Selective hydrogenation of α-pinene to: Cis -pinane over Ru nanocatalysts in aqueous micellar nanoreactors

Article abstract of DOI:10.1039/c6ra13099b

d-α-Tocopheryl polyethylene glycol 1000 succinate (TPGS-1000) stabilized Ru(0) nanoparticles were prepared and characterized. These nanoparticles were employed to selectively hydrogenate α-pinene to cis-pinane. With a small amount of Na₂CO

Full text of DOI:10.1039/c6ra13099b

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Selective Hydrogenation of αꢀPinene to Cisꢀpinane over Ru Nanocatalysts in Aqueous
Micellar Nanoreactors
Shengli Hou,a Congxia Xie,*a Fengli Yu,a Bing Yuana and Shitao Yub
aState Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering,
Qingdao University of Science and Technology, Qingdao, 266042, China. E-mail: xiecongxia@126.com;
Fax: +86 532 8402 3927; Tel: +86 532 8402 3927
bCollege of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, 266042,
China.
Abstract：Dꢀαꢀtocopheryl polyethylene glycol 1000 succinate (TPGSꢀ1000) stabilized Ru(0) nanoparticles
were prepared and characterized. These nanoparticles were employed to selectively hydrogenate αꢀpinene to
cisꢀpinane, with a small amount of Na2CO3 present, reaction rates could be increased significantly, and the
reaction media could be readily recycled. TEM, CLSM, IR and leaching experiments were employed to
quantify the advantages of catalytic system. The procedure is environmentally friendly. It offers a reference
for the catalytic hydrogenation of other hydrophobic natural products.
Key words: αꢀpinene; hydrogenation; micelle; nanoreactor; water
1. Introduction
Metal nanoparticles have attracted a growing interest in the international scientific community in the
last few decades. Such nanoparticles exhibit many novel chemical and physical properties that differ from
their bulk counterparts.1 These novel properties are attributed to the surface effect, quantum size effect and
quantum tunneling effect. They are widely used in various kinds of catalytic reactions and fuel cells due to
their large surface area, small size and highꢀactivity,2 such as hydrogenation, hydrogenolysis, oxidation,
biomass conversion and FischerꢀTropsch synthesis reaction.3 Therefore, synthesis of metal nanoparticles has
attracted a tremendous amount of attention from researchers over the past two decades. Micellar method is
one of important ways widely used to prepare nanoꢀmetal catalyst with accurately controlled particle sizes
and shapes.4
Micelles formed by addition of amphiphiles to an aqueous reaction medium. The hydrophilic chains
extended into the water and the lipophilic chains pointed to the interior. Then the surfactant assembled into
micelles with a lipophilic 'core'.5 For the fabrication of metal nanoparticles, the use of micelles for
nanoparticle synthesis has attracted much interest because of its potential advantages. The reaction is
restricted in the lipophilic core, the growth of the obtained metal nanoparticles can be controlled by the size
of the nonpolar core, and the metal nanoparticles can be stabilized. Metal nanoparticles protected by
micelles can be thought as micellar catalysis.6 Micellar catalysis in water can play an important contributing
role in green chemistry. Early review papers on the subject of micellar catalysis date back to the late 1970s.7
Since that time, this area has witnessed a rapid growth. Today, many valued reactions take place within the
lipophilic core of selfꢀaggregated nanomicelles. This type of catalyst, which combines the advantages of
both homogeneous and heterogeneous catalysts, can be regarded as a ‘‘semiꢀhomogeneous catalyst’’: the
metal nanoparticles are heterogeneous in nature but, similar to homogeneous catalysts, their high degree of
dispersion in the solvent facilitates the access of reactant molecules.8 Furthermore, a high local
concentration of the reactants in the nanometerꢀsized micelles leads to accelerated transformations and
increased selectivities.
Compared to traditional “soaps”, the vitamin E derived amphiphiles TPGS offer outstanding properties
in terms of reactivity, selectivity, and catalyst recycling in many transitionꢀmetalꢀcatalyzed reactions.9 Such
as asymmetric hydrosilylation reactions, crossꢀcouplings, Trifluoromethylation of heterocycles , asymmetric
intramolecular hydrocarboxylations. Could all be carried out in ≤5 wt % TPGS/water micellar catalysis at
room temperature.10 However, nanoparticles prepared using TPGS micelles as a stabilizer have seldom been

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reported. In this paper, we designed a catalyst of Ru(0) nanoparticles protected by TPGSꢀ1000 micelles in
aqueous solution. The catalytic system was used in the hydrogenation of αꢀpinene to produce cisꢀpinane,
which is highly desirable materials for fine chemistry, numerous derivatives, such as linalool,
dihydromyrcenol and other terpenes series spices have been prepared from such intermediates.11
2. Results and discussions
2.1 Impact of surfactant concentration
Table 1 Screening of the weight percent of TPGSꢀ1000 for the conversion of αꢀpinene a
Size (nm) c
Zeta
Weight percent Average Diameter
Viscosity
(Cp) d
Conv.
(%)
Select.
(%)
Entry
potencial
(mV)
ꢀ
(%)
(nm) b
TPGS
Ru(0)/TPGS
1
2
3
4
5
0
ꢀ
ꢀ
ꢀ
1.12
1.28
1.32
1.43
1.50
6.0
96.7
98.5
99.2
99.0
98.9
0.25
0.50
1.00
1.50
2.63 ± 0.15
2.47 ± 0.07
2.19 ± 0.09
2.63 ± 0.16
12.3
10.6
10.5
10.0
17.3
15.1
14.2
13.6
12.2
23.5
29.3
31.2
52.5
61.3
59.0
50.7
a
b
Conditions: substrate (2 mmol), catalyst (0.01mmol), H2 (0.5 MPa), temperature (50 °C), time (1.5 h). Measured using
c
TEM, the results represent error margin of 95% confidential intervals. TEM images were shown in Fig. S3. Measured
using DLS. d Measured at 16 °C.
The results from varying the amount of TPGSꢀ1000 from 0 to 1.5 weight percent for the reaction were
shown in Table 1. The conversion and selectivity were poor in the control experiment performed ‘on water’
(entry 1), this confirmed the importance of micellar catalysis in facilitating hydrogenation in aqueous media.
The conversion of αꢀpinene increased with the weight percent of TPGSꢀ1000 (entries 2 and 3), and
decreased with further increased in TPGSꢀ1000 percent (entries 4 and 5). This could be ascribed to a
combination of the following factors: On one hand, Ru(0) particle size decreased with increasing the amount
of TPGSꢀ1000, and the catalyst activity increased. Ru(0)/TPGSꢀ1000 micelle size also decreased from 17.3
nm to 13.6 nm, the zeta potential of Ru(0)/TPGSꢀ1000 micelle increased with increasing the amount of
TPGSꢀ1000, these leaded Ru(0) nanoparticles became more stable.12 Furthermore, the number of micelles
increased with the amount of TPGSꢀ1000 rose further, the interfacial area between two phases would also
increase.13 As a result, the reaction was accelerated. However, the decreased activity observed as the weight
percent of TPGSꢀ1000 exceeded a certain value. It was probably due to micelle sizes were decreased, which
left less room for the substrate to diffuse into micelles. Furthermore, the viscosity of the mixture was
increased with the amount of TPGSꢀ1000 increased (1.25 Cp to 1.50 Cp), which may have interfered with
the impingement of the reactant molecules.14 Hence, we selected 0.5% TPGSꢀ1000/H2O for subsequent
experiments.
2.2 Effects of metal salts on αꢀpinene hydrogenation
Table 2 Effects of metal salts on the conversion of αꢀpinene a
Weight
(mg)
2.5
Size
(nm) b
15.9
16.3
17.1
19.7
39.8
18.7
Conv.
(%)
Select.
(%)
Entry
Salt
1
2
3
4
5
H3O40PW12
CoCl2
50.3
51.5
58.0
99.7
99.1
95.6
99.0
98.8
99.1
98.8
98.3
97.5
2.0
NaCl
2.0
Na2CO3
Na2CO3
NaOH
2.0
10.0
2.0
6
a
b
Conditions: substrate (2 mmol), catalyst (0.01mmol), H2 (0.5 MPa), temperature (50 °C), time (1.5 h). Measured using
DLS.

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It is known that the size of TPGSꢀderived micelles can be strongly increased in the presence of metal
salt, leading to faster reactions in crossꢀcoupling and ringꢀclosing olefin metathesis.15 To establish whether
this effect can also be utilized in hydrogenation reaction catalyzed by Ru(0) nanoparticle catalyst, dynamic
light scattering (DLS) measurements of a Ru(0)/TPGSꢀ1000 solution were carried out (Table 2).
Interestingly, the addition of salt to the Ru(0)/TPGSꢀ1000 solution made the micellar sizes become larger.
But the impact on the conversion was different. When adding a small amount of acidic salts and neutral salts,
the conversion were decreased (entries 1,2,3 compared with entry 3 in Table 1). However, the limited
conversion could be increased dramatically by adding 2mg of Na2CO3 (entry 5) or NaOH (entry 7). With
further increased the amount of Na2CO3, the conversion and selectivity were declined to some degree (entry
6). Attempted to figure out the cause of the increased conversion, we designed a series of experiments to
explore this phenomenon. The research started with the evaluation of pH effects on the hydrogenation of
αꢀpinene over Ru catalyst (As Ru catalyst was resistant to corrosion and oxidation, it provided reliable
results over the whole pH range) 16. Experiments were carried out at a pH range from 0.51 to 12.9, adjusted
with HCl to obtain an acidic and NaOH for a basic reaction medium. The results were presented in Fig. 1.
Without the addition of acid or base the reaction mixture has an initial pH of 2.1. A decrease of the pH to
0.95 and 0.51 led to a decrease of the conversion from 61.3% to 47.5% and 43.1%. By a sharp contrast,
changed the pH to higher values of 10.2 and 11.2 induced a markedly increased in the conversion of
αꢀpinene. Even higher pH of 11.5 and 12.9 led to a gradually reduced conversion was also observed.
Nevertheless, the conversion of αꢀpinene was almost unchanged when the reaction system was free of
TPGSꢀ1000 micelles. These results indicated high pH had no directly impact on Ru(0) nanoparticles and
αꢀpinene, but it was favorable to hydrogenation of αꢀpinene in micellar system. The Ru(0) nanoparticles
before the reaction (Fig.S3) and after the reaction (Fig. 4B) were measured using TEM, the results showed
that there were almost no change in the sizes of the Ru(0) nanoparticles after adding 2 mg Na2CO3 in the
hydrogenation (pH=11.2). Furthermore, The infrared spectroscopy was also used to characterize the change
of Ru(0) nanoparticles under higher pH (Fig. 2). The appearance of the shoulder peak around 1600 cmꢀ1
suggested that the interactions between Ru(0) and TPGSꢀ1000 were enhanced, and Ru(0) nanoparticles
became more stable in the micelles.17 Hence, the acceleration by Na2CO3 could be ascribed to a combination
of the following factors: On one hand, TPGSꢀ1000 micelles were enlarged, and the internal volume capacity
of the lipophilic ‘core’ increased. This could accommodate more substrates in the reaction. On the other
hand, Na2CO3 was added to the reaction system and provided an alkaline environment. Alkaline
environment was favorable to hydrogenation of αꢀpinene in micellar system. Furthermore, alkaline
conditions could improve the mobility of lipophilic chain. The mass transfer rate between aqueous phase and
lipophilic ‘core’ was accelerated, which was affects both reaction rate and product separation procedure.18 In
addition, after Na2CO3 was added in the hydrogenation reaction, Ru(0) nanoparticles were uniformly
dispersed in aqueous (Fig S4), provided an effective catalytic system. Moreover, αꢀpinene also dispersed
evenly in the semiꢀhomogeneous catalyst during hydrogenation (Fig S5). The biphasic interface area was
significantly increased, and the energy barrier of phase transfer was effectively minimized.19 The
combination of the aforementioned factors created highly advantageous conditions for acceleration of the
reaction.
100
80
60
40
0.5% TPGSꢀ1000/Water
Water
9
6
3
0
0
2
4
6
8
10
12
14
pH

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Fig. 1
Conversion of the hydrogenation of αꢀpinene over Ru(0) nanoparticles as a function of pH. Reaction condition: H2
(0.5 MPa), temperature (50 °C), time (1.5 h), 0.5 mol% Ru.
TPGSꢀ1000
Ru(0)/TPGSꢀ1000
Ru(0)/TPGSꢀ1000
(With Na CO )
2
3
3000
2500
2000
1500
1000
500
Wavelength (cmꢀ1)
Fig. 2
IR spectra of the Ru(0)/TPGSꢀ1000.
2.3 Recyclability of the catalyst
100
80
1 mL
TPGS solution
60
40
20
0
50 °C
2.5 h
0.5 MPa
2.0 mg Na2CO3
50
°
C
1.5 h
0.5 MPa
0
2
4
6
8
10
12
14
16
18
No. of recycle
Recycling of the aqueous reaction mixture for the conversion of αꢀpinene
Fig. 3
Another characteristic of this process was the opportunity to recycle the contents of the entire reaction
mixture. When the reaction was complete, the product was extracted with a single organic solvent (ethyl
acetate) in reactor (5×0.5 mL). The organic phase was removed with syringe to ensure the product and any
starting materials were removed as completely as possible (Fig. S5). Remaining in the water were the
surfactant, Na2CO3, Ru(0) catalyst and thimbleful of ethyl acetate. Addition of fresh αꢀpinene (0.2730 g) and
H2 (0.5 MPa), a new reaction could be conducted as described for the initial run. As illustrated in Fig. 3, this
process could be repeated more than ten times with a small decrease in conversion. However, when there
was no Na2CO3 in the mixture, this process could be repeated only one cycle. The reduced conversion may
be related to the particle size of the Ru(0) nanoparticles. TEM was employed to measure particle size and
shape. As shown in Fig. 4, after one catalytic cycle, the particle size of the Ru(0) nanoparticles increased,
and aggregation occurred without Na2CO3 in the mixture. Some of Ru(0) nanoparticles gathered into clumps
(Fig. 4A), presumably leading to the decrease in catalytic activity during the recycling experiments. Ru(0)
nanoparticles gathered into clumps also could be observed in the catalyst phase after reaction (Fig. S4).
Nevertheless, when Na2CO3 was added in the mixture and the reaction system was in alkaline environment,
Ru(0) nanoparticles displayed a homogeneous distribution after one catalytic cycle (Fig. 4B). Even after
fifteen cycles, only a small part of Ru(0) nanoparticles aggregation occurred (Fig. 4C). Furthermore, this
gradually decreased conversion was probably due to diffusion of ethyl acetate into the micelles, which left
less room for the substrate.20 This problem can be solved either by removal of excess ethyl acetate under
reduced pressure or by extending the time for phase separation. We also tested the catalyst for Ru(0)
leaching, which is an important criterion in terms of recyclability. 1 g of extraction was dissolved in aqua
regia and tested by ICPꢀAES analysis. Over the 1 cycle, no Ru(0) leaching was observed. Only 0.2% of the
Ru(0) catalyst was lost in the 17 cycles. This result demonstrated the high stability of the Ru(0) catalyst in

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hydrogenation of αꢀpinene under alkaline environment.
Fig. 4
TEM images of Ru(0) nanoparticles in various recycle stages. A: after 1 cycle without Na2CO3. B: after 1
cycle. C: after 15 cycles.
2.4 Hydrogenation of αꢀpinene at room temperature
Moreover, unlike most hydrogenation of αꢀpinene that occur best at high temperatures and high H2
pressure,21 these procedure also could proceed smoothly at normal pressure and room temperature. The
results of αꢀpinene hydrogenation at ambient temperature were summarized in Table 3. The conversion of
αꢀpinene was poor at ambient temperature (entry 1), but increased significantly with Na2CO3 was added
(entries 2 and 3). Some examples previously studied by Nowicki and coꢀworkers were directly compared
(entries 4 and 5).22 The TOF were improved, while reaction times are comparable. We also compared to a
classical catalyst such as Pd/C (entry 6), Ru(0)/TPGSꢀ1000 had a better result on selectivity for cisꢀpinane
than Pd/C. This illustrated that Ru(0)/TPGSꢀ1000 was an efficient catalyst for hydrogenation of αꢀpinene
under alkaline environment.
Table 3 Direct comparisons with other catalysts at room temperature a
P
(MPa)
0.1
T
(°C)
20
Time
(h)
12
5
Conv.
(%)
Select.
(%) b
98.6
98.5
98.7
95
TOF
(hꢀ1) c
3.7
Entry
Catalyst
Salt
Reaction medium
1
2
Ru(0)
Ru(0)
Ru(0)
Ru(0)
Ru(0)
Pd/C
ꢀ
TPGSꢀ1000 d
TPGSꢀ1000 d
TPGSꢀ1000 d
MeꢀγꢀCD d
HEA16Cl d
water
22.0
Na2CO3
0.1
20
64.3
25.7
22.1
11.1
3.3
3
Na2CO3
0.1
20
9
99.5
4 e
5 e
6 f
ꢀ
ꢀ
ꢀ
0.1
20
9
>99.9
>99.9
57.2
0.1
20
30
9
88
0.1
20
80.8
ꢀ
a
b
Conditions: substrate (2 mmol), catalyst (0.01mmol), Na2CO3 (2 mg). Selectivity: The percentage of cisꢀpinane in the
c
d
product. Turnover frequency defined as converted number of mol of substrate per mol of ruthenium per hour. The
e
f
surfactant was dissolved in water. The data were referenced from literature 22. 2 mg of Pd/C substituted for
Ru(0)/TPGSꢀ1000 in the hydrogenation.
2.5 The mechanism of reaction
Scheme 1
Mechanism of the hydrogenation of αꢀpinene

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During the preparation of Ru(0) nanoparticles, amphiphilic surfactant TPGSꢀ1000 was dissolved in
aqueous. The hydrophilic heads were extended into the water. The lipophilic ends were pointed to the
interior and shielded from water. Then, the amphiphilic surfactant TPGSꢀ1000 was assembled into spherical
micelles with lipophilic cores (Scheme 1(a)). When the hydrophilic Ru(III) was reduced to lipophilic Ru(0),
Ru(0) nanoparticles were embedded into the lipophilic cores and protected by micelles (Scheme 1(b)). In the
procedure of hydrogenation of αꢀpinene, αꢀpinene and H2 were easily solubilized in the lipophilic cores.5 In
this microꢀcircumstance, micelles could be considered as nanoreactors. Reactions were divided and limited
into the nanoreactors ((Scheme 1(c)).23 These nanoreactors were beneficial for promoting the substrate to
interact with the Ru(0) catalyst, and the reaction was accelerated.24 Furthermore, αꢀpinene dispersed evenly
in the semiꢀhomogeneous catalyst (Fig S5), and the biphasic interface area was significantly increased.
Under such conditions, the energy barrier of phase transfer was effectively minimized, and the reaction rate
was greatly improved.
In an effort to determine the source of hydrogen atoms in the product pinanes, hydrogenation of
αꢀpinene was carried out under our standard conditions replacing water with D2O (Scheme S1).
Hydrogenation in D2O afforded the nondeuterated cisꢀpinane and transꢀpinane. This indicated that both
hydrogen atoms were arose from H2, and the aqueous medium plays no role in providing H2. In addition,
this also suggested that hydrogenation of αꢀpinene were took place in the lipophilic core between the metal
and the hydrogenꢀcontaining micelles ((Scheme 1(c)).5
2.6 The scope of the catalyst
Table 4 Scope of the hydrogenation in TPGSꢀ1000 micelles a
TOF
(hꢀ1)
Conv.
(%) b
Time
(h)
Entry
Substrate
Octene
Salt
Product (Select. %) b
1
2
ꢀ
84.1
99.9
81.2
99.9
95.6
99.9
5.2
Octane (>99)
Octane (>99)
1.0
1.0
1.5
1.5
1.5
2.0
1.5
1.5
1.5
1.5
1.5
1.5
1.0
1.0
168.2
199.8
108.3
133.2
127.5
99.9
Na2CO3
3
Toluene
Styrene
ꢀ
Methylcyclohexane (>99)
Methylcyclohexane (>99)
4
Na2CO3
5
ꢀ
Ethylbenzene (91.3)
Ethylbenzene (6.4)
Ethylcyclohexane (8.7)
Ethylcyclohexane (93.6)
6
Na2CO3
7
Nitrobenzene
Nitrobenzene
Phenol
ꢀ
Aniline (>99)
Aniline (>99)
Aniline (98.8) Cyclohexylamine (1.2)
Aniline (>99)
Cyclohexanol (98.5) Cyclohexanone (1.5)
Cyclohexanol (>99)
6.9
8
Na2CO3
10.6
85.8
98.9
89.5
99.9
77.3
99.9
14.1
9 c
10 c
11
12
13
14
ꢀ
114.4
131.9
119.3
133.2
154.6
Na2CO3
ꢀ
Na2CO3
ꢀ
Cyclohexene
Cyclohexane (>99)
Cyclohexane (>99)
Na2CO3
199.8
a
b
Conditions: substrate (2 mmol), Ru(0) catalyst (0.01mmol), H2 (0.5 MPa), temperature (50 °C), Na2CO3 (2 mg).
Determined by GCꢀMS. c Ru(0) was replaced with Pd(0), the detailed information was placed in S1.2.
Lastly, to further demonstrate the potential for this mild, green catalyst to be used in other
hydrogenation reactions, an expanded study was undertaken examining several types of arene derivatives
and terminal olefin. The results were summarized in Table 4. Remarkably, all the hydrogenation reactions
were accelerated by Na2CO3. Cyclohexene (entries 13,14) and octene (entries 1,2) were completely
transformed in a very short time. Some interesting results in term of selectivity have been obtained in the
hydrogenation of styrene, the aromatic ring was hydrogenated after added Na2CO3 (entry 6). This indicated
that the regioselective hydrogenation of the exocyclic C–C double bond in the case of styrene was possible
(entry 5 and entry 6). In the hydrogenation process of nitrobenzene, Ru(0) (entries 7,8) nanoparticles were

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inert to these hydrogenation conditions. Pd(0) (entries 9,10) has much higher activity than Ru(0)
nanoparticles.25 Interestingly, the aqueous phase was clear and colorless after hydrogenation of phenol (entry
11). This was presumably due to the phenol destroyed the structure of micelles. Ru(0) nanoparticles were
absolutely aggregated into clumps and deposited in the bottom of the reactor. However, this damage was
greatly weakened after added Na2CO3 (entry 12). Ru(0) nanoparticles were dispersed evenly in the aqueous
phase after reaction.
3
Conclusions
In summary, hydrogenation of αꢀpinene could be performed in water typically at room temperature by
employing nanoreactors formed from the commercial available surfactant TPGSꢀ1000. This procedure was
environmentally friendly, limited amounts of water were used as the reaction media, and only an inꢀreactor
extraction with a minimal amount of a single, recoverable organic solvent. Additionally, with as little as 2
mg of Na2CO3 present, reaction rates could be increased significantly, and the reaction media could be
readily recycled. Lastly, these reactions took place in high yields and stereoselectivity, thereby offering
considerable potential for applications to other hydrophobic natural products in hydrogenation reactions.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No.
31270615) and Taishan Scholar Program of Shandong.
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DOI: 10.1039/C6RA13099B
ABSTRACT
Dꢀαꢀtocopheryl polyethylene glycol 1000 succinate (TPGSꢀ1000) stabilized Ru(0)
nanoparticles were prepared and characterized. These nanoparticles were employed to
selectively hydrogenate αꢀpinene to cisꢀpinane, with a small amount of Na2CO3
present, reaction rates could be increased significantly, and the reaction media could
be readily recycled. TEM, CLSM, IR and leaching experiments were employed to
quantify the advantages of catalytic system. The procedure is environmentally
friendly. It offers a reference for the catalytic hydrogenation of other hydrophobic
natural products.
Hydrogenation of αꢀpinene were took place in the lipophilic core between the
metal and the hydrogenꢀcontaining micelles.