Ru nanoparticles supported graphene oxide catalyst for hydrogenation of bio-based levulinic acid to cyclic ethers

Article abstract of DOI:10.1016/j.cattod.2015.09.042

Ruthenium nanoparticles supported on graphene oxide (GO) catalysts have been evaluated for bio-based levulinic acid (LA) hydrogenation to produce vapor-phase cyclic ethers in a fixed-bed reactor. It was found that using the GO supported Ru nanoparticles (Ru/GO) produced additional hydrogenation products - cyclic ethers (54%) and γ-valerolactone (GVL; 41%), while Ru on carbon (Ru/C) catalysts gave only GVL with 100% LA conversion. To improve the yield of cyclic ethers, an additional two-step hydrogenation of LA via GVL was successfully carried out. This provided GVL as a second feedstock from which the Ru/GO catalyst could produce cyclic ethers such as methyltetrahydrofuran (MTHF) and tetrahydrofuran (THF). Ru on GO catalysts showed a 92% selectivity of predominantly cyclic ethers, including a 77% selectivity of MTHF through two steps process. Such a remarkable enhancement in activity and selectivity of LA hydrogenation over Ru/GO can be attributed to the well-dispersion of Ru nanoparticles, as well as favorable interaction with GO in the presence of oxy-functional groups of GO. In order to evaluate the active sites on the catalyst, they were characterized using different characterization techniques such as Raman, XRD, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), temperature-programmed desorption of NH₃ (TPD), electron microscopy (TEM and SEM) and H₂-chemisorption and N₂ adsorption.

Full text of DOI:10.1016/j.cattod.2015.09.042

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Ru nanoparticles supported graphene oxide catalyst for
hydrogenation of bio-based levulinic acid to cyclic ethers
Pravin P. Uparea, Maeum Leea, Su-Kyung Leea, Ji Woong Yoona, Jongyoon Baea,
Dong Won Hwanga,c, U-Hwang Leea,c, Jong-San Changa,b,∗, Young Kyu Hwanga,c,∗
a Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-Ro, Yuesong,
Daejeon 305-600, South Korea
b Department of Chemistry, Sungkyunkwan University, Suwon 440-476, South Korea
c Department of Green Chemistry, University of Science and Technology (UST), 217 Gajeong-Ro, Yuseong, Daejeon 305-350, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2015
Received in revised form 9 September 2015
Accepted 11 September 2015
Available online xxx
Ruthenium nanoparticles supported on graphene oxide (GO) catalysts have been evaluated for bio-based
levulinic acid (LA) hydrogenation to produce vapor-phase cyclic ethers in a fixed-bed reactor. It was found
that using the GO supported Ru nanoparticles (Ru/GO) produced additional hydrogenation products –
cyclic ethers (54%) and ␥-valerolactone (GVL; 41%), while Ru on carbon (Ru/C) catalysts gave only GVL
with 100% LA conversion. To improve the yield of cyclic ethers, an additional two-step hydrogenation of
LA via GVL was successfully carried out. This provided GVL as a second feedstock from which the Ru/GO
catalyst could produce cyclic ethers such as methyltetrahydrofuran (MTHF) and tetrahydrofuran (THF).
Ru on GO catalysts showed a 92% selectivity of predominantly cyclic ethers, including a 77% selectivity
of MTHF through two steps process. Such a remarkable enhancement in activity and selectivity of LA
hydrogenation over Ru/GO can be attributed to the well-dispersion of Ru nanoparticles, as well as favor-
able interaction with GO in the presence of oxy-functional groups of GO. In order to evaluate the active
sites on the catalyst, they were characterized using different characterization techniques such as Raman,
XRD, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), temperature-
programmed desorption of NH3 (TPD), electron microscopy (TEM and SEM) and H2-chemisorption and
N2 adsorption.
Keywords:
Ru/graphene oxide
Levulinic acid
GVL
Hydrogenation
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
These could be used for the production of conventional hydrocar-
bon fuels [2] as well as for non-fuel petrochemical products [3].
Current world energy needs strongly depend on non-renewable
fossil resources, and these resources are in mature stages [1]. There
are some crucial issues related to the use of these fossil resources,
including global warming, dramatic increase in oil prices, and their
storage and transportation [1]. These issues have stimulated the
development of alternative and renewable resources to substitute
for fossil resources. Biomass is a promising alternative candidate
resource with the potential for long-term availability because it
is the only renewable source of platform chemicals such as lev-
ulinic acid (LA), formic acid (FA), succinic acid (SA), and lactic acid.
Biomass-derived LA is an attractive and versatile platform chemical
for value added products such as methyltetrahydrofuran (MTHF),
␥-valerolactone (GVL), 1,4-Pentanediol (1,4-PDO), 1-Pentanol, and
esters [3–6]. The flexibility of LA is due to the presence of active
functional groups, ketones and carboxylic acids, together [6]. The
applications of MTHF and GVL are well known, DOE (US Depart-
ment of Energy) approved MTHF as a P-series fuel and considered
it as an alternative for gasoline. Additionally, MTHF has other appli-
cations such as solvent for electrolyte formulation, and as adhesives
[7]. Tetrahydrofuran (THF) is also a well-known solvent in used in
organic and polymer industries [7]. Due to its pleasant herbal scent,
there is a large demand for GVL in the perfume and food/fragrance
industries [1,2,8,9].
∗
Corresponding author at: Catalysis Center for Molecular Engineering, Korea
Very few references are reported for the synthesis of MTHF
from LA, mostly in batch type reactors using higher hydrogen
pressure (>100 bar) [9,10]. However, some reports also exist for
the synthesis of MTHF from GVL in liquid phase [11]. We also
Research Institute of Chemical Technology (KRICT), 141 Gajeong-Ro, Yuesong,
Daejeon 305-600, South Korea.
E-mail addresses: jschang@krict.re.kr, jschang020@skku.edu (J.-S. Chang),
ykhwang@krict.re.kr (Y.K. Hwang).
http://dx.doi.org/10.1016/j.cattod.2015.09.042
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: P.P. Upare, et al., Ru nanoparticles supported graphene oxide catalyst for hydrogenation of bio-based
levulinic acid to cyclic ethers, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.042

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have reported a direct synthesis of MTHF (89%) from LA at low
H2 pressure (25 bar) in vapor phase by Ni-promoted Cu–SiO2
nanocomposite catalyst [4]. Likewise MTHF synthesis, liquid phase
and higher hydrogen pressure were mostly used for the GVL pro-
duction from LA [1,12,13]. Upare et al. have recently reported the
selective production of GVL from LA at lower H2 pressure [14,15].
In this work, noble metal catalysts like Ru, Pd and Pt supported on
carbon were investigated for the synthesis of GVL from LA [15].
Among the catalysts listed, Ru on carbon catalyst was found to
be superior in terms of GVL selectivity. Higher dispersion of Ru
nanoparticles was observed to be the main reason for its spe-
cific activity in the production of GVL from LA. A higher degree
of Ru-nanoparticle dispersion in Ru-supported catalysts has been
attributed for the active sites with specific hydrogenation activity
[13,15–17]. Different reaction parameters were also investigated to
improve the activity of Ru/C and to achieve additional hydrogena-
tion products (e.g., MTHF or 1,4-PDO). However, GVL was the main
product obtained (>96%) under all the reaction conditions [15]. This
means Ru/C has some limitations for the production of MTHF at
lower hydrogen pressure (<25 bar). This could be one reason why
higher hydrogen pressures were used in previous investigations
[9–11].
2.2. Preparation of the Ru/GO catalysts
Ru on GO was prepared by the wetness impregnation method. In
a typical procedure, the desired amount of GO (BET 248.0 m2/g) was
soaked in an aqueous solution containing an appropriate amount
of RuCl3·xH2O and 100 ml of double-distilled water. In order to get
fine dispersion of Ru particles, the resultant mixture was sonicated
for 1 h. Sonication could help in the exploitation of GO, and allow
space for easy metallic dispersion. After sonication, the mixture was
aged for 12 h at room temperature with constant stirring. Water
was removed using a rotary evaporator at 70 ◦C for 2 h under vac-
uum. The resultant powder was then dried at 120 ◦C overnight, and
reduced in a fixed bed reactor by a flowing hydrogen stream (in 10%
N2) at 5 cc/min and 450 ◦C for 2 h. The amount of ruthenium load-
ing on the GO was confirmed using ICP analysis and TEM-EDX. A
similar procedure was applied for the preparation of 5% Ru/C, using
activated carbon (768.0 m2/g) as support.
2.3. Catalysts testing for LA hydrogenation
Levulinic acid (LA) hydrogenation was carried out in a con-
ventional fixed-bed down-flow reactor (made of stainless steel
tubing 30 cm in length and with 1 cm outer diameter) packed with
1.0 g of reduced catalyst in the center of the flow reactor. The
hydrogenation of LA was conducted at 265 ◦C with hydrogen pres-
sure from 10 to 25 bar. 10 wt% of LA in 1, 4-dioxane solvent was
introduced into the reactor using a liquid metering pump (Lab
Alliance Series 3) with the desired WHSV (weight hourly space
velocity). The samples were collected at different intervals from
a cold-water-condensation trap equipped with a reactor, and then
subjected to analysis using a gas chromatograph (Donam GC, DS
6200) equipped with FID and a capillary column (Cyclosil B-5). From
GC-MS analysis, as well as from the retention times of the authentic
samples on the gas chromatogram, we confirmed the presence of
LA hydrogenation products including GVL, AL, MTHF, THF, PDO, and
5-hydroxy pentanoic acid (␥-hydroxylvaleric acid). The conversion
of LA and selectivity for products were calculated using Eqs. (1) and
(2), respectively.
In the last decade, chemically derived GO containing sp2 car-
bon arrays have received special interest in various fields. This
is because of the potential for functionalization and modification
of GO as polymer composites, energy materials, sensors, transis-
tors, and biomedical related applications [18], as well as to the
presence of reactive oxygen functional groups (e.g., carboxylic,
epoxy, and hydroxide) [19]. Some research groups have also used
this material as a catalyst for chemical conversion of organic
compounds [19,20] exploiting the active oxy-functional groups
presented on the surface of GO. Recently, magnetic nanoparti-
cles (M-NPs) supported on carbonaceous material have attracted
great interest in catalysis [21]. The high thermal stability, exten-
sive surface area [22], and the presence of oxy-functional groups
on GO facilitate easy dispersion of metallic nanoparticles [23].
The use of GO has been reported in the synthesis of noble-metals
(e.g., Pt, Ru, and Pd) supported on GO for different purposes [24].
Among these, Bong et al. [25], and Janiak et al. [20], have success-
fully investigated the use of Ruthenium-nanoparticle supported
on graphene as catalyst for hydrogenation reactions. Recently,
Du et al. has researched the catalytic activity of mono-dispersed
Ru/graphene material for hydrogen generation from hydrogenol-
ysis of ammonia [26]. However, there was no previous report
about found in the literature regarding to the utilization of Ru on
multilayered GO for hydrogenation reactions. Such active types
of material could be promising catalysts for chemical conver-
sion. Herein, we demonstrate the activity of Ru/GO catalysts for
vapor-phase production of cyclic ethers from LA via and GVL hydro-
genation. This newly developed catalyst can produce additional
hydrogenation products continuously at lower moderate hydrogen
pressure.
Conversion of LA(mol %)
Initial mole of LA − unreacted mole of LA
=
× 100
(1)
(2)
Initial mole of LA feed
Product selectivity(mole %)
Mole of desired product
=
× 100
Total moles of all products
Carbon ballence(%)
(5 × moles of GVL) + (5 × moles of MTHF) + (5 × moles of PDO)
+ (4 × moles of MTHF) + (1 × moles of CH4)
5 × moles of LA converted
=
× 100
(3)
2. Experimental
2.1. Materials
2.4. Characterization of the catalysts
The substance RuCl3·xH2O was obtained from Sigma–Aldrich
and used in the preparation of the catalyst. Levulinic acid (99.0%,
Alfa Aesar), GVL (99.9%, Aldrich), other reference compounds like
MTHF, Anjelica-lactone (AL), 1,4-PDO (1,4-Pentanediol), and 1-
pentanol were purchased from Sigma–Aldrich. The solvent 1,
4-dioxane (HPLC grade, 99.9%, Samchun Chemical) was used as
received.
The amount of Ru loading was measured using inductively cou-
pled plasma analysis (ICP, Jarrell-Ash, USA). The structural variation
of the catalysts before (freshly reduced) and after hydrogenation
of LA, was determined using X-ray diffractometer (Rigaku, Japan)
with Cu K␣ radiation. X-ray photoelectron spectroscopy (XPS) data
of the catalysts were recorded using a spectrometer (Model: Kratos
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Analytical, Ltd., A Shimadzu Group Company). This instrument was
equipped with an AXIX Nova hemispherical electron analyzer and
a duel-core X-ray source working with Al K␣ at 150 W, 30 mA, using
C1s as energy reference (284.4 eV). The microstructural properties
were determined using a transmission electron microscope (Tecnai
G2 Retrofit). The morphology of the catalysts was determined using
a JEOL 6700 scanning electron microscope (SEM). Temperature-
programmed reduction (TPR) measurements were carried out in a
U-shaped quartz reactor, using 5% H2 in a He-gas flow of 50 ml/min
with 100 mg catalyst. Hydrogen consumption was monitored using
online mass spectrometry (Model: BEL-CAT, BEL Japan, Inc.). The
same instrument was used for the pulse hydrogen chemisorption.
The pre-treatment of the catalyst was performed in a flow of H2 at
250 ◦C for 4 h, followed by degassing in flowing Ar at 350 ◦C. Pulse
chemisorption was carried out at 0 ◦C [27]. Raman measurements
were carried out using a Bruker IFS 100/S spectrometer with an
excitation wavelength of 532 nm, in the back-scattering geometry
with a resolution of 4 cm−1. Determination of different functional
groups in the catalyst was performed using FT-IR spectroscopy
(Nicolet-6700). The surface area was evaluated using BET N2 sorp-
tion measurement (Micrometrities, TriStar) experiments, using the
same instrument equipped with TCD detector that was used for the
NH3-temperature program desorption.
method to enhance the activity of a Ru-based catalyst system used
for LA hydrogenation, and to obtain additional hydrogenation prod-
ucts (MTHF and THF). The structural and morphological properties
of the catalysts, as well as the active sites present, were evaluated
using a variety of characterization techniques.
3.1. Catalyst characterization
In the last decade, Raman spectroscopy has become a well-
known, nondestructive technique that can provide direct insight
into electron–phonon interactions for structural characteriza-
tion of graphitic/carbon materials [18,28–33]. Raman is familiar
method for structural investigation of graphene-related materi-
als [18,29]. The Raman spectra of GO, graphite, carbon, and 5%
Ru/GO, are shown in Fig. S1. The peaks at 1560 cm−1 and 1360 cm−1
are the hallmarks for the G-band and D-band, respectively, in
graphitic/carbon materials [30,31]. The G-band peak correspond-
ing to the E2g phonon was centered at 1560 cm−1 in the spectra
(Fig. S1), and the D-band peak at 1360 cm−1 (Fig. S1) indicated
graphene edges and/or subdomain boundaries of sp2 carbon atoms
with a constant number of graphene layers in GO [31]. There was no
sharp peak observed at 2693 cm−1, which is the hallmark of mono-
layer graphene. However, a less intense, broad peak was observed
in the range 2400–2800 cm−1, due to the multilayer morphology of
graphene. It can be very hard to distinguish the multilayer peak
profile in Raman spectra, if there are more than five graphene
layers [32]. Thus, it is reasonable that the GO utilized herein
consisted of multilayer graphene sheets and, that this multilayered
3. Results and discussion
As discussed in the introduction, Ru/GO with different load-
ing (3–7 wt% Ru) was prepared using the wetness impregnation
Fig. 1. SEM images of 5% Ru/GO catalyst.
Fig. 2. EDS mapping images of 5% Ru/GO catalyst.
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Fig. 3. TEM images of (a) 3% Ru/GO, (b) 5% Ru/GO and (c) 7% Ru/GO catalysts.
morphology was maintained after loading with the Ru nanopar-
ticles. These results are analogous with the results from SEM and
TEM analysis. In the SEM images (Fig. 1) of 5% Ru/GO, the multilayer
morphology of the GO is clearly observed where the multilayer
graphene sheets are uniformly well arranged in an array. SEM-EDS
mapping of 5% Ru/GO was performed to check the dispersion of the
Ru nanoparticles (see images in Fig. 2). In the EDS mapping images,
clear dispersion of the Ru nanoparticles can be observed on and
inside the GO layers.
Similarly, TEM analysis was also conducted for all the catalyst
samples to investigate the dispersion of the metal particles. In Fig. 3,
the TEM images of Ru/GO provide confirmation of the multilayer
GO morphology, which was confirmed by SEM and Raman analysis.
Fine and uniform dispersion of Ru nanoparticles can be observed
on the low-resolution images (50 nm) (Fig. 3). The average size
of the Ru particles was determined for all the catalysts and their
distribution in some interval range (0.5 nm) was made by consid-
ering the bunch (∼100 Ru particles) of particles on the images at
higher resolutions. The amount of Ru and the size of the Ru par-
ticles were confirmed via TEM-EDX analysis. The average particle
size of Ru was found to be around 1–2 nm. They were homoge-
nously dispersed on the GO surface with good interaction. Probably,
sonication during preparation of the catalysts helped achieve high
dispersion of the ruthenium on the GO support. As discussed earlier,
well dispersed metallic Ru nanoparticles on a carbon support are
considered as active sites for vapor-phase LA hydrogenation to yield
other products [15]. In the current study, the layered morphology
of the GO support, as well as the presence of oxy-functional groups
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154 oC
(002)
(100)
(101)
(e)
(d)
(c)
(b)
601 oC
299 oC
(a)
(a)
(b)
10
20
30
40
50
60
70
80
2 Theta/degree
100
200
300
400
500
600
700
800
Temperature(oC)
Fig. 4. X-ray diffraction (XRD) patterns of reduced catalysts. (a) GO, (b) 3% Ru/GO,
(c) 5% Ru/GO, (d) 7% Ru/GO and (e) 5% Ru/C catalysts.
Fig. 5. TPR profile of (a) fresh Ru/GO and (b) fresh GO catalysts.
Table 1
H2 pulse chemisorptions results of catalysts at 0 ◦C.
TPR profiles for fresh Ru/GO catalyst (Fig. 5a) showed three
reduction peaks, centered at 154 ◦C, 299 ◦C, and (a major one) at
601 ◦C. In a previous study, the TPR profiles of fresh Ru/C catalyst
were reported and showed three reduction peaks for Ru◦ cen-
tered at 169 ◦C (minor), 240 ◦C (major), and 270 ◦C (major) [15].
This means that most of the ruthenium oxide species in Ru/GO
get reduced at lower reduction temperatures. In Fig. 5a, the low-
temperature reduction peak is due to the reduction of highly
dispersed ruthenium oxide species on the GO support [27,35–37].
Another reduction peak observed at 299 ◦C in the TPR profile is
due to the transformation of strongly interactive RuO2 species
to metallic ruthenium [36]. However, the area and intensity of
the low-temperature reduction peak was higher than the middle
one at 299 ◦C. As discussed above, the particle size of Ru in the
Ru/GO catalysts was around 1–2 nm. As per earlier work [17,38],
the strong interaction and higher dispersion of RuO2 nanoparticles
on carbonaceous support could be due to their interaction with
oxy-functional groups on GO. Komanoya et al. proposed the func-
tionalized structure of Ru in mesoporous carbon by assuming a
ruthenium particle size of >1.2 nm [39], and they believe that Ru
functionalization with carbon is the main reason for the higher
metallic dispersion and interactions with the support. Hence, it is
reasonable that the strong interaction of ruthenium in Ru/GO is due
to the functionalization involving the oxy-functional groups in GO.
Fu et al. demonstrated that these highly dispersed small nanoparti-
cles are responsible for the enhanced catalytic activity due to their
active surface atoms and homogenous distribution [17], which was
also observed by Cao et al. [16]. From TPR analysis, it is clear that the
Ru/GO catalyst allows temperature reduction in the temperature
range of 154–299 ◦C, which is close to the actual reaction temper-
ature (265 ◦C) adopted for effective hydrogenation of LA. The peak
assigned at higher reduction temperature of 601 ◦C is mainly due to
the reduction of the surface oxy-functional groups on GO [33,36].
This was also seen on the TPR profile of core GO samples, but not on
the TPR profile of Ru/C. This is a clear indication of the presence of
Catalyst
Metallic S/A (m2/g)
H2 uptake (mmol/g)
Total dispersion (%)
3% Ru/GO
5% Ru/GO
7% Ru/GO
5% Ru/C
2.63
2.97
2.89
1.09
0.023
0.027
0.033
0.016
8.76
10.96
11.29
6.44
not only helped to achieve well-dispersed Ru particles, but also
improved the hydrogenation activity of the Ru/GO catalyst for pro-
ducing MTHF and THF. These TEM analysis results were correlated
with the results of XRD analysis.
The XRD patterns of the reduced Ru/GO and Ru/C catalysts used
in LA hydrogenation are shown in Fig. 4. The XRD patterns of both
Ru/GO and Ru/C catalysts did not show intense peaks for the metal-
lic Ru, but crystalline traces of metallic Ru were observed in on the
XRD patterns of both catalysts at 2ꢀ of 38.4◦, 42.2◦, and 44.0◦. These
XRD peak positions of metallic Ru were indexed by JCPDS card (no.
06-00663) [16,33]. However, two main broad peaks at 2ꢀ (25.78◦
and 43.38◦) were observed in the X-ray diffraction pattern of all
the catalysts, including GO (Fig. 4a). These are the characteristic
peaks of short range ordering of carbon species [13,33]. There were
hardly any differences found between the XRD patterns of Ru/C
and Ru/GO with 3–7 wt% ruthenium loading. It has been reported
that the XRD pattern does not show a crystalline diffraction peak
for small Ru particles (2–5 nm), due to effective dispersion on the
support [34–36]. In this regard, the XRD results are almost a match
with the TEM analysis results of all the catalysts synthesized.
H2 pulse chemisorption of both catalysts was performed to
investigate Ru dispersion and the amount of H2 uptake. The H2
chemisorption results are presented in Table 1. Among these cat-
alysts, Ru/GO exhibited greater metallic Ru dispersion and greater
metallic surface area compared to Ru/C. Yan et al. investigated
the dispersion of Ru catalysts on the carbon and Al2O3 supports
using H2-chemisorption, and calculated the average Ru particle size
around 2–3 nm by assuming spherical ruthenium particles for both
supports [13]. It is noteworthy that greater metallic dispersion and
greater metallic surface area are considered important factors for
enhancing the catalytic activity of Ru/GO in LA/GVL hydrogenation
for MTHF production. The amount of H2 uptake for both catalysts
was determined by study of the H2 chemisorption. Not surprisingly,
0.027 mmol/g of H2 was adsorbed on 5% Ru/GO, which was greater
than the H2 uptake of Ru/C (0.016 mmol/g). This higher amount of
H2 uptake by Ru/GO could be good evidence an indication of its
superior catalytic activity. By taking these results into account, a
clear relationship between catalytic activity and metal dispersion
was noticed.
oxy-functional groups (e.g., COOH,
C O C ) on the GO surface.
As discussed before, the oxy-functional groups on the GO surface
did not reduce during LA hydrogenation at 265 ◦C and during Ru
reduction at 400 ◦C. As per literature, greater dispersion of the
metallic nanoparticles, and their good interactions, allow metal-
lic reduction at lower temperature than the nominal temperature
[40]. Interestingly, the low-temperature reduction of ruthenium
nanoparticles was also seen in the TPR profile of Ru/GO. This means
that well dispersed Ru◦ nanoparticles having good interaction with
GO, increase the hydrogenation of LA to form MTHF. This increase
is accounted for by the Ru species most active in LA hydrogenation,
and their interaction with the oxy-functional groups.
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Table 2
6
The catalytic activity of Ru/C and Ru/GO in LA and GVL hydrogenation.
Catalyst
aSABET
(m2/g)
bAcidity
(mmol/g)
Selectivity (mol%)*
461.08 eV
483.38 eV
483.14 eV
MTHF
THF
GVL
AL
C1
2
5.5
18.2
0
3% Ru/GO
5% Ru/GO
7% Ru/GO
5% Ru/C
240
263
241
748
263
3.87
4.04
4.13
2.12
4.04
33.4
48.0
42.2
2
2.3 62.3
5.5 41
18.7 20.9
0
16
0
0
0
1
–
460.98 eV
(a)
(b)
97
–
c5% Ru/GO
69
15
Reaction conditions: catalyst: 1.0 g; feed: 10% of LA in 1, 4-dioxane; temperature:
265 ◦C; H2 pressure: 25 bar; WHSV: 0.512 h−1; TOS: 50 h (note: conversion was 100%
for all of catalysts).
a
BET surface area.
NH3-TPD (total acidic strength).
Catalytic results from GVL hydrogenation.
b
455
460
465
470
475
480
485
490
c
*
B.E. (eV)
C1 paraffin (i.e. CH4).
Fig. 6. X-ray photoelectron spectroscopy (XPS) spectra of Ru 3P3/2 and 3P1/2 profile
of (a) reduced 5% Ru/GO and (b) reduced 5% Ru/C catalysts.
different reaction conditions in vapor phase. In contrast, the cat-
alytic activity of Ru/GO for LA hydrogenation was significantly
improved by changing the layered, oxy-functional support to GO
instead of amorphous carbon.
In order to better confirm the metallic Ru species on the catalyst
surface, XPS of Ru 3P3/2 and Ru 3P1/2 orbitals in the B.E. range of
455.0 eV to 490.0 eV were performed, and the results are shown in
Fig. 6. The XPS binding energies for metallic Ru 3P3/2 was 461.0 eV,
and for Ru 3P1/2 was 483.2 eV [41]. The reduced catalysts of Ru/GO
and Ru/C showed two strong signals (at 461.08 eV and 460.98 eV,
respectively) in the 3p3/2 region, and 483.38 eV and 483.14 eV in
the 3P1/2 region, respectively. This clearly indicated the presence
of metallic ruthenium [42] in the reduced catalyst. The small shift
of the BE may have been caused by the strong interaction between
the metallic Ru◦ and its support.
Ru/GO catalysts with different loading from 3 wt% to 7 wt% were
tested for LA hydrogenation and compare their activity with the
activity of 5% Ru/C catalyst. The comparative results of Ru/C and
Ru/GO in the hydrogenation of LA are presented in Table 2. All Ru
supported catalysts exclusively gave 100% LA conversion, but prod-
ucts distribution was found to be different according to Ru loading
and support. The catalyst 3%Ru/GO converted 100% of LA into 35.7%
of cyclic ethers (33.4% of MTHF and 2.3% of THF), 62.1% of GVL, and
remain 2.2% of hydrocarbons selectively at 25 bar of H2 pressures
and under the other similar reaction conditions given below in
Table 2. MTHF selectivity (48.0%) was observed to be increased with
5% Ru/GO, but it gets decreased to 42.2% by use of 7% Ru/GO cata-
lyst was used due to formation of further dehydration and cracking
products such as THF (18.74%) and methane (18.13%). The 5% Ru
loading on GO was considered as optimal catalyst to get MTHF, but
it has also produced 41% of GVL, 5.5% of THF and 5.46% of methane.
These further hydrogenation products were not produced over 5%
Ru/C catalyst which only made 97% of GVL and 2% of MTHF. As dis-
cussed earlier [15], Ru/C catalysts have some limitation and it can
only produces GVL at all previously tested reaction conditions. But,
MTHF and other products were obtained by GO supported catalysts.
However, both Ru/GO and Ru/C catalysts produced only GVL with
100% selectivity at atmospheric pressure. It should be noted that
GC peak for the hydrocarbon (i.e. CH4) were observed on the GC
chromatogram of gaseous product mixture during LA/GVL hydro-
genation over Ru/GO, but were not observed over Ru/C catalyst.
Total acidic strength for both catalysts have been evaluated through
NH3-TPD analysis, and presented in Table 2 (acidity distribution
is presented in Table S1, supporting) along with BET surface area.
Among them, Ru/GO catalysts existed with higher acidic strength
of around 4.0 mmol/g than Ru/C 2.12 mmol/g. This higher acidic
strength of Ru/GO is mainly due to the presence of oxy-functional
groups on GO surface. It means higher acidity present in Ru/GO is
boosting the hydrogenation activity of highly dispersed nanopar-
ticles in further hydrogenation to MTHF and MTHF decomposition
into of THF and hydrocarbons (C1).
The characteristic binding energy (BE) for the Ru 3d5/2 energy
level is not clear because of the overlapping of the carbon 1s (C 1s)
due to the carbonaceous surface. De-convolution of the overlapped
portion in the C 1s XPS spectra was performed to determine the
presence of carbonaceous functional groups on GO as well as on
the carbon surface (shown in Fig. S2, supporting information). The
BE peak at 284.6 eV is due to the presence of non-oxygenated or
aromatic carbon species C C or C C (sp2), and BE peaks at 285.5,
286.6, 287.7, and 289.9 EV are indicative of oxygenated or aliphatic
(sp3) hydroxyl (C OH), epoxy (C
O C), carbonyl (C O), and car-
boxylic ( COOH) carbon species respectively. This was confirmed
by previously reported XPS information for carbonaceous mate-
rial [42]. These oxy-functional species are also observed in the XPS
pattern of activated carbon. The XPS peak area and intensity of the
oxy-functional groups in GO are higher than those in activated car-
bon. The presence of oxy-functional groups was further confirmed
by FT-IR spectra (see Fig. S3). XPS results are a near match with the
FT-IR analysis. This could be the main reason why the acidity of GO
catalysts is higher than carbon. From the XPS and FT-IR investiga-
tions, the presence of oxy-functional groups in GO help increase
hydrogenation to form more MTHF. However, additional detailed
study will be needed to clarify how the noble metal species catalyze
the hydrogenation reaction.
3.2. Hydrogenation of levulinic acid
Noble-metal based catalysts consisting of ruthenium are known
to be the most active catalysts for the hydrogenation of aliphatic
carbonyl compounds [15,43]. The surface area, pore size and distri-
bution, metal dispersion, crystallinity, composition of the support,
reproducibility of the catalyst precursors, SMSI (strong metal sup-
port interaction), hydrogen spillover, and trace elements with
modifying effects, are factors governing the performance of the
catalysts in the hydrogenation reaction. As discussed in the intro-
duction, Ru/C can produce only GVL (97%) and MTHF (2%) under
The favorable factors promote for the further enhancement of
catalytic activity of Ru/GO in LA/GVL hydrogenation, which were
found from the material characterization results. Better perfor-
mance of Ru/GO catalysts than Ru/C was attributed due to the fine
dispersion of metallic Ru nanoparticles (1–2 nm) on layered sup-
ports, good interaction of metallic Ru◦ with support and higher
acidic strength due to presence of acidic oxy-functional groups
(
COOH,
C
O
C
,
OH, etc.) on GO surface.
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100
80
60
40
20
100
(b)
(a)
80
Conversion
MTHF
Conversion
MTHF
GVL
GVL
60
40
20
0
THF
C1 paraffin
1,4-PDO
THF
C1 paraffin
1,4-PDO
0
0
0
2
4
6
8
10
12
5
10
15
20
25
WHSV (h-1)
H2 Pressure (bar)
100
80
60
40
20
Conversion
(c)
MTHF
GVL
THF
C1 paraffin
1,4-PDO
0
260
270
280
290
300
Temperature (oC)
Fig. 7. Effect of reaction parameters on LA hydrogenation over 5% Ru/GO catalyst: effect of (a) H2 pressure at temperature of 265 ◦C and WHSV of 0.512 h−1, and (b) WHSV
at temp. of 265 ◦C and H2 pressure of 25 bar and (c) temperature at H2 pressure of 25 bar and WHSV of 0.512 h−1. Reaction conditions: catalyst: 1.0 g; feed: 10% of LA in 1,
4-dioxane; and TOS: 50 h.
3.2.1. Effect of reaction parameters: hydrogen pressure, WHSV,
and temperature
for cyclic ethers was improved with increasing reaction tempera-
ture. This trend was similar to that observed for change in pressure
(Fig. 7a). At a higher temperature (300 ◦C), 61% cyclic ethers (42%
MTHF and 19% THF) were obtained, along with 20% GVL and 18%
other hydrocarbons. The selectivity for MTHF (50%) was slightly
improved at 275 ◦C, compared to the selectivity (48%) obtained at
265 ◦C. Hence, the reaction temperature of 275 ◦C and H2 pressure
25 bar are considered as the optimum conditions to get a higher
selectivity of MTHF, and 300 ◦C is the optimum temperature to
obtain higher selectivity for cyclic ethers, from LA.
The effect of H2 pressure, WHSV, and temperature on the LA
hydrogenation activity by Ru/GO has been studied and is shown in
Fig. 7. At atmospheric pressure, Ru/GO produced 100% GVL selec-
tively, with 100% LA conversion, which was the same as the Ru/C
results at atmospheric pressure. However, the selectivity for GVL
was decreased when the hydrogen pressure was increased from
1 to 25 bar. At the higher pressure, 54% cyclic ethers (48% MTHF
and 6% THF) and 41% GVL were the major products. It is notewor-
thy that the LA conversion was almost 100% under all the different
reaction conditions. Recently, the direct synthesis of MTHF in vapor
phase hydrogenation of LA over Cu/SiO2 catalyst was reported [4],
in which the MTHF selectivity stabilized above a hydrogen pressure
of 25 bar. Here also, the selectivity for MTHF marginally increased
(>1%) above 25 bar of H2 pressure over Ru/GO. Likewise, the effect of
WHSV was also studied (from 0.512 h−1 to 12 h−1) for the reaction
with 5% Ru/GO catalyst, to check the products trend. It is notewor-
thy that only Ru/GO provided 100% conversion of LA under all the
tested reaction conditions. At higher WHSV (12.0 h−1), 100% GVL
selectivity was obtained, and 48% MTHF selectivity was received at
0.512 h−1 WHSV. In the middle range of WHSV (1.0–11.0), 1,4-PDO
was obtained as a minor product (<20%), this is an important deriva-
tive of LA and is an intermediate in the formation of MTHF and
THF [4,44]. In order to find the optimum reaction temperature, the
effect of temperature (265–300 ◦C) on LA hydrogenation was inves-
tigated at the optimized hydrogen pressure (25 bar), over Ru/GO.
The results are presented in Fig. 7c. Interestingly, the selectivity
3.2.2. GVL hydrogenation to MTHF over 5% Ru/GO
In order to improve the yield of MTHF in a two-step process,
vapor phase hydrogenation of intermediate GVL by 5% Ru/GO was
carried out in a fix-bed reactor using reaction conditions similar to
those discussed for LA hydrogenation. The catalytic results are pre-
sented in Table 2. The catalyst 5% Ru/GO successfully produced 91%
of cyclic ethers (69% MTHF and 16% THF), and 15% of other hydro-
carbons from intermediate GVL at 100% conversion. As mentioned
before, Ru/GO gave 54% (MTHF and THF) of cyclic ethers and 41% of
GVL selectively during continuous hydrogenation of LA. Interest-
ingly, the overall selectivity for MTHF in the two-step process will
be around 77–78%.
Here also, the effect of temperature (265–300 ◦C) as well as the
effect of pressure (1– 25 bar) on GVL hydrogenation was investi-
gated (results in Fig. 8a). In this study, the reaction temperature
of 265 ◦C was found to be the optimum for higher selectivity of
MTHF, because the selectivity for THF and CH4 increased with
Please cite this article in press as: P.P. Upare, et al., Ru nanoparticles supported graphene oxide catalyst for hydrogenation of bio-based
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100
100
(a)
(a)
80
60
80
60
Conversion
Conversion
MTHF
MTHF
THF
C1 paraffin
1,4 PDO
THF
C1 paraffin
1,4 PDO
40
20
0
40
20
0
0
20
40
60
80
100
260
270
280
290
300
Temperature (oC)
Time (h)
100
80
60
40
20
0
100
80
60
40
20
0
Conversion
MTHF
(b)
(b)
THF
GVL
C1 paraffin
1,4-PDO
Conversion
MTHF
THF
C1 paraffin
1,4 PDO
0
5
10
15
20
25
0
20
40
60
80
100
H2 pressure (bar)
Time (h)
Fig. 8. Effect of reaction parameters on GVL hydrogenation over 5%Ru/GO catalyst:
Effect of (a) temperature at H2 pressure of 25 bar and (b) pressure at 265 ◦C. Reaction
conditions: catalyst: 1.0 g, WHSV of 0.512 h−1; feed: 10% of LA in 1, 4-dioxane; and
TOS: 50 h.
Fig. 9. Stability of 5% Ru/GO catalyst for (a) GVL hydrogenation over Ru/GO and
(b) LA hydrogenation over Ru/GO. 1.0 g of catalyst, feed: 10% of LA in 1, 4-dioxane;
temperature = 265 ◦C, H2 pressure = 25 bar, WHSV = 0.512 h−1, TOS = 50 h.
In order to check the robustness of the synthesized catalyst sys-
tem, time on stream studies (Fig. 9) were conducted for both LA
and GVL hydrogenation. Of great interest, 5% Ru/GO exhibited long-
term stability (more than 100 h) without any sign of deactivation.
By considering these catalytic results, Ru/GO is considered as an
efficient catalyst system for complete continuous hydrogenation
of LA, as well as for producing high selectivity of MTHF from GVL
at 100% conversion level.
Scheme 1. Plausible pathway for LA hydrogenation to products using Ru/GO.
The probable route (Scheme 2) for LA hydrogenation to MTHF
and THF has been confirmed by the catalytic results obtained, GCMS
analysis results, and previous records [4,10,15,44]. As discussed
before, that GVL is the main intermediate in the synthesis of MTHF
has been confirmed. For MTHF synthesis two routes are prominent.
One is hydrogenation of the carbonyl group of GVL to MTHF over
metallic sites, and the other one is hydrogenation of GVL to 1,4-
PDO over metallic sites, and then its dehydration to MTHF by acidic
sites. As per the literature, acidic sites are responsible for dehydra-
tion [45] as well as cracking reactions [46]. At this stage, records for
the formation of THF from hydrogenation of LA were not seen in
the literature. Hence, MTHF and 1,4-PDO were treated at 265 ◦C and
25 bar of H2 pressure, in a fixed-bed reactor over 5% Ru/GO catalyst.
In the case of MTHF decomposition, a lesser amount of THF (1.6%)
was produced along with a major amount of alkanes (22%, mainly
pentane) at 24% MTHF conversion. However, for the 1,4-PDO con-
version, 58% MTHF, 21% THF, and 20% C1 paraffin were obtained.
This means that the formation of THF from 1,4-PDO is considerably
more than from MTHF in continuous reaction systems over Ru/GO
catalysts.
increasing reaction temperature. Higher reaction temperatures
strongly affected the selectivity for cyclic ethers as well as for
MTHF. Cyclic ether selectivity of 88% and 85% occurred at 275 ◦C and
300 ◦C, respectively. Hence, the temperature of 265 ◦C was found to
be optimum to get MTHF and cyclic ethers from GVL at 25 bar of
hydrogen pressure.
At lower hydrogen pressure, 1,4-PDO was obtained as major
product instead of MTHF (Fig. 8) with Ru/GO catalysts. As dis-
cussed above, 1,4-PDO is an intermediate for the formation of MTHF
(Scheme 1) and THF [10,44]. Ru/GO produced 83% 1,4-PDO with
lower GVL conversion (69%) at hydrogen pressure of 10 bar. The
1,4-PDO selectivity (92%) was improved at atmospheric pressure,
but GVL conversion (48%) decreased, and MTHF was only a minor
product found at lower-pressure reaction conditions. Nevertheless,
controlling product selectivity is considered a challenging issue in
catalysis. As per catalytic results, the present catalyst system allows
control of product selectivity by varying the reaction parameters.
This could be one of the important features of the Ru/GO catalyst
system, a feature not observed for the Ru/C catalyst.
Please cite this article in press as: P.P. Upare, et al., Ru nanoparticles supported graphene oxide catalyst for hydrogenation of bio-based
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Scheme 2. Reaction route for LA hydrogenation to products.
4. Conclusions
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Hwang, J.-S. Chang, ChemSusChem 4 (2011) 1749–1752.
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[6] V.M. Ghorpade, M.A. Hanna, Industrial applications for levulinic acid, in:
G.B.G.B. Campbell, C. Webb, S.L. McKee (Eds.), Cereals, Springer, New York,
1997, pp. 49–55;
The Ru/GO catalyst worked successfully for two-step hydro-
genation of LA/GVL to produce 92% total cyclic ethers (77% MTHF
and 15% THF) under moderate reaction conditions. The enhanced
catalytic activity of Ru/GO over Ru/C for LA hydrogenation to
form cyclic ethers, is attributed to better dispersion of metal-
lic nanoparticles of Ru on GO than on carbon, stronger metal
support interaction, and higher acidity due to the presence of func-
tional groups (-carboxyl, -epoxy, -hydroxyl, etc.) on the GO surface.
Hence, Ru/GO can be considered an effective catalyst for continuous
production of cyclic ethers from LA.
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The authors acknowledge the financial support received from
the R&D Program of Institutional Research Program of KRICT (KK-
1501-C0) and partly by the Korea Research Council for Industrial
Science and Technology (ISTK) of the Republic of Korea, Grant num-
ber B551179-11-03-00. P.P. Upare is grateful to KRICT for financial
support under the Ph.D and post-doctoral research program.
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Please cite this article in press as: P.P. Upare, et al., Ru nanoparticles supported graphene oxide catalyst for hydrogenation of bio-based
levulinic acid to cyclic ethers, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.042