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autoxidation processes via a radical chain mechanism [11]. In addi-
tion, the enhancement in conversion caused by monometallic gold
is limited when compared to autoxidation. This apparent dual
nature of gold in cyclohexane oxidation prompted us to perform
further studies in an attempt to achieve more efficient catalysts
and gain deeper insights into both gold catalysis and catalytic aer-
obic oxidation when using gold-based alloy catalysts. In addition,
a mechanistic study on promoted autoxidation is warranted due
cobalt-mediated radical polymerisation [12].
partial pressure. Re-usability testing was also performed in an iden-
tical glass reactor. Cyclohexane (8.5 g) and an additional amount
of 1 wt% Au–Pd/MgO (6 mg) were added into the batch reaction
and catalytic oxidation was carried out at 140 ◦C and 3 bar O2 for
Au–Pd/MgO catalyst was tested under the same reaction conditions
(6 mg Au–Pd/MgO, 8.5 g cyclohexane, 140 ◦C and 3 bar O2 for 17 h).
The obtained reaction mixture was analysed by gas chromatogra-
phy [19–21]. Subsequent re-usability tests were carried out on the
same material by following the same procedure.
Previous studies have shown that the promising properties
of Au can be manipulated by the addition of Pd to form Au–Pd
bimetallic alloys as green catalysts [13–17]. Herein we report that
Au–Pd nanoparticles supported on MgO can improve both conver-
sion and product selectivity in the cyclohexane oxidation reaction
as compared to the commercial cobalt naphthenate promoter.
Furthermore, the improvements afforded by the MgO-supported
Au–Pd particles are effective over a wide range of nominal Au:Pd
compositions. Investigation of the reaction mechanism by means
of electron paramagnetic resonance (EPR) spin trapping experi-
ments on the catalytic decomposition of cyclohexyl hydroperoxide
(CHHP) are used to elucidate the origin of these beneficial effects.
2.4. EPR experiments
X-band continuous wave (CW) EPR spectra were recorded at
room temperature in deoxygenated cyclohexane using a Bruker
EMX spectrometer operating at 100 kHz field modulation in an
ER4119HS cavity. The typical instrument acquisition parameters
were: centre field 3487 G, sweep width 100 G, sweep time 55 s, time
constant 10 ms, power 5 mW, and modulation amplitude of 1 G.
Quantitative spectral analyses were carried out using the WinSim
software [22,23]. The spin trapping experiments were performed
using the following procedure: 5,5-dimethyl-1-pyrroline N-oxide,
hereafter labelled DMPO (0.1 mL of 0.1 M solution in cyclohex-
ane), was added to the substrate (0.1 mL of 2.5 molar % solution
of CHHP in cyclohexane) in an EPR sample tube. The mixture was
deoxygenated by bubbling N2 for 1 min prior to recording the
EPR spectrum in order to enhance the signal (by removing dis-
solved molecular oxygen which broadens the line-widths) [24].
catalysts, de-oxygenation was carried out at room temperature,
5 min after the mixing of the catalyst with the reaction mix-
ture. Cyclohexyl hydroperoxide was synthesised by a Grignard
reagent–oxygen reaction [23,25]. Finally, a solution of 2.5 mol%
cyclohexyl hydroperoxide in cyclohexane was obtained.
2. Experimental
2.1. Chemicals
HAuCl4, PdCl2 and other chemicals were purchased from Aldrich
and used without further purification unless otherwise specified.
2.2. Catalyst preparation
1 wt% Au–Pd/MgO catalysts were prepared by using a modified
sol-immobilisation method as reported previously [18]. The desired
amount of HAuCl4 and PdCl2 were added to 800 mL water. After stir-
ring for 15 min, 1.3 mL PVA solution (0.01 g/mL) was added and the
solution was stirred for an additional 15 min. Subsequently, 3.3 mL
of a freshly prepared NaBH4 solution (0.1 M) was added to generate
the Au–Pd nanoalloy particles. After reduction for 45 min, the MgO
support (1.98 g) was added to immobilise the nanoparticles. After
filtration and washing, the solid obtained was dried (110 ◦C, 16 h)
before use. The relative amount of gold and palladium salts used
was varied in order to obtain a systematic series of supported cat-
alysts with different molar ratios of Au-to-Pd, ranging from 20:1 to
1:20. Mono-metallic Au- or Pd-supported catalysts were also pre-
pared for comparative purposes, using the same total metal loading
(i.e. 1 wt%).
2.5. Electron microscopy characterisation
Samples of catalysts were prepared for TEM/STEM analysis by
dry dispersing the catalyst powder onto a holey carbon TEM grid.
Bright field (BF) imaging experiments were carried out on JEOL
2000FX TEM operating at 200 kV. High-angle annular dark field
(HAADF) imaging experiments were carried out using a 200 kV
JEOL 2200FS scanning transmission electron microscope equipped
with a CEOS aberration corrector. This latter microscope was also
equipped with a Thermo-Noran X-ray energy dispersive spec-
troscopy (XEDS) system for compositional analysis.
3. Results and discussion
2.3. Catalyst testing
3.1. Morphologies and catalytic performance of bimetallic
catalysts
Catalytic oxidation of cyclohexane (Alfa Aesar, 8.5 g, HPLC grade)
was carried out in a glass bench reactor using 6 mg of catalyst. The
reaction mixture was magnetically stirred at 140 ◦C and 3 bar O2
for 17 h. Samples of the reaction mixture were analysed by gas
chromatography (Varian 3200) with a CP-Wax 42 column. Adipic
acid was converted to its corresponding ester for quantification
purposes, and chlorobenzene was added as an internal standard.
Furthermore, CBrCl3 (150 mg) was used as a carbon centred rad-
ical scavenger for studying the mechanisms in these oxidation
reactions. CBrCl3 was added into the reactor prior to the catalytic
oxidation under identical reaction conditions. The product distri-
bution as a function of reaction time was monitored by studying
a systematic series of reaction batches subjected to different reac-
tion times under the same conditions of temperature and oxygen
(with Au:Pd molar ratio of 1:1) is shown in Fig. 1a. The mean size of
the Au–Pd nanoparticles was found to be ca. 5.0 nm (Fig. 1b). Aber-
ration corrected STEM-HAADF imaging (Fig. 1c) and XEDS analysis
of individual particles (Fig. 1d) in the bimetallic materials indicate
that the supported particles are in fact homogenous Au–Pd ran-
dom alloys. For comparison, the BF-TEM images of mono-metallic
Au and Pd-catalysts are also shown in Fig. 1e and f, respectively.
All three catalysts have rather similar mean diameters of 5.0 nm
(Au–Pd), 5.3 nm (Au) and 4.2 nm (Pd), respectively, confirming that