Nanoporous carbons derived from MOFs as metal-free catalysts for selective aerobic oxidations

Article abstract of DOI:10.1039/c6ta00324a

Metal-free nanocarbon catalysts have attracted much attention in recent years for their advantages of corrosion resistance, no heavy metal pollution and environmental friendliness as compared to metal-based catalysts. However, a facile preparation of heteroatom doped mesoporous carbon with a high catalytic efficiency is challenging. Here we report the development of a new kind of metal-free catalyst comprised of highly graphitized N-doped nanoporous carbons from direct carbonization of metal-organic frameworks. After metal etching, large specific surface areas and pore volumes, as well as high contents of sp²-bonded carbons are realized in the obtained carbons at the same time. As metal-free catalysts, these nitrogen-doped carbon materials exhibit excellent catalytic performances and robust stability in a series of oxidation reactions including aerobic oxidation of cyclohexane and toluene as well as oxidative coupling of amines. Systematic characterizations suggest that the accessible mesopores generated by chemical etching, and the homogeneous distribution of doped graphitic-type nitrogen should be responsible for the unprecedented performance of these carbon catalysts.

Full text of DOI:10.1039/c6ta00324a

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Nanoporous Carbons Derived from MOFs as Metal-Free Catalysts
for Selective Aerobic Oxidations
Received 00th January 20xx,
Accepted 00th January 20xx
Xi Wang, Yingwei Li*
DOI: 10.1039/x0xx00000x
Metal-free nanocarbon catalysts have attracted much attention in recent years for their advantages of corrosion
resistance, no heavy metal pollution and environmental friendliness as compared to metal-based catalysts. However, a
facile preparation of heteroatom doped mesoporous carbon with a high catalytic efficiency is challenging. Here we report
the development of a new kind of metal-free catalyst comprised of highly graphitized N-doped nanoporous carbons from
direct carbonization of metal-organic frameworks. After metal etching, large specific surface areas and pore volumes, as
www.rsc.org/
2
well as high contents of sp -bonded carbons are realized in the obtained carbons at the same time. As metal-free catalysts,
these nitrogen-doped carbon materials exhibit excellent catalytic performances and robust stability in a series of oxidation
reactions including aerobic oxidations of cyclohexane and toluene as well as oxidative coupling of amines. Systematic
characterizations suggest that the accessible mesopores generated by chemical etching, and the homogeneous
distribution of doped graphitic-type nitrogen should be responsible for the unprecedented performance of these carbon
catalysts.
materials are related to the high surface areas and pore
volumes, different hybridizations of nanocarbons that can
produce distinct electronic structures, and tunable electron
1
. Introduction
Selective oxidation of hydrocarbons in liquid phase to prepare
corresponding oxygen-containing derivatives, such as alcohols,
ketones, aldehydes, acids and esters, is a kind of extremely
important and challenging chemical reactions. Among these
transformations, catalytic oxidation of cyclohexane to
manufacture KA oil and adipic acid, which are pivotal
intermediates in the preparation of chemical products such as
features by doping with other elements (such as B, N, and
2
6,27
P).
Cao et al. reported that higher conversions were
2
1
-8
observed when sp carbons were used as catalysts, indicating
that sp carbons are more active than those of sp
2
3
hybridization. It can be attributed to their higher activity in
catalysing the decomposition of cyclohexyl hydroperoxide
28
9
intermediates. Further research demonstrated that activity
enhancement was achieved in heteroatoms-doped carbon
polyamide, nylon, drugs, polyester and resin, has long been of
interest to researchers. In the present cyclohexane oxidation
manufacturing process, strong oxidative HNO3 is involved,
causing serious equipment corrosion and emission of nitric
oxides. Cyclohexane oxidation through a non-HNO route
would be an attractive but challenging route. So far, the
reported catalytic systems are mostly based on metal catalysts,
27
2
nanotubes. The results suggest that heteroatom-doped sp
carbons may be promising catalysts for the oxidation of
cyclohexane in the liquid phase. Although few carbon-based
catalysts have been currently developed for the oxidation of
cyclohexane, limitations that hinder the practical application of
10
3
1
1-21 these catalysts are still remaining. For example, the
preparation of graphene and carbon nanotube suffers from
the drawback of great consumption of the volatile precursor
through tedious preparation (e.g., CVD). Treating the activated
carbon with reactive nitrogen-containing atmosphere yields
low nitrogen loadings. Besides, calcination of nitrogen-
containing polymers generally requires the use of hard
such as Au, Co, W, metalloporphyrin and molecular sieves.
Undoubtedly, the presence of metals incurs cost problems and
metal contaminants in the final products, particularly for the
pharmaceutical reagents. Therefore, the development of
highly selective and cost-effective heterogeneous metal-free
catalysts for the oxidation of cyclohexane is highly desirable.
Recently, carbon materials have shown great potential as
sustainable catalysts for oxidation of cyclohexane with
comparable activity and selectivity to conventional metal
templates (e.g. SiO ) or sophisticated procedures.
2
Currently, metal-organic frameworks (MOFs) built with
metal ions and oxygen or nitrogen-containing organic ligands
22-28
catalysts.
The outstanding catalytic behaviors of these
have gained particular attention as an emerging class of
porous materials, mainly because of their controllable
framework structures, ordered channels, and various
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China. E-mail: liyw@scut.edu.cn
29
topologies with the stable and regular pore structures.
†
Electronic Supplementary Information (ESI) available: Catalyst characterization
Motivated by these excellent properties and high carbon
and reaction results. See DOI: 10.1039/x0xx00000x
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contents in MOFs, an enormous amount of research has been under vacuum at 150 °C. The crystallographic date for ZIF-67,
View Article Online
DOI: 10.1039/C6TA00324A
focused on attempting to achieve metal-carbon composites by ZIF-8 and Co-MOF-71 were listed and compared in Table S1.
using MOFs as both sacrificial template and alternative carbon
Co/CN catalyst was synthesized by direct pyrolysis of ZIF-67.
30-40
precursor.
The pyrolysis of MOFs has been demonstrated In a typical preparation, 1 g of pre-synthesized ZIF-67 was
as a facile route to synthesize porous N-doped carbon pyrolyzed at a particular temperature for 10 h with a heating
materials with high nitrogen contents. Until now, several rate of 1 ºC/min under argon atmosphere. The prepared
30,33,34
35
36,37
typical MOFs, such as MOF-5,
Al-PCP, and ZIF-8,
catalyst was denoted as Co/CN-X, where X indicated the MOF
have been used as promising templates, yielding highly pyrolysis temperature. ZnO/CN-X and Co/C-X were prepared
microporous metal-carbon composites with excellent following the same procedures as above except using ZIF-8 and
performances in gas adsorption, electrochemical capacitance, Co-MOF-71 as sacrificed template, respectively. It is
sensing, etc. However, seldom attention was paid on liquid- noteworthy that metal ions with a reduction potential lower
phase catalysis probably due to the micropores of these MOF- than -0.27 V will produce metal oxide nanoparticles during
derived materials that would hinder mass-transfer of reactants thermolysis under inert atmosphere, whereas metal ions with
and products. To the best of our knowledge, there are a reduction potential of -0.27 V or higher present in MOFs
47
2+
currently no reports on the use of MOF-derived carbons for always form metallic nanoparticles. Therefore, Zn ions
heterogeneous catalysis. (with a reduction potential of -0.76 V) are unable to form the
As a continuation of our recent development of novel and respective metallic nanoparticles like Co ions (-0.27 V).
2+
efficient MOF-derived nanocomposites for liquid-phase
The as-synthesized Co/CN-X was immersed in aqua regia for
41-46
heterogeneous catalysis,
we report here, for the first time, 24 h to remove the Co component. Next, the sample was
that N-doped mesoporous carbon materials derived from isolated by centrifugation and washed several times with
pyrolysis-etching of ZIF-67 represent excellent catalysts for deionized water and absolute ethanol before drying under
aerobic catalytic oxidations. Through the adjustment of vacuum at 150 °C overnight. The whole procedure above was
calcination temperature, the sizes of mesopores surrounded repeated five times in order to ensure complete removal of Co
by highly graphitized carbon shells, which are created by component. The final sample obtained was denoted as 67-CN-
etching of the in-situ generated Co nanoparticles could be X, where 67 indicated the sample was originated from ZIF-67.
controlled. Moreover, these catalysts are easily separated and 8-CN-X originated from ZIF-8 and 71-C-X originated from Co-
can be conveniently reutilized, which would be promising MOF-71 were obtained following the same procedures.
materials to replace conventional metal catalysts for various
2.2.Materials characterization
catalytic oxidation reactions.
Powder X-ray diffraction patterns of the samples were
obtained on a Rigaku diffractometer (D/MAX-IIIA, 3 kW)
employing Cu Kα radiation (λ = 0.1543 nm) at 40 kV, 30 mA at
room temperature. Raman spectra were recorded on a
LabRAM Aramis Raman Spectrometer (HORIBA Jobin Yvon).
Elemental contents were measured using an Elementar Vario
EL III. The metal contents of the samples were determined
quantitatively by atomic absorption spectroscopy (AAS) on a
HITACHI Z-2300 instrument. N2 adsorption/desorption
isotherms were measured at 77 K with Micromeritics ASAP
2
. Experimental
All chemicals were purchased from commercial sources
(Sigma-Aldrich, Alfa Aesar, and others) and used without
further purification. All solvents were analytical grade and
distilled prior to use.
2.1.Materials preparation.
In a typical synthesis of ZIF-67, cobalt nitrate hexahydrate
2
020 instrument. Before the measurements, the samples were
degassed at 423 K for 12 h. X-Ray photoelectron spectroscopy
XPS) measurements were performed on a Kratos Axis Ultra
(
(
0.45 g, 1.55 mmol) was dissolved in 3 mL of deionized water
0.52 mol/L), and 2-methylimidazole (5.5 g, 67 mmol) was
4
2
(
dissolved in 20 mL of deionized water (3.35 mol/L). The two
solutions were mixed and stirred for 6 h at room temperature.
The resulting purple precipitates were collected by
centrifugation and filtration, washed with water and methanol
subsequently for 3 times, and finally dried under vacuum at
−
9
DLD system with a base pressure of 10 Torr. The size and
morphology of materials were investigated by high-resolution
scanning electron microscopy (HR-SEM, S-3700N of HITACHI)
equipped with an energy dispersive X-ray spectroscopy (EDS)
and high-resolution transmission electron microscopy (HR-TEM,
C/M300 of Philips) operated at 30 kV and 200 kV respectively.
8
0 °C for 24 h. ZIF-8 was synthesized following the same
procedure except using zinc nitrate hexahydrate instead of
cobalt nitrate hexahydrate. In the synthesis of Co-MOF-71, a
solid mixture of Co(NO ) ·6H O (0.4 g, 1.37 mmol) and 1,4-
2.3.Procedure for the oxidation of cyclohexane or toluene
3
2
2
The oxidation of cyclohexane was carried out in a Teflon-lined
stainless steel autoclave equipped with a pressure gauge and a
magnetic stirrer. Typically, a mixture of cyclohexane (1.17 g),
cyclohexanone (32.5 mg as initiator), catalyst (20 mg) and
solvent acetone (0.8 g) was introduced into the reactor at
room temperature. Air in the autoclave was purged several
benzenedicarboxylic acid (230 mg, 1.37 mmol) was dissolved in
a single binary solution of DMF (10 mL) and ethanol (2.5 mL).
The resulting solution was transferred into a Pyrex tube, frozen
2
9
in a liquid N bath, evacuated (200 mTorr), flame-sealed and
2
then heated at 100 °C for 15 h. The resulting pink/purple
crystals of Co-MOF-71 were collected by filtration, washed
with DMF subsequently for 3 times, and finally dried overnight
times with O . Then, the reaction began by starting the
2
2
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agitation (1000 r/min) when oxygen was regulated to 1.5 MPa
after the reaction temperature was reached. After reaction,
the reactor was cooled to 0 °C. The gas phase composition was
analyzed by a GC equipped with a TCD detector and a packed
column (carbon molecular sieve TDX-01), which indicated that
no gaseous product was produced in the reactions. The
products in the solution were quantified and identified by GC-
MS analysis (Shimadzu GCMS-QP5050A equipped with a 0.25
mm × 30 m DB-WAX capillary column).
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The oxidation of toluene was carried out following the same
procedure except with different starting materials. In the case
of toluene oxidation, toluene (2 mL) and catalyst (20 mg) were
loaded into the reactor. And then autoclave was flushed
several times with O , pressurized with O (1 MPa), and then
Figure 1. Schematic illustration of the synthesis procedure for N-doped carbons.
2
2
heated to 160 °C and held at this temperature for 16 h. The
products were analyzed on the GC-MS instrument with an
external standard of anisole.
For the recyclability test, the reactions were performed
maintaining the same reaction conditions as described above,
except using the recovered catalyst. At the end of catalytic
reaction, the mixture was centrifuged and the solid was
recovered, which was thoroughly washed with acetonitrile and
ethyl acetate, dried at 80 °C and then reused in a next run.
2
.4.Procedures for the Oxidative Coupling of Amines
In a typical procedure, catalyst (20 mg) was introduced into a
5 mL Schlenk tube. The tube was vacuumed and purged with
1
oxygen three times. Amines (5 mmol) were then added using a
syringe under the oxygen atmosphere. Finally the tube was
connected with an oxygen balloon. Subsequently, the tube was
stirred with a magnetic bar at the desired temperature. After
completion of the reaction, the solid catalyst was isolated by
centrifugation. The analyses of reaction products were
performed on GC and confirmed by GC-MS.
Figure 2. Powder XRD patterns of the porous N-doped carbons derived from ZIF-67.
Crystal structures of the obtained ZIF-67, Co-based materials
(Co/CN-X, where X indicates the MOF pyrolysis temperature)
and nanoporous carbons (67-CN-X) were characterized by
wide-angle X-ray diffraction (XRD). These carbon materials
exhibited similar characteristic diffractions with two broad
peaks at approximately 2θ = 24° and 44° (Figure 2),
respectively, corresponding to the carbon (002) and (101)
3
. Results and discussion
2
planes that are typical of graphitic carbons (i.e., sp -bonded
The facile two-step synthetic strategy for the preparation of N-
doped nanoporous carbons is illustrated in Figure 1. Typically,
ZIF-67 was used as sacrificial template and precursor to
prepare Co-based materials through thermal annealing at high
temperatures under a flow of argon without any complicated
steps or addition of any other carbon and nitrogen sources. In
the as-synthesized Co-carbon composites, highly dispersed
metallic Co nanoparticles were embedded in the N-doped
carbon. After that, the obtained black powder was washed
thoroughly with an acid solution to remove the formed cobalt
nanoparticles. Homogeneously distributed pores were in-situ
generated in the locations that ever belong to the Co
nanoparticles, resulting accessible N sites that could be utilized
for catalytic reactions. For comparison, carbons derived from a
MOF without cobalt (e.g., ZIF-8) and another MOF without
nitrogen (e.g., Co-MOF-71) were prepared as well. The
crystallographic structure and date for the ZIF-67, ZIF-8, and
Co-MOF-71 have been listed and compared in Table S1.
48, 49
carbon).
Similar observations could also be obtained in
carbons derived from Co-MOF-71 and ZIF-8 (Figure S1). The
empirical parameter (R) was used to evaluate the degree of
graphitization, which is defined as the ratio of height of the
50, 51
(002) peak to the background.
The graphitization degree of
6
7-CN-X increased with elevating the calcination temperature.
Additionally, as compared to the XRD patterns of Co/CN-600,
which denotes the product of ZIF-67 carbonized at 600 °C
without acid treatment, the peaks of cobalt disappeared in the
patterns of 67-CN-600 samples (Figure 4d), signifying cobalt
removal after acid treatment and the final products would be
metal-free carbon materials.
Raman spectra of the obtained porous carbons are shown in
-1
Figure 3. A typical D band roughly at 1353 cm arising from
2
defects on the graphitic lattice of the disordered sp -
hybridized carbon, in this case, caused by the incorporation of
-1
N heteroatoms and a G band at 1588 cm from the vibration
mode to the movement in opposite directions of two carbon
atoms in a single graphene sheet, indicating the presence of
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Figure 3. Raman spectra of the Co-based materials and porous N-doped carbons
derived from ZIF-67.
5
2-55
crystalline graphitic carbon,
could be observed for all the
samples. The relative ratios of G to D bands (I /I ) are
G
D
Figure 4. SEM images of ZIF-67 (a), Co/CN-600 (b) and 67-CN-600 (c), and the
important parameters to illustrate the degree of graphitization. corresponding XRD patterns (d). Inset of (d) shows the XRD pattern of 67-CN-600 in the
range of 10 - 40°.
The I /I ratio for 67-CN-600 was smaller than that of 67-CN-
G
D
7
00, 67-CN-800 and 67-CN-900, demonstrating fewer graphitic
crystallites and meanwhile more N-doping atoms at the edges
of the nanoporous carbon, which might be beneficial to
enhance the catalytic activity. Furthermore, a characteristic
broad and weak 2D band was observed at approximately 2700
-
1
cm for all samples except 67-CN-900, suggesting the
56-58
existence of few-layered graphene.
The appearance of
narrower and sharper 2D band in the 67-CN-900 indicated that
highly graphitic structures were well-developed probably due
to the catalytic effect of Co on the graphitization of carbon
under higher calcination temperatures. Compared with the Co-
based material Co/CN-600, peaks ascribed to vibration of Co
-
1
-1
atoms in the range of 100 cm to 1000 cm disappeared in all
7-CN-X samples, confirming again the complete removal of
6
cobalt by acid treatment. And this could be further verified by
the complete loss of magnetism in 67-CN-600 (Figure S2b).
The surface morphologies of the ZIF-67, Co-based materials
and nanoporous carbons were characterized by scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM). The pristine ZIF-67 clearly showed the
typical rhombic dodecahedral shape with particle sizes of
around 300 nm (Figure 4a). After decomposition and Figure 5. HRTEM images of Co/CN-600 (a, c), and the 67-CN-600 (b, d) after chemical
etching. Nanopores are formed (as shown by dotted red lines) due to the removal of
cobalt nanoparticles.
carbonization of the frameworks at 600 ºC, the obtained
Co/CN-600 exhibited similar particles but rougher surface due
to the formation of nanopores (Figure 4b). In addition, we also
was embedded in graphitic carbon tightly (Figure 5c). As the
MOF pyrolysis temperature increased, the Co nanoparticles
tended to aggregate gradually (Figure 6a, 6c and 6e). However,
after acid treatment, the Co nanoparticles were completely
removed from the carbon matrix (Figure 5b). Lots of
nanopores (as shown by dotted lines in Figure 5d) over the
entire particle with a well-developed graphitic structure were
in-situ generated from the space left by etching Co
nanoparticles. Similar observations could also be obtained in
carbons derived from Co-MOF-71 (Figure S3). In contrast, 8-
CN-600 synthesised from non-cobalt MOF was fabricated by
observed the formation of a small amount of carbon
nanotubes which might be catalyzed by the generated Co
5
9,60
nanoparticles.
Throughout the acid treatment, the yielded
6
7-CN-600 inherited the morphology of the Co/CN-600
without any change in the average particle size and rough
surface, indicating a good retention of the nanopores (Figure
4
c).
HR-TEM was applied in order to further observe the
differences in the microstructure between the Co/CN-X and
7-CN-X. Figure 5a shows that the metallic Co nanoparticles
were highly dispersed in the carbons and each Co nanoparticle
6
4
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Figure 6. HRTEM images of Co/CN-700 (a), Co/CN-800 (c), Co/CN-900 (e) and the
2
Figure 7. N adsorption/desorption isotherms (a, c, e, and g) and pore-size distribution
obtained 67-CN-700 (b), 67-CN-800 (d), 67-CN-900 (f) after chemical etching.
(b, d, f, and h) of the samples.
randomly assembling nano-sized carbon sheets (Figure S4d), all the obtained samples. Unlike the Co/CN-X materials,
which coincided with the Raman and XRD results (Figures S1 hysteresis between adsorption and desorption branches could
and S5).
be clearly observed at medium pressures for 67-CN-X samples,
The HRTEM image (Figure 5d) at the edge of a carbon demonstrating the generation of cage-type mesopores
nanoparticle of 67-CN-600 clearly showed the presence of randomly connected with small windows after etching. The
several graphene layers stacked in parallel with an adjacent almost vertical tails at the relative pressures close to 1.0 were
interlayer distance of approximately 0.34 nm, corresponding indicative of the presence of macroporosity. As can be seen
to the (002) crystal plane of graphitic carbons. More than 20 from the pore size distribution spanning from meso- to macro-
well-developed graphite layers were readily produced when pores, essentially no peaks were observed for all the Co/CN-X
the calcination temperature was increased to 900 °C (Figure samples in this range. However, 67-CN-X samples all possessed
6
f), which is consistent with the XRD and Raman meso- and macro-pores in the same range and most of them
characterization. Furthermore, as shown in Figure S6, the were located in the region of mesopores. For instance, 67-CN-
elemental mapping images on a single carbon particle 600 displayed a pore size distribution with a peak centering at
obtained by the high-angle annular dark field scanning TEM ca. 7.7 nm (Table 1) attributed to the removal of Co
(
HAADF-STEM) revealed that a large quantity of N atoms were nanoparticles, which was in agreement with the TEM results.
homogeneously incorporated into the carbon matrix. Similar observations could also be obtained in carbons derived
Meanwhile, the absence of cobalt signals indicated the acid from Co-MOF-71 (Figure S7b). Notably, neither mesopores nor
treatment by aqua regia could totally remove the Co cores.
macropores were present for 8-CN-600 synthesised from non-
Nitrogen adsorption-desorption isotherms at 77 K were cobalt MOF ZIF-8 (Figure S7d), which coincided with the TEM
measured to determine the surface areas and pore size observation (Figure S4). In addition, as shown in Figure 7, the
distributions of Co/CN-X and 67-CN-X materials (Figure 7 and pore size of the 67-CN materials increased with the ZIF-67
Table 1). The steep increase in N uptake at low relative pyrolysis temperature.
2
pressures indicated the presence of microporous structures for
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Table 1. Characterization results of the materials.
a
b
c
d
e
S
BET
2
S
meso
2
Pore volume
Mesopore volume
Mesopore size
Sample
-1
-1
3
-1
3 -1
(
m g )
330
362
352
398
295
337
249
283
364
376
341
395
(m g )
49
(cm g )
0.37
0.65
0.41
0.86
0.40
0.89
0.26
0.47
0.23
0.26
0.32
1.38
(cm g )
0.21
0.51
0.25
0.72
0.28
0.78
0.20
0.42
0.03
0.05
0.26
1.33
(nm)
Co/CN-600
7-CN-600
Co/CN-700
7-CN-700
Co/CN-800
7-CN-800
Co/CN-900
7-CN-900
ZnO/CN-600
-CN-600
Co/C-600
1-C-600
-
6
90
62
7.7
-
13.7
-
16.0
-
28.5
-
-
6
115
87
136
95
134
7
6
6
8
20
208
282
-
7
7.6
a
b
S
BET is calculated by the Brunauer-Emmett-Teller equation.
S
meso is the mesopore surface area obtained from the BJH cumulative specific adsorption area of pores of
c
d
1
.70-300.00 nm in diameter. Total pore volume is determined by using the adsorption branch of the N
2
isotherm at P/P
0
= 0.99. Mesopore volume is obtained from
e
the BJH cumulative specific adsorption volume of pores of 1.70-300.00 nm in diameter. Mesopore diameter is determined from the local maximum of the BJH
2
distribution of pore diameters obtained in the adsorption branch of the N isotherm.
2
The total surface areas, pore volumes, and mesopore the graphitic carbon plane and bonded with three sp carbon
surface areas and volumes were calculated and the results are atoms. The fitting results for the N 1s spectra of the as-
summarized in Table 1. Compared with the Co/CN-600 (330 prepared 8-CN-600 and 67-CN-X were combined with
2
-1
2 -1
m g ), 67-CN-600 (362 m g ) showed a slight increase in BET elemental analysis results and summarized in Figure 8b. The
surface area after acid treatment. Similar tendency was also pyridinic-N species was the dominant N-containing functional
observed in terms of mesopore surface area. The results group in the 8-CN-600 sample. However, the graphitic-N
indicated that the mesoporous structure contributed to the dominated the N species in the 67-CN-X samples, implying the
incremental pore volumes and surface areas of 67-CN-X catalytic effect of Co on the graphitization of carbon. The XPS
samples. For example, the ratio of Vmeso/Vtotal for Co/CN-600 results displayed the detail of nitrogen conversion: imidazole
increased from 0.57 to 0.78 after etching (Figure S8b). The group mainly converted into the graphitic-N and its percentage
variation was similar for the surface area, and the highest increased as the carbonation temperature enhanced. 2
3
value of Smeso/SBET was 0.47 for 67-CN-900 (Figure S8a). For 71-
The previous Raman results showed that typical sp and sp
CN-600, this observation was more obvious with a value of carbons were obtained (Figure 3). XPS was further used to
3
0
.71, probably due to the etching of larger cobalt particles.
The X-ray photoeletron spectroscopy (XPS) survey spectrum sp carbons and oxygen-containing functionalities. Figure S10
of Co/CN-600 (Figure S9) showed mainly the peaks of four shows the C 1s XPS spectra of the carbon materials. The
elaborate on the surface species, such as sp -type defects in
2
6
4
elements (i.e., C, N, O, and Co) present in the composite. Two spectra were deconvoluted into five major peaks. The peaks2
strong peaks at 793.5 and 778.7 eV could be assigned to Co at 284.6 eV and 285.2 eV corresponded to the C-C bonds of sp
61
3
2
p1/2 and Co 2p3/2 of metallic Co, respectively. However, after and sp carbon, respectively. The peak centered at 285.7 eV
2
etching, only the peaks assigned to C, N, and O elements were was assigned to the sp carbon containing nitrogen atoms (C=N
reserved. All the peaks assigned to Co disappeared completely, bond). The carbon atoms bonded to hydroxyl groups (-OH) and
demonstrating that there was no detectable cobalt (lower than carbonyl groups (-C=O) were indicated by peaks at 286.4 and
0
.1 at%, the detection limit of XPS) on the surfaces of the 288.5 eV, respectively. Furthermore, as the pyrolysis
carbons.
We further investigated the chemical composition and
temperature raised from 600 to 900 °C, the contents of carbon
nitrogen content in the samples by high-resolution XPS. As
shown in Figure 8b, the total nitrogen contents of the 67-CN-X
samples decreased distinctly with raising the pyrolysis
temperature. This decline indicated that the high temperature
facilitated the combination of nitrogen and oxygen atoms to
generate gaseous oxides during the carbonization process. The
N 1s broad peak could be deconvoluted into three peaks, i.e.,
pyridinic-N (398.6 eV), pyrrolic-N (400.7 eV) and graphitic-N
6
2, 63
(
401.6 eV), respectively (Figure 8a).
Pyridinic-N substituted
2
a carbon atom in the C ring and bonded with two sp carbon
6
Figure 8. XPS spectra of the N 1s narrow scan and nitrogen contents of the N-doped
samples.
atoms. Pyrrolic-N was in a five-membered ring and contributed
two electrons to the π system. Graphitic-N was located inside
6
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Table 2. Catalytic performances of carbon materials in the aerobic cyclohexane
oxidation.
to 900 °C, the reactivity and selectivity to adipic acid over the
resulting 67-CN-X materials decreased gradually. However, 8-
View Article Online
DOI: 10.1039/C6TA00324A
a
CN-600, which had comparable nitrogen content but different
nitrogen species comparing with 67-CN-600, also exhibited a
reduced activity (Table 2, entry 8).
The conversions of cyclohexane are plotted against the
amounts of the different nitrogen species on the materials
b
Selectivity (%)
b
(Figure S11). The amount of a certain nitrogen species in an N-
Entry
Catalyst
Conversion (%)
KA
64
36
95
30
27
29
32
38
39
—
AA
21
50
—
doped carbon sample was estimated by combination of the
surface concentration (mole fraction) of the nitrogen species2
measured by XPS and the BET surface area of the sample (m
1
2
3
4
5
6
7
8
9
—
6
8
71-C-600
Pd/C
−
1
g ). The conversion was found to increase almost linearly with
the amount of graphitic nitrogen species. This is in accordance
with the result of Watanabe et al. using nitrogen-doped
18
48
34
23
15
32
9
67-CN-600
67-CN-700
67-CN-800
67-CN-900
8-CN-600
71-C-900
Co/CN-600
67-CN-600
61
62
59
58
52
47
—
70
activated carbon catalysts for aerobic oxidation of alcohols.
Similar increasing trends could also be seen in the correlation
between the conversion and the amounts of pyridinic and
pyrrolic nitrogen species; however, the scatter was much
larger for pyridinic and pyrrolic nitrogen species than for
graphitic nitrogen species. The slight deviation of graphite-type
nitrogen for the relationship in Figure S11a suggests there
might be other active sites on the surface of N-doped carbon
samples. As mentioned in the introduction, Cao and co-
1
0
0.1
0.3
c
1
1
—
—
2
a
workers reported that sp carbons were more active than
Reaction conditions: cyclohexane (1.5 ml), acetone (0.8 g), cyclohexanone (32.5
b
3
mg), butanone (0.2 g), catalyst (20 mg), 1.5 MPa O
and corresponding selectivities were determined by GC-MS by using butanone as
an internal standard. 10% p-benzoquinone was added.
2
, 125 ºC, 8 h. Conversions those of sp hybridization in aerobic oxidation of
28
cyclohexane. Bielawski et al. first disclosed the remarkable
ability of graphene oxide with rich oxygen-containing
functionality to catalyse the oxidation of a variety of alcohols
to ketones/aldehydes, alkenes to diones, and alkynes to the
c
in the 67-CN-X samples increased from 78.7 to 95.8 wt% (Table
S2) along with the reduction of N or O atoms linked directly to
the C atoms. As a result, the ratio of sp -C/sp -C increased
from 2 to 18 and less defect sites derived from the restacking
of C atoms were originated, which can remarkably enhance
the graphitization degree.
The as-synthesized materials were employed as catalysts for
the aerobic cyclohexane oxidation. KA oil and AA were
produced as the main products under the investigated
conditions (Table 2). Some side-products, such as 6-
hydroxyhexanoic acid and ε-caprolactone, were also detected
in some cases but the quantities were very low. The blank
experiment without any catalyst gave a conversion of 6% with
a cyclohexanone/cyclohexanol (K/A) ratio of approximately 1:1
71
2
3
corresponding hydrates. In addition, Fujita et al. recently
indicated that pyridinic nitrogen species were active for the
72
aerobic oxidation of xanthene. Compared with the graphitic
nitrogen species, from the present results these carbon,
oxygen or pyridinic nitrogen surface species might catalyse the
aerobic oxidation as well but would be less active.
The comparison suggests that graphitic nitrogen might be
critical for the catalytic activity of the 67–CN materials in C-H
oxidation. This claim was further verified by the improved
activity of nitrogen-doped 67-CN-900 as compared to 71-C-900
(Table 2, entries 7 and 9), where the introduced nitrogen was
mainly at the graphitic sites (Figure 8). To eliminate the
influence of cobalt on the catalytic performance, a control
experiment was carefully conducted to rule out the
involvement of metal impurities. As shown in Table 2, there
was hardly no conversion over the Co/CN-600 (entry 10),
indicating that cobalt suppressed the reaction instead of
promoting it.
(
Table 2, entry 1), as a result of the autoxidation reaction,
agreeing with the non-catalytic mechanism of C6H12
oxidation.
65-67
A conversion of 8% (Table 2, entry 2) was
obtained over 71-C-600, which is comparable to that on
supported Pd catalyst (Table 2, entry 3). To our delight, the 67-
CN-600 material displayed remarkably improved catalytic
activity with a conversion of 48% as well as a greatly increased
selectivity (61%) to adipic acid (Table 2, entry 4). This would
highlight the effectiveness of nitrogen doping in tuning the
reactivity. To our knowledge, the activity of 67-CN-600 is
higher than that of most of the solid catalysts reported yet,
It is widely suggested that radical intermediates are involved
23
in the liquid-phase aerobic oxidation of cyclohexane.
Addition of radical scavenger (e.g., p-benzoquinone) could be a
way to test the possible involvement of radical species in our
system. With the presence of p-benzoquinone, the reaction
was almost fully suppressed to give a conversion as low as
11, 12, 15, 17, 68, 69
such as supported gold catalysts and zeolites.
∼
0.3% under the investigated conditions (Table 2, entry 11).
The activity and yield of the target product appeared to be
more in line with the nitrogen content (Table 2, entries 4-7).
When the MOF thermolysis temperature increased from 600
This result strongly demonstrated the involvement of radical
species in our reaction system. On the basis of the above
findings, a possible reaction pathway for the liquid-phase
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View Article Online
DOI: 10.1039/C6TA00324A
Figure 9. Schematic representation of the mechanism for cyclohexane aerobic oxidation over 67-CN-600. The simplified model is displayed by both top view and side view. The
gray, blue, red and green balls refer to carbon, nitrogen, oxygen and hydrogen atoms, respectively.
aerobic oxidation of cyclohexane into adipic acid over the 67- from autoxidation to heterogeneous oxidation, which was
CN-600 catalyst was proposed (Figure 9). CNTs and graphene even higher than supported Pd nanoparticles under similar
have been reported to be able to stabilize cyclohexoyl conditions (Table 3, entry 7). When O was replaced by N gas
2
2
hydroperoxide radicals, thereby increase the rate of the (Table 3, entry 3), only traces of toluene was transferred to
23, 25
autocatalytic reaction.
Therefore, the larger compensating benzaldehyde with a selectivity of >99% presumably due to the
positive charge of the ortho-carbon than the meta- and para- absorbed O on the surface of 67-CN-600.
2
−
carbons modified by the graphitic-type nitrogen dopant made
Surface-captured superoxide radical anions (•O ) would be
2
it a superior position for the adsorption of reactive oxygen involved in the reaction pathway, as previously reported in the
25
species such as peroxide. After the formation of the peroxide oxidation of ethylbenzene, although future work is highly
species (step 1), the ortho-carbon atom moved out of the required to reveal the reaction mechanism. The formation of
graphene plane and formed
a tetrahedral structure. such species could be further confirmed by the addition of 10
Subsequently, the as-formed peroxide species reacted with mol% p-benzoquinone (Table 3, entry 4), which served as a
adsorbed cyclohexane (CyH) to produce the intermediate radical scavenger in the reaction. As expected, the reaction
cyclohexyl hydroperoxide (CHHP) (step 2). The produced was almost fully suppressed under these conditions,
cyclohexyl hydroperoxide then underwent decomposition to demonstrating the involvement of radical species in our
form cyclohexanone and cyclohexanol (step 3). Finally, system. The effect of nanopores on suppressing the over-
cyclohexanone and cyclohexanol further reacted with peroxide oxidation of benzaldehyde and thus promoting its selectivity
species to produce adipic acid.
was better verified by testing the N-doped carbon catalysts
Importantly, the nitrogen-doped 67-CN-600 catalysts could
be easily recovered by filtration with only little compromise of
catalytic performances (Figure S12), indicating that the active
graphitic nitrogen were stable during the oxidative
transformation of cyclohexane under the investigated
conditions. Because of its high activity, excellent selectivity and
recyclability, 67-CN-600 showed great potential for efficient
cyclohexane conversion as well as C-H bonds activation.
To demonstrate the wide applicability of the MOF-derived
nitrogen doped carbons for heterogeneous oxidations, we
further investigated the catalytic performances of 67-CN-600
a
Table 3. Catalytic performances of carbon materials in the aerobic toluene oxidation.
b
Selectivity (%)
Conversion
Entry
Catalyst
b
(
%)
31
6
OH
C=O acid ester
1
2
—
29
21
43
-
7
-
67-CN-600
67-CN-600
67-CN-600
8-CN-600
71-C-600
Pd/C
-
-
-
-
-
-
>99
>99
>99
>99
>99
23
for the oxidation of toluene by using O as the only oxidant in
2
c
3
trace
trace
2
-
-
the absence of solvent. When O is used, the critical point of
2
this transformation is to promote selectivity of the catalyst to a
single product among all the possible oxidation products. In
the experiments, the reactor was connected with an oxygen
cylinder, guaranteeing continuous oxygen supply. Under this
condition, autoxidation of toluene into a mixture of benzyl
alcohol, benzaldehyde, benzoic acid, and benzyl benzoate
could take place without any catalyst at 160 °C (Table 3, entry
d
4
-
-
5
6
7
-
-
1
-
-
11
3
46
a
b
1
), while the selectivity was poor. Interestingly, 67-CN-600
2
Reaction conditions: toluene (2 mL), catalyst (20 mg), O (1 MPa), 160 °C, 16 h.
Conversions and selectivities were determined by GC-MS by using anisole as an
could suppress the autoxidation of toluene efficiently and gave
an extremely high selectivity (>99%) to benzaldehyde (Table 3,
entry 2) possibly due to the variation in oxidation mechanism
c
d
external standard. Reference experiments in the absence of O
2
.
10% p-
benzoquinone was added.
8
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a
Table 4. Oxidative coupling of amines to imines.
(
para>meta>ortho; Table 4, entries 2–4). Halogenated and
View Article Online
DOI: 10.1039/C6TA00324A
heteroatom-containing amines were also well tolerated in this
catalytic system (Table 4, entries 5-7). Remarkably, the system
was also capable of selectively catalysing the oxidative
coupling of aliphatic amines, achieving good yields to the
corresponding imines (Table 4, entries 8–9).
b
Yield
%)
Entry
Substrate
Product
(
1
2
3
4
5
6
7
>99
>99
91
4. Conclusions
In summary, we have demonstrated that MOF-derived
nanoporous carbon materials could be used as a new kind of
metal-free catalyst for heterogeneous catalytic oxidations, as
exemplified by the excellent activities, selectivities, and
substrate scopes achieved in the aerobic oxidation of
cyclohexane and toluene as well as oxidative coupling of
amines to imines. Moreover, the catalysts are highly stable and
could be recycled for a number of times without loss of
catalytic efficiency. The nanoporous N-doped carbon materials
are prepared by simple thermolysis of N-containing MOFs
without any additional templates or nitrogen sources. High
specific surface areas, large pore volumes and abundant
mesopores could be in-situ generated after chemical etching
of the metals. This three-dimensional nanoporous structure
provides a highly accessible surface area full of active sites for
catalytic applications. Detailed characterizations reveal that
pyrolysis temperatures influence the microstructures and
surface compositions of the materials, and the incorporated
82
92
92
78
c
8
80
2
graphitic-type nitrogen sites in sp -carbon layer are pivotal for
c
9
81
the investigated C-H bond activation reactions. The present
work might open a new avenue for the design of new carbon-
(1 based metal-free catalysts from MOF thermolysis and chemical
a
Reaction conditions: benzylamine (5 mmol), catalyst (67-CN-600, 20 mg), O
bar), 100 °C, 12 h. Determined by GC-MS with anisole as an external standard.
4 h.
2
b
c
etching for various heterogeneous catalytic applications.
2
with varied pore volumes. The less porous 8-CN-600 was not
effective enough to promote oxidation of toluene (Table 3,
entry 5), indicating that the generated pores enhanced the
activity by providing more accessible active sites. The excellent
performances of N-doped carbons are suggested to be related
Acknowledgements
We thank the National Natural Science Foundation 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 Guangdong Natural Science
Foundation (2013B090500027) for financial support.
2
to the ability of sp -carbon to host the reactive oxygen species
2
2,25,73
with the introduction of graphitic-nitrogen.
The more
nitrogen there was at graphitic sites in the catalyst, the higher
the catalytic activity, as we observed. This claim was further
verified by the reduced activity of 71-C-600, where no
graphitic nitrogen was introduced (Table 3, entry 6).
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA00324A
Graphical Abstract
A new kind of metal-free catalyst comprised of highly graphitized N-doped nanoporous carbons
from direct carbonization of metal-organic frameworks (MOFs) turn out to be excellent metal-free
catalysts for a series of liquid-phase oxidation reactions including aerobic oxidations of
cyclohexane and toluene as well as oxidative coupling of amines.