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S. Ghadamgahi et al.
systematic studies of the effects of reaction parameters in
these types of catalysts are relatively scarce [7, 8].
prepared by ultrasonically dispersing the relevant material
in methanol, depositing a drop of the suspension onto holey
carbon-film-coated 300-mesh copper grid and then allow-
ing the solvent to evaporate under vacuum for *2 h.
Particle diameter distributions were determined by using
ImageJ open-source software to measure and count at least
100 particles in high magnification micrographs.
Although there are reports of the use of such catalysts in
base-free benzyl alcohol oxidation in the literature [9], the
majority rely on the use of base to achieve high conversions
under mild conditions [10, 11]. In 2011, Prati et al., high-
lighted that ‘‘…selectivity of the reaction can be strongly
affected by the presence of a base, whose function is still
under investigation’’ [9]. In regards to the catalytic oxidation
of benzyl alcohol by immobilized Au nanoparticles in the
presence of a base: Zhu et al., found that the use of base as
well as the type of base played a key role in the activity of Au
particles immobilized on activated carbon for benzyl alcohol
oxidation in toluene [12], Dimitratos et al. reported 48 %
conversion over 6 h at 160 °C under solvent free conditions
using Au immobilized on carbon [13]. In comparison, Isida
and co-workers achieved better than 99 % oxidation of
benzyl alcohol by Au particles immobilized on activated
carbon in the presence of K2CO3 over 1 h at 80 °C, in
striking contrast to just 12 % conversion over 3 h under the
same conditions without base [14]. Finally, recent, topical
investigations focused on studies of Au nanoparticles
immobilized on custom-made, high-surface-area carbona-
ceous supports as catalysts in benzyl alcohol oxidation,
utilized base to facilitate reaction [15, 16].
Oxidation reactions were carried out in a 50-mL stain-
less-steel pressurized batch reactor. The Teflon reactor
liner was charged with 1.25 mmol anisole (internal chro-
matography standard), 50 mg of 1.0 wt% Au101/AC, up to
2.5 mmol benzyl alcohol (reactant), up to 2.5 mmol of
K2CO3 and a magnetic stirring bar, and then made up a
volume of 25 mL with methanol. The reactor was purged
five times, pressurised to 5.03 0.07 bar with oxygen and
then heated on a magnetic stirring (750 rpm) hotplate over
*20 min to the target temperature 2 °C. After the
desired reaction period, the reactor was cooled in ice for
*1 h to ensure that volatiles had condensed, after which
the remaining gas was vented. The catalyst was separated
by centrifugation (5000 rpm, 15 min) and the product
mixtures were analysed using a Dionex high-performance
liquid chromatography (HPLC) system fitted with a Luna
5 l C18 reverse-phase column and a UV detector. The
conversion (C) and selectivity (S) were calculated as:
Â
Ãꢀ
This paper focusses on the effects of systematic alter-
ations of the reaction conditions on the activity and
selectivity of catalysts made using Au101 as a precursor,
deposited and activated on commercially available acti-
vated carbon (NoritÒ) designed specifically for the fabri-
cation of catalysts, in the aerobic oxidation of benzyl
alcohol in the presence of K2CO3.
C ¼ ðnreactÞiÀ ðnreactÞf ðnreactÞi 100 %
.h
i
À
Á
Sprod
¼
nprod
ðnreactÞiÀ ðnreactÞf  100 %
f
where nreact and nprod represent the molar amounts of
reactants and products and the subscripts i and f indicate
the initial and final states of the reaction, respectively. Each
experiment was repeated at least three times in order to
ensure reproducibility.
2 Experimental Section
3 Results and Discussion
Au101 was synthesized according tothe methodofWeareetal.
[17],anditsidentitywasconfirmedby1HNMRinCDCl3.The
TEM images and size-distribution data for ‘‘as-synthe-
sized’’ Au101 (Fig. 1a) indicate a mean particle diameter of
1.64 0.05 nm (uncertainty is two standard errors of the
mean), including about 11 % with diameters greater than
2 nm [5]. This is just slightly larger than the core diameter
of 1.5 0.5 nm determined for the precursor Au101 par-
ticles [17]. After immobilization, the mean diameter of the
gold-containing particles increased to 2.62 0.12 nm
(Fig. 1b), the standard deviation of the distribution nearly
doubled and only *16 % of the particles had diameters
less than 2 nm. These indications of particle aggregation
are consistent with observations where deposition of Au101
onto oxides resulted in particle size increases to
2.0–2.7 nm (depending on loading) for Au101/TiO2 [19]
and *2.2 nm for Au101/WO3 [20]. However, on SiO2,
supported gold catalyst, hereafter designated 1.0 wt% Au101
/
AC, was fabricated by stirring (750 rpm) *5 g of NoritÒ
SX1G activated carbon powder (Cabot Corporation; BET
surface area of ca. 800 m2/g) with *50 mg of Au101 in
200 mL of dichloromethane until the solvent became
colourless (*2 h), indicating (near-) complete deposition of
nanoparticles on to the support. The product was centrifuged,
washed several times with CH2Cl2, dried under vacuum for a
day, washed in hot toluene then heated in static air at 100 °C
for3 hfollowinga protocoldescribed byLopez-Sanchezetal.
[18]. Catalyst that was not used immediately was stored under
N2 and in the dark at -4 °C.
High-resolution transmission electron microscopy
(TEM) was performed using a Philips CM20 operating at
200 kV in bright-field mode. Samples for TEM study were
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