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PLAZA CP-1000, Sibata). Distillation of substrates or drying of sam-
ples were performed with a glass oven (B-585 Kugelrohr, BUCHI).
The isolation of products was performed with a single channel auto-
mated flash chromatography system (Smart Flash EPCLC AI-580S, Ya-
mazen). NMR spectra were recorded on a Bruker biospin Avance III
spectrometer (1H, 400 MHz; 13C, 100 MHz) using 5 mm tubes. Chemi-
cal shifts (d) were reported in ppm downfield from SiMe4 (solvent,
CDCl3). GC analyses were performed on Shimadzu GC-2025
equipped with a Stabilwax capillary column (internal diameter=
0.25 mm, length=30 m) and with a flame ionization detector, or on
a Shimadzu GC-17 A with an InertCap 17 capillary column (internal
diameter=0.25 mm, length=30 m). Mass spectra were recorded on
a spectrometer (GCMS-QP2010 SE, Shimadzu) equipped with an In-
ertCap 17 MS capillary column (internal diameter=0.25 mm,
length=30 m) at an ionization voltage of 70 eV.
Conclusions
Hexagonal SrMnO3 (SMO) could activate O2 to form a superoxo
species, which resulted in an efficient heterogeneous catalyst for
the selective liquid-phase oxidation of various types of organic
substrates. This study shows the importance of developing hetero-
geneous catalysts that can activate O2 under mild conditions, and
further elucidation of the catalytic mechanism is now in progress.
Experimental Section
Materials
Toluene (Kanto Chemical) was pretreated with molecular sieves (3 A)
that were evacuated at 2508C for 3 h.[22] Substrates and products
(TCI, Kanto Chemical, and Aldrich) were used as-received, and 2g
and 3a were purified according to reported procedures.[23] Oxygen-
18O2 (97 atom%) was purchased from ISOTEC. Reagents such as
Sr(NO3)2 (Kanto Chemical), Mn(NO3)2·6H2O (Kanto Chemical), citric
acid (Kanto Chemical), ethylene glycol (Kanto Chemical), NaBD4
(ACROS), and CDCl3 (ACROS) were used as-received. OMS-2 and
Mn12O12(OAc)16(H2O)4 were synthesized and characterized according
to procedures given in the literature.[24] 1-Deutero-1-phenylethanol
was synthesized by the reaction of acetophenone with NaBD4.[25]
Synthesis and characterization of SMO
Hexagonal SrMnO3 was synthesized by the polymerized complex
method. Citric acid (15.4 g, 80 mmol) and ethylene glycol (25.1 mL,
450 mmol) were added to an aqueous solution (50 mL) containing
Sr(NO3)2 (2.11 g, 10 mmol) and Mn(NO3)2·6H2O (2.87 g, 10 mmol).
After complete dissolution was achieved, the resulting solution was
evaporated at 333 K to reduce the water content. The transparent
solution was heated at 463 K with continuous stirring to accelerate
polymerization, whereby it finally gelled into a transparent brown
resin. The resultant resin was heated at 523 K for 20 min and 573 K
for an additional 20 min to give a black powder, which is hereafter
referred to as the precursor. The precursor was calcined at 923 K for
5 h on an Al2O3 plate in air. After cooling to room temperature, the
samples were collected and washed with water (ꢀ2 L) followed by
evacuation at room temperature for 3.5 h to give the hexagonal
SrMnO3 catalyst. Yield: 1.70 g (89%). Elemental analysis calcd (%) for
SrMnO3: Sr 45.98, Mn 28.83; found: Sr 45.25, Mn 27.65.
Instruments
XRD patterns were recorded on a diffractometer (Ultima IV, Rigaku;
CuKa, l=1.5405 ꢁ, 40 kV–40 mA) equipped with a high-speed 1-di-
mensional detector (D/teX Ultra, Rigaku). Diffraction data were col-
lected in the range of 2q=10–808 at 0.028 steps with a scan rate of
208minÀ1. Nitrogen adsorption–desorption isotherms were mea-
sured at 77 K with a surface-area analyzer (Nova-4200e, Quantach-
rome). Prior to measurement, the samples were heated at 423 K for
1 h under vacuum to remove physisorbed water. The BET surface
areas were estimated over the relative pressure (P/P0) range of 0.05–
0.30. FTIR spectra were obtained at a resolution of 4 cmÀ1 by using
a spectrometer (FT/IR-6100, Jasco) equipped with an extended KBr
beam splitting device and a mercury cadmium telluride (MCT) detec-
tor. A total of 256 scans were averaged for each spectrum. Raman
spectra were recorded on a spectrometer (NRS-3200, Jasco) with ex-
citation at 532 nm using a green laser. ICP–AES analyses were per-
formed with a Shimadzu ICPS-8100 spectrometer. Differential ther-
mal analysis (DTA) and thermogravimetric (TG) measurements were
performed with a differential thermal analyzer (TG8120, Rigaku). Io-
dometric titration was performed with a Mettler Toledo Easy Pro Ti-
trator System. SMO (ꢀ10 mg) was added to a mixture of 0.5m HCl
aq. (12 mL) and 2m KI aq. (5 mL), and the resulting solution was ti-
trated with 0.01m Na2S3O3 aq.[26] X-ray photoelectron spectroscopy
(XPS) analysis was performed with JEOL JPC-9010 MC using MgKa ra-
diation (1253.6 eV) at 10 kV and 25 mA. Samples were pressed into
pellet and fixed on a double-stick carbon tape. The binding energies
were calibrated using the C1s band at 284.6 eV. The spectrum was
fitted and evaluated by the XPS Peak 4.1 program, whereas the
background was subtracted using Shirley function. The deconvolut-
ed Mn2p spectrum of SMO shows three peaks with binding ener-
gies of 641.9, 642.9, and 644.6 eV, which corresponds to MnIII, MnIV,
and shakeup peak, respectively.[27] The morphology of the samples
was examined using SEM (S-5200, Hitachi). Liquid-phase catalytic oxi-
dation was
Procedure for catalytic oxidation
The catalytic oxidation of various substrates was conducted in a
30 mL glass vessel containing a magnetic stirring bar. All products
were identified by comparison of the GC retention time, mass spec-
tra, and NMR spectra with those of authentic samples. A typical pro-
cedure for catalytic oxidation was as follows: SMO (0.1 g), alcohol
(1 mmol), toluene (2 mL), O2 (1 atm), and an internal standard (naph-
thalene) were charged into the reaction vessel. The reaction solution
was heated at 353 K and periodically analyzed using GC. After the re-
action was completed, the catalyst was separated by filtration. The
analytically pure product was then isolated using a flash chromatog-
raphy separation system with silica gel (pore size 60 ꢁ, particle size
30 mm) and n-hexane/ethyl acetate as an eluent. The products are
1
known and were identified by comparison of their H and 13C NMR
signals with the literature data. The separated SMO was washed with
acetone (40 mL) and water (500 mL), and then dried under vacuum
before recycling. The amounts of surface Mn species were estimated
assuming that the (110) plane is a surface structure because of the
abundant population of Mn species on the (110) plane. The
amounts of surface Mn were estimated using this hypothesis and
the BET surface area of SMO (25 m2 gÀ1) to be 193 mmol·gÀ1
.
Procedure for IR measurements
performed with an organic synthesizer (ALHB-80 & DTC-200HZ-3000,
Techno Applications) or a liquid phase organic synthesizer (CHEMIST
Samples were pressed into self-supporting disks (20 mm diameter,
0.1 g), placed in an IR cell attached to a closed glass-circulation
&
ChemCatChem 2016, 8, 1 – 8
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