Mild water-promoted ruthenium nanoparticles as an efficient catalyst for the preparation of cis-rich pinane

Article abstract of DOI:10.1039/c5ra15283f

Ruthenium (Ru) nanoparticles were prepared using polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer (P123) micelles in water as a stabilizing agent. The P123-Ru micellar catalyst was first used in the hydrogenation of α-pinene to pinane, and the selectivity for cis-pinane reached 98.9%. This result is attributed to the formation of vesicles. The isolated catalyst phase could be used seven times with no treatment, and its catalytic activity and selectivity were almost unchanged. The preparation process of the catalyst and hydrogenation reaction of α-pinene was under mild and environmentally friendly conditions. This research offers an effective method for the hydrogenation of α-pinene and provides a reference for other hydrophobic natural products in hydrogenation reactions.

Full text of DOI:10.1039/c5ra15283f

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Mild waterꢀpromoted ruthenium nanoparticles as an efficient
catalyst for the preparation of cisꢀrich pinane
a
a
a
b
Shengli Hou, Congxia Xie,* Hua Zhong and Shitao Yu
State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and
Molecular Engineering, Qingdao University of Science and Technology, Qingdao,
a
2
66042, China. E-mail: xiecongxia@126.com; Fax: +86 532 8402 3927; Tel: +86
32 8402 3927
5
b
College of Chemical Engineering, Qingdao University of Science and Technology,
Qingdao, 266042, China.
Abstract：
Ruthenium (Ru) nanoparticles were prepared using polyoxyethyleneꢀ
polyoxypropyleneꢀpolyoxyethylene triblock copolymer (P123) micelles in water as
the stabilizing agent. The P123ꢀRu micellar catalyst was first used in the
hydrogenation of αꢀpinene to pinane, and the selectivity for cisꢀpinane reached 98.9%.
This result is attributed to the formation of vesicles. The isolated catalyst phase could
be used seven times with no treatment, and its catalytic activity and selectivity were
almost unchanged. The preparation process of catalyst and hydrogenation reaction of
αꢀpinene was under mild and environmentally friendly conditions. This research
offers an effective method for the hydrogenation of αꢀpinene and provides a reference
for other hydrophobic natural products in hydrogenation reactions.
Keywords: αꢀpinene, hydrogenation, micelle, water, Ru nanoparticles.
1
. Introduction
Biomass is the only renewable carbon resource that can be converted into liquid
chemicals and liquid fuels, biomass and biomass derived compounds holds great
1
potential in synthesis of high valueꢀadded chemicals. For instance, αꢀpinene, mainly
2
obtained from pine trees, is valuable raw materials for fine organic synthesis.
Cisꢀpinane is produced by the hydrogenation of αꢀpinene, it can be used for the
synthesis of specialty chemicals and pharmaceuticals such as linalool,
3
dihydromyrcenol and other terpene series spices. As the hydrogenation of other
4
,5
6,7
biomass, Pd/C and other supported metallic catalysts are commonly applied in the
hydrogenation of αꢀpinene. However, conventional hydrogenation technology yields a
mixture of cisꢀ and transꢀ pinanes (Scheme 1), furthermore, the processes for such
reactions are usually conducted under high reaction temperatures and pressures, and
8,9
the catalysts are generally not suitable for reuse. Recently, Pd/C was used to
catalyze hydrogenation of αꢀpinene under carbon dioxide of high pressure. The results
showed that although the yield of pinane was very high, the selectivity of cisꢀpinane
was poor. In order to obtain superior product selectivity, the hydrogenation reaction
medium and catalyst are key factors.

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H
2
H
2
αꢀpinene
cisꢀpinane
transꢀpinane
Scheme 1. Hydrogenation of αꢀpinene
It has been known since the 1980s that water as the reaction medium can greatly
10
accelerate reactions. Thus, scientists have been extensively exploring this usage.
11,12
Some researchers
have determined that the hydrophobic effect and the
donorꢀacceptor hydrogen bonding ability of water enhance productivity and
enantioselectivity and catalyst recycling in several catalytic reactions. Our team
13
investigated the effect of water on the hydrogenation of αꢀpinene catalyzed by RuCl .
3
The results showed that water significantly promoted the hydrogenation; the rate of
reaction and the molar ratio of cisꢀ and transꢀpinane were significantly improved.
However, the catalyst was not suitable for reuse.
Amphiphilic surfactants tend to mediate between the two phases in the presence
of water and immiscible organic species. When the amount of surfactant is more than
a certain minimum concentration (CMC) in water, nanoscale micelles can be formed
by the aggregation of monomers. Similar to enzymes, nanoscale micelles can cause an
14
acceleration of chemical reaction, which has been recognized since 1975.
Subsequent studies have determined that surfactants can improve catalytic activity
and chemoselectivity in addition to the reusability of catalysts through the formation
1
5,16
of vesicles or micelles in aqueous solutions.
The activity and selectivity depended
1
7
strongly on the property of the surfactants in a given chemical reaction. Because
weak inhibition effects occur by unfavorable interactions of functional groups of the
surfactant, which compete with the substrate for the binding site of the catalyst, the
distribution of substrate and product within the microheterogeneous medium strongly
affects both the reaction rate and the separation procedure of the products. Block
copolymers with the type (PEO) ꢀ(PPO) ꢀ(PEO) have many excellent properties,
such as lack of toxicity, low price, and wide range of solubilities. These nonꢀionic
amphiphiles can be used for micellar reactions similar to surfactants. Toshio Sakai and
coꢀworkers reported that 10 nm gold nanoparticles were obtained at ambient
temperature by simply mixing an aqueous hydrogen tetrachloroaureateꢀ(III) hydrate
1
8
n
m
n
19
2
0
4 2
(HAuCl ·3H O) solution with an aqueous Pluronic block copolymer (P123) solution.
We designed the catalyst of Ru nanoparticles protected by P123 micelles in aqueous
solutions. The catalytic system was used in the hydrogenation of αꢀpinene to produce
cisꢀpinane, and the mechanism of the reaction was explored. The reaction conditions,
such as temperature, hydrogen pressure, reaction time and so on, were also optimized
herein.
2
2
. Experimental
.1 Materials

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Aꢀpinene (purity: 98%) was supplied by Jiangxi Hessence (China) Chemicals Co.
Ltd., RuCl3 (Ru content≥37.5%) was supplied by Aladdin (China) Industrial
Corporation, PdCl , RhCl and PtCl were supplied by CIVIꢀChem (China) Industrial
2
3
4
Corporation, Ru(NO)(NO
Ru
surfactants were purchased from SigmaꢀAldrich (China) Co.. H
3
)
3
was supplied by Alfa Aesar (China) Chemicals Co. Ltd..
3
(CO)12 was supplied by Tokyo Chemical Industry Co. Ltd.. P123 and other
(purity≥99.9%) was
2
supplied by Heli (China) Co.. Water was double distilled and deionized before use.
Other reagents were of analytical grade purity.
2
.2 Preparation of Ru nanoparticles
In a typical experiment, P123 (PEO20ꢀPPO70ꢀPEO20, Mw: 5800, 30mg), RuCl
3
(
(
2.1 mg, 0.01 mmol) and water (2 mL) were placed in a stainless steel autoclave
75mL) equipped with a Teflonꢀliner. The mixture was stirred for 2 h at 25 °C. Then
the autoclave was sealed, and the air in the autoclave was replaced four times with 1
MPa H . The autoclave was pressurized to 0.5 MPa H and placed in a 40 °C water
2
2
bath. After stirring for 1 h, the reactor was cooled to ambient temperature and vented.
The obtained dark homogeneous solution was used directly for the hydrogenation of
αꢀpinene.
2
.3 Hydrogenation of αꢀpinene
In a typical experiment, αꢀpinene (0.272 g, 2 mmol) was added into the catalytic
system as described above (section 2.2). The autoclave was sealed, and the air in the
autoclave was replaced four times with 1 MPa H . Then H was admitted to the
2
2
system at a constant pressure up to 0.7 MPa. The mixture was stirred for 2 h at 40 °C.
After the reaction, the products were extracted by nꢀheptane and analyzed by using
gas chromatography (GC). The GC analysis was performed in a GC9790 (Fuli) gas
chromatography equipped with a flame ionization detector (FID) detector and an OV
1
701 (50 m, 0.25 mm i.d.) column. Under these conditions, the conversion of
αꢀpinene was 99.9%, and the selectivity of cisꢀpinane was 98.9%. For the recycling
procedure, the nꢀheptane remaining in the autoclave was evaporated under a vacuum,
and the αꢀpinene was charged into the autoclave for subsequent recycling.
2
.4 Characterization
The reaction time was determined according to ultravioletꢀvisible absorption
spectra (UVꢀVis, Varian Cary 500); 1 mL of the micellar solution was diluted to 20
mL with distilled water before measurement. The UVꢀVis absorption spectra are
shown in Fig. S1.
The particle sizes and shapes of the Ru nanoparticles were measured by
transmission electron microscopy (TEM, Hitachiꢀ7650). The specimens were
prepared by placing a drop of the micellar dispersion on a copper grid and then
evaporating the solvent. The particle diameters were measured from the enlarged
TEM photographs. A particle size distribution histogram was obtained on the basis of
the measurements of about 400 particles.
The valence of Ru nanoparticles was tested by Xꢀray photoelectron spectroscopy

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(XPS, Kratos Axis Ultra DLD). A mono Al Kα (hv=1486.6 eV) Xꢀray source was
used at a power of 150 W (15 kV). Binding energies were calibrated by using the C1s
hydrocarbon peak at 284.60 eV. The samples were prepared by drying the Ru
nanoparticles by rotator evaporation. The results are shown in Fig. S2.
The amount of Ru leaching during the reaction was measured by inductively
coupled plasmaꢀatomic emission spectrometry (ICPꢀAES, Prodigy XP, Leeman,). A
certain amount of sample was dissolved in 5mL aqua regia. The mixture was then
transferred to a 10 mL volumetric flask, diluted to 10mL, and tested by ICP analysis.
The mean diameter of the P123 micelles containing Ru nanoparticles was
determined by dynamic light scattering (DLS, Nano ZS90, Malvern). The analysis of
the recorded correlation functions was conducted by using the cumulant method.
The vesicleꢀmicroreactor photographs were captured by using a confocal laser
scanning microscope (CLSM, TCSꢀSP5ꢀII, Leica).
3
3
Results and discussions
.1 Influence of various molecular weights of P123
A series of Ru nanoparticles was prepared with different molecular weights of
P123 under the same conditions. Their properties and selectivity upon hydrogenation
of αꢀpinene were investigated, the results are listed in Table 1.
Table 1 Influence of various molecular weight of P123 on the reaction results
molecular
weight
Average
Average
c
Conversion (%) Selectivity (%)
a
b
diameter (nm)
diameter (nm)
1
2
5
8
100
900
800
400
ꢀ
ꢀ
27.1
60.1
90.3
89.5
88.2
95.9
97.1
98.4
98.4
98.1
3.15±0.7
2.8±0.4
2.8±0.4
3.0±0.5
26.3
23.5
20.4
22.4
13000
Reaction conditions: P = 1.0 MPa, T = 40 °C, t = 1 h. αꢀpinene: 2 mmol, metal precursor: RuCl
3
(
0.01 mmol), surfactants: 30 mg, reaction medium: water (2 mL)
a]:measured by TEM (Fig 2). [b]: measured by DLS (Fig 3). [c]: Selectivity: The percentage of
[
cisꢀpinane in the product.
The conversion of αꢀpinene was low when the molecular weight of P123 was
1
100. Because the chain length of P123 was too short to stabilize the particles, the Ru
20,21
nanoparticles aggregated and precipitated.
With an increase in the molecular
weight, P123 provided effective protection against agglomeration, thus enabling
increases in the conversion of αꢀpinene and the selectivity for cisꢀpinane. When the
molecular weight of P123 was 5800, the conversion of αꢀpinene reached the
maximum, 90.3%. Further increases in the molecular weight of P123, only slightly
reduced the conversion of αꢀpinene and selectivity for cisꢀpinane. The results may be
related to the particle size of nanoparticles and the diameter of micelles in the reaction.

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The diameter of P123ꢀRu micelles was tested by DLS. As shown in Table 1, the
average diameter of all the micelles was about 20ꢀ30nm. Because micelles were in
thermodynamic equilibrium where monomers rapidly exchanged among aggregates,
these differences were too small to affect the reaction. The particle size of Ru
nanoparticles was tested by TEM; the results are shown in Table 1 and Fig.1.All of
the particles had small sizes of about 2.0ꢀ4.0 nm in average. Ru nanoparticles were
dispersed evenly in the system, as shown in Fig.1 (B). However, as described in Fig.1
(
A), (C), and (D), some of nanoparticles gathered into clumps, which decreased the
2
2,23
numbers of active sites of the catalyst and reduced the catalytic activity.
Therefore,
the conversion of αꢀpinene decreased. All of the nanoparticles were extremely stable
without precipitation for at least six months at room temperature, because the
amphiphilic block copolymers were able to selfꢀassemble into micelles in the aqueous
solution and on the surface of nanoparticles. These micelles could be exploited as
24,25
nanocontainers to protect the Ru nanoparticles.
The vesicleꢀmicroreactor formed
during the reaction of hydrogenation of αꢀpinene was observed by CLSM, as shown
in Fig. 2. Dispersion for vesicles in samples in (B), (C), and (D) was better than that
for (A); those in (B), with a molecular weight of 5800, had the best distribution. These
results are consistent with those of TEM and catalytic reactivity. Thus, we selected
the molecular weight of 5800 (P123) as the stabilizer for subsequent experiments. We
also explored the use of several commercially available surfactants; our experiment
showed that P123 is the best among those tested (Table S1).
Fig.1 TEM images of Ru nanoparticles protected by P123 with various molecular weights
A: 2900, B: 5800, C: 8400, D: 13000

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Fig.2 CLSM images of vesicles during the hydrogenation of αꢀpinene with various molecular
weights of P123.
A: 2900, B: 5800, C: 8400, D: 13000
3
.2 Influence of various reaction media
A series of Ru nanoparticles protected by P123 was prepared with various
reaction media. Their size and conversion of αꢀpinene were shown in Table 2.
Table 2. Hydrogenation of αꢀPinene by Ru nanoparticles dispersed in various reaction media
Reaction
medium
Average diameter
Conversion
Selectivity
TOF
Entry
[b]
ꢀ1
(nm)
ꢀ
(%)
ꢀ
(%)
ꢀ
(h )
1
2
nꢀheptane
carbon
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
tetrachloride
ethanol
3
4
5
3.8±0.5
2.7±0.4
3.2±0.4
6.0
95.2
97.3
98.6
6
ethyl acetate
methanol
ethanol:
17.5
33.2
17.5
33.2
6
2.4±0.4
36.0
97.8
36.0
[a]
water(1:1)
water
7
2.8±0.4
99.9
98.9
99.9
ꢀ1
ꢀ1
TOF: Turnover frequency measured in [mol product] [mol metal] h .
Reaction conditions: P = 1.0 MPa, T = 40 °C, t = 2 h, αꢀpinene: 2 mmol, metal precursor: RuCl
0.01 mmol), surfactants: P123 (Mw: 5800, 30 mg), reaction medium: 2 mL.
a]: 1mL ethanol and 1 mL water. [b]: measured by TEM, TEM images are shown in Fig. S3.
3
(
[
As shown in Table 2, RuCl
3
in nonpolar reaction media such as nꢀheptane and
CCl
4
(entries 1 and 2) cannot be reduced to Ru nanoparticles. In polar organic
reaction media such as ethanol, ethyl acetate, and methanol and in an aqueous
solution, varied diameters of nanoparticles were obtained. Despite their similar
particle sizes, the stability of Ru nanoparticles and the conversion of αꢀpinene in the
aqueous reaction medium were markedly better than those in the organic reaction
media. When the concentration of P123 in the aqueous solution was higher than its
26
CMC, the hydrophilic PEO blocks extended into the water, and the hydrophobic

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PPO blocks pointed to the interior. Then, the polymer assembled into a spherical
2
7,28
micelle with a core
that protected the Ru nanoparticles. In the hydrogenation
reaction, microreactors were formed. αꢀpinene was dissolved in the hydrophobic
interior of the microreactors, and Ru nanoparticles were enriched in the interface of
29
the microreactors. It was beneficial to promote the substrate to transfer to the
interface and to contact the Ru nanoparticles catalyst, thereby accelerating the
reaction rate. We accordingly selected water as the reaction medium for subsequent
experiments. The amount of water influence on the catalytic activity and selectivity of
Ru nanoparticles was also studied; the data are plotted in Table S2.
3
.3 The influence of various metal precursors
We sequentially examined the catalytic performances of various metal precursors
in the P123 system; the results are shown in Table 3. Ru (CO)12 displayed bad
3
solubility in water and negligible reaction. Nearly identical catalytic reactivity was
observed for three different metal precursors: RhCl and RuCl
3
, PtCl
4
3
. Although all
exhibited better performance than PdCl and Ru(NO)(NO ) ,the selectivity obtained
2
3 3
in the presence of RuCl , 98.6%, was markedly better than that with other catalysts.
3
This result could be related to the higher surface hydrophilicity caused by adsorbed
30
Cl, which increased the mobility of interface between two phases, thereby increasing
2
7,29
the mass transfer rate and the reaction rate.
precursor for subsequent experiments.
3
We hence selected RuCl as the metal
Table 3. Hydrogenation of αꢀpinene catalyzed by nanoparticles derived from various metal
precursors.
ꢀ
1
Entry
Metal precursors
PdCl
RhCl
PtCl
Conversion (%)
Selectivity (%)
TOF (h )
173.2
182.4
184.0
181.0
163.2
7.2
1
2
3
4
5
6
2
86.6
91.2
92.0
90.5
81.6
3.6
91.2
96.1
96.2
98.6
96.9
0
3
4
RuCl3
3 3
Ru(NO)(NO )
3
Ru (CO)12
ꢀ1
ꢀ1
TOF: Turnover frequency measured in [mol product] [mol metal] h .
Reaction conditions: P = 1.0 MPa, T = 40 °C, t = 1 h, αꢀpinene: 2 mmol, metal precursor: 0.01
mmol, surfactants: P123 (Mw: 5800, 30 mg), reaction medium: water (2 mL).
3
.4 Influence of P123 concentration on αꢀpinene hydrogenation
The influence of P123 concentration on the catalytic activity and selectivity of
Ru nanoparticles was studied; the data are plotted in Fig. 3. When the P123
ꢀ3
ꢀ1
concentration increased from 0 to 2.6 × 10 mol L , the conversion of αꢀpinene rose
from 21.7% to 92.8%. Conversions decreased with further increases in P123
concentration. Interestingly, the selectivity for cisꢀpinane increased from 93.5% to
ꢀ3
ꢀ1
9
8.9% as the concentration of P123 increased from 0 to 2.6 × 10 mol L , and no
obvious decrease for selectivity was noted when the concentration of P123 further
ꢀ3
ꢀ1
increased to 5.3 × 10 mol L .

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0
1
2
3
4
5
6
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Conversion
Selectivity
0
1
2
3
4
5
6
ꢀ3
ꢀ1
P123 concentration / 10 mol L
Fig.3. Influence of P123 concentration on αꢀpinene hydrogenation
Reaction conditions: P = 1.0 MPa, T = 40 °C, t = 1 h, αꢀpinene: 2 mmol, metal precursor: RuCl
0.01 mmol), surfactants: P123 (Mw: 5800), reaction medium: water (2 mL).
3
(
To explain this phenomenon, we performed a series of experiments; the results
are shown in Table S3. These phenomena are attributed mainly to the formation of
P123 micelles. After the concentration of P123 reached its CMC, the number of
micelles increased along with the concentration of P123. In the progression of the
reaction, the number of vesicleꢀmicroreactors increased. As a result, the interfacial
area between two phases enlarged during the reaction. The local concentration of
αꢀpinene in the vesicles increased significantly, and the concentration of Ru
nanoparticles increase obviously in the interfacial layer. These conditions are more
favorable for the coordination of αꢀpinene with Ru nanoparticles; thus, the reaction
was accelerated. However, the decreased reactivity observed as the P123
concentration exceeded a certain value is due likely to the dilution effect of the
substrate in the micelles and an increase in the viscosity of the solution, which may
31ꢀ33
have interfered with the impingement of reactant molecules.
concentration of P123 in the reaction was 2.6 × 10 mol L .
Therefore, the best
ꢀ3
ꢀ1
3
.5 The mechanism of reaction
In the procedure of catalyst preparation, P123 was added to the water. When the
concentration reached its CMC, the hydrophilic PEO blocks extended into the water,
and the hydrophobic PPO blocks pointed to the interior. Then, the polymer assembled
into a spherical micelle with a core, as described in Scheme 2 (a). With the addition of
3
+
0
3 2
RuCl and H to the mixture, Ru was reduced in certain conditions to Ru , and Ru
nanoparticles were subsequently formed and were protected in the core by the P123
micelles (Scheme 2 (b)).
In the procedure of reducing the substrate, the vesicles were formed in the
reaction system when the hydrogenation of αꢀpinene occurred. The structures of
internal and external phases were separated by the vesicles. As described in Scheme 2,
every vesicle can be considered as a microreactor in the reaction. The Ru
nanoparticles protected by P123 micelles in the vesicleꢀmicroreactor system were
34
similar to the enzymes in the cells. A special role was played by the

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2
vesicleꢀmicroreactor for the separated reaction spaces. The bulk αꢀpinene and H were
easily solubilized in the hydrophobic interior of the vesicleꢀmicroreactor; thus, the
local concentration of αꢀpinene in vesicles was extremely high. The Ru nanoparticles
35
were enriched in the interface of the vesicleꢀmicroreactor. In this microꢀcircumstance,
the distance of αꢀpinene from the interior of the vesicle to the interface was shortened,
which was beneficial for promoting the substrate to contact with the Ru catalyst.
Furthermore, this structure significantly increased the biphasic interface area, and the
energy barrier of phase transfer was effectively minimized. The combination of the
aforementioned factors created a highly advantageous condition for acceleration of
36
the reaction. In addition, the special spatial structure of the P123 micelles protected
the Ru nanoparticles. This allowed only the endo surface of αꢀpinene with a small
space steric to contact with the catalyst, which increased the selectivity of cisꢀpinane.
In addition, compared with other precious metals (Table 3), Ru is more suitable for
1
3
the highly selective hydrogenation of αꢀpinene.
Scheme 2. Mechanism of hydrogenation of αꢀpinene
3
.6 Optimal conditions of αꢀpinene hydrogenation
Temperature /
°
C
Pressure of H /MPa
2
3
0
40
50
60
0.0
00
0.5
1.0
1.5
2.0
2.5
100
1
00
100
90
80
70
60
50
40
1
90
80
70
60
50
40
95
95
90
85
80
75
70
65
90
85
80
75
70
65
0.004
0
.5
1.0
1.5
2.0
2.5
0.006
0.008
0.010
0.012
0.014
0.016
Time / h
Amount of catalysts / mmol
Fig. 4 The influence of reaction time, temperature, pressure of H and catalyst amount on the
2
reaction
Reaction conditions: P = 0.5ꢀ2.0 MPa, T = 30ꢀ60 °C, t = 0.5ꢀ2.5 h, αꢀpinene: 2 mmol, metal
3
precursor: RuCl (0.005ꢀ0.015 mmol), surfactants: P123 (Mw: 5800, 30mg), reaction medium:
water (2 mL).

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To further study the influence of the catalyst on the hydrogenation of αꢀpinene,
several parameters such as temperature, H pressure, reaction time, and catalyst
2
amount were studied; the results are shown in Fig.4. The conversion of αꢀpinene
increased with increasing temperature. When the temperature was higher than 40°C,
the influence of temperature on the conversion was not obvious. Chou and
3
7
coꢀworkers demonstrated that low temperature favors the production of cisꢀpinane
from αꢀpinene. Thus, we selected the temperature of 40°C for subsequent experiments.
2
The conversion and selectivity increased with increasing H pressure. However, the
use of high pressure would increase the capital costs for the hydrogenation plant
facility. Examination of the catalyst amount revealed 0.01mmol as the best value.
Considering the analysis results of the four factors, the optimum conditions were
determined to be: P = 0.7 MPa, T = 40 °C, t = 2 h, 0.01 mmol RuCl , 2 mmol
3
αꢀpinene. Furthermore, the influence of stirring rate on αꢀpinene hydrogenation was
also studied, the data are plotted in Table S4. Our experiment showed that 500 r/min
is the best stirring rate. Under these conditions, the conversion of αꢀpinene was 99.9%,
and the selectivity of cisꢀpinane was 98.9%.
3
.7 Recyclability of the catalyst
1
00
100
90
80
70
60
50
9
0
8
0
conversion
selectivity
7
0
6
0
5
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
No. of recytcle
Fig. 5 Catalyst recycle data for the hydrogenation of αꢀpinene promoted by Ru nanoparticles
Reaction conditions: P = 0.7 MPa, T = 40 °C, t = 2 h. [substrate] : [catalyst]=200 : 1, αꢀpinene: 2
3
mmol, metal precursor: RuCl (0.01 mmol), surfactants: P123 (Mw: 5800, 30 mg), reaction
medium: water (2 mL).
After the reaction, the product phase was separated from the catalyst phase. The
catalyst was directly recycled by the addition of fresh αꢀpinene. As shown in Fig. 5,
the catalyst can be reused more than seven times with no significant decline in
selectivity or activity. Nearly no appreciable Ru leaching into the organic phase was
observed, as indicated by ICP analysis results of 0.79ppm. The selectivity remained
almost unchanged during the experiments. These results occurred because P123
18
micelles protected the Ru nanoparticles from poisoning and accumulation. Therefore,
the new catalytic system of P123ꢀRu shows excellent stability.
After eight catalytic cycles, the aggregation of Ru nanoparticles was confirmed
by TEM (Fig.S4). The freshly prepared Ru nanoparticles had an average particle size
of 2.8 ± 0.4 nm, and the nanoparticles displayed a homogeneous distribution. After

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DOI: 10.1039/C5RA15283F
eight catalytic cycles, the particle size of the Ru nanoparticles increased, and
aggregation occurred. After 14 reaction cycles, an average size particle size of 4.0 ±
0
.5 nm was observed, presumably leading to the decrease in catalytic activity during
the recycling experiments.
The content of Ru leaching determined by ICP was 5.0 ppm (Table S5) after 14
catalytic cycles, which might be attributed to damaged micelles. Ru nanoparticles
without the protection of P123 micelles were exposed to water, and they easily
entered the product phase. With the loss of Ru catalyst, the catalytic activity gradually
decreased. Furthermore, the residual extractant may also have an impact on catalytic
activity.
This theory might partly account for the loss of activity as well as the possible
deactivation of the catalyst during the recycling procedure. As described in Table 2,
the catalytic activity was very poor when Ru nanoparticles were dispersed in
nꢀheptane instead of water. When 0.5ꢀ2.0 mL excess nꢀheptane was added to the
prepared catalytic system of P123ꢀRu in water, the conversation of αꢀpinene
decreased 6.3% and 24.4%, respectively (Table S6). These results indicate that the
residual nꢀheptane in the emulsions likely affects the assembling behavior of
polymerꢀbased catalysts, resulting in negative effects for the reduction of substrates.
Furthermore, this substance destroys the structure of microꢀreactors to some extent.
4
Conclusions
In summary, a new mild and environmentally beneficial methodology has been
developed for efficient hydrogenation of αꢀpinene in a aqueous medium based on the
generation of Ru nanoparticles protected by P123 micelles. This new catalytic system
has been demonstrated to be suitable for environmentally clean reduction of αꢀpinene.
The hydrogenation of αꢀpinene by the P123ꢀRu micellar catalyst in the aqueous phase
resulted in effective conversion of αꢀpinene and selectivity for cisꢀpinane under very
mild conditions. The catalyst can be reused more than seven times with no significant
decline in selectivity or activity. The P123 micelles can protect the Ru nanoparticles
from poisoning and accumulation. This study provides a reference for the study of
other hydrophobic natural products in hydrogenation reactions.
Acknowledgements
This work was financially supported by the National Natural Science Foundation
of China (No. 31270615).
Notes and references
1
. X. R. Liu, X. C. Wang, S. X. Yao, Y. J. Jiang, J. Guan and X. D. Mu, RSC Adv.,
014, 4, 49501.
. P. MäkiꢀArvela, B. Holmbom, T. Salmi and D. Y. Murzin, Cat. Rev., 2007, 49,
97.
2
2
1
3
4
. S. Tanielyan, N. Biunno and R. Bhagat, Top Catal., 2014, 57, 1564.
. V. A. Semikolenov, I. I. Ilyna and I. L. Simakova, Appl. Catal. A: General, 2001,
2
11, 91.

RSC Advances
Page 12 of 14
View Article Online
DOI: 10.1039/C5RA15283F
5
6
. I. L. Simakova, Y. Solkina, I. Deliy, J Wärnå and D. Y. Murzin, Appl. Catal. A:
General, 2009, 356, 216.
. M. L. Casella, G. F. Santori, A. Moglioni, V. Vetere, J. F. Ruggera, G. M.
Iglesias and O. A. Ferretti, Appl. Catal. A: General, 2007, 318, 1.
. S. H. KO and T. C. Chou, Can. J Chem. Eng., 1994, 72, 862.
. D. Chouchi, D. Gourgouillon, M. Courel, J. Vital and M. N. Ponte, Ind. Eng.
Chem. Res., 2001, 40, 2551.
7
8
9
. A. Milewska, A. B. Osuna, I. M. Fonseca and M. N. Ponte, Green Chem., 2005, 7,
7
26.
1
1
0. D. C. Rideout and R. Breslow, J. Am. Chem. Soc., 1980, 102, 7816.
1. M. B. Gawande, V. D. B. Bonifácio, R. Luque, P. S. Branco and R. S. Varma,
Chem. Soc. Rev., 2013, 42, 5522.
1
1
2. A. Chanda and V. V. Fokin, Chem. Rev., 2009, 109, 725.
3. X. Yang, S. W Liu, C. X. Xie, S. T. Yu and F. S. Liu, Chin. J. Catal., 2011, 32,
6
43.
1
4. J. H. Fendler and E. J. Fendler, Catalysis in Micellar and Macromolecular
Systems, Academic Press, New York, 1975.
1
1
5. A. Cavarzan, A. Scarso and G. Strukul, Green Chem., 2010, 12, 790.
6. L. M. Wang, N. Jiao, J. Qiu, J. J. Yu, J. Q. Liu, F. L. Guo and Y. Liu, Tetrahedron,
2
010, 66, 339.
7. S. L. Huang, K. R. Voigtritter, J. B. Unger and B. H. Lipshutz, Synlett, 2010, 13,
041.
8. M. Schwarze, J. S. MilanoꢀBrusco, V. Strempel, T. Hamerla, S. Wille, C. Fischer,
W. Baumann, W. Arlt and R. Schomäcker, RSC Adv., 2011, 1, 474.
1
1
2
1
2
2
2
2
2
2
9. P. Alexandridis, Curr. Opin. Colloid Interface Sci., 1996, 1, 490.
0. T. Sakai and P. Alexandridis, Langmuir, 2004, 20, 8426.
1. M. H. Liu, M. F. Han and W. W. Yu, Environ. Sci. Technol., 2009, 43, 2519.
2. W. Liu, X. Yang and W. Huang, J. Colloid Interface Sci., 2006, 304, 160.
3. M. Liu, B. He, H. Liu and X. Yan, J. Colloid Interface Sci., 2003, 263, 461.
4. Y. Lin and P. Alexandridis, J. Phys. Chem. B., 2002, 106, 10834.
5. P. Alexandridis and R. J. Spontak, Curr. Opin. Colloid Interface Sci., 1999, 4,
1
30.
6. P. Alexandridis, J. F. Holzwarth and T. A. Hatton, Macromolecules, 1994, 27,
414.
2
2
2
2
7. T. Dwars, E. Paetzold and G. Oehme, Angew. Chem. Int. Ed., 2005, 44, 7174.
8. C. G. P. H. Schroen, M. A. C. Stuart, K. V. D. V. Maarschalk, A. V. D. Padt and
K. V. Riet, Langmuir, 1995, 11, 3068.
2
3
3
3
9. M. S. Goedheijt, B. E. Hanson, J. N. H. Reek, P. C. J. Kamer and P. W. N. M. van
Leeuwen, J. Am. Chem. Soc., 2000, 122, 1650.
0. C. Milone, G. Neri, A. Donato, M. G. Musolino and L. Mercadante, J. Catal.,
1
996, 159, 253.
1. F. Wang, H. Liu, L. F. Cun, J. Zhu, J. G. Deng and Y. Z. Jiang, J. Org. Chem.,
005, 70, 9424.
2. F. Trentin, A. M. Chapman, A. Scarso, P. Sgarbossa, R. A. Michelin, G. Strukul
2

Page 13 of 14
RSC Advances
View Article Online
DOI: 10.1039/C5RA15283F
and D. F. Wass, Adv. Synth. Catal., 2012, 354, 1095.
33. J. H. Li, X. F. Li, Y. P. Ma, J. S. Wu and J. G. Deng, RSC Adv., 2013, 3, 1825.
34. G. L. Sorella, G. Strukula and A. Scarso, Green Chem., 2015, 17, 644.
35. L. Wang, H. X. Ma, L. Song, L. Li and H. D. Wang, RSC Adv., 2014, 4, 1567.
36. H. Y. Fu, M. Li and H. Chen. J. Mol. Catal. A: Chem., 2006, 259, 156.
37. S. H. KO and T. C. Chou, Ind. Eng. Chem. Res., 1995, 34, 457.

RSC Advances
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DOI: 10.1039/C5RA15283F
ABSTRACT
Ruthenium (Ru) nanoparticles were prepared using polyoxyethyleneꢀ
polyoxypropyleneꢀpolyoxyethylene triblock copolymer (P123) micelles in water as
the stabilizing agent. The P123ꢀRu micellar catalyst was first used in the
hydrogenation of αꢀpinene to pinane, and the selectivity for cisꢀpinane reached 98.9%.
This result is attributed to the formation of vesicles. The isolated catalyst phase could
be used seven times with no treatment, and its catalytic activity and selectivity were
almost unchanged. The preparation process of catalyst and hydrogenation reaction of
αꢀpinene was under mild and environmentally friendly conditions. This research
offers an effective method for the hydrogenation of αꢀpinene and provides a reference
for other hydrophobic natural products in hydrogenation reactions.
In the procedure of catalyst preparation, P123 was added to the water. When the
concentration reached its CMC, the hydrophilic PEO blocks extended into the water, and the
hydrophobic PPO blocks pointed to the interior. Then, the polymer assembled into a spherical
3 2
micelle with a core, as described in Scheme (a). With the addition of RuCl and H to the mixture,
3
+
Ru was reduced in certain conditions to Ru0, and Ru nanoparticles were subsequently formed
and were protected in the core by the P123 micelles (Scheme (b) ).
In the procedure of reducing the substrate, the vesicles were formed in the reaction system
when the hydrogenation of αꢀpinene occurred. The structures of internal and external phases were
separated by the vesicles. As described in Scheme, every vesicle can be considered as a
microreactor in the reaction. The Ru nanoparticles protected by P123 micelles in the
vesicleꢀmicroreactor system were similar to the enzymes in the cells. A special role was played by
2
the vesicleꢀmicroreactor for the separated reaction spaces. The bulk αꢀpinene and H were easily
solubilized in the hydrophobic interior of the vesicleꢀmicroreactor; thus, the local concentration of
αꢀpinene in vesicles was extremely high. The Ru nanoparticles were enriched in the interface of
the vesicleꢀmicroreactor. In this microꢀcircumstance, the distance of αꢀpinene from the interior of
the vesicle to the interface was shortened, which was beneficial for promoting the substrate to
contact with the Ru catalyst. Furthermore, this structure significantly increased the biphasic
interface area, and the energy barrier of phase transfer was effectively minimized. The
combination of the aforementioned factors created a highly advantageous condition for
acceleration of the reaction. In addition, the special spatial structure of the P123 micelles protected
the Ru nanoparticles. This allowed only the endo surface of αꢀpinene with a small space steric to
contact with the catalyst, which increased the selectivity of cisꢀpinane. In addition, compared with
other precious metals, Ru is more suitable for the highly selective hydrogenation of αꢀpinene.