A. Hamasaki et al. / Applied Catalysis A: General 469 (2014) 146–152
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synthesis, thus inexpensive bulk supply is possible. Part of the
2.4. Preparation of Au/Co3O4
cobalt oxide support is reduced to Co(0) by spillover hydrogen
arising from the effects of Au nanoparticles. The reduced metal
binds CO to form a cobalt carbonyl equivalent. Pd nanoparticles
nanoparticles, and are therefore expected to play dual roles as a
Pd catalyst and a co-catalyst for promotion of the reduction of a
cobalt oxide support. Hidai and co-workers reported carbonylation
reactions employing a combination of a homogeneous Pd catalyst
and metal carbonyls [26–30]. We anticipated that PdO/Co3O4
would act as a suitable catalyst for the formylation of aryl halides
under conditions that are free of external hydride and additional
ligands.
Au/Co3O4 was prepared by coprecipitation method [20–25]. A
200 mL aqueous solution of HAuCl4•4H2O (0.42 g, 1.0 mmol) and
Co(NO3)2•6H2O (5.5 g, 19 mmol) was poured into 200 mL of aque-
ous solution of Na2CO3 (2.8 g, 26 mmol). The mixture was stirred for
2 h at room temperature. The precipitate was washed with distilled
water several times and dried at 100 ◦C for overnight. The obtained
catalyst was calcined at 400 ◦C for 4 h.
2.5. XAFS spectra of PdO/Co3O4
Pd K-edge and Co K-edge X-ray absorption fine structure (XAFS)
measurements were carried out at BL14B2 of SPring-8 (Hyogo,
Japan). The XAFS samples were ground with boron nitride in an
agate mortar and made as pellets. The storage ring energy was
8 GeV with a typical current of 99.5 mA. Pd K-edge (24.3 keV) XAFS
spectra of Pd/Co3O4, Pd foil, Pd(NO3)2 and PdO were measured
using a Si(311) double crystal monochromator in transmission
mode. Co K-edge (7.7 keV) XAFS spectra of Co foil, Co(NO3)2 and
CoO were measured using a Si(1 1 1) double crystal monochromator
in transmission mode. Ionization chambers were used to measure
the intensity of the incident and transmitted X-rays and the Quick
scan technique (QXAFS) was used in this measurement.
2. Experimental
2.1. General remarks
1H and 13C NMR spectra were recorded on a JEOL JNM-ECS400
spectrometer. 1H assignment abbreviations are the following; sin-
glet (s), doublet (d), triplet (t), and multiplet (m). GC analysis was
carried out using Agilent GC 6850 series II equipped with J&W HP-1
column (length 30 m, I.D. 0.32 nm). Column chromatography was
performed on silica-gel (Kanto Chemicals, Silica gel 60 N, spherical,
neutral; particle size 40–100 m). Formylation reactions were car-
ried out using a 50 mL stainless steel autoclave reactor (Toyo Koatsu
Co., Ltd.) with a glass tube equipped inside. High angle annular
dark-field scanning transmission electron microscopy (HAADF-
STEM) observations were performed by using JEOL JEM-3000F
operating at 300 kV. Powder X-ray diffraction (XRD) data were
collected by Rigaku Multiflex X-ray diffractometer with Cu K␣ radi-
ation. The contents of Pd and Co in the reaction mixtures were
measured by using microwave plasma-atomic emission spectrom-
etry (MP-AES) on Agilent 4100MP-AES system. TiO2 was supplied
from Catalysis Society of Japan, as Japan Reference Catalyst JRC-
TiO-4. CeO2 was purchased from Daiichi Kigenso Kagaku Kogyo
Co., Ltd. Co2(CO)8 (Kanto chemical Co., Inc.) was sublimated under
reduced pressure before use. All other reagents were used in com-
mercial grade. The authentic samples of aldehydes 2 and reduced
by-products 3 were purchased from Wako Pure Chemical Indus-
tries, Ltd., Tokyo Chemical Industry Co., Ltd., Kanto Chemical Co,
Inc., Nacalai Tesque, Inc. and Sigma–Aldrich, Inc.
2.6. Typical procedure for formylation of aryl halides catalyzed by
PdO/Co3O4 (Table 2)
The suspension of 5 wt% PdO/Co3O4 (20 mg, 1.6 mol% Pd atom
and 47 mol% Co to substrate) in dioxane (1 mL) in a stainless
autoclave attached a glass tube was stirred at 120 ◦C under H2
atmosphere (2 MPa) for 2 h. After cooling to RT, aryl halide 1
(0.5 mmol), K2CO3 (0.5 mmol) and dioxane (1 mL) were added and
stirred at 140 ◦C for 20 h under CO:H2 = 3:1 atmosphere (4 MPa).
The reaction mixture was analyzed by GC using tridecane (30 L,
0.1 mmol) as an internal standard after filtration through celite to
remove the catalyst. The products 2 and 3 were identified by com-
parison of the retention time of GC with the authentic samples. The
aldehydes 2a–f and 2h–k were also characterized by 1H and 13C
NMR after chromatographic purifications. Other aldehydes were
found to be difficult to isolate due to low yields and volatilities.
Benzaldehyde (2a). 1H NMR (CDCl3, 400 MHz): ␦ 10.00 (s, 1H),
7.86 (d, J = 6.9 Hz, 2H), 7.61 (t, J = 7.3 Hz, 1H), 7.51 (t, J = 7.6 Hz, 2H);
13C NMR (CDCl3, 100 MHz): ␦ 192.5, 136.5, 134.5, 129.8, 129.1.
p-Tolualdehyde (2b). 1H NMR (CDCl3, 400 MHz): ␦ 9.93 (s, 1H),
7.75 (d, J = 7.8 Hz, 2H), 7.30 (d, J = 8.2 Hz, 2H), 2.41 (s, 3H); 13C NMR
(CDCl3, 100 MHz): ␦ 192.1, 145.6, 134.3, 129.9, 129.8, 21.9.
3,5-Dimethylbenzaldehyde (2c). 1H NMR (CDCl3, 400 MHz): ␦
9.92 (s, 1H), 7.46 (s, 2H), 7.23 (s, 1H), 2.37 (s, 6H); 13C NMR (CDCl3,
100 MHz): ␦ 192.8, 138.8, 136.7, 136.3, 127.6, 21.1.
2.2. Preparation of Co3O4
Co3O4 was prepared by precipitation method. A 200 mL aque-
ous solution of Co(NO3)2•6H2O (5.8 g, 20 mmol) was poured into
200 mL of aqueous solution of Na2CO3 (2.8 g, 26 mmol). The
aqueous mixture was stirred for 2 h at room temperature. The pre-
cipitate was washed with distilled water several times and dried at
100 ◦C for overnight. The obtained catalyst was calcined at 400 ◦C
for 4 h.
o-Tolualdehyde (2d). 1H NMR (CDCl3, 400 MHz): ␦ 10.24 (s, 1H),
7.77 (d, J = 7.3 Hz, 1H), 7.45 (t, J = 7.6 Hz, 1H), 7.33 (t, J = 7.6 Hz, 1H),
7.23 (d, J = 7.3 Hz, 1H), 2.65 (s, 3H); 13C NMR (CDCl3, 100 MHz): ␦
192.9, 140.7, 134.2, 133.7, 132.1, 131.8, 126.4, 19.7.
p-Anisaldehyde (2e). 1H NMR (CDCl3, 400 MHz): ␦ 9.85 (s, 1H),
7.81 (d, J = 8.7 Hz, 2H), 6.97 (d, J = 9.2 Hz, 2H), 3.86 (s, 3H); 13C NMR
(CDCl3, 100 MHz): ␦ 191.0, 164.7, 132.1, 130.0, 114.4, 55.7.
m-Anisaldehyde (2f). 1H NMR (CDCl3, 400 MHz): ␦ 9.95 (s, 1H),
7.42 (m, 2H), 7.36 (s, 1H), 7.15 (m, 1H), 3.83 (s, 3H); 13C NMR (CDCl3,
100 MHz): ␦ 192.2, 160.2, 137.9, 130.1, 123.6, 121.6, 112.1, 55.5.
p-Phenylbenzaldehyde (2h). 1H NMR (CDCl3, 400 MHz): ␦ 10.05
(s, 1H), 7.95 (d, J = 8.7 Hz, 2H), 7.74 (d, J = 6.4 Hz, 2H), 7.63 (d,
J = 6.9 Hz, 2H), 7.48 (t, J = 7.3 Hz, 2H), 7.41 (t, J = 7.1, 1H); 13C NMR
(CDCl3, 100 MHz): ␦ 192.0, 147.3, 139.8, 135.3, 130.4, 129.1, 128.6,
127.8, 127.5.
2.3. Preparation of PdO/Co3O4 and other palladium
heterogeneous catalysts
PdO/Co3O4 was prepared by impregnation method. Co3O4
(0.50 g, 2.1 mmol) were impregnated with Pd(NO3)2 (0.052 g,
0.22 mmol) aqueous solution and stirred for 30 min at room tem-
perature. After impregnation, H2O was removed by vacuum-freeze
drying or evaporation. The obtained catalyst was dried at 100 ◦C
for overnight and calcined at 400 ◦C for 4 h. PdO/TiO2 and PdO/CeO2
were prepared by same method of PdO/Co3O4 except for using TiO2
or CeO2 instead of Co3O4, respectively.