Structural evolution of ZIF-67-derived catalysts for furfural hydrogenation

Article abstract of DOI:10.1016/j.jcat.2020.10.014

Zeolitic imidazolate framework-67 (ZIF-67) can be converted to metallic Co nanoparticles supported on N-doped carbon (Co/NC) through reduction. However, its unique properties, including extremely high surface area, isoreticular pore structure, and regular metal–organic network, disappear after high-temperature (>500 °C) reduction. Aggregated CoO_x particles reduce the number of surface-active sites, resulting in poor catalytic activity. If the original ZIF-67 structure is maintained after the high-temperature reduction, promoting the uniform distribution of active sites in the porous carbon, the catalytic performance can be further improved. Herein, the correlation between the catalytic furfural hydrogenation performance, Co/NC morphology, and oxidation state of Co was investigated as a function of the H₂ reduction temperature and time. The reduction of ZIF-67 at 400 °C for 6 h yields a highly dispersed Co/NC catalyst, while preserving the overall morphology. The resulting Co/NC-400-6 catalyst exhibits the highest activity, promoting high selectivity toward 2-methylfuran. The product selectivity can be further altered by incorporating Cu into ZIF-67 to produce furfuryl alcohol. With proper H₂ treatment to minimize the damage to the intrinsic surface area and pore structure, metal–organic frameworks can be utilized as high-performance heterogeneous catalysts by maximizing the distribution of active sites.

Full text of DOI:10.1016/j.jcat.2020.10.014

Journal Pre-proofs
Structural Evolution of ZIF-67-Derived Catalysts for Furfural Hydrogenation
Jun Gyeong Lee, Sinmyung Yoon, Euiseob Yang, Jae Hwa Lee, Kyung Song,
Hoi Ri Moon, Kwangjin An
PII:
S0021-9517(20)30420-6
https://doi.org/10.1016/j.jcat.2020.10.014
YJCAT 13940
DOI:
Reference:
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Journal of Catalysis
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Revised Date:
Accepted Date:
15 June 2020
1 October 2020
12 October 2020
Please cite this article as: J. Gyeong Lee, S. Yoon, E. Yang, J. Hwa Lee, K. Song, H. Ri Moon, K. An, Structural
Evolution of ZIF-67-Derived Catalysts for Furfural Hydrogenation, Journal of Catalysis (2020), doi: https://
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Structural Evolution of ZIF-67-Derived Catalysts for Furfural
Hydrogenation
Jun Gyeong Lee,† Sinmyung Yoon,† Euiseob Yang,† Jae Hwa Lee,‡ Kyung Song,§ Hoi Ri
Moon,‡,* Kwangjin An†,*
† School of Energy and Chemical Engineering, Ulsan National Institute of Science and
Technology (UNIST), Ulsan 44919, Republic of Korea
‡Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST),
Ulsan 44919, Republic of Korea
§Materials Testing and Reliability Division, Korea Institute of Materials Science (KIMS),
Changwon 51500, Republic of Korea
*
To whom correspondence should be addressed (E-mail: kjan@unist.ac.kr,
hoirimoon@unist.ac.kr)
1

Abstract
Zeolitic imidazolate framework-67 (ZIF-67) can be converted to metallic Co nanoparticles
supported on N-doped carbon (Co/NC) through reduction. However, its unique properties,
including extremely high surface area, isoreticular pore structure, and regular metal-organic
network, disappear after high-temperature (>500 ℃) reduction. Aggregated CoOx particles
reduce the number of surface-active sites, resulting in poor catalytic activity. If the original
ZIF-67 structure is maintained after the high-temperature reduction, promoting the uniform
distribution of active sites in the porous carbon, the catalytic performance can be further
improved. Herein, the correlation between the catalytic furfural hydrogenation performance,
Co/NC morphology, and oxidation state of Co was investigated as a function of the H2
reduction temperature and time. The reduction of ZIF-67 at 400 ℃ for 6 h yields a highly
dispersed Co/NC catalyst, while preserving the overall morphology. The resulting Co/NC-400-
6 catalyst exhibits the highest activity, promoting high selectivity toward 2-methylfuran. The
product selectivity can be further altered by incorporating Cu into ZIF-67 to produce furfuryl
alcohol. With proper H2 treatment to minimize the damage to the intrinsic surface area and
pore structure, metal-organic frameworks can be utilized as high-performance heterogeneous
catalysts by maximizing the distribution of active sites.
Keywords: Metal-organic framework (MOF); Zeolitic imidazolate framework-67 (ZIF-67);
Furfural hydrogenation; Catalyst; in situ characterization
2

1. Introduction
The conversion of biomass to produce value-added chemicals as well as bio-fuels can
provide an alternative energy source to fossil fuels [1-3]. Furfural (FAL) is one of the major
platform chemicals decomposed mainly from hemicellulose after the pyrolysis of
lignocellulosic biomass [4-16]. FAL can be further upgraded by catalytic conversion reactions,
including hydrogenation and hydrodeoxygenation [4-8]. The catalytic hydrogenation of FAL
involving both C=C and C=O double bonds has been widely utilized as a probe reaction to
evaluate the performance of catalysts, due to its distinct reaction pathways. Through
decarbonylation, FAL is converted to furan (FN) and tetrahydrofuran (THF). When the
aldehyde group is hydrogenated, furfuryl alcohol (FA) is firstly produced, after which
tetrahydrofurfuryl alcohol (THFA) is obtained by additional hydrogenation of the furan ring.
By the hydrogenolysis of FA, 2-methylfuran (MF) is also generated. Various noble metal
catalysts, such as Pd, Pt, Ru, and their alloys with second transition metals, are utilized for FAL
hydrogenation [4,5,12-16]. While noble metal catalysts exhibit high activity, non-noble-metal-
based catalysts, such as Co, Cu, and Ni are developed to increase the activity and durability,
due to their well-defined structures and high dispersion in supports [9-11]. Schüth’s group
reported that Co3O4 supported on mesoporous carbon catalysts exhibited high catalytic
performance in FAL hydrogenation [10]. A recent study showed that mesoporous CoO
exhibited very high activity and distinct selectivity toward MF in FAL hydrogenation [11].
To create well-defined catalytic sites and promote high dispersion in the support, thermally
reduced catalysts derived from metal-organic frameworks (MOFs) have been widely utilized
as efficient catalysts [17-24]. As a porous material, MOF consists of organic linkers and metal
ions or clusters, which are arranged regularly to form a well-defined porous structure with a
large surface area [25-33]. Thus, it is suitable for use as a heterogeneous catalyst but also plays
parallel roles as both a template and a precursor to produce nanostructured materials such as
3

metal nanoparticles, porous carbons, and their composites [34-40]. In particular, zeolitic
imidazolate framework-67 (ZIF-67), a sodalite-type MOF produced by the regular arrangement
of Co2+ and 2-methylimidazole (2-MIM), is a good candidate for the production of Co-based
catalysts [41-50]. The unit components of ZIF-67 are regularly arranged, and its isoreticular
structure can provide the framework for the highly dispersed N-doped carbon-supported Co
catalyst (Co/NC). When the original ZIF-67 is reduced by H2 at a high temperature, the Co ions
in ZIF-67 are transformed into metallic Co, while the 2-MIM organic linkers become N-doped
carbons. Recently, Gong et al. reported the application of ZIF-67-derived N-doped carbon
nanotubes (CNTs) containing Co nanoparticles as a useful catalyst for the selective
hydrogenation of biomass-derived compounds [51]. Although the catalytic activity in the
hydrogenation of aldehyde, ketone, carboxyl, and nitro groups was meaningful due to the
presence of Co nanoparticles confined with Nx species (Co–Nx), the overall morphology of
ZIF-67 collapsed due to the high-temperature reduction (900 ℃). It is expected that if the active
Co species are uniformly distributed without aggregation in the porous carbon network,
maintaining the original ZIF-67 structure with a large surface area, the catalytic performance
will be further improved. Although many studies have been reported using ZIF-67 to use a
Co/NC catalyst, few studies have observed changes in the catalyst to maximize the surface area
while minimizing the size of Co nanoparticles during the high temperature reduction process
in a systematic manner.
To improve the dispersion of Co while maintaining the original morphology of ZIF-67, we
prepared various ZIF-67-derived catalysts by H2 reduction at different temperatures and over
different times. The structural evolution of ZIF-67 was monitored in real time by in situ X-ray
diffraction (XRD) and transmission electron microscopy (TEM) with a specific cell capable of
characterizing the catalyst at high temperatures up to 1000 °C under an H2 atmosphere.
Structural, chemical, and morphological characterizations were also performed to determine
4

the most active Co/NC catalyst as a function of the reduction temperature and time. Through
catalytic FAL hydrogenation, the correlation between the catalytic performance and the
structure, porosity, and Co oxidation state of Co/NC catalysts were investigated. The ZIF-67-
derived Co/NC catalysts promoted MF as the major product in the FAL hydrogenation (Scheme
1); however, upon the incorporation of Cu nanoparticles into the ZIF-67 network, the product
selectivity was changed toward FA due to the preferential hydrogenation of the carbonyl group
of FAL. This study provides insights into the optimal method for maintaining the structure of
ZIF-67, which involves promoting the uniform dispersion of Co metals under various reduction
conditions. In addition, the principle of varying the catalytic activity and selectivity in the
biomass-upgrading process over MOF-derived catalysts is also investigated.
2. Experimental
2.1 Catalyst Preparation
ZIF-67 was synthesized in an aqueous solution using a reported method with slight
modification [52,53]. Briefly, 0.6 g of cobalt(Ⅱ) acetate tetrahydrate (Co(CH3COO)2∙4H2O,
98%, Junsei) was dissolved in 5 mL of deionized (DI) water, while a solution containing 2.24
g of 2-MIM (C4H6N2, 97%, Alfa Aesar) in 5 mL of DI water was prepared separately. After
sonication for complete dissolution, the two solutions were mixed with stirring, after which
they were kept for 5 h without stirring at room temperature. The resulting purple precipitate
was collected by centrifugation and washed with ethanol. After drying at 50 ℃ for 24 h in an
oven, the resulting powder was reduced by 4% H2/Ar at a heating rate of 100 ℃ h−1 at different
temperatures and over different times. During the reduction, ZIF-67 was converted to Co/NC.
The resulting Co/NC catalyst was denoted by Co/NC-R-T (where R and T represent the
reduction temperature and time, respectively). After the reduction, all the catalysts were stored
5

in a vacuum desiccator to prevent oxidation of the Co nanoparticles by air. Cu-incorporated
ZIF-67 catalysts with different Cu loadings (x-Cu/ZIF) were prepared by metal-organic
chemical vapor deposition (MOCVD) [54]. Briefly, copper(II) acetylacetonate, in the target
concentration, was mixed with 0.2 g of ZIF-67 and heated to 180 ℃ at a rate of 2.7 ℃ min−1.
The mixture was kept at 180 ℃ for 1 h for the sublimation of the copper precursor to occur;
thereafter, it was further heated to 350 ℃ (2 ℃ min−1) and held at this temperature for 1 h. The
resulting x-Cu/ZIF product was washed with ethanol several times. No reduction was
performed on the Cu/ZIF catalyst to avoid the formation of metallic Co from ZIF-67.
2.2 Characterization
The structural characterization of ZIF-67 and its reduced products (catalysts) was carried out
by Scanning electron microscope (SEM, Hitachi S-4800 microscope), Transmission electron
microscopy (TEM, Jeol JEM-2100F) with Energy-dispersive X-ray spectroscopy (EDS,
Oxford instrument, X-Max 80 T), N2 physisorption by the Brunauer–Emmett–Teller (BET,
BEL Sorp-max) method, and X-ray diffraction (XRD, PANalytical X’Pert Pro) using a Cu Kα
radiation source (λ = 0.154056 nm). The surface chemical property was characterized using an
X-ray photoelectron spectroscopy (XPS, Thermo Fisher system) equipped with an Al Kα
radiation source. An in situ X-ray diffractometer (Rigaku) equipped with a D/teX Ultra 250
detector using Cu Kα radiation (λ = 0.154056 nm) was used to collect diffraction data during
the thermo-programmed reduction from 25 to 800 °C (heating rate of 5 °C min−1) under a flow
of 4.0 vol% H2 in Ar (30 mL min−1) [11,12]. An in situ TEM study was carried out using a
micro electro mechanical system (MEMS) device-based heating holder cell (Climate S3+,
Dens solutions) equipped an aberration-corrected scanning transmission electron microscope
(JEOL JEM-2100F) at 200 kV. The sample was deposited on a silicon nitride transmission
electron microscope membrane and assembled in a heating holder cell. The in situ microscope
6

holder was inserted into the main body of the microscope and maintained under 1 bar of H2 by
a flow of 5.0 vol% H2 in Ar (1 mL min−1). The local heating on the MEMS device cell was
achieved by the Joule heating of a micro-patterned metal heater to control the temperature up
to 600 °C. The analysis was carried out at different temperatures after maintaining these
conditions for 30 min. X-ray absorption spectroscopy (XAS) was carried out at the beamline
6D of the Pohang Accelerator Laboratory (PAL, South Korea). An electron beam energy of 3.0
GeV and an incident X-ray current of 300 mA were achieved, and an Si(111) double-crystal
monochromator was used to filter the incident photon energy. The XANES spectra of the Co
K-edge region were also collected in the transmission detection mode. The IFEFFIT (Athena)
software was used to remove the background and normalize the XAS spectrum. For
comparison, Co3O4 and CoO, purchased from Sigma-Aldrich (99.99%), and metallic Co foil
were used as the reference materials. The composition of Co in the catalyst was determined by
inductively coupled plasma–optical emission spectrometry (ICP–OES, Varian, 700-ES). The
concentration of Co (or Cu) was calibrated by a multi-standard solution (Inorganic ventures).
The digestion of the catalyst was carried out by dissolving the catalyst in aqua regia for 3 days.
The solution was diluted with DI water and filtered to remove a carbon support prior to
measurement. CO-chemisorption was measured by BELCAT II (MicrotracBEL Corp.).
Pretreatment of the catalyst was performed at 180 ℃ in 5% H2/He for 1 h. After the
pretreatment, the temperature was lowered to the measurement temperature (−80 ℃). The
concentration of CO-pulse gas was 5% CO/He. A 30 sccm of the CO/He mixture was injected
into the catalyst for 10 s in one pulse.
2.3 Catalytic activity evaluation
For the catalytic FAL hydrogenation, a 100 mL scaled stainless-steel batch reactor was used
(Chemresys Co.). One gram of FAL (C5H4O2, 99%, Acros), dissolved in 20 mL of 2-propanol,
7

(C3H8O, 99.5%, Alfa Aesar) was added to the reactor in the presence of the catalyst (10‒20
mg) [11,12]. Before initiating the reaction, the solution was purged with H2. The initially set
H2 gas pressure was 20 bar at room temperature. The reaction proceeded at 180 ℃ for 5 h with
stirring at 600 rpm. A gas chromatograph (GC, Agilent 7820A) equipped with a flame
ionization detector (FID) using a DB-Wax column (30 m × 0.320 mm × 0.25 µm) was used to
identify and quantify the product. The conversion of FAL and the product selectivity were
calculated using the following formulas:
mol FAL consumed
mol furfural fed
Conversion (%) =
Selectivity (%) =
× 100
mol product formed
Σ mol products formed
× 100
mol FAL converted
Reaction rate (min−1) =
the number of exposed metal sites ∙ time
The FAL conversion was calculated by the mole of FAL converted per gram Co catalyst per
hour, and the amount of Co in the catalyst was determined by the ICP–OES results. The
reaction rate was determined by the FAL conversion of less than 30‒40% under the controlled
reaction parameters, resulting in a kinetically-controlled regime. The number of exposed metal
sites of Co and Cu was calculated as a result of CO-chemisorption.
3. Results and discussion
Supported metallic Co acts as an active species in many heterogeneous catalytic reactions
rather than as Co complexes with high oxidation states [41-50]. Various supported Co catalysts
are simply prepared by the reduction of Co precursors using H2. The H2 reduction of ZIF-67
yields a Co/NC catalyst, where the Co ions of ZIF-67 are reduced to metallic Co nanoparticles,
and the 2-MIM organic linkers become N-doped carbon. The morphology of ZIF-67 varies
depending on the reduction temperature and time. When the reduction of ZIF-67 was carried
8

out at different temperatures, the change in the morphology became more pronounced. As the
reduction temperature increased to 200, 400, 500, and 600 ℃, the size of the Co nanoparticles
increased. Beyond 400 ℃, the original polyhedral ZIF-67 structure changed to a more irregular
shape with Co nanoparticles larger than 50‒100 nm (Fig. S1). The structural evolution of the
ZIF-67-derived catalysts was further visualized by in situ TEM in an H2 environment. Video
and snapshots of the ZIF-67 structural changes were obtained using an in situ gas cell, equipped
with a spherical aberration coefficient-corrected transmission electron microscope at 200 kV
(Fig. 1). The ZIF-67 loaded in the gas cell was heated from room temperature to 600 ℃ with a
flow of 5% H2/Ar, and there was no apparent change in the observed morphology of ZIF-67
up to 500 ℃. However, at temperatures above 500 ℃, the polyhedral form suffered severe
shrinkage due to the Co clusters created in the ZIF-67 core. A video showing the evolution of
the morphology of ZIF-67 as the temperature rises is provided in supplementary materials.
These results directly showed that the rapid decomposition of the ZIF-67 structure at high
temperatures induces severe aggregation of the resultant nanoparticles. Thus, it was established
that the optimum reduction temperature is 400 ℃, where the ZIF-67 begins to collapse slowly
while maintaining its original polyhedral shape.
Fig. 2 shows the transformation of ZIF-67 to Co/NC-400-6 after H2 reduction at 400 ℃ for
6 h. The original ZIF-67 has a uniform polyhedral structure with a smooth surface, as evidenced
by SEM (Fig. 2a). The average outer diameter of ZIF-67 was 501.4 nm. After reduction, the
polyhedral ZIF-67, with a smooth surface, changes into a porous structure consisting of small
clusters while retaining its original size. Fig. 2b shows that the overall morphology of the
Co/NC-400-6 catalyst was preserved after the reduction. The N2 adsorption–desorption
isotherms show that the original surface area of ZIF-67 (1458 m2 g−1) was significantly
decreased to about 200 m2 g−1 after the reduction at 400 ℃ (Table S1 and Fig. S2). The surface
9

areas of Co/NC-400-2 and Co/NC-400-8 reduced to 219 and 202 m2 g–1, respectively. However,
the surface area of Co/NC-400-12 was significantly reduced to 132 m2 g–1 due to the severe
reducing environment. The Co weight percent (wt%), determined by ICP–OES, and the
average Co particle size of the catalyst are provided in supplementary materials, Table S2 and
Fig. S3. As the MOF structure was collapsed after the reduction, the relative concentration of
Co was increased with the same amount of catalyst. ZIF-67 contains 28.0 wt% Co, however,
the reduced Co/NC-400 catalyst contained 41–55 wt% Co. Scanning TEM (STEM) imaging
and the corresponding elemental mappings with energy-dispersive X-ray spectroscopy (EDS)
results indicated that Co, C, and N derived from ZIF-67 were uniformly dispersed within the
framework (Fig. 2b). When the reduction time at 400 ℃ was increased to 2, 4, 6, 8, and 12 h,
the changes in the ZIF-67 structure were clearly confirmed by TEM, as shown in Fig. 3. The
average particle size of Co was determined to be 3.4–5.0 nm for Co/NC-400-2, Co/NC-400-4,
and Co/NC-400-6 catalysts (Fig. S4). However, the Co size of Co/NC-400-12 increased to 20.8
nm due to high temperature agglomeration (Fig. S3). Interestingly, the reduced Co species act
as a catalyst for the growth of CNTs. After the reduction at 400 ℃ for 12 h, the production of
CNTs was significantly enhanced due to the growth of Co-catalyzed carbon (Fig. 3e).
The structural transformation of ZIF-67 into the Co/NC catalyst was monitored by in situ
XRD under a 4% H2/Ar atmosphere (Fig. 4a-b). The original ZIF-67 shows a representative
XRD pattern due to its well-organized framework. As the temperature increases from 25 to 800
℃, the ZIF-67 structure begins to collapse above 400 ℃. At the same time, a diffraction peak
corresponding to Co(111) nanoparticle appears (Fig. 4a). When the H2 reduction was
conducted at 400 ℃, the Co(111) peak begins to appear noticeably over 4 h in the XRD pattern
(Fig. 4b). As the reduction time further increases over 8 h, traces of the characteristic MOF
pattern disappear, while the intensity of the Co(111) peak increases. The presence of the non-
sharp and weakened XRD peaks reveals that the overall structure of ZIF-67 did not
10

significantly collapse up to 6 h under the 400°C reduction condition, which is consistent with
the TEM results (Figure 3, Fig. S4). The Co/NC catalysts obtained after the H2 reduction at
400 ℃ for different reduction times are characterized by X-ray photoelectron spectroscopy
(XPS) and X-ray absorption near edge structure (XANES) measurements to confirm the
distribution of the Co oxidation states (Fig. 4c-d). After reduction, the metallic Co phase, which
is invisible in the original ZIF-67 spectra, becomes noticeable for Co/NC-400-2, Co/NC-400-
4, and Co/NC-400-6 in the XPS profiles (Fig. 4c). To investigate the change in Co metal
content by increasing the reduction time, Co K-edge XANES spectra of Co/NC-400 catalysts
were obtained (Fig. 4d). The Co K-edge XANES spectra show the transition from Co 1s to Co
4p through a mixed state with O 2p, which is sensitive to the oxidation state of Co [11]. The
degree of reduction of Co was determined by fitting the XANES spectra with the reference
spectra obtained from commercial Co3O4 and Co0. The change of oxidation state from Co3O4
to Co0 is evidenced by the reduced white-line intensity (at 7730 eV based on Co3O4 reference)
and increased pre-edge area (< 7720 eV based on Co0 reference). The normalized XANES
spectra of Co/NC-400-2, Co/NC-400-4, and Co/NC-400-6 show a clear tendency to increase
the proportion of Co0 with increasing reduction time. As the reduction time increases, the trend
of increasing Co0 content is the same as the trend of Co particle growth (Table S2). From these
results, it was confirmed that ZIF-67 was converted to Co/NC without destroying its original
morphology until 6 h after H2 reduction at 400 ℃, and the obtained Co/NC product had a
metallic Co phase.
A liquid-phase FAL hydrogenation reaction was carried out over the ZIF-67-derived
catalysts in a high-pressure autoclave reactor [11,12]. To investigate the catalytic activity and
selectivity toward the FAL hydrogenation, 1 g of FAL, dissolved in 2-propanol with the
catalyst, was pressurized with 20 bar of H2, followed by a reaction at 180 ℃ for 5 h. Under the
current reaction conditions, FAL hydrogenation has two distinct reaction pathways: the
11

hydrogenation of the aldehyde group and decarbonylation (Fig. 5a). When the carbonyl group
is removed from FAL, FN is produced. By subsequent hydrogenation, the produced FN is
converted to THF. On the contrary, the aldehyde group of FAL is hydrogenated to produce FA,
which is further converted to either THFA or MF. MF is produced by the hydrogenolysis of
FA. In previous reports, Co-containing catalysts showed relatively high selectivity toward MF
when there was sufficient conversion [11]. Fig. 5b and Table 1 summarize the FAL conversion
and product selectivity over the Co/NC-400 catalysts derived by the H2 reduction of ZIF-67.
The conversion was 11.9% in the absence of a catalyst under the reaction condition of 180 ℃,
20 bar H2, and 5 h. In the absence of a catalyst, FAL can pyrolyze at 180 ℃. When ZIF-67 (10
mg) was used as a catalyst, it showed similar conversion with the blank reaction (11.4%), but
the selectivity was different. When the amount of ZIF-67 was further increased to 40 and 200
mg under the same reaction conditions, the activity did not change significantly (Table 1). It
was found that the Co2+ species connected by organic linkers in the ZIF-67 structure exhibited
negligible activity against FAL hydrogenation. Nevertheless, ZIF-67 acted as a catalyst, unlike
catalystless pyrolysis. As mentioned earlier, it was confirmed that the reduction of ZIF-67 at
400 ℃ generated Co/NC catalysts, and the original morphology of the MOF was retained;
consequently, the activities of various Co/NC catalysts with different H2 reduction times at 400
℃ were investigated for FAL hydrogenation. As the reduction time increased from 2 to 12 h,
the activity gradually increased, and the maximum activity was exhibited by Co/NC-400-6 (Fig.
5b). As the reduction time is increased further, the activity is decreased without change of
selectivity. In spite of the higher portion of Co in the unit mass (Table S2), the highly reduced
Co/NC-400 catalysts show lower activity because of aggregation of Co during reduction
treatment, decreasing the number of active sites (Fig. S3). The conversion of the Co/NC-400-
6 catalyst was 99.4%, resulting in a high MF selectivity (49.5%). In a previous study, Co-based
catalysts exhibited high activity and high MF selectivities because the CoO and Co surfaces
12

provided excellent sites for FAL with high adsorption energies of -2.39 and -1.90 eV,
respectively [11]. Due to the easy cleavage of the C–OH bond of FA, as an adsorption
intermediate, on CoO(111) compared to the case on Co(111), the CoO site can facilitate the
selective hydrogenation of FAL to FA and subsequent MF production. It was also reported that
the reduced phases of CoO and Co showed higher FAL conversion than that of the original
mesoporous Co3O4 [11]. From these results, it is clear that the Co/NC catalysts derived from
ZIF-67 have abundant Co phases in the N-doped carbon supports, which are responsible for
the high FAL activity and MF selectivity. However, the as-prepared ZIF-67 exhibits poor
adsorption for FAL because the organic linkers and Co2+ species have very strong bonds. The
Co/NC-400 catalysts have different reaction rates depending on the reduction treatment time,
as shown in Fig. 5b; however, the overall selectivities toward MF, FA, and FN with small
products were very similar. The adsorbed FAL undergoes a comparative reaction between
decarbonylation and aldehyde hydrogenation, producing FA and FN, respectively. The
produced FA is converted to MF as a final product after the hydrogenolysis of FA. Further
experiments confirm these reaction pathways. Fig. 6 shows the change in the FAL conversion
and selectivity as a function of the reaction times for Co/NC-400-2 and Co/NC-400-6. With
limited reaction times, the conversion for Co/NC-400-2 was lower than that for Co/NC-400-6;
however, as the reaction time increased to 10 h, both catalysts reached 100% conversion with
similar MF selectivities of 54.7% and 57.5% for Co/NC-400-2 and Co/NC-400-6, respectively
(Fig. 6). Over 4 h, the maximum selectivity of FA was reduced at the expense of MF and FN
production, demonstrating that the conversion from FA to FAL was reversible to produce FN.
In addition, after the complete conversion, MF and FN proved to be the last two main products
among the minor saturated products of the furan ring, such as THF and THFA, generated over
the Co/NC-400 catalysts.
13

Recyclability tests were conducted 4 times to confirm whether the catalyst could be re-used.
A solution of 10 mg of Co/NC-400-6, 20 mL of 2-propanol and 1 g of FAL was charged with
20 bar of H2 and reacted at 180 °C for 3 h. The reaction solution was also taken every 30
minutes during the reaction to track the time-dependent conversion. The used catalysts were
washed with ethanol several times and dried at 80 ℃. After the 3 h reaction, complete
conversion of FAL was obtained. The reaction rate was also calculated by an aliquot taken after
the first 30 min during the reaction to remain in a kinetically-controlled regime. The reusability
results including FAL conversion and reaction rate are shown in Table S3 and Fig. S5–S7.
After the first reaction cycle for 3 h, 98.2% FAL conversion was achieved. Conversion
gradually decreased in subsequent cycles. As a result, the conversion of the reaction after the
4th cycle was 83.0%. It is due to the deactivation of catalysts caused by the aggregation of Co
nanoparticles. The reaction rate of Co/NC-400-6 as the most active form of the Co/NC-400
series decreased as Co nanoparticles become more agglomerated. When the morphology of the
Co nanoparticles of the catalyst obtained after the 3rd recycle reaction was observed by TEM,
no distinct change in size was observed (Fig. S6). The Co particle size was calculated by
Scherrer's equation using the XRD spectrum in Figure S7. As the number of regeneration
reactions increased from 1 to 4, the size of the Co nanoparticles increased from 4.3 to 7.7 nm
(Table S4). The reaction rate decreased significantly after the 4th regeneration reaction (Table
S3), and this trend can be clearly observed in the TEM image in Figure S6. Through these
results, it was proven that the Co/NC-400-6 catalyst was sufficiently reusable, but the activity
decreased as the size of Co nanoparticles increased as the number of regeneration increased.
The product selectivity of Co/NC-400 catalysts can be altered by the incorporation of foreign
atoms. Cu is a well-known catalyst that promotes high FA selectivity in FAL hydrogenation
by the preferential hydrogenation of the carbonyl group of FAL [55,56]. We prepared Cu-
incorporated ZIF-67 catalysts to investigate the synergetic effect of Cu in FAL hydrogenation.
14

Fig. 7a shows the TEM images of Cu-incorporated ZIF-67 catalysts with different
concentration of Cu (x-Cu/ZIF). As the Cu concentration was increased from 0.5 to 5 wt%, the
original polyhedral structure of ZIF-67 was retained after the Cu incorporation, and the Cu
cluster was very small in size. Cu particles were not confirmed by TEM images. However, the
5-Cu/ZIF catalyst, which is a slightly contracted form of ZIF-67, displayed significant Cu
clustering (Fig. 7a). The Cu-incorporated ZIF-67 catalysts prepared by the MOCVD have
highly dispersed Cu nanoparticles within the ZIF-67 structure; thus, the original pore structure
of the catalysts is maintained, as evidenced by EDS, XRD, and N2 adsorption–desorption
isotherms (Fig. S8–10). In the catalytic FAL hydrogenation, the Cu/ZIF catalyst series exhibit
higher selectivity toward FA (~ 50%) than the Co/NC catalysts, due to the role of Cu in the
preferential hydrogenation of the carbonyl group of FAL. The overall selectivities were similar
for all the Cu/ZIF catalysts used; however, the reaction rate decreased as the content of Cu in
ZIF-67 increased (Fig. 7b). Since the reaction rate is normalized by the Cu content of the
catalyst, 0.5-Cu/ZIF exhibited the highest reaction rate, due to the highly dispersed Cu
nanoparticles in ZIF-67. To avoid the formation of metallic Co from ZIF-67, we used Cu/ZIF
catalysts without reduction. When the Cu/ZIF with 10 % of Cu was reduced at 300 ℃ for
different times, there was no significant changes in conversion and selectivity for FAL
hydrogenation (Fig. S11). It is also found that there was no evidence of CuCo alloy formation
during the synthesis or reaction. Scanning TEM image and the corresponding EDS elemental
mappings of as-prepared Cu/ZIF catalyst show small Cu particles that are distinguished from
Co (Fig. S8). The spent Cu/ZIF catalyst series shows a characteristic peaks of ZIF-67 in XRD
(Fig. S9). This indicates that Cu was evenly distributed without aggregation over the ZIF-67
and the entire morphology was preserved after the FAL hydrogenation. To compare the
catalytic activity according to the support materal, the 10-Cu/ZIF catalyst was compared with
10 wt% Cu-supported silica (mesoporous silica, MCF-17) and carbon (activated charcoal,
15

Sigma-Aldrich) catalysts. The 10-Cu/ZIF catalyst afforded a FAL conversion of 73.9%,
whereas 10-Cu/MCF and 10-Cu/AC showed only 4.5% and 3.9% conversions, respectively,
under the same reaction conditions (Table 1). Due to the high surface area and isoreticular pore
structure of ZIF-67 (Table S1), the catalytic performance of the Cu/ZIF catalyst in FAL
hydrogenation was further improved, compared to the performances of the Cu catalysts with
other supports [57]. For further comparison, Cu/NC catalysts were prepared by the controlled
H2 reduction of HKUST-1 [58]. When FAL hydrogenation was carried out over Cu/NC
catalysts reduced by H2 at different reduction temperatures at 20 bar of H2 at 180 ℃ for 5 h,
HKUST-250-6 after reduction at 250 °C for 6 h shows the highest FAL conversion with 72.5 %
of FA selectivity (Fig. S12). The FAL conversion of HKUST-300-6 was significantly lower
than HKUST-250-6, due to the high reduction temperature and the corresponding aggregation
of Cu nanoparticles.
4. Conclusion
To determine the best catalyst for FAL hydrogenation, various ZIF-67-derived Co/NC
catalysts were prepared by H2 reduction at different temperatures and over different times. The
structural evolution of ZIF-67 was monitored by various in situ characterization techniques
under an H2 atmosphere. The structural, chemical, and morphological characterizations
indicated that the most active Co/NC catalyst, which retained the original shape of ZIF-67 and
produced uniformly dispersed Co nanoparticles in the N-doped carbon support, was prepared
by H2 reduction at 400 ℃. As the reduction time of Co/NC-400 catalyst increased, the fraction
of Co0 increased, resulting in high catalytic activity and MF selectivity. It was revealed that
Co/NC-400-6 exhibited the highest FAL conversion, and most of the ZIF-67-derived Co/NC
catalysts promoted MF as the major product. When Cu species were incorporated into the ZIF-
67 network (Cu/ZIF), the selectivity was changed toward FA as the major product due to the
16

preferential hydrogenation of the carbonyl group of FAL. In this study, we can clearly correlate
the Co particle size, oxidation state, and the corresponding catalytic performance of FAL
hydrogenation. It was concluded that FAL activity was maximized only when the original form
of ZIF-67 was maintained without agglomeration of Co nanoparticles. The catalyst with
optimized reduction treatment improved the yield of the biomass-upgrading reaction, and the
selectivity of the catalyst was changed through the introduction of external elements. This
approach is expected to be widely used for the development of high-performance MOF-derived
catalysts without requiring precious metals in the future.
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) and UNIST (1.200035.01). We
acknowledge Suhye Park in Namotec for technical support in in situ TEM study. H.R.M.
acknowledges the support from NRF funded by the Ministry of Science and ICT (NRF-
2016R1A5A1009405). K.S. acknowledges the support from the Korean government (MSIT)
(Grants No. NRF-2018R1A2B6008258) and the Fundamental Research Program of the Korea
Institute of Materials Science (KIMS) (Grant No. PNK7070).
17

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Scheme 1. Schematic illustrations of the transformation of ZIF-67 into a Co-supported N-
doped carbon catalyst (Co/NC) by H2 reduction.
Fig. 1. In situ TEM images showing the structural evolution of ZIF-67 to Co/NC as a function
of temperature in the H2 environment. The as-prepared ZIF-67 was deposited on a TEM cell
consisting of a SiNx membrane window, and the temperature was increased from room
temperature to 800 ℃ under 4% H2/Ar. The images were obtained from the recorded video.
The bar represents the 0.2 μm scale.
22

Fig. 2. SEM and HRTEM images and corresponding elemental EDS maps of (a) ZIF-67 and
(b) Co/NC-400-6 after the H2 reduction at 400 ℃ for 6 h.
23

Fig. 3. TEM images showing the structural evolution of ZIF-67 as a function of the H2
reduction time at 400 ℃: (a) Co/NC-400-2, (b) Co/NC-400-4, (c) Co/NC-400-6, (d) Co/NC-
400-8, and (e) Co/NC-400-12. The bar represents the 100 nm scale.
24

(a)
(b)
Co (111)
Co (111)
800
600
400
12 h
8 h
4 h
200
100
25
0 h
25 ℃
2θ
2θ
(c)
(d)
ZIF-67
Co/NC-400-2
Co/NC-400-4
Co/NC-400-6
Co2+
Co3+
1.5
ZIF-67
Co/NC-400-2
Co/NC-400-4
1.0
0.5
0.0
Co0
1.5
1.0
0.5
0.0
Co3O4
Co0
Co/NC-400-6
7700 7710 7720 7730 7740 7750 7760
812
804
796
788
780
772
7700
7710
7720
7730
7740
7750
7760
Binding energy (eV)
Photon energy (eV)
Fig. 4. (a-b) In situ XRD patterns of ZIF-67 under different reduction conditions: (a) reduction
by 4% H2/Ar in the temperature range of 25–800 ℃ at a rate of 5 ℃ min−1 and (b) reduction by
4% H2/Ar at 400 ℃ for up to 12 h. (c) XPS and (d) normalized Co K-edge XANES spectra of
ZIF-67-derived catalysts after reduction at 400 ℃ for different reduction times (2, 4, and 6 h).
25

(a)
+ H2
Tetrahydrofuran
- CO
+ H2
Furan
+ H2
Tetrahydrofurfuryl
alcohol
Furfural
+ H2
Furfurylalcohol
- H2O
2-Methylfuran
100
80
60
40
20
0
10.0
8.0
6.0
4.0
2.0
0.0
THFA
(b)
FA
THF
FN
MF
Fig. 5. (a) Illustration of the reaction pathways in FAL hydrogenation and the major products.
(b) Reaction rate(◆ ) and product selectivity of FAL hydrogenation as a function of the H2
reduction time over ZIF-67 and Co/NC-400 catalysts.
26

100
80
60
40
20
0
Conversion
(a)
MF
FN
FA
Others
8
0
2
4
6
Reaction time (h)
10
100
80
60
40
20
0
(b)
Conversion
MF
FN
FA
Others
0
2
4
6
8
10
Reaction time (h)
Fig. 6. Change in the FAL conversion and selectivity as a function of the reaction time for ZIF-
67-derived catalysts: (a) Co/NC-400-2 and (b) Co/NC-400-6.
27

100
80
60
40
20
0
1
(a)
(b)
0.5-Cu/ZIF
1-Cu/ZIF
0.8
0.6
0.4
0.2
0
FA
200 nm
MF
5-Cu/ZIF
3-Cu/ZIF
FN
0.5-Cu/ZIF 1-Cu/ZIF 3-Cu/ZIF 5-Cu/ZIF
Fig. 7. (a) TEM images of the Cu-incorporated ZIF-67 catalysts with different Cu loadings.
The bar represents the 200 nm scale. (b) Reaction rate and selectivity of the x-Cu/ZIF catalysts.
28

Table 1. Results of catalytic FAL hydrogenation.
Selectivity(%)
Conversion
Catalyst
(%)
FN
THF
MF
FA
THFA Others
Blank a
11.9
11.4
10.9
18.4
25.3
64.2
99.4
10.1
59.6
52.7
19.2
21.3
19.4
24.1
73.9
4.5
66.2
50.1
45.5
45.8
31.1
25.0
23.6
71.8
33.5
27.3
48.2
47.6
55.2
43.8
27.5
57.9
57.3
0.0
4.4
7.0
4.5
1.9
0.0
0.0
0.0
0.0
0.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
33.8
45.4
47.5
44.5
61.4
35.9
22.4
28.2
23.2
27.1
51.8
52.4
44.8
54.0
60.3
42.1
42.7
0.0
0.0
0.0
0.0
0.0
0.9
0.9
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
5.1
0.0
3.4
3.7
0.0
2.4
1.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
ZIF-67
ZIF-67 b
0.0
ZIF-67 c
0.0
Co/NC-400-2
Co/NC-400-4
Co/NC-400-6
Co/NC-400-6 d
Co/NC-400-8
Co/NC-400-12
0.5-Cu/ZIF
1-Cu/ZIF
5.6
34.9
49.5
0.0
40.9
43.4
0.0
0.0
3-Cu/ZIF
0.0
5-Cu/ZIF
2.2
10-Cu/ZIF
10-Cu/AC
10-Cu/MCF
12.2
0.0
3.9
0.0
Reaction conditions: FAL: 1 g, isopropanol: 20 mL, H2: 20 bar, reaction temperature: 180 °C,
reaction time: 5 h, catalysts: ZIF-67 (10 mg), Co/NC-R-T (10 mg), x-Cu/ZIF (20 mg), 10-
Cu/AC (20 mg), and 10-Cu/MCF (20 mg).
a Without catalyst.
b 40 mg of the catalyst is used.
c 200 mg of the catalyst is used.
d Reaction test was conducted under 20 bar of N2.
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