity of Pd-MCM(1.39)

surpasses that of Pd-MCM(5.85) for each reaction and the

catalytic activities of both Pd-MCMs substantially exceed

those of the silica-supported Pd catalysts, it also seems that

the structural order of the MCM-41 host exerts a beneficial

effect on the catalytic performance. It may be assumed that

the regular character of MCM-41 has an influence on the

nucleation and the growth of the Pd particles and hence it

favours the formation of active ensembles participating in

the catalytic reaction.

4. Conclusions

Pd-MCM-41 materials were prepared via simultaneous

self-assembling and Pd particle generation. The Pd contents

of the samples selected for further investigations proved to

be 1.39 and 5.85%. XRD characterization gave evidence that

the mesoporous structure of the MCM-41 host was

maintained after the formation of Pd particles, although

the introduction of Pd was found to reduce the crystallinity

of the MCM-41 framework. The mean particle diameters

determined from TEM measurements were 23 and 10 nm for

Pd-MCM(1.39) and Pd-MCM(5.85), respectively, indicating

that the preparation procedure did not ensure size control of

the Pd particles. Although some of the smallest particles of

Pd-MCM(5.85) are presumed to be embedded in the

mesopores, the Pd-MCMs can essentially be regarded as

MCM-41-supported Pd catalysts.

The Pd-MCM samples were tested as catalysts for the

liquid-phase hydrogenations of both terminal and internal

alkynes under mild conditions. For the semihydrogenations

of terminal alkynes (phenylacetylene and 3-butyn-1-ol),

both Pd-MCMs exhibited marked hydrogenation activities

and high alkene selectivities. The catalytic performance of

Pd-MCM(1.39) observed for the reaction of phenylacetylene

was superior to that of Pd-MCM(5.85). For the hydrogena-

tion of 3-butyn-1-ol, the 100% selectivity of alkene

formation revealed the complete absence of hydrogenolysis

and overhydrogenation.

The turnover rates for the stereoselective hydrogenations

of the internal alkynes (4-octyne and 1-phenyl-1-butyne)

were found to decrease on increasing the dispersion of Pd-

MCM, unlike the marked stereoselectivities of (Z)-alkene

formation, which displayed no systematic variation with the

particle size. The pronounced specific activity obtained for

the hydrogenation of 4-octyne, together with a marked (Z)-

alkene stereoselectivity at high conversion, confirm that Pd-

MCM(1.39) is an extremely efficient catalyst for this

reaction. The substantial Z stereoselectivities and the limited

overhydrogenation observed for the Pd-MCMs may be due

to the predominance of high-coordination terrace sites as

active species for both samples.

The evidence that the same sequence for the turnover

rates of the reactants was established for both samples

indicates that the Pd-MCMs exhibit a similar catalytic

behaviour, which is not appreciably affected by the Pd

loading and the crystallite size.

The finding that the catalytic activities of the Pd-MCMs

significantly exceed those of silica-supported Pd catalysts

for the hydrogenations of all the reactants implies that the

structural order of the mesoporous host material has a

beneficial effect on the catalytic performance.

Acknowledgement

Financial support of the National Scientific Research

Foundation through OTKA Grants T 047390 and T 042603

is gratefully acknowledged.

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