Heteroatom-doped Carbon Spheres from Hierarchical Hollow Covalent Organic Framework Precursors for Metal-Free Catalysis

Article abstract of DOI:10.1002/cssc.201700979

Covalent organic frameworks (COFs) with hollow structures hold great promise for developing new types of functional materials. Herein, we report a hollow spherical COF with a hierarchical shell, which serves as an effective precursor of B,N-codoped hierarchical hollow carbon spheres. Benefiting from the synergistic effects of hierarchical porosity, high surface area, and B,N-codoping, the as-synthesized carbon spheres show prospective utility as metal-free catalysts in nitroarene reduction. A mechanistic hypothesis is proposed based on theoretical and experimental studies. Boron atoms situated meta to pyridinic N atoms are identified to be the main catalytic active sites. The anti-aromaticity originating from the codoping of B and pyridinic N atoms, not charge distribution and deformation energy, is confirmed to play a pivotal role in the catalytic reaction.

Full text of DOI:10.1002/cssc.201700979

Accepted Article
Title: Hierarchical Hollow Covalent Organic Frameworks-derived
Heteroatom-doped Carbon Spheres for Metal-free Catalysis
Authors: Liuyi Li, Lu Li, Caiyan Cui, Hongjun Fan, and Ruihu Wang
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To be cited as: ChemSusChem 10.1002/cssc.201700979
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10.1002/cssc.201700979
ChemSusChem
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Hierarchical Hollow Covalent Organic Frameworks-derived
Heteroatom-doped Carbon Spheres for Metal-free Catalysis
Liuyi Li,[a] Lu Li,[b] Caiyan Cui,[a] Hongjun Fan*[b] and Ruihu Wang*[a]
Abstract: Covalent organic frameworks (COFs) with hollow
availability, environmental acceptability, corrosion resistance
10]
structures hold great promises for developing new types of functional
and unique surface properties.[5f,
The ordered porous
materials. Herein, we report
a
hollow spherical COF with
a
structures, high concentrations of carbon species and tunable
chemical compositions in COFs make them intrinsic precursors
for self-templated synthesis of porous carbons.[11] Importantly,
benefiting from the precise integration of heteroatom-containing
building blocks and flexible synthetic strategies, the heteroatoms
doping into carbons can be achieved through in situ
carbonization of COFs, which will greatly ameliorate surface
electrical and chemical properties of carbon materials to realize
new functions.[12] Accordingly, the COF-derived porous carbons
can not only enlighten more intriguing applications, but also
provide a platform to understand the intrinsic role of dopants in
metal-free catalysis.
hierarchical shell, which serves as an effective precursor of B,N-
codoped hierarchical hollow carbon spheres. Benefiting from the
synergistic effect of hierarchical porosity, high surface area and B,N-
codoping, the as-synthesized carbon spheres show the prospective
utility as metal-free catalysts in nitroarene reduction. A mechanistic
hypothesis is proposed based on theoretical and experimental
studies. B atoms meta to pyridinic N atoms are identified to be the
main catalytic active sites. The anti-aromaticity originated from the
codoping of B and pyridinic N atoms, not charge distribution and
deformation energy, is corroborated to play a pivotal role in the
catalytic reaction.
Herein, we report the template-free synthesis of a hollow
spherical COF with hierarchical structures (H-COF), and
demonstrate its prospective utility as a precursor for the
fabrication of nitrogen (N) and boron (B)-codoped hollow carbon
spheres (BN-HCS). The as-synthesized BN-HCS exhibit
excellent catalytic activity, high selectivity, superior stability and
broad substrate tolerance in metal-free catalytic reduction of
nitroarenes. Moreover, the theoretical investigations together
with experimental results indicate that the synergistic effects
between B atoms and pyridinic N atoms are beneficial for their
promising catalytic properties.
Introduction
Covalent organic frameworks (COFs), emerging as a class of
new crystalline porous materials,[1] have shown great potential
applications in gas storage,[2] semicoductors,[3] proton
conduction[4] and catalysis,[5] owing to their high surface area,
tunable compositions and diverse structural topologies.[6]
Besides chemical composition and inner porosity, the
morphology of porous materials also plays an important role in
their performances.[7] In the context, hollow structures with
spherical shape and hierarchical architecture are of great
importance in catalysis because of their high surface/volume
ratios and kinetically favorable open structure.[8] The formation of
such COFs may integrate crystallinity and porosity with hollow
and hierarchical features, thus providing a unique opportunity to
develop new types of highly tailorable materials. Although
significant advances in the synthesis of functionalized COFs
have been achieved, hollow COFs with hierarchical shell
structures have been rarely reported.[9]
Results and Discussion
H-COF was synthesized by the solvothermal reaction of 4-
formylphenylboronic acid and 1,3,5-tris(4-aminophenyl)-benzene
(Figure 1). Fourier-transform infrared (FTIR) spectrum of H-COF
exhibits the disappearance of the hydroxyl strething band at
3210 cm-1, the carbonyl stretching band at 1672 cm-1, and the N-
H stretching band at 3300 cm-1. Meanwhile, the characteristic
stretching bands of a six-membered B3O3 boroxine ring and an
imine group at 711 and 1622 cm-1 (Figure S1), respectively,
emerged in the FTIR spectra. These data unambiguous indicate
that H-COF are formed with dual linkages. Solid-state 13C NMR
spectroscopy confimed the presence of the imine carbon at 158
ppm (Figure S2).[13]
The morphology of H-COF was investigated by field-
emission scanning electron microscopy (SEM) and transmission
electron microscope (TEM) analyses. The SEM images show
that H-COF is composed of uniform hollow microspheres with a
diameter of 2.12 ± 0.29 μm (Figure 2b and c), and the shell
thickness is approximately 500 nm (Figure S3). The hollow
structure of H-COF was observed clearly from the TEM images
(Figure 2d and e). Close inspection of the marked orange region
demonstrates that the microspheres consist of the randomly
oriented nanosheets with the thickness of 40 nm. Energy-
Recently, porous carbon materials have captured
considerable attention in metal-free catalysis due to their wide
[a]
Dr L. L, Li,[+] Dr C. Y. Cui, Prof R. H. Wang
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter, Chinese
Academy of Sciences
350002, Fuzhou, China
E-mail: ruihu@fjirsm.ac.cn
[b]
Dr L. Li,[+] Prof H. J. Fan
State Key Laboratory of Molecular Reaction Dynamics
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
116023, Dalian, China
E-mail: fanhj@dicp.ac.cn
These authors contributed equally to this work.
[+]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201700979
ChemSusChem
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dispersive spectroscopy (EDX) line-scanning analysis across the
hollow microsphere reveals that C, N, B and O atoms are
distributed uniformly in the shells (Figure S4). To our knowledge,
such hollow configuration of COFs has not been reported
heretofore. Powder X-ray diffraction (XRD) pattern of H-COF
indicates highly crystalline nature of the hierarchical hollow
microspheres (Figure S5), which is in good agreement with the
simulated pattern of the eclipsed model.[14] The Brunauer-
Emmett-Teller (BET) surface areas of H-COF is 16 m2 g-1
(Figure S6), the relatively low surface area is mainly owing to the
blockage of the surface pore channels by small COF
nanoparticles (vide infra in time-dependent experiemnt of COF
900, respectively. Their SEM images show that the hollow
spherical morphology of the precursor H-COF is well retained
(Figure 2 and Figure S11), which is critical to hold the porosity,
rigidity and uniform distribution of the heteroatoms. The
diameters of the resultant carbon spheres and the thickness of
their nanosheets become smaller and thinner, respectively, than
those of the parent H-COF owing to the framework shrinkage
during carbonization.[16] By increasing pyrolysis temperature, the
average outer diameters in BN-HCS-700, BN-HCS-800 and BN-
HCS-900 decrease to 2.05 ± 0.31, 1.95 ± 0.35 and 1.78 ± 0.3
μm, respectively, and the thicknesses of their nanosheets are
reduced to 36.3 ± 5, 26.5 ± 3.5 and 25 ± 3.8 nm, respectively.
formation). The structure of H-COF showed high thermal stability. TEM images of BN-HCS-800 further confirm the maintenance of
Thermogravimetric analysis (TGA) shows that H-COF is stable
up to 450 oC (Figure S7).
the hollow structure (Figure 2d). The shell of hollow spheres is
composed of nanosheets with thin thickness (Figure 2e and
2f).[17] The EDX elemental mapping images of BN-HCS-800
demonstrate uniform distribution of the B and N atoms in the
hollow carbon shell (Figure 2g), which benefits from the
atomically homogeneous distribution of N and B in H-COF.[18]
B
and N codoping into the hollow carbon is also confirmed by FTIR
and Raman spectra. In FTIR spectra of BN-HCS, the peaks
around 780 and 1050 cm-1 are attributed to B-N and B-C
stretching bands, respectively (Figure S12).32,33 The Raman
spectra of BN-HCS exhibit two characteristic peaks around 1335
and 1590 cm-1 (Figure S13), corresponding to the D and G
bands, respectively. The ID/IG ratios in BN-HCS-700, BN-HCS-
800 and BN-HCS-900 gradually increase from 2.72 to 2.91 to
3.10, suggesting more defects are generated in the carbons as a
result of pyrolysis at high temperatures and/or heteroatoms
doping into the sp2-carbon framework. In their XRD patterns, all
characteristic peaks of H-COF completely disappeared after
carbonization (Figure S14 and S15). The BET surface areas of
BN-HCS-700, BN-HCS-800 and BN-HCS-900 are 420, 558 and
570 m2 g-1 (Figure 3a), respectively. It is worth noting that the as-
synthesized carbon materials show a significant enhancement in
their surface area compared with H-COF precursors. This is
mainly ascribed to the considerable mass-loss of C, N and O in
building blocks, leaving a hollow carbon network.[19]
Figure 1. Synthesis of H-COF (a), SEM (b, c) and TEM (d, e) images for H-
COF.
The time-dependent experiments showed that the formation
of hierarchical H-COF follows the Ostwald ripening process.[9, 15]
As shown in Figure S8, the condensation of 4-
formylphenylboronic acid and 1,3,5-tris(4-aminophenyl)-benzene
generated firstly large amount of nanoparticles, which quickly
assembled into large microspheres to decrease the surface
energy. In the continuous solvothermal reaction, the inner
crystallites dissolve and migrate out by virtue of the reversibility
of the imine bond and boroxine ring formation, thereby
generating hollow cavities inside the spheres. Meanwhile, the
layered stacking structure induces the growth of crystalline
nanosheets outside the microspheres, producing hierarchical
hollow microspheres. FTIR spectra of the isolated samples at
different reaction time are identical with that of H-COF, indicating
the same chemical compositions in these samples (Figure S9).
Apart from reaction time, the solvents also play a decisive role in
modulating the morphology of COFs. By adjusting the volume
ratio of dioxane to mesitylene, nonhollow COFs microspheres
(NH-COF) can be obtained (Figure S10).
Considering the ordered porous structures, high
concentrations of carbon species and the atomically
homogeneous distribution of N and B in H-COF, the feasible
utility of H-COF in the self-templated synthesis of B,N-doped
porous carbons was investigated. The pyrolysis of H-COF at 700,
800 and 900 oC under N2 flow gave rise to porous carbons,
which are denoted as BN-HCS-700, BN-HCS-800 and BN-HCS-
Figure 2. SEM images for BN-HCS-700 (a), BN-HCS-800 (b) and BN-HCS-
900 (c). TEM images (d, e) high-resolution TEM (f) and EDX element mapping
(g) for BN-HCS-800.
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The detailed chemical compositions and bonding
configurations of B and N atoms in BN-HCS were examined by
X-ray photoelectron spectroscopy (XPS) analysis. The surface
contents of C, N and B are consistent with the results of
elemental analysis (Table S1). Their C 1s XPS spectra show
four binding energy peaks at 284.4, 285.5, 286.5 and 288.6 eV,
corresponding to the sp2 carbon, C-B, C-N and C-O,
respectively (Figure S16a). The high-resolution B 1s XPS
spectra consist of two main peaks at 190.2 and 191.1 eV, which
are assigned to B-C and B-N-C, respectively (Figure 3b).[20] The
weak shoulder peak of B 1s at 192.1 eV is related to BC2O
species in BN-HCS-700 and BN-HCS-800,[21] while no obvious
peak of B-O species was observed in BN-HCS-900. The
deconvolution of the N 1s peak suggests the presence of
pyridinic N (398.6 eV), pyrrolic N (399.7 eV), graphitic N (401.0
eV) and pyridinic N-oxide (405.3 eV) (Figure 3c). Among the BN-
HCS materials, the N/C atom ratio in BN-HCS-800 is the lowest
(Figure S16b), but it possesses the highest B/N ratio (Figure
S16c). These clearly indicate that most of B atoms are bonded
to C atoms in the form of BC3 species in BN-HCS-800, while B
atoms predominately generate N-B-C species in BN-HCS-900
(Figure 3d) owing to the ready formation of B-N bond at higher
temperature.[22] In combination with the bonding configurations
and the content of N atoms on the surface of BN-HCS (Figure
S16e), BN-HCS-800 and BN-HCS-900 are dominated by
separated and bonded B and N,[23] respectively. Therefore, the
structural properties of the B,N-codoped carbon spheres can be
reasonably tailored by changing the pyrolysis temperature of H-
COF.
free carbon catalysts.[25] Hydrazine hydrate is a readily available
hydrogen carrier and reductant that is commonly applied in the
metal-catalyzed reduction of nitroarenes.[26] The catalytic
performances of BN-HCS-700, BN-HCS-800 and BN-HCS-900
with hydrazine as the reductant were examined by the reduction
of nitroarenes to anilines, which is usually performed using metal
catalysts.[27] Kinetic profiles of nitrobenzene reduction reveal that
full conversion of nitrobenzene and quantitative selectivity of
aniline can be achieved in 3 h (Figure 4a), and BN-HCS-800
presents the highest catalytic activity. Notably, these BN-HCS
catalysts display a remarkable enhancement in catalytic activity
compared with nonhollow carbon spheres derived from NH-COF
(Figure S17a), which indicates that the morphology of carbon
materials is one of the important factors in the design of metal-
free catalytic systems. BN-HCS-800 demonstrated competitive
catalytic activity relative to reduced graphene oxide[28], N-doped
activiated carbon[29]
, graphene oxide and graphite powder
catalysts under identical reaction conditions (Figure S17 b).
Figure 4. (a) Kinetic profiles of nitrobenzene conversion and aniline formation
for BN-HCS-700, BN-HCS-800 and BN-HCS-900; (b) recycling test of BN-
HCS-800 in nitrobenzene reduction. Reaction condition: BN-HCS (10 mg),
nitroarene (1 mmol), ethanol (1 mL), hydrazine hydrate (1 mL), 100 oC, 4 h.
The catalytic recyclability of BN-HCS-800 was further
evaluated. The solid catalyst was readily recovered and reused
for 10 runs without obvious loss of activity (Figure 4b). The
chemical states and morphology of BN-HCS-800 were well
retained after consecutive ten runs (Figure S18 and S19),
verifying the structural stability of BN-HCS in the catalytic
reactions. The generality of the catalytic system was also
explored. When various nitroarenes were employed as
substrates, the corresponding amine products were obtained in
excellent yields regardless of their electronic and steric nature
(Table 1).
Considering that the catalytic activity of BN-HCS has no
direct correlation with the surface area and/or heteroatoms
concentration (Figure S20), density functional theory (DFT)
calculations were performed to elucidate the effects of B and N
codoping on the catalytic properties. It is known that the
adsorption of hydrazine onto the catalyst surface is the rate-
determining step in the nitrobenzene reduction reaction.[10b, 30]
The energy of hydrazine adsorption (Ead) on various graphene
models, including pure, N-doped, B-doped and N,B-codoped
Figure 3. N2 adsorption-desorption isotherms (a), B 1s XPS spectra (b) and N
1s XPS spectra (c) for BN-HCS-700, BN-HCS-800 and BN-HCS-900. Atom
content (at.%) of B-C and B-N on the surface of BN-HCS-700, BN-HCS-800
and BN-HCS-900 (d).
Since the replacement of high-cost precious metal catalysts
with metal-free carbon materials is of great importance in
catalysis,[24] it is highly desirable to develop new types of metal-
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10.1002/cssc.201700979
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graphenes, were chosen as criteria to examine their catalytic
activity (Figure S21-S24). The optimal Ead values of various
graphene models are shown in Figure 5a. Ead values in B,N(p)-
codoped graphenes are lower than those in other surveyed
models, which mainly result from their highly preferred N-B
adsorption patterns. Among them, the lowest Ead of -1.21 eV
was observed in the model containing a B atom meta to a
positive NICS, which is consistent with the Ead value of the B,N-
codoped graphene models. Therefore, the strong anti-
aromaticity facilitates the adsorption of hydrazine, and thus
increases the catalytic activity in the nitrobenzene reduction.
On the basis of the experimental and theoretical
investigations,
a
plausible mechanism for nitrobenzene
reduction is proposed (Figure 5d). In the initial step, the
hydrazine molecules are adsorbed at B atoms meta to pyridinic
N atoms on the surface of carbon materials along with the
adsorption of nitrobenezene. The oxidation of hydrazine and the
reduction of nitrobenzene take place synchronously, producing
N2 and aniline product. It should be noted that the catalytic
performance is not directly related to the absolute amount of B
and N. The catalytic activity could not be enhanced by just
increasing the B and N contents unless the B-C-N active sites
are accessible to the substrates. Therefore, a homogeneous
distribution of active sites on the walls of the open nanopores is
highly desirable to achieve enhanced catalytic activities. In our
case, the B-C-N active sites, which inherit from the atomic-level
distribution of N and B atoms in H-COF, in conjunction with the
hierarchical hollow structure of BN-HCS lead to the superior
catalytic performance.
Table 1. Nitroarenes reduction using BN-HCS-800 as a catalyst .a
Entry
1 b
Substrate
Product
Conv.(%)
Sel.(%)
100
100
2 b
100
100
3 b
100
100
4
96
97
5
100
100
100
99
6
100
100
7 c
100
100
8
9 d
10
100
Figure 5. (a) Ead of hydrazine on N- and/or B-doped graphene models; (b)
optimized configuration of hydrazine adsorbed on B(m)N(p)-graphene;
Calculated NICSs (ppm) for the ring including both B and N atoms in B,N-
codoped graphene models (c); Schematic mechanism for nitrobenzene
reduction (d). The negative and positive NICSs values indicate aromaticity and
anti-aromaticity, respectively. B(o), B(m) and B(p) stand for B atom ortho,
meta, and para to N atom, respectively; N(p), N(g), N(pr) and G stand for
pyridinic N, graphitic N, pyrrolic N and graphene, respectively.
a Reaction conditions: BN-HCS-800 (10 mg), nitroarene (1 mmol), ethanol (1
mL), hydrazine hydrate (1 mL), 100 oC, 4 h, the products were detected by
GC. b 24 h; c 2 h. d without BN-HCS-800.
pyridinic N atom, namely B(m)N(p)-graphene (Figure 5b). We
found that the configurations of the B,N(p)-codoped graphene
were changed significantly by the adsorption of hydrazine via the
B-N bond, which suggests less aromaticity of heterocycles is
probably favorable for the hydrazine adsorption. Thus, the
aromaticity of the BN ring in B,N-codoped graphene models was
further estimated by the nucleus-independent chemical shifts
(NICSs).[31] All BN(g) rings are aromatic, while all BN(p) rings are
anti-aromaticity, suggesting that the existence of pyridinic N
decreases the aromaticity (Figure 5c). Clearly, B(m)N(p)-
graphene shows the strongest anti-aromaticity with the most
Conclusions
Hierarchical hollow carbon spheres have been constructed for
the first time from COFs. As effective metal-free catalysts, the
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where Etotal is the total energy of the system with adsorbed N2H4, Esubstrate
and EN2H4are the energies of the investigated substrates (graphene or
doped graphene clusters) and isolated hydrazine, respectively.
as-synthesized hollow carbon materials with the retention of
hollow spherical morphology and unique hierarchical structure
exhibit remarkable catalytic properties in reduction reaction of
nitroarenes. Based on experimental results and DFT
calculations, B atoms meta to pyridinic N atoms are proposed to
be the predominant catalytic active sites. This work has
demonstrated the synthesis of hierarchical hollow COFs spheres
and their feasible utility in the preparation of heteroatom-doped
porous carbons for metal-free catalysis. This offers a new
inspiration for the fabrication of COFs-derived carbon materials
with tunability and functionality for advanced catalytic
applications.
A further analysis of decomposition energy and interaction energy of
two fragments in various coordination complexes was carried out. The
energies of the hydrazine and B,N-codoped graphene models in the
coordination complexes were evaluated through single-point calculations.
These single point energies, together with the energy of the respective
fragments in their optimized geometry, allow for the estimation of the
deformation energies of the two fragments, Edef(N2H4) and Edef(Substrate)
.
Such single-point energies of the fragments and the electronic energy of
coordination complexes were used to estimate the interaction energy Eint
between two fragments.
Synthesis of H-COF: 1,3,5-Tris(4-aminophenyl)-benzene (100 mg, 0.28
mmol) and 4-formylphenylboronic acid ( 130 mg, 0.87 mmol) were
dissolved in solution of 1,4-dioxane/mesitylene (1/1 v/v, 10 mL) in a 100
mL thick walled pressure bottle. The bottle was tightened by the screw
cap and then sonicated for 1 min. The reaction mixture was heated at
120 °C for 3 d. The precipitate was isolated by filtration and washed with
anhydrous 1,4-dioxane. The resultant material was purified by Soxhlet
extraction using dichloromethane, dried in vacuo at 80 °C for 12 h to
afford H-COF as a yellowish powder. Yield: 105 mg (54%). Anal. calcd.
for (C15H10BNO)n: C, 77.97; H, 4.36; N, 6.06; found: C, 72.59; H, 5.07;
N, 5.75.
Experimental Section
Materials and methods: Chemical reagents were commercially
available and used without further purification. 1H and 13C NMR spectra
were recorded on a Bruker AVANCE III NMR spectrometer at 400 and
100 MHz, respectively, using tetramethylsilane (TMS) as an internal
standard. Solid-state 13C CP/MAS NMR was performed on a Bruker SB
Avance III 500 MHz spectrometer with a 4-mm double-resonance MAS
probe. FTIR spectra were recorded with KBr pellets using Perkin-Elmer
Instrument. Raman spectra were recorded on a Renishaw inVia system
by using 633 nm laser. Thermogravimetric analysis (TGA) was carried
out on NETZSCH STA 449C by heating samples from 30 to 900 oC in a
dynamic nitrogen atmosphere with a heating rate of 10 °C∙min-1. Powder
X-ray diffraction (XRD) patterns were recorded in the range of 2θ = 3-40o
on a desktop X-ray diffractometer (RIGAKU-Miniflex II) with Cu Kα
radiation (λ = 1.5406 Å). Nitrogen adsorption and desorption isotherms
were measured at 77 K using a Micromeritics ASAP 2020 system. The
samples were degassed at 150 oC for 10 h before the measurements.
Surface areas were calculated from the adsorption data using Brunauer-
Emmett-Teller (BET) equation. The calculation of the pore size
distribution was done using the quenched-solid density functional theory
(QSDFT) equilibrium model. Field-emission scanning electron
microscopy (SEM) was performed on a JEOL JSM-7500F operated at an
accelerating voltage of 3.0 kV. Transmission electron microscope (TEM)
was obtained with TECNAI G2 F20. X-ray photoelectron spectroscopy
(XPS) measurements were performed on a Thermo ESCALAB 250
spectrometer, using non-monochromatic Al Kα x-rays as the excitation
source and choosing C 1s (284.8 eV) as the reference line. Elemental
analyses were performed on an Elementar Vario MICRO Elemental
analyzer.
Synthesis of BN-HCS-700, BN-HCS-800 and BN-HCS-900: H-COF
(100 mg) was pyrolyzed at 700, 800 and 900 oC, respectively, with a
heating rate of 5 oC min-1 under N2 atmosphere in a ceramic boat for 2 h.
The resultant black material was treated with water at 70 oC overnight,
followed by washing with water and methanol, and dried at 80 oC in
vacuo for 6 h to afford BN-HCS-700, BN-HCS-800 and BN-HCS-900,
respectively.
Typical procedure for reduction of nitroarenes: Nitroarene (1 mmol),
hydrazine hydrate (1 mL), BN-HCS catalyst (10 mg) and ethanol (1 mL)
were placed in a Schlenk flask (20 mL), and the reaction mixture was
stirred at 100ꢀoC for a given time. After the reaction, the resultant mixture
was transferred into a tube and the solid was separated by centrifugation.
The organic phase was analyzed with GC to determine conversion and
selectivity. The identity of products was confirmed by comparison with
GC retention times of commercial materials and literature NMR
spectroscopic data.
Recyclability tests for reduction of nitrobenzene: After the first run
reaction was finished, the residual solid was recovered by centrifugation
and washed with ethanol, followed by centrifugation. The processes were
repeated twice, and the resultant solid was directly used for the next run
with the same amount of fresh nitrobenzene, hydrazine hydrate and
ethanol.
Computational Method: All calculations were carried out with Gaussian
09 program. The M06-2X/cc-pVDZ method was utilized for geometry
optimizations and vibrational frequency calculations. The energy of
hydrazine adsorption (Ead) on various graphene models, which is the
rate-determining step of the nitroarenes reduction reaction, was used as
the criteria to estimate catalytic activity of each catalyst. For pure and N-
doped graphene, hydrazine is adsorbed through the weak N-H···C
hydrogen bond, and their Ead values are less than -0.5 eV. Although
hydrazine may be also adsorbed by the additional N-B dative bond in B-
doped and B,N-codoped graphene, this adsorption pattern is less
preferred. Thus, their Ead values are close to those in pure and N-doped
graphenes. Ead is defined as:
Acknowledgements
The authors appreciate the financial assistance from the
National Natural Science Foundation of China (21403238,
21673224 and 21673241), the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB20000000
and XDB17010200), and Major Project of Fujian Province
(2014H0053).
Ead  Etotal (Esubstrate  EN H
)
4
2
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10.1002/cssc.201700979
ChemSusChem
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Hierarchical
organic frameworks ( H-COFs) have
been prepared and used as
hollow
covalent
Liuyi Li, Lu Li, Caiyan Cui, Hongjun Fan*
and Ruihu Wang*
a
Page No. – Page No.
precursor for the fabrication of
heteroatom-doped porous carbons.
Hierarchical Hollow Covalent Organic
Frameworks-derived Heteroatom-
doped Carbon Spheres for Metal-free
Catalysis
Originated
porosity and crystallinity in H-COF, the
as-synthesized carbons show
from
of
hierarchical
promising performances in metal-free
reduction of nitroarenes.
This article is protected by copyright. All rights reserved.