Highly selective hydrogenation of Α-pinene in aqueous medium using PVA-stabilized Ru nanoparticles

Article abstract of DOI:10.1016/j.mcat.2017.10.020

Polyvinyl alcohol (PVA)-stabilized ruthenium nanoparticles in water were synthesized and characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and confocal laser scanning microscope (CLSM). The Ru-PVA catalyst was highly effective for the hydrogenation of α-pinene, exhibiting higher selectivity for cis-pinane as well as high conversion. The selectivity could reach approximately 99%. The catalyst phase could be recycled at least eight times without obvious loss in the catalytic activity and selectivity. Furthermore, both the preparation of catalyst and hydrogenation of α-pinene were under mild experimental conditions. Using water as the reaction medium and PVA as the stabilizing agent made the hydrogenation process environmentally friendly. This catalytic system may also be used to convert a range of cyclenes and long chain alkenes, in particular aromatic compounds to corresponding hydrogenated products.

Full text of DOI:10.1016/j.mcat.2017.10.020

Molecular Catalysis 444 (2018) 62–69
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Research Paper
Highly selective hydrogenation of ␣-pinene in aqueous medium using
PVA-stabilized Ru nanoparticles
Xiaoyan Wang , Fengli Yu , Congxia Xie , Shitao Yub
a
a
a,∗
a
State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and
Technology,No.53 Zhengzhou Road, Qingdao 266042, People’s Republic of China
b
College of Chemical Engineering, Qingdao University of Science and Technology,No. 53 Zhengzhou Road, Qingdao 266042, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 April 2017
Received in revised form
Polyvinyl alcohol (PVA)-stabilized ruthenium nanoparticles in water were synthesized and character-
ized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and confocal
laser scanning microscope (CLSM). The Ru-PVA catalyst was highly effective for the hydrogenation of
2
1 September 2017
␣-pinene, exhibiting higher selectivity for cis-pinane as well as high conversion. The selectivity could
Accepted 16 October 2017
reach approximately 99%. The catalyst phase could be recycled at least eight times without obvious loss
in the catalytic activity and selectivity. Furthermore, both the preparation of catalyst and hydrogenation
of ␣-pinene were under mild experimental conditions. Using water as the reaction medium and PVA
as the stabilizing agent made the hydrogenation process environmentally friendly. This catalytic system
may also be used to convert a range of cyclenes and long chain alkenes, in particular aromatic compounds
to corresponding hydrogenated products.
Keywords:
Polyvinyl alcohol (PVA)
Ru nanoparticles
Water
␣
-Pinene
Hydrogenation
© 2017 Elsevier B.V. All rights reserved.
1
. Introduction
As a part of a sustainable supply chain to fine chemicals,
Pd/C [12]. Palladium colloidal catalyst was also effective for hydro-
genation of ␣-pinene [13]. The conversion of ␣-pinene exceeded
99%, while the selectivity for cis-pinane was 81.3%. Mark et al.
found that ruthenium was a stereoselective catalyst for the hydro-
genation of ␣-pinene to cis-pinane [14]. Ru nanoparticles protected
by amine-functionalized magnetite nanoparticles (Fe O /NH /Ru)
biomass-derived substrates have attracted more and more atten-
tion [1,2]. Among them, turpentine is a kind of natural aromatic oil
originated from pine trees with the largest output in the world. As
the major component of turpentine, ␣-pinene plays an important
role in chemistry and industry of forest products [3,4]. Hydrogena-
tion of ␣-pinene yields a mixture of cis-pinane and trans-pinane.
Pinane is a vital chemical intermediate that can be applied in the
synthesis of valuable fragrance compounds and drug components
such as rose musk, linalyl acetate and vitamin E [5,6]. Compared
with trans-pinane, cis-pinane possesses higher reaction activity.
Therefore, the content of cis-pinane is required to be as high as pos-
sible so as to ensure the quality of subsequent products in industrial
applications. To obtain cis-rich pinane, the challenge is to design
and develop a novel catalytic system with optimal efficiency and
selectivity [7].
3
4
2
were utilized to catalytic hydrogenation of ␣-pinene [15]. Under
the optimal conditions, the ␣-pinene conversion and the selectivity
for cis-pinane could reach 98.6% and 98.1%, respectively. Nonethe-
less, for the forementioned catalytic systems, the catalysts were
easy to be deactivated and/or harsh reaction conditions were usu-
ally required such as high temperature and pressure.
Colloidal methods are widely applied in preparing metallic
nano-catalysts, and the particle sizes and shapes can be accurately
controlled [16]. Such colloids can be used directly as catalysts after
dispersion in liquid reaction media [17]. During colloid preparation,
many polymers acting as stabilizers have been utilized to prevent
particle agglomeration and crystal growth [18–20]. With the fur-
ther studies in these years, it is widely reported that stabilizers
interact with the surface of nanoparticles and compete with reac-
tant molecules for the binding sites of the catalysts [21,22]. Acting
as stabilizers, some polymers can improve the catalytic efficiency
as well as chemoselectivity of catalysts [23]. It is demonstrated that
the particle growth is controlled by the chemical structure [24] and
the concentration of the polymer stabilizers [25]. Polyvinyl alcohol
During the last decades, researchers have been continually seek-
ing cis-rich selective catalytic systems, including unsupported and
supported metallic catalysts [8–11]. Nickel nanoparticles applied
in the hydrogenation of ␣-pinene showed good catalytic activity,
and the selectivity to cis-pinane (94.3%) was higher than that with
(
PVA), a kind of water-soluble surfactant, is an excellent stabi-
∗ Corresponding author.
E-mail address: xiecongxia@126.com (C. Xie).
lizer for colloid preparation. It is nontoxic, commercially available,
http://dx.doi.org/10.1016/j.mcat.2017.10.020
2
468-8231/© 2017 Elsevier B.V. All rights reserved.

X. Wang et al. / Molecular Catalysis 444 (2018) 62–69
63
Table 1
stirred at ambient temperature for 2 h until PVA and RuCl3 were
completely dissolved in water. Then the reactor was sealed. The
Catalytic performance of different catalysts on hydrogenation of ␣-pinene.
Entry
Catalyst
Conversion/%
bSelectivity/%
cTOF/h−1
air in it was replaced 4 times with 1.0 MPa H2. The mixture was
◦
reacted at 50 C for 1 h under 0.5 MPa H . After the reaction was
1
2
3
4
5
6
7
Pd/C (10 wt%)
Ru/C (5 wt%)
Ru-PVP
Ru-PSS
Ru-PEG
26.4
44.2
20.2
30.4
43.8
99.9
44.3
97.3
95.6
94.9
95.6
97.0
98.9
98.4
88
147
67
101
146
376
218
2
completed, the obtained dark colloid was used directly for the cat-
alytic hydrogenation of ␣-pinene or other substrates without any
further separation or washing.
Ru-PVA
a
Ru-PVA
2.3. TEM analysis
Reaction conditions: P = 2.0 MPa, T = 343 K, t = 3 h, The molar ratio of ␣-pinene to Ru
nanoparticles (S/C) = 1000:1.
Ru particle size distribution was determined by using TEM
Hitachi-7650) that was operated at 300 kV. Typically a drop of
colloidal suspension was dispersed on a copper grid covered by
a carbon film and dried at ambient temperature.
a
Reaction conditions: P = 2.0 MPa, T = 325 K, t = 2 h. S/C = 1000:1.
(
b
Selectivity: calculating formula
mole of cis-pinane formed
the total amount of the products
(
cis-pinane selectivity%=
× 100%) was based on Ref. [15].
c
TOF: turnover frequency measured in [mol product] per [mol metal] per h.
2
.4. XPS analysis
relatively inexpensive, and possesses good bio-compatibility [26].
Coating of particle surfaces with PVA can prevent their agglomer-
ation, and give rise to evenly dispersed nanoparticles [27,28].
In this paper, Ru/PVA colloids were prepared using the chem-
ical reduction method with hydrogen. Then the nanoparticulate
system was applied in the hydrogenation of ␣-pinene. The effects
of PVA molecular weight and reaction medium on the reaction
were emphatically investigated. Some characterization techniques,
such as transmission electron microscopy (TEM), X-ray photoelec-
tron spectroscopy (XPS) and confocal laser scanning microscope
The XPS spectra for evaluating the surface chemical states were
recorded by use of AlKa (1486.7 eV) X-ray source and an Axis Ultra
system (Kratos Axis Ultra DLD, UK). During the measurements, the
ultimate vacuum was 1 × 10 Torr. The binding energies were cali-
brated by setting the C1 s peak at 284.60 eV. For sample preparation,
the Ru nanoparticles were isolated by centrifugation in ethanol
and washed with water several times followed by centrifugation at
−
9
3
500 rpm over 5 min. The powder isolated was dried under reduced
pressure.
(
CLSM) were used to obtain more detailed information about the
Ru nanoparticles and the reaction system in terms of size distri-
bution and colloidal properties. Furthermore, the scheme of the
formation of Ru nanoparticles and hydrogenation of ␣-pinene was
explored. Several parameters, such as temperature, reaction time
and hydrogen pressure, were also evaluated.
2
.5. Ru leaching analysis
The content of Ru nanoparticles in product phase was deter-
mined by means of inductively coupled plasma-atomic emission
spectrometry (ICP-AES). The Ru leaching analysis was performed
using a Prodigy XP ICP-AES instrument (Leeman, U.S.A.). The sam-
ples were digested in 5 mL aqua regia and then diluted to 10 mL
before analysis.
2
. Experimental
2.1. Materials
RuCl3 was purchased from Aladdin Industrial Corporation (Ru
2
.6. CLSM analysis
content ≥ 37.5%). 10 wt% Pd/C, 5 wt% Ru/C, PtCl , RhCl and PdCl2
4
3
were purchased from CIVI-Chem Industrial Corporation (China).
-Pinene (purity ≥ 98%) was purchased from Jiangxi Hessence
The pictures of emulsion droplets were obtained through a TCS-
␣
SP5-II CLSM instrument (Leica, German).
Chemicals Company Limited (China). PVA (average molecu-
lar weight = 47,000-205,000) were supplied by Sigma-Aldrich
Company. Poly (styrene sulfonate) (PSS, Mw: 8000), polyvinyl
pyrrolidone (PVP K15, Mw: 10000) and polyethylene glycol (PEG,
Mw: 70000) were purchased from Aladdin Industrial Corporation.
Hydrogen (purity ≥ 99.9%) was purchased from Qingdao Heli Com-
pany Limited (China). All chemicals and reagents used were of
analytical grade. Double distilled and deionized water was used
throughout the work.
2.7. Hydrogenation of ˛-pinene or other substrates
Hydrogenation of ␣-pinene or other substrates was also
performed in the 75 mL stainless steel autoclave. In a typical experi-
ment, ␣-pinene (1.360 g, 10 mmol) was added into the as-prepared
Ru-PVA colloids as depicted in Section 2.2. The mixture reacted
◦
at 70 C under 2.0 MPa H2 and was stirred for 3 h. When the
reaction was completed, the products were extracted using n-
heptane before injection into a GC 9790 gas chromatograph (Fuli,
China) which was equipped with an OV-1701 capillary column
2.2. Ru-PVA colloid preparation
(
50 m × 0.25 mm × 0.25 ␮m) and a flame ionization detector (FID).
After n-heptane remaining in the reactor was removed, the fresh
-pinene was added into the autoclave for the next cycle.
In a typical experiment, RuCl3 (2.1 mg, 0.01 mmol), PVA (Mw:
8,000, 15 mg) and water (3.0 mL) were added into a 75 mL stain-
less steel autoclave equipped with a teflon liner. The mixture was
7
␣
Table 2
Effect of PVA molecular weight on hydrogenation of ␣-pinene.
TOF/h 1
−
PVA Molecular weight
Average particle size of Ru nanoparticles/nm
Conversion /%
Selectivity /%
4
7
1
2
7000
8000
45000
05000
4.6
2.4
4.3
5.6
44.7
60.6
31.9
33.4
98.3
98.7
98.1
98.0
505
685
360
377
Reaction conditions: P = 2.0 MPa, T = 343 K, t = 1 h. S/C = 1000:1, mass of PVA: 15 mg, solvent: water (3 mL).

6
4
X. Wang et al. / Molecular Catalysis 444 (2018) 62–69
Fig. 1. The TEM images (left) and the corresponding particle size distribution histograms (right) of Ru nanoparticles stabilized by PVA with different molecular weight: (a)
7000, (b) 78000, (c) 145000, (d) 205000.
4

X. Wang et al. / Molecular Catalysis 444 (2018) 62–69
65
3
. Results and discussions
disdribution of Ru nanoparticles, the interaction between PVA and
the surface of the nanoparticles could block some active sites and
influence the catalytic performance as well [22].
3.1. Effects of catalysts on hydrogenation of ˛-pinene
Only prolonging the reaction time and other experimental con-
ditions unchanged, we continued to explore the effect of PVA
molecular weight on hydrogenation of ␣-pinene, and the results
were described in Table S2. Ru nanoparticles had the highest cat-
alytic activity under any circumstances when the molecular weight
of PVA was 78000. Therefore, PVA with the molecular weight of
78000 was selected as the stabilizer for the following experiments.
Commercially available Pd/C and Ru/C were selected as the stan-
dard for the comparation on the performance of other catalysts.
Three common hydrophilic polymers such as poly (styrene sul-
fonate) (PSS, Mw: 8000), polyvinyl pyrrolidone (PVP K15, Mw:
1
0000) and polyethylene glycol (PEG, Mw: 70000) were substituted
for PVA as the stabilizers of Ru nanoparticles. The preparation of
Ru-PSS, Ru-PVP and Ru-PEG was the same as that of Ru-PVA. The
results were shown in Table 1.
3.3. Effects of reaction medium on the formation of Ru
It was observed that both the conversion and selectivity were
superior when the hydrogenation of ␣-pinene was catalyzed by Ru-
PVA. Although the difference in the selectivity to cis-pinane is not
obvious, it is crucial in the perfume industry because the purity of
cis-pinane has a direct effect on the quality of downstream prod-
ucts. When ␣-pinene hydrogenation was catalyzed by the other
five catalysts (Entries 1–5), the conversion was poor instead. For
Ru-PEG, serious aggregation of Ru nanoparticles after the catalytic
hydrogenation of ␣-pinene was observed. So it indicated that Ru-
PVA performed exceptionally well compared with them. When the
temperature and reaction time was 325 K and 2 h, respectively, the
nanoparticles and hydrogenation of ˛-pinene
Four kinds of PVA-stabilized Ru nanoparticles were prepared
with different solvents. Then they were applied in the hydrogena-
tion of ␣-pinene and the results were shown in Table 3.
It was observed that the ␣-pinene conversion increased with
the increase of solvent polarity, and the selectivity for cis-pinane
all exceeded 97%. The maximum conversion could be obtained in
particular with water as solvent. On the one hand, water could pro-
mote the hydrogenation of ␣-pinene [31]. On the other hand, it
might be attributed to the hydrophilic nature of PVA and the for-
mation of emulsion droplets during the hydrogenation of ␣-pinene.
It can be confirmed by the following CLSM image as shown in Fig. 2.
The emulsion droplets were spherical particles with a narrow size
distribution, and an average size of about 1.26 ␮m was obtained.
The emulsion reaction medium is favorable for the improvement
of the mass-diffusion limitation in liquid multiphase systems that
was caused by reactant molecules being located in the different
phases [32]. For hydrogenation of ␣-pinene, although H2 diffusion
was limited because of its low concentration in the liquid phase
␣
-pinene conversion decreased to 44.3% (Entry 7), however, the
selectivity to cis-pinane was still as high as 98.4%. For Pd/C, Ru-PVP
and Ru-PSS, we can also increase the reaction temperature and/or
prolong reaction time to enhance the ␣-pinene conversion to 44%
or so. Thus the selectivity to cis-pinane can be compared at identical
conversion levels. Nevertheless, the selectivity for cis-pinane would
decrease with the dramatically increasing temperature, which was
much disadvantageous for the hydrogenation of ␣-pinene [15,29].
Compared with other PVA-stabilized metallic nanoparticles such as
Rh-PVA, Pd-PVA and Pt-PVA, Ru-PVA showed the best performance.
Both the conversion of ␣-pinene and the selectivity for cis-pinane
were the highest (Table S1). Therefore, the above results proved
that Ru-PVA had excellent catalytic performance.
[
33], especially transport of ␣-pinene was improved. Each emulsion
droplet could act as a microreactor. Each microreactor was capa-
ble of affording higher local concentration of ␣-pinene near the Ru
nanoparticles, and thus exhibiting a remarkable rate acceleration
for the hydrogenation of ␣-pinene in water. Therefore, in water
medium, the Ru catalysts showed the optimum catalytic activity.
3.2. Effects of PVA molecular weight on the formation of Ru
nanoparticles and hydrogenation of ˛-pinene
3
.4. Effects of PVA concentration on hydrogenation of ˛-pinene
The catalytic activity and selectivity of Ru nanoparticles sta-
bilized by PVA with different molecular weight from 47000 to
Fig. 3 shows the influence of PVA concentration on the cat-
2
05000 were investigated on hydrogenation of ␣-pinene. As shown
alytic performance of Ru nanoparticles. It was observed that the
in Table 2, no obvious difference in the selectivity to cis-pinane
was observed when PVA molecular weight changed. However, the
conversion increased initially and then declined with the increas-
ing PVA molecular weight. The results may be partially related to
the particle size and distribution of Ru nanoparticles. As shown in
Fig. 1 (A), when PVA molecular weight was 47000, Ru nanoparticles
dispersed evenly but with a larger size of 4.6 nm. It indicated that
the Ru nanoparticles further grew up to larger particles when the
chain length of PVA was not long enough, which decreased spe-
cific surface areas of the catalyst. As depicted in Fig. 1 (B), when
PVA molecular weight was 78000, the average size of Ru nanopar-
ticles was 2.4 nm. It was the smallest as indicated in Table 2 and
Fig. 1. In addition, the size distribution of Ru nanoparticles in Fig. 1
conversion of ␣-pinene increased when PVA concentration rose
−
5
−1
from 0 to 6.42 × 10 mol L . However, with the PVA concentra-
tion further increasing, the conversion declined. On the one hand,
the phenomenon was attributed to the steric stabilization of PVA
[
34]. The polymer chains surrounding the Ru0 particles sterically
afforded the most effective inhibition in particle nucleation and
−
5
−1
growth at PVA concentration of 6.42 × 10 mol L . With further
addition of PVA, the rate of nucleation decreased because the PVA
chains present in the solution interfered with the formation of Ru
particles [25], which to some extent resulted in the loss of catalytic
activity. Moreover, when excess stabilizers were added, the num-
ber of active sites decreased because the polymer blocked part of
(
7
B) was narrower than those in the other figures. Therefore, PVA-
8000 could provide optimum protection against agglomeration
Table 3
Hydrogenation of ␣-pinene by Ru nanoparticles dispersed in different solvents.
so that the ␣-pinene conversion reached a maximum, 60.6%. With
PVA molecular weight further increasing, some of nanoparticles
aggregated into nanoclusters as shown in Fig. 1 (C) and (D). It was
because just like PVP, the higher the PVA molecular weight was, the
kinetic behavior of chains was more complex in the solution [24].
Thus the closer capped Ru within longer chains of PVA could be
aggregated more easily as the Ru3+ were reduced and grown, which
decreased the catalytic activity [30]. Besides the particle size and
TOF/h−1
Solvent
Conversion/%
Selectivity/%
n-heptane
ethyl acetate
ethanol
0
–
–
18.6
21.1
60.6
97.3
97.1
98.7
210
238
685
3+
water
Reaction conditions: P = 2.0 MPa, T = 343 K, t = 1 h. S/C = 1000:1, mass of PVA (Mw:
78000) = 15 mg, solvents: 3 mL.

6
6
X. Wang et al. / Molecular Catalysis 444 (2018) 62–69
favored the high concentration of substrate around catalytic active
sites [32]. The droplets might consequently act as microreactors
and this micro-circumstance could probably promote the substrate
to contact with ruthenium catalyst [38]. Moreover, the biphasic
interface areas were remarkably increased by the presence of emul-
sion droplets, and the mass-diffusion limitation of the hydrophobic
substrate with the hydrophilic catalysts was effectively diminished.
The cooperative effects of the above factors would accelerate the
reaction. Compared with Rh, Pd and Pt, Ru had the highest selectiv-
ity for hydrogenation of ␣-pinene to cis-pinane as shown in Table
S1. The special configuration of the adsorbed PVA on Ru nanopar-
ticles, although difficult to determine [39], probably favored the
formation of cis-rich products. Therefore, the excellent selectivity
for cis-pinane was obtained.
3.6. The effects of reaction conditions on ˛-pinene hydrogenation
Reaction temperature and hydrogen pressure were varied
Fig. 2. Particle size distribution and CLSM image of emulsion system during the
hydrogenation of ␣-pinene.
respectively under different reaction time. The results were
reported in Fig. 5. It clearly indicated that the conversion of ␣-
pinene increased in most cases with an increase of the temperature
(Fig. 5(a)). As shown in Fig. 5(b), ␣-pinene conversion increased
active sites, which was in agreement with many previous stud-
ies [22,35,36]. On the other hand, the declined reactivity when the
PVA concentration exceeded a certain value might be due to a dilu-
tion effect on substrate in the emulsion droplets. Besides, it was
observed that the viscosity of the solution increased, which might
enhance the G-L mass transfer limitation and interfere with the
contact of reactant molecules [37]. With regard to the selectivity for
cis-pinane, the values remained nearly constant with the increase
of PVA concentration as described in Fig. 3(b). So the optimal PVA
◦
◦
◦
remarkably when the temperature rose from 50 C to 70 C. At 80 C,
the conversion was approaching 100%. However, the selectivity
for cis-pinane slightly decreased, which was much disadvanta-
geous for hydrogenation of ␣-pinene [15,29]. In addition, higher
reaction temperature would increase the production cost for the
◦
factories. Therefore, 70 C was the optimal reaction temperature.
As depicted in Fig. 5(c), an increase in H2 pressure from 1.0 MPa
to 2.0 MPa resulted in a significant increase in the conversion of
␣-pinene. However, the conversion increased very slowly at more
than 2.0 MPa. It is probably related with the steric stabilization
of PVA to the Ru nanoparticles. The ligation of PVA with Ru sur-
face atoms and the physical occupation around the nanoclusters
makes the active sites of Ru nano-catalysts limited [19]. During the
process of ␣-pinene hydrogenation, the amount of the adsorption-
state hydrogen originally increased obviously with an increase in
concentration was 6.42 × 10− mol L
5
−1
.
3.5. The scheme of reaction
During the formation of Ru nanoparticles, PVA can stabilize
them in part by ligation of Ru surface atoms and also by physically
occupying space around the nanoclusters. Thereby PVA sterically
prevented the aggregation of Ru nanoparticles [19] as depicted in
Fig. 4(a). The XPS spectra of Ru nanoparticles demonstrated that
H pressure. At more than 2.0 MPa, the number of the atomic hydro-
2
gen absorbed on the active sites of the Ru species nearly reached its
saturation value. Therefore, the conversion increased very slowly.
Considering that high pressure put forward higher demands to the
industrial equipment, the optimal H2 pressure was determined to
be 2.0 MPa. It was also observed that the conversion increased as
reaction time was prolonged (Fig. 5(d)). When the reaction time
was 3 h, the conversion reached nearly 100% and the selectiv-
ity for cis-pinane was still as high as before. Based on the above
3
+
0
8
8.5% Ru was reduced to Ru as shown in Fig. S1.
When the hydrogenation of ␣-pinene occurred with stirring, the
organic phase ␣-pinene dispersed in the catalyst phase in the form
of emulsion droplets (Fig. 2). The internal phase was separated from
external phase by the droplets, which resulted in a large reaction
area. At a microscopic level, the PVA-stabilized catalysts were pos-
sibly assembled at the interface of the emulsion droplets, which
Fig. 3. The effect of PVA concentration on ␣-pinene hydrogenation. Reaction conditions: P = 2.0 MPa, t = 1 h, 10 mmol ␣-pinene, 0.01 mmol RuCl3, PVA (Mw: 78000), 3 mL
water, (a) T = 323 K, 333 K, 343 K, (b) T = 333 K.

X. Wang et al. / Molecular Catalysis 444 (2018) 62–69
67
Fig. 4. Scheme on catalytic hydrogenation of ␣-pinene.
Fig. 5. Effects of reaction conditions on hydrogenation of ␣-pinene. Reaction conditions: S/C = 1000:1, (a) P = 2.0 MPa, T = 323–353 K, t = 1–3 h; (b) P = 2.0 MPa, T = 323–353 K,
t = 2 h; (c) P = 1.0–4.0 MPa, T = 343 K, t = 2 h; (d) P = 1.0–4.0 MPa, T = 343 K, t = 1.0–4.0 h.
results, when S/C = 1000:1, the optimum reaction conditions were
as follows: P = 2.0 MPa, T = 343 K, t = 3 h. Under the aforementioned
conditions, the conversion of ␣-pinene could reach 99.9%, and the
selectivity for cis-pinane was 98.9%, higher than the previous stud-
ies [12,15,31,40].
sion of 90.1%. As shown in Fig. 6, it was possible to recover and reuse
Ru-PVA at least eight times without significant loss in its catalytic
performance. After the 8th cycle, both the conversion and selectiv-
ity remained as high as 89.8% and 98.2%, respectively. On the one
hand, PVA could protect the Ru nanoparticles from poisoning and
agglomerating [19]. On the other hand, it was probably due to the
formation of emulsion droplets. Acting as microreactors, they could
further decrease the contact of the nanoparticles and thus protect
them from agglomeration. Therefore, the Ru-PVA catalyst showed
good repeatability. The conversion declined gradually after eight
cycles, which might be attributed to the trace loss of catalysts. ICP
analysis showed that 0.90 ppm Ru nanoparticles leached into the
n-heptane phase after the 1 st cycle. In addition, the aggregation
of some nanoparticles also to some extent caused a decline in cat-
alytic activity. However, after the 13th cycle, the conversion still
3.7. The reusability of the catalyst
An attractive feature of the present emulsion catalytic system
lies in the easy separation of product by extraction due to the low
solubility of Ru-PVA in n-heptane. Water-soluble catalysts in aque-
ous biphasic system can be easily reused [41]. A new reaction could
be conducted by adding fresh ␣-pinene into the catalytic system
after the product phase was separated from the catalyst phase. The
reusability experiments of Ru-PVA catalyst started with a conver-

6
8
X. Wang et al. / Molecular Catalysis 444 (2018) 62–69
Fig. 6. The reusability of Ru-PVA catalyst. Reaction conditions: 2.0 MPa H2, 338 K, 3 h, S/C = 1000:1.
Table 4
Hydrogenation of other substrates catalyzed by Ru-PVA colloids.
−
Entry
Substrate
Product
Reaction time/h
1.0
Conversion/%
99.9
TOF/h 1
1
1129
2
1.5
99.9
753
3
4
1.5
2.0
99.9
99.7
753
563
5
6
2.0
2.0
99.9
99.8
565
564
Reaction conditions: P = 2.0 MPa, T = 343 K, the molar ratio of substrate to catalyst = 1000:1.
exceeded 70%. It is worth noting that the selectivity for cis-pinane
was almost unchanged during the whole cycle process.
tion reduction of RuCl3 dispersed in PVA solution. PVA-stabilized
Ru nanoparticles were applied in the catalytic hydrogenation
of ␣-pinene, showing excellent performance, especially the high
selectivity to cis-pinane. Through steric effect, PVA could protect
the Ru nanoparticles from poisoning and aggregation. Even after
eight cycles, no obvious loss in both catalytic activity and selectivity
was observed.
3.8. Hydrogenation of other substrates catalyzed by Ru-PVA
colloids
In order to demonstrate the possibility for a general applica-
In addition, water was used as the reaction medium, and the
hydrogenation process was environmentally friendly. Both the syn-
thesis of the catalyst and the hydrogenation of ␣-pinene were
convenient under mild conditions. Therefore, the developed cata-
lyst system has good prospects for industrial application. Moreover,
the catalyst is also efficient for the hydrogenation of other sub-
strates, such as olefinic and aromatic compounds, which provides
a promising and general strategy for the hydrogenation of other
natural hydrophobic products.
tion of this emulsion catalytic system, a wide range of substrates
were tested with Ru-PVA catalyst. The results were summarized in
Table 4. To our delight, Ru nanoparticles showed high activity for
either cyclenes or long chain alkenes. Particularly, Ru nanoparticles
exhibited superior catalytic activity for the reduction of aromatic
olefins, possibly due to the deposits of water-insoluble product as
an oil on the water surface. Therefore, the products could be sep-
arated easily from the aqueous phase containing Ru nanocatalyst
[
42].
4
. Conclusions
Acknowledgements
In summary, Ru nanoparticles with relatively small diameters
and narrow size distribution were prepared by simple hydrogena-
We are grateful for the financial support from the Taishan
Scholar Program of Shandong Province (No.ts 201511033), the

X. Wang et al. / Molecular Catalysis 444 (2018) 62–69
69
National Natural Science Foundation of China (No.31270615),
and the Natural Science Foundation of Shandong Province, China
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