Platinum catalyst on polysiloxane microspheres with N-chelating groups

Article abstract of DOI:10.1016/j.molcata.2016.09.016

Elastic and durable, magnetic and non-magnetic polysiloxane microspheres containing a large number of SiOH groups were obtained by a simple and cheap emulsion process. N-chelating ligands were grafted on these microspheres by the condensation of their silanol groups with N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane. A platinum catalyst was immobilized on these microspheres. At every stage of the microspheres modification they were characterized by spectroscopy methods as well as by SEM microscopy. The catalyst appears only in the form of Pt(II) complex. The Pt(0) form was obtained by reducing this complex using sodium borohydride. The catalytic activity of the obtained catalysts was compared using model reactions, which were the hydrosilylation of phenylacetylene and hydrogenation of cinnamaldehyde. In both of these reactions the new platinum catalysts were recycled several times with the retention of their high catalytic activity. The hydrogenation of cinnamaldehyde leads to two products hydrocinnamaldehyde (HCA) and 3-phenyl-1-propanol (PP). The comparison of the rate of formation of the products with the rate of HCA and cinnamyl alcohol (CA) hydrogenation indicates that neither HCA nor CA are intermediate in the main pathway of the formation of PP.

Full text of DOI:10.1016/j.molcata.2016.09.016

Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Platinum catalyst on polysiloxane microspheres with N-chelating
groups
Piotr Pospiecha,∗, Julian Chojnowskia, Urszula Mizerskaa, Grzegorz Cempurab
a Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Lodz, Poland
b AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 June 2016
Received in revised form 29 August 2016
Accepted 14 September 2016
Available online 14 September 2016
Elastic and durable, magnetic and non-magnetic polysiloxane microspheres containing a large num-
ber of SiOH groups were obtained by a simple and cheap emulsion process. N-chelating ligands were
grafted on these microspheres by the condensation of their silanol groups with N-(2-aminoethyl)-3-
aminopropylmethyldimethoxysilane. A platinum catalyst was immobilized on these microspheres. At
every stage of the microspheres modification they were characterized by spectroscopy methods as well
as by SEM microscopy. The catalyst appears only in the form of Pt(II) complex. The Pt(0) form was obtained
by reducing this complex using sodium borohydride. The catalytic activity of the obtained catalysts was
compared using model reactions, which were the hydrosilylation of phenylacetylene and hydrogenation
of cinnamaldehyde. In both of these reactions the new platinum catalysts were recycled several times
with the retention of their high catalytic activity. The hydrogenation of cinnamaldehyde leads to two
products hydrocinnamaldehyde (HCA) and 3-phenyl-1-propanol (PP). The comparison of the rate of for-
mation of the products with the rate of HCA and cinnamyl alcohol (CA) hydrogenation indicates that
neither HCA nor CA are intermediate in the main pathway of the formation of PP.
Keywords:
Polysiloxane microspheres
Heterogeneous Pt catalysis
Cinnamaldehyde hydrogenation
Hydrosilylation,
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
There are, at least, two main problems in the use of platinum
catalysts, one is their allergic and carcinogenic action and the other
The transition metal catalysts such as complexes of palladium,
platinum, rhodium or ruthenium, play an important role in many
industrial processes. Among them, platinum catalysts have been
commonly used for years. In these catalysts platinum can exist in
an oxidation state 0, II and IV. All these forms are used in many
organic syntheses in both industrial processes and research labo-
ratories. Platinum is well-known as the catalyst of isomerization,
cyclization, dehalogenation, hydrogenation and continues to be
number one in hydrosilylation [1–3]. The last two reactions are
particularly important. Hydrosilylation is one of the basic reactions
in organosilicon chemistry making the synthesis of great number
of organosilicon compounds relevant to modern technology and
medicine possible [4,5]. Hydrogenation, in turn, is a very important
reaction in manufacturing oil, fats pharmaceuticals and petrochem-
ical products [6,7].
is their price [8,9]. That is why a lot of research has been done in
industrial and academic laboratories to improve the activity, selec-
tivity, recoverability and recyclability of platinum catalysts. One
way to achieve these goals is the immobilization of a homogenous
catalyst on solid support also known as heterogenization. In paral-
lel, scientists continue to explore a design of new ligands to obtain
more active and selective catalysts.
Polymers are often considered as catalyst carriers because they
allow to achieve the two aforementioned goals, being supports with
well-defined structure and containing functional groups used as
ligands [10–13]. Considering forms of polymer topology a cross-
linked network seems to be the best choice for a catalyst carrier
[14]. Bidentate nitrogen ligands are attractive for the application in
transition metal catalysts, such as a platinum complex, because of
a high affinity of nitrogen to these metals [15]. Polysiloxanes used
as support for these catalysts give many advantages. The flexibility
and mobility of polysiloxane chain allow to adjust its conformation
to environment thus giving reactants a better access to catalytic
centres. They are also easily modified by the introduction of func-
tional groups which can work as ligands for transition metals [8,16].
The polymer microspheres are a suitable form for catalyst carri-
ers because the shape of particles, their size and chemical structure
∗
Corresponding author at: Centre of Molecular and Macromolecular Studies,
Department of the Engineering of Polymer Materials, Sienkiewicza 112, 90-363 Lodz,
Poland.
E-mail address: piotrp@cbmm.lodz.pl (P. Pospiech).
http://dx.doi.org/10.1016/j.molcata.2016.09.016
1381-1169/© 2016 Elsevier B.V. All rights reserved.

P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
403
give them valuable hydrodynamic properties and the ease of isola-
tion from a reaction mixture. Among spherical particles used as
catalyst supports the most common are TiO2, Al2O3 and Fe3O4,
coated silica or modified silica alone [17–21] while among the poly-
mer particles the most often explored are polystyrene and chitosan
[22–24]. Many works were devoted to composite microspheres
whose construction allows to improve their thermal resistance
and give them magnetic or hydrophobic-hydrophilic properties.
Microspheres exclusively built from polysiloxane chains are a new
interesting material which may find many uses [25–27]. The ability
for an easy modification on their synthesis level e.g. forming mag-
netic particles, smooth introduction of functional groups and the
aforementioned properties of polysiloxanes provide an attractive
multi-purpose material which can be successfully used as catalyst
carriers.
60 ◦C for 4 min, then heated to 240 ◦C at a rate of 12 ◦C/min. Temper-
ature program for hydrosilylation: the column was kept at 60 ◦C for
2 min, then heated to 150 ◦C at a rate of 5 ◦C/min and then heated
to 280 ◦C at a rate of 10 ◦C/min.
2.3. Synthesis of cross-linked polysiloxane microspheres (M1)
The preparation of polysiloxane microspheres was precisely
described earlier [29]. Regular microspheres, of diameters rang-
ing from 3.5 to 30 ␮m (mean value 14.7 ␮m), were analyzed by
29Si and 13C MAS NMR, elemental analysis and SEM. They con-
tained 5.1 mmol/g SiOH. 29Si MAS NMR (␦ in ppm at maximum):
−67.3 MeSi(OSi)3; −57.4 MeSi(OH)(OSi)2; −37.8 MeSi(H)(OSi)2;
−21.0 MeSi(CH2)(OSi)2; +7.2 Me2Si(CH2)(OSi) and Me3SiO. 13C
MAS NMR (␦ in ppm at maximum): −3.5, −0.9, +0.6 CH3; +8.5 CH2.
Elemental analysis in wt%: C-24.03%, H-7.1.
We describe here the synthesis of magnetic or non-
magnetic polysiloxane microspheres, their modification
by
commercially
available
and
cheap
3-(2-amino-
2.4. Synthesis of magnetic cross-linked polysiloxane microspheres
(M2)
ethylamino)propylmethyldimethoxysilane and immobilization of
platinum on them. We demonstrated that the obtained Pt(0) and
Pt(II) complexes heterogenized on these microspheres are effi-
cient and reusable catalysts in hydrogenation and hydrosilylation
reactions.
The preparation of polysiloxane microspheres was carried out
according to the procedure similar to that described previously
[30]. The suspension of iron oxide nanoparticles in isopropanol was
added to the mixture before emulsification. The exact description
of the synthesis was placed in Supporting information. The micro-
spheres (6.4 g–82.8% yield) with diameters in the range of 4.2 to
52.5 ␮m (mean 18.9 ␮m) were acquired. They were analyzed by
29Si and 13C MAS NMR, IR, elemental analysis and SEM. They con-
tained 3.8 mmol/g SiOH. 29Si MAS NMR (␦ in ppm at maximum):
−67.8 MeSi(OSi)3; −58.5 MeSi(OH)(OSi)2; − 38.5 MeSi(H)(OSi)2;
−21.4 MeSi(CH2)(OSi)2; +5.9 Me2Si(CH2)(OSi) and Me3SiO. 13C
MAS NMR (␦ in ppm at maximum): −5.2, −3.6, −2.2, −1.0 CH3; +6.6
CH2, +23.5 C-CH3, +74.2 CH. Elemental analysis in wt%: C-23.64,
H-5.61
2. Experimental
2.1. Chemicals
Toluene, acetone, ethanol, polyvinyl alcohol (Mn 7.2 × 104)
and dioxane (POCh, analytical grade), cinnamaldehyde (Aldrich,
99%), cinnamyl alcohol (Aldrich, 98%), hydrocinnamylaldehyde
(TCI, >93%), dimethylphenylsilane (Fluorochem, 98%), phenylacety-
lene (ABCR, 98%), potassium tetrachloroplatinate(II) (ABCR, 99.9%),
N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (ABCR,
97%), undecane (Aldrich, >99%) were used as received. Iron (III)
chloride, Iron(II) chloride, hydrochloric acid (solution 38%) and
ammonium hydroxide (solution 25%) (Chempur, analytical grade)
were used as received. Polyhydromethylsiloxane (PHMS) was pur-
chased in ABCR under name HMS 991, Mn 4.7 × 103 g/mol as
confirmed by SEC. 1,3-Divinyltetramethyldisiloxane (ABCR, 97%).
Pt(0) Karstedt catalyst containing 20 w/w% Pt was kindly offered by
Momentive Performance Materials GmbH Leverkusen. Magnetite
nanoparticles were obtained by coprecipitation of Fe2+/Fe3+ in an
alkaline solution using known method [28].
2.5. Preparation of 3-(2-aminoethyl)aminopropyl functionalized
microspheres (M1N, M2N)
The
groups
microspheres
were obtained
with
by
pendant
condensation
aminoethylamino
reaction of
silanol groups in microspheres with N-(2-aminoethyl)-3-
aminopropylmethyldimethoxysilane. The reaction was carried out
using the procedure similar to that described in Ref. [30]. Micro-
spheres (M1-8.12 g, M2-1.54 g) were suspended in the toluene
(20 and 10 mL respectively) and the methoxysilane (10 and 2 mL
respectively) was slowly added by syringe to the stirred mixture
and the stirring was continued for 48 h. The microspheres were
separated and repeatedly washed with toluene, dried in vacuum
and analyzed. The [3-(2-aminoethyl)aminopropyl]siloxane groups
replaced almost completely the initial hydroxyl groups, as shown
by 29Si MAS NMR.
2.2. Analytical methods
29Si and 13C MAS NMR spectroscopy, Scanning Electron
Microscopy, FIR, XPS spectroscopy and Elemental Analysis were
precisely described earlier [29]
2.2.1. Transmission electron microscopy
Transmission electron microscopy (TEM) and scanning-
transmission electron microscopy (STEM) analyses were conducted
using thin foils utilizing FEI Tecnai G2 20 TWIN microscope
equipped with EDS of EDAX. In order to make TEM images of the
crosssection of microspheres they were embedded in Araldite 502
(epoxide resin) and cut with an ultramicrotome (Power Tome XL,
Boeckeler Instruments, Inc.).
2.5.1. M1N microspheres
29Si MAS NMR (␦ in ppm at maximum): −69.2 MeSi(OSi)3; −60.9
MeSi(OH)(OSi)2; −23.8 MeSi(CH2)(OSi)2; +5.6 Me2Si(CH2)(OSi)
and Me3SiO. 13C MAS NMR (␦ in ppm at maximum): −4.4, −2.8
CH3; +7.1, +13.1Si-CH2, +21.0C-CH2-C, 39.6H2N-CH2, 50.7NH-CH2,
OCH3.
2.5.2. M2N microspheres
2.2.2. Gas chromatography
29Si MAS NMR (␦ in ppm at maximum): −69.5 MeSi(OSi)3; −60.7
MeSi(OH)(OSi)2; −23.9 MeSi(CH2)(OSi)2; +5.2 Me2Si(CH2)(OSi)
and Me3SiO. 13C MAS NMR (␦ in ppm at maximum): −5.2, −3.7 CH3;
+6.1, +11.9 Si-CH2, +20.7 C-CH2-C, 38.3 H2N-CH2, 49.6 NH-CH2,
OCH3,
Gas chromatographic analysis was performed on a Hewlett
Packard 5890 II apparatus (TCD detector) equipped with a HP-
50+ column (30 m × 0.53 mm × 1 ␮m) using helium as a carrier gas.
Temperature program for hydrogenation: the column was kept at

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P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
Fig. 1. 29Si MAS NMR spectra of microspheres.
Scheme 1. Modification of microspheres by N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane and immobilization of platinum.
complexes were characterized by 29Si and 13C MAS-NMR spec-
troscopy, FIR, SEM, XRF and XPS.
Elemental analysis M1N (in wt.%): C 32.95, H 8.08, N 7.19; M2N
(in wt.%): C 31.00, H 7.83, N 4.48.
2.6.1. M1N Pt
29Si MAS NMR (␦ in ppm at maximum): −69.2 MeSi(OSi)3; −59.9
MeSi(OH)(OSi)2; −23.4 MeSi(CH2)(OSi)2; +5.8 Me2Si(CH2)(OSi)
and Me3SiO. 13C MAS NMR (␦ in ppm at maximum): −3.3, CH3; +6.9,
+12.4 Si-CH2, +19.9 C-CH2-C, +40.2 H2N-CH2, +44.1 OCH3, +50.6
NH-CH2; FIR (in cm−1 at maximum): 350 Pt-Cl; 267, 243, 208 Pt-N;
Elemental analysis (in wt.%): C 20.53, H 6.2, N 4.67.
2.6. Immobilization of platinum onto microspheres
The M1N and M2N microspheres were dispersed in diox-
ane/phosphate buffer mixture and solution of platinum salt K2PtCl4
(0.103 g) in phosphate buffer (pH-7.2) (7 mL), was added drop-
wise. The proportion of mol(Pt)/mol(ligands) has been used as 1/6.
The suspension was shaken for tens hours at room temperature.
The obtained platinum complexes immobilized on microspheres
(M1N Pt and M2N Pt) were isolated from the solution by filtra-
tion, washed successively with several solvents (water, acetone,
toluene) and then dried under vacuum. The complexes M1N Pt and
M2N Pt were light grey and brown in colour respectively. These
2.6.2. M2N Pt
29Si MAS NMR (␦ in ppm at maximum): −67.0 MeSi(OSi)3; −58.8
MeSi(OH)(OSi)2; −21.5 MeSi(CH2)(OSi)2; +7.9 Me2Si(CH2)(OSi)
and Me3SiO. 13C MAS NMR (␦ in ppm at maximum): −2.3, CH3;
+8.8, +14.6 Si-CH2, +22.1, +28.5 C-CH2-C, +42.6 H2N-CH2, +50.8 NH-

P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
405
Fig. 4. SEM photograms.
2.7. Reduction of Pt(II) complex (M1N Ptr)
Fig. 2. HAADF-STEM image of the investigated area and EDX spectra from the area
of the particles.
M1N Pt catalyst was reduced to convert Pt(II) complex into
Pt(0) similar to the procedure described in Ref. [31]. M1N Pt (0.1 g)
was dispersed in ethanol (1 mL), then mixed with NaBH4 (0.1 g) in
deionized water (7 mL) and shaken at room temperature for two
days. The resulting product was filtered, washed with water and
EtOH respectively and dried under vacuum to obtain the dark grey
platinum(0) complex.
CH2, +76.6 CH2, FIR (in cm−1 at maximum): 339 Pt-Cl; 259, 240, 208
Pt-N. Elemental analysis (in wt.%): C 24.18, H 6.78, N 3.42.
Fig. 3. 13C MAS NMR spectra of microspheres.

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P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
Scheme 2. Hydrosilylation reaction.
2.8. Studies of hydrosilylation
Phenylacetylene (0.16 mL, 1.5 mmol), dimethylphenylsilane
(0.23 mL, 1.5 mmol), toluene (0.5 mL) and undecane (0.07 mL) were
placed in a screw cap vial and then Pt complex containing ca.
0.02 mmol Pt was added. The reaction mixture was heated to 100 ◦C
with a mild shaking. Samples were withdrawn at certain time
intervals and analyzed by gas chromatography using undecane as
the internal standard. The conversion was determined based on
the amount of consumed phenylacetylene. After the reaction the
mixture was cooled down and the catalyst was separated by sedi-
mentation, washed by acetone, dried and used for the next reaction
cycle. The products were isolated by distillation under reduced
pressure and characterized by 1H, 13C and 29Si NMR.
2.8.1. Dimethylphenylstyrylsilane
1H (500 MHz, CDCl3): ı 0.69 (s, −CH3), 6.85, 6.88 (d, CH-Si),
7.21, 7.25 (d, CH-Ph), 7.48-7.85 (m, phenyl); 13C (500 MHz, CDCl3,
␦ in ppm at maximum): −2.46 (-CH3), 126.38, 126.57, 127.93,
128.38, 128.61 (Carom), 133.84 ( CH-Si) 133.94 (Si-Carom),138.13
(Si-Carom), 138.45 (Carom C), 145.39 ( CH-Ph); 29Si (500 MHz,
CDCl3, ␦ in ppm at maximum): −10.0
2.8.2. Dimethylphenyl(1-phenylvinyl)silane
1H (500 MHz, CDCl3): ı 0.62 (s, CH3), 5.88 (s, CH2), 6.20 (s,
CH2), 7.34-7.77 (m, phenyl); 13C (500 MHz, CDCl3, ␦ in ppm at
maximum): −2.08 (-CH3), 126.68, 127.10, 128.09, 128.32 (Carom),
129.04 ( CH2), 134.14 (Si-Carom), 138.44 (Si-Carom), 144.33 (C-
Fig. 5. XPS spectra N 1s of M1N Pt before and after 5 cycles of the hydrosilylation
reaction and for M2N Pt before the reaction.
C
arom), 151.17 ( C(Si)-C); 29Si (500 MHz, CDCl3, ␦ in ppm at
maximum): −7.9
2.8.3. Bis-(1,2-dimethylphenylsilyl)ethylbenzene
1H (500 MHz, CDCl3): ı 0.29 (s, CH3), 0.41 (s, −CH-), 0.51 (s,
−CH2-), 6.87–7.74 (m, phenyl); 13C (500 MHz, CDCl3, ␦ in ppm at
maximum): 0.38 (-CH3), 125.87, 126.70, 127.80, 128.92 (Carom),
134.05, 134.49 (Si-Carom), 139.16, 139.85 (Si-Carom), 149.08 (C-
2.9. Studies of cinnamaldehyde hydrogenation
The hydrogenation reactions were carried out in Autoclave Engi-
neers B030SS using the procedure similar to that described in Ref.
[29]. The exact description of the synthesis was placed in Support-
ing information.
C
arom); 29Si (500 MHz, CDCl3, ␦ in ppm at maximum): −10.5
Ph(CH3)2Si-CHPh-, 14.6 Ph(CH3)2Si-CH2-

P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
407
Table 1
Conversion of dimethylphenylsilane and selectivity in catalytic runs in the hydrosilylation reaction.
catalyst
Cycle
Complex [g]
(mol of Pt)
Conversion
[%]
TON
Dimethylphenyl
(1-phenylvinyl)silane
[%]
trans dimethylphenyl-
styrylsilane
[%]
consecutive
addition [%]
M1N Pt
1
3
5
1
0.0501 (2.12 × 10−5
)
)
39.8a
94.6
99.4
85.0b
28
66
69
58.0
35.1
33.8
30.4
29.3
64.9
66.2
64.3
64.8
0.0
0.0
0.0
5.9
M1N Pt after reduction
0.05
(2.14 × 10−5
)
3
5
1
3
5
7
10
1
1
97.1
66.2
68.2
16.2
65.0
67.9
67.9
67.9
67.9
787.4
31.4
28.8
33.4
33.7
30.6
28.6
29.3
51.5
27.6
68.6
64.6
66.6
66.3
63.7
65.1
64.0
46.9
66.4
3.5
6.6
0.0
0.0
5.7
6.3
6.8
1.6
6.0
100.0
23.9a
95.8
100.0
100.0
100.0
100.0a
97.2c
M2N Pt
0.0507 (2.15 × 10−5
K2PtCl2
Karstedt
0.008 (1.93 × 10−5
0.0017 (1.8 × 10−6
)
)
a
After 22 h.
After 360 min.
After 5 min.
b
c
3. Results and discussion
3.1. Synthesis of magnetic microspheres
3.3. Structural studies of platinum immobilized on microspheres
Platinum complex immobilized on the microspheres using
bidentate nitrogen ligands was very conveniently synthesized by
reacting the previously modified microspheres (M1N, M2N) with
potassium chloroplatinate (Scheme 1). The obtained complexes
were characterized by standard analytical techniques.
We developed a method of synthesis of all-polysiloxane micro-
spheres having magnetic properties and containing an easy to
modification SiOH groups. The method is based on cross-linking
of polyhydromethylsiloxane in aqueous emulsion [30]. Magnetic
properties of these microspheres were gained by the introduc-
tion of iron oxide nanoparticles produced according to a published
method [28]. They were added to the PHMS/DVTMDS mixture
before its emulsification using the previously described procedure
[30]. We observed that the incorporation of magnetite into micro-
spheres slightly influenced the SiH/SiOH ratio. The analysis of the
29Si MAS NMR spectrum (Fig. 1) indicated that in the standard
method of the preparation of non-magnetic microspheres a large
number of SiOH was produced (5.1 mmol/g) whereas the addition
of Fe3O4 caused a moderate decrease of SiOH (3.8 mmol/g).
The TEM investigations, of the microsphere cross-section
showed that the iron in the microspheres appeared in the form
of particle clusters (Fig. 2). The SEM photograms indicated that the
shape and surface structure of the microspheres were not changed
after the introduction of magnetite (Fig. 4).
The 29Si MAS NMR spectrum was not changed after the
immobilization of platinum but widening of carbon signals neigh-
bouring with N atoms was observed in 13C MAS NMR spectrum
(Figs. 1 and 3), which was an effect of platinum bonding to the
nitrogen ligand. SEM photographs (Fig. 4) gave us information that
the surface of microspheres was not changed after the modifica-
tion and immobilization of platinum. We suppose that platinum is
distributed in the whole microsphere volume.
The detailed information about M1N Pt and M2N Pt complex
structure was provided by XPS and FIR analysis. In both cases
the analysis of the XPS spectra (Table A in supp. inf.) confirmed
that platinum occurred only in the form of Pt(II). Binding ener-
gies observed for Cl, C, O and Si were in accordance with earlier
data [16,32] (Table A supp. inf.) In both complexes binding ener-
+
gies observed for N pointed to the presence of the NH4 structure
or hydrogen bonds [33,34] (Fig. 5). The analysis O1s XPS (signal
BE 533.5, Table A, Fig. A in supp. inf.) indicates the presence of
water which suggests the existence of hydrogen bonds. It is also
confirmed by a simple simulation DFT (Scheme A supp. inf.), which
proves that the structure in which platinum is coordinated to nitro-
gen by a hydrogen bonded molecule of water is more stable.
A far IR spectroscopy confirmed existence of Pt-N and Pt-Cl
bonds. The characteristic bands with maxima around 330 cm−1
belonged to Pt-Cl and the bands at 208, 235, 260 cm−1 originated
from Pt-N [35–37] (Fig. C supp. inf.).
3.2. Modification of microspheres
The presence of reactive silanol groups on the microspheres
gives a broad possibility of the functionalization of these particles
by organofunctional chloro- or alkoxysilanes. The microspheres
with N-chelating ligands were obtained by the condensation
of silanol groups on the spheres with N-(2-aminoethyl)-3-
aminopropylmethyldimethoxysilane (Scheme 1). The reaction was
carried out in the suspension of microspheres in toluene with an
excess of the silane reagent.
3.4. Studies of the hydrosilylation reaction
The analysis of the 29Si MAS NMR spectrum indicated that most
of hydroxyl functions on polysiloxane were consumed. During the
reaction we observed a disappearance of remaining SiH groups by
their dehydrogenative condensation with the silanol groups (Fig. 1).
The structure of the obtained product was also confirmed by 13C
MAS NMR spectrum (Fig. 3). The amount of the ligands bonded to
microspheres, determined by elemental analysis, was 2.56 mmol
per gram for native microspheres and 1.74 mmol per gram for their
magnetic form.
The hydrosilylation is the most important reaction in silicone
industry and the platinum catalysts are most commonly used.
Hence the choice of model reaction was obvious. Hydrosilylation
of dimethylphenylsilane with phenylacetylene can give a whole
range of products and therefore may be used for the examining of
the catalyst selectivity (Scheme 2).
Activity and selectivity of the reaction was determined by the GC
analysis of samples withdrawn from the reaction system. After the
reaction, the catalyst was separated from the mixture, washed with

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P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
for a several hours also increases the activity of the hydrosilylation
reaction (Fig. B supp. inf.), as this operation removes water from
the complex. However, after the formation of the final structure of
complex the activity remains constant (Fig. 6, Table 1 and Table B in
supp. inf.). The SEM-EDX analysis determined a loss of platinum for
subsequent cycles, and showed a gradual loss of platinum during
subsequent cycles of the reaction (∼20% after 5 cycles). The great-
est loss was observed in the case of the catalyst M2N Ptr (>40%
after five cycles). The catalytic activity of the heterogenized plat-
inum complex was comparable to that of K2PtCl4. On the other hand
the homogeneous Karstedt catalyst was much more active than the
others.
The use of phenylacetylene as the reactant in the hydrosily-
lation provides information about the selectivity of the adopted
platinum complex. In this reaction at least four main products
may be formed: dimethylphenylstyrylsilane (cis and trans form),
dimethylphenyl(1-phenylvinyl)silane and a product of the consec-
utive addition to olefin bond. The ratio of trans dimethylphenyl-
styrylsilane to dimethylphenyl(1-phenylvinyl)silane was about
70:30 for all the three used catalysts. In the cases of M2N Ptr
catalyst we also observed the formation of the consecutive addi-
tion product (Scheme 2, Table 1) which is in accordance with the
results obtained for Karstedt catalyst. We also observed the consec-
utive addition product using M2N Pt catalyst but only in the last
two cycles, which may indicate a gradual reduction of platinum
in subsequent cycles. The selectivity of the studied heterogenized
catalysts is different from that observed for K2PtCl4 and Karstedt
catalysts where the ratio of the obtained ␣ and ␤-(Z) products of
the addition is about 50:50.
3.5. Studies of the hydrogenation reaction
3.5.1. Substrate conversion
The commonly used hydrogenation reaction of cinnamaldehyde
was chosen as the second model catalytic reaction for the testing
of activity and selectivity of the platinum heterogenized on micro-
spheres (Scheme 3). The catalyst activity was compared to that of
K2PtCl4 and Karstedt catalyst. The reaction was performed under
the fixed conditions and the conversion and ratio of the products
was compared in five cycles of the reaction. In the case of M1N Pt
catalyst the full conversion of cinnamaldehyde in the experimen-
tal runs was achieved in 60–90 min (Fig. 7). The rate of substrate
conversion in the subsequent cycles slowly decreased which could
be caused by gradual leaching of the platinum from the complex.
Moreover, the catalytic activity of heterogenized platinum com-
plex was comparable to those operating in the presence of K2PtCl4
or Karstedt catalyst, see Table 2.
The opposite effect appeared when M2N Pt catalyst was used.
In any of the five cycles of reaction after 6 h the full conversion
of cinnamaldehyde was not achieved. In all the five cycles after a
120 min conversion was only at the level of about 25–35%, although
the rate of reaction gradually increased in subsequent cycles.
Fig. 6. Hydrosilylation runs.
acetone, dried and reused. Catalytic activities of M1N Pt, M2N Pt
and M1N Pt after reduction of platinum to Pt(0) (M1N PtR) were
compared to those of K2PtCl4 and Karstedt complexes.
3.5.2. Analysis of products
Hydrogenation of cinnamaldehyde is a commonly used model
for testing various catalysts, because it provides information about
the regioselectivity in the hydrogenation of olefinic and carbonyl
functional groups. In this reaction three main products may be
formed: hydrocinnamaldehyde (HCA), cinnamyl alcohol (CA), and
␥-phenylpropanol (PP). Yields of these products depend on the type
of catalyst, catalyst support, co-catalyst and reaction conditions
[38–41].
Using our platinum catalysts in the cinnamaldehyde hydrogena-
tion reaction we observed two major products HCA and PP. The
chemoselectivity of reactions was not changed with consecutive
cycles (Table 2). The ratio of the HCA:PP was as 60:40 for the M1N Pt
The activity of M1N Pt and M2N Pt complexes significantly
increased in consecutive cycles (Fig. 6, Table 1). Instead, M1N PtR
was active at a high level in all cycles (Fig. 6). These changes of
activities were not related to the oxidation state of platinum which
was confirmed by Pt XPS analysis of the catalysts before and after
five cycles of the reaction (Table A supp. inf.). The reason for this
strange behaviour was probably the formation of the hydrogen
bonds between water and nitrogen as mentioned above (see also
supp. inf., Scheme A). These structures can be created during the
immobilization process and are less active. Additionally, anneal-
ing of the catalyst at a temperature above 100 ◦C under vacuum

P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
409
Scheme 3. hydrogenation reaction.
Fig. 7. Cinnamaldehyde hydrogenation runs.
Table 2
Conversion of cinnamaldehyde and selectivity in catalytic runs in the hydrogenation reaction.
catalyst
Cycle
Complex [g]
(mmol of Pt)
Conversion
[%]
TON
Hydrocinnamaldehyde cinnamyl alcohol
␥-Phenylpropanol
[%]
[%]
[%]
M1N Pt
1
2
3
4
5
1
2
3
4
5
1
1
0.0575 (0.0279)
100.0*
100.0
100.0
98.7
710.6
57.9
55.6
58.00
59.4
61.9
89.0
77.1
77.0
77.7
78.0
52.2
35.28
0.00
0.00
0.00
0.00
0.00
0.00
1.9
2.1
1.8
2.0
13.7
38.46
42.1
44.4
42.00
40.6
38.1
11.0
21.0
20.9
20.5
20.0
34.1
26.26
0.0466 (0.0226)
0.0992 (0.0421)
99.6
876.8
354.3
M2N Pt
30.3**
45.2
51.4
80.6
0.089 (0.0377)
70.3
823.0
678.0
1085.6
K2PtCl2
Karstedt catalyst
0.0103 (0.0248)
0.0331 (0.017)
84.7**
92.8*
*
After 120 min.
After 360 min.
**
catalyst and 77:21 for the M2N Pt catalyst. In the case of M2N Pt
catalyst the formation of a small amount of cinnamyl alcohol (c.a.
2%) was observed after 6 h. The reaction selectivity using K2PtCl4
or Karstedt catalyst was significantly different from that using plat-
inum on microspheres.
The formation of products was followed together with the con-
version of cinamaldehyde. PP was expected to arise as a consecutive
product of the transformation of HCA and/or CA (Scheme 3). How-
ever, the kinetic runs displayed in Fig. 8 clearly showed that PP and
HCA are formed in parallel. A very rapid transformation of CA into
PP was excluded (see Fig. D in sup. inf.). Therefore the most of PP

410
P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
Scheme 4. The proposed mechanism of cinnamaldehyde hydrogenation.
with the SiOH groups. The nitrogen chelating groups pendant to
polysiloxanes chain on these microspheres may serve as ligands
for the effective formation of platinum complexes. This new
Platinum(II) catalyst, immobilized on the polysiloxane micro-
spheres, shows a high catalytic activity in hydrosilylation and
hydrogenation reactions. This complex is stable under the reaction
conditions and may be reused. The Pt(0) form of this catalyst,
which may be generated by reducing the Pt(II) complex with
sodium borohydride, is also a stable and effective catalyst.
The Pt(II) and Pt(0) catalysts showed excellent conversion and
recyclability in the hydrosilylation of dimethylphenylsilane and
Pt(II) in the hydrogenation of cinnamaldehyde. The selectivity
towards the formation of ␤(Z) substituted compound is achieved in
the hydrosilylation process. A fairly high yield of hydrocinnamalde-
hyde may be obtained in the hydrogenation of cinnamaldehyde.
The complex with Pt(0) form was more active but less selective
than platinum (II). The influence of the presence of iron nanoparti-
cles is observed. The magnetic complex is slightly less active in the
hydrogenation process than the platinum on microspheres without
iron.
Fig. 8. Conversion of cinnamaldehyde and increase of products in time for M1N Pt
catalyst.
is formed directly from cinnamaldehyde without the intermediacy
of CA and HCA according to the mechanistic pathway analogous to
that found for the hydrogenation of cinnamaldehyde catalysed by
palladium [29]. The key intermediate in this mechanism is complex
III (Scheme 4), which may be transformed to PP or HCA.
Acknowledgement
The research was supported by NSC (National Science Centre),
Grant no. 2012/07/N/ST5/01980.
4. Conclusions
Appendix A. Supplementary data
Magnetic and non-magnetic polysiloxanes microspheres
obtained by the emulsion process of PHMS and containing a large
number of SiOH groups may be efficiently functionalized by reac-
tions of N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.09.
016.

P. Pospiech et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 402–411
[22] H. Zhao, J. Xu, T. Wang, Appl. Catal. A 502 (2015) 188–194.
411
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