Selective transformation of renewable furfural catalyzed by diverse active species derived from 2D co-based metal-organic frameworks

Article abstract of DOI:10.1016/j.jcat.2017.06.017

We report a facile method for gaining two different types of highly active species from the same 2D Co-based MOFs, which can effectively catalyze the selective transformation of biomass-derived furfural (FUR) in alcohols. Removal of the coordinated water molecules from the Co-based MOFs at 300?°C creates the open metal centers as catalytic active species (designated as ACS-I catalyst). Increasing the pyrolysis temperature to 700?°C, the derived multi-element carbon-matrix nanocomposite from the MOFs (designated as ACS-II catalyst) also shows highly catalytic performance. Both catalyst ACS-I and ACS-II exhibit high reactivity (84.9% conv.) and excellent selectivity (ca. 99.0%) in the oxidative condensation of FUR with n-propanol to produce 3-(furan-2-yl-)-2-methylacrylaldehyde (2) in the presence of molecular oxygen. The particular evidence for the role of metal Co centers in ACS-I and ACS-II is originated from the catalyst characterization and control experiments, in which Ni-I and Ni-II catalysts derived from isomorphrous 2D Ni-based MOFs show no catalytic activity on the transformation of FUR under the similar conditions.

Full text of DOI:10.1016/j.jcat.2017.06.017

Journal of Catalysis 352 (2017) 480–490
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective transformation of renewable furfural catalyzed by diverse
active species derived from 2D co-based metal-organic frameworks
⇑
⇑
Liangmin Ning, Shengyun Liao , Xuguang Liu, Linhao Yu, Xuli Zhuang, Xinli Tong
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, No. 391 Binshuixi
Road, Tianjin 300384, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a facile method for gaining two different types of highly active species from the same 2D Co-
based MOFs, which can effectively catalyze the selective transformation of biomass-derived furfural
(FUR) in alcohols. Removal of the coordinated water molecules from the Co-based MOFs at 300 °C creates
the open metal centers as catalytic active species (designated as ACS-I catalyst). Increasing the pyrolysis
temperature to 700 °C, the derived multi-element carbon-matrix nanocomposite from the MOFs (desig-
nated as ACS-II catalyst) also shows highly catalytic performance. Both catalyst ACS-I and ACS-II exhibit
high reactivity (84.9% conv.) and excellent selectivity (ca. 99.0%) in the oxidative condensation of FUR
with n-propanol to produce 3-(furan-2-yl-)-2-methylacrylaldehyde (2) in the presence of molecular oxy-
gen. The particular evidence for the role of metal Co centers in ACS-I and ACS-II is originated from the
catalyst characterization and control experiments, in which Ni-I and Ni-II catalysts derived from isomor-
phrous 2D Ni-based MOFs show no catalytic activity on the transformation of FUR under the similar
conditions.
Received 28 February 2017
Revised 11 June 2017
Accepted 13 June 2017
Keywords:
Metal-organic frameworks
Catalytic species
Oxidative condensation
Furfural
Molecular oxygen
Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction
sent. Incorporation of redox metals into molecular sieves affords
potentially interesting heterogeneous oxidation catalysts such as
The growing concerning of the exhaustion of fossil resource and
environmental deterioration has provoked the extensive interest
for obtaining fine chemicals from the inexpensive and abundant
biomass [1–5]. Furfural (FUR), a typical bio-based platform chem-
ical, is regarded as versatile and important building blocks for gen-
erating a series of value-added products [6,7]. Ongoing research is
mainly focusing on the catalytic transformation processes of fur-
fural such as selective oxidation, oxidative esterification, partial
hydrogenation, aldol condensation and oxidative condensation
[8–11]. In particular, the carbon-chain-growth condensation is of
great importance for the catalytic upgrading of furfural to produce
a desirable gasoline component and stabilize furanic compounds
[12]. The oxidative condensation of FUR with aliphatic alcohols-
O2 is emerging as a tandem process, which can impel two carbon
molecules together to produce longer hydrocarbon chains using
as low volatile liquid transport fuels [13]. In the previous report,
the efficient transformation of FUR often relies on the catalysis of
precious metallic Pt/Al2O3 [14]. Thus, developing the novel cat-
alytic system with earth abundant metal is still challenging at pre-
TS-1, in which Ti was in tetrahedral positions [15,16]. However,
Roger A. Sheldon, et al. found some redox metal molecular sieves
such as CrAPO-5 and CoAPO-5 are by no means heterogeneous cat-
alysts during the course of oxidation of cyclohexane, in which the
metal leaches to form an active homogeneous catalyst because
metal was attached to defect sites of silicates [17–20].
Metal-organic frameworks (MOFs) have recently emerged as a
novel class of tunable molecular materials with great potential
applications. The key strategies for achieving catalytic active spe-
cies using MOFs are as follows. (i) The catalytic active species
(e.g. metal or metal oxide nanoparticles) were immobilized into
the MOFs matrix [21]. There are several approaches are available,
such as encapsulating the active species in the pores and cages
[22], depositing metallic nanoparticles [23], and post-synthetic
covalent modification [24,25]. (ii) Intrinsic catalytic active sites
originating from the inorganic nodes and organic linkers can serve
as the intriguing platform for developing reusable single site cata-
lysts [26–29]. However, there are a limited number of MOFs with
free coordinated sites available [30,31]. Removal of coordinated
solvent from the metal nodes is a promising method to activation
of the metal nodes [32]. (iii) Metal or metal oxide nanoparticles
derived from the MOFs through thermolysis under the controllable
atmosphere were used as catalysis [33–35]. A comparison of the
⇑
Corresponding authors.
E-mail addresses: 18202587221@163.com (S. Liao), tongxinli@tju.edu.cn (X.
Tong).
http://dx.doi.org/10.1016/j.jcat.2017.06.017
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
481
aforementioned strategies indicates the latter two will probably be
simple and cost-economic. However, the selection of metal nodes
and the controlled growth of MOFs are the key factors for design
and synthesis such catalysts. The discovery of earth-abundant
transition metal catalyst such as Co, Ni and Cu is becoming increas-
ingly critical for economically producing the commercial fine
chemicals owing to the high cost and scarcity of some precious
metals such as Ru, Pd and Au [36–39]. However, in some limited
reports on the application of MOFs for biomass valorization, the
earth-abundant metals such as Co, Ni-based MOFs never have been
used, presumably due to the catalytic active sites are often occu-
pied by the donor atoms from the organic ligands [40–42]. Solvent
removal might be a promising way for initiating the catalytic activ-
ity of the metal nodes [32]. To the best of our knowledge, there is
no report on using this approach combined with cobalt(II) ions as a
strategy for the transformation of biomass [43–45]. In addition,
using metal oxide nanoparticles derived from MOFs precursors as
catalysts for biomass valorization remains unexplored [46–48].
Herein, we perform the oxidative condensation of FUR, a typical
biomass-based platform compound, using the simple MOFs-
derived material as heterogeneous catalyst. The 2D Co-based MOFs
([Co(tia)(H2O)2]n) are evaluated as potential catalytic species for
the selective transformation of FUR in the green alcohol-O2 system
[13]. For the first time, two different types of Co-based active spe-
cies were obtained using the same 2D Co-based MOFs. One cat-
alytic species with the intrinsic open-metal active sites
(designated as ACS-I catalyst) was generated through removal of
coordinated water molecules at 300 °C under argon atmosphere,
promoting the oxidative condensation of FUR and n-propanol
where a 63.4% conversion of FUR and 99% product selectivity are
obtained. With the higher pyrolysis temperature at 700 °C, another
derived active nanocomposite (designated as ACS-II catalyst) is
produced and it catalyzes more efficiently the oxidative condensa-
tion in which the FUR conversion and selectivity of desired product
are increased up to 84.9% and 99.7%, respectively.
electron microscope (SEM: JSM-6301F, JEOL) equipped with a
JED-2300 (JEOL) EDXS spectrometer for chemical analysis and
transmission electron microscope (TEM: JEM-2100, JEOL). X-ray
photoelectron spectra (XPS) were recorded on a KRATOS AXIS
165 with a dual X-ray anode (Mg and Al) and all XPS spectra were
recorded using the MgKa line. Inductively Coupled Plasma Optical
Emission Spectrometry (ICP-OES:Varian 700-ES) was used to mea-
sure the metal content of the catalytic materials.
2.2. Preparation of catalysts
2.2.1. Synthesis of 2D Co-based and Ni-based MOFs
2D Co-based MOFs has been synthesized using the similar pro-
cedure published earlier [49,50]. Briefly, Co(NO3)2Á6H2O
(0.2 mmol, 0.0582 g) or Ni(NO3)2Á6H2O (0.2 mmol, 0.0582 g) and
the aqueous solution of H3ctia (15 mL) (H3ctia = 5-(4-carboxyl-
1H-1,2,3-triazol-1-yl) isophthalic acid, 0.2 mmol, 0.0554 g) whose
pH value was adjusted to 3.5 by NaOH was added into a
Teflon-lined pressure vessel, and then the mixture was heated
to 170 °C and kept at 170 °C for 72 h. After cooling to room tem-
perature at a rate of 2.5 °C/h. Red needle-shaped crystals suitable
for X-ray analysis were directly obtained, collected, washed with
water and ethanol, and then dried in the air. The formula for the
obtained 2D Co-based MOFs is [Co(tia)(H2O)2]n (tia2À = 5-(1H-1,2
,3-triazol-1-yl)isophthalate). Yield about 99% (based on Co(NO3)2Á
6H2O). Anal.Calcd (%) for [Co(tia)(H2O)2]n (325.98): C, 36.83; H,
2.78; N, 12.88. Found: C, 36.50; H, 2.67; N, 12.80. Crystallographic
data for the as-obtained MOFs are given in Table S1. Selected
bond lengths and angles are given in Table S2 in supporting
information.
The formula for the as-synthesized 2D Ni-based MOFs is [Ni(tia)
(H2O)2]n. Yield about 98% (based on Ni(NO3)2Á6H2O). Anal. Calcd
(%) for [Ni(tia)(H2O)2]n (324.98): C, 36.83; H, 2.78; N, 12.89. Found:
C, 36.90; H, 2.69; N, 12.04. Crystallographic data for the as-
obtained MOFs are given in Table S1. Selected bond lengths and
angles are given in Table S3 in supporting information.
2. Experimental
2.1. Materials and methods
2.2.2. Preparation of the active species
As-obtained 2D Co-based MOFs were added in a quartz boat,
and placed in the tubular furnace. The MOFs was heated up at dif-
ferent temperatures from 100 to 800 °C for 4 h under a continuous
argon flow of 50 mL minÀ1. After cooling to room temperature, the
derived catalytic materials were obtained. (The derived catalytic
materials at 300 °C and 700 °C were designated as ACS-I and
ACS-II, respectively).
2.1.1. Materials
All reagents and solvents for synthesis and analysis were
commercially available and used as received. H3ctia (H3ctia =
5-(4-carboxyl-1H-1,2,3-triazol-1-yl) isophthalic acid) has been
synthesized according to the literature [49].
2.1.2. General methods for characterization
The preparation methods of Ni-I and Ni-II catalysts were similar
with those for preparing ACS-I and ACS-II catalysts.
IR spectra were recorded in the range of 4000–400 cmÀ1 on a
Perkin-Elmer spectrometer with KBr pellets. Elemental analyses
for C, H, and
N were carried out on a Model 2400 II,
Perkin-Elmerelemental analyzer. X-ray powder diffraction (XRD)
intensities of the different samples were measured on a Rigaku
2.3. The catalytic oxidative condensation of furfural
D/max-IIIA diffractometer (Cu K
a
, k = 1.54056 Å). TGA experiments
All oxidative condensation experiments were performed in a
120 mL stainless steel autoclave equipped with the magnetic
stirring and a temperature controller. A typical procedure for
oxidation condensation of FUR with n-propanol is as follows. FUR
(0.1 g, 1 mmol), n-propanol (15 mL), the Co-based catalyst
(25 mg) and Cs2CO3 (25 mg) were added into the autoclave. After
the reactor was sealed, the pure oxygen was pumped to replace
the atmosphere for several times. Then under the pressure of
0.3 MPa, the mixture was preheated to the set temperature with
magnetic stirring and kept for a certain time. After the autoclave
was cooled down and the excess gas was released, the as-
obtained mixture was analyzed by GC and GC–MS (The details
are shown in the supporting information).
were performed in flowing N2 on a NETZSCH TG 209 instrument
with a heating rate of 10 °C/min. Single crystal diffraction data
for Co-based and Ni-based MOFs were collected with a Bruker
SMART APEX CCD instrument with graphite monochromatic MoK
a
radiation (k = 0.71073 Å). The data were collected at 293(2) K. The
absorption corrections were made by multi-scan methods. The
structure was solved by direct methods and refined by full-
matrix least-square methods on all F2 data with the program
Olex2. The quantitative analyses of the products were determined
on a GC apparatus with FID detector or on the Agilent 6890/5973
Gas Chromatograph-Mass Spectrometer (GC–MS) instrument.
The morphology of catalytic materials was obtained by scanning

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L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
ture is above 600 °C. Furthermore, TGA plots of the initial ACS-I
3. Results and discussion
and used ACS-I were summarized in Fig. S10 in supporting infor-
mation. It can be seen that the plots of used ACS-I almost over-
lapped with the fresh ACS-I.
3.1. Preparation and characterization of the catalytic active species
FTIR spectra of the 2D Co-MOFs are shown in Fig. 3. We tenta-
tively assigned the characteristic peaks at 3619 cmÀ1 and
3167 cmÀ1 to the -O-H stretch from the coordinated water and
H-bonds in the initial 2D Co-based MOFs. The characteristic peaks
Single X-ray diffraction analysis of the as-obtained 2D Co-based
MOFs shows six-coordinated Co(II) centers were bridged by the
tia2À ligands in three directions to afford 2D layer in crystallo-
graphic bc plane (tia2À = 5-(1H-1,2,3-triazol-1-yl) isophthalate
(see Fig. 1). The distance between two adjacent layers is about
3.2 Å (see Scheme 2). The apical positions of the metal centers were
occupied by two coordinated water molecules. A method for the
thermal treatment of the as-synthesized MOFs was used to pro-
duce active spices for cascade oxidative condensation of furfural
with n-propanol at different temperature under Ar atmosphere
(see Scheme 1 and Table 1). We found that there is no catalytic
activity below 150 °C (Table 1, Entries 1 and 2). The conversion
of FUR and the selectivity of 2 are respectively 22% and 84.8% when
the treatment temperature was increased to 200 °C (Table 1, Entry
3). Both the conversion of FUR (63.4%) and the selectivity of 2
(99.5%) are increased by further increasing the temperature to
300 °C (Table 1, Entry 5). Next, when the temperature was elevated
from 300 °C to 700 °C, the conversion of FUR and selectivity of 2
was first decreased, then increased (Table 1, Entries 5–10). We
found the performance of the derived materials at 700 °C was best,
giving 84.9% conversion of FUR and over 99% selectivity of 2
(Table 1, Entry 10). Increasing the temperature to 800 °C resulted
in lower conversion and selectivity (Table 1, Entries 11 and 12).
It is obvious that two different high active species were obtained
from 2D Co-based MOFs by heating under different temperatures.
The as-obtained active species was designated as ACS-I catalyst at
300 °C and ACS-II catalyst at 700 °C, respectively (Scheme 2).
In order to reveal the relationship between the structures and
the performance of active species, various techniques such as
TGA, IR, XRD, SEM TEM, ICP and XPS were applied to characterize
the catalysts. The TGA curves of 2D Co-based MOFs shows a first
stage of weight loss from 40 to 300 °C, which is presumably due
to the release of the coordinated water molecules (Fig. 2). The
observed weight loss (14.34%) is slightly higher than the calculated
value (11.1%). The second weight loss of 2D Co-based MOFs begins
1630 cmÀ1
, , , ,
1380 cmÀ1 1247 cmÀ1 1080 cmÀ1 470 cmÀ1 and
441 cmÀ1 were corresponding to the stretch of the ACOOÀ, C@N,
CAN, CoAO and CoAN. The intensity of the peak at 3169 cmÀ1
decreased upon heating presumable due to the removal of coordi-
nated water. The characteristic peaks of ACOOÀ, C@N, CAN, CoAO
and CoAN were observed for the derived ACS-I, suggesting the
metal–organic moieties in crystallographic bc plane can be kept
integrity. All of these are consistent with the results from TGA
experiment. FTIR spectra of used ACS-I and ACS-II display their
characteristic peaks didn’t change after being recycled for five
times (see Fig. S11 and S12 in Supporting Information).
The XRD patterns of the derived species at 150 °C, 200 °C and
250 °C or the as-synthesized 2D Co-based MOFs match well with
the computer-simulated patterns of crystal structure (Fig. 4). For
the patterns of the derived ACS-I at 300 °C exhibited two peaks
at 9.6° and 13.6° were agreement well with the 2D Co-based MOFs,
which verified the metal–organic framework in crystallographic bc
plane was still kept integrity. The disappeared peaks of the derived
materials with the heat treatment temperature above 300 °C sug-
gested the skeleton collapse of MOFs. When the pyrolysis temper-
ature was at 800 °C, the new peaks appeared at 44.3°, 51.5°, and
76.0° are correspondent to the diffraction of (111), (200) and
(220) plane of Cobalt, which indicates that
a single face-
centered cubic (fcc) crystalline -Co phase (PDF 89-4307) was pre-
a
sent on the carbon support surface after heat treatment [51]. The
peaks at 9.6° and 13.6° of ACS-I disappeared and no diffraction
peaks can be observed after being used in the reaction media
(see Fig. S13 in supporting information). The crystallinity of the
ACS-I species was thoroughly destroyed, even if TEM and SEM
show the morphology of ACS-I unchanged after being recycled
for several times. As far as ACS-II, XRD patterns did not change
after being recycled for five times (see Fig. S14 in supporting
information).
at 320 °C, corresponding to the decomposition of the ligand tia2À
The organic framework collapsed thoroughly when the tempera-
.
Fig. 1. Crystallographic structure of the as-synthesized 2D Co-based MOFs.
Scheme 1. Oxidation of transformation of FUR with n-propanol-O2 catalysed by the active species derived from 2D Co-based MOFs.

L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
483
Scheme 2. The routes for producing the catalytic active species using 2D Co-based MOFs.
Table 1
The catalytic efficiency of the active species acquired with different heat treatment temperature.a
c
Entry
Temperature (°C)
Conv. (%)b
Sel. (%)
2
3
Others
1
2
3
4
5
6
7
8
0
13.3
13.5
22.0
44.4
63.4
41.8
36.6
63.7
81.6
84.9
51.0
27.5
81.2
85.5
84.8
99.1
99.5
91.6
99.1
92.7
92.2
99.7
90.2
90.8
8.5
5.9
7.1
0.9
0.5
8.3
0.9
0
10.3
8.6
8.1
0
0
0
150
200
250
300
350
400
500
600
700
750
800
0
7.3
7.8
0
0
0
9
0
10
11
12
0.3
9.8
9.2
a
b
c
Reaction conditions: FUR (0.1 g), catalyst (25 mg), n-propanol (15 mL), Cs2CO3 (25 mg), O2 (0.3 MPa), 140 °C and 4 h.
The conversion of 1 was determined by the internal standard technique using GC.
The selectivity was acquired by the area normalization method on GC; 3 represents propyl furan-2-carboxylate; 2 represents 3-(furan-2-yl-)-2-methylacrylaldehyde.
HRSEM images of 2D Co-based MOFs and the materials derived
open unsaturated coordinated Co(II) centers by removal the coor-
dinated water molecules. The reactants including furfural and n-
propanol are intercalated into the layers and the reaction can be
promoted by the open unsaturated coordinated Co(II) center. Obvi-
ously, the active sites in ACS-I are well-confined and uniformly dis-
persed in 2D layers, which can pave the way for us to design the
single-site heterogeneous catalysts. (ii) The ACS-II catalyst is
multi-element nanocomposites including Co, O, N and C dispersed
on the carbon matrix. The catalytic active sites are centered on the
metal cobalt. These conclusions were further supported by the XPS
measurements and the following control catalytic experiment.
XPS characterization of ACS-I and ACS-II relies on the careful
analysis of peaks associated with C1s, N1s, O1s and Co2p3/2, posi-
tioned around 285, 400, 532 and 778 eV, respectively (Figs. 8 and
9). The C1s level spectrum of ACS-I was nicely fitted with five com-
ponents at 283.97, 284.48, 285.13, 286.15 and 289.08 eV, which
are associated with the adsorption ACAH (on the triazol ring),
ACAH (on the benzene ring), ACAN, AC@C and ACOOA, respec-
tively (Fig. 8a). The best fitted peaks of O1s at 531.96 and
532.64 eV could be assigned the adsorption of AC@O and ACoAO,
at 300 °C, 500 °C and 700 °C were shown in Fig. 5. ACS-I species
maintained the cuboid block shape of the synthesized crystal
(compared Fig. 5a and b). Meanwhile, the images of the ACS-I spe-
cies highlight the lamellar structure. In contrast, the ACS-II was
spherical nanoparticles distributed on some cotton-shaped matrix,
the particle size falls within the range of 0–30 nm (Fig. 5c and d).
At the same time, the morphology and structure of the ACS-I
and ACS-II were further investigated by TEM (Fig. 7(a) and (d)).
The HRSEM images of the fresh and used ACS-I show the lamellar
structure unchanged after being recycled for five times (Figs. 5
(b) and 6(a), (b)). The TEM images of them also verified the mor-
phology of used ACS-I can be kept integrity well after being recy-
cling for several times (Fig. 7(a)–(c)). The HRSEM and TEM of
used ACS-II show the size of the particles agglomerated slightly
after being recycling for several times (Figs. 5(d), 6(c), (d), and 7
(d)–(f)).
The aforementioned structural information of the derived mate-
rials from TGA, IR, XRD, SEM and TEM experiments reveals the
facts as follows: (i) The active sites in ACS-I catalyst are the created

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L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
cules didn’t affect the ligand tia2À and the coordinated modes of
Co centers.
In contrast, the best fitted peaks of C1s, N1s, O1s and Co2p3/2 in
ACS-II catalyst is totally different from ACS-I (Fig. 9). The best fit-
ted peaks of C1s at 284.2 and 284.9 eV could be assigned the
adsorption of graphite, while the peaks at 285.6 and 288.5 eV can
be attributed to the adsorption of ACAN and C@O (Fig. 9a). Nota-
bly, two different components of N1s at 401.50 and 401.60 eV
appeared in ACS-II catalyst (Fig. 9b). For O1s, the best fitted peaks
positioned at 531.49, 531.86 and 532.48 eV (Fig. 9c). The binding
energy of Co2p3/2 in the ACS-I is higher than ACS-II. The peaks at
781.08, 781.71 and 783.35 eV indicates the existence of CoAO,
CoAN and CoAC bonds (Fig. 9d).
XPS was also used to analyze the used ACS-I and ACS-II. It was
found that the structure and composition of them remain
unchanged after five recycle runs (see Figs. S15, S16, S17 and S18
in supporting information).
In addition, IR, TGA, XRD, XPS, SEM, TEM and ICP has been per-
formed on the fresh and used Ni-I, Ni-II catalysts derived from 2D
Ni-based MOFs at 300 °C and 700 °C, respectively (see Figs. S19-
S27 and Table S7 in supporting information). All of the data shows
the structure of Ni-I, Ni-II are very similar to the ACS-I and ACS-II,
but the low catalytic activity for oxidation of FUR with alcohol may
be due to the difficulty for forming Ni (III). Another reason for
interpretation the poor activities of Ni-I, Ni-II is that Ni show no
activity during the course oxidation of furyl alcohol (FA) to FUR,
very important step for producing 2 (see Scheme S1: the mecha-
nism of oxidation condensation of FUR and Table S8: the controlled
oxidation experiments of furyl alcohol in supporting information)
[13].
Fig. 2. TGA plots of 2D–Co-based MOFs.
3.2. ACS-I and ACS-II species for catalytic oxidation transformation of
FUR with alcohol
The prepared ACS-I and ACS-II species were used as the cata-
lysts for oxidative condensation of FUR with n-propanol-O2. The
initial experiments were carried out to determine the synergistic
effect between the catalysts and additives (Table 2). Negligible
conversion was observed without any catalysts (Table 2, Entry 1).
When ACS-I and ACS-II were used as the catalyst, low conversion
and poor selectivity were obtained (Table 2, Entries 2 and 4). To
our delight, the reactivity was greatly enhanced by adding Cs2CO3.
The ACS-I catalyst combined with Cs2CO3 as additive afforded
63.4% conversion of FUR and over 99% selectivity for 2. The ACS-
II catalyst with Cs2CO3 as additive afforded 84.9% conversion of
FUR and over 99% selectivity for 2 (Table 2, Entries 3 and 5). The
isomorphous 2D Ni-based MOF was used to produce Ni-I and Ni-
II species under the heat treatment temperature of 300 °C and
700 °C, respectively. The poor catalytic performance of Ni-I and
Ni-II catalysts in the presence of Cs2CO3 further proved that the
active sites in ACS-I and ACS-II are the metal Co centers, not the
organic framework or the carbon matrix (Table 2, Entries 7 and 8).
In order to further improve the reactivity and selectivity, we
screened several basic or acidic additives (Table 3). The moderate
conversion of FUR and selectivity of the desired 2 were obtained
in the presence of Li2CO3 and Na2CO3 (Table 3, Entries 1 and 2).
61.2% conversion of FUR with selectivity of 40.0% for 2 in ACS-I sys-
tem and 60.8% conversion of FUR with selectivity of 88.5% for 2 in
ACS-II system suggested K2CO3 was not the ideal additive for pro-
ducing 2 (Table 3, Entries 3 and 4). Similarly, the strong basic addi-
tive NaOH may promote the transformation of FUR with 68.0% and
88.1% conversion in ACS-I and ACS-II system, respectively. But
60.2% and 83.3% selectivity of the desired product in ACS-I and
ACS-II system are unsatisfactory (Table 3, Entries 5 and 6). The
organic basic (butylamine) and acidic additive (H3PO3) were also
giving the poor selectivity for 2 (Table 3, Entries 7 and 8).
Fig. 3. FTIR spectra of the derived materials at different temperature.
Fig. 4. XRD patterns of the derived materials at different temperature.
respectively (Fig. 8c). Two best fitted peaks of Co2p3/2 at 778.14
and 778.81 eV are corresponding to the adsorption of ACoAN
and ACoAO, respectively (Fig. 8d). The XPS spectra of ACS-I also
verified that the behaviour for removal coordinated water mole-

L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
485
Fig. 5. HRSEM images of the 2D-Co-based MOFs and the derived materials. (a) Initial as-synthesized 2D Co-based MOFs; (b) ACS-I species; (c) The materials derived at 500 °C;
(d) ACS-II species.
Fig. 6. HRSEM images of ACS-I and ACS-II: (a) ACS-I after one recycle run; (b) ACS-I after five recycle runs; (c) ACS-II after one recycle run; (d) ACS-II after five recycle runs.
Next, the influence of reaction time on the conversion of FUR
and selectivity of were investigated at 140 °C (shown in
Fig. 10). In both systems, prolonged reaction time resulted in
increasing the conversion and decreasing the selectivity. Decreas-
ing of the selectivity of 2 might be ascribed to the further conden-
sation of 2 by prolonging reaction time.
We then turned our interest to test the generality of oxidation
of FUR with other alcohols (Fig. 11). 74.9% and 83.3% conversion
of FUR can be obtained using the ACS-I + Cs2CO3 catalytic system
and the ACS-II + Cs2CO3 catalytic system with ethanol,
respectively. However, comparing with n-propanol, the selectivity
of 3-(furan-2-yl-)acrylaldehyde decreased to 93.9% and 83.2%.
2

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L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
Fig. 7. TEM images of ACS-I and ACS-II: (a) fresh ACS-I; (b) ACS-I after one recycle run; (c) ACS-I after five recycle run; (d) fresh ACS-II; (e) ACS-II after one recycle run; (f)
ACS-II after five recycle run.
Fig. 8. XPS analysis for ACS-I: (a) C1s; (b) N1s; (c) O1s; (d) Co2p3/2
.

L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
487
Fig. 9. XPS analysis for ACS-II: (a) C1s; (b) N1s; (c) O1s; (d) Co2p3/2
.
Table 2
Control experiments of the oxidative condensation of FUR with n-propanol-O2.a
Entry
Catalytic system
Conv. (%)b
Sel. (%)c
2
3
Others
1
2
3
4
5
6
7
8
Cs2CO3
ACS-I
ACS-I + Cs2CO3
ACS-II
ACS-II + Cs2CO3
No
4.1
3.7
4.2
99.5
2.8
99.7
–
89.2
75.8
0.5
86.3
0.3
–
21.5
20.9
7.1
20.0
0
10.9
0
–
5.3
38.2
15.6
63.4
21.7
84.9
1.8
d
Ni-I + Cs2CO3
Ni-II + Cs2CO3
25.5
4.2
73.2
40.9
d
a
b
c
Reaction conditions: FUR (0.1 g), catalyst (25 mg), n-propanol (15 mL), Cs2CO3 (25 mg), O2 (0.3 MPa), 140 °C and 4 h.
The conversion of FUR was determined by the internal standard technique using GC.
The selectivity was acquired by the area normalization method on GC; 3 represents propyl furan-2-carboxylate; 2 represents 3-(furan-2-yl-)-2-methylacrylaldehyde.
Ni-I and Ni-II catalysts are isostructural with ACS-I and ACS-II, respectively.
d
With ACS-I + Cs2CO3 catalyst, the low conversion of FUR in moder-
ate selectivity of desired product in iso-proanol and n-butanol can
be contributed to the steric hindrance of alcoholic molecule during
reaction. While with ACS-II + Cs2CO3 catalyst, about 45% conver-
sion of FUR and 65% selectivity of the desired product can be
acquired in isopropanol and n-butanol.
Recyclability of ACS-I and ACS-II catalysts shows that (Fig. 12)
the selectivity of 2 displayed no significant change after being recy-
cled five times. But the conversion of FUR slightly drops a bit dur-
ing each of the five steps. In order to confirm whether the metal Co
leaches and the observed catalysis is truly heterogeneous in nature,
the following experiments have been performed. (i) Inductively
Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used
to measure the Co content in solution and solid after each recycling
run for ACS-I and ACS-II. The results are summarized in Fig. 12,
Table S4 in supporting information. No Co content in solution
was detected by ICP-OES. The amount of Co in the solid remains
the same during each of the steps. (ii) Homogeneous cobalt salts
was used to catalyze this transformation and found the activities
of the homogeneous cobalt are very poor. Especially, in order to

488
L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
Table 3
Effect of various additives on the oxidative condensation of FUR with n-propanol-O2 catalyzing by ACS-I and ACS-II catalysts.a
Entry
Additive
In ACS-I system
In ACS-II system
Conv. (%)b
Sel. (%)c
2
Conv. (%)b
Sel. (%)c
2
3
Others
3
Others
1
2
3
4
5
6
7
8
Li2CO3
Na2CO3
K2CO3
Cs2CO3
NaOH
KOH
Butylamine
H3PO3
18.1
28.9
61.2
63.4
68.0
78.0
76.6
59.5
69.8
27.6
40.0
>99
60.2
74.3
12.7
0
30.2
0
0
0.5
0
2.5
75.8
57.0
0
28.5
31.2
60.8
84.9
88.1
77.4
60.4
49.3
15.2
21.8
88.5
>99
83.3
79.5
15.9
0
18.1
0
7.7
0.3
16.7
0
66.7
78.2
3.8
0
0
20.5
0
72.4
60.0
0
39.8
23.2
11.5
43.0
84.1
0
100
a
b
c
Reaction conditions: FUR (0.1 g), catalyst (25 mg), n-propanol (15 mL), Cs2CO3 (25 mg), O2 (0.3 MPa), 140 °C and 4 h.
The conversion of FUR was determined by the internal standard technique using GC.
The selectivity was acquired by the area normalization method on GC; 3 represents propyl furan-2-carboxylate; 2 represents 3-(furan-2-yl-)-2-methylacrylaldehyde.
Fig. 10. Influence of reaction time on the oxidative condensation of FUR with n-propanol-O2 to 2 with the derived catalysts. (a) ACS-I + Cs2CO3; (b) ACS-II + Cs2CO3. Reaction
condition: FUR (0.1 g), catalyst (25 mg), n-propanol (15 mL), Cs2CO3 (25 mg), O2 (0.3 MPa), 140 °C.
Fig. 11. Oxidation condensation of FUR with some typical alcohol lover the derived catalysts. (a) ACS-I + Cs2CO3; (b) ACS-II + Cs2CO3. Reaction condition: FUR (0.1 g), catalyst
(25 mg), alcohol (15 mL), Cs2CO3 (25 mg), O2 (0.3 MPa), 140 °C, 4 h. dthe reaction time: 10 h.
preclude the phenomenon of catalyst-inhibitor conversion, the
activity of homogeneous cobalt salts was tested in low concentra-
tion and no narrowness domain was observed (see Table S5 in sup-
porting information) [15,20]. (iii) ACS-I and ACS-II ware treated in
n-propanol (15 mL) under the reaction conditions in the absence of
FUR. After centrifugation, the filtrate was fed into the reaction
medium. No catalytic activity was observed (see Table S6 in sup-
porting information). So even if a little of Co beyond the limit of
detection leaches but is not an active homogenous catalyst for
the transformation of FUR. In addition, the aforementioned data

L. Ning et al. / Journal of Catalysis 352 (2017) 480–490
489
Fig. 12. Recycle tests of the ACS-I and ACS-II catalysts in the oxidative condensation of FUR with n-propanol-O2. Reaction condition: FUR (0.1 g), catalyst (25 mg), n-propanol
(15 mL), Cs2CO3 (25 mg), O2 (0.3 MPa), 140 °C, 4 h.
of the used ACS-I and ACS-II from IR, TGA, XRD, XPS, SEM, TEM and
ICP help us to assure that ACS-I and ACS-II are truly heterogeneous
catalyst for oxidation condensation of FUR with alcohol.
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