6
0
N. Candu et al. / Journal of Catalysis 352 (2017) 59–66
In comparison with Pd, other transition metals are considered
sonicated at 700 W for 1 h in water and the unsuspended particles
were removed by centrifugation to obtain fl-G dispersed in water.
Water was removed by freeze-drying at room temperature to
obtain fl-G as powder.
much less general and efficient catalysts to promote coupling reac-
tions [26,27]. Specifically in the case of Au, although the number of
studies is still very scarce, Suzuki-Miyaura and Sonogashira CAC
coupling reactions have been reported [28–33].
The lower catalytic activity of Au NPs does not fit well with the
somehow unexpected ability of small Au NPs (<5 nm) to catalyze
the coupling of chlorobenzene and phenylboronic acid [28].
Chlorobenzene exhibits low reactivity for cross coupling using Pd
catalysts. In the case of Au catalysts, the yield of biphenyl
decreases from 87% to 76% and 10% when the size of NPs increases
from 1 to 2 and 5 nm and no coupling occurs for samples with Au
NP size larger than 10 nm [28]. Chaudhary et al. showed that the
support plays a crucial role in the catalytic activity of Au NPs as
Suzuki-Miyaura catalysts, basic alkaline earth oxides being the
most appropriate ones among a variety of other metal oxides
2
.1.2. Au NPs deposition on fl-G (0.1 wt%)
fl-G obtained from pyrolysis of alginate as described above
(
100 mg) was added to ethylene glycol (40 mL) and the suspension
was sonicated at 700 W for 1 h to obtain dispersed fl-G. HAuCl
0.2 mg) was added to the suspension of ethylene glycol containing
4
(
fl-G and Au metal reduction was then performed at 120 °C for 24 h
under vigorous magnetic stirring (500 rpm). The Au/G was recov-
ered by filtration and washed thoroughly with water and with ace-
tone. The Au/G was dried in a vacuum desiccator at 110 °C to
remove the remaining water. Analysis of gold present on the pow-
ders was determined by ICP-OES (Agilent Technologies, 700 Series)
by suspending the powders in aqua regia at room temperature for
[
29]. Thus, using MgO as a support for Au NPs, high biphenyl yield
for iodobenzenes and bromobenzenes were reached, but for
chlorobenzenes, poor yields were obtained. The catalytic activity
of Au NPs-graphene oxide nanocomposite has also been reported
for Suzuki-Miyaura coupling reaction of chlorobenzene and
phenylboronic acid [30]. Interestingly, the reaction of phenyl-
boronic acid with bromobenzene and chlorobenzene in the pres-
ence of Au NPs/graphene oxide nanocomposite was reported to
be 95% and 96% yield of biphenyl, respectively, while the use of
iodobenzene gave 38% of the yield. Therefore, some unexpected
lower activity of iodobenzene observed and attributed to the struc-
ture and properties of the support.
Considering the present interest in Au catalysis [34] and the
contrasting reports on the reactivity order of aryl halides, it is still
pertinent to provide a rationalization of the relative reactivity of
aryl halides in Au catalysis and what can be the influence of the
preferential crystal morphology, facet orientation and strong graft-
ing on the graphene sheet on the activity of Au NPs.
In the present manuscript the existing data about the activity of
supported Au NPs have been complemented by showing that Au
grafted on graphene may exhibit three orders of magnitude higher
activity than an analogous Pd-graphene catalyst and that Au-
graphene presents an opposite reactivity order than Pd-graphene
for aryl halides, catalyzing the coupling of chlorobenzene with
phenylboronic acid with higher yields than the couplings of bro-
mobenzene and iodobenzene. This reverse product yield of Au vs.
Pd has not been reported in the literature so far and it will be
shown to derive from the strong poisoning effect of iodide, and
in lesser extent by Br, on Au NPs. A rationale for this poisoning
effect is proposed based on kinetics study and reported quantum
chemical calculations on models of these Au NPs.
Another aspect of our study is to show that 111-facet oriented
Au nanoplatelets supported on G exhibit, in spite of their much
large particle size, about one order of magnitude higher catalytic
activity in the Suzuki-Miyaura coupling than analogous small size
Au NPs prepared by the polyol method and supported on G lacking
this strong grafting and orientation. In fact, herein we report activ-
ity for Au platelets with particle size 20–40 nm that according to
the literature should be catalytically inactive [28].
3
h and analyzing the Au content of the resulting solution. Pd/G
and Pt/G were prepared by following the same procedure using
Pd(OAc) (5.5 mg) and Na PtCl O (6.0 mg), respectively.
Á6H
2
2
6
2
2
(
.1.3. Synthesis of oriented Au NPs over few-layers graphene films
Au/fl-G)
.5 g of chitosan from Aldrich (low molecular weight) was dis-
0
solved in water by adding acetic acid (0.23 g). Impurities accompa-
nying the commercial sample of chitosan were removed by
filtrating the solution through 0.45
lm syringe. The viscous solu-
tion (500 L) was cast as nanometric film on quartz plates
l
2
(
2 Â 2 cm ) spinning at a rate of 4000 rpm during 1 min. Then,
the resulting chitosan film was immersed in a HAuCl solution
0.01 mM) during 1 min to adsorb Au. Similar procedure was fol-
4
(
2
+
2À
6
lowed to incorporate Pd and PtCl
PdCl and Na PtCl , respectively. Pyrolysis of the resulting alginate
using 0.01 mM solutions of
2
2
6
films containing the transition metals was performed under argon
atmosphere heating according to the following temperatures and
times: ramp 5 °C/min up to 900 °C with a holding time of 2 h. Anal-
ysis of metals present on the films was made by immersing the
plates at room temperature into aqua regia for 3 h and determining
the Au or Pt content of the resulting solution by ICP-OES.
2.2. General procedure for the Suzuki coupling of phenylboronic acid
with halobenzenes
All reagents were purchased from Sigma-Aldrich and used as
received without further purification. To a solution of halobenzene,
(
2
2 mmol) in 20 mL of deionized H O was added to phenylboronic
acid (2.4 mmol), NaOH (8 mmol) and catalyst. The catalyst was
suspended by sonication in the case of Au/G powders or introduced
2
in the reactor as 1 Â 1 cm film on quartz in the case of Au/fl-G. The
resulting mixture was stirred under reflux at 80 °C for 24 h. The
reaction mixture was filtered, then extracted with diethyl ether
(3 Â 10 mL) and the combined organic layer was dried over Na
2
-
SO , filtered and concentrated. All samples were derivatized to
4
their trimethylsilyl ethers with BSTFA+TMCS (99:1) before analysis
on GC-MS. The products were analyzed and identified using GC-MS
(
THERMO Electron Corporation instrument).
2
. Experimental section
The analysis of the reaction products was carried out by GC-MS
THERMO Electron Corporation instrument), Trace GC Ultra and
(
2
2
.1. Catalysts preparation
DSQ, TraceGOLD: TG-5SilMS column with the following charac-
m working with a temperature
teristic: 30 m  0.25 mm  0.25
l
.1.1. Synthesis of few-layers graphene (fl-G)
Sodium alginate (Sigma) was pyrolyzed under argon
program (50 °C (2 min) to 250 °C at 10 °C/min (Hold 10 min) for
a total run time of 32 min) at a pressure of 0.38 Torr with He as
the carrier gas. MS identification of the products: Biphenyl: GC-
atmosphere heating according to the following program: annealing
at 200 °C for 2 h holding time, ramp at 10 °C/min up to 900 °C with
6
+
MS, (m/z): 154 (M , 100), 128 (6), 115 (4), 76 (14), 63 (5), 51 (6);
+
h
holding time. The resulting carbonaceous residue was
o-iodobiphenyl: GC-MS, (m/z): 280 (M , 33), 152 (100), 127 (14),
Candu, Natalia
