Huang et al.
Dehydration of Phenylboronic Acid to Boroxine Catalyzed by Au Nanoclusters
n
face-fused biicosahedral Au23 core encapsulated by a shell
of fifteen gold atoms. The catalytic studies of dehydra-
tion of phenylboronic acid to boroxine indicated that Au38
nanocluster showed more stability and higher activity than
Au25 nanocluster.
2.4. Characterization
Electrospray ionization (ESI) mass spectra were recorded
using a Waters Q-TOF mass spectrometer equipped with
Z-spray source. The source temperature was kept at 70 C.
ꢀ
The sample was directly infused into the chamber at
5
ꢁL/min. The spray voltage was kept at 2.20 kV and
2. EXPERIMENTAL DETAILS
the cone voltage at 60 V. UV-vis spectra of the Au (dis-
n
solved in CH Cl ꢂ were acquired on SHIMADZU UV-
2
.1. Preparation of Au25 Nanoclusters
2
2
2700 diode array spectrophotometer at room temperature.
0
.4 mmol HAuCl · 3H O dissolved in 5 mL nanopure
The catalytic products were analyzed by a gas chromato-
graph (Agilent Technologies: 6890N).
4
2
water, and 0.47 mmol TOAB (tetraoctylammonium bro-
mide) dissolved in 10 mL toluene, were combined in a
2
5 mL tri-neck round bottom flask. After 15 min stirring,
3
. RESULTS AND DISCUSSION
the aqueous was removed using a 10 mL syringe. The
toluene solution of gold(III) cooled down to 0 C in an ice
bath over a period of 30 min. 0.17 mL PhCH CH SH was
added. After the solution turns to clear, an aqueous solu-
ꢀ
The atom packing structures of Aun nanoclusters are
critical for identification catalytically active species and
understanding the correlation of structural properties
with catalytic properties. The formation of one-sized
monodisperse Aun nanoclusters in the above experi-
mental conditions and the obtaining of single crystals
allow us to determine their total structures by electro-
spray ionization mass spectrometry (ESI-MS) and X-ray
2
2
tion of 4 mmol NaBH was quickly added. After 12 hours
4
reaction, 20 mL ethanol was added to separate Au25 clus-
ters from TOAB. Au25 nanoclusters were obtained after
removing the supernatant.
1
4–16
2
.2. Preparation of Au38 Nanolcusters
crystallography.
Au25 and Au38 nanoclusters have been
found that they possess a non-crystalline atomic pack-
ing structure and no translational symmetry in the metal
core unlike crystalline larger Au nanoparticles that adopt
0
.3 mmol HAuCl · 3H O was first dissolved in 20 mL
4
2
methanol and 1.2 mmol phenylethanethiol (PhCH CH SH)
was dissolved in 10 mL nanopure water. The two solu-
2
2
Delivered by Publishing Technology to: Chinese University of Hong Kong
typical face centered cubic (fcc) structure, and accord-
ꢀ
tions were then mixed and then cI oP o: l e2 d2 1t o. 2 03 5. C7 .0 .38 3m Om no :l Fri, 08 Jan 2016 15:04:30
ingly exhibit unique and interesting electronic properties.
Copyright: American Scientific Publishers
NaBH4 dissolved in 6 mL nanopure water was added
to the suspension. After one hour reaction, the pre-
cipitation (Au:SR) was collected by centrifugation and
washed with methanol. The above Au:SR clusters were
dissolved in 4 mL nanopure water. Then 6 mL acetone and
In detail, Au (or denoted as Au (SR) , SR is thiolate
2
5
25
18
ligant.) nanocluster features a centered icosahedral Au13
kernel encapsulated by an exterior gold shell comprised
of twelve gold atoms in six extended staples of –S–Au–
1
5ꢀ16
S–Au–S–, denoted as Au (S) below.
Viewed along
2
3
8
mL phenylethanethiol were added and the PhCH CH SH
2 2
the three mutually perpendicular C axes of the icosahe-
2
phase forms the upper layer and the aqueous solution of
Au:SR clusters forms the bottom layer. The solution was
heated to 80 C and kept for several hours. After 3 hours,
the isolated phenylethanethiol phase was washed with
ethanol and acetone to remove excess phenylethanethiol
and byproducts. Au38 nanoclusters were extracted with
toluene.
dron, the twelve exterior gold atoms form six pairs and
are situated around the ± x, ± y, and ± z axes, respec-
tively (Fig. 1(A) inset). Another view of the Au (SR)
ꢀ
2
5
18
structure is an Au13 icosahedral kernel capped by six sta-
ple motifs of –S–Au–S–Au–S– along the ± x, ± y, and
±
z axes. Apparently, the twelve exterior gold atoms form
an open shell on the Au icosahedron. An icosahedron
1
3
has 20 triangular faces (Au ꢂ, but only twelve of them are
3
2
.3. Catalytic Test
face-capped and leave eight Au triangular faces uncapped.
3
0
.66 mmol barium hydroxide was dissolved in 10 mL
The entire Au25 cluster is protected by eighteen ligands
containing S and the entire particle exhibits a quasi-D2h
symmetry.
nanopure water in a round bottom flask and the solution
was heated to 80 C. 0.44 mmol phenylboronic acid was
ꢀ
added to the flask and stirred for 5 min. 2 mg Au catalyst
Au38 (or denoted as Au (SR) ꢂ nanocluster is com-
n
38
24
in 10 mL toluene was added to initiate the reaction. After
the solution was stirred for 12 hours at 80 C, the product
posed of an icosahedral Au core protected by fifteen gold
atoms shell (Fig. 1(B)). Au38 has a face-fused biicosahe-
23
ꢀ
17
was extracted with hexane. The product enters the organic
layer and the excess boronic acid remains in the aqueous
layer. The organic layer was washed twice with saturated
dral Au23 (13 + 13 − 3 = 23) core. The exterior Au shell
comprises 15 Au atoms face-capped onto the Au faces of
3
the Au23 core; note that some Au faces are left uncapped.
3
NaCl solution and dried over MgSO powders. After filtra-
tion, hexane was removed in a rotary evaporator to obtain
the final product.
Each exterior Au atom has one shorter Au–Au contact
(3.04–3.12 Å) to the biicosahedron and two longer ones
4
(3.18–3.29 Å) to the other two Au atoms on the Au face
3
J. Nanosci. Nanotechnol. 13, 5088–5092, 2013
5089