Efficient and selective aerobic oxidation of alcohols catalysed by MOF-derived Co catalysts

Article abstract of DOI:10.1039/c5gc02082d

A simple and highly efficient nanostructured catalyst system comprising magnetic Co nanoparticles stabilized by N-doped carbon composite (Co/C-N) was synthesized by one-pot thermal decomposition of a Co-containing MOF. The catalysts were characterized by temperature programmed desorption (TPD), N₂ physical adsorption, powder X-ray diffraction (PXRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electronic microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The catalytic activity of Co/C-N materials was investigated in the selective aerobic oxidation of alcohols in neat water under an atmospheric pressure of air and base-free conditions. As compared to those prepared by traditional impregnation methods, Co/C-N exhibited an efficient performance with significantly improved catalytic activities. Besides conferring high activity and selectivity to the target products, the proposed catalytic system featured a broad substrate scope for both aryl and alkyl alcohols. Furthermore, the magnetically recoverable Co/C-N catalyst could be easily separated from the reaction system by using an external magnetic field and reused at least five times without any significant loss in catalytic efficiency under the investigated conditions.

Full text of DOI:10.1039/c5gc02082d

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Efficient and selective aerobic oxidation of
alcohols catalysed by MOF-derived Co catalysts†
Cite this: DOI: 10.1039/c5gc02082d
Cuihua Bai, Aiqin Li, Xianfang Yao, Hongli Liu and Yingwei Li*
A simple and highly efficient nanostructured catalyst system comprising magnetic Co nanoparticles stabil-
ized by N-doped carbon composite (Co/C–N) was synthesized by one-pot thermal decomposition of a
Co-containing MOF. The catalysts were characterized by temperature programmed desorption (TPD), N
2
physical adsorption, powder X-ray diffraction (PXRD), Raman spectroscopy, transmission electron
microscopy (TEM), scanning electronic microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).
The catalytic activity of Co/C–N materials was investigated in the selective aerobic oxidation of alcohols
in neat water under an atmospheric pressure of air and base-free conditions. As compared to those pre-
pared by traditional impregnation methods, Co/C–N exhibited an efficient performance with significantly
improved catalytic activities. Besides conferring high activity and selectivity to the target products, the
proposed catalytic system featured a broad substrate scope for both aryl and alkyl alcohols. Furthermore,
the magnetically recoverable Co/C–N catalyst could be easily separated from the reaction system by
using an external magnetic field and reused at least five times without any significant loss in catalytic
efficiency under the investigated conditions.
Received 3rd September 2015,
Accepted 21st October 2015
DOI: 10.1039/c5gc02082d
www.rsc.org/greenchem
free conditions using molecular oxygen (ideally air) as the
oxidant and green media (e.g., water) as the solvent.
Introduction
Selective oxidation of alcohols to carbonyl compounds is one
Containing a magnetic element in heterogeneous catalysts
of the most important transformations in organic chemistry, is additionally desirable owing to their facile and efficient
1
5
and a variety of efficient methodologies have been developed magnetic separation after reactions. Up to now, although a
1
–3
4
to date. In many cases, noble metal catalysts such as gold,
few magnetically heterogeneous catalysts have been developed
5
6
7
palladium, platinum, and ruthenium show excellent activi- for the synthesis of carbonyl compounds from alcohol oxi-
ties and selectivities for this chemical process. However, dation, they suffer from inherent problems such as the require-
because of the high price and limited reserves of noble metals, ment of base additives, and the uses of alkyl hydroperoxide as
1
3,14b,16
it is urgent to reduce their usage. Therefore, some non-noble terminal oxidants and volatile organic solvents.
The
8
9
10
11
12
transitional metals, e.g., Co, Mn, Fe, V, and Cu, have development of a cost-effective and magnetic catalyst for the
been explored for the oxidation of alcohols, among which the selective oxidation of alcohols in an environmentally benign
development of efficient and recyclable heterogeneous cata- solvent such as water employing air as the oxidant still
1
b
lysts has received wide interest. Nevertheless, when using remains a great challenge.
non-noble metals as heterogeneous catalysts, their efficiencies
Herein, we report a novel, highly efficient, and reusable Co-
are generally lower than the noble catalysts in terms of activity based heterogeneous catalyst for the synthesis of carbonyl
13
and substrate scope. Consequently, a large excess of base compounds from the aerobic oxidation of alcohols. To the best
additives or explosive oxidizing agents (e.g., peroxy acids, and of our knowledge, this work represents the first example of a
metal oxidants) are employed to promote the activity and highly active magnetic catalyst for oxidation of alcohols in neat
1
4
selectivity of this transformation. In terms of economy and water under an atmospheric pressure of air and base-free con-
environmental friendliness, it is highly desirable to catalyze ditions. The as-synthesized Co/C–N composites exhibit an out-
alcohol oxidation by reusable non-noble catalysts under base- standing performance in the aerobic oxidation of alcohols,
with significantly improved catalytic activities as compared to
those prepared by traditional impregnation methods. Besides
conferring high activity and selectivity to the target products,
the proposed catalytic system features a broad substrate scope
for both aryl and alkyl alcohols. Furthermore, the catalyst
State Key Laboratory of Pulp and Paper Engineering, School of Chemistry and
Chemical Engineering, South China University of Technology, Guangzhou 510640,
China. E-mail: liyw@scut.edu.cn
†
Electronic supplementary information (ESI) available: Additional characteriz-
ation and reaction results. See DOI: 10.1039/c5gc02082d
could be easily recycled by using an external magnetic field.
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The Co-based catalyst was prepared simply by one-pot
Alcohol oxidations were performed under base-free con-
thermal decomposition of cobalt-containing metal–organic ditions using an atmospheric pressure of air as the oxidant.
frameworks (MOFs). MOFs are known as a new class of The reaction results are summarized in Table 2. First, the as-
porous materials comprising metal ions and organic linkers, synthesized Co/C–N materials were examined as catalysts for
which have exhibited a number of potential applications the aerobic oxidation of 1-phenylethanol in toluene at 100 °C.
in many fields such as gas storage/separation, catalysis, Under the conditions, although the conversions were some-
1
7–21
sensing, and biomedicine.
More recently, MOFs have what low, it was noteworthy that the ketone selectivities
been considered as alternative precursors to construct new achieved by the Co/C–N composites were all 100% (Table 2,
metal oxides or carbon nanomaterials due to their orderly entries 1–5). Among the five Co/C–N materials, Co/C–N700,
2
2
structures, high porosities, and isolated metal nodes.
i.e., the material prepared at 700 °C, was the most active for
Porous materials with high surface areas and porosities that 1-phenylethanol oxidation (Table 2 entry 3). Using the Co/
could be utilized in many practical applications (i.e., hetero- C–N700 catalyst, different solvents were tested, and water was
geneous catalysis, electrochemistry, gas adsorption, and
2
3–26
sensors) have been successfully derived from MOFs.
However, the reports on the employment of such MOF-
derived materials for liquid-phase catalytic oxidation reac-
tions are scarce.
Table 2 Optimization of reaction conditions for oxidation of 1-phenyl-
ethanol to acetophenone
2
3c,27
Results and discussion
a
a
[
Co
9
(btc)
6
(tpt)
2
(H
2
O)15]·solvent (MOF 1) (btc = 1,3,5-benzene- Entry
2,4,6-tris(4-pyridyl)-1,3,5-triazine) was
Catalyst
Solvent
Conv. (%)
Yield (%)
tricarboxylate, tpt
=
1
2
3
4
5
6
7
8
9
1
Co/C–N500
Co/C–N600
Co/C–N700
Co/C–N800
Co/C–N900
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Co/C–N700
Toluene
Toluene
Toluene
Toluene
Toluene
Acetonitrile
DMF
DMA
Dimethyl benzene
H O
2
DMSO
n-Heptane
n-Hexane
Acetone
Ethanol
10
12
43
24
19
28
39
37
26
65
<5
41
24
30
52
84
98
68
8
10
12
43
24
19
28
39
37
26
65
<5
41
24
30
52
84
98
68
<5
selected as the MOF precursor (Fig. S1†) due to the mixed
rigid triangular ligands in the MOF, highly symmetrical struc-
2
8
ture, and good thermal stability. The organic linkers in the
MOF structure would be carbonized gradually following the
slow heating procedure, and the resulting carbon composite
could prevent serious aggregation of Co to facilitate the syn-
thesis of monodisperse nanostructured porous Co/C–N
0
materials. To disclose the effect of the N element on the pro- 11
3
−
2−
12
perties of the derived nanomaterials, [Co
3
(H
2 6
O) (btc )(btc )-
1
1
3
4
1
−
(
btc )]·2H O (MOF 2) was also selected as the precursor to
2
2
9
prepare the N-free Co/C materials (Fig. S2†). Considering 15
b
1
1
1
6
7
8
2
H O
2
H O
2
H O
that MOF 1 begins to decompose when the temperature
increases to ca. 400 °C (Fig. S3†), the applied calcination temp-
c
c,d
c,e
2
8
eratures are varied from 500 to 900 °C. The as-synthesized 19
2
H O
materials are denoted as Co/C–Nx, where x represents the MOF
pyrolysis temperature. The Co contents in the Co/C–Nx
materials were about 30–45 wt%, while 53.2% of Co was found determined by GC-MS. 48 h. 110 °C, 48 h. 5 mol% Co. Filled with
in the Co/C700 (Table 1).
Reaction conditions: 1a (0.5 mmol), catalyst (10 mol% Co), solvent
a
(
2 mL), air (1 bar), 100 °C, 24 h. Conversion and yield were
b
c
d
e
2
N (1 bar).
Table 1 Properties of Co-based materials
Content (wt%)
Particle size (nm)
S
BET
Pore volume
Pore size
(Å)
Sample
(m g−1)
2
(cm g
3
−1
)
C
a
N
a
H
a
Co
b
XRD
TEM
1
10
241
256
159
180
165
4
0.01
0.07
0.12
0.13
0.20
0.11
0.01
0.05
8.9
5.0
4.9
6.2
6.4
6.1
—
42.0
64.5
61.2
54.4
53.6
52.8
34.0
32.4
7.2
1.8
1.5
1.2
1.0
1.0
—
2.4
0.8
0.7
0.7
0.6
0.6
3.0
0.5
19.8
31.0
35.9
42.7
44.2
45.2
18.3
53.2
—
—
5.4
6.6
9.8
10.3
35.8
—
Co/C–N500
Co/C–N600
Co/C–N700
Co/C–N800
Co/C–N900
2
5.5
6.4
9.6
10.2
18.1
—
Co/C700
34
8.8
—
30.2
95.6
a
b
Measured by elemental analysis. Measured by AAS.
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shown to be the best one among the solvents investigated was also pyrolyzed at 700 °C to afford Co/C700. Interestingly,
Table 2, entry 10). the Co/C700 catalyst showed a much lower activity and pro-
Inspired by these intriguing results, we further optimized duced acetophenone in only 34% yield (Table 3, entry 10)
(
the oxidation reaction conditions. When the reaction time was under identical conditions, demonstrating the importance of
prolonged to 48 h, acetophenone could be obtained in 84% N-doping in the composite for the oxidation reaction. These
yield (Table 2, entry 16). A slight increase of temperature to control experiments suggested that the synergic interactions
1
10 °C led to an almost quantitative transformation (Table 2, between the C–N composite and Co played an important role
entry 17). Note that the reaction also proceeded smoothly in determining the high activity of Co/C–N700 in the oxidation
when the Co usage was decreased to 5 mol% (Table 2, entry of alcohols.
1
8). However, it was evident that the oxidant (i.e., air) was
To elucidate reasons for the remarkable catalytic activity of
crucial for this oxidation process (Table 2, entry 19). Thus, the Co/C–N materials in aerobic alcohol oxidation, we exam-
after screening the reaction parameters, the optimized reaction ined the composites with a series of characterization tech-
conditions could be obtained as follows: 10 mol% Co/C–N700 niques. Basicity is reported to play a crucial role in promoting
as the catalyst, H O as the solvent, air as the oxidant, and at the activity of liquid phase oxidation of alcohols with mole-
2
1
4c,30
1
10 °C for 48 h.
cular oxygen.
Here, the basic properties of Co/C–N700 and
-TPD (Fig. 1). In general, the
A series of carbon- or Co-based materials were prepared as Co/C700 were investigated by CO
2
reference catalysts to verify the unique catalytic characteristics strength of basic sites may be determined from CO2 deso-
of the Co/C–N composites in aerobic oxidation. It can be seen rption temperatures, i.e., the higher the desorption tempera-
that in the absence of any catalyst, the reaction hardly occurred ture, the stronger the basicity. A few desorption peaks were
(
Table 3, entry 1). Similarly, a very low yield was obtained when observed in the CO -TPD curve of Co/C–N700. The peak at
2
solely using activated carbon as the catalyst (Table 3, entry 2). ca. 245 °C was attributed to the basic site associated with weak
Notably, the C–N composite, which was derived from the syn- chemical adsorption of CO molecules. Co/C–N700 exhibited
2
thesized Co/C–N700 after being immersed in aqua regia to broad peaks within the temperature range of 330–530 °C, with
remove Co, showed some activity in the oxidation of 1-phenyl- the main desorption located at about 460 °C and one shoulder
ethanol (Table 3, entry 3). Commercial Co-based catalysts peak at ca. 380 °C. In contrast, only one sharp peak (at
including metallic Co powder, Co oxides, and salt were all ca. 445 °C) was observed for the Co/C700. These results indi-
essentially inactive for this transformation under the investi- cated that pyrolysis of MOFs with N-containing ligands could
2
gated conditions (Table 3, entries 4–7). The MOF 1 precursor enhance the basicity of the resulting materials. The CO -TPD
also showed poor activity toward ketone formation (Table 3, curves of Co/C–Nx materials prepared at different pyrolysis
entry 8). A conversion of 15% was obtained when using Co/C temperatures are summarized in Fig. S4.† It can be seen that all
as the catalyst, which was prepared by a traditional impreg- the curves showed a main desorption peak at around 450 °C,
nation method (Table 3, entry 9). For comparison, MOF 2 that and Co/C–N700 had the most amount of basic sites. The results
has a similar structure as MOF 1 but containing no N elements suggested that the high activity of Co/C–N700 could be mainly
attributed to the strong basicity of the material.
The specific surface areas and porosities were analyzed by
N
2
adsorption–desorption isotherms at 77 K and the results
Table 3 Aqueous oxidation of 1-phenylethanol catalyzed by various
catalysts
are summarized in Table 1. Both MOFs 1 and 2 exhibited a
non-porous structure as reported in the literature.
28,29
But
after carbonization, the obtained materials possessed high
specific surface areas. As compared to MOF 1, remarkable
2
−1
increases in the specific surface areas from around 10 m g
2
−1
3
−1
to 256 m g and total pore volumes from 0.01 cm g to
a
a
Entry
Catalyst
Conv. (%)
Yield (%)
1
2
3
—
C
C–N
Co
CoO
<5
6
55
<5
5
<5
<5
<5
15
34
<5
<5
55
<5
5
<5
<5
<5
13
34
b
4
5
6
7
8
Co
O
3 4
Co(NO ) ·H O
MOF 1
Co/C
Co/C700
3 2 2
c
9
10
2
Reaction conditions: 1a (0.5 mmol), catalyst (10 mol% Co), H O
a
(
2 mL), air (1 bar), 110 °C, 48 h. Conversion and yield were
b
determined by GC-MS. The synthesized Co/C–N700 was immersed in
aqua regia to remove Co. Co/C was prepared by an impregnation
c
method.
2
Fig. 1 CO -TPD profiles of Co/C–N700 (a) and Co/C700 (b).
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ca. 0.20 cm g−1 were observed for the Co/C–N composites could be seen at 1315 cm−1 due to the disorder induced fea-
3
3
1,32
−1
(Table 1). The lower surface area observed on the Co/C–N700 tures caused by lattice defects.
The G band at 1590 cm
was probably attributed to the decomposition of the micro- represents the first-order scattering of the E2g vibrational
3
1,32
porous structure. For the Co/C700 derived from MOF 2, the mode within aromatic carbon rings.
The relative ratio of D
specific surface area and pore volume were, however, not band to G band (I /I ) in the Raman spectra of Co/C–N500,
D
G
enhanced as obviously as observed for the Co/C–N materials.
Co/C–N600, Co/C–N700, Co/C–N800 and Co/C–N900 materials
XRD patterns of the Co/C–N materials prepared at different was 2.53, 2.49, 1.71, 1.24, and 1.20, respectively, indicating
pyrolysis temperatures are shown in Fig. 2. The weak peak at that the crystallization degree of graphitic carbon was
ca. 25° could be assigned to a typical (002) interlayer of graph- increased with an increase in pyrolysis temperature, which was
ite-type carbon sheets, while the peaks at around 44.2°, 51.5°, in good agreement with those observed in XRD measurements.
7
5.8°, 92.2°, and 97.6° were assignable to the well-crystallized Moreover, the appearance of the 2D band indicated that gra-
Co with a face-centered cubic (fcc, Fm ˉ3 m (225), a = 0.355 nm) phitic structures were well developed in the Co/C–N800 and
3
0
structure (JCPDS no. 15-0806). At lower carbonization temp- Co/C–N900 composites. Notably, a similar profile was also
eratures, only one peak at ca. 44.2° was observed. More intense observed for the Co/C700 (Fig. 3).
diffraction peaks were detected at elevated temperatures,
SEM and TEM images were taken to investigate the mor-
implying the formation of a Co phase with a higher crystalliza- phology and structure of the materials. As seen in Fig. 4, with
tion degree. Interestingly, the Co/C700 nanocomposite showed an increase in the pyrolysis temperature, the Co/C–N surfaces
similar peaks as Co/C–N700 (Fig. S5†). However, the mean par- were distorted with rougher surfaces, indicating the decompo-
ticle size (ca. 30.2 nm) of metallic cobalt in Co/C700 (Table 1), sition and carbonization of the MOF frameworks. Interestingly,
which was calculated from the broadening of the Co(111) nanotubes were observed on the Co/C–N900 (Fig. S6†). SEM
3
1
plane using the Scherrer equation, was much bigger than images of Co/C700 also showed rough surfaces as the Co/
that in Co/C–N700 (ca. 9.6 nm). C–Nx. From TEM images (Fig. 5), it can be seen that Co was
Fig. 3 shows the Raman spectra of Co/C–N and Co/C700 highly dispersed on the N-doped carbon in Co/C–N700 with a
nanocomposites. For the MOF-derived materials, the D band uniform particle size of 9.8 ± 2.6 nm, while Co nanoparticles
in the Co/C700 material were seriously aggregated with much
bigger clusters (>100 nm). A high-resolution TEM image of Co/
C–N700 confirmed the existence of crystallized Co nano-
particles tightly surrounded by graphitized carbon. The Co
nanoparticles tended to aggregate gradually with an increase
Fig. 2 Powder XRD patterns of (a) Co/C–N500, (b) Co/C–N600, (c) Co/
C–N700, (d) Co/C–N800, and (e) Co/C–N900.
Fig. 4 SEM images of (a) MOF 1, (b) Co/C–N500, (c) Co/C–N600, (d)
Fig. 3 Raman spectra of (a) Co/C700, (b) Co/C–N500, (c) Co/C–N600,
d) Co/C–N700, (e) Co/C–N800, and (f) Co/C–N900.
Co/C–N700, (e) Co/C–N800, (f) Co/C–N900, (g) MOF 2, and (h) Co/
C700.
(
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Fig. 6 N 1s spectra for (a) Co/C–N700, (b) Co/C–N800, and (c) Co/
C–N900.
The above characterization results suggested that the intro-
duction of N-containing ligands would be the key to facilitate
the formation of uniformly dispersed Co particles. Overall,
N-doping in the materials promoted the basic properties and
played an important role in reducing the particle size of Co,
which may lead to materials with excellent performances in
catalytic reactions.
The substrate scope of the selective alcohol oxidation over
the Co/C–N materials was investigated under an atmospheric
pressure of air and base-free conditions (Table 4). Different
secondary benzylic alcohols bearing electron-donating or
Fig. 5 TEM images and the size distribution of Co particles for Co/
C–N700 (a, d), Co/C700 (b), and Co/C (c).
-withdrawing functional groups afforded the corresponding
substituted ketones in good to excellent yields within 48 h
in pyrolysis temperature (Fig. S7†). These results suggested (entries 2–7). Diphenylmethanol (1h), which is sterically hin-
that the thermolysis temperature and N-containing ligands dered, was converted to the corresponding ketone (2h) in 97%
played important roles in reducing the particle size of Co yield (entry 8). 1-(Naphthalen-1-yl)-ethanol also underwent oxi-
particles.
dation smoothly and gave 2i in 68% yield (entry 9). The Co/
To gain further insight into the catalyst structure and C–N700 catalyst was also active in the aerobic oxidation of 2,3-
especially the nitrogen location, XPS characterization was per- dihydro-1H-inden-1-ol (1j), furnishing 2j in 81% yield (entry
formed on the materials. Three distinct peaks were observed 10). Notably, even 1-phenylprop-2-en-1-ol (1k) bearing activated
in the N 1s spectra of Co/C–Nx with electron binding energies double bonds also worked well and afforded the desired oxi-
of 398.8, 400.5, and 402.8 eV, which were attributed to pyri- dation product in ca. 52% yield (entry 11). Moreover, a range
dine-type, pyrrole-type, and ammonium species nitrogen, of aliphatic alcohols were suitable for the formation of the
30,34
respectively (Fig. 6).
In Co/C–N700, the percentage of pyridi- corresponding ketones 2j–o, achieving moderate to good yields
nic nitrogen bonded to metal was higher than that in the other within 60 h (entries 12–15). It was noteworthy that excellent
two samples, i.e., Co/C–N800, and Co/C–N900. Such a coordi- selectivities to the target oxidation products were obtained for
nation interaction between the nitrogen and cobalt was advan- all the investigated substrates. Although in some cases the con-
tageous in preventing a serious aggregation of Co in the course version was not satisfying, it could be significantly improved
of MOF pyrolysis, in good accordance with the TEM obser- simply by prolonging the reaction time, without any remark-
vations. In the cobalt region for all samples, only two main able changes in selectivity. For example, 2-butyl alcohol was
peaks at ca. 793.5 and 778.7 eV were observed, characteristics for efficiently oxidized to 2-butanone in excellent yield (95%) after
33
Co 2p1/2 and Co 2p3/2 of metallic Co (Fig. S8†). Deconvolution 96 h of reaction (entry 16). The present catalytic system was
revealed that the Co region at ca. 778.7 eV consisted of three also applicable to the oxidation of primary alcohols, giving the
peaks at 778.7 eV, 780.1 eV, and 782.7 eV, respectively (Fig. S9†). corresponding aldehydes 2p–t in excellent yields (Table 4,
The observation suggested that some of the Co atoms on the entries 17–21).
surface could have some interaction with the nitrogen or carbon
atoms surrounding the Co nanoparticles.
Finally, we investigated the reusability of the highly active
Co/C–N700 catalyst in the aerobic oxidation of 1-phenylethanol
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Table 4 Aqueous oxidation of various secondary alcohols catalyzed by
Co/C–N700
Table 4 (Contd.)
a
a
Entry Substrate
Product
Conv. (%) Sel. (%)
a
a
Entry Substrate
1
Product
Conv. (%) Sel. (%)
d
1
1
1
7
8
9
92
90
96
87
>99
>99
>99
>99
98
>99
65
>99
>99
97
d
d
d
2
3
20
d
21
86
>99
4
5
6
7
85
77
96
72
99
99
Reaction conditions: substrate (0.5 mmol), Co/C–N700 (10 mol% Co),
a
H O (2 mL), air (1 bar), 110 °C, 48 h. Yield was determined by
2
b c d
GC-MS. 60 h. 96 h.
2
H O (0.5 mL), 65 h.
>99
>99
in aqueous medium. After the reaction, the catalyst was dis-
persed in water, but it was easily separated with the assistance
of an external magnet (Fig. S11†). AAS analysis of the reaction
solution confirmed that the content of Co in the solution was
below the detection limit, indicating that there was no signifi-
cant leaching of the active Co during the reaction process. The
used catalyst was washed several times with acetic ester and
alcohol, and then dried in air. The catalytic activity of the used
catalyst was significantly reduced (Table S1†). The PXRD data
8
9
98
68
99
>99
(
Fig. S12†) indicated that the diffraction peaks of the Co phase
apparently decreased in strength. However, after being reduced
in H at 400 °C for 1 h, the PXRD profile was similar to the
2
1
1
0
1
81
60
>99
88
fresh one (Fig. S12†), and no apparent Co aggregation could be
observed (Fig. S13†). As shown in Table S2,† upon hydrogen
treatment, the catalyst could be reused at least five times
without any significant loss of activity for the aerobic oxidation
of 1-phenylethanol under the investigated conditions.
b
b
b
1
1
1
2
3
4
72
67
70
>99
>99
>99
Conclusions
In summary, we have developed a novel, highly efficient, and
reusable Co-based heterogeneous system for selective aerobic
oxidation of alcohols. The Co/C–N composites are synthesized
by one-step thermolysis of Co-containing MOFs under an inert
atmosphere. It was found that the N element could be intro-
duced into the carbon matrix during pyrolysis of the MOFs
with N-containing ligands, and the presence of N is beneficial
for the improvement of basicity and metal dispersion of the
Co-carbon nanocomposites. The Co/C–N materials exhibit an
excellent performance in the aerobic oxidation of alcohols in
b
c
1
1
5
6
59
95
>99
>99
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neat water under an atmospheric pressure of air and base-free solid phase. The solid was collected by filtration, and dried in
conditions. Besides conferring high activity and selectivity to air.
the target products, the proposed catalytic system features a
Synthesis of Co/C–N materials by thermolysis of MOF
broad substrate scope for both aryl and alkyl alcohols. Further- 1. The typical procedure for the preparation of the Co/C–N
more, the catalyst could be easily recycled by using an external materials is described as follows: 0.5 g of 1 was pyrolyzed at a
−
1
magnetic field and reused. The present reaction system might high temperature for 8 h with a heating rate of 1 °C min at
provide a simple, cost effective, and environmentally friendly room temperature under an argon atmosphere. The prepared
procedure for the preparation of carbonyl compounds from material was denoted as Co/C–Nx, where x indicated the MOF
selective alcohol oxidation.
pyrolysis temperature.
Synthesis of Co/C700 by thermolysis of MOF 2. MOF 2
(
7
0.5 g) was placed in a tubular furnace and calcined up to
00 °C under an argon gas flow at a heating rate of 1 °C min .
−
1
Experimental
General information
Maintaining the targeted-temperature for 8 h, the resulting
solid was cooled to room temperature to give Co/C700.
All starting materials and solvents were obtained from com-
mercial suppliers and used without further purification.
Atomic absorption spectroscopy (AAS) was performed on a
Hitachi Z-2300 instrument. Elemental analysis was performed
on an Elementar Vario EL III equipment. Powder X-ray diffrac-
tion patterns of the samples were obtained on a Rigaku diffr-
actometer (D/MAX-IIIA, 3 kW) using Cu Kα radiation (40 kV,
Synthesis of C–N. The prepared Co/C–N700 was dispersed
in aqua regia. After 12 h, the supernatant was decanted and
replaced with fresh aqua regia. The exchange process was
repeated several times. The solid was washed several times
with water and collected by centrifugation, and finally dried at
100 °C overnight in an oven. The obtained material was
denoted as C–N.
3
0 mA, λ = 0.1543 nm). XPS data were obtained on an Axis
Synthesis of Co/C by impregnation. An aqueous solution of
Co(NO ) ·H O (0.7 g) was added dropwise to the activated
carbon. The sample was aged at room temperature for 24 h
and then washed twice with deionized water (30 mL). The as-
synthesized sample was dried under vacuum at 100 °C for 2 h
Ultra DLD using Mono Al Kα (1486.6 eV, 10 mA × 15 kV) as an
X-ray source.
The size and morphology of materials were characterized by
using a scanning electronic microscope (SEM, 1530 VP of LEO)
equipped with an energy dispersive X-ray detector (EDX, Inca
3
2
2
and then was treated in a stream of H
yield Co/C.
2
at 400 °C for 2 h to
3
00 of Oxford), and a high-resolution transmission electron
microscope (HR-TEM, C/M300 of Philips). BET surface area
and pore size measurements were performed with N
adsorp- Catalytic reaction
tion/desorption isotherms at 77 K on a Micromeritics ASAP
020 instrument. Mass spectra were recorded on a GC-MS
spectrometer (Shimadzu GCMS-QP5050A equipped with a
.25 mm × 30 m DB-WAX capillary column). CO -TPD data
2
Procedures for the aerobic oxidation of alcohols. Alcohols
0.5 mmol), H O (2 mL), and catalyst (Co 10 mol%) were
2
(
2
added into the Schlenk tubes. The reaction mixture was stirred
at 110 °C and under atmospheric conditions for 48 h. After
completion of the reaction, 100 µL of n-hexadecane as the
internal standard was added. Then, the products were
extracted with ethyl acetate from the liquid mixture, sub-
sequently subjected to GC-MS analysis. The products were
isolated by preparative TLC using n-hexane/ethyl acetate (20 : 1,
v/v) as an eluent.
0
2
were obtained on a Micromeritics AutoChem II 2920 instru-
ment. H NMR and C NMR spectra were recorded on a
Bruker Avance III 400 spectrometer using CDCl as the solvent
and tetramethylsilane (TMS) as the internal standard.
1
13
3
Catalyst preparation
Synthesis of MOF 1. In a typical synthesis, Co(NO ) ·6H O
3
2
2
(
464 mg) was added to 48 mL 1 : 1 : 1 (v/v) mixture of N,N′-di- Recycling of catalyst
methylmethanamide (DMF), ethanol and water containing tpt
124 mg) and H btc (168 mg) under stirring in a vial. The vial
The recyclability of the Co/C–N700 catalyst was tested for the
aerobic oxidation of 1-phenylethanol maintaining the same
reaction conditions as described above, except for using the
recovered catalyst. Each time, the catalyst was isolated from
the reaction mixture by magnetic separation at the end of cata-
lytic reaction, thoroughly washed with acetic ester and alcohol
(
3
was then sealed and allowed to stand at 100 °C. After heating
for 24 h, red cubic shaped crystals were formed. The crystals
were collected by filtration, washed with DMF and ethanol and
then dried in air. Powder X-ray diffraction analysis indicated
that the resultant compound exhibited the same crystalline
structure as that prepared by Bu et al. (Fig. S1†).
2
and then dried in air, treated in H at 400 °C for 1 h before
being reused.
Synthesis of MOF 2. A mixture of 1,3,5-benzenetricarboxylic
acid (3 mmol), cobalt(II) chloride hexahydrate (6 mmol), pot-
assium hydroxide (3 mmol) and water (5 mL) was added to a
Acknowledgements
2
5 mL Teflon container. The container was then sealed and
placed in an oven heated at 180 °C for 3 days. After cooling, We thank the National Natural Science Foundation of China
dark violet needle crystals were obtained as a homogeneous (21322606, 21436005, and 21576095), the State Key Laboratory
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of Pulp and Paper Engineering (2015TS03), the Doctoral
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