1184
A. Zhang et al. / Catalysis Communications 12 (2011) 1183–1187
2
.2. Preparation of catalyst
The procedure to prepare Co-ZIF was similar to that reported
previously [32]. It was generally prepared by a solvent-thermal
method. Co(OAc) ·4H O (6.7 mmol) was dissolved in cyclohexanol
10 ml), and then a cyclohexanol solution containing 1.7 mmol
2
2
(
imidazole (20 ml) was added. The heterogeneous mixture was stirred
at room temperature for 12 h, and then was placed into a Teflon-lined
autoclave (40 ml). The autoclave was sealed and heated at 413 K for
2
4 h. After the mixture was cooled to room temperature, violet
crystals (named Co-ZIF) were collected and washed with ethanol.
2
.3. Characterization of catalyst
The powder XRD patterns were collected on a PANalytical X'Pert
MPD theta–theta system using Cu Kα radiation in continuous scan mode
−1
ranging from 5° to 90° at a scanning rate of 0.2°s . Data were analyzed
using the PANalytical X'Pert HighScore Plus software package.
The FT-IR spectra were recorded on a Bruker Tensor27 spectrometer
−1
in the domain 400–4000 cm . The spectra averaged over 200 scans
were refined by subtracting the spectrum of KBr used as background.
The determination of the unit cell and the data collection for the
crystal of Co-ZIF were carried out on a Bruker SMARTCCD diffractometer
with graphite-monochromated Mo Kα radiation (λ=0.71073 Å) at
Fig. 1. Single-crystal structure of Co-ZIF.
regarded as supramolecular isomer. It was constructed from the Co ion
and imidazole ligand. The framework Co-ZIF contained only a
crystallographically unique cobalt (II) ion and two independent imid-
azolate link-ages. Within the framework, the cobalt ion was linked into
boat- and chair-like 6-rings. This unit (each accommodated a
cyclohexanol molecule) was linked to form an infinite 3D net with 1D
channel in Co-ZIF.
2
93 K. The structure was solved by direct methods and refined by full-
matrix least-squares methods, which were performed using the
SHELXL-97 software package, and the result was almost the same as
the document [32].
2
.4. Catalytic reaction
FT-IR spectrum of Co-ZIF was presented in Fig. 2 (0 cycle), and
−
1
−1
all peaks were assigned: 3108 cm
v(Ph–H); 2926 cm
v(C–H);
−
1
−1
The activity and selectivity of the catalyst were investigated in the
1658, 1603 cm v(C–C); 1482 cm (ring stretching of imidazolate)
−
1
−1
−1
epoxidation of various olefins with molecular oxygen using isobutyr-
aldehyde as reductant and acetonitrile as solvent. In a typical
experiment, a 50 ml round bottom flask equipped with an efficient
water condenser and a magnetic stirrer was kept in a constant
temperature water bath. Then olefin (8 mmol), isobutyraldehyde
1316 cm
δ(C–H); 1233 cm
(ring vibration); 1164 cm
(ring
−1
−1
−1
breathing); 1084 cm δ(C–H) 952, 831 cm (ring bending); 753 cm
γ(C–H); 666 cm
−1
(torsion). XRD spectrum of Co-ZIF was also dis-
played, and most peaks were assigned in Fig. 3 (0 cycle). The above
characterizations of Co-ZIF could indicate that the prepared crystal
material was Co-ZIF material as reported before.
(
32 mmol), catalyst (1 mg) and acetonitrile (16 ml) were added to the
flask. The reaction was started by bubbling O at atmospheric pressure
2
into the reaction mixture at the rate of 7–10 ml/min. The reaction
mixture was stirred vigorously for 3 or 5 h. Then the catalyst was
separated by the centrifugation of reaction mixture, and the liquid
organic phase was analyzed quantitatively using a gas chromatograph
3.2. Catalytic activity studies
The ability of the prepared Co-ZIF catalyst was investigated in the
epoxidation of cyclooctene with molecular oxygen and isobutyralde-
hyde in acetonitrile. The results were presented in Table 1. In the
blank test, only 30.6% conversion and 40.2% selectivity were obtained.
(
Agilent 7890) equipped with a hydrogen flame ionization detector, a
capillary column of 30 m length with OV1701 stationary phase, a
programmed oven (temperature range 323–493 K) and N as a carrier
2
2 2
When Co(OAc) ·4H O simply used as the catalyst, it showed the
gas. Reaction products were also confirmed by analyzing the reaction
mixture with a gas chromatograph−mass spectrometer having a
programmed oven (temperature range 353–523 K), He as a carrier gas.
The selectivity, yield of epoxides, and the conversion of the olefins
were calculated on the basis of gas chromatography using toluene as
the internal standard. The turnover frequency for epoxide formation
TOFepo =nepo/(nCo·h) [molesepoxide formed ·mol Co−1
h
−1
]) was also
(
calculated.
Cycle reactions of Co-ZIF were carried out in this work, and
cyclooctene was used as the test substrate. In a typical cycle experiment,
Co-ZIF was filtrated from the reaction mixture after each experiment,
washed with ethanol and acetonitrile repeatedly, dried in the air, and
then used in the next run under identical conditions.
3
. Results and discussion
3
.1. Characterization of Co-ZIF
Cobalt (II) imidazolate reported here was crystalline material. X-ray
single-crystal diffraction was displayed in Fig. 1. Co-ZIF could be
Fig. 2. FT-IR spectra of the fresh Co-ZIF and the recovered ones.