Polysiloxane microspheres functionalized with imidazole groups as a palladium catalyst support

Article abstract of DOI:10.1002/aoc.3446

Polysiloxane microspheres containing a large number of silanol groups were obtained by an emulsion process of modified polyhydromethylsiloxane. N-substituted imidazole groups were grafted on these microspheres by the silylation of their silanol groups with N-[γ-(dimethylchlorosilyl)propyl]imidazole hydrochloride. The progress of the reaction was monitored using ²⁹Si and ¹³C magic angle spinning (MAS) NMR and its impact on microsphere morphology was studied using scanning electron microscopy (SEM). The usefulness of the imidazole-functionalized microspheres as a support for a metal catalyst was demonstrated by their reaction with PdCl₂(PhCN)₂. In this way a new heterogenized catalyst, Pd(II) complex with imidazole ligands supported on polysiloxane microspheres, was generated. This catalyst, MPd, was characterized using ¹³C and ²⁹Si MAS NMR, X-ray photoelectron, Fourier transform infrared and far-infrared spectroscopies, X-ray diffraction, SEM-energy-dispersive X-ray spectroscopy and wide-angle X-ray scattering. The catalyst appears in two structures, as Pd(II) complex and Pd(0) nanoclusters. Its catalytic activity was tested using a model reaction, the hydrogenation of cinnamaldehyde, and compared with that of an analogous complex operating in a homogeneous system. MPd showed a high activity in the promotion of hydrogenation of cinnamaldehyde. The activity in the substrate conversion was stable at least in five cycles of this reaction. The main product was hydrocinnamaldehyde which could be obtained with a yield above 70%. A mechanism of the reaction is proposed.

Full text of DOI:10.1002/aoc.3446

Full paper
Received: 9 October 2015
Revised: 16 December 2015
Accepted: 30 December 2015
Published online in Wiley Online Library: 11 February 2016
(wileyonlinelibrary.com) DOI 10.1002/aoc.3446
Polysiloxane microspheres functionalized with
imidazole groups as a palladium
catalyst support
a
a
a
a
Piotr Pospiech *, Julian Chojnowski , Urszula Mizerska , Tomasz Makowski ,
b
b
Krzysztof Strzelec and Natalia Sienkiewicz
Polysiloxane microspheres containing a large number of silanol groups were obtained by an emulsion process of modified
polyhydromethylsiloxane. N-substituted imidazole groups were grafted on these microspheres by the silylation of their silanol
2
9
groups with N-[γ-(dimethylchlorosilyl)propyl]imidazole hydrochloride. The progress of the reaction was monitored using Si
1
3
and C magic angle spinning (MAS) NMR and its impact on microsphere morphology was studied using scanning electron micros-
copy (SEM). The usefulness of the imidazole-functionalized microspheres as a support for a metal catalyst was demonstrated by
their reaction with PdCl (PhCN) . In this way a new heterogenized catalyst, Pd(II) complex with imidazole ligands supported on
2 2
13 29
polysiloxane microspheres, was generated. This catalyst, MPd, was characterized using C and Si MAS NMR, X-ray photoelec-
tron, Fourier transform infrared and far-infrared spectroscopies, X-ray diffraction, SEM–energy-dispersive X-ray spectroscopy
and wide-angle X-ray scattering. The catalyst appears in two structures, as Pd(II) complex and Pd(0) nanoclusters. Its catalytic
activity was tested using a model reaction, the hydrogenation of cinnamaldehyde, and compared with that of an analogous com-
plex operating in a homogeneous system. MPd showed a high activity in the promotion of hydrogenation of cinnamaldehyde. The
activity in the substrate conversion was stable at least in five cycles of this reaction. The main product was hydrocinnamaldehyde
which could be obtained with a yield above 70%. A mechanism of the reaction is proposed. Copyright © 2016 John Wiley & Sons,
Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: polysiloxane microspheres; heterogeneous Pd catalysis; cinnamaldehyde hydrogenation; imidazole ligand
contact with an external reagent molecule, so much so that the mi-
crospheres are able to absorb organic compounds.
Introduction
There has recently been a growing interest in the use of micro-
spheres as catalyst supports because the shape and size of these
particles give them adequate hydrodynamic properties and make
In the research reported here, polysiloxane microspheres were
functionalized with imidazole which was further used as a ligand in
the formation of a palladium complex. The heterogenized palladium
catalyst thus obtained was characterized, and its ability to promote
hydrogenation reactions was tested using a model reaction system.
Imidazole is known as a good ligand in various transition metal cata-
[
1]
them suitable for easy isolation from reaction systems. Micro-
spheres find broad applications in various areas of science, medi-
[2]
cine and technology. Polysiloxanes are very attractive materials
for the fabrication of microspheres. Macromolecules of these poly-
mers are very flexible and mobile which allow the adjustment of
the chain conformation to suit the environment. These features
give them a high permeability of many organic substances and
together with their amphiphilic character endow them with inter-
[5]
lytic systems. Monodentate imidazole ligands are able to form
complexes with palladium containing two, three or four imidazole
[6]
groups bonded to the metal atom. Imidazole is able to form rela-
tively strong bonding to metals and its complexes with metals show
[7]
high stability. In addition, imidazole toxicity is low and it is more
easily degradable than commonly used phosphine ligands which
[
3]
esting surface properties. Due to their inorganic skeleton, they
have high thermal stability and their side groups can be easily
modified. Although there is an extensive literature on polysiloxane
composite particles, very little research has been devoted to whole
polysiloxane microspheres. We have recently worked out a method
of synthesis of polysiloxane microspheres containing a large
number of reactive silanol groups which make easy the
[8]
makes its metal complexes more environmentally friendly.
*
Correspondence to: Piotr Pospiech, Centre of Molecular and Macromolecular
Studies, Engineering of Polymeric Materials, Sienkiewicza 112, 90-363 Lodz,
Poland. E-mail: piotrp@cbmm.lodz.pl
[
4]
functionalization of these particles. Unlike the rigid cage-
structured particles obtained using sol–gel techniques, these
microspheres are built of linear polysiloxane chains cross-linked
by flexible diethylenedisiloxane bridges. Due to this structure, func-
tional groups on polymers of these particles may easily come into
a Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences,
Sienkiewicza 112, 90-363, Lodz, Poland
b Lodz University of Technology, Institute of Polymer and Dye Technology,
Stefanowskiego 12/16, 90-924, Lodz, Poland
Appl. Organometal. Chem. 2016, 30, 399–407
Copyright © 2016 John Wiley & Sons, Ltd.

P. Pospiech et al.
Palladium complexes belong to the most common transition
metal catalysts. They are widely used to promote hydrogenations
of organic compounds which are crucial processes in the food
diffraction patterns were recorded in the 2θ range from 10° to 46°
and are presented as functions of angle 2θ. Typical time of acquisi-
tion was 50 s. The samples were prepared as powder and were irra-
[
9]
and petrochemical industries. They are also used in the catalysis
i
diated at an incident angle (α ) of 0.020°.
[
10]
of carbon–carbon bond-forming coupling
Buchwald–Hartwig, amination, carbonylation and oxidation
as well as in the
Infrared (IR) spectroscopy
[11]
reactions. Although homogeneous catalysis by palladium com-
plexes dominates in academic research, the application of these
systems in practice is limited. The reasons are difficulties in the sep-
aration of expensive and often toxic catalysts from reaction
mixtures, which leads to the contamination of products with traces
of catalyst. Heterogenization of palladium greatly facilitates the sep-
aration of the catalyst and its recycling thus reducing costs and
The Fourier transform IR spectra were collected, with a resolution of
ꢀ
1
4 cm , using a Jasco 6200 spectrophotometer equipped with an
attenuated total reflectance (ATR) accessory. Each spectrum was
ꢀ
1
the average of 64 scans in the range 600–4000 cm . Far-infrared
(FIR) spectra were measured using a Bruker Vertex 70v spectrome-
ter equipped with Si beam splitter and Globar was used as FIR
source. For each sample, 256 scans were accumulated at a resolu-
[
12]
ꢀ1
making the chemical process more environmentally friendly.
tion of 4 cm , with scanner velocity at 2.5 kHz.
Gas chromatography
Experimental
GC analysis was performed using a Hewlett Packard 5890 II appara-
tus equipped with a TCD detector and an HP-50+ column (30 m ×
Chemicals
0.53 mm × 1 μm). Injector and detector temperature was 210°C.
4
Toluene, acetone, ethanol, poly(vinyl alcohol) (PVA; M
n
= 7.2 × 10 g
mol ) and dioxane (POCh, analytical grade), cinnamaldehyde
Aldrich, 99%), cinnamyl alcohol (Aldrich, 98%) and hydrocinnamy-
Carrier gas was helium. Temperature programme: the column was
ꢀ
1
ꢀ
1
kept at 50°C for 2 min, then heated to 200°C at a rate of 5°C min
.
(
Thermogravimetric analysis (TGA)
laldehyde (TCI, >93%) were used as received.
Polyhydromethylsiloxane (PHMS) was purchased from ABCR un-
TGA was performed with a TA Instruments 2950 TGA HR analyser at
a rate of 10 K min under nitrogen.
3
ꢀ1
ꢀ
1
der the name HMS 991, M
exclusion chromatography.
DVTMDS; 97%) was from ABCR. Pt(0) Karstedt catalyst containing
n
= 2.0 × 10 g mol as confirmed by size
1,3-Divinyltetramethyldisiloxane
X-ray photoelectron spectroscopy (XPS)
(
2
0% (w/w) Pt was kindly offered by Momentive Performance
XPS spectra of all samples were acquired at room temperature
using an ESCALAB-210 (VG Scientific, UK) spectrometer and a
non-monochromated Al Kα (hν = 1486.6 eV) X-ray source, operated
at 300 W (15 kV, 20 mA). Survey scan (0–1350 eV) was acquired at
the pass energy of hemispherical analyser of 75 eV, with a step of
Materials GmbH Leverkusen. Triethylamine (POCh, >99%) was
purified by standard methods.
limidazole)palladium(II) chloride (PdCl (mim) ) were prepared from
PdCl (ABCR, 99.9%) and respectively benzonitrile (ABCR, 99%) or
[
13]
PdCl (PhCN)2 and bis(methy-
2
2
2
[
14]
2
[
15]
1
-methylimidazole (Merck Millipore, for synthesis).
dimethylchlorosilyl)propyl]imidazole hydrochloride was prepared
from chlorodimethylsilane and allylimidazole according to a
N-[γ-
0.4 eV. Detailed spectra in all regions were acquired at a pass en-
(
ergy of 25 eV and step of 0.1 eV. Data processing was performed
using Avantage Data System (Thermo Electron). Data smoothing
procedure was applied and the nonlinear Shirley background was
subtracted. Binding energy (BE) scale was referenced to the C 1s
peak of C–Si bond of siloxane according to Beamson and Briggs
[
16]
method described previously.
Analytical methods
[
17]
data with BE = 284.38 eV.
Magic angle spinning (MAS) NMR spectroscopy
29
X-ray fluorescence (XRF) analysis
Si MAS NMR spectra were recorded with a Bruker DSX 400 spec-
trometer working at 59.627 MHz. The spectra were acquired with
cross-polarization, applying 90 ms pulses, 6 s pulse delay and 3
ms contact time, with samples in 4.0 mm zirconia rotors spinning
at 8 kHz. The peak positions were referenced to the signal of
The palladium content was determined by XRF spectroscopy using
an energy-dispersive XRF spectrometer (Canberra, model 1510).
The samples were homogenized with colloidal silica in the propor-
tion of microspheres–Pd complex to SiO of 1:20 using MoO as the
2
3
1
3
8 8
Q M .
C MAS NMR spectra were obtained with the Bruker DSX
internal standard (0.5 wt%). Obtained powder was then pressed
ꢀ
2
4
00 spectrometer working at 100.61 MHz using 4 mm ZrO rotors,
2
into tablets having a surface density of 0.0478 g cm . The tablets
were excited using an Am-241 source of photons at 60 keV. Pd-K
and Mo-K photons were detected using a semiconductor Si(Li)
detector.
spinning rate if 8 kHz, contact time of 2 ms and repetition delay
of 2 s. The acquisition was collected with SPINAL 16 decoupling
sequence.
Scanning electron microscopy (SEM)–energy-dispersive X-ray spectroscopy
Elemental analysis
(
EDX)
Elemental analysis for nitrogen, carbon and hydrogen was per-
formed using an automatic EuroVector model 3018 analyser.
SEM images were obtained with a JEOL JSH 5500 LV microscope in
high-vacuum mode at an accelerated voltage of 10 kV. Samples
were coated with a fine gold layer (about 10 nm thick) using an
ion coating JEOL JFC 1200 apparatus. The surfaces of the analysed
samples were studied using EDX.
Synthesis of cross-linked polysiloxane microspheres
The preparation of polysiloxane microspheres was carried out
[
4]
according to a procedure described previously. A solution of
PHMS (48.02 g) and DVTMDS (10.08 g) in dioxane (40 ml) was
Wide-angle X-ray scattering
ꢀ
6
X-ray diffraction (XRD) measurements were performed using a
source of Cu Kα radiation, operating at 30 kV and 50 mA. The
stirred at 45°C with Pt(0) Karstedt complex (4.7 x 10 mol of Pt)
for 8 min. After this time, 80 ml of isopropanol was added and
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 399–407

Polysiloxane microspheres as a palladium catalyst support
29
13
the solution was introduced into 400 ml of degassed water with
beige colour. MPd was characterized using Si and C MAS NMR
1.63 g of dissolved PVA heated at 45°C. The obtained mixture was
spectroscopy, TGA, ATR, FIR, SEM-EDX, XRD, XRF and XPS.
29
homogenized for 3 min using a high-speed MPW-120 homogenizer
set at 7500 rpm. The formed emulsion was introduced to 2800 ml
of the aqueous solution of PVA having the same concentration
and temperature as the solution used in the homogenization. The
suspension was stirred (150 rpm) at 45°C for two days. Micro-
spheres were isolated by centrifuging, and washed several times
with water and freeze-dried in vacuum. Regular spherical particles,
Si MAS NMR (δ at maximum, ppm): ꢀ67.6 MeSi(OSi)
MeSi(OH)(OSi) )(OSi) ; +7.2 Me Si(CH )(OSi) and
; ꢀ22.6 MeSi(CH
SiO. C MAS NMR (δ at maximum, ppm): ꢀ3.1, ꢀ0.8 CH
+8.7 CH ; +14.3 CH CH CH -Im; +24.2 CH CH CH -Im; +49.9
CH CH CH -Im; +118.1, +128.0, +137.9 CH imidazole ring. IR (at max-
3
; ꢀ58.9
2
2
2
2
2
1
3
Me
3
3
;
2
2
2
2
2
2
2
2
2
2
ꢀ
1
imum, cm ): 623 NH; 653 ring; 767 Si–C str; 835 Si–C rock; 916 Si–OH
str; 1009 Si–O str; 1097, 1252 CH sym def; 1408 Si–OCH(CH ) str;
3
3 2
5
2.01 g (88% yield), with diameters ranging from 3.5 to 30 μm
1358, 1444, 1519 CN imidazole ring; 2957 C–H str. FIR (at maximum,
ꢀ
1
(
mean value 14.7 μm) were obtained. They were analysed using
cm ): 543 siloxane matrix; 342 Pd–Cl; 308 Pd–N. Elemental analysis
(%): C, 31.56; H, 6.35; N, 5.90. TGA: 233°C – loss 0.4%; 279°C – loss
7.6%; 480°C– loss 34.7%; residue 45.13 wt%.
2
9
13
Si and C MAS NMR and IR spectroscopies, TGA, elemental anal-
ꢀ
1
ysis and SEM. They contained 8.0 mmol g SiOH.
2
9
Si MAS NMR (δ at maximum, ppm): ꢀ67.3 MeSi(OSi) ; ꢀ57.4
3
MeSi(OH)(OSi) ; ꢀ37.8 MeSi(H)(OSi) ; ꢀ21.0 MeSi(CH )(OSi) ; +7.2
2
2
2
2
Reduction of Pd(II) Complex
13
Me Si(CH )(OSi) and Me SiO. C MAS NMR (δ at maximum, ppm):
2
2
3
ꢀ
1
ꢀ
3.5, ꢀ0.9, +0.6 CH ; +8.5 CH . IR (at maximum, cm ): 763 Si–C
MPd catalyst was reduced to convert Pd(II) complex into Pd(0)
3
2
[
18]
str; 836 Si–C rock; 905 Si–OH str; 1010 Si–O str; 1268 CH sym def;
according to a procedure described previously.
MPd (0.5 g)
(0.47 g) in
3
1407 CH asym def; 2160 Si–H str; 2966 C–H str; 3355 O–H str. Ele-
was dispersed in ethanol (3 ml), then mixed with NaBH
4
3
mental analysis (%): C, 24.03; H, 7.1.
deionized water (35 ml) and shaken at room temperature for 20 h.
The resulting product was filtered, washed successively with EtOH
2
and Et O and dried under vacuum to afford the dark grey Pd(0)
complex.
Preparation of imidazole-functionalized microspheres (M-Im)
The reaction of silanol groups in the microspheres with N-[γ-
(
dimethylchlorosilyl)propyl]imidazole hydrochloride [(PrIm)Me
2
SiClꢁ
Studies of cinnamaldehyde hydrogenation
[16]
HCl], obtained according to a previous description, was carried
[4]
out using the procedure similar to that described elsewhere. The
reaction was performed in a suspension of microspheres in dried
methylene chloride at room temperature. Microspheres (6.1 g, con-
The hydrogenation reactions were carried out in an Autoclave
Engineers B030SS reactor with 300 ml glass test-tube. The reactor
was charged with 50 ml of 96% ethanol, 2.5 ml of toluene and
2.5 ml of cinnamaldehyde and a suitable amount of solid catalyst
taining ca 49 mmol SiOH) were suspended in a mixture of CH
60 ml) and triethylamine, used as HCl acceptor (35 ml). The mixture
was slowly added to the solution of the hydrochloride (17.089 g,
.0782 mol) in CH Cl (20 ml). After the addition the mixture was
stirred for 10 h. Volatile components, i.e. solvent, excess triethylamine
and excess [(PrIm)Me SiCl], were distilled off under reduced pressure
2 2
Cl
(
(MPd or PdCl (mim) ) was introduced. The reactor was flushed
2 2
three times with pure nitrogen and then the hydrogen was intro-
duced. The hydrogen pressure was adjusted to a suitable value
(4 MPa). Reaction was carried out at 65°C for 2 h with continuous
stirring (400 rpm) and was monitored at certain time intervals by
GC analysis of withdrawn samples using toluene as the internal
standard. Then the mixture was cooled and the catalyst was sepa-
rated by decantation, washed with acetone, dried under vacuum
and used for the next reaction.
0
2 2
2
and a new portion of methylene chloride (10 ml) was introduced. The
suspension was washed with water and microspheres were isolated
by centrifugation. This operation was repeated several times. Then
M-Im was dried in a vacuum of 0.2 mmHg and analysed using Si
and C MAS NMR and IR spectroscopies, TGA, elemental analysis
and SEM.
29
13
2
9
Results and discussion
Si MAS NMR (δ at maximum, ppm): ꢀ67.4 MeSi(OSi) ; ꢀ58.6
3
MeSi(OH)(OSi) ; ꢀ22.4 MeSi(CH )(OSi) ; +7.4 Me Si(CH )(OSi) and
2
2
2
2
2
Modification of microspheres by grafting imidazole to
polysiloxane chains
1
3
Me SiO. C MAS NMR (δ at maximum, ppm): ꢀ3.1, ꢀ0.8 CH ; +7.1
3
3
CH2; +13.9 CH CH CH -Im; +24.6 CH CH CH -Im; +48.7
2
2
2
2
2
2
CH CH CH -Im; +117.7, +127.6, +135.9 CH imidazole ring. IR (at
We worked out the method of synthesis of polysiloxanes micro-
spheres containing a large number of SiOH groups. The method
2
2
2
ꢀ
1
[4]
maximum, cm ): 622 NH; 661 ring; 771 Si–C str; 835 Si–C rock;
06 Si–OH str; 1015 Si–O str; 1252 CH sym def; 1407 Si–OCH(CH3)2
9
is based on formation of an emulsion of microspheres from modi-
fied PHMS. The cross-linking of the polysiloxane in aqueous emul-
sion, which solidifies the polymer droplets, proceeds in parallel
with the hydrolysis of SiH groups producing a large number of SiOH
groups. The presence of reactive silanol groups gives a broad pos-
sibility for the functionalization of these particles. The microspheres
with imidazole groups were obtained by the silylation of the silanol
groups on the spheres with N-[γ-(dimethylchlorosilyl)propyl]imid-
azole hydrochloride (Scheme 1). The reaction was carried out in
the suspension of microspheres in methylene chloride with an
excess of the imidazole reagent and triethylamine used as HCl
acceptor.
3
str; 1357, 1444, 1507 CN imidazole ring; 2957 C–H str. Elemental
analysis (%): C, 38.41; H, 6.89; N, 6.83. TGA: 198.9°C – loss 1.06%;
372.9°C – loss 53.8%; residue 45.13 wt%.
Immobilization of palladium onto microspheres
The M-Im microspheres were suspended in toluene and a solution
of palladium complex PdCl (PhCN) in toluene, used in a proportion
2 2
of 1:4 mol(Pd)/mol(imidazole), was slowly introduced. The suspen-
sion was shaken for a few days at room temperature. The colour
of the solution faded from deep to light brown. The obtained palla-
dium complex heterogenized on microspheres (MPd) was sepa-
rated from the solution by filtration, rinsed a few times with
toluene and then dried under vacuum. The complex was a light
29
The analysis of the Si MAS NMR spectrum (Fig. 1) indicates that
most of the hydroxyl functions on polysiloxane are silylated. The
silylation leads to the introduction of imidazole groups bonded
Appl. Organometal. Chem. 2016, 30, 399–407
Copyright © 2016 John Wiley & Sons, Ltd.
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P. Pospiech et al.
Scheme 1. Modification of microspheres by N-[γ-(dimethylchlorosilyl)propyl]
imidazole hydrochloride (1, native microspheres; 2, [(PrIm)Me
SiClꢁHCl]; 3,
M-Im).
2
Figure 3. TGA curves: (A) native; (B) after modification (M-Im); (C) after
immobilization of Pd (MPd).
cleavage of cross-links in the silylation process also explains the
observation that the ability of microspheres to absorb organic sol-
vents increases after the modification. Native microspheres absorb
about 12 wt% of toluene but after the silylation the absorption in-
creases to above 60 wt% which might be beneficial for palladium
immobilization. The increased absorption capacity indicates that
the silicon network in the microspheres is loosened which allows
for a better contact of the palladium reagent with the ligand
grafted on polysiloxane. Hence an enhancement of the efficiency
of complex formation on the microspheres and an increase in its
catalytic activity could be expected. The loosening of the network,
which probably occurs mostly in the surface layer of the micro-
spheres, is manifested by a tendency of sticking of microspheres
during the modification. Round traces seen on microspheres
(Fig. 4(B)) are formed as a result of the temporary sticking to other
spheres.
2
9
Figure 1. Si MAS NMR spectra of microspheres: (A) native; (B) after
modification; (C) after immobilization of Pd.
by N-propyldimethylsiloxane bridge to the polysiloxane chain
13
which is confirmed from the C MAS NMR spectrum (Fig. 2(B)).
The disappearance of the remaining SiH groups during the reaction
as a result of their dehydrogenative condensation with the silanol
groups is also observed using Si MAS NMR (Fig. 1(B)) and IR spec-
troscopies. The amount of imidazole groups bonded to micro-
29
Structural studies of palladium immobilized on microspheres
ꢀ
1
spheres, determined using elemental analysis, is 4.88 mmol g
.
The well-known ligand-exchange process was applied to immobi-
lize palladium on the polysiloxane microspheres functionalized by
The thermal properties of the microspheres after the modifica-
tion are significantly changed, which might be a result of the cleav-
age of Si–O–Si stiff bridges between polysiloxane chains caused by
imidazole. PdCl (PhCN)2 was used as the precursor with labile
2
benzonitrile ligands which can exchange at room temperature in
[
19]
the presence of the hydrochloride in the modification process.
The largest weight loss occurs at about 390°C, while the unmodified
microspheres mostly decompose at 400–500°C (Fig. 3). The
Figure 4. SEM images: (A) native microspheres; (B) with imidazole (M-Im);
(C) with palladium (MPd); (D) MPd after five cycles of hydrogenation
reaction.
1
3
Figure 2. C MAS NMR spectra of microspheres: (A) native; (B) after
modification; (C) after immobilization of Pd.
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Appl. Organometal. Chem. 2016, 30, 399–407

Polysiloxane microspheres as a palladium catalyst support
[
20]
the presence of a stronger ligand.
The obtained product was
13
characterized using several analytical techniques. C MAS NMR
spectra allowed the observation of widening of imidazole ring sig-
nals which indicates that palladium is bonded to the nitrogen li-
gand (Figs. 1(C) and 2(C)). The amount of palladium bonded to
the surface of the polysiloxane carrier was estimated using XRF
and SEM-EDX. The palladium content found by the XRF method is
4.91 wt% that makes 86% of the total amount of palladium used
in the immobilization reaction. The EDX analysis gives a surface
layer content of only 15.6 wt% of the total palladium used. The dif-
ference in the palladium content measured by SEM-EDX and XRF
indicates that a large proportion of palladium is inside the micro-
spheres. Small aggregates of Pd(0) nanoparticles can be seen in
the SEM micrographs (Fig. 4) and TEM micrographs (Fig. 2 in
supporting information).
The information provided by XPS and FIR allows a deeper under-
standing of the palladium complex structure. Analysis of the XPS
spectra (Fig. 5; Table 1) indicates that palladium occurs in two basic
forms, Pd(0) and Pd(II). The observed positions of the Pd 3d5/2 peaks
(
binding energy of 338.04 eV) are close to that found for Pd(II)
[
21]
reported by other authors. Nearly half of the total amount of pal-
ladium is in Pd(0) form (binding energy of 335.62 eV). This may be
caused by the presence, undetectable by NMR, of an amount of
SiH groups which induces the reduction of PdCl
dal Pd particles (Fig. 5). It has been reported that imidazole deriv-
atives might stabilize catalytically active Pd(0) nanoparticles.
2
(PhCN)
2
to colloi-
[22]
[23]
Binding energies observed for Cl, N and Si are in accordance with
earlier data which are helpful for the identification of the structure
23b,24
of the complex (Table 1).
Additional XPS spectrum and bind-
ing energies observed for O 1s (Fig. 1 in supporting information)
exclude the presence of palladium oxide.
FIR spectroscopy is very good tool for the identification of transi-
tion metal complexes, but the bands originating from palladium
bonded to microspheres are very weak. Nevertheless, FIR analysis
ꢀ
1
permitted us to identify bands with maxima at 383 and 341 cm
Figure 5. Pd 3d XPS spectra of MPd before and after five cycles
hydrogenation reaction.
ꢀ
1
as belonging to Pd–Cl and those at 285, 245 and 110 cm as orig-
6
b,25
inating from Pd–N.
This result indicates that the palladium is
bonded with chlorine and imidazole ring in a way similar to that
in the complex used in the homogeneous system (Fig. 6).
microsphere structure by the formation of bridges between
polysiloxane chains by imidazole palladium complexes.
It is interesting to note that the presence of palladium improves
the thermal stability of microspheres (Fig. 3). Although the decom-
position begins at a similar temperature to that of native micro-
spheres, the main decomposition, which occurs above 300°C,
leads to significantly smaller weight loss, about 40%. This increase
in the stability could be explained by the stiffening of the
Studies of hydrogenation reaction
Substrate conversion
The hydrogenation reaction of cinnamaldehyde was selected as the
model catalytic reaction for the testing of the activity and selectivity
Table 1. Binding energies (BE, in eV) and the shares (in %) of individual XPS peak components before and after hydrogenation reaction
Pd 3d5/2
BE (eV)
N 1s
BE (eV)
Cl 2p3/2
BE (eV)
Si 2p
% atom BE (eV) % atom
Si–O
C 1s
BE (eV)
C–Si; C–N;
% atom
% atom
% atom
δ+
Pd (II);
Pd–Cl;
Pd–N
Pd
;
–N¼
–NH– –N–Pd
Pd–Cl C–Cl
Pd (0)
C
arom
C¼N;
C–OH
MPd
338.04
0.58 397.93
0.53
0.4
3.1
4.0
1.7
2.4
197.97
1.4 102.11 11.1
0.6 102.72 5.6
284.70
284.70
38.3
12.7
3
35.62
399.50
200.44
286.09
286.01
4
00.95
MPd after 5
hydrogenation
reactions
338.15
0.1
399.46
198.08
0.26 102.55 11.0
0.13 103.16 5.5
38.58
12.8
336.6
0.31
0.58
401.13
199.68
335.19
Appl. Organometal. Chem. 2016, 30, 399–407
Copyright © 2016 John Wiley & Sons, Ltd.
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P. Pospiech et al.
in many cycles of the heterogenation reaction. Moreover, the cata-
lytic activity of the heterogenized palladium complex is at least
comparable to that in homogeneous systems in the presence of
2 2
PdCl (mim) (Table 2).
The rate of the substrate conversion in the first cycle is slightly
lower, as compared with those in subsequent cycles. In addition,
a small induction period is seen in the first run which disappears
in consecutive runs. These observations point to an increase in
the catalyst activity on coming to the second cycle. This increase
may be explained by the change of the palladium complex struc-
ture during the hydrogenation. The fraction of the catalyst in the
Pd(0) form increases during the reaction and it is more active in
the transformation of the substrate than the Pd(II) complex. In order
to confirm this conclusion, the structure of the palladium complex
after five cycles was investigated using XPS and XRD analyses.
The XPS spectra show that after the reaction the major fraction of
2 2
Figure 6. FIR spectra of MPd and PdCl (mim) catalysts.
of the obtained MPd heterogenized catalyst. The catalyst activity
was compared to that of PdCl (mim) complex operating in homo-
2
2
δ+
geneous systems. Measured cinnamaldehyde conversions and
ratios of the reaction products, after a fixed period of the reaction
performed under the same conditions, were compared. Five cycles
of the reaction were carried out with the catalyst supported on
palladium was as Pd(0) but some fractions of metastable Pd spe-
21a
cies (BE = 336.6 eV) also are observed. The content of Pd(II) signif-
icantly decreases but the total amount of Pd changes only slightly.
This means that the palladium appears in catalytically active Pd(0)
nanoclusters which are formed under the catalysed reaction condi-
tions (Table 1). It had been found previously that imidazole stabi-
microspheres. In all the experimental runs at 65°C and H
of 4 MPa the full conversion of cinnamaldehyde is achieved in
0–45 min (Fig. 7; Table 1 in supporting information). The substrate
conversion is quantitative and is held in five consecutive cycles
Table 2) which allows the assumption that MPd might be reused
2
pressure
23b
3
lizes Pd(0) nanoparticles making them catalytically very active.
The silicon signal in the XPS spectrum is not changed after the reac-
tion, but the fraction of Cl, as shown by XPS, is significantly
decreased which corresponds to the change of the palladium
surroundings.
(
The XRD diffractograms of MPd before and after the hydrogena-
tion reaction are shown in Fig. 8. In the spectrum of MPd, containing
mainly Pd(II), there is a broad signal close to 2θ = 20°, originating
from polysiloxane matrix. A peak seen at about 2θ = 39° is attributed
to crystalline phase corresponding to metallic palladium of [Pd(0)
(
111)]. The diffractogram of MPd recorded after five cycles of the
[26]
hydrogenation exhibits an enhancement of this signal.
The MPd catalyst is reduced by the reaction with sodium borohy-
dride to change the Pd(II) complex into pure Pd(0) catalyst form.
This reduced catalyst shows significantly higher activity which
allows its use at a lower concentration and to obtain higher catalytic
yield (Table 2).
2 2
We believe that the homogeneous complex PdCl (mim) also is
reduced to Pd(0) in the initial stage of the reaction and all the
hydrogenation reaction was catalysed almost exclusively by Pd(0)
Figure 7. Cinnamaldehyde hydrogenation runs with various complexes.
Table 2. Conversion of cinnamaldehyde after 2 h and selectivity in selected catalytic runs of the hydrogenation reaction
a
Catalyst
Cycle Complex
Time
(min)
Conversion
(%)
TON
Hydrocinnamaldehyde γ-Phenylpropanol Ethylcinnamyl ether
(g) (mmol of
(%)
(%)
(%)
Pd)
MPd
2
5
1
1
0.1392 (0.0747)
0.0713 (0.0383)
0.0396 (0.0212)
0.031 (0.0908)
30
98.9
100.0
100.0
87.0
152.5
228.1
229.6
451.8
516.5
517.3
667.8
922.5
934.7
163.3
217.2
218.8
38.6
37.1
34.6
73.0
69.8
68.3
77.2
73.2
72.3
83.9
77.8
75.9
60.0
61.5
62.5
27.0
29.2
30.0
22.8
26.8
27.7
16.0
20.3
19.9
0.0
0.5
1.4
0.0
0.4
0.7
0.0
0.0
0.0
0.0
1.0
2.2
60
1
1
1
1
20
30
60
99.5
20
30
99.7
MPd after reduction
71.4
80
98.7
20
30
100.0
74.7
2 2
PdCl (mim)
60
99.3
20
100.0
a
2
Reaction conditions: 65°C, 4 MPa of H , 2.5 ml of cinnamaldehyde and 2.5 ml of toluene in 50 ml of ethanol as solvent.
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Appl. Organometal. Chem. 2016, 30, 399–407

Polysiloxane microspheres as a palladium catalyst support
It is fully eliminated when the hydrogenation is performed using
the catalyst totally reduced to the Pd(0) form (Table 2). The
chemoselectivity of the reaction catalysed by MPd is changed with
consecutive cycles (Table 2). The yield of HCA gradually increases in
subsequent cycles from about 38% in the second cycle to about
69% in the fifth cycle and to 74% in the presence of the catalyst
reduced to the pure Pd(0) form (Table 2; and Table 2 in supporting
information). These yield variations are in a good accordance with
the change in the oxidation state of palladium. Since Pd(II) being
the dominant form in the first cycle undergoes gradual reduction
in consecutive cycles, it is concluded that it favours the formation
of the fully reduced product and is responsible for the formation
of the side product CEE. Using MPd catalyst in pure Pd(0) form
allows one to obtain a fairly high yield of HCA which is a particularly
useful chemical in the pharmaceutical industry.
Figure 8. XRD patterns of MPd catalyst: fresh and after recycling five times.
Mechanism of hydrogenation on Pd(0)
which is in accord with the conspicuous induction period of the re-
action (Fig. 7). Reduction of PdCl (mim) to Pd(0) is additionally con-
firmed by a dark precipitate which appears in the reaction mixture.
The hydrogenation reaction was carried out following both the sub-
strate conversion and the formation of products. Results are
displayed in Fig. 9. The most interesting feature of the kinetics of
the formation of products is that PP is formed already in the initial
period of the reaction in parallel with the formation of HCA (Fig. 9),
while CA does not appear at all. Such course of the reaction could
be expected only if the intermediate product to PP was CA very rap-
idly transformed into PP in a consecutive step. However, this is not
the case. Separate experiments, in which cinnamaldehyde and CA
2
2
Analysis of products
Hydrogenation of cinnamaldehyde is a convenient model for test-
ing palladium catalysts in homogeneous and heterogeneous
systems as it may give information about the competition between
the hydrogenation of olefin and carbonyl functions. This reaction
is well known to lead to the formation of three main products,
hydrocinnamaldehyde (HCA), cinnamyl alcohol (CA) and
γ-phenylpropanol (PP), depending on the catalyst and the reaction
conditions. Using palladium catalysts, HCA and PP are usually major
are simultaneously subjected to reaction with H , show that CA is
2
reduced relatively slowly at a rate comparable to that of
cinnamaldehyde (Fig. 10).
The observed kinetics may be explained assuming that HCA and
CA are not intermediates in the main stream of the reaction leading
to PP. Hydrogenations of olefinic and aldehyde functions in
cinnamaldehyde substrate mostly occur in one formal step.
Although it should be admitted that the kinetics do not exclude
the partial formation of PP via HCA and CA intermediates. The pro-
posed mechanism which is in agreement with all these observa-
tions is shown in Scheme 3. The key intermediate is Pd(IV)
complex III which may be transformed into Pd(IV) complex IIIa
which further undergoes reductive elimination giving HCA. The
other possibility is transformation of the key complex with hydro-
gen addition into Pd(IV) complex IV, which, after reductive elimina-
tion, produces PP.
[
27]
products.
70–80 mol%) product using palladium nanoparticles on
Galletti and co-workers obtained HCA as the main
(
2
7b
polyketone.
ligands immobilized on silica also led to HCA with selectivity above
0%. Zhao et al. demonstrated that a platinum catalyst gave CA
Binuclear palladium complexes with phosphine
2
7c
9
as major product, while using a palladium catalyst predominantly
2
7d
led to HCA.
Testing our palladium catalysts, we observe two major products
of the cinnamaldehyde reaction, HCA and PP, which are accompa-
nied by a small amount of cinnamylethyl ether (CEE). The latter is
formed as a result of a side reaction between ethanol and aldehyde
[
28]
(Scheme 2). This side product was also observed by Milone et al.
Scheme 2. Hydrogenation and synthesis of ether (1, cinnamaldehyde; 2,
CA; 3, HCA; 4, CEE; 5, PP).
Figure 9. Conversion of cinnamaldehyde and increase of products with
time for reduced MPd catalyst.
Appl. Organometal. Chem. 2016, 30, 399–407
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P. Pospiech et al.
cinnamaldehyde in one formal step. The mechanism of the main
stream of the reaction involves an intermediate complex which de-
composes either to HCA or to PP while HCA and CA are not
intermediates.
Acknowledgments
The research was supported by NSC (National Science Centre),
grant no. 2012/07/N/ST5/01980. The authors thank Przemyslaw
Sowinski from CMMS PAS for obtaining the TEM image.
Figure 10. Cinnamaldehyde and CA hydrogenation. (Reaction conditions:
5°C, reduced MPd catalyst (0.0054 mmol Pd), 4 MPa of H , 2.5 ml of
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