Ruthenium nanoparticles on colloidal carbon spheres: An efficient catalyst for hydrogenation of ethyl lactate in aqueous phase

Article abstract of DOI:10.1016/j.catcom.2013.05.023

A ruthenium catalyst supported on glucose-derived carbon spheres was prepared and characterized by TEM, XPS, FTIR TG-DTG and XRD. The Ru/CSP catalyst showed an excellent catalytic performance for the hydrogenation of ethyl lactate to 1, 2-propanediol in water. The catalyst was recycled for six times without the marked loss in both the activity and selectivity. The steric hindrance and electronic effects of substrates on the hydrogenation were tested. It is postulated that the high performance of this novel catalyst system is related to the cooperation between the hydroxyl groups of support surface and water as a solvent.

Full text of DOI:10.1016/j.catcom.2013.05.023

Ruthenium Nanoparticles on Colloidal Carbon Spheres: An Efficient Catalyst
for Hydrogenation of Ethyl Lactate in aqueous phase
Qin Yang, Jie Zhang, Lei Zhang, Haiyan Fu, Xueli Zheng, Maolin Yuan, Hua
Chen, Ruixiang Li
PII:
DOI:
Reference:
S1566-7367(13)00196-9
doi: 10.1016/j.catcom.2013.05.023
CATCOM 3518
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
5 April 2013
16 May 2013
27 May 2013
Please cite this article as: Qin Yang, Jie Zhang, Lei Zhang, Haiyan Fu, Xueli Zheng,
Maolin Yuan, Hua Chen, Ruixiang Li, Ruthenium Nanoparticles on Colloidal Carbon
Spheres: An Efficient Catalyst for Hydrogenation of Ethyl Lactate in aqueous phase,
Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.05.023
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

ACCEPTED MANUSCRIPT
Ruthenium Nanoparticles on Colloidal Carbon Spheres: An Efficient Catalyst
for Hydrogenation of Ethyl Lactate in aqueous phase
Qin Yang, Jie Zhang, Lei Zhang, Haiyan Fu, Xueli Zheng, Maolin Yuan，Hua Chen,
Ruixiang Li*
Key Lab of Green Chemistry and Technology, Ministry of Education, College of
Chemistry, Sichuan University, Chengdu, Sichuan 610064, P. R. China
* Corresponding authors
Tel: 86-28-85412904;
Fax: 86-28-85412904
Email: liruixiang@scu.edu.cn (R.X. Li)
1

ACCEPTED MANUSCRIPT
Abstract
A ruthenium catalyst supported on glucose-derived carbon spheres was prepared
and characterized by TEM, XPS, FTIR TG-DTG and XRD. The Ru /CSP catalyst
showed an excellent catalytic performance for the hydrogenation of ethyl lactate to 1,
2-propanediol in water. The catalyst was recycled for six times without the marked
loss in both the activity and selectivity. The steric hindrance and electronic effects of
substrates on the hydrogenation were tested. It is postulated that the high performance
of this novel catalyst system is related to the cooperation between the hydroxyl groups
of support surface and water as a solvent.
Keywords: Ruthenium; Aqueous-phase hydrogenation; Ethyl lactate; Carbon spheres
1. Introduction
1，2-propanediol (1,2-PDO) is an important industrial chemical, which is widely
used as solvent or a raw material to produce polymers, resins, and functional fluids [1].
A commercial route to synthesize 1, 2-PDO is the hydration of propylene oxide
derived from propylene by either a chlorohydrin process or a hydroperoxide process,
which is unfavorable from green chemistry point of view [2, 3]. Thus, to develop a
clean and economical synthetic approach of 1, 2-PDO is necessary. Ethyl lactate can
be easily obtained by the fermentation of renewable sources such as agricultural crops
and biomass streams. The hydrogenation of ethyl lactates would lead to the formation
of 1, 2-PDO [4-8], which provides an eco-friendly alternative to the petroleum-based
process for 1, 2-PDO production. Hydrogenation of carboxylic acids and esters to the
2

ACCEPTED MANUSCRIPT
corresponding alcohols is often carried out under a high hydrogen pressure
(20-30MPa) and reaction temperature (473-573K) [9, 10]. Therefore, various attempts
have been paid to developing some highly efficient catalyst systems that can work
under the mild conditions [11-13]. Louessard et al. [11] first reported that Ru-Sn/SiO2
was effective for the hydrogenation of ethyl acetate to ethanol. In addition, Luo et al.
[4, 5] discovered that the introduction of tin to ruthenium catalyst was an effective
method to improve the activity and selectivity for the hydrogenation of ethyl lactate to
1, 2-PDO. Feng et al. [14] investigated the effect of supports, and Ru/SiO2 showed the
highest activity for the hydrogenation of ethyl lactate to 1, 2-PDO. Hwang et al. [15]
studied the vapor-phase hydrogenation of ethyl lactate over copper-silica
nanocomposites and discovered the dispersion of Cu on silica was a major factor
affecting the catalytic activity in the reaction. In our previous study [6], it was found
that the hydrogenation of ethyl lactate was easy to achieve over RuB/TiO2 in aqueous
solution, but the amorphous RuB particles were liable to crystallization to lose
catalytic activity. Furthermore, we found that the synergic effect between surface
hydroxyl groups of AlOOH and water could drastically promote the catalytic behavior
of the Ru-Pt/AlOOH catalyst in the hydrogenation of methyl propionate to propanol
[16]. Unfortunately, the quick crystal growth of the carrier AlOOH in water resulted
in dropping of the catalyst’s activity, causing a poor reusable catalyst.
Recently, a new carbon material, prepared by the hydrothermal treatment of
low-cost glucose, has aroused many interests [17]. The preparative process belongs to
“green chemistry” because it does not cause contamination to the environment and the
3

ACCEPTED MANUSCRIPT
reactant is safe. Moreover, according to the research about glucose-based carbon
spheres (CSP), the outer layer offers numerous functional groups, such as carboxylic,
aldehyde, and hydroxyl groups. Therefore, the noble metal nanoparticles can be easily
loaded on the carbon sphere and stabilized by the surface groups [17, 18]. And they
were used for Suzuki-Miyaura reactions [19] and the reduction of methyl orange [20].
We herein report the carbon spheres supported ruthenium nanoparticles to catalyze the
hydrogenation of ethyl lactate in water. The as-obtained Ru/CSP catalyst exhibits the
highly catalytic activity and selectivity. The recycling potential of the Ru/CSP catalyst
is also assessed.
2. Experimental Section
2.1. Materials and methods
All the reagents (A.R.) were commercially available and used as received. The
procedure for preparing carbon spheres was in accordance with the reported method
by Sun et.al. [17].
The loading of ruthenium was analyzed by inductively coupled plasma (ICP, IRIS
Intrepid). Transmission electron microscopy (TEM) was carried out in Tecnai G2 F20
S-TWIN. The X-ray photoelectron spectra (XPS) were recorded with a Kratos
XSAM800 spectrometer. And the IR spectra were recorded on a NEXUS 670. All
liquid samples were analyzed by gas chromatography (Agilent 6890) equipped with a
capillary column PEG -20 M (30 m × 0.25 mm). n-Decane was used as internal
standard. Reactants and products were identified by comparison with the standard
4

ACCEPTED MANUSCRIPT
samples and GC-MS.
2.2. Catalytic test
The hydrogenation was carried out in a 60 ml steel autoclave equipped with a
magnetic stirrer and electric temperature controller. In a typical experiment, the
catalyst (88.0 mg), ethyl lactate (0.5 ml), and distilled water (3 ml) were added to the
autoclave. It was purged with H2 three times, pressurized with H2 to the designed
pressure. After the autoclave was heated to the designed temperature, the stirring rate
was set at a constant of 1500 rpm.
The conversion of ethyl lactate and selectivity of 1, 2-PDO is defined as following:
ethyl lactate consumed
×100%
Conversion of ethyl lactate (%) = ethyl lactate initially charged
carbon in specific product
carbon in all detected products
Selectivity (%) =
×100%
3. Results and discussion
3.1 Catalyst characterization
The TEM images of Ru /CSP catalyst were shown in Fig.1. The synthesized Ru
nanoparticles had an average diameter of ~3 nm and the size of carbon spheres was
typically ~500 nm. It was indicated that the highly dispersed Ru nanoparticles have
been supported on the surface of carbon spheres through an in-situ reduction process
in ethanol, and no aggregation was observed.
The carbon spheres were prepared from glucose under hydrothermal condition at
180℃, leading to aromatization and carbonization [21-23]. To confirm the existence
5

ACCEPTED MANUSCRIPT
and kinds of these surface groups, FTIR spectra of carbon spheres, Ru /CSP and Ru
/CSP after catalysis were showed in Fig.2. As shown, the samples exhibited the
absorption peaks at 1710 and 1620 cm-1, which were attributed to C=O and C=C
vibrations, respectively, supporting the concept of aromatization of glucose during
hydrothermal treatment. Moreover, the strong band at 3450 cm-1 implies the existence
of a large number of residual hydroxyl groups on their surface before and after
catalysis.
The binding energy of Ru/CSP is at 280.5 eV in XPS spectra (Fig.3), which is close
to the binding energies of Ru0, clearly indicating the formation of zero valent
ruthenium [24]. The binding energies at 284.6 eV, 286.3 eV and 288.5 eV,
corresponding to C-C、C-H, C-O、O-H and C=O of carbon spheres further indicated
the existence of carboxylic, aldehyde, and hydroxyl groups on the surface of carbon
spheres.
3.2 Effect of solvent
The effect of solvents on the hydrogenation of ethyl lactate was shown in Table 1.
It can be seen that the catalysts reduced in ethanol, 2-propanol, or 1-butanol show
little difference in both activity and selectivity when the reaction was carried out in
water (Table, entries 2-4), 91.8%~93.7% conversions of ethyl lactate were obtained
with excellent selectivity towards 1,2-PDO (ranging from 92.3%~95.4%). However,
the conversion obviously decreased to 74.9% when using Ru/CSP pretreated by H2 as
catalyst under identical condition (Table 1, entry 1). In our previous study [16], the
6

ACCEPTED MANUSCRIPT
carrier (γ-Al2O3) of catalyst (Ru-Pt/γ-Al2O3) could be transformed into OH-rich
boehmite (AlOOH) under hydrothermal condition to obtain a highly active catalyst
for the hydrogenation of methyl propionate. Since carbon spheres are hydroxyl-rich
carbonaceous materials, there was no need to generate hydroxyl groups through
hydrothermal treatment. So the reduction of Ru/CSP with H2 in water was not an
optimum process. When the hydrogenation was performed in alcohols (Table 1,
entries 5 to 8), both the conversion and the selectivity to 1, 2-propanediol were
reduced due to the serious transesterification reaction between substrate and solvent
alcohols. Beside the transesterification product, the desired 1, 2-PDO was rare.
Therefore, the best catalytic result was obtained in water as solvent. In this case,
93.7% of ethyl lactate was converted with selectivity of 95.4% to 1, 2-PDO (Table 1,
entry 2), which was better than the reported Ru-Sn bimetallic catalysts for the
hydrogenation of ethyl lactate [4, 5]. In addition, for Ru/charcoal, moderate
conversion of ethyl lactate (70.9%) and selectivity to 1, 2-PDO (85.4%) were
observed. The colloidal ruthenium nanoparticles exhibited markedly high selectivity
(97.8%), whereas the conversion of ethyl lactate was conservative (80.5%). These
results clearly demonstrated that the prepared Ru/CSP exhibited high catalytic
performance in the presence of both water and support.
ICP-AES analysis showed that 9.2 ug of Ru was found in the filtrate. In order to
understand whether it is the ruthenium species leached into the solution that catalyzed
the reaction, a filtrate was used in the hydrogenation of ethyl lactate (entry 10e). As
low as 5% conversion testified that the Ru/CSP played the catalytic role.
7

ACCEPTED MANUSCRIPT
3.3 Effect of steric and electronic factors
The effect of the steric and electronic factor of substrates for the hydrogenation of
carboxylic ester was investigated on Ru/CSPs. The results were shown in Table 2.
Firstly, several alkyl propionates were hydrogenated with 5.0 MPa H2 at 453K for
8h (Table 2, entries 1-4). As can be seen, increasing the bulkiness of the alkyl group
obviously decreased the conversion, possibly due to the larger alkyl group, which is
no favorable for the adsorption of the substrate on the catalyst. On the other hand, it
also slowed down the hydrogenolysis of alkyl group. Therefore, the hydride species
selectively attacked the carbonyl group (Table 2, entries 1-4). And the selectivity
towards desired propanol increased from 62.1% to 91.5% when the alkyl changing
from methyl to isopropyl group. Then, ethyl acetate and ethyl trifluoroacetate were
also subject to the identical condition (Table 2, entries 5, 6). It was found that the
strong electron-withdrawing –CF3 attached to the carbonyl group greatly increased the
conversion and the selectivity toward 2,2,2-trifluroethanol to ›99% and 94.7%,
respectively. These results implied that the electronic property on carbonyl carbon of
esters was the key factor for both the activity and the selectivity of hydrogenation.
It is noteworthy that a significant amount of acid was also produced besides the
desired product propanol. In order to clarify the reason for the formation of acid, N2
was introduced to replace H2 during the reaction of methyl propionate. In this case,
propionic acid was still detected in the product, possibly resulted from the hydrolysis
pathway. Furthermore, after the initial hydrogenation reaction, we detected methane,
ethane and propane for different hydrogenated substrate in the vent gas. These results
8

ACCEPTED MANUSCRIPT
proved the occurrence of hydrogenolysis during the hydrogenation process.
Therefore, we speculated that there were three possible reaction routes for the
substrate esters (Schem1). Firstly, ruthenium species activated hydrogen. Meanwhile,
the basic oxygen atom in water molecule was easy to combine with the activated
hydrogen molecule through hydrogen bond and promotes the heterolytic splitting of
the activated hydrogen molecule to give a ruthenium-hydride species [25, 26]. The
hydrogen bond between carbon sphere and substrate molecule promoted not only the
adsorption of substrate, but also the activation of C=O bonds. Afterwards, the hydride
on ruthenium attacked the positively charged carbon on carbonyl group （C ） to
Ⅰ
form the intermediate aldehyde (Route ①), which was quickly hydrogenated to give
both the alcohol and the regeneration of the active catalyst. Meanwhile, the hydride
might also attack the carbon in alcohol (C ) to form acid and alkane (Route ②). The
Ⅱ
hydrolysis of ester occurs inevitably on the account of the fact that the hydrogenation
is carried out in the aqueous solvent (Route ③). The acid could be further
hydrogenated.
When the reaction was completed, the product was separated from the catalyst by a
simple centrifugation. The catalyst was reused after washing with distilled water three
times. The data, showed in Figure.4, indicated that the activity of the catalyst and the
selectivity to 1, 2-PDO decreased slightly after six catalytic cycles. The results
reflected that the Ru nanoparticles could be stabilized by the surface groups on the
carbon spheres. Probable reasons for the loss of activity and selectivity were leaching
of ruthenium active species in each cycle or somewhat aggregation of Ru
9

ACCEPTED MANUSCRIPT
nanoparticles（Fig.1 c, d）.
4. Conclusions
The Ru /CSP catalyst prepared by a simple in-situ reduction route exhibits good
activity and selectivity in water for hydrogenation of ethyl lactate without a second
metal or any additive. The selectivity of 95.4% to 1, 2-PDO could be obtained with
conversion of 93.7% of ethyl lactate over Ru /CSP at 423K under 5 MPa of H2 for 8h.
It can be concluded that the surface groups presented on the carbon spheres are
effective for preventing the aggregation of the ruthenium nanoparticles during the
reaction. Both the surface groups on the carbon spheres and water as the solvent are
important to this catalytic system. The increase of the steric hindrance disfavored the
reaction, and the electron-withdrawing group on the substrate was beneficial to the
reaction. Meanwhile, there may be three pathways for hydrogenation of ester in
aqueous solvent: hydrogenation, hydrogenolysis and hydrolysis. This environmentally
friendly catalyst could be reused for up to six times with a slight decrease in activity
and selectivity.
Acknowledgements
This work was funded by the Natural Science Foundation of China (No. 21072138),
and was supported by the innovation foundation of Sichuan University.
10

ACCEPTED MANUSCRIPT
References
[1] M. Balaraju, V. Rekha, P. S. Sai Prasad, B. L. A. Prabhavathi Devi, R. B. N.
Prasad, N. Lingaiah, Appl. Catal. A: Gen. 354 (2009) 82–87.
[2] J. Haveren, E. L. Scott and J. Sanders, Biofuel. Bioprod. Bior. 2 (2008) 2, 41–57.
[3] J. Chaminand, L. a .Djakovitch, P. Gallezot, P. Marion, C. Pinel and C. Rosier,
Green Chem. 6 (2004) 359–361.
[4] G. Luo, S. R. Yan, M. H. Qiao, J. H. Zhuang, K. N. Fan, Appl. Catal. A: Gen 275
(2004) 95-102.
[5] G. Luo, S. R. Yan, M. H. Qiao, K. N. Fan, Appl. Catal. A: Gen 332 (2007) 79-88.
[6] G. Y. Fan, Y. Zhang, Y. F. Zhou, R. X. Li, H. Chen, X. J. Li, Chem Lett. 37 (2008)
852-853.
[7] L. Huang, Y. L. Zhu, H. Y. Zheng, M. X. Du, Y. W. Li, Appl Catal A: Gen. 349
(2008) 204-211.
[8] J. J. Xue, F. Cui, Z. W. Huang, J. L. Zuo, J. Chen, C. G. Xia, Chin. J. Catal. 33
(2012) 1642-1649.
[9] R. G. Wall, US4113662 (1987).
[10] N. Masanori, S. Toru, EP1586549A1 (2005).
[11] P. Louessard, J. P. Candy, J. P. Bourneville, J. M. Basset, Structure and Reactivity
of Surface, Elsevier, Amsterdam, 1989. p. 591.
[12] Z. Zhang, J. E. Jacjson, D. J. Miller, Appl. Catal. A: Gen. 219 (2001) 89-98.
[13] Y. Hara, K. Endou, Appl. Catal. A: Gen. 239 (2003) 181-195.
[14] J. Feng, W. Xiong, Y. Jia, J. B. Wang, D. R. Liu, R. X. Qin, Reaction Kinetics,
11

ACCEPTED MANUSCRIPT
Mechanisms, and Catalysis, 104 (2011) 89-97.
[15] P. Kasinathan, D. W. Hwang, et al. , Appl. Catal. A: Gen. 451 (2013) 236-242.
[16] Y. F. Zhou, H. Y. Fu, X.. J. Zheng, R .X. Li, Catal. Commun. 11 (2009) 137-141.
[17] X. M. Sun, Y. D. Li, Angew. Chem. Int. Ed. 43 (2004) 597-601.
[18] L. Kong, X. Lu, X. Bian, W.Zhang, C.Wang. Langmuir 26 (2010) 5985-5990.
[19] C. B. Putta, S. Ghosh, Adv. Synth. Catal. 353 (2011) 1889-1896.
[20] L. R. Kong, X. F. Lu, X. J. Bian, W. J. Zhang, C. Wang, Appl. Mater. Inter. 3
(2011) 35-42.
[21] T. Sakaki, M. Shibata, T. Miki, H. Hirosue, N. Hayashi, Bioresour. Technol. 58
(1996) 197 – 202.
[22] G. C. A. Luijkx, F. van Rantwijk, H. van Bekkum, M. J. Antal, Jr. Carbonhydrate
Res. 272 (1995) 191 – 202.
[23] Q. Wang, H. Li, L. Q. Chen, X. J. Huang, Solid State Ionics 152-153 (2002) 43 –
50.
[24] Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation,
1992.
[25] V. M. Desphande, W. R. Patterson, C. S. Narasimhan, J. Catal. 121 (1990) 165.
[26] Y. Pouilloux, F. Autin, C. Guimon, J. Barrault, J. Catal. 176 (1998) 215.
12

ACCEPTED MANUSCRIPT
The list of Table, Figure, and Scheme captions
Fig.1. TEM and HRTEM images of the Ru/CSP
Fig.2. FTIR spectra of carbon spheres
Fig.3. XPS spectra of fresh Ru/CSP
Table 1 Effect of solvent on the hydrogenation of ethyl lactate over Ru/CSP
Table 2 The hydrogenation performance of the Ru/CSP for different substrates
Schem1.The proposed mechanism
Fig.4. Recycling experiments for the hydrogenation of ethyl lactate
13

ACCEPTED MANUSCRIPT
Fig.1. TEM and HRTEM images of the Ru/CSP catalyst before used (a,b), after six
catalytic cycles(c,d).
14

ACCEPTED MANUSCRIPT
Fig.2. FTIR spectra of carbon spheres: A) sample unloaded with metal, B) Ru/CSP
catalyst, and C)Ru/CSP catalyst after catalysis
15

ACCEPTED MANUSCRIPT
Fig.3. XPS spectra of fresh Ru/CSP
16

ACCEPTED MANUSCRIPT
Table 1 Effect of solvent on the hydrogenation of ethyl lactate over Ru/CSP
Catalyst .a
Entry Reductant
Solvent in
hydrogenation
Water
Conversion%
Selectivity
1,2-PDOb TREFc PAd
1
2
H2
74.9
93.7
93.7
91.8
51.1
2.4
91.2
95.4
92.5
92.3
6.2
-
8.7
4.6
7.5
7.8
0.2
1.2
4.9
7.2
22.0
5.5
Ethanol
2-Propanol
1-Butanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Water
-
3
Water
-
4
Water
-
5
Methanol
Ethanol
93.6
6
98.7
49.3
9.8
-
7
2-Propanol
1-Butanol
1,4-Dioxane
Water
10.6
27.0
0.8
45.8
8
83.1
9
77.9
94.5
-
-
10e
5.0
Reaction condition: a 423 K; 5.0 MPa H2; 8 h; 3 ml H2O; 1% Ru catalyst;
b1,2-propanediol; c Transesterification; d propionic acid; e reused filtrate.
17

ACCEPTED MANUSCRIPT
Table 2 The hydrogenation performance of the Ru/CSP for different substrates. a
Entry
Substrates
Conversion (%)
Selectivity (%)
n-propanol
PA e
1
2
3
4
5
6
Methyl propionate
Ethyl propionate
n-propyl propionate
Isopropyl propionate
Ethyl acetate
51.2
31.0
62.1b
80.7b
87.6b
91.5b
84.0c
94.7d
37.8
19.3
11.5
8.5
24.5
13.8
42.9
16.0
5.2
Ethyl trifluoroacetate
＞99%
a
b
Reaction condition: 453 K, 5.0 MPa H2, 8 h, 3.0 ml H2O, 2% Ru catalyst;
selectivity to propanol; c selectivity to ethanol; d selectivity to 2,2,2-trifluoroethanol;
e propionic acid.
18

ACCEPTED MANUSCRIPT
R1C OOH + R2C H3
Ⅰ
Ⅱ
R1
Hydrogenolysis
Route ②
O
C
Ⅰ
O
R2C H2OH
①
Ⅱ
Route ①
Hydrolysis
C H2R2
Ⅱ
H
O
②
+
Route ③
H
H
R1C H2OH
Ⅰ
H
H
R1C OOH
O
Ⅰ
+
R2C H2OH
Ⅱ
Ru
CSP
Schem1.The proposed pathway in the hydrogenation of ester in aqueous-phase.
19

ACCEPTED MANUSCRIPT
Fig.4. Recycling experiments for the hydrogenation of ethyl lactate
20

ACCEPTED MANUSCRIPT
Highlights:
1 Ethyl lactate was efficiently hydrogenated to 1, 2-PDO in H2O over Ru/CSP catalyst.
2 The catalyst was recycled for six times without marked activity loss.
3 The possible hydrogenation pathway was suggested.
21