Catalytic transfer hydrogenation of levulinate esters to γ-valerolactone over supported ruthenium hydroxide catalysts

Article abstract of DOI:10.1039/c4ra08074b

Production of γ-valerolactone (GVL) from levulinate esters and several alcohols as hydrogen donors via a catalytic transfer hydrogenation (CTH) process was performed over supported ruthenium hydroxide catalysts. Among the catalysts examined, Ru(OH)_x supported on high-surface-area, anatase TiO₂ containing highly-dispersed ultrasmall Ru(OH)_x nano-clusters was found to be the most active catalyst, which converted levulinate esters to GVL in an almost quantitative yield under mild reaction conditions with low catalyst loading. Addition of heterogeneous bases could afford a faster reaction rate in GVL production by accelerating the following intramolecular dealcoholation step. The catalyst was reusable over repeated catalytic cycles without loss of catalytic performance, making this material a potential candidate for efficient GVL production under ambient reaction conditions.

Full text of DOI:10.1039/c4ra08074b

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PAPER
Catalytic transfer hydrogenation of levulinate
esters to g-valerolactone over supported
ruthenium hydroxide catalysts†
Cite this: RSC Adv., 2014, 4, 45848
Yasutaka Kuwahara,*ab Wako Kaburagi and Tadahiro Fujitania
a
Production of g-valerolactone (GVL) from levulinate esters and several alcohols as hydrogen donors via a
catalytic transfer hydrogenation (CTH) process was performed over supported ruthenium hydroxide
x 2
catalysts. Among the catalysts examined, Ru(OH) supported on high-surface-area, anatase TiO
containing highly-dispersed ultrasmall Ru(OH) nano-clusters was found to be the most active catalyst,
x
which converted levulinate esters to GVL in an almost quantitative yield under mild reaction conditions
with low catalyst loading. Addition of heterogeneous bases could afford a faster reaction rate in GVL
production by accelerating the following intramolecular dealcoholation step. The catalyst was reusable
over repeated catalytic cycles without loss of catalytic performance, making this material a potential
candidate for efficient GVL production under ambient reaction conditions.
Received 4th August 2014
Accepted 15th September 2014
DOI: 10.1039/c4ra08074b
www.rsc.org/advances
cellulose, glucose, fructose, as well as 5-hydroxymethylfurfural
Introduction
19–21
22–27
(HMF), in water
and alcohols,
respectively (Scheme 1).
With increasing concern about the strong dependence on The former hydrogenation reaction is generally conducted by
36
2
8–33
33–35
33
petroleum and the associated adverse effects on the environ- using various metal catalysts such as Ru,
ment, worldwide efforts have recently been devoted to devel- Cu,
Pd,
Pt, Ir,
(>30 bar).
2
37,38
however the systems require high-pressure H
opment of advanced technologies to produce fuels and The latter CTH process is an alternative viable option for scal-
chemicals from biomass resources, which is a so-called able GVL production, since the reaction proceeds under milder
“
biomass renery”. The key reason of this trend is that conditions without the need of ammable, high-pressure H
biomass is an abundant, renewable and carbon-neutral source, and alcohols are used as inexpensive and recyclable H-donors.
2
39
and is available worldwide. A number of studies in this area Moreover, in an actual process, the production of levulinate
have demonstrated that lignocellulosic biomass has a great esters from carbohydrates in alcohol media allows higher
1
–7
potential to be converted into fuels and a variety of chemicals.
Of various chemicals synthesized from lignocellulosic biomass, tion of LA in water,
g-valerolactone (GVL) has been regarded as one of the most potentially be reused as H-donors in the following CTH process.
product yields and easier product separation than the produc-
22,40
and the spent alcohol media can
8
9
valuable molecules, since it can be used as a fuel additive,
Therefore, levulinate esters are better alternative intermediates
solvent for biomass processing, as well as an ideal precursor for the energy-saving, cost-effective manufacture process of
alkenes,
and other valu- GVL.
Current route to produce GVL from cellu-
Several catalytic systems have recently been reported for the
losic biomass includes (i) the hydrogenation (using molecular production of GVL via a CTH process. Dumesic et al. reported a
10
11,12
13–15
for the production of alkanes,
able chemicals.
16,17
H
2
) or (ii) the catalytic transfer hydrogenation (CTH) (using CTH process to convert LA and its esters into GVL using ZrO
2
18
ꢀ
alcohols) of levulinic acid (LA) and its esters, which are catalyst and isopropanol/isobutanol as H-donors at 150
C
41
intermediate compounds produced by acid-catalyzed solvolysis under pressurized inert gas ow. Analogously, Sun and Lin
of various carbohydrate fractions of lignocelluloses, such as et al. investigated catalytic activity of hydrated zirconia,
ZrO(OH)
supercritical ethanol.
ular sieves (Zr-beta) has recently been reported to be more active
2 2
$xH O, for a CTH reaction of ethyl levulinate in a
42,43
a
Microporous zirconosilicate molec-
Research Institute for Innovation in Sustainable Chemistry, National Institute of
Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba,
44,45
Ibaraki, 305-8565, Japan
b
and stable catalysts compared with bulk zirconia.
However,
Division of Materials and Manufacturing Science, Graduate School of Engineering, the series of these precedent works have demonstrated that the
Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan. E-mail:
CTH process over Zr-based catalysts suffer from drawbacks such
as high catalyst loading, relatively high reaction temperature/
pressure, moderate GVL selectivity and appreciable catalyst
deactivation during the reaction. Apart from these, Guo and Fu
kuwahara@mat.eng.osaka-u.ac.jp; Fax: +81-6-6105-5029; Tel: +81-6-6879-7458
†
2
Electronic supplementary information (ESI) available: XRD data, CO -TPD data,
TEM image and XAFS analysis data of the materials, in situ FTIR spectra and
reaction kinetics data. See DOI: 10.1039/c4ra08074b
45848 | RSC Adv., 2014, 4, 45848–45855
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Scheme 1 Production route of g-valerolactone (GVL) from lignocellulosic biomass.
et al. reported a room-temperature CTH reaction using Raney Ni
Ruthenium hydroxide (Ru(OH) ) as a baseline sample affor-
x
46
preactivated in 2-propanol solution; however, the catalyst is ded 47% GVL yield in 24 h of reaction time (entry 10), while
sensitive to air and a long-time storage of catalyst in alcohol neither RuO2 nor Ru metal showed activity in this reaction
media is needed, which are critical drawbacks in a practical (entries 11 and 12). When the same amount of Ru was used,
large-scale synthesis of GVL. Therefore, development of an Ru(OH)
alternative catalytic system to efficiently produce GVL from sions compared to the unsupported Ru(OH)
levulinate esters under ambient reaction conditions is still a supported catalysts even under mild reaction conditions with
matter of concern. only 0.8 mol% of the catalysts. In this reaction conditions using
x
supported on TiO
2
particularly gave higher ML conver-
x
and the other
Herein, we report a CTH reaction of levulinate esters to 2-PrOH as a H-donor, byproducts associated with trans-
produce GVL over ruthenium hydroxides supported on basic esterication (e.g., isopropyl levulinate), that are frequently
ꢀ
41
oxides under ambient reaction conditions (90 C, atmospheric identied over bulk zirconia catalysts, were hardly observed. An
pressure) and with low catalyst loading (0.8 mol%). Ru is the important aspect is given from Ru(OH) supported on different
cheapest metal among noble metals, and its hydroxide form is Al supports; Ru(OH) supported on g-Al
expected to act as an efficient heterogeneous catalyst for this GVL yield (64%) than that supported on a-Al
x
O
2 3
x
2
O
3
gave a superior
(38%) in 24 h of
2
O
3
reaction owing to its approved catalytic ability in the CTH reaction, although both provided similar conversions of ML (70–
47,48
reaction of carbonyl compounds.
x 2 3
In the present catalytic 78%) (cf. entries 4 and 5). Ru(OH) /a-Al O mainly afforded 4-
system, GVL was produced through sequential CTH and deal- hydroxy pentanoic acid methyl ester (4-HPM), which is a reaction
coholation reactions, being catalyzed by ruthenium hydroxide intermediate to be transformed to GVL via an intramolecular
and basic sites of oxides, respectively. In particular, highly- dealcoholation. This difference is probably due to different
2 3
dispersed ruthenium hydroxide species supported on high- number of basic site, i.e., abundant basic sites present on g-Al O
ꢁ
1
surface-area, anatase TiO
2
showed a high catalytic activity for (0.122 mmol g ) more efficiently promote the dealcoholation
(0.023
-TPD data, see Fig. S2 in ESI†). Nevertheless,
supported on typical basic supports such as MgO and
this reaction. The catalyst could afford an almost quantitative step of 4-HPM to produce GVL than those on a-Al
yield of GVL under the optimal conditions and was readily mmol g ) (for CO
reusable over multiple cycles without loss of catalytic Ru(OH)
2 3
O
ꢁ
1
2
x
performances.
CeO were almost inactive for this reaction (entries 7 and 8), and
2
those supported on ZrO and Mg–Al hydrotalcite (HT) showed
2
activities almost comparable to that of unsupported Ru(OH)x
(
entries 6 and 9), suggesting that the catalytic activity is also
inuenced by crystalline structure of support materials.
Among the catalysts tested, Ru(OH) catalysts supported on
TiO provided the highest conversions of ML, which far out-
performed that given by unsupported Ru(OH) . Among three
Results and discussion
GVL production via a CTH process
x
2
x
In the initial studies, Ru(OH) catalysts supported on a variety of
x
metal oxides were prepared and tested in the CTH reaction of
methyl levulinate (ML) using 2-PrOH as a hydrogen donor. The
X-ray diffraction (XRD) patterns of supported ruthenium
hydroxide catalysts were identical to those of the parent
different types of TiO supports (anatase TiO (TiO (A), ST-01,
2
2
2
2
ꢁ1
Ishihara Sangyo Co., Ltd., SBET ¼ 339 m
g
), anatase and
rutile TiO (TiO (AR), P25, AEROSIL Japan Industries, S
BET
¼ 55
2
2
2
ꢁ1
m g ), and rutile TiO
2
(TiO
2
(R), TTO-55, Ishihara Sangyo Co.,
Ltd., SBET ¼ 47 m g )), TiO possessing anatase crystalline
phase gave apparently higher GVL yields (76–80%, entries 1–2)
compared to pure rutile TiO (49%, entry 3). This is also
supports and no diffraction peaks assignable to Ru(OH)
x
and
2
ꢁ1
2
RuO were observed in all cases (see Fig. S1 in ESI†), suggesting
2
that ruthenium species are highly dispersed on the support
surface. Table 1 summarizes the catalytic results together with
Ru loading and the surface area determined by the BET (Bru-
nauer–Emmett–Teller) method from N adsorption isotherms.
The BET surface areas of the supported ruthenium hydroxide
catalysts were close to those of the parent supports in all cases.
2
attributed to the presence of larger amount of basic sites on
ꢁ1
anatase TiO crystals (0.091–0.119 mmol g ) than that on rutile
2
ꢁ1
2
TiO crystals (0.057 mmol g ), which can promote the intra-
2
molecular dealcoholation step (see Fig. S2 in ESI†).
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a
x
Table 1 CTH reaction of methyl levulinate over various supported Ru(OH) catalysts
b
2
ꢁ1
c
c
Entry
Catalyst
Ru loading (wt%)
S
BET (m g
)
Conversion (%)
GVL yield (%)
1
2
3
4
5
6
7
8
9
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
x
/TiO
/TiO
/TiO
/a-Al
/g-Al
/ZrO
/CeO
/MgO
/HT
2
2
2
(A)
(AR)
(R)
0.83
0.82
0.84
0.84
0.84
0.83
0.83
0.85
0.85
—
336
53
46
6.7
236
70
130
346
87
86
>99
>99
78
70
52
1.2
1.3
76
80
49
38
64
48
0.6
0.8
43
47
x
x
x
x
x
x
x
x
x
2
O
O
3
2
3
2
2
50
52
10
11
12
13
63
RuO
Ru metal
TiO (A)
2
—
—
—
n.d.
n.d.
339
No reaction
No reaction
No reaction
2
a
c
ꢀ
b
Reaction conditions: catalyst (Ru 0.8 mol%), methyl levulinate (1 mmol), 2-PrOH (5 mL), 90 C, 24 h, Ar. Determined by BET method.
Determined by GC using biphenyl as the internal standard.
Next, the effects of Ru loading on the catalytic activity were XAFS analysis of Ru(OH) /TiO
x
2
studied by altering the weight percentage of Ru metals on the
catalysts (0.8, 2 and 4 wt% Ru). The XRD patterns remained
unchanged upon increasing the Ru loadings (see Fig. S1 in
ESI†). Fig. 1 compares ML conversion and GVL yield at 3 h of
To clarify the local structure of ruthenium species on the TiO2
supports, X-ray absorption ne structure (XAFS) measurement
was carried out. Fig. 2 shows Ru K-edge X-ray absorption near
edge structure (XANES) spectra and radial distribution func-
reaction on Ru(OH)
Ru loadings. When TiO
support, the activity monotonically decreased as Ru loading
increased from 0.82 to 4.2 wt%. In contrast, Ru(OH) supported
x
/TiO
2
(A) and Ru(OH)
x
/TiO
2
ꢁ1
(AR) with varied
3
tions (RDF) obtained from the k -weighted extended XAFS
2
2
(AR) (SBET ¼ 55 m g ) was used as a
(EXAFS) oscillations of Ru(OH)
x 2
/TiO (AR) together with those of
Ru(OH) as a reference sample. It is worth noting that pure
x
x
Ru(OH) has one-dimensional chain-like structure consisting of
x
2
ꢁ1
on high-surface-area TiO
continual activity increase as increasing the Ru loading, and 4%
2
(A) (SBET ¼ 339 m
g ) showed a
RuO6 octahedral units connected by two oxygen atoms with
48
each other. The Ru K-edge XANES spectra of Ru(OH) /TiO (AR)
x
2
Ru(OH) /TiO (A) afforded the highest activity (89% yield of GVL
x
2
are almost identical to that of Ru(OH)
x
, showing that the Ru
ꢀ
with >99% conversion at 90 C aer 24 h of reaction; for detailed
atoms are likely present on TiO (AR) surface in ruthenium(III)
2
reaction kinetics, see Fig. 5). This result indicates a strong
activity dependence on surface area of TiO supports.
2
hydroxide form (Fig. 2(A)). In the RDF, two distinct peaks were
˚
observed at around r ¼ 1.6 and 2.7 A (the phase shi
Fig. 1 Comparison of methyl levulinate (ML) conversion and GVL yield
at 3 h of reaction on (a) Ru(OH) /TiO (AR) and (b) Ru(OH) /TiO (A) with Fig. 2 (A) Ru K-edge XANES spectra and (B) Ru K-edge radial distri-
x
2
x
2
different Ru loadings. Reaction conditions: catalyst (Ru 0.8 mol%), ML bution functions of Ru(OH)
x
/TiO
2
(AR) catalysts with varied Ru content
ꢀ
(
1 mmol), 2-PrOH (5 mL), 90 C, 3 h, Ar.
x
((a) 0.8 wt%, (b) 2 wt% and (c) 4 wt%) and (d) Ru(OH) .
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uncorrected), which correspond to Ru–O and Ru/Ru bonds,
respectively (Fig. 2(B)). From curve-tting analysis, the coordi-
nation number (C.N.) of the rst Ru–O shell was determined to
be about six in all cases (Table 2), again proving that Ru atoms
have octahedral coordination similar to that of Ru(OH)
However, the intensity of the second Ru/Ru shell signals was
obviously weakened compared to that of pure Ru(OH) . Curve-
tting analysis of this signal showed that the interatomic
x
.
x

distance between two neighboring Ru atoms was unchanged
˚
(
3.11 ꢂ 0.01 A), but the C.N. was varied from 0.91 to 1.1 by
increasing the Ru loading from 0.82 to 4.2 wt% (Table 2), sug-
gesting the generation of more aggregated Ru(OH) species at
x
higher Ru loadings. With the increase in the C.N.s, the catalytic
activity monotonically decreased, indicating that highly-
dispersed ruthenium hydroxide species is the main active
sites for the present CTH reaction.
Fig. 3 (A) Ru K-edge XANES spectra and (B) Ru K-edge radial distri-
x 2
bution functions of Ru(OH) /TiO (A) catalysts with varied Ru content
(
(a) 0.8 wt%, (b) 2 wt% and (c) 4 wt%) and (d) Ru(OH) .
x
A similar analysis was also performed on Ru(OH)
x 2
/TiO (A).
The XANES spectra of Ru(OH) /TiO (A) catalysts were very
x
2
similar to that of Ru(OH) at all Ru loading levels (Fig. 3(A)),
x
indicating the formation of ruthenium(III) hydroxide species on
anatase TiO crystals. In the RDF, the C.N. for the Ru–O shell at
2
Although the cause of the increased catalytic activity on
Ru(OH) /TiO (A) cannot be fully understood from this aspect
x
2
r ¼ 1.6 was estimated to be approximately six; however, the
(other congurational factors and the balance between Ru(OH)x
intensity of the Ru/Ru shell signals were considerably weaker
sites and support oxides may also affect simultaneously), it
appears certain that highly-dispersed ultrasmall ruthenium
hydroxide nano-clusters act as the main active sites for the
present CTH reaction.
compared to pure Ru(OH)
and a shrinkage of Ru/Ru interatomic distance was observed
Fig. 3(B)). Curve-tting analysis of the Ru/Ru shell signals
x 2
or even that supported on TiO (AR),
(
showed that the C.N. is varied from 0.44, 0.68 and 0.76 by
increasing the Ru loading to 0.83, 2.1 and 4.0 wt%, respectively.
These values are apparently lower than those observed on
Effect of base additives
Ru(OH)
x
/TiO
2
(AR) catalysts. Transmission electron microscope
/TiO (A) exhibited Ru clusters with
The reaction kinetics in the GVL production from ML and 2-
propanol over 4% Ru(OH) /TiO (A), the best catalyst among
examined, is shown in Fig. 4 (le, top), which reveals that the
present GVL production route involves sequential two reaction
steps; (i) a CTH reaction to reduce ML to 4-HPM by Ru(OH)
catalyst and (ii) the following intramolecular dealcoholation
reaction of 4-HPM to produce GVL by basic sites, where the
latter step dominates the GVL production rates. To obtain a
(TEM) image of 4% Ru(OH)
x
2
x
2
the size of less than 3 nm throughout the particle surface (see
Fig. S3 in ESI†). These combined analyses indicate that Ru
species on TiO (A) surface are present mainly as highly-
2
x
dispersed ultrasmall ruthenium hydroxide nano-clusters and
that a high-surface-area TiO can offer a surface environment
2
suitable for generating such a ruthenium hydroxide species.
a
x 2
Table 2 EXAFS curve-fitting parameters of over Ru(OH) /TiO catalysts with varied Ru loadings
b
2
ꢁ1
c
d
˚
Catalyst
Ru loading (wt%)
S
BET (m g
)
Shell
R (A)
C.N.
0
2
4
0
2
4
.8% Ru(OH)
x
/TiO
2
(AR)
0.82
2.1
4.2
0.83
2.1
4.0
—
53
59
61
Ru–O
Ru/Ru
Ru–O
Ru/Ru
Ru–O
Ru/Ru
Ru–O
Ru/Ru
Ru–O
Ru/Ru
Ru–O
Ru/Ru
Ru–O
Ru/Ru
2.02
3.10
2.02
3.12
2.01
3.11
2.02
2.86
2.02
2.97
2.01
3.00
2.02
3.11
6.2
0.91
6.1
1.0
6.1
1.1
5.6
0.44
5.6
0.68
5.5
% Ru(OH)
x
x
/TiO
2
(AR)
(AR)
% Ru(OH)
/TiO
2
.8% Ru(OH)
x
/TiO
2
(A)
336
349
214
63
% Ru(OH)
x
x
/TiO
2
(A)
(A)
% Ru(OH)
/TiO
2
0.76
6.3
1.2
Ru(OH)
x
a
3
ꢁ1
b
c
d
˚
k -Weighted EAXFS, 3.0 # k (A ) # 11.0. Determined by BET method. Average interatomic distance. Average coordination number.
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Scope of substrates and alcohols
Using the optimized catalyst, 4% Ru(OH) /TiO (A), we examined
x
2
the scope of substrates and alcohols in the CTH process (Table
). When ML was used as a starting material, other secondary
3
alcohols such as 2-BuOH and cyclohexanol also efficiently
afforded GVL with moderate yields (entries 5 and 6), but 2-PrOH
afforded the highest GVL yield under the same reaction condi-
tions (for reaction kinetics, see Fig. S4 in ESI†). In situ FTIR
ꢀ
measurement with a ow of 2-propanol vapor in He at 90 C
conrmed the generation of covalently-bound iso-propoxyl
groups, indicative of dissociative adsorption of 2-propanol
over 4% Ru(OH) /TiO (A) catalyst (for in situ FTIR difference
x 2
spectra, see Fig. S5 in ESI†). In the present CTH reaction, the
thus formed alkoxy species are considered to easily undergo the
b-H elimination, and two H-atoms remained on the Ru site are
considered to readily react with ML to give 4-HPM via the
46,49
formation of 6-ring transition state.
On the other hand,
primary alcohols such as methanol, ethanol and 1-propanol did
not act as efficient H-donors under the reaction conditions
examined in this study (entries 1–3). This is due to the difficulty
46
in b-hydride elimination from primary alcohols.
GVL was successfully synthesized from ethyl levulinate (EL)
and butyl levulinate (BL) in good to excellent yields (entries 7
ꢀ
and 8); the reaction of EL and BL at 90 C for 24 h with 0.8 mol%
Fig. 4 Effect of base additives in GVL production from ML with 2-
Ru resulted in 71% and 63% yields of GVL with >99% conver-
sions, respectively. With 4.0 mol% Ru catalyst, EL and BL
afforded 88% and 81% GVL yields, respectively. In both cases,
the reaction proceeded with the same manner as that of ML, i.e.,
levulinate esters underwent a CTH step to produce the corre-
sponding 4-hydroxypentanoates, followed by intramolecular
dealcoholation to form GVL. As for the reaction of LA, 64% GVL
PrOH over 4% Ru(OH)
triethylamine. Reaction conditions: catalyst (Ru 0.8 mol%), ML (1
mmol), 2-PrOH (5 mL), 90 C, additive (1 mmol or 100 mg), Ar. Others
x
/TiO
2
(A) catalyst. HT ¼ hydrotalcite, TEA ¼
ꢀ
may contain byproducts associated with transesterification reaction.
faster GVL production rate by accelerating the dealcoholation yield was attained aer 24 h of reaction with 4.0 mol% Ru
step, addition of bases was also examined.
catalyst, while GVL was scarcely produced from LA when 0.8
Comparison of reaction kinetics in Fig. 4 clearly reveals that mol% Ru catalyst was used (entry 9). This is primarily due to the
heterogeneous bases such as MgO and hydrotalcite (HT) act as
effective co-catalysts, affording GVL in almost quantitative
yields (>98%) within 24 h of reaction (Fig. 4, middle), whereas
Table 3 CTH reaction of levulinate esters and levulinic acid with
the ruthenium hydroxides supported on MgO and HT showed
various alcohols as hydrogen donors over 4% Ru(OH)
catalyst
x
/TiO
2
(A)
subpar catalytic activities for this reaction. The GVL production
rates were as fast as that given with 4.0 mol% Ru catalyst (Fig. 4,
right top), demonstrating the efficacy of base addition. Under
the presence of MgO or HT, the formation of 4-HPM was
signicantly suppressed and GVL was dominantly produced.
This is due to the prominent basicity of the heterogeneous
bases to effectively promote the dealcoholation step of 4-HPM to
form GVL (see Fig. S2 in ESI†). On the other hand, addition of
a
c
c
Entry
Substrate
Alcohol
Conversion (%)
GVL yield (%)
b
typical homogeneous bases such as Na
2
CO
3
and triethylamine
1
ML
ML
ML
ML
ML
ML
EL
MeOH
EtOH
1-PrOH
2-PrOH
2-BuOH
CyOH
2-PrOH
2-PrOH
2-PrOH
0
2.2
2.0
>99 (100)
84
57
0
0.8
0.6
89 (99)
67
54
2
3
4
5
6
7
8
9
(TEA) scarcely affected GVL yield, but rather reduced the
conversion rate of ML, although the formation of 4-HPM was
somehow suppressed (Fig. 4, bottom). They are likely to interact
d
d
directly with Ru(OH)
to their homogeneities. In particular, in the presence of
Na CO , appreciable amounts of byproducts associated with
x
sites and interrupt the CTH reaction due
d
d
>99 (100)d
71 (88)d
BL
LA
>99 (100)
63 (81)
2
3
d
d
10.8 (80)
3.5 (64)
transesterication reaction were formed owing to its strong
basicity.
a
Reaction conditions: catalyst (Ru 0.8 mol%, 20 mg), substrate (1
b c
ꢀ
ꢀ
mmol), alcohol (5 mL), 90 C, 24 h, Ar. At 60 C. Determined by
GC. Ru 4.0 mol% (100 mg).
d
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2
ꢁ1
2
ꢁ1
strong adsorption of acidic LA on basic Ru(OH)
x
sites and
S
BET ¼ 10 m g ) and g-Al
2
O
3
(JRC-ALO-4, SBET ¼ 210 m g
)
possibly due to the water molecules generated during the were supplied from Sumitomo Chemical Co., Ltd. and the
reaction. The reaction rate decreased in the order of: ML > EL > Catalysis Society of Japan, respectively. ZrO (RC-100, S
¼ 112
2
BET
2
ꢁ1
2
ꢁ1
BL [ LA (for reaction kinetics, see Fig. S3 in ESI†), showing
m
g
) and CeO2 (SBET ¼ 131 m
g ) were obtained from
2
ꢁ1
that the present catalytic system is especially effective for the Daiichi Kigenso Kagaku Kogyo Co., Ltd. MgO (SBET ¼ 45 m g
)
2
ꢁ1
transformation of levulinate esters to GVL.
and Mg–Al hydrotalcite (SBET ¼ 67 m g ) were commercially
supplied from Wako Pure Chemical Ind., Ltd.
Recyclability of catalyst
Catalyst preparation
Since the lifetime of the catalysts has vital effect on the cost of
the overall process, reusability and stability of the catalysts are
of great importance for practical applications. To evaluate the
reusability and stability of the catalysts, catalyst recycle tests for
the CTH reaction of ML with 2-PrOH were performed over four
successive cycles using 4% Ru(OH) /TiO (A) catalyst. Aer each
catalytic run, the catalyst was separated from the reaction
mixture by ltration, washed with acetone and water, dried
under vacuum, and then subjected to next catalytic run. As
Supported Ru(OH)
x
catalysts were prepared by precipitation
solution according to a previously
method in aqueous RuCl
3
48
reported method. Typically, 2.0 g of metal oxide support pre-
ꢀ
treated at 550 C for 3 h in air was mixed with an aqueous RuCl3
solution (8.3 mM) for 15 min with magnetic stirring. The
solution pH was adjusted to 13.2 by addition of an aqueous
NaOH solution (1 M), followed by stirring at room temperature
for 24 h. The solid was then ltered, washed with a copious
amount of water, and dried under vacuum to afford supported
x
2
x 2
shown in Fig. 5, the prepared 4% Ru(OH) /TiO catalyst was at
x x
Ru(OH) as a black powder. Pure Ru(OH) was synthesized with
a similar procedure except for the addition of oxide support.
least four times reusable without loss of catalytic activity and
selectivity (89 and 99% GVL yields aer 24 h of reaction with 0.8
and 4.0 mol% Ru catalyst, respectively, even in the 4th use). ICP
analysis of the solution aer the reaction showed that elution of
Ru species is negligible. XAFS analysis of the spent catalyst
conrmed that the conguration of Ru species was unchanged
upon the reaction (Fig. S6 in ESI†). These results show that
Ru(OH) /TiO (A) is a highly effective and recyclable heteroge-
x 2
neous catalyst for the CTH reaction of levulinate esters to give
GVL under mild reaction conditions.
Characterization
Powder XRD patterns were recorded on a Rigaku RINT 2000
diffractometer with CuK
˚
a
radiation (l ¼ 1.54056 A, 40 kV to 40
mA). Nitrogen adsorption–desorption isotherms were measured
at ꢁ196 ꢀC by using Micromeritics ASAP 2020 system. The
ꢀ
samples were degassed at 200 C under vacuum for 4 h prior to
the measurements to vaporize physisorbed water. The specic
surface area was calculated by the BET (Brunauer–Emmett–
Teller) method by using adsorption data ranging from P/P
0
¼
Experimental
0.05 to 0.30. Elemental analysis was performed by Sumika
Chemical Analysis Service. Ru loadings were determined by
inductively coupled plasma atomic emission spectrometry (ICP-
AES).
Materials
All commercially available chemical reagents for material
synthesis and catalytic tests were purchased from (Wako Pure
Chemical Ind., Ltd.) and used without further purication. A
variety of metal oxides were used as supports for Ru(OH)
catalyst; anatase TiO (ST-01, S
Ru K-edge X-ray absorption spectra were recorded in uo-
rescence mode at room temperature at BL01B1 facility of
SPring-8, Hyogo, Japan. All EXAFS data were examined by using
REX2000 soware (Rigaku), by which Fourier transformation of
x
2
ꢁ1
¼ 339 m g ) and rutile TiO2
2
2
BET
ꢁ1
(
TTO-55, SBET ¼ 47 m g ) were supplied from Ishihara Sangyo
3
k -weighted normalized EXAFS data (FT-EXAFS) was performed
2
ꢁ1
Co., Ltd. Anatase and rutile TiO
2
(P25, SBET ¼ 55 m g ) was
(AKP-3000,
ꢁ1
˚
over the range 3.0 < k (A ) < 11.0 to obtain the radial structure
function (RDF). For the curve-tting analysis, the empirical
phase shi and amplitude functions for Ru–O and Ru/Ru were
extracted from the data for RuO2.
2 3
supplied from AEROSIL Japan Industries. a-Al O
Procedures for catalytic reactions
In a typical reaction, catalyst (Ru 0.8 mol%), levulinate ester (1
mmol) and alcohol (5 mL) were added to a quartz glass reactor
equipped with a reux condenser and magnetically stirred at 90
ꢀ
C under 1 atm of Ar. Aer the allotted reaction time, a portion
of the reaction mixture was withdrawn by ltration and then
analyzed by a gas chromatograph (Shimadzu GC-14B) with a

ame ionization detector equipped with a capillary column
(
ULBON HR-20 M; 0.53 mm ꢃ 30 m; Shinwa Chemical Ind.,
Ltd.). Conversion of levulinate ester and yields of GVL and other
products were determined by using biphenyl as an internal
standard. For recycle test of catalyst, the spent catalyst was
x 2
Fig. 5 Recyclability test of 4% Ru(OH) /TiO (A) catalyst. Reaction
conditions: catalyst (Ru 0.8 or 4.0 mol%), methyl levulinate (1 mmol),
ꢀ
2
-PrOH (5 mL), 90 C, 24 h, Ar.
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This work was nancially supported by the Research Institute
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gratefully acknowledge Dr Yu Horiuchi (Osaka Prefecture Univ.,
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