Synthesis of silanized magnetic Ru/Fe3O4@SiO2 nanospheres and their high selectivity to prepare: Cis -pinane

Article abstract of DOI:10.1039/c6ra14692a

(3-Aminopropyl)-triethoxysilane (APTS) and (3-mercaptopropyl)-trimethoxysilane (MPTS) grafted SiO₂-coated iron oxide (Fe₃O₄@SiO₂) magnetic supports were prepared. Various noble metal nanoparticles such as Ru, Pt, Rh and Pd were directly grown on the surfaces of the magnetic supports with ultrasmall and nearly monodisperse sizes, especially the APTS grafted Fe₃O₄@SiO₂ (Fe₃O₄@SiO₂/APTS) nanospheres. The Fe₃O₄@SiO₂/APTS/Ru catalyst demonstrated a high catalytic activity towards the hydrogenation of α-pinene to form cis-pinane. The conversion of α-pinene was 99.1% and the selectivity for cis-pinane reached 97.6% at 120 °C after 4 h. The Fe₃O₄@SiO₂/APTS/Ru catalyst was easily separated from the reaction mixture when applying an external magnetic field, and the separated Fe₃O₄@SiO₂/APTS/Ru had good reusability. After being reused eight times, the α-pinene conversion and the cis-pinane selectivity also can reach 97.8% and 96.9%, respectively.

Full text of DOI:10.1039/c6ra14692a

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Synthesis of silanized magnetic Ru/Fe3O4@SiO2
nanospheres and their high selectivity to prepare
cis-pinane†
Cite this: RSC Adv., 2016, 6, 81310
a
b
b
a
b
*
Yue Liu, Lu Li, Shiwei Liu, Congxia Xie and Shitao Yu
(3-Aminopropyl)-triethoxysilane (APTS) and (3-mercaptopropyl)-trimethoxysilane (MPTS) grafted SiO2-
coated iron oxide (Fe3O4@SiO2) magnetic supports were prepared. Various noble metal nanoparticles
such as Ru, Pt, Rh and Pd were directly grown on the surfaces of the magnetic supports with ultrasmall
and nearly monodisperse sizes, especially the APTS grafted Fe3O4@SiO2 (Fe3O4@SiO2/APTS)
nanospheres. The Fe3O4@SiO2/APTS/Ru catalyst demonstrated a high catalytic activity towards the
hydrogenation of a-pinene to form cis-pinane. The conversion of a-pinene was 99.1% and the selectivity
for cis-pinane reached 97.6% at 120 ꢀC after 4 h. The Fe3O4@SiO2/APTS/Ru catalyst was easily separated
from the reaction mixture when applying an external magnetic field, and the separated Fe3O4@SiO2/
APTS/Ru had good reusability. After being reused eight times, the a-pinene conversion and the cis-
pinane selectivity also can reach 97.8% and 96.9%, respectively.
Received 6th June 2016
Accepted 19th August 2016
DOI: 10.1039/c6ra14692a
www.rsc.org/advances
poly(ethylene oxide) triblock copolymer P123 (PEO20–PPO70–
PEO20, average molecular weight 5800) micelles signicantly
1. Introduction
improved the molar ratio between cis-pinane and trans-pinane
produced from the hydrogenation of a-pinene.10 However, as
a non-ionic surfactant, stabilizer P123 brought to the production
of an emulsion, consisting in the oil-soluble product and the
water containing the Ru. As a result, it was difficult to separate
the product from the emulsied mixture. Although an extraction
method could be used to separate the product from the emulsion
system, the use of an extracting agent could destroy the stability
of the nanoparticles. These considerations indicate the impor-
tance of developing an inert support to stabilize the Ru nano-
particles and to nd a convenient separation method for the
products of the a-pinene hydrogenation reaction.
Recently, immobilizing noble metal nanocatalysts on Fe3O4
magnetic support has drawn a great deal of attention due to
their good separation performance in the presence of an
appropriate magnetic eld.11,12 Considering the Fe3O4 particles
are reactive especially in an acid environment and thereby to
lose their magnetic properties, silica as a protecting shell was
usually used to coat the Fe3O4 particles to form a core–shell
(Fe3O4/SiO2) structure.13,14 Meanwhile, the silica shell can
prevent the aggregation of the Fe3O4 particles and provide
numerous surface Si–OH groups for further modication.15,16
The metal nanocatalysts can be immobilized on the magnetic
support through two approaches. The one is to deposit the
presynthesized metal nanoparticles onto the support by chem-
ical adsorption.17 This approach allows the ne control over the
size and shape of the metal nanoparticles, but the whole oper-
ation steps are tedious, and moreover, the adsorbed nano-
catalysts are easy to be leached from the magnetic support
The hydrogenation of a-pinene can produce a mixture of cis-
pinane and trans-pinane. cis-Pinane is an important industrial
intermediate and widely used for the synthesis of specialty
chemicals and pharmaceuticals such as linalool, dihy-
dromyrcenol and other terpene series species.1 cis-Pinane is
industrially prepared through the hydrogenation of a-pinene in
the presence of a catalyst, such as Pd/C or Pt/SiO2;2–4 however,
both catalysts show a low selectivity towards cis-pinane. The
identication of highly active and selective catalysts is a crucial
aspect for the efficiency of the process. Ru-based catalysts have
been found to show good catalytic performance towards hydro-
genation of compounds such as lactic acid,5 glucose,6 benzene7
and cinnamaldehyde.8 Ru-based catalysts are also used in the
hydrogenation of a-pinene. RuCl3 dissolved in water exhibits
good catalytic properties, with 99.7% conversion of a-pinene and
96.3% selectivity towards cis-pinane at 160 ꢀC for 5 h.9 The main
drawback observed in the use of this catalyst is its deactivation
over time, due to, the agglomeration of Ru particles originating
from the hydrogenation of RuCl3. The use of Ru nanoparticles
protected by poly(ethylene oxide)-b-poly(propylene oxide)-b-
aKey Laboratory of Sensor Analysis of Tumor Marker Ministry of Education, College of
Chemistry and Molecular Engineering, Qingdao University of Science and Technology,
53 Zhengzhou Road, Qingdao 266042, People's Republic of China
bCollege of Chemical Engineering, Qingdao University of Science and Technology, 53
Zhengzhou Road, Qingdao 266042, People's Republic of China. E-mail:
yushitaoqust@126.com; Fax: +86 532 84022719; Tel: +86 532 84022719
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra14692a
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during the catalytic reaction. The other is to directly grow metal temperature for 2 h, using a mechanical stirring system. The
nanoparticles on the magnetic support using the pregraed homogeneous mixture was transferred to a 200 mL Teon-lined
organic functional molecules as a stabilizer.18 This protocol is stainless-steel autoclave and the reaction was carried out at 200
facile to conduct and the amount of catalyst on the magnetic ꢀC for 10 h. The reacted mixture was separated by applying an
support is relatively small. APTS and MPTS with a well dened external magnetic eld. The obtained magnetite particles were
structure, amine and mercapto groups were considered to gra washed three times with water and dried at 60 ꢀC for 4 h under
from the surface of Fe3O4/SiO2, respectively.19 Therein, APTS a 500 Pa pressure, obtaining Fe3O4 spheres.28
and MPTS can act as robust stabilizer for supporting metal
In the second step, the SiO2 shell coating was applied on the
nanoparticles.20,21 At the same time, APTS–metal and MPTS– surface of the Fe3O4 spheres through a typical solution sol–gel
metal catalysts have been found to show the high catalytic method as follows.17 1.4 g of Fe3O4, 5 mL of ammonia solution,
activity towards hydrogenation,22,23 Heck reaction,24,25 and 70 mL of water and 280 mL absolute ethanol were dispersed for
Suzuki reaction,26,27 and so on. Comparatively few investigations 15 min at 30 C, using a mechanical stirring system. Then, 4.0
ꢀ
on the comparing the metal nanoparticles immobilized effect mL of TEOS was added dropwise under vigorous stirring and
and stability of the APTS and MPTS, which is the target of this le to react for 8 h at 30 ꢀC. Fe3O4@SiO2 was obtained aer
work.
separation in the presence of an external magnetic eld, three
Herein, APTS and MPTS functionalized magnetic Fe3O4/SiO2 cycles of washing with water, and drying at 60 ꢀC for 4 h under
were designed and synthesized to in situ growth of metal 500 Pa pressure. It was found that the thickness of the silica
nanoparticles. Metal nanoparticles were shown to be deposited shell depends on the amount of TEOS introduced. When 2.0 mL
with high dispersion and stability, especially when APTS was of TEOS were used, the thickness of the silica layer obtained was
used as the affinity ligand. In addition, this kind of catalyst can 25.6 nm, which could be observed by transmission electron
efficiently catalyze the hydrogenation of a-pinene reaction. microscopy (TEM).
When the reaction is completed, the catalyst can be conve-
In the third step, 0.4 g of Fe3O4@SiO2, 0.3 mL of APTS was
niently recovered by applying a permanent magnet externally. dissolved in 140 mL of isopropyl alcohol at 70 ꢀC for 8 h under
In particular, the Fe3O4@SiO2/APTS/Ru catalyst provided vigorous stirring. The obtained solid material was dried in
ꢀ
excellent reactivity and reusability in the hydrogenation of vacuum at 60 C for 4 h. Fe3O4@SiO2/APTS was obtained aer
a-pinene.
separation in the presence of an external magnetic eld, three
cycles of washing with water, and drying at 60 ꢀC for 4 h under
500 Pa pressure. Fe3O4@SiO2/MPTS were prepared using
a similar method.
2. Experimental
2.1. Materials
In the fourth and nal step, Ru nanoparticles were loaded
onto the Fe3O4@SiO2/APTS, using RuCl3 as the precursor. 0.2 g
of Fe3O4@SiO2/APTS, 0.1 g of RuCl3, and 50 mL of ethanol were
dispersed for 30 min in an ultrasonic oscillator. Following the
dispersion, 50 mL of 0.14 mol Lꢁ1 NaBH4 solution, used as
reducing agent, were dropped into the mixture, which reacted at
room temperature for 2 h. Fe3O4@SiO2/APTS/Ru was separated
by applying a magnet, washing ve times with water, and drying
at 60 ꢀC for 4 h under 500 Pa pressure.29 The Fe3O4@SiO2/APTS/
Ru nanospheres were analyzed by inductively coupled plasma-
atomic emission spectrometry (ICP-AES) to determine the
content of Ru nanoparticles. Fe3O4@SiO2/MPTS/Ru, Fe3O4@-
SiO2/MPTS/Pd, Fe3O4@SiO2/MPTS/Rh, Fe3O4@SiO2/MPTS/Pt,
Fe3O4@SiO2/APTS/Pd, Fe3O4@SiO2/APTS/Rh and Fe3O4@SiO2/
APTS/Pt nanospheres were prepared using a similar method.
The chemicals used in the course of the work were: a-pinene
(C10H16, Sigma-Aldrich, 97%), Pd/C (Aldrich, 10%), Ru/C
(Aldrich, 10%), ruthenium(III) chloride (RuCl3, Macklin, Ru
content >37.5%), palladium(II) chloride (PdCl2, Sigma-Aldrich,
99%), platinum(II) chloride (PtCl2, Sigma-Aldrich, Pt content
>73.0%), rhodium(III) chloride hydrate (RhCl3, Sigma-Aldrich,
Rh content >38.5%), ferric chloride hexahydrate (FeCl3$6H2O,
Sigma-Aldrich, 99%), anhydrous sodium acetate (NaAc, Merck
Millipore, 99%), ethylene glycol ((CH2OH)2, Macklin, 99%),
tetraethylorthosilicate (TEOS, Aldrich, 98%), ammonia solution
(NH3$H2O, Macklin, 99%), ethanol (C2H5OH, Sigma-Aldrich,
99%), sodium borohydride (NaBH4, Macklin, 96%), (3-
aminopropyl)-triethoxysilane (APTS, Aladdin, 99%), (3-
mercaptopropyl)-trimethoxysilane (MPTS, Aladdin, 99%),
sodium citrate (Na3C6H5O7, Sigma-Aldrich, 99%), isopropyl
alcohol ((CH3)2CHOH, Sigma-Aldrich, 99%), hydrogen (H2,
Qingdao Airichem Specialty Gases & Chemicals Co. Ltd., 99.99
wt%). All of these materials were commercially available and
were used without further purication.
2.3. Measurements and characterizations
Some samples of the intermediates and end-products were
characterized using TEM, Fourier transform infrared spectros-
copy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spec-
troscopy (XPS), ICP-AES, and quantum design vibrating sample
magnetometer. TEM analyses were performed using a JEOL-
2.2. Preparation of Fe3O4@SiO2/APTS/Ru nanospheres
The preparation of Fe3O4@SiO2/APTS/Ru was carried out in four 1200 microscope, operated at 100 kV. FT-IR spectra were
steps, as shown in Scheme 1. To begin, the magnetite particles recorded in the wavelength range of 4500–400 cmꢁ1 on a Nicolet
were prepared by a robust solvothermal reaction. 2.6 g of 510P FT-IR spectrometer using the KBr method. XRD patterns
FeCl3$6H2O, 4.8 g of NaAc, and 1.0 g of trisodium citrate were were collected on a Rigaku D/max-2400 diffractometer with Cu-
accurately mixed with 80 mL of ethylene glycol at room Ka anode radiation at 40 kV and 100 mA in the 2q range of 10–
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Scheme 1 Synthesis route to Fe3O4@SiO2/APTS/Ru (Pd/Pt/Rh) and Fe3O4@SiO2/MPTS/Ru (Pd/Pt/Rh) nanospheres.
90ꢀ with a scan speed of 2ꢀ minꢁ1. XPS was carried out on a PHI- and WALL is the total amount of the products, including cis-
5702 instrument (Thermo Fisher Scientic-UK; X-ray source: pinane and trans-pinane.
monochromated Al Ka, hn ¼ 1486.6 eV; X-ray energy: 15 kV, 150
W, binding energies: C 1s hydrocarbon peak at 284.60 eV). ICP-
AES was carried out using a Varian Vista Pro ICP-AES instru-
ment running at 1200 W forward power. Before analysis, the
3. Results and discussion
3.1. Effects of catalysts on reaction results
samples were dissolved in
5 mL aqua regia. Magnetic
The work started with the screening of catalysts. The reaction
results for the different catalysts are given in Table 1. It was seen
that, when no catalyst was used, the hydrogenation reaction did
not take place at all (entry 1). It is well known, Pd/C and Ru/C
demonstrated the best performance towards the hydrogena-
tion of the carbon–carbon double bond and were therefore used
as the standard against which the behavior of all the other
catalysts was compared.30,31 The Pd/C catalyst showed poor
catalytic performance, with both a-pinene conversion and cis-
pinane selectivity below 84% (entry 2). Ru/C showed excellent
catalytic properties towards hydrogenation. The a-pinene
conversion and cis-pinane selectivity were over than 94%,
respectively (entry 3). The above results indicate that Ru-based
catalysts display a higher catalytic activity than the Pd-based
catalyst; however, the reusability of Ru/C was poor due to the
loss of the catalyst might happen during its reclamation. More
importantly, the aggregation of the Ru nanoparticles might also
happen aer each cycle, and thus the surface area of the catalyst
as well as the catalytic sites were decreased.32 To solve this
problem, Ru nanoparticles were tried to immobilize on
magnetic support in order to greatly improve dispersibility and
recyclability.
measurements were performed on a vibrating sample magne-
tometer at room temperature in an applied magnetic eld
sweeping from ꢁ15 to 15 kOe. The Brunauer–Emmett–Teller
(BET) method was utilized to calculate the specic surface areas
(SBET) using adsorption data in the relative pressure range
0.005–0.250.
2.4. Catalytic reaction
Mixtures of different amounts of a-pinene and catalyst was
reacted at 120 ꢀC for 4 h under 4 MPa H2 partial pressure in
a 100 mL stainless steel reactor with mechanical stirring. Aer
the reaction was completed, the catalyst was separated from the
product phase by applying an external magnet. The product
phase was analyzed using gas chromatography (GC) to deter-
mine the obtained conversion of a-pinene and the selectivities
towards the different products. The catalyst layer was reused
directly in the recycle experiments to examine its reusability. All
experiments were repeated ve times in order to determine the
reproducibility of the results.
Samples of the products layer were characterized qualita-
tively using a gas chromatography-mass spectrometer (GC-MS)
on an HP6890/5973 GC-MS equipped with an AT-5 capillary
column (30 m ꢂ 0.25 mm ꢂ 0.25 mm). Quantitative analyses
were carried out using GC on an HP6890 GC equipped with
a ame ionization detector (FID) and an AT-5 capillary column
(30 m ꢂ 0.25 mm ꢂ 0.25 mm). The temperatures of the injector
and the detector were 250 ꢀC and 260 ꢀC, respectively. The
temperature program was as follows: holding at 80 ꢀC for 2 min,
increasing to 180 ꢀC at a rate of 4 ꢀC minꢁ1, and nally holding
at 180 ꢀC for 2 min. The qualitative analysis was conducted on
the basis of the holding time of the peak. The content of reac-
tants and products was directly obtainable by the GC chem-
station system, according to the area of each chromatograph
Table 1 Effects of catalysts on the hydrogenation resultsa
Entry
Catalyst
C/%
S/%
1
2
3
4b
5
6
7
8
Blank
Pd/C
Ru/C
Ru/C
1.0 ꢃ 0.1
70.6 ꢃ 0.2
82.2 ꢃ 0.3
96.1 ꢃ 0.1
95.6 ꢃ 0.2
95.7 ꢃ 0.4
97.6 ꢃ 0.3
96.8 ꢃ 0.1
95.2 ꢃ 0.4
83.9 ꢃ 0.1
94.9 ꢃ 0.2
78.6 ꢃ 1.2
45.3 ꢃ 0.5
99.1 ꢃ 0.3
95.6 ꢃ 0.2
76.3 ꢃ 0.5
Fe3O4/Ru
Fe3O4@SiO2/APTS/Ru
Fe3O4@SiO2/MPTS/Ru
Fe3O4@SiO2/Ru
a
a-Pinene 2.72 g, n(a-pinene) : n(metal active site) ¼ 150, H2 4 MPa, 120
peak. The a-pinene conversion was determined as a function of ꢀC, 4 h. C: conversion of a-pinene. S: selectivity toward cis-pinane.
b
Reused aer ve runs. Entries 6–8: the thicknesses of the silica layer
the mass of a-pinene consumed in the reaction. The product
were about 25.6 nm. Entries 2–8: the loading of metal nanoparticles
was 10 wt%.
selectivity was calculated by Ws/WALL ꢂ 100, where Ws is the
amount of the product for which selectivity is being evaluated
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Fig. 1 XPS spectrum of Fe3O4@SiO2/APTS/Ru (a and a0) and Fe3O4@SiO2/MPTS/Ru (b and b0).
Among all the Fe3O4 supported catalysts, the non- Fe3O4@SiO2/APTS/Ru and Fe3O4@SiO2/MPTS/Ru contained
encapsulated shell Ru/Fe3O4 catalysts showed the worst cata- 8.09 wt% and 8.12 wt% Ru. However, the size and distribution
lytic performance (entry 5). This was due to the fact that the of Ru nanoparticles on the Fe3O4@SiO2 support were depen-
pure Fe3O4 spheres had no functional groups on their surface, dent on the affinity ligands used. When APTS was employed,
hindering adsorption of the metal active center.33 The silica smaller Ru nanoclusters (2.35 ꢃ 0.7 nm) and narrower cluster
shell was modied by APTS and MPTS catalysts showed the best size distribution were obtained in TEM micrographs. MPTS
catalytic performance for hydrogenation. In particular, Fe3- gave rise to Ru nanoparticles of 3.75 ꢃ 1.7 nm. When no affinity
O4@SiO2/APTS/Ru and Fe3O4@SiO2/MPTS/Ru showed an ligands were used, although a greatly many of the Ru nano-
excellent catalytic activity towards hydrogenation: the a-pinene particles were well located on the particles' surface, some
conversion reached 99.1%, 95.6% and the cis-pinane selectivity serious aggregates of Ru nanoparticles are also obviously
was 97.6%, 96.8%, respectively (entries 6 and 7). In order to observed on the surface of Fe3O4@SiO2 (Fig. S2†). Therefore,
explain the good catalytic performance of Fe3O4@SiO2/APTS/Ru Fe3O4@SiO2/APTS/Ru showed the higher conversion than Fe3-
and Fe3O4@SiO2/MPTS/Ru, XPS, FT-IR spectra and TEM anal- O4@SiO2/MPTS/Ru and Fe3O4@SiO2/Ru.19
yses were carried out. The results are shown in Fig. 1, S1† and 2.
In order to further explain the size and distribution of nano-
As shown in Fig. 1, the peaks present at 280.2 eV (assigned to Ru particles dependent on the affinity ligands used. Pd, Rh or Pt
3d5/2), 461.5 eV (assigned to Ru 3p3/2), and 484.5 eV (assigned to nanoparticles were loaded on the Fe3O4@SiO2/APTS and Fe3-
Ru 3p1/2) suggested the presence of Ru(0) in its elementary O4@SiO2/MPTS nanospheres and used to catalyze the hydroge-
state.34 In addition, the numerous groups were modied on the nation reaction, respectively (Table 2). It was seen that,
Fe3O4@SiO2 shell. They were functionalized with mercapto and
amine functionalities as revealed by the FT-IR spectra, which
have been known to have strong affinity with transition metal
nanoparticles (Fig. S1†). The functional mercapto and amine
groups can serve as anchors to immobilize Ru nanoparticles on
the silica layer surface and facilitate the dispersion of Ru
nanoparticles by preventing them from aggregation in the
process of reaction.35 Numerous monodispersed Ru nano-
particles were therefore uniformly distributed on the silica shell
of the Fe3O4@SiO2 catalyst and nearly all the Ru nanoparticles
were connected to the Fe3O4@SiO2 spheres (Fig. 2). As a result,
Fe3O4@SiO2/APTS/Ru and Fe3O4@SiO2/MPTS/Ru nanospheres
made the catalytic active center Ru(0) highly dispersed and
offered the reaction a large area interface, thus enhancing
hydrogenation.36 ICP-AES elemental analyses showed that the
Table 2 Effects of catalysts on the hydrogenation results by the
various metal nanoparticlesa
Entry
Catalyst
C/%
S/%
1
2
3
4
5
6
Fe3O4@SiO2/APTS/Pd
Fe3O4@SiO2/APTS/Rh
Fe3O4@SiO2/APTS/Pt
Fe3O4@SiO2/MPTS/Pd
Fe3O4@SiO2/MPTS/Rh
Fe3O4@SiO2/MPTS/Pt
87.1 ꢃ 0.2
89.3 ꢃ 0.2
92.5 ꢃ 0.1
84.3 ꢃ 0.3
85.9 ꢃ 0.1
88.1 ꢃ 0.3
82.6 ꢃ 0.1
87.7 ꢃ 0.3
90.1 ꢃ 0.1
81.9 ꢃ 0.1
86.9 ꢃ 0.2
88.9 ꢃ 0.1
a
a-Pinene 2.72 g, n(a-pinene) : n(metal active site) ¼ 150, H2 4 MPa, 120
ꢀC, 4 h. C: conversion of a-pinene. S: selectivity toward cis-pinane. The
thicknesses of the silica layer were about 25.6 nm. The loading of
metal nanoparticles was 10 wt%.
Fig. 2 TEM and particle distributions of Fe3O4@SiO2/APTS/Ru (a and a0) and Fe3O4@SiO2/MPTS/Ru (b and b0).
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Fe3O4@SiO2/APTS/Pd (Rh or Pt) and Fe3O4@SiO2/MPTS/Pd (Rh and 8.12 wt% Ru, respectively. As shown in Fig. 4 and S5,† the
or Pt) catalyst showed poor catalytic performance, with both a- catalysts, Fe3O4@SiO2/APTS/Ru and Fe3O4@SiO2/MPTS/Ru, can
pinene conversion and cis-pinane selectivity below 93%. Fe3- be reused more than four times with no signicant decline in
O4@SiO2/APTS/Pt showed excellent catalytic properties towards selectivity or activity. Nearly no appreciable Ru leaching into the
hydrogenation in Table 2. The a-pinene conversion and cis-
pinane selectivity were only about 93%, respectively (entry 3). The
above results indicate that Fe3O4@SiO2/APTS/Ru catalysts display
a higher catalytic activity than the above catalysts. TEM results for
Fe3O4@SiO2/APTS/Ru (Fig. 2a) showed that the Ru nanoparticles
with smaller average size (2.35 nm), which was favorable for
hydrogenation.36 As shown in Table 2, the results were Fe3O4@-
SiO2/APTS/Pd (Rh or Pt) showed the higher conversion than
Fe3O4@SiO2/MPTS/Pd (Rh or Pt). This may be due to the fact that
in the products obtained in the present work, when APTS was
employed, smaller Pd (Rh or Pt) nanoparticles and narrower
cluster size distribution were obtained than MPTS (Fig. 3).
In this study, Fe3O4@SiO2/APTS/Ru nanospheres could be
easily recovered by an external magnet as is shown in Fig. S3† and
quick dispersibility through a slight shake when the magnetic
eld was removed (Fig. S4†). For comparison, Fe3O4@SiO2/
MPTS/Ru nanospheres was also separated and recycled in the
hydrogenation reaction. The used Ru content in the mixed
solution was kept the same for both Fe3O4@SiO2/APTS/Ru and
Fe3O4@SiO2/MPTS/Ru nanospheres in the recycling experiments.
Fig. 4 The reusability of Fe3O4@SiO2/APTS/Ru. Reaction conditions:
ICP-AES elemental analyses showed that the Fe3O4@SiO2/APTS/
a-pinene 2.72 g, Fe3O4@SiO2/APTS/Ru 0.13 g (the thicknesses of the
Ru and Fe3O4@SiO2/MPTS/Ru nanospheres contained 8.09 wt%
silica layer were about 25.6 nm, Ru 10 wt%), H2 4 MPa, 120 ꢀC, 4 h.
Fig. 3 TEM and particle distributions of Fe3O4@SiO2/APTS/Pt (A and A0), Fe3O4@SiO2/MPTS/Pt (a and a0), Fe3O4@SiO2/APTS/Rh (B and B0),
Fe3O4@SiO2/MPTS/Rh (b and b0), Fe3O4@SiO2/APTS/Pd (C and C0) and Fe3O4@SiO2/MPTS/Pd (c and c0).
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organic phase was observed, as indicated by the ICP-AES analysis nanospheres own higher catalytic activity. In our study, APTS
results of 0.79 and 0.76 ppm, respectively. Their conversions then might have served as a stronger affinity ligand than MPTS to
decreased gradually in subsequent runs to 96.6% and 83.9% at provide for higher Ru dispersion and to more effectively prevent
run eight cycles. As shown in Fig. 5b that Ru nanoparticles in the agglomeration, and growth of Ru nanoparticles during the
Fe3O4@SiO2/MPTS/Ru catalysts grew bigger aer eight runs than hydrogenation of a-pinene.
the fresh. This could have led to the reduced conversions aer
numerous recycling. While the Ru nanoparticles remained highly
3.2. Effects of different amounts of Ru nanoparticles on the
reaction
dispersed on Fe3O4@SiO2/APTS with an average size of 3.2 ꢃ 0.5
nm was observed aer eight reaction cycles (Fig. 5a). As a result,
Fe3O4@SiO2/APTS/Ru with various loading amounts of Ru nano-
particles was synthesized through a similar process as the one
described earlier. The amount of Ru used during loading was 5
the catalyst of Fe3O4@SiO2/APTS/Ru nanospheres also had larger
surface area, which could offer more catalytic sites toward
hydrogenation of a-pinene.37 Therefore Fe3O4@SiO2/APTS/Ru
wt%, 8 wt%, 10 wt%, or 12 wt%. The Ru content in Fe3O4@SiO2/
APTS/Ru was determined by the ICP-AES technique, and the
results are reported in Table 3. As shown in Table 3 (entries 1–5),
the measured Ru content in the Fe3O4@SiO2/APTS/Ru catalyst was
less than that of the actual feed in all experiments. This may be due
to the fact that some Ru was not loaded on the silica shell in the
preparation process, or that some Ru, loaded on the outer surface
of the silica shell, was lost during the washing process (entries 6–
8). Fe3O4@SiO2/APTS/Ru with different amounts of Ru was used to
catalyze the hydrogenation of a-pinene (Table 3). It was found that
the conversion of a-pinene increased with increasing amounts of
Ru. When the Ru nanoparticles content was 10%, the hydroge-
nation of a-pinene was effectively catalyzed (entry 4). This could be
explained by the fact that, with increasing Ru dosage, Fe3O4@SiO2/
APTS/Ru offers much more active sites, enhancing hydrogena-
tion.38 A further increase in the Ru nanoparticle content above 10%
brought no additional benet to the hydrogenation reaction. This
value was therefore selected as optimal.
Fig. 5 TEM of Fe3O4@SiO2/APTS/Ru (a) and Fe3O4@SiO2/MPTS/Ru (b)
after eight runs of a-pinene hydrogenation.
Table 3 Effects of different amounts of Ru nanoparticles on the
hydrogenation reactiona
Rub/wt%
Ruc/wt%
C/%
S/%
3.3. Effects of the thickness of the silica layer on the reaction
Entry
In order to get Fe3O4@SiO2 with different thicknesses of the silica
layer, the amount of TEOS introduced when coating the spheres
was tuned. Ru nanoparticles were loaded onto the obtained
Fe3O4@SiO2 with different thicknesses of the silica layer and
used to catalyze the hydrogenation reaction. The results are re-
ported in Table 4. It was seen that the amount of TEOS was the
key to the nal thickness of the silica layer obtained. Reducing
the amount of TEOS from 4.0 mL to 1.0 mL, brought to a decrease
in the shell thicknesses from 40.1 to 18.2 nm (Fig. 6). The
1
2
3
4
5
8
3.91
6.12
8.09
9.86
9.82
8.12
8.09
75.9 ꢃ 0.5
91.3 ꢃ 0.4
99.1 ꢃ 0.3
99.2 ꢃ 0.1
—
96.1 ꢃ 0.2
96.3 ꢃ 0.3
97.6 ꢃ 0.3
97.4 ꢃ 0.2
—
10
12
10
10
10
5d
6e
7f
—
—
—
—
a
a-Pinene 2.72 g, Fe3O4@SiO2/APTS/Ru 0.13 g, H2 4 MPa, 120 ꢀC, 4 h.
b
The thickness of the silica layer was 25.6 nm. Ru content in the
loading process.
c
Ru content determined by ICP-AES in the
d
Fe3O4@SiO2/APTS/Ru. Fe3O4@SiO2/APTS/Ru not washed with water thickness of the silica layer could therefore be easily controlled by
e
during the preparation process. Fe3O4@SiO2/APTS/Ru washed three
times with water during the preparation process. Fe3O4@SiO2/APTS/
Ru washed ve times with water in the preparation process.
varying the amount of TEOS added. In addition, when the
thickness of the silica layer was about 25.6 nm, the Ru nano-
f
particles displayed morphological regularity, monodispersity and
Table 4 Effect of the thickness of the silica layer on the hydrogenation reactiona
Entry
TEOS/mL
Thickness/nm
SBET/m2 gꢁ1
Rub/wt%
C/%
S/%
1
2
3
4
4.0
3.0
2.0
1.0
40.1 ꢃ 1.1
33.2 ꢃ 1.2
25.6 ꢃ 1.1
18.2 ꢃ 0.9
2.2
4.1
5.9
7.5
5.61
7.21
8.09
8.21
91.6 ꢃ 0.4
94.9 ꢃ 0.2
99.1 ꢃ 0.1
99.1 ꢃ 0.2
95.8 ꢃ 0.3
96.1 ꢃ 0.3
97.6 ꢃ 0.3
97.4 ꢃ 0.1
a
a-Pinene 2.72 g, Fe3O4@SiO2/APTS/Ru 0.13 g, (10.0 wt% of Ru), H2 4 MPa, 120 ꢀC, 4 h. b Ru content determined by ICP-AES in Fe3O4@SiO2/APTS/
Ru. SBET: BET surface area.
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Fig. 6 TEM of Fe3O4@SiO2 with different thickness of the silica layer. The amount of TEOS were 4.0 mL (a), 3.0 mL (b), 2.0 mL (c), 1.0 mL (d),
respectively.
size uniformity (Fig. 2). The thickness of the Fe3O4@SiO2/APTS/
Ru silica shell had a signicant effect on the hydrogenation
reaction. When increasing the thickness of the silica layer from
18.1 to 40.1 nm, the catalytic performance of Fe3O4@SiO2/APTS/
Ru gradually decreased. This may be due to the fact that an
increase in the thickness of the silica layer brought to a decrease
in the surface area of Fe3O4@SiO2 available to the reaction and
a lower loading of Ru nanoparticles.39
Based on the above results, the mechanism of a-pinene
hydrogenation in the presence of Fe3O4@SiO2/APTS/Ru was
proposed. In the initial stage, a-pinene can be easily absorbed
on the surface of the catalyst due to the lipophilicity of the APTS.
H2 is activated by the electronically supported Ru. The bulky
gem-dimethyl group inhibits absorption of the b-face of a-
pinene on the surface of the catalyst. Hydrogenation therefore
takes place preferentially from the a-face of the molecule,
increasing thus selectivity towards cis-pinane.40
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4. Conclusions
The controlled silica layer of Fe3O4@SiO2 was modied by APTS
and MPTS. Various noble metal such as Ru, Pt, Rh and Pd nano-
clusters were shown to be deposited with high dispersion and
stability, especially when APTS was used as the affinity ligand.
Fe3O4@SiO2/APTS/Ru exhibited an outstanding catalytic perfor-
mance with 99.1% conversion of a-pinene and 97.6% selectivity for
cis-pinane. In addition, when the reaction was completed, Fe3-
O4@SiO2/APTS/Ru and Fe3O4@SiO2/MPTS/Ru can be conveniently
recovered by applying a permanent magnet externally. The results
showed that Fe3O4@SiO2/APTS/Ru had excellent catalytic activity
and renewable activity in the hydrogenation of a-pinene.
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Acknowledgements
This work was nancially supported by the Taishan Scholar
Program of Shandong, the National Natural Science Foundation
of China (31270615). The authors are grateful for the nancial
support.
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