Double-active sites cooperatively catalyzed transfer hydrogenation of ethyl levulinate over a ruthenium-based catalyst

Article abstract of DOI:10.1016/j.mcat.2017.09.026

Presently, designing high-performance catalyst systems for the sustainable production of chemicals through biomass conversions is of significant importance for large-scale practical application. As for heterogeneous catalysts, the high dispersion of active species can play a vital role in guaranteeing their superior performance. In this regard, the combination of active species with a favorable support matrix is crucial for achieving highly dispersive character of active species and the formation of cooperation between them. Herein, we first synthesized a novel ruthenium-based catalyst, Ru/Zn-Al-Zr layered double hydroxide (Ru/ZnAlZr-LDH), which was employed in the transfer hydrogenation of biomass-derived ethyl levulinate (EL) into γ-valerolactone (GVL) using 2-propanol as hydrogen donor. Extensive characterizations revealed that the interaction between Ru species and the ZnAlZr-LDH matrix helped enhance the dispersion of Ru species on the LDH and also determined the nature of electron-rich Ru species. Furthermore, a cooperative effect between double-active sites on the catalyst, e.g. a large amount of surface hydroxyl groups and highly dispersive electron-rich Ru species, was beneficial to the formation of both activated six-membered ring transition state and active ruthenium-hydride species in the course of EL transfer hydrogenation, thereby resulting in an unparalleled activity with a fastest GVL formation rate of 1250 μmol g_cat^?1 min^?1 to date, with respect to other Zr- or Ru-based catalysts previously reported.

Full text of DOI:10.1016/j.mcat.2017.09.026

Molecular Catalysis 442 (2017) 181–190
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Editor’s choice paper
Double-active sites cooperatively catalyzed transfer hydrogenation of
ethyl levulinate over a ruthenium-based catalyst
∗
∗
Zhi Gao, Guoli Fan , Lan Yang, Feng Li
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of
Chemical Technology, Beijing, 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 July 2017
Received in revised form 4 September 2017
Accepted 23 September 2017
Presently, designing high-performance catalyst systems for the sustainable production of chemicals
through biomass conversions is of significant importance for large-scale practical application. As for
heterogeneous catalysts, the high dispersion of active species can play a vital role in guaranteeing their
superior performance. In this regard, the combination of active species with a favorable support matrix
is crucial for achieving highly dispersive character of active species and the formation of cooperation
between them. Herein, we first synthesized a novel ruthenium-based catalyst, Ru/Zn-Al-Zr layered dou-
ble hydroxide (Ru/ZnAlZr-LDH), which was employed in the transfer hydrogenation of biomass-derived
ethyl levulinate (EL) into ␥-valerolactone (GVL) using 2-propanol as hydrogen donor. Extensive charac-
terizations revealed that the interaction between Ru species and the ZnAlZr-LDH matrix helped enhance
the dispersion of Ru species on the LDH and also determined the nature of electron-rich Ru species. Fur-
thermore, a cooperative effect between double-active sites on the catalyst, e.g. a large amount of surface
hydroxyl groups and highly dispersive electron-rich Ru species, was beneficial to the formation of both
activated six-membered ring transition state and active ruthenium-hydride species in the course of EL
transfer hydrogenation, thereby resulting in an unparalleled activity with a fastest GVL formation rate of
Keywords:
Ruthenium
Double-active sites
Cooperative effect
Catalytic transfer hydrogenation
Ethyl levulinate
−1
−1
1
250 ␮mol gcat min to date, with respect to other Zr- or Ru-based catalysts previously reported.
2017 Elsevier B.V. All rights reserved.
©
1
. Introduction
Currently, with the rapid development of economy and human
conversion. GVL may be widely transformed into various impor-
tant chemicals [16], such as suitable reaction solvents for biomass
upgrading [17–19], excellent food and fuel additives [20,21], and
liquid fuel precursors [22–24].
society, the consumption of non-renewable fossil resources is
increasing year by year. Meanwhile, the use of fossil resources also
brings out serious environmental pollution problems and global cli-
mate change. Correspondingly, exploring a wide range of new, safe,
environmentally friendly and sustainable resources and energy
sources is of significant importance. As an ideal alternative to
fossil resources, abundant renewable raw biomass especially can
be converted into various liquid transportation fuels and valu-
able chemicals [1–3]. For example, cellulose may be transformed
into many kinds of desired platform molecules [4–10]. Among
them, levulinic acid is regarded as one of the most important
biomass-derived platform compounds due to its excellent reac-
tive nature. Specially, hydrogenating levulinic acid or its esters can
generate another important platform molecule, ␥-valerolactone
Due to easy deactivation and low stability of precious metal
catalysts, strong acidity and low volatility of levulinic acid, and
need of high hydrogen pressure, large-scale production of GVL
through the hydrogenation of levulinic acid is limited. Compara-
bly, hydrogenating ethyl levulinate (EL) to produce GVL is regarded
as an economical and eco-friendly alternative, because high yields
of EL can be directly achieved through ethanolysis of lignocel-
lulose biomass and GVL product is more easily separated and
purified [25,26]. Meanwhile, considering many drawbacks of exter-
nal molecular hydrogen obtained from non-renewable substances,
such as low utilization, poor security, and difficulty in storage and
transportation, using formic acid as hydrogen source for the pro-
duction of GVL has been investigated [27,28]. In spite of high GVL
yields, the corrosive nature of formic acid, as well as harsh reac-
tion conditions, limits its practical application. Therefore, exploring
suitable hydrogen donors for the production of GVL is highly desir-
able.
(
GVL) [11–15], which is identified as a key sustainable biomass
∗ Corresponding author.
E-mail addresses: fangl@mail.buct.edu.cn (G. Fan), lifeng@mail.buct.edu.cn
In recent years, non-corrosive alcohols have been utilized as
both hydrogen donors and solvents in the transfer hydrogenation
(
F. Li).
http://dx.doi.org/10.1016/j.mcat.2017.09.026
468-8231/© 2017 Elsevier B.V. All rights reserved.
2

1
82
Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
of EL to produce GVL [29–34], considering the fact that alcohols
can be produced from renewable biomass resources and dehy-
drogenated aldehydes or ketones formed are valuable chemicals.
In this regard, different Zr-based catalysts (e.g., Zr-Beta zeolite
100 mL deionized water to form a mixed base solution. Then, above
salt and base solutions were simultaneously added to a colloid
mill, rapidly mixed at a rotor speed of 3000 rpm for 2 min. The
resulting suspension was centrifuged and washed with deionized
◦
[
29], metal hydroxides [30,31], ZrO2 [32], Zr-containing catalyst
water until pH value reaches 7.0, and then dried at 70 C overnight.
with a phenate [33]) exhibit good catalytic performance in the EL
transfer hydrogenation using alcohols as hydrogen donors, mostly
via Meerwein–Ponndorf–Verley (MPV) reduction pathway. Despite
many efforts, in most cases, current catalytic systems often require
long reaction time, large amounts of catalysts, or high pressure of
inert gas, in order to facilitate the dehydrogenation of alcohols and
promote the reaction equilibrium, thus leading to high energy con-
sumption and even catalyst leaching. As for transfer hydrogenation
of EL, the present limited design strategy of heterogeneous catalysts
hinders the rational improvement of their catalytic performances.
Thanks to the versatility and flexibility of microstruc-
tures and compositions, layered double hydroxides (LDHs,
For comparison, pure M Al-LDH (M = Zn, Mg, Ni, or
1
1
Co; M /Al molar ratio = 75:25), pure ZnAlZr-LDH (Zn/Al/Zr
1
molar
ratio = 75:15:10),
Ru/ZnAl-LDH
(Ru/Zn/Al
molar
ratio = 1.5:75:25), M /ZnAlM -LDH (M = Ru, Au, or Pd; M = Zr or
2
3
2
3
Sn; M /Zn/Al/M3 molar ratio = 1.5:75:15:10) samples also were
2
synthesized according to the same procedure as for Ru/ZnAlZr-LDH.
In addition, Zr(OH)4 supported Ru catalyst with the Ru loading
of about 0.9 wt%, Ru(OH) /Zr(OH) , was prepared by incipient
3
4
wetness impregnation using the Zr(OH)4 support. And, metallic
0
0
Ru also was loaded on the ZnAlZr-LDH to obtain Ru /ZnAlZr-
LDH sample with the Ru loading of 0.9 wt.%. First, ZnAlZr-LDH
powder (0.5 g) was added into 100 mL of RuCl ·3H O aqueous
3
2
2+
Mx3+(OH)2] Ax/n ·mH O), well known as a class of highly
x+
n−
[
M1
solution (10 mM). Then, the obtained suspension was stirred at
room temperature for 2 h. Subsequently, 17 mL of NaBH4 solution
(0.16 M) was added into the above suspension under ice water
bath and maintained for 6 h under nitrogen atmosphere. Finally,
−x
2
ordered two-dimensional layered clay materials, have been widely
used as catalyst supports in various heterogeneous catalytic pro-
cesses [35–37]. Especially, it was reported that regular hydroxyl
arrays on positively charged two-dimensional brucite-like sheets
of LDHs as Brønsted base sites could promote the MPV reactions
of aldehydes with alcohols [30,38], as well as other organic reac-
tions including condensation [39,40], transesterification [41], and
isomerization [42].
0
Ru /ZnAlZr-LDH obtained was centrifuged, washed with deionized
◦
water and dried at 70 C overnight under vacuum.
2.2. Characterization
Herein, we first synthesized a new ruthenium-based cata-
lyst (Ru/ZnAlZr-LDH) and reported its unparalleled activity in
the EL transfer hydrogenation to produce GVL and mechanistic
understanding of the promotion effect. An extensive investigation
indicated that the immobilization of Ru species on the ZnAlZr-LDH
matrix could induce the formation of highly dispersed electron-rich
Ru species due to the strong interaction between Ru species and
surface hydroxyl groups of ZnAlZr-LDH, and the resulting catalyst
gave a GVL yield of 98% only within 10 min, along with a fastest GVL
formation rate of 1250 ␮mol gcat min to date, with respect to
other Zr- or Ru-based catalysts previously reported. The activity
of the catalyst was higher than those exhibited by bare ZnAl-
LDH, Zr substituted ZnAl-LDH, and ZnAl-LDH supported Ru catalyst.
Such unprecedented catalytic performance of Ru/ZnAlZr-LDH was
ascribed to a cooperative effect between double-active sites on
the catalyst, e.g. a large amount of surface hydroxyl groups of the
ZnAlZr-LDH and highly dispersive electron-rich Ru species, which
was beneficial to the formation of both activated six-membered
ring transition state and active ruthenium-hydride species in the
course of transfer hydrogenation, thereby greatly improving the
dehydrogenation of isopropanol and the hydrogenation of EL.
Moreover, the activity of as-synthesized ruthenium-based hetero-
geneous catalyst kept almost unchanged after it was reused more
than six times. Until now, this is the first report about the appli-
cation of such highly efficient LDH supported Ru catalyst for the
transfer hydrogenation of EL to produce GVL.
X-ray diffraction (XRD) patterns of samples were recorded at
room temperature using Shimadzu XRD-6000 diffractmeter with
graphite-filtered Cu K␣ source (␭ = 0.15418 nm) at a scanning rate
◦
◦
◦
of 10 /min, and 2␪ angle ranging from 3 to 70 .
Elemental analysis of samples was carried out on a Shimadzu
ICPS-7500 inductively coupled plasma atomic emission spectro-
scope (ICP-AES). The samples were dissolved in nitrohydrochloric
acid before measurements.
N2 adsorption-desorption isotherms of samples were obtained
−1
−1
◦
on a Micromeritics ASAP 2020 sorptometer apparatus at −196 C.
The total specific surface areas were determined by the multipoint
Brunauer-Emmett-Teller (BET) method.
High-resolution transmission electron microscopy (HRTEM)
were measured using a JEOL 2100 operated at an accelerating volt-
age of 200 kV. High-angle annular dark-field scanning transmission
electron microscopy with energy-dispersive X-ray spectroscopy
(HAADF-STEM-EDX) measurements of the sample was investigated
using a JEOL2010F instrument.
X-ray photoelectron spectroscopy (XPS) measurements were
performed using a VG ESCALAB 2201 XL spectrometer with
monochromatic Al K␣ X-ray radiation (1486.6 eV photons). The
binding energy calibration of all spectra was referenced to the C
1s signal at 284.6 eV.
The number of basic sites was obtained using a method based
on the irreversible adsorption of organic acids with different pKa
values, e.g. acrylic acid (pKa = 4.2) and phenol (pKa = 9.9) [44,45].
In situ Fourier transform infrared (FT-IR) spectra of EL or pyri-
dine adsorption were recorded using a Thermo Nicolet 380 FT-IR
spectrometer. The sample (30 mg) pressed into a self-supporting
2
. Experimental
wafer was placed into an evacuable IR cell with CaF windows, and
2
◦
◦
2.1. Synthesis of catalysts
then evacuated at 100 C for 1 h. After cooling to 30 C, EL or pyri-
dine was introduced and held for 1 h. Finally, physically adsorbed
EL or pyridine was removed by evacuation under vacuum.
Ru/ZnAlZr-LDH sample was synthesized through
a sepa-
rate nucleation and aging steps method previously developed
by our group [43]. In a typical experiment, first, RuCl ·3H O,
2.3. Catalytic transfer hydrogenation of EL
3
2
Zn(NO ) ·6H O, Al(NO ) ·9H O and Zr(NO ) ·5H O with
3
2
2
3
3
2
3
4
2
a
Ru/Zn/Al/Zr molar ratio of 1.5:75:15:10 were dissolved
The liquid phase transfer hydrogenation of EL was performed
in a 100 mL stainless steel autoclave under magnetic stirring. In
a typical experiment, EL (2.8 mmol), isopropanol (131 mmol) and
the catalyst (0.1 g) were charged into the reactor. Before each run,
in 100 mL deionized water to obtain a salt solution, while
2
−
3+
3+
4+
Na CO
[
and
NaOH
([CO3 ] = 2([Al ] + [Ru ] + [Zr ]);
2
3
OH ] = 1.6([Zn2+] + [Al3+] + [Ru3+] + [Zr4+])) were also dissolved in
−

Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
183
diffractions, indicating that the introduction of Ru, Zr or Sn leads
to the decreased crystallinity of LDH phases, as well as the for-
mation of small LDH particle with reduced size. No characteristic
diffractions assignable to metallic Ru and RuO phases are observed
2
in Ru-containing samples, suggesting that Ru species should be
highly dispersed on the surface of LDHs or exist in bulk phase in
the form of other chemical states. Further, typical HRTEM images
of the representative Ru/ZnAlZr-LDH sample clearly depict that the
sample has the sheet-like feature of LDH materials (Fig. 2a, b). No
aggregates of small particles possibly related to Ru species can be
observed. HAADF-STEM-EDX images (Fig. 2c, d) reveal the uniform
surface distributions of Zn, Al, Zr and Ru elements, indicative of
highly dispersive nature of these elements on the sample. In addi-
tion, as for Ru-containing samples, the Ru loadings confirmed by
ICP-AES are about 0.9 wt%, in good accordance with the theoreti-
cal values. Moreover, Table 1 shows that the BET specific surface
2
area of Ru/ZnAlZr-LDH (119 m /g) is higher than other samples
2
2
2
(
42 m /g for ZnAl-LDH, 57 m /g for Ru/ZnAl-LDH and 93 m /g for
ZnAlZr-LDH). As evidenced in Fig. 1, Ru/ZnAlZr-LDH and Ru/ZnAl-
LDH samples present relatively weakened diffraction intensities
compared with ZnAlZr-LDH and ZnAl-LDH sample, respectively. It
demonstrates that the introduction of Ru leads to the decreased
crystallinity of LDH phases, as well as the formation of LDH crystals
with the reduced particle size. Therefore, a smaller particle size of
Ru/ZnAlZr-LDH than ZnAlZr-LDH can give rise to a higher specific
surface area of Ru/ZnAlZr-LDH sample.
Fig. 1. XRD patterns of (a) ZnAl-LDH, (b) Ru/ZnAl-LDH, (c) ZnAlZr-LDH, (d)
Ru/ZnAlZr-LDH and (e) Ru/ZnAlSn-LDH.
the reactor was sealed and flushed with 2 MPa N2 for ten times to
remove the air. Then, the reactor was initiated by stirring with a
rate of 900 rpm at a designated temperature under N atmosphere.
2
3.2. Catalytic performance of catalysts
After reaction, the autoclave was cooled to room temperature and
depressurized carefully. The reaction products were filtered to
obtain a clear solution and then analyzed using Agilent GC7890 B
gas chromatograph equipped with a flame ionization detector and a
DB-wax capillary column (30.0 m × 250 ␮m × 0.25 ␮m). The prod-
ucts were identified by comparison with known standards, and
toluene was used as an internal standard to quantitatively analyze
liquid products with a standard deviation less than 2%. In each case,
the mass balance was above 97%.
Preliminary, we examined the EL transfer hydrogenation over
different pure base LDH supports with various binary metal cations.
The catalytic results are presented in Table 2. Obviously, the EL
transfer hydrogenation using isopropanol as hydrogen source can
perform over the ZnAl-LDH support; it affords an EL conversion of
◦
2
(
4% with a GVL selectivity of 65% after reaction for 30 min at 200 C
entry 1). The major by-product is isopropyl levulinate formed
through the transesterification of EL with isopropanol. Noticeably,
other LDH supports of MgAl-LDH, NiAl-LDH and CoAl-LDH exhibit
much less catalytic activity for the reaction (entries 2–4). Among
them, remarkably, ZnAl-LDH support shows the best activity.
Further, the catalytic performances of various Zr and/or Ru-
containing catalysts for the EL transfer hydrogenation were tested.
When Zr is introduced into the lattice of brucite-like layers, an
improved yield of GVL by 20% can be achieved over the ZnAlZr-
LDH (entry 5). In the presence of Ru/ZnAl-LDH, the EL conversion
and the GVL selectivity sharply increase to 78% and 90% (entry 6),
respectively, indicating a promotional effect of the introduction of
Ru into the catalyst on the EL transfer hydrogenation. Compared
with Ru/ZnAl-LDH, Ru/ZnAlZr-LDH sample with the same Ru load-
3
. Results and discussion
3.1. Structural characterization
Fig. 1 illustrates the XRD patterns of ZnAl-LDH, Ru/ZnAl-LDH,
ZnAlZr-LDH, Ru/ZnAlZr-LDH and Ru/ZnAlSn-LDH samples. In all
cases, typical characteristic diffractions indexed to (003), (006),
(
012), (015), (018), (110) and (113) planes of carbonate-type
hydrotalcite-like materials can be found, indicating the success-
ful synthesis of LDH materials [46,47]. Compared with ZnAl-LDH
sample, other samples present relatively weakened intensities of
Table 1
Textural properties and surface basicity of different samples.
SBETa (m /g)
2
Vpb (cm /g)
3
dpc (nm)
Total base sites (mmol
acrylic acid/g)
Distribution of base sites
Samples
Medium-strength and
strong base sites (mmol
phenol/g)
Weak base sitesd
(mmol/g)
ZnAl-LDH
ZnAlZr-LDH
Ru/ZnAl-LDH
Ru/ZnAlZr-LDH
Ru/ZnAlSn-LDH
42
93
57
119
128
0.36
0.62
0.54
0.26
0.31
35.8
27.4
32.3
6.5
4.42
4.83
4.61
5.02
5.10
0.14
0.19
0.17
0.23
0.24
4.28
4.64
4.44
4.79
4.86
6.2
a
BET specific surface area.
Total pore volume.
Mean pore diameter.
b
c
d
Weak base sites = total base sites-medium strength and strong base sites.

1
84
Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
Fig. 2. HRTEM (a,b) and HAADF-STEM image (c) of representative Ru/ZnAlZr-LDH sample with the EDX mapping of Zn-K, Al-K, Zr-K and Ru-K (d).
ing of 0.9 wt% shows enhanced catalytic performance with a high
EL conversion of 89% and a GVL selectivity of 94% (entry 7). More-
over, it is found that Ru /ZnAlZr-LDH sample exhibits a catalytic
dispersed on the Ru/ZnAlZr-LDH in two cases of the Ru loadings
of 0.6 and 0.9 wt%. If the amount of EL charged is decreased from
2.8 mmol to 1.4 mmol, an extremely high GVL yield of 98% can be
achieved over the Ru/ZnAlZr-LDH with the Ru loading of 0.9 wt%
after 10 min (entry 12).
0
performance similar to that of ZnAlZr-LDH (entry 8), indicating
0
that Ru species are not catalytically active sites in the present
catalytic reaction. Previously, it was reported that surface acid-
base property of catalysts could significantly affect the activity and
selectivity in the transfer hydrogenation of carbonyl compounds
Since Ru-containing catalysts show much higher catalytic
performances than corresponding Ru-free LDH samples and
Ru/ZnAlSn-LDH that has almost the same quantity of surface
basic sites as Ru/ZnAlZr-LDH (Table 1), we speculate that the
enhanced reaction activity of Ru/ZnAlZr-LDH may be correlated
with the favorable electronic structure of Ru species. Further,
noticeably, several reference samples, Au/ZnAlZr-LDH, Pd/ZnAlZr-
[
32]. As for LDH materials, surface acidity mainly originates from
metal cations, which are considered as Lewis acidic sites. In order to
identify the effect of surface acid-base property on the catalytic per-
formance, Ru/ZnAlSn-LDH sample also was synthesized. Because
the electronegativity of Sn4+ cation is almost equal to that of Zr
cation, it is expected that the strength and type of surface acidic
4+
LDH, Ru(OH) /Zr(OH)4 and mechanical mixed sample of Ru(OH)3
3
and ZnAlZr-LDH, present much lower activity than Ru/ZnAlZr-LDH
under similar reaction conditions (entries 13–16). It seems that
the kinds of supported metals, the modes of the interaction of Ru
species with supports, and the types of supports greatly impact on
the catalytic activity.
From the above results, it is clearly indicated that Ru/ZnAlZr-
LDH catalyst exhibits an excellent activity for the EL transfer
hydrogenation to produce GVL, demonstrating that its much higher
activity may be related to special interaction between Ru species
and the ZnAlZr-LDH support. Moreover, to carry out a rigorous
comparison of the activities of different catalysts, the mass specific
activity (MSA) of catalysts was determined by normalizing the ini-
tial reaction rate to the total amount of catalyst without the effects
of products or adverse reactions. As shown in Fig. 3, obviously, the
MSA value for pure ZnAlZr-LDH is much lower than those for Ru-
4+
sites should keep unchanged if Zr ions in brucite-like layers are
4
+
replaced by Sn ions. Moreover, the total number of base sites on
the Ru/ZnAlSn-LDH is slightly higher than that on the Ru/ZnAlZr-
LDH (Table 1). However, the catalytic reaction result indicates that
Ru/ZnAlSn-LDH exhibits a less activity than Ru/ZnAlZr-LDH in spite
of a similar GVL selectivity (entry 9).
In addition, when the dosage of Ru/ZnAlZr-LDH is increased
from 0.1 g to 0.2 g, the EL conversion and the GVL selectivity reach
9
4% and 95% after reaction for a very short time of 10 min, respec-
tively (entry 10), due to the increased amount of active sites. If the
Ru loading is decreased from 0.9 wt% to 0.6 wt%, both the EL con-
version and the GVL selectivity over the Ru/ZnAlZr-LDH are slightly
reduced to 90% and 92%, respectively, after 10 min (entry 11). It
demonstrates that surface catalytically active sites should be highly

Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
185
Table 2
The catalytic results of the transfer hydrogenation of EL over different samples.
Time (min) Temp. ( C) Zr: EL (mol%) Ru:EL (mol%) Conv.a (%) Select.b (%)
◦
RGVLc (␮mol g min
−1
−1
)
Yield of acetone (%)
Entry Catalysts
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
ZnAl-LDH
MgAl-LDH
NiAl-LDH
30
30
30
30
30
30
30
30
30
10
10
10
10
10
10
30
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
90
–
–
–
–
–
–
–
–
24
20
16
9
49
78
89
51
81
94
90
100
42
44
63
54
100
94
65
63
57
50
72
90
94
71
93
95
92
98
81
96
90
76
97
95
88
85.0
100
97
146
118
85
2.9
2.7
2.0
1.2
6.3
10.1
11.8
6.5
10.4
12.5
12.0
13.8
5.7
5.8
8.5
7.3
–
CoAl-LDH
42
ZnAlZr-LDH
Ru/ZnAl-LDH
Ru/ZnAlZr-LDH
3.4
–
3.5
3.5
–
7.0
7.0
14.0
7.0
7.0
7.0
3.4
88.1
45.3
–
–
329
655
781
338
703
1250
1163
686
476
591
794
383
81
0.3
0.3
0.3
0.3
0.6
0.4
1.2
–
0
Ru /ZnAlZr-LDH
Ru/ZnAlSn-LDH
Ru/ZnAlZr-LDH
Ru/ZnAlZr-LDH
Ru/ZnAlZr-LDH
d
0
1
2
3
4
5
6
7
8
9
0
1
2
e
f
d
Au/ZnAlZr-LDH
Pd/ZnAlZr-LDHd
–
Ru(OH)3/Zr(OH)4d
Ru(OH)3-ZnAlZr-LDH
0.6
0.3
–
–
4.2
–
g
h
Zr-HBA
60
60
1440
180
540
720
Zr(OH)4i
207
6
94
62
14
–
–
–
–
j
Ru(OH)x/TiO2
100
100
100
100
k
Zr-MOFs
130
80
150
18.0
–
–
Raney Nil
–
–
Cu-Ni/Al2O3m
–
a
b
c
d
e
f
◦
Reaction conditions: reaction temperature, 200 C; catalyst, 0.1 g; EL, 2.8 mmol; isopropanol, 131 mmol; N2 atmosphere.
Major by-product was isopropyl levulinate.
GVL formation rate.
Catalyst, 0.2 g.
Catalyst with the Ru loading of 0.6 wt%, 0.2 g.
Catalyst with the Ru loading of 0.9 wt%, 0.2 g; EL, 1.4 mmo.
Mechanical mixture of Ru(OH)3 and ZnAlZr-LDH; the amount of Ru, 0.9 wt%.
The data from ref. [33].
The data from ref. [30]; catalyst, 0.2 g; EL, 1 mmol; isopropanol 100 mmol.
The data from ref. [31].
g
h
i
j
k
l
The data from ref. [48].
The data from ref. [49].
The data from ref. [50].
m
Table 3
Catalytic hydrogenation performance over Ru/ZnAlZr-LDH catalyst.a
Hydrogen donors
ethanol
Conv. (%)
Select. To GVL (%)
Yield of GVL (%)
52
34
100
62
92
92
93
93
48
31
93
58
1
2
-butanol
-butanol
cyclohexanol
a
Conditions: Ru/ZnAlZr-LDH, 0.2 g; EL, 2.8 mmol; solvent, 131 mmol; reaction
◦
time, 10 min; reaction temperature, 200 C; N2 atmosphere.
GVL formation rate over the Ru/ZnAlZr-LDH, which is calculated
based on the moles of GVL produced to the total amount of cata-
lyst within the entire reaction time period (Table 2), is about 15.4,
6
.0, 13.3, and 208 times as those over Zr-HBA [33], Zr(OH)4 [30],
Zr-MOFs [48] and Ru(OH)x/TiO2 [31] previously reported, respec-
tively, although there are much lower Zr or Ru contents in the
Ru/ZnAlZr-LDH (Table 2). Furthermore, the GVL formation rate of
Ru/ZnAlZr-LDH is also much higher than those of Raney Ni and Cu-
Ni/Al O catalysts reported previously [49,50]. The results strongly
2
3
confirm the much higher intrinsic activity of the present Ru/ZnAlZr-
LDH. As shown in Fig. 4, the GVL yield over the Ru/ZnAlZr-LDH
reaches the maximum (about 97%) after a reaction time of 50 min.
It demonstrates that 50 min is the optimum reaction time under the
Fig. 3. Comparison of MSA values over different catalysts. Reaction conditions: reac-
tion temperature, 200 C; catalyst, 0.1 g; EL, 2.8 mmol; isopropanol, 131 mmol; N2
atmosphere; Ru loading: 0.9 wt%.
◦
◦
present reaction conditions (i.e. catalyst, 0.1 g; 200 C; EL, 2.8 mmol;
isopropanol, 131 mmol).
Different hydrogen donors including primary alcohols (ethanol
and 1-butanol) and secondary alcohols (2-butanol and cyclohex-
anol) also were examined in the transfer hydrogenation of EL
(Table 3). Noticeably, in the presence of 2-butanol, Ru/ZnAlZr-LDH
yields a complete EL conversion. In contrast, a much lower EL
conversion of 34% is obtained in the case of 1-butanol. Regard-
less different EL conversions, the GVL selectivity remains almost
containing catalysts. Among them, Ru/ZnAlZr-LDH possesses the
−
1
−1
highest MSA value (1.32 mmol g min ). It means that the high
activity of ZnAlZr-LDH alone seems unlikely. The improved trans-
fer hydrogenation activity of Ru/ZnAlZr-LDH should be attributed
to the introduction of highly dispersed Ru species on the surface
of ZnAlZr-LDH. Despite different reaction conditions, the attained

1
86
Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
Fig. 6. Change in the yield of acetone with different samples. Conditions: catalyst,
◦
.2 g; isopropanol, 131 mmol; 2 h; 200 C; N2 atmosphere.
0
Fig. 4. Yield of GVL over the Ru/ZnAlZr-LDH as a function of reaction time in the
transfer hydrogenation of EL. Reaction conditions: reaction temperature, 200 C;
catalyst, 0.1 g; EL, 2.8 mmol; isopropanol, 131 mmol; N2 atmosphere.
present Ru-based catalyst exhibited an unprecedented high perfor-
mance in the transfer hydrogenation of EL, with respect to other Zr-
or Ru-based catalysts previously reported.
◦
3.3. Catalytic reaction mechanism
In the present transfer hydrogenation process, easy dehydro-
genation of the hydroxyl group in isopropanol can greatly facilitate
the hydrogenation of EL to generate GVL. In order to find out
the origin of catalytic performance, the dehydrogenation ability
of different samples was investigated. As shown in Fig. 6, in the
absence of EL substrate, the yields of acetone obtained over ZnAl-
LDH and ZnAlZr-LDH are much lower than those over Ru/ZnAl-LDH,
Ru/ZnAlZr-LDH and Ru/ZnAlSn-LDH, strongly mirroring that the
dehydrogenation ability of Ru-free samples are lower than that
of Ru-containing samples. Moreover, Ru/ZnAl-LDH, Ru/ZnAlZr-LDH
and Ru/ZnAlSn-LDH almost deliver an identical yield of acetone,
demonstrating that the partial replacement of Al to Zr or Sn
does not contribute to the dehydrogenation of isopropanol in the
present catalytic systems. Previously, however, it was reported that
the conversion of isopropanol to acetone/hydrogen could not be
observed in the absence of EL [51]. Our result further demonstrates
that the existence of Ru species in the Ru/ZnAlZr-LDH is highly
beneficial to the dehydrogenation of isopropanol, thus enhancing
catalytic activity of the catalyst.
Commonly, surface basicity of catalysts is good for the trans-
fer hydrogenations [40,52], because dehydrogenation process can
proceed through the abstraction of the hydrogen in isopropanol
on basic sites. The method based on irreversible adsorption of
organic acids including acrylic acid and phenol has been used to
determine the amount of surface basic sites of uncalcined LDH
materials. As shown in Table 1, the number of total basic sites in
the Ru/ZnAl-LDH (4.61 mmol acrylic acid/g) is slightly higher than
that in the ZnAl-LDH (4.42 mmol acrylic acid/g), probably due to
the fact that only basic sites on the external surface of the cata-
lyst can be absorbed. Similarly, ZnAlZr-LDH sample containing Zr
in the brucite-like layers has a larger number of surface basic sites in
comparison to ZnAl-LDH, due to higher surface area of ZnAlZr-LDH.
It demonstrates that high surface area of LDH materials facilitates
the exposure of surface hydroxyl groups on the brucite-like lay-
ers. Moreover, all the samples possess a small quantity of surface
medium-strength and strong basic sites. However, among all sam-
ples, the change in the number of basic site is much smaller than
that of the specific surface area. Meanwhile, in situ FT-IR spectra
Fig. 5. Recycling property of Ru/ZnAlZr-LDH catalyst. Conditions: catalyst, 0.2 g;
◦
2
00 C; 10 min; isopropanol, 131 mmol; EL, 1.4 mmol (high yield level) or 2.8 mmol
(
low yield level); N2 atmosphere.
unchanged in all cases. This suggests that the nature of hydrogen
donors cannot affect the selectivity in the present catalytic systems.
Fig. 5 show the stability of the Ru/ZnAlZr-LDH catalyst deter-
mined by six consecutive catalytic tests at two different EL
conversion levels. After each recycling experiment, the used cat-
◦
alyst was separated by filtration, dried at 70 C for 12 h, and reused
in the next reaction. It is noted that no obvious decline in GVL
yields is observed at two different conversion levels after it is
reused for six successive recycling runs. Possible Ru leaching was
also investigated by ICP-AES and the results reveal that the leach-
ing of ruthenium in the solution after six consecutive cycles is
negligible. The above results indicate the excellent stability and
reusability of the Ru/ZnAlZr-LDH catalyst, owing to the strong inter-
actions between Ru species and the ZnAlZr-LDH matrix. To date, the

Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
187
Table 4
Effect of different additives on the transfer hydrogenation of EL over the Ru/ZnAlZr-
a
LDH.
Additives
Conv. (%)
Select. to GVL (%)
benzoic acid
pyridine
33
89
85
92
a
◦
Conditions: 200 C; 10 min; isopropanol, 131 mmol; EL, 2.8 mmol; catalyst,
.2 g; N2 atmosphere; additive, 0.5 g.
0
face hydroxyl groups on the lattice of Ru/ZnAlZr-LDH as basic sites
in the present catalytic process. In addition, if pyridine is added
into the reaction system to block surface acidic sites, the catalytic
activity of Ru/ZnAlZr-LDH is slightly decreased. Previously, it was
reported that surface Lewis acid sites could strongly improve the
MPV hydride transfer reaction [40,53–55]. Conventionally, Zr and
Sn oxides are considered as strong Lewis acid solids. However, our
above results clearly demonstrate that in the case of Ru/ZnAlZr-
LDH, Zr4+ species should be located within the brucite-like layers
3+
4+
through substituting Al ions in the LDH. Thus, such Zr species
is not suitable to provide Lewis acid sites, and surface acidity of
Ru/ZnAlZr-LDH catalyst is not a main driving force in the present
EL transfer hydrogenation.
On the other side, it is well-known that the electronic prop-
erty of active metal centers in catalysts plays a crucial role for a
broad range of heterogeneous reactions. In particular, in the case of
the hydrogenation process of carbonyl-containing compounds, the
backbonding of d electrons of electron-rich metal sites to ␲ orbitals
of the carbonyl group can improve the adsorption and the activation
of the C O bond [56]. In order to obtain the basic clues concern-
ing the interaction between Ru species and different LDH matrixes,
XPS characterization of Ru-containing samples was performed. As
shown in Fig. 8, clearly, the binding energy (BE) value of Ru species
increases gradually from 281.4 eV for Ru/ZnAlZr-LDH to 281.7 eV
for Ru/ZnAlSn-LDH and 282.0 eV for Ru/ZnAl-LDH, reflecting that
Ru species exist in the form of Ru(OH)3 in all samples [57,58].
Especially, a larger amount of electron-rich Ru3+ species can form
in the Ru/ZnAlZr-LDH. Such electron-rich state of Ru species with
the increased electron cloud density should be correlated with
the presence of strong interaction between Ru3+ species and the
ZnAlZr-LDH matrix. Interestingly, we also observe a negative cor-
relation between the activity of catalysts and the BE values of Ru3+
species. The lower the BE value, the higher the activity. Ru/ZnAlZr-
LDH with a lowest BE value exhibits a highest activity. The kind
of electron-rich Ru3+ species probably facilitate ␲-back bonding to
Fig. 7. In situ FT-IR spectra (B) of pyridine adsorbed on different samples: (a) ZnAl-
LDH, (b) Ru/ZnAl-LDH, (c) Ru/ZnAlZr-LDH and (d) Ru/ZnAlSn-LDH.
of pyridine adsorbed on different samples were measured (Fig. 7)
−
1
reveal that no obvious vibration bands at about 1454 cm
1
and
−
1
540 cm related to surface Lewis and Brønsted acidic sites are
observed in each case, indicative of the lack of surface acidity.
The above results reflect that surface acid-base property of present
LDH-based catalyst systems should not be responsible for the dif-
ference in their transfer hydrogenation performances.
Further, two poisoning experiments were carried out by intro-
ducing extra additives into the reaction system to gain more insight
into the mechanism for the transfer hydrogenation of EL and the
reaction results are shown in Table 4. In the presence of benzoic
acid in the reaction system, the EL conversion is sharply decreased
to 33%. The benzoic acid may react with isopropanol to generate
ethers, thus restraining the conversion of EL. However, the molar
ratio of benzoic acid to isopropanol is only 0.03. Therefore, the rapid
decrease in the EL conversion should mainly be ascribed to the
blocking of surface basic sites, indicating that the crucial role of sur-
Fig. 8. XPS of Ru 3d region for different supported Ru-containing samples (A) and used Ru/ZnAlZr-LDH (B) after reaction.

1
88
Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
the catalyst is greatly beneficial to the activation of the C O bond.
This may be mainly assigned to the electron-rich state of Ru species
in the Ru/ZnAlZr-LDH, which promotes the electron transfer from
the Ru d orbital to the C O 2␲* anti-bonding orbital, resulting in
the weakness of the C O bond strength. To quantitatively discuss
the effect of the interaction between the catalyst surface and EL, a
correlation between the positions of C O bands (IR results) and the
MSA value for transfer hydrogenation of EL by different catalysts is
presented in the inset in Fig. 9. Notably, the MSA value linearly
increases with the decrease in the wavenumber of C O stretching
band for EL adsorbed on catalysts. The above results demonstrate
that the C O bond of EL coordinated to electron-rich state of Ru
species has lower the bond order and thus Ru/ZnAlZr-LDH catalyst
exhibits the higher reactivity toward the EL hydrogenation.
Based on above characterizations and experimental results,
a plausible mechanism for the transfer hydrogenation of EL to
GVL over the Ru/ZnAlZr-LDH catalyst is tentatively proposed
(Scheme 1). In general, the transfer hydrogenation of EL proceeds
through two reaction routes: the one follows MPV reduction path-
way involving an activated six member ring transition state related
to surface hydroxyl groups [38,60], and the other one obeys a metal
hydride route related to surface highly dispersive Ru species [61].
For MPV reaction route, firstly, isopropanol is adsorbed on sur-
face hydroxyl groups on the LDH matrix and further dissociated to
the corresponding alkoxide 1. Secondly, EL molecule coordinates
with the alkoxide to form a six-membered ring transition state 2.
Thirdly, the hydride from the alkoxide is transferred to the carbonyl
in EL via a pericyclic mechanism. Subsequently, the new carbonyl
dissociates and gives the intermediate species 3, while acetone is
released. Fourthly, the intermolecular ligand exchange between
intermediate species 3 and another isopropanol from solution leads
to the generation of ethyl 4-hydroxypentanoate (4-HPE) and alkox-
ide 1. Finally, 4-PHE formed in this reaction system is rapidly
converted to GVL through an intramolecular transesterification.
In the case of metal hydride route, firstly, ruthenium alkox-
Fig. 9. In situ FT-IR spectra of EL adsorbed on different catalysts. The Inset shows the
correlation between IR band positions of EL adsorbed on catalysts and MSA values
for transfer hydrogenation of EL by different catalysts.
the carbonyl group in EL and thus the adsorption and activation of
the C O bond. Further, the XPS result of recovered Ru/ZnAlZr-LDH
shows that the binding energy of Ru species for 3d 5/2 core level
3+
still is about 281.4 eV. It demonstrates that Ru species in the cat-
alyst cannot be reduced to metallic Ru0 species by isopropanol or
in situ formed active hydrogen species during reaction process.
Further, to understand the origin of the unparalleled catalytic
activity of Ru/ZnAlZr-LDH catalyst in the transfer hydrogenation
of EL, the interaction between the catalyst surface and C O group
in EL was studied by in situ FT-IR spectra of EL adsorbed on
different samples at room temperature (Fig. 9). For ZnAl-LDH sam-
ꢀ
ide species 1 is formed via the ligand exchange occurring
between surface ruthenium hydroxide and isopropanol. Then, ␤-
ꢀ
hydride elimination occurs in species 1 , and ruthenium hydride
ꢀ
species 2 and acetone are formed. Subsequently, the reaction
between species 2 and EL substrate results in the formation of
ketone–hydride species 3 . The hydride transfer from ruthenium
hydride to the carbonyl carbon of EL in species 3 gives other
−
1
ꢀ
ple, the adsorption at 1728 cm
is assigned to the stretching
ꢀ
vibration of the C O bond in adsorbed EL [59]. Noticeably, the
wavenumber for the stretching vibration of the C O bond decreases
progressively in the order of ZnAl-LDH > Ru/ZnAl-LDH > Ru/ZnAlSn-
LDH > Ru/ZnAlZr-LDH, reflecting that Ru/ZnAlZr-LDH interacts
more strongly with carbonyl group of the EL than other samples.
Therefore, the introduction of simultaneous Ru and Zr species into
ꢀ
ꢀ
alkoxide species 4 . At last, similar to the MPV route, GVL can be
produced through the intermolecular ligand exchange between
ꢀ
alkoxide species 4 and isopropanol and following intramolecular
transesterification process.
Scheme 1. Proposed possible mechanisms for the transfer hydrogenation of EL to GVL over the Ru/ZnAlZr-LDH.

Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
189
Table 5
Transfer hydrogenation of different carbonyl compounds over the Ru/ZnAlZr-LDH catalyst.
a
Entry
Substrates
Products
Conversion (%)
100
Selectivity (%)
1
90
2
3
100
100
92
93
4
5
6
100
100
100
96
94
97
7
100
91
a
◦
Reaction conditions: 200 C; 30 min; catalyst, 0.2 g; substrate, 2.8 mmol; isopropanol, 131 mmol; N2 atmosphere.
As evidenced by poisoning experiments shown in Table 4, sur-
face basic sites are highly essential for the high catalytic activity
of catalysts. Moreover, the fact that the catalytic performance of
Ru/ZnAlZr-LDH is much higher than that of ZnAlZr-LDH demon-
GVL. By combination of Ru species with the ZnAlZr-LDH matrix,
the electronic structure of Ru3+ species is relationally modified.
XPS analysis confirms the electron-rich nature of Ru3+ species
in the form of Ru(OH) . Ru/ZnAlZr-LDH catalyst presents an
3
3
+
strates that Ru species also plays a key role in enhancing the
unprecedented activity with a high GVL yield of 98% only after
reaction for 10 min, and yields a fastest GVL formation rate of
3+
catalytic activity. Surface basic sites and Ru species, which are
responsible for MPV reduction pathway and metal hydride route,
respectively, are equally important for the present reaction. There-
fore, two reaction routes have an equal status. In the present
catalytic system, Ru/ZnAlZr-LDH provides both favorable electron-
−
1
−1
1250 ␮mol gcat min to date, with respect to other Zr- or Ru-
based catalysts previously reported. Such unprecedented high
performance of Ru/ZnAlZr-LDH can be attributable to a coopera-
tive effect between double-active sites on the catalyst, e.g. a large
amount of surface hydroxyl groups and highly dispersive electron-
rich Ru species, which is beneficial to the activation of the hydroxyl
group in isopropanol and the C O bond in EL, thereby greatly facil-
itating the formation of activated six-membered ring transition
state and active ruthenium-hydride species in the course of transfer
hydrogenation processes. Moreover, as-synthesized Ru/ZnAlZr-
LDH catalyst can be reused at least six times without obvious lose
in the activity. The present readily available, highly efficient and
stable ruthenium-based catalyst possesses promising potential for
the practical production of commercially important chemicals from
biomass-derived carbonyl compounds in terms of economic and
environmental sustainability.
3
+
rich Ru species and abundant hydroxyl groups on the catalyst
surface, which can construct the most effective cooperative effect
between them. As a result, the above proposed reaction mecha-
nisms of catalytic transfer hydrogenation of EL reasonably explain
the excellent catalytic performance of Ru/ZnAlZr-LDH catalyst.
3.4. Feasibility of the Ru/ZnAlZr-LDH catalyst
To verify the catalytic transfer hydrogenation ability of as-
synthesized Ru/ZnAlZr-LDH catalyst, the feasibility of Ru/ZnAlZr-
LDH was tested using several biomass-derived carbonyl com-
pounds as substrates. As shown in Table 5, Notably, Ru/ZnAlZr-LDH
can significantly catalyze these aldehydes and ketones to cor-
responding alcohols with high conversions (100%) and high
selectivities to target alcohol products (90–97%) within a very short
time of 10 min. Therefore, Ru/ZnAlZr-LDH shows great potential as
an effective and versatile catalyst for the transfer hydrogenation of
carbonyl compounds in terms of its excellent reactivity, reusability
and feasibility.
Acknowledgments
We gratefully thank the financial support from National Natural
Science Foundation of China and the Fundamental Research Funds
for the Central Universities (buctrc201528).
References
4
. Conclusions
[
1] C.O. Tuck, E. Pérez, I.T. Horváth, R.A. Sheldon, P.M. Oliakoff, Science 337 (2012)
95–699.
6
In summary, we develop a new Ru/ZnAlZr-LDH catalyst with
[2] M. He, Y. Sun, B. Han, Angew. Chem. Int. Ed. 52 (2013) 9620–9633.
[3] Y. Wang, Y. Miao, S. Li, L. Gao, G. Xiao, Mol. Catal. 436 (2017) 128–137.
an unparalleled activity in the transfer hydrogenation of EL to

1
90
Z. Gao et al. / Molecular Catalysis 442 (2017) 181–190
[
[
[
4] Q. Li, P. Man, L. Yuan, P. Zhang, Y. Li, S. Ai, Mol. Catal. 431 (2017) 32–38.
5] J. Jae, W. Zheng, R.F. Lobo, D.G. Vlachos, ChemSusChem 6 (2013) 1158–1162.
6] G.-H. Wang, J. Hilgert, F.H. Richter, F. Wang, H.-J. Bongard, B. Spliethoff, C.
Weidenthaler, F. Schüth, Nat. Mater. 13 (2014) 293–300.
[34] Z. Yang, Y.-B. Huang, Q.-X. Guo, Y. Fu, Chem. Commun. 49 (2013) 5328–5330.
[35] F. Li, X. Duan, Struct. Bonding. 119 (2006) 193–223.
[36] G. Fan, F. Li, D.G. Evans, X. Duan, Chem. Soc. Rev. 43 (2014) 7040–7066.
[37] P.J. Sideris, U.G. Nielsen, Z. Gan, C.P. Grey, Science 321 (2008) 113–117.
[38] W. Hao, W. Li, X. Tang, X. Zeng, Y. Sun, S. Liu, L. Lin, Green Chem. 18 (2016)
1080–1088.
[
[
7] J. Mitral, X. Zhou, T. Rauchfuss, Green Chem. 17 (2015) 307–313.
8] H. Luo, D.F. Consoli, W.R. Gunther, Y. Román-Leshkov, J. Catal. 320 (2014)
1
98–207.
[39] M.J. Climent, A. Corma, S. Iborra, K. Epping, A. Velty, J. Catal. 225 (2004)
316–326.
[
9] Y. Yang, G. Gao, X. Zhang, F. Li, ACS Catal. 4 (2014) 1419–1425.
[
[
[
10] S. Zhu, Y. Xue, J. Guo, Y. Cen, J. Wang, W. Fan, ACS Catal. 6 (2016) 2035–2042.
11] O.A. Abdelrahman, A. Heyden, J.Q. Bond, ACS Catal. 4 (2014) 1171–1181.
12] V. Mohan, V. Venkateshwarlu, C.V. Pramod, B.D. Raju, K.S.R. Rao, Catal. Sci.
Technol. 4 (2014) 1253–1258.
[40] M. Shirotori, S. Nishimura, K. Ebitani, Catal. Sci. Technol. 4 (2014) 971–978.
[41] E. Li, Z.P. Xu, V. Rudolph, Appl. Catal. B: Environ. 88 (2009) 42–49.
[42] M. Shirotori, S. Nishimura, K. Ebitani, Catal. Sci. Technol. 6 (2016) 8200–8211.
[43] Q. Hu, G. Fan, F. Li, J. Catal. 340 (2016) 184–195.
[44] R. Ionescu, O.D. Pavel, R. Bîrjega, R. Z a˘ voianu, E. Angelescu, Catal. Lett. 134
(2010) 309–317.
[
[
13] D.M. Alonso, S.G. Wettstein, J.A. Dumesic, Green Chem. 15 (2013) 584–595.
14] D. Sun, A. Ohkubo, K. Asami, T. Katori, Y. Yamada, S. Sato, Mol. Catal. 437
(
2017) 105–113.
[45] O.D. Pavel, R. Z a˘ voianu, R. Bîrjega, E. Angelescu, Catal. Commun. 12 (2011)
[
[
15] M. Al-Naji, A. Yepez, A.M. Balu, A.A. Romero, Z. Chen, N. Wilde, H. Li, K. Shih, R.
Gläser, R. Luqueb, J. Mol. Catal. A: Chem. 417 (2016) 145–152.
16] I.T. Horvath, H. Mehdi, V. Fabos, L. Boda, L.T. Mika, Green Chem. 10 (2008)
845–850.
[46] S. Sankaranarayanan, A. Sharma, K. Srinivasan, Catal. Sci. Technol. 5 (2015)
1187–1197.
2
38–242.
[47] C. Rudolf, B. Dragoi, A. Ungureanu, A. Chirieac, S. Royer, A. Nastro, E. Dumitriu,
Catal. Sci. Technol. 4 (2014) 179–189.
[
[
17] Z.-Q. Duan, F. Hu, Green Chem. 14 (2012) 1581–1583.
18] E.I. Gurbuz, J.M.R. Gallo, D.M. Alonso, S.G. Wettstein, W.Y. Lim, J.A. Dumesic,
Angew. Chem. Int. Ed. 52 (2013) 1270–1274.
19] S.G. Wettstein, D.M. Alonso, Y. Chong, J.A. Dumesic, Energy Environ. Sci. 5
[48] A.H. Valekar, K.-H. Cho, S.K. Chitale, D.-Y. Hong, G.-Y. Cha, U.-H. Lee, D.W.
Hwang, C. Serre, J.-S. Chang, Y.K. Hwang, Green Chem. 18 (2016) 4542–4552.
[49] Z. Yang, Y.-B. Huang, Q.-X. Guo, Y. Fu, Chem. Commun. 49 (2013) 5328–5330.
[50] B. Cai, X.-C. Zhou, Y.-C. Miao, J.-Y. Luo, H. Pan, Y.-B. Huang, ACS Sustainable
Chem. Eng. 5 (2017) 1322–1331.
[51] Z. Xue, J. Jiang, G. Li, W. Zhao, J. Wang, T. Mu, Catal. Sci. Technol. 6 (2016)
5374–5379.
[52] Q. Hu, G. Fan, L. Yang, X. Cao, P. Zhang, B. Wang, F. Li, Green Chem. 18 (2016)
2317–2322.
[53] L. Bui, H. Luo, W.R. Gunther, Y. Román-Leshkov, Angew. Chem. Int. Ed. 52
(2013) 8022–8025.
[54] Y. Zhu, G. Chuah, S. Jaenicke, J. Catal. 227 (2004) 1–10.
[55] C. Jiménez-Sanchidrián, J.M. Hidalgo, J.R. Ruiz, Appl. Catal. A: Gen. 303 (2006)
23–28.
[56] Y.-K. Lee, S.T. Oyam, J. Catal. 239 (2006) 376–389.
[57] M. Matsushita, K. Kamata, K. Yamaguchi, N. Mizuno, J. Am. Chem. Soc. 127
(2005) 6632–6640.
[58] M. Kotani, T. Koike, K. Yamaguchi, N. Mizuno, Green Chem. 8 (2006) 735–741.
[59] J.M. Tukacs, D. Király, A. Strádi, G. Novodarszki, Z. Eke, G. Dibó, T. Kégl, L.T.
Mika, Green Chem. 14 (2012) 2057–2065.
[
(
2012) 8199–8203.
[
20] H. Heeres, R. Handana, D. Chunai, C.B. Rasrendra, B. Girisuta, H.J. Heeres,
Green Chem. 11 (2009) 1247–1255.
21] J.Q. Bond, D. Wang, D.M. Alonso, J.A. Dumesic, J. Catal. 281 (2011) 290–299.
22] D.M. Alonso, J.Q. Bond, J.C. Serrano-Ruiz, J.A. Dumesic, Green Chem. 12 (2010)
[
[
9
92–999.
23] J.Q. Bond, D.M. Alonso, D. Wang, R.M. West, J.A. Dumesic, Science 327 (2010)
110–1114.
[
[
1
24] J.-P. Lange, R. Price, P.M. Ayoub, J. Louis, L. Petrus, L. Clarke, H. Gosselink,
Angew. Chem. Int. Ed. 49 (2010) 4479–4483.
25] C. Chang, G. Xu, X. Jiang, Bioresour. Technol. 121 (2012) 93–99.
26] L. Peng, L. Lin, H. Li, Ind. Crops Prod. 40 (2012) 136–144.
27] X. Du, L. He, S. Zhao, Y. Liu, Y. Cao, H. He, K. Fan, Angew. Chem. Int. Ed. 123
[
[
[
(
2011) 7961–7965.
28] L. Deng, J. Li, D.-M. Lai, Y. Fu, Q.-X. Guo, Angew. Chem. Int. Ed. 48 (2009)
529–6532.
[
6
[
[
29] J. Wang, S. Jaenicke, G.-K. Chuah, RSC Adv. 4 (2014) 13481–13489.
30] X. Tang, H. Chen, L. Hu, W. Hao, Y. Sun, X. Zeng, L. Lin, S. Liu, Appl. Catal. B:
Environ. 147 (2014) 827–834.
[60] R. Cohen, C.R. Graves, S.T. Nguyen, J.M.L. Martin, M.A. Ratner, J. Am. Chem. Soc.
126 (2004) 14796–14803.
[
[
[
31] Y. Kuwahara, W. Kaburagi, T. Fujitani, RSC Adv. 4 (2014) 45848–45855.
32] M. Chia, J.A. Dumesic, Chem. Commun. 47 (2011) 12233–12235.
33] J. Song, L. Wu, B. Zhou, H. Zhou, H. Fan, Y. Yang, Q. Meng, B. Han, Green Chem.
[61] K. Yamaguchi, T. Koike, M. Kotani, M. Matsushita, S. Shinachi, N. Mizuno, J.
Chem. Eur. 11 (2005) 6574–6582.
1
7 (2015) 1626–1632.