Efficient conversion of furfural into cyclopentanone over high performing and stable Cu/ZrO2 catalysts

Article abstract of DOI:10.1016/j.apcata.2018.05.030

Currently, biomass transformation to produce high value-added chemicals and liquid biofuels is attracting more and more interest by the virtue of its importance in the sustainable development of human society. Herein, we reported the conversion of furfural (FFA) into cyclopentanone (CPO) in water over high performing and stable Cu/ZrO₂ catalysts prepared by our developed one-pot reduction-oxidation method. It was demonstrated that surface structures and catalytic performances of catalysts could be delicately adjusted by varying the calcination temperatures for catalyst precursors. Especially, an appropriate calcination temperature of 500 °C could significantly enhance the interactions between surface Cu species and the ZrO₂ support, thus greatly facilitating the formation of Cu⁺-O-Zr-like structure at the metal-support interface, and the resulting Cu/ZrO₂ catalyst showed a superior catalytic performance with a high CPO yield of 91.3% under mild reaction conditions (i.e. a low hydrogen pressure of 1.5 MPa and 150 °C) to other metal oxides supported copper catalysts prepared by the conventional impregnation. It was revealed that in addition to surface acidic sites, surface Cu⁺/(Cu°+Cu⁺) ratio also played a key role in promoting the formation of CPO in the present Cu/ZrO₂ catalytic system.

Full text of DOI:10.1016/j.apcata.2018.05.030

Accepted Manuscript
Title: Efficient conversion of furfural into cyclopentanone over
high performing and stable Cu/ZrO2 catalysts
Authors: Yifeng Zhang, Guoli Fan, Lan Yang, Feng Li
PII:
S0926-860X(18)30260-6
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.05.030
APCATA 16679
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
15-3-2018
8-5-2018
25-5-2018
Please cite this article as: Zhang Y, Fan G, Yang L, Li F, Efficient conversion of
furfuralintocyclopentanoneoverhighperformingandstableCu/ZrO2 catalysts, Applied
Catalysis A, General (2018), https://doi.org/10.1016/j.apcata.2018.05.030
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Efficient conversion of furfural into cyclopentanone over
high performing and stable Cu/ZrO2 catalysts
Yifeng Zhang, Guoli Fan, Lan Yang, Feng Li*
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center
for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing,
100029, China.
* Tel.: +8610-64451226; fax: +8610-64425385. E-mail: lifeng@mail.buct.edu.cn
Graphical abstract
1

Highlights
 High performing and stable supported Cu-based catalysts were facilely fabricated.
 Strong interactions between Cu species and the ZrO2 support were formed.
 Highly efficient conversion of furfural into cyclopentanone was achieved.
 Surface Cu+-acidic site synergistic effect on the CPO formation was confirmed.
ABSTRACT: Currently, biomass transformation to produce high value-added chemicals and
liquid biofuels is attracting more and more interest by the virtue of its importance in the
sustainable development of human society. Herein, we reported the conversion of furfural (FFA)
into cyclopentanone (CPO) in water over high performing and stable Cu/ZrO2 catalysts prepared
by our developed one-pot reduction-oxidation method. It was demonstrated that surface
structures and catalytic performances of catalysts could be delicately adjusted by varying the
calcination temperatures for catalyst precursors. Especially, an appropriate calcination
temperature of 500 °C could significantly enhance the interactions between surface Cu species
and the ZrO2 support, thus greatly facilitating the formation of Cu+-O-Zr-like structure at the
metal-support interface, and the resulting Cu/ZrO2 catalyst showed a superior catalytic
performance with a high CPO yield of 91.3 % under mild reaction conditions (i.e. a low hydrogen
pressure of 1.5 MPa and 150 °C) to other metal oxides supported copper catalysts prepared by
the conventional impregnation. It was revealed that in addition to surface acidic sites, surface
Cu+/(Cu0+Cu+) ratio also played a key role in promoting the formation of CPO in the present
Cu/ZrO2 catalytic system.
KEYWORDS: Copper; Zirconia; Metal-support interactions; Surface structure; Hydrogenation.
1. Introduction
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With the decrease of global fossil resources, energy crisis caused by the increasing
consumption of fossil fuels (coal and oil) and the resulting environmental pollution are becoming
more and more serious all over the world. As a kind of renewable and sustainable resource and
the best alternative to fossil fuels, biomass has attracted broad research attention, because its
efficient transformation into important chemicals and high value-added liquid biofuels can not
only decline the dependence on fossils but also reduce environmental pollution [1-3]. For
example, furfural (FFA), a most important and significant biomass-derived platform compound
from hemicellulose and xylose [4-9], can be converted through hydrogenation/hydrogenolysis
processes to produce various chemicals, nonpetroleum-derived liquid fuels, and fuel additives
(e.g. furfuryl alcohol (FAL), tetrahydrofurfuryl alcohol (THFA), 2-methylfuran, tetrahydrofuran,
γ-valerolactone, and pentadiol) [10-17].
Cyclopentanone (CPO), as an important intermediate, is used to manufacture a wide range
of chemicals (e.g. medicines, herbicides, pesticides, and perfumes) [18], fuel precursors and
high-density fuels through condensation-hydrogenation process [19,20]. In industry, CPO is
mainly produced by the pyrolysis of adipic acid and its derivatives or the direct oxidation of
cyclopentene [21-23]. However, the former simultaneously produces large numbers of pollutants,
while the latter always affords low CPO yields under harsh reaction conditions including high
reaction temperatures and/or high pressures. Fortunately, it was recently reported that CPO could
be obtained by the hydrogenation of biomass-derived FFA in neat water over various supported
noble metal catalysts [24-28], especially, along with a high yield of 92.1% over the Pd-Cu/C
catalyst under a hydrogen pressure of 3.0 MPa at 160 °C [26], and even a quantitative conversion
of FFA into CPO over the Au/TiO2 under a hydrogen pressure of 4.0 MPa [27]. As for bimetallic
Pd-Cu/C catalyst, it was shown that the synergy between Pd0-Cu+ species contributed to a good
catalytic activity [26]. Very recently, Ru-based catalyst has been reported to efficiently catalyze
the FFA hydrogenation to generate CPO under a hydrogen pressure of 1.0 MPa [28]. Taking into
consideration high costs of precious metals, non-precious copper, nickel and bimetallic Cu-Ni
catalysts also were developed for the above reaction [29-34]. For non-precious metal catalysts,
3

however, relatively high reaction temperatures and/or hydrogen pressures adopted greatly limit
their large-scale industrial application. Therefore, developing high performing non-precious
metal catalytic systems for the synthesis of CPO has important implications for practical
applications.
To achieve the high CPO selectivity in the FFAhydrogenation, it is necessary for catalytically
active sites on catalysts to be able to significantly promote the rearrangement of formed FAL
intermediate to obtain CPO. It was reported that interfacial Cu+ species formed in the case of Cu-
based catalysts could function as active sites activating the C=O bond in FFA to some extent
[29,35], while surface Lewis acidic sites favored the FAL rearrangement [36]. On the other side,
it is well known that the metal-support interactions can greatly affect the catalytic performance
of catalysts. For example, the strong interactions between copper species and metal oxides could
improve the catalytic performances in large numbers of hydrogenation reactions [37-40].
Recently, we also reported that supported Cu catalysts showed good catalytic performances in
the transformation of γ-valerolactone to valerate esters with the help of surface Cu+ species
[41,42].
In this contribution, we synthesized a series of Cu/ZrO2 catalysts by our developed one-pot
reduction-oxidation route for the transformation of FFA to produce CPO, and especially
investigated the effect of the calcination temperature for catalyst precursors on the surface
structures and catalytic performances of the resulting copper-based catalysts to unravel the role
of metal-support interactions in affecting the catalytic performance. It was shown that as-formed
Cu/ZrO2 catalyst at a calcination temperature of 500 °C exhibited an excellent catalytic
performance under mild reaction conditions (e.g. a quite low hydrogen pressure of 1.5 MPa
and150 °C), along with a quite high CPO yield of 91.3 %, indicative of the superiority to other
metal oxides supported copper catalysts prepared by the conventional impregnation. High
catalytic efficiency of the catalyst mainly resulted from the occurrence of strong metal-support
interactions (SMSIs) facilitating the formation of surface Cu+ species, as well as favorable
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surface acidity. To the best of our knowledge, there is no report about using such high performing
and stable Cu/ZrO2 catalyst for the conversion of FFA to produce CPO up to now.
2. Experimental section
2.1 Preparation of Cu/ZrO2 catalysts
A series of Cu/ZrO2 catalysts were prepared by our developed one-pot reduction-oxidation
method [41]. Firstly, Zr(NO3)4·5H2O (0.01mol, 4.293g) and Cu(NO3)2·3H2O (0.01mol, 2.416g)
were dissolved into 80 ml of deionized water to form a salt solution A, while NaBH4 (0.2 mol,
7.59g) was dissolved into 80 ml of deionized water to obtain a solution B. Subsequently, above
two solutions were simultaneously added to a colloid mill, and at once mixed rapidly at a rotor
speed of 3000 rpm for 3 min. Afterwards, the resulting suspension was transferred to a Teflon-
lined autoclave reactor and aged at 150 °C for 48 h. The obtained precipitate was washed with
deionized water until pH = 7.0 and dried at 70 °C overnight. Then, the obtained solid (denoted
as CuO/ZrO2-R) was calcined at different temperatures of 400, 500 or 600 °C under air
atmosphere and held for 6 h to obtain calcined CuO/ZrO2-x catalyst precursors (x represents the
calcination temperature). At last, CuO/ZrO2 samples were reduced in 10% v/v H2/Ar atmosphere
at 300 °C for 2h to gain Cu/ZrO2-R and Cu/ZrO2-x catalysts. In addition, pure ZrO2-500 support
also was synthesized according to the similar procedure to that for CuO/ZrO2-500 without the
addition of Cu(NO3)2·3H2O. For comparison, different metal oxides supported Cu catalysts with
the similar copper loadings (i.e. Cu/ZrO2-im, Cu/MgO-im, Cu/ZnO-im and Cu/Al2O3-im) were
synthesized by the conventional impregnation method.
2.2 Characterization
Shimadzu XRD-6000 diffractometer with a graphite-filtered Cu Kα source (λ= 0.15418 nm) at
40 kV and 30 mA was utilized to obtain X-ray diffraction (XRD) patterns of samples.
Elemental analysis was carried out using a Shimadzu ICPS-7500 inductively coupled plasma
atomic emission spectroscopy (ICP-AES).
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Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100
electron microscope. N2 adsorption-desorption experiments of samples were performed on a
Micromeritics ASAP 2020 sorptometer apparatus.
Hydrogen temperature-programmed reduction (H2-TPR) and ammonia temperature-
programmed desorption (NH3-TPD) experiments were carried out on a Thermo Fisher TPDRO-
1100 chemical adsorption instrument with a thermal conductivity detector (TCD). For H2-TPR,
the sample (100 mg) was degassed under Ar flow at 200 °C for 2h. Afterwards, TPR
measurement was executed in a flow of 10% H2/Ar (30 mL min-1) with a heating rate of 10 °C
min-1 from 50 to 800 °C. For NH3-TPD, the sample (100 mg) was reduced in a flow of 10%
H2/Ar atmosphere at 300 °C for 2h with a ramping rate of 5 °C min-1, and then cooled to room
temperature. Subsequently, the sample was degassed at 300°C for 1h under He flow and followed
by treatment with a 5% NH3/He flow for 1.5 h at 100 °C. Then, chemisorbed NH3 was desorbed
by heating at a ramping rate of 10 °C min-1 up to 900 °C.
X-ray photoelectron spectra (XPS) was recorded on a Thermo VG ESCALAB250 X-ray
photoelectron spectrometer using a monochromatic Al Kα radiation of 1486.6 eV photons. The
graphite C1s peak at 284.6 eV was used to calibrate all binding energies.
Active copper surface areas of samples were determined by N2O titration using a
Micromeritics ChemiSorb 2920 instrument. Firstly, a H2-TPR process under a 10% H2/Ar flow
(40mL min-1) from 50 to 300 °C was conducted on the calcined sample (100 mg). Secondly, after
the reduced sample was oxidized with a flow of 10% N2O/N2 (40mL min-1) at 70 °C for 1h,
residual N2O was removed under an Ar flow. Finally, H2-TPR was carried out again from room
temperature to 300 °C. Cu surface area was obtained based on spherical shape of metal particles
and surface concentration of 1.47 ×1019 Cu atoms on per m2.
In situ infrared spectra (IR) of pyridine (Py) adsorption was collected on a Thermo Nicolet
380 FT-IR spectrometer. First, the self-supporting wafer of reduced sample (30 mg), which was
placed into an evacuable IR cell with CaF2 windows, was evacuated at 200 °C for 1h under argon
6

flow. Then, pyridine was introduced and held for 1h. Finally, IR spectra were recorded after
physiosorbed pyridine was removed by evacuation.
2. 3 Catalytic FFA hydrogenation tests
Typically, FFA (5 mmol, 0.48 g), catalyst (0.05 g) and ultrapure water (15 ml) were placed
into a stainless-steel autoclave reactor with a magnetic stirrer, and then the reactor was sealed
and flushed with pure hydrogen for ten times. Subsequently, the reactor was charged with pure
hydrogen under atmospheric pressure. After that, the reactor was heated to certain reaction
temperature (typically 150 oC) at a heating rate of 10 °C/min and then hydrogen was further fed
into the reactor up to certain reaction pressure (typically 1.5 MPa). The reaction was initiated by
stirring at a speed of 1000 rpm. During the reaction, the hydrogen pressure was kept constant by
supplying hydrogen continuously from gas cylinder through the pressure regulator to compensate
for the consumed hydrogen in the reactor. After the reaction, the reactor was rapidly cooled to
room temperature and then depressurized. The organic products were extracted by
dichloromethane from the liquid and then analyzed by gas chromatograph (Agilent 7890B)
equipped with a DB-WAX capillary column (30.0 m × 250 μm × 0.25 μm) and identified as
compared with known standards using dodecane as an internal standard. The used catalyst was
recycled from the reaction solution by centrifugation and washed several times with deionized
water and ethanol before the next cyclic test.
3. Results and discussion
3.1 Structural characterization
Fig.1A presents the XRD patterns of different CuO/ZrO2 catalyst precursors. In each case,
four diffractions appearing at 2θ of about 30.2°, 35.2°, 50.4° and 60.2° are indexed as the (011),
(110), (020) and (121) planes of tetragonal ZrO2 (t-ZrO2) phase (JCPDS 50-1089), respectively.
As the calcination temperature is elevated, no obvious change in diffractions assignable to t-ZrO2
is found, indicative of the formation of stable t-ZrO2 phase in all precursors. Interestingly, for
7

CuO/ZrO2-R sample, one can see another diffraction at about 43.2°, which is assigned to the
(111) plane of metallic Cu phase (JCPDS 04-0836). In the XRD patterns of CuO/ZrO2 samples,
no diffractions related to metallic Cu can be observed; While a new diffraction corresponding to
the (111) plane of CuO phase (JCPDS 05-667) appear at about 38.5° due to the oxidation of
metallic Cu and is enhanced gradually with the elevated calcination temperature. As shown in
Fig.1B, for reduced Cu/ZrO2 samples, in addition to those for t-ZrO2 phase, the intensive (111)
diffraction for metallic Cu phase can be observed, owing to the complete reduction of CuO phase
in precursors. The average size of Cu0 particles was calculated based on Scherrer equation (Table
1). The Cu particle size in the Cu/ZrO2-R is the smallest (20.2 nm), whereas that in the Cu/ZrO2-
600 is largest (30.9 nm).
Further, the microstructure of reduced Cu/ZrO2 samples were clearly presented by TEM
observations. TEM images (Fig.2) depict that large quantities of small-sized black Cu NPs are
dispersed on the surface and with the elevated calcination temperature, more serious
agglomeration of NPs can be observed. Obviously, the average Cu particle sizes in Cu/ZrO2
samples estimated by TEM are much smaller than those based on the XRD results (Table 1). In
general, the smaller the particles, the lower the order of the atoms, the more difficult it is to be
detected by XRD. Even if the samples contain only a small number of large-sized particles, the
intense diffraction originating from large particles may overshadow that of small-sized particles
in the XRD patterns [43]. As further evidenced by TEM images (Fig.S1), a few large-sized Cu
particles can be detected in Cu/ZrO2 samples. As it is known, smaller metal particles commonly
exhibit higher hydrogenation activities compared with larger ones owing to the larger active
metal surface areas and low activation energies for dissociation of hydrogen [44]. For example,
it was reported that the smaller copper particles in Cu-based catalyst showed higher catalytic
activities in the hydrogenation of dimethyl oxalate to methyl glycolate [45]. Therefore, we
consider that in our case, a large amount of small-sized Cu particles should be more important
in terms of catalytic activity, compared with a few large-sized Cu particles.
8

In addition, all Cu/ZrO2 samples show type-IV N2 adsorption-desorption isotherms,
reflecting the presence of mesoporous structure (Fig.S2) [46]. The pore diameter distributions
reveal that the average pore diameter of samples is slightly increased with the elevated
calcination temperature, indicative of the occurrence of particle agglomeration to some extent.
As summarized in Table 1, the BET specific surface area of reduced Cu/ZrO2 samples decreases
gradually with the elevated calcination temperature, due to the enhanced crystallization of t-ZrO2
phase in samples, as well as more serious agglomeration of particles at higher calcination
temperatures. Meanwhile, it is noted that metallic copper surface area for Cu/ZrO2-R determined
based on N2O titration is higher (23.6 m2/g) than those for other Cu/ZrO2 samples, in good
accordance with the above XRD results.
3.2 Redox behaviors and surface electronic states of samples
The redox behaviors of CuO/ZrO2 precursors were investigated by H2-TPR analysis. As
displayed in Fig.3, CuO/ZrO2-R sample presents a broad peak in the temperature range of 125-
210 °C, which is associated with the reduction of surface highly dispersed Cu2+ species. With the
elevated calcination temperature to 400 °C, in addition to a relatively small peak for surface Cu2+
species, another large reduction peak appears at about 215 °C, mirroring the presence of a large
amount of bulk CuO. In the case of CuO/ZrO2-500, two reduction peaks for surface highly
dispersed Cu2+ species and bulk CuO obviously shift to higher temperatures of 180 °C and 235 °C,
respectively. The reduced reducibility of Cu2+ species for CuO/ZrO2-500 strongly reflects the
enhanced interactions between Cu2+ species and the ZrO2 support, probably due to the formation
of the Cu-O-Zr-like structure at the metal-support interface [41,42]. As the calcination
temperature is further elevated to 600 °C, the reduction peak area for highly dispersed Cu2+
species is significantly decreased, despite a slight increase in the reduction temperature, mainly
owing to the sintering and accumulation of surface Cu2+ species and thus the formation of large-
sized bulk CuO in the sample. The above results demonstrate that an appropriate calcination
9

temperature of 500 °C is good for the enhanced interactions between Cu2+ species and the ZrO2
matrix.
To determine surface electronic states of copper species on Cu/ZrO2 samples, XPS data were
obtained (Fig.4A). Clearly, in each case, Cu 2p3/2 and Cu 2p1/2 signals appear at about 932.6 eV
and 952.5 eV, respectively, and no satellite peaks are found in the binding energy range of 940-
945 eV. This means that Cu2+ species should be reduced to Cu0 species and/or Cu+ species [47].
To further discriminate between Cu+ and Cu0 species, the Cu LMM XAES of samples was
recorded (Fig.4B). Deconvolution of the spectra clearly reveals the coexistence of Cu0 and Cu+
species in all samples, based on two different kinetic energy values (about 916.2 eV for Cu+ and
918.6 eV for Cu0, respectively) [48,49]. Noticeably, surface Cu+/(Cu0+Cu+) ratio calculated by
XAES subpeak fitting increases gradually with the calcination temperature up to 500 °C, due to
the formation of more Cu+-O-Zr-like structure at the metal-support interface thereby significantly
stabilizing Cu+ species [41]. However, the relative proportion of surface Cu+ species in the
Cu/ZrO2-600 inversely decreases, which is attributable to the weakened metal-support
interactions originating from the presence of a large amount of large-sized bulk CuO species in
the CuO/ZrO2-600 precursor.
Previous studies have demonstrated that surface acid-base properties of catalysts could
greatly affect the FFA hydrogenation to form CPO [27,36]. Especially, surface weak acidic sites
were beneficial to the improvement in the CPO selectivity, whereas strong acidic or basic sites
was not conducive to the formation of CPO. To determine surface acidity of Cu/ZrO2 samples,
in situ IR spectra of adsorbed pyridine was recorded (Fig.5A). In each case, several absorption
bands appearing at 1400-1700 cm-1 are correlated with pyridine adsorption on Lewis acidic sites
(LA: 1450 cm-1, 1612 cm-1) and Brønsted/Lewis acidic sites (LA+BA: 1498 cm-1), as well as on
H-bonding adsorbed pyridine (HB: 1595cm-1,1601cm-1) [50]. Notably, with the elevated
calcination temperature, the intensities of all the absorption bands are reduced progressively and
almost completely disappear in the case of Cu/ZrO2-600, indicative of the reduced surface acidity.
Further, surface acidity of samples was analyzed by NH3-TPD (Fig.5B). In each case, there is a
10

broad desorption peak in the range of 100-550 °C. Fine deconvolution of the broad peak depicts
that two desorption subpeaks in the range of 100-350 °C are assigned to NH3 desorption on weak
acidic sites, while another peak to NH3 desorption on medium-strength acidic sites [51,52]. For
pristine ZrO2-500 sample, besides three desorption subpeaks below 500 °C, two small desorption
subpeaks above 500 °C are associated with strong Lewis acidic sites, confirming that Cu+ species
should exist in the form of Cu+-O-Zr structure at Cu0-ZrO2 interface, thus suppressing the
formation of strong acidic sites. In addition, the quantitative result (Table 1) reveals that the
amount of total acidic sites decreases with the calcination temperature. Interestingly, the amount
of weak acidic sites on Cu/ZrO2-400 and Cu/ZrO2-500 samples is almost equal, probably because
surface Cu+ species may function as Lewis acidic sites. Moreover, NH3-TPD of pure Cu powder
prepared by the one-pot reduction-oxidation method in the absence of Zr(NO3)4·5H2O was
performed (Fig.S3). According to the metallic surface area-normalized basis, the amount of NH3
adsorbed on the copper powder with the copper surface area of 85 m2/g is about 0.35 mol/m2.
Considering the actual metallic copper surface area of 18.5 m2/g, a tiny amount of NH3 (0.0065
mmol/g) can be adsorbed on metallic copper sites in the case of Cu/ZrO2-500. It further confirms
that surface acidic sites on Cu/ZrO2 samples mainly come from the ZrO2 support and the metal-
support interface.
3.3 Catalytic FFA hydrogenation
The FFA hydrogenation to produce CPO in aqueous solution is a quite complicated process
(Fig.S4) [25,26], which usually can be divided into two types of reaction processes:
hydrogenation and rearrangement. Firstly, the C=O bond in FFA can be selectively hydrogenated
to form FAL intermediate. Secondly, FAL further rearranges in aqueous solution. Commonly, 4-
hydroxycyclopentenone can be first produced through the FALrearrangement, but it is extremely
unstable in the reaction system and quickly converts to 2-cyclopentenone (2-CPEO). Thirdly, 2-
CPEO is further hydrogenated to generate CPO. At last, CPL is produced by the deep
hydrogenation of CPO. Meanwhile, THFA can be generated through the reduction of C=C bond
11

in FAL, besides small amounts of oligomers produced by the reaction between FFA and FAL. As
a result, to achieve a high CPO selectivity, surface active sites on catalysts should enable the
promotion of the FALrearrangement and prevent the C=C bond hydrogenation or polymerization
reactions.
The change of FFAconversion and product selectivities over different Cu/ZrO2 catalysts with
the reaction time is presented in Fig.6. It is worthwhile to note that over Cu/ZrO2-R and Cu/ZrO2-
400, a complete conversion of FFA is achieved within 30 min, indicative of their extremely high
catalytic activity. In the cases of Cu/ZrO2-500 and Cu/ZrO2-600, however, the complete
conversion of FFA needs a longer reaction time of 2 h. During FFA hydrogenation, surface Cu0
sites are mainly responsible for dissociation of H2 [35]. Meanwhile, surface Cu+ species were
reported to enable the adsorption and activation of the C=O bond in carbonyl compounds
[29,35,36], probably preventing the formation of THFA through the hydrogenation of furan ring
in FFA molecule in our case. Recently, Hronec et al. also reported that Cu+ species played an
important role in gaining excellent catalytic activity in the FFA hydrogenation into CPO because
of the interaction between Cu+ species and the C=O bond in FFA [26]. It suggests that metallic
copper species are not the sole important catalytically active sites in the FFA hydrogenation, and
Cu+ site can function as another active site. Therefore, the higher hydrogenation activities of
Cu/ZrO2-R and Cu/ZrO2-400 catalysts should be attributed to surface synergy between Cu0 and
Cu+ species, especially the presence of more highly dispersed Cu0 species (Table 1).
Correspondingly, in the case of Cu/ZrO2-600, a reduced hydrogenation activity may be
associated with both the smaller copper surface area and the smaller amount of surface Cu+
species. As shown in Table 2, despite the high activity, a lower CPO selectivity of 45.7 %, as
well as higher FAL and CPL selectivities (37.7 % and 15.9 %, respectively), is obtained over the
Cu/ZrO2-R after a reaction time of 4 h, in comparison with other catalysts. It suggests that during
multi-step FFA hydrogenation processes, surface Cu0 species are beneficial to the hydrogenation
of the C=O bond in substrates (i.e. FFA and CPO intermediate). For Cu/ZrO2-400 and Cu/ZrO2-
500 catalysts, initially formed FALintermediate can rapidly convert into CPO in the initial period
12

of 2 h. Among these tested catalysts, Cu/ZrO2-500 catalyst affords a highest CPO selectivity of
91.3% and a lowest FAL selectivity of 1.4 % after 4 h. It illustrates that the FAL rearrangement
can procced rapidly once FAL is formed. However, with the further increase in the reaction time
from 4 h to 8 h, the CPO selectivity declines gradually, which mainly results from the deep
hydrogenation of CPO and thus the formation of CPL. Compared with those over Cu/ZrO2-400
and Cu/ZrO2-500 catalysts, the conversion rate of FFA over the Cu/ZrO2-600 is slower, thus
leading to a decreased CPO selectivity after 4 h. Interestingly, for Cu/ZrO2-600, the CPO
selectivity almost keeps unchanged with the reaction time from 4h to 8h, due to the equal rates
between FAL conversion and CPL formation at this time originating from the lower
hydrogenation activity of Cu/ZrO2-600 catalyst.
To identify the intrinsic catalytic performance of different Cu/ZrO2 catalysts, the turnover
frequency (TOF) value of FFA conversion was determined according to the rate of FFA
consumption per number of exposed surface metallic copper sites in the initial reaction stage of
15 min (FFA conversions were controlled to less than 30 % by reducing the amount of catalyst
to 0.01 g under the same reaction conditions) (Table S1). As shown in Fig.7A, the TOF value
increases gradually with the increase in the metallic copper surface area. The significant
difference in the TOF value among Cu/ZrO2 catalysts strongly demonstrate that metallic copper
species are not the sole catalytically active sites in the FFA hydrogenation. For the FFA
hydrogenation process, the carbonyl oxygen of FFA with lone pair electrons can adsorb on
electrophilic Cu+ sites on the surface of Cu/ZrO2 catalysts, thereby promoting the activation of
the C=O bond in FFA [26,29,36]. Correspondingly, besides surface metallic copper species,
surface Cu+ species can greatly improve the activity of the present Cu/ZrO2 catalysts. Further,
the variations in the CPO selectivity at different reaction time as a function of surface
Cu+/(Cu0+Cu+) ratio on different Cu/ZrO2 catalysts are plotted (Fig.7B). Notably, despite
different reaction times, the CPO selectivity presents a good linear trend with surface
Cu+/(Cu0+Cu+) ratio, indicating that surface Cu+ species may facilitate the FAL rearrangement.
Moreover, the initial rate of CPO formation at low conversion levels was determined by using a
13

reduced amount of catalyst (Table S1). As shown in Fig.7C, the higher surface Cu+/(Cu0+Cu+)
ratio, the faster the initial rate of CPO generation. Obviously, the highest initial rate of CPO
formation is obtained over the Cu/ZrO2-500. It indicates that the formation of CPO is closely
related to Cu+ species. Under the same reaction conditions, however, no CPO can be generated
over the Cu/ZrO2-600 after a reaction time of 15 min, due to its poor hydrogenation activity. The
above results confirm that surface Cu+/(Cu0+Cu+) ratio on Cu/ZrO2 catalysts can play a key role
in promoting the formation of CPO product.
Furthermore, the effect of the reaction temperature on the FFA conversion and product
selectivities over the Cu/ZrO2-500 was explored. As shown in Fig.8A, one can see that in
addition to target CPO product, a large amount of FAL intermediate is produced at a low reaction
temperature of 130 °C. With the increasing reaction temperature to 150 °C, Cu/ZrO2-500 delivers
a high CPO yield of 91.3 %. According to the literature [53], the ionization constant of water
(pKa) at 150°C is about 11.6, indicating that the high temperature is beneficial to the dissociation
of water as an endothermic reaction. Moreover, it was reported that the high reaction
temperatures could contribute to the rearrangement of FFAto 4-hydroxycyclopentenone(4-HCP)
[54], another intermediate, during the FFA hydrogenation into CPO. It is expected that an
additional effect of the acidity of water should be substantial at the high reaction temperature of
150 oC, thus promoting the formation of CPO to a large extent in our case. This is consistent with
our experimental results that high reaction temperature is conducive to accelerate the FAL
rearrangement and promote the CPO formation. Further increase in the reaction temperature,
however, leads to a gradual decline in the CPO selectivity, due to the easier deep hydrogenation
o
of CPO. Therefore, it is concluded that the appropriate reaction temperature of 150 C is
beneficial to the FAL rearrangement and the generation of CPO product. Moreover, we further
investigated the effect of hydrogen pressure on the FFA conversion and product selectivities
(Fig.8B). Noticeably, with the increasing hydrogen pressure from 0.5 to 1.5 MPa, the CPO
selectivity increases gradually from 54.5 % to 91.3 %, whereas the selectivities of FAL and 2-
CPEO intermediates present an opposite trend, despite the formation of a small amount of CPL.
14

As the hydrogen pressure is further increased to 2.0 MPa, the CPO selectivity begins to decline
due to the generation of more CPL. As a result, the high hydrogen pressure can facilitate the
hydrogenation of both the C=O and C=C bonds in reactants and intermediates.
The stability of Cu/ZrO2-500 also was examined (Fig.S5), the catalyst can maintain a high
activity in the FFA hydrogenation after five successive cycles, along with high conversions of
above 99 %, and the CPO selectivity is decreased only by about 2.5%. The XRD patterns, Cu 2p
XPS and Cu LMM XAES (Fig.S6) of used Cu/ZrO2-500 catalyst reveal that there is almost no
change in the XRD patterns of Cu/ZrO2-500 after five successive runs, and the relative
proportion of surface Cu+ species in total copper species also almost keeps unchanged. In
addition, elemental analysis by ICP-AES shows that the decrease in the Cu content in the catalyst
is only 0.4 wt % after five runs (Table S2), indicating that there is a slight loss of the active
composition due to a slight solubility of active Cu species in the reaction medium, thereby
causing a slight decrease in the activity. And, TEM images of the used catalyst show that the size
of Cu particles does not change significantly after five runs (Fig.S6). The above results
demonstrate the good stability of Cu/ZrO2-500 catalyst.
3.4 Mechanistic study on the FFA hydrogenation
To study the reaction mechanism of the FFA hydrogenation over Cu/ZrO2 catalysts, a series
of controlled reactions were further tested (Table 3). In the FFA hydrogenation, the FAL
rearrangement is a key step in generating target CPO product. Therefore, the hydrogenation of
FAL as substrate also was examined over different catalysts (Table 3, entries 1-4). In the absence
of any catalysts, a quite low FAL conversion of 2.5 % is obtained (Table 3, entry 1). One can see
that pure ZrO2-500 and Cu/ZrO2-R afford the FALconversions of 8.1 % and 65.7 %, respectively,
along with the CPO selectivities of 36.9 % and 67.5 % (Table 3, entries 2 and 3). Cu/ZrO2-500
(Table 3, entry 4) yields a greatly increased FAL conversion of 97.8 %. To determine the effect
of surface acidic sites on the above reaction, surface Brønsted/Lewis acidic sites on the Cu/ZrO2-
500 catalyst was blocked by pyridine. First, the catalyst was soaked in pyridine for 15 min.
15

Subsequently, the catalyst was purged with nitrogen gas and then dried at room temperature for
12 h under vacuum. Finally, the reaction was carried out over the poisoned catalyst. Noteworthy,
after surface acidic sites on the Cu/ZrO2-500 was blocked by pyridine, the FFA conversion
slightly decreases but the CPO selectivity drops rapidly from 62.7 % to 40.3 % (Table 3, entries
5 and 6). It demonstrates that surface acidity of catalysts is beneficial to the formation of CPO.
The above results further illustrate that both Cu+ species and surface acidity can promote the FAL
rearrangement and thus the production of CPO.
For comparison, other different metal oxides (i.e. ZrO2, Al2O3, MgO, and ZnO) supported
copper catalysts were synthesized by the conventional impregnation (Fig.S7, S8 and S9). As
shown in Fig S8, surface acidity of ZrO2 is significantly higher than those of amphoteric Al2O3
and ZnO [43]. As expected, basic MgO presents a rather weak acidity. As shown in Table 4, the
CPO selectivities over Cu/ZrO2-im and Cu/Al2O3-im are much lower than that over the present
Cu/ZrO2-500, which should be associated with less surface Cu+ species on Cu/ZrO2-im and
Cu/Al2O3-im. Meanwhile, compared with Cu/ZrO2-im and Cu/Al2O3-im, Cu/ZnO-im affords a
much lower selectivity to CPO, due to its weaker surface acidity. Especially, almost no CPO can
be produced over the Cu/MgO-im, which is corelated with the absence of surface Cu+ species
and acidic sites. Although Cu/ZnO-im possesses the similar amount of surface Cu+ species to
that on the Cu/Al2O3-im, Cu/ZnO-im delivers a much lower CPO selectivity. The above results
further demonstrate that both favorable surface acidity and abundant Cu+ species on catalysts
can significantly promote the FAL rearrangement to form CPO in the FFA hydrogenation.
Based on the above characteristic and experimental results, a possible catalytic mechanism
for the FFA hydrogenation to CPO over Cu/ZrO2 catalysts is proposed (Fig.9). Firstly, surface
highly dispersed Cu0 species as active centers can dissociate molecular hydrogen and thus
provide available active hydrogen species for reducing the C=O bond in FFA to generate FAL.
Meanwhile, Cu+ species as electrophilic centers can polarize the C=O bond of FFA through the
isolated electron pair of carbonyl oxygen. In a way, surface Lewis acidic sites can serve as
electron acceptors to accept the electron on the carbonyl group, which facilitates the attack of
16

active hydrogen species to the C=O bond and not to the C=C double bond in furan ring [55].
Subsequently, the FAL rearrangement can proceed through the self-dissociation of water to
generate 2-CPEO intermediate. During FAL rearrangement, weak Lewis acidic sites are
beneficial to the formation and stability of formed carbocation species [56], while Cu+ species
are beneficial to the stability of formed oxycation species [26,54]. Finally, CPO is produced by
the hydrogenation of 2-CPEO. As a result, unique surface synergistic effect between Cu+ species
and acidic sites on Cu/ZrO2 catalysts is the key to achieve the high CPO selectivity in the present
catalytic system.
4. Conclusions
In summary, series of Cu/ZrO2 catalysts synthesized by our developed one-pot reduction-
oxidation route were employed in the conversion of FFA to produce CPO in water. By adjusting
the calcination temperature of catalyst precursors, surface structures and catalytic performances
of Cu/ZrO2 catalysts could be delicately regulated. It was found that the Cu/ZrO2 catalyst
obtained at the calcination temperature of 500 °C exhibited a superior catalytic performance with
a high CPO yield of 91.3 % to other supported Cu/ZrO2 ones under mild reaction conditions (i.e.
a low hydrogen pressure of 1.5 MPa and 150 °C). Combining a series of structural
characterizations with catalytic experimental results, we revealed that an appropriate calcination
temperature of 500 °C could significantly improve the interactions between Cu species and the
ZrO2 support, thus forming Cu+-O-Zr-like structure at the metal-support interface. Surface
synergy between acidic sites and Cu+ species on Cu/ZrO2 catalysts greatly promoted the
rearrangement of FAL intermediate and thus the formation of high-yield target CPO product in
the present catalytic system. The present findings provide a high performing supported copper
catalyst for the conversion of FAL into CPO in water, while as-constructed surface synergistic
effect between Cu+ species and acidic sites open a new idea for the design of other metal catalysts
for the biomass transformations through catalytic hydrogenation/hydrogenolysis processes. Such
17

readily available, highly efficient and stable Cu-based heterogeneous catalyst possesses
promising potential for the industrial application.
Acknowledgments
This study was funded through National Natural Science Foundation of China
(21776017;21521005) and Fundamental Research Funds for the Central Universities
(buctrc201528).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.apcata.xxxx.
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Table
Table 1 The compositional and structural properties of samples.
b
c
d
+ e
Samples
Content (wt.%) a SBET
D111
SCu
SCu
Cu+/ (Cu0+Cu+) Specific acidity f
21

Cu
25.9
26.1
26.0
26.7
Zr
(m2/g) (nm) (m2/g)
(m2/g)
13.9
19.9
18.5
4.5
Ratio e
0.37
(mmol/g)
0.46 (0.30) g
0.32 (0.21)
0.30 (0.22)
0.14 (0.13)
Cu/ZrO2-R
Cu/ZrO2-400
Cu/ZrO2-500
Cu/ZrO2-600
23.5
24.8
24.5
25.1
115
108
93
20.2
24.0
25.2
30.9
23.6
21.6
18.5
6.0
0.48
0.50
58
0.43
a
b
c
Determined by ICP-AES; Specific surface area calculated by the BET method; Average
crystallite size of metallic Cu particles estimated from XRD patterns; d Metallic copper surface
area determined by N2O titration; e Determined by Cu LMM XAES analysis; f Density of total
acidic sites determined by NH3-TPD; g Density of weak acidic sites determined by NH3-TPD.
Table 2 Catalytic results of Cu/ZrO2 catalysts for the FFA hydrogenation. a
Selectivity (%)
FAL
2-CPEO Others b
Time
(h)
Conv.
(%)
Catalysts
CPO
16.0
45.7
19.2
89.9
18.2
91.3
8.9
CPL
0
Cu/ZrO2-R
0.5
4.0
0.5
4.0
0.5
4.0
0.5
4.0
100
100
100
100
77.1
100
75.5
100
82.8
0.6
0
0.6
0.7
2.3
3.1
1.7
2.1
1.1
1.4
15.9
0
37.7
Cu/ZrO2-400
Cu/ZrO2-500
Cu/ZrO2-600
72.4
6.1
0.9
6.8
1.4
3.0
0.8
4.2
0
1.9
73.3
3.8
0
1.4
87.0
71.2
3.5
23.1
a Reaction conditions: catalyst, 0.05 g; FFA, 5 mmol; H2O, 15 ml; reaction temperature, 150 °C;
hydrogen pressure, 1.5 MPa. b Oligomers by-product.
Table 3 Catalytic results of Cu/ZrO2 catalysts for the FAL or FFA hydrogenation. a
Entry
Catalysts
Substrates Conv.
Selectivity (%)
22

(%)
CPO
CPL FAL 2-CPEO Others
1
blank
FAL
FAL
FAL
FAL
FFA
FFA
2.5
8.1
7.1
0
--
--
30.3
2.1
0
62.6
61.0
7.6
2
ZrO2-500
36.9
67.5
77.6
62.7
40.3
0
3
Cu/ZrO2-R
Cu/ZrO2-500
Cu/ZrO2-500
Cu/ZrO2-500
65.7
97.8
98.2
95.9
24.9
18.5
0.7
0.2
-
4
-
0.5
9.2
2.5
3.4
5 b
6 c
25.6
54.9
1.8
2.1
a
Reaction conditions: catalyst, 0.05 g; FAL or FFA, 5 mmol; H2O, 15 ml; reaction
temperature, 150 °C; hydrogen pressure, 1.5 MPa; reaction time, 4 h. b Reaction time, 1.5 h.
c catalyst was poisoned by pyridine before reaction; reaction time, 1.5h.
Table 4 Catalytic results of metal oxides supported copper catalysts for FFA hydrogenation a
Cu+/ (Cu0+Cu+)
Ratio d
Selectivity (%)
b
c
D111
SCu
Conv.
(%)
Entry
Catalyst
(nm)
(m2/g)
CPO
91.3
67.0
42.9
0.6
CPL
3.8
1.0
1.5
0.3
0
FAL Others
1
2
3
4
5
Cu/ZrO2-500
Cu/ZrO2-im
Cu/Al2O3-im
Cu/MgO-im
Cu/ZnO-im
25.2
28.6
27.2
25.7
34.8
18.5
12.6
11.3
14.7
9.2
0.50
0.33
0.29
0
100
90.8
99.7
100
1.4
3.5
13.1
5.1
18.9
50.5
98.5
86.3
0.6
0.30
63.2
3.9
9.8
a Reaction conditions: catalyst, 0.05 g; FFA, 5 mmol; H2O, 15 ml; reaction temperature, 150 °C;
b
hydrogen pressure, 1.5 MPa; reaction time, 4h. Average crystallite size of metallic Cu
c
particles estimated from XRD patterns; Metallic copper surface area determined by N2O
titration; d Determined by XAES analysis.
Figure captions
23

Fig.1 XRD patterns of CuO/ZrO2 precursors (A) and Cu/ZrO2 samples (B) obtained at different
calcination temperatures: (a) room temperature, (b) 400 °C, (c) 500 °C and (d) 600 °C.
Fig.2 TEM images of Cu/ZrO2-R (A), Cu/ZrO2-400 (B), Cu/ZrO2-500 (C) and Cu/ZrO2-600 (D)
samples.
Fig.3 H2-TPR profiles of CuO/ZrO2-R (a), CuO/ZrO2-400 (b), CuO/ZrO2-500 (c) and CuO/ZrO2-
600 (d).
Fig.4 Cu 2p XPS (A) and Cu LMM XAES (B) of Cu/ZrO2-R (a), Cu/ZrO2-400 (b), Cu/ZrO2-500
(c) and Cu/ZrO2-600 (d) samples.
Fig.5 in situ IR spectra of adsorbed pyridine (A) and NH3-TPD curves (B) over pristine ZrO2-
500 (a) and Cu/ZrO2-R (b), Cu/ZrO2-400 (c), Cu/ZrO2-500 (d) and Cu/ZrO2-600 (e)
samples.
Fig.6 Change in the conversion and product selectivities in the FFA hydrogenation with the
reaction time over Cu/ZrO2-R (A), Cu/ZrO2-400 (B), Cu/ZrO2-500 (C) and Cu/ZrO2-600
(D). Reaction conditions: reaction temperature, 150 °C; hydrogen pressure, 1.5 MPa.
Fig.7 The variation in the TOF value of FFA conversion as a function of metallic copper surface
area for Cu/ZrO2 catalysts (A), the variations in the CPO selectivity as a function of surface
Cu+/(Cu0+Cu+) ratio on Cu/ZrO2 catalysts at different reaction times (B), and the variations
in the initial rate of CPO formation as a function of surface Cu+/(Cu0+Cu+) ratio in the
reaction stage of 15 min (C). Reaction conditions: reaction temperature, 150 °C; hydrogen
pressure, 1.5 MPa.
Fig.8 Change of FFA conversion and product selectivities over the Cu/ZrO2-500 in the FFA
hydrogenation at different reaction temperatures (A) and different hydrogen pressures (B).
Fig.9 Proposed reaction mechanism for the catalytic FFA hydrogenation to CPO over Cu/ZrO2
catalysts.
24

25

26

27

28

29