A. Taketoshi et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 12 (2013) 1413
KMnO4, Fe(NO3)2¢6H2O, Co(NO3)2¢6H2O, Ni(NO3)2¢6H2O,
Cu(NO3)2¢6H2O, La(NO3)3¢6H2O, NaClO3, Na2CO3, NaOH,
KOH, and HNO3 were used as received. As gold precursors,
tetrachloroauric(III) acid tetrahydrate (H[AuCl4]¢4H2O) and
acetylacetonato(dimethyl)gold(III) ([Au(acac)(CH3)2]) were
purchased from Kishida Reagents Chemicals Co., Ltd. and
Tri Chemical Laboratories Inc., respectively. Bis(ethylene-
diamine)gold(III) trichloride ([Au(en)2]Cl3) was prepared in a
similar manner to the previous literature.30 As noble metal
precursors, palladium(II) chloride, bis(acetylacetonato)palla-
dium(II), and bis(acetylacetonato)platinum(II) were purchased
from Wako Pure Chemical Industries, Ltd. Tetraammine-
platinum(II) dichloride hydrate was purchased from Kojima
Chemicals Co., Ltd. 4-Bromophenyl methyl sulfide, 4-fluoro-
phenyl methyl sulfide, 4-methoxyphenyl methyl sulfide (Tokyo
Chemical Industry Co., Ltd.), 4-chlorophenyl methyl sulfide,
mesitylene, methyl phenyl sulfide (thioanisole) (Wako), di-
benzothiophene (Kanto Chemical Co., Inc.) and 1,2-dichloro-
benzene (Aldrich) were purchased and were used without any
further purification.
because MnO2¹x and MnO2 are negatively charged in aqueous
solution of pH 7 (1 wt % Au loading). Briefly, [Au(en)2]Cl3
(21.2 mg) was dissolved in 9 mL of H2O (5.5 mmol L solu-
¹1
tion). The pH of the solution was adjusted to 9 by adding
aqueous NaOH solution and heated to 70 °C. Subsequently,
manganese oxide (1.00 g) was dispersed and stirred under the
control of pH at 8 to 10 at 70 °C for 1 h. The precipitate was
washed with H2O, filtered, dried at 120 °C overnight, and then
calcined in air at 300 °C for 4 h.
Palladium and platinum on manganese oxide were prepared
by SG and by DP in the same procedures as gold catalysts
(1 wt % loading).
Characterization.
The BET specific surface area was
obtained from nitrogen adsorption measurements. The samples
were pretreated under vacuum at 200 °C for 2 h. Then, N2 ad-
sorption isotherms were measured at 77 K with a SHIMADZU
Tristar. X-ray diffraction (XRD) patterns were obtained by
using a Rigaku RINT-TTR III diffractometer operated at 50
kV and 300 mA with Cu Kα radiation. Transmission electron
microscope (TEM) and high-angle annular dark-field scanning
TEM (HAADF-STEM) observations were carried out by using
a JEOL JEM-3200FS operating at 300 kV. X-ray photoelectron
spectroscopy (XPS) experiments were conducted by using a
SHIMADZU ESCA-3400 spectrometer. The Mg Kα source was
operated at 10 kV and 10 mA. The binding energy scale was
referenced to the C 1s peak (285 eV) arising from adventitious
carbon in the sample.
The amount of surface excess oxygen was measured by
iodometric titration.36 A mixture of weighed amount of sample
(0.02-0.03 g), KI (0.02 g), and 0.1 M aqueous HCl solution
(5 mL) was shaken for 5 min. The suspension was filtered and
washed with ethanol and distilled water. The filtrate con-
taining iodine was titrated with a sodium thiosulfate solution
(1.0 © 10¹3 M).
Metal Oxides.
V2O5 was prepared by calcination of
NH4VO3 in air at 300 °C for 4 h.
MnO2¹x was prepared by neutralization. An aqueous solution
of Mn(NO3)2¢6H2O (0.1 M, 400 mL) was rapidly added into an
aqueous solution of Na2CO3 (0.1 M, 480 mL) at room temper-
ature. After stirring for 1 h, the suspension was centrifuged and
the precipitate was repeatedly washed with distilled water until
the pH reached a steady value of around 8. The precipitate was
filtrated, dried at 120 °C overnight, and then calcined in air at
300 °C for 4 h.
MnO2_A was prepared by reduction of KMnO4 with HCHO
and KOH.31 MnO2_B was prepared by oxidation of Mn(NO3)2
by NaClO3 and HNO3.31 For the preparation of MnO2_C, an
aqueous solution (125 mL) of Mn(NO3)2¢6H2O (75 mmol) was
added to an aqueous solution (125 mL) of KMnO4 (50 mmol)
and NaOH (150 mmol) at room temperature and the dispersion
was stirred for 1 h.31 The precipitate was repeatedly washed
with distilled water until the pH reached a steady value of
around 9. The precipitate was collected by filtration, dried at
120 °C overnight, and then calcined in air at 300 °C for 4 h.
For the preparation of MnO2_D, an aqueous solution (30 mL)
of MnCl2¢4H2O (50 mmol) was added slowly to an aqueous
solution (100 mL) of KMnO4 (37 mmol) at room temperature
and the dispersion was stirred for 30 min. The precipitate was
repeatedly washed with distilled water until the pH reached
a steady value of around 4. The precipitate was collected by
filtration, dried at 120 °C overnight, and then calcined in air at
300 °C for 4 h.
Gold Catalysts. Gold on Fe2O3, Co3O4, NiO, CuO, and
La2O3 were prepared by coprecipitation (CP) (Au/base metal =
1/19).32 Gold on acidic metal oxides such as Al2O3, SiO2, V2O5,
MoO3, and WO3 were prepared by solid grinding (SG) (1 wt %
Au loading).33 Gold on TiO2, ZnO, ZrO2, SnO2, and CeO2, the
point of zero charge of which was above 7, were prepared by
deposition-precipitation (DP) (1 wt % Au loading).34 Gold on
MgO was prepared by DP in the presence of Mg citrate to
control the size of gold NPs (1 wt % Au loading).35 Gold on
manganese oxide (MnO2¹x and MnO2) were prepared by SG
and by DP with [Au(en)2]Cl3 cation complex as a precursor,
Catalytic Tests.
To an autoclave was charged methyl
phenyl sulfide (118 ¯L, 1.0 mmol), 1,2-dichlorobenzene (5.0
mL), catalyst (40 mg), and a magnetic stirring bar. The auto-
clave was purged and filled with O2 until the gauge pressure
reached 0.5 MPa. The reaction mixture was stirred at 100 °C for
36 h. The mixture was extracted with chloroform and filtered.
1
The filtrate was analyzed by H NMR (JEOL 300 MHz) using
mesitylene as an internal standard.
Results and Discussion
Support Screening for Gold Catalysts. Various kinds of
supported gold catalysts have been screened for the oxidation
of methyl phenyl sulfide (1) in 1,2-dichlorobenzene at 100 °C
for 24 h under 6 atm of oxygen (Table 1). Methyl phenyl
sulfoxide (2) was obtained in 38% yield with a high selectivity
above 97% by using Au/MnO2¹x (Entry 1). The metal oxides
which work as oxidation catalysts by the Mars-van Krevelen
mechanism such as V2O5 can be a support (Entry 2).37 Gold
on Co3O4 and on CeO2 also produce sulfoxide 2, but the reac-
tion is very slow (Entries 3 and 4). Only gold deposited on
manganese oxides exhibited significantly high activity and high
selectivity to sulfoxide 2.
Effect of Deposition of Noble Metals on Manganese
Oxides. To clarify the role of gold in this reaction, other noble
metals such as Pd and Pt were deposited on MnO2¹x by DP and