Preparation, characterization and evaluation of a series of heterogeneous platinum catalysts immobilized on magnetic silica with different acid ligands

Article abstract of DOI:10.1007/s11243-019-00348-w

Platinum was immobilized on magnetic silica gel by means of boronic, nitric, carboxylic or sulfuric acid ligands to give four heterogeneous Pt nano-catalysts, designated as Fe₃O₄@SiO₂-BA@Pt, Fe₃O₄@SiO₂-NA@Pt, Fe₃O₄@SiO₂-CA@Pt and Fe₃O₄@SiO₂-SA@Pt, respectively. Particles of these mono-dispersible Pt catalysts were 10–20?nm in size and could be separated for recycling by means of a magnet. Fe₃O₄@SiO₂-BA@Pt (0.174?mmol/g Pt) showed the best catalytic activity and selectivity, which were better than Speier’s catalyst. Its turnover numbers were up to 1.7 × 10⁶ and 1.1 × 10⁶ for hydrosilylation of 1-hexene or styrene, respectively. This material could also catalyze the hydrosilylation of a broad range of alkenes and alkynes as substrates and methyldichlorosilane or triethoxysilane as silanes. Similar yields of 1-hexyl-methyldichlorosilane at the first and eighth runs (96.5% and 95.2%, respectively), together with a final Pt content of 0.171?mmol/g indicated the outstanding stability of Fe₃O₄@SiO₂-BA@Pt under the catalytic reaction conditions.

Full text of DOI:10.1007/s11243-019-00348-w

Transition Metal Chemistry
https://doi.org/10.1007/s11243-019-00348-w
Preparation, characterization and evaluation of a series
of heterogeneous platinum catalysts immobilized on magnetic silica
with diferent acid ligands
Laiming Li1 · Youxin Li1,2 · Aschenaki Assefa1 · James J. Bao1
Received: 8 May 2019 / Accepted: 31 July 2019
© Springer Nature Switzerland AG 2019
Abstract
Platinum was immobilized on magnetic silica gel by means of boronic, nitric, carboxylic or sulfuric acid ligands to give
four heterogeneous Pt nano-catalysts, designated as Fe3O4@SiO2-BA@Pt, Fe3O4@SiO2-NA@Pt, Fe3O4@SiO2-CA@Pt
and Fe3O4@SiO2-SA@Pt, respectively. Particles of these mono-dispersible Pt catalysts were 10–20 nm in size and could be
separated for recycling by means of a magnet. Fe3O4@SiO2-BA@Pt (0.174 mmol/g Pt) showed the best catalytic activity and
selectivity, which were better than Speier’s catalyst. Its turnover numbers were up to 1.7×106 and 1.1×106 for hydrosilylation
of 1-hexene or styrene, respectively. This material could also catalyze the hydrosilylation of a broad range of alkenes and
alkynes as substrates and methyldichlorosilane or triethoxysilane as silanes. Similar yields of 1-hexyl-methyldichlorosilane
at the frst and eighth runs (96.5% and 95.2%, respectively), together with a fnal Pt content of 0.171 mmol/g indicated the
outstanding stability of Fe3O4@SiO2-BA@Pt under the catalytic reaction conditions.
Introduction
Thus, magnetic catalysts have much potential in industrial
homogeneous catalysis [8]. Magnetic nanoparticles, espe-
cially Fe3O4 encapsulated with silica (Fe3O4@SiO2), are
emerging as interesting supports for the immobilization of
metal catalysts [9, 10]. In contrast to many traditional sup-
ports, Fe3O4@SiO2 nanoparticles allow for the distribution
of active sites throughout the outer surface, which is benef-
cial for high efciency in catalytic reactions [11].
Heterogeneous catalysts have several well-known advan-
tages over homogeneous catalysis, including good stability,
easy recovery and recycling, and low manufacturing costs
[1, 2]. However, their applications in industry remain lim-
ited by issues of catalytic efciency, recycling times and
commercial supply [3]. Hence, much efort in catalysis
research is directed toward obtaining practical and efcient
heterogeneous catalysts [4]. In this context, magnetic nano-
particles have attracted much attention for the immobiliza-
tion of metal catalysts due to their high stability and useful
magnetic properties [5–7]. Magnetic heterogeneous catalysts
can often be easily recovered from solution using a magnet,
avoiding time- and solvent-consuming recovery procedures.
Metallic platinum has been widely used as a catalyst in
the industrial and laboratory-scale oxidation of toluene,
hydrosilylation of alkenes and alkynes, hydrogenolysis of
glycerol, decomposition of formaldehyde, sulfuric acid,
etc. [12–17]. For example, Karstedt’s catalyst and Speier’s
catalyst can both catalyze hydrosilylation [18]. However,
in many cases, the amounts of Pt are low and losses of Pt
from the nanoparticles are apparent during recycling, which
adversely afect catalytic activity [19, 20]. Thus, there is a
requirement for novel and efcient heterogeneous catalysts
which can obtain comparable activities and selectivities
to homogeneous Pt catalysts [21]. Several approaches for
the preparation of Pt catalysts on a magnetically retriev-
able phase have been reported, including a Pt-based mag-
netically separable catalyst (Pt/SiO2/Fe3O4) modifed with
cinchonidine [22], Fe3O4/SiO2-Pt/Au/Pd magnetic nano-
catalysts with multifunctional hyperbranched polyglycerol
amplifers [23] and platinum impregnated on magnetite [13].
* Youxin Li
lyx@tju.edu.cn
1
Tianjin Key Laboratory for Modern Drug Delivery
and High-Efciency, Collaborative Innovation
Center of Chemical Science and Engineering, School
of Pharmaceutical Science and Technology, Tianjin
University, Tianjin 300037, China
2
School of Pharmaceutical Science and Technology, Tianjin
University, Room 412-8, Building No. 24, 92 Weijin Road,
Nankai District, Tianjin 300072, China
Vol.:(0123456789)
1 3

Transition Metal Chemistry
Nevertheless, a number of drawbacks remain, including high
manufacturing costs, poor stability and aggregation to plati-
num clusters resulting in undesired side reactions, such as
isomerization and hydrogenation [24–26].
by Yongyuan Distilled Water Manufacturing Centre (Tian-
jin, China). All other reagents were obtained from Tianjin
Jiangtian Chemical Technology Co. (Tianjin, China).
In this paper, we report on our eforts to prepare four
magnetic heterogeneous Pt catalysts based on magnetic
silica nanoparticles with boronic, nitric, sulfuric or car-
boxylic acid groups, designated as Fe3O4@SiO2-BA@Pt,
Fe3O4@SiO2-NA@Pt, Fe3O4@SiO2-SA@Pt and Fe3O4@
SiO2-CA@Pt, respectively. To the best of our knowledge,
there are no previous reports of magnetic heterogeneous Pt
catalysts immobilized through these acid groups, except for
carboxylic acids as reported by our group [27, 28]. After
characterizing the four Pt catalysts, their catalytic stabilities
and activities were investigated. The Pt catalyst immobilized
through boronic acid proved to be most promising and was
further used to catalyze hydrosilylation of a range of alkenes
and alkynes as substrates, and methyldichlorosilane or tri-
ethoxysilane as silanes.
Instrumentation
Pt contents of these materials were assayed with a HITACHI
180-80 polarized Zeeman atomic absorption spectrophotom-
eter (AAS) (HITACHI, Ltd., Japan). Particle morphology
was investigated using a Hitachi S2800 scanning electron
microscope (SEM) and a JEM-2100HR (JEOL, Japan)
transmission electron microscope (TEM) operating at an
accelerating voltage of 200 kV. X-ray difraction analysis
(XRD) results were obtained on a D/MAX-2500 X-ray dif-
fractometer (Rigaku Corporation, Japan), using Cu Kα radia-
tion, operated at 40 kV, 30 mA and room temperature. X-ray
photoelectron spectroscopy (XPS) data were obtained under
ultrahigh vacuum (1 × 10−8 Pa) on an UIVAC-PHI 1600
ESCA spectrometer with an Al anode (Al Kα=1486.6 eV).
Ultraviolet–visible (UV–Vis) absorption spectra were
obtained with a U-3900 ultraviolet–visible spectrophotom-
eter (Hitachi, Japan). Gas chromatography was carried out
with a GC2020 chromatograph equipped with a 30 m cap-
illary column with a stationary phase of 5% phenyl–95%
methyl polysiloxane, under the following conditions: initial
temperature 60 °C for 3 min; ramp 10 °C min−1; fnal tem-
perature 260 °C for 5 min; injector and detector temperature
260 °C; a sample of 0.8 μL was injected and the split ratio
was 1/30. The magnetic behaviors of these materials were
assayed with a vibrating sample magnetometer (VSM) LDJ-
9600 (USA).
Experimental section
Reagents and chemicals
Unless otherwise stated, all chemicals were of analytical
grade or better. FeSO4.7H2O (>99%) was purchased from
Tianjin Jiezheng Chemical Trade Co. (Tianjin, China).
Fe2(SO4)3 (>99%) was obtained from Tianjin Yuanli Chemi-
cal Co. (Tianjin, China). Ammonium hydroxide (28%, w/w),
sodium silicate, hydrochloric acid and methanol were pur-
chased from Tianjin Kemiou Chemical Reagent Co. (Tian-
jin, China). 3-Glycidyloxypropyltrimethoxysilane (GPTMS)
was obtained from Shanghai Macklin Biochemical. Co.
(Shanghai, China). 3-Nitroaniline, 3-aminophenylboronic
acid monohydrate, 3-aminophenylsufonic acid and 4-amin-
benzoic acid were purchased from Shanghai Bid pharmatech
Ltd. (Shanghai, China). Chloroplatinic acid (H2PtCl6·6H2O)
was purchased from Tianjin Fengchuan Chemical Reagent
Technologies Co. (Tianjin, China). Triethoxysilane and
methyldichlorosilane were obtained from Zhejiang Kaihua
Synthetic Co. (Zhejiang, China). 1-Hexene was obtained
from Shanghai Aladdin Biological Technology Co. (Shang-
hai, China). 1-Hexyl-methyldichlorosilane was purchased
from CNW Technologies Co. (Germany). Allyl glycidyl
ether, styrene and 5-hexen-2-one were obtained from Nine-
dinn chemistry Co. (Shanghai, China). Diphenylacetylene,
trans-β-methylstyrene, α-methylstyrene, 1-phenyl-1-butyne,
allylbenzene, 4-phenyl-1-butene, phenylacetylene and (E)-
4-phenyl-3-buten-2-one were purchased from Beijing Ouhe
Technology Co. (Beijing, China). Platinum standard solution
was purchased from Beijing Beina Chuanglian Biotechnol-
ogy Institute (Beijing, China). Distilled water was supplied
Preparation of the catalysts
Scheme 1 illustrates the preparation of these immobilized
Pt nano-catalysts.
Magnetite (Fe3O4) nanoparticles were prepared by chemi-
cal coprecipitation of Fe3+ and Fe2+ in ammonium hydrox-
ide solution, based on the reported methods [29, 30]. The
surface of the Fe3O4 nanoparticles was then covered with
SiO2 (SiO2 encapsulated Fe3O4 nanoparticles, Fe3O4@SiO2)
according to the published method [31].
To prepare epoxy propyl modifed Fe3O4@SiO2 (Fe3O4@
SiO2-EP), a dispersion of Fe3O4@SiO2 powder (500 mg) in
toluene (50 mL) was ultrasonically irradiated for 20 min.
After adding γ-glycidoxypropyltrimethoxysilane (4 mmol),
the mixture was boiled at refux under nitrogen for 24 h. The
product was separated with a magnet and washed sequen-
tially with dichloromethane (50 mL), acetone (50 mL),
dichloromethane (50 mL) and diethyl ether (50 mL).
Finally, the solid product was dried under vacuum at 80 °C
overnight.
1 3

Transition Metal Chemistry
Scheme 1 Synthesis of four immobilized Pt nano-catalysts on magnetic silica nanoparticles through boronic acid, nitric acid, sulfuric acid or
carboxylic acid groups
Next, 3-aminophenylboronic acid monohydrate
(APBA·H2O, 1.012 g), 3-nitroaniline (1.1064 g), 3-ami-
nophenylsufonic acid (1.3079 g) or 4-aminbenzoic acid
(1.2456 g) were each dissolved in deionized water (100 mL).
A portion of Fe3O4@SiO2-EP (1.0 g) was added to each
solution and dispersed by ultrasonic irradiation. After adjust-
ing the pH to 8.5 using NaOH solution, the mixture was
mechanically stirred for 24 h at room temperature. Each
product was then separated, washed and dried as described
above.
required silane, the mixture was continually heated and
stirred for a certain time, then cooled to room tempera-
ture. The catalyst was separated with an external magnet,
and the solution was assayed by GC using n-decane as an
internal standard.
Results and discussion
Finally, a solution of H2PtCl6ˑ6H2O (0.2 g) in ethanol
(10 mL) was mixed with a portion of the acid-derivatized
nanoparticles (1.0 g) and the mixture was vigorously stirred
for 9 h at 60 °C under nitrogen. The resulting catalysts
(Fe3O4@SiO2-BA@Pt, Fe3O4@SiO2-NA@Pt, Fe3O4@
SiO2-SA@Pt and Fe3O4@SiO2-CA@Pt) were magnetically
separated, washed with deionized water (3 times) and etha-
nol (3 times) and dried under vacuum at 80 °C overnight.
Preparation of the heterogeneous catalysts
It is well known that anions such as boronate, nitrate, sul-
fate and carboxylate can interact with Pt cations by ionic
attraction. Such interactions are highly dependent on the
pH and ionic strength of solution, often making it difcult
to immobilize platinum ions. Moreover, these anions will
2−
not attract other negatively charged species such as PtCl6
.
Hence, research to date has focused on heterocyclic supports
or materials such as TiO2 and Al2O3 [32–34]. Following
our discovery that carboxylic acids can immobilize platinum
catalysts through coordination interactions [27, 28], we have
investigated the feasibility of immobilizing Pt salts through
coordination interactions with boronic, nitric or sulfuric
acids. For comparison, carboxylic acid-modifed magnetic
silica gel was also used to immobilize Pt.
Catalytic hydrosilylation reactions
All catalytic reactions were performed without nitrogen
protection. A portion of the catalyst and 10 mmol of the
alkene/alkyne were placed in a 10 mL fat-bottomed tube
equipped with a refux condenser, and the mixture was
stirred 30 min at 60 °C. After adding 15 mmol of the
1 3

Transition Metal Chemistry
Fe3O4@SiO2-BA could immobilize 0.174 mmol/g Pt
(33.95 mg/g), about three times as much as Fe3O4@SiO2-
NA (0.055 mmol/g Pt) and fve times as much as Fe3O4@
SiO2-CA (0.032 mmol/g). Contrary to our expectations, we
found no detectable Pt on the sulfate-based Fe3O4@SiO2-
SA support. Previous reports have shown that the –B(OH)2
group can form a stable ring with cis-diols [35]. Thus, we
guess that the –B(OH)2 group can form a chelate ring with
Pt to anchor so much Pt catalyst.
Catalyst characterization
The surface morphology and size of the Pt-containing nano-
particles were characterized using TEM (Fig. 1a–d). The
images showed that the average particle size was 10–20 nm,
within the range for nanoparticles. Furthermore, the micro-
graphs showed a spherical shape for these materials, while
dry powder images showed aggregation of the nanoparticles.
To avoid damage to the TEM instrument, high-resolution
TEM images of these materials were not obtained. In aque-
ous suspension, all four samples appeared uniform and
mono-dispersed. Figure 1e shows a typical image.
The wide-angle XRD patterns of these catalysts are
shown in Fig. 2a. Almost identical XRD patterns were
obtained for Fe3O4@SiO2-BA@Pt, Fe3O4@SiO2-NA@Pt,
Fe3O4@SiO2-CA@Pt and Fe3O4@SiO2-SA@Pt compared
with Fe3O4. Characteristic difraction peaks were observed
The quantities of Pt in these materials were assayed
by AAS, and the results are shown in Table 1. The
Fig. 1 TEM micrograph of a Fe3O4@SiO2-BA@Pt, b Fe3O4@SiO2-NA@Pt, c Fe3O4@SiO2-CA@Pt, d Fe3O4@SiO2-SA@Pt and e microscopic
observation of the synthetic magnetic material dispersed in water
Table 1 The amount and
Entry
Catalyst
AAS results
XPS analysis
Pt4+ (%)
valence state of Pt on functional
magnetic nanoparticles
Pt (mg g−1
)
Pt (mmol g−1
)
Pt2+ (%)
Pt0 (%)
1
2
3
4
Fe3O4@SiO2-BA@Pt
Fe3O4@SiO2-NA@Pt
Fe3O4@SiO2-CA@Pt
Fe3O4@SiO2-SA@Pt
33.95
10.76
6.32
–
0.174
0.055
0.032
–
30.07
6.02
29.09
–
42.41
87.63
48.02
–
27.52
6.35
22.89
–
1 3

Transition Metal Chemistry
at 2θ of 30.2°, 35.5°, 43.3°, 53.8°, 57.3° and 62.8°, cor-
responding to the (220), (311), (400), (422), (511) and
(440) difraction planes of Fe3O4, respectively. These peaks
match with the magnetic cubic structure of Fe3O4 (JCPDS
65-3107) [36]. No obvious SiO2 peaks were observed in
the XRD patterns, indicating the amorphous nature of the
SiO2 coating. No obvious peaks of metallic Pt were detected,
indicating that the amount of Pt was too low and/or the Pt
particles were highly dispersed on these supports.
(a)
35.5
30.2
62.8
43.3
57.3
Fe
O
@SiO -SA@Pt
3
4 2
53.8
Fe
Fe
O
@SiO -CA@Pt
3
4
2
O
@SiO -NA@Pt
3
4 2
Fe
Fe
O
O
@SiO -BA@Pt
3
3
4
2
The Scherrer formula was used to calculate the crystallite
size of these materials [37, 38]:
4
Kꢀ
D =
10
20
30
40
50
2
60
70
80
90
ꢁ cos ꢂ
o
where D is the crystallite size (nm), λ is the X-ray wave-
length (k=1.542 Å) (Cuka), β is the full width at half-maxi-
mum height (FWHM) of the line, K is the Scherrer constant
(K=0.94) [39], and θ is the Bragg difraction angle (°). As
can be seen, the four immobilized catalysts (Fe3O4@SiO2-
BA@Pt, Fe3O4@SiO2-NA@Pt, Fe3O4@SiO2-CA@Pt and
Fe3O4@SiO2-SA@Pt) exhibited higher crystallite size than
that of Fe3O4. This phenomenon is most likely due to intro-
duction of the SiO2 shell and functional groups. It also could
be the result of aggregation of the materials after drying
under vacuum at 80 °C overnight followed by fne grinding
in a mortar by hand [40].
(b)
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2-EP
80
60
40
Fe3O4@SiO2-BA@Pt
Fe3O4@SiO2-NA@Pt
Fe3O4@SiO2-CA@Pt
Fe3O4@SiO2-SA@Pt
20
0
-20
-40
-60
-80
The magnetic properties of the catalysts were investigated
on a VSM in an applied magnetic feld from − 10,000 to
10,000 Oe. The hysteresis loops of Fe3O4, Fe3O4@SiO2,
Fe3O4@SiO2-EP, Fe3O4@SiO2-BA@Pt, Fe3O4@SiO2-
NA@Pt, Fe3O4@SiO2-SA@Pt and Fe3O4@SiO2-CA@Pt
are shown in Fig. 2b. The saturated magnetization decreases
systematically due to the surface coating of the Fe3O4 nano-
particles with silica [41], consistent with the TEM results
(Fig. 1). The magnetization saturations of the catalysts were
about 22.0 emu g−1 such that they could easily be separated
from solution with a magnet.
-10000
-5000
0
5000
10000
Magnetic Field (Oe)
(c)
Chloroplatinic acid solution
Fe3O4@SiO2-BA@Pt
Fe3O4@SiO2-NA@Pt
Fe3O4@SiO2-CA@Pt
Fe3O4@SiO2-SA@Pt
264
UV spectrometry was used to characterize the solution
before and after immobilizing Pt on these acid-modifed
nanoparticles. The UV absorption curves (Fig. 2c) showed
a maximum at 264 nm for the original chloroplatinic acid
solution, corresponding to Pt4+ [42]. After preparation of
Fe3O4@SiO2-BA@Pt and Fe3O4@SiO2-CA@Pt, this peak
was signifcantly decreased, and a new peak was observed
at 235 nm, corresponding to Pt2+ [43]. Similarly, after prep-
aration of Fe3O4@SiO2-NA@Pt, the peak at 264 nm was
dramatically decreased and a new peak at 235 nm and a
minimum at 225 nm indicated that most of the Pt4+ was
changed to Pt2+ or Pt0. For Fe3O4@SiO2-SA@Pt, the UV
spectrum of the residual solution of Pt was similar to the
original, indicating very little uptake of Pt.
235
200
250
300
Wavenumber (nm)
350
400
Fig. 2 a XRD patterns of Fe3O4, Fe3O4@SiO2-BA@Pt, Fe3O4@
SiO2-NA@Pt, Fe3O4@SiO2-CA@Pt and Fe3O4@SiO2-SA@Pt. b
Room temperature magnetization curves of Fe3O4, Fe3O4@SiO2,
Fe3O4@SiO2-EP,
Fe3O4@SiO2-BA@Pt,
Fe3O4@SiO2-NA@Pt,
Fe3O4@SiO2-CA@Pt and Fe3O4@SiO2-SA@Pt. c The UV absorp-
tion curves of the chloroplatinic acid solution before and after immo-
bilization
The XPS spectrum of the Fe3O4@SiO2-BA@Pt nanopar-
ticles is shown in Fig. 3a. Peaks corresponding to O, Si, C,
1 3

Transition Metal Chemistry
were assigned to Pt 4f5/2 and Pt 4f7/2, respectively [44]. The
valence states of platinum in these materials are shown in
Table 1. It was found that about 30% exists in the form of
Pt4+ and 70% as Pt2+ and Pt0 in both Fe3O4@SiO2-BA@Pt
and Fe3O4@SiO2-CA@Pt. In Fe3O4@SiO2-NA@Pt, more
than 90% is in the form of Pt2+ and Pt0, which might be
formed through passivation processes during preparation of
the catalyst [45]. Overall, these observations confrm that
Pt nanoparticles were successfully immobilized on the sur-
faces of Fe3O4@SiO2-BA@Pt, Fe3O4@SiO2-NA@Pt, and
Fe3O4@SiO2-CA@Pt.
(a)
Fe
Fe
O @SiO -SA@Pt
4 2
3
O
@SiO -CA@Pt
2
3
4
Fe
O @SiO -NA@Pt
4 2
3
Fe
O
@SiO -BA@Pt
4 2
3
Catalytic performance
1000
800
600
400
200
0
The catalytic activities of the nanoparticles were tested
for the hydrosilylation of 1-hexene with methyldichlorosi-
lane, and the results were compared with those obtained for
Speier’s catalyst. The results of these experiments are sum-
marized in Table 2. Only a little 1-hexene was observed for
Fe3O4@SiO2-BA@Pt (0.5%) and Speier’s catalyst (1.6%).
Meanwhile, the proportion of the β-adduct, 1-hexyl-methyl-
dichlorosilane, was 98.4% for Fe3O4@SiO2-BA@Pt which
was signifcantly higher than for Speier’s catalyst (78.5%).
Fe3O4@SiO2-NA@Pt and Fe3O4@SiO2-CA@Pt also
showed good selectivity (98.2 and 99.0%, respectively),
together with good conversion (80.4 and 75.4%, respec-
tively). For Fe3O4@SiO2@Pt (immobilizing Pt on Fe3O4@
SiO2) and Fe3O4@SiO2-EP@Pt, some 1-hexene was con-
verted, but the conversion ratios were low and the selectivi-
ties were poor. Compared with Speier’s catalyst (6.2×103),
the turnover frequencies (TOF) for 1-hexene were good
for Fe3O4@SiO2-BA@Pt (9.3 × 103), Fe3O4@SiO2-NA@
Pt (8.9 × 103) and Fe3O4@SiO2-CA@Pt (7.3 × 103). In
addition, we increased the amount of reactants in order to
obtain the turnover number (TON). The reaction of 1-hex-
ene (120 mmol) with methyldichlorosilane (192 mmol) was
catalyzed using 0.3 mg of Fe3O4@SiO2-BA@Pt (0.052 μmol
Pt), Fe3O4@SiO2-NA@Pt (0.016 μmol Pt) or Fe3O4@SiO2-
CA@Pt (0.010 μmol Pt) at 12 h. Under these conditions, the
regioselectivity was still around 99%, while TON was up to
1.67×106 for Fe3O4@SiO2-BA@Pt, 1.23×106 for Fe3O4@
SiO2-NA@Pt and 7.29×105 for Fe3O4@SiO2-CA@Pt.
The catalytic activities of these nanoparticles were also
tested for the hydrosilylation of styrene with methyldi-
chlorosilane. The results are summarized in Table 2 and
are similar to those for hydrosilylation of 1-hexene. When
Fe3O4@SiO2-BA@Pt was used as catalyst, the substrates
were completely converted with highest TOF. Lower yields
were obtained with Fe3O4@SiO2-NA@Pt (84.5%) and
Fe3O4@SiO2-CA@Pt (65.5%), which was similar to Spei-
er’s catalyst (68.1%). We also increased the amount of sty-
rene (130 mmol) and methyldichlorosilane (192 mmol) to
allow for calculation of TON, giving values of 1.09×106 for
Binding Energy (eV)
(b)
Pt2+
Fe3O4@SiO2-SA@Pt
Fe3O4@SiO2-CA@Pt
Fe3O4@SiO2-NA@Pt
Fe3O4@SiO2-BA@Pt
Pt4+
75
Pt0
80
70
65
Binding Energy (eV)
Fig. 3 a XPS spectra of the elemental survey scans of Fe3O4@SiO2-
BA@Pt, Fe3O4@SiO2-NA@Pt, Fe3O4@SiO2-CA@Pt and Fe3O4@
SiO2-SA@Pt. b XPS spectra of the Fe3O4@SiO2-BA@Pt, Fe3O4@
SiO2-NA@Pt, Fe3O4@SiO2-CA@Pt and Fe3O4@SiO2-SA@Pt show-
ing Pt 4f5/2 and Pt 4f7/2 binding energies
B, Pt and Fe were observed. The peaks from Si 2s (157.9 eV)
and Si 2p (108.2 eV) indicated that the silica layer had been
successfully coated onto the Fe3O4 nanoparticles [30]. The
B1s peak (202.0 eV) verifed that boronic acid groups were
successfully bonded onto the surface of Fe3O4@SiO2. The
XPS spectrum of Fe3O4@SiO2-CA@Pt showed the same
elements as for Fe3O4@SiO2-BA@Pt, except for B. Simi-
larly, the XPS spectrum of Fe3O4@SiO2-NA@Pt showed
extra N rather than B. No Pt signals were observed for
Fe3O4@SiO2-SA@Pt, which further verifed the failure in
immobilization of Pt through the sulfate groups. The XPS
data also indicated the oxidation state of Pt in these materi-
als. In Fig. 3b, the binding energies of Pt at 75.8 and 71.4 eV
1 3

Transition Metal Chemistry
Table 2 Hydrosilylation of 1-hexene/styrene and methyldichlorosilane catalyzed by various catalysts
TOF (103 h−1
)
Entry
Catalyst
Conversion rate (%)
Selectivity (β/α)
a
b
a
b
a
b
1
2
3
4
5
6
7
Speier’s catalyst
Fe3O4@SiO2@Pt
Fe3O4@SiO2-EP@Pt
Fe3O4@SiO2-BA@Pt
Fe3O4@SiO2-NA@Pt
Fe3O4@SiO2-CA@Pt
Fe3O4@SiO2-SA@Pt
98.4
48.4
76.5
99.5
80.4
75.4
–
68.1
37.4
51.1
100
84.5
65.5
–
78:22
68:32
80:20
100:0
98:2
99:1
–
55:45
60:40
60:40
60:40
60:40
60:40
–
6.2
0.8
4.5
9.3
8.9
7.3
–
3.5
0.2
2.2
9.4
9.1
6.8
–
a: Hydrosilylation of 1-hexene and methyldichlorosilane. b: hydrosilylation of styrene and methyldichlorosilane. Reaction conditions: 10.0 mmol
1-hexene/styrene, 15 mmol methyldichlorosilane, temperature: 60 °C, reaction time: 4 h, the mol ratio of Pt to 1-hexene/styrene: 1:10,000. TOF
was calculated by moles of product per molar Pt per hour
0.3 mg Fe3O4@SiO2-BA@Pt (0.052 μmol Pt), 8.21×105 for
0.3 mg Fe3O4@SiO2-NA@Pt (0.016 μmol Pt) and 7.43×105
for 0.3 mg Fe3O4@SiO2-CA@Pt (0.010 μmol Pt) after 12 h.
Hence, Fe3O4@SiO2-BA@Pt shows the best performance,
with signifcant catalytic ability and selectivity.
methyldichlorosilane was replaced by triethoxysilane, the
selectivity for 5-hexen-2-one increased to 91.4% (entries 3
and 4). Similar selectivity (above 95.0%) for the β-adduct
was observed in hydrosilylation of allyl glycidyl ether and
methyldichlorosilane or triethoxysilane (entries 5 and 6).
Compared with the hydrosilylation of styrene and methyl-
dichlorosilane (yield 100%, selectivity 60%), the β-adduct
yield for hydrosilylation of styrene and triethoxysilane was
down to 56.6%, while selectivity was slightly improved
to 77.8% (entry 7). These results indicate that catalysis
with Fe3O4@SiO2-BA@Pt gave good activity and selectiv-
ity, even when methyldichlorosilane was substituted with
triethoxysilane.
As the most effective catalyst, Fe3O4@SiO2-BA@Pt
was investigated further. To examine the substrate scope of
Fe3O4@SiO2-BA@Pt, we employed eight aromatic alkenes
and alkynes and their hydrosilylation with methyldichlorosi-
lane was investigated. The results of these experiments are
listed in Table 3. In most cases, Fe3O4@SiO2-BA@Pt exhib-
ited good catalytic activity. For phenylacetylene (entry 7),
allylbenzene (entry 1) and α-methylstyrene (entry 3), excel-
lent conversion rates of 82–98% were obtained. Comparing
with allylbenzene (entry 1) and 4-phenyl-1-butene (entry
4), the results suggest that the conversion rates of aromatic
alkenes decreased with increasing alkyl chain length. For
trans-β-methylstyrene (entry 2), (E)-4-phenyl-3-buten-2-one
(entry 5), 1-phenyl-1-butyne (entry 6) and diphenylacetylene
(entry 8), the conversion rates of internal aromatic alkenes/
alkynes were lower than for terminal aromatic alkenes/
alkynes.
A crucial beneft regarding this magnetic catalyst is
its straightforward separation; we therefore investigated
the recyclability of Fe3O4@SiO2-BA@Pt, using the reac-
tion of 1-hexene with methyldichlorosilane. On comple-
tion of the frst reaction, the catalyst was separated with a
magnet and the decantate was analyzed. Fresh substrates
were then added to the recycled catalyst, and a further
run was performed. In this way, Fe3O4@SiO2-BA@Pt was
reused eight times under the following conditions: reaction
temperature 60 °C, reaction time 4 h, mole ratio between
1-hexene and saline 1:1.5, and mole ratio of Pt to 1-hexene
1:10,000. The results are shown in Table 5. The yields
of 1-hexyl-methyldichlorosilane were 96.5% for the frst
run and 95.2% for the eighth. No Pt was detected in the
product solutions. The amount of Pt after the eight experi-
ments (0.171 mmol/g) was close to the original amount
(0.174 mmol/g) for Fe3O4@SiO2-BA@Pt, indicating
excellent stability for this catalyst.
We also employed other alkenes to evaluate the cata-
lytic activity and regioselectivity of Fe3O4@SiO2-BA@Pt,
giving the results listed in Table 4. The catalyst showed
excellent performance in these reactions. The selectiv-
ity for the β-adduct was above 99.0% in the reaction of
1-hexene with both methyldichlorosilane and triethoxysi-
lane (entries 1 and 2). The yield of the β-adduct was up
to 85.3% in hydrosilylation of 5-hexen-2-one and methyl-
dichlorosilane, with a conversion rate of 89.4%. When
1 3

Transition Metal Chemistry
Table 3 Catalytic activity
of Fe3O4@SiO2-BA@Pt
for the hydrosilylation of
methyldichlorosilane and
aromatic alkene or alkyne
Conversion
rate (%)
Entry
1
Alkene
Silane
Time/h
Methyldichlorosilane
4
4
85
Methyldichlorosilane
Methyldichlorosilane
2
3
4
5
6
74
82
65
47
48
4
4
4
6
Methyldichlorosilane
Methyldichlorosilane
Methyldichlorosilane
Methyldichlorosilane
Methyldichlorosilane
7
8
6
6
98
30
Reaction conditions: 10 mmol alkene or alkyne, 15 mmol methyldichlorosilane, temperature: 60 °C,
reaction time: 4 h, the mol ratio of Pt to alkene or alkyne was 1:8000
Table 4 Hydrosilylations of
alkene and methyldichlorosilane
or triethoxysilane
Selectivity
Time Conversion
(%)
Entry
Alkene
Silane
(h)
rate (%)
β-Adduct
1
2
Methyldichlorosilane
Triethoxysilane
4
4
98.3
99.4
>99.0
>99.0
3
4
5
6
Methyldichlorosilane
4
4
4
4
89.4
81.2
96.4
96.8
85.3
Triethoxysilane
91.4
Methyldichlorosilane
Triethoxysilane
>95.0
>95.0
7
Triethoxysilane
6
77.8
56.6
Reaction conditions were same as Table 3
1 3

Transition Metal Chemistry
14. Peng R, Li S, Sun X, Ren Q, Chen L, Fu M, Wu J, Ye D (2018)
Appl Catal B 220:462–470
Table 5 Recyclability of
Runs
Yield (%)
the Fe
3O4@SiO2-BA@
15. Zhu S, Gao X, Zhu Y, Cui J, Zheng H, Li Y (2014) Appl Catal B
158–159:391–399
Pt in catalyzing the
1
2
3
4
5
6
7
8
96.5
98.6
97.3
93.2
94.5
91.4
93.5
95.2
hydrosilylation of 1-hexene and
methyldichlorosilane
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were prepared under mild conditions to produce heteroge-
neous Pt catalysts. The preparation method was simple and
used readily available reagents. Characterization showed
these Pt catalysts are nanoparticles with an average size
of 10–20 nm. In catalyzing hydrosilylation of alkenes and
alkynes, three of these catalysts, especially Fe3O4@SiO2-
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