Base-Free Direct Oxidation over Supported Au Catalysts
Au L3-edge X-ray absorption fine structure (XAFS) measurements
were carried out at BL14B2 of SPring-8 (Hyogo, Japan).[14] 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. Au L3-edge (11.9 keV) XAFS spectra of Au/NiO
and Au foil were measured using a Si(311) double crystal mono-
chromator 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 measure-
ment. The energy calibration was performed by taking the pre-
edge peak of Cu-K edge X-ray absorption near edge structure
(XANES) spectra of Cu-foil at 8980.3 eV. The spectral analysis was
carried out by the XAFS analysis softwares, Athena and Artemis.[15]
The extraction of the extended X-ray absorption fine structure
(EXAFS) oscillation from the spectra, normalization by edge-jump,
and Fourier transformation were performed by Athena. The curve-
fitting analysis was carried out in R-space by Artemis. k-Range was
2–10 ꢁÀ1 and r-range was 1.9–3.4 ꢁ. In the curve-fitting analysis,
the backscattering amplitude, the phase shift, and the mean-free
path of the photoelectron were calculated by FEFF8.4, and then
the other parameters, that is, the number of neighbouring atoms,
the interatomic distance between the absorbed atom to the neigh-
boring atom, the Debye–Waller factor, and the absorption edge
energy were treated as fitting parameters. The intrinsic loss factor
was obtained by the curve-fitting analysis of the EXAFS data of the
Au-foil.
*
Figure 5. Time-yield curves for the oxidation of 1-octanol over Au/NiO. : 1-
~
~
*
octanol, : 1-octanal, : octanoic acid, octyl octanoate. Reaction condi-
tions: 1-octanol (2.7 mmol), Au/NiO (Au 3 mol%), H2O (3 mL), 1,4-dioxane
(9 mL), O2 (0.5 MPa), 1008C.
Conclusions
We demonstrate the base-free oxidation of 1-octanol to octa-
noic acid and octyl octanoate over supported Au NPs. The
product selectivity could be tuned by selecting the nature of
the support. Au on NiO proved to be a highly active catalyst
for the aerobic oxidation of 1-octanol to octanoic acid, with se-
lectivities above 90%. Characterization by TEM and XAFS re-
vealed that the conventional CP method affords small Au NPs
deposited on nanometer-sized NiO particles. In contrast, Au/
CeO2 produces octyl octanoate directly from 1-octanol with
a selectivity of 82% in a high yield. To the best of our knowl-
edge, this is the first report on the Au-catalyzed base-free aero-
bic oxidation of poorly reactive aliphatic alcohols to carboxylic
acids and esters as major products. Long aliphatic carboxylic
acids and their alkyl esters are high-value chemicals, finding
use as surfactants, lubricants, emulsifiers, and so on. These re-
sults may open the door to new green processes for producing
high-value carboxylic acids and esters.
Conversions and product yields were analyzed by gas chromato-
graphy using SHIMADZU GC-14B with a Shinwa Chemical ULBON
HR-1 capillary column (0.53 mm i.d., 30 m) using anisole as an in-
ternal standard. Qualitative analysis was performed with a GC–MS
(SHIMADZU PARVUM2 and GC-2010 with a Shinwa Chemical
ULBON HR-1 capillary column, 0.25 mm i.d., 30 m).
Catalyst preparation: Gold on NiO was prepared by the co-precipi-
tation (CP) method.[11] Briefly, an aqueous solution (200 mL) con-
taining Ni(NO3)2·6H2O (5.5 g, 19 mmol) and HAuCl4·4H2O (0.4 g,
1 mmol) was rapidly added into 0.1m Na2CO3 (240 mL) at 708C,
and the reaction mixture was stirred at 708C for 1 h. The precipi-
tate was washed with water, filtered, dried in air at 1008C over-
night, and then calcined in air at 3008C for 4 h to obtain Au/NiO.
Gold on Co3O4 was prepared in a similar manner to Au/NiO but at
room temperature for 3 h. Gold on CeO2 was prepared by the dep-
osition–precipitation (DP) method.[16] Gold on Al2O3 was prepared
by the solid grinding (SG).[17]
Typical experiments for the aerobic oxidation of 1-octanol: to an au-
toclave was charged 1-octanol (1.26 mL, 8 mmol), H2O (3 mL), Au
catalyst (Au 0.1 mol%), and a magnetic stirring bar. The autoclave
was purged and filled with O2 until the pressure reached 0.5 MPa.
The reaction mixture was stirred at 1008C for 18 h. After the reac-
tion, the mixture was extracted with Et2O and the organic layer
was dried over Na2SO4 and filtered. The filtrate was analyzed by
GC–MS and GC by using anisole as an internal standard.
Experimental Section
Materials: Tetrachloroauric acid tetrahydrate (HAuCl4·4H2O) was
purchased from Tanaka Kikinzoku KKK. and used as received.
Reagent grades Co(NO3)2·6H2O and Ni(NO3)2·6H2O were purchased
from Wako Pure Chemical. Sodium carbonate, 1-octanol, and ani-
sole were purchased and used as received. Octyl octanoate was
synthesized according to the literature.[13] High purity nanoparticu-
late CeO2, TiO2 (P-25), and Al2O3, were supplied by Daiichi Kigenso
Kagaku Kogyo, Nippon Aerosil Co., Ltd., and Mizusawa Chemicals,
respectively.
Acknowledgements
Instruments: In order to check the presence of tiny Au clusters,
high angle annular dark-field scanning transmission electron mi-
croscopy (HAADF-STEM) observations were performed by using
a JEOL JEM-3000F operating at 300 kV. Specific surface area of cat-
alysts was calculated from the nitrogen adsorption measurements.
The samples were pretreated under vacuum at 2008C for 2 h and
then N2 adsorption isotherms were obtained at 77 K with a SHI-
MADZU Tristar.
This work was financially supported by JST-CREST, JST Research
for Promoting Technological Seeds (A), and a Grant-in-Aid for
Young Scientists (B) (21750160) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan. The synchrotron
radiation experiments were performed at the BL14B2 in the
SPring-8 with the approval of the JASRI (2009B1007).
ChemSusChem 2012, 5, 2243 – 2248
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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