Dioxygen oxidation of 1-phenylethanol with gold nanoparticles and N-hydroxyphthalimide in ionic liquid

Article abstract of DOI:10.1016/j.molcata.2013.02.007

Gold nanoparticles (Au-NPs) of 8 nm average diameter were obtained by thermal reduction under nitrogen from KAuCl₄ in the presence of n-butylimidazol dispersed in the ionic liquid (IL) 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIm⁺BF₄^-). Characterization of the Au-NP was done by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Catalytic activities of the Au-NP/IL dispersion were evaluated in the oxidation of 1-phenylethanol at 100 and 160 °C under 4 bar pressure of dioxygen in a base-free system. Au-NP in combination with the radical initiator N-hydroxyphthalimide (NHPI) showed good conversion and selectivity for the oxidation of 1-phenylethanol to acetophenone through formation of an α-hydroxy carbon radical. The concomitant side products di(1-phenylethyl)ether and di(1-phenylethyl)peroxide were rationalized by an equilibrium due to the IL matrix of the α-hydroxy carbon radical with the 1-phenylethoxy radical. Maximum turnover number was ～5200 based on the total number of moles of gold but a factor of about six larger, TON ≈ 31 300, when only considering the Au-NP surface atoms. The fraction (N _S/N_T) of exposed surface atoms (N_S ≈ 2560) for an average 8 nm Au-NP (having N_T ≈ 15 800 atoms in a ～17-shell icosahedral or cuboctahedral particle) was estimated here as 0.16.

Full text of DOI:10.1016/j.molcata.2013.02.007

Journal of Molecular Catalysis A: Chemical 372 (2013) 72–78
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Dioxygen oxidation of 1-phenylethanol with gold nanoparticles and
N-hydroxyphthalimide in ionic liquid
Hassan Hosseini-Monfared∗,1, Hajo Meyer, Christoph Janiak∗
Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Gold nanoparticles (Au-NPs) of 8 nm average diameter were obtained by thermal reduction under
nitrogen from KAuCl4 in the presence of n-butylimidazol dispersed in the ionic liquid (IL) 1-n-butyl-
3-methylimidazolium tetrafluoroborate (BMIm+BF4−). Characterization of the Au-NP was done by
transmission electron microscopy (TEM) and dynamic light scattering (DLS). Catalytic activities of the
Au-NP/IL dispersion were evaluated in the oxidation of 1-phenylethanol at 100 and 160 ◦C under
4 bar pressure of dioxygen in a base-free system. Au-NP in combination with the radical initiator N-
hydroxyphthalimide (NHPI) showed good conversion and selectivity for the oxidation of 1-phenylethanol
to acetophenone through formation of an ␣-hydroxy carbon radical. The concomitant side products
di(1-phenylethyl)ether and di(1-phenylethyl)peroxide were rationalized by an equilibrium due to the
IL matrix of the ␣-hydroxy carbon radical with the 1-phenylethoxy radical. Maximum turnover number
was ∼5200 based on the total number of moles of gold but a factor of about six larger, TON ≈ 31 300, when
only considering the Au-NP surface atoms. The fraction (NS/NT) of exposed surface atoms (NS ≈ 2560) for
an average 8 nm Au-NP (having NT ≈ 15 800 atoms in a ∼17-shell icosahedral or cuboctahedral particle)
was estimated here as 0.16.
Received 30 November 2012
Received in revised form 10 February 2013
Accepted 11 February 2013
Available online xxx
Keywords:
Gold nanoparticles
Oxidation catalysis
Dioxygen
Ionic liquids
Alcohol
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
energy [8]. As a result of their colloidal instability, many nanopar-
ticles need to be stabilized via additional (capping) agents such
Selective oxidation of alcohols to carbonyl compounds is a fun-
damental transformation in organic synthesis because the target
molecules can be obtained directly in one-pot sequences. Alcohols
are important precursors for various chemicals and in particular,
the oxidation of alcohols and polyols is of interest owing to the
large array of biological hydroxy-derivatives [1–3]. Choice of the
as surfactants or polymers, which provide a steric, electrostatic or
electrosteric particle stabilization [9]. For catalytic applications of
NPs strong surface protection is, however, undesirable as it hinders
substrate access and interaction with the surface catalytic sites.
Instead ligand-free NPs should be advantageous for a high activ-
ity. Ionic liquids (ILs) can stabilize M-NPs on the basis of their ionic
nature [10], high polarity, high dielectric constant and supramolec-
ular network without the need of additional protective ligands [11].
ILs can therefore function both as stabilizer and solvent for the
preparation of small (<5 nm) and (generally) kinetically stabilized
M-NPs [4,12,13].
The past decade has seen an explosive growth in catalytic reac-
tions studied in ILs. Often the IL enables more efficient reactions
compared with standard organic solvents and catalysts show good
or even enhanced activities when applied in ionic liquids [14–16].
ILs are interesting in the context of green catalysis [17] which
requires that catalysts be designed for easy product separation from
the reaction products and multi-time efficient reuse/recycling [18].
The Pd-metal-catalyzed oxidation of benzyl alcohol with 1 atm O2
in the IL BMIm+X− gave good conversion to benzaldehyde, albeit
only for X = BF4 and not for X = Cl or Br and with Pd amounts of
equal or larger than 2.8 mol% [19].
oxidants determines the practicabili−ty and efficiency of the oxida-
2−
tion reactions. Oxidants, like MnO4 and Cr2O7
produce toxic,
environmentally and economically unacceptable by-products or
like NaOCl have a low (≤30 wt.%) active oxygen content. In this con-
text, O2 (or air) is the most attractive oxidants because of its high
contents of active oxygen species (100% for dioxygenase-type, 50%
for monooxygenase-type) and co-production of only water.
Nanoparticles (NPs) are of high interest in catalysis [4–6]. The
small size of nanoparticles results in a large fraction of surface
atoms [7]. Interaction of unprotected small particles will, however
lead to agglomeration or aggregation from the cohesive surface
∗
Corresponding authors. Tel.: +49 2118112286.
E-mail addresses: monfared@znu.ac.ir (H. Hosseini-Monfared),
janiak@uni-duesseldorf.de (C. Janiak).
Oxidation by gold is of timely interest for green processes requir-
ing stable, selective and non-toxic heterogeneous catalysts, as well
1
Permanent address: Department of Chemistry, University of Zanjan, 45195 313,
Zanjan, Iran.
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.02.007

H. Hosseini-Monfared et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 72–78
73
as air or O2 as oxidants [20–22]. Gold is cheaper than platinum,
palladium and most other noble metals used as catalyst. One of
the unique features of gold catalysis is the kinetic aspect of cor-
relating the turnover frequency strongly to the size of metallic
gold particles. In particular, many investigations on the liquid-
phase oxidation of polyols, alcohols and carbohydrates indicate
that only small gold particles are catalytically active [23]. A size
threshold in gold catalytic activity has been found resulting in
complete inactivity for particles with diameters >∼2 nm [20]. Com-
pared to other common catalysts, like the platinum group metals,
the outstanding properties of gold catalysis are also represented
by high selectivity which allows discrimination within chemical
groups and geometrical positions, leading to superior yields in the
desired products. Among many examples, glycols can be oxidized
to monocarboxylates [24], and unsaturated alcohols to unsatu-
rated aldehydes [25]. From its biocompatibility, availability and
easy recovery, gold appears as an exciting catalyst for sustain-
able processes based on the use of clean reagents, particularly O2.
Considering the influence of various parameters in the aerobic oxi-
dation of alcohols by gold based catalysts, it can be concluded that
a prominent role in the activity and selectivity is played by the
solvent [26].
Unsupported nanogold was described for the aerobic oxida-
tion of stilbene and cyclohexene in methylcyclohexane [27] and
the aerobic oxidation of alcohols [28]. Gold nanoparticles have
been used mostly as supported catalysts for oxidation applica-
tions [14,20,30,29,26]. The unique catalytic activity of supported
Au nanoparticles has been ascribed to various effects including
thickness/shape, the metal oxidation state, and support effects [31].
Benzyl alcohol is oxidized selectively to benzaldehyde with high
yield by molecular oxygen over a reusable nano-sized gold cata-
lyst supported on U3O8, MgO, Al2O3 or ZrO2 in the absence of any
solvent with only little formation of benzylbenzoate [32].
20 (S)TEM and FEI TITAN 80-300 probe CS-corrected STEM. All sam-
ples for HAADF-STEM were prepared by dropping a small amount
of Au-NP/IL dispersion on a carbon coated copper grid. After 5 min
the excess of the IL was removed by dipping the grid into a water
bath for 2 min. Finally the grid was plasma cleaned for 10 s.
Gas chromatography measurements were conducted using a
Perkin Elmer headspace GC HS6 with a flame ionization detec-
tor (FID) and a PEG capillary column (25 m long × 0.32 mm inner
diameter × 1.0 ␮m film thickness). The GC program used was: oven
temperature 120 ◦C, injector temperature 170 ◦C, carrier gas N2
pressure 200 kPa. The conversion was analyzed by adding a drop
of the mixture into a headspace GC sample vial with 1 mL of water.
The addition of water as a non-electrolyte can enlarge the activity
coefficient of organic components; thereby increases their detec-
tion sensitivity through the increase in peak area. The FID does
not detect the water itself [38]. In order to verify the results of
the headspace GC analyses we isolated the reaction products from
selected experiments using standard procedures and confirmed
their purity by 1H NMR. Conversion and selectivity were calcu-
lated with respect to the substrate. The products were identified
with Thermo Trace DSQ GC-MS and 1H NMR. 1H NMR spectra were
collected on a Bruker Avance DRX 500 spectrometer (500 MHz) or
a Bruker Avance DRX 200 spectrometer (200 MHz) as CDCl3 solu-
tions.
2.2. Synthesis of gold nanoparticles in ILs [12]
Au-NP was prepared by thermal decomposition and reduction
from KAuCl4 in an ionic liquid (cf. Scheme 1) under nitrogen in
a glass vessel which was connected to an oil bubbler. In a typical
experiment KAuCl4 (57.6 mg, 0.152 mmol) was dissolved/dispersed
(during 48 h) under nitrogen at room temperature in the pres-
ence of n-butylimidazole (1.5 equiv. for each Cl atom, 0.914 mmol)
in the ionic liquid 1-n-butyl-3-methylimidazolium tetrafluorob-
orate (BMIm+BF4−) (3.0 g, 2.5 mL, density 1.2 g/mL). The solution
was slowly heated to 230 ◦C for 18 h under magnetic stirring. The
solution color immediately changed from yellow to dark red (dark
brown-red), indicating the formation of gold nanoparticles. The
decomposition process gave a 1.0 wt.% or 60.8 ␮mol/mL disper-
sion of gold (30 mg in 3.0 g IL). During the decomposition process,
a white haze of n-butylimidazolium chloride was formed. After
cooling to room temperature under nitrogen, an aliquot of the
ionic liquid was collected under nitrogen atmosphere for TEM and
dynamic light scattering (DLS) characterization.
From the viewpoints of atom economy and environmental con-
cern, developing noble metal catalysts and using molecular oxygen
or air as the oxidant prevail as an attractive green technology
[33,34]. Here we report to the best of our knowledge for the first
time oxidation of an alcohol with gold nanoparticles (Au-NPs) in an
ionic liquid (Scheme 1).
2. Experimental
2.1. Materials and instrumentation
KAuCl4 was obtained from STREM, n-butylimidazole (p.a.)
from Aldrich, N-hydroxyphthalimide (NHPI) from ACROS, the
ionic liquid (IL) 1-n-butyl-3-methylimidazolium tetrafluoroborate
(BMIm+BF4−) from IoLiTec (H2O content ꢀ 100 ppm; Cl− con-
tent ꢀ 50 ppm). All manipulations were done by using Schlenk
techniques under nitrogen. The ionic liquid was dried under a
high vacuum (10−3 mbar) for several days to avoid hydrolysis of
2.3. Catalytic dioxygen oxidation of alcohol with Au-NP/IL
The oxidation reactions were carried out in a glass inlay of a
100 mL steel autoclave. The autoclave was conditioned by evac-
uation and re-filling with dioxygen. All autoclave loading was
carried out under air. In a typical experiment, 5.0 mL (41.3 mmol)
of 1-phenylethanol was added to the reactor with 0.1 mL Au-
NP/BMIm+BF4− dispersion (6.1 ␮mol Au) and NHPI (1 mmol). After
purging with O2, the reactor was pressurized to 4 bar and placed
in a thermostated oil bath at a chosen temperature of 100 or
160 ◦C (see Table 1). Stirring rate was 850 rpm. The O2 consump-
tion over time was monitored online with a Büchi pressflow gas
controller (Büchi pbc). After 24 h the reactor was depressurized
and the product mixture was analyzed by gas chromatography and
1H NMR as follows: (i) by taking a sample with a Pasteur-pipette
from the reaction mixture without removing the NP/IL dispersion
and after adding 1 mL water injecting it into headspace GC; (ii)
by evacuating the substrate/product from the NP/IL under vac-
uum (0.003 mbar, 200 ◦C) into a clean cold trap and injecting into
headspace GC; (iii) by removing the substrate/product from NP/IL
by extraction with petroleum ether or diethyl ether (3× 1 mL) and
BMIm+BF4 to HF [18,35–37].
−
FT-IR (Fourier transform infrared) measurements were carried
out on a Bruker TENSOR 37 IR spectrometer in a range from 4000
to 500 cm−1 with ATR technique.
A Malvern Zetasizer Nano-ZS was used for the dynamic light
scattering (DLS) measurements working at 633 nm wavelength.
The resolution of the DLS instrument is 0.6 nm. Care was taken for
choosing the right parameters, such as the index of refraction of Au
(0.11) with absorption of 0.1 at this wavelength. Samples were pre-
pared by dilution of the metal/IL dispersion (10 ␮L; 1.0 wt.% of Au)
in n-butylimidazole (2 mL, 99% p.a.; particle free) and was placed
in a glass cuvette before measurement.
HAADF-STEM (high angle annular dark field scanning transmis-
sion electron microscopy) images were taken with FEI TECNAI G2
F

74
H. Hosseini-Monfared et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 72–78
Scheme 1. Summary of the work reported here with two catalytic mechanisms depending on Au-NP/IL or Au-NP/IL/NHPI catalysis (Au-NP = gold nanoparticle, IL = ionic
liquid, NHPI = N-hydroxyphthalimide, PINO = phthalimide N-oxyl (PINO) radical).
Table 1
Oxidation of 1-phenylethanol with Au-NP and O2.a
No.
Catalyst
Temp. (◦C)
Conversion
(%)
Products (selectivity%)b
Acetophenone
TONc
TONsurface
∼Ether
∼Peroxide
d
1
2
3
4
5
6
7
8
9
None
NHPI
Au-NP/IL
100
100
100
100
100
100
160
160
160
160
160
160
No reaction
12
3
100
–
–
–
–
–
–
200
70
880
810
1200
420
5280
4860
100
100
69
47
74
86
68
61
58
48
Au-NP/ILe
Au-NP/IL/NHPI
Au-NP/IL/NHPI
None
2
13
60f
31
44
50
43
77
69
15
27
13
7
16
19
29
28
15
27
13
7
16
19
14
25
NHPI
Au-NP/IL
3390
970
5220
940
20 340
5820
31 320
5640
10
11
12
Au-NP/ILg
Au-NP/IL/NHPI
Au-NP/ILh/NHPI
a
Conditions: catalyst 0.1 mL Au-NP/IL dispersion (6.1 ␮mol Au), 5.0 mL 1-phenylethanol (41.35 mmol) giving a molar substrate/Au ratio of 6780, O2 pressure 4 bar, NHPI
(N-hydroxyphthalimide) 1 mmol, reaction time 24 h, unless noted otherwise. Each catalytic reaction was carried out at least twice to ensure reproducibility.
b
Products were acetophenone, di(1-phenylethyl)ether Ph(CH3)CH
TON = mol(products)/mol(Au) for the conversion reached after 24 h.
O CH(CH3)Ph and di(1-phenylethyl)peroxide Ph(CH3)CH O O CH(CH3)Ph.
c
d
e
f
TONsurface = [mol(products)/mol(Au)]/(NS/NT) with the fraction of surface atoms NS/NT = 0.16 or ∼1/6 (see estimation in text).
0.2 mL Au-NP/IL dispersion, 12.2 ␮mol Au, molar substrate/Au ratio = 3390.
1.0 mL 1-phenylethanol (8.27 mmol), molar substrate/Au ratio = 1356.
0.3 mL Au-NP/IL dispersion, 18.3 ␮mol Au, molar substrate/Au ratio = 2260.
0.5 mL Au-NP/IL, 30.5 ␮mol Au, molar substrate/Au ratio = 1356.
g
h
injecting the into headspace-GC. The reaction mixture (organic
phase + NP/IL) could also easily be analyzed by 1H NMR without
separation of NP/IL (see Fig. S1 in supporting information). In the
1H NMR spectrum of the reaction mixture there was no overlap
between substrates/products and IL peaks. This way it was also
ensured that no product was lost or the substrate/product ratio
changed due to distillation or extraction. Alternatively, for 1H NMR
analysis the substrate/product can be separated from the NP/IL by
extraction with diethyl ether (3× 1 mL) which is also preferred in
order to try to reuse the NP/IL.
red-purple, indicating an agglomeration process which is presum-
ably caused by the generated free HCl acid (cf. Scheme 1). In the
absence of air, Au-NP are effectively stabilized by the IL without
the need of additional capping ligands [12]. When 0.1 mL of the
Au-NP/IL dispersion was added to 5 mL of 1-phenylethanol for the
catalytic reaction (see below) no precipitation was detectable in
the presence of air and after a 24 h reaction at 100 or 160 ◦C.
To ensure reproducibility each catalytic reaction was carried out
at least twice.
3. Results and discussion
Gold nanoparticles with an average diameter of 8 nm
(Figs. 1 and 2) were produced in the ionic liquid 1-n-butyl-3-
methylimidazolium tetrafluoroborate (BMIm+BF4−) as reported
in the literature (Scheme 1) [12]. The synthesis of Au-NP was
carried out under nitrogen in a Schlenk vessel in the carefully
dried and deoxygenated ionic liquid. Upon exposure to air, the
Fig. 1. Example of a histogram of the number-size-distribution for Au-NP (synthe-
sized as 1 wt.% in BMIm+BF4−) from dynamic light scattering (DLS) with medium
diameter (hydrodynamic diameter with standard deviation ꢀ) of (8 2) nm from 10
measurements.
yellow-orange Au dispersion in BMIm+BF4 forms a red-purple
−
precipitate, indicating particle aggregation. Without the presence
of n-butylimidazole the Au-NP/IL dispersion also quickly turns

H. Hosseini-Monfared et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 72–78
75
Scheme 2. Formation of Au nanoparticles by thermal decomposition and reduction
of KAuCl4 in the IL BMIm+BF4−. The formed BHIm+Cl− was analyzed by elemental
analysis and 1H NMR. Au-NP formation is suggested through the formation of an
Au–carbene intermediate with subsequent reductive decomposition.
As for Pd-NPs, we suggest here the formation of N-heterocyclic
carbene (NHC)–Au species as the origin of the formation of Au-
NP (Scheme 2) [39,40]. We also suggest that the formation of
these Au–NHC intermediates slows down the aggregation process.
In the presence of n-butylimidazole, the released HCl is bound
as an imidazolium salt (precipitating as a white haze), similar
to the IL matrix. This prevents the formation of an acidic reac-
tion medium, which would destabilize the Au-NP and lead to
clustering.
3.1. Catalytic activities of Au-NP
The Au-NP catalytic potential was evaluated in the oxidation of
1-phenylethanol with dioxygen under pressure of 4 bars. Auto oxi-
dation of 1-phenylethanol was not observed under 4 bar pressure
of dioxygen at 100 ◦C (Table 1, entry 1). However, in the presence of
the radical initiator N-hydroxyphthalimide (NHPI) the alcohol was
oxidized with 12% conversion after 24 h (Table 1, no. 2). Oxidation
of 1-phenylethanol by using low or high concentration of Au-NP
in IL alone (6.1 ␮mol, 12.2 ␮mol) occurred with 100% selectivity to
acetophenone but the conversion (2–3%) was very low (Table 1,
nos. 3 and 4).
Au-NP/IL with the radical initiator NHPI increased the conver-
sion only upon lowering the alcohol substrate to gold ratio. In
addition the combination of Au-NP/IL with NHPI decreased the
0.30
Au-NP/IL
/NHPI
0.24
0.18
NHPI
0.12
none
25
0.06
0.00
0
5
10
15
Time [h]
20
Fig. 2. Examples of high-angle annular dark field scanning transmission electron
microscopy (HAADF-STEM) images of Au-NP in BMIm+BF4−. Particle size determi-
nation from TEM gave an average diameter (8 3) nm. The larger bright spot near
the center in the middle and bottom TEM pictures arises from the focused electron
beam during the image scan. This is clearly illustrated with the zoomed-in TEM at
the bottom, where the bright near-center spot was not yet present in the middle
TEM picture.
Fig. 3. Example curves (corresponding to Table 1, no. 7 (red), 8 (black) and 11
(green)) for the dioxygen gas uptake (volume at 4 bar pressure and 293 K ambi-
ent temperature) over time as a direct measure of the time–activity profile. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of the article.)

76
H. Hosseini-Monfared et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 72–78
selectivity (Table 1, nos. 5 and 6). Besides acetophenone also di(1-
phenylethyl)ether and di(1-phenylethyl)peroxide were formed
(Eq. (1)). Aliphatic and aromatic peroxides are used as curing agent
for siloxane polymers [41]. A lower molar ratio of 1-phenylethanol
to gold understandably increased the conversion (no. 6 versus no.
5).
The formulae given in Ref. [42] and the supporting information
of Ref. [43] (ꢂ missing there) are not denoted with b in the
denominator. However, this constant b must be added there to
account for the free space between the atom spheres in the metal
packing.
OH
Au-NP/IL
NHPI
O2
O
O
O
O
+
+
di(1-phenylethyl)ether
di(1-phenylethyl)peroxide
1-phenylethanol
acetophenone
(1)
Oxidation of 1-phenylethanol with O2/Au-NP was very sensitive to
temperature. In the absence of the Au-NP catalyst oxidation pro-
ceeded up to 31% conversion with O2 (Table 1, no. 7) or to 44%
with O2/NHPI only (no. 8) at 160 ◦C. The presence of Au-NP/IL
(without NHPI) yielded a conversion of 50% at 160 ◦C (Table 1,
no. 9) with about 70% selectivity for acetophenone. Increasing the
amount of Au-NP/IL unexpectedly lowered the conversion (no.
10 versus no. 9) and also the selectivity. The highest conver-
sion with TON of 5220 was achieved by Au-NP/IL/NHPI system
(Table 1, no. 11). Using a higher amount of Au-NP in the Au-
NP/IL/NHPI system again resulted in a lower conversion and
selectivity (Table 1, no. 12). As part of our study we used vari-
ous amounts of Au-NP/IL while keeping the NHPI amount (1 mmol)
constant. With Au-NP/IL in any proportion relative to NHPI, the
acetophenone selectivity always was lower than 100%. A near
optimal condition in terms of acetophenone selectivity for the
Au-NP/IL/NHPI system was obtained when 0.10 mL Au-NP/IL dis-
persion (with 6.1 ␮mol Au) was used with 1 mmol NHPI for 5.0 mL
(41.35 mmol) 1-phenylethanol (entries nos. 5 and 11). Turnover
numbers in oxidation reactions often start from less than 100
[42].
ꢀ
ꢁ
3
D
NT
=
(4)
2rb
Eq. (4) is from Ref. [44] (given there as d/datom = b(NT)1/3).
With the values for gold (ꢁ = 19.32 g/cm3, Ar = 196.966 g/mol,
b = 1.105 for Au with fcc packing, Vg = 0.01256 nm3, r = 0.1442 nm)
Eq. (2), (3) or (4) give NT = 15 820 atoms for Au-NP of 8 nm mean
diameter.
The total average atom number NT of 15 820 is close to the
magic atom number of Nm = 14 993 for an icosahedron or cubocta-
hedron with m = 17 shells (Nm = (1/3)(2m − 1)(5m2 − 5m + 3)) [45].
The number of surface atoms for a 17-shell icosahedron or cuboc-
tahedron is NS = 2562 (NS = 10m2 − 20m + 12) [45]. The fraction of
exposed or surface atoms NS/NT ≈ 2560/15 800 is 0.16 or 16% on
average. Thus, here the TON based on surface atoms will be a fac-
tor of about six larger than the TON based on the total number of
moles of gold. In view of the Au-NP size dispersion the number
of surface atoms, that is, their fraction can, however, only be an
estimate.
For each reaction the dioxygen gas uptake was followed over
time (Fig. 3). The gas uptake behavior with NHPI and Au-NP/IL/NHPI
shows a high reactivity during the first 1 or 3 h, respectively. After
this period, the curves level off. Yet, even after 24 h the reaction
slowly continues. A comparison of the curve profiles and slow oxy-
gen uptake in Fig. 3 after about 4–5 h is suggestive that after this
time the catalyst activity of NHPI and Au-NP/IL/NHPI has mostly
ceased and that the ongoing oxidation could be due to dioxygen
alone. A gold catalyst poisoning might be responsible for the Au-
NP/IL/NHPI plot leveling off. Such a poisoning could occur through
coordination of oxygen or product species to the active gold sites
(cf. Schemes 3 and 4).
Except for the last two entries nos. 11 and 12 with the Au-
NP/IL/NHPI system at 160 ◦C one can note an equimolar formation
of di(1-phenylethyl)ether and di(1-phenylethyl)peroxide (Table 1)
(see below for a suggested explanation).
The turnover number (TON) for metal nanoparticle catalyst is
usually based on the number of moles of the metal used. Typi-
cally, only the surface atoms or even a fraction of it are catalytically
active in a dispersed (heterogeneous) nanoparticle. Thus, the TON
calculated from only the surface atoms can be one order of mag-
nitude larger than the TON based on the total amount of metal
for nanoparticles having more than 15 shells. Most likely, only a
fraction of the surface atoms will be catalytically active, such as
corners, edges or defect sites. Therefore, the TON based on sur-
face atoms will still be an underestimate of the true, unknown TON
[42].
For the Au-NP/IL catalytic systeme (without NHPI) work-up
of the reaction mixture was carried out by vacuum distillation
From TEM and DLS we obtained an average Au-NP diameter of
8 nm. From this average nanoparticle diameter D the total number
of metal atoms (NT) in the nanocrystal can be calculated according
to Eq. (2), (3) or (4):
ꢀ ꢁ
3
NAꢁV
Ar
4
3
D
NT
=
and V =
ꢂ
(2)
2
with NA = Avogadro’s number (6.022 × 1023 mol−1), ꢁ = metal den-
sity, and Ar = relative atom mass (in g/mol) [42].
4ꢂ(D/2b)3
NT
=
(3)
3Vg
with b = 1.105 for closed packed (cp), face-centered cubic or
hexagonal-cp (fcc or hcp) crystallographic structures and 1.137
for body-centered cubic (bcc) structures, Vg = volume of a single
metal atom according to Vg = (4/3)ꢂr3, r = atomic (metal) radius.
Scheme 3. Possible dehydrogenation mechanism [cf. 50,51] of 1-phenylethanol
catalyzed on the metal surface by Au-NP/IL to form acetophenone as major prod-
uct and the side products di(1-phenylethyl)ether and di(1-phenylethyl)peroxide
(ads = adsorbed).

H. Hosseini-Monfared et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 72–78
77
Scheme 4. Possible reaction pathways (adapted from Ref. [58]) for the aerobic oxidation of 1-phenylethanol catalyzed by Au-NP/IL/NHPI to form acetophenone as major
product and the side products di(1-phenylethyl)ether and di(1-phenylethyl)peroxide. Steps (a)–(e) are described in the text.
(0.003 mbar, 200 ◦C) or by extraction with diethyl ether (3×
1 mL) in order to leave only the 0.1 mL Au-NP/IL dispersion
in the reaction vessel and to test for recyclability. Fresh 1-
phenylethanol substrate was then added but the conversion in the
second run consistently reached at most only about 50% (160 ◦C,
4 bar) or about 25% (160 ◦C, 1 bar) of the conversion in the first
run.
The consumed O2 volumes in Fig. 3 agree with the observed
conversions (Table 1) for the suggested mechanism (cf. Scheme 3)
assuming ideal gas behavior for the calculation. The uptake of
0.07(1) L after 24 h for O2 alone (without NHPI and Au-NP, red curve
in Fig. 3) translates into 11.5(1.6) mmol O2 which is to be com-
pared with 31% conversion of 41.35 mmol alcohol into 12.8 mmol
of products and H2O2. The O2 uptake of 0.16(1) L after 24 h with
NHPI (black curve in Fig. 3) corresponds to 26(2) mmol O2 which
is significantly higher than the 18.2 mmol products upon 44% con-
version. This reflects additional O2 needed for the NHPI-to-PINO
(re)generation. The O2 consumption of 0.28(1) L after 24 h with Au-
NP/IL/NHPI (green curve in Fig. 3) equals 46(2) mmol, again more
than 31.8 mmol products at 77% conversion due to additional O2
for the PINO (re)generation.
The autoxidation of primary and secondary alcohol with dioxy-
gen produces carbonyl compounds and hydrogen peroxide as the
primary products (Scheme S1 in supporting information) [46].
Alcohols are generally not autoxidized as readily as olefins or alde-
hydes, mainly owing to the high rate of termination of ␣-hydroxy
peroxy radicals.
Gold nanoparticles on solid supports are well known in alco-
hol oxidations, often under alkaline conditions, yet a mechanism
is often not discussed and considered elusive [32,47]. A metal-
assisted hydrogen abstraction from the hydroxyl group, which is
favored under basic conditions, is seen as the initial step [48]. For
example, it is generally accepted that benzaldehyde is formed from
benzyl alcohol by dehydrogenation on the gold surface [49,50]. In
the classical dehydrogenation mechanism on metal surfaces, the
alcohol dehydrogenates in two steps. The adsorbed oxygen (O) or
OH species react with the adsorbed H atoms (Scheme 3) [50,51].
The intermediate 1-phenylethoxy radical explains formation of the
minor ether and peroxide products. Thermal desorption analysis
on nanoporous gold confirmed the possible adsorptions of O2 and
1-phenylethanol [28].
The efficient aerobic oxidation of various types of organic com-
pounds has been carried out using N-hydroxyphthalimide (NHPI)
as a key radical generator in recent years [52,53]. It is believed that
the phthalimide N-oxyl (PINO) radical is generated in situ from the
reaction of O2 and NHPI [54]. In the presence of Au-NP the role
of Au-NP probably lies in the activation of O2 through chemisorp-
tion for the reaction with NHPI. The superoxo- or peroxo species,
[Au]n
O O, facilitates the production of PINO similar to reported
NHPI/Co(II) systems [55–57] (Scheme 4(a)).
Further, the PINO radical abstracts the hydrogen atom from
1-phenylethanol to produce the ␣-hydroxy carbon radical and
regenerate NHPI (Scheme 4(b)). The resulting ␣-hydroxy carbon
radical reacts with O2 to form a peroxy radical (Scheme 4(c)), which
is further converted to acetophenone, thereby regenerating the
PINO radical from NHPI (Scheme 4(d)). A similar mechanism has
been reported for the oxidation of alcohols to ketone or acid with
O2/NHPI/CuBr [58]. The PINO radical can, of course, also be regener-
ated through the reaction with Au-NP/O2 (cf. Scheme 4(a)) or with
O2 alone in the absence of Au-NP which explains the higher O2 con-
sumption than is needed for the alcohol to ketone conversion alone
(see above).
The ␣-hydroxy carbon radical in equilibrium with the 1-
phenylethoxy radical can also react to the minor ether and peroxide
products as sketched in Scheme 4(e). The largely equimolar for-
mation of di(1-phenylethyl)ether and di(1-phenylethyl)peroxide
suggests that they are coming from the same species. Since both
the peroxide and ether are also produced in the absence of NHPI
and Au-NP/IL at 160 ◦C (Table 1) and since there is no report on the
formation of these products by the other metal-catalyzed oxidation
of alcohols with NHPI/O2 [52], we conclude that these compounds
are produced because of partial stabilization of the 1-phenylethoxy
radical in equilibrium with ␣-hydroxy carbon radical by the ionic
liquid.

78
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Appendix A. Supplementary data
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found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2013.02.007.
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