Mesoporous mixed CuCo oxides as robust catalysts for liquid-phase furfural hydrogenation

Article abstract of DOI:10.1016/j.apcata.2018.12.010

A series of highly ordered mesoporous CuCo oxide catalysts with a controlled composition are successfully synthesized by nanocasting from mesoporous silica, KIT-6 template. Liquid-phase furfural (FAL) hydrogenation is carried out to find the optimal composition of the CuCo oxide catalysts to achieve the best catalytic performance. As-prepared mesoporous mixed CuCo oxides exhibit a high surface area (60?135 m² g^?1) and a well-defined ordered mesostructure with homogenous dispersion of Cu and Co. Among various compositions of Cu_xCo_y oxides (x = 1–9, y = 1–x) studied, the Cu₁Co₅ oxide catalyst shows the highest conversion in the hydrogenation of FAL, which is superior to those achieved with mesoporous monometallic oxides, CuO and Co₃O₄. While 2-methylfuran is produced from furfuryl alcohol via aldehyde hydrogenation and subsequent hydrogenolysis, the formation of 2-methylfuran increased with a decrease in the Cu/Co ratio of the CuCo oxide catalyst. The mixed CuCo oxide catalyst is readily reduced under the reaction environment to produce metallic CuCo as the active species. The synergistic interactions between Cu and Co in the mixed CuCo oxide catalysts play an important role in the outstanding catalytic performance for FAL hydrogenation, which could not be achieved with either of the monometallic catalysts or their physical mixtures. The excellent stability and recyclability of mesoporous mixed CuCo oxide catalysts as well as the exceptionally high activity, surpassing those of the monometallic oxides, render them promising as a low-cost and efficient catalyst for the industrial upgrading of biomass-derived FAL.

Full text of DOI:10.1016/j.apcata.2018.12.010

Accepted Manuscript
Title: Mesoporous mixed CuCo oxides as robust catalysts for
liquid-phase furfural hydrogenation
Authors: Chinh Nguyen-Huy, Hojeong Lee, Jihyeon Lee, Ja
Hun Kwak, Kwangjin An
PII:
S0926-860X(18)30606-9
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.12.010
APCATA 16914
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Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
29 July 2018
30 November 2018
12 December 2018
Please cite this article as: Nguyen-Huy C, Lee H, Lee J, Kwak JH, An K, Mesoporous
mixed CuCo oxides as robust catalysts for liquid-phase furfural hydrogenation, Applied
Catalysis A, General (2018), https://doi.org/10.1016/j.apcata.2018.12.010
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Mesoporous mixed CuCo oxides as robust catalysts
for liquid-phase furfural hydrogenation
Chinh Nguyen-Huy,‡ Hojeong Lee,‡ Jihyeon Lee, Ja Hun Kwak,*, and Kwangjin An*
School of Energy and Chemical Engineering, Ulsan National Institute of Science and
Technology(UNIST), Ulsan 44919, Republic of Korea
‡These authors contributed equally.
*Correspondence should be addressed (E-mail: kjan@unist.ac.kr)
1

Graphical abstract
100
80
60
40
20
0
MF
FA
FAL
FR
Cu1Co5
mixture
Cu
Co
Cu1Co5
2

Highlights

As-prepared mesoporous mixed CuCo oxides exhibit a high surface area and well-defined
ordered mesostructure with homogenous dispersion of Cu and Co.
Among various compositions of CuxCoy (x = 1‒9) studied, the Cu1Co5 catalyst shows the
highest conversion in furfural hydrogenation, which is superior to those observed with
mesoporous monometallic oxides, CuO and Co3O4.




In terms of selectivity, the formation of 2-methylfuran increases with a decrease in the
Cu/Co ratio in the CuCo catalyst.
The mixed CuCo oxide catalyst is readily reduced under the reaction environment to
produce metallic CuCo as the active species.
Excellent stability and recyclability of mesoporous mixed Cu1Co5 oxide catalysts as well
as an exceptionally high activity, surpassing those of the monometallic oxides make them
promising as low-cost efficient catalysts for industrial upgradation of biomass-derived
furfural.
Abstract
A series of highly ordered mesoporous CuCo oxide catalysts with a controlled composition are
successfully synthesized by nanocasting from mesoporous silica, KIT-6 template. Liquid-
phase furfural (FAL) hydrogenation is carried out to find the optimal composition of the CuCo
oxide catalysts to achieve the best catalytic performance. As-prepared mesoporous mixed
CuCo oxides exhibit a high surface area (60‒135 m2‧ g-1) and a well-defined ordered
mesostructure with homogenous dispersion of Cu and Co. Among various compositions of CuxCoy
oxides (x = 1‒9) studied, the Cu1Co5 oxide catalyst shows the highest conversion in the
3

hydrogenation of FAL, which is superior to those achieved with mesoporous monometallic oxides,
CuO and Co3O4. While 2-methylfuran is produced from furfuryl alcohol via aldehyde
hydrogenation and subsequent hydrogenolysis, the formation of 2-methylfuran increased with a
decrease in the Cu/Co ratio of the CuCo oxide catalyst. The mixed CuCo oxide catalyst is readily
reduced under the reaction environment to produce metallic CuCo as the active species. The
synergistic interactions between Cu and Co in the mixed CuCo oxide catalysts play an important
role in the outstanding catalytic performance for FAL hydrogenation, which could not be achieved
with either of the monometallic catalysts or their physical mixtures. The excellent stability and
recyclability of mesoporous mixed CuCo oxide catalysts as well as the exceptionally high
activity, surpassing those of the monometallic oxides, render them promising as a low-cost and
efficient catalyst for the industrial upgrading of biomass-derived FAL.
Key words: mesoporous oxides, CuCo, furfural, hydrogenation, selectivity
4

1. Introduction
With the increasing demand for chemicals and fuels, it is imperative to seek and find
sustainable alternatives for petroleum-based products [1]. Biomass, the only renewable source
of carbon-neutral energy, can be used for the production of fuels and value-added chemicals,
while addressing the need to limit the usage of declining fossil resources and related
environmental issues [2,3]. Specifically, oxygen-containing chemicals are highly attractive
owing to their high added value and the lower consumption of hydrogen for their production.
Furfural (FAL) has been identified as one of the most important biomass-derived molecules,
which are produced from hemicelluloses via acid-catalyzed hydrolysis and dehydration in a
large scale [4]. Some of the important chemicals derived from FAL include furfuryl alcohol
(FA), 2-methylfuran (MF), furan (FR), and tetrahydrofuran (THF). Heterogeneous catalysts
are widely used for FAL conversion, owing to their higher stability and the ease of separation
from the raw materials as compared to homogeneous catalysts. To date, many groups have
utilized a variety of metals such as Au [5,6], Pt [7-10], Pd [11-13], Ru [14,15], Ir [16], and their
alloys with a second transition metal [17-24] for FAL hydrogenation, because of their high
activities. However, the high cost and scarcity of the precious metals limit their practical
application. Therefore, the development of efficient catalysts based on non-precious metals is
important for commercializing biomass-derived products.
Ordered mesoporous materials are widely used as catalysts as well as supports owing to their
attractive properties, such as a high surface area, uniform and tunable pore size and shape, and
their availability in various structures and compositions [25,26]. Moreover, mesoporous
materials have a highly crystalline framework with highly uniform pores, which renders them
ideal catalysts for understanding the catalytic performance related to their structure,
composition, and porosity [27]. Highly ordered mesoporous transition metal oxides can be
5

easily prepared via the nanocasting method, in which mesoporous silica materials such as SBA-
15 or KIT-6 are used as hard templates [28-32]. Owing to the versatility of the method and
various structures accessible by the use various templates, mesoporous metal oxides with well-
ordered pores are widely used for many catalytic studies [33-42]. However, the availability of
mesoporous metal oxides, which are usually required to be reduced before using as catalysts,
is very limited owing to their low structural stability under H2-rich conditions. Often, under
severe reducing conditions, the oxides are converted into metals, resulting in severe
aggregation or collapse of the mesoporous framework [43]. In order to overcome this problem,
mixed metal oxides might be used as alternatives, because bimetallic structures or alloys not
only strengthen the material but also provide new active species which are not available in
monometallic compounds [1, 44-54]. For instance, a highly stable bimetallic Cu–Co catalyst
shows high activity in FAL hydrogenation, owing to the strong interaction between Cu and Co
in the well-mixed oxide structure [54]. The combination of Cu metal and CoOx is shown to be
effective as a bi-metallic Cu–Co catalyst for FAL hydrogenation, since the C–O bond can be
broken using the early transition metal oxides such as CoOx, ReOx, and MnOx [55].
Herein, we report the synthesis of highly ordered mesoporous mixed oxides of CuCo via the
nanocasting approach using cubic Ia3d mesoporous silica, KIT-6, as a hard-template. Catalytic
properties of the mixed CuCo oxides in liquid-phase FAL hydrogenation were evaluated in
comparison with those of the corresponding monometallic oxides, CuO and Co3O4. The catalysts
were characterized via various techniques, including transmission electron microscopy (TEM),
X-ray diffraction (XRD), N2-adsorption, X-ray photoelectron spectroscopy (XPS), and
temperature-programmed reduction using H2 (H2-TPR), to understand not only the structural
and chemical properties of the mesoporous mixed CuCo oxides prepared with controlled
compositions but also real active species. The catalytic activity and selectivity in the FAL
6

hydrogenation over various mesoporous CuCo oxide catalysts were investigated to determine the
optimal composition of the catalyst. Further, distinct reaction pathways that affect the product
selectivity and catalyst recyclability are also studied.
2. Experimental
2.1 Synthesis of Catalysts
2.1.1 Silica KIT-6 template
KIT-6 silica was synthesized according to a previously reported method of the cooperative
assembly described elsewhere [28,31]. Briefly, 27 g of Pluronic P123 (PEO20−PPO70−PEO20)
(Sigma-Aldrich) was dissolved in a 2 L polypropylene bottle containing 980 mL of water and
43.5 mL of concentrated HCl. Then, 33.3 mL of butanol was added to the solution at 35 °C,
with vigorous stirring. The mixture was stirred continuously for further 6 h. After that, 62.2
mL of tetraethoxysilane (Sigma-Aldrich) was added under vigorous stirring and the mixture
was maintained at the same temperature, overnight. The hydrothermal process was then
performed at 80 °C under static condition. The solid was filtered, dried overnight at 90 °C, and
calcined at 550 °C for 6 h.
2.1.2 Synthesis of mesoporous oxides catalysts
Mesoporous oxides with controlled Cu and Co contents were synthesized by the nanocasting
method using mesoporous silica, KIT-6, as illustrated in Scheme 1. Briefly, metal precursors,
Cu(NO3)2·2.5H2O and Co(NO3)2·6H2O (Sigma-Aldrich), were dissolved in 6 mL water. In all
cases, the concentrations of the total metal ions, Cu2+ and Co2+ + Co3+, were kept constant at 4
M. In general, 6 g of KIT-6 silica was dispersed in 50 mL of toluene and the metal precursor
solution was added to the mixture. After the evaporation of toluene, an ammonia solution was
vaporized onto the remaining powder to catalyze the creation of an ordered oxide framework.
7

The ammonia-driven deposition is well-known to be an efficient method to generate a well-
dispersed mesoporous Cu-based oxide. The products, mesoporous oxides with various Cu–Co
compositions, were finally calcined at 300 °C for 5 h at the heating rate of 2 °C·min-1. The
silica templates were completely removed using 2 M aqueous NaOH solution which was pre-
heated to 90 °C. The precipitates were collected by centrifugation and washed with water. After
4 cycles of washing steps with both NaOH solution and water, the mesoporous Cu–Co oxides
were collected, followed by drying at 50 °C. The obtained mesoporous Cu–Co oxides are
denoted as CuxCoy, where, x and y represent the relative ratio of Cu and Co.
2.2. Catalysts characterization
Several physical and chemical techniques were employed to characterize the mesoporous
mixed CuCo oxide catalysts. Surface properties of mesoporous materials were characterized
by the Brunauer–Emmett–Teller (BET) method by nitrogen physisorption in a BEL SORP-
max unit. BET specific surface area was calculated in the range of p/p0 = 0.05–0.2. A pore
diameter was calculated by the BJH method from the adsorption branches. A pore volume
obtained by the BJH method. The chemical composition of the catalysts was verified by
inductively coupled plasma optical emission spectroscopy (ICP-OES; Varian 700-ES). XRD
patterns were recorded on a PANalytical X’Pert PRO diffractometer using Cu Kαradiation (λ
= 0.154056 nm). The surface morphology was characterized by field-emission scanning
electron microscopy (FE-SEM, Nova NanoSEM 230). Structural information was obtained by
TEM (JEOL JEM-1400). High-resolution TEM (HR-TEM, JEOL JEM-2100F) and dark field
and bright field images were also obtained. H2-TPR was carried out using Auto ChemⅡ2920
(Micromeritics Instruments Co., USA). X-ray photoelectric spectroscopy (XPS) analysis was
carried out on a K-alpha (ThermoFisher) system equipped with an Al Kα X-ray radiation
8

source. In situ powder XRD data were recorded on a SmartLab (Rigaku) X-ray diffractometer
equipped with a D/teX Ultra 250 detector using Cu Kαradiation (λ= 1.54184 Å).
2.3. Reaction procedure and product analysis
Mesoporous mixed CuCo oxide catalysts (10‒20 mg) were taken in a 100 mL stainless-steel,
high-pressure reactor containing FAL (1 g, Acros, 99%) and isopropanol (20 mL, Sigma-
Aldrich). After flushing with hydrogen, the reactor was pressurized with hydrogen to 20 bar.
The temperature of the reactor was then increased to 180 °C and maintained there with stirring
at 600 rpm for 5 h. After the completion of the reaction, the resulting liquid product was
collected and analyzed by gas chromatography (GC) using a flame ionization detector (FID;
Agilent 7820A) equipped with a capillary column (DB-Wax, 30 m length, 0.32 mm internal
diameter, and 0.25 μm film thickness). The following program was repeated: hold at 50 °C for
3 min, increase the temperature to 50–100 °C at the rate of 10 °C‧ min-1 and hold for 3 min,
and increase the temperature to 100–200 °C at the rate of 25 °C‧ min-1 and hold for 3 min. The
products, FA (Sigma-Aldrich, 98%), FR (Sigma-Aldrich, 99%), THF (Alfa Aesar, 99%), and
MF (Sigma-Aldrich, 99%), were analyzed. Finally, the conversion of FAL and product
selectivity were calculated as follows:
mol furfural consumed
Conversion (%) 
Selectivity (%) 
100
100
mol furfural fed
mol productformed
mol furfural consumed
3. Results and discussion
3.1. Characterization of the catalyst
9

Fig. 1 shows the high angle annular dark-field scanning TEM (HAADF-STEM), bright-field (BF)
STEM, and HR-TEM images of mesoporous mixed CuxCoy oxides. All five materials (Cu5Co1,
Cu3Co1, Cu1Co1, Cu1Co3, and Cu1Co5 oxides) show a highly ordered pore structure (Fig. 1a–e).
Upon examining many particles, the presence of the highly ordered mesoporous structure
throughout the matrix is confirmed, which suggests a high level of infiltration of the template by the
two precursors. Because KIT-6 has three-dimensional cubic symmetry, spherical replicas of the
CuCo oxide replica were formed. Fig. 2a shows an FE-SEM image of Cu1Co5 oxide. Apparently,
the well-ordered porous structures of Cu1Co5 oxide are clearly observed. HAADF-STEM, and EDS
mapping images of the Cu1Co5 are shown in Fig. 2b–e. They reveal that the highly ordered
mesoporous Cu1Co5 oxide contains well-mixed Cu and Co. Point scan EDS (Fig. S1) and EDS
mapping (Fig. S2) analysis also confirm homogenous dispersion of Cu and Co in Cu1Co3 oxide.
To characterize the surface components of the mesoporous CuCo oxides, XPS was performed. Fig.
3 shows the XPS Co 2p and Cu 2p spectra of CuCo with various compositions. The binding energies
of Co 2p3/2 and Co 2p1/2 are observed at 779 and 794 eV, respectively, corresponding to the
characteristic spin–orbital splitting of 15.2 eV reported previously [56, 57] (Fig. 3a). After
deconvolution, Co3+ and Co2+ peaks are observed at ∼779.4 and 781.0 eV, respectively, which
are in good agreement with the binding energies of spinel Co3O4 (∼779.5 eV for Co3+ and
∼780.8 eV for Co2+) [41]. Satellite peaks with a relatively low intensity located between the
Co 2p3/2 and Co 2p1/2 peaks indicate that Co ions are present in a partial spinel-type lattice
arrangement and the Co3+ ions are mixed with Co2+ ions [58]. The asymmetric Co 2p1/2 peak
confirms the existence of both Co2+ and Co3+ ions. The quantitative surface compositions in
terms of Co2+/Co3+ ratios are listed in Table 1. The surface Co2+/Co3+ molar ratio of the mixed
CuCo structures is much higher than that of Co3O4 (0.75) and increased with a decrease in the
Cu/Co ratio (1.69 for Cu5Co1 and 2.41 for Cu1Co5). However, as the content of Co is further
10

increased to Cu1Co7 and Cu1Co9, the Co2+/Co3+ ratios decreased to 2.01 and 1.84, respectively.
Fig. 3b shows the XPS spectra of Cu 2p and deconvolution of the Cu 2p3/2 peaks of the mixed
CuCo oxide series. Two major peaks of Cu 2p3/2 and Cu 2p1/2 are located at 933.3 and 953.1
eV, respectively. The splitting energy is 19.8 eV, which represents the formation of Cu2+ and
the value agrees well with that reported previously [58]. The satellite peaks at the high binding
energy originate from the shake-up transition due to the ligand–metal 3d charge transfer in the
presence of Cu2+. It is commonly recognized that the photoelectron peak at approximately
932.3–932.4 eV of Cu 2p3/2 is usually from tetrahedral Cu+. The components at 933–934 eV
with their satellites are due to octahedral Cu2+. In all the mesoporous mixed CuCo oxides,
tetrahedral Co2+ ions are dominant. These results suggest the surface structure of the CuCo to
be as follows: Co2+ ions are partially substituted by Cu2+ ions in the mixed CuCo oxide
structures [59, 60], while the entire structure contains well-mixed Cu and Co oxides,
maintaining a homogeneous distribution of Cu and Co according to their composition.
The composition of the as-prepared mesoporous CuCo oxides was characterized by XRD (Fig.
4a). XRD patterns of monometallic oxides could be indexed to Co3O4 (JCPDS 42-1467) and CuO
(JCPDS 45–0937). The well-resolved diffraction peaks in Fig. 4a are attributed to the high
crystallinity of the resulting mesoporous oxides. Depending on the composition of CuxCoy (x = 1‒
9), the corresponding XRD peaks are similar or intermediate between those of Co3O4 and CuO,
indicating the successful formation of mesoporous mixed oxides, with the maintenance of the
original composition of the precursors. In the XRD patterns of the oxide catalysts, no metallic
species, Co, Cu, or CoCu alloy, are identified. However, the enlarged XRD patterns of the CuCo
oxide catalysts (Fig. 4b) clearly show that the peaks corresponding to CuO are shifted to the positive
direction, indicating an incorporation of Cu2+ ions into the spinel structure of Co3O4 with the
formation of CuxCoyO4 compound, which is in good agreement with the XPS results. N2
11

adsorption–desorption isotherms of the mesoporous CuO, Co3O4, and CuxCoy oxides exhibit a
typical type-IV characteristic with an apparent hysteresis loop at a relative pressure (p/p0) of
0.4–0.9, indicating a well-ordered mesoporous structure with uniform pore sizes (Fig. S3).
Table 1 summarizes the BET specific surface area, mean pore diameter, and pore volume of
the mesoporous oxides and their relative Cu/Co ratio determined by ICP-OES. The Cu/Co
ratios determined by ICP-OES match well with the actual precursor ratios used. The as-
prepared mixed CuCo oxides and Co3O4 have high surface areas (60‒135 m2‧ g-1), which are
much higher than that of CuO (46 m2‧ g-1). The increase in the Co content is associated with
an increase in the surface area and pore volume (Table 1). The high surface areas and pore
volumes of the replicated CuCo oxides are expected to enhance their catalytic activity.
To investigate the reduction and structural evolution of the mixed CuCo oxides, H2-TPR
experiments were carried out. In general, when metal species interact with carriers, thus
creating new surface compounds or altering their chemical states, the temperature at which
they are reduced changes. Fig. 4c shows the TPR patterns of the CuxCoy with various Cu/Co
ratio (x/y = 5/1, 3/1, 1/1, 1/3, 1/5, 1/7, and 1/9) and the monometallic oxide counterparts. The
H2-TPR profile of Co3O4 displays two peaks at 206 and 363 °C due to hydrogen consumption.
These peaks are ascribed to the two-step reduction of Co3O4 to Co(0), which takes place via
the intermediate, CoO [61]. On the contrary, CuO shows only one peak at 275 °C,
corresponding to the reduction of CuO to Cu(0) [62]. The mixed CuCo oxides show different
behaviors in the H2-TPR, depending on the ratio of Cu/Co, owing to strong interactions
between Cu and Co. This characteristic indicates that the H2-TPR proceeds through the
reduction of both segregated (CuO) and mixed oxides, which has been observed in many
previous studies on bimetallic CuCo catalysts [60,63,64]. Overall, the H2-TPR profiles reveal
that the main peak of the CuCo shifts to a lower temperature, as the content of Cu is increased
12

(Fig. 4c). Interestingly, the trend of the peak shifts in H2-TPR correlates with the ratio of Cu/Co
in the bimetallic compounds, however they are different from that of the monometallic CuO or
Co3O4. Even with the addition of less than 10% of the counterpart element to the oxide, the
characteristic peak position is appreciably changed from that of the monometallic oxide. For
example, the Cu1Co9 oxide containing 10% Cu exhibits a strong peak at 240 °C, which is
different from that of Co3O4 with two weak reduction peaks. The interaction between Cu and
Co results in totally different reduction behavior compared to the monometallic compound.
3.2. Catalytic hydrogenation of FAL
Liquid-phase hydrogenation of FAL was carried out at 180 °C under 20 bar H2 over
mesoporous mixed CuCo oxides with controlled compositions as well as monometallic CuO and
Co3O4 catalysts to evaluate the influence of the chemical composition of the catalysts on their
catalytic activity. With the total amount of the catalysts kept the same in the FAL hydrogenation
reaction, the mixed oxides of CuxCoy with appropriate Cu/Co ratios show much higher FAL
conversion than the monometallic oxide of CuO and Co3O4 (Fig. 5a). As-prepared mesoporous
Co3O4 shows very low conversion of FAL (5.3%) with a mass activity of 0.011 mol‧ h-1‧ g-1 (Table
2, Entry 9), and the product selectivity is toward FR and FA. This result indicates that crystalline
Co3O4 is not an efficient catalyst for FAL hydrogenation, despite its high surface area and a well-
ordered pore structure. Compared to Co3O4, CuO exhibits a higher activity in FAL hydrogenation
(36.6% of FAL conversion), with a mass activity of 0.076 mol‧ h-1‧ g-1 (Table 2, Entry 1). Upon
introducing the co-metal in mixed oxides, the activity of the resulting mixed CuCo oxides increased
further, beyond those of the monometallic oxides (Fig. 5a and Table 2). Over the Cu1Co5 catalyst, a
full conversion (100%) was achieved with a highest mass activity of 0.208 mol‧ h-1‧ g-1. With a
further increase in the content of Co, FAL conversions catalyzed by Cu1Co7 and Cu1Co9 catalysts
obviously declined (38.3 and 18.4%, respectively). The increase in the activity of the mixed CuCo
13

oxide catalysts can be attributed to the hydrogen spill-over effect, whereby the aldehyde
hydrogenation reaction takes place, which was also suggested by Srivastava et al [54]. Incorporation
of Co as a co-metal of CuO creates well-mixed CuCo oxides owing to strong interactions between
Cu and Co, as is evident from XRD, XPS and H2-TPR analyses. In terms of the selectivity, CuO
exhibits a selectivity toward FA, while the formation of MF increases with a present in the Cu/Co
ratio (x/y = 5/1 to 1/5), and the Cu1Co5 catalyst shows the highest selectivity towards MF (62%). In
order to further investigate the superior activity of the mixed oxides catalysts, control experiments
were conducted with physical mixtures of mesoporous Co3O4 and CuO with the same Cu/Co ratios
as the mixed CuCo oxides under the same reaction conditions. For all catalysts composed of
physical mixtures, less than 20% FAL conversion was obtained (Fig. 5a). Fig. 5b shows selectivity
of CuO, Cu5Co1, and Cu1Co5 catalysts was obtained by FAL hydrogenation reaction at the
same conversion rate. As the concentration of Co increases in the mixed CuCo oxide catalysts,
the selectivity toward MF is further increased. The selectivity toward MF was highest in the
Cu1Co5 catalyst (62%). Although product selectivity varies with reaction conditions and time,
high FA and MF selectivity are due to Cu and Co characteristics, respectively. On a catalyst
consisting of a physical mixture of individual CuO and Co3O4 with controlled ratios, the surface
normalized reaction rate decreases as the relative fraction of Cu decreases, indicating that the
overall activity of the physically mixed catalyst is dependent on the concentration of CuO (Fig.
5c and Table 2). However, the as-prepared CuCo oxide catalyst exhibits a higher reaction rate
than those of physical mixtures with the same Cu/Co ratio. In Fig. 5c, the reaction rate does not
vary significantly from pure CuO to Cu1Co5 catalyst, and decreases to pure Co3O4. The overall
activity of the CuCo oxide catalyst was determined by the concentration of exposed Cu.
However, this value does not follow the trend of Cu/Co ratios which obtained from XPS
analysis (Table 1). In addition, the different selectivity in the CuCo oxide catalyst compared to
pure CuO (Fig. 5b) indicates that the activity of the CuCo oxide originated from not only the
14

concentration of exposed Cu but also Co. The synergistic interactions between Cu and Co
within the mixed CuCo catalysts further enhanced the reaction rate in FAL hydrogenation.
3.3. Active species and reaction mechanism
To clarify the active metal species for FAL hydrogenation over mixed CuCo oxides catalysts,
in situ XRD study was conducted under a flow of 100 vol.% H2 (10 mL‧ min-1) at 180 ºC. Fig.
6 presents time-resolved XRD patterns for CuO, Cu5Co1, and Cu1Co5 catalysts, indicating
different reduction behaviors under hydrogen atmosphere at 180 ºC. The pure mesoporous CuO
was converted to metallic Cu after 40 min (Fig. 6a), while the mixed Cu5Co1 catalyst was
immediately transformed from CuO to metallic Cu (Fig. 6b). It demonstrates that the reduction
of Cu5Co1 was faster than CuO under the same operating condition, which was consistent with
H2-TPR results. Similarly, the Co rich Cu1Co5 catalyst changed the structures as a function of
reduction time at 180 ºC (Fig. 6c). As the time increased, the corresponding peak intensity of
Co3O4 decreased with increasing intensity of the metallic Co, indicating that the Co3O4 phase
was substantially reduced under hydrogen at 180 ºC. XRD patterns of spent catalysts after the
reaction for 5 h are shown in Fig. 7. XRD peaks corresponding to metallic Cu and Co are
observed in spent catalysts, demonstrating that the catalysts were already reduced during the
reaction. In addition, we performed a blank test in the presence of isopropanol and H2 (20 bar)
over Cu1Co5 and Cu5Co1 catalysts without the addition of FAL under the same reaction
conditions. The XRD patterns of the spent Cu1Co5 and Cu5Co1 catalysts (Fig. S4) demonstrate
that the catalysts were already reduced by having metallic Cu and Co species during the
reaction, which provided active sites for FAL hydrogenation.
The chemically and structurally combined CuCo oxide catalysts were reduced by hydrogen
under the reaction conditions, which was confirmed by the highly reduced bimetallic CuCo
phase as the active species. The synergistic interactions between Cu and Co within the mixed
15

CuCo catalysts further enhanced the reaction rate in FAL hydrogenation. It is concluded that
the distinct bimetallic CuCo phase derived from the CuCo oxide catalyst plays a crucial role in
their outstanding catalytic performance in the FAL hydrogenation which not be achieved with
either of the monometallic catalysts or their physical mixtures.
In order to understand the specific reaction mechanism, FAL and FA hydrogenation were
carried out over Cu1Co5 catalyst. The overall conversion of both FAL and FA, and selectivity
as a function of reaction time are shown in Fig. 8. As the reaction proceeds, the FAL conversion
increases with the reaction time (Fig. 8a). After 3 h, approximately 70% of FAL is converted,
while the yield of FA reaches the maximum (36%), which then decreases gradually. After 9 h,
FAL is completely consumed and the yields of FR and MF increased up to 27 and 60%,
respectively, over the entire reaction time. These results indicate that FAL is rapidly converted
to FA as a main intermediate with maximum selectivity at 3 h, and then, the yield of MF
increases via hydrogenolysis at the expense of FA beyond 3 h. When FA is used as a feed over
Cu1Co5 catalyst, a high yield in both MF and FR are observed (Fig. 8b). MF and FR production
increases with increasing FA conversion. MF is mainly produced by hydrogen-assisted
dehydration of FA or a dehydrogenation pathway through a methoxy intermediate [65]. FA can
be directly decomposed into FR, however, alternatively and more preferably, FA is first
dehydrogenated to FAL, followed by FAL decarbonylation to the FR proposed by Vorotnikov
and co-workers [65]. As the FA conversion is attained to 100% shown in Fig. R3b, the FR yield
also gradually increases at the expense of MF. It demonstrates a reversible reaction, in which
a small fraction of MF is converted to FA again, and FA produces FR via decarbonylation.
Vorotnikov et al. also concluded that FR was the only species not to form directly from MF
due to the high energy for the preferred pathway [65]. In the current FA hydrogenation, after
the conversion reached 100%, the FR yield is further increased, while the MF yield is
16

decreased, indicating that the reversed transformation of MF to FR occurs through the FA
formation. A steady increase of FR over reaction time indicates that FR is not an intermediate
in any of the hydrogenation/dehydrogenation reaction of FAL or FA, consistent with a previous
study [65]. Based on these results, we found that there are two competitive reaction pathways
for the FAL hydrogenation over the Cu1Co5 catalyst, as illustrated in scheme 2: aldehyde
hydrogenation towards FA and subsequent hydrogenolysis to MF, and decarbonylation of FAL
to FR.
The stability and recyclability of mixed CuCo oxide catalysts was tested by reusing both
Cu5Co1 and Cu1Co5 through seven catalytic cycles. After centrifugation and washing with
solvent (isopropanol), the used catalyst could be readily used in the next catalytic cycle. As
shown in Fig. 9, the Cu1Co5 catalyst shows a full conversion of FAL in the first 2 runs and the
conversion gradually decreases in the next consecutive catalytic runs up to 4 cycles, while the
Cu5Co1 catalyst shows a steady decrease in FAL conversion. The decreased activity could be
due to the deposition of organic reactants/products and/or possible carbons on the catalyst
surface during the reaction. To remove the possible deposits, the catalysts were dried at 100
°C, followed by calcination in air at 300 °C for 2 h. By this step, the potentially adsorbed or
deposited organic/carbon species were removed without catalyst collapse. As expected, the
catalyst could be regenerated after the 4th cycle, and the conversion of FAL was recovered,
since the organic molecules deposited were burned during the calcination and the surface active
sites of the catalyst were recovered from the coke.
4. Conclusion
We developed a series of highly ordered mesoporous mixed CuCo oxide catalysts via the
nanocasting approach using silica templates. The prepared CuCo replicas possess high surface
areas, which are higher than that of mesoporous CuO. For the liquid-phase hydrogenation of
17

FAL, the optimized Cu1Co5 catalyst shows the best catalytic performance (100 % FAL conversion)
in comparison with the mesoporous monometallic oxides, CuO and Co3O4. The FAL
hydrogenation over mixed CuCo oxide catalysts follows two reaction pathways: aldehyde
hydrogenation toward FA and then hydrogenolysis to MF, and decarbonylation toward FR.
The formation of MF increased with a decrease in the Cu/Co ratio. Physical mixtures of mesoporous
Co3O4 and CuO with the same Cu/Co ratios as the mixed CuCo oxides showed much less activity
than the mixed oxides catalysts under the same reaction conditions. The chemically and
structurally combined CuCo oxide catalysts were reduced by hydrogen under the reaction
conditions, by producing the highly reduced bimetallic CuCo phase as the active species.
Combined results reveal that the synergistic interaction between Cu and Co in the mixed CuCo
catalysts plays a crucial role in the outstanding catalytic performance in the FAL hydrogenation.
Moreover, the recovered mixed CuCo catalyst shows good regenerability during consecutive
catalytic runs. The efficient and versatile mixed CuCo oxides are not only promising catalysts for
effectively upgrading biomass-derived FAL, but also provide a useful guidance for the rational
design of low-cost catalysts without using precious metals.
ACKNOWLEDGEMENTS: This research was supported by Basic Science Research
Program (2018R1A1A1A05079555) and Technology Development Program to Solve Climate
Changes (2017M1A2A2087630) of the National Research Foundation of Korea (NRF) funded
by the Ministry of Science and ICT, and MOTIE(KIAT_N0001754).
18

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21

Table 1. Results of nitrogen adsorption-desorption, ICP-OES, and XPS analyses of CuCo oxides.
Nitrogen adsorption-desorption ICP-OES
XPS
Catalyst
Surface area (SBET),
Total pore
Pore size (BJH),
(nm)
Cu/Co Theoretical
Cu/Co
Co2+/Co3+
(m2‧ g-1)
volume (cm3‧ g-1)
CuO
46
60
8
0.09
0.24
0.34
0.54
0.59
0.35
0.58
0.45
0.48
-
-
5
-
-
Cu5Co1
Cu3Co1
Cu1Co1
Cu1Co3
Cu1Co5
Cu1Co7
Cu1Co9
Co3O4
16
14
17
18
12
19
15
15
4.72
2.90
0.89
0.35
0.18
0.13
0.11
-
1.64
1.79
1.02
0.48
0.21
0.15
0.15
-
1.69
1.71
1.72
2.27
2.41
2.01
1.84
0.75
94
3
129
135
117
120
119
124
1
0.33
0.2
0.13
0.1
-
22

Table 2. Results of catalytic hydrogenation of FAL over mesoporous CuCo oxides a
Yield (%)
FA
Mass reaction rate Area reaction rate
Entry Catalyst
Conversion (%)
[mol‧ h-1‧ g-1]
[mol‧ h-1‧ m-2]
Cyclo-
pentanol
FR
MF
Other
1
2
3
4
5
6
7
8
9
CuO
36.6
37.4
62.9
82.5
87.5
100
0.076
0.078
0.131
0.172
0.182
0.208
0.079
0.038
0.011
1.65
1.30
1.39
1.33
1.35
1.78
0.66
0.32
0.09
11.1
10.1
19.0
26.1
27.6
20.5
9.5
1.4
5.2
7.2
16.5
19.8
38.5
4.6
1.9
-
23.3
20.0
34.3
37.3
37.6
38.1
24.2
12.1
1.6
1.4
2.0
2.4
2.5
2.4
1.5
-
-
-
Cu5Co1
Cu3Co1
Cu1Co1
Cu1Co3
Cu1Co5
Cu1Co7
Cu1Co9
Co3O4
-
-
-
1.4
-
38.3
18.4
5.3
4.5
-
-
3.0
-
-
a
Reaction conditions: FAL: 1 g, isopropanol: 20 mL, catalyst: 10 mg, H2: 20 bar, reaction temperature: 180 °C, reaction time: 5 h, stirring
speed: 600 rpm.
23

List of Figure
Fig. 1. (a–e) HAADF-STEM, (f–j) BF-STEM, and (k–o) HR-TEM of mesoporous CuxCoy
oxides: (a,f,k) Cu5Co1, (b,g,i) Cu3Co1, (c,h,m) Cu1Co1, (d,i,n) Cu1Co3, and (e,j,o) Cu1Co5.
Insets show illustrations of mesoporous CuxCoy: Co in pink and Cu in blue.
Fig. 2. (a) SEM and (b) HAADF-STEM images of mesoporous Cu1Co5 oxides. (c–e) EDS
mapping performed at an acceleration voltage of 200 kV: O (K line) in red, Cu (K line) in blue,
and Co (K line) in pink.
Fig. 3. (a) Co 2p and (b) Cu 2p XPS spectra of mesoporous CuCo oxides.
Fig. 4. (a) XRD patterns and (b) their enlarged areas of mesoporous CuCo oxides with different
composition.
Fig. 5. (a) FAL conversion and selectivity as a function of reaction time over mesoporous CuCo
catalysts with controlled composition (* indicates a catalyst consisting of a physical mixture of
individual CuO and Co3O4 mixed at a controlled rate). (b) FAL conversion and selectivity after
the 10 h reaction over CuO, Cu5Co1, and Cu1Co5 catalysts. (c) Surface normalized reaction
rate. Reaction conditions: FA (1 g), isopropanol (20 mL), catalyst (10 mg), H2 (20 bar), reaction
temperature (180 °C), stirring speed (600 rpm).
Fig. 6. In situ XRD patterns of (a) CuO, (b) Cu5Co1, and (c) Cu1Co5 under hydrogen atmosphere
at 180 ºC.
Fig. 7. XRD patterns and enlarged XRD patterns of fresh and spent catalyst: (a,c) Cu1Co5, (b,d)
Cu5Co1.
Fig. 8. (a) FAL and (b) FA conversion and selectivity as a function of reaction time over mixed
Cu1Co5 oxide catalyst. Reaction conditions: FAL or FA (1 g), isopropanol (20 mL), catalyst
(10 mg), H2 (20 bar), reaction temperature (180 °C), stirring speed (600 rpm).
24

Fig. 9. Recycling test for FAL conversion over Cu1Co5 and Cu5Co1 catalysts. (* indicates that
the catalyst was reactivated by calcination in air flow at 300 °C for 2 h). Reaction conditions:
FAL (1 g), isopropanol (20 mL), catalyst (20 mg), H2 (20 bar), reaction temperature (180 °C),
stirring speed (600 rpm).
25

Cu1Co3
Cu3Co1
Cu1Co1
Cu1Co5
Cu5Co1
a
c
d
e
b
100nm
100nm
200nm
100nm
100nm
f
h
g
i
j
100nm
100nm
100nm
200nm
100nm
m
n
l
k
o
Cu5Co1
Cu1Co5
10nm
10nm
10nm
10nm
10nm
Fig. 1.
26

Fig. 2.
27

a
Co 2p
2p3/2
Co2+
2p1/2
Co3+
Cu5Co1
Cu3Co1
Cu1Co1
Cu1Co3
Cu1Co5
Cu1Co7
Cu1Co9
Co3O4
790
800
780
770
Binding energy (eV)
Cu 2p
Satellite peak
b
2p3/2
Cu2+ Cu+
CuO
2p1/2
Cu5Co1
Cu3Co1
Cu1Co1
Cu1Co3
Cu1Co5
Cu1Co7
Cu1Co9
950
940
930
960
Binding energy (eV)
Fig. 3.
28

Fig. 4.
29