In Situ Generation and Stabilization of Accessible Cu/Cu2O Heterojunctions inside Organic Frameworks for Highly Efficient Catalysis

Article abstract of DOI:10.1002/anie.201913811

Heterostructural metal/metal oxides are the very promising substituents of noble-metal catalysts; however, generation and further stabilization of accessible metal/metal oxide heterojunctions are very difficult. A strategy to encapsulate and stabilize Cu/Cu₂O nanojunctions in porous organic frameworks in situ is developed by tuning the acrylate contents in copper-based metal–organic frameworks (Cu-MOFs) and the pyrolytic conditions. The acrylate groups play important roles on improving the polymerization degree of organic frameworks and generating and stabilizing highly dispersed and accessible Cu/Cu₂O heteronanojunctions. As a result, pyrolysis of the MOF ZJU-199, consisting of three acrylates per ligand, generates abundant heterostructural Cu/Cu₂O discrete domains inside porous organic matrices at 350 °C, demonstrating excellent catalytic properties in liquid-phase hydrogenation of furfural into furfuryl alcohol, which are much superior to the non-noble metal-based catalysts.

Full text of DOI:10.1002/anie.201913811

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Accepted Article
Title: In situ Generation and Stabilization of Accessible Cu/Cu2O
Heterojunctions inside Organic Frameworks for Highly Efficient
Catalysis
Authors: Chuan-De Wu, Kai Chen, and Jia-Long Ling
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913811
Angew. Chem. 10.1002/ange.201913811
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10.1002/anie.201913811
Angewandte Chemie International Edition
RESEARCH ARTICLE
In situ Generation and Stabilization of Accessible Cu/Cu2O
Heterojunctions inside Organic Frameworks for Highly Efficient
Catalysis
Kai Chen[a], Jia-Long Ling[a] and Chuan-De Wu*[a]
Abstract: Heterostructural metal/metal oxides are the very
promising substituents of noble-metal catalysts; however, generation
and further stabilization of accessible metal/metal oxide
heterojunctions are very difficult. Herein, a strategy to in situ
encapsulate and stabilize Cu/Cu2O nanojunctions in porous organic
frameworks is developed by tuning the acrylate contents in copper-
based metal-organic frameworks (Cu-MOFs) and the pyrolytic
conditions. The acrylate groups play important roles on improving
the polymerization degree of organic frameworks, and generating
and stabilizing highly dispersed and accessible Cu/Cu2O
heteronanojunctions. As a result, pyrolysis of the MOF ZJU-199,
consisting of three acrylates per ligand, generates abundant
heterostructural Cu/Cu2O discrete domains inside porous organic
matrices at 350 oC, demonstrating excellent catalytic properties in
liquid-phase hydrogenation of furfural into furfuryl alcohol, which are
much superior to the non-noble metal-based catalysts in the
literature. This work provides a promising pathway for developing
highly efficient non-noble metal catalysts in practical applications.
decreased transition energy barriers during catalysis.5 However,
the exposed active metallic copper is easily oxidized to form
CuOx coating layers, leading to the heterostructural Cu/CuOx
interfaces inaccessible, and thus to heavily deteriorate the
catalytic properties.6
Metal-organic frameworks (MOFs) are a class of porous
crystalline materials, constructed from the coordination bond
connections between multidentate organic linkers and metal
ions/clusters, which have been extensively used to fabricate
porous MNPs@carbon composites under high temperature
pyrolytic conditions.7 MNPs@carbon composites exhibited high
catalytic efficiency in numerous reactions, attributed to the in situ
formed graphitic carbons that provide good electron mobility to
promote charge separation/transfer. However, extreme high
annealing temperature often resulted in severe collapse of the
pristine skeletons of MOFs and subsequent aggregation of in
situ formed MNPs, leading to the formation of thermally stable
homogeneous phases or the heterojunction interfaces
inaccessible to reactant molecules during catalysis.8
Introduction
Noble metal catalysts (e.g. Pd, Pt, Au and Rh) are highly active
in numerous chemical processes, attributed to their unique
electron and orbital properties.1 However, the high-cost and
scarcity of noble metals heavily hindered their practical
applications. A tremendous effort has been paid on developing
the alternatives with earth-abundant metal sources, including
metal/metal oxide nanoparticles (MNPs); however, their catalytic
properties remain much inferior to those of noble catalysts.2 In
accordance with the unique electron donating and accepting
properties of noble metals, the most promising alternative
catalysts based on earth-abundant metals should consist of
heterojunctions, in which one could act as the redox-active
center while the other one should perform as the Lewis acid or
base site, which would synergistically work to mimic the electron
and orbital properties of noble metals.3
Scheme 1. Schematic illustration of the preparation of porous organic
frameworks, consisting of in situ generated heterostructural Cu/CuOx
nanoparticles, for liquid phase hydrogenation of FAL into FOL.
Among numerous earth-abundant metal/metal oxides, Cu-
based nanomaterials are highly attractive, because the copper
element could form valence-mixed heterostructures, such as
Cu/Cu2O, Cu2O/CuO and Cu/Cu2O/CuO.4 In contrast to the
isolated single phase of Cu or CuOx, the Cu/CuOx
heterostructures could re-localize the electron densities of
different redox-active sites on the interfaces, resulting in
It has been demonstrated that cinnamic acid could be
decarboxylated to produce styrene and further polymerized at
o
200-270 C in the presence of CuSO4 catalyst.9 Therefore, the
Cu-MOFs, consisting of multiple acrylate moieties in organic
linkers, should be an ideal class of porous templates for the
fabrication of porous organic frameworks under suitable pyrolytic
conditions.10 The polystyrene networks might perform as robust
physical barriers to prevent the migration of in situ formed
copper nanospecies during pyrolysis, resulting in abundant
accessible Cu/CuOx nanojunctions inside porous organic
matrices. As shown in Scheme 1, the sizes, compositions and
heterostructures of in situ generated Cu/CuOx nanoparticles
inside porous organic frameworks are indeed systematically
tunable by adjusting the constituents and functionalities of
organic linkers in Cu-MOFs and controlling the annealing
[a]
K. Chen, J.-L. Ling and Prof. Dr. C.-D. Wu
State Key Laboratory of Silicon Materials
Department of Chemistry
Zhejiang University
Hangzhou 310027, P. R. China
E-mail: cdwu@zju.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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There also appears an obvious C=O vibration band at 1670 cm-1,
which is in line with the TG results, indicating that there are
multiple cross-linking pathways for the organic residues under
pyrolytic conditions. The characteristic peak of exocyclic C=C
vibration at 980 cm-1 gradually decreases upon elevating the
temperature, while the peak at 1385 cm-1 assigned to the
aliphatic C-H bending vibration of methylene groups gradually
increases. Moreover, the decrease of the vibration intensities for
exocyclic C=C and carboxylate groups are highly synchronized,
indicating that the acrylate moieties in ZJU-199 were first
decarboxylated to generate highly reactive styrene species, and
subsequently thermal-polymerized to form 3D polystyrene
networks under annealing conditions. UV-Vis spectra of the
chloroform eluents of annealed samples showed that there is no
signal ascribed to the aromatic compounds, indicating that the
organic residues are highly polymerized. The organic residues
suffered from severe carbonization when the temperature was
raised to 500 oC and fully carbonized at 800 oC, as confirmed by
Raman spectroscopy (Figure S2).
conditions, which realized high catalytic efficiency, selectivity
and stability in liquid phase hydrogenation of furfural (FAL) into
furfuryl alcohol (FOL).
Results and Discussion
The Cu-MOF ZJU-199 (ZJU = Zhejiang University) is built from
[Cu2(COO)4] paddle-wheel secondary building units (SBUs) and
benzene-1,3,5-tri-β-acrylate (BTAC) linkers, sharing the same
tbo topology and coordination modes with the Cu-MOF HKUST-
1.11 There are three acrylate moieties per ligand, indicating that
ZJU-199 should be an ideal precursor for the preparation of
porous organic frameworks consisting of in situ generated
Cu/CuOx nanojunctions (Figure S1 and Scheme S1). We first
monitored the thermal behaviors of ZJU-199 by TG-MS
measurement under inert N2 atmosphere (Figure 1a). The
o
weight loss before 150 C should be assigned to the release of
encapsulated solvent and coordinated water molecules. Upon
raising the temperature, a weight loss of 22.1 wt% was observed
o
in the range of 250-400 C. Based on the overlap of DTG curve
and corresponding CO2 mass plot, the weight loss should be
originated from decarboxylation of organic linkers. The weight
loss is smaller than the content of exact carboxylate groups
(28.6 wt%) in ZJU-199, indicating that there should occur other
decomposition pathways besides of decarboxylation, such as
trapping part of the carboxylate moieties in the form of carbonyl
or ester moieties in the residues, because there is no obvious
emission of CO2 gas afterwards (Scheme S2).12 Another weight
loss from 400 to 500 oC should be attributed to further
dehydrogenation and condensation of organic moieties. When
raising the temperature to 800 oC, the retained residue is of 51.0
wt%, closing to the sum of calculated masses for copper
element (20.8 wt%) and decarboxylated moieties (33.1 wt%) in
ZJU-199, indicating no obvious organic weight loss occurred
during the carbonization process. In contrast, a remarkable
weight loss (41.0 wt%) is observed in the temperature range
o
from 250 to 400 C for HKUST-1, which is greatly higher than
the calculated value (31.4 wt%) for the carboxylate groups
o
(Figure 1b). When the temperature was raised to 800 C, only
26.2 wt% of residual mass was obtained for HKUST-1, which is
slightly higher than the copper content (22.8 wt%). The above
results clearly demonstrated that the presence of acrylate
moieties in organic ligands could markedly prevent the
evaporation of organic moieties under pyrolytic conditions, which
might fabricate physical barriers to inhibit the agglomeration of in
situ formed MNPs.
Figure 1. TG, DTG and CO2-MS curves for (a) ZJU-199 and (b) HKUST-1.
The nature of annealed samples was characterized by
power X-ray diffraction (PXRD) studies (Figure 2b).
According to the TG results, ZJU-199 was pyrolyzed at
different temperatures for 2 h under N2 atmosphere to prepare
Cu/CuOx@organic framework composites, denoted as ZJU-199-
T (T = temperature). FT-IR spectra show that there is a broad
decarboxylation process for ZJU-199 from 250 to 350 oC (Figure
2a). The characteristic signals of aromatic moieties in
decarboxylated ZJU-199-350 appear at 1595 and 1440 cm-1.
o
Decomposition of the ZJU-199 skeleton was initiated at 250 C,
as there appeared broad and weak characteristic peaks
assigned to Cu2O (JCPDS, Card No. 05-0667, 36.4o), and very
weak peaks ascribed to metallic Cu (JCPDS, Card No. 04-0836,
43.2 and 50.5o). Interestingly, distinct phase transformation was
observed when gradually increasing the annealing temperature.
Cu2O represents the major phase in ZJU-199-300 and ZJU-199-
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350 along with small amounts of metallic copper species. The
Cu2O species was gradually reduced to form Cu0 when the
annealing temperature was raised to 400 oC. Deep condensation
of the organic residue at higher temperature would further
reduce the Cu2O species into metallic copper species. The
broad diffraction peaks of copper species in ZJU-199-T indicate
that the Cu/Cu2O particle sizes are very small, according to the
Scherrer equation (Table S1). SEM images show that thermal
treatment of ZJU-199 did not change the appearance of the
octahedral morphology (Figure S3). TEM images show that the
nanoparticles are uniformly distributed in the organic frameworks
with average diameters of less than 10 nm in ZJU-199-350,
300-400 oC), while the vibration peaks of ZJU-36-T are similar to
those of ZJU-199-T. These results revealed that the acrylate
moieties in organic linkers played important roles to preserve the
3D networks of the parent MOFs for the formation of organic
frameworks under pyrolytic conditions. SEM images show that
there are meso/macro cracks on the external surfaces of the
annealed HKUST-1 samples, while the pristine morphology was
almost reserved for ZJU-35 and ZJU-36 at low annealing
temperature (< 400 oC) (Figures S9-S11). It is worth noting that
ZJU-35-800 formed melted-like particles with round-edged
octahedral morphology. This result should be ascribed to only
one acrylate group per ligand in ZJU-35, which tends to form
flexible 1D polystyrene chains that are easily shrunk at high
temperature.14 TEM images show that the diameters of NPs in
different pyrolyzed samples are in the order of ZJU-199 < ZJU-
36 < ZJU-35 < HKUST-1 under the same pyrolytic conditions,
indicating that the acrylate moieties in Cu-MOFs could prevent
the aggregation of NPs by forming physical barriers under
pyrolytic conditions (Figures S12-S14).
o
which were slightly increased to 13 nm at 800 C (Figures 2c,
2d and S4).
Figure 3a shows the solid-state 13C magic-angel spinning
(MAS) NMR spectra of copper-etched Cu-MOF-350, covering
the aliphatic carbons between 0 and 80 ppm and aromatic rings
from 100 to 150 ppm. The missing of the exocyclic vinyl group at
ca. 110 ppm and the gradual growth of the broad γ peak
centered at 36.5 ppm, assigned to the tertiary and secondary
aliphatic carbon, further confirmed that the vinyl groups are
highly polymerized.15 Meanwhile, the α peak intensity for
HKUST-1-350 and ZJU-35-350 is relatively low, indicating that
the decarboxylated benzene species is less reactive for the
formation of coupled C-C bonds. The absence of the peak
ascribed to the C=O moiety indicates that the decarboxylative
coupling process might be the major reaction under the pyrolytic
conditions.
Figure 2. (a) FT-IR spectra and (b) PXRD patterns of annealed samples of
ZJU-199 at different temperatures. TEM images and the corresponding
histograms of metal particle size distributions for (c) ZJU-199-350 and (d) ZJU-
199-800.
To understand the roles of acrylate moieties on the
formation of porous organic frameworks and the in situ
generation of Cu/CuOx heterostructures, we further studied the
thermal behaviors of HKUST-1, ZJU-35 and ZJU-36, consisting
of zero, one and two acrylate moieties per ligand, respectively,
which are isostructural to ZJU-199 (Scheme S1 and Figure
S5).11b,13 As shown in Figure S7, all of them are fully
decarboxylated at 350 oC, and the retained organic residue
mass is in the order of ZJU-199 > ZJU-36 > ZJU-35 > HKUST-1,
which are proportional to the contents of acrylate groups,
indicating that the acrylate groups could effectively inhibit the
loss of decarboxylated organic moieties during pyrolysis. FT-IR
spectrum shows that the phenyl ring vibration signals of
decarboxylated organic moieties are hardly recognized for
HKUST-1-300 (Figure S8). There appear the predominant
peaks at 752 and 694 cm-1 attributed to the out-of-plane bending
vibration bands of substituted phenyl groups for ZJU-35-T (T =
Figure 3. (a) Solid-state 13C MAS NMR spectra, and (b) Cu 2p, (c) O 1s and
(d) C 1s XPS spectra of Cu-MOF-350.
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To further understand the roles of acrylate groups in the
pyrolytic process, we studied the thermal behaviors of MOF-143,
High resolution TEM (HRTEM) images show that there are
two kinds of lattice fringes with inter-planar spacing of 0.209 and
0.181 nm assigned to the (111) and (200) of metallic Cu,
respectively, where the (111) fringes of Cu2O (d = 0.246 nm) are
located at the external surfaces of Cu NPs in HKUST-1-350
(Figure 4a).4d Accompanying increase of the acrylate contents
in organic ligands, the ratio of metallic copper to Cu2O in the
NPs gradually decreases, which generated core-shell structures
consisting of metallic Cu cores covered with thin Cu2O layers in
ZJU-35-350 and ZJU-36-350 (Figure 4b and 4c). As a
consequence, it is difficult to discriminate the different copper
species by LMM spectra. Cu2O became the major species in the
biphasic Cu/Cu2O nanojunctions in ZJU-199-350 (Figure 4d). It
is interesting that metallic copper is located at the edge of the
multi-component heteronanostructures, resulting in highly
exposed Cu0 sites and Cu/Cu2O interfaces. Since the Cu/Cu2O
nanojunctions would generate unique local electrostatic fields at
the interfaces, ZJU-199-350 might exhibit unexpected electronic
properties that do not possess by discrete individuals. When the
annealing temperature was increased to 800 oC, the copper
species was highly reduced and aggregated, which resulted in
heavy decrease of the accessible Cu/Cu2O nanojunction
interfaces (Figure S24). N2 sorption measurements revealed
a
Cu-MOF built from [Cu2(COO)4] SBUs and 1,3,5-
benzenetribenzoate (BTB) (Figures S16 and S17).16 FT-IR
spectrum of MOF-143-350 shows that there appear an evident
aromatic signal at 1598 cm-1, and an emerged peak at 1734 cm-1
assigned to the ester groups, illustrating the successful
reservation of the organic moieties in the pyrolyzed sample
(Figure S18). However, there also appear the characteristic
vibration bands of monosubstituted phenyl moieties at 760 and
697 cm-1, indicating insufficient cross coupling of the
decarboxylated BTB moieties. This conclusion was confirmed by
monitoring the UV-Vis spectrum of the chloroform eluate of
MOF-143-350, which presents the absorption bands of aromatic
compounds ascribed to the decarboxylated BTB monomer
(1,3,5-triphenylbenzene) and coupled oligomers (Figure S19).
PXRD profile shows that metallic copper represents the major
phase in MOF-143-350 with an average diameter of 19.5 nm
(Figures S20 and S21). These characterizations revealed that
pyrolysis of MOF-143 would retain most of the organic mass,
attributed to the large molecule weight of BTB ligand. However,
the decarboxylated BTB moieties were poorly polymerized,
which could not effectively inhibit the migration of in situ
generated copper species, and resulted in large aggregated NPs. that ZJU-199-350 has hierarchical porous structure, which is
These results further confirmed the important roles of acrylate
groups in ZJU-199 on improving the polymerization degree, and
impeding the agglomeration of in situ formed MNPs under
annealing conditions.
beneficial for mass transport during catalysis (Figures S25 and
S26; Table S1).
X-ray photoelectron spectroscopy (XPS) was utilized to
study the surface compositions and chemical states of the in situ
formed copper species in the 350 oC treated samples (Figure 3).
All samples displayed similar Cu 2p XPS spectra, where the
main peaks appear in the region around 933 eV, corresponding
to the 2p3/2 peaks of Cu element. Fittings of the Cu 2p3/2 peaks
resulted two components with binding energy peaks located at
934.8 and 932.7 eV that are assigned to the surface CuII and
CuI/Cu0 species, respectively.17 The shake-up satellite peak at
943 eV further confirmed the existence of CuII species in the
pyrolyzed samples. Restricted by the low crystallinity of CuII
nanospecies, no obvious diffraction peak for CuO or Cu(OH)2
was found in the PXRD patterns. Because it is difficult to
distinguish Cu0 and CuI from the XPS spectra, we attempted to
make a distinction by Cu LMM Auger spectra. Although only a
broad peak at 916.5 eV for CuI species could be directly
observed, we could figure out the gradual decrease of the Cu0
peak intensity at 918.2 eV, accompanying increase of the
acrylate contents in organic ligands (Figure S22). Combined
with the XPS data, we calculated the [(Cu0+CuI)/CuII] atomic
ratios for all samples (Table S1). An obvious decrease tendency
of the [(Cu0+CuI)/CuII] atomic ratio was observed when
increasing the acrylate contents in the pristine MOFs, which is
consistent with the PXRD results.
Figure 4. HRTEM images of (a) HKUST-1-350, (b) ZJU-35-350, (c) ZJU-36-
350 and (d) ZJU-199-350 (the inserts show the schematic representations of
Cu/Cu2O heterostructures; color scheme: Cu, green; Cu2O, red).
Biomass is an ideal class of alternatives to fossil fuels as a
feedstock of fine chemicals and fuels due to its abundance and
sustainability.18 Furfural (FAL), as one of the major biomass
platform chemicals, can be primarily produced from acid-
catalyzed hydrolysis of hemicellulose and then converted into
diverse chemicals and fuels.19 Furfuryl alcohol (FOL),
a
hydrogenated product of FAL, is an important intermediate in
fine chemicals and polymer industries.20 The commercial
production of FOL from FAL is usually performed over toxic
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10.1002/anie.201913811
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copper chromite under harsh conditions.21 Hence, there is
growing need for developing environment- and eco-friendly
catalysts that could efficiently convert FAL into FOL with high
activity and selectivity under mild conditions. The interesting
heterostructures of Cu/Cu2O nanojunctions in the pyrolyzed Cu-
MOFs prompted us to evaluate the catalytic application in
hydrogenation of FAL into FOL (Table 1). Obviously, the pristine
Cu-MOFs could not initiate the FAL hydrogenation reaction with
very low yield of FOL under the catalytic conditions of 1 MPa H2
and 130 oC. Meanwhile, HKUST-1 and ZJU-35 derived
composites could not effectively promote the FAL hydrogenation
reaction, because most of the Cu/Cu2O nanojunction interfaces
in the severely aggregated copper NPs are inaccessible. As
expected, great boost of the catalytic performance was achieved
for ZJU-36-350 and ZJU-199-350 with 78.2 and 97.0% FOL
yields, respectively. It is worth noting that ZJU-199-350 exhibits
the highest turnover frequency (TOF) in the FAL hydrogenation
under mild conditions, compared with the earth-abundant metal-
based catalysts in the literature (Table S2). The high catalytic
efficiency should be attributed to the small particle sizes (<10 nm)
of Cu/Cu2O nanojunctions with highly exposed interfaces inside
porous organic matrices.
Table 1. Hydrogenation of FAL into FOL catalyzed by various Cu-MOFs and
pyrolyzed samples.[a]
Entry
1
Catalyst
Conv. (%)[b]
2.7
Yield (%)
1.3
Sel. (%)[b]
50.2
66.7
43.9
54.1
82.4
68.7
69.1
24.3
84.2
68.7
99.1
47.2
34.1
94.6
94.9
88.3
86.0
27.5
27.2
97.4
>99
HKUST-1
2
HKUST-1-300
HKUST-1-350
HKUST-1-400
HKUST-1-500
HKUST-1-800
ZJU-35
3.1
2.0
3
9.8
4.3
4
11.4
7.7
6.2
5
6.3
6
5.5
3.8
7
5.8
4.0
8
ZJU-35-300
ZJU-35-350
ZJU-35-400
ZJU-35-500
ZJU-35-800
ZJU-36
19.2
23.4
38.5
8.2
4.7
9
19.7
26.4
8.1
10
11
12
13
14
15
16
17
18
9.7
4.6
6.4
2.2
ZJU-36-300
ZJU-36-350
ZJU-36-400
ZJU-36-500
ZJU-36-800
ZJU-199
74.9
82.4
41.1
15.8
8.5
70.9
78.2
36.3
13.6
2.3
It has been demonstrated that the electron-deficient copper
species (CuI/II) could perform as the Lewis acid sites to activate
the carbonyl moieties by side-bond interaction.22 The Cu0
species is able to dissociate the absorbed H2 molecules to form
active hydrogen atoms, which easily react with the neighboring
activated -C=O groups to produce FOL. As there are multiple
nanojunction (Cu/Cu2O) interfaces on
a single NP, the
19
20
21
22
23
24
6.5
1.8
cooperation between Cu0 and adjacent CuI/II Lewis acid sites
would significantly boost the catalytic performance (Scheme
S3).23 In order to confirm this assumption, the catalytic
performance of FAL hydrogenation over ZJU-199-400 was also
carried out, in which the Cu0 was the major species with similar
particle sizes in ZJU-199-350 (Figure S27). As expected, the
FOL yield (29.3%) is dramatically decreased for ZJU-199-400,
indicating the important roles of CuI/II Lewis acid sites in the FAL
hydrogenation reaction. Apart from the high concentration of
CuI/II species, the abundant accessible Cu/Cu2O heterojunction
interfaces should be also accounted for the high catalytic activity
of ZJU-199-350, compared with ZJU-36-350. When the
ZJU-199-300
ZJU-199-350
ZJU-199-400
ZJU-199-500
ZJU-199-800
71.9
97.1
34.2
7.8
70.1
97.0
29.3
4.7
85.7
60.3
23.3
12.3
2.9
[a] Reaction conditions: 1.0 mmol FAL, 25 μmol catalyst (based on Cu), 2 mL
isopropanol, 1 MPa H2, 130 oC, 3 h. [b] Determined by GC in the presence of
n-octanol as the internal standard.
To comprehensively understand the reaction mechanism, a
series of FAL hydrogenation experiments were conducted in the
presence of ZJU-199-350 catalyst (Figure 5). Figure 5a shows
the influence of reaction temperature on the catalytic
performance of ZJU-199-350 in the range of 110-140 oC under 1
MPa hydrogen pressure for 3 h. The FOL yield greatly increases
o
annealing temperature was increased to 500 and 800 C, only
little FOL was produced for the annealed products of ZJU-36
and ZJU-199. Since negligible particle size or phase difference
could be observed for the NPs in ZJU-199-400 and ZJU-199-
500, the deteriorated catalytic activity should be resulted from
the deep-condensation of organic moieties that could block the
access of FAL molecules to the heterojunction interfaces. FT-IR
spectra revealed that dehydrogenation and carbonization at 500
oC could destroy all of the functional groups in the organic
supports, which resulted in weak metal-support interactions to
lead to deterioration of the catalytic efficiency.
o
from 52.7% at 110 oC to 97.0% at 130 C with retained FOL
selectivity of >99%. Further elevating the reaction temperature to
140 oC did not improve the FAL conversion. The effect of
reaction time on the FAL hydrogenation was also investigated
(Figure 5b). The FOL yield increases remarkably when
prolonging the reaction time from 1 to 3 h with steady FOL
selectivity of >99%. The H2 pressure is another important factor
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that dramatically affected the catalytic results in the
hydrogenation reaction. The FOL yield boosted 6-fold and 11-
fold when the H2 pressure was increased from 0.1 to 0.5 and 1
MPa at 130 oC for 3 h, respectively (Figure 5c). Further increase
of the H2 pressure could not obviously improve the catalytic
results. We also tested the catalytic performance of ZJU-199-
350 in various solvents, including water, toluene, methanol and
isopropanol (Figure 5d). Among these solvents, water resulted
in a low FOL yield of 18.6%, which should be ascribed to the
reversed surface properties of hydrophilic water and
hydrophobic organic matrices that could prevent the accessibility
of FAL molecules to catalytic centers. The moderate FAL
conversion and FOL yield in toluene should be originated from
the poor H2 solubility in nonpolar solvents.24 For the protic
organic solvents, ZJU-199-350 exhibits superior catalytic
performances in isopropanol (97.0% FOL yield) and methanol
(95.8% FOL yield), because hydrogen bonding between the
carbonyl groups in FAL and the terminal hydroxyl groups in
alcohols would help activation of the FAL molecules.22
reaction temperature, and thus to greatly increase the reaction
rate.
Figure 6. Kinetic studies of FAL hydrogenation over ZJU-199-350 catalyst: (a)
reaction rates of ZJU-199-350 at different temperatures (the inset represents
the Arrhenius plot for the FAL hydrogenation). Reaction conditions: 2.5 mmol
FAL, 5 mL isopropanol, 25 μmol catalyst, 1 MPa H2, 100-130 oC, 2 h. (b) The
recyclability of ZJU-199-350 for the hydrogenation of FAL. Reaction
conditions: 1.0 mmol FAL, 25 μmol catalyst, 2 mL isopropanol, 1 MPa H2, 130
oC, 3 h.
Figure 5. Effect of reaction conditions on hydrogenation of FAL over ZJU-199-
350: (a) 1.0 mmol FAL, 25 μmol catalyst, 2 mL isopropanol, 1 MPa H2, 3 h; (b)
1.0 mmol FAL, 25 μmol catalyst, 2 mL isopropanol, 1 MPa H2, 130 oC; (c) 1.0
mmol FAL, 25 μmol catalyst, 2 mL isopropanol, 130 oC, 3 h; (d) 1.0 mmol FAL,
25 μmol catalyst, 2 mL solvent, 1 MPa H2, 130 oC, 3 h.
We further evaluated the stability and recyclability of ZJU-
199-350 for the FAL hydrogenation reaction. The catalyst
showed stable activity and selectivity for five successive runs in
the FAL hydrogenation (Figure 6b). FT-IR spectrum of the spent
catalyst demonstrated that the organic framework was well
preserved after catalysis, and no obvious metal leaching was
found as revealed by ICP-OES (Figure S28 and Table S3).
Compared with the fresh ZJU-199-350, the spent catalyst
exhibits similar XRD profile (Figure S29). XPS results showed
that the CuII content slightly decreased after catalysis, indicating
that partial CuI/II species was reduced during the hydrogenation
reaction (Figure S30). It is worth noting that no obvious particle
agglomeration, phase separation or core-shell Cu/Cu2O
heterojunction formation was observed in the spent ZJU-199-
350, indicating the important roles of the organic framework
matrices on stabilizing the Cu/Cu2O heterostructures by serving
as the physical barriers (Figure S31).
To obtain more insight into the hydrogenation reaction over
ZJU-199-350, the kinetic behaviors were subsequently studied
(Figure 6a). The reaction performed at low temperature of 100-
o
130 C achieved low conversions of FAL, which are beneficial
for the calculations of TOF and initial reaction rates (r0) of the
reaction. On the basis of the r0 at different temperatures, the
apparent activation energy (Ea) for ZJU-199-350 was estimated
to be 57.4 kJ/mol, calculated based on the Arrhenius plot. This
value is higher than the FAL adsorption energy (<30 kJ/mol),
indicating that the reaction was controlled by the surface
kinetics.25 The high Ea value indicates that there is strong
interaction between FAL molecules and copper sites, which
could assist the adsorption and activation of FAL to promote the
hydrogenation reaction. Meanwhile, the strong FAL adsorption
over copper catalysts could be greatly diminished by raising the
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10.1002/anie.201913811
Angewandte Chemie International Edition
RESEARCH ARTICLE
[7]
(a) X. Jing, C. He, L. Zhao, C. Duan, Acc. Chem. Res. 2019, 52, 100-
109; (b) K. Lu, T. Aung, N. Guo, R. Weichselbaum, W. Lin, Adv. Mater.
2018, 30, 1707634; (c) G. Maurin, C. Serre, A. Cooper, G. Férey, Chem.
Soc. Rev. 2017, 46, 3104-3107; (d) H. Wang, X. Dong, V. Colombo, Q.
Wang, Y. Liu, W. Liu, X.-L. Wang, X.-Y. Huang, D. M. Proserpio, A.
Sironi, Y. Han, J. Li, Adv. Mater. 2018, 30, 1805088; (e) B. Liu, H.
Shioyama, T. Akita, Q. Xu, J. Am. Chem. Soc. 2008, 130, 5390-5391;
(f) L. Jiao, Y. Wang, H.-L. Jiang, Q. Xu, Adv. Mater. 2018, 30, 1703663;
(g) S. J. Yang, S. Nam, T. Kim, J. H. Im, H. Jung, J. H. Kang, S. Wi, B.
Park, C. R. Park, J. Am. Chem. Soc. 2013, 135, 7394-7397; (h) Z. Bao,
G. Chang, H. Xing, R. Krishna, Q. Ren, B. Chen, Energy Environ. Sci.
2016, 9, 3612-3641; (i) B. Y. Guan, X. Y. Yu, H. B. Wu, X. W. Lou, Adv.
Mater. 2017, 29, 1703614; (j) Y.-Z. Chen, R. Zhang, L. Jiao, H.-L. Jiang,
Coord. Chem. Rev. 2018, 362, 1-23; (k) Q.-G. Zhai, X. Bu, X. Zhao, D.-
S. Li, P. Feng, Acc. Chem. Res. 2017, 50, 407-417; (l) X. Lian, Y. Fang,
E. Joseph, Q. Wang, J. Li, S. Banerjee, C. Lollar, X. Wang, H.-C. Zhou,
Chem. Soc. Rev. 2017, 46, 3386-3401.
Conclusion
In summary, we successfully regulated the particle sizes, phase
compositions and nanostructures of the encapsulated Cu/Cu2O
nanojunctions in organic frameworks derived from Cu-MOFs by
tuning the ligand structures and pyrolytic conditions. Promotion
of the polymerization degree of decarboxylated organic moieties
was achieved by introducing exocyclic acrylate groups in organic
linkers of Cu-MOFs. The highly polymerized porous organic
matrices could efficiently prevent the agglomeration of in situ
generated NPs under annealing conditions, which formed stable
Cu/CuOx nanojunctions with abundant accessible interfaces
inside porous organic matrices. The facilely prepared
Cu/Cu2O@organic framework composite material ZJU-199-350
exhibits superior catalytic activity, selectivity and stability in
hydrogenation of FAL into FOL. This work provides a unique
perspective for the generation of accessible metal/metal oxide
nanojunctions inside organic frameworks derived from MOFs for
highly efficient heterogeneous catalysis.
[8]
(a) W. Bak, H. S. Kim, H. Chun, W. C. Yoo, Chem. Commun. 2015, 51,
7238-7241; (b) K. Zhao, Y. Liu, X. Quan, S. Chen, H. Yu, ACS Appl.
Mater. Interfaces 2017, 9, 5302-5311; (c) X. Liu, D. Xu, L. Zhang, ACS
Sustainable Chem. Eng. 2017, 5, 7800-7811; (d) Y. Wang, Y. Lü, W.
Zhan, Z. Xie, Q. Kuang, L. Zheng, J. Mater. Chem. A 2015, 3, 12796-
12803; (e) H. Niu, S. Liu, Y. Cai, F. Wu, X. Zhao, Microporous
Mesoporous Mater. 2016, 219, 48-53; (f) A. K. Kar, R. Srivastava, Inorg.
Chem. Front. 2019, 6, 576-589; (g) K. Yang, Y. Yan, H. Wang, Z. Sun,
W. Chen, H. Kang, Y. Han, W. Zhang, X. Sun, Z. Li, Nanoscale 2018,
10, 17647-17655.
Acknowledgements
We are grateful for the financial support of the National Natural
Science Foundation of China (grant nos. 21525312 and
21872122).
[9]
E. Scott, F. Peter, J. Sanders, Appl. Microbiol. Biotechnol. 2007, 75,
751-762.
[10] K. Chen, C.-D. Wu, Angew. Chem. Int. Ed. 2019, 58, 8119-8123.
[11] (a) L. Zhang, C. Zou, M. Zhao, K. Jiang, R. Lin, Y. He, C.-D. Wu, Y. Cui,
B. Chen, G. Qian, Cryst. Growth Des. 2016, 16, 7194-7197; (b) S. S.-Y.
Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams,
Science 1999, 283, 1148-1150.
Keywords: Metal-organic frameworks • Organic framework
materials • Composite materials • Heterojunctions • Catalysis
[12] (a) R. Dabestani, P. F. Britt, A. C. Buchanan, Energy Fuels 2005, 19,
365-373; (b) N. M. Sánchez, A. de Klerk, Thermochim. Acta 2018, 662,
23-40.
[1]
[2]
[3]
(a) H.-L. Liu, F. Nosheen, X. Wang, Chem. Soc. Rev. 2015, 44, 3056-
3078; (b) J. Wang, G. Dong, Chem. Rev. 2019, 119, 7478-7528; (c) D.
Pflӓsterer, A. S. K. Hashmi, Chem. Soc. Rev. 2016, 45, 1331-1367.
(a) D. Wang, D. Astruc, Chem. Soc. Rev. 2017, 46, 816-854; (b) P.
Gandeepan, T. Müller, D. Zell, G. Cera, S. Warratz, L. Ackermann,
Chem. Rev. 2019, 119, 2192-2452.
[13] G.-Q. Kong, Z.-D. Han, Y. He, S. Ou, W. Zhou, T. Yildirim, R. Krishna,
C. Zou, B. Chen, C.-D. Wu, Chem. Eur. J. 2013, 19, 14886-14894.
[14] H. Gu, C. Ma, C. Liang, X. Meng, J. Gu, Z. Guo, J. Mater. Chem. C
2017, 5, 4275-4285.
(a) C. Ray, T. Pal, J. Mater. Chem. A 2017, 5, 9465-9487; (b) B. Zhang,
J. Liu, J. Wang, Y. Ruan, X. Ji, K. Xu, C. Chen, H. Wan, L. Miao, J.
Jiang, Nano Energy 2017, 37, 74-80; (c) H. Robatjazi, H. Zhao, D. F.
Swearer, N. J. Hogan, L. Zhou, A. Alabastri, M. J. McClain, P.
Nordlander, N. J. Halas, Nat. Commun. 2017, 8, 27; (d) R. Boppella, C.
H. Choi, J. Moon, D. H. Kim, Appl. Catal. B: Environ. 2018, 239, 178-
186.
[15] (a) R. V. Law, D. C. Sherrington, C. E. Snape, Macromolecules 1996,
29, 6284-6293; (b) I. van Zandvoort, E. J. Koers, M. Weingarth, P. C. A.
Bruijnincx, M. Baldus, B. M. Weckhuysen, Green Chem. 2015, 17,
4383-4392.
[16] H. Furukawa, Y. B. Go, N. Ko, Y. K. Park, F. J. Uribe-Romo, J. Kim, M.
O’Keeffe, O. M. Yaghi, Inorg. Chem. 2011, 50, 9147-9152.
[17] J. P. Espinós, J. Morales, A. Barranco, A. Caballero, J. P. Holgado, A.
R. González-Elipe, J. Phys. Chem. B 2002, 106, 6921-6929.
[18] H. Wang, J. Male, Y. Wang, ACS Catal. 2013, 3, 1047-1070.
[19] (a) L. T. Mika, E. Cséfalvay, Á. Németh, Chem. Rev. 2018, 118, 505-
613; (b) W. Leitner, J. Klankermayer, S. Pischinger, H. Pitsch, K.
Kohse-Höinghaus, Angew. Chem. Int. Ed. 2017, 56, 5412-5452.
[20] K. Yan, G. Wu, T. Lafleur, C. Jarvis, Renew. Sustain. Energy Rev. 2014,
38, 663-676.
[4]
(a) M. B. Gawande, A. Goswami, F.-X. Felpin, T. Asefa, X. Huang, R.
Silva, X. Zou, R. Zboril, R. S. Varma, Chem. Rev. 2016, 116, 3722-
3811; (b) N. K. Ojha, G. V. Zyryanov, A. Majee, V. N. Charushin, O. N.
Chupakhin, S. Santra, Coord. Chem. Rev. 2017, 353, 1-57; (c) S.
Zhang, Y. Ma, H. Zhang, X. Zhou, X. Chen, Y. Qu, Angew. Chem. Int.
Ed. 2017, 56, 8245-8249; (d) X. Chang, T. Wang, Z.-J. Zhao, P. Yang,
J. Greeley, R. Mu, G. Zhang, Z. Gong, Z. Luo, J. Chen, Y. Cui, G. A.
Ozin, J. Gong, Angew. Chem. Int. Ed. 2018, 57, 15415-15419; (e) P.
Ling, Q. Zhang, T. Cao, F. Gao, Angew. Chem. Int. Ed. 2018, 57, 6819-
6824; (f) S.-C. Qi, X.-Y. Qian, Q.-X. He, K.-J. Miao, Y. Jiang, P. Tan,
X.-Q. Liu, L.-B. Sun, Angew. Chem. Int. Ed. 2019, 58, 10104-10109; (g)
R. T. Yang, A. J. Hernández-Maldonado, F. H. Yang, Science 2003,
301, 79-81.
[21] B. M. Nagaraja, V. S. Kumar, V. Shasikala, A. H. Padmasri, B.
Sreedhar, B. D. Raju, K. S. R. Rao, Catal. Commun. 2003, 4, 287-293.
[22] W. Gong, C. Chen, Y. Zhang, H. Zhou, H. Wang, H. Zhang, Y. Zhang,
G. Wang, H. Zhao, ACS Sustainable Chem. Eng. 2017, 5, 2172-2180.
[23] G.-P. Zhan, C.-D. Wu, Sci. Bull. 2019, 64, 385-390.
[24] M. J. Taylor, L. J. Durndell, M. A. Isaacs, C. M. A. Parlett, K. Wilson, A.
F. Lee, G. Kyriakou, Appl. Catal. B: Environ. 2016, 180, 580-585.
[25] S. Sitthisa, T. Sooknoi, Y. Ma, P. B. Balbuena, D. E. Resasco, J. Catal.
2011, 277, 1-13.
[5]
[6]
(a) R. Zhang, L. Hu, S. Bao, R. Li, L. Gao, R. Li, Q. Chen, J. Mater.
Chem. A 2016, 4, 8412-8420; (b) H. Xu, J.-X. Feng, Y.-X. Tong, G.-R.
Li, ACS Catal. 2017, 7, 986-991; (c) Y. Song, H. Wang, G. Liu, H.
Wang, L. Li, Y. Yu, L. Wu, J. Catal. 2019, 370, 461-469.
J. Song, J. Li, J. Xu, H. Zeng, Nano Lett. 2014, 14, 6298-6305.
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10.1002/anie.201913811
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
A strategy to generate and stabilize
Cu/Cu2O nanojunctions inside porous
organic frameworks is developed by
controlled pyrolysis of metal-organic
Kai Chen, Jia-Long Ling and Chuan-De
Wu*
Page No. – Page No.
frameworks, which results in
composite material that consists of
easily accessed Cu/Cu2O
heterojunctions inside porous organic
matrices, exhibiting excellent catalytic
properties in hydrogenation of furfural
into furfuryl alcohol.
a
In situ Generation and Stabilization of
Accessible Cu/Cu2O Heterojunctions
inside Organic Frameworks for Highly
Efficient Catalysis
((Insert TOC Graphic here))
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