Liquid phase hydrogenation of phenylacetylene over Pd and PdZn catalysts in toluene: Effects of alloying and CO2 pressurization

Article abstract of DOI:10.1039/c4ra02220c

Liquid-phase hydrogenation of phenylacetylene was investigated with Pd and PdZn alloy catalysts in toluene compressed by CO₂. The influence of alloying and dense phase CO₂ on the rate of conversion and the product selectivity was exa

Full text of DOI:10.1039/c4ra02220c

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Liquid phase hydrogenation of phenylacetylene
over Pd and PdZn catalysts in toluene: effects of
alloying and CO2 pressurization†
Cite this: RSC Adv., 2014, 4, 24922
Hiroshi Yoshida,a Toshiaki Zama,a Shin-ichiro Fujita,a Joongjai Panpranotb
a
*
and Masahiko Arai
Liquid-phase hydrogenation of phenylacetylene was investigated with Pd and PdZn alloy catalysts in
toluene compressed by CO2. The influence of alloying and dense phase CO2 on the rate of conversion
and the product selectivity was examined. With either Pd or PdZn the partially hydrogenated product of
styrene was produced in high selectivity but the selectivity to styrene obtained with the latter was larger
than that with the former when compared at the same conversion level. After the initial substrate was
fully consumed, the styrene was fully hydrogenated to ethylbenzene for both catalysts but more slowly
with the PdZn alloy catalyst. The presence of a dense phase of CO2 (1.5 MPa) enhanced the rate of
phenylacetylene hydrogenation and the selectivity to styrene for the Pd and PdZn catalysts. Possible
reasons for the positive effects of alloying and dense phase CO2 on the partial hydrogenation of
phenylacetylene to styrene are discussed.
Received 14th March 2014
Accepted 19th May 2014
DOI: 10.1039/c4ra02220c
www.rsc.org/advances
of alkynes.1 For example, Parvulescu et al. prepared SiO2-
ˆ
1. Introduction
embedded Pd, Au, and PdAu colloids and tested their catalytic
performance in the liquid phase hydrogenation of 3-hexyn-1-ol
at 20 ꢀC.18 They showed that the alloying of Pd with Au was
effective for enhancing the rate of hydrogenation and the
selectivity to cis-3-hexen-1-ol. Scott et al. investigated the
hydrogenation of 3-hexyn-1-ol over poly(vinylpyrrolidone)-
stabilized PdAu bimetallic particles in 1-butyl-3-methyl-imida-
zolium hexauorophosphate and indicated that the catalytic
performance depended on the composition.19 Panpranot et al.
prepared several PdAu catalysts by the combination of incipient
wetness impregnation and deposition-precipitation methods.20
The authors applied those PdAu catalysts for the hydrogenation
of 1-heptyne in toluene at 30 ꢀC and discussed the relationship
between the catalytic performance (activity and selectivity) and
structural features.
In the present work, Pd and PdZn alloy catalysts supported
on ZnO were prepared according to the previous works21,22 and
applied for the liquid phase selective hydrogenation of a model
alkyne of phenylacetylene in toluene compressed by CO2.
Earlier Rodriguez made a detail study on the electronic and
chemical properties of PdZn surface.23 It is noted that the
electronic perturbations induced by the alloying of Pd with Zn
decrease its CO chemisorption ability by weakening the Pd(4d)–
CO(2p) bonding interactions. This allows us to expect that the
PdZn surface is different from the Pd surface in the adsorption
of the substrate of phenylacetylene and its hydrogenated
product of styrene and then in the catalytic activity for the
hydrogenation of these reacting species. The present authors
have investigated the catalytic performance of these Pd and
Selective hydrogenation of alkynes to alkenes is one of the
important hydrogenation reactions in industry.1,2 An alkyne is
partially hydrogenated to the corresponding alkene and the
alkene so formed is also hydrogenated to the fully hydrogenated
alkane. It is desirable to suppress the full hydrogenation for the
selective production of the alkene at a high conversion level.
There is a trend that the adsorption of alkynes is stronger than
that of alkenes and the hydrogenation of alkenes is difficult to
achieve when the initial alkynes remain unreacted in the reac-
tion mixture; when the alkynes are almost consumed, the
alkenes may then be adsorbed and hydrogenated. It is therefore
required that the desired catalysts should adsorb alkynes but
not alkenes.3 The selective hydrogenation of alkynes is still a
challenging task from industrial and academic points of view.
Several previous authors investigated the liquid-phase hydro-
genation reactions using different alkynes such as pentyne,4,5
hexyne,6 heptyne,7–11 octyne,12 phenylacetylene,13–16 and
1-phenyl-1-pentyne.17 The most common catalysts used in
industry and academia are Pd-based catalysts. Several authors
used bimetallic Pd catalysts and examined the impact of alloy-
ing on the catalytic performance in the selective hydrogenation
aDivision of Chemical Process Engineering, Faculty of Engineering, Hokkaido
University, Sapporo 060-8628, Japan. E-mail: marai@eng.hokudai.ac.jp
bCenter of Excellence of Catalysis and Catalytic Reaction Engineering, Department of
Chemical Engineering, Chulalongkorn University, Bangkok 10330, Thailand
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02220c
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PdZn catalysts with respect to the total conversion and the temperature for 1 h and increasing temperatures at a rate of 5 K
product selectivity for the hydrogenation of phenylacetylene. To minꢁ1 to 500 ꢀC to obtain PdZn alloy catalysts.21,22
our knowledge only a few works were reported so far on the
X-ray diffraction (XRD) patterns of ZnO-supported Pd
hydrogenation of alkynes using PdZn alloy catalysts in the samples so prepared were measured on JOEL JDX-8020 using Cu
literature. Engels et al. used PdZn alloy catalysts different in the Ka radiation through a Ni lter to determine the phases (Pd,
composition for hydrogenation of phenylacetylene in methanol PdZn) present therein and estimate the sizes of Pd and PdZn
at 50 ꢀC.24 They showed that the hydrogenated product of crystallites by Pd (111) (2Q ¼ 40.1ꢀ) and PdZn (111) (2Q ¼ 41.2ꢀ)
styrene was exclusively produced during the course of reaction diffraction line broadening, respectively, with the Scherrer
up to 100% conversion and the further hydrogenation of styrene equation. The surface of Pd and PdZn supported on ZnO was
started to occur aer the initial substrate was fully consumed. examined by X-ray photoelectron spectroscopy (XPS) measure-
Tew et al. showed the effectiveness of PdZn alloy catalyst for the ment. The sample was ground in a mortar, pressed onto a
gas-phase hydrogenation of 1-pentyne to 1-pentene at temper- sample holder, and measured by a JEOL JPS-9200 without such
atures of 45–100 ꢀC.25 The present authors have studied the a pretreatment as Ar sputtering. The charge-up shi correction
selective hydrogenation of phenylacetylene in toluene that is of binding energy for Pd was made by setting the C 1s binding
compressed by CO2. It is demonstrated that CO2 can act as a energy at 248.5 eV. The amount of exposed Pd species in the
reaction promoter enhancing the reaction rate and controlling catalyst was determined by CO pulse chemisorption at 50 ꢀC
the product selectivity in several reactions.26,27 These actions are using a BEL-METAL-1 system. About 150 mg sample was heated
ascribable to interactions of CO2 molecules with organic in a quartz cell at 100 ꢀC for 30 min in H2 and the cell was cooled
substrates, intermediates, and/or catalysts in liquid phases. For to 50 ꢀC in He. Then 1 cm3 CO (1% in He) was pulsed over the
example, in liquid-phase hydrogenation of aromatic nitro sample until the TCD signal for the effluent gas became
compounds, the desired aniline products can be obtained in unchanged.
100% selectivity at any conversion level up to 100% conver-
sion.28,29 The CO2 molecules dissolved in the liquid phase
2.2. Liquid-phase hydrogenation
interact with nitro group and decrease its reactivity while the
Liquid-phase hydrogenation of phenylacetylene and styrene
interactions with CO2 increase the reactivity of intermediates
(partially hydrogenated product) was conducted according to
such as nitrosobenzene and N-phenylhydroxylamine. There is
the previous works.28–31 A stainless steel reactor (100 cm3) was
another effect of CO2 that the dissolution of CO2 changes the
loaded with a catalyst sample 10 mg and toluene 5 cm3, the
liquid properties from pure organic liquid to dense phase CO2
remaining air was replaced with H2 by purging a few times, and
uid, facilitating the dissolution of a gaseous reactant of H2. As
the mixture was compressed by H2 at 0.6 MPa. The reactor was
a result, the nitro compounds can be hydrogenated more
heated to 100 ꢀC and maintained for 30 min for in situ reduction
rapidly and the aniline products can be produced in almost
of catalyst. Then, phenylacetylene 0.5 cm3 and toluene 25 cm3
100% selectivity in the presence of compressed CO2. These
were added to the reactor at ambient temperature, it was cooled
interesting results have motivated us to study the impact of CO2
to 0 ꢀC in an ice-water bath, 0.6 MPa H2 was introduced, and the
pressurization on the selective hydrogenation of phenyl-
mixture was stirred by a magnetic stirrer. When CO2 was used, it
acetylene with Pd and PdZn alloy catalysts.
was added to the reactor with a liquid pump (JASCO SCF-Get) to
a partial pressure of 1.5 MPa aer the introduction of H2. Aer
the reaction, the reactor was depressurized slowly to ambient
pressure and the liquid mixture was analyzed by a gas chro-
2. Experimental
matograph (GL Science Micro GC CP4900). The total conversion
of phenylacetylene was determined from the amounts of the
substrate measured before and aer reaction. The selectivity to
a product was determined from the amount (mole) of the
product formed divided by the total amount (mole) of all the
products detected.
2.1. Catalyst preparation and characterization
Impregnation and coprecipitation methods were used to
prepare Pd/ZnO catalyst samples according to the previous
works.21,22 For the former, a commercial ZnO powder (Kanto
Chemical) was impregnated with a Pd(NO3)2 solution (Tanaka
Kikinzoku), dried at 100 ꢀC overnight, heated in air at 5 K minꢁ1
to 500 ꢀC, and calcined for 3 h. The loading of Pd was 1, 10, and
30% by weight. For the latter, a mixed solution of Pd(NO3)2 and
2.3. High-pressure FTIR and phase behavior measurement
Zn(NO3)$6H2O (Wako Pure Chemical), in which Pd : Zn ¼ 1 : 3 Molecular interactions of phenylacetylene and styrene with CO2
in mole, was prepared and placed in an water bath at 70–80 ꢀC. in the liquid phase were examined by an in situ high pressure
Then, drops of another solution of Na2CO3 (Wako) was added FTIR (JASCO FTIR-620) in attenuated total reection (ATR)
slowly to this solution until pH of the mixture became to 8. The mode at ambient temperature. The formation and adsorption of
precipitate formed was ltered, washed with distilled water, CO from H2 and CO2 over Pd and PdZn catalysts at high pres-
separated by centrifuge, and dried at 100 ꢀC. The resultant solid sures were examined by FTIR in transmittance mode using a
materials were calcined in the same manner as mentioned different laboratory-made cell. A catalyst (20 mg) and SiO2
above. The loading of Pd was 30% by weight. Those ZnO-sup- powder (20 mg) were ground in a mortar and the mixture was
ported Pd samples were reduced in 4% H2 (in N2) at ambient pressurized at 50 MPa for 1 h to prepare a catalyst pellet. The
temperature for 1 h to prepare Pd catalysts or at ambient pelletized catalyst sample was placed in a high-pressure FTIR
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cell (JASCO). The cell volume was 1.5 cm3 and the optical
pathway was 1 cm. The cell was purged by H2 gas (99.99%) a few
times to remove the air at room temperature. In situ FTIR
measurements were made at a H2 pressure of 0.6 MPa and at
different CO2 pressures up to 3 MPa using the JASCO FTIR-620
spectrometer with a resolution of 2 cmꢁ1. The spectra collected
in the presence of 0.6 MPa H2 and CO2 at different pressures
was used as a background. The extent of volume expansion of
the reaction liquid phase pressurized by CO2 was examined with
a high pressure view cell equipped with two sapphire windows,
through which the liquid mixture was examined by naked eye.
The details of those experimental setups and procedures were
described elsewhere.30,32
3. Results and discussion
The ZnO-supported Pd and PdZn catalysts prepared and used in
the present work are listed in Table 1, which includes the metal
crystallite sizes determined by XRD and the numbers of exposed
Pd species determined by CO chemisorption. XRD patterns
obtained are presented in ESI (Fig. S1†). Those catalysts were
used to study the liquid phase selective hydrogenation of
phenylacetylene.
3.1. Hydrogenation with Pd/ZnO and PdZn/ZnO catalysts
prepared by impregnation
Features of the hydrogenation of phenylacetylene in toluene
were examined with several Pd/ZnO catalysts different in the
reduction temperature and the Pd loading (catalysts # 1–6 of
Table 1). The reaction results obtained are given in Fig. 1. The
substrate was hydrogenated to styrene and it was further
hydrogenated to ethylbenzene. Fig. 1 shows that the rate of
phenylacetylene conversion with Pd catalysts is larger than that
with PdZn alloy catalysts. For the PdZn catalysts, the selectivity
to styrene was high irrespective of the large difference in the
PdZn crystallite size (Table 1), which was larger than 90% at
almost complete conversion. For the Pd catalysts, in contrast,
the selectivity to styrene became lowered as the Pd loading was
increased. For the 30 wt% Pd catalyst, the selectivity was smaller
than 85% at a higher conversion. It was observed that the
Fig. 1 Conversion – time and styrene selectivity – conversion profiles
in hydrogenation of phenylacetylene over Pd (B) and PdZn alloy (C)
catalysts including different amounts of Pd on ZnO. Pd loading: (a)
30 wt%, (b) 10 wt%, (c) 1wt%.
styrene was further hydrogenated to ethylbenzene at a longer
reaction time for the PdZn and Pd catalysts, similar to the
previous result of Engels et al.,24 but more gradually for the
former (ESI, Fig. S2†). In addition, styrene was used as a starting
substrate instead of phenylacetylene. Table 2 shows the results
obtained with either styrene or phenylacetylene. The conversion
Table 1 ZnO-supported Pd catalysts prepared and used in the present work
Preparation
method
Pd loading
(wt%)
Reduction
Chemical
state
Crystallite
sizeb (nm)
CO adsorbedc
Catalyst no.
temperaturea (ꢀC)
(mmol gꢁ1
)
1
2
3
4
5
6
7
8
Impregnation
Impregnation
Impregnation
Impregnation
Impregnation
Impregnation
Coprecipitation
Coprecipitation
30
30
10
10
1
1
30
30
Room
500
Room
500
Room
500
Room
500
Pd
PdZn
Pd
PdZn
Pd
PdZn
Pd
3.8
115
4.4
42
4.5
19
0.30
na
0.26
na
0.10
na
1.0
na
4.4
28
PdZn
a
Reduced in a 4% H2 stream at ambient temperature for 1 h for Pd; at ambient temperature and at increasing temperatures with a rate of 5 K minꢁ1
to 500 ꢀC for PdZn. b Determined by X-ray diffraction line broadening. c Amount of CO chemisorbed per 1 g of Pd/ZnO catalyst. na: not adsorbed.
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Table 2 Activity of ZnO-supported Pd catalysts prepared by impregnation for hydrogenation of phenylacetylene and styrenea
Conversion (%)
Phenylacetylene
TOFb (sꢁ1
)
Catalyst no.
Pd loading (wt%)
Chemical state
Styrene
Phenylacetylene
Styrene
1
2
3
4
5
6
30
30
10
10
1
Pd
PdZn
Pd
PdZn
Pd
PdZn
35.9
33.1
37.1
28.2
30.3
29.7
35.2
6.5
42.3
21.3
26.2
15.8
0.30 (4.5)
9.9
1.2 (5.2)
9.2
10.0 (11.1)
44.0
0.16 (2.1)
0.94
0.67 (2.9)
3.4
4.3 (4.6)
11.4
1
a
Reaction conditions: reaction time: 10 min for phenylacetylene, 20 min for styrene. b The amounts of exposed Pd species were estimated from the
results of XRD and CO chemisorption. TOF values determined from CO chemisorption results are given in parenthesis. The estimation of the
amounts of exposed Pd species is explained in ESI.
of styrene in 20 min was comparable to that of phenylacetylene Rodriguez;23 Pd species in the PdZn alloy crystallites was more
in 10 min for the Pd/ZnO catalysts. For the PdZn/ZnO ones, in electron-decient compared to that in the Pd ones. These
contrast, the styrene was hydrogenated even more slowly electron-decient Pd sites on the PdZn crystallites are likely to
compared to the phenylacetylene. This may explain the higher be more benecial for the adsorption of alkyne (phenyl-
selectivity to styrene in hydrogenation of phenylacetylene with acetylene) and alkene (styrene), which may explain the larger
the PdZn alloy catalysts. In a previous work,11 it was shown that TOF values of the hydrogenation of these substrates over the
when Pd was alloyed with Au on a support of TiO2, the activity to PdZn alloy catalysts (Table 2). This effect should be more
the hydrogenation of 1-heptyne was increased while the selec- signicant for the phenylacetylene than the styrene, in which
tivity to the partially hydrogenated product of 1-heptene was the former alkyne is more easily adsorbed than the latter
deceased. The Au species were assumed to act as an electronic alkene.3 The surface of PdZn alloy on Pd (111) surface may
promoter for Pd. The inuence of Au for Pd/TiO2 is different expose both Pd and Zn atoms in ordered structures.33,34 The
from that of Zn for Pd/ZnO observed in the present work.
authors speculate that the surface of nano-sized PdZn alloy
Table 2 also shows turnover frequency (TOF) values for crystallites is a mosaic-like structure exposing Pd and Zn atoms
hydrogenation of phenylacetylene and styrene, in which the and so the catalytic function of exposed Pd species are unlikely
amounts of exposed Pd species were determined from the to change with the crystallite size, which is responsible for the
results of XRD and CO chemisorption. Table 2 indicates that the higher selectivity to styrene irrespective of the Pd loading.
TOF values of the hydrogenation of phenylacetylene and styrene Carturan et al. reported that the formation of b Pd–H is facili-
were comparable for the monometallic Pd catalysts. For the tated over larger Pd crystallites and this promotes the hydro-
PdZn alloy catalysts, these TOF values were enhanced but to a genation of styrene to ethylbenzene.35 In addition, one can
larger extent for the hydrogenation of phenylacetylene. Fig. 2 speculate that the surface of larger Pd crystallites is less elec-
shows XPS spectra collected, indicating that the binding energy tron-decient, which decreases the difference in the adsorption
(BE) of Pd 3d5/2 was 335.2 eV for the Pd catalysts (Fig. 2a) and the between phenylacetylene and styrene; namely, the difference in
BE was shied to 335.9 eV by the formation of PdZn alloy the specic hydrogenation activity for these alkyne and alkene
(Fig. 2b). Namely, the BE of Pd 3d became higher when Pd was is decreased, resulting in an increase in the selectivity to the
allowed with Zn, in accordance with the previous results of alkene, styrene, for larger Pd crystallites, as observed (Fig. 1).
3.2. Hydrogenation with Pd/ZnO and PdZn/ZnO catalysts
prepared by coprecipitation
The above results demonstrate that the PdZn alloy catalysts are
more selective to the partial hydrogenation of phenylacetylene
to styrene as compared to the Pd ones but less active (Fig. 1). To
improve the performance of the former catalyst, coprecipitation
was used to prepare Pd/ZnO samples, in which Pd loading was
30 wt% (catalyst # 7 and 8 of Table 1). The XRD patterns of these
catalysts reduced at ambient temperature and 500 ꢀC are given
in ESI (Fig. S1†). Fig. 3 compares the performance between the
Pd/ZnO catalysts prepared by coprecipitation and impregna-
tion. The catalyst prepared by coprecipitation was highly active
Fig. 2 XPS spectra of (a) Pd on ZnO and (b) PdZn alloy on ZnO samples
than that by impregnation while keeping the high selectivity to
styrene. This may result from its smaller PdZn crystallite size, 28
prepared by impregnation (Im) and coprecipitation (Co) followed by
the reduction at ambient temperature and at 500 ꢀC, respectively.
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Fig. 4 Influence of the presence of 1.5 MPa CO2 (C) on the activity
and selectivity in phenylacetylene hydrogenation for Pd (a) and PdZn
alloy (b) catalysts that were prepared by coprecipitation with a Pd
loading of 30 wt% and reduced at ambient temperature and 500 ꢀC.
Fig. 3 Conversion – time and styrene selectivity – conversion profile
in hydrogenation of phenylacetylene hydrogenation over 30 wt% Pd
loaded ZnO catalysts prepared by coprecipitation (C) and impreg-
nation (B) for Pd (a) and PdZn (b) catalysts reduced at ambient
temperature and 500 ꢀC.
reactions.26,27 For the present cases of phenylacetylene and
styrene in toluene, however, no signicant interactions of CO2
with the alkynyl and alkenyl groups were observed as conrmed
by ATR-FTIR (ESI Fig. S3 and S4†). Other possible factors are
interactions of CO2 with supported Pd and PdZn crystallites.
Previously high-pressure UV/Vis absorption was measured for
nm, compared to 115 nm for the corresponding catalyst
prepared by impregnation (Table 1).
3.3. Hydrogenation in the presence of dense phase CO2
To examine the inuence of CO2 pressurization, the hydroge- silver particles dispersed on the surface of a quartz plate in CO2
nation of phynylacetylene was further studied in the presence of at high pressures.36 The optical absorption was found to change
dense phase CO2, in which the Pd/ZnO catalysts prepared by with CO2 pressure and the properties of the metal particles were
coprecipitation were used (catalyst # 7 and 8 of Table 1) because probably changed through interactions with CO2 molecules
of their better performance compared to those prepared by accumulating on their surface. These interactions could occur
impregnation. The hydrogenation of phenylacetylene was con- for the present supported Pd and/or PdZn crystallites and
ducted in toluene pressurized by CO2 at a pressure of 1.5 MPa. change their catalytic functions. For the hydrogenation over
The inuence of CO2 on the catalytic performance is shown in noble metal catalysts in the presence of dense phase CO2, CO is
Fig. 4. It is noteworthy that the rate of hydrogenation and the oen formed through the reverse water gas shi reaction of H2 +
selectivity to styrene can be improved in the presence of 1.5 MPa CO2 / H2O + CO, which is adsorbed on their surface and
CO2 for either Pd or PdZn catalyst. Under the conditions used, changes their catalytic performance.30,32,37,38 The formation and
the volume of the liquid phase was observed to expand to some adsorption of CO on Pd and PdZn catalysts was examined by in
small extent in the presence of 1.5 MPa CO2. Dissolution of a situ FTIR in transmission mode. Fig. 5 shows FTIR spectra
certain amount of CO2 promotes the dissolution of the gaseous collected in the presence of 0.6 MPa H2 and CO2 at high pres-
reactant of H2 and this may cause the increase in the rate of sures. No CO adsorption was observed for the alloyed PdZn
phenylacetylene conversion. The concentration of phenyl- catalyst at 1.5 MPa CO2 (used in the reaction runs) and a higher
acetylene in toluene used was 0.13 mol dm3 and, at such a low CO2 pressure of 3.0 MPa. For the Pd catalyst, an absorption
substrate concentration, the positive effect of the promoted band appeared at 1900–1800 cmꢁ1, assignable to CO species
dissolution of H2 into the liquid phase should surpass the adsorbed. The absorption band on Pd/ZnO is located at lower
negative effect of the dissolution of CO2 as a diluent. It was frequency compared to a Pd/Al2O3 sample under similar pres-
previously indicated that the presence of dense phase CO2 sure conditions.32 Fig. 5a indicates that the spectra collected at
causes interactions of CO2 molecules with certain functional different CO2 pressures are not so different. For the Pd/ZnO
groups such as carbonyl, nitro, and cyano groups of substrates catalyst, the type of CO adsorption (linear, bridge) and the
and intermediates and this results in changes in the rate of structure of Pd sites for CO adsorption are unclear. However, a
reaction and the product selectivity for hydrogenation and other possibility is given here; if CO could preferentially be adsorbed
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effects of CO2 in the liquid-phase hydrogenation of
cinnamaldehyde.
4. Conclusions
Compared to ZnO-supported Pd catalyst prepared by impreg-
nation and reduced at ambient temperature, alloyed PdZn/ZnO
ꢀ
one reduced at 500 C is less active but more selective to the
hydrogenation of phenylacetylene to styrene in toluene. For the
latter, the selectivity to styrene does not depend on the amount
of Pd loaded (1, 10, 30 wt%) so much but it decreases with
increasing Pd loading for the former. The chemical state of
active Pd species on the surface of PdZn crystallites is unlikely to
change with the Pd loading due to its nature that Pd and Zn
species are regularly exposed irrespective of the Pd loading (the
size of PdZn crystallites). In contrast, the properties of Pd
species on Pd crystallites may depend on their size changing
with the Pd loading. The TOF value (per exposed Pd atom) for
the PdZn alloy catalysts is signicantly higher than that of the
Pd ones. The difference between PdZn and Pd catalysts is more
signicant for the hydrogenation of phenylacetylene than for
that of styrene, resulting in a higher selectivity to styrene in the
hydrogenation of phenylacetylene over the PdZn catalysts. The
enhanced TOF for the PdZn catalysts may result from electronic
and/or morphological modication of Pd species by alloying
with Zn at a high reduction temperature of 500 ꢀC. The activity
of PdZn alloy catalyst can be improved by using coprecipitation
instead of impregnation, which decreases the size of PdZn
crystallites formed and hence increases the number of exposed
Pd species, while keeping its higher selectivity to styrene. The
simple pressurization of the reaction mixture by CO2 (1.5 MPa)
enhances the rate of phenylacetylene conversion and the
selectivity to styrene for both Pd and PdZn catalysts. The
dissolution of H2 into the liquid phase is promoted by the
dissolution of CO2 and this may be a reason for the enhance-
ment of the rate of phenylacetylene hydrogenation. No inter-
actions of CO2 with the alkynyl and alkenyl C–C bonds were
detected by in situ high pressure FTIR. The formation and
Fig. 5 High-pressure FTIR spectra of CO formed and adsorbed on Pd
(a) and PdZn alloy (b) in the presence of 0.6 MPa H2 and at different
CO2 pressures given.
on highly coordinated Pd sites (less electron-decient sites) and adsorption of CO from CO2 and H2 at high pressures were
low coordinated Pd sites (more electron-decient sites) could be observed by the FTIR for the Pd catalyst but not for the alloyed
effective for the adsorption and hydrogenation of phenyl- PdZn one. Possible reasons for the roles of CO2 as a reaction
acetylene and styrene, the relative reactivity of styrene could be promoter were given.
lowered and this would enhance the selectivity to styrene,
similar to the effect of Pd crystallite size as discussed above.
Further physicochemical study is needed to have a deep insight
Acknowledgements
into those possible direct impacts of CO2 on the Pd and PdZn This work was partially supported by JSPS (Japan Society for the
crystallites.
Promotion of Science) – NRCT (National Research Council of
Another example in which the presence of dense phase CO2 Thailand) bilateral program and Grant-in-Aid for JSPS Fellows
has positive effects on both the reaction rate and the product 242667.
selectivity is the hydrogenation of an a,b-unsaturated aldehyde
of cinnamaldehyde to the corresponding unsaturated alcohol of
cinnamyl alcohol over supported Pt catalysts.39,40 For this
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