Magnetically separable mesoporous silica-supported palladium nanoparticle-catalyzed selective hydrogenation of naphthalene to tetralin

Article abstract of DOI:10.1002/aoc.5204

A novel magnetically separable mesoporous silica-supported palladium catalyst was designed and prepared for the selective hydrogenation of naphthalene to tetralin, which is an important transformation from a practical viewpoint. In the catalyst, Pd nano grains were dispersed uniformly and protected within the mesoporous silica shells being coated on the Fe₃O₄ core, so that the durability of the catalyst could be significantly improved.

Full text of DOI:10.1002/aoc.5204

Received: 1 July 2019
DOI: 10.1002/aoc.5204
Revised: 26 July 2019
Accepted: 4 August 2019
F U L L P A P E R
Magnetically separable mesoporous silica‐supported
palladium nanoparticle‐catalyzed selective hydrogenation
of naphthalene to tetralin
Yonghui Yang1,2 | Bolian Xu1,3 | Jie He2 | Jianjun Shi2 | Lei Yu4
| Yining Fan1,3
1 Key Laboratory of Mesoscopic Chemistry
A novel magnetically separable mesoporous silica‐supported palladium catalyst
was designed and prepared for the selective hydrogenation of naphthalene to
tetralin, which is an important transformation from a practical viewpoint. In
the catalyst, Pd nano grains were dispersed uniformly and protected within
the mesoporous silica shells being coated on the Fe3O4 core, so that the dura-
bility of the catalyst could be significantly improved.
of Ministry of Education, School of
Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China
2 School of Chemical Engineering, Anhui
University of Science and Technology,
Huainan 232001, China
3 Nanjing University‐Yangzhou Chemistry
and Chemical Engineering Institute,
Yangzhou 211400, China
KEYWORDS
4 School of Chemistry and Chemical
Engineering, Yangzhou University,
Yangzhou 225002, China
hydrogenation of naphthalene, magnetically separable catalyst, mesoporous silica, palladium, tetralin
Correspondence
Lei Yu, School of Chemistry and Chemical
Engineering, Yangzhou University,
Yangzhou 225002, China.
Email: yulei@yzu.edu.cn
Yining Fan, Key Laboratory of Mesoscopic
Chemistry of Ministry of Education, School
of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China.
Email: ynfan@nju.edu.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 21773107;
Priority Academic Program Development
(PAPD) of Jiangsu Higher Education
Institutions; Jiangsu Provincial Six Talent
Peaks Project, Grant/Award Number:
XCL‐090; Natural Science Foundation of
Jiangsu Province, Grant/Award Number:
BK20181449; Natural Science Foundation
of Yangzhou, Grant/Award Number:
YZ2016137
1 | INTRODUCTION
industrial viewpoints.[1] In this regard, partial hydroge-
nation is a practical method to consume naphthalene
Naphthalene is an ecotoxic molecule that abundantly
exists in coal tar. Converting this chemical into useful
products is meaningful from both environmental and
because the product tetralin generated following this
reaction is a useful industrial intermediate that is com-
prehensively used as a solvent in industrial production,
Appl Organometal Chem. 2019;e5204.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5204

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YANG ET AL.
as an intermediate in agricultural chemical synthesis,
and as a good hydrogen donor in fuel cells.[2]] In indus-
trial catalysis, a variety of metal catalysts, such as Pd,[3]
Pt,[[4] Rh,[5] Ru,[6] Ir,[7] and Ni,[8] have been developed
for the hydrogenation/dehydrogenation reactions, with
alumina, zeolite, or activated carbon usually employed
as the catalyst support.[9] However, with the dwindling
supplies of the metal resources in recent years, the cost
of the catalyst is increasing. Therefore, developing easily
recyclable and reusable catalyst for the transformation is
crucial for industrial application purpose.
2.2 | Preparation of core–shell
microspheres and catalyst
2.2.1 | Synthesis of Fe3O4 particles
About 2.7 g of FeCl3·6H2O and 7.2 g of anhydrous sodium
acetate were added to 100 mL ethylene glycol to obtain a
homogeneous yellow solution under magnetic stirring.
The solution obtained was then placed in a Teflon‐lined
stainless‐steel autoclave (200 mL) and sealed. After heating
at 200 °C for 8 hr, the solution was cooled down to room
temperature. The prepared Fe3O4 particles were magneti-
cally separated, subsequently washed with water and etha-
nol, and dried in vacuum at 60 °C for 12 hr before use.
Nano catalysts have attracted much attention owing to
their high efficiency as well as the green features of the
reaction processes free of ligands or additives.[10] In field
applications, magnetic materials, such as Fe3O4, have
been widely used as the support for catalytic metal nano-
particles (NPs) to achieve convenient magnetic separation
as they enable more efficient catalyst recovery process,
compared with the traditional homogeneous or heteroge-
neous catalysts.[11] However, aggregation of active metal
NPs may result in catalyst deactivation and is a major
issue hindering their industrial application. To overcome
this limitation, the use of magnetically separable mesopo-
rous silica materials has been investigated,[12] which
afford additional opportunities to improve the related
magnetic nano catalysts, in which the catalytic sites can
be well protected within mesoporous silica shells to
enhance the durability of the catalyst. However, there is
no report on using a magnetic catalyst for naphthalene
hydrogenation yet, let alone the application of magneti-
cally separable mesoporous catalysts for the reaction. In
most studies from our group, green reactions were uti-
lized for synthesizing useful fine chemicals,[13] and in this
work, we chose to study the selective hydrogenation of
naphthalene to tetralin due to its great significance in
various industries. Recently, we developed a novel mag-
netically separable mesoporous silica‐supported palla-
dium nano catalyst (Fe3O4@SiO2@mSiO2–Pd) for the
transformation.
2.2.2 | Synthesis of Fe3O4@SiO2@mSiO2
core–shell microspheres
Fe3O4 particles weighing 0.10 g were immersed in
50 mL aqueous HCl (0.1 mol/L) for 10 min under
ultrasonication conditions. The Fe3O4 particles were
then washed with deionized water and dispersed in a
mixture of ethanol (80 mL), deionized water (20 mL),
and aqueous ammonia (1.0 mL; 28 wt%) under
ultrasonication. Then, 0.03 g of TEOS was added to this
mixture, which was subsequently stirred at room tem-
perature for 6 hr. The microspheres thus obtained were
separated and washed with ethanol and water, and re‐
dispersed in a mixed solution containing 0.30 g of
CTAB, 80 mL of deionized water, 1.00 g of aqueous
ammonia (28 wt%), and 60 mL of ethanol. The mixture
was homogenized for 0.5 hr to obtain a uniform disper-
sion. TEOS (0.40 g) was then added dropwise into this
dispersion with continuous stirring. After 6 hr, the prod-
uct was magnetically separated and washed with etha-
nol and water to remove nonmagnetic by‐products.
Finally, CTAB templates in the materials were removed
by extraction. During the process, the purified micro-
spheres were re‐dispersed in 60 mL of acetone and
heated at a reflux temperature for 48 hr to remove the
CTAB template. The aforementioned extraction process
was repeated three times, and the microspheres
obtained were washed with deionized water to produce
the Fe3O4@SiO2@mSiO2 microspheres.
2 | EXPERIMENTAL
2.1 | Materials
Iron (III) chloride hexahydrate, ethylene glycol, anhy-
drous sodium acetate, anhydrous ethanol, aqueous
ammonia solution (28%), tetraethoxysilane (TEOS),
cetyltrimethylammonium bromide (CTAB), acetone,
hydrochloric acid, naphthalene, tridecane, and palladium
chloride (PdCl2) were purchased from Beijing Chemical
Reagent Company (Beijing, China). All chemicals were
of analytical grade and used directly without further puri-
fication. Deionized water was used in all experiments.
2.2.3 | Preparation of
Fe3O4@SiO2@mSiO2–Pd
Fe3O4@SiO2@mSiO2 microspheres were immersed in an
aqueous hydrochloric acid solution of PdCl2 (PdCl2 con-
centration was 5.6 × 10−3 mol/L and HCl concentration
was 0.05 mol/L) at room temperature overnight. In this

YANG ET AL.
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process, about 2 wt.% of Pd was incorporated into the
materials. The materials were initially dried in vacuum
at 110 °C for 5 hr, and then calcined in air at 500 °C for
5 hr. The calcined catalyst powders were then reduced
in flowing mixed H2–N2 gas containing 6% of H2
(80 mL/min; standard temperature and pressure), with
the temperature gradually increased from room tempera-
ture to 300 °C at the heating rate of 2 °C/min and then
held at 300 °C for 2 hr.
2.3 | Evaluation of catalytic activity
The Fe3O4@SiO2@mSiO2–Pd‐catalyzed hydrogenation
reaction of naphthalene was performed in an autoclave
using tridecane as the solvent at 240 °C (pressure under
6.5 MPa). Reaction details such as amounts of solvent,
reactant and catalyst, and the reaction time are listed in
Table 1. The reaction mixture was stirred at a rate of
600 rpm. The reaction products were analyzed with an
Agilent 6890 N gas chromatograph equipped with a flame
ionization detector. The catalyst could be magnetically
separated after the reaction, and washed with tridecane
before using it in subsequent reactions under the same
conditions.
2.2.4 | Characterization of materials
X‐ray diffraction (XRD) patterns of the catalyst powder
were recorded on a Philips X’pert PRO diffractometer
with Cu‐Ka radiation (40 kV, 40 mA). Nitrogen
adsorption–desorption isotherms were measured at 77 K
with a Micromeritics Tristar 3020 analyzer. Before mea-
surements, the samples were degassed in vacuum at
200 °C for at least 6 hr. The Brunauer–Emmett–Teller
(BET) method was utilized to calculate the specific sur-
face areas. Using the Barrett–Joyner–Halenda (BJH)
model, the pore volumes and pore size distributions were
derived from the desorption branches of isotherms.
Transmission electron microscopy (TEM) analysis was
performed on a JEM‐2100 (HR) microscope operated at
200 kV. The specimens were prepared by ultrasonically
suspending the sample in ethanol and placing a drop of
the solution on a copper grid with amorphous carbon
film. Magnetic properties of the samples were measured
using the SQUID magnetic property measurement system
(MPMS‐XL‐7).
3 | RESULTS AND DISCUSSION
3.1 | Preparation of catalyst
The core–shell‐structured Fe3O4@SiO2@mSiO2–Pd cata-
lyst was synthesized via the method illustrated in
Figure 1. In the process, the uniform magnetic Fe3O4
microspheres (I) were initially prepared in a stainless‐
steel autoclave by applying a surfactant‐free solvothermal
method. They were then deposited with double layers of
silica through the sol–gel approach with TEOS as well
as the surfactant‐templating approach using CTAB as
the template.[14] After removing CTAB by acetone extrac-
tion, the uniform mesoporous silica phase with cylindri-
cal channels at the outer layer of the material
was
formed,
affording
the
well‐dispersed
Fe3O4@SiO2@mSiO2 microspheres (II) that were then
TABLE 1 Catalytic performance of the mesoporous Fe3O4@SiO2@mSiO2–Pd catalyst in the hydrogenation of naphthalenea
Entry
Naphthalene (g)
Catalyst (g)
t (hr)
Conversion (%)b
Selectivity (%)
Tetralin (%)
Decalin (%)
1
2
3
4
5
6
5
5
0.1
0.3
0.3
0.3
0.3
0.3
3
3
8
8
3
3
19.1
36.2
65.1
36.3
97.9
99.8
100
100
100
100
96.9
78.5
0
0
5
0
10
2
0
3.1
21.5
1
aThe reactions were performed in 120 mL of tridecane solvent.
bConversion ratio of naphthalene.

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YANG ET AL.
FIGURE 1 Diagram for the preparation of the core–shell‐structured Fe3O4@SiO2@mSiO2–Pd catalyst. CTAB, cetyltrimethylammonium
bromide; TEOS, tetraethoxysilane
immersed into a palladium salt solution to load Pd2+
.
Fe3O4@SiO2@mSiO2–Pd, both Fe3O4 and amorphous sil-
ica signals emerged (Figure 2a, curve c). Although induc-
tively coupled plasma‐mass spectrometry (ICP‐MS)
analysis indicated that the material contained about
1.96% of Pd, the characteristic peaks of Pd were not
observed in its XRD pattern (Figure 2a, curve c), showing
that the Pd NPs were well protected and dispersed uni-
formly inside the mesoporous silica shells. In the low‐angle
XRD patterns of the materials (Figure 2b, curves a–d), the
(100) diffusion peaks at 2θ = 2.4° demonstrated that both
Fe3O4@SiO2@mSiO2 and Fe3O4@SiO2@mSiO2–Pd pos-
sessed mesoporous structures, which might be formed in
the outer silica shell.[14]
Upon hydrogenation of the mixture, Pd2+ was reduced
to Pd NPs, thus eventually generating the
Fe3O4@SiO2@mSiO2–Pd catalyst (III). The unique
microstructure of the outer layer mesoporous channels
in this material might facilitate adsorption and release
of large‐size guest objects during catalytic reactions.
3.2 | Characterization of catalyst
3.2.1 | X‐ray diffraction analysis
XRD analysis was performed to characterize the as‐
prepared materials (Figure 2). In the wide‐angle XRD pat-
tern, signals of the high‐crystalline cubic spinel structure of
the as‐prepared Fe3O4 agreed well with the standard Fe3O4
(cubic phase) XRD spectrum (Figure 2a, curve a, JCPDS
89–0688).[14,15] The XRD pattern of Fe3O4@SiO2@mSiO2
microspheres prepared by the Stöber process is similar to
that of Fe3O4 (Figure 2a, curve b vs. a), but an obvious dif-
fraction peak at 2θ = 15°–25°, which was attributed to
amorphous silica, was observed. In the XRD pattern of
3.2.2 | N2 adsorption–desorption
experiments
N2 adsorption–desorption experiments of the materials
were performed and the isotherms are depicted in
Figure 3. A linear increase was found in the amount of
nitrogen adsorbed at a low relative pressure, which could
be classified as a type of H2 hysteresis loop according to
FIGURE 2 (a) Wide‐angle X‐ray diffraction (XRD) patterns and (b) low‐angle XRD patterns of Fe3O4 particles (curve a),
Fe3O4@SiO2@mSiO2 microspheres (curve b), Fe3O4@SiO2@mSiO2–Pd catalyst (curve c), and pure mesoporous shell mSiO2 (curve d)

YANG ET AL.
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FIGURE 3 (a) N2 adsorption–desorption isotherms and (b) pore‐size distributions of Fe3O4 (curve a), Fe3O4@SiO2@mSiO2 (curve b), and
Fe3O4@SiO2@mSiO2–Pd (curve c)
the IUPAC classification of physisorption isotherms
and hysteresis In the isotherms,
loops.[16]
can be seen, the Fe3O4 particles were dispersed in a
monolayer and had similar mean diameters (about
380 nm). They also had a spherical morphology and
contained a cluster of magnetite NPs measuring about
20 nm, which are consistent with the result obtained
using the Scherrer equation (Figure 4a). After treatment
with the sol–gel approach based on the hydrolysis and
condensation of TEOS, the mesoporous silica layer‐
both Fe3O4@SiO2@mSiO2 and Fe3O4@SiO2@mSiO2–Pd
exhibited IV‐type curves (Figure 3a, curves b and c). As
calculated by the BJH method, the mesoporous size distri-
bution of the materials exhibited sharp peaks centering
on about 2.4–2.6 nm uniformly (Figure 3b, curves b
and c). The specific surface areas of Fe3O4,
Fe3O4@SiO2@mSiO2, and Fe3O4@SiO2@mSiO2–Pd were
calculated to be 30, 313, and 302 m2/g, respectively,
according to the BET method (see Table S1 in the
supporting information). The related BJH desorption
covered
core–shell‐structured
Fe3O4@SiO2@mSiO2
microspheres were obtained. The silica layer could be
observed as the light borders around the Fe3O4 particles
in the TEM image (Figure 4b). In Fe3O4@SiO2@mSiO2–
Pd, the Pd NPs were embedded tightly within the silica
layer on the surface of Fe3O4 core, and they were
observed as small black dots in the light areas in the
TEM image (Figure 4c).
cumulative
volumes
of
the
aforementioned
materials were 0.06, 0.26, and 0.24 cm3/g, respectively
(see Table S1). The results showed that the silica coating
dramatically enhanced both specific surface area
(313 vs. 30 m2/g) and pore size (0.26 vs. 0.06 cm3/g) of
the materials, whereas the loading of Pd NPs partially
blocked the mesoporous channels and led to
slight decrease of the specific surface area (302 vs. 313
m2/g) and pore size (0.24 vs. 0.26 cm3/g) of the materials.
a
3.2.4 | Magnetic features of the catalyst
The magnetic hysteresis loops indicated that the Fe3O4‐
cored materials possessed superparamagnetism (Figure 5
a). Both magnetization and demagnetization curves
were consistent. No hysteresis phenomenon was
observed, with the remnant magnetization and coerciv-
ity equaling zero. The maximum saturation magnetiza-
tion of the as‐prepared Fe3O4 was 82.2 emu/g
(Figure 5a, curve a). The hysteresis loops of
3.2.3 | Transmission electron microscopy
analysis
The TEM images of Fe3O4, Fe3O4@SiO2@mSiO2, and
Fe3O4@SiO2@mSiO2–Pd are presented in Figure 4. As
FIGURE 4 Transmission electron microscopy images: (a) Fe3O4, (b)Fe3O4@SiO2@mSiO2, and (c) Fe3O4@SiO2@mSiO2–Pd

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YANG ET AL.
FIGURE 5 (a) Vibrating‐sample
magnetometer hysteresis loops: Fe3O4
(curve a), Fe3O4@SiO2@mSiO2 (curve b),
and Fe3O4@SiO2@mSiO2–Pd (curve c).
(B) Photo of the separation process of the
Fe3O4@SiO2@mSiO2–Pd catalyst
Fe3O4@SiO2@mSiO2 and Fe3O4@SiO2@mSiO2–Pd were
substantially in accordance, showing that loading of Pd
NPs did not affect the magnetic features of the material.
Maximum saturation magnetizations of the mesoporous
materials were 50.4 and 47.6 emu/g, respectively
(Figure 5a, curves b and c). As shown in Figure 5b,
the Fe3O4@SiO2@mSiO2–Pd catalyst could be easily
separated from the reaction liquid by a 30‐sec external
magnetic treatment.
3.3 | Fe3O4@SiO2@mSiO2–Pd‐catalyzed
hydrogenation of naphthalene to tetralin
3.3.1 | Evaluation of catalyst performance
The hydrogenation reactions of naphthalene were per-
formed over the Fe3O4@SiO2@mSiO2–Pd material to
evaluate its catalytic activity (Table 1). Upon heating 5 g
of naphthalene with 0.1 g of catalyst in 120 mL of
FIGURE 6 Recycling catalytic
performance in hydrogenation of
naphthalene over Fe3O4@SiO2@mSiO2–
Pd catalyst (reaction conditions: 120 mL of
tridecane, 1 g of naphthalene, 0.3 g of the
catalyst, T = 240 °C, p = 6.5 MPa)

YANG ET AL.
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tridecane solvent at 240 °C (pressure under 6.5 MPa H2)
for 3 hr, tetralin was obtained with 100% selectivity.
About 19.1% of naphthalene was converted during the
process (Table 1, entry 1). The reaction could be acceler-
ated using higher volumes of the catalyst and extending
the reaction time, which will increase the naphthalene
conversion ratio (Table 1, entries 2 and 3). However,
enhancing the initial concentration of naphthalene
reduced its conversion ratio (Table 1, entry 4). By con-
trast, the conversion ratio of naphthalene was enhanced
in reactions with reduced substrate concentration
(Table 1, entries 5 and 6). The reaction with initial naph-
thalene concentration of 0.017 g/mL was found to be
more preferable, yielding 96.9% tetralin; additionally,
3.1% of the over‐reduction by‐product decalin was pro-
duced (Table 1, entry 5).
ACKNOWLEDGEMENTS
This work was supported by National Natural Science
Foundation of China (21773107), Natural Science Foun-
dation of Yangzhou (YZ2016137), Natural Science Foun-
dation of Jiangsu Province (BK20181449), Jiangsu
Provincial Six Talent Peaks Project (XCL‐090), and Prior-
ity Academic Program Development (PAPD) of Jiangsu
Higher Education Institutions.
ORCID
Lei Yu https://orcid.org/0000-0001-5659-7289
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How to cite this article: Yang Y, Xu B, He J, Shi
J, Yu L, Fan Y. Magnetically separable mesoporous
silica‐supported palladium nanoparticle‐catalyzed
selective hydrogenation of naphthalene to tetralin.
Appl Organometal Chem. 2019;e5204. https://doi.
org/10.1002/aoc.5204