Oxidation of benzylic compounds by gold nanowires at 1 atm O2

Article abstract of DOI:10.1039/c0cc01568g

Ultra-thin gold nanowires (GNWs), with uniform surface crystallinity, appear much more active than gold nanoclusters in benzylic compound oxidation. Ethylbenzene oxidation was used as a model reaction and a radical chain reaction mechanism was revealed by the DFT method.

Full text of DOI:10.1039/c0cc01568g

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Oxidation of benzylic compounds by gold nanowires at 1 atm O2w
Lei Hu,a Xueqin Cao,a Jianhui Yang,b Min Li,a Haiyan Hong,a Qingfeng Xu,a Jianfeng Ge,a
Lihua Wang,a Jianmei Lu,*a Liang Chen*b and Hongwei Gu*a
Received 25th May 2010, Accepted 11th October 2010
DOI: 10.1039/c0cc01568g
Ultra-thin gold nanowires (GNWs), with uniform surface
crystallinity, appear much more active than gold nanoclusters
in benzylic compound oxidation. Ethylbenzene oxidation was
used as a model reaction and a radical chain reaction mechanism
was revealed by the DFT method.
Fringes associated with the gold lattice appear clear, in which
the axial direction is (111) (Fig. 1B inset).
The oxidation of styrene was used to evaluate the activity
of the GNW catalyst. Table 1 compares the catalytic
performance of the GNWs for the partial oxidation of styrene
in different solvents at 100 1C for 20 h. The gold content
determined using inductively coupled plasma mass spectro-
scopy (ICP-MS) was 0.0252% (wt%), which was four times
lower than the nanocluster catalyst. It was interesting to find
that the solvent has an important effect on product formation.
Using xylene or chlorobenzene as solvent, aldehyde is the major
product with B70% selectivity. In contrast, epoxy is the major
product in DMF or 1, 4-dioxane, with a selectivity as high as
63.3%. In 1,4-dioxane, the styrene conversion is 81.7% and the
aldehyde and epoxy yield is 31.4 and 44.0%, respectively. In the
styrene oxidation reaction, molecular oxygen was firstly
absorbed on the surface of the gold nanowires to form active-
oxygen. Both solvent and styrene can react with this active-
oxygen through t wo pathways present in the reaction: (1) if the
solvent firstly reacted with this active-oxygen, a free radical
intermediate would be formed. This intermediate is not stable,
and can form peroxide in an oxygen environment. This per-
oxide acts as an oxidant in the styrene oxidation reaction and an
epoxy compound is the major product.9 (2) If the solvent is
more stable than styrene, styrene can react with the active-
oxygen and aldehyde is the major product.5 When using xylene
as a solvent, methylbenzaldehyde, a new oxidation product, was
detected by GC-MS. This product is formed from the partial
oxidation of xylene (ESIw). A free-radical reaction mechanism
was further demonstrated by adding 1-mercaptohexadecane to
the reactant. No oxidation products were achieved under the
same reaction conditions. 1-Mercaptohexadecane served two
important roles: (1) capping on the surface of gold nanowires to
inhibit the interaction of oxygen and the nanowire surface;
(2) as a free-radical quencher to stop the reaction.
Because of their importance in the manufacture of dyes,
flavorants, perfumes, pharmaceuticals and fine chemicals,1
oxidation reactions involving active gold, such as catalytic
alcohol oxidation,2 the direct synthesis of azobenzene,3 and
low temperature CO oxidation,4 have been of considerable
interest over the past decade. Lambert’s group5 reported that
ultra-small gold nanoclusters have relatively high catalytic
activity and selectivity as evaluated by styrene oxidation under
molecular oxygen. Dapurkar et al.6 reported that ketone
formation from benzylic compounds could be achieved using
supported gold nanoparticle catalysts at 1 atm O2. Recently, it
has been found that the nanomaterials’ catalytic abilities and
product selectivity are dependent on the catalysts’ surface
crystallinity.7 Here, we investigated the catalytic performance
of ultra-thin gold nanowires (GNWs), a new morphology of
active gold with single surface crystallinity, and found that
these one-dimensional gold nanocatalysts are more active than
gold nanoclusters and that the catalytic cycles undergo a
radical chain oxidation mechanism.
GNWs with an average diameter of 1.4 nm were prepared
following a reported methodology8 with minor modification.
HAuCl4Á4H2O (0.2 mmol) in 3 ml hexane and 1 ml oleylamine
was injected into 9 ml oleylamine at room temperature while
stirring vigorously. The temperature was raised to 80 1C to
remove the hexane and the mixture turned colorless in 5 min.
The reaction was maintained at 80 1C for 4 h without stirring.
The GNWs were collected by precipitation and centrifugation.
Fig. 1A shows representative transmission electron micro-
scopy (TEM) images of the as-synthesized GNWs. The nano-
wires are a few micrometres in length with an average diameter
of approximately 1.4 nm, as calculated from the analysis of
high-resolution TEM images in many different regions.
a Key Laboratory of Organic Synthesis of Jiangsu Province, Key
Laboratory of Absorbent Materials and Techniques for Environment,
College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou, 215123, China.
E-mail: lujm@suda.edu.cn, hongwei@suda.edu.cn;
Fax: +86-65880905; Tel: +86-65880905
b Institute of Materials Technology & Engineering,
Chinese Academy of Sciences, Ningbo, 315201, China
w Electronic supplementary information (ESI) available: Details of
GNWs synthesis and catalyst testing; GC spectra; Computational
details and the coordinates of GNWs are provided (5 pages). See
DOI: 10.1039/c0cc01568g
Fig. 1 Transmission electron microscope images of ultra-thin Au
nanowires. (Inset: high resolution TEM image of nanowires.) Experi-
ments were performed with a Schlenk system.
c
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Chem. Commun., 2011, 47, 1303–1305 1303

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Table 1 Catalytic performance of GNWs based on the partial
oxidation of styrenea
Yield (%)c
Selectivity (%)c
Entryb
Conversion (%)
a
b
a
b
c
1
2
3
4
5
6
7
23.2
81.7
22.7
39.5
3.9
6.5
14.7
44.0
4.4
9.4
1.2
27.9
38.5
70.5
62.5
69.3
50.6
87.2
63.3
53.9
19.2
23.8
30.7
5.7
8.8
1.6
10.3
13.7
—
31.4
16.0
24.7
2.7
14.5
14.8
28.6
17.0
1.6
1.68
3.8
0.9
9.9
a
Reactions are carried out with 1.25 g styrene and 0.4 mg GNWs.
1. DMF, 2. 1,4-dioxane, 3. m-xylene, 4. p-xylene, 5. chlorobenzene,
b
c
6. heptane, 7. toluene; Calculated by GC-MS using decane as an
internal standard.
Table 2 summarizes the catalyst activities in the oxidation of
various benzylic compounds at 1 atm O2 where the benzylic
compounds were used as both solvent and reactant. Ethylben-
zene, 4-nitroethylbenzene and iso-propylbenzene formed a
ketone with 21, 23 and 47% conversion, respectively. GNWs
show a higher selectivity (100%) as compared with Dapurkar’s
results6 (ethylbenzene oxidation with 85% selectivity).
Fig. 2 Illustration of ethylbenzene oxidation on the GNWs.
than on the Au (111) surface.11 The roughness of the curved
surface increases the reactivity of the GNW by altering the
electronic structures.12 The dissociated atomic oxygen on the
GNW is highly reactive and is able to activate the C–H bond.
Indeed, the extraction of the most active hydrogen atom to
form an OH radical turns out to be exothermic, with a reaction
energy of 0.12 eV. The required activation barrier is as low as
0.32 eV.
To establish the reaction mechanisms of ethylbenzene
oxidation on GNWs, we performed DFT calculations
(Fig. S1w) based on a coaxial multi-shell GNW model according
to the work of Kondo10. The saturated ethyl group is too inert
to interact with the GNW surface. Ethylbenzene can therefore
only be physisorbed on the GNW, with the benzene ring
perpendicular to the GNW surface. C–H activation
(3.79 eV) in the ethyl group is extremely difficult in the gas
phase or on the gold surface and the reaction is expected to be
triggered by the endothermic dissociation of O2 on the GNW
with a reaction energy of À0.11 eV. Furthermore, the corre-
sponding activation barrier is 1.01 eV, which is 0.25 eV lower
After removal of the first hydrogen, the ethylbenzene radical
is able to bind with either the OH adsorbed on the GNW
(Fig. 2, cycle 1) or with an O2 molecule (cycle 2). However, the
strong interaction (2.08 eV) between the OH and the GNW
reduces the probability of cycle 1 occurring. Furthermore, this
step will consume the active oxygen atoms and rapidly termi-
nate the catalytic reactions since O2 dissociation is rather
difficult. On the other hand, O2 molecules can be strongly
bound to the ethylbenzene radical, with an adsorption energy
(Ea) of 1.23 eV. In return, the O2 bond strength is significantly
weakened, as indicated by the bond length elongation from
1.23 to 1.38 A. The hanging oxygen in O2 is activated to
interact with the GNW with an Ea of 0.44 eV. Meanwhile,
cleavage of the C–H bond is also promoted as the second
hydrogen atom can be captured by the OH radical to form an
H2O molecule (Ea = 1.2 eV). As a consequence, the C–O bond
transforms to the CQO double bond, which subsequently
dissociates the attached O2 to yield a new oxygen atom on
the GNW. The associated reaction energy is calculated to be
0.94 eV. As such, no net atomic oxygen is consumed in cycle 2.
Only trace amounts of initial O2 dissociation can successfully
trigger the catalytic reactions.
Table 2 Catalytic results of the partial oxidation of alkylbenzenea
Selectivity (%)
a
b
Entry
R
R1
R2
Conversion (%)
1
2
3
4
5
6
7
8
9
10
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
1.3
4.3
1.3
3.4
1.0
0.6
95.2
62.1
100
82.4
100
100
100
83.5
70.8
62.9
4.8
37.9
0
17.6
0
0
o-CH3
m-CH3
p-CH3
p-Cl
p-OCH3 –H
–H
p-NO2
–H
–H
–CH3 21.1
–CH3 23.1
–CH3
0
16.5
29.2
37.1
p-OCH3 –H 1.3
–H –CH3 –CH3 47.9
Generally, the reactivity of benzylic compounds correlates
with their C–H bond strength.13 Table 2 shows that toluene or
xylene form corresponding aldehydes with low conversions.
Compared with the weakest C–H bond strength of
a
Reactions are carried out with 1.25 g alkylbenzene and 0.5 mg
GNWs.
c
1304 Chem. Commun., 2011, 47, 1303–1305
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ethylbenzene (3.79 eV), the bond strength of toluene is much
higher at 3.97 eV. In summary, we successfully synthesized
GNWs with single surface crystallinity, and these GNWs show
high activities for oxidation reactions at 1 atm O2. Our DFT
calculations reveal that catalysis proceeds via a radical
chain oxidation mechanism. Our findings present new
opportunities for gold catalysis and should provide significant
benefit towards understanding the catalytic mechanisms of
nanomaterials.
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c
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Chem. Commun., 2011, 47, 1303–1305 1305