Fabrication of Ultrathin 2D Cu-BDC Nanosheets and the Derived Integrated MOF Nanocomposites

Article abstract of DOI:10.1002/adfm.201806720

Freestanding 2D nanosheets with many unprecedented properties have been used in a myriad of applications. In this work, 2D copper-bearing metal-organic frameworks (MOFs; viz., Cu-BDC) nanosheets are successfully fabricated via a facile and benign methodology through using Cu₂O nanocubes (≈60 nm) as a confined metal ion source and 1,4-benzenedicarboxylic acid (H₂BDC) as an organic linker. The Cu₂O nanocubes gradually release Cu⁺ ions which are further oxidized by the dissolved oxygen and serve as nutrients for construction of 2D frameworks. In contrast, the conventional solvothermal synthesis with copper salt exclusively yields bulk Cu-BDC with edge dimensions of 2–10 μm. Interestingly, the as-prepared Cu-BDC nanosheets show ultrathin thickness, oriented growth, and excellent crystallinity, which can be exploited as a platform for the design of a series of 2D-integrated nanocatalysts by loading various metal nanocrystals such as Au, Ag, Pt, and Ru, with 3-mercaptopropionic acid as molecular link. In addition, it is found that Cu-BDC/M composites with highly accessible active sites on the surface exhibit high catalytic activity in several condensation reactions between benzaldehyde and primary amines. The findings offer an alternative strategy for rational design and synthesis of 2D MOF nanosheets and the derived 2D nanocomposites for catalytic applications.

Full text of DOI:10.1002/adfm.201806720

FULL PAPER
2D MOFs
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Fabrication of Ultrathin 2D Cu-BDC Nanosheets
and the Derived Integrated MOF Nanocomposites
Guowu Zhan,* Longlong Fan, Feigang Zhao, Zhongliang Huang, Bin Chen, Xin Yang,
and Shu-feng Zhou
important to note that, differing from the
Freestanding 2D nanosheets with many unprecedented properties have been
used in a myriad of applications. In this work, 2D copper-bearing metal-
organic frameworks (MOFs; viz., Cu-BDC) nanosheets are successfully fab-
ricated via a facile and benign methodology through using Cu2O nanocubes
(≈60 nm) as a confined metal ion source and 1,4-benzenedicarboxylic acid
(H2BDC) as an organic linker. The Cu2O nanocubes gradually release Cu+
ions which are further oxidized by the dissolved oxygen and serve as nutri-
ents for construction of 2D frameworks. In contrast, the conventional solvo-
thermal synthesis with copper salt exclusively yields bulk Cu-BDC with edge
dimensions of 2–10 µm. Interestingly, the as-prepared Cu-BDC nanosheets
show ultrathin thickness, oriented growth, and excellent crystallinity, which
can be exploited as a platform for the design of a series of 2D-integrated
nanocatalysts by loading various metal nanocrystals such as Au, Ag, Pt, and
Ru, with 3-mercaptopropionic acid as molecular link. In addition, it is found
that Cu-BDC/M composites with highly accessible active sites on the surface
exhibit high catalytic activity in several condensation reactions between
benzaldehyde and primary amines. The findings offer an alternative strategy
for rational design and synthesis of 2D MOF nanosheets and the derived 2D
nanocomposites for catalytic applications.
traditional methods, copper ions from
insoluble solid materials are gradually
released, and then consumed as nutri-
ents for MOF growth. However, most of
the products were irregular in geometric
shapes; therefore, it is a great challenge
to control the shape of MOFs, particularly
the 2D nanosheets.[12–14]
Cu-BDC (where BDC2− = 1,4-benzen-
edicarboxylate) as a typical copper-bearing
MOFs has attracted many research inter-
ests in the past years.[15–20] As shown in
Figure 1a–d, the building block unit in
Cu-BDC framework contains two 5-coor-
dinate copper cations bridged in a paddle-
wheel-type configuration and to which the
terephthalate ligands (BDC2−) are coordi-
nated in a bidentate bridging fashion.[16]
Besides the carboxylic ligands, one
terminal dimethylformamide (DMF) is
also coordinated with the copper ions.[21]
Therefore, similar to HKUST-1, copper
sites in Cu-BDC are ready to lose their
reversibly coordinated DMF solvent upon
thermal activation, which leads to open
coordination sites and thereby work well as heterogeneous
Lewis acid catalysts towards a rich variety of reactions including
aerobic homocoupling of arylboronic acids,[22] Friedlander reac-
tion,[23] three-component couplings of amines, aldehydes, and
alkynes,[24] acetylation of alcohols,[25] cyclization reaction,[26]
1. Introduction
Metal-organic frameworks (MOFs) with networks of metal
ions coordinated to multidentate organic molecules have been
extensively studied in the past two decades.[1–4] According to
provisional statistics, more than 20 000 different MOFs have
been reported.[5] Different from the straightforward synthesis
of MOFs by using well-soluble salts as metal ion source, some
special examples have been recently reported to use Cu-based
solid materials as metal ion source to produce Cu-bearing
MOFs,[6–11] such as copper hydroxide, copper hydroxysul-
fate, copper electrode, cuprous oxide, or even copper net. It is
[27]

direct oxidative C C coupling,
and the like. Accordingly,
Cu-BDC has received intense attention similar to the traditional
Cu-based nanostructured materials in catalysis.[28] Considering
that the crystal structure of Cu-BDC belongs to monoclinic,
C2/m space group (a = 11.4 Å, b = 14.3 Å, c = 7.8 Å, and
β = 108°),[16] the intrinsic structure anisotropy is possible for
2D growth of the MOFs. Therefore, herein, we present a new
synthetic strategy for Cu-BDC nanosheets by using Cu2O nano-
cubes as a copper ion source at benign conditions.
Prof. G. Zhan, L. Fan, F. Zhao, Z. Huang, B. Chen, Dr. X. Yang,
Prof. S.-f. Zhou
College of Chemical Engineering
Huaqiao University
668 Jimei Boulevard, Xiamen, Fujian 361021, P. R. China
E-mail: gwzhan@hqu.edu.cn
However, although the pristine MOFs own some catalytic
abilities,[29] the nanocomposites of MOFs and metal nanopar-
ticles (MNPs) have emerged as sustainable alternatives to the
conventional heterogeneous catalysts,[30–35] wherein MOFs
serve as support materials or encapsulation shells.[13,36,37] For
instance, microporous MOFs can provide a large surface area
for loading MNPs working similarly to the mesoporous carbon
and silica, where the interactions between the MNPs and the
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201806720.
DOI: 10.1002/adfm.201806720
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Figure 1. a) Dicopper(II) paddle-wheel building block for Cu-BDC frameworks. Copper, oxygen, carbon, and hydrogen atoms are shown in blue, red,
gray, and white, respectively. Cu–Cu distance is 0.263 nm. b–d) Different views of a portion of the Cu-BDC structure, wherein guest molecules were
omitted for clarity. The crystallographic data were obtained from The Cambridge Crystallographic Data Centre (CCDC-687690).[16] e) Schematic illustra-
tion of the synthesis of Cu-BDC nanosheets and the deviated nanocatalysts, starting from Cu2O nanocubes as raw materials. MPA = 3-mercaptopro-
pionic acid, and R-NBH4 = tetrabutylammonium borohydride.
MOFs are very important for preventing the agglomeration
of the MNPs in catalysis applications. For MNP@MOF-type
composites, the average size of the embedded MNPs is highly
dependent on pore dimensions in MOFs, and due to that
the pores can act as a scaffold to direct the in situ growth of
MNPs.[38] In fact, the integration of MOFs and MNPs into a
single nanostructure will create synergetic effects, and the
multifunctional catalysts are capable of catalyzing cascade reac-
tions. For example, core–shell structured Pd@IRMOF-3 were
highly efficient in a cascade reaction, where IRMOF-3 was used
as the alkaline catalysts for Knoevengel condensation, and Pd
cores could catalyze the selective hydrogenation of the products
from the Knoevengel condensation reaction.[39]
As shown in Figure 1e, in brief, our synthetic strategy involves
the redispersion of Cu2O in ethanol, followed by addition of an
1,4-benzenedicarboxylic acid (H2BDC) solution (dissolved in
DMF). Cu2O nanocubes would gradually release Cu+ ions which
were further oxidized by the dissolved oxygen to construct 2D
frameworks. Furthermore, the obtained Cu-BDC nanosheets were
explored as support materials of incorporating various MNPs (e.g.,
Au, Ag, Pt, and Ru NPs) with the assistance of 3-mercaptopropionic
acid (MPA) as molecular linkers. Finally, the workabilities of
Cu-BDC/M nanocatalysts were examined in the condensation
reactions between benzaldehyde and primary amines.
2. Results and Discussion
2.1. Synthesis of Cu-BDC Nanosheets and Bulk Cu-BDC
To begin with, Cu2O nanocubes with an average edge side of
60 nm were synthesized,[40] as shown in Figure 2a. Afterward,
by simply mixing the Cu2O nanocubes (suspension in ethanol)
with an H2BDC solution (dissolved in DMF) at room tempera-
ture for 4 h, the desired product (i.e., Cu-BDC nanosheets) could
be obtained. Such chemical and structural transformations
were clearly observed from the color change of the mixture:
from yellow to blue (insets in Figure 2). The morphology,
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Figure 2. a) Representative TEM image of Cu2O nanocubes; b,c) HAADF-STEM images and d–f) TEM images at different magnifications of the Cu-BDC
nanosheets prepared by using Cu2O as the copper ion source. g) STEM image and the corresponding EDX elemental maps of the Cu-BDC nanosheets.
Insets in (a) and (b) are the photographs of Cu2O nanocubes and Cu-BDC nanosheets ethanolic suspensions, respectively.
structure, porosity, and composition of the as-prepared Cu-BDC
nanosheets were fully characterized by various techniques. First,
the high-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM; Figure 2b,c) and transmis-
sion electron microscopy (TEM; Figure 2d–f) images clearly
show that the product consists of very thin lamellae with square
morphology and a mean edge length of ≈300 nm. The high-
resolution TEM of the Cu-BDC nanosheets cannot be clearly
recognized, likely due to that MOFs are unstable under the expo-
sure of the energetic electron beam.[39] Energy-dispersive X-ray
(EDX) elemental maps show that the elements C, O, and Cu are
distributed uniformly throughout the whole sheet (Figure 2g).
On the contrary, as shown in Figure S1a,b in the Supporting
Information, the conventional solvothermal synthesis with
Cu(NO3)2 as copper ion source yields bulk MOF crystals with
edge dimensions of serval micrometers (2–10 µm), which will
be mentioned as Cu-BDC bulk materials hereafter. The compar-
isons between Cu-BDC nanosheets and bulk-type Cu-BDC were
further examined by other techniques in the following sections.
Moreover, when using big Cu2O microparticle (size of 0.4 µm)
as the copper ion source, the obtained Cu-BDC exhibits nano-
particles shape instead of nanosheet shape (see Figure S1c,d,
Supporting Information). The results indicate that the sup-
plying manner of copper ion source will greatly affect the shapes
of Cu-BDC. For instance, Cu(NO3)2 is a well-soluble metal salt
which leads to homogeneously distributed nucleation sites of
Cu-BDC in the whole solution. In contrast, Cu2O nanocubes are
spatially inhomogeneous copper ion sources across the solution,
which can ideally initiate nucleation and subsequent growth
of MOF seed crystals within isolated domains. Therefore, the
shapes of Cu-BDC products from using Cu2O as copper ion
source are dependent on the oxidative dissolution rate of Cu2O
and the coordination rate of Cu2+ and BDC2−. We believe that
the growth mechanism of Cu-BDC nanosheets can be divided
into three consecutive steps: (1) gradual release of Cu+ ions
from Cu2O nanocubes; (2) oxidation of Cu+ to Cu2+ ions by the
dissolved oxygen; and (3) nucleation-growth of Cu-BDC to con-
struct 2D frameworks by coordination between Cu2+ and BDC2−.
It is worth noting that the copper ions for constructing Cu-BDC
are absolutely provided by Cu2O nanocubes.
2.2. Characterizations of Cu-BDC Nanosheets
X-ray diffraction (XRD) experiments were followed to confirm
whether our product is crystallographically pure Cu-BDC. First,
as shown in Figure 3a, the experimental pattern of bulk-type
MOFs (viz., with copper nitrate as copper ion source) matched
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Figure 3. a) XRD patterns of Cu2O nanocubes and Cu-BDC products (nanosheets or bulk materials prepared by using Cu2O or Cu(NO3)2 as copper ion
sources). The theoretical pattern of Cu-BDC labeled “simulated” is the predicted powder pattern based on the single-crystal structural assignment. b,c)
A hypothetical view of the desolvated Cu-BDC structure (i.e., without DMF solvent) on the (2 0 −1) plane (colored in blue) and the packing diagram
along [2 0 −1] axis illustrating the stacking of lamellar structures that are covalently bonded.
well with the predicted powder diffraction pattern of Cu(BDC)
(DMF), in which all the Bragg diffractions were detected, thus
revealing that no oriented growth was found in the bulk-type
Cu-BDC. In contrast, XRD patterns of the Cu-BDC nanosheets
exhibited mainly (2 0 −1) and (4 0 −2) crystallographic
planes, suggesting that these planes are all perpendicular
to the stacking direction of the layers or say that the flat top
and bottom surfaces of Cu-BDC nanosheets are bound by
[2 0 −1] facets.[20] No XRD peaks belonging to Cu2O was found
in the MOF products, indicating the total conversion of Cu2O
to Cu-BDC. As depicted in Figure 3b,c, the MOF structure
extends on the (2 0 −1) crystallographic plane with identical
Cu dimer units linked to BDC2− linkers to yield 2D layers.[16,41]
These sheets are then connected through weak stacking inter-
actions between the adjacent layers (e.g., hydrogen-bonding).
On the basis of the above results, it can be concluded that the
usage of Cu2O as copper ion source plays an important role in
the construction of Cu-BDC nanosheets with oriented growth.
Fourier-transform infrared spectroscopy (FTIR) spectra of
Cu-BDC nanosheets are displayed in Figure 4a, in which the
characteristic bands are associated with the organic ligands, DMF
solvent, and the copper coordination groups in the frameworks.
For instance, the observed bands at 1578, 1501, 1399, 1156,
and 1017 cm−1 are all associated with benzene rings of H2BDC
bands at 1624 and 1439 cm−1 belonging to the antisymmetric and
symmetric stretch modes, respectively. The splitting Δ between
the two ν(COO) frequencies is 185 cm−1, indicating a biden-
tate configuration of the carboxylate ligand.[43] The C O Cu
 
stretching exhibits the characteristic band at 1110 cm−1.[44] The
band at 1666 cm−1 is assigned to the carbonyl group in DMF.[22]

Other observed bands are attributed to the moisture or the C H
characters (mainly in the fingerprint region). The atomic force
microscopy (AFM) image (2 µm × 2 µm) in Figure 4b shows sev-
eral Cu-BDC nanosheets with thickness below 6 nm. Considering
that Cu-BDC nanosheets are oriented along [2 0 −1] and the inter-
layer distance between (2 0 −1) planes is about 5.2 Å, the stacking
layers in the thin nanosheets should be less than 10 layers. The
thermal stability of Cu-BDC nanosheets was studied by using
thermogravimetric analysis (TGA). As shown in Figure 4c, the
TGA profile taken in the air (or nitrogen) of Cu-BDC nanosheets
suggests the MOF structures are stable up to 300 °C. The coordi-
nated DMF molecule was liberated by heating to 150 °C, where
the weight loss accounting of 24 wt% corresponds well to the
loss of one DMF molecule per monomer. And the weight loss
occurred in the temperature range of 300 and 330 °C is due to
the decomposition of BDC2− linkers from the MOFs. The evolved
gases during the TGA experiments include water, CO2, and NO2,
as illustrated by the TGA coupled with FTIR characterization in
Figure 4d. The weight calculation indicates that the mole ratio
ligand.[42] And the COO group is characterized by the two

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Figure 4. Characterizations of Cu-BDC nanosheets. a) FTIR spectrum of Cu-BDC nanosheets, in which the symbols represent the specific organic groups
discussed in the main text. b) 2D height AFM image (2 µm × 2 µm) of several Cu-BDC nanosheets (where the thickness data were acquired by measuring
the height profiles along the tracks shown in the micrograph). The white spots in the AFM image indicate corrugated layers of small nanosheets. c) TGA
profiles of Cu-BDC nanosheets in air and N2. d) 3D-FTIR spectra of the evolved gases from TGA analysis using air. e) N2 sorption isotherms at 77 K of
Cu-BDC nanosheets and Cu-BDC bulk material under different activation conditions, in which NS(3), NS(2), and NS(1) represent Cu-BDC nanosheets
activated at 120, 200, and 250 °C for 12 h prior to the tests, respectively. f) XPS core-level spectrum of Cu 2p3/2 in Cu-BDC nanosheets sample.
of Cu2+:BDC ligand:DMF is around 1:1:1, with an empirical
formula of [Cu(BDC)(DMF)]. This result is similar to the TGA
results of Cu-BDC bulk materials (Figure S2, Supporting Infor-
mation), indicating that both MOFs share the same composition.
The porosity of the desolvated Cu-BDC nanosheets and
bulk-type Cu-BDC was measured by N2 sorption–desorption
isotherms at 77 K, as displayed in Figure 4e. The Cu-BDC
nanosheets exhibit Brunauer−Emmett−Teller (BET) and
Langmuir specific surface areas of 372 and 491 m2 g−1, respec-
tively, both of which are slightly higher than those values from
Cu-BDC bulk materials (372 vs 318, and 491 vs 471). And
the micropore volume and micropore area in the Cu-BDC
nanosheets were calculated as 0.136 mL g−1 and 293 m2 g−1
from the corresponding t-plot analysis. Moreover, we found that
an activation treatment was necessary to reach the maximal sur-
face area and pore volume which indicates that the evolution of
porosity proceeds upon heat treatments by removing the DMF
guest molecules. As shown in Table S1 in the Supporting Infor-
mation, the BET surface area gave the following values: 16, 117,
and 372 m2 g−1 for the Cu-BDC nanosheets activated at 120, 200,
and 250 °C for 12 h prior to the tests, respectively. Therefore, it
was believed that the pores in the MOFs can only be detected by
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N2 upon thermal desolvation of Cu-BDC to eliminate the DMF
solvents.[16] As shown in Figure 4f, for Cu-BDC nanosheets
sample, the characteristic XPS peak of Cu 2p3/2 was observed
at a binding energy (BE) of 934.5 eV, together with the presence
of two well-known “shake-up satellites,” suggesting the distribu-
tion of all divalent Cu(II) species in MOFs. The result also indi-
cates that the copper ions from Cu2O precursor were oxidized to
Cu2+ before the coordination with organic linkers (i.e., BDC2−).
2.3. Fabrication of Cu-BDC/M Integrated Nanocomposites
To demonstrate the structural advantages of Cu-BDC
nanosheets as support materials for heterogeneous catalysts,
we separately deposit Au, Ag, Pt, and Ru NPs onto the MOF
nanosheets. First, we started with introducing Au NPs on
the Cu-BDC nanosheets, that is, the MOFs nanosheets were
mixed with a gold precursor (e.g., aqueous HAuCl4), followed
Figure 5. Characterization of Cu-BDC/Au catalysts. a–c) TEM images, d–f) HRTEM images, g–i) HAADF-STEM images, and j) the corresponding EDX
elemental maps of Cu-BDC/Au nanocatalysts. Inset in (d) shows the corresponding size distribution of gold nanoparticles. Refer to the corresponding
EDX pattern and more TEM images in Figures S4–S7 in the Supporting Information.
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by a reductive conversion of Au3+ to Au NPs by the reductants
(e.g., R-NBH4). In order to enhance the interaction between
Au NPs and the MOFs, MPA was used which contained a car-
boxyl group on one end and a thiol group on the other end (see
Figure S3, Supporting Information). During the synthesis, the
carboxylate anion on MPA could chelate with the coordinatively
unsaturated Cu2+ on the exterior surface of MOFs nanosheets;
meanwhile, the thiol group could bind to the Au NPs. After
the immobilization of Au NPs onto Cu-BDC nanosheets, the
morphology of Cu-BDC nanosheets could be maintained, and
Au NPs were uniformly dispersed, as confirmed by TEM, high-
resolution TEM (HRTEM), HAADF-STEM, and EDX elemental
mapping images (Figure 5). The thiolate-protected MNPs were
very small with an average size of 2.2 nm which were distrib-
uted on the external surface of the MOF nanosheets. Since the
nanosheets are very thin, the loading positions of the NPs are
derived from mostly external surface. This is one of the reasons
why abundant NPs were observed in the edges of nanosheets
under the TEM and STEM observations. However, as the
surface free energies of lower coordinated cations at edges
are higher than that of cations on the bottom or top face on
nanosheets,[45] NPs are easier to be deposited on the edge of the
nanosheet rather than on the faces. The d space of the lattice
measured at 0.232 nm in Figure 5f gives evidence for Au (111)
planes. The existence of S signal from the EDX elemental maps
also verifies the molecular binder role of MPA (see Figure 5j).
Furthermore, we found the amount of gold loading on the
Cu-BDC support was highly dependent on the amount of
MPA, even using the same amount of HAuCl4 precursors. For
instance, the gold loadings on Cu-BDC/Au catalysts were 5.5,
7.9, and 9.1 wt%, by adding 0, 0.15, and 0.3 mL MPA (0.1 m)
during the synthesis, respectively (under any other identical
condition), which represents a higher metal efficiency when
using MPA as a molecular linker.
Likewise, other catalytic metal species were introduced
on Cu-BDC nanosheets in a similar manner. As illustrated
in Figures 6–8, Pt NPs, Ru NPs, and Ag NPs were separately
deposited on the exterior surface of Cu-BDC nanosheets.
As expected, by the assistance of MPA, the sizes of these MNPs
are quite small, which is highly desirable for heterogeneous
catalysis. As shown, the average sizes of Pt NPs, Ru NPs, and
Ag NPs were 2.5, 3.1, and 4.3 nm, respectively. Moreover,
Figure 6. Characterizations of Cu-BDC/Pt catalysts. a–c) TEM images, d–f) HAADF-STEM images, and g) the corresponding EDX elemental maps of
Cu-BDC/Pt nanocatalysts. Inset in (c) shows the corresponding size distribution of Pt NPs. Referring the corresponding EDX pattern in Figure S8 in
the Supporting Information.
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Figure 7. Characterization of Cu-BDC/Ru catalysts. a–e) TEM images, f) HAADF-STEM image, and g) the corresponding EDX elemental maps of
Cu-BDC/Ru nanocatalysts. Inset in (e) shows the corresponding size distribution of Ru NPs. Refer to the corresponding EDX pattern in Figure S9 in
the Supporting Information.
XPS characterizations also confirm the successful preparation
of supported metal catalysts (Figure 9). All of the XPS results
reveal the formation of MNPs in the zerovalent state (metallic
state).[46–49] It was found that these values are slightly higher
than those of bulk metals, probably due to the metal-to-sulfur
charge transfer as the result of thiol−metal bonds at the surface
of the thiol-capped NPs.[46–49] Figure 9e shows the XPS spectra of
the Cu-BDC/Au in S 2p regions. The fitting results indicate that
S 2p3/2 and S 2p1/2 peaks centering at BEs of 162.6 and 163.8 eV,
respectively, are mainly characterized to S–M (M = metals: Au,
Pt, Ag, or Ru).[46] Moreover, the characteristic XRD peaks due
to MNPs in the MOFs/M hybrid were indistinguishable, indi-
cating the formation of very small MNPs.[39] In addition, the
formation of MNPs was accompanied by a partial degradation
of MOF structure as the reductant tetrabutylammonium boro-
hydride (R-NBH4) would lead to the partial reduction of copper
ions in the MOFs (see Figure 9f). This is also the reason why
the characteristic XRD peaks due to MOFs shift towards lower
angles after the loading of MNPs (shown in Figure S11a, Sup-
porting Information), suggesting an expansion of the cell
volume due to the partial reduction of the copper ions. We also
try to use other conventional reductants (such as NaBH4 and
H2 (at 250 °C)) to prepare Cu-BDC/M composite. XPS spectra
of the two comparison samples together with the sample pre-
pared by using R-NBH4 reductant are shown Figure S11b,c in
the Supporting Information. Similarly, the copper ions in the
MOFs were partially reduced when using NaBH4 as reductant,
and most of the copper ions were reduced when using H2 at
high-temperature heating. This is due to the standard reduc-
tion potential of the gold precursor (AuCl4− + 3e− → Au + 4Cl−)
that is about 1.00 V (vs standard hydrogen electrode), while the
standard reduction potential of Cu2+ in MOFs is about 0.34 V.[50]
The calculated amounts of Cu0 in the composites from the
XPS analysis were 17%, 55%, and 78%, respectively, by using
R-NBH4, NaBH4, and H2 (at 250 °C) as reductants. Therefore,
it was found that R-NBH4 is a relatively mild reductant as
compared with other conventional reductants.
2.4. Catalytic Applications of Cu-BDC/M Nanocomposites
Schiff bases continue to play an important role in organic and
biological chemistry,[51–54] which can be synthesized from the
acid-catalyzed condensation of a primary amine with a carbonyl
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Figure 8. Characterization of Cu-BDC/Ag catalysts. a–f) TEM images and g) the corresponding EDX elemental maps of Cu-BDC/Ag nanocatalysts.
Inset in (f) shows the corresponding size distribution of Ag NPs. Refer to the corresponding EDX pattern in Figure S10 in the Supporting Information.
compound (e.g., aldehyde). As mentioned before, Cu-BDC
contains coordinatively unsaturated metal centers, which are
potential as Lewis acid sites in condensation reactions. And the
combination of MNPs with Cu-BDC nanosheets is expected to
exhibit improved catalytic activity based on synergetic effects
from the two components. Therefore, the as-synthesized
Cu-BDC and Cu-BDC/Au were used to catalyze the condensation
reactions of benzaldehyde with three different primary amines
(e.g., ethanolamine, aniline, and 4-aminothiophenol). For com-
parison, blank experiments without catalysts showed inferior
activity under the studied conditions, indicating the impor-
tance of Lewis acid sites. When the studied amine was aliphatic
amine with short carbon chain (viz., ethanolamine), the reac-
tion rates over both Cu-BDC and Cu-BDC/Au were quite fast.
As shown in Tables S2 and S3 in the Supporting Information,
the product yielded from the condensation of ethanolamine and
benzaldehyde are higher than 96% at a short reaction time of
15 min, for using Cu-BDC or Cu-BDC/Au as catalysts.
Figure 10b,c and the corresponding GC spectra in Figures S12
and 13, Supporting Information). This trend is the same for
using both aromatic amines (aniline and 4-aminothiophenol)
as substrates. However, as 4-aminothiophenol was used instead
of aniline, the reaction rate was enhanced a lot. The result sug-

gests that the electron-donating para substituents ( SH) on ani-
line would increase the reaction rate, which is consistent with a
previous report.[55] When the reduced Cu-BDC sample prepared
by heating in H2 at 250 °C (Cu2+ → Cu0) was utilized as a cat-
alyst (Figure S11b, Supporting Information), it exhibited very
low conversion of benzaldehyde (21%), suggesting that partially
coordinated Cu2+ sites are critical for the catalytic reaction. This
phenomenon is similar to the study by Mobinikhaledi et al. who
used Cu(II) salt[56] as a homogeneous catalyst for Schiff bases
synthesis. The effect of gold loading amount on the catalytic
performance can be found in Figure 10d. Obviously, a higher
gold loading led to a higher yield of imine product, which fur-
ther confirms that synergetic effects were raised by the integra-
tion of 2D MOF support and Au NPs. Similarly, the loading of
Au nanoparticles on reduced Cu-BDC also enhanced the cata-
lytic performance. And it is believed that thiol group (from the
MPA) only exists in the space between Cu-BDC flat surface and
gold nanoparticles, which is similar to layer-by-layer assembly
method to deposit Au NPs on a flat substrate functionalized
It was found that the reaction rate became much slower
when two different aromatic amines were tested. The reaction
scheme is displayed in Figure 10a. Impressively, it was observed
that the Cu-BDC/Au greatly increased the reaction rate as com-
pared with pristine Cu-BDC nanosheets as catalysts, especially
in the initial stage of the reaction (reaction time <60 min; see
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Figure 9. XPS spectra of Cu-BDC/M nanocatalysts for the identification of the metal chemical states. a) Au 4f, b) Ag 3d, c) Pt 4f, d) Ru 3p,
e) S 2p, and f) Cu 2p3/2 XPS core-level spectra of in the Cu-BDC/M nanocatalyst samples. M = Au in (a,e,f), M = Ag in (b), M = Pt in (c), and
M = Ru in (d).
with thiol group.[57] Moreover, Figure S14 in the Supporting
Information shows the catalytic performance of the other three
catalysts (viz., Cu-BDC/Ag, Cu-BDC/Pt, and Cu-BDC/Ru) for
condensation reactions between benzaldehyde and aniline.
It was found that loading of Au NPs on Cu-BDC nanosheets
exhibited superior catalytic performance than loading other
NPs on Cu-BDC.
We also compare the catalytic activities of Cu-BDC
nanosheets and Cu-BDC bulk material in condensation of
benzaldehyde with aniline in toluene. The equilibrium con-
version of benzaldehyde was 70% and 50%, respectively, for
Cu-BDC nanosheets and Cu-BDC bulk material, indicating that
the nanosheet structure of Cu-BDC benefits the formation of
more accessible active sites on the external surface (i.e., par-
tially coordinated Cu2+ and Au NPs) and the reaction species
over bulk Cu-BDC involve longer molecular diffusion distance.
The recyclability of the catalysts has also been checked for both
condensation reactions between benzaldehyde and aromatic
amines. As shown in Figure 10e, the designed Cu-BDC/Au can
be repeatedly used for at least five consecutive reaction cycles
with only slight activity loss. And the slight activity loss was
caused by (i) slight decrease of Au loading in the spent cata-
lysts (i.e., 8.9 and 8.5 wt% in the two cases), where 9.1 wt% of
Au loading was present in the fresh catalyst, and (ii) the loss of
solid catalyst during the repeated experiments. The condensa-
tion reactions studied in our work are only simple demonstra-
tions for examining the workability of our designed catalysts,
and we believed that the obtained Cu-BDC/Au can also serve
as catalysts for other types of acid-catalyzed reactions reported
in the literature, such as Knoevenagel condensation reaction,
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Figure 10. a) Condensation reactions between benzaldehyde and aromatic amines, in which R = H for aniline and R = SH for 4-aminothiophenol.
b,c) Results of experiments with condensation between benzaldehyde and b) aniline and c) 4-aminothiophenol over pristine Cu-BDC and Cu-BDC/
Au catalysts, respectively. In the sample of Cu-BDC/Au, gold loading amount was determined as 9.1 wt% by ICP–OES. d) The effect of Au loading
amount on the product yield from condensation between benzaldehyde and aniline. And e) catalytic performance of recycling Cu-BDC/Au catalysts in
condensation reactions between benzaldehyde and aromatic amines (aniline or 4-aminothiophenol).

C H arylation reaction, hydroxylation and nitration of aryl
halides, cycloaddition of benzyl azide and phenylacetylene,
4. Experimental Section
Chemicals and Materials: The following chemicals were used as
three-component coupling (amines, aldehydes, and alkynes to
received without further purification: copper (II) nitrate trihydrate
(Merck, 99.5%), copper (II) chloride dihydrate (Aldrich, 99%), H2BDC
(Aldrich, 98%), gold (III) chloride trihydrate (Sigma-Aldrich, 99.9%),
potassium tetrachloroplatinate (II) (Sigma-Aldrich, 98%), silver nitrate
(Merck, 99.0%), ruthenium (III) chloride hydrate (Aldrich, 99%), MPA
(Lancaster, 99%), tetrabutylammonium borohydride (R-NBH4, 98%,
Sigma-Aldrich), polyvinylpyrrolidone (PVP; Fluka, K-30), l-ascorbic acid
(Aldrich, 99%), sodium hydroxide (Merck, 99%)), aniline (Alfa Aesar,
99%), benzaldehyde (Sigma-Aldrich, 99%), 2-aminoethanol (Sigma-
Aldrich, 99%), 4-aminothiophenol (Sigma-Aldrich, 97%), n-dodecane
(Alfa Aesar, 99%), toluene (J.T. Baker, 99.5%), N,N′-DMF (Merck,
99.8%), methanol (Fisher, 99.99%), and ethanol (Fisher, 99.99%).
Deionized water was used for all experiments.
Synthesis of Cu2O Nanocubes: Specifically, 0.1 mmol of CuCl2 and 0.1 g
of PVP were dissolved in 40 mL of water, followed by a dropwise addition
(30 µL s−1) of 2.5 mL of NaOH (0.2 m). Then, the solution was stirred
magnetically for 5 min, followed by a dropwise addition (10 µL s−1) of
2.5 mL of ascorbic acid (0.1 m). After the addition, the mixture was further
stirred for another 5 min. The product was recovered by centrifugation
and then washed with ethanol twice. Finally, the yellow-colored Cu2O
nanocubes were redispersed into 10 mL of ethanol for the future use.
Synthesis of Cu-BDC Nanosheets: 0.5 mmol of H2BDC was fully
dissolved into 5 mL of ethanol and 5 mL of DMF. To this solution, 10 mL
of the above prepared Cu2O suspension (in ethanol) was added. The
form the corresponding propargylamines), etc.[58]
3. Conclusion
In summary, we have demonstrated that 2D Cu-BDC
nanosheets can be synthesized by using Cu2O nanocubes as
copper ion source at mild conditions (i.e., room temperature,
atmosphere pressure, and short reaction time), which is very
different from the conventional top-down or bottom-up syn-
thetic strategies. The as-prepared 2D Cu-BDC nanosheets with
oriented growth are desirable host materials for constructing
integrated catalysts. For instance, Au, Pt, Ag, and Ru NPs with
uniform size (<5 nm) can be specifically deposited on the exte-
rior surface of Cu-BDC nanosheets with the assistance of MPA
as a molecular linker. Furthermore, the obtained Cu-BDC/
Au nanocatalysts exhibited excellent catalytic activity in Schiff
bases synthesis reactions, and it was believed that synergistic
effects between Cu-BDC and Au NPs raised during the catalytic
reactions.
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formation of Cu-BDC nanosheets was carried out at room temperature
for 4 h, wherein Cu2O nanocubes gradually released Cu+ ions which
were further oxidized by the dissolved O2 to construct 2D frameworks.
Finally, the blue-colored products were collected via centrifugation,
followed by washing with methanol, and finally the solid was redispersed
in 40 mL of methanol for the future use.
Chun Zeng for helpful discussions on this project. This work was
supported by the Scientific Research Funds of Huaqiao University
(No. 18BS102 & 16BS501) and the Program for Innovative Research
Team in Science and Technology in Fujian Province University.
Synthesis of Cu-BDC Bulk Materials: Bulk-type Cu-BDC was prepared
by using Cu(NO3)2 as copper ion source according to a previously
reported method.[22] Specifically, equimolar quantities of copper nitrate
(7.5 mmol) and H2BDC (7.5 mmol) were dissolved in 150 mL of DMF.
Then, this solution was solvothermally treated at 110 °C for 36 h. After
the reaction, the products were collected via centrifugation, followed by
washing with ethanol/DMF twice, drying at 60 °C overnight.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
Synthesis of Cu-BDC/Au-Supported Nanocatalysts: For
a general
synthesis, 1 mL of HAuCl4⋅3H2O methanolic solution (0.010 m) was
mixed with 40 mL of the above-prepared Cu-BDC methanolic suspension.
The mixture was stirred for 40 min at room temperature, followed by an
addition of 0.3 mL of MPA methanolic solution (0.1 m). After stirring for
5 min, 0.3 mL of R-NBH4 methanolic solution (0.25 m) was injected into
the above mixture. (Note that R-NBH4 solution was used around 8 min
just after the preparation.) The mixture was stirred for 30 s, followed by
centrifuging and washing with ethanol twice. Other MNPs (M = Pt, Ag,
and Ru) loading on Cu-BDC nanosheets were synthesized in a similar
way but changing the corresponding metal precursors and the amount
of MPA and R-NBH4. Complete descriptions of the synthetic parameters
can be found in the Supporting Information.
Evaluation of Catalytic Activities: In a typical experiment, 30 mg of
catalysts was first dispersed into 10 mL of toluene solvent by sonication,
then a mixture containing 1 mmol of benzaldehyde, 1 mmol of aniline
(or 4-aminothiophenol, or ethanolamine), and 0.1 mL of n-dodecane as
an internal standard was added to this catalyst suspension. The reaction
mixture was stirred at 80 °C. The reaction rate was monitored by
withdrawing aliquots from the reaction mixture at different time
intervals, and the liquid after removing catalysts was immediately
analyzed by gas chromatography (GC) and GC–mass spectrometry
(GC–MS).
Characterization Techniques: Morphologies of samples were
characterized by field-emission scanning electron microscopy (FESEM;
JSM-6700F), TEM (JEM-2010), HAADF-STEM (JEM-2100F), and AFM
(Bruker Dimension Icon). The crystallographic information was analyzed
by XRD (Bruker D8 Advance) equipped with a Cu Kα radiation source
(λ = 1.5406 Å). The elemental mapping was done by EDX (Model 7426;
Oxford Instruments). The surface composition and oxidation state of
the samples were further analyzed by X-ray photoelectron spectroscopy
(XPS; AXIS-HSi; Kratos Analytical). All binding energies were referred to
2D MOFs, catalysis, metal-organic frameworks, nanosheets, Schiff bases
Received: September 23, 2018
Revised: November 18, 2018
Published online:
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