Supported Au nanoparticles as efficient catalysts for aerobic homocoupling of phenylboronic acid

Article abstract of DOI:10.1039/c2cc31115a

Au nanoparticles with small sizes (1-4 nm) were effectively formed on Mg-Al mixed oxides (Au/MAO), which showed superior catalytic performances and good recyclability in aerobic homocoupling of phenylboronic acid.

Full text of DOI:10.1039/c2cc31115a

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COMMUNICATION
Supported Au nanoparticles as efficient catalysts for aerobic
homocoupling of phenylboronic acidw
Liang Wang,a Wei Zhang,b Dang Sheng Su,b Xiangju Mengc and Feng-Shou Xiao*c
Received 15th February 2012, Accepted 14th April 2012
DOI: 10.1039/c2cc31115a
Au nanoparticles with small sizes (1–4 nm) were effectively
formed on Mg–Al mixed oxides (Au/MAO), which showed
superior catalytic performances and good recyclability in aerobic
homocoupling of phenylboronic acid.
One of the most important discoveries in the field of synthetic
organic chemistry is the coupling reactions for the formation
of carbon to carbon bonds, because the coupling products of
biaryls are widely applied for the production of pharmaceutical and
fine chemicals.1–7 Particularly, homocoupling of phenylboronic
acid has been attracting much attention recently.4–7 Normally,
Fig. 1 TEM images of Au/MAO. Inset: enlarged TEM image.
this reaction is catalyzed by Pd2+ species.4 But in recent years,
Au catalysts, which are currently a hot topic in catalysis,5–17 have
performed well in homocoupling of phenylboronic acid.5–8 For
example, Corma and co-workers reported that Au3+ cations
supported on CeO2 show excellent catalytic performances in
homocoupling of phenylboronic acid;5 Primo and co-workers
showed that Au nanoparticles rich in Au3+ and Au+ are effective
catalysts for this reaction.7 In these cases, gold cations are
catalytically active sites for the homocouplings.5–8
a MAO support, with narrow size distribution of 1.0–4.0 nm.
The high-resolution TEM images of Au nanoparticles show
obvious Au lattices, with a lattice spacing of 0.239 nm,
associated with {111} Au (Fig. 1b).9c These results indicate
that the Au species are present as metallic Au nanoparticles on
the MAO support. Additionally, in comparison with the XRD
patterns of the MAO support, Au/MAO gives additional
peaks at 38.71, 64.71, and 77.71, which are associated with
(111), (220) and (311) incidences of metallic Au (Fig. S1, ESIw).15a
The XRD results are in good agreement with the results obtained
by TEM, confirming that Au nanoparticles are successfully formed
on the MAO support. Fig. 2 shows Au4f XPS spectra of the
Au/MAO samples with various treatments. The as-synthesized
Au/MAO gives a binding energy of Au4f7/2 of 84.5 eV (Fig. 2a),
with a shift of 0.5 eV from the typical Au0 (84.0 eV).15,16
This shift is reasonably attributed to the interaction between
Au nanoparticles and the MAO support.15,16
Additionally, Tsukuda and co-workers also reported water-
soluble Au nanoparticles as homogeneous-like catalysts exhibiting
excellent catalytic properties in the homocoupling of phenylboronic
acid.8 Here, we demonstrate that Au nanoparticles formed on
Mg–Al mixed oxides (Au/MAO, molar ratio of Mg/Al of 3) as
efficient heterogeneous catalysts are highly active in homocoupling
of phenylboronic acid. Importantly, the Au/MAO has excellent
recyclability and a new mechanism has been proposed.
The Au/MAO catalyst was prepared by anion-exchange of
AuCl4À, followed by calcination at 400 1C for the formation of
Au nanoparticles. In order to completely eliminate Au+ or
Au3+ species, the Au/MAO was treated in NaBH4 solution
(see ESIw). The Au loading is 0.7 wt% as analyzed by
inductively coupled plasma spectroscopy (ICP). Fig. 1 shows
typical transmission electron microscopy (TEM) images of
Au/MAO, where the Au nanoparticles are highly dispersed on
Table 1 presents catalytic activities and selectivities of
various Au catalysts in homocoupling of phenylboronic acid.
Generally, an inorganic base (e.g. K2CO3) is necessary to
activate the phenylboronic acid.5–8 However, the Au/MAO
catalyst shows very high activities under base-free conditions
(Table 1, entries 1–4), and the MAO support is completely
inactive in this homocoupling under the same reaction conditions
(Table 1, entry 12). For example, Au/MAO gives the conversion
of phenylboronic acid higher than 99.5% with diphenyl yields of
88.0–90.0% in MeOH or EtOH solvent (Table 1, entries 1 and 2).
These results indicate that the Au/MAO catalyst is very efficient.
In addition, the Au/MAO catalyst is very stable, and the
Au leaching was negligible in the reaction. For example, after
treating Au/MAO in MeOH for 12 h and filtrating the solid
catalyst, the Au concentration in the liquor is less than 5 ppb
a State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun 130012, China
b Department of Inorganic Chemistry, Fritz Haber Institute of the
Max Planck Society, Faradayweg 4-6, Berlin 14195, Germany
c Key Lab of Applied Chemistry of Zhejiang Province, Zhejiang
University, Hangzhou 310028, China. E-mail: fsxiao@zju.edu.cn
w Electronic supplementary information (ESI) available: Experimental
details, XRD patterns and IR spectra. See DOI: 10.1039/c2cc31115a
c
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Fig. 3 Dependences of catalytic conversion on time in homocoupling
of phenylboronic acid over (a) Au/MAO catalyst in toluene solvent,
(b) Au/MAO catalyst in anhydrous toluene solvent, (c) Au/MAO
catalyst poisoned by triethoxyethylsilane in anhydrous toluene solvent.
The reaction conditions were the same as those given in Table 1.
0.6 eV from that of the as-synthesized Au/MAO (Fig. 2b).
This shift might be attributed to the interaction between Au
nanoparticles and phenylboronic acid such that the boronic acid
groups were transmetalated by Au nanoparticles.17 Interestingly,
when molecular oxygen was introduced into the reaction system,
the Au/MAO gave an Au4f7/2 binding energy of 84.5 eV (Fig. 2c)
again. These results confirm that the molecular oxygen should
interact with Au nanoparticles.
Fig. 2 Au4f XPS spectra of (a) as-synthesized Au/MAO, (b) Au/MAO
treated with phenylboronic acid in the absence of oxygen, (c) after (b),
exposure to molecular oxygen.
Table 1 Homocoupling of phenylboronic acid on Au catalystsa
Notably, there are H2O molecules18 and hydroxyl groups on
the MAO support in the reaction system. To investigate the
necessity of H2O molecules for the reaction, catalytic conversion
over the Au/MAO catalyst in the presence of anhydrous toluene
was tested. As shown in Fig. 3, when the reaction time reached
3 h, the Au/MAO catalyst with anhydrous toluene gave a
conversion of 9.6% (Fig. 3b), which is much lower than that
(34.1%, Fig. 3a) of the Au/MAO catalyst with conventional
toluene. Upon addition of 0.5 mL of water to the reaction system,
the high conversion of phenylboronic acid (86.3%) on Au/MAO
could be obtained after reaction for 11 h (Fig. 3b). These results
suggest that a small amount of water is necessary for the homo-
coupling. Additionally, the product distribution influenced by H2O
molecules in the reaction system was also investigated. In conven-
tional toluene solvent, the molar ratio of biphenyl and phenol in
the products was 13.9 (88.7%/6.4%, Table 1, entry 3), which is very
similar to the value of 14.1 (22.6%/1.6%, Table 1, entry 8) in
anhydrous toluene solvent, suggesting that a small amount of
H2O in the reaction system almost does not influence the
product distribution. However, when the water solvent was used,
the molar ratio of biphenyl and phenol in the products was 1.2
(53.4%/44.6%, Table 1, entry 4), suggesting that a large amount of
water strongly influences the phenol selectivity. This phenomenon
might be assigned to that more hydroxyls are favorable for the
formation of phenols by a side-reaction (step 2, Scheme S2, ESIw).
In contrast, when the Au/MAO catalyst was poisoned by triethoxy-
ethylsilane (Scheme S1 and Fig. S2, ESIw) to remove most of the
surface hydroxyl groups, it still gave relatively low conversion
(24.6%) even if water was introduced and the reaction time was
long enough (Fig. 3c). These results suggest that hydroxyl groups
on Au/MAO play a critical role in the homocoupling.
Yieldb (%)
Conversion of
Ph–Ph Ph–OH Ph–B(OH)2c (%)
Entry Catalyst Solvent
1
2
3
4
Au/MAO MeOH
Au/MAO EtOH
Au/MAO Toluene
Au/MAO H2Od
Au/MAO MeOH
Au/MAO MeOH
90.0 10.0
88.0 12.0
88.7 6.4
53.4 44.6
90.4 9.6
85.0 12.9
>99.5
>99.5
95.1
98.0
>99.5
97.9
5e
6f
7g
8
Au/MAO Deoxidized MeOH Trace Trace o1.0
Au/MAO Anhydrous toluene 22.6 1.6 24.2
Trace 2.5
9
10
Au/SiO2 MeOH
2.5
Au/
Al2O3
MeOH
12.3 3.5
15.8
11
12
Au/TiO2 MeOH
MAO MeOH
7.0
—
0.3
h
—
7.3
—
h
h
a
Reaction conditions: 1 mmol of phenylboronic acid, 6 mL of solvent,
30 mg of Au catalyst, 1.5 MPa of O2, 100 1C for 12 h, dodecane as internal
standard. b GC yields. c Conversion of Ph–B(OH)2 for the yield of
products. d The mixture stirred at room temperature for 1 h before reaction.
e Reused. f Recycled 4 times. g N2 instead of O2. h Undetectable.
by ICP analysis. Very interestingly, after recycling 4 times, the
Au/MAO catalyst is still very active, giving the conversion of
97.9% (Table 1, entries 5 and 6). These results indicate that the
Au/MAO catalyst has excellent recyclability.
To understand the role of oxygen in the reaction, oxygen
was replaced by nitrogen. As a result, no products were
detected in the homocoupling (Table 1, entry 7). This result
suggests that oxygen is necessary for this reaction. It is worth
mentioning that when treated in deoxidized toluene containing
phenylboronic acid in the absence of oxygen, the Au/MAO
showed an Au4f7/2 binding energy of 83.9 eV, with a shift of
Based on the experimental evidence and the suggested
mechanism in the literature,4,5,8,14a,17 we proposed the homo-
coupling of phenylboronic acid over the Au/MAO catalyst as
c
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Chem. Commun., 2012, 48, 5476–5478 5477

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Notes and references
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Scheme 1 The proposed new mechanism for aerobic homocoupling
of phenylboronic acid over a Au/MAO catalyst.
follows (Scheme 1): (1) the phenylboronic acids should interact
with the surface hydroxyl groups of MAO to form boric acid
and phenyl species on Au nanoparticles with negative charge
(Aun2À, Au4f XPS in Fig. 2b); (2) the phenyl species on the
Au nanoparticles interact with each other to form a biphenyl
product, where the homocoupling occurs. However, if the phenyl
species on the Au nanoparticles interact with hydroxyl groups, a
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18 The H2O molecules were mainly from the solvents (higher than
4 mmol mLÀ1 of H2O) without any anhydrous treatment.
2Ph–B(OH)2 + H2O + 1/2O2 - Ph–Ph + 2B(OH)3
Furthermore, Au nanoparticles formed on SiO2, Al2O3 and
TiO2 with similar Au loadings were prepared, but they show
much lower conversions of phenylboronic acid (2.5–15.8%,
Table 1, entries 9–11) than Au/MAO, which possibly are due
to the difference in Au particle sizes and hydroxyl groups. These
works are currently under investigation. Moreover, various
substituted phenylboronic acids were employed in the homo-
coupling over Au/MAO. As shown in Table S1 (ESIw), the
catalyst worked well with different substrates, giving the desired
coupling products with high conversion (83.7–99.5%).
In summary, we prepared Au nanoparticles on MAO, which
shows high activities and excellent recyclability in aerobic
coupling of phenylboronic acid under base-free conditions.
Catalytic tests and XPS characterizations show that molecular
oxygen, hydroxyls on MAO, and H2O in a reaction system
play important roles in this reaction, therefore a possible
mechanism is proposed. Considering the importance of hetero-
geneous nanosized Au catalysts and homocouplings, it is
expected that this work could have great potential for the
production of pharmaceutical and fine chemicals that need
further exploration.
This work is supported by the National Natural Science
Foundation of China (20973079 and U1162201) and State
Basic Research Project of China (2009CB623501).
c
5478 Chem. Commun., 2012, 48, 5476–5478
This journal is The Royal Society of Chemistry 2012