A covalent organic framework-based route to the: In situ encapsulation of metal nanoparticles in N-rich hollow carbon spheres

Article abstract of DOI:10.1039/c6sc01659f

Metal nanoparticles (NPs) encapsulated in hollow nanostructures hold great promise for a variety of applications. Herein, we demonstrate a new concept where covalent organic frameworks (COFs) doped with metal cations can be readily used as novel precursors for the in situ encapsulation of metal NPs into N doped hollow carbon spheres (NHCS) through a controlled carbonization process. The obtained Pd@NHCS composites show a significantly enhanced catalytic activity and selectivity in the hydrogenation of nitrobenzene in ethanol and oxidation of cinnamyl alcohol compared with that of the conventional Pd/N-C and commercial Pd/C catalysts. The excellent catalytic performance should be related to the synergism of the porous hollow spheric structure, highly dispersed Pd NPs, and uniform distribution of N dopants on the materials. We believe that this newly developed methodology could be extended to the synthesis of other metal NPs@NHCS composites for a variety of advanced applications.

Full text of DOI:10.1039/c6sc01659f

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A covalent organic framework-based route to in-situ
encapsulation of metal nanoparticles in N-rich hollow carbon
spheres
Received 00th January 20xx,
Accepted 00th January 20xx
Liyu Chen,a Lei Zhang,a Zhijie Chen,a Hongli Liu,a Rafael Luque,*b and Yingwei Li*a
DOI: 10.1039/x0xx00000x
Metal nanoparticles (NPs) encapsulated in hollow nanostructures hold great promise for a variety of applications. Herein,
we demonstrate a new concept that covalent organic frameworks (COFs) doped with metal cations can be readily used as
novel precursors for in-situ encapsulation of metal NPs into N doped hollow carbon spheres (NHCS) through a controlled
carbonization process. The obtained Pd@NHCS composites show significantly enhanced catalytic activity and selectivity in
the hydrogenation of nitrobenzene in ethanol and oxidation of cinnamyl alcohol compared with that of conventional Pd/N-
C and commercial Pd/C catalysts. The excellent catalytic performance should be related to the synergism of porous hollow
spheric structure, highly dispersed Pd NPs, and uniform distribution of N dopants on the materials. We believe this newly
developed methodology could be extended to the synthesis of other metal NPs@NHCS composites for a variety of
www.rsc.org/
advanced
applications.
synthesis requires the employment of an easily removable
material (e.g., SiO2) as the hard template for the coating and
subsequent pyrolysis of nitrogen containing precursors (e.g.,
dopamine, and resorcinol–formaldehyde) to afford a N-rich
carbon shell.7 Then NHCS could be obtained after the removal
of siliceous components by treatment with HF solution, which
is then used as supports for the immobilization of metal NPs
by impregnation or coprecipitation. Although this traditional
synthetic route has been widely used, it involves tedious and
complicated steps, and requires hazardous reagents (i.e., HF)
for the removal of hard templates and excess reducing
reagents for the reduction of metal precursors. Moreover,
postloading of metal NPs to the support would lead to
insufficient attachment of the NPs to the support.3c Therefore,
it is highly desirable to develop a simple and efficient strategy
to integrate metal NPs into NHCS for the design of advanced
nanocatalysts.
Introduction
Hollow-structured porous materials, with large internal voids
and porous shells, have promising applications in a variety of
fields, such as gene therapy, confined catalysis, adsorption and
storage.1 Among such systems, hollow carbon spheres (HCS)
are very attractive owing to their outstanding features of low
density, good thermal stability, and high permeability.2
Recently, considerable interests have been raised to employ
HCS as nanoreactors by encapsulating catalytically active metal
nanoparticles (NPs) in the porous shell of HCS.3 The shell could
function as a barrier to prevent encapsulated metal NPs from
coalescence, and the pore in the shell connecting the hollow
core can provide a highway network for the free diffusion of
reactant molecules to access the metal active sites.4 Moreover,
it has been demonstrated that incorporation of heteroatoms
(e.g., nitrogen, boron, sulphur and phosphorus) into the
carbon lattice can significantly improve its chemical and
electrical properties, and also enhance the interactions
between the carbon support and the embedded metal NPs to
trigger superior catalytic performance.5
Recently, solid-state pyrolysis of porous organic networks,
such as metal-organic frameworks (MOFs)8 and covalent
organic frameworks (COFs),9 has emerged as a simple, efficient
and promising approach for the fabrication of nanostructured
materials. Especially, COFs are only composed of organic
skeletons, making them outstanding templates for facile
preparation of porous carbons.10 By carefully choosing
construction units, COFs with various morphographies and
functions could be synthesized, thus allowing to control the
shape and composite of the final nanostructured materials via
thermolysis.11 Nevertheless, to the best of our knowledge, the
employment of COF-based materials as pyrolysis sacrificial
precursors for NHCS supported metal NPs composites has
never been explored.
Considerable efforts have been devoted to exploring
effective strategies for the preparation of metal NPs
encapsulated within N doped HCS (NHCS).6 Typitally, the
a.Key Laboratory of Fuel Cell Technology of Guangdong Province, School of
Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou 510640 (China), E-mail: liyw@scut.edu.cn
b.Departamento de Química Orgánica, Universidad de Córdoba, Edif. Marie Curie,
Ctra Nnal IV-A, Km 396, E14014, Córdoba (Spain), E-mail: q62alsor@uco.es
Electronic Supplementary Information (ESI) available: Experimental details and
catalysts characterization. See DOI: 10.1039/x0xx00000x
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In this work, we report a novel one-step synthesis strategy exists a strong coordination between Pd(OAc)2 with the imine
for in-situ fabrication of NHCS encapsulated metal NPs by groups of LZU1.
using metal ion doped COF as precursor. As a proof of concept,
we chose LZU1,12 a hollow spheric COF, as the NHCS template.
In this COF, the eclipsed nitrogen atoms in adjacent layers can
serve as coordination sites for transition cations such as Pd2+
(Scheme 1). During the annealing process under an inert
atmosphere, Pd cations will be reduced to form Pd NPs,
meanwhile the N-containing organic linkers will serve as both
carbon and nitrogen sources for the formation of NHCS.
Through this rational design, the high nitrogen doping in the
NHCS support could geometrically and electronically modify
the in-situ growth of Pd NPs and the hollow structure could
facilitate the diffusion of substrate, endowing the obtained
material with superior catalytic capability. As demonstrated in
the hydrogenation of nitrobenzene and oxidation of cinnamyl
alcohol, this unique Pd@NHCS hybrid material exhibits
significantly improved catalytic performances in terms of
activity and selectivity as copmpared to the Pd/N-C and
commercial Pd/C catalysts under identical conditions.
Figure 1. (a-b) TEM images of PdII-LUZ1. (c-d) TEM images of Pd@NHCS(500). (e)
HRTEM image of Pd NPs. (f-i) EDS elemental mapping of a Pd@NHCS(500) particle
Scheme 1. Schematic illustration of the fabrication of Pd@NHCS. Color coding: grey,
(green: C; blue: N and red: Pd).
carbon; blue, nitrogen; orange, Pd precursors or NPs.
As observed from the thermogravimetric analysis (TGA)
curves (Figure 3a), PdII-LZU1 begins to decompose when the
Results and discussion
temperature is increased to ca. 500 °C under argon. Therefore,
the PdII-LZU1 composites were annealed at 500 °C for 3 h with
LZU1 was synthesized following a previously reported protocol
with slight modifications.12 Pd(II)-doped LZU1 (PdII-LUZ1) was
a heating rate of 1 °C/min to produce the final composites,
facilely prepared through a simple impregnation of LZU1 with
palladium acetate. Field emissions canning electron
microscopy (FESEM) and transmission electron microscope
(TEM) images show that PdII-LZU1 is mainly composed of
hollow spheres with a diameter of about 250 nm and a shell
thickness of ca. 40 nm (Figure S1a, and Figure 1a-b). X-ray
photoelectron spectroscopy (XPS) measurements were
performed to investigate the intermolecular interaction
between the COF and Pd(OAc)2. For the Pd 3d 5/2 spectrum,
one peak at 337.8 eV was observed for PdII-LZU1, characteristic
of divalent Pd ions (Figure S2). This value shifts negatively in
comparison with that of free Pd(OAc)2.13 In addition, the
parent LZU1 contains only one kind of N, i.e., Schiff base
nitrogen (Figure 2a). After the immobilization of Pd(OAc)2, a
new N 1s peak at ca. 400 eV was observed. The shift of the N
1s peak toward a higher binding energy reflects a decrease in
the electron density of N. These results indicate that there
which is denoted as Pd@NHCS(500). The carbonized products
show similar morphology as compared to the precursor (Figure
S1b). The diameter of the spheres and the thickness of the
shell measured from the TEM images are ca. 200 nm and 30
nm, respectively (Figure 1c-d). The observed smaller particles
in comparison with the precursors are reasonable as a
consequence of volume shrinking after high-temperature
heating. The corresponding dark field scanning transmission
electron microscopy (DFSTEM) images (Figure S3) clearly show
hollow shells. The Pd NPs are uniformly distributed with an
average size of 6 nm, mostly located within the carbon shell. A
representative high-resolution TEM image of Pd-in-UiO-67
displays distinct lattice fringes with d spacings of 0.22 nm,
corresponding to the (111) lattice spacing of face-centered
cubic Pd (Figure 1e). The surface of Pd is not coated by carbon
layers, indicating that Pd NPs are embedded within the pores
or supported on the surface of NHCS, thus accessible to the
reactants. The energy dispersive spectrometer (EDS) mapping
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images (Figure 1f-i) reveal the uniform distribution of C, N and generation of large amounts of defects because of the high
Pd elements all over the support. The Pd content in the nitrogen doping percentage in the resulting composites.
Pd@NHCS(500) is 2.4 wt% as measured by atomic absorption
N2 adsorption–desorption experiments were performed at
spectroscopy (AAS). Elemental analysis results further reveal 77 K to measure the surface area and porosity of the resultant
that Pd@NHCS(500) is composed of C, H and N elements, and NHCS supported Pd composites. The adsorption/desorption
the nitrogen content is as high as 11.0 wt% (Table S1).
isotherms are shown in Figure 3c. Pd@NHCS samples show a
Futher increasing the annealing temperature to 600 °C, typical adsorption curve of type I plus IV with steep increases
some particles with a broken outer shell were observed (Figure at low relative pressures and an obvious hysteresis loop in the
S4). The as-formed Pd NPs show some aggregation, with an P/P0 range of 0.8−1, revealing the presence of micro-, meso-
average size of 19 nm. Elevating the annealing temperatures and macropores. These Pd@NHCS materials have
also leads to a decrease in nitrogen content wihile an increase Brunauer−Emmeꢀ−Teller (BET) surface areas of 450−550 m2
in Pd content of the resultant samples (Table S1).
g−1 and pore volumes of 0.19−0.24 cm3 g−1 (Table S2). The
porous structure of NHCS supports would benefit the
transportation of molecules to embeded Pd NPs in catalytic
transformations.
Figure 2. (a) XPS N 1s of LUZ1 before and after the immobilization of Pd(OAc)2. High-
resolution (b) Pd 3d spectra, (c) N1s spectra, and (d) C1s spectra for Pd@NHCS(500)
and Pd@NHCS(600).
Figure 3. (a) TGA curve of PdII-LUZ1. (b) Powder XRD patterns for Pd@NHCS(500) and
Pd@NHCS(600). (c) Nitrogen adsorption isotherms for Pd@NHCS(500) and
Pd@NHCS(600). (d) Raman spectra for Pd@NHCS(500) and Pd@NHCS(600).
The crystalline phases and structures of the materials were
examined by powder X-ray diffraction (XRD) and Raman
analysis. The characteristic XRD peaks (Figure 3b) at 40.1, 46.7,
68.2, 82.2 and 86.7 may be assigned to the (111), (200), (220),
XPS measurements were conducted to investigate the
(311) and (222) reflections of Pd (JCPDS 65-6174), respectively. electronic and structural properties of Pd@NHCS composites.
No peaks for other possible impurity were detected, indicating As illustrated in Figure 2b, Pd@NHCS(500) contains ca. 60%
high purity of the synthesized materials. Increasing the Pd0 as revealed by the Pd 3d 5/2 XPS peaks at 335.2 and 337.4
calcination temperature enhances the intensity of the Pd eV, relatively a less percentage of Pd0 (ca. 43%) was observed
diffraction peak, suggesting
a bigger size and higher for Pd@NHCS(600). The N 1s spectra of the Pd@NHCS
crystallinity degree of Pd NPs as the calcination temperature composites can be deconvoluted into two peaks at 398.6 and
increases, which is in good agreement with that observed in 399.9 eV, respectively, which are consistent with pyridinic N
TEM analysis. A broad and weak XRD peak around 29.5° was and quaternary N in the carbon texture (Figure 2c). The ratio
detected (Figure 3b), confirming the existence of amorphous of graphitic nitrogen to pyridinic nitrogen in Pd@NHCS is
carbon. In addition, the Raman spectra show two broad bands increased when the pyrolysis temperature is enhanced (e.g.,
(Figure 3d), in which the G band at
in-plane vibration of aromatic carbon atoms, while the D band graphitization degree at elevated temperatures. The C 1s XPS
at
1313 cm-1 is a disorder induced Raman feature caused by spectra of the Pd@NHCS composites present in Figure 2d can
∼
1584 cm-1 indicates the 1.3 for 500 °C and 2.2 for 600 °C), implying a higher
∼
the nonperfect crystalline structure.14 The relative ratio of D be deconvoluted into two major peaks. The peak at 284.6 eV
band to G band (ID/IG) in the Raman spectra of Pd@NHCS(500) corresponds to the sp2 graphitic carbon species, while the
and Pd@NHCS(600) is 1.30 and 1.16, respectively. The D band other one at 285.4 eV may be assigned to the sp2-carbon
appears to be stronger than the G band, implying the
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containing nitrogen atoms, suggesting the presence of demonstrated that Pd/N-C(500) had a similiar Pd loading
nitrogen functionalities on the surface of NHCS. (2.1 wt%) to that of Pd@NHCS(500). In the hydrogenation of
The catalytic efficiencies of the as-synthesized nanohybrid nitrobenzene, Pd/N-C(500) exhibited a moderate activity and
materials were evaluated using the hydrogenation of produced aniline in 89% yield in 120 min. The substantially
nitrobenzene to aniline as model reaction.15 The reactions lower catalytic activity for Pd/N-C(500) as compared to
were performed under atmospheric pressure of H2 and room Pd@NHCS(500) indicates that the hollow structure and porous
temperature (25 °C), and the reaction profiles are shown in shell of the NHCS may facilitate the substrate to access Pd NPs
Figure 4. It was found that hydrogenation of nitrobenzene in thus showing an enhanced reactivity.
alcohols would afford not only aniline, but also imine, N-
On the other hand, Pd(OAc)2 was impregnated onto a
ethylaniline and tertiary amine as by-products coming from commercial carbon followed by annealing at 500 °C for 3h to
the further N-alkylation of amine with alcohol.16 Pure afford Pd/C(500). Under the similar reaction conditions,
NHCS(500) gave essentially no conversion under the reaction Pd/C(500) showed the worst activity, giving only 89%
condition, implying the need of a metal to perform the conversion in 120 min. Interestingly, Pd/C(500) exhibited a
hydrogenation of nitrobenzene. Pd@NHCS(500) showed a very different product distribution comparing with those of
high activity with a complete conversion of nitrobenzene and Pd@NHCS and Pd/N-C. As shown in Table S3, the selectivity to
88% aniline selectivity within only 50 min. Pd@NHCS(600) was aniline was very low (12%), while mono- or di-N-alkylation
less active than Pd@NHCS(500), requiring a much longer time product became the main product over the Pd/C(500) catalyst.
(ca. 100 min) to obtain
a quantitative conversion of To gain more in-depth information about the different
nitrobenzene. The relatively poor performance may result catalytic behaviors, the Pd@NHCS(500) and Pd/C(500)
from the larger particle size of Pd NPs in Pd@NHCS(600) (as materials were characterized by TEM, AAS and XPS. The as-
demonstrated by the TEM analysis). Nevertheless, the prepared Pd NPs in Pd/C(500) have a similar size distribution,
Pd@NHCS(600) catalyst could also achieve a similar high Pd loading (2.0 wt%) and percentage of Pd0 as that of
selectivity to aniline as Pd@NHCS(500) (Figure 4b).
Pd@NHCS(500) (Figures S5b and S6). However, interestingly,
the binding energy of Pd 3d is shifted to a higher value by
approximately 0.5 eV. This could be due to the lack of electron
donating dopant (e.g., N) in the carbon support, resulting in
more positively charged Pd NPs on the Pd/C(500).
In situ attenuated total reflection infrared (ATR-IR)
spectroscopy was further employed to gain some molecular
insights into the surface adsorption property of the catalysts
for ethanol. Initially, N2-saturated ethanol adsorption on
different catalyst surfaces was investigated. The difference
spectra of adsorbed ethanol on Pd@NHCS(500) is displayed in
Figure S7. In this plot, spectrum of the pristine Pd@NHCS(500)
(Figure S8) was subtracted. Bands located in the region of
1200–1500 cm-1 are characteristics of C–H stretchings for
methyl and methylene groups in ethanol.18 These bands show
no obvious change in intensity with time, implying a weak
absorption of ethanol on the Pd@NHCS(500) material.
However, for Pd/C(500), these bands gradually become
weaker with time, suggesting that ethanol might adsorb
strongly on the Pd/C(500). Nitrogen functionalities on NHCS
could act as coordination donor ligands to tune the electronic
properties of Pd NPs to exhibited a comparatively higher
negative charge to that of Pd/C(500) (Figure S6). Considering
alcohols may be adsorbed on the Pd surface via the lone
electron pair of the oxygen atom, electron-enriched Pd in
Pd@NHCS(500) may depress the adsorption of ethanol while
positively charged Pd in Pd/C(500) facilitates this adsorption.19
Therefore, it is reasonable that ethanol could undergo N-
alkylation with aniline when using Pd/C(500) as catalyst, while
the alkylation reaction is supressed over the Pd@NHCS(500)
catalyst.
Figure 4. (a) Nitrobenzene conversions and (b) selectivities to aniline versus time in the
hydrogenation of nitrobenzene over the Pd@NHCS(500), Pd@NHCS(600), Pd/C-N(500)
and Pd/C(500). (c) Effect of Pd loading on the catalytic performance of Pd@NHCS(500).
(d) Recyclability of 2.4% Pd@NHCS(500) in the hydrogenation of nitrobenzene.
To illustrate the benefit of the hollow spheric structure and
N dopant to the catalytic performance, we also prepared Pd
NPs supported on N doped carbon and commercial carbon
materials for comparison. First, N doped carbon was prepared
according to our previous report.17 The N-C material was then
impregnated with Pd(OAc)2 and treated at 500°C for 3 h under
argon. The resulting material is denoted as Pd/N-C(500). TEM
image shows that Pd/N-C(500) has a comparable average size
of Pd NPs to that of Pd@NHCS(500) (Figure S5a). AAS analysis
The proposed protocol could be applied for the preparation
of Pd@NHCS with different Pd loadings, simply by changing
the impregnated amounts of Pd(OAc)2 on COFs. As measured
by AAS, we could achieve different Pd loadings (e.g., 2.4%,
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3.6% and 5.0%) on NHCS. As shown in Figure S9, these samples
showed comparable size distribution of Pd NPs. These
composites show similar catalytic activities for nitrobenzene
hydrogenation when a same amount of Pd (1 mol% to
substrate) was used, indicating the loading of Pd on NHCS
would not play a critical role in the activity (Figure 4c).
Acknowledgements
This work was supported by the NSF of China (21322606,
21436005, and 21576095), the Doctoral Fund of Ministry of
Education of China (20120172110012), Fundamental Research
Funds for the Central Universities (2015ZP002 and
2015PT004), and the Guangdong NSF (2013B090500027).
The stability and reusability are of great importance for the
practical application of
a heterogeneous catalyst. After
reaction, the catalyst was separated by centrifugation. AAS
analysis of the reaction solution showed that the
concentration of palladium in solution was below the
detection limit, suggesting that the leaching of active Pd during
the reaction was negligible. The used catalyst was thoroughly
washed with ethanol, and then dried under vacuum to remove
the residual solvent. Results included in Figure 4d reveals that
Pd@NHCS(500) could be regenerated and reused at least five
times in the subsequent reactions without significant lose of
catalytic activity and selectivity. The metal contents of the
reused catalysts are almost the same as the fresh one, as
determined by AAS. TEM images also show that no apparent
Pd aggregation could be observed on the recycled catalysts
(Figure S10). These results demonstrate that the highly active
Pd@NHCS(500) catalyst is stable and reusable under the
investigated conditions.
Notes and references
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Conclusions
In summary, we have demonstrated a novel and facile in-situ
strategy to synthesize metal NPs encapsulated in the shell of
nitrogen doped hollow carbon spheres (NHCS) by using metal
ions doped COFs as the precursors through a simple annealing
process. The obtained Pd@NHCS composites exhibit
unprecedent catalytic performances in terms of activity and
selectivity that are much superior to conventional Pd/N-C and
commercial Pd/C catalysts in the hydrogenation of
nitrobenzene and oxidation of cinnamyl alcohol. The obtained
excellent catalytic properties should be originated from the
synergism of porous hollow spheric structure, highly dispersed
Pd NPs, and uniform distribution of N dopants. This proposed
protocol can simplify the synthesis process with the
elimination of hard templates and significant reduction of
wastes as compared to other conventional methodologies,
which might be extended to the preparation of other metal
NPs@NHCS composites for potential applications in a variety
of fields including heterogeneous catalysis demonstrated here.
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