A stable and effective Ru/polyethersulfone catalyst for levulinic acid hydrogenation to γ-valerolactone in aqueous solution

Article abstract of DOI:10.1016/j.cattod.2014.01.020

A cross-linked sulfonated polyethersulfone supported Ru nanoparticle catalyst was prepared for highly hydrogenation of levulinic acid (LA) into γ-valerolactone (GVL) at mild conditions (3.0 MPa H₂ and 70 °C) in aqueous solution. X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations show the formation of highly dispersed small (～3 nm) Ru clusters on the surface of polyethersulfone. Infrared spectroscopy (IR) and acid-base titration indicate the presence of sulfonic groups without the influence of the deposition of Ru species. Polyethersulfone consisted of cross-linked electron -withdrawing group SO₂, maintained its intrinsic thermal stability during the hydrogenating reaction process, and its swelling property promoted the adsorption of LA in aqueous solution. The synchronization of sulfonic groups as active sites for esterification process and metal sites for hydrogenation promoted the hydrogenation reactivity.

Full text of DOI:10.1016/j.cattod.2014.01.020

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A stable and effective Ru/polyethersulfone catalyst for levulinic acid
hydrogenation to ␥-valerolactone in aqueous solution
Yirong Yaoa, Zhiqiang Wangc, Sheng Zhaoa, Danhong Wanga,
Zhijie Wub,∗, Minghui Zhanga,∗∗
a Key Laboratory of Advanced Energy Materials Chemistry (MOE), College of Chemistry, Nankai University, Tianjin 300071, PR China
b State Key Laboratory of Heavy Oil Processing and the Key Laboratory of Catalysis of CNPC, China University of Petroleum, Beijing 102249, PR China
c Tianjin Key Laboratory of Water Environment and Resources, Tianjin Normal University, Tianjin 300387, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A cross-linked sulfonated polyethersulfone supported Ru nanoparticle catalyst was prepared for highly
hydrogenation of levulinic acid (LA) into ␥-valerolactone (GVL) at mild conditions (3.0 MPa H2 and 70 ◦C)
in aqueous solution. X-ray diffraction (XRD) and transmission electron microscopy (TEM) characteriza-
tions show the formation of highly dispersed small (∼3 nm) Ru clusters on the surface of polyethersulfone.
Infrared spectroscopy (IR) and acid-base titration indicate the presence of sulfonic groups without
the influence of the deposition of Ru species. Polyethersulfone consisted of cross-linked electron -
withdrawing group SO2 , maintained its intrinsic thermal stability during the hydrogenating reaction
process, and its swelling property promoted the adsorption of LA in aqueous solution. The synchronization
of sulfonic groups as active sites for esterification process and metal sites for hydrogenation promoted
the hydrogenation reactivity.
Received 6 November 2013
Received in revised form
23 December 2013
Accepted 10 January 2014
Available online xxx
Keywords:
Levilinic acid
␥-Valerolactone
Ru catalyst
Hydrogenation
Polyethersulfone
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Since the reported catalytic hydrogenation of LA to GVL over Pt
catalysts [24], the novel metal (Pd, Ru, Rh, Ir, etc) and transitional
Efficient conversion of biomass feedstocks to fuels and chem-
icals has been regarded as a promising strategy to maintain the
sustainability in fuel supply and chemical production [1,2]. Lev-
ulinic acid (LA) is a well known product of hexose acid hydrolysis
and it is inexpensively obtained by the decomposition of cellu-
lose feedstock of glucose. Consequently, it is an attractive starting
material for the production of many useful C5 based compounds
including ␥-valerolactone (GVL) [2–20], methyltetrahydrofuran
[21], ␦-aminolevulinic acid and diphenolic acid [22]. Specially, GVL
is widely applied as food additives, solvents or intermediates to pro-
duce useful chemicals, and a new platform molecule for gasoline
and diesel fuels [23]. Thus, the production of GVL from LA hydro-
genation represents a highly desirable process towards enhancing
the sustainability of the chemical industry and earns much atten-
tion [2–21,24,25].
metal (Ni and Cu) based catalysts have been used [2–25]. Christian
et al. [25] employed Raney Ni to hydrogenate LA at 220 ◦C, with
4.8 MPa of H2 pressure to obtain 94% yield of GVL. Manzer et al. [26]
investigated the activities of different metal catalysts supported
on activated carbon such as Ir, Rh, Pd, Ru, Pt, Re and Ni at 150 ◦C,
5.5 MPa of H2 using 1, 4-dioxane as solvent. The Ru/C catalyst was
sieved to be the best choice with ∼100% conversion and >97% selec-
tivity of GVL after 4 h hydrogenation. Upare et al. [21] carried out
the gas hydrogenation of LA over carbon supported Ru, Pt and Pd at
265 ◦C and 1–25 bar H2, and also pointed out the best performance
on Ru/C catalyst with ∼100% conversion and ∼100% selectivity. Liu
[2] prepared Ru/C catalyst to conduct the hydrogenation reaction
at a low temperature (130 ◦C, 1.2 MPa H2) and obtain 92% conver-
sion and 99% selectivity of GVL using 5 wt.% of LA in methanol. At
present, LA hydrogenation is usually carried out in organic solvents
[2–26], and the aqueous hydrogenation of LA over noble metal
catalysts has been examined by Dumesic [27] and Pinel [28]. Sur-
prisingly, in the green solvent (water), the yield of LA became low
(<90%) in dilute solution (5 wt.%, 15 MPa H2) [28] or concentrated
aqueous solution (50 wt.%, 3.5 MPa H2) [27]. Moreover, the Ru/C
catalyst showed slow deactivation with time on stream (from 90
to 68% conversion after 106 h), and regeneration treatment of the
∗
Corresponding author at: China University of Petroleum (Beijing), College of
Chemical Engineering, Beijing, China. Tel.: +86 10 89733235; fax: +86 10 89734979.
∗∗
Corresponding author.
E-mail addresses: zhijiewu@cup.edu.cn, zhijie.wu@hotmail.com (Z. Wu),
zhangmh@nankai.edu.cn (M. Zhang).
http://dx.doi.org/10.1016/j.cattod.2014.01.020
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Yao, et al., A stable and effective Ru/polyethersulfone catalyst for levulinic acid hydrogenation to
␥-valerolactone in aqueous solution, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.020

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O
esterification
HO
O
O
OH
GVL
-
Ã
H2
Hydroxyvaleric acid
O
O
O
HO
Levulinic acid
O
OH
H3C
Pseudo-
Levulinic acid
O
H2
esterifiaction
HO
O
OH
(enol form)
O
O
O
Levulinic acid
-
Á
GVL
AL
Scheme 1. Proposed reaction routes of LA hydrogenation.
catalyst under flowing H2 at 673 K for 2 h only allowed for partial
recovery of the initial catalytic activity (83% conversion) [27]. These
literatures together show that there is still a challenge in develop-
ing a high active and stable Ru catalyst for the LA hydrogenation in
aqueous solution.
stability of sulfonic acid. Here, SPES was designed to disperse small
Ru nanoparticles for the hydrogenation of LA in order to obtain a
new bifunctional catalyst (Ru/SPES) with high activity and good
stability.
Shown in Scheme 1 are the reaction pathways in LA hydrogena-
tion based on the literature [2–27]. The formation of GVL required
the reduction and esterification steps which require the metal sites
and acid sites respectively [7–27]. Here, we propose a strategy of
incorporation of acid sites into the Ru metal catalyst to promote
the esterification reactivity and then the hydrogenation activity in
LA hydrogenation. Galletti et al. [8] demonstrated that mixing acid
catalysts with Ru metal catalysts promoted the hydrogenation of
LA. Among the cationic exchange resins (Amberlyst 70 and 15), nio-
bium phosphate and niobium oxides, Amberlyst A70 was shown to
be the best acid catalyst with high yield of GVL (99%) using a lot of Ru
metal catalyst (1 meq) at mild hydrogenation conditions (4.7 wt.%
LA, 70 ◦C and 3.0 MPa H2) and good recycling runs (5 times). Besides
Amberlyst A70, they found the combination of Ru/C (0.01 meq) and
niobium phosphate leads to a high yield (83%) of GVL at mild con-
ditions (1.63 wt.% LA in products of giant reed hydrolysis, 70 ◦C
and 0.5 MPa H2) [29]. We expect that the use of acid catalysts as
supports to disperse Ru metal should be a better strategy to syn-
chronize the acid sites and metal sites as a bifunctional catalyst for
the hydrogenation of LA because of the simultaneous access of both
sites. Indeed, Luo et al. [16] used the metal oxides or zeolites as sup-
ports for Ru metal, and found the high promotion by ZSM-5 zeolites.
However, the leaching of acid sites (Al atoms) of ZSM-5 occurred in
the acidic conditions using organic solvent. Thus, the choice of acid
support becomes challenge to the design of bifunctional catalysts.
[30]
2. Experimental
2.1. Chemicals and reagents
The cross-linked sulfonated polyethersulfone (SPES) were syn-
thesized using m-trihydroxybenzene as a cross-linker as reference
[31]. Amberlyst 15 was purchased from Alfa Aesar Company, Ltd.
and used as received. Ruthenium trichloride (Ru > 37%), SiO2 and
carbon were purchased from KRS Fine Chemical Co. Ltd. (Tianjin,
China) and used as received. Levulinic acid (98%, from GF Co., Tian-
jin, China) was employed without any purification.
2.2. Catalyst preparation
2.2.1. Synthesis of Ru/SPES
2.0 wt.% Ru/SPES was prepared by a modified impregnation
method. Typically, 0.027 g RuCl3 was firstly dispersed into 50 mL
distilled water. After dissolved completely, 0.5 g polyethersulfone
was added under constantly stirring. The mixed solution was kept
at 70 ◦C for 6 h. The water was dried using rotary evaporator at 50 ◦C
and then reduced at 130 ◦C for 6 h in H2 flow (100 cm−3 g−1 min−1).
The resulted Ru/SPES was directly re-dispersed into 20 mL water in
a stainless steel autoclave for reaction.
In our previous work [31], we have synthesized a new sta-
ble solid acid: sulfonated polyethersulfone (SPES), which showed
high activity and good thermal stability in esterification process
because of its swelling property and the introduction of SO2
group. After catalyst dwelling in reaction system, the volume of cat-
alyst enlarged and pores in catalyst became loose, which increased
contact area between reactants and catalytic active sites, thus
improved the catalytic performance. The catalyst introduced pull-
electron SO2 group, which increased the acid strength and the
2.2.2. Synthesis of Ru/SiO2 and Ru/C
2.0 wt.% Ru/SiO2 and Ru/C were prepared as referenced cata-
lysts. In details, 0.5 g SiO2 (200 cm2 g−1) or carbon (1850 cm2 g−1
)
was added to 50 mL 0.002 mol L−1 RuCl3 aqueous solution. Then, the
above solution kept stirring for 4 h at room temperature. After dried
by rotary evaporator at 50 ◦C, the Ru/SiO2 sample was obtained by
calcination treatment at 500 ◦C in air for 3 h and then reduced at
500 ◦C under H2 flow (100 cm−3 g−1 min−1) for 4 h, and Ru/C was
directly reduction by H2 at the same condition without calcination.
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2.3. Catalyst characterization
The structure of the catalysts was characterized by X-ray diffrac-
tion (XRD) on a Bruker D8 FOCUS (Bruker AXS GmbH, Cu Ka,
˚
ꢀ = 1.54178 A) instrument. The particle size distribution of Ru clus-
ters was determined by transmission electron microscope (TEM, FEI
Tecnai G2, operated at 200 kV) by counting at least 400 crystallites.
The surface area weighted cluster diameters, dTEM, were calculated
using dTEM = ˙nid3/˙nid2. The metal loading or metal and sulfur
i
i
contents in reaction solution were analyzed by inductively coupled
plasma atomic emission spectrometry (ICP-AES) on an IRIS Intrepid
spectrometer. Infrared spectroscopy (IR, FTS6000Varian) was used
to prove successful introduction of sulfonate group. The acidity of
sample was determined by acid-base titration: 0.1 g samples were
equilibrated in 20 mL of 0.1 mol L−1 NaOH solution for 12 h to neu-
tralize acid, titrated excess NaOH with calibrated 0.02 mol L−1 HCl
solution. The acidity density of Amberlyst 15 (6.9 mmol g−1) was
determined by acid-base titration.
2.4. Levulinic acid hydrogenation
Fig. 1. XRD patterns of supports and their corresponding supported Ru samples.
LA hydrogenation reaction was carried out in a three-phase
30 mL glass tube in 50 mL stainless steel autoclave to avoid the
effect of stainless steel on hydrogenation [32]. In a typical process,
LA was diluted with distilled water (5.0 wt.% LA). The dosage of cat-
alyst was 5.0 wt.% based on the quantity of LA. After that, the reactor
was purified with hydrogen three times to eliminate air, then pres-
surized to 3.0 MPa and heated to 70 ◦C. After 2 h reaction, the reactor
was quenched down to room temperature. The liquid-products
were performed with a gas chromatograph (GC, FULI 9790) using a
capillary column (40 m × 3 mm × 0.25 ␮m) with stationary phase
of diethyl polysiloxane. The flame ionization detector (FID) was
operated at 250 ◦C. The following temperature program was used
5.6 g) enlarged about ten times after swelling in water at room
temperature for 2 h. Based on equation of the swelling degree
(Q = (m–m0)/m0, where m is the mass of swollen catalyst and m0
is the initial mass of sample) [33], swelling degree of Ru/SPES in
water was 13.2. This suggests that the pores in Ru/SPES catalyst
would become loose after catalyst dwelling in reaction system [34],
which increases contact area between reactants and catalytic active
sites, thus improve the catalytic performance shown in the next
Section below.
Fig. 3 illustrates the infrared spectroscopy of Ru/SPES catalyst.
The absorption bands at 1475 and 1600 cm−1 are attributed to the
stretching vibration of C C and that at 2940 cm−1 is assigned to
in analysis: 60 ◦C for 1 min and increased to 220 ◦C (1.67 ◦C s−1
)
for 5 min. Quantitative determination of Ru and sulfur leaching in
liquid-products was measured by ICP-AES.
C
H stretching vibration, which indicates the presence of aromat-
ics [35]. The adsorption bands at 1088 and 1255 cm−1 are ascribed
to the introduction of sulfonate group to the polymer [36]. The
absorption band at 610 cm−1 assigned to OH bending vibration
bonding to SO3H groups [37]. The introduction of pull-electron
SO2 increases the acid strength and the stability of sulfonic acid.
The acidity density of catalyst was 3.8 mmol g−1 measured by acid-
base titration.
The thermal stabilities of catalysts were investigated and shown
in Fig. 4. It can be seen from the results that the Ru/SPES exhibited
the weight loss at 45–130, 270–410 and 480–560 ◦C. These three
peaks can be assigned to desorption of water, decomposition of sul-
fonic groups, and destruction of polymer framework, respectively.
The sulfonic acid group in SPES began to decompose at 270 ◦C, and
the rate reached the maximum at 345 ◦C. This suggests that Ru/SPES
shows good thermal stability due to the chemical environment of
the sulfonic group in main line of polymer (SPES contained electron
withdrawing group SO2 ) [31], and the deposition of Ru clusters
on SPES did not change the stability.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of supported Ru catalysts and the supports are
shown in Fig. 1. Two broad peaks around 2ꢁ = 20◦ and 40◦ are found
in SPES sample, indicating the amorphous structure of polymer the
same as those of SiO2 and carbon supports. After the deposition of
Ru metal, a peak ascribed to (1 1 1) plane of Ru metal (PDF No. 06-
0663) was observed on SiO2 samples, suggesting the formation of
large Ru metal crystals. However, no obvious peak corresponding
Ru metal could be observed on Ru/SPES and Ru/C samples, suggest-
ing the presence of highly dispersed tiny Ru clusters. Unlike SiO2
support, the high dispersion of Ru on carbon should be benefited
from a much higher surface area of carbon.
TEM images and particle size distribution graph of Ru/SPES were
displayed in Fig. 2. Ru nanoparticle aggregates are randomly dis-
tributed over the surface of SPES (Fig. 2a). HRTEM image shows the
irregular morphology (Fig. 2b) of aggregates. The average size of Ru
particles (assuming spherical particles) was 3.0 nm, in agreement
with the XRD pattern results.
It is well known that the activity of catalytic reactions was
influenced by accessibility of reactants to active sites. Here the
parameters of swelling support (SPES) could change in solvent,
and affect of the adsorption capability of SPES in aqueous solution.
The absorption of distilled water over Ru/SPES was investigated.
Compared with initial state (volume: 0.5 mL; weight: 0.4 g), the
volume and weight of catalyst (wet volume: 5.7 mL, wet weight:
3.2. Catalytic performance
For the LA hydrogenation, mild conditions (low temperature,
low pressure, and short time, etc.) should be better to reduce the
leach of active metal (i.e. Ru [16]) because of the high acidity of
LA. Here, LA hydrogenation was carried in a stainless steel auto-
clave at 70 ◦C for 2 h with 2.0 wt.% Ru/SPES. The evaluation results
of catalysts were summarized in Fig. 5. Ru/SiO2 catalyst shows a low
conversion (22.2%) of LA and a high selectivity (∼98%) of GVL. The
conversion increases to 54.5% using Ru/C with smaller Ru cluster
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Fig. 3. Infrared spectra of Ru/SPES catalyst.
reactivity of Ru metal using acid catalyst [8]. Unlike the Ru/SiO2 and
Ru/C catalysts, Ru/SPES has much higher conversion (87.9%) of LA,
which should be benefited from the high dispersion of Ru clusters
and the acidity of SPES.
In order to verify the process for LA hydrogenation in presence
of acid catalyst, we collected products reacting for 0.5 h, 1 h and
2 h, respectively, to analysis contents by GC–MS. In all the reac-
tions, LA and GVL were detected, but theoretical intermediates
such as ␥-hydroxyvaleric acid and angelicalactone (␣-AL) were
not determined (Scheme 1). ␥-Hydroxyvaleric acid is very instable,
which can react with alcohol to ester under acid condition shortly
[2,5,38,39]. Therefore, we considered reaction processes as follows:
LA hydrogenated to ␥-hydroxyvaleric acid and then esterificated to
GVL, which is in accordance with the research of Galletti [8].
A very low conversion (3.4%) of LA was observed on SPES sup-
ports, which confirms the facilitation of SPES for the hydrogenation
of LA, the same as the work of Galletti [8]. This demonstrated the
role of acid catalyst involving intermolecular lactonization and keto
Fig. 2. (a) TEM images, (b) HRTEM images and (c) particle size distribution graph of
Ru/SPES catalyst.
sizes and thus more surface active metal sites. Mixing the Ru/SiO2
or Ru/C catalyst with Amberlyst 15 indeed increases the conver-
sion of LA to 40.4% or 68.0% respectively, and the selectivity of GVL
change little. This clearly confirms the promotion of hydrogenation
Fig. 4. Thermogravimetric (TG) curves of Ru/SPES.
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Fig. 7. XPS spectra of Ru/SPES catalyst.
Fig. 5. LA conversion and GVL selectivity using different Ru supported catalysts.
(Histogram in black: conversion of LA; Histogram in gray: selectivity of GVL.
Ru/SiO2*: Ru/SiO2 and Amberlyst15 as co-catalysts; Ru/C*: Ru/C and Amberlyst15
as co-catalysts; Ru/SPESNa: Ru/SPES neutralized by NaOH aqueous solution;
Ru/SPESNa*: Ru/SPESNa and Amberlyst15 as co-catalysts).
specials and sulfur species in the reaction solution could not be
detected by ICP characterization, suggesting no leaching of Ru/SPES
at mild reaction conditions. Interestingly, the conversion of LA
increases from 87.9% to 92.1% after the first hydrogenation, and
gradually to 97.2% at fourth run (Fig. 6). Moreover, the selectiv-
ity almost stays the same during recycling. The reason may be
explained from the surface properties of small Ru clusters. Because
of the high activity of Ru clusters in the air or oxygen atmosphere,
the transfer of fresh Ru/SPES catalysts into the hydrogenation reac-
tion inevitably contact oxygen to form some surface ruthenium
oxides (Fig. 7). The binding energies of Ru were respectively 280.0,
281.5, 284.0 and 285.3 eV ascribed to Ru 3d5/2 and Ru 3d3/2. The val-
ues at 280.0 and 284.0 eV indicate the Ru0 species on the surface of
catalyst, while 281.5 and 285.3 eV suggest the formation for ruthe-
nium oxychloride [42] from air exposition [43] during drying and
storing process. These oxides could be reduced gradually during LA
hydrogenation, and the conversion increases gradually. These data
show the good stability of Ru/SPES in the hydrogenation of LA.
hydrogenation in LA hydrogenation. Here, we neutralized the acidic
sites of Ru/SPES via ion-exchange using 0.1 mol L−1 NaOH aque-
ous solutions (Ru/SPESNa), the conversion decreases from 87.9%
to 64.6%. Mixing the Ru/SPESNa with Amberlyst 15 led to 76.8%
conversion, still lower than the Ru/SPES even the same acidity
density of the catalyst were controlled. These data together show
that deposition of metal clusters on acid supports is better than
a mechanical mixture of metal and acid sites during the LA hydro-
genation. Besides the acceleration esterification process (Scheme 1)
by the acidic groups ( SO3H) in SPES [40,41], the swelling property
of SPES facilitates more LA reactants access to the acid sites and
metal sites [34].
Metal leaching is ascribed to the deactivation of catalyst in LA
hydrogenation [16]. During the five recycles of catalysts, the Ru
4. Conclusion
We reported a new bifunctional catalyst, cross-linked sul-
fonated polyethersulfone supported Ru nanoparticles, for the
selective hydrogenation of levulinic acid to ␥-valerolactone. High
dispersed small (∼3 nm) Ru clusters were formed over polyether-
sulfone via impregnation. The synchronization of surface acidic
groups (SO3H) of polyethersulfone for esterification process and
metal sites for hydrogenation led to high reactivity, selectivity
and stability. Compared with the metal catalysts mechanical mix-
ing with acid catalysts, the swelling property of polyethersulfone
promoted the adsorption capability of levulinic acid, and thus
increased the accessibility of reactants to both metal sites and acid
sites. The leaching of metal sites was avoided using polyethersul-
fone as supports, which improve the stability of catalyst.
Acknowledgments
This work was supported by the Natural Science of Foundation
China (21206192, 21303088, 51103105), the Science Foundation
of China University of Petroleum-Beijing (YJRC-2013-38), the Open
Project of Key Lab Adv Energy Mat Chem (Nankai Univ), and the
Doctoral Program of Higher Education of China (20120007120010).
Fig. 6. LA conversion and GVL selectivity of Ru/SPES catalyst during recycling ((His-
togram in black: conversion of LA; Histogram in gray: selectivity of GVL).
Please cite this article in press as: Y. Yao, et al., A stable and effective Ru/polyethersulfone catalyst for levulinic acid hydrogenation to
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Please cite this article in press as: Y. Yao, et al., A stable and effective Ru/polyethersulfone catalyst for levulinic acid hydrogenation to
␥-valerolactone in aqueous solution, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.01.020