Metal-Organic Framework-Templated Porous Carbon for Highly Efficient Catalysis: The Critical Role of Pyrrolic Nitrogen Species

Article abstract of DOI:10.1002/chem.201504867

Metal-free catalysts are of great importance and alternative candidates to conventional metal-based catalysts for many reactions. Herein, several types of metal-organic frameworks have been exploited as templates/precursors to afford porous carbon materials with various nitrogen dopant forms and contents, degrees of graphitization, porosities, and surface areas. Amongst these materials, the PCN-224-templated porous carbon material optimized by pyrolysis at 700°C (denoted as PCN-224-700) is composed of amorphous carbon coated with well-defined graphene layers, offering a high surface area, hierarchical pores, and high nitrogen content (mainly, pyrrolic nitrogen species). Remarkably, as a metal-free catalyst, PCN-224-700 exhibits a low activation energy and superior activity to most metallic catalysts in the catalytic reduction of 4-nitrophenol to 4-aminophenol. Theoretical investigations suggest that the content and type of the nitrogen dopant play crucial roles in determining the catalytic performance and that the pyrrolic nitrogen species makes the dominant contribution to this activity, which explains the excellent efficiency of the PCN-224-700 catalyst well.

Full text of DOI:10.1002/chem.201504867

DOI: 10.1002/chem.201504867
Full Paper
&
Porous Materials |Very Important Paper|
Metal–Organic Framework-Templated Porous Carbon for Highly
Efficient Catalysis: The Critical Role of Pyrrolic Nitrogen Species
+
+
[a]
Gang Huang , Li Yang , Xiao Ma, Jun Jiang,* Shu-Hong Yu, and Hai-Long Jiang*
Abstract: Metal-free catalysts are of great importance and
alternative candidates to conventional metal-based catalysts
for many reactions. Herein, several types of metal–organic
frameworks have been exploited as templates/precursors to
afford porous carbon materials with various nitrogen dopant
forms and contents, degrees of graphitization, porosities,
and surface areas. Amongst these materials, the PCN-224-
templated porous carbon material optimized by pyrolysis at
content (mainly, pyrrolic nitrogen species). Remarkably, as
a metal-free catalyst, PCN-224-700 exhibits a low activation
energy and superior activity to most metallic catalysts in the
catalytic reduction of 4-nitrophenol to 4-aminophenol. Theo-
retical investigations suggest that the content and type of
the nitrogen dopant play crucial roles in determining the
catalytic performance and that the pyrrolic nitrogen species
makes the dominant contribution to this activity, which ex-
plains the excellent efficiency of the PCN-224-700 catalyst
well.
7
008C (denoted as PCN-224-700) is composed of amorphous
carbon coated with well-defined graphene layers, offering
a high surface area, hierarchical pores, and high nitrogen
Introduction
afford highly developed internal surface areas and large
porosities, which facilitate the adsorption and concentration of
catalytic substrates, minimize diffusion limitations, and
catalyst deactivation, thus greatly meriting the conversion.
On the other hand, the surface chemistry of the carbon ma-
terial plays a decisive role in its catalysis by offering active sites
that can chemisorb the reactants and provide surface inter-
mediates of sufficient strength. Doping with hetero atoms into
carbon materials has been demonstrated to be an effective
method to manipulate the surface chemistry, tailor the elec-
tronic properties, and make local changes to the elemental
Catalysis is an important and widely investigated subject both
in fundamental and industrial chemistry. The heart of catalytic
chemistry is centered on the design of catalysts, in which
metal, metal oxide, metal complexes, and so forth are un-
doubtedly the most vibrant for many paramount reactions in
modern chemistry. However, metal catalysts with limited re-
serves are normally expensive and often not stable enough. To
achieve green and sustainable chemistry, it is highly desirable
to develop metal-free catalysts. Carbon materials, as one class
of important metal-free catalyst, have recently sparked intense
interest and been extensively studied in many reactions, such
as the oxygen reduction reaction (ORR) in fuel cells and the ox-
[
2]
composition, thus improving the catalytic performance.
Among the diverse potential dopants in carbon materials, ni-
trogen atoms are considered to be an excellent choice due to
their comparable atomic size and five valence electrons, which
is beneficial for the formation of strong valence bonds with
carbon atoms. Many routes, such as electrical-joule heating in
an ammonia atmosphere, thermal treatment in an ammonia
atmosphere, hydrothermal reaction with urea, and so forth,
have been developed to dope nitrogen atoms into various
[
1]
idation and reduction reactions with various substrates.
Carbon materials, which possess different dimensionalities (0
to 3D) and mainly include fullerene and its derivatives, carbon
nanotubes, graphene (oxides), and porous carbon, are cheap,
stable, recyclable, biocompatible, environmentally friendly, and
readily available. Porous carbon materials in 3D structures
[
3]
carbon materials. Depending on the synthetic techniques, an
inhomogeneous distribution of nitrogen dopants and a subsur-
face region of nitrogen doping at a very limited depth are
common, whereas the highly uniform nitrogen doping
+
+
[
a] G. Huang, L. Yang, X. Ma, Prof. Dr. J. Jiang, Prof. Dr. S.-H. Yu,
Prof. Dr. H.-L. Jiang
Hefei National Laboratory for Physical Sciences at the Microscale
CAS Key Laboratory of Soft Matter Chemistry
Collaborative Innovation Center of Suzhou Nano Science and Technology
School of Chemistry and Materials Science
University of Science and Technology of China
Hefei, Anhui 230026 (P.R. China)
throughout the whole materials remains
challenge.
a
significant
To obtain porous carbon materials with a homogeneous
distribution of nitrogen atoms, metal–organic frameworks
E-mail: jianglab@ustc.edu.cn
[
4]
(
MOFs) could be ideal templates and precursors. As a class of
jiangj1@ustc.edu.cn
+
crystalline porous material, MOFs consisting of metal ions/clus-
ters and rigid organic linkers have been demonstrated to be
[
] These authors contributed equally to this work.
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201504867.
[
5–8]
functional materials for a wide variety of applications.
Re-
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cently, MOFs have been exploited to prepare porous carbon
materials with exceptionally high specific surface areas by
means of pyrolysis at high temperature in an inert atmos-
[
9,10]
phere,
and the MOF-derived porous carbon materials have
been applied to energy-related applications, such as in super-
capacitors, the oxygen reduction reaction (ORR), lithium-ion
[9,10]
batteries, and so forth.
The periodic arrangement of differ-
ent atoms in highly ordered MOFs as precursors/templates
offers congenital conditions for homogeneous atomic distribu-
tion. Therefore, MOFs based on nitrogen-containing organic
ligands would enable the resultant nitrogen atoms to be well
distributed throughout the porous carbon matrix upon
pyrolysis. In addition, the resultant nitrogen percentages are
readily tunable by controlling the nitrogen content of the
ligands/MOFs or the subsequent pyrolysis temperatures.
Bearing this approach in mind, three representative MOFs
(
i.e., MOF-5 without nitrogen atoms, ZIF-8, and PCN-224 con-
Scheme 1. Schematic illustration for the synthesis of nitrogen-doped porous
carbon materials by the pyrolysis of different MOFs.
[11]
taining different types of nitrogen species) were selected as
hard templates and precursors to afford porous carbon materi-
als, namely, MOF-5-T, ZIF-8-T, and PCN-224-T (T=pyrolysis tem-
perature), upon pyrolysis. The content and form of the nitro-
gen dopant involved in the resultant porous carbon materials
and their surface areas are dependent on the MOF templates
and pyrolysis temperatures. The porous carbon material ob-
tained by pyrolysis of PCN-224 at 7008C exhibits excellent cat-
alytic performances and an even lower apparent activation
energy than most metallic catalysts in the hydrogenation of 4-
nitrophenol (4-NP). To elucidate the mechanism behind the
high catalytic efficiency of PCN-224-700, first-principles simula-
tions at the density functional theory (DFT) level were per-
formed. The charge polarization effect, molecule adsorption,
and electron migration abilities of three model structures of
nitrogen-doped graphene (NG) were examined. It is revealed
that the accumulation of a high positive charge density by the
pyrrolic nitrogen structure in PCN-224-700 results in a strong
metal-like property, which greatly enhances the catalytic ability.
This result is in good agreement with the highest pyrrolic
nitrogen content and best catalytic activity of PCN-224-700 of
all the investigated porous carbon-based catalysts.
peak in the ZIF-8- and PCN-224-templated samples shifts
toward a comparatively sharp peak at 268, in stark contrast to
the broad peak at approximately 238 of the MOF-5-templated
carbon matrix. This outome suggests an improved degree of
graphitization in the former carbon materials. The result is fur-
ther supported by Raman spectroscopic analysis. The degree
of graphitization of carbon (R) can be evaluated by the ratio of
integrated intensity of the D and G bands ( n˜ =1360 and
À1
1580 cm , respectively; R=I /I ) As expected, PCN-224-700
D
G
has the lowest R value, which indicates a higher degree of
graphitization relative to the other carbon materials (Fig-
[
12]
ure 1a). Nitrogen sorption experiments were performed to
examine the pore textures and surface areas of the MOF-de-
rived porous carbon materials at different pyrolysis tempera-
tures (see Figure 1b–d and Table S1 in the Supporting Informa-
tion). Amongst these materials, MOF-5-1000 has the highest
2
À1
BET surface area of 2084 m g . The BET surface areas of ZIF-8-
2
À1
1000 and PCN-224-700 are 1289 and 1089 m g , respectively,
thus representing the highest values among their respective
MOF-derived carbon materials. In addition, pore size distribu-
tion analysis indicates that the carbon materials derived from
ZIF-8 are almost microporous, whereas both MOF-5- and PCN-
Results and Discussion
2
24-templated carbon materials generally possess hierarchical
Synthesis and characterization
pores (see Figure S3 in the Supporting Information), which, in
addition to the large surface area and pore volume, would be
beneficial because of the exposure of active sites and rapid
transportation of the catalytic substrates/products.
The three representative MOFs, that is, MOF-5, ZIF-8, and PCN-
[11]
2
24, were prepared based on previous reports, and their
purity was shown by using powder X-ray diffraction (PXRD;
see Figure S1 in the Supporting Information). The MOFs under-
Scanning electron microscopy (SEM) for the pyrolysis prod-
ucts MOF-5-1000, ZIF-8-1000, and PCN-224-700 shows that the
morphology of the original MOFs can be mostly retained, al-
though the sizes shrink to some extent after pyrolysis (see Fig-
ure S4 in the Supporting Information). Given the absence of ni-
trogen atoms in pristine MOF-5, the pyrolysis product should
be based on carbon only. In comparison, elemental analysis
shows that the nitrogen contents of ZIF-8-1000 and PCN-224-
700 are 4.41 and 3.08 wt%, respectively, in agreement with the
high nitrogen contents of their parent MOFs (the theoretical
went pyrolysis at different temperatures in a N atmosphere,
2
followed by acid etching and washing to remove the residual
metal (oxide) to give porous carbon materials (Scheme 1),
which are denoted as MOF-5-T, ZIF-8-T, and PCN-224-T,
respectively.
The PXRD patterns for the MOF-templated carbon materials
exhibit two peaks at around 25 and 448 indexed to the charac-
teristic (002) and (101) diffraction patterns of carbon (see Fig-
ure S2 in the Supporting Information). The considerable (002)
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Figure 2. The a) TEM and b) HRTEM images of PCN-224-700. c,d) Elemental
mapping images of the carbon and nitrogen atoms, respectively, in PCN-
Figure 1. a) Raman spectra of MOF-5-1000, ZIF-8-1000, and PCN-224-700. Ni-
trogen adsorption and desorption isotherms for b) MOF-5-T, c) ZIF-8-T, and
d) PCN-224-T (T=600, 700, 800, 900, and 10008C). The high-resolution N 1s
XPS spectra for e) ZIF-8-1000 and f) PCN-224-700.
224-700.
Catalytic activity evaluation
nitrogen contents of ZIF-8 and PCN-224 are 24.40 and
The above results demonstrate the well-defined porous struc-
ture, high surface area, and homogeneous nitrogen doping for
the MOF-templated carbon materials, which thus encouraged
us to investigate the catalytic reduction of 4-nitrophenol
(4-NP; one of the most universal organic pollutants in waste
waters from agricultural and industrial sources) to 4-aminophe-
4
.10 wt%, respectively). High-resolution X-ray photoelectron
spectroscopy (XPS) for N 1s in ZIF-8-1000 and PCN-224-700
showed that there are three types of nitrogen species: pyridin-
ic, pyrrolic, and graphitic nitrogen (see Figure 1e,f and
Table S2 in the Supporting Information). Notably, the ratio of
diverse types of N species is different in these materials; that
is, the percentage of pyrrolic nitrogen atoms in ZIF-8-1000 is
much lower than in PCN-224-700 (30.48 vs. 45.45%, respective-
ly) possibly due to the fact that PCN-224 has a high pyrrole
content. In addition, no high-contrast particles can be ob-
served in the transmission electron microscopy (TEM) image
nol (4-AP) by NaBH in water under ambient conditions. The
4
reaction progress was observed by the naked eye (see Fig-
ure S7 in the Supporting Information), and the reduction kinet-
ics were recorded by using UV/Vis absorption spectroscopic
analysis, which showed a decreasing characteristic peak at l=
400 nm for the 4-NP ion and an increasing characteristic peak
at l=300 nm for the 4-AP ion. The conversion from a nitro
into an amino compound has great industrial value, and this
reduction reaction represents an important probe of the cata-
(
see Figure S5 in the Supporting Information), thus revealing
the complete removal of metal (oxide) species upon acid etch-
ing, which is in accordance with the results of the PXRD and
thermogravimetric (TG) analysis (see Figures S2 and S6, respec-
tively, in the Supporting Information). High-resolution TEM
images of PCN-224-700 clearly present an amorphous carbon
structure coated by well-defined graphene layers on the edge
of the rodlike structure (Figure 2b). The amorphous internal
carbon atoms are in agreement with the fair intensity of the
[
13]
lytic activity, which usually uses metallic catalysts. However,
very few metal-free catalysts are highly active for this
[
14]
reaction.
It is well known that the reaction does not proceed in the
absence of a catalyst. Upon the introduction of the MOF-tem-
plated catalyst, the reaction proceeds effectively. The MOF-5-T
(Tꢀ7008C) matrix has negligible activity, possibly due to the
relatively low surface area, low degree of graphitization, and
lack of nitrogen dopant in the catalyst. At higher pyrolysis tem-
peratures, the product can catalyze the conversion of 4-NP and
the catalytic activity increases with the pyrolysis temperature
(see Figure S8a in the Supporting Information), which could be
explained by the larger surface area, pore volume, and im-
proved degree of graphitization in the catalyst obtained at
higher temperatures. Similar to MOF-5-T, ZIF-8-T with a pyrolysis
(
002) peak in the PXRD pattern mentioned above, whereas the
graphene layer coatings could be crucial to the catalytic
behavior. Elemental mapping indicates that PCN-224-700 is
mainly composed of carbon and nitrogen species (Fig-
ure 2c,d), in which nitrogen atoms are dispersed uniformly
throughout the carbon matrix, thus demonstrating the niche
of the MOF precursor.
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[
17]
temperature lower than 7008C is almost inactive. Although the
resultant carbon materials ZIF-8-T show evident activity upon
pyrolysis at elevated temperatures, the reaction kinetics are
even significantly lower than for MOF-5-T prepared at the
same temperature (see Figure S8b in the Supporting Informa-
tion). This result can be attributed to the only microporosity
that exists in ZIF-8-T, which is detrimental to the transportation
of the catalytic substrate/product, thus slowing the catalytic re-
action kinetics. Unexpectedly and in contrast to the previous
two examples, the pyrolysis product of PCN-224 exhibits con-
siderable activity, even at 6008C, and PCN-224-700 possesses
the best activity among all the obtained carbon materials,
possibly due to the high pyrrolic nitrogen content, hierarchical
porosity, and good degree of graphitization (see Figure S8c in
the Supporting Information), as described above. The activities
of the PCN-224-templated products gradually decreases with
the pyrolysis temperatures, which could be dominated by the
surface area.
catalyst; and 4) the heteroatom involved in the structure. To
understand the effect of the degree of graphitization further,
the activities of commercial graphite and activated carbon ma-
terials were investigated in the catalytic reduction of 4-NP to
4-AP. The activated carbon material in the amorphous state is
almost inactive, although its surface area is very high
2
À1
(1396 m g ), whereas the nonporous graphite material shows
a fair catalytic behavior (see Figure 3a and Figure S9 in the
Supporting Information), possibly due to the delocalized
bonds that exist in the graphitized catalyst. For comparison,
graphene oxide (GO) was also investigated with respect to the
catalytic reaction, and its activity was higher than graphite but
lower than all the MOF-templated carbon materials (Figure 3a).
In addition, the heteroatom dopant, namely, the nitrogen
atom, can change the electron cloud density of the delocalized
bond and thus improve the catalytic activity.
In fact, these factors would synergistically affect the activity
of a given catalyst. For example, MOF-5-1000 possesses an
even better activity than ZIF-8-1000; although the former ma-
terial does not contain the nitrogen heteroatom as catalytic
active site, it has a higher surface area and pore volume and
contains hierarchical pores. The much higher activity of PCN-
224-700 relative to MOF-5-1000 could be mainly ascribed to
the absence of the nitrogen atom active sites in the latter cata-
lyst. However, the mechanism behind the lower activity of ZIF-
8-1000 relative to PCN-224-700 becomes rTHER complicated
because the degrees of graphitization and nitrogen contents
of these materials are similar, whereas the surface area of the
former is even higher. Given the significantly different forms of
nitrogen in these two catalysts, we assume that the nature of
the nitrogen species involved in the structure plays a critical
role in the activity.
Among the different MOF-templated carbon materials, it is
clear that PCN-224-700 has a much higher activity than MOF-5-
1
000 and ZIF-8-1000 (Figure 3a). As expected, the reaction
over PCN-224-700 presents pseudo-first-order kinetics and the
À3 À1
reaction rate constant is up to k=5.3”10 s , which, to the
best of our knowledge, is higher than all the reported metal-
[
14]
[15]
free catalysts and even superior to many metallic catalysts.
Given that the PCN-224-700 catalyst is prepared at high tem-
peratures, it is very stable and thus can be readily recycled and
reused. The three reaction runs displayed in Figure 3b show
almost the same catalytic activity curves, thus demonstrating
the perfect stability and recyclability of PCN-224-700. To calcu-
late the apparent activation energy (E ), the reaction rate con-
a
stant of the PCN-224-700 obtained in the range of 274–298 K
À1
was calculated to be 26.4 kJmol (Figure 3b inset), which is
[
15c,d,16]
lower than most of previously reported metallic catalysts.
Theoretical investigation
With such a low activation energy, PCN-224-700 gives an even
better performance than most metallic catalysts.
To elucidate the mechanism further behind the high catalytic
efficiency of PCN-224-700, especially the influence of the nitro-
gen form, nitrogen-doped graphene (NG) models were built to
investigate their structural and electronic properties before
and after interaction with the 4-NP molecule. Geometry opti-
mization calculations give three stable NG structures, which
consist of separate pyrrolic, pyridinic, and graphitic rings (Fig-
À
ure 4a). The adsorption of a 4-NP molecular ion on those
[
14b]
three NG systems through the hydroxy and nitro groups
À
were simulated, with the optimized NG@4-NP structures dis-
played in Figure 4b,c, respectively. The pyrrolic NG system in-
volves the bonding of a nitrogen atom to two carbon atoms
Figure 3. a) Catalytic conversion of 4-NP over activated carbon, graphite,
graphene oxide, MOF-5-1000, ZIF-8-1000, and PCN-224-700. b) The catalytic
conversion of 4-NP for three cycles over PCN-224-700 (inset: Arrhenius plots
of the rate constant of the reduction of 4-NP by PCN-224-700).
(
C1 and C2) and one hydrogen atom (H), the pyridinic NG
system has only two NÀC bonds (C1 and C2), and the graphitic
NG system contains three NÀC bonds (C1, C2, and C3).
Normally, carbon atoms in graphene hold negative or neu-
tral charges. However, when an nitrogen atom is doped, the
ortho-carbon and ortho-hydrogen atoms around that nitrogen
It is generally accepted that catalytic activity connected to
the structure of carbon-based porous catalysts is influenced by
the following factors: 1) high surface area of the catalyst, thus
enabling the active sites to be more accessible; 2) porosity and
pore sizes, as large pores merit the transportation of catalytic
substrates/products; 3) degree of graphitization of the carbon
[
18]
atom could accumulate positive charges. Thus, we examined
the charge distribution present. As expected, the nitrogen
atoms tend to accumulate negative charges to result in posi-
tive charges on those atoms around the nitrogen atom
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À
inevitably promotes the activation of the 4-NP ion because its
reduction to the 4-AP form is driven by positive charges.
Meanwhile, the pyridinic NG structure can donate more posi-
À
tive charges (0.18 e) to the 4-NP molecular ion through the
nitro-group adsorption (Table 2), whereas its adsorption energy
À
is still much lower than the pyrrolic NG@4-NP system.
One of the advantages of metallic catalysts is good electric
conductivity, which helps to deliver energetic charge to trigger
chemical reactions. It is well known that graphene has excel-
[
19]
lent electric conductivity, and our NG systems might also
maintain the ability to deliver charge to drive the reduction of
À
[20]
the 4-NP ion. We display the calculated partial density of
states (PDOS) for the different regions in the NG systems in
Figure 5. We defined region 2 as the part that contains the
Figure 4. a) The optimized structures of pyrrolic NG, pyridinic NG, and
À
graphitic NG systems. b,c) The adsorption structures of 4-NP molecular
ions on three NG systems through the hydroxy and nitro moieties, respec-
tively. The black, middle gray, light gray, and crossed circles stand for the N,
C, H, and O atoms, respectively.
Table 1. The charges of ortho-carbon, ortho-hydrogen, and nitrogen
[
a]
atoms of three NG systems.
Pyrrolic NG Charge [e] Pyridinic NG Charge [e] Graphitic NG Charge [e]
N
À0.56
0.17
0.17
N
À0.44
0.22
0.22
–
N
À0.31
0.22
0.21
C1
C2
H1
C1
C2
–
C1
C2
C3
0.46
0.23
[
a] The positive electron number is the unit of the charge.
(
Table 1). The strongest charge polarization occurs in the pyr-
rolic NG system, as reflected by the number of positive charges
on the C1, C2, and H atoms. It is known that positive charges
help to attract the terminal oxygen atoms (often negative
À
charged) of the 4-NP ion, thereby facilitating adsorption. This
behavior explains the fact that the contact distance between
À
the pyrrolic NG system with the 4-NP ion is significantly small-
er than the other two cases. Therefore, the pyrrolic NG struc-
ture, with the strongest positive polarization charges around
the nitrogen atom, exhibits the biggest adsorption energies
À
for the 4-NP ion in the hydroxy- or nitro-attaching situations
Figure 5. The computed partial density of states (PDOS) of different regions
in the a) pyrrolic NG, b) pyridinic NG, and c) graphitic NG systems. Region 2
contains the doped nitrogen species, whereas regions 1 and 3 are the non-
doped graphene parts aside region 2.
(
Table 2). Consequently, strong coupling of electronic struc-
À
tures was found in the pyrrolic NG@4-NP system. It can be
seen that the NG part donates a positive charge of approxi-
À
mately 0.10–0.13 e to the 4-NP ion (Table 2). This behavior
doped nitrogen species, whereas regions 1 and 3 are non-
doped with graphene parts aside region 2. Obviously, the
PDOS features of regions 1–3 in the pyrrolic NG system are
similar to each other and are all close to that of pure gra-
phene. This scenrio means that the pyrrolic NG system still
maintain the same electron transportation ability as graphene,
which is helpful in catalytic applications by efficiently deliver-
ing an abundant electron source. In contrast, the PDOS of re-
gions 1 and 3 are very different to region 2 in the pyridinic
and graphitic NG system, thus implying the breakdown of the
Table 2. The adsorption energies (Ead) and charges being donated to the
À
À
4
-NP ion by the NG system (DCharge) in the three NG@4-NP model
systems, in which the molecules are attached to the NG structure
through the hydroxy or nitro groups.
Structure
Hydroxy attachment
Nitro attachment
E
ad [eV]
DCharge [e]
E
ad [eV]
DCharge [e]
À
pyrrolic NG@4-NP
0.94
0.27
0.73
0.10
0.06
0.03
0.82
0.30
0.73
0.13
0.18
0.09
À
À
pyridinic NG@4-NP
graphitic NG@4-NP
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electron transportation channels from region 1 to 3. Therefore,
the pyrrolic NG structure achieves the advantages of metal cat-
alysts without including metallic atoms by offering adsorption
sites for further catalytic reactions and providing efficient chan-
nels to deliver energetic charge for these reactions. These re-
sults are consistent with previous reports that have shown that
the doped nitrogen atoms could induce a local charge density
product were obtained on a Japan Rigaku DMax-gA rotation anode
X-ray diffractometer equipped with graphite monochromatized
CuKa radiation (l=1.54 Š). TG analysis was performed on a TGA
Q5000 integration thermal analyzer at a heating rate of 108C in an
air atmosphere. UV/Vis spectra of the samples were recorded on
a UV/Vis spectrophotometer (TU-1810, Beijing Pgeneral, China).
The nitrogen content was measured on a VarioELIII elemental ana-
lyzer. Raman-scattering spectra were recorded on a Renishaw
[14b]
+
and create metal-like properties.
System 2000 spectrometer by using Ar for excitation (l=
5
14.5 nm). XPS measurements were performed on a Thermo ESCA-
LAB 250 high-performance electron spectrometer by using mono-
chromatized AlKa (hn=1486.7 eV) as the excitation source. Nitro-
gen sorption measurements were conducted on a Micromeritics
ASAP 2020 system at 77 K. Prior to the measurements, the carbon
materials were activated at 1208C in a dynamic vacuum for 24 h.
Electrochemical experiments were performed on a CHI 760E elec-
trochemical workstation in a 2m solution of H SO . Before the
Conclusion
Three different types of representative MOF materials, that is,
MOF-5, ZIF-8, and PCN-224, have been exploited as hard tem-
plates to obtain metal-free porous carbon materials. The resul-
tant carbon materials have exhibited high surface areas, micro-
porous/hierarchically porous structures, and various degree of
graphitizations; furthermore, these materials possess different
types of nitrogen species in the frameworks, depending on the
pyrolysis temperature and parent MOF structure. The experi-
mental results have clearly shown that the MOF-templated
carbon materials are very active toward the catalytic reduction
2
4
measurements, the capacitor cell was evacuateded for 30 min in
a 2m H SO electrolyte.
2
4
Preparation of MOFs
Synthesis of MOF-5: The synthesis of MOF-5 was based on a
[11b]
modified previous report.
Typically, terephthalic acid (0.51 g)
and triethylamine (0.85 mL) were dissolved in DMF (40 mL) and
Zn(OAc) ·2H O (1.7 g) was dissolved in DMF (50 mL). The two
solutions were mixed and vigorously stirred for 2.5 h at room tem-
perature. The resultant particles were collected by centrifugation
and immersed in DMF overnight. The precipitate was separated
of 4-NP to 4-AP in the presence of NaBH under ambient con-
4
2
2
ditions. The catalytic performance is highly dependent on the
structural character of the resultant porous carbon catalysts.
The PCN-224-700 matrix outperformed all the other counter-
parts and presents a great potential for industrial chemistry be-
cause of its excellent activity, stability, and recyclability. Theo-
retical investigations have suggested that the content and
type of the nitrogen dopants are important for the catalytic
conversion. Three types of nitrogen species, especially the pyr-
rolic nitrogen species, have shown a strong ability to absorb 4-
NP ions, activate the nitro groups, and deliver energetic charg-
es. Thus, is has been found that the pyrrolic nitrogen species
makes the greatest contribution to the high catalytic per-
formance, which is in consistent with the high pyrrolic nitro-
gen contents and excellent catalytic efficiency of the PCN-224-
and immersed in CHCl , which was exchanged twice in 2 days. The
3
resultant product was collected by centrifugation and dried under
vacuum overnight at room temperature.
Synthesis of ZIF-8: The synthesis of ZIF-8 was based on a modified
[11e]
previous report.
Typically, zinc nitrate hexahydrate (1.68 g) was
dissolved in methanol (80 mL) and 2-methylimidazole (3.70 g) was
dissolved in methanol (80 mL). The two solutions were mixed and
vigorously stirred for 24 h at room temperature. The resultant par-
ticles were collected by centrifugation, washed with methanol
(
3”), and dried under vacuum overnight at 808C.
Synthesis of PCN-224: The synthesis of PCN-224 was based on
[11f]
a modified previous report.
Typically, ZrCl4 (0.15 g), H TCPP
2
7
00 catalyst. These results shed light on the component and
(
0.05 g), and benzoic acid (1.75 g) in DMF (10 mL) were ultrasoni-
structure of doped carbon catalysts and the influence of these
factors on the activity. This study can pave the way to rational
design and the development of metal-free carbon materials for
highly efficient catalysis.
cally dissolved in a Teflon-lined stainless-steel autoclave (20 mL).
The autoclave was sealed and maintained at 1208C for 24 h. After
cooling to room temperature, the solid was collected by filtration,
washed with DMF (3”) and acetone (1”), and immersed in
acetone for over 12 h. The mixture was centrifuged and dried
under vacuum overnight.
Experimental Section
Synthesis of porous carbon materials
Materials and instrumentation
Synthesis of MOF-5-T: Typically, MOF-5 powder (0.50 g) was trans-
ferred into a tube furnace. The furnace was heated at 600, 700,
800, 900, and 10008C, respectively, with a heating rate of
All chemicals were obtained from commercial sources and used
without further purification. Tetrakis(4-carboxyphenyl)porphyrin
À1
À1
(
H TCPP) was prepared according to the procedures described in
2
58Cmin in a nitrogen-gas flow (flow rate: ꢁ60 mLmin ). The
furnace was held at this temperature for 1 h and allowed to cool
to room temperature to yield a black powder. The obtained
powder was washed with an aqueous solution of HCl (3”, except
for the powder pyrolyzed at 900 and 10008C, in which the gener-
ated ZnO was reduced by carbon to give evaporative Zn at such
the Supporting Information. Distilled water with a specific resist-
À1
ance of 18.25 MWcm was obtained by using reversed osmosis
followed by ion-exchange and filtration (Cleaned Water Treatment
Co., Ltd., Hefei, China). SEM was performed on a Zeiss Supra 40 ap-
paratus at an acceleration voltage of 5 kV. TEM was performed on
a Hitachi H-7650 transmission electron microscope at an accelera-
tion voltage of 120 kV. High-resolution TEM and elemental map-
ping were performed on a JEOL-2100F transmission electron micro-
scope at an acceleration voltage of 200 kV. PXRD patterns of the
[9a,b]
a high temperature)
to remove the Zn and/or ZnO species. The
black powder was collected by centrifugation, washed with dis-
tilled water (3”) and ethanol (3”), and dried under vacuum at
1208C.
Chem. Eur. J. 2016, 22, 3470 – 3477
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Synthesis of ZIF-8-T: Typically, ZIF-8 powder (0.50 g) was trans-
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Received: December 3, 2015
Published online on February 3, 2016
1
Chem. Eur. J. 2016, 22, 3470 – 3477
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim