Palladium complex catalyst immobilized on epoxy support under supercritical conditions

Article abstract of DOI:10.1016/j.crci.2014.01.011

The aim of this research was to obtain a new-generation complex catalyst under supercritical carbon dioxide. A new complex catalyst based on epoxy resin cured with multifunctional polythiourethane was prepared. The use of polythiourethane as a hardener allowed us to introduce linking groups into the structure of the polymer without further functionalization of the resin. Additionally, the use of supercritical CO₂ enabled a more accurate and better distribution of the metal complex in the polymer matrix. The presence of the functional groups allowed us to obtain a catalyst wherein the metal centers had a different electronic structure and various degrees of oxidation, so that such a system was characterized by its high selectivity. The catalytic properties of the prepared catalysts were tested in the hydrogenation reaction. Research methods like time-of-flight secondary-ion mass spectrometry (ToF-SIMS), infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and nitrogen BET surface area measurements were used to characterize polymeric support and heterogenized catalysts.

Full text of DOI:10.1016/j.crci.2014.01.011

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Full paper/Memoire
Palladium complex catalyst immobilized on epoxy support
under supercritical conditions
*
Natalia Ba˛czek, Krzysztof Strzelec
Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland
A R T I C L E I N F O
A B S T R A C T
Article history:
The aim of this research was to obtain a new-generation complex catalyst under
supercritical carbon dioxide. A new complex catalyst based on epoxy resin cured with
multifunctional polythiourethane was prepared. The use of polythiourethane as a
hardener allowed us to introduce linking groups into the structure of the polymer without
further functionalization of the resin. Additionally, the use of supercritical CO2 enabled a
more accurate and better distribution of the metal complex in the polymer matrix. The
presence of the functional groups allowed us to obtain a catalyst wherein the metal centers
had a different electronic structure and various degrees of oxidation, so that such a system
was characterized by its high selectivity. The catalytic properties of the prepared catalysts
were tested in the hydrogenation reaction. Research methods like time-of-flight
secondary-ion mass spectrometry (ToF–SIMS), infrared spectroscopy (IR), X-ray photo-
electron spectroscopy (XPS), scanning electron microscopy (SEM) and nitrogen BET
surface area measurements were used to characterize polymeric support and hetero-
genized catalysts.
Received 25 November 2013
Accepted after revision 16 January 2014
Available online xxx
Keywords:
Supported catalysts
Heterogeneous catalysis
Microporous materials
Hydrogenation
Palladium
Polythiourethanes
Epoxy resins
ß 2014 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
improve their biocatalytic properties [3]. Such resins were
found to have a high capacity and selectivity towards
Recently, there has been a growing interest in exploring
transition metal complex catalysts immobilized on poly-
meric organic matrices. Polymer-supported transition
metal complexes have started to be looked at as good
alternatives to typical inorganic carriers. The main
advantage of polymer supports is the chemical, physical
and morphological structure of these materials and its
influence on the catalytic properties of the metal complex
immobilized on them. The use of epoxy resins as carriers of
catalysts has been limited so far to the use of the metal
complex as the polymerization initiator and the precursor
of the catalytic centres in the cured resin [1,2] and to the
application of various matrices functionalized with epoxy
groups for the immobilization of proteins and enzymes to
metal-ion species. For example, a sulfur analogue of the
glicidyl methacrylate resin was synthesized by Moore et al.
[4], in which the oxirane group was replaced by a thiirane
ring. The ring-opening reaction of the thiirane group with
amines or other nucleophiles generates a thiol group at the
b
-carbon atom. Such immobilization of ligands produces
ion-exchange resins that may be used in wastewater
treatment and in the regeneration of metal salts in
industrial processes. In these ion-exchange resins, not
only the metal binds to the ligand, but also the thiol group
takes part in metal-ion binding, thereby acting as
additional donor site. Crudden et al. showed that thiol-
functionalized mesoporous silicates are efficient Pd
scavengers and that the resulting Pd-encapsulated materi-
als catalyze the Suzuki–Miyaura and Mizoroki–Heck
coupling reactions with virtually no leaching of Pd. Several
heterogeneity tests were performed on the catalyst under
Suzuki–Miyaura conditions and indicate that the main part
*
Corresponding author.
E-mail address: krzysztof.strzelec@p.lodz.pl (K. Strzelec).
1631-0748/$ – see front matter ß 2014 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2014.01.011
Please cite this article in press as: Ba˛czek N, Strzelec K. Palladium complex catalyst immobilized on epoxy support
under supercritical conditions. C. R. Chimie (2014), http://dx.doi.org/10.1016/j.crci.2014.01.011

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of the catalysis (> 95%) occurs on the support via a truly
heterogeneous catalyst. Studies are ongoing to determine
the structure of the catalyst and the scope of its activity [5].
Magnetic resins with amine and mercaptan as chelating
groups were prepared by suspended condensation poly-
merization of 2-chloroethoxymethyl thiirane with dia-
mines. The resins show a high affinity for noble-metal ions
and Hg(II), and predominantly adsorbed Pd(II) or Hg(II)
with Cu(II), Zn(II), and Mg(II) coexisting [6].
prepared from epoxy resin cured with thiol-terminated
polythiourethane. These novel effective curing agents were
synthesized from low-molecular-weight di- and multi-
functional mercaptans and diisocyanates [13]. The oligo-
meric polythiourethanes, apart from their good curing
characteristic of epoxides, allow one to introduce new
binding ligands into the structure of polymers. The
immobilization of the homogeneous catalyst was carried
out under supercritical conditions. The use of supercritical
CO2 will enable a more accurate and better distribution of
the metal complex in a polymer matrix. The activity of these
new catalysts was checked in the hydrogenation reaction.
In our earlier works, we proposed a similar approach
and used thiol-functionalized epoxy resins as coatings for
magnetic cores in encapsulation processes. This novel type
of supports for metal complex catalysts can be easily
separated by the use of a magnetic field. Our experiments
have shown that the microporous structure of the polymer
supports in the hydrogenation of cinnamaldehyde seems
to have higher impact on the reaction activity and
selectivity as compared with the influence of the chemical
structure of the supports. The morphology parameters,
such as pore-size distribution and specific surface area
depends on the type of thiol used as resin hardener [7].
In recent years, scCO2 had many applications in many
fields of chemistry. Supercritical fluids have a unique and
valuable potential for the enhanced processing of many
materials. The applications of supercritical fluids to
polymer processing are known. The ability of supercritical
carbon dioxide to swell and plasticize polymers is crucial
to the impregnation, extraction, and modification of
polymeric materials [8]. Supercritical fluid dyeing tech-
nology has received attention in the textile industry due to
increasing environmental concerns. The solubilities of
three novel disperse azo dyes in supercritical carbon
dioxide were measured. The results show that this method
is very promising for the development of supercritical
processes for dyeing applications based on these dispersed
dyes [9]. A thin film of polymer was fabricated on a
functionalized silicon wafer through self-assembled
monolayers (SAMs) of perfluorophenyl azide derivatives
(PFPA-silane) with covalent bonds by photochemical
reaction. The SAMs were formed in supercritical carbon
dioxide and the immobilization of polymers was per-
formed by UV irradiation. The results indicate that scCO2 is
a good solvent for silylation reactions, better than common
organic solvents such as toluene [10]. Supercritical fluids
allow the synthesis of many types of particles since the
solvent’s chemical and physical properties can be varied
with temperature or pressure, both of which can affect the
degree of supersaturation and nucleation. Hakuata
reported methods for the formation of fine particles using
supercritical fluids CO2 and water [11]. Carbon dioxide in
its liquid or supercritical state (scCO2) has a prodigious
potential as an environmentally benign reaction medium
for sustainable chemical synthesis. Since the mid 1990s,
rapidly increasing research efforts have shown that scCO2
can replace conventional and potentially hazardous
solvents in a wide range of processes. There is also
increasing evidence that the application of scCO2 can
broaden the scope of catalytic synthetic methodologies
[12].
2. Experimental
2.1. Materials
As epoxy resin, a high-purity bisphenol A diglycidylether
D.E.R.TM332 (DER), epoxy equivalent 170 (viscosity 4000–
6000 mPaÁs at 25 8C), The Dow Chemical Company, USA, was
used. A multifunctional polythiourethane hardener (MPTU)
was prepared by the reaction of hexamethylene diisocya-
nate (HDI) and pentaerythritol tetrakis (3-mercaptopropio-
nate) (SIGMA) in excess molar quantity (by 2 moles) using a
previously reported method [13]. Homogeneous catalyst
PdCl2(PhCN)2 was prepared from PdCl2 (Sigma, 99%) and
benzonitrile (Sigma, 99%) in petroleum ether (Sigma) [14].
Toluene (POCH), trans-cinnamylaldehyde (Aldrich), ethanol
(POCH) were used for the hydrogenation reaction, without
further purification.
2.2. Support preparation
Three grams of epoxy resin (DER) and 1.2
g of
polythiourethane (MPTU) were placed in a flask and mixed
until a homogenous consistency was obtained. The mixture
was transferred into a Teflon1 mold and cured at tempera-
tures from 298 to 423 K over a time period of 1 h to 48 h. The
cured resin was frozen by immersion in liquid nitrogen and
mechanically ground to powder. The particle sizes were
about 0.5 mm, which was determined by sieve analysis.
2.2.1. Preparation of the catalyst
On the prepared support, we immobilized the palla-
dium complex PdCl2(PhCN)2 from a saturated toluene
solution through a ligand-exchange process under super-
critical conditions. The supercritical immobilization set-up
is represented in Fig. 1. It is mainly composed of a 100 ml
stainless steel autoclave equipped with a thermostat, a
temperature controller and a pressure indicator. In a
typical experiment, 0.5 g of the support and 15 ml of a
saturated solution of the palladium complex were
introduced into the glass tube, which was placed at the
bottom of the autoclave. The autoclave was heated to the
set temperature and filled with liquid CO2 through a high-
pressure pump until the desired pressure has been
reached. The immobilization was carried out for 5 h at
300 K and at a pressure of 90 bar. The supercritical phase,
containing the catalyst precursor, diffuses inside the
polymer matrix during a pre-established impregnation
Herein, we reported the synthesis of some efficient
heterogenized palladium catalysts based on supports
Please cite this article in press as: Ba˛czek N, Strzelec K. Palladium complex catalyst immobilized on epoxy support
under supercritical conditions. C. R. Chimie (2014), http://dx.doi.org/10.1016/j.crci.2014.01.011

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3
electrons. All binding energies (BEs) were referenced to the
C 1s neutral carbon peak at 284.6 eV.
2.4. Study of Pd leaching
ToF–SIMS measurements were performed using an
ION-ToF GmbH instrument (ToF-SIMS IV) equipped with a
25-keV pulsed Bi+ primary ion gun in the static mode
(primary ion dose of about 1.5Á1011 ions/cm2). The
analyzed area corresponds to a square of 500 Â 500 mm
in the case of secondary-ion mass spectra collecting and
surface imaging. For each sample, three spectra from
different surface areas were recorded. To obtain the plain
surface of catalysts (then better mass resolution could be
achieved), powder samples were tableted before the
measurements. In order to compare the quantity of
palladium present on the surface of the catalysts before
and after the reaction, the number of counts of the selected
ions obtained from the collected mass spectra was
normalized on the basis of the number of total counts.
2.5. Morphology studies
The morphology of the samples was investigated with a
scanning electron microscope JEOL 5500 LV, working under
high vacuum and with an accelerating voltage of 10 kV. An
energy-dispersive X-ray microscope (SEM–EDX), JEOL JSM
840A, Japan, was used to observe the elemental distribution.
The bright points indicate Pd-rich areas. Pore-size distribu-
tion parameters were determined by application of the BET
method on a Sorptomatic 1900 FISONS Instrument.
Fig. 1. Scheme of supercritical immobilization equipment; photograph of
the autoclave with the thermostated bath: (1) CO2 bottle; (2, 4) pressure
gauge; (3) cooling bath cryostat; (5) liquid pump; (6) needle valve; (7)
temperature sensor; (8) glass tube; (9) thermostated bath; (10)
autoclave; (11) pressure reduction valve; (12) solvent trap.
duration. At the end of this period, CO2 is vented out of the
cell through the depressurization valve in isothermal
conditions. For comparison, the immobilization process
2.6. Catalyst test
The reactions were carried out in a 100 ml Parr
microreactor model 4593 with a glass liner. The reactor
was charged with 0.2 g of supported palladium catalyst, 5 ml
of 96% ethanol, 0.25 ml of toluene, and 0.25 ml of
cinnamaldehyde. The reactor was flushed several times
with pure hydrogen and then the hydrogen pressure and
temperature were adjusted to the required level, 40 bar and
75 8C, respectively. The progress of the reaction and product
distribution were routinely and quantitatively analyzed by
gas chromatography (GC) using a Hewlett-Packard 5990 II
gas chromatograph equipped with a thermal conductivity
detector. The GC column was HP-50þ (crosslinked 50% Ph
was also carried out by
a standard immobilization
procedure. The mixture of the same amount of Pd complex
solution and support was stirred at room temperature for
48 h. The products of both processes were filtered off,
washed with toluene (3 Â 20 ml) to remove the quantity of
complex that was non-chemically bounded to the polymer
and finally dried under vacuum at 50 8C for 8 h.
2.3. Spectroscopy characterization
The FT-IR spectra were measured with a BIORAD FT-IR
175 C spectrophotometer under an air atmosphere. XPS
measurements were made on a VG ESCALAB 210 spectro-
Me silicone) 30 m  0.63 mm  1.0
mm film thickness. The
products were identified by matching their retention times
with those of authentic samples. In the recycling tests, after
a reaction was complete, the catalyst was filtered off from
the reaction mixture, washed three times with 20 ml of hot
toluene, dried under vacuum and used for the next reaction.
meter with a Mg Ka (hn = 1253.6 eV) excitation from an X-
ray tube (reduced power 10 kV, 10 mA). The pressure in the
spectrometer chamber was about 10–9 mbar. The samples
were pressed to pellets under a pressure of 100 kbar for
10 min before these measurements. The C 1s, N 1s, O 1 s, Cl
2p and Pd 3d core level spectra were recorded. The
analyzer pass energy was set at 20 eV. A take-off angle of
908 was used in all XPS studies. Curve fitting was
performed using the ECLIPSE data system software. This
software describes each of the components of a complex
envelope as a Gaussian–Lorentzian sum function. The
background was fitted using a non-linear model function
proportional to the integral of the elastically scattered
3. Results and discussion
3.1. Characterization of the polymer supports and
immobilized Pd catalysts
To determine the physicochemical properties of the
polymer matrices and polymer-supported catalysts, we
Please cite this article in press as: Ba˛czek N, Strzelec K. Palladium complex catalyst immobilized on epoxy support
under supercritical conditions. C. R. Chimie (2014), http://dx.doi.org/10.1016/j.crci.2014.01.011

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Fig. 2. FT–IR spectra of (a) DER/MPTU/Pd/scCO2; (b) DER/MPTU/Pd; c) DER/MPTU.
used a variety of analytical methods. These included IR and
X-ray photoelectron spectroscopy (XPS) and the nitrogen
BET adsorption method. The chemical structure of the
polymers was confirmed by FT–IR. Fig. 2 shows IR spectra
of the epoxy resin cured with multifunctional polythiour-
ethane (MPTU) and catalyst DER/MPTU/Pd/scCO2. The
characteristic peaks at 1507 cmÀ1 and 1104 cmÀ1 confirms
the presence of thiourethane linkages. The lack of the
absorption peaks of the S–H vibration at 2545 cmÀ1 and of
the isocyanate group at 2272 cmÀ1 demonstrates the
quantitative polyaddition of thiol and isocyanate groups.
There is a small peak at 1716 cmÀ1 and there are no free
hydroxyl groups only for this catalyst because in this case
the homogeneous catalyst was immobilized on the carrier
under supercritical conditions. The shifts and clear changes
palladium catalyst is consistent with the results from XPS
measurements. X-ray photoelectron spectroscopy analysis
gives a better indication of the chemistry of the surface of
the heterogenized catalyst particle, whereas the other
analysis methods are more relevant to the composition of
the particles as a whole. XPS analyses the outer surface of
particulates and enables significant information to be
extracted concerning the coordination of a catalytic metal
to the polymer support and the mechanism of the catalytic
reaction. The binding energy peak of Cl 2p3/2 in the region
of 198.8–199.0 eV observed in the XPS spectra (Table 1)
indicates the presence of the PdCl2 structure. As a result of
the immobilization of PdCl2(PhCN)2, nearly 5% of PdCl
bonds and 1.32% of the divalent palladium bonds emerged.
Minor fractions of Pd nanoclusters were also observed,
which formed due to the lateral overlapping of the d
in the intensity of ether absorption bands at 1011 cmÀ1
1024 cmÀ1 1085 cmÀ1 and 1233 cmÀ1 after catalyst
,
,
immobilization suggest that ether linkage generated with
the opening of each epoxy ring or introduced with a
polythiourethane hardener is one of the active center of
polymer network (Fig. 3a).
The stretching vibration of the C 5 O group of the
urethane combined with strong absorption at N–H at
1257 cmÀ1 and the presence of characteristic urethane
bands at 1606 cmÀ1 attests to the urethane crosslinking
process (Fig. 3a). The free hydroxyl groups remaining in the
condensed resin provide adequate points for epoxy cross-
linking via urethane linkages by the extremely reactive
isocyanate groups released through dissociation of thiour-
ethane moieties by the heat of the curing reaction. The
shifts in the intensity ratio of these bands indicate an
interaction between the nitrogen atom and the transition
metal. The change in intensity of bands at 1104 cmÀ1 and
1507 cmÀ1 shows that the oxygen and nitrogen atoms of
the thiourethane moiety can participate in metal coordi-
nation (Fig. 3b). The IR spectra of the polymer-supported
Fig. 3. Possible modes of coordination of the Pd(II) complex by the DER/
MPTU support.
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Table 1
Binding energy (eV) values inferred from XPS measurements.
Sample
N1s
O1s
Pd 3d5/2
Pd(0)
Cl 2p3/2
d
+
Nimine
Namine
Pd
Pd(II)
DER/MPTU
402.16
406.18
406.20
405.17
405.18
400.37
406.68
406.69
403.66
403.67
531.61
535.21
535.26
534.25
534.27
–
–
–
–
DER/MPTU/Pd/scCO2 before use
DER/MPTU/Pd/scCO2 after use
DER/MPTU/Pd before use
DER/MPTU/Pd after use
337.45
336.44
336.34
335.34
–
339.2
339.1
340.2
340.1
198.91
198.82
198.79
198.78
336.8
–
335.1
atomic orbitals. They are not stable under normal
conditions, but their prolonged life is related to the
protection afforded by the polymer matrix. The XPS data
summarized in Table 1 show that three possible coordina-
tion sites participate in the binding of Pd to the epoxide
support. The XPS spectrum of DER/MPTU/Pd/scCO2 and
DER/MPTU/Pd catalysts show that palladium appears
predominantly in the form of Pd(II). It indicates that this
polymeric support chains might stabilize such a catalyti-
cally active monovalent metastable species with a binding
energy larger than that of the Pd metal. The recovered DER/
alcohols is an important step in the preparation of various
fine chemicals such as fragrances for the perfume industry.
This reaction can give three products, namely hydrocinna-
maldehyde (HCALD), cinnamyl alcohol (CALC), and hydro-
cinnamyl alcohol (Scheme 1). The mostly desired product is an
a,
b-unsaturated alcohol that is difficult to produce in the
catalytic hydrogenation of -unsaturated aldehydes because
a,b
the hydrogenation of C 5 C bonds in the presence of noble
metals is thermodynamically and kinetically favored over the
hydrogenation of the C 5 O bonds. The results (Table 2) show
that the morphology, the chemical structure of the polymeric
matrix and the conditions of immobilization influence the
selectivity of this reaction, which strongly depends on the
types of binding groups present in the polymer.
MPTU/Pd/scCO2 and DER/MPTU/Pd catalysts after
a
hydrogenation reaction contained also some minor frac-
tion of Pd(0). Since the epoxide matrix cured with the
polythiourethane hardener possesses coordination sites of
different coordination abilities, it seems very likely that the
zero-valent palladium, Pd(0), comes from the reduction of
Pd(II) coordinated to the carbonyl oxygen, the weaker of
the three coordination sites, whereas new species formed
after each reaction run, Pd clusters, come from the
reduction of Pd(II) coordinated to the nitrogen atoms.
The studies show that the catalyst’s selectivity depends
on the size distribution of the metal immobilized on the
carriers. For DER/MPTU/Pd/scCO2, the distribution of the
metal complex in the polymer matrix was better, which
SEM micrographs confirmed. Fig. 4 shows the elemental
distribution for the cases of DER/MPTU/Pd/scCO2 and DER/
MPTU/Pd. The micrograph of DER/MPTU/Pd/scCO2 indi-
cates a uniform distribution of the palladium particles. A
less regular distribution is observed for the DER/MPTU/Pd
catalyst (Fig. 4a,d). Different size distributions of the metal
particles, clearly visible on lower magnification SEM–EDX
images (Fig. 4c,d), influenced by the morphology of the
support, can be an explanation of the unequal catalyst
performances of the DER/MPTU/Pd catalyst, for which the
immobilization was carried out without the use of
supercritical conditions. The results presented in Table 2
show that for catalysts DER/MPTU/Pd/scCO2 and DER/MPTU/
Pd, there is a significant difference between the product’s
distribution between the 1st and 5th runs. This is probably
caused by an easier access to the active spots in the open
pores of the catalyst. DER/MPTU/Pd/scCO2 catalyst selectiv-
ity was slightly lower than for the DER/MPTU/Pd catalyst,
which might be caused by the release of the interior of the
metal from the swollen matrix during the reaction. The
conventionally prepared catalyst showed higher selectivity
to HCALC, whereas the DER/MPTU/Pd/scCO2 catalyst had
higher selectivity to CALC, which is more desirable.
3.2. Catalytic test
To confirm the high activity of the prepared catalyst and
the impact of the structure of the polymer support on
stability, the hydrogenation of cinnamaldehyde (Scheme 1)
at a pressure of 40 bar and at 70 8C was carried out. The
selective hydrogenation of
a,b-unsaturated carbonyl
compounds into their corresponding
a,b-unsaturated
Recycling the immobilized palladium catalysts five
times demonstrated their high stability, despite the
changes that occur in their structure during the catalytic
reaction. The selective mercury poisoning was used to
exclude the possibility of catalyzing the reaction by
unbound palladium metal, which is the result of elution
of the complex from the surface of the carrier. This method
is commonly used to define the nature of homogeneous or
Scheme 1. Hydrogenation of cinnamaldehyde.
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under supercritical conditions. C. R. Chimie (2014), http://dx.doi.org/10.1016/j.crci.2014.01.011

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Table 2
The hydrogenation of cinnamaldehyde over the DER/MPTU/Pd/scCO2 and DER/MPTU/Pd.
Catalyst
Run
Intensity of 106Pd+
Conversion [%]
HCALD [%]
HCALC [%]
CALC [%]
DER/MPTU/Pd/scCO2
1
5
1
5
7.8 Â 10À3
6.4 Â 10À3
4.5 Â 10À3
2.9 Â 10À3
100
98
10
3
60
80
62
94
30
17
31
3
DER/MPTU/Pd
97
7
95
3
heterogeneous reaction [15–18]. The DER/MPTU/Pd/
scCO2 and DER/MPTU/Pd catalysts were exposed to
mercury and then subjected to the catalytic tests. The
catalytic activity was similar to that against mercury
poisoning. For DER/MPTU/Pd/scCO2, we observed a loss
of activity after mercury poisoning, which means that
the use of scCO2 enables also the introduction of
chemically unbounded complexes into the matrix of
the support. This test was also done for the homo-
geneous complex PdCl2(PhCN)2. In this case, the addi-
tion of Hg(0) to the homogeneous reaction mixture
caused the reaction to stop. Mercury poisoning tests
showed that catalysis was not associated with the
leached precursor complexes or in situ formed palla-
dium nanoclusters, which would redeposit to the
polymer support after the reaction.
Fig. 4. SEM micrographs of the surface of the (a and c) DER/MPTU/Pd/scCO2; (b and d) DER/MPTU/Pd.
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Table 3
Characteristics of polymer support and supported Pd catalysts by the BET methods.
DER/MPTU
DER/MPTU/Pd/scCO2
DER/MPTU/Pd
Surface area (m2 gÀ1
)
155
2
102
1.9
75
Mean pore diameter (nm)
Volume of pores (%)
macro w > 50 nm
1.9
10.9
12.3
55
11.1
54
meso 2 < w < 50 nm
micro w < 2 nm
56.7
32.4
36.8
0.33
0.36
34.9
0.31
0.32
Pore specific volume (cm3 gÀ1
)
0.22
0.25
Porosity (vol volÀ1
)
3.3. Catalyst stability
complex blocked on the DER/MPTU support. We are aware
of the poor accuracy of the BET method for low-porosity
materials like the highly crosslinked epoxy resin. With
regard to the swelling behavior of the resin, especially
when the catalytic reaction is performed under solid-liquid
heterogeneous conditions, it is important to realize that
the dimension of the pores can change enormously.
Therefore, in future work, we will apply additional
techniques such as inverse steric exclusion chromatogra-
phy (ISEC) and pycnometry to study the morphology of
these supports and catalysts in the swollen state.
The stability of the supported catalyst was tested
during repeated catalysis runs. Recycling the immobilized
palladium catalysts five times demonstrated their high
stability. The time-of-flight secondary-ion mass spectro-
metry (ToF-SIMS) was applied to investigate the deactiva-
tion process and to observe the deterioration of palladium
distribution on the catalyst surface after the 1st and the 5th
uses of the catalysts. ToF–SIMS method permits the
evaluation of the chemical composition of the catalyst
surface, of the nature of interactions between the metallic
phases and the support as well as the distribution of the
metal. For the DER/MPTU/Pd/scCO2 and DER/MPTU/Pd
catalysts, a drop of the surface accessible Pd was observed
(Table 2) after the 5th run. A small drop of the amount of
surface accessible palladium was observed, especially for
DER/MPTU/Pd/scCO2; however the drop of the Pd content
was at a safe level and did not significantly affect the
activity.
4. Conclusion
The results of this study have demonstrated that the
prepared polymer support meets the requirements for
organic supports and can be used for the preparation of
palladium catalysts. Additionally, the use of supercritical
conditions cause a better distribution of the homogeneous
catalyst in a matrix of polymeric support and influence the
activity of the catalyst prepared in scCO2 The immobiliza-
tion in scCO2 conditions is a simple method and allows a
high concentration of metal in the carrier matrix as
compared to the traditional method of immobilization. The
immobilization in scCO2 can be used even for carriers with
low sorption capacity, because metal can get into the deep
layers of the matrix, which is not available with standard
immobilization. Thus, it could find a possible use as a
convenient procedure for the preparation of industrial
catalysts. The use of polythiourethane as a curing agent
enables one to obtain supports that do not need any further
functionalization. The investigated catalyst showed good
activity and selectivity during prolonged use in the
cinnamaldehyde hydrogenation reaction.
3.4. Structure porosity studies
Our studies showed that the morphology of a polymer
support can greatly improve and affect the activity and
selectivity of the heterogenized catalyst. Steric demands
for a catalytic center bound to a support can be different
from those of its homogeneous analogues. The structure
porosity of the support and the conditions of the
immobilization process affect the accessibility of the
interior of the swollen resin by increasing diffusional
restrictions for the reaction species. The BET nitrogen
adsorption method was used to investigate the structure
porosity of the investigated materials (Table 3). For all
supports, the immobilization of the palladium precursor
resulted in a similar loss in surface area and a correspond-
ing decrease in mean pore diameter. The BET data shows
that the original structure porosity of the polymer supports
has been changed after the attachment of the Pd complex.
The catalysts DER/MPTU/Pd/scCO2 and DER/MPTU/Pd
shows a pore-size distribution with a lower volume of
micropores, and these catalysts had micropores and a
greater volume of meso- and macropores. In addition, a
significant decrease in specific surface area after the
immobilization of metal was observed for the carrier (for
DER/MPTU/Pd/scCO2, 155 m2/g before immobilization to
102 m2/g after immobilization). Such a significant decrease
in the specific surface area was caused by the metal
Acknowledgement
This work was supported by the National Science
Centre (Poland)—grant No. 0255/B/H03/2011/40.
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