Cascade catalytic transfer hydrogenation-cyclization of ethyl levulinate to γ-valerolactone with Al-Zr mixed oxides

Article abstract of DOI:10.1016/j.apcata.2015.10.049

A series of Al-Zr mixed oxides with different molar ratios were prepared by co-precipitation method and utilized to catalyze conversion of ethyl levulinate (EL) to γ-valerolactone (GVL). Those as-prepared catalysts were characterized by XRD, SEM, N₂ adsorption-desorption, XPS, TG, NH₃-TPD, CO₂-TPD and pyridine-IR. The introduction of Al into ZrO₂ was demonstrated to enlarge surface areas, as well as increase the number of acid and base sites that were catalytically active for transfer hydrogenation. Effects of Al-Zr molar ratio, calcination temperature, reaction temperature and time, catalyst dosage, and alcohols used as hydrogen donors on the catalytic performance were investigated. A high GVL yield of 83.2% at EL conversion of 95.5% could be achieved at 220 °C in 4 h over Al₇Zr₃-300 using 2-propanol as hydrogen donor and solvent. Poisoning experiments verified that acid-base sites played a synergic role in producing GVL from EL. Moreover, the Al-Zr mixed oxide catalyst was able to be reused for several times with a slight loss in its catalytic activity.

Full text of DOI:10.1016/j.apcata.2015.10.049

Applied Catalysis A: General 510 (2016) 11–19
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Cascade catalytic transfer hydrogenation–cyclization of ethyl
levulinate to ꢀ-valerolactone with Al–Zr mixed oxides
Jian He, Hu Li, Ye-Min Lu, Yan-Xiu Liu, Zhi-Bing Wu, De-Yu Hu, Song Yang∗
Center for Research and Development of Fine Chemicals, State-Local Joint Engineering Laboratory for Comprehensive Utilization of Biomass, State Key
Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering (Ministry of Education), Guizhou University, Guiyang 550025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2015
Received in revised form 30 October 2015
Accepted 30 October 2015
Available online 11 November 2015
A series of Al–Zr mixed oxides with different molar ratios were prepared by co-precipitation method
and utilized to catalyze conversion of ethyl levulinate (EL) to ꢀ-valerolactone (GVL). Those as-prepared
catalysts were characterized by XRD, SEM, N2 adsorption–desorption, XPS, TG, NH3-TPD, CO2-TPD and
pyridine-IR. The introduction of Al into ZrO2 was demonstrated to enlarge surface areas, as well as increase
the number of acid and base sites that were catalytically active for transfer hydrogenation. Effects of
Al–Zr molar ratio, calcination temperature, reaction temperature and time, catalyst dosage, and alcohols
used as hydrogen donors on the catalytic performance were investigated. A high GVL yield of 83.2% at
EL conversion of 95.5% could be achieved at 220 ◦C in 4 h over Al7Zr3-300 using 2-propanol as hydrogen
donor and solvent. Poisoning experiments verified that acid–base sites played a synergic role in producing
GVL from EL. Moreover, the Al–Zr mixed oxide catalyst was able to be reused for several times with a
slight loss in its catalytic activity.
Keywords:
ꢀ-Valerolactone
Ethyl levulinate
Mixed metal oxide
Catalytic transfer hydrogenation
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
The catalytic hydrogenation–cyclization of levulinic acid or its
esters, capable of producing from carbohydrates [26–30], is a key
The depletion of fossil fuel reserves and environmental deteri-
oration have stimulated researchers around the world to search
for renewable resources to produce fuels and chemicals [1,2].
Considerable attention has been laid on the production of value-
added chemicals and biofuels from biomass resources because of its
renewable, abundant and widely distributed properties [3–6]. Var-
ious valuable compounds such as 5-hydroxymethylfurfural (HMF)
[7–10], levulinic acid [11–13], 5-(ethoxymethyl) furfural (EMF)
[14–16], sorbitol [17–18], and ꢀ-valerolactone (GVL) [19] have
been demonstrated to be capable of producing from biomass in
recent years. Among those compounds, GVL with high boiling
point (207 ◦C) and flash point (96 ◦C), as well as low melting point
(−31 ◦C) and vapor pressure (3.5 kPa at 80 ◦C) is considered as a
promising fuel additive [20]. More importantly, a number of value-
added chemicals (e.g., methyl pentenoate [21] and ꢁ-caprolactam
[22]), and bio-based liquid fuels such as 2-methyltetrahydrofuran
[23], valeric esters [24], and long chain liquid alkenes [25] were
able to be produced from GVL.
step in production of GVL from biomass (Scheme 1) [31]. Unlike lev-
ulinic acid, alkyl levulinates possess relatively lower boiling points,
as well as easy recovery and acid-free characteristics. Moreover,
relatively higher yields of alkyl levulinate can be achieved from
acid-catalyzed alcoholysis of carbohydrates [32–35]. In this respect,
the catalytic conversion of alkyl levulinates to GVL seems to be
more attractive, wherein hydrogenation of 2-ketone group to 2-
CH2OH is considered to be an important step, and various hydrogen
sources including hydrogen, formic acid and alcohols are involved
[36,37]. In the atmosphere of hydrogen gas, metal catalysts like Ru
[38–41], Pd [42,43], Ag [44], Cu [45,46], Ni [47,48], Co [49] and Mo
[50] particles have been demonstrated to be efficient for convert-
ing levulinic acid or its esters to GVL. However, some drawbacks
are suffered when H2 is used as the H-donor. For example, the pro-
cedure for catalyst preparation is somehow complicated, as it used
to be reduced at the atmosphere of hydrogen at high temperatures.
Moreover, the storage/usage of H2 is expensive and unsafe. As an
alternative way, more attention has been paid to use formic acid as
H-donor, which is able to be produced from biomass-derived sugars
[51–56]. However, acid-resistant catalysts bearing high selectivity
to decompose formic acid into H2 rather than CO and H2O are highly
demanded. In this case, only limited number of noble or non-noble
∗
Corresponding author. Fax: +86 851 88292170.
E-mail address: jhzx.msm@gmail.com (S. Yang).
http://dx.doi.org/10.1016/j.apcata.2015.10.049
0926-860X/© 2015 Elsevier B.V. All rights reserved.

12
J. He et al. / Applied Catalysis A: General 510 (2016) 11–19
metal-based heterogeneous or homogeneous catalysts can satisfy
these conditions.
Chuandong Chemical Reagent Company. All chemicals were used
as received without further purification.
Recently, the use of alcohols as hydrogen donor for the hydro-
genation/hydrogenolysis of biomass derivatives such as furfural,
HMF, and sugars to produce hydrogenates [57,58] has attracted
a widespread attention, which is mostly ascribed to the sustain-
able catalytic process with alcohols obtainable from biomass [59].
Besides, the carbonyl products (e.g., acetone), which are formed
from oxidation of alcohols, can be separated by simple distillation
and reused elsewhere. Catalytic production of GVL from biomass
derivatives through Meerwein–Ponndorf–Verley (MPV) reduction
using alcohol as hydrogen donor, on the other hand, is highly
chemo-selective reduction of carbonyl groups to alcoholic hydroxyl
groups in organic chemistry [60]. This reaction pathway is gen-
erally called a catalytic transfer hydrogenation (CTH) process. In
comparison to use H2 or formic acid as hydrogen source, the cat-
alytic transfer hydrogenation process offers many advantages. For
example, inexpensive and abundant alcohol can be used as both
hydrogen donor and solvent. What is more, MPV reduction reaction
does not rely on zero valence metal catalysts, especially precious
metals [60]. Various catalysts such as ZrO2 [61,62], Raney Ni [63],
Zr-Beta [64,65], ZrO(OH)2·xH2O [66], in situ generated ZrO(OH)2
[67], Ru(OH)x/TiO2 [68] and Zr-HBA [69] have been explored for
the production of GVL from levulinic acid and its esters. But, it is
still necessary to improve the stability and reactivity of the cata-
lysts. Thus, developing low-cost and more efficient catalysts for GVL
production from levulinic acid and its esters by the CTH reaction is
significant.
Mixed metal oxides are often preferable for heterogeneous
catalysis due to their low cost and easy regeneration features
[70,71]. On the other hand, the physical–chemical properties of
mixed metal oxides such as surface area, thermal and chemical sta-
bility, and acid density are superior to their individual components
[72–76]. Zr-based catalysts were reported to be efficient for MPV
reduction [77–79], and Lewis acid or base sites were demonstrated
to be responsible for the high activity of the reduction reaction
[80,81]. Moreover, increasing acidity or basicity of heterogeneous
catalysts is beneficial for the lactonization reaction to produce GVL
[68,82]. Inspired by above findings, present work aims to incor-
porate aluminum into zirconia by co-precipitation method, so as
to increase the Lewis acid or base contents, thus enhancing the
producibility of GVL from ethyl levulinate. Gratifyingly, the com-
bination of alumina with zirconia increases contents of both acid
sites and base sites and even enlarges specific surface areas of the
obtained catalysts. Importantly, all Al–Zr mixed oxides exhibit bet-
ter catalytic activity than sole ZrO2 and Al2O3, in which the mixed
catalyst with a Zr/Al molar ratio of 3/7 shows the highest perfor-
mance.
2.2. Catalyst preparation
Al–Zr mixed metal catalysts were prepared by a co-precipitation
method with slight modifications [83], In a general procedure,
ZrOCl2·8H2O (18 mmol) and Al(NO3)3·9H2O (42 mmol) were dis-
solved in deionized water (120 mL) at room temperature. To the
resulting transparent solution, aqueous ammonia (25–28%) was
dropwise added under vigorous stirring, and the final pH value of
the slurry was adjusted to 9. The white gel solution was aged at
room temperature for 12 h, then filtered and washed with deion-
ized water until no chloride ion was detected with AgNO3 solution.
The resulting precipitate was dried at 100 ◦C for 10 h, and then cal-
cined at a specified temperature with a temperature gradient of
1 ◦C/min in air for 4 h to afford the final catalyst (7:3). By using the
identical method, Al–Zr mixed oxides with different metal ratios
were prepared, and denoted as AlxZr10−x-T (T on behalf of differ-
ent calcination temperature). For comparison, sole Al2O3 and ZrO2
catalysts were also synthesized in the same procedure.
2.3. Catalysts characterization
Powder X-ray diffraction (XRD) patterns were recorded using
Bruker D8 Advance X-ray diffractometer with Cu K␣ radiation
(ꢂ = 1.54056 nm) and the 2ꢃ angle between 5 and 80◦. Scan-
ning electron microscopy (SEM) images were obtained using
field-emission scanning electron microscopy (FESEM; JEOL, EOL,
JSM-6700F, 5 kV). NH3-TPD and CO2-TPD were performed on
an AutoChem 2920 chemisorption analyzer. The specific surface
areas (Brunauer–Emmett–Teller) of the catalysts were obtained
from nitrogen adsorption–desorption measurements, which were
conducted on a Micromeritics ASAP2020 instrument, and the
mean pore sizes and pore volumes were calculated from the
Barrett–Joyner–Halenda method. Thermogravimetry (TG) analysis
was determined by NETZSCHSTA 429 instrument in the range of
room temperature to 800 ◦C with a heating rate of 10 ◦C/min under
N2 atmosphere (flow rate: 30 mL/min). The property of acid was
measured by FT-IR of adsorbed pyridine, which was performed by
PE Company. XPS was performed on Shandong TRW technology Co.,
LTD. with Thermo ESCALAB 250Xi instrument. The concentration of
Al and Zr in the filtrate after removal of Al7Zr3-300 was measured
by ICP-AES. Elemental analyses (Vario EL III, Elementar) were used
to measure the carbon content of used Al7Zr3-300 catalysts.
2.4. Typical procedure for catalytic conversion of ethyl levulinate
(EL) to GVL
2. Experimental
Catalytic conversion of EL to GVL was performed in a 25 mL
stainless steel autoclave with Teflon lined reactor under magnetic
stirring, and placed in a temperature-controlled oil bath. As a typ-
ical run, 1 mmol EL, 5 mL alcohol as hydrogen donor and solvent,
and 0.0721 g metal oxide catalyst were charged into the reactor,
the vessel was sealed and zero time was taken as soon as the stain-
less steel autoclave placed into the preheated oil bath at a fixed
temperature with stirring. When the reaction finished, the reactor
2.1. Materials
Ethyl levulinate (99%), sec-butyl alcohol (AR, 98%), GVL (98%)
and ZrOCl2·8H2O (98%) were purchased from Shanghai Aladdin
Industrial Inc. Al(NO3)3·9H2O (AR, ≥99%), 2-propanol (AR, ≥99.7%)
and other reagents used in the work were obtained from Chongqing
Scheme 1. The pathway for producing GVL from levulinic acid (LA) or its esters.

J. He et al. / Applied Catalysis A: General 510 (2016) 11–19
13
The injector temperature and detector temperature was 250 ◦C,
270 ◦C, respectively; oven temperature programmed from 60 ◦C
(keeping for 1 min) to 230 ◦C at a rate of 10 ◦C /min and held
for 5 min. n-Butyl alcohol was used as an internal standard in
GC analysis. Liquid products of the reaction solution were iden-
tified by GC–MS (Agilent 6890-5973) with DB-17 capillary column
(60 m × 0.25 mm × 0.25 ␮m).
3. Results and discussions
3.1. Catalyst characterization
XRD patterns of different aluminum zirconium catalysts are
shown in Fig. 1a. For ZrO2-600 samples, a series of sharp crys-
talline peaks occurred at 2ꢃ = 28.23◦, 30.38◦, 31.46◦, 34.48◦, 50.37◦,
60.05◦, indicating that ZrO2-600 exhibits a mixture of monoclinic
(2ꢃ = 28.23◦, 31.46◦), tetragonal (2ꢃ = 30.38◦, 34.48◦) and cubic
phase (2ꢃ = 50.37◦, 60.05◦) [84–87]. In the case of Al2O3-600, three
crystalline peaks around 37.1◦, 46.2◦ and 66.6◦ were observed. A
peak at 2ꢃ = 37.1◦ is assigned to the Al2O3 single phase material,
and two peaks at 46.2◦ and 66.6◦ are characteristic peaks of the
cubic ꢀ-alumina, which is the reflection of X-rays from the (4 0 0)
and (4 4 0) crystal planes, respectively [84–89].
It can be clearly observed that XRD peaks of Al–Zr mixed oxides
are very different from those of single oxides (i.e., ZrO2 and Al2O3),
and only one broad band around 31.7◦ was observed (Fig. 1a), pos-
sibly demonstrating that alumina and zirconia components were
combined uniformly. No apparent diffraction peaks except a wide
band around 31.7◦ was observed in the case of Al7Zr3 catalysts
calcined at different temperatures (Fig. 1b), which indicated the
existence of homogeneous Al–Zr bi-component. Furthermore, SEM
images illustrate that the as-prepared catalysts were distributed
irregularly with particle sizes around 1–2 ␮m (Fig. S1).
Thermal property of Al7Zr3-300 was investigated by TG analysis.
As shown in Fig. S2, the first loss (about 9.38%) below 200 ◦C can
be assigned to water molecules physical adsorbed on the surface of
the catalyst. The second weight loss ranging from 350 ◦C to 500 ◦C
is ascribed to the decomposition of the hydroxyl groups remaining
in the Al–Zr mixed oxide [62,84,90–92].
N2 sorption isotherms of all those as-prepared catalysts exhib-
ited a typical type IV pattern with an H3-type hysteresis loop (Fig.
S3), indicating that irregular mesoporous structures are existent
among those samples. BET surface areas of prepared catalysts were
obtained from N2 adsorption–desorption, as present in Table 1.
The BET surface areas of the Al–Zr mixed oxides calcined at
600 ◦C increase with increasing Al content (Table 1, entries 1–5),
and Al7Zr3 calcined at 300 ◦C has the largest BET surface area
(199.8 m2/g) (Table 1, entry 6). Pore size distributions of those sam-
ples are shown in Fig. S4. It can be observed that the mean pore sizes
Fig. 1. XRD patterns of different aluminum zirconium catalysts calcined at 600 ◦
(a); XRD patterns of Al7Zr3 catalysts calcined at different temperatures (b).
C
was rapidly cooled to room temperature in water bath, and samples
were collected and analyzed by gas chromatography (GC).
2.5. Sample analysis
EL conversions and yields of the product GVL were quantita-
tively analyzed on the basis of standard sample curves by gas
chromatography (GC, Agilent 6890) equipped with FID detector
and HP-Innowax column (30 m × 0.32 mm × 0.25 ␮m). 1 ␮L injec-
tion volume; 1.0 mL/min Nitrogen carrier gas; 20:1 split ratio;
Table 1
BET surface areas, and acid strength and density of as-prepared catalysts.
Entry
Sample
BET surface area (m2/g)a
Acid strength distribution (%)b
Total acid density
(mmol/g)b
<300 ◦
C
300–500 ◦
C
>500 ◦
C
1
2
3
4
5
6
7
8
Al2O3-600
Al7Zr3-600
Al5Zr5-600
Al3Zr7-600
ZrO2-600
Al7Zr3-300
Al7Zr3-450
Al7Zr3-750
147.3
120.7
76.1
50.1
22
199.8
157.3
125.5
65.5
88.4
83.3
91.7
38.7
20
34.5
11.6
16.7
8.3
61.3
80
0
0
0
0
0
0
10.5
0
0.58
0.509
0.36
0.24
0.15
1.98
0.57
0.43
84.6
86
4.9
14
a
The BET surface areas were determined by N2 adsorption–desorption.
Acid strength distribution and total acid density were measured by NH3-TPD.
b

14
J. He et al. / Applied Catalysis A: General 510 (2016) 11–19
and the pore volumes of those as-prepared catalysts range from 3.9
to 11.3 nm and 0.04 to 0.24 cm3 g−1, respectively (Table S1).
XPS spectra of Al (2p) and Zr (3d) for Al7Zr3-600, Al5Zr5-600
and Al3Zr7-600 are illustrated in Fig. S5. Al (2p) spectra of three
catalysts are located at around 74 eV, demonstrating the valence
of aluminum is +3 in the studied Al–Zr nixed oxides [93]. Zr (3d)
spectra of three catalysts located at around 182 eV and 184 eV cor-
responding to Zr 3d5/2 and Zr 3d3/2 confirm the valence of zirconium
is +4 in this Al–Zr mixed oxides [94]. Surface Al/Zr molar ratios
of three different molar ratios samples were measured from XPS
quantitative measurements (Table S1), which are close to theoret-
ical ratios.
The acid strength and density of those as-prepared catalysts
were evaluated by NH3-TPD, as shown in Fig. 2 and Table 1. A blank
NH3-TPD was conducted to calibrate TCD signals (Fig. S6). It can be
obviously seen that all those prepared catalysts mainly possessed
weak and moderate acid sites (Fig. 2 and Table 1), and the acid
density of Al–Zr mixed oxides calcined at 600 ◦C increases with the
increase of Al content (Table 1, entries 1–5). And it is evident to
note that Al7Zr3 calcined at 300 ◦C possesses a higher amount of
acid sites (1.98 mmol/g) than the rest (Table 1, entry 6).
Fig. 3. The adsorbed pyridine-IR spectra of Al2O3-600, Al7Zr3-600 and ZrO2-600
(desorption at 250 ◦C).
The base strength and content of those prepared catalysts
(including a blank one) were also determined by CO2-TPD (Fig. S7).
Similar to acidity, the basicity of those catalysts mainly belongs to
weak and moderate base sites (Fig. S7 and Table S2), and the base
density of the Al–Zr mixed oxides calcined at 600 ◦C increases with
increasing Al content (Table S2, entry 1–5). It is also clear to note
that the Al7Zr3 calcined at 300 ◦C possesses the largest amount of
base sites (1.35 mmol/g) among those samples (Table S2, entry 6).
Thus, the addition of Al is not only in favor of enlarging surface
area but also beneficial to enhance acid and base amount for Al–Zr
mixed oxides calcined at 600 ◦C. And Al7Zr3 calcined at 300 ◦C has
the highest BET surface area (199.8 m2/g) and the largest amount
of acid sites (1.98 mmol/g) and base sites (1.35 mmol/g) of those
catalysts. It is speculated that two reasons may account for Al7Zr3
calcined at 300 ◦C possessing the highest amount of acid sites and
base sites. On the one hand, Al7Zr3 calcined at 300 ◦C bearing higher
surface area is more accessible to NH3 and CO2 gases, leading to the
increase in acid and base density, respectively. On the other hand,
the amount of acid ( OH) and base (O2−) sites was reported to
decrease with the increase of calcination temperatures for sole ZrO2
and Al2O3 [62,95,96], as well as mixed oxides [97,98]. Therefore,
the moderate calcination temperature of 300 ◦C facilitates Al–Zr
mixed oxides to bear more acidic OH groups and basic sites (e.g.,
O2− anions), besides high surface areas, thus rendering the catalyst
more accessible to NH3 and CO2.
It was reported that the MPV reduction could be promoted by
the synergetic effect of Lewis acid and base [99]. Thus, the acid
properties of Al–Zr mixed oxides calcined at 600 ◦C were further
evaluated by FT-IR spectra of pyridine adsorption. As shown in
Fig. 3, the band at 1452 cm−1, which is characteristic of Lewis acid,
can be found in the spectra of those three catalysts. No characteris-
tic band of Bronsted acid, located at 1540 cm−1, was observed; and
the band at 1494 cm−1 further confirmed the presence of Lewis acid
species. Therefore, it can be concluded that those samples calcined
at 600 ◦C contain Lewis acid sites without measurable amounts of
Bronsted acid sites. Notably, the content of Lewis acid is signifi-
cantly enhanced with the addition of Al to ZrO2 (Fig. 3). In addition,
the band around 1618 cm−1, assigned to other pyridine adsorption
peak [100], is also detected.
The acid type of Al7Zr3-300 catalyst was also measured by FT-IR
spectrum of pyridine adsorption (Fig. S8). The band at 1535 cm−1
is characteristic of Bronsted acid, which can be resulted from the
remaining hydroxyl groups of Al7Zr3-300 catalyst. The result is
consistent with the deduction that the moderate calcination tem-
perature of 300 ◦C facilitates Al–Zr mixed oxides to bear more acidic
OH groups and basic sites (e.g., O2− anions), while high calcination
temperature (e.g., 600 ◦C) will decompose the hydroxyl groups, as
demonstrated by TG analysis.
3.2. Effect of the ratio of aluminum zirconium
In order to obtain the appropriate ratio of aluminum to zirco-
nium for efficient production of GVL from EL in 2-propanol, Al–Zr
Fig. 2. NH3-TPD profiles of different aluminum zirconium catalysts calcined at
600 ◦C (a); NH3-TPD profiles of Al7Zr3 calcined at different temperatures (b).

J. He et al. / Applied Catalysis A: General 510 (2016) 11–19
15
Fig. 5. (a) Effect of Al7Zr3 calcined at different temperatures on the catalytic con-
version of EL into GVL. Reaction conditions: 1 mmol EL, 5 mL 2-propanol, 0.072 g
catalyst, T = 180 ◦C, t = 6 h. (b) The GVL yields, selectivities and formation rates of
Al7Zr3 calcined at different temperatures when the EL conversion maintained at
around 10%. Reaction conditions: 1 mmol EL, 5 mL 2-propanol, 0.072 g catalyst,
T = 180 ◦C.
Fig. 4. (a) Effect of different ratios of aluminum to zirconium mixed oxides cal-
cined at 600 ◦C on the catalytic conversion EL into GVL. Reaction conditions: 1 mmol
EL, 5 mL 2-propanol, 0.072 g catalyst, T = 180 ◦C, t = 6 h. (b) The GVL yields, selectivi-
ties and formation rates of different ratios of aluminum to zirconium mixed oxides
calcined at 600 ◦C when the EL conversion maintained at around 10%. Reaction
conditions: 1 mmol EL, 5 mL 2-propanol, 0.072 g catalyst, T = 180 ◦C.
mixed oxides calcined at 600 ◦C were selected for initial evaluation,
and results are given in Fig. 4a. It can be clearly seen that the cat-
alytic performance of Al7Zr3 (56.8% EL conversion and 44.9% GVL
yield) is screened to be superior to the rest counterparts (i.e., Al3Zr7
and Al5Zr5) at 180 ◦C for 6 h. It seems the catalytic activity of those
catalysts is increased with the increase of Al content, which is in
agreement with the increased number of acid and base sites as well
as surface area, as clearly demonstrated in Table 1 (entries 1–5),
Table S2 (entries 1–5), and Fig. S9. For sole Al2O3-600, although the
density and surface area of Al2O3 are higher than others especially
ZrO2-600, the GVL yield cannot compare with others. To shed light
on this phenomenon, the reaction mixtures catalyzed by Al2O3-600
and ZrO2-600 (at 180 ◦C for 6 h) were both detected by GC–MS (Fig.
S10). It was found that Al2O3-600 other than ZrO2-600 was more
prone to facilitate the formation of 2-propyl levulinate (10% vs. 2%
yield). In comparison to EL, 2-propyl levulinate was much more dif-
ficult to be converted into GVL [31], which is also consistent with
the work of Dumesic’s group [61].
Furthermore, yields, selectivities, and formation rates of GVL in
the presence of various mixed oxides with different Al/Zr molar
ratios reacting at 180 ◦C were measured at a consistent EL conver-
sion of around 10%. As can be seen from Fig. 4b, Al7Zr3-600 shows
the highest catalytic activity among those catalysts with respect
to GVL formation rate, yield, and selectivity. It is indicated that the
addition of aluminum into zirconia not only increases the GVL yield
and selectivity but also enhances the GVL formation rate. Therefore,
the Al7Zr3 catalyst was selected for subsequent studies.
3.3. Effect of Al7Zr3 calcined at different temperature
It was reported that morphological structure of zirconia directly
affected its catalytic activity in the production of GVL [62]. Herein,
the catalytic activity of Al7Zr3 calcined at different temperatures
was tested for the catalytic conversion of EL into GVL in 2-propanol
at 180 ◦C for 6 h. It can be seen from Fig. 5a that the calcination
temperature of Al7Zr3 has a significant influence on the synthesis
of GVL, and Al7Zr3 calcined at 300 ◦C exhibited superior perfor-
mance with 82.9% conversion of EL and 67.9% yield of GVL to others,
which is probably related to the catalyst (calcined at 300 ◦C) pos-
sessing relatively higher surface area (199.8 m2/g) and larger acid
(1.98 mmol/g) and base amount (1.35 mmol/g) (Table 1, Table S2,
and Fig. S11).
Similarly, the yields, selectivities, and formation rates of GVL
in the presence of Al7Zr3 calcined at different temperatures react-
ing at 180 ◦C were also measured at a consistent EL conversion of
around 10%, and results are present in Fig. 5b. It can be seen that GVL
yields and selectivities of Al7Zr3 catalysts with different calcination
temperatures were not significantly different when EL conversion
maintained around 10%. However, Al7Zr3 calcined at 300 ◦C showed
the highest GVL formation rate among various Al7Zr3 catalysts,
indicating that appropriate adjustment of acid and base sites of
Al7Zr3 materials could accelerate the reaction rate. Thus, the Al7Zr3
catalyst calcined at 300 ◦C was selected for further studies.

16
J. He et al. / Applied Catalysis A: General 510 (2016) 11–19
Fig. 6. Effect of reaction temperature and time on production of GVL from EL. Reac-
tion conditions: 1 mmol EL, 5 mL 2-propanol, and 0.072 g Al7Zr3-300.
Fig. 7. Effect of different alcohols as hydrogen donors on production of GVL from EL.
Reaction conditions: 1 mmol EL, 5 mL alcohol, 0.072 g Al7Zr3-300 catalyst, T = 220 ◦C,
t = 4 h.
3.4. Effect of reaction temperature and time
S1. The main by-product was identified to be 1 (2-propyl levuli-
nate) formed by transesterification between EL and 2-propanol,
while the intermediate 4-HPE was not detectable, indicating that
the catalytic conversion of 4-HPE into GVL was quite fast. All these
observations are consistent with a previous study by Dumesic et al.
[61]. Meanwhile, other by-products like 2 and 3 (Scheme S1) were
also detected, supporting the proposed reaction routes (Scheme
S1). It was worthy mentioned that the GVL yield increased by only
5.3% when the catalyst dosage further increased (equal to the mass
of EL), indicating that the GVL yield marginally increased but more
catalyst was utilized. From an economical and practical point of
view, the mass ratio of EL and catalyst was set at 2:1 and selected
for subsequent experiments.
The effect of reaction temperature and time on the production
of GVL from EL was studied, and all experiments were conducted
at 180, 200, and 220 ◦C with the reaction time in the range of 1 to
10 h, respectively. The results are shown in Fig. 6, it can be seen that
both reaction temperature and time show a remarkable influence
on the conversion of EL into GVL. For example, only 12.7% yield of
GVL was obtained when the reaction was carried out at 180 ◦C for
1 h; whereas the GVL yield could reach 72.7% after reacting for 10 h.
However, GVL yield was elevated from 50.5% to 83.2% as the reac-
tion temperature increased from 180 to 220 ◦C within 4 h. It seems
that a long time is needed to acquire higher yield when the reac-
tion is conducted at a lower temperature since EL being converted
to GVL proceeds an intermediate ethyl 4-hydroxypentanoate (4-
HPE) through the reversible MPV reduction (Scheme 1); while both
reaction rates and GVL yields are elevated at higher temperatures
(Fig. 6), which may favor the reversible reaction moving to the
right direction. At the same time, it was found that beyond 4 h of
reaction time, the yield of GVL was increased a little. The yield of
GVL was increased by only 4.4% when the time increased from 4
to 10 h. Those observations suggest that the reaction time beyond
4 h hardly affects the GVL yield. From an economical point of view,
220 ◦C and 4 h was thus selected as optimal temperature and time,
respectively, for further investigation.
3.6. Effect of different alcohol as hydrogen donor
It is well known that hydrogen source is an important factor for
the production of GVL from EL through the CTH process. The effect
of various alcohols, including methanol (MeOH), ethanol (EtOH), 2-
propanol (2-PrOH), sec-butyl alcohol (2-BuOH) and cyclohexanol
(CylOH), were thus examined to acquire the optimum hydrogen
donor. As given in Fig. 7, it is clearly found that secondary alco-
hols show the better performance than primary alcohols, which
may be related to the reduction potential of alcohols [66,101].
Only 3.4% yield of GVL was obtained when methanol was used as
hydrogen source. Cyclohexanol tends to adsorb around the active
sites of the mixed oxides, which is prone to interfere the contact
between active sites and EL [101], thus showing inferior EL conver-
sion to 2-propanol and sec-butyl alcohol, but with high selectivity
owing to the less active sites accessible for side reactions. There-
fore, 2-propanol or sec-butyl alcohol was determined to be the best
hydrogen donor in this catalytic system.
3.5. Effect of catalyst dosage
The effect of catalyst dosage on GVL yield from EL was studied
and results are presented in Fig.S12. Those experiments were con-
ducted through changing the mass ratio of EL to the catalyst in the
range 4:1 to 1:2 and other reaction conditions kept constant. Obvi-
ously, EL conversion and GVL yield were increased with increasing
the catalyst dosage at first, which should be mainly attributed to
more available number of catalytically active sites. When the mass
ratio of EL and catalyst decreased from 4:1 to 2:1, the conversion
of EL and yield of GVL increased from 76.3% and 65.8% to 95.5% and
83.2%, respectively. When the mass ratio of EL and catalyst reached
to 1:1, 88.6% yield of GVL at complete EL conversion was achieved.
However, further increase of catalyst dosage led to the GVL yield
decreased to 83.7% although the EL was still completely converted.
It was proposed that the excess use of the catalyst at a high tem-
perature of 220 ◦C would facilitate the formation of by-products as
detected by GC–MS (Fig. S13), thus resulting in the decrease of GVL
yield. Reaction pathways were thus deduced, as shown in Scheme
3.7. Poisoning and leaching experiments
To investigate the effect of Lewis acid and base sites on the pro-
duction of GVL from EL, piperidine and benzoic acid were added
into the reaction system to poison acid [102] and base sites [66],
respectively. As shown in Table S3, it is clearly seen that the addi-
tion of either piperidine or benzoic acid sharply reduces the yield
and formation rate of GVL as well as TOF value, illustrating the syn-
ergic role of acid–base sites of Al7Zr3-300 in the production of GVL
from EL.

J. He et al. / Applied Catalysis A: General 510 (2016) 11–19
17
three successive runs exhibits a lower GVL yield, which may be
resulted from insoluble organics deposited on the surface of the cat-
alyst. Fig. S16 illustrates the TG analyses of fresh Al7Zr3-300, used
Al7Zr3-300 catalysts (after four times) calcined at 300 ◦C for 4 h or
10 h. 24.5% of loss weight in total was observed for used Al7Zr3-
300 catalyst (after four times) calcined at 300 ◦C for 4 h, which was
larger than that of fresh Al7Zr3-300. It was indicated that the deac-
tivation of Al7Zr3-300 after successively used four times could be
ascribed to insoluble organics or polymer deposits on the used cata-
lyst, and calcination of the catalyst for 4 h at 300 ◦C may be not fully
enough to remove the organic matters. By prolonging the calcina-
tion time from 4 to 10 h, the total loss weight of the used Al7Zr3-300
catalyst was less (Fig. S16), which demonstrated that insoluble
organics or polymer deposited on the used catalyst could be signif-
icantly removed after longer calcination time at 300 ◦C. Moreover,
the deduction could be further confirmed by the carbon residues
and BET surface areas of those fresh and used catalysts (Table S4 and
Fig. S17). The concentrations of both acid and base sites of the used
Al7Zr3-300 catalyst after four times calcined at 300 ◦C for 10 h were
evaluated by NH3-TPD and CO2-TPD, respectively (Fig. S18). The
total acid and base density of the regenerated Al7Zr3-300 catalyst
(i.e., the Al7Zr3-300 catalyst was recovered in the fourth cycle and
calcined at 300 ◦C for 10 h) was 1.87 and 1.28 mmol/g, respectively,
which were close to those of the fresh Al7Zr3-300 catalyst (total
acid and base density: 1.98 and 1.35 mmol/g, respectively). There-
fore, carbon deposits, leading to the deactivation of the Al7Zr3-300
catalyst after four successive cycles, could be removed by regen-
eration of the catalyst at a calcination temperature of 300 ◦C for
10 h.
Fig. 8. Recyclability of Al7Zr3-300 on the synthesis of GVL from EL. Reaction condi-
tions: 1 mmol EL, 5 mL 2-propanol, 0.072 g Al7Zr3-300, T = 220 ◦C, t = 4 h.
To verify whether the catalytic process was heterogeneous or
homogeneous, the Al7Zr3-300 catalyst was removed from the reac-
tion solution by filtration after reacting for 2 h at 220 ◦C. The filtrate
without solid catalyst was continued to react at 220 ◦C for another
2–8 h. As shown in Fig. S14, stable yields of GVL (around 49%) were
observed after removal of the Al7Zr3-300 catalyst, while a sharp
increase in GVL yield was detected in the presence of the cata-
lyst for reacting an additional 2 h. Moreover, the ICP analysis of the
solution demonstrates that below 0.5 ppm zirconium and 0.7 ppm
aluminium anions leached into the alcoholic solution. These results
demonstrated that Al7Zr3-300 was heterogeneous in the produc-
tion of GVL from EL.
The catalytic performance of Al7Zr3-300 was further compared
with those of reported catalysts in the production of GVL from
levulinic acid or its esters in alcohols. As shown in Table S5, the
Al7Zr3-300 catalyst was found to exhibit comparable catalytic
activity with other catalysts. In this respect, Al7Zr3-300 prepared
through this cheap and facile approach shows a promising poten-
tial to be used a heterogeneous catalyst for converting biomass
derivatives into GVL.
3.8. Catalyst recycling
It is of great importance and significance to prepare a catalyst
with good recyclability. The catalytic stability of the Al7Zr3-300
catalyst was investigated for the conversion of EL to GVL in 5 mL
2-propanol under the same conditions. After the reaction was fin-
ished, the reused catalyst was collected by centrifugation, washed
with water, ethanol and acetone three times in the ultrasonic
respectively, dried at 100 ◦C for 2 h and subsequently calcined at
300 ◦C for 4 h, then the recovered catalyst was used for the next
run under the identical reaction conditions. The results are pre-
sented in Fig. 8, a visibly decrease in yield of GVL and conversion of
EL are found in four consecutive cycles. It was reported that some
organic compounds, which were considered as by-products, would
be formed in the production of GVL from EL through CTH process
[66]. We inferred that some of insoluble organics or polymers were
generated from those organic compounds including by-products,
unreacted EL, synthesized GVL, and some intermediates through a
series of reactions such as aldol addition and condensation. After
reusing for four consecutive cycles, the regenerated Al7Zr3-300 by
calcination at 300 ◦C for 10 h exhibited GVL yield (82.4%) as high as
the fresh catalyst (83.2%).
4. Conclusions
In this study, a series of Al2O3–ZrO2 catalysts were prepared by
a simply co-precipitation method, and the morphologies and cat-
alytic activities of those prepared catalysts were characterized and
investigated for conversion of EL to GVL. Various reaction param-
eters, such as Al/Zr mole ratios, calcination temperature, reaction
temperature and reaction time, catalyst dosage, and different alco-
hols as hydrogen donor, have been studied. Al7Zr3 calcined at 300 ◦C
was found to exhibit the highest catalytic activity for the produc-
tion of GVL from EL. A high GVL yield of 83.2% with an excellent
EL conversion of 95.5% was obtained when 2-propanol was used
as hydrogen donor at 220 ◦C for 4 h. Notably, a maximum GVL
yield of 88.6% could be achieved while the mass ratio of EL and
Al7Zr3-300 reached 1:1. It could be concluded that the addition of
Al to ZrO2 enlarged the surface area as well as increased the num-
ber of catalytic effective acid and base sites on those catalysts for
producing GVL from EL. Moreover, the increased contents of acid
sites along with the introduction of Al species into mixed oxides
were almost assigned to Lewis acid sites, which could promote the
CTH process for GVL production. Poisoning and leaching experi-
ments verified that the acid–base properties might contribute to
superior activity of Al7Zr3-300 in the production of GVL from EL,
and that the catalytic system was truly heterogeneous. Further-
more, a slight loss on catalytic activity could be observed after
the catalyst was reused four times, which was demonstrated to
be caused by the insoluble organic species deposited on the sur-
To shed light on the deactivation of Al7Zr3-300 after succes-
sively used for four times and its regeneration through calcination
at 300 ◦C for 4 or 10 h, XRD, TG analyses, N2-adsorption–desorption,
elemental analyses and TPD were used to characterize the fresh
and used Al7Zr3-300 catalysts. Fig. S15 shows the XRD patterns of
the fresh and used Al7Zr3-300 catalysts, it is clearly seen that the
Al7Zr3-300 catalyst after one run almost shows no changes in com-
parison with fresh Al7Zr3-300; nevertheless, the Al7Zr3-300 after

18
J. He et al. / Applied Catalysis A: General 510 (2016) 11–19
face of Al7Zr3-300 catalyst, as clarified by XRD, TG analyses, N2
adsorption–desorption, and elemental analyses. Fortunately, the
regeneration of recovered Al7Zr3-300 by calcination at 300 ◦C for
10 h showed comparable activity to the fresh counterpart. There-
fore, the Al–Zr mixed oxide catalysts described herein with simple
and cost-effective preparation procedure, high catalyst activity, and
acceptable reusability show a great potential for production of GVL
from biomass derivatives.
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the Natural Science Foundation of China (No. 21576059) and
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