Highly efficient transformation of Γ-valerolactone to valerate esters over structure-controlled copper/zirconia catalysts prepared via a reduction-oxidation route

Article abstract of DOI:10.1016/j.apcata.2017.06.032

Design and development of novel and efficient catalysts are crucial but challenging for the catalytic conversion of biomass and derivatives to fuels and chemicals. In this paper, a novel separate nucleation and aging steps assistant reduction-oxidation strategy was developed to synthesis CuO/ZrO₂ complex precursor with homogeneously distributed Cu and Zr components, which can be used as an ideal precursor for the synthesis of highly dispersed Cu/ZrO₂ catalyst. Characterization results revealed that homogeneous dispersion of CuO, high surface area of ZrO₂ support with controlled porous structure, and strong interaction between CuO and ZrO₂ in CuO/ZrO₂ precursor could lead to the enhanced Cu dispersion and the formation of Cu⁺ active centers. The synthesized Cu/ZrO₂ catalysts exhibited excellent catalytic performance (85.4% conversion of GVL and 98.0% selectivity of pentyl valerate) in the catalytic transformation of GVL to valerate esters, more efficient than that of Cu/ZrO₂-CP and Cu/ZrO₂-CH catalysts prepared via co-precipitation and chemisorption hydrolysis methods, respectively. The superior catalytic performance was mainly attributed to both the cooperation of Cu⁰ and Cu⁺ species and the highly dispersed surface Cu⁰, thereby improving the adsorption and polarization of C[dbnd]O bond in GVL and the following dissociation of H₂ to produce active hydrogen for the hydrogenation step during the catalytic transformation of GVL. Moreover, such copper-based catalysts exhibited potential applications in the exploitation and utilization of biomass resources with significantly enhanced efficiency.

Full text of DOI:10.1016/j.apcata.2017.06.032

Accepted Manuscript
Title: Highly efficient transformation of ␥-valerolactone to
valerate esters over structure-controlled copper/zirconia
catalysts prepared via a reduction-oxidation route
Authors: Shanshan Liu, Guoli Fan, Lan Yang, Feng Li
PII:
S0926-860X(17)30281-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.06.032
APCATA 16294
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
16-3-2017
16-6-2017
22-6-2017
Please cite this article as: Shanshan Liu, Guoli Fan, Lan Yang, Feng Li, Highly
efficient transformation of ␥-valerolactone to valerate esters over structure-controlled
copper/zirconia catalysts prepared via a reduction-oxidation route, Applied Catalysis
A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.06.032
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Highly efficient transformation of γ-valerolactone to valerate esters
over structure-controlled copper/zirconia catalysts prepared via a
reduction-oxidation route
Shanshan Liu, 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,
P.O. Box 98, No.15 Beisanhuan Donglu, Beijing 100029, P.R. China. Tel.: 8610-64451226;
Fax: 8610-64425385.
Email：fangl@mail.buct.edu.cn (G.Fan); lifeng@mail.buct.edu.cn (F.Li)
Graphical abstract
1

Highlights
 Novel reduction-oxidation route for the synthesis of Cu/ZrO2 catalysts is established
 Metal-support interaction is responsible for the formation of surface Cu+
 Porous structure of ZrO2 support inhibits the aggregation of Cu species
 Cooperation of Cu0 and Cu+ is crucial for achieving high catalytic performance
Abstract
Design and development of novel and efficient catalysts are crucial but challenging for
the catalytic conversion of biomass and derivatives to fuels and chemicals. In this paper, a
novel separate nucleation and aging steps assistant reduction-oxidation strategy was developed
to synthesis CuO/ZrO2 complex precursor with homogeneously distributed Cu and Zr
components, which can be used as an ideal precursor for the synthesis of highly dispersed
Cu/ZrO2 catalyst. Characterization results revealed that homogeneous dispersion of CuO, high
surface area of ZrO2 support with controlled porous structure, and strong interaction between
CuO and ZrO2 in CuO/ZrO2 precursor could lead to the enhanced Cu dispersion and the
formation of Cu+ active centers. The synthesized Cu/ZrO2 catalysts exhibited excellent
2

catalytic performance (85.4% conversion of GVL and 98.0% selectivity of pentyl valerate) in
the catalytic transformation of GVL to valerate esters, more efficient than that of Cu/ZrO2-CP
and Cu/ZrO2-CH catalysts prepared via co-precipitation and chemisorption hydrolysis methods,
respectively. The superior catalytic performance was mainly attributed to both the cooperation
of Cu0 and Cu+ species and the highly dispersed surface Cu0, thereby improving the adsorption
and polarization of C=O bond in GVL and the following dissociation of H2 to produce active
hydrogen for the hydrogenation step during the catalytic transformation of GVL. Moreover,
such copper-based catalysts exhibited potential applications in the exploitation and utilization
of biomass resources with significantly enhanced efficiency.
Keywords: copper/zirconia catalyst; reduction-oxidation route; metal-support interaction;
valerate esters; biomass-conversion.
1. Introduction
Urged by the decreasing fossil resources and growing environmental concerns,
tremendous efforts have been devoted to exploiting economically efficient and environmental
friendly technologies to use biomass as feedstock for the production of bio-based platform
molecules and convert these molecules into desired value-added chemicals [1-4]. Among these
molecules, γ-valerolactone (GVL), a colorless liquid with a sweet and herbaceous odor,
obtained from C6 and C5 sugars through levulinic acid (LA) intermediate [5-12], has been
identified as a promising fuel additive; meanwhile, it can also be used as one of the most
promising platform intermediate to produce biofuels [13-15], solvents [16-18], polymers [19,
20], and fine chemicals [21-24].
Among the fine chemicals generated by the catalytic transformation of GVL, alkyl
valerates have been extensively studied and used as oxygenic fuel components for gasoline
and diesel fuels because of their unique properties such as low polarity, higher energy density,
and less miscible with water [5, 25]. Lange et al did some pioneering works and reported that
3

GVL can be converted directly into pentyl valerate over Pt/TiO2 or Pd/TiO2 catalyst at 275-
300 °C with unsatisfactory selectivity [26]. Sun et al reported an efficient bi-functional base-
metal catalyst (Co/HZSM-5) for the conversion of GVL in ethanol solvent with ethyl valerate
selectivity up to 64.2% [27]. Recently, Cu-based catalysts, a kind of candidates for the
substitution of noble-metal catalysts, have exhibited extraordinary catalytic performance in
the catalytic transformation of GVL. For example, Ravasio et al developed a chemisorptions-
hydrolysis method for the preparation of Cu/SiO2 catalysts and tested their catalytic activity
for one-pot transformation of GVL into pentyl valerate at 250 °C using pentanol as solvent.
They found that small Cu particles were acted as Lewis acid sites to activate GVL and were
primarily responsible for the excellent catalytic performance with GVL conversion over 91%
and pentyl valerate selectivity up to 92 % [28]. Chan-Thaw et al explored Cu/SiO2-ZrO2
catalysts and investigated their catalytic performance for the production of pentyl valerate and
ethyl valerate from GVL. They found that the use of ethanol as solvent resulted the extra
formation of ethyl 4-ethoxy pentanoate and pentenoic esters byproducts, thus leading the low
selectivity of ethyl valerate [29]. Up to now, the overall yield of valerate esters over Cu-based
catalyst is still low and many processes are required to be integrated urgently.
In the past decades, Cu-based catalysts have been prepared by various methods including
oxalate gel-coprecipitation method [30], co-precipitation [31, 32], impregnation [33] and
chemisorption hydrolysis [34]. Despite the above-mentioned methods which were used to
prepare Cu-based catalyst with improved catalytic performance, in most cases, their synthetic
process often suffers from drawbacks such as low time efficiency, complicated fabrication
process, high fabrication cost, use of toxic reagent, and poor controllability on the dispersion
and durability of Cu nanoparticles, etc. Therefore, it is urgent to design and develop novel
strategies for the construction of highly dispersed Cu-based catalyst with enhanced catalytic
activity and stability.
As one of the most widely used catalyst supports, ZrO2 has a high surface area for the
dispersion of metal nanoparticles, a porous structure for the diffusion and transfer of reactants
and products, and a large amount of surface Zr4+-O2- acid-base centers and oxygen vacancies
for tailoring the catalytic performance [35-38]. Therefore, adjustments and optimizations of
the textural properties of ZrO2, as well as the metal-support interaction between Cu and ZrO2
4

support, are key factors to control the dispersion, surface chemical state, and durability of Cu
active components, which are essential to enhance the catalytic performance and economic
feasibility of the Cu/ZrO2 heterogeneous catalyst.
Based on the above consideration, we herein developed a significant strategy for the
synthesis of Cu/ZrO2 catalysts with highly dispersed Cu nanoparticles and large amount of
Cu+ catalytic centers through the reduction of evenly distributed CuO/ZrO2 complex precursor,
which was prepared by a novel separate nucleation and aging steps assistant reduction-
oxidation route. This strategy exhibits remarkable merits: i) basic and reductive reaction
conditions supplied by NaBH4 and unique reduction-oxidation route can synthesize ZrO2 and
CuO synchronously, simplifying the preparation process; ii) explosive nucleation process in
the colloid mill results in the formation of ultrafine nuclei, H2 bubble generated by the
hydrolysis of NaBH4 in the aging step maintains the reaction system in a sustained turbulent
state, and the unique reaction condition is benefit for the formation of CuO/ZrO2 precursor
with homogeneously distributed Cu and Zr components; iii) homogeneous dispersion of CuO,
strong interaction between CuO and ZrO2, and porous structure of ZrO2 support inhibit the
aggregation of Cu species and lead to the formation of Cu+ active centers during the reduction
process. Herein, the initial NaBH4 concentration plays an important role in determining the
dispersion of Cu nanoparticles, surface areas of Cu0, surface Cu+/(Cu0 + Cu+) molar ratio,
surface acidity, and textural properties of ZrO2 support. The correlation between the structure
and catalytic performance of the prepared Cu/ZrO2 catalyst was studied preliminarily in the
transformation of GVL to corresponding alkyl valerates. It was found that the surface Cu+
acted as active centers for the absorption and polarization of the carboxylic group in GVL, Cu0
undertakes the dissociation of H2 to produce active hydrogen for the hydrogenation step, and
the cooperation of Cu+ and Cu0 is mainly responsible for its excellent catalytic performance.
Furthermore, as formed Cu/ZrO2 catalyst also showed high efficiency in the one-pot
conversion of levulinic acid (LA) to pentylvalerate.
2. Experimental
2.1 Synthesis of catalyst precursors
5

A series of CuO/ZrO2 precursors were prepared by a novel separate nucleation and aging
steps assistant reduction-oxidation route. Typically, solution A: 0.5209 g Cu(NO3)2·3H2O and
4.2933 g Zr(NO3)4·5H2O were dissolved in 40 mL of deionized water to give a mixed salt
solution. Solution B: NaBH4 was dissolved in 40 mL of deionized water to form another
solution with a designed NaBH4/(Cu + Zr) molar ratio of x (x = 10, 15, 20, 25). Solutions A
and B were simultaneously added to a colloid mill with the rotor speed set at 4000 rpm and
mixed for 5 min. Then, the mixed slurry was sealed in a Teflon-lined autoclave and aged at
150 °C for 24 h, respectively. Subsequently, the resulting suspension was washed thoroughly
with deionized water until pH = 7.0, and dried at 70 °C for 24 h. The obtained precursors were
denoted as CuO/ZrO2-x, in which x represents the molar ratio of NaBH4/(Cu + Zr).
For comparative study, CuO/ZrO2-CP and CuO/ZrO2-CH precursors were also prepared.
As for CuO/ZrO2-CP, 0.5209 g Cu(NO3)2·3H2O and 4.2933 g Zr(NO3)4·5H2O were dissolved
in deionized water to give a mixed salt solution, and the above salt solution was titrated with
Na2CO3 (0.2 M) solution under vigorous stirring at room temperature until pH = 10.0. The
resulting suspension was aged at 60 °C for 10 h, and then centrifuged and washed with
deionized water until pH = 7.0. The solid was dried at 70 °C for 24 h, and calcined in air at
400 °C for 4 h. CuO/ZrO2-CH was prepared by a chemisorption hydrolysis method. Typically,
1 g commercial ZrO2 was ultrasonically dispersed into a 250 mL 0.007M [Cu(NH3)4]2+
solution, aged at 0 °C for 6 h, and then slowly diluted with 100 mL deionized water. The
obtained suspension was separated by centrifugation, washed with deionized water, dried at
70 °C for 24 h, and calcined in air at 400 °C for 4 h. In addition, CuO/SiO2-CH, CuO/MoO3-
CH, and CuO/Al2O3-CH precursors were also prepared by the above chemisorption hydrolysis
method using commercial acidic SiO2, MoO3, and Al2O3 as support under the similar reaction
conditions, respectively.
2.2 Synthesis of Cu-based heterogeneous catalysts
The above synthesized catalyst precursors were placed in a tube-furnace reactor, and
reduced in 10 % H2/N2 atmosphere at 300 °C for 2 h at a ramp rate of 2 °C min-1. The catalysts
obtained from CuO/ZrO2-x, CuO/ZrO2-CP, CuO/ZrO2-CH, CuO/SiO2-CH, CuO/MoO3-CH,
and CuO/Al2O3-CH were denoted as Cu/ZrO2-x, Cu/ZrO2-CP, Cu/ZrO2-CH, Cu/SiO2-CH,
6

Cu/MoO3-CH, and Cu/Al2O3-CH, respectively. All the catalysts were designed with an ideal
Cu loading of 10 wt%.
2.3 Characterization
Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance, at
room temperature using Cu-Kα radiation (40 kV, 30 mA, and Kα =1.5418 Å) between 20° and
80° with a scanning rate of 10° min-1.
Elemental analysis for metal ions in samples was performed using a Shimadzu ICPS-
7500 inductively coupled plasma atomic emission spectroscope (ICP-AES).
Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vector 22
spectrometer using the KBr pellet technique (1mg of sample in 100 mg of KBr).
Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were
carried out on a JEOL 2100 operated at an accelerating voltage of 200 kV. High-angle annular
dark-field scanning TEM-energy-dispersive X-ray spectroscopy (HAADF–STEM–EDS)
images were recorded on a JEOL2010F instrument combined with an X-ray energy dispersive
spectroscopy (EDS) system.
Low-temperature N2 adsorption-desorption measurements were carried out at 77 K using
a Micromeritics ASAP 2020 instrument. The Brunauer-Bmmett-Teller (BET) and nonlocal
Density Functional Theory (NL-DFT) models were used to determine the total specific surface
area and the pore size distribution of the samples, respectively.
X-ray photoelectron spectroscopy (XPS) was recorded on a Thermo VG ESCALAB250
X-ray photoelectron spectrometer using Al Kα X-ray as the excitation source. The binding
energies were calibrated based on the graphite C1s peak at 284.6 eV. X-ray induced Auger
spectra (XAES) were carried out on a PHI Quantera SXM using Al Kα X-rays as the excitation
source.
Hydrogen temperature-programmed reduction (H2-TPR) and N2O titration were
performed on a Mircomeritics ChemiSorb 2720, and a thermal conductivity detector (TCD)
was used to determine the amount of hydrogen and N2O consumption during the test. 40 mg
catalyst precursor was placed in a quartz tube and degassed in Ar atmosphere at 200 °C for 2
7

h. The catalyst precursor was then heated in 10 % H2-Ar at a rate of 30 mL min-1 with a heating
rate of 10 °C min-1 from room temperature up to 800 °C. The active copper surface area was
measured by the N2O oxidation method. Firstly, the catalyst precursor was reduced at 300 °C
for 4 h under 10 % H2-Ar flow (30 mL min-1). After cooling to 70 °C, the gas was switched to
10 % N2O/N2 with a flow rate of 40 mL min-1 for 2 h. The amount of H2 and N2O consumption
was determined by TCD. Copper surface area was calculated by assuming a spherical shape
of copper metal particles with a surface Cu density of 1.47×1019 copper atoms per m2.
NH3-TPD experiments were performed on a Micromeritics Chemi-Sorb 2720 instrument.
The sample (0.1 g) was loaded into the quartz tube, degassed by heating in He at 200 °C for 1
h. The adsorption of NH3 was carried out at 100 °C in an NH3-He mixture (5 vol% NH3) for
1.5 h, and then the adsorbed NH3 was desorbed by raising the temperature up to 900 °C at a
rate of 10 °C min-1.
2.4 Catalytic tests
Liquid phase catalytic evaluation was performed in a 100 mLhigh pressure reaction kettle
equipped with timing heating system and a magnetic stirrer. Typically, 1 mL GVL, 0.1 g
catalyst, and 20 mL alcohol solvent were charged into the reactor. Prior to the reaction, air was
flushed out of the reactor, and H2 was fed into the reactor at the designated pressure. Then the
reactor was placed into an electric heating system which was preset to the certain reaction
temperature. The reaction was initiated by starting the stirring and performed for a period of
time. After reaction, the reactor was cooled with an ice-water bath for 30 min, and
depressurized carefully. The products were quantified by GC (Agilent GC-7890B equipped
with DB-WAX capillary column, 30.0 m × 250 μm × 0.25 μm) with a flame ionization detector
by the internal standard method using dodecane. The conversion and selectivity were obtained
through at least three parallel experiments. In each case, the carbon balance, calculated based
on the mass difference between carbon in the introduced reactant and that in products was
above 96 %. To investigate the reusability of the catalysts, the catalyst was removed from the
reaction mixtures, washed with alcohol and acetone, and dried at 70 °C for 12 h. Then, fresh
substrate and the recovered catalyst were employed in the repeated test.
8

3. Results and Discussion
3.1 Structural characterizations
Previously, we developed a novel strategy for the synthesis of fundamentally and
technologically important metal oxide (ZrO2, CeO2, TiO2) with high surface area, controlled
pore structure, and uniform size distribution [39]. The established strategy involves rapid
nucleation and the following kinetically controlled hydrothermal process, which combines the
merits such as simplicity, versatility, commercial feasibility, etc. Here, we expanded the
strategy to prepare homogeneously distributed CuO/ZrO2 composite, an ideal precursor for
the synthesis of highly dispersed Cu/ZrO2 catalysts. Figure 1 shows the XRD patterns of
CuO/ZrO2-20 precursor (prepared with the NaBH4/(Cu + Zr) molar ratio of 20) and the
resulting Cu/ZrO2-20 catalyst after reduction. The XRD pattern of CuO/ZrO2-20 precursor
presents a series of intensive and broad (101), (110), (112), (211), and (220) diffractions of t-
ZrO2 crystalline phase (JCPDS No. 42-1164), and no diffractions related to CuO species were
detected, indicating the homogeneous distribution nature and small particle size of CuO phase.
As a contrast, new sharp Cu (111) diffraction at 43.3° appears in the XRD pattern of Cu/ZrO2-
20 catalyst, indicative of the presence of a face-centered cubic (fcc) metallic copper phase
(JCPDS No. 04-0836). The average crystal size of metallic Cu particles estimated by the
Scherrer formula according to the Cu (111) plane is about 6.2 nm. The textural property of
CuO/ZrO2-20 precursor and Cu/ZrO2-20 catalyst was investigated by low-temperature N2
adsorption-desorption measurements (Figure S1). Both CuO/ZrO2-20 precursor and Cu/ZrO2-
20 catalyst exhibit a combined isotherm of types I and IV with an apparent hysteresis loop in
the relative pressure (P/P0) range of 0.4-0.8, reflecting the presence of micro/mesoporous
structure. Notably, compare with CuO/ZrO2-20 precursor, the Cu/ZrO2-20 catalyst shows
comparable surface area (168 m2 g-1 vs. 153 m2 g-1) and slightly decreased total pore volume
(0.2216 cm3 g-1 vs. 0.2178 m3 g-1). Pore size distribution of Cu/ZrO2-20 catalyst reveals that
most of the pores lie in the diameter range of 1-10 nm, is similar to that of CuO/ZrO2-20
precursor, reflecting high textural stability during the reduction transformation process.
The morphology and structure of representative Cu/ZrO2-20 catalyst was characterized
by TEM and HRTEM. As shown in Figure 2a, ZrO2 support presents a porous sponge-like
9

framework, which is composed of interconnected spherical particle of about 3±2 nm in size.
Due to the similar bright-dark image contrast, it causes trouble for us to distinguish the Cu
particle from the ZrO2 support. A typical HRTEM image of Cu/ZrO2-20 (inset in Figure 2a)
demonstrates the (111) planes of the Cu component with a d-spacing of 0.209 nm, and the
(101) planes of the ZrO2 support with a d-spacing of 0.296 nm [39, 40]. Meanwhile, the blurry
boundaries between metallic Cu and ZrO2 support further confirm the strong interaction
between them and the homogeneously dispersed Cu particles. Further microstructural
information and visual Cu dispersive nature of the synthesized Cu/ZrO2-20 catalyst were
obtained from the HAADF-STEM-EDS observations. As shown in Figure 2b, semi-spherical
Cu nanoparticles with an average size of 6~8 nm are distinguishable by the different contrast
compared to that of ZrO2 support. The elemental mapping images of O, Cu, and Zr depict the
uniform distribution of Cu and Zr in the Cu/ZrO2-20 catalyst (Figure 2c-e). Furthermore, the
STEM-EDS line scan spectra of one individual particle in Figure 2g depict that Cu, O, and Zr
lines show a similar Gaussian distribution along the red line in Figure 2f, indicating the
homogeneous distribution and close contact in the Cu/ZrO2-20 catalyst.
Series Cu/ZrO2-x catalysts were also prepared with different initial NaBH4/(Cu + Zr)
molar ratio. XRD patterns of CuO/ZrO2-x precursors were shown in Figure S2. For the sample
of CuO/ZrO2-25, an additional small CuO (111) diffraction peak can be observed at around
38.7° with respect to that of other CuO/ZrO2-x samples (JCPDS no. 05-0661), indicating the
formation of large CuO nanoparticles in CuO/ZrO2-25 sample. Meanwhile, the XRD pattern
and TEM image of the Cu/ZrO2-25 catalyst further confirm the formation of large Cu particles
after reduction (Figure S3).
In our synthetic strategy, NaBH4 acts as both reductant and alkaline reagent to control the
formation of CuO/ZrO2 catalyst precursor. Here, after mixing of Zr4+ and Cu2+ cations with
NaBH4 reductant in a vigorously stirring colloid mill reactor, nucleation process of Cu through
the reduction of Cu2+ cations is completed within a very short time, due to the energetic
collision and strong hydraulic shear force in the highly turbulent liquid film between the stator
and rotor of the colloid mill. Growth of Cu nuclei hardly takes place due to the quite short
residence time in the colloid mill, and thus agglomeration of the Cu nuclei is inhibited and
their sizes remain at a minimum. The XRD pattern of reduced intermediate collected from the
10

colloid mill only reveals the presence of amorphous phase (Figure 3a). In order to further
reveal the formation mechanism, the obtained intermediate was aged for 3 h at room
temperature under N2 atmosphere. As shown in Figure 3b, the reduced sample exhibits broad
(111), (200), and (220) diffractions of face-centered cubic (fcc) metallic copper phase, and no
characteristic diffractions corresponding to metallic Zr (JCPDS No. 34-0657) or ZrO2 are
detected. The above results indicate a rapid nucleation of Cu in the colloid mill. In tandem
with the above experimental observations, a possible formation mechanism is proposed, and
the chemical reactions involved are believed to proceed as follows (eqs 1-4) [41, 42]:
-
-
2Cu2+ + BH4 +4H2O → 2Cu↓ + B(OH)4 +2H2↑ +4H+
(1)
(2)
(3)
(4)
BH4 (aq) + 4H2O → H3BO3 (aq) + 4H2 + OH-(aq)
-
2Cu + O2 + 2H2O →2Cu(OH)2
4Zr4+ +4Cu(OH)2 +16OH-→4CuO-ZrO2↓ +12H2O
In the initial step, the Cu2+ cations are rapidly reduced to metallic Cu0 nuclei in the colloid
mill by the reductive NaBH4 (eq 1). Simultaneously, OH- anions can be released into the
reaction system by the hydrolysis of NaBH4, and thus raising the pH value of the aqueous
reaction solution (eq 2). The newly formed Cu0 nuclei are oxidized into Cu(OH)2 by the trace
amount of dissolved oxygen under alkaline hydrothermal conditions (eq 3). With the increase
of the reaction time, the newly formed Cu(OH)2 diffuses into the reaction system and reacts
with Zr4+ anions through a dehydration reaction under hydrothermal conditions (eq 4). Finally,
through a structural transformation, CuO/ZrO2 complex precursor can be obtained, in which
the Cu and Zr components are evenly distributed on an atomic level without segregation of
“lakes” of separate phrase. However, higher NaBH4 concentration may compel the reaction to
proceed through a different reaction path as follows: (eq 5-6) [39]
Zr4+ +4OH-→ Zr(OH)4
(5)
(6)
Zr(OH)4 +Cu(OH)2→CuO-ZrO2↓ +3H2O
The higher NaBH4 concentration leads to the release of more sufficient OH- anions in the
reaction system, Zr4+ cations can also react with OH- anions to form Zr(OH)4 nuclei (eq 5),
and CuO/ZrO2 precursor can be obtained via the dehydration of the formed Cu(OH)2 and
Zr(OH)4 nuclei (eq 6). It should be note that eqs 4-6 may proceed concurrently. Namely, the
formation path and properties of the CuO/ZrO2 precursors can be simply controlled by varying
11

the initial NaBH4 concentration.
Textural properties of series Cu/ZrO2-x catalysts were characterized by low-temperature
N2 adsorption-desorption measurements and listed in Table 1. Under current condition, a
higher initial concentration of NaBH4 leads to a higher pH value of the reaction system due to
the hydrolysis of NaBH4, which is benefit for the formation and stabilization of t-ZrO2 with
higher crystallinity and larger particle size. Hence, the BET surface areas of the Cu/ZrO2-x
catalyst show a decline trend with increasing the NaBH4 concentration. The surface acidity of
Cu/ZrO2-x catalysts was also investigated via NH3-TPD measurements. As shown in Figure
S4, all the test samples exhibit a broad desorption peak at temperature range of 50-300 °C and
a sharp desorption peak at temperature range of 300-450 °C, which can be assigned to the
different weak acid sites and moderate strength acid sites, respectively. According to
quantitative analysis on NH3-TPD results of Cu/ZrO2-x catalysts, the total acid sites decrease
with increasing the initial concentration of NaBH4 (Table 1), probably caused by the increased
particle size and decreased surface defects of the t-ZrO2 support. The above results indicate
that the textural properties and the surface acidity of the obtained catalysts can be simply
modulated by changing the NaBH4 concentration during the preparation process. However, in
order to get a more accurate influence mechanism of the NaBH4 concentration on the prepared
Cu/ZrO2-x catalyst, systemic theoretical and experimental investigations still need to be done.
H2-TPR measurements were carried out to determine the reducibility of CuO/ZrO2-x
precursors. Previously studies suggested that the reduction of highly dispersed CuO to Cu0
may involve a two reduction step: Cu2+→Cu+→Cu0. Due to the strong metal-support
interaction, the second process of the reduction requires a higher reduction temperature than
the first step [43, 44], and thus Cu+ active species can be formed under mild reduction
condition [45, 46]. As shown in Figure 4, there are three reduction regions for all CuO/ZrO2-
x precursors in the temperature range of 150-230 °C, indicating the existence of different
surface chemical state of Cu species in samples. For CuO/ZrO2-10 sample, three peaks at
around 175 °C, 185 °C, and 205 °C can be ascribed to the reduction of highly dispersed surface
Cu species (peak I), reduction of Cu-O-Zr to Cu+-O-Zr structure (peak II), and CuO in bulk
(peak III), respectively. For CuO/ZrO2-15 and CuO-ZrO2-20 samples, peak I and peak II shift
to higher temperatures slightly, and the intensity and peak area ratio of II exhibits an increasing
12

trend compared to that of CuO/ZrO2-10 sample. In our synthetic strategy, the floating rate and
size of the in situ generated hydrogen bubbles increase with increasing the concentration of
NaBH4 during the reaction process, and the violent dynamical perturbation caused by the
inosculation and blast of hydrogen bubbles is benefit for the dispersion of Cu species. The
highly dispersed Cu species can interact strongly with the ZrO2 support, which makes Cu
species much more difficult to reduce, and thus leading to slightly elevated reduction
temperatures. The increased peak area ratio of II is probably caused by the formation of more
Cu-O-Zr structure in the CuO/ZrO2 precursor with increasing the dispersion of Cu species.
Reduction peaks of bulk CuO (peak III) in CuO/ZrO2-15 and CuO-ZrO2-20 sample shift to
lower temperatures, which can be ascribed to the enhancement of the dispersion of CuO in
bulk, probably caused by the enhanced dynamical perturbation in the reaction system. With
the further increase in the NaBH4 concentration, the CuO/ZrO2-25 sample shows a reduced
peak area ratio of II and increased peak area ratio of III compare to that of CuO/ZrO2-20
sample, reflecting the reduced amount of Cu-O-Zr structure and increased amount of bulk
CuO in CuO/ZrO2-25 sample. In this case, though higher NaBH4 concentration may result in
more intensely dynamical perturbation, the increased pH value of the reaction system compels
the formation of CuO/ZrO2 precursor to proceed through a different reaction path, leading to
the formation of large amount of bulk CuO on the ZrO2 support. In addition, the formation of
Cu-O-Zr structure in the prepared CuO/ZrO2 precursor can be further confirmed by the
appearance of additional shoulder peak at 853 cm-1 with respect to that of pure ZrO2 support
(Figure S5). Furthermore, the appearance of an additional reduction peak in the TPR result of
CuO/ZrO2-20 in the temperature range of 350-600 °C further verified the stepwise reduction
of highly dispersed CuO (Figure S6). The above results demonstrate the unique reaction route
and condition is benefit for the formation of Cu species with relatively high dispersion and
strong metal-support interaction.
XPS and H2-N2O characterizations were carried out to confirm and quantify the
surface/near-surface chemical states and Cu species in the series Cu/ZrO2-x catalysts. As
shown in Figure 5, for all Cu/ZrO2-x catalysts, the appearance of a Cu 2p 3/2 signal at ca.933-
935 eV and the absence of a shake-up satellite peak at 940-945 eV confirm the reduction of
Cu2+ species [42]. The coexistence of Cu0 and Cu+ species in Cu/ZrO2-x catalysts can be
13

confirmed by the appearance of a broad and asymmetric Auger kinetic energy peak in the Cu
LMM XAES. The kinetic energy at ca. 916.2 eV and 918.6 eV are assigned to Cu+ species and
Cu0 species, respectively. As listed in Table 1, it is interesting to note that the surface area of
Cu0, Cu+/(Cu0 + Cu+) molar ratio, and surface Cu (including Cu0 and Cu+) all increase with
the value of x initially and decreases afterwards, and reach their maximum when value of x is
set at 20. The variation trend of the XPS results in Cu/ZrO2-x catalysts is somewhat consistent
with the above H2-TPR results.
3.2 Catalytic performance of Cu/ZrO2 catalysts
In consideration of the fact that the as-synthesized Cu/ZrO2 catalyst has a large amount
of exposed active adsorption sites and reaction centers originating from high dispersion of Cu
active components, as well as surface acidity and porous crystalline structure of ZrO2 support,
the synthesized Cu/ZrO2 catalyst may show excellent catalytic performance in some important
biomass conversion reactions. To investigate the catalytic performance of the synthesized
Cu/ZrO2 catalyst, the catalytic transformation of GVL to pentyl valerate (PV) was chosen as a
probe reaction.
The effects of different reaction conditions on the catalytic transformation of GVL were
investigated firstly over Cu/ZrO2-20 catalyst using pentanol as solvent. As shown in Figure
S7a, with the extension of reaction time, the conversion of GVL increases from 26.4 % to 75.7%
with relatively high PV selectivity (>98.0 %) in the initial 5 h reaction period at 230 °C. The
conversion reaches up to 85.4% with almost unchanged selectivity at 10 h. However, further
increase the reaction time to 12 h, only results in a slightly increased GVLconversion (86.2%).
If the reaction temperature is elevated from 180 °C to 230 °C, the conversion of GVLincreases
sharply from 15.5 % to 85.4 % because of the increased reaction rate, and the PV selectivity
always maintains at a high level (98.0 %). With further increasing the reaction temperature up
to 250 °C, the conversion of GVL remains constant, selectivity of PV drops to 97.2 %, 2-
methyltetrahydrofuran (2-MTHF) with a selectivity of about 2.6% can be detected as the main
byproduct (Figure S7b). We speculate that high reaction temperature is favor for the formation
of 1, 4-pentanediol (1,4-PDO) from the hydrogenolysis of GVL and the sequential dehydration
cyclization of 1,4-PDO to 2-MTHF [50,51]. Higher H2 pressure is preferable for the
14

conversion of GVL and the PV selectivity; nevertheless, 1,4-PDO is produced as the main
byproduct when the H2 pressure exceeded 2 MPa (Figure S7c). Hence, without additional
remarks, 10 h of reaction time, 230 °C of reaction temperature, 1.5 MPa H2 pressure are chose
as the optimized reaction conditions and performed in the following texts.
The catalytic performance of series Cu/ZrO2-x catalysts for the transformation of GVL to
valerate ester was also investigated. The conversion of GVL, PV selectivity, and reaction rate
r (calculated by the rate of GVL consumption per catalyst mass in the initial reaction state with
GVL conversion less than 30%) were listed in Table 2. Among the series prepared Cu/ZrO2-x,
Cu/ZrO2-20 catalyst exhibited the best catalytic performance and the highest reaction rate, and
the r value increased with the order of Cu/ZrO2-25<Cu/ZrO2-10<Cu/ZrO2-15<Cu/ZrO2-20. In
addition, the catalytic performance of Cu/ZrO2-CP and Cu/ZrO2-CH catalysts was also
investigated under identical reaction conditions (Table 2 entries 5, 6), the conversion of GVL
and the PV selectivity were far less than that of Cu/ZrO2-x catalysts. The low catalytic
performance over the Cu/ZrO2-CP and Cu/ZrO2-CH catalysts should be assigned to the low
dispersion of Cu active components (Table 1 entries 5, 6), which may be caused by the poor
interaction between the CuO and ZrO2 support, and the high mobility of active Cu species in
the CuO/ZrO2-CP and CuO/ZrO2-CH precursors during synthesis and following reduction
processes.
To clarify the origin of different catalytic performance over the synthesized Cu/ZrO2-x
catalysts, systematical and comprehensive studies were carried out via correlating the
physicochemical data listed in Table 1 with their catalytic performance in the catalytic
transformation of GVL (Table 2). There is no much difference on the actual Cu loadings of
series Cu/ZrO2-x; therefore, Cu loadings is not the origin of their different GVLtransformation
activities. Meanwhile, higher surface area does not correspond to superior catalytic activity,
indicative of that the texture property is not the key role in determining their catalytic
performance. According to literature, in most cases, surface acidity of the catalyst plays an
important role in the adsorption and activation of C=O bond in GVL, and dehydration of pentyl
4-hydroxylvalerate intermediate [52]. However, in our case, there is a weak relationship
between the surface acidity and the performance of the Cu/ZrO2-x catalysts, high total surface
acid sites does not correlate with the high catalytic performance. It is well known that pyridine
15

can poison both the Brönsted acid sites and the Lewis acid sites. Hence, to further gain insight
into the effect of the surface acid sites on the catalytic transformation of GVL, the poisoning
test was carried out by adding pyridine in the reaction system. After the surface acid sites of
the Cu/ZrO2 being poisoned by pyridine, the GVL conversion dropped slightly to 73.2 % with
a 95.4 % PV selectivity (Table 2, entry 7), indicating the surface acidity is benefit for the
catalytic transformation of GVLbut not the dominate factor under our experimental conditions.
Furthermore, other acidic oxides such as SiO2, MoO3, and Al2O3 were also used as support to
prepare Cu-based heterogeneous catalyst for the catalytic transformation of GVL (Table 2,
entries 8-10), and the GVL conversion and PV selectivity over these catalysts were inferior
than that over Cu/ZrO2-CH catalyst, reflecting the positive effect of the porous ZrO2 support
on the catalytic transformation of GVL.
As to hydrogenation catalyzed by Cu-based catalyst, the surface Cu0 plays a crucial role
in the activation and dissociation of hydrogen. Figure 6 shows the variations of the reaction
rate constant r in the catalytic transformation of GVL as functions of the amount of surface
Cu0 and surface Cu. Indeed, the highest r value of 69.6 mmol gcat-1 h-1 is achieved over Cu/ZrO2-
20 with highest amount of surface Cu0; however, the value of r does not correlate well with
the surface Cu0 (Figure 6a). For example, r value over Cu/ZrO2-25 is lower than that of
Cu/ZrO2-10 though it has larger surface Cu0, indicating that the surface Cu0 is not a sole key
factor governing the performance of the catalysts. Notably, it can be found that there is a good
relationship between the amount of surface Cu and the r value (Figure 6b), suggesting surface
Cu+ also plays an important role in the catalytic transformation of GVL to some extent. To
further confirm the role of Cu+ species in the catalytic transformation of GVL, Cu/ZrO2-20-
600 catalyst without surface Cu+ species was prepared through the reduction of CuO/ZrO2-20
precursor at 600 °C, and the sole presence of Cu0 species could be confirmed by the appearance
of single symmetrical peak at kinetic energy of 918.5 eV in the Cu LMM XAES of Cu/ZrO2-
20-600 catalyst (Figure S8). The conversion of GVL and PV selectivity over Cu/ZrO2-20-600
catalyst drop significantly to 33.7% and 63.1%, respectively (Table 2 entry 11), even worse
than that of Cu/ZrO2-25 catalyst though they have similar surface area, surface Cu0, and total
acid sites (Table 1, entries 4 and 7). On the basis of the above catalytic results, we conclude
that high dispersion of Cu nanopaticles, as well as cooperation of surface Cu0 and Cu+ are
16

crucial for achieving high catalytic performance in the catalytic transformation of GVL under
our experimental condition.
A possible mechanism for the catalytic transformation of GVL in pentanol over the
Cu/ZrO2 catalysts is also proposed (Figure 7). Firstly, C=O bond in GVL can be absorbed and
polarized by the surface electrophilic Cu+ species, and the formed electropositive carbonyl C
atom is benefit for the nucleophilic addition of electronegative hydroxyl group in pentanol,
leading to the formation of pentyl 4-hydroxylvalerate intermediate. Secondly, pentyl
pentenoate can be sequentially formed by the fast dehydration of pentyl 4-hydroxylvalerate
[29, 53]. Finally, the highly dispersed surface Cu0 active center dissociates H2 to produce
active hydrogen for the final hydrogenation of pentyl pentenoate to generate PV product.
Cooperation between the surface Cu+ and highly dispersed Cu0 endows the prepared Cu/ZrO2
catalyst with excellent catalytic performance in the transformation of GVL.
Heterogeneous catalysts often suffer from a decline of catalytic performance caused by
the aggregation and leaching loss of active component, and the reusability of heterogeneous
catalysts is a critical factor in practical catalytic application. Consequently, the reusability of
the Cu/ZrO2-20 catalyst was also investigated representatively. In our case, Cu/ZrO2-20
catalyst shows a slightly reduced catalytic performance (80.0 % of GVL conversion and 97.6 %
PV selectivity) after four cycles (Figure 8). Systematic characterizations of the recovered
Cu/ZrO2-20-RE catalyst were carried out, and the relevant structural and component data were
listed in Table 1 (Entry 8). It is revealed that the structure of the catalyst does not change after
recycling four times, and no obvious agglomeration and growth of Cu particles can be
observed (Figure S9 and Figure S10A). Elemental analysis by ICP-AES demonstrates that the
Cu leaching loss is only about 1 wt% after recycling four times. In addition, Cu LMM XAES
result shows that surface Cu+/(Cu0 + Cu+) ratio decreases from 0.51 to 0.47 after four
consecutive runs. (Figure S10B; Table 1, entry 8). We speculate that the slight decline of
catalytic performance over the recovered Cu/ZrO2-20-RE catalyst is mainly caused by the
leaching loss of Cu component and the decline of surface Cu+ active sites.
To explore the range of suitable solvents for this catalytic system and the application in
preparation of various alkyl valerates, series alcohols were investigated as solvents for the
catalytic transformation of GVL over the Cu/ZrO2-20 catalyst. The conversion of GVL
17

exhibits a downward trend with the increase in the alkyl chain of the primary alcohol (Table
3, entries 1-4), and the selectivity to alkyl valerates shows a trend from decline to rise. For
short-chain alcohols such as methanol and ethanol, strong polarity and weak steric hindrance
are benefit for the ring-opening and addition processes, and thus leads to high GVL conversion
and excellent alkyl valerates selectivity. For propanol and butanol, the subdued polarity may
be responsible for the declined conversion of GVL, the increased steric hindrance may inhibit
the dehydration and hydrogenation process of the formed 4-hydroxy-valerate intermediate,
and propyl 4-hydroxylvalerate and butyl 4-hydroxylvalerate can be detected as main
byproducts, respectively, leading to declined vlaerate selectivity. Though pentanol has weaker
polarity and larger steric hindrance than that of butanol, the intermiscibility of GVL in pentyl
is higher than that in butanol, which may be benefit for the dehydration and hydrogenation of
the formed pentyl 4-hydroxylvalerate intermediate, resulting in higher pentyl valerate
selectivity. In addition, the high selectivity of hexyl vlaerate in hexanol further confirms the
above speculation to some extent (Table 3, entry 5). The conversion of GVL and selectivity to
valerate ester in secondary alcohol and cyclitol is lower than that in primary alcohol (Table 3,
entries 6-8). For isopropanol and isoamyl alcohol, 4-hydroxy-valerate can be detected as the
main byproduct, reflecting the steric hindrance has a negative effect on the dehydration and
hydrogenation of the formed 4-hydroxy-valerate intermediate [54]. For cyclohexanol, in
addition to the detection of cyclohexyl 4-hydroxylvalerate, 2-MTHF can also be detected as a
byproduct, which may be ascribed to the hydrogenolysis of GVL. Having established the
catalytic transformation route for the production of valerate ester from GVL and alcohol, we
have explored the possibility of a straight forward production of valerate ester from levulinic
acid (LA), a biomass derived platform molecule. We have performed a “one-pot” process that
combines the catalytic hydrogenation of LA to GVL and catalytic transformation of GVL to
PV over the Cu/ZrO2-20 catalyst. The result shows that LA totally converts to GVL and the
selectivity to PV reaches up to 58.7%. In future, the reaction conditions will be optimized to
gain a higher yield of the target product.
4. Conclusion
18

In summary, a cost-effective and facile strategy for the synthesis of CuO/ZrO2 complex
precursor has been established herein. The unique reduction-oxidation route, explosive
nucleation process, and sustained perturbation at the aging step are benefit for the formation
of CuO/ZrO2 complex nanocrystals uniformly distributed on an atomic level, which can be
used as an ideal precursor for the synthesis of Cu/ZrO2 catalyst with highly dispersion of Cu
and large amount of Cu+ catalytic centers. It was found that the formation path and properties
of CuO/ZrO2 precursors, as well as the surface Cu+/(Cu0 + Cu+) molar ratio of Cu/ZrO2 catalyst
can be simply manipulated by varying the initial NaBH4 concentration. The prepared Cu/ZrO2
catalysts process remarkable stability and efficiency in the catalytic transformation of GVL to
pentyl valerate, which may attribute to the cooperation between Cu+ and Cu0 originating from
enhanced metal-support interaction and highly dispersed CuO nanopareticles of the CuO/ZrO2
precursors. The correlation between the catalytic performance and the surface Cu active
components demonstrated that Cu+ is the active site for the adsorption and polarization of
GVL, while Cu0 undertakes the hydrogenation of intermediates. As formed Cu/ZrO2 catalyst
was also efficient in straight forward production of valerate ester from biomass derived
platform molecule. The present study could inspire the development of readily available,
stable, highly efficient Cu-based catalyst with superior performance in the flexible conversion
of bio-derived platform molecules in terms of economic and environmental sustainability.
Acknowledgments
This work is financially supported by the National Natural Science Foundation of China
and the Fundamental Research Founds for the Central Universities (buctrc 201528).
Supplementary material
Supplementary data associated with this article can be found, in the online version, at xxxx
Low-temperature N2 adsorption-desorption and pore size distribution of the CuO/ZrO2-
20 precursor and Cu/ZrO2-20 catalyst; XRD patterns of series CuO/ZrO2-x precursors; XRD
patterns of series of CuO/ZrO2-x precursor (a) and TEM image of Cu/ZrO2-25 catalyst (b);
19

NH3-TPD profiles of series Cu/ZrO2-x catalysts; FT-IR spectra of pure ZrO2 (a) and
CuO/ZrO2-20 precursor (b); H2-TPR spectra of CuO/ZrO2-20 precursor; Effect of reaction
parameters on the catalytic transformation of GVL over Cu/ZrO2-20 in pentanol: reaction time
(a), reaction temperature (b), H2 pressure (c). Reaction conditions: 1 mLGVL, 20 mLpentanol,
0.1 g catalyst; Cu 2p XPS spectra and Cu LMM XAES (inset) of Cu/ZrO2-20-600 catalyst;
TEM image of the recovered Cu/ZrO2-20-RE; XRD patterns (A) and Cu LMM (B) of
Cu/ZrO2-20 catalyst: before (a) and after (b) reaction.
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22

Table
Table 1. The structural and component data of different catalysts.
d
Cu
(wt %)a
SBET
(m2 g-1)b
Total acid sites
(mmol g-1)c
D111
SCu
(m2 g-1)e
Cu+/
(Cu0 + Cu+)f
Surface Cu0
(μmol g-1)e
Surface Cu
(μmol g-1)g
Entry Samples
(nm)
1
2
3
4
5
6
7
8
Cu/ZrO2-10
9.6
9.5
9.7
9.9
9.9
9.6
9.7
9.6
192
158
153
168
104
53
0.88
0.79
0.61
0.52
0.54
0.37
0.43
0.64
7.8
6.0
6.8
7.4
6.2
3.6
2.1
6.8
8.0
0.44
0.45
0.51
0.39
0.34
0.26
0.05
0.47
145
164
179
151
89
258
298
363
248
134
70
Cu/ZrO2-15
7.4
6.2
8.7
8.5
10.3
9.0
6.6
Cu/ZrO2-20
Cu/ZrO2-25
Cu/ZrO2-CPh
Cu/ZrO2-CHi
Cu/ZrO2-20-600j
Cu/ZrO2-20-REk
52
152
157
166
190
174
358
a Determined by ICP-AES.
b Specific surface area calculated by the BET method.
c Determined by NH3-TPD analysis.
d Crystallite size based on XRD line broadening of the (111) plane for Cu metal.
e Metallic copper surface area determined by H2-N2O titration.
f Determined by Cu XAES analysis.
g Deduced from metallic copper surface area and Cu XAES results.
1

h Cu/ZrO2-CP catalyst prepared by co-precipitation method
i Cu/ZrO2-CH catalyst prepared by chemisorption hydrolysis method.
j Cu/ZrO2-20-600 catalyst reduced in H2 at 600 °C for 4 h.
k Cu/ZrO2-20 catalyst recycled for 4 times.
2

Table 2. Transformation of GVL to pentyl valerate over various catalysts. a
Selectivity (%)
Entry Catalysts
Conversion (%)
r (mmol gcat-1 h-1)b
2-
Others
PV
MTHF
0.9
1
Cu/ZrO2-10
Cu/ZrO2-15
Cu/ZrO2-20
Cu/ZrO2-25
Cu/ZrO2-CP
Cu/ZrO2-CH
Cu/ZrO2-20-pyc
Cu/SiO2-CH
Cu/MoO3-CH
Cu/Al2O3-CH
39.5
60.9
85.4
66.2
37.5
25.4
73.2
9.3
98.1
97.7
98.1
97.8
62.1
56.7
95.4
7.1
1.0
1.1
0.9
1.0
9.5
13.6
2.4
1.0
0.7
1.1
11.6
0.8
44.4
52.1
69.6
36.2
24.9
18.3
59.6
5.0
2
1.2
3
1.0
4
1.2
5
28.4
29.7
2.2
6
7
8
91.8
83.0
88.5
25.3
1.6
9
24.2
14.5
16.3
10.4
63.1
97.6
17.1
9.9
10
11
12
Cu/ZrO2-20-600 33.7
Cu/ZrO2-20-REd 83.00
23.6
65.5
a
Reaction conditions: 1 mL GVL, 20 mL pentanol, 0.1 g catalyst, reaction temperature 230 °C, reaction
time10 h, and 1.5 MPa H2 pressure.
b
Calculated by the rate of GVL consumption per catalyst mass in the initial reaction state with GVL
conversion less than 30%.
c Cu/ZrO2-20 catalyst poisoned by pyridine.
d Cu/ZrO2-20 catalyst reused for 4 times.
1

Table 3. Catalytic performance of Cu/ZrO2-20 catalyst for the transformation of GVL in
different alcohol solvents. a
Entry
Solvent & reactant
Products
Conversion
Selectivity
O
1
95.6%
92.4%
OH
O
O
2
3
4
5
92.4%
90.0%
87.6%
72.5%
90.0%
74.9%
75.2%
94.8 %
OH
OH
O
O
O
O
O
OH
O
O
6
69.3%
41.9%
OH
O
O
7
8
82.5%
79.0%
52.7%
45.5%
OH
O
OH
O
a Reaction condition: 1 mLGVL, 20 mLsolvent, 0.1 g catalyst, reaction temperature 230 °C, reaction time10
h, and H2 pressure 1.5 MPa.
2

Figures and captions
Figure 1 XRD patterns of CuO/ZrO2-20 precursor and corresponding Cu/ZrO2-20 catalyst
after reduction.
Figure 2 HRTEM images (a) of Cu/ZrO2-20 catalyst; HAADF-STEM image (b) of Cu/ZrO2-
20 with EDX mappings of O (c), Cu (d) and Zr (e); High-magnification STEM
image (f); EDX line spectra of CuK, ZrK, and OK along the red line in f (g).
3

Figure 3 XRD patterns for reduced immediate products in the colloid mill (a) without aging
and (b) aging for 3 h at room temperature under N2 atmosphere.
Figure 4 H2-TPR spectra of CuO/ZrO2-x precursors prepared with different initial
NaBH4/(Cu+Zr) molar of x: a=10, b=15, c=20, d=25.
4

Figure 5 Cu 2p XPS spectra (A) and Cu LMM XAES (B) of Cu/ZrO2-x with different initial
NaBH4/(Cu+Zr) molar of x: a=10, b=15, c=20, d=25.
Figure 6 The r value as a function of surface Cu0 (a) and surface Cu (b) for catalytic
transformation of GVL over series Cu/ZrO2-x catalyst.
5