Transfer hydrogenation of methyl levulinate into gamma-valerolactone, 1,4-pentanediol, and 1-pentanol over Cu-ZrO2 catalyst under solvothermal conditions

Article abstract of DOI:10.1016/j.catcom.2015.12.019

Diverse products were obtained from transfer hydrogenation of methyl levulinate (ML) by adjusting different factors under solvothermal conditions. Excellent yields of gamma-valerolactone (GVL, 75%), 1,4-pentanediol (1,4-PDO, 39%), and 1-pentanol (1-PAO, 13.8%) could be obtained using zirconium-supported copper catalyst calcined at 550 °C and under optimal conditions.

Full text of DOI:10.1016/j.catcom.2015.12.019

Transfer hydrogenation of methyl levulinate into gamma-valerolactone,
1,4-pentanediol, and 1-pentanol over Cu–ZrO2 catalyst under solvothermal
conditions
Yongquan Yang, Xingliang Xu, Weijian Zou, Huijuan Yue, Ge Tian,
Shouhua Feng
PII:
DOI:
Reference:
S1566-7367(15)30173-4
doi: 10.1016/j.catcom.2015.12.019
CATCOM 4536
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
3 November 2015
17 December 2015
18 December 2015
Please cite this article as: Yongquan Yang, Xingliang Xu, Weijian Zou, Huijuan Yue,
Ge Tian, Shouhua Feng, Transfer hydrogenation of methyl levulinate into gamma-
valerolactone, 1,4-pentanediol, and 1-pentanol over Cu–ZrO2 catalyst under solvothermal
conditions, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.12.019
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ACCEPTED MANUSCRIPT
Transfer hydrogenation of methyl levulinate into
gamma-valerolactone, 1,4-pentanediol, and 1-pentanol over
Cu–ZrO2 catalyst under solvothermal conditions
Yongquan Yang, Xingliang Xu, Weijian Zou, Huijuan Yue, Ge Tian, Shouhua Feng
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun, Jilin, People’s Republic of China
Corresponding author:
Ge Tian
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry
College of Chemistry
Jilin University
Changchun
Jilin
People’s Republic of China
Tel.: +86−431−85168604
Fax: +86−431−85168624
E-mail: tiange@jlu.edu.cn
Highlights
1. A simple solvothermal method was introduced into the sequential conversion of methyl
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levulinate (ML).
2. Various yields of diverse products were obtained by tuning a series of factors.
3. Catalyst 2.5-CZ-550 showed a better performance for the conversion of ML, because
of its rich acid content and Cu0 species on the surface.
Abstract
Diverse products were obtained from transfer hydrogenation of methyl levulinate (ML) by
adjusting different factors under solvothermal conditions. Excellent yields of
gamma-valerolactone (GVL, 75%), 1,4-pentanediol (1,4-PDO, 39%), and 1-pentanol
(1-PAO, 13.8%) could be obtained using zirconium-supported copper catalyst calcined at
550°C and under optimal conditions.
Keywords: solvothermal conditions, isopropanol, copper–zirconium catalysts, methyl
levulinate, transfer hydrogenation
1. Introduction
With the decline of fossil fuels, numerous scientists have recently focused their
attention toward seeking new energy resources and strategies to meet the increasing
demand of energy. At the same time, bio-refinery also attracts researchers’ attention. One of
the degradation products of biomass1,2 is levulinic acid (LA), which is a versatile raw
material and platform molecule3 with tremendous market efficiency, because the available
clean products such as gamma-valerolactone (GVL), 1,4-pentanediol (1,4-PDO), and
1-pentanol (1-PAO) are streaming down, with applications in the fields of agriculture,
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cosmetics, and dyestuff.4,5
Synthesis of GVL from LA is the first step of hydrogenation, posing a challenge to
researchers with the combined effect of the carbonyl and ester groups on selective
hydrogenation. The high stability of ring structure in GVL indeed inhibits some
hydrogenation catalysts6; therefore, the yield of 1,4-PDO depends on the activity of
catalysts as well as some strict reaction conditions. Numerous studies have been conducted
since the early years of the 20th century to explore the catalytic process of producing
1,4-PDO from LA and its ester, including homogeneous catalysis7,8 and heterogeneous
catalysis.9–11 However, use of noble metal catalysts is not suitable for commercial
application, because of their high cost. Thus, studies on transition metal oxide catalysts
attracted research attention.12–17 Recently, non-noble metal catalyst Cu–ZrO2 has been
found to exhibit a unique feature for transfer hydrogenation of LA and its ester,15–17 because
of its strong interaction force between Cu and ZrO2, unique structure, and moderate
acid–base property.15,18,19 In this study, which aims at the transfer hydrogenation of methyl
levulinate (ML), we conduct experiments with a special hydrogen source, isopropanol, for
its accessibility and operability in a simple solvothermal treatment20 with Cu-ZrO2,
providing a possible route to produce derived chemicals. It is interesting to note that we
have obtained not only GVL and 1,4-PDO, but also 1-PAO, a critical product undergoing
three steps of hydrogenation. The hypothetical reaction sequence for the conversion of ML
into GVL, 1,4-PDO, and 1-PAO is shown in Scheme 1, where steps 1 and 2 involve copper
coupling effect and transfer hydrogenation, respectively, and step 3 represents diol
hydrogenolysis. It was generally accepted that, through the coupling of dehydrogenation
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and hydrogenation on Cu-based catalysts, a sequence of alcohol was efficiently used as
hydrogen source.21–24 Detailed process parameters on the reaction and the
copper–zirconium catalysts were characterized.
2. Experimental
A detailed description of the materials, synthesis, characterization, catalytic test, and
product analysis is presented in Supporting Information (SI) M1–M5.
3. Results and discussion
3.1 Hydrogenation of ML catalyzed by various Copper–Zirconium (CZ) catalysts
Initially, we assume Cu/Zr molar ratio as a variable and conduct a study on the
catalytic performance of the corresponding R-CZ-T catalysts. (The preparation of catalyst
is shown in SI; we defined our H2-activated catalyst as R-CZ-T, where R denotes the Cu/Zr
molar ratio and T denotes the calcination temperature.) With a reaction temperature of
200°C (a better parameter for 1,4-PDO formation discussed hereinafter) and reaction time
of 12 h, we found that the optimum Cu/Zr molar ratio is 2.5:1 (Table 1, entry 1–5, 8). In
this context, 2.5-CZ-550 had a better performance than other R-CZ-T catalysts, producing
39% yield of 1,4-PDO. Similarly, on the basis of this Cu/Zr molar ratio, a series of
activated catalysts upon different calcination temperatures ranging from 400 to 700°C
(preparation of catalyst shown in SI) were tested (Table 1, entry 6–10). From the yield of
1,4-PDO (Table 1, entry 1–10), we conducted a further study on the catalytic performance
of 2.5-CZ-550 (Section 3.2).
3.2 Effects of the reaction conditions
In order to investigate the maximum yield of single product, tunable factors (e.g., reaction
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temperature, reaction time, and H-donor) were taken into account. The optimum reaction
time for GVL, 1,4-PDO, and 1-PAO was 12 h, and their corresponding optimum reaction
temperatures were 120, 200, and 240°C, found by solvothermal methods (Fig. S1 (a–c) SI
M6, Table 1, entry 8, 11–14). When the temperature was increased to 240°C, the number of
by-products formed, such as 1,7-octanediol and gamma-octanoic lactone, increased,
thereby resulting in a sharp decrease in the yield of the main product. Under optimum
reaction conditions, GVL, 1,4-PDO, and 1-PAO could be achieved with yields of 75%,
39%, and 13.8%, respectively. During hydrogenation, trials on different H-donors were
conducted, and the result is shown in Table 1 (entry 8, 15–20). It can be concluded that
secondary alcohol is a more suitable substitute for H2 than primary alcohol. To the best of
our knowledge, isopropanol performs better than other H-donors, because of its better
utilization of H atoms and good H-balance during the catalytic process.25,26
3.3 Catalyst characterization
Results of characterizations are displayed in SI M6.
X-ray diffraction (XRD) patterns of Cu–ZrO2 catalysts calcined at elevated
temperatures from 400 to 700°C and the corresponding H2-activated Cu–ZrO2 (R-CZ-T)
catalysts are depicted in Figure 1. After exposure to H2, the appearance of indexed
diffraction lines at 2θ = 43.5° (111), 50.6° (200), and 74.3° (220) indicates the presence of
the crystalline phase of metallic Cu in Figure 1(B). A weak diffraction peak of Cu2O (2θ =
36.5°) was found, partly due to slight oxidization of the sample, matching the results of
X-ray photoelectron spectroscopy (XPS) analysis data in Figure S2. A progressive
transformation of the crystalline phase of ZrO2 was identified, from amorphous ZrO2
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(A-ZrO2) to tetragonal ZrO2 (T-ZrO2) and finally monoclinic ZrO2 (M-ZrO2).18 It is
obvious that only weak and broad diffraction peaks of CuO in Figure 1(A) can be observed
for the catalyst calcined at 400 and 500°C; however, when the temperature was increased to
550°C, the peaks become strong and narrow, and simultaneously, diffraction peaks of
T-ZrO2 (2θ = 30.2° and 60.4°) appear. When the temperature was further increased to
600°C, the peaks become more stronger and A-ZrO2 completely transforms into T-ZrO2,
and, to some degree, the sample confronts sintering after further calcination at 700°C. As
shown in Table 2, with increasing calcination temperature from 400 to 700°C, the BET
surface areas of the Cu–ZrO2 catalysts decreased sharply from 61 to 28 m2 g−1, which
confirmed that calcination temperature affects specific surface area of
coprecipitation-derived Cu-ZrO2 materials.18 The results presented in Figure 1 and Table 2
confirm that, as the temperature increases, crystallinity increases with the decrease of BET
surface areas of the samples. Thus, comprehensive factors will be considered because the
properties of surface areas and crystallinity significantly affect the activity of Cu–ZrO2
catalysts.
Figure S2 presents the NH3-TPD profiles of various activated catalysts. The figure
indicates that 2.5-CZ-400 and 2.5-CZ-500 exhibit an abundant weak acidic site at around
223°C and an incisive strong acidic site at around 609°C, whereas 2.5-CZ-600 and
2.5-CZ-700 exhibit a lower amount of weak acidic site and a medium strong acidic site at
around 337°C. Because of the higher calcination temperature, acidic sites would transit and
sintering finally results in loss of acid content as well as surface area.27 It is surprising that
2.5-CZ-550 possesses the most abundant acidic sites (Table S1) and more acid content
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(Table 2) than other R-CZ-T samples. The rich acidic sites and acid content in the
copper-containing catalysts have a high promotion on the hydrogenolysis mechanism of
lactone and some categories of esters.17,28
XPS analysis data of 2.5-CZ-T are shown in Figure S3(a–e). From the Cu 2p spectra,
probably because of its high activity on surface, in addition to Cu0 (around 932.3 eV29),
Cu2O (933.5 eV29) and CuO (935.5 eV30) passivation layers were detected. Data on the
fraction of Cu species (Table 2) were in good agreement with the catalytic performance of
2.5-CZ-T (Table 1, entry 6–10).
Inductively coupled plasma (ICP) spectroscopy analysis of 2.5-CZ-T samples (Table
S2) reflects the experimental Cu/Zr molar ratio. H2-TPR (temperature-programmed
reduction) and thermogravimetric/differential thermal analysis (TG-DTA) results of
2.5-CZ-550 catalyst are shown in Figure S4 and S5, respectively. It is evident from the
figure that the H2-TPR profile of the catalyst exhibited a broad band of H2 consumption at
207°C, because of the sequential reduction of CuO to Cu via Cu2O. No distinct
decomposition was found before 900°C (Fig. S5 (A)), and an exothermic peak was
observed at around 180°C for the process of conversion of Cu and Cu2O to CuO in DTA.
Finally, catalytic performance, XRD patterns, and XPS analysis of the recycled 2.5-CZ-550
catalysts were tested and the results are shown in Figures S6–S8, respectively. Although
encountering inevitable leaching of copper particles, CZ catalyst still exhibited a good
performance, producing 17% yield of 1,4-PDO in the third run at 200°C.
4. Conclusions
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Cu–ZrO2 catalyst calcined at 550°C with 2.5:1 Cu/Zr molar ratio exhibits a higher activity
for transfer hydrogenation of ML, because it possesses more abundant acid content (1.603
mmol/g) and Cu0 species (28%) than other CZ catalysts. By tuning a series of factors (e.g.,
reaction temperature, reaction time, and H-donor), various yields of the diverse products
were obtained. As a H-donor, isopropanol becomes the best substitute for H2, making the
reaction more operable with 2.5-CZ-550 catalyst. Further experiments to understand the
mechanism and improve the yields of main product under relatively mild solvothermal
conditions are ongoing.
Acknowledgments
This study was supported by grants from the National Natural Science Foundation of
China (No. 20901026 and 21401070), Youth Foundation of Jilin Province (No.
20130522128JH and 20140520078JH), and S&T Development Program of Jilin Province
(Nos. 20140101115JC).
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Scheme:
Scheme 1 Hypothetical reaction sequence for the conversion of ML into GVL, 1,4-PDO,
and 1-PAO over Cu–ZrO2 catalyst
Figure:
Fig.1 XRD patterns of (A) 2.5-CZ catalysts after calcination and (B) further exposure to H2
List of tables:
Table 1 Hydrogenation of ML over various R-CZ-T catalysts
Table 2 Physicochemical properties of 2.5-CZ-T catalysts calcined at different temperatures
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OH
O
O
O
O
H
+
O
GVL
step 1
O
ML
s
OH
H
t
e
p
+
Cu-ZrO2
step 3
2
O
OH
OH
OH
H
+
O
OH
1-PAO
1,4-PDO
Scheme 1 The hypothetical reaction sequence for the conversion of ML into GVL, 1,4-PDO
and 1-PAO over Cu-ZrO2
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Fig.1 XRD patterns of the (A) 2.5-CZ catalysts after calcination and (B) further exposure to
H2(2.5-CZ-T catalysts)
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Table 1 Hydrogenation of ML over various R-CZ-T catalystsa
Entry
Sample
Solvent
T [°C]
Yield（%）
GVL
54
55
46
53
0
1,4-PDO
1-PAO
1
Cu-550
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
2-PrOH
MeOH
200
200
200
200
200
200
200
200
200
200
80
19
32
34
28
0
0
0
2
6-CZ-550
3
2-CZ-550
0
4
1-CZ-550
0
5
ZrO2-550
0
6
2.5-CZ-400
2.5-CZ-500
2.5-CZ-550
2.5-CZ-600
2.5-CZ-700
2.5-CZ-550
2.5-CZ-550
2.5-CZ-550
2.5-CZ-550
2.5-CZ-550
2.5-CZ-550
2.5-CZ-550
41
41
47
50
61
0
32
31
39
37
17
0
0.5
0.6
1.1
0.3
0
7
8
9
10
11b
12
13
14
15
16
17
18
19
20
0
120
160
240
200
200
200
200
200
200
75
55
10
14
64
49
40
55
0
0
0
34
9
0
13.8
0
0
EtOH
7
0
1-PrOH
7
0
2.5-CZ-550 1-BuOH
2.5-CZ-550 2-BuOH
0
0
27
0
0
2.5-CZ-550
H2O
0
a 100% ML conversion, reaction conditions: solvent 20 mL, ML 0.1 mL, catalyst 0.2 g,
reaction time 12 h
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b 0% ML conversion
Table 2 Physicochemical properties of 2.5-CZ-T catalysts calcined at different temperatures
Entry
Sample
SBET
Vporc
Acid
content
Binding Energy (eV)
& fraction of Cu
(m2/g)
(cm3/g)
(mmol/g)a species (%)b
1
2
3
2.5-CZ-400
2.5-CZ-500
2.5-CZ-550
61
59
55
0.088
0.096
0.137
1.453
1.561
1.603
933.5 (55)
935.5 (45)
933.5 (55)
935.5 (45)
932.4 (28)
933.5 (40)
935.5 (32)
932.3 (20)
933.5 (48)
935.3 (32)
933.5 (100)
4
5
2.5-CZ-600
2.5-CZ-700
41
28
0.147
0.156
0.742
0.716
a Total acid content was estimated by NH3-TPD test.
b Binding energy and fraction of Cu species were tested by XPS analysis.
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Graphical abstract
Relatively mild and operational solvothermal conditions were used for hydrogenation of
ML to GVL, 1,4-PDO, and 1-PAO over zirconium-supported copper catalysts.
16