Conversion of ethyl levulinate to γ-valerolactone catalyzed by the new Zr-containing organic–inorganic hybrid catalysts

Article abstract of DOI:10.1002/jccs.201800020

Three novel heterogeneous Zr-containing organic–inorganic hybrid catalysts were prepared for the transfer hydrogenation of biomass-derived ethyl levulinate (EL) to γ-valerolactone (GVL) with isopropyl alcohol (IPA) as the hydrogen donor and were characterized by Fourier transform infrared (FT-IR), Temperature-programmed desorption (TPD), Thermogravimetry (TG), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). On their basis, the effects of temperature, time, the amount of catalyst, and the solvent were investigated on the yield of GVL. A GVL yield of 96.5% with 99% EL conversion was obtained using Zr-Pg, which was prepared by the reaction of ZrCl₄ and phloroglucinol (Pg) in N,N-dimethylformamide (DMF) at room temperature. The high catalytic activity may be attributed to the strong basicity, which was increased by the sufficient quantity of phenate in the structure of Zr-Pg. Meanwhile, Zr-Pg could be efficiently and easily recycled at least five times without any notable loss of activity and selectivity. Through the results of comparative experiments and characterization, we propose a plausible reaction mechanism for the high activity of Zr-Pg.

Full text of DOI:10.1002/jccs.201800020

Received: 14 January 2018
Revised: 7 May 2018
Accepted: 17 May 2018
DOI: 10.1002/jccs.201800020
A R T I C L E
Conversion of ethyl levulinate to γ-valerolactone catalyzed by the
new Zr-containing organic–inorganic hybrid catalysts
Ruiying Wang1 | Jianjia Wang1 | Huimin Zi1 | Haijun Wang1 | Yongmei Xia2 | Xiang Liu1
1Key Laboratory of Synthetic and Biological
Three novel heterogeneous Zr-containing organic–inorganic hybrid catalysts were
prepared for the transfer hydrogenation of biomass-derived ethyl levulinate (EL) to
γ-valerolactone (GVL) with isopropyl alcohol (IPA) as the hydrogen donor and
were characterized by Fourier transform infrared (FT-IR), Temperature-pro-
grammed desorption (TPD), Thermogravimetry (TG), X-ray diffraction (XRD),
Brunauer-Emmett-Teller (BET), energy dispersive X-ray spectroscopy (EDX),
Colloids, Ministry of Education, School of
Chemical and Material Engineering, Jiangnan
University, Wuxi, Jiangsu, China
2State Key Laboratory of Food Science &
Technology, Wuxi, Jiangsu, China
Correspondence
Haijun Wang, Key Laboratory of Synthetic and
Biological Colloids, Ministry of Education, School
of Chemical and Material Engineering, Jiangnan
University, Wuxi 214122, Jiangsu, China.
Email: wanghj329@outlook.com
scanning electron microscopy (SEM), and transmission electron microscopy
(TEM). On their basis, the effects of temperature, time, the amount of catalyst, and
the solvent were investigated on the yield of GVL. A GVL yield of 96.5% with
99% EL conversion was obtained using Zr-Pg, which was prepared by the reaction
of ZrCl4 and phloroglucinol (Pg) in N,N-dimethylformamide (DMF) at room tem-
perature. The high catalytic activity may be attributed to the strong basicity, which
was increased by the sufficient quantity of phenate in the structure of Zr-Pg. Mean-
while, Zr-Pg could be efficiently and easily recycled at least five times without any
notable loss of activity and selectivity. Through the results of comparative experi-
ments and characterization, we propose a plausible reaction mechanism for the high
activity of Zr-Pg.
Funding information
This work was financial supported by the MOE &
SAFEA for the 111 Project (B13025), Grant/
Award Number: B13025
KEYWORDS
ethyl levulinate, hydrogen donor, transfer hydrogenation, Zr-Pg, γ-valerolactone
produced from cellulose by acid-catalyzed hydrolysis and
alcoholysis.[9]
1
| INTRODUCTION
Biomass as a kind of renewable resources to produce energy
has great prospects for the energy crisis caused by the fast
and huge consumption of fossil fuels.[1,2] The key is the cat-
alytic conversion of biomass into highly valuable platform
chemicals to realize the biorefinery industry.[3] Ligno-
cellulose not only is abundant and low cost but also can
be easily converted to diverse molecules, among which
γ-valerolactone (GVL) has a bright future in industrial appli-
cations.[4]GVL can be used as a fuel additive for gasoline,[5]
a green solvent,[6] and a precursor for the production of other
chemicals.[7] GVL can be obtained from levulinic acid
(LA) or levulinate esters through the hydrogenation
reaction.[8] And LA and levulinate esters can be easily
In the past few years, many processes have been devel-
oped for the conversion of GVL from LA and levulinate
esters. GVL has been synthesized through LA hydrogenation
using various metal catalysts (such as Ru,[10] Pd,[11] Ni,[12]
Cu,[13] Co,[14] Pt,[15] Ir,[16] and Rh[17]) in the presence of
hydrogen. Currently, the main strategies for the production
of GVL are those using formic acid (FA) or alcohols as the
hydrogen source. However, there are some disadvantages
using FA as the H donor, such as the use of precious metals
(e.g., Pd, Pt, Ru, and Au), the use of environmentally
unfriendly solvents, harsh reaction conditions, and the use of
corrosive acids.[18]
The use of alcohols as the H donor to realize the catalytic
transfer hydrogenation (CTH) of LA or its esters to GVL is a
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2018;1–9.
http://www.jccs.wiley-vch.de
1

2
WANG ET AL.
typical Meerwein–Ponndorf–Verley (MPV) reduction.[19]
Many researchers have paid attention to alcohols as
H donors because of their inexpensive, plentiful, environ-
mentally friendly characters and their low reduction poten-
tial.[20] In a short period, the CTH of LA or its esters to
GVL has been mainly based on zirconium as the catalyst,
such as ZrO2,[19] ZrO(OH)2,[21] ZrAlOx,[22] and ZrFeOx
[23]
as heterogeneous catalysts, for the production of GVL.
Recently, some efficient catalysts containing both Zr4+ and
organic acids groups have been reported. Song and
coworkers designed Zr-HBA as the catalyst for the CTH of
ethyl levulinate (EL) and achieved 94.4% yield of GVL with
isopropyl alcohol (IPA) as the hydrogen source.[24] Valekar
and coworkers found that zirconium-based metal–organic
frameworks exhibit high catalytic activity in the presence of
IPA.[25] Kuwahara and workers reported the CTH of EL to
GVL over sulfonic acid-functionalized UiO-66.[26] Xue and
coworkers used the zirconium-cyanuric acid coordination
polymer (Zr-CA) as the CTH reaction catalyst of LA and
obtained almost quantitative yield.[27] The excellent catalytic
activity of these catalysts may mostly be attributed to the
acidic and basic sites in their structure. Generally, the
increase in the catalyst's basicity could benefit the CTH
reaction.[28]
In this work, we synthesized three new organic–
inorganic hybrid Zr-containing catalysts for the CTH of EL
using IPA as the H donor. Among them, Zr-Pg (phlorogluci-
nol) was found to have strong acid–basic pair sites and effi-
cient activity for the conversion of EL into GVL. The highly
active, selective, and stable Zr-Pg for the reaction may be
due to the large number of phenate groups in the structure.
IPA as the H donor exhibited great hydrogen generation
capacity because of its low reduction potential. We selected
EL as the feedstock because the catalysts prepared were
sensitive to acid and did not block the basic sites as LA does.
To the best of our knowledge, there have been no earlier
reports based on Pg, hydroquinone (Hq), and inositol (Is) as
building blocks for this reaction.
FIGURE 1 SEM images of (a) Zr-Pg and (c) Zr-Is. TEM images of (b) Zr-
Pg and (d) Zr-Is
and d). The SEM and TEM of Zr-Hq were also investigated.
(Figure S1, Supporting Information).
2.1.2
| Fourier transform infrared (FT-IR) spectroscopy
The prepared catalysts were characterized by FT-IR spec-
troscopy, and the results are shown in Figure 2. The typical
asymmetric vibrational band at 1,598 cm−1 and the symmet-
ric vibrational band at 1,500 cm−1 of the Zr-Hq phenate
groups are clearly seen, which are narrow compared to those
of Zr-Pg, indicating that there was more coordination
between Zr4+ and the phenate groups in the structure of Zr-
Pg (Figure 2(1) and (2)).[29] The above results also support
the fact that the phenate groups of Pg and Hq were coordi-
nated to the Zr4+ ions successfully. Similarly, the Zr-Is alco-
hol hydroxyl peak at 1,100 cm−1 was smaller than that of Is,
which can be the evidence that Is was coordinated to the Zr4
+
ions successfully (Figure 2(1) and ESI S4(a)). The broad
peak at 3,402 cm−1 was assigned to the stretching vibration
of O–H (which did not participate in the coordination) and
absorbed water.[30] Is, Hq, and Pg were also investigated by
FT-IR (Figure S4).
2
|
RESULTS AND DISCUSSION
2.1.3
| Thermal analysis
Thermogravimetry analysis (TGA) curves of Zr-Pg and Zr-Is
can be used to analyze the stability of the catalyst, as shown
in Figure 3. For the catalyst Zr-Pg, a small weight loss was
noticed in the range 40–100 C, which was due to the
removal of physically adsorbed water on the surface of the
2.1 | Catalyst characterization
2.1.1
| SEM and TEM analysis
ꢀ
Scanning and transmission electron microscopy (SEM and
TEM) were carried out to examine the surface morphology
and internal composition of the prepared catalysts. Typical
surface SEM and TEM images of the Zr-Pg particles are
shown in Figure 1a and b, respectively. It can be observed
that Zr-Pg has no uniform shape and that the gaps between
the aggregated particles result in the porosity of the catalyst.
Meanwhile, SEM and TEM characterization showed that
Zr-Is had spherical shape and a porous structure (Figure 1c
catalyst. Meanwhile, the weight loss at the higher tempera-
ꢀ
ture (100–180 C) is due to the removal of the trapped water
in the catalyst’s pores. Furthermore, fast weight loss of the
ꢀ
catalyst occurred in the temperature range 180–260 C,
which can be attributed to the organic linkers. Also, the cata-
ꢀ
lyst Zr-Pg showed good thermal stability below 180 C
(Figure 3a). It is obvious that the catalyst Zr-Is is very

WANG ET AL.
3
(1)
(2)
(3)
(a)
(b)
(c)
4000
3500
3000
2500
2000
1500
1000
500
20
40
60
80
Wavenumber/cm-1
2-Theta (Degree)
FIGURE 2 FT-IR spectra of (1) Zr-Is, (2) Zr-Hq, and (3) Zr-Pg
FIGURE 4 Wide-angle powder XRD patterns of (a) Zr-Is, (b) Zr-Pg, and
(c) Zr-Hq
unstable compared to Zr-Pg, as can be seen from the curves
(Figure 3b). The reason for this is the ability of Pg to coordi-
nate with metal ions more strongly than Is. The TGA curves
of Zr-Hq were also studied by thermal analysis (Figure S5).
(a)
0.4
105
2.1.4
| Wide-angle X-ray diffraction (XRD) study
0.2
90
The XRD technique was also used to characterize the syn-
thesized catalysts (Figure 4). The diffraction patterns showed
that the catalysts Zr-Pg, Zr-Hq, and Zr-Is had no obvious
crystal structure. The low degree of crystallinity of the Zr-Pg
catalyst resulted from the only weak coordination groups
(phenate) in the structure of the as-prepared compound.
Thus, it can be concluded that there were many defects or
irregular connectivities in Zr-Pg. The pattern showed one
broad diffraction peak at 2θ = 25.0ꢀ, suggesting that Zr-Pg
was amorphous (Figure 4a). Similarly, the curves of Zr-Hq
and Zr-Is provided evidence for their amorphous nature
(Figure 4b and c).
TG
0.0
75
-0.2
-0.4
-0.6
60
DTG
45
30
100
200
300
400
500
600
Temperature (°C)
(b)
2.1.5
| Elemental analysis of the prepared catalysts
0.2
The surface element composition of Zr-Is, Zr-Hq, and Zr-Pg
catalysts were characterized by energy dispersive X-ray
spectroscopy (EDX). The images are shown in Figure 5, in
which the peaks of C, O, and Zr are clearly seen. The results
proved that Zr-Pg possessed a higher amount of Zr and O
than Zr-Is and Zr-Hq. Meanwhile, the amount of Cl in Zr-Pg
was less than that in Zr-Hq. This means that fewer Cl atoms
would affect the reaction when Zr-Pg is used as the catalyst.
The elemental compositions of the zirconium materials were
also examined through inductively coupled plasma-atomic
emission spectroscopy (ICP-AES). The elemental analysis
data are listed in Table S1 (Supplementary Information).
100
DTG
0.0
90
80
70
TG
-0.2
-0.4
-0.6
100
200
300
400
500
600
2.1.6
| N2 sorption studies
Temperature (°C)
The nitrogen adsorption–desorption isotherms and pore size
FIGURE 3 TG-DTG analysis of (a) Zr-Pg (a) and (b) Zr-Is
distributions of Zr-Pg and Zr-Hq samples are shown in

4
WANG ET AL.
(a)
Zr
82
80
78
76
74
72
70
68
66
O
C
Na
Cl
(a)
Zr
O
Cl
C
N
(b)
Zr
C
0.2
0.4
0.6
0.8
1.0
O
Cl
(c)
Relative pressure (p/p0)
(b)
0
200
400
600
800
1000
Binding Energy (ev)
65
60
55
50
45
40
35
FIGURE 5 EDX spectra of (a) Zr-is, (b) Zr-Hq, and (c) Zr-Pg
Figure 6a and b. According to the International Union of
Pure and Applied Chemistry (IUPAC) classification, all
three types of materials have type-IV isotherms and meso-
porous structure (pore diameter between 2 and 15 nm).[31]
Meanwhile, the pore size of the Zr-Pg was well distributed
and centered at ~2.5 nm, while the pore size of the Zr-Is was
distributed and centered at ~7.5 nm (see the inset of
Figure 6). The surface area, average pore diameter, and pore
size distribution are given in Table 1. A decrease in the sur-
face area was observed for Zr-Hq and Zr-Pg from 232.9 to
170.7 m2/g and in pore volume from 0.124 to 0.100 cm3/g,
the but average pore diameter increased from 2.81 to
3.12 Å. In addition, the surface area, pore volume, and pore
size of the catalyst Zr-Is are also displayed. The low BET of
Zr-Is is due to the use of water as the solvent.[32]
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
FIGURE 6 Nitrogen physisorption isotherms of (a) Zr-Pg and (b) Zr-Is
2.1.8
| Conversion of EL to GVL by various catalysts
The effects of different catalysts on the conversion of EL to
GVL were investigated, and the results are shown in
Table 2. It is obvious that EL cannot be transformed without
any catalysts (Table 2, entry 1). Zr-Hq had satisfactory cata-
2.1.7
| Acidity and basicity analysis of Zr-Pg and Zr-Hq
The acidity of Zr-Pg and Zr-Hq was examined by the NH3-
TPD (temperature-programmed desorption) method, and the
results are shown in the Figure 7A. It suggested that Zr-Pg
had higher acidity than Zr-Hq. The Lewis acidic sites mainly
resulted from Zr4+, which was beneficial to the activation of
the carbonyl groups in EL and thus promoted the occurrence
of the reaction.[27] In addition, the results of CO2-TPD char-
acterization suggested that Zr-Pg had much stronger basicity
than Zr-Is (Figure 7B). The strong basicity of Zr-Pg mainly
resulted from the phenate groups, which could be helpful for
the dissociation of IPA to the corresponding hydrogen and
alkoxide, thus promoting the reactivity.[24, 28b] The above
explains the difference in catalytic activity of Zr-Pg and
Zr-Is.
ꢀ
lytic activity for the conversion of EL at 170 C for 7 hr,
with a GVL yield of 90.1% (Table 2, entry 3). Zr-Pg had bet-
ter catalytic activity, mainly attributed to the more phenate
groups in the structure of Zr-Pg compared to Zr-Hq
(Table 2, entry 2). A GVL yield of 72.5% with 83% EL
conversion was achieved by Zr-Is, which may be due to the
TABLE 1 Physical properties of Zr-Hq and Zr-Pg
Sample
type
Surface
Pore
Average pore
diameter (nm)
Entry
area (m2/g)
volume (cm3/g)
1
2
3
Zr-Pg
Zr-Hq
Zr-is
232.9
170.7
41.0
0.124
0.100
0.025
2.81
3.12
18.97

WANG ET AL.
5
TABLE 2 Catalytic activity of different catalysts on the conversion of EL
to GVLa
(A)
40
Temp. Time Conversion Yield
Entry Catalyst
(ꢀC)
170
170
160
170
170
170
150
180
150
(hr)
(%)
(%)
Ref.
1
2
None
Zr-Pg
7
0
0
—
30
20
10
0
7
99
93.8 This work
93.5 This work
90.1 This work
72.5 This work
58.2 This work
11
7
98
3
Zr-Hq
Zr-is
96
4
7
83
(a)
5b
6c
7c
8
ZrCl4
7
97
[33]
ZrO2
16
9
100
99
84.7
[26]
UiO-66-S60
Zr-HA
93.0
(b)
[34]
11
12
1
99.5
100
100
84.2
9d
10Cu-5Ni/Al2O3 150
Zr-PhyA 200
97.0
[35]
[28]
10
98.0
a
Reaction conditions: 0.1 g of EL, 5 mL of IPA, 0.2 g catalyst, 0.17 g
200
300
400
500
naphthalene.
b
c
d
Temperature(°C)
0.03 mmol Zr.
S60: 60 mol fraction of sulfonated ligand in initial gel.
Hydrogen donor: 2-BuOH.
(B)
-0.7
amount of catalyst were investigated for the conversion of
EL to GVL.
-0.6
-0.5
-0.4
-0.3
(a)
(b)
2.2.1
| Effect of the solvent
It is obvious that solvents with different hydrogen donation
abilities performed differently on the conversion of EL to
GVL using the catalyst Zr-Pg. The result was consistent with
the finding that the GVL yield changed obviously when dif-
ferent solvents were used as the reaction media. Therefore,
we studied the impact of different solvents on the GVL yield
using the catalyst Zr-Pg; the results are shown in Table 3.
From the table, we find that the catalyst Zr-Pg shows the
best catalytic activity with IPA as the hydrogen donor under
the same reaction conditions; almost all EL was transformed
with 93.8% GVL yield (Table 3, entry 3). Meanwhile, 93.1%
yield of GVL was achieved when the reaction was carried
out with 3-pentanol, and a 81.5% yield of GVL was obtained
with CyOH (Table 3, entry 4 and 7). From the reactivity and
a practical point view, IPA was selected as H donor for the
reaction. Obviously, 1-BuOH and 2-BuOH were less effi-
cient solvents in this reaction, with the furfural yield of 36.3
and 51.9%, respectively (Table 3, entry 5 and 6). In addition,
200
400
600
800
Temperature(°C)
FIGURE 7 (A) NH3-TPD spectrum of (a) Zr-Pg and (b) Zr-Is.
(B) CO2-TPD spectrum of (a) Zr-Pg and (b) Zr-Is
weaker basicity of Zr-Is (Table 2, entry 4). Meanwhile, the
reaction also occurred with ZrCl4 alone as the catalyst, indi-
cating that free Zr4+ has catalytic activity (Table 2, entry 5).
The prepared catalysts were also compared with other
recently reported catalysts (Table 2, entries 6–10). It could
be concluded that the final GVL yield and conversion of EL
were nearly at the same level or slightly lower than other
reported catalysts.
TABLE 3 Effect of different solvents on the conversion of EL to GVLa
Entry
Solvent
MeOH
EtOH
EL conversion (%)
GVL yield (%)
1
2
3
4
5
6
7
7
9
5.3
8.2
IPA
99
88
56
76
98
93.8
81.5
36.3
51.9
93.1
CyOH
2.2 | Conversion of EL to GVL by Zr-Pg
1-BuOH
2-BuOH
3-Pentanol
Based on the above studies, Zr-Pg showed the best catalytic
activity for the conversion of EL to GVL. Therefore, the
conversion of EL to GVL was sstudied using Zr-Pg. Further-
more, the influence of the solvent, temperature, time, and the
a
Reaction conditions: 0.1 g of EL, 5 mL of solvent, 0.2 g of catalyst,
T = 170 ꢀC, t = 7 hr.

6
WANG ET AL.
100
80
60
40
20
0
90
80
70
60
50
40
30
140 °C
150 °C
160 °C
170 °C
180 °C
0.10
0.15
0.20
0.25
0.30
Amount of catalyst (g)
5
6
7
8
9
FIGURE 9 Effect of the amount of catalyst on EL conversion and GVL
yield. Reaction conditions: 0.1 g of EL, 5 mL of IPA, 0.17 g naphthalene,
T = 170 ꢀC, t = 8 hr
Time (h)
FIGURE 8 Influence of the reaction temperature and time on the
conversion of EL to GVL. Reaction conditions: 0.1 g of EL, 5 mL of IPA,
0.2 g of catalyst, 0.17 g naphthalene
summary, 0.25 g of catalyst was found to be the appropriate
amount under the reaction conditions.
the lowest catalytic activity was seen in GVL yield when
MeOH and EtOH were used as solvents, which was consis-
tent with earlier reported works (Table 3, entry 1 and 2).[35]
2.3 | Reusability of the catalyst
The catalyst Zr-Pg was recovered from the resulting reaction
mixture and washed with an ethanol/water (95:5) system and
2.2.2
| Effect of reaction temperature and time
ꢀ
During the progress of the reaction producing of GVL from
EL, the reaction temperature has a profound effect on the
conversion of EL (140–180 C) and the reaction time also
was important (5–9 hr); the results are shown in Figure 8. In
a mild temperature system at 140 C, EL conversion rate
was relatively low, affording only 59.2% GVL yield for 9 hr
in IPA. Higher GVL yield of 95.5% was obtained with the
increase in reaction temperature and time. The yield of GVL
dried at 60 C in an oven. The reaction temperature and time
ꢀ
were maintained at 170 C and 8 hr, respectively. The cata-
ꢀ
lyst was recycled five times for the conversion of EL to
GVL, with only a slight decrease in the catalytic activity
under the optimum reaction conditions, as shown in
Figure 10. When the catalyst was used for the first time,
97.5% GVL yield was obtained from EL conversion. After
repeated use, the yield of GVL did not significantly
decrease, demonstrating that Zr-Pg is stable under the opti-
mum reaction conditions in this study. Besides, the catalyst
reused five times was characterized by SEM, TEM, FTIR,
and XRD, as shown in Figure 11. As can be seen, the
ꢀ
ꢀ
was nearly independent of the temperature after 170 C and
reaction times beyond 8 hr. IPL (isopropyl levulinate) is the
major side product at lower reaction temperatures. The
improvement of the GVL yield with raising reaction temper-
ature is because IPL undergoes CTH like EL more effec-
tively at higher temperatures. Therefore, 170 C and 8 hr
were selected as the optimum parameters for the reaction.
ꢀ
100
80
60
40
20
0
2.2.3
| Effect of the amount of catalyst
In order to optimize the catalytic activity of Zr-Pg, the influ-
ence of the amount of catalyst on the conversion of EL to
GVL was studied at 170 C for 8 hr. The results are shown
ꢀ
in Figure 9. As can be seen, when a small amount of catalyst
(0.10 g) was added, the conversion of EL and the yield of
GVL reached 92 and 82.7%, respectively. With increasing
amounts of the catalyst, the catalyst activity significantly
improved. The maximum GVL yield of 97.3% was obtained
when the amount of catalyst was 0.25 g. However, the yield
of GVL showed a downward trend when the amount of cata-
lyst was increased further, mainly because the more number
of active sites accelerated the formation of by-products
(mainly isopropyl levulinate) in the reaction process. In
1
2
3
4
5
Run No
FIGURE 10 Reusability of the catalyst on the conversion of EL into GVL.
Reaction conditions: 0.1 g of EL, 0.25 g of catalyst, 5 mL of IPA, 0.17 g
naphthalene, T = 170 ꢀC, t = 8 hr

WANG ET AL.
7
100
95
90
85
80
75
70
(a)
(b)
(a)
(b)
(c)
(d)
Used
Used after five times
Fresh
4
6
8
10
12
Fresh
20
Reaction time (h)
4000
3500
3000
2500
2000
1500
1000
500
40
60
80
Wavenumber/cm-1
2-Theta (Degree)
FIGURE 12 (a) Time versus yield plots for the CTH reaction of EL
catalyzed by Zr-Pg. Reaction conditions: 0.1 g of EL, 0.25 g of catalyst,
5 mL of IPA, T = 170 ꢀC, 0.17 g naphthalene. (b) GVL yields upon
removing the solid catalyst after 6 hr and continued for up to 12 hr
FIGURE 11 (a) SEM and (b) TEM images. (c) FTIR graphs and (d) XRD
patterns of the used catalyst
the carbonyl group of EL as reported on GVL formation over
the Zr-containing catalysts (step 2). Then, EL accepts the
hydrogen from the dissociation of IPA to form 4-HPE
(4-hydroxypetanoate) via the formation of a six-membered
transition (step 3).[28b, 33] Step 4 releases the initial active
sites. Meanwhile, six-membered ring transition state via
Brönsted acid (–OH)-catalyzed intramolecular dealcoholization
can be converted to GVL as the reported GVL formation pro-
cesses (step 5).[26] Thus, the –OH groups present in the frame-
work of Zr-Pg play a critical role in the selection of GVL.
structure and properties of Zr-Pg did not change after five
runs. These results further provided evidence that the cata-
lyst was stable in the reaction system.
2.4 | Leaching and heterogeneity of the catalyst
To study the leaching and heterogeneity of the catalyst,
two reactions were carried out: one was the CTH reaction
of EL always catalyzed by Zr-Pg, and the other in which
the reaction the solid catalyst was removed after 6 hr and
the reaction continued up to 12 hr. The results are shown
in Figure 12. The GVL yield did not further increase after
the catalyst was removed. The conclusion is that the active
sites in Zr-Pg were not leached into the reaction mixture.
Besides, the metal leaching was estimated by ICP-AES
analysis of the mother liquor. The amount of Zr leached
from Zr-Pg (0.25 g) was 1.325 ppm under the reaction
condition in which 0.1 g EL and 5 mL IPA as solvent
3
| EXPERIMENTAL
3.1 | Materials
Inositol (99%), hydroquinone (99%), ZrOCl2 (98%), and
NaOH were purchased from Sinopharm Chemical Reagent
Co. Ltd (Shanghai, China). ZrCl4 (98%) was purchased from
Aladdin Industrial Inc. (Shanghai, China). Pg (99%) was
ꢀ
were used at 170 C for 8 hr. The above can be the evi-
dence that the reaction proceeded with heterogeneous
catalysis.
OH
O
O
O
H
O
Zr
O
O
O
Zr
Zr
2.5 | Reaction mechanism
O
O
Zr
H
O
O
O
O
Step 1
O
O
O
O
Zr
On the basis of the characterization of NH3-TPD and CO2-
TPD techniques and previously reported literature (Figure 6),
we propose the possible reaction mechanism for EL conversion
to GVL in Scheme 1.[24,26,28b,33,34] According to the obtained
experimental results, we conclude that the Zr-Pg catalyst pos-
sesses acidic sites (Zr4+, –OH) and basic sites (O2−). In the first
step, IPA is adsorbed on the Zr-O embedded in the Zr-Pg
framework, and Zr4+–O2− acid–base pair sites interact IPA and
dissociate them to the corresponding alkoxide and hydrogen, in
which the O2− as basic sites are helpful to the deprotonation of
IPA (step 1).[24,26] Subsequently, the acidic sites Zr4+ activate
Zr-Pg
Zr
OH
O
EtOH
O
O
OEt
OEt
Step 2
Step 4
Step 5
O
O
O
+
Intramolecular
delcoholization
O
OEt
O
OEt
O
O
O
O
H
Step 3
O
Zr
O
O
Hydrogen transfer
O
Zr
Zr
O
O
O
Zr
O
H
H
O
O
O
O
O
O
O
O
Zr
Zr
SCHEME 1 Plausible reaction mechanism for EL conversion to GVL
using Zr-Pg as catalyst

8
WANG ET AL.
ꢀ
ꢁ
obtained from Energy Chemical Industrial Inc. (Shanghai,
China). N,N-Dimethylformamide (DMF), triethylamine, and
other solvents were AR grade and were also purchased from
Sinopharm Chemical Reagent Co. Ltd. Deionized water was
Moles of EL
Starting moles of EL
EL conversion ðmol%Þ = 1−
× 100:
ð1Þ
produced with
a laboratory water-purification system
(RO DI Digital plus).
4
| CONCLUSIONS
3.2 | Synthesis of Zr-Pg and Zr-Hq
A series of Zr-containing organic–inorganic hybrid catalysts
were tested for the conversion of EL into GVL in order to
select the best catalytic activity. A GVL yield of 97.3% with
99% EL conversion was achieved under the optimum condi-
tion when the Zr-Pg used as catalyst, which was synthesized
by the reaction of ZrCl4 and Pg. The results of the experi-
ments and characterization indicated that Zr-Pg possessed
both acidic and the basic sites, which explained the excellent
performance of this catalyst. Besides, Zr-Hq and Zr-Is were
synthesized and used to understand the origin of the active
sites. The Zr-Pg catalyst could be recycled at least five times
without any apparent loss of catalytic activity. In addition,
the possible reaction mechanism of the catalyst was pro-
posed. We believe that the catalyst has great potential appli-
cations to be used as a heterogeneous catalyst in the MPV
reduction of various substrates due to their green prepara-
tion, high activity, and good reusability.
Pg (1.0 g; 8 mmol) and ZrCl4 (1.4 g; 6 mmol) were dis-
solved in DMF at room temperature in a 250-mL round
flask. Triethylamine was added dropwise to the mixture until
the solution became clear in 30 min. Then, the mixture was
stirred for 6 hr at room temperature. The mixture gradually
became pink from white and then turned into brown with
stirring. The ꢀbrown mixture was also aged for 3 hr in a water
bath at 60 C. Finally, an earth color precipitate was
obtained by filtration (washed in DMF, ethanol, and ethyl
ꢀ
ether three times, respectively) and dried at 60 C for 12 hr.
The Zr-Hq catalyst with Hq was synthesized using a similar
route as for Zr-Pg.
3.3 | Synthesis of the Zr-Is
The Zr-Is was prepared according to the literature method
with slight modification.[34] Is (0.18 g; 1 mmol) and NaOH
(0.24 g; 6 mmol) were dissolved in 60 mL deionized water.
ZrOCl2ċ8H2O (0.99667 g; 3 mmol) was dissolved in
20 mL deionized water. Then the latter solution was drop-
wise added to the first solution. After that, the mixture was
stirred for 6 hr at room temperature. A white precipitate
could be obtained by centrifugation, which was washꢀed
with water followed by ethanol and dried at 60 C
overnight.
ACKNOWLEDGMENT
This work was financially supported by the MOE & SAFEA
for the 111 Project (B13025).
ORCID
Haijun Wang
http://orcid.org/0000-0003-2857-5861
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How to cite this article: Wang R, Wang J, Zi H,
Wang H, Xia Y, Liu X. Conversion of ethyl levulinate
to γ-valerolactone catalyzed by the new Zr-containing
organic–inorganic hybrid catalysts. J Chin Chem Soc.
2018;1–9. https://doi.org/10.1002/jccs.201800020
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