Synthesis of a highly active amino-functionalized Fe3O4@SiO2/APTS/Ru magnetic nanocomposite catalyst for hydrogenation reactions

Article abstract of DOI:10.1002/aoc.4686

An amino-functionalized silica-coated Fe₃O₄ nanocomposite (Fe₃O₄@SiO₂/APTS) was synthesized. The Fe₃O₄@SiO₂ microspheres possessed a well-defined core–shell structure,

Full text of DOI:10.1002/aoc.4686

Received: 3 September 2018
DOI: 10.1002/aoc.4686
Revised: 9 October 2018
Accepted: 14 October 2018
F U L L P A P E R
Synthesis of a highly active amino‐functionalized
Fe3O4@SiO2/APTS/Ru magnetic nanocomposite catalyst for
hydrogenation reactions
Yue Liu1
| Mingxin Lv1 | Lu Li3 | Hailong Yu1 | Qiong Wu1 | Jinhui Pang3 |
Yuxiang Liu1 | Congxia Xie2 | Shitao Yu1 | Shiwei Liu1
1 College of Chemical Engineering,
An amino‐functionalized silica‐coated Fe3O4 nanocomposite (Fe3O4@SiO2/
Qingdao University of Science and
Technology, 53 Zhengzhou Road, Qingdao
266042, People's Republic of China
2 State Key Laboratory Base of Eco‐
chemical Engineering, College of
APTS) was synthesized. The Fe3O4@SiO2 microspheres possessed a well‐
defined core–shell structure, uniform sizes and high magnetization. An
immobilized ruthenium nanoparticle catalyst (Fe3O4@SiO2/APTS/Ru) was
obtained after coordination and reduction of Ru3+ on the Fe3O4@SiO2/APTS
nanocomposite. The Ru nanoparticles were not only ultra‐small with nearly
monodisperse sizes but also had strong affinity with the surface of
Fe3O4@SiO2/APTS. The obtained catalyst exhibited excellent catalytic perfor-
mance for the hydrogenation of a variety of aromatic nitro compounds, even
at room temperature. Moreover, Fe3O4@SiO2/APTS/Ru was easily recovered
using a magnetic field and directly reused for at least five cycles without signif-
icant loss of its activity.
Chemistry and Molecular Engineering,
Qingdao University of Science and
Technology, 53 Zhengzhou Road, Qingdao
266042, People's Republic of China
3 College of Marine Science and Biological
Engineering, Qingdao University of
Science and Technology, 53 Zhengzhou
Road, Qingdao 266042, People's Republic
of China
Correspondence
Shitao Yu and Shiwei Liu, College of
Chemical Engineering, Qingdao
University of Science and Technology, 53
Zhengzhou Road, Qingdao 266042,
People's Republic of China.
KEYWORDS
amino‐functionalized, core–shell structure, Fe3O4@SiO2/APTS/Ru, hydrogenation
Email: yushitaoqust@126.com;
liushiweiqust@126.com
Funding information
Key R&D project of Shandong; Open
Fund of the Biomass‐energy and Materials
Key Laboratory of Jiangsu, Grant/Award
Number: JSBEM201802; National Natural
Science Foundation of China, Grant/
Award Number: 31700517; Taishan
Scholars Projects of Shandong, Grant/
Award Numbers: 2017GGX70102 and
ts201511033
1 | INTRODUCTION
ready availability and high surface area properties.[1–3]
The larger exposed surface area of the active component
Noble metal nanoparticles have shown remarkable
potential for numerous applications in electronic, chemi-
cal, biological and catalytic fields due to their robustness,
of a nanoparticle catalyst enhances markedly the contact
between reactants and catalyst and mimics homogeneous
catalysts.[4,5] However, owing to their high surface
Appl Organometal Chem. 2019;e4686.
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LIU ET AL.
energies, metallic nanoparticles are normally unstable
and tend to coagulate upon sintering during catalytic
reactions.[6,7] In order to overcome these problems, exten-
sive studies have been performed to immobilize noble
metal nanoparticle catalysts. Numerous methods have
been developed to immobilize metal nanoparticles on a
large variety of solid supports, such as microporous poly-
mers,[8] mesoporous silica,[9] amorphous silica[10] and
carbon nanofibers.[11] However, these methods suffer
from tedious and high‐cost separation via centrifugation
or filtering.
More recently, magnetic core–shell nanocomposites
have been widely used as supports for metal nanoparti-
cles.[12–14] Composites with magnetic cores and func-
tional shell structures have received particular attention
because the magnetic cores allow the composites to be
conveniently collected or separated using an external
magnet.[15–18] Functional shells, such as SiO2 and C, can
be applied to immobilize metal nanoparticles.[19] Our
team synthesized dense carbon shells embedded in
Fe3O4 spheres through the hydrothermal treatment of
glucose. However, we found that the Fe3O4@C micro-
spheres tend to aggregate during the preparation process.
Silica has been mostly used as a coating material because
it can efficiently inhibit hydrogen ions from passing
through.[20] However, silica shells have few functional
groups for immobilizing metal nanoparticles on their sur-
face and must be functionalized before being used as sup-
ports.[21] Significant attention has been focused on amino
functionalization, because amino groups are well known
to stabilize metal nanoparticles against aggregation dur-
ing catalytic reactions.[22] (3‐Aminopropyl)triethoxysilane
(APTS) has been grafted on the surface of silica shells,
where the amino groups of APTS can act as robust stabi-
lizers for supporting metal nanoparticles, thus increasing
reusability.[23] Amino‐functionalized Fe3O4@SiO2 shows
excellent catalytic activity in the selective oxidation of
alcohols and the Suzuki reaction.[24,25] Ruthenium‐based
catalysts have attracted much attention owing to their
versatile roles in many catalytic reactions, including the
hydrogenation of levulinic acid,[26] CO2 hydrogenation
to formic acid,[27] oxidation of methanol[28] and CO2
methanation.[29]
FIGURE 1 FT‐IR spectra of Fe3O4 (a), Fe3O4@SiO2 (b) and
Fe3O4@SiO2/APTS/Ru (c)
the surface of amino‐functionalized Fe3O4@SiO2.
Fe3O4@SiO2/APTS/Ru exhibits excellent catalytic perfor-
mance for the hydrogenation of a variety of aromatic
nitro compounds, even at room temperature. When the
reaction is completed, through a simple magnetic separa-
tion, the core–shell Fe3O4@SiO2/APTS/Ru nanocatalyst
can be easily recovered with a magnetic field and reused
at least five times without significant reduction of cata-
lytic performance.
2 | RESULTS AND DISCUSSION
2.1 | Structural characterization of
Fe3O4@SiO2 and Fe3O4@SiO2/APTS/Ru
microspheres
Figure 1 shows the Fourier transform infrared (FT‐IR)
spectra of the various as‐prepared magnetic particles.
The absorption band at 586 cm−1 in Figure 1a is ascribed
to Fe─O─Fe vibrations, which also confirms the presence
of Fe3O4. In comparison with Figure 1a, the sharp peak at
1086 cm−1 in Figure 1b can be assigned to the Si─O─Si
vibrations, whereas the peaks at ca 1629 and 3432 cm−1
are attributed to the absorbed water and hydroxyl groups,
respectively. The Si─O bending vibration mode of the
silanol group is seen at 959 cm−1. After modification with
APTS (Figure 1c), the peaks observed at 2960 cm−1 can be
Herein, we report the synthesis of a magnetic, amine‐
functionalized catalyst using APTS as the modifying
reagent to immobilize Ru nanoparticles (S1, Scheme 1).
Ru nanoparticles are not only ultra‐small with nearly
monodisperse sizes but also have a strong affinity with
SCHEME 1 Synthesis route to
Fe3O4@SiO2/APTS/Ru nanospheres

LIU ET AL.
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assigned to the C─H bonds from the silane APTS. Contri-
butions from the ─NH2 group were probably overlapped
with vibration bands related to silanol groups and
adsorbed water.[30] This result suggests that the surface
modification of the Fe3O4@SiO2 core–shell nanospheres
was successfully accomplished.
FIGURE 2 TEM images of Fe3O4 (a), Fe3O4@SiO2 (b) and Fe3O4@SiO2/APTS/Ru (c), and Ru particle distribution (d). TEM image of
Fe3O4@SiO2/Ru (e), and Ru particle distribution (f)

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LIU ET AL.
Figure 2 shows transmission electron microscopy
particle surface, some major aggregates of Ru nanoparti-
cles were also obviously observed on the surface of
Fe3O4@SiO2.
(TEM) images of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2/
APTS/Ru, as well as the particle distributions of Ru. It
is easily observed in Figure 2a that the diameter of the
Fe3O4 particles was ca 270 nm with a narrow size distri-
bution. As shown in Figure 2b, the core–shell structure
of Fe3O4@SiO2 can be clearly seen, with a uniform silica
shell deposited on the Fe3O4 particles with a thickness of
ca 28 nm. As illustrated in Figure 2c, numerous
monodispersed Ru nanoparticles (2.5 nm) were uniformly
decorated on the surface of Fe3O4@SiO2, which was
mainly due to the amino groups effectively preventing
the aggregation of the Ru nanoparticles, ensuring that
the Ru nanoparticles were not easily lost when reused
in liquid‐phase catalytic reactions.[31] Figure 2e shows
that when no affinity ligands were used, although many
of the Ru nanoparticles were located on the Fe3O4@SiO2
The crystallinity and phase composition of the
resulting products were investigated using powder X‐ray
diffraction (XRD). Figure 3 displays the wide‐angle XRD
patterns of the samples. As shown in Figure 3a, all the
peaks were in agreement with the face‐centered cubic
structure of Fe3O4. The diffraction peaks of Fe3O4 can
be indexed to cubic Fe3O4 (JCPDS: 65‐3107). After coat-
ing with SiO2, a new broad peak at ca 23° appeared
because of the existence of amorphous silica. For the
Fe3O4@SiO2/APTS/Ru samples, in addition to the Fe3O4
diffraction peaks, a tiny peak at 44° was distinguishable,
owing to the characteristic diffraction of Ru(0). The
XRD results indicated that the Ru nanoparticles had been
successfully loaded onto the surface of the Fe3O4@SiO2/
APTS microspheres. The average size of the Ru crystal-
lites, calculated using the Scherrer equation, was ca
2.5 nm, in good agreement with the average particle size
derived from TEM analysis.[32]
To better understand the composition of
Fe3O4@SiO2/APTS/Ru, we carried out X‐ray photoelec-
tron spectroscopy (XPS) analysis. Figure 4 shows the
XPS spectra of the surface of the Fe3O4@SiO2/APTS/Ru.
In Figure 4, the Ru binding energy of Fe3O4@SiO2/
APTS/Ru exhibited three strong peaks centered at 280.2,
461.5 and 484.5 eV, which were assigned to Ru 3d5/2
,
Ru 3p3/2 and Ru 3p1/2, respectively. The results from
XPS revealed that Ru(0) existed in its elementary state.
This was consistent with the XRD results.[33]
The magnetic properties of Fe3O4, Fe3O4@SiO2 and
Fe3O4@SiO2/APTS/Ru nanoparticles were investigated
using a vibrating sample magnetometer. The results are
shown in Figure 5. All samples showed hysteresis loops
without any detectable remanence, reflecting the
superparamagnetic properties of the samples. The satura-
tion magnetization values of Fe3O4@SiO2 and
FIGURE 3 XRD patterns of Fe3O4 (a), Fe3O4@SiO2 (b) and
Fe3O4@SiO2/APTS/Ru (c)
FIGURE 4 XPS spectra of Fe3O4@SiO2/APTS/Ru

LIU ET AL.
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FIGURE 6 Photographs of catalyst dispersed in reaction mixture
(a) and magnetic separation of catalyst from reaction medium (b)
FIGURE 5 Magnetization curves of Fe3O4 (a), Fe3O4@SiO2 (b)
and Fe3O4@SiO2/APTS/Ru (c)
the Fe3O4@SiO2/APTS/Ru nanoparticles showed fast sep-
aration under an applied magnetic field and quick disper-
sion through a slight shake when the magnetic field was
removed.
Fe3O4@SiO2/APTS/Ru were 43.6 and 41.1 emu g−1
,
respectively. The decrease in the saturation magnetiza-
tion suggested the presence of some Ru particles on the
surface of the magnetic supports. Even with this reduc-
tion in the saturation magnetization, the catalyst was still
efficiently and easily separated from solution with the
help of an external magnetic force. As a result of the
superparamagnetic properties and high magnetization,
2.2 | Catalyst testing for hydrogenation
reactions
Here, we chose the hydrogenation of a variety of aromatic
nitro compounds as a model to further study the
TABLE 1 Hydrogenation of various substrates catalyzed by Fe3O4@SiO2/APTS/Rua
Entry
Substrate
Product
Time (min)
Yield (%)
TOF (h−1
)
TON
1
40
>99
>99b
96c
86d
198
197
192
174
131
131
128
116
2
3
4
5
6
40
40
40
40
40
95
190
196
196
197
198
127
130
130
131
131
>99
98
>99
>99
7
8
40
40
>99
98
197
196
131
130
aReaction conditions: Fe3O4@SiO2/APTS/Ru, 0.05 g; loading of metal nanoparticles, 10 wt%; H2, 1 atm; room temperature; substrate, 5.0 mmol; solvent, 5 ml.
bYield after five runs.
cHydrogenation with 10% Ru/C; the amount of catalysts was decided according to the same metal loading.
dYield after five runs catalyzed by Ru/C.

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LIU ET AL.
FIGURE 7 TEM images of Fe3O4@SiO2/APTS/Ru (a) and Ru/C (b) after five runs of hydrogenation
performance of the Fe3O4@SiO2/APTS/Ru nanospheres
(S1). The reactions were carried out in ethanol at room
temperature and under 1 atm of H2. As evident from
Table 1, we observed that the catalyst was very active
for the hydrogenation reaction under mild conditions.
The catalytic hydrogenation of aromatic nitro compounds
produced yields of over 95%. In contrast, commercial 10%
Ru/C was used to catalyze the hydrogenation reaction of
nitrobenzene. The first yield was equal to that for
Fe3O4@SiO2/APTS/Ru. The good catalytic activity of the
Fe3O4@SiO2/APTS/Ru nanospheres was derived from
their ultra‐small sizes and highly dispersed Ru nanoparti-
cles. The ultra‐small sizes and highly dispersed Ru nano-
particles resulted in a high specific surface area, allowing
the aromatic nitro compound ions to easily access the Ru
nanoparticles that serve as an electron relay in the system
for a reductant, thus leading to a high rate of reduc-
tion.[34] Furthermore, the reusability of Fe3O4@SiO2/
APTS/Ru was assessed in catalyzing the hydrogenation
of nitrobenzene. After completion of the reaction, the cat-
alyst was recovered in a facile manner from the reaction
mixture using a permanent magnet (Figure 6). A satisfac-
tory yield (>99%) was still obtained even after the catalyst
had been reused five times, the nitrobenzene conversion
being 99.7% and the aniline selectivity 99.3%. The good
reusability of Fe3O4@SiO2/APTS/Ru was attributed to
the strong stability of the Ru nanoparticles on
Fe3O4@SiO2/APTS, which originates from the presence
of multiple amino functional groups on the
Fe3O4@SiO2/APTS shell, and to the very limited loss of
the Ru nanoparticles. The loading of the amino groups
in the catalyst was 2.1%, determined through the XPS
characterization technique. After the fifth reaction cycle,
the loading of the amino groups was 2.0%, nearly no loss
of amino groups. In addition, the catalyst was completely
separated by applying an external magnetic field. No
appreciable Ru leaching into the organic phase was
observed after the fifth reaction cycle, as indicated by
the inductively coupled plasma atomic emission spec-
trometry (ICP‐AES) analysis result of 0.79 ppm. The
TEM image (Figure 7a) of the catalyst obtained after the
fifth cycle of the reaction did not show a significant
change in catalyst morphology. The Ru nanoparticles
remained highly dispersed on Fe3O4@SiO2/APTS with
an average size of 3.2 0.5 nm. In our study, APTS might
have served as a strong affinity ligand for high Ru disper-
sion, effectively preventing the agglomeration of Ru
nanoparticles during the reaction.
For comparison, Ru/C was also separated by
centrifuging and recycling in the hydrogenation reaction.
The Ru content adopted during the recycling experiments
in the mixed solution was kept the same for both the
Ru/C catalyst and the Fe3O4@SiO2/APTS/Ru composite
microspheres. As clearly evident from Table 1, the yield
after five runs using Ru/C was only 86%, the nitroben-
zene conversion was 88.1% and the aniline selectivity
was 97.6%. This result was probably due to the aggrega-
tion of the Ru nanoparticles, which might have happened
after each cycle (Figure 7b). In addition, the Ru content
in the catalyst was significantly reduced according to
ICP‐AES analysis, from 7.6 to 3.1%, indicating that the
active components were lost during the cycling. Thus,
the surface area of the catalyst, as well as the catalytic
sites, decreased.
3 | CONCLUSIONS
An amino‐modified core–shell structure catalyst nano-
composite, Fe3O4@SiO2/APTS/Ru, was prepared. Ru

LIU ET AL.
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catalytic performance for the hydrogenation of a variety
of aromatic nitro compounds, even at room temperature,
giving yields of over 95%. Furthermore, Fe3O4@SiO2/
APTS/Ru was easily recovered using a magnetic field
and directly reused for at least five cycles without signifi-
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ACKNOWLEDGEMENTS
This work was financially supported by the Taishan
Scholars Projects of Shandong (ts201511033), the
National Natural Science Foundation of China
(31700517), the Open Fund of the Biomass‐energy and
Materials Key Laboratory of Jiangsu (JSBEM201802),
the Key R&D project of Shandong (2017GGX70102),
13th National 5‐year R&D plans in rural areas
(2016YFD0600804). The authors are grateful for the
financial support.
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[21] P. Panja, P. Das, K. Mandal, N. R. Jana, ACS Sustain. Chem.
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The authors declare that they have no conflict of interest.
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How to cite this article: Liu Y, Lv M, Li L, et al.
Synthesis of a highly active amino‐functionalized
Fe3O4@SiO2/APTS/Ru magnetic nanocomposite
catalyst for hydrogenation reactions. Appl
Organometal Chem. 2019;e4686. https://doi.org/
10.1002/aoc.4686
SUPPORTING INFORMATION
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