Biosynthesized ruthenium nanoparticles supported on carbon nanotubes as efficient catalysts for hydrogenation of benzene to cyclohexane: An eco-friendly and economical bioreduction method

Article abstract of DOI:10.1016/j.apcata.2014.07.015

Ru/carbon nanotubes (CNTs) catalysts synthesized by an environmentally benign and economical bioreduction approach were applied to the hydrogenation of benzene to cyclohexane under mild conditions without adding any solvent. The catalysts were characteriz

Full text of DOI:10.1016/j.apcata.2014.07.015

Applied Catalysis A: General 484 (2014) 154–160
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Biosynthesized ruthenium nanoparticles supported on carbon
nanotubes as efficient catalysts for hydrogenation of benzene to
cyclohexane: An eco-friendly and economical bioreduction method
∗
Yao Ma, Yangqiang Huang, Youwei Cheng , Lijun Wang, Xi Li
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 May 2014
Received in revised form 26 June 2014
Accepted 9 July 2014
Available online 17 July 2014
Ru/carbon nanotubes (CNTs) catalysts synthesized by an environmentally benign and economical biore-
duction approach were applied to the hydrogenation of benzene to cyclohexane under mild conditions
without adding any solvent. The catalysts were characterized by a variety of techniques including BET,
TEM, HAADF-STEM -EDX, XRD, XPS, FTIR, TG, and DTG. The effect of various preparation parameters,
such as Ru loading, preparation temperature and calcination temperature on the catalytic performance
◦
◦
were systematically analyzed and the optimum conditions were found to be 2 wt%, 60 C, and 500 C,
Keywords:
Biosynthesis
Ruthenium nanoparticles
Ruthenium catalysts
Benzene hydrogenation
◦
respectively. In terms of reaction conditions, the combination of reaction temperature at 80 C under
the pressure of 4 MPa, and the time length of 0.5 h proved to be optimum, under which the cycohexane
−1
yield of 99.97% along with the TOF value of 6983.09 h were achieved. Such results could match or even
outweigh those reported in the literature. Furthermore, efforts were also made to probe the stability of
catalysts. In a word, the merits of excellent durability, sound productivity and preferable reusability grant
the catalysts a promising future in the industrial application.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
the dominant production process in industry, although cyclohex-
ane can be obtained from the separation of petroleum distillate
[8]. There are lots of researchers [9,10] who use the catalytic
benzene hydrogenation reaction to evaluate the performance of
the hydrogenation catalysts. The use of homogeneous catalysts
will inevitably encounter the separation problems which could be
very sophisticated and money-consuming during the production
process. Thus, to seek more economical heterogeneous catalysts
becomes the primal concern for researchers in this field. The IFP
process uses nickel-based heterogeneous catalysts at temperature
With the increasing industrial demands of low-aromatic diesel
fuels, the development of effective catalysts for hydrogenation of
arenes is of critical importance [1,2]. Poor fuel with high aromatic
content produces a low cetane number in diesel fuel and a high
smoke point in jet fuel. There is also evidence that particulate emis-
sions in diesel exhaust gases correlate with the aromatic content
in the fuel [2]. More and more stringent environmental laws and
regulations have been enforced to bring down aromatics content
to controllable level, which boosts new catalytic processes for aro-
matic saturation.
The hydrogenation of benzene to cyclohexane is one of the
most important compounds hydrogenation reactions practiced in
industry [3–5]. Cyclohexane is a valuable chemical used in the
manufacture of nylon 6 and nylon 66, which constitute about
◦
above 200 C under 50 bar [11,12]. These forcing conditions will
lead to poor life-time performance, additionally, higher temper-
ature will create favorable conditions for the side reactions such
as isomerization and hydrocracking which result in selectivity of
cyclohexane decline. Recently, several types of noble metals such
as Ru, Rh, Pt, Pd, etc. have arose extensive interests and been used
extensively as heterogeneous catalysts in the hydrogenation of
benzene to cyclohexane [13–15]. In particular, considerable studies
have focused on Ru-based catalysts which have extensively been
investigated in hydrogenation processes. Nowicki et al. reported
new colloidal solutions of Me-CD protected Ru(0) nanoparticles
which were prepared by reduction of RuCl3 with sodium boro-
hydride [16]. They found these nanoheterogenous system present
9
0% of all polyamides. In addition, cyclohexane is an excellent
and nontoxic solvent for cellulose ether, wax, asphalt and rubber
[
6,7]. Nowadays, catalytic benzene hydrogenation to cyclohexane is
∗ Corresponding author. Tel.: +86 571 8795 2210; fax: +86 571 8795 1227.
E-mail addresses: ywcheng@zju.edu.cn, uway@163.com (Y. Cheng).
http://dx.doi.org/10.1016/j.apcata.2014.07.015
926-860X/© 2014 Elsevier B.V. All rights reserved.
0

Y. Ma et al. / Applied Catalysis A: General 484 (2014) 154–160
155
interesting activity for aromatic ring hydrogenation. Ozkar and co-
workers prepared intrazeolite ruthenium(0) nanoclusters catalyst
which exhibited excellent catalytic activity in the hydrogenation of
benzene to cyclohexane [17].
various preparation and reaction conditions on the reaction was
also included.
2. Experimental details
The deposition-precipitation (DP) technique has long been used
to synthesize Ru-based catalysts, which is not only energy inten-
sive (because of numerous stringent conditions) but also hostile
to environment (due to the use of toxic solvents or additives
such as surfactant stabilizers and auxiliary capping agents). Com-
pared to the conventional methods, bioreduction approach (“green
chemistry”) based on microorganisms or plants extract [18] is eco-
friendly, cost-efficient and can be served as an alternative method
to the DP technique to produce Ru nanoparticles. Biosynthesis of
noble metal nanoparticles as an intersection of nanotechnology
and biotechnology has gathered increasing interests in the last
decade. Song and Kim reported the biosynthesis of Ag nanopar-
ticles by using five plant leaf extracts (Pine, Persimmon, Ginkgo,
Magnolia and Platanus) which could be used in various areas closely
related with people’s daily life such as cosmetics, foods and medical
applications [19]. Zhan et al. successfully prepared Au nanoparti-
cles by biogenic fabrication methods and they further immobilized
the biosynthesized Au nanoparticles on TS-1 support which can
act as bioreduction catalysts in vapor phase propylene epoxidation
2.1. Materials
Carbon nanotubes (CNTs) were purchased from Bema Environ-
mental Science and Technology Co. Ltd., Cacumen Platycladi (CP)
was obtained from Zhejiang university hospital and other chemical
reagents mentioned were analytic grade from Sinopharm Chemical
Reagent Co. Ltd. and used directly without pretreatment.
2.2. Catalyst preparation
A series of Ru-based catalysts were prepared by the adsorption-
reduction (AR) technique employing plant biomass extract [28].
Firstly, to obtain the CP leaf extract, screened powder CP leaf of
1 g dosage was dispersed in 100 mL deionized water under stirring
for 4 h. The extract was then filtrated and used for the synthesis of
RuNPs. In a typical catalyst preparation procedure, an appropriate
amount of dried CNTs were immersed in aqueous RuCl3 solution
◦
(50 mL, 2.2 mM) for 1 h in an oil bath (60 C) under magnetic stir-
[
20,21]. Although there are fewer attempts to apply the biosysthe-
ring. Afterward, 30 mL CP extract was added immediately into the
mixture solution under stirring. After another 5 h, the products
were filtered, washed thoroughly with deionized water, dried at
sized nanoparticles to catalytic system, it has been proved to be
a promising approach for the fabrication of novel heterogeneous
catalysts.
◦
◦
60 C overnight in a vacuum oven, and then calcined at 500 C for
3 h in the atmosphere of nitrogen. To investigate the influence of
preparation parameters, different preparation conditions (Ru load-
Owning to their unique properties and surfaces, Carbon nano-
tubes (CNTs) are suitable for many potential applications as
promising carbon materials and solid supports for heterogeneous
catalysts [22–25]. CNT-supported metallic nanoparticles exhibit
remarkably high catalytic activities for hydrogenation of aromatic
compounds. Pan and Wai developed a simple one-pot sonoche-
mical method for the preparation of rhodium catalysts supported
on CNTs which exhibited high catalytic activity of benzene and
its derivatives hydrogenation without solvent under mild con-
ditions [26]. Guo et al. reported Pt-based mono and bimetallic
catalysts supported on CNTs by microwave-assisted polyol reduc-
tion method (MAPR) [27]. These catalysts were successfully applied
to the selective hydrogenation of cinnamaldehyde to cinnamal
alcohol. Effective dispersion of the nanocatalysts in organic sol-
vents is one obvious reason favoring the CNT-supported metallic
nanoparticles for catalytic hydrogenation reactions [25]. It becomes
clear that, carbon nanotubes possess specific characteristics such as
remarkable electronic properties, particular adsorption properties
and high resistance to abrasion, all of which usher in a brighter
prospect for such materials.
Hydrogenation of benzene to cyclohexane under mild con-
ditions is also worthy of discussion from the perspective of
energy and environmental considerations. In this paper, we probe
into the biosynthesis of the Ru nanoparticles using the Cacumen
Platycladi (CP) extract, which plays a dual role in reducing and
protecting agent without any other additive and the immobi-
lization of Ru nanoparticles on the nanoscale materials carbon
nanotubes. The prepared catalysts were characterized by dif-
ferent techniques, including low temperature N2 physisorption,
transmission electron microscopy (TEM), high-angle annular dark
field-scanning transmission electron microscopy (HAADF-STEM)
with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction
◦
ing, 0.5–3.0 wt%; preparation temperature, 30–90 C; calcination
◦
temperature, 200–700 C) were implemented. To meet the needs of
the characterization, the Ru nanoparticles (RuNPs) were prepared
by reducing the metal precursor with the CP leaf extract free of the
supports.
2.3. Catalyst characterization
Brunauer–Emmet–Teller (BET) specific surface areas were mea-
sured by N2 adsorption at liquid N2 temperature in an ASAP 2020
analyzer. Transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HRTEM) and high-angle annular
dark field-scanning transmission electron microscopy (HAADF-
STEM) with energy dispersive X-ray spectroscopy (EDX) images
were obtained with a FEI Tecnai G2 F20 S-TWIN microscope oper-
ated at 200 kV. The specimens were prepared by ultrasonically
suspending the sample in ethanol for 0.5 h. Size distribution of
the resulting NPs was estimated on the basis of TEM micrographs
with the assistance of SigmaScan Pro software (SPSS Inc., Version
4.01.003). The X-ray diffraction (XRD) analyses were performed
on a Shimadzu powder X-ray diffractometer with Cu K␣ radia-
◦
tion at 40 kV and 30 mA using the scanning angle 2ꢀ from 10
◦
◦
◦
−1
to 90 , at a step of 0.02 and rate of 2 min . XPS experiments
were performed on a VG ESCALAB MARK II equipment. Monochro-
matic radiation from an Mg K␣ (BE = 1253.6 eV) X-ray source was
used for excitation. FTIR spectra were recorded on a Nicolet 5700,
where the samples were ground with KBr and pressed into the
wafer. Thermogravimetric (TG) and differential thermogravimet-
e
ric (DTG) analysis were measured with a METTLER TGA/SDTA 851
thermobalance. The sample was heated from room temperature to
◦
◦
(
XRD), X-ray photoelectron spectroscopy (XPS), fourier trans-
900 C at a heating rate of 10 C/min under a high purity nitrogen
flow of 100 mL/min.
form infrared spectroscopy (FTIR), thermogravimetric (TG), and
differential thermogravimetric (DTG). The performance of the
bioreduction Ru-based CNTs catalysts was evaluated by the hydro-
genation of benzene to cyclohexane without any solvents under
mild conditions. In order to optimize the reaction parameters
for maximum yield of cyclohexane and TOF value, the effect of
2.4. Catalyst evaluation
The hydrogenation of benzene to cyclohexane was carried out
in a magnetically stirred 100 mL stainless steel high pressure

1
56
Y. Ma et al. / Applied Catalysis A: General 484 (2014) 154–160
Table 1
900
750
600
450
Physical properties of synthesized catalysts.
−1
)a
Vp (cm3
−1
)b
Dp (nm)c
Sample
SBET (m2
g
g
a
b
c
d
e
f
1
2
3
4
5
6
Carbon nanotube
1.0% Ru/CNTd
1.5% Ru/CNT
2.0% Ru/CNT
2.5% Ru/CNT
3.0% Ru/CNT
221
216
204
194
177
176
1.44
1.05
1.13
0.93
1.00
0.92
15.3
16.0
17.7
18.6
18.4
19.7
a
b
c
300
150
0
Specific surface area.
Pore volume.
Average pore diameter.
d
◦
Preparation conditions: preparation temperature 60 C, calcination temperature
◦
5
00 C.
0
.0
0.2
0.4
0.6
0.8
1.0
batch-type reactor. In a typical procedure, 3 mL neat benzene and
0 mg of the biosynthesized catalysts were placed in the reac-
5
Relative Pressure (P/P0)
tor. The reactor was then sequentially sealed with flowing N2
and H2 at 4 MPa pressure to replace the air. Upon completion of
the reaction, the reactor was quickly cooled in a water bath and
depressurized with a connected pressure regulation. Liquid sam-
ples were centrifuged to separate the catalysts from the mixture.
Gas chromatography (Ke Xiao, GC 1690) was used for the analy-
sis of products of hydrogenation of benzene using N-hexane as the
internal standard substance. The yield of cyclohexane and turnover
frequency (TOF) value was calculated and used to evaluate the
activity. TOF was calculated as moles of benzene consumed per
surface of Ru site per hour. Repeated reaction runs with the same
catalyst batch delivered yield/TOF values that were reproducible
within ± 2%.
0
.05
a
b
c
d
e
f
0.04
0
.03
.02
0
0.01
0.00
3
. Results and discussion
1
10
100
3.1. Characterization of the supports and catalysts
Pore Diameter (nm)
The nitrogen physisorption experiments conducted for the pure
Fig. 1. Nitrogen adsorption–desorption isotherms and pore distributions isotherms
for different catalysts varying Ru loading: (a) 0 wt%, (b) 1 wt%, (c) 1.5 wt%, (d) 2 wt%,
e) 2.5 wt%, and (f) 3 wt% (preparation conditions: preparation temperature 60 C,
calcination temperature 500 C).
CNT and the Ru/CNT catalysts have shown their mesoporous
structure. The physical properties of synthesized catalysts were
presented in Table 1. The application of the unique structure of
carbon nanotubes will significantly reduce mass transfer limita-
tion which might be encountered in the micropores structure such
as activated carbon [29]. Evidently, Table 1 indicated that the BET
surface and the pore volume decreased slightly after the immo-
bilization of the Ru nanoparticles into the channels of carbon
nanotubes. As shown in Table 1, the BET surface plunged from
◦
(
◦
distribution of 3.06 ± 0.67 nm. Typical HAADF micrographs (Fig. 2e
and f) confirmed the successful loading of the ruthenium nanopar-
ticles. At different spots in the Ru/CNT catalyst, the presence of
ruthenium, copper, molybdenum and carbon could be traced by
the EDX while the Cu and Mo were introduced by the copper grid.
X-ray diffraction diagrams were recorded for a fresh CNT sup-
port, a calcined 2 wt% Ru/CNT, and an as-prepared RuNPs (Fig. S1
of Supplementary Information). The CNT sample showed the typ-
2
−1
2
0
21 to 176 m g
while the pore volume ranged from 1.44 to
3
−1
.92 cm g after Ru nanoparticles were introduced. Fig. 1 showed
the N adsorption/desorption isotherms and the pore size distribu-
2
tions for different Ru-based catalysts. Their adsorption isotherms
were apparently classified as the types IV with an obvious hystere-
◦
◦
sis loop at the high P/P , according to BDDT classification, revealing
ical characteristic peaks at 2ꢀ of 26.5 and 42.3 , indicating the
existence of the hexagonal graphite structure. These peaks were
identified by JCPDS-ICDD card: 65-6212 [34,35]. After incorpora-
tion of ruthenium in the graphite structure, there were no specific
diffraction peaks of ruthenium in the XRD pattern of 2 wt% Ru/CNT.
The catalysts preserved the main structure of the carbon nanotubes
0
typical mesoporous characteristics of the material [30]. As illus-
trated from the pore size distribution, the incorporation of the
RuNPs led to the increase of the average pore diameters from 15.3
to 19.7 nm which was probably due to the extension role of the
ruthenium nanoparticles [31].
The morphology and particle size of the bioreduction catalysts
were studied by TEM. Fig. 2 represented the typical TEM images of
the RuNPs and the size distribution. As illustrated, plenty of uniform
nanoparticles showed a well-defined spherical shape and were dis-
tributed well on the carbon nanotubes. The high-resolution TEM
image in Fig. 2d showed d-spacing of 2.06 A˚ , 2.14 A˚ and 2.34 A˚ ,
corresponding to (1 0 1), (0 0 2) and (1 0 0) facets of Ru respec-
tively (ICDD-PDF-#65-7645) [32,33]. The histogram of their size
distribution in Fig. 2b indicated that there exist nanoparticles mea-
sured between 1.5 and 5.5 nm and showed a relatively narrow size
◦
revealing the strong reflections of C structures (2ꢀ = 42.3 ), a low
percentage of the ruthenium content and the high dispersion of
the RuNPs on the support. In order to confirm the formation of Ru
nanoparticles, they were prepared by reduction of the metal pre-
cursor with the CP leaf extract without introducing the supports
and were characterized by the XRD analysis. Diffuse peaks were
observed for a homogeneous distribution of quiet small Ru metal
nanoparticles. All of the characteristic peaks of the metallic Ru were
shown in the pattern. The diffraction angle of the peak at 44.1, 38.5,
and 42.2, corresponding to the distance value (2.06 A˚ , 2.34 A˚ and

Y. Ma et al. / Applied Catalysis A: General 484 (2014) 154–160
157
Fig. 2. Representative TEM images of Ru/CNT catalysts with different magnifications (a, b, and c). The inset in (b) indicates the size distribution of the ruthenium nanoparticles
thereof. (d) A typical HRTEM micrograph of the Ru nanoparticles. (e) HAADF STEM images with specific spots and (f) the analysis of EDX (preparation conditions: Ru loading
◦
◦
2
wt%, preparation temperature 60 C, calcination temperature 500 C).
2
(
.14 A˚ ) were due to the (1 0 1), (1 0 0), and (0 0 2) plane of Ru metal
ICDD-PDF-#65-7645) respectively [32,33].
In order to establish the surface composition and chemical
or
C OH in the plant extract were responsible for the reduc-
tion of ruthenium ions. Moreover, these functional groups could be
adsorbed on the surface of the Ru nanoparticles to avoid agglom-
eration in which was similar to the mechanism of conventional
stabilizer such as PVP [40]. To a large extent, these functional groups
might be ascribed to polyols such as flavones and reducing sug-
ars in the extract. From the results of FTIR analyses, the active
state of the bioredcution Ru nanoparticles, X-ray photoelectron
spectroscopy (XPS) measurements were performed. The narrow
scan spectra within the Ru 3d region (280–290 eV) showed highly
overlapping Ru 3d and C 1s peaks (Fig. S2 of Supplementary Infor-
mation). Since the C 1s experimental peak was too complicated
to deconvolve, only a single contributing peak of 284.6 eV was
applied for simplicity [36]. Out of our utmost interests, the peaks
at 283.7 eV and 280.7 eV were assigned to Ru 3d3/2 and Ru 3d5/2
spin-orbit in the zerovalent state respectively. And the presence of
Ru(IV) component probably resulted from air exposure during the
XPS sample preparation [37]. During the reaction process under
hydrogen atmosphere, the Ru nanoparticles were predominantly
in the zerovalent state. The XPS measurements demonstrated the
successful biosynthesis of Ru nanoparticles through a simple pro-
cedure of reducing RuCl3 with CP extract.
b
a
FTIR analysis of the CP extract before and after bioreduction
was carried out to identify the possible functional groups respon-
sible for the reduction of ruthenium chloride and stabilization of
the Ru nanoparticles. As shown in Fig. 3, the intensity of six bands
−
1
at 3425, 2921, 1610, 1407, 1046 and 636 cm was considerably
attenuated after the bioreduction. The absorption bands at 2921
−
1
and 1610 cm
H) and ꢁ(
may be attributed to bending vibration of ı(O H) [38]. Similarly,
were associated with the stretching vibration of
C C), respectively, while the band at 1407 cm
4
000
3500
3000
2500
2000
1500
-1
1000
500
−
1
ꢁ
(
C
Wavenumber (cm )
−1
the band at 1046 cm may be assigned to the stretching vibra-
tion of ꢁ( OH) [39]. Therefore, the functional groups of C
Fig. 3. Typical FTIR spectra of Cacumen Platycladi leaf extract before bioreduction
a), and after bioreduction (b) of ruthenium chloride.
C
C H
(

1
58
Y. Ma et al. / Applied Catalysis A: General 484 (2014) 154–160
7
6
000
000
9
8.9%
Yield of cyclohexane
1
00
1
00
a
b
TOF
8
6
4
0
0
0
9
8
7
6
5
0
0
0
0
0
5000
000
3000
9
4.7%
0
.10
0.05
.00
c
4
8
3.8%
0
-
-
-
0.05
0.10
0.15
2
1
000
000
a
20
0
c
b
1
50
300
450
600
750
900
0
.5
1.0
1.5
2.0
2.5
3.0
o
Temperature ( C)
Ruthenium loading (wt%)
150
300 450
600
750
900
Fig. 5. Effect of Ru loading on the yield of cyclohexane and turnover frequency
o
◦
◦
Temperature ( C)
values (conditions: preparation temperature 60 C, calcination temperature 500 C,
◦
benzene 3 mL, catalyst 0.05 g, reaction temperature 80 C, reaction pressure 4 MPa,
reaction time 0.5 h).
Fig. 4. TG and DTG profiles of (a) CNT support, (b) Ru/CNT catalyst with calcination
◦
temperature of 500 C (preparation conditions: Ru loading 2 wt%, preparation tem-
◦
perature 60 C), and (c) Ru/CNT catalyst without calcination (preparation conditions:
◦
Yield of cyclohexane
Ru loading 2 wt%, preparation temperature 60 C).
TOF
7000
100
6
5
4
000
000
000
biomolecules responsible for the biosynthesis of the Ru nanoparti-
cles might be flavones and reducing sugars.
The TG and DTG curves of CNT support, calcined Ru/CNT cata-
lyst and uncalcined Ru/CNT catalyst were presented in Fig. 4. From
the TG analysis, the presence of some plant biomass residual was
confirmed. As shown in the picture, the plant biomass weighed
8
0
60
40
3000
1
5.1% on uncalcined catalysts in contrast to only 4.2% on calcined
2
1
000
000
catalysts. Actually, the amount of residual plant biomass was large
when compared with other reported supports [28], demonstrating
the superior adsorption property of the multiwalled carbon nano-
tubes. This plant biomass could act as the protective substances
around the Ru nanoparticles. Results from the DTG analysis proved
that the decomposition temperatures of the residual plant biomass
were around 280 C, 560 C, and 730 C. Although the plant biomass
of the Cacumen Platycladi could prevent the metal nanoparticles
from aggregation, we expected that proper calcinations treatment
would contribute to the exposure of the active Ru surface. Hence,
an appropriate calcination treatment was recommended following
the results of TG and the evaluation parts.
2
0
0
30
45
60
75
90
o
Preparation temperature ( C)
◦
◦
◦
Fig. 6. Effect of preparation temperature on the yield of cyclohexane and turnover
frequency values (conditions: Ru loading 2 wt%, calcination temperature 500 C,
benzene 3 mL, catalyst 0.05 g, reaction temperature 80 C, reaction pressure 4 MPa,
◦
◦
reaction time 0.5 h).
illustrated in Fig. 6. As shown, the preparation temperature was a
sensitive factor which had an effect on the ultimate catalytic perfor-
mance. When the preparation temperature was as low as 30 C, the
◦
3.2. Optimization of catalysts preparation conditions
−1
cyclohexane yield was 82.45% and the TOF value was 5718.84 h
.
The cyclohexane yield and TOF value increased nearly linearly with
the preparation temperature increasing. The yield of cyclohexane
3
.2.1. Influence of Ru loading
The effects of Ru loading (0.5–3 wt%) on the catalytic activity of
−1
and the TOF value were 99.97% and 6983.09 h , respectively, at
the bioreduction catalysts towards benzene hydrogenation were
shown in Fig. 5. It is a well-known fact that various metal load-
ing has remarkably different effects on the catalyst performance
◦
the preparation temperature of 60 C. The cyclohexane yield and
TOF value oppositely decreased when preparation temperature of
◦
6
0–90 C is introduced. In general, the preparation temperature is
[
41]. The Ru/CNT catalyst with the lowest Ru loading of 0.5 wt%
corresponding with the reduction rate of the metal nanoparticles
42]. In our preparation system, the yield of cyclohexane and the
presented a poor performance with only 21.32% the yield of cyclo-
[
−1
hexane and 1451.10 h TOF value. The low metal loading along
with the Ru active site insufficient might decrease the catalytic
reactivity. Increasing the catalyst loading to 2 wt% gave rise to cyclo-
hexane yield of 99.97% and TOF value of 6983.09 h . However, too
much load of Ru (Ru loading of 3 wt%) might result in loss of catalytic
activity due to the larger particle size of the ruthenium nanoparti-
cles. Consequently, the optimal Ru loading is 2 wt% at which the Ru
active sites and the particle size were well balanced.
TOF value first showed an upward trend and then a downward
◦
one. This led us to believe that 60 C accompanied with a suitable
reduction rate and homogeneous nucleation rate was a moderate
preparation temperature that should be recommended.
−
1
3.2.3. Influence of calcination temperature
As shown in Fig. 7, the catalytic performance of benzene hydro-
genation strongly depended on calcination temperature. As is
known that calcination temperature plays an important role in the
activation of the catalysts [43]. Appropriate calcination treatment
could well balance between the ruthenium active sites exposed to
the reactants and the stability of the Ru/CNTs catalysts, due to the
prevention of Ru from agglomeration. Thus the conclusion drawn
3.2.2. Influence of preparation temperature
The influence of the preparation temperature on the cyclo-
hexane yield and the TOF value was investigated. The catalytic
performance versus preparation temperature (30–90 C) was
◦

Y. Ma et al. / Applied Catalysis A: General 484 (2014) 154–160
159
7
6
5
4
000
000
000
000
1
00
Yield of cyclohexane
1
1
1
15
10
05
7
500
TOF
8
6
4
2
0
0
0
0
0
7000
6
6
5
5
500
000
500
000
100
9
9
8
5
0
5
3000
2
1
000
000
80
4500
No cal. 200
300
400
500
600
O
700
fresh
1
2
3
4
5
6
Calcination temperature ( C)
Number of Cycles
Fig. 7. Effect of calcination temperature on the yield of cyclohexane and turnover
Fig. 8. The catalytic performance of recycling the bioreduced Ru catalyst (condi-
tions: Ru loading 2 wt%, preparation temperature 60 ◦C, calcination temperature
◦
frequency values (conditions: Ru loading 2 wt%, preparation temperature 60 C,
◦
500 ◦C, benzene 3 mL, catalyst 0.05 g, reaction temperature 80 ◦C, reaction pressure
benzene 3 mL, catalyst 0.05 g, reaction temperature 80 C, reaction pressure 4 MPa,
reaction time 0.5 h).
4 MPa, reaction time 0.5 h).
from Fig. 7 showed that the catalysts without calcination were inert
to catalytic activity which could be caused by the excessive plant
biomass around the Ru nanoparticles. The data clearly indicated
that the yield of cyclohexane increases steadily and continuously
from 200 C to 500 C (from a low value of 13.45–99.97%). A suit-
able calcination of the catalysts would get rid of partial biomass
3
.3. Durability of the bioreduction catalysts
The noble metal heterogeneous catalysts always suffer from
rapid deactivation and the leaching problem [44,45]. In addition
to the remarkable catalytic performance, the recyclability of the
catalyst was also studied. The hydrogenation run was performed
as described in Section 2.4. The catalysts after reaction were sepa-
rated by centrifugation at a speed of 4000 rpm and the supernatant
liquid was carefully removed by use of a syringe. The recycled
catalyst was repeatedly washed by deionized water and ethanol.
Fig. 8 illustrated the catalytic performance during six consecu-
tive cycles and revealed that there was no significant decrease in
the cyclohexane yield and the TOF value which indicated remark-
able stability of the bioreduction Ru catalysts. The TEM analysis
was introduced to elucidate the nature of the reused catalysts. As
was seen from the pictures (Fig. S3 of Supplementary Informa-
tion), the nanoparticles in the recycled catalysts remained at the
◦
◦
(
accounted for 9.9 wt% of the total catalyst weight according to the
TG analyses) and made plenty of metal active sites exposed to the
reactants. At the same time, the Ru/CNTs catalysts calcinated at
◦
5
00 C still contained 4.2% plant biomass according to the TG anal-
yses. The yield of cyclohexane got the highest value of 99.97% when
◦
the calcination temperature was 500 C, and it decreased to 72.56%
◦
as the calcination temperature increased to 700 C. At this point,
higher calcination temperature might remove most of the resid-
ual 4.2% plant biomass. It was this process that might lead to the
◦
agglomeration of the RuNPs. Hence, 500 C was considered to be
the optimum calcination temperature of the catalysts.
Table 2
The comparison of hydrogenation of benzene to cyclohexane catalyzed by Ru catalysts.
T ( C)a
◦
P (MPa)b
t (h)c
Yield (%)d
TOF (h )e
−1
Ref.
Catalyst
Ru/CNTs
80
4.00
0.5
99.97
99.94
100
99.50
35.60
100
>99
18
93
100
90
99
100
100
6983
2747
400
200
102
10
2475
450
664
120
1040
610
50
This work
f
Ru/C
Ru/MMT
40
4.00
2.5
5.0
3.5
4.0
2.0
2.0
7.0
1.0
2.0
4.0
2.0
2.1
[46]
g
Ru/C
h
Ru/Al2O3
Ru/Methylated cyclodextrins
20
60
0.10
6.00
[16]
[47]
Ru/MOFi
j
Ru/MOF
Ru/C
k
Ru/PVPy
120
22
25
25
25
5.00
0.28
0.29
3.00
3.00
[37]
[14]
[48]
[49]
[50]
Ru/intrazeolite
(
0)
Ru /HAp
l
Ru/C composites
Ru/CNT nanocompositesm
476
a
Reaction temperature.
Reaction pressure.
Reaction time.
Yield of cyclohexane.
b
c
d
e
TOF was calculated as moles of benzene transformed per mole of active component per hour.
The commercial 5% Ru/C catalyst from Aladdin Co. Ltd., China.
From ICI CO., Japan.
f
g
h
i
From XiAA Catal. Chem. Tech. Co. Ltd., China.
The Ru/MOF catalyst synthesized in supercritical CO2-methanol solution.
The Ru/MOF catalyst synthesized in aqueous solution.
The commercial Ru/C catalyst.
The Ru/C catalyst synthesized by thermal decomposition from Ru acetylacetonate.
The Ru/CNT catalyst synthesized in supercritical water.
j
k
l
m

1
60
Y. Ma et al. / Applied Catalysis A: General 484 (2014) 154–160
identical state (3.12 ± 0.55 nm) and without obvious agglomer-
ation. The excellent catalysts stability might be ascribed to the
residue plant biomass (validated in the TG studies) as adhered to
RuNPs, it could be served as a protecting agent and thus was able
to prevent the leaching of the metal nanoparticles.
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