Magnetically recyclable Ru immobilized on amine-functionalized magnetite nanoparticles and its high selectivity to prepare cis-pinane

Article abstract of DOI:10.1016/j.molcata.2016.09.007

cis-Pinane was efficient and selective prepared through the hydrogenation of α-pinene base on Ru nanoparticles stabilized by amine-functionalized magnetite nanoparticles (Fe₃O₄/NH₂/Ru). The effects o`f changing carbon chain length on amine-functionalized catalyst formation have also been investigated. The characterization results showed that Ru nanoparticles could be efficient loaded by 1, 6-hexanediamine functionalized magnetite nanoparticles. At the same time, Fe₃O₄/1, 6-hexanediamine/Ru had good superparamagnetic properties and that the introduction of the amine-functionalized improved the monodispersity, morphological regularity and size uniformity of the Ru nanoparticles. The Fe₃O₄/1, 6-hexanediamine/Ru catalyst was completely recoverable with the simple application of an external magnetic field and the catalytic efficiency showed no obvious loss for the hydrogenation of α-pinene even after ten repeated cycles.

Full text of DOI:10.1016/j.molcata.2016.09.007

Journal of Molecular Catalysis A: Chemical 424 (2016) 269–275
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Magnetically recyclable Ru immobilized on amine-functionalized
magnetite nanoparticles and its high selectivity to prepare cis-pinane
a
b
b
a
b,∗
Yue Liu , Lu Li , Shiwei Liu , Congxia Xie , Shitao Yu
a
Key Laboratory of Sensor Analysis of Tumor Marker Ministry of Education, College of Chemistry and Molecular Engineering, Qing dao University of Science
and Technology, 53 Zhengzhou Road, Qingdao 266042, People’s Republic of China
College of Chemical Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao 266042, People’s Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2016
Received in revised form 30 August 2016
Accepted 4 September 2016
Available online 7 September 2016
cis-Pinane was efficient and selective prepared through the hydrogenation of ␣-pinene base on
Ru nanoparticles stabilized by amine-functionalized magnetite nanoparticles (Fe3O4/NH2/Ru). The
effects o‘f changing carbon chain length on amine-functionalized catalyst formation have also been
investigated. The characterization results showed that Ru nanoparticles could be efficient loaded by 1, 6-
hexanediamine functionalized magnetite nanoparticles. At the same time, Fe3O4/1, 6-hexanediamine/Ru
had good superparamagnetic properties and that the introduction of the amine-functionalized improved
the monodispersity, morphological regularity and size uniformity of the Ru nanoparticles. The Fe3O4/1,
Keywords:
Fe3O4/1, 6-hexanediamine/Ru
Magnetic separation
6
-hexanediamine/Ru catalyst was completely recoverable with the simple application of an external
magnetic field and the catalytic efficiency showed no obvious loss for the hydrogenation of ␣-pinene
even after ten repeated cycles.
␣
-Pinene
Hydrogenation
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
molecular weight 5800) micelles distinct improved the molar ratio
between cis-pinane and tran-pinane produced from the hydrogena-
tion of ␣-pinene [9]; 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 emulsified mix-
ture. 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 con-
siderations indicate the importance of developing an inert support
to stabilize Ru nanoparticles and to find a convenient separation
method for the products of the ␣-pinene hydrogenation reaction.
cis-Pinane is a distinctly important industrial intermediate and
widely used for the synthesis of specialty chemicals and phar-
maceuticals such as dihydromyrcenol, linalool and other terpene
series species [1]. Conventionally, cis-pinane is industrially pre-
pared through the hydrogenation of ␣-pinene in the presence of
some noble metal catalysts, such as Pd/C or Pt/C [2,3]; however,
both catalysts show a low selectivity towards cis-pinane. The iden-
tification of highly active and selective catalysts is a crucial aspect
for the efficiency of the process. Ru-based catalysts have been found
to exhibit good catalytic performance towards hydrogenation of
compounds such as levulinic acid [4], phenol [5], aldimines [6]
and ␥-valerolactone [7]. Ru-based catalysts are also used in the
hydrogenation of ␣-pinene. RuCl3 dissolved in water showed good
catalytic properties, with 99.7% conversion of ␣-pinene and 96.3%
selectivity towards cis-pinane [8]. The main drawback observed
in the use of this catalyst was its deactivation over time due
to the agglomeration of Ru nanoparticles originating from the
Recently, immobilizing noble metal nanocatalysts on Fe O4
3
magnetic support have been widely used due to their good separa-
tion performance in the presence of an appropriate magnetic field
[10–18]. Pure Fe O4 particles, lacking functional groups on their
3
surface, hardly adsorb any metal nanoparticles [19]. Considering
the above limitations, coating materials such as silica, polymers,
and carbon should therefore be used as shells, to add the neces-
hydrogenation of RuCl . The use of Ru nanoparticles protected
sary functional groups to the Fe O4 spheres [20–22]. However,
3
3
by poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene
oxide) triblock copolymer P123 (PEO20–PPO70–PEO20, average
a drawback is that the coating materials need further chemical
functionalization, and the process is relatively complicated. There-
fore, developing a simple and facile approach to modify magnetic
nanoparticles is highly desirable. Very recently, much attention has
been focused on amine functionalization, since amines are well
known to stabilize nanoparticles against aggregation without dis-
∗ Corresponding author.
E-mail address: yushitaoqust@126.com (S. Yu).
http://dx.doi.org/10.1016/j.molcata.2016.09.007
1
381-1169/© 2016 Elsevier B.V. All rights reserved.

2
70
Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 269–275
Scheme 1. Synthesis route of Fe3O4/1, 6-hexanediamine/Ru magnetite nanoparticles.
turbing their desirable properties [23–25]. The synthesis process
was simple and facile to obtain amine-functionalized magnetic
nanoparticles, allowing an easy loading of noble metal nanopar-
ticles on its surface. At the same time, amine-functionalized
magnetic nanoparticles catalysts have been found to show the
high catalytic activity towards hydrogenation [26,27], Heck reac-
tion [28,29], Suzuki reaction [30,31] and so on. In the present work,
pressure [32]. The Fe O /1, 6-hexanediamine/Ru particles were
3 4
analyzed by inductively coupled plasma-atomic emission spec-
trometry (ICP-AES) to determine the content of Ru nanoparticles.
Fe O /1, 6-hexanediamine/Pd, Fe O /1, 6-hexanediamine/Rh and
3
4
3
4
Fe O /1, 6-hexanediamine/Pt particles were prepared using a sim-
3
4
ilar method.
Fe O /1, 6-hexanediamine/Ru nanoparticles were successfully pre-
3
4
2
.3. Characterizations
pared using 1, 6-hexanediamine as the functional materials in the
hydrothermal treatment. The catalyst obtained in this way was
used in the hydrogenation of ␣-pinene.
Some samples of the intermediates and end-products were
characterized using transmission electron microscopy (TEM), X-ray
diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR),
ICP-AES, X-Ray photoelectron spectroscopy (XPS) and quantum
design vibrating sample magnetometer. TEM analyses were car-
ried out using a JEOL-1200 microscope, operated at 100 kV. FT-IR
2
. Experimental
2.1. Materials
−
1
spectra were recorded in the wavelength range of 4500–400 cm
The chemicals used in the course of the work were: ␣-
on a Nicolet 510P FT-IR spectrometer using the KBr method. XRD
patterns were collected on a Rigaku D/max-2400 diffractometer
with Cu-Ka anode radiation at 40 kV and 100 mA in the 2␪ range
of 10–90 with a scan speed of 2 /min. XPS was carried out on a
PHI-5702 instrument (Thermo Fisher Scientific-U. K; X-ray source:
monochromated Al Ka, hv = 1486.6 eV; X-ray energy: 15 kV, 150 W;
Binding energies: C1s hydrocarbon peak at 284.60 eV). ICP-AES was
performed using a Varian Vista Pro ICP-AES instrument running at
pinene, RuCl , PdCl , FeCl ·6H O, anhydrous sodium acetate,
3
2
3
2
ethylene glycol, sodium citrate, ethanol, 1, 2-ethylenediamine, 1,
4
1
-butanediamine, 1, 6-hexanediamine, 1, 8-octanediamine and 1,
2-dodecanediamine are of analytical grade and purchased from
◦
◦
Shanghai Chemical Corp. Hydrogen (99.99 wt%) were obtained
from Qingdao Airichem Specialty Gases & Chemicals Co. Ltd. All
of these materials were commercially available and were used
without further purification. Deionized water was used for all
experiments.
1
200 W forward power. Before analysis, the samples were digested
in a mixture of HF, HCl and HNO . Magnetic measurements were
3
collected on a vibrating sample magnetometer at room tempera-
ture in an applied magnetic field sweeping from −15 to 15 kOe.
2
.2. Synthesis of Fe O -NH -Ru magnetite nanoparticles
3
4
2
The preparation of Fe O /1, 6-hexanediamine/Ru was carried
3
4
2
.4. Catalytic reaction
out in two steps, as shown in Scheme 1. To begin, the amine-
functionalized magnetite particles were prepared by a versatile
solvothermal reaction. 1.0 g of FeCl ·6H O, 2.0 g of anhydrous
The formula on hydrogenation of ␣-pinene is shown in
3
2
Scheme 2. Mixtures of different amounts of ␣-pinene and catalyst
was reacted at 160 C for 5 h under 5 MPa H2 partial pressure in a
1
reaction was completed, the catalyst was separated from the prod-
uct phase by applying an external magnet. The product phase was
analyzed using gas chromatography (GC) to determine the obtained
conversion of ␣-pinene and the selectivity towards cis-pinane. The
catalyst layer was reused directly in the recycle experiments to
sodium acetate and 6.5 g 1, 6-hexanediamine were accurately
mixed with 30 mL of ethylene glycol at room temperature for 2 h,
using a mechanical stirring system. The homogeneous mixture
was transferred to a 50 mL Teflon-lined stainless-steel auto-
◦
00 mL stainless steel reactor with mechanical stirring. After the
◦
clave and the reaction was carried out at 200 C for 8 h. The
reacted mixture was separated by applying an external mag-
netic field. The obtained magnetite particles were washed three
◦
times with water and dried at 60 C for 4 h under a 500 Pa
pressure, obtaining Fe O /1, 6-hexanediamine particles [23].
3
4
Fe O /1, 2-ethylenediamine, Fe O /1, 4-butanediamine, Fe O /1,
3
4
3
4
3
4
8
-octanediamine and Fe O /1, 12-dodecanediamine particles were
3 4
prepared using a similar method.
In the second and final step, Ru nanoparticles were loaded
onto the Fe O /1, 6-hexanediamine particles, using RuCl as the
3
4
3
precursor. 0.2 g of Fe O /1, 6-hexanediamine, 0.1 g of RuCl and
3
4
3
5
0 mL of ethanol were dispersed for 40 min in an ultrasonic oscilla-
tor. Following the dispersion, 50 mL of 0.13 mol/L NaBH4 solution
used as reducing agent were dropped into the mixture, which
reacted at room temperature for 4 h. Fe O /1, 6-hexanediamine/Ru
3
4
was separated by applying an external magnetic field, washing
◦
five times with water and drying at 60 C for 4 h under 500 Pa
Scheme 2. The formula on hydrogenation of ␣-pinene.

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 269–275
271
Fig. 1. XPS spectrum of the Fe3O4/1, 6-hexanediamine/Ru.
examine its reusability. All experiments were repeated five times
in order to determine the reproducibility of the results.
lyst was used, the hydrogenation reaction did not take place at
all (Entry 1). The Pd/C and PdCl2 showed poor catalytic perfor-
mance, with both ␣-pinene conversion and cis-pinane selectivity
below 85% (Entries 2 and 3). Ru/C and RuCl3 showed excellent cat-
alytic properties towards hydrogenation. The ␣-pinene conversion
and cis-pinane selectivity were over than 95% and 94%, respec-
tively (Entries 4 and 5). The above results indicate that Ru-based
catalysts display a higher catalytic activity than the Pd-based cat-
alyst; however, the reusability of RuCl3 was poor (Fig. S1) due
to the aggregation of the Ru nanoparticles which occurred after
each cycle (Fig. S2) [35] and to the difficulty of separating the
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 capil-
lary column (30 m × 0.25 mm × 0.25 ␮m). Quantitative analyses
were carried out using GC on an HP6890 GC equipped with a
flame ionization detectoran (FID) and an AT-5 capillary column
(
30 m × 0.25 mm × 0.25 ␮m). 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
1
◦
◦
◦
◦
80 C at a rate of 4 C/min and finally holding at 180 C for 2 min.
product from the product. Among all the Fe O4 supported cat-
3
The qualitative analysis was conducted on the basis of the hold-
ing time of the peak. The content of reactants and products was
directly obtainable by the GC chemstation system, according to the
area of each chromatograph peak. The ␣-pinene conversion was
determined as a function of the mass of ␣-pinene consumed in the
reaction. The product selectivity was calculated by Ws/WALL × 100,
where Ws is the amount of the product for which selectivity is being
evaluated and WALL is the total amount of the products, including
cis-pinane and trans-pinane.
alysts, the non-functionalized magnetite nanoparticles Pd/Fe O4
3
and Ru/Fe O4 catalysts showed the poorest catalytic performance
3
(Entries 7 and 8). This was due to the fact that the pure Fe O4
3
spheres had no functional groups on their surface hindering adsorp-
tion of the metal active center [19]. The amine-functionalized
catalysts showed the best catalytic performance for hydrogenation
(Entries 9–12). In particular, Fe O /1, 6-hexanediamine/Ru showed
3
4
an excellent catalytic activity towards hydrogenation: the ␣-pinene
conversion reached 99.2% and the cis-pinane selectivity was 97.1%
(Entry 9). In order to explain the good catalytic performance of
Fe O /1, 6-hexanediamine/Ru, FT-IR spectra, XPS and TEM anal-
3
4
3
. Results and discussion
yses were carried out. The results are shown in Figs. S3, 1 and 2
As shown in Fig. 1, the peaks present at 280.1 eV (assigned to
.
3
.1. Effects of catalysts on reaction results
Ru 3d5/2), 461.3 eV (assigned to Ru 3p3/2), and 484.4 eV (assigned
to Ru 3p1/2) suggested the presence of Ru(0) in its elementary
state [36]. In addition, Fe O /1, 6-hexanediamine contains numer-
The work started with the screening of catalysts. Pd/C and
3
4
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 cata-
lysts was compared [33,34]. The reaction results for the different
catalysts are given in Table 1. It was seen that when no cata-
ous amines groups (Fig. S3) and these functional groups served as
anchors to immobilize Ru nanoparticles and facilitate their dis-
persion by preventing them from aggregating in solution [37].
From the TEM observations (Fig. 2b), numerous monodispersed
Table 1
Effect of catalysts on the hydrogenation results.a
Entry
Catalyst
C/%
1.2 ± 0.1
S/%
1
2
3
4
5
No catalyst
Pd/C
PdCl2
Ru/C
RuCl3
RuCl3
Fe3O4/Pd
Fe3O4/Ru
Fe3O4/1,6-hexanediamine/Ru
Fe3O4/1,6-hexanediamine/Pd
Fe3O4/1,6-hexanediamine/Rh
Fe3O4/1,6-hexanediamine/Pt
69.3 ± 0.2
78.1 ± 0.5
80.2 ± 0.3
94.9 ± 0.1
95.8 ± 0.3
95.1 ± 0.2
79.6 ± 0.2
95.3 ± 0.4
97.1 ± 0.2
80.6 ± 0.1
87.3 ± 0.4
90.8 ± 0.3
83.6 ± 0.2
85.0 ± 0.3
95.8 ± 0.3
97.9 ± 0.2
81.2 ± 0.3
43.5 ± 0.1
49.7 ± 0.5
99.2 ± 0.3
86.3 ± 0.2
88.9 ± 0.3
93.2 ± 0.5
b
6
7
8
9
1
1
1
0
1
2
a
◦
␣
-pinene 2.7 g, n(␣-pinene): n(metal active site) = 130, H2 5 MPa, 160 C, 5 h. C: conversion of ␣-pinene. S: selectivity towards cis-pinane. C and S determined by GC.
Entries 2, 4, 7–12 the loading of metal nanoparticles was 10 wt%.
b
Reused after five runs.

2
72
Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 269–275
Fig. 2. TEM of Fe3O4/1, 6-hexanediamine (a), TEM and particle distributions of Fe3O4/1, 6-hexanediamine/Ru (b and b’).
Table 2
Effect of different carbon chain length on the hydrogenation result.
a
Entry
Catalyst
N/wt%b
Ru/wt%c
C/%
S/%
1
2
3
4
5
Fe3O4/1, 2-ethylenediamine/Ru
Fe3O4/1, 4-butanediamine/Ru
Fe3O4/1, 6-hexanediamine/Ru
Fe3O4/1, 8-octanediamine/Ru
Fe3O4/1, 12-dodecanediamine/Ru
3.12
2.91
2.74
2.32
2.08
5.95
6.13
8.15
6.51
5.94
85.1 ± 0.1
93.6 ± 0.2
99.2 ± 0.3
95.1 ± 0.1
83.6 ± 0.1
95.9 ± 0.1
96.8 ± 0.2
97.1 ± 0.2
96.7 ± 0.1
96.6 ± 0.1
a
◦
␣
-pinene 2.7 g, catalyst (Ru 10 wt%) 0.16 g, H2 5 MPa, 160 C, 5 h.
N content determined by XPS.
Ru content determined by ICP-AES. C: conversion of ␣-pinene. S: selectivity towards cis-pinane. C and S determined by GC.
b
c
Ru nanoparticles were uniformly decorated on the Fe O /1, 6-
short, amine group cling to the surface of Fe O , a Ru nanoparticle
3 4
3
4
hexanediamine and nearly all the Ru nanoparticles (particles size
.95 ± 0.3 nm) were connected with Fe O /1, 6-hexanediamine. As
will cover several amine group. The amine group will have a bigger
space by increasing the length of carbon chain, which increases the
Ru loading. Their properties upon the hydrogenation of ␣-pinene
were investigated. It was found that the conversion of ␣-pinene
increased with increasing the length of carbon chain when the
length of carbon chain less than six (Entries 1–3). Upon increasing
the length of carbon chain, the conversion of ␣-pinene decreased.
When the length of carbon chain was six, the hydrogenation of
␣-pinene was effectively catalyzed.
1
3
4
a result, Fe O /1, 6-hexanediamine/Ru made the catalytic active
3
4
center Ru(0) highly dispersed and offered the reaction a large area
interface, thus enhancing hydrogenation [38]. When Pd, Rh or Pt
nanoparticles were loaded on the Fe O /1, 6-hexanediamine and
3
4
used to catalyze the hydrogenation reaction, the results were not
satisfying. This may be due to the fact that in the products obtained
in the present work, the Ru nanoparticles with smaller average size
(
1.95 ± 0.3 nm) were evenly dispersed on the surface of Fe O /1, 6-
3
4
hexanediamine (Fig. 2b and b’) [39]. As shown in Fig. S4, the average
diameter of Pd, Rh and Pt on the Fe O /1, 6-hexanediamine used
3.3. Effects of different amounts of Ru nanoparticles on the
3
4
reaction
here was about 3.0 ± 0.2 nm.
Fe O /1, 6-hexanediamine/Ru with various loading amounts of
3
4
3
.2. Effects of different carbon chain length on the reaction
Ru nanoparticles was synthesized through a similar process as
the one described earlier. The amount of Ru used during load-
ing was 3 wt%, 6 wt%, 10 wt%, 13 wt% and 15 wt%. The Ru content
Different carbon chain length was used to prepare a series of
amine-functionalized magnetite nanoparticles. Ru nanoparticles
were loaded onto the obtained amine-functionalized magnetite
nanoparticles with different carbon chain length and used to
catalyze the hydrogenation reaction. The results are reported in
Table 2. The used amine group content in the mixed solution was
kept the same for the different carbon chain functionalized cata-
lysts. XPS analyses showed that the amine group decreased upon
increasing the length of carbon chain. This may be due to the
fact that long-chain carbon molecules are relatively stable in the
reaction and result in less amine groups modified in the Fe O
in Fe O /1, 6-hexanediamine/Ru was determined by the ICP-AES
3
4
technique and the results are reported in Table 3. Fe O /1, 6-
3
4
hexanediamine/Ru with different amounts of Ru was used to
catalyze the hydrogenation of ␣-pinene (Table 3). It was found that
Table 3
Effects of different amounts of Ru nanoparticles on the hydrogenation reaction.a
Entry
Ru/wt%b
Ru/wt%c
C/%
S/%
3
4
1
2
3
4
5
3
6
10
13
15
2.52
4.94
8.15
10.79
12.63
78.6 ± 1.5
85.1 ± 0.4
99.2 ± 0.3
99.3 ± 0.2
99.2 ± 0.2
95.1 ± 0.6
96.3 ± 0.3
97.1 ± 0.2
97.2 ± 0.1
97.1 ± 0.1
nanoparticle [40]. As shown in Table 2, with the decrease of amine
group that less amine groups served as anchors to immobilize Ru
nanoparticles, the measured Ru content in the catalyst was reduced
(
Entries 3–5). This was attributed to less amine groups served as
a
◦
␣
-pinene 2.7 g, Fe3O4/1, 6-hexanediamine/Ru 0.16 g, H2 5 MPa, 160 C, 5 h. C:
anchors to immobilize Ru nanoparticles. Interestingly, when the
length of carbon chain less than six the measured Ru content was
less than Fe O /1, 6-hexanediamine/Ru (Entries 1–3). This can be
conversion of ␣-pinene. S: selectivity towards cis-pinane.
b
Ru content in the loading process.
Ru content determined by ICP-AES in the Fe O /1, 6-hexanediamine/Ru. C and
c
3
4
3
4
explained by the fact that when the length of carbon chain too
S determined by GC.

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 269–275
273
Scheme 3. The proposed mechanism of the hydrogenation of ␣-pinene in the presence of Fe3O4/1, 6-hexanediamine/Ru.
the conversion of ␣-pinene increased with increasing amounts of
3.4. The effects of the reaction conditions on the reaction results
Ru. When the Ru nanoparticles content was 10%, the hydrogena-
tion of ␣-pinene was effectively catalyzed (Entry 3). This could
be explained by the fact that with increasing Ru dosage, Fe O /1,
Fig. 3 shows the effects of the reaction conditions on the hydro-
genation of ␣-pinene. It was found that the conversion of ␣-pinene
increased upon increasing the dosage of catalyst. When the cat-
alyst dosage was 4%, the conversion of ␣-pinene was 98.6%. This
was attributed to a high Ru dosage, Fe O /1, 6-hexanediamine/Ru
3
4
6
-hexanediamine/Ru offers much more active sites enhancing
hydrogenation [41]. A further increase in the Ru nanoparticle con-
tent above 10% brought no additional benefit to the hydrogenation
reaction. This value was therefore selected as optimal.
3
4
offers much more active sites enhancing hydrogenation [43]. After
that, no further increase in the reaction conversion of ␣-pinene
upon increasing the catalyst dosage could be obtained. With the
enhancement of temperature, the conversion of ␣-pinene dramat-
ically increased, however, the selectivity for cis-pinane decreased.
The reason might be that the higher temperature the more fierce
of the electron shaking so the conversion of ␣-pinene increased
and reduced the selectivity of the reaction by increasing the num-
Based on the above results, the mechanism of ␣-pinene hydro-
genation in the presence of Fe O /1, 6-hexanediamine/Ru was
3
4
proposed and is shown in Scheme 3. In the initial stage, ␣-pinene
can be easily absorbed on the surface of the catalyst. H2 is acti-
vated by the electronically supported Ru. The bulky gem-dimethyl
group inhibits absorption of the ␤-face of ␣-pinene on the surface
of the catalyst. Hydrogenation therefore takes place preferentially
from the ␣-face of the molecule, increasing thus selectivity towards
cis-pinane [42].
◦
ber of accessible reaction pathways [44]. Therefore, 100 C was
the optimal reaction temperature. The effect of reaction time on
◦
Fig. 3. The effects of the reaction conditions on the reaction results. Reaction conditions: catalyst: Fe3O4/1, 6-hexanediamine/Ru, P = 2.0–6.0 MPa, T = 80–120 C, t = 4.0–6.0 h,
␣
-pinene: 2.7 g, m(catalyst): m(␣-pinene)/% = 2–6.

2
74
Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 269–275
Fig. 4. Reaction scheme for the recycling of catalyst.
the reaction is also shown in Fig. 3. It was found that the conver-
sion approximately trended to be stable when the reaction time
exceeded 5.0 h. Therefore, the optimal reaction time was 5.0 h.
The conversion increased with aggrandizing H pressure. However,
2
the use of high pressure would increase the capital costs for the
hydrogenation plant facility. Therefore, the optimal H2 pressure of
4
.0 MPa was chosen. Based on the above results, the optimum reac-
◦
tion conditions were obtained as follows: P = 4.0 MPa, T = 100 C,
t = 5.0 h, m(catalyst):m(␣-pinene) = 4%. Under these conditions, the
conversion of ␣-pinene and the selectivity of cis-pinane were 98.6%
and 98.1%, respectively.
Fig. 5. Photograph of the catalyst dispersed in reaction mixture (a) and magnetic
separation of the catalysts from the reaction medium (b).
3.5. The reusability of the catalyst
The recycling of the catalyst is shown in Fig. 4. Fe O /1, 6-
3
4
hexanediamine/Ru was easily recovered by an external magnet
when the reaction was finished, and dispersed quickly through
a slight shake when the magnetic field was removed (Fig. 5).
The recovered Fe O /1, 6-hexanediamine/Ru was directly used
3
4
in the recycling experiments to investigate its reusability. Recy-
cling experiments were conducted under the optimized reaction
conditions and the results shown in Fig. 6. It was seen that the
Fe O /1, 6-hexanediamine/Ru could be reused ten times without
3
4
any obvious decrease in the reaction conversion and selectivity of
the products under the given conditions. Therefore, the Fe O /1,
3
4
6
-hexanediamine/Ru catalyst showed excellent reusable perfor-
mance in the hydrogenation of ␣-pinene. The good reusability of
Fe O /1, 6-hexanediamine/Ru is attributed to the strong stabil-
3
4
ity of Fe O /1, 6-hexanediamine to the Ru nanoparticles, which
3
4
originates from the presence of multiple amines functional groups
on the surface of Fe O /1, 6-hexanediamine and the very limited
3
4
loss of the Fe O /1, 6-hexanediamine/Ru that were separated by
3
4
applying an external magnetic field. Fe O /1, 6-hexanediamine/Ru
own the higher saturated magnetization value (61.6 emu g ) and
the superparamagnetic properties (Fig. S5). Fig. 7 showed the
3
4
Fig. 6. The reusability of Fe3O4/1, 6-hexanediamine/Ru. Reaction conditions: ␣-
−
1
◦
pinene 2.7 g, Fe3O4/1, 6-hexanediamine/Ru 0.11 g (Ru 10 wt%), H2 4.0 MPa, 100 C,
h.
5
Fig. 7. TEM and particle distributions of unused Fe3O4/1, 6-hexanediamine/Ru (a and a’) and repeatedly used Fe3O4/1, 6-hexanediamine/Ru for ten times (b and b’).

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 269–275
275
TEM images of unused Fe O /1, 6-hexanediamine/Ru and the
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Fe O /1, 6-hexanediamine/Ru for ten repeatedly used. It was seen
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[
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that numerous monodispersed nanoparticles were still uniformly
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.15 wt% and 59.61 wt%. Nearly no appreciable Ru and Fe leaching
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to form cis-pinane. Fe O /1, 6-hexanediamine/Ru had good
3
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superparamagnetic properties and that the introduction of the
amine-functionalized improved the monodispersity, morphologi-
[
[
cal regularity and size uniformity of the Ru nanoparticles. Fe O /1,
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-hexanediamine/Ru exhibited an outstanding catalytic perfor-
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Fe O /1, 6-hexanediamine/Ru was easily separated from the
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[
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-hexanediamine/Ru exhibit good reusability.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in
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