Role of polyaniline morphology in Pd particles dispersion. Hydrogenation of alkynes in the presence of Pd-polyaniline catalysts

Article abstract of DOI:10.2478/s11696-013-0383-z

Two polyaniline (PANI) samples of various molecular masses were used for the preparation of palladium catalysts (with 2 mass % of Pd). The physicochemical features of starting polyanilines were found to substantially affect the size and extent of palladium nanoparticles aggregation. Strongly aggregated large palladium particles appeared in the PANI sample of more compact morphology (PANI-H), higher crystallinity and lower specific surface area. Pd nanoparticles of a definitively smaller size were formed in the more amorphous PANI sample of looser morphology (PANI-L) and the extent of particles aggregation was markedly lower. The catalytic properties of Pd/PANI samples were studied in a liquid phase hydrogenation of unsaturated triple bond (C≡C) in alkynes reactants, phenylacetylene, and cyclohexylacetylene. The 2 mass % Pd/PANI-L catalyst prepared using polymer of less compact texture exhibited much higher activity in both reactions. In the presence of the 2 mass % Pd/PANI-L catalyst, alkene products were formed with a high selectivity (approximately 90 %) attained at the almost complete conversion of alkynes. This highly selective hydrogenation of the C≡C to the C=C bond was related to the presence of an electroactive polymer, PANI, in close proximity with Pd active sites. Polyaniline could have a role in a steric effect as well as in a modification of adsorptive properties of Pd centres.

Full text of DOI:10.2478/s11696-013-0383-z

Chemical Papers 67 (8) 1087–1095 (2013)
DOI: 10.2478/s11696-013-0383-z
ORIGINAL PAPER
Role of polyaniline morphology in Pd particles dispersion.
Hydrogenation of alkynes in the presence of Pd-polyaniline catalysts
a
a
a
b
Robert Kosydar, Monika Goral, Alicja Drelinkiewicz*, Jaroslav Stejskal
aInstitute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland
bInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,
Heyrovsky Sq. 2, 162 06 Prague, Czech Republic
Received 29 August 2012; Revised 16 January 2013; Accepted 22 January 2013
Two polyaniline (PANI) samples of various molecular masses were used for the preparation of pal-
ladium catalysts (with 2 mass % of Pd). The physicochemical features of starting polyanilines were
found to substantially affect the size and extent of palladium nanoparticles aggregation. Strongly
aggregated large palladium particles appeared in the PANI sample of more compact morphology
(
PANI-H), higher crystallinity and lower specific surface area. Pd nanoparticles of a definitively
smaller size were formed in the more amorphous PANI sample of looser morphology (PANI-L) and
the extent of particles aggregation was markedly lower. The catalytic properties of Pd/PANI sam-
—
ples were studied in a liquid phase hydrogenation of unsaturated triple bond (C—C) in alkynes
—
reactants, phenylacetylene, and cyclohexylacetylene. The 2 mass % Pd/PANI-L catalyst prepared
using polymer of less compact texture exhibited much higher activity in both reactions. In the
presence of the 2 mass % Pd/PANI-L catalyst, alkene products were formed with a high selectivity
(
approximately 90 %) attained at the almost complete conversion of alkynes. This highly selective
—
—
hydrogenation of the C—C to the C—C bond was related to the presence of an electroactive poly-
mer, PANI, in close proximity with Pd active sites. Polyaniline could have a role in a steric effect
as well as in a modification of adsorptive properties of Pd centres.
—
ꢀ
c 2013 Institute of Chemistry, Slovak Academy of Sciences
Keywords: polyaniline, palladium, hydrogenation, phenylacetylene, cyclohexylacetylene
Introduction
et al., 1995). Unpaired electrons and positive charges
delocalised over the entire conjugation length of the
PANI could provide an environment similar to a π-
donor ligand (Hirao, 2002; Higuchi et al., 1996; Ku-
mar et al., 1995). Electron interactions between PANI
and palladium nanoparticles resulted in a change in
the reduction–oxidation properties of PANI examined
by the cyclic voltammetry technique (Park et al.,
2004). These interactions were also predicted by theo-
retical calculations performed by quantum chemistry
methods (Bia ꢀl ek, 2006). In the light of these obser-
vations, it might be expected that the PANI matrix
could influence the reactivity of Pd-centres in a cat-
alytic reaction. This phenomenon was observed for
palladium–polypyrrole catalysts (Drelinkiewicz et al.,
Polyaniline (PANI) is one of the conductive poly-
mers studied most extensively due to its environmen-
tal stability, controllable electrical conductivity and
interesting chemical properties (Kang et al., 1998).
PANI undergoes protonation with acids to afford the
conducting “salt”; the polymer also readily partici-
pates in redox reaction. Both these processes have
been involved in the synthesis of PANI-transitions
metals composites. Nitrogen groups, particularly the
imine (—N—) ones, could also be considered as po-
tential centres for the coordination of metal ions, such
as Ag, Cu, Pd. In the interaction with metal, the sys-
tem of π-conjugated bonds could be involved (Kumar
*Corresponding author, e-mail: ncdrelin@cyf-kr.edu.pl

1
088
R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
mol L 1) of mole ratio NaCl (POCH, Poland)/PdCl2
equal to 2.5 : 1 and pH of approximately 3–4 was used.
Incorporation of the palladium was achieved by expo-
sure of the PANI sample, at ambient temperature, to
the appropriate volume of the PdCl2 solution to ob-
tain 2 mass % Pd in the final catalysts. The suspension
was stirred up to the total disappearance of palladium
ions in the supernatant liquid (24 h), as confirmed by
the colorimetric method. After the incorporation of
Pd, the solids were filtered, washed with water until
−
2
004) in the hydrogenation of acetophenone as well
as for platinum–polyaniline catalysts (Steffan et al.,
2
008) in the hydrogenation of unsaturated aldehydes.
In our previous work, laboratory-prepared PANI
samples were used for the preparation of Pd-PANI cat-
alysts. These studies concentrated on the interactions
2+
between PANI and Pd complexes. In particular, the
role of the composition of palladium complexes was ex-
amined, whereas the role of polymer morphology was
not taken into consideration. This appears to be an
important effect in the light of growing interest in the
PANI-Pd, Pt, Ru, composites which are seen to be
potential candidates as electrocatalysts in the fuel cell
technology (Antolini & Gonzalez, 2009). Accordingly,
the present experiments were undertaken using two
samples of PANI with different textural and morpho-
logical properties.
−
◦
the Cl anions were absent, and dried at 80 C for 4 h.
Characterisation of catalysts
Specific surface area was determined in accordance
with the standard Brunauer–Emmet–Teller (BET)
method from the nitrogen adsorption–desorption iso-
therm using a Quantasorb Jr. (Ankersmit, USA).
Prior to sorption measurements, the samples were pre-
The present studies sought to evaluate the role
of PANI morphology in the dispersion of palladium
particles in the Pd/PANI catalysts. The catalytic
behaviour of Pd/PANI samples in the hydrogena-
tion of unsaturated triple bond in alkyne reactants,
namely phenylacetylene (Phac) and cyclohexylacety-
lene (Cyac), was evaluated. These reactions were cho-
sen as both are standard examples of alkyne to alkene
and alkane hydrogenations. Moreover, they represent
a class of processes of interest to the pharmaceuti-
cal and fine chemical industries. In order to achieve
selective hydrogenation of alkyne to alkene, metal
sites must exhibit appropriate electronic and/or mor-
phological properties (Molnár et al., 2001). The high
selectivity to alkene was related to the increased
Pd electron density leading to the decreased alkene
adsorption. In effect, greater selectivity to alkene
was achieved by employing electron donor modifiers
such as N-bases (quinoline, pyridine, ammonia). Very
promising results were also obtained for Pd and Pt
catalysts supported by polymers with N-bases as the
constitutional units (Michalska et al., 1998; Arena et
al., 1996; Telkar et al., 2004; Nadgeri et al., 2008).
This type of modification appears to be very durable
because, unlike commonly used modifiers, the N-units
of the polymer cannot be removed or lost during the
catalytic reaction.
◦
treated under vacuum at 80 C for 16 h. The X-ray
diffraction (XRD) studies were carried out using an
HZG-4 diffractometer (Panalytical X‘Pert PRO, The
Netherlands) with CuKα radiation. The morphology
of the reduced catalysts was studied by means of a
Field Emission Scanning Electron Microscope JEOL
JSM – 7500 F (Japan) equipped with the energy dis-
persive EDS system. Two detectors were used and the
images were recorded in two modes. The secondary
electron detector provided secondary electrons images
(SEI) and the backscattered electron detector pro-
vided backscattered electrons (BSE COMPO) micro-
graphs.
Hydrogenation experiments
Hydrogenation experiments were carried out in
−
1
a “batch” type glass reactor (volume of 0.2 L
)
under a constant atmospheric pressure of hydrogen
◦
at a temperature of 22 C as previously described
(Drelinkiewicz et al., 1999). Stirring of the reaction
mixture in the reactor was carried out by using an
agitation device with an agitation frequency control.
Pentanol (Sigma–Aldrich) was used as the solvent.
Prior to the hydrogenation experiment (in a typical
procedure), the catalyst was placed inside the reac-
tor by wetting it with an excess of solvent. Nitrogen
Experimental
(
Linde Gaz, Poland) (15 min) and subsequently hy-
General
drogen (Linde Gaz) (30 min) were passed through
the reactor. Next, the catalyst was placed in con-
tact with the hydrogenated solution and the reac-
tion was started by switching on the agitation de-
vice. The progress of the hydrogenation was mon-
itored by measuring the volume of hydrogen con-
sumed against the reaction time. The reactor was
linked to a system of gas-burettes and an electronic
device for registering the volume of hydrogen con-
sumed in the reaction. Samples of solutions were
withdrawn from the reactor via a sampling tube at
Two commercially available (Sigma–Aldrich, USA)
samples of PANI base in the emeraldine oxidation
state, differing in average molecular mass of polymer,
with a more compact morphology (PANI-H) sample
of Mw = 100000 and a polymer with looser morphol-
ogy (PANI-L) sample of Mw = 10000 were used as the
matrices for palladium. The catalysts were prepared
as previously described (Drelinkiewicz et al., 1998). A
−
3
PdCl2 (Sigma–Aldrich) aqueous solution (4.6 × 10

R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
1089
appropriate time intervals and analysed by the GC
method.
Products analysis was performed using a gas chro-
matograph PE Clarus 500 (USA) equipped with a
flame ionisation detector under the following condi-
tions: capillary column Elite-5 MS (30 m × 0.25 mm ×
0
.25 µm coating) with helium (Linde Gaz) as a car-
−
1
rier gas (flow rate of 1 mL min ) and injection
temperature of 250 C. Product separation was ob-
tained using temperature ramps of 80 C (1 min),
1
◦
◦
◦
−1
◦
5 C min to 220 C (3 min). Commercially available
(
Fluka, Germany; Sigma–Aldrich) analytical stan-
dards of reagents, phenylacetylene, styrene, ethyl-
benzene, cyclohexylacetylene, cyclohexylethylene, and
cyclohexylethane were used in the chromatographic
analysis.
Fig. 1. XRD diffraction pattern of: 1 – PANI-H and 2 – PANI-
Typically, the hydrogenation tests were carried out
at 22 C using 40 mL of substrate solution of 0.046
L samples.
◦
−
1
mol L concentration and the catalyst concentration
−
1
of 0.75 g L . The hydrogenation was performed up
to the point at which the hydrogen uptake was com-
pleted. Introductory experiments were performed in
which the rate of agitation was changed. Those con-
ditions under which the rate of hydrogen uptake did
not depend on an agitation rate were used in further
hydrogenation tests.
mer can be seen. The most characteristic diffractions
◦
are the peaks at 2θ equal to 16.0 and at 2θ equal
◦
◦
to 25.4 . The former, at 2θ equal to 16.0 , has been
attributed to the periodicity along the polymer chain
and the one at 2θ equal to 25.4 to the periodicity per-
pendicular to the polymer chain (Murugesan & Sub-
ramanian, 2003). This demonstrates that the PANI in
the PANI-H sample exhibits a higher degree of crys-
tallinity.
◦
Results and discussion
The scanning electron micrographs of the samples
studied are shown in Fig. 2. The data in the litera-
ture revealed that the morphology of PANI powder
could vary because it is the result of the polymerisa-
tion conditions. Accordingly, spherical particles, the
rice-grain morphology, coral-like cylindrical morphol-
ogy, nanotubes, nanofibres, and nanowires could be
observed as well as fibrous network structures. Fig. 2
shows that both PANI samples consist of granular mi-
croparticles almost spherical in shape and 200–500 nm
in size. However, the samples differ essentially in the
degree of microparticles agglomeration. In the PANI-
H sample, consisting of a polymer with the higher
molecular mass, spherical microparticles of the poly-
mer are intensively agglomerated forming large ag-
gregates (Fig. 2). This affords the highly compact
morphology of the PANI-H sample. The polymer mi-
croparticles in the PANI-L sample are markedly less
agglomerated; as a consequence the morphology of
this PANI is less compact. Apart from these morpho-
logical differences, the PANI-H and PANI-L samples
also differ in textural features such as specific surface
area (Table 1). The more compact morphology of the
PANI-H sample results in a specific surface area of
It is widely known that the physico-chemical prop-
erties of PANI such as morphology and crystallinity
are determined by the molecular mass of the poly-
mer (Stejskal et al., 1998; Li et al., 2004). In general,
the degree of crystallinity of PANI, which is associated
with the supra-molecular organisation of PANI chains,
increases as the molecular mass of the PANI grows.
The present work studied two samples of PANI in the
form of an emeraldine base differing in their molecu-
lar masses. The morphology and textural properties of
the starting polyanilines were characterised by sorp-
tion (BET), X-ray diffraction, and scanning electron
microscopy techniques.
The reflections originating from crystalline and
amorphous polymers appear in the diffraction pat-
tern of the PANI-base. However, a broad halo at 2θ at
◦
around 20 associated with amorphous polymer dom-
◦
inates, whereas the diffractions at 2θ equal to 16 and
5 originating from the crystalline polymer are defini-
◦
2
tively weaker (Pouget et al., 1991). The XRD diffrac-
tion patterns registered for the PANI-H and PANI-L
samples are displayed in Fig. 1.
In the XRD pattern of the PANI-L sample, the
◦
2
−1
broad halo centred at 2θ of approximately 20 strongly
26.8 m g , while the looser structure of the PANI-
L sample gives a higher specific surface area equal to
predominates, thereby showing the domination of an
amorphous polymer. On the other hand, in the XRD
pattern of the PANI-H sample, besides the broad
diffraction associated with an amorphous polymer, a
set of diffractions originating from the crystalline poly-
2
−1
32.8 m g
.
After the incorporation of palladium, the specific
surface area of both samples decreases, albeit to var-
ious extents. The decrease is more pronounced in the

1
090
R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
Table 1. Specific surface area of samples and results obtained in hydrogenation of phenylacetylene (Phac) and cyclohexylacetylene
Cyac)
(
Hydrogenation of Phac
Hydrogenation of Cyac
Surface area
Initial activity · 10−
3
Selectivitya
Initial activity · 10−3
Selectivitya
Sample
m2
g
−1
mol min
−1 −1 b
g
%
−1 −1 b
mol min g
cat
%
cat
PANI-H
mass % Pd/PANI-H
PANI-L
mass % Pd/PANI-L
26.8
16.8
32.8
13.8
–
0.02
–
–
no data
–
–
0.03
–
–
no data
–
2
2
0.83
89.7
1.32
90.6
a) Calculated at 94 % alkyne conversion; b) the rate per 1 g of catalyst.
Fig. 2. SEM micrographs of starting polyanilines; PANI-H (a), PANI-L (b).
case of the PANI-L sample exhibiting a less compact
structure. This may suggest that a looser morphol-
ogy of the polymer permits easier penetration of the
palladium particles throughout the polymer sample.
As previously described (Drelinkiewicz et al.,
1
998), treatment of the PANI base with a palladium
precursor salt could lead directly to the deposition of
Pd-metal particles due to an oxidation–reduction reac-
2+
tion between the Pd species and the nitrogen groups
of PANI. It is well known that various aquachlorocom-
plexes of palladium ions coexist in the aqueous solu-
−
tion of PdCl2 consisting of Cl ions (excess of HCl or
NaCl added). At a given concentration of palladium
ions, the type of ligands in the coordination sphere
of palladium is determined by the concentration of
−
Cl ions. In the PdCl2 precursor solution of low acid-
ity (pH ≈ 3–4) used in this work, the [PdCl2(H2O)2]
complex was dominant. Under these conditions, pal-
ladium ions were reduced to the Pd-metal which was
deposited in the PANI matrix.
Fig. 3. XRD diffraction patterns of; 1 – 2 mass % Pd/PANI-H
and 2 – 2 mass % Pd/PANI-L catalysts.
In the present work, the composition of the palla-
dium precursor solution was identical with the previ-
ous one, whereas the polyanilines samples were of vari-
ous morphologies. Thus, the role of PANI morphology
in the deposition of Pd-particles and, in particular,
its role in the dispersion of palladium particles can be
observed.
The XRD diffraction patterns of catalysts are
shown in Fig. 3. In the diffraction of the 2 mass %
Pd/PANI-H catalyst, well-resolved small diffractions

R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
1091
Fig. 4. SEM micrographs of 2 mass % Pd/PANI-H (a) and 2 mass % Pd/PANI-L (b) catalysts (magnification × 5000).
◦
◦
at 2θ = 40.1 and 46.0 originating from the crys-
talline Pd metal appear. From the broadening of the
◦
diffraction peak at 2θ = 40.1 , the average diameter
of Pd particles is evaluated to be approximately 10–
1
5 nm. On the other hand, no signals of crystalline Pd
can be seen in the XRD diffraction of the 2 mass %
Pd/PANI-L catalyst. This suggests the presence of
Pd-nanoparticles of a size below the detection limit
of the XRD technique (below 2 nm).
Fig. 4 shows the SEM micrographs of 2 mass %
Pd/PANI-H and 2 mass % Pd/PANI-L catalysts reg-
istered at the same magnification (× 5000). A com-
parison of these micrographs clearly demonstrates the
crucial role of polymer morphology in the dispersion
of Pd particles. The white spots corresponding to the
palladium particles can be observed in the micrograph
of the 2 mass % Pd/PANI-H catalyst, i.e. the sample
consisting of the polymer of more compact morphol-
ogy (Fig. 4). On the other hand, in the micrograph
of the 2 mass % Pd/PANI-L sample, only a few in-
sufficiently distinct white spots can be seen, showing
that the particles are too small to be observed dis-
tinctly in the image registered at this relatively low
magnification.
Thus, the compact morphology of the polymer in
the 2 mass % Pd/PANI-H catalyst results in the for-
mation of relatively large palladium particles which
are randomly distributed throughout the polymer
grains (Fig. 4). They are of various shapes and their
size is within a range of 100–200 nm. The SEM im-
age registered at a higher magnification (× 20 000)
shows these particles to be of irregular shape, which
suggests that they represent the aggregates composed
of smaller Pd nanoparticles. This observation is con-
sistent with the XRD diffraction (Fig. 3) revealing
the particles of 10–15 nm in size. Thus, the com-
pact morphology of the PANI-H sample gives rise to a
strong aggregation of palladium nanoparticles leading
to large Pd aggregates (Fig. 5).
Fig. 5. SEM micrograph of 2 mass % Pd/PANI-H catalyst
(magnification × 20000).
The SEM images registered for the 2 mass %
Pd/PANI-L sample (Figs. 4 and 6) demonstrate that
the looser morphology of the more amorphous PANI
affords a better dispersion of the palladium in this
catalyst. The particles of Pd are more homogeneously
distributed throughout the polymer grains, as well as
being of definitively smaller sizes.
A more detailed examination (images registered at
magnification as high as × 400000) also reveals that in
the 2 mass % Pd/PANI-L catalyst the nanoparticles
observed are the aggregates composed of primary Pd
nanoparticles. However, these primary particles form-
ing the aggregates are of a very small size because no
diffractions arising from the Pd-crystalline can be ob-
served in the XRD pattern of 2 mass % Pd/PANI-L
catalyst (Fig. 3).
To summarise, the present results indicate that the
morphology of the PANI sample exhibits a crucial role

1
092
R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
Fig. 6. SEM micrographs of 2 mass % Pd/PANI-L catalyst; magnification (× 100000) (a) and (× 400000) (b).
in dispersion of the palladium particles formed in the
palladium–PANI composites. The use of the PANI
sample with a high molecular mass (PANI-H) and,
thus, compact morphology results in large Pd parti-
cles. The particles of a definitively smaller size are
formed when the polymer exhibits a less compact and
more amorphous structure, i.e. the sample exhibits a
lower molecular mass (PANI-L). This result is of inter-
est given that no data on the role of PANI morphology
in the dispersion of metal particles have previously
been reported.
here is consistent with the observations of other au-
thors.
Hydrogenation experiments
The results in our previous work showed the very
promising performance of Pd/PANI catalysts in selec-
—
tive hydrogenation of the unsaturated triple (C—C)
—
bond in the 2-butyne-1,4-diol reactant (Drelinkiewicz
et al., 2008). The present work investigates the hy-
drogenation of other alkyne reactants, namely pheny-
lacetylene (Phac) and cyclohexylacetylene (Cyac).
Special attention is directed towards the selective hy-
The results obtained by electron microscopy and
XRD techniques show that the primary Pd nanopar-
ticles in both the Pd-polyaniline composites studied
are less or more aggregated.
—
—
drogenation of the C—C to the C—C bonds.
—
Catalytic tests were performed using both 2
mass % Pd/PANI-L and 2 mass % Pd/PANI-H cata-
lysts. The obtained catalysts’ activity and selectivity
towards alkene are given in Table 1. Polyanilines sam-
ple, the support for metal particles, did not exhibit
activity in the hydrogenation test. It is why no ac-
tivity data are given for the initial PANI samples in
Table 1.
According to the literature, particles of noble met-
als (Pd, Pt, Rh, etc.) within a very broad size range
from 1–2 nm up to 100–200 µm could appear in
the PANI composites prepared either by electrochem-
ical or chemical procedures (Amaya et al., 2007;
O’Mullane et al., 2004; Hasik et al., 2009; Antolini &
Gonzalez, 2009; Kim & Park, 2008). Moreover, some
reports in the literature showed aggregation of these
metal particles. The aggregation effect was particu-
larly prominent in the case of nanoscale particles. For
instance, Amaya et al. (2007) reported that, in the Pd–
PANI composite (prepared by chemical procedure),
apart from the particles of 5–15 nm in diameter, ag-
gregated particles which formed a number of agglom-
erates of various shapes and sizes also appeared. The
large clusters of platinum (up to 50 nm in diameter)
formed in the Pt–PANI composites were identified by
O’Mullane et al. (2004) as being composed of primary
particles of 1–2 nm in size. The aggregation of bimetal-
lic Pt–Ru particles, ranging from 3 nm up to 8 nm, to
form various aggregates was also observed by Kim and
Park (2008) in the Pt–Ru–PANI composites prepared
electrochemically. Thus, the effect of the Pd nanopar-
ticles’ aggregation observed in the composites studied
As a measure of catalyst activity, an initial rate
−
1
of alkyne hydrogenation (mol min ) related to the
−
1
−1
catalyst mass (expressed as mol alkyne min
g
) is
cat
assumed. In both the reactions studied, the 2 mass %
Pd/PANI-L catalyst is much much more active and
its activity is distinctly higher (≈ 40-times) than that
of the 2 mass % Pd/PANI-H sample, as shown in Ta-
ble 1. This activity relation is consistent with the elec-
tron microscopy data which demonstrate that the pal-
ladium particles in the 2 mass % Pd/PANI-L catalyst
are better dispersed.
Catalytic hydrogenation of the alkyne reactant is
a consecutive process wherein alkene is an intermedi-
ate product and alkane is formed via hydrogenation
of the C—C bond in the alkene molecule. In order
to achieve the highly selective formation of alkene, a
catalyst needs to be able to inhibit the hydrogena-

R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
1093
slowly. This is demonstrated by the products distri-
bution plots, and also by the shape of the hydrogen
uptake curves, especially for the Cyac substrate.
After consumption of 1 eq. of hydrogen (corre-
sponding to the complete hydrogenation of alkyne to
alkene hydrogenation) a break appears on the hydro-
gen uptake curve showing that further hydrogenation
of C—C bonds in alkenes to form alkanes is a very
slow reaction. The rate is distinctly lower than those
—
of C—C in the alkynes reactants. As a result, in the
—
presence of the 2 mass % Pd/PANI-L catalyst at al-
most complete conversion of the alkyne reactants (at
9
4 % alkyne conversion), the selectivity towards alkene
is very high, attaining approximately 90 % (Table 1).
The substantially lower activity of the 2 mass %
Pd/PANI-H catalyst renders observation of the selec-
tivity effect difficult. Because of the very low rate of
alkyne hydrogenation on the 2 mass % Pd/PANI-H
catalyst, the conversions achieved even after 5 h re-
action time were very low (9 % and 15 % for Phac
and Cyac hydrogenation, respectively). As a result,
the content of the products formed was very low, thus
rendering observation of the alkane product particu-
larly difficult.
Fig. 7. Hydrogenation of phenylacetylene in the presence of 2
mass % Pd/PANI-L catalyst. Hydrogen uptake (ꢀ) and
reagents concentration change against reaction time;
phenylacetyle (
◦
), styrene (ꢁ), and ethylbenzene (ꢂ).
To summarise, hydrogenation of the unsaturated
—
triple C—C bond in both the alkyne reagents to the
—
C—C bond in the corresponding alkene products is a
highly selective reaction on the 2 mass % Pd/PANI-L
catalyst. Unfortunately, the very low activity of the 2
mass % Pd/PANI-H catalyst prevents correct obser-
vation of the selectivity effect.
The highly selective formation of alkenes attained
on the 2 mass % Pd/PANI-L catalyst can be at-
tributed to the presence of PANI in close proxim-
ity to the Pd particles. The role of the PANI could
be considered in terms of its electronic and steric
effects. The selectivity-promoting effect observed for
nitrogen-containing polymers was attributed to mod-
ification of the adsorption properties of metal cen-
tres, leading to inhibited adsorption of the interme-
diate product, alkene (Christodoulakis et al., 2009;
Klasovsky et al., 2009). By this concept, the al-
most entirely selective hydrogenation of 2-butyne-1,4-
diol and phenylacetylene to the corresponding alkenes
obtained due to the presence of poly(vinylpyridine)
and oligoaramides (oligo-p-phenyleneterephtalamide,
OPTA) matrices (Arena et al., 1996; Telkar et al.,
2004) was explained. The former polymer consists of
pyridine units, the latter (OPTA) has both unsatu-
rated aromatic rings as well as –NH2 groups. In the
presence of the latter Pd/OPTA catalyst, the selectiv-
ity of styrene formation was as high as 94–96 % when
almost all the phenylacetylene underwent the reaction
(Arena et al., 1996).
Fig. 8. Hydrogenation of cyclohexylethylene in the presence
of 2 mass % Pd/PANI-L catalyst. Hydrogen uptake
(
ꢀ) and reagents concentration change against reaction
time; cyclohexylacetylene (
and cyclohexylethane (ꢂ).
◦
), cyclohexylethylene (ꢁ)
tion of C—C in the alkene intermediate product up
to the almost complete hydrogenation of the unsatu-
—
rated C—C in the alkyne reactant. Consequently, the
—
selectivity towards alkene is commonly considered at
conversions of alkyne reactants of approximately 80–
9
0 %.
The hydrogen consumption curves and products
distribution graphs for hydrogenation of Phac and
Cyac in the presence of 2 mass % Pd/PANI-L cat-
alyst are plotted in Figs. 7 and 8, respectively. From
the very beginning of the reaction, the hydrogenation
—
of C—C in Phac and Cyac yielding alkene products
—
predominates. However, when almost all the alkyne
reactants have been consumed, further hydrogenation
of the alkenes thus produced, the styrene and cyclo-
hexylethylene to the saturated products proceeds very
The rate of hydrogenation of alkynes in the present
work was independent of the agitation frequency. This
confirmed that the results obtained were independent
of mass transport resistances. Due to the very small

1
094
R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
catalyst particles (Fig. 2), liquid–solid mass trans-
fer resistances also do not have any influence. Thus,
the effect of PANI on the reactivity of Pd-centres
through a moderation of the active sites–reagents’ in-
teractions could be taken into consideration. However,
these interactions could be partially limited as, in the
crystallinity, morphology, and specific surface areas,
catalysts of the same palladium content (2 mass %
Pd) were prepared. The dispersion of palladium par-
ticles in the Pd/PANI catalysts was found to depend
on the morphology of the starting polymer. The com-
pact morphology of the PANI-H sample produced Pd
nanoparticles 10–15 nm in size. They were strongly
aggregated, forming large irregularly shaped agglom-
erates. The loose morphology of the PANI-L sample
resulted in Pd nanoparticles of markedly smaller size
which were noticeably less aggregated. The 2 mass %
Pd/PANI-L catalyst exhibited a very promising cat-
alytic performance in the hydrogenation of unsatu-
2
mass % Pd/PANI-L catalyst, the Pd particles are ag-
gregated to some extent. On the other hand, the activ-
ity/selectivity pattern in the hydrogenation of alkyne
reactants (2-butyn-1,4-diol) on Pd, Pt, Rh, Ru cata-
lysts was reported to be dependent on the structural
units of the polymer, i.e. the nature and number of
functional groups (polyvinylpyrrolidone, poly(vinyl al-
cohol), poly(methylvinyl ether) in close proximity to
metal particles (Telkar et al., 2004). The surrounding
functional groups of the polymer impeded the reac-
tants from reaching the metal surface and, by changing
the polymer, the selectivity pattern was altered. The
authors noted that the high selectivity towards alkene
may be related to the fact that re-adsorption of the
alkene was inhibited due to the polymeric functional
groups surrounding the metal nanoparticles. These ef-
fects could also be taken into account in the activ-
ity/selectivity behaviour of the Pd/PANI catalysts
studied. Moreover, the polymer could also sterically
affect the access of the reagents to the catalytically
active centres, in a similar manner to that observed
by other authors (Telkar et al., 2004).
—
—
rated triple C—C bonds to the double C—C bonds
—
in alkynes, phenylacetylene and cyclohexylacetylene.
—
The unsaturated triple C—C bond was almost en-
—
tirely selectively reduced to the double bond C—C
with almost total conversion of alkynes due to the
complete suppression of alkane formation. The advan-
tageous performance of the Pd/PANI catalysts could
be explained by complex effects constituting a modi-
fication of the adsorptive properties of Pd-centres due
to the presence of the electroactive matrix consisting
of N-groups and a system of delocalised π-electrons.
The polymer could also exhibit a steric effect affecting
access of the reagents to the active sites.
Acknowledgements. The investigations were financially
supported by the Polish Committee for Scientific Research
(NCN) grant 2011/01/D/ST5/04917.
The selectivity changes in the hydrogenation of
alkyne reactants were frequently correlated with
the presence of the β-hydride phase of palladium
References
(
β-PdH). The formation of β-PdH promoted over-
hydrogenation to form alkane. However, it appears
from the literature (Molnár et al., 2001) that the role
of the β-hydride phase in over-hydrogenation is still
unclear as various properties of palladium particles
such as surface morphology, crystallinity, and size and
shape could influence its formation. Moreover, our pre-
vious studies (Drelinkiewicz et al., 2004) revealed that
the trend towards the β-hydride phase formation by
palladium particles dispersed in the matrix of elec-
troactive polymer, polypyrrole differed from that ob-
served for conventional, alumina-supported palladium
catalysts. These literature data clearly demonstrate
the complex nature of the role of the polymer in the ac-
tivity/selectivity pattern of the reaction catalysed by
metal particles dispersed in a polymer matrix. Thus,
the advantageous performance of the Pd/PANI cata-
lyst could be ascribed to an effect of the electroactive
polymer matrix consisting of N-groups and a system
of delocalised π-electrons. The steric effect of the poly-
mer in the vicinity of the active Pd centres as well as
the modification of Pd-centres reactivity needs to be
taken into consideration.
Amaya, T., Saio, D., & Hirao, T. (2007). Template synthesis
of polyaniline/Pd nanoparticle and its catalytic application.
Tetrahedron Letters, 48, 2729–2732. DOI: 10.1016/j.tetlet.
2007.02.059.
Antolini, E., & Gonzalez, E. R. (2009). Polymer supports for
low-temeperature fuel cell catalysts. Applied Catalysis A:
General, 365, 1–19. DOI: 10.1016/j.apcata.2009.05.045.
Arena, F., Cum, G., Gallo, R., & Parmaliana, A. (1996).
Palladium catalysts supported on oligomeric aramides in
the liquid-phase hydrogenation of phenylacetylene. Journal
of Molecular Catalysis A: Chemical, 110, 235–242. DOI:
10.1016/1381-1169(96)00188-4.
Bia ꢀl ek, B. (2006). Computational studies of the interactions be-
tween emeraldine and palladium atom. Surface Science, 600,
1679–1683. DOI: 10.1016/j.susc.2005.12.059.
Christodoulakis, K. E., Palioura, D., Anastasiadis, S. H., &
Vamvakaki, M. (2009). Metal nanocrystals embedded within
polymeric nanostructures: Effect of polymer-metal com-
pound interactions. Topics in Catalysis, 52, 394–411. DOI:
10.1007/s11244-008-9172-2.
Drelinkiewicz, A., Hasik, M., & Choczy n´ ski, M. (1998). Prepa-
ration and properties of polyaniline containing palladium.
Materials Research Bulletin, 33, 739–762. DOI: 10.1016/
s0025-5408(98)00042-7.
Drelinkiewicz, A., Hasik, M., & Kloc, M. (1999). Liquid-phase
hydrogenation of 2-ethylanthraquinone over Pd/polyaniline
catalysts. Journal of Catalysis, 186, 123–133. DOI: 10.1006/
jcat.1999.2493.
Conclusions
Drelinkiewicz, A., Waksmundzka, A., Makowski, W., Sobczak,
J. W., Król, A., & Zi ˛e ba, A. (2004). Acetophenone hy-
Using PANI samples differing in molecular masses,

R. Kosydar et al./Chemical Papers 67 (8) 1087–1095 (2013)
1095
drogenation on polymer–palladium catalysts. The effect
of polymer matrix. Catalysis Letters, 94, 143–156. DOI:
Molnár, A., Sárkány, A., & Varga, M. (2001). Hydrogenation
of carbon-carbon multiple bonds: chemo-, regio- and stereo-
selectivity. Journal of Molecular Catalysis A: Chemical, 173,
185–221. DOI: 10.1016/s1381-1169(01)00150-9.
Murugesan, R., & Subramanian, E. (2003). Effect of organic
dopants on electrodeposition and characteristics of polyani-
line under the varying influence of H2SO4 and HClO4 elec-
trolyte media. Materials Chemistry and Physics, 80, 731–
739. DOI: 10.1016/s0254-0584(03)00127-5.
Nadgeri, J. M., Telkar, M. M., & Rode, C. V. (2008). Hy-
drogenation activity and selectivity behaviour of supported
palladium nanoparticles. Catalysis Communications, 9, 441–
446. DOI: 10.1016/j.catcom.2007.07.023.
O’Mullane, A. P., Dale, S. E., Macpherson, J. V., & Unwin,
P. R. (2004) Fabrication and electrocatalytic properties of
polyaniline/Pt nanocomposites. Chemical Communications,
2004, 1606–1607. DOI: 10.1039/b404636f.
Park, J. E., Park, S. G., Koukitu, A., Hatozaki, O., & Oyama, N.
(2004). Electrochemical and chemical interactions between
polyaniline and palladium nanoparticles. Synthetic Metals,
141, 265–269. DOI: 10.1016/s0379-6779(03)00410-7.
Pouget, J. P., Jozefowicz, M. E., Epstein, A. J., Tang, X., &
MacDiarmid, A. G. (1991). X-ray structure of polyaniline.
Macromolecules, 24, 779–789. DOI: 10.1021/ma00003a022.
Steffan, M., Klasovsky, F., Arras, J., Roth, C., Radnik, J.,
Hofmeister, H., & Claus, P. (2008). Carbon-carbon double
bonds versus carbonyl group hydrogenation: Controlling the
intramolecular selectivity with polyaniline-supported plat-
inum catalysts. Advanced Synthesis & Catalysis, 350, 1337–
1348. DOI: 10.1002/adsc.200800035.
10.1023/b:catl.0000020539.31128.d4.
Drelinkiewicz, A., Zi e˛ ba, A., Król, A., Sobczak, J. W., &
Grzywa, M. (2008). Polyaniline supported Pd and Pt cat-
alysts. Role of metal in hydrogenation of 2-butyne-1,4-diol.
Polish Journal of Chemistry, 82, 1717–1732.
Hasik, M., Turek, W., Nyczyk, A., Stochmal, E., Bernaisk, A.,
Sniechota, A., & So ꢀl tysek, A. (2009). Application of con-
jugated polymer–platinum group metal composites as het-
erogeneous catalysts. Catalysis Letters, 127, 304–311. DOI:
10.1007/s10562-008-9679-y.
Higuchi, M., Imoda, D., & Hirao, T. (1996). Redox behavior of
polyaniline-transition metal complexes in solution. Macro-
molecules, 29, 8277–8279. DOI: 10.1021/ma960761f.
Hirao, T. (2002). Conjugated systems composed of transition
metals and redox-active π-conjugated ligands. Coordina-
tion Chemistry Reviews, 226, 81–91. DOI: 10.1016/s0010-
8
545(01)00436-2.
Kang, E. T., Neoh, K. G., & Tan, K. L. (1998). Polyani-
line: polymer with many interesting intrinsic redox
states. Progress in Polymer Science, 23, 277–324. DOI:
0.1016/s0079-6700(97)00030-0.
A
1
Kim, S., & Park, S. J. (2008). Electroactivity of Pt–Ru/polyani-
line composite catalyst-electrodes prepared by electrochemi-
cal deposition methods. Solid State Ionics, 178, 1915–1921.
DOI: 10.1016/j.ssi.2007.12.074.
Klasovsky, F., Claus, P., & Wolf, D. (2009). Influence of prepa-
ration parameyters on the performance of colloid-derived oxi-
dic palladium catalysts for selective hydrogenation of colloid-
derived oxidic palladium catalysts for selective hydrogenation
of C–C triple bonds. Topics in Catalysis, 52, 412–423. DOI:
Stejskal, J., Riede, A., Hlavatá, D., Prokeš, J., Helmstedt, M.,
& Holler, P. (1998). The effect of polymerization temper-
ature on molecular weight, crystallinity, and electrical con-
ductivity of polyaniline. Synthetic Metals, 96, 55–61. DOI:
10.1016/s0379-6779(98)00064-2.
1
0.1007/s11244-008-9173-1.
Kumar, S., Verma, R., Venkataramani, B., Raju, V. S.,
Gangadharan, S. (1995). Sorption of platinum, palla-
&
dium, iridium, and gold complexes on polyaniline. Sol-
vent Extraction and Ion Exchange, 13, 1097–1121. DOI:
Telkar, M. M., Rode, C. V., Chaudhari, R. V., Joshi, S. S., &
Nalawade, A. M. (2004). Shape-controlled preparation and
catalytic activity of metal nanoparticles for hydrogenation
of 2-butyne-1,4-diol and styrene oxide. Applied Catalysis A:
General, 273, 11–19. DOI: 10.1016/j.apcata.2004.05.056.
10.1080/07366299508918320.
Li, J., Fang, K., Qiu, H., Li, S. P., & Mao, W. M. (2004). Mi-
cromorphology and electrical property of the HCl-doped and
DBSA-doped polyanilines. Synthetic Metals, 142, 107–111.
DOI: 10.1016/j.synthmet.2003.08.014.
Michalska, Z. M., Ostaszewski, B., Zientarska, J., & Sobczak,
J. W. (1998). Catalytic hydrogenation of alkadienes and
alkynes by palladium catalysts supported on heterocyclic
polyamides. Journal of Molecular Catalysis A: Chemical,
129, 207–218. DOI: 10.1016/s1381-1169(97)00212-4.